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Cathodic reduction of oxygen on copper and brass
EkcImchinics Acta. Vol 24. pp. 131-I 38 8 PergnmonPressLtd. 1979. Printed in Gnat
CATHODIC
Britain.
REDUCTION OF OXYGEN AND BRASS* K. BALAKRISHNAN
Central Electrochemical
ON COPPER
and V. K. VENKATE~AN
Research Institute, Karaikudi,
India
(Received 21 October 1977 ; and in revised form 4 July 1978) Abstracl - In this investigation, the electrochemical reduction of oxygen on copper and brass has been studied using the ring-disc electrode technique in solutions containing chloride, sulphatc and nitrate anions
and ammonium cation The E-I curves obtained in the forward and reverse direction of polarization for copper and brass in NaCL, Na2SOI and NH*CI solutions are similar in nature and show two waves. However in (NH,),SO, solution two waves are not observed in the reverse direction. The rate of oxygen reduction is the highest in ammonium sulphate solution for both copper and brass whereas it is the lowest in NH&Cl in the case of copper. In Na$O, and NaCL solutions, the rate of oxygen reduction is higher on copper than on brass. Apparent Tafel slopes for oxygeu reduction obtained for copper and brass vary from 65 mV to 240 mV depending upon the medium. The two steps observed with copper disc electrode have been identified as due to the mduction of oxygen to hydrogen peroxide and reduction of H 202 to OH - ion or water depending upon the pH of the solution. In acid chloride and sulphate media no H,Oz was detected, which suggests direct reduction to H,O. The diagnostic plots of1,{1, us w- “’ employed by Bock& et al indicate that in NaTSO* the reduction ofoxygen to H20z takes place in a parallel reaction whereas in (NH,),SO, both direct reduction of O2 to water (or OH- ion) and the reduction through intermediate H,Oz occur.
INTRODUCTION
Oxygen
reduction assumes importance processes since in neutral solutions
in
cor-
and in the absence of any other depolarizer the corrosion rate is controlled by the kinetics of oxygen reduction reaction. While the oxygen reduction has been extensively studied on noble metals and a few active metals such as nickel and silver with the help of rotating-ring disc electrode[1,2], no such detailed study seems to have been carried out on copper and brass. In the present study some aspects of oxygen reduction on copper and brass in solutions containing different anions have been examined using the rotating ring-disc electrode in order to determine (i) whether the anions, in addition to affecting the dissolution reactions, also affect the cathodic reduction of oxygen, and (ii) whether the reduction passes through the formation of peroxides. rosion
the working electrode. The forward scan was thereby started at -400mV and ended at - 1350 mV. The potentials at which anodic dissolution starts in different solutions are given in Table 1. aEsULTs
reduction General characteristics ofE-i CIITU~S. The E-i curves for oxygen reduction in copper and brass in sodium chloride, sodium sulfate, ammonium chloride and ammonium sulfate solutions are given in Figs 1-4 for both forward and reverse directions of polarization. in Oxygen
cell and measuring and control circuits were the same as those used for anodic dissolution[3]. OSN solutions were used in all cases unless otherwise stated. The experiments were carried out at 3OOBrpm except in cases where the effect of rotation speed was studied. The potential scanning All the experiments were rate was 200mV.min-I. carried out at 30 f O.l”C. It may be pointed out that to avoid any initial dissolution of copper or brass, the electrode assembly was introduced into the cell contaming the respective oxygen saturated solution such that a potential of - 400 mV was already imposed on electrode
-
-I 160 -
EXPERIMENTS The ring-disc
Lb)
la1 -1295
assembly,
J
-1025
-
ti ui >
-890
-
z ‘0 E ?!
-755-
r?
0
I 29
I 50
I 69
Current
ber, 1977 in Sofia, Bulgaria. EA24-2-B
mA
Fig. 1. E-i curvefor oxygenreductionon copper and brassin sodium chloride solution: (a) Brass; (b) copper.
* Paper presentedin the ISE meeting held during Septem131
132
K. BALAKRISHNANAND V. K. VENKATESAN Table
1. Potential
(EA) at which anodic dissolution
starts in deaerated and oxygenated
solutions -.-
S. No. 1. 2.
Metal .~~_ Copper Copper
3. 4.
Copper CoPper
5. 6. 7.
Copper
s* 9.
Brass Brass
Medium -~-“0.5N NaCl OSN NaCl
10.
BKW?S
g;gr;“,‘, 0.5 N NH:Cl (oxygenated) 0.5 N NazSO+ 0.4N NaCl + 0.1 N HCI 0.4N Na2S04 (or 0.4 N NaCI) + 0.1 N NaOH 0.5 N NaCl OSN NaCI (Oxygenated) OSN NH&I
11. 12. 13. 14.
Brass Brass Brass Brass
0.5 N 0.4N 0.4N 0.4N
Copper Copper
r [a)
-1240
(bl
-270 -225 -90 - 225 -400 - 390 - 180 -470 (at -400mV -400 -400 -610 -640
120A)
_.-
direction. In the case of ammonium chloride solution, two waves are observed in the reverse direction, whereas in ammonium sulphate solution, only a hump is observed which disappears in purified solution. It can also be seen that the rate of oxygen reduction is much higher in all cases in the reverse direction. The effect of purity of the solution on the rate of oxygen‘reduction is shown in Fig. 2b which indicates that even in highly purified solutions, the two waves in the reverse direction are distinct, ie, they are not due to impurity effects. However, in insufficiently purified solutions, oxygen reduction is more rapid. Yet another factor is the minimum in the reverse E-i CUNC, especially in sodium chloride and sodium sulphate solutions. The possibility of the first wave be&g due to oxide reduction was examined by subjecting the disc to cathodic polarisation at - 1400 mV (us see) in the same solution for a period of 20min and then immediately starting the cathodic polarization from
I (0)
Curreni
EA(mV) us see -270 - 130
Na,SOa NaCt + 0.1 N HCI NaCl + 0.1 N NaOH Na,SO, + 0.1 N NaOH
all the cases, it may be seen that, at potentials approximately greater than - 700 mV, the reduction reaches a limiting value, the magnitude of the limiting current depending on the nature of the medium. That red&on of oxygen is under pure diffusion control is shown in Fig. 5, in which (&) is plotted against o”’ for copper and brass for two different media. It can be seen that in the case of sodium chloride and sodium sulphate solutions, two distinct waves are obtained in the reverse direction both for copper and brass electrodes, whereas in the forward direction, both waves are distinct only for brass in sodium sulfate. Even in this case, the height of the first wave in the forward direction is very much less than that in the reverse
-1375
-~
(bl
mA
Fig. 2. E-i curve for oxygen reduction on copper and brass in sodium sulpbatc solution. (a) Copper; (b) Brass: 1. No. P.E. (AX.); 2. Double recrystallized; 3. Prselcctrolyeis-100 h; 4. Pre-electrolysis46 h; 5. REV. OF 1; REV. OF 2 and 3.
Current
mA
on copper and brase in Fig. 3. E-i curve for oxygen redtiioq ammonium chloride solution. (a) Copper ; (b) Brass.
