Fundamentals of Metallic Corrosion in Fresh Water - 3
I think the easiest way to understand polarization is to look at each
cathode material as a catalyst for the reduction of oxygen. It is generally accepted
that the reduction of oxygen does not occur directly, and that hydrogen atoms from water
plate out on the cathode. It is these hydrogen atoms that react with the oxygen. For
the purpose of this discussion, the exact mechanism is immaterial. The net result is
that oxygen is reduced, and hydroxyl ions are formed. Those metals that polarize the
least (platinum, graphite) are good catalysts. Silver, copper, nickel, and their alloys
are fair catalysts, while stainless steel, which has very large polarization, is a poor
catalyst. According to this view, oxygen over-voltage is the activation energy for the
cathode reaction.
Engineers are used to thinking of energy in terms of watt-hours
rather than volts. In a chemical reaction the total number of electrons (quantity of
current) is known, so that the net electrical work (or energy) is:
nFE
where n is the number of electrons per molecule, F is the Faraday,
which is 96,500 columbs per equivalent, and E is the voltage. The oxygen over-voltage
on copper at a current density of 0.001 amp/cm2 has been reported to be 0.42
volts. Since 4 electrons are involved in the reduction of oxygen, the activation energy
is 4 x 96500 x 0.42 or 162120 calories per mole.
The significance of the activation energy is illustrated in Figure 2.
The required activation energy acts as a barrier that must be overcome. The higher this
barrier, the fewer molecules will have energies that equal or exceed this value and the
slower the reaction will proceed. It should be noted, however, that even if the metal w
ere a perfect catalyst (i.e. activation energy is zero), oxygen molecules in the vicinity
of the cathode would become depleted and the rate of the corrosion reaction would still
be limited by the relatively slow rate at which the oxygen molecules diffuse to the
surface of the metal.
It is instructive to make some changes in the galvanic cell and
observe the results. A decrease in temperature causes current output to decrease I to
2% per degree F. Since the viscosity of water increases with a drop in temperature, the
diffusion and migration of ions and molecules is retarded. When the electrolyte freezes,
there is a dramatic decrease in current.
Current increases as dissolved oxygen concentration is increased.
This effect is very nearly proportional except in weak electrolytes. Current increases
proportionately with salt concentration depending somewhat on the specific ions that are
created when the salt dissolves. For example, sodium chloride will increase the current
more than the same weight of sodium sulfate.
Except as it may affect the Langelier Index, pH has little effect
over the ranges normally found in well waters (Figure 3). If the pH is above 10, the
hydroxyl ion concentration is so high that it tends to reverse the cathode reaction:
1/202 + H20 + 2e- = 20H-
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