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How is a Euro coin like a wet cell?

2004

I recently read an article about 1-Euro and 2-Euro coins causing nickel allergies in people. It said that the nickel alloy in the center of the coin (nickel brass) made a sort of battery (wet cell) with a slightly different nickel alloy on the edge of the coin (copper-nickel). Sweat being the electrolyte. My question is, which alloy serves as the anode (Ni+ donater) and which serves as the cathode? I'd like to relate the article to my 4th grade curriculum, which talks about copper-zinc wet cells, with copper as the anode.

Thanks for your help!

Sarah M.
teacher - Santa Rosa, California, USA

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2004

This is an amazingly complex question on corrosion. The copper nickel has a composition of 75/25 Cu/Ni, whilst the nickel brass is 75/20/5 as Cu/Zn/Ni. Each alloy is capable of releasing nickel in its own right, with copper forming the cathode and nickel the anode; that is, the nickel will release electrons to form the Ni(2+) ion whilst the copper will absorb them. The circuit, as you correctly say, will be between the copper and nickel, with the sweat as the electrolyte. The corrosion will take place in or very near to where the copper and nickel are in contact. However, what is interesting about your question is that the rate of release of nickel from the coins is less than that you would see from just Cu75/Ni25. This suggests that the Cu/Ni alloy is acting cathodically to the Cu/Zn/Ni and that the nickel brass is acting as a sacrificial anode to the copper nickel. This, of course, makes sense, because the presence of zinc in the brass will generate the anodic reaction, thereby polarising the system and making the copper-nickel the cathode. There is an excellent paper published by Paul-Guy Fournier, Thomas R. Govers and Anne Brun and it is available on the internet at www.europhysics.com/full/23/article6/article6.html This should explain what is going on with the coins, but be aware, it does suggest that the current EU coins are less likely to cause nickel allergies than the old national coins.

I hope this helps you, but it may be too difficult for 4th Graders - perhaps it will put your skills at teaching to the test, by making you get a very complex concept over to the children!

Good luck.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK