FACTORS AFFECTING THE CORROSIVITY OF SOILS

10.2 FACTORS AFFECTING THE CORROSIVITY OF SOILS

Among the factors that control corrosivity of a given soil are (1) porosity (aera- tion), (2) electrical conductivity or resistivity, (3) dissolved salts, including depo- larizers or inhibitors, (4) moisture, and (5) pH. Each of these variables may affect the anodic and cathodic polarization characteristics of a metal in a soil. A porous soil may retain moisture over a longer period of time or may allow optimum aeration, and both factors tend to increase the initial corrosion rate. The situation is more complex, however, because corrosion products formed in an aerated soil may be more protective than those formed in an unaerated soil. In most soils, particularly if not well - aerated, observed corrosion takes the form of deep pitting. Localized corrosion of this kind is obviously more damaging to a pipeline than

a higher overall corrosion rate occurring more uniformly. Another factor to be considered is that, in poorly aerated soils containing sulfates, sulfate - reducing bacteria may be active; these organisms often produce the highest corrosion rates

BUREAU OF STANDARDS TESTS

Aeration of soils may affect corrosion not only by the direct action of oxygen in forming protective fi lms, but also indirectly through the infl uence of oxygen reacting with and decreasing the concentration of the organic complexing agents or depolarizers naturally present in some soils. In this regard, the benefi cial effect of aeration extends to soils that harbor sulfate - reducing bacteria because these bacteria become dormant in the presence of dissolved oxygen. Corrosion rates tend to increase with depth of burial near the soil surface, but not invariably. Bureau of Standards tests [6] on steel specimens exposed 6 – 12 years and buried

30 – 120 cm (12 – 48 in.) below the surface showed greater corrosion at greater depth for fi ve soils, but showed the reverse trend for two soils.

A soil containing organic acids derived from humus is relatively corrosive to steel, zinc, lead, and copper. The measured total acidity of such a soil appears to

be a better index of its corrosivity than pH alone. High concentrations of sodium chloride and sodium sulfate in poorly drained soils, such as are found in parts of southern California, make the soil very corrosive. Macrogalvanic cells or “ long -

line ” currents established by oxygen concentration differences, by soils of differ- ing composition, or by dissimilar surfaces on the metal become more important when electrical conductivity of the soil is high. Anodes and cathodes may be thousands of feet, or even miles, apart. A poorly conducting soil, whether from lack of moisture or lack of dissolved salts or both, is, in general, less corrosive than a highly conducting soil. But conductivity alone is not a suffi cient index of corrosivity; in accordance with the principles of electrode kinetics discussed in Chapter 5 , anodic and cathodic polarization characteristics of a corroding metal in the soil are also a factor.

From studies at the National Physical Laboratory in the United Kingdom, it was concluded that the corrosivity of soil towards ferrous metals could be esti- mated by measuring the resistivity of the soil and the potential of a platinum electrode in the soil with respect to a saturated calomel reference electrode [7] . Soils of low resistivity ( < 2000 Ω - cm) were considered to be aggressive. Soils in which the potential at pH 7 was low [ < 0.40 V (S.H.E.), or < 0.43 V if the soil is

clay] were considered to provide a suitable environment for sulfate - reducing bacteria and to be aggressive. Borderline cases by these two criteria were resolved by considering that a water content of more than 20% causes a soil to be aggressive.