Soil Environments

The corrosivity of soils is affected by water, the degree of aeration, pH level, temperature, salt content, and biological activity. The soil’s particle size plays a role in that smaller particle sizes will hold water, and have less oxygen content; while the opposite exists for large particle sizes. Loose soil will also have greater oxygen content. The oxygen content is highest near or at the soil’s surface and decreases with increasing depth. Soils normally have pH levels in the range 5 to 8. At these levels, the corrosion rate is not significantly affected. However, acidic soils will increase the rate of attack on most metals, and will affect other factors such as microbiological activity. Salt content affects soils in the same manner as atmospheric and water environments, with the major corrosive species once again being chloride and sulfate ions. Sulfate reducing bacteria is the most detrimental microorganism to metal corrosion. Like the water environment, a measure of the soil’s electrical resistivity is a general indication of its corrosiveness. Lower resistivity equates to higher corrosivity. Table 7 lists soil resistivity ratings based upon resistivity. Mappings of soil resistivities are used to determine where to lay underground piping. It is beneficial to route pipelines through soil with like resistivities and ideally, high resistivities. Care must be taken when laying pipelines in the vicinity of other structures, such as railways, to minimize their exposure to stray currents.