Electrodeposition of lustrous zinc coatings from sulfate electrolyte

Article abstract of DOI:10.1007/s11167-005-0012-x

The electrodeposition of lustrous zinc coatings from a sulfate electrolyte containing ZnSO₄, Na₂SO₄, buffer additives, and 2-butyne-1,4 diol was studied.

Full text of DOI:10.1007/s11167-005-0012-x

Russian Journal of Applied Chemistry, Vol. 77, No. 8, 2004, pp. 1268 1272. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 8, 2004,
pp. 1284 1288.
Original Russian Text Copyright
2004 by Medvedev, Makrushin.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Electrodeposition of Lustrous Zinc Coatings
from Sulfate Electrolyte
G. I. Medvedev and N.A. Makrushin
Novomoskovsk Institute, Mendeleev Russian University of Chemical Engineering,
Novomoskovsk, Tula oblast, Russia
Received November 25, 2003
Abstract The electrodeposition of lustrous zinc coatings from a sulfate electrolyte containing ZnSO4,
Na2SO4, buffer additives, and 2-butyne-1,4 diol was studied.
Sulfate zinc-plating electrolytes are now used for
coating of articles of a simple shape. To obtain high-
quality coatings, various organic substances, such as
dextrin, condensation products, and various mixtures,
are added to the electrolyte [1]. At present, zinc is
electrodeposited using electrolytes with luster-forming
additives, which are organic substances of a rather
complex composition [2 4].
ac bridge at a frequency of 30 kHz in a series equiv-
alent circuit. As the working electrode served a zinc-
coated platinum wire situated at the center of a plati-
num-plated cylinder. The pH value of the near-elec-
trode layer was measured by the method described in
[7]. The electrodeposition was performed at 18 25 C
with and without agitation of electrolytes with a
magnetic stirrer.
In this communication, we report the results ob-
tained in a study of Zn electrodeposition from a sul-
fate electrolyte in the presence of 2-butyne-1,4-diol
(BD), which is widely used in deposition of lustrous
nickel and zinc coatings [3, 5]. The electrolyte con-
The influence exerted by the BD concentration,
nature of the buffer solution, and current density ic on
the outward appearance of the coatings is illustrated
in Table 1. As can be seen, lustrous coatings are ob-
tained in H3BO3- and Al2(SO4)3-containing electro-
1
1
tained (g l ): ZnSO4 7H2O 200 250, Na2SO4
lytes at a BD concentration of 30 45 ml l , and
10H2O 50 100, Al2(SO4)3 18H2O, H3BO3, and
aminoacetic acid 25 30; 2-BD (35 wt % solution)
1 70 ml l . A 10 12- m-thick zinc layer was de-
posited onto unpolished St.3 steel. The polarization
curves were measured in the potentiodynamic mode
with a P-5828 potentiostat. The leveling power P of
the electrolytes was determined by a direct method in-
volving a profilographic measurement of the sample
surface with a sine-shaped microprofile and calcula-
tion by the formula [6]
in an electrolyte containing aminoacetic acid, at 60
70 ml l . The ic range in which lustrous coatings are
1
1
formed varies with the nature of the buffer solution.
It is the widest in a Al2(SO4)3-containing electrolyte
2
(ic = 1 6 A dm ). It should be noted that high-
quality lustrous coatings are obtained solely in an
agitated electrolyte, and lusterless and rough coating
are formed without agitation.
The pH value of the electrolytes should be within
3 4. At pH < 3, the ic range in which lustrous coat-
ings are formed is narrower and the current efficiency
(CE) is lower. At pH > 4, semilustrous coatings are
formed, and the electrolyte operation is unstable.
The current efficiency was studied in the electrolytes
ensuring formation of lustrous coatings. It was found
that, in electrolytes containing BD, Al2(SO4)3, and
H0
Hi
a
2
P = 2.3
hav log
,
where a is the amplitude of the wave of the sine-
shaped microprofile; hav , the average thickness of
the coatings (10 m); and H0 and Hi, the initial and
final wave amplitudes of the sine-shaped profile, re-
spectively.