133
Cathodic reduction of oxygen on copper and brass -1185
-fa)
- 1245
(b)
-I
-1050 -
-915 -780
-
-645
-
IlO
-975
I
-840
-705
-510 -570
-435
-300
Current
mA
Fig. 4. E-i curvetar oxygen reductionon copperand brass in ammoniumsulphatesolution.(a) Brass; (b) Capper. - 400 mV (Fig. 6). Even in this case, where one may expect at least a partial reduction of the surface oxide, only a slight infkxion corresponding to the first wave is observed in the forward direction. This fact confirms the earlier observation that the first wave becomes notice&e only on a partially or completely oxide-free surfacq. EIct of pH. The effect of pH on the reduction of oxygen has been studied for brass in chloride solution (0.5 N), at constant ionic strength. It can be seen from Fig. ? that with d&crease in pH to a value of 4.0 the first wave appears in the forward direction and also the minimum of the second wave progressively decreases leading to a well-detied overall wave. However, at still lower pH values, the two waves overlap to give a single wave, indicating a change in the mechanism. The fact that the reduction of H+ ions does not interfere with reduction of oxygen in highly acidic solutions is demonstrated in Fig. 8, where E-i curves for both forward and reverse directions are compared in the
G Fig. 5. L,iiting_difhkon current for O2 reduction on copper and brass i VJ /ru@pm): -O-O- Copper in NaCl ; -COBrassin (NH&SOL.
I
2
Current
3
4
5
mA
Fig. 6. ElTecLof pre-cathodic polarisation of copper ekcReverse - - --: Without precathodic trode : Forward ---; polarization: F&ward -O-OP; Reverse -0 0 0 0. presence and absence of oxygen. It may be observed in the case of hydrogen evolution that (i) the forward and reverse E-i curyes show no difference, (ii) in the case of HCl, hydrogen overvoltage on brass is higher than that on copper, values being the same on copper and brass in H,SO,. In addition, the reduction of NO; in the presence and absence of oxygen was examined on copper and brass. Figure 9 shows that the reduction of NO; on copper is much faster than on brass on an initially oxide covered surface (forward direction) and that it is the same on reduced surface (reverse direction). However, in presence of oxygen, the total reduction of NO; and oxygen on copper is very much higher than on brass. In the case of alkaline solutions of chloride and sulfate (pH = 13.0), (Fig. 10). the two waves are distinct in the forward and reverse directions. HOWever, the inflections are shifted by about 2OOmV in the negative direction. Even in the reverse direction, the oxygen reduction current is fairly small compared to that at lower pH, indicative of incomplete reduction of the surface oxide. Ring-disc experiments. In order to determine whether the two steps observed during oxygen reduction are due to formation of H,OZ and OH- or H,O, experiments were carried out, with a copper disc and platinum ring electrode, the latter being nsed to detect hydrogen peroxide by oxidation under limiting current conditions. It has been shown that oxidation of H,Oz can be carried out on the platinum ring electrode in the different solutions under limiting conditions over a wide potential range[3a]. The variation of ring currents with copper disc electrode potential for oxygen reduction in alkaline Na2S04, (NHJ2S04 and NH&l are shown in Fig. 11. In alka%ne sodium sulpbate (Fig. 1 la), the variation of ring current with disc potential is not significant.
134
K. BALAKRISHNAN AND V. K. VENKATESAN I. pH =2.0 2.pH=I 0
pH=3 0
pti=40
!JH=50
_
_ _
m
1
_ _
::
+ -270 &+-
4-44
* current
I
I
I
I
2
3
mA
of pH an the E-i curve for oxygen reduction on brass in sodium chloride solution.
Fig. 7. Effect
Current
mA
Fig. 8. Comparison of regions of hydrogen
evolution in deareated solution and oxygen reduction on copper and brass in acid chloride and suIphate solutions : A. Hjdrogen evolution : 0.4 N NaCl + 0.1 N HCI; 0.4N Na2S0., + 0.1 Brass -&-A-; H$O,: Copper -o-O-; Brass - X-X-; Copper -A-A- : B. Oxygen reduction : Acid chloride : Cop per; Forward ~-; Reverse -------I Brass: Forward and reverse -A-A-: Acid sulphate : Copper -a-m-; Brass
The higher ring current observed in this case has to be attributed to the fact that, in alkaline solutions, higher limiting currents were observed. It may fuIfher be seen from Fig. llb, that the amount of H102 produced in ammonium chloride and ammonium sulphate solutions is much less than in sodium chloride and sodium sulphate solutions and that ammonium sulphate gives rise to a smaller amount of I&O2 than ammonium chloride solution. The diagnostic plots by Damjanovic et al have beq used to examine whether hydrogen peroxide is formed as an intermediate or in a parallel reaction. Figure 12 shows the plot of IJI, us UI-‘~* at two different in ammonium sulphate solution. A linear potentials relationship with an intercept greater than N is observed. the other
In the case of sodium sulphate solution, on hand, I,Jl, is a constant at diffkrent rpm for
the different potentials indicated in the figure (Fig. 13). I (al
-b-A-.
I(b)
-485 I
I
I
I
I
I
2
3
4
5
6
7
Current
I
mA
Fig. 9. Reduction of nitrate ion on copper and brass in deaerated and oxygenated sodium nitrate solutions. Deaerated solution: Brass: Forward -_, ; Reverse t ; Copper: Forward and reverse ----; Oxygenated solution : Copper: Forward -X-X-: Reverse -0-C-; Brass: Forward -A-A-;
2
-350
Reverse -A-A-.
02 Current
mA
Fig. 10. E-i curve for oxygen reduction in alkaline solution. (a) Copper; (1) Na2S04; (2) NaCl: (b) Brass; (1) Na,SB,;
(2) NaCI.
Cathodic reduction of oxygen on copper and brass
135
I:
IC
.
0
-“I-‘
r
”
5
I
04
1
-350 05
1
r
IO
Ring
(, , , 005
current
01
I
0.22
I 0.26
I
030
_I L w 2 .sec2
Fig. 13. Plots of I,,/I.vsw-112for oxygen reductionat copper -670mV -A-A-; disc in NalSO.. -530 mV -O-O-; -705mV-O-n-; -775mV-x-x-.
mA
Fig. 11. Variation of ring current (H,Oz) oxidation) with copper disc potential during oxygen reduction. (a) 0.4N Na,SO, + 0.1NNaOH; (b) 1. OSN (NH,),SOL; 2. 0.5N NH&I. Experiments in pure solutions. Since it is known that, in certain cases, hydrogen peroxide is formed only in impure solutions, a few experiments have been carried out in sodium sulphate and ammonium sulphate solutions which have been pre-electrolysed for 100 hat a cd of 0.01 mA cm-‘. It is observed that even in these cases hydrogen peroxide is formed indicating that oxygen reduction on copper occurs through the formation of hydrogen peroxide irrespective of the purity of the solutions. DISCUSSION
Cathodic reduction ofoxygen
I 0 ti3
on copper and brass
General characteristics. The importance of kinetic studies on oxygen reduction on metals from the point of view of corrosion is well-known. The studies on effect ofcertain anions as well as ammonium ion on the anodic dissolution of copper and brass especially with regard to the mechanism of dissolution are published elsewhere[3c]. In the case of corrosion processes, the changes in dissolution rates in the presence of different anions are interpreted in terms of the effect of the anions on anodic dissolution and to some extent in terms of variation in oxygen solubility
Fig. 12. Plots of I&, 0sW- r” for oxygen reductionat copper discin (NH&SO,-O-O-49OmV; -@-@-56OmV.
