October 30, 2007
Determination of Inhibition Effects of Various Imidazole Derivatives on Copper Corrosion in 0.1M HCl Media
By Bereket, G (Aldemir), S Pakdil; Ogretir, C
In the present study the inhibitive behaviour of 1H-imidazole, 1- methyl imidazole, imidazolidin-2-one, 4-(imidazole-1- yl)acetophenone, 4-(imidazole-1-yl) phenol and L-hystidine on the corrosion of copper in 0.1 M HCl has been investigated by potentiodynamic polarisation and polarisation resistance techniques. Except for 2-imidazolidone, the tested imidazole derivatives were found to behave as cathodic inhibitors. 1-methyl imidazole, 4- (imidazole-1-yl) acetophenone, 4-(imidazole-1-yl) phenol and L- hystidine were adsorbed on copper surface according to Temkin isotherm model. By using semi-empirical quantum chemical methods, physical parameters such as charge on the heteroatoms of imidazole derivatives q^sub n^, energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and dipole moments were calculated. Possible correlations between these calculated physical parameters and inhibitor efficiencies were considered. Finally, the mechanistic information relating to the inhibition process was elucidated. Keywords: Copper corrosion, Imidazole, Hydrochloric acid, Electrochemical methods
Copper and its alloys have excellent thermal conductivity and good mechanical workability. For these reasons they are being widely used in heating and cooling systems. However, scale and corrosion product accumulations cause a reduction in heating efficiency of the system. Thus, periodic descaling and cleaning with acid pickling solutions are necessary. Hydrochloric and sulphuric acid are widely used for pickling of copper and its alloys and corrosion inhibitors are necessary to control metal dissolution and to minimise acid consumption during this process. Organic species are generally recognised as effective inhibitors for the corrosion of metals and for their alloys. The efficiency of an organic compound may be associated with adsorption at the metal/solution interface. According to the dominant force, adsorption can be due to physisorption, chemisorption or a combination of these.1 Imida/ole derivatives are five membered, two nitrogen atom containing heterocyclic rings2 and are well known for their strong adsorption on gold,3 silver4,5 and copper.6 They are known to be non-toxic organic compounds acting as copper corrosion inhibitors in acid.7,8
The electronic structure of the organic inhibitors has a key influence on the corrosion inhibition efficiency of the metal. The search for new corrosion inhibitors has been a subject of long and active effort.9 For a proper selection of an inhibitor, the mechanistic information of inhibition process is needed. Furthermore a systematic approach is needed for characterisation of the interaction between the organic molecule and the metal alloy. One approach is to elucide the interaction mechanism or pathway of the inhibition process by molecular orbital calculations of the relevant parameters and some studies using this approach have been reported.10-12
The present article focuses on the investigation of inhibition efficiencies of imidazole derivatives for copper corrosion in hydrochloric acid solution using electrochemical methods. The theoretical consideration of correlation between the molecular structures of the studied imidazole derivatives and their inhibition efficiencies were also studied using semi-empirical quantum chemical calculation methods.
Using a systematic approach, N-substituted imidazole derivatives, 1H-imidazole (I), 1-methyl imidazole (MI), 4-(imidazole-1- yl)acetophenone (AI), 4-(imidazole-1-yl) phenol (PI), were chosen and these can be considered to be a homologous series of compounds having different substituent in N(3) position (R= H, -CH^sub 3^, - C^sub 6^H^sub 4^OH, -C^sub 6^H^sub 4^OCH^sub 3^). The structure of the compounds as given in Fig. 1.
Owing to the fact that stable tautomeric forms play an important role in structure reactivity relationships, 4-(imidazole-1-yl) acetophenone (AI), 4-(imidazole-1-yl) phenol (PI) and imidazolidin- 2-one (D), were also chosen to investigate these effects.
