- Email: [email protected]
On the cathodic overvoltage on aluminium in cryolite-alumina melts—I
Efearochimica
Acra
ON
1978, Vol. 2.3, pp. 223-231.
THE
Pergmon
Press.
Printed
in Great
Britain
CATHODIC OVERVOLTAGE ON ALUMINIUM CRYOLITE-ALUMINA MELTS-I
Electrochemistry
JOMAR THONSTAD* and SV@RRE ROLSETH Laboratories and the Engineering Research Foundation, Technology, University of Trondheim, Norway (Received 17 February
1977; and in$nalform
Norwegian
IN
In&it&e of
13 June 1977)
Abstract-When a constant cathodic current is applied to an aluminium electrode in a Na3A1F6-AlaO, melt at lOlO”C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at cds above 2A/cm2 and the evolution of sodium gas was observed at very high cds. Steady state measurements yielded straight y vs log i plots with a slope of -0.23 V/decade, the overvoltage being -0.19 V at 1 A/cm’. The overvoltage decreased markedly when th? melt was stirred. Potential decay measurements yielded linear log time plots with slopes of 0.0145 V/decade. The charge-transfer resistance ias determined by double pulse measurements to be 0.0031 ohm cm’. AC impedance measurements gave a similar result. The charge transfer overvoltage accounts for only about two percent of the total overvoltage, the rest is apparently diffusion controlled. The cathodic overvoltage in industrial aluminium cells is of similar magnitude as found in the laboratory investigations.
INTRODUCTlON
studies of the cathode reaction at aluminium in cryolite melts were centered on the question of’ whether sodium or aluminium is the primary cathodic product[l]. In case of primary sodium formation the aluminium would be formed in a secondary chemical reaction, Early
3Na + AlF, = 3NaF + Al.
(1)
This equilibrium is shifted to the right in cryolite. Sodium is in the gaseous state at around 1000°C. The solubility of sodium in liquid aluminium at this temperature appears to be about 0.13 wt% Na (extrapolated[2]) at 1 atm sodium pressure, while the equilibrium content in the presence of molten cryolite is only about 0.02 wt%[3]. The notion of primary sodium formation was based on the fact that aluminium in the melt is bound in anionic complexes of the type AlFa- and AIF;, while sodium is present as free cations[l]. Furthermore, gas bubbles that are seen to escape from the melt after electrolysis, burn with the typical sodium culouring, and it was suggested that they consisted of sodium gas[4]. However, excess sodium present should react according to (1) and the flame colouring could also be the result of burning hydrogen bubbles containing sodium salt vapour. More recent studies show that the activity of sodium at the cryolite composition is between 0.04 and 0.056[3, 5, 61 refered to liquid sodium as standard state. The potential difference between the deposition potential of aluminium and that of sodium gas at’ 1 atm can then be calculated. M
=
RT In F
%! &a
(2)
where i& is the activity of liquid sodium at 1 atm sodium pressure[7]. AE is -0.24 to -0.2OV at 223 E.A.23/3-E
1010°C whereas data for cryolite saturated with alumina[8] yield a value of -0.28 V. The difference de-
creases with increasing temperature and with increasing CR (cryolite ratio = mol NaF/mol AIF,). As will be shown the cathodic overvoltage in cryolite melts saturated with alumina reaches -0.2V at -1 A/cmz, and the formation of 1 atm sodium gas can therefore be ruled out at moderate cds. It is conceivable that sodium-containing intermediate cathode products are formed. Aluminium is soluble in cryolite to
-0.1
wt% Al[9,
lo].
The
nature
of the dissolved
metal is not clear, but it appears to consist of sodium as well as aluminiumlp, lo], either in the form of neutral species, or ions and trapped electrons or of subvalent species such as Al l Support for the idea of intermediates is found in cathodic polarization studies on molybdenum and platinum. Antipin[ll] observed four plateaux on polarization curves for molybdenum in cryolite, but only two plateaux when aluminium was present. Kubik et nl[12] and Kubik and Malinovsky[13] observed several arrests on potential decay curves, and the existence of the A12+ species was proposed. Silny et ul[14] found two plateaux on cathodic polarization curves on platinum and they were ascribed to formation of aluminium and sodium. The curves obtained with aluminium electrodes by galvanostatic polarization and by potential decay are smooth, as shown by Piontelli et aE[lS] and by the present authors.
Cathodic
overvoltage
Antipin[ll]
suggested that one of the sections of
the polarization curve for a molybdenum cathode exhibited charge-transfer overvoltage and another diffusion overvoltage. The first attempt to determine cathodic overvoltage on aluminium was reported by Drossbach in 1936[16] but tie overvoltage was found. Extensive studies were later performed by Piontelii
224
JOMARTHON~TADAND SVERRE ROL~ETH
> . F
0
0.01
a03
0.1
0.3
1.0
3.0
i i A.&i2 Fig. 1. Semi-logarithmicplots of cathodic overvoltage on aluminium in cryoiite-alumina melts. l_Lozhkin and Popov[24]; 2-This work: 3-This work,, stirrer rotating at 500 rpm, 1 cm above the cathode. et The arrows indicate in which direction the potential would change with time; 4-Borisoglebski al[26].
et aI[17-19, 151. The charge-transfer overvoltage on the aluminium cathode was found to be only a few millivolts at normal cds, whereas an appreciable “residual” overvoltage was observed, ranging from 100 to 400 mV at 1 A/cm’. A blockage effect (cathode effect) appeared at cds of the order of lOA/cn?, due to the formation of a film of sodium gas at the electrode[15]. Aluminium was assumed to be the primary cathodic product at moderate cds, and the “residual” overvoltage was attributed mainly to enrichment of NaF near the cathode surface[l-/l. The transport number of the sodium ion in cryolite is close to unity[20,2i]. The fact that sodium carries the current while aluminium is being deposited leads to a shift towards higher CR’s near the cathode. Emf data show that the aluminium electrode becomes increasingly negative when the CR increases[22], and the residual overvoltage can be explained in this way, as suggested by Piontelli et aZ[23]. Lozhkin and Popov[24,25] presented overvoltage data for the aluminium cathode in cryolite-alumina melts in the form of Tafel plots, as shown in Fig. 1, curve 1. The results were interpreted as a two-step charge-transfer controlled reaction. Borisoglebski et ~I[261 observed an arrest on the cathodic polarization curve for aluminium and interpreted it as a limiting current for the discharge of trivalent aluminium. As shown in Fig. 1, curve 4, the data can also be fitted into a semi-logarithmic plot. In the present work the cathodic overvoltage on aluminium in cryolite-alumina melts was studied using non-steady state as well as steady state techniques. A subsequent paper treats the steady state data in more detail and a model is presented to illustrate the origin of the overvoltageC22J.
EXPERIMENTAL The experimental cell used in steady-state, galvanostatic and potential decay measurements is shown in Fig. 2. The cell was kept under a nitrogen atmosbhere within the uniform temperature zone of a wire-wound furnace. The cryolite-alumina melt was contained in a graphite crucible. Two identical aluminim-n electrodes housed in sintered alumina cups of 16 mm i.d. served as working electrode and reference electrode. The graphite crucible served as the
Fig. 2. Experimental cell. A-Molybdenum wires; B--Sintered alumina tubes; C-Graphite crucible; D-Cryolite melt; E-Sintered alumina cups; F-Aluminium.
