Gold as an etching resist
I'm doing a lot of artistic etching in brass these days. I now have gold plating equipment and typically start with a layer of nickel strike and plate cyanide-based gold on top of that.
I figured gold would make an effective resist so I could etch areas around it and then blacken the etched areas.
Problem is, the gold seems useless in either ferric chloride (some hydrochloric acid added) or peroxide/sulphuric etchants. It always lifts up or becomes perforated with holes.
The gold is pulse plated on at anything from 8v to 15v (I tried a lot of experimenting) at 10kHz and 50% duty cycle. Thicker does not seem to help much, as well as being expensive.
Does anyone have insight into this problem? Pulse plating is supposed to make a layer virtually free of pinholes, I would have assumed gold to be totally corrosion-free under these circumstances.
Due to my prior experience in photolithography, I can understand some of your problems. The acidic nature of the etching solution, depending on its strength, may be responsible for the undercutting and lifting problem. I am not surprised with pinholes especially if the coating is thin. I can suggest a couple of experiments.
The simplest might be using a lacquer or two-part epoxy as a mask instead of gold. Proper curing and drying may solve the problem. If that does not work, try these on gold plating. The last suggestion is to use another plate like nickel under the gold plate.
- Fort Collins, Colorado
Right, actually I'm starting by nickel plating with nickel strike to prevent the gold from eventually diffusing into the brass (not a problem in this short of a term, I understand).
I tried putting a resist on top of the gold, this does prevent the gold's decay. At first I was afraid the acid was aggressively attacking the nickel and undercutting the gold from the side, but it is not so.
The acid is not reacting with the gold itself- there are flecks floating around afterwards. Kind of like toxic Goldshlauger.
Problem is, I can't combine this with another resist effectively. The only two options I can see are:
1. Plate the whole thing beforehand, apply resist and watch the acid strip the gold. It's expensive and the gold not under the resist does impair the acid's progress enough to cause problems.
2. Plate the peaks by hand after the resist is removed. The etched areas are still going to be conductive as well, so I would need a very fine brush to do this. I don't see it as a practical option.
3. Plate through a positive mask, strip the mask, apply a negative mask over the gold, and etch. This is going to be quite time-consuming and will increase the defect and mortality rates dramatically. It's my only viable option I see, however.
YOU WILL IMPROVE THE ETCHING RESISTANCE BY INCREASING THE NICKEL THICKNESS TO ABOUT 10 microns.
THE UNDERCUTTING MAINLY ARISES FROM A GALVANIC ACTION BETWEEN THE BRASS AND THE GOLD AND THE GREATER THE DISTANCE BETWEEN THE BRASS AND THE GOLD THE BETTER - HENCE GREATER NICKEL THICKNESS!
THE GALVANIC ACTION IS GREATER THE GREATER THE DIFFERENCE IN ELECTRODE POTENTIAL OF THE TWO METALS. e.g. GOLD/COPPER IS BAD, GOLD/NICKEL AND NICKEL/COPPER NOT TOO BAD. TIN/NICKEL IS GOOD. etc.Peter Vivian
- PROCESS SERVICES Ltd, LONDON U.K.
I'll give that a try. I've still got questions though- ferric chloride can readily eat nickel as well as brass. Of course, it doesn't eat gold, that flakes out into the acid. I'd assumed a thicker nickel would undercut just as soon an only take a tiny bit longer to get to the brass.
I'm pretty much an amateur in understanding galvanic corrosion. So gold is subject to this as well? I thought that wouldn't happen in a generally nonreactive metal like gold.
One other issue- I'm using Woods nickel strike, not bright nickel (though I do have some, but not mixed up). I've tried all sorts of cleaning methods to be sure the gold's not having plating problems. Could I be missing something here?
I experimented with gold as a resist in the 1970's and gave it up because the etching process plated the gold. Why go to the expense of applying gold only to hide it under a thin plate. There are other resists that will reduce your cost. Ronald TharpRonald Tharp
The Sterling Karet - Blowing Rock, North Carolina
September 19, 2011
You may want to consider something that is not cyanide based such as an Iodine based gold etch. I'm not sure is it would work with your exact project but I know that Union Etchants makes a few that could be a possibility for your project. Hope this helps.
- Woburn, Massachusetts
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