Thermally converted oxide microstructure on ferritic stainless steel with Cu/Ni dual-layer coating for SOFC interconnect applications

Abstract

Cu/Ni dual-layer coating is applied to improve the performance of SUS 430 steel as solid oxide fuel cells (SOFCs) interconnects. Cu layer is deposited by electroplating, followed by magnetron sputtering Ni layer on top of Cu layer. The coated steels are evaluated in air at 800 °C in correspondence with SOFC cathode environment. Results showed that a considerable amount of Ni diffuses into the steel substrate, while Fe and Cr diffuse from the steel substrate into the coating during the initial oxidation. A (Ni,Fe)₃O₄ spinel layer is initially formed on the scale surface. Cu diffuses outwards and eventually dopes into (Ni,Fe)₃O₄ spinel to form (Ni,Fe,Cu)₃O₄ spinel. The oxide scale evolved into a dual-layer structure of an inner Cr₂O₃ layer and an outer layer of NiO mixed with spinels after 5 weeks of oxidation. Preoxidation treatment in 800 °C air for electroplated Cu coated steel prior to depositing Ni layer improves oxidation resistance and significantly decreases Cr outward diffusion after 5 weeks of oxidation. Moreover, the oxide scales formed on both Cu/Ni and CuO/Ni coated steels maintain low area specific resistance (ASR) after 5 weeks of oxidation.