Tungum® (UNS C69100) vs. 316 Stainless Steel (UNS S31600) Performance in Hydrogen Systems

Author Ruben Muro – Global Sales Manager, T2 Alloys Limited

February 2026

The global energy sector is undergoing a fundamental transition as governments, operators, and technology developers seek alternative sources of fuel capable of delivering prolonged, low-cost energy while supporting aggressive renewable energy and decarbonization targets. Among the available options, hydrogen has emerged as a critical energy carrier for enabling large-scale energy storage, transportation, and conversion across multiple industries.

As hydrogen infrastructure expands rapidly, the volatility, diffusivity, and embrittlement potential of hydrogen demand a rigorous reassessment of the materials currently used to contain, transport, and control this energy source. Ensuring that hydrogen systems are designed with materials that provide long-term safety, reliability, and resistance to degradation is essential if hydrogen is to become a viable cornerstone of the future energy mix.

Austenitic stainless steels such as 316/316L have historically been selected for hydrogen service based on general corrosion resistance, previous automotive specifications, and availability. However, extensive research and service experience have shown that 316 stainless steels can exhibit measurable degradation in the presence of hydrogen, particularly under conditions involving pressure cycling, welding, and cryogenic operation.

Given the inherent characteristics of hydrogen and the documented response of 316 stainless steel, alternative materials with proven hydrogen compatibility warrant serious consideration. Tungum® (UNS C69100), a copper-based alloy with exceptionally low hydrogen solubility and outstanding resistance to fatigue and localized corrosion, represents a tubing material that should be evaluated as part of a safe, reliable, and durable hydrogen infrastructure for decades to come.

This paper presents a technical comparison of Tungum tube and 316 stainless steels for hydrogen service, with emphasis on hydrogen solubility, fatigue performance, and corrosion resistance.

Figure 1 – Hydrogen Solubility Comparison

Copper-based alloys such as Tungum exhibit much lower hydrogen solubility than austenitic stainless steels, significantly reducing hydrogen embrittlement risk.[1,2,6]

Figure 2 – Fatigue S–N Curves in Hydrogen

Hydrogen exposure significantly degrades fatigue strength in 316 stainless steels, while Tungum maintains stable fatigue behavior due to minimal hydrogen absorption. [1,8]

Figure 3 – Pitting Corrosion Depth Comparison

Laboratory and field exposure testing in chloride environments demonstrate significantly reduced pit depth in Tungum compared with 316 stainless steels. [2,4]

Microstructural Influence on Hydrogen Compatibility

Hydrogen interaction with metals is strongly influenced by microstructure. Iron-based alloys contain numerous “trap” sites including dislocations, grain boundaries, inclusions, and weld-induced defects. These features promote hydrogen accumulation and embrittlement mechanisms.

Copper-based alloys such as Tungum exhibit a homogeneous FCC matrix with limited hydrogen trapping capability. Hydrogen solubility and diffusivity are extremely low, resulting in negligible mechanical property degradation even under hydrogen exposure.

Figure 4 – Hydrogen Absorption Comparison

Relative hydrogen trap density in copper-based versus iron-based alloys. Lower trap density in Tungum significantly reduces embrittlement susceptibility. [1,2,5]

Expanded Fatigue Crack Initiation and Growth Analysis

Hydrogen-assisted fatigue degradation occurs through accelerated crack initiation and enhanced crack growth rates. In austenitic stainless steels, hydrogen promotes localized plasticity at the crack tip, reducing fatigue life under cyclic stress.

Published studies demonstrate that hydrogen-charged 316 stainless steel exhibits reduced endurance limits and steeper S–N slopes compared with inert environments. Welds and cold-worked regions are particularly susceptible.

Tungum’s resistance to hydrogen uptake effectively eliminates hydrogen-assisted fatigue mechanisms, preserving fatigue margins even under aggressive cycling.

Detailed Pitting and Crevice Corrosion Mechanisms

Pitting and crevice corrosion are often life-limiting for stainless steels in chloride containing environments. Breakdown of the very delicate chromium oxide passive film initiates localized attack, which can propagate rapidly under stagnant conditions. This allows for aggressive pitting and crevice corrosion.

Copper alloys form stable surface films that do not rely on chromium passivation. Tungum’s alloy chemistry suppresses localized corrosion initiation and limits pit propagation, as evidenced by significantly shallower pit depths in laboratory and field exposure testing.

Side-by-Side Material Comparison for Hydrogen Service

Note: This table is compiled from multiple published sources; no single publication presents a direct alloy-to-alloy comparison.

