Z. Gary Yang

Materials Properties Database for Selection of High-Temperature Alloys and Concepts of Alloy Design for SOFC Applications

To serve as an interconnect / gas separator in an SOFC stack, an alloy should demonstrate the ability to provide (i) bulk and surface stability against oxidation and corrosion during prolonged exposure to the fuel cell environment, (ii) thermal expansion compatibility with the other stack components, (iii) chemical compatibility with adjacent stack components, (iv) high electrical conductivity of the surface reaction products, (v) mechanical reliability and durability at cell exposure conditions, (vii) good manufacturability, processability and fabricability, and (viii) cost effectiveness. As the first step of this approach, a composition and property database was compiled for high temperature alloys in order to assist in determining which alloys offer the most promise for SOFC interconnect applications in terms of oxidation and corrosion resistance. The high temperature alloys of interest included Ni-, Fe-, Co-base superalloys, Cr-base alloys, and stainless steels. In the US alone, there are hundreds of commercial compositions produced, over 250 of which are listed in Appendix A. Two initial criteria (oxidation resistance and oxide scale electrical conductivity) were used to reduce the list of alloys to manageable proportions. Thermal expansion and fabrication characteristics were then considered to further reduce the list of stainless steels. Due to their outstanding oxidation resistance and their potential to be used in SOFC components that can exclude alumina scales from the stack electrical path, alloys with a sufficient amount of aluminum were classified into a separate alumina-forming alloy category. The down-selected compositions (approx. 130 in number) and their characteristics and/or applications are listed in the Selected Alloy Compositions tables (Appendix B). Following the down-selection of alloy compositions, materials properties of interest corresponding to the their functional requirements in SOFC stacks were compiled in a tabular form (Appendix C). For comparison, the properties of selected noble metals and intermetallics were also collected and compiled and are listed in a separate table in Appendix C. Analysis of the pertinent literature indicated that, for a wide variety of alloys, there remains a lack of information on specific materials properties. Also, we have observed a large scatter in the reported database. For those cases, we employed general alloying principles as a tool of choice to approximate the unavailable data and to evaluate the reliability and consistency of collected data. Though numerous high temperature alloys look promising, it is anticipated that there will be few, if any, “off the shelf” alloy compositions which could completely satisfy the materials requirements as an interconnect, especially for a long term in a specific SOFC design. Therefore, some concepts of alloy design, including composition, constitution, and structure, as well as their effects on properties relevant to SOFC applications, are elaborated in an attempt to provide guidance for modification of current compositions and development of new alloys. Acknowledgement: This work was funded by the U.S. Department of Energy’s National Energy Technology Laboratory (NETL) under the Core Technology Program (CTP) of the Solid-State Energy Conversion Alliance (SECA).

Observations on the oxidation of Mn-modified Ni-base Haynes 230 alloy under SOFC exposure conditions

The commercial Ni-base Haynes 230 alloy (Ni-Cr-Mo-W-Mn) was modified with two increased levels of Mn (1 and 2 wt per cent) and evaluated for its oxidation resistance under simulated SOFC interconnect exposure conditions. Oxidation rate, oxide morphology, oxide conductivity and thermal expansion were measured and compared with commercial Haynes 230. It was observed that additions of higher levels of Mn to the bulk alloy facilitated the formation of a bi-layered oxide scale that was comprised of an outer M3O4 (M=Mn, Cr, Ni) spinel-rich layer at the oxide – gas interface over a Cr2O3-rich sub-layer at the metal – oxide interface. The modified alloys showed higher oxidation rates and the formation of thicker oxide scales compared to the base alloy. The formation of a spinel-rich top layer improved the scale conductivity, especially during the early stages of the oxidation, but the higher scale growth rate resulted in an increase in the area-specific electrical resistance over time. Due to their face-centered cubic crystal structure, both commercial and modified alloys demonstrated a coefficient of thermal expansion that was higher than that of typical anode-supported and electrolyte-supported SOFCs.