| 1. |
- Alnegren, Patrik, 1988, et al.
(författare)
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Degradation of ferritic stainless steels under conditions used for solid oxide fuel cells and electrolyzers at varying oxygen pressures
- 2016
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 110, s. 200-212
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Tidskriftsartikel (refereegranskat)abstract
- Four commercial ferritic stainless steels were tested at 850 °C in oxygen pressures ranging from 10-4 to 1 atm, in order to investigate the isolated effect of oxygen pressure on corrosion, in the context of solid oxide electrolysis cells. The oxidation rates of all steels were essentially independent of oxygen partial pressure, which indicates n-type behavior. FIB/SEM analysis revealed that the grain size of the oxides was found to decrease at lower oxygen pressures. Volatile Cr species evaporation in pure oxygen was significantly lower than what has been reported for simulated solid oxide fuel cell environments with humid air.
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| 2. |
- Alnegren, Patrik, 1988, et al.
(författare)
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Inhibiting chromium evaporation and oxide scale growth on SOFC metallic interconnects by nano coatings
- 2014
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Ingår i: 20th World Hydrogen Energy Conference, WHEC 2014. - 9780000000002 ; 1:2014
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Konferensbidrag (refereegranskat)abstract
- High chromium ferritic steel is today the most commonly considered material for SOFC interconnectors due to many desirable properties, such as matching thermal expansion coefficient with other cell components but most importantly better machinability and price compared to ceramic alternatives. Yet there are some obstacles that need to be addressed before long term stability of a ferritic steel interconnector based fuel cell stack can be realized. First of all the electrical conductivity needs to remain high throughout the fuel cell stack operating life time and thus the formed oxide layers need to be electrically conductive and thin. Secondly, volatilization of chromium from the oxide scale of metallic interconnects causes rapid degradation due cathode poisoning. In the current study both oxidation and chromium evaporation of ferritic steel substrates are investigated in controlled atmospheres that simulates the environments of an operating SOFC stack. Samples coated with nanometer scale dual coatings of Co and Ce were tested. The dual coating substantially increased the performance of the ferritic substrates by i) significantly reducing oxidation rate, ii) increasing scale adherence and iii) diminishing chromium evaporation by 90 % via the formation of a Co-Mn-spinel cap layer.
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| 3. |
- Alnegren, Patrik, 1988, et al.
(författare)
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Severe dual atmosphere effect at 600 °C for stainless steel 441
- 2016
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 301, s. 170-178
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Tidskriftsartikel (refereegranskat)abstract
- AISI 441 foils of 0.2 mm thickness were exposed in a dual atmosphere setup in which one side was exposed to air -3% H2O and the other to Ar -5% H2 - 3% H2O. The experiment was performed at 600 °C and was referenced against exposures in air +3% H2O on both sides. The exposure conditions were chosen to simulate the conditions of an interconnect in intermediate temperature solid oxide fuel cell stacks (IT-SOFC). A strong dual atmosphere effect was observed: local breakaway corrosion was discovered after only 1000 h on samples exposed to dual atmospheres. After 3000 h iron oxide had propagated to cover the entire surface area of the sample. In comparison, the samples exposed in single atmosphere formed thin protective chromia scales on both sides even after 3000 h of exposure.
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| 4. |
- Alnegren, Patrik, 1988, et al.
(författare)
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Temperature dependence of corrosion of ferritic stainless steel in dual atmosphere at 600–800 °C
- 2018
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 392, s. 129-138
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Tidskriftsartikel (refereegranskat)abstract
- The ferritic stainless steel AISI 441 (EN 1.4509) is exposed for 1000 h to air - 3% H2O on one side and to Ar - 5% H2 – 3% H2O on the other at temperatures 600, 700, and 800 °C. Conditions are chosen to mimic the environment of metallic interconnects in an operating solid oxide fuel cell (SOFC). At 600 °C, ∼25 μm thick Fe2O3/(Fe,Cr)3O4 forms on large parts of the air side of the samples. Reference samples exposed to air - 3% H2O on both sides form thin protective layers of (Cr,Mn)3O4/Cr2O3 at the same temperature. At higher temperatures, 700 and 800 °C, all samples form protective layers of (Cr,Mn)3O4/Cr2O3 regardless of exposure to single or dual atmosphere. It is concluded that corrosion resistance in a dual atmosphere has an inverse dependence on temperature. Different hypotheses for the underlying cause for the dual atmosphere effect are discussed and compared to the experimental data.
