Corrosion of NiTi Wires with Cracked Oxide Layer

Improving fatigue lifetime of NiTi stents remains to stay in the center of medical device research worldwide. Since components of implanted gastrointenstinal NiTi stents are exposed to combined mechanical loading and corrosion in biological fluids, mechanical as well as corrosion properties of surface oxide layers are of special importance in the context of their fatigue performance. This work focusses evaluation of corrosion resistance of superelastic NiTi wires with specific surface oxide layers deformed to constant strain levels in defined biological fluids. Medical grade superelastic NiTi wires undergoing martensitic transformation (austenite B2 - R-phase - martensite B19’) are submerged into various biological fluids and prestrained to constant strain levels in the range (0% -8%). Corrosion resistance is evaluated using electrochemical impedance spectroscopy (EIS) method utilizing an application of point defected model (PDM). This model considers the effect of vacancies in the substrate and their annihilation at the grain boundaries, precipitates and dislocations, and incorporation into the outer layer of the oxide structure in bilayer oxide. The model takes into account the effect of martensitic transformation on the absorption of hydrogen, which arises at the interface passive bilayer. Information about chemical composition and quality of the surface oxide layers are evaluated using the XPS system NanoEsca prior the corrosion test as well as after the test in relation to local corrosion attack. Based on the obtained results, protective properties of surface oxide layers are discussed. It is shown that they depend both on the composition and quality of the layer as well as deformation and phase state.