Thermodynamic, electrochemical and quantum chemical investigation of some Schiff bases as corrosion inhibitors for mild steel in hydrochloric acid solutions

Corrosion Science 52 (2010) 933–942

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Thermodynamic, electrochemical and quantum chemical investigation of some Schiff bases as corrosion inhibitors for mild steel in hydrochloric acid solutions Ishtiaque Ahamad a, Rajendra Prasad b, M.A. Quraishi a,* a b

Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Uttar Pradesh, Varanasi 221 005, India Department of Chemistry, Faculty of Science, Banaras Hindu University, Uttar Pradesh, Varanasi 221 005, India

a r t i c l e

i n f o

Article history: Received 29 July 2009 Accepted 7 November 2009 Available online 13 November 2009 Keywords: A. Mild steel A. Acid solutions B. Weight loss B. Polarization C. Acid corrosion

a b s t r a c t The corrosion inhibition of mild steel in 1.0 M HCl solution by four Schiff bases was investigated using weight loss and electrochemical measurements and quantum chemical calculations. All compounds showed >90% inhibition efficiency at their optimum concentrations. The activation energy (Ea) of corrosion and other thermodynamic parameters were calculated to elaborate the mechanism of corrosion inhibition. The adsorption of the inhibitors on the mild steel surface follows Langmuir isotherm model. Polarization studies indicated that all studied inhibitors are mixed type. The computed quantum chemical properties viz., electron affinity (EA) and molecular band gap (DEMBG) show good correlation with experimental inhibition efficiencies. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Mild steel is widely used in a variety of industrial applications such as petroleum industries, power plants, etc. Hydrochloric acid solutions are widely used for pickling, descaling, acid cleaning, oilwell acidizing, etc. [1]. The use of inhibitor is one of the best known methods of corrosion protection. The efficiency of inhibitor depends on the nature of environment, nature of metal surface and electrochemical potential at the interface, and the structural feature of inhibitor, which includes number of adsorption centres in the molecule, their charge density, the molecular size, and mode of adsorption, formation of metallic complexes and the projected area of inhibitor on the metal surface [2–4]. Organic compounds containing hetero-atoms such as N, O and S have been reported efficient corrosion inhibitors for metals and alloys [5–9]. In acidic environments, organic compounds with more than one hetero-atoms containing pi-electrons exhibit high inhibiting properties by providing electrons to interact with metal surface [10]. It has been reported that Schiff bases show more inhibition efficiency than corresponding amines and carbonyl compounds [11]. Recently more emphasis is being paid in the investigation of Schiff bases as corrosion inhibitors in acidic media due to their enhanced inhibiting action and ease of their synthesis from relatively cheap raw materials [12–17]. The quantum chemical calculations can complement the experimental investigations or even predict with some confidence into * Corresponding author. Tel.: +91 930 7025126; fax: +91 542 2368428. E-mail addresses: [email protected], [email protected] (M.A. Quraishi). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.11.016

experimentally unknown properties. There has been increasing use of the density functional theory (DFT) methods in the applications related to organic and bioorganic compounds [18]. The advancement in methodology and implementations has reached a point where predicted properties of reasonable accuracy can be obtained from DFT calculations. In this line, it has been reported [19] that energy of highest occupied molecular orbital (EHOMO) is often associated with the electron donating ability of a molecule. The higher the value of EHOMO of the inhibitor, the greater is the ease of offering electrons to the unoccupied d orbital of metallic iron, and the higher is the inhibition efficiency of the inhibitors. Similarly lower the value of energy of lowest unoccupied molecular orbital (ELUMO), the easier is the acceptance of electrons from metallic iron surface. Thus higher EHOMO and lower ELUMO values generally enhance the inhibition efficiency. Moreover, smaller value of HOMO–LUMO energy gap (DEH–L) for an inhibitor, higher the inhibition efficiency of that inhibitor [20]. However, it is fundamentally more appropriate to compare electron donating and accepting ability of a molecule with ionization potential (IP) and electron affinity (EA), respectively. Therefore, it is worth to compute these properties theoretically. In the present investigation four new corrosion inhibitors, namely, N0 -(phenylmethylene) isonicotinohydrazide (INHB), N0 -(2-hydroxybenzylidene) isonicotinohydrazide (INHS), N0 -(furan-2-ylmethylene) isonicotinohydrazide (INHF) and N0 -(3-phenylallylidene) isonicotinohydrazide (INHC), have been synthesized to investigate their inhibition effect on the corrosion of mild steel in molar hydrochloric acid solutions. Fig. 1 shows the chemical structure of four synthesized Schiff bases. The inhibition performance is

