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244 C h a p t e r 7 C o r r o s i o n F a i l u r e s , F a c t o r s , a n d C e l l s 245
same corrosion conditions, logically would be expected to last as long
as the original section. However, the new section will usually fail
sooner than expected unless it is electrically insulated from the
remainder of the system.
7.4.8 Microbial Corrosion Cells
The anaerobic corrosion of iron was noted in the nineteenth century
and many theories were proposed about its mechanism. Decades of
scientific research projects and investigations on the complex
influence of microbes on increasing or decreasing corrosion rates
have provided a much deeper insight in the role microorganisms play
on the life of systems exposed to waters and grounds where they
proliferate (see Chap. 10 for more details).
Microorganisms tend to attach themselves to solid surfaces,
colonize, proliferate, and form biofilms which may in turn produce an
environment at the biofilm/metal interface radically different from
the bulk environment in terms of pH, dissolved oxygen, organic and
inorganic species. Since the biofilm tends to create nonuniform surface
conditions, localized attack might start at some points on the surface
leading to localized corrosion, usually in the form of pitting [22].
When microorganisms are involved in the corrosion of metals, the
situation is more complicated than for an abiotic environment, because
microorganisms not only modify the near-surface environmental
chemistry via microbial metabolism but also may interfere with the
electrochemical processes occurring at the metal–environment interface.
Many industrial systems are likely to contain various structures where
MIC and biofouling may cause serious problems: open or closed cooling
systems, water injection lines, storage tanks, residual water treatment
systems, filtration systems, different types of pipes, reverse osmosis
membranes, potable water distribution systems and most areas where
water can stagnate.
One noteworthy example that has taken an alarming proportion
in recent years is plugging and corrosion of emergency fire sprinkler
systems that are typically assumed to be functional until one
attempts to use them, often during an emergency situation.
Numerous reports in the past decade have described the rapid
development of pinhole sized leaks and highly obstructive interior
growth developments in sprinkler system piping, fittings, and
supply tanks. Some occurrences have been reported after less than
one year of system service. In many of these cases, the cause has
been attributed to MIC [23].
Such failures can take two forms. First is the failure of a system to
hold water. This is most often seen in the development of the pinhole-
sized leaks, often considered to be a sure sign of MIC infection. This
is also typically the only concern in many treatment investigations.
Second, and more concerning, is the failure of a system to achieve
its designed purpose: that of fire control. Several systems with MIC