Corrosion characterization of microarc oxidation coatings formed on Mg-7Li alloy

Ceramic coatings with thickness of 27 μm were fabricated on Mg-7Li alloy in Na₂SiO₃-C₆H₁₈O ₂₄P₆ solution by microarc oxidation (MAO). The morphology and phase composition of MAO coatings were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion behavior of the bare and MAO coated Mg-7Li alloy was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the MAO coatings were composed of MgO, Li₂O, and Mg₂SiO ₄, and there existed some micropores on the coating surface with a diameter of 3-20 μm. The corrosion potential (E_corr) and corrosion current density (I_corr) of the MAO coated alloy were about -1.4761 V and 7.204 × 10^-7 A/cm², respectively. The E _corr of the MAO coated alloy increased by 109.6 mV and its I _corr decreased by three orders compared with that of the bare Mg-7Li alloy. The EIS plots indicated that the impedance of the MAO coated alloy was 15 times higher than that of the bare alloy. The fitting parameters showed that the resistance of the MAO coatings was far greater than that of the bare alloy. The dense intermediate layer and the transition layer of the MAO coatings acted as a barrier to hinder the proceeding of solution permeation, remarkably improving the corrosion resistance of the Mg-7Li alloy.