Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys.[1]: p.1265 [2] The coating serves as a corrosion inhibitor, as a primer to improve the adherence of paints and adhesives,[2] as a decorative finish, or to preserve electrical conductivity. It also provides some resistance to abrasion and light chemical attack (such as dirty fingers) on soft metals.[2]
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 complex composition including chromium salts, and a complex structure.[2]
The process is sometimes called alodine coating, a term used specifically[2] in reference to the trademarked Alodine process of Henkel Surface Technologies.[3]