Enhanced internal oxidation as trigger for breakaway oxidation of Fe-Cr alloys in gases containing water vapor

Binary Fe-Cr alloys were studied with respect to isothermal oxidation behavior at 900 degrees C in Ar-O-2, Ar-H2O and Ar-H-2-H2O gas mixtures. Chromium in solution exhibits a higher tendency to become internally oxidized in the gases containing water vapor than in the Ar-O-2 gas because of hydrogen dissolution in the alloy. This mechanism explains why Fe-Cr alloys with intermediate Cr contents (e.g., 10-20 wt.%) exhibit protective chromia-rich scale formation in dry gases but tend to form breakaway-type Fe-rich oxides in wet gases. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.