in different solutions. If the anions and ammonium ion have also some influence on the kinetics of oxygen reduction, then the net effect of the anions on the corrosion process would naturally depend on its influence on cathodic reduction of oxygen and on the anodic dissolution of metal. So, a complete understanding of the role of these anions in promoting corrosion would warrant a detailed study of the kinetics of oxygen reduction on these metals in presence of various salts. The nature of the E-i curves for oxygen reduction is the same both on copper and brass. Table 1 indicates that the anodic current in the cathodic potential range studied here is either zero or very small in the forward direction, though, in the reverse direction, there may he a slight anodic current at less negative potentials. Further, a reference to the work of Miller and Bellavance[4] indicates that for pure copper in 0.1 M HCl + 1 M NaCl saturated with oxygen, the partial anodic current is nearly zero at -2250 mV (USsee) and at -210mV it is 268 PA/cm’. However, for brass in the same medium the partial anodic cd is zero only at about - 365 mV (W see) and is of the order of 738 PA/cm2 at - 297 mV (vs see). Pickering and Byrne[S] report a value of about 0.1 mA/cm’ for zinc from alpha brass in the polarization range of -343 to -843 mV (see) and nearly zero for Cu up to - 193 mV (see) in Na$SO., solution (pH = 5). The E-i curves for oxygen reduction on copper and brass given in Figs l-3 indicate that, in the forward direction, there are two regions with differing slopes. In the reverse polarization, two distinct steps are observed in all the cases. In the case of sodium sulphate, however, even in the forward direction, two steps are observed. The higher rate of oxygen reduction on both copper and brass in the reverse direction can be attributed to faster reduction on an oxide-free surface than on oxide-covered surfaces, as has been reported by many workers[6,7] in the case of platinum. In the reverse direction, even if the oxide is not completely reduced, the surface can be considered to be at least partially free of oxide. In the case of oxygen reduction in nitrate solution (Fig. 9) a few interesting facts are observed. In deaerated nitrate solution, the reduction of nitrate takes place at lower over-voltages on copper than on brass on the oxide covered surface (forward polari-
K.
136
BhLhKmhN
AND
xation), but at the same potentials (in the reverse direction) on an oxide-free surface. However, in presence of oxygen, the total reduction of nitrate and oxygen is much higher on copper than on brass in both the directions. In presence of oxygen it therefore, appears that it is not possible to effect sufficient reduction of the oxide present on brass. Further, the reduction of oxygen itself is inhibited in presence of nitrate. These facts suggest that the corrosion rate of copper in neutral nitrate solution is much higher than that of brass. Corrosion experiments carried out on copper and brass in oxygenated 0.5 N sodium nitrate solution for 15 days at 30 f 2°C indicated a corrosion rate of 29.0mg/dm2/day for copper, whereas it was only 4.5 mg/dm”/day for brass. This is also supported by visual observation of the surfaces of copper and brass : the former was highly discoloured, whereas the other remained bright. The cause of the appearance of the minimum in the reverse current curve merits some attention. As seen in Fig. 2b, this is not due to impurities in the solution, which only increase the reduction rate without affecting the characteristics of the minimum. Further, the difference between the forward and reverse polarirations is less when the electrode is subjected to precathodic polarization, indicating that the lower reduction (Fig. 1) in the forward direction is only due to the presence of oxide. Likewise, the difference in the E-i curves in the forward and reverse directions in fairly acidic solution (pH = 2.0) (Fig. 7) is negligible. Further, with decrease in pH, there is a gradual change in the nature of the E-i curve leading to the absence of the two distinct steps. However, in alkaline solutions, the two steps become prominent in both forward and reverse dir&ions. The minimum in the E-i curve may perhaps be connected with a high level of adsorption of chloride or sulphate on an oxidefree surface. In the reverse direction, decreasing cathodic
V.K.
VENKATFSAN
polarization will facilitate the adsorption of anion on a surface relatively free from oxide leading to a decrease in current. At less cathodic potentials, adsorption of oxygen decreases the adsorption of anions leading again to an increase in current. When oxide or chetnisorbed oxygen is formed at decreasing cathodic overpotential, the current again falls. This explanation is speculative in character, but it is known that adsorption of chloride ion on platinum surface covered with chemisorbed oxygen is inhibited and that the adsorption is a time-dependent process[S]. In the reverse direction, even if the oxide is not completely reduced, the surface can be considered to be partially free from oxide. It is known that cations like Ag+ and Cu2+ retard the rate of oxygen reduction. It may be observed here that the region of the minimum and of the corresponding maximum lies at potentials more negative than the dissolution potential (Table 1) at least in a few cases. Figure 5 shows that cathodic reduction of oxygen in the solutions considered is under diffusion control. The Tafel plots for the activation-diffusion control have been obtained by plotting log i/i,-; us potential (E) for both copper and brass for various media and the Tafel slopes are summarized for both forward and reverse direction of polarization in Table 2. The slopes for the two waves, wherever they occur, are indicated as (1) and (2). It can be seen that on copper, the slopes are higher in neutral chloride solution than in acid or alkaline solution. The values observed for the reverse polarization, except in the case of ammonium salt solutions where nearly the same values are obtained in both the cases, are generally smaller. It may be pointed out here that the apparent Tafel slopes for oxygen reduction reported for different metals in various solutions mostly range from -4RT/F to RT/F. The variation of ring current due to H,O, oxidation with disc potentials (Figs not given) showed that, in
Table 2. Apparent Tafel slopes for oxygen reduction on copper and brass Apparent sto&, in rnv Sl. No. 1. 2. 3.
Metal/alloy
5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Copper Copper CopPer Copper copper Copper Copper Copper Brass Brass Brass Brass Brass Brass
15.
Brass
16. 17. 18.
Brass Brass Brass
4.
