ARTICLE pubs.acs.org/IECR
Combined Electrochemical and Quantum Chemical Study of Some Diamine Derivatives as Corrosion Inhibitors for Copper Aysel Yurt* and G€ozen Bereket Department of Chemistry, Faculty of Arts and Science, Eskis-ehir Osmangazi University, 26480 Eskis- ehir, Turkey ABSTRACT: The effects of 1,2-diaminoethane (DE), 1,6-diaminohexane (DH), 1,8-diaminooctane (DO), and hydrazine (H) on the electrochemical behavior of copper in 0.1 M H2SO4 were investigated using the potentiodynamic polarization and electrochemical impedance spectroscopy methods at 298 K. A significant decrease in the corrosion rate of copper was observed in the presence of the investigated inhibitors. The potentiodynamic polarization data indicated that the inhibitors were of mixed type, but the cathodic effect was more pronounced. Electrochemical measurements showed that the inhibition efficiencies increased with increasing inhibitor concentration and followed the order DO > DH > H > DE. This reveals that the inhibitive actions of the inhibitors are mainly due to adsorption on the copper surface. Adsorption of these inhibitors follows a FloryHuggins adsorption isotherm. The correlation between the inhibition efficiencies of inhibitors and their molecular structures has been investigated using quantum chemical parameters obtained by AM1 semiempirical method. Calculated quantum chemical parameters indicate that diamine compounds are adsorbed on copper surface by chemical mechanism.
1. INTRODUCTION Because of its excellent electrical and thermal conductivities and good mechanical workability, copper is a widely used metal with extensive industrial application. It is commonly used as a material in heating and cooling systems. Because deposition of scale and corrosion products in the equipment causes a decrease in thermal conductivities, periodic descaling and cleaning in sulfuric acid (or hydrochloric acid) pickling solutions is necessary. Because of their aggressiveness, inhibitors are used to eliminate the undesirable destructive effect of the pickling acids and prevent the dissolution of metal. Many organic compounds containing nitrogen, oxygen, and/or sulfur atoms have been used to inhibit the corrosion of copper.14 The inhibiting actions of organic compounds are usually attributed to their interactions with the metal surface through adsorption. The adsorption of these compounds onto the metal surface depends on the nature and surface charge of the metal, the chemical composition of electrolytes, and the molecular structure and electronic characteristics of the inhibitor molecules. Hydrazine is known to be an effective corrosion inhibitor but is highly toxic inorganic compound.57 On the other hand, it is known that amines are very effective inhibitors for metals and alloys in different corrosive media. The relatively high water solubility of low-molecular-weight amines is an advantage for their use as corrosion inhibitors.8 The protective properties of amine derivatives depend on their ability to reduce corrosion rate and are enhanced at higher electron densities around the nitrogen atoms. Applications of corrosion inhibitors in acidic solution by amine derivatives such as N-phenyl-1,4-phenylenediamine for copper,9 primary aliphatic amines for steel,10 and diamine derivatives for steel11 have been reported. The action of inhibitors depends on the specific interaction between the functional groups and the metal surface. Theoretical approaches provide means of analyzing these interactions, and many such reports have been published in this area.1215 r 2011 American Chemical Society
The aim of the present work was to investigate the effects of three diamine derivatives—1,2-diaminoethane, 1,6-diaminohexane, and 1,8-diaminooctane—as alternative inhibitors to hydrazine for the corrosion of copper in 0.1 M H2SO4 solutions. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were employed to evaluate the corrosion rate of copper and the inhibition efficiencies of these compounds. The relationships between the inhibition efficiency of the used compounds in 0.1 M H2SO4 and some quantum chemical parameters such as highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies, charge density of adsorption centers, dihedral angles, and dipole moments have been also investigated by quantum chemical calculations.
