Corrosion and protection characteristics of zinc and manganese phosphate coatings

Abstract

The corrosion and protection characteristics of zinc and manganese phosphate coatings in aqueous solutions are investigated by means of physical methods and electrochemical measurements. The results show that the insulation property of zinc phosphate coating is better than that of manganese phosphate, but the porosity of the former is inferior to that of the latter. The anodic current of coated samples decreases and polarization resistance increases compared to the substrate. The corrosion of phosphated steel exhibits the characteristics of no diffusion in acidic solution, a finite-length diffusion in neutral medium, and a semi-infinite diffusion in alkahne solution. Chemical dissolution is the primary form in the failure of phosphate coatings, which is induced by the electrochemical corrosion of the substrate. The protection ability of phosphate coatings mainly depends on their barrier performance.