What causes hydrogen embrittlement? In the absence of
residual stress or external loading, environmental hydrogen
embrittlement is manifested in various forms, such as
blistering, internal cracking, hydride formation, and reduced
ductility. With a tensile stress or stress-intensity factor
exceeding a specific threshold, the atomic hydrogen interacts
with the metal to induce subcritical crack growth leading to
fracture. In the absence of a corrosion reaction (polarized
cathodically), the usual term used is hydrogen- assisted
cracking (HAC) or hydrogen stress cracking (HSC). In the
presence of active corrosion, usually as pits or crevices
(polarized anodically), the cracking is generally called
stress-corrosion cracking (SCC), but should more properly be
called hydrogen-assisted stress-corrosion cracking (HSCC).
Thus, HSC and electrochemically anodic SCC can operate
separately or in combination (HSCC). In some metals, such as
highs-strength steels, the mechanism is believed to be all, or
nearly all, HSC. The participating mechanism of HSC is not
always recognized and may be evaluated under the generic
heading of SCC.
