Carburisation of ferritic Fe–Cr alloys by low carbon activity gases

Model Fe–Cr alloys were exposed to Ar–CO2–H2O gas mixtures at 650 and 800 °C. At equilibrium, these atmospheres are oxidising to the alloys, but decarburising (aC ≈ 10−15 to 10−13). In addition to developing external oxide scales, however, the alloys also carburised. Carbon supersaturation at the scale/alloy interface relative to the gas reflects local equilibrium: a low oxygen potential corresponds to a high pCO/pCO2 ratio, and hence to a high carbon activity. Interfacial carbon activities calculated on the basis of scale–alloy equilibrium are shown to be in good agreement with measured carburisation rates and precipitate volume fractions, providing support for the validity of the thermodynamic model

Development of ferritic steels for application as interconnect materials for intermediate temperature solid oxide fuel cells (SOFCs)

Fuel cells are considered as new energy production systems. High temperature fuel cells has experimented a large interest due to its high efficiencies much higher than that encountered for low temperature fuel cells. However, the materials requirements for high operation temperature makes that some kinds of materials are under development. One ofthese critical components are the interconnect materials. These are subjected at two different atmospheres, i.e. air and $H^{2}$-$H^{2}$0 mixtures. The present work examines the possibility of using commercial alloys and the suitability of different model alloys developed exclusive as construction materials for SOFC interconnect. It was confirmed that none of the available commercial steels seem to be suitable for such application. Therefore, large efforts were put on the development of a new interconnect material. From this work, two suitable alloy compositions are given, which derived into two commercial (Crofer 22APU) and semi-commercial (JS3) batches that are currently under real SOFC tests. The obtained results confirm that high Cr contents are needed to overcome the high operation temperatures, ca. 800°C. At the same time, suitable additions of minor alloying elements are needed reducing the formation of volatile chromium species (Mn), strengthening the nearsurface zone of the alloy (Ti) and improving the scale adhesion (La). Additionally, other minor alloying elements, such as Al and Si may affect strongly the oxidation behaviour for these FeCrNIn(Ti/La) type alloys, thus they should be strongly controlled

Metallic materials in solid oxide fuel cells

Fe-Cr alloys with variations in chromium content and additions of different elements were studied for potential application in intermediate temperature Solid Oxide Fuel Cell (SOFC). Recently, a new type of FeCrMn(Ti/La) based ferritic steels has been developed to be used as construction material for SOFC interconnects. In the present paper, the long term oxidation resistance of this class of steels in both air and simulated anode gas will be discussed and compared with the behaviour of a number of commercial available ferritic steels. Besides, in-situ studies were carried out to characterize the high temperature conductivity of the oxide scales formed under these conditions. Main emphasis will be put on the growth and adherence of the oxide scales formed during exposure, their contact resistance at service temperature as well as their interaction with various perovskite type contact materials. Additionally, parameters and protection methods in respect to the volatilization of chromia based oxide scales will be illustrated