Corrosion behavior on orthopedic NiTi alloy with nanocrystalline/amorphous surface

In order to enhance the surface wear resistance and nitrogen diffusion during plasma treatment, orthopedic NiTi alloy is subjected to surface mechanical attrition treatment (SMAT) and a nanocrystalline and partial amorphous structure is fabricated in the surface layer. It is found that hardness in the surface layer is notably improved. The corrosion behavior is systematically studied in a 0.9% NaCl physiological solution by electrochemical methods. Potentiodynamic polarization measurements indicate that the corrosion resistance of SMAT NiTi with the surface nanocrystalline and partial amorphous structure is significantly enhanced compared to the bare NiTi with coarse grains. Both corrosion potential (Ecorr) measurements and electrochemical impedance spectroscopy (EIS) reveal that a passive oxide layer is readily formed on the SMAT NiTi during early immersion in the 0.9% NaCl solution. When the passive oxide layer has stabilized after long exposure in the 0.9% NaCl solution, corrosion induced by Cl− begins to degrade the passive oxide film. The observed corrosion behavior of SMAT NiTi is considered to be associated with the surface nanocrystalline and amorphous structure.