Effects of a small addition of Mn on the corrosion behaviour of Zn in a mixed solution

The effects of a small addition of Mn (0.4 wt%) on the corrosion behaviour of pure Zn (99.995 wt%) in a mixed solution (0.1 M NaCl + 0.1 M Na2SO4 + 0.01 M NaHCO3, pH 8.4) were investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and X-ray photoelectron spectroscopy (XPS). The electrochemical impedances of both Zn and Zn–0.4Mn have been successfully fitted with a suitable EIS equivalent circuit model. Fitted impedance results revealed that 0.4 wt% Mn improved both the pore resistance and charge transfer resistance of Zn in the mixed solution. As a result, both anodic and cathodic reaction rates were reduced. X-ray photoelectron spectroscopy (XPS) analysis showed that the corrosion films formed in the mixed solution consisted of zinc oxide (ZnO), zinc hydroxide (Zn(OH)2) and zinc hydrozincite (Zn5(CO3)2(OH)6). The role of small addition of Mn is that it promotes the precipitation of hydrozincite in the pores of corrosion film. An “alleviation of local acidification” mechanism is proposed to explain the investigated results.