Oxidation mechanisms of metallic carrier materials for gas separation membranes in power generation systems with carbon dioxide capture
Materialien und Werkstoffe der Sinterprozesse und der generativen Fertigungsverfahren
Zusammenfassung der Projektergebnisse
Assuming a further decrease of nuclear power and before energy supply by extensive use of renewable sources is available, the only available bridging technology can be provided by conventional power plants. The undesired increase in CO2 emissions can be counteracted by carbon dioxide capture and storage (CCS) technologies. The most efficient way to reach this technological goal is by introducing gas separation techniques using ceramic membrane technology. However, until now hardly any investigations have been carried out on the development of metallic support structures for the assembly of the gas separation systems. The metallic carriers must possess high corrosion resistance in service environments with varying concentrations of O2, N2, H2, H2O, CO2 and CO at temperatures up to 1000°C. The present project aims at an investigation of the fundamental aspects of the oxidation mechanism in such complex service environments. Based on the technological requirements, the alloys to be studied will be so called “marginal” alumina forming materials (alloy 601, 214, 224) on the basis of NiCrAl alloys. The experimental methods used to study the oxidation processes and the microstructural characterization of the corrosion products formed in the mixed gas environments include the use of thermogravimetric analyses (TGA) in combination with SEM/EDX/WDX, X-ray diffraction (XRD), secondary neutrals mass spectrometry (SNMS), glow discharge optical emission spectroscopy (GDOES, and especially conventional and high resolution TEM investigate the scale formation mechanisms and microstructural changes in the bulk alloy on various length scales. During 800°C exposure Alloy 602 CA formed a protective external Al2O3 scale independent of the test gas used mainly due to the beneficial effect of surface cold work while Haynes 214 showed initially Al2O3 formation but after 1000 h exposure in Ar-20%O2 significant internal oxidation of aluminum due to the formation of γ´-precipitates. It was suggested that the aluminum concentration in γ is crucial for the formation of an Al2O3 layer and not the bulk alloy concentration. In addition, also the concentration of other alloying elements affects the formation of an external Al2O3 scale. In contrast to the behavior at 800°C, Alloy 602 CA exhibited at 900 – 1100°C internal Al2O3 nodules, whose amount increased with increasing exposure temperature, whereas Haynes 214 formed after initial internal oxidation of aluminum a continuous Al2O3 layer in Ar-20%O2. A mechanism describing the oxidation processes in Haynes 214 at 800 – 1100°C in Ar-20%O2 was proposed. Exposure of Haynes 214 in different H2O containing gases revealed an increased tendency to internal oxidation of aluminum in the absence of O2 in the test gas possibly due to hydrogen diffusion into the alloy. Furthermore, a lower pO2 was proposed to suppress or decrease the growth rate of NiO to a larger extent than that of Al2O3 and therefore promote the formation of an external Al2O3 scale. Investigations on Haynes 224 showed that, despite its lower aluminum content compared to Haynes 214 the material exhibited after 72 h oxidation at 800 – 1100°C always an external Al2O3 formation with different amounts of Cr-rich transient oxide. It was proposed that the addition of iron, results in a lower amount of Al-rich γ´-precipitates in Haynes 224 compared to Haynes 214 thus increasing the tendency to external Al2O3 formation. Studies with a number of model alloys showed that the effect of gas composition on protective alumina formation was most pronounced for alloys with low Cr contents. This explains why alloy 214 was more sensitive to gas composition variations than 602 and 224. When exposing an alloy with high Al content (e.g. 10%), Al depletion due to alumina growth finally results in a critical Al content which results in breakaway oxidation. This critical Al content is lower than that found when using bare specimens which can be explained by a newly identified scale healing mechanisms occurring after scale damage as a result of thermal cycling.
Projektbezogene Publikationen (Auswahl)
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Oxidation Mechanisms of Alumina Forming Alloys in Environments Relevant to Gas Separation Membranes in Power Generation Systems, Gordon Research Conference “High Temperature Corrosion”, New London, NH, USA, 21.-26.07.2013
M. Schiek, L. Niewolak, L. Singheiser, R. Vaßen, W. J. Quadakkers
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Oxidation Mechanisms of Alumina Forming Alloys in Environments Relevant to Gas Separation Membranes in Power Generation Systems. International Collaborative Workshop on “Degradation of Materials in Extreme Environments”, Pittsburgh, PA, USA, 18-19.07.2013
M. Schiek, R. Iskandar,L. Niewolak, J. Mayer, W. J. Quadakkers
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TEM Studies of Complex Oxide Layers on High Temperature Alloys, International Collaborative Workshop on “Degradation of Materials in Extreme Environments”, Pittsburgh, PA, USA, 18- 19.07.2013
R. Iskandar, J. Mayer
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Oxidation Mechanisms of Alloy 602 CA in Different Gas Atmospheres Relevant to Gas Separation Membranes, European Corrosion Congress 2014, EUROCORR 2014, Pisa, I, 08.-12.09.2014
M. Schiek, L. Niewolak, R. Vaßen, W. J. Quadakkers
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Oxidation Mechanisms of Marginal Alumina Forming Alloys in Environments Relevant to Gas Separation Membranes, 9th International Conference on “Microscopy of Oxidation”, Nottingham, GB, 14.-16.04.2014 [Best-Poster Award]
M. Schiek, L. Niewolak, L. Singheiser, R. Vaßen, W. J. Quadakkers
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Oxidation Mechanisms of Metallic Carrier Materials for Gas Separation Membranes, Dissertation, Nov. 2015, Ruhr-Universität Bochum
M. Schiek
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Scale formation of alloy 602 CA during isothermal oxidation at 800–1100 C in different types of water vapor containing atmospheres, Oxidation of metals 84 (5-6) (2015) 661-694
M Schiek, L Niewolak, W Nowak, GH Meier, R Vaßen, WJ Quadakkers
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External α-Al2O3 scale on Ni-base alloy 602 CA–Microstructural evolution, Corrosion Science 127 (2017) 27-38
R Pillai, A Chyrkin, T Galiullin, E Wessel, D Grüner, WJ Quadakkers
