Page 457 - Corrosion Engineering Principles and Practice

424 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 425

Special handling of coupons after removal from the system being
monitored is crucial to ensure that subsequent laboratory tests
provide representative information. Biofilms in particular are highly
sensitive to dehydration, exposure to air, temperature, mechanical
damage, and other gross environmental changes that can occur
during removal and transport of the coupons [12].
Examination of coupons for microbial populations can be
performed either directly or indirectly, using histological embedding
techniques to preserve and remove the biofilm. Although fairly
involved, the embedding technique offers several advantages over
direct observation in that the biofilm and corrosion products are
preserved for future analysis. Environmental scanning electron
microscopy (ESEM) may also be utilized to examine biofilms on test
coupons. However, exopolymers and corrosion products often
obscure the presence of cells, making quantification and identification
difficult with this method.
Monitoring Microbiologically Influenced Corrosion Effects
The presence of a biofilm on a metallic surface can greatly alter the
local corrosion processes. In addition to the electrochemical changes
that directly affect corrosion, biofilms can also modify other readily
measured characteristics such as pressure drop or heat transfer
resistance. Monitoring such microbiological influences can provide a
useful indicator that a biofilm is present and that action should be
taken to mitigate potential MIC.

Deposition Accumulation Monitors Methods for monitoring deposits
can provide an indication of the accumulation of biofilm and other
solids on surfaces or in orifices. For example, monitoring the pressure
drop across an orifice provides a simple method for continuous
monitoring of deposit accumulation and biofilm accumulation. The
main disadvantage of the pressure drop technique is that it is not
specific to the biofilm buildup since it detects the total scaling and
deposition effects in a line [12].
These measurements can be made on actual operating units on
line but they may also be done using model heat exchanger units or
instrumented pipe loops run in parallel to system flow. Figure 10.12
shows such an instrumented pipe loop test unit with five parallel,
instrumented pipe runs. Water flow from the target system is diverted
through this unit, so that conditions are representative of the actual
operating system [18].
Measurements of friction factors and heat exchange efficiencies
can indicate fouling. While all sorts of deposits can affect flow and
heat transfer in an operating industrial system, biofilms are
especially effective. A 165-pm thick biofilm shows 100 times the
relative roughness of a calcite scale and a thermal conductivity close
to that of water, that is, almost 100 times less than carbon steel [9].