Electrochemical and Dft Studies of 8 hydroxyquinoline as Corrosion

ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2013, Vol. 49, No. 2, pp. 229–239. © Pleiades Publishing, Ltd., 2013.

PHYSICOCHEMICAL PROBLEMS OF MATERIALS PROTECTION

Electrochemical and Dft Studies of 8hydroxyquinoline as Corrosion Inhibitor For Az61 Magnesium Alloy In Acidic Media D. Seifzadeh*, S. HamzedoustHasankiadeh, and A. N. Shamkhali Department of Applied Chemistry, Faculty of Science, University of Mohaghegh Ardabili, Ardabil, Iran Email: [email protected], [email protected] Received February 7, 2012

DOI: 10.1134/S2070205113020123 1

1. INTRODUCTION

During the past few years a considerable increase on the use of magnesium alloys has been noticed, namely for applications in the automotive, aerospace and elec tronic industries. The growing interest on the use of Mg alloys is due to their high strength/weight ratio, good cast ability, easy machining and good recycling possibil ities. The wrought MgAlZn alloys, such as AZ31, AZ61 and AZ91 have been found applications in the automotive industry, mainly in the production of struc tural components. However high chemical activity and poor corrosion resistance of magnesium alloys limits its widespread application [1]. Thus the corrosion resis tance of these alloys should be enhanced by the suitable protection method. Among many corrosion protection methods, the use of inhibitors is one of the most practi cal methods for protecting metals or alloys from corro sion [2, 3]. Corrosion protection of magnesium alloys by inhibitors is not fully studied. There are a few impor tant reports in the literature about the corrosion inhibi tion of commercial purity magnesium foil in sodium chloride [4] and also the corrosion inhibition of AZ91D alloy in ASTM D138487 corrosive solution [5]. 8Hydroxyquinoline (8HQ) is a widely used chelat ing agent in analytical chemistry. 8HQ is an organic compound containing nitrogen and oxygen hetero atoms and also aromatic rings as the basic requirements for corrosion inhibition action. Figure 1 shows the chemical structure of the 8hydroxyquinoline. Corro sion inhibition effect of 8HQ has been studied by sev eral researchers. Tang et al. [6] have been investigated the corrosion inhibition of the cold rolled steel by 8HQ in sulfuric acid solution. Song et al. [7] have been found that 8HQ can inhibit aluminum alloy corrosion in 3.5% chloride solution. Gao et al. [8] have been studied the corrosion inhibition effect of 8HQ on AZ91D in ASTM standard solution and its synergistic effect with sodium dodecylbenzenesulphonate. However, there is no report in the literature about the corrosion inhibition of magnesium alloys in acidic media by 8HQ or other

compounds and also there is no report about corrosion inhibition of AZ61 alloy. The aim of the present work is to study the inhibition properties of 8HQ on the corrosion of AZ61 magne sium alloy in hydrochloric acid and sulfuric acid solu tions using potentiodynamic polarization, Electro chemical Impedance Spectroscopy (EIS) and weight loss methods. Also the inhibition mechanism has been investigated using several experimental and theoretical methods including Fourier Transform Infrared Spec trometry (FTIR), Diffuse Reflectance Spectroscopy (DRS) and Density Functional Theory (DFT). More over the morphology of AZ61 samples after immersion in corrosive solutions was investigated by Scanning Electron Microscopy (SEM) in the presence and absence of inhibitor. 2. EXPERIMENTAL DETAILS 2.1. Materials AZ61 magnesium alloy samples were used as sub strate. The chemical composition of the used alloy determined by Energy Dispersive XRay Spectroscopy (EDAX) and is given in Table1 (The relative EDAX spectrum has not been shown). The acid solutions (0.1 M HCl and 0.1 M H2SO4) as corrosive media were made from analytical grade 37% HCl and 98% H2SO4 (both of them purchased from Merck) respectively. Doubledistilled water was used to solution prepara tion. AZ61 samples were mounted in polyester (purchase from TABA chemical company) in such a way that only 0.5 cm2 of samples was in contact with test solutions.

1 The article is published in the original.

N OH Fig. 1. Chemical structure of 8Hydroxy Quinoline.

229

230

SEIFZADEH et al.

