Corrosion Mechanisms
9
2.2.1 Dissimilar Electrode Cells
Dissimilar electrode cells may be formed when two dissimilar metals
are in contact or due to the heterogeneity of the same metal surface. The
Daniel cell is an example of the former. In practice, a copper pipe connected to a steel pipe or a bronze propeller in contact with the steel hull
of a ship provides an example of this type of corrosion cell. This is often
referred to as galvanic coupling, in which the less noble metal becomes
the anode. Galvanic corrosion is discussed further in Chapter 3. A cold
worked metal in contact with the same metal annealed leads to a similar
situation (i.e., the cold worked metal remaining anodic). On the same metal
surface, such type of cell formation may result from dissimilar phases and
impurities, grain boundaries, differentially strained areas, and scratches
or abrasions. In a single crystal, the different crystal faces differ in their
electrochemical characteristics because of the difference in their atomic
orientation and, as a result, one crystal face tends to become anodic compared to the others.
2.2.2 Concentration Cells
Concentration cells are formed when the electrodes are identical but are in
contact with solutions of differing composition. A salt concentration cell forms
when one electrode is in contact with a concentrated solution and the other
electrode with a dilute solution. On short-circuiting, the electrode in contact
with the dilute solution will be anodic (refer to Figure 2.1). The local variation
Cathode
Cu
Cu
Anode
–
+
Dilute CuSO 4
Conc. CuSO 4
FigurE 2.1
Salt concentration cell.
9
2.2.1 Dissimilar Electrode Cells
Dissimilar electrode cells may be formed when two dissimilar metals
are in contact or due to the heterogeneity of the same metal surface. The
Daniel cell is an example of the former. In practice, a copper pipe connected to a steel pipe or a bronze propeller in contact with the steel hull
of a ship provides an example of this type of corrosion cell. This is often
referred to as galvanic coupling, in which the less noble metal becomes
the anode. Galvanic corrosion is discussed further in Chapter 3. A cold
worked metal in contact with the same metal annealed leads to a similar
situation (i.e., the cold worked metal remaining anodic). On the same metal
surface, such type of cell formation may result from dissimilar phases and
impurities, grain boundaries, differentially strained areas, and scratches
or abrasions. In a single crystal, the different crystal faces differ in their
electrochemical characteristics because of the difference in their atomic
orientation and, as a result, one crystal face tends to become anodic compared to the others.
2.2.2 Concentration Cells
Concentration cells are formed when the electrodes are identical but are in
contact with solutions of differing composition. A salt concentration cell forms
when one electrode is in contact with a concentrated solution and the other
electrode with a dilute solution. On short-circuiting, the electrode in contact
with the dilute solution will be anodic (refer to Figure 2.1). The local variation
Cathode
Cu
Cu
Anode
–
+
Dilute CuSO 4
Conc. CuSO 4
FigurE 2.1
Salt concentration cell.