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10CATALYSIS FOR RENEWABLE ENERGY
SOME TETRAZOLIC COMPOUNDS AS CORROSION INHIBITORS FOR COPPER IN NITRIC ACID MEDIUM
M. Mihit*, L. Bazzi*, R. Salghi**, B. Hammouti***, S. El Issami*, E. Ait Addi****
*Faculté des Sciences d'Agadir, Laboratoire Environnement & Matériaux, Equipe de Chimie Physique Appliquées, BP 8106, 80000 Agadir,
Morocco. Telephone 00 212 28 09 57 Fax 00 212 28 22 01 00, E-mail: lahcen_bazzi@yahoo.fr
**Ecole National des Sciences Appliquées d'Agadir, Laboratoire d'Ingénierie des Procédés de l'Energie & de l'Environnement, BP 1136, 80000
Agadir, Morocco. Telephone 00 212 28 22 83 13 Fax 00 212 28 23 20 07, E-mail: salghi@ensa-agadir.ac.ma
***Faculté des Sciences d'Oujda, Laboratoire de Chimie Appliquée & Environnement,B. P 717, Oujda, Morocco. Telephone 00 212 68 63 22 73 Fax 00 212 36 500 603, E-mail: hammoutib@gmail.com
****Ecole Supérieure de Technologie d'Agadir, Département Bio-industrie, BP 33/S, 80000 Agadir, Morocco.
Telephone 00 212 28 23 25 83 Fax 00 212 28 22 78 24, E-mail: habib@esta.ac.ma
**Author for correspondence, Pr. R. Salghi, Ecole National des Sciences Appliquées d'Agadir, BP 1136, Agadir, Morocco.
Phone 00 212 28 22 83 13 Fax 00 212 28 23 20 07, E-mail salghi@ensa-agadir.ac.ma
Received: 31 July 2007; accepted: 25 Aug 2007
The corrosion inhibition of copper in 0.1 M HNO3 was studied by gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements by the use of tetrazolic compounds such as 1,2,3,4-tetrazole (TTZ), 1-phenyl-1,2,3,4-tetrazole (PT), 5-amino-1,2,3,4-tetrazole (AT) and 1-phenyl-5-mercapto-1,2,3,4-tetrazole (PMT). The addition of these compounds acts differently on the kinetic of corrosion process of copper in nitric medium. The comparative study indicates that PMT is the best inhibitor. Its inhibition efficiency increases while its concentration increases in solution to attain 97 % at 10-3 M. Electrochemical measurements show that both polarization and transfer resistances increase with PMT concentration. The adsorption of PMT on the copper surface followed the Frumkin isotherm model. The results obtained from the various methods are in good agreement.
Keywords: thermodynamic analysis in alternative energy, corrosion, inhibition, copper, nitric acid, tetrazole
Organization: Faculty of Science Agadir Morocco. He was awarded a PhD degree in Corrosion field in 2006.
Main range of scientific interests: Corrosion and the inhibition of the corrosion of copper and its alloys by organics and inorganics compounds. Pubications: More than 10 papers in corrosion.
Mohammed Mihit
Lahcen Bazzi
Organization: Faculty of Science Agadir Morocco. He was awarded a PhD degree in Corrosion Science in 1995. He is currently a teacher and researcher in the Laboratory of Materials & Environment.
Education: University Ibn Zohr, Faculty of Science, PhD degree in corrosion (1995). Ecole Normale Supérieure Rabat, Doctorat (DES), 1987.
Experience: University Ibn Zohr, Faculty of Science Agadir, Professor B (2003 - to date).
University Ibn Zohr, Faculty of Science Agadir, Professor A (2000-2003).
University Ibn Zohr, Faculty of Science Agadir, lecturer Professor (1995-1999).
University Ibn Zohr, Faculty of Science Agadir, Assistant Professor (1988-1994).
Main range of scientific interests: The research interest covers corrosion inhibition of aluminium, tin
and tinplate in industrial water by oxoanions, tetrazole, pyrazole and aminoacid and ester compounds.
Additionally, he works on pesticide degradation in water, soil, fruits and vegetables.
Pubications: He has published more than 40 papers in corrosion and sensors fields.