2. EXPERIMENTAL PROCEDURES A conventional three-electrode cell was used for electrochemical measurements. A platinum sheet and a (Ag|AgCl|Cl) electrode served, respectively, as the counter and reference electrodes. The working electrode was prepared from a cylindrical copper rod (99.94%) embedded in a Teflon holder with polyester, so that only the circular cross section (0.1935 cm2) of the rod was exposed. Before each experiment, the electrode was polished with a sequence of emery papers of different grades (600, 800, 1200), washed with doubly distilled water, and degreased with acetone. Test solutions were prepared from analytical-grade chemicals and doubly distilled water. The formulas of the investigated diamine compounds are listed in Table 1. Received: February 23, 2011 Accepted: May 12, 2011 Revised: May 3, 2011 Published: May 12, 2011 8073
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Table 1. Molecular Structures of the Investigated Inhibitors inhibitor
molecular structure
abbreviation
1,8-diaminooctane
H2N(CH2)8NH2
DO
1,6-diaminohexane
H2N(CH2)6NH2
DH
1,2-diaminoethane
H2N(CH2)2NH2
DE
hydrazine
H2N-NH2
H
Table 2. Electrochemical Parameters for Copper in 0.1 M H2SO4 Solutions Containing Investigated Diamine Compounds Obtained by the Tafel Extrapolation Method inhibitor
3. RESULTS AND DISCUSSION 3.1. Polarization Measurements. Figure 1 shows representative anodic and cathodic polarization curves of copper in 0.1 M H2SO4 solutions in the absence and presence of various concentrations of DO. In the presence of inhibitors, the Ecorr values slightly shifted toward the negative direction in comparison to the results obtained in the absence of the inhibitor; this effect was more evident at higher concentrations. Both the anodic and cathodic current densities were decreased by the inhibitors, but the cathodic curves were affected to a greater extent. These results suggest that all tested inhibitors suppressed both the anodic and cathodic reactions, although mainly the cathodic one.16 The three distinct regions that appeared in the polarization curves of the copper were the active dissolution region (apparent
icorr
βa
(μA cm2)
(mV dec1)
IE θ
(%)
0.1
60.6
—
DO
1 102
25
10.0
63.7
0.971
97.1
5 103
6
12.7
54.8
0.964
96.4
1 103 5 104
2 2
16.2 18.3
49.8 49.7
0.954 0.948
95.4 94.8
1 104
15
57.6
46.0
0.835
83.5
1 102
2
23.8
51.2
0.932
93.2
5 103
10
41.5
46.2
0.881
88.1
1 103
7
54.0
49.1
0.846
84.6 73.4
DE
Electrochemical impedance (EIS) measurements and potentiodynamic polarization and linear polarization studies were carried out using a CHI 604 electrochemical analyzer. The cell was open to air, and all measurements were conducted at 298 K. The working electrode was first immersed into the test solution for 45 min to establish a steady-state open-circuit potential. EIS measurements were performed at corrosion potentials (Ecorr) over a frequency range of 100 kHz 0.05 Hz with a signal amplitude perturbation of 5 mV. Potentiodynamic polarization studies were performed at a scan rate of 0.5 mV s1 in the potential range from 300 to þ300 mV relative to the corrosion potential. Experiments were always repeated at least three times. Theoretical calculations were carried out at the restricted HartreeFock (RHF) level using AM1, semiempirical selfconsistent fieldmolecular orbital (SCFMO) methods in the HyperChem Release 8.0 program implemented on an Intel Core 2 Duo computer using a relative permittivity of 78.4 corresponding to water.
Ecorr (mV)
—
DH
H
Figure 1. Tafel polarization curves of copper in 0.1 M H2SO4 solutions containing various concentrations of DO.
C (mol L1)
21
350
—
5 104
9
93.0
53.5
0.734
1 104
16
134.8
54.5
0.615
61.5
1 102 5 103
5 12
85.0 96.0
51.9 48.0
0.757 0.726
75.7 72.6
1 103
16
100.0
46.5
0.714
71.4
5 104
14
148.0
52.4
0.577
57.7
1 104
14
203.7
56.8
0.420
42.0
1 102
5
37.9
41.8
0.892
89.2
5 103
1
40.0
40.9
0.856
85.6
1 103
3
51.6
44.3
0.853
81.3
5 104 1 104
6 7
95.6 136.5
45.3 50.9
0.727 0.651
72.7 60.1
Figure 2. Comparison of the inhibition efficiency data for the investigated inhibitors: (1) DO, (2) DH, (3) DE, and (4) H obtained by the Tafel extrapolation method for copper in 0.1 M H2SO4.
