The effect of electrochemical adsorbates on texture and morphology development during zinc and zinc-cobalt electrodepositions

Article abstract of DOI:10.1016/j.electacta.2008.01.085

Electrochemical impedance spectroscopy (EIS) investigation of zinc electrodeposition at 10 mA cm^-2 indicated the possibility of hydrogen adsorption during deposition process. This causes low angle pyramidal {1 1.5} and {1 0.3} texture component via lateral bunching and results the morphology of ridges type. Conversely, at the same current density used in zinc-cobalt deposition the hydrogen adsorption is less pronounced due to adsorption of cobalt and/or cobalt-contained species during the growth stage. This leads to a strong inhibition resulted in a different type of growth, called Field-Oriented Texture . This kind of growth promoted columnar grain growth results in a large number of elongated crystals almost normal to the substrate surface followed by some blocky crystals on top of them. This develops prism planes as a major texture component besides some basal and low angle pyramidal planes.

Full text of DOI:10.1016/j.electacta.2008.01.085

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 4674–4678
The effect of electrochemical adsorbates on texture and morphology
development during zinc and zinc–cobalt electrodepositions
K. Raeissia,∗, A. Saatchia, M.A. Golozara, A. Tufania, J.A. Szpunarb
a Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran
b Department of Mining, Metals and Materials Engineering, McGill University, M.H. Wong Building, 3610 University, Montreal, Canada H3A 2B2
Received 14 November 2007; received in revised form 21 January 2008; accepted 28 January 2008
Available online 7 February 2008
Abstract
Electrochemical impedance spectroscopy (EIS) investigation of zinc electrodeposition at 10 mA cm−2 indicated the possibility of hydrogen
adsorption during deposition process. This causes low angle pyramidal {1 1.5} and {1 0.3} texture component via lateral bunching and results the
morphology of ridges type. Conversely, at the same current density used in zinc–cobalt deposition the hydrogen adsorption is less pronounced
due to adsorption of cobalt and/or cobalt-contained species during the growth stage. This leads to a strong inhibition resulted in a different type
of growth, called “Field-Oriented Texture”. This kind of growth promoted columnar grain growth results in a large number of elongated crystals
almost normal to the substrate surface followed by some blocky crystals on top of them. This develops prism planes as a major texture component
besides some basal and low angle pyramidal planes.
© 2008 Elsevier Ltd. All rights reserved.
Keywords: Texture; Morphology; Zinc electrodeposition; Zinc–cobalt electrodeposition; Adsorption
1. Introduction
getically favorable, because the edge energy per atom is always
higher than the surface energy [4,5]. According to Fisher, the
bunching is possible by the presence of blocking adsorbates [3].
Inzinc-alloyelectrodeposition, someelectrochemicalspecies
rather than hydrogen would form and adsorb onto the substrate
surface [6]. In zinc–cobalt electrodeposition, reduction of cobalt
(or cobalt-contained species) and its adsorption is expected to
occur strongly at the early stage as well as during the deposi-
tion at low overpotentials [7]. This adsorption can affect the
zinc deposition mechanism and stimulate the hydrogen evo-
lution strongly [7]. In contrary to zinc electrodeposition, it is
reported that in the presence of cobalt ions, the mechanism of
hydrogen evolution involves a diffusion-controlled process [7].
In addition, a strong inhibition for hydrogen evolution due to
the coverage of adsorbed cobalt and/or cobalt-contained species
can be achieved.
The aim of this work is to study the intermediate adsorp-
tion and its consequence inhibition effect on texture formation
in zinc and zinc–cobalt electrodeposition. For this purpose,
electrochemical impedance spectroscopy (EIS) and orientation
distribution function (ODF) methods were used to find the pos-
sibility of adsorption and to evaluate the texture components of
coatings, respectively.
During the electrodeposition of some metals and alloys near
redoxpotential, thesurfacecouldbecoveredbyadsorbedspecies
such as hydrogen and alloy element-contained species. The
adsorbed hydrogen atoms, which are capable of continually
regenerating on the surface, can lead to relaxation during the zinc
electrodepositionatlowoverpotentials[1]. Thus, themechanism
of electrodeposition can be strongly affected by the adsorbates
[1,2].
In zinc electrodeposition, adsorbed hydrogen ZnH+ad can act
asaninterfaceinhibitorpreventingdirectincorporationofadions
(adatoms) into the microsteps. This increases two-dimensional
nucleation frequency [1,2] thereby encouraging the lateral
bunching growth of zinc [3]. Multiplication of the microsteps
to macrosteps is the so-called “bunching”. Fisher has described
bunching growth mechanism of microsteps, which are regen-
erated by two-dimensional nucleation. It is mentioned that the
process of coalescence of microsteps to form macrosteps is ener-
∗
Corresponding author. Fax: +98 311 3912752.
E-mail address: k raeissi@cc.iut.ac.ir (K. Raeissi).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.01.085

