| 1. |
- Sand, Tommy, 1982, et al.
(författare)
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Effective Reduction of Chromium-oxy-hydroxide Evaporation from Ni-Base Alloy 690
- 2019
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 92:3-4, s. 259-279
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Tidskriftsartikel (refereegranskat)abstract
- The corrosion of the Ni-base alloy 690 (60Ni, 30Cr and 10Fe) in humidified air was studied at 500–800 °C, and the rate of CrO2(OH)2 volatilization was measured quantitatively as a function of exposure time using a denuder technique. Different gas velocities were employed in exposures with a maximum duration of 200 h. Corrosion morphology was investigated by SEM/EDX using BIB-milled cross sections. The rate of chromium volatilization increased with increasing temperature and gas velocity. The rate of volatilization decreased with exposure time. Two oxide scale morphologies were observed, depending on temperature and gas velocity. In the 500–700 °C range, the scale consisted of chromia-rich corundum-type oxide, while exposures with high gas velocities at 800 °C produced an entirely different type of scale that included a Ni-rich and Cr-poor cap layer. The latter scale morphology is suggested to result from extensive chromium depletion of the alloy substrate which triggers a new mode of oxidation involving formation of NiCr spinel oxide. Continued volatilization of CrO2(OH)2 causes the NiCr spinel to decompose into a Ni-rich oxide that forms a cap layer on the scale surface. This cap layer is very efficient in decreasing the rate of chromium volatilization, allowing the chromium levels in the substrate to recuperate. We show that volatilization of chromium (VI) from the alloy can be mitigated by an oxidation pre-treatment that allows the Ni-rich cap layer to form.
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| 2. |
- Sand, Tommy, 1982, et al.
(författare)
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Exploring the Effect of Silicon on the High Temperature Corrosion of Lean FeCrAl Alloys in Humid Air
- 2021
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 95:3-4, s. 221-238
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Tidskriftsartikel (refereegranskat)abstract
- A new approach to reduce the chromium and aluminium concentrations in FeCrAl alloys without significantly impairing corrosion resistance is to alloy with 1-2 wt.% silicon. This paper investigates the "silicon effect" on oxidation by comparing the oxidation behavior and scale microstructure of two FeCrAl alloys, one alloyed with silicon and the other not, in dry and wet air at 600 degrees C and 800 degrees C. Both alloys formed thin protective oxide scales and the Cr-evaporation rates were small. In wet air at 800 degrees C the Si-alloyed FeCrAl formed an oxide scale containing mullite and tridymite together with alpha- and gamma-alumina. It is suggested that the reported improvement of the corrosion resistance of Al- and Cr-lean FeCrAl's by silicon alloying is caused by the appearance of Si-rich phases in the scale.
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| 3. |
- Sand, Tommy, 1982, et al.
(författare)
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A Comparison of the Oxidation and Nitridation Properties of Selected Chromia- and Alumina-Forming Alloys at 800 degrees C
- 2022
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 98:1-2, s. 163-178
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Tidskriftsartikel (refereegranskat)abstract
- Three FeCrAl alloys and two chromia-formers (a stainless steel, and a Ni-base alloy) have been exposed in four environments (dry air, air + 20% H2O, 20% H-2 + 20% H2O + Ar and 95% N-2 + 5% H-2) for 168 h at 800 degrees C. The corroded samples were investigated by SEM/EDS, XRD and gravimetry, and the formation of CrO2(OH)(2)(g) was measured as a function of time using a denuder technique. The Fe-base alloy formed a Cr-rich protective oxide scale in dry air and wet air but suffered break-away oxidation in 20% H-2 + 20% H2O + Ar. In contrast, the Ni-base alloy suffered extensive NiO formation and internal oxidation in dry air and wet air but formed a protective chromia scale in 20% H-2 + 20% H2O. All three FeCrAl alloys formed protective alumina scales in dry air, wet air and 20% H-2 + 20% H2O + Ar. The FeCrAl alloy Kanthal APMT was severely nitrided in the 95% N-2 + 5% H-2 environment due to defects in the oxide scale associated with RE-rich inclusions which allowed nitrogen to enter the alloy. In contrast, the two Cr-lean FeCrAl alloys Kanthal EF101 and Kanthal EF100 did not suffer nitridation at all.