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ft
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Rachid Salghi
Belkhir Hammouti
Souad El Issami
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El Habib Ait Addi
Organization: Ecole Nationale des Sciences Appliquées of Agadir. He was awarded a PhD degree in Corrosion field in 1999. He is lecturer professor, his researcher is in the Laboratoire d'Ingénierie des procédés de l'Energie et de l'environnement (LIPEE). He is expert in the pesticide field. Education: Ecole Nationale des Sciences Appliquées Agadir, Diplome d'habilitation (2005), Université Mohamed Premier, Faculté des Sciences d'Oujda, PhD degree in Chemistry Sciences (1997-1999). Université Mohamed Premier, Faculté des Sciences d'Oujda, Diplôme d'Etude Supérieures in Chemistry Sciences (1994-1997).
Experience: Ecole Nationale des Sciences Appliquées d'Agadir, Lecturer professor (2005 - to date). Ecole Superieure de Technologie d'Agadir, Asistant professor (2001-2005). Ministère de l'Agriculture, Etablissement Autonome de Contrôle et de Coordination des Exportations; Chef of laboratory (2000-2001). Ministère de l'Agriculture, Etablissement Autonome de Contrôle et de Coordination des Exportations; Responsible of laboratory (1997-2000).
Main range of scientific interests: The research interest covers as well corrosion inhibition of aluminium, lead, tin and tinplate in industrial water by oxoanions, tetrazole, pyrazole and aminoacid and ester compounds as pesticide degradation in water, soil, fruits and vegetables. Pubications: More than 30 papers in corrosion and pesticide.
Organization: University of Mohammed 1st Faculty of Science Oujda. He was awarded a PhD degree in Corrosion Science in 1994. He is currently a professor and the Director of the Laboratory of Applied Chemistry and Environment.
Education: University of Mohammed 1st Faculty of Science Oujda, Faculty of Science, Ph.D. degree
in corrosion (1994). Ecole Normale Supérieure Rabat, Doctorat (DES), 1987.
Experience: University of Mohammed 1st Faculty of Science Oujda Professor B (2003 - to date).
University of Mohammed 1st Faculty of Science Oujda, Professor A (1999-2003).
University of Mohammed 1st, Faculty of Science Oujda, lecturer Professor (1994-1998). University of
Mohammed 1st Faculty of Science Oujda, Assistant Professor (1988-1994).
Main range of scientific interests: Prof. Hammouti's research interest covers acidity sensors and corrosion inhibition of iron, steel, lead, aluminium and copper in mineral acids by tetrazole, pyrazole, polymer, thiophene, pyridine, peptide, aminoacid and ester compounds. Additionally his research interest is moreover directed towards the field of the pesticides and selective electrodes. Pubications: More than 100 papers in corrosion and sensors fields.
Organization: Faculty of Science Agadir Morocco. She was awarded a PhD degree in Corrosion Science in 2005. She is currently a teacher and researcher in the Laboratory of Materials & Environment.
Education: University Ibn Zohr, Faculty of Science, PhD degree in corrosion (2005). University Ibn Zohr, Faculty of Science, Doctorat (DES), 1994.
Experience: University Ibn Zohr, Faculty of Science Agadir, lecturer Professor (2005 - to date). University Ibn Zohr, Faculty of Science Agadir, Assistant Professor B (2003-2005). University Ibn Zohr, Faculty of Science Agadir, Assistant Professor A (1995-2003).
Main range of scientific interests: The research interest covers as well corrosion inhibition of aluminium, lead, tin and tinplate in industrial water by oxoanions, tetrazole, pyrazole and aminoacid and ester compounds as pesticide degradation in water, soil, fruits and vegetables. Pubications: More than 12 papers in corrosion.
Organization: Ecole Supérieure de Technologie of Agadir. He was awarded a PhD degree in
Corrosion Science in 2005. He is currently an assistant professor.
Education: University Ibn Zohr, Faculty of Science, PhD degree in corrosion (2005).
Experience: Ecole Superieure de Technologie d'Agadir, Asistant professor (2006 - to date).
Ministère de l'Agriculture, Etablissement Autonome de Contrôle et de Coordination des Exportations;
Chef of laboratory (2001-2006). Ministère de l'Agriculture, Etablissement Autonome de Contrôle et
de Coordination des Exportations (1995-2001).
