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The oxidation behavior of Fe-16Cr steels in N^sub 2^-12 vol pet H2O was studied at 850 °C. The oxide scale was compact and had excellent adhesion to the substrate; moreover, there were three layers of different compositions existing in the scale. To gain an insight into the transport mechanism, two-stage oxidation was carried out in N^sub 2^-12 vol pet H^sub 2^^sup 16^O and followed in N^sub 2^-12 vol pet H^sub 2^^sup 18^O gas mixtures. The oxygen isotope profiles in oxide scales were determined by secondary ion mass spectrometry. The results showed that oxidation in water vapor proceeded by outward chromium transport, especially, the oxidation involved inward transport of water molecules.
DOI: 10.1007/s11661-009-9986-y
© The Minerals, Metals & Materials Society and ASM International 2009
Ferritic 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 high- temperature exposure.'xf>1 Therefore, it is important to clarify the oxide scale formation and oxidation mech- anism. 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.'7'81 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,19, lul the internal oxidation behavior in steam,'11·12' the hydrogen defect mechanism.1'31 the catalytic oxidation mechanism facilitated by microcracks or microchannels."4' and the evaporation of chromium volatile species.115 I7' 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, we used H^sub 2^^sup 18^O/H^sub 2^^sup 16^O isotopie labeling, and this method using labeled oxidations is known to be the most powerful for determining the direction of mass transport in thin oxides.'181 This article reports the oxidation behavior ofthe SUS 430 stainless steel in N2-12 vol pel HiO gas mixtures at 850 0C. It was found that the oxide scales grow not only by...