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O CONHN

CH

CONHN

N

H

CH

N

INHB

CONHN

CH

INHS

O

N

CONHN

CH

CH

CH

N

INHF

INHC

used to study the corrosion behaviour. All electrochemical experiments were performed in Gamry electrochemical cell with three electrodes connected to Gamry Instrument Potentiostat/Galvanostat with a Gamry framework system based on ESA400. Gamry applications include software DC105 for corrosion and EIS300 for EIS measurements, and Echem Analyst version 5.50 software package for data fitting. The mild steel of 1 cm2 was the working electrode, platinum electrode was used as an auxiliary electrode, and standard calomel electrode (SCE) was used as reference electrode. All potentials were measured versus SCE. Tafel curves were obtained by changing the electrode potential automatically from 250 to +250 mV versus corrosion potential (Ecorr) at a scan rate of 1 mVs1. EIS measurements were carried out in a frequency range from 100 kHz to 10 mHz under potentiostatic conditions, with amplitude of 10 mV peak-to-peak, using AC signal at Ecorr. All experiments were measured after immersion for 30 min in 1.0 M HCl with and without addition of inhibitors.

Fig. 1. Chemical structures of the tested Schiff bases.

evaluated by weight loss, polarization curves and electrochemical impedance spectroscopy (EIS). Several isotherms are tested for their relevance to describe the adsorption behaviour of the compounds studied and differences in behaviour are explained on the basis of structural properties. Theoretical calculations have performed by full geometry optimization of inhibitors using the lowest energy geometrical configuration at the level of density functional theory. Further, we have computed molecular band gap (DEMBG) using time dependent density functional theory (TD-DFT) method. 2. Experimental 2.1. Chemicals and materials The Schiff bases were synthesized by the condensation of isonicotinohydrazide (INH) and an appropriate aldehyde in methanolic solutions. Stock solutions of the synthesized Schiff base were prepared in 10:1 ratio water:ethanol mixture to ensure solubility. The corrosion tests were performed on mild steel specimens with a composition (in wt.%) C: 0.076, P: 0.012, Si: 0.026, Mn: 0.192, Cr: 0.050, Cu: 0.135, Al: 0.023, Ni: 0.050 and Fe balance. Square specimens with dimensions 2.5  2.0  0.025 cm were used for weight loss measurements. For electrochemical measurements, the exposed surface area of mild steel specimens was 1.0 cm2. The surface pre-treatment was carried out by polishing with 600, 800, 1000 and 1200 grit emery paper, followed by washing with double distilled water and finally degreased with acetone and dried at room temperature. All chemicals were of analytical reagent grade and were used without further purification. The solutions were prepared using double distilled water and all experiments were carried out in unstirred solutions. 2.2. Weight loss measurements The weight loss measurements were carried out in a glass vessel containing 100 mL of 1.0 M HCl with and without addition of different concentrations of different inhibitors at temperature 35 °C for 3 h immersion time. The specimens were withdrawn, rinsed with doubly distilled water, washed with acetone, dried and weighed. The experiments were done in triplicate and the average value of the weight loss was noted. 2.3. Electrochemical measurements Potentiodynamic polarization resistance and AC-electrochemical impedance spectroscopy (EIS) are the techniques which were

2.4. Theoretical calculation procedure All calculations were performed with the Gaussian 03 package [21]. The lowest energy geometrical configuration of Schiff bases were obtained at DFT level using Kohn–Sham approach. The vertical IP and vertical EA were computed using unrestricted Kohn– Sham formalism. The molecular band gap was computed as first vertical electronic excitation energy from the ground state using the TD-DFT approach as implemented in Gaussian 03. For all DFT calculations, the PBE1 hybrid functional [22] that includes 25% of exact exchange was used. We have chosen PBE1 hybrid functional because it has many times proved its efficiency on a wide range of compounds, and it generally provides accurate results on ground and excited-state properties, including charge-transfer transitions [23]. It is notable that PBE1 functional is derived from the Perdew–Burke–Erzenrh of PBE exchange–correlation functional, but it is not fitted on any experimental data. The split valence 631G** basis sets [24] were used for all atoms. However, the solution components and surface competition reactions have been neglected [25]. 3. Results and discussion 3.1. Weight loss measurements 3.1.1. Effect of inhibitor concentration The effect of addition of Schiff bases (namely INHB, INHS, INHF and INHC) at different concentrations on the corrosion of mild steel in 1.0 M HCl solution was studied by weight loss measurements at 35 °C. The highest concentration is sometimes limited by the solubility of the compound. If at two consecutive concentrations the inhibitor efficiency showed no further increase, higher concentrations were not tested. The inhibition efficiency (E%) and corrosion rate (CR, mm year1) were calculated according to the Eqs. 1 and 2 [26], respectively.

Eð%Þ ¼

w0  wi  100 w0

ð1Þ

where w0 and wi are the values of corrosion weight losses of mild steel in uninhibited and inhibited solutions, respectively.

