Interaction of sealing material mica with interconnect steel for solid oxide fuel cells application at 600 °C

In the last few years, a lot of effort has been undertaken to reduce the operation temperature of solid oxide fuel cells (SOFCs). Intermediate temperatures in the range of 600–650 °C are thought to be highly attractive due to the expected reduction of high-temperature corrosion and interdiffusion phenomena. Furthermore, each stack operated at higher temperatures has to pass this temperature range during thermal cycling. In this work, a study has been carried out to investigate the interaction between interconnect steel (DIN 1.4760) and vermiculite mica paper filled with talc at 600 °C. Mica paper has been implemented recently as a new sealing material for SOFC stacks, aiming to replace established but brittle glass ceramics. Corrosion experiments were conducted at 600 °C under dual gas atmosphere conditions as well as in air. The interconnect steel showed the formation of non-protective oxide scales in contact with mica, especially in the presence of air. The morphology of oxide scales was investigated by SEM/EDX. Possible reasons for this unexpected result are discussed. The complete encapsulation of mica by embossed and welded sheets made of FeCrAlY-steel (DIN 1.4767) as well as the pre-oxidation of interconnect steel were investigated as solutions to this problem. Pre-oxidation was found to be a promising measure to protect the interconnect steel from breakaway oxidation in contact with mica.

► Abnormal oxidation behaviour of interconnect steels at 600 °C in contact with mica.
► Breakaway oxidation preferentially on the air side starting from single grains.
► Pronounced in case of coarse grained steels and locally increased air humidity.
► Contact with mica not mandatory, but volatile species obviously support the effect.
► Pre-oxidation of steel at 800 °C in air is a reliable protection for at least 400 h.