Popov K.I., Djokic S.S., Grgur B.N. Fundamental Aspects of Electrometallurgy

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Chapter 3

density. The value of the deposition overpotential is larger than in the case of

cadmium and the crystallisation overpotential is lower, resulting in a

decrease in the zero nucleation zone radius, and hence a considerably larger

nucleation rate. A further decrease in the exchange current density value, as

in the case of Ni, leads to the situation shown in Fig. 3.10. A surface film is

formed, but it is porous, probably due to hydrogen co-deposition.

3. Surface Morphology of Metal Electrodeposit

Hence, a decrease in the value of the exchange current density of the

deposition process enhances thin surface metal film formation on inert

substrates due to an increase in the nucleation rate and a decrease in the

radius of the zero nucleation zones. As a result of this, a compact surface

metal film is formed with a smaller quantity of electrodeposited metal, and

its coarseness and porosity decrease with a decreasing exchange current

density. On the other hand, at sufficiently negative equilibrium potentials

and low hydrogen overpotential for an inert substrate, decreasing the

exchange current density of the deposition process can produce a porous

deposit due to hydrogen co-deposition.

3.1.5

Deposition from complex salt solutions

The silver deposits obtained from a nitrate bath at overpotentials

corresponding to an initial current densities of and from the ammonium

complex salt bath at an overpotential corresponding to the current density of

are presented in Fig. 3. 11

. The solutions are the same as in section

3.1.2.2. It can be seen that the deposit obtained from the nitrate bath consists

of a small number of nuclei and that boulders are formed even at

which leads to the formation of a non-compact deposit.

On the other hand the deposit obtained from the ammonium complex salt

bath is microcrystalline.

The poor microthrowing power of deposits obtained from nitrate

solutions at smaller current densities can be explained in the following way.

Chapter 3

For the deposition overpotential is given by:

according to Eq. 2.41, where:

and, for and from Eqs. 3.11 and 3.12.

Thus, at low current densities, poor microthrowing power is expected. In

the ammonium complex salt solution, For and Eqs. 3.11,

3.12 and 2.37 are valid which means that N > 0. Hence, for deposition at low

current densities, decreasing lead to increasing coverage of an inert

substrate for a given quantity of deposited metal.

3.1.6 Deposition in the presence of adsorbed additives

3.1.6.1

Inorganic compounds

In order to illustrate the effect, silver was deposited onto Ag and Pt

substrates from aqueous solutions containing in 100 g

3. Surface Morphology of Metal Electrodeposits

Fig.

3.12a

and in

Fig 3.12b. Galvanostatic and potentiostatic deposition

conditions were applied in an open type cell

20-22

Silver deposits formed from a containing electrolyte on to a Pt

substrate with galvanostatic current pulses are shown in Fig. 3.12b. At a

current density of (i.e. under the optimal film deposition

conditions, as determined in Ref. 20), almost complete surface coverage was

achieved even with a charge quantity of

This is probably due to the possibility of further nucleation occurring

immediately next to the already existing nuclei, as a result of the smaller

values of the radii of the nucleation exclusion zones. Obviously, this is due

to the decrease of the exchange current density for the deposition process.

For comparison, in phosphate-free nitrate solution, a compact Ag film had

not been deposited even after had been passed through the

cell, as can be seen from Fig. 3.12 a.

3.1.6.2

Organic compounds

The electrolyte used in all experiments was a solution of

in to which was added

poly(oxyethylene alkylphenol) (9.5 mol ethylene oxide)

The overpotential-log(current density) plot is given in Fig. 3.13. A

well-defined Tafel line characterised by and

Chapter 3

was observed at higher potentials also. This phenomenon is explained

by the formation of a film of the organic additive which completely covers

the cathode at sufficiently negative potentials

24,25

. Tafel linearity was also

observed over a short overpotential range at low overpotentials. The values

of and obtained in this case are close to

the values expected for deposition from a pure solution

It has recently been shown

that the optimum plating overpotential and

current density are determined by the upper limit of the validity of the

Tafel equation for the deposition process (see also section 3.2.1.3.1). In

this case, as can be seen in Fig. 3.13, the optimum deposition

overpotentials are about 530 mV and 40 mV in the presence and the

absence of adsorption of the aditive respectively. Cadmium deposits

thick obtained at 40 mV and 530 mV are shown in Fig. 3.14. It can be seen

that the deposits obtained at 40 mV have a large grain size, whereas those

obtained at 530 mV are fine grained, due to the larger overpotential.

3. Surface Morphology of Metal Electrodeposits

3.1.7

Conclusions

A surface metal film on an inert substrate is formed by the coalescence of

growing grains developed from corresponding nuclei, as is illustrated in Fig.

3.15, whereby the surface properties of the inert substrate are changed to

those of the electrodeposited metal.

Chapter 3

It is obvious that the larger nucleus density, the thinner is the thickness of

the metal film required to isolate the substrate from the solution. At the same

time a thinner surface film will be less coarse than a thicker one. This means

that a smoother and thinner surface film will be obtained at larger deposition

overpotentials and nucleation rates, i.e. by electrodeposition processes

characterized by high cathodic Tafel slopes and low exchange current

densities.

