Page 87 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 65

Cracks grow by slip dissolution because of diffusion of active water molecules,
halide ions to the crack tip, followed by rupture of the protective oxide film because
of strain concentration and fretting contact between the crack faces. This is followed
by dissolution of the fresh surface and growth of oxide on the bare surface. In the
alternate mechanism of HE, the steps involved are (i) diffusion of water molecules
or hydrogen ions to the crack tip; surface diffusion of adsorbed atoms to preferential
surface locations, absorption and diffusion to critical locations in the microstructure
such as grain boundaries, regions of high triaxiality ahead of crack tip or void. Under
cyclic loading, fretting contact between the mating crack faces, pumping of aqueous
medium to the crack tip by crack walls, and continuous blunting and resharpening of
the crack tip by reverse loading influence the rate of dissolution (79).
Fatigue crack initiation in commercial alloys occurs on the surface or subsurface
and is usually associated with surface defects or discontinuities such as nonmetallic
inclusions, notches, and pits. For low-stress, high-cycle fatigue, crack initiation spans
a large part of the total lifetime. For high-strength steels in sodium chloride, the sulfide
inclusions served as sites for corrosion pits and subsequent fatigue crack initiation.
Corrosion was formed by selective dissolution of MnS inclusions. Cathodic polariza-
tion suppresses the dissolution rate and prevents formation, but hydrogen effects can
increase the crack growth rates of well-defined cracks (79).
The materials and corrosive environments have been classified (81) into the fol-
lowing three groups on the basis of surface corrosion conditions:

1. Active dissolution conditions.
2. Electrochemically passive conditions.
3. Bulk surface films, such as three-dimensional oxides.

In the active dissolution group, emerging PSBs are preferentially dissolved. This
dissolution attack results in mechanical instability of the free surface and generation
of new and larger PSBs, followed by localized corrosive attack, resulting in crack
initiation. Under passive conditions, the relative rates of periodic rupture and refor-
mation of the passive film control the extent to which corrosion reduces the fatigue
resistance. When bulk oxide films are present on the surface of the metal sample,
rupture of the film by PSBs leads to preferential dissolution of the fresh metal that is
produced (79).

1.7.38 Crack Propagation
The environment can affect crack propagation in CF, leading to an increase in crack
growth rate. Three types of crack growth behavior have been documented (73, 82).
Figure 16.4 on page 199 of Uhlig’s Corrosion Handbook (79) shows the sigmoidal
variation of the fatigue crack growth as a function of stress intensity factor range on
a log–log scale under purely mechanical loading conditions. The typical variation of
the crack velocity, da/dt as a function of applied stress factor K is shown on a log–log
scale in the figure for growth of cracks in metallic materials under sustained loading in
the presence of an environment. It is clear from this figure that the environment has no