There is a need for numerical models capable of predicting local accumulation of hydrogen near stress concentrators and crack tips to prevent and mitigate hydrogen assisted fracture in steels. The experimental characterisation of trapping parameters in metals, which is required for an accurate simulation of hydrogen transport, is usually performed through the electropermeation test. In order to study grain size influence and grain boundary trapping during permeation, two modelling approaches are explored; a 1D Finite Element model including trap density and binding energy as input parameters and a polycrystalline model based on the assignment of a lower diffusivity and solubility to the grain boundaries. Samples of pure iron after two different heat treatments - 950C for 40 minutes and 1100C for 5 minutes - are tested applying three consecutive rising permeation steps and three decaying steps. Experimental results show that the finer grain microstructure promotes a diffusion delay due to grain boundary trapping. The usual methodology for the determination of trap densities and binding energies is revisited in which the limiting diluted and saturated cases are considered. To this purpose, apparent diffusivities are fitted including also the influence of boundary conditions and comparing results provided by the constant concentration with the constant flux assumption. Grain boundaries are characterised for pure iron with a binding energy between 37.8 and 39.9 kJ/mol and a low trap density but it is numerically demonstrated that saturated or diluted assumptions are not always verified, and a univocal determination of trapping parameters requires a broader range of charging conditions for permeation. The relationship between surface parameters, i.e. charging current, recombination current and surface concentrations, is also studied.

中文翻译：

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模拟氢渗透通过纯铁进行捕获和表面现象表征

需要能够预测氢在应力集中器和裂纹尖端附近的局部积累的数值模型，以防止和减轻钢中的氢辅助断裂。金属中捕获参数的实验表征是准确模拟氢传输所需的，通常通过电渗透测试进行。为了研究渗透过程中晶粒尺寸的影响和晶界俘获，探索了两种建模方法；包括陷阱密度和结合能作为输入参数的一维有限元模型和基于对晶界分配较低扩散率和溶解度的多晶模型。使用三个连续上升渗透步骤和三个衰减步骤测试经过两种不同热处理（950℃ 40 分钟和 1100℃ 5 分钟）的纯铁样品。实验结果表明，由于晶界俘获，更细的晶粒微观结构促进了扩散延迟。重新审视了用于确定陷阱密度和结合能的常用方法，其中考虑了有限的稀释和饱和情况。为此，拟合表观扩散系数，包括边界条件的影响，并将恒定浓度提供的结果与恒定通量假设进行比较。晶界的特征是结合能在 37.8 和 39 之间的纯铁。9 kJ/mol 和低捕集密度，但数值证明饱和或稀释假设并不总是得到验证，捕集参数的单义确定需要更广泛的渗透充电条件。还研究了表面参数，即充电电流、复合电流和表面浓度之间的关系。