It is known that, for the imidazole group in its structure, L- histidine (H) exhibits inhibitive behaviour for metal. Furthermore hystidine, an aminoacid, is widely considered as nontoxic. L- histidin, however, has a different structure than the remaining imidazole derivatives. Hence only electrochemical studies were performed, and it was not included in the calculation for the systematic studies between structure and inhibitor efficiency.
1 Names, abbreviations and structures of studied imidazole derivatives
The investigation of the inhibiting properties of the imidazole derivatives was performed using potentiodynamic polarisation. The experiments were carried out using 99-9%Cu as the electrode material. A cylindrical copper rod was inserted in a Teflon tube leaving only a cross section (0-1968 cm^sup 2^) exposed. The electrode surface was abraded with sequence of emergy paper of different grades, immersed in concentrated nitric acid for 30 s and washed with double distilled water.
2 E-log i curves of copper in 0.1M HCl+XM 4-(imidazol-1-yl) acetophenone
Measurements were made in a conventional electrolytic cell with a platinum counter electrode using a silver-silver chloride (Ag | AgCl | Cl^sup -^) electrode as reference electrode. The electrode was immersed in solutions consisting of 0.1M hydrochloric acid with organic inhibitors at concentrations ranging from 0.0005 to 0.01M. Potentiodynamic experiments were conducted at 20[degrees]C using a CHI604 Electrochemical analyser.
The working electrode was first immersed into the test solution for 45 min to establish a steady state open circuit potential. Potentiodynamic polarisation studies were then performed at a scan rate 0.5 mV s^sup -1^ in the potential range of E=E^sub corr+-300 mV. Linear polarisation measurements were carried out in a potential + 20 mV versus corrosion potential at a scan rate 0.05 mV s^sup - 1^. The measurements were conducted under aerated conditions.
Theoretical calculations were carried out at the restricted Hartree-Fock level using standard AMI, PM3 and MNDO semi-empirical self-consistent field molecular orbital methods using the MOPAC 7.0 code package.13 Initial estimates for the geometries of all the structures were obtained by using the molecular mechanics programme Chemoffice Pro14 followed by full optimisation of all geometrical variables without any symmetry constraint, using the semi-empirical AMI, PM3, MNDO quantum chemical methods implemented in the MOPAC 7.0 programme.
Results and discussion
Potentiodynamic polarisation measurements
Figure 2 shows typical cathodic and anodic polarisation curves for copper in 0.1M HCl solution in the absence and presence of various concentrations of AI. Similar polarisation curves were obtained in 0.1M HCl with various concentrations of AI, I, D, MI, PI, H.
Electrochemical corrosion parameters obtained by the Tafel Extrapolation method are depicted in Table 1, where variables beta^sub a^ and beta^sub c^ given here are the anodic and cathodic Tafel slopes respectively.
The corrosion currents i^sub corr^, obtained in 0.1M HCl solution with AI, PI, MI and H were found to be lower than those obtained without the imidazole derivatives except at their lower concentrations when, in some cases, they are acting as activators (Table 1). Increase in concentrations of AI, PI, MI and H cause a shift of corrosion potentials E^sub corr^, in the negative direction as well as variation to the Tafel slopes. This indicates that the inhibition of copper corrosion in 0.1M HCl solution by the studied imidazole derivatives is under cathodic control. The increase in inhibition efficiency with concentration of imidazole can be explained by the increasing adsorption of these compounds on the copper surface.
The inhibition efficiency of 1H-imidazole is seen to change irregularly with concentration. A similar behaviour for this compound was also observed for the corrosion inhibition of copper in H^sub 2^SO^sub 4^.8 Imidazoline-2-one, on the other hand, has a greatly reduced inhibition efficiency and, at some concentrations, acts as an activator. Such behaviour can be understood by the presence of a stable tautomeric form of D, which will be discussed in the quantum chemical part of the present paper.
The constant f depends on the intermolecular interaction in the adsorption layer and on the heterogeneity of the surface. If f has a positive value, then mutual attraction of the molecules occurs; if it is negative, then repulsion occurs.
