Conversion coatings are coatings for metals where the part surface is converted into the coating with a chemical or electro-chemical process. Examples include chromate conversion coatings, phosphate conversion coatings, bluing, black oxide coatings on steel, and anodizing. They are used for corrosion protection, increased surface hardness, to add decorative color and as paint primers.  Conversion coatings may be very thin, on the order of 0.00001". Thick coatings, up to 0.002", are usually built up on aluminium alloys, either by anodizing or chromate conversion.