Environmental Engineering Reference
In-Depth Information
Figure 3.16
Crevice corrosion of stainless steel in seawater due to barnacles attach-
ment.
3.5.4 Practical Examples
The practical examples of crevice corrosion or deposit corrosion are quite com-
monplace. Deposits of dirt and leaves on tin roofs often lead to leak formation.
Similarly, a welding debris inadvertently left on the inside wall of a stainless
steel digester filled with demineralized water produced leakage during a shut-
down period. Rapid attack with red rust developed under a stainless steel bolt
that had fallen to the bottom of a 18-8 stainless steel tank containing a hot saline
solution in a dyeing plant has been reported [7]. Flange faces underneath gaskets,
particularly fibrous gaskets having a wick action, are prone to attack by crevice
corrosion.
Figure 3.16 illustrates crevice corrosion of stainless steel in seawater as a
result of barnacles attachment to the surface [8].
3.6 INTERGRANULAR CORROSION
Intergranular corrosion is a preferential attack on the grain boundary phases or
the zones immediately adjacent to them. Little or no attack is observed on the
main body of the grain. The continuous attack along the grain boundary can
readily be detected under microscope in a metallographically polished and etched
specimen (Fig. 3.17). The surface of the affected stainless steel part shows visible
rusting. The alloy loses its mechanical strength like a brick wall having its mortar
degenerated. The alloy tends to disintegrate and in the extreme case the grains
fall out.
Since the grain boundary region is an area of crystallographic mismatch
between the orderly structures within the adjacent grains, it is chemically slight-
