Elecrodeposition of lustrous zinc coatings from sulfate electrolyte

Article abstract of DOI:10.1134/S1070427207080101

Electrodeposition of lustrous zinc coatings from a sulfate electrolyte containing ZnSO₄, Na₂SO₄, buffer additives, and fixers (products of phenol-formaldehyde condensation) was studied.

Full text of DOI:10.1134/S1070427207080101

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2007, Vol. 80, No. 8, pp. 1316 1321.
Pleiades Publishing, Ltd., 2007.
Original Russian Text
pp. 1276 1281.
G.I. Medvedev, N.A. Makrushin, V. Khamun’ela, 2007, published in Zhurnal Prikladnoi Khimii, 2007, Vol. 80, No. 8,
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Elecrodeposition of Lustrous Zinc Coatings
from Sulfate Electrolyte
G. I. Medvedev, N. A. Makrushin, and V. Khamun’ela
Novomoskovsk Institute, Mendeleev Russian University of Chemical Engineering, Novomoskovsk,
Tula oblast, Russia
Received December 19, 2006; in final form, March 2007
Abstract Electrodeposition of lustrous zinc coatings from a sulfate electrolyte containing ZnSO , Na SO ,
4
2
4
buffer additives, and fixers (products of phenol-formaldehyde condensation) was studied.
DOI: 10.1134/S1070427207080101
Lustrous zinc coatings on articles of simple con-
figuration are deposited from sulfate electrolytes con-
taining various brightening additives (aldehydes,
ketones, sulfo compounds, and various condensation
products of complex composition [1 4]).
where a is the sine wave amplitude of the micro-
profile; h , average thickness of the coating (10 m);
av
H and H , initial and final amplitudes of the sinusoi-
0
i
dal microprofile, respectively.
The capacitance of the electric double layer was
measured in the course of electrolysis with a P-5021
ac bridge at a frequency of 30 kHz in a series equiv-
alent circuit. A zinc-coated platinum wire placed at
the center of a platinum-coated cylinder served as
the working electrode. The pH value in the near-elec-
trode layer was measured by the method described
in [6]. The degree of surface filling with organic ad-
ditives, Q, was estimated using the polarization curves
In this study, we examined the electrodeposition
of zinc from a sulfate electrolyte containing a fixer,
product of phenol-formaldehyde condensation with
the structural formula
OH
OH
CH2
CH2
[7] by the formula
CH OH
2
n
Q = 1
exp( E/b),
The study was performed in an electrolyte con-
1
where E is the potential shift in cathodic polariza-
tion, and b is the Tafel constant found from the polar-
ization curves plotted in the semilog scale.
taining (g l ): ZnSO 7H O 200 250, Na SO
4
2
2
4
1
0H O 50 100, Al (SO ) 18H O, H BO , amino-
2 2 4 3 2 3 3
acetic acid 25 30, and fixer 0.5 5, at 18 25 C and
pH 2 5. Zinc plates served as anodes. A 6 15- m-
thick zinc layer was deposited onto unpolished St.3
steel. Polarization curves were recorded with a P-5828
potentiostat. The luster was measured on a FB-2
photoelectric luster meter. The leveling power P of
the electrolytes was determined by a direct profile
measurement of the sample surface with a sinu-
soidal microprofile. P was calculated by the formu-
la [5]
The internal stresses and microhardness of the coat-
ings were determined as described in [8].
The results obtained in a study of the influence
exerted by the fixer concentration, nature of the buffer
additive, and current density on the outward appear-
ance of the coatings show that dark gray coatings
with a rough surface are formed from an electrolyte
containing Al (SO ) buffer additive and a fixer at
2
4 3
2
i = 1 11 A dm . At the same time, lustrous coatings
c
P = 2.3a/2 h log(H /H ),
are formed from an electrolyte containing H BO
av
0
i
3
3
1316

