Pitting of Iron by Chloride in Borate Buffer Solution: Role of the Anodic Oxide Film

The anodic behavior of Fe in borate buffer solution has been studied in the presence and absence of added , and the results compared and discussed. Passivation of Fe in Cl⁻‐free borate buffer results in a stable film only after an anodic charge of 2.9 mC cm⁻² is passed, independent of potential. Below 1.5 mC cm⁻² the film is not stable to open‐circuit breakdown in pure borate buffer solution and does not possess a fully formed outer layer. Because of the absence of the outer layer, Fe²⁺ can be transported across the film. Between 1.5 and 2.9 mC cm⁻² there is a transition region of developing stability. The results suggest that the anodic charge, and therefore the thickness, not the potential, is the important parameter in the stability of the passive film on Fe. In solution containing Cl⁻, Fe is susceptible to pitting only within the transition region. It is possible to define , the critical anodic charge for pitting, which is found to be independent of potential. A constant is consistent with the experimentally observed dependencies of the pitting potential on [Cl⁻] and the induction time on potential.