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Electrodeposition of alloys: XXI electrodeposition of Pb-Sn alloys and their X-ray structure
Surface and Coatings Technology, 38 (1989) 299
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310
299
ELECTRODEPOSITION OF ALLOYS: XXI* ELECTRODEPOSITION OF Pb-Sn ALLOYS AND THEIR X-RAY STRUCTURE Y. N. SADANA and Z. H. ZHANG Metal Finishing Research Center, Department of Chemistry, Laurentian University, Sud bury, Ontario P3E2C6 (Canada) (Received June 8, 1988)
Summary The electrodeposition of Pb—Sn alloys from an aqueous solution containing diethylenetriaminepentaacetates of the metals could be a regular, equilibrium or irregular process depending on the experimental conditions. The percentage of tin in the deposit increased with an increase in pH but decreased with an increase in temperature or stirring of the electrolyte. The X-ray phase structure of the electrodeposited alloys and the effect of annealing on the structure are reported.
1. Introduction Recently we [1] proposed a new electrolyte for the electrodeposition of Pb—Sn alloys and also reviewed the different types of baths that have been reported for their codeposition. The new bath, based on the diethylenetriaminepentaacetates (DTPAs) of the metals, was successfully used to obtain deposits over a wide range of composition by varying the solution composition and current density. We now report the effect of the plating variables temperature, pH and stirring on the composition of the alloy deposits and current efficiency. The X-ray structure of the electroplated alloys is also reported and compared with that of the thermal alloys.
2. Experimental procedure The plating baths were prepared by mixing calculated volumes of Pb—DTPA, Sn—DTPA and DTPA stock solutions, the preparation of which is described below.
*For part XX see ref. 2. 0257-8972/89/$3.50
© Elsevier
Sequoia/Printed in The Netherlands
300
The Pb—DTPA stock solution was prepared by dissolving yellow lead oxide (64.6 g) in 1 M DTPA solution (290 ml) with gentle heating. The solution was filtered and diluted with doubly distilled water to 1000 ml. The Sn—DTPA solution was prepared via copper carbonate. Copper carbonate (58 g) was dissolved in 1 M DTPA solution (500 ml). After the copper carbonate was completely dissolved, pure tin metal powder was slowly added to the solution. The solution was kept hot until the blue color disappeared. The solution was filtered and diluted with doubly distilled water to 1000 ml. The copper content in the solution was determined by the atomic absorption (AA) method and was less than 2 ppm. Lead and tin in the stock solutions were also analyzed using the AA method. The DTPA stock solution was prepared by dissolving DTPA power in a minimum amount of 1:1 (v/v) aqueous ammonia solution with constant stirring. The concentration of the DTPA solution was 1 M. The pH of this solution was 4.17. Alloy deposition was studied from three solutions, B, C and D, the compositions of which are given in Table 1. Solutions B and C were prepared from Sn—DTPA, Pb—DTPA and DTPA stock solutions whereas bath D was made from tin(II) chloride, Pb—DTPA and DTPA solutions. The details of the preparation of the deposits, their chemical and X-ray analyses have been described before [1, 2]. TABLE 1 Composition of the solutions for Pb—Sn alloy deposition Bath
B C D
Sn
Pb
DTPAa
(g l’)
(mol l’)
(g 1’)
(rnol 1_i)
(g 11)
(mol l’)
10.0 15.0 20.0
0.084 0.126 0.169
20.0 15.0 5.0
0.091 0.072 0.024
140.0 140.0 140.0
0.356 0.356 0.356
Sn (wt.%) in bath
pH
33.33 50.00 66.67
4.20 4.38 —
a~ ammonium salt.
3. Results and discussion 3.1. Effect of temperature The influence of temperature on the composition of the alloy deposits and current efficiency was examined in two baths: in one (bath B) the effect of current density on the composition of the deposit was as expected from the diffusion considerations, and in the other (bath C) the phenomenon of “inversion current density” was observed til. The baths B and C contained 33.3 metal% Sn and 50.0 metal% Sn (100X concentration (g 11) of tin! concentration (g Y’) of tin plus lead) in the solution respectively. Temperatures of 25, 50 and 73 °Cwere employed, and the results are shown in
301
Figs. 1 and 2. In general an increase in temperature reduced the percentage of tin in the deposit at all current densities. In solution B at 73 °C(Fig. 1), the behavior was very typical of normal alloy deposition and three regions could be discerned: (i) a low current density range in which the alloy deposit was rich in lead and the deposit composition stayed almost constant; (ii) a middle current density range in which the tin content of the deposit increased rather quickly with an increase in current density and (iii) a high current density range in which again the alloy composition stayed constant with increasing current density.