Cathodic
overvoltage on aluminium
22s
pre-melted prior to the experiments and were saturated with alumina, when not otherwise stated. The experiments were performed at 1010°C + 3”. RESULTS
Galvanostatic measurements
Fig. 3. Cell for double pulse measurements. A--Cryolite melt; &Graphite crucible; C-Aluminium cathode; D-Aluminium anode; E-Threaded boron nitride cmcible; F-Graphite supporter; G-Steel tube, current conductor; HSintered alumina tube, I-Molybdenum wire, current conductor.
counter electrode, or a third aluminium electrode could be used. The molybdenum wires which made up the electrical contact to the aluminium electrodes were sheathed in alumina tubes. When melts unsaturated with alumina were used, the sheaths and the cups were made of boron nitride. The tips of the molybdenum wires which dipped into the metal would pick up aluminium, forming solid intermetallic compounds at the surface. Some molybdenum would also dissolve in the liquid aluminium, the saturation concentration being -3 ~t”/~ Mot27]. Comparison with a pure aluminium electrode showed that this contamination had no noticeable effect on the potential of the aluminium electrode. After an equilibration period of about 30min the potential difference between two aluminium electrodes was less than 5 mV. The cd was calculated on the basis of the cross sectional area of the cup containing the working electrode. The actual aluminium surface area was somewhat larger, since it had a concave shape facing downwards. because of the high interfacial tension[l] and because the electrolyte wets the sintered alumina better than does aluminium. The Ohmic resistance was determined with a single pulse technique, whereby the Ohmic voltage drop could be distinguished during the first microsecond of the pulse. The cell used for double pulse measurements, which is shown in Fig. 3, was so constructed to minimize the inductance of the circuit. The surface area of the aluminium counter electrode was far larger than that of the cathode, so that the polarization of the counter electrode could be neglected. The melts were made up of hand-picked Greenland cryolite, reagent grade alumina and super purity aluminium. The melts were
When a constant cathodic current was applied to an aluminium electrode of the type shown in Fig. 2, the potential began to decrease, ie become more negative; at first rapidly and then more slowly, as appears from Fig. 4. A stable level was reached after a certain time, being less than 5 min below 0.2 A/LX?, and more than 1 h between 0.2 and 1 A/cm2 and less than 15 min at higher cds. These time lags varied considerably from one experiment to the other, and a semistable state was usually reached more quickly, after 2, 15 and 10min respectively for the cd range quoted. The potential was rather unstable at high cds. A potential peak often occurred shortly after a high cds was applied, as can be seen in Fig. 4. Similar maxima were previously reported by Piontelli et al[lS]. Plots of the potential us the square root of time yielded straight lines, as shown in Fig. 5. This behaviour is indicative of a diffusion controlled process. A peculiar feature observed on potential-time curves at high cds was the pronounced potential oscillations. In a potentiostatic arrangement the current would oscillate correspondingly. Potential-time curves observed on an oscilloscope are shown in Fig. 6. Oscillations were seldom observed below 1 A/cm’. Both the amplitude and the frequency of the oscillations increased with increasing cd and the time lag before the oscillations started decreased with increasing cd. The potential oscillations normally had the rather irregular appearance shown in Fig. 6, but occasionally a distinct sawtooth pattern developed, being quite similar to what is observed with small gasevolving electrodes.
_.,,.I/I I r’
s-*.*I’ ~ I< I
.0.3
-0.05
:
‘k----y-’
01 3 0
2clo-
400 timls
fxo
a
Fig. 4. Change in potential of aluminium electrode when a constant cathodic current was applied. The cds in A/cm2 are indicated on the curves.
226
JOMAR THONSTAD AND SVERRE ROLSETH
(1) is probably satisified at the electrolyte surface. As the cd increases the CR near the cathode will increase, as indicated above, and the solubility of metal in the so melt in the diffusion layer will also increase[9], that larger quantities of metal will dissolve. At the same time the vapour pressure of sodium will increase with increasing potential, as appears from (2). Eventually a stage may be reached where gaseous sodium of 1 atm pressure is formed.
-0.2
> ~
-0.1
F
Visual observations
I
0
I
I
I
I
!
I
80
PO
0
l/2 if2 t 1s
overvoltage as a function of the square root of time when a cd of 0.6A/cmZ was applied.
Fig. 5. Cathodic
Oscillations are not uncommon in diffusion controlled processes due to fluctuations in the diffusion layer thickness. Here, however, pronounced oscillations occurred only at overvoltages greater than around -200 mV, ie in the potential range where sodium formation becomes thermodynamically feasible. Such formation therefore needs to be considered. Any sodium gas formed would probably react with the melt according to (1) before reaching the surface of the melt. Experiments with aluminium cathodes in LiJAIFs-Alz03 melts at 840°C gave oscillations at high cds similar to those shown in Fig. 6. In lithium cryolite the main cathodic product at high cds may be liquid lithium metal. Due to the low density of lithium compared to that of the melt the buoyancy would carry it away from the electrode, again giving rise to oscillations. Potential-time curves for aluminium in cryolite showed no discrete potential jump that could be attributed to a transition from discharge of aluminium to discharge of sodium. If such a transition takes place it apparently occurs gradually, and equilibrium
_ ~_--
Visual observation of an aluminium cathode was difficult because of the presence of a dense metal fog[l], and the formation of gas bubbles at the cathode could not be positively confirmed. Observations were more easily made at a 3.5 mm dia iron rod dipping 1 cm into the melt. The furnace was open at the top and a light beam was focused on the electrode. Greyish clouds of metal fog were seen to leave the cathode and the fog became increasingly denser as the cd was increased. At -5 A/cm’ the fog turned reddish, possibly due to overheating. At above 20A/cm2 violent current fluctuations occurred, as big bubbles of sodium evolved and burned with a cracking sound. This phenomenon in analogous to the “cathode effect” on aluminium electrodes, as described by Piontelli et al[15]. Sodium gas evolution will be favoured by high teniperatures and high CR’s. Tests at 1150°C indicated that gas bubbles appeared at - 2.5 A/cm2. A vigorous evolution of sodium was observed in pure NaF melts at 1010°C at cds above 1.5A/cmz. At lower cds the metal apparently dissolved in the melt. The cd at which sodium evolution occurred increased slightly when a few percent AIFj was added to the melt. Occasionally gas bubbles were observed at cds lower than those mentioned here and are thought to be due to hydrogen, originating from moisture in the melt. Steady-state
experiments:
the effect of stirring
Current-voltage diagrams were recorded by increasing the current stepwise and noting the potential
_w
I__
-0.05 0
20 limeIs
Fig. 6. Oscilloscope
LO Iimr/s
recording of the working electrode/reference potential when connecting currents with cds as indicated on the curves.
constant
Cathodic overvoltage
227
on aluminium
transfer controlled. The linear “Tafel” behaviour was maintained but the overvoltage was considerably lower, as shown in Fig. 1, curve 3 and the time needed to reach steady state was reduced considerabiy. The effect of stirring increased when the distance to the electrode was varied between 2 cm and 0.2 cm. Potential decay
r------l
I
Fig.
