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Evaluation of corrosion inhibition of brass-118 in artificial seawater by benzotriazole using Dynamic EIS
Corrosion Science 51 (2009) 2573–2579
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Evaluation of corrosion inhibition of brass-118 in artificial seawater by benzotriazole using Dynamic EIS Husnu Gerengi a,*, Kazimierz Darowicki b, Gozen Bereket c, Pawel Slepski b a
Department of Chemistry, Kaynaslı Vocational College, Duzce University, 81900 Kaynaslı-Duzce, Turkey Corrosion and Materials Engineering, Gdansk University of Technology, 11/12, 80-952 Gdansk, Poland c Department of Chemistry, Faculty of Arts and Science, Eskisßehir Osmangazi University, 26480 Eskisehir, Turkey b
a r t i c l e
i n f o
Article history: Received 24 February 2009 Accepted 25 June 2009 Available online 1 July 2009 Keywords: A. Brass B. Dynamic Electrochemical Impedance Spectroscopy (DEIS) C. Corrosion inhibitors C. Artificial seawater C. Benzotriazole
a b s t r a c t The paper presents the results of corrosion behaviour of brass-118 in artificial seawater and the inhibitor effect of benzotriazole (BTA) by using a novel method called dynamic electrochemical impedance spectroscopy (DEIS). This method allows the tracing of the dynamics of the corrosion and the inhibition process based on the evaluation of electrical parameters of the equivalent circuit. Instantaneous impedance spectra recorded up to 10 h show that an exposure of few hours is not enough for the determination of inhibition efficiency. The results indicate the usefulness of DEIS technique in the field of inhibitor research. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Copper alloys represent an important category of nonferrous alloy which is widely used as a material in various heating and cooling systems [1,2]. Brass has been widely used for shipboard condenser, power plant condenser and petrochemical heat exchanger [3,4]. Majority of marine propellers from the smallest to the largest are made from copper and copper alloys [5]. Brass118 is especially used in applications such as cladding for ships’ hulls, legs of oil rig platforms and sea water intake screens [6]. Even though copper and copper alloys are corrosion resistant, they are prone to corrode in solutions that contain oxygen and high concentration of chloride, sulphate, sulphide and nitrate ions [7,8]. One of the most important methods in corrosion protection is to use inhibitor. Inhibition of copper corrosion in different media by organic inhibitor has been widely investigated [9–13]. Benzotriazole (BTA) is one of the most efficient inhibitor for copper and its alloys in aqueous chloride media and has been subject of numerous scientific studies in last 50 years [14–18]. Nature of the formation of protective layers in the presence of BTA was investigated by electrochemical technique, atomic force micros-
* Corresponding author. Tel.: +90 5053987953; fax: +90 3805442812. E-mail address: [email protected] (H. Gerengi). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.06.040
copy (AFM) and X-ray photo electron spectroscopy (XPS) [19]. Inhibitor action of BTA on the corrosion of brass in 3% NaCl was also studied by electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA) [20]. It has been generally assumed that the effectiveness of BTA has been related to the formation of a [Cu+BTA]n film which is considered to be insoluble and polymeric [21–23]. Brass has been widely studied in 3.5% NaCl media where it has been observed that the chloride ion has a strong influence on copper corrosion mechanism [24]. However according to laboratory test, these solutions do not always reproduce the corrosion loses accurately in natural seawater, because, in addition to chlorides, other impurities in the salt composition can pronouncedly effect the corrosion [25]. Therefore, in this investigation it is proposed to study the corrosion inhibition of brass-118 with BTA in artificial seawater. Since the corrosion process is one of an electrochemical nature, the electrochemical methods employed play an important role in determination of efficiency of corrosion protection when inhibitors are used. Recently, Darowicki and co-workers [26,27] described a new impedance technique termed dynamic electrochemical impedance spectroscopy (DEIS) which can be used under non-stationary conditions [28]. Such a method of impedance measurement is completely novel which has never been employed in evaluating the corrosion inhibition by using of inhibitor. The aim of this work is to investigate electrochemical behaviour of brass-118 in artificial seawater and study inhibitor effect of BTA using dynamic impedance technique.
