Cathodic deposition of zinc from dilute solutions onto a rotating disc electrode

Article abstract of DOI:10.1023/B:RJAC.0000030355.85549.d6

The cathodic deposition of zinc onto a rotating disc electrode was studied. The influence exerted by various parameters on the process was examined. The observed phenomena were explained in terms of corrosion and the positive difference-effect.

Full text of DOI:10.1023/B:RJAC.0000030355.85549.d6

Russian Journal of Applied Chemistry, Vol. 77, No. 2, 2004, pp. 222 225. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 2, 2004,
pp. 226 229.
Original Russian Text Copyright
2004 by Kolosnitsyn, Yapryntseva.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Cathodic Deposition of Zinc from Dilute Solutions
onto a Rotating Disc Electrode
V. S. Kolosnitsyn and O. A. Yapryntseva
Institute of Organic Chemistry, Ufa Scientific Center, Russian
Academy of Sciences, Ufa, Bashkortostan, Russia
Received May 22, 2003
Abstract The cathodic deposition of zinc onto a rotating disc electrode was studied. The influence exerted
by various parameters on the process was examined. The observed phenomena were explained in terms of
corrosion and the positive difference-effect.
Purification of sewage to remove heavy-metal ions
is a problem of current importance for many industries.
Various methods can be used for sewage purification,
but electrochemical methods, such as electrocoagula-
tion, electroflotation, electrodestruction, and electro-
deposition of metals, are of particular interest. The
electric treatment makes it possible to purify wastewa-
ter of varied composition and dispersity without rais-
ing the content of salts in a water being purified and,
rather frequently, with precipitates not formed at all
or formed in a considerably decreased amount [1].
Among the electrochemical methods mentioned above,
the electrolysis is of special interest, since it can be
performed without use of chemicals in the course of
purification and yields a metal in the form of a target
product, which simplifies its further processing.
EXPERIMENTAL
Zinc was electrodeposited in a glass cell with a ro-
tating cathode. The cell volume was 0.5 l. As cath-
odes were used discs made of stainless steel, sheet
2
copper, and aluminum. The cathode area was 160 cm .
Semi-discs made of titanium gauze coated with ru-
thenium(IV) oxide served as anodes. Prior to the ex-
periments, the cathodes were trimmed with an emery
paper and washed in the electrolyte solution.
The electrodes were polarized in the galvanostatic
mode with a B5-49 source of stabilized current and
voltage. The voltage was monitored with a V7-27 A/1
voltmeter.
The rate of rotation of the disc electrode was de-
termined and controlled using a tachometer.
The electrodeposition of metals from dilute solu-
tions has its own specific features, which consist in
that the process rate is limited by delivery of a re-
agent to the electrode surface. This problem can be
solved by using a rotating disc electrode instead of
an immobile electrode. As shown previously [2], us-
ing a rotating disc cathode allows the rate of copper
electrodeposition from dilute solutions to be increased
by a factor of 5 10, with a high current efficiency
(CE) retained.
In this study, we continue to examine the electro-
Fig. 1. Effect of the rotation rate
ciency CE by zinc in cathodic deposition from dilute so-
on the current effi-
deposition of metals onto a rotating disc electrode and
the cathodic deposition of zinc from solutions with
lutions. Supporting electrolyte 0.5 M Na SO , c
Zn
2+
=
2
4
1
1
concentrations of up to 1 g l .
100 mg l , pH 6, copper cathode.
1
070-4272/04/7702-0222 2004 MAIK Nauka/Interperiodica

CATHODIC DEPOSITION OF ZINC FROM DILUTE SOLUTIONS
The electrolyte solutions were prepared by the
223
gravimetric method by dissolving the appropriate dry
salts in distilled water. The current efficiency was
determined from the difference of the zinc concentra-
tions before and after the electrolysis. The content of
zinc(II) was determined by direct complexometric
titration.
The duration of electrolysis was calculated on
the basis of the initial concentration of zinc ions in
the solution, so that the amount of electricity passed
through the cell corresponded to the complete re-
covery of metal.
Fig. 2. Effect of the pH of the initial solution on the
CE by zinc in cathodic deposition. Supporting electrolyte
1
2
0.5 M Na SO , c
2
+
= 100 mg l , i = 1 mA cm
52.3 rad s , copper cathode.
,
2
4
Zn
1
=
The dependence of CE on the rate of cathode rota-
tion is shown in Fig. 1. It can be seen that CE is 5%
for the immobile electrode. As the rotation rate is
1
raised to 52.3 rad d , CE increases to 35 40%.
With the rotation rate raised further, CE somewhat
decreases.
The pH value of the electrolyte solutions also af-
fects the current efficiency. The dependence of CE on
the solution pH passes through a maximum at pH 7
(
Fig. 2).
Irrespective of the initial concentration of zinc ions,
the dependences of CE on current density also pass
through a maximum (Fig. 3), whose position shifts to
higher current densities as the concentration of zinc
ions in the electrolyte increases.
With a stainless steel electrode, no zinc is depos-
ited onto the cathode, whereas with copper and alu-
minum electrodes, the cathodic deposition of zinc is
rather effective (Fig. 4).
To account for the dependences obtained, it is nec-
essary to note that the electrodeposition of metals,
which stand to the left of hydrogen in the electro-
chemical series, is almost always complicated by
a concurrent reaction of hydrogen evolution. In addi-
tion, electronegative metals can displace hydrogen
from water, i.e., they may be subject to corrosion with
a hydrogen and oxygen depolarization. Therefore,
the CE by zinc in cathodic deposition will be deter-
mined by the relative rates of three processes: zinc re-
covery, hydrogen evolution, and corrosion of zinc
from the electrode surface. It is also necessary to note
that amphoteric metals, such as zinc, exhibit a positive
difference-effect in cathodic polarization, which orig-
inates from destruction of a protective zinc oxide film
upon alkalization of the near-cathode layer of the so-
lution through cathodic evolution of hydrogen.
Fig. 3. Current efficiency CE by zinc in cathodic deposition
from dilute solutions with various concentrations vs. cur-
rent density i. Supporting electrolyte 0.5 M Na SO ,
2
4
1
The flux N of a solution across the boundary dif-
fusion layer, which appears on a rotating cathode and
pH 6,
= 52.3 rad s , copper cathode. Zinc concentra-
1
tion (mg l ): (a) 5, (b) 100, and (c) 1000.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 2 2004

