Page 434 - 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

10.2.6 Corrosion of Materials Other Than Steel

Cast Iron
The corrosion rates of pure iron, wrought iron, or mild steel are quite
similar in soil environments. However, as described under graphitic
corrosion in Chap. 6, when grey cast iron corrodes only a network of
graphite particles may be left behind by the dealloying process. Some
cast iron water mains that are almost one century old are in this
condition and continue to adequately serve their purpose. Once the
cast iron is graphitized, the exterior becomes an extremely noble
electrode in any galvanic couple. Thus, care should be used in
attaching uncoated or unprotected cast iron to other metals in the
soils. Cast iron may also suffer pitting and depending on soil
corrosivity and the presence of bacteria can lead to complete
perforation of the pipe wall as in Fig. 10.5.
Coating of the exterior of cast iron pipe with bituminous or lower
oil coatings has often been used to provide a measure of protection.
More recently tape wrapping with self adherent tapes have been
found to be a very cost-economical method of providing an efficient
barrier between the metal and a corrosive soil Fig. 10.6. CP can then
be applied provided the pipe sections are electrically joined.

FIGURE 10.5 A corrosion pit that led to the perforation and leak of a water
main. (Courtesy of Drinking Water Services, City of Ottawa)