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

Подождите немного. Документ загружается.

222

Chapter 9

A positive influence of water is attributed to a formation of dischargable

hydroxo-complexes, according to the reaction:

Discharge of these hydroxo-complexes is then given by the equation:

In the real systems, however, the discharge mechanism is more

complicated and involves side reactions such as evolution of HBr

formation and a possible polymeraization of the solvent and impurities.

Electrodeposited aluminum is of a higher purity (99.5 to 99.999 %) than the

anode, since the impurities and alloying elements are insoluble in the

electrolyte. Impurities are continuously eliminated by filtration of the

electrolyte. The pure aluminum layer becomes pore-free at a thickness

greater than The thickness of electrodeposited aluminum can reach

if some special applications are required. The aluminum coating has

the very good ductility, corrosion resistance, and when electropolished it is

convenient for the production of mirrors of high optical quality.

As mentioned above, the second group includes metals, which can be

electrodeposited from aqueous solutions (i.e., Cu, Zn, Ni, Co, Ag, Au etc..).

The electroplating of these metals from non-aqueous solutions at the present

does not have significant industrial applications, and therefore is mostly of

academic value. The research shows that electrodeposition of this group of

metals takes place in polar types of solvents (solvents that have active

functional groups such as CO, and in which, the same

type of salts used in the aqueous solutions are dissolved (providing they have

sufficient solubility). As examples of studied systems, electrodeposition of

various metals, such as Zn, Pb, In and Sn, from formamide, acetamide,

glycerol, ethanol, acetone etc. should be mentioned.

The results do not

show any advantage over electroplating of these metals from aqueous

solutions. Usually, deposition of these metals from non aqueous solutions

produces deposits of poor quality and maintaining of solution chemistry

encounters experimental difficulties.

9. Electroplating and Surface Finishing

223

Electrodeposition of metals from liquid ammonia has also been studied

(Ag, Cu, Pb, Hg etc.). However, no results of a practical value were obtained.

Any metals from this group can readily be deposited from aqueous solutions.

Electrodeposition from non-aqueous electrolytes is attractive for several

metals and alloys that cannot be deposited from aqueous solutions.

Significant results have been obtained with aluminum and its alloys.

Improvements in electrodeposition from non-aqueous electrolytes will

continue to grow due to desirable physico-chemical characteristics of

coatings (e.g., aluminum or its alloys) or reactivity of substrate with aqueous

solutions (e.g., uranium). Developments in other areas of technology (energy

conversion, advanced batteries, electronics etc..) may lead to requirements

for materials with specific properties and to advancement in the field of

electrodeposition of metals and alloys from non-aqueous electrolytes.

9.5

ELECTROPLATING FROM ROOM

TEMPERATURE MOLTEN SALTS

As with plating from non-aqueous electrolytes, electrodeposition of metals

from molten salts has attracted the attention of researchers from two different

reasons. The first is related to the search of platability of metals from molten

224

Chapter 9

salts, especially those that cannot be plated from aqueous solutions or non-

aqueous types of solutions. For those metals which are platable from aqueous

solutions e.g. zinc, the search for a convenient molten salts electroplating is

often related to a target to avoid the simultaneous hydrogen evolution reaction

and to obtain coatings of more desirable properties. In the electrodeposition of

zinc from aqueous solutions, the simultaneous hydrogen evolution reaction

significantly affects the embrittlement and sometimes reduces current

efficiency of the process. As a result an aprotic-plating bath is required.

Although there are well-established molten salts electrolyses, particularly

those related to the electrowinning of metals such as aluminum, magnesium,

sodium etc., attention in this section is directed towards the room

temperature molten salts electrodeposition. In this case, the hydrogen

reaction can be avoided, or deposition of metals that are not platable from

aqueous solutions may occur. The room temperature molten salts are based

on anhydrous They are analogous to the high temperature melts

with a difference that the NaCl is replaced with an aromatic

organic chloride. This replacement of NaCl results in lowering the melting

point well below room temperature, sometimes as low as – 50 °C. Several

types of organic aromatic chlorides have been investigated. This includes 1-

methyl-3-ethyl-imidazolium chloride (MEIC), l-(l-butyl) pyridinium

ammonium chloride (BPAC), 1, 2-dimethyl-3-propyl-imidazolium chloride

(DMPIC) etc. The most studied aromatic organic chloride for the room

temperature molten salts has been the MEIC. Room temperature molten salts

can be obtained from the combination of anhydrous and MEIC.

