Stress Corrosion Cracking
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Metals are useful in engineering structures because of their strength,
ductility, and durability. Ductility is extremely important as it allows the
material to deform in response to loading thus redistributing the stresses. In
some cases, however, chemical interactions with the environment can reduce the
ductility of metals so that they behave more like brittle materials when
subjected to stress.
-Caustic
Stress Corrosion Cracking
-Chloride Stress Corrosion Cracking - CSCC
Definition.
Stress corrosion
cracking is
the intergranular or transgranular cracking of a material due to the combined action
of tensile stress and a specific environment.
Mechanism.
Even after many years of intensive study, the exact mechanism
of stress corrosion cracking remains a matter of extensive disagreement and study.
It is commonly attributed to the rupture of protective films at the
tips of pits
or pre-existing cracks due to the applied stress. In many cases, the materials appear
to be totally resistant to corrosion in a given environment until stresses are applied.
They then crack catastrophically without any sign of other
corrosion attack.
Examples.
Many
materials, particularly
high strength materials, are susceptible to stress corrosion cracking when exposed
to a specific environment. For example, cold worked brass, which is found in ammunition
cartridges, is susceptible to stress corrosion cracking when exposed to an environment
containing ammonia. In chloride containing environments, titanium alloys,
aluminum
alloys, and high strength
stainless steels are susceptible and specific alloys,
which are resistant to stress corrosion cracking, should be used. The stresses required
to initiate and propagate cracking are often low and many failures occur due to
residual stresses rather than applied stress.
Appearance.
Stress corrosion
cracking must
be evaluated using microscopic examination of the cracked sections. The cracking
is often branched. Stress corrosion cracking
can occur in the presence of other
forms of corrosion attack or without the presence of other visible attack.
Significant Measurements.
In general, alloys known to be susceptible to stress corrosion cracking should be avoided. In some cases, special heat treatments can minimize the susceptibility to stress corrosion cracking. Many tests have been developed to test the susceptibility of metals to stress corrosion cracking. All of these combine mechanical loading, often in the presence of a pre-existing crack, and exposure to the specific environment of interest. For alloys with limited sensitivity to stress corrosion cracking, critical stresses can be defined below which stress corrosion will not occur. The structural analysis and manufacturing processes required to accommodate these critical stress criteria are often very complex and the use of highly resistant materials is recommended.
GUIDELINES FOR THE SELECTION OF METALLIC MATERIALS FOR STRESS CORROSION CRACKING RESISTANCE IN SODIUM CHLORIDE ENVIRONMENTS
