Thermodynamic, chemical and electrochemical investigations of 2-mercapto benzimidazole as corrosion inhibitor for mild steel in hydrochloric acid solutions

3.1 Weight loss tests

Gravimetric measurements of steel were investigated in 1.0 M HCl in the absence and presence of various 2MBI concentrations at 6 h of immersion and at 308 K. The inhibition efficiency (α%) was calculated by the following relation:

Table 1 collects the corrosion rates and Fig. 1 shows the variation of inhibition efficiencies evaluated from weight loss measurements for different inhibitor concentrations in 1.0 M HCl. The corrosion rate decreases with 2MBI concentration and in turn the inhibition efficiency (α%) increases to attain 98%. From weight loss measurements, we can conclude that 2MBI is an excellent inhibitor.

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Figure 1

Variation of inhibition efficiency (α%) with the concentration of 2MBI for mild steel in 1.0 M HCl.

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3.2 Polarisation measurements

The polarisation curves of steel recorded in 1.0 M HCl in the absence and presence of 2MBI at different concentration at 308 K are presented in Fig. 2. The extrapolation method for the polarisation curves was applied and the data for corrosion potential (E_corr), corrosion current density (I_corr), cathodic Tafel slopes (b_c) and percentage inhibition efficiency (η%) are show in Table 2. The relation determines the inhibition efficiency (η%):

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Figure 2

Potentiodynamic curves for mild steel in 1.0 M HCl in the presence of various 2MBI concentrations.

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As it is show in Fig. 2 and Table 2, cathodic current–potential curves give rise to parallel Tafel lines indicating that the hydrogen evolution reaction is under activation controlled. The cathodic current density decreases with the concentration of 2MBI however, a slight effect is observed on the anodic portions. This result indicates that 2MBI is adsorbed on the metal surface on the cathodic sites and hence inhibition occurs.

These results demonstrate that the hydrogen reduction is inhibited and that the inhibition efficiency increases with inhibitor concentration to attain 97 at 10⁻³ M of 2MBI.

We remark that the inhibitor acts on the anodic portion and the anodic current density is reduced (Fig. 2). It seems also that the presence of the inhibitor change slightly the corrosion potential values in no definite direction. These results indicated that 2MBI acts as a mixed-type inhibitor.

3.3 Effect of 2MBI concentration on the corrosion rate of steel at various temperatures

The effect of temperature on the anti-corrosion effectiveness of 2MBI, was studied at various 2MBI concentrations in the temperature domain (313–353 K) after 1 h of immersion. The collected curves in Fig. 3 show the evolution of corrosion rate (W) with 2MBI concentration (C) at different temperatures.

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Figure 3

Variation of corrosion rate with the concentrations of 2MBI for mild steel in 1.0 M HCl at different temperatures.

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Fig. 3 indicates that at a given 2MBI concentration the corrosion rate of steel increased with temperature. The increase is more pronounced at low concentrations. The results also indicate that for a given temperature, the corrosion rate of steel decreased with increasing inhibitor concentration. The values of inhibition efficiency obtained from the weight loss for different inhibitor concentrations and at various temperatures in 1.0 M HCl are given in Table 3 and Fig. 4. It is clear that inhibition efficiency increased with increase in inhibitor concentration. The maximum value of inhibition efficiency (α%) obtained for 10⁻³ M 2MBI is 98% at 308 K.

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Figure 4

Variation of inhibition efficiency (α%) with concentration of 2MBI for mild steel in 1.0 M HCl at different temperatures.

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The inhibition efficiencies decrease slightly with increasing temperature indicating that higher temperature dissolution of steel predominates on adsorption of 2MBI at the metal surface.

3.4 Apparent activation energy (E_a) and pre-exponential factor (A)

In order to obtain more details on the corrosion process, activation kinetic parameters such energy (E_a), enthalpy $(\mathrm{\Delta }{H}_{\text{a}}^{\circ })$ and entropy $(\mathrm{\Delta }{S}_{\text{a}}^{\circ })$ evaluated from the effect of temperature using Arrhenius law (Eq. (3)) and the alternative formulation of Arrhenius equation (Eq. (4)) (Aljourani et al., 2009):

Fig. 5 shows Arrhenius plot of the corrosion rate for mild steel in 1.0 M HCl in the presence of different concentrations of 2MBI. Straight lines are obtained. The activation energies are estimated from the slopes (−E_a/R). Additionally The straight lines obtained from the transition state plot of the corrosion rate for mild steel in 1.0 M HCl in the presence of different concentrations of 2MBI (Fig. 6) permit the calculation of $\mathrm{\Delta }{H}_{\text{a}}^{\circ }$ and $\mathrm{\Delta }{S}_{\text{a}}^{\circ }$ , respectively, from slopes $(-\mathrm{\Delta }{H}_{\text{a}}^{\circ }/R)$ and intercepts $(\ln R/\mathit{Nh}+\mathrm{\Delta }{S}_{\text{a}}^{\circ }/R)$ . E_a, $\mathrm{\Delta }{H}_{\text{a}}^{\circ }$ and $\mathrm{\Delta }{S}_{\text{a}}^{\circ }$ are collected in Table 4.