C R ðmm=yÞ ¼

87:6  w ATD

ð2Þ

where w is corrosion weight loss of mild steel (mg), A the area of the mild steel specimen (cm2), T is the exposure time (h) and D the density of mild steel (g cm3). The values of inhibition efficiencies (E%) and corrosion rates (CR) obtained from weight loss measurements for different concentra-

I. Ahamad et al. / Corrosion Science 52 (2010) 933–942 Table 1 Corrosion parameters for mild steel in 1.0 M HCl in absence and presence of different concentrations of Schiff bases obtained from weight loss measurements. Inhibitor concentration (ppm)

Weight loss (mg cm2 h1)

E (%)

CR (mm y1)

Blank

7.00

–

77.9

INHB 50 100 150 200 250 300

4.70 2.73 1.37 1.17 1.00 0.50

32.9 60.9 80.5 83.3 85.7 92.9

52.3 30.4 15.2 12.9 11.1 5.6

INHS 50 100 150 200 250 300

3.37 2.43 1.70 1.43 0.87 0.67

51.9 65.2 75.7 79.5 87.6 90.5

37.5 27.1 18.9 15.9 9.6 7.4

INHF 50 100 150 200 250 300

3.70 2.10 1.07 0.67 0.53 0.43

47.1 70.0 84.8 90.5 92.4 93.8

41.1 23.4 11.9 7.4 5.9 4.8

INHC 50 100 150 200

0.47 0.20 0.10 0.03

93.3 97.1 98.6 99.5

5.2 2.2 1.1 0.4

tions of INHB, INHS, INHF and INHC in 1.0 M HCl are given in Table 1. It is clear that E% increased with increasing inhibitor concentration, reached at a maximum value and further increase in inhibitor concentration did not cause any significant change in the efficiency. While corrosion rate decreased with increasing inhibitor concentration. The corrosion inhibition can be attributed to the adsorption of the Schiff bases at the mild steel/acid solution interface [27]. The maximum inhibition efficiency (E%) of 99.5% was observed for INHC at 200 ppm concentration, while other inhibitors exhibit their maximum efficiency at 300 ppm concentration (Table 1). Good performance of Schiff bases as corrosion inhibitors for mild steel in 1.0 M HCl solutions can be attributed to the presence of heterocyclic ring/aromatic ring and >C@N group in their structures [28]. The inhibition efficiency of Schiff bases followed the order INHC > INHF > INHB > INHS. The inhibitor INHC gives the best performance. This can be explained on the basis of the presence of >C@C< in conjugation with azomethine group (>C@N–). This extensive delocalized p-electrons favour its greater adsorption on

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the mild steel surface, thereby giving rise in very high inhibition efficiency (99.5%) at concentration as low as 200 ppm [10]. The better performance of INHF (93.8%) than INHB (92.9%) is due to the presence of furan heterocyclic ring in INHF molecule, since iron has better coordination affinity towards oxygen and nitrogen bearing ligands [29]. The relatively poor performance of INHS may be probably due to the presence of ortho hydroxyl group which prevents its flat orientation on the metal surface causing less adsorption and thereby less inhibition [30]. In addition, relatively poor performance of INHS might also be due to the existence of intramolecular H-bonding (@N. . .H–O–). Similar interpretation has been reported in the literature [17]. 3.1.2. Effect of immersion time In order to assess the stability of inhibitive behaviour of inhibitors on a time scale, weight loss measurements were performed in 1.0 M HCl in absence and presence of inhibitors at optimum concentrations for 3–8 h immersion time at temperature 35 °C. The variation of E% with immersion time is shown in Fig. 2. It is evident from Fig. 2 that INHC retained 98.9% of its inhibition capability even after 8 h, in comparison to the 93.8%, 93.2% and 90.7% inhibition retained by INHB, INHF and INHS, respectively. As it is seen in Fig. 2, the inhibition performance of all studied Schiff bases did not show any significant change with increase in immersion time from 3 to 8 h. One could notice that inhibition efficiency of INHB shows a slight increase from 92.8% to 93.8%. This increase in E% is probably due to the increased adsorption of INHB molecules with time. From these results, we can conclude that all Schiff bases under investigation are efficient corrosion inhibitors for mild steel in molar hydrochloric acid solution. 3.1.3. Effect of temperature and thermodynamic activation parameters In order to study the effect of temperature on the inhibition efficiencies of Schiff bases, weight loss measurements were carried out in the temperature range 35–65 °C in absence and presence of inhibitors at optimum concentrations. The various corrosion parameters obtained are listed in Table 2. The fractional surface coverage h can be easily determined from the weight loss measurements by the ratio E%/100, where E% is inhibition efficiency and calculated using relation 1. The data obtained suggest that all Schiff bases get adsorbed on the mild steel surface at all temperatures studied and corrosion rates increased in absence and presence of inhibitors with increase in temperature in 1.0 M HCl solutions. In acidic media, corrosion of metal is generally accompanied with evolution of H2 gas; rise in temperature usually accelerates the corrosion reactions which results in higher dissolution rate of the metal. Inspection of Table 2 showed that corrosion rate increased with increasing temperature both in uninhibited and inhibited solutions, and the values of inhibition efficiency for INHC (99.5–

Fig. 2. Variation of inhibition efficiency with immersion time in 1.0 M HCl in presence of optimum concentrations of: (1) INHB, (2) INHS, (3) INHF and (4) INHC.

