Page 60 - Corrosion Engineering Principles and Practice

40 C h a p t e r 3 C o r r o s i o n E l e c t r o c h e m i s t r y 41

In neutral waters the anodic corrosion of some metals like aluminum,
zinc, or magnesium develops enough energy to split water directly as
illustrated in Fig. 3.4 and Eq. (3.15).

−
+
Water splitting cathodic reaction: 2H O(l) + 2e − → H 2 + 2OH (3.15)
2
The change in the concentration of hydrogen ions or increase in
hydroxyl ions can be shown by the use of pH indicators, which
change color and thus can serve to demonstrate and locate the
existence of surfaces on which the cathodic reactions in corrosion are
taking place. There are several other cathodic reactions encountered
during the corrosion of metals. These are listed below:
Oxygen reduction:
+
(acid solutions) O + 4H + 4e → 2H O (3.16)
−
2 2
(neutral or basic solutions) O + 2H O + 4e → 4OH (3.17)
− −
−
2
2
Hydrogen evolution: 2H + 2e → H g) (3.5)
(
−
+
2
Metal ion reduction: Fe 3+ + e → Fe 2+ (3.18)
−
Metal deposition: Cu 2+ + 2e → Cu(s) (3.19)
−
Hydrogen ion reduction, or hydrogen evolution, has already
been discussed. This is the cathodic reaction that occurs during
Mg 2+

H
e – O H O H

H +

e –
H 2

H +
H

O H

Magnesium

FIGURE 3.4 Electrochemical reactions occurring during the corrosion of
magnesium in neutral water.