Understanding the effect of bromides on the stability of titanium oxide films based on a point defect model

The effect of donor density, ND on the critical localized breakdown potential, Eb of the passivity on Ti was investigated in bromide-containing aqueous solutions. Oxide films of approximately fixed thickness were galvanostatically grown on Ti in 0.5 M H2SO4 at various current densities. The ND was estimated using Mott–Schottky analysis. The stable localized breakdown induced by bromides manifested itself in potentiodynamic current-potential curves by a sudden current increase at Eb. The passive film on Ti is n-type and oxygen vacancies were considered as majority defects. The Eb tends to decrease as the oxygen vacancy and bromide concentration increases. Additionally, the Eb of anodically grown films was found to increase with the potential scan rate, inversely of the behavior observed for the native oxide on Ti, but in concert with corresponding relationships followed by other metals/alloys and predicted by the point defect model (PDM). Physico-electrochemical processes leading to growth and breakdown of passivity on Ti can be understood in terms of a PDM. The higher susceptibility of titanium oxide to local breakdown in presence of bromides than chlorides is interpreted by considering electrochemical reactions occurring at the TiO2|solution interface and their effect on film surface and bulk properties upon increasing the potential during the positive-going potential scan. Finally, it should be noted that a further understanding on the mechanism of bromide-induced passivity breakdown on Ti might be useful to achieve optimized anodization conditions for ordered oxide nanostructures with desired properties in electrolytes other than fluorides and perhaps at lower anodization potentials.

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► Galvanostatically grown titanium oxide films of fixed thickness but of a different donor density were prepared and characterized by Mott–Schottky analysis.
► The validity of the point defect model (PDM) was investigated and found that PDM can explain the bromide-induced localized breakdown of the passivity on titanium.
► The PDM may account for the higher susceptibility of titanium oxide to bromides than chlorides by considering additional surface reactions resulting from the oxygen evolution reaction and electroactivity of bromides.