1

CHAPTER I

INTRODUCTION

1.1 Electrodeposition

Electroplating is a form of surface modification, in which metal is deposited

electrochemically from a solution of its own ions. The current enters and leaves the

electrolyte via two conducting electrodes, which are called the ‘anode’ and the ‘cathode’

respectively. At each electrode, there are more than one reactions occurring. These are as

follows:

At the anode

M M n+ + ne- (metal dissolution)

2H2O O2 + 4H+ + 4e- (oxygen evolution)

M2+ M3+ + e- (oxidation)

M MO (passivation)

If metal dissolution is the major reaction, it is said to have good efficiency.

At the cathode

Mn+ + ne- M (metal deposition)

2H2O +2e- H2 + 2OH- (hydrogen evolution)

M3+ + e- M2+ (reduction)

If the deposition process is the major process it is said to have a good current efficiency.

1.1.1 Metal deposition from simple salt solutions

During the electro deposition of metals, the cations in simple salt solutions are

present in hydrated form, having the same valency and oxidation states as the anhydrated

form. Thus, the first step in the deposition process is the removal of the water molecules,

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Figure 1.1 Sequence of events occurring during cathodic deposition (a)

hydrated metal ion in bulk Solution (b) hydrated metal ion

enters diffusion layer(c) hydrated metal ion enters electric

double layer

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which occur at the diffusion layer (Figure 1.1). If the energy required to dehydrate the

water molecule is high, then the deposition process will need a high activation

polarization and high over potential before the actual discharge occurs. As the ion

migrates across the diffusion layer, the water molecules orient themselves in the direction

of the electric field and then are lost as the ion enters the electric double layer. When the

ion has lost its hydration shell, it is discharged and adsorbed onto the cathode as an ad-

atom, which then diffuses across the cathode surfaces to a growth point [1-4]. At the

growth point, the atom becomes incorporated into the growing crystal lattice of the

deposit on the cathode.

1.1.2 Metal deposition from complex salt solutions

Metals such as copper, zinc, cadmium and silver are commonly deposited from

complex ion bath. For example, silver cyanide solution is used to plate silver on a

substrate. The complexed ion consists of a positively charged ion surrounded by

negatively charged anions. The overall charge of the complex is negative. One suggested

mechanism is that the deposition proceeds via a complex of lower coordination number

where the complexed ion will have a neutral or positive charge. This type of complex

ion, with lower coordination number, will be present in a small amount, and its formation

will be rate-determining step [5]. The possible mechanisms for the discharge of metal ion

from its complex are

(a) The decomposition of the complex present in the electrolyte into its components

Eg., [Ag(CN)2]- Ag+ +2CN-

Ag+ + e- Ag

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(b) The formation of positively charged complex from the existing negative complex

and the discharge of the positive complex as a whole

2[Ag (CN) 2]- [Ag2CN]+ + 3CN-

[Ag2CN]+ + e- Ag2CN

Ag2CN Ag+ + AgCN-

Using Tafel equation, which decides electro kinetic parameters (described in the

subsequent sub section)

a b log iη = ±=

Where η - Over voltage a & b are Tafel slopes

The i0 (exchange current density) can be calculated. The value of ‘b’ indicates

whether there is a change in the mechanism or not during the electro deposition process.

In certain case ‘b’gives information about the way in which molecules are adsorbed on

the surface [6]. The value of ‘i0’ is a measure of the rate of the reaction. It has been

shown that the calculated α value gives a reliable estimate of an irreversible process [7].

In an electrode process involving multistep electron transfer, Tafel plot diagnose the

mechanism by taking different ‘b’ values. It is found that when ’b’= 120± 10 mV for

majority of the multistep metal deposition process, charge transfer is the rate controlling

step.

1.2 Factors influencing electrodeposition process

The process of metal deposition is affected by (1) current density (2) temperature

(3) hydrodynamic conditions (4) pH of the solution (5) nature of the metal (6) bath

composition (7) addition agents (8) crystallography (9) impurities present in the solution.

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1.2.1 Current density

Generally increase in current density decreases the crystal size. With increase in

current density, the rate of formation of nuclei will be increased and the crystal size is

decreased [8]. Increase in current density produce spongy or burnt deposits that usually

contain hydroxides or basic salts. The concentration of the metal (and that of metal ions)

in the diffusion layer decreases and the polarization increases while increasing the current

density. The limiting current density in a given solution has been used to compute the

composition and thickness of the diffusion layer and vice versa.

1.2.2 Temperature

In general, an increase in bath temperature causes an increase in the crystal size.

Increase in bath temperature increases solubility and thereby the transport number, which

in turn leads to increased conductivity of the solution. It also decreases the viscosity of

the solution, thereby replenishing the double layer relatively faster. High bath

temperature usually decreases less adsorption of hydrogen on the deposits and thereby

reduces stress and tendency toward cracking.

