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Electroplating a key as a science project

In helping my son with his science fair project of copper plating brass keys he decided to reverse the polarity of the cathode and anode (copper strip to negative brass key to positive). We assumed since copper was the more active metal that nothing would happen with the polarity reversed. What occurred is the copper strip turned black and the key is light gray. What happened?

Larry N.
student - Berthoud, Colorado, USA
2004

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Your son had an interesting thought, Larry. The copper-brass battery that you create here does tend to resist the plating current (sort of like putting one battery out of several in backwards). but the difference in electromotive potential between copper and brass is not nearly enough to resist the voltage provided by the battery.

It is hard to be explicit about why you get various black smuts on the anode and cathode, but smuts of this type are usually very finely divided dots of copper powder (copper looks black when very finely divided). Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004

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The "brass"key is not brass--It is a very very thin brass plate on another metal. Your son Stripped the brass off of the key. Copper Oxide in one form is Black. A co deposit of stuff that is black is normally referred to as "smut" . It is undesirable, so no one wastes money trying to figure out what it is/was.

James Watts
- Navarre, Florida
2004

The battery used in the project was a 6v lantern battery. Once the key was removed from the solution and allowed to dry it came out a light copper color. The copper strip on the other hand actually grew in thickness and does appear to have smut deposits. Thanks for the responses. His project was good enough that he was invited to the regionals this month.

Larry N.
- Berthoud, Colorado
2004

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Hi, Larry. 6 volts is far too much. 1-1/2 volts will plate better and with less smut. See our "How Electroplating Works" FAQ for how to do copper plating with no chemicals beyond vinegar ⇦in bulk on eBay or Amazon [affil links] .

Glad to hear of your son's success!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
October 23, 2008