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Long term, cyclic oxidation studies of three high -Cr, ferritic steels were carried out at 800 8C and 900 8C in air. It was found that with decreasing sample thickness the life time of the mentioned alloys decreases due to breakaway phenomena. This effect is caused by faster exhaustion of the chromium reservoir from the bulk alloy in case of thinner components. The observed life time limits can be predicted with reasonable accuracy by a theoretical model, using oxide growth rate parameters, initial alloy Cr content and critical Cr content required for protective chromia scale formation. In the calculation it has, however, to be taken into account that the oxidation rates of the steels increase with decreasing specimen thickness.

The oxidation behaviour of two ferritic steel variants was studied at 800 °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 18O‐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 °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.

For planar solid oxide fuel cell (SOFC) designs chromia forming ferritic steels are being considered as possible construction materials for the interconnections. In accordance with SOFC market requirements it is in many cases necessary to reduce the size of the fuel cell and thus the thickness of the interconnect. Therefore, long term, cyclic oxidation studies of ferritic interconnector steels were carried out at 800°C and 900°C in air thereby putting main emphasis on the effect of specimen thickness on the oxidation behaviour. It was observed that with decreasing sample thickness the life time of the mentioned alloys decreases due to breakaway phenomena. This effect is caused by the smaller chromium reservoir in the alloy in case of thinner components. The observed life time limits can be predicted with reasonable accuracy by a theoretical model, using oxide growth rate parameters, initial alloy Cr content and critical Cr content for protective chromia scale formation. It also has to be taken into account that the oxidation rates of the steels increase with decreasing specimen thickness.