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H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
2. Experimental approach
2.2. Method: Dynamic electrochemical impedance spectroscopy (DEIS)
2.1. Materials
DEIS is a measurement technique of electrode impedance that is characterized by two major factors. First of them is excitation signal, which is not generated frequency-by-frequency as it takes place in majority of commercial impedance systems but is a composition of elementary sinusoidal signals. Such approach enables considerable decrease of excitation time required to acquire single impedance spectrum. Second factor distinguishing DEIS among other techniques is the way of acquiring of impedance data. Short Time Fourier Transformation (STFT) is used for that purpose. It provides information not only about amplitudes and phase shifts of individual frequency components but also about their time-localization. Detailed information regarding principles of DEIS measurement in galvanostatic mode was presented recently by Slepski et al. [30]. Generation of current multisinusoid perturbation signal was performed with a National Instruments Ltd. PCI-6120 card. The same card was used to record the current and voltage signals. Autolab PGSTAT 30 equipment was used to supply galvanostatic condition and also as a voltage–current converter. Measurements were carried out in zero-DC current conditions. The perturbation signal consisted of a package composed of current sinusoids of the frequency ranges from 4.5 kHz to 700 mHz. Amplitude of multisinusoid excitation signal was selected individually for each experiment to ensure that peak-to-peak amplitude of response signal is not higher than 10 mV after 10 h of measurement, when analyzed system reveals the highest impedance. The low limit of measurement frequency depends on the length of analyzing window (10 s) [31]. In other words; the low-frequency limit depends on the time scale of performed analysis.
Electrochemical measurements were carried out in a three-electrode type cell with separate compartments for the reference electrode (Ag/AgCl), the counter (Pt) and the working (Brass-118) electrode of area 0.2 cm2. The surface of the working electrode was prepared by grinding with abrasive papers of 400–1800 grades. It was then rinsed with distilled water and then degreased with acetone. During measurements the solution was stirred with a magnetic stirrer (500 rot/min). In each case, BTA was added after 10 min of the starting of the experiment in order to investigate how it interacted with the specimen. Brass-118 had the composition; (wt.%): 79 Cu, 1 Zn, 1 Mn, 10 Al, 5 Fe and 4 Ni. Chemical Composition of the artificial seawater is given in Table 1 [29]. The pH of solution was 8.10 and electrical resistivity was (q) 25 X cm as measured by Nilsson electrical resistance conductance meter model 400. BTA used was of commercial grade (Roanal, Budapest) [Fig. 1]. Table 1 Composition of the artificial seawater. Component
Concentrations (g/l)
NaCl MgCl2 Na2SO4 CaCl2 NaHCO3 KBr H3BO3
24,530 5,200 4,090 1,160 0,201 0,101 0,027
3. Results and discussion
Fig. 1. Structures of benzotriazole.
DEIS spectra at OCP were recorded for brass-118 in artificial seawater without and with 0.0100, 0.0134, 0.0201 and 0.0206 M BTA. Ten minutes after starting of each experiment, requisite
Fig. 2. DEIS result of brass-118, without inhibitor in artificial seawater.
H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
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Fig. 3. DEIS result of brass-118, with 0.0100 M BTA in artificial seawater.
quantity of BTA was quickly introduced in the corrosion cell so as to have the desired concentration in artificial seawater. Figs. 2–6 illustrate impedance spectra achieved for Brass-118 in artificial seawater without inhibitor, with 0.0100, 0.0134, 0.0201 and 0.0268 M of BTA. Particular impedance spectra, achieved in case of presence of BTA in the solution are characterized by flattened semicircle shape in range of high frequencies terminated with a tail in range of low frequencies. Evaluation of impedance
spectra is visible with 10 h of exposure of samples. The highest changes in shape of impedance spectra take place for first 2–5 h of exposition, where increase of magnitude of high-frequency semicircle takes place and low-frequency range becomes apparent. In case of DEIS spectra recorded in artificial seawater without BTA (Fig. 2) changes of impedance spectra are small. In examined case impedance spectra achieved by means of DEIS technique are limited in the lowest frequency range to be able to
Fig. 4. DEIS result of brass-118, with 0.0134 M BTA in artificial seawater.
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H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
Fig. 5. DEIS result of brass-118 with, 0.0201 M BTA in artificial seawater.
Fig. 6. DEIS result of brass-118, with 0.0268 M BTA in artificial seawater.
record impedance changes in time induced by BTA presence in the solution. However, this may induce issues with proper interpretation of acquired impedance spectra. Additional measurement with classic EIS technique was performed ranging up to 10 mHz frequencies. This experiment was carried out 8 h after injection of BTA (0.0268 M) to maintain condition of system stationary. Acquired impedance spectrum that was presented on Fig. 7 demonstrates the presence of 2 semicircles.
