Corrosion, pronounced kuh ROH zhuhn, is the destruction of a material caused by the chemical action of a gas or liquid. Corrosion occurs chiefly in metals, but it may also affect ceramics. Rust is the most familiar form of corrosion.
In most cases, corrosion involves two related chemical reactions—oxidation and reduction. In oxidation, the atoms of a metal give up electrons to a liquid or a gas. In reduction, part of the same metal or an adjoining metal captures these electrons from the liquid or gas. The electrons that flow from one metal to the other form an electric current. In this sense, corrosion is an electrochemical process.
There are many types of corrosion. One kind, called localized attack, occurs on small areas of bare metal and produces holes or cracks. Another type, uniform corrosion, attacks much larger areas, such as the surface of an aluminum pot or a copper roof. It 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 type of corrosion and its severity depend on the chemical makeup of the metal and of the corrosive agent. 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. Corrosion tends to be more severe if the corrosive agent hits the metal at a high speed. If the agent contains solid particles, the corrosion is even worse and is called erosion-corrosion.
Most metals and polymers react with oxygen or other gases, particularly at elevated temperatures. Metals and ceramics may disintigrate; polymers may become brittle. Materials are also attacked by corrosive liquids, leading to premature failure. The engineer faces the challenge of selecting materials or coatings that prevent these reactions and permit operation in extreme environments.
Causes of corrosion are the subject of extensive investigation by industry. Almost any type of corrosion can manifest itself under widely differing operating conditions. Also, different types of corrosion can occur simultaneously. It is not uncommon to see crack growth from stress corrosion to be accelerated by crevice corrosion, for example.
The following may be considered as different types of corrosion: