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topic 0219

Introduction to galvanic corrosion

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I am looking for information on galvanic corrosion.Can you help me, please?

Thank you

Carl Boucher

Hello, Carl.

Different metals have different EMFs, i.e., tendency to dissolve into ionic form. Those with a strong tendency are called active metals or base metals; those with a weaker tendency are called noble metals.

If two different metals are in an environment capable of conveying ions (basically if they are wet) they comprise a battery. If the two metals are touching, or connected with a metallic circuit, the circuit is complete, and the battery can discharge - that is, the more active metal dissolve into ionic form. Carbon-zinc batteries, nickel-cadmium batteries, etc. are examples where the electricity generated by galvanic corrosion is harnessed as useful power.

The way to minimize galvanic corrosion is to pick two metals that are close to each other in activity, break the ionic circuit, or break the metallic circuit. The latter is not possible, of course, when speaking of a plated finish. Breaking the ionic circuit usually involves keeping the parts dry and salt-free.

Sometimes galvanic corrosion is good. For example, when zinc is used to protect steel highway guard rails or ship hulls.

AESF has an "Illustrated Lecture" (booklet and slides) on "Metallic Corrosion", which is a good introduction.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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