Metallic interconnects for SOFC: competitive effect of water vapour and oxygen on the oxidation behaviour of ferritic stainless steel.
Résumé
Due to their low environmental impact, their high efficiency and their fuel flexibility, Solid Oxide Fuel Cells are studied for many years in order to supply electricity without fossil sources. The need of interconnect to create a stack (providing electrical connection between anode and cathode) and seal every single cell (having to avoid any contact between air and fuel) appears as a crucial point. Ferritic Stainless Steel (FSS) like Crofer22APU are considered as good candidates for interconnects. These chromia forming alloys have to be operational in the service conditions: 800°C in air (cathode side) and in humidified hydrogen (anode side). The performance of the interconnect stainless steels is then limited by the oxide scale formation, low electronic conductivity of this scale and eventual volatility of chromium oxide. Previous researches demonstrated than the corrosion behaviors of La2O3 and Y2O3-coated and uncoated FSS in air and in H2/10%H2O are completely different [1, 2]. In anode atmospheres, the corrosion rate is higher, the alloys tend to form whiskers-type oxide morphologies, the porosity of the oxide scale is increased and the diffusion of Fe and Mn in the scale is promoted. However, one of the main parameters in interpreting the differences in oxidation behavior between air and wet hydrogen is that both the oxygen partial pressure and the water vapour partial pressure are different. Thus, the goal of this fundamental study is to realize oxidation of uncoated and La2O3 and Y2O3-coated Crofer22APU and Fe30Cr in O2, O2/H2O and Ar/H2O at 800°C and to compare these results with previous results (in air and in H2/H2O) in order to determine the exact influence of water vapor and oxygen partial pressure on corrosion behavior. Kinetics were registered for 100 h at 800°C and the corrosion products were carefully analysed by SEM, EDX, XRD, SIMS.