Revised by BoatUS editors in April 2012

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. A 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 metals typically involved, corrosion damage is primarily confined to the zinc itself when properly installed.

Zincs should be impurity-free to work properly. Typically the better zincs are cast or machined from virgin alloy, not scrap, by North American zinc producers. The contact points on shaft zincs should be solid copper to provide better contact and superior conductivity. All hardware should be stainless steel with nuts cast in place so they will stay in the zinc if unscrewed.

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 an indicator that there is an electrical leakage problem that should be located and corrected, although different factors, such as where and how you use the boat and how often you run it, can also result in increased zinc deterioration.  For example, if your boat is kept in water of high salinity and you use it very frequently (with water rushing over the zinc), erosion will probably occur faster than with a boat sitting in fresh water, seldom used.

Better shaft zincs are typically made with socket head cap bolts designed to be tightened with an Allen wrench. Allen wrenches are easier to use underwater than slotted head fasteners and you can buy an Allen wrench for your bolt with a T handle and make the job easier. Thin pencil zincs for engines are easily replaced with an adjustable wrench.


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