Page 433 - Corrosion Engineering Principles and Practice

402 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 403

rods all possibly interconnected. This can cause much difficulty, both
in causing corrosion when no protective measures are taken, and
again as a complicating factor when CP is applied. Further problems
may arise because of the presence of many paved and unpaved areas
that set up troublesome aeration cells. Furthermore, the usual
construction activity ultimately results in badly mixed soils. Soil
resistivity maps, covering entire plant areas, are useful both in
anticipating trouble areas and in planning control measures.

Well Casings
Although a well casing may be considered as merely a vertical
pipeline, it also has several unique features. It costs much more per
unit length, so that leak repair is enormously expensive, if at all
possible. The corrosive exposure is also complicated by the variety of
soil strata through which it passes. Normally, these are much more
varied than would be found on an equal length of horizontal pipeline
and casings are usually bare. Any investigation is severely hampered
by the fact that only one end of the pipe is accessible, so that most
conventional surveys cannot be conducted.
Underground Tanks
Underground tanks usually are large enough to contact more than
one stratum of soil, so they may be subject to concentration cell action.
They are almost always subject to oxygen cell attack, even when
under pavement. Often, there are fittings of different metals, and
seldom is the coating as good as that used on pipelines. One of the
major problems in dealing with these structures is that it is difficult to
justify an engineering study for each tank, yet they cannot all be
treated alike.
Steel Piling
An important difference with steel piling is that a few pits or even
holes have little effect on its structural strength. Consequently, much
more corrosion can be tolerated than with pipelines or any other
vessels for which leaks cannot be tolerated. Piling is almost always
bare, vertical, and hence subject to the same kinds of cells that attack
oil well casings. Bonding often may be a problem because individual
piles may not be interconnected electrically, a condition that makes
both investigation and protection a problem.
Transmission and Communication Towers
Electrical lines often have built-in problems because of the common
use of extensive copper ground wire. The galvanic cells thus created
can be ruinous. While this problem can be avoided by not
interconnecting, there is still a problem with the corrosion of a large
number of small units, which makes both study and protection
difficult (see Chap. 7 for some examples).