Application of electric equivalent circuit containing two (QR) circuits in a form of R(Q(R(QR))) in impedance data analysis was not satisfactionary. Parameter v2 describing quality of fitting procedure, defined as [32]:
v2 ¼
" n X ðZ 0i ðxi ; ~ pÞ ai Þ2 2
i¼1
a2i þ bi
þ
ðZ 00i ðxi ; ~ pÞ bi Þ2 2
a2i þ bi
# ð1Þ
H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
2577
where: xi, ai, bi - experimental data points, ~ p - parameters associated with a model, Z 0i , Z 00 - calculated point, was equal to 4 103. Essential differences between impedance values measured and theoretical appeared in low-frequency range, where diffusion transport processes manifest most often. Stirring of the solution in time of experiment excludes presence of diffusion restriction between the solution and sample surface. In the presence of BTA sample surface covers with polymer film Cu(I)-BTA that limits corrosion process. In such case diffusion through finite layer can occur which is defined in electric equivalent circuit as O parameter. Fitting procedure of acquired impedance to electric equivalent model R(Q(RO)) presented on Fig. 7 confirmed this assumption, v2 parameter was equal to 1 104. Fig. 7. Nyquist plot acquired for brass-118 with 0.0268 M BTA in artificial seawater. (h) experimental, (- - -) R(Q(R(QR))) model, (-) R(Q(RO)) model.
Fig. 8. An electrical circuit of R(Q(RW)) model.
Fig. 9. Exemplary impedance spectra acquired for Brass-118 with 0.0268 M BTA in artificial seawater with DEIS technique. (h) experimental, (-) R(Q(RW)) model.
Fig. 11. Surface heterogeneity (n) vs. time of exposure in artificial sea water in the presence of different concentration of BTA based on the DEIS results. ( 0.0268 M BTA, r 0.0201 M BTA, 4 0.0134 M BTA, s 0.0100 M BTA, } no inh.).
Fig. 10. Charge transfer resistance evaluation upon ten hours exposure to artificial sea water in the presence of different concentration of BTA based on the DEIS results. ( 0.0268 M BTA, r 0.0201 M BTA, 4 0.0134 M BTA, s 0.0100 M BTA, } no inh.).
Fig. 12. Q parameter of the CPE element vs. time of exposure in artificial sea water in the presence of different concentration of BTA based on the DEIS results. ( 0.0268 M BTA, r 0.0201 M BTA, 4 0.0134 M BTA, s 0.0100 M BTA, } no inh.).
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Table 2 Change of %IE, n and Q (Ssncm2) values for the corrosion brass-118 in artificial seawater in the presence of BTA after 1, 3, 5, 6 and 10 h. Concentrations
No inhibitor 0.0100 M BTA 0.0134 M BTA 0.0201 M BTA 0.0268 M BTA
(1 h)
(3 h)
(5 h)
(10 h)
%IE
n
Q
%IE
n
Q
%IE
n
Q
%IE
n
Q
%IE
n
Q
– 57.7 60.9 80.9 86.2
0.62 0.85 0.81 0.85 0.83
2.12 105 1.88 106 1.52 106 1.49 106 1.13 106
– 73.5 83.3 85.1 87.5
0.68 0.84 0.85 0.87 0.87
2.80 105 2.46 106 1.42 106 5.11 107 4.02 107
– 80.8 84.5 90.1 91.3
0.70 0.83 0.86 0.88 0.89
3.30 105 2.56 106 4.69 107 3.93 107 3.48 107
– 83.3 85.6 91.3 92.1
0.74 0.85 0.87 0.88 0.88
2.48 105 1.23 106 4.91 107 3.37 107 3.09 107
– 74.1 81.9 88.6 90.3
0.76 0.84 0.86 0.89 0.92
2.36 105 1.14 106 4.87 107 3.22 107 2.78 107
Impedance spectra acquired by means of DEIS technique contain only partial frequencies range in which the diffusion type mentioned above manifests. Taking into consideration that at the initial phase diffusion element O is indistinguishable from a Warburg impedance (W), all impedance data were analyzed with the use of simplified R(Q(RW)) model (Fig. 8). Key Rs resistance of electrolyte in bulk Rct charge transfer resistance at the metal surface W Warburg impedance Q constant phase element
Fig. 9 presents fitting procedure of exemplary impedance spectra acquired with DEIS technique to applied electric equivalent circuit. In the analyzed electric equivalent circuit, Q describes the nonideal behaviour of capacitance. The impedance of constant phase element (CPE) is given by:
Z CPE ¼ ½Q ðjxÞn 1
ð2Þ