2
H3BO3, CE varies at ic = 1 6 A dm within 93
95%. At the same time, in an electrolyte containing
2
The capacitance of a double electric layer was
measured in the course of electrolysis with a P-5021
BD and aminoacetic acid at ic = 4 8 A dm , CE
decreases from 94 to 89%.
1070-4272/04/7708-1268 2004 MAIK Nauka/Interperiodica

ELECTRODEPOSITION OF LUSTROUS ZINC COATINGS FROM SULFATE ELECTROLYTE
1269
Table 1. Influence of the BD concentration, nature of a buffer additive, and current density on the outward appearance
1
of the coatings [electrolyte (g l ): ZnSO4 7H2O 200, Na2SO4 10H2O 50; pH 3.5; mechanical agitation]
1
1
2
Buffer solution, 30 g l
Al2(SO4)3
BD, (35 wt% solution, ml l )
i, Adm
Outward appearance of coating
1 5
6 10
11 29
1 8
1 8
1 4
5 8
1 6
7 8
1 4
5 7
8
1 8
1 8
1 5
6 8
1 5
6 7
8
Lusterless
Silvery
Silvery with luster at edges
Lusterous
Lustrous with burnt ports
Lusterous
30 40
41 45
Lustrous with lusterless stripes
Lustrous with overheating
Lusterless
H3BO3
1 5
6 10
11 29
Silvery
Silvery with overheating
Lusterous
30 40
Lustrous with lusterless stripes
Lusterous with burnt ports
Lusterless
Aminoacetic acid
1 5
6 30
40 50
1 8
1 8
1 8
Silvery
Silvery with luster at edges
Silvery
60 70
1
2 3
4 8
9
Semilustrous
Lusterous
Lustrous with burnt ports
In the zinc-plating electrolytes, cathodic polariza-
tion curves were measured on a rotating-disc electrode.
It can be seen from Fig. 1 that BD leads to an increase
in the cathodic polarization Ec (curves 1, 2). As
the rate of rotation of the disc electrode is raised from
200 to 2000 rpm in a BD-containing electrolyte, Ec
increases (curves 2 4).
tion rate of a disc electrode is adopted as a basis for
classification of additives with respect to their level-
ing power. If the polarization increases with rotation
rate, then it would be expected that the additive will
produce a leveling effect under the given conditions
The inhibiting influence exerted by BD on the Zn
electrodeposition is due to its adsorption on the elec-
trode surface. This is confirmed by measurements of
the capacitance of the double electric layer. In an elec-
trolyte without BD, the dependence of the capacitance
2
on the potential has a minimum (20 F cm ) at
E = 0.8 V (Fig. 1, curve 5). In the presence of BD,
the capacitance of the double electric layer decreases
2
to 9 F cm at E = 0.8 V (curve 6). At more elec-
tronegative potentials, an increase in the capacitance
is observed owing to desorption of a part of luster-
forming additive from the electrode surface [8].
Fig. 1. Current density i and capacitance C of the double
electric layer vs. the electrode potential relative to a stan-
c
Analysis of the cathodic polarization curves shows
that the extent to which the Zn electrodeposition is
hindered depends on the rate at which BD is delivered
to the cathode surface: the higher the rate of delivery
of the additive to the cathode surface, the stronger
the hindrance to the process. As is known [6], such
a dependence of the polarization curves on the rota-
dard hydrogen electrode E in the zinc-plating electrolyte.
1
Electrolyte (g l ): ZnSO 7H O 200, Na SO 10H O 50,
4
2
2
4
2
Al (SO ) 18H O 30; pH 3; the same for Fig. 2;
2
4 3
2
(1, 5) Electrolyte, (2 4, 6) electrolyte + BD (35 wt % solu-
1
tion) 35 ml l . Curves 1 4 were obtained on the rotat-
ing-disc electrode at a rotation rate (rpm): (1, 2) 0,
(3) 200, and (4) 2000.
