Cathodic protection is based on Le Chatlier's Principle. If you pumped in electrons to the anodic reaction, they would theoretically pile up on the right side, pushing the equation to the left, or to the reagent side. This would slow the corrosion process down. This is the converse of the aformentioned increasing rate scenario. If you take electrons away it speeds up, but if you add electrons, it slows down. So cathodic protection is simply providing electrons to the threatened metal from an external source to slow the dissolution of that metal. The threatened material corrodes because it serves as the anode; in cathodic protection you create another stronger anode which makes the metal in question a cathode. Without the electron reinforcements, the metal would just lose electrons in attempt to return to a more stable state.
A modified galvanic cell system serves as simplified cathodic protection example. Imagine two beakers filled with a weak acid; one has a zinc strip in it and the other has a copper strip. Both metals are losing electrons via oxidation reactions. In other words, the electrolyte environment is corroding both metals.
Cu -> Cu2+ + 2e (anodic)
Zn -> Zn2+ + 2e (anodic)
And in both cases the hydrogens from the acid are taking the electrons.
4H+ + O2 + 4e -> 2H2O (cathodic) for copper
2H+ + 2e -> H2 (cathodic) for zinc
If we then added an electrical connection between the metal strips, we could measure that one of the electrodes' potential become different than the other. The copper becomes a cathode and the zinc becomes an anode. In other words, the zinc wants to lose electrons more than the copper. So the zinc essentially provides the copper with all the electrons it needs and its overall rate of corrosion decreases while the rate of corrosion for the zinc increases. The zinc is now cathodically protecting the copper just as Dow's Galvanum cathodically protects the steel components of the Hurricane Barrier. The Galvanum anodes purchased by the Corps in 1967 protected the steal components by providing them with enough electrons to maintain their mass despite the corrosive forces of the harbor.
Conclusions
