Microstructure and protection characteristics of the naturally formed oxide films on Mg–xZn alloys

• The oxide films on Mg–xZn alloys consist of similar chemical composition.
• The higher Zn content results in the thicker but higher defect of the oxide films.
• The oxide films exhibit different protection performance under various potentials.

The naturally formed oxide films on Mg–2Zn and Mg–5Zn alloys were investigated by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. The oxide films on the both alloys present a similar chemical composition, consisting of surface layer of basic magnesium carbonate and MgO following with MgO and ZnO, but the oxide film on Mg–5Zn is thicker and contains more defects. The protection performance of the oxide film on Mg–5Zn is worse under open circuit potential but better in a suitable anodic potential scope compared with that on Mg–2Zn alloy.