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Geometric influences on the spontaneous passivation of copper in alkaline hexacyanoferrate(III) solutions under conditions of free convection
75
J. Electroanal. Chem.. 213 (1986) 75-84
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
GEOMETRIC INFLUENCES ON THE SPONTANEOUS OF COPPER IN ALKALINE HEXACYANOFERRATE(II1) UNDER CONDITIONS OF FREE CONVECTION
PASSIVATION SOLUTIONS
J.M. KijHLER, A. WXEGAND and A. LERM Akudemte der ~~se~chu~ten 6900 Jena (G. 13.R.j
der DDR, Phystkulisek-Technisches
~~~t~tut, ~elmh~~tzweg
4,
{Received 13th September 1985: in revised form 15th May 1986)
ABSTRACT The passivation of copper proceeds in the presence of an oxidizing agent without an external current. Cyclovoltammetric, chronopotentiomet~c and open-circuit potential measurements show, that in alkaline solution the passivation rate increases with decreasing hydroxide and increasing hexa~~ofe~at~II1) concentration. The rate increases also by the formation of mixed potentials with NiCrSiO, surfaces and by reduction of the surface area. These effects can be explained by an increase of the diffusion-controlled cathodic partial current in comparison to the anodic partial current. The influence of surface area on the passivation rate is important on surfaces with an extension of about or smaller than the thickness of the diffusion layer.
Electrochemical processes which take place under open-circuit conditions are complex processes. Under quasi-steady-state conditions the overlap of two or more elecrochemical processes can often be described by adding the time-independent partial current potential curves [1,2]. In this way, the investigation and prediction of dissolution rates in “currentless” etching processes of thin metallic mono- and double-layer systems is possible [3-51. Beside the dissolution behaviour, the passivation behaviour is of interest in many practical cases of complex electrochemical processes. The anodic behaviour of copper has been investigated in a wide potential range. Several studies show that anodic passivation may occur both in alkaline [6-lo] and in acid [11,12] solutions. In alkaline solutions copper anodes form Cu,O, Cu(OH),, complex ions (HCuO;) and Cu,O, with increasing potential [6]. At lower potentials a base layer of Cu,O is formed by a dissolution-precipitation mechanism under participation of Cu(OH); [8], Cu(OH), appears at potentials between -0.27 and -0.25 V (vs. ~22-0728/86/$03.50
0 1986 Eisevier Sequoia S.A.
SCE). This Cu(OH), layer grows by precipitation of Cu*+ ions which are formed in the porous base layer of Cu *O. The passive range at potentials around - 0.25 V (vs. SCE) is due to the formation of a passivating layer of CuO [9,10]. At high potentials the formation of Cu(II1) occurs, whereby Cu,03 decays under evolution of oxygen ]I31. Activated copper behaves in alkaline hexacyanoferrate(II1) solutions (HCF solutions) under open-circuit conditions like an activated electrode in oxidizing-agentfree solutions under galvanostatic anodic dissolution. In alkaline solutions of HCF copper passivates spontaneously. The rate of passivation, which is connected with the rate of dissolution before passivation, depends on the hydroxide and HCF concentrations. It can be estimated by chronopotentiometric measurements [14]. Our investigations have shown (as will be described below), that a series of reactions proceeds on activated copper after insertion in alkaline HCF solution. The lifetime of transition states depends not only on the concentrations but also on geometric factors which influence the cathodic partial current. EXPERIMENTAL
Thin films of copper (8 pm) were used as samples. The surface proportion dependence was measured at double-layer systems of copper and a non-stoichiometric NiCrSiO, base layer. These layers were prepared by high-rate sputtering. Patterns were prepared photolithographically. A resist mask was used for covering in measurements of small surface areas. Electrochemical measurements were carried out by means of a PS 4 potentiostat (Meinsberg) and a PV 3 generator (Meinsberg). A platinum plate was used as reference electrode. The copper surfaces were activated by currentless rinsing in hydrochloric acid and in aqueous solutions of thiourea. The measurements were accomplished on vertically oriented samples in unstirred aqueous solutions of HCF and NaOH at 333 K. RESULTS
AND DISCUSSION
Electrode processes The cyclovoltammogram (Fig. 1) shows three anodic peaks and one cathodic peak. The small first peak below -0.8 V (vs. Pt) is due to the formation of Cu(I), while the second peak is caused by the formation of Cu(I1). Both the first and the second peak are in agreement with the cyclovoltammograms observed in HCF-free solutions [9]. The baseline (broken line in Fig. 1) represents a constant cathodic partial current. It has a constant distance from the abscissa (i = O-line) up to a potential higher than - 0.2 V (vs. Pt). The distance between the baseline and the abscissa is proportional to the concentration of HCF in the solution. The potential-independent reduction of HCF is responsible for the deviation of the baseline from the abscissa.
