Comparison of corrosion resistance and apatite-forming ability of NiTi treated by different low-temperature methods

A number of low-temperature soft methods recently attempted by the authors for preparing TiO2 coating on NiTi intermetallic are described. These methods include hydrothermal oxidation in water, sol–gel coating with steam post-treatment, low-voltage DC anodization in acetic acid, AC anodization in a calcium and phosphate containing medium, and electrodeposition. In all these methods, the temperature reached by the bulk of the substrate did not exceed 200 °C. The characteristics of the coatings acquired using various techniques are summarized. The increases in corrosion resistance for various samples in Hanks’ solution (a simulated body fluid commonly used to assess in vitro corrosion resistance) obtained from polarization study and electrochemical impedance spectroscopy are compared and discussed. The apatite-forming ability of the treated samples was also studied.