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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
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