Most construction projects involve interaction with the earth in one form or another. Almost all types of infrastructure, from above ground structures such as buildings and roadways, to buried structures such as undergrounds pipes, vessels and utilities, rely on the soil as a means of stability and support. Unfortunately, many soils, depending on their composition, can be potentially corrosive to metallic materials.
The severity of corrosion that can be caused by a particular soil type is dependent on a variety of factors, including the soil's chemical properties, environmental conditions and the properties of the metal in contact with the soil. In this article, we shall look at the causes of corrosion in soils and the factors that contribute to the varying degree of soil corrosiveness in detail.
What is Soil Corrosion?
Simply put, soil corrosion is an electrochemical process whereby complex chemical reactions between the soil and the contacting metal result in the formation of corrosion products and deterioration of the metal. (Read our primer in the article An Introduction to Soil Corrosion.)
Corrosion is a natural redox (reduction-oxidation) reaction that converts a refined metal to its more chemically stable state. Three components must be present for corrosion to occur: an anode (the buried metal in contact with the soil), a cathode (oxygen) and an electrolyte (the moisture found in the soil). The degree of corrosiveness and the rate of corrosion differ between different soil types and properties.
Factors that Influence Soil Corrosion
The key factors that influence the severity and rate of corrosion of soils are:
Aeration refers to the amount of air within the voids of the soil particles. A higher degree of aeration (higher porosity) lowers the tendency for the formation of corrosion. Well-aerated soils promote higher rates of evaporation and retain less water, thus reducing the amount of electrolyte available for the corrosive redox reaction to take place.
The amount of aeration in soils is directly related to the soil particle size and gradation. For example, sandy soils, due to their relatively large particle size, possess better aeration and allow for quicker drainage and evaporation of moisture that clayey soils.
As mentioned previously, one of the key elements necessary for corrosion to occur is an electrolyte, which is responsible for facilitating the transfer of electrons between the anode and the cathode. The drier the soil, the less electrolyte present to facilitate the corrosion process.
Soil resistivity is directly related to the soil's moisture content and the levels of soluble salts in the soil. (Learn more about soluble salts in the article Five Key Factors in Understanding the Role of Soluble Salts in Coatings Failures.) Increasing the moisture content lowers the soil resistivity. Since corrosion is an electrochemical process that involves electric potentials and the transfer of electrons, high levels of soil resistivity obstruct the corrosion process while soils with low resistance levels are deemed more corrosive. Sandy soils drain easily and therefore are considered to be least corrosive, while by contrast, clayey coils retain electrolytes (moisture) and are considered to be at the higher end of the corrosive spectrum.
Dissolved salt content
Although the presence of water in soils enables oxidation and by extension corrosion, the process can be greatly accelerated by the presence of dissolved salts. Dissolved chloride salts in water increase the conductivity of the electrolyte (due to an increased number of dissociated ions) and enhance the number of electrolysis reactions. Generally, soils with chloride and sulfate levels below 100 ppm and 200 ppm respectively are considered mildly corrosive.
Soil acidity (pH level)
pH (potential of hydrogen) is a numerical scale used to measure the acidity or alkalinity of a solution. The scale ranges from 1 to 14, with 7 being the neutral (neither acidic nor alkaline) point. Values below 7 indicate acidity, with 1 being the most acidic and values above 7 indicate alkalinity, with 14 being the most alkaline.
The pH levels of soils vary widely, with values ranging anywhere from 2.5 to 10. A neutral pH of 7 in soils is considered to be ideal to minimize the potential for corrosion. Soils with pH values below 5 are considered to be aggressive and can lead to increased corrosion rates and premature pitting of metals. The inherent pH of a given soil can fluctuate due to environmental factors such as rainfall.
The soil's resistivity is affected by the atmospheric temperature. As the temperature decreases, the resistivity of the soil increases, and hence the corrosive potential of the soil decreases. As pore water freezes at 0°C (32°F), its resistivity increases abruptly. A subsequent decrease in the temperature results in an almost exponential increase in soil resistivity. Therefore, the formation of corrosion is highly unlikely in sub-zero temperature environments.
Methods to Reduce Soil Corrosion
There are several methods available to mitigate the potential for soil corrosion, which include cathodic protection, sacrificial anodes and protective coatings. More information about these methods is available in the related articles listed below. All of these methods are highly effective and can help avoid costly repairs and replacement in the long term.
It is almost impossible in most land-based construction projects to avoid interacting with the surrounding soil. It is therefore important to understand the soil corrosion phenomenon and the key factors that contribute to the process.