Effect of strain on the corrosion resistance of nitinol and stainless steel in simulated physiological environment

Electropolished SE-508 NiTi and passivated 316L stainless steel wires were deformed from 0% to 10% strain and were then corrosion tested in Hank's physiological solution. Results show that the breakdown potential and repassivation potential of NiTi were not affected by the strain. The breakdown and repassivation potentials were approximately 1080 mV vs. SCE and 980 mV vs. SCE, respectively for the Nitinol samples, independent of strain levels. However, the breakdown potential for the stainless steel wires decreased significantly as a function of strain. The breakdown potentials ranged from 770 mV vs. SCE for unstrained samples to 510 mV vs. SCE for the 10% strain specimens. Stainless steel specimens strained up to 8% exhibited a repassivation potential of approximately 70 mV vs. SCE. Stainless steel samples that were deformed to 10% strain did not show any repassivation potential during the reverse scan of the polarization test. Localized corrosion (pitting) initiated at the maximum strain area (in tension) on the stainless steel specimens.