Jan 18, 1995 - Abstract-Zinc-nickel binary alloys have been successfully electrodeposited onto steel sheets from baths containing zinc acetate, nickel acetate ...
Pergamon 0013-4686(95)00327-4
Electrochimica Acra. Vol. 41. No. 9. pp. 1413 1418. 1996 Copyright (> 1996 Published by Elswier Smnce Ltd. Printed in Great Britain. All rights reserved 0013-4686196 $15.00 + 0.00
ELECTROPLATING OF ZINC-NICKEL BINARY FROM ACETATE BATHS S. S. Am
EL REHIM,* E. E.
Department
of Chemistry,
FOUAD,
Faculty
ALLOYS
S. M. ABD EL WAHAB and HAMDY. H. HASSAN of Science, Ain Shams University,
Cairo, Egypt
(Received 18 January 1995; in reuisedform 25 July 1995) Abstract-Zinc-nickel binary alloys have been successfully electrodeposited onto steel sheets from baths containing zinc acetate, nickel acetate and acetic acid (pH 4.4-4.6). The potentiodynamic cathodic polarization, cathodic current efficiency, morphology and composition of the deposits were determined for a variety of bath composition, temperature, current density and superimposed ac on dc. The baths are characterized by high cathodic current efficiency for co-deposition. The co-deposition shows an anomalous behaviour with zinc being the preferentially deposited metal. X-ray tests revealed the presence of a single y-phase (rich-zinc alloys) with body centered cubic structure. Copyright 0 1996 Published by Elsevier Science Ltd. Key words: zinc, nickel, electroplating,
binary
alloys.
INTRODUCTION Electroplating of Zn-Ni binary alloys has been applied to the production of highly corrosion resistant alloy plated steel mainly for automotive body panels[ 1, 21, aerospace[3] and electronics[4] industries. This alloy can be electrodeposited from various types of baths such as chloride containing ammonium chloride[5], sulphate containing boric acid[6], pyrophosphate[7] and sulphate[8] baths. In most cases, the electrodeposition of Zn-Ni alloys is a co-deposition of anomalous plating types, that is the electrochemically less noble zinc metal deposits preferentially on the cathode with respect to nickel the more noble metal. The mechanism which permits this anomaly still remains unknown. Several authors[9-I l] attributed the anomalous behaviour to the formation of zinc hydroxide on the cathode surface which suppresses the discharge of nickel ions. The zinc hydroxide formation arises from the local increase in pH due to the hydrogen evolution. However, Nicol and Philip[lZ] and Swathirajan[13] suggested that underpotential deposition (UDP) of the less noble metal on the cathode surface suppresses the deposition of the more noble metal. Continuing our research work on the individual electroplating of nickel[14] and zinc[l5] from acetate baths, we have likewise electroplated Zn-Ni binary alloys from the same baths under different plating conditions.
EXPERIMENTAL Experiments were carried out on solutions taining Zn(CH,COO), 2H,O, Ni(CH,C00)2 * Author to whom correspondence
conand
should be addressed. 1413
CH,COOH (glacial). All solutions were freshly prepared from A. R. chemicals and doubly distilled water. The pH of each solution was measured using a lseibolo Wien pH meter. The experimental set-up for electrodeposition consisted of a Perspex rectangular cell provided with a plane parallel steel sheet cathode and a platinum sheet anode. Each electrode had dimensions of 2.5 x 3 cm and filled the cross section of the cell. The volume of the solution was 150ml. Before each run, the cathode was mechanically polished with a 600 mesh emery paper and washed with distilled water, rinsed with ethanol and then weighed. Direct current (dc) was supplied by a 12V battery. The plating time was 15min. Experiments were performed at the required temperature f0.5”C with the help of an air thermostat. In some experiments a sinusoidal alternating current (UC) was superimposed on dc. The ac was supplied by an UC generator type GF 20 Rc and connected directly to the cathode and the anode. To achieve separation of ac and dc circuits, a capacitor (1OOpF) was introduced into the ac circuit and an inductor (10.5 H) into the dc circuit. After electrolysis, the cathode was withdrawn from the cell, washed thoroughly with distilled water, dried in dissicator and then weighed. The chemical composition of the deposited alloy was determined in its solid state by the use of energy dispersed X-ray system type LINK EDS 800/500. Cathode current efficiency (ccc) was calculated from the deposited mass and the total coulombs passed. Potentiodynamic cathodic polarization (E/i) curves were measured in the rectangular cell. A potentioscan (Wenking Model POS 73) was used as dc source. Potentials were measured against saturated calomel electrode (see). To avoid contamination the reference electrode was connected to the working steel electrode via a bridge provided with Luggin capillary and filled with solution under test. The capillary was placed as near as possible to the cathode
