Multiscale Photoelectrochemical Studies on Oxidized Duplex Stainless Steels

International audiencePhotoelectrochemical characterizations at macroscopic, microscopic and mesoscopic scales were performed on thermally oxidized duplex stainless steel samples, with special attention paid to the differences exhibited by the oxides formed on austenitic and ferritic grains. Whereas duplex stainless steel oxidized in water vapour exhibited overall n-type semiconduction after 5 min oxidation, or n-type semiconduction combined with an insulating behaviour after 1 h oxidation at 850 A degrees C, only overall p-type semiconduction was shown by samples oxidized in oxygen. Mesoscopic experiments allowed to put in evidence that the oxides grown on austenitic grains showed a lower chromium content in Fe2-x Cr (x) O-3. On the other hand, the contrast in the photoelectrochemical images could be explained by the ratio of the photocurrents measured on austenitic grains to the ones measured on ferritic grains. Atmosphere-dependent changes in the relative chromia fraction in austenitic and ferritic grains were also evidenced

Photoelectrochemical investigations on individual ferritic and austenitic grains of a duplex stainless steel oxidized in water vapour.

International audienceThis work presents a multiscale photoelectrochemical characterization of oxide layers, and aims mainly at introducing the mesoscopic scale (ca 30 mu m). For the first time, photocurrent energy spectra could be recorded on individual ferritic and austenitic grains of a unique 2205 duplex stainless steel sample oxidized for 5 minutes at 650 degrees C under 20% water vapour in nitrogen. These results allowed us to explain contrasting issues in the photoelectrochemical images obtained from the oxidized sample. Moreover, the bandgap energies obtained by fitting these individual mesoscopic photocurrent energy spectra with a novel approach developed in our laboratory, showed that all oxide scales were constituted of Fe(2)O(3) and Cr(2)O(3) and of an Fe(2-x)Cr(x)O(3) solid solutions, but that the x-value of the latter was different depending on the metallurgic phase of the oxidized substrate. The latter results were shown to be in agreement with those of additional Raman analyses of the oxidized austenitic and ferritic grains

Photoelectrochemistry of Oxidation Layers: A Novel Approach to Analyze Photocurrent Energy Spectra

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

Water vapour effects on the oxidation of chromia-forming alloys

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Scale growth during short term oxidation of stainless steels at high temperatures

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Identification by photoelectrochemistry of oxide phases grown during the initial stages of thermal oxidation of AISI 441 ferritic stainless steel in air or in water vapour

International audienceRoom temperature photoelectrochemistry was used to characterise oxide phases grown during the initial stages of oxidation of the ferritic stainless steel AISI441 at 650°C and 850°C in synthetic air or in water vapour. Grazing incidence X-ray diffraction and Raman spectroscopy were additionally used to discuss PEC results. Haematite Fe2O3 (~2.0 eV), chromia Cr2O3 (3.0 and 3.5 eV) and their mutual solid solution (~ 2.5 eV) were detected by their respective bandgap values determined from photocurrent vs. energy curves. The Cr/Fe ratio of the films increased with time/temperature and was higher in air-grown than in H2O-grown oxides. Observation of photocurrent vs. potential curves indicated that chromia was N-type in all specimens, resulting from thermodynamic equilibrium with the metallic substrate and not with the gas phase