Localized Breakdown of the Niobium Anodic Oxide Film by Bromides

The growth of the anodic oxide passive film on Nb and its stability were studied in aqueous bromide media. Potentiodynamic polarization up to 12 V_SCE showed that passive Nb loses its stability resulting in either metastable or stable pitting corrosion depending on the bromide concentration and the potential scan rate. On the contrary, Nb remains passive in the more aggressive chloride media where most passive metals and alloys are often susceptible to localized breakdown. This unusual bromide-induced localized breakdown of passive Nb occurs for bromide concentrations higher than 0.25 M at a critical breakdown potential (E_b). Stable active dissolution in pits was observed at potentials higher than E_b. Το get an insight into the physicochemical processes responsible for the susceptibility of Nb to the bromide-induced localized corrosion, the oxide growth and its breakdown are discussed in terms of a point defect model (PDM). Experimental relationships between E_b and either the bromide activity or the potential scan rate agree with the PDM predictions. The bromide-induced localized breakdown of passive Nb seems to be associated with the absorption of bromides preferably on lattice anion vacancies at the oxide|electrolyte interface leading through a series of processes to vacancy condensation at the Nb|oxide interface.