In simple terms, corrosion processes may be considered as reactions of metals with species in the environment to form chemical compounds.
The international standard definition of corrosion is as follows:
"Physicochemical interaction between a metal and its environment which results in changes in the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part". (ISO 8044-1986)
Note that the term "corrosion" applies to the process, not to the result, which is "corrosion damage", deterioration or effect. Implicit in the concept of corrosion as a process is the rate of corrosion reaction; implicit in the damage caused is the extent and nature of the damage in relation to the function of the systems concerned.
A broader, but widely accepted alternative definition, from the International Union of Pure and Applied Chemistry (IUPAC) encompasses the degradation of non-metals as well as metallic materials, as follows:
"Corrosion is an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in consumption of the material or in dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered. Exclusively physical or mechanical processes such as melting or evaporation, abrasion or mechanical fracture are not included in the term corrosion."