Influence of Microscopic Surface Features on Localized Corrosion Initiation in Nitinol

The susceptibility of pitting in Nitinol has been described in literature to be dependent on numerous factors (e.g., surface processing, surface damage, etc.). Although these studies provide insight into key factors that influence corrosion performance at the device level, it is not well understood what features cause pit initiation locally on the Nitinol surface. Some studies have suggested that pitting initiates at inclusions due to heterogeneous local conditions and possible galvanic effects (Shabalovskaya, 2008). However, pitting resistance was shown to be similar in devices with lower and higher oxide inclusions suggesting that pitting is not influenced by inclusions (Nagaraja, 2022). Furthermore, surface damage has been shown to act as the “weak link” on the surface resulting in pit initiation sites, but not in all instances. Therefore, the objective of this study is to elucidate the local features that may impact pit initiation in Nitinol.

Electropolished Nitinol with different microstructural purities and types of surface damage will be tested in this study to understand the impact of microscopic features on pit initiation. Specimens will be imaged with scanning electron microscopy (SEM) to identify inclusions and damaged regions. A miniaturized electrochemical cell will be used to perform localized potentiodynamic tests on these features. Post-test SEM will be performed to understand pitting morphology and location relative to pre-test SEM images. Breakdown potentials for different microstructural purities and surface damage types will be compared. The study findings will allow for better understanding of the factors that influence pit initiation in Nitinol medical devices.