In the present study the effect of O_2 and H_2O content on the oxidation behaviour of the 9percent Cr steel P91 was studied in the temperature range 600-800 deg C. The oxidation rates under the various experimental conditions were determined by in-situ thermogravimetry. It is shown that in dry oxygen a protective scale growth occurs with an oxidation rate controlled by diffusion in the scale. In presence of water vapour, after an incubation period, the scales become non-protective, as a result of a change of the oxidation limiting process. The water vapour effect is especially apparent in the temperature range 600-700 deg C, wherease at higher temperatures hardly any effect was found. The destruction of the protective scale by water vapour does not only depend on H_2O content but also on the H_2O/O_2-ratio.
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