Hydrogen-induced cracking (HIC) is caused by nascent hydrogen atoms (Ho), usually produced in aqueous hydrogen sulphide (H2S).
Hydrogen atoms produced, for example, by the corrosion reaction of iron, usually combine to form hydrogen gas molecules.
However, in the presence of sulphide or cyanide, the hydrogen recombination reaction is poisoned so that the nascent hydrogen atoms diffuse into the steel rather than recombining on the metal surface to form hydrogen gas.
Hydrogen atoms that enter the pipeline can cause embrittlement and failure. Usually HIC failures occur within a few weeks of the pipeline being put in service.
