Corrosion of Ferritic Stainless Steels Used in Solid Oxide Fuel Cells

Licentiate thesis, 2018

Solid oxide fuel cells (SOFC) are energy conversion systems with clean emissions (depending on the fuel used) and high electrical efficiencies, which could potentially replace conventional conversion systems such as combustion engines. However, other issues, such as high costs and limited lifetime, must be resolved before widespread commercialization of SOFCs can be achieved.

One of the major cost factors and the component that reduces the lifetime of SOFCs immensely, is the interconnect, which electrically connects multiple fuel cells to form a stack. Interconnects are typically made of ferritic stainless steels, and, due to high temperatures (600 °C – 900 °C) and the aggressive environments prevalent in fuel cells, the interconnects corrode over time. The protective Cr2O3 that forms on ferritic stainless steels at high temperatures mitigates the corrosion process to a certain extent. However, this oxide layer leads to two other issues: (i) vaporization of hexavalent Cr species, which leads to cathode poisoning and (ii) an increase in the electrical resistance of the interconnect caused by a continuously growing oxide layer. Both these problems can be reduced to a certain extent with coatings. Especially spinel coatings have been proven highly effective at decreasing Cr-evaporation.

The first part of this work examines the influence of the Co3O4 spinel coating on area specific resistance (ASR). It was found that the Co3O4 thickness of Crofer 22 APU, which was exposed in air for 500 h at 600 °C, did not significantly impact the ASR, and, instead, the main contributor to overall resistance was the thermally grown Cr2O3.

The second part of this work focuses on the long-term stability of Ce/Co-coated (10 nm Ce/640 nm Co) AISI 441. Coated AISI 441 was exposed by AB Sandvik Materials Technology for up to 37 000 hours at 800 °C in laboratory air. Subsequent analysis showed very low Cr-evaporation rates compared to uncoated AISI 441, and ASR values below 40 mΩcm2, suggesting that, under these conditions, the coating is effective in reducing Cr-evaporation rates and Cr2O3-growth rates even after long exposure times.

The last part of the work analyzes the influence of pre-oxidation on the dual atmosphere effect at 600 °C with regard to two parameters: the pre-oxidation time and the pre-oxidation location. It was demonstrated that longer pre-oxidation times for AISI 441 result in extended resistance against dual atmosphere corrosion on the air-facing side. It was also found that the pre-oxidation layer on the hydrogen-facing side is more important for corrosion resistance in dual atmosphere than the pre-oxidation layer on the air-facing side.