Chromate conversion coating

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.

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 as it dries, typically in 24 hours or less. Curing can be accelerated by heating to 70 °C (158 °F), 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.

Chemistry

The chromate coating process starts with a redox reaction between the hexavalent chromium and the metal. In the case of aluminum, for example,

Cr⁶⁺
+ Al⁰ → Cr³⁺
+ Al³⁺

The resulting trivalent cations react with hydroxide ions in water to form the corresponding hydroxides, or a solid solution of both hydroxides:

Cr³⁺
+ 3 OH⁻
→ Cr(OH)
₃

Al³⁺
+ 3 OH⁻
→ 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.

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 Cr
₂O
₃, or mixed (III)/(VI) oxide, with very little Al
₂O
₃. 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 Cr⁶⁺
, 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.