Page 436 - 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

thickness of the galvanized coating. Additional protection can be
afforded by so-called duplex systems, in which additional paint
coatings are applied to galvanized steel.

Lead
At one time, lead was used extensively as sheathing for telephone
cables, but it has been phased out in favor of plastics in most
applications, in great part due to the toxicity of lead ions. Although
lead is amphoteric*, a reasonable corrosion resistance was shown in
most soils. The corrosion resistance of lead and lead alloys in soils is
generally regarded as being in-between those of steel and copper. The
corrosion resistance of buried lead sheathing for power and
communication cables has usually been satisfactory. Caution needs to
be exercised in soils containing nitrates and organic acids such as
acetic acid. Excessive corrosion is also found under highly alkaline
soil conditions. Silicates, carbonates, and sulfates tend to retard
corrosion reactions by their passivating effects on lead.
Stainless Steels
Stainless steels are rarely used in soil applications, as their corrosion
performance in soil is generally poor and not better than bare steel.
Localized corrosion attack is a particularly serious concern. The
presence of chloride ions and concentration cells developed on the
surface of these alloys tends to induce localized corrosion damage.
Since pitting tends to be initiated at relatively high corrosion potential
values, higher redox potentials increase the localized corrosion risk.
Common grades of stainless steel and even the very highly alloyed
versions are certainly not immune to MIC.

Copper and Alloys
Copper is generally considered to have good resistance to corrosion in
soils. Corrosion concerns are mainly related to highly acidic soils and the
presence of carbonaceous contaminants such as cinder. Sulfides, often
produced by SRBs, also greatly increase the risk of corrosion damage.
In the case of brasses, consideration must be given to the risk of
dezincification, especially at high zinc levels. Soils contaminated with
detergent solutions and ammonia also pose a higher corrosion risk for
copper and copper alloys. Additional corrosion protection for copper
and copper alloys is usually considered only in highly corrosive soil
conditions. CP, the use of acid-neutralizing backfill such as limestone,
and protective coatings can be used in these applications.

Concrete
Concrete is extensively used underground as footings, piers, tanks,
piping, and so forth. The material is normally used with reinforcing steel.

* Amphoteric signifies that the metal is attacked by either acidic or basic
environments.