Application of composite coatings as protection/contacting layers for metallic high-chromium-content SOFC interconnect material

Oxidation of the surface of metallic chromium oxide forming metallic interconnect (MIC) can cause up to one third of the total SOFC stack degradation during the long-time operation at elevated (750 - 850 °C) temperatures. The application of protective coatings is the most effective method not only for reduction of the growth of oxide scales but also for prevention of evaporation of Cr-containing species from MIC and of the poisoning of the air electrode. Two approaches to form the protective layers on the surface of CFY interconnect material with high chromium content (~ 94 %) have been tested. The CuNiMn-spinel (CNM) coatings were deposited using the wet powder spraying (WPS) of the slurries. As an alternative approach physical vapour deposition (PVD) method was used to apply thin metallic films on the surface of MIC and to form the protection layer by in-situ oxidation under the stack relevant conditions. The experiments were carried out at first using the model samples of different geometries to evaluate the properies and efficiency of the coatings. Composite pastes with addition of perovskite powders were also tested, because the CNM layers densify at SOFC operating conditions and shrinkage during long-term operation can cause the decrease of the contact area between the components and accelerate the degradation of the stack performance. The experiments have shown that the perovskite additive can efficiently reduce the shrinkage compared to the pure CNM material and match it well to the shrikage of other stack components. Moreover, the perovskite additive do not deteriorate the electrical properties of the composite since the perovskites have electrical conductivity comparable to CNM. The PVD coatings were tested in combination with CNM containing contacting layers applied by screen printing to reduce the chromium release rate. The experiments have shown a good compatibility and mechanical stability between the contacting layer and PVD protective coating during operation and thermal cycling. The materials and composites have been characterized by scanning electron microscopy (SEM/EDX), optical dilatometry and electrical conductivity measurements. Finally, the most promising material combinations obtained for model samples were transferred to SOFC stacks MK35x and evaluated under real operation condition.