Theoxidation kinetics and chemical composition distribution over the depth of the metal-oxide layer at low-temperature (300 degrees C) gas oxidation of the Fe-18Cr alloy at different degrees of system's vacuuming (0.0001-1 Torr) have been studied by the methods of spectral ellipsometry, Raman spectroscopy, and Auger spectroscopy with layer-by-layer etching. A strong activating effect of the oxygen pressure (0.02 Torr) corresponding to the active-passive transition with an oxidation rate extremum has been recorded. It is accompanied by redistribution of the chemical composition over the depth of the surface oxide related both to direct oxidation of iron and chromium at the oxide-gas interface and to exchange reactions (such as substitution reactions) of iron and chromium oxides in the bulk of the oxide layer. Initially, as a result of direct oxidation, a structurally sensitive layer of iron and chromium oxides rapidly grew on the surface in an amount insufficient for passivation and, subsequently, during the diffusion of chromium from the metal and the reaction of substitution of chromium with iron oxides, chromium oxides of chromium sesquioxide type and chromium spinel were formed in the bulk of the iron oxide layer. At passivation of the alloy, the decrease of the rate of oxide growth at further increase of the oxygen pressure is related to the localization of the formation of protective chromium oxides in a thin heterogeneous layer when the front of reaction of magnetite with chromium shifted from the oxide-gas to the metal-oxide interface.
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