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Figure 5

Arrhenius plot of the corrosion rate for mild steel in 1.0 M HCl in the presence of different concentrations of 2MBI.

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Figure 6

Transition state plot of the corrosion rate for mild steel in 1.0 M HCl in the presence of different concentrations of 2MBI.

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Previous studies (Aljourani et al., 2009; Elachouri et al., 1996; Ferreira et al., 2004) showed that compared with the activation energy in the absence of inhibitor, higher values for E_a were found in the presence of inhibitor. Other studies (Banerjee and Misra, 1989; Ferreira et al., 2004) showed that in the presence of 2MBI, the activation energy was lower than that in its absence.

It was clear that (see Table 4) the values of E_a in the presence of the 2MBI are higher than that in the uninhibited acid solution (59.39 kJ mol⁻¹).

The decrease of inhibition efficiencies with increasing temperature and the increase of E_a in the presence of the inhibitor indicate the physical adsorption mechanism (Popova et al., 2003).

Fig. 7 indicates that the activation energy increases with the concentration of 2MBI. In the same way, the pre-exponential factor increases with the concentration of 2MBI (as shown in Table 4). The pre-exponential factor and activation energy varies in the same way. From Eq. (3), it can be seen that at a certain temperature, the value of the steel corrosion rate is jointly decided by the apparent activation energy and pre-exponential factor. As a whole, the steel corrosion rate basically decreases with an increase in concentration of 2MBI. As can be seen from Table 4, it was clear that both E_a and A increased in the presence of 2MBI (Mu et al., 2004).

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Figure 7

Variation of activation parameters E_a and $\mathrm{\Delta }{H}_{\text{a}}^{\circ }$ with 2MBI concentration for mild steel in 1.0 M HCl.

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The variation of activation parameters E_a and $\mathrm{\Delta }{H}_{\text{a}}^{\circ }$ with 2MBI concentration is shown in Fig. 7. From the data obtained in Table 4, it seems that E_a increases with the inhibitor concentration. The positive sign of the enthalpy ( $\mathrm{\Delta }{H}_{\text{a}}^{\circ }$ ) reflects the endothermic nature of the steel dissolution process (Table 4). We remark that E_a and $\mathrm{\Delta }{S}_{\text{a}}^{\circ }$ values vary in the same way with the inhibitor concentration. This result permit to verify the known thermodynamic relation (Gomma and Wahdan, 1995) between E_a and $\mathrm{\Delta }{S}_{\text{a}}^{\circ }$ as shown also in Table 4:

The positive values of entropies ( $\mathrm{\Delta }{S}_{\text{a}}^{\circ }$ ) in the presence of inhibitor imply that the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that an increase in disordering takes place on going from reactants to the activated complex (Elachouri et al., 1996).

3.5 Adsorption isotherm and thermodynamic parameters

Assuming the corrosion inhibition was caused by the adsorption of 2MBI, and the values of surface coverage (θ) for different concentrations of 2MBI in 1 M HCl were evaluated from weight loss measurements using the Sekine and Hirakawa’s method (Sekine and Hirakawa, 1986):

In order to get a better understanding of the electrochemical process on the metal surface, adsorption characteristics are also studied for 2MBI. This process is closely related to the adsorption of the inhibitor molecules (Hackerman and Sudbury, 1950; Hackerman, 1962) and adsorption is known to depend on the chemical structure (Cheng et al., 1999; Ateya et al., 1984; Akiyama and Nobe, 1970). Adsorption isotherms are very important in determining the mechanism of organic electrochemical reactions. The most frequently used adsorption isotherms are Langmuir, Temkin and Frumkin.

In hydrochloric acid solution, the organic compound follows the Langmuir adsorption isotherm. This is as follows:

From the values of surface coverage, the linear regressions between C/θ and C are calculated and the parameters (adsorption coefficients, slopes, and linear regression coefficients) are listed in Table 5. Fig. 8 shows the relationship between C/θ and C at various temperatures. These results show that the linear regression coefficients (r) and the slopes are almost equal 1.000, indicating that the adsorption of inhibitor onto steel surface agrees the Langmuir adsorption isotherm.

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Figure 8

Curve fitting of the corrosion data for mild steel in 1.0 M HCl in the presence of different concentrations of 2MBI to the Langmuir isotherm at different temperatures.

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In addition, the equilibrium constant of the adsorption process (K) decreases with increasing temperature (Table 5). It is well known that K designates the adsorption power of inhibitor onto the steel surface; clearly, 2MBI gives higher values of K at lower temperatures, indicating that it was adsorbed strongly onto the steel surface. Thus, the inhibition efficiency decreased slightly with the increase in temperature as the result of the improvement for the desorption of 2MBI from the steel surface.

