Influence of initial conditions on trap-controlled hydrogen diffusion withcalculations of effective diffusion coefficients

An effective diffusion coefficient for trap-controlled diffusion such as hy drogen in solid materials is generally determined considering the character istics of trapping sites. However, it is possible that the initial conditio ns (initial hydrogen concentrations at normal and trapping sites) also infl uence the diffusion phenomena. To investigate the influence, we derive a tw o-dimensional analytic solution of a diffusion equation with an arbitrary n umber of trapping terms, and verify the model by comparing with the, experi ments of hydrogen evolution from cylindrical specimens at two different ini tial conditions. The comparison shows that the experimental results can be explained fairly well using the present solution. We next calculate effecti ve diffusion coefficients by varying initial conditions for the transient e volution from a cylindrical specimen, and show that the coefficients are mu ch influenced by the initial conditions, which cannot be explained using th e local equilibrium assumption. The effective diffusion coefficient determi ned with the local equilibrium assumption is shown to be a kind of average value of those calculated in the present study.