Tafel region), the transition region, and the limiting current region. The dissolution kinetics of copper in sulfuric acid solutions were investigated by Zhang et al., who found that the anodic dissolution of copper takes place in two steps17 2Cu þ H2 O f Cu2 O þ 2Hþ þ 2e
ð1Þ
Cu2 O þ 2Hþ f 2Cu2þ þ H2 O þ 2e
ð2Þ
The cathodic corrosion reaction in aerated acidic solution is O2 þ 4Hþ þ 4e f 2H2 O
ð3Þ
The cathodic polarization curve display a limiting diffusion current due to the reduction of dissolved oxygen at more negative potentials. As can be seen from Figure 1, a hump is observed at potentials close to Ecorr, especially at lower inhibitor concentrations. It was not possible to evaluate the cathodic Tafel slope, because a hump was present at potentials 2030 mV more 8074
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Figure 3. Nyquist plots of copper in 0.1 M H2SO4 solutions containing (a) various concentrations of DO and (b) 1 102 M concentration of all diamines.
Table 3. Electrochemical Parameters of Copper Obtained by EIS Methods in 0.1 M H2SO4 Solutions Containing the Investigated Diamine Compounds inhibitor
Figure 4. Electrochemical equivalent circuit diagram for metalelectrolyte interface.
negative than Ecorr that completely prevented linear extrapolation to Ecorr of the cathodic polarization curves. This anomaly was verified by Moretti and Quartarone for the acidic corrosion of copper.18,19 In any case, it was possible to evaluate icorr values by extrapolation of the anodic Tafel slope to Ecorr. The electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), and anodic Tafel slope (βa) obtained from the polarization curves and the corresponding inhibition efficiency (IE) values at different inhibitor concentrations are reported in Table 2. The percentage inhibition efficiency at different inhibitor concentrations was calculated from the equation
C
Ecorr
Rs
Rt
(mol L1)
(mV)
(Ω cm2)
(Ω cm2)
IE θ
(%)
—
0.5
2
493
—
—
DO
1 102 5 103
25 6
28.9 23.1
6705 5156
0.926 0.904
92.6 90.4
1 103
2
15.6
3542
0.861
86.1
5 104
2
17.3
3069
0.839
83.9
1 104
15
13.9
976
0.495
49.5
21
1 102
2
18
3066
0.839
83.9
5 103
10
9
1516
0.675
67.5
1 103
7
9.3
1493
0.669
66.9
5 104 1 104
9 16
8.7 20.8
1242 1021
0.603 0.517
60.3 51.7
1 102
5
8.7
1318
0.626
62.6
5 103
12
8.7
756
0.348
34.8
ð4Þ
1 103
16
4.9
656
0.249
24.9
5 104
14
13.8
607
0.188
18.8
where icorr* and icorr are the corrosion current densities in the presence and absence of inhibitor, respectively. Inspection of the IE values reveals that inhibition efficiency increased with increasing concentration of additive (Table 2 and Figure 2). This suggests that all compounds were adsorbed on the metal surface by blocking the active sites of the metal surface. The increase in inhibition efficiency observed at higher inhibitor concentrations indicates that more inhibitor molecules were adsorbed on the metal surface, thus providing wider surface coverage and that these compounds acted as adsorption inhibitors. The inhibition efficiency values of the examined diamine derivatives follow the order DO > DH > H > DE. 3.2. Electrochemical Impedance Spectroscopy Measurements. Figure 3a shows representative Nyquist plots of copper in 0.1 M H2SO4 solutions in the presence and absence of various concentrations of DO. As can be seen from Figure 3, each curve for copper has only one capacitive loop represented by one slightly depressed semicircle with its center below the real axis. Deviation from ideal semicircles can be attributed to the inhomogeneities of the copper surface, as well as mass-transport processes.20 This result indicates that the adsorbed diamine
1 104
14
10
555
0.126
12.6
1 102
5
7.6
1672
0.705
70.5
5 103 1 103
1 3
5.8 7.8
1329 1201
0.629 0.589
62.9 58.9
5 104
6
7.6
1048
0.529
52.9
1 104
7
10.4
916
0.462
46.2
IE ð%Þ ¼ ½1 ðicorr =icorr Þ 100
DH
DE
H
