The energy transition aimed at achieving net-zero carbon emissions requires the integration of hydrogen into the global energy mix as a vector for the storage and transport of renewable energy. In this context, the repurposing of existing natural gas transportation infrastructure represents a strategic solution to accelerate its large-scale implementation. However, the phenomenon of hydrogen embrittlement poses critical issues that may compromise the structural integrity of pipelines. This doctoral research project aims to investigate the effects of cathodic hydrogen on an API 5L X52 steel extracted from an Italian pipeline that has been in service since the 1970s with a view to assessing its suitability for future use in hydrogen applications. The experimental activity involved the base material and the girth weld in order to analyze the effect of microstructure on susceptibility to embrittlement. Hydrogen was introduced by electrochemical charging by optimizing parameters such as current density (J = -10 mA/cm²) and electrolyte composition (0.01 mol/L H2SO4 and 2.00 g/L CH4N2S) at the laboratories of the Istituto Italiano della Saldatura-Ente Morale. The fusion zone accumulated a greater amount of hydrogen compared to both the base material and the heat affected zone. Despite the base material exhibiting larger grains than the heat affected zone, it absorbed a greater quantity of hydrogen due to its peculiar microstructure, characterized by alternating ferrite and pearlite bands and by the widespread presence of elongated manganese sulfides, which acted as trapping sites. The effect of hydrogen on mechanical properties was evaluated with fracture mechanics tests using notched tensile and bending specimens (SENT and SENB). The base material showed the greatest reduction in fracture toughness; however, the three materials exhibited similar behavior in hydrogen environment. Fractographic analysis performed by electron backscattered diffraction provided further insight into the embrittlement mechanisms. At Cranfield University, hydrogen permeation was investigated on base material, weld metal, and stressed base material specimens using a Devanathan–Stachurski cell. The base material exhibited the highest hydrogen diffusion coefficient but also promoted local hydrogen accumulation phenomena leading to surface blistering. In contrast, the weld metal was able to trap more hydrogen, showing slower diffusion and the absence of blistering. In the stressed metal, it was observed that the amount of absorbable hydrogen increased as strain intensity increased. Finally, an innovative experimental setup for the direct measurement of hydrogen permeation was designed and developed, based on the use of a mass spectrometer as a detector for hydrogen flux.
Assessment of the Repurposing Potential of Natural Gas API 5L X52 Steel Pipelines for Hydrogen Transport
CATALANO, FLAVIO
2026-06-29
Abstract
The energy transition aimed at achieving net-zero carbon emissions requires the integration of hydrogen into the global energy mix as a vector for the storage and transport of renewable energy. In this context, the repurposing of existing natural gas transportation infrastructure represents a strategic solution to accelerate its large-scale implementation. However, the phenomenon of hydrogen embrittlement poses critical issues that may compromise the structural integrity of pipelines. This doctoral research project aims to investigate the effects of cathodic hydrogen on an API 5L X52 steel extracted from an Italian pipeline that has been in service since the 1970s with a view to assessing its suitability for future use in hydrogen applications. The experimental activity involved the base material and the girth weld in order to analyze the effect of microstructure on susceptibility to embrittlement. Hydrogen was introduced by electrochemical charging by optimizing parameters such as current density (J = -10 mA/cm²) and electrolyte composition (0.01 mol/L H2SO4 and 2.00 g/L CH4N2S) at the laboratories of the Istituto Italiano della Saldatura-Ente Morale. The fusion zone accumulated a greater amount of hydrogen compared to both the base material and the heat affected zone. Despite the base material exhibiting larger grains than the heat affected zone, it absorbed a greater quantity of hydrogen due to its peculiar microstructure, characterized by alternating ferrite and pearlite bands and by the widespread presence of elongated manganese sulfides, which acted as trapping sites. The effect of hydrogen on mechanical properties was evaluated with fracture mechanics tests using notched tensile and bending specimens (SENT and SENB). The base material showed the greatest reduction in fracture toughness; however, the three materials exhibited similar behavior in hydrogen environment. Fractographic analysis performed by electron backscattered diffraction provided further insight into the embrittlement mechanisms. At Cranfield University, hydrogen permeation was investigated on base material, weld metal, and stressed base material specimens using a Devanathan–Stachurski cell. The base material exhibited the highest hydrogen diffusion coefficient but also promoted local hydrogen accumulation phenomena leading to surface blistering. In contrast, the weld metal was able to trap more hydrogen, showing slower diffusion and the absence of blistering. In the stressed metal, it was observed that the amount of absorbable hydrogen increased as strain intensity increased. Finally, an innovative experimental setup for the direct measurement of hydrogen permeation was designed and developed, based on the use of a mass spectrometer as a detector for hydrogen flux.
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