ELECRODEPOSITION OF LUSTROUS ZINC COATINGS
1317
Table 1. Effect of buffer additives, fixer concentration, and current density on the outward appearance of zinc coatings
1
[
electrolyte composition (g l ): ZnSO 7H O 200 and Na SO 10H O 50; pH 3 4, T = 20 C]
4 2 2 4 2
Buffer additive, g l 1
Fixer concentration, g l 1
Current density, A dm 2
Coating appearance
HBO , 30
1 5
Silvery
Silvery with burn-on
Silvery
3
0
1
.5
2
6
11
1
2 10
Lustrous
1
1
1
Lustrous with burn-on
Silvery
2
5
8
11
3
Lustrous
Semilustrous
Semilustrous with burn-on
Lustrous
3
5
4
6
9
1
4 6
Semilustrous
7
11
Semilustrous with burn-on
Aminoacetic acid 30
1 5
Silvery
Silvery with burn-on
Lustrous
Lustrous with burn-on
Lustrous
0
1
.5
2
6
1
9
1
8
8
11
4
3
5
4
5 8
Semilustrous
Semilustrous with burn-on
Lustrous
9
1
3
6
9
11
2
5
8
Semilustrous
Silvery
Silvery with burn-on
11
and aminoacetic acid. As can be seen in Table 1,
the outward appearance of the coatings varies in this
case with the fixer concentration and with the cathode
current density.
Cathodic polarization curves were measured in
the zinc-plating electrolytes. As can be seen in Fig. 2,
the presence of a fixer enhances the cathodic polariza-
tion (Fig. 2, curves 1, 2). The inhibition effect of
the fixer on the electrodeposition of zinc is due to its
The widest range of current densities at which lus-
trous coatings are formed is observed at fixer con-
1
centrations of 1 2 g l . In this case, the current densi-
2
ties vary in the range from 2 to 10 A dm in an elec-
2
trolyte containing H BO , and from 1 to 8 A dm in
3
3
an electrolyte containing aminoacetic acid. At higher
1
fixer concentrations (c = 3 5 g l ), the range of cur-
rent densities at which lustrous coatings are deposited
is markedly narrower.
In the electrolytes studied, the pH value should be
within the range 3 4. At pH < 3, the i range provid-
c
ing deposition of lustrous coatings and the current ef-
ficiency decrease. At pH > 4 lustrous coatings are de-
posited, but the electrolyte exhibits an unstable behav-
ior. The current efficiency by zinc in the electrolytes
yielding lustrous coatings varies from 85 to 97%. In
the electrolyte containing aminoacetic acid, the cur-
rent efficiency is the highest (Fig. 1, curves 1, 2).
Fig. 1. Current efficiency CE by zinc vs. the current
1
density i . Electrolyte composition (g l ): ZnSO 7H O
c
4
2
200, Na SO 10H O 50, fixer 1; pH 3.5. (1) Electro-
2
4
2
1
lyte + aminoacetic acid, 30 g l ; (2) electrolyte + H BO ,
3
3
1
30 g l
.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 8 2007

1318
MEDVEDEV et al.
tive potentials, which is due to partial desorption of
the fixer from the electrode surface [9].
The electrolytes yielding lustrous zinc coatings
exhibit the leveling effect. The leveling power varies
from 0.2 to 0.4, depending on the cathode current
density. The throwing power of the electrolytes is
8
12%. The microhardness of the zinc deposits var-
2
ies within the range 80 100 kg mm . The lustrous
zinc coatings have small internal stresses (IS = 100
2
1
50 kg cm ).
As is known [3], the deposition of lustrous coat-
ings is due to smoothing of the submicrometer sur-
face roughness. For a dull coating to be formed, it
suffices that an additive is adsorbed on the cathode
surface and the electrocrystallization overpotential
is raised, whereas formation of a lustrous coating,
requires that an adsorption layer of a certain com-
position should be present on the cathode surface.
Such layers can be formed from organic substances
and products of secondary reactions in the near-
cathode layer. In the case of a simultaneous metal
deposition and hydrogen evolution, the adsorption
layer may consist of hydroxides and other basic com-
pounds [3].
Fig. 2. Current density i and electric double layer capac-
c
itance C vs. the electrode potential E in a zinc-plating
c
1
electrolyte. Electrolyte composition (g l ): ZnSO 7H O
4
2
2
00, Na SO 10H O 50, H BO 30, pH 3.5. (1), (3) Elec-
2 4 2 3 3
1
trolyte, (2), (4) electrolyte + fixer, 1 g l
.
The suggestion that zinc hydroxides are formed in
the near-cathode space is confirmed by the results of
pH measurements in the near-cathode layer (pH ). As
s
2
can be seen from Fig. 3, pH varies at i = 1 10 A dm
s
c
from 4 to 6, depending on the nature of the buffer
additive. In the electrolyte containing aminoacetic
acid, the range of pH variation is the narrowest (cur-
ve 4). It should be noted that the pH of hydrate for-
mation in the zinc-plating electrolyte is 6.4. These
results suggest that presence of zinc hydrates in
the near-cathode layer is not necessary for lustrous
coatings to be deposited. Apparently, lustrous coatings
are deposited if a highly dispersed adsorption layer is
formed on the surface [3].
Fig. 3. Dependence of pH in the near-cathode layer
s
on the current density i in a zinc-plating electrolyte.
c
1
Electrolyte composition (g l ): ZnSO 7H O 200,
4
2
Apparently, buffer additives control the dispersity
of the adsorption layer. In their presence, the adsorp-
tion layer may change because of the formation of
difficultly soluble zinc borates in the near-cathode
space. It is not improbable that complex compounds
of orthoboric and aminoacetic acids with zinc ions, as
well as intracomplex (chelated) compounds of H BO
Na SO 10H O 50, fixer 1. (1) Electrolyte; (2) electro-
lyte + H BO , 30 g l ; (3) electrolyte + Al (SO ) 18H O,
0 g l ; (4) electrolyte + aminoacetic acid, 30 g l .
2
4
2
1
3
3
2
4 3
2
1
1
3
adsorption on the cathode surface. This is confirmed
by the results obtained in measuring the capacitance
of the electric double layer. In the electrolyte contain-
ing no fixer, the capacity-vs.-potential dependence
3
3
with the fixer, can be formed.
2
has a minimum (C = 20 mF cm ) at a potential of
An important issue in elucidating the mechanism of
formation of lustrous coatings, is determining the de-
gree of filling of the cathode surface with the ad-
sorbed additive. The dependences of the degrees of
luster (curve 1) and surface filling (curve 2) on the cur-
0
.8 V (Fig. 2, curve 3). Introduction of a fixer into
the electrolyte lowers the double layer capacitance to
mF cm 2 at E = 0.8 V (Fig. 2, curve 4). An in-
crease in the capacitance is observed at more nega-
9
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 8 2007

