Comparative hydrogen embrittlement behavior of alloy 725 by electrochemical and thermal gaseous pre-charging

Structural materials can be embrittled when exposed to hydrogen-rich environments. To investigate hydrogen embrittlement behavior, researchers often employ electrochemical or gaseous pre-charging to introduce hydrogen prior to mechanical testing, but whether these methods are equivalent remains an open question. We evaluate both methods in alloy 725 by comparing surface crack formation during tensile testing within a scanning electron microscope, and characterizing interior cracks distribution post-mortem via X-ray computed tomography. On the surface, gaseous pre-charged samples exhibited significantly fewer secondary cracks than electrochemically pre-charged samples; internally, high local hydrogen concentration at the center alone did not promote high density of secondary cracks with gaseous pre-charging. These differences are attributed to the low hydrogen diffusivity in face-centered cubic materials, which produces distinct concentration gradients under each pre-charging conditions. These findings suggest that electrochemical and gaseous pre-charging are not equivalent, with implications for damage modeling of structural components in different hydrogen service environments.