The Effects of Nitinol Purity and Surface Finish on Corrosion Susceptibility

Previous studies have investigated the effects of inclusion/size distribution on fatigue performance [Launey et al., 2014, Robertson et al., 2015]. However, it is unclear whether corrosion resistance is impacted by reduction in inclusion size and distribution. Although there has yet to be agreement, previous literature has suggested that pitting corrosion initiates at inclusions [Bai et. al., 2011, Shabalovskaya et al., 2004]. Therefore, the objective of this study is to determine the impact of different nitinol microstructural purities on pitting corrosion resistance and nickel ion release.

A generic transcatheter heart valve frame was manufactured from three different nitinol microstructural purities; each Nitinol material conforms to the specification in ASTM F2063. Since corrosion resistance depends on manufacturing and surface processing [Decker et al., 2011, Sullivan et al., 2015], valve frames were processed to have different surface finishes (e.g., electropolishing, chemical etching). These devices were subjected to ASTM F2129 testing and real-time immersion in PBS to 60 days in order to capture the pitting and general corrosion resistance.

This study provides a comprehensive investigation into whether medical device corrosion is impacted by inclusion size and distribution. Objective evidence is gained through evaluation of the pitting and general corrosion susceptibility of heart valve frames manufactured from available nitinol microstructural purities and surface finishes.