1. Overview of Hydrogen Transmission Pipeline Construction Status

1.1 Global Construction of Pure Hydrogen & Hydrogen-Blended Pipelines

Hydrogen, as a green, efficient, and sustainable secondary energy source, has become a key part of the new energy system. The construction of hydrogen transmission pipelines—both for pure hydrogen and hydrogen-blended natural gas—has made significant progress worldwide, though scale varies by region.

In Europe, the total length of completed hydrogen transmission pipelines is approximately 1,770 km. The United States leads with a more extensive network, boasting 2,720 km of hydrogen pipelines, while Canada has around 150 km of such infrastructure. For hydrogen-blended natural gas projects, Europe has taken early steps: France launched the GRHYD project in 2014, injecting hydrogen into local natural gas networks at a ratio not exceeding 20% (with a 6%-20% blend ratio for natural gas-powered buses). Italy’s national natural gas network company adopted a 5%-10% hydrogen blend ratio in 2019, and the UK conducted research on 20% hydrogen blending the same year. Australia initiated a hydrogen-blended natural gas demonstration project in 2020 with a 10% blend ratio, and the UK’s Energy Networks Association (ENA) announced a target to achieve 20% hydrogen blending in natural gas pipelines by 2023.

In China, the total length of completed hydrogen transmission pipelines is about 400 km. Pilot projects for hydrogen-blended natural gas pipelines have also been launched, including offshore hydrogen-blended pipeline projects, natural gas hydrogen-blending demonstration projects in multiple regions, and a pilot project for hydrogen-blended natural gas carbon reduction. Most existing domestic pure hydrogen pipelines operate at a maximum pressure of 5 MPa and primarily use low-strength seamless steel pipes. One exception is the hydrogen-blended pipeline (with 68% hydrogen content) that uses longitudinal submerged arc welded steel pipes, with strength equivalent to Grade 20 steel.

1.2 Challenges in Hydrogen Pipeline Scaling

Compared with oil and natural gas pipelines, hydrogen pipelines account for a very small proportion in terms of mileage, transmission pressure, and throughput. This is mainly due to two factors: high costs and hydrogen damage to materials. Data shows that the cost of hydrogen pipelines in the United States is more than twice that of natural gas pipelines. However, with the future increase in hydrogen pipeline pressure levels and construction scale, the cost of hydrogen transmission is expected to approach that of natural gas.

2. Development of Hydrogen Transmission Pipeline Technical Standards

2.1 Domestic Hydrogen System Standards

Domestic standards related to hydrogen systems are categorized into three main areas, though most do not cover buried long-distance hydrogen pipelines:

• Safety standards for hydrogen systems: Such as GB/T 29729—2013 Basic Requirements for Hydrogen System Safety and GB 4962—2008 Safety Technical Code for Hydrogen Use.
• Standards for hydrogen stations and hydrogen refueling stations: Including GB 50177—2005 Design Code for Hydrogen Stations and GB 50516—2010 (2021 Edition) Technical Code for Hydrogen Refueling Stations.
• Standards for hydrogen storage and transmission systems: GB/T 34542 Hydrogen Storage and Transmission Systems (8 parts in total, with Parts 4-8 yet to be released).

GB/T 34542.1—2017 Hydrogen Storage and Transmission Systems – Part 1: General Requirements specifies general requirements for overall design, safety accessories, installation and commissioning, operation management, and risk assessment of hydrogen storage and transmission systems. It applies to systems with a working pressure of ≤140 MPa and an ambient temperature range of -40°C to 65°C. Additionally, GB/T 34542.2—2018 Part 2: Test Methods for Compatibility of Metallic Materials with Hydrogen Environment and GB/T 34542.3—2018 Part 3: Test Methods for Hydrogen Embrittlement Sensitivity of Metallic Materials provide testing and evaluation standards for hydrogen-exposed materials.

2.2 International Hydrogen Pipeline Standards

Major international standards for hydrogen pipelines cover system design, material requirements, and testing methods, with distinct scopes of application:

2.2.1 ASME B 31.12—2019 Pipeline Systems for Hydrogen

This standard applies to long-distance transmission pipelines, distribution pipelines, and service pipelines that transport hydrogen and liquid hydrogen from manufacturing facilities to points of use. It covers industrial and long-distance pipelines but excludes pressure vessels designed per ASME Boiler and Pressure Vessel Codes, pipeline systems with temperatures above 232°C or below -62°C, systems with pressure exceeding 21 MPa, pipelines with water vapor content >20 mg/L, and pipelines with hydrogen volume fraction <10%.

