Page 51 - Reliability and Maintainability of In service Pipelines

40 Reliability and Maintainability of In-Service Pipelines

combination of factors, including cyclic stresses and a favorable environment for
corrosion, is more damaging than fatigue alone, which is usually the case in pipe-
lines. The usage of protective coatings and designing pipes with a focus on reduc-
ing stresses that concentrate on particular areas, avoiding spontaneous changes
and removing potential causes of cyclic stresses can reduce the risk of corrosion.
However, in pipelines where fatigue is an issue, failure is inevitable.
Another inevitable case of failure is stress corrosion cracking (SCC), which
consists of many functions that cause and accelerate this type of cracking. Pipes
that are subject to corrosive environments are prone to the formation of cracks on
the surface, which grow and cause spontaneous failure. Although metal pipes are
ductile and undergo tensile stress on a regular basis, SCC occurs, particularly
when combined with elevated temperatures. This occurrence is more common in
pipes made from alloys, such as galvanized iron, than pure metals. It is observed
that temperature fluctuations can alter the temperature gradient and accelerate the
rate of corrosion. This is very common in pipes that carry fluids of varying tem-
peratures and hot water pipes, caused by thermogalvanic corrosion. Vapor caused
by the evaporative action in pipes with temperature variances create an almost
constant moist environment and a thriving atmosphere for corrosion.
Stress corrosion cracking is also a known problem for pressure pipes derived
from stainless steel, especially common in chemical and petrochemical plants.
The presence of aggressive agents favors the corrosive environment and stainless
steel is particularly susceptible to corrosion when temperatures are exceedingly
high. Although stainless steel contains carbon content of 0.03 wt%, which is
significantly lower than most metal alloys, along with a high chromium content,
which allows resistivity to corrosion, this effect is counteracted when exposure to
high temperatures of 415 850 C. This causes a change in the microstructure of

stainless steel which becomes susceptible to precipitation of chromium carbides,
causing sensitization. The formation of these carbides cause a depletion in the
chromium content of sensitized steels, resulting in a rapid change in its structural
composition, allowing an increased susceptibility to failure.
However, for stainless steels, environments that cause cracking are specific as
not all environments cause SCC. The ability of stainless steels to resist corrosion
is due to the formation of a passive layer which occurs in oxidizing environments.
This occurs when the steel consists of a 10.5% minimum chromium content.
However, corrosion affecting stainless steels is different to corrosion of other
metals. For corrosion to occur in stainless steels a change or break down of the
passive layer in environments that present nonoxidizing conditions is required.
Environments containing caustic chemicals, changes in pH, and temperature, as
well as contamination, fabrication, and maintenance processes can affect the pas-
sive layer and the corrosion rate, as well as the type of corrosion that takes place
in stainless steels.