Abnormal Growth Transport in Oxide Scales on Fe-16Cr Steels in Water Vapor
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itic steels are considered to be candidates as interconnects for solid oxide fuel cells.[1–4] One issue associated with the use of steels in this application, however, is the high electrical resistance that arises from the growth of an oxide scale on the alloys during hightemperature exposure.[5,6] Therefore, it is important to clarify the oxide scale formation and oxidation mechanism. In recent years, a large number of studies have reported that the presence of water vapor in the exposure environment can accelerate the oxidation rate of steels when compared with a dry environment.[7,8] However, there is no general agreement concerning the growth mechanism for oxidation of steels in water vapor. Several mechanisms have been proposed to explain the enhanced oxidation behavior in the presence of water vapor, e.g., the dissociation mechanisms,[9,10] the internal oxidation behavior in steam,[11,12] the hydrogen defect mechanism,[13] the catalytic oxidation mechanism facilitated by microcracks or microchannels,[14] and the evaporation of chromium volatile species.[15–17] It can be seen that the effect of water vapor on accelerating the oxidation of steels is quite complicated and not consistent. To clarify the oxidation mechanisms,
JIA GONG, B. DENG, C. ZHONG, and D.M. SUN, Ph.D. Candidates, and J. LI and Y.M. JIANG, Professors, are with the Department of Material Science, Fudan University, Shanghai 200433, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted April 1, 2009. Article published online September 3, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
we used H218O/H216O isotopic labeling, and this method using labeled oxidations is known to be the most powerful for determining the direction of mass transport in thin oxides.[18] This article reports the oxidation behavior of the SUS 430 stainless steel in N2-12 vol pct H2O gas mixtures at 850 °C. It was found that the oxide scales grow not only by cation transport to the scale/gas interface, but also by inward transport of water molecules. The material selected for experiments was 430SS. The chemical composition is listed in Table I. The samples for the oxidation experiments cut from plate in the form of coupons with a thickness of 1.0 to 1.3 mm and surface area of 10 9 15 mm were mechanically abraded with silicon carbide papers (up to 2000grit number). After polishing with diamond paste to a 1.5-lm finish, the samples with mirrorlike surfaces were ultrasonically cleaned with distilled water and acetone. The oxidation exposures were carried out at 850 °C in N2-12 vol pct H2O gas mixtures. For analysis of the scale growth mechanisms, a two-stage oxidation experiment was carried out. In this experiment, the samples were oxidized for 100 minutes in N2-12 vol pct H216O; the second stage of oxidation was continued for 100 minutes in N2-12 vol pct H218O gas mixtures. In these tests, nitrogen was bubbled through a glass container containing H2O/H218O at 48 °C. The total flow rate was fixed to ensure laminar flow over the samples at a velocity o
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