Zinc anodes are placed on hulls, propeller shafts, rudders, trim tabs, outboard engines, stern drives, and in the cooling system of most inboard engines to protect their metal parts from galvanic corrosion.
Galvanic corrosion occurs when dissimilar metals, such as an aluminum stern drive and a bronze through-hull, are placed in an electrolyte solution such as seawater. Dissimilar metals are those metals which have different corrosion potential (measured in volts) as determined by the Galvanic Series, and the most common scenario in boating is current passing from the more noble bronze to the less noble aluminum. This transfer of current results in deterioration of the less noble metal. The addition of a zinc actually increases current activity between the metals, but because zinc is the least noble of the three metals in this example, corrosion damage is primarily confined to the zinc itself when properly installed. Zincs must be impurity-free to work properly.
Zincs should be replaced when they have been reduced to about one half their original size or weight. Inspect them carefully during your annual haul out and replace them accordingly. If your zincs need replacement more often than once a year it is a strong indicator that you have an electrical leakage problem that should be located and corrected.
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