Page 447 - Corrosion Engineering Principles and Practice

416 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 417

The stickiness of slime formers traps all sorts of particulates that
might be floating by, which, in dirty water, can result in the impression
that the deposit or mound is an inorganic collection of mud and
debris. Slime formers can be efficient “scrubbers” of oxygen, thus
preventing oxygen from reaching the underlying surface. This creates
an ideal site for SRB growth.

10.3.3 Monitoring Microbiologically Influenced Corrosion
An effective biocorrosion mitigation program needs to include
corrosion monitoring as a periodic or continual means of assessing
whether program goals are being achieved. Monitoring techniques
that detect the presence of microbes, especially on metallic surfaces,
can provide an early indication of incipient MIC or the potential for
MIC. A number of methods for the detection of microorganisms,
including specific types of organisms and estimates of their numbers
and activity, have been developed [12].
The first biocorrosion monitoring systems were focused on
assessing the number of microbes per unit volume of water sampled
from a system. Data obtained with these systems were combined
with electrochemical corrosion measurements, using electrical
resistance (ER) or linear polarization resistance (LPR) probes in
addition to coupon weight loss measurements. The problem with this
approach is that the number of free-floating planktonic organisms in
the water does not correlate well with the organisms present in
biofilms on the metal surface where the corrosion actually takes place.
An effective monitoring scheme for controlling both biofouling and
biocorrosion should include data gathering using as many of the
following techniques as possible [14]:
• Sessile bacterial counts, by either conventional biological
techniques or optical microscopy of organisms present in the
biofilm, on the metal surface.
• Direct observation of the community structure of the biofilm.
Several types of probe systems are commercially available for
holding and inserting metallic coupons into the system.
Examination of the biofilm has been done by scanning
electron microscopy, epifluorescence optical microscopy, or
confocal laser scanning microscopy.
• Identification of the microorganisms found in both the process
water and on the metal surface.
• Surface analysis to obtain chemical information on corrosion
products and biofilms.
• Evaluation of the morphology of the corrosive attack on the
metal surface after removal of biological and corrosion
product deposits with conventional macrophotography,
stereomicroscopy, optical and scanning electron microscopy,
or other metallographic techniques.