Environmental Engineering Reference
In-Depth Information
Figure 3.48
Corrosion tunnel models. (a) Initiation of a crack by the formation of
corrosion tunnels at slip steps and ductile deformation and fracture of ligaments. (b) Disso-
lution of tunnels to flat-slot formation.
of hydride then forms in the region of crack tip and the process is repeated,
resulting in discontinuous crack growth.
Hydrogen-assisted crack growth in the manner described above has been sug-
gested as the SCC mechanism for ferritic steels, nickel-base alloys, austenitic
stainless steels, aluminum alloys, and intermetallic compounds. The most proba-
ble source for atomic hydrogen to enter the metal is the cathodic reduction of
hydrogen ions accompanying anodic dissolution in aqueous environments. Mate-
rials with impurities like sulfur, phosphorus, antimony, and tin segregated at grain
boundaries have been found to be more susceptible to hydrogen-induced cracking
because these impurities act as hydrogen evolution poisons. On the other hand,
the growing stress corrosion cracks have been effectively stopped in low-strength
ductile alloys by cathodic polarization, refuting the validity of hydrogen-assisted
