Composite Cu-LaFeO3 Coating on High Cr Ferritic Stainless Steels for IT-SOFC Interconnects

Abstract

Lowering the operating temperatures of Solid Oxide Fuel Cells (SOFCs) at Intermediate Temperature (IT) range makes it possible to use cheap and robust metallic interconnects in place of the traditional ceramic parts. Ferritic stainless steels are ideal choice for SOFC interconnects due to mechanical properties and Thermal Expansion Coefficients (CTE) compatibility with the other parts of the cells, although they are notoriously subjected to various degradation processes in the operating environment. The growth of insulating chromia scales and the evaporation of chromium towards the cathode material are unacceptable issues that usually requires the application of protective coatings to ensure long-term stability of the stack performances. In last years several ceramic materials, particularly spinels and perovskites, have been investigated as protective layers, by applying different techniques for coating layer formation (1). Apart from conventional coating methods, a novel passivation technique based on spontaneous chemical reactions occurring in alkaline molten carbonate baths has been recently proposed by present authors (2,3). Dense and well-adhering La-Fe perovskite layers on austenitic and low chromium ferritic stainless steels have been obtained with this method. Evidence has been also provided that chemical composition of the salt bath may strongly affect the coating growth mechanism, allowing a morphology-tuned synthesis of the passivation layer. In this work, the influence of copper oxide addition to the molten carbonate bath is evaluated, with the aim of enhancing the electronic conductivity of the coating by producing a copper-perovskite composite layer. The passivation of the steel during the formation process is evaluated by means of in-situ electrochemical methods. Morphology and phase composition of the coatings are studied with Scanning Electron Microscopy and X-Ray diffraction analysis. The experimental results show that, under specific molten salt bath compositions, reduction of copper oxide and deposition of rounded metallic copper particles occurs on the coating surface simultaneously with the growth of the typical cubic-shaped LaFeO3 perovskite crystals. The Figure shows a typical evolution of the coating structure over the synthesis time. For over-exposed synthesis (i.e. 48h) the initially deposited metallic copper particles dissolve again in the bath leaving hollow perovskite grains. Bath chemical composition and synthesis time can therefore be tailored to obtain an effective Cu decorated LaFeO3 perovskite coating surface completely covering the steel substrate and with homogeneous dispersion of the Cu particles. The effect of adding copper on the LaFeO3coating will be examined for IT-SOFC interconnect applications. Acknowledgements This work was partially supported by the European Commission through the FP7 Fuel Cells and Hydrogen Joint Undertaking, Grant Agreement 325331 (Steel Coatings For Reducing Degradation in SOFC, SCORED 2.0). References Shaigan N, Qu W, Ivey DG, Chen W. A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources. 2010 Mar;195(6):1529–42.Frangini S, Masci A, Zaza F. Molten salt synthesis of perovskite conversion coatings: A novel approach for corrosion protection of stainless steels in molten carbonate fuel cells. Corros Sci. Elsevier Ltd; 2011 Aug;53(8):2539–48.Frangini S, Zaza F, Masci A. Molten carbonate corrosion of a 13-Cr ferritic stainless steel protected by a perovskite conversion treatment: Relationship with the coating microstructure and formation mechanism. Corros Sci. Elsevier Ltd; 2012 Sep;62:136–46. Figure 1