Civil Engineering Reference
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Fe
2
+
(OH)
−
O
2
H
2
O
(OH)
−
Fe
2
+
Anode
Cathode
−
−
−
−
−
−
FIGURE 6.1
Corrosion process on the steel surface.
Anodic Reaction
2e
−
(6.1)
The electrons from the anodic reaction will accumulate on other parts of the
steel because another reaction uses the electrons with oxygen and water: this is
called the cathodic reaction.
Fe
2
+
+
Fe
→
Cathodic Reaction
1
2
O
2
→
2e
−
+
2OH
−
+
H
2
O
(6.2)
Equation (6.2)
shows that hydroxide ions (OH
−
) are present in a cathodic
reaction. The OH
−
increase the alkalinity and reduce slightly the effect of car-
bonates or chlorides. It is important to keep in mind that water and oxygen are
the main reasons for corrosion.
As shown in the above equations and
Figure 6.1
, anodic and cathodic reac-
tions are the first step in corrosion because the OH
−
will react with ferrous iron
(Fe
2+
), as shown in
Equation (6.3
). This reaction produces ferrous hydroxide,
which will react, as shown in
Equation (6.4)
, with oxygen and water and pro-
duce ferric hydroxide, Fe(OH)
3
, and the last component, which is the hydrate
ferric oxide (rust). The chemical term for rust, Fe
2
O
3
⋅
H
2
O, is shown in
Equation (6.5)
. This chemical reaction is shown graphically in
Figure 6.1
.
Fe
2
+
+
2OH
−
→
Fe
ð
OH
Þ
2
(6.3)
4Fe
ð
OH
Þ
2
+
O
2
+
2H
2
O
4Fe
ð
OH
Þ
3
(6.4)
→
⋅
2Fe
ð
OH
Þ
3
→
Fe
2
O
3
H
2
O
+
2H
2
O
(6.5)
Saturated Fe(OH)
3
is nearly neutral in pH. A magnetic hydrous ferrous fer-
rite, Fe
3
O
4
⋅
nH
2
O, often forms a black intermediate layer between hydrous
Fe
2
O
3
and FeO. Hence rust films normally consist of three layers of iron oxides
in different states of oxidation.
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