Effect of anodization and annealing on corrosion and biocompatibility of NiTi alloy

Anodization of NiTi alloy was carried out on a biomedical grade NiTi alloy in phosphoric acid based electrolyte and subjected to post treatment by annealing. The morphology of anodized surface was porous with irregularly distributed pores and annealing resulted in the formation of nodules of titanium oxides on the porous structure. Studies showed that the corrosion potential shifted to more noble direction after annealing and there was one order decrease in corrosion current density compared to as-anodized surface. Cyclic potentiodynamic polarization behavior of mechanically polished and anodized surfaces was almost similar and both the surfaces exhibited positive hysteresis loop on reversing the scan direction. The formation of negative hysteresis loop in the case of annealed anodized alloy showed that the surface was capable of retaining its passive behavior even at higher potentials. Dynamic impedance spectroscopy studies carried out on annealed anodized surfaces showed that on increasing the potential from open circuit potential (ocp) to higher potentials (0.8 V), the thickness of the oxide layer also increased, but behaved as a leaky capacitor. In terms of bioactivity, the entire surface of the annealed sample was covered with hydroxyapatite after 12 days of immersion in Hanks' solution and the nickel released (370 ppb) was comparatively less than bare NiTi surface (633 ppb)