The influence of secondary phase carbide particles on the passivity behaviour of NiTi shape memory alloys

The current investigation aims at studying the passivity behaviour of NiTi shape memory alloys with different levels of secondary phase titanium carbide (TiC) particles in an electrolyte of 0.9% sodium chloride at 37 °C. The influence of carbides and thermo‐mechanical treatment/cold working on the passivity breakdown is highlighted. The polarisation studies on the as‐cast and cold worked NiTi with high (0.05 wt%) and low (0.005 wt%) carbon levels show a significant difference in oxide stability. The alloy with extremely low carbon content shows a higher breakdown potential. Higher carbon levels result in higher density of larger TiC and these carbide/matrix interfaces are more susceptible to pitting. The qualitative behaviour of passive layer formed at 0.5 V on the cold worked NiTi alloy with different carbon levels was ascertained by electrochemical impedance spectroscopy (EIS). The oxide on the NiTi alloy with high (0.05 wt%) carbon levels showed lower resistance and poor stability at this condition.