Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media

Egyptian Journal of Petroleum (2015) xxx, xxx–xxx

H O S T E D BY

Egyptian Petroleum Research Institute

Egyptian Journal of Petroleum www.elsevier.com/locate/egyjp www.sciencedirect.com

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Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media Ahmed A. Farag *, M.A. Migahed, A.M. Al-Sabagh Department of Petroleum Applications, Egyptian Petroleum Research Institute (EPRI), 1 Ahmed El-Zomor St., Nasr City, 11727 Cairo, Egypt Received 12 January 2014; accepted 10 June 2014

KEYWORDS Adsorption; Carbon steel; Corrosion; Electro and nonelectrochemical methods

Abstract In this study, the inhibition effect of 2,20 -(heptane-1,7-diylbis(azanylylidene)bis-(metha nylylidene))diphenol (HAMD) on carbon steel corrosion in 0.5 mol L1 H2SO4 solution was studied. Weight loss and electrochemical techniques such as open circuit potential (OCP), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to inspect the efficiency of HAMD as corrosion inhibitor. Scanning electron microscopy (SEM) and energy dispersion X-ray (EDAX) were used to characterize the steel surface. Polarization measurements indicated that, the studied inhibitor acts as mixed-type inhibitor. The adsorption of HAMD molecules on the carbon steel surface obeys Langmuir adsorption isotherm. ª 2015 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

1. Introduction Carbon steel is used in mass amounts in marine applications, chemical processing, petroleum production and refining, construction and metal processing equipment, despite its relatively high cost [1,2]. These applications usually induce serious corrosive effects on equipments, tubes and pipelines made of iron and its alloys [3,4]. Therefore, the prevention of metals used in petroleum field and industrial applications from corrosion is vital that must be dealt with especially in acid media. The development of corrosion inhibitors is based on organic compounds containing nitrogen, oxygen, sulfur atoms, * Corresponding author. Tel.: +20 (202) 01009270941 (mobile), +20 (202) 22747917 (work). E-mail address: [email protected] (A.A. Farag). Peer review under responsibility of Egyptian Petroleum Research Institute.

and multiple bonds in the molecules that facilitate adsorption on the metal surface [5]. The existing data show that organic inhibitors act by the adsorption on the metal surface and protective film formation [6]. The adsorption of organic inhibitors at the metal/solution interface takes place through the replacement of water molecules by organic inhibitor molecules [7,8]. Most of the commercial inhibitor formulations consist of aldehydes and amines [9]. Due to the presence of the –C‚N– group in the molecule, Schiff bases are considered as good corrosion inhibitors. Some researches revealed that the corrosion inhibition efficiency of the Schiff bases is better than the corresponding amines and aldehydes [10]. The Schiff base has been previously reported as an effective corrosion inhibitor for steel, copper and aluminum [11–14]. The adsorbed species protect the metal from the aggressive medium, which causes deterioration of the metal. Adsorption process depends not only on the nature and the charge of the metal but also on the chemical structure of the inhibitor.

http://dx.doi.org/10.1016/j.ejpe.2015.07.001 1110-0621 ª 2015 Production and hosting by Elsevier B.V. on behalf of Egyptian Petroleum Research Institute. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: A.A. Farag et al., Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media, Egypt. J. Petrol. (2015), http://dx.doi.org/10.1016/j.ejpe.2015.07.001

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The objective of the present work is to investigate the inhibiting action of 2,20 -(heptane-1,7-diylbis(azanylylidene)bis-(methanylylidene))diphenol (HAMD) on carbon steel corrosion in 0.5 mol L1 H2SO4 solution using chemical and electrochemical techniques. Scanning electron microscopy (SEM) and energy dispersion X-ray (EDAX) were used to characterize the steel surface. The thermodynamic parameters such as the adsorption equilibrium (Kads) and the free energy  of adsorption (DGads ) are calculated and discussed.

were abraded with different grades of emery papers, washed with distilled water, degreased with acetone, dried and kept in a desiccator. The specimens were accurately weighted and then immersed in solution containing 0.5 mol L1 H2SO4 solutions with and without various concentrations of HAMD. After 4 h exposure, the specimens were taken out, rinsed thoroughly with distilled water, dried and weighted accurately. Three parallel experiments were performed for each test. The average weight loss, DW (mg) was calculated using the following equation [16]:

2. Materials and experimental techniques

DW ¼ W1  W2

2.1. Materials

where W1 and W2 are the average weight of specimens before and after exposure, respectively.

