Preparation and performance of coating on rare-earth compounds-immersed magnesium alloy by micro-arc oxidation

A composite ceramic coating containing Y2O3–ZrO2–MgO (YSZ–MgO) was prepared on AZ91D magnesium alloy, which was immersed in Y(NO3)3 aqueous solution as pretreatment, by micro-arc oxidation (MAO) process. The morphology, elemental and phase compositions, corrosion behavior and thermal stability of the coatings were studied by SEM, EDX, XRD, electrochemical corrosion test, high temperature oxidation and thermal shock test. The results show that the coating mainly consists of ZrO2, Y2O3, MgO, Mg2SiO4, and MgF2. Among these compounds, Y2O3 accounts for 26.7% of (Y2O3 + ZrO2). The thickness of YSZ–MgO coating is smaller than that of ZrO2–MgO coating, but its compactness and surface roughness are better than those of ZrO2–MgO coating. YSZ–MgO coating has a good corrosion resistance, and its corrosion rate in 5% NaCl aqueous solution is lower than that of ZrO2–MgO and only about 8.5% of that of AZ91D magnesium alloy. After oxidation at 410 °C, the mass gain of AZ91D magnesium alloy presents a linear increase with the oxidation time. The YSZ–MgO coating and ZrO2–MgO coating can remarkably decrease the oxidation mass gain. The oxidation mass gain of YSZ–MgO coating is lower than that of ZrO2–MgO coating, especially during a long oxidation period. The thermal shock resistance of YSZ–MgO coating is superior to ZrO2–MgO coating.