corrosion

Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with its environment. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases. Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material 's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.
Corrosion Process
Elements are rarely found in a pure metal state. Rather they are found in chemical combinations with one or more nonmetallic elements. These chemical combinations are commonly known as ore. Significant energy must be expended to reduce the ore to pure metal. This energy can be applied via metallurgical or chemical means and is done so in the process called smelting and refining. Additional energy may be used in the form of cold-working or heating and casting to the pure metal into a working shape. Corrosion, which can be simply defined as rust, is the tendency for metals to revert to their natural, lower energy state of ore. Metallic corrosion includes both

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