Corrosion Fatigue of Superelastic NiTi Wires Subjected to Various Thermomechanical Loadings

Superelastic NiTi devices and implants suffer from random corrosion fatigue failure when transforming cyclically in body fluids. This work deals with in-situ electrochemical corrosion studies of superelastic NiTi wires and springs subjected to various thermomechanical loadings in simulated body fluid environment. Electrochemical parameters obtained from preliminary results show that localized corrosion strongly depends on the kinetics of surface repassivation competing with mechanical processes controlled by strain rate during cycling. In-situ measurements of electrochemical corrosion on thermomechanical loaded superelastic NiTi wires reveal previously unknown mechanism of corrosion fatigue in the phase transforming NiTi alloys. We applied several in-situ test methods, such as open circuit potential, potentiodynamic and potentiostatic polarization, electrochemical impedance spectroscopy to characterize e.g. the passive layer, surface oxides, interface between austenite and stress induced martensite. The obtained results are discussed in view of the structure, defects and phase and chemical composition of the mechanically loaded surface matrix/oxide layers.