The electronic properties of chromia scales grown between 800°C and 900°C on chromium metal and chromia-forming ferritic stainless steels were determined using room temperature PhotoElectroChemistry (PEC) experiments and the relative importance of the n- and p-character of the scales could be assessed. According to the thermodynamic previsions of defects structures, the external part of all the scales grown in oxygen exhibits band gap energy around 3.5 eV, with a marked p-type character on chromium and a possibly n-type behaviour on stainless steels. On the contrary, the internal part of the scales is always n-type, with predominant interstitial chromium defects. A major change appears when chromium or stainless steels are oxidised in water vapour-argon mixtures, where the absence of a p‑type semiconductor in the scales could be evidenced. Hydrogen defects are thought to be responsible of this particular behaviour which leads to a strong reduction of residual stresses due to increased high temperature relaxation. Moreover, the inversion of the growth direction resulting from high mobility of the OH defects makes the chromia scales grown in water vapour more adherent than when grown in oxygen.
International audienceIn the course of the last 30 years, photoelectrochemical techniques have been shown to be useful tools to characterize oxidation layers. Analyzing photocurrent versus applied potential plots, or, more often, photocurrent versus photon energy spectra, has actually allowed to identify the presence in the oxidation layers of one or several semiconducting components. However, up to now, when applied to photocurrent energy spectra of complex oxide scales, the usual analysis of these spectra provided only more or less qualitative information on the nature (through bandgap energies) and, in favourable cases, on the semiconducting type, of the oxides present in the scale. The novel approach discussed here to the description of the photocurrent resulting from several contributions under modulated light conditions, allowed for robust fitting of experimental photocurrent energy spectra, and to extract from the latter more quantitative information
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