77
422M HCF 2,2 M NoOH
E/!/h
PII-
Fig. 1. Cyclovoltammogram of copper in alkaline HCF solution.
According to Vet&r, a diffusion-~ntrolled process can be distinguished from a reaction-controlled process by investigating the influence of convection in the solution on the intensity of the electrode process. The intensity of a diffusion-controlled process increases with increasing convection of the solution [El]. The stirring of an alkaline HCF solution (0.25 M HCF, 2.1 M NaOH) strongly influences the poteniostatic reduction (-0.75 V vs. Pt) of HCF on a copper electrode. This means that the cathodic partial process in the passivation of copper in the absence of external current must be diffusion-controlled. The deviation of the baseline disappears in the range of the two corresponding peaks (III/IV). The amount of charge, which is represented by the anodic and cathodic peaks, is equal. The shape and sire of the two peaks remain constant in subsequent cyclovoltammet~c cycles. It must be concluded that these peaks are due to the oxidation and reduction of an unchanged layer which is adsorbed at the electrode surface. The position of the two peaks near the Pt potential and in the range of disappearance of the base line deviation must be interpreted as the oxidation of hexacyanoferrate(II), and the reduction of the anodically formed hexacyanofe~ate(II1) which is contained in the Cu(I1) surface film. The two corresponding peaks reflect a partially reversible electrode process. Unlike the oxidation and reduction of a copper hexacyanoferrate film on glassy carbon, which shows symmetrical oxidation and reduction peaks [16], the difference of the maximum potentials of the two peaks must be related to a slow transport process. A comparison between the amounts of charge of the second (Qrr) and the third (Qm) anodic peak gives information about the nature of the adsorbed layer. Qn represents the Cu(I1) formed. QnI represents the oxidizable hexacyanoferrate(l1) which is contained in the adsorbed layer. The quotient 2Qrn/Qn is equal to the proportion of hexacyanoferrate(I1) in the adsorbed layer and Cu(I1) formed in the second transition state (second peak). Considering the low solubility of Cu*+ in alkaline solutions it must be assumed that most of the Cu(I1) formed during the
78
Fig. 2. Relation between the quotient of exchanged charges of the third (Q,,t) and second (Qrr) transition states and the concentration of HCF in solutions of 2.5 M NaOH (323 K) under consideration of cathodic partial current.
second transition state is contained in the adsorbed layer. Consequently the quotient 2QdQu represents approximately the proportion of Cu(I1) and hexacyanoferrate(I1) in the adsorbed layer. The quotient Q,,,/Q,, increases with increasing concentration of HCF in the solution (Fig. 2). It seems that HCF is built non-stoichiometrically into the layer which is formed in the range of the second anodic peak by the Cu(I1) species. At temperatures around 320-330 K and at higher concentrations of HCF (O-34-0.75 M), the quotient QIII/QII reaches a value of 0.25. This value is identical to the inverse ratio of the number of electrons which are exchanged by electrochemical formation of Cu,Fe(CN), from Cu(0) and oxidation of the hexacyanoferrate(I1) to hexacyanoferrate(II1) in this compound. An alternative interpretation is to assume that hexacyanoferrate does not form a precipitate but is dissolved in occluded soluble material in the adsorbed layer of copper hydroxide or oxide. A distinction between these explanations could be made by Raman spectroscopic measurements of the electrode surface. The electrode processes are clearly reflected by galvanostatic measurements. Three transition states are observed in anodic galvanostatic polarisation (Fig. 3). The first is due to Cu(I) formation, the second to Cu(I1) formation, and the third is caused by the oxidation of hexacyanoferrate(I1) contained in the adsorbed layer. In agreement with the literature data of copper behaviour in HCF-free solutions [S], the duration of the first transition state increases only slightly with increasing hydroxide concentration, while the duration of the second transition state T increases with the square of the hydroxide concentration, which can be explained by a linear dependence of the chronopotentiometric constant K on the hydroxide concentration. Under open-circuit conditions only two transition states are observed (Fig. 3, curve 5). The “infinite duration of the third transition state” is caused by the fact
19
2s M NOOH O,L5M HCF 333 K
I
20
LO
s t/s
60
-
Fig. 3. Anodic chronopotentiograms of activated copper in alkaline HCF solutions. 27 mA/cm2; (3) 17 mA/cm2; (4) 6.7 mA/cm2; (5) without anodic polarisation.
