An investigation of (NaPO3)6 effects and mechanisms during micro-arc oxidation of AZ31 magnesium alloy

Micro-arc oxidation (MAO) in an environmentally friendly alkaline silicate electrolyte containing various concentrations of (NaPO3)6 has been used to produce oxide coatings on an AZ31 magnesium alloy. The variations of voltage, current and microdischarges with the treating time were recorded. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize microstructures and compositions of the coatings. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in the 3.5 wt.% NaCl solution were used to evaluate the corrosion behaviors of the coated samples. EIS conducted in the working electrolyte was used to test electrochemical properties of the passive films formed in the initial stage during the MAO process. The results show that (NaPO3)6 concentration significantly influences the initial passive film and hence the voltage rise rate and microdischarges. Further, the corrosion resistance of the samples decreases with increase of (NaPO3)6 concentration due to complex evolution in the microdischarges and the microstructures.