Electrochemical frequency modulation as a new technique for monitoring corrosion inhibition of iron in acid media by new thiourea derivative

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1 Electrochimica Acta 51 (2006) Electrochemical frequency modulation as a new technique for monitoring corrosion inhibition of iron in acid media by new thiourea derivative S.S. Abdel-Rehim b, K.F. Khaled a,, N.S. Abd-Elshafi a a Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt b Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt Received 27 June 2005; received in revised form 16 September 2005; accepted 17 September 2005 Available online 15 November 2005 Abstract The electrochemical frequency modulation EFM technique provides a new tool for electrochemical corrosion monitoring. EFM is a nondestructive technique as electrochemical impedance spectroscopy (EIS). EFM technique was used in comparison with the traditional dc and ac techniques. Results obtained by EFM technique were shown to be in agreement with other electrochemical techniques. With EFM technique, a corrosion rate can be obtained instantaneously in very short time which makes this technique ideal in online corrosion monitoring. New synthesized thiourea derivative named 1,3-diarylidenethiourea (DAT) was examined as a new corrosion inhibitor for iron in 1 M hydrochloric acid solution Published by Elsevier Ltd. Keywords: Electrochemical frequency modulation; Electrochemical impedance spectroscopy; Tafel extrapolation; Iron; Acid corrosion inhibition 1. Introduction Several electrochemical techniques are used to determine corrosion rates in metal samples. Linear polarization resistance (LPR) technique, Tafel extrapolation and electrochemical impedance spectroscopy (EIS). Simplicity is an obvious advantage of LPR and Tafel extrapolation methods; however, their application is hindered by systematic errors caused by arbitrary assumptions and simplifications where the system must be polarized over a wide potential range, so the measurement is time consuming and the electrode surface affected by the measurement [1]. The most important sources of error are the following: (a) erroneous estimation of the Tafel slopes or their determination by extrapolation; (b) determination of the polarization resistance using finite amplitude; (c) current potential characteristics differing from the Butler Volmer relationship. Electrochemical Frequency Modulation (EFM) is a non-destructive corrosion measurement technique that can directly give values of the corrosion current without a prior knowledge of Tafel constants. Like EIS, it is a small signal ac technique. Unlike EIS, however, two sine waves (at different frequencies) are applied Corresponding author. Tel.: ; fax: address: khaled.f@mailcity.com (K.F. Khaled). to the cell simultaneously. Because current is a non-linear function of potential, the system responds in a non-linear way to the potential excitation. The current response contains not only the input frequencies, but also contains frequency components which are the sum, difference, and multiples of the two input frequencies. In corrosion research, it is known that the corrosion process is non-linear in nature, a potential alteration by one or more sine waves will generate responses at more frequencies than the frequencies of applied signal. Current response cannot be measured at zero harmonic and intermodulation frequencies. Measuring the direct current (dc) at frequency zero is called the Faraday rectification technique. This technique can be used for corrosion rate measurements if at least one of the Tafel parameters is known. The corrosion rate and both Tafel parameters can be obtained with one measurement by analyzing the harmonic frequencies [2]. Virtually no attention has been given to the intermodulation or electrochemical frequency modulation; however, EFM has been shown that this non-linear response contains enough information about the corroding system so that the corrosion current can be calculated directly. The great strength of the EFM is the causality factors which serve as an internal check on the validity of the EFM measurement [3]. With the causality factors the experimental EFM data can be verified /$ see front matter 2006 Published by Elsevier Ltd. doi: /j.electacta

