Metallic materials for solid oxide fuel cells and electrolysers - Mitigating high temperature corrosion
Doctoral thesis, 2023

Solid oxide cells (SOCs) are high-temperature energy conversion devices that have great potential due to their high efficiency, low operating costs, and flexibility. SOCs can produce electricity from a variety of fuels as solid oxide fuel cells (SOFCs), and they can convert electricity to fuels as solid oxide electrolyser cells (SOECs). However, the wide-spread commercialisation of this technology is hindered by high system cost, lack of durability, and poor performance stability during long-term operation. Owing to the high-temperature operation and aggressive environment of SOCs, metallic materials used for interconnects and balance of plant (BOP) components are subject to corrosion. Interconnects are typically made of ferritic stainless steel (FSS), which forms a protective chromia scale at high temperatures. The degradation mechanisms, such as Cr (VI) evaporation and chromia scale growth, lead to electrode poisoning and increased electrical resistance, which degrade cell performance.

The primary objective of this thesis is to develop alternative materials and understand the degradative mechanisms so as to effectively reduce the costs and improve the performances of metallic materials in SOC systems. The Cr evaporation, oxide scale growth, the microstructural evolution of the oxide scale, and the area-specific resistances are investigated for the selected materials. The majority of the thesis is focused on Ce/Co coatings. Ce/Co-coated, low-cost, commercial FSS (AISI 441, AISI 430, and AISI 444) are compared to tailor-made Crofer 22 APU in air-side atmospheres. Ce/Co-coated steels are further investigated under dual-atmosphere conditions. The Ce/Co coating is compared to various coatings from research laboratories and universities world-wide. Furthermore, the underlying causes for the improvement in the oxidation resistances of FSS that occur in the presence of the reactive element Ce (in the Ce/Co coating) are investigated, and a new mechanism is proposed. Finally, a model to predict the lifetimes of the coated steels is proposed. Moreover, a new coating system, the Ce/FeNi coating, is proposed as an alternative to the Ce/Co coating. The Ce/FeNi coating is found to be more effective than Ce/Co coating in reducing chromia scale growth.

While research on metallic materials for SOC has centred on the interconnects, the metallic materials used in BOP components, which can be a significant source of volatile chromium species, have been largely neglected. Five metallic materials (AISI 441, AISI 444, A197/Kanthal® EF101, alloy 800H, and alloy 600) are examined for potential usage in BOP components. The oxidation and Cr evaporation behaviours of these materials are discussed and correlated to the observed microstructures.


High-temperature corrosion

MCO coating


Balance of Plant

Chromium evaporation


KB-salen, Kemigården 4, Chalmers
Opponent: Dr. Jan Van Herle, Senior Scientist, EPFL, Switzerland


Mareddy Reddy

Chalmers, Chemistry and Chemical Engineering, Energy and Material

Reevaluating the Cr Evaporation Characteristics of Ce/Co Coatings for Interconnect Applications

ECS Transactions,; Vol. 103(2021)p. 1899-1905

Paper in proceeding

Reddy, M. J., Krogsgaard T., Froitzheim, J. and Svensson, J. E. Re-visiting the behaviour of reactive elements in the Ce/Co coatings of ferritic stainless steels

Reddy, M. J., Goebel, C., Valot, A., Svensson, J. E., Frandsen, H. L., and Froitzheim, J. Comparison of (Mn, Co)3O4 vs (Fe, Ni)3O4 spinel coatings for stainless steel interconnects in solid oxide electrolysis

The hydrogen economy, in which hydrogen is used as a primary energy carrier, has gained attention as a solution to global energy and environmental problems. Green hydrogen can be produced using renewable electricity to split water molecules into hydrogen and oxygen. Hydrogen is abundant and versatile and can power various applications, from vehicles to homes, and it can be used to decarbonise hard-to-abate sectors. Hard-to-abate sectors are those which cannot be electrified and instead rely on fossil fuels, such as steel, cement, fertiliser production, and heavy-duty transport.

Solid oxide cells (SOCs) are expected to play an essential role in the hydrogen economy. They can both produce electricity from fuel (in fuel cell mode) and produce fuel from electricity (in electrolyser mode). Thus, they form a closed-loop system that generates electricity on demand using hydrogen that was previously produced from water using renewable energy sources. High operating temperatures, typically 600°–900°C, make the SOCs highly efficient. However, the metallic materials of SOC systems, primarily used in the interconnects that connect the individual cells to form a SOC stack, and the Balance of Plant, which supports the functioning of the SOC stack, degrade at these high operating temperatures.

This thesis aims to develop alternative materials that can effectively reduce costs and improve the performances of the metallic materials in SOC systems. Ferritic stainless steels used to make interconnects form a protective Cr2O3 scale at high temperatures to prevent the material from further oxidation. However, the Cr2O3 scale at the air electrode forms volatile Cr species, thereby poisoning the air electrode. The growth of the Cr2O3 scale increases the electrical resistance across the interconnect, which decreases the efficiency of the SOC. Speciality steels such as Crofer 22 APU have been developed to mitigate these issues. These steels are expensive, and their degradation is still too high for long-term operation. Protective coatings based on (Co,Mn)3O4 spinel were used to reduce chromium evaporation and improve the oxidation resistance of the interconnects. The present work explores Ce/Co-coated, low-cost steels, such as AISI 441 and AISI 430, as alternatives to Crofer 22 APU. A new coating system with improved performance is developed for application in solid oxide electrolyser cells.

Towards a sustainable society: Developing metallic materials to advance solid oxide technology

VINNOVA (2021-01003), 2021-05-01 -- 2024-08-31.

Low Cost Interconnects with highly improved Contact Strength for SOC Applications (LOWCOST-IC)

European Commission (EC) (EC/H2020/826323), 2018-12-01 -- 2021-12-31.

Driving Forces

Sustainable development

Subject Categories

Materials Engineering

Chemical Engineering

Areas of Advance


Materials Science


Chalmers Materials Analysis Laboratory



Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5298



KB-salen, Kemigården 4, Chalmers


Opponent: Dr. Jan Van Herle, Senior Scientist, EPFL, Switzerland

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