Solution
1
Forward
NaCl 238 & 155 NaCl (pH = 1) 120& 135 NaCl (pH = 13) 100 Na,SO, 125 Na,SO, (pH = I) 155 pi”2 (pH = 13) 85 132 (NH*,)W, 135& 225 N&l 188 & I25 NaCl (pH = I) 148 NaCl (pH = 13) 67 Na,SO, (A.R.) Na$O, (DC.) 135 & 145 Na,SC, (PH) 125& 180 Il@J hl Na,SO, 205 & 170 (PH = t) NaCl (pH = 4) 110 NH,Ct 137 WH4W34 175
Reverse
2
3
87 75 85
45 32 62 157 & 115 40 37 200 & 260 5.5
65 80 8r 140
38 60 62 48
-
4 67 110 50 35 142 60 58 32 78 72
157 & 113
-
40 90 225 & 125
190 -
Cathodic reduction of oxygen on copper and brass sodium chloride and sodium sulphate, the ring current and then reaches a limiting value at a potential less cathodic than that corresponding to the limiting disc current for oxygen reduction. Further, the ring current in sodium chloride solution is slightly higher than that in sodium sulphate solution, which is in agreement with the results of Kozawa[X], who has reported that in presence of chloride, the percentage production of HaOl is much higher than in the presence ofsulphate. The same trend is observed in the case of ammonium chloride and ammonium sulphate solutions, though the H,Oz detected at the ring is still less in these solutions when compared to the corresponding sodium salt. However, in alkaline sulphate solution, the ring currents corresponding to H,O* are much higher than in any of the other solutions studied indicating the stabilization of H,OI as OIH- in alkaline solution (Fig. 11). Diagnostic plots. The diagnostic plots of Damjanovic et al have been used for establishing the mechanism of oxygen reduction as far as the formation of H,O, is concerned. Figure 12 shows the plots of i,/I, USO- I” for oxygen reduction on copper in ammonium sulphate solution for two different potentials. It may be seen that both the slopes and intercepts change with potential indicating that H,O, is formed as intermediate in these solutions during oxygen reduction. But the slopes are negative. Negative slopes have been reported by Genshaw for oxygen reduction on platinum in pure H$O.+. But the diagnostic equation
first increases with potential
x+1 1,/i, = 7
x+2 + 7
k ~ WII2
requires that the slope of such plots must be positive since k’which is proportional to the rate constant must be positive. Such a negative slope was in fact reported by Genshaw[lO] for oxygen reduction on platinum in H2S04 over a certain potential range. This was explained by assuming the following probable path for the reduction of oxygen to H202 0,+2H+H,O, which is preceded by the step H,O+
+e-+H+H*O
He assumed that the rate of hydrogen ion discharge becomes small in comparison to the rate of oxygen reduction and hence 1.,/I, becomes very large. A similar negative slope ha5 also been reported on gold electrode in acidic solutions by Genshaw er nl[ lo] at more positive potentials. This was explained on the basis that O2 reduction takes place under diffusion control on oxide patches whereas H202 is formed at bare electrodes. It can be seen from the Figs 1-4 that the reduction potentials are less cathodic in the presence of ammonium ions than in the corresponding sodium salt solution. In sodium sulphate solution, the lJ1, 11s w- “’ plots (Fig. 13) are parallel to the x-axis at all potentials and the intercepts which are greater than i/N depend on potential. This condition confirms the equation IdlJI, = x + i/N. These faqts indicate that in this case H202 is produced in a parallel reaction and does not react
further. forward Thus, solution, viz
I37
The above experiments correspond to the polarization curves. the results show that, in ammonium sulphate all of the following three reactions take place, 0,
+4H+
+4e--+2H,O
HIOl + 2H* + 2e +
2H10
However, oxygen reduction on copper in sodium sulphate solutions takes place in the parallel steps, one involving direct reduction to H,O or OH-, the H,O, formed in the alternative step being stable. Since the E-i curves for copper and brass are similar, it may be expected that a similar mechanism is also applicable for brass. It may further be noted (Figs l-4) that the influence of ammonium ions can be explained on the basis of change in zero charge potential or by the explanation given by Myuller and Sobol[ll] and Balashova and Kuleznovar121 and Tarasevich et . al[13,14] for the influence of cations on the oxygen reduction on platinum but it will be only speculative at this stage. J
&fect
ofanion
and ammonium ion an oxygen reduction
The effect of anion and pH on the rate of oxygen reduction seems to depend on the nature of the metal. On copper, the oxygen reduction current (Figs 14 and 15) increases in the order (NH&SO+ > H,SO, > NaCl > HCI > Na,SO, > NH,Cl whereas for brass, it increases in the order (NH&SO+ > H2S01 > HCl _ Na#O, > NH,CI > NaCl in the range -350 to - 500 mV on an oxide-covered surface (forward polarization). In the case of oxide-f&e surface, it depends on the potential range. The highest reduction current observed in ammonium sulfate solution points out that NH: ion favours the oxygen reduction probably by causing a change in zero charge potential to more positive value. Such a shift can increase the rate of reduction has been indicated by Bockris et al[lS, 161. However, in pre+nce of chloride, as in the cases &f NH,CI, the influence of NH: ion is not felt due to adsorption of chloride ions.
0
I
I
I
2
t 30 Current
I
I
I
I
2
I
3
mA
14. E&A of diRerent salts on oxygen reduction on copper (a) Forward; (b) Reverse). -O-O(NHJ2SO~; -we Na,S04 ;-A-A- NaCl ; -A-A(NH&l) ; -O-OFii.
HCl; -I-m-
H2S04.
138
K. BALAKRISF~NANANO V. K. VENKATMN
2
3
/
0 Current
and
of rate of reduction of oxygen on copper
brass
With regard to the effect of a common anion on oxygen reduction on copper and brass, it is observed (Fig. 16) that, in sodium chloride solution, oxygen reduction is much faster on copper than on brass (Fig. 16c) which may perhaps be a reason for the reported [17] higher rate of corrosion of copper when compared to brass. In the case of sulphate solutions, oxygen reduction on copper is much higher at potentials above -450 mV, whereas beIow that,it is sIightly higher on brass (Fig. 16b). In the case ofammonium chloride and ammonium sulphate, the reduction rate is nearly the same both on copper and brass at all potentials (Fig. 16a). Acknowledgement - The authors thank Dr. H. V. K. Udupa, Director. Central Electrochemical Research Institute,
1(bl
0
I
I
,
2
0
I
I
I
I
I
20
I
2
Current
mA
Fig. 16. Rate of oxygen reduction on copper and brass. A: NH&l (1 and 2 copper and Brass); 3 and 4 (NH&SO, (Copper
and Brass); B: Na,SO,'(l - copper; 2 - J3raes);C: N&l (I - Brass; 2 - copper).
3
mA
Fig. 15. Effect of different salts on oxygen reduction on brass (a) Forward -0-ON&l; -A-ANH&I ; -A-A(NH&SO, ; -o-a-
Comparison
2
; (b) Reverse). -0 -@ HCl; -m-m-
Na2SOL ;
H,SO,.
Karaikudi, for his keen interest in the work. The authors are also thankful to Messrs. K. R. Ramakrishnan and Y. Mahadeva Lyer for their help in electronic Instrumentation. REFERENCES
1. A. Damjanovic in Modern Aspects of Eiectrochemisfry (Edited by J. O’M Bockris and B. E. Conway), Vol. 5, p, 369. Plenum Press, New York (1969); T. P. Hoare, Elecrrochemistry of Oxygen. Interscience, New York (1968). 2. A. Damjanovic, M. A. Genshaw and J. O’M Bockris, J. &em. Phys. 45, 4057 (1966). Ph.D. Thesis, Madurai University, 3. a. K. Balakrishnan, lndia (1975); b. K. Balakrishnan and V. K. Venkatesan, Trans. SAEST (India] 12,4,221 (1977). c. K. Balakrishnan and V. K. Venkatesan, Werkstoflund Korrosion 29, (2) 113 (1978). 4. B. Miller and M. F. Etellawance, J. elecrrochem. Sot. 119, 11, 1510 (1972). H. W. Pickering and Byrne, ibid. 116, (11) 1492 (1969). Acra 11, :: A. Damjanovic andJ. O’M Bockris,Elecrrochim. 376 (1956). 7. Mueller and L. N. Nekrasov, J. elecrrounalyr. C&m. 9, 282 (1965). 8. A. Kozawa, J. electronnofyt. C&m. 8, 20 (1964). University, 9. M. A. Gensbaw, PbD. thesis, Pennsylvania U.S.A. (1966). and J. O’M Bockris, J. 10. M. A. Genshaw, A. Damjanovic electroannlvr. C&m. 15, 173 (1967). 11. L. Myulle; and V. V. Soboi, So;. Electrochem. 1, 92 (1965). and M. I. Kuleznova, Sou. Elec12. N. A. Balashova trmhem. 1, 128 (1965). 13. M. R. Tarasevich, G. 1. Zakharin and R. M. Smimova, Sov. Electroch. 8, 611 (1972). 14. V. S. Vilinskaya and M. R. Tarasevich, Sov. Electrochem. 9, 1123 (1973). R. J. Mannan and A. Damianovic, J. 15. J. O’M Bock& them. Phys. 48, 5. 1898 (1968). 16. J. O’M Bock& and S. Srinivasan Fuel Cells. McGrawHill, New York (1977). 17a. P. L. Annamalai et al, Chemical Age of India 19, 4, 288 (1968); b. B. Sanyal and T. K. Gorover, Prac. Symp. on Corrosion of Metals, p. 54. at Kanpur, India (1962).
