by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Hydrogen Embrittlement and Seal Permeation

What Every Engineer Needs to Know

The hydrogen economy is scaling fast, with global demand for hydrogen expected to double between 2021 and 2030 (Statista). And the growing demand for fuel cells, electrolyzers, pipelines, and green energy storage is all driving demand for hydrogen-compatible hardware.

Here is the problem engineers are facing: hydrogen is uniquely aggressive. It is the smallest molecule in existence, and it attacks both metallic components and elastomeric seals through. That is why it is dangerous for engineers to treat hydrogen as a typical gas and attempt to reuse designs from natural gas or nitrogen service.  Doing so is just setting up a system for premature and potentially catastrophic failure.

Given the importance of the topic, the blog post focuses on hydrogen embrittlement and seal permeation, including the causes and industry best practices for addressing their effects in a hydrogen sealing solution.

Hydrogen Embrittlement in Metal Seals

At metal surfaces, H₂ dissociates into atomic hydrogen at metal surfaces. This atomic hydrogen then diffuses into the metal’s crystal lattice, where it accumulates at grain boundaries, dislocations, and stress concentrations.

While the metal’s yield strength remains unchanged, its ductility and fracture toughness decrease. The metal part can then fail without warning at stresses far below what a standard tensile test would suggest is safe. This phenomenon is particularly dangerous. 

The metals most susceptible to hydrogen embrittlement are high-strength steels above ~1,000 MPa; BCC crystal structures (ferritic and martensitic steels); and hardened fasteners like Grade 12.9, which are notorious. On the other hand, the more resistant metals include austenitic stainless steels (304L, 316L) and nickel-based alloys, as their FCC crystal structure resists hydrogen diffusion more effectively.

One of the best practices for addressing hydrogen embrittlement in metal seals is to use materials with reduced strength levels where conditions allow. For example, steel with an ultimate tensile strength of 800 MPa often outperforms 1,400 MPa steel when it is used in H₂ service. And for wetted components, engineers often opt for austenitic stainless steel. 

Another approach is to control hydrogen-generating manufacturing processes (acid pickling, electroplating) per ASTM A143. Parts can also be baked at about ~190°C within hours of processing to drive out the absorbed hydrogen.

Finally, minimizing stress concentrations on metal seals is another effective approach. The impact of these stress concentrations is significantly greater in hydrogen service.

Seal Permeation and Rapid Gas Decompression with Elastomeric Seals

Elastomeric seals can also be sensitive to hydrogen, but through a different mechanism. Because of their small atoms, hydrogen dissolves into and diffuses through elastomeric seal materials at rates far exceeding other common gases. The permeation is made worse in the presence of elevated pressure and temperature, where a seal can become fully saturated with hydrogen.

When system pressure drops during shutdown or a relief event, dissolved hydrogen tries to exit the elastomer faster than it can diffuse out of the surface, a process known as  RGD (Rapid Gas Decompression). The result is the elastomeric seals experiencing internal nucleation, blistering, and explosive tearing from the inside out. In short, a seal that survives thousands of cycles can be destroyed by a single fast depressurization event.

The most direct way to mitigate the effects of elastomeric seal permeation is to use a material that effectively resists it. Several options are summarized below.

  • FFKM (Kalrez, Perlast) — Excellent: Best all-around performance in H₂ service; highest cost
  • PTFE (spring-energized seals) — Good: Very low permeation rate; requires a different design approach than standard elastomeric seals
  • EPDM — Good: Surprisingly strong RGD resistance; widely used in fuel cell systems
  • FKM (Viton) — Moderate: Adequate at lower pressures but evaluate carefully for high-pressure cycling applications
  • NBR / Silicone — Avoid: High hydrogen permeability; not suitable for H₂ service

Other best practices include controlling the decompression rate by utilizing engineered bleed-down or staged depressurization. In addition, face seals instead of radial seals tend to work better in high-pressure applications. Backup rings are another tool to use, limiting extrusion and preferving the seal geometry. Finally, design the seals in compliance with NORSOK M-710 or ISO 23936-2. Keep in mind that data sheets alone are not sufficient for high-pressure hydrogen duty

Conclusion

Neither hydrogen embrittlement nor seal permeation is a mystery as both are well-characterized and manageable. The engineers who will struggle are those who reach for familiar materials and assume the physics is the same. Here at Advanced EMC, we encourage engineers to know the mechanisms behind these failure modes related to hydrogen service, then select deliberately and test accordingly. And Advanced EMC is here to help you every step of the way. Contact one of our seal solution experts today to explore what your options are and put industry best practices to use.

