Hydrogen gas infrastructure
by Sara McCaslin Sara McCaslin No Comments

The Two Failure Modes That Define Hydrogen Sealing

Hydrogen infrastructure, including electrolyzers, refueling stations, fuel cells, storage and distribution, is rapidly scaling, and seal manufacturers are responding with dedicated hydrogen product lines. But hydrogen is not just “another gas to seal against.” Its molecular size and behavior create two distinct failure modes that do not appear with conventional gases, and a seal rated “hydrogen compatible” on a generic datasheet can still fail in service if the selection does not account for the specific duty cycle.

Failure Mode One: Permeation

Hydrogen has an extremely small molecular size. So small, in fact, that it can diffuse through elastomers far more readily than larger gas molecules; this is a diffusion problem rather than a chemical attack problem: the elastomer is simply not an effective barrier, as opposed to degrading. 

Permeation is a serious issue in hydrogen sealing solutions. It reduces system efficiency by causing hydrogen loss, poses serious safety risks in enclosed spaces, and can, over time, contribute to internal damage as the gas accumulates within the polymer matrix.

FKM generally exhibits lower hydrogen diffusivity than NBR or EPDM, though all common elastomers permeate to some degree. Keep in mind that this is a matter of degree, not absolute barrier vs. non-barrier. There is, however, a better polymer barrier material.

PTFE is virtually inert to hydrogen with markedly lower permeation than any common elastomer, which is why it has become the default choice as pressure and criticality increase

Failure Mode Two: Rapid Gas Decompression (RGD)

Rapid gas decompression (RGD) is the failure mode most specific to hydrogen sealing and the one most likely to be underappreciated by engineers accustomed to conventional gas sealing. In this failure mode, hydrogen molecules that have diffused into the elastomer during sustained high-pressure exposure need time to escape when pressure drops; if depressurization happens faster than the gas can diffuse back out, it becomes trapped and forms internal voids, blisters, or cracks as it expands within the material.

This is also an instance where the failure mode is not solely a chemical compatibility issue. A material can be chemically compatible with hydrogen yet still fail catastrophically under rapid depressurization (RGD) if the duty cycle involves fast pressure drops. This highlights the importance of understanding the kinetics of gas escape and the mechanical response of the seal material.

This is why RGD damage can range from subsurface blistering to structural rupture, especially in high-pressure hydrogen systems such as fueling stations, electrolyzer connections, and pipeline seals. Recognizing these failure modes is essential for preventing field failures during pressure cycling, which are common in applications like rapid fueling or frequent pressure adjustments.

Material Selection Is Not One-Size-Fits-All

Here are some suggested material selections.

EPDM (Ethylene Propylene Diene Monomer)

This is often the default for lower-demand applications, and specifically for alkaline electrolyzers, offering good general hydrogen compatibility and low-temperature flexibility at a reasonable cost. It exhibits excellent low-temperature flexibility, a good compression set, and resistance to alkaline media. However, it has a higher hydrogen gas permeation rate than some other options, namely fluoropolymers such as PTFE. It is primarily used with alkaline electrolyzers (AEL/AEM), water/coolant loops, low-pressure distribution, and low-temperature static seals.

FKM (Fluoroelastomer / Viton)

FKM is favored where chemical resistance matters more broadly (e.g., acidic environments in PEM electrolyzer balance-of-plant). However, it is incompatible with certain hydrogen-adjacent process chemistries, where EPDM may be required instead. It exhibits very low hydrogen permeation compared with synthetic rubbers and has excellent thermal stability across a wide temperature range. However, it becomes brittle at extremely low temperatures, which can lead to leakage during cold thermal cycling. Note that FKM and EPDM are not interchangeable substitutes for each other. It is often used with PEM fuel cells and PEM electrolyzers (oxygen-side/stack sealing), high-temperature gas processing, and high-pressure static connections.

HNBR (Hydrogenated Nitrile Butadiene Rubber )

This material is often specified for fueling-station dispenser seals where low-temperature performance (down to around -40°C) matters, since dispensing frequently involves pre-cooled hydrogen. It has excellent mechanical strength, wear resistance, and extrusion resistance. It is also highly resistant to RGD when compounded properly and retains its flexibility at low temperatures. However, it has a moderate hydrogen permeation rate, and its continuous-use temperature is capped at 300°F. HNBR is commonly used for high-pressure hydrogen valves, compressors, sour gas blends, and pipeline seals, as well as dynamic applications subject to rapid cycling

PTFE (Polytetrafluoroethylene)

PTFE is the default across nearly all pressure tiers once RGD resistance and minimal permeation become priorities. It is inert, low-permeation, and functional across an unusually wide temperature range, though it comes with its own design considerations. It is non-elastic in the sense that it has no inherent memory. It is also prone to creep under continuous pressure loads. Note that it is typically used in energized seal configurations rather than as a standalone elastomer substitute. . It is often used with spring-energized dynamic shaft/stem seals in high-pressure valves, cryogenic liquid hydrogen valves, compressor packing rings, and anti-extrusion back-up rings..

Material Selection and the Duty Cycle

Material selection must consider the actual duty cycle, including pressure cycling frequency, depressurization rates, and temperature swings, rather than relying solely on a generic ‘hydrogen compatible’ rating. For example, two systems operating at the same peak pressure may have vastly different RGD risks depending on their depressurization speed and cycling frequency, which directly impact seal longevity.

Conclusion

Hydrogen sealing involves two key failure modes, permeation and RGD, and understanding these helps engineers feel more equipped to select appropriate materials as infrastructure scales. If you need further assistance finding an effective, reliable sealing solution for your system, contact the sealing experts here at Advanced EMC.

by Daniel Mays Daniel Mays 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.