Outline Curriculum (5 lectures) Each lecture ? 45 minutes

1
Outline Curriculum (5 lectures)Each lecture ? 45
minutes

• Lecture 1 An introduction in electrochemical
  coating
• Lecture 2 Electrodeposition of coating
• Lecture 3 Anodizing of valve metal
• Lecture 4 Electroless deposition of coating
• Lecture 5 Revision in electrochemical coating

2
Lecture 2 of 5Electrodeposition of Coating
3
Electrochemical Surface Engineering

• An electro-chemical reaction
• Cathode Metals/alloys coatings
• Anode Soluble or insoluble
• Conductive solution ionic species
• Transfer of electrons

4
An example of electroplating of copper
Power Supply
e-
Copper Anode
Steel Cathode
Main reaction Cu2 2e- ? Cu
5
Other possible electrochemical reactions

• At the cathode

Electrodeposition of copper Cu2 2e- ?
Cu Hydrogen evolution 2H 2e- ? H2
At the anode
Soluble anode Dissolution of copper Cu ? 2e- ?
Cu2 Insoluble anode Oxygen evolution H2O ? 2e-
? 2H 0.5 O2
Overall reaction
Cu2 H2O ? Cu 2H 0.5 O2
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Definition Electron transfer reactions

• Oxidizing agent n e- Reducing agent
• Oxidizing agents get reduced
• Reducing agents get oxidized
• Oxidation is a loss of electrons (OIL)
• Reduction is a gain of electrons (RIG)

OILRIG
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Typical steps in the electroplating of metals

1. Cleaning with organic solvent or aqueous
   alkaline to remove dirt or grease.
2. Is the surface is covered by oxides as a result
   of corrosion, clean with acid.
3. Rinse with water to neutralise the surface.
4. Electroplate metals under controlled condition.
5. Rinse with water and dry.
6. Additional step heat treatment in air or vacuum
   environment

8
What is the Job of the Bath?

• Provides an electrolyte
• to conduct electricity, ionically
• Provides a source of the metal to be plated
• as dissolved metal salts leading to metal ions
• Allows the anode reaction to take place
• usually metal dissolution or oxygen evolution
• Wets the cathode work-piece
• allowing good adhesion to take place
• Helps to stabilise temperature
• acts as a heating/cooling bath

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Typically, What is in a Bath?e.g., Watts Nickel

• Ions of the metal to be plated, e.g.
• Ni2 (nickel ions) added mostly as the sulphate
• Conductive electrolyte
• NiSO4, boric acid, NiCl2
• Nickel anode dissolution promoter
• NiCl2 provides chloride ions
• pH buffer stops cathode getting too alkaline
• Boric acid (H3BO3)
• Additives
• Wetters, levellers, brighteners, stress
  modifiers..

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Current efficiency

• pH changes accompany electrode reactions wherever
  H or OH- ions are involved.
• In acid, hydrogen evolution occurs on the surface
  of cathode. This will result in a
  localised increase in pH near the surface of the
  electrode.
• In acid, oxygen evolution occurs on the surface
  of anode. This will result in
  a drop of pH near the surface of the electrode.
• pH buffer stops the cathode getting too alkaline.
• Boric acid (H3BO3)

2H 2e- ? H2
H2O ? 2e- ? 2H 0.5 O2
H
Cathode
H2
OH?
H2O ? H OH?
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Current efficiency

• Is the ratio between the actual amount of metal
  deposit, Ma to that calculated theoretically from
  Faradays Law, Mt.

12
Parameters that may influence the quality of
electrodeposits

• Current density (low to high current)
• The nature of anions/cations in the solution
• Bath composition, temperature, fluid flow
• Type of current waveform
• the presence of impurities
• physical and chemical nature of the substrate
  surface

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An example of Current vs. Potential Curve for
electroplating of metal
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Typical Recipe and ConditionsWatts Nickel

• Component Concentration/g L-1
• Nickel sulphate 330
• Nickel chloride 45
• Boric acid 40
• Additives various
• Temperature 60 oC
• pH 4
• Current density 2-10 A dm-2

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Faradays Laws of Electrolysis

• Amount of material amount of electrical energy

n amount of material q electrical charge z
number of electrons F Faraday constant
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Faradays Laws of Electrolysis Expanded
Relationship
n amount of material w mass of material M
molar mass of material I current t time z
number of electrons F Faraday constant
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Current, Current density, Surface area
j current density mA cm-2 I current A A
surface area of the electrode cm2
jelectroplate electroplating current density
(metal electroplate) jcorrosion corrosion
current density (metal corrosion/dissolution)
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Faradays Laws of Electrolysis Average thickness
w weight (mass) of metal M molar mass of
metal I current t time z number of
electrons F Faraday constant x thickness of
plating
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Faradays Laws of Electrolysis Average deposit
thickness
The thickness of plate depends on - the current
(I) - the time for which it passes (t) - the
exposed area of the work-piece (A) - a constant
(M/rAzF) which depends on the metal and the
bath
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Faradays Laws of Electrolysis Question - Nickel
Plating
Nickel is plated from a Watts bath at a current
density of 3 A dm-2. The current efficiency is
96. The molar mass of nickel is 58.71 g
mol-1. The density of nickel is 8.90 g cm-3. The
Faraday constant is 96 485 C mol-1. What will be
the averaged plating thickness in 1 hour?
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Faradays Laws of Electrolysis Answer - Nickel
Plating
Assume that the reaction is Ni2 2e-
Ni So, two electrons are involved for every Ni
atom, and z 2 The current density used in
plating nickel is 96 of the total current,
i.e., 0.96 x 3 A dm-2.
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Faradays Laws of Electrolysis Answer - Nickel
Plating
The average deposit thickness is given by
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Summary

• Electrodeposition is a versatile coating
  technique.
• There is a high degree of control over deposit
  thickness.
• Many metals can be electroplated from aqueous
  baths.
• So can some alloys, conductive polymers and
  composites.
• Rates of electroplating can be expressed via
  Faradays Laws of electrolysis.
• Thank you for your attention!