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Abstract

High-carbon steel alloys are currently the preferred material for hydrogen transport through pipelines. However, the different steel grades show a certain permeability for hydrogen, and are thus prone to embrittlement. Grain boundaries in ferritic microstructures play a dual role in the context of hydrogen embrittlement: on the one hand, they act as H traps and thus reduce the amount of mobile H in the system. On the other hand, exactly this trapping is expected to promote hydrogen enhanced decohesion at the grain boundaries. In order to influence the segregation process as well as the cohesive properties of interfaces in ferrite, one needs to understand in detail the relationship between strain, carbon and hydrogen solubility, and cohesive strength. We present the results of ab-initio studies of H segregation in Fe single crystal {001} and {111} cleavage planes, as well as at a Σ5 symmetrical tilt grain boundary. We determine the solution energy as a function of tractions normal to the interface for different loading and relaxation schemes. While the chosen method clearly affects the quantitative results, the qualitative findings are the same: In relaxed as well as strained microstructures, H tends to accumulate at the grain boundary. While it reduces the surface energies, and hence the work of separation, there is no significant impact of H on the transgranular or intergranular fracture stress.