1
I
0
600 stirring
7. Overvoltage
800 rntdrpm
I 1200
us speed of rotation of a propeller1 em above the cathode surface. A/cm’; 24.3 A/C&.
shaped stirrer located l-l.4
after 2-3 min. The two minute intervals were sufficient to reach a semi-stable state when the current was increased in steps of 2&30% of the preceding value. Longer time intervals shifted the curve slightly in the direction indicated by the arrows on curves 2 and 3 in Fig. 1. For the same reason curves recorded when decreasing the current stepwise showed slightly lower potentials at high cds and slightly higher values at intermediate cds compared to the ascending curves, Nearly all results could be represented by linear semilogarithmic plots. However, at cds above l-2A/c& the experimental points tended to lie below the straight line. The electrolyte was stirred by a propeller 1 cm above the electrode. The overvoltage decreased with increasing stirring rate as shown in Fig. 7 showing clearly that the cathode process is at least partly mass
The slow rise in potential when a constant current was connected, had its counterpart in a slow decay when the current was disconnected. Such potential decays have been studied for charge-transfer controlled processes[28, 291, where plots of E vs log time are linear with slopes equal to the Tafel slope, except in cases with high coverage of intermediates, where the slope will be less[29]. The theory is not well established for diffusion controlled processes, but it appears that the potential decay will be s10w[30,31]. After steady-state electrolysis had been established the current was interrupted electronically, the potential decay recorded on a storage oscilloscope, and several measurements were fitted together to cover the entire time scale. Plots of E us log t exhibited one straight line in some cases, but curves with two or even three straight sections were more common, as depicted in Fig. 8. Slopes in the range 0.01-0.05 V/decade could be found with no apparent dependence on the initial cd. These slopes are far lower than the slopes of the Fig. 1, which were polarization curves in -0.23 V/decade in cryolite. As indicated, such a behaviour is to be expected for a diffusion controlled process, or possibly for charge-transfer with a high concentration of intermediates. Double pulse measurements The double pulse method[32] allows for a direct determination of the charge-transfer overvoltage. It was found to be essential to keep the inductance of
-0.3
2 Fig. 8. Potential
log 11 s decay curves of different shapes for various initial cds. I-MO-A/cm’; 34.75
A/cm’M-65
A/cm’.
2-1.5
A/cm’;
JOMARTHONSTAD
22x
the circuit as low as possible and to shield the wiring, as in Fig. 3. The Ohmic voltage drop was compensated in a bridge circuit, and the overvoltage could then be read directly off the screen of a storage oscilloscope. The measurements were carried out in cryolite melts containing 5% AlzOj at 1010°C and were performed with variable lengths (rr) of the first pulse. Due to the contribution of diffusion overvoltage, the total overvoltage will vary with this time t, as follows[33] : RTi2 q=nF’-+20
4N Q+’
l/2
SVERRE ROLSETH
AND
constant, the other terms having their usual meaning. The results were plotted vs t:” and extrapolated back to zero time to find the charge-transfer over-voltage. The compensation of the Ohmic resistance was critical, since the Ohmic resistance of the circuit was 120 times higher than the charge-transfer resistance. This explains the scatter in the experimental results shown in Fig. 9. The curves were fitted by the least square method. Since the measurements were carried out in the range of law overvoltages where overvoltage and cd are proportional, the charge-transfer resistance (R,) can simply be derived as
2
R, = q/i,.
where i, is the cd of the second pulse and N is a
A set of experimental
data is given in Table 1.
I
0
-E -2
0 n
“8
F
8
3&S
@
____--
01
r35
1
2 112 t/2 t tcls (a1
1
2
112 iI21 l.Ei
3
3
( b) Fig. 9 (a) and (b). Double pulse measurements. The overvoltage at the beginning of the second pulse as a function of the square. root of the duration (ti) of the first pulse. The cds of the second pulse in mA/em’ are indicated on the curves.
(4)
229
Cathodic overvoltage of: aluminium
0.02 r-----
Hz
(a)
c:
0.01
d
0 I#
Id
I
1
ld
20
100
IO
!I? (b)
Fig. 10 (aj(c). Faraday impedance split-up into the resistive (R.) and capacitive (l/w0 terns and plotted vs the inverse square root of the angular frequency. The ac current was superimposed on a dc current of (+0.82 A/co?; (b)-O.l2A/cm’; (c)--O A/cm*.
230
JOMM
Table 1. Experimental including the cathodic
to zero time (q(tl -
THONSTAD AND SVERRE ROLSETH
data for double pulse measurements cd (ia), the overvoltage extrapolated 0)) and the calculated charge-transfer resistance (R,)
i2
mA/cm’
?ICr- 01 mV
- 0.4 -0.6 - 1.0 -1.4 - 2.4 - 1.8 -4.0 -4.95 -6.60
135 170 345 435 690 770 1410 I.540 1920
Rr
Ohm cm’ 0.0030 0.0035 0.0029 0.0032 0.0035 0.0023 0.0028 0.0032 0.0034
-i
The exchange cd can be calculated from the expression RT 1 i. = _._ (5) nF R; Depending on the value of n, the exchange current will be 36, 24 or 12 A/cm2 for n being 1, 2 or 3 respectively. For a cd of 1 A/cm’ the charge-transfer overvoltage will be only - 3.5 mV. Compared to the total overvoltage of - 190 mV, given in Fig. 1, curve 2, it is clear that the charge-transfer overvoltage is almost negligible. Impedance
measurements
The experimental cell was similar to that shown in Fig. 2 except for the current conductors which were arranged closely parallel and shielded so as to diminish the inductance and the pick-up of noise. The cryolite meft was saturated with alumina at 1010°C. The dc electrolysis current was varied from zero up to 1.2 A/IX?. An ac current of low amplitude (-2 mV) and with frequencies varying from 1 Hz to 200 kHz was superimposed. The QCimpedance and the phase angle between the working electrode and the reference were determined as outlined in a previous paper[34]. To obtain reliable results the magnitude of the impedances should not be much smaller than the Ohmic resistance of the cell, which was around 0.1 Ohm. Here, however, this was not the case and the results are somewhat uncertain. The results for the charge-transfer resistance (R,) were quite scattered, ranging from 0.003 to 0.015 Ohm cm’. The lower values are in agreement with the findings of the double pulse method which is considered to be superior. The capacitive term (~/CL) was in most cases close to zero at low frequencies. It tended to exhibit a maximum at - 3000 Hz, as appears from Fig. 10(a), the reasons for which are not clear. The curves shown in Fig. 10(a) were most commonly observed, but the shapes shown in (b) also occurred. The drop in the resistive term at low frequencies can hardly be related to any particular reaction mechanism. No systematic variation in the impedance data were found when the cd of the dc electrolysis current was varied. At the beginning of an experiment and at zero dc current the type of curves shown in Fig. 10(c) was
Fig. 11. Cathodic potential decay in a 150 kA industrial aluminium cell when the current was interrupted. obtained; they are typical for a diffusion controlled process. Measurements conducted at zero dc current later in an experiment would invariably give curves of the types shown in Figs 10(a) and (b). Only a tentative explanation can be given for this behaviour. Provided the dissolved metal acts as an intermediate product in the process, and provided that equilibrium was not yet established with respect to dissolution of metal, such a behaviour seems possible. Attempts to determine the double layer capacitance of aluminium were unsuccessful. The very low chargetransfer resistance indicate that appreciable current will flow when the electrode is polarized, and this renders determination of double layer capacitance difficult. Impedance measurements on aluminium in cryolite were also attempted by Vetyukov et aI[35]. The changes in resistance with frequency were found to be smatl and unstable. Therefore, the measurements were carried out with an Al-Cu electrode in a NaClLKCl melt containing i-3 wt% Na3AlFs. The exchange cd (io) in this system was -2A/em’, and extrapolation to pure aluminium in pure cryolite at 965°C was reported to yield i, = 20_04A/cm* for II = 3.
