Growth mechanisms and electrical conductivity of oxide scales on ferritic steels proposed as interconnect materials for SOFC's

The oxidation behaviour of two ferritic steel variants was studied at 800 degrees C in air and the results were compared with data obtained for two Cr-based materials. The mechanisms of scale formation were investigated for oxidation times ranging from a few minutes up to 6,000 hours. A number of conventional analysis techniques such as optical metallography, scanning electron microscopy, and X-ray diffraction were used for scale characterization, in combination with two-stage oxidation studies using an (18)O-tracer. It was found that the growth rates of the scales on the two steels were not only governed by the main scale forming alloying elements, Cr and Mn, but to a substantial extent by minor additions of Si and Al. At the test temperature of 800 degrees C these latter elements affect scale formation, although they are not directly incorporated in the surface scales. Independent of a detailed alloy composition, the conductivities of the scales on the ferritic steels were found to be higher than those of the surface scales formed on the Cr-based materials studied.