Effect of thermal oxidation treatment on surface characteristic and corrosion resistance of Ni-Ti shape memory alloy

In this study, a mechanically polished Ni-Ti alloy was subjected to isothermal oxidation (TO) in N₂-20vol.%O₂ at temperatures ranging from 300 to 800°C. TO-treated surfaces were then characterized by field emission type scanning electron microscopy (FE-SEM), energy dispersive X-ray spectrometer (EDX), X-ray electron spectroscopy (XPS) and X-ray diffraction (XRD). Electrochemical corrosion tests were also carried out using a three electrochemical cell connected to a computer driven potentiostat. Results showed that Ni-Ti alloy exhibited different oxidation behavior depending on the treatment temperatures. A Ni free layer was observed in the oxide layer for the specimens TO-treated at temperatures 500°C and above. Specimens TO-treated at temperatures ranging from 300 to 500°C showed higher corrosion resistance compared to that of the surface finished by polishing. This was because the TO-treatment created a thick oxide layer with an amorphous structure. Specimens TO-treated at above 600°C showed very low passive current densities, but lower pitting potential. This was because these samples were covered with crystalline oxide. Consequently, the TO-treatment at 500°C produced a smooth protective nickel free oxide layer, which contributes to good biocompatibility of Ni-Ti implants.