Metallic supported Solid Oxide Fuel Cells (SOFCs) are considered to be a cost-effective and competitive alternative to state-of-the-art all-ceramic SOFCs. In substituting the supporting ceramic by a porous metal-layer, the advantages of the ductile alloy can be exploited and the mechanical strength is improved. However, this new technology originates different problems, which have to be solved before the product is ready for the market. The most important issue is corrosion, inevitably taking place at the operating conditions. The growing oxide layer will continuously fill the pores and a sufficient gas-diffusion through the layer may be one limiting factor for the long-term applicability of metallic supported SOFCs. In order to understand the implications of corrosion on the gas-diffusion, a modelling study on a microstructural level was pursued. The metallic support was measured with X-ray tomography and reconstructed into a computational geometry. Different surface representations (stair-step, smooth) were analysed and their influence on the results assessed. A geometrical evaluation tool was programmed that determines e.g. porosity distribution, averaged pore diameter, number of pores. An enhanced corrosion model based on Wagner's theory was implemented in OpenFOAM which describes the growth of the oxide thickness depending on corrosion rate constants, which can be easily retrieved by measurements. The model is applied on complex 3-D microstructures, where also the shrinkage of the alloy, due to consumption of Cr-ions for the oxide formation, is taken into account as a boundary condition. The transient oxide growth and its corresponding change of the microstructure impedes the gas-diffusion. This was assessed by determining the change of the concentration over-potential, which results from a decreased mass transport. Furthermore, the effective diffusion coefficient was computed, which is an important input parameter for simulations on cell- and stack-level. It has been shown in this work that the application of periodic boundary conditions for the lateral walls increase the accuracy of the solution dramatically, compared to the state-of-the-art symmetry approach, and that more reliable results can be obtained from smaller geometries. In addition to that, it is now possible to evaluate the change of the microstructure due to corrosion and its transient influence on the gas-diffusion, which allows predicting the degradation of the SOFC caused by corrosion of the metallic support.