(Invited) Cr Volatility, Protective Coatings and SOFC Interconnect Testing

High chromium ferritic steels are today the most commonly considered material for SOFC interconnects due to a combination of desirable properties, such as matching thermal expansion coefficient with other cell components but most importantly better machinability and price compared to ceramic alternatives. Yet, the high operating temperature (600-900°C) causes some materials degradation issues that need to be addressed in order achieve the desired long term stability over operating periods of several 10.000h. The main function of the interconnect is to separate the fuel compartment from one cell from the air compartment of the neighboring cell and to conduct electricity between adjacent cells. Consequently the electrical conductivity needs to remain high throughout the fuel cell stack operating life time and thus the formed oxide layers need to be electrically conductive and thin. Secondly, volatilization of chromium from the oxide scale of metallic interconnects has shown to cause detrimental effects on the cathode in SOFCs. In the current study oxidation behavior, chromium evaporation and oxide scale resistance of ferritic steel substrates are investigated in controlled atmospheres that simulate the environment of an operating SOFC stack. Additionally samples coated with nanometer scale dual coatings of Co and Ce were tested. The dual coating substantially increased the performance of the ferritic substrates by i) significantly reducing oxidation rate, ii) increasing scale adherence, iii) diminishing chromium evaporation by 90 % via the formation of a Co-Mn-spinel cap layer and iv) substantially reducing the area specific resistance.