ABSTRACT

Aqueous corrosion of metaJs and alloys takes place in a variety of environments containing water. A classification based on the environments has often been described in different textbooks. Thus, the terms atmospheric corrosion, marine co"osion, and soil corrosion are in wide use. While these terms serve to describe certain typical characteristics of corrosion pertaining to an environment, corrosion attacks having common features are not unlikely in different environments. Such a classification is useful in assessing the extent and type of attack with the changing conditions of the environment. For example, atmospheric corrosion of steels decreases drastically at a relative humidity of below 60%, the increase in salinity above a certain level increases localized attack, and so on. Some definite forms or types of attack are evident irrespective of environments and it is convenient to classify corrosion on the basis of these forms. Broadly, corrosion can be classified as general and localized. The latter takes different forms and can be described as (1) galvanic or two-metal corrosion, (2) pitting, (3) crevice corrosion, (4) intergranular corrosion, (5) dealloying or selective leaching, (6) erosion corrosion, and (7) corrosion cracking. Localized attack is an outcome of metallurgical heterogeneities (as in galvanic corrosion, intergranular corrosion, and selective leaching), local aggravation of corrosive conditions (as in pitting and crevice corrosion), and involvement of mechanical factors (as in erosion corrosion and corrosion cracking). Because of its localized nature, attack is often intense leading to the premature failure of metallic components. These different forms of corro-

sion are described in subsequent sections. Biologically influenced corrosion has been dealt with in a separate section.