Thermodynamic study of metal corrosion and inhibitor adsorption

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Materials Chemistry and Physics 110 (2008) 145–154

Thermodynamic study of metal corrosion and inhibitor adsorption processes in mild steel/1-methyl-4[4(-X)-styryl pyridinium iodides/hydrochloric acid systems Ehteram A. Noor ∗ , Aisha H. Al-Moubaraki King Abd El-Aziz University, Girls’ College of Education, Department of Chemistry, Jeddah-9470, Saudi Arabia Received 25 August 2007; received in revised form 20 January 2008; accepted 21 January 2008

Abstract The corrosion and inhibitor adsorption processes in mild steel/1-methyl-4[4 (-X)-styryl] pyridinium iodides (X: H, CH3 and OCH3 )/hydrochloric acid systems was studied at different temperatures (25–60 ◦ C) by means of hydrogen evolution (HE) and weight loss (WL) measurements. It was found that the studied compounds exhibit a very good performance as inhibitors for mild steel corrosion in 1.5 M HCl. Results show that the inhibition efficiency increases with decreasing temperature and increasing concentration of inhibitors. Good agreement between the results obtained from hydrogen evolution and weight loss measurements was appeared. It has been determined that the adsorption for the studied inhibitors on mild steel complies with the Langmuir adsorption isotherm at all studied temperatures. The kinetic and thermodynamic parameters for mild steel corrosion and inhibitor adsorption, respectively, were determined and discussed. On the bases of thermodynamic adsorption parameters, comprehensive adsorption (physisorption and chemisorption) for the studied inhibitors on mild steel surface was suggested. A good correlation between the substituent type and the inhibition efficiency of inhibitors through the application of Hammet relationship was obtained. Results show that with increasing the donor property of the substituent, the inhibition efficiency of the inhibitor is increased in the order: I-H < II-CH3 < III-OCH3 . © 2008 Elsevier B.V. All rights reserved. Keywords: Mild steel; Corrosion; Thermodynamic; Inhibitor; Adsorption; Hydrochloric

1. Introduction To remove undesirable scales (e.g., mill scale, rust) the particular piece of metal is immersed in a suitable acid solution, called an acid pickling bath. The acid solution attacks and dissolves the scale. Once the scale is dissolved, the acid is then free for further attack upon the metal. In order to reduce the metal dissolution and the consumption of acid, corrosion inhibitors are added to the pickling solution. As known, Hydrochloric acid (HCl) is the most important pickling acid. It is used with concentrations from 5 to 15 mass % at temperature up to 80 ◦ C [1]. These required inhibitors that remain effective even under such severe conditions. Accordingly, the method of acid corrosion inhibition must be evaluated according to the parameters of the particular corrosion system. The selection of appropriate inhibitors mainly depends on the type of acid, its concentration, temperature, the ∗

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presence of dissolved inorganic and/or organic substances even in minor amounts and, of course, on the type of metallic material exposed to the action of acidic solution [1]. Generally speaking, corrosion inhibitors are found to protect steel corrosion in acid solutions by adsorbing themselves on steel surface. Adsorption is a separation process involving two phases between which certain components can become differentially distributed. Adsorption can be described by two main types of interaction [2]: • Physisorption, involves electrostatic forces between ionic charges or dipoles on the adsorbed species and the electric charge at the metal/solution interface. The heat of adsorption is low and therefore this type of adsorption is stable only at relatively low temperatures. • Chemisorption, involves charge sharing or charge transfer from the inhibitor molecules to the metal surface to form a coordinate type bond. In fact, electron transfer is typically for transition metals having vacant low-energy electron

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E.A. Noor, A.H. Al-Moubaraki / Materials Chemistry and Physics 110 (2008) 145–154

