Effects of NaAlO2 on structure and corrosion resistance of microarc oxidation coatings formed on AM60B magnesium alloy in phosphate-KOH electrolyte

Microarc oxidation coatings on AM60B magnesium alloy were prepared in phosphate-KOH electrolytes with and without NaAlO2 addition. The effect of NaAlO2 on the characteristic of breakdown voltage in different concentrations of NaAlO2 has been studied. The compositions, structure and morphologies of the oxide coatings formed in different concentrations of NaAlO2 were determined by energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The corrosion resistance of the oxide coatings was evaluated in 3.5 wt.% NaCl solution using potentiodynamic polarization tests. The results showed that the solution conductivity increased while the values of breakdown voltage decreased with the increase of concentration of NaAlO2. In both cases the coatings contained Mg, Al, O and P as well as a trace amount of Na and K, and were mainly composed of MgO and spinel MgAl2O4. The addition of NaAlO2 into the base electrolyte resulted in the increase of spinel MgAl2O4 in the oxide coating. Furthermore, the number and size of micropores on top of the surface decreased with the increasing concentration of NaAlO2, though the surface roughness increased with the NaAlO2 addition. The coating formed in the electrolyte containing 8.0 g/L NaAlO2 exhibited the highest corrosion resistance in NaCl solution.