Chromate conversion coating
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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. Note that industry has always measured hexavalent chromium in chromate conversion coatings in units of mass per area, yet legislated limits are given in weight percent; there is currently no method available to measure hexavalent chromium as a percentage of the total chromate coating mass.
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, and 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 aluminium which has been chromated per Mil-C-5541.
See also
- Zinc chromate primer (paint)
External links
- [Overview of different chromating processes]
- [yellow] and [green] chromating chemistry on aluminum
- [MIL-C-5541, Chromate conversion coating specification for aluminum.]
- [MIL-C-17711B, Chromate conversion coating specification for zinc and zinc alloy casting and hot dipped galvanized parts]
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