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Corrosion of copper in both aqueous ethylenediamine and its complexes with some transition metal ions
Materials
Chemistry
and Physics 46 ( 1996) 61-66
Corrosion of copper in both aqueous ethylenediamine and its complexes with some transition metal ions A. El-Sayed, F. Rashwan, F. El-Cheikh Department
of Chemistry, Received
Sohag Faculty 4 August
of Science,
1995; accepted
South- Valley University, 13 December
Sohag, Egypt
1995
Abstract The corrosion of copper in both 0.2 M ethylenediamine (en) /buffer and buffer containing complexes of some transition metal ions with ethylenediamine has been studied at the open-circuit potential (E,,,) using impedance spectroscopy. Impedance parameters, such as the polarization resistance R,, solution resistance R, and electrode capacitance C, were computed and compared. In all solutions investigated, the results showed that the electrode capacitance C gradually increased, whereas both R, and R, decreased as the time of immersion was increased. This behaviour has been attributed to a continuous change in the electrode surface area as a result of Cu dissolution. On the basis of the evaluatedexperimental results, it is established that therates ofelectrodecorrosion in thecaseof [Cd(en),]*‘, [Zn(en),]*’ and [Ni( en)z]2’ complexes are comparable. On the other hand, the rate of corrosion of Cu in the presence of [Cu(en),]” complex is found to be much higher (7.0 times). The data showed that the corrosion of Cu decreases in the following order: en > > Cu complex > > Cd complex > Zn complex > Ni complex. Keywords:
Impedance;
Copper;
Corrosion;
Ethylenediamine
complexes
1. Introduction
2. Experimental
Impedance spectroscopy (IS) has been widely used in the study of corrosion phenomena [ I], The study of corrosion and corrosion inhibition of Cu and its alloys is of great interest because of their wide applications in mechanical and chemical environments. Numerous additives of both organic and inorganic compounds have been used as corrosion inhibitors for Cu in various media [2]. Ethylenediamine (en) was used as a bath for the electroplating of Cu in which certain organic and inorganic additives were used to improve the quality of the electroplates [ 3-51. Also, ethylenediamine has been used as a corrosion inhibitor of Cu in acid solutions [ 6,7]. Balezin et al. [ 81 studied the corrosion of Cu in water containing ethylenediamine and monoethanolamine. They found that the corrosion of the metal increases with increasing concentration of both solutions. The corrosion of Cu was also investigated in ethylene glycol-water mixtures containing chloride ions [ 91. As far as we have been able to establish, there are very few publications relating to the corrosion of Cu in aqueous ethylenediamine. The present study was undertaken to understand more of the corrosion behaviour of Cu in aqueous ethylenediamine containing some metal-ion complexes using the modern impedance technique.
2.1. Electrolytes
0254-0584/96/$15.00
0 1996 Elsevier
Science S.A. All rights reserved
Doubly distilled water and analytical grade reagents were used in all experiments without further purification. All measurements were made in solutions having the same ionic strength, consisting of 0.2 M ethylenediamine, 0.1 M NH&l and 0.1 M KCl. The metal-ion complexes were prepared prior to each experiment with a 2: 1 ratio of both ethylenediamine and the chloride salt of the metal in the buffer solution. The pH of each solution was measured just before the experimental run using a Fisher 230 A pH/ion meter. The pH values were 10.2, 8.3, 8.0, 7.5 and 7.4 for the en/buffer, Cd-en/ buffer, Zn-enlbuffer, N&n/buffer and Cu-enlbuffer solutions, respectively. 2.2. Copper specimen All measurements were made using a Cu sheet as the working electrode, with a constant surface area of 4.0 cm* (99.9% electrolytic laminae, Prolabo) . Prior to each measurement, the electrode was polished with No. 600 emery paper and then degreased in pure ethanol, washed in running doubly
A. El-Sayed
et al. /Materials
Chemistry
and Physics 46 (199G) 61-66
(b)
(4
W
Fig. 1. Impedance diagrams for the Cu electrode at the corrosion potential after 40 min immersion in chloride buffer at 25 “C: (a) en; (b) Cu-en complex; (c) Cd+n complex; (d) Zwen complex.
distilled water, dried and weighed before being inserted in the polarization cell.
3. Results 3.1. Behavionr in en/b@er sohtion
2.3. Polarization
cell and measurements
A conventional three-electrode cell was used with 1.Ocm2 Pt sheet as the counter electrode, which was separated from the main cell compartment using a glass sinter, The reference electrode was a saturated calomel electrode (SCE) , to which all potential values are referred. The cell description has been given elsewhere [ 101. Prior to each experimental measurement, the solution under investigation was made oxygen free by passing prewashed pure nitrogen though it for a sufficient duration of time. The impedance spectroscopic measurements were carried out in the frequency range from 1.O to 0.005 Hz [ 111. Five data points were taken for each decade of frequency, or for each change in frequency by a power of ten. An amplitude of 10 mV (peak to peak) for the a.c. signal was used for all IS experiments. An EG&G Princeton Applied Research model 273 potentiostat/galvanostat, coupled with an EG&G Princeton Applied Research model 5208 two-phase lock-in analyzer, was controlled using an IBM PC 30 computer. Model 378 a.c. impedance software, from EG&G Princeton Applied Research, was used for all IS measurements. The cell temperature was kept constant (25.0 I 1.0 “C) using a Haake N3 ultrathermostat. Each experiment was repeated until the reproducibility of the results was satisfactory. Scanning electron microscope (SEM) observations were made using a JEOL JSM 5300 instrument.
