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Comments on mechanisms of copper electrodissolution in chloride media
EkcmAimica Acta Vd. 38. No. 18, pp 27814783.1993 Fvintcd in Gmal Britain.
0013-4686/93%.00 + o.00 0 1993.perpmon Rcu Ltd.
SHORT COMMUNICATION
COMMENTS ON MECHANISMS OF COPPER ELECTRODISSOLUTION IN CHLORIDE MEDIA C. Dxs~uts,*
0. R. MA-~
M. M. MUSIANI$ and B. TRIBOLLET*
* UPR 15 CNRS 4 place Jussieu 75252 Paris, France t PEMM/COPPE/UFRJ, Caixa Postal 68505.21945 Rio de Janeiro, Brazil $ IPELP/CNR, Corso Stati Uniti 4,3X)20 Camin (Padova), Italy (Received 15 February 1993; in revisedform 5 July 1993) Abstract-The aim of this communication is to point out some overlap and discrepancies which have appeared in recent literature concerning copper electrodissolution in chloride media.
1. INTRODUCPION Copper electrodeposition in chloride media has been extensively investigated mainly in hydrochloric acid, but as no influence of pH was reported[l], a common mechanism may hold for both acidic and neutral solutions. The present analysis is limited to copper dissolution as Cu(1). This involves an active dissolution domain followed by a plateau current i,, both influenced by mass transport. The current increase beyond the plateau accounts for the onset of dissolution as Cu(II) and will be ignored here. Copper dissolves anodically in Cl- media below 1 M to yield CuCl; (above this concentration, complexes with a higher number of chloride ligands are formed)[2]. In the region of active dissolution even near the open circuit potential, the current is partially controlled by mass transport with[2-61: z-1 =
1,’
+(.&-l/2*
In [2, 11, 121, the first step was assumed fast and at pseudo-equilibrium. Based on this hypothesis, the calculated current shows a dependence of the coefllcient a in equation (1) on [Cl-], and potential in agreement with experiments. In contrast, I, is predicted to be proportional to [Cl-]: whereas experimental results show a linear relationship. For low current densities, Deslouis et aZ.[5] assumed that: (i) the first step [equation (2)] is no longer at pseudo-equilibrium; (ii) coverage 6 by CuCl is small (this assumption was implicit); and (iii) the mass transport control is only due to CuCl; removal from the interface. They obtained the following equation:
F
-= i
0)
For high overpotentials, a current plateau is observed with iL proportional to Q”’ but lower than the Levich current calculated by considering a transport limitation by Cl- ions only. 2 ACI’IVE DISSOLUTION PLATEAU
BELOW THE
Although some works consider a single electrochemical step[l, 4, 6-101, most authors agree on a two-step sequence where CuCl is an insoluble absorbed species[2,5,1 l-163: cu+c1-
-CuCl+ek’ k-1
CuCl + Cl- +
(2) c&l;
(3)
1 k,[Cl-1,
k_,k_2BCUC,2_
+
+
ex
k,k,CCl-12,
exp
k,WJ-12, &uar
*
(4)
This equation has the advantage with respect to previous ones of being able to explain the current dependence on chloride concentration, potential and angular velocity of the rde. Apparently unaware of [5J, Crtmdwell, starting from the same hypotheses, obtained recently equation (4) under an identical form[15, 163. He also experimentally verified its validity in the whole active dissolution range below the plateau One critical point concerns the nature of the diffusing species involved in the mass transport control. According to equations (2) and (3), they can be either Cl- or C&l;, respectively, flowing towards the
2781
2782
C. DESLOUIS et al.
metal and away from it. Their fluxes are related to the current by : i F
-CD
; = DC,_
cucz~-
CcucGllJ 6CUCIZ- ’
w~-l,
(5)
- ccl-101
.
