Electrochemical response of MoO2-Al2O3 oxide films via plasma electrolytic oxidation

The present investigation looked into the role of current frequency on the structural feature and electrochemical response of MoO2-Al2O3 hybrid oxide films formed on Al-Mg-Si alloy subjected to plasma electrolytic oxidation (PEO). The current hybrid coatings were formed in an alkaline-silicate electrolyte containing sodium molybdate with respect to alternate current frequencies, such as 60, 500, and 1200 Hz. The results of microstructural observations showed that the oxide film produced at 1200 Hz was relatively more compact and less porous than those produced at 60 and 500 Hz. This was attributed mainly to the incorporation of MoO2 whose tendency was observed to be increase as current frequency increased in this study. Accordingly, the potentiodynamic polarization tests revealed that the sample coated at 1200 Hz exhibited superior corrosion behavior to those coated at 60 and 500 Hz. Such electrochemical response underlying the corrosion characteristics of the present alloy was discussed in the context of equivalent circuit model taking the roles of outer and inner layers, and interface between oxide film and substrate into account.