Oxygen-mediated inhibition of gaseous hydrogen embrittlement in pipeline steels: sub-size specimen testing and coupled diffusion-damage modeling

Abstract

Hydrogen embrittlement (HE) poses a major challenge for safe hydrogen transport, particularly in repurposed natural gas pipelines. Trace oxygen additions can mitigate HE, yet their effectiveness remains insufficiently understood. This study examines the influence of 50–500 vppm oxygen in gaseous HE on a modern E355 mod. steel using sub-size tensile specimens. Tests were performed at $100$ and $200$ bar, with strain rates of $1\times 10^{-5}$ and $5\times 10^{-4}$ s⁻¹. In pure hydrogen, the reduction of area decreased from 80% in air to 55% ($100$ bar, fast rate), 45% ($200$ bar, fast rate), and 40% ($100$ bar, slow rate), indicating severe embrittlement. Oxygen additions progressively recovered ductility, with inhibition effectiveness rising from 35–50% (50 vppm and $100$ bar) to 80% (500 vppm and $200$ bar). Fractography revealed reduced hydrogen-induced surface cracking and enhanced ductile features with increasing oxygen content. Finite-element simulations employed a modified nonlocal GTN model coupled with hydrogen diffusion, extended here with an ad hoc diffusion boundary condition to represent oxygen-induced inhibition. To our knowledge, this is the first FE framework to explicitly account for environmental inhibition effects. The model successfully reproduced key experimental trends, including oxygen-mediated ductility recovery, strain-rate-sensitive stress drops, and the transition from surface- to internally-initiated damage. This integrated experimental-modeling approach provides a mechanistic basis for interpreting oxygen-mediated inhibition, quantifies the beneficial effect of trace oxygen, and establishes a foundation for predictive assessments of hydrogen uptake and damage evolution in steels under potential pipeline inhibition conditions.