Mesoscale finite element phase field modelling of liquid lithium corrosion of steels

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Liquid lithium is a proposed coolant for the STEP fusion reactor because of its ability to remove heat efficiently, as a liquid metal, but with the added advantage that Li can breed tritium from neutron interactions. One of the proposed materials that the liquid lithium will be in contact with is grade P91 steels. These steels have been used extensively in the fossil fuels and nuclear industry and they have both reasonable corrosion/oxidation resistance due to the Cr present but more importantly have high creep strength at elevated temperatures required by STEP. P91 grades are ferritic-martensitic steels with a highly complex microstructure that contains a variety of both precipitates and inclusions. These are well known to influence the creep properties but are likely to have a role to play in the corrosion of the steel by liquid lithium. At Imperial we have recently been developing a model, within the finite element phase field method framework, that can generate polarisation curves similar to those observed experimentally during the measurements of corroding metals. These models will be developed to study single grain, bi-crystals and grains with a single second phase particle embedded, of the various types found in P91 steels, to assess which particles are critical in the presence of Li and control the corrosion behaviour of these steels. These models will be developed in parallel with the localised corrosion experiments that will take place at UKAEA of P91 steels in liquid lithium. The models once developed for simple cases will also attempt (subject to experimental data availability) to include the effect of radiation damage through hardening and potentially grain boundary chemical segregation effects in these steels on the corrosion behaviour in liquid Li.