Page 175 - Corrosion Engineering Principles and Practice

150 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 151

It is therefore a good yardstick to define the state of knowledge at the
time such a list is created.
These corrosion forms could also have been organized on the basis
of other factors than their visible appearance or inspectability. The
degree of localization, for example, would be a way to organize
corrosion problems as a function of the surface selectivity of attack.
Another method to organize corrosion problems would be by using
metallurgical features of importance such as the grain structure of a
metallic material. A common type of corrosion attack for which the
grain structure is important is intergranular or intercrystalline corrosion
during which a small volume of metal is preferentially removed along
paths that follow the grain boundaries to produce what might appear
to be fissures or cracks. Intergranular and transgranular corrosion
sometimes are accelerated by tensile stress. In extreme cases, the cracks
proceed entirely through the metal, causing rupture or perforation.
This condition is known as stress corrosion cracking (SCC).
While the types of corrosion identified in Fig. 6.1 are described
individually in the following sections it should be recognized at the
onset that during any damaging corrosion process these types often
act in synergy. The unfolding of a crevice situation, for example, will
typically create an environment favorable for pitting, intergranular
attack, and even cracking.
The actual importance of each corrosion type will also differ
between systems, environments, and other operational variables.
However, there are surprising similarities in the corrosion failure
distributions within the same industries as can be seen by comparing
Figs. 6.2(a) and(b). Both corrosion failure distribution charts represent

High temperature,
Wear, erosion, 3%
cavitation, 6%
Pitting, 5%
Intergranular SCC,
6% General
corrosion,
33%

Transgranular
SCC, 19%

Intergranular
Others, 10% corrosion,
Hydrogen Corrosion fatigue, 4%
embrittlement, 3% 11%
(a)
FIGURE 6.2 Failure-statistics of large chemical process plant in Germany
(a), and in the United States (b).