July 28, 2013

All about noble sacrifices

WHAT DO YOU KNOW about the Galvanic Series? It sure doesn’t sound very exciting, unless you are a proton or an electron, but it’s of vital importance to boaters. It’s an indication of what metal on your boat is likely to eat some other metal, perhaps a bit that’s keeping the vessel afloat.

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:




        Mild steel

        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.

Today’s Thought
True science teaches, above all, to doubt and to be ignorant.
— Manuel de Unamano, The Tragic Sense of Life

Money isn’t everything. In fact, at the end of the month you’ll find it’s nothing at all.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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