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Electrochemical behavior of brass in 0.1 M NaCl
Applied Surface Science 193 (2002) 167–174
Electrochemical behavior of brass in 0.1 M NaCl M. Kabasakalog˘lu, T. Kıyak, O. S¸endil, A. Asan* Faculty of Arts and Science, Department of Chemistry, Gazi University, 06500 Ankara, Turkey Received 19 February 2002; accepted 4 April 2002
Abstract The corrosion behavior of brass in 0.1 M NaCl was investigated with voltametric technique. For comparison the behaviors of pure components of the alloy was investigated under the same condition. Evaluation of the solution analysis with the voltametric curves shows that initial dissolution takes place at about 1.0 V (SCE) as ZnO and zinc ions. The dissolution of copper as ions is prevented by the exchange reaction between them. This prevention continues up to CuCl formation potential. Even in the passivity region of brass, dissolution of zinc continues with anodic polarization. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Zinc; Copper; Brass; Electrochemical behavior; 0.1 M NaCl
1. Introduction Copper and its alloys are widely used in many industrial fields, especially in marine applications. Its corrosion behavior is mainly a process of dezincification. Corrosion and passivity of brass have been interpreted in terms of a dezincification process and the preferential dissolution of zinc that leaves a copper-rich porous residue. Despite the many studies reported, the reaction mechanism at different potentials is yet to be clarified [1–9]. There is still a controversy about the nature of corrosion product layers, and their growth rates are the determining factor in the corrosion behavior of brass. The aim of our work is to clarify the nature of the corrosion mechanism and corrosion product layers grown on brass in a 0.1 M NaCl solution. This was carried out by taking the voltametric current–potential curves of copper and zinc, the alloying elements of *
Corresponding author. Tel.: þ90-312-212-6030x3116; fax: þ90-312-212-2279. E-mail address: [email protected] (A. Asan).
brass, at 0.1 M NaCl aqueous solution and comparing these curves with those obtained for brass. The results of the analysis of the solutions after prolonged anodic polarization together with the voltametric curves were highly useful in elucidating the corrosion mechanism.
2. Experimental The brass electrode was prepared from a 1 mm thick brass sheet with a composition of Cu 70% and Zn 30%, it was cut as a strip and sealed with epoxy resin. The copper and zinc electrodes were prepared from an electrolytically pure copper wire (99.999%) and a pure zinc strip (99.99%) in the same manner. The electrodes were mechanically polished with fine grain emery paper of 1200 grade under water flow, washed with distilled water and acetone and dried prior to the experiments. Open surface areas of brass, copper and zinc electrodes were 0.12, 0.06 and 0.06 cm2, respectively. The potential of the working electrode was measured against a saturated calomel electrode (SCE) with a Luggin Haber capillary tip. The counter electrode
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was a Pt wire and all the three electrodes were mounted on a conventional Pyrex glass cell. The current–potential curves were recorded using a model 363 potentiostat/galvanostat (EG&G Princeton Applied Research) coupled with a Wenking model scan generator and an IBB recorder. Constant potential electrolysis was also performed in 0.1 M NaCl (pH ¼ 6:0 0:1) solutions at different potentials. The soluble ionic species were detected with PU 9258 Model Atomic Absorption Flame Spectrometer.
3. Results and discussion To elucidate the equilibrium reactions responsible for the corrosion potential of brass in 0.1 M NaCl, it is necessary to study the electrochemical reaction of copper and zinc before this potential is attained. 3.1. Potentiodynamic polarization of zinc Fig. 1 shows successive i–E curves obtained in 0.1 M NaCl solution with zinc electrode in the potential range
between 2.0 and 0.2 V at 200 mV/s. All the curves were taken from negative to positive potential values. The surface of the electrode is passive till 1.15 V in the first scan when it is oxidized from 2.0 V to positive potentials. The second and following scans taken at higher scan rates contain a side step between 1.35 and 1.100 V. This is the formation region of ZnO [10–12], this oxide is reduced at a peak in the cathodic region. The presence of this peak in the cathodic region confirms the fact that the anodic dissolution takes place with the formation of ZnO as well as zinc ions. The ZnO film formed on the zinc surface in air has better coverage properties than that formed during the anodic polarization, and its pit formation potential has more positive values than that of its counterpart formed in anodic polarization as in the first scan in Fig. 1. This shows that if the film formed on the zinc surface in air is disrupted during the corrosion process, its reformation occurs at a greater negative potential, the first process is most probably the formation of adsorbed ZnOads upon the electrode surface which converts into stable ZnO
Fig. 1. Successive anodic and cathodic polarization curves of zinc between 2.0 and 0.2 V. Scan rate: 200 mV/s. ( ) first scan, (—) successive scans, scan numbers increase with the arrow direction.