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1270
MEDVEDEV, MAKRUSHIN
only slightly, the leveling effect cannot be stronger.
At the same time, at too high concentrations of
the additive, the leveling power decreases because of
the termination of the diffusion control over the rate
of delivery of the additive and its inhibition effect.
Thus, it was established that a luster-forming additive
BD to a zinc-plating electrolyte provides a leveling
effect because of its being simultaneously a leveling
additive.
As is known [3], the formation of lustrous coatings
is due to the leveling of submicrometer irregularities
on the surface. In this case, it is sufficient that the ad-
ditive should be adsorbed on the cathode surface, with
the overvoltage of electrocrystallization increased, for
a lusterless coating to be formed. At the same time, it
is necessary that, for lustrous coatings to be obtained,
an adsorption layer of a certain composition should be
present on the cathode surface. Layers of this kind can
be formed from organic substances and products of
secondary reactions in the near-cathode layer. In the
case of a simultaneous deposition of metal and evolu-
tion of hydrogen, the adsorption layer may consist of
hydroxides and other basic compounds [3].
Fig. 2. Leveling power P of the zinc-plating electrolyte vs.
(1) current density
i
and (2) BD concentration
c
=
c
(35 wt % solution). Mechanical agitation: (1) c
BD
1
2
35 ml l and (2) i = 4 A dm
.
c
The assumption that hydroxo compounds of zinc
are formed in the near-cathode space is confirmed by
measurements of pHs in the near-cathode layer. As
can be seen from Fig. 3, the pH value varies from 3.5
2
to 6 at ic = 1 8 A dm (depending on the type of
a buffer solution). The pHs varies to the smallest ex-
tent in an electrolyte containing aminoacetic acid
(curve 4). It should be noted that hydrates are formed
in a zinc-plating electrolyte at pH 6.4. All these facts
suggest that the presence of hydroxo compounds of
zinc in the near-cathode layer is not a necessary con-
dition for obtaining lustrous deposits. Apparently,
lustrous coatings are formed in the case when a highly
dispersed adsorption layer is formed on the electrode
surface [3].
Fig. 3. pH in the near-cathode layer vs. the current density
1
i
in the zinc-plating electrolyte. Electrolyte (g l ):
c
ZnSO 7H O 200, Na SO 10H O 50, and BD (35 wt %
solution) 35 ml l . (1) Electrolyte; (2, 3, 4) 1 + H BO ,
Al (SO ) 18H O. Aminoacetic acid 30 g l , respectively.
4
2
2
4
2
1
3
3
1
2
4 3
2
[6]. Profilographic measurements on a gently sloping
sine-shaped profile showed that surface leveling does
occur in the course of Zn electrodeposition.
Evidently, the dispersity of the adsorption layer is
controlled by buffer solutions. In their presence, the
adsorption layer can change because of the formation
of both Al(OH)3 and sparingly soluble zinc borates in
the near-cathode layer. It is not improbable that com-
plex compounds are formed by orthoboric and amino-
acetic acids with zinc ions, and intracomplex chelate
compounds, by H3BO3 with BD and products of its
reduction [3].
The dependences of P on ic and BD concentration
are shown in Fig. 2. It can be seen that the P ic and
P C dependences pass through a maximum at ic =
2
4 A dm (curves 1, 2). The run of these dependences
can be understood in terms of the adsorption-diffusion
leveling theory [6]. Raising ic first leads to an increase
in the leveling power. Then, at relatively high current
densities, the concentration of the additive on the sur-
face and its inhibition effect will decrease even at
microprojections owing to the high rate of surface re-
newal, which must diminish the efficiency of leveling.