Tests were performed on a carbon steel of the following composition (wt.%): 0.07% C, 0.24% Si, 1.35% Mn, 0.017% P, 0.005% S, 0.16% Cr, 0.18% Ni, 0.12% Mo, 0.01% Cu and the remainder Fe.

2.5. Electrochemical measurements

2.2. Inhibitor The used Schiff base inhibitor (Fig. 1) in this study was synthesized through condensation reaction between 0.5 mol of 2hydroxybenzaldehyde and 0.25 mol of 1,7-heptanediamine in three-necked round flask with a condenser, thermometer and a magnetic bar to produce 2,20 -(heptane-1,7-diylbis(azanylylidene))bis(methanyl-ylidene))diphenol (HAMD). The mixture was allowed to cool. Then, the obtained precipitate was recrystallized by ethanol [15]. The structure of this product was confirmed by FT–IR and 1H NMR spectroscopy as shown in Figs. 2 and 3, respectively. IR bands (KBr): m = 1619 cm1 (C‚N); m = 3461 cm1 (OH); m = 2921 cm1 (aliphatic CH). The 1H NMR spectrum (Fig. 3) 1H NMR (DMSO) d: 1.29 (s, 6H, CH2); 1.65 (s, 4H, CH2); 3.71 (s, 4H, CH2); 4.77 (s, 2H, OH); 7.08 (s, 2H, Ar–H); 7.08 (d, 2H, Hz, Ar–H); 7.52 (s, 2H, Ar–H); 8.56 (s, 2H, CH‚N). 2.3. Solutions The corrosion tests were performed in 0.5 mol L1 H2SO4 solution in the absence and presence of various concentrations of the inhibitor. 0.5 mol L1 H2SO4 solution was prepared by dilution of 98% H2SO4 with distilled water. The concentrations of the inhibitor employed were varied from 1 · 105 to 1 · 103 mol L1. For each experiment, a freshly prepared solution was used under air atmosphere without stirring. All chemicals were of analytical reagent grade (Merck) and were used without further purification.

The electrochemical measurements were carried out using Volta lab 40 (Tacussel-Radiometer PGZ301) potentiostate and controlled by Tacussel corrosion analysis software model (Voltamaster 4) under static condition. A conventional three electrode cylindrical glass cell was used, equipped with a saturated calomel electrode (SCE) as the reference electrode and a platinum electrode as the auxiliary electrode. The working electrode is carbon steel, which was cut from a cylindrical rod covered by Teflon leaving only 0.5 cm2 of the surface area exposed to corrosive solution. Before each experiment, the corrosion potential was allowed to stabilize for 0.5 h in test solution to establish the steady state open circuit potential (Eocp). Potentiodynamic polarization curves were obtained by changing the potential automatically from 900 to 200 mV versus open circuit potential (OCP) with the scan rate of 2 mV s1. Electrochemical impedance spectroscopy (EIS) was measured in the frequency range of 100 kHz–10 mHz with a perturbation of 10 mV amplitude at the OCP. 2.6. Surface morphology studies The surface morphology of the steel specimens was examined after exposure to 0.5 mol L1 H2SO4 in the absence and presence of 1 · 103 mol L1 of HAMD inhibitor. JEOL 5410 scanning electron microscope SEM (Japan) was used for this investigation. Energy dispersive analysis of X-rays (EDAX) system was attached with a JEOL 5400 scanning electron microscope to characterize the elements of the film formed on the steel surface. 3. Results and discussion 3.1. Weight loss measurements

2.4. Weight loss measurements Weight loss experiments were carried out using specimens having the dimensions of 7.0 cm · 2.0 cm · 0.3 cm. The specimens

OH

HO N

The corrosion rate, A (mg cm2 h1) of carbon steel specimens after 4 h exposure to 0.5 mol L1 H2SO4 solution with and without the addition of various concentrations of HAMD inhibitor was calculated and the obtained data are listed in Table 1. The corrosion rates (A), surface coverage (h) and inhibition efficiency (gw) of each concentration were calculated using the following equations [17]:

N

A¼ Figure 1

ð1Þ

Chemical structures of the inhibitors.