compounds effectively inhibited the anodic dissolution of the metal and that the corrosion behavior was controlled by charge transfer.21 The equivalent circuit model used to fit the experimental results is shown in Figure 4.22 The impedance parameters such as solution resistance (Rs) and charge-transfer resistance (Rt) derived from the Nyquist plots and the inhibition efficiencies (IEs) of all examined inhibitors are given in Table 3. Percentage inhibition efficiencies [IE (%)] were calculated using the formula IE ð%Þ ¼ ½ðRt Rt Þ=Rt 100
ð5Þ
where Rt* and Rt are values of the charge-transfer resistance with and without inhibitor, respectively. 8075
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Figure 6. Plot of FloryHuggins adsorption isotherm of DH obtained using surface coverage values calculated from the Tafel polarization results.
the electrochemical parameters obtained from potentiodynamic polarization and EIS measurements, respectively Figure 5. Comparison of the inhibition efficiency data for (1) DO, (2) DH, (3) DE, and (4) H obtained by the potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS) methods for copper in 0.1 M H2SO4.
From Figure 3b, comparing the impedance behavior of copper in H2SO4 solution with and without diamine derivatives, one can see that the corrosion of copper is obviously inhibited in the presence of additive. It can be seen from Figure 3b and Table 3 that the charge-transfer resistance (Rt) obtained in the presence of DO is a maximum (6705 Ω) with an inhibition efficiency of 92.6%. The quantitative results can be seen in Table 3; the inhibition performances of these four inhibitors increased with increasing concentration. This behavior reveals that the diamine compounds adsorbed on the copper surface and acted as adsorption inhibitors. The inhibition efficiencies calculated from ac measurements (EIS) show the same trend as those observed from dc polarization measurements. Figure 5 shows comparative inhibition efficiency data for the investigated inhibitors at optimal concentrations obtained by Tafel extrapolation and electrochemical impedance for copper in sulfuric acid. 3.3. Adsorption Mechanism of Inhibitors. Tables 2 and 3 indicate that all examined inhibitors are effective corrosion inhibitors for copper in sulfuric acid. The increase in inhibition efficiencies with increasing concentration of the studied diamine derivatives shows that the inhibition activity is due to adsorption on copper. The adsorption process of the inhibitors is influenced by the molecular structures and electronic properties of the organic compounds, the nature and surface charge of the metal, the distribution of charge in the molecules, and the type of aggressive medium.23 The adsorption of an organic molecule at a metal/solution interface can be represented as a substitution adsorption process between the organic molecules in the aqueous solution [Org(sol)] and the water molecule on the metallic surface [H2O(ads)]24,25 OrgðsolÞ þ xH2 OðadsÞ a OrgðadsÞ þ xH2 OðsolÞ
ð6Þ
where x is the size ratio representing the number of water molecules replaced by one molecule of organic adsorbate. Basic information on the interaction between the inhibitor and the metal surface can be provided from the adsorption isotherms. To obtain the adsorption isotherm, the surface coverage (θ) values of the inhibitors were calculated from eqs 7a and 7b using
θ ¼ ½1 ðicorr =icorr Þ
ð7aÞ
θ ¼ ½1 ðRt =Rt Þ
ð7bÞ
where icorr* and icorr are the corrosion current densities with and without inhibitor, respectively, and Rt* and Rt are the chargetransfer resistance values with and without inhibitor, respectively. Thus, several adsorption isotherms were tested for the description of the adsorption behavior of studied compounds, and it was found that the adsorption of the studied diamine derivatives on the copper surface in H2SO4 solution obeys the FloryHuggins adsorption isotherm given by the equation26,27 logðθ=CÞ ¼ log Kads þ x logð1 θÞ
ð8Þ
