Environmental Engineering Reference
In-Depth Information
It is to be noted that although the actual dissolution process of metal is taking
place through anodic reaction, cathodic reaction is equally important in the whole
operation. The electrons liberated by anodic reaction are consumed in the ca-
thodic process. A corroding metal does not accumulate any charge. It therefore
follows automatically that these two partial reactions of oxidation and reduction
must proceed
simultaneously and at the same rate
to maintain this electroneu-
trality. Some basic concepts of corrosion control also evolve from this simple
electrochemical picture:
Metal dissolution can be retarded by retarding the ca-
thodic process; metal dissolution can also be retarded or stopped altogether by
the supply of electrons to the corroding metal from any external source
. The
latter forms the basis of cathodic protection (Section 4.4).
The picture as presented for the corrosion of zinc in hydrochloric acid is
equally applicable for the corrosion of any other metal, say, iron or aluminum
in hydrochloric acid or any other acidic medium in the absence of oxygen. In
all cases, the metal will enter into the solution as ions through anodic process,
i.e.,
Fe
→
Fe
2
2e
(2.4)
Al
→
Al
3
3e
(2.5)
with the liberation of hydrogen gas through cathodic process. The anodic reaction
for any corroding metal can thus be written in the general form:
M
→
M
n
n
e
(2.6)
where
n
is the valency of the metal involved.
Cathodic reaction varies from medium to medium, but here also some general-
ization is possible. In acid media in the absence of oxygen in the examples dis-
cussed, the cathodic reaction is one of hydrogen evolution:
2H
2e
→
H
2
(2.3)
However, if oxygen is present in acid solutions, the dominant cathodic reaction
is that of oxygen reduction:
O
2
4H
4e
→
2H
2
O
(2.7)
Contamination of heavy salts such as FeCl
3
and CuCl
2
is common in some acids
used industrially. Dissociation of these salts produces higher valence ions such
as Fe
3
and Cu
2
, which are also electron acceptors and get reduced to lower
valency states, i.e.,
Fe
3
e
→
Fe
2
(2.8)
Cu
2
e
→
Cu
(2.9)
