Correlations between the Growth Mechanism and Corrosion Resistance of Plasma Electrolytic Oxidation Coatings on AZ31B Magnesium Alloy

摘要

In the present work, plasma electrolytic oxidation (PEO) coatings with different microstructures werefabricated on AZ31B magnesium alloy. The thickness, morphology, phase composition and elementaldistribution of the obtained coatings were characterized by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), and energy dispersive spectroscopy (EDS). The corrosion behaviors of PEOcoatings were measured by potentiodynamic polarization curves and electrochemical impedancespectroscopy (EIS). The results show that a composite dielectric coating is obtained with similar contentsof MgO and MgF 2 (MgO-MgF 2 coating). During the discharge in the PEO process, by taking advantageof these two dielectrics’ electrical performance and physical properties to influence dielectricbreakdown, and utilizing the effect of blocking and heating in the two dielectrics, the molten materialsejected from the discharge channels decrease and more remains in the discharge channels; therefore, thethickness of the compact layer of the MgO-MgF 2 coating increases. Consequently, the coating thicknessis approximately 3.5 μm, which is twice that of the coating with MgO as the main component (MgOcoating). Furthermore, the results of electrochemical experiments demonstrate that the corrosionpotential of the MgO-MgF 2 coating is 0.91 V larger than that of the MgO coating. As evaluated by EIS,the impedance of the inner compact layer is increased by more than 5 times from 7.652×10 5 Ω·cm 2 forthe MgO coating to 4.933×10 6 Ω·cm 2 for the MgO-MgF 2 coating. These results indicate that the growthmechanism of the MgO-MgF 2 coating can increase the thickness of the compact layer and effectivelyimprove the corrosion resistance.