DISCUSSION
The results presented in this paper indicate that the observed cathodic overvoltage on aluminium in cryolite can be treated as diffusion overvoltage, apart from a small contribution of charge-transfer overvoltage. As will be outlined in more detail in a subsequent paper[22], the electrolyte in the vicinity of
Cathodic overvoltage on aluminium the cathode becomes enriched with respect to NaF and depleted with respect to AIF,. This change in CR shifts the reversible emf of the aluminium electrode in the negative direction. Only the impedance data do not seem to fit into this model. In fact, the UC impedance does not give any information as to the nature of the major part of the overvoltage. As indicated above the potentialdetermining reaction may comprise the constituents of the dissolved metal as intermediate products. During electrolysis the melt near the cathode may be supersaturated with respect to dissolved metal, and the alternating current may then have no apparent effect. A model of the diffusion layer and a calculation on theoretical overvoltage curves based on the reversible emf will be treated in a subsequent paper[22]. Cathodic
overvoltage in industrial aluminium
cells
The electrolyte used in industrial cells normally has the composition CR = 2.7, 5 wt% CaF,, l-7% A1103. There is convection in the metal and in the electrolyte due to magnetic fields[36], anodic gas evolution. The
thermal
gradients
and the
REFERENCES 1. K. Griotheim,
2. 3. 4. 5. 6. 7. 8. 9. 10.
Il. 12. 13. 14.
metal-electrolyte interface is disturbed by ripples[37] and occasionally by larger waves[38]. Accurate overvoltage measurements in industrial cells would be difficult to carry out, and two simple procedures were tried instead, ie in 150 kA cells which operated with a cathodic cd of around 0.5 A/c&. An iron rod, the upper part of which was shielded by boron nitride, was aluminized by cathodic polarization or by dipping it into the metal pool. The potential jump occurring when the rod made contact with the metal was recorded. The most reproducible results were obtained when the electrode was kept so close to the metal that occasional contacts were made due to the metal undulation. The potential jump was on average, -0.14v. The long-time stability of the iron electrode was poor. Therefore, in a second set of experiments an aluminium electrode enclosed in a boron nitride tube was used to determine the potential decay of the aluminium cathode when the electrolysis current was dis-
27.
connected. The current was lowered gradually to 90 kA and then abruptly to zero. The time available until electrolysis was resumed varied from 20 to 400 s.
2x
As shown in Fig. 11 the potential decay ranged from 0.03 to 0.1 V. The variation in starting potentials was probably due to instability of the reference electrode. On the basis of these measurements it can be concluded that the cathodic overvoltage in industrial aluminium cells is of a magnitude similar to that found in laboratory cells.
15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
29. 30. 31. 32. 33. 34. 35.
Acknowledgements-Financial support from the Royal Norwegian Council for Scientific and Industrial Research is gratefully acknowledged. The authors are indebted to Mr. Johan Gulbransen for experimental assistance and to Mr. Svein Hove who carried out steady-state measurements as part of his graduation the&.
231
36. 37. 38.
C. Krohn. M. Malinovskv, K. Matiasovsky and J: Thonstad, Aluminium Ele&aZysis. Aluminium Verlag, Diisseldorf (1977). C. E. Ransley-&d H. Neufgld, i. Inst. Metals 78, 25 (1950). E. W. Dewing, Met. Trans. 3, 495 (1972). E. Griinert, 2. Elektrochem. 48, 393 (1942). M. M. Vetyukov and V. B. Vinokurov, Tsvet. Met 44, (6), 24 (1971). H. Ginsberg and S. Wilkening, Metall 27, 787 (1973). M. M. Makansi, C. H. Muendel and W. A. Selke, J. phys. Chem. 59, 40 (1955). K. Yoshida and E. W. Dewing, Met. Trans. 3, 683 (1972). J. Thonstad, Can. J. Chem. 43, 3429 (1965). M. M. Vetyukov and V. B. Vinokurov, Fiz. Khim. Elektrokhim. Rasplau. Solei Shlakov, p. 367. Naukova Dumka, Kiev (1969). L. N. Antipin, Z. Fiz. Khim. 29, 1668 (1955). C. Kubik, K. Matiasovsky, M. Malinovsky and J. Zeman, Electrochim. Acta 9, 1521 (1964). C. Kubik and M. Malinovskv. Chem Zvestv_ 21., 920 (1967); 22, 34 (1968). -’ A. Silnv. M. Malinovskv and K. Matiasovskv._- Chem. &-sty-23, 561 (1969). R. Piontelli, B. Mazza and P. Pedeferri, Electrochim. Met. 1, 217 (1966). P. Drossbach, Z. Elektrochem. 42, 65 (1936). R. Piontelli and G. Montanelli, Alluminio 22, 699 (1953). R. Piontelli and G. Montanelli. Alluminio 25. 79 (19561. R. Piontelli, Metailurgia Ital. 52, 469 (196Oj. . ’ W. B. Frank and L. M. Foster, J. __ ~hvs. Chem. 61. 1531 (1957). A. Tual and M. Rolin, Electrochim. Acta 17, 1945 (1972). .I. Thonstad and S. Rolseth, EIectrochim. Acta 23, 231, (1977). R. Piontelli, B. Mazza and P. Pedeferri, Alluminio 34, 623 (1965). L. N. Lozhkin and A. P. Popov, .I. appl. Chem. USSR 41, 2277 (1968). L. N. Loibkin and A. P. Popov, Fir. Khim. Elektrokhim. Rasp,lav. Solei Shlakov, _ a. 303. Khimiva, _ Leningrad (1968). Yu. V. Borisoglebski, M. M. Vetyukov and V. S. Vinokurov. Tsvet. Met. 44. (101. 37 119731. M. Hansen; Constitution of &&y Alloys; 2nd supplement, (Edited by F. A. Shunk), p. _ 30. McGraw Hill. New York (1966). G. Armstrong and J. A. V. Butler, Trans. Faraday Sac. 29, 1261 (1933). B. E. Conway and P. L. Bourgault, Trans. Faraday Sot. 58. 593 (1962). D. C. &ahar&, .I: phys. Chem. 57, 257 (1953). W. T. Scott, J. them. Phvs. 23. 1936 (19551. H. Gerischer and M. Krause, Z. phyi. Ch&. N.F. 10, 264 (1957). H. Matsuda, A. Oka and P. Delahay, J. Am. them. Sot. 81, 5077 (1959). J. Thonstad, Electrochim. Acta 15, 1581 (1970). M. M. Vetyukov, D. S. Dyblin and Yu. V. Borisoglebski, Soui~t Electrochem. 8, 332 (1972). K. Griotheim. C. Krohn. K. Naeumann and K. TPrrklep, Met. ‘Trans. 1, 31j3 (1970). M. F. Doeramadzhi and S. E. Gefter. Sou. J. Non Ferrous Met. 6, (9), 54 (1965). T. Sele, Light Metals, Proceedings of the 106th AIME Annual Meeting, p, 7 Atlanta, (1977).
