Corrosion performance of plasma electrolytic oxidized AZ31 magnesium alloy in silicate solutions with different additives

In this work, plasma electrolytic oxidation coatings were produced on AZ31 magnesium alloy. The electrolytes used were mainly composed of silicate, silicate with addition of phosphate or fluoride, respectively. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX) analysis, X-ray diffraction (XRD), and potentiodynamic polarization tests were carried out to investigate the microstructure, element composition, phase composition and corrosion resistance of the coatings. The results showed that the coating formed in silicate electrolyte was mainly composed of MgO, Mg2SiO4 and had a dense structure and low thickness. After the addition of fluoride, MgF2 was formed and both the coating thickness and corrosion resistance were increased. The addition of phosphate also quickened the film growth rate, but the corrosion resistance of the coating was reduced instead due to the loose and porous microstructure.