Fundamentals of Metallic Corrosion in Fresh Water   -   4

At pH values below 4, hydrogen ion replaces oxygen as the oxidizing agent and the cathode reaction becomes:

2H⁺ + 2e^- = H₂

From the fact that nearly all of the voltage drop in the galvanic cell occurs at the water cathode interface, the effects of changing the geometry of the cell may be predicted. If the anode and cathode are moved closer to each other, there is little increase in current. As the cathode is withdrawn from the solution, the current is found to be proportional to the cathode area that remains submerged. If the anode area is reduced, there is only a small decrease in current.

The practical consequences of these area effects are important. In nearly all commercial gate valves there are brass rings in the cast iron body, but because the brass cathode area is very small compared to the iron anode, galvanic corrosion is negligible. Note that a protective coating placed on the anode must be perfect, or very severe pitting will occur at any pinholes. On the other hand, if the cathode is coated with an insulating film, galvanic corrosion can be greatly reduced even with a relatively poor coating.

Figure 4 illustrates that, in a highly conductive electrolyte, corrosion will be greater and will extend for a considerable distance from the cathode, while in a poorly conducting electrolyte, corrosion will be localized near the cathode. Thus it is possible for perforation to occur more rapidly in the less-conductive solution although the total amount of corrosion is much less in the weak electrolyte.

When the metals comprising the cathode are changed, the principal effect is to change polarization. If the copper cathode is replaced with stainless steel, for example, the initial current is about the same, but after a few hours the galvanic current from the stainless is only a small fraction of that from copper. Thus a stainless steel screen connected to a mild steel casing would cause far less corrosion on the mild steel than would a bronze or Monel screen.

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