224
KOLOSNITSYN, YAPRYNTSEVA
At the same time, at high flow velocities of the
electrolyte, the cathodically formed alkali is removed
from, and the oxidant (e.g., oxygen) delivered to
the cathode surface. As a result, the surface of fresh-
ly deposited zinc has enough time to be covered
with a protective oxide film. This explains why CE
increases when the rate of cathode rotation is raised to
5
2.3 rad s 1 (Fig. 1). However, as the rotation rate
increases further, CE decreases somewhat, which is
probably due to the fact that, at higher flow velocities
of the electrolyte, the corrosion becomes the so-called
impingement corrosion. In this case, the passivating
films, which were formed and existed before the onset
of corrosion, are torn off the metal surface by a liquid
jet, so that the metal without a protective film under-
goes intense corrosion [5].
Fig. 4. Variation of zinc concentration c
Zn2+
in the
course of electrolysis with cathodes made of different
materials. Supporting electrolyte 0.5 M Na SO , pH 6,
2
4
1
=
52.3 rad s . Cathode: (1) copper and (2) aluminum.
(
) Duration of electrolysis.
is at rest with respect to it, is directly proportional
to the diffusion coefficient D of the solution and to
the gradient of the solution concentration c and in-
versely proportional to the thickness of the boundary
diffusion layer : [3]:
The shape of the obtained CE-vs.-pH curve (Fig. 2)
can be accounted for by the influence of acidity on
processes of hydrogen evolution and zinc corrosion,
which occur simultaneously at the cathode.
N = D c/ .
(1)
It is known that the dependence of the hydrogen
overvoltage on the solution pH passes through a max-
imum at pH 7 [6]. Because zinc is an amphoteric met-
al, the rate of its corrosion also depends on the solu-
tion pH. The corrosion rate is the lowest in neutral
solutions and increases dramatically in acid and al-
kaline solutions [5, 7].
In turn,
can be calculated as follows [4]:
=
1.61D1/3 1/6 1/2,
(2)
where is the kinematic viscosity of the electrolyte
2
1
solution (cm s ), and
rotation of the electrode (s ).
is the angular velocity of
1
Thus, the appearance of a peak in the CE-vs.-pH
curve (Fig. 2) is due to the fact that, in acid and al-
kaline solutions, the processes of zinc corrosion and
hydrogen evolution prevail over the cathodic deposi-
tion of zinc.
The concentration difference is given by
c = c0
c ,
(3)
c
where c is the concentration of ions in the solution
0
The effect of current density on the CE by zinc in
cathodic deposition was studied in solutions with ini-
tial pH 6. The appearance of extrema in the CE i
dependences is probably due to the fact that, at low
rates of the cathodic process, the pH of the near-
electrode layer increases, but does not exceed 7. Under
these conditions, the rates of hydrogen evolution and
zinc corrosion are the lowest. Therefore, the CE by
zinc in its cathodic recovery increases with current
density at low polarizing currents. After the current
densities at which pH 7 is established in the near-elec-
trode layer are attained, the rates of cathodic hydrogen
evolution and zinc corrosion start to increase. There-
fore, CE by zinc starts to decrease after a certain
polarizing current density is reached.
bulk, and c is the concentration of ions at the elec-
c
trode surface.
The latter concentration can be calculated as fol-
lows:
cc = c0
i /(DFz ),
(4)
c
i
where i is the current flowing to the cathode, and
c
zi is the ion charge.
Substituting the expression for the boundary layer
thickness into Eq. (4), we obtain
cc = c0
1.62ic 1/6/(D1/3Fzi 1/2).
(5)
According to (5), the concentration of ions at the
cathode surface increases with the rotation rate. This
allows electrolysis to be performed at higher current
densities.
Presumably, the shift of the peak positions to
higher current densities, observed in the CE i de-
pendences as the concentration of zinc(II) increases, is
due to acceleration of the cathodic recovery of zinc
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 2 2004

CATHODIC DEPOSITION OF ZINC FROM DILUTE SOLUTIONS
225
owing to a decrease in the overvoltage. In this case,
the rates of other concurrent processes remain virtual-
ly unchanged.
increases and then decreases as the current density
becomes higher. The pH value of the electrolyte solu-
tions, at which the CE by zinc is at a maximum in
electrodeposition from dilute solutions with concen-
When zinc is deposited onto the surface of metals
that have a more positive standard potential than zinc,
it forms with them short-circuited galvanic microcells
1
tration of up to 1 g l , is 6 7.
REFERENCES
[8], in which the more electropositive metal is a cath-
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a soluble anode. The dissolution of zinc is limited by
the cathodic evolution of hydrogen, which, in turn, is
dependent on the overvoltage of hydrogen evolution
on a given metal.
1
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tion on metals that constitute stainless steel (such
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aluminum. Therefore, the rate of electrochemical cor-
rosion of zinc on the surface of a stainless steel elec-
trode is such that we do not observe its deposition.
At the same time, the deposition of zinc onto copper
and aluminum is rather effective, with the CE by zinc
in cathodic deposition being as high as 30 40%.
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CONCLUSION
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The study performed demonstrated that the deposi-
tion of zinc from dilute aqueous solutions onto a ro-
tating disc electrode is complicated by intense cor-
rosion. On an immobile cathode, the current efficiency
does not exceed 5%. On a rotating electrode, the cur-
rent efficiency by zinc grows with the rotation rate,
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2.3 rad s . Irrespective of the initial concentration
1
5
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RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 2 2004