Don't let it happen to you...Dec 19 '02 Write an essay on this topic.
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The Bottom Line It is harmful to metals and machines.
Corrosion is the destruction of a material caused by the chemical action of a gas or a liquid. Corrosion occurs mostly in metals, but it may also affect ceramics. Iron oxide, also known as rust, is the most familiar form of corrosion. In rust, a reddish–brown substance forms on iron and steel that are exposed to either moist air or water containing dissolved oxygen. There are many types of corrosion. One type, called localized attack, occurs on small areas of bare metal and does little damage, such as creating holes or cracks. Another kind, uniform corrosion, attacks much larger areas, such as the surface of an aluminum pot or a copper roof. This uniform corrosion can be beneficial. On copper roofs, for example, such corrosion produces a thin, greenish film called patina that protects the surface against further rapid corrosion. The corrosion of metals such as aluminum, tin, and zinc usually stops after a thin layer of metal oxide forms on the exposed surface of the metal. This layer blocks oxygen from reaching the metal. This same principle applies to iron when it combines with oxygen. A thin, almost invisible, coating of iron oxide forms that prevents further rusting when water vapor is not in the air. When water is present, however, the oxide that forms is bulky and porous, thereby allowing oxygen continued access to the iron below. The type of corrosion and its severity depend on the chemical makeup of the metal and of the corrosive agent. The corrosive agent is the chemical that causes a material to corrode. Other major factors include stresses in the metal, the temperature of the corrosive agent, and the speed at which the agent moves against the metal. If the corrosive agent hits the metal at high speed, the corrosion is more destructive. If the agent contains solid particles, the corrosion is even worse and is called erosion corrosion. Corrosion takes place at a much faster rate in heavily industrialized areas that have high levels of sulfur and nitrogen pollutants in the atmosphere. These compounds combined with moisture in the air produce extremely corrosive acids. If rust and other types of corrosion are not prevented, nails, bridges, buildings, oil rigs, and other objects made from metal will slowly corrode away, causing our society great complications. Since iron is very cheap, easily available, and the most common useful metal, knowledge of preventing rust is very important.  |
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