High temperature oxidation of Fe–Cr alloy in O2–H2–H2O atmospheres; microstructure and kinetics

The oxide microstructure and oxidation kinetics of the alloy Fe0.78Cr0.22 were examined at 1173 K in air and in 7% H2/93% Ar, both containing 1 or 12% water vapour. Oxide whiskers and ridges were observed on the chromia scales grown under reducing conditions, while they were absent from the chromia scales grown in air. This difference is caused by vaporization of chromium species from the scale grown in air. The growth mechanism of whiskers is discussed. The chromia scales were found to adhere much better to the alloy when grown in hydrogen/argon than in air. This is probably caused by a scavenge of impurities under reducing conditions. The growth rate of the scales is independent of the oxygen activity, which is explained by an interstitial growth mechanism of the chromia scales. The growth rate decreases with increasing water content in air. This is caused by a simultaneous vaporization of chromium species from the scale during the growth. In contrast to steady-state growth of chromia scales, the breakaway oxidation is highly affected by the oxygen activity in the reaction atmosphere. A low oxygen activity can protect the alloy from the catastrophic oxidation behaviour observed at larger oxygen activities. Tuning of oxygen activity may be useful for long-term oxidation of Fe–Cr alloys under reducing conditions.