The diffusion and trapping of hydrogen in steel

Abstract The mobility of dissolved hydrogen in an iron lattice having a population of extraordinary, or trapping, sites for hydrogen is analyzed under the assumption of local equilibrium between the mobile and the trapped populations. It is shown that at low coverage of the trapping sites the usual solutions of the diffusion equations can be used to analyze the experimental results and that the effective diffusivity is a function of trap density and of the magnitude of the trap depth. In the domain of coverage in which the activity of trapped hydrogen is not linear in the fractional occupation of the trap population, the concept of phenomenological diffusivity becomes non-operational and the diffusion equation must be solved with terms for sources and sinks, as done by McNabb and Foster. Values for trap number and depth are obtained by applying the relevant equations to selected data on a variety of steels with and without cold work. The results lead to the suggestion that in the absence of cold work the solid-solid interfaces are as or more important than dislocations as loci of trapping sites, and that in cold-worked steels, microcrack surfaces are more important than dislocations for hydrogen trapping.