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The role of addition agents in the electrodeposition of Ni-Mn-Zn alloys from a sulphate bath
Surface and Coatings Technology, 28 (1986) 129 - 137
129
THE ROLE OF ADDITION AGENTS IN THE ELECTRODEPOSITION OF Ni-Mn-Zn ALLOYS FROM A SULPHATE BATH R. KASHYAP, S. K. SRIVASTAVA and S. C. SRIVASTAVA Department of Chemistry, University of Lucknow, Lucknow (India) (Received May 17, 1985)
Summary The modifications caused by the addition agents dextrin, glucose, 1serme, j3-alanine and glycine during the electrodeposition of Ni—Mn—Zn alloy have been investigated as a function of various plating variables. The electrolytic bath contained 20 g 1~nickel sulphate, 80 g 1’ manganese sulphate, 20 g 1’ zinc sulphate, 30 g l’ ammonium sulphate, 18 g 1_i thiourea and 0.8 g 1_i ascorbic acid. Bright light grey, smooth and unevenly fine-grained deposits were generally formed in all cases except for j3-alanine which gave blackish-grey, uneven and coarsely crystalline deposits. The nickel content in the deposits was found to increase with the addition of dextrin, glucose or 1-serine and was decreased on adding 13-alanine or glycine to the bath. In contrast, the zinc content followed the reverse trend under similarconditions. The percentage of manganese, however, was always smaller when glucose, $3-alanine or glycine were added to the bath but was greater with the addition of dextrin or l-serine. Further, the alloy composition also changed with increasing concentration of these agents. The cathode current efficiency was found to increase linearly in each case as the current density was increased and remained almost unaffected by the presence of the above additives at a given current density. Dextrin, glucose and l-serine were observed to lower the cathode polarization at a given current density; 13-alanine and glycine caused the value of the cathode polarization to increase. Comparatively large values for the throwing power were thus obtained when dextrin, glucose or l-serine was added to the bath. Attempts have been made to explain the results on the basis of adsorption and other theoretical aspects of electrodeposition.
1. Introduction The inclusion of small amounts of certain organic substances [1, 2] in the depositing solution generally leads to morphological changes in electrodeposits as well as influencing their physicomechanical characteristics [3, 4]. Several such addition agents [5 15] are reported to have been used to modi-
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fy the nature of the electrodeposits in three-component alloys containing nickel or manganese. However, no systematic effort has been made to explore the choice of these agents to achieve certain desired results. The present study is concerned with the effects of the addition agents dextrin, glucose, l-serine, 13-alanine and glycine on the morphology and composition of the alloy, the cathode current efficiency, the cathode polarization and the throwing power during the electrodeposition of an Ni—Mn—Zn alloy from a sulphate bath. The electrodeposition of this alloy under various plating conditions has been described elsewhere [16].
2. Experimental details Thin coatings of the Ni—Mn—Zn alloy were deposited at 30 °Cand a pH of 3.7 by electrolysing the electrolytic solution containing dextrin, glucose, l-serine, 13-alanine or glycine as an addition agent and were analysed to determine their percentage compositions and cathode current efficiencies using procedures reported elsewhere [16]. The cathode potentials were measured to an accuracy of ±0.001V against a saturated calomel electrode. The steady state value of the electromotive force of the cathode—calomel electrode combination with and without the passage of a definite current was recorded on a vernier potentiometer using a sensitive lamp-and-scale galvanometer as the zero-point indicator. The difference between the potentials attained under given plating conditions was taken as the cathode polarization The reported values correspond to the hydrogen scale. The throwing power N was calculated from the specific resistance p of the electrolyte and the Tafel slope b of the plot of the cathode polarization against the logarithm of the current density by using Gardam’s formula [17]: ~.
b N=
—
The morphological changes in the deposits caused by the addition agents were examined using photomicrographs.
