Chromate conversion coatings are mainly used to slow the effects of corrosion on aluminium, zinc, magnesium and their alloys. These coatings are so named because surface film is formed by a chemical reaction which converts the metal surface.
These conversion coatings form an ideal substrate for paints and powder coating, providing a clean, essentially inert surface which exhibits excellent adhesion properties. Most organic coatings applied directly to aluminium surfaces will not adhere well, and if subjected to any deformation, they will tend to flake off exposing the bare aluminium. Scratching off the paint surface also provides a nucleation site for aluminium corrosion leading to further undercutting of the coating.
A pre-treatment is therefore required which can be applied to aluminium to provide a suitable basis for subsequent coatings. Chromate conversion coatings have been developed which can be used on aluminium alloys and which are compatible with most paint systems.