Table 1 Names, structures and abbreviations of the studied inhibitors Inhibitor

Structure

Abbreviation

1-Methyl-4[4 (-H)-styryl] pyridinium iodide

I-H

1-Methyl-4[4 (-CH3 )-styryl] pyridinium iodide

II-CH3

1-Methyl-4[4 (-OCH3 )-styryl] pyridinium iodide

III-OCH3

orbital. Chemisorption is typified by a much stronger adsorption energy than physical adsorption. Such a bond is therefore more stable at higher temperatures. Thermodynamic adsorption parameters and kinetic corrosion parameters are a useful tool for clarifying the adsorption behaviour of an inhibitor. Earlier work for Riggs and Hurd [3] revealed that from a comparison of activation energies of uninhibited and inhibited corrosion reaction, heats of inhibitor adsorption may obtained. However, It was found that while positive heat of adsorption, Hads. > 0 (endothermic process) is attributed unequivocally to chemisorption [4,5], a negative heat of adsorption, Hads. < 0 (exothermic process) may involve either physisorption [6] or chemisorption [7,8] or mixture of both processes (comprehensive adsorption) [9–11]. It is well known that the effect of temperature on the inhibited acid–metal reaction is highly complex, because many change occur on the metal surface such as rapid etching and desorption of inhibitor and the inhibitor itself may undergo decomposition and/or rearrangement. It was found that few inhibitors with acid–metal systems have specific reactions which are effective at high temperature as (or more) they are at low temperature [12–14]. The object of the present work is to evaluate the corrosion kinetic parameters of mild steel and the adsorption thermodynamic parameters of three selected inhibitors (Table 1) in mild steel/inhibitor/1.5 M HCl system. The choice of these compounds is based on molecular structure considerations, i.e., these are organic compounds having the same adsorption centers but they only differ in the substituent type at the para position of phenyl group and hence if a difference in the protective properties is observed, it should be predominately attributed to a difference in the electronic effect of the substituent type. Two chemical methods were employed to carry out the measurements, these are hydrogen evolution (HE) and weight loss (WL) methods.

2. Experimental details 2.1. Materials preparation 1-Methyl-4[4 (-X)-styryl] pyridinium iodides (Table 1) were synthesized as previously reported in the literature [15]. The purity of the studied compounds was checked by chemical analysis, melting point determination and spectral data. Prior all measurements, the steel samples (P: 0.02%, Mn: 0.370%, S: 0.030%, Mo: 0.010%, Ni: 0.039%, C: 0.210%, Si: 0.170 and the remainder is iron) are abraded with a series of emery papers from 400 to 1200 grade. The specimens are washed thoroughly with bidistilled water, degreased with acetone and dried with air. The aggressive solution (1.5 M HCl) is prepared by dilution of analytical grade 37% HCl with bidistilled water in absence and presence of the studied inhibitors in the concentration range from 5 × 10−5 to 5 × 10−4 M.

2.2. Methods 2.2.1. Hydrogen evolution measurements (HE) 50 ml of tested solution were placed in flask of Mylius type [16] and a degreased and weighed mild steel sample was introduced into the solution. The time was recorded and H2 evolved was collected in the calibrated tube by the downward displacement of water over time interval of 90 min. A plot of H2 evolved per unit area against time in absence and presence of different concentrations of the studied inhibitors produced a straight lines. The slope of this lines gave the rate of H2 evolved (ρHE , ml cm−2 min−1 ). Inhibition efficiency is calculated using the following equation:

IEHE (%) =

o ρHE − ρHE o ρHE

× 100

(1)

o and ρHE are the corrosion rates in absence and presence of a certain where ρHE concentration of inhibitor, respectively.

2.3. Weight loss measurements (WL) At the end of each experiment of HE, the sample was withdrawn from the tested solution, washed thoroughly with bidistilled water followed by acetone and dried with air, then weighed again. The rate of mass loss was calculated (ρWL , g cm−2 min−1 ) as follows: ρWL =

Wb − Wa S · t∞

(2)


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Fig. 1. Dependence of mild steel corrosion rate (ρHE and ρWL ) on the concentration of I-H in 1.5 M HCl at different temperatures. where Wb and Wa are the specimen weight before and after immersion in the tested solution, respectively, S is the surface area of the specimen and t∞ is the end time of each experiment. The IEWL (%) values can be calculated from WL data by using equation similar to Eq. (1):

IEWL (%) =

o ρWL − ρWL o ρWL

× 100

(3)

All the experiments were conducted in aerated, stagnant solutions at temperatures in the range 25–60 ◦ C.

3. Results and discussion 3.1. Effect of concentration of the inhibitor on corrosion rate of mild steel Fig. 1 represents the corrosion rate (ρHE and ρWL ) as a function of I-H concentration at different temperatures. Similar curves were obtained for the two other inhibitors and not shown.

Table 2 Inhibition efficiencies for different concentrations of I-H, II-CH3 and III-OCH3 at different temperatures Inhibitor concentration (Cinh. , M)

IE(%) I-H

II-CH3

III-OCH3

HE

WL

HE

WL

HE

WL

5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

72.89 92.72 96.16 97.51

72.5 92.11 96.02 96.44

79.36 94.84 97.47 98.23

78.49 94.37 96.31 96.55

89.4 95.88 98.23 –

86.23 95.45 97.99 99.04

30◦ 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

70.67 89.88 94.01 95.35

69.54 88.11 93.97 95.58

75.19 94.01 95.3 96.23

74.61 93.81 95.64 96.46

82.91 93.31 97.82 –

82.71 94.79 97.4 98.56

40◦ 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

60.97 79.05 88.42 94.48

59.22 79.18 89.64 93.03

64.8 87.78 94.67 95.4

63.68 88.03 93.79 94.72

74.19 91.43 94.94 96.65

73.13 90.52 96 97.8

50◦ 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

41.85 65.62 82.49 88.16

40.56 65.4 82.69 86.56

43.44 76.42 87.96 91.51

43.98 75.49 87.46 89.81

52.76 77.16 89.3 94.09

54.7 78.16 91.05 95.01

60◦ 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

33.42 57.47 73.07 84.14

32.07 56.32 75.9 82.13

37.11 64.26 82.11 87.41

35.15 62.08 80.85 85.48

41.76 66.47 84.25 92.03

42.16 66.39 84.49 92.89

25◦

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Table 3 IE (25–60◦ ) % values for I-H, II-CH3 and III-OCH3 at different concentrations Ihibitor concentration (Cinh., M)

IE (25–60◦ ) (%) I-H

5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

II-CH3

III-OCH3

HE

WL

HE

WL

HE

WL

39.47 35.25 23.09 13.37

40.43 35.79 20.12 14.31

42.25 30.58 15.36 10.82

43.34 32.29 15.46 11.07

47.64 29.41 13.98 –

44.07 29.06 13.5 6.15

It is clear that at certain experimental temperature, corrosion rate of mild steel decreases with an increase in concentration of inhibitor, and at concentrations above 1.0 × 10−4 M, the value of corrosion rate does not change basically with increasing concentration except that at 60 ◦ C. In absence and presence of a certain concentration of inhibitor, the corrosion rate of mild steel increases with rise in temperature, obeying the Arrhenius type reactions. It was reported that the rate for iron corroding in acid solutions is approximately doubles for every 10 ◦ C rise in temperature [17].

3.3. Corrosion kinetic parameters Arrhenius suggested the famous equation which evaluate the temperature dependence of the rate constant as follows [18]: log ρ = log A −

∗ Eapp.

2.303RT

(5)

∗ is the apparent activaHere, A is the frequency factor and Eapp. tion energy, R is the gas constant (R = 8.314 J mol−1 K−1 ) and T is the absolute temperature. Eq. (5) predicts that a plot of

3.2. Effect of concentration of inhibitor on the inhibition efficiency Inhibition efficiencies calculated for mild steel in 1.5 M HCl from HE and WL data in the presence of different concentrations of the studied inhibitors at each experimental temperature are listed in Table 2. Inspection of the data in this table reveal that good consistency between the results obtained from HE and WL measurements. As observed, the inhibition efficiency increases with increasing the concentration of each inhibitor, while it decreases with an increase in temperature. Such behaviour can be interpreted on the basis that the inhibitors exert their action by adsorbing themselves on the metal surface and an increase in temperature resulted in the desorption of some adsorbed inhibitor molecules leading to a decrease in the inhibition efficiency. However, at certain temperature and certain concentration, the inhibitive action of the studied inhibitors can be given in the following increasing order: I-H < II-CH3 < III-OCH3 To evaluate the stability of the studied inhibitors over the experimental range of temperature, IE (%) values for different concentrations of the studied inhibitors are calculated as follows: IE(%) = IE(25◦ ) − IE(60◦ )

(4)

IE (%) value represents the overall decay in the IE% with increasing temperature from 25◦ to 60 ◦ C. All estimated IE (%) values were listed in Table 3. The data in this table showed that at concentrations more than 1.0 × 10−4 M, the investigated inhibitors become more stable with increasing temperature and under these conditions, inhibitor III-OCH3 is the most efficient inhibitor.

