In anode‐supported solid oxide fuel cells (SOFCs), air break‐in on the anode side can result in reoxidation of metallic nickel. The volume expansion caused by Ni oxidation generates stresses within the substrate, the anode and the electrolyte. Those stresses exceed the stability of the components, potentially promoting crack growth. Therefore, either the SOFC degrades continuously after each redox‐cycle or the membrane electrode assembly (MEA) fails completely if the electrolyte cracks.
The influence of several reoxidation parameters on the mechanical integrity of Ni–YSZ‐anodes after reoxidation was investigated using different types of samples. All samples were SFEs (substrate–functional layer–electrolytes), consisting of Ni–YSZ‐substrate, Ni–YSZ‐anode and YSZ‐electrolyte. Investigations were carried out on freestanding SFEs and SFEs attached to steel plates (Crofer22APU, Thyssen Krupp V. D. M., Material Data Sheet No. 4046, Edition of December 2006) with a glass sealing.
The results show a big influence of the degree of oxidation, homogeneity of oxidation, the operating temperature and the incident flow on the behaviour and the mechanical integrity of the reoxidised SFEs. The time of oxidation and the gas flow rate were influencing parameters, whereas the influence of the porosity was insignificant. The behaviour of the SFEs upon reoxidation also changes dramatically when comparing freestanding samples with attached samples.
Today’s challenging requirements on thermal power plant cycle efficiency favor plants with steam temperatures up to 630°C. Cost competition, however, confines the application of particularly cost-intensive materials causing high manufacturing effort such as Ni-based alloys. Alternatives are martensitic steels containing less than 10% Cr. Due to the low Chromium content, these materials are less oxidation-resistant compared to 12% Cr-steels. Increased load-cycling requirements resulting from varying renewable energy production may result in decreased plant efficiency due to increased scaling and spallation of oxide layers during thermal transients. Hence, it is beneficial to protect such components against oxidation using coatings.
Investigations on various coating systems for their potential to serve as oxidation protection are described in this paper. These investigations consisted of a short-term screening program to identify the most promising coatings followed by an extensive test program including long-term steam exposure at high temperatures, thermal cycling, solid particle erosion tests as well as tests under operating conditions on samples and blades.
The test program revealed which coatings appeared to be the most promising solution for power plant applications, showing excellent oxidation protection capability of the base material in steam at high temperatures, structural stability upon thermal cycling and good solid-particle erosion protection. Tests under operational conditions have proven the functionality and stability of the coatings.
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