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topic 3670

Diffusing the galvanic couple


(1999)

I have a question about galvanic behavior of metals that are explosion bonded together.

I have been told by the fabricator that explosion bonding creates a bimetallic couple that eliminates galvanic corrosion. On the face of it, this does not seem possible:

Assume V=IR, and in the presence of an electrolyte, individual metals have galvanic potential with each other. With a non-zero voltage, there will be current inversely proportional to the resistance. If the metal is explosion bonded, there is nearly zero resistance, and insanely close to zero voltage. Does this mean the galvanic couple is ineffective as a driver for corrosion, or that the current is extremely high (yielding a fairly active behavior)?

Can you steer me to some appropriate literature?

Charlie Brady
Bremerton, Washington


(1999)

Charlie:

Unless the explosion bonding creates a high resistance interface (which I don't believe is true), then galvanic corrosion will not be eliminated. Galvanic corrosion occurs from current flow through the two metals and through the electrolyte. Thus, both metals must be directly connected and in contact with the same electrolyte. The more conductive these two paths are, the greater the corrosion rate. The driving force for corrosion, of course, is the difference in electrochemical potential for the two metals in the electrolyte.

Any basic corrosion text will have a description of the process. You can also find some good descriptions of the mechanism on the web.

larry hanke
Larry Hanke
materials testing laboratory
Minneapolis, Minnesota





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