4.16.3.2.5.1 Aluminum

The permeability of hydrogen isotopes through alu­minum is smaller than that through most metals. This is due to a very low intrinsic solubility and a moderate hydrogen diffusivity. Young and Scully99 reviewed the diffusivity of hydrogen in aluminum and discussed the reasons for the wide range of reported values. A majority of hydrogen present in aluminum alloys at lower temperatures is trapped at defects and alloying content tends to increase the solubility for hydrogen in aluminum slightly.99,170,171

Application of aluminum alloys in fusion reactors is limited by their neutron activation, low melting temperature (933 K for pure aluminum or signifi­cantly lower for some alloys), the formation of embrittling intermetallics when layered on other metals, and the difficulty of joining aluminum alloys (because of the native oxide layer). The thermal limitation is significant because precipitates in con­ventional aluminum alloys coarsen or dissolve above 450 K and most conventional alloys are not very resistant to creep deformation. Few studies of alumi­num alloys have been made to date for use at tem­peratures greater than about 450 K. Because many aluminum-containing intermetallics and aluminum oxide also have low hydrogen permeability and do not have the same thermal limitations as metallic aluminum, it has been proposed that they be used as hydrogen isotope barriers.123

4.16.3.2.5.2 Nickel

Nickel alloys are described in Chapter 2.08, Nickel Alloys: Properties and Characteristics. The per­meation of hydrogen and its isotopes in nickel has been extensively studied. The results from Louthan et al.10 provide a good estimate of the transport properties. They found that cold-worked nickel dis­plays higher permeability to hydrogen, speculating that diffusion is enhanced along dislocation networks. Louthan4 also measured permeation in high-pressure gaseous hydrogen isotopes, showing that the trans­port is not dependent on concentration (and that Sievert’s law is appropriate at elevated pressure). Normalization of nickel’s permeation4 and diffusion3 by the mass ratio (eqn [3]) has been shown to provide good agreement for all the three isotopes.

4.16.3.2.5.3 Copper

There are numerous reports on the permeability of hydrogen in copper; the gas permeation results of Begeal103 are suggested here. Caskey and co — workers172 found that the diffusivity of hydrogen in copper depends on the oxygen content, and conse­quently, that the effective diffusivity can be much lower than the lattice diffusivity. Nevertheless, the reported diffusivity is consistent among a number of reports.3,103,172-174 Copper is also described in Chapter 4.20, Physical and Mechanical Properties of Copper and Copper Alloys.