Applicable Codes and Standards

ASME B31.12 – Hydrogen Piping and Pipelines

ASME BPVC Section VIII, Div. 1 & 2

ISO 11114-2 – Metallic Materials for Hydrogen Gas Service

ISO/TR 15916 – Hydrogen Safety Considerations

CSA CHMC 1 – Hydrogen Compatibility Test Methods

EIGA Doc 121 – Materials Selection for Hydrogen Service

Documented Performance Degradation and Failures of 316 Stainless Steel in Hydrogen Service

While 316/316L stainless steel is frequently specified for hydrogen systems due to its general corrosion resistance and austenitic structure, extensive experimental evidence demonstrates that it is not immune to hydrogen-induced degradation. Under conditions representative of hydrogen service—high pressure, cyclic loading, welding, cold work, and cryogenic temperatures—measurable reductions in fatigue life, fracture toughness, and crack resistance have been documented.

Case Study 1 – Hydrogen-Assisted Fatigue Cracking in Welded 316L

Experimental testing of welded 316L stainless steel joints exposed to hydrogen environments has shown significant reductions in fatigue strength compared with uncharged specimens. Weld metal and heat-affected zones exhibited accelerated crack initiation and propagation under cyclic loading. Fractographic analysis revealed hydrogen assisted cracking features within the weld bead, demonstrating that welding-induced microstructural heterogeneity exacerbates hydrogen susceptibility.

Ref.) NASA White Sands Test Facility (WSTF). *Hydrogen Compatibility of Materials*, NASA TM‑102411, 2019.

Case Study 2 – Hydrogen-Assisted Crack Growth in Pre-Cracked 316L

Specimens Laboratory investigations on pre-cracked 316L stainless steel specimens tested in hydrogen environments have demonstrated enhanced crack growth rates and multi-crack formation at the crack tip. Hydrogen presence altered the local deformation mechanisms, reducing flaw tolerance and increasing the likelihood of unstable crack propagation under applied stress.

Ref.) Ono, K., Somerday, B., et al. “Hydrogen-Assisted Fatigue and Crack Growth in Austenitic Stainless Steels.” *International Journal of Hydrogen Energy*, 2005.

Case Study 3 – Fracture Mode Transition in 316 Stainless Steel Exposed to Hydrogen

Mechanical testing of 316 stainless steel in high-pressure hydrogen gas has shown a transition in fracture surface morphology from ductile microvoid coalescence to quasi cleavage and intergranular features. This transition indicates hydrogen-mediated suppression of intrinsic toughening mechanisms, even in fully austenitic microstructures traditionally considered resistant to embrittlement.

Ref.) ASME. *ASME B31.12 Hydrogen Piping and Pipelines*, Nonmandatory Appendix A, 2022 Edition.

These documented behaviors are particularly relevant for hydrogen piping and tubing systems subjected to pressure cycling, welding, and offshore or cryogenic service. In contrast, copper-based alloys such as Tungum exhibit negligible hydrogen uptake, eliminating these degradation pathways.

Summary

Hydrogen infrastructure places exceptional demands on metallic materials due to hydrogen diffusion, pressure cycling, cryogenic temperatures, and environmental exposure. Material selection directly impacts system safety, reliability, and life-cycle cost.

This white paper demonstrates that Tungum® (UNS C69100), an aluminum-nickel-silicon copper alloy, offers superior resistance to hydrogen-related degradation mechanisms when compared with 316 stainless steel (UNS S31600). Tungum exhibits much lower hydrogen solubility, minimal embrittlement risk, stable fatigue performance under hydrogen cycling, and exceptional resistance to pitting and crevice corrosion in chloride environments.

For hydrogen tubing systems—particularly those operating offshore, in cryogenic service, or under frequent pressure cycling—Tungum represents a technically robust and lower risk alternative to 316 stainless steel.

References

1. San Marchi, C., Somerday, B.P., Hydrogen Compatibility of Structural Materials, Sandia National Laboratories.

2. ASM Handbook, Volume 13A: Corrosion—Fundamentals, Testing, and Protection.

3. ASM Handbook, Volume 19: Fatigue and Fracture.

4. Oldfield, J.W., Todd, B., Corrosion Science, Vol. 38, 1996.

5. Lynch, S.P., Corrosion Reviews, Vol. 30, 2012.

6. NIST Monograph 177, Cryogenic Materials Data Handbook.

7. NASA White Sands Test Facility. Hydrogen Compatibility of Materials, NASA TM‑102411, 2019.

8. Ono, K., Somerday, B., International Journal of Hydrogen Energy, 2005.

9. ASME B31.12 Hydrogen Piping and Pipelines, 2022.

10.ISO 11114‑2:2020.

11.ISO/TR 15916 – Hydrogen Safety Considerations.

12.CSA CHMC 1 – Hydrogen Compatibility Test Methods.