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Table 2 Various corrosion parameters for mild steel in 1.0 M HCl in absence and presence of optimum concentrations of Schiff bases at different temperatures. Temp (°C)

1 M HCl CR (mm y1)

Inhibitor INHB

35 45 55 65

77.9 107.6 162.5 208.5

INHS

INHF

h

E%

CR (mm y1)

h

E%

CR (mm y1)

h

E%

CR (mm y1)

h

E%

5.6 9.6 33.0 73.1

0.929 0.91 0.799 0.649

92.9 91.0 79.9 64.9

7.4 11.9 38.6 86.1

0.905 0.889 0.763 0.587

90.5 88.9 76.3 58.7

4.8 7.8 29.7 72.7

0.938 0.928 0.817 0.651

93.8 92.8 81.7 65.1

0.4 0.7 1.5 2.2

0.995 0.993 0.991 0.989

99.5 99.3 99.1 98.9

98.9%) remains almost constant while the inhibition efficiency of other Schiff bases (INHB, INHS and INHF) decreased with temperature. These results confirm that INHC is the best inhibitor as compared to the other Schiff bases in the range of temperature studied. A decrease in inhibition efficiencies with the increase in temperature in the case of INHB, INHS and INHF might be due to weakening of physical adsorption. In order to calculate activation parameters for the corrosion process, Arrhenius Eq. (3) and transition state Eq. (4) were used [31]:

  Ea C R ¼ A exp  RT CR ¼

INHC

CR (mm y1)

ð3Þ

    RT DH  DS exp  exp Nh RT R

ð4Þ

where CR is the corrosion rate, A is the Arrhenius pre-exponential factor, Ea the activation energy for corrosion process, h the Planck’s constant, N the Avogadro’s number, R the universal gas constant, T the absolute temperature, DH the enthalpy of activation and DS the entropy of activation. The apparent activation energies (Ea) at optimum concentration of Schiff bases were determined by linear regression between log CR and 1/T (Fig. 3) and the result is shown in Table 3. All the linear regression coefficients were close to 1, indicating that the mild steel corrosion in hydrochloric acid can be elucidated using the kinetic model. Inspection of Table 3 showed that the values of Ea

determined in 1.0 M HCl containing INHB, INHS, INHF and INHC are higher (52.7–87.8 kJ mol1) than that for uninhibited solution (29.2 kJ mol1). The increase in the apparent activation energy may be interpreted as physical adsorption that occurs in the first stage [32]. Szauer and Brand explained that the increase in activation energy can be attributed to an appreciable decrease in the adsorption of the inhibitor on the mild steel surface with increase in temperature. As adsorption decreases more desorption of inhibitor molecules occurs because these two opposite processes are in equilibrium. Due to more desorption of inhibitor molecules at higher temperatures the greater surface area of mild steel comes in contact with aggressive environment, resulting increased corrosion rates with increase in temperature [33]. Fig. 4 showed a plot of log (CR/T) versus 1/T. The straight lines are obtained with a slope (DH° = slope/2.303 R) and an intercept of log(R/Nh + DS°/2.303 R) from which the values of DH° and DS° are calculated and are given in Table 3. Inspection of these data revealed that the thermodynamic parameters (DH° and DS°) for dissolution reaction of mild steel in 1.0 M HCl in the presence of all inhibitors are higher (50.0–85.1 kJ mol1) than that of in the absence of inhibitors (26.6 kJ mol1). The positive signs of DH° reflect the endothermic nature of the mild steel dissolution process suggesting that the dissolution of mild steel is slow [34] in the presence of inhibitors. One can notice that Ea and DH° values vary in the same way (Table 3). This result permits to verify the known thermodynamic reaction between the Ea and DH° as shown in Table 3 [35]:

Fig. 3. Arrhenius plots of log CR vs. 1/T for mild steel in 1.0 M HCl in the absence and the presence of inhibitors at optimum concentrations of: (1) 1.0 M HCl, (2) INHB, (3) INHS, (4) INHF and (5) INHC.

