Environmental Engineering Reference
In-Depth Information
Figure 3.8
Schematic representation of cyclic polarization to determine pitting poten-
tial,
E
pit
, and protection potential,
E
prot
.
creasing positively charged substrate. In the absence of chlorides, the passive
film dissolves slowly, according to:
FeOOH
H
2
O
→
Fe
3
3OH
(3.1)
where FeOOH represents the passive film. Chloride ion catalyzes the liberation
of Fe
3
according to the reactions [4]:
FeOOH
Cl
→
FeOCl
OH
(3.2)
FeOCl
H
2
O
→
Fe
3
Cl
2OH
(3.3)
Accumulation of relatively thick chloride salt ''islands'' has been observed
on the surface of iron at the critical pitting potential [5] and FeOCl approxi-
mates the composition of the salt islands. These reactions proceed to remove
the passive film at a preferred site until a pit is initiated through direct anodic
dissolution to Fe
2
. The preferred sites may arise from a surface scratch, from
an emerging dislocation or other defects, or from random variations in solution