Fig. 1 (a) presents the Nyquist complex plane plot for the IS of Cu electrode in 0.2 M aqueous ethylenediamine containing 0.1 M NH&l and 0.1 M KCl. From the plot, it is clear that the experimental results show an extreme deviation from the usual semicircle shape. This abnormal behaviour may be attributed to continuous changes in the electrode surface as a result of the relatively high rate of metal dissolution during IS measurements [ 121. The appearance of the IS diagram under the real axis may be ascribed to adsorption of ethylenediamine on the electrode surface in a step just prior to Cu dissolution. In this case, calculation of the electrode capacitance, solution resistance and polarization resistance is precluded. At the instant of immersion, the observed corrosion potential (E,,,) was = - 540 mV versus SCE, which implies that the Cu electrode is very active in the en/buffersolution. After immersion for 4.5 h, the open circuit potential (O.C.P.) reaches a steady-state value ( - 500 mV) , This shift in E,,, to a less negative potential is attributed to some inhibition effects [ 131, Hence, it is suggested that the instantly formed [WenM2+ complex is readily chemisorbed on the electrode surface [ 141. 3.2. Behaviour in the presence of (Crl(en),]”
complex
The IS for the Cu electrode in the buffer solution containing [Wen)J*+ complex shows a capacitive and less resistive semicircular behaviour (Fig. 1(b) ), The electrode capacitance C,, was calculated making use of the Bode format of IS (Fig. 2). Both Ihe charge transfer
A. El-Sayed
+
,-. m : ” CD -10+ -1.5
Chemistry
+ t *+t
184 -4B
et al. /Materials
,
+t
+t i
I
t
1
ft
tL,.L++
,
1
t t
t
tt tf , + t tttt+ 1
ttt+
1
I
I
1
I
lug Frecwnccj (Hz) Fig. 2. Bode plot of the Cu electrode in the Cu-en complex after 40 min immersion.
resistance R, and the solution resistance R, were determined using the Nyquist diagrams (Table 1). In addition, the corrosion current density (i,,,) data are calculated using the charge transfer resistance R, obtained from impedance measurements, together with the Stern-Geary equation: ice, =
b&c
63
and Physics 46 (1996) 61-66
processes, respectively. Assuming that b,= 60 mV and b, = 120 mV, the values of i,,, are given in Table 2. In the presence of [ Cu( en) 2] ‘+ complex, the weight loss of the Cu electrode after 4.5 h immersion was 2.0 mg, in comparison with 8.0 mg weight loss for the same immersion time in en/buffer solution only. This indicates that the corrosion of the electrode increases with increasing concentration of free en. Also, it is suggested that the [ Cu( en) J2’ complex is probably chemisorbed 1.141 at the electrode surface, leading to some corrosion inhibition of the metal. Unlike the behaviour in en/buffer, the constancy of E,,, at - 37Oi 1 mV in the presence of the [Cu(en),]‘+ complex implies that the equilibrium between the electrode and its ion in solution is attained in a very short time. As shown in Table 1, both R, and R, decrease from 6.3 and 33.1 R at the instant of immersion to 2.8 and 16.5 0, respectively, after 4.5 h. These data, in addition to the regular increase in the electrode capacitance and corrosion current density with time (Table 2), are indicative of the continuous changes in the electrode surface as a result of its dissolution. 3.3. Behaviour in the presenceof [Cd(en),]‘+ complex
2.303 Rd(b,+b,)
where R, = (dEldi) = 0, A is the electrode surface area, and b, and b, are the Tafel slopes of the anodic and cathodic
Fig. 1(c) represents the Nyquist complex plane plot for the IS of Cu electrode in [ Cd(en),] 2f complex/buffer solu-
Table 1 Impedance parameters of the Cu electrode in complexes of en with some transition-metal ions
t
lWen),l*+
(tin)
R,(fin)
R, (0)
-Ec, (mV)
R, (a)
R, (0)
-&xc (mv)
Rs (a)
R, (a)
-&m (mv)
R,(fl)
R, Cfi)
-L Mv
10 40 50 70 90 110 130 150 200 260
33.1 21.6 24.0 23.0 21.5 19.8 18.5 18.0 17.0 16.5
6.3 6.0 5.2 4.5 4.0 3.5 3.1 3.0 2.9 2.8
370 370 370 371 371 370 370 370 370 370
388 360 320 300 272 216 189 165 94 90
338 125 88 75 45 25 11 8 6.3 4.8
415 411 410 408 406 403 402 401 400 400
733 581 302 265 230 200 175 156 140 125
1105 525 115 75 68 40 10 8 I 6.5
400 395 393 393 393 393 392 392 392 392
440 440 373 315 300 270 240 230 180 166
2100 1950 1200 930 800 550 435 350 240 205
380 363 346 340 337 335 333 332 332 322
lWenM*
Table 2 Corrosion current density i,,
Kn(enM2+
[Ni(en)J”
(PA cm-*) of Cu in complexes of en with some transition-metal ions
t (mm)
[We&l*+
lWen),l*’
Pn(en),12+
[Ni(en)J”
10 40 50 70 90 110 130 150 200 260
690 125 836 966 1087 1243 1403 1450 1500 1553
13 35 49 58 97 174 395 543 690 906
4 8 38 58 64 109 435 543 621 669
2 2.2 3.6 4.1 5.4 8.0 10 12 18 21
A. El-Sayed
64
et al. /Materials
Chemistry
and Physics 46 (1996) 61-66
65
60
I-
t
t 30
L 0
50
100
150 Time(mh.)
200
250
300
Fig. 3. Variation of the Cu electrode capacitance with time at the corrosion potential at 25 “C in (a) G-en complex; (b) Cd-en complex; (c) Zn-en complex; (d) Ni-en complex.
