Chromate conversion coating or alodine coating is a type of

conversion coating A conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound. These coatings are commonly applied to protect the part ...

used to passivate steel,

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc 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 periodi ...

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

copper Copper is a chemical element with the symbol Cu (from la, cuprum) 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 pinkis ...

silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...

titanium Titanium is a chemical element with the Symbol (chemistry), 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, resista ...

magnesium Magnesium is a chemical element with the 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 of the periodic ta ...

, and

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

alloys. The coating serves as a

corrosion inhibitor In chemistry, a corrosion inhibitor or anti-corrosive is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that comes into contact with the fluid. The effectiveness ...

, as a primer to improve the adherence of paints and adhesives, as a decorative finish, or to preserve electrical conductivity. 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

screw A screw and a bolt (see '' Differentiation between bolt and screw'' below) are similar types of fastener typically made of metal and characterized by a helical ridge, called a ''male thread'' (external thread). Screws and bolts are used to f ...

s, 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 In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...

, 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 physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...

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 Hexavalent chromium (chromium(VI), Cr(VI), chromium 6) is chromium in any chemical compound that contains the element in the +6 oxidation state (thus hexavalent). Virtually all chromium ore is processed via hexavalent chromium, specifically the ...

compounds, such as chromates and dichromates. The widely used

Cronak process The Cronak process is a conventional chromate conversion coating process developed in 1933 by The New Jersey Zinc Company. It involves immersing a zinc or zinc-plated article for 5 to 15 seconds in a chromate solution, typically prepared from sodiu ...

for zinc and cadmium consists of 5–10 seconds of immersion in a room-temperature solution consisting of 182 g/ L

sodium dichromate Sodium dichromate is the inorganic compound with the formula Na2 Cr2 O7. However, the salt is usually handled as its dihydrate Na2Cr2O7·2 H2O. Virtually all chromium ore is processed via conversion to sodium dichromate and virtually all compou ...

(Na₂Cr₂O₇ · 2H₂O) and 6 mL/L concentrated sulfuric acid.

Chemistry

The chromate coating process starts with a

redox reaction Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

between the hexavalent chromium and the metal. In the case of aluminum, for example, : + ⁰ → + The resulting trivalent cations react with

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. I ...

ions in water to form the corresponding

hydroxides Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

, or a

solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The wor ...

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 The nanoscopic scale (or nanoscale) usually refers to structures with a length scale applicable to nanotechnology, usually cited as 1–100 nanometers (nm). A nanometer is a billionth of a meter. The nanoscopic scale is (roughly speaking) a lo ...

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 Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands ar ...

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 consists 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 Galvanization or galvanizing ( also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged ...

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 term chromic acid is usually used for a mixture made by adding concentrated sulfuric acid to a dichromate, which may contain a variety of compounds, including solid chromium trioxide. This kind of chromic acid may be used as a cleaning mixtu ...

. 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 Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is an inorganic compound with the formula CrO3. It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name. This compound is a dark-purple s ...

, 20-30% potassium tetrafluoroborate , 10-15%

potassium ferricyanide Potassium ferricyanide is the chemical compound with the formula K3 e(CN)6 This bright red salt contains the octahedrally coordinated 3−.html" ;"title="e(CN)6sup>3−">e(CN)6sup>3− ion. It is soluble in water and its solution shows some g ...

, 5-10% potassium hexafluorozirconate , and 5-10%

sodium fluoride Sodium fluoride (NaF) is an inorganic compound with the formula . It is used in trace amounts in the fluoridation of drinking water, in toothpaste, in metallurgy, and as a flux. It is a colorless or white solid that is readily soluble in water. I ...

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 Barium nitrate is the inorganic compound with the chemical formula Ba( NO3)2. It, like most barium salts, is colorless, toxic, and water-soluble. It burns with a green flame and is an oxidizer; the compound is commonly used in pyrotechnics. Man ...

, sodium silicofluoride and

ferricyanide Ferricyanide is the anion e(CN)6sup>3−. It is also called hexacyanoferrate(III) and in rare, but systematic nomenclature, hexacyanidoferrate(III). The most common salt of this anion is potassium ferricyanide, a red crystalline material that ...

. 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 The military specification referred to as MIL-DTL-5541, "Military Specification, Chemical Conversion Coatings on Aluminum and Aluminum Alloys" covers chemical conversion coatings formed by the reaction of chemical conversion materials with the sur ...

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 structure, w ...

Steel

Steel and

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

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 Cathodic protection (CP; ) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. A simple method of protection connects the metal to be protected to a more easily corroded "sacrific ...

of the underlying steel from

rust Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO( ...

Phosphate coatings

Chromate conversion coatings can be applied over the

phosphate conversion coating Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of iron, zinc, or manganese phosphates, to achieve corrosion resistance, lubrication, or as a foundation for subsequent coatings or pai ...

s often used on

ferrous In chemistry, the adjective Ferrous indicates a compound that contains iron(II), meaning iron in its +2 oxidation state, possibly as the divalent cation Fe2+. It is opposed to " ferric" or iron(III), meaning iron in its +3 oxidation state, suc ...

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. However, these do not seem to provide the long-term corrosion protection of the traditional formula. 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 Unio ...

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,iIssues 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