Material Compatibility

The mechanical properties of all materials are detrimentally affected by exposure to hydrogen. The magnitude of the deterioration depends on the material, the specific environment, e.g., hydrogen pressure and temperature, and the mechanical loading. Therefore, materials exposed to hydrogen must be carefully selected and designs must account for the resulting deterioration of mechanical properties.

Exposure of metals to hydrogen can lead to embrittlement, which can be manifested as significant losses in tensile strength, ductility, and fracture toughness as well as accelerated fatigue crack growth. This can result in failure of pressure containing components. See Hydrogen Embrittlement for more information about deterioration of mechanical properties of metals exposed to hydrogen.

Ideally, testing (i.e., direct exposure of the material to hydrogen) and analysis is done to assure that the material will perform as expected at planned operating conditions as well as worst-case conditions. If testing isn't practical, referring to existing material selection guidance found in the literature may provide the needed assurance. See Evaluating Metals for more information about ways to evaluate the suitability of metals for hydrogen service.

Metals that are less resistant to deterioration when exposed to hydrogen may still be used when analysis of pressure containing components shows that the mechanical loading is acceptable. The generally accepted demonstration method is fracture mechanics. See Component Design for more information about designing metallic components for hydrogen service.

Materials commonly used in hydrogen gas service for specific ranges of material characteristics (e.g., chemical composition and strength), mechanical loading, and environmental conditions are:

• Austenitic stainless steels
• Aluminum alloys
• Low-alloy ferritic steels
• C-Mn ferritic steels
• Copper alloys

Materials commonly avoided in hydrogen service are:

• High strength ferritic and martensitic steels
• Gray, malleable, and ductile cast irons
• Nickel alloys
• Titanium alloys

Consideration of toughness at low temperature is needed for liquid hydrogen service. Industry uses the Charpy impact test, which measures the amount of energy absorbed by a material during fracture, to judge a material's fracture toughness at the cold condition.

Some materials change from ductile to brittle behavior as their temperature is lowered, and this can occur at temperatures much higher than cryogenic temperatures.

Materials exhibiting low-temperature embrittlement (i.e., lack of toughness) should not be used for cryogenic service.

Embrittlement of sealing materials is also an important concern since most elastomers are not suitable for cryogenic service.

Materials commonly used in liquid hydrogen service are:

• Austenitic stainless steels
• Aluminum alloys

A small leak at a joint, if ignited, can result in a flame that impinges on another component. Therefore, components should be fire resistant, i.e., be able to withstand heat from a fire for a short period of time without failing.

• Copper joined by soldering or brazing should be avoided
• Aluminum alloys may also fail (melt) when exposed to a hydrogen jet fire

Nonmetallic sealing materials like PTFE gaskets and valve stem seals will likely fail when exposed to a hydrogen jet fire.

If these non-fire-resistant components are used, they can be protected from exposure to external fire by insulation or shielding.

In addition to not being fire resistant, polymers are inferior to metals with respect to permeation of hydrogen.