tion. The evaluated IS parameters are listed in Table 1, and the capacitance is represented by line b in Fig. 3. These results show a drastic decrease in both R, and R,, from 338 and 388 fl at the beginning of the experiment to only 4.8 and 90.0 a, respectively, at the end of the experimental run (after 4.5 h) . These data, along with the straightforward increase in the electrode capacitance (line b in Fig. 3) and i,,, (Table 2)) confirm the view mentioned before concerning the corrosion process occurring on the electrode surface.
Fig. 4. Nyquist plots for the Cu electrode at the corrosion potential as a function of immersion time at 25 “C: (a) 10 min; (b) 40 min; (c) 50 min; (d) 70 min; (e) 90 min; (f) 110 min; (g) 130 min; (h) 150 min; (i) 200 min; (j) 260 min.
from the plots, the IS behaviour is a somewhat flattened semicircle, but is not very far from the ideal shape. The extracted data contained in Table 1 demonstrate that both R, and R, decrease rapidly from 2100 and 440 s1 to 205 and 166 Q respectively, after 4.5 h immersion, while a regular increase in the electrode capacitance with time is obtained (line d in Fig. 3). Generally, these results are very similar to the data computed in the presence of Zn-en complex.
3.4. Behaviour in the presenceof [Zn(en)J2 + complex
4. Discussion
The Nyquist plane diagram for IS and Cu electrode in the complex/buffer solution is presence of [Zn(en),]‘+ depicted in Fig. 1 (d) . The computed IS parameters and i,,, values are listed in Tables 1 and 2, respectively. The variation of the electrode capacitance with time is represented by line cinFig.3. The results demonstrate a behaviour similar to that observed in the last case, where both R, and R, decrease, from 1105 and 733 Sz to 6.5 and 125 a, respectively, after 4.5 h immersion. Also, a straightforward relationship between the time of immersion and the electrode capacitance is registered (line c in Fig. 3).
The linear variation of the electrode capacitance with time (Fig. 3) for all solutions investigated, i.e., [Cu(en)J2”, [Cd(en),]‘+, [Zn(en),]** and [Ni(en)2]2” complexes, suggests corrosion stimulation of Cu as a result of continuous changes in the electrode surface area [ 151. The rates of corrosion in the presence of [Cd(en),12’, [Zn(en)212”, [zn(en>212+ and [Ni(en)2]2’ complexes are found to be comparable, whereas that in the case of [ Cu(en),] 2’ is much higher (see Fig. 3 and Table 2). This unique behaviour in the case of Cu-en may be interpreted in view of the liability characterizing most of the transition metal ion complexes. It is found with en and Cu2* that [Cu(en),(H20),12’ and are readily formed in aqueous solutions [Cu(en)UW)d2 [ 161. On the basis of this foundation, the following equilibrium can be suggested for our case:
3.5. Behaviour in the presenceof [Ni(en)2]2’ complex
Fig. 4 represents the Nyquist complex plane diagrams for the Cu electrode in buffer solution containing [ Ni( en) *] *+ complex for various intervals of immersion time. As shown
[Cu(en)2(H20)2]2’ [Cu(en)(H20)4]2+
+2H,Oti ten
A. El-Sayed
et al. /Materials
Chemistry
Since en molecules are consumed on the surface of the electrode, a shift of the equilibrium point to the right must occur, and hence water-ligand substitution becomes very possible.
Fig. 5. SEM micrographs of a Cu electrode sheet surface under different experimental conditions (magnification: X 1000) : (a) the electrode surface before immersion (uncorroded surface) ; (b) the electrode surface after 1 h immersion in N&en/buffer solution; (c) the same conditions as in (b) but after 5 h immersion time; (d) the electrode surface after 5 h immersion in en/buffer solution only.