1.2.3 Agitation

In general, agitation of the plating solution replenishes metal salts or ions at the

cathode and reduces the thickness of the diffusion layer. It sweeps away the gas bubbles,

which may otherwise cause pits. Agitation helps increasing the operating current density

and thereby permits a higher operating current density.

1.2.4 pH of the solution

pH control to maintain process optimization is a vital parameter in a plating

operation. pH of the bath influences the hydrogen evolution voltage, the precipitation of

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basic inclusion, the decomposition of the complex or hydrate from which the metal is

deposited, and the extent of adsorption of additives. In a complex bath, pH may

influence equilibrium between various ligands process. When the anode is insoluble,

oxygen evolution takes place at the anode

2H2O O2 + 4H+ + e-

On the other hand, hydrogen evolution at the cathode is accompanied by the production

of hydroxide ion:

2H2O+ 2e- 2OH- + H2

In a neutral bath, if the current efficiency is greater at the anode than at the

cathode, the bath becomes more alkaline. If the electrode efficiencies are similar, the pH

of the bath remains unchanged. Hence change in pH of a plating bath is a good indication

of electrode efficiencies. In certain conditions precipitation of metal hydroxides may

occur locally within the cathodic double layer, which get codeposited with the plated

metal and give defective deposit while increasing the pH due to hydrogen evolution. Thus

buffers are necessary to minimize these pH changes.

1.2.5 Nature of the metal

Based on the magnitude of polarization, metals can be classified into three groups.

Group I consists of metals like Ag, Pb, Cd, Sn etc., which are deposited at an

overpotential of a few milli volts and rough deposits are obtained with grain size greater

than 10-3 cm. Group III consists of metals like Ni, Co, Fe etc., which are associated with

a large electrochemical polarization and are deposited as dense deposits with the grain

size of around 10-5 cm. Cu, Bi, and Zn constituting the II group are intermediate metals

with respect to the overpotential value and grain size of the deposit. The interaction of

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the substrates with the deposited metal differs and this affect the kinetics of nucleation

and hence the number of grains per unit area [9].

1.2.6 Nature of the electrolyte and bath composition

The nature of the electrolyte has a significant effect on the properties of the

deposit. In general, metal deposits obtained from simple salts are rough and coarse

grained. Complexing anions and ligands generally shift the equilibrium potential to more

negative values by reducing the activity of the free ions. This results in fine-grained

deposits with fewer tendencies to form dendrites.

Most of the metals used in plating can be deposited from more than one type of

bath, each of which may be suited for a certain applications. In each type of bath, there is

a fairly wide range of composition and concentration. Generally an increase in the metal

concentration under given conditions decreases the cathode polarization, and as

predicted, increases the crystal size. But the additives in a plating solution play a more

important role in the crystal size and its distribution.

1.2.7 Addition agents

Addition agents are substances, which are intentionally added to metal depositing

solutions in small quantities to produce a beneficial change in the properties of the

deposit [10]. Generally their effects seem to be out of proportion to their concentration

[11]. Additives affect deposition and crystal building processes as adsorbates at the

surface of the cathode. There are two basic types of adsorption: chemisorption and

phisorption. Adsorbed additives affect the kinetics of electrodeposition and the growth

mechanism by changing the concentration of growth sites on a surface, the concentration

of adions on the surface, the diffusion coefficient D, and the activation energy of surface

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diffusion of adions. These addition agents decrease the interfacial tension at the metal-

solution boundary due to adsorption on the surface resulting in the reduced grain size of

the crystals [12].

In electrodeposition process additives can be added in the form of leveling

agents, stress reliever, wetting agent and brighteners. Leveling agent is added to reduce

the surface roughness during deposition. Theories of leveling are based on (i) the

correlation between an increase in the polarization produced by the leveling agents, [13]

(ii) preferential adsorption of leveling agent on high point (peak or flat surfaces) (iii) and

inhibition of deposition at these points [14,15]. Stress relievers are selectively adsorbed

or occluded and change the crystal orientation and reduce the stress [16]. Wetting agents

are used to wet the ‘cathode surface ‘although in practice, their role is to reduce the

surface tension of water so that hydrogen bubbles may more easily get detached from the

surface. Brightener modifies the deposits by preferential surface adsorption. There are

three possible mechanisms for bright deposition: (i) diffusion-controlled leveling (ii)

grain refining (iii) randomization of crystal growth [17].

1.2.8 Crystallisation process

There are two basic types of formulation of coherent deposits [14, 18] Viz., layer

growth and three dimensional (3D) crystallites growth (Figure 1.2). In the layer growth

mechanism a crystal enlarges by a spreading of discrete layers (steps), one another across

the surface. In this case growth layer, a step, is a structure component of coherent

deposit. Steps, or growth layers, are the structural components for the construction of a

variety of growth forms in the electrodeposition of metals. (E.g., columnar crystals,

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whiskers and fiber texture). Many monoatomic steps can unite (bunch, coalesce) to form

a polyatomic step.

