Corrosion of Ferritic Stainless Steel Interconnects for Solid Oxide Cells – Challenging Operating Conditions
Doctoral thesis, 2018
This thesis seeks out the potentially demanding operating conditions for solid oxide cells and focuses on investigating the effect of changing the environment on the degradation of ferritic stainless steels. Tests in which steel coupons were exposed to different atmospheres were performed to simulate the degradation of an interconnect inside an operating solid oxide cell. The effect of operating solid oxide fuel cells in electrolysis mode was specifically investigated, which means that interconnects were exposed to pure oxygen instead of ambient air and higher steam content on the fuel side. It was found that at 850 °C, ferritic stainless steels with 18-26% chromium content did not oxidize faster when the oxygen pressure was increased. However, the microstructure of the formed oxide scales on the steels was found to depend on oxygen concentration which caused oxide spallation for some steels at lower oxygen pressures.
Experiments in hydrogen with high steam content, representing the other side of the interconnect, revealed an increase in the oxidation rate of the steel if the chromium content in the steel was too low, due to a change of the oxidation mechanism. Dilution of the same atmosphere with argon changed the oxidation mechanism to more protective behavior, which led to new insights in designing relevant simulated solid oxide cell fuel side conditions. It was also found that the oxidation rate of ferritic stainless steels in fuel side atmosphere can be significantly reduced by the physical vapor deposition (PVD) of cerium onto the surface. Even with applied cerium, however, steels with lower chromium content might still be at risk of rapid oxidation due to iron-rich oxide formation.
A close-to-reality atmosphere was also simulated by exposing a ferritic steel simultaneously to air on one side and hydrogen on the other, which resulted in severely accelerated corrosion at 600 °C. Areas of up to 30 µm thick iron oxide were formed on the air side after 1000 h and grew to cover most of the surface after 3000 h. This dual atmosphere effect was concluded to have an inverse relation to temperature since accelerated corrosion was not observed at 700 and 800 °C. In addition, it was found that the corrosion resistance could be improved if the steel was pre-oxidized in air before exposure to dual atmosphere.
high temperature electrolysis
high temperature oxidation
Chalmers, Chemistry and Chemical Engineering, Energy and Material
Degradation of ferritic stainless steels under conditions used for solid oxide fuel cells and electrolyzers at varying oxygen pressures
Corrosion Science,; Vol. 110(2016)p. 200-212
P. Alnegren, M. S. Kiranmayee, J. Froitzheim, J.-E. Svensson, Influence of Absolute Pressure of H2/H2O on Corrosion of Ferritic Stainless Steel at 850 °C
P. Alnegren, Jan Grolig, J.-E. Svensson, J. Froitzheim, Reduced Oxidation of Ferritic Stainless Steels at 850 °C in high H2O/H2 by Coating with Cerium
Severe dual atmosphere effect at 600 °C for stainless steel 441
Journal of Power Sources,; Vol. 301(2016)p. 170-178
Temperature dependence of corrosion of ferritic stainless steel in dual atmosphere at 600–800 °C
Journal of Power Sources,; Vol. 392(2018)p. 129-138
Areas of Advance
Other Chemical Engineering
Metallurgy and Metallic Materials
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4429
KC-salen, Kemigården 4
Opponent: Paolo Piccardo, Associate Professor, Department of Chemistry and Industrial Chemistry, University of Genoa, Italy