Electrochemical and surface characterization of a nickel-titanium alloy

For clinical implantation purposes of shape memory metals the nearly equiatomic nickel-titanium (NiTi) alloy is generally used. In this study, the corrosion properties and surface characteristics of this alloy were investigated and compared with two reference controls, AISI 316 LVM stainless steel and Ti6A14V. The anodic polarization curves, performed in Hanks' solution at 37 degrees C, demonstrated a passive behaviour for the NiTi alloy. A more pronounced difference between the corrosion and breakdown potential, i.e. a better resistance to chemical breakdown of passivity was found for the NiTi alloy compared to AISI 316 LVM. X-ray electron spectroscopy (XPS) and scanning electron microscopy (SEM) were undertaken to study the elemental composition and structure of the surface films prior to, and after immersion in Hanks' solution. The passive film on the NiTi alloy consists of a mainly TiO2-based oxide with minimal amounts of nickel in the outermost surface layers. After immersion in Hanks' solution the growth of a calcium-phosphate layer was observed. The passive diffusion of nickel from the NiTi alloy, measured by atomic absorption spectrophotometry reduced significantly in time from an initial release rate of 14.5 x 10(-7) microg cm(-2) s(-1) to a nickel release that could not detect anymore after 10 days. It is suggested that the good corrosion properties of the NiTi alloy and the related promising biological response, as reported in literature, may be ascribed to the presence of mainly a TiO2-based surface layer and its specific properties, including the formation of a calcium-phosphate layer after exposure to a bioenvironment.