26,_
(6)
At currents low enough, it is obvious that i/F < D,,_[CI-],/26,,_ and therefore [Cl-],, x [Cl-], . In addition, in a second paper[lfl, Deslouis et al. derived an impedance model valid at low currents and showed by EHD impedance measurements in this current range, that the Schmidt number (SC = v/ D) approximates 2000. At higher currents, Smyrl[4] obtained by UCimpedance measurements a SC value of 1500 which does not completely exclude the existence of a concentration gradient of Cl- as the Schmidt number for Cl- is around 500[18]. More recently[18], a Schmidt number of 2000 was measured by EHD impedance at all currents below the plateau. Considering also that the experiments of Crundwell show a dependence of the current on potential, angular velocity and [Cl-], in agreement with equation (4), this equation is likely to be valid at any current below the plateau.
3. PLATEAU
F
1
k-,
BN’
D
_
Cl
ccl-l,
D
6, represents here a common value of the diffusion layers for species B and D. This is a rough approximation for Cl- and CuCl; as Da_ N 4D,,,_ and therefore, a,,_ kr 1.66,,,,- . The current associated to equations (2) and (3) of the mechanism corresponds to steady-state conditions and excludes the existence of oscillations, otherwise considered in the treatment of Pearlstein et a1.[13, 143 and induced by the moving boundaries of the CuCl layer. For steady-state conditions and in the considered [Cl-] range, the diffusion rate (and here either Cl- or CuCl; can be considered because we just need an order of magnitude) D/S, is about 5 x 10m3cm s- ‘, ie 500 times larger that the velocity of the receding copper metal surface by considering
FJ-la,
(x > 6,, in the electrolyte); = D,
(8)
ccl-l,, - W-ICI
film
(7)
-
hl-
sol
6,
(0 c x < 6,, in the film); N cucl~-
(9)
CCuCLlat
Lczz - s CuCl2(as [CuCl;],
= 0).
(10)
The Nernst approximation is applied to equations (8) and (10) but 6,,_ and 6cuc,I_ are obtained through integration of the convective diffusion equation for the relevant species. From equation (2) now assumed irreversible in this potential range, one has:
Ncz-
CZI L-+x--E2 2
=
NC,-
NC,_
REGION
In the plateau region, though the existence of a porous CuCl layer is widely accepted, very few models were proposed in the literature. In 1985, Pearlstein et aI.[13, 143 considered the two-step mechanism [equations (2) and (3)], where CuCl precipitates as a porous film, reaction described by equation (3) is occurring at the filmsolution interface, and Cl- diffuses through the film to react at the copper surface according to equation (2), supposed as irreversible (k_ 1 = 0). With formally the same scheme, but presented as a very general one where the metal, the reacting ion (species B instead of Cl-) and the produced ion (species D instead of C&l;) are undefined, Crundwell[ZO] calculated the current expression as: ZL -=
an average current density of 102mAcm-2. Then the system is stable as shown by all reported data for similar experimental conditions. Of course, the presence of moving boundaries should be taken into account when modulating either the potential (cc impedance) or the rde angular velocity (EHD impedance). With the same hypotheses as above, and without the simplification on the values of the diffusion layer thicknesses, the equation of the current was recently calculated in [19]. As the result is somewhat different from the expression of the current given in [20], we will briefly recall the calculation of the current given in [19]. In fact, let us consider a CuCl layer formed on the copper surface with a thickness 6,. In the following, D, refers to the diffusion coefficient of Cl- in the porous film. The different fluxes (of Cl- in the solution and through the film, and of CuCl; from the filmsolution interface toward the bulk) have been written as absolute values, but in the balances they have been taken as influxes or outfluxes. One then has:
= i = k,[Cl-1,
.
exp
(11)
film
N cuc,I_ represents the outflux of CuCl; but also the production rate of Cl- at x = 6, from equation (3). Hence : N cuc,z- =
MT-I,, - k-,lWXl~.
Conservation of the influx and outflux of Clx = 6, yields: + Ncuc12- = NC,-
NC,film
. ad
(12) at
(13)
Mass balance of CuCl (production removal at x = 6,) provides :
at x = 0 and
= Ncuczr
(14)
NC,-
9
film
2783
Short Communication
but is lower than the Levich current only controlled by the diffusion of Cl- ions and defined as: = 2Nha,-. SC.1
W)
NCI-
. _
FJh,-Ccl-I,
IL -
From equations (9) and (11) one has:
W-lo =
f@&.
