Chromate conversion coating

Chromate conversion coating is a type of conversion coating used to passivate steel, aluminum, zinc, cadmium, copper, silver, magnesium, and tin alloys. It is primarily used as a corrosion inhibitor, primer, decorative finish, or to retain electrical conductivity. The process is named after the chromate found in chromic acid, also known as hexavalent chromium, the chemical most widely used in the immersion bath process whereby the coating is applied. However, hexavalent chromium is toxic, thus, highly regulated, so new, non-hexavalent chromium-based processes are becoming more readily available at a commercial level. One alternative contains trivalent chromium. In Europe the RoHS (Restriction of Hazardous Substances) Directive is commonly referred to regarding elimination of hexavalent chromium in electrical and electronic equipment, and the REACH ("Registration, Evaluation, Authorisation and Restriction of Chemicals") Directive to wider applications including chromate conversion coating processes, paint primers and other preparations.

Chromate conversion coatings are commonly applied to everyday items such as hardware and tools, and can usually be recognized by their distinctively iridescent, greenish-yellow color.

Chromate conversion coatings on an aluminium substrate are known by the following terms: chemical film,yellow iridite, and the brand names Iridite and Alodine (in the UK Alocrom). It is also commonly used on aluminum alloy parts in the aircraft industry.

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