Hydrogen damage mechanisms of metallic, non-metallic materials, coatings and composite materials was reviewed and where possible, guidelines for hydrogen service of such materials, were identified. This review provided information necessary for the appropriate selection and qualification of materials that will be used in HYDRA plants.

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Metallic materials

The behavior of metallic materials in hydrogen was addressed starting from an initial description of the phenomenon of hydrogen adsorption and permeation in the crystalline structure of all metals as atomic hydrogen. The following different damage mechanisms were then explored in depth:

• High temperature hydrogen attack;

• Formation of hydrides;

• Hydrogen-induced blistering and cracking;

• Hydrogen embrittlement.

A particular focus was made on the hydrogen embrittlement mechanism for which, in the first instance, a qualitative description of the damage mechanism was provided. Furthermore, the effects of pressure, temperature and impurities present in the hydrogen gas were considered as well as the effects of the mechanical and microstructural properties of the materials subjected to gaseous hydrogen environment.

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Non-metallic materials

As for metallic materials, the hydrogen behavior of non-metallic materials was addressed starting from the description of the phenomenon of hydrogen permeation in the polymer matrix and then its permeability was analyzed when influenced by:

• Properties of the polymers;

• Operating conditions.

The damage mechanisms were discussed taking into account not only the interaction of polymers with hydrogen, but also other factors that contribute to the degradation of the polymeric material.

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Coatings

Within the hydrogen damage mechanisms, the role of coatings in preventing and limiting hydrogen embrittlement was explored, acting as real physical barriers against hydrogen permeation. The metallic, polymeric and ceramic coatings were analyzed in detail.

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Composite materials

The behavior of composite materials in hydrogen has been studied in view of their use in the construction of hydrogen storage tanks. The characteristics of the individual components of these materials have been analyzed. Metallic or polymeric liners necessary for hydrogen storage tanks have also been considered.

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Suitability of metallic and polymeric materials in hydrogen

The guidelines reported in ASME B31.12-2023 have been taken as reference for the evaluation of the suitability of metallic materials in hydrogen. These guidelines establish precautionary considerations regarding the service of metals in hydrogen and are useful for the purpose of correct engineering design.

Regarding the compatibility of polymeric materials, unlike metals, there is currently no extensive literature available and there are no specific standards. The evaluation of compatibility has been defined by referring to some standards for hydrogen applications and to the experience gained in this field.