Environmental Engineering Reference
In-Depth Information
2
Aqueous Corrosion: Fundamentals
2.1 ELECTROCHEMICAL NATURE OF AQUEOUS
CORROSION
Corrosion in aqueous solutions is by far the most common of all corrosion pro-
cesses. Aqueous medium is provided by water, seawater, and various process
streams in industry. Moisture in atmosphere and water in soil account for the
aqueous corrosion in these media. In all of these cases, water is hardly present
in pure form. Rather, various salts and gases remain dissolved in it and their
dissociation renders the water somewhat conducting. For all practical purposes,
it acts as an electrolyte. The chemical nature of this electrolyte may be acidic,
alkaline, or neutral.
2.1.1 Electrochemical Reactions
Aqueous corrosion can best be exemplified by the attack of zinc by hydrochloric
acid. When a piece of zinc is placed in hydrochloric acid, vigorous reaction fol-
lows with the formation of zinc chloride and liberation of hydrogen gas. The
reaction is chemically represented by
Zn
HCl
→
ZnCl
2
H
2
(g)
↑
(2.1)
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