Strain induced localized corrosion of NiTi, NiTiCo and NiTiCr alloys in 0.9% NaCl

Shape memory and super elastic alloys are commonly used in biomedical and engineering areas, due to their higher elastic deformation characteristics and low elastic module when in martensitic state. For biomaterial applications, the alloy must exhibit adequate corrosion resistance and biocompatibility, especially in chloride environments. The addition of ternary elements in NiTi alloys aim to improve the mechanical properties. Addition of Co increases the elastic limit and reduce the transformation temperature while Cr additions increase the yield strength. However, it was demonstrated that this modification can affect the corrosion resistance of the raw materials. This study aims to assess the corrosion and strain induced corrosion resistance of NiTi alloys modified by Co and Cr additions in the presence of 0.9% NaCl solution. Ternary alloys were compared to NiTi binary alloys, when unstrained and strained within the elastic regime where martensitic transformation is induced. Electrochemical impedance spectroscopy (EIS) and anodic polarization tests were performed on both conditions. Straining electrode corrosion tests were performed under constant electrochemical potential being the electrochemical response registered. Tests using wire samples as straining working electrodes permitted the assessment of the correlation between deformation and the anodic current of the alloys immersed in 0.9% NaCl solution. It was concluded that, despite the mechanical benefits provided by the addition of ternary elements, these additions increased the susceptibility to localized corrosion and the pitting corrosion susceptibility enhanced by stress and corresponding strain.