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There is a need for numerical models capable of predicting local accumulationof hydrogen near stress concentrators and crack tips to prevent and mitigatehydrogen assisted fracture in steels. The experimental characterisation oftrapping parameters in metals, which is required for an accurate simulation ofhydrogen transport, is usually performed through the electropermeation test. Inorder to study grain size influence and grain boundary trapping duringpermeation, two modelling approaches are explored; a 1D Finite Element modelincluding trap density and binding energy as input parameters and apolycrystalline model based on the assignment of a lower diffusivity andsolubility to the grain boundaries. Samples of pure iron after two differentheat treatments - 950C for 40 minutes and 1100C for 5 minutes - are testedapplying three consecutive rising permeation steps and three decaying steps.Experimental results show that the finer grain microstructure promotes adiffusion delay due to grain boundary trapping. The usual methodology for thedetermination of trap densities and binding energies is revisited in which thelimiting diluted and saturated cases are considered. To this purpose, apparentdiffusivities are fitted including also the influence of boundary conditionsand comparing results provided by the constant concentration with the constantflux assumption. Grain boundaries are characterised for pure iron with abinding energy between 37.8 and 39.9 kJ/mol and a low trap density but it isnumerically demonstrated that saturated or diluted assumptions are not alwaysverified, and a univocal determination of trapping parameters requires abroader range of charging conditions for permeation. The relationship betweensurface parameters, i.e. charging current, recombination current and surfaceconcentrations, is also studied.

Simulation of hydrogen permeation through pure iron for trapping and surface phenomena characterisation