Corrosion Mechanisms
13
The hydrogen and oxygen are also shown in the diagram by the dotted lines.
The hydrogen line represents the equilibria:
2H + 2e =H in acid solutions
+
–
2
(2.9)
or
2
2
2
2
H O
e
H
OH in neutral or alkaline solu
2 +
=
−
+
−
t tions
(2.10)
These two reactions are equivalent and their pH dependence of single electrode potential is represented by:
E
E
pH
H /H
O
H /H
+ 2
+ 2
=
−0 059
.
(2.11)
at pH = 0; that is, for [H + ] = 1, E
O
H+/H 2
= 0 and the slope is −0.059V. Similarly,
for oxygen equilibrium with water the corresponding reactions at lower and
higher pH are:
O
H
e
H O
+
2
2
4
4
2
+
+
=
−
(2.12)
and
O
H O e
OH
2
2
2
4
4
+
+
=
−
−
(2.13)
The pH dependence of single electrode potential is represented by:
E
E
pH
2
2
2
2
/H O
O
O /H O
O
=
−0 059
.
(2.14)
at pH = 0, E
V
O
O /H O
2
2
=1 226
.
and at pH = 1 (i.e., for [OH − ] = 1), E
V
O
O /H O
2
2
= 0 401
.
.
Here again, the slope of the line is −0.059V. Water is stable in the area designated
by these two lines. Below the hydrogen line it is reduced to hydrogen gas, and
above the oxygen line it is oxidized to oxygen.
The potential–pH diagram shows three clear-cut zones:
1. Immunity zone. Under these conditions of potential and pH, iron
remains in metallic form.
2. Corrosion zone. Under these conditions of potential and pH, iron corrodes, forming Fe 2+ or Fe 3+ or HFeO 2
− .