Composite Cu-LaFeO3Conversion Coatings on a 18Cr Ferritic Stainless Steel for IT-SOFC Interconnects: Effect of Long-Term Air Exposure at 700°C on Cr Diffusion Barrier and Electrical Properties

In our previous study copper oxide additions were used to accelerate the formation of perovskite LaFeO 3 conversion coatings on stainless steels from molten carbonate baths. Incorporation of copper particles into the growing coating was an additional effect resulting in the formation of a composite Cu-LaFeO 3 structure. In continuation to our previous study, the aim of this work is to report the effect of copper additions on long-term stability and performance of perovskite conversion coatings under IT-SOFC interconnect conditions. To this end, a Cu-LaFeO 3 coated K41 18Cr ferritic stainless steel was examined in air at 700°C up to 1000 h. In order to simulate properly the situation of a real IT-SOFC cell, Area Specific Resistance (ASR) and Cr-barrier properties of the coated steel were evaluated simultaneously with a special coating characterization setup. Studies were conducted by comparison with single-phase LaFeO 3 coatings obtained in a molten carbonate bath similar to that used for the formation of the composite Cu-LaFeO 3 coatings but without the addition of copper oxide. Copper addition did not change the general morphology of the perovskite coating, which remains a multi-layer coating, being composed of an outer LaFeO 3 crystalline layer, a middle Fe-rich oxide and two inner Fe-Cr rich oxide layers. However, copper was beneficial in promoting a thinner and more stable coating structure along with finer perovskite grain size. These structural improvements were further confirmed by the results obtained with electrical measurements that showed a better ASR behavior of the Cu-LaFeO 3 coatings. On the other hand, no relevant copper effects could be detected on the coating oxidation stability and on the Cr-barrier properties of the perovskite conversion coatings. Both LaFeO 3 and Cu-LaFeO 3 coatings showed similarly high coating stability and excellent Cr-barrier capability in experiments conducted at 700°C up to 1000 h. In definitive, dual-phase Cu-LaFeO 3 seem more promising systems for IT-SOFC interconnects than single-phase LaFeO 3 conversion coatings, although further improvements in ASR electrical properties are needed.