ASME B 31.12—2019 requires that the maximum operating pressure of all materials listed in Table GR-2.1.1-2 shall not exceed 21 MPa, unless material performance under hydrogen conditions meets ASME B&PVC Div.3. It recommends using API Spec 5L PSL2 X42/X52 steel pipes and covers three types of steel pipes: electric resistance welded pipes, seamless steel pipes, and submerged arc welded pipes. Compared with natural gas pipeline standards, this standard adds a material performance factor, which increases the required design wall thickness of steel pipes.

2.2.2 CGA G-5.6—2005 (R2013) Hydrogen Pipeline Systems

Developed by the Compressed Gas Association (Europe), this standard applies to transmission and distribution systems for gaseous pure hydrogen and gaseous hydrogen mixtures, with a temperature range of -40°C to 175°C and a total pressure limit. For stainless steel, it specifies that the hydrogen partial pressure shall be above 0.2 MPa. It also sets special requirements for ultra-high-purity hydrogen pipelines (purity ≥99.995%).

2.2.3 AIGA 033/14 Hydrogen Pipeline Systems

Released by the Asian Industrial Gases Association (AIGA), this standard is adapted from CGA G-5.6, ensuring consistency with European standards for regional application.

2.2.4 Hydrogen-Exposed Material Testing Standards

International testing standards for hydrogen-exposed materials include:

• ISO 11114-4:2017 Mobile Gas Cylinders – Compatibility of Cylinder and Valve Materials with Gas Contents – Part 4: Test Methods for Selecting Hydrogen Embrittlement-Resistant Steels
• ASME B&PVC Div.3—2019 ARTICLE KD10 Special Requirements for Vessels for Hydrogen Service
• ANSI/CSA CHMC 1—2014 Test Methods for Evaluating Material Compatibility in Compressed Hydrogen Applications – Metals
• ASTM G 142—1998 (2016) Standard Test Method for Determining Sensitivity of Metals to Embrittlement in Hydrogen-Containing Environments at High Pressure, High Temperature, or Both
• ASTM G 129—2021 Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking
• ASTM F 1459—2006 (2017) Standard Test Method for Determining the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HGE)

3. Impact of Hydrogen on Pipeline Materials

The key technical challenges in developing hydrogen transmission pipe materials include hydrogen embrittlement (mainly affecting metallic materials) and hydrogen permeability (mainly affecting non-metallic materials such as polyethylene). Hydrogen embrittlement in steel pipes manifests as delayed fracture under stress and is influenced by environmental, material, and stress factors. Three basic conditions are required for hydrogen embrittlement fracture: sufficient hydrogen content, hydrogen-sensitive metallographic structure, and sufficient triaxial stress.

3.1 Hydrogen Compatibility of Metallic Materials

Domestic and international research has focused on high-pressure hydrogen environment compatibility testing of materials such as 4130 Cr-Mo low-alloy steel, 300-series austenitic stainless steel, 6061 aluminum alloy, and API Spec 5L X42~X80 pipeline steel. A domestic university has established China’s first database for the compatibility of domestic metallic materials with high-pressure hydrogen environments.

Studies on X80 pipeline steel (via slow strain rate tensile tests, fracture toughness tests, and fatigue crack growth tests) show that hydrogen does not significantly change the material’s elastic modulus, yield strength, or tensile strength, but it significantly reduces plasticity and fracture toughness while accelerating fatigue crack growth rate. For X52 pipeline steel, the maximum reduction in area shrinkage rate reaches 80.4%, and the maximum reduction in elongation rate is 52.3%. Additionally, research on X60, X80, and X100 pipeline steel indicates that as material strength increases, hydrogen embrittlement sensitivity rises significantly when the hydrogen charging current density exceeds a certain threshold—highlighting the need to control current density during cathodic protection of buried pipelines.

3.2 Influence of Hydrogen Pressure and Purity on Material Performance

Hydrogen concentration and pressure directly affect material failure risk: pipelines are less prone to severe fracture when hydrogen concentration is below 50%, and hydrogen-induced crack propagation at defects is unlikely when pipeline pressure is below 2 MPa. As hydrogen pressure increases, material sensitivity to hydrogen embrittlement rises.

