Soil Electrical Resistivity

Soil electrical resistivity indicates the relative capability of the soil to carry electrical current and is a main indicator in determining corrosiveness of the soil. This is generally recognised as the most significant soil characteristic with regard to corrosivity of the soil. Soil resistivity can change dramatically with moisture content. Soil, which has a high resistivity when it is dry, can have substantially lower resistivity when it is wet or saturated depending on factors such as pH and chemical content.

The soil corrosiveness is classified based on soil electrical resistivity by the British Standard BS-1377 as follows:

The above table does not take pipe wall thickness into account, as it is not a significant factor generally when considering corrosion rates. Corrosion will initially occur at many points, gradually reducing as some anodic (more electro-negative) areas become cathodic (more electro-positive) in relation to stronger anodes. The strong anodic areas become fewer in number but increasingly active so that penetration rates increase. Once the pitting pattern is established, the rate of penetration depends on many factors; the build-up of corrosion product tends to reduce rates, the increase in cathodic area to increase rates, shape of pit formed will also affect rates, to say nothing of the external environment factors. Thus, while a thicker wall will lengthen the time to penetration it may not be in proportion to the increase in wall thickness.