Page 92 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

70 INTRODUCTION AND FORMS OF CORROSION
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An example is the SCC of stainless steel at 200 C in a caustic solution or in aerated
chloride solution where no traces of dissolution are visible on the crack face. The
three conditions, namely, tensile stress, susceptible sample material, and a corrosive
environment are the conditions necessary for stress corrosion to take place (73, 90).
For instance, SCC of metals has been by far the most prevalent cause of failure of
steam generator components in pressurized water reactors (PWRs) to an extent of
69% of all cases, piping in boiling water reactors (59.7%) and PWRs (23.7%). More
than 60% of inspected steam turbines in nuclear power plants have disks with stress
corrosion cracks (91).
The two classic examples of SCC are the seasonal cracking of brass and the caus-
tic embrittlement of steel. Seasonal cracking refers to SCC of brass cartridge cases.
Cracks were observed during the period of heavy rainfall along with hot weather in
the tropics. This intergranular SCC was attributed to the internal stresses in ammo-
nia solution that resulted from the decomposition of organic matter in the presence
of oxygen and humidity. Many explosions of riveted boilers occurred in the early
steam-driven boilers at the tubes of riveted furnaces because of the fact that some
areas were subjected to cold working during riveting operations. Carbon steel sub-
jected to a stress close to the elastic limit and exposed to hot concentrated alkali
solutions or nitrate solutions are susceptible to SCC. SCC was observed in rivets
used in water boilers although the furnace water was treated with alkalis to minimize
corrosion. Crevices between rivets and the boiler plate of the furnace allowed boiler
water to concentrate until the alkali content was sufficient to reach the pH required
to cause cracking (17).

1.8.5 Morphology
Failed samples appear macroscopically brittle and exhibit highly branched
cracks that propagate transgranularly and/or intergranularly, depending on the
metal/environment combination. Transgranular stress corrosion crack propagation
is often discontinuous on the microscopic scale and occurs by periodic jumps
of the order of a micrometer. Intergranular cracks propagate continuously or
discontinuously, depending on the system (17, 73).
Intergranular and transgranular cracking often occur simultaneously in the same
alloy. Such transitions in crack modes are observed in alloys with large amounts of
nickel, iron chromium, and brasses. In corrosion under tension, ruptures are fragile
and are sometimes characterized by the presence of cleavages, in particular, in the
case of HE (17).
Cleavage is a brittle fracture that occurs along specific crystallographic planes.
Cleavage has a well-defined crystallographic orientation and it is easy to recognize
its occurrence by optical microscopy as it exhibits brilliant and flat fracture facets that
are related to the dimension of the grain size of the material of interest. Examination
with a scanning electron microscope shows flat fracture facets showing cleavage steps
and river patterns caused by the crack moving through the crystal along a number of
parallel planes that form a series of plateaus and connecting ledges.