Schiff bases as corrosion inhibitor for aluminium in ...

Corrosion Science 54 (2012) 251–259

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Schiff bases as corrosion inhibitor for aluminium in HCl solution Serpil Sß afak a, Berrin Duran a,⇑, Aysel Yurt a, Gülsßen Türkog˘lu b a b

Eskisßehir Osmangazi University, Faculty of Science and Letters, Department of Chemistry, 26480 Eskisßehir, Turkey Anadolu University, Faculty of Science, Department of Chemistry, 26470 Eskisßehir, Turkey

a r t i c l e

i n f o

Article history: Received 18 May 2011 Accepted 19 September 2011 Available online 22 September 2011 Keywords: A. Aluminium B. EIS B. Polarisation C. Acid corrosion

a b s t r a c t Three Schiff bases named 1,5-bis[2-(2-hydroxybenzylideneamino)phenoxy]-3-oxopentane (D1), 1,5bis[2-(5-chloro-2-hydroxybenzylideneamino)phenoxy]-3-oxopentane (D2) and 1,5-bis[2-(5-bromo2-hydroxybenzylideneamino)phenoxy]-3-oxopentane (D3) were synthesized and their inhibitive capabilities on the aluminium corrosion in 0.1 M HCl were investigated by means of electrochemical impedance spectroscopy, Tafel polarisation and scanning electron microscopy techniques. Results showed that, compounds under study exhibit inhibitor properties and adsorption of these compounds was found to accord with Temkin adsorption isotherm. Polarisation curves indicated that the studied Schiff bases were cathodic inhibitor and the effectiveness of these inhibitors decreased in the order of D3 > D2 > D1. Quantum chemical calculations were performed to provide further insight into the inhibition efficiencies determined experimentally. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Aluminium and aluminium alloys represent an important category of materials due to their high technological value and wide range of industrial applications, especially in aerospace and household industries [1]. Owing to these applications of aluminium and its alloys, considerable attention has been devoted to the corrosion behaviour of these materials in various aggressive environments [2–6]. It is well known that aluminium is usually protected by a thin oxide film which has been formed either spontaneously (native film) or deliberately (e.g. anodic films). The solubility of the oxide film is negligible in neutral solutions (pH interval 4.0–8.5) at room temperature, whereas heavy corrosion is observed both in highly acidic and alkaline media [2,7]. Acidic solutions are used for pickling, chemical and electrochemical etching of aluminium. Therefore, inhibition of aluminium corrosion in acidic media has a great importance [8,9]. One of the most common methods to protect metals against acid corrosion is the use of organic compounds containing functional groups and p electrons in their structure, as inhibitors [10–12]. In the literature, several Schiff bases have reported as effective corrosion inhibitors for different metals and alloys in acidic media [13–15]. Increasing popularity of Schiff bases in the field of corrosion inhibition science based on the ease of synthesis from relatively inexpensive starting-materials and their eco-friendly or low toxic properties [16,17]. The high inhibitory performance of these

⇑ Corresponding author. Tel.: +90 222 239 3750/2868; fax: +90 222 239 3578. E-mail address: [email protected] (B. Duran). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.09.026

compounds results from the substitution of different heteroatoms (e.g. N, O, Cl, Br) and p electrons in their structure besides the presence of imine functional group [12,14]. These molecules normally form very thin and persistent adsorbed films that lead to a decrease in the corrosion rate due to the slowing down of anodic, cathodic reaction or both [18]. Inhibitors act through a process of surface adsorption, so the efficiency of an inhibitor depends on the characteristics of the environment in which it acts, the nature of the metal surface and electrochemical potential at the interface. The structure of inhibitor itself, which includes the number of adsorption active centres in the molecule and their charge densities, the molecule size, the mode of adsorption and the protected area of the inhibitor on the metal surface, has also effect on the efficiency of an inhibitor [19,20]. This work represents just one part of a systematic research project related to investigation of relationship between inhibition efficiency and the structure of inhibitors for acidic corrosion of different substrates. Another series of synthesized Schiff bases was tested for mild steel and related study has been recently reported in literature [21]. Selected molecules for investigation are big enough (molecular weight: 496–650 g/mol), therefore it was considered to cover more surface area of metal and to enhance the adsorption ability. Also, due to the selected molecules have phenyl groups, oxygen, nitrogen and halogen atoms which are assumed to be additional adsorption centers, it is expected to show good corrosion inhibition efficiency. At the same time, substitution effects of chlorine and bromine atoms on compound’s inhibition action to be compared by selected compounds. The effect of the molecular structure on the chemical reactivity has been object of great interest in several disciplines of chemistry. In this respect, quantum chemical calculations have been widely

