Inhibitor synthesis

Benzimidazole and its derivatives are an important class of bioactive molecules. We have redone the reaction of Azzaoui and al. ¹⁸, who have obtained the isoxazolyl-methylbenzimidazole1 from the action of hydroxylamine hydrochloride on 4Z-(2-oxopropylidene)-2,3,4,5-tetrahydro-1 H-1,5-benzodiazepin-2-one1 (Scheme 1).

To a solution of 4Z-(2-oxopropylidene)-1,2,4,5-tetrahydro-1H-1,5-benzodiazepin-2-one 1 (10 mmol) in ethanol, it was added hydroxylamine hydrochloride (10 mmol) and all was refluxed for a period of 2h. After neutralization with NaHCO₃, the formed residue was recrystallized from ethyl acetate to afford the title compound 2 as colorless crystals. The compound is characterized by ¹H-NMR (DMSO-d₆): 2.36 (s,3H,CH₃), 4.26 (s,2H, CH₂), 6.23 (s,1H) and 7.00-7.60 (m,CH_aromatique).

Scheme 1 Synthesis of 2-((5-methyl-isoxazol-3-yl)-methyl)-benzimidazole. 

Preparation of electrodes and test solution

Corrosive solutions were prepared by dilution of a 37% HCl analytical reagent grade with doubly distilled water. The tested concentrations ranged from 10^-3 to 10^-6 M. Steel strips contained 0.36 wt.% C, 0.66 wt.% Mn, 0.27 wt.% Si, 0.02 wt.% S, 0.015 wt.% P, 0.21 wt.% Cr, 0.02 wt.% Mo, 0.22 wt.% Cu, 0.06 wt.% Al and the remainder iron.

The working area of 1 cm² was subsequently ground with 200 and 1200 grit grinding papers, cleaned by distilled water and ethanol at hot air. The effect of temperature on the inhibition efficiencies of the inhibitor was tested from 303 to 333 K.

Electrochemical experiments

The effect of 2-((5-methyl-isoxazol-3-yl)-methyl)-benzimidazole (MMB) on the carbon steel corrosion was studied using electrochemical techniques: electrochemical impedance spectroscopy (EIS) and polarization potentiodynamic (PDP), in the concentration range of 10^-3 to 10^-6 M, at 303 K.

The electrochemical experiment consisted of a three electrode electrolytic cell made by a platinum foil as counter electrode, saturated calomel as reference electrode and carbon steel as working electrode, with an exposed area of 1 cm².

The carbon steel specimen was immersed in a test solution for 30 min, until a steady-state (open circuit) potential was achieved using a type PGZ100 potentiostat. EIS measurements were performed with a frequency range from 100 KHz to 10 mHz and an amplitude of 10 mV with 10 points per decade.

The polarization curves were recorded by polarization from -600 to -200 mV/SCE, with a scan rate of 0.5 mV/s.

The data obtained by PDP and EIS methods were analyzed and fitted using graphing and analyzing impedance software, version EcLab.

Effect of inhibitor concentration

Potentiodynamic polarization curves

Fig. 2 present the polarization curves for carbon steel in a 1 M HCl solution and in the presence of our compound tested at 303 K. Table 1 regroups the parameters derived from PDP plots. The values of η_PDP(%) are calculated by the following formula:

(1)

where and are the corrosion current densities in uninhibited and inhibited media, respectively.

Figure 2 Potentiodynamic polarization curves for carbon steel in the absence and presence of different MMB concentrations. 

Table 1 Tafel polarization parameters for carbon steel in a 1 M HCl solution, in the absence and at different concentrations of MMB at 303 K. 

Analysis of the PDP curves indicates that the addition of MMB decreases both cathodic and anodic current densities with the increase in the inhibitor concentration. The presence of the inhibitor tested in the aggressive medium causes a slight change in the shift of E_corr values in all tested concentrations, suggesting that MMB can be classified as a mixed type inhibitor ¹⁹, with no change in the metal corrosion mechanism ²⁰.

