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topic 26438

Electroplating theory: questions about anodes and membranes


(2003)

Q. Hello,

I've been reading a little about electrochemistry. Far from the M.E.'s turf yeah? Well, electroplating seems to be a very practical science. There's one catch to all this valuable information: In every experiment I have seen, the anode is of the material that you want to plate onto the cathode. But the salt bath contains that metal ion already. Why is it necessary to not only have the metal ions that you want plated to the cathode in the salt bath, but also have the metal you want plated as the anode as well. This seems redundant, because as I have learned it it is the metal ion in the salt bath that plates to the cathode and not really the anode metal. It seems as though the anode's only purpose is to complete the circuit.

Thanks in advance for taking the time to respond.

Joshua Montgomery
M.E. Student - Norman, Oklahoma


(2003)

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.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


Gold Plating Technology

(2004)

Q. My question is along the lines as Josh's about electroplating. I am looking into electroplating lasers with gold. I've found several online websites that sell gold plating kits with stainless steel anodes. I've also found kits or companies that sell platinized titanium or niobium anodes. I only found one website that sold pure gold anodes. I understand why gold anodes might not be practical, but is there an advantage to using platinized niobium or titanium anodes over stainless steel anodes?

Aditee Dabholkar
- Urbana, Illinois, USA


(2004)

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).

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


February 12, 2009

Q. What is the use of membranes in electrolysis when the desired products are electroplated/ deposited at respective electrodes?

parvatalu damaraju
- Mumbai, India


February , 2009

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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


January 19, 2012

Q. How do I plate the inside of a small brass tube with gold? I understand that you must use nickel plating first and then gold.

Mel Morrison
- Somerset, Massachusetts USA


January 20, 2012

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,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



January 10, 2013

Q. I need to calculate the minimum voltage need to plate, here is my set up and what I understand:
Anode stainless steel, from what I have read SS will always have a passive coating of Cr2O3.
Cathode 12um NiCr
solution Sifco 5355 non cyanide Gold solution, PH = 8.5, concentration = 0.125m. From a previous post by Ted Mooney, I take it that SS will not dissolve in this case. 5355-> HKo, C2H8N2, Na2SO3, Mercaptosuccinic acid, Au; all in roughly equal proportions of 1-5%.
I understand how to use the Nernst/BV equations. What I don't understand is which formal potential to use. Au|Au+, e- has formal potential of 1.8V. I assume that is for the cathode half reaction, but what do I use for the anode half reaction.
I have not been able to find anything about Cr2O3 formal potential. Is it relevant? I think stainless steel is non-sacrificial. Is this true?
Although I have a basic understanding of BV/Nernst, a description of the overall reaction and the key players in the solution, (in addition to Au), would be helpful if time allows.
Thanks so much for your help.

Jerry Gregoire
- Bozeman, Montana


January 15, 2013

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,

Daniel Montanes
- Canuelas, Buenos Aires, Argentina


January 17, 2013

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

Lyle Kirman
consultant - Cleveland, Ohio


January 21, 2013

thumbs up signDaniel, 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.

Jerry Gregoire
- Bozeman



Gold anodes for a gold plating bath

October 30, 2016

Q. What happens when 18k or 14k gold is used as the anode during plating operations in a cyanide bath?

Hugo van Niekerk
H.v. N. Goldsmith - South Africa


October 2016

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,

pic of Ted Mooney
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Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



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