ANALYSIS OF ALTERNATIVES non-confidential report
Legal name of applicant(s): ZF Luftfahrttechnik GmbH
Submitted by: ZF Luftfahrttechnik GmbH
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 ANALYSIS OF ALTERNATIVES
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.
ii Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
CONTENTS
1. SUMMARY ...... 1 2. ANALYSIS OF SUBSTANCE FUNCTION...... 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
iii Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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
iv Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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
v Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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
vi Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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
vii Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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 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 CFRP Carbon-fibre-reinforced polymer CSR Chemical Safety Report 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 EHS Environmental Health and Safety
viii Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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 Pyr. Liq. Pyrophoric liquid
ix Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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
x Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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. Chromium trioxide rinsing after phosphating is a passivation process after phosphate conversion coating (phosphating). It fulfils two Chromium trioxide rinsing after requirements by using only one process step (removal of drag-out phosphating) comprising liquids and residuals of former processes adhering to the substrate and passivation of the surface by enhancing corrosion resistance.
xi Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
Term Definition 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 program contracts, based on that military or space development program.
Main treatment The purpose of the surface treatment is primarily for, but not limited to, corrosion protection. The main treatment occurs after the pre-treatment
xii Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
Term Definition and before the post-treatment. Examples include conversion coating, 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. 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.
xiii Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
Term Definition 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.
xiv Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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 steps 1 that may be applied to a number of different metal substrates (e.g. aluminium, steel, zinc, magnesium, titanium, alloys and composites with metallic areas). 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 conditions 2. 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
1 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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
2 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES only be estimated. Decreased corrosion protection performance would necessitate shorter inspection intervals, with a substantial impact on associated maintenance costs.
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 certification, qualification 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.
3 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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
4 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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.
5 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
2. ANALYSIS OF SUBSTANCE FUNCTION
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 date ³
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).
2.2. Uses of chromium trioxide Chromium VI containing substances have been widely used since the mid of 20 th 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 CrO 3 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 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;
6 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
- 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.
7 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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
8 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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)
9 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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)
10 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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)
11 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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.
12 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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,
13 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES no complete chromium trioxide-free process chain is industrially available though for key applications providing all the required properties to the surfaces for all applications.
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 Removing surface contamination Spray - 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 to anodising further processing.
Pre-treatment processes - 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 Brush (CAA) 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
14 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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) CCC) - Post-treatment processes 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 ZF Luftfahrttechnik GmbH
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 (HNO 3) 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.
16 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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 substrate or metal coatings are basically the same. According to Vasques et
17 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES al (2002) and Zhao et al (2001), aluminium exposed to a CCC solution results in the simultaneous oxidation of Al to Al 3+ and the reduction of Cr(VI) to Cr(III) as follows: (1) 2 Al 2 Al 3+ + 6e –
3+ + (2) 2 Al + 3 H 2O Al 2O3 + 6 H
2- + - (3) 2(CrO 4) + 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 (CrO 4 or Cr 2O7 ) with a lower valent form that is insoluble and creates an extremely protective film (Cr 2O3 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 (Al 2O3) 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).
18 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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 (Al 2O3). 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 ZF Luftfahrttechnik GmbH
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 + HCrO 4 AlOHCrO 4 + OH - - (2) (AlO(OH))2 + HCrO 4 (AlO) 2CrO 4 +OH +H 2O
20 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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) Al 2O3 + H 2O 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.
21 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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 ZF Luftfahrttechnik GmbH 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.
23 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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.
24 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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 100 Thermal Cycling -180- +200 °C (ECSS Q-ST-70-04)
25 Use number: 4 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES
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 m /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
26 Use number: 4 ZF Luftfahrttechnik GmbH ANALYSIS OF ALTERNATIVES
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 m /125 in² (Internal specifications) Resistivity For slurry coatings: < 10 /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 ZF Luftfahrttechnik GmbH
ANALYSIS OF ALTERNATIVES the most relevant test method is cross-cut test according to ISO 2409 with requirement of GT0 dry and