Cr2O3 scale growth rates on metallic interconnectors derived from 40,000 h solid oxide fuel cell stack operation

Abstract

The ohmic resistance caused by Cr2O3 scale formation on metallic interconnects (MICs) can significantly contribute to the overall degradation of SOFC stacks. For this reason oxide scale growth on Cr5Fe1Y2O3 (CFY) and Fe22Cr0.5Mn (Crofer) was investigated by scanning electron microscopy (SEM) from post-test samples that were either exposed to air at 850 °C (furnace) or operated in Hexis planar SOFC-stacks under dual atmospheres (anode and cathode conditions) at temperatures around 900 °C. The study includes unique test results from a stack operated for 40,000 h. To analyze inhomogeneity in scale thicknesses a dedicated statistical image analysis method has been applied. SEM images were used to compare the structural phenomena related to MIC oxidation at different sample locations. The observed differences between different sample locations may relate to locally different conditions (temperature, pO2, H2O/O2-ratio). Cr2O3 scale growth on the anode side is found to be approximately twice as fast in comparison to the scale growth on cathode side. Finally, based on our time lapse analyses with extensive sampling it can be concluded that reliable predictions of scale growth requires statistical analyses over a period that covers at least a quarter (10,000 h) of the required SOFC stack lifetime (40,000 h).