Aluminium alloys are widely used in aircraft construction, extrusions for building, skins for caravans, commercial vehicles and beverage cans. In each case they are protected by organic surface coatings (paints) often interior and exterior. Prior to coating, the metal is pretreated to improve corrosion protection, normally with a chromate treatment.

¢momate treatments normally contain hexavalent chromium (chromates CrO₄^-2) and an accelerator to speed up the reaction, they work at pH 1-2 and temperatures of 20-40°C and normally follow the process acid attack upon the metal.

2A1 + 6H⁺  ->  2A1³ + 3H₂

Reduction of hexavalent chromium to tri valent chromium formation of hydrated aluminium oxide. Formation of aluminium chromate.

2A1³⁺ + 3CrO₄^2-  ->  A1₂ (CrO₄)₃

These surfaces may be exposed by themselves or are suitable to take surface coatings.

Anti corrosive pigments in a primer work in a similar manner. When the paint film is penetrated by moisture, these pigments slowly release chromate ions, because of their low but measurable solubilities, these chromate ions reacting at the metal surface during service of the paint film, and when initially applied the acid attack upon the metal causing the chromate ions present to react at the metal surface in conjunction with the phosphate ions formed, from the phosphoric acid.