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| 2. |
- Hansson, Kristina, 1971, et al.
(författare)
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On the Mechanism of MoSi2 Pesting in the Temperature Range 400–500°C
- 2000
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Ingår i: Ceramic Engineering and Science Proceedings. - 0196-6219 .- 1940-6339. ; 21:4, s. 469-476
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of a MoSi2 composite was studied in the temperature range 400–500°C. The peak pesting temperature was identified and the oxidation kinetics was observed up to a holding time of 4000 hours. The reaction kinetics was tracked using thermogravimetric analysis :IS well iis oxide thickness nieasurenients. A detailed analysis of the morphology and composition of the oxide was performed using SEM and EDX. The peak pesting temperature was found to be 470°C. The reaction kinetics in static air could be described with two consecutive parabolic oxidation curves, one up to 500 hours and another between 500–4000 hours. With a higher water vapor coiitent the kinetics showed linear behwior. MoO3 evaporation took place during the entire duration of the oxidation. The oxide appeared to grow by inward diffusion of oxygen to the oxidehulk interface.
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| 3. |
- Hansson, Kristina, 1971, et al.
(författare)
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Oxidation behaviour of a MoSi2-based composite in different atmospheres in the low temperature range (400–550 °C)
- 2004
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 1873-619X .- 0955-2219. ; 24:13, s. 3559-3573
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation characteristics of a MoSi2-based composite within the temperature range of 400–550 °C were investigated. The effects of temperature and water vapour on oxidation were examined. The oxidation kinetics were studied using a thermobalance, while the morphology and composition of the oxides were examined using XRD, ESEM/EDX, and SEM/EDX.The peak oxidation rates in dry O2 and View the MathML source were found to occur at temperatures of approximately 510 and 470 °C, respectively. Within the temperature range of accelerated oxidation (400–500 °C), the oxidation rate in View the MathML source was substantially higher than that in dry O2. At higher temperatures, the oxidation rate decreased, and the magnitude of the decrease was steeper and occurred at a lower temperature for View the MathML source (510 °C) than for O2 (550 °C). Furthermore, the rate of depletion of molybdenum (Mo) from the oxide scales during oxidation increased with increasing temperature and water vapour content. It appears that Mo loss is a key process influencing the protective properties of the oxide layer on the MoSi2 composite. A potential mechanism for the different oxidation behaviours in O2 and View the MathML source is proposed.
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| 4. |
- Hansson, Kristina, 1971, et al.
(författare)
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The beneficial effect of water vapour on the oxidation at 600 and 700 °C of a MoSi2-based composite
- 2005
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Ingår i: JOURNAL OF THE EUROPEAN CERAMIC SOCIETY. ; 25:1, s. 1-11
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation characteristics of a MoSi2-based composite in O2 and O2+10% H2O at 600 and 700 °C were investigated. The effects of temperature and water vapour on oxidation were examined. The oxidation kinetics were studied using a thermobalance and furnace exposure, while the morphologies and compositions of the oxides were examined using XRD, ESEM/EDX, and SEM/EDX. We propose that oxidation proceeds by the initial formation of MoO3 crystals and amorphous SiO2 on the surface. The MoO3 is then evaporated; as volatile (MoO3)3 species in O2 and additional MoO2(OH)2 species in O2+10% H2O, which results in a porous, Mo-depleted oxide. However, the pores in the Mo-depleted SiO2 scale heal, and a protective crystalline scale is established eventually. The vapour pressures of the abovementioned volatile species increase with temperature and/or water vapour content in the atmosphere, which leads to accelerated Mo depletion from the oxide scale. A shorter time elapses before the oxide scale is transformed into the relatively Mo-free protective SiO2 scale, which results in less oxide being formed. Thus, the formed scale becomes thinner in O2+10% H2O than in O2. Thereby the Mo removal is beneficial when water vapour is added to the exposure atmosphere.
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| 5. |
- Hansson, Kristina, 1971, et al.
