Environmental Engineering Reference
In-Depth Information
lurgical Aspects of SCC'') brings about substantial change in the microchemistry
of the grain boundary with respect to the bulk alloy. These segregated solutes
or precipitated phases may act as anode in the local cell or, by acting as an
efficient cathode, may cause the dissolution to be localized on the immediately
adjacent matrix. The role of stress here is to keep the crack open for the accessibil-
ity of the corrosive medium to the crack tip. The stress concentration at the crack
tip producing yielding has also been considered to accelerate dissolution due to
the so-called mechanochemical effect [24]. Intergranular stress corrosion crack-
ing (IGSCC) of sensitized stainless steel in various environments, including high-
purity water, of carbon steels exposed to nitrate, hydroxide, or carbonate-bicar-
bonate solutions and of aluminum alloys exposed to chloride solutions have been
explained in these terms. However, a propensity for intergranular corrosion is no
guarantee for IGSCC. It has been shown that although the nickel-base superalloy
IN718 and Ticolloy show intergranular corrosion in NaCl solution, in deaerated
solutions they may crack transgranularly [25].
Strain-generated active path mechanism
. The SCC in alloys that do not have
a preexisting path has been explained by dissolution of strain-generated active
path. There are two distinct models:
film rupture
[26] and
slip-step dissolution
[27]. According to the film rupture model, the localized plastic deformation at
the crack tip ruptures the passivating film, exposing the bare metal which dis-
solves rapidly, resulting in crack extension. The film heals and the cycle is re-
peated. It is also assumed by some that once the propagation starts, the crack tip
remains bare as the rate of film rupture exceeds the rate of repassivation. Ac-
cording to the slip-step dissolution model, the local plastic deformation produces
a slip step and the bare slip step sustains dissolution until the next repassivation.
Figure 3.47 gives the schematic representation of these two models.
Since many of the stress corroding systems are associated with film formation
and the SCC occurs under some sort of borderline passivity condition, the models
involving film rupture have received considerable support. However, controversy
persists with regard to a crack propagation by continued dissolution. Fracture
surface features such as crystallographic cleavage and crack arrest marks for
transgranular SCC, and well-defined grain boundary for intergranular SCC, often
matching with those on the opposing fracture surface, are indicative of a brittle
mechanical cleavage with very little dissolution.
A corrosion tunnel model
[28] has emerged where dissolution and mechanical
fracture have been combined. It assumes that a fine array of small corrosion
tunnels forms at the emerging slip steps. These tunnels grow in diameter and
length until the stress in the remaining ligaments causes ductile deformation and
fracture (Fig. 3.48a). However, such a model should produce a grooved fracture
surface with evidence of microvoid coalescence at the broken ligaments, which
have not been observed experimentally. So this model has been modified subse-
quently [29] by the suggestion that the application of a tensile stress results in
