Abstract. 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 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 the corrosion behavior of Nickel-Titanium shape memory alloy have been investigated in this study. The 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 ground and polished, then the 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 the polarization curves technique in NaF solution at different concentrations (0.5, 1,1.5, 2) wt,%. Results of the corrosion test indicated that the corrosion current density after chemical oxidation decreased from 0.303 for the bare sample to 0.074 in 0.5% NaF solution, which indicates that the TiO2 layer on the NiTi SMA surface is protective after chemical oxidation. Keywords. Nickel-titanium alloys, Chemical oxidation, Corrosion resistance.
Conclusions Sample sintering at 950°C for 6 hours is efficient to obtain complete sintering and transform Ni and Ti into alloy structure. All samples (cooled inside the furnace) compacted at 600 MPa and sintered at 950°C for 6 hours gave a three-phase structure (NiTi cubic phase, NiTi monoclinic phase, and hexagonal Ni3Ti phase) at room temperature. TiO2 layer was formed on the sample surface treated with 10% H2O2 solution. It is clear that the oxide layer on NiTi SMA is a continuous layer without any cracks which indicates that the layer is protective.