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structure. The increase in the magnitude of the current during the reverse scan of the first curve in the same potential region in the anodic region proves that the pits formed at the pits formation potential do not become passive again. Anodic polarization curves in the successive scans coincide, but the cathodic ones do not. The reason for this is that the oxide is not fully reduced in the cathodic region since there is inadequate time for the reduction of oxide at the scan rate employed. This film forms a crystallization nuclei for the new ZnO layers to be formed in the successive scan, which causes an increase in the cathodic peak. The dissolution of
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Zn2þ continues in the pores of ZnO film as it is formed. 3.2. Potentiodynamic polarization of copper Fig. 2a–d show the anodic and cathodic polarization curves of copper obtained with successive scans in 0.1 M NaCl up to different potentials. In Fig. 2a the copper electrode was anodically oxidized up to 0.65 V starting from 1.75 V and then cathodically reduced. In copper the anodic current starts to flow at 0.85 Vat a scan rate of 200 mV/s. As the number of scans is increased, one observes
Fig. 2. Successive i–E profiles for copper at 200 mV/s in 0.1 M NaCl solution between 1.75 V and (a) 0.65 V, (b) 0.35 V, (c) 0.15 V, (d) þ0.05 V. Dark and dashed lines show first and second scans, respectively. Scanning rate: 200 mV/s.
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that the charge passing in the anodic and cathodic regions increases, and there appears a layer at which the peak potential shifts towards negative values. If the data are compared with similar studies in the literature, it becomes clear that the potential ranges between 0.85 and 0.65 V, where Cu/CuOH or Cu/Cu2O equilibrium is established in alkaline solutions [13–17]. Oxidation potential of copper to Cu2O and corresponding reactions are [18]: ECu2 O=Cu ¼ þ0:471 0:059 pH ðV; SHEÞ þ
The first step is CuCl þ Cl ¼ CuClads þ e followed by a physical conversion second step CuCl þ Cl ! CuCl2 and third rate determining step Cu þ Cl ¼ CuCl2 . This mechanism proposed for acidic media is also valid for NaCl medium. Second scan in Fig. 2d shows C0III step attributed to the reduction of CuO to Cu2O. At þ0.05 V oxidation of the Cu2O layer to CuO in the pores of the film corresponding to the equilibrium of the reaction ECu2 O=CuO ¼ þ0:669 0:059 pH ðV; SHEÞ
Cu2 O þ 2H þ 2e ¼ 2Cu þ H2 O
2CuO þ 2Hþ þ 2e ¼ Cu2 O þ H2 O
According to the first equation formation potential of Cu2O in 0.1 M NaCl solution ðpH ¼ 6:0 0:1Þ is about 0.13 V (SCE); 0.65 V is more negative from this value. In Fig. 2 hydrogen evolution occurs between 1.75 and 0.65 V. So surface pH may be higher than 6:0 0:1, for example, it has an alkaline pH of 12 due to hydrogen evolution. Peak CI can be explained as the reduction of chemisorbed CuOH layer as depicted by other researchers in 0.1 M alkaline solution [12–16]. Successive scans thicken the chemisorbed CuOH layer. It gets much more stable and reduces at more negative potentials. Fig. 2b shows that the oxidation takes place up to 0.35 V. There is an anodic limitation in the anodic region, while there is a peak CII that appears at more positive potentials than the first CI peak in the cathodic region. CII peak may be the Cu2O reduction peak which is formed from the conversion of CuOH to Cu2O. The anodic oxidation up to 0.15 V increases both peak currents and peak areas or the cathodic charge (Fig. 2c). The oxidation up to þ0.05 V is depicted in Fig. 2d. There is a rapid increase in current starting from 0.15 V and there appears a new reduction peak CIII in the scan reversed from þ0.05 to 0.15 V in the pit formation potential in chloride media. The dissolution takes place as CuCl2 . This may form CuClads with the reaction given below. CIII is the reduction peak of CuCl to Cu. Since CuCl2 forms CuCl with Cu according to the reaction given below, the surface is covered again. This causes the passage of lower amount of current in the second scan in both the cathodic and anodic direction:
For the 0.1 M NaCl solution (pH ¼ 6:0), CuO ! Cu2 O reduction potential is 0.065 V (SCE) that is very close to the switching potential of þ0.050 V. The shapes of the polarization curves are highly affected by the initial potential and the scan rate employed. In Fig. 3 starting potential for polarization was 1.0 V. Hydrogen evolution is not rigorous as in Fig. 2. So CuOH and Cu2O films are not formed as in alkaline surface of copper electrode (Fig. 2a and b). Anodic current begins to pass from 0.15 V and the following reactions take place: Cu þ Cl ¼ CuClads þ e CuCl þ Cl ! CuCl2
and
CuCl2 þ Cu ¼ 2CuCl þ e According to Moreau’s [19] study to investigate the behavior of copper in acidic media, there are three distinct steps during the anodic polarization.
