Journal of The Electrochemical Society, 159 (11) F725-F732 (2012) F725 0013-4651/2012/159(11)/F725/8/$28.00 © The Electrochemical Society Behavior of Interconnect Steels in Carbon Containing Simulated Anode Gas of Solid Oxide Fuel Cells L. Niewolak, z E. Wessel, T. H¨ uttel, C. Asensio-Jimenez, L. Singheiser, and W. J. Quadakkers z Forschungszentrum J¨ ulich, Institute of Energy Research (IEF-2), 52425 J¨ ulich, Germany The corrosion behavior of three commercially available high chromium ferritic steels and one austenitic steel was tested in CO- containing simulated anode gas of a Solid Oxide Fuel Cell. Aim of the study was to evaluate the suitability of selected steels for interconnects in SOFC’s with an operating temperature around 600 ◦ C. Main emphasis was put on the effect of a nickel contact layer on the materials behavior in the high simulated anode gas. In spite of the high carbon activity in the gas (a C >1) the steels in the as ground condition were found to form protective, chromium rich surface oxide scales and did not show indications for metal dusting or internal carburization, up to the maximum test times of 300 h. Presence of a metallic nickel layer resulted in carbon uptake of the steels, whereby the relative amounts increased with decreasing steel chromium content. Interdiffusion between ferritic steel and nickel layer lead to austenite formation, with or without σ-phase formation. Diffusion of chromium into the nickel layer resulted in formation of a thin chromia based surface scale on the initial nickel coating. This effect impedes the transfer of carbon from the gas into the nickel coated steel. © 2012 The Electrochemical Society. [DOI: 10.1149/2.033211jes] All rights reserved. Manuscript submitted July 6, 2012; revised manuscript received August 2, 2012. Published September 10, 2012. A Solid Oxide Fuel Cell (SOFC) is an electrochemical device that converts the chemical energy in fuels into electrical energy by utilizing the natural tendency of oxygen and hydrogen to react. 1 Compared to other fuel cell systems a main advantage of the SOFC is its ability to use not only hydrogen but also presently available fossil fuels (such as methane, butane or even reformed gasoline and diesel), thus reducing operating costs and increasing flexibility. In an SOFC system the single cell is constructed of an electrolyte (e.g. yttria stabilized zirconia -YSZ) arranged between a porous anode (e.g. Ni/ZrO 2 -cermet) and cathode (e.g. (La,Sr)MnO 3 ). 2–5 In the anode substrate supported planar SOFC designs the components are assembled in flat stacks, with air and fuel flowing through channels commonly built into the so called metallic interconnects. In an SOFC stack the metallic interconnect thus provides the separation of the gas atmospheres, the electrical connection between the various single cells and it acts as current collector. 5–12 In recent years a number of new ferritic steels such as Cro- fer 22 APU or ZMG 232 5–15 were developed for application as construction material for SOFC interconnects. As the development of the steels was focused on SOFC operating temperatures around 800 ◦ C, they contain high chromium contents of approximately 22 wt%. This composition assures the required long term oxidation resistance 5–15 and a coefficient of thermal expansion (CTE) simi- lar to that of the ceramic parts of the SOFC, especially the anode substrate. 5–15 Lowering the SOFC operation temperature to for instance 600 ◦ C would have a number of advantages such as reducing the rate of interaction of the interconnect with cathode and anode side con- tact materials, 11–15 reducing the formation of deleterious volatile Cr- containing species 15,16 and it potentially offers the possibility to obtain higher cell efficiencies provided that suitable electrolyte and electrode materials prevail. In the frame of the European project SOFC600 17 the potential suitability of a number of commercially available fer- ritic and austenitic steels for application in SOFC’s with an operating temperature of 600 ◦ C was investigated. The results were summarized in a previous paper of the present authors describing the oxidation behavior of selected steels in air and in the simulated anode gases Ar-4%H 2 -2%H 2 O and N 2 -CO-CO 2 -H 2 O. 18 The present paper presents the behavior of three selected high- chromium (18–23 wt%) ferritic steels and one austenitic (25 wt%) steel in a high-carbon activity, simulated anode gas at 600 ◦ C. The main emphasis was put on the degradation behavior and the mi- crostructural stability during exposure at 600 ◦ C. Additionally the effect of commonly used nickel contacting on the behavior of the in- terconnect in carbon-containing simulated anode gas was addressed. z E-mail: l.niewolak@fz-juelich.de; j.quadakkers@fz-juelich.de For this purpose a model system consisting of nickel coated inter- connects steels were investigated under the same conditions as the non-coated steels. Possible implications of the experimental findings for the materials behavior under real operating conditions will be discussed. Experimental The investigations included three ferritic steels with Cr contents varying between 18 and 22 wt% and an austenitic steel with a Cr content of approximately 25 wt%. The chemical compositions of the studied alloys as determined by ICP-OES are shown in Table I. All steels prevailed in form of sheets with a thickness of 0.4–2 mm. Reasons for selection of these steels as potentially suit- able interconnect materials for 600 ◦ C applications were given in Reference 18. For the oxidation tests samples with nominal dimensions of 10 × 20 mm were cut from the prevailing sheets. All samples were subsequently ground with SiC abrasive papers down to 1200 grit surface finish. For all experimental conditions two samples of each steel were used. The first set of samples was exposed in the as ground condition. The second set of specimens was ground and subsequently electroplated with a nickel layer of approximately 5–10 μm thickness using a so called Watt’s bath. 19 Prior to elec- troplating the samples were etched for 30 s in 0.2 M HCl. The etch- ing step removes the passive oxide layer present on the steels and thus ensures better adhesion between substrate and coating. The Ni- layer was applied to simulate the reactions which may occur between the commonly used Ni-current collector and the interconnect in an SOFC. 20,21 The coated and non-coated specimens were exposed in separate, isothermal 300 h test runs in simulated SOFC anode gas Ar – 9.2%CO – 3.7%H 2 – 0.2%H 2 O (vol.%) at 600 ◦ C. The composition of this test gas (in the following designated as SAG) was selected to simulate the high carbon and low oxygen activity commonly prevailing in the anode side environment of a SOFC if reformed hydrocarbons are being used as fuel. Using an argon diluted gas was dictated by laboratory safety regulations with respect to maximum allowable CO and H 2 contents. The test gas differs in its chemical composition from that prevailing in a real SOFC anode environment, however, as will be shown later the equilibrium activities of carbon and oxygen are in both gases virtually the same. The oxidation experiments were conducted in a specially de- signed horizontal furnace facility. The reactor tube and billet container were made of sintered alumina (Alsint 99.7% Al 2 O 3 ) to avoid con- tamination of the specimen surfaces with volatile silicon containing species which may occur if reactor tubes made of silica glass or silica ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.94.249.180 Downloaded on 2014-04-29 to IP