Air and wet air oxidation of 9Cr-3Co-3W creep resistant ferritic steel at 650 °C

The new experimental creep resistant ferritic steel of the 9Cr-3Co-3W type was oxidised at 650 degrees C in air and wet air. The oxidation kinetics was measured by intermittent weight measurement. The scales formed were analysed using techniques of XRD, SEM and EDS. The results showed that the oxidation rate was more than a magnitude faster in wet air than in air. The oxidation kinetics in air obeyed the parabolic rate law of oxidation only in a limited oxidation period of up to 1726 h whereas it did not follow any power rate law of oxidation in wet air. The steel cannot form a protective Cr2O3 scale either in air or in wet air at 650 degrees C. Instead, the scale formed in air consisted of an outer (Fe0.6Cr0.4)(2)O-3 layer and an inner Cr-rich (Fe,Cr)(2)O-3 layer containing Cr2O3 particles, but in wet air it consisted of an outer Fe3O4 layer and an inner (Fe,Cr)(3)O-4 layer.