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Influence of chloride ions on the formation of interphase layers in weak acid zinc electrolytes
Influence of Chloride Ions on the Formation of Interphase Layers in Weak Acid Zinc Electrolytes by Larisa Grincevichene and Romanas Vishomirskis, Institute of Chemistry, Vilnius, Lithuania, and Saba Jakobson and David Crotty, MacDermid Inc., Waterbury, Conn.
I
t was determined’-” that in diluted acid zinc electrolytes the rate of zinc electrodeposition depends mainly on the interphase layers that are formed on a nonpolarized electrode due to interaction of the zinc cathode with the solution and electrolysis products. In order to check this proposition it is useful to investigate the role of interphase layers in more concentrated solutions.
EXPERIMENTAL The investigation was carried out in a solution containing 0.25 M ZnCl, and 0.50 M NaCl, pH = 4.8. The characterization of cathode polarization was carried out using a potentiodynamic procedure with a potentiostat and electrochemical cell coupled with a recording potentiometer. The sweep rate (S) was from 2 X IO-’ to 2 X 10-l V/set. The cathodes and anodes were constructed of pure zinc (99.99+%) with an exact cathode area of 0.2 cm* defined by masking with Teflon. The anode area was calculated to be approximately 10 cm*. The cathode potential was measured against a saturated silver chloride electrode, and the values were recalculated on the hydrogen scale. A thermostat was used to maintain the 20-6O”C temperature. Prior to each potentiostatic run and in order to insure that the cathode surface was chemically clean, the cathode surface was lightly scrubbed with Vienna soda (CaO/MgO = l/l) and rinsed in deionized water. Using the potentiodynamic curves, the kinetic parameters for transfer coefficient and constant rate were measured. The transfer coefficient was calculated using the following equation: E PI* - E, = 0.046/o n,
(1)
where METAL FINISHING
. AUGUST
1997
E, = Peak potential E,, = Half peak potential (Y = Transfer coefficient na = 2 Peak current (ip) for a nonreversible process was calculated using the following equation:4 i, = 2.99 x IO5 nA(ar~,J’~ CDm SIR (2) where na = 2, (;Y= 0.32 for chloride ions5 D = 7.23 X 10e6 cm2/sec for [Zn2’] ions6 A = 0.2 cm2, C = 0.25 X 10e3 M and 0.75 X lo-’ M for concentrated solution Current peak for a reversible process was calculated using the following equation:4 i, = 3.70 X lo5 X n3’2ACD”2Si’2 (3)
where n=2 A = 0.2 cm2 C = 0.25 X 1O-3 M and 0.75 X 10e3 M for concentrated solution The diffusion overpotential was calculated using the following equation:7 n, = (RT/nF)ln [ 1 -i/i,]
(4)
where i = Cathode current density i, = Limiting current The rate constant (K,) was calculated using the following equation: E, = Eo - (RT/on,F) (0.78-lnK, + In VDB)
(5)
where E, = Standard potential of system (Zn/ Zn2+) B = an,FV/RT The influence of ohmic resistance on the potential was determined using the procedure worked out by Mechinskas et al.* The same procedure was 0 Copyright Elsevier Science Inc.
used measuring the capacity of the double layer of zinc electrodes. The authors are grateful to P. Mechinskas for consulting while measuring and interpreting the results. The measured ohmic resistance was 0.6 f 0.1 ohm/ cm*. Calculated on this basis, the growth of polarization in the range of used current densities is 12 f 3%. The capacity of the double layer for nonpolarized electrode was 15 f 5 pF/cm* and during the electrolysis 30 f 10 pF/cm2. The rise of capacity during the electrolysis may be caused by the formation of interphase layers on the cathode surface.
