Hydrogen embrittlement
reduction in ductility of a metal exposed to hydrogen
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are generally less susceptible.
The essential facts about the nature of hydrogen embrittlement have been known since the 19th century.
Hydrogen embrittlement is maximised at around room temperature in steels, and most metals are relatively immune to hydrogen embrittlement at temperatures above 150 °C. Hydrogen embrittlement requires the presence of both atomic ("diffusible") hydrogen and a mechanical stress to induce crack growth, although that stress may be applied or residual. Hydrogen embrittlement increases at lower strain rates. In general, higher-strength steels are more susceptible to hydrogen embrittlement than mid-strength steels.
Metals can be exposed to hydrogen from two types of sources: gaseous dihydrogen and atomic hydrogen chemically generated at the metal surface.
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