This title appears in the Scientific Report :
2001
Untersuchungen zur Chromfreisetzung aus metallischen Interkonnektorwerkstoffen für die Hochtemperaturbrennstoffzelle
Untersuchungen zur Chromfreisetzung aus metallischen Interkonnektorwerkstoffen für die Hochtemperaturbrennstoffzelle
Metallic interconnect materials (chromium-based alloys and chromium-containing steels) form volatile chromium species under operating conditions of the Solid Oxide Fuel Cell. These volatile Cr(VI) species are reduced to solid Cr(lII) compounds at the triple phase boundary cathode/electrolyte/oxidant...
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Personal Name(s): | Gindorf, Christian (Corresponding author) |
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Contributing Institute: |
Werkstoffstruktur und Eigenschaften; IWV-2 |
Imprint: |
Jülich
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
2001
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Physical Description: |
IV, 100 p. |
Dissertation Note: |
Aachen, Techn. Hochsch., Diss., 2001 |
Document Type: |
Book Dissertation / PhD Thesis |
Research Program: |
Werkstoff- und Bauteilentwicklung für die Hochtemperatur-Brennstoffzelle |
Series Title: |
Berichte des Forschungszentrums Jülich
3853 |
Subject (ZB): | |
Publikationsportal JuSER |
Metallic interconnect materials (chromium-based alloys and chromium-containing steels) form volatile chromium species under operating conditions of the Solid Oxide Fuel Cell. These volatile Cr(VI) species are reduced to solid Cr(lII) compounds at the triple phase boundary cathode/electrolyte/oxidant after penetrating the cathode material, leading to a degradation of the electrochemical properties of the cell. In this work the vaporization of volatile chromium species was studied by the transpiration method. Transpiration experiments were carried out under equilibrium as well as nonequilibrium conditions. Equilibrium studies were carried out in air, using Cr203(s) (powder) at different water partial pressures (between 0.7 mbar and 0.3 bar) and temperatures (between 400 °C and 950 °C). Thermodynamic calculations based on the measured temperature dependence yielded an enthalpy of reaction of OrH94s = 122.85 kJ/mol for the formation of the volatile chromium species Cr02(OH)2(g) (dominating in the vapour over Cr203(S) under these experimental conditions) . Transpiration experiments under non-equilibrium conditions were carried out at a water vapour pressure of p(H20) = 0.02 bar at temperatures of 850 °C and 950 °C. The chromium vaporization rate of metallic interconnect materials forming different oxide scales as well as the chromium vaporization of different coated alloy samples (perovskite coatings on Cr5FelY203 and X10CrAl18 substrates) were determined. Perovskite coatings are used as a diffusion barrier against volatile chromium(VI) species vaporizing from the interconnect surface . In this work the chromium retention potential of vacuum plasma sprayed coatings (scale thickness 25-30 [,m) was investigated by comparison of chromium transport rates of coated and uncoated interconnect samples. As a result of this experiments a chromium retention of more than 99 % was found for protective perovskite coatings (e.g. Lao.9Sro.,Cr03) under cathodic operating conditions of the SOFC. Investigations of the scale morphology of perovskite coatings revealed a time dependent densification process. Due to that densification process possible values of more than 99 % were observed after different annealing times. The vaporization of volatile chromium(VI) species from coated interconnect specimens is caused by cracks and pores in the protective scale. This was confirmed by 53Cr tacer experiments with protective perovskite scales based on lanthanum chromfite which were carried out to obtain chromium self diffusion coefficients . For the scale composition of Lao.9Sro.1CrO3 at a temperature of 1000 °C a chromium diffusion coefficient of 2.59 - 10-16 cm2/s was determined. Considering a scale thickness of 30 [tm, from this diffusion coefficient, a break through time of 550 years can be calculated. According to these results, grain boundary diffusion as well as bulk diffusion are too slow to compete with gas phase transport of chromium through the protective scale. Considering the results of this work it is possible to reduce the chromium deposition rate in the cathode by a factor of more than two orders of magnitude if the SOFC operating temperature is decreased by 100 °C from 950 °C to 850 °C and if proper substrate/coating combinations are used |