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25°C
£40
°
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2:~~Ti°~°
2) Current density (A/dm Fig. 1. Effect of temperature on the alloy composition in bath B (pH, 4.20 - 4.40; medium stirring). AB is the composition reference line corresponding to 33.3 wt.% Sn in the solution.
SC 25°C
Current density (Aldm2) Fig. 2. Effect of temperature on the alloy composition in bath C (pH, 4.20; medium stirring). AB is the composition reference line corresponding to 50.0 wt.% Sn in the solution.
302
In the first region, reduction of lead ions was the predominant reaction at the cathode surface and an increase in current density increased the rate of deposition of lead till it became diffusion controlled. The current density at that point was the limiting current density of lead deposition, ‘ljm(pb), and had a value of 1.75 A dm2 at 73 °C.With a decrease in the temperature of plating, ‘ljm(pb) decreased. At 50 °Cit was about 1.1 A dm2, and at 25 °Cit was apparently less than 0.5 A dm2, the lowest current density employed in the present study, and was not observed. The increase in the value of ‘ljm(pb), which reflects an increase in the concentration of the free (or aquated) Pb2~ions with increasing temperature is understandable because dissociation of the ligated lead ions would increase with increase in temperature. When the current density was increased beyond ‘1im(pb)~ the rate of deposition of tin and/or hydrogen increased, resulting in a higher tin content in the deposit. With a further increase in current density the deposition of tin increased till it became diffusion controlled. At 73 °Cthis occurred at 7.0 A dm2 which was the limiting current density of alloy deposition, ‘lim(alloy) At this current density both lead and tin depositions were diffusion controlled and the alloy composition corresponding to this current density was the limiting alloy composition. The limiting current densities and compositions at different temperature of solution B are given in Table 2. Another interesting feature of the bath B was the fact that the percentage of tin (the less noble metal) in the limiting composition at 25 °C was higher than that in the bath; the codeposition was thus irregular. At 50 °C, the percentage of tin in the bath and the limiting deposit composition were the same (equilibrium codeposition). At 73 °Cthe codeposition became of the regular type and the percentage of the less noble metal in the deposit was less than that in the bath. This complex effect of temperature on Pb—Sn alloy deposition may arise from the fact that the deposition potentials of the two metals in DTPA solutions are very close: —0.924 V and —0.880 V for tin and lead respectively. In solution C, the effect of temperature on the composition of the alloy deposits was similar to that in solution B, and an increase in temperature reduced the percentage of tin in the deposit. The inversion current density TABLE 2 Limiting current densities and compositions at different temperatures of solution B Temperature (°C)
‘ljm(Pb)
‘lim(alloy)
(A dm2)
(A dm~)
Limiting deposit composition (Wt.% Sn)
25 50 73
<0.5 1.10 1.75
2.00 5.10 7.00
47 33 27
303
reported previously [1] at 25 °Cwas observed at 50 °Cbut not at 73 °C.The alloy deposits obtained at 25 and 50 °Ccontained more tin than that in the bath, but at 73 °Cthe percentage of tin in the deposit was lower than that in the bath, and the temperature of equilibrium deposition in bath C was apparently above 50 °Cbut below 73 °C. The behavior at 73 °Cin bath C was similar to that in bath B and the percentage of tin in the deposit was less than that in the solution. The limiting current densities of the alloy deposition and the limiting composition of the alloy at different temperatures of solution C are given in Table 3. The ‘Iim(alloy) in both solutions increased with an increase in temperature. This may result from an increase in the concentration of free metallic ions in the solution owing to enhanced dissociation of the complex ions at high temperatures. The effect of temperature on the current efficiency is shown in Fig. 3. The current efficiency either increased or stayed constant when the temperature was increased from 25 to 50 °Cbut the increase in the temperature to 73 °Creduced the current efficiency at all current densities except at 8.0 A dm2. 3.2. Effect of pH The effect of the pH of the electrolyte on the composition of the deposit and current efficiency was studied in the bath D containing 66.7 metal% Sn. This bath was prepared from tin(II) chloride and the diethylenetriaminepentaacetates of lead and ammonium. The pH of the solution was adjusted by adding aqueous ammonia or hydrochloric acid. Electrodepositions at pH 3.05, 4.21 and 5.03 were studied. Deposition at pH values higher than 5.03 could not be studied because increasing the pH of the solution above this value resulted in the precipitation of tin from the solution. The experimental results are listed in Fig. 4. An increase in pH tended to increase the percentage of tin in the deposit. The interesting feature of the effect of pH was that the curves c, d, e and f (current densities, 2.0 12.0 A dm2) in Fig. 4 intersect at the point X. This point corresponds to the pH 4.21 which may be called the “inversion -
TABLE 3 Limiting current densities of alloy deposition and limiting composition of the alloy at different temperatures of solution C Temperature (°C)
‘lim( Pb)
‘1im(a11o~~)
(A dm2)
(A dm
25 50 73
<0.5 1.10 2.10
2.0 4.0 5.1
)
Limiting deposit composition (Wt.% Sn) 77 59 37
304 100
~04~6~70 Temperature
80 (°C)
Fig. 3. Effect of temperature on the current efficiency of alloy deposition in bath B 2 curve b, 1.0 A dm2 curve c, (pH, 4.20 - 4.40; medium stirring): curve a, 0.50 A dm 2.0 A cm2 curve d, 3.0 A dm~2 curve e, 4.0 A dm2 curve f, 8.0 A dm2.
