Soil corrosion is a complex phenomenon, with a multitude of variables involved. Chemical reactions involving almost each of the existing elements are known to take place in soils, many of which are not yet fully understood. The relative importance of variables changes for different materials, making a universal guide to corrosion impossible. Variations in soil properties and characteristics across three dimensions can have a major impact on corrosion of buried structures.
The response of carbon steel to soil corrosion depends primarily on the nature of the soil and certain other environmental factors, such as the availability to moisture and oxygen. These factors can lead to extreme variations in the rate of the attack. For example, under the worst condition a buried vessel may perforate in less than one year, although archaeological digs in arid desert regions have uncovered iron tools that are hundreds of years old. (reference)
Some general rules can be formulated. Soils with high moisture content, high electrical conductivity, high acidity, and high dissolved salts will be most corrosive. The effect of aeration on soils is somewhat different from the effect of aeration in water because poorly aerated conditions in water can lead to accelerated attack by sulfate-reducing anaerobic bacteria.
The effect of low levels of alloying additions on the soil corrosion of carbon steels is modest. Some data seems to show a small benefit of 1%Cu and 2.5% Nion plain carbon steel. The weight loss and maximum pit depth in soil corrosion can be represented by an equation of the form:
Z = a·tm
Z - either the weight of loss of maximum pit depth
t - time of exposure
a and m - constants that depend on the specific soil corrosion situation.
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