TY  - THES
AU  - Schiek, Martin
TI  - Oxidation Mechanisms of Metallic Carrier Materials for Gas Separation Membranes
VL  - 316
PB  - Universität Bochum
VL  - Dr.
CY  - Jülich
M1  - FZJ-2016-02859
SN  - 978-3-95806-138-5
T2  - Schriften des Forschungszentrums Jülich Reihe Energie & Umwelt / Energy & Environment
SP  - 148 S.
PY  - 2016
N1  - Universität Bochum, Diss., 2016
AB  - A major goal of research worldwide is the development of new concepts for reducing CO$_{2}$ emissions in power plants and other industry sectors. Very promising concepts are the “carbon capture and storage” (CCS) technologies. These contain gas separation units requiring the use of gas separation membranes. The need to reduce membrane thickness in combination with a high susceptibility for mechanically-induced failure of the ceramic membrane material makes the use of a metallic support structure for the membranes necessary. The requirements for the supports can be fulfilled by $\alpha$-Al$_{2}$O$_{3}$ forming NiCrAl-base alloys. The membrane carriers have to withstand different aggressive atmospheres at temperatures up to 1000°C. As the formation of a protective Al$_{2}$O$_{3}$ surface scale is crucial for the operation of the membrane carrier, the present study deals with the influence of temperature and gas atmosphere on the selective oxidation of aluminum thereby using the commercially available alloys 602 CA and Haynes 214 with different aluminum contents of 2.4 wt.-% and 4.5 wt.-%, respectively. As these amounts are far below the high aluminum contents of MCrAlY alloys (10 – 13 wt.-% Al) considered as promising materials for the useas support structure, the investigation of possible effects of gas composition and/ortemperature on the protective Al2O3 scale formation was possible in relatively short-termexperiments (up to 1000 h). The studies were focused on the temperature range 800 – 1100°C in different atmospheres. During 800°C exposure Alloy 602 CA formed a protective external Al$_{2}$O$_{3}$ scale independent of the test gas used mainly due to the beneficial effect of surface cold work while Haynes 214 showed initially Al$_{2}$O$_{3}$ formation but after 1000 h exposure in Ar-20%O$_{2}$ significant internal oxidation of aluminum due to the formation of $\gamma$´-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%O$_{2}$. A mechanism describing the oxidation processes in Haynes 214 at 800 – 1100°C in Ar-20%O$_{2}$ was proposed. Exposure of Haynes 214 in different H$_{2}$O containing gases revealed an increased tendency to internal oxidation of aluminum in the absence of O$_{2}$ in the test gas possibly due to hydrogen diffusion into the alloy. Furthermore, a lower pO$_{2}$ was proposed to suppress or decrease the growth rate of NiO to a larger extent than that of Al$_{2}$O$_{3}$ and therefore promote the formation of an external Al$_{2}$O$_{3}$ 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 Al$_{2}$O$_{3}$ 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 $\gamma$´-precipitates in Haynes 224 compared to Haynes 214 thus increasing the tendency to external Al$_{2}$O$_{3}$ formation.
LB  - PUB:(DE-HGF)11
UR  - https://juser.fz-juelich.de/record/809969
ER  -