Environmental Engineering Reference
In-Depth Information
Figure 3.45
Effect of applied potential on time of fracture of α brass in ammoniacal
copper sulfate solutions with different copper contents.
stress corrosion cracks, then the crack velocities should be related to the current
density at the crack tip through Faraday's law according to:
i
a
M
ZF
v
(3.6)
ρ
where
i
a
is the current density,
M
the molecular mass of the metal,
Z
the valency
of the sovated ions, F is Faraday's constant and
is the density of the metal. A
straight-line relationship between crack velocity and current density, conforming
to Eq. 3.6, has actually been observed in many systems supporting the postulate
(Fig. 3.46). The deviation can be attributed to dissolution in the crack initiation
process alone and a mechanical fracture propagation step.
ρ
Mechanisms of SCC
Any mechanism of SCC should explain the characteristic features of the process,
i.e., the susceptibility of an alloy in a limited number of environments, the role
of stress, and the nature of cracking apart from the influence of various metallurgi-
cal and environmental factors in the cracking process. Since SCC essentially
involves a corrosive environment, mechanisms based on localized dissolution of
metal have been postulated. On the other hand, the brittle fracture feature of SCC
has led to the postulation of mechanisms based on cleavage. There are several
