Effect of voltage on properties of microarc oxidation films prepared in phosphate electrolyte on Zr–1Nb alloy

A thin protective film on Zr–1Nb alloy was fabricated by AC microarc oxidation (MAO) in phosphate electrolyte. The morphology, microstructure and phase constituents of MAO films under different discharge voltages were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Raman spectroscopy. Their surface roughness and microhardness were measured. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate corrosion behaviors of films. The equivalent circuits of EIS plots for bare Zr–1Nb alloy and MAO films were proposed. It was found that all of films at different discharge voltages consisted of the monoclinic ZrO2 and tetragonal ZrO2 phases, but the content of monoclinic ZrO2 was much higher than that of tetragonal ZrO2 phase. Microhardness of MAO films is over 570 HV, which is higher than that of Zr–1Nb alloy substrate with 245 HV. The film fabricated in higher voltage shows lower corrosion current density and higher impedance, which displays better corrosion resistance. The EIS results identify that the inner compact layer of MAO films plays a key role in improving the corrosion resistance of Zr–1Nb alloy.