Effect of Specimen Thickness on the Oxidation Rate of High Chromium Ferritic Steels: The Significance of Intrinsic Alloy
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Effect of Specimen Thickness on the Oxidation Rate of High Chromium Ferritic Steels: The Significance of Intrinsic Alloy Creep Strength Cristina Asensio-Jimenez • Leszek Niewolak • Heike Hattendorf • Bernd Kuhn • Pawel Huczkowski Lorenz Singheiser • Willem Joseph Quadakkers
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Received: 18 July 2012 / Published online: 24 December 2012 Ó Springer Science+Business Media New York 2012
Abstract Previous studies revealed that initial sample thickness affects the growth rate of oxide scales formed during 800 or 900 °C air exposure. The effect is partially related to differences in depletion of minor alloying additions such as Mn, Ti, La in thick and thin specimens. However, it has previously been proposed that the specimen thickness dependence is partially governed by differences in creep strength of thick and thin substrates. To investigate this hypothesis, discontinuous air oxidation experiments were carried out with the Laves phase strengthened ferritic steel Crofer 22 H at 800 °C. Comparing the data for solution annealed and preaged (500 h, 900 °C) materials it could be shown that intrinsic creep strength of the alloy substantially affects oxidation rates. The observations can qualitatively be explained by assuming the relaxation of oxide growth stresses by plastic deformation of the metallic substrate to be an important parameter affecting the kinetics of oxide scale growth. Keywords
Ferritic steel Creep strength Oxidation rate Laves phase
Introduction High chromium ferritic steels are considered as construction materials for interconnects in solid oxide fuel cells (SOFC) [1–4]. The first group of high-chromium ferritic C. Asensio-Jimenez L. Niewolak (&) B. Kuhn P. Huczkowski L. Singheiser W. J. Quadakkers Research Centre Ju¨lich, Institute of Energy and Climate Research IEK-2, 52425 Ju¨lich, Germany e-mail: [email protected] W. J. Quadakkers e-mail: [email protected] H. Hattendorf ThyssenKrupp/VDM, 58762 Altena, Germany
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Oxid Met (2013) 79:15–28
steels, such as Crofer 22 APU [5, 6] which were especially designed for SOFC applications, was optimized mainly with respect to low oxidation rates, high electrical conductivity of the surface oxide scales, reduced Cr evaporation and improved workability. This could be achieved by specifying steel compositions with careful control of minor alloying additions and impurities. More specifically, the concentrations of the commonly used de-oxidants aluminium and silicon were specified at very low levels, beneath 0.1 %. In this way, the formation of electrically insulating sub layers of alumina and/or silica beneath the surface scales (consisting of chromia and outer Cr/Mn spinel) during service at temperatures in the range 600–800 °C could be prevented [3, 7]. Also the adverse effect of the internal silica and alumina on the external scale growth due to formation of outwardly protruding metallic nodules was suppressed by the very low aluminium and silicon contents [3, 7, 8]. In Ref. [8, 9] it was found that the air oxidation rates of
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