ANALYSIS OF ALTERNATIVES non-confidential report
Legal name of applicant(s): LANXESS Deutschland GmbH in its legal capacity as Only Representative of LANXESS CISA (Pty) Ltd.; Atotech Deutschland GmbH; Aviall Services Inc.; BONDEX TRADING LTD in its legal capacity as Only Representative of Aktyubinsk Chromium Chemicals Plant, Kazakhstan; CROMITAL S.P.A. in its legal capacity as Only Representative of Soda Sanayii A.S.; Elementis Chromium LLP in its legal capacity as Only Representative of Elementis Chromium Inc; Enthone GmbH.
Submitted by: LANXESS Deutschland GmbH in its legal capacity as Only Representative of LANXESS CISA (Pty) Ltd.
Substance: Chromium trioxide EC No: 215-607-8, CAS No: 1333-82-0
Use title: Surface treatment for applications in the aeronautics and aerospace industries, unrelated to Functional chrome plating or Functional chrome plating with decorative character
Use number: 4
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Disclaimer
This document shall not be construed as expressly or implicitly granting a license or any rights to use related to any content or information contained therein. In no event shall applicant be liable in this respect for any damage arising out or in connection with access, use of any content or information contained therein despite the lack of approval to do so.
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CONTENTS
1. SUMMARY ...... 1 2. INTRODUCTION ...... 6 2.1. The substance ...... 6 2.2. Uses of chromium trioxide ...... 6 2.3. Purpose and benefits of chromium trioxide ...... 6 3. ANALYSIS OF SUBSTANCE FUNCTION...... 8 3.1. Usage ...... 8 3.2. Surface treatment processes ...... 13 3.2.1. Pre-treatment processes ...... 15 3.2.1.1 Functional cleaning ...... 15 3.2.1.2 Pickling and etching ...... 15 3.2.1.3 Deoxidising ...... 16 3.2.1.4 Stripping ...... 17 3.2.2. Main surface treatment processes...... 17 3.2.2.1 Chemical conversion coating (including phosphate conversion coating: phosphating) ...... 17 3.2.2.2 Chromic acid anodising (CAA) ...... 18 3.2.2.3 Passivation of stainless steel ...... 19 3.2.2.4 Sacrificial coatings ...... 19 3.2.2.5 Slurry (diffusion) coatings ...... 20 3.2.3. Post-treatment processes ...... 20 3.2.3.1 Sealing after anodizing ...... 20 3.2.3.2 Passivation of metallic coatings ...... 21 3.2.3.3 Chromium trioxide rinsing (after phosphating) ...... 21 3.3. Key chromium trioxide functionalities in surface treatment processes ...... 21 3.3.1. Pre-treatments - key functionalities ...... 22 3.3.1.1 Functional cleaning, pickling, etching ...... 22 3.3.1.2 Deoxidising ...... 22 3.3.1.3 Stripping of inorganic finishes (e.g. conversion coatings, anodic coatings) ...... 22 3.3.1.4 Chemical stripping of organic coatings (e.g. primers, topcoats and specialty coatings) ...... 23 3.3.2. Key functionalities of chromium trioxide-based main processes & post-treatments ...... 23 3.3.3. Key functionalities in the aerospace sector ...... 25 4. ANNUAL TONNAGE...... 29 4.1. Annual tonnage band of chromium trioxide ...... 29 5. GENERAL OVERVIEW ON THE SPECIFIC APPROVAL PROCESS IN THE AEROSPACE SECTOR ...... 30 5.1. General overview ...... 30 5.2. Development and qualification ...... 33 5.2.1. Requirements development ...... 33 5.2.2. Technology development ...... 34 5.2.3. Qualification ...... 36 5.2.4. Certification ...... 37 5.2.5. Implementation / industrialisation ...... 38 5.2.6. Examples ...... 40 6. IDENTIFICATION OF POSSIBLE ALTERNATIVES ...... 41 6.1. Description of efforts made to identify possible alternatives ...... 41 6.1.1. Research and development ...... 41 6.1.2. Data searches ...... 42 6.2. Consultations ...... 42 6.3. List of possible alternatives ...... 42 7. SUITABILITY AND AVAILABILITY OF POSSIBLE ALTERNATIVES ...... 44 7.1. Main Processes & Post-treatments ...... 44 CATEGORY 1 ALTERNATIVES ...... 44 7.1.1. ALTERNATIVE 1: Acidic surface treatments ...... 44 7.1.1.1 Substance ID and properties ...... 44 7.1.1.2 Technical feasibility ...... 45 7.1.1.3 Economic feasibility ...... 51 7.1.1.4 Reduction of overall risk due to transition to the alternative ...... 51 7.1.1.5 Availability (R&D status, timeline until implementation) ...... 51
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7.1.1.6 Conclusion on suitability and availability for acidic surface treatments ...... 52 7.1.2. Alternative 2: Cr(III)-based surface treatments ...... 53 7.1.2.1 Substance ID and properties ...... 53 7.1.2.2 Technical feasibility ...... 53 7.1.2.3 Economic feasibility ...... 58 7.1.2.4 Reduction of overall risk due to transition to the alternative ...... 58 7.1.2.5 Availability (R&D status, timeline until implementation) ...... 58 7.1.2.6 Conclusion on suitability and availability for Cr(III)-based processes ...... 59 7.1.3. ALTERNATIVE 3: Silane/Siloxane and sol-gel coatings ...... 59 7.1.3.1 Substance ID and properties ...... 59 7.1.3.2 Technical feasibility ...... 61 7.1.3.3 Economic feasibility ...... 64 7.1.3.4 Reduction of overall risk due to transition to the alternative ...... 64 7.1.3.5 Availability (R&D status, timeline until implementation) ...... 64 7.1.3.6 Conclusion on suitability and availability for Silane/Siloxane and sol-gel coatings ...... 65 7.1.4. ALTERNATIVE 4: Water-based post-treatments (Hot water sealing, Rinsing after Phosphating)...... 65 7.1.4.1 Substance ID and properties ...... 65 7.1.4.2 Technical feasibility ...... 66 7.1.4.3 Economic feasibility ...... 69 7.1.4.4 Reduction of overall risk due to transition to the alternative ...... 69 7.1.4.5 Availability (R&D status, timeline until implementation) ...... 69 7.1.4.6 Conclusion on suitability and availability for Water-based post-treatments ...... 69 CATEGORY 2 ALTERNATIVES ...... 70 7.1.5. ALTERNATIVE 5: Manganese-based processes ...... 70 7.1.5.1 Substance ID and properties ...... 70 7.1.5.2 Technical feasibility ...... 70 7.1.5.3 Economic feasibility ...... 72 7.1.5.4 Reduction of overall risk due to transition to the alternative ...... 72 7.1.5.5 Availability (R&D status, timeline until implementation) ...... 72 7.1.5.6 Conclusion on suitability and availability for manganese-based processes ...... 73 7.1.6. ALTERNATIVE 6: Magnesium rich primers ...... 73 7.1.6.1 Substance ID and properties ...... 73 7.1.6.2 Technical feasibility ...... 74 7.1.6.3 Economic feasibility ...... 76 7.1.6.4 Reduction of overall risk due to transition to the alternative ...... 76 7.1.6.5 Availability (R&D status, timeline until implementation) ...... 76 7.1.6.6 Conclusion on suitability and availability for Mg-rich primers ...... 76 7.1.7. ALTERNATIVE 7: Molybdates and Molybdenum-based processes...... 77 7.1.7.1 Substance ID and properties ...... 77 7.1.7.2 Technical feasibility ...... 77 7.1.7.3 Economic feasibility ...... 78 7.1.7.4 Reduction of overall risk due to transition to the alternative ...... 78 7.1.7.5 Availability (R&D status, timeline until implementation) ...... 79 7.1.7.6 Conclusion on suitability and availability for molybdates and molybdenum-based processes ...... 79 7.1.8. ALTERNATIVE 8: Organometallics (Zr- and Ti-based products) ...... 79 7.1.8.1 Substance ID and properties ...... 79 7.1.8.2 Technical feasibility ...... 80 7.1.8.3 Economic feasibility ...... 81 7.1.8.4 Reduction of overall risk due to transition to the alternative ...... 81 7.1.8.5 Availability (R&D status, timeline until implementation) ...... 82 7.1.8.6 Conclusion on suitability and availability for organometallics ...... 82 7.1.9. ALTERNATIVE 9: Electrolytic paint technology ...... 82 7.1.9.1 Substance ID and properties ...... 82 7.1.9.2 Technical feasibility ...... 83 7.1.9.3 Economic feasibility ...... 84 7.1.9.4 Reduction of overall risk due to transition to the alternative ...... 84 7.1.9.5 Availability (R&D status, timeline until implementation) ...... 85 7.1.9.6 Conclusion on suitability and availability for Electrolytic paint technology ...... 85 7.1.10. ALTERNATIVE 10: Zinc-nickel electroplating ...... 85 7.1.10.1 Substance ID and properties ...... 85
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7.1.10.2 Technical feasibility ...... 86 7.1.10.3 Economic feasibility ...... 86 7.1.10.4 Reduction of overall risk due to transition to the alternative...... 87 7.1.10.5 Availability (R&D status, timeline until implementation) ...... 87 7.1.10.6 Conclusion on suitability and availability for zinc-nickel electroplating ...... 87 7.1.11. ALTERNATIVE 11: Benzotriazole-based processes, e.g. 5-methyl-1H-benzotriazol ...... 87 7.1.11.1 Substance ID and Properties ...... 87 7.1.11.2 Technical feasibility ...... 88 7.1.11.3 Economic feasibility ...... 88 7.1.11.4 Reduction of the overall risk due to transition to the alternative ...... 88 7.1.11.5 Availability (R&D status, timeline until implementation) ...... 88 7.1.11.6 Conclusion on suitability and availability for benzotriazole-based processes...... 89 7.2. Pre-treatments ...... 89 7.2.1. Inorganic acids ...... 89 7.2.1.1 Substance ID and properties ...... 89 7.2.1.2 Technical feasibility ...... 89 7.2.1.3 Economic feasibility ...... 93 7.2.1.4 Reduction of overall risk due to transition to the alternative ...... 93 7.2.1.5 Availability (R&D status, timeline until implementation) ...... 93 7.2.1.6 Conclusion on suitability and availability for inorganic acids ...... 94 8. OVERALL CONCLUSIONS ON SUITABILITYAND AVAILABILITY OF POSSIBLE ALTERNATIVES ... 95 9. REFERENCES...... 97 APPENDIX 1 – INITIAL LIST OF POTENTIAL ALTERNATIVES TO CHROMIUM TRIOXIDE CONTAINING SURFACE TREATMENTS ...... 99 APPENDIX 2 – GENERAL INFORMATION AND THE RISK FOR HUMAN HEALTH AND THE ENVIRONMENT FOR RELEVANT SUBSTANCES ...... 102 APPENDIX 2.1: MAIN PROCESSES AND POST-TREATMENTS ...... 102 APPENDIX 2.2: PRE-TREATMENTS: CLEANING, PICKLING, ETCHING-CR(VI)-FREE ALTERNATIVES .. 119 APPENDIX 2.3: SOURCES OF INFORMATION ...... 123
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List of Figures:
Figure 1: Surface Treatment Processes steps where chromium trioxide might be involved...... 2 Figure 2: Illustration of the development, qualification, certification and industrialisation process required in the aerospace sector...... 4 Figure 3: ATR 600 aircraft & Gulfstream V aircraft (UTC Aerospace Systems – Propeller Systems, 2014) ...... 9 Figure 4. Schematic illustration of typical corrosion findings in an aircraft fuselage. (Airbus Group, 2014)...... 10 Figure 5: Propellers mounted on an aircraft. (UTC Aerospace Systems – Propeller Systems, 2014) ...... 10 Figure 6: Undercarriage - landing gear, examples (Rowan Technology Group, 2005) ...... 11 Figure 7: Gas Turbine Engine sketch example PW4000 92 inch fan engine (www.pw.utc.com/Content/PW400094_Engine/img/B-1-4-1_pw400094_cutaway_high.jpg, 2014)...... 11 Figure 8: Emergency door damper for a civil aircraft. (UTC Aerospace Systems – Propeller Systems, 2014) ...... 11 Figure 9: Surface treatment processes steps where chromium trioxide might be involved ...... 13 Figure 10: Schematic anodic coating after acidic anodising (Hao & Cheng, 2002) ...... 19 Figure 11: Illustration of the qualification, certification and industrialisation processes...... 31 Figure 12: Illustration of the technology development and qualification process. (EASA, 2014; amended) ...... 36 Figure 13: Morphology of aluminium surfaces treated with TSA, CAA and PSA (Airbus Fast report, 2009)...... 45 Figure 14: Formation of boehmite during hot water sealing of anodised aluminium surface (Hao & Cheng, 2000) .. 66 Figure 15: Aluminium test panels after 144 cycles accelerated cyclic, acidified salt spray test according to ASTM G85, Method A2, A: Al with chromate-based sealing, B: Cross-scribed Al with chromate-based sealing, C: Al with hot water sealing, D: Cross-scribed Al with hot water sealing. (GE Aviation, 2014) ...... 67 Figure 16: Cathodic protection by the Sacrificial Anode method (Pathak et al, 2012) ...... 74 Figure 17: Surface morphology of the scratched coating samples after immersion in 3%NaCl solution for 150 days: A) 50Mg; B) 40Mg 10Al; C) 30Mg 20Al; D) 20Mg 30Al; E)10Mg 40Al; F)50Al (Wang et al, 2013) ...... 75 Figure 18: Electrodeposition process, cathodic and anodic deposition (from Pawlik M. 2009) ...... 82 Figure 19: Components of an electrocoat conveyor process (Pawlik, 2009) ...... 83
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List of Tables
Table 1: Overview of key potential alternatives for main surface treatments ...... 4 Table 2: Substance of this analysis of alternatives...... 6 Table 3: Corrosion prone areas on different types of aircraft ...... 8 Table 4: Overview of surface treatment processes indicating most important application methods, purpose, as well as example products. This is not intended to be an exhaustive list...... 14 Table 5: Key requirements within the aviation sector...... 25 Table 6: Technology Readiness Levels – Overview (US Department of Defence, 2011, adapted 2014)...... 31 Table 7: List of main treatment alternatives categorised ...... 43 Table 8: List of pre-treatment alternatives categorised...... 43 Table 9: Overview on the acids used in the different surfaces treatment processes ...... 45 Table 10: Process relevant criteria of CAA, PSA and TSA (Airbus Fast report, 2009) ...... 46 Table 11: Sector specific overview on chromium trioxide-based surface treatments processes where Cr(III)-based technics are evaluated ...... 53 Table 12: Some commonly used alkoxysilane precursors for sol-gel coatings ...... 60 Table 13: Chromium trioxide-based surface treatments processes where sol-gel coatings may be an alternative ...... 61 Table 14: Chromium trioxide-based surface treatments processes where water-based post-treatments may be an alternative ...... 66 Table 15: Chromium trioxide-based surface treatments processes where manganese-based products may be an alternative ...... 70 Table 16: Chromium trioxide-based surface treatments where Mg rich primers may be an alternative ...... 74 Table 17: Chromium trioxide-based surface treatments where molybdate-based processes may be an alternative .... 77 Table 18: Chromium trioxide-based surface treatments where fluorotitanic and fluorozirconic-based products may be an alternative ...... 80 Table 19. Chromium trioxide-based surface treatments where electrocoat systems may be an alternative ...... 83 Table 20: Chromium trioxide-based surface treatment processes where Zinc-Nickel electroplating may be an alternative ...... 86 Table 21: Chromium trioxide-based surface treatment processes where benzotriazoles may be an alternative ...... 88 Table 22: Overview on the replacement substances used in the different pre-treatment processes ...... 89
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Abbreviations AA2024 Aluminium alloy, most commonly used in the aerospace sector ACF Airbus Chromate Free Al Aluminium Acute Tox. Acute Toxicity AMMTIAC Advanced Materials, Manufacturing, and Testing Information Analysis Center Asp. Tox. Aspiration hazard ASTM American Society for Testing Materials AoA Analysis of Alternatives app. approximately Aquatic Acute Hazardous to the aquatic environment Aquatic chronic Hazardous to the aquatic environment with long lasting effects BSA Boric-Sulphuric Acid anodizing CAA Chromic Acid Anodizing Carc. Carcinogenicity CAS unique numerical identifier assigned by Chemical Abstracts Service (CAS number) CCC Chemical Conversion Coatings Cd Cadmium CFRP Carbon-fibre-reinforced polymer CMR Carcinogenic, Mutagenic and Toxic to Reproduction CPVC Critical Pigment Volume Concentration Cr Chromium Cr(III) Trivalent Chromium Cr(VI) Hexavalent Chromium CRES Corrosion resistant stainless steel CSR Chemical Safety Report CTAC Chromium Trioxide REACH Authorization Consortium DT&E Development, Test and Evaluation EASA European Aviation Safety Agency EC unique numerical identifier of the European Community (EC number) e.g. exempli gratia, for example
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EHS Environmental Health and Safety EMI Electromagnetic Interference EN European Norm EPA Environmental Protection Agency ESA European Space Agency EU European Union Eye Dam. Serious eye damage Eye Irrit. Eye irritation Flam. Liq. Flammable liquid Flam.sol. Flammable solid HITEA Highly Innovative Technology Enablers for Aerospace HVOF High Velocity Oxy Fuel ISO International Organization for Standardization IVD Ion Vapour Deposition Me Metal Met. Corr. Substance or mixture corrosive to metals Mg Magnesium Mil-DTL United States Military Standard MoCC Molybdate-based conversion coatings MRL Manufacturing Readiness Level MRO Maintenance, Repair and Operations MSDS Material Safety Data Sheet Muta. Germ cell mutagenicity NASA National Aeronautics and Space Administration NDSU North Dakota State University Ni Nickel NSST Neutral Salt Spray Test OEM Original Equipment Manufacturer OT&E Operational Test and Evaluation Ox. Liq. Oxidising liquid Ox. Sol. Oxidising solid PSA Phosphoric Sulphuric Acid Anodizing
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Pyr. Liq. Pyrophoric liquid Pyr. Sol. Pyrophoric solid QPL Qualified Products List REACH Registration, Evaluation, Authorisation and Restriction of Chemicals R&D Research and Development Repr. Reproductive toxicity Resp. Sens. Respiratory sensitiser RoHS Directive on Restriction of Hazardous Substances SAA Sulphuric Acid Anodizing SEA Socio Economic Analysis Self-heat. Self-heating substances or mixture Self-react. Self-reactive substances or mixture Skin. corr. Skin corrosion Skin. Sens. Skin sensitisation Skin irrit. Skin irritation Sn Tin SST Salt Spray Test STC Supplemental Type Certificate STOT RE Specific target organ toxicity, repeated exposure STOT SE Specific target organ toxicity, single exposure SVHC Substance of Very High Concern Ti Titanium TRL Technology Readiness Level TSAA Tartaric-Sulphuric-Acid-Anodizing US United States VOC Volatile Organic Compounds VTMS Vinyl trimethoxysilane
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Glossary
Term Definition The ability of a material to spontaneously repair small amounts of chemical or mechanical damage that exposes areas of metal without any Active corrosion inhibition surface protection (“self-healing properties”). This functionality is advantageous and enhances service life duration of parts, maintenance intervals and on-flight security of air travellers. Parameter describes the tendency of dissimilar particles or surfaces to Adhesion promotion cling to one another (for example adhesion of coating to substrate, adhesion of paint to coating and/or substrate). This terms comprises civil and military applications of aviation and Aerospace space industry. Electrolytic oxidation process in which the surface of a metal, when anodic, is converted to a insulating coating having desirable protective Anodizing or functional properties. The anodic film formation is mainly driven by the applied voltage. Chromic acid anodizing is one example of anodizing. Typical method for surface treatment of parts. May also be referred to Bath as dipping or immersion. None-bath methods include wiping, spraying, and pen application. The process where two parts are joint together by means of a bonding Bonding material; an adhesive sometimes in combination with a bonding primer and a conversion or anodizing treatment. Potential alternative provided to the Aerospace OEM for their Candidate alternative evaluation. These have already been evaluated in the labs of formulators. Alternative considered promising, where considerable R&D efforts Category 1 alternative have been carried out within the different industry sectors. Alternative with clear technical limitations which may only be suitable Category 2 alternative for niche applications and not as a general alternative. Alternative which has been screened out at an early stage of the Category 3 alternative Analysis of Alternatives and which is not applicable for the use defined here. Verification that an aircraft or spacecraft and every part of it complies Certification with all applicable airworthiness regulations and associated Certification Specifications (specs). Parameter is defined as the ability of solid materials to resist damage by Chemical resistance chemical exposure. When brought in contact with water, chromium trioxide forms two acids and several oligomers: chromic acid, dichromic acid, and oligomers of Chromic acid chromic acid and dichromic acid. For the purpose of this document the terms chromic acid is synonymous with a mixture containing chromium trioxide and water. This is intended to be in line with ECHA Q&A #805.
Chromium trioxide rinsing after Chromium trioxide rinsing after phosphating is a passivation process phosphating) after phosphate conversion coating (phosphating). It fulfils two requirements by using only one process step (removal of drag-out
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Term Definition comprising liquids and residuals of former processes adhering to the substrate and passivation of the surface by enhancing corrosion resistance. Surface preparation for subsequent processing including removal of dirt Cleaning and oil. The term has some overlap with the definitions of pickling and deoxidising. A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be Coating decorative, functional, or both. A coating may be a paint, a lacquer or a metal (e.g. hard chrome, cadmium coating, zinc-nickel coating) or an inorganic substance. Chemical process applied to a substrate producing a superficial layer containing a compound of the substrate metal and an anion of an Conversion Coating environment. Note that within the surface finishing industry a conversion coating is sometimes referred to as a passive coating or passivation. Means applied to the metal surface to prevent or interrupt oxidation of the metal part leading to loss of material. This can be a metal conversion Corrosion protection coating or anodizing, a pre-treatment, paint, water repellent coating, sealant, liquid, adhesive or bonding material. The corrosion protection provides corrosion resistance to the surface. Structural zone (like assembly, component) to which a given Counterpart assembly/part is fitted. Deoxidising is a pre-treatment step required to activate the surface prior to further processing i.e. to remove surface oxides. The term Deoxidising deoxidising is often used interchangeably with pickling. Very little metal is removed during deoxidising. Removal of residue that is often left over from etching processes. Desmutting Desmutting is often grouped with cleaning, deoxidizing or pickling. Process that changes surface as well as removes material. This term has Etching significant overlap with the term pickling. After having passed qualification and certification, the third step is to implement or industrialise the qualified material or process in all Implementation relevant activities and operations of production, maintenance and the supply chain. In-service evaluations are common practice to validate accelerated corrosion results obtained in the laboratory to determine correlation In-service evaluation between accelerated corrosion testing and when used on operating aircraft. Legacy part A legacy part shall mean any part of an end product for aerospace which is manufactured in accordance with a type certification applied for before the earliest sunset date (including any further supplemental or amended type certificates or a derivative) or for defence and space which is designed in accordance with a military or space development contract signed before the earliest sunset date, and including all production, follow-on development, derivative and modification
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Term Definition program contracts, based on that military or space development program. The purpose of the surface treatment is primarily for, but not limited to, corrosion protection. The main treatment occurs after the pre-treatment and before the post-treatment. Examples include conversion coating, Main treatment anodizing and passivation of stainless steel. Sometimes conversion coating and anodizing are followed by painting; in which case these can be regarded as the pre-treatment and the painting as the main treatment. Portion of a specification that controls which materials may be used in Materials control the process. Products that have met all requirements may be added to this list by the OEM. Process providing corrosion protection to a substrate or a coating. Note that within the surface finishing industry a passive or passivation Passivation coating is often referred to as a conversion coating. Both terms are used in this document. Is designed to both remove embedded iron/steel particles from stainless Passivation of stainless steel steel and oxidise the surface chromium in the alloy to augment its natural corrosion resistant passive oxide layer. Metallic coatings applied on steel (such as cadmium, zinc, zinc-nickel, or aluminium) need to be passivated for corrosion protection. Passivation of metallic coatings Technically, this kind of passivation is a conversion coating and the process is a post-treatment applied after the application of the non- chromium metal coating. A non-chromium trioxide conversion process containing metal phosphates used mainly for some ferrous substrates and is generally Phosphating used a key for subsequent painting, oiling or lubrication films. It sometimes requires a chromium trioxide-based post-treatment (rinsing after phosphating). Pickling is the removal of oxides or other compounds from a metal surface by chemical or electrochemical action. The term pickling is not used consistently with the surface finishing industry and is often Pickling referred to as the following processes: cleaning, scale removal, scale conditioning, deoxidizing, etching, and passivation of stainless steel. This term has overlap with the term Etching. Post-treatment processes are performed after the main surface treatment Post-treatment process to enhance corrosion protection. Pre-treatment processes are used to remove contaminates (e.g. oil, grease, dust), oxides, scale, and previously applied coatings (e.g. Pre-treatment electroplated coatings, anodize coatings, conversion coatings, paint). The pre-treatment process must also provide chemically active surfaces for the subsequent treatment. A series of surface treatment process steps. The individual steps are not stand-alone processes. The processes work together as a system, and Process chain care should be taken not to assess without consideration of the other steps of the process. In assessing candidate alternatives for chromium trioxide, the whole process chain has to be taken into account.
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Term Definition OEM validation and verification that all material, components, equipment or processes have to meet or exceed the specific performance Qualification requirements which are defined in the certification specifications documented in technical standards or specifications. Numerous aerospace applications require an electrical conductive Resistivity coating for the respective use. Classification and labelling information of substances and products reported during the consultation being used for alternatives / alternative Risk reduction processes are compared to the hazard profile of the used chromium trioxide. Business partnership in which costs and benefits are shared amongst all participating partners. The intention is to rely on the commercial success, while reducing the risk of loss. For the aerospace industry, risk- Risk sharing partners sharing arrangements where made with suppliers to reduce investments and the dependence on loans. The suppliers are responsible for design activities, development and manufacture of major components or systems. Material to fill gaps or joints or to exclude the environment in order to prevent electrochemical corrosion between two parts with dissimilar Sealant material composition (metal-metal and metal-carbon composite) or crevice corrosion. This can be applied by means of spatula, extrusion, brush or spray. For a high corrosion resistance micropores of the anodized surface have Sealing to be closed by a post-treatment step (sealing after anodizing). Removal of coatings prior to rework. Differentiation based on the kind Stripping of coating removed (stripping or inorganic finishes, stripping or organic coatings). This Use includes processes that convert the surface of an active metal or coat metal surfaces by forming/incorporating a barrier film of Surface treatment for complex chromium compounds that protects the metal from corrosion applications in the aeronautics and provides a base for subsequent treatments such as painting or and aerospace industries, bonding. This includes integrated process systems where chromium unrelated to Functional chrome trioxide is used in a series of pre/main/post-treatments. Pre-treatment plating or Functional chrome includes processes such as chemical polishing, stripping, dexodizing, plating with decorative character pickling and etching of metals. Main-treatment includes processes such (hereafter referred to as surface as conversion coatings, passivation and anodizing, deposition and other treatment for the aero sector) surface treatments where a chromium trioxide-based solution is used. Post-treatment includes processes such as rinsing, staining and sealing for final surface protection.
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1. SUMMARY This Analysis of Alternatives (AoA) forms part of the Application for Authorisation (AfA) for the use of chromium trioxide in the surface treatment of metals. The use, as defined, covers a number of surface treatment processes and steps1 that may be applied to a number of different metal substrates (e.g. aluminium, steel, zinc, magnesium, titanium, alloys and composites with metallic areas). The use is also intended to cover the downstream use of chromic acid and dichromic acid (in-line with ECHA Q&A #805). Surface treatment aims to modify the surface of a substrate so that it performs better under conditions of use. Surface treatment processes using chromium trioxide typically involve immersion of the metal component in each of a series of treatment baths containing chemical solutions or rinses under specific operating conditions2. Different chemicals and operating conditions are specified for individual surface treatment processes (see Figure 1) in order to effectively treat different substrates and/or confer specific performance characteristics to the treated article. The relevant surface treatment processes which the AfA covers, the characteristics of chromium trioxide and its critical functionality in each of the treatment processes are introduced at chapter 3. The aerospace sector specify surface treatment with chromium trioxide in order to meet strict performance criteria necessary for regulatory compliance and for public safety, as described further below and in chapter 5. This summary aims to shortly explain why use of chromium trioxide in surface treatment is essential to the aerospace sector. It describes the steps and effort involved in finding and approving a replacement for chromium trioxide in these applications and evaluates potential alternatives in detail (chapter 6 and 7).
Chromium trioxide-based surface treatment systems Chromium trioxide has been used for more than 50 years to provide surface protection to critical components and products within the aerospace sector, where the products to which they are applied must operate to the highest safety standards in highly demanding environments for extended time periods. Surface treatments based on chromium trioxide have unique technical functions that confer substantial advantage over potential alternatives. These include: - Outstanding corrosion protection and prevention for nearly all metals under a wide range of conditions; - Active corrosion inhibition (self-healing, e.g. repairing a local scratch to the surface); - Excellent adhesion properties to support application of subsequent coatings or paints; and - Excellent chemical and electrical resistivity. The chemistry behind chromium trioxide surface treatment systems and processes is complex. Surface treatment processes typically involve numerous steps, often including several important pre- treatment and post-treatment steps as well as the main treatment process itself. Figure 1 provides an overview of the applications included in the application for authorisation according to the process steps. These steps are almost always inter-related such that they cannot be separated or individually modified without impairing the overall process or performance of the treated product.
1 See Chapter 3 for detail. 2 Occasional localised ‘touch-up’ of a surface may be made by brush or a pen-stick, as explained in the CSR
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Use Description Surface Treatment of Metal Parts
Pre-treatment Post-treatment
• Cleaning Treatment • Sealing after Anodizing • Pickling/Etching • Conversion Coating • Rinsing • Deoxidising • Anodizing • Passivation of metallic • Stripping coatings on steel
Figure 1: Surface Treatment Processes steps where chromium trioxide might be involved This means that while the use of chromium trioxide may be specified at different points in the process, it cannot be entirely replaced in the process without impacting the technical performance of the final article. The implications of this are important as chromium trioxide-free alternatives for some individual steps are available and used by industry. However, where this is the case, chromium trioxide is mostly specified in one of the other steps within the overall surface treatment system. As of today, no complete chromium trioxide-free treatment system, providing all the required properties to the surfaces of all articles in the scope of this application, is industrially available. This means it is imperative to consider the surface treatment system as a whole, rather than the step involving chromium trioxide on its own, when considering alternatives for such surface treatment systems. Furthermore, components that have been surface treated with chromium trioxide typically represent just one of many critical, inter-dependent elements of a component, assembly or system. In general, chromium trioxide-based surface treatment is specified as one element of a complex system with integrated, often critical performance criteria. Compatibility with and technical performance of the overall system are primary considerations of fundamental importance during material specification.
Use of chromium trioxide in surface treatment for the aerospace sector Chromium trioxide-based surface treatments are specified in the aerospace sector because they provide superior corrosion resistance and inhibition, improved paint adhesion, low electrical contact resistance and/or enhanced wear-resistance (see chapter 3.3). These characteristics are essential to the safe operation and reliability (airworthiness) of aircraft and spacecraft which operate under extreme environmental conditions. These structures are extremely complex in design, containing millions of highly specified parts, many of which cannot be easily inspected, repaired or removed. Structural components (e.g. landing gear, fasteners) and engine parts (e.g. internal components for gas turbines) on aircraft are particularly vulnerable to corrosion. Chromium trioxide surface treatment processes and performance have been successively refined and improved as a result of many decades of research and experience in the sector, and reliable data is available to support their performance. While corrosion cannot be totally prevented, despite the highly advanced nature of chromium trioxide-based coating systems in place today, there is also extensive experience, amassed over decades, on the appearance and impact of corrosion to support its effective management in these systems. On the other hand, while several potential alternatives to chromium trioxide, predominantly Cr(III)- and mineral acid-based systems, are being investigated for different processes, substrates and treatment steps, results so far do not support reliable conclusions regarding their performance as part of such complex systems, in demanding environments and test conditions representative of in-service situations. These potential alternatives do not support all the properties of chromium trioxide-based surface treatment systems, and their long-term performance can currently
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Identification and evaluation of potential alternatives for the aerospace sector An extensive literature survey and consultation with aerospace industry experts was carried out to identify and evaluate potential alternatives to chromium trioxide. A total of 33 potential alternatives for all parts of the process chain were identified. 11 potential alternatives (including processes and substances for all parts of the process chain) are a focus for ongoing research and development (R&D) programs and are examined in further detail in this report. Here, a candidate alternative is defined as a potential alternative provided to the aerospace manufacturer for evaluation following initial evaluation by the formulator. Table 1 at the end of this section summarises the main findings of the AoA for the aerospace sector. The various main treatment and post-treatments processes and identified potential or candidate alternatives are discussed in chapter 7.1, while the pre-treatments processes are discussed in chapter 7.2. In summary, the analysis shows there are no technically feasible alternatives to chromium trioxide- based surface treatment systems for all key applications in the aerospace sector. Several potential alternatives are subject to ongoing R&D, but do not currently support the necessary combination of key functionalities to be considered technically feasible alternatives.
Ongoing development of potential alternatives for the aerospace sector Assuming a technically feasible potential alternative is identified as a result of ongoing R&D, extensive effort is needed beyond that point before it can be considered an alternative to chromium trioxide within the aerospace industry. Aircraft are one of the safest and securest means of transportation, despite having to perform in extreme environments for extended timeframes. This is the result of high regulatory standards and safety requirements. The implications for substance substitution in the aerospace industry are described in detail in a report prepared by ECHA and European Aviation Safety Agency (EASA) in 2014, which sets out a strong case for long review periods for the aerospace sector based on the airworthiness requirements deriving from European Union (EU) Regulation No 216/2008. Performance specifications defined under this Regulation drive the choice of substances to be used either directly in the aircraft or during manufacturing and maintenance activities. It requires that all components, equipment, materials and processes incorporated in an aircraft must be certified, qualified and industrialised before production can commence. The process is illustrated in Figure 2. This system robustly ensures new technology and manufacturing processes can be considered ‘mission ready’ through a series of well-defined steps only completed with the actual application of the technology in its final form (and under mission conditions). When a substance used in a material, process, component, or equipment needs to be changed, this extensive system must be followed in order to comply with airworthiness requirements. The system for alternative development through qualification, certification, industrialization and implementation within the aviation sector is mirrored in the defence and space sectors. The detailed process involved in qualification, certification and industrialisation, and the associated timeframes, are elaborated in chapter 5. Of course, these steps can only proceed once a candidate alternative is identified. Referring to experience, it can take 20 to 25 years to identify and develop a new alternative, even assuming no drawbacks during the various stages of development of these alternatives. Experience over the last 30 years already shows this massively under-estimates the replacement time for chromium trioxide-based surface treatments. Taken together, available evidence clearly shows that no viable alternative for chromium trioxide in surface treatments is expected for at least the next 12 or even 15 years.
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Figure 2: Illustration of the development, qualification, certification and industrialisation process required in the aerospace sector. As a further consideration, while the implications of the development process in the aeronautic and aerospace sectors are clearly extremely demanding, specification of an alternative, once available, can be built into the detailed specification for new aircraft types (and new spacecraft). This is not the situation for existing aircraft types, for which production and/or operation may still be ongoing. Production, maintenance and repair of these models must use the processes and substances already specified following the extensive approval process. Substitution of chromium trioxide-based surface treatment for these ‘legacy’ craft introduces yet another substantial challenge; re-certification of all relevant processes and materials. In practice, it will be impractical and uneconomical to introduce such changes for many such aircraft types. In this context, the scale and intensity of industry- and company- wide investment in R&D activity to identify alternatives to chromium trioxide surface treatment systems is very relevant to the findings of the AoA. Serious efforts to find replacements for chromium trioxide have been ongoing within the aerospace industry for over 30 years and there have been several major programs to investigate alternatives to chromium trioxide in the aerospace sector over the last 20 years. The level of industry investment to date in such activity is estimated to be at least €100million.
Table 1: Overview of key potential alternatives for main surface treatments
Potential Alternative Technical failure - Corrosion resistance not proved for the range of substrates Acidic surface treatments - Does not cover the broad range of different substrates in general - No reproducible results of corrosion resistance on all kind of substrates Organometallics (zirconium and titanium- - No active corrosion inhibition based products) - Adhesion of coating to substrate not sufficient - Corrosion requirements not met Molybdates and Molybdenum-based - No active corrosion inhibition processes - No conductive coating (no resistivity) - Difficult process control - No stand-alone corrosion protection Silane/Siloxane and Sol-gel coating - No conductive coating (resistivity not sufficient) - Limitations to geometry of parts (no complex parts) - Inconsistent corrosion results - Limited active corrosion inhibition Cr(III)-based surface treatments - Inconsistent adhesion (coating to substrate) results with poor quality reproducibility at current stage Manganese-based processes - Corrosion resistance insufficient
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Concluding remarks A large amount of research over the last 30 years has been deployed to identify and develop viable alternatives to chromium trioxide-based surface treatment. Due to its unique functionalities and performance, it is challenging and complex to replace surface treatments based on chromium trioxide (or Cr(VI)-ions derived from chromium trioxide) in applications that demand superior performance for corrosion and/or adhesion to deliver safety over extended periods and extreme environmental conditions. Several potential alternatives to chromium trioxide such as Cr(III)- and mineral acid- based systems, are under investigation for the aerospace industry. However, based on experience and with reference to the status of R&D programs, alternatives are not foreseen to be commercially available for key applications in this sector for at least 12 or 15 years. As a result, a review period of 12 years was selected because it coincides with best case (optimistic) estimates by the aerospace industry of the schedule required to industrialise alternatives to chromium trioxide. It also reflects the duration of the standard long review period indicated by ECHA.
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2. INTRODUCTION
2.1. The substance The following substance is subject to this analysis of alternatives:
Table 2: Substance of this analysis of alternatives.
# Substance Intrinsic property(ies)1 Latest application date² Sunset date³
Chromium trioxide Carcinogenic (category 1A) 1 EC No: 215-607-8 21.03.2016 21.09.2017 Mutagenic CAS No: 1333-82-0 (category 1B)
1 Referred to in Article 57 of Regulation (EC) No. 1907/2006 ² Date referred to in Article 58(1)(c)(ii) of Regulation (EC) No. 1907/2006 3 Date referred to in Article 58(1)(c)(i) of Regulation (EC) No. 1907/2006
This substance is categorized as substance of very high concern (SVHC) and is listed on Annex XIV. Adverse effects are discussed in the Chemical Safety Report (CSR). When brought in contact with water, chromium trioxide (EC No 215-607-8) forms two acids and several oligomers: Chromic acid (EC No 231-801-5), Dichromic acid (EC No 236-881-5), oligomers of chromic acid and dichromic acid (further referred as "Chromic acids and their oligomers"). This AoA discusses many situations where this is the case. For the purpose of this document the terms chromic acid is synonymous with a mixture containing chromium trioxide and water.
