The iron sulfide detected in the crack is the anodic reaction
product, generated by ferrous ions (Fe2þ) and sulfur ions (S2).
Other ions (HS, Hþ) are produced inevitably in the corrosion
reactions. After crack growth, the previous accumulated hydrogen
atoms would be redistributed, so the hydrogen content in steel is
relatively reduced. But sufficient hydrogen can be provided by the
cathodic reaction occurring in the crack. Meanwhile, the formation
reaction of molecular gaseous hydrogen (Eq. (4)) could be retarded
due to the presence of H2S or HS. As a consequence, it provides a
steady stream of hydrogen atoms to diffuse into steel for the
growth of a crack branch or a new crack by corrosion reactions in
the crack. The embrittled regions were closely associated with
hydrogen content in the specimen [4]. Corrosion reactions carried
out along the crack and promote the formation of new cracks, and
this cyclic process continues until the sample fails.