(författare)
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The influence of water vapor on the oxidation of MoSi2 at 450 °C
- 2001
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Ingår i: Materials Science Forum. - 1662-9752 .- 0255-5476. ; 369-372, s. 419-426
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of a MoSi2 composite was studied in dry air, oxygen and oxygensaturated with 10% water vapour at 450°C. The kinetics were investigated using TGA as well as oxide thickness measurements. Detailed analyses were performed on the morphology and composition of the oxide using XRD, ESEM, SEM, and EDX. It is shown that the oxidation rate increases drastically in the presence of water vapour, and the growth of Mo03 crystals on the oxide surface increases considerably. The different regions in the oxide cross-section are Mo-depleted compared with the corresponding regions in the bulk when oxidised in oxygen saturated with 10% water vapour. However, the samples oxidised in dry oxygen only shows Mo-depletion in some outer parts of the oxide. Accelerated growth of the MoSi2-oxide layer during exposure in 02+10%H20 compared to that in 02 can be related to the fact that more volatile Mo-species form in the presence of water vapour, resulting in a substantial loss of Mo03 from the inner part of the oxide. The voids left behind are not healed by the silica at this low temperature, which leaves the oxide with an open structure. As a result, the oxidation rate increases.
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| 6. |
- Hellström, Kristina M, 1971, et al.
(författare)
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Oxidation behaviour of a (Mo, W)Si2-based composite in dry and wet oxygen atmospheres in the temperature range 350–950◦C
- 2009
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Ingår i: Journal of the European Ceramic Society. - : Elsevier BV. - 1873-619X .- 0955-2219. ; 29:10, s. 2105-2118
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of a (Mo, W)Si2-based composite was investigated in the temperature range (350–950 °C). The influence of temperature and water vapour on the oxidation was examined. The kinetics was studied using a thermobalance whereas the morphology and composition of the oxides were examined using X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive X-ray (EDX). Focused ion beam (FIB) milling was performed on some of the oxide scales which allowed us to look at a non-mechanically disturbed scale/oxide in cross-section. Rapid oxidation was found to occur in the 550–750 °C temperature range. The mass gains were significantly larger in O2 than in O2 + 10%H2O. The different mass changes in the two exposure atmospheres were attributed to the higher vapour pressure of the volatile MoO2(OH)2 and WO2(OH)2 species in O2 + 10%H2O than that of (MoO3)3 and (WO3)3 in dry O2. The peak mass gain was found to occur at a temperature of about 750 °C in O2 and 650 °C in O2 + 10%H2O. At temperatures above 850 °C, especially when water vapour is present, the removal of Mo and W from the oxide scales is rapid enough to allow partial healing of the silica, causing the oxidation rate to drop. At 950 °C in O2 + 10%H2O, a protective SiO2 scale could be re-established quickly and maintained, causing the oxidation to essentially cease.
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| 7. |
- Pompe, Robert, et al.
(författare)
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Evaluation of ceramic sacrificial coatings for improved alkaliinduced corrosion protection in biofuel-fired boilers
- 2013
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Ingår i: NACE - International Corrosion Conference Series. - 0361-4409. - 9781627481458 ; , s. (9 pp)-
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Konferensbidrag (refereegranskat)abstract
- In the combustion of bio-based fuels the critically exposed burner parts in small boilers are typically uncooled and are usually made of FeCrNi alloys. These materials can suffer attack from the ashes because of the formation of alkali chromate. The reaction depletes the protective oxide in chromia, leading to accelerated corrosion. Selected "acidic" ceramic coatings based on Ti, Si, B and P have been evaluated for use as sacrificial layers to prevent the initial reaction of alkali and chromium. An accelerated oxidation test method including mild thermal shock has been utilized that previously proved useful to provide application-relevant results. A comparison of coated and uncoated specimens was performed with an austenitic high temperature steel as a substrate. The results indicate that the alkali released from the ash reacts with the respective "acidic" elements in the deposited coatings. This reaction has promoted initial formation of a thin and continuous chromium-rich protective oxide sub-layer. In addition, the oxide scale formed on the coated specimens appeared more coherent and crack-free. A significant enrichment of Ni at the steel-oxide interface also occurred that can promote high temperature corrosion resistance.
