Material selection for mobile hydrogen pipelines, particularly tube trailers, is critical to ensuring safety, durability, and efficiency in hydrogen transportation. The primary challenge lies in mitigating hydrogen permeation, which can lead to material degradation, embrittlement, and potential failure. Three key material categories are commonly evaluated for this application: polymer liners, aluminum alloys, and composite materials. Each has distinct advantages and limitations in terms of hydrogen resistance, mechanical properties, and compliance with industry standards such as NACE MR0175.
Polymer liners are widely used in hydrogen tube trailers due to their low permeability and flexibility. Materials such as high-density polyethylene (HDPE) and polyamide (PA) exhibit strong resistance to hydrogen diffusion, reducing the risk of gas loss and material weakening. HDPE, for instance, has a hydrogen permeability coefficient in the range of 1.5 to 3.0 × 10⁻¹³ cm³·cm/cm²·s·Pa, making it a reliable barrier. However, polymers generally lack the mechanical strength to withstand high pressures alone, necessitating reinforcement with metal or composite outer layers. Another limitation is temperature sensitivity; most polymers degrade or lose structural integrity at elevated temperatures, restricting their use in high-temperature environments.
Aluminum alloys offer a balance between lightweight properties and hydrogen compatibility. Alloys such as AA 6061 and AA 7075 are commonly used due to their low susceptibility to hydrogen embrittlement compared to steel. Aluminum’s natural oxide layer provides additional protection against hydrogen permeation, with permeability rates significantly lower than those of uncoated steel. However, aluminum is not entirely immune to hydrogen effects. Prolonged exposure to high-pressure hydrogen can still lead to localized embrittlement, particularly in welded or highly stressed regions. Additionally, aluminum’s lower strength compared to steel or composites may require thicker walls to meet pressure vessel standards, increasing weight and reducing payload capacity in mobile applications.
Composite materials, particularly carbon fiber-reinforced polymers (CFRP), are increasingly favored for high-pressure hydrogen storage and transport. These materials combine high strength-to-weight ratios with excellent hydrogen resistance. The permeability of CFRP is highly dependent on the matrix resin used; epoxy resins, for example, exhibit permeability coefficients around 1.0 × 10⁻¹⁴ cm³·cm/cm²·s·Pa, outperforming many metals and polymers. Composites also resist hydrogen embrittlement, as the reinforcing fibers are typically inert to hydrogen diffusion. However, challenges include high manufacturing costs, susceptibility to impact damage, and the need for rigorous quality control during fabrication to prevent delamination or microcracks that could compromise integrity.
NACE MR0175 standards provide critical guidelines for material selection in hydrogen service, particularly concerning sulfide stress cracking and hydrogen-induced cracking. While originally developed for oil and gas applications, these standards are often referenced for hydrogen systems due to similarities in failure mechanisms. Materials must undergo rigorous embrittlement testing, including slow strain rate tests (SSRT) and sustained load tests, to evaluate their performance under hydrogen exposure. Aluminum alloys and composites typically perform well in these tests, whereas some high-strength steels may fail due to their susceptibility to hydrogen-assisted cracking.
Embrittlement testing is essential for validating material suitability. SSRT involves exposing tensile specimens to hydrogen environments while measuring elongation and fracture characteristics. A reduction in ductility indicates susceptibility to embrittlement. For aluminum alloys, testing often focuses on welded joints and heat-affected zones, where microstructural changes can increase vulnerability. Composites are tested for interfacial bonding strength between fibers and matrix, as debonding can accelerate permeation.
In summary, polymer liners excel in minimizing hydrogen permeation but require structural support. Aluminum alloys offer a lightweight, embrittlement-resistant solution but may lack the strength for ultra-high-pressure applications. Composites provide the best combination of strength and permeation resistance but come with higher costs and manufacturing complexities. Adherence to NACE MR0175 and thorough embrittlement testing ensures that selected materials meet the rigorous demands of mobile hydrogen transport.
The choice between these materials ultimately depends on operational requirements, including pressure ratings, weight constraints, and lifecycle costs. Advances in material science continue to improve the performance of all three categories, with ongoing research focused on enhancing permeation resistance and durability for the growing hydrogen economy.