2.2. Uses of chromium trioxide Chromium VI containing substances have been widely used since the mid of 20th century. The major uses of chromium trioxide in CTAC for surface treatment for the aero sector are as follows: - Pre-treatment processes (e.g. Functional Cleaning, Pickling, Etching, Deoxidizing, Stripping of various substrates such as Aluminium, Magnesium, steel); - Passivation processes (e.g.: of various types of steel, cadmium, aluminium, magnesium and zinc substances and coatings, alloyed or not); - Chemical conversion coating (CCC) (e.g.: CCC by dip process, brush process or pre- treatment to provide paint adhesion and corrosion protection, CCC by dip process and/or brush process –or with no paint applied afterwards); - Chromic acid anodising (CAA) including associated CrO3 processes (CAA with and without chromium trioxide sealing for corrosion protection of aluminium components; chromium trioxide sealing after chromium trioxide-free anodization); - Sacrificial and diffusion coatings for corrosion protection (e.g.: inorganic aluminium-based slurry coating steels; slurry aluminide coating for sulphidation protection); and - Rinsing after phosphating. An overview of the respective surface treatment processes and their applications can be found in Table 4.
2.3. Purpose and benefits of chromium trioxide Chromium trioxide offers a broad range of functions, mainly based on the characteristics of the Cr(VI) compound. It has been widely used for over 50 years in the industry in various applications. The multifunctionality of chromium trioxide provides major properties to the surfaces treated with the
6 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES respective process. The following key functionalities for the aerospace sector are discussed in more detail in chapter 3.3: - Corrosion resistance: excellent corrosion protection and prevention to nearly all metals in a wide range of environments; - Active corrosion inhibition: when a coating is damaged, e.g. by a scratch exposing the base material to the environment, the solubility properties of chromium trioxide allow diffusion to the exposed area and inhibit corrosion; - Adhesion promotion (adhesion to subsequent coatings or paint), - Providing low electrical contact resistance (resistivity); - Enhancing wear resistance; - Delivering optimal layer thickness (for the respective treatment and purpose); - Enhancing chemical resistance; - Providing biostatic properties; and - Inhibiting the growth and proliferation of biological organisms. Several alternatives are being tested to substitute chromium trioxide. It is a challenge to find a substitute which meets all requirements for a product, for each use, and specific applications while also being technically and economically feasible. Many alternatives are already qualified for some applications and substrates, but none of them provide all the key properties of chromium trioxide as defined in the following sections.
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3. ANALYSIS OF SUBSTANCE FUNCTION Chromium trioxide is used in the aerospace industry in surface treatment as illustrated in the following sections.
3.1. Usage Surface treatment is aimed to modify the surface to adapt it to specific use conditions. The main uses of chromium trioxide-based surface treatments in the aerospace sector are providing better corrosion resistance, improved paint adhesion, low electrical contact resistance or enhanced wear resistance to surfaces. This is achieved by a process chain which combines initial pre-treatment process steps preparing the surfaces for a subsequent coating, the main process providing the protective coating itself and post-treatment process steps. The complexity of an aircraft, spacecraft and the airworthiness requirements make corrosion resistance, improved paint adhesion, low electrical contact resistance or enhanced wear resistance a very challenging task. Inspection and quality control is very difficult and heavy maintenance is required almost every 10 years and some parts (e.g. structural components) cannot be removed. Metal surfaces and metal parts can be affected from corrosion by a broad variety of factors, such as - Temperature; - Humidity; - Salinity of the environment; - Industrial environment - Geometry of parts - Surface conditions; - Erosion; - Radiation; - Impurities; - Stress; - Pressure; - Biological growth; - Accumulated liquid; - Operational fluids; and - Galvanic coupling (e.g. at fasteners adjacent to dissimilar metals). All the factors listed above can occur alone or in combinations under certain environments at different parts of an aircraft or spacecraft. Not all components of an aircraft are equally susceptible to corrosion, especially vulnerable components are known to include structural components such as the skin originating at lap joints as well as fasteners and fastener holes, landing gear, other structural components, and engine components. Other major areas susceptible to corrosion include where moisture and liquids are entrapped, such as under fairings. For spacecraft, external parts exposed to harsh environments (e.g. at launch pad in Kourou) interstage skirts and pyrotechnic equipment are susceptible to corrosion. However, corrosion prone areas vary with the type of aircraft that are listed exemplarily in the following table. Table 3: Corrosion prone areas on different types of aircraft
Civil Aircraft/Spacecraft Fighter Aircraft Helicopters Main landing gear Main landing gear Main rotor head assembly Nose landing gear Nose landing gear Tail rotor assembly Rudder and elevator shroud areas Missile and gun blast areas Transmission housing of dynamics
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Civil Aircraft/Spacecraft Fighter Aircraft Helicopters Aileron and flap track area, flap Leading edges, hinge lines and air Main rotor blades and leading edges tracks and trailing edges ducts Access and freight doors Cockpit frames Fuselage Control cables Wing fold areas EMI/ Lightening Strike Shielding EMI/ Lightening Strike Shielding Engine intake areas Galley and Lavatory EMI/ Lightening Strike Shielding Cargo Areas LO Coatings Pyrotechnic equipment Interstage skirts Anywhere liquid can accumulate Fuselage in general and in particular inside aircraft, seat tracks, cargo systems, wingskin, etc.
Importantly, in this demanding environment corrosion may still occur with the highly developed Cr(VI)-containing coating systems. For currently used coatings, decades of extensive experience exists relating to the appearance and impacts of corrosion. Without a well-developed Cr(VI)-free alternative, corrosion will certainly increase, as these alternative coatings do not offer all the crucial properties of Cr(VI)-based coating systems and their long-term performance can currently only be estimated. Likely, the corrosion issues would not appear suddenly but only after several years, when hundreds of aircraft are delivered. Re-equipping, if possible, would cost hundreds of million €. As a consequence, decreased corrosion protection performance may lead to shorter inspection intervals, which has a significant impact on the maintenance costs for aircraft. Furthermore, for secure adaptation of the inspection intervals a detailed knowledge of the alternatives is a prerequisite. Some of the corrosion prone areas, as well as further examples on parts requiring corrosion protection are illustrated in Figures 3-8 below:
Figure 3: ATR 600 aircraft & Gulfstream V aircraft (UTC Aerospace Systems – Propeller Systems, 2014)
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Figure 4. Schematic illustration of typical corrosion findings in an aircraft fuselage. (Airbus Group, 2014).
Figure 5: Propellers mounted on an aircraft. (UTC Aerospace Systems – Propeller Systems, 2014)
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Figure 6: Undercarriage - landing gear, examples (Rowan Technology Group, 2005)
Figure 7: Gas Turbine Engine sketch example PW4000 92 inch fan engine (www.pw.utc.com/Content/PW400094_Engine/img/B-1-4-1_pw400094_cutaway_high.jpg, 2014).
Figure 8: Emergency door damper for a civil aircraft. (UTC Aerospace Systems – Propeller Systems, 2014)
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Different kinds of corrosion occur at these prone areas with some of the most common being illustrated in the following paragraphs. Fastener and fastener holes are well known to be susceptible areas for different kinds of corrosion. Galvanic, filiform and crevice corrosion can occur at fasteners in contact with the aircraft skin (dissimilar metal). Stress corrosion cracking is also applicable as fasteners have to withstand stresses or loads. Furthermore, fasteners may be susceptible to hydrogen embrittlement and should be slightly cathodic to the material they are joining to. Corrosion fatigue and stress corrosion cracking may develop around fastener holes due to stress concentration at a single point in the hole, and also on structural components which have to withstand stress and are exposed to corrosive environments. Exfoliation corrosion may occur in materials that are susceptible to this form of corrosion (such as crevices of thick extruded or rolled aluminium plate). The potential for exfoliation corrosion to occur is increased at unprotected panel edges where end-grain is exposed. This is also true for other exposed metal areas such as countersinks. Fretting corrosion occurs when overlapping metallic joints are subject to repeated or cyclic relative movement and where a corrosive environment is present. Treating surfaces susceptible to corrosion with Cr(VI)-containing products provides, in combination with the correct choice of material, the required corrosion prevention properties and functionality. Any structural detail where there is an unsealed gap between adjacent components where moisture can become entrapped (like a joint) is highly susceptible to corrosion. Protection can be provided by priming these surfaces adequately and use of a water-displacing/barrier working like a sealant. Again the use of Cr(VI) has proved to be most effective for this purpose. Electrical systems are often subject to corrosion at wires, connectors and contacts, especially where moisture or humid environments are present. Highly corrosive environments are also present in aircraft engines during operations caused by high temperatures and the presence of corrosive gases, high temperature oxidation/corrosion and liquids. Accelerated forms of corrosion can be found at the engine air inlet where airborne solids or rain erosion can damage the metal and coating surfaces. Similar highly corrosive environments are present in helicopter components such as rotor heads, and main and tail rotor blades. There are many more areas which are prone to corrosion in an aircraft. The following list provides a rough overview but is far from being complete: - Bilge areas where wastes of all kinds are collected (e.g. hydraulic fluids, water, dirt); - Control surface actuating rods and fittings may corrode as a consequence of coupling with dissimilar metals, being damaged, or deteriorated protective coatings; - Undercarriage bays (e.g. area of the wheel wells) are affected by debris from the runway; - Battery compartments and vent openings due to battery spillage; - Fuel tanks due to the ingress of moisture and resulting microorganisms that can reside in fuel; - Engine exhaust trail areas affected by exhaust gases; - Galley and lavatory areas are affected by spilled foods and human waste; and - Cargo areas collect all kinds of miscellaneous corrosive materials brought in by the cargo containers (e.g. mud, salt, oils, water, livestock waste, chemical spills, food products, etc.).
In this introduction, only examples of corrosion were presented. However, there is often a combination of certain properties required for these parts.
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3.2. Surface treatment processes Surface treatment of metals is a complex step by step process in many industry sectors. For operations with high performance surfaces in demanding environments, the use of Cr(VI)-containing components is essential to ensure the long-term (over decades) quality and safety of the end product. As specifically illustrated in Figure 9, there are various steps within the whole surface treatment process. These are classified into pre-treatment processes (for an adequate preparation of the substrate for subsequently applied process steps), process steps (main process), and in post-treatment processes (which mostly have to be applied for final surface protection). Some examples are listed in Table 4, but the table is not exhaustive.
Use Description Surface Treatment of Metal Parts
Pre-treatment Treatment Post-treatment • Cleaning • Sealing after Anodizing • Conversion Coating • Pickling/Etching • Rinsing • Passivation • Deoxidising • Passivation of metallic • Anodizing • Stripping coatings on steel
Figure 9: Surface treatment processes steps where chromium trioxide might be involved
Only the combination of adequate pre-treatments, main process step and post-treatment leads to a well-prepared surface providing all necessary key requirements for the respective applications (as described in detail in chapter 3.3.2. To be clear, the use of chromium trioxide in at least one process step is crucial to ensure the quality of the product and to meet the requirements of the industry. Although single process steps can be assessed individually, they cannot be seen as stand-alone processes but as part of a whole process chain. Consequently, when assessing alternatives for chromium trioxide-based surface treatments, the whole process chain and the performance of the end product has to be taken into account. While R&D on replacement technologies in surface treatments has been ongoing for decades, industry has developed and has already partly qualified alternate treatments for special applications. Therefore, it is possible that for individual process steps, either pre-treatments, main process steps and post-treatments steps, chromium trioxide-free alternatives are already on the market and industrially used. However, it is crucial to consider the following points: - In each case, the performance of the alternative materials/techniques must - importantly - be evaluated as part of a whole system (Figure 9); - Any change of single steps in the process chain of surface treatments will require component and/or system level testing and evaluation, (re)qualification and implementation into the supply chain; and - Current approvals for most coating systems still incorporate at least one layer prepared with chromium trioxide, but mostly several layers where Cr(VI)-based treatments are used. We therefore clearly state that for a thorough assessment of replacement technologies, it is mandatory to include the whole process chain (including pre- and post-treatments), taking into consideration that for all steps involved, Cr(VI)-free solutions have to be developed, which in combination are technically equivalent to the current Cr(VI) containing treatments. For few applications from the aerospace sector, where corrosion risk is low, first complete Cr(VI)-free solutions exist. Currently,
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Table 4: Overview of surface treatment processes indicating most important application methods, purpose, as well as example products. This is not intended to be an exhaustive list.
Process Application Purpose Product/Substrate examples
Cleaning of copper and copper Bath Surface preparation for subsequent - Functional alloys, magnesium and Wipe processing. cleaning magnesium alloys Spray Removing surface contamination - Electropolishing of steel Removal of mechanically deformed Pickling of stainless steel layers, oxides or other compounds from - - Activation of steel before Cd
a metal surface by chemical or Pickling/ Bath plating electrochemical action. Etching Spray Pre-treatment of Al alloys, Cu Removal of material selectively to - reveal a surface or the surface - Pre-treatment of Carbon- properties. reinforced Composite parts
Deoxidising is a pre-treatment step Pre-treatment of Al alloys prior Deoxidising Bath required to activate the surface prior to - treatment processes - to anodising further processing. Pre Pre-treatment of metallic Removal of metal sulphide and other - Desmutting Bath substrates after pickling, etching complexes after etching and deoxidising Stripping of organic and Bath Stripping is the removal of a coating - inorganic material, such as paint Stripping Wipe from the component substrate or an from steel or hard anodic undercoat. Brush coating from Al alloy, Mg alloy
Bath Chemical process that introduces a Spray Chemical chemical coating or changes the Aluminium Conversion Wipe surface of the substrate to improve the - Magnesium Coating Brush substrate properties (e.g. corrosion - Mg housing (CCC) Coil resistance, or promote adhesion of - coating subsequent coatings) Electrolytic oxidation process in which - Structural parts for aerospace equipment and systems Chromic acid Bath the surface of a metal, when anodic, is anodising converted to an oxide having desirable - Metallic airframe components (CAA) Brush protective or other functional - Small parts: fasteners, connector properties. shells Remove embedded iron/steel particles Passivation of from stainless steel to restore its Stainless Steel Bath - stainless steel natural corrosion resistant passive - Any other product examples
Main process oxide layer. Sacrificial coating: Application of a thin layer where the metallic ingredient - Sacrificial Coating - Inorganic Sacrificial Spray in the coating has lower value of aluminium-based slurry coating coatings electrode potential than the substrate to on steels – basecoat plus be protected. sealcoat
Diffusion coating: A process based on Slurry the coating material diffusing into the - High temperature oxidation and (diffusion) Spray substrate at high temperatures. corrosion resistant coating for coatings Application of coatings by spraying a Turbine components slurry onto a clean, prepared surface
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Process Application Purpose Product/Substrate examples
and then baking that layer of slurry to - Inorganic aluminium-based produce a stable coating film that is slurry coating on steels – well bonded to the substrate. basecoat plus sealcoat
Sealing after Bath Sealing of the porous anodic coating - Anodised aluminium surfaces anodising Brush providing protective properties for corrosion resistance Passivation of metallic Chemical process applied to a - Metallic coatings on steel such coatings substrate producing a superficial layer as Cd coatings, Zinc coatings, Bath (Post- containing a compound of the substrate Zinc coatings, Zinc-Nickel treatment metal and an anion of an environment. coatings)
treatment processes CCC) -
Post Rinsing after Sealing of a phosphated surface for Substrates treated with Bath - phosphating corrosion protection Phosphate Conversion Coating
3.2.1. Pre-treatment processes A number of pre-treatments are necessary to prepare the surface of the substrates for the subsequent process steps. Adequate preparation of the base metal is a prerequisite: adhesion between a coating and the substrate depends on the force of attraction at molecular levels. Therefore, the surface of the metal must be absolutely free of contaminants, corrosion and other foreign matter until the main treatment process is finished. Additionally, homogeneous formation of anodic films or passivation is influenced by the pre-treatment. Inhomogeneous surfaces will result in unpredictable corrosion performance. A number of different pre-treatments are in place depending on the respective subsequent process and its functionality. The pre-treatments discussed below are based on the use of chromium trioxide, while chromium trioxide-free pre-treatments are discussed in chapter 7 (evaluation of alternatives).
3.2.1.1 Functional cleaning All surfaces have to be prepared for subsequent processing by removing dirt, soil, scale and oxide layers. Chromium trioxide functional cleaning solutions are used for achieving best results. Functional cleaning is not a stand-alone process but part of a process chain. Presence of those contaminants influences the appearance of the subsequent layer and increases the susceptibility for fatigue and pit corrosion. For electropolishing, chromium trioxide is used for buffering and assisting in rinsing off the solution after completing the process. The process is used to remove flaws or debris from the surface of a metal substrate. Electropolishing can also be described as a “reverse plating” process as it is carried out in a blended chemical electrolyte bath using a combination of rectified current. For electropolishing of martensitic stainless steel, the use of chromium trioxide is mandatory. It improves the fatigue behaviour of structural parts and effectively eliminates impurities from previous thermal treatments. Control of pH can only be done by chromium trioxide.
3.2.1.2 Pickling and etching There is considerable overlap between these terms in the industry. Pickling/Etching is the removal of mechanically deformed layers, oxides or other compounds from a metal surface by chemical or electrochemical action. Etching is also used to remove smear in order to ensure the detection by dye penetrant inspection of cracks or other defects formed during metal shaping such as machining or forming. Typically it removes 0.5 to 3 µm of the substrate, and it is required because the initially
15 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES unprotected metal surfaces exposed to atmospheric conditions are continuously oxidised. These oxides interfere with subsequent processes (e.g. in passivation the required optimal corrosion protection could not be reached). With regard to aluminium alloys, pickling/etching is additionally used to enhance the surface for adhesive bonding for subsequent processing. Pickling/etching is not a stand-alone process but part of a process chain. Chromium trioxide is used as it is a strong oxidising agent offering the strongest reduction behaviour and therefore also the best result for tailoring of surfaces. In addition, the use of chromium trioxide-based pickling/etching solutions has a very low impact on the fatigue properties of the treated substrate. Strongly alkaline solutions containing chromium trioxide may also be used for specific metals. A specialized application is when processing stainless steel before bonding. This chromium trioxide- based surface treatment is used for the removal of contamination from the steel surface prior to bonding to create a clean surface and to ensure adequate adhesion of the subsequent bonded layer. The process consists of several steps, the main one being an electrolytic process and a subsequent desmutting step which uses chromium trioxide loaded products. The process removes material/contaminations from the surface and creates a surface topography beneficial for subsequent bonding. The process enables stainless steel to be bonded to other materials independent of type.
3.2.1.3 Deoxidising Deoxidising is a pre-treatment step required to activate the surface prior to further processing. Deoxidising is not a stand-alone process but part of a process chain. Deoxidising is an intermediate process step after degreasing and cleaning prior to subsequent process steps, such as anodizing or conversion coatings. Chromium trioxide-based deoxidising is able to fulfil a number of different purposes, depending on the initial pre-treatment and the required subsequent process. As an example, both the pre-treatment processes degreasing and etching are performed under alkaline conditions while a subsequent anodizing is performed under acidic conditions. Therefore, the surface has to be neutralized after the pre-treatments prior to the anodizing process. As a further purpose, deoxidising also removes potentially remaining oxides from the metal surface and activates the surface for the subsequent processes. The chromium trioxide-based deoxidiser solutions often comprise additives such as nitric acid (HNO3) and hydrofluoric acid (HF), depending on the specific purpose. According to Harvey et al (2008), these deoxidisers remove around 1 µm of the surface during the treatment, including the surface oxide and the majority of the intermetallic particles leaving the surface with a chromium containing oxide. Mode of action for Aluminium pre-treatment by cleaning, pickling/ etching/ deoxidising: The general mode of action for cleaning, pickling, etching and deoxidising is basically the same. The purpose of all these steps is the removal of oxides and of certain amounts of the base metal from the surface. There are numerous chemical reactions involved with the different constituents of the deoxidising solution, the aluminium oxide layer on the part surface, and the alloying elements (e.g. Cu, Si, Mg, Zn) of the substrate. While many of the reactions are intended there are also many unintended reactions. The role of chromates in the deoxidiser solution is primarily to provide uniform removal of aluminium and alloy metals and activate the surface for subsequent processing.
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3.2.1.4 Stripping Stripping is the removal of a coating from the component substrate or an undercoat. Chromium trioxide-based stripping solutions can be used for a wide range of substrates and coatings, while it turned out when considering alternatives, the stripping process always has to be adapted taking into account the substrate (metal, specific alloy type) and the kind of coating to be removed. Therefore, stripping can be divided into two main processes as there are stripping of inorganic finishes (such as hard anodic coating from Al alloys) and stripping of organic finishes such as primer and/or paint (from steel, aluminium, nickel/cobalt alloys, titanium, magnesium, corrosion resistant stainless steel CRES). Stripping is used to remove the coating (for example an anodic coating applied by CAA) without attacking the aluminium substrate itself. This process is used for rework, maintenance and repair operations, when the coating has to be removed and repaired, for example as partial repair. Stripping of coatings, which had been applied more than 10 years ago requires higher temperatures while “younger” coatings are more easily removed. Stripping is not a stand-alone process but part of a metal pre-treatment process chain.
3.2.2. Main surface treatment processes In the following chapter, various main treatment processes that are applied on surfaces within different industry sectors are described. A summary can be found in Table 4.
3.2.2.1 Chemical conversion coating (including phosphate conversion coating: phosphating) Chemical Conversion Coating (CCC) is a chemical or electrolytic process applied to a substrate producing a superficial layer containing a compound of the substrate metal and the process chemistry. In general CCC form an adherent, fixed, insoluble, inorganic crystalline or amorphous surface film of complexes from oxides and chromium trioxide or phosphates as an integral part of the metal surface by means of a chemical reaction between the metal surface and the immersion solution. CCC is usually carried out by immersion of the product in an acidic bath with an aqueous solution containing dissolved chromium trioxide or phosphate salts together with an acid such as sulphuric acid or nitric acid. CCC can also be applied by spray or wipe techniques, more common in repair and non-stationary operations. The thickness of the coating typically is between 0.05 – 2 µm. There are two main classes of products which are subject to a CCC treatment, the first are products made of aluminium and its alloys (Al CCC) and magnesium and its alloys (Mg CCC). The second are metallic coatings such as aluminium-based coatings, zinc-based coatings, zinc-nickel-coatings and cadmium coatings applied on metallic substrate (such as steels, stainless steels, aluminium, copper etc.) and on composites substrates where CCC is to provide corrosion protection. This process is further referred to and discussed as passivation of metallic coatings. Indeed, other metals can also be subject to conversion coatings. CCC are used throughout the aerospace sector on a wide variety of components and equipment such as steel landing gear components, fasteners, electrical connectors and enclosures. Mode of action: Various research efforts have been made leading to considerable understanding of the mechanism of metal corrosion and the inhibition provided by chromium trioxide as conversion coatings over the last decades. The formation of a CCC includes several steps, typically starting with a deoxidising and degreasing pre-treatment to prepare the substrate’s surface, followed by the main process step of dipping the substrate in the chromium trioxide solution (Vasques et al, 2002). The chemical reactions provided below are specific to aluminium substrate, however, the general mode of action for magnesium or titanium substrate or metal coatings are basically the same. According to
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Vasques et al (2002) and Zhao et al (2001), aluminium exposed to a CCC solution results in the simultaneous oxidation of Al to Al3+ and the reduction of Cr(VI) to Cr(III) as follows: (1) 2 Al 2 Al3+ + 6e–
3+ + (2) 2 Al + 3 H2O Al2O3 + 6 H
2- + - (3) 2(CrO4) + 10 H +6e 2 Cr(OH)3 + 2 H2O. The result is a protective layer based on chromium trioxide anions, absorbed in the pores of the aluminium oxide layer. As residual chromium trioxide is retained in the CCC, it provides active corrosion inhibition to the surface by diffusion into local defects and altering the local environment. Any comparison of an alternative for chromium trioxide in CCC must take this unique property into consideration (Zhao et al, 2001). According to Zhao et al (2001), the mechanisms for the inhibition of metal alloy dissolution 2- 2- are that the chromium trioxide is a very soluble, higher-valent, oxidizing ion (CrO4 or Cr2O7 ) with a lower valent form that is insoluble and creates an extremely protective film (Cr2O3 or Cr(OH)3). The degree of corrosion resistance of conversion coatings is generally proportional to the coating thickness (Deresh, L., 1991).
3.2.2.2 Chromic acid anodising (CAA) CAA is an electrolytic oxidation process in which the surface of a metal, when anodic, is converted to an insulating coating having desirable protective or other functional properties. The oxide layer partly grows into the substrate and partly grows onto the surface. The total oxide thickness after anodising is between 3 and 60 µm, while the thickness after hard anodising is up to 300 µm. With regard to maximum thickness, dimensional constraints by design have to be respected. CAA comprises a number of different process steps including pre-treatment and post-treatments. Anodising is used to increase corrosion and wear resistance as well as adhesion for subsequent processes. Substrates that can be treated by anodising include aluminium alloys, titanium, magnesium, niobium, zirconium, hafnium and tantalum. The main commercial application is the treatment of aluminium to create alumina (Al2O3) on the surface (RPA Report, 2005). CAA is performed in an acidic solution containing chromium trioxide and in some cases other acids. The parts to be treated form the anode electrode of an electrical circuit, the respective cathode is inert. The electric current can be varied which leads to oxidation of the base metal at the anode with the formation of aluminium oxides on the surface. Some of the aluminium is dissolved, as ions, into the process bath, which leads to bath losses and the need to replace some of the bath solution. Anodised aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness and still have low corrosion resistance that can be improved by applying suitable sealing substances. Anodic films are generally much stronger and more adherent than most types of paint and metal plating, but also more brittle. This makes them less likely to crack and peel from aging and wear, but more susceptible to cracking from thermal stress. The unsealed anodised surfaces provide a good paint adhesion to subsequent layers, but need to be sealed or primed for providing a good corrosion protection (Fast Report, 2009). Chromic Acid Anodising is mainly used for aerospace and military applications (RPA Report, 2005).
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The thickness of a CAA surface is typically in a range between 0.5 to 18 µm. This is significantly thicker than the natural oxide surface of an untreated aluminium surface (which is typically 0.005 µm) and thinner than surfaces created by non-chromium trioxide Sulphuric Acid Anodising (SAA) (which is typically 15 µm and greater).
Figure 10: Schematic anodic coating after acidic anodising (Hao & Cheng, 2002)
Mode of action: CAA improves the corrosion resistance of aluminium or aluminium alloy surfaces by anodic treatment in an electrolytic bath by forming aluminium oxide (Al2O3). The oxidation and reduction process is analogous to CCC, but the anodic film formation is mainly driven by the applied voltage during the anodizing cycle (refer to equations 1 to 3 above).
3.2.2.3 Passivation of stainless steel A material is considered passivated when it shows a high resistance to corrosion in an environment in that one would normally expect corrosion to occur. Stainless steel is considered a material that naturally passivates because it contains chromium as an alloying element that forms a very thin chromium oxide layer on the surface of the stainless steel. This thin chromium oxide layer is responsible for passivating stainless steel. A properly passivated stainless steel can resist corrosion in humid air and salt water. However, if the passive oxide is damaged or destroyed, then passivation is the process used to restore or reform the passive oxide layer on stainless steel alloys, and this passive oxide layer is critical to make stainless steel corrosion resistant. One method that damages the passive oxide layer is by machining or forming stainless steel with steel tools. These steel tools leave small particles of iron embedded in the stainless steel part. A stainless steel part with embedded iron particles would quickly form rust spots if subjected to high humidity or salt spray conditions. Passivation of stainless steel removes embedded iron/steel particles from stainless steel to restore its natural corrosion resistant passive oxide layer.
3.2.2.4 Sacrificial coatings Sacrificial Coating is a thin film protective coating comprising metal particles and an inorganic binder. These systems have been used for many years for critical structural parts of gas turbines used for aerospace and power generation applications. They are applied by spray guns or brushes and are heat cured afterwards. Heat cured coatings have a lower value of electrode potential than metal substrate for most applications. The metal content is preferentially aluminium allowing the coating to provide sacrificial corrosion protection to low alloy steels and specialist steel alloys, where a corrosion sensitive alloying and heat treatment provides high steel strength for low weight components. The coating behaves as the anode in a galvanic cell, this releases an electron flow into the substrate material turning it into the cathode and thus preventing corrosion. Sacrificial coatings are unique in
19 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES their activity at high temperatures, where corrosion protection, high temperature oxidation resistance, chemical resistance, abrasion resistance, and good flexibility are combined. The combination of powdered metal and inorganic binder provide a level of heat resistance that is associated with ceramic materials. Sacrificial coatings are unique in their combination of heat resistance and a level of flexibility that is equivalent to thin, often sheet metal components. The process is normally performed at elevated temperatures in a controlled chamber.
3.2.2.5 Slurry (diffusion) coatings Diffusion coating is a process in which metal components that will be subjected to high temperature conditions and highly corrosive environments are coated with a non-corrosive material. Slurry (diffusion) coatings can be described as metallic paints with Cr(VI) present in the matrix of the coating. The most widely used slurry coatings are chromium-, aluminium- or silicon-based materials. Substrate materials are mainly steels (including carbon, alloy and stainless steels) and refractory metals, among other alloys. Chromium trioxide in these systems combines excellent corrosion protection, wear resistance and active corrosion inhibition. These systems have been used for many years for critical structural parts of propellers (as hubs, domes, points where the blades are attached), as gas turbine engine components, and as power generation components. Furthermore, they are used as cadmium replacement. They are applied by spray guns or brushes and are heat cured afterwards. Primer layers represent one part of a multi-layer coating (i.e. metallic ceramics coatings) and are covered by (a) further layer(s), which can be a sealant or topcoat.
3.2.3. Post-treatment processes A number of different, chromium trioxide-based post-treatment processes can be applied to the surfaces as described below. Colourings and primers are not part of this AfA.
3.2.3.1 Sealing after anodizing The surfaces of substrates after anodizing are naturally porous, the coating cannot provide the required corrosion resistance without further treatment (Hao, L. & Cheng, B.R., 2000), therefore a sealing post-treatment is necessary for a broad variety of sectors and applications. Sealing is often performed in a hot aqueous chromate solution (typically > 95°C but below the solution’s boiling point) using either sodium dichromate, potassium dichromate or a mixture. For some applications, hot water or salts of other metals are used for sealing. Chromium trioxide- conversion coating solutions can also be used for the purpose of sealing after anodizing. Mode of action: The sealing after anodizing step is performed with a dichromium trioxide solution comprising chromium trioxide, sodium dichromate, potassium dichromate, or a mixture thereof. During the sealing, chromium trioxide and hydroxides precipitate in the pores of the previously anodized oxide layer and are hydrated. By this process, the pores are closed and an adequate wear resistance and corrosion resistance is provided to the surface. The hydration process (in the course of sealing after anodizing) is pH-dependent, but in all cases, the chromate is absorbed to the anodized aluminium surface. Depending on the pH, Cr(VI)-based sealing forms either aluminium oxychromate (equation 1) (at pH <6) or aluminium dioxychromate (equation 2) in the coating micropores (Steele, L.S, & Brandewie, B., 2007).
- - (1) AlOOH + HCrO4 AlOHCrO4 + OH - - (2) (AlO(OH))2 + HCrO4 (AlO)2CrO4 +OH +H2O
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The final step closes the pores by contact with hot water and locks in the chromium trioxide in the pores according to equation (3):
(3) Al2O3 + H2O 2 AlO(OH) The hydrated aluminium oxide (boehmite) has a larger volume than aluminium oxide, therefore the pores are closed.
3.2.3.2 Passivation of metallic coatings Metallic coatings, such as cadmium, zinc, zinc-nickel, aluminium or Al coated by Ion Vapour Deposition (IVD), applied on metallic substrate (such as steels, stainless steels, aluminium, copper etc.) and on composites substrates need to be passivated for corrosion protection. Technically, this kind of passivation is a conversion coating (refer to chapter 3.2.2.1). Passivation is a post-treatment after the application of the non-chromium metal coating. Different kinds of metal coatings are used depending on the respective functionality of the coated part. Cadmium plated parts, for instance, are used due to the unique properties of cadmium required in many high performance aerospace applications; for example the most commonly used aerospace fastener plating material is cadmium. Cadmium is galvanic compatible with aluminium parts and other metallic or composite substrates and protects ferrous components in aluminium assemblies from corrosion (sacrificial corrosion protection). Cadmium plated steel needs to be heat-treated to prevent hydrogen embrittlement. The corrosion resistance of Cd platings is up to 2000 h in salt spray tests, which can be further enhanced (doubled) by chromium trioxide passivation.
3.2.3.3 Chromium trioxide rinsing (after phosphating) Chromium trioxide-based rinsing after phosphating is a passivation process after phosphate conversion coating (phosphating). A chromium trioxide rinsing step fulfils two requirements by using only one process step. First, the rinsing removes the drag-out comprising liquids and residuals of former processes adhering to the substrate to ensure that there is no deterioration of the substrate by residuals (RPA Report, 2005). Additionally, by processing at a temperature between 70 and 80°C, the surface is passivated and the corrosion resistance of the surface is enhanced. This enhancement is necessary for phosphate conversion coatings because these coatings have a naturally occurring porosity which would negatively affect the corrosion resistance of the coated surface without any post-treatment (Narayanan, S., 2005). The rinsing is performed by immersing the product in the rinsing solution. Mode of action: The rinsing step is necessary for the adhesion of subsequent paint coatings to the phosphated surface. The rinsing leads to the deposition of water-insoluble chromium trioxide salts which reduce the porosity of the treated surface by about 50%. Additionally, the chromium trioxide solution etches protruding crystals of the phosphate coating to provide a plain surface for subsequent painting. The rinsing is performed with chromium trioxide because the removal is most effective by using this solution (Narayanan, 2005).
3.3. Key chromium trioxide functionalities in surface treatment processes An overview on the key functionalities and the performance requirements of chromium trioxide in the respective surface treatment is provided in the paragraphs below, subdivided into pre-treatment processes, main processes and post-treatment processes. During the consultation phase, the key functionalities for chromium trioxide within this use were identified taking the whole surface treatment processes into account. Nevertheless, the most important key functionality for all the main processes and post-treatment processes is corrosion resistance.
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It should be noted that while the numerical values reported for key requirements here have been supplied by industry, they are not necessarily the same for all companies. Furthermore, requirements for individual applications may also vary.
3.3.1. Pre-treatments - key functionalities As stated in chapter 3.2.1., a clear demarcation between the processes of pickling, etching, cleaning, and deoxidizing does not exist. When comparing specifications from different sources, the terminology is not always consistent from one document to another. It can be stated that, with all these processes, adequate surface preparation for subsequent processes can be achieved by removal of surface residues. The main difference is that for cleaning, a less aggressive chemistry is used for light scale removal or removal of other contaminants like shot peen residue. As the aim of the pre-treatment processes is to prepare the surfaces for subsequent process steps, the key functionalities are not always the same as for the main process or the post-treatments discussed in chapter 3.3.2.
3.3.1.1 Functional cleaning, pickling, etching The key functionality of cleaning, pickling and etching is the adequate removal of oxide and debris from a metal surface (e.g. Aluminium, Magnesium, Cadmium). For pickling and etching, selective removal of certain amounts of base material or removal of surface defects is required for surface activation. This process is controlled by the etch rate. The careful control of this step influences the quality of the subsequent coating layer. After these pre-treatments, the processed parts shall be free of pits, corrosion products, discolouration, uneven etching, increased surface roughness or other defects that would prohibit further chemical processing. This can be checked by visual inspection or penetrant inspection (American Society for Testing Materials (ASTM) E 1417). The etching rate has to be adequately chosen depending on the metal substrate. Under-etching or over-etching has to be avoided so that the key functionalities of the subsequent layer are not affected (for example: poor adhesion resulting in cracks and blistering). The etch rate is typically controlled by measuring the weight before and after on a witness coupon at a regular interval. In addition, treated surfaces shall be free of intergranular attack in a defined excess or end grain pitting in distinct limits. Further it is important that the baths can be used for a long timeframe with manageable maintenance. Additionally, the racks carrying the parts are usually used in the overall process chain and therefore have to be compatible with the chemicals used in the subsequent process steps.
3.3.1.2 Deoxidising With a chromium trioxide-based deoxidising process step, the key quality criteria is to provide surface activation. The deoxidising causes a metal removal of the substrate, which shall not exceed certain limits (physical measurement using a micrometre). By applying a deoxidising solution, the metal is attacked and end grain pitting and intergranular attack may be caused. Deoxidising shall neither cause end grain pitting nor intergranular attack in certain excess and depth. Furthermore, the appearance of the deoxidised surface (after rinsing) is visually inspected. It has to be a water break free surface without streaks or discolourations and no pitting or selective attack to the substrate, no non-rinseable residuals, or contamination from the deoxidising solutions shall be observed on the surface.
3.3.1.3 Stripping of inorganic finishes (e.g. conversion coatings, anodic coatings) Specifications require no hydrogen embrittlement during 200 h of sustained load according to ASTM F 519, although the effect of the hydrogen can be removed to a certain extent by subsequent de-
22 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES embrittlement heat treatment. End grain pitting and intergranular attack negatively influencing the substrate quality, shall not exceed a ratio of (surface) pit size to pit depth of 6:1, which is tested according to ASTM F 2111. Stripping of the inorganic coating may have an impact to a shot peen compressive layer, which can affect the fatigue properties. However, no more than 10% of the thickness of the shot peen layer shall be removed, which is tested by a micrometre. As a certain amount of the base material may be removed by stripping, it must be guaranteed that the parts still conform to the drawing after stripping. This is tested by post processing inspection measurements. Stripping of an inorganic finish from Ti alloys may leave a notable hydrogen content on the substrate, which is tested according to ASTM E 1447. The hydrogen content can cause hydrides to precipitate, which then can lead to embrittlement and cracking under stress. Other parameters which may affect the substrate are residual stress, surface roughness and fatigue. However, as the reason for performing the stripping process is the need to repair a coating, these parameters cannot be worse than for the original (defect) coating. Stress corrosion cracking of the substrate is tested by exposing test specimens for 4.0 ±0.5 h in a molten salt bath. The specimens are rejected if the material shows pitting, cracking or rough etching. Chromium trioxide-based stripping solutions can be used for a wide range of substrates and coatings, while it turned out when considering alternatives, the stripping process always has to be adapted taking into account the substrate (metal, specific alloy type) and the kind of coating to be removed.
3.3.1.4 Chemical stripping of organic coatings (e.g. primers, topcoats and specialty coatings) Chromium trioxide is used as active ingredient for stripping, although the most common active ingredients in solvent brush on paint strippers include methylene chloride (dichloromethane), benzyl alcohol, formic acid or hydrogen peroxide. Different active ingredients are useful for different paint systems. Chromium trioxide used in lower concentrations would still possibly contribute to paint removal, but other active ingredients would provide most of the paint removal action. The important role of chromium trioxide is to mitigate pitting and galvanic corrosion during the stripping process. For the corrosion resistance of substrates where paint became stripped, various tests are carried out described as follows. Sandwich corrosion is tested according to ASTM F 1110. Immersion corrosion is tested according to ASTM F 483. The fluid should neither cause corrosion nor a weight change of any test panel, specific for the respective kind of substrate. Dissimilar metals corrosion is tested by immersing coupled dissimilar metal components together. The test is failed if the panels exhibit pitting, etching or corrosion products. Adhesion is tested according to the International Organization for Standardization (ISO) 2409, mesh peel test, long beam test and other tests depending on the process specific requirements. When stripping off paint from metal substrates, hydrogen embrittlement may occur. No hydrogen embrittlement should be observed during 200 h of sustained load according to ASTM F 519. No cracks are allowed to occur on the surface after stripping of paint. The fatigue properties are tested according to ASTM E 466 (5 stripping cycles) and no degradation should occur. For composite materials, no evidence of wet media penetration shall be detected after thermographic inspection. The surface roughness of the stripped surface is tested with a profilometer.
