Composite Cu-LaFeO3 Conversion 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

Abstract

In our previous study copper oxide additions were used to accelerate the formation of perovskite LaFeO3 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-LaFeO3 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-LaFeO3 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 LaFeO3 coatings obtained in a molten carbonate bath similar to that used for the formation of the composite Cu-LaFeO3 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 LaFeO3 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-LaFeO3 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 LaFeO3 and Cu-LaFeO3 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-LaFeO3 seem more promising systems for IT-SOFC interconnects than single-phase LaFeO3 conversion coatings, although further improvements in ASR electrical properties are needed.