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A preliminary in situ ESR investigation of the structure of the iron electrode | 1 M NaOH electrolyte interface
JOUntC~L OF
ELSEVIER
Journal of Electroanalytical Chemistry 405 (1996) 241-243
Preliminary note
A preliminary in situ ESR investigation of the structure of the iron electrode I 1 M NaOH electrolyte interface L. Beden *, P. Crouigneau, B. Beden, C. Lamy Laboratoire de chimie I, "Electrochimie et Interactions", URA CNRS 350, Universit~ de Poitiers, 40 avenue du recteur Pineau, 86022 Poitiers, France
Received 7 November 1995
Keywords: ESR spectroscopy; Corrosion; Iron electrode; Spectroelectrochemistry
1. Introduction Despite intense scientific research over many years, the characterisation and the determination of the nature of corrosion products of iron remains a subject of wide controversy. A great variety of experimental techniques, both ex situ [ 1 - 3 ] and in situ [4-13] have been used. To our knowledge, electron spin resonance spectroscopy (ESR) does not seem to have been considered so far. Thus, in this work, we have focused our attention on the possibility of using ESR as an in situ technique for the characterisation of the oxides formed on the surface of a coating iron electrode immersed in an alkaline environment. As the magnetic properties of the main iron compounds are known to give intense ESR signals [14], using ESR spectroscopy as a suitable spectroelectrochemical technique appeared relevant. Combined ESR spectroscopy and electrochemical techniques have already been used to investigate electrode processes at the electrode lelectrolyte interface [15]. ESR spectroscopy can be used to give information on paramagnetic species, and is characterized by a very high sensitivity which allows investigation o f surface species.
layer of iron was electrochemically deposited. Best deposits were obtained when the coating was realised at a controlled intensity ( j = - 6.5 /xA cm -2 of geometric area) in a 0.15 M iron(II) sulphate bath. There is a good agreement between Fig. 1 (cyclic voltammogram of the iron coating recorded in the three-electrode ESR spectroelectrochemical cell) and the standard CVs found in the literature [6,7], recorded under the same experimental conditions, i.e. 1 M NaOH, potential sweep rate of 50 mV s - =, same potential limits and room temperature. The reference electrode was Hg IHg2SO4 IK2SO 4 (sat) (MSE). Once the coating was achieved, the electrode was transferred to the three-electrode ESR spectroelectrochemical cell. The 1 M NaOH solution was first deaerated with N 2 bubbling and the electrode potential was held for 10 min. in the H 2 evolution in order to reduce the native oxides formed in air [8] during the transfer procedure. ESR measurements were carried out immediately after. CV OF AN IRON COATING 8 III 6
/~15t~
2
I
,//
"~-~
cyc~
IV
2. Experimental -2
The experimental set up included a V A R I A N E3 X-band spectrometer (wavelength = 3 cm) with a 100 kHz magnetic field modulation frequency, and Hi-Tek waveform generator and potentiostat. As working electrode a P t l A u electrode was used (a substrate which gives no ESR signals) on which a thin
-6 -6 i I -ZOO -1.75
U
vl
V 15
i
v
-1.50
i
I -1.25
i
I J 1 -1.00 -0.75
i -0.50
.
i -0,25
,
i 0.00
, 025
E I V(MSE)
Fig. l. Cyclic voltammogramsof an iron coating electrode in 1 M NaOH electrolytic solution, recorded in the ESR spectroelectrochemical cell at different cycles (as indicated); v = 50 mV s- l, room temperature.
* Corresponding author. Elsevier Science S.A. SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 4 7 0 -
0 mV/s; 1M NaOH
-4
1
L. Beden et a l . / Journal o f Electroanalytical Chemistry 405 (1996) 241-243
242
IO0
400
A
2oo
/~__~/b
0
90 80
I: E~1.70VtMSE
~
~
H=1190 G
"oo
2 : E ~ 1 . 4 5 V{MSE)
7O
3: E ~ 1 . 2 0 ~ M S E I
"E
60
4:EL1.05
V{MSE)
5:E,,-0.80V(MSE)
-1200
-1600 6:E=-0.20V~MSlE
H'=800 G 1lth c~e
-1400 ,
I
411
2O
E~1.70V(MSE i
i
i
!
i
1000
2000
3000
4000
5000
H/Gauss Fig. 2. " I n s i t u " ESR spectra o f an iron coated electrode in 1 M N a O H electrolytic solution recorded at various applied potentials (as indicated)
during the first potential cycle after immersion of the electrode under potentiostatic control at - 1.70 V (MSE). Applied magnetic field scanned between 500 and 5000 Gauss.
