Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in H2O,Oxide-Growth Mechanisms,and Scale Adherence |
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| Authors: | Hultquist Gunnar Tveten Bent Hörnlund Erik |
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| Affiliation: | (1) Department of Materials Science and Engineering, Division of Corrosion Science, Royal Institute of Technology, S-100 44 Stockholm, Sweden;(2) Centre for Materials Science, University of Oslo, Gaustadalléen 21, N-0371 Oslo, Norway |
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| Abstract: | Oxidation of 1-mm thick chromium samples with 7 to <1 wt. ppm hydrogenhas been studied at 900°C in a closed reaction chamber. The gases usedwere O2, H2O, and gas mixtures of O2+H2Oat a total pressure of about 20 mbar. By means of oxygen labeling intwo-stage oxidations, H216O followed by H218O,the position of oxide growth within the oxide scale was determined withsecondary-ion mass spectrometry (SIMS). The results are consistent withvisual observations and scanning electron microscopy (SEM), which showsflat oxides with good adherence when formed in H2O. Whenreplacing O2 with H2O in the atmosphere, the oxidationrate increases by a factor of two with an increase of oxide growth at boththe inner and outer part of the oxide. An increased metal (cation) transportis observed when 5 wt. ppm hydrogen is present in the chromium metal priorto oxidation in both O2 and H2O. This is detrimentalfor the adherence of the oxide scale. The uptake of hydrogen in H2Oexposure for 1600 min was measured to 1.5 wt. ppm. In exposures toO2+H2O mixtures, no H2 is formed and no netwater is consumed as long as O2 is present. A plausible reactionscheme based on an experiment with H218O is presentedto explain this observation. |
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| Keywords: | chromium hydrogen content two-stage oxidation scale adherence |
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