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S. Sßafak et al. / Corrosion Science 54 (2012) 251–259

used to investigate the molecule in its electronic structure level and to interpret the experimental results. The inhibition property of a compound has been often correlated with molecular properties. Therefore, it is worthwhile to compute these features theoretically [22,23]. Although there are quite number of reports concerning investigations of Schiff bases as corrosion inhibitors for iron, mild steel and zinc in acidic media [15,24,25], relatively few study were devoted to protection of aluminium with Schiff bases [8,26,27]. Thus, the present study was undertaken to compare the inhibitive abilities of three selected Schiff bases, namely 1,5-bis[2-(2-hydroxybenzy lideneamino)phenoxy]-3-oxopentane (D1), 1,5-bis[2-(5-chloro-2hydroxybenzylideneamino)phenoxy]-3-oxopentane (D2) and 1,5bis[2-(5-bromo-2-hydroxybenzylideneamino)phenoxy]-3-oxopentane (D3), on the corrosion of aluminium in 0.1 M HCl solution at room temperature by using electrochemical impedance spectroscopy (EIS) and Tafel polarisation techniques. In addition, this study aimed at describing the adsorption mode of the studied compounds and explaining differences in behaviour based on structural properties. 2. Experimental 2.1. General procedure for synthesis of Schiff bases The used Schiff base compounds were synthesized from 1:2 mol ratios of 1,5-bis(o-aminophenoxy)-3-oxopentane and the appropriate aldehydes (2-hydroxy benzaldehyde, 5-chloro-2-hydroxybenzaldehyde and 5-bromo-2-hydroxybenzaldehyde) through a condensation reaction in methanolic media then recrystallised in methanol–THF mixture. Chemicals used in synthesis were purchased from Merck and used as received. The resulting salicylaldimine Schiff bases were shown in Fig. 1. 2.2. Sample preparation For the investigation, aluminium alloy (which is used in cooling pipes) rod with the chemical composition of Fe (0.5%), Zn (0.005%), Si (0.2%), Ti (0.005%), Cu (0.1%) and Al (remainder) was used. Cylindrical rod with a 20 cm length of aluminium was mounted in a Teflon tube and polyester was used to fill the space between Teflon and aluminium electrode. The circular cross sectional area of the aluminium rod exposed to the corrosive medium, used in electrochemical measurements was 0.2884 cm2. Prior to any test, the exposed area of aluminium working electrode was mechanically ground by Forcipol 1V grinder/polisher (USA) with a sequence of silicon carbide emery papers of different grits (800, 1000 and 1200) and degreased ultrasonically in ethanol, then rinsed by ultra pure water.

100 lM. The entire three compounds are insoluble in water. Hence, stock solutions of the compounds were prepared by adding 2% (v/ v) of dimethyl sulphoxide (DMSO) as co-solvent to ensure solubility. In order to eliminate the effect of DMSO, 0.1 M HCl solution containing 2% DMSO was used as blank solution and corrosion rate of aluminium was determined in this solution, at first. Chemicals used in corrosion experiments were of analytical grade (Fluka) and used without further purification; ultra pure water was used to prepare test solutions. 2.4. Electrochemical measurements Electrochemical measurements were carried out in a typical three-electrode glass corrosion cell at room temperature with an aluminium working electrode, a Pt wire counter electrode and Ag/AgCl as reference. The reference electrode was connected to a Luggin capillary to minimise IR drop. Before starting the experiments, test solutions were degassed with ultra pure nitrogen bubbling for 45 min to avoid any reactions with dissolved oxygen and deaerating was continued during the measurements. All test solutions were continuously stirred at constant rate. The electrochemical measurements were conducted after 45 min of immersion time in experimental solution to attain a stable open circuit potential (Eocp) value. Electrochemical measurements were performed using Gamry Reference 600 potentiostat/galvanostat/ZRA system (Wilmington, USA) which was interfaced to a personal computer to control the experiments and the data were plotted using Echem Analyst 5.50 software. Impedance measurements were carried out in frequency range from 100 kHz to 0.5 Hz with amplitude of 10 mV peak to peak using AC signals at open circuit potential. Analysis of the impedance spectra and fitting of the experimental results to equivalent circuits was performed using ZSimpWin 3.21 programme, which allowed the chi-square value (v2, i.e. the sum of the square of the differences between theoretical and experimental points) to judge quality of equivalent circuit fitting. Tafel polarisation curves were obtained by changing the electrode potential automatically from 250 to +600 mV at open circuit potential with a scan rate of 0.5 mV s1. In order to ensure reproducibility of the curves and spectrums, each experiment was repeated three times and the most reproducible results were given in this manuscript. 2.5. Surface analysis The morphology of the corroded surface of each specimen was monitored by using a Jeol JSM 5600LV model scanning electron microscope. All micrographs of the corroded specimens have a magnification of 300 to provide a constant view.

