Post-test Characterization of Metallic Materials and Adjacent Components in an SOFC Stack After 34,000 h Operation at 700 °C

The present paper describes the electrochemical results and the post exposure microstructural characterization by means of SEM/EDS of a four layer SOFC stack after 34,000 h operation under load at 700 degrees C, showing low voltage degradation rate less than 0.3% per 1,000 h. Emphasis was put on the behavior of the ferritic interconnect steel and its interaction with glass sealants as well as contacting and coating materials. The interconnect steel had formed thin protective chromia scales. Gas distributing steel foils of 200 mu m showed in some locations breakaway type oxidation. It was related to local overheating caused by locally occurring combustion of a defective cell. The interaction of the steel with the glass sealant showed good adhesion. Interdiffusion at the joint between nickel contact and steel interconnect at the anode side resulted in minor oxide formation on the Ni wires and in austenite formation in the steel without formation of sigma-FeCr formation at the steel/austenite interface. The steel showed excellent compatibility with the Manganese-Cobalt-Ferrite (MCF) chromium retention coatings. Chromium enriched phases were found near the interface between MCF coating and perovskite contact layer. Excess glass sealing material interacted with the contact layer without showing obvious detrimental effects.