These chloroaluminate salts have a high ionic conductivity, good thermal

stability, a wide electrochemical window and adjustable Lewis acidity. The

mixtures are liquid at room temperature (about 25 °C) over the

range of composition from 40 to 67 mol. %

In the acidic melts, metal ions are believed to be only weekly complexed

or solvated by ions and thus can be reduced at more positive

potentials. These melts contain a molar excess of In the alkaline

melts, the metal ions are strongly complexed by chloride ions and exist as

discrete anionic chloride complexes, e.g. where z is the valence

of the metal ion. The alkaline melts contain a molar excess of MEIC. The

formation of strong anionic chloride complexes in alkaline region makes the

metal ions more difficult, or in some cases impossible to reduce within the

electrochemical window of the melt. Consequently, most of the studies on

the electrodeposition of metals from room temperature chloroaluminate

melts are carried out in the Lewis acidic composition region of the melt,

which contains a molar excess of

The Lewis acidity of and organic chloride donor (RCl) mixtures is

a function of the molar ratio, N, according to the equation:

9. Electroplating and Surface Finishing

225

Melts with the ratio N>1 (molar excess of are Lewis acidic, while

those with N<1 (molar excess of RC1) are Lewis basic. The equilibrium

constant for the above equation is estimated from the potentiometric

titration

at In acidic melts, the dominant anionic species are

and The electrochemical potentials are determined by

oxidation and reduction. Deposition of aluminum proceeds according

to the reaction:

Pure metals such as palladium, gold, tin, mercury, lead and zinc, have

been electrodeposited from the acidic chloroaluminate melts.

The

electrodeposition of some transition metals is complicated by the

codeposition of aluminum. The formation of alloys such as Co-Al, Cu-Al,

Ni-Al, Cr-Al, Fe-Al is observed at potentials several hundreds milivolts

more positive than the potential at which the bulk deposition of aluminum

occurs.

48-52

Experiments are performed in a glove box with an inert atmosphere

(usually nitrogen). To remove the protonic or other impurities, the

melt is purified by pre-electrolysis of the melt at constant current

density for several days, while the electrolyte is stirred. The electrolyte is

then filtered to remove aluminum particles, which are produced during the

pre-electrolysis step.

Metal-ions (i.e. Zn, Co, Cu etc.) are introduced in the melt by an anodic

dissolution of corresponding metals. For the electrodeposition process

materials such as glassy carbon, platinum, tungsten etc. are used as working

electrodes (cathodes). To maintain the concentration of the metal ion in the

melt constant, the counter electrodes (anodes) are usually prepared from the

same metal. For the electrochemical measurements, the aluminum reference

electrodes are commonly used.

The current efficiency of the plating process frequently achieves 100 %.

The surface morphology of deposited films can vary from smooth and dense

to nodular, porous and dendritic deposits, depending on the experimental

conditions.

Due to experimental difficulties there are not, at the present, commercial

applications of the room temperature molten salts for the electroplating

purposes. Although very promising results are obtained by far, significant

amount of research and engineering should be carried out in the future, in

order to apply these processes on industrial scale.

226

Chapter 9

9.6 ELECTROPOLISHING

Electropolishing is defined as a process of anodic dissolution of metals or

alloys in an appropriate solution resulting in production of improved

morphology and geometry of the surface and a shiny, bright and smooth

appearance. Technical advantages of the electropolishing include a reduction

in coefficient of friction, an increase in the magnetic susceptibility of some

magnetic materials, an increase in corrosion resistance and excellent

reflectance. In addition, electropolishing is widely used in the metallography

for the microscopic investigation of crystallographic structure of metals and

alloys. Some theoretical aspects of electropolishing are discussed in the

section 3.2.3.

