Galvanic corrosion begins when two metals far apart in the Galvanic series are connected under water by a conductor. They form a rudimentary electric cell, in which electrolytic corrosion eats away the less “noble” metal. The process even takes place out of water, between, for example, the aluminum in a mast and the stainless-steel screws holding a fitting in place.
Metals close together in the Galvanic Series have little or no reaction with each other, but the farther apart they are, the more vigorous the corrosion. For example, a copper nail that falls into the seawater bilge of an aluminum boat will eventually eat a hole right through it, as the less noble metal, aluminum, corrodes away.
Here is a shortened version of the Galvanic Series, showing the metals most used on boats. It starts with the least noble metals, the ones that will be sacrificed, and ends with the most noble metals, which will be spared:
Stainless steel (active)
Stainless steel (passive)
Note that stainless steel will corrode almost as fast as mild steel in its active state, when it is in still water with no access to oxygen. But when it has a ready supply of oxygen from air or from water, it is passive.
True science teaches, above all, to doubt and to be ignorant.— Manuel de Unamano, The Tragic Sense of Life
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