In the 3D crystallites growth mechanism the structural components are 3D

crystallites, coherent deposit is built up as a result of coalescence of these crystallites.

The growth sequence of electrodeposition via nucleation coalescence consists of four

stages: (1) formation of isolated nuclei and their growth to TDC (3D crystallites), (2)

coalescence of TDC, (3) formation of linked network, and (4) formation of continuous

deposit. The difference in crystal structure during deposition stems from the potential

dependence of the nucleation and growth processes. Competition between nucleation and

growth processes is strongly influenced by the potential of the cathode [19].

1.2.8.1 Development of columnar microstructure

The columnar microstructure is perpendicular to the substrate surface, as shown in

figure.1.2. This microstructure composed of relatively fine grains near the substrate but

then changes to a columnar microstructure with much coarser grains at greater distances

from the substrate. The low surface energy grains grow faster than the high-energy ones.

This rapid growth of the low-surface-energy grains at the expense of the high-energy

grains results in an increase in mean grain size with increased thickness of deposit and

the transition from a fine grain size near the substrate to a coarse, columnar grain size.

1.2.9 Effect of impurities and their control

Similar to the action of addition agents, impurities in the plating solution have

significant effect on the performance of the plating solution and on the properties of the

plated coatings [20]. Organic impurities, introduced into the plating solution as surface

greases or lubricants or breakdown products of addition agents, can be easily removed by

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Substrate

(a)

(b)

(c)

Figure 1.2 Schematic representation of layer growth (a, b)

and the nucleation-coalescence mechanism (c)

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treatment with activated charcoal. Metallic impurities cause major problem and can be

removed by dummy electrolysis. These impurities reduce the current efficiency, quality

and alter the growth morphology.

1.3 Electrode kinetics

The theory of electrode kinetics is based on two hypotheses:

(i) that any electrochemical reaction can be divided into two or more oxidation or

reduction reactions

(ii) that there will be no net accumulation of electrical charge during an electrochemical

reaction.

1.3.1 Electrode potential

When a metal is dipped into solution, which contains salts and ions of that metal,

equilibrium is set up between the metal and the solutions. That is between the forward

tendency:

M M n+ + ne-

and the opposite tendency for the ions to lose their charge and hence deposit onto the

metal:

Mn+ + ne- M

At one atmospheric pressure and unit activity, the potential assumed by the metal

is called standard electrode potential, Eo. The standard electrode potential of a metal is an

indication of its basicity or nobility. The magnitude of the potential between a metal and

a solution of its ions (Mn+ + ne- M ) is given by the Nernst equation:

(0.059 log noE E M

η)+

= +

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Where E is the electrode potential of an electrochemical cell, E0 is the standard electrode

potential, and n is the number of electrons. The temperature is at 25o C. In general, the

potential of a metal electrode becomes more positive when the concentration of the ions

of that metal increases, and vice versa.

1.3.2 Polarisation

The change in potential of an electrode as result of electrolysis is known as

‘polarization’ of that electrode. The change in potential is known as the ‘overpotential’.

There are three contributing factors to the overall reaction overpotential, these being the

concentration polarization ŋc, activation polarization ŋa, and the ohmic polarization ŋo.

(a) Concentration polarization

The polarization that is caused by the changes in local concentration of metal salts

and ions is known as ‘concentration polarisation’. When deposition at the cathode has

initiated and proceeded for a given time, the concentration of metal ions near the cathode

decreases and if deposited ions are not replenished as fast as they are consumed. The

change in concentration lowers the equilibrium potential as given by the Nernst equation

l o g ec

b

aR tn F a

η = (1.1)

Where ae is the activity of the depositing ion next to the electrode, ab is the

activity in the bulk of the solution, and n is the number of electrons involved in the

discharge of the ion. The rate of reaction is thus limited by diffusion and the limiting rate

is the limiting current density, iL. It is significant during an electrodeposition. For the

anodic reaction (metal dissolution), it is negligible. In many electrodeposition processes,

it could be a rate-determining step. The limiting current density iL, is an important

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quantity associated with concentration polarization. The higher the limiting current

density, the greater is the amount of metal deposited in a given time. The limiting current

density is given by:

(1 )

oL

i

D i n F Ci

t δ=

− (1.2)

Di is the diffusion coefficient of ion’i’, Co is the bulk concentration of ion ‘i’, δ is the

thickness of the diffusion layer adjacent to the electrode in which the concentration

gradient of electrolyte occurs, and ti is the transport number of ion ‘i’.