S. S. ABD EL REHIMet al.
1414
surface. The E/i curves were recorded using an X-Y recorder (series 2000, Ominographic). The morphology of the deposited alloys was examined using a scanning electron microscope (JEOL SEM T200). X-ray analysis of the deposits was carried out using a Philips diffractometer (40 kV, 25 mA) with Ni filter and Cu radiation.
RESULTS
AND
DISCUSSION
1. Cathodic polarization Figure 1 shows the potentiodynamic cathodic polarization curves for the electrodeposition of Ni, Zn and Zn-Ni alloys under similar conditions. These curves were swept from the rest potentials into the negative direction with a scan rate of 1 mV s- 1 and at 20°C. The polarization curves for the deposition of Zn and Zn-Ni alloy exhibit a small cathodic peak C prior to their respective deposition potentials at which steep activations are observed. This cathodic peak C is assigned previously[15] to discharge the H+ ions. The drop in the cathodic current beyond the peak potential of the peak C indicates suppression of hydrogen evolution. Such behaviour could be attributed to the formation of an inhibitory Zn hydroxide formed and adsorbed on a cathode surface. It is probable that some of the evolution sites for hydrogen are occupied by adsorbed Zn hydroxide. In hydrogen evolution in the presence of Zn ions, Zn hydroxide formed by hydrolysis due to the predominant hydrogen evolution within the region of peak C. It seems that the hydrolysis of Zn ions begins at the peak potential of peak C. It is generally accepted that the single deposition of Zn and Ni occurs via a two-step mechanism in which one electron is transferred in each step[16, 171. For Zn-Ni co-deposition, it is probable that the deposition of each metal occurs independently through two successive one-electron transfer steps. The deposition can proceed for a metal M (Zn or Ni)
Zn Ni
1.0 -
I’
0.8 p!
E 0.6 -
?5 ._
0.4 -
0.2 0.0
_--_-700
-900
-* -1100
, -1300
E (mV) Fig. 1. Calculated (-----) and experimental (---) polarization curves for electrodeposition of Zn from 0.1 M zinc acetate, Ni from 0.15 M nickel acetate and Zn-Ni alloys from 0.1 zinc acetate + 0.15 M nickel acetate. CH,COOH = 0.2 M, scan rate = 1 mV s- ’ and T = 20°C.
as follows: M’++e-+M+ M+ +e-++M where the intermediate M+ may or may not be adsorbed on the electrode surface. On the other hand, Chassaing et aZ.[18] suggested a reaction mechanism for the electrodeposition of Zn-Ni alloys in chloride bath. The specific feature of this mechanism is the formation of a mixed intermediate surface compound (ZnNi,;,), adsorbed on the cathode surface. Such an intermediate can be either spontaneously decomposed on the alloy surface as (ZnNi),:,
+ e- -+ Zn + Ni
or included as a whole into the deposit. Regarding the data of Fig. 1, it is seen that the polarization curve for Ni deposition lies at considerable more positive potentials than that of Zn indicating that Ni is the nobler metal. Moreover, the polarization curve for Zn-Ni co-deposition lies between those of the parent metals. Therefore, one can except preferential deposition of Ni and production of rich-Ni alloys. However, this expectation is not valid with the present system and the so-called anomalous co-deposition is observed according to Brenners classification[ 191 and rich-Zn alloys are always obtained. Nevertheless, with the aid of the experimentally determined alloy composition over the range of the current density used, the actual (calculated) partial polarization curves for each metal during co-deposition could be computed using the procedure recommended