Soil Variables

Several variable have been identified to have an influence on corrosion rates in soil.

Water, in liquid form, represents the essential electrolyte required for electrochemical corrosion reactions. A distinction is made between saturated and unsaturated water flow in soils. The latter represents movement of water from wet areas towards dry soil areas. Saturated water flow is dependent on pore size and distribution, texture, structure, and organic matter.

The oxygen concentration decreases with increasing depth of soil. In neutral or alkaline soils, the oxygen concentration has an important effect on corrosion rate due to its participation in the cathodic reaction. However, in the presence of certain microbes (such as sulfate reducing bacteria) corrosion rates can be very high, even under anaerobic conditions. Excavation can obviously increase the degree of aeration in soil, compared with the undisturbed state.

Soils usually have a pH range of 5-8. In this range, pH is generally not considered to be the dominant variable affecting corrosion rates. More acidic soils obviously represent a serious corrosion risk to common construction materials such as steel, cast iron and zinc coatings. Soil acidity is produced by mineral leaching, decomposition of acidic plants (for example coniferous tree needles), industrial wastes, acid rain and certain forms of micro-biological activity. Alkaline soils tend to have high sodium, potassium, magnesium and calcium contents. The latter two elements tend to form calcareous deposits on buried structures with protective properties against corrosion. The pH level can affect the solubility of corrosion products and also the nature of microbiological activity.

Resistivity has historically been used as a broad indicator of soil corrosivity. Since ionic current flow is associated with soil corrosion reactions, high soil resistivity will arguable slow down corrosion reactions. Soil resistivity generally decreases with increasing water content and the concentration of ionic species. Soil resistivity is by no means the only parameter affecting the risk of corrosion damage. A high soil resistivity alone will not guarantee absence of serious corrosion.

The redox potential essentially is a measure of the degree of aeration in a soil. An 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.

Chloride ions are generally harmful, as they participate directly in anodic dissolution reactions of metals and their presence tends to decrease the soil resistivity. They may be found naturally in soils as a result of brackish groundwater and historical geological sea beds (some waters encountered in drilling mine shafts have chloride ion levels comparable to sea water) or from external sources such as de-icing salts applied to roadways. The chloride ion concentration in the corrosive aqueous soil electrolyte will vary, as soil conditions alternate between wet and dry.

Compared to the corrosive effect of chloride ion levels, sulfates are generally considered to be more benign in their corrosive action towards metallic materials. However, concrete may be attacked as a result of high sulfate levels. The presence of sulfates does pose a major risk for metallic materials in the sense that sulfates can be converted to highly corrosive sulfides by anaerobic sulfate reducing bacteria.

Microbiologically influenced corrosion (MIC) refers to corrosion that is influenced by the presence and activities of microorganisms and/or their metabolites (the products produced in their metabolism). Bacteria, fungi and other microorganisms can play a major part in soil corrosion. Spectacularly rapid corrosion failures have been observed in soil due to microbial action and it is becoming increasingly apparent that most metallic alloys are susceptible to some form of MIC. (more)