The concentration over potential can be expressed in terms of current density, I, as follows

2.3 (1 )logcL

RT inF i

η −= (1.3)

The concentration polarization decreases with increasing temperature, due to an

increased diffusion velocity. It is also reduced by agitation because of the reduced

diffusion layer thickness and increased iL.

(b) Activation polarization

When a potential is applied to an electrode kept in an electrolyte, the current will

not flow immediately and this continues till a critical potential, namely the discharge

potential (or deposition potential) is reached. Beyond this, the current rises with the

potential obeying ohm’s law. This initial sluggishness of reaction is due to activation

over potential, which is a measure of the extent of the activation required for the

particular reaction. When the rate of reaction or a series of reactions are determined as a

slow electrochemical step, it is said to be under activation control. The rate of reaction is

given by equations

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0 e x pan F ni iR T

α= (1.4)

0

(1 )expcnFn

RTα−

=i i (1.5)

Where Ic is the symmetry coefficient representing the fraction of overpotential

affecting the oxidation process.

Since the current applied to polarize the electrode surface away from the

equilibrium is the net rate reaction,

i.e. iapp = ia – ic, then

i app = ia – ic

0 0(1 )exp expapp

nFn nFni i iRT RT

α α− −= − (1.6)

For positive or negative over potentials, equation. (1.3) can be expanded. For

simplicity, there are three cases to be considered:

(i) For small values of ia (<10 i0), and hence also of η,a linear relationship is found

between η and i

aa c t

b

iR Tn F i

η = (1.7)

(ii) For ia >10 io, a logarithmic relationship is obtained according to Tafel equation.

Thus, the reaction rate and overpotential for activation is given by

logact aa b iη = + (1.8)

Where ‘a’ and ‘b’ are the Tafel constants, and ‘b’ is the ‘Tafel slope ‘of a

reaction. The constants ‘a’ and ‘b’ are characteristic for each electrode process and b is

about 0.12 V. The exchange current density i0, varies from 10-3 to 10-12 A/cm2 for

various metals.

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(iii) For ia >> i0, deviation from the Tafel slope is obtained due to concentration and /or

ohmic polarization.

The activation polarization is strongly dependent on the composition of the

solution, particularly its anion content and additives. It is independent of the electrolyte

but decreases rapidly with increasing temperature, since the exchange current density

increases with temperature.

( c ) Ohmic polarization

‘Ohmic polarization’ is due to an ohmic resistance in a film, for example oxide

film or an absorbed film from the solution opposing the current flow, and thus causing an

ohmic potential drop. This ohmic polarization is given by:

r IRfη = (1.9)

Where Rf is the film resistance for the entire electrode surface, and I is the current

flow. ηr disappears in a few microseconds when the current stops.

1.3.3 Mass transport

If cathodic polarization is carried out too quickly, the ions cannot diffuse

sufficiently rapidly from the bulk of the solution to the surface, hence limiting current

density is reached, which results in a formation of a diffusion layer, in which either

ligands or metal ions vary in concentration from zero on the ‘metal’ side of this layer, to

the bulk value of the ‘solution’ side.

The mass transfer i.e., movement of particles in an electrolyte, is achieved by

migration, convection and diffusion.

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1.3.3.1 Migration

Migration is the movement of charged species due to a potential gradient. There is

always a potential gradient between the two electrodes in a cell when current is flowing.

It is a purely electrostatic phenomenon and in systems with a high concentration of inert

electrolyte, it need not contribute significantly to the transport of reactant and product,

since the inert electrolyte carries most of the charge.

1.3.3.2 Convection

Convection is the movement of species due to an external mechanical force. In

practice, convection is usually achieved by stirring or agitation of the electrolyte or by

flowing it through the cell.

1.3.3.3 Diffusion

Diffusion is the movement of species due to a concentration gradient. Conversion

of reactant to product at an electrode surface will always cause the concentration of the

reactant at the electrode to be lower than that of the bulk and hence further reactant will

diffuse the electrode.

1.3.4 Anodic dissolution

Anodic dissolution of metals takes place during corrosion, electrorefining and

electrowinning and electroplating of metals. Anodic dissolution is often complicated by

the formation of some insoluble oxides, hydroxides and salts, which retard the process.

The rate of anodic dissolution also depends upon the ionic composition of the electrolyte,

its pH and the presence of complexes. At sufficient positive potentials oxygen is liberated

at the anode. [21-23].

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In anodic process and in aqueous solutions, many metals get oxidized to different

degrees (depending upon potential). The mechanism of metal dissolution in active,

passive and tran passive states is differentiated by characteristic potential for each state

(Figure1.3). Ordinarily the specific adsorption of anions and water molecules on an anode

is viewed as the onset of the formation of salt hydroxide or a hydrated complex

respectively. During formation of multicharged ions the anode process takes place in

steps, one after the other involving a single electron electrochemical step. The first stage

usually proceeds with a minimum overpotential compared to the next. It has been

recorded that films of oxides, hydroxides and salts are absent on a metal situated in an

active state.

