Inhibitory effect of Atlas cedar essential oil on the corrosion of steel in 1 m HCl

2.1 Materials and solutions

Tests were performed on S300 steel (Maghreb Steel, Casablanca, Morocco) specimens with the composition (wt.%) C (0.15%), Mn (1.25%), Si (0.05%), and Fe (98.55%). The aggressive hydrochloric acid solution was prepared by dilution of analytical grade 37% HCl with bidistilled water.

2.2 Preparation of essential oil

The essential oil of Atlas cedar (Cedrus atlantica) was obtained from wood waste by steam distillation using Clevenger device. The essential oil was extracted from a mixture of water/oil three times by diethyl ether, and the resulting ether layer was dried over anhydrous sodium sulfate. Then, the diethyl ether was evaporated under vacuum, and the essential oil obtained (0.7% yield) was stored at low temperature to avoid its evaporation and decomposition. Filtration of this essential oil by chromatography on silica gel using hexane/ethyl acetate as eluent allowed fractions hydrocarbons, ketones, and alcohols. Indeed, elution with hexane leads to a hydrocarbon portion, which represents 75% of the composition and consists mainly of α-, β-, and γ-himachalenes in ratios of 70%, 20%, and 8%, respectively, and the ar-himachalene in small amounts of 2% (El Haib et al., 2011; Loubidi et al., 2014). Increasing the polarity of the eluent (hexane/ethyl acetate: 99/1) is the oxygenated fraction mainly consisting of α-atlantones isomers, 6, 7 and other oxygenated compounds 5 and 8. This fraction represents 20% of the composition (Figure 1) (Karroumi, 2015; Mazoir, 2016). The last fraction containing sesquiterpenic alcohols has been obtained with hexane/ethyl acetate 95/5 eluent. This fraction represents 10% of the composition of this oil (Aberchane, 2004). It should be noted that different fractions contain other unidentified products in low percentages, and represented products are extensively studied in the literature (Karroumi et al., 2015).

Figure 1:

Different families of essential oil of Atlas cedar.

2.3 Electrochemical tests

The electrochemical study was performed in aerated solutions with a PGZ100 potentiostat controlled by Voltamaster 4. The electrochemical study was performed with a conventional three-electrode cell, where the counter electrode was a sheet of platinum with 2-cm² surface area, and Ag/AgCl was used as reference electrode. The working electrode was cut from S300 steel rod with a cross-section area of 0.76 cm². Before each experiment, the surface of the working electrode was polished with 180, 500, and 1200 grit emery papers, until surface like a mirror was obtained and degreased with acetone on ultrasonic device and rinsed abundantly with distilled water. The polarization curves were obtained with a scan rate of 1 mV/s from −0.8 to −0.2 V vs. (Ag/AgCl). The EIS was measured with frequencies ranging from 100 kHz to 0.01 Hz at open circuit potential by applying sine wave voltages of 0.01-V peak-to-peak values. Before all experiments, the potential was stabilized in the test solution at open circuit potential (OCP) during 30 min. The effect of temperature on the inhibitor performance was examined in a temperature range from 293 to 323 K.

2.4 SEM observation and EDX analysis

Surface analysis of steel was carried out in the absence and presence of the inhibitor using a TESCAN VEGA3-EDAX SEM with accelerating voltage of 20 kV. The steel coupons were analyzed after 2 h of immersion time in the absence and presence of inhibitors at 1 g/l. The elemental composition was determined using energy dispersive X-ray spectroscopy (EDX).

3.1 Potentiodynamic polarization curves

Figure 2 shows the potentiodynamic polarization curves for the steel in 1 m hydrochloric acid solution, containing different concentrations of Atlas cedar essential oil at 293 K after 30 min of immersion.

Figure 2:

Potentiodynamic polarization plots of steel immersed in 1 m HCl in the presence of different concentrations of essential oil of Atlas cedar.

Table 1 presents the electrochemical kinetic parameters such as corrosion current density (i_corr), corrosion potential (E_corr), cathodic and anodic Tafel slopes (bc and ba), and inhibition efficiency (η%); the parameters were determined by extrapolation method of the experimental curves.

The inhibition efficiency (η%) was calculated from:

where i_corr and i′_corr are corrosion current density without and with the oil.

It is observed that the cathodic reaction typified by the hydrogen evolution is inhibited and the inhibition efficiency increases as the essential oil concentration increases (Gunasekaran & Chauhan, 2004). It is clear from the results that the addition of essential oil causes a decrease in cathodic Tafel slope. This change in cathodic Tafel plots obtained in Figure 2 indicates that the mechanism of H⁺ reduction (Eq. 4) is affected by the essential oil addition (Znini et al., 2012). Also, the corrosion potential slightly shifts to a cathodic domain (≈17–26 mV) in the presence of the inhibitor. In the anodic domain, it can be seen that the polarization curves display the same characteristics. These general trends are confirmed by the data in Table 1. It is noted that if the displacement in corrosion potential is more than 85 mV with respect to corrosion potential of the blank, the inhibitor can be considered as a cathodic or anodic type (Singh et al., 2016a,b). From the outcome, the maximum shift was 26 mV, which suggested that the inhibitor is a mixed type. From Table 1, it is obvious that the inhibition efficiency increases with increasing inhibitor concentration to attain 80.65% at 2.0 g/l. Consequently, essential oil is a good inhibitor and acts as a mixed type with a strong predominance of cathodic character by blocking the surface area (Bouyanzer et al., 2006).