K. Raeissi et al. / Electrochimica Acta 53 (2008) 4674–4678
4675
2. Experimental procedure
Substrates were prepared from a commercial cold rolled low
carbon steel sheet with a thickness of 1 mm. Specimens were
sealed in a stainless steel foil sack and annealed at 880 ◦C for
3 h. Specimens were then mechanically polished using 600-
grit abrasive SiC paper and subsequently electropolished in a
solution of 95 vol% acetic acid and 5 vol% perchloric acid for
2.5–3 min. After electropolishing, the specimens were washed
with distilled water and soaked in 10% sulfuric acid solution for
20 s. Specimens were then washed again with distilled water and
immediately placed in the electroplating bath.
The bath compositions used were ZnSO4·7H2O (620 g l−1
)
Fig. 1. Cross section of the ODF of steel substrate sheet at ϕ2 = 45◦.
plus Na2SO4 (75 g l−1) for zinc deposition and ZnSO4·7H2O
(620 g l−1)andCoSO4·7H2O(125 g l−1)plusNa2SO4 (75 g l−1
)
for zinc–cobalt deposition. The pH of the baths was adjusted to 2
using dilute sulfuric acid solution. Deposition was conducted in
a standard corrosion cell, with two graphite counter electrodes
and a saturated calomel reference electrode (SCE), at a current
of 10 mA cm−2. The reference electrode was placed close to the
cathode surface via a luggin capillary, filled with bath solution.
The temperature of the cell was kept at 25 2 ◦C. The current
density of 10 mA cm−2 was found suitable for both zinc and
zinc–cobalt electrodeposition, as it provides a large amount of
surface coverage by adsorbates.
An EG&G computer controlled potentiostat/galvanostat
model 263A was used to maintain the current density at
10 mA cm−2. In order to produce a constant coating thickness of
5 ␮m, theplatingtimewassetto1100 s. AnEG&GACresponser
(model 1025) was coupled with potentiostat/galvanostat to read
AC impedance resistance. AC impedance measurements were
taken using a conventional cell with a platinum counter electrode
and a SCE as the reference electrode, with a similar arrangement
as in the standard corrosion cell.
dent from this figure using the location map depicted in Fig. 3.
The pyramidal texture component is essentially produced due
to epitaxial growth of zinc on steel substrate grains [8–10]. It
is believed to be strongly under the influence of ␥ fiber texture
intensity of steel substrate [11]. Epitaxial growth of zinc can
be developed by constructing the Burger’s orientation relation-
ship between the {0 0.2} planes of zinc crystallites and {1 1 0}
terraces of the surface of steel substrate created by electropol-
ishing [12,13]. In this way, zinc atoms are incorporated one after
another into microsteps or kinks of substrate surface [8–10].
Fig. 4 shows the morphology of zinc deposited at
10 mA cm−2. The coating consists of thin-layered hexagonal
plates aligned in a specific way on each grain of substrate.
This would make a specific kind of morphology called “ridges
morphology” which is an evidence for epitaxial growth [8,9,11].
Fig. 5 shows the Nyquist plot of zinc deposition. This AC
impedance reading was performed at −1.05 V. This potential
Philips XL30 scanning electron microscope was used to
observe the morphology of the deposits. A Siemens D500
diffractometer was used to determine substrate and coating
texture. The diffractometer was operated using Cu K␣ at an
accelerating voltage of 40 kV and a current of 40 mA. For the
coatings, three pole figures were measured for the planes of
{0 0.2}, {1 0.0} and {1 0.1} using the reflection technique in five
deg polar and angular intervals. The ODFs were then calculated
using “TexTools” software.
3. Results and discussion
Fig. 1 shows cross section of the ODF of steel substrate sheet
at ϕ2 = 45◦. The main texture component is the so-called ␥ fiber
[9] that is characterized by ꢀ1 1 1ꢁ directions parallel to normal
of the steel sheet. According to Fig. 1, a strong {1 1 1} ꢀ1 1 0ꢁ
and {1 1 1} ꢀ1 1 2ꢁ orientations can be considered for the steel
substrate grains.
Fig. 2 shows cross sections of the ODF of electrodeposited
zinc at ϕ2 = 30◦ and 55◦. These cross sections were selected
because they can reveal all the texture components sufficiently.
A sharp non-fiber texture component, which involves pyramidal
Fig. 2. Cross sections of the ODF of electrodeposited zinc at ϕ2 = 30◦ and 55◦.
¯ ¯
¯ ¯
{1 1.5} ꢀ3 3.1ꢁ and pyramidal {1 0.3} ꢀ3 2.1ꢁ component is evi-