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| 5. |
- Falk Windisch, Hannes, 1984, et al.
(författare)
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Chromium vaporization from mechanically deformed pre-coated interconnects in Solid Oxide Fuel Cells
- 2015
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 297, s. 217-223
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Tidskriftsartikel (refereegranskat)abstract
- Cathode poisoning, associated with Cr evaporation from interconnect material, is one of the most important degradation mechanisms in Solid Oxide Fuel Cells when Cr2O3-forming steels are used as the interconnect material. Coating these steels with a thin Co layer has proven to decrease Cr vaporization. To reduce production costs, it is suggested that thin metallic PVD coatings be applied to each steel strip before pressing the material into interconnect shape. This process would enable high volume production without the need for an extra post-coating step. However, when the pre-coated material is mechanically deformed, cracks may form and lower the quality of the coating. In the present study, Chromium volatilization is measured in an air-3% H2O environment at 850 C for 336 h. Three materials coated with 600 nm Co are investigated and compared to an uncoated material. The effect of deformation is investigated on real interconnects. Microscopy observations reveal the presence of cracks in the order of several μm on the deformed pre-coated steel. However, upon exposure, the cracks can heal and form a continuous surface oxide rich in Co and Mn. As an effect of the rapid healing, no increase in Cr vaporization is measured for the pre-coated material.
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| 6. |
- Falk Windisch, Hannes, 1984, et al.
(författare)
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Co- and Ce/Co-coated ferritic stainless steel as interconnect material for Intermediate Temperature Solid Oxide Fuel Cells
- 2017
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 343, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Chromium species volatilization, oxide scale growth, and electrical scale resistance were studied at 650 and 750 °C for thin metallic Co- and Ce/Co-coated steels intended to be utilized as the interconnect material in Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFC). Mass gain was recorded to follow oxidation kinetics, chromium evaporation was measured using the denuder technique and Area Specific Resistance (ASR) measurements were carried out on 500 h pre-exposed samples. The microstructure of thermally grown oxide scales was characterized using Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscopy (STEM), and Energy Dispersive X-Ray Analysis (EDX). The findings of this study show that a decrease in temperature not only leads to thinner oxide scales and less Cr vaporization but also to a significant change in the chemical composition of the oxide scale. Very low ASR values (below 10 m? cm2) were measured for both Co- and Ce/Co-coated steel at 650 and 750 °C, indicating that the observed change in the chemical composition of the Co spinel does not have any noticeable influence on the ASR. Instead it is suggested that the Cr2O3 scale is expected to be the main contributor to the ASR, even at temperatures as low as 650 °C.
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| 7. |
- Falk Windisch, Hannes, 1984, et al.
(författare)
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Determination of the oxide scale growth mechanism using 18O-tracer experiments in combination with Transmission Electron Microscopy and nanoscale Secondary Ion Mass Spectrometry
- 2018
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Ingår i: Materials Characterization. - : Elsevier BV. - 1044-5803. ; 136, s. 128-133
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Tidskriftsartikel (refereegranskat)abstract
- Two-stage 18 O 2 / 16 O 2 exposures can be used to investigate the effect that alloying elements, secondary phases, or surface treatments have on the high temperature oxidation behaviour of certain materials. During subsequent exposures to 16 O 2 - and 18 O 2 -rich atmospheres, 16 O- and 18 O-rich layers are formed. Analysis of the layers using Seco ndary Ion Mass Spectrometry (SIMS) depth profiling allows for conclusions to be drawn about the oxide scale growth mechanism. The conclusions are, however, not entirely unambiguous due to the limited lateral resolution of the technology. Rough surface topography and the thickness variation of the oxide scale over the analysed volume add to the ambiguity of the findings. In this study, an Fe-20%Cr alloy was exposed to both 18 O- and 16 O-rich environments at 850 °C. Two methods were used to analyse the thermally grown Cr 2 O 3 scale: (1) traditional SIMS depth profiling and (2) preparation of a cross-sectional lamellae for Transmission Electron Microscopy (TEM), which, subsequently, was analysed in a NanoSIMS. The NanoSIMS 16 O and 18 O elemental maps were then superimposed on the TEM image. In comparison with traditional SIMS depth profiling, the nanoSIMS elemental maps reveal detailed information about local oxide growth in different parts of an oxide scale. Moreover, a clear 16 O/ 18 O interface can be seen in the nanoSIMS maps, which is not the case in the sputter depth profiles. The findings of this study show that the aforementioned issues associated with sputter depth profiling can be eliminated by mapping a cross-section of an oxide scale using high resolution nanoSIMS.
