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The measurement and interpretation of the macrostress generated during the electrodeposition of copper
0019-4686/79/0201~191
rm.cw
THE MEASUREMENT AND INTERPRETATION OF THE MACROSTRESS GENERATED DURING THE ELECTRODEPOSITION OF COPPER J.A.
HARRISON
and P. J.
STRONACH
School of Chemistry University, Newcastle-on-Tyne,
England
(Received 7 June 1978) Abstract - Measurements of the time dependence of the macrostress, generated in thick layers of copper and HCI solutions, are presented. The results are discussed in terms of the electrode kinetics of the appropriate electron transfer reactions
deposited from H,SO,
NOMENCLATURE
A
b D F F M ;
area of electrode width of strip pen-recorder deflection faraday restoring force current plated length of strip atomic weight overall number of electrons transferred stress time thickness of strip thickness of deposit density standard rate of the first electron transfer[l]
contribute to the intrinsic stress are crystal coalescence, occluded matter, generation of dislocations. The importance of these various possibilities depend on the system. This subject area has been reviewed in detail[3-63. In the case of Cu deposition from pure solutions probably only crystal coalescence and the need to be considered as generation of dislocations possibilities.
Morphology
of Cu deposits
For Cu deposition from sulphate baths SafranekE7) has stated that columnar deposits with some (110) plane orientation are observed. In Cl- containing aqueous solution a rough powdery deposit is observed S]. On adding Cl- ions to a solution containing S04-I no effect is observed until a critical concentration is reached[9]. It has been suggested[9] that a CuCl deposit can also be formed. However this has not been confirmed.
In a previous paper[l] we have discussed in detail the electrode kinetics of the electron transfer step in the reduction of Cu2+ onto Cu, Zn2+ onto Zn, and Cd’* onto Cd. It has not yet proved possible to obtain much information about the following electro crystallization steps. However on a rough metal surface it is likely that the growth of the surface is controlled primarily by the electron transfer step, that is by the primary distribution. An important property of massive electrodeposits is the macrostress. It is the purpose of the present note to present the result of measuring the macrostress during the deposition of Cu and to compare the results with the electrode kinetic information. Cu is a very suitable system for this purpose as the rate of Cu deposition from H,SO, solution onto a rough surface is some ten times slower than from HCl solution (probably from the CuCl+ complex). This allows the balance between the rate of the interfacial reaction and diffusion to be changed in the steady state by changing the solution from which the metal is deposited.
Measurement
of stress
The most widely used technique to measure stress depends on the bending of a metal strip which is plated on one side only as used by Stoney[lO]. The equations used to calculate the stress have been discussed by Soderberg and Graham[ 111 and Gabe and West[12]. Various methods have been used to determine the degree of bending of the strip[13-231. Hoar and Arrowsmith[21] used a cathode strip which was rigidly clamped at one end. The bending of the strip was prevented by the magnetic field of a coil into which an armature, attached to the free end of the strip, protrudes. By varying the current flowing through the c&l the strip can be held in its original position and a correlation is made between the current flowing through the coil and the stress of the deposit. Other methods which have been used include the ‘Spiral Contractometer’ of Brenner and SenderofT[24] the ‘Stressometer’ of Kushner[25] and a dilatometric method[26,27]. A useful compilation of steady state stress measurements are given by Safranek[7]. The method used, in the present work, was based on that of Hoar and Arrowsmith[21] and a diagram of the apparatus is shown in Fig. 1. One side of a mild steel strip was insulated by coating it with “Lacomit”. A small square of steel was attached perpendicular to the
Stress in electrodeposits A distinction has been made in the literature between induced and intrinsic stress[Z]. Induced stress arises due to a difference in structure between deposit and substrate and is a maximum at the deposit substrate interface. Superimposed on this is the intrinsic stress in the deposit. The various effects which can 191
192
J. A.
HARRISON AND P.J.STRONACH
FS
maintain the strip in its unplated position. The stress (S) is given by:
PSU
cv
A2
A3
S-g
M
where S is the stress (kg cm-*), 1is the plated length of the strip (cm), b is its breadth (cm) and t’ its thickness (cm), F’ is the restoring force applied to the strip (kg) and x the thickness of the deposit (cm). Assuming that the current density and current efficiency is 100°/0 the thickness of the deposit (x) can be related to the time of plating and together with equation (I) gives S=-
2l’F’nFp t’itM
of the stress measuringequipment. the connectionto the working electrode. light sensitivediodes which sense the balance point.