previously[ 181. The partial polarization curves (dotted) for Zn and Ni are given in Fig. 1. It is seen that the partial polarization curve for Zn is shifted to more positive values (depolarization) while that of Ni is shifted to more negative direction. This denotes that the electrodeposition of Ni tends to be suppressed in the presence of inhibitory Zn hydroxide just as in the case of hydrogen. Accordingly, Ni does not begin to deposit at its equilibrium potential but requires an extra potential[9]. This negative shift in the partial polarization of Ni amplifies electrode potential in causing anomalous co-deposition. Figure 2 shows the effect of a bath composition on the potentiodynamic polarization curves for Zn-Ni co-deposition. An increase in Zn or Ni content in the bath decreases the co-deposition potential. The reverse is observed by increasing the acid concentration in the bath. The charge corresponding to the cathodic peak C slightly increases with increasing the acid concentration. Increasing the acidity of the bath may cause an increase in the buffering capacity resulting in larger hydrogen evolution. Some of the evolved hydrogen may adsorb on the cathode surface and results in an increase in co-deposition potential and consequently an inhibition of alloy deposition. Figure 3 illustrates that an increase in the temperature of the bath significantly increases the reduction charge of the peak C and shifts the codeposition potential of the alloy to more positive direction. These changes may be due to the decrease in the activation overpotentials of both hydrogen
Electroplating
of zinc-nickel
binary
alloys
1415
l.O-
o.ak
0.8 -
h
0.6
-
E P ”
0.4
-
p!
-700
-900
-1100
-1300
E WV) Fig. 2. Potentiodynamic polarization curves for Zn-Ni alloys from baths containing (1) 0.1 M zinc acetate, 0.15 M nickel acetate, 0.2M acetic acid, (2) 0.2 M zinc acetate, O.lSM nickel acetate, 0.2M acetic acid, (3) 0.1 M zinc acetate, 0.25 nickel acetate, 0.2 M acetic acid, (4) 0.1 M zinc acetate, 0.15 M nickel acetate, 0.3 M acetic acid, scan rate = 1 mV s- ‘, T = 20°C.
evolution and alloy co-deposition. In addition, an increase in temperature leads to an increase in the rates of diffusion of reducible ions to the cathodic diffusion layer and consequently the concentration overpotentials also decrease. Figure 4 depicts the influence of superimposed sinusoidal UC on dc on the course of cathodic polarization features. Superimposed ac on dc decreases the co-deposition potential of the alloy and promotes hydrogen evolution. Such effects of UC can be explained in terms of asymmetric polarizability of the electrode. It seems that, under identical conditions, the anodic polarization of the electrode is greater than its cathodic one. This means that the anodic half cycle of UC is more effective than the cathodic half cycle. Therefore, the average value of cathodic potential under the influence of superimposed ac becomes less negative than that corresponding to dc alone[15, 203. Moreover, the shift in 1 .o
r
E (mV) Fig. 3. Effect of temperature on potentiodynamic polarization curves for electrodeposition of Zn-Ni alloys from bath containing 0.1 M zinc acetate, 0.15 M nickel acetate, 0.2 M acetic acid. pH = 4.6, scan rate = mV s- ‘. (1) 20°C (2) 35°C (3) 45°C (4) 55°C (5) 70°C.
E (mV) Fig. 4. Effect of alternating current density, i,, , on potentiodynamic polarization curves for Zn-Ni alloy deposition from bath as given in Fig. 3, (I) i, = 0, (2) i., = 0.66Adm-*, (3) i, = 0.99Adm-‘, (4) i, = 1.32Adm-*. Frequency = 50 Hz. Scan rate = 1 mV s- ‘, T = 20°C.
