Page 429 - Corrosion Engineering Principles and Practice

398 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 399

Cathode Scale
A third auxiliary effect is the deposition of compounds from the soil
on the buried system. This effect is extremely pronounced in seawater,
where a cathode scale resembling hard glossy enamel can build up
over time and grow as thick as 2 cm. When deposited in seawater, this
material is a complex mixture of calcium and magnesium oxides,
hydroxides, and carbonates. The composition of this hard deposit
depends upon CP current density, among other factors (see Chap. 8
for details on calcareous deposits).
In fresh or brackish water cathode scale composition is even more
variable than the scale formed in seawater since its formation is based
on highly variable ion concentrations. This is also true in soils.
Some soils will not form a visible scale, although there is enough
calcium present in most soils to favor scale formation. Often, when a
system has been polarized by CP, no scale will be visible until the
system surface dries out. It then appears as a whitish coloration. On a
buried system which has not been under CP, cathode scale is
frequently found in an irregular mottled pattern; this makes the
actually active cathodic areas visible to the eye.
Pitting
When a buried system that has been in corrosive soil without adequate
protection for some time is examined, it is usually found that by far
the greater part of the area is unaffected. Where corrosion has taken
place, it is in the form of pits, which are relatively small areas where
the attack has been deep (Fig. 10.3).
In general, there will always be one spot on any buried system
where all of the conditions combine to give the highest rate of
penetration. This hot spot is exactly where the first pit through
perforation will occur. Since the underground system is invisible to
the observer, the resulting leak will be the first place where corrosion
will make its presence known.

10.2.5 Examples of Buried Systems

Pipelines
Pipelines carrying oil, gas, and water are surely the most considerable
assets buried in soils from the deepest subsea exploitation fields to
the most remote tropical regions of the world. The subject of both
external and internal gas pipeline corrosion is covered in many details
in Chap. 12 and many details of the corrosion management of water
lines are discussed in Chap. 8.
Some pipelines deteriorate slowly, and in certain cases pipeline
life has been reliably targeted at 70 years or more. Other pipelines
have been built which have exhausted their useful life after one year
of operation. Apart from the quality of the construction, coatings,