Phosphate coating is typically used as a pretreatment prior to coating or painting, increasing corrosion protection, and improving lubricity (the friction properties of sliding components). Phosphate coatings are also widely utilized as an application to threaded parts and then top-coated with oil (P&O) to prevent galling and to inhibit rust.
The phosphating process is a reaction that occurs on a metal substrate when the metal is put in contact with process solution through immersion. The primary benefits from phosphate coating are strong adhesion and corrosion protection. Typically, phosphate coatings are used on steel parts but can also be effective on aluminum.
Manganese phosphate coatings are utilized for corrosion protection, anti-galling and lubricity. There are various phosphate coatings available. Among these, manganese phosphate coatings are the hardest, while providing superior corrosion and abrasion protection. In comparison to zinc phosphate coatings, manganese phosphate coatings offer extended wear protection, effective even after the breaking in of the coated components.
These coatings are applied only by immersion. Uses for manganese phosphate applications include bearings, bushings, and fasteners. Use of manganese phosphate is especially useful on parts that will be subjected to sliding or rotating, such as screw threads.
Zinc phosphate coatings are also available and are mainly used for rust proofing on ferrous metals. Zinc phosphate coatings can be applied by either immersion or spraying. Zinc phosphate is a lighter alternative to manganese phosphate, but provides excellent resistance to harsh environments that can rapidly degrade components.