Page 440 - Corrosion Engineering Principles and Practice

408 C h a p t e r 1 0 C o r r o s i o n i n S o i l s a n d M i c r o b i o l o g i c a l l y I n f l u e n c e d C o r r o s i o n 409

be noted in Fig. 10.1. The mechanisms potentially involved in MIC
are summarized as
• Cathodic depolarization, whereby the cathodic rate limiting
step is accelerated by microbiological action
• Formation of occluded surface cells, whereby microorganisms
form “patchy” surface colonies. Sticky polymers attract and
aggregate biological and nonbiological species to produce
crevices and concentration cells, the basis for accelerated attack
• Fixing of anodic reaction sites, whereby microbiological
surface colonies lead to the formation of corrosion pits, driven
by microbial activity and associated with the location of these
colonies
• Underdeposit acid attack, whereby corrosive attack is
accelerated by acidic products of the MIC “community
metabolism,” principally short-chain fatty acids.
In order to influence either the initiation or the rate of corrosion in
the field, microorganisms usually must become intimately associated
with the corroding surface. In most cases, they become attached to
the metal surface in the form of either a thin, distributed film, or a
discrete biodeposit. The thin film, or biofilm, is most prevalent in
open systems exposed to flowing seawater, although it can also occur
in open freshwater systems. Such thin films start to form within the
first 2 to 4 hours of immersion, but often take weeks to mature. These
films will usually be spotty rather than continuous but will
nevertheless cover a large portion of the exposed metal surface [10].
Biodeposits differ from distributed films and may be up to several
square centimeters covering typically only a small fraction of the total
exposed metal surface, possibly leading to localized corrosion effects.
The organisms in these deposits have generally a large effect on the
chemistry of the environment at the metal/film or the metal/deposit
interface without having any measurable effect on the bulk electrolyte
properties. However, organisms will occasionally be concentrated
enough in the environment to influence corrosion by changing the
bulk chemistry. This is sometimes the case in anaerobic soil
environments, where the organisms do not need to form either a film
or a deposit in order to influence corrosion [10].
10.3.1 Planktonic or Sessile
Microorganisms that are attached to a surface are termed sessile
organisms and these are most often present as a consortium or
community of organisms, collectively referred to as a biofilm.
Complex assemblages of various species may occur within both
planktonic and sessile microbial populations. The environmental
conditions largely dictate whether these microorganisms exist in
a planktonic or sessile state.