3. Results Comparatively smooth and fine-grained coatings were produced by adding any of the above surfactants to the electrolytic bath. The exact morphological changes caused by these additives are given in Table 1. It can be seen that the addition of dextrin, glucose, l-serine or glycine favours the formation of light grey, fairly smooth, bright and fine-grained deposits, whereas uneven and coarsely crystalline blackish-grey deposits are formed in the presence of 13-alanine under similar electrolytic conditions. The variation in the deposit composition produced by these additives is demonstrated in Table 2 where the nickel content is seen to increase on
131 TABLE 1 Summary of the morphologies of the alloys deposited in the absence and in the presence of addition agents
Sample number
Addition agenta
Current density
Morphology of the deposit
2)
(A dm 1
None
1.0
2
None
2.0
3 4
Dextrin Dextrin
1.0 2.0
5
Glucose
1.0
6
Glucose
2.0
7
l-Serine
1.0
8
l-Serine
2.0
9
~3-Alanine
1.0
10 11
j3-Alanine Glycine
2.0 1.0
12
Glycine
2.0
Light grey uneven fine-grained compact deposit Dark grey uneven deposits with localized regions of comparatively large crystals Blackish-grey smooth fine-grained deposit Dark grey uneven crystalline deposit with scattered regions of larger crystals Bright and fairly smooth, fine-grained uneven compact deposit Blackish-grey uneven coarsely crystalline deposits having scattered regions of grains larger than found in sample 5 Blackish-dark-grey smooth microcrystalline uneven deposit Blackish-dark-grey deposit, rougher than sample 7 with scattered regions of larger crystals Light grey uneven crystalline deposit with scattered compact regions of larger grain size than found in sample 4 Dark grey uneven coarsely crystalline deposit Blackish-grey smooth fine-grained uneven deposit with localized regions of larger crystals Blackish-grey uneven smooth crystalline deposit
Bath composition (g 1’): NiSO 4, 20; ZnSO4, 20; MnSO4, 80; (NH4)2S04, 30;thiourea, 18; ascorbic acid, 0.8. Temperature, 30 °C;pH, 3.7. aConcentration of addition agents, 1 g l’.
the addition of dextrin, glucose or l-serine but decreases on the addition of /3-alanine or glycine at a given current density. In contrast, the zinc content followed the reverse trend under similar conditions. Further, the percentage of manganese increases on the addition of dextrin or l-serine, but with glucose, 13-alanine or glycine it decreases. The alloy composition has also been found to be a function of the additive concentration, as is evident from Table 3. The nickel percentage in the alloy films increases with increasing concentration of dextrin, glucose or l-serine and with u3-alanine or glycine it decreases. In contrast, the reverse trend is observed in the case of zinc. The manganese content decreases when the concentration of glucose, /3-alanine or glycine in the electrolytic bath is increased, whereas it increases with increasing concentration of dextrin or
132 TABLE 2 Effect of addition agents on the deposit composition Addition agenta
Metal in the deposit (wt.%)
None Dextrin Glucose 1-Serine I3-Alanine Glycine
Ni
Mn
Zn
8.53 8.74 8.94 8.74 8.13 8.33
0.56 0.58 0.52 0.58 0.44 0.52
90.91 90.68 90.54 90.68 91.43 91.15
Bath composition: as for Table 1. 2. Temperature, 30of°C;pH, 3.7;agents, current1 gdensity, 2.0 A dm~ aConcentration addition
TABLE 3 Effect of the addition agent concentration on the deposit composition Addition agent
None Dextrin Glucose l-Serine 3-Alanine Glycine
Concentration (gl1)
Metal in the deposit (wt.%) Ni Mn
Zn
1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0
8.53 8.74 8.94 8.94 9.35 8.74 8.94 8.13 7.92 8.33 8.13
90.91 90.68 90.42 90.54 90.13 90.68 90.42 91.43 91.68 91.15 91.43
0.56 0.58 0.64 0.52 0.52 0.58 0.64 0.44 0.40 0.52 0.44
Bath composition: as for Table 1. Temperature, 30 ~C; pH, 3.7; current density, 2.0 A dm2.
l-serine. No significant morphological changes in the electroplates are observed as the current density is increased in the presence of these agents. However, the percentage composition of the deposits varies as shown in Table 4. The amounts of nickel and manganese in the alloy invariably increase with increasing current density, whereas the zinc content decreases monotonically. The cathode current efficiency remained almost unaffected by the
presence of the addition agents at a given current density (see Table 5). However, the current efficiency gradually increases linearly as the current density is increased with and without the addition of these agents. A maximum value of 69.36% was obtained with glucose at a current density of 3.0 A dm~2.
