Page 442 - Corrosion Engineering Principles and Practice

410 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 411

a biofilm composed of immobilized cells and their extracellular
polymeric substances builds up on the surface.
The growing biofilm increasingly prevents the diffusion of
dissolved gases and other nutrients coming from the bulk
environment. These changing conditions become inhospitable to
some microorganisms at the base of the biofilm and eventually
many of these cells die, for example, on the internal wall of a water
handling system. As the foundation of the biofilm weakens, shear
stress due to adjacent fluid flow may cause sloughing of cell
aggregations exposing the bare surface to the bulk fluid in localized
areas (Fig. 10.9, Stage 5). The exposed areas are subsequently
recolonized and new microorganisms and their exopolymers are
woven into the fabric of the existing biofilm (Fig. 10.9, Stage 6). This
phenomenon of biofilm instability occurs even when the physical
conditions in the bulk liquid remain constant. Thus, biofilms are
constantly in a state of flux [11].
Depending on the type of industrial system, planktonic organisms
may include, besides bacteria, unattached algae, diatoms, fungi, and
other microorganisms present in a system bulk fluids. In most cases,
it is planktonic bacteria that are the focus of monitoring for MIC using
microbiological detection techniques since system fluids are generally
easier to sample than metallic surfaces. Unfortunately, the levels of
planktonic bacteria present in the liquids are not necessarily indicative
of MIC problems or their severity [12].
Monitoring sessile organisms either requires that the system be
regularly opened for sampling or that accommodations be made in
the system to allow for regular collection or on-line tracking of
attached organisms while the system continues to operate. Because
the presence of viable sessile organisms rarely correlates to the
corrosivity of an environment, it is a good practice to use additional
methods that directly determine the presence of active MIC. At
best, the detection of viable planktonic bacteria may serve as an
indicator that living microorganisms are present in a particular
system, some of these organisms being capable of participating in
the microbial attack.
10.3.2 Microbes Classification
One useful microbe classification consists in describing microorgan-
isms according to their oxygen tolerance [10]:
• Strict (or obligate) anaerobes, which will not function in the
presence of oxygen
• Aerobes, which require oxygen in their metabolism
• Facultative anaerobes, which can function in either the
absence or presence of oxygen
• Microaerophiles, which use oxygen but prefer low levels