Mechanism of Hydrogen-induced Cracking

HIC occurs when H atoms diffusing through a linepipe steel become trapped as H2 molecules at inhomogeneities in the steel. A planar, gas-filled defect is created, which grows parallel with the vessel surfaces as it traps more diffusing H atoms. If the 

The H atom source is normally the cathodic reaction of an acid corrosion mechanism occurring at the internal vessel surface, i.e., the reduction of hydrogen ions, H+  

The H atoms produced by the cathodic reaction are adsorbed onto the steel surface. They may combine to form a H2 gas molecule, which enters the internal vessel environment. Alternatively, they may become absorbed into the steel  

H2 gas formation Hads + Hads — H2 (H2 evolved from steel surface) 

H atom absorption Hads — HabS (H atom enters steel) 

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