Main range of scientific interests: The research interest covers corrosion inhibition of tin and tinplate in industrial water by oxoanions, tetrazole, pyrazole and aminoacid and ester compounds, pesticide persistence and the dissipation of their residues in water, soil, fruits and vegetables and chemical quality of the industrial wastewater. Pubications: More than 16 papers in corrosion and pesticide.
Introduction
Numerous investigations have been performed on the corrosion inhibition of copper and its alloys by the use of different organic compounds [1-9]. Among the practical interest of these later are the chemical bond that linked
the inhibitors and the metal. Therefore, the organic inhibitors provide often an excellent protection against corrosion phenomenon. This protection was explained by the presence of heteroatoms (S, N, P, Se) in the molecular structure of the inhibitors [8-12].
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The addition of an inhibitor at certain concentration minimises the direct interaction between the metal and corrosive agents. In some cases, the coordination of the inhibitor molecules to the surface is weak, and their presence in the corrosive solutions required maintaining the desired concentration of these agents to attain a minimal protection of the metal [10-13]. As some inhibitor molecules also serve as reducing agents for the copper oxidation products, they must be continuously added to avoid other possible type of corrosion [14]. In the last few decades, numerous investigations were performed by using the traditional electrochemical methods [15-17], but recently, many research groups try to understand processes taking place at electrode surfaces by using in situ different accelerated corrosion techniques coupled by surface sensitive techniques like quartz crystal microbalance (QCM) and scanning probe microscopy (SPM) [18-22]. These in situ techniques have fostered a better understanding of dissolution/deposition of metals and adsorption of different ions by measuring mass differences in order of nanogram per surface area or visualizing the surface morphological changes in nanometer or even atomic scale. Related to this issue, several investigations [23-27] reported that the inhibition effectiveness might be due to the formation of thin layers of copper-inhibitor complexes. It's found also that 5-mercapto-1-phenyl-tetrazole (PMT) was an efficient inhibitor against the dissolution of various metals in different acid mediums [5, 6, 28]. This molecule adsorbs probably through the sulphur atom or through coordination with nitrogen from the tetrazole ring [29]. Ye et al. [30] demonstrate also that copper surface is normally coated with cuprous oxide layer (Cu2O), and PMT forms protective barriers of inert, insoluble and long-lasting polymeric Cu(I) complex coating on the Cu2O substrate. The aim of this paper is to study the corrosion behaviour of copper in 0.1 M HNO3 in the presence and in the absence of the tetrazolic compounds at 25 °C by the use of gravimetric measurements combined with linear potential scan voltammetry (I-E) and electrochemical impedance spectroscopy (EIS) methods.
Experimental methods
Materials
The material used was prepared from copper (99.99 %). The specimens used in the gravimetric tests have a rectangular form (2x3 cm). For the electrochemical studies, the electrode was provided in rod form and was embedded in epoxy resin in a Teflon holder with electrical contact being achieved by means of a copper wire threaded into the base of the metal sample. The surface of the specimen was 0.35 cm2. Prior to each test, the exposed surfaces were abraded successively with different emery paper up 1200 grade, washed thoroughly with acetone and rinsed with double distilled water. The experiments were carried out in aerated 0.1M HNO3 solution which was prepared using analytical grade
reagent (65 %) and double distilled water. All experiments have been performed at temperature 25 ± 1 °C. Tetrazolic compounds tested (at 10-3M) were 1,2,3,4-tetrazole (TTZ), 1-phenyl-1,2,3,4-tetrazole (PT), 1-phenyl-5-mercapto-1,2,3,4-tetrazole (PMT), and 5-amino-1,2,3,4-tetrazole (AT). They are Aldrich commercial products (> 99 % purity). Their molecular structures are shown in Fig. 1.
2 1 2 1 2 1
TTZ AT PT PMT
Fig. 1. Molecular structures of organic compounds tested
Gravimetric and electrochemical measurements Gravimetric measurements were carried out in a double walled glass cell equipped with a thermostat-cooling condenser. All experiments were carried out under total immersion in 75 ml of test solutions at temperature 25 °C. Mass loss was recorded by an analytical balance with 0.1 mg accuracy at 72 hours of immersion. Steady-state polarization experiments were conducted with a potentiostat PGP 201. The polarization curves were obtained by means of the linear potential sweep of 60 mV min-1 after the working electrode was left under free corrosion (for about 30 min) to reach a stable potential. The electrochemical measurements were performed in a conventional three-electrode electrochemical cell (Tacussel Standard CEC/TH). Saturated calomel electrode (SCE) and platinum electrode are used as reference and auxiliary electrodes, respectively.
