Effect of Geometry and Composition of Cr Steels on Oxide Scale Properties Relevant for Interconnector Applications in Solid Oxide Fuel Cells (SOFCs) (2006)

A number of high-Cr ferritic steels have been investigated as possible construction materials (interconnectors) for Solid Oxide Fuel Cells (SOFCs). The mentioned materials have the advantage of a higher electronic conductivity, lower cost and easier fabrication than so far used lanthanum chromite-based ceramics. A large number of ferritic steels are commercially available in a wide range of compositions, however it seems that none of them can fulfil all requirements for the SOFC interconnector application. Therefore the main emphasis was put to the investigation of the high temperature properties of recently introduced high chromium ferritic steels especially designed for SOFC applications. The scale formation mechanisms were investigated during oxidation times ranging from a few minutes up to 6000 hours. For scale characterization a number of conventional analysis techniques such as optical metallography, scanning electron microscopy and X-ray diffraction were used in combination with two-stage oxidation studies using 18O-tracer. It was found that the growth rates of the scales 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 temperatures of 800°C and 900°C these latter elements affect the scale formation although they are not directly incorporated in the surface scales.SOFC market requirements lead in many cases to the demand for a reduction of the fuel cell size and/or weight and thus of the interconnector thickness. Therefore, the main emphasis was made to investigate changes in the oxidation behaviour in the case of thin components. It was found that with decreasing sample thickness the lifetime of the mentioned steels 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 lifetime 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 of the Cr-reservoir exhaustion it has, however, to be taken into account, that during air exposure the oxidation rates increase with decreasing specimen thickness. The possible explanation of this effect is discussed on the basis of scale formation mechanisms involving microcrack formation in the surface oxide scale and depletion of major and minor alloying additions in the bulk alloy. The electrical conductivity of the interconnect is a crucial property for SOFC application whereby the conductivity of the chromium based oxide scale which forms during high temperature service has to be taken into account in the overall conductivity value. Therefore experimental data concerning the electrical conductivity of the surface oxide scales formed in the temperature range 600-800°C on the investigated ferritic steels have been determined. The data are correlated with oxide scale morphologies and scale formation mechanisms and the results are compared with those obtained for two “pure chromia” forming materials.