Fig. 1. Diagram
WE Sl, 52 Cl,C2 Al A2, A3
CV
PSU M SE
coils through which current flows to maintain the strip position. potentiostat connected as a galvanostat with the current controlled by R and the potential by V+ Servo amplifier controlled by Si, S2
where i is the applied current (A), t is the time of plating (s), M is the atomic weight of the deposited metal, A is the area of the cathode which has been plated (cm’), n is the number of electrons involved in the deposition reaction, F is the Faraday (coulombs mole- ‘) and p is the density of the deposit (gmcm- 3). For the strips used in these experiments it was found that the pen-recorder gave a 1 V deflection for an applied force of 50 x 10m6kg acting at the top of the strip. The thickness of the strip was 10-l cm and the breadth was 1.25 cm. The plated length was 8 cm. The plating current was measured in mA and when the approximate values were inserted into equations (1) and (2) the stress, for a copper deposit, could be expressed as :
constantvoltage unit.
stress reading meter unit voltage comparator
secondaryelectrode
top of the strip and this carried a length of wire which passed through the two coils. This square served as a means of detecting the zero deflection point as, when the cathode was clamped into position, it was positioned between two light-sensitive detectors. If this square moved from its original position, a current was caused to pass through the ceils by a conventional servo amplifier type circuit in order to bring it back to zero. A pen-recorder was connected to the instrument in order to continuously monitor the current flowing through the coils. The instrument was calibrated by applying a known force to the insulated metal strip and measuring the recorder deflection corresponding to the current flowing through the coils which was required to maintain the strip in its original position. The deflection was a linear function of the force. Measurements were carried out at various cds by plating on to an area of 10cm2 of the strip from solutions of CuS04 (0.5 M) in H,SO, (1 M) and CuCl, (0.5 M) in HCI (1 M). The current was maintained using a potentiostat connected to operate galvanostatically. The mild steel strip acted as a cathode and a sheet of copper foil was used as the reference electrode. Culculation
of the stress
Hoar and Arrowsmith[Zl] derived an equation to relate the stress of a deposit to the force required to
s=
2.89 x lo40
it
(3)
Values of D, the recorder deflection (V), i, the plating current (mA), and t. the time of plating @in), can now be substituted into (3) to give the stress of the deposit at any time (t) in kg cm-‘.
Stress results Copper was plated on to the mild steel strips from sulphate and chloride solutions. The deflection of the pen-recorder was monitored on the paper chart. The stress at any particular time of plating was calculated using equation (3) and Fig. 2 shows the variation of stress with time. It is quite noticable that the behaviour in SOisolution is different to that observed for the Cl- case. For the SO:- case an initial increase in tensile stress is observed at short plating times which becomes constant at longer times. The initial stress is probably due to the different lattice structures of the deposit and the substrate. At greater deposit thicknesses the stress due to the actual deposit is the observed superimposed on to this original stress, indicating that the stress of the deposit is negligible. In the Cl- case an initial increase in tensile stress (of greater magnitude than for the SOi- case) is again observed, but at longer plating times a compressive stress is observed superimposed on to this original stress. At fairly long times the stress seems to level off, but as the deposits from Cl - solution are very powdery, this is probably due to the deposit fahing away from the electrode.
The measurement
.
and interpretation
primary distribution. A higher initial stress is observed the lower the cd passing. This seems to indicate that initially the surface roughness is such that the interfacial rate is more important than diffusion. Ultimately with time or cd diffusion becomes the most important limiting process and stress becomes less or disappears. An alternative explanation for the departure from the initial stress with time could be incorporation of solid, however this seems to be less likely as under theseconditions, if the Nernst equation determines the surface concentration of the soluble CuCl species, it is not expected to exceed the solubility. When (krh)l is very low as in SOf- solution the initial stress is much lower than in Cl- and has a constant value and is independent of current density. On the basis of the comparison with Cl- solutions it seems that the measured stress can be considered to be a combination of the “induced” stress and the intrinsic stress, which is a property of the metal being deposited. In the case of Cu deposition from SOi- because of the very much lower electrochemical rate the conditions of the primary distribution are retained up to long times and the measured stress is simply the induced stress with negligible intrinsic stress. It seems that the measurement of stress as a function oftime brings some insight into how the deposit is built up. Much more detailed measurements of stress, structure and electrode kinetics will be necessary to understand the phenomena which take place during the formation of thick layers of metals.