Table 1 Electrochemical parameters for copper in 0.1M HCl in absence and presence of various concentrations of I, Al, Pl, D, Ml, H obtained by Tafel extrapolation
3 Graph of theta versus log c for 4-(imidazol-1-yl) acetophenone in 0.1M HCl
4 E-log i curves for copper in 0.1M HCl+XM 4-(imidazol-1-yl) acetophenone
Polarisation resistance measurements
The determination of the polarisation resistance of copper in HCl with addition of investigated imidazole compounds was studied. Typical polarisation resistance curves for copper in 0-1M HCl solution in the presence and the absence of inhibitor AI are shown in Fig. 4. Table 3 shows the corrosion parameters and degree of protection for copper in HCl obtained by polarisation resistance method. Table 2 Thermodynamic parameters of adsorption obtained by Tafel extrapolation method for Al, Pl, Ml, H derivatives on copper in 0.1M HCl solution
Table 3 Electrochemical parameters for copper in 0.1M HCl in presence of various concentrations of I, Al, Pl, D, Ml, H obtained by polarisation resistance method
Quantum chemical study
To investigate the mechanism of inhibition, the authors have searched the relations between quantum chemical parameters and the experimentally obtained inhibitor efficiencies. Since tautomeric forms of organic molecules play an important role in structure reactivity relationships, the authors have concentrated primarily on the stable forms of the molecules AI, PI and D (Fig. 6).
5 Graph of theta versus log c for 4-(imidazol-1-yl) acetophenone in 0.1M HCl
In order to find a correlation between the molecular structure and their effectiveness in inhibiting copper in 0.1 M HCl, the energy of highest occupied (HOMO) and the lowest unoccupied (LUMO) molecular orbitals, the distribution of charges on the individual atoms of each molecule, dipole moments were collected and are shown in Table 5.
Aqueous phase semi-empirical calculated (AM1, PM3, MNDO) relative stability data for two potential tautomeric group substituted imidazole derivatives along with their model compounds (fixed models) are given in Table 6. Quantum chemical parameters of stable tautomer forms have been used in the correlation with inhibitor efficiency and quantum chemical parameters.
Molecular geometry plays an important role in the adsorption and conjugation of molecules. For this reason the values of dihedral angles were calculated by energy minimisation. The calculated values of dihedral angles for AI and PI given in Table 7 indicate that both of them are planar.
From the pK^sub a^ values16 of Table 8 it can be assumed that in acidic media the studied molecules were protonated and therefore a study of any possible correlation between the inhibitor efficiencies and positive charge density on the N(3) atom was carried out. The best correlation was obtained using the aqueous phase PM3 calculation (Fig. 7).
Table 4 Thermodynamic parameters of adsorption obtained by polarisation resistance method for Al, Pl, Ml, H on copper in 0.1M HCl solution
Table 5 Aqueous phase AM1, PM3, MNDO data for I, D, Ml, Pl, Al
6 Tautomerism for a 4-(imidazoH-yl) acetophenone, b 4-(imidazol- 1-yl) phenol and c imidazoline-2-one
7 Graph of inhibitor efficiency versus qN3 quaternary aqueous phase PM3 calculations
Chemisorption of organic molecules occurs due to chealation on the metal surface. Thus, electron donation from the molecule into unoccupied d orbitals on the metal surface will occur with the reverse process forming the antibonding orbital. The higher the HOMO energies E^sub H^ of the molecule, the easier for its electrons to move into metallic unoccupied d orbitals, and the higher is the expected inhibitor efficiency. Similarly, the lower the LUMO energies E^sub L^ of the molecule, the easier is the acceptance of electrons from metallic d orbitals and, again, the higher is the expected inhibitor efficiency.17
Observation of the plots for inhibition efficiencies versus E^sub H^ (Fig. 8) and E^sub L^ (Fig. 9) respectively show that this is the case. In order to maintain consistency of the calculation the PM3 method is used throughout this discussion. Another important point concerns the gap between the E^sub L^ and E^sub H^ for each molecule in a homologous series. This gap can be used as a characteristic quantity for metallic complexes. Cherry et al.18 have used the concept of the E^sub L^-E^sub H^ gap in developing a theoretical model of capable of qualitatively explaining the structural stability and conformation in many molecular systems. Furthermore, the energy gap may be related to the redox potential and the electrical resistivities of the complexes.19
As can be seen from the inhibitor efficiency versus E^sub L^- E^sub H^ graphs, the inhibitor efficiency increases as the E^sub L^- E^sub H^ values decrease (Fig. 10), which indicates the stability of the formed complexes. Variation of inhibitor efficiency versus E^sub H^ and E^sub L^ graphs may be explained by the donor behaviour of the imidazole molecule.
