Model-based prediction of the ohmic resistance of metallic interconnects from oxide scale growth based on scanning electron microscopy

Abstract

The increase of ohmic losses caused by continuously growing Cr2O3 scales on metallic interconnects (MICs) is a major contribution to the degradation of SOFC stacks. Comparison of measured ohmic resistances of chromium- (CFY) and ferritic-based alloy (Crofer) MICs at 850 °C in air with the growth of mean oxide scale thicknesses, obtained from SEM cross section images, reveals a non-trivial, non-linear relationship. To understand the correlation between scale evolution and resulting ohmic losses, 2D finite element (FE) simulations of electrical current distributions have been performed for a large number of real oxide scale morphologies. It turns out that typical morphologies favor nonhomogeneous electrical current distributions, where the main current flows over rather few “bridges”, i.e. local spots with relatively thin oxide scales. These current-“bridges” are the main reason for the non-linear dependence of ohmic losses on the corresponding oxide scale morphology. Combining electrical conductivity and SEM measurements with FE simulations revealed two further advantages: it permits a more reliable extrapolation of MIC-degradation data over the whole stack lifetime and it provides a method to assess the effective electrical conductivity of thermally grown Cr2O3 scales under stack operation.