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The anodic behaviour of zinc in saturated solution of zinc sulphate
Elccrrochimfca Acta, 1972, Vol. 17.
pp. 99 to 105.
Pcrpamon Press. Printed in Northern Ireland
THE ANODIC BEHAVIOUR SATURATED SOLUTION SULPHATE” K.
SCHWABE and H.-B.
OF ZINC IN OF ZINC L&K
Institut fur Elektrochemie und Physikalische Chemie, Technische Universit& Dresden, Deutsche Demokratische Republik asZn and Y3 marked zinc sulphate, we have shown with a zinc rotating disk Abstract-Using electrode, that in saturated solution of zinc sulphate, after formation of a pore-free salt layer, the anodic reaction consists in the direct reaction of the metal with sulphate ion into solid salt. We determined the rate of exchange of zinc and sulphate ions between the saturated solution and the salt. From the cd after formation of the pore-free passive layer, and from resistance measurements, we deduce that above a very thin layer of anhydrous zinc sulphate there exists a layer of ZnS01.7 H40, which is O-6 to 1 x 10L times thicker. The specific resistance of the anhydrous salt at 25°C is pI = (2.4 f 15) x lOI1 Sl.cm and that of the ZnSOa.7H,0 layer pr = (4-O f l-1) X IO8a-cm. R&um&Employant zinc sulfate, mar@ de 6sZn et Y3 avec une electrode en forme de disque rotante dans une solution saturee de zinc sulfate, nous reuissions de montrer qu apres la formation d’une couche du se1 non poreuse, la reaction anodique consiste en transformation dire&e entre les metaux et l’ion sulfate en se1 solid. Nous avons determine la velocite d&change entre les ions de zinc et de sulfate en solution saturee et les ions en sel. Determinant le courant apres la formation de la couche passive non poreuse et la resistance, nous avons constatt qu’il existe une couche de ZnSO&. 7Hs0 dune epaisseur (0,6-l) 1 lo4 fois plus volumineuse qu’une petite couche au-dessous, consistant de zinc sulfate non aqueuse. La resistance specifique du se1 non aqueux, mesurke B 25°C est b = (2,4 31 1,5) - loll ohm.cm et celle-lzl de la couche de ZnS04.7Hs0 est pl = (4,0 & 1,l) log a.cm. einer rotierenden Zinkscheibenelektrode wurde unter Verwendung von 65Zn und a% markiertem Sulfat nachgewiesen, da0 in gesattigter Zinksulfatlosung nach Ausbildung einer porenfreien Satzschicht die Anodenreaktion in einer direkten Umsetzung des Metalls mit dem Sulfation zu festem Salz besteht. Dazu wurde die Austauschgeschwindigkeit zwischen Zn- und Sulfationen in der gesattigten Losung und denen im Salz bestimmt. Aus der Stromstarke nach Ausbildung der porenfreien Passivschicht und Widerstandsmessungen ergab sich, daB tiber einer sehr diinnen Schicht wasserfreien Zinksulfats eine (0,6 - 1) - lOa mal dickens Schicht von ZnSO,. 7H,O liegt. Der SpezifischeWiderstand des wasserfreienSalzes betragt bei 25°C pw = (2,4 f 1,5) * lOI1 Ohm.cm, der des ZnS04.7H40 pr = (4,0 f 1,l) * lo6 Ohm-cm.
Znsammenfassung-An
INTRODUCTION
IT HAS BEEN shown1 that with anodic dissolution of zinc in neutral or acid concentrated solutions of zinc sulphate, a salt layer is formed on the electrode, and this Iayer inhibits the anodic dissolution in a similar way as an oxide layer on aluminium. On zinc in acid solution of zinc sulphate the existence of an oxide passive layer was found to be impossible. z It was assumed that after the formation of a nearly pore-free salt layer the anodic reaction consists in direct transformation of the metal with the anions into solid salt,s Zn + S0,2- --f ZnSO,(s) + 2 e-. Investigating the anodic process on a zinc rotating disk electrode with radioactive zinc (6sZn) and S0,2(s5S) in saturated ZnSO, solution, we have succeeded in showing that after an anodic process of dissolution at the beginning, followed by precipitation of the salt from the supersaturated solution, a direct reaction of the metal with sulphate ion occurs. * Manuscript received 19 October 1970. 99
K.
loo
EXPERIMENTAL
SCHWABE
TECHNIQUE,
and H.-B.
