Surface Characterization and Corrosion Resistance of Generation I-III Nitinol with Various Surface Finishes
Although improvements in fatigue performance with increasing Nitinol microstructural purity have been previously characterized (Robertson et al. 2015, Pelton et al. 2017), there is limited information on whether corrosion resistance is impacted by reductions in inclusion size and distribution. Therefore, the objective of this study is to characterize the surface oxide for different Nitinol microstructural purities and determine its influence on corrosion susceptibility.
To assess the surface oxide, X-ray photoelectron spectroscopy (XPS) and Electrochemical impedance spectroscopy (EIS) are performed on Nitinol with a variety of compositions and surface finishes to compare their impedance. Generation I - Superelastic (SE508), Generation II - Extra Low Interstitial (ELI), and Generation III - Electron Beam Refined (EBR) Nitinol with chemically etched (CE), electropolished (EP), and black oxide (BO) surface finishes are analyzed. The capacitance and resistance of the primarily TiO2-comprised surface films are measured to calculate the film thicknesses to corroborate results from sputtering depth profile analysis.
In addition, potentiodynamic measurements and Scanning Electron Microscopy are performed to determine whether pitting occurs at inclusions and if corrosion resistance is increased for higher purity Nitinol for different surface finishes. The corrosion susceptibility is correlated to surface properties (e.g., film thickness and resistance) to assess the influence of surface treatment and inclusion content on pitting corrosion resistance.