Environmental Engineering Reference
In-Depth Information
Figure 3.51
Film-induced cleavage mechanism. A schematic illustration of events
during the propagation of transgranular stress corrosion cracks by cleavage. Figures a-c
represent a section at the crack tip, whereas Figs. d-f represent a plan view of a semicircu-
lar crack radiating from the initiation site.
∆
x
indicates the crack advance distance per
event.
reduction, rather than increase in diffusivity, and thus it becomes indistinguish-
able from earlier surface energy models.
The proposed mechanisms of SCC thus differ immensely from one another
with some suggesting an exclusive role of dissolution and others giving dissolu-
tion a marginal role or no role at all in the cracking process. While the proponents
of a particular mechanism have produced enough evidence in support of the
mechanism, observations not conforming to the mechanism have also been cited
in plenty. Though there have been attempts from time to time to find a unified
theory explaining all features of SCC in all of the alloy-environment systems,
it appears more reasonable to believe that the mechanism varies from system to
system. It is also probable that no particular mechanism operates exclusively in
one system but that more than one mechanism is at play. Parkins [43] has pro-
posed that the different mechanisms of stress corrosion should be considered as
occurring within a continuous spectrum, with a gradual transition from one to
the others as the dominance of corrosive processes is replaced by stress or strain
leading to a brittle fracture, which is shown in Table 3.4.
Figure 3.50
Tarnish rupture models. (a) Alternate tarnish formation and rupture lead-
ing to transgranular cracking. (b) Penetration of tarnish along the grain boundary and its
rupture causing intergranular cracking.