Electrochemical impedance spectroscopy (EIS) was carried out with a voltalab PGZ 100 electrochemical system at Ecorr after immersion in solution. After determination of the steady-state current at a given potential, sine wave voltage (10 mV) peak to peak, at frequencies between 100 kHz and 10 mHz was superimposed on the rest potential. Computer programs (Voltamaster4) automatically controls the measurements performed at rest potentials after 30 min of exposure. The impedance diagrams were given in the Nyquist representation.
Results and discussion
Comparative study
The effect of the addition of tetrazolic compounds on the corrosion of copper in aerated nitric acid solution was studied by gravimetric measurements after 72 hours of immersion period and at 25 °C. The concentration of these inhibitors of 10-3 M was chosen to compare the inhibition efficiency of the four compounds toward the
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generalised corrosion. The value of the inhibition efficiency (EG/%) was determined using the equation (1).
W -W'
Eg =-corrw corr x 100,
(1)
where Wcorr and W'corr (mg-h-1-cm-2) are the corrosion rate in absence and presence of the inhibitor, respectively.
The corrosion data for different inhibitors tested are reported in Table 1.
Table 1
Corrosion rate of copper in 0.1 M HNO3 with and without tetrazo le at 10-3 M, and the corresponding inhibition efficiency
Solution Wcorr / Mg cm"2-h_1 Eg / %
Blank 3.65 -
TTZ 2.50 31
AT 1.51 58
PT 0.20 94
PMT 0.09 97
addition of mixed inhibitors in solution does not change corrosion potential significantly because they inhibit both the anodic and cathodic reactions. Small changes in potentials can be a result of the competition of the anodic and the cathodic inhibiting reactions, and of the metal surface condition.
Table 2 gives the values of the associated electrochemical parameters.
Table 2
Electrochemical parameters of copper in 0.1 M HNO3 with and without tetrazole at 10"3 M, and the corresponding inhibition efficiencies
Solutio n Ecorr / mV (SCE) Icorr / ^ cm-2 RP / ß 2 cm ßc / mV dec-1 Ei / % ERp / %
Blank -63 4.3 4050 -85 - -
TTZ -49 3.0 5525 -73 30 27
AT -54 1.7 11065 -74 60 63
PT -51 0.3 92500 -75 94 96
PMT -27 0.2 133320 -75 95 97
The inhibition efficiency (Ei / %) was determined using the equation (2) :
E{ = '1 'corr x 100
(2)
According to this data, it is clear that the addition of tetrazolic compounds reduces the corrosion rate of copper in nitric acid solution. The inhibition efficiency was found to depend on the nature of substituents. Potentiokinetic polarization curves were plotted for copper in 0.1M HNO3 solution in the presence of TTZ, PMT, AT or PT at 10-3 M (Fig. 2).
-1000 -800 -600 -400 -200 0 E/mV (SCE)
200 400
Icorr and ¡Corr are the uninhibited and inhibited corrosion current densities, respectively, determined by extrapolation of the cathodic Tafel lines to corrosion potential (ECorr).
From Table 2, it was clearly seen that cathodic slope was found equal indicating that the reduction of hydrogen did not modified in the presence of the inhibitors tested. Thus, the presence of tetrazolic compounds at 10-3 M leads to decrease in the values of Icorr, which was particularly significant in the case of PMT. The inhibition properties of tetrazolic compounds were evaluated also by polarization resistance method. Fig. 3 presents plots of the current densities as a function of over-voltage near the corrosion potential Ecorr.
3
< A
Fig. 2. Polarization curves of copper in 0.1 M HNO3 with and without tetrazole at 10'3M
A linear region with apparent Tafel was observed. The cathodic reaction was activation-controlled and the addition of the compounds tested decreased the current densities in large anodic and cathodic domains of potential. This result indicated that the compounds studied acted as mixed-type inhibitors. Generally, the
-15 -10 -5 0 5 E-Ecorr vs SCE /mV
10
15
Fig. 3. Linear polarization curves of copper in 0.1M HNO3 with and without tetrazole at 10'3M
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Table 2 gathered the corresponding values of polarization resistance Rp and the inhibition efficiency (ERt / %) which is calculated using the equation (3).
