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 oxidation and exposure to oxygen in the environment and electrochemical processes meaning the metal forms corrosion cells on its surface that greatly accelerate the transformation of metal back to the ore state and involves both chemical reactions and the flow of electrons. A basic electrochemical process that drives the corrosion of metals is galvanic action, where current is generated internally by physical and chemical reactions occurring among the components of the cell. Resistance to corrosion
Some metals are more intrinsically resistant to corrosion than others. There are various ways of protecting metals from corrosion including painting, hot dip galvanizing, and combinations of these. Intrinsic chemistry
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 on Earth and have long been valued. More common "base" metals can only be protected by more temporary means. Some metals have naturally slow reaction kinetics, even though their corrosion is thermodynamically favorable. These include such metals as zinc, magnesium, and cadmium. While corrosion of these metals is continuous and ongoing, it happens at an acceptably slow rate. An extreme example is graphite, which releases large amounts of energy upon oxidation, but has such slow kinetics that it is effectively immune to electrochemical corrosion under normal conditions. Passivation
Passivation refers to the spontaneous formation of an ultrathin film of corrosion products known as passive film, on the metal's surface that act as a barrier to further oxidation. The chemical composition and microstructure of a passive film are different from the underlying metal. Types of corrosion
It occurs when two different metals have physical or electrical contact with each other and are immersed...
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