:
.
t
.
i
-1.OO
.
i
-075
.
i
.
i
-0.50 -025
.
i
O.OO
.
0.25
E I V(MSE)
7:EL1.30VIMSEI
8:E~150V(M,S~
-
-ZOO -1.75 -1.50 -1.25
Fig. 4. Dependence of the ESR line intensity on potential at two different applied magnetic fields. Two examples are given; curve I : H = 1190 Gauss, first potential cycle; curve 2: H = 800 Gauss, 1 lth potential cycle.
of a given ESR line was recorded while the potential of the working electrode was scanned (Fig. 4). The two types of experiments were found to be informative and complementary.
3. Discussion Using the same equipment, two different types of experiments were carried out: first at fixed applied potentials (selected at characteristic points of the voltammogram) the ESR spectra were recorded during magnetic field scanning (Figs. 2 and 3), then at a fixed magnetic field the intensity 70
I
|
I
A 60
50 10:E~1.70V~MSE} 11:E~1.40V~S~
40
12:E~1.20
r-
V(MSE}
13:E~1.05V
20
~
14:E--0.80
V(MSIS
15:E=~"0.10
VIMSEI
16:E~1.30V~MSFJ 17:E~1.50V{MSEI
10 18:E~1.70V(M6~ 0
i 1000
i 2000
i 3000
i 4000
i 5000
H/Gauss Fig. 3. " I n s i t u " ESR spectra o f an iron coated electrode in 1 M N a O H electrolytic solution recorded at various applied potentials (as indicated) after 30 rain o f pretreatment by O R C at 50 m V s - l within the same potJential limits. Applied magnetic field scanned between 500 and 5000 Gauss.
3.1. In situ ESR spectra recorded at various given potentials
ESR spectra numbers 1 to 9 (Fig. 2) were recorded at various potentials during the first voltammetric cycle, starting from the lower limit. All spectra are characterized by an intense signal at ca. 700 Gauss, labelled A in the figure, likely to be due to the contribution of the iron coating substrate as its intensity depends on the thickness of the deposit. This signal shifts slightly with potential, reaching ca. 800 Gauss at the uppermost potential limit. A shoulder, B, hardly visible on curve 1, occurs markedly at - 1 . 4 5 V / M S E (curve 2). The maximum, first located at ca. 1000 Gauss, gradually shifts down to 950 Gauss at - 1 . 0 5 V, then totally disappears at more positive potentials. This shoulder is likely to be correlated with the first anodic peak of the voltammogram (peak I in Fig. 1). According to the literature [8], it is around this range of potentials that Fe(OH) 2 is formed. This latter Species is supposed to be further oxidised as iron(III) species at more positive potentials. The transformation of Fe(OH) 2 can be followed in situ on the ESR spectra, first by a decay of the intensity of the shoulder (curves 4 and 5), then by a nearly complete disappearance for potentials positive to - 0 . 8 V / M S E . It must be stressed, however, that no changes in the ESR spectra, which would correspond to iron(III) formation, are visible in the passive region (curves 6 and 7). This agrees with the first cycle of the voltammogram (Fig. 1) in which shoulder II (supposed to be Fe304 formation) and peak III (normally assigned to FeOOH species in the literature) are
L. Beden et aL / Journal of Electroanalytical Chemistry 405 (1996) 241-243
not yet developed. Also interesting is that during the reverse potential scan of the first cycle, no shoulder reappears in the ESR spectra at potentials more negative than - 1.6 V / M S E . This suggests that the irreversibility of the electrochemical process is even stronger than that usually concluded from the voltammetric peaks. The situation is different after several oxidation-reduction cycles (ORC), i.e. after growth of the supposedly associated voltammetric peaks III and V. In Fig. 3, ORC conditions, at 50 mV s - J , were applied to the working electrode for 30 min. and new ESR spectra were recorded (curves 10 to 18). There is still the appearance of a very clear shoulder, B, at - 1 . 4 V / M S E , which follows the same potential evolution as during the first cycle. But more interesting is that a broad ESR signal, C, is seen at around 2000-2500 Gauss (curves 13 to 16), i.e. roughly in the potential domain where iron(III) species are formed. 3.2. In situ experiments carried out at constant magnetic field