where C is the concentration of the inhibitor, x is the number of water molecules replaced by one inhibitor molecule, and Kads is the equilibrium constant for the adsorption process. A typical plot of log θ/C against log (1 θ) for DH, including a regression line, is given in Figure 6. The equilibrium constant (Kads) obtained from the FloryHuggins isotherm was used to calculate the standard free energy of adsorption (ΔGads) as follows ΔGads ¼ RT lnð55:5Kads Þ
ð9Þ
where 55.5 mol 3 dm3 is the molar concentration of water in the solution. The thermodynamic parameters for the adsorption process obtained from the adsorption isotherms for the studied molecules are reported in Table 4. The values of x are more than unity, indicating that each molecule of the inhibitor is attached to more than one active site of the copper surface and replaced by more than one water molecule.28 As can be seen from Tables 24, the number of water molecules replaced by inhibitor molecule is inversely proportional to the inhibition efficiencies of the diamine derivatives. This result reveals that the degree of surface coverage obtained from the adsorption of one DO molecule can be achieved by the adsorption of more than one DE molecule onto the copper surface. Adsorption of one DO molecule on the copper surface mechanically screens more than two active sites of the metal from the action of the corrodent, because of its larger molecular size. The large negative values of ΔGads ensure the spontaneity of the adsorption process and the stability of the adsorbed layer on the copper surface, as well as a strong interaction between the molecules and the metal surface, whereas 8076
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Table 4. Thermodynamic Parameters of Adsorption Obtained by Tafel Polarization Measurements for the Studied Diamine Compounds on Copper in 0.1 M H2SO4 Solution inhibitor
Kads (dm3 mol1)
ΔGads (kJ mol1)
x
DO
1.51 105
39.50
1.85
Table 5. Calculated Quantum Chemical Parameters of the Studied Diamine Derivatives Inhibitor
DO
DH
DE
H
IE
0.971
0.932
0.757
0.892
EH(eV) EL(eV)
10.1097 3.4444
10.1142 3.4446
10.1696 3.4918
10.5950 3.4001 13.9951
DH
6.18 104
37.28
2.57
DE
4.41 104
36.45
4.14
EL EH (eV)
13.5541
13.5608
13.6614
H
5.93 104
37.18
2.60
μ (Debye)
2.794
2.651
2.682
2.963
qN(1)
0.4111
0.4124
0.4111
0.3407
qN(2)
0.4186
0.4186
0.4094
0.3406
the higher values of the equilibrium constants reveal the filmforming capabilities of the inhibitors on the copper surface.29 The adsorption isotherms confirm that the first step of the inhibition mechanism of diamine derivatives in acidic environments is the adsorption of the molecules on the copper surface through the nitrogen atom of the compounds, thus forming an adsorption layer that acts as a barrier to the solution and enhances the protection of the metal surface. Compounds can be adsorbed onto the copper surface by a chemical or physical mechanism. Diamine derivatives can be present in 0.1 M H2SO4 solution in protonated form. Thus, protonated molecules might be physisorbed on the cathodic sites of the copper and inhibit corrosion by blocking the reduction of dissolved oxygen. In addition, all diamine compounds can be chemisorbed onto the copper surface through the interaction of the unshared electron pairs of the N atoms of the inhibitor molecules with the metal. It is well-known that ΔGads values on the order of 20 kJ mol1 or lower indicate physisorption, whereas those on the order of 40 kJ mol1 or higher involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption).30,31 The values of ΔGads for DO, DH, DE and H are in the range of 36.45 kJ mol1 < ΔGads < 39.50 kJ mol1. These values suggest that the adsorption of the inhibitor molecules onto the metal surface can be through both physisorption and chemisorption. In addition, values of the adsorption free energy, ΔGads, very close to 40 kJ mol1 shows that chemisorption makes the major contribution whereas physisorption makes a minor contribution in the inhibition mechanism of diamines on copper surfaces in acidic solution.32,33 It is known that chemical adsorption results from the interaction of unshared electron pairs in the molecule with the metal surface, whereas physical adsorption results from electrostatic interactions between the charged centers of the molecules and the charged metal surface, which results in a dipole interaction between the molecule and the metal surface. Therefore, the adsorption and inhibition mechanism of the studied compounds depends on the molecular structure and electronic characteristics of the inhibitor molecule. This idea is discussed in the next section in terms of quantum chemically calculated molecular parameters. 