Protective Coatings of Metallic Interconnects for IT-SOFC Application

Abstract

The development of high-performing planar solid oxide fuel cell (SOFC) stacks operating at intermediate temperature (700–850°C) is based on thin-electrolyte anode supported cells (ASCs) and interconnects made by ferritic stainless steels. These metallic materials match very well the thermal expansion behavior of the ASCs and can be manufactured and formed using cheaper and easier processes than ceramics or chromium alloys. Nevertheless, some problems remain to be solved with these components as the performance degradation due to the oxide scale growth at the cathodic contact surface and the evaporation of volatile Cr-containing species, which poisons the cathodic materials. Both effects strongly limit the stack performance compared to single cells and increase the degradation rate with time. Providing the steel composition is carefully controlled, the above problems can be limited and some special ferritic stainless steels have been developed in the past years for SOFC application. Unfortunately, no commercial alloy is still able to satisfy the limit in degradation rate required for stationary applications (SECA target is <0.25% upon 1000h on a minimum service life of 40,000h). To achieve these goals a further improvement of composition should be required but this cannot be easily obtained in a cost-effective large-scale metallurgical production. An alternative and probably simpler way is to coat the surface of the steel with a protective layer with the twofold aim to limit Cr evaporation and to develop a conductive scale. In the present work, the effect of different oxide coatings on the chromium evaporation rate and on the contact resistance of ferritic stainless steel has been investigated. To obtain a conductive layer, spinel compositions containing Co, Mn, and Cu have been considered. Steels surfaces have been spray-coated using alcoholic suspensions, and the microstructural evolution of the interface between the metallic substrate and oxide layers has been investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy line-scan analysis for exposure at high temperature. The variation with time of the area-specific resistance at 800°C has been recorded up to 1000h. The evaporation rate of Cr-containing species has been also evaluated by a qualitative method.