(1) 45 mA/cm’;
(2)
that the oxidation process which takes place in the third transition state is the back-reaction fo the HCF reduction, which leads to oxidizing processes at the copper electrode under open-circuit conditions. Examining the first two transition states, we conclude that the spontaneous reduction of HCF on the copper surface and the oxidation of copper, which is connected with the HCF reduction, proceeds like an electrode process under galvanostatic conditions. Increasing the HCF concentration acts in the same way as increasing the galvanostatic current density i,. This fact is well illustrated by means of the chronopotentiometric equation, where r represents the duration of a transition state: i,fi=K
(1)
equation must be completed by the addition of an anodic partial current density i, which is due to the cathodic partial current density of HCF reduction i_:
This
i,fi+i+fi=K
Under open-circuit conditions the polarisation-current-density i,h=K
(2)
term disappeares: (3)
This means that the inverse transition times under open-circuit conditions ril depend on the square of the concentrations of oxidizing agent (HCF). Inverse chronopotentiometric plots of the passivation of copper in alkaline solutions of 0.15 M HCF show the connections between the transition times, the anodic polarisation currents 1r, and the concentration of hydroxide ions in the case of the simultaneous reduction of HCF (Fig. 4). All chronopotentiometric straight lines go through the ordinate at the same point, which marks the intensity of the
80
Ul
a?
r-
w/s-b
Fig. 4. Connection between the transition times 7 aud the anodic polarisation current in the galvanostatic passivation of copper in alkaline solutions of 0.15 M HCF.
cathodic partial current. This intersection point is dependent on the concentration of HCF in the solution. The steepness of the plots increases with increasing hydroxide concentration, as a result of a non-steady-state diffusion process of hydroxide to the electrode surface. In the case of no external current, increasing transition times result in increasing hydroxide concentrations with constant concentration of HCF (I,, = 0).
Splitting of the cathodic partial current If the surface of copper is in direct electrical contact with a NiCrSiO, surface, an acceleration of the passivation is observed. The passivation speed increases with increasing size of the NiCrSiO, surface ANiCrSiO,,if the size of the copper surface remains constant (Fig. 5). Up to the potential of Cu(I1) formation, any anodic processes on NiCrSiO, are negligible in alkaline HCF solutions. After a fast passivation, which concerns only the top layer of the alloy, NiCrSiO, remains passive up to the passive region of copper. But on this NiCrSiO, surface the cathodic processes can occur. The reduction of HCF is effusion-eontroll~ up to the potential of the corresponding peaks of HCF reduction and oxidation (peaks III and IV) of the cyclovoltammograms. Since the reduction of HCF before copper passivation proceeds under diffusion control both on copper and on NiCrSiO,, the density of the cathodic partial current on NiCrSiO, is equal to the density of the cathodic partial current on copper. The cathodic partial current splits into components which are proportional to the size of the surface areas. Under open-circuit conditions the transition time TVdepends on the square of the copper surface size (Fig. 5). This dependence can be explained by means of a
81
20
Lo
60
4.i
AC" + &crsio, Fig. 5. Influence of cathodically efficient area (AC- + ANiCrLO,) passivation in alkaline HCF solution under open-circuit conditions.
on the transition
time of copper
chronopotentiometric equation: The anodic partial current density i, is the quotient of the anodic partial current I, and the anodic surface A,. The cathodic partial current I_ is the product of the cathodic partial current density i_ and the cathodic surface A _. I+ = i-A-/A+
(4
The insertion of eqn. (4) into eqn. (3) leads to a surface quotient-containing chronopotentiometric equation: i-6
= KA+/A_
(5)
Using the ~~onopotentiome~c results, the transition time which depends on HCF concentration and copper surface size was estimated. The estimated functions are represented by the lines in Fig. 5. They show that the experimental results are in agreement with the chronopotentiometric equation (5). Passivaticm of small areas
Observing the speed of passivation of copper in small areas under open-circuit conditions, we noticed an acceleration of,passivation in comparison with large areas. The passivation rate increases with decreasing surface area.
82
b
Fig. 6. (a) Shape of the diffusion layer in diffusion-controlled processes on small areas. (b) A simple model of the enlarged cathodic efficient area in the diffusion-controlled cathodic partial process on small areas.
It seems that this effect is due to the similar size of the extension of copper areas and.the thickness of the diffusion layer. The experimental results could be interpreted by a simple model. Measuring the passivation of isolated small squares of copper with the linear extension I, a diffusion layer with the thickness d in the neighbourhood of the metal must be considered. The diffusion to the surface occurs not only in the direction perpendicular to the electrode surface. Horizontal diffusion components contribute to the transport of HCF to the electrode due to the influence of the edge (Fig. 6a). Such an influence acts like an enlarged area in diffusion-controlled processes. The apparent magnification of the area in a diffusion-controlled process depends on the thickness of the diffusion layer d and the linear extension of the squares I. An approach to real conditions is possible by the following simple model: The apparent enlarged area is assumed to be a square with linear extensions
v 1
2
3m-’
1-pwn-l-
Fig. 7. Size effect on transition time T,, in the passivation solutions under open-circuit conditions.
