Effects of PEG6000 on microstructure and corrosion resistance of plasma electrolytic oxidation coatings on magnesium alloy under different voltages

Plasma electrolytic oxidation (PEO) coatings were prepared on AZ91D magnesium alloy in aqueous silicate electrolytes without and with polyethylene glycol 6000 (PEG6000). Effects of PEG6000 concentration on microstructures and corrosion resistance of coatings under two voltages were studied. The static contact angles of the coatings were investigated by the contact angle meter, and their anti-corrosion properties were evaluated using the potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The results show that, after adding PEG6000 into the electrolyte, the thicknesses of the coatings decrease somewhat, and the chemical and phase compositions of the coatings almost remain unchanged, but the content of the deposited phase slightly increases. Meanwhile, the surface roughness of the coatings decreases and their compactness adds by augmenting the adsorptive uniformity of anion ions on anode/electrolyte interface. Therefore, the contact angle of the coating increases and its corrosion resistance is significantly strengthened. As the concentration of PEG6000 grows, the anti-corrosion performance of the coating firstly improves and then deteriorates. When PEG6000 is 5 g/L at low voltage but is 10 g/L under high voltage, the corrosion resistance of the coating is the best.