Ferritic stainless steel interconnect plates are widely used in planar solid oxide fuel cell and electrolysis cell stacks. During stack production and operation, nickel from the Ni/yttria stabilized zirconia fuel electrode or from the Ni contact component layer diffuses into the interconnect plate, causing transformation of the ferritic phase into an austenitic phase in the interface region. This is accompanied with changes in volume, and in mechanical and corrosion properties of the interconnect plates. 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 (DIffusion Controlled TRAnsformation) software. The kinetics of inter-diffusion and austenite formation was explored in detail. The simulation was further validated by comparing with experiments. The results show that after 2000 h at 800°C Ni diffuses more than 100 μm deep into Crofer 22 APU. Along with the Ni diffusion, part of the ferritic steel with 50–60 μm in thickness has transformed into the austenitic phase. Growth of the austenite phase in commercial interconnect materials was predicted to take place under practical stack operation conditions.