L&K
RESULTS
AND
INTERPRETATION
1. Measuring cell and electrode Our measuring cell was a simple lo-ml beaker covered by a perforated plug to restrict the evaporation of water. The cell was placed in a thermostat. The volume of the exchange solution was 5 ml. The anode was a zinc rotating disk electrode, in order to show up any following diffusion process that might occur, and to eliminate its effect. The electrode was covered with a coat of epoxide resin. When operating in organic solvents, to study the migration of SOd2- in the layer, the epoxide resin was protected against the solution by a cover of Teflon. The horizontal disk cathode was placed between the anode and the solid salt phase on the bottom in order to protect the passive layer from mechanical loss. 2. Electrical
circuit
The anode was passivated by a cd of 60 mA/cm 2. After formation of the passive layer, the total voltage was kept constant at 30 V. The cd was registered by a shunted mV-compensating recorder. To determine specific resistances, tabloids of anhydrous zinc sulphate and of ZnS0,.7 H,O were contacted with mercury on both sides and the resistance was calculated from the voltage drop and the cd (Fig. 1).
I-
I
FIG. 1. Unit for measurement of resistance. Rx = resistance of test material, RN = measuring resistance, E = Vibron electrometer s coaxial cable.
Since the cd was about 10B9 A, the connexions were made of coaxial cables. The input resistance of the Vibron electrometer serving to measure the voltage drop was more than 1016 CL 3. Radiation
measurement
The impulse sequence of the test material containing ?S was fixed under a micaend-window-counter (mica-end-window 1.7 mg/cm”) and the activity of the test material, containing 65Zn, was measured with a NaI scintillation crystal.
The anodic behaviour of zinc in saturated solution of zinc sulphate
101
4. Measurement
of exchange rate of Zn2+ and SOp2- between solid salt and solution without polarization
The ion exchange between the rehydrated zinc-marked and sulphate-marked passive layer and a saturated zinc-sulphate solution was studied. Small amounts of the solution were taken off in definite intervals. Considering the measured impulse rate, the activity increase in the solution could be followed quantitatively. With variation of the rotation speed of the rotating disk electrode it was shown that the exchange rate is not influenced by a following diffusion process in the solution and the exchange is not simulated by a slow dissolution of the layer. The exchange rate of sulphate was found to be ~~o,~- = 3 x lo-* mol/min per cm2. The exchange rate in the investigated range of 200 min was independent of the layer thickness and from the exchange time. The exchange rate of zinc was inversely proportional to the square root of the exchange time and was independent of the thickness of the layer. vzn = a _ t-If2
with
1.8 . 1O-6 < a<
1 . 1O-5 mol/min1’2 per cm2.
We assume that the larger and time-dependent exchange rate of Zn2+ as compared with SOa2- is caused by a higher mobility in the crystal. The decrease with time occurs because after equilibrium between the surface of the salt and the solution is achieved, the supply of active Zn2+ ions from the interior of the crystal decreases. 5. Measurement
of exchange rate of Zn2+ and SOa2- with anodic polarization
With anodic polarization of the zinc electrode, no transference of active ions into the solution was observed. This behaviour is caused by the growth of a second inactive salt layer from the solution on the passive layer. This layer prevents exchange between the layer below and the solution. Anodic polarization causes additional zinc sulphate in form of a sulphate complex to arrive at the anodic surface by transference4 and here a thin supersaturated layer of the solution is produced. From this solution layer, continuously, depending on the cd, inactive zinc sulphate grows on the protective layer. This mechanism can be demonstrated experimentally. 6. Measurement
of charge transport in the zinc sulphate Iayer
It is interesting to investigate the mechanism of charge transfer in the layer. If pores are available ion transfer would be preferred, because of the low resistance of the pores. In the solid salt, transfer by defects may occur by S042- or Zn2+. This phenomenon was studied by the following method. 6.1. Participation of sulphate ions in charge transport in the layer. A radioactive layer was produced on an inactive sulphate layer on the zinc anode by polarization in 35S-marked zinc sulphate solution. After polarization, a solution of dithizone in a mixture of acetone-dimethylformamide (1: 1), was used to dissolve the complete layer in small portions. From counter rate measurements in the single portions, the proportion of active suIphate was found to be a function of the distance from the electrode surface. If no S042- participates in current transport in the layer, a sharp boundary would occur. The layer close to the metal surface would not show any
K.
