X-ray photoelectron spectroscopy, XPS, or ESCA was used to analyse a passive film formed on 18-8 stainless steel. ESCA spectra obtained from the passive surface treated at various potentials in 1N H2SO4 have shown distinct peaks of oxygen (OK), carbon (CK), and chromium (CrLII, CrLIII). Peaks of iron (FeLII, FeLIII) in the spectra also were observed clearly, but those of nickel (NiLII, NiLIII) were weak. The ratio of the peak height of CrLIII to one of FeLIII demonstrates a substantial enrichment of chromium in the passive film compared with the mechanicaly polished surface. Nickel is found not to be enriched. Two kinds of passive films are found, their chromium ratios depending on the formation potential. One is formed in a more basic potential region than a critical potential of 0.40 V (vs. SCE), showing an increase of chromium ratio with increase in the potential. The other having a higher and nearly constant the ratio is produced in the more noble potential region than the critical potential.
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Okamoto et al. (1974) studied this question.