Formation of zinc–zinc phosphate composite coatings by cathodic electrochemical treatment

Abstract

The formation of zinc–zinc phosphate composite coatings by cathodic electrochemical treatment and evaluation of its corrosion resistance is addressed in this paper. The cathodic phosphating process offers some unique advantages—it requires no specific addition of accelerator in the bath, it is capable of producing good quality coatings even at low temperature, it permits deposition of coatings of desired thickness, thus offering benefits in terms of energy savings, decrease in processing cost and improvement in plant life. Being a cathodic process, there is no iron dissolution and no ferric phosphate sludge formation, which renders it an eco-friendly process. Based on the amount of coating mass as a function of process variables and the potential-time measurement, a pictorial model is proposed for the deposition of zinc phosphate coating. The surface morphology of the coatings exhibits plate-like crystals. The corrosion behaviour of cathodically phosphated mild steel substrate in 3.5% sodium chloride solution exhibits the stability of these coatings, which last for a week with no red rust formation. This is due to the presence of a composite layer of zinc and zinc phosphate that acts as a mechanical barrier against further corrosion for a considerably longer time. Being a cathodic process, the possibility of hydrogenation of steel is the major limitation of this methodology.