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I'm back for a few days, and cannot resist a temptation such as a scientific question/answer.

For more details, the corrosion process in work here is galvanic corrosion. All metals can be put on a scale of corrosion-readiness, with gold on top. When two metallic parts are electrically connected (as when both of them are inmersed in water), one of them will concentrate all the corrosion, while the other will remain protected.

The corrosion process itself consists in electrons flowing from the corroded part to the protected part, leaving ionized atoms getting away from the metal.

In marine applications, Zinc is used as easy-to-corrode metal, because it protects all metals usually employed in metallic contructions (steel alloys included).

In the image below, Copper is more corrosion-resistant than steel, so steel is corroded. But Steel is mechanically more resistant than copper, so the structure may weaken. The solution is to use sacrificail Zinc connection between both metals. It will protect both pipes, but will require periodic maintenance (easy: to replace the sacrificial Zinc part).
schematical representation of galvanic corrosion process

an anodic Zinc sacrificial part

by Xavier in Paris on Mon Jan 25th, 2010 at 03:51:39 AM EST
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