ABSTRACT
All conventional phosphating solutions are dilute phosphoric acid-based solutions of one or more alkali metal/heavy metal ions, which essentially contain free phosphoric acid and primary phosphates of the metal ions contained in the bath. Several phosphating baths have been developed with surfactants as one of the additives. The formation of soluble primary ferrous phosphate leads to a concurrent local depletion of free acid concentration in the solution resulting in a rise in pH at the metal/solution interface. This change in pH alters the hydrolytic equilibrium which exists between the soluble primary phosphates and the insoluble tertiary phosphates of the heavy metal ions present in the phosphating solution, resulting in the rapid conversion and deposition of insoluble heavy metal tertiary phosphates. Phosphate coatings have been used in a wide variety of applications, e.g. the use of phosphate coatings for corrosion protection, as a base for paint, to provide wear resistance and as an aid in cold forming of steel.
