Page 422 - Corrosion Engineering Principles and Practice

390 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 391

lead to the formation of macro corrosion cells. Therefore, for
structures such as pipelines, the merit of a corrosion risk classification
based on an absolute value of soil resistivity is limited. Soil resistivity
can be measured by a few traditional techniques such as the Wenner
four-pin method described in Chap. 5 or, more recently, by
electromagnetic measurements. The latter allows measurements in a
convenient manner and at different soil depths. Another option for
soil resistivity measurements is the soil box method, also described
in Chap. 5, in which a sample is taken during excavation, preferably
in the immediate vicinity of the buried structure being investigated.

Redox potential
The redox potential is essentially a measure of the degree of aeration
in a soil. A high redox potential indicates a high oxygen level. Low
redox values may provide an indication that conditions are conducive
to anaerobic microbiological activity. Sampling of soil will obviously
lead to oxygen exposure, and unstable redox potentials are thus likely
to be measured in disturbed soil.

Chlorides
Chloride ions generally participate in the dissolution reactions of
many metals. Furthermore, their presence tends to decrease the soil
resistivity. Chlorides may be found naturally in soils as a result of
brackish groundwater and historical geological seabeds or come from
external sources such as deicing salts applied to roadways.
Sulfates
Sulfate ions are generally considered to be more benign in their direct
corrosive action toward metallic materials than chlorides. However,
concrete may be attacked as a result of high sulfate levels. The
presence of sulfates also poses a major risk for metallic materials since
these ions are nutrients to SRBs that convert these benign ions into
highly corrosive sulfides.

10.2.3 Soil Corrosivity Classifications
For design and corrosion risk assessment purposes, it is desirable to
estimate the corrosivity of soils without necessarily conducting
exhaustive corrosion testing. Corrosion testing in soils may be
complicated by the need of long exposure periods since buried
structures are usually expected to last for several decades during which
many soil conditions may be encountered. Considering the complexity
of the parameters affecting soil corrosion, it is obvious that the use of
relatively simple soil corrosivity models is bound to be inaccurate.
One of the simplest classifications is based on a single parameter,
soil resistivity. Table 10.3 shows the generally adopted corrosion
severity ratings. Sandy soils are high on the resistivity scale and
therefore are considered to be the least corrosive. Clay soils, especially