where j is an imaginary number, Q is the frequency independent real constant, x = 2pf is the angular frequency (rad/s), f is the frequency of the applied signal, n is the CPE exponent for whole number of n = 1, 0, 1 CPE is reduced to the capacitor (C), resistance (R) and inductance (L). The value of n = 0.5 corresponds to Warburg impedance (W). The dispersion of the capacitive semicircle is related with surface heterogeneity due to surface roughness or inhibitor adsorption and formation of porous layer [33] and thus n serves as a measure of surface heterogeneity. Figs. 10–12 shows the evaluation of Rct, n and Q with time for brass-118 without and with 0.0100, 0.0134, 0.0201, 0.0268 M BTA. It can be seen from those figures that the system is not at equilibrium at the beginning of initial 5 h. From Fig. 8, it is evident that Rct increases considerably in the presence of BTA and the value of Rct increases as the concentration of BTA increases. Moreover, at each concentration Rct tends to attain almost a constant value after 5 h. This indicates the improvement of corrosion inhibition by BTA with time. Similarly, the evaluation of Q parameter that has the meaning of frequency distributed double layer capacitance with time (Fig. 12) confirms improvement of corrosion inhibition with time. Variation of parameter n of the constant phase element Q with time (Fig. 11) shows that it increases with the increase of BTA concentration. This can be attributed to decrease in surface heterogeneity thus improvement in corrosion inhibition. The parameter n also reached almost constant value after 5 h indicating that the system was not stationary until 5 h. The percentage inhibition efficiency (IE%) was calculated from the charge transfer resistance (Rct) values by using following equation:
IEð%Þ ¼
(6 h)
ðRct Þ1 ðRctðinhÞ Þ1 ðRct Þ1
100
ð3Þ
where the Rct(inh) and Rct are the charge transfer resistance values with and without inhibitor. Calculated IE%, n and Q values at differ-
ent times (after 1, 3, 5, 6 and 10 h), are shown in Table 2. Results from this Table show that IE%, n and Q values are almost constant after 5 h which indicates that system becomes stationary after 5 h. The value of Q from Table 2 is 3.93 107 Ssncm2 at the end of 5 h in the presence of 0.0201 M BTA while in the absence of BTA the value of Q is 1.49 106 Ssncm2. This low interfacial capacitance in the presence of BTA has been reported by others [34,35] and was attributed to the formation of polymeric type film comprising Cu(I)-BTA units. It is generally assumed that adsorption of BTA leads to formation of polymeric cuprous complex [36]. However; a certain time is needed for the building up this polymeric complex. Attained constant values of circuit parameters Rct, n and Q are related with the complete build up polymeric species at the end of 5 h. However Rct and n values increase where Q values decrease with the increase of BTA concentrations which are indicative of increase inhibition in other ways formation of higher amount of polymeric complex. 4. Conclusions The application of dynamic electrochemical spectroscopy technique for studying inhibition effect of BTA for brass-118 in artificial seawater allows to observe variation of Rct, n and Q with time. The change of these electrical parameters with time shows that system reaches to stable state after 5 h. Thus, the calculated inhibition efficiencies should be more realistic when the system is in the stable state (e.g. at the end of 5 h). Result shows that BTA has excellent inhibition properties on the corrosion of brass-118 in artificial sea water. However; DEIS method is more informative since it gives information about time when BTA has stable maximum inhibition effect. Thus, DEIS method should be used before each inhibitor testing method. References [1] [2] [3] [4] [5] [6] [7]
[8]
[9] [10] [11] [12] [13] [14] [15] [16]
E. Stupnisek-Lisac, A. Loncarik Bozic, I. Cafuk, Corrosion 54 (1998) 713. A.M. Zaky, Br. Corros. J. 36 (2001) 59. R.F. North, M.J. Pryar, Corros. Sci. 10 (1970) 297. G. Quartarone, G. Moretti, J. Bellami, Corrosion 54 (1998) 606. T.H. Rogers, Marine Corr. Book, 1968, p. 116, ISBN: T000009540. The Institute of Materials, Minerals and Mining. Available from:
H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579 [17] S. Ferina, M. Loncar, M. Metikoš-Hukovic´, in: Proceeding of the 8th Symposium on Corrosion Inhibitors Ferrara-Italy, 1995, p. 1065. [18] F. Chaouket, A. Srhiri, A. Benbachir, A. Frignani, in: Proceeding of the 8th Symposium on Corrosion Inhibitors Ferrara-Italy, 1995, p. 1031. [19] T. Kosec, I. Milosev, B. Pihlar, Appl. Surf. Sci. 253 (2007) 8863. [20] A. Naguib, F. Mansfeld, Corros. Sci. 43 (2001) 2147. [21] J.B. Cotton, I.R. Scholes, Br. Corros. J. 2 (1967) 1. [22] G.W. Poling, Corros. Sci. 10 (1970) 359. [23] F. El-Taib Heakal, S. Haruyama, Corros. Sci. 20 (1980) 887. [24] G. Kear, B.D. Barker, F.C. Walsh, Corros. Sci. 46 (2004) 109. [25] V.S. Sinyavskii, V.D. Kalinin, Prot. Met. 41 (2005) 317. [26] K. Darowicki, K. Krakowiak, P. Slepski, J. Electroanal. Chem. 33 (2005) 575. [27] K. Krakowiak, K. Darowicki, P. Slepski, Electrochim. Acta 50 (2005) 2699.