Electropolishing as an anode process, is similar to electromachining,

however there are significant differences between them. For instance,

electropolishing is usually carried out from unstirred, concentrated acidic

solutions as electrolytes, at lower current densities, and with the electrode

separations of at least 1 cm. The quality of electropolished surfaces depends

on electrochemical conditions including anodic polarization, electrolyte

composition and microgeometry. It is determined by the appearance,

measurements of profiles with optical profilometers, and also using

microscopic techniques. In terms of the surface roughness, the two types of

electropolishing are distinguished. The first, commonly called anodic

levelling or smoothing, refers to the elimination of the surface roughness

with a height of more than The second type is called anodic

brightening and is referred to the elimination of surface roughness less than

However, this distinguishing between the smoothing and brightening

is a very approximate simplification, since there is no simple relationship

between measurements of profile and brightness.

A schematic presentation of the anodic current density – potential

relationship, during the electropolishing process is given in Figure 9.9. Four

distinguishable regions on this curve can be seen: AB, BC, CD and DE. In

the region AB, the anode dissolves. Under these conditions the surface

microroughness does not disappear. The electropolishing takes place under

mass transfer conditions, at limiting current density and in the potential

range between and The surface of the electrode becomes smooth and

microroughness decreases. When the potential approaches the value of

the surface of the metal becomes bright. An increase in the potential above

leads to a rapid increase in the current density, causing in this way an

increase in the roughness of the electrode surface, due to metal dissolution

and simultaneous oxygen evolution reaction.

9. Electroplating and Surface Finishing

227

A decrease in the microroughness during electropolishing is a

consequence of current distribution at the electrode surface. The surface of

an electrode profile, with distinguished protrusions and depressions is

schematically presented in Figure 9.10. It seems that the protrusions are

more accessible to the current flow than depressions. Therefore, under the

conditions of electropolishing, protrusions will dissolve faster, thus leading

in this way to a decrease in the microroughness. Thermodynamically, it is

more probable that the preferential transformation of solid phase (i.e., metal

crystals) into the ionic (solvated) phase would take place at protrusions than

in depressions. This is due to smaller energy of transformation of ions from

solid to solvated phase at protrusions than in the depressions. In practice, the

anodic dissolution frequently deteriorates microtopography due to unequal

localized etching.

The formation of a passive oxide layer at the anode surface plays crucial

role during the electropolishing.

53-55

Electrolytes used for electropolishing

contain substances (for example which form an oxide passive film,

and acids, i.e. or which dissolve this passive film. Typical

formulations used in the electropolishing of aluminum, copper and stainless

steel are given in table 9.3.

228

Chapter 9

Electropolishing is carried out in concentrated aqueous solutions of

phosphoric acid, sulfuric acid or their mixtures, and sometimes in

combinations of perchloric and acetic acids. There are also formulations in

which, methanol is used instead of water.

If the rate of passive film formation is less then the rate of dissolution, the

anode surface is rather etched, leading to an increase in the microroughness,

9. Electroplating and Surface Finishing

229

due to non-uniform coverage with oxide passive film. On the other hand,

when the rate of passive oxide film formation is more than the rate of

dissolution, the film thickness increases covering the anode surface. In this

way the electropolishing is not achieved. It is obvious that the

electropolishing occurs when the rates of the passive film formation and

dissolution are comparable.

The levelling of the electrode surface during electropolishing is a

consequence of a non-uniform passivation of protrusions and depressions. It

seems that the depressions are better covered with passive films than the

protrusions, leading in this way to faster dissolution of protrusions than

depressions. Passivation of protrusions to a lesser degree is explained by an

increased chemical activity, due to a faster rate of diffusion of metal ions on

protrusions than on depressions.

The rate of anodic levelling is equal to the difference in dissolution rate

between protrusions and depressions on a rough surface. It is determined by

the predominant current distribution on the surface profile. Consequently,

the rate of anodic levelling is dependent on geometrical, electrochemical and

hydrodynamic parameters.

53,57

While the influence of the geometry of the surface remains qualitatively

the same, the charge transfer overvoltage (secondary current distribution)

tends to reduce the rate of anodic levelling.