It is obvious that the discussion concerning the effect of the value of

exchange current density on the nucleation exclusion zone radius is not

connected to the mechanism of surface film formation but to the mechanism

of nucleation itself and the saturation nucleus number density. Papers

dealing with three-dimensional growth and related phenomena

are mainly

concerned with the determination of the mechanism of the formation of a

surface film and are unimportant from the point of view of the estimation of

the deposit thickness required to isolate a substrate from an electrolyte

solution. For this purpose, the saturation nucleus density, or better to say the

distribution of the distances between nearest neighbours is much more

important. It is obvious that half of the largest distance between nearest

neighbours

8,29

(as illustrated by Fig. 3.15) is the radius to which each grain

must grow to produce a nonporous thin metal film. It is clear that the

distribution of the distances between nearest neighbour crystallites is the

most important dependence in the treatment of the thin metal film formation

on an inert substrate. From the corresponding histograms, as shown by

Milchev et al

8,29

, it is possible to estimate the radius of the nucleation

exclusion zones, as well as the maximum distance between nearest

neighbour crystallites, which determines the thickness of a deposit required

to isolate the substrate from the electrolyte solution, as illustrated in Fig.

3.15. If the distance between the nearest neighbour crystallites is smaller

than the grain radius the deposit will overlap resulting in coarse deposit

growth initiation.

Apart from the nucleation density, the preferential orientation of the

nuclei is important in surface metal film formation. As the deposit becomes

free of the influence of the substrate structure on thickening, instead of the

formation of a randomly oriented grain structure, a preferred crystal

orientation can develop, which gives a definite texture to the cross section of

the deposit

. Texture can be expressed in terms of degree of orientation of

the grains constituting the deposit.

It is to note a the theoretical approach to the problems of deposit

orientation was successfully developed by Pangarov et al

3-5

. Using this

3. Surface Morphology of Metal Electrodeposits

theory, it was possible to determine the preferred orientation as a function of

overpotential from silver single crystals

to nickel and iron thin films

3,5

3.2 THICK COMPACT METAL

ELECTRODEPOSITS

After formation of a thin surface metal film on an inert substrate further

deposition takes place in the same way as on a massive electrode of the

metal, the ions of which were reduced to form the electrodeposit. The final

thickness of the electrodeposited surface layer varies from the order of ten

micrometers in electroplating, to many times larger in electrowinning and

refining of metals. The surface of deposits obtained in electroplating must be

smooth and bright, in the other cases the surfaces of the deposit surface has

to be as smooth as possible. How this can be achieved will be revealed in

this section.

3.2.1

Coarse surfaces

3.2.1.1

Mathematical models

Any solid metal surface that represents a substrate for metal deposition

possesses a certain roughness. In addition, it may appear coarse or smooth,

and this is not necessarily related to the roughness. Fig. 3.16 shows cases of

surfaces with a) the same roughness and profoundly different coarseness and

b) vice versa.

Chapter 3

It is the level of coarseness which determines the appearance of metal

deposits, while even with considerable roughness, if below the visual level,

the surface may appear smooth.

It is convenient to define the surface coarseness as the difference in

thickness of the metal at the highest and lowest points above an arbitrary

reference plane facing the solution. In early models used to describe the

surface by periodic functions this is equal to twice the amplitude of the

function

31,32

Historically, it was first established that under certain conditions of

dissolution the surface coarseness tends to decrease

. Krichmar

was the

first to point out that in some cases of deposition, under conditions

somewhat analogous to those in which the coarseness decreases, the opposite

effect occurs; i.e., in prolonged cathodic reduction, under conditions at

which, the process is close to being under complete diffusion control,

amplification of the surface coarseness occurs.

Taking a sinusoidal profile for the electrode surface:

Krichmar

obtained the relationship:

for where

and

In the above equations a is the wavelight of the sinusoidal profile, F is

the Faraday constant, H is the local elongation, is the initial amplitude of

the sinusoidal profile, is the amplitude of the sinusoidal profile at time

t, j is the current density, is the limiting diffusion current density, V is the

molar volume of metal, n is the number of electrons, Q is the quantity of

3. Surface Morphology of Metal Electrodeposits

electricity, t is the time, x is the co-ordinate normal to the plane of the

electrode and is the thickness of the diffusion layer.

Simpler mathematics were used in another, independently derived theory

of the same phenomenon put forward by et al

, and Diggle et al

. A

somewhat simplified treatment will be given here.

Consider the model of the surface irregularity shown in Fig. 3.17. The

surface irregularity is buried deep in the diffusion layer, which is

characterized by a steady linear diffusion to the flat portion of the surface.

The current densities at the various parts of the surface are as follows.

a) At the flat part of the surface, the limiting diffusion current density is

that for steady-state linear diffusion, i.e.,

where D is the diffusion coefficient and is the bulk concentration of the

depositing ions.

b) At the side of an irregularity, even when a possible lateral diffusion

flux supplying the depositing ions is neglected, the current density, at any

point of height must be larger than the current density, j, at the flat part of

surface.