Rp'-Rp
Erp=^RpRL-x 100
(3)
Rp and R 'p are the polarization resistance values without and with inhibitor, respectively.
From polarization resistance measurements, the weakest value of Rp is found for copper in 0.1 M HNO3 without inhibitor. However, the addition of the tetrazole in solution leads to an increase in the polarization resistance values. In fact, the value of Rp is 5525 Q cm2 in the case of TTZ and increases for AT (11065 Q cm2), PT (92500 Q cm2) and PMT (Rp = 133320 Q cm2). This in turn leads to a decrease in corrosion current density Icorr values because this later is inversely proportional to Rp.
Furthermore, corrosion behaviour of copper, in acidic solution with and without addition of tetrazole at 10-3 M, is investigated by electrochemical impedance spectroscopy (EIS) measurement at corrosion potential (Ecorr) and after 30 min of immersion (at 25 °C). The corresponding Nyquist diagrams are shown in Fig. 4.
a s
N
706050403020100-
■ Blank . TTZ
/
0 12 3 4
10 20 30 40 50 60 Zre /KQcm2
70
Fig. 4. Nyquist plots obtained for copper in 0.1 M HNO3 without and with tetrazole at 10-3M (at Ecorr)
The charge-transfer resistance (Rt) values are calculated from the difference in impedance at low and high frequencies, as suggested by Tsuru et al. [31]. The double layer capacitance (Cdl) was obtained, at the frequency fm at which the imaginary component of the impedance is maximum (-Zm.), using the equation (4):
1
C =-
Cdl 2nfmR •
(4)
The impedance parameters derived from this investigation are mentioned in Table 3.
Table 3
Impedance parameters for copper in 0.1 M HNO3 with and without addition of tetrazolic inhibitors at 10-3 M, and the corresponding inhibition efficiency
Solution Rs / ñ cm2 Rt / ñ cm2 fm / HZ Сл / M.F cm-2 Er, / %
Blank 14 409б 0.б3 б1.б -
TTZ 1б б020 0.б3 41.9 32
AT 18 13250 0.4 30.0 б9
PT 21 90770 0.2 8.7 95
PMT 22 135170 0.2 5.9 97
The inhibition efficiency (ERt / %) got from the chargetransfer resistance is calculated using the following equation:
ERt =
R -R R
-x 100
(5)
Rt and R't are the charge-transfer resistance values without and with inhibitors, respectively. As we notice, Fig. 4, the impedance diagram, corresponds to the blank, shows perfect semi-circle indicating a charge transfer process mainly controlling the corrosion of copper. The addition of tetrazolic compounds enhances the value of the transfer resistance in acidic solution. EIS study confirms that the tested compounds are efficient inhibitors. From the impedance data reported in Table 3, the values of Rt increase according to the sequence TTZ < AT < < PT < PMT. This behaviour reveals the adsorption of inhibitors on the copper surface, and an increasing in the values of corrosion inhibition efficiency. The value of double-layer capacitance decreases in the presence of tetrazole. This decrease confirms the adsorption of these compounds on the metal surface leading to a film formation on the copper surface. The inhibition efficiency obtained from EIS measurement are close to those deduced from polarization and gravimetric methods.
m
c
C3
g
o L
5 -
4-
3 -
Blank
TTZ
AT
PT
PMT
-2 -1 0 1 2 3 4 5 6 Log(2pf/Hz)
Fig. 5. Bode plots of copper in 0.1 M HNO3 containing different tetrazolic compounds at 10-3M (at Ecorr)
13
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As the impedance diagrams obtained in the case of PT and PMT inhibitors, the semi-circles are more nonperfect. In this subject, several studies showed that the semi-circle is imperfect while the inhibition effect of the inhibitors is important [32-36]. Fig. 5 shows the Bode plots on the electrodes made of these samples.
The EIS results for the liquid and steam-treated samples can be explained by the simple equivalent Randles circuit [37], Fig. 6, which consist of a solution resistance Rs in series with a component composed of parallel film
capacitance Cdi and transfer resistance R, [38].