Fig. 2. (a) Arrhenius and (b) transition state plots for mild steel corrosion rates (ρWL ) in 1.5 M HCl in absence and presence of different concentrations of I-H.


149

Table 4 Corrosion kinetic parameters for mild steel in 1.5 M HCl in absence and presence of different concentrations of the investigated inhibitors Inhibitor concentration (Cinh. , M)

A (g cm−2 min−1 )

∗ Eapp. (kJ mol−1 )

H* (kJ mol−1 )

S* (J mol−1 K−1 )

I-H 0.0 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

3.93 × 103 8.81 × 106 4.17 × 109 43.1 × 1010 4.75 × 109

42.43 64.74 82.81 84.17 81.64

40.13 62.44 80.13 81.38 79.35

−183.96 −119.8 −69.75 −71.3 −80.17

II-CH3 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

4.68 × 107 3.82 × 1010 1.29 × 109 2.70 × 108

69.45 89.63 82.28 78.68

67.15 87.14 79.59 76

−105.92 −50.84 −79.52 −92.52

III-OCH3 5.0 × 10−5 1.0 × 10−4 2.5 × 10−4 5.0 × 10−4

6.24 × 108 7.66 × 1010 2.22 × 1010 7.87 × 109

76.95 91.93 90.78 89.8

74.65 89.21 88.48 87.12

−84.36 −45.57 −54.68 −64.45

log ρ vs. 1/T should be a straight line. The slope of the line ∗ /2.303R) and the intercept of the line extrapolated is (−Eapp. (1/T = 0) gives log A. Note that ρ and A must have the same units. On the other hand, the change of enthalpy (H* ) and entropy (S* ) of activation for the formation of the activation complex in the transition state can be obtained from the transition-state equation [18]: ρ R S ∗ H ∗ log = log + − (6) T hN 2.303R 2.303RT where h is Planck’s constant and N is Avogadro’s number. A plot of log ρ/T vs. 1/T gives straight line. The slope is (−H* /2.303R) and the intercept is [(log(R/hN)) + (S* /2.303R)], from which the values of H* and S* are calculated, respectively. By using the experimental corrosion rate values obtained from weight loss measurements (ρWL ) for mild steel in 1.5 M HCl in absence and presence of different concentrations of inhibitor I-H, plots in accordance with Eqs. (5) and (6) are obtained and illustrated in Fig. 2. Similar plots for the two other inhibitors were obtained and not shown. All the corrosion kinetic ∗ , H* and S* ) are estimated and listed in parameters (Eapp. Table 4 and the following notes can be written: ∗ • The value of Eapp. for mild steel corrosion in uninhibited 1.5 M HCl solution (42.43 kJ mol−1 ) is in the same order of magnitude of some literature data for steel in some acid solutions [19–21]. ∗ • In general, Eapp. values for the inhibited solutions (in the studied range of inhibitor concentration) are higher than that for the uninhibited one, indicating a strong inhibitive action for the studied compounds by increasing the energy barrier for the corrosion process, emphasizing the electrostatic character of the inhibitor’s adsorption on mild steel surface. ∗ • Eapp. value for the inhibited systems increases with inhibitor concentration until it reaches a maximum value at concentration of 2.5 × 10−4 M for I-H and at concentration of

