Papers by Keyword: Scale Growth Kinetics

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Authors: Shan Tang, Xi Tang, Z.D. Xiang
Abstract: The new experimental creep resistant ferritic steel of the 9Cr-3Co-3W type was oxidised at 650 °C in air and wet air. The oxidation kinetics was measured by intermittent weight measurement. The scales formed were analysed using techniques of XRD, SEM and EDS. The results showed that the oxidation rate was more than a magnitude faster in wet air than in air. The oxidation kinetics in air obeyed the parabolic rate law of oxidation only in a limited oxidation period of up to 1726 h whereas it did not follow any power rate law of oxidation in wet air. The steel cannot form a protective Cr2O3 scale either in air or in wet air at 650 °C. Instead, the scale formed in air consisted of an outer (Fe0.6Cr0.4)2O3 layer and an inner Cr-rich (Fe,Cr)2O3 layer containing Cr2O3 particles, but in wet air it consisted of an outer Fe3O4 layer and an inner (Fe,Cr)3O4 layer.
Authors: Xi Tang, Zhi Dong Xiang, H. Pan, Ze Xi Yuan, Run Wu, X. Chen
Abstract: This study was carried out to determine the minimum Al content needed to form an Al2O3 scale on creep resistant ferritic steels at 650 °C. Two steels differing mainly in Al content were oxidized in air at 650 °C for 3000 h. One of the steels contained 2.3 wt% Al and the other 1.9 wt% Al. Oxidation resistance of the two steels was also compared with that of the commercial P92 steel at the same temperature. The oxidation was monitored by weight gain measurement. XRD, SEM and EDS techniques were used to analyze the scale formed on the surface of the steels. For the steel containing 2.3 wt% Al, a continuous Al2O3 scale was observed after 3000 h of oxidation and growth of the scale was parabolic with an extremely low rate constant of 0.00058 mg cm-2 h-1/2. For the steel containing 1.9 wt% Al, however, only a non-protective scale was formed, which exhibited a layer structure that consisted of an outermost porous Fe2O3 layer, followed by a relatively dense intermixed Fe2O3 and FeCr2O4 inner layer and then by an internal oxidation layer containing voids, Al2O3 and un-reacted metal particles in addition to Fe and Cr oxides; growth of this type of non-protective scale followed the logarithmic kinetics Δmt = klln(αt + 1) for oxidation times up to 3000 h.
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