133 TABLE 4 Effect of current density on the deposit composition in the absence and in the presence of addition agents Metal Metal in the deposit (wt.%) at various current densities
2
1.0 A dm
Addition
1.5 A dm2
2.0 A dm2 2.5 A dm2
3.0 A dm2
agenta
Ni Mn Zn
6.50 0.24 93.26
7.72 0.40 91.88
8.53 0.56 90.91
9.35 0.72 89.93
10.36 0.90 88.74
None None None
Ni Mn Zn
6.71 0.26 93.03
7.92 0.44 91.64
8.74 0.58 90.68
9.55 0.76 89.69
10.57 0.90 88.53
Dextrin Dextrin Dextrin
Ni Mn Zn
6.91 0.26 92.83
7.92 0.40 91.68
8.94 0.52 90.54
9.55 0.72 89.73
10.77 0.90 88.33
Glucose Glucose Glucose
Ni Mn Zn
6.71 0.32 92.97
7.92 0.40 91.68
8.74 0.58 90.68
9.75 0.76 89.49
10.57 0.90 88.53
l-Serine l-Serine l-Serine
Ni Mn Zn
6.30 0.26 93.44
7.52 0.40 92.08
8.13 0.44 91.43
9.14 0.64 90.22
10.16 0.84 89.00
j3-Alanine j3-Alanine j3-Alanine
Ni Mn Zn
6.30 0.26 93.44
7.52 0.40 92.08
8.33 0.52 91.15
9.14 0.64 90.22
9.96 0.76 89.28
Glycine Glycine Glycine
Bath composition: as for Table 1. Temperature, 30 °C;pH, 3.7. a~ncentration of addition agents, 1 g 11. TABLE 5 Effect of addition agents on the cathode current efficiency as a function of current density Addition
agent None Dextrin Dextrin Glucose Glucose l-Serine l-Serine 13-Alanine 13-Alanine Glycine Glycine
~
Concen-
1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0
Cathode current efficiency
(%)
at various current densities
1.OAdm2
1.5Adm2
2.OAdm2
2.5Adm2
3.OAdm2
66.68 66.70 66.73 66.72 66.81 66.71 66.74 66.67 66.65 66.67 66.65
67.40 67.42 67.43 67.42 67.48 67.42 67.45 67.39 67.01 67.39 67.35
67.79 67.81 67.83 67.82 67.86 67.81 67.83 67.75 67.73 67.78 67.75
68.45 68.47 69.15 68.46 68.51 68.48 68.73 68.44 68.20 68.44 68.40
69.27 69.29 69.32 69.31 69.33 69.29 69.32 69.25 69.04 69.23 69.04
Bath composition as for Table 1. Temperature, 30 °C;pH, 3.7.
134
The variation in the cathode polarization with current density during electrodeposition of the alloy both in the presence and in the absence of an
additive in the electrolytic bath is shown in Table 6. The cathode polarization decreased in the presence of dextrin, glucose or 1-serine at a given current density; with 13-alanine and glycine it was found to increase slowly at current densities above 2.0 A dm2. Furthermore, in each case it has been observed to shift to more negative values as the current density is increased. The Tafel relation (see Fig. 1) was found to be satisfied in the presence of the above addition agents in the electrolytic bath and hence Gardam’s formula was applied to calculate the throwing power N. It was comparatively high in the presence of dextrin, glucose or l-serine (see Table 7). 0.98 0.97
~
-
0.96~
.
0.92 0.91
I
0.0
I
I
0.1 0.2 0.3 0.6 LOG CURRENT DENSITY
0.5
Fig. 1. Variation in the cathode polarization with the logarithm of the current density at 30 °C (bath composition as for Table 1) in the absence and in the presence of addition agents (1 g 1_i): curve 1, glucose; curve 2, l-serine; curve 3, dextrin; curve 4, none; curve 5, glycine; curve 6, j3-alanine.
4. Discussion The electrodeposition of alloys from aqueous solutions may be satisfactorily explained on the basis of the simultaneous discharge of cations. It has been suggested by Antropov [18] that in general the polarization should be low if there is only one electron taking part in the discharge step or when the process proceeds through successive one-electron steps. A high polarization may result if the discharge of metallic ions involves two-electron transfer in a single step. The electrochemical discharge process may be facilitated by the presence of an addition agent through the formation of a
135
00c~I~0
a~
~
~ 00000
a a 00000
11111
a a
-~N~
—
Lr~L0~0
~
~o~0
11111 a
-a kc,ca
-~
~ 00000
11111
C
g
I
S
a
~
~
.~
o
a
~
~,
a
:~ ~
oa. a
a 00000
~
I
C
a
u
~
a
III’’
aC) a
a a
a a
a a
I I I
a a
C
a~
a C)
0W,-4t-t’ 0 N 0 C) N
ao
a, ~
~ ~
0 0 0 0 0
a
00
0
a
-a a 0
0~a
~
F.
L)u~—
—4~—iC’1C~C’~
aaç~ ~
136 TABLE 7 Effect of addition agents on the resistivity, the Tafel slope and the throwing power
Addition agenta
Resistivity
Thfel slope b
Throwing power N
(~lcm)
(V)
(A cm~
None Dextrin Glucose l-Serine 13-Alanine Glycine
15.97 16.10 17.12 17.21 15.77 15.95
0.0625 0.0636 0.0773 0.0700 0.0600 0.0609
0.00196 0.00198 0.00226 0.00203 0.00190 0.00191
1)
Bath composition 0 as for Table 1. Temperature, 30 C; pH, 3.7. aconcentration of addition agents, 1 g L’.