of small areas (/2) of copper in alkaline HCF
83
of I+ 2d (Fig. 6b). The real surface of the diffusion layer consists of surfaces of cylindrical and spherical segments with radius d. The overall diffusion surface must be considerably larger than (I + 2d)‘. But the mean diffusion way is longer than d, which is caused by the non-linear concentration gradients. These two geometric simplifications of the model have contrary influences on the deviation from reality. and l/l was found by measuring the A linear connection between 1/4h passivation of small areas at no external current with linear extensions between 0.33 and 1.9 mm (Fig. 7). This fact agrees with the proposed model, if 1’ is inserted as the anodic area and (I + 2d)* is inserted as the apparent cathodic efficient area in eqn. (5):
(7)
Following eqn. (9) we were able to estimate the quotients i-K_’ which agree with the chronopotentiometrically estimated data. The value of d was nearly 0.22 mm in solutions of 0.3 M HCF and of 0.45 M HCF, independent of the HCF concentration. This value lies in the range of the thickness of diffusion layers under free convection conditions [16]. The anodic partial current is almost unaffected by the thickness of the diffusion layer or other edge effects. Under galvanostatic conditions the change in electrode processes occurs before the expanding diffusion front reaches the border of the diffusion layer. If the expanding diffusion front reached this plane, a change of electrode processes would not occur. That is why the anodic processes are influenced less by surface size than the cathodic processes. Edge effects in anodic partial processes of open-circuit passivation concern smaller areas than cathodic partial processes, if both processes are diffusion controlled. CONCLUSIONS
In the absence of external current, passivation of copper takes place in alkaline solutions, if an oxidizing agent, for instance HCF, is present. The duration of the transition state in the absence of external current after the insertion of an activated copper electrode in alkaline HCF solutions increases with decreasing HCF and increasing hydroxide concentrations.
84
During the transition state an adsorbed layer is formed which contains Cu(I1) and hexacyanoferrate(II), whereby the proportion of these two components depends on the concentration of HCF in the solution. The oxidizing agent acts as a constant anodic polarization current in the copper passivation in consequence of the diffusion control of the HCF reduction. The passivation process in the absence of external current can be described by a superposition of a steady-state diffusion-controlled cathodic partial process and a non-steady-state diffusion-controlled anodic partial process. The shortening of transition times in the case of the presence of a direct electrical contact with NiCrSiO, electrodes is due to the predominance of the cathodic partial process on NiCrSiO,. The transition times of copper in the case of splitting of the cathodic partial current can be described by considering the ratio of the areas in the chronopotentiometric equation. The strong dependence of the transition times in the absence of returned current on the ratio of the areas is due to the connection between transition times and current densities in the chronopotentiometric equation. The decrease of transition times with decreasing extension of small areas is due to the role of the edge in diffusion processes. The connection between transition times and the extension of small areas [(0.33 mm)‘-(2 mm)2] can be described using a very simple model. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
C. Wagner and W. Traud, Z. Elektrochem., 44 (1983) 391. M. Spiro and P.L. Freund, J. Electroanal. Chem.. 144 (1983) 293. J.J. Kelly and G.J. Keel, J. Electrochem. Sot., 125 (1978) 860. J.J. Kelly and G.J. Keel, Philips Tech. Rundsch., 38 (1979) 178. A. Lerm, S. Unger, A Wiegand and G. Dal], Hermsdorfer Tech. Mitt., 24 (1984) 2029. H.P. Leckie, J. Electrochem. Sot., 117 (1970) 1478. D.D. Macdonald, J. Electrochem. Sot., 121 (1974) 651. D. Ashworth and D. Fairhurst, J. Electrochem. Sot., 124 (1977) 506. D.W. Shoesmith, T.E. Rummery, D. Owen and W. Lee, J. Electrochem. Sot., 123 (1976) 790. D.W. Shoesmith, S. Sunder, M.G. Bailey, G.J. Wallace and F.W. Stanchell, J. Electrochem. Sot., 143 (1983) 153. L. Kiss, M.L. Varsanyi and A. Bosquez, Magy. Kern. Foly., 87 (1981) 540. L. Kiss, A. Bosquez and M.L. Varsanyi, Acta Chim. Acad. Sci. Hung., 108 (1981) 108. S.M. Abd El Haleem and G.B. Aleya, J. Electroanal. Chem., 117 (1981) 309. M. Kahler, A. Lerm and A. Wiegand, Z. Chem., 25 (1985) 156. K.J. Vetter, Elektrochemische Kinetik, Springer, Berlin Gattingen Heidelberg, 1961, p. 171. L.M. Siperko and T. Kuwana, J. Electrochem. Sot., 130 (1983) 396.
