The effectiveness of permeation barriers is often assessed by the permeation reduction factor (PRF), which is the ratio of the effective permeability of specimen without a barrier to an equivalent specimen with a barrier; thus, the greater the PRF value, the more effective the permeation barrier.

4.16.3.3.1 Oxides

Most metals form a native oxide layer in the presence of oxygen. Generally, oxides have very low permeabil­ities for hydrogen isotopes, and native oxide layers may reduce permeability by about an order of magni­tude.175 The coefficient of thermal expansion is often very different from that of the underlying metal. This can cause cracks and spalling, which can reduce the effectiveness of the barrier. In environments that are aqueous or are subjected to elevated temperatures in the presence of oxygen, the oxide layer may be replen­ished and may even grow and coarsen. In addition to native oxides, an oxide layer might be deposited onto the base metal or the base metal might be dipped or
otherwise coated with a second metal, which forms a low-permeability oxide. Such coatings may reduce hydrogen permeability by five orders of magnitude or more.176 Chromia, alumina, and rare-earth oxides have been studied extensively.

The low dissociation pressure of Cr2O3 makes it a common native oxide on steels when allowed to form at elevated temperatures and relatively low oxygen partial pressures.177 Chromia is a better barrier (offering a permeation reduction of about an order of magnitude)175,178 than various Cr2MO4 spinels that may also form (M = Ni, Fe, Co).177 Chromia is also present in mixed oxides in chemical densified coatings, which help to give reduction factors of four orders of magnitude.179

Aluminum forms a self-passivating native oxide that has been shown to be resistant to hydrogen iso­tope permeability, because of a very low solubility for hydrogen. Because this layer is very thin (~4 nm) and the hydrogen permeability of the base metal is very low, it remains debatable whether this amorphous native oxide, which can be grown by anodization, has a lower permeability than aluminum or not.180,181 Cleaned stainless steel may be hot-dipped in alumi­num (forming both a relatively pure surface layer and mixed aluminides between the surface and substrate) and then oxidized. This hot-dip aluminizing proces­sing is simple and generally forms coatings that have excellent adhesion properties (although substantially different thermal expansion coefficients),182 which reduce permeation rates by at least one order of
magnitude, and sometimes more than five orders of magnitude.175,183

The basic properties of hydrogen transport in alu­mina have been characterized and are presented in Figures 18 and 19. Roy and Coble185 hot-isostatically pressed high-purity (>99.99%) alumina powders and charged the dense alumina with hydrogen at elevated temperatures to determine solubility. Fowler et a/.184 obtained diffusion coefficients for single-crystal, poly­crystalline, and powdered alumina, and for alumina that was doped with MgO. They observed faster diffu­sion in powdered specimens, suggesting that the grain boundaries may provide short-circuit diffusion paths. They also noted that the diffusivity of MgO-doped alumina was four to five orders of magnitude greater than that of pure alumina. This suggests that the purity of barrier coatings matters a great deal and transmuta­tion of barriers in a fusion environment may increase the permeability from the ideal case measured in the laboratory.

Yttria and erbia have been deposited on specimens through a number of physical deposition techniques, including plasma spray, arc deposition, and sol-gel deposition.186-188 The advantage of these oxides is not the magnitude of permeation reduction (one to three orders of magnitude), but their high thermal and mechanical stability in a reducing atmosphere.