complex in the solution owing to the oxidation of the agent by the following mechanism: M2~+ AA —f (M—AA)~ (M—AA)~+ e
—*
M
+
AA
where M metal and AA addition agent. The complex, after being adsorbed on the cathode surface, may accept an electron at the active site to discharge the metal ion which is finally incorporated in the deposit. The addition agent is thereby released and can reform the complex. The reaction proceeds thus, accelerating continuously. This may cause the polarization to decrease, as a result of which uniform fine and bright deposits are produced, as has been observed in the case of dextrin, glucose and l-serine (see Table 6). A comparatively high value of polarization with f3-alanine or glycine in the bath may be attributed to the preferential adsorption of these additives or one of their decomposition products in the electroplates. Such adsorption changes the nature of the cathode surface by blocking the various growth sites, leading to an increased concentration of anions in the vicinity of the cathode and thereby resulting in an increased polarization at a given current density. This increased polarization is probably responsible for enhancing the formation of new crystal nuclei on the metal surface by suppressing the growth of existing crystallites. As a consequence smooth and bright deposits are formed in the presence of these additives as has been discussed by Brenner [1]. Further, a gradual shift in the cathode polarization to more negative values with increasing current density in each case may be caused by the increased rate of discharge of metal ions at the cathode. Since, at higher current densities, the rate of discharge of metal ions at the cathode is limited by diffusion, more hydrogen ions are simultaneously discharged, which makes the cathode polarization become more negative (see Table 6).
Preferential adsorption of one metal ion over another at various faces and growth sites of the cathode surface in the presence of the various addi-
137
tives can cause the deposit composition to vary. The discharge of a particular metal ion may be blocked as a result of this preferential adsorption on a specific growth site and it may consequently lead to an evening out of the growth rate for that metal. Furthermore, the observed increase in cathode current efficiency with increasing current density in the presence of these agents in the electrolytic
bath (see Table 5) may be due to the utilization of comparatively more current for the deposition of the alloy than for the discharge of the hydrogen ions which result from the ionization of the solvent molecules. The throwing power obtained for the deposition of the alloy in the presence of glucose is significantly higher than that obtained in the presence of the other agents, as is evident from Table 7. It follows that relatively uniform coatings should be formed in the former case and in fact this has been found experimentally. Acknowledgment One of the authors (R.K.) is grateful to the Council of Scientific and Industrial Research, New Delhi, for providing financial assistance.
References 1 A. Brenner, Electrodeposition of Alloys: Principles and Practice, Vol. I, Academic Press, New York, 1963. 2 E. Raub, Z. Elektrochem., 55 (1951) 146 - 151. 3 S. Ichioka, Jpn. Patent 75,06,416 (Cl. C25D, C22 C, HOIF) (1975). 4 N. I. Shklovskaya, I. B. Murashova, 0. F. Bondarenko and V. I. Marochkin, Probi. Elektrokhim. Korroz. Met., 1 (1977) 30 - 34. 5 T. S. Vanaja and T. L. Ramachar, Electroplat. Met. Finish., 26 (8) (1973) 15 - 17, 20 - 23. 6 0. Petrosaviciute, B. Stulpinas and E. Siusa, Nauchn. Tr. Vyssh. Uchebn. Zaved. Lit. SSR, Khim. Khim. Tekhnol., (1975) 207 - 211. 7 W. H. McMullen and T. J. Mooney, Ger. Patent 2,630,980 (Cl. C25D3/56) (1977); U.S. PatentAppl. 594, 214 (1975). 8 J. R. Clauss, A. R. Tremmel and C. N. Adamowicz, U.S. Patent 3,806,429 (CL 204-41 C23b) (1974). 9 R. P. Dambal and T. L. Ramachar, Electroplat. Met. Finish., 25 (4) (1972) 10 - 18. 10 V. B. Singh, J. Appl. Electrochem., 9 (3) (1979) 285 - 289. 11 R. Sivakumar and T. L. Ramachar, Met. Finish. J., 18 (206) (1972) 65 - 70. 12 0. Petroseviciute, B. Stulpinas and E. Siusa, Liet. TSR Aukst. Mokyklu Mokslo Darb., Chem. Chem. TechnoL, 19 (1978) 38 - 43. 13 R. K. Shukla, S. K. Jha and S. C. Srivastava, Surf. Technol., 17(1982) 157. 14 S. N. Srivastava, R. Kashyap and S. C. Srivastava, Acta Chim. Acad. Sci. Hung., 117 (1) (1984)3 -9. 15 S. K. Jha, R. K. Shukla and S. C. Srivastava, Surf. Technol., 21 (1984) 383 - 389. 16 R. Kashyap, S. N. Srivastava and S. C. Srivastava, J. App!. Electrochem., 15 (1985) 23 - 27. 17 G. E. Gardam, Trans. Faraday Soc., 34 (1938) 698. 18 L. I. Antropov, Theoretical Electrochemistry, Mir, Moscow, 1972, p. 496.