100
90
80
a -J .-~
_‘(~
a0~ a
60
e
f d C
C C
40
b
30 a
20
I
3
4 pH of the bath
5
Fig. 4. Effect of pH on the alloy composition (bath composition: Sn (as SnCl 2), 20.0; 2 curve h, 1.0 A dm~2 2.0 A dm2 d, 3.0 A 25 dm2 curve a,e, 0.50 4.0 A Pb, 10.0; DTPA, 140.0 curve g l’; c, medium stirring;curve temperature, °C:curve A dm~2 dm curve f, 1 2.0 A dm2.
305
pH”. Below this pH, tin metal was less noble, but above this pH, tin behaved as the nobler metal. This type of behavior in alloy depositions has been reported previously [3]. The current efficiency decreased as the pH was increased from 3.05 to 5.03. The effect was more pronounced at low current densities. The results are shown in Fig. 5. This decrease in current efficiency may result from the fact that a pH would increase the relative concentration of the complex ions and decrease the concentration of the free metallic ions. 3.3. Effect of stirring The effect of stirring on the deposit composition and the current efficiency was investigated in solution C. The results are shown in Fig. 6. Stirring did not have a significant effect on the composition of the deposit when the current density was 4.0 A dm~2or higher. At lower current densities, stirring reduced the tin content in the deposit. The current efficiency of alloy deposition increased when the solution was stirred. 3.4. The Tafel relations during alloy deposition The cathode potential increased, i.e. became less negative on increasing the temperature at constant current density. An increase in temperature
100
90
a
b
80 C
70
d
0
60
50
c
5)
(~)
40 30~ 20 10
3
4 pH of
5 the
bath
Fig. 5. Effect of pH on the current efficiency of alloy deposition (temperature, 25 °C; medium stirring): curve a, 0.50 A dm2 curve b, 1.0 A dm2 curve c, 3.0 A dm2 curve d, 12.0 A dm2.
:
306 100
3
Current density (A/diTI Fig. 6. Effect of stirring on the aiioy composition (—) and the current efficiency (—---—) of alloy deposition in bath C (temperature, 25 °C; pH, 4.10): curve a, no agitation; curve b, medium agitation; curve a’, no agitation; curve b’, medium agitation.
would increase the dissociation of the complex ion in solution and move the deposition potential in the positive direction. Sadana and Zhang ~1] showed that the existence of inversion current density was reflected in the crossing over of the partial current density— cathode potential curves of the individual metals during alloy deposition. In the present study, it was observed that changing the temperature of the electrolyte had an effect on the inversion current density. Thus in solution C, inversion current density was present at 50 °C but not at 73 °C. This prompted us to examine the effect of temperature on the Tafel relations (cathode potential vs. logarithm of partial current density of metal deposition) of individual lead and tin during Pb—Sn alloy deposition in the solution B and C and the results are shown in Figs. 7 and 8. These were constructed from the observed current density—cathode potential curve of alloy deposition. In solution B in which inversion current density was not observed at room temperature [1], the Tafel lines of lead and tin at 50 and 73 °C(Fig. 7) did not intersect at these elevated temperatures and the behavior was similar to that reported for room temperature. The lines for tin were to the left of those for lead, and the deposition of lead in this solution was preferential. For the bath C, in which Sadana and Zhang ~1] observed inversion current density at room temperature, it was observed that the Tafel lines at 50 °C, like those at 25 °C ~1], crossed at point C (Fig. 8) and inversion current density was observed at 50 °C. At 73 °C,however, the Tafel lines did not intersect and at this temperature, inversion current density was not present.
307
02
~
0 6
1g1 (A/din2) -1.2 0 0.
a ‘a °
~
a
-1.1
‘2
(A/din2) Fig. 7. Effect of temperature on the Tafel relation of single metals during alloy deposition in solution B: curve a, tin; curve b, lead; (1) 50 °C;(2)73 °C.
