Effect of electrolytic polishing on corrosion fatigue strength of TiNi shape memory alloy

Abstract

The corrosion fatigue strength of a TiNi shape memory alloy (SMA) is crucial for ensuring safety when the material is used as a sensor or actuator element in a severely corrosive environment, such as sea water. Furthermore, in the medical field, the corrosion fatigue strength in a blood vessel is important because it determines the product life of medical devices, such as self-expanding stents. In this study, we improved the corrosion fatigue strength of a TiNi SMA wire by electrolytic polishing. The main findings are as follows: (1) Electrolytic polishing in 25 wt% nitric acid and 75 wt% methanol at 218 K after mechanical polishing significantly reduces the surface roughness of the TiNi SMA wire. (2) The electrolytically polished TiNi SMA wire has a thin passive layer of TiO2 on its surface. (3) The corrosion fatigue life of the electrolytically polished TiNi SMA wire is much longer than that of a conventional TiNi SMA wire. (4) The fatigue crack on the electrolytically polished TiNi SMA wire initiates at a surface micro-imperfection.