At a too low concentration of the additive in the elec-
trolyte, when the Zn electrodeposition is inhibited
Of high importance in obtaining lustrous coatings
is the hydrodynamic mode in which a luster-form-
ing additive is delivered to the cathode surface and
decomposition products are removed from the near-
cathode space, as also noted in [9].
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ELECTRODEPOSITION OF LUSTROUS ZINC COATINGS FROM SULFATE ELECTROLYTE
1271
It has been shown [10] that, in the course of elec-
trolysis, the luster-forming additive BD undergoes
complex transformations, which strongly affects the
quality of the coatings obtained. A gas-chromato-
graphic analysis of the BD-containing electrolyte
showed that, as the electrolysis duration increases,
2-butene-1,4-diol, 2-butane-1,4-diol, 2.6-octene-1,8-
diol, 2-butenol, and butanol are accumulated owing
to diol reduction. The influence exerted by luster-
forming additives in acid electrolytes may be associ-
ated with their reduction in the near-cathode layer
[11]. This is accompanied by a deceleration of the hy-
drogen evolution and the corresponding change in
the structure of the adsorption layer, as well as by
a decrease in the degree of hydrogen saturation of
the metal [12].
Table 2. Change in the total energy of the products in
comparison with the total energy of the starting substances
Reac-
tion
E,
kJ mol
Reaction equation
1
(1) Zn(OH)2 + e = ZnOH + OH
(2) ZnOH+ + e = ZnOH
271
897
1051
579
(3) [ZnBD]2+ + e = Zn+ + BD
(4) [Zn(OH)BD]+ + e = ZnOH + BD
(5) [Zn(OH)2BD] + e = ZnOH + OH + BD
337
1
of the cluster with a hydrated zinc ion (200 kJ mol ).
It is noteworthy that the BD molecule forms no stable
system with the cluster without Zn2+ ions.
Alkalization may result in that mixed aqua hydroxo
complexes of varied composition with BD are formed
in the near-cathode layer. With account taken of the
fact that BD molecules displace H2O molecules from
the aqua complexes with Zn2+, the most probable is
the formation in the near-cathode layer of the following
species: Zn(OH)2, Zn(OH)+, [ZnBD]2+, [Zn(OH)BD]+,
and [Zn(OH)2BD]. The highest energy of interaction
with a Zn49 cluster is observed for Zn(OH)+ species.
The presence of a BD molecule in a complex species
somewhat decreases the energy of such an interaction
and increases the distance between the Zn2+ ion and
the cluster surface. One of the reasons for such a be-
havior may be the spatial structure of the complexes
being formed, which creates steric hindrance to ap-
proach of Zn2+ ions to the cluster.
To elucidate the mechanism of the influence ex-
erted by BD on the Zn electrodeposition, the system
consisting of hydrated ions present in the electrolyte
and BD molecules was subjected to a quantum-chem-
ical analysis. The calculation was performed using
the semiempirical chemical method PM3 [13, 14].
We calculated the coordination numbers of hydration
for ions and BD molecules. According to the calcu-
lation performed, these numbers are 5 for Zn2+, 3 for
H3O+, 8 for SO24 , and 9 for BD. Introduction of
H3O+ and SO42 ions into a Zn2+-containing aqueous
solution results in their additional interaction. It was
shown that each BD molecule displaces two H2O
molecules from the hydration sheaths of Zn2+ ions,
thus forming stable complexes with the ions. It was
established that the interaction of Zn2+ ions with
BD molecules is energetically more favorable than
the formation of the hydration sheath of an ion. It is
the most probable, with account of the composition of
the zinc-plating sulfate electrolyte, that complex ions
[Zn(H2O)x (BD)y ]2+ (x = 1 5 and y = 1 3) exist in
the bulk of the electrolyte.
