Chromate conversion coating or alodine coating is a type of conversion coating used to passivate

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...

, cadmium,

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

titanium Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

, and tin alloys. The coating serves as a corrosion inhibitor, as a primer to improve the adherence of

paint Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are ...

s and adhesives, as a decorative finish, or to preserve

electrical conductivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

. It also provides some resistance to abrasion and light chemical attack (such as dirty fingers) on soft metals. 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 complex composition including chromium salts, and a complex structure. The process is sometimes called alodine coating, a term used specifically in reference to the trademarked Alodine process of Henkel Surface Technologies.

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. The coating is soft and gelatinous when first applied, but hardens and becomes

hydrophobic In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, thu ...

as it dries, typically in 24 hours or less. Curing can be accelerated by heating to , 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. 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 (Na₂Cr₂O₇ · 2H₂O) and 6 mL/L concentrated

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

Chemistry

The chromate coating process starts with a redox reaction between the hexavalent chromium and the metal. In the case of aluminum, for example, : + ⁰ → + The resulting trivalent cations react with hydroxide ions in water to form the corresponding hydroxides, or a

solid solution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

of both hydroxides: : + 3 → : + 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. 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 , or mixed (III)/(VI) oxide, with very little . Typically the process variables are adjusted to give a dry coating that is 200-300 nm thick. 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. 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 , 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.

Substrates

Zinc

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. 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. 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. As of 1995, Henkel's Alodine 1200s commercial formula for aluminum consisted of 50-60% chromic anhydride , 20-30% potassium tetrafluoroborate , 10-15% potassium ferricyanide , 5-10% potassium hexafluorozirconate , and 5-10% sodium fluoride 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. Iridite 14-2 is a chromate conversion bath for aluminum. Its ingredients include chromium(IV) oxide, barium nitrate, sodium silicofluoride and ferricyanide. In the aluminum industry, the process is also called chemical film or yellow iridite, Commercial trademarked names include ''Iridite'' and ''Bonderite'' (formerly known as ''Alodine'', or ''Alocrom'' in the UK). 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 alloy Magnesium alloys are mixtures of magnesium (the lightest structural metal) with other metals (called an alloy), often aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structur ...

Steel

Steel and

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

cannot be chromated directly. Steel plated with zinc or zinc-aluminum alloy may be chromated. Chromating zinc plated steel does not enhance zinc's cathodic protection of the underlying steel from rust.

Phosphate coatings

Chromate conversion coatings can be applied over the phosphate conversion coatings often used on

ferrous In chemistry, iron(II) refers to the chemical element, element iron in its +2 oxidation number, oxidation state. The adjective ''ferrous'' or the prefix ''ferro-'' is often used to specify such compounds, as in ''ferrous chloride'' for iron(II ...

substrates. The process is used to enhance the phosphate coating.

Safety

Hexavalent chromium compounds have been the topic of intense workplace and public health concern for their carcinogenicity, and have become highly regulated. 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; 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. In Europe, the

RoHS The Restriction of Hazardous Substances Directive 2002/95/EC (RoHS 1), short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Uni ...

and REACH Directives encourage elimination of hexavalent chromium in a broad range of industrial applications and products, including chromate conversion coating processes.

References

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. 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. Robert Peter Frankenthal (2002):
Corrosion Science: A Retrospective and Current Status in Honor of Robert P. Frankenthal
' Proceedings of an international symposium. Occupational Exposure to Hexavalent Chromium, US Dept. of Labor, OSHA Federal Register # 71:10099-10385, 28 Feb 2006. New surface treatment for aluminum. Anthony, J. Iron Age (1946), 158(23), 64-7.
accessed 2009-03-27 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. MacDermid MSDS for Iridite 14-2, Product number 178659. 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. 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. 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. 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.

External links

an

chromating chemistry on aluminium {{DEFAULTSORT:Chromate Conversion Coating Coatings Corrosion prevention Chromium