A study of the features of a compact metal dissolution reveals that ionization of a

metal proceeds in a known order i.e., later wise. In terms of energy, the pure

polycrystalline surface of (anodic) metal is not uniform. The atoms and groups of atoms,

most weakly bonded to other atoms of the crystal lattice possess excess free energy.

Therefore the surface layer comprises active centers. Anions, water molecules and

surface-active substances are adsorbed on these active centers. The larger the number of

defects in the crystal lattice the greater the number of active centers and faster the

reaction rate.

When an anode potential becomes still noble, the bond between the adsorbed

particle and the metal is strengthened; the particles are deformed on the electrical field

and on attaining some critical potential value, sharp retardation of the anode process sets

in (area b, in figure 1.3). Anodic current decreases to ip (current in passivated state). It

is of paramount importance to establish the causes of anodic passivation. Passivity can

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be eliminated by agitation, raising the temperature, and by ligand concentration in the

solution, or by introducing a suitable depassivator.

The potential at which an anodic current is maximum is known as the initial

potential of passivation, while the current at which E =Eccp is called critical current

passsivation. At E < Eact, the current increases, and E act is termed as Flade potential.

The magnitude of the Flade potential depends upon the nature of the metal, pH,

electrolyte composition, and the operating temperature.

1.3.5 Theory of electron transfer

1.3.5.1 Simple electron transfer

A simple electron transfer reaction scheme may be represented as

Ox + ne ↔ R

The net current of such a chemically reversible process may be given by

. ....of ox b R

oI nFAK C nFAK C= (1.10)

Where ‘I’ is the net current flow, n is the number of electrons involved, ’F’ is the

charge per mole, ‘Coox’ and C0 R’ are the surface concentrations (activity coefficients

would be assumed to be unity) of oxidant and reductant respectively, Kf and Kb are the

charge transfer rate constants for the forward and backward reactions respectively. The

specific feature of the electro chemical kinetics when compared to chemical kinetics is

that the electrochemical rate constants Kf and Kb are potential dependent. A part β [24]

of the applied electrochemical free energy nFE activates the forward reaction and another

part (1- β) retards the reverse reaction.

.o n F E R Tf fK K e β−= (1.11)

(1 ).o nFE RT

b bK K e β−= (1.12)

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Figure 1.3 Anodic polarization curve characterizing transition of metal to a state

of passivation. a- active region; b-region of transition from active state

to passive; c- passivation state; d-region of anodic liberation of

oxygen. ϕP- Faraday potential; ϕo2 – potential of oxygen evolution.

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At standard electrode potential E0 both the reaction rate constants would be equal.

This rate constant (Kh0) is the standard heterogeneous rate constant.

(1 ). .o nFE RT o nFE RT of b hK e K eβ β− −= K= (1.13)

Equation (1.10) may now be expresses in terms of Kh0

( ) (1 ) ( )o oo o nF E E RT o nF E E RTh ox Ri nFAK C e C eβ β− − − − = − (1.14)

The symmetry factor (β) and its significance has been discussed in detail by

many authors [25-27]. For more involved reaction schemes, a more general factor

describing the potential dependence of current is employed. This is the transfer

coefficient α. The current expression for such reaction is

( ) (1 ) ( )oo o nF E E RT o nF E E RTh ox RnFAK C e C eα α− − − − = −

o

i (1.15)

At equilibrium both the forward and backward currents must be equal (the

exchange current density i0) and the net current would be zero. In such a situation the

surface concentration of the electro active species would be equal to its bulk

concentration, cb ox and cb R. Hence

( ) (1 ) ( ). .o o

eq eqnF E E RT nF E E RTo b o bh ox h R onFAK C e nFAK Ce iα α− − − −= = (1.16)

Nernst equation may be obtained from (1.16) since this is the rate expression at

the equilibrium condition.

( ) (1 ) ( )oeq eqnF E E RT nF E E RTb b

ox RC eα α− − − −=

o

C e (1.17)

Therefore ( )oeq

bnF E E RT R

box

Ce

Cα− − = (1.18)

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Therefore lnb

o Req b

ox

CRtE EnF C

= − (1.19)

Using equation (1.16) and (1.18) a new relation between i0 and K0

h may be obtained

i n (1 ). .oh ox RFAK C Cα α−= (1.20)

Now dividing (1.14) by (1.20) and suitable rearrangement using (1.18), we may

obtain the most general current potential relationship for simple electron transfer reaction

where η, the over voltage is equal to (E-Eeq)

(1 ).

o onF RT nF RTox R

o b box R

C Ci i e e

C Cα η α η− −

= −

(1.21)

Several simplifications of this general expression are possible. For example if we

assume that the surface concentration is equal to bulk concentration i.e.

