Desirable Features of a Non-Chromate Conversion Coating Process by K

Technical Article Desirable Features of a Non-Chromate Conversion Coating Process by K. Sampath* An innovative constraints-based modeling approach was used to identify desirable features of a low-cost Table 1 replacement to chromate conversion coating process Typical chemical composition for aluminum alloys. Identifi ed features included Of a chromate conversion coating bath2 use of a coating bath containing appropriate cations Chemical Type of Compound Amount relative to the position of the aluminum substrate in Formula the EMF series and with ionic radii and coordination Orthophosphoric acid H PO 63.7 g/L (8.5 oz/gal) numbers comparable to those of Al(III), Cr(III) and 3 4 Sodium fl uoride NaF 4.5 g/L (0.6 oz/gal) Cr(VI) ions, availability of cations in multiple valence Chromic oxide CrO 9.7 g/L (1.3 oz/gal) states, decreasing solubility of reaction products with 3 increasing pH and their ability to form waters of (i.e., gel-like) undergoes hardening, becomes hydropho- hydration. Limited experiments showed that a coat- bic, less soluble in water and more abrasion resistant. ing bath containing Mn(VI) ions provided conversion The resulting conversion coating on the aluminum surface coatings with adequate promise, thereby validating offers corrosion resistance and an anchoring surface for the utility of this novel modeling approach. organic fi nishes, such as a paint topcoat. The chemical composition of the chromate conversion Both chromate conversion coatings and paints based on coating is indefinite as it contains varying amounts of chromates are widely used in preventing or mitigating reactants, reaction products, waters of hydration and other corrosion-related problems in structures and components anions, such as fl uorides and phosphates. In the presence made of aluminum alloys.1 These surface coatings resist of corrosive conditions, the hexavalent Cr(VI) ions that are corrosion and wear, and also promote paint adhesion. incorporated into the coating leach out, and possibly tie-up Commonly, these protective coatings are produced by the free electrons and thereby provide exceptional corro- immersion, electrostatic spraying, swabbing or brushing sion resistance. techniques that use either chromates (as potassium or stron- However, solutions containing Cr(VI) ions have been tium chromate), dichromates (as potassium dichromate), determined to be carcinogens. The U.S. Environmental phosphates (as orthophosphoric acid) or fl uorides (as either Protection Agency (EPA) has included hexavalent chromium sodium or potassium fl uoride or hydrofl uoric acid). on its list of toxic chemicals for voluntary replacement, and Occasionally, electrolytic methods are also used to has promulgated strict air emission and waste disposal obtain protective surface coatings, using chromic, sulfuric standards to curtail the use of hexavalent chromium. These or oxalic acid as the electrolyte. These methods are also developments have created a need for alternative chemical known as anodizing treatments. conversion coating compounds that do not contain Cr(VI) When a chromate conversion coating is applied to an ion, but either meet or exceed the protective properties dis- aluminum alloy component, the substrate surface reacts played by the hexavalent chromium-based compounds. with the chemical solution. The metal surface is converted Furthermore, from legacy and cost standpoints, any into a tight adherent coating, all or part of which consists alternative must involve only minor modifi cations to the of an oxidized form of the substrate metal. The chromium present process stream so that it becomes readily accept- ions in the solution are partially reduced from a higher to a able. However, while a cost-effective and environmentally lower valency state (e.g., from hexavalent Cr(VI) to triva- benign alternative that provides protection comparable to lent Cr(III)), with a concurrent rise in the pH of the coating that of chromium is needed, how does one fi nd such an bath. When dried, the coating that is initially gelatinous alternative? Currently, no known rational methods are available for identifying alternatives to Cr(VI). Clearly, the identifi cation of potential alternatives to Nuts & Bolts: chromates must be based on a fundamental understand- What This Paper Means to You ing of the corrosion-inhibiting mechanism. Although the Recent events have made the search for hexavalent chromate use of chromates as corrosion inhibitors has evolved over substitutes all the more important. But what exactly are the features of a low-cost replacement conversion coating process for * Corresponding Author: aluminum alloys? The author takes a novel modeling method to Dr. Krishnaswamy Sampath sift through all the possibilities. This interesting approach gives