Structure, corrosion resistance and apatite-forming ability of NiTi alloy treated by micro-arc oxidation in concentrated H2SO4

Titanium oxide coatings were prepared on NiTi alloys by MAO treatment in concentrated H2SO4 electrolyte using DC power supply. The surface of the coating exhibited a typical MAO porous and rough structure. XPS analysis indicated that the coatings were mainly consisted of O, Ti, and a trace amount of Ni, and the concentration of Ni was greatly reduced compared to that of the NiTi substrate. The curves of potentiodynamic polarization indicated that the corrosion resistance of NiTi alloy was significantly improved by micro-arc oxidation. The coatings were tightly adhesive to the substrates with the bonding strength higher than 45 MPa. The SBF soaking test result showed that a kind of bone-like apatite was observed on the MAO coating after soaking for 28 days, suggesting that MAO treatment in concentrated sulfuric acid electrolyte was an effective way to improve the bioactivity of NiTi alloys.