[wpaladin]

This is probably what OP meant:

"They are made from a metal alloy with a more "active" voltage (more negative reduction potential / more positive electrochemical potential) than the metal of the structure. The difference in potential between the two metals means that the galvanic anode corrodes, so that the anode material is consumed in preference to the structure." https://en.wikipedia.org/wiki/Galvanic_anode