Investigating temperature effects on short- and long-term hydrogen permeation in API 5L X65Q steel

This study investigates the effects of temperature (25–70 °C) and microstructural trapping on hydrogen permeation in API 5L X65Q pipeline steel using electrochemical permeation. Short-term permeation suggests diffusion primarily governed by Fick's law, modulated by microstructural trapping. A trap binding energy of 28.4 kJ/mol and trap density of 6.84 x 10 26 sites/m 3 were derived under low trap occupancy regime and validated with a one-dimensional finite element model. On the other hand, long-term results reveal an abnormal ‘double rise’ transient, characterized by a first pseudo-steady state plateau, followed by a secondary increase in the current density, reaching a second-steady state plateau. This phenomenon is attributed to micro-porosities (inclusion density 25.3 particles/mm 2 , porosity fraction 0.0102 %) from inclusion-matrix debonding. The final current density decline is linked to hydrogen-induced damage, confirmed by SEM analysis. This study highlights the overlooked role of micro-porosities around inclusions in long-term permeation behaviour in pipeline steel for hydrogen applications.