Information about zinc nickel and tin zinc electrochemistry

A. Hi, Richard. Electrodeposition is very simple in concept: If you move electrons from the anode to the cathode with a battery, rectifier, or other power supply, then metal will be reduced at the anode to positively charged ions as you "steal" their electrons; and they'll dissolve into solution and migrate over to the cathode, where they will re-join those electrons and be re-oxidized into metal. Faraday's Law of Electrolysis will give you numbers on this: 96,485 coulombs or ampere-seconds will transfer one gram equivalent weight of metal.

But as you attempt to plate out an alloy, the problem you will run into is that one of the metals will be much more noble than the other and will very readily plate out while your voltage is too low to plate out virtually any of the more active metal. The deposit will be 99.99% noble metal and almost zero active metal. You can then look at the Nernst equation and realize that the only way you'll get the active metal to plate out is to reduce the concentration of the noble metal to the merest wisp. But if you attempt to do this by reducing the concentration of the noble metal to a wisp, you will exhaust the solution of that metal nearly instantly. So actually, it just isn't practical to electroplate alloys like zinc-nickel or tin-zinc out of simple acids.

So what is actually done is that complexing agents are added which "tie up" the more noble metal. In this way, you can keep plenty of the noble metal dissolved into the solution as a complex, while the number of "free" ions is very low, so you can get the Nernst equation to work the way you need it to.

In the case of the tin-zinc bath, cyanide can be used as the complexing agent to tie up the tin and allow you to plate a workable tin-zinc alloy. But for the zinc-nickel bath and for cyanide-free tin-zinc baths, the practical complexing agents are (to the best of my knowledge) all proprietary trade secrets that people can't share with you.

A technical literature search will generate papers that will reveal the chemistry behind the early attempts at zinc-nickel and tin-zinc plating, although I would think that nothing has been published on the most successful proprietary solutions that are finding commercial use. Good luck.

A. Hi Mahesh. Please introduce yourself because it is very difficult to know where to start when we don't know what you're up to. If you just need to get a couple of steel tubes plated, the way it is normally done is to send them to a plating shop -- some people know this and some don't. If you must do the plating in-house for some reason, the way it is normally done is to purchase a brand name tin-zinc plating process from a plating process supplier -- again, some people don't realize that plating solutions are rarely formulated from commodity chemicals by the end user.

If, however, you are doing research into plating solution formulation rather than building a product, and these carbon steel tubes are just a substrate you've chosen to use for your experiments, please see if you can use flat sheets instead because you won't get any plating on the inside of your tubes and they may contaminate your solutions.

1. My experience includes designing a plating plant where we tin-zinc plated acres of steel weldments and cast iron, but we used an Atotech proprietary bath that was cyanide based, and cyanide may be inappropriate for your environment. Further, the process required filming of the anodes, which might be problematic in your situation, and our desired ratio was 78-22%.

Try to get access to a copy of Brenner's "Electrodeposition of Alloys" through a university library. It is almost 1400 pages and summarizes almost everything ever learned about alloy plating up to its publication date, and includes references to almost every technical article on the subject. It has a 35-page chapter on formulations for tin-zinc plating, plus dozens of other references to it scattered throughout the volumes.

2. If you want your deposit to be 60-40, the ratio of tin to zinc anodes will ideally be 60-40 for balance and equilibrium. Further, maintaining a 60-40 ratio in the anodes will serve as a double-check that your deposit is 60-40 because if you are replacing anodes in a different ratio you are not depositing at 60-40 as you think :-)
3. The current density will probably be in the 30 ASF range.
4. I would suggest simply putting your cathode between two rows of anodes as is done in production plating plants and the anode-cathode ratio will be acceptable. Good luck.

Regards,