RESULTS AND DISCUSSION The rest potential of Zn/Zn*+ in diluted solutions is almost equilibrium.3,2 With the rise of zinc concentration the differences between the equilibrium and rest potentials increase. The main reason-zinc corrosion in acid solutions-is hydrogen and oxygen depolarization. The rest potential of zinc in 0.25 M ZnCl, + 0.5 M NaCl is -0.78 V, which is close to equilibrium potential.2 When a zinc electrode is polarized, an essential inhibition of 0.07 V in the initial stage is observed (Fig. 1, Curve 1). If the polarization is measured a second time without first withdrawing the cathode from the electrolyte, the cathode polarization is lower, especially in the initial stage (Fig. 2, Curve 1). At high current density the difference is only 0.01 V (Figs. 1 and 2, Curve 1). Figures 1 and 2 show that i, decreases when the sweep rate decreases. The results obtained show that the decrease of polarization measured the second time without withdrawing the cathode from the electrolyte is not con41
“6P 40 3020
10
0.1
0.3
v
Figure 1. Potentiodynamiccurrent density/ potential (&/A&) curves during the deposition of zinc on a mechanicallypolished electrode from the electrolyte containing 0.25 M ZnCi, + 0.5 M NaCi at various sweep rates (Wsec)(1: 2 x 10-l; 2: 1 x 10-l; 3: 5 x lo-2; 4: 2 x lo-2; 5: 1 x 10p2; 6: 1 x 10e3; 7: theoretical curve, calculated according to Equation 4). netted with the change of electrode surface; because when the sweep rate
-5 3 40
30 20 10
0.1
0.3
V
Figure 2. Potentiodynamiccurrent density/ potential (i&%&J curves during the deposition of zinc on a mechanically polished electrode plated the second time without withdrawing the cathode from the eiectroiyte containing 0.25M &Cl, + 0.5 M NaCiat various sweep rates (l-6 are the same as in Fig. 1). 42
of potential is high, the magnitude of i, is almost the same (Figs. 1 and 2, Curve 1). It is possible to suggest that this difference in polarization in initial stages is due to the condition of the electrode surface. This proposition can be confirmed by experiments with zinc electrodes coated additionally from acid zinc electrolyte. In this case no increase in polarization in the initial stages in observed and the polarization curve is the same, as in the case when a polarization is measured the second time on mechanically polished electrode without withdrawing. If the polarization is measured at the lowest sweep rate of potential (Fig. 1, Curve 5 and 6) in the area of i,, the active surface increases 15% (Fig. 2, Curves 5 and 6). The measured polarization is essentially higher than diffusion polarization, calculated according to Equation 4 (Fig. 1, Curve 7). At 40°C the rate of cathode process is higher in the i, area and almost unchanged in the low current density area. When the sweep rate is very low an essential growth of cathode surface is observed and it becomes impossible to fix the i, (Fig. 3). At 60°C the depolarization is low and a further rise of it is observed (Fig. 4). The analysis of temperature influence on the cathode process is characterized by temperature coefficient (K,),2 which shows a very high magnitude at A+ = 0.1 V (20-40°C). At higher A+, the K, decreases and gets the numbers that are common for diffusion limitations (Fig. 5). When the temperature rises from 40 to 60°C K, decreases till A+ = 0.1 V and there is a slight increase in the i, area. The magnitude and the character of K, change on the cathode potential suggest that the rise of temperature not ‘only increases the rate of zinc electrodeposition but has an influence on the conditions of the surface of the cathode. When zinc concentration is three times higher, the magnitude of i, at the highest sweep rate increases 2.5 times and at the lowest sweep rate 4-5 times (Fig. 6). This phenomenon can be stipulated by the growth of the active cathode surface in the i, area. The common polarization is almost independent of zinc concentration (Figs. 1 and 6). When sodium ions are replaced by
60 50 40 30 20 10
0.1
0.3
V
Figure 3. Potentiodynamiccurrent density/ potential (&/A&) curves during the deposition of zinc from the electrolyte,containing 0.25 M ZnCi, + 0.5 54NaCi at various sweep rates (l-6 are the same as in Fig. 1, t q 4O'C). potassium or ammonium ions, the i, and i, are slightly increased (Figs. 7 and S), and potassium ions are more effective. In concentrated solutions (Fig. 9) ammonium ions increase i, and i, essentially. On the basis of polarization measurements the dependence of i, from the sweep rate was established (Fig. 10). The experimental curve (Fig. 10, Curve 3) does not begin from the initial coordinate, and the angle is essen
Figure 4. Potentiodynamiccurrent density/ potential (QA+,J curves during the deposition of zinc from the electrolyte,containing 0.25 M ZnCi, + 0.5 M NaCi at various sweep rates (l-6 are the same as in Figure 1, t = 60%). METAL FINISHING
l
AUGUST 1997
I
t was determined’-” that in diluted acid zinc electrolytes the rate of zinc electrodeposition depends mainly on the interphase layers that are formed on a nonpolarized electrode due to interaction of the zinc cathode with the solution and electrolysis products. In order to check this proposition it is useful to investigate the role of interphase layers in more concentrated solutions.
