(Invited) In-Situ Detection of Breakaway of Fe-17% Stainless Steel in Water Vapor Containing Air By Synchrotron Radiation X-Ray Diffraction

Stainless steels containing more than 16 mass% of Cr show slow oxidation after protective Cr2O3 film has covered the surface at high temperature. For longer exposure, however, oxidation accelerates after non-protective Fe oxide formed. It is important to explain the cause of the transition from protective to non-protective oxidation. Mainly two models have been proposed: mechanical failure and chemical failure of Cr2O3. Objective of this study is to make clear which mechanism is predominant for such a transition on a Fe-17% stainless steel (SS430). Because the transition occurs suddenly at high temperature, no experimental study has made so far. In this study, an in-situ measurement of oxide crystal structure on SS430 stainless steel was carried out at 1373 K using an intense synchrotron X-ray source and the following results were obtained. The surface of the steel was initially covered with Cr2O3, which was then converted to FeCr2O4, and finally Fe3O4 and Fe2O3 formed on SS430 stainless steel. These results indicated that the transition on SS430 stainless steel is due to the chemical failure, in which Cr depletion beneath Cr2O3 layer and the subsequent ionization of Fe took place. Mechanical failure of Cr2O3 can not explain the transition.