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[28] S. Nagarajan, S. Tamilselvi, N. Rajendran, Meter. Corros. 58 (2007) 33. [29] K. Darowicki, H. Gerengi, G. Bereket, P. Slepski, A. Zielenski, Korozyon 14 (2006) 3, ISSN: 1306-3588. [30] P. Slepski, K. Darowicki, K. Andrearczyk, J. Electroanal. Chem., in press. Available from:. [31] K. Darowicki, J. Orlikowski, A. Arutunow, J. Solid State Eletrochem. 8 (2004) 352. [32] B. Yeum, ZsimpWin Version 3.00 – user manual, Princeton Applied Research, 2002. [33] A. Popova, M. Christov, Corros. Sci. 48 (2006) 3208. [34] M. Fenelon, C.B. Breslin, J. Appl. Electrochem. 31 (2001) 509. [35] S. Mamasß, T. Kıyak, M. Kabasakalog˘lu, A. Koç, Meter. Chem. Phys. 93 (2005) 41. [36] R. Youda, H. Nishihara, K. Aramaki, Electrochim. Acta 35 (1990) 1011.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Evaluation of corrosion inhibition of brass-118 in artificial seawater by benzotriazole using Dynamic EIS Husnu Gerengi a,*, Kazimierz Darowicki b, Gozen Bereket c, Pawel Slepski b a
Department of Chemistry, Kaynaslı Vocational College, Duzce University, 81900 Kaynaslı-Duzce, Turkey Corrosion and Materials Engineering, Gdansk University of Technology, 11/12, 80-952 Gdansk, Poland c Department of Chemistry, Faculty of Arts and Science, Eskisßehir Osmangazi University, 26480 Eskisehir, Turkey b
a r t i c l e
i n f o
Article history: Received 24 February 2009 Accepted 25 June 2009 Available online 1 July 2009 Keywords: A. Brass B. Dynamic Electrochemical Impedance Spectroscopy (DEIS) C. Corrosion inhibitors C. Artificial seawater C. Benzotriazole
a b s t r a c t The paper presents the results of corrosion behaviour of brass-118 in artificial seawater and the inhibitor effect of benzotriazole (BTA) by using a novel method called dynamic electrochemical impedance spectroscopy (DEIS). This method allows the tracing of the dynamics of the corrosion and the inhibition process based on the evaluation of electrical parameters of the equivalent circuit. Instantaneous impedance spectra recorded up to 10 h show that an exposure of few hours is not enough for the determination of inhibition efficiency. The results indicate the usefulness of DEIS technique in the field of inhibitor research. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Copper alloys represent an important category of nonferrous alloy which is widely used as a material in various heating and cooling systems [1,2]. Brass has been widely used for shipboard condenser, power plant condenser and petrochemical heat exchanger [3,4]. Majority of marine propellers from the smallest to the largest are made from copper and copper alloys [5]. Brass118 is especially used in applications such as cladding for ships’ hulls, legs of oil rig platforms and sea water intake screens [6]. Even though copper and copper alloys are corrosion resistant, they are prone to corrode in solutions that contain oxygen and high concentration of chloride, sulphate, sulphide and nitrate ions [7,8]. One of the most important methods in corrosion protection is to use inhibitor. Inhibition of copper corrosion in different media by organic inhibitor has been widely investigated [9–13]. Benzotriazole (BTA) is one of the most efficient inhibitor for copper and its alloys in aqueous chloride media and has been subject of numerous scientific studies in last 50 years [14–18]. Nature of the formation of protective layers in the presence of BTA was investigated by electrochemical technique, atomic force micros-
* Corresponding author. Tel.: +90 5053987953; fax: +90 3805442812. E-mail address: [email protected] (H. Gerengi). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.06.040
copy (AFM) and X-ray photo electron spectroscopy (XPS) [19]. Inhibitor action of BTA on the corrosion of brass in 3% NaCl was also studied by electrochemical impedance spectroscopy (EIS) and electrochemical noise analysis (ENA) [20]. It has been generally assumed that the effectiveness of BTA has been related to the formation of a [Cu+BTA]n film which is considered to be insoluble and polymeric [21–23]. Brass has been widely studied in 3.5% NaCl media where it has been observed that the chloride ion has a strong influence on copper corrosion mechanism [24]. However according to laboratory test, these solutions do not always reproduce the corrosion loses accurately in natural seawater, because, in addition to chlorides, other impurities in the salt composition can pronouncedly effect the corrosion [25]. Therefore, in this investigation it is proposed to study the corrosion inhibition of brass-118 with BTA in artificial seawater. Since the corrosion process is one of an electrochemical nature, the electrochemical methods employed play an important role in determination of efficiency of corrosion protection when inhibitors are used. Recently, Darowicki and co-workers [26,27] described a new impedance technique termed dynamic electrochemical impedance spectroscopy (DEIS) which can be used under non-stationary conditions [28]. Such a method of impedance measurement is completely novel which has never been employed in evaluating the corrosion inhibition by using of inhibitor. The aim of this work is to investigate electrochemical behaviour of brass-118 in artificial seawater and study inhibitor effect of BTA using dynamic impedance technique.
