Effect of Al Content on the Microstructure and Corrosion Resistance of Plasma Electrolytic Oxidation (PEO) Coatings on Mg-Al Alloys

Effect of Al on the microstructure and corrosion resistance of plasma electrolytic oxidation (PEO) coating of Mg-Al alloys was investigated by means of microscopy observation and electrochemical measurement. The outer layer of PEO coating was composed of MgO and Mg₂SiO₄. While, the inner layer on α phase, eutectic phase and β phase was mainly consisted of MgO and MgF₂, Mg₂SiO₄, Mg₂SiO₄ and MgF₂, respectively. Al content had two influences on the corrosion resistance of PEO coating of Mg-Al alloys, on one hand, the MgF₂ composition of the inner layer with the volume fraction of β phase, indicating the enhancement of corrosion resistance. On the other hand, the interface of inner layer above the eutectic phase and β phase increased with Al content, on which corrosion preferentially initiated on it due to high misfit degree and difference of Femi level, leading to the decrease of the corrosion resistance. In brief, with the increase of Al content, the corrosion resistance of PEO coating of Mg-Al alloys increased and then decreased.