Table 1. Chemical composition of the AZ61 magnesium alloy Alloy elements

Al

Zn

Mn

Si

Cu

Fe

Mg

Wt %

5.920

0.490

0.150

0.037

0.003

0.007

remainder

Prior to all measurement, magnesium alloy samples were polished successively with a sequence of emery papers of different grades (400, 800 and 1500 grits). Before the testes, the samples were carefully degreased with ethanol, rinsed with double distilled water and finally dried by compressed air. 2.2. Methods 2.2.1. Electrochemical tests. Potentiodynamic polarization and EIS tests were carried out using BHP2061 and pAUTOLAB2 Potentiostat–Gal vanostat instruments respectively. Electrochemical experiments were performed using the three compo nent electrochemical cell consisting of mounted AZ61 samples as working electrode, platinum sheet as counter electrode and saturated Ag/AgCl electrode as reference electrode. For polarization measurements, the poten tial was scanned with a scan rate of 5 mVs–1 in the cathodic to anodic direction, so that the maximum over voltage was ±200 mV. Corrosion current (Icorr) was cal culated using the Stern–Geary equation [9]: Ba × Bc (1) I corr = . 2.303R p ( B a + B c ) To calculate polarization resistance (Rp), linear region of polarization curves (overvoltage lower than ±20 mV) was used. Cathodic and anodic Tafel slopes (Bc and Ba) were calculated from the Tafel regions (over voltage more than ±50 mV) of cathodic and anodic branch of polarization curves respectively. EIS mea –1.1

E, V vs Ag/AgCl

–1.2 –1.3 –1.4 –1.5 –1.6 –1.7 –1.8 –1.9 –2.0 –2.1 –8

Blank 0.001 M 0.005 M 0.01 M 0.05 M 0.1 M

surements were carried out in the frequency range of 10 kHz–0.05 Hz, at the open circuit potential, by applying 5 mV sine wave ac voltage. EIS data were pre sented in the Nyquist and Bode plot forms. To calculate the EIS parameters the Zview2 fitting software was used. All polarization and EIS experiments were per formed after 1 h immersion of working electrode in cor rosive solution under atmospheric conditions. All experiments were carried out at certain temperature [2]. The temperature was fixed by MEMMERT digital thermostat. The volume of test solutions for each exper iment was 100 ml. 2.2.2. Weight loss. The gravimetric measurements were carried out at the definite time interval of 2 h at room temperature using a Sartorius (TE214S) analyti cal balance. Weight loss measurements were performed using the alloy specimens with a size of 0.6 cm × 0.5 cm × 0.25 cm. Before each experiment, the samples were abraded using emery papers (grades 400, 800, 1500), washed with distilled water, degreased with ethanol and finally dried by compressed air. The initial weight of each specimen was noted before immersion. Then the samples were immersed in 100 ml of 0.1 M HCl solution without and with different concentrations of the inhib itor. After 2 h of immersion, the samples were washed, dried and reweighed [10]. 2.2.3. Analysis and characterization methods. The surface morphology of the AZ61 magnesium alloy sam ples in the absence and presence of 8HQ was observed by SEM (LEO, VP 1430) instrument. EADX analysis was used to determine the chemical composition of the film formed on alloy surface in the presence of inhibitor using the same instrument. The protective film formed on the magnesium alloy surface also analyzed by the FTIR (Shimadzu, 8400 S) and DRS (Shimadzu, 4100S) methods. 2.2.4. Density Functional Theory. The structures of 8hydroxyquinoline and also its possible complexes with magnesium cations were opti mized by B3LYP hybrid functional [11] and TZVP basis set [12, 13] by Firefly (PCGamess) program [14, 15]. 3. RESULTS AND DISCUSSIONS

–7

–6

–5

–4 –3 logI, A

–2

–1

Fig. 2. Typical polarization curves for corrosion of AZ61 magnesium alloy in 0.1 M HCl in the absence and presence of different concentrations of 8HQ.

0

3.1. Electrochemical Tests 3.1.1. Potentiodynamic polarization. The typical potentiodynamic polarization curves of AZ61 alloy in 0.1 M HCl in the presence and absence of 8HQ at dif ferent concentrations after 1 h immersion are shown in Fig. 2. Various corrosion parameters such as corrosion current density (Icorr), corrosion potential (Ecorr),

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 2 2013

ELECTROCHEMICAL AND DFT STUDIES OF 8HYDROXYQUINOLINE

231

Table 2. Polarizatio parameters for AZ61 magnesium alloy in 0.1 M HCl containing various concentrations of 8HQ at 298 K C, M