J. Electroanal. Chem.. 213 (1986) 75-84
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
GEOMETRIC INFLUENCES ON THE SPONTANEOUS OF COPPER IN ALKALINE HEXACYANOFERRATE(II1) UNDER CONDITIONS OF FREE CONVECTION
PASSIVATION SOLUTIONS
J.M. KijHLER, A. WXEGAND and A. LERM Akudemte der ~~se~chu~ten 6900 Jena (G. 13.R.j
der DDR, Phystkulisek-Technisches
~~~t~tut, ~elmh~~tzweg
4,
{Received 13th September 1985: in revised form 15th May 1986)
ABSTRACT The passivation of copper proceeds in the presence of an oxidizing agent without an external current. Cyclovoltammetric, chronopotentiomet~c and open-circuit potential measurements show, that in alkaline solution the passivation rate increases with decreasing hydroxide and increasing hexa~~ofe~at~II1) concentration. The rate increases also by the formation of mixed potentials with NiCrSiO, surfaces and by reduction of the surface area. These effects can be explained by an increase of the diffusion-controlled cathodic partial current in comparison to the anodic partial current. The influence of surface area on the passivation rate is important on surfaces with an extension of about or smaller than the thickness of the diffusion layer.
Electrochemical processes which take place under open-circuit conditions are complex processes. Under quasi-steady-state conditions the overlap of two or more elecrochemical processes can often be described by adding the time-independent partial current potential curves [1,2]. In this way, the investigation and prediction of dissolution rates in “currentless” etching processes of thin metallic mono- and double-layer systems is possible [3-51. Beside the dissolution behaviour, the passivation behaviour is of interest in many practical cases of complex electrochemical processes. The anodic behaviour of copper has been investigated in a wide potential range. Several studies show that anodic passivation may occur both in alkaline [6-lo] and in acid [11,12] solutions. In alkaline solutions copper anodes form Cu,O, Cu(OH),, complex ions (HCuO;) and Cu,O, with increasing potential [6]. At lower potentials a base layer of Cu,O is formed by a dissolution-precipitation mechanism under participation of Cu(OH); [8], Cu(OH), appears at potentials between -0.27 and -0.25 V (vs. ~22-0728/86/$03.50
0 1986 Eisevier Sequoia S.A.
SCE). This Cu(OH), layer grows by precipitation of Cu*+ ions which are formed in the porous base layer of Cu *O. The passive range at potentials around - 0.25 V (vs. SCE) is due to the formation of a passivating layer of CuO [9,10]. At high potentials the formation of Cu(II1) occurs, whereby Cu,03 decays under evolution of oxygen ]I31. Activated copper behaves in alkaline hexacyanoferrate(II1) solutions (HCF solutions) under open-circuit conditions like an activated electrode in oxidizing-agentfree solutions under galvanostatic anodic dissolution. In alkaline solutions of HCF copper passivates spontaneously. The rate of passivation, which is connected with the rate of dissolution before passivation, depends on the hydroxide and HCF concentrations. It can be estimated by chronopotentiometric measurements [14]. Our investigations have shown (as will be described below), that a series of reactions proceeds on activated copper after insertion in alkaline HCF solution. The lifetime of transition states depends not only on the concentrations but also on geometric factors which influence the cathodic partial current. EXPERIMENTAL
Thin films of copper (8 pm) were used as samples. The surface proportion dependence was measured at double-layer systems of copper and a non-stoichiometric NiCrSiO, base layer. These layers were prepared by high-rate sputtering. Patterns were prepared photolithographically. A resist mask was used for covering in measurements of small surface areas. Electrochemical measurements were carried out by means of a PS 4 potentiostat (Meinsberg) and a PV 3 generator (Meinsberg). A platinum plate was used as reference electrode. The copper surfaces were activated by currentless rinsing in hydrochloric acid and in aqueous solutions of thiourea. The measurements were accomplished on vertically oriented samples in unstirred aqueous solutions of HCF and NaOH at 333 K. RESULTS
AND DISCUSSION
Electrode processes The cyclovoltammogram (Fig. 1) shows three anodic peaks and one cathodic peak. The small first peak below -0.8 V (vs. Pt) is due to the formation of Cu(I), while the second peak is caused by the formation of Cu(I1). Both the first and the second peak are in agreement with the cyclovoltammograms observed in HCF-free solutions [9]. The baseline (broken line in Fig. 1) represents a constant cathodic partial current. It has a constant distance from the abscissa (i = O-line) up to a potential higher than - 0.2 V (vs. Pt). The distance between the baseline and the abscissa is proportional to the concentration of HCF in the solution. The potential-independent reduction of HCF is responsible for the deviation of the baseline from the abscissa.
