High temperature (800 °C) oxidation of AISI 441 stainless steel with Mn–Co contact layers for SOFC stacks

Abstract

Characteristics of stainless steel grade AISI 441 deposited with Mn, Co and 10 wt% La0·6Sr0·4CoO3-δ (LSC) powder mixtures as cathode contact materials in solid oxide fuel cell (SOFC) stacks are studied. Microstructures of both uncoated and coated steel are examined after ageing at 800 °C for 10,000 h in air. It is found that the free surfaces of both uncoated and Mn–Co coated (Mn/Co ratio = 1:2 and 2:1) steel coupons suffer severe oxide scale spallation. In contrast, on the contact surfaces of uncoated coupons Mn–Cr–Fe spinels develop; and Mn–Co spinels grow on Mn–Co and Mn–Co-LSC coated (Mn/Co ratio = 1:1) coupons. Precipitates of Ti, Si and Nb oxides segregate at the steel/scale interface, weakening scale adherence. Area specific resistance (ASR) of steel/powder/La0·6Sr0·4Co0·2Fe0·8O3-δ (LSCF) assemblies is measured at 800 °C for 5043 h in air. The Mn–Co-LSC mixtures (Mn/Co ratio = 1:1 and 1:2) display close ASR values of 21–24 mΩ cm2, less than the value of 71 mΩ cm2 from the Mn–Co mixture (Mn/Co ratio = 2:1). This is attributed to the lower intrinsic resistivity of the derived Mn–Co oxides as well as good contact with steel and the LSCF wafer.