Fig. 3. Anodic polarization of copper between 1.0 and þ0.15 V. Scanning rate: 200 mV/s.
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3.3. Potentiostatic polarization of brass During corrosion of brass in 0.1 M NaCl solution, the formation of Cu2O, CuO and Cu(OH)2 compounds upon the surface of brass can be visibly observed due to their different colors. The comparison of the polarization curves of brass with those of copper show that the reactions which take place during the anodic polarization of brass is similar. In order to investigate the effect of zinc in the corrosion mechanism of brass the polarization was initiated from the same potential of 1.75 V. Fig. 4a was obtained at a scan rate of 200 mV/s with the final potential of 0.15 V. When the anodic region of the first scan curve is compared with the first scan curve of Fig. 2c, it can be seen that the anodic step corresponding to the formation of copper oxides is absent in the former. The cathodic
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peak observed at 1.6 V in the cathodic reverse scan is a small replica of the peak observed with zinc electrode in the same medium and conditions. As a result of this comparison we can conveniently claim that brass is in a passive state due to the ZnO layer present on its surface before the experiment which protects the surface during the first anodic polarization. After being reduced at 1.65 V, we can say that zinc gives dissolution and oxidation steps around 1.15 V in the second scan in the anodic region. Since there are not only zinc atoms upon the surface and the formation of passivating ZnO layer is facilitated by the local cathode effect of Cu atoms present on the surface, the current does not continue to be constant. The presence of this peak in the successive cathodic scans indicates that ZnO forms upon the surface after each scan. No CuCl reduction peak was observed in the first scan for copper. If the potential is kept at 0.15 V for 2 min, one can see the reduction peak of CuCl. The decrease in the dimension of zinc dissolution peak in the third scan after the reduction of CuCl and ZnO reveals that CuCl is not fully reduced upon the surface. However this layer does not prevent the oxidation of ZnO since the ZnO reduction peak is observed in a more distinctive way during the third scan. Fig. 4b illustrates the case reversed from þ0.05 V. The difference between Fig. 4a and b is that there is an additional anodic peak at 0.21 V before brass dissolution as CuCl2 and a cathodic peak at 1.25 V, which are attributed to Cu2O formation and reduction. Fig. 5 shows the initial potential and scan rate effect on the shape of the polarization curves. At higher scan rates only CuCl reduction peak is apparent as in Fig. 3, while at lower scan rate the steps related to all the copper oxides and especially those corresponding to the reduction of CuCl are separately observed. The polarization curves in Fig. 5 show quite a similarity with those obtained for copper at low scan rates. In a potential range between 1.0 and þ0.2 V, copper and brass electrodes show the same electrochemical properties.
4. Discussion Fig. 4. Polarization of brass between 1.75 V and (a) 0.15 V and (b) þ0.05 V in 0.1 M NaCl solution.
The comparison of the polarization curves of zinc and brass in a range between 1.75 and 1.0 V show
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Fig. 5. The polarization curves of brass obtained in 3% NaCl at the scan rates of 4 and 100 mV/s.
that the protective layer upon the surface is ZnO. The disruption of this layer by any reason causes corrosion by the transfer of the zinc ions into the solution. If the corrosion potential of the metal in a medium is close to the potential at which the copper oxides are formed, the corrosion products formed are the copper oxides.
Since the medium contains chloride, there is insoluble CuCl on the surface and CuCl2 in solution. This hypothesis is supported by the electrolysis data carried out at a potential range between 0.6 and þ1.0 V for various periods. Figs. 6 and 7 show the change of the charge passed with potential and time and solution analysis during the electrolysis respectively. The solution analyses revealed that copper and zinc passes into solution at every potential employed. The graphs, which depict the change of charge against the potential, and time in electrolysis, are taken as the anodic polarization curves of brass. This graph reveals that copper ions begin to pass into the solution starting from the potential at which the zinc ions begin to pass into the solution. Looking at the polarization curve of brass at slow scan rate and charge–potential relation, the increase in current begins at 0.0 V. The relation between the charge passed and time and potential can be explained as follows: 0.6 V is the potential where active dissolution of zinc takes place (Fig. 1). The dissolution of copper in 0.1 M NaCl takes place as Cu þ Cl ! CuClads þ e and Cu þ 2Cl ! CuCl2 þ e That is why the amount of copper in the first 20 min at 0.6 V is higher than those in 40 and 60 min. The
Fig. 6. The change of the charge passed with potential and time during the electrolysis. Electrode potential was kept constant at different potentials for 20, 40 and 60 min.