77
422M HCF 2,2 M NoOH
E/!/h
PII-
Fig. 1. Cyclovoltammogram of copper in alkaline HCF solution.
According to Vet&r, a diffusion-~ntrolled process can be distinguished from a reaction-controlled process by investigating the influence of convection in the solution on the intensity of the electrode process. The intensity of a diffusion-controlled process increases with increasing convection of the solution [El]. The stirring of an alkaline HCF solution (0.25 M HCF, 2.1 M NaOH) strongly influences the poteniostatic reduction (-0.75 V vs. Pt) of HCF on a copper electrode. This means that the cathodic partial process in the passivation of copper in the absence of external current must be diffusion-controlled. The deviation of the baseline disappears in the range of the two corresponding peaks (III/IV). The amount of charge, which is represented by the anodic and cathodic peaks, is equal. The shape and sire of the two peaks remain constant in subsequent cyclovoltammet~c cycles. It must be concluded that these peaks are due to the oxidation and reduction of an unchanged layer which is adsorbed at the electrode surface. The position of the two peaks near the Pt potential and in the range of disappearance of the base line deviation must be interpreted as the oxidation of hexacyanoferrate(II), and the reduction of the anodically formed hexacyanofe~ate(II1) which is contained in the Cu(I1) surface film. The two corresponding peaks reflect a partially reversible electrode process. Unlike the oxidation and reduction of a copper hexacyanoferrate film on glassy carbon, which shows symmetrical oxidation and reduction peaks [16], the difference of the maximum potentials of the two peaks must be related to a slow transport process. A comparison between the amounts of charge of the second (Qrr) and the third (Qm) anodic peak gives information about the nature of the adsorbed layer. Qn represents the Cu(I1) formed. QnI represents the oxidizable hexacyanoferrate(l1) which is contained in the adsorbed layer. The quotient 2Qrn/Qn is equal to the proportion of hexacyanoferrate(I1) in the adsorbed layer and Cu(I1) formed in the second transition state (second peak). Considering the low solubility of Cu*+ in alkaline solutions it must be assumed that most of the Cu(I1) formed during the
78
Fig. 2. Relation between the quotient of exchanged charges of the third (Q,,t) and second (Qrr) transition states and the concentration of HCF in solutions of 2.5 M NaOH (323 K) under consideration of cathodic partial current.
second transition state is contained in the adsorbed layer. Consequently the quotient 2QdQu represents approximately the proportion of Cu(I1) and hexacyanoferrate(I1) in the adsorbed layer. The quotient Q,,,/Q,, increases with increasing concentration of HCF in the solution (Fig. 2). It seems that HCF is built non-stoichiometrically into the layer which is formed in the range of the second anodic peak by the Cu(I1) species. At temperatures around 320-330 K and at higher concentrations of HCF (O-34-0.75 M), the quotient QIII/QII reaches a value of 0.25. This value is identical to the inverse ratio of the number of electrons which are exchanged by electrochemical formation of Cu,Fe(CN), from Cu(0) and oxidation of the hexacyanoferrate(I1) to hexacyanoferrate(II1) in this compound. An alternative interpretation is to assume that hexacyanoferrate does not form a precipitate but is dissolved in occluded soluble material in the adsorbed layer of copper hydroxide or oxide. A distinction between these explanations could be made by Raman spectroscopic measurements of the electrode surface. The electrode processes are clearly reflected by galvanostatic measurements. Three transition states are observed in anodic galvanostatic polarisation (Fig. 3). The first is due to Cu(I) formation, the second to Cu(I1) formation, and the third is caused by the oxidation of hexacyanoferrate(I1) contained in the adsorbed layer. In agreement with the literature data of copper behaviour in HCF-free solutions [S], the duration of the first transition state increases only slightly with increasing hydroxide concentration, while the duration of the second transition state T increases with the square of the hydroxide concentration, which can be explained by a linear dependence of the chronopotentiometric constant K on the hydroxide concentration. Under open-circuit conditions only two transition states are observed (Fig. 3, curve 5). The “infinite duration of the third transition state” is caused by the fact
19
2s M NOOH O,L5M HCF 333 K
I
20
LO
s t/s
60
-
Fig. 3. Anodic chronopotentiograms of activated copper in alkaline HCF solutions. 27 mA/cm2; (3) 17 mA/cm2; (4) 6.7 mA/cm2; (5) without anodic polarisation.
