Effect of Al content on the microstructure and corrosion resistance of plasma electrolytic oxidation (PEO) coatings on Mg-AI alloys

摘要

Effect of Al on the microstructure and corrosion resistance of plasma electrolytic oxidation (PEO) coating of Mg-AI alloys was investigated by means of microstructure observation and electrochemical measurement. The PEO coatings on Mg-AI alloy consisted of two layers, including an inner layer near the coating/substrate interface and an outer layer. The outer layer was composed of MgO and Mg_2SiO_4. While, the inner layer on different phases showed distinguishing microstructure, the inner layer on a phase, eutectic phase and β phase was mainly consisted of MgO and MgF_2, Mg_2SiO_4, Mg_2SiO_4 and MgF_2, respectively. Therefore, the Al content had two influences on the corrosion resistance of PEO coating of Mg-AI alloys, on one hand, the volume fraction of β phase increased with Al content, and then increased the MgF_2 composition of the inner layer, enhancing of corrosion resistance of PEO coating. On the other hand, the increasing Al content evoked the increase of eutectic phase, due to high misfit degree and difference of Femi level, corrosion preferentially initiated on the interface of the PEO coating above the eutectic phase and (3 phase, which decreased the corrosion resistance of PEO coating. In brief, with the increase of Al content, the corrosion resistance of PEO coating of Mg-AI alloys increased and then decreased.