The Influence of Humidity Content on Ferritic Stainless Steels Used in Solid Oxide Fuel Cell under Dual Atmosphere Conditions at 600°C

In the last few years, the interest in the development and application of green energy harvesting, storage and production technologies, boosted the research activities to fulfill the demand of better performing, more reliable and cost-effective materials. In the field of Solid Oxides Fuel Cells (SOFC), many efforts have been spent on the interconnects, that are used to electrically connect single cells to obtain stacks for higher output voltages. Interconnects are exposed simultaneously to fuel gas (i.e., H 2 ) and oxidizing (humid air) atmospheres (dual atmosphere) at a working temperatures in the range between 600°C and 800°C. Nowadays, chromia forming ferritic stainless steel (FSS) has been selected as material for interconnects due to its ease of fabrication, affordability and electrical conductivity. In air the material forms a protective chromia layer that reduces further corrosion. However, in dual atmosphere conditions, FSS commonly show fast and unexpected degradation, on the air side, especially at 600°C. The mechanism behind the dual atmosphere effect is still not fully clear, but previous works highlighted the importance of pre-oxidation time, exposure temperature, humidity and surface finishing. Until now, major efforts have been spent in studying the long-term behaviour, with the aim to identify a long-lasting interconnect material. To successfully overcome the effects of dual atmosphere-induced corrosion, it is of fundamental importance to investigate how the corrosion phenomenon starts and evolves. In this work, the initial stages of dual atmosphere breakaway corrosion at 600°C were investigated for selected FSS alloys. The samples were exposed under controlled conditions which resemble those of SOFC (Ar-5%H 2 + 3%H 2 O//FSS//air + 3%H 2 O). The oxide scales were further investigated by means of Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) analysis. Experiments with different exposure times were performed (i.e., up to 12 hours) and the influence of temperature ramping was studied. Several parameters were evaluated, like the effects of chromium content, the role of sample thickness and the influence of trace elements in different alloys and different batches of the same alloy. Implications of grain orientation at the surface, as well as of pre-oxidation treatment were investigated too.