2574
H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
2. Experimental approach
2.2. Method: Dynamic electrochemical impedance spectroscopy (DEIS)
2.1. Materials
DEIS is a measurement technique of electrode impedance that is characterized by two major factors. First of them is excitation signal, which is not generated frequency-by-frequency as it takes place in majority of commercial impedance systems but is a composition of elementary sinusoidal signals. Such approach enables considerable decrease of excitation time required to acquire single impedance spectrum. Second factor distinguishing DEIS among other techniques is the way of acquiring of impedance data. Short Time Fourier Transformation (STFT) is used for that purpose. It provides information not only about amplitudes and phase shifts of individual frequency components but also about their time-localization. Detailed information regarding principles of DEIS measurement in galvanostatic mode was presented recently by Slepski et al. [30]. Generation of current multisinusoid perturbation signal was performed with a National Instruments Ltd. PCI-6120 card. The same card was used to record the current and voltage signals. Autolab PGSTAT 30 equipment was used to supply galvanostatic condition and also as a voltage–current converter. Measurements were carried out in zero-DC current conditions. The perturbation signal consisted of a package composed of current sinusoids of the frequency ranges from 4.5 kHz to 700 mHz. Amplitude of multisinusoid excitation signal was selected individually for each experiment to ensure that peak-to-peak amplitude of response signal is not higher than 10 mV after 10 h of measurement, when analyzed system reveals the highest impedance. The low limit of measurement frequency depends on the length of analyzing window (10 s) [31]. In other words; the low-frequency limit depends on the time scale of performed analysis.
Electrochemical measurements were carried out in a three-electrode type cell with separate compartments for the reference electrode (Ag/AgCl), the counter (Pt) and the working (Brass-118) electrode of area 0.2 cm2. The surface of the working electrode was prepared by grinding with abrasive papers of 400–1800 grades. It was then rinsed with distilled water and then degreased with acetone. During measurements the solution was stirred with a magnetic stirrer (500 rot/min). In each case, BTA was added after 10 min of the starting of the experiment in order to investigate how it interacted with the specimen. Brass-118 had the composition; (wt.%): 79 Cu, 1 Zn, 1 Mn, 10 Al, 5 Fe and 4 Ni. Chemical Composition of the artificial seawater is given in Table 1 [29]. The pH of solution was 8.10 and electrical resistivity was (q) 25 X cm as measured by Nilsson electrical resistance conductance meter model 400. BTA used was of commercial grade (Roanal, Budapest) [Fig. 1]. Table 1 Composition of the artificial seawater. Component
Concentrations (g/l)
NaCl MgCl2 Na2SO4 CaCl2 NaHCO3 KBr H3BO3
24,530 5,200 4,090 1,160 0,201 0,101 0,027
3. Results and discussion
Fig. 1. Structures of benzotriazole.
DEIS spectra at OCP were recorded for brass-118 in artificial seawater without and with 0.0100, 0.0134, 0.0201 and 0.0206 M BTA. Ten minutes after starting of each experiment, requisite
Fig. 2. DEIS result of brass-118, without inhibitor in artificial seawater.
H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
2575
Fig. 3. DEIS result of brass-118, with 0.0100 M BTA in artificial seawater.
quantity of BTA was quickly introduced in the corrosion cell so as to have the desired concentration in artificial seawater. Figs. 2–6 illustrate impedance spectra achieved for Brass-118 in artificial seawater without inhibitor, with 0.0100, 0.0134, 0.0201 and 0.0268 M of BTA. Particular impedance spectra, achieved in case of presence of BTA in the solution are characterized by flattened semicircle shape in range of high frequencies terminated with a tail in range of low frequencies. Evaluation of impedance
spectra is visible with 10 h of exposure of samples. The highest changes in shape of impedance spectra take place for first 2–5 h of exposition, where increase of magnitude of high-frequency semicircle takes place and low-frequency range becomes apparent. In case of DEIS spectra recorded in artificial seawater without BTA (Fig. 2) changes of impedance spectra are small. In examined case impedance spectra achieved by means of DEIS technique are limited in the lowest frequency range to be able to
Fig. 4. DEIS result of brass-118, with 0.0134 M BTA in artificial seawater.
