Chromate conversion coating is a type of conversion coating used to passivate aluminum alloys. It is primarily used as a corrosion inhibitor, primer, decorative finish, or to retain electrical conductivity. Below is the most common specification for chromate conversion.
Class 1A chemical conversion coatings are intended to provide corrosion resistance when left unpainted as well as to improve adhesion of paint finish systems on aluminum and aluminum alloys. Coatings of this type may be used, for example, on tanks, tubing and component structures where paint finishes are not required for the interior surfaces, but are required for the exterior surfaces.
Class 3 chemical conversion coatings are intended for use as a corrosion preventive film for electrical and electronic applications where lower electrical resistance is needed relative to Class 1A coatings.
The primary difference between Class 1A and Class 3 coating is thickness, since current passes more readily through a thinner current resistant barrier (coating). Because Class 3 coatings are thinner, they are more susceptible to corrosion than Class 1A coatings. If painting areas surrounding electrical contacts, Class 3 coatings will improve adhesion of paint systems on aluminum and aluminum alloys.