Table 3 Activation parameters Ea, DH° and DS° for the mild steel dissolution in 1.0 M HCl in the absence and the presence of different Schiff bases at optimum concentrations. Inhibitor

R2

Ea (kJ mol1)

DH° (kJ mol1)

DS° (J mol1 K1)

K (M1)

DG° (kJ mol1)

1 M HCl INHB INHS INHF INHC

0.994 0.977 0.969 0.952 0.992

29.159 77.438 87.787 81.828 52.713

26.566 74.705 85.078 79.136 50.021

122.832 10.431 42.696 23.184 90.765

– 9815.8 7651.7 10845.2 249749.0

– 33.833 33.195 34.088 42.122

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Fig. 4. Arrhenius plots of log CR/T vs. 1/T for mild steel in 1.0 M HCl in the absence and the presence of inhibitors at optimum concentrations of: (1) 1.0 M HCl, (2) INHB, (3) INHS, (4) INHF and (5) INHC.

DH ¼ Ea  RT

ð5Þ

On comparing the values of the entropy of activation (DS°) in Table 3, it is clear that positive entropy of activation is obtained in the presence of INHB, INHS and INHF, while negative value (less negative than DS° value in free 1.0 M HCl solution) is observed in the case of INHC. Such variation is concerned with the phenomenon of ordering and disordering of the inhibitor molecules at the electrode surface and could be explained as follows. The adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasi-substitution process between the organic compounds in the aqueous phase and water molecules at the electrode surface [36]. The adsorption of inhibitors on the mild steel surface is accompanied by desorption of water molecules from the surface. Thus, while the adsorption process for the inhibitor is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent. The thermodynamic values obtained are the algebraic sum of the adsorption of organic molecules and desorption of water molecules [37]. Hence, the gain in entropy is attributed to the increase in solvent entropy and to more positive water desorption enthalpy [38]. The positive values of DS° also suggest that an increasing in disordering takes place in going form reactants to the metal/solution interface [39], which is the driving force for the adsorption of inhibitors onto the mild steel surface. 3.1.4. Adsorption isotherm The adsorption isotherm can be determined by assuming that inhibition effect is due mainly to the adsorption at metal/solution interface. Basic information on the adsorption of inhibitors on the metal surface can be provided by adsorption isotherm. In order to

obtain the isotherm, the fractional surface coverage values (h) as a function of inhibitor concentration must be obtained. The values of h can be easily determined from the weight loss measurements by the ratio E%/100, where E% is inhibition efficiency obtained by weight loss method. So it is necessary to determine empirically which isotherm fits best to the adsorption of inhibitors on the mild steel surface. Several adsorption isotherms (viz., Frumkin, Langmuir, Temkin, Freundlich) were tested and the Langmuir adsorption isotherm was found to provide the best description of the adsorption behaviour of the studied Schiff bases. The Langmuir isotherm is given by following equation [40]:

h=ð1  hÞ ¼ KC

ð6Þ

where K is the equilibrium constant of the adsorption process and C is the molar concentration of inhibitor. The plot of log (h/1  h) versus log C gave a straight line as shown in Fig. 5. The linear regression coefficients (R2) are almost equal to 1, confirming that the adsorption of studied Schiff bases in 1.0 M HCl solution follows the Langmuir’s adsorption isotherm. The free energy of adsorption (DG°) is related to the adsorption constant (K) with following equation [41]:

K¼

  1 DG exp 55:5 RT

ð7Þ

where the value 55.5 is the concentration of water in solution expressed in mol L1[41]. The values of K and DG° were calculated at 35 °C and are listed in Table 3. The large negative values of DG° confirmed the spontaneity of the adsorption process and stability of the adsorbed layer on the mild steel surface. Moreover, the high value of DG° obtained for INHC indicated that this compound is

Fig. 5. Langmuir isotherm plots for adsorption of Schiff bases on the mild steel in 1.0 M HCl in the presence of optimum concentrations of: (1) INHB, (2) INHS, (3) INHF and (4) INHC.

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Table 4 Polarization parameters for mild steel in 1.0 M HCl in the absence and the presence of different Schiff bases at optimum concentrations. Inhibitor

Concentration (ppm)

Ecorr (mV/SCE)

Icorr (lA cm2)

ba (mV dec1)

bc (mV dec1)

E (%)