2.3. Solution preparation

2.6. Quantum chemical calculations

The measurements were performed in 0.1 M HCl in the absence and presence of Schiff bases in the concentration range from 10 to

Quantum chemical calculations were performed with complete geometry optimisation by using standard Gaussian 03W [28] software package, at semi-empirical level using AM1 [29], PM3 [30], MNDO [31] methods. 3. Results and discussion 3.1. Electrochemical impedance measurements

Fig. 1. The general structure of investigated Schiff bases.

The inhibition efficiencies of D1–D3 on aluminium were examined by electrochemical impedance spectroscopy. The impedance spectra of aluminium in 0.1 M HCl solution in the absence and presence of five different concentrations of Schiff bases were recorded. Fig. 2 shows the impedance spectra in Nyquist format.


The impedance diagrams display one single capacitive loop represented by slightly depressed semi-circle for all studied compounds. This capacitive loop indicates that the corrosion of aluminium in 0.1 M HCl solution is mainly controlled by charge transfer process and formation of a protective layer on the metal surface. Deviations from the ideal semi-circle are generally attributed to the frequency dispersion as well as inhomogeneities, roughness of metal surface and mass transport process [32–34]. The impedance response of aluminium in HCl changes with the addition of Schiff bases into the test solutions and this change more pronounced with increasing inhibitor concentration. The diameter of the capacitive loop increases as the concentration of inhibitor rises, this increase indicates adsorption of inhibitor molecules on the metal surface [35]. On the other hand, the similar nature of impedance diagrams obtained in the absence and presence of Schiff bases reveal that the addition of inhibitors does not change the mechanism for the dissolution of aluminium in HCl [36–38]. All experimental spectra were fitted with an appropriate equivalent circuit to find the parameters, which describe and being con-

253

sistent with the experimental data. Fig. 3 depicts the proposed equivalent circuit, which consists of a solution resistance Rs in series to the constant phase element CPE (Q) and the charge transfer resistance Rct while CPE is parallel to Rct. Same equivalent circuit was proposed in the literature for acidic corrosion of aluminium in presence of Schiff bases [8]. The use of CPE-type impedance has been extensively stated by previous reports [15,39,40]. The impedance of CPE is described as [22,40]:

Z CPE ¼

1 1 Y 0 ðjxÞn

ð1Þ

where Y0 is the magnitude of the CPE, j is the imaginary unit, x is the angular frequency and n is the phase shift which gives details about the degree of surface inhomogeneity. For ideal electrodes, the CPE is equal to an ideal capacitor when n = 1. The convincing fitting results of Bode plots of D1 compound for R(QR) equivalent circuit were shown in Fig. 4 just as an representative example and according to this equivalent circuit, the experimental data were found to be suitably well fitted to simulated data. Similar fitting results were also obtained for D2 and D3 compounds. The values of fitted parameters, inhibition efficiencies (g), surface coverage degrees (h) and fitting quality values (v2) for all tested inhibitors were presented in Table 1. Inhibition efficiencies were calculated using the following equation, where Rct and R0ct are the charge transfer resistance of aluminium with and without inhibitors, respectively [41]

