The electrochemical behavior of nitinol in simulated gastric fluid

Increased use is being made of nitinol for implants that are exposed to gastric fluid. However, few corrosion studies have involved nitinol in an appropriate acidified chloride solution. In this work, the electrochemical behavior of electropolished (EP) nitinol was examined in simulated gastric fluid, the corresponding neutral solution with the same concentration (0.6%) of NaCl, and 0.9% NaCl. Cyclic potentiodynamic polarization was used to evaluate the susceptibility to pitting corrosion, while electrochemical impedance spectroscopy was used to examine the passive oxide film. The potentiodynamic tests showed that the susceptibility of EP nitinol to pitting corrosion is affected by chloride concentration and pH. Acidification, in particular, resulted in the susceptibility being markedly higher in gastric fluid compared with that in the corresponding neutral NaCl solution. The impedance data could be fitted using a parallel resistance-capacitance (as a constant phase element) circuit associated with the oxide film. The thickness of the oxide was determined from the capacitive component and found to be little affected by chloride concentration. In contrast, acidification increased the solubility of the oxide enough to decrease the thickness of the film from 5.3 nm in 0.6% NaCl to 4.2 nm in gastric fluid. The resistivity of the oxide obtained from the resistance was affected by chloride concentration (0.7 × 10¹¹ and 1.7 × 10¹¹ Ω m in 0.9% and 0.6% NaCl, respectively) and particularly by pH (6.3 × 10¹¹ Ω m in gastric fluid). The resistivity values suggest that the oxide was more defective in the neutral solutions. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B:2394-2400, 2017.