1 M HCl INHB INHS INHF INHC

– 300 300 300 200

448.0 488.0 462.0 484.0 476.0

1090.0 145.0 156.0 121.0 82.0

66.0 56.6 49.0 75.8 102.4

97.7 129.8 98.4 181.5 194.3

– 86.7 85.7 88.9 92.5

more strongly adsorbed on the mild steel surface in 1.0 M HCl than the other inhibitors. This is in good agreement with the range of the inhibition efficiency values obtained from both weight loss and electrochemical techniques. It is well known that values of DG° of the order of 20 kJ mol1 or lower indicate a physisorption [42] while those more negative than 40 kJ mol1 involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [43]. As can be seen in Table 3, the DG° value for INHC is 42.1 kJ mol1 clearly indicates its chemical adsorption on the mild steel surface. Chemical adsorption of INHC can also be supported by its temperature independent adsorption on the mild steel surface (Table 2). The range of calculated DG° values for rest inhibitors are ranging between 33.2 and 34.1 kJ mol1. This indicated that adsorption of these inhibitors on the mild steel surface may involve complex interactions: both physical adsorption and chemical adsorption [44]. The possible adsorption mechanism is: (i) electrostatic interaction between the charged inhibitor molecules and charged mild steel surface. This process is called physical adsorption, (ii) direct adsorption on the basis of donor–acceptor interactions between the lone pairs of electrons of hetero-atoms, p-electrons of benzene and heterocyclic rings and the vacant d-orbitals of iron surface atoms. This process is called chemical adsorption, (iii) indirect adsorption of the charged inhibitor molecules on the mild steel surface through a synergistic effect with chloride ions from hydrochloric acid solution. 3.2. Potentiodynamic polarization measurements The corrosion potential (Ecorr), corrosion current density (Icorr), and anodic (ba) and cathodic (bc) slopes are obtained by the anodic and cathodic regions of the Tafel plots. The corrosion current density (Icorr) can be obtained by extrapolating the Tafel lines to the corrosion potential [45] and the inhibition efficiency (E%) values were calculated from the relation:

Eð%Þ ¼

Icorr  IcorrðiÞ  100 Icorr

ð8Þ

where Icorr and Icorr(i) are corrosion current densities obtained in absence and presence of inhibitors, respectively.

Table 4 represents all corrosion parameters including inhibition efficiency of the Schiff bases obtained from Tafel polarization studies. The polarization curves for mild steel in the absence and in the presence of inhibitors INHB, INHS, INHF and INHC at optimum concentrations are given in Fig. 6. The parallel cathodic Tafel lines (Fig. 6) suggested that the addition of inhibitors to the 1.0 M HCl solution do not modify the hydrogen evolution mechanism and the reduction of H+ ions at the mild steel surface which occurs mainly through a charge transfer mechanism [46]. The change in the values of bc in the presence of inhibitors clearly indicates the effect of the Schiff base compounds on the kinetics of hydrogen evolution. The shift in the anodic Tafel slope (ba) values may be due to the adsorption of chloride ions/or inhibitor modules onto the mild steel surface [47]. It can be noticed that in anodic domain for potential higher than 300 mV/SCE, the presence of Schiff bases did not change the current-vs.-potential characteristics and inhibitors start to desorb (Fig. 6). This potential can be defined as the desorption potential. Similar behaviour have been already reported for other organic compounds [48,49]. The behaviour of inhibitors at potentials greater than 300 mV/SCE could be associated to the significant dissolution of mild steel. This dissolution results in desorption of the adsorbed film of inhibitors on the surface of the electrode in 1.0 M HCl media. In this case desorption rate of inhibitors is raised more than its adsorption. However, inhibitors influenced anodic reaction at potentials lower than 300 mV/SCE. This result showed clearly that the inhibition of the mild steel corrosion is under cathodic and anodic control, i.e. mixed type. It is clear from Table 4 that the corrosion current density (Icorr) values decreased considerably in the presence of Schiff bases at optimum concentrations while no definite trend was observed in the shift of Ecorr values. The studied Schiff base compounds act as corrosion inhibitors suppressing both anodic and cathodic reaction by getting adsorbed on the mild steel surface blocking the active sites, and these results suggested that the addition of inhibitors reduces the anodic dissolution and also retards the cathodic hydrogen evolution reaction, indicating that theses inhibitors exhibit cathodic and anodic inhibition effects [50]. Therefore, all four studied Schiff bases can be classified as mixed inhibitors in1.0 M HCl. The values of inhibition efficiency obtained by polarization curves measurements followed the order INHC > INHF > INHB > INHS.

Fig. 6. Tafel plots for mild steel in 1.0 M HCl containing optimum concentration of various Schiff bases: (1) 1.0 M HCl; (2) INHS, (3) INHB, (4) INHF and (5) INHC.

I. Ahamad et al. / Corrosion Science 52 (2010) 933–942

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Fig. 7. (a) Nyquist plots and (b) Bode plots of mild steel in 1.0 M HCl in absence and presence of optimum concentration of various Schiff bases: (1) 1.0 M HCl, (2) INHS, (3) INHB, (4) INHF and (5) INHC.

