Corrosion is the destructive attack upon a metal by its environment or with sufficient damage to its properties, such that it can no longer meet the design criteria specified. Not all metals and their alloys react in a consistent manner when in contact with corrosive fluids. One of the common intermediate reactions of a metal surface is achieved with oxygen, and those reactions are variable and complex. Oxygen can sometimes function as an electron acceptor and cause cathodic depolarization by removing the “protective” film of hydrogen from the cathodic area. In other cases, oxygen can form protective oxide films. The long-term stability of these films also varies: some are soluble in the environment, others form more stable and inert passive films. Electrochemically, a metal surface is in the active state (the anode), i.e., in which the metal tends to corrode, or is being corroded. When a metal is passive, it is in the cathodic state, i.e., the state of a metal when its behavior is much more noble (resists corrosion) than its position in the emf series would predict. Passivity is the phenomenon of an (electrochemically) unstable metal in a given electrolyte remaining observably unchanged for an extended period of time.
Metallic Materials Pure metals and their alloys tend to enter into chemical union with the elements of a corrosive medium to form stable compounds similar to those found in nature. When metal loss occurs in this way, the compound formed is referred to as the corrosion product and the metal surface is spoken of as being corroded.
Corrosion is a complex phenomenon that may take any one or more of several forms. It is usually confined to the metal surface, and this is called general corrosion. But it sometimes occurs along defective and/or weak grain boundaries or other lines of weakness because of a difference in resistance to attack or local electrolytic action.
In most aqueous systems, the corrosion reaction is divided into an anodic portion and a cathodic portion, occurring simultaneously at discrete points on metallic surfaces. Flow of electricity from the anodic to the cathodic areas may be generated by local cells set up either on a single metallic surface (because of local point-to-point differences on the surface) or between dissimilar metals.
Nonmetallics As stated, corrosion of metals applies specifically to chemical or electrochemical attack. The deterioration of plastics and other nonmetallic materials, which are susceptible to swelling, crazing, cracking, softening, and so on, is essentially physiochemical rather than electrochemical in nature. Nonmetallic materials can either be rapidly deteriorated when exposed to a particular environment or, at the other extreme, be practically unaffected. Under some conditions, a nonmetallic may show evidence of gradual deterioration. However, it is seldom possible to evaluate its chemical resistance by measurements of weight loss alone, as is most generally done for metals.
