Electrochemical Performance of Passive Film Formed on Ti–Al–Nb–Zr Alloy in Simulated Deep Sea Environments

Titanium alloy Ti–Al–Nb–Zr has high specific strength and becomes a promising structural material used in the deep sea. The excellent corrosion resistance of the alloy is derived from the protective passive film formed on its surface. By now, full agreement on interpretation of the anti-corrosion performance of the film in marine environment, especially in the deep sea, has not been reached. In this work, the electrochemical performance of two-surface-state Ti–Al–Nb–Zr alloys which are treated by mechanical polishing and anodizing pre-passivation in the simulated shallow sea, 1000 m and 3000 m deep sea environments, is investigated. By interpreting the electrochemical kinetic parameters, it is found that the dominant cathodic process becomes hydrogen evolution in simulated deep sea environments, but the reduction rate is restrained by high hydrostatic pressure, which arrests the passivation of the alloy. Assisted by X-ray photoelectron spectroscopy analysis, it is found that the passive film mainly consists of titanium oxides. There are intermediate oxides with non-stoichiometric ratio involved in the film formation due to the low dissolved oxygen concentration and low temperature. The results of Mott–Schottky and electrochemical impedance show that the film has n-type semiconducting property with oxygen vacancies as the main point defects. The anti-corrosion performance in simulated deep sea environments is one order of magnitude lower than that in the simulated shallow sea environment. However, from 1000 to 3000 m, the corrosion resistance is reduced very slightly. In the inner layers of the passive film and the passive film formed in simulated deep sea environments, the proportion of low-valence titanium oxides is relatively high. The doping of low-valence titanium (Ti(II) or Ti(III)) results in a porous structure and ion permeability of the passive film, as well as relatively low corrosion resistance.