When the concentration

overvolatge is present (tertiary current distribution), two cases are

distinguished. Below the limiting current, potential and mass transport

influence the current distribution, and, therefore, the rate of levelling. This

situation is of a little interest for the practical applications. At the limiting

current density, the current distribution depends only on mass transport.

For the anodic levelling and anodic brightening terms macrosmoothing

and microsmoothing were introduced by Edwards.

The macrosmoothing

results from local differences in the current distribution on the surface profile

or the concentration of the transport limiting species, and is preceded by

microsmoothing. Macrosmoothing is a consequence of a higher current

distribution on protrusions, which causes higher distribution rates.

Microsmoothing results from the surface kinetics and passivation

behavior, due to suppression of the influence of surface defects and

crystallographic orientations on the dissolution process. The microsmoothing

occurs when dissolution of metal is mass transport controlled, which

corresponds to the limiting current plateau. In most electropolishing systems,

the rate of transport of dissolution products from the anode into the bulk

solution determines the limiting current.

The rate of levelling at the limiting current density may theoretically be

predicted on the basis of Nernst diffusion layer model. The Nernst diffusion

layer, for a triangular profile with the height, is presented in Figure

230

Chapter 9

9.11. The broken line represents the outer limit of the Nernst diffusion layer

for an ideal microprofile. In an ideal case, when the diffusion layer

should follow the surface profile as indicated by the broken line. Under these

conditions, the current density is uniform and only the geometrical levelling

should occur. However, due to local hydrodynamics, the situation is more

complicated, since the ratio between and can change significantly. For

this case a detailed modelling of the hydrodynamic perturbation, caused by

the surface microprofile is required for a quantitative prediction of the rate of

levelling.

In the aqueous solutions, during the high dissolution rate, the surface

concentration of dissolution products is in a reasonable agreement with the

saturation concentration of the corresponding metal salt. In the concentrated

acid type solutions the surface concentration of products of

dissolution significantly exceeds the saturation concentration, which

probably causes formation of metastable species at the anode surface. During

the electropolishing, a low water concentration may reduce the limiting

current by decreasing the saturation of metal ions produced due to

dissolution.

There is general agreement among researchers that electropolishing

occurs when the reaction rate is controlled by mass transfer. In an attempt to

explain the electropolishing process the following mechanisms appeared in

the literature:

(i)

(ii)

salt – film mechanism, and

acceptor mechanism.

9. Electroplating and Surface Finishing

231

The salt – film mechanism is based on an assumption that the surface

concentration of the metal ions due to dissolution is very high that exceeds

solubility and causes the precipitation of a salt film. The rate of reaction is

then determined by the rate of diffusion of metal ions away from the

electrode surface.

In the acceptor mechanism, the metal ions produced from the dissolution

remain on the electrode surface until they are complexed by an “acceptor”

species, which include either an anion or water. The rate of reaction, for this

case, is determined by mass-transfer of the acceptor to the electrode surface.

Consistent with the acceptor mechanism, the reactions describing dissolution

of copper may be presented as follows:

and

The experimentally obtained limiting current plateaus for electropolishing of

copper in concentrated phosphoric acid (85 % solution) at different rotation

speed are presented in Figure 9.12.

As this figure shows, the limiting

current plateaus extend over a potential range of 1 V. Above the current

plateau, the oxygen evolution reaction most probably takes place. According

to the results presented in Figure 9.12, the value of the limiting current

density depends on the rotation speed. An increase in the rotation speed

leads to an increase in the limiting current density.

In Figure 9.13 are presented dependencies of limiting current density on

the square root of rotation speed for temperatures 30 °C, 40 °C and 90 °C.

Based on their experimental results , Vidal and West supported the

acceptor mechanism theory, in which the limiting current density is given by

the Levich equation:

where is the limiting current density, F is the Faraday constant, is the

diffusion coefficient of the acceptor species (probably water), is the

kinematic viscosity, is the rotation speed and is the concentration of the

acceptor species. They assumed that the concentration of the acceptor is