Modeling of Ni Diffusion Induced Austenite Formation in Ferritic Stainless Steel Interconnects

Interconnect plates, made of ferritic type stainless steel, are widely used in planar solid oxide fuel cell (SOFC) or electrolysis cell (SOEC) stacks. These interconnect plates serve as current collector and separator for the neighboring fuel and oxygen electrode compartments of two adjacent cells. An intimate contact between the cell component and the interconnect (IC) plate is therefore essential to ensure optimum cell and stack performance. During stack production and operation, inter-diffusion across the cell – IC interface takes place, which under certain circumstances introduces adverse effects on the electrical, mechanical, and corrosion properties of the IC plates. One representative example is diffusion of nickel from the Ni/YSZ fuel electrode or from the Ni contact component into the IC plate, while iron and chromium from the steel diffuse in the reverse direction. Diffusion of Ni into the steel causes transformation of the ferritic BCC phase into the austenitic FCC phase in the interface region, accompanied with changes in volume and in mechanical and corrosion properties of the IC plates. Very few studies have been devoted to investigate this process experimentally. In this work, kinetic modeling of the inter-diffusion between Ni and FeCr based ferritic stainless steel was conducted, using the CALPHAD (CALculation of PHAse Diagrams) approach with the DICTRA software. The kinetics of inter-diffusion and austenite formation was explored in full detail, as functions of steel composition, thickness of Ni contact component or IC plate, temperature and time. The simulation was further validated by comparing with previously obtained experimental results. Growth of the austenite phase in commercial interconnect materials such as Crofer 22 APU and Crofer 22H was predicted under practical SOFC and SOEC operation conditions. The present work provides a proper account of the thermodynamics and kinetics of Ni-IC inter-diffusion and in addition provides input to further analysis of associated changes in the mechanical and corrosion properties of the IC plates.