Environmental Engineering Reference
In-Depth Information
3
Aqueous Corrosion: Forms
3.1 CLASSIFICATION OF AQUEOUS CORROSION
Aqueous corrosion of metals and alloys takes place in a variety of environments
containing water. A classification based on the environments has often been de-
scribed in different textbooks. Thus, the terms
atmospheric corrosion, marine
corrosion
, and
soil corrosion
are in wide use. While these terms serve to describe
certain typical characteristics of corrosion pertaining to an environment, corro-
sion 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 conve-
nient 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 corro-
sion, (4) intergranular corrosion, (5) dealloying or selective leaching, (6) erosion
corrosion, and (7) corrosion cracking. Localized attack is an outcome of metallur-
gical heterogeneities (as in galvanic corrosion, intergranular corrosion, and selec-
tive 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-
47