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| 4. |
- Sand, Tommy, 1982, et al.
(författare)
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Efficacy of an external chromia layer in reducing nitridation of high temperature alloys
- 2022
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 197
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Tidskriftsartikel (refereegranskat)abstract
- Six high temperature alloys have been exposed in N2/H2 environments at 900 °C. In order to study the efficacy of a chromia barrier layer against nitrogen ingress, experiments were performed in two environments having the same N2/H2 ratio but slightly different water content, chromia formation being spontaneous in one case only. The samples were evaluated by SEM/STEM/EDX, XRD, gravimetry and GD-OES. The presence of an external chromia scale reduced nitridation of the alloy by 50–95%. Furthermore, in the presence of a continuous alumina layer no nitridation of the alloy was detected.
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| 5. |
- Sand, Tommy, 1982
(författare)
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High Temperature Corrosion Behaviour of Ni-base Alloys and FeCrAl Alloys – The Influence of Water Vapour
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electricity production, transportation, and manufacturing industry are some of the largest sources of greenhouse gas emissions. In many cases, these processes are carried out at high temperature and energy efficiency is limited by material degradation, so-called ‘high-temperature corrosion’. Understanding material degradation at high temperature is of the utmost importance in making these processes more energy efficient, thereby reducing greenhouse gas emissions. One of the corrosive species at high temperature is water vapour, which is present in many industrial processes. Exposure of chromia-forming alloys in air that contains water vapour can result in the formation of volatile chromium-containing compounds from the chromia scale that protects the metal surfaces. This reduces the life times of the components and leads to the uncontrolled release of poisonous hexavalent chromium species. Strategies to mitigate the formation of volatile chromium-oxy-hydroxide species include: coating the metal surfaces; and adding alloying elements that promote the formation of an oxide scale that is less susceptible to the formation of volatile species. This thesis explores how Ni-base alloys and FeCrAl alloys, form oxide scales that exhibit low degrees of evaporation. The studies were carried out with a denuder technique to measure evaporation rates in the temperature range of 500-800 °C in an environment that consisted of air with 20-40 vol.% water. In-depth analyses of the formed oxide scales were performed using electron microscopy and X-ray techniques. The studied Ni-base alloy contained approximately 30 wt.% chromium, 60 wt.% nickel, and 10 wt.% iron, and formed a chromium-rich oxide scale in the studied environments. Under the most severe condition, i.e., 800 °C and a high gas velocity, the initially very high evaporation rate decreased rapidly with exposure time. Examination of the oxide scale after 200 hours showed that an essentially chromium free and nickel-rich oxide had formed as a result of extensive chromium depletion in the oxide and near-surface region of the alloy. It is concluded that the formation of a Ni-rich cap layer drastically reduces the evaporation rate, which leads to the recovery of chromium levels in the near-surface region of the substrate, thereby allowing a new chromia layer to form at the metal/oxide interface. The final oxide scale was shown to be highly resistant to both evaporation and further oxidation. One of the two studied FeCrAl alloys contained rather low concentrations of chromium (~12 wt.%) and aluminium (~4 wt.%), such that it had good structure stability, weldability, and formability. This alloy also contained about 1.3 wt.% silicon, to increase oxidation resistance. Under all the studied conditions (600-800 °C, dry and wet air), protective alumina scales were formed, showing very low levels of evaporation of chromium-oxy-hydroxides. From the exposure in wet air at 800 °C, a significantly high level of silicon was found in the oxide scale. GIXRDmeasurements indicated the presence of mullite and tridymite in the scale. The results described in thesis increase our knowledge of oxide formation on Ni-base alloys and FeCrAl alloys in wet air and can be used for guidance when selecting alloys in environments that cause the evaporation of chromium-oxy-hydroxides in the temperature range of 500-800 °C.
