Hydrogen transport: An integrated materials–integrity–systems review of legacy natural gas pipeline conversion to pure hydrogen service

Repurposing high-pressure natural gas pipelines for hydrogen is increasingly viewed as a technically viable and often economically preferable alternative to newbuild corridors for medium to long-distance, high-utilization transport, provided hydrogen-specific degradation, integrity, and regulatory issues are explicitly addressed. This review screens about 260 records and synthesizes roughly 80 key studies on hydrogen embrittlement, hydrogen-induced cracking, sulfide stress corrosion cracking, microbiologically influenced corrosion, and hydrogen-affected fatigue in vintage API 5L X42–X70 steels, welds, and heat-affected zones. It reports fracture-toughness reductions of up to ∼50% and fatigue crack growth rate increases of up to an order of magnitude in severe hydrogen environments, and evaluates coatings, polymeric liners, and metallic cladding as mitigation options, distinguishing widely deployed non-structural measures such as epoxy and PE liners from higher-performance barriers for critical or pure-hydrogen corridors. It outlines a hydrogen-specific integrity workflow that integrates in-line inspection, fracture-mechanics-based fitness-for-service and engineering critical assessments, and probabilistic models to define segment-level safe flaw sizes, operating envelopes, and reliability-based inspection and rehabilitation plans. On the systems side, techno-economic and environmental evidence indicates that, beyond distances of a few hundred kilometers and at moderate-to-high load factors, repurposed assets can achieve substantially lower levelized transport costs and lifecycle emissions than newbuild hydrogen pipelines or carrier-based options, while hydrogen’s low volumetric energy density and high diffusivity reshape compressor requirements, line-pack behavior, and leakage control. Finally, the review proposes a risk-based, multi-objective planning framework to guide decisions on when to repurpose, derate, or replace specific legacy pipelines.