Development of a Gaseous Hydrogen Permeation Method: Effects of Palladium-Coated Charging Surfaces and Partial Permeation Transients

Hydrogen diffusion plays a central role in the susceptibility of steels to hydrogen embrittlement, yet reported diffusivity values often exhibit significant variability due to differences in experimental methodology. Gaseous hydrogen permeation experiments, while more representative of service conditions, are highly sensitive to surface boundary conditions and trapping effects, which can bias the diffusivity obtained from analysis. In this work, a gaseous hydrogen permeation methodology was developed at the High-Performance Powertrain Materials Laboratory (HPPM) at the University of British Columbia, Okanagan. A dedicated gas management system (GMS) was implemented to enable controlled pressure step transients, allowing partial permeation transients to be collected under gaseous charging conditions. This approach was applied to commercially pure iron and API 5L X60 pipeline steel to evaluate diffusion and trapping behaviour across materials with differing microstructural complexity. The results demonstrate that diffusivity obtained from transients spanning the full charging–discharging range (0–2 MPa) reflects an effective parameter influenced by reversible hydrogen trapping, whereas transients measured over incremental pressure steps (1–2 MPa) provide a more consistent estimate of lattice-controlled diffusion. The application of palladium coatings to the charging surface was found to promote hydrogen entry, reducing surface impedance effects and further improving agreement with Fickian diffusion behaviour.