DW St

ð2Þ

Please cite this article in press as: A.A. Farag et al., Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media, Egypt. J. Petrol. (2015), http://dx.doi.org/10.1016/j.ejpe.2015.07.001

Adsorption and inhibition behavior of a novel Schiff base

FT–IR chart of HAMD.

Figure 2

Figure 3

3

1

H NMR chart of HAMD.

Table 1 Corrosion parameters for carbon steel in 0.5 mol L1 H2SO4 in the absence and presence of different concentrations of HAMD, obtained from weight loss measurements at 303 K. Inhibitor conc. (mol L1)

A (mg cm2 h1)

h

gw (%)

Blank 1 · 105 5 · 105 1 · 104 5 · 104 1 · 103

9.00 2.70 1.71 1.26 0.86 0.58

– 0.700 0.810 0.860 0.905 0.936

– 70.0 81.0 86.0 90.5 93.6

while corrosion rate decreased. This could be due to the inhibitor molecules effect acting by adsorption on the metal surface [18]. The variation of gw and inhibitor concentrations in 0.5 mol L1 H2SO4 solution at 303 K is shown in Fig. 4. When the concentration of inhibitors is less than 1 · 103 M, the gw increased sharply with an increase in concentration, while a further increase causes no appreciable change in performance. The reason for the high inhibition efficiency of the studied HAMD toward carbon steel may be due to the presence of the imine group (–C‚N–), oxygen atoms and aromatic benzene rings in HAMD molecule. 3.2. Potentiodynamic polarization curves

h¼

Auninh  Ainh Auninh

gW ¼

Auninh  Ainh  100 Auninh

ð3Þ

ð4Þ

where S is the surface area of specimens (cm2), and t is the immersion time (h), Auninh and Ainh are corrosion rates in the absence and presence of inhibitor, respectively. It is clear that gw increased with increasing inhibitor concentration,

The potentiodynamic polarization curves of carbon steel in 0.5 mol L1 H2SO4 solution with various concentrations of HAMD are shown in Fig. 5. The values of associated electrochemical parameters, i.e., corrosion potential (Ecorr), corrosion current density (icorr), cathodic Tafel slope (bc), anodic Tafel slope (ba) and inhibition efficiency (g) were calculated from these curves and given in Table 2. The inhibition efficiency (gp) was calculated from polarization measurements according to the relation [19]:

Please cite this article in press as: A.A. Farag et al., Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media, Egypt. J. Petrol. (2015), http://dx.doi.org/10.1016/j.ejpe.2015.07.001

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A.A. Farag et al. 100

Inhibition efficiency (%)

95

90

85

80

75

70 0

Figure 4

1

2

3

4

5 C (M)

6 10-4

7

8

9

10

11

Relationship between the inhibition efficiency and inhibitor concentration for carbon steel after 4 h immersion at 303 K.

-350

Blank -5 1×10 -5 5×10 -4 1×10 -4 5×10 -3 1×10

-400

E vs SCE/mV

-450 -500 -550 -600 -650 -700 -750 -3.0

-2.5

-2.0

-1.5

-1.0

-0.5

logi/mA.cm

0.0

0.5

1.0

1.5

-2

Figure 5 Polarization plots of carbon steel electrode obtained in 0.5 mol L1 H2SO4 solution and containing various concentrations of HAMD at 303 K.

