PNNL

Abstract

In previous work, (Mn,Co)3O4 (MC) spinels have been systematically investigated and applied as protection layers on a variety of candidate SOFC interconnect steels. In recent years, the primary emphasis has been on the application of spinel coatings to AISI 441 ferritic stainless steel, which is being investigated as an interconnect alloy by PNNL in collaboration with Allegheny Ludlum and NETL. AISI 441 is prepared via conventional melt metallurgy and is therefore less expensive that other candidate steels, such as Crofer22APU, which utilize vacuum processing to reduce the Si content to very low levels. Additions of Nb and Ti to the AISI 441 tie up residual Si in Laves phase at grain boundaries, eliminating the need for expensive processing. To mitigate scale adhesion issues, a Ce-modified MC (Ce-MC) coating (e.g., Ce0.02Mn1.49Co1.49O4) was developed, which combines the advantages of rare earth (RE) surface treatment or additions, and the MC spinel coating. The stability and electrical performance of AISI 441 coated with Ce-MC protection layers were validated in short-term and long-term (1 year) testing. These tests indicated that the spinel protection layers helped minimize the interfacial electrical resistance and mitigate the scale growth beneath the spinel coatings on AISI 441. In addition, the Ce-modified coatings improved the scale adherence and overall interconnect surface stability.