129
THE ROLE OF ADDITION AGENTS IN THE ELECTRODEPOSITION OF Ni-Mn-Zn ALLOYS FROM A SULPHATE BATH R. KASHYAP, S. K. SRIVASTAVA and S. C. SRIVASTAVA Department of Chemistry, University of Lucknow, Lucknow (India) (Received May 17, 1985)
Summary The modifications caused by the addition agents dextrin, glucose, 1serme, j3-alanine and glycine during the electrodeposition of Ni—Mn—Zn alloy have been investigated as a function of various plating variables. The electrolytic bath contained 20 g 1~nickel sulphate, 80 g 1’ manganese sulphate, 20 g 1’ zinc sulphate, 30 g l’ ammonium sulphate, 18 g 1_i thiourea and 0.8 g 1_i ascorbic acid. Bright light grey, smooth and unevenly fine-grained deposits were generally formed in all cases except for j3-alanine which gave blackish-grey, uneven and coarsely crystalline deposits. The nickel content in the deposits was found to increase with the addition of dextrin, glucose or 1-serine and was decreased on adding 13-alanine or glycine to the bath. In contrast, the zinc content followed the reverse trend under similarconditions. The percentage of manganese, however, was always smaller when glucose, $3-alanine or glycine were added to the bath but was greater with the addition of dextrin or l-serine. Further, the alloy composition also changed with increasing concentration of these agents. The cathode current efficiency was found to increase linearly in each case as the current density was increased and remained almost unaffected by the presence of the above additives at a given current density. Dextrin, glucose and l-serine were observed to lower the cathode polarization at a given current density; 13-alanine and glycine caused the value of the cathode polarization to increase. Comparatively large values for the throwing power were thus obtained when dextrin, glucose or l-serine was added to the bath. Attempts have been made to explain the results on the basis of adsorption and other theoretical aspects of electrodeposition.
1. Introduction The inclusion of small amounts of certain organic substances [1, 2] in the depositing solution generally leads to morphological changes in electrodeposits as well as influencing their physicomechanical characteristics [3, 4]. Several such addition agents [5 15] are reported to have been used to modi-
0376-4583/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
130
fy the nature of the electrodeposits in three-component alloys containing nickel or manganese. However, no systematic effort has been made to explore the choice of these agents to achieve certain desired results. The present study is concerned with the effects of the addition agents dextrin, glucose, l-serine, 13-alanine and glycine on the morphology and composition of the alloy, the cathode current efficiency, the cathode polarization and the throwing power during the electrodeposition of an Ni—Mn—Zn alloy from a sulphate bath. The electrodeposition of this alloy under various plating conditions has been described elsewhere [16].
2. Experimental details Thin coatings of the Ni—Mn—Zn alloy were deposited at 30 °Cand a pH of 3.7 by electrolysing the electrolytic solution containing dextrin, glucose, l-serine, 13-alanine or glycine as an addition agent and were analysed to determine their percentage compositions and cathode current efficiencies using procedures reported elsewhere [16]. The cathode potentials were measured to an accuracy of ±0.001V against a saturated calomel electrode. The steady state value of the electromotive force of the cathode—calomel electrode combination with and without the passage of a definite current was recorded on a vernier potentiometer using a sensitive lamp-and-scale galvanometer as the zero-point indicator. The difference between the potentials attained under given plating conditions was taken as the cathode polarization The reported values correspond to the hydrogen scale. The throwing power N was calculated from the specific resistance p of the electrolyte and the Tafel slope b of the plot of the cathode polarization against the logarithm of the current density by using Gardam’s formula [17]: ~.
b N=
—
The morphological changes in the deposits caused by the addition agents were examined using photomicrographs.