1.0 × 10−4 M for II-CH3 and III-OCH3 after which, lim∗ ited decrease in Eapp. value is observed (Fig. 3) with further ∗ increase in inhibitor concentration. The decrease in the Eapp. value at higher level of inhibitor efficiency was reported in the literature [9,10,22]. According to Riggs and Hurd [3] the decrease in apparent activation energy at higher level of inhibition arise from a shift of the net corrosion reaction from that on the uncovered surface to one involving the adsorbed sites directly. In fact one has to take into account the effect of frequency factor (A). Some researches [7–10] have shown that for most corrosion reactions, the tendency of variation in frequency factor is similar to that in apparent activation energy; for the studied systems, similar phenomenon has been observed (Table 4). ∗ ), ¯ app. • The average value of apparent activation energies (E for I-H, II-CH3 and III-OCH3 are: 78.34, 79.25 and 80.03 kJ mol−1 , respectively. According to these values one

Fig. 3. Dependence of apparent activation energy on the concentration of the investigated inhibitors.

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can give the inhibitive character of the studied inhibitors in the following order: I-H < II-CH3 < III-OCH3 ∗ • As observed, for all cases Eapp. > H ∗ by a value which approximately equal to RT. From the thermodynamic and kinetic point of view, the unimolecular reactions is characterized by following equation [23]: ∗ Eapp. − H ∗ = RT

(7)

Hence, mild steel sample corrodes in 1.5 M HCl solutions either in absence or presence of different concentrations of the studied inhibitors by a unimolecular reaction. • The negative values for S* in the inhibited and uninhibited systems implies that the activation complex in the rate determining step represents association rather than dissociation step, meaning that a decrease in disorder takes place on going from reactant to the activated complex [24]. 3.4. Thermodynamic adsorption parameters Adsorption isotherms provide information about the interaction among the adsorbed molecules themselves and also their

interactions with the electrode surface. For the studied inhibitors, it was found that the experimental data obtained from WL measurements fitted Langmuir’s adsorption isotherm, which is given by: Cinh. Θ−1 =

1 + Cinh. K

(8)

where Θ (Θ = IE %/100) is the surface coverage and K is the equilibrium constant of the adsorption process. This model for Langmuir adsorption isotherm has been used extensively in the literature for various metal/inhibitor/acid solution systems [5,8,10,11,25–29]. Fig. 4 represents the Langmuir adsorption isotherm for the studied inhibitors at different temperatures with slopes of unity. From the intercept of the straight lines, K values can be obtained and related to the free energy of adsorption, Gads. by: K = −log CH2 O − (Gads. /2.303RT )

(9)

where CH2 O is the molar concentration of water molecules at the electrode/electrolyte interface. Thermodynamically, Gads. is related to the enthalpy and entropy of adsorption process, Hads. and Sads. , respectively,

Fig. 4. Lngmuir adsorption isotherm for the investigated inhibitors at different temperatures.


Fig. 5. Dependence of log K on 1/T for mild steel/inhibitor/1.5 M HCl systems.

Fig. 6. Dependence of IEWL (%) of the investigated inhibitors on Gads. at different temperatures.

by the famous equation: Gads. = Hads. − TSads.

(10)

From both Eqs. (9) and (10) the following equation can be written [30]: Hads. Sads. log K = −log CH2 O + − (11) 2.303R 2.303RT

•

•

A plot of log K vs. 1/T gives a straight lines as shown in Fig. 5. The slope of these lines is −Hads. /2.303R and the intercept is (− log CH2 O + Sads. /2.303R), from which the values of Hads. and Sads. were calculated, respectively. All estimated values of K, Gads. , Hads. and Sads. were recorded in Table 5 and interpreted as follows: • The high values for equilibrium constant of adsorption are good indicative for a strong adsorption of the studied inhibitors on mild steel surface. • The calculated values of Gads. are negative which indicate that the adsorption of inhibitor’s molecules on the metal surface is a spontaneous process. This behaviour can be seen obviously in Fig. 6, showing the relation between the average value of the inhibition efficiency, IEWL (%), for the studied inhibitors and their Gads. values at different temperatures. As observed, the studied inhibitors obey the general rule that the effectiveness of corrosion inhibition increases with increasing the negative value of Gads. . • Generally, values of Gads. around −20 kJ mol−1 or lower are consistent with the electrostatic interaction between charged molecules and the charged metal surface (physisorption); those around −40 kJ mol−1 or higher involve charge sharing or transfer from organic molecules to the metal surface to form a coordinate type of metal bond (chemisorption) [5]. In the present work, the calculated Gads. values are almost slightly less negative than −40 kJ mol−1 indicating that the adsorption of inhibitor molecules is not merely physisorption