CATHODIC
Britain.
REDUCTION OF OXYGEN AND BRASS* K. BALAKRISHNAN
Central Electrochemical
ON COPPER
and V. K. VENKATE~AN
Research Institute, Karaikudi,
India
(Received 21 October 1977 ; and in revised form 4 July 1978) Abstracl - In this investigation, the electrochemical reduction of oxygen on copper and brass has been studied using the ring-disc electrode technique in solutions containing chloride, sulphatc and nitrate anions
and ammonium cation The E-I curves obtained in the forward and reverse direction of polarization for copper and brass in NaCL, Na2SOI and NH*CI solutions are similar in nature and show two waves. However in (NH,),SO, solution two waves are not observed in the reverse direction. The rate of oxygen reduction is the highest in ammonium sulphate solution for both copper and brass whereas it is the lowest in NH&Cl in the case of copper. In Na$O, and NaCL solutions, the rate of oxygen reduction is higher on copper than on brass. Apparent Tafel slopes for oxygeu reduction obtained for copper and brass vary from 65 mV to 240 mV depending upon the medium. The two steps observed with copper disc electrode have been identified as due to the mduction of oxygen to hydrogen peroxide and reduction of H 202 to OH - ion or water depending upon the pH of the solution. In acid chloride and sulphate media no H,Oz was detected, which suggests direct reduction to H,O. The diagnostic plots of1,{1, us w- “’ employed by Bock& et al indicate that in NaTSO* the reduction ofoxygen to H20z takes place in a parallel reaction whereas in (NH,),SO, both direct reduction of O2 to water (or OH- ion) and the reduction through intermediate H,Oz occur.
INTRODUCTION
Oxygen
reduction assumes importance processes since in neutral solutions
in
cor-
and in the absence of any other depolarizer the corrosion rate is controlled by the kinetics of oxygen reduction reaction. While the oxygen reduction has been extensively studied on noble metals and a few active metals such as nickel and silver with the help of rotating-ring disc electrode[1,2], no such detailed study seems to have been carried out on copper and brass. In the present study some aspects of oxygen reduction on copper and brass in solutions containing different anions have been examined using the rotating ring-disc electrode in order to determine (i) whether the anions, in addition to affecting the dissolution reactions, also affect the cathodic reduction of oxygen, and (ii) whether the reduction passes through the formation of peroxides. rosion
the working electrode. The forward scan was thereby started at -400mV and ended at - 1350 mV. The potentials at which anodic dissolution starts in different solutions are given in Table 1. aEsULTs
reduction General characteristics ofE-i CIITU~S. The E-i curves for oxygen reduction in copper and brass in sodium chloride, sodium sulfate, ammonium chloride and ammonium sulfate solutions are given in Figs 1-4 for both forward and reverse directions of polarization. in Oxygen
cell and measuring and control circuits were the same as those used for anodic dissolution[3]. OSN solutions were used in all cases unless otherwise stated. The experiments were carried out at 3OOBrpm except in cases where the effect of rotation speed was studied. The potential scanning All the experiments were rate was 200mV.min-I. carried out at 30 f O.l”C. It may be pointed out that to avoid any initial dissolution of copper or brass, the electrode assembly was introduced into the cell contaming the respective oxygen saturated solution such that a potential of - 400 mV was already imposed on electrode
-
-I 160 -
EXPERIMENTS The ring-disc
Lb)
la1 -1295
assembly,
J
-1025
-
ti ui >
-890
-
z ‘0 E ?!
-755-
r?
0
I 29
I 50
I 69
Current
ber, 1977 in Sofia, Bulgaria. EA24-2-B
mA
Fig. 1. E-i curvefor oxygenreductionon copper and brassin sodium chloride solution: (a) Brass; (b) copper.
* Paper presentedin the ISE meeting held during Septem131
132
K. BALAKRISHNANAND V. K. VENKATESAN Table
1. Potential
(EA) at which anodic dissolution
starts in deaerated and oxygenated
solutions -.-
S. No. 1. 2.
Metal .~~_ Copper Copper
3. 4.
Copper CoPper
5. 6. 7.
Copper
s* 9.
Brass Brass
Medium -~-“0.5N NaCl OSN NaCl
10.
BKW?S
g;gr;“,‘, 0.5 N NH:Cl (oxygenated) 0.5 N NazSO+ 0.4N NaCl + 0.1 N HCI 0.4N Na2S04 (or 0.4 N NaCI) + 0.1 N NaOH 0.5 N NaCl OSN NaCI (Oxygenated) OSN NH&I
11. 12. 13. 14.
Brass Brass Brass Brass
0.5 N 0.4N 0.4N 0.4N
Copper Copper
r [a)
-1240
(bl
-270 -225 -90 - 225 -400 - 390 - 180 -470 (at -400mV -400 -400 -610 -640
120A)
_.-
direction. In the case of ammonium chloride solution, two waves are observed in the reverse direction, whereas in ammonium sulphate solution, only a hump is observed which disappears in purified solution. It can also be seen that the rate of oxygen reduction is much higher in all cases in the reverse direction. The effect of purity of the solution on the rate of oxygen‘reduction is shown in Fig. 2b which indicates that even in highly purified solutions, the two waves in the reverse direction are distinct, ie, they are not due to impurity effects. However, in insufficiently purified solutions, oxygen reduction is more rapid. Yet another factor is the minimum in the reverse E-i CUNC, especially in sodium chloride and sodium sulphate solutions. The possibility of the first wave be&g due to oxide reduction was examined by subjecting the disc to cathodic polarisation at - 1400 mV (us see) in the same solution for a period of 20min and then immediately starting the cathodic polarization from
I (0)
Curreni
EA(mV) us see -270 - 130
Na,SOa NaCt + 0.1 N HCI NaCl + 0.1 N NaOH Na,SO, + 0.1 N NaOH
all the cases, it may be seen that, at potentials approximately greater than - 700 mV, the reduction reaches a limiting value, the magnitude of the limiting current depending on the nature of the medium. That red&on of oxygen is under pure diffusion control is shown in Fig. 5, in which (&) is plotted against o”’ for copper and brass for two different media. It can be seen that in the case of sodium chloride and sodium sulphate solutions, two distinct waves are obtained in the reverse direction both for copper and brass electrodes, whereas in the forward direction, both waves are distinct only for brass in sodium sulfate. Even in this case, the height of the first wave in the forward direction is very much less than that in the reverse
-1375
-~
(bl
mA
Fig. 2. E-i curve for oxygen reduction on copper and brass in sodium sulpbatc solution. (a) Copper; (b) Brass: 1. No. P.E. (AX.); 2. Double recrystallized; 3. Prselcctrolyeis-100 h; 4. Pre-electrolysis46 h; 5. REV. OF 1; REV. OF 2 and 3.
