| Abstract | Bromide-induced pitting of Fe is triggered by a critical state of the passive oxide film. The state corresponds to a specific thickness of the oxide which depends on the concentration of bromide ions, but not the anodizing potential. In addition, pitting will only occur if the critical state is established in < 10 s. The critical thickness for pitting is linearly dependent on the logarithm of the concentration of halide ions. Similar slopes are obtained for chloride and bromide ions, however, the critical thickness for Br- is greater than for Cl- at the same concentration, reflecting the more aggressive nature of the Cl-. Once development of the passive film has passed the critical stage, pitting becomes very unlikely, even if the potential is held well above the pitting potential. Partial cathodic reduction experiments demonstrate that a thinner film does not pit more rapidly than a thicker film; instead more charge must pass for the thinner film to reach the critical state prior to pitting. |
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