Page 437 - Corrosion Engineering Principles and Practice

406 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 407

Concrete is stable in most soils but serious corrosion can occur from a
number of sources. When appraising the potential corrosivity of a soil
toward concrete, both the soil chemistry and the effect of groundwater
must be considered. The soil must be judged aggressive if

1. pH is less than 6
2. Sulfate or sulfide content is high
3. Magnesia content is high
Sulfate and magnesium ions can be corrosive to concrete even in
neutral soils. Reactions occur with the calcium salts in the concrete to
destroy the cohesion and produce a soft, porous mass. A Type-V
Portland will resist the sulfate more capably, but aluminate cements
are typically used to prevent magnesium salt attack.
Organic compounds, particularly esters, can rapidly degrade the
usual concrete. Detergents can accelerate the rate of degradation. The
freezing of absorbed water in the concrete can cause spalling or
cracking. Thus, when proposing the use of concrete underground one
should be attentive to water levels in the ground, the selection of the
proper concrete, the proper cure of the material, the density of the
finished product, the cleanliness of the water and sand used, the
depth of coverage of the reinforcing metal, the possible need to seal
the exterior, and the need to maintain the reinforcing metal as an
electrically continuous structure if CP is considered.
The surface can be treated chemically with fluoride treatments or
sodium silicate washes to densify and harden the exterior as a method
of corrosion protection. Bituminous coatings are often applied to seal
the exterior. Epoxy coatings are most compatible with the concrete as
a coating, patching compound, or adhesive.

Polymeric Materials
Polymeric materials or “plastics” of construction have revolutionized
many of the underground materials applications. Although certainly
not immune to failure, the use of plastics negates the pitting, galvanic
action, and other forms of localized attack experienced when using steel
in buried applications. The smooth interior of tubing material will
normally remain clean for the easy conveyance of liquids or gases. In
diameters of 15 cm or less, the laying of pipe in the ground can be
accomplished at the pace of a slow walk, which is an important economic
advantage when combined with the savings from unneeded external
protection.
The three major polymeric materials used in soils are
thermoplastics: polyvinyl chloride (PVC), acrylonitrile-butadiene-
styrene (ABS), and polyethylene (PE) of various densities. These
materials are not attacked by the concentration of acids, alkalies, or
solvents encountered in soil environments. Mechanical support of
the materials by soil at a constant temperature overcomes two of the