In order to elucidate the phenomena and mechanisms of oxidation of Fe-(9, 17%)Cr alloys in Ar-10%H2O atmosphere at 1 000-1 400 K, the thickness and the morphology of scales formed were investigated.
The scales usually consisted of the outer scale and the subscale. The oxide phases of these scales were identified as FeO and the mixtures of FeO and FeCr2O4, respectively. With the increase in Cr content in the alloys the rate of the total oxidation decreased, but the rate of the subscale formation increased.
At all the temperatures parabolic kinetics was observed in the subscale formation, and its apparent activation energies were estimated to be 141 kJ/mol for Fe-9%Cr alloy at temperature above 1 100 K and 214 kJ/mol for Fe-17%Cr alloy at temperature above 1 000 K, respectively. The rate determining diffusion elements were considered to be Fe in FeO for the former alloy and Fe and Cr in α phase for the latter alloy.
As for the subscale configurations, in Fe-17%Cr alloy only a continuous subscale formation was observed, whereas in Fe-9%Cr alloy the characteristic subscale was observed to form periodically. The periodic growth of FeO and FeCr2O4 can be explained by a Liesegang phenomenon, because the kinetics of the subscale formation is parabolic and the subscale formed periodically obeys JABLCZYNSKI'S relationship.