3.3.2. Key functionalities of chromium trioxide-based main processes & post-treatments As already stated, the described main processes and post-treatments rely on the use of chromium trioxide due to a number of key functionalities, which are described in detail below.
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Corrosion resistance / active corrosion inhibition Corrosion describes the process of oxidation of a metallic material due to chemical reactions with its surroundings, such as humidity, but also corrosive electrolytes. In this context, the parameter corrosion resistance means the ability of a metal aircraft part to withstand gradual destruction by chemical reaction with its environment. For the aerospace sector, this parameter is one of the most important since meeting its minimum requirements plays a key role in assuring the longest possible life cycle of aircraft and all the implicit parts, the feasibility of repairing and maintenance activities and most importantly, the safety of all air travellers. Especially the AA2024 aluminium alloy, most commonly used in the aerospace sector, contains app. 5% of Cu as alloying element to provide the material strength. But Cu as noble element acts as bulid in corrosion driver. Inhibition of the Cu is mandatory for long-term corrosion stability. The corrosion resistance requirements vary within the aerospace sector and are dependent on the metal substrate (aluminium alloy, steel type), the coating thickness and the respective surface treatment process. Corrosion inhibiting components can be categorized according to basic quality criteria which are inhibitive efficiency, versatility and toxicity. Ideally, the component is applicable in all surface treatment processes, compatible with subsequent layers, and performs effectively on all major metal substrates. Furthermore it has to guarantee product stability (chemically and thermally) and has to reinforce the useful coating properties. The ability of a material to spontaneously repair small amounts of chemical or mechanical damage is known as an active corrosion inhibition or self-healing property. If this characteristic is given for a certain material, it is tremendously advantageous and will enhance service life duration of parts, maintenance intervals and on-flight security of air travellers. The requirements for active corrosion inhibition are varying within the aerospace sector and are depending on the metal substrate and the respective surface treatment process. The active corrosion inhibition of chromium trioxide-based surface treatments are generally tested in line with the corrosion resistance based on the same test methods and requirements, as the active corrosion inhibition of a coating is a characteristic feature. Adhesion promotion (adhesion to subsequent coatings or paint) Depending on the final functions of the parts, they may be coated with decorative or protective layers (such as paint). In this analysis, the parameter adhesion describes the tendency of dissimilar particles or surfaces to cling to one another. Regarding the aerospace industry, many parts are exposed to harsh environmental conditions, in contact with other metallic parts or have to withstand strong mechanical forces. The requirements for adhesion vary within the aerospace sector and depend on the specific coating thickness and the function and location of the part. Chemical resistance This parameter is defined as the ability of solid materials to resist damage by chemical exposure. Especially for aerospace applications, it is highly important that all parts withstand contact with different chemicals like hydraulic fluids, de-icing fluids, greases, oils and lubricants. The chemical modification of protective coatings or the metal parts themselves could escalate maintenance costs and sacrifice to some extent travel safety. The requirements for chemical resistance are varying within the aerospace sector and are depending on the metal substrate, the coating thickness and the respective surface treatment process. A general high-end chemical resistance provided by the chromium trioxide-based surface treatment is typically tested in line with the corrosion resistance based on the same test methods and requirements. More specific tests on chemicals are performed in addition.
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Layer thickness The thickness of the layers or coatings on the substrate are also crucial for the performance of the parts. Not meeting the specified requirements of this parameter could lead to deficiencies for other related characteristics like corrosion and chemical resistance, improper adhesion of coatings to the substrate or increased fatigue properties. Resistivity Electrical resistivity is a property that quantifies how a given material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. Many aerospace applications require electrically conducting materials for static discharge, electromagnetic interference shielding, and electrical bonding (e.g. lightning strike protection). A material that provides good electrical bonding between all joints is required to assist in controlling and shielding against electrical effects. In addition, electrical bonding combined with corrosion resistance is also a common requirement for metal-based aerospace assemblies. Eddy current test is used to detect surface & subsurface defects, corrosion in aircraft structures, fastener holes and bolt holes. Surface defects and Resistivity testing are performed by high frequency; for detection of sub- surface defects, low frequency method is carried out.
3.3.3. Key functionalities in the aerospace sector In Table 5 selected quantifiable requirements of the key functionalities for the main process steps and the post-treatments are listed to give a short overview on the widespread range of requirements. The selection was made regarding the most relevant process related key functionality. A more detailed description is given in the subsequent paragraphs. In particular, extended requirements with higher in-service relevance apply for some applications that may not be described in the table below.
Table 5: Key requirements within the aviation sector Quantifiable key Process Requirements (not exhaustive) functionality 2 – 24 h (ferretic, precipitation hardened CRES, ISO 9227, ASTM B117) Corrosion resistance 96 – 750 h (austenitic CRES, ISO 9227, ASTM B117) 2 - 500 h martensitic steel Passivation of Adhesion to subsequent GT0 dry, GT1 wet after 168 h (Cross-Cut Test ISO 2409), stainless steel layer partly immersion for 14 days No induction of hydrogen embrittlement shall be observed after heat treatment. This is tested via Tensile test Embrittlement (EN2832) or slow bending test EN2831 or other testing procedures. Depending on thickness and type of plating. Corrosion resistance 336 h-1000 h (ISO 9227, ASTM B117) Depending on the type of coating: Layer thickness < 1 µm, 2 to 7 µm, 7 to 12 µm, 10 to 20 µm Passivation of metallic coatings Adhesion to subsequent Paint adhesion: GT0 dry, GT1 wet after 168 h (Cross-Cut (Post-treatment layer Test ISO 2409), partly immersion for 14 days CCC) no alteration of the coating after immersion in chemicals Chemical resistance and no corrosion after 750 h Salt Spray Test (SST) No loss of coating or passivation system after immersion Temperature resistance in liquid nitrogen at -196°C
Aerospace sector Aerospace 100 Thermal Cycling -180- +200 °C (ECSS Q-ST-70-04)
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Quantifiable key Process Requirements (not exhaustive) functionality For unpainted parts: 168-336 h (ISO 9227, ASTM B117, United States Military Standard MIL-DTL-81706 with AA2024 unclad as reference) Corrosion resistance Chemical / For painted parts: length from scratch <2-3 mm after 960 Chromium trioxide h (Filiform corrosion test, EN3665), some companies conversion coating require <0.5 mm. - Aluminium alloys Adhesion to subsequent GT0 dry, GT1 wet after 168 h (Cross-Cut Test ISO 2409), layer partly immersion for 14 days Chemical resistance 168-750 h (ISO 2812, ISO 2409, BS3900 Part G5). Resistivity ≤ 5 mΩ/in² (MIL-DTL- 5541 F) Comparison with chromium trioxide protection system with and without varnish (ISO 9227): Without varnish: 30 h with no pits in SST (ISO 9227), 15 Corrosion resistance cycles with no pits for internal ageing cycles. With varnish: 336 h with no pits in SST (ISO 9227) - internal ageing cycles: No pits after 15 cycles Chemical/ Chromium trioxide Layer thickness < 3 µm (microscopic evaluation) conversion coating GT0-1 (with primer 12-20 µm, with varnish 60-100 µm Adhesion to subsequent - Magnesium and/or another varnish 12-25 µm) at initial and after 14 layer days in demineralised water (ISO 2409) No impact after immersion 24 h in products oils, fluids, Chemical resistance greases, no impact after degreasing with different products Comparison with chromium trioxide system (internal Resistivity protocol) For unpainted parts: 336-2000 h (ISO 9227, ASTM B117) for equipment and structural parts (AA2024) For painted parts: 3000 h (ISO 9227, ASTM B117) Chromic acid Corrosion resistance anodising (CAA) or length from scratch <2-3 mm, no blisters in the surface, chromate free max 1,25 mm blisters from artificial scratch after 960 h anodizing with (Filiform corrosion test, EN3665), some companies subsequent Sealing require <0.5 mm. after Anodizing or Layer thickness 2-7 µm with subsequent After immersion in various fluids, oils or grease: corrosion paint after Chemical resistance resistance in Salt Spray equivalent to chromium trioxide Anodizing system Adhesion to subsequent GT0 dry, GT1 wet after 168 h in demineralized water layer (Cross-Cut Test ISO 2409) For sacrificial coatings: No signs of softening, blistering or lifting after 20 cycles between salt spray and high heat. (internal specifications) No signs of breakdown or excessive corrosion after 750 h Sacrificial and Corrosion resistance exposure to sulphur oxides in high salt environments Slurry (diffusion) (Internal specifications) coatings For slurry coatings: 1000 h (unscribed) 500 h (scribed, ASTM B117) Adhesion to subsequent Classification number 2 (BS3900 Part E6 cross cut test) layer
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Quantifiable key Process Requirements (not exhaustive) functionality No signs of softening, blistering or lifting (BS3900 Part Chemical resistance G5) Hardness Scratch hardness >1000 VH (BS3900 Part E2) For sacrificial coatings: ≤ 15 mΩ/125 in² (Internal specifications) Resistivity For slurry coatings: < 10 Ω/in² (Internal specifications) Compliant with substrate material through a 120 degree Flexibility bend
Chromium trioxide Corrosion resistance 2 h (ISO 9227) Rinsing after Adhesion to subsequent phosphating GT0 dry after 168 h (Cross-Cut Test, ISO 2409) layer
Passivation of stainless steel For passivation of stainless steel, the minimum requirements are highly depending on the kind of passivated stainless steel. The aerospace industry indicated minimum requirements regarding corrosion resistance of 2 – 24 h for ferritic and precipitation hardened CRES and 96 h - 750 h for austenitic CRES as well as 2 h - 500 h for martensitic steel. The tested items shall not show evidence of red rust, nor significant white rust (coating corrosion products). No induction of hydrogen embrittlement shall be observed after heat treatment (tested via tensile test (EN2832) or slow bending test EN2831 or other testing procedures. For steels with 1100 MPa≤ UTS < 1450 MPa, a heat treatment temperature of 190°C for 8 h can be used. Steels with UTS > 1450 MPa require 23 h heat soak at 190°C. No crack or failure shall be observed during the test. Chromium trioxide rinsing after phosphating on steels Steel surfaces where rinsing after phosphating was applied have to withstand 2 h in SST (ISO 9227) and must demonstrate sufficient paint adhesion according to ISO 2409 (GT0). More demanding test programmes are currently under development, ensuring that the quality criteria from the aerospace sector can be met with the alternative treatments. Passivation of metallic coatings (post-treatment CCC) For passivation of metallic coatings on steel, the aerospace industry indicated minimum requirements regarding corrosion resistance ranging between 96 h and 1000 h SST (depending on the thickness of the coating) according to ISO 9927 (with no show of red rust) for zinc, cadmium and aluminium substrates. The tested items shall neither show evidence of red rust, nor significant white rust (coating corrosion products) before 96 h SST. Adhesion of the coating to the substrate is assessed by the cross cut test according to ISO 2409. If paint is applied on passivated surfaces, it has to fulfil GT0 under dry conditions, respectively GT1 under wet conditions. Temperature resistance is analysed according to internal standards. The surface has to withstand shrink fitting using liquid nitrogen. No loss of coating or passivation system should be observed after immersion in liquid nitrogen at -196°C. Chemical conversion coatings - aluminium Regarding conversion coated surfaces, the aerospace industry provided a minimum requirement between 168-336 h without the appearance of corrosion for SST performed according to ISO 9227 and ASTM B117. When filiform corrosion is tested on painted parts according to EN3665, the requirement for some companies is length from scratch <0.5 mm, while most companies require <2- 3 mm after 960 h. The layer thickness must not exceed 1 µm. Regarding subsequent paint adhesion,
27 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES the most relevant test method is cross-cut test according to ISO 2409 with requirement of GT0 dry and
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4. ANNUAL TONNAGE
4.1. Annual tonnage band of chromium trioxide The annual tonnage band for the use of chromium trioxide in surface treatment for the aero sector is <100 tonnes per year.
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5. GENERAL OVERVIEW ON THE SPECIFIC APPROVAL PROCESS IN THE AEROSPACE SECTOR
5.1. General overview Much has already been written about the airworthiness and approvals process in the aerospace industry in the document “An elaboration of key aspects of the authorisation process in the context of aviation industry“ published in April 2014 by ECHA and European Aviation Safety Agency (EASA). The document makes a strong case for justification of long review periods for the aerospace sector. In this section we identify key points from the ECHA EASA “elaboration” document and add additional detail and justification for long review periods with specific regard to chromium trioxide. Some of the key points identified in the “elaboration” document are: - “The aerospace industry must comply with the airworthiness requirements derived from EU Regulation No 216/2008 in Europe, and with similar airworthiness requirements in all countries where aeronautical products are sold.” - “All components, from seats and galleys to bolts, equipment, materials and processes incorporated in an aircraft fulfil specific functions and must be certified, qualified and industrialised.” In addition the new materials must be developed and evaluated prior to these three steps. - “If a substance used in a material, process, component, or equipment, needs to be changed, this extensive process [of development, qualification, certification and industrialization] has to be followed in order to be compliant with the airworthiness requirements.” - “Although the airworthiness regulations (and associated Certification Specifications) do not specify materials or substances to be used, they set performance specifications to be met (e.g. fire testing protocols, loads to be sustained, damage tolerance, corrosion control, etc.). These performance specifications will drive the choice of substances to be used either directly in the aircraft or during the manufacturing and maintenance activities.” - The development (TRL (Technology Readiness Level) 1-6) process “is an extensive internal approval process with many different steps from basic technology research up to technology demonstration in a lab environment.” - “Depending upon the difficulty of the technical requirements [qualification] can easily take 3-5 years. After initial laboratory testing, each specific application must be reviewed, which means additional testing for specific applications / parts. Airworthiness Certification begins at this same time, this certification can take from 6 months to years. Additional time is needed for production scale-up and development of a supply chain.”
Each one of these points is of significant importance for the aerospace sector with regards to chromium trioxide. Further elaboration will be made within this section. The last bullet point highlights that it can take a significant period of time to develop and implement new alternatives. It should be noted that in the case of chromium trioxide, the stated time needed for taking an alternative from the development phase through qualification, certification and implementation has been significantly underestimated. Efforts to find replacements for chromium trioxide have been ongoing within the aerospace industry for over 30 years. In this time some successful substitutions have been made, but large challenges remain. Efforts thus far to identify equivalents for substances with critical, unique properties like corrosion inhibition have proven that there are no ‘drop-in’ replacement substances for Cr(VI). Depending on the specific application and performance requirements, many more years may be required before alternatives are identified and implemented.
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In this section the general process for alternative development through qualification, certification, industrialization and implementation within the aerospace sector is described. This process is also followed closely by the military and space sectors. The long-term operations of aviation applications are similar to the space industry. In the case of the space industry, the lifecycle depends on the type of spacecraft (military or civil launchers, satellites etc.), but it can be at least as complex and challenging as aircraft regarding: lifecycle duration, environment exposure and high requirements. The space sector is a highly valuable sector and a major political concern, as the EU wants to have its own capacity to access to space (for example major programs such as Ariane 5, Vega, and all satellite programs for earth observation and telecommunications). Apart from the complexity of the supply, the aerospace sector faces particular unique challenges related to the operating environment, compliance with the airworthiness requirements and spacecraft requirements and the longevity of an aircraft and spacecraft that constrain its ability to adopt changes in materials and processes in the short, medium or even longer terms. Because of the stringent requirements for qualification and certification, a formal process for technology readiness and manufacturing readiness is followed. The process for qualification, certification and industrialization as described in the ECHA EASA “elaboration” document is shown in Figure 11.
Figure 11: Illustration of the qualification, certification and industrialisation processes.
This diagram is perhaps overly simplified and doesn’t indicate the significant level of research and development work required prior to achieving qualification. As stated in the “elaboration” document “This process is an extensive internal approval process with many different steps from basic technology research up to technology demonstration in a lab environment.” The actual process followed by Original Equipment Manufacturers (OEMs) in the aerospace sector more closely follows the framework for Technology Readiness Levels (TRLs) and Manufacturing Readiness Levels (MRLs) originally developed by National Aeronautics and Space Administration (NASA). OEMs usually adapt this TRL/MRL approach resulting in individual versions which are considered proprietary and cannot be presented here. The NASA version is shown in Table 6. Table 6: Technology Readiness Levels – Overview (US Department of Defence, 2011, adapted 2014).
TRL# Level Title Description Lowest level of technology readiness. Scientific research begins to be translated into applied research and development 1 Basic principles observed and reported (R&D). Examples might include paper studies of a technology’s basic properties. Invention begins. Once basic principles are observed, practical Technology concept and/or application applications can be invented. Applications are speculative, and 2 formulated there may be no proof or detailed analysis to support the assumptions. Examples are limited to analytic studies. Analytical and experimental critical Active R&D is initiated. This includes analytical studies and 3 function and/or characteristic proof-of- laboratory studies to physically validate the analytical concept predictions of separate elements of the technology. Examples
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TRL# Level Title Description include components that are not yet integrated or representative. Basic technological components are integrated to establish that Component and/or breadboard they will work together. This is relatively “low fidelity” 4 validation in laboratory environment compared with the eventual system. Examples include integration of “ad hoc” hardware in the laboratory. Fidelity of breadboard technology increases significantly. The basic technological components are integrated with reasonably Component and/or breadboard 5 realistic supporting elements so they can be tested in a validation in relevant environment simulated environment. Examples include “high-fidelity” laboratory integration of components. Representative model or prototype system, which is well beyond that of TRL 5, is tested in a relevant environment. System / subsystem model or prototype Represents a major step up in a technology’s demonstrated 6 demonstration in a relevant environment readiness. Examples include testing a prototype in a high- fidelity laboratory environment or in a simulated operational environment. Prototype near or at planned operational system. Represents a System prototype demonstration in an major step up from TRL 6 by requiring demonstration of an 7 operational environment actual system prototype in an operational environment (e.g., in an aircraft, in a vehicle, or in space). Technology has been proven to work in its final form and under expected conditions. In almost all cases, this TRL represents Actual system completed and qualified the end of true system development. Examples include 8 through test and demonstration developmental test and evaluation (DT&E) of the system in its intended weapon system to determine if it meets design specifications. Actual application of the technology in its final form and under Actual system through successful mission conditions, such as those encountered in operational 9 mission operations test and evaluation (OT&E). Examples include using the system under operational mission conditions.
In general the TRL assessments guide engineers and management in deciding when a candidate alternative (be it a material or process) is ready to advance to the next level. Early in the process, technical experts establish basic criteria and deliverables required to proceed from one level to the next. As the technology matures, additional stakeholders become involved and the criteria are refined. As specific applications are targeted as initial implementation opportunities, design and certification requirements are added to the criteria. Many more factors have to be taken into account prior to making a decision about transition of technology or replacing a material. A formal gate review process has been established by some companies to control passage between certain levels in the process. A similar set of guidelines for MRLs exist for the management of manufacturing risk and technology transition process. MRLs were designed with a numbering system similar and complementary to TRLs and are also intended to provide a measurement scale and vocabulary to discuss maturity and risk. It is common for manufacturing readiness to be paced by technology or process readiness. Manufacturing processes require stable product technology and design. Many companies combine the aspects of TRLs and MRLs in their maturity assessment criteria, as issues in either the technology or manufacturing development will determine production readiness and implementation of any new technology. Referring back, now, to Figure 11, the general process steps can be loosely correlated to the steps in the TRL and MRL frameworks. Qualification begins after TRL 6 when technology readiness has been demonstrated. Certification begins around TRL 9 at the latest and may be performed in parallel with
32 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES qualification. Industrialization/Implementation are not tracked on the NASA TRL scale, but some OEMs refer to this phase as TRL 10. As previously stated, what is missing from the diagram is the necessary and significant work that is performed before reaching technology readiness at TRL 6. The following sections describe the highlights of the entire process from definition of needs before technology development begins through to implementation. The emphasis here is to provide a description of the general process while highlighting the inherent complexities. One additional point to keep in mind when reviewing the process description that follows is that there is no guarantee that the initial process to identify an alternative for a substance is successful. Failure is possible at every stage of the TRL process. The impact of failure can be significant in terms of time.
5.2. Development and qualification
5.2.1. Requirements development A need for a design change may be triggered due to many reasons. The one of interest here is when a substance currently used for production of aerospace parts is targeted for sunset (e.g. chromium trioxide). Completely removing one substance may impact various parts and systems on an aircraft and spacecraft and may involve many different processes with different performance requirements. Once a substance is identified to be targeted by a regulation, a first step is to identify the materials and processes containing the specific substance. Most companies rely upon the information provided by the chemical manufacturer in the safety data sheet (SDS). This information source has many limitations when used for substance identification including: lack of reporting due to protection of proprietary data; reporting large concentration bands to protect specific formulary data; different disclosure requirements based upon country (articles exemption, thresholds, de minimis, specific substance classifications, etc.) to name a few. After identifying the materials and processes and associating them with specifications and other design references, parts get identified along with applications and products potentially impacted. This is the first step in order to assess the impact for the company. This work requires contributions from numerous personnel from various departments of an aerospace company like Materials & Processes, Research & Development, Engineering, Customers Service, Procurement, Manufacturing, Certification, including affiliates in other countries and Risk Sharing Partners. Current production aircraft may have been designed 20 to 30 years ago (or more) using design methods and tools that are not easily revisited, nor were they necessarily standardized between OEMs. Checking and changing the drawings implies updating, e.g. creating the drawings under the new formats and tools, which can involve a tremendous amount of design work. Note: When a new design is needed e.g. to remove a substance, it may not be compatible with the existing one, this means that spare parts designs of the original materials/configurations may need to be preserved in order to be able to produce spare parts for the aircraft using the original (baseline) configuration. This is an additional impact to be taken into account. Once a substitution project is launched, technical specialists, from engineering and manufacturing departments, must define the requirements that the alternatives have to fulfil. Alternatives must satisfy numerous requirements. In many cases requirements are identified that introduce competing technical constraints and lead to complex test programmes. This can limit the
33 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES evaluation of alternatives. For instance, for some materials, dozens of individual engineering requirements with similar quantities of industrial requirements may be defined. Categories of technical requirements may include: - Materials and processes requirements (e.g. corrosion resistance, adhesion strength), - Design requirements (e.g. compatibility of the component’s geometry complexity and with the coating application technique), - Industrial requirements (e.g. robustness and repeatability), - Environment, Health & Safety (EHS) requirements.
Definition of needs itself can be complex and requires significant timeframe. The complexity can be due to: - Different behaviour of the substitute compared to original product: new requirements may be defined. In this case, sufficient operational feedback to technically understand the phenomenon and to reproduce it at laboratory scale is a must in order to be able to define acceptance criteria. - Requirements may come from suppliers and have an impact on the design. - EHS regulations evolution. Once initial technical requirements are defined, potential solutions can then be identified and tested. The timeframe for initial requirements development can last up to 6 months. Note that requirements may be added and continue to be refined during the different levels of maturity.
5.2.2. Technology development The development process (typically TRL 4-6) is complex, and several years are often necessary before reaching development phase end (TRL 6). The following points explain why it may be long and complex: - Developing solutions usually necessitates several testing phases before meeting the numerous requirements, which often induce several loops to adjust the formulation / design. - Some tests are long lasting (e.g. some corrosion tests last 3000 h or longer). - In some cases, potential alternatives are patented, preventing multiple sources of supply, which is an obstacle to a large supply-chain deployment due to increases in legal costs and in some cases a reduction in profitability for the business. - When no 1 to 1 replacement solution is available, each alternative process must individually be considered to determine for which specific quantitative application it is suitable. This work represents a significant resources mobilization, especially in term of drawings update and implementation of alternatives which due to the multiple work streams takes longer with higher costs. Moreover, spare parts and maintenance processes redesign may result in complex management both at the OEM and the Airlines. Additionally, substances regulations are evolving throughout the long research and development phase and life cycle of aircraft, which is another challenge for OEMs. There is a risk that significant investments could be made to develop and qualify alternative solutions involving substances with low EHS impacts identified at that point in time. Solutions may be developed and finally qualified, however, in the meantime, EHS constraints on those substances increased to a point where they now meet the SVHC criteria. - When the suppliers have no “off the shelf” solutions, they have to develop new ones considering the list of requirements that are often highly complex to combine (see the description of requirements in the above paragraph).
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- Drawings impact: The replacement of a material / process may impact the complete design of a part. Additionally, the mating part/counterpart functionality must be analysed too (materials compatibility, dimensional compatibility, stress compatibility). This may lead to redesign of the complete part plus mating parts. - Process instructions shall be elaborated. The description of the development process is included in the qualification section of the ECHA EASA “elaboration” document. The text is reproduced here for continuity. “Qualification precedes certification and is the process under which an organisation determines that a material, process, component or equipment have met or exceeded specific performance requirements as documented in a technical standard or specification. These specifications, often abbreviated as spec(s), contain explicit performance requirements, test methods, acceptance testing, and other characteristics that are based upon the results of research, development and prior product experience. The industry relies upon standards issued by government-accredited bodies, industry or military organisations, or upon company-developed proprietary specs. Most materials and process specifications include either a “Qualified Products List” (QPL) or “Materials Control” section that identifies products that have met the requirements. Application and use of these qualified products must be assessed and certification implications addressed before being used on aircraft hardware. OEMs rely upon the expertise of the chemical formulators to provide viable candidates to test against specific material and process specs.”
It is important to note that many iterations of these formulas are rejected in the formulator’s laboratory and do not proceed to OEM evaluation. Formulators estimate 2 to 5 years before candidates are submitted to OEMs. “Once candidate(s) are developed, the OEM evaluates candidates by performing screening testing. If the candidate passes screening, testing is expanded to increase the likelihood that the preparation will pass qualification. If the candidate fails, which is often the case, material suppliers may choose to reformulate. It is not uncommon to iterate multiple times before a candidate passes screening. In some technically challenging areas, over 100 formulations have been tested with no success. This phase of development can take multiple years depending upon the material requirements. For those materials that pass screening, production scale-up, development of process control documents, manufacturing site qualifications, and extensive qualification testing is required to demonstrate equivalent or better performance to that which is being replaced. This phase of the process can also result in formulation or manufacturing iterations and may take several additional years. Depending on the complexity of the change and the criticality of the application (for example, fire protection or corrosion prevention have high safety implications and require development and testing against multiple, rigorous performance standards), re-certification may be required. The industry is ultimately limited by the material formulators’ willingness to expend their resources to develop alternative materials and technologies to be tested.”
The small volumes of materials sold, demanding performance requirements, and tightly controlled manufacturing processes for aviation customers provide insufficient incentive for reformulation in some cases. When material formulators are not willing to reformulate their materials, new sources need to be sought.
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Figure 12: Illustration of the technology development and qualification process. (EASA, 2014; amended)
“This process [TRL 1-6 development] is an extensive internal approval process with many different steps from basic technology research up to technology demonstration in a lab environment. Depending upon the difficulty of the technical requirements, these initial steps can easily take 3-5 years. After initial laboratory testing, each specific application must be reviewed, which means additional testing for specific applications / parts. Airworthiness Certification begins at this same time, this certification can take from 6 months to years. Additional time is needed for production scale-up and development of a supply chain.”
It should be noted that the timeframes for development and qualification stated in the “elaboration” document have been combined and may be understated in the case of chromium trioxide. Depending on the application and the complexity of material and process requirements, this process can easily take multiple years. As noted in the “elaboration” document the timeframe for development alone is typically a minimum of 3 to 5 years. Our experience with replacement of the substance addressed in this dossier is that the development takes much longer. For typically successful projects the duration is 3 to 5 years. For unsuccessful projects the development goes through repeated iterations and has taken over 30 years and still continues with limited success.
5.2.3. Qualification Only after a technology has demonstrated TRL 6, do the OEMs begin the qualification. All material, components, equipment or processes have to meet or exceed the specific performance requirements which are defined in the Certification Specifications documented in technical standards or specifications as described in chapter 5.2.4. These are issued by military organisations, government- accredited bodies, industries or upon company-developed proprietary specifications. Products which have met all requirements are included in the documents as “Qualified Products List” (QPL) or in the “Materials Control” section. The main reasons for qualification are: - To fulfil requirements by the Airworthiness Authorities (EASA); this is the first level of the Aircraft Certification Pyramid. - To ensure that only Approved, reliably performing Materials, Parts and Processes are used to produce Aircraft Components and Systems.
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- To ensure that the product, the process or method is compliant with the Industry Regulations and Aircraft Manufacturer requirements to fulfil a specified function. - To provide a level of confidence and safety. - To ensure consistent quality of products and processes. - To ensure Supplier control, and to guarantee production and management system robustness, throughout the Supply Chain. The qualification process is mandatory to demonstrate compliance with airworthiness and certification requirements: the qualification process ensures that the technical and manufacturing requirements documented in the relevant material and/or process specifications are met. The qualification process comprises several steps before materials/processes are qualified. Even if most showstoppers are identified during the development phase, process confirmation/production verification are performed during the qualification phase. In case of failure, product qualification will be cancelled and the development phase must start again from the beginning. Based upon OEM experience, the time period needed to pass the qualification process is estimated to be on the order of 8 years and can be even longer when major test failures occur. This is one of the main challenges for chromium trioxide replacement. Depending upon the materials, processes and criticality of the applications being evaluated, in-service evaluation and monitoring will be required and can extend to 15 years or more depending upon application.
5.2.4. Certification This next step is to certify that an aircraft and every part of it complies with all applicable airworthiness regulations and associated Certification Specifications (specs). This step is also well described in the “elaboration” document and is reproduced here for continuity. “Certification is the process under which it is determined that an aircraft, engine, propeller or any other aircraft part or equipment comply with the safety, performance environmental (noise & emissions) and any other requirements contained in the applicable airworthiness regulations, like flammability, corrosion resistance etc. Although the airworthiness regulations (and associated Certification Specifications) do not specify materials or substances to be used, they set performance specifications to be met (e.g. fire testing protocols, loads to be sustained, damage tolerance, corrosion control, etc.). These performance specifications will drive the choice of substances to be used either directly in the aircraft or during the manufacturing and maintenance activities. Some examples of performance requirements are the following: - Resistance to deterioration (e.g. corrosion) Environmental damage (corrosion for metal, delamination for composites) and accidental damage during operation or maintenance. - Corrosive fluids - Hydraulic fluids; Blue water systems (toilet systems and areas); leakage of corrosive fluids/substances from cargo. - Microbiological growth in aircraft fuel tanks due to moisture/contamination in fuel cause severe corrosion. Such corrosion debris has the potential to dislodge from the fuel tanks, migrate through the fuel system, and lead to an in-flight engine shutdown. - Resistance to fire – Flammability Requirements Fire-proof and fire-resistance. Aircraft elements are expected to withstand fire for a specified time without producing toxic fumes; this leads to using products like flame retardants, insulation blankets, heat protection elements in hot areas (e.g. around engines). The primary certification of the aircraft (or engine and propeller) is granted to the manufacturer by the Competent Aviation Authority of the “State of Design” which is typically the authority of the state where the manufacturer of the aircraft (or engine or propeller) is officially located (EASA in the case of aircraft designed and manufactured in the EU and European Free Trade Association countries).
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Aircraft that are exported to other countries will have to be certified (validated) also by the authority of the “State of Registry”. Manufacturers work with the certification authorities to develop a comprehensive plan to demonstrate that the aircraft meets the airworthiness requirements. This activity begins during the initial design phase and addresses the aircraft structure and all systems in normal and specific failure conditions (e.g. tire failure, failure of structural components, hydraulics, electrical or engines). The tests needed to demonstrate compliance, range from thousands of coupon tests of materials, parts and components of the airplane, up to tests that include the complete aircraft or represents the complete aircraft (system). The performance and durability of the various materials have to be confirmed while the behaviour of the parts, components and the complete airplane will have to be tested in the applicable environmental and flight conditions including various potential damage or failure conditions. For a new Type Certificate, this overall compliance demonstration covers several thousands of individual test plans of which some will require several years to complete. Often, after the initial issuance of the Type Certificate, the tests that have the objective to demonstrate durability of the aircraft during its service life, will continue. All the different aspects covered by the Type Certificate together define the “approved type design” which includes, among other aspects, all the materials and processes used during manufacturing and maintenance activities. Each individual aircraft has to be produced and maintained in conformity with this approved type design. Changes to the approved type design may be driven by product improvements, improved manufacturing processes, new regulations (including those such as new authorisation requirements under REACH, customer options or the need to perform certain repairs. When new materials or design changes are introduced, the original compliance demonstration will have to be reviewed for applicability and validity, in addition to a review of potential new aspects of the new material or design change that could affect the airworthiness of the aircraft. Depending on the change, this review could be restricted to coupon or component tests, but for other changes this could involve rather extensive testing. E.g. changes in protective coatings could affect not only the corrosion resistance but could also affect the friction characteristics of moving components in actuators in the different environmental conditions, changing the dynamic behaviour of the system, which in the end affects the dynamic response of the airplane. Before the new material or design change can be introduced on the aircraft, all test and compliance demonstrations have to be successfully completed and approved by the Competent Authority. This approval results in the issuance of a Supplemental Type Certificate (STC), change approval or repair approval. It is important to note that, according to the EU Regulation No 216/2008, EASA is the design competent authority for civil aircraft only. Any other aircraft (e.g. military, fire-fighting, state and police aircraft) will have to follow similar rules of the corresponding State of Registry. To be able to maintain and operate an aircraft the responsible organisations must be approved by the competent authority and compliance is verified on a regular basis. Maintenance of an aircraft requires that the organization complies with specific procedures and materials described in the maintenance manuals which are issued by and the responsibility of the OEMs.” As noted in the “elaboration” document, in optimal cases certification can take as little as 6 months but typically will take several years. The duration really depends on the specific material and application.
5.2.5. Implementation / industrialisation An aircraft consists of several million parts which are provided by thousands of suppliers or manufactured internally by OEMs. Significant investment, worker training and manufacturing
38 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES documentation may be required to adapt the manufacturing processes which sometimes require changes in existing facilities or the construction of new facilities. The industrial implementation is usually scheduled to follow a step wise approach to minimize the technical risks and benefit from lessons learned. This implies that the replacement is not implemented in one shot in all plants and at all suppliers but stepwise. Each OEM may own several plants, e.g. up to 20 manufacturing sites / final assembly lines worldwide for some of them. Furthermore, the implementation of an alternative process may induce new development and modification in the complete process flow. The following text is reproduced from the “elaboration” document and describes the process for implementation of an alternative: “Industrialisation is an extensive step-by-step methodology followed in order to implement a qualified material or process throughout the manufacturing, supply chain and maintenance operations, leading to the final certification of the aerospace product. This includes re-negotiation with suppliers, investment in process implementation and final audit in order to qualify the processor to the qualified process. Taking into account that an aircraft is assembled from several million parts provided by several thousand suppliers, this provides an indication of the complexity for the industrialisation stage of replacement materials/processes, and the supply chain which provides these parts. Special challenges are: - Low volumes limit influence on changes to suppliers’ materials / processes - Procurement & insertion of new equipment - Scale-up & certification of new process - Incompatibility of coatings could be a risk. - Re-negotiation of long term agreements with suppliers*. - Increased complexity of repairs – Multiple different solutions for different applications as a substitute for a single, robust process. For example, currently all aluminum parts can be repaired with one chromated conversion coating. In some specific cases, the future state could require different conversion coatings for each aluminum alloy and application environment. Since different alloys are not easily distinguishable on the shop floor, ensuring that the proper repair procedures are used will be much more difficult. If alternate means of compliance approvals are requested for repair facilities or airlines, regulatory agencies are unlikely to have adequate knowledge or technical data to make informed assessments.
The operating environment, longevity of the aircraft, supply chain complexity, performance and above all airworthiness requirements are some of the considerations which can constrain the ability of the industry to make changes and adopt substitutes in the short, medium or long term.” *Changes to the design or manufacturing may require re-negotiations with suppliers which can be time-consuming, especially when long-term contracts are concerned. The supply chain is complex in the aviation industry; it includes but is not limited to chemical manufacturers, importers, distributors, formulators, component manufacturers, OEMs, Airline operators, and aftermarket repair and overhaul activities. The timeframe for implementation and industrialisation is unknown. Simple changes may take 18 months to 5 years. Our experience with replacement of the substance addressed in this dossier is that full implementation and industrialization has yet to be accomplished. Implementation by airlines and Maintenance, Repair and Operations (MROs) further requires that an alternative is approved by the OEM and made available in the maintenance documents.
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When the alternative process is included in the maintenance documents, challenges described above have to be faced by airlines and MRO to implement the alternative. Here, for operating supplies and testing time frames, another 3 years might be necessary, depending on complexity of the alternative. When more alternative processes have to be established simultaneously, as it is currently the case for TSA and Boric-Sulphuric Acid anodizing (BSA), more than 5 years might be necessary to fully implement the alternatives.
5.2.6. Examples In 2003, RoHS (Directive on Restriction of Hazardous Substances, 2002/95/EC) was adopted by the EU and took effect on July 2006. This Directive triggered companies to substitute lead-based solder in electronic assemblies and all subsequent changes in the product designs and manufacturing processes: Basic research was started in the example company in 2003 with the selection and tests of alternative lead-free solder. The Research Program is still running in 2014 and the qualification and industrialization phase is ongoing: Components (IC’s, connectors, printed circuit boards etc.) had to be changed due to the higher soldering temperature that all materials have to withstand with lead-free solder, and most of the manufacturing equipment had to be replaced by new ones. This fundamental replacement of lead (Pb) for aerospace and military applications with harsh environmental conditions will take more than 15 years in total to be deployed up to TRL 9. Work on a replacement for CAA began in 1982. The initial driver for this R&D effort was to reduce emissions of Cr(VI) and comply with federal and local clean air regulations in the US. Initial requirements were identified and four candidate solutions were evaluated. One candidate solution was down selected in 1984. Qualification testing began in 1985. A process specification for boric sulphuric acid anodizing was released in 1990. In 1991 and 1992 industrialization began as several Boeing facilities began producing parts using the BSA process. One outside supplier also began processing parts to the Boeing specification in 1992. Evaluation of additional applications continued into the mid-1990s. In 2015, industrialization of the BSA alternative for CAA is still not complete. Many Boeing suppliers are shared with other OEMs and industries impeding the conversion to BSA from CAA because they must continue to support multiple customer requirements. Note that for unprimed parts a dilute chromium trioxide seal is still required to provide required corrosion resistance. Work is ongoing to develop alternatives for this application. It is also worth noting that boric acid is now being proposed for Annex XIV requiring authorisation. Should this happen alternatives may need to be developed for BSA. Other OEM solutions will need to be evaluated, qualified and certified by Boeing.
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6. IDENTIFICATION OF POSSIBLE ALTERNATIVES
6.1. Description of efforts made to identify possible alternatives To prepare the authorisation of chromium trioxide, the industry consortium CTAC (Chromium Trioxide Authorisation Consortium) of 150+ members was launched in 2012. The aim of CTAC was to efficiently gather and analyse all necessary information for the three pillars of the authorisation dossier (CSR, AoA, SEA).