Two examples of results are given in Fig. 4. They represent the dependence of the ESR line intensity on applied potential. The first curve (top of Fig. 4) was recorded at 1190 Gauss during the first voltammetric cycle. Characteristic features are observed at - 1.45 V and ca. - 1 . 1 V during the positive going potential scan, and - 1 . 3 V during the backward potential scan. They are labelled a, b and c respectively. Interestingly, a and b correspond to peak I and shoulders III and IV of the first cycle of the voltammogram, but c has no corresponding feature. It occurs 200 mV positive of peak V, i.e. much before the electrochemical reduction peak. Here, the irreversibility of the electrochemical processes associated with peaks III and V is demonstrated again. The second curve is an example of results obtained after the 1 lth potential cycle, the magnetic field being fixed at 800 Gauss. a has almost disappeared, while b and c are shifted positively. In particular b coincides quite accurately with peak IV but, at 800 Gauss, peak III is not detected. These two curves demonstrate the possibilities and the interest of working at a fixed magnetic field and recording the ESR line intensity during a potential scan. The main advantage of the technique is that it keeps potentiostatic control of the electrode surface. For a complete survey of the problem, experiments need to be carried out at close intervals of the magnetic field, over the range 500 to 5000 Gauss. Then a real mapping of the surface, i.e. a three-dimensional diagram containing all related information be-
243
tween magnetic absorbance, magnetic field and potential can be obtained. Such work is currently in progress. 4. Conclusion
These preliminary studies show that ESR is a suitable technique for the in situ study of the formation of oxide layers at the surface of an iron coated electrode in contact with an alkaline medium. The interpretation, i.e. the assignment to given species of the obtained signals is at the moment difficult. The lack of references adds to the difficulty of interpreting spectra for ferri/ferromagnetic species. But it is definitely demonstrated that the ESR spectra are potential dependent and that they are correlated to the main voltammetric features. Similarly, by working at a fixed magnetic field, a mapping of the surface can be reconstructed. Currently the work continues in several areas, it is, for instance, important to monitor the thickness of the deposited iron layer more accurately. An iron coating which is too thick gives too intense an ESR signal (originating from iron itself), which masks the potential dependent features. Conversely, too thin a coating is probably not homogeneous enough to ensure reproducible results. It is also important to try to correlate the in situ ESR signals with those due to reference compounds. References [1] C.L. Foley, J. Kruger and C.J. Bechtodt, J. Electrochem. Soc., 114 (1967) 994. [2] R.W. Revie, B.G. Baker and J.O'M. Bockris, J. Electrochem. Soc., 122 (1975) 1460. [3] S.C. Tjong and E. Yeager, J. Electrochem. Soc., Acc. Brief Com., 128 (1981) 2251. [4] W.E. O'Grady, J. Electrochem. Soc., 127 (1980) 555. [5] J.O'M. Bockris, Corros. Sci., 32 (1991) 1105. [6] A. Bewick, M. Kalaji and G. Larramona, J. Electroanal. Chem., 318 (1991) 207. [7] G. Laramona and C. Gutierrez, J. Electrochem. Soc., 136 (1989) 2171. [8] C.A. Melendres, M. Pankuch, Y.S. Li and R.L. Knight, Electrochim. Acta, 37 (1992) 2747. [9] J.C. Rubim, J. Chem. Soc., Faraday Trans. I, 85(12) (1989) 4247. [10] C. Johnston, Vibrational Spectroscopy, 1 (1990) 87. [11] H. Neugebauer, A. Moser, P. Strecha and A. Neckel, J. Electrochem. Soc., 137 (1990) 1472. [ 12] W. Tschingei, H. Neugebauer and A. Neckel, J. Electrochem. Soc., 137 (1990) 1475. [13] A. Hugot-le Goff, J. Flis, N. Boucherit, S. Joiret and J. Wilinski, J. Electrochem. Soc., 137 (1990) 2684. [14] P. Pascal in Nouveau trait6 de chimie min~rale, Vol. XVII, 1st edn., Masson et Cie, Paris, (1967). [15] C. Lamy and P. Crouigneau, J. Electroanal. Chem., 150 (1983) 545.