Cbl
l
6000~
Acknowledgement acknowledged.
I
~ The support
of this work by S.R.C. is
REFERENCES
x 1.
I
193
of the macrostress
t, min
Fig. 2. Plots of stress (S) OStime (t) for copper deposition from
(a) CuSO, (0.5 M) in HzSOI (1 M) at (x ) 5 mA a-‘, (0) 10mAcm-2,(~)20mAcm-~,(~)30mAcm-~;(b)CuCI~ (0.5M)inHCI(1M)at(I)1mAcm~2,(~)5mAcm-2,(01 10 mA cm-‘, (0) 20 mA cm-‘, (0) 30 mA cm-‘. It is interesting to note that the variation of stress with current density appears to be very small in contrast to the effects observed by other workers. CONCLUSIONS
The measured stress seems to correlate with the measured rate of the electron transfer reaction. When (klh), , is higher see [ 1) as in the case of deposition from the Cl- solution the initial (induced stress) is higher and the final stress is lower. The reason for the initially higher stress could be expected to be connected with the balance between the interfacial electrochemical rate, the surface roughness and the rate of diffusion. The first two points are usually expressed as the
J. A. Harrison,
D. R. Sandbach and P. J. Stronach,
Electrmkim ACk 24. 179 (19791. Abscheidung und Elektro2. H. Fischer, Elektrolytische kristallisation von Metallen. Berlin, Springer-Verlag (1954). 3. E. Raub and K. Muller, Fundamentals of Metal Deposition Elsevier (1967). 4. J. B. Kushner, Metal Finish 56, 46, 82, 56, 52, (1958). 5. J. B. Kushner, Proc. Congr. Int. Lrnion Electrodeposition. Su&ace Finish, 8th 1972 (Pub. 1973) p. 334. 6. W. Weil, Plating 1231 (1970); 50, 137 (1971). 7. W. H. Safranek, The Properties o( Electrodeposited Metals and Alloys. Elsevier (1974). 8. D. W. Hardesty, J. ehxtrockem. SK 117, 168 (1970). 9. W. H. Gauvin and C. A. Winkler, .I. electrochem. Sac. 99. 71 (1952). IO. G. Stoney, Proc. R. Sot. A 82. 1972 (1909). 72 (1909). If. K. G. Soderberg and A. K. Graham. Proc. Am. Electroplaters Sot. 3;1. 74 (1947). 12. R. Gabe and J. M. West, Trans. Inst. Metal Finish. 40,6 (1963). 13. k. A. ‘Vuilleumier, 1. elecfrochem. Sec. 42, 99 (19221. 14. V. Kohlschutter and E. A. Vuilleumier, Z. EIektrochemie 24, 300 (1918). 15. V. T. Vagramyan and Yu. S. Petrova, The Mechanical Properties of Electrolytic Deposits, New York, Consultants Bureau (1962). 16. R. H. D. Barklie and H. J. Davies, Proc. Inst. Mech. Eng. 731 (1930). 17. A. W. Hothersall, C. J. Leadbeater, J. electrodepositors’ Tech. Sot. 14, 207 (1938).
194 18. M. Ya. Popereka
19. 20. 21. 22.
J. A. HMRJSQN AND P. J.
and V. V. Koshmanov, 1. appl. Chem. (Russ.) 38, 2011 (1%5). C. Marie, J. Than, Cmpt. Rend. 193, 31 (1931). P. A. Jacquet, Trans. e&rorhem. Sot. 64,393 (1934). T. P. Hoar, D. I. Arrowsmith, Tram. In.@. Metal Finbh. 34, 354 (1957). J. E. Stareok. E. J. Seyb, A. C. Tulumello, Plating 41,117l (1954).
STRONACH
23. W. M. Phillips,
F. L. Clifton,
Proc. Am. Elecrroplaters
sot. 34.97 (1947). 24. A. Brenner, S. Senderoff, J. Res. Nat. Bur. Stand. 42, 105 (1949). 25. J. B. Ku&m, Metal Finish. 41, 1145 (1954). 26. J. H. C~eghorn, K. S. A. Gnanasckaran, D. J. Hall, Metal Finish J. 18, 92 (1972). 27. M. Perakh, SvrJI Technol. 4, 527, 538 (1976).
rm.cw
THE MEASUREMENT AND INTERPRETATION OF THE MACROSTRESS GENERATED DURING THE ELECTRODEPOSITION OF COPPER J.A.
