[tubman]

You have to think of the cooling system as a very large, very weak wet cell battery. Current flows from the anode to the cathode through the electrolyte (the coolant). The anode is a reducing agent because its behavior will reduce ions at the cathode. Anode mass decreases as the reacting anode material becomes aqueous. Since electricity always wants to take the easiest path, supplying an anode of a lesser noble value protects the rest of the system. Previously, I had a "battery" with the cast iron as the cathode and the aluminum heads as the anode; now, I have one with the cast iron as the cathode and the magnesium chunk as the anode. Thus, the electrons and ions are being taken from the magnesium chunk rather than the heads. Electricity is a bully; it only picks on the weakest kid (anode). Since the electrolyte (the coolant) is involved in the whole process, only it has to be in contact with both the anode and cathode, which do not need to be in direct contact.

From some of the comments here, I get the impression that some think that the material of the anode doesn't make much difference. It does. The further apart two metals are in the Galvanic Series, the better the battery is. Remember taking an old "D" cell apart when you were a kid? The center pole was graphite, while the case was zinc. Look at the table I supplied, do a little math, and I think you can see why those old "D" cells are rated at 1.5 volts. Similarly, you will see that zinc and aluminum alloys are quite close together on the table, and will produce a much smaller voltage difference than aluminum and magnesium, thus producing a smaller current and anode erosion (less protection).

Keep thinking batteries and electricity, not chemistry and some "voodoo" process.

I hope this helps a bit.