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Corrosion is the destructive reaction of a metal with another material, e.g. oxygen, or in an extreme pH environment (either acidic or basic).
The corrosion product is a mix of oxide and salts of the original metal.
Corrosion is the primary means by which metals deteriorate. Most metals corrode when in constant contact with moisture in the air, acids, bases, salts and oils.
Corrosion specifically refers to any process involving the deterioration or degradation of any metallic element or compound. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery. When metal atoms are exposed to an environment containing water molecules they can give up electrons. This effect can be concentrated locally to form a pit or, sometimes, a crack, or it can extend across a wide area to produce general wastage.
Corrosion can be divided into two categories: dry corrosion or wet corrosion.
Resistance to corrosion
The materials most resistant to corrosion are those for which corrosion is thermodynamically unfavorable. Any "corrosion" products of gold or platinum tend to decompose spontaneously into pure metal, which is why these elements can be found in metallic form in nature, and is a large part of their intrinsic value.
Other metals are protected from corrosion by slow reaction kinetics. These include such metals as zinc, magnesium and cadmium. While these metals easily corrode, they do so relatively slowly. An extreme example of this effect is semimetallic graphite, which releases large amounts of energy upon oxidation but does not corrode at all under normal conditions.
Given the right conditions, a thin film of corrosion products can form on a metal's surface spontaneously, acting as a barrier to further oxidation. When this layer stops growing at less than a micrometre thick under the conditions that a material will be used in, the phenomenon is known as passivation.
Passivation in air and water at moderate pH is seen in such materials as aluminium, stainless steel, titanium, and silicon. However, this passivating film is destroyed under certain conditions, causing corrosion to proceed. These conditions vary depending on the material: high pH for aluminum, low pH or the presence of chloride ions for stainless steel, high temperature for titanium (in which case the oxide dissolves into the metal, rather than the electrolyte) and fluoride ions for silicon.
On the other hand, sometimes unusual conditions can bring on passivation in materials that normally don't display this phenomenon, as the alkaline conditions in concrete do for steel rebar.
Theory
One way to understand the structure of metals on the basis of particles is to imagine an array of ions sitting in a "gas" of free electrons. Coulombic attraction holds these oppositely-charged particles together, but there are other sorts of negative charge which could neutralize the metal ions, such as the negative ions (anions) in an electrolyte. For a given ion at the surface of a metal, there is a certain amount of energy to be gained or lost by dissolving into the electrolyte or becoming a part of the metal, based on how strongly attractive the electron gas or anions are. This strongly depends on a host of variables, including the types of ions involved and their concentrations, and on how many electrons are at the metal's surface. In turn, corrosion processes strongly affect all of these variables, so that a reshuffling of electrons and ions tends to produce a state of local equilibrium.
See also
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