In the transition toward a low-carbon energy system, hydrogen is increasingly recognised as a key energy carrier due to its versatility, environmental neutrality, and capacity to integrate renewable sources across industrial, transport, and residential sectors. Repurposing existing pipeline infrastructure for hydrogen transport offers immediate availability and cost advantages, yet hydrogen exposure can impair the fatigue performance of pipeline steels, potentially leading to brittle failures. Moreover, internal oxide layers that naturally form during long-term service may further influence fatigue behaviour, raising questions about the suitability of vintage steels for hydrogen applications.

This thesis examines the hydrogen-assisted fatigue behaviour of a vintage API 5L X52 pipeline steel, with particular focus on internal oxides formed after 16 years of natural-gas service. Optical Microscopy, surface-roughness measurements, and Raman Spectroscopy revealed a thick (36 ± 17 to 100 ± 27 μm), adherent, multi-layered oxide composed of magnetite (Fe3O4) and goethite (α-FeOOH), with local hematite (Fe2O3). Based on these findings, near-representative artificial oxide layers (AOLs) were produced with galvanostatic anodisation and validated for controlled in-situ gaseous fatigue testing.

Fatigue experiments in 100 bar H2 and N2 environments showed that hydrogen reduced fatigue life by ∼40% in polished specimens and ∼43% in AOL-coated specimens. The comparable magnitude of this reduction indicates that the presence of oxide layers does not impose an additional hydrogen-related penalty on fatigue life. AOLs decreased fatigue life by 52-54% relative to the polished surface condition, for nitrogen and hydrogen environments, respectively. This reduction highlights the accelerated effect of oxides on (hydrogen-assisted) fatigue degradation. Crack initiation was estimated using displacement-signal derivatives, ex-situ Alternating Current Potential Drop (ACPD), and Paris-law propagation calculations. While each method introduced uncertainties, results consistently indicated that hydrogen shortened the initiation phase, with oxide layers amplifying this effect by facilitating local hydrogen ingress through microcracks and pitting. Paris-law estimates a similar large reduction in crack-propagation cycles under hydrogen compared with nitrogen. At the same time, ex-situ ACPD measurements confirmed that most fatigue life was consumed during initiation. SEM fractography revealed a transition from ductile fracture in nitrogen to brittle transgranular fracture in hydrogen, independent of surface condition.

The novelty of this work lies in its combined, experimentally isolated assessment of hydrogen exposure and internal oxide layers, which are typically studied separately. The demonstrated hydrogen/oxide synergy represents a critical, previously overlooked factor in fitness-for-service evaluations of repurposed pipelines. Overall, the findings indicate that the combined presence of hydrogen and oxide layers markedly reduces fatigue life, underscoring the importance of surface condition and oxide integrity for the safe reuse of aged steel infrastructure for hydrogen transport.