3.3. Electrochemical impedance measurements The impedance measurements were carried out after immersion for 30 min in 1.0 M HCl solutions in absence and presence of optimum concentrations of Schiff bases. Fig. 7 shows typical Nyquist and Bode plots for mild steel in 1.0 M HCl in the absence and presence of studied inhibitors at optimum concentrations. The Nyquist plots show a depressed capacitive loop in the high frequency (HF) range and an inductive loop in the lower frequency (LF) range. The HF capacitive loop can be attributed to the charge transfer reaction and time constant of the electric double layer and to the surface inhomogeneity of structural or interfacial origin, such as those found in adsorption processes [51]. The LF inductive loop may be attributed to the relaxation process obtained by adsorption of the species like Clads and H+ads on the electrode surface [52–55]. It may also be attributed to the re-dissolution of the passivated surface at low frequencies [55,56]. From Bode plots, it could be seen that the Bode-phase plots (Fig. 7b) give one time constant for all inhibitors. Therefore, in the studied frequency range, an equivalent structure model (equivalent circuit) was proposed in order to fit and analyze the obtained EIS data (Fig. 8). In addition, the deviations of phase angle from 90° and of the slope in the impedance modulus plot (Fig. 7b) from 2 expressed that the system did not behave like an ideal capacitor [57]. Fig. 8 is a circuit generally used to model the iron/acid interface [58,59] where there was only one time constant in Bode plots. In this case, the time constant associated with the relaxation process of the electric double layer dominated, and the obtained Rct was a sum of the electric double layer and the adsorption layer effects because they could not be split up [60]. In this equivalent circuit (Fig. 8), Rs is the solution resistance and Rct is the charge-transfer resistance whose value is a measure of electron transfer across

the surface and is inversely proportional to corrosion rate [61] and CPE the constant phase element. The constant phase element, CPE, is introduced in the circuit instead of a pure double layer capacitor (Cdl) to give a more accurate fit [62,63]. The CPE element is used to explain the depression of the capacitance semi-circle, which corresponds to surface heterogeneity resulting from surface roughness, impurities, dislocations, grain boundaries, adsorption of inhibitors, formation of porous layers [64–67], etc. The impedance of the CPE is expressed as:

Z CPE ¼

1 Y 0 ðjxÞn

ð9Þ

where Y0 is the magnitude of the CPE, j is the imaginary number (j2 = 1), n is the phase shift and x (= 2pf) is the angular frequency for which imaginary component of impedance is maximum. The CPE describes an ideal capacitor when n = 1. The factor n is a valuable criterion of the nature of the metal surface and reflects the microscopic fluctuations of the surface [68] and the value of n usu-

Fig. 8. Equivalent circuit for impedance analysis.

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Table 5 Electrochemical parameters for mild steel in 1.0 M HCl in the absence and the presence different Schiff bases at optimum concentrations. Inhibitor

Concentration (ppm)

RS (X cm2)

Rct (X cm2)

n

Y0 (106 X1 cm2)

Cdl (lF cm2)

E(%)

1 M HCl INHB INHS INHF INHC

Nill 300 300 300 200

0.879 0.714 0.638 1.032 0.662

12.24 56.88 44.70 95.99 229.94

0.841 0.848 0.848 0.824 0.822

282.2 140.9 142.2 138.4 68.4

97.67 58.78 57.28 54.64 27.83

– 78.5 72.6 87.2 94.7

ally lies between 0.50 and 1.0 [69]. In addition, the double layer capacitances, Cdl, for a circuit including a CPE were calculated by using the following equation [55,70]:

C dl ¼ Y 0 ðxmax Þn1

ð10Þ

where xmax = 2pfmax, fmax is the frequency at which the imaginary component of the impedance is maximum. According to the above-mentioned equivalent circuit, our experimental data were fitted very well. The inhibition efficiencies, E%, of the tested inhibitors were calculated from the Rct values at optimum concentrations using the following equation:

E% ¼

RctðiÞ  Rctð0Þ  100 RctðiÞ

puted quantum chemical properties such as EHOMO, ELUMO, DEH–L, vertical IP, vertical EA, DEMBG, and number of transferred electrons (DN) are listed in the Table 6. The number of transferred electrons (DN) was calculated using the following equation [2,72]:

DN ¼

vFe  vinh 2ðgFe þ ginh Þ

ð13Þ

where vFe and vinh denote the absolute electronegativity of iron and the inhibitor molecule, respectively; gFe and ginh denote the abso-

ð11Þ

where Rct(0) and Rct(i) are the charge-transfer resistance values in absence and presence of inhibitor, respectively. The electrochemical parameters RS, Rct, Y0, Cdl and E% are listed in Table 5. Inspection of Fig. 7a reveals that addition of the inhibitors increases the capacitive loop diameter of the Nyquist plots without affecting their characteristic features. This means that the inhibition action of these inhibitors is due to their adsorption on the metal surface without altering the corrosion mechanism. The data of Table 5 showed that the magnitude of Rct increased while that of Cdl decreased with addition of inhibitors in 1.0 M HCl at optimum concentrations. The double layer between the charged metal surface and the solution is considered as an electrical capacitor. The adsorption of inhibitors on the electrode surface (mild steel) decreases its electrical capacity as they displace the water molecules and other ions originally adsorbed on the surface. The decrease in this capacity with inhibitors may be attributed to the formation of a protective adsorption layer on the electrode surface [53]. The thickness of this protective layer (d) is related to Cdl in accordance with Helmholtz model, given by the following equation [71]:

