The influence of current density on the morphology and corrosion properties of MAO coatings on AZ31B magnesium alloy

Abstract

Micro-arc oxidation (MAO) coatings were prepared on AZ31B magnesium alloy using alkaline silicate electrolyte at different current densities (0.026, 0.046 and 0.067 A/cm2). Field Emission Scanning Electron Microscopy (FESEM) analysis of the coating revealed an irregular porous structure with cracked morphology. Compositional analysis carried out for MAO coating showed the presence of almost an equal amount of Mg and O (34 wt.%) apart from other elements such as F, Si and Al. The cross-sectional FESEM images clearly portrayed that the MAO coating was dense along with the presence of very few fine pores. The surface roughness (Ra) of the coatings increased with an increase in the current density. Potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) studies were carried out for both the bare and MAO coated AZ31B Mg alloy in 3.5% NaCl solution. The corrosion potential (Ecorr) and corrosion current density (icorr) values obtained for the bare substrate were − 1.49 V and 46 μA/cm2, respectively. The coating prepared at 0.046 A/cm2 exhibited the lowest icorr value of 7.79 × 10− 10 A/cm2 and highest polarization resistance (41.6 MΩ cm2) attesting to the better corrosion resistance of the coating compared to other samples. EIS results also indicated almost similar corrosion behavior for the MAO coatings. Mott–Schottky analysis showed n-type and p-type semiconductor behavior for the oxide layer present on the bare magnesium alloy and MAO coatings respectively.