HARRISON
and P. J.
STRONACH
School of Chemistry University, Newcastle-on-Tyne,
England
(Received 7 June 1978) Abstract - Measurements of the time dependence of the macrostress, generated in thick layers of copper and HCI solutions, are presented. The results are discussed in terms of the electrode kinetics of the appropriate electron transfer reactions
deposited from H,SO,
NOMENCLATURE
A
b D F F M ;
area of electrode width of strip pen-recorder deflection faraday restoring force current plated length of strip atomic weight overall number of electrons transferred stress time thickness of strip thickness of deposit density standard rate of the first electron transfer[l]
contribute to the intrinsic stress are crystal coalescence, occluded matter, generation of dislocations. The importance of these various possibilities depend on the system. This subject area has been reviewed in detail[3-63. In the case of Cu deposition from pure solutions probably only crystal coalescence and the need to be considered as generation of dislocations possibilities.
Morphology
of Cu deposits
For Cu deposition from sulphate baths SafranekE7) has stated that columnar deposits with some (110) plane orientation are observed. In Cl- containing aqueous solution a rough powdery deposit is observed S]. On adding Cl- ions to a solution containing S04-I no effect is observed until a critical concentration is reached[9]. It has been suggested[9] that a CuCl deposit can also be formed. However this has not been confirmed.
In a previous paper[l] we have discussed in detail the electrode kinetics of the electron transfer step in the reduction of Cu2+ onto Cu, Zn2+ onto Zn, and Cd’* onto Cd. It has not yet proved possible to obtain much information about the following electro crystallization steps. However on a rough metal surface it is likely that the growth of the surface is controlled primarily by the electron transfer step, that is by the primary distribution. An important property of massive electrodeposits is the macrostress. It is the purpose of the present note to present the result of measuring the macrostress during the deposition of Cu and to compare the results with the electrode kinetic information. Cu is a very suitable system for this purpose as the rate of Cu deposition from H,SO, solution onto a rough surface is some ten times slower than from HCl solution (probably from the CuCl+ complex). This allows the balance between the rate of the interfacial reaction and diffusion to be changed in the steady state by changing the solution from which the metal is deposited.
Measurement
of stress
The most widely used technique to measure stress depends on the bending of a metal strip which is plated on one side only as used by Stoney[lO]. The equations used to calculate the stress have been discussed by Soderberg and Graham[ 111 and Gabe and West[12]. Various methods have been used to determine the degree of bending of the strip[13-231. Hoar and Arrowsmith[21] used a cathode strip which was rigidly clamped at one end. The bending of the strip was prevented by the magnetic field of a coil into which an armature, attached to the free end of the strip, protrudes. By varying the current flowing through the c&l the strip can be held in its original position and a correlation is made between the current flowing through the coil and the stress of the deposit. Other methods which have been used include the ‘Spiral Contractometer’ of Brenner and SenderofT[24] the ‘Stressometer’ of Kushner[25] and a dilatometric method[26,27]. A useful compilation of steady state stress measurements are given by Safranek[7]. The method used, in the present work, was based on that of Hoar and Arrowsmith[21] and a diagram of the apparatus is shown in Fig. 1. One side of a mild steel strip was insulated by coating it with “Lacomit”. A small square of steel was attached perpendicular to the
Stress in electrodeposits A distinction has been made in the literature between induced and intrinsic stress[Z]. Induced stress arises due to a difference in structure between deposit and substrate and is a maximum at the deposit substrate interface. Superimposed on this is the intrinsic stress in the deposit. The various effects which can 191
192
J. A.
HARRISON AND P.J.STRONACH
FS
maintain the strip in its unplated position. The stress (S) is given by:
PSU
cv
A2
A3
S-g
M
where S is the stress (kg cm-*), 1is the plated length of the strip (cm), b is its breadth (cm) and t’ its thickness (cm), F’ is the restoring force applied to the strip (kg) and x the thickness of the deposit (cm). Assuming that the current density and current efficiency is 100°/0 the thickness of the deposit (x) can be related to the time of plating and together with equation (I) gives S=-
2l’F’nFp t’itM
of the stress measuringequipment. the connectionto the working electrode. light sensitivediodes which sense the balance point.
