Effect of temperature and pH on the corrosion resistance of nitinol

Few studies have investigated the effects of temperature on the corrosion resistance of Nitinol. This is surprising, since it is well known that temperature may affect the corrosion behavior of metals by increasing their corrosion rate and/or affecting their resistance to localized corrosion (pitting). Similarly, most corrosion studies of implant devices have investigated the corrosion resistance of Nitinol at a physiological pH of 7.4 without further study of the effect of other pH values. However, the literature reports that physiological pH can range from 1 to 9 depending on the type of body fluids. Therefore, the goals of this study were to assess the influence of temperature and pH on the corrosion resistance of Nitinol. Electropolished NiTi and passivated 316L stainless steel wires were tested in Hank's physiological solution at temperatures ranging from 10 degrees C to 80 degrees C and pH varying from 1 to 9. The cyclic potentiodynamic tests performed in this study follow ASTM standard F2129. Results show that the ability of Nitinol to repassivate is significantly reduced by an increase in temperature. No other significant effects were observed by a change in temperature. Stainless steel corrosion rate, on the other hand, was significantly affected by an increase in temperature while the other parameters remained very stable. Study of the effect of pH revealed that except for a change in the potential for oxygen evolution, the corrosion resistance of NiTi was not significantly affected. In contrast, the results for stainless steel indicate that both uniform corrosion rate and localized corrosion are affected by a decrease in pH.