and Physics 46 (1996)
61-66
65
It is also found [ 161 that Cu complexes are among the class of metal-ion complexes that are characterized by very high rate constants for water-ligand exchange (k= 108-1010 s-i). The approximate rate-constant values for the other three metal-ion complexes under investigation are found to be 4.0 X 108, 4.0 X lo7 and 1.5 X lo4 s-l for the Cd, Zn and Ni complexes, respectively. According to the foregoing discussion, the following order for the corrosion rates of the electrode can be understood: Cu complex > > Cd complex > Zn complex > Ni complex. The observed EC,, values at the instant of electrode immersion are found to be -540, -430, -418, - 380 and - 370 mV versus SCE for the en, Cd, Zn, Ni and Cu solutions, respectively. These data suggest that the activity of the Cu electrode decreases in the sequence en system > Cd system > Zn system > Ni system > Cu system. However, the IS results show that the corrosion rate of the electrode in the presence of Cu-en complex must lie directly behind that of free en in the latter sequence. Since the Cu electrode in the case of the Cu-en system is immersed in its own ions, fast equilibrium must be established, and hence E,,, will reach a steady-state value in a very short time. Thus, we believe that the real E,,, value for the Cu-en complex should lie between -540 and -430mV. From the thermodynamic point of view, the corrosion rates of the Cu electrode according to the literature values of the stability constants for the complexes under investigation should follow the sequence Cd complex > Zn complex > Ni complex > Cu complex; hence the K values listed are 2.9 X 105, 5.9 X 16, 3.3 X lo7 and 4.6 X lOlo, respectively [ 171. It is clear that a high stability constant implies reduction of the free-ligand concentration in the solution, and consequently a low corrosion rate must be observed. This view is in accordance with the IS experimental results, which imply that the corrosion of Cu decreases in the order Cd complex > Zn complex > Ni complex. The deviation of the corrosion rate in the presence of the Cu-en system from this sequence may be ascribed to its high rate constant, characterizing the water-ligand substitution process, as discussed before. The fact that the corrosion rate of copper metal increases regularly with time (Table 2)) so that it increases by lo-, 70and 180-fold after 260 min for the Ni, Cd and Zn complexes, respectively, confirms the idea suggested that copper complexes enhance the corrosion of copper metal. As time passes, more and more copper complexes accumulate in solution (due to copper corrosion), replacing the complexes of Ni, Zn and Cd (Cu complexes being the most stable), and the corrosion rate becomes more and more pronounced. For the corrosion of Cu metal in the presence of Cu-en complex, it was found that the rate increased by only 2.25 times. By comparing the SEM micrographs for the Cu electrode surface before immersion (uncorroded specimen, Fig. 5 (a) ) with that obtained after 1 h immersion in Ni-en complex/ buffer solution (Fig. 5(b) ) , one may observe a significant difference. The clear scratches that characterize the uncorroded surface, as a result of its sheet-polishing nature, seem
66
A. El-Sayed et al. /Materials Chetnisrry and Physics 46 (1996) 61-66
to be partially covered with corrosion products. The progress of corrosion, upon increasing the immersion time to 5 h, can be confirmed by observing the development of the electrodesurface coverage given in Fig. 5 (c) . These observations support the data listed in Tables 1 and 2 concerning increase in the capacitance of the electrode double layer and i,,, with time. Finally, Fig. 5 (d) shows a SEM micrograph of the Cu surface in en/buffer free metal-ion complex. The micrograph shows almost no scratches on the surface as a result of more corrosion products. Under these circumstances and as mentioned before, the highest interaction rate between en and the electrode surface was obtained. The SEM observations again coincide with the results of the impedance measurements.
5. Conclusions In en/buffer solution only, the rate of Cu corrosion was pronounced relative to that observed in the presence of the metal-ion complexes under investigation. The corrosion rates were found to decrease in the order en complex > > Cu complex > Zn complex > Ni complex. The IS parameters indicated that the corrosion rate of Cu in both en/buffer and metal-ion complexes/buffer solutions was time dependent. The corrosion rates in the presence of Cd, Zn and Ni complexes were found to be comparable, whereas that in case of Cu complex was much higher (7 times). This behaviour could be explained in terms of the liability characterizing most of the transition-metal complexes. Hence it was found that Cu complexes exhibit higher rate constants for substitu-
tion of inner-sphere water ligands in various aqua ions, which implies a continuous supply of free en to the electrode surface. The impedance results are confirmed by scanning electron microscope observations. References [I] J.R. MacDonald (ed.), Impedance Spectroscopy, Wiley, New York, 1987, Ch. 4. [2] G. Trabanelli and V. Carassitti, in M.G. Fontana and R.W. Stnehle (eds.), Advances irt Corrosion Science and Techo[ogy, Vol. 1, Plenum, New York, 1970, ps 147. [3] SM. Beloglazov and AS. Milushkin, Korroz. Zashch. Met., 3 ( 1977) 126. [4] AS. Milushkin and SM. Beloglazov, Korroz. Zmhch. Met., 3 (1977) 118. [5] S.I. Berezina, Yu.G. Voitsekhoskii, A,V. Ilyasov and B.G. Yavishev, Prikl. Elektrokhitn., 1-2 ( 1973) 62. [6] AS. Fouda and A.K. Mohamed, J. Elecrrochettr. Sot. India, 39 (4) (1990) 244. [7] C.P. Yang and CF. Wu, Ta T’uug Hwelr Pao, 9 (1979) 139. [8] S.A. Balezin, F.B. Glikina, E.G. Zak and O.V. Mynsnikova, UC/~.Znp., Mosk. Gas. Pedagog, Inst., 303 ( 1969) 2 IO, [9] P.M. May, I.M. Ritchieand E.T. Tan,J, Appl. Electrochetn., 21 (1991) 358. [lo] S.S. Abd El-Rchim, A. El-Sayed and A.A. Samnhi, Sutf Coat. Techtzol., 27 (1986) 205. [ 111 A.M. Zayed and A.A. Sagues, Corros. Sci., 30 (1990) 1025. [ 121 J.N. Arkuszewska and M. Kramczyk, Corros. Sci., 33 (1992) 861. [ 131 F.H. El-Hajjar and F.M. Al-Kharafi, Corros. Sci., 28 (1988) 163. [ 141 MS. Abdel-Aal and M.H. Wahdan, Br. Corros. J., 23 (1988) 25. [ 151 J. Vosta and N. Hackerman, Corros. Sci., 30 (1990) 949. [ 161 F.A. Cotton, G. Wilkinson and P.L. Gnus, Basic Inorganic Chemistry, Wiley, New York, 2nd edn., 1987, pp, 181,510. [ 171 J.A. Dean, Lang’s Handbook of Chemistry, 12th edn., McGraw-Hill, New York, 1979, pp, 5-58.
