Chromating is the conversion coating process of depositingan oxide layer over a metal surface to enable the metal to react with oxide layer. This forms a layer of passivated metal chromate on the surface.
Chromium phosphating of alumium and its alloys.
• Chromate - Phosphate process
the solutions are mixtures of acids, mainly containing chromates and phosphates with some additives. The coatings, produced on the aluminium are complex and move akin to oxides rather than crystalline salts. The colour of coating is a blue-green or olive green colour, the thinner coatings being more suitable for paint pretreatment. The solutions can be used by immersion
• Phosphate process
Fascinated aluminium assemblies may include steel components, typical examples being found in the car industry, electric panel where aluminium panel may be joined to steel frame before painting. It is necessary, therefore, to use a conversion coating which is compatible with both metals. Phosphate coating have been well tried for treating steel, and they have been adapted for use with aluminium as well. The solutions can be used by immersion. But are commonly applied by spray on an automatic line.