C dl ¼

ee0 A d

ð12Þ

where e is the dielectric constant of the medium and e0 is the permittivity of free space (8.854  1014 F cm1) and A is the effective surface area of the electrode. Data in Table 5 showed that the Rs values are very small compared to the Rct values. The most pronounced effect and the highest Rct value (229.9 X cm2) was obtained by inhibitor INHC at 200 ppm concentration. The lowest Rct value (44.7 X cm2) was obtained by inhibitor INHS. The high Rct values are generally associated with slower corroding system [9,25,67]. The order of the E% obtained for the Rct values is INHC > INHF > INHB > INHS. It is worth noting from Table 5 that the percentage inhibition efficiencies obtained from impedance measurements are comparable and run parallel with those obtained from weight loss and potentiodynamic polarization measurements. 3.4. Quantum chemical calculations The full optimized minimum energy geometrical configurations of Schiff bases under investigation are shown in Fig. 9. The com-

Fig. 9. Optimized structures of (a) INHB, (b) INHS, (c) INHF and (d) INHC.

941

I. Ahamad et al. / Corrosion Science 52 (2010) 933–942 Table 6 Computed quantum chemical parameters for the Schiff bases with inhibition efficiency (obtained from weight loss method). Inhibitor

EHOMO (eV)

ELUMO (eV)

IP (eV)

EA (eV)

DEH–L (eV)

DEMBG (eV)

DN

E%

INHB INHS INHF INHC

6.511 6.305 6.182 6.149

1.709 1.785 1.827 1.973

8.021 7.849 7.824 7.532

0.326 0.260 0.405 0.683

4.802 4.520 4.355 4.177

2.506 2.531 2.230 1.897

0.378 0.395 0.340 0.435

92.9 90.5 93.8 99.5

lute hardness of iron and the inhibitor molecule, respectively. These quantities are related to electron affinity (EA) and ionization potential (IP) as follows:

v ¼ ðI þ AÞ=2

ð14Þ

g ¼ ðI  AÞ=2

ð15Þ

the molecular band gap (DEMBG) and electron affinity (EA) increase the inhibition efficiency.

Acknowledgement For iron atom, a theoretical v value of 7 eV mol1 and g value of 0 eV mol1 were used [2] to calculate the fraction of electrons transferred, DN, from inhibitor to the iron atom. In literature it has been reported that the values of DN show inhibition effect resulted from electrons donation [2,72]. According to Lukovits’s study [72], if the value of DN < 3.6, the inhibition efficiency increased with increasing electron donating ability of inhibitor at the metal surface. Also it was observed [73] that inhibition efficiency increased with increase in the values of DN. However, our study reveals that there is no regular trend in the inhibition efficiency by increasing values of DN. The data in Table 6 revealed that quantum chemical parameters viz. EHOMO, ELUMO and DEH–L are not in good correlation with experimental inhibition efficiency. However, electron affinity shows good correlation with experimental inhibition efficiency. Decreasing order of EA is INHS > INHB > INHF > INHC. In order to make good correlation with observed inhibition efficiency we calculate molecular band gap (DEMBG) which can be defined as the excitation energy of the lowest electronic state in the molecule. The computed values of DEMBG are given in Table 6 and decreasing order of it is INHS > INHB > INHF > INHC. We observe that smaller value of DEMBG, higher the inhibition efficiency and vice versa. Thus we can conclude that the electron affinity (EA) and molecular band gap (DEMBG) are good quantities to correlate with experimental inhibition efficiencies of inhibitors.

4. Conclusions (1) All the studied Schiff bases showed good inhibition properties for the corrosion of mild steel in 1.0 M HCl solutions, and the inhibition efficiency increased with increasing the concentration of the inhibitors. The inhibiting efficiencies of Schiff bases followed the order INHS < INHB < INHF < INHC. (2) Based on the electrochemical measurements, all the Schiff bases functioned as mixed-type inhibitors. On the basis of properties of EIS Bode diagrams, one equivalent structure model was selected which could fit the experimental data very well. (3) The inhibiting efficiencies determined by Tafel polarization, EIS measurements and weight loss measurements are in good agreement. (4) The adsorption of Schiff bases on the mild steel surface in 1.0 M HCl obeyed the Langmuir adsorption isotherm model. (5) The negative sign of the DG° and DH° indicated that the adsorption of Schiff bases on the mild steel surface in 1.0 M HCl is spontaneous and exothermic. (6) Data obtained from quantum chemical calculations were correlated to the experimentally obtained inhibition efficiencies. It is found for studied Schiff bases that the lower

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