Effects of Nitinol Microstructural Purity on Localized and Uniform Corrosion Susceptibility

The effects of reductions in Nitinol inclusion size and area fraction on corrosion resistance have yet to be elucidated, particularly for very high microstructural purity Nitinol. Therefore, the objective of this study was to determine the impact of various Nitinol microstructural purities on pitting resistance and nickel ion release. Generic heart valve frames were manufactured with different vacuum arc remelting methods (VAR, H.P. VAR, and VAR/EBR) and surface finishes (thermal oxide, chemical etch, electropolish). Frames were subjected to pitting corrosion testing and 60-day immersion in a simulated physiological environment. Microstructural analysis found the maximum inclusion length for VAR/EBR tubing was 2 times smaller than H.P. VAR and 8 times smaller than standard VAR tubing. Despite these differences, pitting corrosion resistance was similar between microcleanliness groups for any of the surface finishes tested. In addition, the nickel release profiles for chemically etched and electropolished heart valve frames were similar for all microstructural purities further demonstrating that reduction in inclusion size did not impact corrosion resistance. Electropolished surfaces possessed better pitting and ion release performance compared to thermal oxide surfaces. These results indicate that the localized and uniform corrosion performance is maintained in higher microstructural purity VAR-based Nitinol for different surface finishes.