3. Chemical conversion coatings

3.1 Different chemical conversions

After degreasing and pickling, chemical conversion coatings (sometimes referred to as "chemical treatment") are produced by simple immersion in an oxidizing aqueous electrolyte for a specified time. The principle is based on chemical reactions of oxidation and precipitation on the surface to form thin layers 50 to 300 nm thick.

The formation mechanism consists mainly of three stages:

• oxidation of the metal substrate leads to the formation of metal cations and is accompanied by the reduction of the oxidant, resulting in an increase in pH near the metal surface. As aluminum alloys rapidly passivate by coating themselves with a thin layer of aluminum oxyhydroxide, it is necessary...