TEM Characterization of Long Term Aged Interconnect Oxide Layers from Real Stack

Abstract

Steel interconnect robustness under SOFC conditions for extended operation remains a challenge. The observation and analysis of the interconnect microstructure aged in real conditions for long periods provides analysis data to help to understand degradation and potentially to design accelerated testing.In this study, TEM observations of the cross section of interconnects aged in real conditions (9000h and 20000h stacks) are presented. The interconnects are Crofer22APU with the air side coated with manganese cobalt oxide (MCO) to prevent chromium evaporation and increase oxidation resistance. FIB-prepared lamella from the inlet and outlet air side are analysed by TEM imaging and EDX. A major difference between inlet and outlet is the position of Mn-rich grains in the chromia scale: at the inlet, Mn-rich grains are located at the metal/chromia interface (figure 1(a)) while at the outlet, Mn-rich crystals are located close but not directly adjacent to the metallic bulk (figure 1(b)). The inlet microstructure has been observed in the literature in an oxidation study of MCO coated steel [1]. However, the outlet case which was observed for both the 9000h and 20000h aged stack samples was not yet reported in the literature, to the best of our knowledge. The difference between the inlet and outlet position was observed as well on a 15000h aged sample by SEM, and may be explained by differences between the inlet and outlet conditions: higher temperature at the outlet, as well as lower oxygen content. In addition to this particular Mn position, Fe particles are also observed in the chromia scale, confirming results previously described in the literature[2]. The analysis of the MCO coating also shows an inner denser layer, as expected after ageing at high temperature[3]. Analysis of the segregation of cobalt oxide at the MCO grain boundaries is also presented.More investigations are necessary to explain the complete oxidation mechanisms of the MCO-coated Crofer interconnect. However, the important role of the coating in interconnect oxidation is shown. The understanding of all reactions and the analysis of interconnects aged in real conditions, on different locations, should help designing more reliable combinations, better predictive models and possibly accelerated tests.[1] S. Molin et al., « Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects », J. Eur. Ceram. Soc., vol. 37, no 15, p. 4781-4791, déc. 2017, doi: 10.1016/j.jeurceramsoc.2017.07.011.[2] L. V. Gambino, N. J. Magdefrau, et M. Aindow, « Microstructural effects of the reduction step in reactive consolidation of manganese cobaltite coatings on Crofer 22 APU », Mater. High Temp., vol. 32, no 1-2, p. 142-147, janv. 2015, doi: 10.1179/0960340914Z.00000000090.[3] B. Talic, V. Venkatachalam, P. V. Hendriksen, et R. Kiebach, « Comparison of MnCo2O4 coated Crofer 22 H, 441, 430 as interconnects for intermediate-temperature solid oxide fuel cell stacks », J. Alloys Compd., vol. 821, p. 153229, avr. 2020, doi: 10.1016/j.jallcom.2019.153229. Figure 1