Regarding galvanic corrosion, each metal possesses its own natural corrosion potential, measured in millivolts: the more “negative” they are, the more prone to corrosion. The goal of the anode, then, is always to be more negative than the metal it’s protecting. The relative energy capacity of zinc is 368 amp hours per pound with a voltage of negative 1,050 millivolts. While that’s adequate in most cases, zinc anodes possess one flaw: When used in brackish or fresh water they are prone to developing a calcareous coating, a whitish material that essentially puts a zinc anode to sleep. Many sailors mistakenly perceive these especially “long-lived” zincs as effective. In reality, they’re anything but; even where anodes are still present, they provide no anodic protection at all. An anode that lasts for an inordinately long time probably isn’t working for any number of reasons, calcareous coating or otherwise.