Metal Dusting Problem with Metallic Interconnects for Solid Oxide Fuel Cell
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Metal Dusting Problem with Metallic Interconnects for Solid Oxide Fuel Cell Z. ZENG and K. NATESAN Argonne National Laboratory Energy Technology Division Argonne, IL 60439, USA ABSTRACT Metallic interconnects in the solid oxide fuel cell (SOFC) are oxidized on the cathode side by air and carburized on the anode side by natural gas. Metallic alloys can be attacked by metal dusting corrosion in carbonaceous gases of high carbon activity in the temperature range of 350-1000°C. Under these conditions, pits form on the alloy surface and can become large holes through the alloy plate, with subsequent disintegration into a powdery mixture composed of carbon, fine particles of metal, and carbide. Fe and Ni-base alloys were tested in carbonaceous gases around the SOFC operating temperature. It was found that the oxide scales on the alloy surface prevent metal dusting corrosion. If the major phase in the oxide scale is chromic oxide, the alloys have good resistance to metal dusting corrosion. However, the alloys are easily attacked if the major phase is spinel. INTRODUCTION Recently, significant progress on reducing the operating temperature of an SOFC has been achieved by reducing the thickness of the electrolyte [1]. The lower operating temperature (550~800°C vs. 1000°C) makes it possible to consider high-temperature alloys as interconnect candidates. The advantage is that a metallic alloy has lower cost, higher electrical conductivity, and higher heat conductivity and is easier to fabricate than the ceramic interconnect presently used. Therefore, research on the metallic interconnect has attracted much attention recently [2,3]. Metallic interconnect acts as a gas separator and distributor; therefore, it works in a dual-gas environment: it faces air on one side, and natural gas on the other. On the side with air, metallic interconnect needs to be protected from oxidation. On the other side with natural gas, the interconnect needs to be protected from the carburizing gas. The metallic interconnect could be carburized at the working temperature of 550~800°C. Moreover, another catastrophic carburization phenomenon, “metal dusting” could occur in an atmosphere containing carbon [4,5]. Pits form on alloy surfaces during the metal dusting corrosion. Metal dusting occurs in environments with carbon activity (ac)>1. Gaseous reactions that lead to or cause metal dusting are: CO+H2=C+H2O 2CO=C+CO2 CH4=C+2H2
ac=K1•PCO•PH2/PH2O ac=K2•P2CO/PCO2 ac=K3•PCH4/P2H2
(1) (2) (3)
Inlet gases in the SOFC could be natural gas (mainly methane) or reformed gases containing H2, CO, CO2, and H2O. The composition of the fuel flow gradually changes from high concentration methane or H2 at the gas inlet to its oxidation products H2+H2O and CO+CO2. The humidity is low at the fuel cell inlet and high at the outlet. The reverse is true for carbon activity
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For the SOFC-gas turbine system, the SOFC would work at high pressure. Carbon activities increase with increasing pressure for reactions 1 and 2. However, ac decreases with increasi
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