Towards dense corrosion-resistant plasma electrolytic oxidation coating on Mg-Gd-Y-Zr alloy by using ultra-high frequency pulse current

Ultra-high frequency (≥10 kHz) was employed to produce plasma electrolytic oxidation (PEO) upon the surface of Mg-8Gd-3Y-0.5Zr alloy (GW83, in wt%) to understand its mechanistic contribution to the growth of PEO. The maximal pore area of the resulting PEO coatings was reduced by about one order of magnitude when frequency was increased from 0.5 kHz (65.8 μm2) to 20 kHz (7.1 μm2), which is attributed to the ten times reduction in single pulse energy. Cross-sectional SEM micrographs and electrochemical impedance spectroscopy confirm that the PEO coatings obtained at low frequency (i.e. 0.5 and 5 kHz) were consisted of an inner barrier layer and an outer porous layer, while ultra-high frequency PEO coatings were divided into three distinct layers: an inner barrier layer, an intermediate compact layer, and an outer porous layer. Moreover, thickness of the effective corrosion barrier layer of PEO coating including inner barrier layer and intermediate compact layer increased as a function of frequency, resulting in high corrosion resistance of the ultra-high frequency PEO coating. Those findings are anticipated to provide new insights to guide design and preparation of high-quality PEO coatings to tackle corrosion challenges of Mg alloys.