ELECRODEPOSITION OF LUSTROUS ZINC COATINGS
rent density are shown in Fig. 4. It can be seen that,
1319
as the cathode current density increases, the degree
of luster of the coatings passes through a maximum,
whereas the degree of filling decreases. At the current
densities studied, the degree of surface filling varies
from 24 to 10%. This shows that lustrous coatings are
formed at a well-defined degree of surface filling.
To elucidate the mechanism by which the fixer af-
fects the electrodeposition of zinc, we carried out
a quantum-chemical study of a system constituted by
hydrated ions present in the electrolyte and fixer mol-
ecules. To find the degree of polymerization, n, of
the fixer molecule, its molecular weight was deter-
mined by the cryoscopic method. The results of these
measurements demonstrated that n = 1. Consequently,
the electrolyte also contains a monomeric product of
phenol-formaldehyde (F) condensation.
Fig. 4. Degree of luster B (1) and degree of the surface
filling Q (2) vs. the current density i .
c
The quantum-chemical calculation was performed
by the semiempirical PM3 method [11, 12]. Compared
with other semiempirical methods and ab initio cal-
culations with different basis sets [13], this method
describes rather well the parameters of intermolecular
interactions. The energy of interaction between the par-
ticles constituting the system under consideration was
calculated as the difference between the total elec-
tronic energy of the system and the sum of the total
electronic energies of its constituents. For example,
2
+
we calculated for the system Zn49 [ZnF] , first,
the energy of interaction between a complex-forming
ion and the ligand in the complex [ZnF]2 ion
2+
Fig. 5. Structure of a Zn complex with a monomeric
product of phenol-formaldehyde (F) condensation and
a water molecule, according to the results of a quantum-
chemical calculation Formaldehyde molecule: hydrogen
atoms, white; carbon, gray; and oxygen, dark gray.
+
Ec = E[ZnF]2+
(EZn2+ + E ),
F
+
[
Zn(OH)F] species is most likely in the near-cathode
and then the energy of interaction of the complex ion
with the cluster
layer. Less probable is the formation of [Zn(OH) F]
2
species.
Ecl = EZn49
[ZnF]2+
(E[ZnF]2+ + EZn49 ).
In the next stage, we carried out a quantum-chem-
ical study of the structure of complexes with formal-
dehyde molecules at the electrode solution interface.
In doing so, we modeled the surface of the zinc elec-
trode with a cluster formed by 49 atoms arranged in
two layers having the hexagonal form. The clusters
were chosen as recommended in [15].
The results of the calculation show that, similarly
to the case of 2-butyne-1,4 diol [14], the interaction of
Zn ions with the formaldehyde molecules is more
energetically favorable than the formation of a hy-
drated shell of the ion. In a sulfate zinc-plating elec-
trolyte, complex ions of composition [Zn(H O) (F) ] ,
where x = 1 5, y = 1 3, are most likely to exist in
2
+
2
+
2
x
y
To simulate the reduction of Zn2+ on the surface of
2
+
the zinc electrode, the zinc cluster system Zn49 Zn
F
the electrolyte bulk.
was calculated. It was shown that the highest interaction
+
In the near-cathode layer, mixed aqua hydroxo com-
plexes of various compositions with formaldehyde mol-
ecules can be formed via alkalization. In view of
the fact that formaldehyde molecules displace water
energy with the Zn49 cluster is observed for Zn(OH)
species. The formaldehyde molecules in the complex
constitute, as they approach the cluster, a stable system
at a distance of 418 pm from the cluster. In this case,
2
+
1
molecules from the Zn aqua complexes (Fig. 5),
the interaction energy is 437 kJ mol , which consider-
+
2+
the formation of Zn(OH) , Zn(OH) , and [ZnF]
ably exceeds the value for interaction of the cluster with
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 8 2007