Ecorr, V

Ba, mV/dec

Bc, mV/dec

Rp, Ωcm2

Icorr, Acm–2

%IE

Blank 0.001 0.005 0.010 0.050 0.100

–1.558 –1.647 –1.703 –1.716 –1.644 –1.619

118 151 160 157 141 71

138 164 174 131 145 157

0.94 1.33 1.38 1.63 2.81 52.21

4.619 × 10–1 2.566 × 10–2 3.619 × 10–2 1.902 × 10–2 1.104 × 10–2 4.066 × 10–4

– 29.32 31.88 42.33 66.54 98.20

Table 3. Corrosion parameters for AZ61 magnesium alloy in 0.1 M H2SO4 containing various concentrations of 8HQ at 298 K

Blank 0.10 0.15 0.20

Ecorr, V

Ba, mV/dec

–1.751 –1.817 –1.757 –1.696

117 139 134 107

Bc, mV/dec

Anodic and cathodic Tafel slopes (Ba and Bc), polariza tion resistance (Rp) and inhibition efficiencies (%IE) are collected in Table 2. The Icorr values were used to cal culate the inhibition efficiency, (%IE), using the fol lowing equation [10]: R p – R °p %IE = × 100. Rp

Rp, Ωcm2

Icorr, Acm–2

%IE

1.15 1.96 2.82 37.10

2.467 × 10 1.566 × 10–2 9.999 × 10–3 7.467 × 10–4

– 41.32 59.22 96.90

148 144 126 158

(2)

Where, Rp and R °p are the polarization resistances in the presence and absence of inhibitor respectively. It is obvious from the Table 2 data that the corrosion current of AZ61 alloy sample significantly decreases after the addition of 8HQ. Inhibition efficiencies increase with 8HQ concentration and reach to maxi mum value, 98.48%, at the presence of 0.1M 8HQ. These results have been proven the inhibition effect of the studied compound on the corrosion of AZ61 alloy in Hydrochloric acid media. The values of both anodic and cathodic Tafel slopes change after the addition of inhibitor. In the case of cathodic reaction, the changes are more obvious. In other word, this compound affects both anodic and cathodic corrosion reactions and therefore can be classified as mixed type inhibitor [16]. The same results obtained for 0.1 M H2SO4 corrosive solution. The related curves are presented in Fig. 3 and also the corresponding corrosion parameters are given in Table 3. It is evident that the inhibition efficiency (%I.E.) obtained in H2SO4 solutions is lower than that obtained in HCl solutions at the same acidic concentration. Thus, it can be concluded that the type of the aggressive acid anion should influence the inhibition effect of 8 HQ. It seems that the specific adsorption of the present aggressive anions have an important role in the inhibi

–2

2–

tion process. In H2SO4 solutions, SO 4 anions can be adsorbed onto the metal surface forming MgSO4 with relatively low solubility which remains on the metal sur face. In fact, MgSO4 covers a considerable portion of the metal surface leaving less space available for inhibi tor molecules to bond or adsorb. The same behavior was observed during other investigations [17]. The pKa values of 8 hydroxyquinoline functional groups (Pyridine and Phenol) are schematically pre sented in Fig. 4 [18]. In the high acidic media (pH < pKa1), the 8 hydroxyquinoline convert to protonated (cathionic) form while in the solutions with intermedi –1.2 –1.3 –1.4 E, V vs Ag/AgCl

C, M

–1.5

Blank 0.1 M 0.15 M 0.2 M

–1.6 –1.7 –1.8 –1.9 –2.0 –2.1 –6

–5

–4

–3 logI, A

–2

–1

Fig. 3. Polarization curves of AZ61 magnesium alloy sam ples in 0.1 M H2SO4 corrosive solution in the absence and presence of different concentrations of 8HQ.


0

232

SEIFZADEH et al.

OH phenol: weakly acidic

N丣 OH H

N丣 OH H

N

••

pyridine: weakly basic

pH < pKa1 pKa29.99

N

N 䊞

OH

O

pKa1 < pH < pKa2

pH > pKa2

pKa15.2 Fig. 5. Different forms of 8Hydroxyquinoline in various solutions.

Fig. 4. pKa values of 8hydroxyquinoline functional groups.

