Corrosion Behavior of Some Titanium Base Alloys in Acid Solutions

Abstract

Corrosion and passivation behavior of new, ternary titanium alloys Ti-10Mo-10Al and Ti-15Mo-5Al in nitric acid solutions of different concentrations (20% and 60%) and temperatures (25degrees, 50degrees, and 75degreesC) is presented in this paper. Electrochemical impedance spectroscopy (EIS) data were obtained at different potentials as Bode plots. The spectra show some deviations from the ideal behavior, which demonstrate the necessity to consider a complex equivalent circuit with two time constants. This equivalent circuit is composed of the uncompensated ohmic resistance of the solution (R-Omega), the double layer capacity (C-dl) in parallel with the polarization resistance (R-p) for a passive film, and a Warburg element (W) associated with a resistance (R-1) for the diffusion processes. The anodic polarization curves present the self-passivation of these alloys. Both titanium and its ternary alloys directly pass into a passive state, due to the fact that, in strong oxidizing nitric acid solutions, the titanium directly oxidizes to Ti4+ ions and spontaneously forms titanium dioxide TiO2. The passivation of the titanium alloys is due to the compact and protective film containing titanium dioxide, TiO2, and some insoluble molybdenum oxide, MoO3.