Outline curriculum 5 lectures each lecture 45 minutes
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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.

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Outline Curriculum (5 lectures) Each lecture  45 minutes

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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


Lecture 2 of 5Electrodeposition of Coating


Electrochemical Surface Engineering

  • An electro-chemical reaction

  • Cathode: Metals/alloys coatings

  • Anode: Soluble or insoluble

  • Conductive solution: ionic species

  • Transfer of electrons


An example of electroplating of copper

Power

Supply

e-

Copper

Anode

Steel

Cathode

Main reaction

Cu2+ + 2e- Cu


Other possible electrochemical reactions

Electrodeposition of copper Cu2+ + 2e- Cu

Hydrogen evolution2H+ + 2e-  H2

At the cathode

At the anode

Soluble anode

Dissolution of copper Cu 2e- Cu2+

Insoluble anode

Oxygen evolutionH2O  2e-  2H+ + 0.5 O2

Overall reaction

Cu2+ + H2O Cu + 2H+ + 0.5 O2


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


Typical steps in the electroplating of metals

  • Cleaning with organic solvent or aqueous alkaline; to remove dirt or grease.

  • Is the surface is covered by oxides as a result of corrosion, clean with acid.

  • Rinse with water to neutralise the surface.

  • Electroplate metals under controlled condition.

  • Rinse with water and dry.

  • Additional step: heat treatment in air or vacuum environment


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


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..


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


Current efficiency

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


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


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


Typical Recipe and ConditionsWatts Nickel

Component Concentration/g L-1

Nickel sulphate330

Nickel chloride 45

Boric acid 40

Additivesvarious

Temperature60 oC

pH4

Current density2-10 A dm-2


Faraday’s Laws of Electrolysis

Amount of material = amount of electrical energy

n = amount of material

q = electrical charge

z = number of electrons

F = Faraday constant


Faraday’s 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


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)


Faraday’s 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


Faraday’s 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


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?


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.


Faraday’s Laws of Electrolysis: Answer - Nickel Plating

The average deposit thickness is given by:


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 Faraday’s Laws of electrolysis.

    Thank you for your attention!


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