A study of oxidation behavior of AZ91D alloy with YSZ coating using EIS

Oxidation behavior of AZ91D magnesium alloy with 3YSZ coating and aluminum interlayer was studied in air at 250 °C using electrochemical impedance spectroscopy EIS, scanning electron microscopy SEM, and X-ray diffraction XRD. The oxidation process was carried out in various duration times from 1 to 10 h. A three-electrode electrochemical cell was employed for all the EIS measurements. Also, to focus on the characteristics of the oxide films, a nonaggressive electrolyte 0.1 M Na2SO4 was used. The EIS data were interpreted with a two-layered model, and the obtained capacitance and resistance are related to the thickness and defectiveness of YSZ coating and oxide film which was formed during the oxidation process. The results showed that after 6 h of oxidation time, the defects are produced in YSZ coating and it helps to increase the thickness of the oxide film. Also, XRD results revealed that the oxide films were mainly composed of Al2O3 and MgAl2O4 phases. The oxide film remained protective during extra oxidation period. In addition, the electrochemical model was supported by SEM observations.