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Fig. 7. Zinc and copper ion concentrations after the electrolysis of brass in 0.1 M NaCl solutions for different periods.
amount of copper dissolved which cannot leave the surface precipitates upon the surface by giving 2CuCl2 þ Zn ! 2Cu þ ZnCl4 2 reaction with zinc atoms and the number of copper atoms on the surface shows an increase. Here zinc provides a cathodic protection to copper. The oscillations in the charge potential curve before 0 mV are due to the anodic dissolution of copper and zinc with the increasing polarization. The steps corresponding to these dissolutions are not as distinctive in the polarization curves (Fig. 4a and b). The reason for this is that the potential range between 0.6 and 0.0 V indicates the formation range of Cu(I) compounds. The current passed oxidizes copper and zinc, while the zinc atoms upon the surface reduce these compounds back to copper. These two phenomena oppose each other and cause the passage of a limiting-like current. Around 0.0 V there is a decrease in both, the amounts of copper and zinc ions passing to
the solution and the amount of charge measured. The polarization curves obtained at high and low scan rates reveal that the surface is covered at this potential. The surface gets passivated with the formation of CuCl as in the case of copper itself. As seen in the polarization curves the reverse scan goes under the forward scan. An increase in the amount of zinc ions is observed again at þ0.2 and þ0.4 V. The amount of charge passing through the circuit also shows an increase. The potentials at which decrease and increase in the charge passed take place and the decrease and increase in the polarization curves show quite a good accordance. Such a change can only be explained by the oxidative reaction of Cu(II) oxide or hydroxide, formed electrochemically under the CuCl film, with zinc in the brass. CuO þ Zn þ 4Cl þ H2 O ! Cu þ ZnCl4 2 þ 2OH CuðOHÞ2 þ Zn þ 2Cl ! Cu þ ZnðOHÞ2 Cl2 2
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Since copper is chemically reduced as it is electrochemically oxidized with above reactions, the amount of zinc ions in the solution increases. At þ0.6 V the amounts of copper and zinc ions passed to the solution are at their minimum. The reason for that is the termination of dissolution with the formation of ZnO as a result the surface gets more basic after the reactions given below:
Acknowledgement
CuO þ Zn ! Cu þ ZnO
[1] H.W. Pickering, P.J. Byrne, J. Electrochem. Soc. 116 (1969) 1492. [2] B. Miller, M.I. Bellavance, J. Electrochem. Soc. 119 (1972) 1510. [3] J.E. Holliday, H.W. Pickering, J. Electrochem. Soc. 120 (1973) 474. [4] A.G. Gad-Allah, M.M. Abou-Romio, M.W. Badawy, H.H. Rehan, J. Appl. Electrochem. 21 (1991) 829. [5] B.S. Kim, T. Piao, S.N. Hoier, S.M. Park, Corros. Sci. 37 (1995) 557. [6] W.A. Badawy, S.S. El-Egamy, A.S. El-Azap, Corros. Sci. 37 (1995) 1969. [7] R.C. Newman, T. Shahrabi, K. Sieradzki, Corros. Sci. 28 (1988) 873. [8] J. Moralles, P. Esparza, G.T. Fernandez, S. Gonzalez, J.E. Garaw, J. Coceres, R.C. Salvarezza, A.J. Arvia, Corros. Sci. 37 (1995) 231. [9] J. Milosev, M. Metikos-Hukovic, Electrochim. Acta 42 (1997) 1537. [10] L. Sziraki, A. Cziraki, I. Gero¨ czs, Z. Vertesy, L. Kiss, Electrochim. Acta 43 (1998) 175. [11] L.M. Abrantes, L.M. Castillo, C. Norman, L.M. Peter, J. Electroanal. Chem. 163 (1984) 209. [12] E.E. Foad El Sherbini, S.S. Abd El Rehim, Corros. Sci. 42 (2000) 785. [13] F.D. Quarto, S. Piazza, C. Sunseri, Electrochim. Acta 30 (1985) 315. [14] M.R.G. De Chialvo, R.C. Salvarezza, D.V. Moll, A.J. Arvia, Electrochim. Acta 30 (1985) 1501. [15] V. Collisi, H-H. Strehblow, J. Electroanal. Chem. 284 (1990) 385. [16] D.M.J.G. Ahern, T.G. Ryan, J. Electrochem. Soc. 137 (1990) 553. [17] G. Bertrand, E. Rocca, C. Savall, C. Rapin, J.-C. Labrune, P. Steinmetz, J. Electroanal. Chem. 38 (2000) 489. [18] M. Pourbaix, in: Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1996. [19] A. Moreau, Electrochim. Acta 26 (1981) 1609.