(16)
From equations (10) and (12) one has
PCl;la, = MCQM-,
:
+ &,,az-/&,,a~-)~ (17)
and from equations (S), (15) and (17), one gets: [cl-],
= F
2&J-
To sum up, in chloride media below 1 M, copper dissolves acccording to a two-step sequence expressed by equations (2) and (3). At all currents below the plateau, hypotheses involving an electrochemical step out of equilibrium, a low coverage value of insoluble CuCl species and a mass-transport control due only to CuCl; are verified and equation (4) is valid. On the plateau, equation (2) is irreversible (with k- 1 = 0) and occurs on the copper surface, whereas the reaction corresponding to equation (3) takes place at the CuCl film-electrolyte interface. The current is then given by:
ccl-l, ‘L F = 26,,- + k_2 6,uc,l-
f
DC,s,_ ccl-l,
(21)
*
Da-
.
(22)
k,G
Below the plateau, the UC and EHD impedance expressions predicted in [17J are in satisfactory agreement with experiments. Based on the hypotheses leading to equation (lo), the calculation of impedance expressions on the plateau may be found in[21].
(18) Also from equations (9), (lo), (15) and by assuming that [Cl-J&lm) = [Cl-]&(solution), one gets the value for the fihn thickness which depends on kinetics as well as on transport parameters in solution according to:
Replacing this expression in equation equation for the current can be derived :
ccl-l,
h. -= F
1 k+2
26,~ Da-
+ k-2 k,
&uc,z-.
(18), the
(20)
G
The divergence between this equation and equation (7) lies in the presence of an additional term in the denominator. A close comparison of the calculations of [19] and [20] shows a difference in the writing of equation (13) which was written in [20] as: N,,_
- NCUCII_= N,_ rum
sol
with the same definition of N,,. From an experimental point of view, the current shows a dependence on SZ”‘, so that:
REFERENCES 1. M. Braun and K. Nobe, J. electrochem. Sot. 126, 166 (1979). 2. H. P. Lee and K. Nobe, J. electrochem. Sot. 133, 2035 (1986). 3. W. D. Bjorndahl and K. Nobe, Corrosion 40,82 (1984). 4. W. Smyrl, J. electrochem.Sot. 135 1556 (1985). 5. C. Deslouis, B. Tribollet, C. Pagura and M. M. Musiani, J. Appl. Electrochem. 1%374 (1988). 6. B. Tribollet and J. Newman, J. electrochem. Sot. 131, 2780 (1984). 7. A. L. Bacarella and J. C. Griess, J. efecrochem.Sot. 120, 459 (1973). 8. T. Hurlen, Acta Chem. Scand. 15, 1231 (1961). 9. C. H. Bonfiglio, H. C. Albaya and 0. A. Cobo, Corr. Sci. 13,717 (1973). 10. L. Brossard, J. electrochem. Sot. 130,403 (1983). 11. A. Moreau, Electrochim.Acta 26, 1609 (1981). 12. A. Moreau, J. P. Frayet, F. Del Rey and R. Pointeau, Electrochim.Acta 27, 1281(1982). 13. H. P. Lee, K. Nobe and A. Pearlstein, J. electrochem. Sot. 132,1031(1985). 14. A. Pearlstein, H. P. Lee and K. Nobe, J. efectrochem. Sot. 132.2159 (1985). 15. F. K. Crundwe& Electrochim Acta 36,2135 (1991). 16. F. K. Crundwell, Electrochim. Acta 37,2707 (1992). 17. C. Deslouis, B. Tribollet, G. Mengoli and M. M. Musiani, J. appl. Electrochem. 1%384 (1988). 18. J. S. Newman, in Electrochemical Systems, 2nd edition. Prentice Hall, Englewood Cliffs, NJ (1991). 19. E. D’elia, 0. E. Bar&, 0. R. Mattos, N. Pebere and B. Tribollet, Proc. of the Symp. ‘Metal Deposition and Dissolution’, Vol. 92-23, The eleetrochem. Sot. 164 (1992). 20. F. K. Crundwell, Electrochim. Acta %,1183 (1991). 21. 0. E. Barcia, 0. R. Mattes, N. Pebcre and B. TriboUet, J. electrochem. Sot. 140.2825 (1993).
