On several boats I've inspected after the damage was done, lead or cast-iron keels had been exposed to seawater due either to faulty paint systems on the metal keels or, in one case, because the owner was racing the boat and had wet-sanded the epoxy barrier coating on the lead down to bare metal. In any of these cases, the anode-to-cathode surface-area relationship had been altered, and insufficient anode area caused the factory-installed anodes to dissolve quite quickly, in a matter of weeks instead of months. Remember that the next bit of metal in the corrosion food chain after the anodes is the drive casing itself, which, made of aluminum, is far more susceptible to anodic action than bronze through-hulls or other fittings.
The Cathodic Protection System**
To fully understand the cause of this corrosion, you need to understand how and why a properly designed cathodic protection system works. The system begins by tying all of the underwater metals together electrically via a bonding system. On most boats, this is accomplished with green insulated wire, sized 8 gauge or larger. What happens next is an electro-chemical process known as polarization of the metals. You see, all metals when submerged in an electrolyte solution (such as salt water) have a voltage potential. Potentials vary depending upon the specific alloy used. By electrically connecting all of the various alloys, once polarization occurs, the voltage potentials equalize. Anodes need to be tied to this same bonding system, and the anode material must have a voltage potential at least 200 millivolts more negative than the rest of the system. The anodes then become the sacrificial component within the system; enough anode mass needs to be added to ensure satisfactory service life, which is generally considered to be six to nine months. At that interval, you need to install new anodes.
Hey, It's the Neighbors**
Essentially what's being created with this bonding system is what we call a galvanic cell, with an anode and cathode connected electrically and both submerged in the same electrolyte solution, in this case, the water around your boat. Think of it as a giant battery. Low-level current flows from the more negative anode to the cathode, but because each element of the cathode has undergone polarization, their potential voltage is equal, so none experiences corrosion. Instead, the anode slowly wears away.