Effects of NaAlO 2 on structure and corrosion resistance of microarc oxidation coatings formed on AM60B magnesium alloy in phosphate–KOH electrolyte

Abstract

Microarc oxidation coatings on AM60B magnesium alloy were prepared in phosphate–KOH electrolytes with and without NaAlO 2 addition. The effect of NaAlO 2 on the characteristic of breakdown voltage in different concentrations of NaAlO 2 has been studied. The compositions, structure and morphologies of the oxide coatings formed in different concentrations of NaAlO 2 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 NaAlO 2. 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 MgAl 2O 4. The addition of NaAlO 2 into the base electrolyte resulted in the increase of spinel MgAl 2O 4 in the oxide coating. Furthermore, the number and size of micropores on top of the surface decreased with the increasing concentration of NaAlO 2, though the surface roughness increased with the NaAlO 2 addition. The coating formed in the electrolyte containing 8.0 g/L NaAlO 2 exhibited the highest corrosion resistance in NaCl solution.