0013-4686/93%.00 + o.00 0 1993.perpmon Rcu Ltd.
SHORT COMMUNICATION
COMMENTS ON MECHANISMS OF COPPER ELECTRODISSOLUTION IN CHLORIDE MEDIA C. Dxs~uts,*
0. R. MA-~
M. M. MUSIANI$ and B. TRIBOLLET*
* UPR 15 CNRS 4 place Jussieu 75252 Paris, France t PEMM/COPPE/UFRJ, Caixa Postal 68505.21945 Rio de Janeiro, Brazil $ IPELP/CNR, Corso Stati Uniti 4,3X)20 Camin (Padova), Italy (Received 15 February 1993; in revisedform 5 July 1993) Abstract-The aim of this communication is to point out some overlap and discrepancies which have appeared in recent literature concerning copper electrodissolution in chloride media.
1. INTRODUCPION Copper electrodeposition in chloride media has been extensively investigated mainly in hydrochloric acid, but as no influence of pH was reported[l], a common mechanism may hold for both acidic and neutral solutions. The present analysis is limited to copper dissolution as Cu(1). This involves an active dissolution domain followed by a plateau current i,, both influenced by mass transport. The current increase beyond the plateau accounts for the onset of dissolution as Cu(II) and will be ignored here. Copper dissolves anodically in Cl- media below 1 M to yield CuCl; (above this concentration, complexes with a higher number of chloride ligands are formed)[2]. In the region of active dissolution even near the open circuit potential, the current is partially controlled by mass transport with[2-61: z-1 =
1,’
+(.&-l/2*
In [2, 11, 121, the first step was assumed fast and at pseudo-equilibrium. Based on this hypothesis, the calculated current shows a dependence of the coefllcient a in equation (1) on [Cl-], and potential in agreement with experiments. In contrast, I, is predicted to be proportional to [Cl-]: whereas experimental results show a linear relationship. For low current densities, Deslouis et aZ.[5] assumed that: (i) the first step [equation (2)] is no longer at pseudo-equilibrium; (ii) coverage 6 by CuCl is small (this assumption was implicit); and (iii) the mass transport control is only due to CuCl; removal from the interface. They obtained the following equation:
F
-= i
0)
For high overpotentials, a current plateau is observed with iL proportional to Q”’ but lower than the Levich current calculated by considering a transport limitation by Cl- ions only. 2 ACI’IVE DISSOLUTION PLATEAU
BELOW THE
Although some works consider a single electrochemical step[l, 4, 6-101, most authors agree on a two-step sequence where CuCl is an insoluble absorbed species[2,5,1 l-163: cu+c1-
-CuCl+ek’ k-1
CuCl + Cl- +
(2) c&l;
(3)
1 k,[Cl-1,
k_,k_2BCUC,2_
+
+
ex
k,k,CCl-12,
exp
k,WJ-12, &uar
*
(4)
This equation has the advantage with respect to previous ones of being able to explain the current dependence on chloride concentration, potential and angular velocity of the rde. Apparently unaware of [5J, Crtmdwell, starting from the same hypotheses, obtained recently equation (4) under an identical form[15, 163. He also experimentally verified its validity in the whole active dissolution range below the plateau One critical point concerns the nature of the diffusing species involved in the mass transport control. According to equations (2) and (3), they can be either Cl- or C&l;, respectively, flowing towards the
2781
2782
C. DESLOUIS et al.
metal and away from it. Their fluxes are related to the current by : i F
-CD
; = DC,_
cucz~-
CcucGllJ 6CUCIZ- ’
w~-l,
(5)
- ccl-101
.
26,_
(6)
At currents low enough, it is obvious that i/F < D,,_[CI-],/26,,_ and therefore [Cl-],, x [Cl-], . In addition, in a second paper[lfl, Deslouis et al. derived an impedance model valid at low currents and showed by EHD impedance measurements in this current range, that the Schmidt number (SC = v/ D) approximates 2000. At higher currents, Smyrl[4] obtained by UCimpedance measurements a SC value of 1500 which does not completely exclude the existence of a concentration gradient of Cl- as the Schmidt number for Cl- is around 500[18]. More recently[18], a Schmidt number of 2000 was measured by EHD impedance at all currents below the plateau. Considering also that the experiments of Crundwell show a dependence of the current on potential, angular velocity and [Cl-], in agreement with equation (4), this equation is likely to be valid at any current below the plateau.
