Corrosion and Mechanical Degradation of Ni-Ti Superelastic Alloy in Neutral Fluoride Solution

Abstract

The corrosion behavior and mechanical degradation of Ni-Ti superelastic alloy in neutral fluoride solution were investigated with a focus on the effect of temperature during immersion. Corrosion was accelerated by increasing the temperature, and mass loss increased with immersion time. Furthermore, Ti ions (Ni ions) preferentially dissolved in the early (later) stage of immersion. The tensile strength of the immersed specimen decreased when the mass loss was approximately 0.040-0.050 mg/mm3. The fracture mechanisms of specimens immersed in 2.0% NaF solution were active path corrosion at room temperature and presumably hydrogen embrittlement at 80 °C. For long-term (e.g., 180 days) immersions at 80 °C, hydrogen embrittlement did not occur, because the absorbed hydrogen diffused to the center of the immersed specimens, rendering the hydrogen distribution comparatively uniform. These findings suggest that the fracture mechanism of immersed specimens can change as a result of the temperature during immersion.