ate (pKa1 < pH < pKa2) and high pH values, the uncharged and anionic forms will be stable respectively. Different forms of the studied compound in various solutions are presented in Fig. 5. In the studied concen trations of hydrochloric acid and sulfuric acid the pH values are significantly lower than the pKa1, thus it is expected that the 8hydroxyquinoline be in protonated form. In this condition, the protonated inhibitor mole 2– cule can be physically adsorbed onto Cl– and SO 4 anions that play an intermediate role between the alloy surface and protonated inhibitor molecule. In other word, it seems that the physical adsorption have a rela tively important role in the corrosion inhibition effect of the 8 hydroxyquinoline. It is generally accepted that Cl– ions have stronger tendency to adsorb on the metals surface in comparison 2– with SO 4 ions and the electrostatic influence on the inhibitor adsorption may be the reason for an increased protective effect in halidecontaining solution. More 2– over, the lesser interference by SO 4 ions with the adsorbed protonated cations may lead to lower adsorp tion and inhibition of acid corrosion. So, the adsorption of inhibitor on the alloy surface is greater from 0.1 M HCl solution, which leads to better inhibition perfor mance than that in 0.1 M H2SO4 [19]. The formation of chemical bond between the mag nesium surface and 8hydroxyquinopline has been reported in several researches [8, 20, and 21]. Thus the chemical bonding may be has an important role in inhi bition process. This idea will investigated in separate sections by different methods. 3.1.2. EIS. The effects of the inhibitor concentra tion on the impedance behavior of AZ61 magnesium alloy in 0.1 M HCl solution have been studied by means of EIS. All experimental data were recorded at the cor

rosion potential, and after 1 h immersion at which point the open circuit potential had reached a stationary value. Figure 6 represents the obtained results in Nyquist plots form. The symbols represent the experi mental data and the solid lines represent the fitting result, generated using the equivalent circuits in Fig. 7. In the presence of inhibitor, all of diagrams are charac terized by two welldefined capacitive loops, at high and medium frequencies, followed by an inductive loop in the lower frequency domain. The high frequency capacitive loop is related with electric double layer while the second capacitive loop originated from the diffusion through the surface film [22–25]. According to Chao and Zhang theory [26], the low frequency inductive loop was attributed to the corrosion nucleation at the initiation stage of localized corrosion. The circuit employed allows the identification of both charge transfer resistance (Rt) and the film resis tance (Rf), the double layer capacitance (CPEdl), the film capacitance (Cf) and finally, the inductive loop resistance (RL) and inductance (L). It is worth mention ing that the double layer capacitance (Cdl) value is affected by imperfections of the surface, and that this effect is simulated via a constant phase element (CPE) [16]. The CPE can be modeled as follows [2]: –α

(3) ( Z CPE = ( jωC ) ). The parameter α quantifies different physical phe nomena like surface inhomogeneousness resulting from surface roughness, inhibitor adsorption, porous layer formation, etc. [19]. The electrochemical parameters of the tested systems are calculated by Zview2 software and collected in Table 4. The corrosion of AZ61 was decreased in the presence of the inhibitor because the total surface resistances (Rct + Rf) were significantly increased in the presence of inhibitor. As the inhibitor concentration increased, the total resistance increased and the value of double layer constant phase element

Table 4. Corrosion parameters for AZ61 magnesium alloy in 0.1 M HCl containing various concentrations of 8HQ at 298 K C, M

Rct, Ω

Blank 0.01 0.05 0.10

4.5 4.8 13.5 93.0

CPEdl, F

Rf, Q

CPEf, F

4.745 × 10–5 4.834 × 10–5 2.182 × 10–5 8.216 × 10–11

– 1.6 8.0 268.5

– 1.676 × 10–3 1.158 × 10–2 2.071 × 10–5

Rtotal(Rct + Rf) 4.5 6.4 21.5 361.5

%IE

RL, Ω

L, H

– 29.69 79.07 98.76

1.5 31.8 42.0 271.1

0.88 2.00 7.14 252.50


ELECTROCHEMICAL AND DFT STUDIES OF 8HYDROXYQUINOLINE (a)

(b)

2.5

3

Not Fitted

Not Fitted Fitted

2

233

2.0

Fitted

1.5

0

1.0

18

20

22

24

26

–Z@

–Z@

1

0.5 0

–1

–0.5 –2 –1.0 17 –3

10

19

Zr(Ω)

21 Zr(Ω)

(c)

(d)

23

25

250 Not Fitted Fitted

6

150

4

100

2

Not Fitted Fitted

200

–Zi(Ω)

–Z@

8

50 0

0 –50 –2 –4 30

–100 35

40

45

50

55

Zr(Ω)

–150 0

100

200 300 Zr(Ω)

400

500

Fig. 6. Nyquist plots for the AZ61 alloy samples in the 0.1 M HCl after addition of inhibitor in different concentrations: Blank (6.a), 0.01 M 8HQ (6.b), 0.05 M 8HQ (6.c), and 0.1 M 8HQ (6.d).