Chemistry
and Physics 46 ( 1996) 61-66
Corrosion of copper in both aqueous ethylenediamine and its complexes with some transition metal ions A. El-Sayed, F. Rashwan, F. El-Cheikh Department
of Chemistry, Received
Sohag Faculty 4 August
of Science,
1995; accepted
South- Valley University, 13 December
Sohag, Egypt
1995
Abstract The corrosion of copper in both 0.2 M ethylenediamine (en) /buffer and buffer containing complexes of some transition metal ions with ethylenediamine has been studied at the open-circuit potential (E,,,) using impedance spectroscopy. Impedance parameters, such as the polarization resistance R,, solution resistance R, and electrode capacitance C, were computed and compared. In all solutions investigated, the results showed that the electrode capacitance C gradually increased, whereas both R, and R, decreased as the time of immersion was increased. This behaviour has been attributed to a continuous change in the electrode surface area as a result of Cu dissolution. On the basis of the evaluatedexperimental results, it is established that therates ofelectrodecorrosion in thecaseof [Cd(en),]*‘, [Zn(en),]*’ and [Ni( en)z]2’ complexes are comparable. On the other hand, the rate of corrosion of Cu in the presence of [Cu(en),]” complex is found to be much higher (7.0 times). The data showed that the corrosion of Cu decreases in the following order: en > > Cu complex > > Cd complex > Zn complex > Ni complex. Keywords:
Impedance;
Copper;
Corrosion;
Ethylenediamine
complexes
1. Introduction
2. Experimental
Impedance spectroscopy (IS) has been widely used in the study of corrosion phenomena [ I], The study of corrosion and corrosion inhibition of Cu and its alloys is of great interest because of their wide applications in mechanical and chemical environments. Numerous additives of both organic and inorganic compounds have been used as corrosion inhibitors for Cu in various media [2]. Ethylenediamine (en) was used as a bath for the electroplating of Cu in which certain organic and inorganic additives were used to improve the quality of the electroplates [ 3-51. Also, ethylenediamine has been used as a corrosion inhibitor of Cu in acid solutions [ 6,7]. Balezin et al. [ 81 studied the corrosion of Cu in water containing ethylenediamine and monoethanolamine. They found that the corrosion of the metal increases with increasing concentration of both solutions. The corrosion of Cu was also investigated in ethylene glycol-water mixtures containing chloride ions [ 91. As far as we have been able to establish, there are very few publications relating to the corrosion of Cu in aqueous ethylenediamine. The present study was undertaken to understand more of the corrosion behaviour of Cu in aqueous ethylenediamine containing some metal-ion complexes using the modern impedance technique.
2.1. Electrolytes
0254-0584/96/$15.00
0 1996 Elsevier
Science S.A. All rights reserved
Doubly distilled water and analytical grade reagents were used in all experiments without further purification. All measurements were made in solutions having the same ionic strength, consisting of 0.2 M ethylenediamine, 0.1 M NH&l and 0.1 M KCl. The metal-ion complexes were prepared prior to each experiment with a 2: 1 ratio of both ethylenediamine and the chloride salt of the metal in the buffer solution. The pH of each solution was measured just before the experimental run using a Fisher 230 A pH/ion meter. The pH values were 10.2, 8.3, 8.0, 7.5 and 7.4 for the en/buffer, Cd-en/ buffer, Zn-enlbuffer, N&n/buffer and Cu-enlbuffer solutions, respectively. 2.2. Copper specimen All measurements were made using a Cu sheet as the working electrode, with a constant surface area of 4.0 cm* (99.9% electrolytic laminae, Prolabo) . Prior to each measurement, the electrode was polished with No. 600 emery paper and then degreased in pure ethanol, washed in running doubly
A. El-Sayed
et al. /Materials
Chemistry
and Physics 46 (199G) 61-66
(b)
(4
W
Fig. 1. Impedance diagrams for the Cu electrode at the corrosion potential after 40 min immersion in chloride buffer at 25 “C: (a) en; (b) Cu-en complex; (c) Cd+n complex; (d) Zwen complex.
distilled water, dried and weighed before being inserted in the polarization cell.
3. Results 3.1. Behavionr in en/b@er sohtion
2.3. Polarization
cell and measurements
A conventional three-electrode cell was used with 1.Ocm2 Pt sheet as the counter electrode, which was separated from the main cell compartment using a glass sinter, The reference electrode was a saturated calomel electrode (SCE) , to which all potential values are referred. The cell description has been given elsewhere [ 101. Prior to each experimental measurement, the solution under investigation was made oxygen free by passing prewashed pure nitrogen though it for a sufficient duration of time. The impedance spectroscopic measurements were carried out in the frequency range from 1.O to 0.005 Hz [ 111. Five data points were taken for each decade of frequency, or for each change in frequency by a power of ten. An amplitude of 10 mV (peak to peak) for the a.c. signal was used for all IS experiments. An EG&G Princeton Applied Research model 273 potentiostat/galvanostat, coupled with an EG&G Princeton Applied Research model 5208 two-phase lock-in analyzer, was controlled using an IBM PC 30 computer. Model 378 a.c. impedance software, from EG&G Princeton Applied Research, was used for all IS measurements. The cell temperature was kept constant (25.0 I 1.0 “C) using a Haake N3 ultrathermostat. Each experiment was repeated until the reproducibility of the results was satisfactory. Scanning electron microscope (SEM) observations were made using a JEOL JSM 5300 instrument.
