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 5

Electrodeposition 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 copperPowerSupply

CopperAnode

SteelCathode

e-

Main reactionCu2+ + 2e- Cu

Other possible electrochemical reactions

At the cathodeElectrodeposition of copper Cu2+ + 2e- CuHydrogen evolution 2H+ + 2e- H2

At the anodeSoluble anodeDissolution of copper Cu 2e- Cu2+ Insoluble anodeOxygen evolution H2O 2e- 2H+ + 0.5 O2

Overall reactionCu2+ + 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

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

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

Cathode

H+

H2

OHH2O H+ + OH

Current efficiency

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

%100M

MefficiencyCurrent

t

a

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 sulphate 330Nickel chloride 45Boric acid 40Additives variousTemperature 60 oCpH 4Current density 2-10 A dm-2

Faraday’s Laws of Electrolysis

Amount of material = amount of electrical energy

zF

qn

n = amount of materialq = electrical chargez = number of electronsF = Faraday constant

]molC[

]C[]mol[ 1

Faraday’s Laws of Electrolysis: Expanded Relationship

zF

qn

zF

It

M

w

n = amount of materialw = mass of materialM = molar mass of materialI = currentt = timez = number of electronsF = Faraday constant

Current, Current density, Surface area

A

Ij

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

F.z

t.I.Mw

w = weight (mass) of metalM = molar mass of metalI = currentt = timez = number of electronsF = Faraday constantx = thickness of platingF.z.A.

t.I.Mx

Faraday’s Laws of Electrolysis: Average deposit thickness

F.z.A.

t.I.Mx

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

Faraday’s Laws of Electrolysis: Question - Nickel Plating

Nickel is plated from a Watts bath ata 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

F.z.A.

t.I.Mx

The average deposit thickness is given by:

)96485)(2)(100)(90.8(

)3600)(396.0)(71.58(123

1

molCcmcmg

sAxmolgx

mcmxcmxx 35104.351054.3 43

• Electrodeposition is a versatile coating technique.Electrodeposition is a versatile coating technique.

• There is a high degree of control over deposit thickness.There is a high degree of control over deposit thickness.

• Many metals can be electroplated from aqueous baths.Many metals can be electroplated from aqueous baths.

• So can some alloys, conductive polymers and composites.So can some alloys, conductive polymers and composites.

• Rates of electroplating can be expressed via Faraday’s Rates of electroplating can be expressed via Faraday’s

Laws of electrolysis.Laws of electrolysis.

Thank you for your attentionThank you for your attention!!

Summary