102
SCHWABE and H.-B.
LiiCK
radioactivity, but the outer parts of the layer should indicate complete specific activity of the active salt until the beginning of the inactive layer. With charge transport by anions in the pores of the layer, however, close to the metal surface a higher activity than that in the middle of the layer should appear, since the pores would carry ions without separation of solid salt. If anions participate in charge transport by defect conduction in the solid material, a continuous drop of activity towards the metal Thickness
of layer
x
105,
mol
0
o-
Q-
o-
10 5
1
KP
0/
I IO
5 Thickness
of layer
x
IO’,
mm
Fxc. 2. Distribution of marked sulphate in the passive layer: Phase 1. Inactive sulphate Iayer grown directly on the zinc electrode. Phase 2. Radioactive marked (%) sulphate layer grown on Phase 1.
surface must be found. Our results (Fig. 2) exclude transport through the pores, but our method does not permit the unequivocal conclusion that there is defect conduction mechanism of SOa2-. In the dissohttion by the DMF-acetone-dithizone mixture it is possible that a displacement of marked SOa2- occurs and therefore a participation of SOa2- in defect transport is simulated: however, this transport does not seem probable in the case of ZnSO,. 6.2. Participation of Zn2+ in charge transport in the layer. After formation of a sulphate layer from a 65Zn marked zinc electrode, the decrease of solution activity can serve for deciding whether the dissolution of zinc to hydrated ions is followed by a secondary precipitation of solid salt from the solution (I), or whether direct formation of solid salt with the anions occurs (II), as we have postulated before: (I) Zn + x H,O G+ Zn2+(x H,O) -+ 2 e, Zn2+(x H,O) + SOa2- --f ZnSO, . 7 H,O(s); (II) Zn + SOa2- $
ZnSO,(s)
+ 2 e-.
The anodic behaviour of zinc in saturated solution of zinc sulphate
103
In (I), Zn2+(x H,O) migrates from the pores of the layer into the solution relatively by quickly, and forms a solid salt hydrate. But in (II), the formed salt is immediately built into the lattice of the layer and the Znw are able to penetrate into the solution only by much slower defect migration in the crystal lattice and by exchange with the ions of the solution. Therefore, in case (II) a much slower increase of the activity of the solution compared with the charge passed should be found than in case (I). However, at the beginning of the anodic charge (I) cannot be excluded, since at this moment no completely pore-free layer exists. The =Zn electrode was produced by electrolytic deposition of BSZn from the 6SZnS04 solution on our rotating disk electrode. The quantity of deposited Zn was always about one order larger than that necessary for the following anodic dissolution at 100 per cent current efficiency. Plotting the charge QL calculated from the radioactivity of the solution into which zinc has been dissolved, as a function of the time t, a proportionality with @/a is found. But when a definite time (tIE) is reached, no further activity increase in the solution occurs (Fig. 3). Plotting the complete charge passed Q, US the root of time (Fig. 4) two straight lines are found. But after
FIG. 3. QL 21stime.
I f f
Fro.
min”
4. Total amount of charge passed us time.
K.
104
SCHWABE
and H.-B. LGCK
the bend of the curve, the charge still increases essentially, although in the solution nearly no activity increase occurs. The charge resulting from the marked Zn2+ in the solution in the first part of the curve is not very reproducible, (increase aJ; it is 20-52 per cent of the charge passed in this time interval. The fluctuation is caused by the impossibility to obtain equal pore density. The remaining charge evidently, in the first part of the increase, is used to complete the layer until all pores are closed. The larger the number of pores and consequently a,, the larger should be the increase drop after all pores are closed. If the increase of the second straight line is signified a,, consequently, the increase of active zinc in the solution would be higher, the larger is (aI - aJ. was found to be
Indeed,
the increase
of the charge
b = 0*7(a, -
QL in the solution
with fi
a2).
