What is Phosphate coatings

Phosphate coatings are used to increase corrosion resistance and improve powder paint adhesion,and are used on steel parts for corrosion resistance, lubricity, or as a foundation for subsequent coatings or painting.It serves as a conversion coating in which a dilute solution of phosphoric acid and phosphate salts is applied via spraying or immersion and chemically reacts with the surface of the part being coated to form a layer of insoluble, crystalline phosphates.Phosphate conversion coatings can also be used on aluminium, zinc, cadmium, silver and tin.

The main types of phosphate coatings are manganese, iron and zinc.Manganese phosphates are used both for corrosion resistance and lubricity and are applied only by immersion. Iron phosphates are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphates are used for rust proofing (P&O), a lubricant base layer, and as a paint/coating base and can also be applied by immersion or spraying.

A phosphate coating is a transition layer in several respects. It is less dense than most metals but more dense than coatings. It has thermal expansion properties which are intermediate between that of the metal and the coating. The result is that phosphate layers can smooth out the sudden changes in thermal expansion which would otherwise exist between the metal and the paint. Phosphate coatings are porous and can absorb the coating. Upon curing, the paint solidifies, locking into the phosphate pores. Adhesion is greatly enhanced.


  1. Combined cleaning and phosphating. 1.0 to 1.5 minutes at 100 degrees F to 150 degrees F.
  2. Water rinse 1/2 minute
  3. Chromic acid rinse or deionized water rinse. 1/2 minute.