Deposition of TiO₂ on NiTi by cathodic synthesis for improving corrosion resistance and apatite-forming ability

TiO2 was fabricated on nickel titanium (NiTi) shape memory alloy via a two-step low-temperature process for enhancing surface properties in orthopedic applications. A peroxotitanium hydrate deposit was first formed on NiTi by cathodic synthesis, and then transformed into TiO2 anatase by hydrothermal treatment in steam. The oxide coating was characterized by scanning electron microscopy, atomic-force microscopy, thin-film X-ray diffractometry, and X-ray photoelectron spectroscopy. The corrosion resistance in Hanks' solution, as evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization tests, was increased by more than two orders of magnitude due to the oxide coating. A layer of hydroxyapatite of a few micrometers in thickness grew on the coated samples after immersion in Kokubo's solution for 14 days. These results indicate that the low-temperature process reported provides a simple route for synthesizing an adherent oxide coating on NiTi with high corrosion resistance and enhanced apatite-forming ability.