Chromate conversion coating

From Wikipedia, the free encyclopedia
  (Redirected from Chromating)
Jump to navigation Jump to search

Chromate conversion coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, magnesium, and tin alloys.[1] 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,[2] the chemical most widely used in the immersion bath process whereby the coating is applied. However, hexavalent chromium is toxic, thus, highly regulated,[3] so new, non-hexavalent chromium-based processes are becoming more readily available at a commercial level.[4] One alternative contains trivalent chromium. In Europe, the RoHS (Restriction of Hazardous Substances) Directive is commonly referred to with regards to the elimination of hexavalent chromium in electrical and electronic equipment, and the REACH ("Registration, Evaluation, Authorisation and Restriction of Chemicals") Directive is typically referenced in the context of wider applications including chromate conversion coating processes, paint primers and other preparations.


Zinc chromate conversion coating on small steel parts.



Chromate conversion coatings on an aluminium substrate are known by the following terms: chemical film,[5] yellow iridite,[5] and the brand names Iridite[5], Bonderite[6] (formerly known as Alodine, in the UK, Alocrom).[7] It is also commonly used on aluminium alloy parts in the aircraft industry.




Phosphate coatings[edit]













Curing can be accelerated by heating up to 70 °C (158 °F), but higher temperature will gradually damage the coating on steel.


  2. ^ Frankenthal, Robert Peter (2002). Corrosion Science: A Retrospective and Current Status in Honor of Robert P. Frankenthal : Proceedings of the International Symposium. ISBN 9781566773355.
  4. ^
  5. ^ a b c
  6. ^ "Aircraft Structures – Alodine Coating" (pdf). Special Airworthiness Information Bulletin (SAIB): HQ-18-09. FAA. February 5, 2018. Retrieved 2018-04-03.

External links[edit]