gEIS ð%Þ ¼

Rct R0ct 100 Rct

ð2Þ

The data of Table 1 show that, the Rs values are very small compared to the Rct values. It should be noted that while Rct values increase, the Q values decrease. The increase of Rct values with increasing inhibitor concentration suggests the formation of a protective layer on the electrode surface. This layer acts as a barrier for mass and charge transfer [42,43]. According to EIS data, the order of the inhibition efficiency is D3 > D2 > D1. 3.2. Tafel polarisation measurements The influence of various concentrations of D1–D3 compounds on the polarisation behaviour of aluminium in hydrochloric acid medium was shown in Fig. 5. At first sight, polarisation curves demonstrate that the curves moved toward lower current density region in presence of Schiff bases. Compared with the blank experiment, cathodic branches of the polarisation curves recorded for each compound are shifted significantly to the direction of the current deduction, which implied that the all used Schiff bases have effect on cathodic reaction of corrosion process. Moreover, it can be seen from the Fig. 5 that, the values of corrosion potential displace to more negative direction with addition of each compound. Due to the negative shifts in the corrosion potentials and noticeable decrease of the cathodic currents by adding compounds into the test solution, three compounds under study can be considered as cathodic inhibitors [44]. Since no linear Tafel regions were observed in anodic branches of polarisation curves recorded in Schiff

Fig. 2. Nyquist plots of aluminium in 0.1 M HCl solution containing different concentrations of the Schiff bases.

Fig. 3. Electrical equivalent circuit used for modelling metal/solution interface in the absence and presence of inhibitors.

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protective ability than the Cl-substituted one and Cl-substituted Schiff base is more protective, compared to its unsubstituted derivative. Similar findings were documented by Aytaç et al. that, bromine including phenolic Schiff base to be more effective inhibitor than its Cl-substituted and unsubstituted derivatives [14]. 3.3. Adsorption isotherm and thermodynamic parameters The efficiency of Schiff base as a corrosion inhibitor mainly depends on its adsorption ability at the metal/solution interface. Therefore, it is essential to know the mode of adsorption and the adsorption isotherm that can give valuable information about the interaction of inhibitor and metal surface. The values of surface coverage (h) calculated for different concentrations of Schiff bases were used to test graphically to various adsorption isotherms including Freundlich, Langmuir, Temkin and Frumkin isotherms. To choose the isotherm that best fit to experimental data, the correlation coefficient (R2) was used and the best fit was obtained from the Temkin isotherm, which can be expressed by the following equation:

Fig. 4. R(QR) equivalent circuit fitting of Bode plots of aluminium in 0.1 M HCl solution containing various concentrations of D1.

base containing solutions, cathodic Tafel lines were extrapolated to the corrosion potential in order to determine polarisation parameters belong to aluminium corrosion in 0.1 M HCl solution with and without additives. Corrosion potentials (Ecorr), corrosion current densities (icorr), cathodic Tafel slope values (bc) deduced from the polarisation curves and corresponding inhibition efficiencies (g) and surface coverage degrees (h) were listed in Table 2. The inhibition efficiencies for different inhibitor concentrations were calculated from the following equation [41], where i and i0 are the corrosion current densities with and without inhibitor, respectively

gP ð%Þ ¼

i0 i 100 i0

expðf hÞ ¼ K ads C

ð4Þ

where Kads is the adsorption equilibrium constant, h is the degree of surface coverage, C is the inhibitor concentration and f is the molecular interaction parameter which depends on the intermolecular interaction in the adsorption layer and heterogeneity of the surface. When f is positive, mutual attraction of molecules occurs and when f is negative, repulsion occurs [47]. Typical plot of h vs. ln C for three compound conferred straight lines as shown in Fig. 6, which suggested that the adsorption of these compounds on metal surface obeyed Temkin adsorption isotherm. From the straight lines, Kads and f parameters were calculated. Kads is related to the standard Gibbs free energy of adsorption DG0ads and is expressed by following equation, where 55.5 is the molar concentration of water in the solution, R is the gas constant (8.314 J K1 mol1) and T is the absolute temperature (K)

