| 1. |
- Goebel, Claudia, 1988, et al.
(författare)
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Influence of pre-oxidation on dual atmosphere effect on AISI 441 interconnects for solid oxide fuel cell applications
- 2017
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. ; 78:1, s. 1559-1563
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Konferensbidrag (refereegranskat)abstract
- In previous studies an extreme dual atmosphere effect on the airfacing side of AISI 441 at 600°C was observed. However indications showed that pre-oxidation of the material might have a beneficial effect on the corrosion resistance in dual atmosphere. To examine this further we pre-oxidized AISI 441 samples for 0 min, 11 min, 45 min, 180 min and 280 min and subsequently exposed these at 600 °C for 500 h under dual atmosphere conditions. Photographs of the air-facing sides were taken throughout exposure to monitor the corrosion behavior. SEM analysis was performed on all samples after exposure.
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| 2. |
- Goebel, Claudia, 1988, et al.
(författare)
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The effect of pre-oxidation parameters on the corrosion behavior of AISI 441 in dual atmosphere
- 2018
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 43:31, s. 14665-14674
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Tidskriftsartikel (refereegranskat)abstract
- Dual atmosphere conditions have been shown to be detrimental for the ferritic stainless steel interconnects used in solid oxide fuel cells (SOFC) under certain conditions. In the present work, we analyze the influence of pre-oxidation on corrosion resistance in dual atmosphere with regard to two parameters: the pre-oxidation time and the pre-oxidation location (pre-oxidation layer on the air-facing side or the hydrogen-facing side). The steel AISI 441 is investigated and pre-oxidation is achieved in air at 800 °C. To examine the influence of pre-oxidation time on corrosion behavior, five different pre-oxidation times are used: 0, 11, 45, 180, and 280 min. The samples are exposed discontinuously to dual atmosphere for 1000 h at 600 °C. Photographs, taken throughout the exposure, show that the pre-oxidation time correlates with the onset of breakaway corrosion. To analyze the influence of pre-oxidation location on corrosion behavior, the samples are pre-oxidized for 180 min, and then a pre-oxidation layer is removed from one side of the sample. Subsequent dual atmosphere exposure at 600 °C for 500 h shows that the pre-oxidation layer on the hydrogen-facing side is more important for corrosion resistance in dual atmosphere than the pre-oxidation layer on the air-facing side.
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| 3. |
- Goebel, Claudia, 1988
(författare)
-
Corrosion of Ferritic Stainless Steels Used in Solid Oxide Fuel Cells
- 2018
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Solid oxide fuel cells (SOFC) are energy conversion systems with clean emissions (depending on the fuel used) and high electrical efficiencies, which could potentially replace conventional conversion systems such as combustion engines. However, other issues, such as high costs and limited lifetime, must be resolved before widespread commercialization of SOFCs can be achieved. One of the major cost factors and the component that reduces the lifetime of SOFCs immensely, is the interconnect, which electrically connects multiple fuel cells to form a stack. Interconnects are typically made of ferritic stainless steels, and, due to high temperatures (600 °C – 900 °C) and the aggressive environments prevalent in fuel cells, the interconnects corrode over time. The protective Cr2O3 that forms on ferritic stainless steels at high temperatures mitigates the corrosion process to a certain extent. However, this oxide layer leads to two other issues: (i) vaporization of hexavalent Cr species, which leads to cathode poisoning and (ii) an increase in the electrical resistance of the interconnect caused by a continuously growing oxide layer. Both these problems can be reduced to a certain extent with coatings. Especially spinel coatings have been proven highly effective at decreasing Cr-evaporation. The first part of this work examines the influence of the Co3O4 spinel coating on area specific resistance (ASR). It was found that the Co3O4 thickness of Crofer 22 APU, which was exposed in air for 500 h at 600 °C, did not significantly impact the ASR, and, instead, the main contributor to overall resistance was the thermally grown Cr2O3. The second part of this work focuses on the long-term stability of Ce/Co-coated (10 nm Ce/640 nm Co) AISI 441. Coated AISI 441 was exposed by AB Sandvik Materials Technology for up to 37 000 hours at 800 °C in laboratory air. Subsequent analysis showed very low Cr-evaporation rates compared to uncoated AISI 441, and ASR values below 40 mΩcm2, suggesting that, under these conditions, the coating is effective in reducing Cr-evaporation rates and Cr2O3-growth rates even after long exposure times. The last part of the work analyzes the influence of pre-oxidation on the dual atmosphere effect at 600 °C with regard to two parameters: the pre-oxidation time and the pre-oxidation location. It was demonstrated that longer pre-oxidation times for AISI 441 result in extended resistance against dual atmosphere corrosion on the air-facing side. It was also found that the pre-oxidation layer on the hydrogen-facing side is more important for corrosion resistance in dual atmosphere than the pre-oxidation layer on the air-facing side.
