{{Short description|Chemical treatment of metals}} {{Distinguish|Chrome plating|Diffusion hardening}}
thumb|right|240px|Zinc chromate conversion coating on small steel parts.
'''Chromate conversion coating''' or '''alodine coating''' is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys.<ref name=busch2001/>{{rp|p.1265}}<ref name=osbo2001/> The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives,<ref name=osbo2001/> as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack (such as bare fingers) on soft metals.<ref name=osbo2001/>
Chromate conversion coatings are commonly applied to items such as screws, hardware and tools. They usually impart a distinctively iridescent, greenish-yellow color to otherwise white or gray metals. The coating has a non-stoichiometric composition that includes chromium salts, arranged in a complex structure.<ref name=osbo2001/>
The process is sometimes called alodine coating, a term used specifically<ref name=osbo2001/> in reference to the trademarked Alodine process of Henkel Surface Technologies.<ref name=henk2009/>
==Process== Chromate conversion coatings are usually applied by immersing the part in a chemical bath until a film of the desired thickness has formed, removing the part, rinsing it and letting it dry. The process is usually carried out at room temperature, with a few minutes of immersion. Alternatively, the solution can be sprayed, or the part can be briefly dipped in the bath, in which case the coating reactions take place while the part is still wet.<ref name=osbo2001/>
The coating is soft and gelatinous when first applied, but hardens and becomes hydrophobic as it dries, typically in 24 hours or less.<ref name=osbo2001/> Curing can be accelerated by heating to {{convert|70|C}}, but higher temperature will gradually damage the coating on steel.
===Bath composition=== The composition of the bath varies greatly, depending on the material to be coated and the desired effect. Most bath formulae are proprietary.
The formulations typically contain hexavalent chromium compounds, such as chromates and dichromates.<ref name=frank2002/>
The widely used Cronak process for zinc and cadmium consists of 5–10 seconds of immersion in a room-temperature solution consisting of 182 g/L sodium dichromate (Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> · 2H<sub>2</sub>O) and 6 mL/L concentrated sulfuric acid.<ref name=edwa1997/>
===Chemistry=== The chromate coating process starts with a redox reaction between the hexavalent chromium and the metal.<ref name=osbo2001/> In the case of aluminum, for example,
:{{chem|Cr|6+}} + {{chem|Al}}<sup>0</sup> → {{chem|Cr|3+}} + {{chem|Al|3+}}
The resulting trivalent cations react with hydroxide ions in water to form the corresponding hydroxides, or a solid solution of both hydroxides:
:{{chem|Cr|3+}} + 3 {{chem|OH|−}} → {{chem|Cr(OH)|3}}
:{{chem|Al|3+}} + 3 {{chem|OH|−}} → {{chem|Al(OH)|3}}
Under appropriate conditions, these hydroxides condense with elimination of water to form a colloidal sol of very small particles, that are deposited as a hydrogel on the metal's surface. The gel consists of a three-dimensional solid skeleton of oxides and hydroxides, with nanoscale elements and voids, enclosing a liquid phase. The structure of the gel depends on metal ion concentration, pH, and other ingredients of the solution, such as chelating agents and counterions.<ref name=osbo2001/>
The gel film contracts as it dries, compressing the skeleton and causing it to stiffen. Eventually shrinkage stops, and further drying leaves the pores open but dry, turning the film into a xerogel. In the case of aluminum, the dry coating comprises mostly chromium(III) oxide {{chem|Cr|2|O|3}}, or mixed (III)/(VI) oxide, with very little {{chem|Al|2|O|3}}. Typically the process variables are adjusted to give a dry coating that is 200-300 nm thick.<ref name=osbo2001/><ref name=lytle1995/><ref name=zhao2001/>
The coating contracts as it dries, which causes it to crack into many microscopic scales, described as "dried mud" pattern. The trapped solution keeps reacting with any metal that gets exposed in the cracks, so that the final coating is continuous and covers the entire surface.<ref name=osbo2001/>
Although the main reactions turn most of the chromium(VI) anions (chromates and dichromates) in the deposited gel into insoluble chromium(III) compounds, a small quantity of them remains un-reacted in the dried-out coating. For example, in the coating formed on aluminum by a commercial bath, about 23% of the chromium atoms were found to be hexavalent {{chem|Cr|6+}}, except in a region close to the metal. These chromium(VI) residues can migrate when the coating is wetted, and are believed to play a role in preventing corrosion in the finished part—specifically, by restoring the coating in any new microscopic cracks where corrosion could start.<ref name=osbo2001/><ref name=lytle1995/><ref name=zhao2001/>
==Substrates==
===Zinc=== {{Redir|Zinc yellow|the RAL color|RAL 1018 Zinc yellow}} Chromating is often performed on galvanized parts to make them more durable. The chromate coating acts as paint does, protecting the zinc from white corrosion, thus making the part considerably more durable, depending on the chromate layer's thickness.<ref name=giga1997/><ref name=rocco2004/><ref name=long2003/>
The protective effect of chromate coatings on zinc is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black. Darker coatings generally provide more corrosion resistance.<ref name=dega2003/> The coating color can also be changed with dyes, so color is not a complete indicator of the process used.
