Electrochemical and corrosion behaviour of sputtered TiNi shape memory films

Electrochemical and corrosion behaviors of TiNi-based shape memory thin films were explored using electrochemical impedance spectroscopy (EIS) and polarization methods in phosphate buffered saline (PBS) solutions at 37°C. Compared with those of electro-polished and passivated bulk NiTi shape memory alloys, the break-down potentials of the sputter-deposited amorphous TiNi films were much higher. After crystallization, the break-down potentials of the TiNi films were comparable with that of the bulk NiTi shape memory alloy. Additionally, variation of composition of the TiNi films showed little influence on their corrosion behaviour. The EIS data were fitted using a parallel resistance-capacitance circuit associated with passive oxide layer on the tested samples. The thickness of the oxide layer for the TiNi thin films was found much thinner than that of bulk NiTi shape memory alloy. During electrochemical testing, the oxide thickness of the bulk alloy reached its maximum at a voltage of 0.6~0.8 V, whereas those of TiNi films were increased continuously up to a voltage of 1.2 V.