Fig. 6. A simple Randles-type equivalent circuit for EIS measurements
The values of the inhibition efficiency obtained from different methods are in good agreement. Therefore, the inhibiting effect was found to increases according to the following sequence: TTZ < AT < PT < PMT. This order is mainly due to the nature of the substituents and its position. It's well known that different substituents on the organic molecule polarize the functional group in a different manner [39] and the inhibition efficiency of organic inhibitor containing heteroatoms should follows the sequence O < N < S < P [40]. The presence of NH2 in AT, phenyl in PT, SH and phenyl in PMT may increase the polarity and the adsorbability of inhibitors on the metal surface. Therefore, the inhibition efficiency of TTZ is about 30 % and increases to 63 % in the cases of AT, then it attain 97 % with PMT. These jumps may be explained by the calculation of the net atomic charges [41]. The net charge of sulphur atom being -0.061 e, is adsorbed on the surface of metal. The presence of both phenyl and SH groups in PMT leads to synergistic intramolecular effect (Table 4) [30, 42-43].
Table 4
Net atomic charges of tetrazolic compounds calculated by PETRA program [41]
Inhibitor Ml) N(2) N(3) N(4) elumo / eV EHOMO/eV AE / eV
TTZ -0.2439 -0.1391 -0.1743 0.1502 - -0.085 -11.413 11.32
AT -0.1658 -0.1101 -0.1176 0.1950 - 0.018 -10.084 10.10
PT -0.2212 -0.1165 -0.1559 0.1852 - -0.765 -9.893 9.128
PMT -0.2258 -0.1132 -0.1701 0.1826 -0.061 -0.800 -9.418 8.618
The calculation shows that N(4) with positive charge does not participate to the adsorption phenomenon. Yan et al. [44] revealed that 2-mercaptobenzoxazole (MBO) is the good inhibitor toward copper corrosion in NaCl solution. Their studies show that the inhibition film (MBO-Cu) causes shift of N1S and S2p binding energies. Consequently, it could be inferred that the presence of mercapto group and nitrogen atom in the molecule could have an important role in the formation of the inhibition film. The computation of some of quantum chemical parameters such as the energies of the molecular orbitals, EHOMO (High Occupied Molecular Orbital Energy) and Elumo (Lowest Unoccupied Molecular Orbital Energy), has been determined for possible relations with the inhibitor efficiencies of the inhibitors tested (Table 4). HOMO energy is often associated with the electron donating ability of a molecule. The less negative HOMO energy and the smaller energy gap (AE = Elumo - Ehomo) are often interpreted by a stronger chemisorption bond and perhaps greater inhibition efficiency [45, 46]. The plot of the inhibition efficiency of tetrazolic compounds against EHOMO showed a linear correlation of slope equal to unity and regression coefficient R = 0.99 (Fig. 7). The similar results were obtained elsewhere [47]. From the results of the comparative study, it's evident that PT and PMT were the excellent corrosion inhibitors
for copper in nitric acid medium. Afterwards of this work, we proposed doing a detailed study of PMT in order to better understand its inhibition mechanism.
Ehomo/ev
Fig. 7. Correlation of HOMO energies with percent inhibition efficiency of tetrazole derivatives
Effect of PMT concentration Table 5 regroups the gravimetric parameters (Wcorr and Eg / %) of copper in 0.1M HNO3 in the presence of PMT at different concentrations after 72 h of period immersion.
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Table 5
Corrosion rate of copper in 0.1 M HNO3 with PMT at different concentration, and the corresponding inhibition efficiencies at T = 25°C
[PMT] Wcorr / Mg cm"2-h_1 Eg / %
Blank 3.65 -
10-9 M 1.83 50
10"8 M 1.45 60
10-7 M 0.65 82
10"6 M 0.50 86
10-5 M 0.38 90
10-4 M 0.20 94
10-3 M 0.09 97
3 210 -1-2 -3
-10 -5 0 5
E-Ecorr /mV (SCE)
10
0.1 M HNO3 was studied also by the use of electrochemical impedance spectroscopy (EIS) measurements. The impedance diagrams and the corresponding electrochemical parameters are shown, respectively, in Fig. 9 and Table 6.
70 6050-
C3 40-
.§ 30-
—■— Blank
—•— 10"M
-A- IO"M
-▼- 10:M
10'M
10JM
--- 10Л1
- — 10JM
According to this data, it's clear that the corrosion rate of copper in the blank is higher in comparison with the blank containing PMT. The addition of 10-9 M PMT into the aggressive medium reduces this corrosion rate by 51 % and reaches 97 % at 10-3 M. The inhibition efficiency of PMT increases as function of its concentration and exceeds 90 % from 10-5 M. Fig. 8 presents the evolution of the current densities as function of the over-voltage near of the corrosion potential £corr.
