Just like any other environment, soils can be corrosive. In corrosion engineering, this becomes an issue because a wide variety of the structures that people and industries rely on are buried under the ground, such as vessels, storage tanks and pipelines.
Although soil corrosion can lead to financial losses and structural failure, there are measures that can be implemented throughout the design and production process to prevent it. In order to do this, it is important to develop a fundamental understanding of the potential of soil to develop corrosion.
What is Soil Corrosion?
Soil corrosion refers to a very complex phenomenon that involves several variables. This may include chemical reactions with certain elements that occur in soils. For instance, variables in soil characteristics and properties can have a huge impact on the corrosion activity that occurs in buried structures.
For instance, carbon steel's response to corrosion in soil basically depends on the soil’s nature and other factors in the environment, such as oxygen and moisture. Such factors are capable of producing intense effects in terms of the corrosion rate and severity of attack. In a worst-case scenario, a buried vessel may perforate within a year.
So what are the key factors in soil corrosion? In general, the following will dictate how corrosive soil will be:
- High electrical conductivity
- High levels of moisture
- High levels of dissolved salts and acidity
There are many other factors to consider when it comes to soil corrosion, and these should be examined very carefully to prevent extreme attacks.
Soil corrosion usually takes place with storage tanks that are stored under the ground, cables, foundations, and distribution and transmission pipelines. But then, any structures that are in contact with earth are considered prone to the damaging effects of soil corrosion.
The Soil Corrosion Problem
The damage of corrosion could result in billions of dollars in expenditures. The most significant portion of losses related to corrosion involves the corrosion of metal or steel structures that are buried in soils. Metals that are in contact with materials such as soil can develop a corrosion potential. In most instances, the rate of corrosion can be so fast and the severity can be so extreme that it results in structural failure.
Thus, the mechanism of soil corrosion should be known, along with the strategies that should be implemented to avoid or mitigate the corrosion of metal when it comes in contact with soil. Essentially, there are basic issues that are significant to soil corrosion. These are as follows:
- Soil corrosivity can be associated with the aeration level, salt content, water retention and acidity of soil, as well as the existence of ionic species in the soil.
- Galvanic processes that occur when metals come in contact with soils.
- The mitigation strategies that are being used in the practice of modern engineering.
- The use of sacrificial anodes to provide protection.
So let's dive a little deeper into these basic factors.
Aeration is described as the amount of air trapped in soil. This is a very essential factor when it comes to corrosion, as it also affects evaporation and water retention rates. Soil that is well aerated is less prone to corrosion because it has higher rates of evaporation and lower water retention. The gradation and the size of the soil particles also play a vital role in determining the degree of aeration.
For instance, sandy soils are more favorable, as these have bigger particles that permit faster evaporation and better aeration after water is introduced to the soil. A fast way to categorize soils according to their aeration is by looking at the color. Yellow, brown and reddish soils have good aeration, while the gray ones are considered unfavorable.
Resistivity and Moisture Content
Soil corrosivity is largely dependent on the moisture content of soils compared to any other variables. Water is considered one of the major elements needed for the process of electrochemical corrosion, along with other components such as metal and oxygen. Essentially, corrosion will not take place when the soil is totally dry. According to various studies, higher moisture content can reduce the soil resistivity, which can in turn increase the potential for corrosion. Bear in mind that when the soil has reached its saturation point, extra moisture will have little or no effect on the resistivity.
Acidity or pH
Soils possess varying levels of acidity, ranging from a pH of between 2.5 and 10. When the pH level reaches 5 or below, fast corrosion rates as well as early pitting in steel or metallic structures are likely to occur. A pH of 7, or neutral, is favorable because it can minimize the possibility of damage due to corrosion. Take note that soil’s intrinsic pH can be affected by other factors such as rainfall.
Redox, or oxidation reduction potential, refers to the activity or strength of oxidizers and reducers in relation to their concentration. Oxidizers accept electrons, while reducers lose electrons. A high redox potential tends to signify high oxygen (O2) concentration, low electron activity and "oxidizing" conditions, while a low O2 concentration means low redox potential, high electron activity and "reducing" conditions.
The Existence of Ionic Species
Chloride ions are harmful to metals and they have a direct role in metal anodic dissolution. The presence of chloride ions in soils could result in reduced resistivity. Chloride ions naturally exist in soils due to geological sea beds and brackish groundwater, but they can also appear as a result of external sources, like de-icing salts.
Compared to chloride ions, sulfates are less corrosive to metal structures. The thing is that sulfates can transform into sulfides, which are highly corrosive due to the action of anaerobic bacteria that could reduce the sulfate.
How to Control Soil Corrosion
Soil corrosion affects almost all structures on Earth. As such, the right measures should be implemented by industries, especially design engineers, in order to avoid or at least prevent the damage that corrosion can cause. Basically, there are three key ways to control soil corrosion:
- Cathodic Protection
Cathodic protection is a corrosion control technique that is typically used to manage the corrosion of metal by transforming it into the cathode within an electrochemical reaction or cell. (For further reading, see Cathodic Protection and Anode Backfills.)
- Sacrificial Anodes
The use of galvanic anodes is one of the most effective ways of protecting various steel structures from the harmful effects of corrosion. A good example is zinc anodes, which are highly capable of providing galvanic protection to metals that are in contact with soils.
- The Use of Protective Coatings
The use of protective coatings on metal structures buried under or in contact with the soil can significantly prevent the occurrence of soil corrosion, depending on the level of protection. (More information is available in An Intro to Pipeline Corrosion and Coatings.)
Generally, soil corrosion can take place in almost all structures that are exposed to soil. Even so, accurate design, favorable production environments and adequate protection levels can prevent the harmful damage that can be brought about by corrosion in soils. (Follow up reading can be found in Corrosion Prevention for Buried Pipelines.)