Conversion coating

A Conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound. These coatings are commonly applied to protect the part against corrosion, to improve the adherence of other coatings, for lubrication, or just for decoration. 
Types.
The most common conversion coating processes for metal parts with industrial use include
Non-metallic substrates.
Conversion coatings have been studied also for non-metallic substrates, e. g. for protection of fluorozirconate glasses used in optoelectronics.
Regulations.
US military specifications for conversion coatings include MIL-DTL-81706 and MIL-DTL-5574.