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Fig. 5. Nyquist plots of zinc deposition at −1.05 V.
with the charge-transfer resistance [1,2]. The two low-frequency
inductive loops correspond to the relaxation of both the coverage
of Znad and ZnH+ in order of decreasing frequencies [1,2].
+
ad
As it was mentioned by Ichino et al. [2], adsorbed hydrogen
covers the entire surface of zinc growing deposit near the zinc
redox potential [2].
The adsorbed hydrogen ZnH+ could be considered as a
ad
blocking adsorbate with an interfacial inhibition behavior [2]
which is commonly necessary for bunching growth [8]. The
bunching only comes about by the fact that younger microsteps
overtake the older ones during the growth and then all microsteps
coalesce to form the macrosteps. For this purpose, the older steps
in their lateral growth must be hindered more than the younger
ones [3]. This succeeds by the time dependency of inhibitor
adsorption [3]. It seems that ZnH+ poses all the mentioned
ad
Fig. 3. location map of some texture components in ODF of hexagonal lattice
requirements to act as an effective interface inhibitor needed for
the bunching growth. Finally, the lateral aggregation of the cre-
ated macrosteps occurs and results in ridges morphology as seen
in Fig. 4. This kind of growth has also been discussed previously
by Itoh et al. [4] and is expected to occur only on surfaces that are
electrolytically polished which comprise microsteps [12,13]. In
this way, a fully epitaxial growth of zinc onto the electropolished
steel substrate can be obtained [8–10]. Eventually, It can be con-
at ϕ2 = 30◦ and 55◦.
corresponds to the current density of 10 mA cm−2 and is read
directly from cathodic scan curve obtained for zinc deposition
in Fig. 6. The AC impedance data shows one capacitive loop at
high frequencies and two possible inductive loops at low fre-
quencies. The high frequency capacitive loop corresponds to
the impedance created by double-layer capacitance in parallel
Fig. 6. Cathodic scan curves for zinc and zinc–cobalt, scan rate = 40 mV s−1
.
Fig. 4. Morphology of the zinc deposited at 10 mA cm−2
.

