Environmental Engineering Reference
In-Depth Information
as cathode. Coupling of metals wide apart in the series should be avoided as
this would produce a large driving force. However, the magnitude of potential
difference does not always give the correct prediction of the magnitude of the
increase in corrosion rate. For example, zinc coupled to platinum will corrode
at a higher rate in an acid medium than when it is coupled to gold, though the
position of gold is higher up in the emf series. (The explanation of such a behavior
lies in kinetic factors. The exchange current density for hydrogen evolution reac-
tion is greater on platinum than on gold and the resulting i corr is therefore higher.)
The situation is described by graphical representation in Fig. 3.1.
The importance of area effect in galvanic corrosion should also be well under-
stood. Since the same corrosion current flows through the anodic and cathodic
areas in the couple, the current density assumes a higher value on a smaller area.
A higher anodic current density essentially means a higher rate of dissolution
and so a small anode-large cathode combination should be avoided by all means.
Overlooking of this simple rule has led to many hazardous consequences in prac-
tice. From the mixed-potential point of view, the area effect can be described by
plotting a graph of log (total current) versus potential as shown in Fig. 3.2. A
1-cm 2 piece of zinc corroding in an acid solution at a rate of i A corrodes at a
higher rate of i B when coupled with a 1-cm 2 piece of platinum. In these cases
current and current densities are equal because electrodes of unit area have been
used. When a 10-cm 2 piece of platinum is coupled with a 1-cm 2 piece of zinc,
the cathode sustains a reduction rate of hydrogen ions 10 times that of the previ-
ous case. In other words, the exchange current on this larger cathode will be 10
times the exchange current on the 1-cm 2 piece. The intersection of the cathodic
polarization curve with the anodic polarization curve in this case yields a corro-
sion current of i C , which is i corr for the 1-cm 2
piece of anode.
Figure 3.1
Corrosion behavior of zinc-gold and zinc-platinum galvanic couples.
Search WWH ::




Custom Search