At the inversion current density, the deposition of both metals was diffusion controlled and the concentration of the metal ions at the cathode— solution interface was almost zero. The concentration of the DTPA ion, however, would be relatively high at the cathode—solution interface, which should hinder the passage through the double layer of the incoming metallic ions diffusing to the cathode surface. The effect on tin, which forms a more stable complex, would be more than on lead and should increase with an increase in current density. This perhaps could be the cause of the observed decrease in the tin content of the deposit with an increase in the current density above the inversion current density. Another contributory factor would be the pH changes in the double layer. At high current densities, owing to rapid evolution of hydrogen at the cathode, there would be an excess of hydroxyl ions in the double layer and the pH at the cathode— solution interface should be quite high. This would also have a hindering effect on the deposition of the metallic ions. The effect would be greater in the case of tin because the solubility product constant for tin(II) hydroxide is lower than that for lead(II) hydroxide [4].
:
308 -1.2
(1)
0.6
-0.2
0.2 2)
0.6
0.0 0.4 igi (A/din2 I
0.8
lg~ (A/dm 5)
‘a 0
(2)
.2
~8
~4
Fig. 8. Effect of temperature on the Tafel relation of single metals during alloy deposition in solution C: curve a, tin; curve b, lead; (1) 50°C;(2) 73 °C.
X-ray structure of Pb—Sn alloy deposits The equilibrium phase diagram as summarized by Hansen and Anderko [51 shows that lead and tin form a eutectic. The two terminal solid solutions are separated by a wide region in which lead and tin phases coexist. The solid solubility of tin in lead was reported as 1.9 wt.% Sn at room temperature and it increased to 18.2 wt.% at the eutectic temperature (183 °C).The solid solubility of lead in tin was only 2.5 wt.% at the eutectic temperature and decreased rapidly as the temperature decreased. The structure of the electrodeposited Pb—Sn alloys was examined as deposited and after annealing. The results are shown in Fig. 9. There was no significant difference between the crystal structures of the thermally prepared alloys and the electrodeposited alloys. 3.5.
3.5.1. As-deposited alloys The solid solubility of tin in lead in electrodeposited alloys was higher than that in thermally prepared alloys and a supersaturated solid solution was formed. In the electrodeposits, the solid solubility of tin in lead at room temperature was at least 6.8 wt.% (11.3 at.%) as compared with 1.9 wt.%
309 Sn cast b
alloys
Pb+Sn room temperature
0
10
20
30
40
50
60
70
80
90
100 Sn
PPb 0
10
Pb 20
30
40
50
+
60
Sn
Cast alloys 183°C
70
80
90
100
at.% Sn electrodeposited PbV/A 0
10
r
Pb+Sn 20
30
~3
PbVA
0
10
40
50
60
alloys, 70
80
90
100 as deposited
Pb+Sn 20
30
40 50 60 at.% Sn
alloys, 70
region
80
90
after
100 annealing(125°C, 100 hours(
not studied
Fig. 9. X-ray structure of Pb—Sn alloy deposits.
in thermal alloys. The solubility of lead in tin was not determined because no deposits with an appropriate composition were obtained in the present study. In the region 11.57 95.28 wt.% Sn (18.6 97.2 at.% Sn), both lead and tin phases were observed. The tin phase was p-Sn phase with a tetragonal structure. No intermediate phase was detected. -
-
3.5.2. Alloys after annealing After the annealing treatment, the supersaturated solid solution decomposed and the solid solubility of tin in lead decreased to 4.69 wt.% (7.9 at.%). Both lead and tin phases were observed in the region 6.76 95.28 wt.% Sn (11.3 97.2 at.% Sn). -
-
4. Conclusions (a) An increase in temperature decreased the tin content of the deposit. (b) The percentage of tin in the deposit increased with an increase in pH. The phenomenon of inversion pH was observed in this system. (c) The codeposition of Pb—Sn alloys could be a regular, equilibrium or irregular process depending upon the experimental conditions. (d) The solid solubility of tin in lead in deposited alloys was higher than the equilibrium value reported for thermal alloys.
310
Acknowledgments The authors wish to thank Dr. C. H. Belanger, Vice President (Academic) of Laurentian University, and the Natural Sciences and Engineering Research Council of Canada for providing financial support for the project. We also thank Mr. Z. Z. Wang, Visiting Scientist from the People’s Republic of China, for helpful discussions.
References Y. N. Sadana and Z. H. Zhang, Surf. Coat. Technol., 34(1988)109-121. Y. N. Sadana and Z. Z. Wang, Surf. Coat. Technol., 37 (1989) 419 - 434. 3 Y. N. Sadana and W. G. Kerr, Met. Finish., 78 (1980) 43. 4 A. F. Clifford, Inorganic Chemistry of Qualitative Analysis, Prentice—Hall, Englewood Cliffs, NJ, 1961, p. 461 - 462. 5 M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1 2
2nd Edn., 1958, p. llO6Y.
