Mechanical and electrochemical consideration for the corrosion behaviour of titanium and some of its alloys used as implant devices

Abstract

Metalographical and microhardness studies completed with electrochemical dc measurements and Electrochemical Impedance Spectroscopy (EIS) measurements have been performed on titanium and its alloysTi-6Al-4V, Ti-1Ni-0.5Mo and Ti-5Al-2.5Fe in order to characterize their passive film and corrosion resistance at 37°C under simulated physiological conditions (Ringer solution) acidulated with HCl at pH=2.5. The impedance spectra were obtained at different potential values between Ecorr and + 0.7 V vs. SCE. Comparation between the passive current densities and the stable passive region were made and the conclusions were related with the alloy structure and the values of microhardness. Analysis of the impedance spectra was done by fitting the experimental data to different equivalent circuits. Two equivalent circuits, with one time constant and two time constants respectively, can be satisfactory used for fitting the spectra: one time constant represents the characteristics of the passive film and the second one is for the charge transfer reactions. The polarization resistance and the double layer capacity were compared at different polarization potentials for the detection of the passive film structure and the roughness of the electrode surface. It can be seen for all the materials that Rp is very high and decreases slightly with the potential: the very high resistance Rp implies a high corrosion resistance which can be attributed to the formation of the protective TiO 2 layer. There is a decrease in the values of the parameter n of the CPE (constant phase element used in the mathematically modelling in order to consider also the electrochemical behaviour of systems which do not correspond exactly to a pure capacitance) related to the rugosity of the electrode surface. With electrochemical methods and surface techniques, a good characterisation of the process can be realised. The obtained results permitted an interpretation of the contribution of alloying elements and of environmental conditions (very acid solutions) to behaviour of the implant and a mechanism of the process is proposed.