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A highly anodic metal such as zinc will corrode much more quickly than a highly cathodic metal such as stainless steel when the two are in contact with each other. The purpose of the zinc anode is to sacrifice itself in order to protect the more valuable metals on your boat, such as the propeller, drive shaft and outdrive.
GALVANIC SERIES OF
This corrosion is called "galvanic corrosion" and is rapidly accelerated when stray current is introduced. This stray current can come from many sources including shore power at your dock or faulty wiring on your or even your neighbors boat.
METALS AND ALLOYS Any one of the below metals and alloys will theoretically corrode while protecting any other that is lower in the series as long as both form part of an electrical circuit:
Galvanic corrosion occurs when two dissimilar metals are in close contact with an electrolyte, a medium through which an electrical current can flow such as water. The rate of corrosion depends upon the differences in electrical potential, or the anodic-cathodic relationship, of the metals as defined by the "Galvanic Series of Metals & Alloys" shown here. When these metals are placed in a conductive liquid such as salt water, current will flow. This is also sometimes referred to as electrolysis. This current removes metal from the most anodic metal which is therefore "sacrificing" itself to the more cathodic metal. Galvanic series relationships are useful as a guide for selecting metals to be joined, and help with the selection of metals having the least tendency to interact galvanically, and indicates the degree of protection needed. To reduce the likelihood of galvanic corrosion, it is recommended that metals are grouped per the Galvanic Series chart. If it is not possible to do so, other recommendations are:
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