Effect of NaAlO2 concentrations on microstructure and corrosion resistance of Al2O3/ZrO2 coatings formed on zirconium by micro-arc oxidation

In this study, Al2O3/ZrO2 composite coatings were prepared on Zr substrates by micro-arc oxidation (MAO) in the NaAlO2-containing electrolytes, and the effect of NaAlO2 concentration on the microstructure, bond strength, microhardness and corrosion resistance of coatings was systematically investigated. The study reveals that the adequate NaAlO2 in the electrolyte (>0.2 M) is essential to the formation of needle-like α-Al2O3 in the coatings, and the amount of α-Al2O3 rises with the increase of the NaAlO2 concentration. m-ZrO2 and t-ZrO2 are present in all of the coatings, but their relative amount largely depends on the amount of Al2O3. It is also found that as the NaAlO2 concentration increases from 0.2 to 0.3 M, the coating becomes denser and thicker, and its bond strength, maximum microhardness and corrosion resistance increases as well. The coating formed at 0.3 M NaAlO2 demonstrates the highest bond strength of 52 MPa, the maximum microhardness of 1600 Hv0.2N and the superior corrosion resistance. However, the overhigh concentration of NaAlO2 (0.35 M) is found harmful to the coating's microstructure and properties.