CuðOHÞ2 þ Zn ! Cu þ ZnðOHÞ2 At higher potentials the concentrations of copper and zinc ions passing to the solution increase as a result of the rupture of the passive layer upon the surface with oxygen evolution.
5. Conclusion Brass metal is covered with ZnO in air. The initial dissolution takes (1.00 V) place as Zn2þ in an aqueous media containing the chloride. As it starts as CuCl2 , the dissolution is prevented by the exchange of copper and zinc. This prevention continues up to CuCl formation potential. CuCl film does not prevent the formation of Cu(II) oxides. The copper oxides initially formed can dissociate by the effect of zinc atoms upon the surface. That is why only the CuCl reduction step is distinct at higher scan rates. At lower scan rates, on the other hand, it is possible to see the reduction step of copper oxides in the cathodic region since the surface zinc atoms are not sufficient enough to give reaction with copper oxides. In conclusion ZnO provides the passivity of brass. Under the conditions that this ZnO layer is ruptured, both copper and zinc atoms pass into the solution. The corrosion mainly continues by the passage of zinc ions into the solution. Even in the passivation region in the polarization curve, the dissolution of zinc continues.
The authors wish to express their gratitude to the Gazi University Research Council for the financial support. References
Electrochemical behavior of brass in 0.1 M NaCl M. Kabasakalog˘lu, T. Kıyak, O. S¸endil, A. Asan* Faculty of Arts and Science, Department of Chemistry, Gazi University, 06500 Ankara, Turkey Received 19 February 2002; accepted 4 April 2002
Abstract The corrosion behavior of brass in 0.1 M NaCl was investigated with voltametric technique. For comparison the behaviors of pure components of the alloy was investigated under the same condition. Evaluation of the solution analysis with the voltametric curves shows that initial dissolution takes place at about 1.0 V (SCE) as ZnO and zinc ions. The dissolution of copper as ions is prevented by the exchange reaction between them. This prevention continues up to CuCl formation potential. Even in the passivity region of brass, dissolution of zinc continues with anodic polarization. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Zinc; Copper; Brass; Electrochemical behavior; 0.1 M NaCl
1. Introduction Copper and its alloys are widely used in many industrial fields, especially in marine applications. Its corrosion behavior is mainly a process of dezincification. Corrosion and passivity of brass have been interpreted in terms of a dezincification process and the preferential dissolution of zinc that leaves a copper-rich porous residue. Despite the many studies reported, the reaction mechanism at different potentials is yet to be clarified [1–9]. There is still a controversy about the nature of corrosion product layers, and their growth rates are the determining factor in the corrosion behavior of brass. The aim of our work is to clarify the nature of the corrosion mechanism and corrosion product layers grown on brass in a 0.1 M NaCl solution. This was carried out by taking the voltametric current–potential curves of copper and zinc, the alloying elements of *
Corresponding author. Tel.: þ90-312-212-6030x3116; fax: þ90-312-212-2279. E-mail address: [email protected] (A. Asan).
brass, at 0.1 M NaCl aqueous solution and comparing these curves with those obtained for brass. The results of the analysis of the solutions after prolonged anodic polarization together with the voltametric curves were highly useful in elucidating the corrosion mechanism.
2. Experimental The brass electrode was prepared from a 1 mm thick brass sheet with a composition of Cu 70% and Zn 30%, it was cut as a strip and sealed with epoxy resin. The copper and zinc electrodes were prepared from an electrolytically pure copper wire (99.999%) and a pure zinc strip (99.99%) in the same manner. The electrodes were mechanically polished with fine grain emery paper of 1200 grade under water flow, washed with distilled water and acetone and dried prior to the experiments. Open surface areas of brass, copper and zinc electrodes were 0.12, 0.06 and 0.06 cm2, respectively. The potential of the working electrode was measured against a saturated calomel electrode (SCE) with a Luggin Haber capillary tip. The counter electrode
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 2 5 8 - 1
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was a Pt wire and all the three electrodes were mounted on a conventional Pyrex glass cell. The current–potential curves were recorded using a model 363 potentiostat/galvanostat (EG&G Princeton Applied Research) coupled with a Wenking model scan generator and an IBB recorder. Constant potential electrolysis was also performed in 0.1 M NaCl (pH ¼ 6:0 0:1) solutions at different potentials. The soluble ionic species were detected with PU 9258 Model Atomic Absorption Flame Spectrometer.