K. Raeissi et al. / Electrochimica Acta 53 (2008) 4674–4678
4677
Fig. 7. Schematic representation of lateral growth of zinc on steel substrate
surface.
Fig. 9. Nyquist plots of zinc–cobalt deposition at −1.08 V.
sidered that the steel substrate microsteps has been transformed
to the zinc deposit macrosteps through bunching and resulted
low angle pyramidal texture components such as (1 1.5) and
(1 0.3), as seen from the ODF results in Fig. 2. Development of
the low angle pyramidal planes could be assumed schematically
as it is drawn in Fig. 7.
with respect to the zinc deposition. The high frequency capac-
itive loop, similar to that observed for zinc, corresponds to
impedance created by the double-layer capacitance in parallel
with the charge-transfer resistance. The inductive loop appeared
at low frequencies corresponds again to relaxation of Zn+
Fig. 8 shows the cross sections of the ODF of electrode-
posited zinc–cobalt. It is noted that the maximum intensity of
the planes is much lower than that of zinc (Fig. 2). Non-fiber
prism {1 1.0} ꢀ0 1.6ꢁ orientation is taken to be account as the
major texture component besides the basal {00.2} and some low
ad
coverage [1,2]. The surprising capacitive loop observed at low
frequencies on the Nyquist plot may reflect the relaxation of
cobalt and/or cobalt-contained species onto the entire growing
surface of deposit. Similar results have also been reported by
previous works on zinc–nickel deposition [14] and zinc deposi-
tion from baths with small amount of lead [2]. The absence of
any inductive loop at low frequencies on the spectrum reflects
the inhibited adsorption of hydrogen ZnH+ad during the coating
growth. As it was seen, hydrogen adsorption was the predomi-
nant adsorbate in zinc deposition (Fig. 5). This could be related
to the high tendency for coverage of the growing surface by
cobalt and/or cobalt-contained species which markedly reduce
the hydrogen adsorption.
¯ ¯
¯ ¯
angle pyramidal {1 1.5} ꢀ3 3.1ꢁ and {1 0.3} ꢀ3 2.1ꢁ orientations.
The AC impedance data for zinc–cobalt deposited at –1.08 V
corresponding to 10 mA cm−2 is seen in Fig. 9. It shows one
capacitive loop at high frequencies, an inductive loop followed
by a capacitive loop at low frequencies. This feature predicts
a different electrochemical behavior for zinc–cobalt deposition
Fig. 10 shows morphology of the zinc–cobalt deposited at
10 mA cm−2. The morphology consists of ␩-phase crystallites
with cobalt percentage of about 1.4 wt% determined by EDAX
analysis. This kind of morphology differs from the zinc mor-
phology (Fig. 4), because a large number of elongated crystals
are grown normal to the substrate without merging, as it is shown
in Fig. 11 with more details. This observation indicates that the
Fig. 8. Cross sections of the ODF of electrodeposited zinc–cobalt at ϕ2 = 30◦
and 55◦.
Fig. 10. Morphology of the zinc–cobalt deposited at 10 mA cm−2
.

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K. Raeissi et al. / Electrochimica Acta 53 (2008) 4674–4678
ing is developed by this kind of growth as a manner described
schematically in Fig. 12. As it was mentioned, the presence of
cobalt and/or cobalt-contained species inhibits the lateral growth
mode and encourages the perpendicular mode of growth.
As it can be seen from Fig. 10, some blocky crystals are also
found on top of the coating, which seems to have no contact
with the steel substrate surface. This indicates that perpendicular
growth supported by the steel substrate is degraded by increasing
the deposit thickness. It seems that random three-dimensional
nucleation has developed these blocky crystals on top of the pris-
matic crystallites. The low intensity basal and pyramidal texture
components represented in the corresponding ODF (Fig. 7) can
be related to the presence of these blocks.
4. Conclusion
Fig. 11. Morphology of the zinc–cobalt deposited at 10 mA cm−2 at higher
magnification.
1. Hydrogen adsorption during zinc electrodeposition process
develops non-fiber low angle pyramidal {1 1.5} and {1 0.3}
texture through lateral bunching growth. This develops ridges
morphology in the deposit.
2. Strong inhibition resulted by the adsorption of cobalt and/or
cobalt-containedspeciesisresponsibleforthehighfrequency
of two-dimensional nucleation which results “Field-Oriented
Texture” type of growth in Zn–Co electrodeposition. A
special kind of morphology consisted of numerous fibers
appearedasalargenumberofelongatedcrystalsnormaltothe
substrate is resulted. This develops non-fiber {1 1.0} prism
as the major texture component in the coating.
Fig. 12. Schematic representation of perpendicular growth of zinc–cobalt on
steel substrate surface.
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another interfacial inhibition is created with fairly strong
inhibitive effect. Therefore, a high degree of two-dimensional
nucleation could occur. This phenomenon prevents lateral
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growth called “Field-Oriented Texture” can be created [3,15].
This kind of growth is based on a high frequency of two-
dimensional nucleation and eventually creates a crystallographic
structure which comprises of numerous crystal fibers which are
grown almost normal to the substrate surface [3,15]. Each fiber
crystal is composed of many macrosteps that are piled upon
another one but cannot merge with each other [3]. Therefore, a
large number of elongated crystals perpendicular to the substrate
is expected to form. The prism texture component in the coat-
[15] R. Winand, Electrochim. Acta 39 (1994) 1091.