6.1.1. Research and development As mentioned earlier in this document, a large amount of research over the last few decades has been commissioned to identify and develop viable alternatives to chromium trioxide and Cr(VI) compounds in general. The unique functionalities of chromium trioxide (explained in detail in chapter 3.3) make it challenging and complex to replace the substance in surface treatment applications where superior corrosion or adhesion properties are required to ensure safe performance in a demanding environment. Numerous research programmes were conducted funded by Europe clean sky (MASSPS, ROPCAS, LISA, DOCT, MUST, MULTIPROJECT) as well as programmes funded by United States Air Force (USAF) or other national funded programmes (e.g. LATEST in UK). Some key research programmes are listed below. Amongst a number of initiatives in that respect, the Airbus Chromate-Free (ACF) project was launched more than 10 years ago with the aim to progressively develop new environmental friendly Cr(VI)-free alternatives to qualified products and processes used in aircraft production and maintenance. Even prior to the launch of ACF, R&D efforts included the objective to remove chromium trioxide use. The ACF project is organised into several topics for the different fields of technologies concerned by the replacement. ACF specially addressed applications where chromium trioxide is used in production or applied to the aircraft; such as CAA, basic primer, and external paints. In addition, bonding primer, jointing compounds, pickling, sealants, chromium-based chemical conversion coatings, passivation of stainless steels, passivation of metallic coatings or alternatives to hard chromium are included in the remit of this project. In synergy with ACF, an Airbus Group chromium trioxide replacement project is also in place. A similar initiative was set up for spacecraft, the Launcher chromate-free (LCF) project. In 2006, Boeing in cooperation with the Department of Defence started a three-year program called “Environmentally Benign Coating System for Department of Defence Substrates” for the development of new chromium trioxide free coating systems, based on rare-earth conversion coatings. Industry is not only working on one-to-one replacements for Cr(VI) applications, but is also reconsidering whole current coating systems. The large investment in innovative coating technologies may lead successively to a paradigm shift within the next decades. As an example, the HITEA (Highly Innovative Technology Enablers for Aerospace) project was initiated in 2012; a 17-member consortium consisting of aerospace OEMs, suppliers, paint application companies and academics with the goal to identify and evaluate suitable alternative systems. In 2014, the tested alternatives are planned to reach TRL 2. After the initial phase, the project will focus on a handful of promising alternatives, where further testing will be undertaken within the next years and completion of the research project is planned for 2015. Qualification (TRL 6) will take up to 5-8 years from now. In 2008, the multi-company project SOL-GREEN was initiated for the development of protective coatings of Al/Mg alloys (Cerda et al, 2011). Since these coatings solutions are not based on
41 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES electrochemical conversion, and so require a complete change in technical approach, the industrial production qualification is expected not before 2025. Phase 1 was finished last year, showing that SOL-GREEN 1 faces some technical issues. Therefore, the main objective of SOL-GREEN 2 is to assess and develop an electrophoresis process to apply the anti-corrosion coatings using a SOL-GEL technique for complex geometry parts. These challenges are currently ongoing and the research is mainly conducted at laboratory scale (TRL 2) at universities and in some partner’s plants. However, as mentioned above, this is a long term solution (10-15 years).
6.1.2. Data searches For the analysis of alternatives, extensive literature and test reports were provided by the technical experts of the CTAC consortium members. Furthermore, searches for publically available documents were conducted to ensure that all potential alternate processes to Cr(VI)-containing applications were considered in the data analysis. In addition to databases for scientific literature, the following programmes were intensively consulted: Toxics Use Reduction Institute, Massachusetts, US (www.turi.org/); The Advanced Materials, Manufacturing, and Testing Information Analysis Center (AMMTIAC: http://ammtiac.alionscience.com/). Searches for SDS for Cr(VI)-containing and chrome-free applications were also conducted. Based on these data, primary scoping led to the development of a generic questionnaire containing potential alternatives to chromium trioxide-based surface treatment processes. As a result of this, additional alternate processes, mentioned by companies from the aerospace sector were included in the initial list of alternatives, which can be found in Appendix 1.
6.2. Consultations A questionnaire was provided to all CTAC consortium members to get an overview of and experience with the alternatives, completeness and prioritisation of critical parameters for their specific processes and the minimum technical requirements. During this survey, additional alternatives have been identified which were included into the aforementioned initial list. At this stage of the data analysis, several alternatives had been screened out after bilateral discussions with the companies, based on confirmation that they might be general alternatives to chromium trioxide-based processes (e.g. for functional chrome plating), but are not applicable for the use defined here. To verify data and obtain more detailed quantitative information, further focused technical questionnaires were sent out and discussed with the CTAC consortium members. In addition, site visits to selected companies were carried out. These were carefully chosen to adequately represent the different uses, industry sectors, countries and the size of companies. Discussions with technical experts followed by a final data analysis led to the formation of a list of alternatives divided into 3 categories, according to their potential to be suitable for the specific use. The most promising alternatives within the here defined use (category 1 and 2) are assessed in detail in the following chapter. Category 3 alternatives are summarised in Appendix 1.
6.3. List of possible alternatives The most promising alternatives are discussed in the following chapter. To allow a better overview of the different parts of the process chain, the assessment is made for main treatments plus post- treatments (Table 7) and pre-treatments (Table 8) separately. The main and post-treatment alternatives are classified according to their relevance; as Category 1 (focus of CTAC members, relevant R&D on these substances ongoing) or Category 2 (discussed
42 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES mainly in literature, clear technical limitations, may only be suitable for other industry sectors or for niche applications but not as general alternative). Category 3 alternatives, which are not applicable for the use defined here, are summarised in Appendix 1.
Table 7: List of main treatment alternatives categorised
Category Alternative Surface treatment with chromium trioxide
Chromate Conversion Coating Acidic surface treatments Passivation of stainless steel Chromic acid anodizing Chromate Conversion Coating Passivation of metallic coatings Cr(III)-based surface treatments Category 1 Sealing after anodizing alternatives Sacrificial and slurry (diffusion) coatings Chromate Conversion Coating Silane/Siloxane and Sol-gel coating Sealing after anodizing Passivation of metallic coatings Sealing after anodizing Water-based post-treatments Rinsing after phosphating Molybdates- and Molybdenum-based Chromate Conversion Coating processes Sealing after anodizing
Organometallics (zirconium and titanium- Chromate Conversion Coating based products) Passivation of metallic coatings Benzotriazoles-based processes, e.g. 5- Chromate Conversion Coating methyl-1H-benzotriazol Category 2 Chromate Conversion Coating alternatives Manganese-based processes Sealing after anodizing Sacrificial and slurry (diffusion) coatings Magnesium rich primers Chromate Conversion Coating (local application) Electrolytic paint technology Chromate Conversion Coating + primer
Zinc-nickel electroplating Passivation of metallic coatings
Table 8: List of pre-treatment alternatives categorised.
Alternative Surface pre-treatment Substrate
Functional cleaning/ Aluminium and aluminium alloys, Steel, Pickling/Etching copper, brass, cadmium Deoxidizing Aluminium and aluminium alloys Inorganic acids (plus additives) Stripping of organic (e.g. paints, lacquers) and Aluminium and aluminium alloys, inorganic finishes (e.g. Magnesium and magnesium alloys Conversion coatings, Anodic coatings) Aluminium and aluminium alloys, steel Hydrogen peroxide activated benzyl Stripping of paint (CRES), nickel/cobalt alloys, titanium alcohol (with acids) and magnesium
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7. SUITABILITY AND AVAILABILITY OF POSSIBLE ALTERNATIVES In the following chapter, possible alternatives are assessed specifically for the industry sector, where they may be potential alternatives. Initially, general process or substance properties are described, followed by the assessment of the technical feasibility, availability and reduction of overall risk. To assess the feasibility of the alternatives, colour-coded summary tables are included in the document. The colours are as follows: - Red: not sufficient - the parameters/assessment criteria do not fulfil the requirements of the respective sector; - Green: sufficient - the parameters/assessment criteria do fulfil the requirements of the respective sector; - Yellow – the parameters/assessment criteria fulfil some requirements for some but not all applications (only used for the assessment of the technical feasibility). The alternative assessments each comprise a non-exhaustive overview of substances used with the alternatives and alternative processes as well as the risk to human health and environment. These tables are provided in Appendix 2.
7.1. Main Processes & Post-treatments
CATEGORY 1 ALTERNATIVES The alternatives assessed in this section are considered the most promising ones, where considerable R&D efforts are carried out within the aerospace sector. Category 1 alternatives were often discussed during the consultation phase. In most cases, they are in early research stages and still showed technical limitations when it comes to the demanding requirements from the aerospace sector, such as corrosion performance. However, some of these possible alternatives may already be qualified and used in other industry sectors or for niche applications, respectively for single process steps of the process chain within aerospace but not as general alternatives to chromium trioxide-containing surface treatment process chains. 7.1.1. ALTERNATIVE 1: Acidic surface treatments 7.1.1.1 Substance ID and properties A variety of inorganic acids and organic acids (tannic, citric, tartaric) are currently under evaluation as alternatives to Cr(VI) in surface treatment processes within the applied use. Research is currently focused on boric acid, sulphuric acid, phosphoric acid, tartaric acid, nitric acid and citric acid. Table 9 provides an overview of which acids are used in the respective surface treatment processes. The most important processes, where inorganic acids are used, are anodizing coating processes of aluminium and magnesium, conversion coating processes of aluminium, zinc, zinc-nickel, cadmium, magnesium and silver, and passivation processes of stainless steel for creating stable oxide coatings, converted oxide layers, or passive films. Ferrous metals like iron or certain steels may cause interferences with subsequent coatings or platings and are therefore considered with alternative methods. An overview of general information on substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.1.
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7.1.1.2 Technical feasibility Table 9 summarizes chromium trioxide-based surface treatment processes where inorganic and organic acids were assessed to be an applicable alternative.
Table 9: Overview on the acids used in the different surfaces treatment processes
Industry sector Surface treatment Substrate / coating Alternative
Boric-sulphuric acid anodizing (BSA) Sulphuric acid anodizing (SAA) Chromic acid Aluminium and Thin film sulphuric acid anodizing (TFSA) anodizing aluminium alloys Tartaric sulphuric acid anodizing (TSA)
Aerospace industry Phosphoric acid-based anodizing (PAA, PSA) Chemical conversion Aluminium and SAA, PAA, PSA + topcoat or sealant coating aluminium alloys Nitric acid passivation Passivation Stainless Steel Citric acid passivation
Chromic acid anodizing General assessment of Cr(VI)-free acidic anodizing treatments: Numerous CTAC members have undertaken R&D to identify suitable alternatives for Cr(VI) containing anodizing. For example, Airbus initiated, amongst other projects, the ACF project to “develop new eco-efficient alternatives to qualified products and processes used in the aircraft production and maintenance using chromium trioxide and offer these new solutions widely, bringing an overall benefit throughout the life cycle of the aircraft, including for maintenance operations” (summarized in the Airbus Fast 45 report 2009). Within the ACF project, two processes for the treatment of aluminium surfaces as alternatives to CAA were investigated - TSA: Tartaric Sulphuric Acid Anodising; and - PSA: Phosphoric Sulphuric Acid Anodising.
TSA and PSA are both electrochemical processes used for the generation of an oxide layer on the aluminium surface for corrosion protection. They can also be applied as surface treatment processes prior to subsequent applications of corrosion-inhibiting primers. Figure 13 presents the morphology of surfaces treated with CAA, TSA and PSA. The morphology of both of the chromium trioxide free anodic processes is a regularly structured open porous aluminium oxide.
TSA CAA PSA Figure 13: Morphology of aluminium surfaces treated with TSA, CAA and PSA (Airbus Fast report, 2009).
PSA is similar to CAA and TSA with a modified morphology specifically for bonding applications. Therefore, PSA was assessed regarding its special structural bonding applications.
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For all three processes, the key performance parameters are corrosion resistance and paint adhesion as well as the quality of the coating regarding its fatigue properties. For the best aluminium anodising result, a balance between adhesion, corrosion resistance, and fatigue properties has to be achieved.
Table 10 summarizes process and performance parameters investigated during the development phase of TSA and PSA. TSA and PSA have shorter process times and lower process temperatures, which lead to a more efficient process by reducing time and energy consumption and simultaneously offer an increase in production capacity.
Table 10: Process relevant criteria of CAA, PSA and TSA (Airbus Fast report, 2009)
TSA CAA PSA
Coating thickness [µm] - depending on material 3-5 3-5 1-5
Process temperature [°C] 37-43 40 28-32
Process time [min] 20-35 45 20-25
Anodizing reduces the fatigue life of the aluminium compared to untreated aluminium. Fatigue life is a generic requirement for structural aluminium parts. All alternative processes must provide similar or better fatigue performance compared to CAA. In addition to the performance parameters, process related parameters such as racking, tank materials, and fungal contamination of the bath must also be considered. As a result of the ACF project, it was shown that both TSA and PSA have the same racking procedure as CAA and no changes need to be taken into account. The TSA and PSA processes may be operated with the same tank installations as CAA with only minor changes, in that tank and piping materials must be resistant to phosphoric and sulphuric acid (which is normally the case for CAA processes). Residuals of CAA do not compromise the performance of TSA or PSA. As the TSA electrolytes are less toxic than the CAA, fungus would be able to grow in the treatment line. Therefore, the installation of preventive measures such as filters and UV-lamps is recommended, which could lead to increased process costs. The phosphoric and sulphuric acids also have a large impact on waste-water treatment processes and installations, as these must be completely replaced. Another consequence is that waste water disposal should be in accordance with local environmental regulations and the required levels for CAA are very different compared to PSA/TSA. Following the Airbus ACF project, TSA is being implemented as the new standard replacing CAA in several Airbus plants and deployed within the supply chain. However, the process is only applicable on certain aluminium alloys. Tartaric sulphuric acid can also be used as a pre-treatment for painting in the aviation sector on high Cu containing Al alloys. Both uses are only sufficient with sealing or the subsequent application of paint. Importantly, phosphoric acid-based anodizing processes cannot be used as an alternative to CAA if subsequent sealing has to be applied, since phosphates effectively inhibit this process step by closing pores resulting from the anodizing process. However, PSA as pre-treatment prior to structural bonding is qualified and implementation is ongoing.(Airbus Fast report, 2009). The anodizing process is typically not a stand-alone process step to provide corrosion resistance to the substrate. Given the nature of an anodic coating, a porous surface with insufficient corrosion resistance is created and this coating must be sealed (or painted) to provide adequate protection to the substrate. Another problem of chromium trioxide-free anodic coatings is that they may be thinner than chromium trioxide-based coatings. Also, as the Cr(VI) component is missing in chromium
46 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES trioxide-free anodic coatings, no self-healing properties are delivered to the surface, which influences the general corrosion protection of the surface, even after sealing. TSA Corrosion resistance: As mentioned, TSA is already qualified as a CAA-replacement by some OEMs and suppliers for anodizing of certain Al alloys and Cr(VI)-painted parts. In general, TSA provides less corrosion resistance than CAA, but reaches the requirements of the aviation sector for aluminium alloys such as clad or unclad Al alloys. At present, the combination of TSA with post-treatment of Cr(VI)-free sealing or painting clearly does not meet the specifications for the whole aerospace sector. When applied with Cr(VI)-containing sealing or paint systems, requirements can be met for certain applications. However, it was stated that not all of the currently qualified Cr(VI)-primers for CAA are compatible with TSA, so that only a few TSA-Cr(VI)-primer combinations are qualified and meet the performance values. This can be confirmed by the results from different companies, reporting that corrosion performance on high copper Al alloys from the 2000 and 7000 series did not reach the required 336 h for sealed equipment when tested according to ISO 9227 or ASTM B117. When filiform corrosion was tested (according to specifications for structural parts) after unsealed anodization of painted parts, length from scratch was ~2 -3 mm after 960 h, which is in line with some specifications but not all. For unpainted parts that were sealed, first pits could be observed after 750 h, which is inferior to CAA. It could be clearly demonstrated that the required subsequent primer/sealing step is an issue, since it is currently less efficient than Cr(VI)-based solutions. Presently, TSA only meets the aerospace requirements in combination with Cr(VI)-containing sealing or paint systems for certain applications. Some processes are already qualified by different companies from the aerospace sector. Paint Adhesion: Here, results are inconsistent as some paint incompatibilities exist, depending on the applications, within the aerospace sector. It was reported that paint adhesion properties on TSA layers meet the requirements for military and most other applications after identification and usage of compatible paints. However, for some other applications, painting adhesion problems remain unsolved. Layer thickness: As stated above, the layer thickness after TSA is comparable to CAA and thinner than SAA, resulting in less impact on fatigue properties. Fatigue: Generally, the impact on fatigue is comparable to or shows less fatigue reduction compared to CAA. Resistivity: Furthermore, electrical resistance is not equivalent to CAA. As a consequence, this might have an impact on the design of TSA-based layer systems. For local repair applications, many trials (using numerous mixture compositions) were performed. Currently, performance results without sealing show a high variability depending on the formulation of the anodising solutions and the time before exposure in the salt spray chamber, which makes it difficult to interpret the process performance. But the results may become promising if the cleaning step before anodising is improved and if a suitable Cr(VI)-free sealing can be added after anodising. SAA Corrosion resistance: SAA is a common alternative method for anodizing of aluminium and aluminium alloys. Its specific porous surface structure makes it ideal for colour finishing. The process is already qualified at certain companies and suppliers for corrosion protection of aluminium alloys in unpainted and painted conditions, but not for fatigue parts and parts of low manufacturing tolerances. The difficulty is that each step of the process (cleaning, anodizing, sealing) influences the
47 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES corrosion performance of the surface. Especially for sealing, no satisfying Cr(VI)-free solution currently exists for most types of aluminium alloys. Results provided by the aviation sector differ depending on the substrate used. Bare corrosion protection of 500 h SST can be met with hot water sealing on certain alloys, which is in line with specifications for equipment. However, corrosion performance on Al alloys from the 2000 series and aluminium castings is currently not in line with the requirements of the aviation sector. Further testing of filiform corrosion (according to specifications for structural parts) after unsealed anodization of painted parts, revealed length from scratch of ~2 -3 mm after 960 h, while after CAA lower values (< 0.5 mm) were observed. For unpainted parts that were sealed after anodization, first pits could be observed after 750 h, while performance of CAA still shows corrosion resistance after 2000 h in the Neutral Salt Spray test (ISO 9227). Layer thickness: Layer thicknesses are significantly higher (10-20 µm) than for CAA (specifications claim 2-7 µm). In consequence, a significant reduction in fatigue strength might be observed. Other parameters: During the consultation, the industry confirmed that beside the above-mentioned criteria, other key parameters such as adhesion, and coefficient of friction are currently not met by SAA for most critical applications. Testing of SAA as a replacement for local repair applications showed the same limitations regarding the influence of each process step. After a Cr(VI)-free but Ni-based sealing, corrosion in NSS is observed after 500 h of exposure (30 pits / dm²) already within the first hours of exposure, which is not acceptable for the industry. Adding components to the SAA solution may be necessary for intensified use on copper containing Al alloys, since copper from the treated alloys deposits on the electrode. This issue is not observed with localized CAA, and must be solved in order to avoid additional maintenance. Phosphoric acid-based anodising processes (PAA, PSA) Corrosion resistance: In general, the use of phosphoric acid-based anodising processes as an alternative would be seriously limited if subsequent sealing has to be applied, since phosphates effectively inhibit the sealing process step. It was reported during the consultation that phosphoric acid-based processes (PSA, PAA) are used and qualified as pre-treatment for structural bonding of Al alloys as a replacement of CAA within the aviation sector. It is not intended to use PSA and PAA in sealed conditions for unpainted parts as they clearly provide less corrosion resistance than CAA. Presently, requirements are met only for certain applications and only in combination with Cr(VI)- containing paint systems. PSA does not meet the requirements for military specifications (MIL-A- 8625) and research is ongoing for these applications. Testing of various mixtures of sulphuric and phosphoric acids as a replacement for local repair applications showed the same limitations as described above. The right balance between these acids has not been found yet to produce an acceptable corrosion performance on a 6 µm non-sealed oxide layer. However, R&D will be extended by reconsidering the sealing process and higher oxide thicknesses. Aqueous mixtures of PAA have been studied in depth and particular process parameters for touch-up are now well established. An acceptable corrosion resistance for no-dip PAA is doubtful for clad aluminium and is not feasible for alloyed aluminium without sealing, where severe corrosion is observed within 24 h. This corrosion protection is clearly insufficient, especially considering that the layer thickness is limited to and cannot exceed 1-2 µm. Bonding properties have not yet been studied with aqueous localized PAA processes even if there is a short time between anodising and bonding (like for bath processing). However, localized PAA has been approved for bonding for several years when applied in the form of a gel (PANTA process).
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In conclusion, phosphoric acid-based anodic coatings are not an alternative to chromium trioxide- based processes, if the anodic coating needs to be sealed which is inhibited by phosphoric acid-based processes.
Other acids used for anodizing It was stated during the consultation phase that BSA and Thin Film Sulphuric Acid Anodizing (TFSA) have been tested and partially implemented as a replacement to CAA in the aviation sector. For BSA, health issues clearly point against its use as an alternative. When applied with Cr(VI)-containing sealing or paint system, requirements can be met for certain applications. However, corrosion performance is not always in line with current specifications from this sector. As stated for TSA, combinations of BSA+primer/sealing will need be reworked and not all of the currently qualified primers for CAA are compatible with BSA. Only a few BSA-primer combinations are qualified and meet performance values. Current issues relate to corrosion performance in tanks and equipment, and the corrosive properties of the BSA electrolyte, making it an inappropriate solution for assemblies. Promising results without sealing were obtained for BSA as a replacement for local repair applications. As corrosion performance is still some way from the requirements of the aviation sector, R&D work continues by evaluating the sealing process and also improving the cleaning step. For TFSA the same issues arise as stated in the section above for SAA. For some parts this process is already in use, with limitations regarding its fatigue properties and low manufacturing tolerance on aluminium. Conclusion: In summary, chromium trioxide-free anodizing processes can only be used as a replacement to CAA for specific applications on certain alloys, but not as technically feasible alternative to chromium trioxide-based surface treatment systems for broad application to key applications in the aerospace sector. They are partly qualified within the aviation sector in combination with a few Cr(VI)-containing primers. Substantial requalification of anodizing+primer/sealing is necessary to identify further combinations and applications where they may be suitable. However, the processes described here do not currently meet some of the requirements for structural, fatigue-sensitive parts where corrosion resistance and layer thickness as crucial performance parameters are insufficient. For suitable surface protection, a sealing or paint system containing Cr(VI) is still required.
Corrosion Compatibility Repair Electrical Adhesion Layer thickness resistance with substrates applications resistance For some
applications
Chemical conversion coating General assessment: The aerospace sector stated that chromium trioxide free-acidic anodizing can be used instead of CCC plus paint but only for specific aluminium parts on special aerospace alloys (such as AA2024 or AA7075), as a final corrosion protection layer, and only in cases where no conductive coating is required. In addition, treating a substrate with an anodizing process affects the fatigue properties of the substrate, which does not happen in case of a conversion coating. Corrosion resistance: It is important to note, that acidic anodizing is not a stand-alone CCC alternative, because a Cr(VI) primer has to be applied on top to achieve adequate corrosion performance. Layer thickness: Compared to a CCC layer, the surface of the alternative (combining an acidic anodized surface with topcoat) is higher resulting in issues with the mechanical dimensions of the treated parts.
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Conductivity: Chromate-free Anodic surfaces do not have conductive properties and anodizing processes are only an alternative for conversion coatings prior to paint applications, where conductivity is not a requirement. In general, conversion coatings are applied to substrates due to the conductive properties of the resulting coating. Conclusion: Anodizing plus topcoat as conversion coating would only be suitable for coatings without required conductivity. As this is a broad requirement of the aerospace sector on conversion coated substrates, the alternative does not fulfil these requirements technically. Additionally, the process is not usable for MRO or deoxidizer applications or as electroless process (e.g. for pipes) due to the non-local process application.
Corrosion resistance Repair application Stand-alone process Resistivity
Passivation of stainless steel General assessment: For the passivation of steels, nitric or citric acid can be used and are already implemented for decades, although they may not be applicable to all kinds of stainless steels. However, research is ongoing. As with chromium trioxide-based passivation, a post-treatment process is necessary for some stainless steels, e.g. high carbon stainless steels such as 440°C, and this is usually performed with chromium trioxide. However, a significant amount of stainless steels which have been applied since the beginning of aircraft production still need to be investigated due to their unavailability for testing (not available in quantity required for the testing; only high volume can be provided): abnormal etching (impact on e.g. end grain pitting/intergranular corrosion) cannot be excluded for some steels. Consequently, general rules of qualification for each production relevant material against specifications shall be established as mitigation and this approach will be implemented in stages. Corrosion resistance: Generally, corrosion resistance test requirements (ISO 9227) for austenitic stainless steel are from 96 h to 750 h and for martensitic stainless steels 2 h to 500 h. It was stated during consultation that nitric acid passivation is qualified for certain austenitic and martensitic stainless steels for particular applications, and, for example, a corrosion resistance of 48 h could be reached for martensitic stainless steel after nitric acid passivation. However, this neither covers all kinds of stainless steels, nor the full suite of corrosion requirements of the aerospace sector. Regarding the passivation of stainless steel with citric acid, variation in solution concentration, exposure time and temperature enables the passivation process for austenitic stainless steels and certain non-austenitic steels. The overall performance may be lower compared to Cr(VI) and for some critical steels (e.g. some martensitic or precipitation hardening CRSs), and post-treatments may be necessary to reach the required specifications (IFAM, 2007). Conclusion: R&D is ongoing within the aerospace sector regarding the passivation of stainless steels with citric and nitric acid to improve the quality of the protective surface. However, uncertainties remain about the performance of acid-based passivation alternatives on certain kinds of stainless steels, as crucial requirements and functionalities have not yet been proven or met.
Corrosion resistance Compatibility with substrates Complex geometry
Not for all types of steel Not for all types of steel
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7.1.1.3 Economic feasibility In general, the acid-based alternatives are determined to be economically equivalent or even less expensive than chromium trioxide-based alternatives because the acids used are available for the industry. Regarding TSA and PSA, these alternatives were stated to have shorter process times and lower process temperatures, leading to a more efficient process and an increased capacity at the same time. However, TSA is more susceptible to contaminations (e.g. fungi). To compensate for this issue, heating the solution, or implementation of UV treatment might be necessary, which has an impact on increasing process costs. Even so, main players in the aviation sector have qualified two different alternatives. From a suppliers’ point of view, it is clearly not economically feasible to install and run two new production lines for these different alternatives in parallel. The passivation of stainless steel with nitric acid is a mature process and already in use at various aerospace companies, for example qualified for certain austenitic and martensitic stainless steels for particular applications. However, for the use as a replacement of chromium trioxide-based passivation, the final process controls are not yet developed and the economic impact of e.g. complex process control, potential process changes for different kinds of stainless steels passivated by different kinds of acids cannot be assessed at the current stage. When applying nitric acid passivation, higher process temperatures are necessary, resulting in slightly higher process costs.
7.1.1.4 Reduction of overall risk due to transition to the alternative As the alternatives are not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. In case of boric acid, the substance is classified as Repr. (Reproductive toxicity) 1B. Boric acid is a SVHC and included on the Candidate list according to REACH Annex XV due to its toxicity for reproduction. Therefore, the use of BSA as alternative to CAA may become time limited by potentially transferring boric acid to REACH authorisation (Annex XIV). Recently, boric acid was included into the 6th draft recommendation of priority substances. Apart from boric acid, tartaric acid constitutes the toxicological worst case scenario and is classified as Acute Tox. (Acute Toxicity) 4, Skin Irrit.(Skin irritation) 2, Skin Sens. (Skin sensitation) 1, Eye Irrit. (Eye irritation) 2, STOT SE (Specific target organ toxicity, single exposure) 3, Eye Dam. (Serious eye damage) 1. A transition to a nitric acid employing alternative would contradict the tendency to reduce the use of this substance in order to avoid NOx emissions. As such, transition from chromium trioxide – which is a non- threshold carcinogen – to one of the above mentioned alternatives would constitute a shift to less hazardous substances. However, as some of the alternate substances used are as well under observation, the replacement has to be carefully evaluated case by case.
7.1.1.5 Availability (R&D status, timeline until implementation) Chromium trioxide-free anodizing alternatives have been part of intensive R&D over a number of years resulting in the qualification and implementation of TSA and BSA as a replacement for CAA for some structural applications (painted and unpainted conditions) within the aviation sector. Substantial time and effort is needed to identify further suitable combinations of chromium trioxide- free anodizing alternatives+primer/sealing, as the newly developed anodizing processes are not compatible with all current primer/sealings, but still compatible for some chromated primers/sealing. Importantly, a subsequent sealing or painting with Cr(VI)- containing substances is still currently necessary to provide the required corrosion resistance to the substrate. The use of phosphoric acid- based anodizing processes as treatment prior to structural metal bonding is either already implemented, or ready for qualification within the aerospace sector. Some companies from the aerospace sector clearly stated that results for chromium trioxide-free anodizing alternatives are currently not in line with the specifications, and development is still at low
51 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES maturity. For these companies, it is anticipated that TRL 6 for additional non-structural applications may be reached within 1-3 years (TSA), while an additional >5 years are estimated prior to qualification (SAA). For SAA, testing on the component level and on breadboards were planned to begin in 2014. It was reported that a company within aerospace is working on a sulphuric acid-based anodizing process followed by a Cr(III) sealing, which also involves a Cr(VI)-free pre-treatment. The process is stated to be at TRL 4 stage. It is currently being tested in two treatment shops, where corrosion and fatigue tests are performed. After qualification, the alternative would need to be tested in other treatment shops to evaluate if the specific performance requirements can be met. Industrial qualification TRL 9 is not expected to be reached before 2022. Overall, these alternative anodizing processes could reach TRL 9 at the earliest in 2020, but for some applications not before 2025. Please note that a complete process chain without Cr(VI)-containing treatment steps meeting the requirements for structural parts and MRO applications is currently not foreseeable. For further development of PSA for additional applications, testing on the component level is planned for 2015 by a company within the aerospace sector. The process is stated to be at TRL 3 stage. It is currently being tested in a test-line environment, where corrosion, adhesion and fatigue tests are performed. Industrial qualification is not expected to be reached before 2020. Overall, these processes could reach TRL 9 at the earliest in 2020, but for some applications not before 2025. For passivation of stainless steel, nitric acid is qualified for the passivation for certain austenitic and martensitic stainless steels for particular applications in the aerospace sector, but not applicable to all types of steel. Citric acid is already in use for some specific applications, although further R&D efforts have to be undertaken as this alternative process is currently not ready for industrial implementation as general alternative. Taken together, the described replacement substances are promising alternatives, but cannot be applied to all kinds of stainless steel so far. Until all different applications can be performed without Cr(VI), it is expected that at least 12 to 15 years are necessary until full implementation of alternative products in the supply chain.
7.1.1.6 Conclusion on suitability and availability for acidic surface treatments In conclusion, TSA, BSA, PAA and TFSA constitute chromium trioxide-free anodizing processes that are partly qualified and industrially available for some companies in the aviation sector, and in some cases (non-structural parts in painted and unpainted conditions, structural metal bonding) are already implemented. Nevertheless, anodizing cannot be seen as a stand-alone process providing corrosion protection to a surface, but only as one process step in an advanced and sophisticated process chain where Cr(VI) remains necessary in various process steps to ensure the quality of the final product for the highly demanding environment of the aerospace sector. Although much R&D effort has been made in previous years, and alternatives replacing single process steps were individually developed, at the current stage no completely chromium trioxide-free process chain is industrially available. The industry is working hard on the replacement of Cr(VI) in all process steps and first successes have been recognised. From an economic perspective, further issues arise for the supply chain when taking into account that there is no one to one replacement for anodizing, but different processes have been developed and implemented within recent years by the main actors in the aviation sector. As stated, major OEMs developed different alternative solutions while sharing some similar subcontractor base. But for suppliers it is simply not feasible to establish two new production lines for these alternatives. As a result, suppliers would have to drop one of the clients or continue with the chromium trioxide-based process which is accepted by both clients. For the passivation of stainless steel, alternatives are under R&D and partly implemented for industrial use. Presently, uncertainties remain about their performance on certain steels, as crucial
52 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES requirements and functionalities have not yet been proven. Here, additional R&D is necessary to reach maturity for all types of stainless steel with Cr(VI)-free alternatives. Anodizing plus topcoat as potential alternative to CCC would only be suitable for coatings without required conductivity. As this is a broad requirement of the aerospace sector on conversion coated substrates, the alternative does not fulfil these requirements technically. Furthermore, the parts have to be painted with a chromated primer. In summary, Cr(VI)-free acidic surface treatments represent a promising alternative which is currently under investigation at most companies and is already implemented at some companies for some applications, but still in combination with Cr(VI)-containing primer or sealing. Additional implementations, for various applications, are not foreseen before 2020, and only if no major drawbacks occur. For the use in structural applications, no completely chromate-free process chain is currently available, nor is an implementation foreseen within the next decade. Therefore, a period of at least 12 to 15 years is necessary until replacement techniques may be industrialised which do not use Cr(VI) in any of the steps involved. 7.1.2. Alternative 2: Cr(III)-based surface treatments
7.1.2.1 Substance ID and properties Cr(III) processes are generally based on the same principle as Cr(VI) processes. However, there are major differences in the distinct chemical composition of the solutions and required additives as well as the operating parameters and ancillary equipment, such as ion exchangers, depending on the kind of surface treatment. In general, predominantly two types of Cr(III) solutions are used: sulphate- and chloride-based. An overview of general information on substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.2.
7.1.2.2 Technical feasibility Table 11 summarizes the chromium trioxide-based surface treatment processes where Cr(III)-based surface treatments were assessed to be an applicable alternative.
Table 11: Sector specific overview on chromium trioxide-based surface treatments processes where Cr(III)-based technics are evaluated
Industry sector Surface treatment Substrate / coating
Passivation of metallic Aluminium deposit and aluminium alloys, Cd, Zn, coatings Zn-Ni deposits Sealing after anodizing Aluminium and aluminium alloys Aerospace industry Aluminium and aluminium alloys, Magnesium and Chemical conversion coating magnesium alloys Sacrificial and slurry Aluminium (diffusion) coatings
Passivation of metallic coatings Substrate compatibility: Passivation is mainly performed on coatings based on aluminium and its alloys, cadmium plated surfaces, and zinc and its alloys. General assessment: It was stated during the consultation phase that R&D is underway on passivation on Zn alloys, Cd and Al coatings. Successful chrome-free passivation after Al coating was
53 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES demonstrated for Al IVD but more work may be required at an industrial scale. Here, CCC solutions may be more applicable as Cr(III) conversion coatings are used on zinc and zinc-nickel alloys in the automotive industry and this technology is being transferred to the aerospace industry. For Zn-Ni- coatings and Al IVD surfaces, new suppliers are qualified with a non-chromium trioxide solution but it is not applicable to the whole supply chain yet, as legacy suppliers still have to use Cr(VI) processes. As Zn-Ni is not necessarily an environmental improvement over cadmium (Zn-Ni coatings with Cr(III) passivation instead of Cd coating with Cr(VI) passivation), new qualifications will not be prioritised. A Cr(III) conversion coating on Zn-plated substrates is not suitable for uses where a high level of corrosion resistance is required. For these purpose, R&D is ongoing and the current TRL level is low (TRL 2-3). Passivation processes with Cr(III)-based products on Cadmium are under investigation, but have a low maturity. As a remark, many proprietary products contain some Co salts which are a SVHC. These cobalt salts are necessary for the corrosion performance. Corrosion resistance: Although successful chromium trioxide-free passivation of Al coatings was demonstrated for Al IVD in some tests, outdoor exposure testing has shown Cr(III) passivation not to be as effective in providing corrosion protection to the coating or exposed steel as Cr(VI)-based passivation, and more development work is thus required. For Cr(III) passivated Zn-Ni coatings, greater amounts of corrosion have been observed during salt spray tests compared to Cr(VI) passivated Zn-Ni. Especially for uses where a high level of corrosion resistance is required, Cr(III) passivation on Zn-plated substrates is not suitable. For these purposes, R&D is ongoing and the current TRL level is low (TRL 2-3). Cr(III)-based passivation of Cd coatings was tested but found to be insufficient, as the requirements regarding corrosion resistance were not met. Some Cr(III) passivated Cd coatings meet company specific corrosion requirements of 750 h salt spray exposure, but only when the passivation solution contained cobalt salts. Without cobalt salts, the corrosion resistance was reduced to 336 h after salt spray exposure which is far below company requirements. Depending on the Cr(III)-based process performed, white and red corrosion are visible after 48 hours, when tested in salt spray exposure. Active corrosion inhibition: Active corrosion inhibition is poorer compared to chromium trioxide- based passivation in some situations. Adhesion of subsequent paint: During outdoor exposure testing, results have shown aluminium on steel exhibiting red corrosion within a scribe after 42 months industrial exposure, whereas there was no red rust in a scribe of Cr(VI) passivated cadmium. Reproducibility: The performance does not consistently meet the requirements since the process reproducibility must be improved. Corrosion and adhesion properties are currently an issue. Layer thickness: Resulting layer thickness ranges between 250-600 nm, which is sufficient for aerospace applications. Cr(III) is also in R&D for local (repair) applications on Zn-Ni alloys. Here, the performance is not comparable to Cr(VI) and therefore not in line with aerospace specifications. While adhesion properties are sufficient, the corrosion requirements are not fulfilled; corrosion is visible during the first 100 h of exposure in NSS. It must be determined if the salt spray failure is due to the coating or the Cr(III) passivation. Better performance is seen for the Zn-Ni immersion process with Cr(III). Although addition of Carcinogenic, Mutagenic and Toxic to Reproduction (CMR) substances like cobalt salts improved the corrosion performance in bath, this approach has not been pursued further. The final process must be economically feasible with no significant additional cost. Conclusion: In summary, passivation processes with Cr(III)-based products are under investigation for various substrates, but currently have a low maturity. During the consultation it was clearly stated
54 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES that Cr(III)-based processes are neither applicable for corrosion protection of Al alloys nor for the passivation of steels.
Active corrosion Corrosion resistance Paint Adhesion Reproducibility Layer thickness inhibition
For Al IVD coatings In some situations
Sealing after anodizing – one process step General assessment: Given the nature of an anodic coating, a porous surface with insufficient corrosion resistance depending on the application is created. This anodic coating necessarily has to be sealed by post-treatment processes (sealing after anodizing) to provide adequate protection to the substrate. It was stated during the consultation that extensive R&D is ongoing on the Cr(III)-based sealing after anodizing alternative within the aerospace sector. Corrosion resistance: Tests performed within the aerospace sector on anodized surfaces sealed with Cr(III) showed varying corrosion protection results depending on the anodizing process used. The test results of Cr(VI)-free sealing with Cr(III)-based conversion product showed that a corrosion resistance performance of 336 in SST can only be achieved under controlled conditions in the laboratory. When tested under industrial conditions, this performance was clearly not reached. This is in line with statements from other companies on Al alloys from the 2000 and 7000 series, where corrosion can already be observed after 100 h. Small amounts of Cr(VI) may drastically increase the performance and the observed differences may be due to Cr(VI) impurities which can be found in commercial Cr(III) products or might come in by contamination from other processes. Suitability needs to be validated in additional tests which are more representative for specific in-service relevant conditions. Resistivity: The sealing aims to clog the pores of anodizing layer to make it perfectly insulating. For local repair, application tests were performed by the aerospace sector with a Cr(III)-based pen on CAA. Corrosion requirements were clearly not met, with first signs of corrosion occurring after 125 h. Additional test series were carried out with various Cr(III) -salts. Cr(III) alone was clearly not sufficient for corrosion resistance and needs a pH stabilizer. Further research demonstrated better results when using a Ni-based mixture containing Cr(III). Corrosion performance varied depending on the anodizing process used. Sealing on CAA led to corrosion protection ranging between 400-500 h on anodised AA2024 T3 and will be tested on other alloys. When applied on Cr(VI)-free SAA, corrosion in NSS is currently observed after 500 h of exposure (60 pits / dm²) already within the first hours of exposure, which is not acceptable for the industry. Although this sealing is not classified as a CMR mixture, it cannot be currently accepted for qualification because of the presence of a Ni salt. Research is ongoing to further decrease the Ni content. Without major drawbacks, TRL 6 may be reached within the next 5 years. Conclusion: Cr(III) sealing after anodizing is a promising alternative. However, as stated by technical experts from industry, although the Cr(III) sealing impregnates the pores of the anodic coating, a certain porosity will remain with current alternative techniques. This limits the corrosion resistance in general. In consequence, further R&D and improvement is necessary and anticipated on Cr(III) sealing. The implementation (TRL 9) at one company for specific applications is foreseen, but not before 2020.
