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.
If the environment is controlled (especially in recirculating systems), corrosion inhibitors can often be added to it. These chemicals form an electrically insulating or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions. Corrosion inhibitors are chemicals that, when added in relatively low concentrations to the environment, reduces the rate of a corrosive process. Which substance that acts like inhibitors depends on both the corrosive environment as well as the alloy. Inhibitors are normally used in closed systems such as automobile radiators and steam boilers. An example of this principle is the use of antifreeze in cars. The effectiveness of an inhibitor depends on several different mechanisms. Some reacts with the chemically active species in the solution while other react with the corroding surface and interfere with the corrosive reaction, or form a thin protective coating. For example, oxygen is generally removed by reductive inhibitors such as amines and hydrazines. In this example, hydrazine converts oxygen, a common corrosive agent, to water, which is generally benign. Many inhibitors are also toxic and are therefore not suitable to use in all applications. Another limitation of inhibitors is that they generally lose the effectiveness when the temperature and concentration of the environment increases.