ELSEVIER
Journal of Electroanalytical Chemistry 405 (1996) 241-243
Preliminary note
A preliminary in situ ESR investigation of the structure of the iron electrode I 1 M NaOH electrolyte interface L. Beden *, P. Crouigneau, B. Beden, C. Lamy Laboratoire de chimie I, "Electrochimie et Interactions", URA CNRS 350, Universit~ de Poitiers, 40 avenue du recteur Pineau, 86022 Poitiers, France
Received 7 November 1995
Keywords: ESR spectroscopy; Corrosion; Iron electrode; Spectroelectrochemistry
1. Introduction Despite intense scientific research over many years, the characterisation and the determination of the nature of corrosion products of iron remains a subject of wide controversy. A great variety of experimental techniques, both ex situ [ 1 - 3 ] and in situ [4-13] have been used. To our knowledge, electron spin resonance spectroscopy (ESR) does not seem to have been considered so far. Thus, in this work, we have focused our attention on the possibility of using ESR as an in situ technique for the characterisation of the oxides formed on the surface of a coating iron electrode immersed in an alkaline environment. As the magnetic properties of the main iron compounds are known to give intense ESR signals [14], using ESR spectroscopy as a suitable spectroelectrochemical technique appeared relevant. Combined ESR spectroscopy and electrochemical techniques have already been used to investigate electrode processes at the electrode lelectrolyte interface [15]. ESR spectroscopy can be used to give information on paramagnetic species, and is characterized by a very high sensitivity which allows investigation o f surface species.
layer of iron was electrochemically deposited. Best deposits were obtained when the coating was realised at a controlled intensity ( j = - 6.5 /xA cm -2 of geometric area) in a 0.15 M iron(II) sulphate bath. There is a good agreement between Fig. 1 (cyclic voltammogram of the iron coating recorded in the three-electrode ESR spectroelectrochemical cell) and the standard CVs found in the literature [6,7], recorded under the same experimental conditions, i.e. 1 M NaOH, potential sweep rate of 50 mV s - =, same potential limits and room temperature. The reference electrode was Hg IHg2SO4 IK2SO 4 (sat) (MSE). Once the coating was achieved, the electrode was transferred to the three-electrode ESR spectroelectrochemical cell. The 1 M NaOH solution was first deaerated with N 2 bubbling and the electrode potential was held for 10 min. in the H 2 evolution in order to reduce the native oxides formed in air [8] during the transfer procedure. ESR measurements were carried out immediately after. CV OF AN IRON COATING 8 III 6
/~15t~
2
I
,//
"~-~
cyc~
IV
2. Experimental -2
The experimental set up included a V A R I A N E3 X-band spectrometer (wavelength = 3 cm) with a 100 kHz magnetic field modulation frequency, and Hi-Tek waveform generator and potentiostat. As working electrode a P t l A u electrode was used (a substrate which gives no ESR signals) on which a thin
-6 -6 i I -ZOO -1.75
U
vl
V 15
i
v
-1.50
i
I -1.25
i
I J 1 -1.00 -0.75
i -0.50
.
i -0,25
,
i 0.00
, 025
E I V(MSE)
Fig. l. Cyclic voltammogramsof an iron coating electrode in 1 M NaOH electrolytic solution, recorded in the ESR spectroelectrochemical cell at different cycles (as indicated); v = 50 mV s- l, room temperature.
* Corresponding author. Elsevier Science S.A. SSDI 0 0 2 2 - 0 7 2 8 ( 9 5 ) 0 4 4 7 0 -
0 mV/s; 1M NaOH
-4
1
L. Beden et a l . / Journal o f Electroanalytical Chemistry 405 (1996) 241-243
242
IO0
400
A
2oo
/~__~/b
0
90 80
I: E~1.70VtMSE
~
~
H=1190 G
"oo
2 : E ~ 1 . 4 5 V{MSE)
7O
3: E ~ 1 . 2 0 ~ M S E I
"E
60
4:EL1.05
V{MSE)
5:E,,-0.80V(MSE)
-1200
-1600 6:E=-0.20V~MSlE
H'=800 G 1lth c~e
-1400 ,
I
411
2O
E~1.70V(MSE i
i
i
!
i
1000
2000
3000
4000
5000
H/Gauss Fig. 2. " I n s i t u " ESR spectra o f an iron coated electrode in 1 M N a O H electrolytic solution recorded at various applied potentials (as indicated)
during the first potential cycle after immersion of the electrode under potentiostatic control at - 1.70 V (MSE). Applied magnetic field scanned between 500 and 5000 Gauss.
