Page 438 - Corrosion Engineering Principles and Practice

406 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 407

major problems associated with the use of plastics: the low modulus
and strength, and the large thermal expansion property. When
installed at sufficient depth to prevent collapse from heavy topsoil
loading by trucks or tractors, for example, these materials will provide
many years of trouble-free service.
When piping installations are made, care should be used in
making up the joints to ensure their integrity. Extra care should be
taken to avoid making notches on the outer surface and the pipe
should be laid on a uniform base free of rocks or any other hard,
irregular pieces.
Other plastics, such as polypropylene, polybutylene, and glass-
reinforced polyester or epoxy, are also used for specific services at
greater expense. The glass-reinforced thermosetting plastics have
been particularly successful as tank materials or the underground
storage of a wide range of products from water to gasoline. Long
exposures in wet soils show only a superficial attack on the exterior
of such thermoset polymers.

10.3 Microbiologically Influenced Corrosion
As mentioned in Chap. 7, microbes are present in almost all
environments. These potentially corrosive agents flourish in a wide
range of habitats and show a surprising ability to colonize water-rich
surfaces wherever nutrients and physical conditions allow.
A significant feature of microbial problems is that they often appear
when conditions are favorable to an exponential growth of the
organisms [9]. Because they are largely invisible, it has taken
considerable time to establish a solid scientific basis for defining their
role in materials degradation. Many engineers still continue to be
surprised that such small organisms can lead to spectacular failures
of large engineering systems.
Figure 10.7 shows the pitted area of a 15-cm circulating water line
from the supply to the auxiliary vacuum pumps. Note the cluster of
hemispherical pits and the long striated grooves due to the MIC
attack. Each pit represents a localized cluster of anaerobic bacteria
that have become destructive to the pipe wall. The grooves are the
results of bacteria attacking along the steel structure probably due to
the drawing process in manufacturing the pipe. Figure 10.8 shows a
pit and perforation of 6.3-cm internal diameter carbon steel pipe
carrying heavy oil. The pit morphology is typical of sulfate reducing
MIC attack.
As illustrated in the form of a pipe cross-section in Fig. 10.1,
MIC is responsible for the degradation of a wide range of materials
[1]. Most metals and their alloys, for example, stainless steels,
aluminum and copper alloys, polymers, ceramic materials, and
concrete can be attacked by microorganisms. The synergistic effect
of different microbes and degradation mechanisms should also