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Phosphating, or conversion coating, is the application of an iron or zinc phosphate coating to the substrate. Conversion coating can be a very critical part of the pretreatment process, adding significantly to the performance of the finished coating.

A phosphate coating converts the metal substrate to a uniform, inert surface, which improves bonding, minimizes the spread of oxidation if the coating is scratchedand improves the overall corrosion resistance of the final part.

A conversion coating can be iron, zinc, polycrystalline, chromate, or manganese phosphate film. They are developed on both ferrous (iron based) and non-ferrous surfaces (zinc, aluminum, terne and manganese).

Parts are subjected to an acidic bath and a chemical conversion forms a complete film on the part surface, changing the chemical and physical nature of the metal surface.