2 3270 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) In this paper, an evaluation for EFM as a new non-linear distortion technique, the electrochemical frequency modulation, was investigated and evaluated as an instantaneous corrosion monitoring technique. The EFM technique will be used here for online monitoring of the corrosion rate of pure iron in absence and presence of a new synthesized organic compound named 1,3-diarylidenethiourea (DAT) as a possible corrosion inhibitor. Corrosion inhibition data will compare with that obtained from traditional techniques like Tafel extrapolation and EIS. 1,3-Diarylidenethiourea (DAT) will be investigated as a new corrosion inhibitor for iron in 1 M HCl solutions at 25 C. Corrosion inhibitors are commonly used to decrease the corrosion attack on metallic materials especially in acidic media [4 16]. 2. Experimental details A mixture of 2-hydroxynaphthalene carboxayldehyde (2 mmol) and thiocarbohydrazide (1 mmol) in absolute ethanol (40 ml) and glacial acetic acid (10 ml) was refluxed for 1 h cooled then poured onto ice. The obtained solid was filtered and crystallized from dioxan to give 1,3-diarylidenethiourea. Fig. 1 shows the preparation method for the studied thiourea derivative. All electrochemical measurements were performed in a typical three-compartment glass cell consisted of the iron rod (Puratronic %) its surface area 0.28 cm 2 as working electrode (WE) (prepared using emery papers of different grit sizes up to 4/0 grit, polished with Al 2 O 3 (0.5 m particle size)), platinum mesh as counter electrode (CE), and a saturated calomel electrode (SCE) as the reference electrode. Solutions were prepared from bidistilled water. The electrode potential was allowed to stabilize for 60 min before starting the measurements. All experiments were conducted at 25 C. The electrolyte solution was made from analytical reagent grad HCl. The electrodes were arranged in such a way that onedimensional potential field existed over the WE surface in solution. To get an impression about the process occurred at the iron/acid interface, Tafel curves were obtained by changing the electrode potential automatically from ( 250 to +250 mv SCE ) Fig. 1. Preparation of 1,3-diarylidenethiourea (DAT). versus open circuit potential with scan rate of 1 mv/s. Also, EIS measurements were carried out in a frequency range of mhz with amplitude of 10 mv peak-to-peak using ac signals at open circuit potential. Electrochemical Frequency Modulation (EFM) carried out using two frequencies 2 and 5 Hz. The base frequency was 1 Hz, so the waveform repeats after 1 s. The higher frequency must be at least two times the lower one. The higher frequency must also be sufficiently slow that the charging of the double layer does not contribute to the current response. Often, 10 Hz is a reasonable limit. In this study, we use a perturbation signal with amplitude of 10 mv for both perturbation frequencies of 2 and 5 Hz. The choice for the frequencies of 2 and 5 Hz was based on three arguments [2]. First, the harmonics and intermodulation frequencies should not influence each other. Second, the frequency should be as low as possible to avoid influence of the capacitive behaviour of the electrochemical double layer. Third, the frequency should be as large as possible to reduce time needed to perform a measurement. While these arguments do not allow the selection of perfect frequencies, the chosen frequencies were considered as reasonable compromise. Measurements were performed with a Gamry Instrument Potentiostat/Galvanostat/ZRA. This includes a Gamry Framework system based on the ESA400, Gamry applications that include DC105 for dc corrosion measurements, EIS300 for electrochemical impedance spectroscopy measurements and EFM140 to calculate the corrosion current and the Tafel constants along with a computer for collecting the data. Echem Analyst 4.0 Software was used for plotting, graphing and fitting data. 3. Results and discussion The result of EFM experiments is a spectrum of current response as a function of frequency. The spectrum is called the intermodulation spectrum and an example is shown in Figs Figs. 2 5 show the corresponding current response in the intermodulation spectrum. The harmonic and intermodulation peaks are clearly visible and are much larger than the background noise. The two large peaks, with amplitudes of about 200 A, are the response to the 2 and 5 Hz excitation frequencies. Those peaks between 1 and 20 A are the harmonics, sums, and differences of the two excitation frequencies. Analysis of these peaks at intermodulation frequencies can reveal the corrosion rate and Tafel parameters. It is important to note that between the peaks the current response is very small. There is nearly no response (<100 na) at 4.5 Hz, for example, the frequencies and amplitudes of the peaks are not coincidences. They are direct consequences of the EFM theory. The corrosion parameters such as inhibition efficiency (E EFM, %), corrosion current density ( A/cm 2 ), Tafel constants and causality factors at different concentrations of DAT in 1 M HCl at 25 C are presented in Table 1. As can be seen from Table 1, the corrosion current densities decrease by increasing the concentrations of DAT. The causality factors in Table 1 indicate that the measured data are of good quality. The standard values for CF-2 and CF-3 are 2.0

3 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) Fig. 2. Intermodulation spectrum for iron in 1 M HCl. Fig. 3. Intermodulation spectrum for iron in 1 M HCl in presence of 10 4 M from DAT molecules. Fig. 4. Intermodulation spectrum for iron in 1 M HCl in presence of M from DAT molecules.

