Effect of Two Steps Phosphating Operation on Protection Properties and Paint Adhesion of Plain Carbon Steel

In this study, a different method which is based on dissolution of substrate i.e. the early stage of phosphating and its effect on the characterization of formed layer in the second step of phosphating was investigated. This method that called two steps phosphating has obvious influence on the formation of dense and very fine crystalline zinc phosphate coating on the plain carbon steel. Dissolution of base metal during initial phosphating and entrapping the small particles of the initial phosphate coating, during removing of phosphate layer with grinding, increases the number of nucleation sites on the surface. Considering time consuming and cost constraints of grinding in industry, in this paper the possibility of using sand blasting operating as a substitution method for removing initial coating was studied. On the other hand, according to the results of corrosion tests such as salt spray, polarization and electrochemical impedance spectroscopy (EIS), improved protective properties have been achieved for the denser coating which is formed by the introduced method. Also, the results of dry and wet adhesion of two diverse paint types, air-drying paint (epoxy systems, polyurethane) and oven coating (alkyd - melamine (Series YS)), showed improving paint adhesion for formed phosphate layer by this innovative method.