278
Fundamentals of Corrosion
Sacrificial metal coatings are characterized by their galvanic action.
Exposure of the base metal, as a result of mechanical damage, polarizes
the base metal cathodically to the corrosion potential of the coating layer,
as shown in Figure 8.12, so that little corrosion takes place on the exposed
base metal. A galvanic couple is formed between the exposed part of the
base metal and the surrounding coating metal. Because sacrificial metals are
more negative in electrochemical potential than iron or steel, a sacrificial
metal acts as an anode and the exposed base metal behaves as a cathode.
Electrode Potential
Log Current Density
i corr
of zinc
coating
i corr
of exposed
iron
i oc
on
iron
i oc
on
zinc
i corr
of uncoated
iron
E corr
of iron
E corr
of zinc
O
2 + 2 H
2 O + 4 e
4 O H –
O
2
+
2
H
2
O
+
4
e
4
O
H
–
F e
F e
2 + +
2 e
Z n
Z n
2 + + 2 e
FigurE 8.12
Cathodic control protection.
TabLE 8.3
Corrosion Products Formed on Various Sacrificial Metal Coatings
Metal
Corrosion Product
Al
Al O , Al O H O, AlOOH, Al OH amorphous Al
2
3
2
3
2
3
2
β
∝
( ) ,
O O 3
Zn
ZnO, Zn(OH) 2 , 2ZnCO 3 3Zn(OH) 2 , ZnSO 4 4Zn(OH) 2 , ZnCl 2 4Zn(OH) 2 ,
ZnCl 2 6Zn(OH) 2
Mn
γ-Mn 2 O 3 , MnCO 3 , γ-MnOOH
Cd
CdO, CdOH 2 , 2CdCO 3 3Cd(OH) 2