Pitting Resistance of Nitinol Stents Before and After Implantation

The corrosion resistance of Nitinol wire stents “as formed”, formed with different pre-treatments, and formed with electropolishing were examined and compared using a standardized cyclic polarization procedure as the test method (ASTM F2129).  Serendipitously an explanted device with stents which appeared to be made from the same Nitinol wire was made available for a parallel examination.  The explant sample wires were found to have excellent corrosion resistance achieving vertex potentials of +850 mV(sce) without pitting in most cases, unlike similar “as formed” stents.  Two formed but unused control stents of the same lot of wire as the explant showed relatively poor corrosion resistance (pitting at +410 mV(sce) on average).  Scanning electron microscopy and x-ray photoelectron spectroscopy were employed to illuminate some differences between the control and the explant.  The results pose the following question: Is the standard cyclic polarization test in a deoxygenated inorganic saline solution, a test optimized to reveal susceptibility to pitting and crevice corrosion, an appropriate test method for stents implanted in richly oxygenated organic sites?