gp ¼

icorrðuninhÞ  icorrðinhÞ  100 icorrðuninhÞ

ð5Þ

where icorr(uninh) and icorr(inh) are uninhibited and inhibited corrosion current densities, respectively. The values of current densities were obtained by the extrapolation of the current– potential lines to the corresponding corrosion potentials. As it can be seen from Fig. 5, the addition of HAMD to the aggressive solution reduces both anodic metal dissolution and also retards cathodic hydrogen evolution reactions. This result is indicative of the adsorption of inhibitor molecules on the active sites of mild steel [20]. The inhibition is more and more

pronounced with the increasing inhibitor concentration. It is also seen clearly from Fig. 5 and Table 2 that, the presence of HAMD in 0.5 mol L1 H2SO4 solution does not remarkably shift the corrosion potential (Ecorr). These results suggest that the HAMD can be classified as mixed-type inhibitor [21]. 3.3. Electrochemical impedance spectroscopy Fig. 6 shows the Nyquist plots for carbon steel in 0.5 mol L1 H2SO4 solutions in the absence and presence of various concentrations of HAMD. The impedance diagrams consist of a

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Adsorption and inhibition behavior of a novel Schiff base

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Table 2 Potentiodynamic electrochemical parameters for the corrosion of carbon steel in 0.5 mol L1 H2SO4 solution in the absence and presence of various concentrations of HAMD at 303 K. Inhibitor conc. (mol L1)

Ecorr (mVSCE)

icorr (mA cm2)

ba (mV dec1)

bc (mV dec1)

h

gp (%)

Blank 1 · 105 5 · 105 1 · 104 5 · 104 1 · 103

537.8 544.3 548.5 548.1 540.5 545.6

4.0032 1.2119 0.9054 0.7333 0.5292 0.2025

215.4 222 209 200.4 175.5 137.7

222 254 240.2 234.7 240 270

– 0.697 0.774 0.817 0.868 0.949

– 69.7 77.4 81.7 86.8 94.9

150 140

Blank -5 1×10 -5 5×10 -4 1×10 -4 5×10 -3 1×10

130 120 110 100

Zi/ohm.cm

2

90 80 70

17.8 Hz

60 50 40 30

1 kHz

20 10

0.4 Hz

0 -10 0

10

20

30

40

50

60

70

80

Zr/ohm.cm Figure 6 303 K.

90

100 110 120 130 140 150

2

EIS for carbon steel electrode obtained in 0.5 mol L1 H2SO4 solution and containing various concentrations of HAMD at

Table 3 Impedance electrochemical parameters derived from the Nyquist plots for carbon steel in 0.5 mol L1 H2SO4 solution in the absence and presence of various concentrations of HAMD at 303 K.

Figure 7 Electrical equivalent circuit used for modeling the interface carbon steel in 0.5 mol L1 H2SO4.

large capacitive loop at high frequencies followed by a small inductive loop at low frequency values. The high frequency capacitive loop is usually related to the charge transfer of the corrosion process and double layer behavior. On the other hand, the low frequency inductive loop may be attributed to the relaxation process obtained by adsorption species like FeSO4 or inhibitor species on the electrode surface. It might be also attributed to the re-dissolution of the passivated surface at low frequencies [22]. Furthermore, the diameter of the capacitive loop in the presence of HAMD is bigger than that in the uninhibited solution and increases with HAMD concentration. This indicates that the impedance of carbon

Inhibitor conc. (mol L1)

Rs (ohm cm2)

Rct (ohm cm2)

Cdl (lF cm2)

h

gi (%)

Blank 1 · 105 5 · 105 1 · 104 5 · 104 1 · 103

1.7 2.3 2.6 3.1 3.6 3.6

9.8 32.4 55.9 61.7 78.3 142.8

348.5 106.0 61.1 55.4 43.7 23.9

– 0.696 0.825 0.841 0.875 0.931

– 69.6 82.5 84.1 87.5 93.1

steel corrosion increases with the inhibitor concentration. The capacitive loops are not perfect semicircles which can be attributed to the frequency dispersion effect as a result of the roughness and inhomogeneous of electrode surface. The data obtained were fitted according to the electrical equivalent circuit diagram suggested in Fig. 7 in order to model the mild

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steel/solution interface. Where Rs represents the solution resistance, Rct represents the charge transfer resistance and CPE represents the constant phase element. CPE was used in place of a double layer capacitance (Cdl) in order to give a more accurate fit to the experimental results [23]. Generally, the use of a CPE is required due to the distribution of relaxation times as a result of inhomogeneities present at a micro- or nano-level, such as the surface roughness/porosity, adsorption, or diffusion. The fitted parameters are listed in Table 3. The

values of charge transfer resistance (Rct) were given by subtracting the high frequency impedance from the low frequency one as follows [24]: 8