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299
ELECTRODEPOSITION OF ALLOYS: XXI* ELECTRODEPOSITION OF Pb-Sn ALLOYS AND THEIR X-RAY STRUCTURE Y. N. SADANA and Z. H. ZHANG Metal Finishing Research Center, Department of Chemistry, Laurentian University, Sud bury, Ontario P3E2C6 (Canada) (Received June 8, 1988)
Summary The electrodeposition of Pb—Sn alloys from an aqueous solution containing diethylenetriaminepentaacetates of the metals could be a regular, equilibrium or irregular process depending on the experimental conditions. The percentage of tin in the deposit increased with an increase in pH but decreased with an increase in temperature or stirring of the electrolyte. The X-ray phase structure of the electrodeposited alloys and the effect of annealing on the structure are reported.
1. Introduction Recently we [1] proposed a new electrolyte for the electrodeposition of Pb—Sn alloys and also reviewed the different types of baths that have been reported for their codeposition. The new bath, based on the diethylenetriaminepentaacetates (DTPAs) of the metals, was successfully used to obtain deposits over a wide range of composition by varying the solution composition and current density. We now report the effect of the plating variables temperature, pH and stirring on the composition of the alloy deposits and current efficiency. The X-ray structure of the electroplated alloys is also reported and compared with that of the thermal alloys.
2. Experimental procedure The plating baths were prepared by mixing calculated volumes of Pb—DTPA, Sn—DTPA and DTPA stock solutions, the preparation of which is described below.
*For part XX see ref. 2. 0257-8972/89/$3.50
© Elsevier
Sequoia/Printed in The Netherlands
300
The Pb—DTPA stock solution was prepared by dissolving yellow lead oxide (64.6 g) in 1 M DTPA solution (290 ml) with gentle heating. The solution was filtered and diluted with doubly distilled water to 1000 ml. The Sn—DTPA solution was prepared via copper carbonate. Copper carbonate (58 g) was dissolved in 1 M DTPA solution (500 ml). After the copper carbonate was completely dissolved, pure tin metal powder was slowly added to the solution. The solution was kept hot until the blue color disappeared. The solution was filtered and diluted with doubly distilled water to 1000 ml. The copper content in the solution was determined by the atomic absorption (AA) method and was less than 2 ppm. Lead and tin in the stock solutions were also analyzed using the AA method. The DTPA stock solution was prepared by dissolving DTPA power in a minimum amount of 1:1 (v/v) aqueous ammonia solution with constant stirring. The concentration of the DTPA solution was 1 M. The pH of this solution was 4.17. Alloy deposition was studied from three solutions, B, C and D, the compositions of which are given in Table 1. Solutions B and C were prepared from Sn—DTPA, Pb—DTPA and DTPA stock solutions whereas bath D was made from tin(II) chloride, Pb—DTPA and DTPA solutions. The details of the preparation of the deposits, their chemical and X-ray analyses have been described before [1, 2]. TABLE 1 Composition of the solutions for Pb—Sn alloy deposition Bath
B C D
Sn
Pb
DTPAa
(g l’)
(mol l’)
(g 1’)
(rnol 1_i)
(g 11)
(mol l’)
10.0 15.0 20.0
0.084 0.126 0.169
20.0 15.0 5.0
0.091 0.072 0.024
140.0 140.0 140.0
0.356 0.356 0.356
Sn (wt.%) in bath
pH
33.33 50.00 66.67
4.20 4.38 —
a~ ammonium salt.
3. Results and discussion 3.1. Effect of temperature The influence of temperature on the composition of the alloy deposits and current efficiency was examined in two baths: in one (bath B) the effect of current density on the composition of the deposit was as expected from the diffusion considerations, and in the other (bath C) the phenomenon of “inversion current density” was observed til. The baths B and C contained 33.3 metal% Sn and 50.0 metal% Sn (100X concentration (g 11) of tin! concentration (g Y’) of tin plus lead) in the solution respectively. Temperatures of 25, 50 and 73 °Cwere employed, and the results are shown in
301
Figs. 1 and 2. In general an increase in temperature reduced the percentage of tin in the deposit at all current densities. In solution B at 73 °C(Fig. 1), the behavior was very typical of normal alloy deposition and three regions could be discerned: (i) a low current density range in which the alloy deposit was rich in lead and the deposit composition stayed almost constant; (ii) a middle current density range in which the tin content of the deposit increased rather quickly with an increase in current density and (iii) a high current density range in which again the alloy composition stayed constant with increasing current density.
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__________________________________
25°C
£40
°
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2:~~Ti°~°
2) Current density (A/dm Fig. 1. Effect of temperature on the alloy composition in bath B (pH, 4.20 - 4.40; medium stirring). AB is the composition reference line corresponding to 33.3 wt.% Sn in the solution.
