Page 107 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 85

of the same corrosion–deformation interactions in the vicinity of grain boundaries
(122).
The enhancement of creep by anodic dissolution is known in the case of copper
in acetic acid (123) and austenitic stainless steels and nickel-based alloys in PWR
environments. The initial vacancy injection from the surface is followed by vacancy
attraction to the dislocations inside, which promotes easier glide, climb, and cross-
ing of microstructural barriers. This mechanism illustrates the corrosion-enhanced
plasticity approach (73).

1.8.10.23 Tarnish Rupture Model In this model, fracture of the film exposes the
fresh bare surface that reacts with the environment to reform the surface film. The
crack propagates by alternating film growth and rupture. This model was first pro-
posed to explain transgranular SCC and then applied in a modified form to explain
intergranular SCC by assuming that the oxide film penetrates along the grain bound-
ary ahead of the crack tip, which may not be the case in all systems (124).

1.8.10.24 Film-Induced Cleavage Models Dealloying and/or vacancy injection
could cause brittle fracture. In this model, the brittle crack initiates in a surface film or
layer and this crosses the film/matrix without loss of speed. The brittle crack contin-
ues in the ductile matrix until it eventually blunts and stops. This model needs better
verification and understanding of surface films and brittle fracture.
1.8.10.25 Adsorption-Induced Brittle Fracture This model is based on the con-
cept that adsorption of environmental species lowers the interatomic bond strength
and the stress required for cleavage (4). This model can satisfactorily explain the sus-
ceptibility of certain alloys for bond cleavage when the alloys are bonded to certain
ions. One of the important factors in support of this mechanism is the existence of
critical potential below which SCC does not occur in some systems. This model of
cracking underlines the relationship between the potential value and the capacity of
adsorption of the aggressive ion. This also explains the prevention of SCC by cathodic
protection. This model is also useful in explaining the rupture of plastic materials or
glass. This model is referred to as the stress-sorption model and similar mechanisms
have been proposed for HE and liquid metal embrittlement. In this model, the crack
propagates in a continuous way at a rate dictated by the arrival of embrittling species
at the crack tip. This model does not explain as to how the crack maintains a sharp
tip in a normally ductile material (125).
1.8.10.26 Decohesion Models Interactions between a localized dislocation array
and the crack tip under an applied stress produce a maximum stress ahead of the
crack tip to which hydrogen is driven under the stress fields from behind the tip.
When the hydrogen concentration reaches a critical value, a microcrack is nucleated
because either the local cohesive strength is reduced, dislocation motion is blocked
in the hydrogen enriched zone, or both. The microcrack arrests about 1 mm ahead
of the original location of the tip, and these processes repeat, ultimately leading
to discontinuous microcracking (73). This phenomenon is sometimes referred to as
hydrogen-enhanced decohesion.