Compatibility with Corrosion resistance Adhesion One-step process Surface properties substrates
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Sealing after anodizing – two-step process General assessment: As stated in the previous paragraph, after applying Cr(III)-based sealing, a certain level of porosity of the sealed surface remains in comparison to the conventional chromium trioxide-based sealing processes. It was also discussed if the corrosion performance of the CCC layer itself might be insufficient. This remaining porosity negatively influences the corrosion resistance of the coating. Several companies within the aerospace sector have performed R&D on this issue showing that the remaining pores can be closed by an additional post-treatment process. Consequently, the whole sealing process must be expanded by using an additional process step. For this process, two post-treatments after CCC are under evaluation: Cr(III) plus rare earth elements, which is discussed below, and Cr(III) plus subsequent hot water sealing discussed in chapter 7.1.4. One example for a post-treatment comprising two process steps is a Cr(III)-based sealing post- treatment plus a rare earth elements-based additional post-treatment. The process is patent protected by one company in the aerospace sector. As stated during the consultation phase, work is currently ongoing to further improve the corrosion resistance of the layer by closing the remaining pores. The process is stated to be technically feasible, development is in TRL 4. To date, the process has been tested in two treatment shops, where corrosion and fatigue tests are performed. After qualification (TRL 6) would be reached, the alternative would need to be tested in further treatment shops to ensure quality and robustness. As stated in chapter 5, generally, from TRL 6 to industrial implementation (TRL 9), at least five years are necessary if no major drawbacks occur. As stated during consultation, it is unlikely that a two-step sealing process will be implemented as a general alternative for the whole aerospace sector. The major technical issue is the implementation of a two-step sealing alternative instead of a one-step chromium trioxide-based process.
Compatibility with Corrosion resistance Adhesion One-step process Surface porosity substrates Adhesion on anodizing with a double step process
is similar to adhesion on CAA sealed
Chemical conversion coating General assessment: The Cr(III) conversion coating is currently the best alternative for CCC with chromium trioxide, and is being implemented at different aerospace companies on selective aluminium alloys and applications. This technology is already qualified at some contractors but currently only for 5000 and 6000 aluminium alloys, not for the 2000 and 7000 AA series. Furthermore, Cr(III)-based solutions are qualified according to MIL-DTL-81706. Generally, companies provided results for several commercially available products with significantly varying performances, depending on the substrate used. So far, the process is not robust enough to meet the requirements for all alloys. The main challenge is to identify and specify the optimum process window. Therefore, these coatings currently remain at low maturity within the aerospace sector. Corrosion resistance: For Aluminium alloys from 5000 and 6000 series, the corrosion resistance is inferior compared to Cr(VI) CCC, but the required 168 h SST (ASTM B117, ISO 9227) can be met with this process. However, it was stated by several companies from this sector that for alloys from the 2000 series (which is the type of alloy most often used in the aerospace industry) and 7000 series, corrosion performance is insufficient for most applications when tested at an industrial scale. Depending on the Cr(III)-based product used, first signs of corrosion already appear after 48 - 100 h in salt spray tests compared to reference at 168 h (ISO 9227). These surface characteristics are
56 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES dependent on the surface preparation. In addition, Cr(III)-based solutions do not exhibit the same active corrosion inhibiting properties as Cr(VI)-based solutions. However, some aerospace companies stated that their Cr(III)-based solutions are in line with their specifications for various alloys from Al, Mg, Ti and also on the 2000 and 7000 series aluminium under narrow and thoroughly controlled process conditions. Furthermore, Cr(III)-based solutions are qualified according to MIL-DTL-81706, meaning that 168 h in SST can be fulfilled on AA2024, although it should be noted that the Cr(III)-based solutions were only applicable under very narrow and controlled process conditions and included Cr(VI)-based pre-treatment. In general, results on the corrosion resistance of Cr(III)-based CCC are inconsistent within the aerospace sector and further R&D is necessary to obtain reproducible results and high qualitative coatings. Adhesion: Where the applied parts are used for bonding applications, adhesion tests are ongoing. Existing results from the industry demonstrate that the adhesion performance is currently inconsistent. Depending on the Cr(III)-based product used, adhesion promotion to subsequent corrosion resistant paints was not in line with the cross-cut requirement of Corrosion Paint Reproducibilit Repair Layer Fatigue Conductivity resistance Adhesion y applications thickness Depending Depending With impact With impact on solution on solution on corrosion on corrosion and and and paint and paint substrate substrate adhesion adhesion 57 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Sacrificial and slurry (diffusion) coatings As an alternative to chromium trioxide containing sacrificial and slurry (diffusion) coatings, Cr(III)- based products do not provide sufficient corrosion protection, heat resilience or flexibility with regard to the specifications of the aerospace industry. Protection and other physical properties may only be sufficient for specific applications on steel and nickel substrates. Corrosion resistance is clearly below the rigorous test requirements consisting of 20 cycles between salt spray and high heat without breakdown or excessive corrosion creep from scribed areas. The most rigorous test for high temperature diffusions coatings consists of 750 h exposure to sulphur oxides in high salt environments. Corrosion resistance Maturity Reproducibility 7.1.2.3 Economic feasibility Against the background of significant technical failure of these alternate systems, no detailed analysis of economic feasibility was conducted. First indications were made, stating that the process is in general economically feasible. Cd-replacement processes are feasible from an economic point of view. 7.1.2.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. As worst case scenario, chromium (III) chloride is classified as Skin Irrit. 2, Eye Irrit. 2, Acute Tox. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. 7.1.2.5 Availability (R&D status, timeline until implementation) For conversion coatings, several products based on Cr(III) are already available on the market and are part of extensive research within the aviation sector. Furthermore, this technology is already qualified at some contractors but currently only for 5000 and 6000 aluminium series alloys, not for the 2000 and 7000 AA series. First Cr(III)-based solutions are qualified according to MIL-DTL- 81706. To date, information is inconsistent with regard to their performance in highly demanding environments. For some applications on specific alloys, first implementations may be expected in 2017, other companies reported that TRL 6 can be reached within the next years if test requirements are fulfilled without major drawback. However, most applications on high copper containing Al alloys are still far away from industrialization. Most importantly, for structural parts, for which highest requirements have to be fulfilled, the maturity of these products is currently low. Deployment into the supply chain would be expected not before 2030. Clearly, none of the available products can be used as a general alternative for all applications on all substrates. For the passivation of metallic coatings, Cr(III)-based alternatives have a low maturity so far and 10 to 15 years may be needed for further R&D / before potential implementation, if no major drawback occurs. Cr(III) solutions as sealing alternatives were stated to be a promising alternative for equipment and structural parts and plans are underway within the aerospace sector to implement Cr(III) sealing of anodized parts. Further research at the laboratory scale is ongoing to improve the final quality of the 58 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES coating and to validate suitability with further testing. TRL 6 is not yet reached and the implementation (TRL 9) at one company is foreseen, but not before 2020. However, the TRL 9 level capability across the whole aerospace sector is highly dependent on a number of factors, such as the commercial availability of the technique, the demonstration of capability for all kinds of aluminium substrates and all kinds of parts, and the maturity of the supply chain. In addition, there are still substantial efforts needed to develop alternatives for the most challenging applications including aircraft components that are especially vulnerable for corrosion and other areas where moisture and liquids are entrapped. For the use in legacy programs and especially vulnerable parts, additional R&D efforts are needed. For these applications, the developmental process will certainly include multiple iterations and testing until successful implementation. Here, at least 12 to 15 years are necessary until full implementation of alternatives for all applications into the supply chain. Additionally, the development of a two-step sealing process comprising Cr(III) sealing after anodizing process is at TRL 4. To date, the process has been tested in two treatment shops, where corrosion and fatigue tests are performed. After qualification (TRL 6) would be reached, the alternative would need to be tested in further treatment shops to ensure quality and robustness. As stated in chapter 5, generally, from TRL 6 to industrial implementation (TRL 9), at least five years are necessary if no major drawbacks occur. As stated during consultation, it is unlikely that a two- step sealing process will be implemented as a general alternative for the whole aerospace sector. The major technical issue is the implementation of a two-step sealing alternative instead of a one-step based process. 7.1.2.6 Conclusion on suitability and availability for Cr(III)-based processes Taking all these extensive R&D efforts for the different processes and the first products on the market into account, it can be stated that Cr(III)-based surface treatments are technically not yet feasible as general alternative to the described chromium trioxide-based surface treatments within the aerospace sector. With regard to conversion coatings, first qualifications on specific alloys were carried out for some applications, while the tested products do not show a sufficient performance on all Al alloys. The major technical limitation is the corrosion performance of the coated substrates, especially on the widely used Al alloys from the 2000 and 7000 series. For the passivation of metallic coatings, Cr(III)-based alternatives have a low maturity so far and 12 to 15 years are needed for further R&D and before potential implementation of the alternative into the supply chain, if no major drawback occurs. With regard to Cr(III)-based sealing, the two-step alternative is not considered to become implemented into the supply chain, while a single-step sealing is under intensive development. For this process, at least 12 to 15 years are necessary until full implementation for all applications into the supply chain. In summary, to date Cr(III) is not an alternative to the current systems in the aerospace sector, but represents a promising alternative. First implementations are foreseen for sealing after anodizing or CCC, but not before 2020, if no major drawbacks occur. For the use as structural applications, no completely chromium trioxide free process chain is currently available, nor is an implementation foreseen within the next decade. 7.1.3. ALTERNATIVE 3: Silane/Siloxane and sol-gel coatings 7.1.3.1 Substance ID and properties Sol–gel protective coatings have shown excellent chemical stability, oxidation control and enhanced corrosion resistance for metal substrates (Wang et al, 2009). Sol-gel describes a wide variety of 59 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES processes. In general, the evaporation of the solvent and the subsequent destabilization of the sols lead to a gelation process and the formation of a transparent film due to the small particle size in the sols. Currently, sol gel technology is expanding rapidly, extensive R&D effort is being made and many new products are appearing on the market, especially since the advent of hybrid and nanocomposite materials. The following description of the sol-gel process is derived from Wang & Bierwagen (2009). By using sol-gel coating processes, a network of oxides is formed on the substrate created by the progressive condensation reactions of molecular precursors in a liquid medium. In general, two main processes - an inorganic and an organic process - can be distinguished. When using the inorganic sol-gel process, a network is formed by a colloidal suspension (usually oxides) and the gelation of the sol (colloidal suspension of very small particles, 1 to 100 nm). The organic approach is the most widely used sol-gel process and generally starts with a solution of monomeric metal or metalloid alkoxide precursors in an alcohol or other low-molecular weight organic solvent (M(OR)n). M is a network-forming metal of transition metal such as Si, Ti, Zr, Al, Fe, B, etc. and R is typically an alkyl group (CnH2n+1). Typical precursors are silanes and siloxanes (=silicones). Some commonly used alkoxysilane precursors for sol-gel coatings are provided in Table 12: Table 12: Some commonly used alkoxysilane precursors for sol-gel coatings Abbreviation Chemical name MTES Methyltriethoxysilane MTMS Methyltrimethoxysilane VTMS Vinyltrimethoxysilane PTMS Phenyltrimethoxysilane APS 3-Aminopropyltrimethoxysilane BTSTS Bis-[3-(triethoxysilyl)-propyl]tetrasulfide TMOS Tetramethylorthosilicate TEOS Tetraethylorthosilicate An overview on general information on substances used within this alternative and the risk to human health and the environment is provided in Appendix 2.1.3. Depending on the geometry (size and shape) of the part to be coated, different technologies such as spraying, immersion, electrodeposion or dip-spin coating can be used for applying a sol-gel coating. The most common and commercially used application technique for sol-gel coatings is dip-spin technology, while spraying technology is the most commoly used technique within the aerospace sector. The dip spin application is performed in a spray chamber using a mesh basket. The parts to be coated with a sol-gel coating via dip spin technology are placed on a conveyor and then metered by weight into a wire mesh basket with a diameter generally between 0.5 to 1.0 m. The basket (full of parts) is then submerged into a coating vat. The basket is then raised out of the coating solution, yet still remains in the vat, and spun up to 600 rpm (revolutions per minute). A drying period, resulting in the formation of the respective sol-gel coating, is necessary after the coating application. The exact drying 60 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES technique varies dependent on different microstructures, quality requirements, and practical applications. 7.1.3.2 Technical feasibility Table 13, summarizes the chromium trioxide-based surface treatment processes where silane or siloxane (mostly applied as sol-gel coatings) and other Sol-Gel coatings are considered to be an alternative. Table 13: Chromium trioxide-based surface treatments processes where sol-gel coatings may be an alternative Industry sector Surface treatment Substrate / coating Sealing after anodizing Aluminium and aluminium alloys Aluminium and aluminium alloys, Mg and Chemical conversion coating Aerospace sector Magnesium alloys Passivation of metallic Cd plated surfaces coatings General assessment: From academic research, it was stated that thin films without the need for machining or melting can be applied. Literature also highlights that complex shapes can be coated with the sol-gel coating process as (Wang et al, 2009). Current challenges with this process include: - Interface properties of sol-gel coatings (adhesion, delamination) determine the quality of the sol-gel coating. General approaches or methods to evaluate these properties have not yet been established, - Processing times are very long and the curing process is performed at high temperatures, - Due to a substantial volume contraction and internal stress accumulation caused by the large amount of evaporation of solvents and water, the coating can easily establish cracks and therefore, the formation conditions of the sol-gel coatings have to be carefully controlled during the drying process (Wang & Bierwagen, 2009). These scientific statements and expectations from Wang and Coworkers (2009) are in contrast to numerous industry experience with sol-gel coatings. The performance of the sol-gel coating is strongly dependent on the properties of the pickling solution and/ or the surface pre-treatment and conditions used prior to the sol-gel coating. As a consequence, adhesion and corrosion protection properties are not only a property of the sol-gel system itself, but are strongly linked to the whole process chain including pre-treatments. Sealing after anodizing General assessment: Sol-Gel technology is intended to be used as alternative for sealing after anodizing. Research is also ongoing on sol-gel coatings to replace the whole anodizing plus sealing process. In general, the same issues can be observed as described above for the conversion coating. The major technical limitation is that currently no solution exists to apply the coating to complex parts with reproducible performance. Sol-gel as an alternative for sealing after anodizing has the major drawback that sol-gel coatings for aerospace applications are predominantly applied via spray process and therefore are not applicable on complex shaped parts. Treatment of complex shaped parts have to be done by immersion. Corrosion resistance: First generation silane-based sol-gel coatings were tested in SST to ASTM B117. The results were neither sufficient nor repeatable, because no consistent performance was able to be provided by the sol-gel coating, as the 336 h corrosion resistance was met but also 61 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES failed for test series at several companies. In summary, it was stated during the consultation that the corrosion resistance of sol-gel coatings intended to replace the anodizing plus sealing is not sufficient at the current stage. In addition, further testing such as the performance of sol-gel coatings in cyclic corrosion testing is necessary. Chemical resistance: The stability of sol-gel coatings against hydraulic fluids and fuel still has to be tested. Conclusion: Currently, sol-gel coatings are not technically feasible alternatives, neither as sealing alternative nor replacing the whole anodizing plus sealing process. The same disadvantages apply as for conversion coating and further R&D is necessary to improve the performacne. A TRL is not defined yet. Curing Corrosion resistance Layer thickness Reproducibility Complex geometry temperatures Chemical conversion coating General assessment: First Sol-Gel applications are used as an alternative to CCC for painting and bolding applications or on the exterior fuselage of aluminum parts with low corrosion risk, due to clad layer. OEM approvals for wings and some detail parts were granted. As corrosion performance of Sol-Gel coatings is very poor (see below), a Cr(VI)-containing corrosion inhibiting primer has to be applied subsequently for adequate performance. Corrosion resistance: Sol-gel chemistries by themselves do not provide stand alone corrosion resistance, therefore; rely on additives or subsequent coatings to provide the corrosion resistance to meet part requirements. Currently there are no known additives to the silane matrix that have shown stand alone corrosion resistance that meet aerospace requirements. In a salt spray test according to ISO 9227, only 72 h without significant corrosion on Al alloys were observed, which is not meeting the specifications of the aerospace sector, especially not for structural parts. For some applications, sufficient corrosion performance can be achieved at the laboratory scale, while other test series have failed. The issue is the poor reproducibility of the current technology. With regard to magnesium substrates, R&D efforts on this alternative have been stopped, as no corrosion protection could be achieved. In general, the active corrosion inhibition of sol-gel coatings shall be improved, as well as the application technique for local purposes, before these coatings can be an alternative to chemical conversion coatings. Fatigue: Since silanes/Sol-gel are a very broad chemistry of coatings, the potential impact on fatigue would have to be assessed on a material by material base. Several companies from the aerospace sector have found silane coatings that could meet similar fatigue performance as conversion coatings for specific applications. Some sol-gel coatings may only be used where good adhesion properties have to be reached (no need for corrosion protection). Adhesion: Several companies from the aerospace sector have found applications on aluminum substrates where specific organo silanes provide adequate and in some cases improved adhesion properties over chromate conversion coatings for specific paint systems and applications. Since these silanes do not provide adequate standalone corrosion performance they are limited to parts that are primed with specific primers though. Layer thickness and conductivity: For layers thicker 1-3 µm, elctrical resistance is not in line with the requirements. For electrical bonding applications, the thickness is too high, leading to electrical 62 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES insulation. Several companies confirmed that the conductive properties of the tested sol-gel coatings do not meet the requirements of the aerospace sector. Application technique/ Coating of complex parts: Sol-gel coatings typically have a relatively short pot/tank life, in the realm of hours, compared to chrome conversion coatings, which depending on the material can be years or decades. Because of this, sol-gel coatings are usually spray applied to parts. This means a change of manufacturing process is required for their implementation. However; they don’t require rinsing, which provides an alternative to immersion processing for large complex parts. However, current technologies showed limitations in applying the coating to complex parts and more sophisticated strategies must be developed to overcome this issue. This is the main objective of project phase 2 of SOL-GREEN. In this project, an electrophoresis process will be assessed and developed to apply anti-corrosion coatings through SOL-GEL technique for complex geometry parts. These challenges are currently ongoing and the research mainly conducted at a laboratory scale. Reproducibility: All these considerations indicate that the process for the industrial application of sol- gel coatings may be complex with limited reproducibility. Process temperature: Several products were already tested by the aerospace sector and found to be insufficient since the curing temperatures are above 100°C, which is clearly not sustainable for preparation of the entire aircraft or spacecraft and the incompatibilities with e.g. the curing temperature of elastomers and/or composite parts. However, further R&D is ongoing on Silane-based coatings applied not via Sol-Gel technique. The sol-gel coatings applied via dip-spin technology may not be applicable for MRO applications, as the parts to be treated would have to be dismantled for treatment if Sol-Gel should be applied. Main challenges are the high curing temperatures and the lifetime of the sol. Conclusion: Although Sol-gel coatings already reached the market for some special parts, they are technically clearly not suitable as general alternative to Chemical conversion coatings at the current stage, mostly due to the insufficient corrosion performance, the high curring temperature, the treament of complex parts and MRO applications. Corrosion Curing Complex Adhesion Layer thickness Reproducibility resistance temperatures geometry Passivation of metallic coatings Corrosion resistance: With regard to sol-gel as a passivation process on Cd plated surfaces, it was stated during the consultation phase within the aerospace sector that the corrosion resistance is not sufficient at the current stage of R&D. Conclusion: Sol-gel coatings as alternative for Cr(VI) passivation on Cd plated surfaces do not provide sufficeint corrosion resistance at the current stage of R&D. As other alternatives are more promising, no further data is available. Corrosion resistance 63 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 7.1.3.3 Economic feasibility Against the background of significant technical failure of these alternate systems, no detailed analysis of economic feasibility was conducted. However, based on the literature research and consultations, there is no indication that the discussed alternative is not economically feasible. 7.1.3.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. In addition, publically available information on specific alternative products was evaluated. However, please note that the exact substance identity and composition of products used in the Sol- Gel process is very often not known as this is confidential business information. Therefore, only the hazard classifications for the Sol-Gel matrix could be taken into account. In a worst case they are classified as Flam. Liq. (Flammable liquid) 3, Acute Tox. 4, Eye Dam. 1, Skin Irrit. 2, Eye Irrit. 2, STOT SE 3, Asp. Tox (Aspiration hazard) 1, Muta. (Germ cell mutagenicity) 1B, Carc. (Carcinogenicity) 1B. The substance vinyl trimethoxysilane (VTMS) constitutes the worst case scenario and is included in the CoRAP (Community Rolling Action Plan), indicating substances for evaluation by the EU Member States in the next three years. The evaluation aims to clarify concerns that the manufacture and/or use of these substances could pose a risk to human health or the environment. As such, a transition from chromium trioxide – which is a non-threshold carcinogen – to one of the above mentioned alternative products could constitute a shift to less hazardous substances. However, as at least one of the alternate substances is itself classified for mutagenicity and carcinogenicity, any replacements will need to be carefully evaluated on a case by case basis. 7.1.3.5 Availability (R&D status, timeline until implementation) Products based on aqueous solutions of zirconium salts, which are activated by an organo-silicon compound, are already approved by several companies within the aerospace sector for special parts (e.g. fuselage, wing). These are primarily for parts and assemblies where the corrosion performance is provided by the subsequent primer and topcoat. They provide good adhesion properties but are insufficient in terms of standalone corrosion protection. Nevertheless, these systems are subject to comprehensive R&D efforts worldwide: - The HITEA project covers many aspects of Cr(VI) replacement in the aerospace sector, also including a thorough analysis of sol gel pre-treatment. In 2014, the tested alternatives are at TRL 2. After the initial phase, the project will focus on a handful of promising alternatives, where further testing is conducted within the next years. Qualification (TRL 6) will take up to 10 years from now. Sol-gel has also been looked at previously as a pre-treatment and was considered not to be adequate as it gave good adhesion properties, but no anti-corrosion protection. It is currently being looked at again in the HITEA project. - The aim of the SOLCOAT project is to develop and improve a sol gel coating for magnesium alloys for industrial manufacture and use. This project is still ongoing. In 2008, the multi-company project SOL-GREEN was initiated for the development of protective coatings of Al/Mg alloys (Cerda et al, 2011). Since these coatings solutions are not based on electrochemical conversion, but determine a complete technological innovation, the industrial production qualification is not expected before 2025. Phase 1 was finished last year, showing that SOL-GREEN 1 faced some technical issues. Besides the main problem of, by far, insufficient corrosion performance, the layer thickness cannot be controlled adequately with the current dip coating processes. Satisfying results were only achieved on specimens, but not on parts due to their 64 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES complex geometries. Therefore, the main objective of SOL-GREEN 2 is to assess and develop an electrophoresis process to apply anti-corrosion coatings through SOL-GEL technique for complex geometry parts. These challenges are currently ongoing, the research was mainly conducted at laboratory scale (TRL 2) at universities and in some partner plants. However, as mentioned above, this is a long term solution since for qualification and implementation, 10-15 years will be needed. 7.1.3.6 Conclusion on suitability and availability for Silane/Siloxane and sol-gel coatings Chromium trioxide-free sol gel systems are an innovative technology which have arisen over a number of decades and are now part of extensive R&D efforts in the aerospace sector and beyond. Their overall performance might be sufficient for some applications, e.g. where standalone corrosion is not required or is provided by other layers of the coating system (e.g. chromated primer), or where sol-gel coatings can be used to improve paint adhesion and for structural adhesive bonding applications). However, various key performance criteria such as corrosion protection, resistivity of the coating, as well as a reproducable coating quality pose significant hurdles to implementation of sol-gel coatings for all applications within the aerospace sector. In summary, Cr(VI)-free sol gel systems are from a technical point of view clearly not equivalent to chromium trioxide-based surface treatments and are currently not a general alternative. While some applications have been implemented or partially implemented, the maturity of the systems for most applications (e.g. standalone corrosion protection) is still in the early research stages (TRL 2). Another 15 years are necessary to develop and implement these systems into the supply chain provided no major drawbacks occur. 7.1.4. ALTERNATIVE 4: Water-based post-treatments (Hot water sealing, Rinsing after Phosphating) 7.1.4.1 Substance ID and properties Water-based post-treatments, such as hot water sealing, can be used as a sealing post-treatment after anodizing of aluminium and as post-treatment (rinsing) after phosphate conversion coating. With regard to the sealing application, the anodizing process leaves a porous oxide layer, which has to be closed to provide adequate corrosion resistance to the substrate. Without high quality sealing, the anodic surface is highly absorbent to all kinds of dirt, grease, oil and stains and is susceptible to corrosion. The anodized part is immersed into boiling, deionized water, with a temperature ranging between 96°C to 100°C. During the sealing process, hydrated aluminium oxide (boehmite AlOOH) is formed in the pores by three intermediate steps (Figure 14). First, hydrated aluminium oxide precipitates as a gel of pseudoboehmite (boehmite with higher content of water), a reaction that can be controlled by diffusion, pH and chemical composition of the sealing solution. In a second step, an increasing pH leads to condensation of the gel forming crystalline pseudoboehmite, which starts to fill up the pores. Finally, the boehmite recrystallizes from the pseudoboehmite. Boehmite has a higher volume than pseudoboehmite, consequently the pores of the anodized surface are sealed. The optimal pH for hot water sealing ranges between 5.5 and 6.5. To enhance the performance of the process, various inhibitors such as silicate and phosphate as well as lithium acetate, potassium fluoride or hydrogen fluoride can be added (Hao & Cheng, 2000). Given the nature of the sealing, this alternative is solely applicable on anodized aluminium and aluminium alloys. 65 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Figure 14: Formation of boehmite during hot water sealing of anodised aluminium surface (Hao & Cheng, 2000) For anodizing processes without stringent surface protection requirements, hot water sealing is an economic and suitable method. Within the aerospace sector, where a highly demanding environment requires long-term corrosion protection, a corrosion resistance of at least 336 h is needed. Replacement solutions for rinsing after phosphate conversion coating on steel have to fulfil two requirements in one process step. As per the classical rinsing step, complete removal of residue from the previous processes must be ensured. Additionally, by using a tempered rinsing (temperature between 70 and 80°C), the surface has to be passivated, which leads to an increased corrosion resistance of the surface. This is especially of importance for phosphated surfaces because these coatings naturally generate a certain porosity which would negatively affect the corrosion performance of the coated surface without any post-treatment (Narayanan, S., 2005). A not exhaustive overview of general information on substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.4. 7.1.4.2 Technical feasibility Hot water sealing is an alternative for Cr(VI)-based sealing after anodising and only applicable on aluminium and its alloys. The process rinsing after phosphating is a crucial treatment step on steels (Table 14). Table 14: Chromium trioxide-based surface treatments processes where water-based post-treatments may be an alternative Industry sector Surface treatment Substrate / coating Sealing after anodizing Aluminium and aluminium alloys Aerospace industry Passivation (rinsing after Steel phosphating) Sealing after anodising – one step Water-based post-treatments, such as hot water sealing, were tested by several companies from the aerospace sector as an alternative to sealing after anodizing on aluminium substrates. 66 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Corrosion resistance: The results on corrosion resistance vary and depend on the origin and quality of the anodic coating (for example chromium trioxide-based (CAA) or sulphate-based (SAA)). A 336 h SST requirement according to ISO 9227, specified as minimum corrosion requirement by several companies within the aerospace sector for specific applications and parts can be met under perfect controlled lab conditions for CAA and as stated during the consultation, also for sealed surfaces on SAA. However, maintaining the same water qualities and anodic film quality (which are key to guaranteeing the requirement is met) under manufacturing conditions is not realistic. Further tests on aluminium (2024, 2818 and 7175 alloys) after chromium trioxide-free anodizing processes, such as SAA, TSA or TFSA clearly did not meet corrosion requirements according to salt spray test ASTM G85 as shown in Figure 15. Figure 15: Aluminium test panels after 144 cycles accelerated cyclic, acidified salt spray test according to ASTM G85, Method A2, A: Al with chromate-based sealing, B: Cross-scribed Al with chromate-based sealing, C: Al with hot water sealing, D: Cross-scribed Al with hot water sealing. (GE Aviation, 2014) Rapid pH variation and water containing inhibitors (e.g. by phosphate, silicate, and chloride) of boiling water make it challenging to maintain water quality, leading to a short bath life. Additionally, hot water sealing considerably reduces the abrasion resistance and hardness of anodic coatings (Hao & Cheng, 2000) and subsequent paint adhesion of hot water sealed anodized coatings was reported to be unacceptable. In general, hot water sealing without additives was found to be insufficient as the corrosion requirements of the companies are either not reached, or only under perfectly controlled lab conditions (not at an industrial scale). Buffers, such as ammonium acetate and other proprietary additives, may be added to the deionized water to improve the sealing quality, and further R&D was performed for hot water sealing processes plus additives. A number of additive solutions are commercially available. Commonly used additives are lithium acetate, potassium fluoride or hydrogen fluoride. It was expected that corrosion performance would increase, but in the tests it remained below the corrosion resistance requirement 67 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES of 336 h SST according to ISO 9227, which is essential for numerous aerospace companies, especially for structural parts. Paint adhesion: The results for paint adhesion are inconsistent and further development on the alternative treatment is necessary. Conclusion: Neither hot water sealing as a stand-alone nor hot water sealing plus additives are able to fulfil the high corrosion requirements, which are essential for numerous aerospace companies, especially for structural parts. Stand-alone Compatibility with Corrosion resistance Paint adhesion Surface porosity process substrates Process control issues, not for all inconsistent requirements Sealing after anodising – two step R&D is currently ongoing with regard to Cr(III) conversion coatings plus an additional hot water sealing step. The disadvantage is that this is a two-step process. Corrosion resistance: The SST performance on Al alloys is inferior compared to a Cr(VI)-based sealing. For some Al alloys, the performance can almost reach 500 h, while for others the corrosion resistance is far from being sufficient. Chemical resistance: Other performance parameters, such as chemical resistance of the surfaces treated by this two-step process, is not yet tested. Corrosion resistance Single process step Chemical resistance Passivation (rinsing after phosphating on steel) General assessment: As stated during the consultation, R&D is currently performed to develop rinsing solutions based on hot water and/or proprietary dips not containing any chromium trioxide. Further critical performance factors are in discussion. Corrosion resistance: To date, surface after rinsing has to withstand 2 h in SST. At the moment, this performance cannot be met with the proposed alternative solutions. Adhesion to paint: To date, surfaces after rinsing with chromium trioxide solutions demonstrate sufficient paint adhesion according to ISO 2409 (GT0), which at the moment cannot be met with the proposed alternative solutions. More demanding test programmes are currently under development, with the aim to ensure that all quality criteria from the aerospace sector can be met with the alternative treatments. Conclusion: At the current stage, chromium trioxide-free rinsing after phosphating solutions based on hot water and/or proprietary dips are under development, but are not meeting the requirements regarding corrosion resistance and paint adhesion. Corrosion resistance Adhesion to paint Surface porosity 68 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 7.1.4.3 Economic feasibility Against the background of significant technical failure of this alternative, no detailed analysis of economic feasibility was conducted. However, based on the literature research and consultations, there is no indication that the discussed alternative is not economically feasible in general. Nevertheless, there are some concerns regarding, for example, a short life time of the hot water bath leading to increased costs for maintenance of the bath, change of bath, and bath chemicals. As the hot water sealing requires a relatively long sealing time, a high energy consumption is caused leading again to increased costs (Hao & Cheng, 2000). 7.1.4.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. In a worst case, potassium fluoride as common additive to water-based post-treatments is classified as Acute Tox. 1 and Skin Corr.(Skin corrosion) 1A. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of the above mentioned alternative products would constitute a shift to less hazardous substances. 7.1.4.5 Availability (R&D status, timeline until implementation) Hot water sealing without additives is an industrially available and qualified process for applications where the corrosion requirements are not as stringent as for most of the aerospace sector. In general, hot water sealing without additives was found to be insufficient as the corrosion requirements of the companies are either not reached at all, or only under perfect controlled lab conditions (not at an industrial scale). Regarding hot water sealing with additives and water-based rinsing solutions after phosphating of steels, the corrosion requirements of the aerospace sector are not fulfilled. For use as sealing, replacement techniques are subject to further R&D work. Development is also ongoing for the application as an alternate rinsing solution. A TRL is not defined yet. At the current stage of development, water-based sealing is technically not equivalent to Cr(VI)-based sealing and is therefore not a general alternative. According to the current very early stage of research, at least 12 to 15 years are necessary until full implementation of the alternative products into the supply chain. 7.1.4.6 Conclusion on suitability and availability for Water-based post-treatments Water-based post-treatments, nominal hot water sealing with and without additives, and water-based rinsing solutions after phosphating of steel are technically no alternative to chromium trioxide-based post-treatments because the corrosion requirements of the aerospace sector are definitely not fulfilled. At the current stage of development, water-based sealing is technically not equivalent to Cr(VI)-based sealing and is therefore not a general alternative. According to the current very early stage of research, at least 12 to 15 years are necessary until full implementation of the alternative products into the supply chain. 69 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES CATEGORY 2 ALTERNATIVES The alternatives assessed in this section are mainly discussed in literature and were mentioned only few times during the consultation phase. In most cases, they are in very early research stages and showed clear technical limitations when it comes to the demanding requirements of the aerospace sector. They currently may only be suitable for specific industry sectors or for niche applications but not as general alternatives to Cr(VI) containing coating systems. 7.1.5. ALTERNATIVE 5: Manganese-based processes 7.1.5.1 Substance ID and properties This alternative comprises all manganese containing processes such as permanganate conversion coatings, permanganate / phosphate conversion coatings and manganese-based sealing processes. In all processes, the permanganate is applied by immersing the substrate in the respective solution. - The permanganate anion MnO4 is a strong oxidising agent which is reduced to manganese (IV) oxide, while elementary metal is oxidised and dissolved from the substrate as cation (Lee et al, 2013). Adding other substances such as phosphate to the conversion solution can enhance the corrosion resistance of the conversion coating (Lee et al, 2002). For use in sealing applications, a manganese salt solution was tested. The pores of the anodic coating are filled with manganese oxide (MnO2) and the manganese oxide deposit grows from the pore base. Deposition is continued until it reaches the outer surface of the coating (Alwitt & Liu, 2001). For replacement of conversion coatings on magnesium and its alloys, conversion coating alternatives based on potassium permanganate are under development within the aerospace sector and two solutions are under investigation: - Solution A with potassium permanganate and potassium dihydrogenphosphate - Solution B with potassium permanganate and dihydrogen hexafluorozirconate. A non-exhaustive overview of general information on Substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.5. 