Behavior of titanium alloys in neutral solutions by EIS and potentiostatic methods

Abstract

Passivity of Ti and Ti-15Mo, Ti-15Mo-5Al alloys in concentrated chlorides solutions and sea water was investigated by electrochemical impedance spectroscopy (EIS) and potentiostatic techniques. The anodic polarization curves do not present the active-passive transition range, Ti and its alloys being passive even from the beginning of the anodic polarization because of the primary pre-existing oxide film. The curves are similar and are characterized by the passive current density; this does not change with the temperature and alloy composition. The molybdenum increases the stability of the passive film enlarging considerably the passive range of the alloys; this fact can be explained by the formation of the insoluble molybdenum chlorides. Electrochemical impedance spectroscopy (EIS) measurements have been performed on titanium alloys in order to characterize the passive films on these alloys. The impedance spectra were obtained at different potential values of +0.2 V; +0.8 V and E-corr Nyquist and Bode diagrams were recorded at these potentials. These diagrams are the same for Ti and its alloys; so, the equivalent circuit is the same for Ti and its alloys. This equivalent circuit contains two time constants: one time constant represents the passive film and double layer, the second is for the,diffusion processes.