Formation of a compact oxide layer on AZ91D magnesium alloy by microarc oxidation via addition of cerium chloride into the MAO electrolyte

Magnesium and its alloys have been used in many industries, but they are reactive and require protection against aggressive environments. In this study, a compact and relatively pore-free oxide coating was formed on AZ91D magnesium alloy to improve its corrosion resistance by means of the microarc oxidation (MAO) process via the addition of CeCl₃ as an additive into the MAO electrolyte. Morphologies and compositions of the coatings were studied with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. It was found that with the addition of CeCl₃ into the MAO electrolyte, the obtained coating showed more uniform morphology compared with the coating produced in the CeCl₃ free electrolyte. EDS analysis confirmed the presence of cerium in the coating formed in the electrolyte with CeCl₃. Polarization tests results showed that the corrosion current density of the coating formed in the CeCl₃ containing electrolyte was less than that of the coating formed in the electrolyte without CeCl₃. Furthermore, electrochemical impedance spectroscopy (EIS) tests indicated that the coating formed in the electrolyte with the addition of CeCl₃ improved the corrosion resistance of the substrate significantly.