Electroplating theory: questions about anodes and membranes

A. This M.E. ended up spending his whole career in plating and metal finishing, so your situation doesn't sound strange to me :-)

Most plating baths use anodes of the same material that you wish to plate with, because that provides an equilibrium condition: metal goes into solution from the anode as metal is removed from solution by plating out onto the cathode. These are generally called soluble anodes.

There are exceptions to the use of soluble anodes. For example, chromium is plated out of chromic acid, using lead anodes, because chromium metal will not dissolve properly from the anodes into the solution. As another example, although gold can be plated using soluble gold anodes, they are not used because most shops would find the cost and security risk of having thousands of pounds of gold anodes hanging around to be inconvenient. For a final example, soluble and insoluble anodes can be employed in the same tank; this could be applicable where soluble anodes are desired for the convenience of running an equilibrium process, but you need auxiliary anodes for throw into deep recesses, and you'd rather they stay of consistent dimension rather than melt away on you.

A. Platinized anodes are expensive, so stainless steel anodes are used whenever they are acceptable, which is on SOME gold plating baths. They may be okay in a cyanide bath, but will not hold up in most acid baths (they'll dissolve).

A. Hi, Parvatalu. Most electroplating processes do not use membranes. But there are certain processes where it is desirable to keep certain ions away from either the anode or the cathode, and membranes can achieve this. For example, in some trivalent chromium plating processes, the anodes are enclosed behind membranes so that trivalent chromium ions cannot reach the anodes where they could be oxidized to hexavalent chromium.

Regards,

Hi, Mel. Yes, it's usually best to do a barrier layer of nickel between a brass substrate and gold plating.

From a theoretical standpoint, the way I.D.s are plated is by having auxiliary anodes run through the I.D. For example, you string the tubing on stainless steel wires which become the anode, but with insulating spacers so the wire doesn't touch the tubing. That is because the plating follows the electricity (per Faraday's Law of Electrolysis) and the electricity essentially follows the path of least resistance per Ohm's Law. Depending on actual dimensions, you'd get little plating inside the tubing without auxiliary anodes. You may also mask the O.D. if desired to keep plating off the outside.

You asked "how do I plate . . .", and while it certainly may be possible for you to do so, electroplating is a service-oriented business more than an OEM process, for many reasons including hazardous waste management rules. So the usual actual answer to your question is that you find a plating shop that offers such a service. Good luck.

Regards,

A. Dear Jerry,

I think (and this I tell you without having used a gold plating bath) that the potential you must use will depend of the system itself, and the Cr2O3 will not be a part of this equation. I suppose the anodic reaction is the formation of some gas, like oxygen, from OH- ions of the solution. Then, you must add the concentrations (so in the Nernst equation you have, the standard potentials of both hemi-reactions and the concentrations of every element involved in them) AND the overpotential. The overpotential will depend of the anode surface, the composition of the solution, and I suppose the temperature.

There you have some complex Nernst equation, but I suppose you have many, many tables that tell you how many volts and amperes should a part of certain geometry have in the bath. If not, sorry but you must experiment and measure... The Amperes should tell you how many gold you have plated and the voltage will be some uncertainty until you gain some experience!

Well, I wrote this based only in my experience with zinc and nickel plating baths, and with some theoretical physicochemistry, so where are the gold platers so to solve Jerry's doubts?

Hope you can learn more about this! Greetings,

A. At a sufficiently high potential, somewhere around 6 volts or a little higher, the stainless steel will start to dissolve.

Daniel, Kyle

Thank you for your responses. My voltage is always <= 5V so the SS anode should not be a problem; however, I am planning on trying Carbon anodes so it may not be an issue.

I think the take home lesson is to run an experiment. I can tell if the electrode is plated by measuring the impedance.

Thanks for your time.

A. Hi Hugo. Hopefully someone else can give you a better answer because in an almost 50-year career in plating I never once actually saw a gold anode used in a gold plating bath. Obviously, over the long term, that 6/24 or 10/24 of trash contaminants will contaminate the plating bath so it works less well; but on a one-off basis it might be possible to successfully gold plate (I don't know).

But I would not want new readers to be misled about how gold plating is done. Generally, you buy a gold plating solution from a vendor specializing in that field, and use non-perishable anodes of stainless steel or platinum clad titanium -- you don't make it up yourself from commodity chemicals. Good luck.

Regards,

A. Hello Rex. Although 316SS contains molybdenum which reduces its tendency to rust, it's not quite a matter of type 316SS being 'special'. Rather, corrosion depends on the environment the material is used in, and there is a range of corrosion resistance. Plain steel is too rust-prone; 316SS is 'good enough'; more corrosion resistant grades of stainless steel and other alloys are available and better still but cost more; 304SS costs less but is more prone to rusting.

Yes, 304SS can probably be 'good enough' if it's really important to you, but for the small amount of material involved in a hobby plating anode it just may not be worth the effort to try to get by with 304SS. Good luck.

Regards,

 Thanks Ted, I will stay with 316. --Rex