ELECTROCHEMICAL AND SURFACE CHARACTERIZATION OF A NICKEL-TITANIUM ALLOY

For clinical implantation purposes of shape memory metals the nearly e quiatomic nickel-titanium (NiTi) alloy is generally used. In this stud y, the corrosion properties and surface characteristics of this alloy were investigated and compared with two reference controls, AISI 316 L VM stainless steel and Ti6Al4V. The anodic polarization curves, perfor med in Hanks' solution at 37 degrees C, demonstrated a passive behavio ur for the NiTi alloy. A more pronounced difference between the corros ion and breakdown potential, i.e. a better resistance to chemical brea kdown of passivity was found for the NiTi alloy compared to AISI 316 L VM. X-ray electron spectroscopy (XPS) and scanning electron microscopy (SEM) were undertaken to study the elemental composition and structur e of the surface films prior to, and after immersion in Hanks' solutio n. 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 l ayer was observed. The passive diffusion of nickel from the NiTi alloy , measured by atomic absorption spectrophotometry reduced significantl y in time from an initial release rate of 14.5 x 10(-7) mu g cm(-2) s( -1) to a nickel release that could not detect anymore after 10 days. I t is suggested that the good corrosion properties of the NiTi alloy an d 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-pho sphate layer after exposure to a bioenvironment. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.