Page 53 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

METALLURGICALLY INFLUENCED CORROSION 31

precipitation of the chromium carbides depletes the matrix of chromium adjacent to
the grain boundary, and the chromium depleted zone undergoes corrosion.
In most cases, intergranular attack arises from compositional dissimilarities rather
than structural defects. One of the forms of corrosion in which structural defects play
a role is SCC. When a large number of stacking faults are present, it is easier for grains
to slip (facilitate shear displacement of one part of grain with respect to another). Thus
the material has a tendency to creep rather than crack to relieve tensile stresses. In
some materials, decreasing the free energy stored in stacking faults can change the
nature of SCC from intergranular to transgranular process (41).
IGC at elevated temperatures is a serious problem in the sulfidation of nickel
alloys. Deep penetration can occur rapidly through the thickness of the alloy. This
type of IGC can be evaluated by (i) X-ray mapping during examination by a scanning
electron microscope equipped with an energy-dispersive X-ray detector and transmis-
sion electron microscopy (4).
IGC of aluminum alloys is controlled by material selection and proper selection
of thermal (tempering) treatments that can affect the amount, size, and distribution of
second-phase intermetallic precipitates. Resistance to IGC is achieved by the use of
heat treatments that cause general uniform precipitation throughout the grain struc-
ture. General guidelines for selecting suitable heat treatments for these alloys are
available (45).
Exfoliation is a form of macroscopic IGC that affects aluminum alloys used in
industrial or marine environments. Corrosion starts laterally from initiation sites on
the surface and in general proceeds intergranularly along planes parallel to the sur-
face. The corrosion products formed in grain boundaries force metal away from the
underlying base material resulting in a layered or flake-like appearance. In certain
materials, corrosion progressing laterally along planes parallel to rolled surfaces is
known as exfoliation, and it generally occurs along grain boundaries – hence IGC.
A layered appearance is a common manifestation of exfoliation (also known as layer
corrosion) resulting from voluminous corrosion products prying open the material
such as aluminum alloys (4).
Exfoliation corrosion is common among high-strength heat-treatable 2xxx and
7xxx alloys. Exfoliation corrosion of Al 6xxx in salt medium has been observed.
Exfoliation corrosion in these alloys is usually confined to relatively thin sections of
highly worked products. Exfoliation corrosion is observed in unalloyed magnesium
above a critical chloride concentration, but this morphology was not seen in mag-
nesium alloys, in which individual grains were preferentially attacked along certain
crystallographic planes. The early stages of this form of attack caused swelling at
points on the surface because of apparent delamination of the magnesium crystals
with interspersed corrosion products. As the attack progressed, whole grains or parts
of grains disintegrated and dropped out, leaving the equivalent of large irregularly
shaped pits (47).

1.5.1.14 Testing of Intergranular Attack This necessitates metallographic exam-
ination (30). The Standard Practice ASTM G110 (1992) involves evaluation of IGC
resistance of heat-treatable aluminum alloys by immersion in a sodium chloride and