Environmental Engineering Reference
In-Depth Information
Numerous cases of SCC in chemical process plants and power generation
plants have been reported in the literature. Oxygenated pure or impure high-
temperature water has caused failures in steam generator shells made of carbon
or low-allow steel [21]. Extensive failures have taken place in austenitic 18Mn-
4Cr steel rotor end-retaining rings in contamination with oxygenated high-tem-
perature water [49]. SCC of austenitic stainless steel pipes caused by the same
environment in boiling water reactors (BWRs) has cost the world's nuclear power
industries as much as $10 billion [50]. SCC of nickel-base alloy 600 in reducing
high-temperature water and supercritical steam in pressurized water reactors
(PWRs) has been reported [51]. Room temperature cracking of sensitized austen-
itic stainless steels produced by polythionic acid was first experienced in catalytic
reformers used in the petroleum industry [52].
3.9.2 Corrosion Fatigue
Corrosion fatigue is the cracking of a metal or alloy under the combined action
of a corrosive environment and repeated or fluctuating stress. As in SCC, succes-
sive or alternative exposure to stress and corrosion does not lead to corrosion
fatigue.
Metals and alloys fail by cracking when subject to cyclic or repetitive stress
even in the absence of a corrosive medium, a phenomenon known as fatigue
failure . The greater the applied stress, the less is the number of cycles required
and the shorter is the time to failure. In steels and other ferrous materials, no
failure occurs for an infinite number of cycles at or below a stress level that is
called the endurance limit or the fatigue limit. In a corrosive medium, the fatigue
limit is lowered or no longer observed, i.e., the failure occurs at any applied stress
if the number of stress cycles is sufficiently large. Corrosion fatigue may thus
also be defined as the reduction in the fatigue life of a metal in a corrosive envi-
ronment. These behaviors are illustrated in Fig. 3.55. In nonferrous metals and
alloys, fatigue limit is not indicated, but nevertheless a corrosive environment
brings down the fatigue life. It may be noted here that unlike SCC, corrosion
fatigue is equally prevalent in pure metals and their alloys.
Corrosion fatigue cracks, like mechanical fatigue cracks, are usually transgran-
ular and are rarely branched (Fig. 3.56). The cracks propagate perpendicular to
the principal tensile stress. Whereas in mechanical fatigue a single crack leads
to failure, corrosion fatigue cracks in an affected component are often numerous.
The fracture surface may show macroscopic beach marks typical of fatigue failure
and corrosion products. Electron microscopy reveals striation marks, each one
indicating the advancement of fatigue crack for each cycle of stress (Fig. 3.57).
Stress Factors
Mean stress, frequency of cyclic stress, and stress amplitude affect corrosion
fatigue. If the mean stress has a tensile value, the growing fatigue crack is held
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