Corrosion behavior of Al-1wt% Mg-0.85wt%Si alloy coated by micro-arc-oxidation using TiO2 and Na2MoO4 additives: Role of current density

The present study reported the formation of the oxide layer incorporating TiO2 and MoO2 particles and their influences on the microstructure and corrosion behavior of the oxide layer formed by micro-arc-oxidation (MAO) with respect to current density. For this purpose, MAO coatings on Al-1wt%Mg-0.85wt%Si alloy sample were performed under an alternating current condition in a silicate-molybdate electrolyte with dispersed TiO2 at three different current densities, such as 100, 150, and 200 mAcm−2. The incorporation of TiO2 would be attained through the combined effects of electrophoresis and physical mixing with the molten oxide during plasma arcing whilst the incorporation of MoO2 would rely on the decomposition of molybdate ions triggered by plasma-assisted electrochemical reaction. Among three different oxide layers produced at three current densities, the oxide layer produced under 150 mAcm−2 exhibited the lowest corrosion current density value of 2.79 × 10−11 Acm−2 as well as the highest polarization resistance value of 1.76 × 109 Ωcm2, which was attributed mainly to the fact that a significant decrease in micro-pore density by the incorporation of particles into the oxide layer was obtained. The electrochemical mechanism affecting corrosion resistance of the MAO-coated samples was elucidated based on the equivalent circuit model consisting of resistor and capacitor elements.