Oxidation of Nitinol and its Effect on Corrosion Resistance

This study explores the phase transformations of oxide formation in NiTi and its effects on corrosion resistance. Electropolished Ni-50.8 at% Ti wires were heat treated between 400 and 1000 °C for 3 to 300 minutes in air. Surface analytical techniques were used to characterize the thickness, composition and phase distribution of the oxide surface layers. The results of this study suggest that oxidation occurs as follows: NiTi+O2 → Ni3Ti + TiO2 → Ni4Ti + TiO2 → Ni + TiO2. The presence, amount, and distribution of these phases depend on both time and temperature. Corrosion behavior of these oxidized wires with respect to the breakdown potential (E_bd) by potentiodynamic polarization tests was investigated. The E_bd dramatically decreases from 1000 mV to below -100 mV vs SCE as the oxide thickness increases from less than 0.01 µm to 10 µm. Above 10 µm, however, the oxide forms a protective barrier and the E_bd increases to 1000 mV. Samples deformed up to 3% strain in bending developed cracks in the protective TiO2 layer and exposed the Ni-rich phases with a concomitant decrease in E_bd to below -100 mV vs SCE. These results will be discussed in terms of processing parameters for medical devices.