(1) 45 mA/cm’;
(2)
that the oxidation process which takes place in the third transition state is the back-reaction fo the HCF reduction, which leads to oxidizing processes at the copper electrode under open-circuit conditions. Examining the first two transition states, we conclude that the spontaneous reduction of HCF on the copper surface and the oxidation of copper, which is connected with the HCF reduction, proceeds like an electrode process under galvanostatic conditions. Increasing the HCF concentration acts in the same way as increasing the galvanostatic current density i,. This fact is well illustrated by means of the chronopotentiometric equation, where r represents the duration of a transition state: i,fi=K
(1)
equation must be completed by the addition of an anodic partial current density i, which is due to the cathodic partial current density of HCF reduction i_:
This
i,fi+i+fi=K
Under open-circuit conditions the polarisation-current-density i,h=K
(2)
term disappeares: (3)
This means that the inverse transition times under open-circuit conditions ril depend on the square of the concentrations of oxidizing agent (HCF). Inverse chronopotentiometric plots of the passivation of copper in alkaline solutions of 0.15 M HCF show the connections between the transition times, the anodic polarisation currents 1r, and the concentration of hydroxide ions in the case of the simultaneous reduction of HCF (Fig. 4). All chronopotentiometric straight lines go through the ordinate at the same point, which marks the intensity of the
80
Ul
a?
r-
w/s-b
Fig. 4. Connection between the transition times 7 aud the anodic polarisation current in the galvanostatic passivation of copper in alkaline solutions of 0.15 M HCF.
cathodic partial current. This intersection point is dependent on the concentration of HCF in the solution. The steepness of the plots increases with increasing hydroxide concentration, as a result of a non-steady-state diffusion process of hydroxide to the electrode surface. In the case of no external current, increasing transition times result in increasing hydroxide concentrations with constant concentration of HCF (I,, = 0).
Splitting of the cathodic partial current If the surface of copper is in direct electrical contact with a NiCrSiO, surface, an acceleration of the passivation is observed. The passivation speed increases with increasing size of the NiCrSiO, surface ANiCrSiO,,if the size of the copper surface remains constant (Fig. 5). Up to the potential of Cu(I1) formation, any anodic processes on NiCrSiO, are negligible in alkaline HCF solutions. After a fast passivation, which concerns only the top layer of the alloy, NiCrSiO, remains passive up to the passive region of copper. But on this NiCrSiO, surface the cathodic processes can occur. The reduction of HCF is effusion-eontroll~ up to the potential of the corresponding peaks of HCF reduction and oxidation (peaks III and IV) of the cyclovoltammograms. Since the reduction of HCF before copper passivation proceeds under diffusion control both on copper and on NiCrSiO,, the density of the cathodic partial current on NiCrSiO, is equal to the density of the cathodic partial current on copper. The cathodic partial current splits into components which are proportional to the size of the surface areas. Under open-circuit conditions the transition time TVdepends on the square of the copper surface size (Fig. 5). This dependence can be explained by means of a
81
20
Lo
60
4.i
AC" + &crsio, Fig. 5. Influence of cathodically efficient area (AC- + ANiCrLO,) passivation in alkaline HCF solution under open-circuit conditions.
on the transition
time of copper
chronopotentiometric equation: The anodic partial current density i, is the quotient of the anodic partial current I, and the anodic surface A,. The cathodic partial current I_ is the product of the cathodic partial current density i_ and the cathodic surface A _. I+ = i-A-/A+
(4
The insertion of eqn. (4) into eqn. (3) leads to a surface quotient-containing chronopotentiometric equation: i-6
= KA+/A_
(5)
Using the ~~onopotentiome~c results, the transition time which depends on HCF concentration and copper surface size was estimated. The estimated functions are represented by the lines in Fig. 5. They show that the experimental results are in agreement with the chronopotentiometric equation (5). Passivaticm of small areas
Observing the speed of passivation of copper in small areas under open-circuit conditions, we noticed an acceleration of,passivation in comparison with large areas. The passivation rate increases with decreasing surface area.
82
b
Fig. 6. (a) Shape of the diffusion layer in diffusion-controlled processes on small areas. (b) A simple model of the enlarged cathodic efficient area in the diffusion-controlled cathodic partial process on small areas.
It seems that this effect is due to the similar size of the extension of copper areas and.the thickness of the diffusion layer. The experimental results could be interpreted by a simple model. Measuring the passivation of isolated small squares of copper with the linear extension I, a diffusion layer with the thickness d in the neighbourhood of the metal must be considered. The diffusion to the surface occurs not only in the direction perpendicular to the electrode surface. Horizontal diffusion components contribute to the transport of HCF to the electrode due to the influence of the edge (Fig. 6a). Such an influence acts like an enlarged area in diffusion-controlled processes. The apparent magnification of the area in a diffusion-controlled process depends on the thickness of the diffusion layer d and the linear extension of the squares I. An approach to real conditions is possible by the following simple model: The apparent enlarged area is assumed to be a square with linear extensions
v 1
2
3m-’
1-pwn-l-
Fig. 7. Size effect on transition time T,, in the passivation solutions under open-circuit conditions.