ð3Þ

According to data of Table 2, corrosion current density of aluminium decreases and inhibition efficiency increases with the increasing inhibitor concentration. The shift of corrosion potentials in the cathodic direction and decrease of corrosion currents with the increase of additive concentration indicate the adsorption of these compounds at the cathodic sites of the aluminium surface [45]. At the same time, change of cathodic Tafel slopes reveals that the inhibitors control the cathodic reaction. Increase in inhibition efficiency with increasing inhibitor concentration indicates that, more inhibitor molecules are adsorbed on the metal surface providing wider surface coverage and these compounds act as adsorption inhibitors [46]. Examination of polarisation data shows that the decrease in inhibition efficiency follows the order: D3 > D2 > D1. It means that Br-substituted Schiff base has more

K ads

1 DG0ads ¼ exp 55:5 RT

! ð5Þ

The thermodynamic parameters derived from Temkin adsorption isotherms for the studied compounds were given in Table 3. Calculated Kads values for D1–D3 compounds are 8.14 106, 8.76 106, 2.15 108, respectively. In literature, it was reported that the high Kads values (> 100 M1) attribute to the stronger and more stable adsorbed layer formation on the metal surface

Table 1 Impedance parameters extracted from the fit to the equivalent circuit for the impedance spectra of aluminium in the absence and presence of the inhibitors. Inhibitor

C (lM)

Rs (X cm2)

Q (S cmn/cm2)

n

Rct (X cm2)

gEIS (%)

h

v2

– D1

– 10 30 50 70 100

0.36 0.94 1.88 1.36 1.63 1.58

3.29 105 2.71 105 2.59 105 2.23 105 2.40 105 1.69 105

1.00 0.96 0.94 0.94 0.94 0.94

144.0 298.6 299.9 322.8 335.2 372.6

– 51.8 52.0 55.4 57.0 61.4

– 0.518 0.520 0.554 0.570 0.614

7.98 103 5.27 104 1.45 103 9.25 104 1.09 103 7.63 104

D2

10 30 50 70 100

1.59 0.77 2.15 2.83 1.64

1.95 105 1.49 105 2.99 105 2.20 105 1.77 105

0.94 0.96 0.93 0.93 0.94

302.9 303.1 335.5 346.4 383.3

52.4 52.5 57.1 58.4 62.4

0.524 0.525 0.571 0.584 0.624

6.73 104 1.11 103 1.99 103 1.90 103 7.88 104

D3

10 30 50 70 100

1.80 2.48 2.26 1.85 1.97

2.95 105 2.13 105 2.12 105 1.85 105 1.92 105

0.94 0.94 0.93 0.93 0.93

328.2 357.4 374.2 385.8 404.3

56.1 59.7 61.5 62.7 64.4

0.561 0.597 0.615 0.627 0.644

1.06 103 1.44 103 1.18 103 1.24 103 1.09 103

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Fig. 6. Typical Temkin adsorption isotherms obtained from potentiodynamic polarisation data for adsorption of studied Schiff bases on the aluminium surface.

[22,48]. It is clear from the table that, relatively high values of Kads reflect the strong interaction between the inhibitor and the aluminium surface. Literature demonstrates that the values of standard Gibbs free energy of adsorption around 20 kJ mol1 or lower (more positive) indicate adsorption with electrostatic interaction between the adsorbent and adsorbate (physisorption), while those around or higher (more negative) than 40 kJ mol1 involve charge sharing between the molecules and the metal (chemisorption) [37,38,49]. The calculated values of DG0abs ranged from 49.38 to 57.49 kJ mol1 for studied Schiff bases and these values point out the adsorption of compounds occur by chemisorption. Moreover, the sign of f parameters is positive which implies that the interaction between the molecules causes an increase in the adsorption energy with the increase of surface coverage [47]. 3.4. SEM analyses

Fig. 5. Potentiodynamic polarisation curves of aluminium in 0.1 M HCl solution in the absence and presence of various concentrations of the Schiff bases.