Current
mA
on copper and brase in Fig. 3. E-i curve for oxygen redtiioq ammonium chloride solution. (a) Copper ; (b) Brass.
133
Cathodic reduction of oxygen on copper and brass -1185
-fa)
- 1245
(b)
-I
-1050 -
-915 -780
-
-645
-
IlO
-975
I
-840
-705
-510 -570
-435
-300
Current
mA
Fig. 4. E-i curvetar oxygen reductionon copperand brass in ammoniumsulphatesolution.(a) Brass; (b) Capper. - 400 mV (Fig. 6). Even in this case, where one may expect at least a partial reduction of the surface oxide, only a slight infkxion corresponding to the first wave is observed in the forward direction. This fact confirms the earlier observation that the first wave becomes notice&e only on a partially or completely oxide-free surfacq. EIct of pH. The effect of pH on the reduction of oxygen has been studied for brass in chloride solution (0.5 N), at constant ionic strength. It can be seen from Fig. ? that with d&crease in pH to a value of 4.0 the first wave appears in the forward direction and also the minimum of the second wave progressively decreases leading to a well-detied overall wave. However, at still lower pH values, the two waves overlap to give a single wave, indicating a change in the mechanism. The fact that the reduction of H+ ions does not interfere with reduction of oxygen in highly acidic solutions is demonstrated in Fig. 8, where E-i curves for both forward and reverse directions are compared in the
G Fig. 5. L,iiting_difhkon current for O2 reduction on copper and brass i VJ /ru@pm): -O-O- Copper in NaCl ; -COBrassin (NH&SOL.
I
2
Current
3
4
5
mA
Fig. 6. ElTecLof pre-cathodic polarisation of copper ekcReverse - - --: Without precathodic trode : Forward ---; polarization: F&ward -O-OP; Reverse -0 0 0 0. presence and absence of oxygen. It may be observed in the case of hydrogen evolution that (i) the forward and reverse E-i curyes show no difference, (ii) in the case of HCl, hydrogen overvoltage on brass is higher than that on copper, values being the same on copper and brass in H,SO,. In addition, the reduction of NO; in the presence and absence of oxygen was examined on copper and brass. Figure 9 shows that the reduction of NO; on copper is much faster than on brass on an initially oxide covered surface (forward direction) and that it is the same on reduced surface (reverse direction). However, in presence of oxygen, the total reduction of NO; and oxygen on copper is very much higher than on brass. In the case of alkaline solutions of chloride and sulfate (pH = 13.0), (Fig. 10). the two waves are distinct in the forward and reverse directions. HOWever, the inflections are shifted by about 2OOmV in the negative direction. Even in the reverse direction, the oxygen reduction current is fairly small compared to that at lower pH, indicative of incomplete reduction of the surface oxide. Ring-disc experiments. In order to determine whether the two steps observed during oxygen reduction are due to formation of H,OZ and OH- or H,O, experiments were carried out, with a copper disc and platinum ring electrode, the latter being nsed to detect hydrogen peroxide by oxidation under limiting current conditions. It has been shown that oxidation of H,Oz can be carried out on the platinum ring electrode in the different solutions under limiting conditions over a wide potential range[3a]. The variation of ring currents with copper disc electrode potential for oxygen reduction in alkaline Na2S04, (NHJ2S04 and NH&l are shown in Fig. 11. In alka%ne sodium sulpbate (Fig. 1 la), the variation of ring current with disc potential is not significant.
134
K. BALAKRISHNAN AND V. K. VENKATESAN I. pH =2.0 2.pH=I 0
pH=3 0
pti=40
!JH=50
_
_ _
m
1
_ _
::
+ -270 &+-
4-44
* current
I
I
I
I
2
3
mA
of pH an the E-i curve for oxygen reduction on brass in sodium chloride solution.
Fig. 7. Effect
Current
mA
Fig. 8. Comparison of regions of hydrogen
evolution in deareated solution and oxygen reduction on copper and brass in acid chloride and suIphate solutions : A. Hjdrogen evolution : 0.4 N NaCl + 0.1 N HCI; 0.4N Na2S0., + 0.1 Brass -&-A-; H$O,: Copper -o-O-; Brass - X-X-; Copper -A-A- : B. Oxygen reduction : Acid chloride : Cop per; Forward ~-; Reverse -------I Brass: Forward and reverse -A-A-: Acid sulphate : Copper -a-m-; Brass
The higher ring current observed in this case has to be attributed to the fact that, in alkaline solutions, higher limiting currents were observed. It may fuIfher be seen from Fig. llb, that the amount of H102 produced in ammonium chloride and ammonium sulphate solutions is much less than in sodium chloride and sodium sulphate solutions and that ammonium sulphate gives rise to a smaller amount of I&O2 than ammonium chloride solution. The diagnostic plots by Damjanovic et al have beq used to examine whether hydrogen peroxide is formed as an intermediate or in a parallel reaction. Figure 12 shows the plot of IJI, us UI-‘~* at two different in ammonium sulphate solution. A linear potentials relationship with an intercept greater than N is observed. the other
In the case of sodium sulphate solution, on hand, I,Jl, is a constant at diffkrent rpm for
the different potentials indicated in the figure (Fig. 13). I (al
-b-A-.
I(b)
-485 I
I
I
I
I
I
2
3
4
5
6
7
Current
I
mA
Fig. 9. Reduction of nitrate ion on copper and brass in deaerated and oxygenated sodium nitrate solutions. Deaerated solution: Brass: Forward -_, ; Reverse t ; Copper: Forward and reverse ----; Oxygenated solution : Copper: Forward -X-X-: Reverse -0-C-; Brass: Forward -A-A-;
2
-350
Reverse -A-A-.
02 Current
mA
Fig. 10. E-i curve for oxygen reduction in alkaline solution. (a) Copper; (1) Na2S04; (2) NaCl: (b) Brass; (1) Na,SB,;
(2) NaCI.
Cathodic reduction of oxygen on copper and brass
135
I:
IC
.