Acra
ON
1978, Vol. 2.3, pp. 223-231.
THE
Pergmon
Press.
Printed
in Great
Britain
CATHODIC OVERVOLTAGE ON ALUMINIUM CRYOLITE-ALUMINA MELTS-I
Electrochemistry
JOMAR THONSTAD* and SV@RRE ROLSETH Laboratories and the Engineering Research Foundation, Technology, University of Trondheim, Norway (Received 17 February
1977; and in$nalform
Norwegian
IN
In&it&e of
13 June 1977)
Abstract-When a constant cathodic current is applied to an aluminium electrode in a Na3A1F6-AlaO, melt at lOlO”C, the potential decreases gradually and linearly with the square root of time to more negative values. Pronounced potential oscillations occurred at cds above 2A/cm2 and the evolution of sodium gas was observed at very high cds. Steady state measurements yielded straight y vs log i plots with a slope of -0.23 V/decade, the overvoltage being -0.19 V at 1 A/cm’. The overvoltage decreased markedly when th? melt was stirred. Potential decay measurements yielded linear log time plots with slopes of 0.0145 V/decade. The charge-transfer resistance ias determined by double pulse measurements to be 0.0031 ohm cm’. AC impedance measurements gave a similar result. The charge transfer overvoltage accounts for only about two percent of the total overvoltage, the rest is apparently diffusion controlled. The cathodic overvoltage in industrial aluminium cells is of similar magnitude as found in the laboratory investigations.
INTRODUCTlON
studies of the cathode reaction at aluminium in cryolite melts were centered on the question of’ whether sodium or aluminium is the primary cathodic product[l]. In case of primary sodium formation the aluminium would be formed in a secondary chemical reaction, Early
3Na + AlF, = 3NaF + Al.
(1)
This equilibrium is shifted to the right in cryolite. Sodium is in the gaseous state at around 1000°C. The solubility of sodium in liquid aluminium at this temperature appears to be about 0.13 wt% Na (extrapolated[2]) at 1 atm sodium pressure, while the equilibrium content in the presence of molten cryolite is only about 0.02 wt%[3]. The notion of primary sodium formation was based on the fact that aluminium in the melt is bound in anionic complexes of the type AlFa- and AIF;, while sodium is present as free cations[l]. Furthermore, gas bubbles that are seen to escape from the melt after electrolysis, burn with the typical sodium culouring, and it was suggested that they consisted of sodium gas[4]. However, excess sodium present should react according to (1) and the flame colouring could also be the result of burning hydrogen bubbles containing sodium salt vapour. More recent studies show that the activity of sodium at the cryolite composition is between 0.04 and 0.056[3, 5, 61 refered to liquid sodium as standard state. The potential difference between the deposition potential of aluminium and that of sodium gas at’ 1 atm can then be calculated. M
=
RT In F
%! &a
(2)
where i& is the activity of liquid sodium at 1 atm sodium pressure[7]. AE is -0.24 to -0.2OV at 223 E.A.23/3-E
1010°C whereas data for cryolite saturated with alumina[8] yield a value of -0.28 V. The difference de-
creases with increasing temperature and with increasing CR (cryolite ratio = mol NaF/mol AIF,). As will be shown the cathodic overvoltage in cryolite melts saturated with alumina reaches -0.2V at -1 A/cmz, and the formation of 1 atm sodium gas can therefore be ruled out at moderate cds. It is conceivable that sodium-containing intermediate cathode products are formed. Aluminium is soluble in cryolite to
-0.1
wt% Al[9,
lo].
The
nature
of the dissolved
metal is not clear, but it appears to consist of sodium as well as aluminiumlp, lo], either in the form of neutral species, or ions and trapped electrons or of subvalent species such as Al l Support for the idea of intermediates is found in cathodic polarization studies on molybdenum and platinum. Antipin[ll] observed four plateaux on polarization curves for molybdenum in cryolite, but only two plateaux when aluminium was present. Kubik et nl[12] and Kubik and Malinovsky[13] observed several arrests on potential decay curves, and the existence of the A12+ species was proposed. Silny et ul[14] found two plateaux on cathodic polarization curves on platinum and they were ascribed to formation of aluminium and sodium. The curves obtained with aluminium electrodes by galvanostatic polarization and by potential decay are smooth, as shown by Piontelli et aE[lS] and by the present authors.
Cathodic
overvoltage
Antipin[ll]
suggested that one of the sections of
the polarization curve for a molybdenum cathode exhibited charge-transfer overvoltage and another diffusion overvoltage. The first attempt to determine cathodic overvoltage on aluminium was reported by Drossbach in 1936[16] but tie overvoltage was found. Extensive studies were later performed by Piontelii
224
JOMARTHON~TADAND SVERRE ROL~ETH
> . F
0
0.01
a03
0.1
0.3
1.0
3.0
i i A.&i2 Fig. 1. Semi-logarithmicplots of cathodic overvoltage on aluminium in cryoiite-alumina melts. l_Lozhkin and Popov[24]; 2-This work: 3-This work,, stirrer rotating at 500 rpm, 1 cm above the cathode. et The arrows indicate in which direction the potential would change with time; 4-Borisoglebski al[26].
et aI[17-19, 151. The charge-transfer overvoltage on the aluminium cathode was found to be only a few millivolts at normal cds, whereas an appreciable “residual” overvoltage was observed, ranging from 100 to 400 mV at 1 A/cm’. A blockage effect (cathode effect) appeared at cds of the order of lOA/cn?, due to the formation of a film of sodium gas at the electrode[15]. Aluminium was assumed to be the primary cathodic product at moderate cds, and the “residual” overvoltage was attributed mainly to enrichment of NaF near the cathode surface[l-/l. The transport number of the sodium ion in cryolite is close to unity[20,2i]. The fact that sodium carries the current while aluminium is being deposited leads to a shift towards higher CR’s near the cathode. Emf data show that the aluminium electrode becomes increasingly negative when the CR increases[22], and the residual overvoltage can be explained in this way, as suggested by Piontelli et aZ[23]. Lozhkin and Popov[24,25] presented overvoltage data for the aluminium cathode in cryolite-alumina melts in the form of Tafel plots, as shown in Fig. 1, curve 1. The results were interpreted as a two-step charge-transfer controlled reaction. Borisoglebski et ~I[261 observed an arrest on the cathodic polarization curve for aluminium and interpreted it as a limiting current for the discharge of trivalent aluminium. As shown in Fig. 1, curve 4, the data can also be fitted into a semi-logarithmic plot. In the present work the cathodic overvoltage on aluminium in cryolite-alumina melts was studied using non-steady state as well as steady state techniques. A subsequent paper treats the steady state data in more detail and a model is presented to illustrate the origin of the overvoltageC22J.
EXPERIMENTAL The experimental cell used in steady-state, galvanostatic and potential decay measurements is shown in Fig. 2. The cell was kept under a nitrogen atmosbhere within the uniform temperature zone of a wire-wound furnace. The cryolite-alumina melt was contained in a graphite crucible. Two identical aluminim-n electrodes housed in sintered alumina cups of 16 mm i.d. served as working electrode and reference electrode. The graphite crucible served as the
Fig. 2. Experimental cell. A-Molybdenum wires; B--Sintered alumina tubes; C-Graphite crucible; D-Cryolite melt; E-Sintered alumina cups; F-Aluminium.