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| 8. |
- Pompe, Robert, et al.
(författare)
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How to screen high temperature steels for optimal use in biofuel-heated burners for residential boilers: Role of alkali
- 2011
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Ingår i: NACE - International Corrosion Conference Series. - 0361-4409. ; , s. 12p-
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Konferensbidrag (refereegranskat)abstract
- Hot parts of burners for residential boilers are subject to irregular heating cycles up to 800 °C and higher, simultaneously exposed to alkali salts, variable oxygen partial pressure - and thermal shock. Fundamental hot corrosion studies under well-controlled conditions, in combination with simple ranking tests simulating the effect of ash deposited on the alloy surface, have been performed. The investigation included four types of high-alloyed steel and two types of ash from wood and bark pellets. The work was complemented with field studies of selected damaged parts. Two mechanisms involving interaction of potassium as well as carbon under intermittent oxygen-deficient conditions with chromium from the alloys have been corroborated. A simple ranking test procedure has also been designed. It proved relevant as a screening method only after incorporating a mild thermal shock in the heat treatment sequence. A significant difference in corrosive effect, particularly on a lower chromium-alloy like 304 L, has been observed for the ash derived from the wood and bark pellets, respectively.
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| 9. |
- Tang, Jun Eu, 1974, et al.
(författare)
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An Investigation of the Microstructure in the Pest Oxide of a MoSi2- Based Composite
- 2000
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Ingår i: Ceramic Engineering and Science Proceedings. - Hoboken, NJ, USA : John Wiley & Sons, Inc.. - 0196-6219 .- 1940-6339. ; 21:4, s. 477-484
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Tidskriftsartikel (refereegranskat)abstract
- The pesting of MoSi2 was investigated by performing detailed microanalysis on the pest oxide layer. The studied material was a MoSi2-Mo5i3-clay composite which had undergone pest-oxidation for 4000 hours at 450°C. Results from detailed SEM and TEM studies, including quantitative EDX analysis, on the various features in the pest oxide are presented. It was found that MoSi2, as well as the MosSi3, transforms immediately into an oxide mixture of MoO3 nanocrystals and amorphous SiO2 with significant loss in molybdenum upon oxidation. With time, part of the oxide mixture cluster into lamellar MoO3 aggregates. These aggregates disappear from the oxide after even longer times, leaving voids in the oxide structure. This allows even quicker depletion of MoO3 from the oxide, leaving dark grey regions containing mostly SO2.
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| 10. |
- Tang, Jun Eu, 1974, et al.
(författare)
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Microanalysis on the oxidation and sulfate attack of partially stabilized zirconia thermal barrier coating
- 2001
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Ingår i: Ceramic Engineering and Science Proceedings. - 0196-6219 .- 1940-6339. ; 22:4, s. 463-470
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Tidskriftsartikel (refereegranskat)abstract
- This aim of this investigation is to study the effect of the presence of Na2SO4 deposits and water vapor on the oxidation of an air plasma-sprayed TBC composed of a partially stabilized ZrO2 top coat with an underlying NiCoCrAlY bond coat. XRD and SEM/EDX were used to analyze the changes in the coatings after oxidation at 1000degreesC for 72 hours in dry or humidified (containing 50% H2O) O-2 atmosphere, with or without Na2SO4 deposited on the top coat. When oxidized in oxygen, bond coat oxide regions, consisting of almost pure aluminum oxide, were formed at the top coat / bond coat and bond coat / substrate interfaces and in the bond coat around the splat lines. When water vapor was present, the top coat / bond coat interface oxide was marginally thicker and included small regions with more chromium, cobalt and nickel. The addition of the salt deposits resulted in some destabilization in the outermost regions of the top coat. The salt deposits also caused the formation of thicker oxide comprising two regions, though this formation was observed only at the top coat / bond coat interface. The first type was a thin inner (i.e. bordering the bond coat) oxide that was mostly aluminum oxide. The other was a much thicker oxide containing higher levels of chromium, cobalt and nickel along with aluminum. In this oxide region, the aluminum level was higher when the chromium level was lower and vice versa.
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