Zinc phosphating is a chemical conversion process that has been widely used in industry for corrosion protection and primer for painting. In this investigation, the effect of activation treatment with nickel acetate on the microstructural evolution and corrosion resistance of the zinc phosphate coating on mild steel was studied. The chemical composition and morphology of the coatings were analyzed via scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The corrosion resistance of the coatings was evaluated using polarization curves and electrochemical impedance spectroscopy (EIS) in an aerated 3.5% NaCl solution. Surface morphology observations revealed that the activation treatment increased the population density and refined the grain size of the coating. The electrochemical results showed better barrier protection characteristics and corrosion resistance for activated phosphate coatings compared with inactivated coatings.