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H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
Fig. 5. DEIS result of brass-118 with, 0.0201 M BTA in artificial seawater.
Fig. 6. DEIS result of brass-118, with 0.0268 M BTA in artificial seawater.
record impedance changes in time induced by BTA presence in the solution. However, this may induce issues with proper interpretation of acquired impedance spectra. Additional measurement with classic EIS technique was performed ranging up to 10 mHz frequencies. This experiment was carried out 8 h after injection of BTA (0.0268 M) to maintain condition of system stationary. Acquired impedance spectrum that was presented on Fig. 7 demonstrates the presence of 2 semicircles.
Application of electric equivalent circuit containing two (QR) circuits in a form of R(Q(R(QR))) in impedance data analysis was not satisfactionary. Parameter v2 describing quality of fitting procedure, defined as [32]:
v2 ¼
" n X ðZ 0i ðxi ; ~ pÞ ai Þ2 2
i¼1
a2i þ bi
þ
ðZ 00i ðxi ; ~ pÞ bi Þ2 2
a2i þ bi
# ð1Þ
H. Gerengi et al. / Corrosion Science 51 (2009) 2573–2579
2577
where: xi, ai, bi - experimental data points, ~ p - parameters associated with a model, Z 0i , Z 00 - calculated point, was equal to 4 103. Essential differences between impedance values measured and theoretical appeared in low-frequency range, where diffusion transport processes manifest most often. Stirring of the solution in time of experiment excludes presence of diffusion restriction between the solution and sample surface. In the presence of BTA sample surface covers with polymer film Cu(I)-BTA that limits corrosion process. In such case diffusion through finite layer can occur which is defined in electric equivalent circuit as O parameter. Fitting procedure of acquired impedance to electric equivalent model R(Q(RO)) presented on Fig. 7 confirmed this assumption, v2 parameter was equal to 1 104. Fig. 7. Nyquist plot acquired for brass-118 with 0.0268 M BTA in artificial seawater. (h) experimental, (- - -) R(Q(R(QR))) model, (-) R(Q(RO)) model.
Fig. 8. An electrical circuit of R(Q(RW)) model.
Fig. 9. Exemplary impedance spectra acquired for Brass-118 with 0.0268 M BTA in artificial seawater with DEIS technique. (h) experimental, (-) R(Q(RW)) model.
Fig. 11. Surface heterogeneity (n) vs. time of exposure in artificial sea water in the presence of different concentration of BTA based on the DEIS results. ( 0.0268 M BTA, r 0.0201 M BTA, 4 0.0134 M BTA, s 0.0100 M BTA, } no inh.).
Fig. 10. Charge transfer resistance evaluation upon ten hours exposure to artificial sea water in the presence of different concentration of BTA based on the DEIS results. ( 0.0268 M BTA, r 0.0201 M BTA, 4 0.0134 M BTA, s 0.0100 M BTA, } no inh.).
Fig. 12. Q parameter of the CPE element vs. time of exposure in artificial sea water in the presence of different concentration of BTA based on the DEIS results. ( 0.0268 M BTA, r 0.0201 M BTA, 4 0.0134 M BTA, s 0.0100 M BTA, } no inh.).
