The permeation of hydrogen and its isotopes through many of the transition metals is lower than that dis­played by iron and the ferritic steels; the notable excep­tions include groups 4 and 5 as well as palladium. Figure 21 shows the permeability of several metals; the diffusivity and the solubility are listed in Table 1 for these metals. In general, the activation energy asso­ciated with permeability (AHs + Ed) is larger for the materials with lower permeability and the permeability tends to converge at elevated temperatures.

We do not attempt to comprehensively review the data for nonferrous metals. However, gas permeation studies are considered the standard for transport prop­erties, particularly studies that report permeability, diffusivity, and solubility. Permeation of tritium through metals and alloys was reviewed by Steward.101

4.16.3.3.5.1 Molybdenum

Several reviews ofthe literature on hydrogen transport in molybdenum have noted variability ofthe transport
parameters.101,106,118 The reported permeability values are relatively consistent between the majority of studies, while the diffusivity and solubility values range over several orders of magnitude. The results of Tanabe et a/.106 are proposed here as they appear to represent nearly upper bounds of both diffusivity and solubility, without overestimating permeability. The study of Tanabe and coworkers also has the advantage that permeability and diffusivity were measured over a wide range of temperature and pressure, confirming the appropriate pressure depen­dencies of permeability and diffusivity for diffusion — limited transport.

4.16.3.3.5.2 Silver

The available data for hydrogen permeation through silver are limited. The diffusivity of hydrogen is reported by Katsuta and McLellan.107 McLellan also reports the solubility ofGroup IB metals from satura­tion experiments.108 Although these saturation experi­ments do not appear to provide reasonable values for other Group IB metals and are not consistent with other reported solubility measurements,217 Stew­ard, nevertheless, suggests estimating the permeability of hydrogen using these reported relationships.108

4.16.3.3.5.3 Platinum

There are relatively few gas permeation studies of platinum. Ebisuzaki et a/.109 report the permeability, diffusivity, and solubility of both hydrogen and

deuterium through single crystals of high-purity plat­inum. The permeability of hydrogen in platinum is similar to that in copper.

4.16.3.3.5.4 Gold

Caskey and Derrick110 report the permeability of deuterium through gold; Begeal103 reports a similar relationship. Diffusivity measurements, however, differ depending on the conditions of the measure­ment and the microstructural state of the gold.110,21 Cold-worked gold tends to give a higher activation for diffusion, suggesting that trapping is active to relatively high temperatures. Caskey and Derrick110 speculate that trapping is related to vacancies.

The diffusivity shown in Table 1 is from Eichenauer and Liebscher,11 while the solubility is estimated from this diffusivity and the permeability reported by Caskey and Derrick.110