7.1.5.2 Technical feasibility Manganese-based processes are assessed as an alternative for the following chromium trioxide-based surface treatments (Table 15). Table 15: Chromium trioxide-based surface treatments processes where manganese-based products may be an alternative Industry sector Surface treatment Substrate / coating Aluminium and aluminium alloy, Magnesium and Chemical conversion coating magnesium alloys Sealing after (chromium Aluminium and aluminium alloys Aerospace Industry trioxide-free) anodizing Sacrificial and diffusion coatings for corrosion Aluminium protection 70 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Chemical conversion coating Corrosion performance: Various developments and test results have been reported in literature for manganese-based coating solutions. When tested on zinc, the addition of aluminium (as aluminium sulphate Al2(SO4)3) and phosphate (as sodium dihydrogenphosphate NaH2PO4) to the permanganate conversion coatings leads to promising corrosion performance compared to chemical conversion coating (Lee et al, 2002). Further investigations regarding the role of permanganate in permanganate/phosphate coatings on certain kinds of magnesium alloys were performed by Lee et al (2013). According to the authors, adding more permanganate to the phosphate solution resulted in a thinner coating with a compact magnesium oxide layer on the tested substrate. The thinner coatings showed less cracks and a higher corrosion resistance than the thicker coating with less permanganate. When tested on AA2024 by continuous salt spray exposure (ASTM B117) for coated specimens with and without cross-hatch mark for about 750 h, coating discolouration along with the presence of corrosion products were noticed (Yoganandan et al, 2012). According to information from the aerospace sector, permanganate conversion coatings on aluminium and its alloys were tested but found not to be sufficient, as the corrosion requirements were not met. For example, commercially available permanganate-based solutions for conversion coatings were tested but the corrosion resistance in salt spray tests is not sufficient and in addition, not comparable to Cr(III) processes which are the main focus of chromium trioxide-free alternatives. With regard to the aerospace sector, manganese-based processes are not usable on high copper containing aluminium alloys (like AA2024) or on titanium and titanium alloys. The process times for manganese-based alternatives are very long compared to chromium trioxide-based processes. With regard to magnesium and its alloys, potassium permanganate conversion coatings are at a very early R&D stage and not all key functionalities have been evaluated yet. It was stated during the consultation that corrosion resistance can currently only be met for one kind of magnesium substrate without varnish. From the initial results tested on solution A (with potassium permanganate and potassium dihydrogenphosphate), the corrosion resistance of the alternative conversion coating was found to be equivalent to the existing chemical conversion coating, while the corrosion resistance of solution B (with potassium permanganate and dihydrogen hexafluorozirconate) was lower compared to the chemical conversion coatings. Layer adhesion on substrate: Adhesion properties are stated to be sufficient for the tested alternatives: GT0-1 at initial and after 14 days in demineralised water (ISO 2409). Layer thickness: The resulting layer thickness of potassium permanganate conversion coatings on magnesium substrate was found to be equivalent to the layer thickness resulting from chemical conversion coatings. Conclusion: In summary, permanganate conversion coatings are not technically an alternative to chemical conversion coatings for the aviation sector, but R&D programs cannot currently totally exclude this option. Corrosion resistance Layer Adhesion Layer thickness Sealing after (chromium trioxide-free) anodizing Corrosion resistance: Literature data indicate that manganese-based sealing provides corrosion resistance in ambient earth atmosphere as well as other features for special applications such as enhanced conductivity, decreased resistivity and stability to withstand high negative bias voltage in a vacuum plasma without arcing (Alwitt & Liu, 2001). However, the aerospace sector stated during 71 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES the consultation that the performance of manganese-based sealings are deemed inferior compared to chromium trioxide-based sealings. Manganese-based alternatives were stated to show improper performance regarding sealing after chromium trioxide free anodizing, but in combination with other compounds promising results may be obtained (CEST, research project). The alternative process is still under R&D. In summary, manganese-based sealing solutions are technically not feasible as alternative to chromium trioxide sealing solutions, but R&D is ongoing to enhance the properties. Corrosion resistance Sacrificial and Slurry (diffusion) coatings As an alternative to chromium trioxide containing sacrificial and slurry (diffusion) coatings, manganese-based products do not provide sufficient corrosion protection, heat resilience or flexibility with regard to the specifications of the aerospace industry. Protection and other physical properties may only be sufficient for specific applications on steel and nickel substrates. Corrosion resistance is clearly below the rigorous test requirements consisting of 20 cycles between salt spray and high heat without breakdown or excessive corrosion creep from scribed areas. The most rigorous test for high temperature diffusions coatings consists of 750 h exposure to sulphur oxides in high salt environments. Corrosion resistance Maturity Reproducibility 7.1.5.3 Economic feasibility Against the background of significant technical failure of these alternative systems, no detailed analysis of economic feasibility was conducted. However, based on the literature research and consultation, there is no indication that the discussed alternative is not economically feasible. With regard to potassium permanganate conversion coatings on magnesium substrate, it was stated during consultation that implementation of these solutions seems to be of equivalent cost compared to current chromium trioxide conversion processes. Solution A is a company specific bath based on easily available products and solution B is a commercial bath. 7.1.5.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. As worst case assumption, Potassium permanganate is classified as Ox. Sol. (Oxidising solid) 2, Acute Tox. 4, Aquatic Acute 1, Aquatic Chronic 1, Skin Corr. 1C. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. 7.1.5.5 Availability (R&D status, timeline until implementation) According to literature research, R&D has been performed over a number of years on permanganate conversion coatings. None of the systems currently fulfils the corrosion requirements of the aerospace sector. During the consultation, it was reported that these coatings are not applicable on high Cu containing Al alloys, which are the most important substrates used within the aerospace sector. Due 72 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES to clear technical limitations, this alternative is not part of R&D efforts within the aerospace sector. A TRL is not yet defined. The final success of permanganate-based alternatives cannot be determined at the moment. Following the technical assessment, the relevance of these systems for aluminium substrate is questionable. With regard to potassium permanganate conversion coatings on magnesium substrate, the above mentioned solution A is a company specific bath based on easily available products and solution B is a commercial bath. At the current stage, none of these solutions are implemented in the supply chain, as none are qualified. It has to be noted that magnesium parts are not standard aerospace substrates, and conversion coating on magnesium is a niche application, which explains that R&D efforts on substitution are more difficult than on other subjects (for example chemical conversion coating on aluminium). In general, R&D effort on potassium permanganate conversion coatings as alternative to chemical conversion coatings on magnesium substrates is in a very early R&D stage and at least 12 to 15 years are necessary until full implementation into the supply chain. 7.1.5.6 Conclusion on suitability and availability for manganese-based processes In literature, few developmental products as replacement systems to CCC have been reported so far. These are not compliant with the minimum corrosion requirements of the civil aerospace industry at the laboratory scale. During the consultation, companies did not report in depth experience with these alternatives, and a TRL is not yet defined. Potassium permanganate conversion coatings as an alternative to chemical conversion coatings on magnesium substrate are in a very early R&D stage, not all key functionalities have been evaluated yet ,and it was stated during the consultation that corrosion resistance can currently only be met for one kind of magnesium substrate without varnish. Thus, it can be concluded that conversion coatings techniques or sealing after anodizing based on permanganate compounds are technically not yet an equivalent to chromium trioxide-based processes and are therefore not a general alternative for aerospace applications. According to the current very early stage of research, another 12 to 15 years are necessary to fully develop and implement these systems into the industrial supply chain. 7.1.6. ALTERNATIVE 6: Magnesium rich primers 7.1.6.1 Substance ID and properties The concept of Mg-rich primers was developed in the early 2000s by North Dakota State University (NDSU) refining the approach for cathodic corrosion protection of Al alloys without the use of chromium trioxide. Magnesium is electrochemically the most active metal in common iron and aluminium alloys and is therefore widely used for corrosion protection to the underlying substrates. Mg-rich primers were stated to be applicable in underground and undersea structures such as ships, submarines and aircraft (Pathak et al, 2012). Their mode of corrosion protection on aluminium alloys is completely different from Cr(VI)-based treatments (Johnson et al, 2007). Magnesium-rich primers were reported by a number of studies for the protection of Al alloys. They are able to provide corrosion protection to the Al substrate by a two-stage mechanism. In the first stage, the Mg particles provide cathodic protection and prevent the aluminium substrate from corrosion. This is caused because magnesium is more electronegative than aluminium and as a result, the Al substrate in this galvanic couple is cathodically polarized while the less noble Mg particles in the coating are anodically dissolved. The sacrificial Mg particles serve as a source of electrical energy (Pathak et al, 2012). 73 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Figure 16: Cathodic protection by the Sacrificial Anode method (Pathak et al, 2012) In the second stage, precipitation of a porous barrier layer of magnesium oxides is observed and corrosion is further inhibited by a barrier mechanism (Wang et al, 2013). The primary Mg-compounds of interest are magnesium oxide (MgO) and magnesium hydroxide (Mg(OH)2) since they are the species that would be present on the pigment surface or as the by-product of cathodic protection of the substrate. Based on their properties, the use of Mg-rich primer leads to a local coating environment with an acid neutralising capability. Since either compound has such poor solubility in water, the local pH under a neutral wet environment would not be raised to any significant amount. These compounds would buffer the local environment to near neutrality (Johnson et al, 2007). Mg-rich coatings, termed as such because they are formulated to ensure that the Mg loading exceeds the critical pigment volume concentration (CPVC) contain Mg particles in physical and electrical contact with each other as well as with the substrate. The protective nature of Mg-rich primers is due to their behaviour as sacrificial anode providing cathodic protection to the underlying iron and/or aluminium alloys (Figure 16) (Pathak, 2012). An overview on general information on commercially available mg rich primer systems used within this alternative and the risk to human health and the environment is presented within Appendix 2.1.6. 7.1.6.2 Technical feasibility Mg rich primers are under investigation as an alternative for local conversion coating applications (Table 16). Table 16: Chromium trioxide-based surface treatments where Mg rich primers may be an alternative Industry sector Surface treatment Substrate / coating Chemical conversion coating Aerospace industry Aluminium and aluminium alloys (local application) Chemical conversion coating (local application) General assessment: Several R&D projects for Mg-rich primers have been described in previously published literature and are still ongoing at academic and industrial scale, specifically for mixtures of Mg particles and epoxies to optimize their performance. 74 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES In current R&D programs, the Mg rich primers were found to perform well on aluminium alloys and on outdoor exposure at various sites across the US. Nevertheless, factors such as reactivity of the Mg particles in the coating, increases in pH, hydrogen gas liberation at coating metal interfaces and primer adhesion have to be taken into account for further R&D (Pathak et al, 2012). It has also been observed that these same Mg-rich primers fail rapidly and exhibit heavy blistering very early in salt spray tests (Bierwagen et al, 2009). A study conducted by Wang et al (2013) partly replaces Mg particles with Al particles and studies the performance of Mg-Al-rich primers (with varying content) on AA2024 aluminium alloy. For the study, aluminium alloy panels were used as substrate and coated with Mg-rich primers with a thickness of the dried primer of about 85 ± 5 µm. Mg-rich primers containing different contents of pure magnesium and aluminium in the epoxy paint were used for the tests. Cross scratch tests with immersion of the panels for 150 days in 3 wt% NaCl solution were carried out to examine the corrosion resistance. The results, provided in Figure 17, show that the addition of Al particles decreased the distribution of Mg particles in the primer surface, followed by less corrosion products of magnesium in the coating. Here, the Mg-Al rich primer with 20% Mg and 30% Al (D) showed better corrosion protection of Aluminium alloy than Mg-rich primer with 50% Mg. Figure 17: Surface morphology of the scratched coating samples after immersion in 3%NaCl solution for 150 days: A) 50Mg; B) 40Mg 10Al; C) 30Mg 20Al; D) 20Mg 30Al; E)10Mg 40Al; F)50Al (Wang et al, 2013) Further research studies have been performed on the behaviour of Mg rich primers for corrosion protection on AA2024-T3. According to Nanna & Bierwagen (2004), salt fog exposure of aluminium coated with Mg rich primers (without topcoat) to atmospheric CO2 solution showed that magnesium forms (unspecified) magnesium carbonate compound Mg5(OH)5•CO3 on the surface within the first 24 h exposure of the surface. After 500 h SO2 salt fog exposure, the magnesium carbonates are replaced by a more dense magnesium hydroxide Mg(OH)2 (brucite). However, during this time, the aluminium remains cathodically corrosion protected. For exposure times > 1300 h, the primer fails and hexahydrite MgSO4∙6H20 is formed on the surface. Corrosion protection: Mg-rich primers are a potential alternative for local chemical conversion coating applications. R&D for the use of Mg-rich primers is ongoing for potential replacement of some conversion coating applications on aluminium alloys. Standard filiform corrosion tests and salt spray tests are satisfactory, but more studies, especially on the long-term stability (for example by crevice corrosion and cyclic testing), need to be performed. Apart from many studies conducted academically, Mg-rich primers are also currently the subject of research in the aerospace industry as potential alternatives to primer/paint systems (not part of this dossier). 75 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Compatibility with substrate: With regard to the substrate, Mg-rich primers might also be usable for other substrates such as titanium alloys, steel, corrosion resistant steel (CRES) and carbon-fiber- reinforced polymer (CFRP) or on chromium trioxide-free anodized surfaces, but the process mechanism has to be further investigated. From the first initial results, Mg rich primers are a promising alternative in relation to the topcoat, but the performance may decrease with the generation of oxides Mg(OH)2. Therefore, further R&D is necessary to investigate long term stabilities and degradation modes of the primer. Corrosion resistance Compatibility with substrate 7.1.6.3 Economic feasibility Against the background of significant technical failure of these alternate systems at the current stage of R&D, no detailed analysis of economic feasibility was conducted. However, based on the literature research and consultations, there is no indication that the discussed alternatives are not economically feasible. 7.1.6.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. Classification and labelling information of substances and products reported during the consultation was checked. In a worst case they are classified as Flam. Liq. 3, Skin Irrit. 2, Eye Irrit. 2, Skin Sens. 1, Aquatic Chronic 2, Acute Tox. 4, Asp. Tox. 1. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these products would constitute a shift to less hazardous substances. 7.1.6.5 Availability (R&D status, timeline until implementation) Mg rich primers are commercially available as paint/primer systems. A first Mg rich primer formulation was marketed recently. It is an Mg-rich corrosion inhibiting chromium-free epoxy primer. It has to be stated that, while the detailed composition of this formulation constitutes confidential business information and is not known, the primer contains “VOC exempt solvents” (Volatile Organic Compounds = halogenated hydrocarbons) which might not comply with national law (e.g. BImSchV) or EU regulation. Furthermore, the NSDU Research Foundation announced in January 2014 that it has concluded a license agreement with the paint manufacturer. The licensing agreement gives the paint manufacturer exclusive rights in marine and automotive markets to further develop and commercialise the patented coatings technology developed at NDSU. The magnesium- rich technology will be used in primers marketed to both the military and civilian automotive and shipbuilding industries. The products are designed to be applied over chromium-free pre-treatments or bare metal, eliminating Cr(VI) entirely from the coating system. Hence, currently no alternative is on the market or is being commercially developed which would meet the requirements of the aerospace industry. R&D for primer applications is at low maturity but already reached TRL stage for some applications. 7.1.6.6 Conclusion on suitability and availability for Mg-rich primers Taking all R&D efforts and the first products on the market into account, it can be stated that Mg rich primers are already qualified for some military purposes (without resistivity as requirements) and might be also used in other sectors (automotive or marine). For the civil aerospace sector, the 76 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES development is at very early laboratory scale. Test results on Mg rich primers provide promising corrosion protection but the long-term stability has not been assessed yet. In summary, to date Mg-rich primers are not an alternative to the current systems in the civil aerospace sector, but represent an alternative which is currently under investigation at the laboratory scale and at an early stage of R&D. For the majority of applications, a TRL was yet not defined. Therefore, at least 12-15 years are necessary until these alternatives may be fully implemented into the supply chain. 7.1.7. ALTERNATIVE 7: Molybdates and Molybdenum-based processes 7.1.7.1 Substance ID and properties The coating industry uses molybdate for various processes. It has been used as corrosion inhibitor (e.g. zinc molybdate, calcium zinc molybdate) since the early 1970s and is also used as a molybdenum-based metal coating. Here, a molybdenum coating can be applied by immersion, creating a Molybdate-based Conversion Coating (MoCC) to form a protective oxide layer on the surface of the metal to be treated, which provides corrosion resistance. Two main types of MoCC are possible: a coating based on sodium molybdate (Na2MoO4•2H2O), and a coating based on a mixture of molybdate and phosphate (e.g. as phosphoric acid). Stabilizer additives such as cerium fluoride are used for the MoCC. A non-exhaustive overview on general information on Substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.7. 7.1.7.2 Technical feasibility Table 17 summarizes the chromium trioxide-based surface treatment where molybdate-based processes were assessed to be a potential alternative. Table 17: Chromium trioxide-based surface treatments where molybdate-based processes may be an alternative Industry sector Surface treatment Substrate / coating Chemical conversion coating Aluminium and aluminium alloys Aerospace industry Sealing after anodizing Aluminium and aluminium alloys Chemical conversion coating General assessment: The MoCC process has to be carefully controlled, as the final corrosion resistance from the treated surface is highly dependent on the substrate, its pre-treatment, and the process conditions. When applied on aluminium, pH values >9 lead to dissolution of aluminium ions from the substrate, dramatically increasing corrosion rates. The performance at neutral pH is only better if an oxide thickening step is performed beforehand. In this case, molybdate can be incorporated into the pores of the Al oxide, forming a protective layer. The performance can be further improved by surface pickling, which helps to uniformly distribute molybdate in the pores. This leads to a reduction of the amount of chloride ions that are the main reason for pit corrosion (Hamdy et al, 2005). In conclusion, the quality and long-term corrosion resistance of MoCC is highly dependent on the chemical conditions of the treatment. The reported data suggest that the influence of molybdates is still unclear and various handicaps exist that limit their application on an industrial scale. The corrosion resistance of conversion coatings based on sodium molybdate or a mixture of molybdate and phosphate show less corrosion resistance in salt spray tests than the conventional chromium trioxide-based treatment. For example, corrosion resistance of sodium molybdate coating 77 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES withstands maximum 72 h in salt spray test (ECHA Annex XV dossier, 2011), while specifications from the aerospace sector require at least 72 h and significantly more, depending on the specific substrate. The corrosion resistance can be improved by additives such as organic layers or stabilizers. For instance, a patent is held by Trumble et al (1999) regarding a molybdate-phosphate mixture for conversion coating added with stabilizers, such as cerium fluoride. With this stabilizer, the corrosion resistance to standard salt spray tests can be improved, with results up to 300 h. The US Environmental Protection Agency (EPA) developed a new type of heteropolymolybdate-based conversion coating for certain aluminium alloys (2024-T3, 6061-T6, and 7075-T6 aluminium alloys) with a corrosion resistance of 336 h in a salt spray test according ASTM B117 (US EPA, 2002). Corrosion resistance: Recent information provided by the aerospace sector confirmed that conversion coatings based on molybdate compounds clearly do not meet the corrosion requirements on Al alloys, especially not the high corrosion resistance required for structural parts. Additionally, they do not provide the self-healing characteristics of chromium trioxide. Resistivity: MoCC are non-conductive coatings, which clearly do not fulfil the requirements of aerospace applications. Conclusion: Presently, MoCC are not technically feasible as an alternative for chemical conversion coatings. Corrosion resistance Active corrosion inhibition Resistivity Sealing after anodizing Corrosion resistance: With regard to the use of molybdate salts for sealing after anodizing of aluminium alloys, the corrosion requirements of aluminium alloys treated with TSA followed by a molybdate-based sealing layer have not been reached. The final coating inspected after 500 h salt spray test according ISO 9227 showed from 4 pits to more than 15 pits depending on aluminium alloy and therefore, the corrosion requirement of no pits after exposure was not met. Conclusion: In conclusion, molybdate salts for sealing after anodizing are not technically feasible as an alternative to chromium trioxide-based sealings. Corrosion resistance Active corrosion inhibition Compatibility with substrates 7.1.7.3 Economic feasibility Against the background of significant technical failure of these alternate systems, no detailed analysis of economic feasibility was conducted. First information was provided that chemical cost can be higher by factor 2. However, based on the literature research and consultations there is no indication that the discussed alternative is not economically feasible. 7.1.7.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. Sodium molybdate is classified as Skin Irrit. 2, Eye Irrit. 2, Acute Tox. 4, STOT SE 3. As 78 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. 7.1.7.5 Availability (R&D status, timeline until implementation) Although intensive R&D has been performed in the past on molybdenum conversion coatings, no industrial implemented molybdate-based coatings have been developed that fulfil the corrosion requirements of the aerospace sector. The final success of molybdate-based alternatives cannot currently be determined. Following the technical assessment, the relevance of these systems is questionable as it was stated that the corrosion performance cannot be reached on aluminium alloys. During the consultation, companies reported that these systems were already screened out at an early stage, as more promising alternatives were chosen for further development. 7.1.7.6 Conclusion on suitability and availability for molybdates and molybdenum-based processes Few developmental products have been reported as replacement systems to CCC so far. All of them failed to meet the requirements of the aerospace industry at the laboratory scale. During the consultation, companies reported that these systems were screened out at early stage, as more promising alternatives were chosen for further development. It can be concluded that conversion coatings based on molybdate compounds are not technically equivalent to Cr(VI)-based products and are, therefore, not a general alternative for aerospace applications on aluminium substrates. Following the technical assessment, the relevance of these systems is questionable. 7.1.8. ALTERNATIVE 8: Organometallics (Zr- and Ti-based products) 7.1.8.1 Substance ID and properties Since the late 1970s, organometallic coatings based upon organo-titanates, organo-zirconates or organo-zircoaluminates have been reported and found some applications in adhesive technology. It is generally considered that they form interfacial primary bonds to the substrate via reaction with surface protons. So far, titanates have not found wide use in adhesive technology but are used in the manufacture of some film laminates. In the last few years, a new generation of environment-friendly conversion coatings based on titanium or zirconium oxides has attracted extensive attention owing to good corrosion and wear resistance. Moreover, the new conversion coatings can operate at lower process temperatures. These Ti/Zr-based conversion coatings have been mostly used on aluminium and magnesium alloys. However, reports applying to zinc have been scarce until now (Guan et al, 2011). Products based on fluorotitanic and fluorozirconic acid were stated as alternatives for conversion coatings in the aerospace sector and the architectural sector. A patent protected chrome-free passivation treatment was developed specifically for aluminium alloys. This commercially available solution is based on hexafluorozirconate and is qualified under MIL-DTL-81706 Class 3 for military purposes. The coating can be used as a final finish and can also serve as a base for paints, high performance topcoats, powder paints, lacquers, or as a base for rubber bonding. A non-exhaustive overview of general information on substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.8. 79 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 7.1.8.2 Technical feasibility Zirconium- and titanium-based products are assessed as an alternative for the following chromium trioxide-based surface treatments (Table 18): Table 18: Chromium trioxide-based surface treatments where fluorotitanic and fluorozirconic-based products may be an alternative Industry sector Surface treatment Substrate / coating Chemical conversion coating Aluminium and alloys, Mg, Ti and their alloys Aerospace industry Passivation on metallic Aluminium and aluminium alloys, Cd plated surfaces, coatings Zinc alloys Chemical conversion coating Substrate compatibility: CCC is mainly performed on aluminium, Mg, Ti and its alloys. Corrosion resistance: Several R&D programs in the military and space sector investigated organometallics as alternative to CCC. Products based on Zr are qualified according to MIL-DTL- 81706 Class 3, meaning a corrosion resistance of 168 h on AA2024 and AA7075 has been demonstrated. In contrast, literature reports that the corrosion performance on low copper alloys might be sufficient, while on alloys from the 2000 and 7000 series, the requirements from the aviation sector are not met (Berry, 2007). Here, surface corrosion and pits were observed within 24 h on AA7075 and AA2219 (ASTM B117), accompanied by poor adhesion results (ESA STM 276, 2008). A commercially available Zr-based conversion coating provides corrosion resistance and adhesion for subsequent painting. According to publically available company specific test results, the product withstands over 1000 h in the Neutral Salt Spray Test (NSST) on various aluminium alloys (5052, 6022, 3003, 1100, 6111, 6061), partly outperforming Cr(VI)-based coatings. Compared to the highest copper containing alloy tested in this trial (Al A356), the product failed the NSST according to ASTM B117. According to further tests report from a supplier, when testing on AA2024, the most common alloy used in the aerospace sector and of a high copper content, corrosion already appeared after less than 24 h in NSST (ASTM B117) (the criterion for failing was four or more pits on 2 of 3 0.155 m² sized panels). This result is clearly insufficient for the demanding environment of the aerospace sector, where corrosion resistance requirements (alloy type depending) start with a minimum of 72 h on Al alloys of the 2000 series. In general, for copper contents > 2% in the aluminium alloy, the product is not passing the specifications. During the consultation phase information provided by several companies from the aerospace sector confirmed that Conversion Coatings based on Zr- or Ti-compounds clearly do not meet the corrosion requirements on AA2024 in bare and in painted conditions (>168 h, ISO 9227, ASTM B117). When tested as proprietary Zr-based touch-up formulation, results were not found to be reproducible or repeatable. Conclusion: Testing performed by the aviation sector confirmed the publically available results, described above, that on copper containing aluminium alloys the corrosion resistance does not reach the required 168 h in ASTM B117 Salt Spray Test. Therefore, conversion coatings based on Zr and Ti are not technically suitable as an alternative for chemical conversion coatings. Active corrosion Corrosion resistance Adhesion Reproducibility inhibition 80 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Passivation of metallic coatings General assessment: As stated during the consultation, a Zr-based organometallic alternative has been tested for the passivation of IVD Al coatings on steel and there are no known problems to treat complex geometric parts with a Zr-based organometallic alternative. For passivation, this alternative is believed not to affect the fatigue properties of the tested IVD Al material. Testing of further coatings (such as zinc, aluminium and cadmium) to be treated with this alternative are under further R&D. Corrosion resistance: No evidence of base metal corrosion (steel, red rust) was determined in the tests and the product met the type II corrosion requirements of 504 h SST (ISO 9227) of the tested company when using the Zr-based alternative coating for passivation on IVD Al. Potentially occurring white rust due to the IVD coating shall not be a cause for rejection (Acorn, research project). Adhesion: When applied on IVD Al, the adhesion properties to paint were found to be not sufficient: they did not meet the GT0 rating (dry), GT1 rating wet (results vary from 1-2 and 2-4 respectively). Layer thickness: The thickness of the Zr-based passivation layer is commonly 0.1 µm. Chemical resistance: The chemical resistance of IVD Al passivated with a Zr-based organometallic alternative to fuel, hydraulic fluid and runway de-icer is meeting the requirements. Conclusion: In summary, Zr-based passivation is not technical feasible as alternative for the passivation of IVD Al coatings. Although the corrosion requirements might be sufficient, the adhesion properties are not in line with current specifications. The passivation of other coatings/substrates (such as Zinc, Aluminium, and Cadmium) is under further R&D. To date, passivation based on Zr and Ti formulation are technically not suitable for applications within the aviation sector. Corrosion Compatibility with Adhesion to paint Layer thickness Chemical resistance resistance substrates 7.1.8.3 Economic feasibility Against the background of significant technical failure of these alternate systems, no detailed analysis of economic feasibility was conducted. However, based on the literature research and consultations there is no indication that the discussed alternative is not economically feasible. However, fluoric acids are very aggressive products in general and investment for modification of the surface treatment line may be needed. Also bath maintainability has to be evaluated. 7.1.8.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. In addition, publically available information on specific alternatives products was evaluated. However, the exact substance identity and composition of products used is very often not known as this is confidential business information of suppliers. As worst case assumption, fluorotitanic acid is classified as Met. Corr. 1, Acute Tox. 2, Skin Corr. 1B, Eye Dam. 1. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. 81 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 7.1.8.5 Availability (R&D status, timeline until implementation) Several products based on these substances are already commercially available and R&D was found to be ongoing on these substances. NASA has implemented an organometallic zirconate coating beneath a primer and topcoat on several aluminium alloys used in solid rocket boosters. Furthermore, there has been some utilisation of Ti/Zr-base coating processes in the architectural, coil coating, and automobile industries. However, in unpainted conditions, inferior corrosion performance on copper- containing aluminium alloys has to be accepted for these applications. For alternate passivation processes on zinc, aluminium and cadmium, R&D efforts are at the early laboratory scale. At the current stage of R&D it is unknown if the technical issues can be solved. Currently, these replacement products clearly do not meet the specifications of the aerospace sector. R&D programs are ongoing within this field but to date, no TRL for aerospace applications has been defined. According to the current early stage of research, another 15 years are necessary to develop and implement these systems into the supply chain. 7.1.8.6 Conclusion on suitability and availability for organometallics Some replacement systems (developmental and commercial products) to CCC have been tested so far by the aviation sector and all failed to meet the corrosion requirements at the laboratory scale. A TRL is not yet defined. Thus, it can be concluded that conversion coatings based on Zr and Ti compounds are not technically equivalent to chromium trioxide-based products and are not a general alternative for aerospace applications. According to the current stage of research, another 15 years might be necessary to develop and implement these systems into the supply chain. 7.1.9. ALTERNATIVE 9: Electrolytic paint technology 7.1.9.1 Substance ID and properties In the electrocoating, or electrodeposition process, after pre-treatment metal parts are dipped into an electrically charged tank of coating formulation. Electrical current is used to apply the coating to a conductive substrate via anodic or cathodic deposition (see Figure 18). Figure 18: Electrodeposition process, cathodic and anodic deposition (from Pawlik M. 2009) Within the military sector, the cathodic electrocoat deposition process is applicable for the deposition of positively or negatively charged paint particles (depending on the substrate and application). After coating, the metal part is moved to the rinse stages. In the final step the coated metal parts are thermally cured (30 minutes at 93 °C) to achieve the final coating properties. Curing parameters (time 82 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES and temperature) can vary according to the substrate, the coated surface, the part thickness and other parameters. According to the supplier, “the electrocoat process can be fully automated and offers increased material utilisation”. A schematic overview of the components of an electrocoat conveyor process can be found in Figure 19. Figure 19: Components of an electrocoat conveyor process (Pawlik, 2009) The substance identity and composition of the electrocoating formulation used in the process is not known as this is proprietary to the supplier. Therefore, only limited information on the risk to human health and environment from this alternative is provided in Appendix 2.1.9. 7.1.9.2 Technical feasibility Table 19. Chromium trioxide-based surface treatments where electrocoat systems may be an alternative Industry sector Surface treatment Substrate / coating Chemical conversion coating Aerospace industry Aluminium and alloys, Mg, Ti + primer Chemical Conversion Coating General assessment: This alternative treatment is investigated to replace the combination "chemical conversion + primer" (when it is applied with a thickness of 12-20 µm) and could also be a replacement for anodizing (when it is applied with a thickness of 5 µm) or the combination "anodizing + primer" (when it is applied with a thickness of 17-30 µm). The performance results which are reported in the following paragraphs are determined within the framework of testing for the military sector. In this sector, some chromium trioxide-free coatings must fulfil specific military standards such as MIL-PRF-85582 class N, which are not comparable to the requirements of the civil aviation industry. Therefore, coatings developed for military purposes are not directly applicable for the general structure for civil aircraft, as the frequency of military planes is very low compared to civil planes running on a daily basis, as well as considerations related to the systems and performance envelope of military aircraft. Based on these daily demands to ensure the airworthiness of civil aircraft, the requirements are much more comprehensive. Data from literature assumed that key performance criteria were met for military applications (corrosion, adhesion, flexibility, chemical resistance against different fluids) on AA2024 and 7075 (Pawlik, 2009). Results of beach exposure testing on corrosion performance are still ongoing (Lingenfelter, 2012). Data on corrosion performance is currently inconsistent. 83 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Corrosion resistance: When comparing literature results from laboratory and military research, tests according to ASTM B117 and EN3665 revealed an insufficient corrosion performance on Al alloys with length from scratch exceeding 3 mm after 3000 h and 720 h, respectively. In contrast, another research program reported average length of blisters at the scratch is 1 mm on clad 2024-T3 and 0.25 mm on bare 2024-T3 (Collinet et al, 2012) after 3000 h-exposure in the filiform corrosion test (EN 3665) and the neutral salt spray test (ISO 9227). After 6000 h in the NSST, results still meet the standard civil aviation requirements in laboratory testing. The system does not provide active corrosion inhibition and is as such no replacement for parts that need corrosion protection including active corrosion inhibition. However, when the aerospace industry shared their experience on this new application, their test results are not consistent with the current research programs. First results from the aerospace/helicopters sector (civil and military) for corrosion resistance after beach exposure (atmospheric corrosion) and accelerated aging have shown lower performances than reference (chemical conversion coating + Cr(VI) primer). Adhesion properties: While in general few issues on adhesion were reported, it has also been noted that electrocoats showed insufficient adherence with various sealants used. Here, adhesion enhancer would be needed. Further tests are needed to improve the performance on this requirement. Other parameter: It was also indicated that higher performances of the electrocoat systems in terms of bending, scratch resistance and corrosion resistance after salt spray exposure compared to reference systems were achieved. It is reported that the electrocoat primer is also applicable for complex-shaped parts and can be coated uniformly. In contrast to the information above, this process does not appear to be suitable for assembled aircraft. A major drawback of this treatment is that as it is a dip coating process, it can only be applied to OEM and MRO parts that can be removed from the aircraft during overhaul. Difficulties were also observed with electrocoat stripping, as it has to be carefully evaluated which products are suitable for this process. Indeed, this technology requires development of a new repair and touch-up process. Therefore, another limitation of this process is that electrocoated parts can currently only be repaired by using conventional Cr(VI)-containing anti-corrosive primers. In conclusion it was confirmed that with this alternative, the current aerospace standards cannot be completely met (especially for conductive requirement, and touch-up & repair process). The produced layers are not conductive and as stated above repairing processes have to be redefined. Corrosion Active Chemical MRO Complex resistance (long corrosion Adhesion resistance applications geometries term) inhibition 7.1.9.3 Economic feasibility Against the background of significant technical failure of these alternate systems at this time, no detailed analysis of economic feasibility was conducted. Life-cycle costs have not been established. There is an initial capital requirement for the baths. However, based on the literature research and consultations, there is no indication that the discussed alternative is not economically feasible. 7.1.9.4 Reduction of overall risk due to transition to the alternative The substance identity and composition of the electrocoating formulation used in the process is not known as this is proprietary to the supplier. The classification of a commercial product was reported 84 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES by the supplier during the consultation as Eye Irrit. 2, and Aquatic Chronic 3 as well as Skin Irrit. 2, respectively. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of the above mentioned alternative inhibitors/products would constitute a shift to less hazardous substances. 7.1.9.5 Availability (R&D status, timeline until implementation) The electrocoat primer technology was originally developed for the automotive industry and is currently being adapted to fulfil requirements of other sectors. An electrocoat primer was commercially launched in 2012 and is qualified to SAE International’s Aerospace Material Specification (AMS) 3144 for anodic electrodeposition primer for aircraft applications (publically available information as of January 2014). SAE International is a global association of technical experts in the aerospace and automotive industries. Their documents serve as recommendations for the transportation sector mainly within the US and Canada but do not carry any legal force. Within the aerospace industry in the EU, R&D efforts have been ongoing for many years, however so far these systems have not passed the early development phase. 7.1.9.6 Conclusion on suitability and availability for Electrolytic paint technology A chromium trioxide-free electrocoat system is available on the market and currently qualified according to AMS 3144. While the overall performance may be sufficient for the military sector, this system is not yet generally qualified for civil aviation. The daily demands particularly with regard to corrosion performance, are much more comprehensive to ensure the airworthiness of civil aircraft. This process is not applicable for assemblies and assembled aircraft. Further technical limitations were highlighted during the consultation phase. In summary, electrocoat primer system shows some important technical limitations which clearly do not qualify them to be a general alternative to chromium trioxide-based coating systems so far. Since these systems are in early research stages (no TRL defined yet), for substitution of chromium trioxide at least 15 years is anticipated until implementation of the alternative products into the supply chain. 7.1.10. ALTERNATIVE 10: Zinc-nickel electroplating 7.1.10.1 Substance ID and properties Electroplating with a zinc-nickel electrolyte forms metallic coatings from zinc and nickel on the substrate (steel) by using the substrate as a cathode and an anode and inducing an electrical current. A number of different zinc electroplating types are commercially available and used, these are - acid zinc type electroplating using aqueous soluble zinc salts, such as zinc sulphate, zinc chloride or zinc acetate with sodium hydroxide or potassium hydroxide as conducting salts; - alkaline zinc electroplating; and - cyanide zinc electroplating. With regard to the applications described below, a commercially available alkaline zinc-nickel electroplating solution is evaluated. As the distinct composition is confidential business information, only general information on a typically used zinc compound, a typically used nickel compound and the risk to human health and environment can be provided in Appendix 2.1.10. 