3. Results and discussion To elucidate the equilibrium reactions responsible for the corrosion potential of brass in 0.1 M NaCl, it is necessary to study the electrochemical reaction of copper and zinc before this potential is attained. 3.1. Potentiodynamic polarization of zinc Fig. 1 shows successive i–E curves obtained in 0.1 M NaCl solution with zinc electrode in the potential range
between 2.0 and 0.2 V at 200 mV/s. All the curves were taken from negative to positive potential values. The surface of the electrode is passive till 1.15 V in the first scan when it is oxidized from 2.0 V to positive potentials. The second and following scans taken at higher scan rates contain a side step between 1.35 and 1.100 V. This is the formation region of ZnO [10–12], this oxide is reduced at a peak in the cathodic region. The presence of this peak in the cathodic region confirms the fact that the anodic dissolution takes place with the formation of ZnO as well as zinc ions. The ZnO film formed on the zinc surface in air has better coverage properties than that formed during the anodic polarization, and its pit formation potential has more positive values than that of its counterpart formed in anodic polarization as in the first scan in Fig. 1. This shows that if the film formed on the zinc surface in air is disrupted during the corrosion process, its reformation occurs at a greater negative potential, the first process is most probably the formation of adsorbed ZnOads upon the electrode surface which converts into stable ZnO
Fig. 1. Successive anodic and cathodic polarization curves of zinc between 2.0 and 0.2 V. Scan rate: 200 mV/s. ( ) first scan, (—) successive scans, scan numbers increase with the arrow direction.
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structure. The increase in the magnitude of the current during the reverse scan of the first curve in the same potential region in the anodic region proves that the pits formed at the pits formation potential do not become passive again. Anodic polarization curves in the successive scans coincide, but the cathodic ones do not. The reason for this is that the oxide is not fully reduced in the cathodic region since there is inadequate time for the reduction of oxide at the scan rate employed. This film forms a crystallization nuclei for the new ZnO layers to be formed in the successive scan, which causes an increase in the cathodic peak. The dissolution of
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Zn2þ continues in the pores of ZnO film as it is formed. 3.2. Potentiodynamic polarization of copper Fig. 2a–d show the anodic and cathodic polarization curves of copper obtained with successive scans in 0.1 M NaCl up to different potentials. In Fig. 2a the copper electrode was anodically oxidized up to 0.65 V starting from 1.75 V and then cathodically reduced. In copper the anodic current starts to flow at 0.85 Vat a scan rate of 200 mV/s. As the number of scans is increased, one observes
Fig. 2. Successive i–E profiles for copper at 200 mV/s in 0.1 M NaCl solution between 1.75 V and (a) 0.65 V, (b) 0.35 V, (c) 0.15 V, (d) þ0.05 V. Dark and dashed lines show first and second scans, respectively. Scanning rate: 200 mV/s.
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that the charge passing in the anodic and cathodic regions increases, and there appears a layer at which the peak potential shifts towards negative values. If the data are compared with similar studies in the literature, it becomes clear that the potential ranges between 0.85 and 0.65 V, where Cu/CuOH or Cu/Cu2O equilibrium is established in alkaline solutions [13–17]. Oxidation potential of copper to Cu2O and corresponding reactions are [18]: ECu2 O=Cu ¼ þ0:471 0:059 pH ðV; SHEÞ þ
The first step is CuCl þ Cl ¼ CuClads þ e followed by a physical conversion second step CuCl þ Cl ! CuCl2 and third rate determining step Cu þ Cl ¼ CuCl2 . This mechanism proposed for acidic media is also valid for NaCl medium. Second scan in Fig. 2d shows C0III step attributed to the reduction of CuO to Cu2O. At þ0.05 V oxidation of the Cu2O layer to CuO in the pores of the film corresponding to the equilibrium of the reaction ECu2 O=CuO ¼ þ0:669 0:059 pH ðV; SHEÞ
Cu2 O þ 2H þ 2e ¼ 2Cu þ H2 O
2CuO þ 2Hþ þ 2e ¼ Cu2 O þ H2 O
According to the first equation formation potential of Cu2O in 0.1 M NaCl solution ðpH ¼ 6:0 0:1Þ is about 0.13 V (SCE); 0.65 V is more negative from this value. In Fig. 2 hydrogen evolution occurs between 1.75 and 0.65 V. So surface pH may be higher than 6:0 0:1, for example, it has an alkaline pH of 12 due to hydrogen evolution. Peak CI can be explained as the reduction of chemisorbed CuOH layer as depicted by other researchers in 0.1 M alkaline solution [12–16]. Successive scans thicken the chemisorbed CuOH layer. It gets much more stable and reduces at more negative potentials. Fig. 2b shows that the oxidation takes place up to 0.35 V. There is an anodic limitation in the anodic region, while there is a peak CII that appears at more positive potentials than the first CI peak in the cathodic region. CII peak may be the Cu2O reduction peak which is formed from the conversion of CuOH to Cu2O. The anodic oxidation up to 0.15 V increases both peak currents and peak areas or the cathodic charge (Fig. 2c). The oxidation up to þ0.05 V is depicted in Fig. 2d. There is a rapid increase in current starting from 0.15 V and there appears a new reduction peak CIII in the scan reversed from þ0.05 to 0.15 V in the pit formation potential in chloride media. The dissolution takes place as CuCl2 . This may form CuClads with the reaction given below. CIII is the reduction peak of CuCl to Cu. Since CuCl2 forms CuCl with Cu according to the reaction given below, the surface is covered again. This causes the passage of lower amount of current in the second scan in both the cathodic and anodic direction:
