The Effects of Chemical Oxidation on Corrosion Behavior of Ni-Ti Alloy

Nickel Titanium intermetallic (NiTi) has been used in several fields (mostly corrosion issues were not a concern) since it was discovered about sixteen years ago. In the last decade, alloys made from NiTi have been widely used in the internal and external biomedical tools and equipment such as nails, fixation plates, bone fracture, self-expanding cardiovascular and urological stents, and orthodontic wires. The effects of chemical oxidation on corrosion behavior of Nickel-Titanium shape memory alloy have been investigated in this study. Technique of powder metallurgy was used to prepare the alloy from the elemental powders of Titanium and Nickel with 600 MPa of compacting pressure. The sintering process was achieved in a 10⁻⁴ torr vacuum atmosphere at 950 °C . XRD analysis exhibited that Ni and Ti were fully transformed into NiTi (both monoclinic and cubic phase) and Ni₃Ti phase. After sintering, the samples are grinded and polished, then samples were surface modified by chemical oxidation. Optical microscopy, scanning and x-ray diffraction techniques were utilized for characterizing the surface samples. The corrosion rate has been studied in vitro by using polarization curves technique in NaF solution at different concentration (0.5, 1,1.5, 2) wt,%. Results of corrosion test indicated that the corrosion current density after chemical oxidation decreased from 0.303 for bare sample to 0.074 in 0.5% NaF solution, which indicates that TiO₂ layer on the NiTi SMA surface are protective after chemical oxidation.