Fig. 1 (a) presents the Nyquist complex plane plot for the IS of Cu electrode in 0.2 M aqueous ethylenediamine containing 0.1 M NH&l and 0.1 M KCl. From the plot, it is clear that the experimental results show an extreme deviation from the usual semicircle shape. This abnormal behaviour may be attributed to continuous changes in the electrode surface as a result of the relatively high rate of metal dissolution during IS measurements [ 121. The appearance of the IS diagram under the real axis may be ascribed to adsorption of ethylenediamine on the electrode surface in a step just prior to Cu dissolution. In this case, calculation of the electrode capacitance, solution resistance and polarization resistance is precluded. At the instant of immersion, the observed corrosion potential (E,,,) was = - 540 mV versus SCE, which implies that the Cu electrode is very active in the en/buffersolution. After immersion for 4.5 h, the open circuit potential (O.C.P.) reaches a steady-state value ( - 500 mV) , This shift in E,,, to a less negative potential is attributed to some inhibition effects [ 131, Hence, it is suggested that the instantly formed [WenM2+ complex is readily chemisorbed on the electrode surface [ 141. 3.2. Behaviour in the presence of (Crl(en),]”
complex
The IS for the Cu electrode in the buffer solution containing [Wen)J*+ complex shows a capacitive and less resistive semicircular behaviour (Fig. 1(b) ), The electrode capacitance C,, was calculated making use of the Bode format of IS (Fig. 2). Both Ihe charge transfer
A. El-Sayed
+
,-. m : ” CD -10+ -1.5
Chemistry
+ t *+t
184 -4B
et al. /Materials
,
+t
+t i
I
t
1
ft
tL,.L++
,
1
t t
t
tt tf , + t tttt+ 1
ttt+
1
I
I
1
I
lug Frecwnccj (Hz) Fig. 2. Bode plot of the Cu electrode in the Cu-en complex after 40 min immersion.
resistance R, and the solution resistance R, were determined using the Nyquist diagrams (Table 1). In addition, the corrosion current density (i,,,) data are calculated using the charge transfer resistance R, obtained from impedance measurements, together with the Stern-Geary equation: ice, =
b&c
63
and Physics 46 (1996) 61-66
processes, respectively. Assuming that b,= 60 mV and b, = 120 mV, the values of i,,, are given in Table 2. In the presence of [ Cu( en) 2] ‘+ complex, the weight loss of the Cu electrode after 4.5 h immersion was 2.0 mg, in comparison with 8.0 mg weight loss for the same immersion time in en/buffer solution only. This indicates that the corrosion of the electrode increases with increasing concentration of free en. Also, it is suggested that the [ Cu( en) J2’ complex is probably chemisorbed 1.141 at the electrode surface, leading to some corrosion inhibition of the metal. Unlike the behaviour in en/buffer, the constancy of E,,, at - 37Oi 1 mV in the presence of the [Cu(en),]‘+ complex implies that the equilibrium between the electrode and its ion in solution is attained in a very short time. As shown in Table 1, both R, and R, decrease from 6.3 and 33.1 R at the instant of immersion to 2.8 and 16.5 0, respectively, after 4.5 h. These data, in addition to the regular increase in the electrode capacitance and corrosion current density with time (Table 2), are indicative of the continuous changes in the electrode surface as a result of its dissolution. 3.3. Behaviour in the presenceof [Cd(en),]‘+ complex
2.303 Rd(b,+b,)
where R, = (dEldi) = 0, A is the electrode surface area, and b, and b, are the Tafel slopes of the anodic and cathodic
Fig. 1(c) represents the Nyquist complex plane plot for the IS of Cu electrode in [ Cd(en),] 2f complex/buffer solu-
Table 1 Impedance parameters of the Cu electrode in complexes of en with some transition-metal ions
t
lWen),l*+
(tin)
R,(fin)
R, (0)
-Ec, (mV)
R, (a)
R, (0)
-&xc (mv)
Rs (a)
R, (a)
-&m (mv)
R,(fl)
R, Cfi)
-L Mv
10 40 50 70 90 110 130 150 200 260
33.1 21.6 24.0 23.0 21.5 19.8 18.5 18.0 17.0 16.5
6.3 6.0 5.2 4.5 4.0 3.5 3.1 3.0 2.9 2.8
370 370 370 371 371 370 370 370 370 370
388 360 320 300 272 216 189 165 94 90
338 125 88 75 45 25 11 8 6.3 4.8
415 411 410 408 406 403 402 401 400 400
733 581 302 265 230 200 175 156 140 125
1105 525 115 75 68 40 10 8 I 6.5
400 395 393 393 393 393 392 392 392 392
440 440 373 315 300 270 240 230 180 166
2100 1950 1200 930 800 550 435 350 240 205
380 363 346 340 337 335 333 332 332 322
lWenM*
Table 2 Corrosion current density i,,
Kn(enM2+
[Ni(en)J”
(PA cm-*) of Cu in complexes of en with some transition-metal ions
t (mm)
[We&l*+
lWen),l*’
Pn(en),12+
[Ni(en)J”
10 40 50 70 90 110 130 150 200 260
690 125 836 966 1087 1243 1403 1450 1500 1553
13 35 49 58 97 174 395 543 690 906
4 8 38 58 64 109 435 543 621 669
2 2.2 3.6 4.1 5.4 8.0 10 12 18 21
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et al. /Materials
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and Physics 46 (1996) 61-66
65
60
I-
t
t 30
L 0
50
100
150 Time(mh.)
200
250
300
Fig. 3. Variation of the Cu electrode capacitance with time at the corrosion potential at 25 “C in (a) G-en complex; (b) Cd-en complex; (c) Zn-en complex; (d) Ni-en complex.
