The lecture discusses electrodeposition of coatings through electrochemical reactions. It describes how electrodeposition works, involving an electrochemical reaction at the cathode and anode. Common reactions include metal deposition, hydrogen evolution, and oxygen evolution. Factors that influence coating quality like current density and bath composition are also covered. Faraday's laws of electrolysis relating electrical charge to material deposition are explained. An example calculation determines the thickness of a nickel coating deposited for 1 hour.

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 5
Electrodeposition 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
Copper
Anode
Steel
Cathode
e-
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

6. 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

7. 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

9. 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..

10. 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
Cathode
H+
H2
OH
H2O  H+ + OH

11. Current efficiency
• Is the ratio between the actual amount of metal
deposit, Ma to that calculated theoretically from
Faradays Law, Mt.
%
100
M
M
efficiency
Current
t
a



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

13. An example of Current vs. Potential Curve
for electroplating of metal

14. Typical Recipe and Conditions
Watts 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

15. Faraday’s Laws of Electrolysis
Amount of material = amount of electrical energy
zF
q
n 
n = amount of material
q = electrical charge
z = number of electrons
F = Faraday constant
]
mol
C
[
]
C
[
]
mol
[ 1



16. Faraday’s Laws of Electrolysis:
Expanded Relationship
zF
q
n
zF
It
M
w

n = amount of material
w = mass of material
M = molar mass of material
I = current
t = time
z = number of electrons
F = Faraday constant

17. Current, Current density, Surface area
A
I
j 
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)

18. Faraday’s Laws of Electrolysis:
Average thickness
F
.
z
t
.
I
.
M
w
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
F
.
z
.
A
.
t
.
I
.
M
x



19. Faraday’s Laws of Electrolysis:
Average deposit thickness
F
.
z
.
A
.
t
.
I
.
M
x


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/AzF)
which depends on the metal and the bath

20. Faraday’s 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?

21. Faraday’s 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.

22. Faraday’s Laws of Electrolysis:
Answer - Nickel Plating
F
.
z
.
A
.
t
.
I
.
M
x


The average deposit thickness is given by:
)
96485
)(
2
)(
100
)(
90
.
8
(
)
3600
)(
3
96
.
0
)(
71
.
58
(
1
2
3
1




mol
C
cm
cm
g
s
A
x
mol
g
x
m
cm
x
cm
x
x 
35
10
4
.
35
10
54
.
3 4
3


 


23. • 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 Faraday’s
Laws of electrolysis.
Thank you for your attention!
Summary