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| 4. |
- Goebel, Claudia, 1988, et al.
(författare)
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Does the conductivity of interconnect coatings matter for solid oxide fuel cell applications?
- 2018
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 383, s. 110-114
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Tidskriftsartikel (refereegranskat)abstract
- The present work aims to quantify the influence of typical interconnect coatings used for solid oxide fuel cells (SOFC) on area specific resistance (ASR). To quantify the effect of the coating, the dependency of coating thickness on the ASR is examined on Crofer 22 APU at 600 °C. Three different Co coating thicknesses are investigated, 600 nm, 1500 nm, and 3000 nm. Except for the reference samples, the material is pre-oxidized prior to coating to mitigate the outward diffusion of iron and consequent formation of poorly conducting (Co,Fe) 3 O 4 spinel. Exposures are carried out at 600 °C in stagnant laboratory air for 500 h and subsequent ASR measurements are performed. Additionally the microstructure is investigated with scanning electron microscopy (SEM). On all pre-oxidized samples, a homogenous dense Co 3 O 4 top layer is observed beneath which a thin layer of Cr 2 O 3 is present. As the ASR values range between 7 and 12 mΩcm 2 for all pre-oxidized samples, even though different Co 3 O 4 thicknesses are observed, the results strongly suggest that for most applicable cases the impact of the coating on ASR is negligible and the main contributor is Cr 2 O 3 .
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| 5. |
- Goebel, Claudia, 1988, et al.
(författare)
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Long-term (4 year) degradation behavior of coated stainless steel 441 used for solid oxide fuel cell interconnect applications
- 2020
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Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753. ; 449
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Tidskriftsartikel (refereegranskat)abstract
- The present work aims to investigate the long-term stability of Ce/Co coated AISI 441 used as an interconnect material in solid oxide fuel cells (SOFC). Being a commercially available alloy the use of AISI 441 would greatly reduce the cost of SOFCs in comparison to tailor-made interconnect materials such as Crofer 22 APU. To analyze the long-term stability Ce/Co coated AISI 441 is exposed in air at 800 °C for up to 38 000 h. Mass gain values are recorded continuously. After 7 000, 23 000, and 35 000 h area specific resistance (ASR) measurements are performed, and cross-sections are prepared and analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX) spectroscopy. Cr-evaporation measurements are conducted on samples exposed for up to 38 000 h.
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| 6. |
- Goebel, Claudia, 1988, et al.
(författare)
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Long term (4 years) performance of Co/Ce coated 441 for SOFC interconnect applications
- 2017
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. ; 78:1, s. 1675-1679
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Konferensbidrag (refereegranskat)abstract
- Co/Ce coated 441 samples were exposed at 800°C in laboratory air for up to 37 000 h and subsequently characterized using different techniques. Mass gain data of all samples were recorded throughout exposure. Cr-evaporation measurements were conducted and compared with unexposed Co/Ce coated AISI 441 as well as unexposed and uncoated AISI 441. Furthermore SEM analysis on BIB cross-sections was performed on 7 000 h, 23 000 h and 35 000 h exposed Co/Ce coated 441.
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| 7. |
- Goebel, Claudia, 1988, et al.
(författare)
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Self-healing properties of Ce/Co-coated stainless steel under simulated intermediate temperature solid oxide fuel cell conditions
- 2021
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Ingår i: Surface and Coatings Technology. - : Elsevier BV. - 0257-8972. ; 428
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Tidskriftsartikel (refereegranskat)abstract
- The interconnects used in solid oxide fuel cells (SOFC) are usually shaped into a corrugated form that creates gas channels. Coatings are applied onto an interconnect to increase its longevity by reducing Cr(VI) evaporation and oxide scale growth. To date many manufacturers first deform the interconnect and then apply the coating. However, the reverse (hereinafter termed pre-coating) would be more cost-effective, because large-scale roll-to-roll coating processes could then be used instead of batch coating processes. The drawback of this method is that cracks are introduced into the coating during deformation. The present work shows that the cracks heal after relatively short exposure times for the often-used Ce/Co coating (10 nm Ce and 640 nm Co) even at low operating temperatures (650 °C and 750 °C). The Cr evaporation rate of pre-coated deformed Ce/Co-coated AISI 441, even though slightly elevated in the beginning of the exposure, decreases and stabilizes to rates that are comparable to that of undeformed Ce/Co-coated AISI 441. SEM micrographs show that the cracks introduced during the shaping of the interconnect heal after roughly 70 h of exposure at 750 °C and 360 h of exposure at 650 °C.