Cathodic
overvoltage on aluminium
22s
pre-melted prior to the experiments and were saturated with alumina, when not otherwise stated. The experiments were performed at 1010°C + 3”. RESULTS
Galvanostatic measurements
Fig. 3. Cell for double pulse measurements. A--Cryolite melt; &Graphite crucible; C-Aluminium cathode; D-Aluminium anode; E-Threaded boron nitride cmcible; F-Graphite supporter; G-Steel tube, current conductor; HSintered alumina tube, I-Molybdenum wire, current conductor.
counter electrode, or a third aluminium electrode could be used. The molybdenum wires which made up the electrical contact to the aluminium electrodes were sheathed in alumina tubes. When melts unsaturated with alumina were used, the sheaths and the cups were made of boron nitride. The tips of the molybdenum wires which dipped into the metal would pick up aluminium, forming solid intermetallic compounds at the surface. Some molybdenum would also dissolve in the liquid aluminium, the saturation concentration being -3 ~t”/~ Mot27]. Comparison with a pure aluminium electrode showed that this contamination had no noticeable effect on the potential of the aluminium electrode. After an equilibration period of about 30min the potential difference between two aluminium electrodes was less than 5 mV. The cd was calculated on the basis of the cross sectional area of the cup containing the working electrode. The actual aluminium surface area was somewhat larger, since it had a concave shape facing downwards. because of the high interfacial tension[l] and because the electrolyte wets the sintered alumina better than does aluminium. The Ohmic resistance was determined with a single pulse technique, whereby the Ohmic voltage drop could be distinguished during the first microsecond of the pulse. The cell used for double pulse measurements, which is shown in Fig. 3, was so constructed to minimize the inductance of the circuit. The surface area of the aluminium counter electrode was far larger than that of the cathode, so that the polarization of the counter electrode could be neglected. The melts were made up of hand-picked Greenland cryolite, reagent grade alumina and super purity aluminium. The melts were
When a constant cathodic current was applied to an aluminium electrode of the type shown in Fig. 2, the potential began to decrease, ie become more negative; at first rapidly and then more slowly, as appears from Fig. 4. A stable level was reached after a certain time, being less than 5 min below 0.2 A/LX?, and more than 1 h between 0.2 and 1 A/cm2 and less than 15 min at higher cds. These time lags varied considerably from one experiment to the other, and a semistable state was usually reached more quickly, after 2, 15 and 10min respectively for the cd range quoted. The potential was rather unstable at high cds. A potential peak often occurred shortly after a high cds was applied, as can be seen in Fig. 4. Similar maxima were previously reported by Piontelli et al[lS]. Plots of the potential us the square root of time yielded straight lines, as shown in Fig. 5. This behaviour is indicative of a diffusion controlled process. A peculiar feature observed on potential-time curves at high cds was the pronounced potential oscillations. In a potentiostatic arrangement the current would oscillate correspondingly. Potential-time curves observed on an oscilloscope are shown in Fig. 6. Oscillations were seldom observed below 1 A/cm’. Both the amplitude and the frequency of the oscillations increased with increasing cd and the time lag before the oscillations started decreased with increasing cd. The potential oscillations normally had the rather irregular appearance shown in Fig. 6, but occasionally a distinct sawtooth pattern developed, being quite similar to what is observed with small gasevolving electrodes.
_.,,.I/I I r’
s-*.*I’ ~ I< I
.0.3
-0.05
:
‘k----y-’
01 3 0
2clo-
400 timls
fxo
a
Fig. 4. Change in potential of aluminium electrode when a constant cathodic current was applied. The cds in A/cm2 are indicated on the curves.
226
JOMAR THONSTAD AND SVERRE ROLSETH
(1) is probably satisified at the electrolyte surface. As the cd increases the CR near the cathode will increase, as indicated above, and the solubility of metal in the so melt in the diffusion layer will also increase[9], that larger quantities of metal will dissolve. At the same time the vapour pressure of sodium will increase with increasing potential, as appears from (2). Eventually a stage may be reached where gaseous sodium of 1 atm pressure is formed.
-0.2
> ~
-0.1
F
Visual observations
I
0
I
I
I
I
!
I
80
PO
0
l/2 if2 t 1s
overvoltage as a function of the square root of time when a cd of 0.6A/cmZ was applied.
Fig. 5. Cathodic
Oscillations are not uncommon in diffusion controlled processes due to fluctuations in the diffusion layer thickness. Here, however, pronounced oscillations occurred only at overvoltages greater than around -200 mV, ie in the potential range where sodium formation becomes thermodynamically feasible. Such formation therefore needs to be considered. Any sodium gas formed would probably react with the melt according to (1) before reaching the surface of the melt. Experiments with aluminium cathodes in LiJAIFs-Alz03 melts at 840°C gave oscillations at high cds similar to those shown in Fig. 6. In lithium cryolite the main cathodic product at high cds may be liquid lithium metal. Due to the low density of lithium compared to that of the melt the buoyancy would carry it away from the electrode, again giving rise to oscillations. Potential-time curves for aluminium in cryolite showed no discrete potential jump that could be attributed to a transition from discharge of aluminium to discharge of sodium. If such a transition takes place it apparently occurs gradually, and equilibrium
_ ~_--
Visual observation of an aluminium cathode was difficult because of the presence of a dense metal fog[l], and the formation of gas bubbles at the cathode could not be positively confirmed. Observations were more easily made at a 3.5 mm dia iron rod dipping 1 cm into the melt. The furnace was open at the top and a light beam was focused on the electrode. Greyish clouds of metal fog were seen to leave the cathode and the fog became increasingly denser as the cd was increased. At -5 A/cm’ the fog turned reddish, possibly due to overheating. At above 20A/cm2 violent current fluctuations occurred, as big bubbles of sodium evolved and burned with a cracking sound. This phenomenon in analogous to the “cathode effect” on aluminium electrodes, as described by Piontelli et al[15]. Sodium gas evolution will be favoured by high teniperatures and high CR’s. Tests at 1150°C indicated that gas bubbles appeared at - 2.5 A/cm2. A vigorous evolution of sodium was observed in pure NaF melts at 1010°C at cds above 1.5A/cmz. At lower cds the metal apparently dissolved in the melt. The cd at which sodium evolution occurred increased slightly when a few percent AIFj was added to the melt. Occasionally gas bubbles were observed at cds lower than those mentioned here and are thought to be due to hydrogen, originating from moisture in the melt. Steady-state
experiments:
the effect of stirring
Current-voltage diagrams were recorded by increasing the current stepwise and noting the potential
_w
I__
-0.05 0
20 limeIs
Fig. 6. Oscilloscope
LO Iimr/s
recording of the working electrode/reference potential when connecting currents with cds as indicated on the curves.
constant
Cathodic overvoltage
227
on aluminium
transfer controlled. The linear “Tafel” behaviour was maintained but the overvoltage was considerably lower, as shown in Fig. 1, curve 3 and the time needed to reach steady state was reduced considerabiy. The effect of stirring increased when the distance to the electrode was varied between 2 cm and 0.2 cm. Potential decay
r------l
I
Fig.