C0 ox= Cb

ox and C0R = Cb R, we obtain

(1 )nF RT nF RT

oi i e eα η α η− − = − (1.22)

For inherently slow electron transfer processes (i0<< I) and at very high

overvoltage values, (η>120 mV), one of the terms in the right of (1.22) would

predominate. For a cathodic process, η is negative and (1.22) becomes

0n F R TI i e α η−= (1.23)

The logarithmic form of this expression is the Tafel relation established in 1905

and widely used in voltammetric work.

02.303 2.303log logRT RTi inF nF

ηα α

= − (1.24)

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For fast electron transfer processes i0 would be very large and hence i/i0 << 1. If

this is assumed to be zero as limiting condition, (1.22) becomes

(1 )

o onF RT nF RTox R

b box R

C Ce e

C Cα η α η− −= (1.25)

Using equation (1.18) an equation similar to the Nernst equation (1.19) but which

contains the surface concentration instead of the bulk concentration is obtained

lno

o Roox

CRTE EnF C

= − (1.26)

The surface concentration would be equal to bulk concentration at equilibrium, or

at very low current densities or when the bulk concentration of electro active species is

substantially higher than the mill molar concentration used in electroanalysis. However,

at low concentrations employed in electroanalytical chemistry the mass transfer effect

necessarily enters into the electroanalytical problem.

1.3.5.2 Linear sweep and cyclic voltammetric studies of simple electron transfer

When large quantity of supporting electrolyte is employed, the mass-transfer of

electro active species by conduction becomes negligible, even if it is charged. In linear

sweep and cyclic voltammetric studies both the electrode and electrolyte are kept

stationary and thus no convective mass-transfer is involved. But since the potential

varies with time, the surface concentration also depends on time and hence Fick’s second

law of diffusion must be solved to estimate the surface concentration Cox (o, t)[28-34]

2

2

( , ) ( , )ox ox oxC x t D C x tt x

δ δδ δ

= (1.27)

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1.3.5.3 Reversible electron transfer

For a reversible electron transfer process, equation (1.26) is used as one of the

boundary conditions. The peak current (ip) for such reversible process at 25oC is given

by

Ip = 2.69 x 105.n3/2 ADox1/2 Cb

ox1ү

/2 (1.28)

This is often called the Randles-sevick equation. In this expression the peak

current is expressed in amperes is the number of electrons involved in the process, Dox is

the diffusion coefficient of the oxidant in cm2.sec-1. The bulk concentration is expressed

in moles.cm-3. The peak potential Ep for a reversible electron transfer process is

independent of sweep rate γ. The difference between Ep and peak potential at half the

peak current Ep/2 is given by the equation

56.5 Ep- Ep/2 = ---- mV at 250C (1.29) n

The cyclic voltammetric characteristics of reversible process may also be obtained

by solving the same equation (1.26) using (1.27) as the boundary condition. In this case

the anodic peak current Ipa is equal to Ip

c , the peak separation Ep= Epa -Ep

c is given by

56.5 Ep

a -Epc = ---- mV at 25 0C (1.30)

n

1.3.5.4 Irreversible electron transfer

For an irreversible electron transfer equation (1.11) is used as one of the boundary

conditions for solving eq.(1.27). The peak current and peak potential for irreversible

processes are given by

Ip = 2.98 x 105. α n1/2 AD1/2 Cboxү

1/2 (1.31)

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1 2( )0.78 ln lno o

P hRT D nFVE E KnF RT

αα

= − + −

(1.32)

Hence Ep shifts with sweep rate according to

dEp 30 ----- = ---- (1.33) d log V αn

Ep and Ep/2 are related by the equation

48 Ep- Ep/2 = --- mV at 250C (1.34) αn

In cyclic voltammetric experiments no anodic peak would be noticed in the

subsequent anodic sweep for an irreversible process.

1.3.5.5 Quasi-reversible process

For a quasi-reversible system the current potential relationship must be derived by

solving equations (1.27) with (1.21) a boundary condition. A complete solution to this

problem is also available [35]. The peak current and peak potential relations are more

involved. At slow sweep rates the peak current relation simplifies to a reversible process

(1.28) and at high sweep rates again it becomes identical to irreversible peak current

relation (1.31)

A very easily measurable parameter of quasi-reversible system is the potential

difference between the cathodic and anodic peak ∆Ep. This would be greater than 59/n

mV for a quasi-reversible process. For each ∆Ep value, Nicholson [36] has tabulated

corresponding Ψ function is given by equation (1.34).

