Material Selection
Material selection for liquid hydrogen service should be based on mechanical properties at the low temperature (e.g., yield and tensile strength, impact strength). Industry typically uses the Charpy impact test to determine the amount of energy absorbed by a material during fracture, which is a measure of the material's toughness.
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 should not be used for cryogenic service.
The large temperature difference between ambient and cryogenic conditions (300° F or more) results in significant thermal contraction of most materials, which should be accommodated for in designs for cryogenic service.
Since many carbon steels are brittle below ambient temperatures, precautions should be taken to prevent cold hydrogen from entering vessels and piping made of carbon steel or other brittle materials. In most applications, liquid hydrogen is vaporized and warmed prior to use.
The following material behavior concerns must be considered when selecting materials for liquid hydrogen service:
- The transition from ductile to brittle behavior at very low temperatures
- The modes of plastic deformation at very low temperatures
- The effect of metallurgical instability and crystalline structure phase transformations on mechanical and elastic properties.
Embrittlement of sealing materials is also an important concern, since most elastomers are not suitable for cryogenic service.
Insulation should be noncombustible or have a self-extinguishing fire rating. Open cell insulating materials such as Styrofoam should be avoided. Formation of liquid air on exposed, adjacent surfaces penetrate into the foam to create a shock sensitive combination capable of explosive yields comparable to fuel-oxidizer mixtures.
References
G-095-2004, ANSI/AIAA Guide to Safety of Hydrogen and Hydrogen Systems, can be purchased from www.aiaa.org.