Page 448 - 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

• Electrochemical corrosion measurements.
• Water quality and redox potential measurements.
• Other types of information specific to each operational
system, including duty cycle and downtime information,
concentrations, and timing for addition of biocides and other
chemical inputs, local sources and nature of pollutants, and
so forth.
Sampling
Samples for analysis are usually obtained by scraping accessible
surfaces. In open systems or on external surface of pipelines or other
underground facilities this can be done directly. For low pressure water
systems, bull plugs, coupons or inspection ports can provide a way to
expose specimens representative of internal surfaces [15]. However,
more sophisticated devices are required for pressurized systems to
allow mounting an assembly on a standard pressure fitting [16].
If the biofilm developing is to be representative of the behavior
in a system it is important for the sampling coupons to be made of a
material similar to the system material and that the coupons be flush-
mounted in the wall of the system to have the same flow effects as
those of the surrounding surface. While pressure fittings allow
inserting coupons directly in process units, these fittings can be
expensive. Pressure vessel codes and accessibility can also restrict
possible locations. For these reasons sidestream installations are
often preferred.
Handling of field samples should be done carefully to avoid
contamination with foreign matter including biological materials.
Many different types of sterile sampling tools and containers are
available and for anaerobic systems, special handling and transport
tools are essential to avoid exposure to oxygen from the air. One
option is to analyze samples on the spot with special kits. Where
transportation to a laboratory is required, Torbal jars or similar
anaerobic containers can be used [17]. In many cases, simply placing
samples directly in a large volume of the process water in a completely
filled screw cap container is adequate.
Processing in the lab should also be done anaerobically using
special techniques or in anaerobic chambers designed for this
purpose. Because viable organisms are involved, processing should
be done quickly to avoid growth or death of cells stimulated or
inhibited by changes in temperature, oxygen exposure, or other
factors [9].
Biological Assessment
Biological assays are performed on liquid samples or on suspensions
of solid deposits to identify and enumerate viable microorganisms,
quantify metabolic or specific enzyme activity, or determine the
concentration of key metabolites [18]. Table 10.9 summarizes some
of the methods that have been used to detect and describe