Fig. 1. Diagram
WE Sl, 52 Cl,C2 Al A2, A3
CV
PSU M SE
coils through which current flows to maintain the strip position. potentiostat connected as a galvanostat with the current controlled by R and the potential by V+ Servo amplifier controlled by Si, S2
where i is the applied current (A), t is the time of plating (s), M is the atomic weight of the deposited metal, A is the area of the cathode which has been plated (cm’), n is the number of electrons involved in the deposition reaction, F is the Faraday (coulombs mole- ‘) and p is the density of the deposit (gmcm- 3). For the strips used in these experiments it was found that the pen-recorder gave a 1 V deflection for an applied force of 50 x 10m6kg acting at the top of the strip. The thickness of the strip was 10-l cm and the breadth was 1.25 cm. The plated length was 8 cm. The plating current was measured in mA and when the approximate values were inserted into equations (1) and (2) the stress, for a copper deposit, could be expressed as :
constantvoltage unit.
stress reading meter unit voltage comparator
secondaryelectrode
top of the strip and this carried a length of wire which passed through the two coils. This square served as a means of detecting the zero deflection point as, when the cathode was clamped into position, it was positioned between two light-sensitive detectors. If this square moved from its original position, a current was caused to pass through the ceils by a conventional servo amplifier type circuit in order to bring it back to zero. A pen-recorder was connected to the instrument in order to continuously monitor the current flowing through the coils. The instrument was calibrated by applying a known force to the insulated metal strip and measuring the recorder deflection corresponding to the current flowing through the coils which was required to maintain the strip in its original position. The deflection was a linear function of the force. Measurements were carried out at various cds by plating on to an area of 10cm2 of the strip from solutions of CuS04 (0.5 M) in H,SO, (1 M) and CuCl, (0.5 M) in HCI (1 M). The current was maintained using a potentiostat connected to operate galvanostatically. The mild steel strip acted as a cathode and a sheet of copper foil was used as the reference electrode. Culculation
of the stress
Hoar and Arrowsmith[Zl] derived an equation to relate the stress of a deposit to the force required to
s=
2.89 x lo40
it
(3)
Values of D, the recorder deflection (V), i, the plating current (mA), and t. the time of plating @in), can now be substituted into (3) to give the stress of the deposit at any time (t) in kg cm-‘.
Stress results Copper was plated on to the mild steel strips from sulphate and chloride solutions. The deflection of the pen-recorder was monitored on the paper chart. The stress at any particular time of plating was calculated using equation (3) and Fig. 2 shows the variation of stress with time. It is quite noticable that the behaviour in SOisolution is different to that observed for the Cl- case. For the SO:- case an initial increase in tensile stress is observed at short plating times which becomes constant at longer times. The initial stress is probably due to the different lattice structures of the deposit and the substrate. At greater deposit thicknesses the stress due to the actual deposit is the observed superimposed on to this original stress, indicating that the stress of the deposit is negligible. In the Cl- case an initial increase in tensile stress (of greater magnitude than for the SOi- case) is again observed, but at longer plating times a compressive stress is observed superimposed on to this original stress. At fairly long times the stress seems to level off, but as the deposits from Cl - solution are very powdery, this is probably due to the deposit fahing away from the electrode.
The measurement
.
and interpretation
primary distribution. A higher initial stress is observed the lower the cd passing. This seems to indicate that initially the surface roughness is such that the interfacial rate is more important than diffusion. Ultimately with time or cd diffusion becomes the most important limiting process and stress becomes less or disappears. An alternative explanation for the departure from the initial stress with time could be incorporation of solid, however this seems to be less likely as under theseconditions, if the Nernst equation determines the surface concentration of the soluble CuCl species, it is not expected to exceed the solubility. When (krh)l is very low as in SOf- solution the initial stress is much lower than in Cl- and has a constant value and is independent of current density. On the basis of the comparison with Cl- solutions it seems that the measured stress can be considered to be a combination of the “induced” stress and the intrinsic stress, which is a property of the metal being deposited. In the case of Cu deposition from SOi- because of the very much lower electrochemical rate the conditions of the primary distribution are retained up to long times and the measured stress is simply the induced stress with negligible intrinsic stress. It seems that the measurement of stress as a function oftime brings some insight into how the deposit is built up. Much more detailed measurements of stress, structure and electrode kinetics will be necessary to understand the phenomena which take place during the formation of thick layers of metals.
