The diffusivity of H was measured at 573 to 923K by using a desorption technique that involved the out-gassing of thermally H-charged cylindrical samples having various grain sizes. It was found that the Arrhenius plot exhibited a change in apparent activation energy for diffusion at about 900K. There also existed a correlation between increasing activation energy and decreasing grain size:

∞ (0vol% voids), 473-903K:     D (cm2/s) = 9.53 x 10-2 exp[-44.3(kJ/mol)/RT]

15mm (0vol% voids), 573-923K:     D (cm2/s) = 4.58 x 10-2 exp[-37.0(kJ/mol)/RT]

4mm (0vol% voids), 573-903K:     D (cm2/s) = 1.52 x 10-1 exp[-53.4(kJ/mol)/RT]

4mm (0.19vol% voids), 573-903K:     D (cm2/s) = 6.00 x 101 exp[-88.7(kJ/mol)/RT]

3mm (0.17vol% voids), 623-903K:     D (cm2/s) = 1.54 x 101 exp[-81.2(kJ/mol)/RT]

3mm (0.50vol% voids), 623-903K:     D (cm2/s) = 4.22 x 102 exp[-106.9(kJ/mol)/RT]

2mm (0.04vol% voids), 623-913K:     D (cm2/s) = 2.11 x 100 exp[-67.0(kJ/mol)/RT]

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 the H at the nodes of grain boundaries. At very large grain sizes, fast diffusion could occur along the grain boundary.

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