EXPERIMENTAL The investigation was carried out in a solution containing 0.25 M ZnCl, and 0.50 M NaCl, pH = 4.8. The characterization of cathode polarization was carried out using a potentiodynamic procedure with a potentiostat and electrochemical cell coupled with a recording potentiometer. The sweep rate (S) was from 2 X IO-’ to 2 X 10-l V/set. The cathodes and anodes were constructed of pure zinc (99.99+%) with an exact cathode area of 0.2 cm* defined by masking with Teflon. The anode area was calculated to be approximately 10 cm*. The cathode potential was measured against a saturated silver chloride electrode, and the values were recalculated on the hydrogen scale. A thermostat was used to maintain the 20-6O”C temperature. Prior to each potentiostatic run and in order to insure that the cathode surface was chemically clean, the cathode surface was lightly scrubbed with Vienna soda (CaO/MgO = l/l) and rinsed in deionized water. Using the potentiodynamic curves, the kinetic parameters for transfer coefficient and constant rate were measured. The transfer coefficient was calculated using the following equation: E PI* - E, = 0.046/o n,
(1)
where METAL FINISHING
. AUGUST
1997
E, = Peak potential E,, = Half peak potential (Y = Transfer coefficient na = 2 Peak current (ip) for a nonreversible process was calculated using the following equation:4 i, = 2.99 x IO5 nA(ar~,J’~ CDm SIR (2) where na = 2, (;Y= 0.32 for chloride ions5 D = 7.23 X 10e6 cm2/sec for [Zn2’] ions6 A = 0.2 cm2, C = 0.25 X 10e3 M and 0.75 X lo-’ M for concentrated solution Current peak for a reversible process was calculated using the following equation:4 i, = 3.70 X lo5 X n3’2ACD”2Si’2 (3)
where n=2 A = 0.2 cm2 C = 0.25 X 1O-3 M and 0.75 X 10e3 M for concentrated solution The diffusion overpotential was calculated using the following equation:7 n, = (RT/nF)ln [ 1 -i/i,]
(4)
where i = Cathode current density i, = Limiting current The rate constant (K,) was calculated using the following equation: E, = Eo - (RT/on,F) (0.78-lnK, + In VDB)
(5)
where E, = Standard potential of system (Zn/ Zn2+) B = an,FV/RT The influence of ohmic resistance on the potential was determined using the procedure worked out by Mechinskas et al.* The same procedure was 0 Copyright Elsevier Science Inc.
used measuring the capacity of the double layer of zinc electrodes. The authors are grateful to P. Mechinskas for consulting while measuring and interpreting the results. The measured ohmic resistance was 0.6 f 0.1 ohm/ cm*. Calculated on this basis, the growth of polarization in the range of used current densities is 12 f 3%. The capacity of the double layer for nonpolarized electrode was 15 f 5 pF/cm* and during the electrolysis 30 f 10 pF/cm2. The rise of capacity during the electrolysis may be caused by the formation of interphase layers on the cathode surface.