3. PLATEAU
F
1
k-,
BN’
D
_
Cl
ccl-l,
D
6, represents here a common value of the diffusion layers for species B and D. This is a rough approximation for Cl- and CuCl; as Da_ N 4D,,,_ and therefore, a,,_ kr 1.66,,,,- . The current associated to equations (2) and (3) of the mechanism corresponds to steady-state conditions and excludes the existence of oscillations, otherwise considered in the treatment of Pearlstein et a1.[13, 143 and induced by the moving boundaries of the CuCl layer. For steady-state conditions and in the considered [Cl-] range, the diffusion rate (and here either Cl- or CuCl; can be considered because we just need an order of magnitude) D/S, is about 5 x 10m3cm s- ‘, ie 500 times larger that the velocity of the receding copper metal surface by considering
FJ-la,
(x > 6,, in the electrolyte); = D,
(8)
ccl-l,, - W-ICI
film
(7)
-
hl-
sol
6,
(0 c x < 6,, in the film); N cucl~-
(9)
CCuCLlat
Lczz - s CuCl2(as [CuCl;],
= 0).
(10)
The Nernst approximation is applied to equations (8) and (10) but 6,,_ and 6cuc,I_ are obtained through integration of the convective diffusion equation for the relevant species. From equation (2) now assumed irreversible in this potential range, one has:
Ncz-
CZI L-+x--E2 2
=
NC,-
NC,_
REGION
In the plateau region, though the existence of a porous CuCl layer is widely accepted, very few models were proposed in the literature. In 1985, Pearlstein et aI.[13, 143 considered the two-step mechanism [equations (2) and (3)], where CuCl precipitates as a porous film, reaction described by equation (3) is occurring at the filmsolution interface, and Cl- diffuses through the film to react at the copper surface according to equation (2), supposed as irreversible (k_ 1 = 0). With formally the same scheme, but presented as a very general one where the metal, the reacting ion (species B instead of Cl-) and the produced ion (species D instead of C&l;) are undefined, Crundwell[ZO] calculated the current expression as: ZL -=
an average current density of 102mAcm-2. Then the system is stable as shown by all reported data for similar experimental conditions. Of course, the presence of moving boundaries should be taken into account when modulating either the potential (cc impedance) or the rde angular velocity (EHD impedance). With the same hypotheses as above, and without the simplification on the values of the diffusion layer thicknesses, the equation of the current was recently calculated in [19]. As the result is somewhat different from the expression of the current given in [20], we will briefly recall the calculation of the current given in [19]. In fact, let us consider a CuCl layer formed on the copper surface with a thickness 6,. In the following, D, refers to the diffusion coefficient of Cl- in the porous film. The different fluxes (of Cl- in the solution and through the film, and of CuCl; from the filmsolution interface toward the bulk) have been written as absolute values, but in the balances they have been taken as influxes or outfluxes. One then has:
= i = k,[Cl-1,
.
exp
(11)
film
N cuc,I_ represents the outflux of CuCl; but also the production rate of Cl- at x = 6, from equation (3). Hence : N cuc,z- =
MT-I,, - k-,lWXl~.
Conservation of the influx and outflux of Clx = 6, yields: + Ncuc12- = NC,-
NC,film
. ad
(12) at
(13)
Mass balance of CuCl (production removal at x = 6,) provides :
at x = 0 and
= Ncuczr
(14)
NC,-
9
film
2783
Short Communication
but is lower than the Levich current only controlled by the diffusion of Cl- ions and defined as: = 2Nha,-. SC.1
W)
NCI-
. _
FJh,-Ccl-I,
IL -
From equations (9) and (11) one has:
W-lo =
f@&.
