Page 85 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

MECHANICALLY ASSISTED CORROSION 63

of carbon steels. CF cracks can occur in the absence of pits and follow boundaries or
prior-austenite grain boundaries (50).
Aluminum alloys exposed to chloride solutions result in CF cracks originating at
pitting sites or sites of IGC. Initial crack propagation is normal to the axis of principal
stress. This is contrary to the behavior of fatigue cracks initiated in dry air, where
initial growth follows crystallographic planes. Initial CF cracking normal to the axis
of principal stress also occurs in aluminum alloys exposed to humid air, but pitting is
not a prerequisite for crack initiation (50).
CF cracks in copper and its alloys initiate and propagate intergranularly. Cu–Zn
and Cu–Al alloys show marked reduction in fatigue resistance in aqueous chloride
solutions. This type of behavior is difficult to distinguish from SCC except that it may
occur in environments that do not cause failures under static stress and in sodium
chloride or sodium sulfate solutions (50).
The CF of low alloy steels in high-temperature waters is an example where crack
tip chemistry has been identified as responsible for environmentally assisted crack-
ing. In some cases, hot water may increase the fatigue crack propagation rate in low
alloy steels. This effect is observed only when the sulphur content of the wrought
steel exceeds 100–150 ppm (74). The cause of environmentally assisted cracking
(EAC) has been attributed to the accumulation of sulfide ions in the crack tip environ-
ment, originating from the dissolution of MnS inclusions bared by the crack advance
(75, 76). In aerated conditions the potential gradient inside the crack tends to inhibit
outward diffusion of sulfide, which leads to environmentally assisted corrosion over
larger loading conditions, particularly at lower frequencies (77, 78).

1.7.36 Mechanism of CF

The mechanism of fatigue in air proceeds by localized slip within grains of the metal
caused by stress. The air adsorbed on the fresh surface exposed at slip steps prevents
rewelding on the reverse stress cycle. Continued slip produces displaced clusters of
slip bands that protrude above the metal surface and corresponding cracks (intrusions)
from elsewhere. The corrosion process may remove barriers to plastic deformation
such as dislocations, induce plastic deformation by reducing surface energy, and
favor slip formation by injecting dislocations along slip planes (Fig. 1.19) (8). After
or during initiation of microcracks, the propagation follows in part because of the
adsorption of oxygen, water, or other ionic species along the partitions of the crack.
The adsorption of oxygen or ionic species reduces the energy of the surface and pre-
vents the welding of the metallic surface during the inverse constraint cycle. The
formation of differential aeration cells because of different concentrations of oxygen
in the localized sites can play a role in the dissolution of the metal at the bottom of
the crack (anode) and hence contribute to the propagation of the crack. A corrosive
medium eliminates the fatigue limit or shortens the life above the fatigue limit.
According to Wang (79) the four stages of fatigue are:

1. Precrack cyclic deformation that includes the formation of persistent slip bands
(PSBs), extrusions, and intrusions.