In high-pressure hydrogen-blended natural gas environments, the fatigue crack growth rate of pipeline steel is approximately one order of magnitude higher than in hydrogen-free environments. For example, the fatigue life of X80 pipelines without hydrogen blending is 22.8 times that of pipelines with 50% hydrogen blending. When hydrogen content is below 5%, hydrogen in coal-based natural gas has little effect on the strength of domestic X80 pipeline steel but reduces plasticity and significantly impairs fatigue crack growth performance—with a more severe degradation effect on the base metal than on welds.

Hydrogen purity and impurity gases also impact hydrogen embrittlement: gases such as O₂, CO₂, and SO₂ hinder hydrogen penetration into metals, reducing hydrogen embrittlement; in contrast, H₂S accelerates hydrogen penetration and exacerbates embrittlement. CO reduces hydrogen adsorption on the surface of X80 steel samples, thereby decreasing hydrogen embrittlement sensitivity—suggesting that adding CO to natural gas during hydrogen blending can lower the risk of hydrogen embrittlement failure in pipeline steel.

3.3 Influence of Material Composition and Microstructure

For high-strength low-alloy steels (commonly used in pipelines), elements such as S, P, Al, Si, and Mn easily form segregation or inclusions during steelmaking or rolling, increasing hydrogen embrittlement sensitivity. Thus, the mass fractions of S and P in steel must be strictly controlled. In contrast, strong carbide-forming elements (Cr, Mo) and active elements (V, Ti, rare earths) reduce hydrogen embrittlement sensitivity. Among high-strength low-alloy steels, Cr-Mo steel and Cr-Ni-Mo steel have better hydrogen embrittlement resistance than carbon-manganese steel.

Microstructure also plays a critical role in hydrogen embrittlement, with susceptibility increasing in the following order: ferrite/pearlite < bainite < low-carbon martensite < martensite-bainite mixture < twinned martensite. Ferrite has the lowest hydrogen embrittlement sensitivity, while martensite has the highest—due to more hydrogen traps in martensite that easily capture hydrogen atoms and cause damage. Needle-like ferrite exhibits better resistance to hydrogen-induced cracking (HIC) than ultra-fine needle-like ferrite, making it an ideal microstructure for oil and gas transmission pipeline steel. For X80 longitudinal submerged arc welded pipes, needle-like ferrite microstructures show better hydrogen embrittlement resistance than polygonal ferrite + bainite microstructures (measured by area shrinkage and tensile displacement loss rates).

3.4 Influence of Service Conditions

Hydrogen transmission pipelines bear complex operating stresses during service, as well as residual stresses from manufacturing or installation—both of which significantly increase hydrogen embrittlement sensitivity. Tensile stress causes hydrogen to accumulate at stress concentration points; in high-pressure hydrogen environments, hydrogen molecules adsorb on the metal surface, dissociate into hydrogen atoms, penetrate the metal, and accumulate in localized regions, leading to hydrogen enrichment.

Welded joints are the weakest links in pipelines: uneven heating during welding causes microstructural inhomogeneity, residual stress, and potential welding defects—all of which increase hydrogen embrittlement sensitivity. Thus, studying hydrogen-induced embrittlement in hydrogen-exposed pipelines (especially at joints) is critical for safe design and operation of high-pressure hydrogen-exposed pipelines.

Hydrogen embrittlement typically occurs below 95°C, with the most severe hydrogen-induced plasticity loss observed near room temperature for most alloys.

4. Evaluation and Testing Methods for Hydrogen Transmission Steel Pipes

4.1 Hydrogen Environment Compatibility Testing

Hydrogen environment compatibility testing includes pre-charged hydrogen testing and high-pressure hydrogen environment testing. The latter better simulates the actual interaction between hydrogen and component stress/deformation in real hydrogen-exposed environments. International institutions have developed high-pressure testing equipment (e.g., a maximum test pressure of 210 MPa and temperature of 190°C), while a domestic university has independently developed China’s first 140 MPa high-pressure hydrogen environment durability test device.

