Effect of CoSO4 on the characteristics of micro-arc oxidation coatings

Oxidation coatings were prepared on the surface of AZ31B magnesium alloy by micro-arc oxidation in the electrolytes with CoSO4 addition varying from 0 to 0.8 g/L. The effects of different concentrations of CoSO4 on the properties and microstructure of the coating were analysed by Scanning Electron Microscope (SEM), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) and Electrochemical Workstation. The results revealed that the addition of different concentrations of CoSO4 caused the voltage variation, which resulted in the changes of microstructure, phase composition and element distribution of coatings. Furthermore, it led to the improvement of coating properties. It was found that the oxidation voltage increased firstly and then decreased. Because of CoSO4 addition, the morphology of the coating was changed. XRD patterns and XPS confirmed that Co3O4, MgO and Mg2SiO4 were generated. The addition of CoSO4 could effectively improve the hardness, corrosion resistance and thermal-shock resistance.