plating, anodizing, & finishing Q&As since 1989
"Spot electroplating" for art project
May 10, 2012
Q. I want to electroplate a small area of a large bronze item. If I seal off a circle of it (about a foot square), fill that area with the salt-solution and run a current from one side to the grounded point at the furthest end, will it work.
A: will the placement of the cathode (i.e., the whole surface, in and out of the sealed area) matter? Will I need to move it around as it reacts? Or will it disperse the current?
I planned to tape it just outside the furthest point on the circle from the anode
B: on that scale will a 12V Sealed lead acid battery work?
How long will I need to get a nice even heavy surface. Will I need a significant amount of solution for that surface area, is solution volume an issue? 2L?
Masters Student, Electronic Art
A. Hi Oliver.
There are scientific rules that govern how electroplating works. Some of the science is very complex and poorly understood, but some of it is very straightforward -- and that part you should probably understand before you try to get into the construction details. Just the old saw that the science must precede the engineering :-)
Electroplating is governed by Faraday's Law of Electrolysis, which in brief, says that plating thickness is proportional to time and current. What you are actually doing in electroplating is getting atoms to move from the anode to the cathode, and the way you do it is by using a battery or DC power source to force the electrons of those atoms to flow through the wiring from the anode to the cathode, which in turns forces the nuclei to migrate through the solution as charged ions from anode to cathode to catch up. There are numbers associated with Faraday's Law, like 96,400 coulombs (ampere-seconds) moves one gram equivalent weight from anode to cathode.
Another thing you know about electricity is that, slightly simplified, it follows the path of least resistance. So, when you combine these two facts, the plating follows the path of least resistance. That is to say that the plating is going to be very heavy on the area of the cathode that is close to the anode, and thin or non-existent on the areas of the cathode which are far from the anode. So, no, it will not disperse the current; and, yes, you would have to move the anode around.
The more difficult part of the science is that the metal doesn't just cleanly deposit in a "nice even surface" as you wish. Considering again the "path of least resistance", as soon as you have a tiny asperity, that's where the current wants to flow, and wants to build a lump :-)
My suggestion is that you investigate "brush plating" and get a brush plating kit and proprietary plating solution (which contains additives to counteract the "lumping"). I think you'll find that 12 volts is much too high, and 1-1/2 to 3 volts is more appropriate. Good luck.
Ted Mooney, P.E.
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