For the 0.1 M NaCl solution (pH ¼ 6:0), CuO ! Cu2 O reduction potential is 0.065 V (SCE) that is very close to the switching potential of þ0.050 V. The shapes of the polarization curves are highly affected by the initial potential and the scan rate employed. In Fig. 3 starting potential for polarization was 1.0 V. Hydrogen evolution is not rigorous as in Fig. 2. So CuOH and Cu2O films are not formed as in alkaline surface of copper electrode (Fig. 2a and b). Anodic current begins to pass from 0.15 V and the following reactions take place: Cu þ Cl ¼ CuClads þ e CuCl þ Cl ! CuCl2
and
CuCl2 þ Cu ¼ 2CuCl þ e According to Moreau’s [19] study to investigate the behavior of copper in acidic media, there are three distinct steps during the anodic polarization.
Fig. 3. Anodic polarization of copper between 1.0 and þ0.15 V. Scanning rate: 200 mV/s.
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3.3. Potentiostatic polarization of brass During corrosion of brass in 0.1 M NaCl solution, the formation of Cu2O, CuO and Cu(OH)2 compounds upon the surface of brass can be visibly observed due to their different colors. The comparison of the polarization curves of brass with those of copper show that the reactions which take place during the anodic polarization of brass is similar. In order to investigate the effect of zinc in the corrosion mechanism of brass the polarization was initiated from the same potential of 1.75 V. Fig. 4a was obtained at a scan rate of 200 mV/s with the final potential of 0.15 V. When the anodic region of the first scan curve is compared with the first scan curve of Fig. 2c, it can be seen that the anodic step corresponding to the formation of copper oxides is absent in the former. The cathodic
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peak observed at 1.6 V in the cathodic reverse scan is a small replica of the peak observed with zinc electrode in the same medium and conditions. As a result of this comparison we can conveniently claim that brass is in a passive state due to the ZnO layer present on its surface before the experiment which protects the surface during the first anodic polarization. After being reduced at 1.65 V, we can say that zinc gives dissolution and oxidation steps around 1.15 V in the second scan in the anodic region. Since there are not only zinc atoms upon the surface and the formation of passivating ZnO layer is facilitated by the local cathode effect of Cu atoms present on the surface, the current does not continue to be constant. The presence of this peak in the successive cathodic scans indicates that ZnO forms upon the surface after each scan. No CuCl reduction peak was observed in the first scan for copper. If the potential is kept at 0.15 V for 2 min, one can see the reduction peak of CuCl. The decrease in the dimension of zinc dissolution peak in the third scan after the reduction of CuCl and ZnO reveals that CuCl is not fully reduced upon the surface. However this layer does not prevent the oxidation of ZnO since the ZnO reduction peak is observed in a more distinctive way during the third scan. Fig. 4b illustrates the case reversed from þ0.05 V. The difference between Fig. 4a and b is that there is an additional anodic peak at 0.21 V before brass dissolution as CuCl2 and a cathodic peak at 1.25 V, which are attributed to Cu2O formation and reduction. Fig. 5 shows the initial potential and scan rate effect on the shape of the polarization curves. At higher scan rates only CuCl reduction peak is apparent as in Fig. 3, while at lower scan rate the steps related to all the copper oxides and especially those corresponding to the reduction of CuCl are separately observed. The polarization curves in Fig. 5 show quite a similarity with those obtained for copper at low scan rates. In a potential range between 1.0 and þ0.2 V, copper and brass electrodes show the same electrochemical properties.
4. Discussion Fig. 4. Polarization of brass between 1.75 V and (a) 0.15 V and (b) þ0.05 V in 0.1 M NaCl solution.
The comparison of the polarization curves of zinc and brass in a range between 1.75 and 1.0 V show
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Fig. 5. The polarization curves of brass obtained in 3% NaCl at the scan rates of 4 and 100 mV/s.
that the protective layer upon the surface is ZnO. The disruption of this layer by any reason causes corrosion by the transfer of the zinc ions into the solution. If the corrosion potential of the metal in a medium is close to the potential at which the copper oxides are formed, the corrosion products formed are the copper oxides.
