Influence of Milling–Electrochemical Polishing on Corrosion Resistance of NiTi Shape Memory Alloy

As an important artificial implant material, the corrosion resistance of NiTi shape memory alloy is closely related to the machined surface quality. In this paper, the multiple analysis methods concerning potentiodynamic polarization, impedance spectrum and corrosion morphology are used to analyze the corrosion resistance of the alloy. The potentiodynamic polarization and impedance spectrum test results show that the conductivity and corrosion current density of electrochemical polishing surface decrease, and the polarization resistance and corrosion potential increase compared with milling. After electrochemical polishing, the surface roughness of the milling sample is decreased, and the NiTi alloy of austenite phase is transformed into TiO(2), which improves the corrosion resistance of the alloy. In addition, there are pitting corrosion, hole corrosion and crevice corrosion morphology on the milling surface, while the pitting corrosion and hole corrosion exist on the electrochemical polishing surface. The corrosion morphology verified the analysis of potentiodynamic polarization and impedance spectrum. The multiple analysis method proposed in this paper can be used as a more accurate evaluation method for the corrosion resistance of alloy surface, avoiding the error of analysis results caused by the impedance spectrum equivalent circuit and potentiodynamic polarization following Tafel relationship.