8

Rct ¼ Zre ðat low frequencyÞ  Zre ðat high frequencyÞ

ð6Þ

The values of electrochemical double layer capacitance (Cdl) were calculated at the frequency, fmax, at which the imaginary component of the impedance is maximal (Zmax) by the following equation [25]:

-530

-540

E vs SCE/mV

-550

B: Blank -5 1: 1×10 -5 2: 5×10 -4 3: 1×10 -4 4: 5×10 -3 5: 1×10

B

-560

1

-570

2 3

-580 5

-590

4

-600 0

200

400

600

800

1000

1200

1400

1600

1800

Time/sec. Figure 8 Variation of the OCP of steel vs. time of immersion obtained in 0.5 mol L1 H2SO4 solution and containing various concentrations of HAMD at 303 K.

12

10

y = 1.0647x + 0.091 R² = 0.999

C/ (M) × 10-4

8

6

4

2

0 0

2

4

6

C (M) Figure 9

8

10

12

10-4

Langmiur adsorption plots of steel in 0.5 mol L1 H2SO4 solution containing HAMD inhibitor at 303 K.

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Adsorption and inhibition behavior of a novel Schiff base

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Figure 10 EDAX and SEM images of carbon steel surface after 4 h immersion in 0.5 mol L1 H2SO4 solution in the (a) absence and (b) presence of 1 · 103 mol L1 HAMD.

Cdl ¼ ð2pfmax Rct Þ1

ð7Þ

The values of percentage inhibition efficiency (gi) were calculated from the values of Rct according to the following equation [26]:



 RctðinhÞ  RctðuninhÞ gi ¼  100 RctðinhÞ

ð8Þ

where, Rct(inh) and Rct(uninh) are the values of the charge transfer resistance in the presence and absence of inhibitor, respectively. As it is shown in Table 3, the addition of HAMD

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reduces the Cdl values and increases Rct. This may be attributed to the gradual replacement of water molecules and other ions originally adsorbed on the surface by the adsorption of inhibitor molecules on the metal surface [27]. The adsorption of inhibitor molecules at the metal/solution interface results in an increase of Rct, which reduces the corrosion rate of carbon steel. 3.4. Open circuit potential (OCP) versus time measurements The variation of OCP of carbon steel was followed as a function of time in aerated 0.5 mol L1 H2SO4 solution in the absence and presence of various concentrations of HAMD. Results obtained are shown in Fig. 8. In inhibited solution the steady state Ecorr drifts to more negative values. According to Riggs [28], the classification of a compound as an anodic or cathodic type inhibitor is feasible when the OCP displacement is at least 85 mV in relation to that one measured for the blank solution. However from Fig. 8, the positive shift in Ecorr is less than 50 mV in the presence of inhibitors. These findings reveal that the investigated inhibitors can be regarded as mixed-type inhibitors [29]. These results are compatible with those obtained from polarization measurements.

Basic information on the adsorption of inhibitor on metal surface can be provided by adsorption isotherm. Several isotherms are employed to fit the experimental data. It is found that the adsorption of HAMD on carbon steel surface obeys the Langmuir adsorption isotherm equation [30]: ð9Þ

where, C is the concentration of inhibitor, Kads the adsorptive equilibrium constant and h is the degree surface coverage (h = g/100) obtained from Table 1. A plot of C/h versus C (Fig. 9) gives a straight line with an average correlation coefficient of 0.9998 and a slope of nearly unity (1.06) suggests that the adsorption of HAMD molecules obeys Langmuir adsorption isotherm. The standard free energy of adsorption of inhibitor (DGads ) on carbon steel surface can be evaluated with the following equation [31]; DGads ¼ RT lnð55:5Kads Þ