SC 25°C
Current density (Aldm2) Fig. 2. Effect of temperature on the alloy composition in bath C (pH, 4.20; medium stirring). AB is the composition reference line corresponding to 50.0 wt.% Sn in the solution.
302
In the first region, reduction of lead ions was the predominant reaction at the cathode surface and an increase in current density increased the rate of deposition of lead till it became diffusion controlled. The current density at that point was the limiting current density of lead deposition, ‘ljm(pb), and had a value of 1.75 A dm2 at 73 °C.With a decrease in the temperature of plating, ‘ljm(pb) decreased. At 50 °Cit was about 1.1 A dm2, and at 25 °Cit was apparently less than 0.5 A dm2, the lowest current density employed in the present study, and was not observed. The increase in the value of ‘ljm(pb), which reflects an increase in the concentration of the free (or aquated) Pb2~ions with increasing temperature is understandable because dissociation of the ligated lead ions would increase with increase in temperature. When the current density was increased beyond ‘1im(pb)~ the rate of deposition of tin and/or hydrogen increased, resulting in a higher tin content in the deposit. With a further increase in current density the deposition of tin increased till it became diffusion controlled. At 73 °Cthis occurred at 7.0 A dm2 which was the limiting current density of alloy deposition, ‘lim(alloy) At this current density both lead and tin depositions were diffusion controlled and the alloy composition corresponding to this current density was the limiting alloy composition. The limiting current densities and compositions at different temperature of solution B are given in Table 2. Another interesting feature of the bath B was the fact that the percentage of tin (the less noble metal) in the limiting composition at 25 °C was higher than that in the bath; the codeposition was thus irregular. At 50 °C, the percentage of tin in the bath and the limiting deposit composition were the same (equilibrium codeposition). At 73 °Cthe codeposition became of the regular type and the percentage of the less noble metal in the deposit was less than that in the bath. This complex effect of temperature on Pb—Sn alloy deposition may arise from the fact that the deposition potentials of the two metals in DTPA solutions are very close: —0.924 V and —0.880 V for tin and lead respectively. In solution C, the effect of temperature on the composition of the alloy deposits was similar to that in solution B, and an increase in temperature reduced the percentage of tin in the deposit. The inversion current density TABLE 2 Limiting current densities and compositions at different temperatures of solution B Temperature (°C)
‘ljm(Pb)
‘lim(alloy)
(A dm2)
(A dm~)
Limiting deposit composition (Wt.% Sn)
25 50 73
<0.5 1.10 1.75
2.00 5.10 7.00
47 33 27
303
reported previously [1] at 25 °Cwas observed at 50 °Cbut not at 73 °C.The alloy deposits obtained at 25 and 50 °Ccontained more tin than that in the bath, but at 73 °Cthe percentage of tin in the deposit was lower than that in the bath, and the temperature of equilibrium deposition in bath C was apparently above 50 °Cbut below 73 °C. The behavior at 73 °Cin bath C was similar to that in bath B and the percentage of tin in the deposit was less than that in the solution. The limiting current densities of the alloy deposition and the limiting composition of the alloy at different temperatures of solution C are given in Table 3. The ‘Iim(alloy) in both solutions increased with an increase in temperature. This may result from an increase in the concentration of free metallic ions in the solution owing to enhanced dissociation of the complex ions at high temperatures. The effect of temperature on the current efficiency is shown in Fig. 3. The current efficiency either increased or stayed constant when the temperature was increased from 25 to 50 °Cbut the increase in the temperature to 73 °Creduced the current efficiency at all current densities except at 8.0 A dm2. 3.2. Effect of pH The effect of the pH of the electrolyte on the composition of the deposit and current efficiency was studied in the bath D containing 66.7 metal% Sn. This bath was prepared from tin(II) chloride and the diethylenetriaminepentaacetates of lead and ammonium. The pH of the solution was adjusted by adding aqueous ammonia or hydrochloric acid. Electrodepositions at pH 3.05, 4.21 and 5.03 were studied. Deposition at pH values higher than 5.03 could not be studied because increasing the pH of the solution above this value resulted in the precipitation of tin from the solution. The experimental results are listed in Fig. 4. An increase in pH tended to increase the percentage of tin in the deposit. The interesting feature of the effect of pH was that the curves c, d, e and f (current densities, 2.0 12.0 A dm2) in Fig. 4 intersect at the point X. This point corresponds to the pH 4.21 which may be called the “inversion -
TABLE 3 Limiting current densities of alloy deposition and limiting composition of the alloy at different temperatures of solution C Temperature (°C)
‘lim( Pb)
‘1im(a11o~~)
(A dm2)
(A dm
25 50 73
<0.5 1.10 2.10
2.0 4.0 5.1
)
Limiting deposit composition (Wt.% Sn) 77 59 37
304 100
~04~6~70 Temperature
80 (°C)
Fig. 3. Effect of temperature on the current efficiency of alloy deposition in bath B 2 curve b, 1.0 A dm2 curve c, (pH, 4.20 - 4.40; medium stirring): curve a, 0.50 A dm 2.0 A cm2 curve d, 3.0 A dm~2 curve e, 4.0 A dm2 curve f, 8.0 A dm2.