2

1 2

( ) .( )

oox R h

ox

D D KD nFV RT

α

ψπ

= (1.35)

25

1.4 Properties of electrodeposits

1.4.1 Adhesion

An important property of electrolytic metal deposits is their adhesion onto the

basis metal. It can be defined as the “condition in which two surfaces are held together by

either valence forces or by mechanical anchoring or both together”. Good adhesion is

promoted by i) Strong bonding across the interfacial region, ii) low stress gradients,

either from intrinsic or applied stress, iii) absence of easy fracture modes, and iv) no long

term degradation modes [37,38]. In general, adhesion can be broken down into the four

categories such as interfacial adhesion, inter diffusion adhesion, Intermediate layer

adhesion and Mechanical interlocking [39]. General methods of testing adhesion of the

electrodeposits have been reviewed by Davies Whittaker [40] and H.Plog [41].

Qualitative tests are simple and have significance for the practicing platter, but are little

useful for precise specifications. The essentials of qualitative testing are based on the

development of tensile and shear stresses at the interface created by some severe local

deformation, created by heating or mechanical working. Adhesion on oxide films is

generally poor.

1.4.2 Stress

Electrolytically deposited metal coatings often have internal stresses, which

strongly affect the mechanical, technological behaviour of the electroplated metals.

Thus, tensile stress can lead to cracking and peeling off the coating, and also considerably

decreases the fatigue strength of parts. This internal stress, which can arise from several

sources, may be either tensile (tendency for the layer to contract) or compressive

(tendency for the coating to expand). It can be divided into two categories (i) stress

26

resulting from distortion due to lattice misfit at the interface between the coating and the

substance; and ii) intrinsic internal stress arising from particular plating conditions and

bath composition [42]. Stress is influenced by the presence of organic compounds in the

solution in addition to the plating variables. Many organic compounds are incorporated in

the formation of commercial plating solutions to act as ‘stress reliever. Occasionally

stress may serve a useful purpose. For example, in the production of magnetic films for

use in high-speed computers, stress in electrodeposited iron; nickel and cobalt

electrodeposits will bring about preferred directions of easy magnetization and other

related effects [43]. Two kinds of stress exist in coatings: differential thermal stress and

residual or instrinc stress [42].

1.4.3 Hardness

Hardness can be defined as the resistance of a metal to plastic deformation.

Hardness is due to the occurrence of one or more of the following phenomena: i). Internal

stress ii). Small grain size iii) incorporation of fine particles such as oxides in the lattice

iv) Preferred orientation. Hardness is usually determined by an indention technique, but

scratch tests and various types of abrasion or rubbing techniques can be employed. The

hardness of electrolytic deposits is measured with the Vickers or Knoop diamond tester

by the resistance offered to penetration when a hardened steel or diamond tipped

penetrator of precise geometry is impressed on to the surface under a known load. The

Vickers hardness number Hv, is calculated from the expression

1.845P Hv = --------- d2

27

Where‘d’ is the length of diagonal of the impression (um) and p is the load applied (g).

The hardness is often high with commercial bright deposits. The maximum hardness

values are attained in the alloy deposits, which frequently have pronounced lattice

defects.

1.4.4 Porosity

The term porosity includes many types of pore or channel, ranging from micro

channels extending from the surface of the deposit to the base metal of the cathode. Such

microscopic pores are not of great importance in case of Zn or Cd on steel because the

basis metal will be protected by the deposit and the deposits only corrode with reference

to the basis metal. However, in the case of tin or lead on steel, the basis metal will be

corroded through the pores and produce rust spots. The porosity of an electrodeposit is

influenced by several factors, which include the plate thickness, deposit structure,

additives and plating solution composition, pH, current density, temperature, anode

behavior, cathode surface and internal stress. There are several methods available to test

the porosity in electrodeposits. These are based on corrosion, gas permeability [44],

photography, audiography and adsorption.

1.4.5 Solderability

The solderability of a device lead refers to the ease and uniformity of wetting the

device lead by solder under the conditions used in the electronics industry. The solder

used in the solderability test contains 60-63 mass percent lead. Tin is the active

constituent promoting wetting and led is an inactive dilutent. In PCB assembly, there are

three aspects of solderability (i) thermal demand, (ii) wettability and (iii) resistance to

soldering heat. The following four factors affecting the solderability such as storage

28

condition, conditions of the platelayer, porosity of the platelayer and intermetallic

compound layer.

1.4.6 Corrosion

Corrosion may be defined as a reaction between a material and its environment

[45-47] This reaction results in a loss of the material either to the environment or to an

intermediate phase of an oxidized character through physical, chemical, or

electrochemical means [48,49]. A major function of electroplated coatings is the

protection of the basis metal against chemical attack by its environment, i.e. against

corrosion. For corrosion the primary variables are the coating metal, its thickness and the

service environment. Metallic coatings are of two types-single metal coatings and alloy

coating. In order to protect corrodible metal like steel several metallic coatings such as

copper, nickel, tin, zinc, cadmium etc. and its alloy have been used [50]. The following

factors are responsible for metallic corrosion. (i) air, temperature and time (ii) moisture

(water) (iii) ions such as halides, H+ or OH- (iv) contact between dissimilar metals and

(v) Stress.

