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 °C in Ar–O2, Ar–H2O and Ar–H2–H2O gas mixtures. Chromium in solution exhibits a higher tendency to become internally oxidized in the gases containing water vapor than in the Ar–O2 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.