Comparing Fig. 4 with Fig. 3 it was found that, even after the bend of the QG/P2 curve, some 65Zn occurs in the solution. Assuming that the discontinuous change of the time dependence of the charge is mostly caused by transition of charge transport through the pores to charge transport through the pore-free salt, it is possible to explain the activity increase found in the soIution even after the bend of the curve, by a small exchange of the active closed pores with the inactive solution. The increase of Q, without pores can be extrapolated from the increases of Q, and QL with fi, but this is not explained here. This extrapolated increase would appear if the condition after the bend of the curve existed from the beginning, ie on our assumption a pore-free layer. In this case, an increase of Qo should be found, immediately after the bend. By extrapolation, the increase of a layer, which a priori is pore-free, is found to be O-65 x lo-’ mol/min1/2. The increase after the bend has practically the same value (O-67 to 0.69) x lo--’ mol/mirP2. The cd, consequently, is inversely proportional to 4 and therefore behaves like diffusion limiting current in the non-stationary case. It seems possible to assume that the cd is determined by diffusion of the charge carrier in the salt layer, Therefore, the cd in presence of a pore-free layer reIated to the geometrical surface is
dQc --jict”dt
al
2 . t-li2
mol/min,
i w 2 x lO+ t-lf2 A. 7. The voltage dependence of the passive current In the passive state, it was noted that the passive current in a non-porous layer depends on the polarization voltage. When the voltage is varied very quickly, eg from 30-20 V, the passive cd decreases suddenly and then reaches a value below the cd at the beginning. The initial value, even after a long period, could not be attained again. Quantitative estimation showed that the resistance of the layer (Rp) changes with the square root of the polarization voltage, R, = bV% + RlO.
The anodic behaviourof zinc in saturatedsolution of zinc sulphate
105
RI0 is the resistance of the complete hydrated layer at 0 V. Since it was evident that Ohm’s law was valid, a simple relation for the extension of the anhydrous part in the layer could be obtained, using the above mentioned equation, x
w-
o-91 . q . u hv-
PJ.~’
where q is the geometrical surface (O-5 cm2), pw the specific resistance of the anhydrous part and p1 the specific resistance of the hydrated part. Since no direct determination of the resistances in the layer was possible, they were measured on compact samples. It was evident, however, that the conductivity in the layer could be compared with those in compact samples only within a definite limit. 8. Determination
of the specz$c resistance
of ZnS0,.7
H,O and ZnSO,
In producing tabloids of zinc sulphate heptahydrate, dehydration by pressure was avoided. The tabloid resistance was measured as mentioned above (Fig. 1). The specific resistance for zinc sulphate heptahydrate was calculated to be p1 = (4-O f
l-l)
x lo6 !Acm.
The determination of the resistance for anhydrous zinc sulphate requires the elimination of all possibility of humidity adsorption, since with surface effects a higher conductivity could be simulated. With careful attention to this, the specific resistance of the anhydrous zinc sulphate was determined as
pw = (2-4 rt: l-5) x 1011CLcm. 9. Estimation of the layer thickness thickness of the complete layer
of the anhydrous
saltproportionateIy
to the
The results of Section 7 show that 91 per cent of the total resistance in the layer (R,) is in the anhydrous layer (R,), R, This gives the proportion
= O-9&
of the anhydrous layer to the complete layer of 6000: 1, xg =
6000x,.
Thus the real passive layer is protected against the electrolyte solution by a hydrated protective layer with thickness 3-4 orders higher than the basis layer. These results co&m our former assumption, that the real passive layer in passivating salt layers is not identical with the thick visible layers, but is a very thin film with high resistance produced directly on the metal. REFERENCES 1. 2. 3. 4.
K. K. K. G.
SCHWABE, Habilitationsschrift,Dresden (1933).
SCHWABE and F. LQHMA~, Z. phys. Chem. 215,158 (1960/61). SCHWABE, Electrochim. Acfu 3, 186 (1960); Werkst. Kerr. 18,961 M. WOLTEN and C. V. KINK, J. Am. them. Sot. 71,576 (1949).
