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Stress Corrosion Cracking
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Stress corrosion cracking (SCC) is the formation of brittle cracks in a normally sound material through the simultaneous action of a tensile stress and a corrosive environment
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Stress corrosion cracking (SCC) is the formation of brittle cracks
in a normally sound material through the simultaneous action of a tensile stress
and a corrosive environment. In most cases, SCC has been associated with the
process of active path corrosion (APC) whereby the corrosive attack or anodic
dissolution initiates at specific localized sites and is focused along specific
paths within the material. In some cases, these are along grain boundaries, in
other cases, the path is along specific crystallographic within the grains.
Quite often, SCC is strongly affected by alloy composition, the concentration of
specific corrodent species, and, to a lesser degree, the stress intensity. In
some cases, this latter point may make the use of test methods based on fracture
mechanics concepts difficult to utilize effectively due to excessive crack
branching and tendencies for nonplanar propagation of cracks.
Furthermore, corrosion film characteristics (i.e., passivation) and local
anodic attack (i.e., depassivation) serve as controlling factors in SCC crack
initiation and growth. Therefore, localized corrosion can promote SCC making
exposure geometry and specimen design important factors. In many cases,
mechanical straining or electrochemical inducements such as crevices or
controlled potential are utilized to overcome the problems and uncertainties of
SCC initiation so that the inherent resistance of the material to SCC can be
obtained at reasonable test duration (see Table 1).
Table 1 - Applied Potentials for SCC in Steel Exposed to
Various Service Environments
| Environment | Potential rate (mV, SCE) | | Nitrate | -250 to +1200 | | Liquid ammonia | -400 to > +1500 | | Carbonate | -650 to -550 | | Hydroxide | -1100 to -850 and +350 to +500 |
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