The evolution of laves phase precipitation in AISI 441 under SOFC operating conditions and the effects on oxide growth
Zimny, Christopher Isadore.
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Recent research on solid oxide fuel cell technologies -- a type of high temperature fuel cell -- has resulted in the reduction of operating temperatures from around 1000°C to an intermediate range of 600-800°C. This reduction in temperature allows previously unviable materials to be investigated for use with this system. Interconnects, the component that separates the anode and cathode of adjacent fuel cells, benefit from this advancement as metals may now be utilized in place of the ceramic interconnects traditionally used in solid oxide fuel cell systems. In separating the anode and cathode, interconnects are necessarily subjected to what is known as dual atmosphere exposure: the exposure to the fuel, H 2, on one side of the material and the oxidant, air, on the other side. These extreme operating conditions have unusual effects on materials. AISI 441 is one promising ferritic stainless steel alloy for use as an interconnect; however, under dual atmosphere exposure, AISI 441 sees accelerated and anomalous oxide growth, exceeding that of similar ferritic stainless steels. This study investigated the oxide and Laves phase precipitate evolution of AISI 441 subjected to various environments including single, dual, and vacuum environments at a temperature of 800°C. Additionally, the relationship between the precipitates and the oxide formation was observed. Analysis was performed with a variety of equipment including a field emission scanning electron microscope. In environments containing oxygen, the oxide thickness of AISI 441 was found to increase over time and saw accelerated oxide formation in the dual atmosphere test, corroborating previous research. This study went on to investigate the formation of Laves phase precipitate Fe 2Nbin the microstructure of AISI 441 as a result of atmosphere. Laves phase materials are under research as hydrogen storage materials, suggesting the existence of these phases may facilitate hydrogen transport and storage in materials used as interconnects. Increased hydrogen transport could potentially explain the accelerated and anomalous oxidation of AISI 441 compared to other ferritic stainless steels. Over the time frames tested, the environments had no apparent effect on the Laves phase precipitation; further testing should investigate Laves phase precipitation between 0 and 1000 minutes.