Corrosion protection behaviour of some ternary titanium alloys in sulphuric acid solutions

The paper presents the anticorrosion behaviour of two new ternary titanium (Ti-10Mo-10Al, Ti-15Mo-5A1) alloys in comparison with the base metal in the diluted and concentrated sulphuric acid solutions at elevated temperatures. The electrochemical impedance spectroscopy, as well as potentiostatic and potentiodynamic techniques have been used to characterise the anticorrosion behaviour. The effect of the solution concentration, temperature, influence of the alloying elements and equivalent circuit has been investigated. Ternary titanium alloys achieve more positive corrosion potentials than titanium. The anodic curves exhibit two current peaks for the critical passivation currents which correspond, respectively, to the same two critical passivation potentials: at the first critical potential the Ti2O3 oxide is formed whereas at the second critical potential this oxide is converted to the compact, protective, TiO2 oxide. The decrease of the critical passivation currents with the molybdenum content in the alloy is due to the superficial accumulation of the beta phase, richer in Mo, which affects cathodically these currents. The increase of current along the passive region with aluminium content is supposed to appear because of the aluminium dissolution in acid medium. Both solution concentration and temperature generate the increase of the critical passivation currents and passive currents. An equivalent circuit with three time constants was fitted.