Environmental Engineering Reference
In-Depth Information
Cathodic
O
2
2H
2
O
4e
→
4OH
(2.11)
As the reactions proceed, the dissolved oxygen in the small volume of stagnated
solution inside the crevice is used up. However, this does not prevent the dissolu-
tion reaction inside the crevice because the electrons reach outside the crevice
through the metal where plenty of oxygen is available for reduction. A sort of
concentration cell (differential aeration) is set up between the crevice area and
the area outside the crevice (Section 2.1.3).
The situation is further aggravated in the presence of chloride ions. The accu-
mulated cations inside the crevice attract the negatively charged chloride anions
from the bulk solution. Hydroxide anions also migrate, but they are less mobile
than chloride ions. The metal chloride formed hydrolyzes to produce metal hy-
droxide and hydrochloride acid:
MCl
H
2
O
→
MOH
HCl
(3.5)
The nascent hydrochloric acid destroys the passive film and accelerates the rate
of dissolution of the metal inside the crevice. The cathodic reduction remains
restricted to the areas outside the crevice that remain cathodically protected. Fig-
ure 3.13 depicts the happenings during the initial and later stages of crevice corro-
sion.
Figure 3.13
Crevice corrosion. (a) Initial stage. (b) Later stage.
