Hydrogen surface activation, trapping, and oxygen inhibition in ARMCO iron and X70 studied using gaseous hydrogen permeation

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Abstract
Although the presence of trace oxygen in gaseous hydrogen has been proposed to mitigate hydrogen embrittlement in existing gas transmission infrastructure, its practical feasibilityremains unproven. Here, the effect of small amounts of oxygen in hydrogen gas was studiedusing gaseous hydrogen permeation through annealed ARMCO iron and X70 pipeline steel.Typically, hydrogen permeation is facilitated using a palladium coating, but practical studyof oxygen inhibition requires a bare metal surface. However, native 3nm surface oxidesblocked hydrogen ingress in bare, ground specimens. Therefore, an in situ hydrogen surfaceactivation protocol was established to promote hydrogen uptake and was validated usingX-ray photoelectron spectroscopy. Hydrogen permeation transients using pure hydrogen wereanalysed with a diffusion-trapping model. The weak trap binding energies were 29.9 kJ/molfor ARMCO iron and 33.1 kJ/mol for X70. The strong trap binding energy for X70 was51.1 kJ/mol, with no indication of strong traps in ARMCO iron. Trapping parameters forX65 pipeline steel were also evaluated from prior research data for comparison. Adding2000 ppm oxygen strongly inhibited hydrogen permeation in ARMCO iron, whereas loweroxygen concentrations of 80 ppm and 160 ppm decreased the hydrogen flux without producinga stable steady state. In contrast, the addition of 2000 ppm oxygen produced negligibleinhibition for X70. The low inhibition effectiveness of X70 was consistent with prior researchusing X65, demonstrating that oxygen inhibition may not be viable for microalloyed pipelinesteels under operating conditions

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