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Electrolytic Alloy Deposition

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Chris

Mon Mar 30 2009, 05:32AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Electrolysis in various fused salt baths is effective at depositing heavy layers of most metals from an anode consisting of the metal onto a cathode substrate. The process proceeds as the anode is oxidized and the oxide dissolves into the melt, where it is reduced to metal at the cathode.

I would like to use a process like this to form solid alloy stock or deposit layers of alloys onto other parts. The problem is that if I started out with an anode made of the alloy itself or added salts to the electrolyte corresponding to the alloy composition, they would plate out at different rates. It would take a great deal of experimentation to determine the different rates at which the metals are deposited, and compensate by adding additional material so the electrolyte does not become enriched with certain metals over others.

My idea is to use several anodes in the form of flat bars each consisting of a pure metal being a constituent of the alloy. These would be arranged around the cathode, which would be spun to achieve radial uniformity of the deposit. To account for the difference in deposition rate (which would still require some experimentation), the bars would be of different widths, but would all be lowered into the melt to the same depth to ensure longitudinal uniformity of the cathode deposit. Current could be monitored for each anode to ensure it is being dissolved at the proper rate, but I believe each anode would have to be connected to the same voltage source to avoid them plating from one to other. It might be possible to use anodes at different depths if there was some kind of vigorous vertical stirring of the electrolyte (possibly with ceramic impellers, gas bubbling, or induced convection by a cooler). This would allow better control over the dissolution rates by adjusting the depth of each anode.

Bored Chemist

Mon Mar 30 2009, 05:53AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Plating alloys is a complicated business. I will try to remember to think about it some more but I don't think you want to connect all the anodes to the same voltage. I think you want to feed them from independent constant-current sorces and I think you will need an electrolyte of exactly the right composition.
Do you have any particular metals in mind?

Chris

Mon Mar 30 2009, 06:27AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

Yeah mostly high chrome tool steels like D2, and stainless steels. Also titanium alloys, and nickel alloys like inconel. One of the major uses I have in mind for this process is forming metal matrix composites.

aonomus

Mon Mar 30 2009, 11:47AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

From what I heard, either rotating the workpiece or moving the counter-electrode are 2 ways industry try to get even coatings of metals deposited, and that its mostly a well guarded secret (one of my profs mentioned this a few weeks back).

Perhaps if you are going to use multiple electrodes, instead of trying to current balance them, just switch each electrode on in sequence and cycle through them so each one gets an equal amount of active time, and space them equally. You could experiment with high current pulses at higher frequency too...

Bored Chemist

Mon Mar 30 2009, 05:16PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

The metal that comes out of solution is not necessarily the one that goes into solution so using different electrode in turn probably won't work well (unless you cheat, plate a layer of one metal, then change the electrolyte and plate a layer of a second metal etc. Then heat the stuff to fuse the metals together- tricky.) Someone has a patent for that idea here.

,M1

Do you know about the Nernst equation?

Chris

Fri May 01 2009, 12:29AM

Registered Member #8 Joined: Thu Feb 02 2006, 04:34AM
Location: Harlowton, MT, United States
Posts: 214

I am familiar with the Nernst equation but there is pretty much no chance of finding the potentials that would apply to fused salt baths off a chart.

It seems to me that if multiple anodes were at different potentials (which they would be if they were on their own constant current sources) that inter-anode deposition would be inevitable. Especially during pulsed current electrolysis (which is essential when plating some metals such as titanium), unpowered anodes of more reactive metals could quickly reduce the salts or oxides of less reactive metals in the electrolyte. This latter problem could possibly be solved by using a baseline voltage on each anode during pulsed electrolysis, but I can still see where inter-anode plating could be a problem.

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