100
90
80
a -J .-~
_‘(~
a0~ a
60
e
f d C
C C
40
b
30 a
20
I
3
4 pH of the bath
5
Fig. 4. Effect of pH on the alloy composition (bath composition: Sn (as SnCl 2), 20.0; 2 curve h, 1.0 A dm~2 2.0 A dm2 d, 3.0 A 25 dm2 curve a,e, 0.50 4.0 A Pb, 10.0; DTPA, 140.0 curve g l’; c, medium stirring;curve temperature, °C:curve A dm~2 dm curve f, 1 2.0 A dm2.
305
pH”. Below this pH, tin metal was less noble, but above this pH, tin behaved as the nobler metal. This type of behavior in alloy depositions has been reported previously [3]. The current efficiency decreased as the pH was increased from 3.05 to 5.03. The effect was more pronounced at low current densities. The results are shown in Fig. 5. This decrease in current efficiency may result from the fact that a pH would increase the relative concentration of the complex ions and decrease the concentration of the free metallic ions. 3.3. Effect of stirring The effect of stirring on the deposit composition and the current efficiency was investigated in solution C. The results are shown in Fig. 6. Stirring did not have a significant effect on the composition of the deposit when the current density was 4.0 A dm~2or higher. At lower current densities, stirring reduced the tin content in the deposit. The current efficiency of alloy deposition increased when the solution was stirred. 3.4. The Tafel relations during alloy deposition The cathode potential increased, i.e. became less negative on increasing the temperature at constant current density. An increase in temperature
100
90
a
b
80 C
70
d
0
60
50
c
5)
(~)
40 30~ 20 10
3
4 pH of
5 the
bath
Fig. 5. Effect of pH on the current efficiency of alloy deposition (temperature, 25 °C; medium stirring): curve a, 0.50 A dm2 curve b, 1.0 A dm2 curve c, 3.0 A dm2 curve d, 12.0 A dm2.
:
306 100
3
Current density (A/diTI Fig. 6. Effect of stirring on the aiioy composition (—) and the current efficiency (—---—) of alloy deposition in bath C (temperature, 25 °C; pH, 4.10): curve a, no agitation; curve b, medium agitation; curve a’, no agitation; curve b’, medium agitation.
would increase the dissociation of the complex ion in solution and move the deposition potential in the positive direction. Sadana and Zhang ~1] showed that the existence of inversion current density was reflected in the crossing over of the partial current density— cathode potential curves of the individual metals during alloy deposition. In the present study, it was observed that changing the temperature of the electrolyte had an effect on the inversion current density. Thus in solution C, inversion current density was present at 50 °C but not at 73 °C. This prompted us to examine the effect of temperature on the Tafel relations (cathode potential vs. logarithm of partial current density of metal deposition) of individual lead and tin during Pb—Sn alloy deposition in the solution B and C and the results are shown in Figs. 7 and 8. These were constructed from the observed current density—cathode potential curve of alloy deposition. In solution B in which inversion current density was not observed at room temperature [1], the Tafel lines of lead and tin at 50 and 73 °C(Fig. 7) did not intersect at these elevated temperatures and the behavior was similar to that reported for room temperature. The lines for tin were to the left of those for lead, and the deposition of lead in this solution was preferential. For the bath C, in which Sadana and Zhang ~1] observed inversion current density at room temperature, it was observed that the Tafel lines at 50 °C, like those at 25 °C ~1], crossed at point C (Fig. 8) and inversion current density was observed at 50 °C. At 73 °C,however, the Tafel lines did not intersect and at this temperature, inversion current density was not present.
307
02
~
0 6
1g1 (A/din2) -1.2 0 0.
a ‘a °
~
a
-1.1
‘2
(A/din2) Fig. 7. Effect of temperature on the Tafel relation of single metals during alloy deposition in solution B: curve a, tin; curve b, lead; (1) 50 °C;(2)73 °C.