(CPEdl) decreased. Increasing of the resistance with inhibitor concentration suggests that more inhibitor molecules are adsorbed or bonded to the metal surface at higher concentration leading to greater surface cover age. Decreasing in the CPEdl was caused by reduction in local dielectric constant and/or by increase in the thick ness of the electrical double layer [2]. In this case, the inhibition efficiencies were calculated using the total resistance as follow: R total – R °total %IE = × 100, R total

(4)

° and Rtotal are the surface total resistance where the R total in the absence and presence of inhibitor respectively. The values of the inhibition efficiencies obtained here

are in good trend agreement with the polarization test results for 0.1 M HCl system. 3.2. Weight Loss Results Weight loss data of AZ61 in 0.1 M HCl in the absence and presence of various concentrations of inhibitor were obtained and are given in Table 5. In this case, the inhibition efficiencies (IE %) were calculated according to [2]: C.R blank – C.R inh (5) IE% = . C.R blank Here, the C.Rblank and C.Rinh are the corrosion rates of the studied samples in 0.1 M HCl solution in the absence and presence of inhibitor at different concen trations respectively. The results show that the inhibi


234

SEIFZADEH et al. (a) CPEdl

Rs

Rt

RL

L

(b) CPEdl

Rs

Rf

Rt

Cf RL

L

Fig. 7. Equivalent circuits used to fitting of the experiment results. Uninhibited solution (a), inhibited solutions (b).

tion efficiencies increase with increasing inhibitor con centration. The results obtained from the weight loss measurements are in good trend agreement with those obtained from the electrochemical methods. 3.3. SEM Images The SEM images of AZ61 samples in the used cor rosive solutions in the absence and presence of 8HQ were observed after 1h immersion. Figure 8 shows the SEM images of AZ61 sample in 0.1 M HCl in the absence (8.a) and presence of inhibitor in three differ ent concentrations (8b,c, and d). It can be seen from Fig. 8a that the alloy surface is strongly damaged in the absence of the inhibitor. The observed cracks may be related with hydrogen evolution. For the inhibited samples, it can be seen that the alloy surface is uniform without obvious defects especially in the presence of high inhibitor concentra tions so that the surface of the metal is absolutely free from any pits and cracks. In the case of the sample immersed in solution with highest inhibitor concentra tion, the polishing scratches are also visible indicating Table 5. Average corrosion rates in the presence and absence of 8HQ in HCl 0.1 M obtained by weight loss Sample number CM , M Average C.R, mg/cm2h 1 2 3 4 5

Blank 0.001 0.005 0.010 0.050

217.460 194.030 193.455 152.245 46.090

%IE * 10.77 11.04 29.99 78.80

the very low corrosion of the sample during the immer sion period. Also in the mentioned case, a part of the specimen is covered by a layer of crystalline chemical compound, indicating the formation of a protective film. This fact may be explained by formation of a com plex between the main alloying element and the 8HQ molecule. This idea will be investigated later. The SEM micrographs of AZ61 samples after 1 h immersion in 0.1 M H2SO4 in the absence and presence of inhibitor at are also shown in Fig. 9. Observation of the surface of the samples demonstrates that the sample immersed blank solution (Fig. 9a) and also the sample immersed in the solution with 0.01 M inhibitor (Fig. 9b) have many cracks on the surface and seems to be completely corroded. In the case of the sample immersed in solution with 0.2 M inhibitor concentra tion (Fig. 9d), the polishing scratches are still visible and the alloy surface is relatively smooth, but the different small pits can be observed on the metal. The formation of pits can be explained by relatively complete covering of alloy surface by protective film and increasing of anodic current on the small uncovered areas. However there are no cracks in this case that indicate the rela tively good protection effect of the 8HQ in the studied corrosive media. In the same inhibitor concentrations (0.1 M), corrosion protection effect of 8HQ in HCl medium is stronger that the H2SO4 solution. This result is in agreement with the electrochemical tests results. 3.4. FTIR Spectra FTIR spectroscopy was used to characterize the film formed on the alloy surface in the presence of 8HQ. Figure 10 presents the FTIR spectra of the original 8 HQ powder and also the film formed on the AZ61 sur face after 72 h immersion in 0.1 M HCl solution con taining 0.1 M 8HQ. Several differences can be observed between the FTIR spectrums of the original powder (Fig.10a) and also the film formed on the alloy surface (Fig. 10b). The increase in the intermolecular hydrogen bond energy from about 3095 to 3155 cm–1 indicates a polymeric association between 8HQ mole cules [8]. Also, the stretching vibration of aromatic C– H at around 3040 cm–1 is weakened. The energy shifts for some absorption peaks were observed in the film compound scrapped from the alloy surface, in compar ison with the original compound. The band at around 1593 cm–1, shows a positive change in energy to 1605 cm–1 and also a considerable increasing in inten sity. In the Fig.10a, the band at around 1508 cm–1 and 1473 cm–1 are associated with the stretching vibration of aromatic CC and C–N in the 8HQ structure. The energies of the mentioned bands are significantly changes after the film formation. Moreover new peaks such as the peak at around 1328 cm–1 are detected in the scrapped film FTIR spectrum. In addition, intensity changes also occur in main bands when films form on AZ61 alloy surface. These facts can be explained by direct bonding between the alloying atoms and 8HQ