GB/T 34542.2—2018 specifies four key test items for metallic material compatibility with hydrogen environments:

4.1.1 Slow Strain Rate Tensile Test

Key indicators include stress-strain curves, yield strength, tensile strength, elongation after fracture, and area shrinkage. The test uses constant displacement rate loading: the strain rate of the gauge length of smooth round bar samples shall not exceed 2×10⁻⁵ s⁻¹, and the strain rate of 25.4 mm sample segments centered on the notch (for notched round bar samples) shall not exceed 2×10⁻⁶ s⁻¹—consistent with ANSI/CSA CHMC 1—2014.

4.1.2 Fatigue Life Test

For force-controlled testing, requirements in GB/T 3075 Metallic Materials – Fatigue Testing – Axial Force Control Method apply; for strain-controlled testing, requirements in GB/T 15248 Metallic Materials – Axial Constant-Amplitude Low-Cycle Fatigue Testing Method or GB/T 26077 Metallic Materials – Fatigue Testing – Axial Strain Control Method apply. The key indicator is the stress/strain-life curve.

4.1.3 Fracture Toughness Test

The test procedure follows GB/T 21143—2014 Metallic Materials – Unified Test Method for Quasi-Static Fracture Toughness, with key indicators including stress intensity factor K, crack tip opening displacement δ, and J-integral.

4.1.4 Fatigue Crack Growth Rate Test

Test equipment complies with GB/T 6398—2017 Metallic Materials – Fatigue Testing – Fatigue Crack Growth Method, with the key indicator being the curve of fatigue crack growth rate (da/dN) versus stress intensity factor range (ΔK). ASTM G 142—1998 (2016) specifies slow strain rate tensile tests, while ISO 11114-4:2017 (Clause 5.2, Method 2) specifies fracture toughness tests in gaseous hydrogen environments (using compact tension (CT) samples, with the stress intensity factor threshold K₁ₕ as the indicator) and hydrogen-induced cracking tests (Clause 5.3, Method 3).

4.2 Hydrogen Embrittlement Sensitivity Testing

Also known as the disk pressure test, this method evaluates hydrogen embrittlement sensitivity of metallic materials in pure hydrogen environments. Standards include GB/T 34542.3—2018, ISO 11114-4:2017 (Clause 5.1, Method 1), and ASTM F 1459—2006 (2017). Sensitivity is assessed using the hydrogen embrittlement sensitivity coefficient (maximum ratio of corrected burst pressure in helium environment to corrected burst pressure in hydrogen environment, p’ₕₑ/p’ₕ₂).

5. Material Selection for Hydrogen Transmission Pipelines

5.1 Leveraging Material Properties to Prevent Hydrogen Embrittlement

GB/T 29729—2013 specifies measures to reduce hydrogen embrittlement sensitivity of metallic materials:

• Control material hardness and strength at appropriate levels;
• Reduce residual stress;
• Avoid or minimize cold plastic deformation of materials;
• Prevent fatigue failure of components under alternating loads;
• Use materials with low hydrogen embrittlement sensitivity (e.g., austenitic stainless steel, aluminum alloy).

The material selection route for hydrogen transmission pipelines is similar to that for acid service pipelines, focusing on:

• Steel cleanliness: Minimize inclusions and impurities;
• Low strength: Reduce hydrogen embrittlement sensitivity;
• Chemical composition design: Lower C, P, and S content; increase Cr, Mo (strong carbide-forming elements) and Cu, Ni (corrosion-resistant elements);
• Microstructural control: Minimize inclusion size and quantity; strictly control non-metallic inclusion morphology, composition segregation, and banded structure;
• Performance requirements: High fracture toughness, low residual stress, and avoidance of stress concentration due to structural design.

Carbon steel is the most commonly used alloy series for hydrogen transmission pipelines. API Spec 5L X52 and lower-grade steel pipes, as well as ASTM A 106 Grade B steel, have been widely used with minimal issues. From the perspective of pipe type, acid-resistant seamless steel pipes are widely used in existing long-distance hydrogen transmission pipelines (per GB/T 29729—2013, which mandates seamless steel pipes for hydrogen, liquid hydrogen, and hydrogen slurry industrial pipelines). Seamless steel pipes have no welds and thus no stress concentration issues.