PTFE Spring Energized Seals for Cryogenic Applications
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Why Elastomer Seals Fail in Cryogenic Environments

Elastomer seals are effective in many industrial applications. They are cost-effective, forgiving to install, and reliable across a surprisingly wide range of conditions. Push them into cryogenic temperatures, though, and you stop troubleshooting a maintenance issue and start managing a system failure.

This post breaks down the five primary failure modes that render elastomers unsuitable for cryogenic environments along with some of the alternative solutions that engineers are implementing in their designs.

Why Elastomers Make Effective Seals

Elastomers are effective as seals primarily because of their polymer chain structure. Long, flexible molecular chains in elastomers are able to compress under load and still spring back, filling the microscopic gaps in a mating surface. That recovery force is what creates the seal. Remove the conditions that keep those chains mobile and flexible, and the entire sealing mechanism falls apart.

Cryogenic Failure Mode 1: Glass Transition and Brittleness

Every elastomer has a glass transition temperature (Tg); below it, polymer chains lose mobility. In short, the material stops behaving like rubber and starts behaving like glass: brittle, fragile, unable to recover from deformation.

For Buna-N (NBR), for example, that glass transition threshold sits around -40°C. Viton (FKM), on the other hand, fares slightly worse, typically losing flexibility above -20°C. Liquid nitrogen service operates at -196°C, while liquid hydrogen reaches -253°C and liquid helium reaches -269°C. Standard elastomers do not just underperform at those temperatures but become dangerous (and expesnive) mechanical hazards. A seal that shatters during assembly, or its first thermal cycle, introduces contamination and creates a safety event, not simply a leakage problem.

Cryogenic Failure Mode 2: Compression Set

A seal works by staying compressed, and compression set is what happens when the material cannot fully return to its original shape after prolonged loading. You can think of it as the seal losing the memory of what it used to look like. Now, cold temperatures accelerate this dramatically. Polymer chain mobility drops near the glass transition point, and a seal that was properly loaded at installation may lose its recovery force entirely after just one deep thermal cycle. Each subsequent cycle makes compression set worse. What starts as an acceptable leak rate at commissioning can quickly drift well outside specification after a handful of cooling and warming cycles, with no visible mechanical failure to indicate the problem. This can quickly lead to catastrophic failure.

Cryogenic Failure Mode 3: Thermal Cycling Fatigue

Thermal cycling fatigue often gets mistaken for compression set, but it operates through a different mechanism. The core problem this failure mode is differential thermal expansion between the seal and the surrounding hardware. Most elastomers expand and contract at rates several times higher than stainless steel or aluminum.

As temperatures fluctuate, the seal and flange expand and contract at different rates. In cryogenic valve applications specifically, where seals may cycle repeatedly between ambient and operating temperatures, this mismatch eventually induces micro-cracking at the seal surface and progressively erodes the contact stress at the sealing interface. The seal may look intact, but the leak rate tells a significantly different story.

Cryogenic Failure Mode 4: Outgassing and Vacuum Contamination

In vacuum environments, the failure mode shifts from mechanical to molecular. Elastomers contain residual plasticizers, curing agents, and processing residues, and under vacuum, these volatiles release trapped molecules into the chamber. For applications in semiconductor fabrication, particle accelerators, or analytical instrumentation, outgassing not only raises base pressure but it simultaneously contaminates process gases or sensitive surfaces. Elastomers rated for vacuum service reduce this problem but cannot eliminate it. Virgin PTFE and high-purity fluoropolymer materials meet NASA and ESA standards for minimal outgassing and perform reliably under ultra-high vacuum conditions.

Cryogenic Failure Mode 5: Permeation

Most engineers think of seal leakage as a gap problem. However, with elastomers, that explanation is actually incomplete. Gas molecules can actually dissolve into the polymer matrix on the high-pressure side and diffuse through the bulk material to the low-pressure side, and all with no gap required.