(1967).
pp. 99 to 105.
Pcrpamon Press. Printed in Northern Ireland
THE ANODIC BEHAVIOUR SATURATED SOLUTION SULPHATE” K.
SCHWABE and H.-B.
OF ZINC IN OF ZINC L&K
Institut fur Elektrochemie und Physikalische Chemie, Technische Universit& Dresden, Deutsche Demokratische Republik asZn and Y3 marked zinc sulphate, we have shown with a zinc rotating disk Abstract-Using electrode, that in saturated solution of zinc sulphate, after formation of a pore-free salt layer, the anodic reaction consists in the direct reaction of the metal with sulphate ion into solid salt. We determined the rate of exchange of zinc and sulphate ions between the saturated solution and the salt. From the cd after formation of the pore-free passive layer, and from resistance measurements, we deduce that above a very thin layer of anhydrous zinc sulphate there exists a layer of ZnS01.7 H40, which is O-6 to 1 x 10L times thicker. The specific resistance of the anhydrous salt at 25°C is pI = (2.4 f 15) x lOI1 Sl.cm and that of the ZnSOa.7H,0 layer pr = (4-O f l-1) X IO8a-cm. R&um&Employant zinc sulfate, mar@ de 6sZn et Y3 avec une electrode en forme de disque rotante dans une solution saturee de zinc sulfate, nous reuissions de montrer qu apres la formation d’une couche du se1 non poreuse, la reaction anodique consiste en transformation dire&e entre les metaux et l’ion sulfate en se1 solid. Nous avons determine la velocite d&change entre les ions de zinc et de sulfate en solution saturee et les ions en sel. Determinant le courant apres la formation de la couche passive non poreuse et la resistance, nous avons constatt qu’il existe une couche de ZnSO&. 7Hs0 dune epaisseur (0,6-l) 1 lo4 fois plus volumineuse qu’une petite couche au-dessous, consistant de zinc sulfate non aqueuse. La resistance specifique du se1 non aqueux, mesurke B 25°C est b = (2,4 31 1,5) - loll ohm.cm et celle-lzl de la couche de ZnS04.7Hs0 est pl = (4,0 & 1,l) log a.cm. einer rotierenden Zinkscheibenelektrode wurde unter Verwendung von 65Zn und a% markiertem Sulfat nachgewiesen, da0 in gesattigter Zinksulfatlosung nach Ausbildung einer porenfreien Satzschicht die Anodenreaktion in einer direkten Umsetzung des Metalls mit dem Sulfation zu festem Salz besteht. Dazu wurde die Austauschgeschwindigkeit zwischen Zn- und Sulfationen in der gesattigten Losung und denen im Salz bestimmt. Aus der Stromstarke nach Ausbildung der porenfreien Passivschicht und Widerstandsmessungen ergab sich, daB tiber einer sehr diinnen Schicht wasserfreien Zinksulfats eine (0,6 - 1) - lOa mal dickens Schicht von ZnSO,. 7H,O liegt. Der SpezifischeWiderstand des wasserfreienSalzes betragt bei 25°C pw = (2,4 f 1,5) * lOI1 Ohm.cm, der des ZnS04.7H40 pr = (4,0 f 1,l) * lo6 Ohm-cm.
Znsammenfassung-An
INTRODUCTION
IT HAS BEEN shown1 that with anodic dissolution of zinc in neutral or acid concentrated solutions of zinc sulphate, a salt layer is formed on the electrode, and this Iayer inhibits the anodic dissolution in a similar way as an oxide layer on aluminium. On zinc in acid solution of zinc sulphate the existence of an oxide passive layer was found to be impossible. z It was assumed that after the formation of a nearly pore-free salt layer the anodic reaction consists in direct transformation of the metal with the anions into solid salt,s Zn + S0,2- --f ZnSO,(s) + 2 e-. Investigating the anodic process on a zinc rotating disk electrode with radioactive zinc (6sZn) and S0,2(s5S) in saturated ZnSO, solution, we have succeeded in showing that after an anodic process of dissolution at the beginning, followed by precipitation of the salt from the supersaturated solution, a direct reaction of the metal with sulphate ion occurs. * Manuscript received 19 October 1970. 99
K.
loo
EXPERIMENTAL
SCHWABE
TECHNIQUE,
and H.-B.
