Hydrogen embrittlement rupture caused

Fig. 1.18 Rupture caused by hydrogen embrittlement [51]. http //www.gewater.com/handbook/ boiler water systems/fig14-10.jspO.

The main mechanisms proposed to account for EAC have been reviewed, and it is now clear that the classical opposition between anodic dissolution and hydrogen embrittlement is not relevant, since (i) very often the crack advance is probably not directly due to anodic dissolution but to periodical micro-ruptures, even though anodic dissolution is often an important or even controlling process, (ii) anodic dissolution and hydrogen may simultaneously play a role in the same system, the localization of hydrogen absorption often being a consequence of local dissolution, (iii) absorbed hydrogen may not only cause local or bulk embrittlement, but it can also promote plasticity. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

---