Improved fracture properties of Ni-Ti superelastic alloy under sustained tensile load in physiological saline solution containing hydrogen peroxide by hydrogen charging

We have attempted to improve the fracture properties of Ni-Ti superelastic alloy under a sustained tensile load in physiological saline solution containing hydrogen peroxide by charging with a small amount of hydrogen, which causes negligible hydrogen embrittlement. For hydrogen charged in the parent phase, no pitting potential is observed in anodic polarization curves for the present test solution; the time to fracture increases under an applied stress even in the elastic deformation region of the martensite phase. Upon aging after hydrogen charging, the pitting potential varies widely and the increase in the time to fracture slightly decreases. Hydrogen charging increases the time until a rapid shift of the corrosion potential in the less noble direction, corresponding to the breakdown of surface oxide films. The final fracture is mainly caused by overloading due to localized corrosion irrespective of hydrogen charging. For hydrogen charged in the martensite phase, i.e., the specimen not subjected to interactions between hydrogen and the stress-induced phase transformations, the time to fracture increases by more than that of the specimen subjected to the interactions. The present study indicates that a small amount of hydrogen charging is effective for inhibiting the localized corrosion of Ni-Ti superelastic alloy under applied stress in physiological saline solution containing hydrogen peroxide, thereby improving the fracture properties. In addition, changes in the hydrogen states and/or defects induced by the interactions between hydrogen and phase transformations probably play an important role in improving the fracture properties.