The electrochemical behavior of nitinol in simulated gastric fluid

Abstract

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 × 10 and 1.7 × 10 Ω m in 0.9% and 0.6% NaCl, respectively) and particularly by pH (6.3 × 10 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.

Pit initiation on nitinol in simulated physiological solutions

“…The susceptibility to pit initiation on the surface of the primary pit was likely higher than that on the as‐received EP nitinol surface because of the local increases in acidity and chloride concentration that generally occur during pit growth. Acidification and increased chloride levels have both been shown to raise the pitting susceptibility of EP nitinol wire . Although the susceptibility to secondary pitting was dependent on inclusion size, these more aggressive conditions would probably result in it being less so compared with the pitting susceptibility of the EP surface in 0.9% NaCl and PBS.…”