The effect of inhibitors on corrosion process was examined by the SEM images of corroded aluminium surface in the absence and presence of inhibitors and these images were given in Fig. 7a–d. Fig. 7a shows the SEM image of aluminium sample after 4 h immersion in 0.1 M HCl solution. This micrograph clearly reveals that the metal surface was strongly damaged in the absence of inhibitor due to the metal dissolution in aggressive solution and large number of cracks with high depth were distributed all over the surface. Fig. 7b–d shows images of the aluminium surfaces after 4 h immersion in 0.1 M HCl solution containing 100 lM of D1–D3 compounds, respectively. At first sight, SEM images ob-

Table 2 Corrosion parameters obtained from Tafel polarisation curves of aluminium in 0.1 M HCl in the absence and presence of different concentrations of Schiff bases at 298 K. Inhibitor

C (lM)

Ecorr vs. Ag/AgCl (mV)

icorr (lA/cm2)

bc (mV)

gP (%)

h

– D1

– 10 30 50 70 100

664.07 730.49 739.04 745.06 744.49 748.90

266.67 149.15 126.89 119.16 96.66 90.15

161.46 153.84 144.92 153.06 147.78 140.91

– 44.0 52.4 55.3 63.7 66.2

– 0.440 0.524 0.553 0.637 0.662

D2

10 30 50 70 100

728.41 742.84 748.62 746.96 744.07

145.83 116.67 114.41 90.00 84.00

144.92 144.23 136.98 138.88 137.93

45.3 56.2 57.1 66.2 68.5

0.453 0.562 0.571 0.662 0.685

D3

10 30 50 70 100

727.91 740.95 747.33 754.50 756.65

116.94 96.00 87.50 83.50 69.20

137.93 131.00 128.75 136.36 144.92

56.1 64.0 67.2 68.6 74.0

0.561 0.640 0.672 0.686 0.740

256


Table 3 Thermodynamic parameters calculated from Temkin adsorption isotherm at 298 K for the studied Schiff bases. Inhibitor D1 D2 D3

Kads (M1) 6

8.14 10 8.76 106 2.15 108

DG0abs (J mol1)

f

49375 49557 57491

10.29 9.99 13.74

tained in the presence of inhibitors show that, the addition of these compounds modifies the features of the layer at the metal surface. As can be seen from Fig. 7b–d, the damage of metal surface has diminished in the presence of inhibitors and rough, corroded aluminium surface displaces to much smooth surfaces; especially D3 compound has the much smooth surface compared to the others. On the other hand, there are much less and very small cracks on the surface of aluminium with D2 and D1 compounds than that of surface immersed in 0.1 M HCl alone. This may be due to protective film formation by adsorption of these compounds on the metal surface that the film is responsible for the inhibition of corrosion. As a result, the inhibitor molecules hinder the dissolution of aluminium by forming organic film on the metal surface and thereby suppress the rate of corrosion, as indicated by SEM images.

Fig. 8. HOMO populations of investigated Schiff bases obtained from AM1, PM3 and MNDO methods.

3.5. Quantum chemical calculations The effectiveness of an inhibitor is related to its spatial molecular structure as well as with its molecular electronic structure [12,50]. Certain quantum chemical parameters such as the energy of the highest occupied molecular orbital (EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO) and LUMO–HOMO gap (DELUMO HOMO) can be related to the interactions of metalinhibitor [38]. In this regard, quantum chemical calculations have proved to be a powerful tool for studying corrosion inhibition mechanism and recently, corrosion publications have contained substantial quantum chemical calculations [51,52]. In this study, quantum chemical calculations were conducted at three different semi-empirical techniques by geometry optimisation of the studied compounds in order to support experimental data and to investigate the relationship between molecular struc-

ture of the Schiff bases and their inhibition effects. Frontier molecular orbital (HOMO and LUMO) theory is useful in predicting the adsorption centres of the inhibitor responsible for the interaction with metal surface [51,53]. The HOMO populations of the studied Schiff bases were shown in Fig. 8. It could be easily seen that, molecules are not fully planar which may result in relatively weak interaction between molecules and metal surface. However, different factors need to be considered for elucidating the orientation of organic molecules on the electrode surface. The atoms and groups of the molecules may interact with the electrode surface depend on the geometry of the inhibitor as well as the nature of their frontier molecular orbitals. Three Schiff bases investigated in the present study consist of symmetrical two parts and contain four benzene rings and two C@N groups. Difference between the structures is related with the substitution of Cl and Br atoms at meta

Fig. 7. SEM micrographs of the aluminium surfaces after 24 h immersion period in 0.1 M HCl solutions in the absence and presence of 100 lM inhibitor.

