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Role of anions in electroplating

Hi,

I am trying to figure out what role anions (chloride, sulphate, nitrate) play in copper plating. Actually, I am more interested in the reverse process (galvanic corrosion), but I figure that both processes are intimately related. I'm trying to figure out why some solutions (i.e. nitric vs. sulfuric vs. hydrochloric) will leach copper and leave other components of an alloy untouched while others will eat at certain components of the alloy (maybe like how zinc can preferentially be leached from brass under certain conditions).

As I see it, different anions can increase the solubility/activity of copper ions in the solution. Depending on the relative charges of the anions, they might also affect the ionic strength of the solution, in turn affecting its electrical resistance. But what I'm really interested in is the difference in the anions' relative ability to complex copper. How does this (I assume that it does) affect electroplating? I think that it would probably alter the standard reduction potentials along with the actual reduction potentials, but I'm not sure of whether there are commonly known rules and trends associated with this (?). I have no idea of how it would affect the kinetics of plating either, any thoughts on that? I haven't been able to find much on this, but I saw one link that mentioned that copper nitrate ⇦this on eBay or Amazon [affil links] baths improve plating at higher current densities and that the best finishes tend to come from these baths. Any idea of why? Most of this is just curiosity, but I've been researching this for a couple of hours now without any solid leads.

If you have any recommendations for good references, I would be happy to have those too!

Thank you!

Wendi Sweet
did grad thesis on Cu electroplating; now working in industry/research - San Diego, California, USA
May 20, 2009

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May 28, 2009

Electroplating essentially requires of the constituent chemicals the following characteristics:
1.The metal that is to be deposited must be sourced as an aqueous soluble salt and as a soluble anode.
2.The reduction potential of the cation(Mz+) must enable only discharge of the metal ions at the cathode.Any overvoltage may promote hydrogen evolution reducing the cathode efficiency so much so no deposition of metal occurs.
3.Similarly at the anode ,any over voltage may promote oxygen evolution or Chlorine evolution(if Cl- ions are present) instead of dissolution of the anode thus reducing the anode efficiency resulting in anode passivity.
4.Presence of Cr+6 ions/NO3- ions in solution may prevent deposition of metal since the current is used up for the reduction of Cr+6 to Cr+3 and No3- to No2-.
5.Formulation include salts for improving conductivity (for eg:NaOH,KCl)and organic additives(usually proprietary) to improve brightness of deposit.
6.Complexation usually reduces the free metal ions' concentration promoting fine-grained deposit for e.g., cyanide zinc plating.Complexation will change the reduction potential.This concept is used in alloy plating.
7.If the nucleation rate is high ,that is, when metal concentration is very high,then coarse/porous deposit or even treeing will occur.eg chemical displacement of copper on mild steel from acid solution results in non-adherent copper deposit.
8.Absorption of CO2 from air by alkaline baths will result in gradual build- up of Carbonate resulting in dull deposits.
9.Whenever brass is leached in acid or alkali,zinc dissolves preferably resulting in dezincification of brass.This is because zinc is anodic to copper.
10.Galvanic corrosion is accelerated or decelerated depending on the difference in oxidation potentials being large or small.
11.Good reference books are:2.Electrochemistry by Samuel Glasstone.
12.Electroplating by Loweinheim [on AbeBooks, eBay, or Amazon affil links]
13.Corrosion Handbook by Uhlig
I hope the info is valid and useful.
Good Luck!

Subramanian Ramajayam
consultant - Bangalore, India

Hey, thanks for the detailed response--very helpful!

Wendi Sweet
- San Diego, California
June 3, 2009