Corrosion of ferritic stainless steels used in solid oxide fuel cells - Investigation of novel coatings in single- and dual-atmosphere conditions

Abstract: Solid Oxide Fuel Cells (SOFCs) are systems that convert chemical energy into electrical energy. Their high electrical efficiency and clean emissions (when H2 is used as fuel) make them a strong candidate for replacing conventional conversion systems, such as combustion engines. However, the high costs and limited life-times of SOFCs have hindered their widespread commercialisation. One of the main costs in a fuel cell stack is for the interconnect, which connects the cells electrically. Interconnects are, nowadays, made of Ferritic Stainless Steel (FSS) and, when exposed to fuel cell operating temperatures, typically between 600℃ and 900℃, they suffer severe corrosion. The Cr2O3 layer that forms on the interconnects upon exposure to high temperatures mitigates the corrosion process to some extent. However, the formation of a chromia layer leads to two major issues: a) the volatilisation of Cr(VI) species, which poisons the cathode; b) an increase in the electrical resistance of the interconnect caused by the continuously growing oxide scale. Both of these issues can be mitigated by using coatings. The optimal candidates are spinel oxide coatings, due to their effectiveness at decreasing Cr evaporation and their high conductivity. The first part of this thesis investigates the efficiency of Cu-based coatings for mitigating Cr(VI) evaporation, as compared to the Co-based coatings currently used. Two different processes are used for the deposition of the coatings: a) Physical Vapour Deposition (PVD); and b) Thermal Spray (TS). It was found that, thin PVD coatings are as efficient as much thicker TS coatings at mitigating Cr(VI) evaporation when exposed at 650℃ in the humid air. Area-specific resistance measurements showed that the PVD Ce/Cu coating is as good as the state-of-the-art PVD Ce/Co. The TS MnCo-oxide (MCO) coating, even if its microstructure differs, also display a good ASR. The second part of this thesis focuses on the abilities of coatings that are exposed under simulated SOFC operating conditions, i.e. the simultaneous exposure to air and H2/H2O, to mitigate the so-called ‘dual-atmosphere effect’. Samples that were coated with Ce/Co on the air-side and uncoated on the fuel-side, as well as samples that were coated with Al or Al2O3 on the fuel-side and uncoated on the air-side were exposed at 600℃ for 3,000 h under dual-atmosphere conditions. It was found that applying a coating on the air-side delays the onset of break-away oxidation, although the best effect was seen for samples with Al and Al2O3 coatings on the fuel-side and the coating combination Al//Ce/Co. A significant reduction of the dual-atmosphere effect was observed once a coating was applied on the fuel-side of the interconnect.