tion. The evaluated IS parameters are listed in Table 1, and the capacitance is represented by line b in Fig. 3. These results show a drastic decrease in both R, and R,, from 338 and 388 fl at the beginning of the experiment to only 4.8 and 90.0 a, respectively, at the end of the experimental run (after 4.5 h) . These data, along with the straightforward increase in the electrode capacitance (line b in Fig. 3) and i,,, (Table 2)) confirm the view mentioned before concerning the corrosion process occurring on the electrode surface.
Fig. 4. Nyquist plots for the Cu electrode at the corrosion potential as a function of immersion time at 25 “C: (a) 10 min; (b) 40 min; (c) 50 min; (d) 70 min; (e) 90 min; (f) 110 min; (g) 130 min; (h) 150 min; (i) 200 min; (j) 260 min.
from the plots, the IS behaviour is a somewhat flattened semicircle, but is not very far from the ideal shape. The extracted data contained in Table 1 demonstrate that both R, and R, decrease rapidly from 2100 and 440 s1 to 205 and 166 Q respectively, after 4.5 h immersion, while a regular increase in the electrode capacitance with time is obtained (line d in Fig. 3). Generally, these results are very similar to the data computed in the presence of Zn-en complex.
3.4. Behaviour in the presenceof [Zn(en)J2 + complex
4. Discussion
The Nyquist plane diagram for IS and Cu electrode in the complex/buffer solution is presence of [Zn(en),]‘+ depicted in Fig. 1 (d) . The computed IS parameters and i,,, values are listed in Tables 1 and 2, respectively. The variation of the electrode capacitance with time is represented by line cinFig.3. The results demonstrate a behaviour similar to that observed in the last case, where both R, and R, decrease, from 1105 and 733 Sz to 6.5 and 125 a, respectively, after 4.5 h immersion. Also, a straightforward relationship between the time of immersion and the electrode capacitance is registered (line c in Fig. 3).
The linear variation of the electrode capacitance with time (Fig. 3) for all solutions investigated, i.e., [Cu(en)J2”, [Cd(en),]‘+, [Zn(en),]** and [Ni(en)2]2” complexes, suggests corrosion stimulation of Cu as a result of continuous changes in the electrode surface area [ 151. The rates of corrosion in the presence of [Cd(en),12’, [Zn(en)212”, [zn(en>212+ and [Ni(en)2]2’ complexes are found to be comparable, whereas that in the case of [ Cu(en),] 2’ is much higher (see Fig. 3 and Table 2). This unique behaviour in the case of Cu-en may be interpreted in view of the liability characterizing most of the transition metal ion complexes. It is found with en and Cu2* that [Cu(en),(H20),12’ and are readily formed in aqueous solutions [Cu(en)UW)d2 [ 161. On the basis of this foundation, the following equilibrium can be suggested for our case:
3.5. Behaviour in the presenceof [Ni(en)2]2’ complex
Fig. 4 represents the Nyquist complex plane diagrams for the Cu electrode in buffer solution containing [ Ni( en) *] *+ complex for various intervals of immersion time. As shown
[Cu(en)2(H20)2]2’ [Cu(en)(H20)4]2+
+2H,Oti ten
A. El-Sayed
et al. /Materials
Chemistry
Since en molecules are consumed on the surface of the electrode, a shift of the equilibrium point to the right must occur, and hence water-ligand substitution becomes very possible.
Fig. 5. SEM micrographs of a Cu electrode sheet surface under different experimental conditions (magnification: X 1000) : (a) the electrode surface before immersion (uncorroded surface) ; (b) the electrode surface after 1 h immersion in N&en/buffer solution; (c) the same conditions as in (b) but after 5 h immersion time; (d) the electrode surface after 5 h immersion in en/buffer solution only.
