Experimental Investigation of the Initial Stage of the Oxidation Mechanism of Co Coating for Solid Oxide Fuel Cell Interconnects at 650 °C

The evolution of Co coating in solid oxide fuel cells (SOFCs) under low and medium temperatures (<700 °C) is different from that under high temperatures. In this context, the oxidation corrosion of 441 ferritic stainless steel (FSS) with a Co coating in air under 650 °C was investigated. The results indicated that the Co coating was oxidized rapidly and a Co₃O₄ spinel layer formed in the initial exposure of 5 min, which improved the oxidation resistance of 441 steel. After oxidation at 650 °C for 120 h, a Cr₂O₃ layer with a thickness of 0.2–0.4 μm was observed on the surface of bare 441 steel, while the surface oxide scale of Co-coated 441 steel samples was composed of an inner Cr₂O₃ and Fe₂O₃ solid solution (0.1–0.3 μm thick), an intermediate (Fe, Co)₃O₄ layer and an outer Co₃O₄ spinel layer, respectively. The Co-coated 441 steel sample exhibited better electrical properties. After oxidation at 650 °C for 120 h, the area-specific resistance (ASR) of the Co-coated steel was 3.73 mΩ·cm², which was 25.4% lower than that of bare 441 steel as 5 mΩ·cm². Furthermore, the thermal growth process and protection mechanism of Co coating at 650 °C were discussed.