There is a reason the HYPER lab is the only academic research lab in the US focused on cryogenic hydrogen: it’s hard.
Recall that hydrogen:
- has the largest flammability limits of any gas (4-94% in air by volume).
- has a very low energy barrier for combustion in air (a grain of sand in a jet has enough kinetic energy to ignite).
- has the highest combustion energy of any fuel by mass (119.96 MJ/kg).
- has one of the lowest boiling points of any fluid (boils near -421°F), highest thermal conductivities, and the highest latent heat (energy required to boil the fluid) per mass (420 kJ/kg) of any cryogen. This combines to mean that hydrogen probably has the highest ability to give you cold burns (frostbite) of any fluid.
- has the largest liquid to vapor volume expansion ratio of any fuel. The room temperature gas occupies 780 times the volume of the liquid at atmospheric pressures. If left confined in a sealed vessel, this expansion will cause the pressure to rise well over 27,000 psi.
- has the lowest mass of any atom or molecule, giving hydrogen the largest thermal de-broglie wavelength, or ability to tunnel through things (i.e. leak), of any atom or molecule (~1 nm near 20 K).
- has the highest ability to embrittle (weaken or even revert to powder) many materials by chemically reacting with elements in alloys.
To make maters even worse, there is little to no information available on cryogenic hydrogen embrittlement of materials. So what are we left to do?
Know what works & know the embrittlement mechanisms
We’ve ran cryogenic hydrogen experiments for years, at low pressures, using copper, aluminum, and brass plumbing materials. Hydrogen really doesn’t chemically react with the elements in those pure metals or alloys. That’s fine until you have to go to higher strengths and pressures — then things get tricky. There are two forms of embrittlement that must be considered at cryogenic temperatures: cryogenic phase change and hydrogen attack.
One of the better books on construction of cryostats (cryogenic vessels for experiments) I’ve seen is Jack Ekin’s Experimental Techniques for Low Temperature Measurement. Ekin was a cryogenics researcher at NIST for decades and now maintains the researchmeasurements.com website. Figure 6-20 from his book, available on the website here. Shows the low temperature fracture toughness of various cryostat materials. As you can see, most high-strength steels and titanium allows go through a glassy phase transition that reduces their toughness to ~25% of room temperature values. Pretty much any steel that is designed for strength goes through a huge reduction in toughness below 100 K. A key exception, Stainless Steel AISI 316 which actually gets significantly stronger at cryogenic temperatures.
The second filter is resilience to hydrogen attack. Sadly, little to no data is available on this at cryogenic temperatures. This is likely because everything is slower and the chemical reaction barriers much higher at cryogenic temperatures, so hydrogen attack mechanisms are mostly damped. However, nearly all cryogenic vessels are at room temperature at one time or another between tests, causing them to be exposed to hydrogen and placing a qualifier on their strength. NASA recently published an excellent report overviewing hydrogen embrittlement in materials. The end result? Nickel content is key. Very little to no reduction in yield strength is observed for steels with nickel content higher than 12.5%. Stainless Steel AISI 316 is one of the few steels with high nickel content, though tends to be expensive.
In the end, if you’re unsure and the design is key — best to call a professional. The HYPER lab is in the process of setting up a fatigue load frame for cryogenic hydrogen testing that will be available for testing cryogenic hydrogen embrittlement of novel materials.