L&K
RESULTS
AND
INTERPRETATION
1. Measuring cell and electrode Our measuring cell was a simple lo-ml beaker covered by a perforated plug to restrict the evaporation of water. The cell was placed in a thermostat. The volume of the exchange solution was 5 ml. The anode was a zinc rotating disk electrode, in order to show up any following diffusion process that might occur, and to eliminate its effect. The electrode was covered with a coat of epoxide resin. When operating in organic solvents, to study the migration of SOd2- in the layer, the epoxide resin was protected against the solution by a cover of Teflon. The horizontal disk cathode was placed between the anode and the solid salt phase on the bottom in order to protect the passive layer from mechanical loss. 2. Electrical
circuit
The anode was passivated by a cd of 60 mA/cm 2. After formation of the passive layer, the total voltage was kept constant at 30 V. The cd was registered by a shunted mV-compensating recorder. To determine specific resistances, tabloids of anhydrous zinc sulphate and of ZnS0,.7 H,O were contacted with mercury on both sides and the resistance was calculated from the voltage drop and the cd (Fig. 1).
I-
I
FIG. 1. Unit for measurement of resistance. Rx = resistance of test material, RN = measuring resistance, E = Vibron electrometer s coaxial cable.
Since the cd was about 10B9 A, the connexions were made of coaxial cables. The input resistance of the Vibron electrometer serving to measure the voltage drop was more than 1016 CL 3. Radiation
measurement
The impulse sequence of the test material containing ?S was fixed under a micaend-window-counter (mica-end-window 1.7 mg/cm”) and the activity of the test material, containing 65Zn, was measured with a NaI scintillation crystal.
The anodic behaviour of zinc in saturated solution of zinc sulphate
101
4. Measurement
of exchange rate of Zn2+ and SOp2- between solid salt and solution without polarization
The ion exchange between the rehydrated zinc-marked and sulphate-marked passive layer and a saturated zinc-sulphate solution was studied. Small amounts of the solution were taken off in definite intervals. Considering the measured impulse rate, the activity increase in the solution could be followed quantitatively. With variation of the rotation speed of the rotating disk electrode it was shown that the exchange rate is not influenced by a following diffusion process in the solution and the exchange is not simulated by a slow dissolution of the layer. The exchange rate of sulphate was found to be ~~o,~- = 3 x lo-* mol/min per cm2. The exchange rate in the investigated range of 200 min was independent of the layer thickness and from the exchange time. The exchange rate of zinc was inversely proportional to the square root of the exchange time and was independent of the thickness of the layer. vzn = a _ t-If2
with
1.8 . 1O-6 < a<
1 . 1O-5 mol/min1’2 per cm2.
We assume that the larger and time-dependent exchange rate of Zn2+ as compared with SOa2- is caused by a higher mobility in the crystal. The decrease with time occurs because after equilibrium between the surface of the salt and the solution is achieved, the supply of active Zn2+ ions from the interior of the crystal decreases. 5. Measurement
of exchange rate of Zn2+ and SOa2- with anodic polarization
With anodic polarization of the zinc electrode, no transference of active ions into the solution was observed. This behaviour is caused by the growth of a second inactive salt layer from the solution on the passive layer. This layer prevents exchange between the layer below and the solution. Anodic polarization causes additional zinc sulphate in form of a sulphate complex to arrive at the anodic surface by transference4 and here a thin supersaturated layer of the solution is produced. From this solution layer, continuously, depending on the cd, inactive zinc sulphate grows on the protective layer. This mechanism can be demonstrated experimentally. 6. Measurement
of charge transport in the zinc sulphate Iayer
It is interesting to investigate the mechanism of charge transfer in the layer. If pores are available ion transfer would be preferred, because of the low resistance of the pores. In the solid salt, transfer by defects may occur by S042- or Zn2+. This phenomenon was studied by the following method. 6.1. Participation of sulphate ions in charge transport in the layer. A radioactive layer was produced on an inactive sulphate layer on the zinc anode by polarization in 35S-marked zinc sulphate solution. After polarization, a solution of dithizone in a mixture of acetone-dimethylformamide (1: 1), was used to dissolve the complete layer in small portions. From counter rate measurements in the single portions, the proportion of active suIphate was found to be a function of the distance from the electrode surface. If no S042- participates in current transport in the layer, a sharp boundary would occur. The layer close to the metal surface would not show any
K.
