Corrosion behavior and protective ability of multilayer Galvanic coatings of Zn and Zn–Mn alloys in sulfate containing medium

The corrosion resistance and protective ability of electrodeposited coatings of Zn, Zn–Mn(6%) and Zn–Mn(11%) alloys as well as of chromated and non-chromated multilayer galvanic systems were investigated in a free aerated medium of 1N Na2SO4 (pH 6.0) at open circuit conditions (OCP–Ecorr) using polarization resistance (Rp) measurements. The anodic behavior of these objects was determined using potentiodynamic (PD) polarization curves. It was established that the corrosion products appeared after the treatment and known as zinc hydroxide sulfates–ZHS–increase the protective ability of the coating (or multilayer system) due to their low product of solubility Ps (for example, Ps of Zn4(OH)6SO4·4H2O has a value of the order 10− 57) and seems to be a reason for better protection toward the iron substrate. Surface morphology and its changes were registered with scanning electron microscopy (SEM).