Permeability:

Permeability (Φ) of gas (involving H₂ or T₂) by materials is a measure of how much gas will migrate across a material wall of provided thickness and area over a given time.   It is a direct function of the ability to solubility and diffuses.  Dimensionally,

Φ(cm³(H_2' STP) . cm(thickness)/cm²(area) . sec.) = D(cm²/sec.) . S (cm³(H_2' STP) / cm³(material)                                                                                     (A-1)

where:

Φ = permeability

D = diffusivity

S = solubility

The subsequent materials are listed in sequence of increasing permeability:  ceramics and graphite, nonhydriding metals, silicas, hydriding metals, and polymers.  The permeability of several other hydrogen-bearing molecules by polymers has been studied. For such molecules, permeability could be well in excess of which for hydrogen by a polymer. This must be considered while handling tritiated water or organic solvents.

Two factors which influence the permeability of a metal are oxides on surface and surface area. Since the permeability of hydrogen by a metal oxide at a provide temperature is commonly sequences of magnitude lower than it is by the metal, a thin surface oxide could markedly decrease the permeability of hydrogen by the material.

For example, if LiD salt is placed in contact with the surface of a stainless steel specimen, the oxide is reduced, permitting increased permeation.  Permeability reduces as oxidation proceeds if a metal undergoes surface oxidation within the presence of steam.