of small areas (/2) of copper in alkaline HCF
83
of I+ 2d (Fig. 6b). The real surface of the diffusion layer consists of surfaces of cylindrical and spherical segments with radius d. The overall diffusion surface must be considerably larger than (I + 2d)‘. But the mean diffusion way is longer than d, which is caused by the non-linear concentration gradients. These two geometric simplifications of the model have contrary influences on the deviation from reality. and l/l was found by measuring the A linear connection between 1/4h passivation of small areas at no external current with linear extensions between 0.33 and 1.9 mm (Fig. 7). This fact agrees with the proposed model, if 1’ is inserted as the anodic area and (I + 2d)* is inserted as the apparent cathodic efficient area in eqn. (5):
(7)
Following eqn. (9) we were able to estimate the quotients i-K_’ which agree with the chronopotentiometrically estimated data. The value of d was nearly 0.22 mm in solutions of 0.3 M HCF and of 0.45 M HCF, independent of the HCF concentration. This value lies in the range of the thickness of diffusion layers under free convection conditions [16]. The anodic partial current is almost unaffected by the thickness of the diffusion layer or other edge effects. Under galvanostatic conditions the change in electrode processes occurs before the expanding diffusion front reaches the border of the diffusion layer. If the expanding diffusion front reached this plane, a change of electrode processes would not occur. That is why the anodic processes are influenced less by surface size than the cathodic processes. Edge effects in anodic partial processes of open-circuit passivation concern smaller areas than cathodic partial processes, if both processes are diffusion controlled. CONCLUSIONS
In the absence of external current, passivation of copper takes place in alkaline solutions, if an oxidizing agent, for instance HCF, is present. The duration of the transition state in the absence of external current after the insertion of an activated copper electrode in alkaline HCF solutions increases with decreasing HCF and increasing hydroxide concentrations.
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During the transition state an adsorbed layer is formed which contains Cu(I1) and hexacyanoferrate(II), whereby the proportion of these two components depends on the concentration of HCF in the solution. The oxidizing agent acts as a constant anodic polarization current in the copper passivation in consequence of the diffusion control of the HCF reduction. The passivation process in the absence of external current can be described by a superposition of a steady-state diffusion-controlled cathodic partial process and a non-steady-state diffusion-controlled anodic partial process. The shortening of transition times in the case of the presence of a direct electrical contact with NiCrSiO, electrodes is due to the predominance of the cathodic partial process on NiCrSiO,. The transition times of copper in the case of splitting of the cathodic partial current can be described by considering the ratio of the areas in the chronopotentiometric equation. The strong dependence of the transition times in the absence of returned current on the ratio of the areas is due to the connection between transition times and current densities in the chronopotentiometric equation. The decrease of transition times with decreasing extension of small areas is due to the role of the edge in diffusion processes. The connection between transition times and the extension of small areas [(0.33 mm)‘-(2 mm)2] can be described using a very simple model. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
C. Wagner and W. Traud, Z. Elektrochem., 44 (1983) 391. M. Spiro and P.L. Freund, J. Electroanal. Chem.. 144 (1983) 293. J.J. Kelly and G.J. Keel, J. Electrochem. Sot., 125 (1978) 860. J.J. Kelly and G.J. Keel, Philips Tech. Rundsch., 38 (1979) 178. A. Lerm, S. Unger, A Wiegand and G. Dal], Hermsdorfer Tech. Mitt., 24 (1984) 2029. H.P. Leckie, J. Electrochem. Sot., 117 (1970) 1478. D.D. Macdonald, J. Electrochem. Sot., 121 (1974) 651. D. Ashworth and D. Fairhurst, J. Electrochem. Sot., 124 (1977) 506. D.W. Shoesmith, T.E. Rummery, D. Owen and W. Lee, J. Electrochem. Sot., 123 (1976) 790. D.W. Shoesmith, S. Sunder, M.G. Bailey, G.J. Wallace and F.W. Stanchell, J. Electrochem. Sot., 143 (1983) 153. L. Kiss, M.L. Varsanyi and A. Bosquez, Magy. Kern. Foly., 87 (1981) 540. L. Kiss, A. Bosquez and M.L. Varsanyi, Acta Chim. Acad. Sci. Hung., 108 (1981) 108. S.M. Abd El Haleem and G.B. Aleya, J. Electroanal. Chem., 117 (1981) 309. M. Kahler, A. Lerm and A. Wiegand, Z. Chem., 25 (1985) 156. K.J. Vetter, Elektrochemische Kinetik, Springer, Berlin Gattingen Heidelberg, 1961, p. 171. L.M. Siperko and T. Kuwana, J. Electrochem. Sot., 130 (1983) 396.