Page 435 - Corrosion Engineering Principles and Practice

404 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 405

FIGURE 10.6 Wrapping a water main pipe with adhesive tape to provide
a barrier against soil corrosion. (Courtesy of Drinking Water Services,
City of Ottawa)

Aluminum
Aluminum alloys are used relatively rarely in buried applications,
although some pipelines and underground tanks have been
constructed from these alloys. Like stainless steels, aluminum alloys
tend to undergo localized corrosion damage in chloride-contaminated
soils. Protection by coatings is essential to prevent localized corrosion
damage. Cathodic protection criteria for aluminum alloys to
minimize the risk of generating undesirable alkalinity are available
(see Fig. 4.14). Aluminum alloys can undergo accelerated attack
under the influence of microbiological effects. Documented
mechanisms include attack by organic acid produced by bacteria
and fungi and the formation of differential aeration cells [8].
Zinc
Zinc may be used as a reference half-cell in soils. However, the main
application of zinc in buried applications is in galvanized steel, for
example, in the fabrication of culverts. Performance may be adequate
unless soils are poorly aerated, acidic, or highly contaminated with
chlorides, sulfides, and other corrosive ions. Well-drained soils with
a coarse texture (the sandy type) provide a high degree of aeration. It
should also be borne in mind that zinc corrodes rapidly under highly
alkaline conditions. Such conditions can arise on the surface of
cathodically overprotected structures. The degree of corrosion
protection afforded by galvanizing obviously increases with the