:
.
t
.
i
-1.OO
.
i
-075
.
i
.
i
-0.50 -025
.
i
O.OO
.
0.25
E I V(MSE)
7:EL1.30VIMSEI
8:E~150V(M,S~
-
-ZOO -1.75 -1.50 -1.25
Fig. 4. Dependence of the ESR line intensity on potential at two different applied magnetic fields. Two examples are given; curve I : H = 1190 Gauss, first potential cycle; curve 2: H = 800 Gauss, 1 lth potential cycle.
of a given ESR line was recorded while the potential of the working electrode was scanned (Fig. 4). The two types of experiments were found to be informative and complementary.
3. Discussion Using the same equipment, two different types of experiments were carried out: first at fixed applied potentials (selected at characteristic points of the voltammogram) the ESR spectra were recorded during magnetic field scanning (Figs. 2 and 3), then at a fixed magnetic field the intensity 70
I
|
I
A 60
50 10:E~1.70V~MSE} 11:E~1.40V~S~
40
12:E~1.20
r-
V(MSE}
13:E~1.05V
20
~
14:E--0.80
V(MSIS
15:E=~"0.10
VIMSEI
16:E~1.30V~MSFJ 17:E~1.50V{MSEI
10 18:E~1.70V(M6~ 0
i 1000
i 2000
i 3000
i 4000
i 5000
H/Gauss Fig. 3. " I n s i t u " ESR spectra o f an iron coated electrode in 1 M N a O H electrolytic solution recorded at various applied potentials (as indicated) after 30 rain o f pretreatment by O R C at 50 m V s - l within the same potJential limits. Applied magnetic field scanned between 500 and 5000 Gauss.
3.1. In situ ESR spectra recorded at various given potentials
ESR spectra numbers 1 to 9 (Fig. 2) were recorded at various potentials during the first voltammetric cycle, starting from the lower limit. All spectra are characterized by an intense signal at ca. 700 Gauss, labelled A in the figure, likely to be due to the contribution of the iron coating substrate as its intensity depends on the thickness of the deposit. This signal shifts slightly with potential, reaching ca. 800 Gauss at the uppermost potential limit. A shoulder, B, hardly visible on curve 1, occurs markedly at - 1 . 4 5 V / M S E (curve 2). The maximum, first located at ca. 1000 Gauss, gradually shifts down to 950 Gauss at - 1 . 0 5 V, then totally disappears at more positive potentials. This shoulder is likely to be correlated with the first anodic peak of the voltammogram (peak I in Fig. 1). According to the literature [8], it is around this range of potentials that Fe(OH) 2 is formed. This latter Species is supposed to be further oxidised as iron(III) species at more positive potentials. The transformation of Fe(OH) 2 can be followed in situ on the ESR spectra, first by a decay of the intensity of the shoulder (curves 4 and 5), then by a nearly complete disappearance for potentials positive to - 0 . 8 V / M S E . It must be stressed, however, that no changes in the ESR spectra, which would correspond to iron(III) formation, are visible in the passive region (curves 6 and 7). This agrees with the first cycle of the voltammogram (Fig. 1) in which shoulder II (supposed to be Fe304 formation) and peak III (normally assigned to FeOOH species in the literature) are
L. Beden et aL / Journal of Electroanalytical Chemistry 405 (1996) 241-243
not yet developed. Also interesting is that during the reverse potential scan of the first cycle, no shoulder reappears in the ESR spectra at potentials more negative than - 1.6 V / M S E . This suggests that the irreversibility of the electrochemical process is even stronger than that usually concluded from the voltammetric peaks. The situation is different after several oxidation-reduction cycles (ORC), i.e. after growth of the supposedly associated voltammetric peaks III and V. In Fig. 3, ORC conditions, at 50 mV s - J , were applied to the working electrode for 30 min. and new ESR spectra were recorded (curves 10 to 18). There is still the appearance of a very clear shoulder, B, at - 1 . 4 V / M S E , which follows the same potential evolution as during the first cycle. But more interesting is that a broad ESR signal, C, is seen at around 2000-2500 Gauss (curves 13 to 16), i.e. roughly in the potential domain where iron(III) species are formed. 3.2. In situ experiments carried out at constant magnetic field
