Chromate conversion coating

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Image:Stampings steel zinc chromate conversion coating.JPG

Chromate Conversion Coatings are a type of conversion coating used to treat aluminum, zinc, cadmium, copper, silver, magnesium and their alloys. The process uses various toxic chromium compounds which may include hexavalent chromium. The industry is developing less toxic alternatives in order to comply with substance restriction legislation such as RoHS.

The most common use for chromating is to impart corrosion resistance to zinc, zinc plated steel, and aluminum parts. It is also used as a primer for subsequent coatings and is especially important in the aircraft industry as untreated aluminum is very difficult to paint or glue. Chromated parts retain their electrical conductivity to varying degrees. The process may be used to add color for decorative or identification purposes.

Chromate coatings are soft and gelatinous when first applied but harden and become hydrophobic as they age. Curing can be accelerated by heating up to 150° F, but higher temperatures will damage the coating. Thickness of coatings vary from a few nanometers to several hundred nanometers.

The strength of some chromate coatings is indicated by color, progressing from clear/blue to yellow, gold, olive drab and black. Darker coatings provide more corrosion resistance. Chromate conversion coatings are common on everyday items such as hardware and tools and usually have a distinctive yellow color. Steel parts are usually plated with zinc or cadmium prior to chromating.

Chem film refers to aluminum which has been chromated per Mil-C-5541.

[edit] See also

Zinc chromate primer (paint)

Iron and steel may also be treated by chromate converstion coatings; for example steel pipelines are frequently treated with a chromic acid dip before being coated with powder epoxy.

[edit] References

Xia L., Akiyama E., Frankel G., McCreerya R., (2000). "Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings - Equilibrium Aspects of CrVI Concentration". Journal of The Electrochemical Society 147: 2556-2562. DOI:10.1149/1.1393568.
Xia L., McCreerya R., (1998). "Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024-T3 Probed by Vibrational Spectroscopy". Journal of The Electrochemical Society 145: 3083-3089. DOI:10.1149/1.1838768.
Kendig M.W., Buchheit R.G. (2003). "Corrosion Inhibition of Aluminum and Aluminum Alloys by Soluble Chromates, Chromate Coatings, and Chromate-Free Coatings". Corrosion 59: 379-400.