0
-“I-‘
r
”
5
I
04
1
-350 05
1
r
IO
Ring
(, , , 005
current
01
I
0.22
I 0.26
I
030
_I L w 2 .sec2
Fig. 13. Plots of I,,/I.vsw-112for oxygen reductionat copper -670mV -A-A-; disc in NalSO.. -530 mV -O-O-; -705mV-O-n-; -775mV-x-x-.
mA
Fig. 11. Variation of ring current (H,Oz) oxidation) with copper disc potential during oxygen reduction. (a) 0.4N Na,SO, + 0.1NNaOH; (b) 1. OSN (NH,),SOL; 2. 0.5N NH&I. Experiments in pure solutions. Since it is known that, in certain cases, hydrogen peroxide is formed only in impure solutions, a few experiments have been carried out in sodium sulphate and ammonium sulphate solutions which have been pre-electrolysed for 100 hat a cd of 0.01 mA cm-‘. It is observed that even in these cases hydrogen peroxide is formed indicating that oxygen reduction on copper occurs through the formation of hydrogen peroxide irrespective of the purity of the solutions. DISCUSSION
Cathodic reduction ofoxygen
I 0 ti3
on copper and brass
General characteristics. The importance of kinetic studies on oxygen reduction on metals from the point of view of corrosion is well-known. The studies on effect ofcertain anions as well as ammonium ion on the anodic dissolution of copper and brass especially with regard to the mechanism of dissolution are published elsewhere[3c]. In the case of corrosion processes, the changes in dissolution rates in the presence of different anions are interpreted in terms of the effect of the anions on anodic dissolution and to some extent in terms of variation in oxygen solubility
Fig. 12. Plots of I&, 0sW- r” for oxygen reductionat copper discin (NH&SO,-O-O-49OmV; -@-@-56OmV.
in different solutions. If the anions and ammonium ion have also some influence on the kinetics of oxygen reduction, then the net effect of the anions on the corrosion process would naturally depend on its influence on cathodic reduction of oxygen and on the anodic dissolution of metal. So, a complete understanding of the role of these anions in promoting corrosion would warrant a detailed study of the kinetics of oxygen reduction on these metals in presence of various salts. The nature of the E-i curves for oxygen reduction is the same both on copper and brass. Table 1 indicates that the anodic current in the cathodic potential range studied here is either zero or very small in the forward direction, though, in the reverse direction, there may he a slight anodic current at less negative potentials. Further, a reference to the work of Miller and Bellavance[4] indicates that for pure copper in 0.1 M HCl + 1 M NaCl saturated with oxygen, the partial anodic current is nearly zero at -2250 mV (USsee) and at -210mV it is 268 PA/cm’. However, for brass in the same medium the partial anodic cd is zero only at about - 365 mV (W see) and is of the order of 738 PA/cm2 at - 297 mV (vs see). Pickering and Byrne[S] report a value of about 0.1 mA/cm’ for zinc from alpha brass in the polarization range of -343 to -843 mV (see) and nearly zero for Cu up to - 193 mV (see) in Na$SO., solution (pH = 5). The E-i curves for oxygen reduction on copper and brass given in Figs l-3 indicate that, in the forward direction, there are two regions with differing slopes. In the reverse polarization, two distinct steps are observed in all the cases. In the case of sodium sulphate, however, even in the forward direction, two steps are observed. The higher rate of oxygen reduction on both copper and brass in the reverse direction can be attributed to faster reduction on an oxide-free surface than on oxide-covered surfaces, as has been reported by many workers[6,7] in the case of platinum. In the reverse direction, even if the oxide is not completely reduced, the surface can be considered to be at least partially free of oxide. In the case of oxygen reduction in nitrate solution (Fig. 9) a few interesting facts are observed. In deaerated nitrate solution, the reduction of nitrate takes place at lower over-voltages on copper than on brass on the oxide covered surface (forward polari-
K.
136
BhLhKmhN
AND
xation), but at the same potentials (in the reverse direction) on an oxide-free surface. However, in presence of oxygen, the total reduction of nitrate and oxygen is much higher on copper than on brass in both the directions. In presence of oxygen it therefore, appears that it is not possible to effect sufficient reduction of the oxide present on brass. Further, the reduction of oxygen itself is inhibited in presence of nitrate. These facts suggest that the corrosion rate of copper in neutral nitrate solution is much higher than that of brass. Corrosion experiments carried out on copper and brass in oxygenated 0.5 N sodium nitrate solution for 15 days at 30 f 2°C indicated a corrosion rate of 29.0mg/dm2/day for copper, whereas it was only 4.5 mg/dm”/day for brass. This is also supported by visual observation of the surfaces of copper and brass : the former was highly discoloured, whereas the other remained bright. The cause of the appearance of the minimum in the reverse current curve merits some attention. As seen in Fig. 2b, this is not due to impurities in the solution, which only increase the reduction rate without affecting the characteristics of the minimum. Further, the difference between the forward and reverse polarirations is less when the electrode is subjected to precathodic polarization, indicating that the lower reduction (Fig. 1) in the forward direction is only due to the presence of oxide. Likewise, the difference in the E-i curves in the forward and reverse directions in fairly acidic solution (pH = 2.0) (Fig. 7) is negligible. Further, with decrease in pH, there is a gradual change in the nature of the E-i curve leading to the absence of the two distinct steps. However, in alkaline solutions, the two steps become prominent in both forward and reverse dir&ions. The minimum in the E-i curve may perhaps be connected with a high level of adsorption of chloride or sulphate on an oxidefree surface. In the reverse direction, decreasing cathodic
V.K.
VENKATFSAN
polarization will facilitate the adsorption of anion on a surface relatively free from oxide leading to a decrease in current. At less cathodic potentials, adsorption of oxygen decreases the adsorption of anions leading again to an increase in current. When oxide or chetnisorbed oxygen is formed at decreasing cathodic overpotential, the current again falls. This explanation is speculative in character, but it is known that adsorption of chloride ion on platinum surface covered with chemisorbed oxygen is inhibited and that the adsorption is a time-dependent process[S]. In the reverse direction, even if the oxide is not completely reduced, the surface can be considered to be partially free from oxide. It is known that cations like Ag+ and Cu2+ retard the rate of oxygen reduction. It may be observed here that the region of the minimum and of the corresponding maximum lies at potentials more negative than the dissolution potential (Table 1) at least in a few cases. Figure 5 shows that cathodic reduction of oxygen in the solutions considered is under diffusion control. The Tafel plots for the activation-diffusion control have been obtained by plotting log i/i,-; us potential (E) for both copper and brass for various media and the Tafel slopes are summarized for both forward and reverse direction of polarization in Table 2. The slopes for the two waves, wherever they occur, are indicated as (1) and (2). It can be seen that on copper, the slopes are higher in neutral chloride solution than in acid or alkaline solution. The values observed for the reverse polarization, except in the case of ammonium salt solutions where nearly the same values are obtained in both the cases, are generally smaller. It may be pointed out here that the apparent Tafel slopes for oxygen reduction reported for different metals in various solutions mostly range from -4RT/F to RT/F. The variation of ring current due to H,O, oxidation with disc potentials (Figs not given) showed that, in
Table 2. Apparent Tafel slopes for oxygen reduction on copper and brass Apparent sto&, in rnv Sl. No. 1. 2. 3.
Metal/alloy
5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Copper Copper CopPer Copper copper Copper Copper Copper Brass Brass Brass Brass Brass Brass
15.
Brass
16. 17. 18.
Brass Brass Brass
4.