235

(а)

10 µm (b)

20 µm

(c)

10 µm

20 µm

(d)

Fig. 8. SEM images of AZ61 magnesium alloy in the 0.1 M HCl in the absence (a) and presence of 0.001 M (b), 0.01 M(c) and 0.1 M (d) of 8HQ after 1 h immersion.

(а)

10 µm

(b)

10 µm

(c)

10 µm

(d)

20 µm

Fig. 9. SEM images of AZ61 magnesium alloy in the 0.1 M H2SO4 in the absence (a) and presence of 0. 1 M (b), 0.15 M (c) and 0.2 M (d) of 8HQ after 1 h immersion. PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 2 2013

236

SEIFZADEH et al.

Table 6. Geometrical parameters of free and protonated 8HQ molecule Free ligand Geometry parameter

value

Geometry parameter

value

C12N16 C3N16 C2C3 C1C2 C2O17

1.33 1.37 1.42 1.38 1.38

O17H18 C12N16C3 C3C2C1 C3C2O17 N16C3C2O17

0.98 118.30 120.20 118.30 0.00

Protonated ligand C12N16 C3N16 C2C3 C1C2 C2O17

1.34 1.38 1.41 1.38 1.38

O17H18 C12N16C3 C3C2C1 C3C2O17 N16C3C2O17

0.97 123.00 119.30 113.80 0.00

molecules via O and N atoms and the formation of pro tective film. 3.5. DRS Studies Solid UVvis absorption spectrums of the original 8HQ powder and also the film scrapped from AZ61 alloy surface after 12 h immersion in 0.1 M HCl con taining 0.1 M inhibitor are presented in Fig. 11. In both spectrums, the noisy and high energy bands in the 220– Table 7. Molecular parameters of free and protonated 8HQ molecule Quantum chemical parameters Parameters HOMO (eV) LUMO (eV) ΔE (eV)

Free ligand –6.1549 –1.8938 4.2611

Protonated ligand –10.7316 –6.9358 3.7958

Mulliken charges Atom N16 O17 C12 C3 C2 C1 C27 C20 C19 C21

Free ligand –0.31 –0.47 –0.11 0.00 0.22 –0.19 –0.11 0.00 0.22 –0.19

Protonated ligand –0.53 –0.52 0.13 0.12 0.25 –0.17 0.13 0.12 0.25 –0.17

240 nm are related with π π* transition. For the original compound, the wide and strong energy band in the 240–420 nm regions is due to n π* charge tran sition. In the case of scrapped film, this energy band covers more wavelength region until 470 nm. More probably, this result is related with the formation of a complex between the Mg atom and the 8HQ [8]. 3.6. Calculation Method Recently, density functional theory (DFT) has been used to analyze the characteristics of the inhibitor/sur face mechanism and to describe the structural nature of the inhibitor on the corrosion process. Furthermore, DFT is considered a very useful technique to probe the inhibitor/surface interaction as well as to analyze the experimental data [27]. The corrosion inhibition of AZ61 magnesium alloy by 8 hydroxyquinoline was attributed to the physical and chemical adsorption of the molecule species. But a correlation between the experimental results and molecular properties has not been created. Thus the density functional theory was applied to study the geometrical and electronic struc tures of the inhibitor and possible complexes. The opti mized structures of free and protonated 8HQ molecule as well as their geometrical parameters were given in Fig. 12 and Table 6 respectively. Furthermore, the molecular parameters such as the Mulliken charges, frontier molecular orbital energies and the energy sepa ration between the HOMO (Highest occupied molecu lar orbital) and LUMO (Lowest unoccupied molecular orbital) orbital were presented in Table 7. The calcu lated Mulliken charges show negative charge on N16 and O17 atoms, therefore they can be considered as the active centers for the adsorption of 8HQ inhibitor on the metal surface. It is well known that the more nega tive the atomic partial charges of the adsorbed center are, the more easily the atoms donate its electrons to the unoccupied orbital of the metal. Partial negative charges of both nitrogen and oxygen atoms on the 8 HQ structure significantly increase after the protona tion process which can promote the adsorption of inhibitor on AZ61 alloy surface. The separation energy, ΔE = (ELUMO – EHOMO), is an important parameter as a function of reactivity of the inhibitor molecule towards the adsorption on metallic surface. As ΔE decreases, the reactivity of the molecule increases leading to increase the inhibition efficiency of the molecule [28]. The calculations indicate that the protonated molecule has the lowest value of ΔE, 3.79 eV, which can facilitate its adsorption on the metal surface and accordingly increases its inhibition effi ciency. This result is in agreement with that obtained by Mulliken charge values. Based on the theoretical and experimental results, different bonding possibilities between the magnesium cathions and the 8HQ ligand in both free and proto nated forms were considered and the optimized struc tures of possible three complexes were presented in



