Abstract
Hydrogen has been seen as a possible energy carrier in the transition toward carbon neutrality. To make large-scale hydrogen use feasible, an extensive distribution network is needed. Pipelines are the most efficient way to transport hydrogen over long distances, and reusing existing natural gas pipelines is considered a cost-effective alternative to building new ones. However, steels used in pipelines can suffer from hydrogen embrittlement, which reduces their ductility, toughness and resistance to cracking. This presents a challenge for safe hydrogen transport, making it essential to understand how hydrogen affects pipeline materials.
This PhD project investigated the effect of hydrogen on the mechanical performance of both modern and older vintage pipeline steels. Four steels were studied: three taken from spare material from in-service natural gas pipelines (grades X60–X65) and one modern X65 steel. The focus was on mechanical properties, fracture toughness, and fatigue crack growth behaviour. Tests were conducted in air and under in situ electrochemical hydrogen charging, which simulates hydrogen exposure during pipeline operation.
A screening programme using slow strain rate tensile testing was first carried out to compare the steels and identify the most and least susceptible to hydrogen embrittlement. These two were then chosen for detailed testing of fracture toughness and fatigue crack growth rate.
Because the tests were performed in seawater with hydrogen charging, conventional measurement cannot operate reliably. To address this, digital image correlation (DIC) was introduced as a non-contact method for tracking crack opening and crack propagation. The main challenge for DIC was the formation of hydrogen bubbles on the specimen surface, which blocked visibility and interfered with image analysis. A new procedure was therefore developed to mitigate this problem, enabling accurate measurements during hydrogen exposure.
Fracture toughness experiments revealed that hydrogen strongly reduced crack resistance compared to tests in air. An unusual behaviour was observed during constant load testing, where cracks that had begun to grow sometimes stopped despite the increasing driving force. Post-test microscopy suggested that this was linked to crack branching in hydrogen-affected zones, which effectively shielded the main crack.
Fatigue crack growth tests were carried out in both air and hydrogen. Hydrogen accelerated crack growth in both modern and vintage steels, but the degree of acceleration depended on frequency and microstructure. At low frequencies, the acceleration was particularly pronounced. Despite these effects, most of the results were still within existing design guidelines for hydrogen pipelines, although the modern steel showed higher crack growth rates than expected at low stress intensity factor ranges.
Overall, this work highlights how hydrogen affects the fracture and fatigue behaviour of different pipeline steels and shows that performance cannot always be predicted from simple screening tests. The findings contribute to the safe design and potential repurposing of existing pipelines for hydrogen transport.