102
SCHWABE and H.-B.
LiiCK
radioactivity, but the outer parts of the layer should indicate complete specific activity of the active salt until the beginning of the inactive layer. With charge transport by anions in the pores of the layer, however, close to the metal surface a higher activity than that in the middle of the layer should appear, since the pores would carry ions without separation of solid salt. If anions participate in charge transport by defect conduction in the solid material, a continuous drop of activity towards the metal Thickness
of layer
x
105,
mol
0
o-
Q-
o-
10 5
1
KP
0/
I IO
5 Thickness
of layer
x
IO’,
mm
Fxc. 2. Distribution of marked sulphate in the passive layer: Phase 1. Inactive sulphate Iayer grown directly on the zinc electrode. Phase 2. Radioactive marked (%) sulphate layer grown on Phase 1.
surface must be found. Our results (Fig. 2) exclude transport through the pores, but our method does not permit the unequivocal conclusion that there is defect conduction mechanism of SOa2-. In the dissohttion by the DMF-acetone-dithizone mixture it is possible that a displacement of marked SOa2- occurs and therefore a participation of SOa2- in defect transport is simulated: however, this transport does not seem probable in the case of ZnSO,. 6.2. Participation of Zn2+ in charge transport in the layer. After formation of a sulphate layer from a 65Zn marked zinc electrode, the decrease of solution activity can serve for deciding whether the dissolution of zinc to hydrated ions is followed by a secondary precipitation of solid salt from the solution (I), or whether direct formation of solid salt with the anions occurs (II), as we have postulated before: (I) Zn + x H,O G+ Zn2+(x H,O) -+ 2 e, Zn2+(x H,O) + SOa2- --f ZnSO, . 7 H,O(s); (II) Zn + SOa2- $
ZnSO,(s)
+ 2 e-.
The anodic behaviour of zinc in saturated solution of zinc sulphate
103
In (I), Zn2+(x H,O) migrates from the pores of the layer into the solution relatively by quickly, and forms a solid salt hydrate. But in (II), the formed salt is immediately built into the lattice of the layer and the Znw are able to penetrate into the solution only by much slower defect migration in the crystal lattice and by exchange with the ions of the solution. Therefore, in case (II) a much slower increase of the activity of the solution compared with the charge passed should be found than in case (I). However, at the beginning of the anodic charge (I) cannot be excluded, since at this moment no completely pore-free layer exists. The =Zn electrode was produced by electrolytic deposition of BSZn from the 6SZnS04 solution on our rotating disk electrode. The quantity of deposited Zn was always about one order larger than that necessary for the following anodic dissolution at 100 per cent current efficiency. Plotting the charge QL calculated from the radioactivity of the solution into which zinc has been dissolved, as a function of the time t, a proportionality with @/a is found. But when a definite time (tIE) is reached, no further activity increase in the solution occurs (Fig. 3). Plotting the complete charge passed Q, US the root of time (Fig. 4) two straight lines are found. But after
FIG. 3. QL 21stime.
I f f
Fro.
min”
4. Total amount of charge passed us time.
K.
104
SCHWABE
and H.-B. LGCK
the bend of the curve, the charge still increases essentially, although in the solution nearly no activity increase occurs. The charge resulting from the marked Zn2+ in the solution in the first part of the curve is not very reproducible, (increase aJ; it is 20-52 per cent of the charge passed in this time interval. The fluctuation is caused by the impossibility to obtain equal pore density. The remaining charge evidently, in the first part of the increase, is used to complete the layer until all pores are closed. The larger the number of pores and consequently a,, the larger should be the increase drop after all pores are closed. If the increase of the second straight line is signified a,, consequently, the increase of active zinc in the solution would be higher, the larger is (aI - aJ. was found to be
Indeed,
the increase
of the charge
b = 0*7(a, -
QL in the solution
with fi
a2).
