Electrochemical stability of orthopedic porous NiTi shape memory alloys treated by different surface modification techniques

The complex surface morphology and large exposed surface area induce electrochemical instability on porous NiTi shape memory alloys in human body fluids. Consequently, leaching of toxic nickel ions from the alloys impede wider applications of the materials in the biomedical fields, especially as bone implants. Electrochemical impedance spectroscopy (EIS) is a useful tool to evaluate the electrochemical stability of surface film in simulated body fluids (SBF) and to identify the most effective surface modification techniques for porous NiTi alloys. In the present work, EIS is employed to characterize porous NiTi alloys that have been modified by various processes in SBF at 37°C to evaluate the relationship between the surface film structure and electrochemical stability. Two different equivalent circuits involving a dual oxide film model with a porous outer layer and an inner barrier layer are proposed to model the experimental data acquired under open-circuit conditions for the control sample (dense NiTi) and porous NiTi alloys, respectively. The modeled results reveal that both chemical treatment and oxygen plasma immersion-ion implantation are effective surface modification techniques to form a protective film with higher electrochemical stability on the surface of porous NiTi alloys. © 2009 The Electrochemical Society.