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material failure may be accelerated by the combined
effect of corrosion and mechanical stress. Two examples
of such processes are stress corrosion cracking and
corrosion fatigue.
The most common type is transgranular
stress corrosion cracking, SCC, which may develop in
concentrated chloride-containing environments. Previously,
it was generally considered that an elevated temperature
was necessary for SCC to occur. In recent years, however,
SCC has been experienced at ambient temperature on standard
grade steels like 304L or 316L that were exposed to
high tensile stresses. In these cases the steel surface
was contaminated with solid salt deposits and the humidity
of the atmosphere was rather high. These two factors
resulted in a thin liquid film saturated with chloride.
Other contaminants, such as H2S, may increase the risk
of SCC in chloride-containing environments. Other environments
that may give rise to SCC, particularly on low alloy
steels, include very alkaline solutions at high temperatures.
A typical SCC attack takes the form of thin, branched
cracks. |
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