85 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 7.1.10.2 Technical feasibility Zinc-nickel electroplated coatings are evaluated as alternative for the passivation of cadmium plated coatings (metallic coatings) on steel. In this regard, the alternative would not only replace the passivation step, but also the cadmium plating (Table 20). Table 20: Chromium trioxide-based surface treatment processes where Zinc-Nickel electroplating may be an alternative Industry sector Surface treatment Substrate / coating Passivation of metallic Aerospace industry Steel with cadmium coating coatings Passivation of metallic coatings Corrosion resistance: It was stated during the consultation that the corrosion resistance of the alternative coating is not sufficient. Chemical resistance: As stated during the consultation, the chemical resistance of the alternative was found to be at least equivalent to Cd coating plus Cr(VI) passivation. Resistivity: The coating does not provide adequate conductive properties at the current stage of R&D and the alternative is not industrially implemented in the supply chain. Fastener applications with lubricant properties need to be developed with an additional lubricant coating on top of the Zinc- Nickel plating. This lubricant coating is not conductive, therefore, in comparison to a Cr(VI) passivated Cd coating, the alternative coating will be problematic for metallisation applications. Adhesion to subsequent layer: The adhesive properties of the alternative was found to be equivalent to a Cr(VI) passivated Cd coating. Layer thickness: As stated during the consolation, the layer thickness of the Zn-Ni-alternative is equivalent to Cr(VI) passivated Cd coating. Repair/Touch-up capability: The current treatment of Cd coatings with a Cr(VI) passivation provides a direct touch-up possibility in cases where the cadmium plating thickness does not conform just after plating (less than minimum) by adding cadmium and re-immersing the parts. This touch-up possibility is currently not generally qualified with zinc-nickel bath and R&D studies are in progress to deal with this problem. Conclusion: Zinc-nickel electroplating as an alternative to replace Cd coatings and the subsequent Cr(VI) passivation is in the process of evaluation, but at the current stage is not technically feasible. More evaluation tests are needed (notably on metallisation problems and on immersion touch-up) for these solutions in order to be able to substitute cadmium plating with zinc-nickel plating. Clearly, for most applications within the aerospace sector, this alternative will not become a realistic substitution as sufficient corrosion resistance will rather be achieved without an additional passivation step. Corrosion Chemical Repair Adhesion Layer thickness Resistivity resistance resistance capabilities 7.1.10.3 Economic feasibility Against the background of significant technical failure of these alternate systems, no detailed analysis of economic feasibility was conducted. However, based on the literature research and consultations there is no indication that the discussed alternative is not economically feasible. This solution requires 86 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES more frequent maintenance, the bath is more expensive and has a shorter lifetime compared to the Cd plating plus Cr(VI) passivation bath. At the moment, zinc-nickel-electroplating is a specific treatment but it is not deployed in the aerospace supply chain, and so would generally induce increased costs (e.g. for standard parts like fasteners). 7.1.10.4 Reduction of overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. Mixtures that include nickel compounds and nickel metal as stated in Appendix 2.1.10 are listed by IARC as a Group 1 chemical with sufficient evidence for mixtures that include nickel compounds and nickel metal of carcinogenicity in humans (IARC, Monographs 100C). Based on the available information on the substances used within this alternative, nickel fluoride (as exemplary nickel substance) would be the worst case with a classification as Carc. 1A, Muta. 2, Repr. 1B, STOT RE (Specific target organ toxicity, repeated exposure) 1, Resp. Sens. (Respiratory sensitiser) 1, Skin Sens. 1, Aquatic Acute 1 and Aquatic Chronic 1. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. However, as the alternate substance used is classified as mutagenic, the replacement must be carefully evaluated on a case by case basis. 7.1.10.5 Availability (R&D status, timeline until implementation) R&D efforts on this alternative are well progressed in comparison with other alternatives. But some issues remain; for example, a solution for metallisation, and immersion touch-up possibility. Some initial applications are beginning to be applied for specific parts in new programs. However, the alternative is not yet implemented in the aerospace supply chain. 7.1.10.6 Conclusion on suitability and availability for zinc-nickel electroplating Zinc-nickel electroplating as alternative to Cd coatings plus Cr(VI) passivation is under evaluation, and might be suitable for some applications in new programs. Technical issues such as insufficient corrosion performance or conductivity need to be addressed. Furthermore, nickel components in the plating solution are classified as CMR substances. In summary, Zinc-nickel electroplating shows some major technical limitations which clearly does not qualify the application to be a general alternative to chromium trioxide-based coating systems so far. Following the technical assessment, the relevance of this applications as general alternative is questionable, as sufficient performance for most applications will not be achieved without a subsequent passivation step. Since these systems are in early research stages (no TRL defined yet), for substitution of chromium trioxide at least 15 years are necessary to pass the approval process of the aerospace sector until implementation into the supply chain. 7.1.11. ALTERNATIVE 11: Benzotriazole-based processes, e.g. 5-methyl-1H-benzotriazol 7.1.11.1 Substance ID and Properties Benzotriazoles, e.g. 5-methyl-1H-benzotriazol are corrosion inhibiting substances commonly used in primers providing copper blocking properties. 5-methyl-1H-benzotriazol forms a complex with copper metal at its surface, leading to stabilization of copper and inhibiting oxidation as long as the layer stays intact. An overview of general information on substances used within this alternative and the risk to human health and the environment is represented within Appendix 2.1.11. 87 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 7.1.11.2 Technical feasibility Benzotriazoles are assessed to be a potential alternative for surface treatments according to Table 21: Table 21: Chromium trioxide-based surface treatment processes where benzotriazoles may be an alternative Industry sector Surface treatment Substrate / coating Aerospace Chemical conversion coating Aluminium and aluminium alloys, copper Chemical conversion coating Regarding benzotriazole as an alternative for chemical conversion coating, it was stated during the consultation phase by the aerospace sector that 1H-benzotriazole was tested in the laboratory as corrosion inhibitor, but is not suitable as stand alone system. When mixed with other stabilizers, such as phosphates (Sr-Mg polyphosphates or Na2HPO4), the results obtained in laboratory tests were better. To date, benzotriazole-mixtures are not technically feasible alternatives to replace chromium trioxide- based conversion coating processes as only laboratory testing has been completed, meaning that the alternative is at a very early TRL stage. Corrosion resistance 7.1.11.3 Economic feasibility Against the background of significant technical failure of this alternative, no detailed analysis of economic feasibility was conducted. However, it was stated that investments are needed for collecting waste containing the 5-methyl-1H-benzotriazol, as it cannot be discharged to the on-site sewage treatment plant due to its complexing properties. Importantly, the former one-step process has to be adapted to a two-step process. 7.1.11.4 Reduction of the overall risk due to transition to the alternative As the alternative is not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. Based on the available information on the substances used within this alternative (see Appendix 2.1.11), 5-methyl-1H-benzotriazol would be the worst case with a classification as Acute Tox. 4, Skin Irrit. 2, Skin Irrit. 2 and STOT SE 3. As such, a transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. 7.1.11.5 Availability (R&D status, timeline until implementation) Benzotriazoles are not a stand-alone corrosion protection system and other inhibitors with Al-oxide stabilisation function are necessary. This has to be further investigated and it was stated during the consultation phase to be under R&D. Furthermore, in case suitable combinations of benzotriazoles with other inhibitors can be found in laboratory tests for subsequent primer/paint applications, functionalities and performance have to be further investigated by end-users in the aerospace sector in additional extended tests on top of the known standard tests to simulate real-life conditions. 88 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Consequently, the exact timeline until qualification/certification of this alternative with benzotriazoles cannot be estimated at the moment but would certainly be considered to take at least 12 to 15 years, if technically possible. 7.1.11.6 Conclusion on suitability and availability for benzotriazole-based processes As a conclusion, benzotriazoles are not a stand-alone replacement for CCC as the corrosion resistance is not sufficient. Further R&D is necessary and if the alternative becomes technically feasible, at least 12 to 15 years are necessary until full implementation of the alternative into the supply chain. 7.2. Pre-treatments After several alternatives for the main surface treatment have been assessed in the former chapters, the following chapters concentrate on alternatives for the pre-treatments like etching, pickling, deoxidizing, and stripping. As already stated in chapter 3.2, only the combination of processes applied in sequence (pre- treatment, main process step and post-treatment) is able to provide the coating with the required functionalities. Although the single process steps can be assessed individually, they are not stand- alone processes but part of a process chain or process flow. For the assessment of alternatives to chromium trioxide, the whole process chain has to be taken into account. 7.2.1. Inorganic acids 7.2.1.1 Substance ID and properties A variety of inorganic acids are currently under evaluation as alternatives to chromium trioxide in surface pre-treatment processes. Research currently focuses on sulphuric acid, phosphoric acid, nitric acid and a mixture of sulphuric acid, nitric acid, and ferric ions from iron sulphate (named “sulfonitroferric acid”). An overview of general information on substances used within this alternative and the risk to human health and the environment is provided in Appendix 2.2. 7.2.1.2 Technical feasibility Table 22 summarizes the chromium trioxide-based surface pre-treatment processes where inorganic acids may be an alternative: Table 22: Overview on the replacement substances used in the different pre-treatment processes Surface treatment Substrate / coating Alternative Aluminium and aluminium alloys, Mixture of sulphuric acid, nitric acid, and ferric ions Functional cleaning/ steel, copper, brass, (“sulfonitroferric acid”) Pickling/Etching cadmium Steel Mineral acids, electrolytic pickling Electropolishing Steel Mineral acids Aluminium and Mixture of sulphuric acid, nitric acid, and ferric ions Deoxidising aluminium alloys, (“sulfonitroferric acid”) Aluminium and Tartaric-nitric acid , Mixture of sulphuric acid, nitric acid, and ferric Stripping of organic aluminium alloys, ions and inorganic finishes Magnesium and magnesium alloys (“sulfonitroferric acid”) 89 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Pickling/etching – pickling/etching of metal surfaces (e.g. aluminium alloys, copper, brass) General assessment: For metallic substrates such as aluminium alloys, alternatives based on either nitric acid, sulphuric acid, phosphoric acid, or combinations together with ferrous ions from iron sulphate (sulfoferric, sulfonitroferric) are in the developmental stage and partly introduced and qualified for pickling at some companies. With this treatment, moderate oxide removal can be performed. These pickling/etching solutions are significantly more aggressive than the Cr(VI)-based one, causing higher degrees of end grain pitting and intergranular attacks. For moderate cleaning purposes, a less aggressive solution is needed. Compared to pre-treatments on steel, where surface characteristics for subsequent bonding are the most critical requirement, the surface preparation of Al and its alloys includes other issues. The treatments applied for preparation of aluminium surfaces currently involve multiple steps containing chromium trioxide, as described in the introduction. Generally, the performance of the subsequent treatments after pickling/etching is strongly linked to the, robust and validated, combination of pre- treatment processes, and to the type / chemical composition of the Al alloy being processed. A mixture of “sulfonitroferric acid” is already qualified and implemented for surface preparation prior CAA, TSA, SAA, PSA and chemical conversion coating on aluminium alloys for some specific applications in the aerospace sector. It is approved prior to fusion welding, prior to brazing, and for removal of foreign metal contamination. Alkaline etching with sodium hydroxide containing additives, and acid etching with solutions of nitric acid containing fluoride from hydrofluoric acid or ammonium bifluoride are acceptable alternatives to chromium trioxide-based etching solutions used for aluminium, and alkaline etching is preferred for etching aluminium alloys. These substances cannot currently be seen as general alternatives though but are only suitable on some metal alloys or for some applications. It was stated during the consultation that these solutions are not suitable for removal of welding and brazing flux. They are clearly not suitable for localized repairs and to steel substrates. For other substrates, such as copper or brass, phosphoric and fluoride acid solutions can be added for heavy oxides removal, and this solutions may become technically feasible after further development. For Al alloys, this treatment is not adequate, due to the comparatively more aggressive character of the treatment, as explained in the first paragraph. Process development and Compatibility: Some of the newly developed and partly implemented alternatives are already in use for some applications within the aerospace industry. Importantly, these alternatives do not prepare the surface equivalent to chromium trioxide-based processes. As a consequence, extensive analysis and adjustments of the whole process chain had to be performed until these products were approved. The main issue is that measurable parameters rarely exist for the performance of the pre-treatment alone, so that standard tests mainly have to be performed on the final coating. At this stage, it very challenging to evaluate which part of the process chain is influencing the surface characteristics of the final coating. Taking all the different applications and alloys used in the aerospace industry into account, it must be stated that substantial efforts are still needed to implement and adjust the existing pre-treatment chain to the chrome-free alternatives, in order to meet the specifications of the aerospace industry. In summary, implementation of alternative pre-treatments is highly complex and depends on multiple factors. As the pre-treatment steps are all linked to each other and to the performance of the final coating, new processes have to be analysed with regard to potential reaction by-products or impurities influencing subsequent steps or different surface characteristics derived from alternative pre- treatments. Pickling/etching – pickling of steel General assessment: During the consultation phase it was stated that R&D is ongoing for the use of mineral acids as alternative to pickling of steels with Cr(VI). The pickling of stainless steel with a 90 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES mixture of nitric acid and hydrofluoric acid is used as a standard process within the aerospace industry, the same applies for pickling of low alloy steels with different mineral acid-based solutions. Currently these alternatives cannot be used for low alloy steels, as the main criteria for these pre- treatments, a sufficiently prepared surface for optimal adhesion of the subsequent layers, is not fulfilled. Additional R&D is necessary to develop appropriate replacement solutions. For pickling before bonding of stainless steels, as well as sulphuric acid, electrolytic pickling can also be applied. The metal substrate is the cathode (or the anode, depending on the steel type and exact process) in an electrolytic cell containing a strong acidic solution, such as sulphuric acid. During the consultation phase the aerospace sector stated that laboratory scale testing demonstrated the technical feasibility of the process step for pickling of stainless steel prior to bonding with chromium trioxide free solutions, but the need for further chromium trioxide-based pre-treatments remains. Industrial up-scaling to demonstrate the suitability of the chromium trioxide free process chain still has to be conducted. In conclusion, neither sulphuric acid pickling of steel before bonding, nor electrolytic pickling are currently alternatives to chromium trioxide-based pickling pre-treatment processes, but the alternatives have shown their technical feasibility in laboratory scale testing and further R&D has to be performed. Electropolishing An alternative is currently under development for Electropolishing, a process used to remove flaws or debris from the surface of a metal substrate. Within this pre-treatment on steel, chromium trioxide is needed for buffering the solution, elimination of impurities from thermal treatments and most importantly, improve the fatigue properties of structural parts. As a replacement technology, several alternative acids (chlorohydric, phosphoric or phosphoric/sulfuric acid) are currently under investigation and are qualified only for localized repair applications on stainless steel. In general, they are not an acceptable solution as the martensitic steel will become fragile after this treatment, generating inter granular corrosion that will lead to part rupture. Deoxidising General assessment: Current R&D studies revealed mineral acid-based deoxidiser solutions as a potential alternative for special applications within the aerospace sector. As previously stated, a mixture of “sulfonitroferric acid” is already qualified and implemented for surface preparation prior to CAA, TSA, SAA, PSA and chromate conversion coating on aluminium alloys for some specific applications in some parts of the aerospace sector. The deoxidising causes a dissolution of aluminium oxides, which is not in line with specifications from the aerospace sector. Furthermore, visual inspection of the appearance of the deoxidised surface (after rinsing) revealed a water break free surface without streaks or discolourations. End grain pitting and intergranular attack: This parameter is within the required ranges for some sectors, but experience in other sectors indicates that failures periodically occur with aged solutions. This can require premature disposal of deoxidising solutions and result in unplanned downtime. Possible rework in the processing facility is also within the required ranges. Corrosion resistance: It was stated during the consultation that these alternatives are within the required ranges for some applications but do not meet the requirements for corrosion protection, when used prior to sealed anodizing or conversion coatings. Corrosion resistance failures occur intermittently with aged solutions in these applications. This can require premature disposal of deoxidising solutions and result in unplanned downtime and possible rework in the processing facility. When corrosion protection is not a requirement (e.g. as pre-penetrant etching/desmutting and deoxidizing prior to unsealed anodizing), this solution might be used as alternative to chromium 91 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES trioxide-based deoxidisers and as stated during consultation, is already implemented for parts of the aerospace sector. Process development and Compatibility: As for alternatives to pickling/etching, the same limitations and concerns apply for the implementation of alternative deoxidizing solutions. As mentioned, some of the newly developed and partly implemented alternatives are already in use for some applications within the aerospace industry. Importantly, these alternatives do not prepare the surface equivalent to chromium trioxide-based processes. As a consequence, extensive analysis and adjustments of the whole process chain had to be performed until these products were approved. The main issue is that measurable parameters rarely exist for the performance of the pre-treatment alone, so standard tests have to be performed mainly on the final coating. At this stage, it very challenging to evaluate which part of the process chain influences the surface characteristics of the final coating. Taking all the different applications and alloys used in the aerospace industry into account, it must be stated that substantial efforts are still needed to implement and adjust the existing pre-treatment chain to the chrome-free alternatives, in order to meet the specifications of the aerospace industry. As reported during the consultation, for localized repair applications on Aluminium alloys, currently no alternatives to chromium trioxide-deoxidizing solutions are available. In summary, the proposed solutions are not suitable as general alternative but can only be used for specific applications (e.g. heavy duty deoxidising) where no corrosion resistance is required. They are not approved for many other applications such as deoxidising of cast and welded parts or light duty deoxidising. Most importantly, these solutions are not generally suitable for adequate surface preparation prior conversion coating on Al alloys where corrosion resistance is a requirement. Stripping of organic and inorganic finishes, conversion coatings or anodic coatings It was stated during the consultation phase that for each kind of inorganic finish, such as hard anodic coatings and conventional anodic coatings, spray coatings and conversion coatings, individual stripping alternatives have to be developed, while the current chromium trioxide-based stripping can be used for all substrates and inorganic finishes without differentiation. For some applications on external metallic and customized surfaces on Al alloys, stripping alternatives based on benzyl alcohol (with or without acid, and / or peroxide), or solutions with formic acid are qualified and used. However, for other applications (like landing gear made from e.g. high strength steels, titanium alloys), these products are currently not suitable. For stripping on Magnesium, R&D is ongoing on tartaric-nitric-based solution. This solution is for stripping of organic and inorganic coatings on magnesium. This solution is currently in laboratory scale evaluation and reformulations are necessary. Initial results (roughness, ability to retreat after stripping, corrosion performance of treatment with using of stripping) are promising but the solution is not enough mature yet for industrial application. General assessment: Current R&D studies revealed a mineral acid-based solution of sulphuric acid and nitric acid with iron sulphate as a potential alternative for the stripping of conversion coatings and anodic coatings from aluminium alloys (but clearly not for the above mentioned hard anodic and thermal spray coatings). These solutions are stated to meet the requirements in general, but with severe limitations. Key parameters: With regard to residual stress, the alternative solutions potentially etch the surface of the parts which can then cause residual stress. A short immersion time of the material to be stripped has to be guaranteed, this is required to avoid impacting residual stress, surface roughness or causing intergranular attack, which can affect the fatigue properties. The short immersion time is also necessary to avoid excessive removal of base material. The corrosion requirements are clearly not met for assemblies with dissimilar metal components. In helicopters, a lot of different metals (mainly 92 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Al alloys and steel types) are assembled and the stripping of paint process with chromium trioxide- free alternatives must be adapted to each individual metal. On an assembled helicopter or airplane, it is not possible to clearly differentiate between the individual metals and therefore, alternative stripping methods would get in contact with materials they are not suitable for. This would increase the risk for galvanic corrosion between the different metals. Besides high strength steel components, (for which the alternative is not applicable in general), other sensitive parts such as windows, pilot tubes, doors, access ports, tires, landing gears, and wheel areas have to be protected prior to the application of the stripping alternative. From a safety point of view, it may be considered that for solutions containing peroxides severe chemical reaction can take place when the process is not properly controlled. For steel substrates, iron can be dissolved from the metal which may lead to explosive decomposition of hydrogen peroxide. In conclusion, these solutions may be technically feasible for some applications but are limited to stripping of conversion coatings and anodic coatings from aluminium alloys, when the respective process control and short immersion time is guaranteed. Nevertheless, it is not a general stripping alternative for the large variety of inorganic coatings and substrates within this use. As reported during the consultation, for localized repair applications, currently no alternatives to chromium trioxide- stripping solutions are available. 7.2.1.3 Economic feasibility For the tested alternatives, no detailed analysis of the economic feasibility was carried out, as they are not qualified throughout the whole aerospace sector and can currently not be seen as general alternatives. It was stated during the consultation phase that electrolytic pickling is expected to be more expensive than chromium trioxide-based pickling processes when considering investments needed. However, based on the literature research and consultations, there is no indication that the discussed alternatives are not economically feasible. 7.2.1.4 Reduction of overall risk due to transition to the alternative As the alternatives are not technically feasible, only classification and labelling information of substances and products reported during the consultation were reviewed for comparison of the hazard profile. Based on the available information on the substances used within this alternative (see Appendix 2.2.), nitric acid would be the worst case with a classification as Ox. Liq. (Oxidising liquid) 3, Skin Corr. 1A, Met. Corr. 1, Skin Irrit. 2, Eye Dam. 1, STOT SE 3. As such, transition from chromium trioxide – which is a non-threshold carcinogen – to one of these substances would constitute a shift to less hazardous substances. 7.2.1.5 Availability (R&D status, timeline until implementation) Several alternatives based on mineral acids are available for all types of pre-treatment steps on different substrates. Still, no suitable alternative is available for all types. Some of the newly developed and partly implemented alternatives are already in use for some applications within the aerospace industry. As stated, these alternatives do not prepare the surface equivalent to chromium trioxide-based processes. As a consequence, extensive analysis and adjustments of the whole process chain had to be performed until these products were approved. The main issue is that measurable parameters rarely exist for the performance of the pre-treatment alone, so standard tests have to be performed mainly on the final coating. At this stage, it is very challenging to evaluate which part of the process chain influences the surface characteristics of the final coating. Taking all the different applications and alloys used in the aerospace industry into account, it must be stated that substantial R&D efforts are still needed to implement and adjust the existing pre-treatment chain to the chrome-free alternatives, in order to meet the specifications of the aerospace industry. 93 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES For pickling/etching/deoxidizing, development of further alternatives is in TRL 3-5. Etching alternatives are estimated to reach TRL 6 in 2018, with subsequent qualification and implementation into the supply chain as pre-treatment for all kind of applications. It was stated during the consultation that R&D on the deoxidizing alternatives is ongoing and currently at TRL 3-5. It is not expected though that the pre-treatment will become technically feasible as an overall replacement for chromium trioxide-based deoxidizing applications. Therefore, no timeline for qualification can be provided. It may become an alternative for special applications where no corrosion resistance is necessary. Further evaluation is necessary to complete evaluation and to incorporation of the alternative in the specifications. A lot of R&D has been performed on alternatives for stripping of inorganic finish. It is noted that for each individual inorganic coating to be stripped, individual stripping alternatives have to be evaluated and additionally, different substrates have to be taken into account. As stated during the consultation, there are no generally suitable non-chrome alternatives for the stripping of hard anodic coating from aluminium alloys and for the stripping of thermal spray coatings (aluminium oxide) from aluminium alloys at the current stage. Even though a R&D program was started in the 1990s, no alternative was found which meets the requirements. Latest R&D studies revealed mineral acid-based solutions as a potential alternative for the stripping of conversion coatings and anodic coatings from aluminium alloys, but not for the above mentioned hard anodic and thermal spray coatings. With regard to the aerospace sector and taking the different assemblies that combine a large variety of metal substrates, types of alloys, and coating possibilities on an aircraft into account, mineral acid- based solutions are currently not viable as a general alternative to chromium trioxide-based stripping of inorganic finish. 7.2.1.6 Conclusion on suitability and availability for inorganic acids In summary, for pickling/etching of steel as pre-treatment processes, alternatives for some applications are technically feasible, while currently no generally acceptable replacement exists. For some steels and other substrates, development is currently of low maturity. For chromium trioxide- free pickling/etching pre-treatment on aluminium and its alloys, a mixture of sulphuric acid, nitric acid, and ferric ions are commercially available and qualified for some anodizing applications on Al alloys. Since the alternative is technically not yet equivalent to the current process, further R&D is necessary. For localized repairs on anodized layers, the etching step needs to be adapted to each Al- alloy. As a consequence, further development on chromium trioxide-free etching/pickling pre- treatment is necessary before the alternative can become broadly deployed throughout the whole aerospace sector, especially as any change of pre-treatment has to be carefully adapted to the subsequent process. Although certain pre-treatments may already be used within the aerospace sector, these are part of a process chain and at the current stage, a Cr(VI) containing subsequent treatment has to be applied to fulfil the respective demands of the aerospace sector. Currently, no complete chromium trioxide-free process chain is industrially available providing all the required properties to the surfaces for all applications. Technical feasibility Economic feasibility Risk reduction Availability Qualified for specific Qualified for specific applications in some applications in some parts parts of the aerospace of the aerospace sector sector 94 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 8. OVERALL CONCLUSIONS ON SUITABILITYAND AVAILABILITY OF POSSIBLE ALTERNATIVES For this Application for Authorisation, an extensive literature survey and consultation with aerospace industry experts was carried out to identify and evaluate potential alternatives to surface treatments of metals with chromium trioxide such as aluminium, steel, zinc, magnesium, titanium, alloys, composites, sealings of anodic films. Chromium trioxide-based surface treatments are specified in the aerospace sector because they provide superior corrosion resistance and inhibition, improved paint adhesion, low electrical contact resistance and/or enhanced wear-resistance. These characteristics are essential to the safe operation and reliability (airworthiness) of aircraft and spacecraft which operate under extreme environmental conditions. These structures are extremely complex in design, containing millions of highly specified parts, many of which cannot be easily inspected, repaired or removed. Structural components (e.g. landing gear, fasteners) and engine parts on aircraft are particularly vulnerable to corrosion. Surface treatment processes typically involve numerous steps, often including several important pre- treatment and post-treatment steps as well as the main treatment process itself. These steps are almost always inter-related such that they cannot be separated or individually modified without impairing the overall process or performance of the treated product. This means that while the use of chromium trioxide may be specified at different points in the process, it cannot be entirely replaced in the process without impacting the technical performance of the final article. The implications of this are important as chromium trioxide-free alternatives for some individual steps are available and used by industry. However, where this is the case, chromates are always specified in one of the other steps within the overall surface treatment system. This means it is imperative to consider the surface treatment system as a whole, rather than the step involving chromium trioxide on its own, when considering alternatives for such surface treatment systems. Furthermore, components that have been surface treated with chromium trioxide typically represent just one of many critical, inter-dependent elements of a component, assembly or system. In general, chromium trioxide-based surface treatment is specified as one element of a complex system with integrated, often critical performance criteria. Compatibility with and technical performance of the overall system are primary considerations of fundamental importance during material specification. A total of 33 potential alternatives for all parts of the process chain were identified and evaluated during the consultation. 11 potential or candidate alternatives (including processes and substances for all parts of the process chain) are a focus for ongoing research and development (R&D) programs and are examined in further detail in this report. Here, a candidate alternative is defined as a potential alternative provided to the aerospace manufacturer for evaluation following initial evaluation by the formulator. While several potential alternatives to surface treatments with chromium trioxide, predominantly Cr(III)- and mineral acid-based systems, are being investigated, substrates and treatment steps, results so far do not support reliable conclusions regarding their performance as part of such complex systems, in demanding environments and real-world situations. These potential alternatives do not support all the properties of chromium trioxide-based surface treatment systems, and their long-term performance can currently only be estimated. Decreased corrosion protection performance would necessitate shorter inspection intervals, with a substantial impact on associated maintenance costs. In summary, the analysis shows there are no technically feasible alternatives to chromium trioxide-based surface treatment systems for key applications in the aerospace sector. Several potential alternatives are subject to ongoing R&D, but do not currently support the necessary combination of key functionalities to be considered technically feasible alternatives. 95 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Assuming a technically feasible potential alternative is identified as a result of ongoing R&D, extensive effort is needed beyond that point before it can be considered an alternative to chromium trioxide within the aerospace industry. Aircraft are one of the safest and securest means of transportation, despite having to perform in extreme environments for extended timeframes. This is the result of high regulatory standards and safety requirements. Performance specifications defined under EU Regulation No 216/2008 drive the choice of substances to be used either directly in the aircraft or during manufacturing and maintenance activities. It requires that all components, equipment, materials and processes incorporated in an aircraft must be certified, qualified and industrialised before production can commence. This approval system robustly ensures new technology and manufacturing processes can be considered ‘mission ready’ through a series of well-defined steps only completed with the actual application of the technology in its final form (and under mission conditions). When a substance used in a material, process, component, or equipment needs to be changed, this extensive system must again be followed in order to comply with airworthiness requirements. The system for alternative development through qualification, certification, industrialization and implementation within the aviation sector is mirrored in the defence and space sectors. These approval steps can only proceed once a candidate alternative is identified. Referring to experience, it can take 20 to 25 years to identify and develop a new alternative, even assuming no drawbacks during the various stages of development of these alternatives. Experience over the last 30 years already shows this massively under-estimates the replacement time for chromium trioxide surface treatments. As a further consideration, while the implications of the development process in the aeronautic and aerospace sectors are clearly extremely demanding, specification of an alternative, once available, can be built into the detailed specification for new aircraft types (and new spacecraft). This is not the situation for existing aircraft types for which production and/or operation will still be ongoing. Production, maintenance and repair of these models must use the processes and substances already specified following the extensive approval process. Substitution of chromium trioxide-based surface treatment for these ‘legacy’ craft introduces yet another substantial challenge; re-certification of all relevant processes and materials. In practice, it will be impractical and uneconomical to introduce such changes for many such aircraft types. In this context, the scale and intensity of industry- and company-wide investment in R&D activity to identify alternatives to chromium trioxide surface treatment systems is very relevant to the findings of the AoA. Serious efforts to find replacements for chromium trioxide have been ongoing within the aerospace industry for over 30 years and there have been several major programs to investigate alternatives to chromium trioxide in the aerospace sector over the last 20 years. A large amount of research over the last 30 years has been deployed to identify and develop viable alternatives to chromium trioxide-based surface treatment. Due to its unique functionalities and performance, it is challenging and complex to replace surface treatments based on chromium trioxide in applications that demand superior performance for corrosion and/or adhesion to deliver safety over extended periods and extreme environmental conditions. Several potential alternatives to chromium trioxide, such as Cr(III)- and mineral acid-based systems, are under investigation for the aerospace industry. However, based on experience and with reference to the status of R&D programs, alternatives are not foreseen to be commercially available for key applications in this sector for at least 12 or 15 years. As a result, a review period of 12 years was selected because it coincides with best case estimates by the aerospace industry of the schedule required to industrialise alternatives to chromium trioxide. It also reflects the duration of the standard long review period indicated by ECHA. 96 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES 9. REFERENCES Acorn Surface Technology Limited. Research project. AMMTIAC (Advances Materials, Manufacturing and Testing Information Analysis Center) (2012): AMMT- 39 – Analysis of Alternatives to Hexavalent Chromium: A Program Management Guide to Minimize the Use of Cr(VI) in Military Systems, AMMT-39 by Lane, R.A., Fink, C. & Grethlein, C Cerda et al. (2011): SOL-GREEN Project: Investigations and development of anti-corrosion solgel coatings for aluminium alloys used in aeronautical industry. Presentation at the 2nd International Conference on Light Metal finishing. CEST (Kompetenzzentrum für elektrochemische Oberflächentechnologie GmbH). Research project. Collinet M., Labouche D. (2012): Characterization of Oxide Layers Formed on Aluminium Alloys during new PPG Electrodeposited Structural Paints. CLEAN SKY Ecodesign – Grant Agreement N°267285 Deresh, L. (1991): Composition and method for producing chromate conversion coatings. EP Patent EP 0451409 A1 Alwitt, R.S., Liu, Y (2001) Electrically conductive anodized aluminium coatings. US Patent 622 82 41 B1 R. Berry, “Iridite NCP non-chromate passivate for aluminium,” DoD Metal Finishing Workshop, May, 2007 ECHA Annex XV (2011): Identification of SVHC. ESA STM-276 (2008): Assessment of Chemical Conversion Coatings for the Protection of Aluminium Alloys A Comparison Of Alodine 1200 with Chromium-Free Conversion Coatings. A.M. Pereira,G.Pimenta B.D. Dunn FAST Report (2009): Flight Airworthiness Support Technology – FAST 45. Airbus Technical Magazine. December 2009 Hamdy, A.S., Beccaria, A.M. & Traverso, P. (2005): Corrosion protection of AA6061 T6-10% Al2O3 composite by molybdate conversion coatings, Journal of Applied Electrochemistry (35), pages 467-472. Hao, L. and Cheng, B.R. (2000): Sealing processes of anodic coatings: past, present and future. Aluminium Finishing 98 (12): 8–18. Harvey, T.G., Hughes, A.E, Hardin, S.G., Nikpour, T., Toh, S.K., Boag, A., McCulloch, D. and Horne, M. (2008): Non-chromate deoxidation of AA2024-T3: Sodium bromate-nitric acid (20-60°C), Applied Surface Sciences 23: 3562-3575. IARC Monographs 100C: Nickel and Nickel compounds: 169-218. http://monographs.iarc.fr/ENG/Monographs/vol100C/mono100C-10.pdf IFAM (Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung) 2007: Evaluation of chromate free passivation for corrosion resistant steel alloys. Research project. IFAM (Fraunhofer-Institut für Fertigungstechnik und Angewandte Materialforschung) 2008: Conductivity Investigation on CCC. Research project. Johnson et al. (2007): Magnesium rich primer for chrome free protection of aluminium alloys. AFRL-RX-WP- TP-2008-4012. 97 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Lee, H.S, Kim, S.K., Sohn, H.J, Kang, T. & Kim, H.J (2002): Electrochemical properties of Permanganate- based conversion coatings on zinc. Meeting abstracts- Electrochemical Society; 2002-2; 16 Electrochemical Society Meeting; 202nd Lee, Y.L., Chu, Y.R., Li, W.C. & Lin, C.S, (2013): Effect of permanganate concentration on the formation and properties of phosphate / permanganate conversion coating on AZ31 magnesium alloy, Corrosion Science, 70, pages 74-81. Lingenfelter T. (2012): Electrocoat process for non-chromate primers in DoD manufacturing. ASETSDefense: Sustainable Surface Engineering for Aerospace and Defense Workshop. PPG Industries, Inc. US EPA (2002): Final Report: New Environmentally Benign Heteropolymolybdate Conversion Coatings for Aluminium Alloys, Minevski, Zoran, Lynntech Inc. Nanna, M.E. & Bierwagen, G.P. (2004): Mg-rich coatings: a new paradigm for Cr-Free corrosion protection of Al Aerospace Alloys, JCT Research, Vol 1, No.2, pages 69-80. Narayanan, S. (2005): Surface pre-treatment by phosphate conversion coatings – a review. Red. Ad. Mater. Sci. 9: 130-177. Pathak, S.S., Mendon, S.K., Blanton, M.D. & Rawlins, J.W., 2012: Magnesium-based sacrificial anode cathodic protection coatings (Mg-rich primers) for aluminium alloys, Metals, 2, pages 353-376. Pawlik, M. (2009): Electrocoat primers for the aerospace industry. PPG Industries, Inc, Presentation. RPA Report (2005): Environmental Risk Reduction Strategy and Analysis of Advantages and Drawbacks for Hexavalent Chromium – Under Framework Contract: CPEC 24. Final Report prepared for the Department for Environment, Food and Rural Affairs. Risk & Policy Analysts Limited, Norfolk, UK. Steele, L.S. and Brandewie, B. (2007): Treated Aluminium Article and method for making same. EP Patent 1780313 A2. Trumble, W.P. & Lawless, P.T. (1999): Molybdenum phosphate based corrosion resistant conversion coatings. Patent, WO 2000026436 A1 Vasques, M.J., Halada, G.P., Clayton, C.R and Longtin, J.P. (2002): On the nature of the chromate conversion coating formed on intermetallic constituents of AA2024-T3. Surface and Interface Analysis 33: 607-616. Wang, D. & Bierwagen, G., 2009: Sol-gel coatings on metals for corrosion protection, Progress in Organic Coatings, 64, pages 327-338. Wang, J., Zuo, Y & Tang, Y., 2013: The study on Mg-Al rich epoxy primer for protection of aluminium alloys, Int. J. Electrochem. Sci, 8, pages 10190-10203. Yoganandan, G and Balaraju, JN and William Grips, VK (2012) The surface and electrochemical analysis of permanganate based conversion coating on alclad and unclad 2024 alloy. Applied Surface Science, 258 (-). Pp. 8880-8888. ISSN 0042-20 Guan, Y., Liu, J.G., Yan, C.W. (2011): Novel Ti/Zr Based Non-Chromium Chemical Conversion Coating for the Corrosion Protection of Electrogalvanized Steel. Int. J. Electrochem. Sci., 6 (2011) 4853 – 4867 Zhao, J., Xia, L., Sehgal, A., Lu, D., McCreery, R.L. and Frankel, G.S. (2001): Effects of chromate and chromate conversion coatings on corrosion of aluminium alloy 2024-T3. Surface and Coatings Technology 140: 51-57. 