257


Table 4 HOMO energy (EHOMO), LUMO energy (ELUMO) and HOMO–LUMO gap (DELUMO HOMO) values obtained from three different semi-empirical methods for D1–D3 compounds. EHOMO (eV)

AM1 PM3 MNDO

8.48 8.47 8.75

D1

D2

D3

ELUMO (eV)

DELUMO HOMO (eV)

EHOMO (eV)

ELUMO (eV)

DELUMO HOMO (eV)

EHOMO (eV)

ELUMO (eV)

DELUMO HOMO (eV)

5.75 5.86 5.27

2.73 2.61 3.48

8.19 8.17 8.23

5.80 5.92 5.31

2.39 2.25 2.92

7.97 7.96 7.91

5.75 5.94 5.42

2.22 2.02 2.49

position of terminal benzene rings for D2 and D3 compounds, respectively. It can be seen in Fig. 8 that, the HOMO distributions obtained from three different calculation methods are similar for each compound, especially AM1 and PM3 methods have given very close results. The HOMO location of D1 compound is distributed throughout the molecule while the HOMO location of D2 compound is generally distributed over the two benzene rings that take place only one side of the molecule. On the other hand, for D3 compound, the highest molecular orbitals localised on the halogen atom substituted terminal benzene ring which takes place only one part of the molecule. Substitution of electronegative chlorine and bromine atoms leads to the withdrawn of electrons by these atoms and causes to localisation of HOMO electrons towards these atoms. According to HOMO distributions of Schiff bases it can be said that, D3 compound carries its rich negative centres in small region, in comparison with D2 and D1 compounds. The parts of the molecules with low HOMO density probably more oriented towards to cathodic sites of the aluminium surface and afterwards adsorption occurs by sharing of electrons. Computed EHOMO, ELUMO and DELUMO HOMO values of studied Schiff bases were listed in Table 4. The HOMO energy indicates the electron-donating ability of the molecule to an appropriated acceptor with empty molecular orbitals (p orbital in aluminium) [38]. Therefore, increase in the values of EHOMO can facilitate the adsorption and improve the inhibition efficiency. The corrosion rate decreases with increasing EHOMO value (less negative). In the same way, low values of the DELUMO HOMO will provide good inhibition efficiencies, because the excitation energy to remove an electron from the last occupied orbital will be low [54]. Quantum chemical parameters listed in Table 4 revealed that D3 has the highest EHOMO and the lowest DE values for all calculation methods compared to the D2 and D1 compounds. EHOMO values of three Schiff bases increased in the order: D1 < D2 < D3 and DELUMO HOMO decreased in the order: D1 > D2 > D3. These results agree with the experimental observations, which imply that D3 compound has better corrosion performance. Similarly, good correlations between the corrosion rate and EHOMO and as well as energy gap have been found in literature [38,55]. The relationship between corrosion inhibition efficiency and HOMO energies for D1–D3 compounds was plotted in Fig. 9 and good correlations were obtained especially from AM1 and PM3 methods. It is clear from the figure that, the inhibition efficiency increased with the EHOMO rising. Moreover, the gap between the HOMO and LUMO energy levels of the molecules was another important factor that should be considered. For three different semi-empirical methods, frontier orbitals (HOMO and LUMO) energy levels of studied molecules were graphically illustrated in Fig. 10 and it is clearly seen that the greater trend of offering electrons to unoccupied low lying orbital of the aluminium belongs to compounds in the order of: D3 > D2 > D1.

minium. A general mechanism for the dissolution of aluminium in acidic medium would be similar to that reported in the literature [8,56,57]

AlðSÞ þ H2 O AlOHads þ Hþ þ e 3þ

AlOHads þ 5H2 O þ Hþ Al 3þ

Al

þ 6H2 O þ 2e

þ H2 O ½AlOH2þ þ Hþ

½AlOH

2þ

þ X ½AlOHX

þ

ðaÞ ðbÞ ðcÞ ðdÞ

The controlling step in the metal dissolution is the complexation reaction between the hydrated cation and the anion present in reaction (d). In the presence of chloride ions the reaction will correspond to:

½AlOH2þ þ Cl ! ½AlOHClþ

ðeÞ

The cathodic hydrogen evolution follows the steps:

Hþ þ e ! Hads

ðfÞ

Hads þ Hads ! H2

ðgÞ

In aqueous acidic solutions, Schiff bases exist either as neutral molecules or in the form of protonated Schiff bases (cations). The interaction of neutral as well as protonated forms on the alumin-

Fig. 9. Correlation of HOMO energy with percentage inhibition efficiency for D1–D3 compounds.