Table 6 Comparison of calculated semi-empirical (AM1, PM3 and MNDO) aqueous phase relative stability (RS) data for Al, Pl and D* in Fig. 6, kcal mol^sup -1^
Table 7 Dihedral angles for Pl and AI*
8 Graph of inhibitor efficiency versus E^sub H^ aqueous phase PM3 calculations
9 Graph of inhibitor efficiency versus E^sub L^ aqueous phase PM3 calculations
10 Graph of inhibitor efficiency versus E^sub L^-E^sub H^ aqueous phase PM3 calculations
Owing to the fact that correlations between inhibitor efficiency and experimental pK^sub a^ of the imidazole derivatives relates the inhibitor efficiency to the basicity of the molecules, graphs of inhibitor efficiency versus pK^sub a^ were constructed (Fig. 11).
In aerated HCl solution, the following mechanism is proposed for the corrosion of copper.20
Table 8 Experimental acidity constants16 of AI, PI, MI, I and D
11 Graph of inhibitor efficiency versus experimental pK^sub a^
As shown in the calculated relative stability data given in Table 6, D was found to exist in the keto form in which no pyridine like nitrogen atom is present in its structure. However, the authors' results as well as previous studies on inhibition effect of imidazole derivatives on corrosion of various metals shows that adsorption of imidazole derivatives occurs through this nitrogen atom.7,8,10,21 Since the semi-empirical calculation indicates the favorability of the keto form, the observed corrosion activation (not inhibition) behaviour of compound D can be explained by the absence of a pyridinium N.
Electrochemical and quantum chemical studies of the effects of imidazole derivatives on the corrosion behaviour of copper in aerated 0.1M HCl have led to the following conclusions.
1. Both Tafel extrapolation and polarisation resistance show that among the studied imidazole derivatives AI, PI, MI and H behave as cathodic inhibitors.
2. Although data obtained from Tafel extrapolation and polarisation resistance methods were not same, the relative trends inhibition efficiencies were in good agreement.
3. The adsorption of the molecule onto copper is consistent with the Temkin adsorption model.
4. Semi-empirical calculations performed in the present study show that the corrosion inhibition behaviour of imidazole derivatives can be correlated to key molecular parameters such as dipole moment, charge density and to the energy difference between the highest occupied and lowest unoccupied molecular orbitals.
5. The experimental and model evidence supports that, in the first stage physical adsorption occurs onto the copper with the sharing of electrons between the adsorbed molecule and the copper surface, after which a chemisorption becomes dominant.
The authors gratefully acknowledge financial support of Scientific Research and Application Centre of Osmangazi University, Eskicehir, Turkey.
(c) 2007 Institute of Materials, Minerals and Mining
Published by Maney on behalf of the Institute
Received 25 December 2006; accepted 16 May 2007
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G. Bereket*, S. Pakdil (Aldemir) and C. Ogretir
Department of Chemistry, Faculty of Arts and Science, Osmangazi University, 26480 Eskisehir, Turkey
* Corresponding author, email [email protected]
Copyright Institute of Materials Sep 2007
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