cathodic polarization may be related to the concentration changes at the electrode surface. The passage of UC through a solution of complex ions may produce periodic concentration changes[21]. These changes depend on the angular frequency and the reaction rate constants[22]. The concentration wavelength, as well as penetration depth, are also dependent on the same parameters. 2. Current eficiencies for the electrodeposition of Zn-Ni alloys The partial cathodic current efficiencies of Zn and Ni in the deposited alloys were determined under different plating variables. The sum of the two partial current efficiencies for a given alloy is the cathodic current effkiency (ccc) of that alloy. Figures 5-10 give the ccc vs the variables studied. Inspection of the data obtained reveals that in most cases the ccc is high denoting a small amount of hydrogen evolution. The partial current effkiency of Zn deposition is always very high compared to those of Ni deposition and hydrogen evolution. Frequently, any plating variable that increases the partial cathodic current efficiency of Zn deposition decreases the partial cathodic efficiencies of both Ni deposition and hydrogen evolution as well. Data of Figs 5-7 illustrate the influence of the bath composition on the ccc of the alloy deposition. The ccc increases with increasing Zn and with decreasing Ni contents in the bath. The later trend is not expected since an increase in the total metal content in the bath must increase cce[19]. Actually, the observed trend is the resultant of a large increase in the partial cathodic current efficiency of hydrogen reduction and a small increase in the partial current efficiency of Ni deposition. The ccc slightly decreases by increasing the current density (Fig. 8) as a result of decreasing the partial current efficiency of Zn deposition. Increasing temperature or superimposing ac on dc decreases the ccc (Figs 9 and 10, respectively) and this trend can be
S. S. ABD EL REHIM et al.
1416
20
.-
0-0
Nid
20
t
Ni
OLL
OI
0.3
0.3
[Zn++], M
[CH$OOH]
Fig. 5. Effect of zinc acetate concentration on current etliciency for alloy deposition (ccc) and on nickel content of alloys (Ni). CRL is the percentage of Ni in the bath. Ni acetic acetate = 0.15 M, acid = 0.2 M pH = 4.6, i = 1.33 A dm-*, T = 2o”C, time = 10 min.
related to the effects of these cathodic polarization curves. 3. Composition of Zn-Ni
parameters
electrodeposited
.-. 80 -
M
Fig. 7. Effect of acetic acid concentration on current efliciency for alloy deposition (ccc) and on Ni content of alloys (Ni). CRL is the percentage of Ni in the bath. Zn acetate = 0.1 M, Ni acetate = 0.15 M, i = 1.32Adm-‘, T = 20°C. time = 10min.
on the
alloys
Figures 5-10 also include the percentage of Ni in the alloy deposits as a function of the plating variables studied. The CRL in these figures stands for the composition reference line which gives the percentage of Ni in the bath. The outstanding features of the present data are the formation of Zn-rich alloy and the nickel content lies below the CRL showing that Zn deposits preferentially, thus causing anomalous co-deposition. During Zn-Ni co-deposition, a local rise in pH near the cathode surface occurs thus leading to the formation and adsorption of Zn hydroxide on the cathode. The adsorbed hydroxide
IOO-
/*
.-.-•
-.
l \.,
80 & J 60 5 r 2 4.
CRL _______-____-_---
.
_-__-_.
Ni
*-o-e
20
t 01 0.0
I 0.1
I
I
I
0.2
0.3
0.4
i (A dm-‘)
Fig. 8. Effect of current density (i) on current elliciency for alloy deposition (ccc) and on nickel content of alloys (Ni), Zn acetate = 0.1 M. Ni acetate = 0.15. acetic acid = 0.2 M. pH = 416, T = 2O”C, time = 1Omin.
CCE \. 100 -
1.
.-.a_. 80 & 2 60e z a” 40-
\ :
CRL __-____--___________~-~~~~-
20
01 0.1’
Ni
20-
I
I
0.2
0.3
[NI++], M Fig. 6. Effect of nickel acetate concentration on current efficiency for alloy deposition (ccc) and on nickel content of alloys (Ni). CRL is the percentage of Ni in the bath. Zn acetate = 0.1 M, acetic acid 0.2 M, pH = 4.6, i = 1.32Adme2, T = 2O”C, time = 1Omin.
.-
.-.01
I
I
I
20
40
60
Temperature
(“C)
Fig. 9. Effect of temperature on current efliciency for alloy deposition (ccc) and on nickel content of alloys (Ni). Conditions as given in Fig. 8.
1417
Electroplating of zinc-nickel binary alloys
with decreasing Ni content in the bath. This can be discussed on the basis that an increase in metal content in the bath tends to oppose the depletion of that metal in the cathodic diffusion layer. Data of Fig. 7 indicate that the increase in the acetic acid concentration decreasing the ccc but enhances slightly the Ni content of the deposits. Therefore, the inhibition of the alloy deposition is connected to some inhibiting influence of adsorbed hydrogen on Zn deposition. Increasing the current density or the bath temperature, or superimposing ac on dc, tends to increase slightly the Ni content of the deposits and therefore decrease the anomalous feature.