237

(a)

490.15 421.21

1593.55 866.93

T, %

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0 4000

895.96 454.12 1579.47

543.34

973.90

1896.23

574.45 636.80 1059.45 1093.82

1434.27 3049.53

1166.26

817.78

1473.02 1223.27 1409.94 1275.38 1508.64 1285.97

710.67 741.86 781.31

1380.95

3000

2000

1500

1000

500400

cm–1 (b)

80 75 70 65

491.77 464.65 421.99

60 55 865.84 897.30

T, %

50 45

974.20 2359.92

575.88 548.98

1605.35

40 35 30

1059.15 1579.50

3049.40

1094.93 1166.51

1434.86

25

1328.39

20 15

1113.09

1224.03

1410.21

805.50 819.01 710.78

1207.47 1284.45

10

781.73 742.52

1471.75 1500.01

5 0 4000

637.81

1139.33

1381.43

3000

2000

1500

1000

500 400

cm–1

Fig. 10. FTIR spectra of the original 8HQ powder (10a) and the film scrapped from the AZ61 alloy surface after 72 h immersion in the inhibited solution with 0.1 M inhibitor concentration (10b). PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 49 No. 2 2013

238

SEIFZADEH et al. 2.0

(a)

1.8

14H

Absorbance

1.6

11C

15H

Mg(8HQ)2 complex

1.4 1.2

13H 8C

12C

28H

24H

36H

21C

34O

23C

19C

25C

20C

4C

16N

9H

3C

1.0

5C

37Mg

0.8

29H

0.6

35N

26C

27C

31H

0.4

2C

22C

30C

6C

18H

32H

7H

33H

Original 8HQ

0.2

(b)

0 200

10H

1C

7H 18H

250

300 350 400 Wavelength, nm

450

500

Fig. 11. Solid UVvis absorption spectra of the original 8HQ powder and the film scrapped from alloy surface after 12 h immersion in 0.1 HCl containing 0.1 M inhibitor.

38H

32H

34O

37Mg

20C

21C

24H 8C

12C

25C

23C

13H 11C

15H

28H 29H

14H

(a)

(c) 15H

32H 9H

13H

33H

5C

8C

10H

14H 4C

30C

31H

1C

20C 22C

18H

17O

9H

8C

5C

14H

15H

15N

5C

7H 6C

23C

13H

9H

10H

Fig. 13. Optimized structures of complex between two free ligands and magnesium (a), complex between two free and protonated ligands and magnesium (b), and complex between two protonated ligands and magnesium(c).

13H

12C

24H

28H

(b)

11C

8C 4C

1C

21C 25C 29H

14H 11C

3C 2C

19C

7H

2C

16N

18H 36H 17O 34O 37Mg

6C

3C

12C 15N

35N

26C

15H

38H

39H

27C

12C

9H

4C

16N

22C

31H

5C

3C

19C

33H 30C

6C

2C

35N

27C

26C

11C

10H

1C 17O 36H

18H

4C

6C

3C

1C

Fig. 13. The values of standard formation energy (ΔEf = Ecomplex – 2Eligand – E Mg2+ ) of all complexes and also their relative instabilities are calculated (Table 8). It seems that the complex formed through two free and

7H

2C

19H 17O 18H

Table 8. Standard formation energies of all possible com plexes and also their relative instabilities Complex

Fig. 12. The optimized structures of free (a) and protonated (b) 8HQ molecules.