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| 8. |
- Goebel, Claudia, 1988
(författare)
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Strategies to Mitigate the Degradation of Stainless-Steel Interconnects Used in Solid Oxide Fuel Cells
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Interconnects are a vital part of solid oxide fuel cells (SOFC), where they electrically connect individual cells to form a fuel cell stack. They are a main contributor to the overall stack cost and the limited life-time of fuel cells, and, therefore, improvements carried out on the interconnect level could further the commercialization of SOFCs. The limited life-time of the interconnect is related to the material used today, ferritic stainless steels (FSS). FSS interconnects are more cost-effective than previously used ceramics, but they degrade under the conditions prevalent in an SOFC: high temperatures between 600 °C and 850 °C, and a p(O2) gradient. Certain corrosion phenomena that occur, such as Cr evaporation and continuous oxide scale growth, negatively impact cell performance due to cathode poisoning and increased electrical resistance, respectively. These phenomena have been found to be effectively mitigated by coatings, such as the (Co,Mn)3O4 (MCO) coating, or reactive element coatings, such as Ce. The present thesis examines these coatings with regard to three aspects: (i) does the semi-conducting spinel coating affect the electrical resistance of the interconnect negatively, or is its conductivity negligible in comparison to the continuously growing Cr2O3 scale below it; (ii) does the coating self-heal if it is cracked even at intermediate temperatures, i.e. 650 °C and 750 °C, or do the cracks persist and increase Cr evaporation; and (iii) is the long-term stability of the state-of-the-art Ce/Co coating (10 nm Ce/640 nm Co) still effective after 35 000 h, or not. The second aspect is not only important to understand corrosion behavior, but it would also allow for large-scale roll-to-roll PVD coating, which is significantly more cost-effective than batch coating. Another corrosion phenomenon that is elucidated within the scope of this work is the dual atmosphere effect. This effect leads to increased corrosion on the air-facing side of the interconnect if the FSS is exposed to a dual atmosphere, i.e. air on one side and hydrogen on the other side, compared to if the FSS is exposed to an air-only atmosphere. A new theory as to why the dual atmosphere effect occurs is proposed, and it is indirectly verified by means of excluding all other possibilities. Factors that influence the dual atmosphere effect are discussed, and it is shown how the dual atmosphere effect could, in part, be mitigated.
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| 9. |
- Goebel, Claudia, 1988, et al.
(författare)
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The influence of different factors on the dual atmosphere effect observed for AISI 441 interconnects used in solid oxide fuel cells
- 2019
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. ; 91:1, s. 2261-2266
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Konferensbidrag (refereegranskat)abstract
- Interconnects used in solid oxide fuel cells are subjected to dual atmosphere corrosion at 600 °C. In this work we show that the dual atmosphere effect is dependent on the thickness of the steel sheet. For this AISI 441 with varying thicknesses was exposed to dual atmosphere at 600 °C for 336 h. Before and after exposure photographs were taken to analyze the progress of corrosion on the air-facing side of the steel samples. Additionally, scanning electron microscopy and energy dispersive x-ray diffraction (SEM/EDX) analysis was conducted on all exposed samples. It was found that thinner samples showed increased breakaway oxidation than thicker samples.
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| 10. |
- Gündüz, Kerem, 1985, et al.
(författare)
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The effect of hydrogen on the breakdown of the protective oxide scale in solid oxide fuel cell interconnects
- 2021
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Ingår i: Corrosion Science. - : Elsevier BV. - 0010-938X. ; 179
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the effect of hydrogen, on the degradation of AISI 441 interconnect, under solid oxide fuel cell operating conditions was investigated between 500−800 °C for 336 h. As a new hypothesis, it is concluded that, hydrogen impedes Cr diffusion, probably in the grain boundaries, leading to the breakdown of the protective oxide scale. This effect is most severe at 600 °C, while at lower or higher temperatures the effect is attenuated. Cr diffusion is enhanced at high temperatures, whereas protective scales can be obtained at low temperatures with a lower amount of Cr.
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| 11. |
- Tomas, Matthieu, 1993, et al.
(författare)
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Cu-based coatings for IT-SOFC applications
- 2019
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Ingår i: ECS Transactions. - : The Electrochemical Society. - 1938-5862 .- 1938-6737. ; 91:1, s. 2291-2298
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Konferensbidrag (refereegranskat)abstract
- Cu and Ce/Cu coated Sanergy HT samples were exposed at 650 °C and 850 °C for 500 h in air + 3 % water vapor and characterized. Mass gain data of all samples were measured throughout exposure. Cr-evaporation measurements were conducted and compared with uncoated Sanergy HT materials. Furthermore, SEM analysis on BIB cross-sections were performed on exposed samples. Results have shown higher mass gain for samples exposed at 850 ºC than 650 ºC as well as higher Cr evaporation. Microstructural analysis shows thinner oxide scales for samples coated with reactive elements.
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