CRL
_____________________-____-_-
.-
20 1.-.-
0
I I
I 2
I
3
I, (Adm-‘) Fig. 10. Effect of ac density (i,,) on current efficiency for alloy deposition (ccc) and on Ni content of alloys (Ni). Conditions as given in Fig. 8. inhibits the hydrogen evolution and Zn deposition via blocking their deposition sites on the cathode. In addition, the inhibition of the hydrogen evolution is also due to its high overvoltage on the Zn surface[9]. Nevertheless, the adsorbed Zn hydroxide permits a high rate of Zn electrodeposition since the adsorbed layer can provide discharging species for Zn deposition. Figures 5 and 6 outline the Ni content in the deposits decreases with increasing Zn content, or
4. Morphology and structure of Zn-Ni alloys
deposited
The morphology of the as deposited Zn-Ni alloys were investigated by means of scanning electron microscopy (SEM). Some of the micrographs are 11. At low current densities given in Fig. (< 0.7 A dm -*) the deposits consist of loosely adherent nodular grains piled together in a random orientation and not uniformly on the surface as shown in (Fig. lla). With an increase in current density, smooth, adherent, compact and fine-grained deposits are obtained on the whole surface (Fig. llb). The uniformity and fineness of the deposits are enhance by increasing temperature. Above 6O”C, the deposits contain some cracks (Fig. 1lc). Superimposed ac on dc also improves the uniformity and fineness of the deposits but causes some cracks (Fig. lid). It is probable that uniform, fine-grained and cracked
Fig. 11. Scanning electron micrographs of Zn-Ni deposits from a bath containing 0.1 M Zn acetate, 0.15 Ni acetate and 0.2 M acetic acid, pH 4.6, time = 10min. Plate (a), i = 0.66AdmmZ, T = 20°C. Plate (b), i = 1.32Adm-*, T = 20°C. Plate (c), i = 1,32Adm- 2, T = 60°C. Plate (d), in the presence of superimposed ac, i = 1.32Adm-‘, T = 2o”C, i,, = 1.32Admm2, frequency 50Hz.
1418
S. S.
ABD EL REHIM et al.
Table 1. X-ray studies of the alloys deposited from a bath containing Zn acetate 0.1 M, Ni acetate 0.15 M, acetic acid 0.2M, i-1.32Adm-‘, 25”C, 10min. Alloy composition % Zn
Ni
Lattice
Lattice parameter (A) (a)
83.6
16.4
bee
8.921
deposits could be the consequence of hydrogen evolution which to some extent is significant at high temperature and in the presence of superimposed UC. Adsorption of hydrogen atom on the cathode surface impedes the grain growth and enhances nucleus generation resulting therefore in uniform and fine grained deposits. In addition, the adsorbed hydrogen may interfere with the normal flow or slip of the lattice planes under stress. If microscopic voids occur in the substrate, hydrogen may accumulate in molecular form at a relatively high pressure, sufficient to develop cracks[23]. X-ray diffraction of the deposits (in as deposited conditions) showed the formation of rich--Zn single y-phase alloys with body-centered cubic structure (Table 1).
CONCLUSIONS Zn-Ni binary alloys were electroplated on steel sheet cathodes from baths containing zinc acetate, nickel acetate and acetic acid. The effects of a bath composition, current density, temperature and superimposed ac on dc, on the cathodic current efficiency of co-deposition, composition, morphology and structure of the deposited alloys were examined. Hydrogen evolution and nickel co-deposition are strongly suppressed in these baths. This is due to the formation and adsorption of Zn hydroxide on the cathode surface. Subsequently, high current efflciency for co-deposition and Zn-rich (y-phase) alloys are obtained and the system falls in the anomalous category. At current densities higher than 0.7 A dm-’ adherent, smooth and compact deposits are plated. Hydrogen evolution appears to play an important part in changing surface appearance and morphology of the deposits.
An explanation has been offered for the various features observed in this study in the light of cathodic polarization phenomenon.
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