Oxide films formed on Fe in normal false(H216Ofalse) and H218O‐normalenriched 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 H216O (or H218O ) solution and then continuing the oxidation at 0.7V in H218O (or H216O ). 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.

The kinetics of passivation of Fe in pH 8.4 borate buffer solution at 0.0 V have been studied in both the presence and absence of 0.5 M Cl-. Cl-has no influence on the decay of the passive current with time over the course of several hours. Oxide films were examined by AES and SIM S for possible Clincorporation into the oxide lattice. Neither technique was able to detect any Cl-within the film. The same is true even above the pitting potential, which probably indicates that Cl-incorporation into the oxide film is not a precursor to pit initiation. It also appears that Cl-does not cause any film thinning.

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