Since the medium contains chloride, there is insoluble CuCl on the surface and CuCl2 in solution. This hypothesis is supported by the electrolysis data carried out at a potential range between 0.6 and þ1.0 V for various periods. Figs. 6 and 7 show the change of the charge passed with potential and time and solution analysis during the electrolysis respectively. The solution analyses revealed that copper and zinc passes into solution at every potential employed. The graphs, which depict the change of charge against the potential, and time in electrolysis, are taken as the anodic polarization curves of brass. This graph reveals that copper ions begin to pass into the solution starting from the potential at which the zinc ions begin to pass into the solution. Looking at the polarization curve of brass at slow scan rate and charge–potential relation, the increase in current begins at 0.0 V. The relation between the charge passed and time and potential can be explained as follows: 0.6 V is the potential where active dissolution of zinc takes place (Fig. 1). The dissolution of copper in 0.1 M NaCl takes place as Cu þ Cl ! CuClads þ e and Cu þ 2Cl ! CuCl2 þ e That is why the amount of copper in the first 20 min at 0.6 V is higher than those in 40 and 60 min. The
Fig. 6. The change of the charge passed with potential and time during the electrolysis. Electrode potential was kept constant at different potentials for 20, 40 and 60 min.
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Fig. 7. Zinc and copper ion concentrations after the electrolysis of brass in 0.1 M NaCl solutions for different periods.
amount of copper dissolved which cannot leave the surface precipitates upon the surface by giving 2CuCl2 þ Zn ! 2Cu þ ZnCl4 2 reaction with zinc atoms and the number of copper atoms on the surface shows an increase. Here zinc provides a cathodic protection to copper. The oscillations in the charge potential curve before 0 mV are due to the anodic dissolution of copper and zinc with the increasing polarization. The steps corresponding to these dissolutions are not as distinctive in the polarization curves (Fig. 4a and b). The reason for this is that the potential range between 0.6 and 0.0 V indicates the formation range of Cu(I) compounds. The current passed oxidizes copper and zinc, while the zinc atoms upon the surface reduce these compounds back to copper. These two phenomena oppose each other and cause the passage of a limiting-like current. Around 0.0 V there is a decrease in both, the amounts of copper and zinc ions passing to
the solution and the amount of charge measured. The polarization curves obtained at high and low scan rates reveal that the surface is covered at this potential. The surface gets passivated with the formation of CuCl as in the case of copper itself. As seen in the polarization curves the reverse scan goes under the forward scan. An increase in the amount of zinc ions is observed again at þ0.2 and þ0.4 V. The amount of charge passing through the circuit also shows an increase. The potentials at which decrease and increase in the charge passed take place and the decrease and increase in the polarization curves show quite a good accordance. Such a change can only be explained by the oxidative reaction of Cu(II) oxide or hydroxide, formed electrochemically under the CuCl film, with zinc in the brass. CuO þ Zn þ 4Cl þ H2 O ! Cu þ ZnCl4 2 þ 2OH CuðOHÞ2 þ Zn þ 2Cl ! Cu þ ZnðOHÞ2 Cl2 2
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Since copper is chemically reduced as it is electrochemically oxidized with above reactions, the amount of zinc ions in the solution increases. At þ0.6 V the amounts of copper and zinc ions passed to the solution are at their minimum. The reason for that is the termination of dissolution with the formation of ZnO as a result the surface gets more basic after the reactions given below:
Acknowledgement
CuO þ Zn ! Cu þ ZnO
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CuðOHÞ2 þ Zn ! Cu þ ZnðOHÞ2 At higher potentials the concentrations of copper and zinc ions passing to the solution increase as a result of the rupture of the passive layer upon the surface with oxygen evolution.
5. Conclusion Brass metal is covered with ZnO in air. The initial dissolution takes (1.00 V) place as Zn2þ in an aqueous media containing the chloride. As it starts as CuCl2 , the dissolution is prevented by the exchange of copper and zinc. This prevention continues up to CuCl formation potential. CuCl film does not prevent the formation of Cu(II) oxides. The copper oxides initially formed can dissociate by the effect of zinc atoms upon the surface. That is why only the CuCl reduction step is distinct at higher scan rates. At lower scan rates, on the other hand, it is possible to see the reduction step of copper oxides in the cathodic region since the surface zinc atoms are not sufficient enough to give reaction with copper oxides. In conclusion ZnO provides the passivity of brass. Under the conditions that this ZnO layer is ruptured, both copper and zinc atoms pass into the solution. The corrosion mainly continues by the passage of zinc ions into the solution. Even in the passivation region in the polarization curve, the dissolution of zinc continues.
The authors wish to express their gratitude to the Gazi University Research Council for the financial support. References