Modelling for hydrogen diffusion in metals with traps revisited

A rigorous concept for diffusion of hydrogen is presented that allows for the role of trapping and/or hydrostatic stress. Nonlinear partial differential equations for the concentration of freely diffusing hydrogen in the lattice and for the total concentration of hydrogen both in the lattice and traps are derived. A generalized chemical diffusion coefficient can be deduced and is compared with reported relations from the literature. Simulation results are presented for different amounts of traps and their different energetic levels (depths), both for hydrogen concentration profiles and the total amount of hydrogen in the case of charging or discharging a cylindrical specimen. A significant charging/discharging asymmetry occurs for high hydrogen concentrations and high amounts of traps of sufficient depth. Comparisons of the chemical diffusion coefficient with experimental results from the literature are also presented.