RESULTS AND DISCUSSION The rest potential of Zn/Zn*+ in diluted solutions is almost equilibrium.3,2 With the rise of zinc concentration the differences between the equilibrium and rest potentials increase. The main reason-zinc corrosion in acid solutions-is hydrogen and oxygen depolarization. The rest potential of zinc in 0.25 M ZnCl, + 0.5 M NaCl is -0.78 V, which is close to equilibrium potential.2 When a zinc electrode is polarized, an essential inhibition of 0.07 V in the initial stage is observed (Fig. 1, Curve 1). If the polarization is measured a second time without first withdrawing the cathode from the electrolyte, the cathode polarization is lower, especially in the initial stage (Fig. 2, Curve 1). At high current density the difference is only 0.01 V (Figs. 1 and 2, Curve 1). Figures 1 and 2 show that i, decreases when the sweep rate decreases. The results obtained show that the decrease of polarization measured the second time without withdrawing the cathode from the electrolyte is not con41
“6P 40 3020
10
0.1
0.3
v
Figure 1. Potentiodynamiccurrent density/ potential (&/A&) curves during the deposition of zinc on a mechanicallypolished electrode from the electrolyte containing 0.25 M ZnCi, + 0.5 M NaCi at various sweep rates (Wsec)(1: 2 x 10-l; 2: 1 x 10-l; 3: 5 x lo-2; 4: 2 x lo-2; 5: 1 x 10p2; 6: 1 x 10e3; 7: theoretical curve, calculated according to Equation 4). netted with the change of electrode surface; because when the sweep rate
-5 3 40
30 20 10
0.1
0.3
V
Figure 2. Potentiodynamiccurrent density/ potential (i&%&J curves during the deposition of zinc on a mechanically polished electrode plated the second time without withdrawing the cathode from the eiectroiyte containing 0.25M &Cl, + 0.5 M NaCiat various sweep rates (l-6 are the same as in Fig. 1). 42
of potential is high, the magnitude of i, is almost the same (Figs. 1 and 2, Curve 1). It is possible to suggest that this difference in polarization in initial stages is due to the condition of the electrode surface. This proposition can be confirmed by experiments with zinc electrodes coated additionally from acid zinc electrolyte. In this case no increase in polarization in the initial stages in observed and the polarization curve is the same, as in the case when a polarization is measured the second time on mechanically polished electrode without withdrawing. If the polarization is measured at the lowest sweep rate of potential (Fig. 1, Curve 5 and 6) in the area of i,, the active surface increases 15% (Fig. 2, Curves 5 and 6). The measured polarization is essentially higher than diffusion polarization, calculated according to Equation 4 (Fig. 1, Curve 7). At 40°C the rate of cathode process is higher in the i, area and almost unchanged in the low current density area. When the sweep rate is very low an essential growth of cathode surface is observed and it becomes impossible to fix the i, (Fig. 3). At 60°C the depolarization is low and a further rise of it is observed (Fig. 4). The analysis of temperature influence on the cathode process is characterized by temperature coefficient (K,),2 which shows a very high magnitude at A+ = 0.1 V (20-40°C). At higher A+, the K, decreases and gets the numbers that are common for diffusion limitations (Fig. 5). When the temperature rises from 40 to 60°C K, decreases till A+ = 0.1 V and there is a slight increase in the i, area. The magnitude and the character of K, change on the cathode potential suggest that the rise of temperature not ‘only increases the rate of zinc electrodeposition but has an influence on the conditions of the surface of the cathode. When zinc concentration is three times higher, the magnitude of i, at the highest sweep rate increases 2.5 times and at the lowest sweep rate 4-5 times (Fig. 6). This phenomenon can be stipulated by the growth of the active cathode surface in the i, area. The common polarization is almost independent of zinc concentration (Figs. 1 and 6). When sodium ions are replaced by
60 50 40 30 20 10
0.1
0.3
V
Figure 3. Potentiodynamiccurrent density/ potential (&/A&) curves during the deposition of zinc from the electrolyte,containing 0.25 M ZnCi, + 0.5 54NaCi at various sweep rates (l-6 are the same as in Fig. 1, t q 4O'C). potassium or ammonium ions, the i, and i, are slightly increased (Figs. 7 and S), and potassium ions are more effective. In concentrated solutions (Fig. 9) ammonium ions increase i, and i, essentially. On the basis of polarization measurements the dependence of i, from the sweep rate was established (Fig. 10). The experimental curve (Fig. 10, Curve 3) does not begin from the initial coordinate, and the angle is essen
Figure 4. Potentiodynamiccurrent density/ potential (QA+,J curves during the deposition of zinc from the electrolyte,containing 0.25 M ZnCi, + 0.5 M NaCi at various sweep rates (l-6 are the same as in Figure 1, t = 60%). METAL FINISHING
l
AUGUST 1997