Hydrogen permeation in pipeline steels: Effects of charging mode, oxygen impurity, surface condition and microstructure

Gaseous hydrogen permeation in pipeline steels is not a single diffusion-controlled process, but a coupled response of entry-side reactions and internal trapping. Here, electrochemical and high-pressure gaseous permeation, TDS, XPS, EBSD, and DFT-assisted analysis were combined for 20, API 5L B, X52, and X52QS steels. Matched steady-state flux tests showed that charging-mode equivalence in flux did not imply equivalent permeation transients or retained hydrogen: acidic electrochemical charging produced faster transients and more releasable hydrogen, whereas 4.0 MPa gaseous hydrogen charging resulted in lower retained hydrogen even after the exposure duration was extended to 96 h. Native Fe oxides and residual oxygen delayed gaseous hydrogen entry, while charging-side Ni plating increased flux by reducing oxide-related interfacial resistance. Pressure effects separated interfacial kinetics from thermodynamic driving force: total pressure mainly accelerated transient establishment, whereas hydrogen partial pressure governed steady flux. Material-system differences controlled trapping/retention after entry, with X52/X52QS showing stronger trap effects.