Effect of α-to-γ transformation on internal oxidation in FeCr-base alloys in dry and wet gases

Journal article, 2025

The transition from internal to external oxidation has been studied in ferritic and austenitic Fe-10Cr base model alloys in Fe/FeO Rhines pack and H2/H2O gas mixtures between 800 and 1100 °C. Ferritic and austenitic microstructures were stabilized by alloying with Mo (ferrite) and Ni (austenite), respectively. The α-to-γ transformation is experimentally demonstrated to be the primary trigger of internal oxidation in Fe-10Cr. Up to 1000 °C ferritic alloys can form external Cr2O3 scales while all austenitic alloys oxidized internally irrespective of the exposure environment. Wagnerian diffusion analysis accurately predicted the threshold of protectiveness (10 wt% at 1000°C) for the ferritic alloy as well as internal oxidation for the austenitic alloys. Water vapor promotes internal oxidation via increasing the effective oxygen permeability, i.e., the interfacial transport in the internal oxidation zone. However, the effect of H2O and/or H2 on the transition from external to internal oxidation is rather limited at 900 °C and above.