and Physics 46 (1996)
61-66
65
It is also found [ 161 that Cu complexes are among the class of metal-ion complexes that are characterized by very high rate constants for water-ligand exchange (k= 108-1010 s-i). The approximate rate-constant values for the other three metal-ion complexes under investigation are found to be 4.0 X 108, 4.0 X lo7 and 1.5 X lo4 s-l for the Cd, Zn and Ni complexes, respectively. According to the foregoing discussion, the following order for the corrosion rates of the electrode can be understood: Cu complex > > Cd complex > Zn complex > Ni complex. The observed EC,, values at the instant of electrode immersion are found to be -540, -430, -418, - 380 and - 370 mV versus SCE for the en, Cd, Zn, Ni and Cu solutions, respectively. These data suggest that the activity of the Cu electrode decreases in the sequence en system > Cd system > Zn system > Ni system > Cu system. However, the IS results show that the corrosion rate of the electrode in the presence of Cu-en complex must lie directly behind that of free en in the latter sequence. Since the Cu electrode in the case of the Cu-en system is immersed in its own ions, fast equilibrium must be established, and hence E,,, will reach a steady-state value in a very short time. Thus, we believe that the real E,,, value for the Cu-en complex should lie between -540 and -430mV. From the thermodynamic point of view, the corrosion rates of the Cu electrode according to the literature values of the stability constants for the complexes under investigation should follow the sequence Cd complex > Zn complex > Ni complex > Cu complex; hence the K values listed are 2.9 X 105, 5.9 X 16, 3.3 X lo7 and 4.6 X lOlo, respectively [ 171. It is clear that a high stability constant implies reduction of the free-ligand concentration in the solution, and consequently a low corrosion rate must be observed. This view is in accordance with the IS experimental results, which imply that the corrosion of Cu decreases in the order Cd complex > Zn complex > Ni complex. The deviation of the corrosion rate in the presence of the Cu-en system from this sequence may be ascribed to its high rate constant, characterizing the water-ligand substitution process, as discussed before. The fact that the corrosion rate of copper metal increases regularly with time (Table 2)) so that it increases by lo-, 70and 180-fold after 260 min for the Ni, Cd and Zn complexes, respectively, confirms the idea suggested that copper complexes enhance the corrosion of copper metal. As time passes, more and more copper complexes accumulate in solution (due to copper corrosion), replacing the complexes of Ni, Zn and Cd (Cu complexes being the most stable), and the corrosion rate becomes more and more pronounced. For the corrosion of Cu metal in the presence of Cu-en complex, it was found that the rate increased by only 2.25 times. By comparing the SEM micrographs for the Cu electrode surface before immersion (uncorroded specimen, Fig. 5 (a) ) with that obtained after 1 h immersion in Ni-en complex/ buffer solution (Fig. 5(b) ) , one may observe a significant difference. The clear scratches that characterize the uncorroded surface, as a result of its sheet-polishing nature, seem
66
A. El-Sayed et al. /Materials Chetnisrry and Physics 46 (1996) 61-66
to be partially covered with corrosion products. The progress of corrosion, upon increasing the immersion time to 5 h, can be confirmed by observing the development of the electrodesurface coverage given in Fig. 5 (c) . These observations support the data listed in Tables 1 and 2 concerning increase in the capacitance of the electrode double layer and i,,, with time. Finally, Fig. 5 (d) shows a SEM micrograph of the Cu surface in en/buffer free metal-ion complex. The micrograph shows almost no scratches on the surface as a result of more corrosion products. Under these circumstances and as mentioned before, the highest interaction rate between en and the electrode surface was obtained. The SEM observations again coincide with the results of the impedance measurements.
5. Conclusions In en/buffer solution only, the rate of Cu corrosion was pronounced relative to that observed in the presence of the metal-ion complexes under investigation. The corrosion rates were found to decrease in the order en complex > > Cu complex > Zn complex > Ni complex. The IS parameters indicated that the corrosion rate of Cu in both en/buffer and metal-ion complexes/buffer solutions was time dependent. The corrosion rates in the presence of Cd, Zn and Ni complexes were found to be comparable, whereas that in case of Cu complex was much higher (7 times). This behaviour could be explained in terms of the liability characterizing most of the transition-metal complexes. Hence it was found that Cu complexes exhibit higher rate constants for substitu-
tion of inner-sphere water ligands in various aqua ions, which implies a continuous supply of free en to the electrode surface. The impedance results are confirmed by scanning electron microscope observations. References [I] J.R. MacDonald (ed.), Impedance Spectroscopy, Wiley, New York, 1987, Ch. 4. [2] G. Trabanelli and V. Carassitti, in M.G. Fontana and R.W. Stnehle (eds.), Advances irt Corrosion Science and Techo[ogy, Vol. 1, Plenum, New York, 1970, ps 147. [3] SM. Beloglazov and AS. Milushkin, Korroz. Zashch. Met., 3 ( 1977) 126. [4] AS. Milushkin and SM. Beloglazov, Korroz. Zmhch. Met., 3 (1977) 118. [5] S.I. Berezina, Yu.G. Voitsekhoskii, A,V. Ilyasov and B.G. Yavishev, Prikl. Elektrokhitn., 1-2 ( 1973) 62. [6] AS. Fouda and A.K. Mohamed, J. Elecrrochettr. Sot. India, 39 (4) (1990) 244. [7] C.P. Yang and CF. Wu, Ta T’uug Hwelr Pao, 9 (1979) 139. [8] S.A. Balezin, F.B. Glikina, E.G. Zak and O.V. Mynsnikova, UC/~.Znp., Mosk. Gas. Pedagog, Inst., 303 ( 1969) 2 IO, [9] P.M. May, I.M. Ritchieand E.T. Tan,J, Appl. Electrochetn., 21 (1991) 358. [lo] S.S. Abd El-Rchim, A. El-Sayed and A.A. Samnhi, Sutf Coat. Techtzol., 27 (1986) 205. [ 111 A.M. Zayed and A.A. Sagues, Corros. Sci., 30 (1990) 1025. [ 121 J.N. Arkuszewska and M. Kramczyk, Corros. Sci., 33 (1992) 861. [ 131 F.H. El-Hajjar and F.M. Al-Kharafi, Corros. Sci., 28 (1988) 163. [ 141 MS. Abdel-Aal and M.H. Wahdan, Br. Corros. J., 23 (1988) 25. [ 151 J. Vosta and N. Hackerman, Corros. Sci., 30 (1990) 949. [ 161 F.A. Cotton, G. Wilkinson and P.L. Gnus, Basic Inorganic Chemistry, Wiley, New York, 2nd edn., 1987, pp, 181,510. [ 171 J.A. Dean, Lang’s Handbook of Chemistry, 12th edn., McGraw-Hill, New York, 1979, pp, 5-58.