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| 6. |
- Sand, Tommy, 1982
(författare)
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High-temperature corrosion properties of chromia- and alumina-forming alloys
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electricity production, transportation, and manufacturing industry are some of the largest sources of greenhouse gas emissions. In many cases, these processes are carried out at high temperature and energy efficiency is limited by material degradation, so-called ‘high-temperature corrosion’. Understanding material degradation at high temperature is therefore crucial for making these processes more energy-efficient, thereby reducing greenhouse gas emissions. For an alloy to resist high-temperature corrosion, it must form a protective, slow-growing and adherent oxide scale on the metal surface. The type of oxide scale formed and how it evolves depend on the composition of the alloy and the operating conditions. Two common oxide scales formed on high-temperature alloys are chromia (Cr2O3) and alumina (Al2O3) scales. In this thesis, the formation and behaviour of these scales were studied. Focus was on two corrosion mechanisms: (i) how different scale microstructures are able to withstand the formation of volatile chromium-oxy-hydroxide; and (ii) how permeable the scales are to nitrogen. The study involved exposures of a wide variety of high-temperature alloys in environments with either high oxygen and water concentrations or high nitrogen concentration and low oxygen activity. In environments with high concentrations of oxygen and water, chromia-forming Ni-base alloys suffered extensive volatilization of chromium-oxy-hydroxide. The resulting chromium depletion of the alloy triggered nickel oxidation which, in turn, caused a NiO layer to form on top of the chromia scale. The NiO ‘cap-layer’ reduced chromium evaporation rate, resulting in a secondary chromia scale being established at the oxide/metal interface. Cr-containing alloys forming alumina scales, showed a very limited evaporation rate of chromium-oxy-hydroxide. In the nitriding environment, the ability of the scale to prevent nitridation was studied. Gravimetric and GD-OES analyses showed that the presence of a chromia scale decreased the nitridation by 50-95%. A beneficial effect was observed for a two-layered scale that contained both chromia and silica, as this gave a nitridation reduction at the higher limit of the interval, i.e., 83-95%. Furthermore, the study showed that alumina scales without macro defects completely block the ingress of nitrogen.
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| 7. |
- Talic, Belma, et al.
(författare)
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Correlating Oxygen Electrode Degradation to Cr Vaporization from Metallic Interconnects in Solid Oxide Cell Stacks
- 2023
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Ingår i: Journal of the Electrochemical Society. - 1945-7111 .- 0013-4651. ; 170:12
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Tidskriftsartikel (refereegranskat)abstract
- Symmetrical cells consisting of La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) oxygen electrodes screen printed on both sides of a Ce0.9Gd0.1O2-δ (CGO) electrolyte were tested at 800 °C while being held close to a piece of Crofer 22 APU alloy. The alloy was either just pre-oxidized or coated with MnCo2O4 and heat treated prior to the exposure test to elucidate the effects of different Cr vaporization rates. Degradation of the symmetrical cells was monitored by electrochemical impedance spectroscopy, and TOF-SIMS, SEM and EDX analysis were used to examine Cr deposition on the electrodes after the exposure. The results show that the degradation rate of the symmetrical cell is directly proportional to the concentration of gaseous Cr(VI)-species, which had been assessed in a previous experiment. The Cr vaporization rate from Crofer 22 APU with a dense MnCo2O4 coating was measured in moisturized air up with up to 40% H2O and found to be invariant with respect to the steam activity. The degradation rate of symmetrical cells was accelerated by humidity in the air, but, noteworthy, this was found also in the absence of a Cr source.
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| 8. |
- Yangui, Aymen, 1987, et al.
(författare)
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Phosphor Thermometry of Alumina-Forming High-Temperature Alloys Using Luminescent Rare-Earth Ions in YAG: Proof of Concept Using a Dispersion of Ce 3+ -Doped YAG Particles in a FeCrAl Alloy
- 2023
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Ingår i: ECS Journal of Solid State Science and Technology. - : The Electrochemical Society. - 2162-8777 .- 2162-8769. ; 12:3
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Tidskriftsartikel (refereegranskat)abstract
- Most high-temperature processes require monitoring and controlling temperature, preferably with high precision and good lateral resolution. Here we evaluate the use of the technique commonly known as phosphor thermometry, which exploits the temperature dependent photoluminescence from an inorganic phosphor, for the determination of the temperature of a composite material consisting of the metallic alloy FeCrAl dispersed with phosphor particles of yttrium aluminum garnet (Y3Al5O12, YAG) doped with a small amount of luminescent Ce3+ ions (YAG:Ce3+). The results show that with some optimization and by changing the dopant ion, YAG based phosphor particles offer a unique opportunity to measure the surface temperature of metal alloys with high precision and high lateral resolution, all the way up to the maximum working temperature of alumina-forming high temperature alloys at ca. 1300 °C.
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