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Table 2 Change of %IE, n and Q (Ssncm2) values for the corrosion brass-118 in artificial seawater in the presence of BTA after 1, 3, 5, 6 and 10 h. Concentrations
No inhibitor 0.0100 M BTA 0.0134 M BTA 0.0201 M BTA 0.0268 M BTA
(1 h)
(3 h)
(5 h)
(10 h)
%IE
n
Q
%IE
n
Q
%IE
n
Q
%IE
n
Q
%IE
n
Q
– 57.7 60.9 80.9 86.2
0.62 0.85 0.81 0.85 0.83
2.12 105 1.88 106 1.52 106 1.49 106 1.13 106
– 73.5 83.3 85.1 87.5
0.68 0.84 0.85 0.87 0.87
2.80 105 2.46 106 1.42 106 5.11 107 4.02 107
– 80.8 84.5 90.1 91.3
0.70 0.83 0.86 0.88 0.89
3.30 105 2.56 106 4.69 107 3.93 107 3.48 107
– 83.3 85.6 91.3 92.1
0.74 0.85 0.87 0.88 0.88
2.48 105 1.23 106 4.91 107 3.37 107 3.09 107
– 74.1 81.9 88.6 90.3
0.76 0.84 0.86 0.89 0.92
2.36 105 1.14 106 4.87 107 3.22 107 2.78 107
Impedance spectra acquired by means of DEIS technique contain only partial frequencies range in which the diffusion type mentioned above manifests. Taking into consideration that at the initial phase diffusion element O is indistinguishable from a Warburg impedance (W), all impedance data were analyzed with the use of simplified R(Q(RW)) model (Fig. 8). Key Rs resistance of electrolyte in bulk Rct charge transfer resistance at the metal surface W Warburg impedance Q constant phase element
Fig. 9 presents fitting procedure of exemplary impedance spectra acquired with DEIS technique to applied electric equivalent circuit. In the analyzed electric equivalent circuit, Q describes the nonideal behaviour of capacitance. The impedance of constant phase element (CPE) is given by:
Z CPE ¼ ½Q ðjxÞn 1
ð2Þ
where j is an imaginary number, Q is the frequency independent real constant, x = 2pf is the angular frequency (rad/s), f is the frequency of the applied signal, n is the CPE exponent for whole number of n = 1, 0, 1 CPE is reduced to the capacitor (C), resistance (R) and inductance (L). The value of n = 0.5 corresponds to Warburg impedance (W). The dispersion of the capacitive semicircle is related with surface heterogeneity due to surface roughness or inhibitor adsorption and formation of porous layer [33] and thus n serves as a measure of surface heterogeneity. Figs. 10–12 shows the evaluation of Rct, n and Q with time for brass-118 without and with 0.0100, 0.0134, 0.0201, 0.0268 M BTA. It can be seen from those figures that the system is not at equilibrium at the beginning of initial 5 h. From Fig. 8, it is evident that Rct increases considerably in the presence of BTA and the value of Rct increases as the concentration of BTA increases. Moreover, at each concentration Rct tends to attain almost a constant value after 5 h. This indicates the improvement of corrosion inhibition by BTA with time. Similarly, the evaluation of Q parameter that has the meaning of frequency distributed double layer capacitance with time (Fig. 12) confirms improvement of corrosion inhibition with time. Variation of parameter n of the constant phase element Q with time (Fig. 11) shows that it increases with the increase of BTA concentration. This can be attributed to decrease in surface heterogeneity thus improvement in corrosion inhibition. The parameter n also reached almost constant value after 5 h indicating that the system was not stationary until 5 h. The percentage inhibition efficiency (IE%) was calculated from the charge transfer resistance (Rct) values by using following equation:
IEð%Þ ¼
(6 h)
ðRct Þ1 ðRctðinhÞ Þ1 ðRct Þ1
100
ð3Þ
where the Rct(inh) and Rct are the charge transfer resistance values with and without inhibitor. Calculated IE%, n and Q values at differ-
ent times (after 1, 3, 5, 6 and 10 h), are shown in Table 2. Results from this Table show that IE%, n and Q values are almost constant after 5 h which indicates that system becomes stationary after 5 h. The value of Q from Table 2 is 3.93 107 Ssncm2 at the end of 5 h in the presence of 0.0201 M BTA while in the absence of BTA the value of Q is 1.49 106 Ssncm2. This low interfacial capacitance in the presence of BTA has been reported by others [34,35] and was attributed to the formation of polymeric type film comprising Cu(I)-BTA units. It is generally assumed that adsorption of BTA leads to formation of polymeric cuprous complex [36]. However; a certain time is needed for the building up this polymeric complex. Attained constant values of circuit parameters Rct, n and Q are related with the complete build up polymeric species at the end of 5 h. However Rct and n values increase where Q values decrease with the increase of BTA concentrations which are indicative of increase inhibition in other ways formation of higher amount of polymeric complex. 4. Conclusions The application of dynamic electrochemical spectroscopy technique for studying inhibition effect of BTA for brass-118 in artificial seawater allows to observe variation of Rct, n and Q with time. The change of these electrical parameters with time shows that system reaches to stable state after 5 h. Thus, the calculated inhibition efficiencies should be more realistic when the system is in the stable state (e.g. at the end of 5 h). Result shows that BTA has excellent inhibition properties on the corrosion of brass-118 in artificial sea water. However; DEIS method is more informative since it gives information about time when BTA has stable maximum inhibition effect. Thus, DEIS method should be used before each inhibitor testing method. References [1] [2] [3] [4] [5] [6] [7]
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