Permeation is especially problematic in cryogenic hydrogen service. Hydrogen is the smallest diatomic molecule in existence and thus diffuses through more materials than almost any other gas. An elastomer that tests acceptably with nitrogen can still fail badly with hydrogen, and the consequences extend beyond leakage: prolonged hydrogen permeation into metal housings contributes to embrittlement of those components over time, as well.

What Engineers Are Specifying Instead of Elastomer Seals

The answer to cryogenic sealing is rarely one-size-fits-all. The fact is the right replacement depends on the specific application, operating fluid, temperature range, and whether the seal is in a static or dynamic interface. On the plus side, several proven options can address sealing issues that elastomers cannot.

PTFE spring-energized seals are widely used in cryogenic hydrogen systems, cryogenic valve assemblies, and space applications where outgassing, permeation resistance, and low-temperature flexibility all matter. The PTFE jacket remains dimensionally stable and chemically inert at temperatures approaching -253°C, while the internal metallic spring energizer is able to maintain a consistent sealing force even as hardware contracts during cooldown. Energizer geometry is selected based on application requirements: helical springs suit low-temperature and vacuum conditions, cantilever designs work well for lighter dynamic loads, and canted coil configurations handle higher-pressure environments. For cryogenic valves in particular, the spring-energized seal design compensates for the dimensional shifts that occur during repeated thermal cycling without requiring retorquing or adjustment.

FEP-encapsulated helical spring O-rings take a different approach to the same problems. A stainless steel flat-wound helical spring core is completely encapsulated in a seamless FEP (fluorinated ethylene propylene) jacket. The FEP provides the necessary chemical inertness and cryogenic flexibility while the spring eliminates compression set by maintaining a consistent sealing load mechanically (independent of what the jacket material does under thermal stress). This type of cryogenic sealing solution is a common choice in valve assemblies, turbopump flanges, and liquid oxygen and liquid hydrogen feed lines in launch systems. The combination of (1) a non-relaxing spring core and (2) a chemically resistant outer jacket makes them very well-suited to applications involving repeated pressure and temperature transitions.

PTFE and its variants, including PCTFE and TFM, are the standard material group for cryogenic seals used in marine loading arms. In these extremely harsh environments, seals must survive continuous exposure to liquefied petroleum gas, liquefied natural gas, liquid oxygen, liquid nitrogen, and liquid hydrogen during critical loading and unloading operations. PTFE and its variants provide extremely broad chemical compatibility, very low friction, self-lubricating behavior, and the dimensional stability necessary to function reliably at temperatures as low as -269°C. Torlon polyamide-imide is another option for applications requiring rigidity and structural stability even at cryogenic extremes. 

Metal seals, including C-rings and other spring-energized metal profiles, solve the outgassing and permeation problems entirely. Metal produces no outgassing and is impermeable to gas diffusion. The tradeoff is real, however: metal seals require tighter flange tolerances, controlled surface finish, and higher seating loads. For ultra-high vacuum or cleanroom environments where those parameters can be controlled, the performance advantage of implementing a metal seal become clear.

Conclusion

None of the solutions presented here is the “perfect” answer to the harsh, demanding environment of cryogenic seals. However, PTFE spring-energized seals offer excellent flexibility and chemical resistance with relatively forgiving installation requirements. FEP-encapsulated designs, on the other hand, share similar chemical properties with PTFE and include the added resilience of a spring core that does not relax over time. PTFE-based seals in their various formulations cover marine, valve, and general cryogenic service across a wide range of media. And metal seals offer the lowest possible leakage rates and zero outgassing, but at the cost of tighter system-level tolerances.

The most common engineering mistake in this design space is treating the elastomer swap as a drop-in substitution. Groove geometry, surface finish requirements, and seating load all must be revisited when switching seal types, and the coefficient of thermal expansion of the replacement material must be accounted for during installation.

If your application involves cryogenic temperatures, hard vacuum, or both, the sealing specialists at Advanced EMC can help identify the right seal type, material, and groove specification for your design. Contact us to get started.