Formation of oxide layers with porous structure on the surface of titanium-nickel alloy by electrochemical anodization

Abstract

In this work the surface morphology and corrosion properties of TiNi alloy modified by electrochemical anodization in 0.9 (wt. %) NaCl solution have been studied. It has been found that the surface of anodized TiNi samples exhibits microporous structures (from ~5 μm to ~500 nm) that form a network of capillaries being reservoirs for a drug delivery system. The mean chemical composition of the surface (≤ 2 μm) layer is described by the formula ~Ti53Ni35O12 (at. %). It indicates on an increase of the thickness of the oxide film and partial removal of nickel from the surface during anodization. According to the voltammetry and impedance spectroscopy, the electrochemical reactions related to hydrogen depolarization, charge transfer and ion diffusion through the pores into the solution occur at the phase boundary “TiNi/electrolyte”. The maximum value of polarization resistance of the anodized samples is ~294 kΩ·cm2, and the density of corrosion currents reaches ~0.39 μA/cm2. The modified samples show electrochemical activity associated with anodic processes occurred in the double electric layer. When the potential reaches ~ +0.39 V (vs. Ag/AgCl), a dielectric film breakdown is observed, accompanied by oxidation of nickel to Ni2+. The proposed mode of the anodization of the TiNi alloys could be recommended for development of implants with a porous structure of the surface and satisfactory corrosion properties.