Oxidation Behaviour of Fe-Cr Based Alloys in Simulated Anode Side Gases of a Solid Oxide Fuel Cell

One of the challenges in improving the performance and reducing the costs of solid oxide fuel cells (SOFC’s) is the development of suitable interconnect materials. Recent research has shown that specially developed Fe-Cr based alloys possess the suitable combination of properties required for SOFC interconnect materials. Two commercial ferritic steels Crofer22APU (ThyssenKrupp VDM), F18TNb and F18TNbY (ArcelorMittal) were selected and evaluated in respect to oxidation behaviour under conditions relevant to the anode side of SOFC’s. Additionally, the electrical conductivity of the surface oxide scales at the SOFC service temperature of 800°C was determined. Isothermal and cyclic oxidation tests were carried out in three types of gases simulating compositions prevailing at the anode side of an SOFC. The gases differed in water vapour content as well as additions of CO and CO₂. The tests were carried out in the temperature range 800–900°C with exposure times up to 3000 h. The results showed that the minor alloying elements Al and Si exhibited a significant effect on oxide scale adherence and area specific resistance (ASR) of the surface oxide scales. The high chromium contents of the steel resulted in formation of oxide scales with excellent protective properties which resulted in excellent resistance against carburization in the CO/CO₂-containing test gas. The high chromium content has, however, as disadvantage that the alloys may be prone to embrittlement by σ-phase formation at lower temperatures. If chromium contents are well above approximately 22%, this σ-phase formation may even occur during 800°C exposure. In general, all tested alloys exhibited acceptable performance for interconnect materials but a substantially decrease in oxidation rate could be achieved with an additionally applied Y₂O₃ coating.