Potential suitability of ferritic and austenitic steels as interconnect materials for solid oxide fuel cells operating at 600 °C

The oxidation behavior of a number of commercially available ferritic and austenitic steels was tested in air and in two simulated anode gases of a solid oxide fuel cell (SOFC) to evaluate the potential suitability as construction materials for interconnects in SOFC's operating at 600 °C. During air exposure all studied materials showed excellent oxidation resistance due to formation of a protective, double layered chromia/spinel surface scale even if the steel Cr content was as low as 17%. However, in the anode side gases the presence of water vapour (and possibly CO/CO2) increased the tendency to form poorly protective Fe-base oxide scales, in combination with internal oxidation of Cr. The occurrence of this adverse effect could be suppressed not only by increased Cr contents of the alloy but also by a small alloy grain size either in the bulk of the material or in the specimen/component surface. The latter can be promoted by cold work e.g. introduced by specimen/component grinding. As high Cr contents may lead to undesired σ-phase formation and defined surface treatments of an interconnect will not be possible in all designs, the relatively low operating temperature of 600 °C, resulting in low Cr diffusivity in the alloy grains, may require the use of a fine grained interconnect material to obtain and sustain protective chromia base surface scale formation during long-term operation.