Deposits applied by electrodeposition protect substrate metals in three

ways.1.cathodic protection, 2) barrier action and 3) environmental modification or

control. Cathodic protection is provided by sacrificial corrosion of the coating, e.g.,

cadmium and zinc coatings on steel. Barrier action involves use of a more corrosion

resistant deposit between the environment and the substrate to be copied. Examples of

this include zinc alloy automotive parts and copper-nickel-chromium and nickel-

chromium systems over steel. An example of environmental modification or control

29

coatings in combination with a no impervious barrier layer is electrolytic tinplate used in

food packaging.

1.4.6.1 Mechanism of corrosion prevention by metallic coatings

According to one theory [51] a macro cell is set up between the anodic and

cathodic portion of the corroding surface. Another theory [52,53] postulates that the

anodic dissolution and cathodic reduction can take place simultaneously on the same

metal and are distributed randomly over the entire metal surface. The combination of

these two theories gives rise to the electrochemical mechanism of corrosion [54]. The

principal steps in the corrosion process are:

Anodic process

M + nH2O Mn+ .nH2O + ne

Cathodic process in which the electrons are accepted by an atom or an ion capable

of being reduced

O2 + 2H2O + 4e- 4 OH- alkaline media

O2 + 4H+ + 4 e- 2H2O acid media

O2 + 2H+ + 4 e- 2OH- neutral media

Correspondingly

2H2O + 2e- H2 + 2OH-

2H+ + 2e- H2 and

H+ + H2O +2e- H2 + OH-

will also represent some cathodic reaction depending on the presence or absence of

oxygen and flow current. The current flow between the anode and cathode of the local

cell takes place in the corrosive medium in contact with the electrodes. Thus the metal

30

continuously goes into solution at anodic sites and depolarization takes place at cathodic

sites, resulting in metal corroding locally at the anode sites and changing of pH in the

region of the cathode.

1.5 Surface Examination

Optical microscopy, electron microscopy, electron probe microanalysis and X-ray

techniques are the tools available for the examination of structure and surface topography

of the electrodeposits.

1.5.1 Optical microscopy

This is suitable for examining surfaces or cross sections of electrodeposits [55],

but is restricted to a magnification of approximately 3000 times and depth of focus is

quite limited. When considering optical microscopy of surface coatings, resolution,

magnification and depth of field are major concerns. Low power metallurgical

microscope, particularly the binocular type is useful for examining relatively large

surface features such as pits and nodules, but is far inferior to the scanning electron

microscope. The metallurgical microscope differs from the biological microscope in that

the specimen being examined is observed using reflected rather than transmitted light.

1.5.2 Electron microscopy

The most frequently used instrument to reveal the structure of electrodeposits is

the scanning electron microscope (SEM). The major advantages of SEM over optical

microscope are resolution and depth of field. Typical point-to-point resolutions of

modern SEM are around 4 nm at accelerating voltages of around 30 KV. The major

advantages of the SEM instrument over the optical microscope are the large depth of

field. This enables rough surfaces to be examined in the SEM. The image appears in

31

focus as long as the feature lies within the depth of field. Typical depths of field of light

optical microscope and a SEM at a magnification of X 100 are 0.5 µm and 400 µm

respectively [56] The main disadvantages of the SEM is that is not possible to produce

electron diffraction patterns. Thus the crystal structure and orientation of grains cannot

be determined. Great care has to be taken when the SEM is used to determine the grain

size.

The transmission electron microscope (TEM) has better resolution than the SEM

and the capability to obtain electron diffraction patterns. By special techniques it is

possible to obtain atomic resolution in the TEM. The main disadvantage of the TEM is

that only thin specimens can be examined. Thus extensive specimen preparation is

required for most deposits. Recently a new class of microscopes has come into use that is

particularly suited for the study of the structure of electrodeposits. They are the scanning

probe microscopes. Their main advantage for the study of electrodeposits is that they can

operate in liquids. They can therefore be used for in situ studies. However, the scanning

probe microscopes cannot be used to observe continually changing processes because it

takes a relatively long time to form an image. They can have resolutions comparable to

TEM.

1.5.3 X-Ray Technique

X-ray techniques have been used to determine lattice parameters and the

orientation of electrodeposits. Of these, the radial scan method, which is essentially a

line scan through the center of a pole figure, is particularly useful for determining the

fiber texture of electrodeposits, which have been found to have a certain amount of

preferred orientation. The lane back reflection method is suitable for determining lattice

32

constants and is more accurate for this purpose than electron diffraction. Information

obtained using X-ray technique involves deeper surface also and hence it is difficult to

study the surface morphology alone.

33

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