ISO 4520 specifies chromate conversion coatings on electroplated zinc and cadmium coatings. ASTM B633 Type II and III specify zinc plating plus chromate conversion on iron and steel parts. Recent revisions of ASTM B633 defer to ASTM F1941 for zinc plating mechanical fasteners, like bolts, nuts, etc. 2019 is the current revision for ASTM B633 (superseded the revision from 2015), which raised required tensile thresholds when confronting hydrogen embrittlement issues and addressed embrittlement concerns in a new appendix.
===Aluminium and its alloys=== For aluminum, the chromate conversion bath can be simply a solution of chromic acid. The process is rapid (1–5 min), requires a single ambient temperature process tank and associated rinse, and is relatively trouble free.<ref name=osbo2001/>
As of 1995, Henkel's Alodine 1200s commercial formula for aluminum consisted of 50-60% chromic anhydride {{chem|CrO|3}}, 20-30% potassium tetrafluoroborate {{chem|KBF|4}}, 10-15% potassium ferricyanide {{chem|K|3|Fe(CN)|6}}, 5-10% potassium hexafluorozirconate {{chem|K|2|ZrF|6}}, and 5-10% sodium fluoride {{chem|NaF}} by weight. The formula was meant to be dissolved in water at the concentration of 9.0 g/L, giving a bath with pH = 1.5. It yielded a light gold color after 1 min, and a golden-brown film after 3 min. The average thickness ranged between 200 and 1000 nm.<ref name=lytle1995/>
Iridite 14-2 is a chromate conversion bath for aluminum. Its ingredients include chromium(IV) oxide, barium nitrate, sodium silicofluoride and ferricyanide.<ref name=macdXXXX/> In the aluminum industry, the process is also called '''chemical film'''<ref name=edgeXXXX/> or '''yellow iridite''',<ref name=edgeXXXX/> Commercial trademarked names include ''Iridite''<ref name=edgeXXXX/> and ''Bonderite''<ref name=saib2018/> (formerly known as ''Alodine'', or ''Alocrom'' in the UK).<ref name=anth194/> The main standards for chromate conversion coating of aluminium are MIL-DTL-5541 in the US, and Def Stan 03/18 in the UK.
===Magnesium=== ''Alodine'' may also refer to chromate-coating magnesium alloys.<ref name=henk2009/>
===Steel=== Steel and iron cannot be chromated directly. Steel plated with zinc or zinc-aluminum alloy may be chromated.<ref name=rocco2004/><ref name=long2003/> Chromating zinc plated steel does not enhance zinc's cathodic protection of the underlying steel from rust.<ref name=edwa1997/>
===Phosphate coatings=== Chromate conversion coatings can be applied over the phosphate conversion coatings often used on ferrous substrates. The process is used to enhance the phosphate coating.<ref name=edwa1997/>
==Safety==
Hexavalent chromium compounds have been the topic of intense workplace and public health concern for their carcinogenicity, and have become highly regulated.<ref name=usdol2006/>
In particular, concerns about the exposure of workers to chromates and dichromates while handling the immersion bath and the wet parts, as well as the small residues of those anions that remain trapped in the coating, have motivated the development of alternative commercial bath formulations that do not contain hexavalent chromium;<ref name=epaXXXX/> for instance, by replacing the chromates by trivalent chromium salts, which are considerably less toxic and provide as good or better corrosion resistance than traditional hexavalent chromate conversion.<ref name="eckles">{{cite web |last1=Eckles |first1=William |last2=Frischauf |first2=Rob |title=Alternatives to the Hexavalent Chromates: An Evolution of Trivalent Chromate Technologies. |url=https://sterc.org/pdf/sf2006/sf0654.pdf |website=Surface Technology Environment Research Center |publisher=Surface Finish Industry Council (SFIC) |access-date=28 June 2024 |date=18 Sep 2006}}</ref>
In Europe, the RoHS and REACH Directives encourage elimination of hexavalent chromium in a broad range of industrial applications and products, including chromate conversion coating processes.
