Effect Of Long-Term Immersion On The Localized Corrosion Resistance Of Nitinol Wire Under Aerated Conditions

Implantable medical devices need to possess sufficient resistance to localized corrosion to withstand degradation during in vivo service. Manufacturers are typically required to demonstrate a device’s corrosion resistance through in vitro testing. A common measure of nitinol’s susceptibility to localized corrosion is given by the breakdown potential (E_b) relative to the rest potential (E_r), as determined by in vitro cyclic potentiodynamic polarization testing. Long-term in vivo exposure, however, may affect the gap between E_b and E_r. This study, which builds on previous research that was presented at SMST 2007, investigates the effect of aeration on E_r, the long-term potential drift (ΔE_r), E_b, and the gap E_b-E_r.

In this study, mechanically-polished nitinol (MP NiTi) wires were immersed in phosphate-buffered saline (PBS) at 37 ºC for periods of up to thirty days prior to performing cyclic potentiodynamic polarization tests. The polarization tests were performed in accordance with ASTM International Standard F 2129: ‘Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices’. Aeration was achieved using laboratory air and a blood-gas mix. Results are presented showing the effect of immersion time and aeration on E_r, ΔE_r, E_b, and E_b-E_r.