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| 8. |
- Jonsson, Torbjörn, 1970, et al.
(författare)
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High-Temperature Oxidation of FeCr(Ni) Alloys : The Behaviour After Breakaway
- 2017
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Ingår i: Oxidation of Metals. - : Springer Nature. - 0030-770X .- 1573-4889. ; 87:3-4, s. 333-341
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Tidskriftsartikel (refereegranskat)abstract
- The oxidation of an austenitic FeCr(Ni) steel at 600 C in H2O ?KCl ? O2/H2 ? H2O ? Ar environments is studied up to 168 h. The oxidationbehaviour after ‘‘breakaway’’ is investigated by microstructural investigations, andthermodynamic calculations (Thermo-Calc) on the FeCr(Ni)–O system are used tointerpret the results. The characteristic ‘‘breakaway’’ scale morphology exhibits anoutward-growing iron oxide and a bottom layer consisting of inward-growing spineloxide and reaction zones containing Cr-rich oxide precipitates and Cr-depletedmetal. The equilibrium calculations show that a miscibility gap appears in the FeCrspinel oxide below 665 C influencing the microstructure of the inward-growingscale. Equilibrium calculations on the FeCrNi–O system show that Ni is notexpected to enter the spinel oxide in low oxygen activity conditions, in accordancewith observations
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| 9. |
- Jonsson, Torbjörn, 1970, et al.
(författare)
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Oxidation After Breakdown of the Chromium-Rich Scale on Stainless Steels at High Temperature: Internal Oxidation
- 2016
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Ingår i: Oxidation of Metals. - : Springer Science and Business Media LLC. - 1573-4889 .- 0030-770X. ; 85:5-6, s. 509-536
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Tidskriftsartikel (refereegranskat)abstract
- Breakaway oxidation of alloy 304L at 600 degrees C was studied in four environments (O-2 + H2O + KCl, O-2 + H2O + SO2 + KCl, H-2 + H2O + Ar, O-2 + K2CO3) for up to 168 h. The resulting scales were investigated by FIB/SEM, SEM/EDX, STEM/EELS, STEM/EDS and oxidation was elucidated by thermodynamic calculations (Thermo-Calc). The initial thin protective scale broke down in all cases. After breakaway, the scale consisted of two layers, i.e. an inward growing spinel/reaction zone and an outward growing iron-rich layer. The general features and microstructure of the scales after breakaway were similar in all environments and were explained in terms of: (1) Different diffusivities of Cr3+ and Fe2+ in the spinel oxide. (2) The appearance of a miscibility gap in the FeCr and FeCrNi spinel oxides. (3) The equilibrium composition of the spinel (at low pO(2) Ni is not present in the spinel).
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| 10. |
- Mortazavi Seyedeh, Nooshin, 1986, et al.
(författare)
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Interplay of water and reactive elements in oxidation of alumina-forming alloys
- 2018
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Ingår i: Nature Materials. - : Springer Science and Business Media LLC. - 1476-4660 .- 1476-1122. ; 17:7, s. 610-617
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Tidskriftsartikel (refereegranskat)abstract
- High-temperature alloys are crucial to many important technologies that underpin our civilization. All these materials rely on forming an external oxide layer (scale) for corrosion protection. Despite decades of research on oxide scale growth, many open questions remain, including the crucial role of the so-called reactive elements and water. Here, we reveal the hitherto unknown interplay between reactive elements and water during alumina scale growth, causing a metastable ‘messy’ nano-structured alumina layer to form. We propose that reactive-element-decorated, hydroxylated interfaces between alumina nanograins enable water to access an inner cathode in the bottom of the scale, at odds with the established scale growth scenario. As evidence, hydride-nanodomains and reactive element/hydrogen (deuterium) co-variation are observed in the alumina scale. The defectrich alumina subsequently recrystallizes to form a protective scale. First-principles modelling is also performed to validate the RE effect. Our findings open up promising avenues in oxidation research and suggest ways to improve alloy properties.
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