==References== <references>
<ref name=osbo2001>Joseph H Osborne (2001): "Observations on chromate conversion coatings from a sol–gel perspective". ''Progress in Organic Coatings'', volume 41, issue 4, pages 280-286. {{doi|10.1016/S0300-9440(01)00143-6}}</ref>
<ref name=busch2001>K.H. Jürgen, Buschow, Robert W. Cahn, Merton C. Flemings, Bernhard Ilschner, Edward J. Kramer, and Subhash Mahajan (2001): ''Encyclopedia of Material – Science and Technology'', Elsevier, Oxford, UK.</ref>
<ref name=frank2002>Robert Peter Frankenthal (2002): ''[https://books.google.com/books?id=O1DcJk1JpCMC&pg=PA430 Corrosion Science: A Retrospective and Current Status in Honor of Robert P. Frankenthal]'' Proceedings of an international symposium. {{isbn|9781566773355}}</ref>
<ref name=usdol2006>Occupational Exposure to Hexavalent Chromium, US Dept. of Labor, OSHA Federal Register # 71:10099-10385, 28 Feb 2006.</ref>
<ref name=epaXXXX>{{Cite web |url=http://www.epa.gov/nrmrl/std/mtb/pdf/web-powdercoatarticleversion1.pdf |title=Archived copy |access-date=2010-09-15 |archive-date=2011-02-05 |archive-url=https://web.archive.org/web/20110205184759/http://www.epa.gov/nrmrl/std/mtb/pdf/web-powdercoatarticleversion1.pdf |url-status=dead }}</ref>
<ref name=edgeXXXX>{{Cite web|url=http://www.engineersedge.com/iridite.htm|title = Iridite Chromate Conversion Coating Finish Mil-C-5541 Specification | Engineers Edge}}</ref>
<ref name=saib2018>{{cite web|url=http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgSAIB.nsf/(LookupSAIBs)/HQ-18-09?OpenDocument|publisher=FAA|date=February 5, 2018|work=Special Airworthiness Information Bulletin (SAIB): HQ-18-09|title=Aircraft Structures – Alodine Coating|format=pdf|access-date=2018-04-03|archive-date=2022-08-15|archive-url=https://web.archive.org/web/20220815105709/https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgSAIB.nsf/(LookupSAIBs)/HQ-18-09?OpenDocument|url-status=dead}}</ref>
<ref name=anth194>New surface treatment for aluminum. Anthony, J. Iron Age (1946), 158(23), 64-7.</ref>
<ref name=henk2009>[http://www.henkelna.com/cps/rde/xchg/henkel_us/hs.xsl/1556_USE_HTML.htm?countryCode=us&BU=industrial&parentredDotUID=0000000GFR&redDotUID=0000000GFR&brand=000000037X Henkel Alodine products home page], accessed 2009-03-27</ref>
<ref name=lytle1995>F. W. Lytle, R. B. Greegor, G. L. Bibbins, K. Y. Blohowiak, R. E. Smith, and G. D. Tuss (1995): "An investigation of the structure and chemistry of a chromium-conversion surface layer on aluminum". ''Corrosion Science'', volume 31, issue 3, pages 349-369. {{doi|10.1016/0010-938X(94)00101-B}}</ref>
<ref name=edwa1997>{{Cite book | last = Edwards | first = Joseph | title = Coating and Surface Treatment Systems for Metals | publisher = Finishing Publications Ltd. and ASM International | year = 1997 | pages = 66–71 | isbn = 0-904477-16-9 }}</ref>
<ref name=dega2003>{{Cite book| last1 = Degarmo | first1 = E. Paul | last2 = Black | first2 = J T. | last3 = Kohser | first3 = Ronald A. | title = Materials and Processes in Manufacturing | publisher = Wiley | page = 792 | year = 2003 | edition = 9th | isbn = 0-471-65653-4}}</ref>
<ref name=macdXXXX>MacDermid MSDS for Iridite 14-2, Product number 178659.</ref>
<ref name=zhao2001>J. Zhao, L. Xia, A. Sehgal, D. Lu, R. L. McCreery, and G. S. Frankel (2001): "Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3". ''Surface and Coatings Technology'', volume 140, issue 1, pages 51-57. {{doi|10.1016/S0257-8972(01)01003-9}}</ref>
<ref name=rocco2004>A. M. Rocco, Tania M. C. Nogueira, Renata A. Simão, and Wilma C. Lima (2004): "Evaluation of chromate passivation and chromate conversion coating on 55% Al–Zn coated steel". ''Surface and Coatings Technology'', volume 179,issues 2–3, pages 135-144. {{doi|10.1016/S0257-8972(03)00847-8}}</ref>
<ref name=giga1997>M. P. Gigandet, J. Faucheu, and M. Tachez (1997): "Formation of black chromate conversion coatings on pure and zinc alloy electrolytic deposits: role of the main constituents". ''Surface and Coatings Technology'', volume 89, issue 3, 1pages 285-291. {{doi|10.1016/S0257-8972(96)03013-7}}</ref>
<ref name=long2003>Z. L. Long, Y. C. Zhou, and L. Xiao (2003): "Characterization of black chromate conversion coating on the electrodeposited zinc–iron alloy". ''Applied Surface Science'', volume 218, issues 1–4, pages 124-137. {{doi|10.1016/S0169-4332(03)00572-5}}</ref>
</references>
==External links== *[https://web.archive.org/web/20070820114000/http://www.alu-info.dk/Html/alulib/modul/A00337.htm Yellow] and [https://web.archive.org/web/20070819111003/http://www.alu-info.dk/Html/alulib/modul/A00338.htm green] chromating chemistry on aluminium
{{DEFAULTSORT:Chromate Conversion Coating}} Category:Coatings Category:Corrosion prevention Category:Chromium