Solution
1
Forward
NaCl 238 & 155 NaCl (pH = 1) 120& 135 NaCl (pH = 13) 100 Na,SO, 125 Na,SO, (pH = I) 155 pi”2 (pH = 13) 85 132 (NH*,)W, 135& 225 N&l 188 & I25 NaCl (pH = I) 148 NaCl (pH = 13) 67 Na,SO, (A.R.) Na$O, (DC.) 135 & 145 Na,SC, (PH) 125& 180 Il@J hl Na,SO, 205 & 170 (PH = t) NaCl (pH = 4) 110 NH,Ct 137 WH4W34 175
Reverse
2
3
87 75 85
45 32 62 157 & 115 40 37 200 & 260 5.5
65 80 8r 140
38 60 62 48
-
4 67 110 50 35 142 60 58 32 78 72
157 & 113
-
40 90 225 & 125
190 -
Cathodic reduction of oxygen on copper and brass sodium chloride and sodium sulphate, the ring current and then reaches a limiting value at a potential less cathodic than that corresponding to the limiting disc current for oxygen reduction. Further, the ring current in sodium chloride solution is slightly higher than that in sodium sulphate solution, which is in agreement with the results of Kozawa[X], who has reported that in presence of chloride, the percentage production of HaOl is much higher than in the presence ofsulphate. The same trend is observed in the case of ammonium chloride and ammonium sulphate solutions, though the H,Oz detected at the ring is still less in these solutions when compared to the corresponding sodium salt. However, in alkaline sulphate solution, the ring currents corresponding to H,O* are much higher than in any of the other solutions studied indicating the stabilization of H,OI as OIH- in alkaline solution (Fig. 11). Diagnostic plots. The diagnostic plots of Damjanovic et al have been used for establishing the mechanism of oxygen reduction as far as the formation of H,O, is concerned. Figure 12 shows the plots of i,/I, USO- I” for oxygen reduction on copper in ammonium sulphate solution for two different potentials. It may be seen that both the slopes and intercepts change with potential indicating that H,O, is formed as intermediate in these solutions during oxygen reduction. But the slopes are negative. Negative slopes have been reported by Genshaw for oxygen reduction on platinum in pure H$O.+. But the diagnostic equation
first increases with potential
x+1 1,/i, = 7
x+2 + 7
k ~ WII2
requires that the slope of such plots must be positive since k’which is proportional to the rate constant must be positive. Such a negative slope was in fact reported by Genshaw[lO] for oxygen reduction on platinum in H2S04 over a certain potential range. This was explained by assuming the following probable path for the reduction of oxygen to H202 0,+2H+H,O, which is preceded by the step H,O+
+e-+H+H*O
He assumed that the rate of hydrogen ion discharge becomes small in comparison to the rate of oxygen reduction and hence 1.,/I, becomes very large. A similar negative slope ha5 also been reported on gold electrode in acidic solutions by Genshaw er nl[ lo] at more positive potentials. This was explained on the basis that O2 reduction takes place under diffusion control on oxide patches whereas H202 is formed at bare electrodes. It can be seen from the Figs 1-4 that the reduction potentials are less cathodic in the presence of ammonium ions than in the corresponding sodium salt solution. In sodium sulphate solution, the lJ1, 11s w- “’ plots (Fig. 13) are parallel to the x-axis at all potentials and the intercepts which are greater than i/N depend on potential. This condition confirms the equation IdlJI, = x + i/N. These faqts indicate that in this case H202 is produced in a parallel reaction and does not react
further. forward Thus, solution, viz
I37
The above experiments correspond to the polarization curves. the results show that, in ammonium sulphate all of the following three reactions take place, 0,
+4H+
+4e--+2H,O
HIOl + 2H* + 2e +
2H10
However, oxygen reduction on copper in sodium sulphate solutions takes place in the parallel steps, one involving direct reduction to H,O or OH-, the H,O, formed in the alternative step being stable. Since the E-i curves for copper and brass are similar, it may be expected that a similar mechanism is also applicable for brass. It may further be noted (Figs l-4) that the influence of ammonium ions can be explained on the basis of change in zero charge potential or by the explanation given by Myuller and Sobol[ll] and Balashova and Kuleznovar121 and Tarasevich et . al[13,14] for the influence of cations on the oxygen reduction on platinum but it will be only speculative at this stage. J
&fect
ofanion
and ammonium ion an oxygen reduction
The effect of anion and pH on the rate of oxygen reduction seems to depend on the nature of the metal. On copper, the oxygen reduction current (Figs 14 and 15) increases in the order (NH&SO+ > H,SO, > NaCl > HCI > Na,SO, > NH,Cl whereas for brass, it increases in the order (NH&SO+ > H2S01 > HCl _ Na#O, > NH,CI > NaCl in the range -350 to - 500 mV on an oxide-covered surface (forward polarization). In the case of oxide-f&e surface, it depends on the potential range. The highest reduction current observed in ammonium sulfate solution points out that NH: ion favours the oxygen reduction probably by causing a change in zero charge potential to more positive value. Such a shift can increase the rate of reduction has been indicated by Bockris et al[lS, 161. However, in pre+nce of chloride, as in the cases &f NH,CI, the influence of NH: ion is not felt due to adsorption of chloride ions.
0
I
I
I
2
t 30 Current
I
I
I
I
2
I
3
mA
14. E&A of diRerent salts on oxygen reduction on copper (a) Forward; (b) Reverse). -O-O(NHJ2SO~; -we Na,S04 ;-A-A- NaCl ; -A-A(NH&l) ; -O-OFii.
HCl; -I-m-
H2S04.
138
K. BALAKRISF~NANANO V. K. VENKATMN
2
3
/
0 Current
and
of rate of reduction of oxygen on copper
brass
With regard to the effect of a common anion on oxygen reduction on copper and brass, it is observed (Fig. 16) that, in sodium chloride solution, oxygen reduction is much faster on copper than on brass (Fig. 16c) which may perhaps be a reason for the reported [17] higher rate of corrosion of copper when compared to brass. In the case of sulphate solutions, oxygen reduction on copper is much higher at potentials above -450 mV, whereas beIow that,it is sIightly higher on brass (Fig. 16b). In the case ofammonium chloride and ammonium sulphate, the reduction rate is nearly the same both on copper and brass at all potentials (Fig. 16a). Acknowledgement - The authors thank Dr. H. V. K. Udupa, Director. Central Electrochemical Research Institute,
1(bl
0
I
I
,
2
0
I
I
I
I
I
20
I
2
Current
mA
Fig. 16. Rate of oxygen reduction on copper and brass. A: NH&l (1 and 2 copper and Brass); 3 and 4 (NH&SO, (Copper
and Brass); B: Na,SO,'(l - copper; 2 - J3raes);C: N&l (I - Brass; 2 - copper).
3
mA
Fig. 15. Effect of different salts on oxygen reduction on brass (a) Forward -0-ON&l; -A-ANH&I ; -A-A(NH&SO, ; -o-a-
Comparison
2
; (b) Reverse). -0 -@ HCl; -m-m-
Na2SOL ;
H,SO,.
Karaikudi, for his keen interest in the work. The authors are also thankful to Messrs. K. R. Ramakrishnan and Y. Mahadeva Lyer for their help in electronic Instrumentation. REFERENCES
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