Stability and Growth of Passive Films on Pure Iron in Borate Buffer as Investigated by 18O/SIMS

Abstract

Oxide films formed on Fe in normal and borate buffer solution were studied by SIMS. The films formed at potentials <0.18V (SHE) were susceptible to time‐dependent thickening upon air exposure as indicated by18O/16O ratios less than the solution enrichment. Above 0.18V the films were resistant to air oxidation. The 18O/SIMS data indicate that <10% of the oxygen in the oxide film formed on air exposure of a cathodically‐reduced surface arises from the electrolyte; >90% comes from oxygen in the air. Cathodic reduction of passive oxide films containing 18O results in a drop in the 18O content to its natural abundance level, indicating that the reduction completely removes the film. The mechanism of growth of passive films was studied by forming an oxide at 0.35V in (or ) solution and then continuing the oxidation at 0.7V in (or ). The distribution of 18O (or 16O) ions through the film suggests that the additional oxide grew by inward oxygen diffusion. On the relationship between film thickess and potential, 18O/SIMS data reveal that there are two distinct linear stages of oxide growth, above and below ∼0.15V.