4 3272 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) Fig. 5. Intermodulation spectrum for iron in 1 M HCl in presence of M from DAT molecules. and 3.0, respectively. The causality factor is calculated from the frequency spectrum of the current response. If the causality factors differ significantly from the theoretical values of 2 and 3. It can be deduced that the measurements are influenced by noise. If the causality factors are approximately equal to the predicted

5 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) Fig. 6. (a) Nyquist plots for iron in 1 M HCl without and with various concentrations of DAT and (b) Bode plots for iron in 1 M HCl without and with various concentrations of DAT. description of a frequency independent phase shift between an applied ac potential and its current response, a constant phase element () is used, which is defined in impedance representation as in Eq. (2) Z(ω) = Z 0 (jω) n (2) where Z 0 is the constant, ω the angular frequency (in rad/s), j 2 = 1 the imaginary number and n is the exponent. Depending on n, can represent resistance (Z 0 = R, n = 0), capacitance (Z 0 = C, n = 1), inductance (Z 0 = L, n = 1) or Table 2 Electrochemical kinetic parameters obtained by EIS technique for iron in 1 M HCl without and with various concentrations of DAT Concentration (M) R ct ( cm 2 ) C dl ( Fcm 2 ) θ E IMP (%) Blank Warburg impedance for (n = 0.5) [15]. So, by use of the concept of we got good fit for the experimental data. For analysis of the impedance spectra containing one capacitive loop the equivalent circuit given in Fig. 7 was used [15]. Figs. 7 and 8 show representative examples for the impedance spectra carried out at E corr on iron exposed to 1 M HCl in absence and presence of DAT. Here, both simulated and measured results fit well. Figs. 7(a) and 8(a) show Nyquist plots recorded after 1 h immersion in 1 M HCl solution. Bode-phase plots (representative example) gave only one capacitive time-constant as in Figs. 7(b) and 8(b). The proposed equivalent circuit, at Figs. 7 and 8 satisfactorily simulates the spectra. Table 4 shows the circuit elements and their values obtained by applying the equivalent circuit in Figs. 7 and 8. The high frequency part of the impedance and phase angle describes the behaviour of an inhomogeneous surface layer, while the low frequency contribution shows the kinetic response for the charge transfer reaction [15]. Double layer capacitances decrease with respect to the blank solution when DAT is added to the solution. This fact may be explained on the basis of adsorption of DAT molecules on iron surface. In order to gain more information about the mode of adsorption of this compound on the iron surface, the experi-

6 3274 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) Fig. 7. (a) Nyquist plot of iron corrosion in 1 M HCl solution and (b) Bode-phase plots of iron corrosion in 1 M HCl solution at E corr. mental data have been tested with several adsorption isotherms. In order to obtain the isotherm, coverage θ calculated from the capacitance measurements using the following equation: θ = 1 C dl C 0 dl where C 0 dl and C dl are the double layer capacitance in presence and absence of DAT, respectively. The plots of log(θ/(1 θ)) versus log C inh yielded a straight line, where C inh is the inhibitor (3) concentration. The best fit was obtained with Langumir adsorption isotherm as in Fig. 9. Fig. 10 represents typical Tafel curves of iron in 1 M HCl in absence and presence of various concentrations of DAT. As the concentration of DAT increases the corrosion current densities decreased and inhibition efficiency increased. Table 3 shows the corrosion kinetics parameters, as corrosion potential (E corr ), cathodic and anodic Tafel slopes (β a, β c ) and corrosion current density (i corr ), obtained by extrapolation of the Tafel lines. The inhibitor efficiency E T (%) was evaluated from Tafel extrapola- Table 3 Electrochemical kinetic parameters obtained by Tafel extrapolation technique for iron in 1 M HCl without and with various concentrations of DAT Concentration (M) I corr ( Acm 2 ) E corr (mv) b a (mv dec 1 ) b c (mv dec 1 ) C.R. (mpy) E T (%) Blank

7 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) Fig. 8. (a) Nyquist plot of iron corrosion in 1 M HCl solution with 500 ppm DAT and (b) Bode-phase plots of iron corrosion in 1 M HCl solution with 500 ppm DAT at E corr. Fig. 9. Langmuir s adsorption plots of iron in 1 M HCl and containing various concentrations from DAT. Fig. 10. Tafel plots for iron in 1 M HCl in absence and presence of various concentrations of DAT.