The rather wide diversity of complex species
in the near-cathode layer suggests a mechanism
of their discharge (Table 2). To qualitatively estimate
the probabilities of the occurring processes, we cal-
culated the change in the total energy of the presumed
reactions in comparison with the total energy of the
initial substances ( E, Table 2). In doing so, we took
into account that the energy necessary for an electron
to be involved in each of the above processes is a con-
stant value. In the given case, the reaction was con-
sidered with and without BD.
In the following stage, the structure of BD at
the electrode solution interface was analyzed by
the quantum-chemical method. In doing so, the sur-
face of the zinc electrode was modeled by a negatively
charged cluster consisting of 49 atoms situated in two
layers having a hexagonal shape. The cluster was
chosen with account of the recommendations made in
[15]. To model the process of Zn2+ reduction on
the surface of the zinc electrode, a system constituted
by a zinc cluster Zn49, Zn2+, and BD was calculated.
It was shown that, when approaching the cluster, the
BD molecules contained in the complex form a stable
system at a distance of 310 pm from the cluster. In
It follows from the data in Table 2 that the most
energetically probable are reactions (2) (4), which
yield a ZnOH species and a Zn+ ion. Apparently, re-
action (3) is the first of these three to occur. The Zn+
ions formed in this case enter into the electrochemical
reaction
Zn+ + e
Zn,
1
this case, the interaction energy is 353 kJ mol ,
and BD molecules may be reduced electrochemically
to give various products [10].
which considerably exceeds the energy of interaction
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1272
MEDVEDEV, MAKRUSHIN
Table 3. Composition of zinc-plating sulfate electrolytes
yielding lustrous coatings with leveled surface (T = 18
25 C, pH 3 4, mechanical agitation)
[Zn(OH)BD]+ are electroactive species owing to al-
kalization. Sulfate electrolytes for obtaining lustrous
zinc coatings with a leveled surface were developed.
1
Components and
working mode
of electrolyte
Component concentration, g l
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Plating), Moscow: Metallurgiya, 1988.
1
2
3
ZnSO4 7H2O
200 250
50 100
Na2SO4 10H2O
Al2(SO4)3 18H2O
H3BO3
Aminoacetic acid
BD (35 wt % solution)
25 30
30 40
25 30
30 40
25 30
60 70
2
Current density, A dm
Current efficiency, %
1 6
1 5
4 8
92 95
93 95
89 94
Thus, the quantum-chemical calculation showed
that it is the most probable that, in a sulfate zinc-plat-
ing electrolyte containing BD, the complex ions
[Zn(H2O)x (BD)y ]2+ are present in the electrolyte bulk
and Zn(OH)+, [ZnBD]2+, and [Zn(OH)BD]+ are elec-
troactive species in the near-cathode space owing to
alkalization.
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As a result of the study performed, electrolytes
of simple composition were developed for obtaining
lustrous zinc coatings with a leveled surface (Table 3).
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CONCLUSIONS
(1) The study of the electrodeposition of zinc from
a sulfate electrolyte containing 2-butyne-1,4-diol and
buffer additives Al2(SO4)3 18H2O, H3BO3, and
aminoacetic acid showed that, depending on the type
of additive, lustrous coatings are formed in an agitated
2
electrolyte at ic = 1 8 A dm .
(2) The system constituted by hydrated Zn2+
ions and 2-butyne-1,4-diol molecules was studied by
the quantum-chemical method. It was shown that
presence of complexes of the type [Zn(H2O)x (BD)y ]2+
in the electrolyte bulk is the most probable. In
the near-cathode layer, Zn(OH)+, [ZnBD]2+, and
no. 3, pp. 320 341.
15. Shapnik, M.S., Elektrokhimiya, 1994, vol. 30, no. 2,
pp. 143 149.
16. Vyacheslavov, P.M. and Shmeleva, N.M., Kontrol’
elektrolitov i pokrytii (Control of Electrolytes and
Coatings), Leningrad: Mashinostroenie, 1965.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 8 2004