Hydrogen Embrittlement

The technical context of hydrogen embrittlement involves the ingress of atomic hydrogen into a susceptible material, often under tensile stress. Sources of hydrogen are varied and include manufacturing processes like electroplating, welding, and pickling, as well as operational environments involving corrosion or high-pressure hydrogen gas. Once inside the metal, hydrogen atoms, being very small, can diffuse rapidly through the crystal lattice. They tend to accumulate at stress concentration sites such as crack tips, grain boundaries, and inclusions.

The HEDE model suggests that this accumulation of hydrogen lowers the cohesive energy required to separate metal atoms, promoting brittle fracture along crystallographic planes or grain boundaries. In contrast, the HELP model posits that hydrogen enhances the mobility of dislocations, leading to intense, localized plastic deformation and the formation of micro-voids that coalesce into a crack. It is now widely believed that both mechanisms can operate, sometimes simultaneously, depending on the material, temperature, and hydrogen concentration. This understanding was a significant novelty, shifting the view of fracture from a purely mechanical process to one heavily influenced by chemical interactions at the atomic level, fundamentally changing how high-strength materials are designed and protected.