3.6. Corrosion inhibition mechanism According to the electrochemical theory of corrosion, aluminium ions go into solution at the anodic sites in amount chemically equivalent to the revolting hydrogen from the cathodic sites of alu-

Fig. 10. HOMO and LUMO energy levels distribution of D1–D3 compounds.

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ium surface should be competitive with the hydrogen ions, that are going to reduce on the metal surface and hydrogen evolution occurs according to steps (f) and (g). It can be considered that adsorption of D3 and D2 compounds on the cathodic sites of aluminium is facilitated due to their partial HOMO distribution, compared to the D1 compound. Among the Schiff bases investigated, D3 has been found to give better activity as corrosion inhibitor followed by D2 and D1. The results of this study confirm that, whenever electronegative groups present, enhance the inhibition nature of compound. 4. Conclusions Three different Schiff bases were synthesized and tested as possible corrosion inhibitors for aluminium in 0.1 M HCl solution by electrochemical techniques. Electrochemical studies showed that when the aluminium surface was treated with Schiff bases, adsorption of inhibitors and inhibition efficiencies tend to increase with increasing inhibitor concentration. The inhibition efficiencies obtained from EIS and Tafel polarisation methods are in reasonable agreement with each other. The polarisation curves demonstrated that all the tested Schiff bases behave as cathodic inhibitor by inhibiting the cathodic hydrogen evolution reaction. The values of Kads and DG0abs suggest strong interaction of inhibitors with aluminium surface and adsorption phenomenon is typical chemical mechanism that follows the Temkin isotherm. The SEM micrographs confirm the protection of aluminium in 0.1 M HCl solution by the studied Schiff bases. Through the quantum chemical calculations it was shown that calculated parameters were correlated with the experimental results and it was found that inhibition efficiency increased with the higher EHOMO and lower DELUMO HOMO values. Comparative study of these inhibitors shows that the inhibition efficiency follows the order: D3 > D2 > D1 and the order of protection effect is the same for both electrochemical and computational methods. The difference in the inhibition efficiencies of three compounds lies in their structure and electronegative bromine and chlorine atoms facilitate the adsorption of molecule on the aluminium surface. Acknowledgements The financial support from the Eskisßehir Osmangazi University Research Fund (Project Number: 200719038) is gratefully acknowledged. Authors also would like to thank to Dr. Murat Duran for providing guidance about theoretical calculations. References [1] A.S. Fouda, A.A. Al-Sarawy, F.S. Ahmed, H.M. El-Abbasy, Corrosion inhibition of aluminium 6063 using some pharmaceutical compounds, Corros. Sci. 51 (2009) 485–492. [2] D. Mercier, M.G. Barthés-Labrousse, The role of chelating agents on the corrosion mechanisms of aluminium in alkaline aqueous solutions, Corros. Sci. 51 (2009) 339–348. [3] A.K. Maayta, N.A.F. Al-Rawashdeh, Inhibition of acidic corrosion of pure aluminium by some organic compounds, Corros. Sci. 46 (2004) 1129–1140. [4] C.M.A. Brett, On the electrochemical behaviour of aluminium in acidic chloride solution, Corros. Sci. 33 (1992) 203–210. [5] R. Grilli, M.A. Baker, J.E. Castle, B. Dunn, J.F. Watts, Localized corrosion of a 2219 aluminium alloy exposed to a 3.5% NaCl solution, Corros. Sci. 52 (2010) 2855–2866. [6] V. Moutarlier, M.P. Gigandet, B. Normand, J. Pagetti, EIS characterisation of anodic films formed on 2024 aluminium alloy, in sulphuric acid containing molybdate or permanganate species, Corros. Sci. 47 (2005) 937–951. [7] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, London, 1966. [8] H. Ashassi-Sorkhabi, B. Shabani, B. Aligholipour, D. Seifzadeh, The effect of some Schiff bases on the corrosion of aluminium in hydrochloric acid solution, Appl. Surf. Sci. 252 (2006) 4039–4047. [9] M. Abdallah, Antibacterial drugs as corrosion inhibitors for corrosion of aluminium in hydrochloric solution, Corros. Sci. 46 (2004) 1981–1996.

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