A new mechanistic model for environmentally assisted cracking (EAC) of metallic materials is presented.
In this model the mechanism of cracking involves selective dissolution or oxidation of metallic
cations in the oxide film and transport of the generated vacancies to the base metal ahead of the crack
tip. The mechanism of cracking is based on interactions of vacancies with dislocations enhancing concentration
of slip to the highly stressed region ahead of the crack tip.
The validity of the model is discussed based on both new experimental and literature data. It is shown
that selective oxidation and, thus, vacancy generation takes place in the same electrochemical potential
range where EAC cracking is observed at different temperatures in different materials such as in stainless
steels, Ni-base alloys and brass. This model can be applied to explain the observed transgranular
stress corrosion cracking (TGSCC), intergranular stress corrosion cracking (IGSCC) and hydrogen
embrittlement (HE) phenomena of these materials used in high temperature water environments.
Several mechanisms and models have been proposed to describe the environmentally assisted cracking
(EAC) principal y in order to develop efficient remedial measures for this problem. There are several
recent reviews describing most of the proposed EAC mechanisms and models in detail (1, 2, 3).
In the slip dissolution mechanism, crack propagation is related by Faraday?s law to oxidation reactions
that occur at the crack tip as the surface film is ruptured by increasing strain in the underlying metal.
Equation with explanation
Ford and Andresen (4, 5) correlating
a remarkable amount of experimental data and using equation [ 1] as the starting point have been able
to derive a similar equation defined in engineering terms, that is applicable for crack propagation in
boiling water reactor (BWR) environments. The maximum crack growth rate predicted by the slip
dissolution model is limited by the bare surface dissolution rate. In some metal-environment systems
crack growth rates much higher than that predicted by the slip dissolution mechanism have been
measured.
Sieradzki and Newman (6) have proposed a model for stress corrosion cracking (SCC) which is based
on a discontinuous crack advance mechanism. In their model a deployed brittle surface layer initiates a
crack, which then for dynamic reasons can extend to the underlying base metal.
Jones (7) proposed a mechanism where vacancy generation reduces strain hardening and accounts for
localized surface plasticity. Vacancies were thought to increase dislocation mobility, alter surface
plasticity and near-surface ductility by weakening the lattice much like hydrogen.
Meletis (8) has proposed based on transmission electron microscope (TEM) observations on nonferrous
FCC metals that localized anodic dissolution generates subsurface vacancies. Vacancies ease
dislocation mobility and modify their configuration. Stress-assisted vacancy transport accumulates
vacancies on { 11O} planes and produces an embrittled zone with reduced fracture toughness.
The selective dissolution - vacancy - creep (SDVC) model presented by Saario et al. (9) for Alloy 600
includes selective dissolution of iron, the production of vacancies due to selective dissolution as well
as deformation localization to a shear band. In the SDVC model for Alloy 600 in high temperature
water environments the rate determining step in cracking is the generation of vacancies by selective
dissolution of cations through the existing passive film. T
