Influence of sulfides on the passivation behavior of titanium alloy TA2 in simulated seawater environments

The influence of sulfides on the passivation behavior of titanium alloy TA2 in simulated seawater environments was studied by electrochemical methods, surface topology and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry analysis reveals that the passivation process of titanium TA2 was mainly composed of one two-electron step for Ti - 2e(-) -> Ti2+ and two one-electron steps for Ti2+ - e(-) - -> Ti3+ and Ti3+ - e(-) -> Ti4+; both of the processes are irreversible electrochemical reactions. The XPS depth profile indicates that the passive film is mainly composed of a TiO2 predominant top layer with a thickness less than 2 nm and a TiO predominant sub-oxide layer with a thickness of approximately 13 nm. Sulfides not only promote the cathodic hydrogen evolution reactions of titanium but also participate in the passive film formation by the formation of TiOS and TiS2; the permeation depth of sulfide ions is approximately 10 nm. When the sulfides concentration is lower than 2 mM/L, the increase of sulfides increases the formation of titanium dioxide, which enhances the corrosion resistance. While the sulfides concentration is higher than 2 mM/L, the fractions of TiOS and TiS2 increase. Thus, the fraction of TiO2 decreases, and, as a result, the corrosion resistance of the film decreases.