1
I
0
600 stirring
7. Overvoltage
800 rntdrpm
I 1200
us speed of rotation of a propeller1 em above the cathode surface. A/cm’; 24.3 A/C&.
shaped stirrer located l-l.4
after 2-3 min. The two minute intervals were sufficient to reach a semi-stable state when the current was increased in steps of 2&30% of the preceding value. Longer time intervals shifted the curve slightly in the direction indicated by the arrows on curves 2 and 3 in Fig. 1. For the same reason curves recorded when decreasing the current stepwise showed slightly lower potentials at high cds and slightly higher values at intermediate cds compared to the ascending curves, Nearly all results could be represented by linear semilogarithmic plots. However, at cds above l-2A/c& the experimental points tended to lie below the straight line. The electrolyte was stirred by a propeller 1 cm above the electrode. The overvoltage decreased with increasing stirring rate as shown in Fig. 7 showing clearly that the cathode process is at least partly mass
The slow rise in potential when a constant current was connected, had its counterpart in a slow decay when the current was disconnected. Such potential decays have been studied for charge-transfer controlled processes[28, 291, where plots of E vs log time are linear with slopes equal to the Tafel slope, except in cases with high coverage of intermediates, where the slope will be less[29]. The theory is not well established for diffusion controlled processes, but it appears that the potential decay will be s10w[30,31]. After steady-state electrolysis had been established the current was interrupted electronically, the potential decay recorded on a storage oscilloscope, and several measurements were fitted together to cover the entire time scale. Plots of E us log t exhibited one straight line in some cases, but curves with two or even three straight sections were more common, as depicted in Fig. 8. Slopes in the range 0.01-0.05 V/decade could be found with no apparent dependence on the initial cd. These slopes are far lower than the slopes of the Fig. 1, which were polarization curves in -0.23 V/decade in cryolite. As indicated, such a behaviour is to be expected for a diffusion controlled process, or possibly for charge-transfer with a high concentration of intermediates. Double pulse measurements The double pulse method[32] allows for a direct determination of the charge-transfer overvoltage. It was found to be essential to keep the inductance of
-0.3
2 Fig. 8. Potential
log 11 s decay curves of different shapes for various initial cds. I-MO-A/cm’; 34.75
A/cm’M-65
A/cm’.
2-1.5
A/cm’;
JOMARTHONSTAD
22x
the circuit as low as possible and to shield the wiring, as in Fig. 3. The Ohmic voltage drop was compensated in a bridge circuit, and the overvoltage could then be read directly off the screen of a storage oscilloscope. The measurements were carried out in cryolite melts containing 5% AlzOj at 1010°C and were performed with variable lengths (rr) of the first pulse. Due to the contribution of diffusion overvoltage, the total overvoltage will vary with this time t, as follows[33] : RTi2 q=nF’-+20
4N Q+’
l/2
SVERRE ROLSETH
AND
constant, the other terms having their usual meaning. The results were plotted vs t:” and extrapolated back to zero time to find the charge-transfer over-voltage. The compensation of the Ohmic resistance was critical, since the Ohmic resistance of the circuit was 120 times higher than the charge-transfer resistance. This explains the scatter in the experimental results shown in Fig. 9. The curves were fitted by the least square method. Since the measurements were carried out in the range of law overvoltages where overvoltage and cd are proportional, the charge-transfer resistance (R,) can simply be derived as
2
R, = q/i,.
where i, is the cd of the second pulse and N is a
A set of experimental
data is given in Table 1.
I
0
-E -2
0 n
“8
F
8
3&S
@
____--
01
r35
1
2 112 t/2 t tcls (a1
1
2
112 iI21 l.Ei
3
3
( b) Fig. 9 (a) and (b). Double pulse measurements. The overvoltage at the beginning of the second pulse as a function of the square. root of the duration (ti) of the first pulse. The cds of the second pulse in mA/em’ are indicated on the curves.
(4)
229
Cathodic overvoltage of: aluminium
0.02 r-----
Hz
(a)
c:
0.01
d
0 I#
Id
I
1
ld
20
100
IO
!I? (b)
Fig. 10 (aj(c). Faraday impedance split-up into the resistive (R.) and capacitive (l/w0 terns and plotted vs the inverse square root of the angular frequency. The ac current was superimposed on a dc current of (+0.82 A/co?; (b)-O.l2A/cm’; (c)--O A/cm*.
230
JOMM
Table 1. Experimental including the cathodic
to zero time (q(tl -
THONSTAD AND SVERRE ROLSETH
data for double pulse measurements cd (ia), the overvoltage extrapolated 0)) and the calculated charge-transfer resistance (R,)
i2
mA/cm’
?ICr- 01 mV
- 0.4 -0.6 - 1.0 -1.4 - 2.4 - 1.8 -4.0 -4.95 -6.60
135 170 345 435 690 770 1410 I.540 1920
Rr
Ohm cm’ 0.0030 0.0035 0.0029 0.0032 0.0035 0.0023 0.0028 0.0032 0.0034
-i
The exchange cd can be calculated from the expression RT 1 i. = _._ (5) nF R; Depending on the value of n, the exchange current will be 36, 24 or 12 A/cm2 for n being 1, 2 or 3 respectively. For a cd of 1 A/cm’ the charge-transfer overvoltage will be only - 3.5 mV. Compared to the total overvoltage of - 190 mV, given in Fig. 1, curve 2, it is clear that the charge-transfer overvoltage is almost negligible. Impedance
measurements
The experimental cell was similar to that shown in Fig. 2 except for the current conductors which were arranged closely parallel and shielded so as to diminish the inductance and the pick-up of noise. The cryolite meft was saturated with alumina at 1010°C. The dc electrolysis current was varied from zero up to 1.2 A/IX?. An ac current of low amplitude (-2 mV) and with frequencies varying from 1 Hz to 200 kHz was superimposed. The QCimpedance and the phase angle between the working electrode and the reference were determined as outlined in a previous paper[34]. To obtain reliable results the magnitude of the impedances should not be much smaller than the Ohmic resistance of the cell, which was around 0.1 Ohm. Here, however, this was not the case and the results are somewhat uncertain. The results for the charge-transfer resistance (R,) were quite scattered, ranging from 0.003 to 0.015 Ohm cm’. The lower values are in agreement with the findings of the double pulse method which is considered to be superior. The capacitive term (~/CL) was in most cases close to zero at low frequencies. It tended to exhibit a maximum at - 3000 Hz, as appears from Fig. 10(a), the reasons for which are not clear. The curves shown in Fig. 10(a) were most commonly observed, but the shapes shown in (b) also occurred. The drop in the resistive term at low frequencies can hardly be related to any particular reaction mechanism. No systematic variation in the impedance data were found when the cd of the dc electrolysis current was varied. At the beginning of an experiment and at zero dc current the type of curves shown in Fig. 10(c) was
Fig. 11. Cathodic potential decay in a 150 kA industrial aluminium cell when the current was interrupted. obtained; they are typical for a diffusion controlled process. Measurements conducted at zero dc current later in an experiment would invariably give curves of the types shown in Figs 10(a) and (b). Only a tentative explanation can be given for this behaviour. Provided the dissolved metal acts as an intermediate product in the process, and provided that equilibrium was not yet established with respect to dissolution of metal, such a behaviour seems possible. Attempts to determine the double layer capacitance of aluminium were unsuccessful. The very low chargetransfer resistance indicate that appreciable current will flow when the electrode is polarized, and this renders determination of double layer capacitance difficult. Impedance measurements on aluminium in cryolite were also attempted by Vetyukov et aI[35]. The changes in resistance with frequency were found to be smatl and unstable. Therefore, the measurements were carried out with an Al-Cu electrode in a NaClLKCl melt containing i-3 wt% Na3AlFs. The exchange cd (io) in this system was -2A/em’, and extrapolation to pure aluminium in pure cryolite at 965°C was reported to yield i, = 20_04A/cm* for II = 3.
