The term conversion coating, as used in the metal-finishing industry, refers to the conversion of a metal's surface into a surface that will more easily accept applied coatings and/or provide for a more corrosion-resistant surface. They are thin (not over 6,000 nm thick on zinc), quickly and easily formed, easily scratched, and, if used to enhance paint adhesion, they are coated shortly after being formed, to prevent degradation of the conversation coating. Conversion coatings for zinc have been in use since the early 1920s and there are a number of different products on the market. The four main types of conversion coatings in common use are based upon: (1) the production of a film of chromium hydroxides and/or oxides; (2) the production of a precipitated film of heavy metal phosphates; (3) the use of various synthetic polymers with or without heavy metal phosphates and/or oxides; and (4) the formation of a manganese oxide/zinc oxide film by use of permanganates.
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