3. Results Comparatively smooth and fine-grained coatings were produced by adding any of the above surfactants to the electrolytic bath. The exact morphological changes caused by these additives are given in Table 1. It can be seen that the addition of dextrin, glucose, l-serine or glycine favours the formation of light grey, fairly smooth, bright and fine-grained deposits, whereas uneven and coarsely crystalline blackish-grey deposits are formed in the presence of 13-alanine under similar electrolytic conditions. The variation in the deposit composition produced by these additives is demonstrated in Table 2 where the nickel content is seen to increase on
131 TABLE 1 Summary of the morphologies of the alloys deposited in the absence and in the presence of addition agents
Sample number
Addition agenta
Current density
Morphology of the deposit
2)
(A dm 1
None
1.0
2
None
2.0
3 4
Dextrin Dextrin
1.0 2.0
5
Glucose
1.0
6
Glucose
2.0
7
l-Serine
1.0
8
l-Serine
2.0
9
~3-Alanine
1.0
10 11
j3-Alanine Glycine
2.0 1.0
12
Glycine
2.0
Light grey uneven fine-grained compact deposit Dark grey uneven deposits with localized regions of comparatively large crystals Blackish-grey smooth fine-grained deposit Dark grey uneven crystalline deposit with scattered regions of larger crystals Bright and fairly smooth, fine-grained uneven compact deposit Blackish-grey uneven coarsely crystalline deposits having scattered regions of grains larger than found in sample 5 Blackish-dark-grey smooth microcrystalline uneven deposit Blackish-dark-grey deposit, rougher than sample 7 with scattered regions of larger crystals Light grey uneven crystalline deposit with scattered compact regions of larger grain size than found in sample 4 Dark grey uneven coarsely crystalline deposit Blackish-grey smooth fine-grained uneven deposit with localized regions of larger crystals Blackish-grey uneven smooth crystalline deposit
Bath composition (g 1’): NiSO 4, 20; ZnSO4, 20; MnSO4, 80; (NH4)2S04, 30;thiourea, 18; ascorbic acid, 0.8. Temperature, 30 °C;pH, 3.7. aConcentration of addition agents, 1 g l’.
the addition of dextrin, glucose or l-serine but decreases on the addition of /3-alanine or glycine at a given current density. In contrast, the zinc content followed the reverse trend under similar conditions. Further, the percentage of manganese increases on the addition of dextrin or l-serine, but with glucose, 13-alanine or glycine it decreases. The alloy composition has also been found to be a function of the additive concentration, as is evident from Table 3. The nickel percentage in the alloy films increases with increasing concentration of dextrin, glucose or l-serine and with u3-alanine or glycine it decreases. In contrast, the reverse trend is observed in the case of zinc. The manganese content decreases when the concentration of glucose, /3-alanine or glycine in the electrolytic bath is increased, whereas it increases with increasing concentration of dextrin or
132 TABLE 2 Effect of addition agents on the deposit composition Addition agenta
Metal in the deposit (wt.%)
None Dextrin Glucose 1-Serine I3-Alanine Glycine
Ni
Mn
Zn
8.53 8.74 8.94 8.74 8.13 8.33
0.56 0.58 0.52 0.58 0.44 0.52
90.91 90.68 90.54 90.68 91.43 91.15
Bath composition: as for Table 1. 2. Temperature, 30of°C;pH, 3.7;agents, current1 gdensity, 2.0 A dm~ aConcentration addition
TABLE 3 Effect of the addition agent concentration on the deposit composition Addition agent
None Dextrin Glucose l-Serine 3-Alanine Glycine
Concentration (gl1)
Metal in the deposit (wt.%) Ni Mn
Zn
1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0
8.53 8.74 8.94 8.94 9.35 8.74 8.94 8.13 7.92 8.33 8.13
90.91 90.68 90.42 90.54 90.13 90.68 90.42 91.43 91.68 91.15 91.43
0.56 0.58 0.64 0.52 0.52 0.58 0.64 0.44 0.40 0.52 0.44
Bath composition: as for Table 1. Temperature, 30 ~C; pH, 3.7; current density, 2.0 A dm2.
l-serine. No significant morphological changes in the electroplates are observed as the current density is increased in the presence of these agents. However, the percentage composition of the deposits varies as shown in Table 4. The amounts of nickel and manganese in the alloy invariably increase with increasing current density, whereas the zinc content decreases monotonically. The cathode current efficiency remained almost unaffected by the
presence of the addition agents at a given current density (see Table 5). However, the current efficiency gradually increases linearly as the current density is increased with and without the addition of these agents. A maximum value of 69.36% was obtained with glucose at a current density of 3.0 A dm~2.
