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- Josefsson, Helena, 1978
(författare)
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Is water a crucial factor in the oxidation of FeCrAl alloys?
- 2005
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- FeCrAl alloys are much used at high temperatures because the formation of a dense and slow-growing alumina scale provides resistance towards oxidation and high temperature corrosion. While the FeCrAl alloys are primarily intended for temperatures above 950ºC, they are sometimes used at lower temperature, for example as supports in automotive catalysts. This study investigates the oxidation behaviour of the FeCrAl alloy Kanthal AF at 500-900ºC. In addition, the influence of water vapour on oxidation is studied. Alloy Kanthal AF was isothermally oxidized in dry O2 and in O2+40% H2O using tube furnaces. Thermobalance exposures were carried out to investigate oxidation kinetics. The oxide scale was investigated by XRD, SEM, EDX, AES, FIB and TEM techniques. Kanthal AF follows parabolic oxidation kinetics in dry O2 >700°C. The parabolic rate constant increases monotonically with temperature. There is no indication of a maximum kp value at 850-975°C as reported in the literature. At all temperatures the initial oxidation rate was higher in O2/H2O compared to dry O2. At 900°C a duplex scale forms in both environments. The bottom part of the scale is dense and consists of columnar α-Al2O3 grains and grows inward. In dry O2 the top layer consists of a mixture of MgAl2O4 and α-Al2O3. In O2 + H2O, the top layer consists of γ-Al2O3 containing some magnesium and iron. In both environments the scale contains a straight and narrow Cr-rich band which is situated between the top and the bottom layer. This band corresponds to the original sample surface. The stabilization of γ-Al2O3 in O2+H2O is attributed to the hydroxylation of the γ-Al2O3 surface. Accordingly, the initial acceleration of oxidation by H2O is attributed to the stabilization of the fast-growing γ-Al2O3. At 900°C RE-rich oxide protrusions form. Patches with thicker oxide (mainly consisting of γ-Al2O3) form around the Y-enriched oxide protrusions. FeCrAl forms α-Al2O3 already at 700°C. This is in contrast to the commonly held view that γ-Al2O3 forms below 950°C. The concentration of Fe and Cr in the scale was below 5at.%. The formation of α-Al2O3 is attributed to the presence of Cr2O3 in the initial oxide. It is argued that corundum nucleates on the isostructural Cr2O3. At 500 and 600°C significant amounts of Fe and Cr are present in the oxide, indicating a mixture of oxides. Water vapour does not influence the amount and distribution of Fe and Cr in the oxide scale.
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| 2. |
- Josefsson, Helena, 1978, et al.
(författare)
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Oxidation of FeCrAl alloys at 500-900 degrees C in dry O-2
- 2005
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Ingår i: MATERIALS AND CORROSION-WERKSTOFFE UND KORROSION 56 (11): 801-805 NOV 2005. - 0947-5117. ; 56(11), s. 801-805
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Tidskriftsartikel (refereegranskat)abstract
- This paper reports on the oxidation of a commercial FeCrAl alloy, Kanthal AF, in the temperature range 500-900 degrees C. The samples are exposed isothermally in dry oxygen for up to 72 h using a thermo-balance. In addition, 168 h exposures are carried out in a tube furnace. The exposed samples are investigated by grazing angle Xray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX), and auger electron spectroscopy (AES). The rate of oxidation increases with temperature, the kinetics being parabolic in the range 700-900 degrees C. At all exposure temperatures, most of the sample surface is covered by a thin smooth base oxide. In addition, RE-rich particles, with a typical size of 1-3 mu m form. At 800 and 900 degrees C patches of thick oxide appear, featuring needle-formed crystallites situated on top of the base oxide. The thick oxide usually forms around Y-rich oxide particles. The concentration of iron and chromium in the oxide decreases with increasing temperature. XRD proves the formation of alpha-Al2O3 already at 700 degrees C, The low temperature of formation of alpha-Al2O3 is attributed to the presence of chromium in the initial oxide. It is proposed that corundum nucleation is facilitated on a surface consisting of the isostructural escolaite, (Cr2O3). After exposure at 900 degrees C AES shows large amounts of Mg in the outer part of the oxide, MgAl2O4 being detected by XRD together with gamma- and alpha-Al2O3.
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| 3. |
- Josefsson, Helena, 1978, et al.
(författare)
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The Formation of Oxide Scale on FeCrAl at 900°C in dry O2 and O2 + 40% H2O
- 2005
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Ingår i: 206th ECS Meeting; Honolulu, HI; United States; 3 October 2004 through 8 October 2004. ; PV 2004-16, s. 56-66
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Konferensbidrag (refereegranskat)abstract
- Polished Kanthal AF samples were oxidized isothermally at 900°C in O2 and in O2+40% H2O for between 1 and 168 hours. The samples were analysed gravimetrically, by Grazing-Incidence X-ray diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray analysis and Auger Electron Spectroscopy profiling. Much of the surface is covered by a thin and smooth base oxide with Mg-rich nodules on top. In addition, patches of thick uneven oxide and RE-oxide protrusions are present on the surface. Exposure to O2+40% H2O results in significantly higher initial oxidation rate than dry O2. The presence of water vapour also influences oxide morphology. The thick oxide patches exhibit needle-like crystals in dry O2 while rounded oxide agglomerates dominate in O2/H2O. The chemical composition of the oxide was the same in O2 and in O2+40% H2O. α-Al 2O3, γ-Al2O3 and MgAl 2O4 form in both environments.
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