Introduction
Corrosion is the steady degradation of metals as a consequence of chemical rejoinders with their surroundings. The capacity to transfer moist unprocessed innate gases via pipelines and other unreachable points is a crucial aspect in the progression of novel gas fields (Sridhar et al. 221). Therefore, it is important to understand corrosion and methods of controlling it for safety and economic reasons. This paper looks at the causes of corrosion in pipelines, methods of controlling corrosion, and the working principles behind these methods.
The Occurrence of Corrosion in Oil and Gas Pipelines
Corrosion is among the key reasons for failure in the onshore transportation of gases and dangerous fluids in the United States. Corrosion also significantly affects gas dissemination mains and facilities as well as oil and gas collection schemes. In the U.S., for example, the yearly costs incurred from the destruction of structural machinery caused by corrosion surpass the harm from natural disasters such as upheavals, floods, storms, hurricanes, and fires. Other countries such as Britain and Germany also report similar findings. According to reports by NACE, it is approximated that all forms of corrosion lead to the loss of $276 billion (Baker and Fessler 1). Of this sum, the deterioration of onshore gas and fluid transportation pipelines accounts for $7 billion.
Causes of Corrosion
A number of factors contribute to the corrosion of pipelines. These factors can be classified as internal and external. Internal factors lead to corrosion of the inner surfaces of pipelines, whereas external (environmental) factors cause the destruction of the outer surfaces of pipelines in what is known as external corrosion. The constituents of the substances that are transported in pipelines greatly determine the occurrence and rate of corrosion. Gases such as carbon dioxide, oxygen, hydrogen sulfide, and condensed water usually determine the rate and extent of corrosion (Sridhar et al. 221).
Carbon dioxide corrosion, which is also known as sweet corrosion, occurs in two steps. The gas dissolves in water to form carbonic acid, which is a weak acid. Carbonic acid subsequently dissociates into bicarbonate ions that react with iron ions in the pipe through electrochemical reactions that yield ferrous carbonate (FeCO3). As fluids run through the pipe, their motion eliminates a section of the ferrous carbonate deposits leading to corrosion. Water hastens the rate of CO2 corrosion by providing a medium for salts to dissolve. Hydrogen sulfide corrosion (sour corrosion) happens as a result of the reaction between iron and hydrogen sulfide to form iron sulfide. This layer peels off as it becomes thicker thereby increasing the rate of corrosion (Nesic 4311).
Recent research shows that certain microorganisms enhance corrosion by their activities (Baker and Fessler 13). The two categories of such microorganisms “are sulfate-reducing bacteria (SRB) and acid-producing bacteria (APB)” (Baker and Fessler 13). Cathodic shielding currents often yield gases such as hydrogen. Bacteria use up these gases thereby encouraging exterior corrosion by destroying the polarity of the pipes. These bacteria can also create acidic biofilms, which entrap electrolytes and acids, inside pipelines. Microbiologically influenced corrosion occurs mostly in liquid pipelines compared to gas pipelines.
External corrosion occurs in pipelines that are either open to the elements or are concealed in the soil. Studies reveal that aspects that improve the soil’s conduction of electricity amplify its corrosive tendencies because the transfer of ions via an electrolyte is mandatory for corrosion to occur. Therefore, aspects such as elevated moisture content, high salt concentrations, and poor drainage influence corrosion. In addition, aeration and soil type (such as clay, which draws protective layers from pipes during expansion and contraction) also cause external corrosion.
Current Methods of Controlling Corrosion in Pipelines and their Modes of Action
Internal Corrosion
Dehydration is an effective method of controlling corrosion, especially in gas pipelines. Dehydration works by getting rid of any water that forms droplets, which allow internal corrosion. Chemical scrubbing agents such as glycol are used to dry gases completely (Baker and Fessler 23). Entrapped moisture can also be removed physically by using scrubbing agents that have cyclone separators to eliminate residual water from the gases (Baker and Fessler 23).
The presence of unbound water can also cause the destruction of liquid pipelines. This can be prevented by treating fluids to eliminate any water traces. Pigging is one of the treatments that eliminate water from fluids by the use of gravity. Salt driers can also be used to remove water in hydrocarbons.
The use of inhibitors is effective in preventing internal corrosion. Inhibitors are substances (mostly chemicals) that prevent corrosion when put in pipelines. Inhibitors work in three main ways. They form a shielding layer from their reaction with the metal surface or adsorb onto the surface of the pipelines. In addition, the interaction of inhibitors with the corrosive agents may reduce their corrosiveness. The efficacy of this method requires that the inhibitors are adequately distributed in all phases present in the pipeline (Achour, Johlman, and Blumer 1).
Coatings are used to prevent corrosion in pipelines. Substances such as plastic linings are fitted into pipes’ inner surfaces. However, allowing any air space between the pipe and the plastic allows in corrosive substances hence reducing the efficacy of the method.
Buffering, which entails introducing a buffering agent such as a mild base, is also used to control corrosion. This method works by elevating the pH of acids and making them alkaline. The trick behind this method is that bases do not harm metals because they do not promote corrosion. Cleaning pigs rid the insides of pipelines of corrosive substances by directing all harmful substances to pig traps for subsequent elimination (Baker and Fessler 24). Finally, the injection of biocides into pipelines helps to eliminate all SRBs and APBs thereby preventing microbiologically influenced corrosion.
External Corrosion
Methods that prevent external corrosion of pipelines apply to pipelines that convey gases and liquids. Cathodic protection is one such technique that prevents exterior corrosion of pipelines. This method entails placing a current from an external source via the soil towards the pipeline. Cathodic protection works by causing the employed current to dominate the local anodes making the whole uncovered exterior of the pipeline act like an electrode with a negative charge. The efficacy of this method lies in the fact that corrosion happens in anodic areas.
However, the amount of current required depends on the size of the uncovered area. For this reason, it is uneconomical to use cathodic protection alone as a means of controlling corrosion as extremely large currents are needed (Baker and Fessler 15). Therefore, coatings are usually utilized alongside cathodic protection (Baker and Fessler 15). Coatings act as barriers between steel and the electrolyte in preventing corrosion.
‘Pipe-in-pipe’ is another technique that prevents corrosion of pipelines. In this method, a pipe made of metal is inserted into another pipe that has insulating wadding on its inner surface. However, this technique is only suitable for short lengths of pipes as pipes may be costly.
Conclusion
Although there are many methods that can be used to prevent corrosion, none is completely effective on its own. Therefore, it is important to use more than one method to minimize the chances of corrosion.
References
Achour, Mohsen, Carolyn Johlman, and David Blumer 2008. Understanding the Corrosion Inhibitor Partitioning in Oil and Gas Pipelines. Web.
Baker, Michael and Fessler R. Raymond 2008, Pipeline Corrosion: Final Report. PDF File. Web.
Nesic, Srdjan. “Key Issues Related to Modelling of Internal Corrosion of Oil and Gas Pipelines.” Corrosion Science. 49.12(2007): 4308–4338. Web.
Sridhar, N., D. S. Dunn, A. M. Anderko, M. M. Lencka, and H. U. Schutt. “Effects of Water and Gas Compositions on the Internal Corrosion of Gas Pipelines— Modeling and Experimental Studies.” Corrosion. 57.3(2001): 221-235. Web.