is 615 Demuth Street Johnstown, PA 15904 information that may point in the right direction. E-mail: [email protected] 38 Plating & Surface Finishing • March 2004 0310~1.IND 38 3/5/04, 2:18:18 PM Table 2 Table 3 Compounds Commonly Found Typical Analysis of a Chromate In Chromate Conversion Coatings3 Conversion Coating on a Zinc Substrate4 Type of Compounds Chemical Formula* Component Content (wt%) Oxides, hydroxides MxOy, Mx(OH)y Cr(VI) 8.7 Chromates MxCrO4, MxCr2O7 Cr(III) 28.2 Cr oxides, hydroxides Cr2O3, CrxOy, CrOOH, Cr(OH)3, Cr2O3nH2O Zn(II) 2.1 Cr chromates Cr(OH)CrO4, Crx(CrO4)y S (SO4) 3.3 Substrate (M*) Derived PO4, F¯ compounds, etc. Na 0.3 Cr with other anions Crx(PO4)y, etc. H2O 19.3 *M = substrate metal. O Balance several decades, much of this use is based on empirical knowl- modeling is based on a critical assessment of various known and edge. To date, a fundamental or molecular level understanding of perceived events that occur during the chromate conversion coating the chemical/physical mechanism(s) by which they operate has not process. These events are perceived primarily to overcome gaps in emerged. For example, Cr(VI) in the coating is believed to provide known knowledge. The above assessment is used to impose a set both corrosion protection and a self-healing ability. Not only is of mutually inclusive or coherent constraints on the experimental it unclear exactly what functional role chromates play, it is not conditions and develop an understanding of the effects of these con- known what chromate replacement complexes need to do to ensure straints on coating quality and performance, with no adverse effects long-life coating systems. There is very little current work that on either the environment or the health and safety of workers. addresses the electrochemical behavior of chromate replacement compounds. Under its Multiple University Research Initiative, the The specifi c objectives of this article, then, are to: Air Force Offi ce of Scientifi c Research (AFOSR) has been sup- porting research projects that address this basic need to understand 1. provide a basic understanding of chromate conversion coating mechanisms of chromate inhibition. Most other efforts to identify processes and coatings based on known and perceived events, chromate alternatives lack a basic mechanistic approach and is 2. describe recent efforts in the development and evaluation of typically focused on industry practices involving trial-and-error non-chromate conversion coating processes, methods followed by comparative or benchmark testing. 3. describe a constraints-based model that allows the development Furthermore, as with chromate conversion coatings, the corro- of a non-chromate conversion coating process and sion inhibitor paint chemistry for aluminum substrates relies on the 4. prescribe a set of experimental constraints that leads to the extensive use of Cr(VI) in the form of strontium chromate, which identifi cation of alternatives to chromates used in the chromate is incorporated into both the surface pretreatment and the primers conversion coating process. as a highly effective corrosion inhibitor. Besides mandating the removal of chromates in conversion coatings and paint chemis- It must be recognized that the alternative chemical compounds tries, the new environmental regulations are also forcing paint identifi ed by this effort also offer themselves for evaluation as materials and processing technology to move away from the use alternatives to chromates used in paint chemistries. of formulation chemistries involving volatile organic contaminants (VOCs) and hazardous air pollutants (HAPs). These ingredients Critical Assessment must soon be substantially reduced or eliminated from the paint Of Chromate Conversion Coating Processes technology base. Although formulation technology has begun to move toward compliance, the available environmentally compliant As mentioned previously, chromate conversion coatings provide paint systems have so far provided less than satisfactory perfor- corrosion resistance and improve paint adhesion. The development mance. Current comparative evaluations show that adhesion, coat- of successful non-chromate conversion coatings requires, as a pre- ing durability, weathering and corrosion protection have suffered. requisite, a critical assessment of the chromate conversion coating process. This understanding must be based on the following issues: Objectives 1. the type of compounds or ions present in chromate conversion In order to develop effective non-chromate conversion coatings and coating baths, paint formulations, we need a fundamental understanding of the 2. the effects of these compounds or ions on different types of chemical and/or physical processes and mechanisms of corrosion substrate

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