Fig. 10(a and b) shows the SEM images and EDAX spectra of the steel surface after immersion in 0.5 mol L1 H2SO4, for a period of 4 h, in the absence and presence of 1 · 103 of HAMD, respectively. The SEM micrographs show that the surface of steel is highly damaged in the uninhibited solution (Fig. 10a). However, a smoother surface is seen in the presence of the inhibitor (Fig. 10b). The EDAX spectra show the characteristic peaks of some of the elements constituting the steel sample after 4 h immersion in 0.5 mol L1 H2SO4 without inhibitor. In the presence of HAMD inhibitor, the EDAX spectra show additional lines of nitrogen and oxygen, due to the adsorbed layer of inhibitor that covered the electrode surface. In addition, the Fe peaks are considerably suppressed relative to uninhibited steel surface sample. This suppression of the Fe lines occurs because of the overlying inhibitor film. These results indicate that the inhibitor molecules hinder the dissolution of steel by formation of a protective film on the steel surface [34]. These results confirm those from polarization and EIS measurements, which suggest that a protective film is formed over the metal surface, and hence retarded both anodic and cathodic reactions [35]. 3.7. Mechanism of Inhibition

3.5. Adsorption isotherm

C 1 ¼ þC h Kads

3.6. Surface examination by SEM and EDAX

ð10Þ

From Eq. (10) the standard free energy was calculated as 33.5 kJ mol1. The negative value of standard free energy of adsorption indicates spontaneous adsorption of HAMD molecules on the steel surface and also the strong interaction between inhibitor molecules and the metal surface [32]. Generally, the standard free energy values of 20 kJ mol1 or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface (physical adsorption); those of 40 kJ mol1 or more negative involves charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate covalent bond (chemical adsorption). The calculated standard free energy of adsorption value is less than 40 kJ mol1. Therefore, it can be concluded that the adsorption is physical adsorption [33].

A clarification of the mechanism of inhibition requires full knowledge of the interaction between the protective organic inhibitor and the metal surface. Since, the carbon steel is positively charged with respect to potential of zero charge (PZC) in H2SO4 solution [36]. In H2SO4 solution, the HAMD exist either as neutral molecules or in the form of protonated cations. HAMD may adsorb on the metal/acid solution interface by one or more of the following ways: (i) electrostatic interaction of protonated HAMD with already adsorbed SO–4 2 ions, (ii) donor–acceptor interactions between the pelectrons of the aromatic ring and vacant d-orbital of surface iron atoms and (iii) interaction between unshared electron pairs of heteroatoms and vacant d-orbital of surface iron atoms [37]. The protonated HAMD may adsorb on surface through synergistic effect with SO2 ions in H2SO4 solution 4 [38]. It is a well known fact that the inhibitors which not only offer d-electrons but also have unoccupied orbitals, so exhibit a tendency to accept electrons from d-orbital of metal to form stable chelates which are considered as excellent inhibitors [39]. In our case, the donation of electrons might place negative charge on the metal which facilitates the transfer of pelectrons from the metal to the remaining p* orbital of the aromatic ring of the inhibitor to form feedback bond. 4. Conclusions According to the results obtained, the following points can be emphasized: 1. The HAMD has excellent corrosion inhibition efficiency. This was attributed to the adsorption of inhibitor molecules and protective film formation on the metal surface.

Please cite this article in press as: A.A. Farag et al., Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media, Egypt. J. Petrol. (2015), http://dx.doi.org/10.1016/j.ejpe.2015.07.001

Adsorption and inhibition behavior of a novel Schiff base 2. The potentiodynamic polarization curves indicated that the HAMD inhibits both anodic metal dissolution and also cathodic hydrogen evolution reactions i.e. it is considered as mixed-type inhibitor. 3. The adsorption of HAMD molecules on the metal surface in 0.5 mol L1 H2SO4 solution obeys Langmuir adsorption isotherm. 4. The standard free energy of adsorption has been found less than 40 kJ mol1 which indicated that the adsorption is physical adsorption. 5. SEM and EDAX techniques showed that the inhibitor molecules form a good protective film on the steel surface.

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Please cite this article in press as: A.A. Farag et al., Adsorption and inhibition behavior of a novel Schiff base on carbon steel corrosion in acid media, Egypt. J. Petrol. (2015), http://dx.doi.org/10.1016/j.ejpe.2015.07.001