At the inversion current density, the deposition of both metals was diffusion controlled and the concentration of the metal ions at the cathode— solution interface was almost zero. The concentration of the DTPA ion, however, would be relatively high at the cathode—solution interface, which should hinder the passage through the double layer of the incoming metallic ions diffusing to the cathode surface. The effect on tin, which forms a more stable complex, would be more than on lead and should increase with an increase in current density. This perhaps could be the cause of the observed decrease in the tin content of the deposit with an increase in the current density above the inversion current density. Another contributory factor would be the pH changes in the double layer. At high current densities, owing to rapid evolution of hydrogen at the cathode, there would be an excess of hydroxyl ions in the double layer and the pH at the cathode— solution interface should be quite high. This would also have a hindering effect on the deposition of the metallic ions. The effect would be greater in the case of tin because the solubility product constant for tin(II) hydroxide is lower than that for lead(II) hydroxide [4].
:
308 -1.2
(1)
0.6
-0.2
0.2 2)
0.6
0.0 0.4 igi (A/din2 I
0.8
lg~ (A/dm 5)
‘a 0
(2)
.2
~8
~4
Fig. 8. Effect of temperature on the Tafel relation of single metals during alloy deposition in solution C: curve a, tin; curve b, lead; (1) 50°C;(2) 73 °C.
X-ray structure of Pb—Sn alloy deposits The equilibrium phase diagram as summarized by Hansen and Anderko [51 shows that lead and tin form a eutectic. The two terminal solid solutions are separated by a wide region in which lead and tin phases coexist. The solid solubility of tin in lead was reported as 1.9 wt.% Sn at room temperature and it increased to 18.2 wt.% at the eutectic temperature (183 °C).The solid solubility of lead in tin was only 2.5 wt.% at the eutectic temperature and decreased rapidly as the temperature decreased. The structure of the electrodeposited Pb—Sn alloys was examined as deposited and after annealing. The results are shown in Fig. 9. There was no significant difference between the crystal structures of the thermally prepared alloys and the electrodeposited alloys. 3.5.
3.5.1. As-deposited alloys The solid solubility of tin in lead in electrodeposited alloys was higher than that in thermally prepared alloys and a supersaturated solid solution was formed. In the electrodeposits, the solid solubility of tin in lead at room temperature was at least 6.8 wt.% (11.3 at.%) as compared with 1.9 wt.%
309 Sn cast b
alloys
Pb+Sn room temperature
0
10
20
30
40
50
60
70
80
90
100 Sn
PPb 0
10
Pb 20
30
40
50
+
60
Sn
Cast alloys 183°C
70
80
90
100
at.% Sn electrodeposited PbV/A 0
10
r
Pb+Sn 20
30
~3
PbVA
0
10
40
50
60
alloys, 70
80
90
100 as deposited
Pb+Sn 20
30
40 50 60 at.% Sn
alloys, 70
region
80
90
after
100 annealing(125°C, 100 hours(
not studied
Fig. 9. X-ray structure of Pb—Sn alloy deposits.
in thermal alloys. The solubility of lead in tin was not determined because no deposits with an appropriate composition were obtained in the present study. In the region 11.57 95.28 wt.% Sn (18.6 97.2 at.% Sn), both lead and tin phases were observed. The tin phase was p-Sn phase with a tetragonal structure. No intermediate phase was detected. -
-
3.5.2. Alloys after annealing After the annealing treatment, the supersaturated solid solution decomposed and the solid solubility of tin in lead decreased to 4.69 wt.% (7.9 at.%). Both lead and tin phases were observed in the region 6.76 95.28 wt.% Sn (11.3 97.2 at.% Sn). -
-
4. Conclusions (a) An increase in temperature decreased the tin content of the deposit. (b) The percentage of tin in the deposit increased with an increase in pH. The phenomenon of inversion pH was observed in this system. (c) The codeposition of Pb—Sn alloys could be a regular, equilibrium or irregular process depending upon the experimental conditions. (d) The solid solubility of tin in lead in deposited alloys was higher than the equilibrium value reported for thermal alloys.
310
Acknowledgments The authors wish to thank Dr. C. H. Belanger, Vice President (Academic) of Laurentian University, and the Natural Sciences and Engineering Research Council of Canada for providing financial support for the project. We also thank Mr. Z. Z. Wang, Visiting Scientist from the People’s Republic of China, for helpful discussions.
References Y. N. Sadana and Z. H. Zhang, Surf. Coat. Technol., 34(1988)109-121. Y. N. Sadana and Z. Z. Wang, Surf. Coat. Technol., 37 (1989) 419 - 434. 3 Y. N. Sadana and W. G. Kerr, Met. Finish., 78 (1980) 43. 4 A. F. Clifford, Inorganic Chemistry of Qualitative Analysis, Prentice—Hall, Englewood Cliffs, NJ, 1961, p. 461 - 462. 5 M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1 2
2nd Edn., 1958, p. llO6Y.