Morphology, structure and corrosion properties of martensitic TiNi alloy modified by electrochemical anodization

Porous structures formed onto TiNi shape memory alloys during anodization may serve as a drug delivery system due to attractive electrochemical, mechanical and biological performances. Thickness, structure and chemistry of the porous layer are determined by conditions of electrochemical treatment and type of electrolyte. In the current work, the studies of the surface morphology, phase composition and corrosion properties of martensitic TiNi alloy subjected to anodization in an ethylene glycol electrolyte were performed. Comprehensive analysis by optic and electron microscopy has revealed that microporous structures (with dimensions of structural elements from ∼500 nm to ∼3 µm) are formed onto the surface of the anodized TiNi samples. According to the XRD data, the TiNi alloy retains a predominantly martensitic (B19'+R) structure after anodization, and the volume fraction of an austenitic B2 phase does not exceed 15 vol%. Transmission electron microscopic analysis has shown that an amorphous-nanocrystalline passive film possessing TiO+TiO2 phase mixture is formed. Electrochemical characterization, carried out by impedance spectroscopy, allows to conclude that a charge transfer and a diffusion of charges into the solution through the pores occur at the interface between the outer porous layer and chlorine-bearing solution. The polarization resistance, calculated by the Tafel extrapolation method, of the anodized samples reaches ∼294 kOhm‧cm2, and the maximum corrosion current density is 0.39 μA/cm2. Proposed regime of anodization can be considered for modification of TiNi rotary instruments exhibiting martensitic structure and surface porosity.