Water Vapour Effect on Ferritic 4509 Steel Oxidation Between 800 and 1000 °C

The 4509 alloy (Fe-18Cr-Nb-Ti) was oxidised in dry and wet air in the 800-1000 A degrees C temperature range. Results showed that the formation of a chromia layer acts as a good diffusion barrier under isothermal conditions at 800 and 900 A degrees C, under 7.5 vol.% water vapour and dry air. Nevertheless, a breakaway is generally observed at 1000 A degrees C, under wet air 7.5 vol.% H2O. It is proposed that the oxidant H+/OH- species react at the internal interface with iron in the chromium-depleted alloy zone. Wustite reacts with Cr2O3 to form FeCr2O4. Outward iron diffusion leads to Fe3O4 and Fe2O3 formation. The chromia scale was consumed by reaction with wustite, but chromia also internally forms owing to a chromium oxidation process with the inner chromium-rich alloy area.