Corrosion characterization of titanium alloys by electrochemical techniques

The electrochemical behavior of Ti–6Al–4V and Ti–6Al–7Nb alloys, commonly used implant materials, particularly for orthopaedic and osteosynthesis applications, was investigated together with that of Ti–13Nb–13Zr alloy in Hank's solution at 37 °C. The aim of present study was to evaluate their corrosion resistance in an artificial physiological solution. This evaluation was carried out through the analysis of the corrosion potential variation with time, potentiodynamic polarization curves, and electrochemical impedance spectroscopy (EIS) tests. Very low current densities were obtained (order of nA/cm2) from the polarization curves, indicating a typical passive behavior for all investigated alloys. The EIS results exhibited capacitive behavior (high corrosion resistance) with phase angles close to −90° and high impedance values (order of 106 Ω cm2) at low and medium frequencies, which are indicative of the formation of a highly stable film on these alloys in the test solution. The obtained EIS spectra indicated two relaxation time constants and their interpretation in terms of an “equivalent circuit” with the circuit elements representing the electrochemical properties of a two-layer oxide, composed of a porous outer layer and a dense inner layer, was in good agreement. The electrochemical impedance spectroscopy (EIS) technique, therefore, was able to provide reliable data for determination of the passive film structure.