133 TABLE 4 Effect of current density on the deposit composition in the absence and in the presence of addition agents Metal Metal in the deposit (wt.%) at various current densities
2
1.0 A dm
Addition
1.5 A dm2
2.0 A dm2 2.5 A dm2
3.0 A dm2
agenta
Ni Mn Zn
6.50 0.24 93.26
7.72 0.40 91.88
8.53 0.56 90.91
9.35 0.72 89.93
10.36 0.90 88.74
None None None
Ni Mn Zn
6.71 0.26 93.03
7.92 0.44 91.64
8.74 0.58 90.68
9.55 0.76 89.69
10.57 0.90 88.53
Dextrin Dextrin Dextrin
Ni Mn Zn
6.91 0.26 92.83
7.92 0.40 91.68
8.94 0.52 90.54
9.55 0.72 89.73
10.77 0.90 88.33
Glucose Glucose Glucose
Ni Mn Zn
6.71 0.32 92.97
7.92 0.40 91.68
8.74 0.58 90.68
9.75 0.76 89.49
10.57 0.90 88.53
l-Serine l-Serine l-Serine
Ni Mn Zn
6.30 0.26 93.44
7.52 0.40 92.08
8.13 0.44 91.43
9.14 0.64 90.22
10.16 0.84 89.00
j3-Alanine j3-Alanine j3-Alanine
Ni Mn Zn
6.30 0.26 93.44
7.52 0.40 92.08
8.33 0.52 91.15
9.14 0.64 90.22
9.96 0.76 89.28
Glycine Glycine Glycine
Bath composition: as for Table 1. Temperature, 30 °C;pH, 3.7. a~ncentration of addition agents, 1 g 11. TABLE 5 Effect of addition agents on the cathode current efficiency as a function of current density Addition
agent None Dextrin Dextrin Glucose Glucose l-Serine l-Serine 13-Alanine 13-Alanine Glycine Glycine
~
Concen-
1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0 1.0 2.0
Cathode current efficiency
(%)
at various current densities
1.OAdm2
1.5Adm2
2.OAdm2
2.5Adm2
3.OAdm2
66.68 66.70 66.73 66.72 66.81 66.71 66.74 66.67 66.65 66.67 66.65
67.40 67.42 67.43 67.42 67.48 67.42 67.45 67.39 67.01 67.39 67.35
67.79 67.81 67.83 67.82 67.86 67.81 67.83 67.75 67.73 67.78 67.75
68.45 68.47 69.15 68.46 68.51 68.48 68.73 68.44 68.20 68.44 68.40
69.27 69.29 69.32 69.31 69.33 69.29 69.32 69.25 69.04 69.23 69.04
Bath composition as for Table 1. Temperature, 30 °C;pH, 3.7.
134
The variation in the cathode polarization with current density during electrodeposition of the alloy both in the presence and in the absence of an
additive in the electrolytic bath is shown in Table 6. The cathode polarization decreased in the presence of dextrin, glucose or 1-serine at a given current density; with 13-alanine and glycine it was found to increase slowly at current densities above 2.0 A dm2. Furthermore, in each case it has been observed to shift to more negative values as the current density is increased. The Tafel relation (see Fig. 1) was found to be satisfied in the presence of the above addition agents in the electrolytic bath and hence Gardam’s formula was applied to calculate the throwing power N. It was comparatively high in the presence of dextrin, glucose or l-serine (see Table 7). 0.98 0.97
~
-
0.96~
.
0.92 0.91
I
0.0
I
I
0.1 0.2 0.3 0.6 LOG CURRENT DENSITY
0.5
Fig. 1. Variation in the cathode polarization with the logarithm of the current density at 30 °C (bath composition as for Table 1) in the absence and in the presence of addition agents (1 g 1_i): curve 1, glucose; curve 2, l-serine; curve 3, dextrin; curve 4, none; curve 5, glycine; curve 6, j3-alanine.
4. Discussion The electrodeposition of alloys from aqueous solutions may be satisfactorily explained on the basis of the simultaneous discharge of cations. It has been suggested by Antropov [18] that in general the polarization should be low if there is only one electron taking part in the discharge step or when the process proceeds through successive one-electron steps. A high polarization may result if the discharge of metallic ions involves two-electron transfer in a single step. The electrochemical discharge process may be facilitated by the presence of an addition agent through the formation of a
135
00c~I~0
a~
~
~ 00000
a a 00000
11111
a a
-~N~
—
Lr~L0~0
~
~o~0
11111 a
-a kc,ca
-~
~ 00000
11111
C
g
I
S
a
~
~
.~
o
a
~
~,
a
:~ ~
oa. a
a 00000
~
I
C
a
u
~
a
III’’
aC) a
a a
a a
a a
I I I
a a
C
a~
a C)
0W,-4t-t’ 0 N 0 C) N
ao
a, ~
~ ~
0 0 0 0 0
a
00
0
a
-a a 0
0~a
~
F.
L)u~—
—4~—iC’1C~C’~
aaç~ ~
136 TABLE 7 Effect of addition agents on the resistivity, the Tafel slope and the throwing power
Addition agenta
Resistivity
Thfel slope b
Throwing power N
(~lcm)
(V)
(A cm~
None Dextrin Glucose l-Serine 13-Alanine Glycine
15.97 16.10 17.12 17.21 15.77 15.95
0.0625 0.0636 0.0773 0.0700 0.0600 0.0609
0.00196 0.00198 0.00226 0.00203 0.00190 0.00191
1)
Bath composition 0 as for Table 1. Temperature, 30 C; pH, 3.7. aconcentration of addition agents, 1 g L’.