98 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES APPENDIX 1 – INITIAL LIST OF POTENTIAL ALTERNATIVES TO CHROMIUM TRIOXIDE CONTAINING SURFACE TREATMENTS ID Alternative Category Reason for Screening out 1 Acidic surface treatments 1 2 (Summarised under 2 Iridite NCP (Al, F, Oxygen) Zr/Ti-based treatments) 3 Manganese-based treatments 1 This is no alternative for surfaces treatment Mineral Tie-Coat (cathodic 4 3 discussed within this dossier Process is mineralization) related to functional chrome applications. 5 Molybdate-based coatings 1 This is no alternative for surfaces treatment 6 Plasma electrolytic oxidation 3 discussed within this dossier Process is related to functional chrome applications. This is no alternative for surfaces treatment 7 Polymer coatings 3 discussed within this dossier --> Process is related to primer applications. Adhesion and corrosion protection poor on Self-Assembling molecule relevant Al alloys, not seen as general 8 3 systems alternative for surfaces treatment discussed within this dossier. 9 Silane/Siloxane 1 10 Sol-gel coatings 1 This process is only applicable on Mg and its Tagnite (inorganic Silica or 11 3 alloys, no general alternative for surfaces vanadate) treatment discussed within this dossier. 12 Cr(III)-based processes 1 13 Zr/Ti-based coatings 1 This is no alternative for surfaces treatment 14 Zn-Tin-based coatings 3 discussed within this dossier. Benzotriazole or 5-methyl-1H- 15 2 benzotriazole 16 Electrolytic paint technology 2 Type I Cadmium (without 1 (summarized under 17 chromate passivation) Cr(III)-based processes) Type II Cadmium with other non 1 (summarized under 18 chromate passivation, nor TCP Cr(III)-based processes) Unalloyed zinc plating being 1 (summarized under 19 investigated/ has been investigated Cr(III)-based processes) (removal of chromate passivation) Passivation of cadmium and cadmium replacement coatings, 1 (summarized under 20 including Al spray & zinc spray Cr(III)-based processes) (using TCP or NCP to replace (Cr(VI) passivation system). 99 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES ID Alternative Category Reason for Screening out 21 Mg rich primer 2 1 (summarized under 22 Inhibited Sol-Gel systems Sol-Gel) 1 (summarized under 23 Hot water sealing Water-based treatments) Post-treatment with silanes 1 (summarized under 24 systems Sol-Gel) 1 (summarized under 25 Hot water sealing plus additives Water-based treatments) Water rinses and/or proprietary 1 (summarized under 26 dips not containing Cr(VI) Water-based treatments) This is no alternative for surfaces treatment 27 Low tin steel 3 discussed within this dossier. This is no alternative for surfaces treatment 28 Ionic liquids 3 discussed within this dossier. - danger to abrase too much of the base metal substrate - not for complex geometries and not for inner diameters - problem overheating the substrate by too much mechanical disturbance - media blasting: not suitable for assemblies with faying surfaces and complex geometries. - surface roughness of the base substrate Mechanical methods: Mechanical difficult to control under mechanical treatment 29 Sanding / shotblasting / media 3 therefore difficult to meet the substrate blasting / grinding & machining specific surface roughness limitations. - thin substrates: difficult to keep the residual stress to the substrate as small as possible when using mechanical treatments. - grinding & machining: problems in limitations to base material removement - affecting the substrate causing hydrogen embrittlement, residual stress (distortion), surface roughening and reducing the fatigue properties of the substrate. Non-chrome deoxidiser solution 30 1 based on Mineral acids or Iron - Not suitable for assemblies with dissimilar 30 Methylene chloride and phenol 3 metals Stripping of organic coatings: -- Not suitable for assemblies with dissimilar 31 Formic acid 3 metals Not allowed for the use with high strength steels -- Not suitable for composite substrates 32 Heat treatments: Heat gun 3 - possible over-heating and damaging of the substrate Hydrogen peroxide activated 33 1 benzyl alcohol with acid 100 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES ID Alternative Category Reason for Screening out 2 (summarized in 34 Potassium permanganate Manganese-based processes) 1 (summarized in 35 Tartaric-nitric acid Inorganic acids) 101 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES APPENDIX 2 – GENERAL INFORMATION AND THE RISK FOR HUMAN HEALTH AND THE ENVIRONMENT FOR RELEVANT SUBSTANCES APPENDIX 2.1: MAIN PROCESSES AND POST-TREATMENTS APPENDIX 2.1.1: ALTERNATIVE 1: Acidic surface treatments Table 1: Substance IDs and properties. Physicochemical Parameter Value Value properties Chemical name and Boric acid (mono Physical state at 20°C and Solid (crystalline, odourless) composition constituent substance) 101.3 kPa No melting point detected below EC number 233-139-2 Melting point 1000°C CAS number 10043-35-3 Density 1.49 g/cm3 IUPAC name Boric acid Vapour pressure 9.90 . 10-8 kPa (25 °C) Molecular formula B(OH)3 Water solubility 48.40 g/L (20°C, pH = 3.6) Flammability Non flammable Molecular weight 61.83 g/mol Flash point - Physico-chemical Parameter Value Value properties Chemical name and Sulphuric acid (mono Physical state at 20°C and Liquid (odourless) composition constituent substance) 101.3 kPa 10.4-10.5°C EC number 231-639-5 Melting point (for pure sulfuric acid) CAS number 7664-93-9 Density 1.83 g/cm3 (20°C, for 100%) IUPAC name Sulfuric acid Vapour pressure 0.49 hPa (20°C) Molecular formula H2SO4 Water solubility Miscible with water Molecular weight 98.08 g/mol Flammability Non flammable Physico-chemical Parameter Value Value properties Orthophosphoric acid Chemical name and Physical state at 20°C and Solid (crystalline, if no water (mono constituent composition 101.3 kPa attached) substance) EC number 231-633-2 Melting point 41.1 °C (101 kPa) CAS number 7664-38-2 Density 1.84 g/cm3 (38°C) IUPAC name Phosphoric acid Vapour pressure 80 Pa (25°C, extrapolated) Molecular formula H3PO4 Water solubility 5480g/ L (cold water, pH= 0.5) Flammability non flammable Molecular weight 98.00 g/mol Flash Point - Physico-chemical Parameter Value Value properties Chemical name and (+)-tartaric acid (mono Physical state at 20°C and Solid (odourless) composition constituent substance) 101.3 kPa EC number 201-766-0 Melting point 171°C (101 kPa) CAS number 87-69-4 Density 1.76 g/cm3 (20°C) IUPAC name Tartaric acid Vapour pressure < 0.005 kPa (20°C) Molecular formula C4H6O6 Water solubility 1390 g/L (20°C; pH = n.a.) 102 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Physicochemical Parameter Value Value properties Flammability Non flammable Molecular weight 150.09 g/mol Flash point - Physico-chemical Parameter Value Value properties Chemical name and Nitric acid (mono Physical state at 20°C and Liquid (fumes in moist air) composition constituent substance) 101.3 kPa EC number 231-714-2 Melting point - 41.60 °C CAS number 7697-37-2 Density 1.51 g/cm3 (20°C) IUPAC name Nitric acid Vapour pressure 9.00 kPa (25°C) Molecular formula HNO3 Water solubility > 1000g /L (20°C, pH= -1) Flammability non flammable Molecular weight 63.01 g/mol Flash point - Physico-chemical Parameter Value Value properties Chemical name and Citric acid (mono Physical state at 20°C and Solid (crystalline) composition constituent substance) 101.3 kPa EC number 201-269-1 Melting point ca.153°C CAS number 77-92-9 Density 1.67 g/cm3 (20°C) 2-hydroxypropane-1,2,3- IUPAC name Vapour pressure 2.21.10-9 kPa (25°C, extrapolated) tricarboxylic acid Molecular formula C6H8O7 Water solubility 592.00 g/L (20°C) Flammability non flammable Molecular weight 192.13 g/mol Flash Point - Table 2: Classification and labelling of relevant substances. Hazard Class Hazard Statement Number Additional Substance Regulatory and CLP and Category Code(s) of classification and Name status Code(s) (labelling) Notifiers labelling comments REACH registered; Included in CLP Boric acid Regulation, Annex VI H360FD (May damage (index number 005-007- (CAS 10043-35- Repr. 1B fertility. May damage n/a - 00-2); 3) the unborn child) Included according to (EC 233-139-2) Annex XVI on the candidate list (SVHC substance) Specific Concentration H314 (Causes severe limits: skin burns and eye REACH registered; Sulphuric acid damage) Skin Corr. 1A: C ≥ (CAS 7664-93- Skin Corr. 1A 15%, H314 Included in CLP H290 (May be n/a Regulation, Annex VI 9) Met. Corr. 1 Skin Irrit. 2: 5% ≤ C < corrosive to metals) (index number 016-020- 15%, H315 (EC 231-639-5) 00-8); Eye Irrit. 2: 5% ≤ C < 15%; H319 H314 (Causes severe REACH registered; Phosphoric acid Skin Corr. 1B skin burns and eye n/a Legal classification. Included in CLP damage) Regulation, Annex VI 103 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Class Hazard Statement Number Additional Substance Regulatory and CLP and Category Code(s) of classification and Name status Code(s) (labelling) Notifiers labelling comments (CAS 7664-38- Additional self- (index number 015-011- H290 (May be 2) Met. Corr. 1 n/a classification according 00-6); corrosive to metals) (EC 231-633-2) to REACH registration; H302 (Harmful if Acute Tox. 4 swallowed) 1,005 notifiers H315 (Causes skin Skin Irrit. 2 classified the substance irritations) as listed in the cells on the left. Additional H317 (May cause an Skin Sens. 1 1,005 1,056 notifiers REACH registered; allergic skin reaction) Tartaric acid confirmed the Not included in CLP H319 (Causes serious classifications: Skin. Regulation, Annex VI; (CAS 87-69-4) Eye Irrit. 2 eye irritation) Irrit. 2, Eye Irrit. 2 and Included in C&L (EC 201-766-0) STOT SE 3. inventory H335 (May cause STOT SE 3 respiratory irritation) H318 (Causes serious Eye Dam. 1 eye damage) Further classifications n/a according to REACH H315 (Causes skin Skin Corr. 1A registration; irritation) H272 (May intensify Ox. Liq. 3 fire; oxidizer) REACH registered; Nitric acid H314 (Causes severe (CAS 7697-37- Skin Corr. 1A skin burns and eye Included in CLP n/a 2) damage) Regulation, Annex VI (index number 007-004- (EC 231-714-2) Additional 00-1) H290 (May be Met. Corr. 1 classification according corrosive to metals) to REACH registration. Self-classification according to REACH registration. H319 (Causes serious Eye Irrit. 2 2362 eye irritation) This classification was also notified by >2000 REACH registered; parties to the C&L Citric acid inventory, Not included in CLP (CAS 77-92-9) Regulation, Annex VI; H315 (Causes skin (EC 201-069-1) Skin Irrit. 2 Included in C&L irritation) Classification notified inventory to the C&L inventory. H318 (Causes serious Eye Dam. 1 271 eye damage) Further 163 notifiers classified the substance H335 (May cause as Eye Dam. 1 only. STOT SE 3 respiratory irritation) APPENDIX 2.1.2: ALTERNATIVE 2: Cr(III)-based surface treatments Table 1: Substance ID and physicochemical properties Physico-chemical Parameter Value Value properties Chemical name and Physical state at 20°C and Chromium(III) sulphate Solid composition 101.3 kPa EC number 233-253-2 Melting point 90 °C CAS number 10101-53-8 Density 3.10 g/cm³ (anhydrous) IUPAC name Chromium(III) sulphate Vapour pressure - 104 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Physico-chemical Parameter Value Value properties Insoluble in water and acids Molecular formula Cr2(SO4)3 Water solubility (anhydrous). Soluble as hydrate Flammability Non-flammable Molecular weight 392.18 g/mol Flash point: - Physico-chemical Parameter Value Value properties Chemical name and Physical state at 20°C and Chromium(III) chloride solid composition 101.3 kPa EC number 233-038-3 Melting point ca. 1150 °C CAS number 10025-73-7 Density 2.87 g/cm³ (25 °C) IUPAC name Chromium(III) chloride Vapour pressure - Molecular formula CrCl3 Water solubility 0.585 g/cm³ Flammability Non-flammable Molecular weight 158.36 g/mol Flash Point - Table 2: Hazard classification and labelling overview Hazard Hazard Class Number Additional Substance Statement Regulatory and CLP and Category of classification and Name Code(s) status Code(s) Notifiers labelling comments (labelling) Currently not REACH Chromium registered; sulphate 1,103 notifiers did not Not included in the CLP (CAS 10101-53- Not classified - 1103 classify the substance. Regulation, Annex VI; 8) Included in C&L inventory (EC 233-253-2) H302 (Harmful Acute Tox. 4 Additional 6 parties if swallowed) notified the substance as Currently not REACH Chromium H315 (Causes Acute Tox 4 (H302) Skin Irrit. 2 registered; chloride skin irritation) only. 41 Further 6 notifiers Not included in the CLP (CAS 10025-73- H319 (Causes Regulation, Annex VI; 7) submitted the Eye Irrit. 2 serious eye classification as Acute Included in C&L inventory (EC 233-038-3) irritation) Tox 4 (H302) and H330 (Fatal if Aquatic Chronic 3 Acute Tox. 1 inhaled) (H412). APPENDIX 2.1.3: OR ALTERNATIVE 3: Silane/Siloxane and sol-gel coatings Table 1: Substance IDs and properties Parameter Value Physicochemical properties Value Chemical name and Methyl trimethoxysilane Physical state at 20°C and liquid composition (MTMS) 101.3 kPa EC number 214-685-0 Melting point < -77 °C CAS number 1185-55-3 Density 0.96 g/cm³ (20°C) IUPAC name trimethoxy(methyl)silane Vapour pressure 7.84 hPa (20°C) Molecular formula C4H12O3Si Water solubility 29 g/L (20°C) 105 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Parameter Value Physicochemical properties Value Flammability flammable Molecular weight 136.05 g/mol Flash point 11.5 °C (1013 hPa) Parameter Value Physicochemical properties Value Chemical name and Vinyl trimethoxysilane Physical state at 20°C and liquid composition (VTMS) 101.3 kPa EC number 220-449-8 Melting point -97 °C CAS number 2768-02-7 Density 0.97 g/cm³ (20 °C) IUPAC name Ethenyl(trimethoxy)silane Vapour pressure 920 Pa (20 °C) Molecular formula C5H12O3Si Water solubility 9.4 g/L (20 °C, pH = 7) Flammability Flammable Molecular weight 148.05 g/mol Flash point 24 °C Table 2: Hazard classification and labelling Hazard Class Hazard Statement Additional Substance Number of Regulatory and CLP and Category Code(s) classification and Name Notifiers status Code(s) (labelling) labelling comments H225 (Highly Classification of Flam. Liq. 2 flammable liquid and REACH registration ; vapour) notified to the C&L 96 inventory by 96 parties. H317 (May cause an Further 93 parties Skin Sens. 1 allergic skin reaction) classified the substance as Flam. Liq. 2 only H225 (Highly Instead of Flam. Liquid Flam. Liq. 2 flammable liquid and 2 and Skin Sens. 1, 296 vapour) notifiers submitted the classification as H315 (Causes skin Skin Irrit. 2 specified on the left to irritation) the C&L inventory. H319 (Causes serious 296 Best Case: Eye Irrit. 2 eye irritation) 64 additional notifiers Methyl submitted the same REACH registered; trimethoxysil classification but Not included in the CLP ane (MTMS) H335 (May cause STOT SE 3 abstained from the Regulation, Annex VI; (CAS 1185- respiratory irritation) classification as STOT 55-3) Information from C&L SE 3. inventory (EC 214- H225 (Highly 685-0) Flam. Liq. 2 flammable liquid and vapour) H226 (Flammable Flam. Liq. 3 liquid and vapour) One or several classification as H315 (Causes skin Skin Irrit. 2 specified on the left irritation) 62 were notified to the H319 (Causes serious C&L inventory by Eye Irrit. 2 eye irritation) another 62 parties in total. H302 (Harmful if Acute Tox. 4 swallowed) H332 (Harmful if Acute Tox. 4 inhaled) H226 (Flammable Classification included Worst Case: Flam. Liq. 3 176 liquid and vapour) in REACH registration. 106 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Class Hazard Statement Additional Substance Number of Regulatory and CLP and Category Code(s) classification and Name Notifiers status Code(s) (labelling) labelling comments Vinyl H332 (Harmful if Acute Tox. 4 REACH registered; trimethoxysil inhaled) ane (VTMS) Not included in the CLP 352 notifiers submitted Regulation, Annex VI; (CAS 2768- H318 (Causes serious Eye Dam. 1 352 the classification as Eye 02-7) eye damage) Information from C&L Dam. 1 only. inventory; (EC 220- 449-8) H315 (Causes skin Skin Irrit. 2 irritation) Included in the CoRAP list of substances: H319 (Causes serious Eye Irrit. 2 eye irritation) Additional - Initial grounds of classifications included concern: Human H335 (May cause Total STOT SE 3 in the C&L inventory health/Suspected respiratory irritation) number of by notifiers in different sensitiser; additional H304 (May be fatal if combinations. Exposure/Wide notifiers: dispersive use; Asp. Tox 1 swallowed and enters 32 notifiers sumitted the airways) 240 Worker exposure; classification as Muta. Exposure of sensitive H340 (May cause 1B and Carc. 1B. population; High Muta. 1B genetic effects) RCR; Aggregated tonnage H350 (May cause Carc. 1B cancer) - Status: ongoing APPENDIX 2.1.4: ALTERNATIVE 4: Water-based Post-Treatments (Hot water sealing, Rinsing after Phosphating) Table 1: Substance ID and physicochemical properties Parameter Value Physicochemical properties Value Chemical name and Potassium fluoride Physical state at 20°C and 101.3 kPa Solid (odourless) composition EC number 232-151-5 Melting point 860°C CAS number 7789-23-3 Density 2.48 g/cm³ IUPAC name Potassium fluoride Vapour pressure 0.13 hPa (752°C) Molecular formula KF Water solubility 923 g/L (20°C) Flammability Non flammable Molecular weight 58.1 g/mol Flash point - Parameter Value Physicochemical properties Value Chemical name and Lithium acetate Physical state at 20°C and 101.3 kPa Solid (colourless) composition EC number 208-914-3 Melting point 283-285°C CAS number 546-89-4 Density 1.25 g/cm³ IUPAC name Lithium acetate Vapour pressure - Molecular formula C2H3O2Li Water solubility 408 g/L (20°C) Flammability Non flammable Molecular weight 65.9 g/mol Flash point - Parameter Value Physicochemical properties Value 107 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Parameter Value Physicochemical properties Value Chemical name and Hydrogen fluoride Physical state at 20°C and 101.3 kPa Colourless gas composition EC number 231-634-8 Melting point - 83.36°C CAS number 7664-39-3 Density 0.97 g/L (20°C) IUPAC name Hydrogen fluoride Vapour pressure - Molecular formula HF Water solubility - Flammability Non flammable Molecular weight 20.01 g/mol Flash point - Table 2: Hazard classification and labelling overview. Hazard Class Hazard Statement Number Additional Substance Regulatory and CLP and Category Code(s) of classification and Name status Code(s) (labelling) Notifiers labelling comments H331 (Toxic if Acute Tox. 3* Potassium inhaled.) REACH registered; fluoride H311 (Toxic in Included in the CLP (CAS 7789-23- Acute Tox. 3* n/a Legal classification contact with skin.) Regulation, Annex VI 3) (index number: 009-005-00- H301 (Toxic if 2) (EC 232-151-5) Acute Tox. 3* swallowed.) Currently not REACH registered; Not included in the CLP Not classified - 47 Lithium acetate Regulation, Annex VI; (CAS 546-89- Information from C&L 4) inventory; (EC 208-914-3) Acute Tox. 4 (H302). 34 Repr. 2 (H361) 21 H330 (Fatal if Acute Tox. 2 * inhaled) Specific Concentration limits: Hydrogen H310 (Fatal in Acute Tox. 1 REACH registered; fluoride contact with skin.) Skin Corr. 1A: C ≥ 7%, H314 Included in the CLP (CAS 7664-39- n/a Regulation, Annex VI H300 (Fatal if Skin Corr. 1B: 1% ≤ 3) Acute Tox. 2 * (index number: 009-003-00- swallowed.) C < 7%, H314 (EC 231-634-8) 1); H314 (Causes severe Eye Irrit. 2: 0,1% ≤ C Skin Corr. 1A skin burns and eye < 1%; H319 damage.) APPENDIX 2.1.5: ALTERNATIVE 5: Manganese-based processes Table 1: Substance ID and physicochemical properties Physico-chemical Parameter Value Value properties Potassium permanganate Chemical name and Physical state at 20°C and (mono constituent Solid (dark-purple or bronze-like) composition 101.3 kPa substance) EC number 231-760-3 Melting point Decomposes <240°C 108 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Physico-chemical Parameter Value Value properties CAS number 7722-64-7 Density 2.7 g/cm3 (20°C) potassium IUPAC name Vapour pressure - oxido(trioxo)manganese Molecular formula KMnO4 Water solubility ≥64 g/L (20°C) Flammability Nonflammable but will accelerate Molecular weight 158.03 g/mol the burning of combustible material. - Flash point Physico-chemical Parameter Value Value properties Aluminium Sulphate Chemical name and Physical state at 20°C and (mono constituent Solid (granules) composition 101.3 kPa substance) No Melting of substance EC number 233-135-0 Melting point (unspecified hydrate) was observed (25°C-550°C). 1.79 g/ cm3 (at 20°C, unspecified CAS number 10043-01-3 Density hydrate) IUPAC name Aluminium sulphate Vapour pressure - Molecular formula Al2(SO4)3 Water solubility > 1000 g/L (20°C, pH = 2.4) Flammability Non flammable Molecular weight 342.15 g/mol Flash point - Physico-chemical Parameter Value Value properties Sodium dihydrogen- Chemical name and Physical state at 20°C and orthophosphate (mono Solid (granules) composition 101.3 kPa constituent substance) EC number 231-449-2 Melting point > 449°C CAS number 7558-80-7 Density 2.36 g/ cm3 (20.5°C) sodium dihydrogen IUPAC name Vapour pressure - phosphate 50.2-52 .0 %w/w Molecular formula NaH2PO4 Water solubility (20°C, pH = 3.6-4.0) Flammability nonflammable- Molecular weight 119.98 g/mol Flash point - Table 2: Hazard classification and labelling overview Hazard Hazard Number Additional Substance Class and Statement of classification and Regulatory and CLP status Name Category Code(s) Notifiers labelling comments Code(s) (labelling) Potassium H272 (May REACH registered; permanganate Ox. Sol. 2 intensify fire; Classification KMnO4 oxidizer) n/a according CLP Included in CLP Regulation, Annex VI (index number 025- (CAS 7722-64- Regulation Annex VI; Acute Tox. 4 H302 (Harmful if 002-00-9); 7) * swallowed) 109 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Hazard Number Additional Substance Class and Statement of classification and Regulatory and CLP status Name Category Code(s) Notifiers labelling comments Code(s) (labelling) (EC 231-760-3) Aquatic H400 (Very toxic Acute 1 to aquatic life) H410 (Very toxic Aquatic to aquatic life Chronic 1 with long lasting effects) H314 (Causes Further classification Skin Corr. severe skin burns according to REACH 1C and eye damage) registration; H290 (May be Classification Met. Corr. 1 corrosive to included in REACH metals) joint registration and submitted by 131 131 notifiers to the C&L H318 (Causes inventory. Eye Dam. 1 serious eye 361 parties notified damage) the substance as Eye Dam. 1 only. Not - 49 classified H302 (Harmful if Acute Tox. 4 swallowed) H318 (Causes Eye Dam. 1 serious eye Aluminium damage) Sulphate REACH registered; Al2(SO4)3 H319 (Causes Not included in CLP Regulation, (CAS 10043- Eye Irrit. 2 serious eye Annex VI; irritation) 01-3) Information from C&L inventory (EC 233-135-0) H315 (Causes Skin Irrit. 2 skin irritation) Additional ~ 180 notifiers submitted H335 (May cause one or several of the ~180 STOT SE 3 respiratory additional irritation) classifications on the left to the C&L Aquatic H400 (Very toxic Acute 1 to aquatic life) H410 (Very toxic Aquatic to aquatic life Chronic 1 with long-lasting effects) H411 (Toxic to Aquatic aquatic life with Chronic 2 long-lasting effects) Information from Sodium REACH registration; dihydrogen- notified to the C&L orthophosphate Not REACH registered; - 502 inventory by 502 NaH2PO4 classified parties (of which 51 Not included in CLP Regulation, (CAS 7558-80- did not classify due to Annex VI; 7) lacking data). Information from C&L inventory (CAS 231-449- H315 (Causes 2) Skin Irrit. 2 77 One or several skin irritation) classification as 110 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Hazard Number Additional Substance Class and Statement of classification and Regulatory and CLP status Name Category Code(s) Notifiers labelling comments Code(s) (labelling) H319 (Causes specified on the left Eye Irrit. 2 serious eye were notified to the irritation) C&L inventory by 77 parties in total. H335 (May cause STOT SE 3 respiratory irritation) APPENDIX 2.1.6: ALTERNATIVE 6: Magnesium rich primers Table 1: Substance ID and physicochemical properties for arsine as worst case scenario. Parameter Value Physicochemical properties Value Chemical name and Mag Rich Primer, Physical state at 20°C and 101.3 kPa liquid composition (commercially available) CAS number Multiple components Density 1.318 g/cm³ Flammability - Molecular structure Multiple components Flash point 35 °C Table 2: Hazard classification and labelling Additional Hazard Class Hazard Statement Number of classification Regulatory and CLP Substance Name and Category Code(s) Notifiers and labelling status Code(s) (labelling) comments H226 (Flammable Flam. Liq. 3 liquid and vapour) H315 (Causes skin Skin Irrit. 2 irritation) Supplier hazard H319 (Causes serious information Eye Irrit. 2 eye irritation) n/a from related SDS for this H317 (May cause an Skin Sens. 1 product. allergic skin reaction) H411 (Toxic to Mg-rich Primer Aquatic Chronic aquatic life with long (commercially 2 lasting effects) available n/a (product is a mixture of product) H302 (Health several substances) hazardous when swallowed) Acute Tox. 4 H312 (Health hazardous by skin contact) H332 (Health hazardous when inhaled) H304 (Can be deadly Asp. Tox. 1 if swallowed or if it penetrates into the respiratory apparatus) 111 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES APPENDIX 2.1.7: ALTERNATIVE 7: Molybdates and Molybdenum-based processes Table 1: Substance ID and properties Physico-chemical Parameter Value Value properties Disodium molybdate Chemical name and Physical state at 20°C and (monoconstituent Solid (crystalline, odourless) composition 101.3 kPa substance) EC number 7631-95-0 Melting point 687.0°C (anhydrous Substance) CAS number 231-551-7 Density 2.59 g/cm3 disodium IUPAC name tetraoxomolybdate Vapour pressure - dihydrate . Molecular formula Na2MoO4 2H2O Water solubility 654.2 g/L Flammability Molecular weight 241. 95 g/mol - Flash point Physico-chemical Parameter Value Value properties Orthophosphoric acid Chemical name and Physical state at 20°C and Solid (crystalline, if no water (mono constituent composition 101.3 kPa attached) substance) EC number 231-633-2 Melting point 41.1 °C (101 kPa) CAS number 7664-38-2 Density 1.84 g/cm3 (38°C) IUPAC name Phosphoric acid Vapour pressure 80 Pa (25°C, extrapolated) Molecular formula H3O4P Water solubility 5480 g/ L (cold water, pH= 0.5) Flammability non flammable Molecular weight 98.0 g/mol Flash point - Physico-chemical Parameter Value Value properties Chemical name and Physical state at 20°C and Cerium trifluoride Pale pink powder composition 101.3 kPa EC number 231-841-3 Melting point 1430°C CAS number 7758-88-5 Density 6.157 g/cm³ IUPAC name cerium(3+) trifluoride Vapour pressure - ≤ 1.1 mg/L (20°C, pH= 4) Molecular formula CeF3 Water solubility ≤ 0.27 mg/L (20°C, pH =7) ≤ 0.077 mg/L (20°C, pH= 9) Flammability Non flammable Molecular weight 197.1 g/mol Flash point - Table 2: Hazard classification and labelling overview. Hazard Class Hazard Statement Number Additional Regulatory and CLP Substance Name and Category Code(s) of classification and status Code(s) (labelling) Notifiers labelling comments Sodium In line with the molybdate Not classified - 163 information submitted (CAS 7631-95-0) with the REACH (EC 231-551-7) registration 163 112 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Class Hazard Statement Number Additional Regulatory and CLP Substance Name and Category Code(s) of classification and status Code(s) (labelling) Notifiers labelling comments H315 (Causes skin notifiers did not classify Skin Irrit. 2 irritation) the substance. Additional 188 notifiers H319 (Causes classified the substance Eye Irrit. 2 serious eye with hazards (see on the irritation) left). 118 H332 (Harmful if 93 notifiers mentioned Acute Tox. 4 inhaled) the single classification: Aquatic Chronic 3. H335 (May cause STOT SE 3 respiratory irritation) H412 (Harmful to Aquatic Chronic aquatic life with 93 3 long lasting effects) H314 (Causes Skin Corr. 1B severe skin burns n/a Legal classification. REACH registered; Phosphoric acid and eye damage) Included in CLP (CAS 7664-38-2) Regulation, Annex VI Additional self- (EC 231-633-2) H290 (May be (index number 015-011- Met. Corr. 1 n/a classification according corrosive to metals) 00-6); to REACH registration; H312 (Harmful in According to the Acute Tox. 4 contact with skin) REACH Registration substance is not H315 (Causes skin classified. Seven Skin Irrit. 2 irritation) notifiers submitted this REACH registered; Cerium fluoride information to the C&L Not included in CLP H319 (Causes inventory. (CAS 7758-88-5) Eye Irrit. 2 serious eye Regulation, Annex VI; 25 (EC 231-841-3) irritation) Included in C&L However, 25 parties inventory H332 (Harmful if Acute Tox. 4 notified the substance inhaled) for various hazards (see H335 (May cause classifications on the STOT SE 3 respiratory left). irritation) APPENDIX 2.1.8: ALTERNATIVE 8: Organometallics (Zr- and Ti-based products) Table 1: Substance ID and properties for an exemplary tungsten carbide-cobalt coating. Physicochemical Parameter Value Value properties Chemical name and Physical state at 20°C and Hexafluorotitanic acid Liquid composition 101.3 kPa EC number 241-460-4 Melting point < 0°C CAS number 17439-11-1 Density 1.675 g/cm³ (25°C) IUPAC name Hexafluorotitanate(2-) Vapour pressure 23 hPa (20 °C) Molecular formula H2F6Ti Water solubility Fully miscible 113 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Physicochemical Parameter Value Value properties Flammability - Molecular weight 163.87 g/mol Flash point - Chemical name and Physical state at 20°C and Hexafluorozirconic acid liquid composition 101.3 kPa EC number 234-666-0 Melting point - CAS number 12021-95-3 Density 1.512 g/cm³ (25 °C) IUPAC name Hexafluorozirconate(2-) Vapour pressure - Molecular formula H2F6Zr Water solubility Fully miscible Flammability - Molecular weight 207.23 g/mol Flash point - Physicochemical Parameter Value Value properties Chemical name and Physical state at 20°C and Zirconium dioxide solid composition 101.3 kPa EC number 215-227-2 Melting point 2680 °C CAS number 1314-23-4 Density 5.77 g/cm³ (20 °C) IUPAC name Zirconium dioxide Vapour pressure - Molecular formula ZrO2 Water solubility < 55 µg/L (20 °C, pH = 6.5) Flammability - Molecular weight 123.22 g/mol Flash point - Table 2: Hazard classification and labelling overview. Hazard Hazard Class Additional Substance Statement Number of Regulatory and CLP and Category classification and Name Code(s) Notifiers status Code(s) labelling comments (labelling) H290 (May be Met. Corr. 1 corrosive to metals) H301 (Toxic if Acute Tox. 3 swallowed) Classification as H311 (Toxic in included in REACH Acute Tox. 3 contact with skin) 33 registration. H331 (Toxic if Acute Tox. 3 inhaled) Fluorotitanic REACH registered; H314 (Causes acid Not included in the CLP Skin Corr. 1B severe skin burns (CAS 17439-11- Regulation, Annex VI; and eye damage) 1) Included in C&L H300 (Fatal if (EC 241-460-4) Acute Tox. 2 inventory swallowed) H310 (Fatal in Acute Tox. 2 contact with skin) Additional 24 notifiers H314 (Causes 24 listed other Skin Corr. 1B severe skin burns classification. and eye damage) H318 (Causes Eye Dam. 1 serious eye damage) 114 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Hazard Class Additional Substance Statement Number of Regulatory and CLP and Category classification and Name Code(s) Notifiers status Code(s) labelling comments (labelling) H330 (Fatal if Acute Tox. 2 inhaled) Iridite NCP Xn R20/21/22 Chrome-free (Acute Tox. 4, passivation for oral, dermal Harmful by aluminium and inhalation) inhalation, in contact with skin Contains 1-5% and if swallowed. Source of information: Information from related of CAS 12021- n/a 95-3 (EC 234- MSDS Iridite NCP MSDS 666-0) Xi (for R36/38 classification of (Skin Irrit. 2 Irritating to eyes substance please Eye Irrit. 2) and skin. see below) H301 (Toxic if Acute Tox. 3 swallowed) H311 (Toxic in Acute Tox. 3 contact with skin) H314 (Causes 76 Fluorozirconic REACH registered; Skin Corr. 1B severe skin burns acid and eye damage) Not included in the CLP (CAS 12021-95- Regulation, Annex VI; H330 (Fatal if 3) Acute Tox. 2 Included in C&L inhaled) (EC 234-666-0) inventory H290 (May be Met. Corr. 1 corrosive to Further classification metals) - according to REACH H331 (Toxic if registration; Acute Tox. 3 inhaled) Not classified - 750 H315 (Causes Skin Irrit. 2 750 notifiers did not Zirconium skin irritation) dioxide classify the substance. H319 (Causes REACH registered; (CAS 1314-23- Additional 73 notifiers Eye Irrit. 2 serious eye 4) 73 did mention human irritation) health hazards, see cells (EC 215-227-2) on the left). H335 (May cause STOT SE 3 respiratory irritation) APPENDIX 2.1.9: ALTERNATIVE 9: Electrolytic paint technology No information on Substance IDs and physicochemical properties of substances used within this alternative are available. No classification and labelling information are available. Due to information from supplier, a commercially available product is classified as Eye Irrit. 2, and Aquatic Chronic 3 and Skin Irrit. 2. 115 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES APPENDIX 2.1.10: ALTERNATIVE 10: Zinc-Nickel Electroplating Table 1: Substance IDs and properties for relevant substances: Physicochemical Parameter Value Value properties Chemical name and Physical state at 20°C Zinc sulphate Solid (white, powder) composition and 101.3 kPa EC number 231-793-3 Melting point Decomposes at 625°C CAS number 7733-02-0 Density 3.35 g/cm³ (monohydrate) IUPAC name Zinc sulphate Vapour pressure - Molecular formula ZnSO4 Water solubility 210 g/L (monohydrate) Flammability - Molecular weight 161.47 g/mol Flash point - Physicochemical Parameter Value Value properties Chemical name and Physical state at 20°C Nickel sulphate Solid (greenish-yellow) composition and 101.3 kPa EC number 232-104-9 Melting point Decomposes at 840°C CAS number 7786-81-4 Density 3.68 g/cm3 at 20°C IUPAC name nickel(II) sulphate Vapour pressure - Molecular formula Ni(SO4)2 Water solubility 293 g/L (at 20°C) Flammability Non flammable Molecular weight 154.8 g/mol Flash point - Table 2: Hazard classification and labelling Hazard No. of Additional Hazard Class Substance Statement Notifiers classification and Regulatory and and Category Name Code(s) (CLP labelling CLP status Code(s) (labelling) inventory) comments H302 (harmful if swallowed) H318 (causes Zinc sulphate Acute Tox. 4 serious eye REACH registered; damage) (CAS 7733- Eye Dam. 1 Included in CLP 02-0) H400 (very toxic Regulation, Annex Aquatic Acute 1 (EC 231-793- to aquatic life) VI (index number Aquatic Chronic 1 3) H410 (very toxic 030-006-00-9); to aquatic life with long lasting effects) H302 (harmful if Specific Nickel Acute Tox. 4 swallowed) Concentration sulphate Skin Irrit. 2 REACH registered; H315 (causes skin limits, M-Factors (CAS 7786- Skin Sens. 1 Included in CLP irriation) Skin Sens. 1; Regulation, Annex 81-4) Acute Tox. 4 H317 (may cause H317: C ≥ 0,01% VI (index number (EC 232-104- Resp. Sens. 1 an allergic skin STOT RE 1; 028-009-00-5); 9) Muta. 2 reaction) H372: C ≥ 1% 116 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard No. of Additional Hazard Class Substance Statement Notifiers classification and Regulatory and and Category Name Code(s) (CLP labelling CLP status Code(s) (labelling) inventory) comments Carc. 1A H332 (harmful if Skin Irrit. 2; Repr. 1B inhaled) H315: C ≥ 20% STOT RE 1 H334 (may cause M=1 allergy or asthma Aquatic Acute 1 STOT RE 1; symptoms or H373: C ≥ 1% Aquatic Chronic 1 breathing STOT RE 2; difficulties if H373: 0,1% ≤ C < inhaled) 1% H341 (suspected of causing genetic defects) H350i (may cause cancer by inhalation) H360D (may damage the unborn child) H372 (cause damage to organs) H400 (very toxic to aquatic life) H410 (very toxic to aquatic life with long lasting effects) APPENDIX 2.1.11: ALTERNATIVE 11: Benzotriazole-based processes, e.g. 5-methyl-1H- benzotriazol Table 1: Substance IDs and properties for relevant substances: Physicochemical Parameter Value Value properties Chemical name and 5-methyl-1H- Physical state at 20°C solid composition benzotriazol and 101.3 kPa EC number 205-265-8 Melting point 80-82 °C CAS number 136-85-6 Density ca. 1.3 g/cm³ (predicted) IUPAC name 5-Methylbenzotriazole Vapour pressure - Molecular formula C7H7N3 Water solubility 6.0 g/L (25 °C) Molecular weight 133.15 g/mol Flammability 210-212 °C Table 2: Hazard classification and labelling Hazard Additional Hazard Class Number Substance Statement classification and Regulatory and CLP and Category of Name Code(s) labelling status Code(s) Notifiers (labelling) comments 5-methyl-1H- H302 (Harmful if Currently not REACH Acute Tox. 4 36 36 notifiers notified benzotriazol swallowed) the substance with registered; 117 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Additional Hazard Class Number Substance Statement classification and Regulatory and CLP and Category of Name Code(s) labelling status Code(s) Notifiers (labelling) comments (6- the single hazard Not Included in CLP methylbenzo- Acute Tox. 4. Regulation, Annex triazole) VI; Included in the (CAS 136-85- C&L Inventory 6) (EC 205-265-8) H302 (Harmful if Acute Tox. 4 swallowed) H315 (Causes Additional 23 Skin Irrit. 2 skin irritation) notifiers classified the substance both Included in the C&L H319 (Causes 23 with Acute Tox. 4 Inventory Eye Irrit. 2 serious eye and with three irritation) additional hazards H335 (May cause (see left). STOT SE 3 respiratory irritation) 118 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES APPENDIX 2.2: PRE-TREATMENTS: CLEANING, PICKLING, ETCHING-CR(VI)-FREE ALTERNATIVES Table 1: Substance IDs and physicochemical properties are presented (not exhaustive): Physico-chemical Parameter Value Value properties Chemical name and Sulphuric acid (mono Physical state at 20°C and Liquid (odourless) composition constituent substance) 101.3 kPa 10.4-10.5°C EC number 231-639-5 Melting point (pure sulfuric acid) 1.83 g/cm3 (20°C, pure sulphuric CAS number 7664-93-9 Density acid) IUPAC name Sulfuric acid Vapour pressure 0.49 hPa (20°C) Molecular formula H2SO4 Water solubility Miscible with water Flammability Non flammable Molecular weight 98.08 g/mol Flash point - Physico-chemical Parameter Value Value properties Orthophosphoric acid Chemical name and Physical state at 20°C and Solid (crystalline, if no water (mono constituent composition 101.3 kPa attached) substance) EC number 231-633-2 Melting/freezing point 41.1 °C (101 kPa) CAS number 7664-38-2 Density 1.84 g/cm3 (38°C) IUPAC name Phosphoric acid Vapour pressure 80 Pa (25°C, extrapolated) Molecular formula H3PO4 Water solubility 5480g/ L (cold water, pH= 0.5) Flammability Non flammable Molecular weight 98.00 g/mol Flash point - Physico-chemical Parameter Value Value properties Chemical name and Nitric acid (mono Physical state at 20°C and Liquid (fumes in moist air) composition constituent substance) 101.3 kPa EC number 231-714-2 Melting/freezing point - 41.60 °C CAS number 7697-37-2 Density 1.51 g/cm3 (20°C) IUPAC name Nitric acid Vapor pressure 9.00 kPa (25°C) Molecular formula HNO3 Surface Tension - Molecular weight 63.01 g/mol Water solubility > 1000g /L (20°C, pH= -1) Flammability Non flammable Molecular structure Flash point - Physico-chemical Parameter Value Value properties Chemical name and Physical state at 20°C and Chromium(III) sulphate Solid composition 101.3 kPa EC number 233-253-2 Melting point 90 °C CAS number 10101-53-8 Density 3.10 g/cm³ (anhydrous) IUPAC name Chromium(III) sulphate Vapour pressure - Insoluble (anhydrous). Soluble as Molecular formula Cr2(SO4)3 Water solubility hydrate. Non-flammable Flammability Molecular weight 392.18 g/mol - Flash point 119 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Physico-chemical Parameter Value Value properties Physico-chemical Parameter Value Value properties Chemical name and Physical state at 20°C and Iron(II)-sulphate Solid composition 101.3 kPa EC number 231-753-5 Melting point > 300°C (decomposes) CAS number 7720-78-7 Density 3.65 g/cm³ IUPAC name Iron(2+) sulfate Vapour pressure - Molecular formula FeSO4 Water solubility Very soluble (>10000 mg/L) Flammability Non flammable Molecular weight 151.9 g/mol Flash point - Physicochemical Parameter Value Value properties Chemical name and Physical state at 20°C and Hydrogen peroxide Liquid (odourless, colourless) composition 101.3 kPa 0.43°C (1013 hPa, for pure EC number 231-765-0 Melting point hydrogen peroxide) 1.44 g/cm3 (calculated for pure CAS number 7722-84-1 Density hydrogen peroxide at 25°C) 2.99 hPa (calculated for pure IUPAC name Hydrogen peroxide Vapour pressure hydrogen peroxide at 25°C) Molecular formula H2O2 Water solubility miscible in water in all proportions Flammability - Molecular weight 34.01 g/mol Flash point: - Physicochemical Parameter Value Value properties Chemical name and Benzyl alcohol Physical state at 20°C and liquid composition 101.3 kPa EC number 202-859-9 Melting point - 15.4°C CAS number 100-51-6 Density 1.04 g/cm3 (22°C) IUPAC name phenyl methanol Vapour pressure 7 Pa (20°C) Molecular formula C7H8O Water solubility 44.0 g/L (50°C) Not considered to be highly Flammability Molecular weight 108.14 g/mol flammable Flash Point - 120 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Table 2: Hazard classification and labelling overview Hazard Hazard Statement Number Additional Substance Class and Regulatory and Code(s) of classification and Name Category CLP status Notifiers labelling comments Code(s) (labelling) Specific Concentration H314 (Causes severe limits: skin burns and eye REACH registered; Sulphuric acid damage) Skin Corr. 1A: C ≥ 15%, (CAS 7664-93- Skin Corr. 1A H314 Included in CLP H290 (may be n/a Regulation, Annex VI 9) Met. Corr. 1 Skin Irrit. 2: 5% ≤ C < corrosive to metals) (index number 016- 15%, H315 (EC 231-639-5) 020-00-8); Eye Irrit. 2: 5% ≤ C < 15%; H319 H314 (Causes severe Phosphoric acid Skin Corr. 1B skin burns and eye n/a Legal classification. REACH registered; (CAS 7664-38- damage) Included in CLP Regulation, Annex VI 2) Additional self- H290 (May be (index number 015- (EC 231-633-2) Met. Corr. 1 n/a classification according to corrosive to metals) 011-00-6); REACH registration; H272 (May intensify Ox. Liq. 3 fire; oxidizer) H314 (Causes severe Skin Corr. 1A skin burns and eye n/a damage) Additional classification H290 (May be Met. Corr. 1 according to REACH REACH registered; Nitric acid corrosive to metals) registration. (CAS 7697-37- Included in CLP 2) Classification notified to Regulation, Annex VI Not classified - 257 the C&L inventory. (index number 007- (EC 231-714-2) 004-00-1) H315 (Causes skin Skin Irrit. 2 irritation) Classification notified to the C&L inventory. H318 (Causes serious Eye Dam. 1 271 eye damage) Further 163 notifiers classified the substance as H335 (May cause Eye Dam. 1 only. STOT SE 3 respiratory irrtation) Currently not REACH Chromium registered; sulphate Not included in the 1103 notifiers did not (CAS 10101-53- Not classified - 1103 CLP Regulation, classify the substance. 8) Annex VI; (EC 233-253-2) Included in C&L inventory Reach registered Iron(II) sulphate H302 (harmful if substance; Harmonised swallowed) classification- Annex (CAS 7720-78- Acute Tox. 4 H315 (causes skin VI of Regulation (EC) 7) Skin Irrit. 2 - irritation) No 1272/2008 (CLP (EC 231-753-5) Eye Irrit. 2 H319 (causes serious Regulation). eye irritation) (Index number: 026- 003-00-7) Hydrogen H271 (May cause fire REACH registered; peroxide Ox. Liqu.1 or explosion; strong Included in CLP (CAS 7722- oxidiser) 550 Liquid Regulation, Annex VI 84-1) H302 (Harmful if (index number 008- Acute Tox. 4 (EC 231-765-0) swallowed) 003-00-9) 121 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES Hazard Hazard Statement Number Additional Substance Class and Regulatory and Code(s) of classification and Name Category CLP status Notifiers labelling comments Code(s) (labelling) H314 (causes severe Skin Corr. 1A skin burns and eye damage) H332 (Harmful if Acute Tox 4. inhaled) H271 (May cause fire Ox. Liqu.1 or explosion; strong oxidizer) H302 (Harmful if Acute Tox. 4 swallowed) H314 (causes severe Skin Corr. 1A skin burns and eye damage) 352 H318 (causes serious Eye Dam.1 eye damage) H332 (Harmful if Acute Tox 4. inhaled) H335 (may cause STOT SE 3 respiratory irritation) H302 (Harmful if Acute Tox. 4 swallowed) 2502 H332 (Harmful if Acute Tox. 4 inhaled) H302 (Harmful if REACH registered; Acute Tox. 4 Benzyl alcohol swallowed) Included in CLP (CAS 100-51-6) Regulation, Annex VI (index number 603- (EC 202-859-9) H312 (Harmful in Acute Tox 4 057-00-5 ) contact with skin.) 352 H318 (Causes serious Eye Dam.1 eye damage) H332 (Harmful if Acute Tox. 4 inhaled) 122 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed. ANALYSIS OF ALTERNATIVES APPENDIX 2.3: SOURCES OF INFORMATION Information on substance identities, physicochemical properties, hazard classification and labelling are based on online data searches. All online sources were accessed between June and September 2014. The main sources are: 1. European Chemicals Agency: http://echa.europa.eu/de/ 2. ChemSpider internet site: http://www.chemspider.com 3. Sigma Aldrich MSDS: http://www.sigmaaldrich.com 4. Alfa Aesar MSDS: http://www.alfa.com/ 5. Chemical Book internet site: http://www.chemicalbook.com 6. Santa Cruz Biotechnology internet site: http://www.scbt.com/ 7. PubChem internet site: http://pubchem.ncbi.nlm.nih.gov 8. ChemBlink internet site: http://www.chemblink.com/ 9. Espimetals.com internet site: http://www.espimetals.com 10. SDS of a commercially available Mg rich primer system. 123 Use number: 4 Copy right protected - Property of Members of the CTAC Submission Consortium - No copying / use allowed.