Hydrogen permeation barriers: Basic requirements, materials selection, deposition methods, and quality evaluation

An effective permeation reduction of gaseous hydrogen isotopes into a metal wall by introducing a barrier is essential in two main fields: prevention of hydrogen-embrittlement in steels and control of the tritium inventory in future nuclear fusion reactors. By far the most important advances and relevant studies originate from the Nuclear Fusion Community where tritium retention is an important issue, influencing safety issues. Stable permeation barriers are searched among materials with the lowest bulk hydrogen solubility and diffusivity. Besides a few specific pure metals, like beryllium and tungsten, dense oxides, nitrides and carbides have been mostly investigated. Coating techniques for preparation of well adhered and perfect barriers are evidently of the same importance as the material selection itself. Most attractive are the techniques where an ad-layer is formed simply by oxidation. Other methods require specific gas environments with strong electric and magnetic fields, which may represent a limit for the ad-layers uniform coverage over large and uneven areas. Evaluation of the achieved barrier performances is another challenging task. Several new methods, which can trace hydrogen isotopes in bulk at very low concentrations, often miss in the determination of their mobility. Also, they do not reveal the role of barrier defects. The classical gas permeation rate method through coated membranes is still the most reliable option to determine the actual Hydrogen Permeation Barrier (HPB) efficiency. At elevated temperature, the hydrogen permeation rate is recorded at the downstream side of a coated membrane exposed to a substantially higher hydrogen upstream pressure. By using modern vacuum instrumentation techniques, even the most effective barriers can be well characterised.