Copper Chemical Plating

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Copper Chemical Plating

Postby savanik » Mon Dec 01, 2014 2:37 pm UTC

I had a weird chemical reaction yesterday that I'm puzzled about.

Over the weekend I was working with copper wire on a jewelry project. I had a set of copper wires twisted into a ring, forged on an anvil by heating it to cherry red in a propane torch, then beating on it with your typical steel hammer. (results here: )

After working it, there was quite a bit of fire scale (copper oxide) built up on the piece, so I dropped it into my acid bath solution (cup of vinegar, about a tablespoon of salt, in a steel pot on the stove) to knock all that off. At a good simmer (about 180 degrees) it took about 5 minutes for the fire scale to completely dissolve. I fished it out of the solution with my steel pliers, washed it off in tap water immediately to prevent oxidation, and noted that the outside layer of copper had taken on a dull, slightly white cast, which I thought would have been part of a red-ox reaction if the copper oxide turned back to copper. I buffed that off, holding it in my pliers, with my dremel with one of those cloth wheel pads on the end, and got a nice copper finish.

After another 15 minutes I noted there was a substantial copper finish plated onto the pliers! No electrical current was used anywhere, and it was at least a little thick - scrubbing at it with the pads didn't pull off the copper finish.

The heck did that come from? I thought perhaps I'd accidentally dissolved some of the copper into the solution, but it was in the acid solution for maybe a few seconds - hardly enough time to really plate something, right? And most everyone online talks about electroplating copper - there weren't any electrical leads anywhere I could think of. I don't think the stove is supposed to conduct electrical energy? I think I would have felt it if the burner was poorly grounded.
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Re: Copper Chemical Plating

Postby Minerva » Mon Dec 01, 2014 4:49 pm UTC

You can easily see the same kind of effect quite easily if you get a piece of say, iron, and put into a solution of copper sulfate or something like that.
It will certainly happen without electricity, for some combinations of metals. Electroplating will typically produce a thicker, more controlled, metal plating, although electroless plating (various chemistry, various metals) is sometimes used in industry, for jewelry or making PCBs or whatever.
A more active metal, such as Fe (most metals actually, since copper is relatively poor as a reducing agent) will reduce the Cu2+ to Cu0 metal. Cu2+ Fe --> Cu + Fe2+
If you use an acid, even vinegar, to clean the oxide off a piece of copper, you do have a solution with dissolved copper ions, even if it may be dilute.
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