Comparing Fig. 4 with Fig. 3 it was found that, even after the bend of the QG/P2 curve, some 65Zn occurs in the solution. Assuming that the discontinuous change of the time dependence of the charge is mostly caused by transition of charge transport through the pores to charge transport through the pore-free salt, it is possible to explain the activity increase found in the soIution even after the bend of the curve, by a small exchange of the active closed pores with the inactive solution. The increase of Q, without pores can be extrapolated from the increases of Q, and QL with fi, but this is not explained here. This extrapolated increase would appear if the condition after the bend of the curve existed from the beginning, ie on our assumption a pore-free layer. In this case, an increase of Qo should be found, immediately after the bend. By extrapolation, the increase of a layer, which a priori is pore-free, is found to be O-65 x lo-’ mol/min1/2. The increase after the bend has practically the same value (O-67 to 0.69) x lo--’ mol/mirP2. The cd, consequently, is inversely proportional to 4 and therefore behaves like diffusion limiting current in the non-stationary case. It seems possible to assume that the cd is determined by diffusion of the charge carrier in the salt layer, Therefore, the cd in presence of a pore-free layer reIated to the geometrical surface is
dQc --jict”dt
al
2 . t-li2
mol/min,
i w 2 x lO+ t-lf2 A. 7. The voltage dependence of the passive current In the passive state, it was noted that the passive current in a non-porous layer depends on the polarization voltage. When the voltage is varied very quickly, eg from 30-20 V, the passive cd decreases suddenly and then reaches a value below the cd at the beginning. The initial value, even after a long period, could not be attained again. Quantitative estimation showed that the resistance of the layer (Rp) changes with the square root of the polarization voltage, R, = bV% + RlO.
The anodic behaviourof zinc in saturatedsolution of zinc sulphate
105
RI0 is the resistance of the complete hydrated layer at 0 V. Since it was evident that Ohm’s law was valid, a simple relation for the extension of the anhydrous part in the layer could be obtained, using the above mentioned equation, x
w-
o-91 . q . u hv-
PJ.~’
where q is the geometrical surface (O-5 cm2), pw the specific resistance of the anhydrous part and p1 the specific resistance of the hydrated part. Since no direct determination of the resistances in the layer was possible, they were measured on compact samples. It was evident, however, that the conductivity in the layer could be compared with those in compact samples only within a definite limit. 8. Determination
of the specz$c resistance
of ZnS0,.7
H,O and ZnSO,
In producing tabloids of zinc sulphate heptahydrate, dehydration by pressure was avoided. The tabloid resistance was measured as mentioned above (Fig. 1). The specific resistance for zinc sulphate heptahydrate was calculated to be p1 = (4-O f
l-l)
x lo6 !Acm.
The determination of the resistance for anhydrous zinc sulphate requires the elimination of all possibility of humidity adsorption, since with surface effects a higher conductivity could be simulated. With careful attention to this, the specific resistance of the anhydrous zinc sulphate was determined as
pw = (2-4 rt: l-5) x 1011CLcm. 9. Estimation of the layer thickness thickness of the complete layer
of the anhydrous
saltproportionateIy
to the
The results of Section 7 show that 91 per cent of the total resistance in the layer (R,) is in the anhydrous layer (R,), R, This gives the proportion
= O-9&
of the anhydrous layer to the complete layer of 6000: 1, xg =
6000x,.
Thus the real passive layer is protected against the electrolyte solution by a hydrated protective layer with thickness 3-4 orders higher than the basis layer. These results co&m our former assumption, that the real passive layer in passivating salt layers is not identical with the thick visible layers, but is a very thin film with high resistance produced directly on the metal. REFERENCES 1. 2. 3. 4.
K. K. K. G.
SCHWABE, Habilitationsschrift,Dresden (1933).
SCHWABE and F. LQHMA~, Z. phys. Chem. 215,158 (1960/61). SCHWABE, Electrochim. Acfu 3, 186 (1960); Werkst. Kerr. 18,961 M. WOLTEN and C. V. KINK, J. Am. them. Sot. 71,576 (1949).
(1967).