1320
MEDVEDEV et al.
the energy necessary for an electron to be involved in
each of the above processes is constant. In the given
case, the reactions involving and not involving form-
aldehyde molecules were considered.
It follows from the data in Table 2 that reactions
2) (4) yielding ZnOH species and Zn ions are the
+
(
most probable from the energy standpoint. Presumably,
of all three reactions, reaction (2) is the first to occur.
The species forming in this case enter into the follow-
ing electrochemical reactions:
ZnOH + e = Zn + OH ,
Zn+ + e
Zn.
Thus, our quantum-chemical calculations demon-
strated that, in a sulfate zinc-plating electrolyte con-
taining formaldehyde molecules, the probability of ex-
2
+
2
+
Fig. 6. Structure of a Zn complex with a monomeric
product of phenol-formaldehyde (F) condensation on
the surface of the Zn cluster, according to the results
istence of complex ions [Zn(H O) (F) ] in the elec-
2 x y
trolyte bulk is the highest, whereas in the near-cathode
+
2+
+
4
9
space, [Zn(OH)] , [ZnF] , and [Zn(OH)F] are elec-
trically active particles owing to alkalization.
of a quantum-chemical calculation.
1
+
a hydrated zinc ion (200 kJ mol ) and a [Zn(OH)F]
The study we performed enabled us to develop
a sulfate zinc-plating electrolyte of simple composi-
tion for deposition of lustrous coatings with smoothed
species. The presence of a formaldehyde molecule in
the complex species makes distance between the zinc
ion and the cluster surface considerably larger. This
may result from the spatial structure of the forming
complexes, which creates, owing to the large size of
the formaldehyde molecule, steric hindrance to ap-
proach of zinc ions to the cluster (Fig. 6). It is note-
surface. The electrolyte composition is as follows
1
(
g l ): ZnSO 7H O 200 250, Na SO 10H O
4
2
2
4
2
5
0 100, H BO 25 30, and fixer 1 2. The process is
3 3
2
performed at ic = 2 10 A dm . The electrolyte
temperature is 18 25 C, pH 3 4, and CE = 84 97%.
2
+
worthy that, without Zn ions, the formaldehyde
molecule forms no stable system with the cluster.
The anodes are made of zinc. Aminoacetic acid (c =
1
2
5 30 g l ) can be taken instead of H BO . In this
3
3
2
The rather wide variety of complex species formed
in the near-cathode layer suggest that their discharge
can occur by different pathways. To assess the prob-
abilities of these pathways, we calculated the change
in the total energy of the reaction products (Table 2)
in comparison with the total energy of the starting
substances. In doing so, we took into account that
case, lustrous coatings are obtained at i = 1 8 A dm
c
with CE = 86 97%.
CONCLUSIONS
(1) A study of zinc electrodeposition from a sulfate
electrolyte containing a fixer (product of phenol-
formaldehyde condensation) and buffer additives
demonstrated that the fixer taken together with H BO
Table 2. Changes in the total energy of the products in
3
3
comparison with the total energy of starting substances
or aminoacetic acid yields lustrous zinc coatings with
2
smoothed surface at i = 1 10 A dm .
c
Reac-
tion
E,
Reaction equation
(2) The system constituting hydrated zinc ions and
fixer (formaldehyde) molecules was studied using
the quantum-chemical method. It was shown that
kJ mol 1
(
(
(
(
(
1)
2)
3)
4)
5)
Zn(OH)2 + e = ZnOH + OH
271
897
874
662
173
2+
complexes of composition [Zn(H O) (F) ] are most
+
2
x
y
ZnOH + e = ZnOH
likely to be present in the electrolyte bulk. In the near-
[ZnF]2+ + e = Zn+ + F
+
2+
+
cathode layer, [Zn(OH)] , [ZnF] , and [Zn(OH)F]
[Zn(OH)F]+ + e = ZnOH + F
are electrically active species owing to alkalization.
[Zn(OH) F] + e = ZnOH +
2
OH + F
(3) Sulfate electrolytes for deposition of lustrous
zinc coatings with smoothed surface were developed.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 80 No. 8 2007

ELECRODEPOSITION OF LUSTROUS ZINC COATINGS
1321
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