Corrosion may be defined as an unintentional attack on a material through reaction with a surrounding medium. The term can refer to a process or to the damage caused by such a process. According to this general definition, materials other than metals, such as ceramics, plastics or concrete, may also be subject to corrosion (or corrode). When no particular reference is made to the material, however, it is normally understood that a metal is being attacked. It is entirely in this limited sense that the term is used in this book. There are good reasons for treating the corrosion of metals separately, apart from deterioration or decay of other materials. Since metals have a high electric conductivity, their corrosion is usually of an electrochemical nature. The chemical deterioration of electrically non-conducting ma­ terials, such as plastics and ceramics, is governed by other physico-chemical principles. It is necessary to devote more attention to metallic corrosion nowadays than earlier, due to 1. An increased use of metals within all fields of technology. 2. The use for special applications, e.g. within the atomic energy field, of rare and expensive metals, whose preservation requires particular precautions. 3. A more corrosive environment due to the increasing pollution of air and water. 4. The use of metallic constructions of more slender dimensions which do not tolerate corrosive attacks to the same extent as did the heavy constructions used in the old days.