The electrochemical behavior of nitinol in simulated physiological solutions.

The electrochemical behavior of nitinol in simulated human bile and phosphate-buffered saline (PBS) was examined using electrochemical impedance spectroscopy. In addition, cyclic potentiodynamic polarization tests were performed in the simulated bile and salt-only bile, and the results compared with those obtained previously in PBS. The potentiodynamic tests showed that electropolishing was effective in providing nitinol with a high resistance to pitting corrosion in the bile solutions, as found in PBS. Differences between the breakdown potential and the corrosion potential indicated that mechanically polished nitinol should be more susceptible to pitting corrosion in simulated bile than in the salt-only bile and PBS. The impedance spectra showed near-capacitive behavior in both PBS and simulated bile, and the data could be fitted by a parallel resistance-capacitance (as a constant phase element) circuit associated with the passive oxide film. The thickness of the oxide was determined from the capacitive component and found to be consistent with surface analytical results reported in the literature. Resistivities obtained from resistance values indicated that compositional features of the oxide were similar for PBS and simulated bile.