a

b

c

Standard formation energy, ev –13.264 –16.108 –16.043 Relative instability 2.844 0 0.065



protonated ligands (c) is a more stable compound with respect to other complexes. 4. CONCLUSIONS The following main conclusions are drawn from the present study: 1. 8hydroxyquinoline acts as an effective corrosion inhibitor for AZ61 magnesium alloy in 0.1 M HCl and also in 0.1M H2SO4 especially in high concentrations. 2. As the inhibitor concentration increases the values of corrosion resistance increase and the values of double layer capacitance decrease. These results can be explained by inhibitor adsorption on the metal surface. 3. The corrosion tests results revealed a better per formance for 8HQ as corrosion inhibitors for AZ61 alloy in HCl than in H2SO4 solutions. 4. SEM images of the alloy were observed in the absence and presence of inhibitor and the results are in good agreement with electrochemical and weight loss methods results in both studied corrosive media. 5. The FTIR and DRS methods were used to study the mechanism of inhibition. It was found that the cor rosion inhibition action is related with formation of protective layer on AZ61 alloy. 6. The optimized structures of the free and proto nated 8HQ and possible complexes as well as their molecular parameters were obtained by DFT theoreti cal methods. REFERENCES 1. Montemor, M.F., Simoes, A.M., and Carmezim, M., J. Appl. Surf. Sci., 2007, vol. 253, p. 6922. 2. Ashassi H., Seifzadeh, D., and Hosseini, M.G., Corro sion Sci., 2008, vol. 50, p. 3363. 3. Ashassi, H., Shaabani, B., and Seifzadeh, D., Electro chim. Acta, 2005, vol. 50, p. 3446. 4. Gao, H., Li, Q., Chen, F.N., et al., Corrosion Sci., 2011, vol. 53, p. 1401. 5. Williams, G., McMurray, H.N., and Grace, R., Elec trochim. Acta, 2010, vol. 55, p. 7824.

239

6. Tang, L., Li, X., Si, Y., et al., Mater. Chem. Phys., 2006, vol. 95, p. 29. 7. Mei, L.S., Liu, Z.H., and Hua, L.J., Trans. Nonferrous. Met. Soc. China, 2007, vol. 17, p. 318. 8. Gao, H., Li, Q., Dai, Y., et al., Corrosion Sci., 2010, vol. 52, p. 1603. 9. Roberge, P.R., Corrosion Inspection and Monitoring, New Jersey: John Wiley, 2007. 10. Zhang, Q. and Hua, Y., Mater. Chem. Phys., 2010, vol. 119, p. 57. 11. Becke, A.D., J. Chem. Phys., 1993, vol. 98, p. 5648. 12. Schaefer, A., Horn, H., and Ahlrichs, R., J. Chem. Phys., 1992, vol. 97, p. 2571. 13. Schaefer, A., Huber, C., and Ahlrichs, R., J. Chem. Phys., 1994, vol. 100, p. 5829. 14. http: //classic.chem.msu.su/gran/firefly/index.html 15. Schmidt, M.W., Baldridge, K.K., Boatz, J.A., et al., J. Comput. Chem., 1993, vol. 14, p. 1347. 16. Bahrami, M.J., Hosseini, S.M.A., and Pilvar, P., Corro sion Sci., 2010, vol. 52, p. 2793. 17. ElNaggar, M.M., Corrosion Sci., 2007, vol. 49, p. 2226. 18. http: //en.wikipedia.org/wiki/Acid_dissociation con stant 19. Li, X., Deng, S., and Fu, H., Corrosion Sci., 2011, vol. 53, p. 302. 20. Zucchi, F., Grassi, V., and Zanotto, F., Mater. Corros., 2009, vol. 60, p. 199. 21. Yang, X., Pan, F., and Zhang, D., Appl. Surf. Sci., 2008, vol. 255, p. 1782. 22. Huang, Y.S., Zeng, X.T., and Hu, X.F., Electrochim. Acta, 2004, vol. 49, p. 4313. 23. Baril, G. and Pebere N., Corrosion Sci., 2001, vol. 43, p. 471. 24. Baril, G., Galicia, G., Deslouis, C., et al., J. Electro chem. Soc., 2007, vol. 154, p. 108. 25. Baril, G., Blanc, C., and Pebere, N., J. Electrochem. Soc., 2001, vol. 148, p. 489. 26. Song, Y., Shan, D., Chen, R., and Han, E.H., Corro 1 sion Sci., 2009, vol. 51, p. 1087. 27. Roque, J.M., Pandiyan, T., and Cruz, J., Corrosion Sci., 2008, vol. 50, p. 614. 28. Issa, R.M., Awad, M.K., and Atlam, F.M., Appl. Surf. Sci., 2008, vol. 255, p. 2433.

SPELL: OK
