Oxidation behavior of Ferritic Alloys as Interconnect of Solid Oxide Fuel Cell (SOFC)
Type of Degreedissertation
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In planar solid oxide fuel cell SOFCs, interconnects separate adjacent single cells, and Cr-containing ferritic stainless steel alloys can be used for interconnects. Oxidation resistance under the SOFC high temperature environment is one of the critical issues for the application of ferritic alloys as the interconnect materials. In SOFCs, there is a H2 concentration difference between the two sides of the interconnect materials, so hydrogen may diffuse through the alloy and aggravate the oxidation of the alloy on the air side. In this work, the oxidation behaviors of alloys 430 and 441 in air with different concentrations of water vapor were studied. The effects of water vapor were different between 430 and 441, which may be attributed to both differences in the alloy composition and alloy grain size. The effects of flow rate on the oxidation behavior of alloys 430 and 441 in dual atmospheres at 800ºC were also investigated. All the oxide scales were composed mainly of a surface (Mn,Cr,Fe)3O4 spinel layer and a Cr2O3 sublayer. For 441, the higher flow rate of Ar-5vol%H2 resulted in the formation of Fe-rich nodules on the air side of sample. To study the effects of alloy grain boundaries on the oxidation behaviors, alloys 430 and 441 with large grains were investigated in dual atmospheres oxidation. Initially, the scale formed on 430 air side contained many Fe-rich nodules along the alloy grain boundaries. Continuous oxidation led to the formation of three different microstructures, including (Fe,Cr)2O3, spinel phase and Cr-rich areas. For 441, only some isolated areas contained Fe-rich nodules on the air side of the sample. Trapping of hydrogen by Fe2Nb at the alloy grain boundaries area might contribute to the lower amount of Fe-rich nodules on 441 oxide scale. To avoid cathode Cr poisoning in SOFCs, ceramic coatings are applied on the Cr-containing ferritic alloys used as interconnect. In this case, ferritic alloys with low Cr concentration could possibly be used. The samples with 13-18% Cr were evaluated by dual atmosphere oxidation experiments. With higher Cr concentration of the uncoated sample, quantities of Fe-rich nodules were reduced. (Mn,Co)3O4-coated ferritic alloys with different Cr levels were studied by long-term oxidation in air at 800ºC. The coated samples with relatively low Cr concentration have more scale spallation. The scale spallation is possibly related with the formation of CoFe2O4, which has a large mismatch of the thermal expansion coefficient with Cr2O3 and (Mn,Co)3O4.