Insight into the Corrosion Resistance of Plasma Electrolytic Oxidation (PEO) Mg Alloys from the Microstructures in PEO Coatings

The microstructures in the thickening of plasma electrolytic oxidation (PEO) coatings and their effects on the corrosion resistance of the PEO Mg alloys were investigated using x-ray tomography in conjunction with electrochemical impedance spectroscopy (EIS). It was found that the micropores would develop and connect with each other in the thickening of PEO coatings, resulting in the increment of the average diameters and the maximum diameters of the micropores. The thickness of the whole PEO coating gradually increased with oxidation time, and it was beneficial to diminish the through-pores in PEO coatings. The PEO Mg alloy prepared by controlling 40 min oxidation time exhibited the best corrosion resistance because of its larger thickness, denser structure, and higher content of the Mg2SiO4 phase in the PEO coating. Based on equivalent circuits for EIS spectra, the limit value of the low-frequency resistance ((R_{{ ext{l}}})) was used to evaluate the corrosion resistance of the PEO Mg alloys. And that this method was verified by the experimental results.