A new look into the electrochemical impedance spectra of plasma electrolytic oxidation coatings on Mg-Zn-Ca alloy in simulated body fluid electrolyte

Electrochemical impedance spectroscopy (EIS) is a very useful electrochemical measurement to estimate the corrosion degradation behavior of Mg alloys in simulated body fluid (SBF). However, conventional interpretation of EIS results relies on the selection of equivalent circuit without associating clear physical meaning of each component to the electrochemical system. In this work, a Mg-Zn-Ca bio-magnesium alloy, with its surface being covered by a plasma electrolytic oxidation (PEO) coating and being further sealing post-treated by a phosphate chemical conversion coating (PCC), was immersed in the SBF. Surface morphology and composition were monitored and compared. Results showed that the EIS being directly fitted using ECs could offer valuable information for qualitative information. However, the high-frequency pseudo-capacitive loop due to the oxide impedance, rather than the double layer, can be interpreted using a Young model. The mid-frequency and low-frequency response of the spectra involves the diffusion and faradaic reactions, but the localized corrosion yields the spectra susceptible to the impact of nonsteady-state fluctuations caused by localized corrosion.