Abstract
Abstract More attention has been paid to the exfoliation of oxide scale on high-temperature heating surface of utility boiler. The oxidation mechanism of HCM12A steel in supercritical water is proposed and the growth of oxide film is simulated. The duplex scale contains an outer magnetite layer and an inner Cr-rich spinel layer. According to the data of Backhaus and Töpfer, the diffusion coefficient values of iron in magnetite layer are discussed and the function of R V, R I {R}_{\text{I}} for oxygen activity can be used for calculation of iron diffusion coefficients in Cr-rich spinel layer. Based on Wagner’s oxidation theory, the oxidation rate constants of HCM12A are calculated at 500 and 600°C in supercritical water, compared with experimental data of the relevant literatures. The oxygen activities at the interfaces of alloy/Cr-rich spinel oxide and magnetite/supercritical water are estimated. The simulation results of weight gain are matched with the test data. The iron diffusion mechanisms inside the magnetite layer and the Cr-rich spinel layer are analyzed. The iron diffusion coefficient at the interface of Cr-rich spinel/magnetite is discontinuous, while the oxygen activity is continuous in the whole double layer. The thickness of oxide scale on inner tube walls of the final superheater coils (T91) of a 600 MW supercritical boiler is calculated by using the calculation method provided by the paper. The modeling results, the measured data, and the calculation results by the method are compared. Accurate calculation of the thickness of the inner and outer oxide scales can provide a necessary basis for predicting the stress and exfoliation of oxide scales.
Highlights
More attention has been paid to the exfoliation of oxide scale on high-temperature heating surface of utility boiler
Based on Wagner oxidation theory, the oxidation rate constants of HCM12A are calculated at 500 and 600°C in supercritical water, and the weight gain of simulation is compared with experimental results in Bischoff et al [5,13]
Known from the literatures [2,3,4,5,6,7,8], a multilayer oxide scale is formed on ferritic–martensitic steels in supercritical water
Summary
Abstract: More attention has been paid to the exfoliation of oxide scale on high-temperature heating surface of utility boiler. It is generally believed that the reaction of supercritical water with ferrite–martensitic steels results in the formation of double-layer oxide scale, the outer layer of magnetite and the inner layer of Cr-rich spinel with the same thickness. The corrosion kinetics of the two alloys are similar, and the corrosion rate in supercritical water is greatly higher than that in steam In both environments, a double oxide structure is formed on the two steels [5]. The oxidation mechanism of HCM12A steel under different environments is presented, the iron diffusion coefficient values in Fe3O4 and Fe2CrO4 are discussed, and the oxygen activities at the interfaces of steel/Cr-rich spinel oxide and magnetite/ supercritical water are estimated. Based on Wagner oxidation theory, the oxidation rate constants of HCM12A (its chemical compositions are listed in Table 1) are calculated at 500 and 600°C in supercritical water, and the weight gain of simulation is compared with experimental results in Bischoff et al [5,13]
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