Environmental Engineering Reference
In-Depth Information
3.
An electronic conductor, the metal itself, through which the electron passes
from anodic site to cathodic site
4.
A cathode, where electron is consumed according to 2H
2e
→
H
2
The picture is analogous to a short-circuited current-producing electrochemical
cell, such as a Daniell cell (Fig. 2.2).
In a Daniell cell, zinc electrode constitutes the anode from which zinc passes
into the ZnSO
4
electrolyte as zinc ions. Electrons released in the dissolution pro-
cess pass through an electronic conductor, a metallic wire, and a high-resistance
voltmeter, on their way to cathode, the copper electrode, where they get con-
sumed by the available cupric ions that plate back as copper on the copper elec-
trode according to Cu
2
Cu. The flow of positive current is assumed
to be from zinc electrode to the copper electrode through the electrolyte, i.e., in
the opposite direction of electron flow. If the resistance is withdrawn and the
zinc and copper electrodes are connected by a metallic wire only, i.e., short-
circuited, zinc will dissolve at a higher rate, electrons will flow through the short-
circuited path, and copper will be deposited at the copper electrode.
If now the short-circuited zinc and copper electrodes are taken from their
respective compartments and placed in hydrochloric acid, zinc will still go into
solution as zinc ion, electrons will flow through the short-circuited path, and in
place of copper deposition a hydrogen evolution reaction will take place at the
copper electrode. The corrosion of zinc in hydrochloric acid (Fig. 2.1) is analo-
gous to this latest situation, except for the fact that the dissolution and reduction
reactions are taking place on the same piece of metal at spatially separated sites.
However, these sites are not eternally fixed. As corrosion proceeds, interchange-
ability of sites leads to a uniform attack on the surface and the piece thins down
2e
→
Figure 2.2
Schematic representation of a Daniell cell.
