| 1. |
- Canovic, Sead, 1979, et al.
(författare)
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Microstructural Investigation of the Initial Oxidation of the FeCrAlRE Alloy Kanthal AF in Dry and Wet O-2 at 600 and 800 degrees C
- 2010
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 1945-7111 .- 0013-4651. ; 157:6, s. C223-C230
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Tidskriftsartikel (refereegranskat)abstract
- The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and 800 degrees C for 72 h in dry O-2 and in O-2 with 10 vol % H2O. The mass gains were 3-5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at 800 degrees C, while no significant effect was observed at 600 degrees C. A thin two-layered oxide formed at 600 degrees C: an outer (Fe, Cr)(2)O-3 corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At 800 degrees C a two-layered oxide formed in both environments. The inner layer consisted of inward grown alpha-Al2O3. In dry O-2 the originally formed outward grown gamma-Al2O3 had transformed to alpha-Al2O3 after 72 h. Water vapor stabilized the outward grown gamma-Al2O3 and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At 800 degrees C the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid O-2.
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| 2. |
- Engkvist, Josefin, 1972, et al.
(författare)
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Oxidation of FeCrAl foils at 500-900 degrees C in dry O-2 and O-2 with 40% H2O
- 2009
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Ingår i: Materials at High Temperatures. - 0960-3409 .- 1878-6413. ; 26:2, s. 199-210
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Tidskriftsartikel (refereegranskat)abstract
- High temperature resistant FeCrAl alloys are frequently used in high temperature applications Such as heating elements and metal based catalytic converter bodies. When exposed to high temperatures an adherent, slowly growing, dense aluminium oxide layer forms on the surface, which protects the underlying alloy from severe degradation. The composition, structure and properties of the formed oxide layer are strongly dependent on the alloy composition, temperature and oxidation environment. In this study, the Sandvik 0C404 FeCrAl alloy, in the form of 50 mu m thick foils, was exposed isothermally in the temperature range 500-900 degrees C for 168 hours in dry O-2 and in O-2 with 40 vol.% H2O. The surface morphology, composition and microstructure of the grown oxide scales were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), grazing incidence X-ray diffraction (GI-XRD), Auger electron spectroscopy (AES), and time of flight secondary ion mass spectrometry (TOF-SIMS). The oxidation process was faster at 900 degrees C than at 500 and 700 degrees C. At 500 degrees C a thin (10-20 nm) mixed oxide of Fe, Cr and Al was formed. Exposure at 700 degrees C resulted ill a similar (40-50 nm) duplex oxide, in both dry O-2 and in O-2 With 40 vol.% H2O. These oxide scales consisted of all inner and an outer relatively pure alumina separated by a Cr-rich band. This type of duplex oxide scale also formed at 900 degrees C with a thin inward growing alpha-Al2O3 at the oxide/metal interface and an outward growing layer outside a Cr-rich band. However, at 900 degrees C the Outward growing layer showed two types of oxide morphologies; a thin smooth base oxide and a much thicker nodular oxide grown on top of substrate ridges. In dry O-2 atmosphere, the main part of this outward growing layer had transformed to alpha-Al2O3. Only in the outer part of the thick oxide nodules, metastable alumina was found. When exposed in the presence of water vapour the main pall of the metastable alumina remained untransformed.
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| 3. |
- Götlind, Helena, 1978, et al.
(författare)
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The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF at 900 degrees C
- 2007
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Ingår i: Oxidation of Metals. - 1573-4889 .- 0030-770X. ; , s. 251-266
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Tidskriftsartikel (refereegranskat)abstract
- The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF is reported. Polished samples were exposed isothermally at 900 degrees C for 1, 24, 72 and 168 h in a well-controlled environment consisting of dry O-2 or O-2 + 40% H2O. The samples were investigated using a combination of gravimetry and several surface-analytical techniques, including XRD, SEM, EDX, FIB, AES and TEM. The presence of water vapor significantly accelerates oxidation during the first 72 h. A two-layered oxide forms in both the dry and wet environments. The bottom layer consists of inward-growing alpha-Al2O3 while the outer layer initially consists of outward-growing gamma-Al2O3. A straight and narrow Cr-enriched band is present at the top of the lower (alpha-Al2O3) oxide, corresponding to the original sample surface. In dry O-2, the top (gamma-Al2O3) layer is converted into a mixture of gamma-Al2-x (Mg,Fe) (x) O-3-(x/2),O- MgAl2O4 and alpha-Al2O3. This transformation does not occur in O-2 + H2O. The initial acceleration of oxidation by H2O is attributed to the stabilization of the outward-growing gamma-alumina layer by the hydroxylation of the gamma-Al2O3 surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.
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| 4. |
- Jonsson, Torbjörn, 1970, et al.
