Characterization of corrosion properties, structure and chemistry of titanium-implanted TiNi shape memory alloy

The near-equiatomic TiNi shape memory alloy passivated through electropolishing and ion implantation with titanium has been studied toward corrosion performance in simulated body fluids (0.9 wt% NaCl, artificial blood plasma). Corrosion rate, nickel oxidation, repassivation and charge transfer has been examined by conventional methods of potentiodynamic polarization, electrochemical impedance spectroscopy and cyclic voltammetry. Experimental validation of the electrochemical results was performed using electron-microscopic (SEM/TEM) techniques and Auger electron spectroscopy. It was revealed that the thickness of the passive layer (TiO + TiO2) could be increased by ~5 times after ion implantation. Regardless of the corrosion environment, the TiNi alloys exhibiting different surface finishes still suffer from pitting corrosion associated with leaching of nickel ions via oxidation reaction. The Ti-implanted alloy shows satisfactory corrosion resistance in comparison with the reference electropolished TiNi alloy. After ion implantation, the dissolution of the surface layer during anodic polarization was restricted due to the formation of the Ni-depleted amorphous sublayer. Auger Ti- and Ni-LMM peaks are found to be shifted to lower energies due to the contribution of NiO and TiO bonding. It has been shown that not the thickness, but rather the structure and phase composition of the oxide layer are main factors responsible for corrosion performance.