Environmental Engineering Reference
In-Depth Information
Figure 3.9
(a) Slow dissolution of passive film forming Fe
3
. (b) Accelerated dissolu-
tion at a soluble salt island forming Fe
3
. (c) Direct anodic dissolution at a pit initiation
site forming Fe
2
.
composition. The sequence of events is described in Fig. 3.9. All initiated pits
are not stable and many become inactive after some growth. However, some get
stabilized.
A stabilized pit starts growing according to the following mechanism as illus-
trated in Fig. 3.10. Since oxygen in the stagnated solution inside the pit is used
up in the cathodic reduction process within a short period, anodic dissolution
concentrates there with accompanying cathodic reduction of oxygen just outside
the pit. A situation of small anode and large cathode prevails. Additionally, as
metal cations accumulate inside the pit and the chloride ions rush to the pit pro-
moting the hydrolysis reaction:
Fe
2
2H
2
O
2Cl
→
Fe(OH)
2
2HCl
(3.4)
the hydrochloric acid produced aggravates the dissolution further. The propaga-
tion thus becomes self-stimulating and along a narrow front. In a way, the sides
of the pits are cathodically protected.
The requirement of a concentrated solution for the initiation and propagation
of a pit is met preferably by a stagnant condition of the solution as well as its
accumulation on the upper side of the component. That is why pits tend to grow