The corrosion inhibition of 2MBI for steel may be well explained by using thermodynamic model, so, the heat, the free energy and the entropy of adsorption are calculated to elucidate the phenomenon for the inhibition action of 2MBI.

According to the Van’t Hoff equation (Tang et al., 2003; Zhao and Mu, 1999):

To obtain the adsorption heat, the linear regression between lnK and 1/T is dealt with, the relationship between lnK and 1/T is shown in Fig. 9. Under the experimental conditions, the adsorption heat could be approximately regarded as the standard adsorption heat ( $\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$ ) (Mu et al., 2004; Zhao and Mu, 1999). The obtained value of $\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$ is −36.26 kJ mol⁻¹. The standard adsorption free energy ( $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ ) is obtained according to the following equation (Khamis, 1990):

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Figure 9

Variation of ln(K) with 1/T for mild steel in 1.0 M HCl in the presence of 2MBI.

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The negative values of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ (Table 5) ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the steel surface (Ali et al., 2005). Furthermore, it is found that $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ slightly increases with temperature.

The negative values of $\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$ also show that the adsorption of inhibitor is an exothermic process (Gomma and Wahdan, 1995). Generally, an exothermic process signifies either physi- or chemisorption while endothermic process is attributable unequivocally to chemisorption (Durnie et al., 1999). In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of a physisorption process is lower than 40 kJ mol⁻¹ while the adsorption heat of a chemisorption process approaches 100 kJ mol⁻¹ (Martinez and Stern, 2002). In the present case; the standard adsorption heat −36.26 kJ mol⁻¹ shows that a comprehensive adsorption (physical adsorption) might occur (Tang et al., 2003). The same results were obtained in previous studies (Trachli et al., 2002; Li and Mu, 2005). The adsorption of inhibitor molecules is accompanied by positive values of $\mathrm{\Delta }{S}_{\text{ads}}^{\circ }$ .

$\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$  = −36.26 kJ mol⁻¹ found by the Van’t Hoff equation, may be also evaluated by the Gibbs–Helmholtz equation, which is defined as follows:

The variation of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ /T with 1/T gives a straight line with a slope that equals $\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$ (Fig. 10). It can be seen from Fig. 10 that $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ /T decreases slightly with 1/T in a linear fashion. The values of $\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$ is negative ( $\mathrm{\Delta }{H}_{\text{ads}}^{\circ }$  = −36.26 kJ mol⁻¹), reflecting the exothermic behaviour of adsorption on the steel surface.

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Figure 10

Variation of $\mathrm{\Delta }{G}_{\text{ads}}^{\circ }$ /T with 1/T for mild steel in 1.0 M HCl in the presence of 2MBI.

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The value of the enthalpy of adsorption found by the two methods such as Van’t Hoff and Gibbs–Helmholtz relations are in good agreement.

3.6 Electrochemical impedance spectroscopy (EIS)

Fig. 11 shows a typical set of Nyquist plots for mild steel in 1.0 M HCl in the absence and presence of various concentrations of 2-mercapto benzimidazole. It is clear from these plots that the impedance response of mild steel has significantly changed after the addition of 2MBI in 1.0 M HCl. The impedance parameters derived from these plots using equivalent circuit described elsewhere (Khaled, 2003) are given in Table 6.

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Figure 11

Nyquist diagrams for steel in 1.0 M HCl in the presence of different of 2MBI concentrations.

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The charge transfer resistance, R_t values are calculated from the difference in impedance at lower and higher frequencies, as suggested by Tsuru and Haruyama (Mansfeld et al., 1981a,b; Tsuru et al., 1978). To obtain the double capacitance (C_dl), the frequency at which the imaginary component of the impedance is maximum (−Z_max) is found and C_dl values are obtained from the equation:

Table 6 indicates that by increasing the concentration of 2MBI, the C_dl values tended to decrease and the inhibition efficiency improved. This decrease in the C_dl, which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, signifying that 2MBI molecules act by adsorption at the metal/solution interface (McCafferty and Hackerman, 1972).

The double layer capacitance C_dl is expressed in the Helmotz model by:

The decrease in C_dl values may be interpreted either by a decrease of local dielectric constant ɛ (McCafferty and Hackerman, 1972) or by the thickness of the adsorbate layer of inhibitor at the metal surface (Bastidas et al., 2000).

The data obtained from EIS are in good harmony with that obtained from potentiodynamic polarisation. The impedance spectra obtained on mild steel in 1.0 M HCl solutions consist of one depressed capacitive loop (one time constant in the Bode-phase representation, not shown here). When Nyquist plot contains a ‘‘depressed semicircle with the center under the real axis’’, such behaviour is characteristic for solid electrodes and often referred to as frequency dispersion that has been attributed to roughness and other inhomogeneities of the solid surface (Khaled, 2003).

Deviation of this kind often referred to as frequency dispersion. Therefore, a constant phase element (CPE) instead of a capacitive element is used in Fig. 2 to get a more accurate fit of experimental data sets using generally more complicated equivalent circuits. The impedance, Z, of CPE has the form (Khaled, 2003):