La-Fe Perovskite Thin Film Coatings of Ferritic Stainless Steels by Surface Chemical Conversion: Dual Atmosphere Oxidation Testing

摘要

One of the challenges to be addressed in order to increase Solid Oxide Fuel Cells (SOFC) stack durability is represented by the corrosion occurring at the interconnect/cathode interface, giving rise to the growth of insulating layers and to Chromium diffusion in the cathode layer. To avoid these issues, protective coatings must be deposited on the interconnect surface. In the last years, Mn-Co spinel layers have been widely studied due to their excellent electrical conductivity and good thermal expansion match with the substrate. The coatings can be obtained by several deposition techniques and each one has different characteristics in terms of costs, scalability and effectiveness. As different approach, a novel passivation technique is currently being developed and studied to produce dense La-Fe perovskite layers to reduce the steel interconnect degradation. The method consists in a chemical conversion of the steel surface occurring in a molten carbonate salt, exploiting spontaneous reactions promoted in the synthesis medium. Perovskite oxides can be considered suitable coating materials due to their sufficiently high electronic conductivity, good thermal matching and low cation mobility. Deposition of perovskite coatings suffers, however, from technological issues related to low sintering capability and relatively high porosity. The proposed conversion technique provides the possibility of obtaining dense perovskite structures for more effective coatings. In this work, a La-Fe perovskite modified commercial 22Cr ferritic stainless steel is studied under dual-atmosphere oxidation test conditions. Morpho-structural evolution during the exposure to typical IT-SOFC conditions (700°C, humid hydrogen at the anode side, humid air at the cathode side) is followed by means of X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (EDX).