Metal pipes are subject to corrosion irrespective of their environment, unless they are protected. Pipes can be found everywhere: buried in the ground, underwater or exposed to the atmosphere.
Without proper protection and regular maintenance, metal pipes will just corrode. Since pipelines serve different purposes such as transporting water and sewage, and also for transporting hazardous products such as chemicals and gas, a structurally sound pipeline is very important. Corrosion primarily weakens a pipeline by attacking its structure, making it susceptible to pits, cracks and fractures.
These seemingly harmless looking pits and cracks can propagate and damage the entire pipeline system, causing irreparable damage and sometimes environmentally damaging consequences.
Why Do Metals Corrode?
This puzzling question is actually very simple – metals corrode because they want to. Metals are more comfortable in the form of compounds such as oxides because they contain less energy and are more stable. During the manufacture of steel, iron is robbed of its oxygen in the blast furnace, putting a lot of energy into the metal in the form of heat. A metal pipe will retain this energy as long as it remains metallic.
However, this energy will always urge the metal to revert to its original ore form from which it was derived. Corrosion causes the pent-up energy in a metal to be released, urging it to revert back to a more stable oxide. While it’s a relief for the metal, we now have a corrosion problem on our hands.
Electric current plays a very important role in corrosion and is usually caused by the corrosion itself, though the opposite also does take place. (Corrosion can be a huge problem in infrastructure. Learn more in Methods for Preventing Corrosion in Infrastructure.)
Causes of Corrosion
Corrosion can be attributed to many reasons. However, there are a few basic reasons why buried pipelines corrode. Here are a few:
When two dissimilar metals are in contact with each other, a voltage can result, causing a flow of current. It is often in replaced sections of otherwise good metallic pipelines that corrosion is more observable. The issue is not with the new pipe, but with the new galvanic cell that was set up when the old pipe was joined to the new pipe of a different material. The problem began when the new pipe became anodic in nature when coupled with an old pipe, resulting in a flow of current. When two different metallic pipes are buried near each other and connected through an electrical conductor such as a geyser, corrosion will occur in the pipeline that acts as the anode.
This galvanic cell can also be formed when the composition of the soils in contact with the pipe are different. Porous soils tend to become cathodic, while impervious soils tend to become anodic. Often during backfilling, a mixture of soils is used to cover up the trench, resulting in such a condition.
Differential Soil Aeration
The bottom of a buried pipe is more likely to corrode than the top because of differential aeration. The top portion of the pipe is more aerated due to oxygen supply from loose top soil. The bottom soil is largely undisturbed and hence less aerated. This difference in aeration also can set up a galvanic cell.
Direct current from any source can also cause corrosion. Such stray currents can travel long distances through a network of pipes and cables. The metal will corrode whenever the current leaves the metal and enters the ground, but when the current enters the metal from the ground, the metal is protected from corrosion.
Function of a Galvanic Cell
To understand, a galvanic cell, it is helpful to understand a simple battery. The battery is basically a small can of zinc filled with an electrolyte. A graphite rod is inserted into the center and acts as the cathode. Zinc has a tendency to corrode because it has more stored energy than graphite.
The energy that lights up a bulb in this circuit is actually provided by the energy released by zinc as it corrodes. Zinc releases its ions into the electrolyte as it corrodes, which travel to the cathode (graphite) and discharge. Electric current is created by the passage of these ions.
A steel pipe buried in moist earth will tend to set up small potentials in some areas of the pipe surface and adjacent areas, because of the slight difference in metal composition of surface condition, and because of the slight difference in the nature of the soil in contact with the pipe. The soil acts as the electrolyte and allows the current to flow from the more negative or anodic areas to the positive or cathodic areas and return through the metal. The metal at the anode will dissolve as pits, while the cathodic portion remains intact.
Protection of Pipes
The primary means of defending against corrosion is by applying protective coatings or linings applied in conjunction with cathodic protection. However, before selecting any method, it is necessary to evaluate the environment in which the pipeline is located. Personnel trained in corrosion control can be engaged to analyze risk factors and present the best way to mitigate corrosion.
Using quality material for installation is a good start to prevent corrosion, but even that will corrode if the pipeline is not regularly monitored and maintained. It is now very well understood that a coated pipeline is better not only at resisting corrosion but can help control corrosion at lesser cost. Corrosion can be stopped in ideal conditions such as when the coating on the pipe is perfect, which is impossible. Some methods to control pipeline corrosion are:
This method is used for buried pipelines and uses direct current to fight corrosion in a metal pipeline. It can also prevent corrosion from beginning in a new pipeline and prevent existing corrosion from getting worse.
These are chemicals that are added to the environment surrounding the affected pipeline in order to curb the attack. These can also help in extending the serviceable life of the pipeline and prevent failure of pipeline.
Materials such as high-density polyethylene (HDPE), polyvinyl chloride (PVC) and glass-reinforced plastic (GRP) are excellent materials that never corrode, are flexible and easy to maintain. These can be used to replace damaged pipelines by trenchless rehabilitation methods such as sliplining and pipe bursting.
Corrosion control cannot be achieved in one sitting; it is a periodic process. Often pipelines are installed and forgotten until maintenance issues such as blockages, leaks or failure begs attention. Regular maintenance can help prevent catastrophic failure, especially in the case of pipelines carrying hazardous material. Sewer pipes are also prone to internal corrosion due to the production of hydrogen sulfide gas.
Regular maintenance checks by means of trenchless investigation methods such as video inspection and closed-circuit television (CCTV) inspection should be carried out to prevent corrosion or stop it at first sight. (For more on the problems hydrogen sulfide can cause, see How to Detect and Control Hydrogen Sulfide Corrosion Problems in Sewer Pipes.)