High-Frequency Welded (HFW) pipes have no obvious weld reinforcement (after deburring) and can undergo full-pipe heat treatment to reduce or eliminate residual stress. Submerged arc welded pipes have weld reinforcement: longitudinal submerged arc welded pipes can reduce residual stress via cold expansion, while spiral submerged arc welded pipes can do so via hydrostatic testing or hydrostatic expansion. Research shows that removing weld reinforcement significantly extends fatigue crack initiation life (by ~1.32 times) and total fatigue life (by ~1.09 times) of samples—highlighting the importance of reducing or removing weld reinforcement for submerged arc welded pipes to lower stress concentration and improve fatigue life.

For non-metallic pipes, polyethylene (PE) is the primary research focus. Although PE pipes have hydrogen permeability issues, the permeation rate is negligible relative to annual transmission volumes.

5.2 Leveraging Internal Coatings to Prevent Hydrogen Damage

Surface functionalization via coating deposition is an effective hydrogen permeation barrier. Ceramic materials are preferred for their low permeability and excellent thermodynamic performance. Current mainstream hydrogen barrier coating materials include ceramic-based coatings (Cr₂O₃, Al₂O₃, Er₂O₃, ZrO₂, Si₃N₄, SiC) and composite coatings (Al-Al₂O₃, Al₂O₃-TiO₂).

For epoxy resin coatings, studies show that hydrogen atoms penetrate epoxy-coated steel samples during electrochemical hydrogen charging and accumulate at the coating-metal interface. When hydrogen pressure exceeds the coating’s adhesion strength, local blistering occurs—with peeling severity increasing as hydrogen charging current density rises. This indicates a positive correlation between hydrogen permeation level of epoxy coatings and hydrogen concentration.

5.2.1 Coating Preparation Methods

Mainstream surface coating preparation methods include:

• Electrochemical methods (electroplating): May promote hydrogen absorption by materials;
• Physical/chemical vapor deposition (PVD/CVD);
• Magnetron sputtering: Used to prepare Al-Al₂O₃ and Al₂O₃-ZrO₂ double-layer composite coatings;
• Ion spraying;
• Molten aluminum dipping.

Innovative coating solutions include ion beam-assisted deposition of Al-Al₂O₃ composite hydrogen barrier coatings, thermal spray composite coatings (anodic metal coating + bottom sealing coating + top corrosion-resistant coating) to prevent hydrogen blistering corrosion, and radio frequency magnetron sputtering to prepare Al films (followed by oxidation to form Al-Al₂O₃ composite coatings).

5.2.2 Coating Applications for Hydrogen Transmission Pipes

Internal coatings for natural gas transmission pipes primarily serve drag reduction purposes, with key indicators focusing on inner surface roughness (typically single-layer coatings formed by curing two-component liquid epoxy coatings, including solvent-based and solvent-free types). For oil country tubular goods (OCTG, including drill pipes), internal coatings are used for corrosion prevention—with standards (e.g., SY/T 0544—2016 Technical Specifications for Internal Coatings of Oil Drill Pipes) mandating two-layer structures (base coat + top coat, including liquid and powder coatings) and significantly thicker dry film thickness than drag-reducing coatings for gas transmission pipes.

For hydrogen transmission pipeline steel pipes, research on hydrogen barrier coatings (e.g., internal PE linings or two-layer epoxy coatings) is recommended to further enhance hydrogen damage resistance.

6. Conclusion

Global research on hydrogen transmission pipeline steel pipes has achieved significant progress in key areas: hydrogen embrittlement mechanisms, hydrogen’s impact on materials, development of technical standards, and development of hydrogen-resistant materials. Both domestic and international markets have completed a certain scale of hydrogen pipeline construction—including pure hydrogen pipelines and hydrogen-blended natural gas pipelines—with pilot projects providing valuable data for industry scaling.

However, compared with mature natural gas pipeline networks, hydrogen pipelines remain limited in scale and proportion—presenting enormous development potential. Future efforts should focus on: (1) Optimizing material composition and microstructure to improve hydrogen embrittlement resistance; (2) Developing low-cost, high-performance hydrogen barrier coatings; (3) Refining technical standards to cover long-distance buried hydrogen pipelines; (4) Reducing construction and operation costs to enhance economic viability. These steps will support the safe, efficient, and large-scale development of hydrogen transmission infrastructure, laying a solid foundation for the global transition to clean energy.