(16)
From equations (10) and (12) one has
PCl;la, = MCQM-,
:
+ &,,az-/&,,a~-)~ (17)
and from equations (S), (15) and (17), one gets: [cl-],
= F
2&J-
To sum up, in chloride media below 1 M, copper dissolves acccording to a two-step sequence expressed by equations (2) and (3). At all currents below the plateau, hypotheses involving an electrochemical step out of equilibrium, a low coverage value of insoluble CuCl species and a mass-transport control due only to CuCl; are verified and equation (4) is valid. On the plateau, equation (2) is irreversible (with k- 1 = 0) and occurs on the copper surface, whereas the reaction corresponding to equation (3) takes place at the CuCl film-electrolyte interface. The current is then given by:
ccl-l, ‘L F = 26,,- + k_2 6,uc,l-
f
DC,s,_ ccl-l,
(21)
*
Da-
.
(22)
k,G
Below the plateau, the UC and EHD impedance expressions predicted in [17J are in satisfactory agreement with experiments. Based on the hypotheses leading to equation (lo), the calculation of impedance expressions on the plateau may be found in[21].
(18) Also from equations (9), (lo), (15) and by assuming that [Cl-J&lm) = [Cl-]&(solution), one gets the value for the fihn thickness which depends on kinetics as well as on transport parameters in solution according to:
Replacing this expression in equation equation for the current can be derived :
ccl-l,
h. -= F
1 k+2
26,~ Da-
+ k-2 k,
&uc,z-.
(18), the
(20)
G
The divergence between this equation and equation (7) lies in the presence of an additional term in the denominator. A close comparison of the calculations of [19] and [20] shows a difference in the writing of equation (13) which was written in [20] as: N,,_
- NCUCII_= N,_ rum
sol
with the same definition of N,,. From an experimental point of view, the current shows a dependence on SZ”‘, so that:
REFERENCES 1. M. Braun and K. Nobe, J. electrochem. Sot. 126, 166 (1979). 2. H. P. Lee and K. Nobe, J. electrochem. Sot. 133, 2035 (1986). 3. W. D. Bjorndahl and K. Nobe, Corrosion 40,82 (1984). 4. W. Smyrl, J. electrochem.Sot. 135 1556 (1985). 5. C. Deslouis, B. Tribollet, C. Pagura and M. M. Musiani, J. Appl. Electrochem. 1%374 (1988). 6. B. Tribollet and J. Newman, J. electrochem. Sot. 131, 2780 (1984). 7. A. L. Bacarella and J. C. Griess, J. efecrochem.Sot. 120, 459 (1973). 8. T. Hurlen, Acta Chem. Scand. 15, 1231 (1961). 9. C. H. Bonfiglio, H. C. Albaya and 0. A. Cobo, Corr. Sci. 13,717 (1973). 10. L. Brossard, J. electrochem. Sot. 130,403 (1983). 11. A. Moreau, Electrochim.Acta 26, 1609 (1981). 12. A. Moreau, J. P. Frayet, F. Del Rey and R. Pointeau, Electrochim.Acta 27, 1281(1982). 13. H. P. Lee, K. Nobe and A. Pearlstein, J. electrochem. Sot. 132,1031(1985). 14. A. Pearlstein, H. P. Lee and K. Nobe, J. efectrochem. Sot. 132.2159 (1985). 15. F. K. Crundwe& Electrochim Acta 36,2135 (1991). 16. F. K. Crundwell, Electrochim. Acta 37,2707 (1992). 17. C. Deslouis, B. Tribollet, G. Mengoli and M. M. Musiani, J. appl. Electrochem. 1%384 (1988). 18. J. S. Newman, in Electrochemical Systems, 2nd edition. Prentice Hall, Englewood Cliffs, NJ (1991). 19. E. D’elia, 0. E. Bar&, 0. R. Mattos, N. Pebere and B. Tribollet, Proc. of the Symp. ‘Metal Deposition and Dissolution’, Vol. 92-23, The eleetrochem. Sot. 164 (1992). 20. F. K. Crundwell, Electrochim. Acta %,1183 (1991). 21. 0. E. Barcia, 0. R. Mattes, N. Pebcre and B. TriboUet, J. electrochem. Sot. 140.2825 (1993).