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Electroplating alloys

Dr. Dmitri Kopeliovich

An alloy coating may be obtained by a co-deposition of the alloy constituents from the electrolyte containing their ions.

Deposited alloys may possess properties and combination of properties, which ate not achievable in the coatings of pure metals.

Electroplating allows to prepare alloys, which can not be produced by melting.

Some metals may be deposited only in form of their alloys. For example electroplating molybdenum (Mo), germanium (Ge) or tungsten (W) is possible only from the electrolytes containing also ions of iron (Fe) or nickel (Ni).

Principles and practice of electroplating alloys are considered in the book of Abner Brenner “Electrodeposition of alloys”.

Role of electrode potentials in electroplating alloys

According to the Faraday’s law for electroplating binary alloy composed of two metals M and A:

WM = IM*t*μM/(nM*F)
WA = IA*t*μA/(nA*F)

Where
WM, WA – weights of the deposited metals M and A;
μM, μA – weights of one mole of the metals M and A;
nM, nA - numbers of electrons transferred by the ions M and A;
F – Faraday’s constant, F = 96485 Coulombs;
t - time.

Then the composition of the deposited alloy (molar concentration of A):

CA = IA/(IMnA/nM + IAnA/nA)

Under equilibrium conditions ions of a metal start to deposit on the cathode surface when its potential is brought below the electrode potential calculated according to the Nernst equation. In the real process the cathode potential, at which the deposition starts is even below the equilibrium Nernst potential due to the electrode Polarization.

If the electrode potentials of the alloy components are different (EM > EA) then the following conditions are possible:

  • Ec > EM > EA (Ec is the cathode potential). Under these conditions no reduction reactions occur. No electric current is passing through the electrolyte.
  • EA < Ec < EM. Only M is deposited. The electric current is a result of the reaction Mn+ + ne- = M.
  • Ec < EA < EM. Both M and A are deposited but the deposition of M is preferable. The electric current is a result of the both reactions: Mn+ + ne- = M and An+ + ne- = A.


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Effect of complexing agent

Electroplating of an alloy composed of metals with different Electrode potentials (more noble metal and more active metal) results in preferential deposition of the metal with higher potential (more noble metal).

In order to obtain an alloy with the composition similar to the ratio of the metals contents in the electrolyte their potentials should be brought closer together.

Complexing (chelating) is a method of approximating (getting closer) electrode potentials of the different metals in the electrolyte by converting the simple ions of more noble metal into complex ions with lower potential.

Complexing agent (chelating agent) is a substance used for complexing particular ions in the electrolyte. Ions of the complexing agent bind the simple metal ions forming complex ions.
Complexing agent not only approximates the metals potentials but it also retains the more noble ions in the solution preventing their immersion deposition.

In order to achieve more stable effect complexing agent is commonly added to the electrolyte solution in an amount higher than it is required by the stoichiometric composition. The complexing agent in non-binded form is called free complexing agent.

A complexing agent may affect on only one of the metals shifting its electrode potential to the negative side when the second metal stays in form of simple ions.
When a complexing agent form complexes with both metal ions potentials of both of them are shifted to the negative side but the difference between them decreases (they are getting closer).
Sometimes two different complexing agents are added for complexing two different metals.

Examples of complexing agents:

  • Potassium cyanide: KCN
  • Potassium tartrate: K2C4H4O6
  • Potassium sodium tartrate (Seignette’s salt or Rochelle salt): KNa2C4H4O6·4H2O
  • Methanesulfonic acid: CH3SO3H
  • Amino acids
  • Phosphoric acid: H3PO3
  • Citric acid: C6H8O7
  • Thiourea: (NH2)2CS

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Effect of operating parameters

  • Electric current density. Commonly increase of current density results in increase of the content of the less noble metal in the deposited alloy.
  • Temperature. Increase in temperature causes decrease of the cathode Polarization. Activation polarization decreases due to intensification of the gaseous Hydrogen formation. Concentration polarization drops as a result of increase of the metal concentration in the Diffusion layer at the cathode surface. Lower cathode polarization at increased temperature results in increase of the content of the more noble metal in the alloy.
  • Agitation. Agitation reduces the thickness of the cathode Diffusion layer, which causes decreased concentration polarization. Lower cathode polarization in agitated cathode or electrolyte results in increase of the content of the more noble metal in the alloy.
  • Additives (addition agents). Additives (brighteners, levelling agents) act similar to complexing agents causing increase of the less noble metal in the alloy. Addition agents are most effective in the electrolytes containing simple (non-comlexed) metal ions.

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electroplating_alloys.txt · Last modified: 2013/12/14 by dmitri_kopeliovich
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