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Corrosion is the deterioration of metals and other materials by chemical reaction. Corrosion of metals is the most common type of corrosion and is a process involving an exchange of electrons between two substances, one of them being the metal. In this process, the metal usually loses electrons, becoming oxidized, while the other substance gains electrons, becoming reduced. For this reason, corrosion is classified as an oxidation-reduction or redox reaction.

While many redox reactions are extremely important and beneficial to society (for example, those that are used to make batteries), the redox reactions involved in corrosion are destructive. In fact, close to $200 billion dollars (or 4% of the gross domestic product) is spent in the United States each year to prevent or repair the damage done by corrosion. Economically, one of the most important metals to corrode is iron and one of its alloys, steel. Almost 20% of the iron and steel produced in the United States each year is used to replace objects that have corroded.

Most metals react with oxygen, but not all metals become subject to corrosion. Many metals react, but their rate of reaction is so slow that no significant corrosion appears visible. In other cases, the metal reacts with oxygen to form a metal oxide which then coats the metal and prevents further reaction with oxygen. A few metals, such as gold and platinum, are very unreactive and they are used in applications where their tendency not to corrode is important.

In general, both oxygen and water are necessary for corrosion to occur. For this reason, corrosion occurs much more slowly in desert climates that are drier. The process of corrosion is accelerated in the presence of salts or acids. In the northern United States and in Canada, winter is especially harsh on cars because there is plenty of water in the form of snow and roads are salted to prevent the formation of ice. Warm temperatures also speed the process of corrosion.

There are many ways in which we can minimize, and sometimes even prevent the occurrence of corrosion. One simple way is to paint the metal surface and prevent oxygen and water from having access to the metal surface. If the paint chips, however, the exposed surface can cause rust to spread beneath the surface of the metal. Another way to protect the metal surface is to coat it with a layer of zinc metal. Zinc is more likely to oxidize but it then forms a coating that adheres firmly to the zinc and prevents further reaction, protecting both the zinc and the metal beneath the zinc. Metals protected by zinc are said to be galvanized. Other metals that form protective coatings can also be used to protect metals that corrode. For example, in stainless steel, chromium or nickel is added to the iron. It is also possible to deposit a thin film of one metal on the surface of another metal. This process is called electroplating. Iron cans are electroplated with tin to form tin cans. The tin is less easily oxidized than the iron. If the shape of the metal object is such that it is difficult to galvanize or to electroplate, a piece of magnesium or zinc can be attached to the object. This piece of attached metal is called a sacrificial anode. It will preferentially corrode, keeping the metal (such as underground tanks or buried pipeline) intact and free from breakage. It becomes necessary to replace the sacrificial anode from time to time, but that is less expensive than replacing the whole pipeline.

Rashmi Venkateswaran

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