30 40 Zre /KQcm2
50
60
70
Fig. 9. Nyquist plots obtained for copper in 0.1M HNO3 with PMT at different concentrations
Table 6
Polarization resistance and impedance parameters of copper in 0.1 M HNO3 in the presence of PMT at various concentrations, and the corresponding inhibition efficiencies
[PMT] Rp/a 2 cm Rt /a cm2 /m/Hz CJfiF cm-2 ERp/% Er, /%
Blank 4050 4096 0.63 61.7 - -
10-9 M 9010 8340 0.63 30.3 55 51
10-8 M 13135 12250 0.63 20.6 69 66
10-7 M 20250 26180 0.40 15.2 80 84
10-6 M 34033 37990 0.35 12 88 89
10-5 M 52600 50070 0.3 10.6 92 92
10-4 M 67500 90450 0.2 8.8 94 95
10-3 M 132320 135170 0.2 5.9 97 97
Fig. 8. Linear polarization curves of copper in 0.1M HNO3 with PMT at different concentration
The corrosion parameters such as the values of
polarization resistance Rp and corresponding inhibition
efficiencies (ERp / %) are shown in Table 6.
From the polarization resistance measurement (Fig. 8,
Table 6), the polarization resistance Rp related to the
copper in 0.1 M HNO3 is the lower value. However, Rp
was found to increase with the rise of PMT
concentration. This result was accompanied with an
increase of the inhibition efficiencies.
The effect of the addition of PMT at different
concentration on the corrosion behaviour of copper in
The impedance diagrams obtained have, generally, a semicircular appearance. It indicates that the corrosion of copper is mainly controlled by a charge transfer process. The interaction of the tested molecule with the metal surface should be competitive with the interaction of the ions in the solution. In this case, the chemisorption has occurred. The adsorption would have occurred through polar centres as nitrogen and sulphur atoms. As for the shape of the impedance diagrams obtained, more nonperfect semi-circulars were found above 10-5M concentration. This explains that PMT is very efficient at high concentrations. The similar results were found elsewhere [32-36].
From the impedance data, we conclude that increasing of PMT concentration provokes an increase of the value of
13
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Rt and consequently the inhibition efficiency values. This behaviour shows the adsorption of PMT on copper surface which is approved also by the decrease of the values of double-layer capacitance as function of the concentration of PMT. This decrease may be due to the film formation on the copper surface [28, 30, 48-50]. Data were tested graphically by fitting to various isotherms. Fig. 10 shows the dependence of the fraction (0) as function of the logarithm of the concentration of PMT, where 0 is the ratio (E/%)/100.
-7 -6 -5
Log ([PMT])/M
Fig. 10. Frumkin isotherm adsorption of PMT on the copper surface in 0.1MHNO3
The obtained plot is consistent with an S-shape adsorbed isotherm for PMT showing an adsorption on the copper surface according to the Fumkin isotherm.
with
9 1 - 9
K = ■
expt
55.5
exp
)= KC
f ^G°ads A
RT
(6)
(7)
where AG°ads is the free enthalpy of adsorption and f is a function of adsorption energy. The average value of k, f and AG°ads calculated from 0 = Log ([PMT]) curve are: f = -8.52; K= 13.5-109 and A AG°ads = -67.7 kJ-mol-1.
The negative value of AG°ads indicates that strongly adsorbed on the copper surface. Literature shows that more negative than -40 kJ mol-1 indicating chemisorption of PMT molecules. Moreover, the inhibition of copper by PMT is often explained by formation of Cu(II)-PMT through its heteroatoms [30].
Conclusion
The inhibiting effect of tetrazolic compounds in 0.1 M HNO3 on copper was studied by various methods. The results are in good agreement and the main conclusions are as follows.
- The corrosion inhibition efficiency (E/%) of tetrazolic compounds follows the sequence: TTZ < AT < PT < < PMT.
- PMT was found to be the best inhibitor for copper.
- The inhibition efficiency of PMT exceeds 90 % above concentration of 10-5 M.
- PMT adsorbs on the copper surface according to the Frumkin adsorption isotherm. The value of AG°ads indicates that PMT adsorbs strongly and spontaneously.
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