3.4. Quantum Chemical Calculations. To investigate the effect of molecular structure on inhibition mechanism and inhibition efficiency, some quantum chemical calculations were performed. Quantum chemical parameters such as the energy of the highest occupied molecular orbital (EH), the energy of the lowest unoccupied molecular orbital (EL), the difference between them (EL EH, also called the HOMOLUMO gap), the dipole moment (μ) of the molecules, and the charge densities on the N atoms (qN) as adsorption centers in the molecules of the diamine compounds were obtained by the AM1 semiempirical method and are reported in Table 5.
There was no direct relationship between the IE value and the dipole moment or the charge densities on the N atoms. However, the inhibition efficiencies of DO, DH, and DE increased as the value of EH increased and as the values of EL and (EL EH) decreased. First-order linear regression analysis was performed for the relationships between IE and the EH, EL, and (EL EH) values of DO, DH, and DE, and the following equations were obtained IE ¼ 35:311 þ 3:398EH
ðr ¼ 0:989Þ
ð10Þ
IE ¼ 15:680 4:274EL
ðr ¼ 0:989Þ
ð11Þ
IE ¼ 26:485 1:883ðEL EH Þ
ðr ¼ 0:987Þ
ð12Þ
The obtained equations and Table 5 show that increasing the value of EH and decreasing the values of both EL and (EL EH) caused a significant increase in the inhibitive action of the studied compounds. It is known that, in chemical adsorption, an increase in EH and a decrease in EL cause a significant increase in the inhibition efficiency of organic compounds.34 On the other hand, the energy gap between EL and EH can be used as a characteristic quantity for metallic complexes.35,14 A lower energy gap indicates a higher stability of the formed complex and, thus, a higher inhibition efficiency. An increase in IE values with a decrease in (EL EH) value indicates the formation of a coordinate covalent bond between the organic molecules and copper. From the positive sign of the coefficient of EH and the negative sign of the coefficient of (EL EH), it can be concluded that the adsorption of these diamine derivatives on the copper surface has a chemical mechanism. Using the quantum chemical calculation results for the studied compounds, graphs of IE (%) versus EL, EH, and (EL EH) were constructed (Figure 7). Because of the differences in the molecular structures of the diamines and hydrazine, hydrazine shows a deviation from linearity in the graphs of IE (%) versus EL, EH, and (EL EH). This result indicates that the inhibition mechanism of hydrazine should be different from that of DO, DH, and DE. The difference might be related to the molecular geometry of adsorption. Coordination between the investigated molecules and the copper electrode surface occurs through the nitrogen atoms of the amine groups. Molecules can be oriented horizontal or vertical with respect to the surface. The orientation of an inhibitor molecule with respect to the metal surface affects the possibility of forming stronger bonds and also the degree of surface coverage. The orientation of the dipole moments of DO, DH, and DE shows that the y components (2.503, 2.195, 2.102) of the dipoles are negative and relatively higher than the x (0.962, 0.711, 0.103) and z (0.784, 1.306, 1.662) components of the dipoles. All of these results suggest that 8077
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Figure 7. Plots of IE (%) versus (a) EL, (b) EH, and (c)EL EH for the studied compounds obtained from AM1 aqueous-phase calculations.