8 3276 S.S. Abdel-Rehim et al. / Electrochimica Acta 51 (2006) Table 4 Circuit elements R p,, and n values for the equivalent circuit in Figs. 7 and 8 of iron in 1 M HCl without and with various concentrations of DAT Concentration (M) Element Value Blank R p E 6S n R p E 6S n R p E 6S n R p E 6S n R p E 6S n R p E 6S n tion method using the following equation: ( E T (%) = 1 I ) I (4) where I 0 and I correspond to uninhibited and inhibited current densities, respectively. Corrosion current density I values decreased by addition of DAT, in 1 M HCl (Table 4). Corrosion potential E corr shifts with no definite trend indicating that DAT acts as mixed type inhibitor in 1 M HCl. Fig. 11. Comparison of inhibition efficiencies obtained with EFM, EIS and Tafel extrapolation. In Fig. 11, the inhibition efficiencies obtained with the three different techniques versus the inhibitor concentration, are shown in a histogram. The three different electrochemical techniques showed similar behaviour. As the inhibitor concentration increased the corrosion rate decreased and the inhibition efficiency increased. Based on these results, the electrochemical frequency modulation technique appears capable of monitoring the corrosion inhibition of iron in 1 M HCl by different concentrations of DAT. 4. Conclusions The following conclusions can be drawn from this study: 1. Electrochemical frequency modulation EFM can be used as a rapid and non-destructive technique for corrosion rate measurements without prior knowledge of Tafel constants. 2. Corrosion rates obtained with this technique were in good agreement with other traditional techniques as Tafel extrapolation and EIS. 3. Causality factors used by EFM are good internal check for verifying the validity of data obtained by this technique. 4. Inhibition efficiencies increases by increasing the inhibitor concentration and inhibition occurs through adsorption of the inhibitor on the iron surface. The adsorption of DAT obeys the Langmuir s adsorption isotherm. 5. Double layer capacitances decreases with respect to the blank solution when this inhibitor is added. This fact may be explained on the basis of adsorption of DAT on iron surface. Acknowledgements The authors wish gratefully acknowledge Prof. Reda Mohammady Abdel-Rahman (D.Sc.) for his help in the preparation of the inhibitor. References [1] R.W. Bosch, W.F. Bogaerts, Corrosion 52 (1996) 204. [2] R.W. Bosch, J. Hubrecht, W.F. Bogaerts, B.C. Syrett, Corrosion 57 (2001) 60. [3] Gamry Echem Analyst Manual, [4] N. Ochoa, F. Moran, N. Pébère, B. Tribollet, Corros. Sci. 47 (2005) 593. [5] P. Ocón, A.B. Cristobal, P. Herrasti, E. Fatas, Corros. Sci. 47 (2005) 649. [6] E.A. Noor, Corros. Sci. 47 (2005) 33. [7] A. Frignani, C. Monticelli, F. Zucchi, G. Trabanelli, Mater. Chem. Phys. 92 (2005) 403. [8] M. Karakuş, M. Şahin, S. Bilgiç, Mater. Chem. Phys. 92 (2005) 565. [9] K. Babic-Samardzija, K.F. Khaled, N. Hackerman, Anti-Corros. Meth. Mater. 52 (2005) 11. [10] S.S. Abdel Rehim, Magdy A.M. Ibrahim, K.F. Khaled, J. Appl. Electrochem. 29 (1999) 597. [11] K.F. Khaled, S.S. Abdel Rehim, N. Hackerman, Ann. Univ. Ferrara 2 (2000) 713. [12] S.S. Abdel Rehim, Magdy A.M. Ibrahim, K.F. Khaled, J. Corros. Prevent. Contr. 3 (2000) 245. [13] S.S. Abdel Rehim, Magdy A.M. Ibrahim, K.F. Khaed, J. Mater. Chem. Phys. 70 (2001) 267.

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