Corrosion of Ferritic Stainless Steel Interconnects for Solid Oxide Cells – Challenging Operating Conditions
Doktorsavhandling, 2018

Solid oxide cells (SOC) have the potential to revolutionize electricity production by being able to both produce electricity with very high efficiency from a variety of fuels or to produce fuels from electricity and abundant raw materials such as water or carbon dioxide. Some material challenges remain to be solved before large-scale commercialization can be achieved. Interconnects made from ferritic stainless steels are key components in solid oxide cells, but the conditions within the cells cause them to degrade from high temperature corrosion.

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.

interconnect

high temperature electrolysis

dual atmosphere

chromium volatilization

fuel cell

high temperature oxidation

SOEC

SOFC

KC-salen, Kemigården 4
Opponent: Paolo Piccardo, Associate Professor, Department of Chemistry and Industrial Chemistry, University of Genoa, Italy

Författare

Patrik Alnegren

Chalmers, Kemi och kemiteknik, Energi och material, Oorganisk miljökemi

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-

Artikel i vetenskaplig tidskrift

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

Artikel i vetenskaplig tidskrift

Fuel cells are devices that silently, and without combustion, produce electricity from hydrogen or other fuels. In addition, they also have the benefit of being significantly more efficient than combustion engines. In recent the years it has also been shown that fuel cells can operate in reverse mode and produce hydrogen gas from electricity and water. This hydrogen production, referred to as electrolysis, can be used for efficient fuel production, especially if electricity from renewable energy production is used. Solid oxide fuel cells are the most efficient types of fuel cells, due to their high operating temperatures of 600-900 °C. However, at these temperatures the metallic parts of the fuel cells degrade due to reaction with the oxygen and the water vapor that are present inside the fuel cells. In this thesis, this degradation process, called corrosion, was studied. Selected steels that are used as materials for fuel cell components were studied in atmospheres that were designed to simulate the environment inside a fuel cell. The purpose was to find how changes in the environment inside a fuel cell affected the corrosion of the steel components. The ambition was to obtain an understanding of the underlying corrosion mechanisms from a chemical perspective that could be used for choosing, or developing, better steels for solid oxide fuel cells and solid oxide electrolysis cells. The findings of this thesis illustrated that the type of steel used in fuel cells needs to be chosen based of the intended operating conditions of the fuel cell. As an example, the corrosion mechanism of the steels changed and the degradation rate was accelerated in atmospheres with high water vapor content when the chromium content of the steels was too low. Additionally, a closer-to-reality atmosphere was simulated by exposing a steel simultaneously to air on one side and hydrogen on the other and it was found that corrosion was significantly accelerated compared to when the steel was exposed to only one of the atmospheres. A solution to reduce the degradation rate of steels in fuel cells was also evaluated. It was found that application of a very thin coating of the element cerium (Ce) onto the steel surface could significantly improve the expected service life of the steel components in solid oxide fuel cells and solid oxide electrolysis cells.

Drivkrafter

Hållbar utveckling

Styrkeområden

Energi

Materialvetenskap

Ämneskategorier

Annan kemiteknik

Metallurgi och metalliska material

Korrosionsteknik

ISBN

978-91-7597-748-5

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4429

Utgivare

Chalmers tekniska högskola

KC-salen, Kemigården 4

Opponent: Paolo Piccardo, Associate Professor, Department of Chemistry and Industrial Chemistry, University of Genoa, Italy