Storing and transporting hydrogen in liquid form presents unique material challenges due to the extremely low temperatures required. Liquid hydrogen (LH2) is maintained at cryogenic temperatures around -253°C (-423°F), which can significantly alter the mechanical and physical properties of containment materials. The primary concerns include hydrogen embrittlement, thermal contraction, and loss of ductility, all of which must be carefully managed to ensure structural integrity and safety. Stainless steels and aluminum alloys are among the most widely used materials for cryogenic hydrogen applications due to their favorable performance under these extreme conditions.

Hydrogen embrittlement is a critical issue when dealing with cryogenic hydrogen systems. This phenomenon occurs when hydrogen atoms diffuse into the metal lattice, reducing ductility and leading to crack propagation under stress. At cryogenic temperatures, the mobility of hydrogen atoms decreases, but the risk of embrittlement remains due to the high solubility of hydrogen in certain metals. Austenitic stainless steels, such as 304 and 316 grades, are particularly resistant to hydrogen embrittlement because their face-centered cubic (FCC) crystal structure limits hydrogen diffusion. Aluminum alloys, particularly the 5000 and 6000 series, also exhibit low susceptibility to embrittlement, making them suitable for LH2 storage and transport.

Thermal contraction is another major consideration. Materials undergo significant dimensional changes when cooled from ambient to cryogenic temperatures. For example, stainless steel contracts by approximately 0.2% to 0.3% in linear dimensions, while aluminum alloys contract by about 0.3% to 0.4%. These contractions must be accounted for in the design of storage tanks and piping systems to avoid mechanical stress, leaks, or structural failure. Differential contraction between joined materials can also create stress concentrations, necessitating careful selection of compatible metals and expansion joints.

Stainless steels are widely used in cryogenic hydrogen systems due to their excellent combination of strength, toughness, and corrosion resistance. Type 304L and 316L stainless steels are common choices because their low carbon content minimizes carbide precipitation, which can reduce toughness at cryogenic temperatures. These alloys retain high impact resistance even at -253°C, a critical property for preventing brittle fracture. Additionally, stainless steels exhibit low thermal conductivity, which helps reduce heat transfer into the LH2, minimizing boil-off losses.

Aluminum alloys offer advantages in weight-sensitive applications, such as aerospace and mobile storage. Alloys like AA 5083 and AA 6061 are frequently used due to their high strength-to-weight ratio and good cryogenic performance. AA 5083, in particular, is known for its excellent weldability and resistance to stress corrosion cracking, making it ideal for large storage tanks. Aluminum’s high thermal conductivity can be a drawback, as it increases heat ingress, but this is often mitigated through advanced insulation techniques.

Material selection must also consider fatigue resistance, as cyclic thermal loading from repeated cooling and warming can lead to crack initiation and growth. Austenitic stainless steels generally exhibit superior fatigue performance at cryogenic temperatures compared to ferritic or martensitic steels. Aluminum alloys also demonstrate good fatigue resistance, though their lower modulus of elasticity can lead to greater elastic deformation under load.

Welding and joining techniques are critical in cryogenic hydrogen systems, as poor weld quality can introduce defects that act as stress concentrators. Stainless steel welds must be carefully controlled to avoid sensitization, where chromium carbides form at grain boundaries, reducing corrosion resistance. Aluminum welds require precise control of heat input to prevent weakening of the heat-affected zone. Post-weld heat treatment may be necessary to relieve residual stresses and restore material properties.

Testing and certification of materials for cryogenic hydrogen service involve rigorous procedures to ensure reliability. Charpy impact tests, fracture toughness evaluations, and fatigue testing are conducted at cryogenic temperatures to validate performance. Standards such as ASTM A240 for stainless steels and ASTM B209 for aluminum alloys provide guidelines for material properties and testing methods.

In summary, the successful use of materials for liquid hydrogen storage and transport hinges on understanding their behavior at cryogenic temperatures. Stainless steels and aluminum alloys are the leading choices due to their resistance to embrittlement, manageable thermal contraction, and overall mechanical robustness. Proper design, fabrication, and testing are essential to ensure these materials perform reliably under the extreme conditions imposed by LH2 systems. Advances in material science continue to refine these alloys and develop new options, further enhancing the safety and efficiency of hydrogen infrastructure.