151

•

•

or chemisorption but obeying a comprehensive adsorption (physical and chemical adsorption). It was observed, limited decrease in the absolute value of Gads. with an increase in temperatures, indicating that the adsorption was somewhat unfavourable with increasing experimental temperature, indicating that physisorption has the major contribution while chemisorption has the minor contribution in the adsorption mechanism. The Hads. values are negative for all studied inhibitors, suggests that the adsorption of inhibitor’s molecules onto metal surface is an exothermic process. In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of Hads. for the physisorption process which is lower than 40 kJ mol−1 while that for chemisorption process approaches 100 kJ mol−1 [11,31]. In the present case, Hads. values are larger than the common physical adsorption heat, but smaller than the common chemical adsorption heat, once again emphasizing that both physical and chemical adsorption take place. The same results were obtained by other authors [9–11]. All values of Sads. are negative. This behaviour might be explained in the following way: - Before the adsorption of inhibitor’s molecules onto the steel surface, inhibitor molecules might freely move in the bulk solution (inhibitor molecules were chaotic), but with the progress in the adsorption, inhibitor molecules were orderly adsorbed onto the steel surface, as a result a decrease in entropy is observed [27]. - From the thermodynamic principles, since the adsorption is an exothermic process, it must be accompanied by a decrease of entropy [32]. According to the increase in the absolute value for Gads. , Hads. and Sads. at each temperature, the inhibitive action of the studied inhibitors can be written in the following order: I-H < II-CH3 < III-OCH3

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Table 5 Thermodynamic adsorption parameters for the investigated inhibitors in mild steel/inhibitor/1.5 M HCl systems at different temperatures Temperature (◦ C)

K × 10−5 (M−1 )

Gads. (kJ mol−1 )

I-H 25◦ 30◦ 40◦ 50◦ 60◦

0.811 0.61 0.349 0.172 0.116

−37.96 −37.87 −37.66 −36.96 −36.99

II-CH3 25◦ 30◦ 40◦ 50◦ 60◦

1.23 0.942 0.525 0.224 0.138

−38.99 −38.97 −38.72 −37.67 −37.47

III-OCH3 25◦ 30◦ 40◦ 50◦ 60◦

1.523 1.22 0.675 0.283 0.159

−39.52 −39.62 −39.38 −38.29 −37.86

3.5. Inhibitor structure and adsorption mechanism The studied inhibitors are organic compounds in the salt form in which the cation being the organic part while the anion being the inorganic part (I− ion). The organic cations have the same adsorption centers, are: • the quaternary N-pyridinium head group and, • the de-localized ␲-electrons of the phenyl ring and the styrene bridge. They differ only in the substituent type in the para position of the phenyl group. Acid corrosion inhibition by such organic cations is strongly influenced by the nature and charge of the metal surface as follows: (i) If the charge of steel surface is negative with respect to the position of the potential of zero charge (PZC), it could be expected that the organic cation would be directly adsorbed on the metal surface. (ii) If the charge of the steel surface is positive with respect to the PZC, it can be assumed that acid anions (Cl− ions in the present work) are specifically adsorbed on the metal surface, creating an excess negative charge towards the solution and favours the adorption of cations. Several studies [14,33–34] treated the steel surface in acid solutions as in case (ii). Anyway, in both cases (i and ii) the net charge on steel surface is negative. The obtained thermodynamic adsorption parameters suggested that the studied organic cations are adsorbed on steel surface in 1.5 M HCl solution by both physisorption and chemisorption. Accordingly, if the studied organic cations is treated as dipole species, the N-pyridinium head group is the

Hads. (kJ mol−1 )

Sads. (J mol−1 K−1 )

