Page 106 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

84 INTRODUCTION AND FORMS OF CORROSION

1.8.10.18 Dissolution Models The model considers that the crack propagates
because of the preferential dissolution at the crack tip giving rise to the formation
of active paths in the material, stresses at the crack tip, and chemical–mechanical
interactions.

1.8.10.19 Film Rupture or Slip Dissolution Model This model assumes the pres-
ence of an active protective film and that the stress opens the crack and ruptures
the film. Localized plastic deformation at the crack tip ruptures the passivating film,
exposing bare metal at the crack tip, and the freshly exposed surface dissolves rapidly.
It is also suggested that the crack tip remains bare as the rate of repassivation is slower
than the rate of film rupture during crack propagation. The low-stress limiting case
of this mechanism is akin to the IGC mechanism, and this model is applicable to
intergranular SCC and not transgranular SCC (4).
The limiting velocity for the crack growth according to the dissolution model may
be written as
da i M
a
=
dt ZFP
where i is the anodic current, M the atomic weight, Z the valence, F the Faraday
a
constant, and P the density of the material. In general, the crack growth rate depends
on the rate at which the film is ruptured and reformed (121).

1.8.10.20 Ductile Mechanical Models Stress concentrations at the base of corro-
sion pits (slots) increase to the level of ductile formation or fracture.
In the corrosion tunneling model, a fine array of small corrosion tunnels are
thought to form at emerging slip steps. The tunnels grow both in diameter and length
until the stress in remaining ligaments result in ductile formation and fracture. It is
suggested that the application of tensile stress results in a change in the morphology
of the corrosion damage from tunnels to thin slots.

1.8.10.21 Adsorption-Enhanced Plasticity Models According to fractographic
studies the cleavage fracture is not an anatomically brittle process, but occurs by
alternate slip at the crack tip in conjunction with the formation of very small voids
ahead of the crack. It is also thought that the chemisorption of environmental species
facilitates the nucleation of dislocations at the crack tip, promoting the shear process
responsible for brittle-like fracture (4).

1.8.10.22 Corrosion-Enhanced Plasticity Models These models are based on a
localizing effect at the very crack tip because of corrosion. Fracture occurs because of
the enhancement in plasticity along one slip system, inducing the formation of pile-up
and a local decrease in cohesion energy because of the presence of hydrogen. The role
of dissolution is to create vacancies that can enhance the plasticity at the crack tip and
create defects in the passivated metals at the slip band emergence needed for hydrogen
absorption. Vacancies may also act as traps for hydrogen and thus increasing the crack
tip concentration. This model has also been applied to intergranular SCC on the basis