The growth mechanism and corrosion resistance of laser-assisted plasma electrolytic oxidation (PEO) composite coating on AZ31B magnesium alloy

In this study, laser-assisted plasma electrolytic oxidation (Laser/PEO) coating was prepared on AZ31B magnesium alloy for corrosion protection, due to insufficient corrosion protection caused by the inherent defects, cracks and poor quality of PEO coatings. The plasma discharge evolution, morphological characteristics, elemental composition during coating growth were characterized by high-speed camera, SEM, EDX, XRD and XPS, respectively. Meanwhile, Mott Schottky (M-S) curves, potentiodynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS) tests characterized the oxygen vacancy defects and corrosion resistance of the coatings. The results demonstrated that laser-assisted irradiation not only induced plasma discharge on the anode surface, but also limited the plasma discharge size in the post-processing stage, which significantly increased the proportion of corrosion-resistant phase Mg₂SiO₄ (the proportion of Mg₂SiO₄ increased from 23.70% to 39.22%), thickness and density in the coating, and obviously reduced the oxygen vacancy defects and microcracks in the coating. As a result, the corrosion resistance of the Laser/PEO coating (9.29(±0.76)×10⁻⁷ A·cm⁻²) was further enhanced in comparation with the PEO coating (3.06(±0.19)×10⁻⁶ A·cm⁻²).