complex in the solution owing to the oxidation of the agent by the following mechanism: M2~+ AA —f (M—AA)~ (M—AA)~+ e
—*
M
+
AA
where M metal and AA addition agent. The complex, after being adsorbed on the cathode surface, may accept an electron at the active site to discharge the metal ion which is finally incorporated in the deposit. The addition agent is thereby released and can reform the complex. The reaction proceeds thus, accelerating continuously. This may cause the polarization to decrease, as a result of which uniform fine and bright deposits are produced, as has been observed in the case of dextrin, glucose and l-serine (see Table 6). A comparatively high value of polarization with f3-alanine or glycine in the bath may be attributed to the preferential adsorption of these additives or one of their decomposition products in the electroplates. Such adsorption changes the nature of the cathode surface by blocking the various growth sites, leading to an increased concentration of anions in the vicinity of the cathode and thereby resulting in an increased polarization at a given current density. This increased polarization is probably responsible for enhancing the formation of new crystal nuclei on the metal surface by suppressing the growth of existing crystallites. As a consequence smooth and bright deposits are formed in the presence of these additives as has been discussed by Brenner [1]. Further, a gradual shift in the cathode polarization to more negative values with increasing current density in each case may be caused by the increased rate of discharge of metal ions at the cathode. Since, at higher current densities, the rate of discharge of metal ions at the cathode is limited by diffusion, more hydrogen ions are simultaneously discharged, which makes the cathode polarization become more negative (see Table 6).
Preferential adsorption of one metal ion over another at various faces and growth sites of the cathode surface in the presence of the various addi-
137
tives can cause the deposit composition to vary. The discharge of a particular metal ion may be blocked as a result of this preferential adsorption on a specific growth site and it may consequently lead to an evening out of the growth rate for that metal. Furthermore, the observed increase in cathode current efficiency with increasing current density in the presence of these agents in the electrolytic
bath (see Table 5) may be due to the utilization of comparatively more current for the deposition of the alloy than for the discharge of the hydrogen ions which result from the ionization of the solvent molecules. The throwing power obtained for the deposition of the alloy in the presence of glucose is significantly higher than that obtained in the presence of the other agents, as is evident from Table 7. It follows that relatively uniform coatings should be formed in the former case and in fact this has been found experimentally. Acknowledgment One of the authors (R.K.) is grateful to the Council of Scientific and Industrial Research, New Delhi, for providing financial assistance.
References 1 A. Brenner, Electrodeposition of Alloys: Principles and Practice, Vol. I, Academic Press, New York, 1963. 2 E. Raub, Z. Elektrochem., 55 (1951) 146 - 151. 3 S. Ichioka, Jpn. Patent 75,06,416 (Cl. C25D, C22 C, HOIF) (1975). 4 N. I. Shklovskaya, I. B. Murashova, 0. F. Bondarenko and V. I. Marochkin, Probi. Elektrokhim. Korroz. Met., 1 (1977) 30 - 34. 5 T. S. Vanaja and T. L. Ramachar, Electroplat. Met. Finish., 26 (8) (1973) 15 - 17, 20 - 23. 6 0. Petrosaviciute, B. Stulpinas and E. Siusa, Nauchn. Tr. Vyssh. Uchebn. Zaved. Lit. SSR, Khim. Khim. Tekhnol., (1975) 207 - 211. 7 W. H. McMullen and T. J. Mooney, Ger. Patent 2,630,980 (Cl. C25D3/56) (1977); U.S. PatentAppl. 594, 214 (1975). 8 J. R. Clauss, A. R. Tremmel and C. N. Adamowicz, U.S. Patent 3,806,429 (CL 204-41 C23b) (1974). 9 R. P. Dambal and T. L. Ramachar, Electroplat. Met. Finish., 25 (4) (1972) 10 - 18. 10 V. B. Singh, J. Appl. Electrochem., 9 (3) (1979) 285 - 289. 11 R. Sivakumar and T. L. Ramachar, Met. Finish. J., 18 (206) (1972) 65 - 70. 12 0. Petroseviciute, B. Stulpinas and E. Siusa, Liet. TSR Aukst. Mokyklu Mokslo Darb., Chem. Chem. TechnoL, 19 (1978) 38 - 43. 13 R. K. Shukla, S. K. Jha and S. C. Srivastava, Surf. Technol., 17(1982) 157. 14 S. N. Srivastava, R. Kashyap and S. C. Srivastava, Acta Chim. Acad. Sci. Hung., 117 (1) (1984)3 -9. 15 S. K. Jha, R. K. Shukla and S. C. Srivastava, Surf. Technol., 21 (1984) 383 - 389. 16 R. Kashyap, S. N. Srivastava and S. C. Srivastava, J. App!. Electrochem., 15 (1985) 23 - 27. 17 G. E. Gardam, Trans. Faraday Soc., 34 (1938) 698. 18 L. I. Antropov, Theoretical Electrochemistry, Mir, Moscow, 1972, p. 496.