The high‐temperature oxidation rate of Fe‐26 Cr alloys proved highly sensitive to the method of surface preparation. An attempt to deduce the nature of the thin oxide films left by the various treatments was made using electrochemical measurements. Electron microscopy and diffraction and parallel experiments with Cr and Fe were carried out for purposes of clarification. Films formed under strongly oxidizing conditions including electropolishing yielded a cathodic reduction curve having an anomalous sharp minimum. It is proposed that such films are nonstoichiometric oxide containing considerable high valency chromium ion and having a high reduction potential and hydrogen discharge potential. The latter is lowered when at a high enough overpotential the chromium at the surface is reduced to a lower valence.
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Caplan et al. (1961) studied this question.