Corrosion is the deterioration of a material due to chemical interaction with its environment. It is natural process in which metals convert its structure into a more chemically-stable form such as oxides, hydroxides, or sulfides. The consequences of corrosion are all too common. Familiar examples include the rusting of automotive body panels and pipings and many tools. Corrosion is usually a negative phenomenon, since it is associated with mechanical failure of an object. Metal atoms are removed from a structural element until it fails, or oxides build up inside a pipe until it is plugged. All metals and alloys are subject to corrosion. Even the noble metals, such as gold, are subject to corrosive attack in some environments.
Protection from Corrosion
As was written, the problem of metallic corrosion is significant. In economic terms, it has been estimated that approximately 5% of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. Therefore, various treatments are used to slow corrosion damage to metallic objects which are exposed to the weather, salt water, acids, or other hostile environments. Since there are many forms of corrosion, there are many ways to stop or mitigate corrosion. In every case, it depends on material to be protected and also on the environment, in which the material is used. Metals may be protected from corrosion by using a metal in an environment in which it is immune, by making a physical barrier between the metal and its environment, by means of an electric current, or by changing the environment.
Cathodic protection is a very effective way of protection from corrosion based on the use of a material that is more anodic than the metal to be protected. The protected metal is electrically connected to another metal that is more reactive in the particular environment. The metal which is less corrosive resistant becomes anode and metal with more corrosive resistance become cathode. The corrosion of the less corrosive resistance is usually increased and attack on more resistant material is decreased. A difference in electrical potential exists between the different metals and serves as the driving force for electrical current flow through the corrodant or electrolyte. Cathodic protection systems are most commonly used to protect buried or submerged metal structures from corrosion, such as steel pipelines and tanks. Magnesium, zinc, and aluminum alloys are common sacrificial anodes. Magnesium anodes are most commonly used for buried soil applications. Zinc is most often used for freshwater and saltwater marine applications. Aluminum alloys are most often used for offshore structures.
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