Formation of Conversion Layers

Conversion layers generally consist of inorganic compounds formed on the metal surface. They are used to increase the corrosion resistance and to improve the paint adhesion of the metal surface. 

Industrially, phosphate layers are the most important and phosphating is used to treat steel, aluminum, and zinc. Chromating produces layers containing trivalent or hexavalent chromium compounds and is mainly used with aluminum and zinc. 

Special oxide layers and inorganic-organic coatings are used for special purposes in strip treatment. 

The surface weight of conversion layers is 0.05-5 g/m. With higher surface weights the flexibility of the layers decreases, which has an adverse effect on the flexural adhesion of the organic coating. 

Zinc phosphating is primarily used for the surface treatment of steel and zinc as well as composites of these metals with aluminum. Aqueous phosphoric acid solutions (pH 2.0-3.6) containing dissolved acidic zinc phosphate, Zn(H2P04)2. are used. 

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