Table 1 shows that the corrosion current densities (i_corr) decrease when the MMB concentration rises in the acidic solution, reaching a value of 97.8 μA/cm² at an inhibitor concentration of 10^-3 M. This i_corr value led to an η_PDP% of about 85.2 % and confirms that MMB is a good inhibitor against carbon steel corrosion in a HCl medium. A slight variation in the values of anodic and cathodic Tafel slopes is observed with the inhibitor presence. These results suggest that the surface blocking effect of the tested adsorbed compound diminishes the anodic and cathodic reactions ²¹.

Electrochemical impedance spectroscopy measurements

Impedance method provides information about the kinetics of the electrode processes and, simultaneously, about the surface properties of the investigated systems. So, the protective effectiveness of MMB on the carbon steel corrosion in the acidic medium (1 M HCl) was studied by EIS method at 303 K, after 30 min of immersion. Fig. 3 shows the influence of MMB concentrations on Nyquist impedance spectra.

Figure 3 Nyquist plot for carbon steel in 1 M HCl, in the absence and presence of different concentrations of MMB at 303 K. 

Figure 4 The equivalent circuit model used to fit the EIS data. 

This figure shows that only single semicircles are observed with the depression at a low frequency, in MMB presence and absence. This depression is probably due to the inhomogeneity and roughness of the carbon steel surface ²⁴. A remarkable increase in the semicircle diameter is observed when the concentration of MMB rises; this behavior is the result of the inhibitor adsorption onto the carbon steel surface ²⁵. The equivalent circuit model shown in Fig. 4 was used to analyze the EIS experiments.Various impedance parameters, namely solution resistance (R_s), charge transfer resistance (R_ct), double layer capacitance (C_dl), inhibition efficiency (η_z%) and surface coverage (θ) were calculated using the equivalent circuit model shown in Fig. 4 and given in Table 2.

Table 2 Electrochemical impedance spectroscopy parameters obtained for carbon steel in a 1 M HCl solution, in the absence and at different concentrations of MMB at 303 K. 

The inhibiting efficiency derived from EIS, η_Z (%), was calculated by the following equation:

(2)

where and are the charge transfer resistance, in the inhibitor absence and presence, respectively.

The double layer capacitance (C_dl) values are calculated using Eq. (3):

(3)

where Q is the CPE constant and n is a coefficient that can be used as a measure of surface inhomogeneity ²⁶.

From Table 2, it can be observed that R_ct value increases with a higher concentration of MMB, while the C_dl values decreases. These results demonstrate that the compound was adsorbed onto the carbon steel surface by the formation of a protective film at the metal/solution interface ^27-29.

The C_dl values diminished with the increase in the MMB concentration. The effect of the dielectric constant of a material on the interface capacitance can be related to the following equation:

(4)

where δ is the film thickness, ε the material dielectric constant and ε₀ is the vacuum permittivity (8.8542 × 10^-14 F cm^-1). Based on the aforementioned, the increase in the MMB concentration reduces the interface dielectric effect. This behavior is due to a substitution process of adsorbed water molecules by MMB molecules, because their dielectric constants are lower than those of water. In this way, a protective film is formed, which diminishes the active sites on the steel surface and, consequently, the rate of the electrochemical corrosion reactions. The η_z% values obtained in the EIS study were in good agreement with those calculated in PDP studies.

Adsorption isotherms

The inhibition efficiency of the corrosion inhibitor mainly depends on the interaction between inhibitor and metal surface, and it can be provided by the adsorption isotherm ³⁰. In order to evaluate MMB adsorption process in a carbon steel surface, Langmuir, Temkin, Freundlich and Frumkin adsorption isotherms were tested.

Figure 5 Plots of the Langmuir adsorption isotherm of the MMB on the carbon steel surface at 303 K. 

