Copper Plating onto Zinc
You win some, You lose some. This is actually a post about a failure to achieve what I was trying to do, but it’s an interesting failure. I was making some zinc etchings (see my earlier post about that) and I thought it would be great if I could then plate the exposed parts of the etching not covered by the resist with copper, which, when the resist was removed, would give me a two-tone metallic effect.
I had been putting a pad of steel wool into the bath to remove the copper from it prior to pouring it down the drain, for environmental reasons. The copper would plate out onto it just fine, but it didn’t have any adhesion— if you weren’t careful taking the steel wool out, the copper would fall right off it. So it seemed that I was close to my goal, if I could just get the copper to adhere to the plates.
Some hours of internet research later, I had a partial success. Commercial copper plating processes use a complex process with very precise parameters of solution strength, electricity, and temperature, as well as proprietary processes such as strike baths and commercial brighteners. There are internet forums devoted to the finer points of this, and I really should have paid more attention in high school chemistry because I can’t follow most of it. But the underlying process is pretty simple.
Dissolve some copper into an acid bath, connect your anode (+) to a copper donor part, and your work piece to the cathode (-). The positively charged copper will attach itself to the negatively charged work piece. The acid is there to help the copper dissolve in the solution, I think.
For my copper anode, I used about a foot of stranded copper wire, with the insulation jacket removed, of course. I used vinegar (acetic acid) for my acid solution, and also added about 3 tablespoons of copper sulfate crystals. I think this is what made the plating work so quickly despite using a weak acid, as otherwise you would have to wait for copper atoms to be pulled off the copper part, transported across the medium, and deposited onto the work piece. For a constant supply of voltage, I used a car battery charger. The 12-14V it supplies is too much voltage, so I used several light bulbs of various denominations to make a voltage divider, with the voltage across the bath between 1 and 2 volts.
The acid will be consumed in the process, so you’ll have to keep adding more to keep the PH down. Also, if you are seeing bubbles or black soot forming on the workpiece, you’re trying to plate too fast– you should reduce the voltage, make the mixture less conductive, or increase the amount of copper dissolved in the bath. An apparently common maxim of plating is that you can’t plate material faster than you put it into the solution. Also, without brighteners you’ll need to immediately clearcoat the copper plate after polishing it if you want it to stay shiny.
The process works pretty well for plating copper or zinc onto steel, but I was completely unsuccessful at plating copper onto zinc. The acid just attacked the zinc and I got a lot of bubbling and soot deposits, but no copper. In the picture below, you can see the bubbles of hydrogen gas coming off the zinc plate.
More research: So I spent a lot of time reading about this, mostly from patents because it turns out copper plating onto zinc is sort of a Thing. Commercial processes involve a cyanide bath, which is not something I can attempt at home due to the fact that it would kill me.
The other mode of attack I’ve been thinking about would be to plate the zinc in an alkaline bath with some other metal, one that would allow me to then plate the copper using the acid bath process. From what I’ve been reading sodium carbonate might work for an alkaline bath, but I’m not sure what metal I would plate with or whether that would really work. Also, we’re getting pretty well outside my grasp of chemistry here. But if you’ve had some success, let me know in the comments!