Environmental Engineering Reference
In-Depth Information
Complete reduction of a contaminated metallic ion giving metal deposition is
also possible:
M
e
→
M
(2.10)
These additional cathodic reactions, if present, invariably accelerate the corrosion
process. A very common example is the acceleration of corrosion of zinc in
commercial grade of hydrochloric acid (called muriatic acid) because of the pres-
ence of FeCl
3
in the acid.
All corrosive media are by no means acidic. Corrosion is widely experienced
in neutral and alkaline aqueous media as well. Oxygen reduction provides the
cathodic reaction in these media:
O
2
2H
2
O
4e
→
4OH
(2.11)
Oxygen plays a big role in aqueous corrosion. Removal or lowering of oxygen
content of the corroding medium has been an age-old practice for corrosion
control.
Since water gets partially dissociated into H
and OH
ions, simple reduction
of water (which is actually the reduction of H
ions) may provide the cathodic
reaction under certain circumstances, even if oxygen is not available:
2H
2
O
2e
→
H
2
2OH
(2.12)
Identification of cathodic reaction helps immensely in the understanding of
corrosion in a particular system and its subsequent control. The familiar ''rust-
ing'' of iron and ferrous alloys in the atmospheric exposure can thus be under-
stood as:
Anodic reaction: Fe
→
Fe
2
2e
(2.4)
Cathodic reaction: O
2
2H
2
O
4e
→
4OH
(2.11)
A combination of the reaction products gives ferrous hydroxide, which in the
presence of oxygen is further oxidized to ferric hydroxide, or rust. It may be
mentioned that the term ''rusting'' is exclusive for ferrous materials only; other
metals corrode but do not rust.
2.1.2 Electrochemical Cell Analogy
Referring back to Fig. 2.1 for the corrosion of zinc in hydrochloric acid, it be-
comes clear that the corroding system has four constituents:
1.
An anode, where metal dissolution takes place according to Zn
→
Zn
2
2e
2.
An electrolyte, the corrodent, into which the metallic ion passes
