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In the transition towards green energy, long distance transportation of gaseous hydrogen by pipeline is considered to be an economically attractive solution. For safe transportation of hydrogen, it is important to conduct relevant mechanical tests in representative hydrogen environments and working conditions of pipelines to demonstrate acceptable hydrogen embrittlement resistance of the steel. One important issue is the testing condition for the ratio of gaseous hydrogen volume over specimen surface area required in mechanical testing since this has not been specified in any testing standards and codes or investigated. This study investigated this issue by performing slow strain rate tensile (SSRT) testing of an X52 pipeline steel in gaseous hydrogen with different ratios of hydrogen volume/specimen surface area, R v/s , ml/mm2. It was found that, with increasing R v/s ratio, the area reduction of the steel decreased and hydrogen embrittlement sensitivity increased in the range of the R v/s ratios investigated. The saturating R v/s ratio appeared to be ∼2.0 beyond which the effect of R v/s ratio on hydrogen embrittlement sensitivity was relatively small. The macroscopic and microscopic observations on both the fracture and side surfaces of the SSRT tested specimens also found that the tendency to cleavage fracture increased with increasing R v/s ratio. The results of the ab initio molecular dynamics (AIMD) simulations performed also predicted a dependence of hydrogen embrittlement on hydrogen volume when R v/s ratio is relatively small. • The effect of the ratio of gaseous hydrogen volume over specimen surface area, R v/s , on HE was investigated the first time in the world. • The threshold R v/s ratio, beyond which the influence of the R v/s ratio on hydrogen embrittlement sensitivity is small, was determined. • Tendency to cleavage fracture increased with increasing R v/s ratio. • The increased tendency to hydrogen embrittlement under higher R v/s ratios was verified by the AIMD simulations. [ABSTRACT FROM AUTHOR]