The diffusivity and solubility of H were measured, at temperatures ranging from 573 to 923K (table 22), by using a desorption technique that involved the out-gassing of thermally H-charged cylindrical samples with various grain sizes. It was found that the Arrhenius plot revealed a change, in the apparent activation energy for diffusion, at about 900K. There also existed a correlation between increasing activation energy and decreasing grain size. A simple model was developed in order to explain the grain-size dependence of the diffusivity. A decrease in diffusivity at lower temperatures, for smaller grain sizes, was attributed to trapping of H at the nodes of grain boundaries. At very large grain sizes, fast diffusion could occur along the grain boundary. No trapping-induced increase in solubility was found.

M.Ichimura, Y.Sasajima, M.Imabayashi: Materials Transactions, 1991, 32[12], 1109-14

Table 22

Diffusion Parameters for H in Al