• The protection ability of diamine compounds was found to lie in the order of DO > DH > H > DE. The protection abilities of the molecules depend on the presence of an alkyl chain and on the length of the alkyl chain in their molecular structure. • The inhibition action of the studied compounds is mainly due to their adsorption on the copper surface. The adsorption process obeys the FloryHuggins adsorption isotherm. • The thermodynamic parameters (Kads, ΔGads) of adsorption for the studied compounds were calculated from their adsorption isotherms. The negative values of ΔGads show the spontaneity of the adsorption. • The inhibition efficiencies of the studied diamines increase when EH increases and when the EL and (EL EH) values decrease. These results indicate the formation of coordinate covalent bonds between the inhibitor molecules and copper • The values of the free energy of adsorption and the relationship between the inhibition efficiency values and the calculated quantum chemical parameters suggest that all of the investigated compounds adsorb on the copper surface through a chemical mechanism.
Figure 8. Schematic illustration of different adsorption modes of (a) diamines and (b) hydrazine on a copper surface.
DO, DH, and DE might be adsorbed horizontally on the copper surface as a result of the interaction between the metal surface and both N atoms in the molecules (Figure 8a).36 On the other hand, the x, y, and z components of the dipole moments of H are 0.01, 0.334, and 2.944, respectively. These results indicate that the vertical adsorption of hydrazine with an angle to the z side most probably can take place through only one N atom (Figure 8b).37 Examination of the experimental and theoretical data shows that the inhibition efficiency values of the investigated compounds follow the order DO > DH > H > DE. The extent of inhibition is directly related to the increase of the adsorption layer, which is a sensitive function of the molecular structure and also the adsorption geometry. The comparative study reveals that the inhibition efficiencies of the studied diamine derivatives are closely related to the number of methyl groups between amine groups. An increase in the carbon chain length appears to cause a considerable increase in the inhibition efficiency; for example, the inhibition efficiency of DO was higher than that of DH. This is due to the adsorption of the molecules through both of the nitrogen atoms of DO, DH, and DE and can also be attributed to the larger molecular size of DO, which ensures greater surface coverage on the copper surface. The higher inhibition efficiencies of DO and DH compared to H can be explained by the presence of methyl groups between the amine groups. The presence of an alkyl chain in the structure of the inhibitors provides flexibility to the molecules, thus increasing the probability of interaction between both N atoms and the metal surface. Wider surface coverage was provided by the adsorption of DO, DH, and DE on the metal surface because of the adsorption through both nitrogen atoms in their molecular structures.
4. CONCLUSIONS • All studied compounds act as mixed-type inhibitors, and their inhibition efficiencies tended to increase with increasing inhibitor concentration.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ90 222 2393750. Fax: þ90222 2393578. E-mail: ayurt@ ogu.edu.tr.
’ ACKNOWLEDGMENT The authors thank Assoc. Prof. Dr. G. K€urkc-€uoglu, Department of Physics, Faculty of Arts and Science, Eskisehir Osmangazi University, for providing the inhibitors used in this study. ’ REFERENCES (1) Ramesh, S.; Rajeswari, S.; Maruthamuthu, S. Corrosion inhibition of copper by new triazole phosphonate derivatives. Appl. Surf. Sci. 2004, 229, 214–225. (2) Li, S.; Chen, S.; Lei, S.; Ma, H.; Yu, R.; Liu, D. Investigation on some Schiff bases as HCl corrosion inhibitors for copper. Corros. Sci. 1999, 41, 1273–1287. (3) Sankarapapavinasam, S.; Ahmed, M. F. Benzenethiols as inhibitors for the corrosion of copper. J. Appl. Electrochem. 1992, 22, 390–395. (4) Zhang, D. Q.; Gao, L. X.; Zhou, G. Inhibition of copper corrosion in aerated hydrochloric acid solution by heterocyclic compounds containing a mercapto group. Corros. Sci. 2004, 46, 3031–3040. 8078
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