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Why can we copper plate iron/steel in alkaline solution but not acid?

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Q. How to plate iron with copper? According to the literature and according to my experience, it is not possible to plate copper onto iron in an acid solution because if I immerse the iron in this solution (the most classic is made up of sulfuric acid & copper sulphate) a "redox spontaneously" begins. The Cu ion steals the electrons from Fe (which has a lower standard reduction potential), is reduced and passes to a metallic structure, the iron oxidizes and goes into solution. This produces a light, insubstantial film of copper on the iron that prevents true strong plating. It is clear that this redox already takes place in a solution of CuSO4 and H2O with an immersed iron rod. Much slower than with sulfuric acid but also clear. Everything has to do with the famous electron affinity and with the different standard reduction potential between the two elements Cu=+0.34 and Fe=-0.76.

So the iron in chemical and galvanic literature to be copper plated must be immersed in a rather strong alkaline copper solution. The question is why does the spontaneous redox in alkaline solution not trigger? Why don't Cu ions oxidize Iron and therefore are reduced? What is preventing this from happening?

Marcello Busini
- Italy Poggibonsi
February 24, 2023

A. Hi Marcello. You explained the problem of trying to plate iron from an acid solution quite well, thanks :-)

But your understanding of it being possible in alkaline plating slightly misses the mark. The reason it is possible is not because of the alkalinity (or only indirectly so); rather the reason is that cyanide copper, pyrophosphate copper, and proprietary alkaline copper solutions "complex" the copper very effectively ...
The standard reduction potentials which you mention apply to solutions of a standard concentration (1.0 normal I think); if you were to reduce the concentration of copper ions a quadrillion-fold, those reduction potentials would be way off. The "Nernst Equation" can be used to find the actual reduction potentials when the copper concentration is almost zero, and you will find that the reduction reaction is then actually in the opposite direction.
Cyanide is able to complex copper so effectively that there are essentially zero copper ions in solution; rather, the copper is effectively plated directly out of the cyanide salt.

As an aside, the same idea is what makes alloy plating possible: a brass alloy for example might seem impossible to electrodeposit because of the great difference in potentials of zinc and copper, but again, there are virtually zero copper ions in solution, the rest being complexed by the cyanide.
Luck & Regards,

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

! Good morning Mr Mooney

Thanks for your kind reply. Forgive me for replying late, but I'm a family man among other things and the things I love to do I do in the few scraps of free time or during the night when I'm at it. Following a video of some gentlemen plating iron with an acidic copper solution I made my first mistake which has already taught me a lot. For now I have created three electrolytes, Wood's nickel, the classic acid copper plating, and an alkaline one of my own invention. By avoiding cyanide (which is perhaps the best) for clear reasons, pyrophosphate remained. But in Italy, accessing certain products is not easy if you are not in the trade (yes, I will succeed, it just takes a little patience). So I made an electrolyte of my own invention and incredibly it works so much that I plated copper acid and also nickel on top. And the plating holds perfectly. Aesthetically it needs to be mechanically polished because it gets a bit dull (lack of a rinse aid I suppose). But for now, that's fine. The first step was to make something that mechanically held up even at the expense of aesthetics. We get to that later.
The explanation to my question was enlightening. I had already guessed a little that there was something that prevented copper from being a free ion and from stealing the electrons from the poor iron who was quiet about his business. But to be honest I thought it was the alkalinity. Coordination complexes and compounds (I hope the tradition is correct) are therefore the explanation. I'll have to study them well.

Thanks again


Marcello Busini
- Poggibonsi Italy
March 14, 2023

A. Hi again. Thanks for the interesting reply. Although Wood's nickel is often used for the purpose of activating/striking stainless steel for adhesion, you can use a nickel strike solution on steel first as a way to enable acid copper plating. The familiar copper plated steel grounding rods are often made by applying a thin layer of nickel plating to the steel followed by a thick layer of acid copper, which allows the process to be done without cyanide or other complexing agents.

Luck & Regards,

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

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