Two examples of results are given in Fig. 4. They represent the dependence of the ESR line intensity on applied potential. The first curve (top of Fig. 4) was recorded at 1190 Gauss during the first voltammetric cycle. Characteristic features are observed at - 1.45 V and ca. - 1 . 1 V during the positive going potential scan, and - 1 . 3 V during the backward potential scan. They are labelled a, b and c respectively. Interestingly, a and b correspond to peak I and shoulders III and IV of the first cycle of the voltammogram, but c has no corresponding feature. It occurs 200 mV positive of peak V, i.e. much before the electrochemical reduction peak. Here, the irreversibility of the electrochemical processes associated with peaks III and V is demonstrated again. The second curve is an example of results obtained after the 1 lth potential cycle, the magnetic field being fixed at 800 Gauss. a has almost disappeared, while b and c are shifted positively. In particular b coincides quite accurately with peak IV but, at 800 Gauss, peak III is not detected. These two curves demonstrate the possibilities and the interest of working at a fixed magnetic field and recording the ESR line intensity during a potential scan. The main advantage of the technique is that it keeps potentiostatic control of the electrode surface. For a complete survey of the problem, experiments need to be carried out at close intervals of the magnetic field, over the range 500 to 5000 Gauss. Then a real mapping of the surface, i.e. a three-dimensional diagram containing all related information be-
243
tween magnetic absorbance, magnetic field and potential can be obtained. Such work is currently in progress. 4. Conclusion
These preliminary studies show that ESR is a suitable technique for the in situ study of the formation of oxide layers at the surface of an iron coated electrode in contact with an alkaline medium. The interpretation, i.e. the assignment to given species of the obtained signals is at the moment difficult. The lack of references adds to the difficulty of interpreting spectra for ferri/ferromagnetic species. But it is definitely demonstrated that the ESR spectra are potential dependent and that they are correlated to the main voltammetric features. Similarly, by working at a fixed magnetic field, a mapping of the surface can be reconstructed. Currently the work continues in several areas, it is, for instance, important to monitor the thickness of the deposited iron layer more accurately. An iron coating which is too thick gives too intense an ESR signal (originating from iron itself), which masks the potential dependent features. Conversely, too thin a coating is probably not homogeneous enough to ensure reproducible results. It is also important to try to correlate the in situ ESR signals with those due to reference compounds. References [1] C.L. Foley, J. Kruger and C.J. Bechtodt, J. Electrochem. Soc., 114 (1967) 994. [2] R.W. Revie, B.G. Baker and J.O'M. Bockris, J. Electrochem. Soc., 122 (1975) 1460. [3] S.C. Tjong and E. Yeager, J. Electrochem. Soc., Acc. Brief Com., 128 (1981) 2251. [4] W.E. O'Grady, J. Electrochem. Soc., 127 (1980) 555. [5] J.O'M. Bockris, Corros. Sci., 32 (1991) 1105. [6] A. Bewick, M. Kalaji and G. Larramona, J. Electroanal. Chem., 318 (1991) 207. [7] G. Laramona and C. Gutierrez, J. Electrochem. Soc., 136 (1989) 2171. [8] C.A. Melendres, M. Pankuch, Y.S. Li and R.L. Knight, Electrochim. Acta, 37 (1992) 2747. [9] J.C. Rubim, J. Chem. Soc., Faraday Trans. I, 85(12) (1989) 4247. [10] C. Johnston, Vibrational Spectroscopy, 1 (1990) 87. [11] H. Neugebauer, A. Moser, P. Strecha and A. Neckel, J. Electrochem. Soc., 137 (1990) 1472. [ 12] W. Tschingei, H. Neugebauer and A. Neckel, J. Electrochem. Soc., 137 (1990) 1475. [13] A. Hugot-le Goff, J. Flis, N. Boucherit, S. Joiret and J. Wilinski, J. Electrochem. Soc., 137 (1990) 2684. [14] P. Pascal in Nouveau trait6 de chimie min~rale, Vol. XVII, 1st edn., Masson et Cie, Paris, (1967). [15] C. Lamy and P. Crouigneau, J. Electroanal. Chem., 150 (1983) 545.