Cbl
l
6000~
Acknowledgement acknowledged.
I
~ The support
of this work by S.R.C. is
REFERENCES
x 1.
I
193
of the macrostress
t, min
Fig. 2. Plots of stress (S) OStime (t) for copper deposition from
(a) CuSO, (0.5 M) in HzSOI (1 M) at (x ) 5 mA a-‘, (0) 10mAcm-2,(~)20mAcm-~,(~)30mAcm-~;(b)CuCI~ (0.5M)inHCI(1M)at(I)1mAcm~2,(~)5mAcm-2,(01 10 mA cm-‘, (0) 20 mA cm-‘, (0) 30 mA cm-‘. It is interesting to note that the variation of stress with current density appears to be very small in contrast to the effects observed by other workers. CONCLUSIONS
The measured stress seems to correlate with the measured rate of the electron transfer reaction. When (klh), , is higher see [ 1) as in the case of deposition from the Cl- solution the initial (induced stress) is higher and the final stress is lower. The reason for the initially higher stress could be expected to be connected with the balance between the interfacial electrochemical rate, the surface roughness and the rate of diffusion. The first two points are usually expressed as the
J. A. Harrison,
D. R. Sandbach and P. J. Stronach,
Electrmkim ACk 24. 179 (19791. Abscheidung und Elektro2. H. Fischer, Elektrolytische kristallisation von Metallen. Berlin, Springer-Verlag (1954). 3. E. Raub and K. Muller, Fundamentals of Metal Deposition Elsevier (1967). 4. J. B. Kushner, Metal Finish 56, 46, 82, 56, 52, (1958). 5. J. B. Kushner, Proc. Congr. Int. Lrnion Electrodeposition. Su&ace Finish, 8th 1972 (Pub. 1973) p. 334. 6. W. Weil, Plating 1231 (1970); 50, 137 (1971). 7. W. H. Safranek, The Properties o( Electrodeposited Metals and Alloys. Elsevier (1974). 8. D. W. Hardesty, J. ehxtrockem. SK 117, 168 (1970). 9. W. H. Gauvin and C. A. Winkler, .I. electrochem. Sac. 99. 71 (1952). IO. G. Stoney, Proc. R. Sot. A 82. 1972 (1909). 72 (1909). If. K. G. Soderberg and A. K. Graham. Proc. Am. Electroplaters Sot. 3;1. 74 (1947). 12. R. Gabe and J. M. West, Trans. Inst. Metal Finish. 40,6 (1963). 13. k. A. ‘Vuilleumier, 1. elecfrochem. Sec. 42, 99 (19221. 14. V. Kohlschutter and E. A. Vuilleumier, Z. EIektrochemie 24, 300 (1918). 15. V. T. Vagramyan and Yu. S. Petrova, The Mechanical Properties of Electrolytic Deposits, New York, Consultants Bureau (1962). 16. R. H. D. Barklie and H. J. Davies, Proc. Inst. Mech. Eng. 731 (1930). 17. A. W. Hothersall, C. J. Leadbeater, J. electrodepositors’ Tech. Sot. 14, 207 (1938).
194 18. M. Ya. Popereka
19. 20. 21. 22.
J. A. HMRJSQN AND P. J.
and V. V. Koshmanov, 1. appl. Chem. (Russ.) 38, 2011 (1%5). C. Marie, J. Than, Cmpt. Rend. 193, 31 (1931). P. A. Jacquet, Trans. e&rorhem. Sot. 64,393 (1934). T. P. Hoar, D. I. Arrowsmith, Tram. In.@. Metal Finbh. 34, 354 (1957). J. E. Stareok. E. J. Seyb, A. C. Tulumello, Plating 41,117l (1954).
STRONACH
23. W. M. Phillips,
F. L. Clifton,
Proc. Am. Elecrroplaters
sot. 34.97 (1947). 24. A. Brenner, S. Senderoff, J. Res. Nat. Bur. Stand. 42, 105 (1949). 25. J. B. Ku&m, Metal Finish. 41, 1145 (1954). 26. J. H. C~eghorn, K. S. A. Gnanasckaran, D. J. Hall, Metal Finish J. 18, 92 (1972). 27. M. Perakh, SvrJI Technol. 4, 527, 538 (1976).