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

1 March 1987

journal article
Published by The Electrochemical Society in Journal of the Electrochemical Society

Vol. 134 (3) , 535-539
https://doi.org/10.1149/1.2100505

Abstract

Oxide films formed on Fe in normal and borate buffer solution were studied by SIMS. The films formed at potentials 18O/¹⁶O ratios less than the solution enrichment. Above 0.18V the films were resistant to air oxidation. The ¹⁸O/SIMS data indicate that 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.

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