−46.47

−28.61 −28.62 −28.66 −28.68 −28.74

−52.65

−45.59 −45.61 −45.65 −45.67 −45.72

−54.51

−49.67 −49.69 −49.73 −49.74 −49.8

positive pole while the de-localized ␲-electrons of the phenyl ring is the negative one, the adsorption of such dipoles species should be stabilized by the participation of physical and chemical adsorption as follows: (1) physisorption: electrostatic interaction between the positively charged N-pyridinium head group and the negatively charged mild steel surface (cathodic area). (2) chemisoption: interaction between the de-localized ␲electrons of the substituted phenyl ring and the vacant, low-energy d-orbitals of Fe surface atoms (anodic area). The former adsorption process (1-physisorption) is enhanced by the co-adsorption of halide ions [35] while the latter (2chemisoption) is run parallel to “the electron-donor property” which was considered to be influenced by the type of the substituent and by the electron availability [36]. Moreover, free rotational behaviour for the phenyl group through the styrene bridge may facilitate the chemisorption process by giving rise to the suitable orientation towards the metal. In previous work for the authors [37], potentiodynamic polarisation measurements for mild steel corrosion in 1.5 M HCl solution in the presence of the investigated inhibitors were obtained at 25 ◦ C, showing the mixed type character for these inhibitors. This result emphasizes the suggested adsorption model. However, this adsorption model was suggested in recent work when similar compounds were used as effective inhibitors for the corrosion of the same steel sample in 2.0 M H3 PO4 solution [21]. 3.6. Effect of substituent type Linear free energy relationships (LFER) have previously been used to correlate the inhibition efficiency of organic compounds with the Hammett substituent constant [38,39]. The


153

electron donor property of the substituent, the adsorption of the inhibitor molecules becomes stronger and more stable, giving the following order: I-H < II-CH3 < III-OCH3 This result is in good agreement with that obtained by other authors [36,39,41,42]. 4. Conclusion The following results can be drawn from this study.

¯ s /K ¯ H ) and Hammett constant for the studied Fig. 7. The LFER between log(K inhibitors in mild steel/inhibitor/1.5 M HCl system.

LFER or Hammett relation is given by [40]: ¯s K log = βσ ¯H K

(12)

¯ H are the average value for ¯ s and K where in the present work, K the equilibrium adsorption constants for the studied inhibitors at different temperatures with and without the substituent, respectively, β is the reaction constant, its magnitude indicates the relative sensitivity of the adsorption process to electron effects and σ is a relative measure of electron density at the reaction center. For those subtituents which attract electrons from the reaction center, the σ value is assigned positive, while for those which have electron-donor property, the σ value is assigned negative. The σ values for the investigated substituents are: 0.00, −0.14 and −0.27. Fig. 7 represents Hammett relation¯ s /K ¯ H ) and σ for the studied inhibitors. As ship between log(K observed from this figure, a straight line of slope β = −1.01 was obtained. The value of β indicates that the adsorption process depends on the donor property of the substituent. Moreover, ∗ ¯ app. Fig. 8 gives good correlation between both E and Hads. and the Hammett constant, emphasizing that with increasing the

• The inhibition efficiency of the studied inhibitors increases with decreasing in temperature and increasing of inhibitor concentration. Good agreement between the results obtained from hydrogen evolution and weight loss measurements is appeared. • The adsorption of the studied inhibitors obeys the Langmuir adsorption isotherm at all investigated temperatures. • At certain concentration and all temperatures the inhibitive action of the studied inhibitors can be given in the following increasing order: I-H < II-CH3 < III-OCH3 ∗ • Eapp. value for all inhibited systems is higher than that of the uninhibited system and it is concentration-dependent. • Thermodynamic adsorption parameters (Hads. , Sads. and Gads. ) show that the studied inhibitors are adsorbed on mild steel surface by an exothermic, spontaneous process. • According to Hads. value for the studied inhibitors, comprehensive (physisorption and chemisorption) adsorption is suggested to occur on mild steel surface. • Hammet relationship shows that the performance of the studied inhibitors depends on the electron donating properties of the substituent and is given in the order:

I-H < II-CH3 < III-OCH3

References

∗ Fig. 8. Correlation between Hemmett’s constant and both of Eapp. and Hads. for the investigated inhibitors.

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