Theories of Corrosion and Corrosion Inhibitors

The word corrosion may be defined as the destruction of metal by chemical or electrochemical action. Destruction by mechanical means is usually called erosion. The rusting of iron is an example of corrosion, while the filing of iron to dust is an example of erosion. Essentially, atmospheric corrosion is the reverse process to that involved in refining metals from their ores. Iron is usually found in nature as iron oxide or iron hydroxide. When it corrodes in air, it returns to iron oxide or iron hydroxide. Copper occurs as the sulfide or basic sulfate. When copper tarnishes, it reverts to the sulfide, and, in certain atmospheres, to the basic sulfate. Because the refining of metal from ore requires the expenditure of energy, the metal is at a higher energy level than the ore, and it is natural that it would try to revert to the form in which it is found in nature. Because iron need only combine with oxygen to form the hydroxide, it is a wonder that we can use iron at all when it is exposed to air. The main reason that it does not destroy itself more quickly than it does is that rust, as it forms on the metal, acts as a barrier between the metal and air, thus slowing down the corrosion process.