A plot of C_inh/θ versus C_inh (Fig. 5) was found to be the best fit, which suggests that MMB adsorption onto the carbon steel surface in the tested medium obeys the Langmuir adsorption. The Langmuir isotherm can be best expressed by the following relation (5):

(5)

where C_inh is the MMB concentration and K_ads is the equilibrium constant for the adsorption−desorption process ³¹. From the intercept of the straight lines in Langmuir plot, the value of the K_ads is calculated and given in Table 3. This value is also related to the standard free energy of adsorption, , by the following equation 6 ^32,33:

(6)

where the constant of 55.5 is the concentration of water in solution mol dm^-3, R is the gas constant and T is the absolute temperature. The parameters of adsorption are regrouped in Table 3.

The high K_ads value obtained in this study indicates the strong interaction between the carbon steel surface and MMB adsorbing molecules ²⁰. The calculated value of is around 39.82 kJ/mol, suggesting that the adsorption mechanism of MMB onto carbon steel in 1 M HCl is probably of a mixed type adsorption: physisorption and chemisorption ³⁴.

Effect of temperature

The effect of temperature on the corrosion inhibition was studied using PDP measurement, in the temperature range from 303 to 333 K, without and with 10^-3 M of MMB during 0.5 h of immersion. The electrochemical parameter extracted by PDP method, included in Table 4 and Fig. 6 (a and b), shows an increase in i_corr with an increase in temperature, and it is more pronounced for the uninhibited solution. We observe that the inhibitor efficiency decreased with a rise in temperature, suggesting that the higher temperature does influence the inhibitor adsorption onto the metal surface.

Figure 6 Effect of temperature on the behavior of carbon steel/ 1 M HCl interface in (a) uninhibited solution and (b) at 10^-3M of MMB. 

Table 4 Temperature influence on the PDP parameters for carbon steel in 1 M HCl, with and without HCl (1M) +10^-3 M of MMB, at a temperature range from 303 K to 333 K. 

The effect of temperature on the corrosion current (i_corr) can be used for determining the activation thermodynamic parameters for the dissolution process, following the Arrhenius equation (Eq.7) and transition state equation (Eq.8) (³⁵,³⁶):

(7)

(8)

where E_a is the activation corrosion energy, ∆H_a is the enthalpy, ∆S_a is the entropy of activation, T is the absolute temperature in Kelvin, h is the Plank constant, N is the Avogadro number and R is the molar gas constant. Arrhenius plots for the corrosion rate of carbon steel are given in Fig. 7. Using the slope of Ln (i_corr) vs. 1/T plots, the values of the apparent activation energy of corrosion (E_a) have been determined (Table 5).

Figure 7 Arrhenius plot of Ln i_corr versus 1000/T for carbon steel corrosion in 1 M HCl + 10^-3 M of MMB. 

The linear regression coefficient was close to 1, showing that carbon steel corrosion in HCl 1 M can be interpreted using the kinetic model. The E_a value calculated in an uninhibited solution (41.74 kJ mol^-1) is in the same order of magnitude as previously described ^{(13, 37)}. The obtained E_a value, in the presence of the tested compound, is 64.62 kJ mol^-1.

E_a value, in the inhibitor presence, is more important than in its absence, indicating physical adsorption. The increase in activation energy explains the slight adsorption of the inhibitor onto the carbon steel surface with increasing temperatures ^38,39. ∆H_a and ∆S_a values evaluated from the slope (∆H_a/R) and the intercept of (Ln(R/Nh) + (∆S_a /R)) of the plots of Ln(i_corr/T) against 1/T (Fig. 8) are listed in Table 5.

Figure 8 Variation of Ln (i_corr/T) versus 1000 /T for blank and 1 M HCl + 10^-3 M of MMB. 

The positive signs of ∆H_a reflect that the CS corrosion process is of an endothermic nature. The data of Table 5 show that ΔH_a value is higher in the presence of the tested inhibitor (61.85 kJ mol^-1) than that in its absence (41.24 kJ mol^-1). ∆S_a value is more positive in a 1 M HCl solution containing MMB than that of the uninhibited solution. Some researches interpreted this behavior by the replacement process of water molecules during the adsorption of the inhibitor molecule onto the electrode surface ^40,41.