(författare)
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Influence of H2O(g) on the Oxide Microstructure of the Stainless Steel 353MA at 900°C in Oxygen.
- 2007
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Ingår i: Journal of the Electrochemical Society. - : The Electrochemical Society. - 1945-7111 .- 0013-4651. ; 154:11, s. C603-C610
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates the impact of water vapor on the corrosion behavior of the austenitic Si-containing FeCrNi steel 353MA at 900°C through a detailed microstructural characterization of the oxide scales formed after 168 h in O2 and in O2 with 40% H2 O. The oxidized samples were investigated by focused ion beam and transmission electron microscopy in combination with energy dispersive X-ray analysis. The microstructural investigation showed that the oxide scales were affected by the presence of water vapor. However, there was no significant difference in scale thickness. In both atmospheres a continuous chromia [Cr-rich (Fe,Cr)2 O3] layer was present beneath the spinel oxides. The influence of water vapor on scale composition is attributed to chromia evaporation by the formation of Cr O2 (OH)2 (g). The ability of the alloy to maintain a continuous chromia layer in spite of chromia evaporation and to avoid breakaway oxidation is attributed to several factors. First, the supply of chromium to the scale by diffusion in the alloy must be rapid. Second, the presence of spinel oxides at the oxide/gas interface may decrease chromia evaporation. Third, the high CrFe ratio in the alloy is suggested to make it difficult to convert the protective chromia to poorly protective hematite. © 2007 The Electrochemical Society.
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| 5. |
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| 6. |
- Jonsson, Torbjörn, 1970, et al.
(författare)
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Oxidation of Fe-10Cr in O-2 and in O-2+H2O environment at 600 degrees C: A microstructural investigation
- 2013
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 75, s. 326-336
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Tidskriftsartikel (refereegranskat)abstract
- Oxidation of Fe-10Cr in dry and wet O-2 was studied at 600 degrees C for up to 168 h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O-2 gives a Cr-rich protective (Fe1-xCrx)(2)O-3 scale. The same protective oxide initially forms in O-2 + H2O environment, but after an incubation period scale breakdown is triggered by CrO2(OH)(2) evaporation that depletes the substrate in Cr and converts (Fe1-xCrx)(2)O-3 to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.
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| 7. |
- 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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| 8. |
- 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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| 9. |
- Liu, Fang, 1975, et al.
(författare)
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Cyclic oxidation of two FeCrAlRE foils at 1100 oC - The influence of the concentration of minor alloying elements on scale microstructure
- 2008
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Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 595-598, s. 707-716
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Konferensbidrag (refereegranskat)abstract
- Two FeCrAlRE alloys, a commercial, 0C404, and a model alloy in the form of thin foils,with different Mn, Nb, Mo and Ti concentrations were subjected to cyclic oxidation in lab air at1100°C. The oxidized samples were studied by gravimetry, Grazing-Incidence X-ray Diffraction(GI-XRD), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-ray(EDX) analysis. The two FeCrAl alloys exhibit different oxidation kinetics; however, both alloyshave the same weight gain after 500 hours exposure. During the early stages the scale consistsmainly of α-Al2O3 together with some oxide particles containing Mn, Al, Fe and Cr formed on thealloys. After 500 hours the 0C404 scale locally also consists of larger polycrystalline regions ofMn-Cr-Al spinel. In addition, Si-rich oxide, chromia and Al-Cr oxide could be observed at themetal/oxide interface.
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| 10. |
- Liu, Fang, 1975, et al.
(författare)
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Early stages of the oxidation of a FeCrAlRE alloy (Kanthal AF) at 900 °C: A detailed microstructural investigation
- 2008
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 50:8, s. 2272-2281
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Tidskriftsartikel (refereegranskat)abstract
- Early stages of the evolution of Al2O3 scales formed on a FeCrAlRE alloy (Kanthal AF) have been investigatedby analytical TEM. The samples were oxidized isothermally at 900 °C in dry O2 or O2 + 40% H2O for1 h or 24 h. All oxide scales exhibited a two-layered structure, with a continuous inward growing a-Al2O3inner layer and an outward growing outer layer. After 1 h, the outer oxide layer consisted of c-Al2O3 inboth environments. After 24 h exposure in dry O2, the c-Al2O3 in the outer oxide layer was partly transformedto a-Al2O3 and spinel oxide (Mg1-xFexAl2O4). In contrast, the c-Al2O3 in the outer layer was nottransformed after 24 h in O2 + 40% H2O, showing that water vapour stabilizes c-Al2O3. All oxide scalescontained a Cr-rich band, a product of the initial oxidation. The inner a-Al2O3 layer is suggested to nucleateon Cr2O3 or Cr2-xFexO3 in the initial oxide.
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