DISCUSSION
The results presented in this paper indicate that the observed cathodic overvoltage on aluminium in cryolite can be treated as diffusion overvoltage, apart from a small contribution of charge-transfer overvoltage. As will be outlined in more detail in a subsequent paper[22], the electrolyte in the vicinity of
Cathodic overvoltage on aluminium the cathode becomes enriched with respect to NaF and depleted with respect to AIF,. This change in CR shifts the reversible emf of the aluminium electrode in the negative direction. Only the impedance data do not seem to fit into this model. In fact, the UC impedance does not give any information as to the nature of the major part of the overvoltage. As indicated above the potentialdetermining reaction may comprise the constituents of the dissolved metal as intermediate products. During electrolysis the melt near the cathode may be supersaturated with respect to dissolved metal, and the alternating current may then have no apparent effect. A model of the diffusion layer and a calculation on theoretical overvoltage curves based on the reversible emf will be treated in a subsequent paper[22]. Cathodic
overvoltage in industrial aluminium
cells
The electrolyte used in industrial cells normally has the composition CR = 2.7, 5 wt% CaF,, l-7% A1103. There is convection in the metal and in the electrolyte due to magnetic fields[36], anodic gas evolution. The
thermal
gradients
and the
REFERENCES 1. K. Griotheim,
2. 3. 4. 5. 6. 7. 8. 9. 10.
Il. 12. 13. 14.
metal-electrolyte interface is disturbed by ripples[37] and occasionally by larger waves[38]. Accurate overvoltage measurements in industrial cells would be difficult to carry out, and two simple procedures were tried instead, ie in 150 kA cells which operated with a cathodic cd of around 0.5 A/c&. An iron rod, the upper part of which was shielded by boron nitride, was aluminized by cathodic polarization or by dipping it into the metal pool. The potential jump occurring when the rod made contact with the metal was recorded. The most reproducible results were obtained when the electrode was kept so close to the metal that occasional contacts were made due to the metal undulation. The potential jump was on average, -0.14v. The long-time stability of the iron electrode was poor. Therefore, in a second set of experiments an aluminium electrode enclosed in a boron nitride tube was used to determine the potential decay of the aluminium cathode when the electrolysis current was dis-
27.
connected. The current was lowered gradually to 90 kA and then abruptly to zero. The time available until electrolysis was resumed varied from 20 to 400 s.
2x
As shown in Fig. 11 the potential decay ranged from 0.03 to 0.1 V. The variation in starting potentials was probably due to instability of the reference electrode. On the basis of these measurements it can be concluded that the cathodic overvoltage in industrial aluminium cells is of a magnitude similar to that found in laboratory cells.
15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.
29. 30. 31. 32. 33. 34. 35.
Acknowledgements-Financial support from the Royal Norwegian Council for Scientific and Industrial Research is gratefully acknowledged. The authors are indebted to Mr. Johan Gulbransen for experimental assistance and to Mr. Svein Hove who carried out steady-state measurements as part of his graduation the&.
231
36. 37. 38.
C. Krohn. M. Malinovskv, K. Matiasovsky and J: Thonstad, Aluminium Ele&aZysis. Aluminium Verlag, Diisseldorf (1977). C. E. Ransley-&d H. Neufgld, i. Inst. Metals 78, 25 (1950). E. W. Dewing, Met. Trans. 3, 495 (1972). E. Griinert, 2. Elektrochem. 48, 393 (1942). M. M. Vetyukov and V. B. Vinokurov, Tsvet. Met 44, (6), 24 (1971). H. Ginsberg and S. Wilkening, Metall 27, 787 (1973). M. M. Makansi, C. H. Muendel and W. A. Selke, J. phys. Chem. 59, 40 (1955). K. Yoshida and E. W. Dewing, Met. Trans. 3, 683 (1972). J. Thonstad, Can. J. Chem. 43, 3429 (1965). M. M. Vetyukov and V. B. Vinokurov, Fiz. Khim. Elektrokhim. Rasplau. Solei Shlakov, p. 367. Naukova Dumka, Kiev (1969). L. N. Antipin, Z. Fiz. Khim. 29, 1668 (1955). C. Kubik, K. Matiasovsky, M. Malinovsky and J. Zeman, Electrochim. Acta 9, 1521 (1964). C. Kubik and M. Malinovskv. Chem Zvestv_ 21., 920 (1967); 22, 34 (1968). -’ A. Silnv. M. Malinovskv and K. Matiasovskv._- Chem. &-sty-23, 561 (1969). R. Piontelli, B. Mazza and P. Pedeferri, Electrochim. Met. 1, 217 (1966). P. Drossbach, Z. Elektrochem. 42, 65 (1936). R. Piontelli and G. Montanelli, Alluminio 22, 699 (1953). R. Piontelli and G. Montanelli. Alluminio 25. 79 (19561. R. Piontelli, Metailurgia Ital. 52, 469 (196Oj. . ’ W. B. Frank and L. M. Foster, J. __ ~hvs. Chem. 61. 1531 (1957). A. Tual and M. Rolin, Electrochim. Acta 17, 1945 (1972). .I. Thonstad and S. Rolseth, EIectrochim. Acta 23, 231, (1977). R. Piontelli, B. Mazza and P. Pedeferri, Alluminio 34, 623 (1965). L. N. Lozhkin and A. P. Popov, .I. appl. Chem. USSR 41, 2277 (1968). L. N. Loibkin and A. P. Popov, Fir. Khim. Elektrokhim. Rasp,lav. Solei Shlakov, _ a. 303. Khimiva, _ Leningrad (1968). Yu. V. Borisoglebski, M. M. Vetyukov and V. S. Vinokurov. Tsvet. Met. 44. (101. 37 119731. M. Hansen; Constitution of &&y Alloys; 2nd supplement, (Edited by F. A. Shunk), p. _ 30. McGraw Hill. New York (1966). G. Armstrong and J. A. V. Butler, Trans. Faraday Sac. 29, 1261 (1933). B. E. Conway and P. L. Bourgault, Trans. Faraday Sot. 58. 593 (1962). D. C. &ahar&, .I: phys. Chem. 57, 257 (1953). W. T. Scott, J. them. Phvs. 23. 1936 (19551. H. Gerischer and M. Krause, Z. phyi. Ch&. N.F. 10, 264 (1957). H. Matsuda, A. Oka and P. Delahay, J. Am. them. Sot. 81, 5077 (1959). J. Thonstad, Electrochim. Acta 15, 1581 (1970). M. M. Vetyukov, D. S. Dyblin and Yu. V. Borisoglebski, Soui~t Electrochem. 8, 332 (1972). K. Griotheim. C. Krohn. K. Naeumann and K. TPrrklep, Met. ‘Trans. 1, 31j3 (1970). M. F. Doeramadzhi and S. E. Gefter. Sou. J. Non Ferrous Met. 6, (9), 54 (1965). T. Sele, Light Metals, Proceedings of the 106th AIME Annual Meeting, p, 7 Atlanta, (1977).