The cathodic hydrogen evolution reaction may be accounted for as follows (Okafor et al., 2012):

However, the first step of adsorption of the inhibitor on the metal surface usually involves replacement of water molecules initially at the metal surface (Zor et al., 2009; Khan et al., 2015):

where Inh_(sol) and Inh_(ads) are the inhibitors in the solution, which are adsorbed on the metal surface, where n is the number of water molecules displaced by the inhibitor.

Subsequently, the inhibitor may then combine with metal on the surface as follows (Okafor et al., 2012):

The effects of different fractions of this essential oil on the corrosion reaction of steel in 1 m HCl were determined by polarization measurements, as presented in Figure 3. It is worth noting that the himachalenes fraction does not show any anti-corrosion activity. However, the oxygenated fraction exhibits remarkable anti-corrosion effect.

Figure 3:

Polarization curves for steel in 1 m HCl with different fractions of inhibitor at 293 K.

The examination in Figure 3 and Table 2 shows that the addition of atlantones or Himachaloles (alcohols) fractions reduces the cathodic current density. This decrease is more pronounced with alcohol inhibitor, which affects mainly both anodic dissolution and cathodic reduction (Bouyanzer et al., 2006). Besides, we remarked that the essential oil and atlantones fraction lead to a slight change in cathodic Tafel slope. Also, the corrosion potential is approximately constant. It is evident from Table 2 that the corrosion potential displacement was around 25 mV compared to the blank. These results reveal that the inhibitor has a mixed-type character (Solmaz, 2010).

The addition of inhibitor leads to change in corrosion process mechanism. One explanation for this phenomenon could be attributed to the adsorption process of active molecules on steel surface leading to the increase of the surface coverage (Halambek et al., 2013). These results indicate that the inhibition efficiency reaches 88% at 1 g/l of Himachaloles (alcohols).

In order to actually understand the electrochemical interfacial phenomena and kinetics of the electrode processes of the system under study, the impedance diagrams are traced.

3.2 Electrochemical impedance spectroscopy

The Nyquist and Bode plots of steel in 1 m HCl solution with and without different inhibitor concentrations at 293 K are presented in Figures 4 and 5, respectively. In the Nyquist plot, the existence of a single semicircle with one time constant in the Bode plots suggests that the only one phenomenon occurring is the charge transfer process (Singh et al., 2016a,b). It is obvious that the increase of the concentration of essential oil leads to an increase in diameter of the semi-circular. The same behavior is observed in Bode modulus plots. This suggests the adsorption of inhibitor molecules on the metal surface (Döner et al., 2011). The examination of Figure 4 indicates that the Nyquist plots do not yield perfect semicircles; this deviation from ideal semicircle is usually attributed to the inhomogeneities of the metal surface, the frequency dispersion, and the mass transport resistance (Aljourani et al., 2009; Solmaz, 2010).

Figure 4:

Nyquist plots of steel immersed in 1 m HCl with and without essential oil at 293 K.

Figure 5:

Bode modulus and phase-angle plots for steel in 1 m HCl with and without different concentrations of essential oil at 293 K.

The general shape of Nyquist and Bode plots is very similar in all cases; this explains that there is no change in the corrosion mechanism with the variation of inhibitor concentration (Reis et al., 2006). The Nyquist plots were analyzed by EC-Lab^® software (Bio-Logic Science Instruments, Seyssinet-Pariset, France) using the equivalent circuit model displayed in Figure 6. The Chi-squared (χ²) values of different plots show the excellent goodness of fit, which validates the circuit proposed.

Figure 6:

Equivalent electrical circuit of the interface of steel/electrolyte.

The impedance parameters, charge transfer resistance, double layer capacitance, and inhibition efficiency are listed in Table 3.

The inhibition efficiency (η%) was calculated from (Eq. 7):

where R_ct and R_ct(inh) are, respectively, the charge transfer resistance values without and with inhibitor.

It is worth noting that the addition of essential oil caused the reduction of double-layer capacitance values and increased the charge transfer resistance values in hydrochloric acid solution. This significant decrease in capacitance can be attributed to adsorption of inhibitor molecules onto steel surface (Dasami et al., 2015).

The corrosion behavior of steel with a different part of the essential oil in 1 m HCl solution is also being explored by Nyquist and Bode plots. Figure 7 shows the Nyquist plots obtained with 1 g/l of Atlantones and Himachaloles (alcohols). It is evident that the diameter of the semicircles in the Nyquist plots increases with the addition of these compounds compared to the blank solution. The same behavior is observed on the Bode modulus plots. From the Bode phase, it could be seen that the plots show one time constant for all inhibitors (Figure 8). According to Table 4, the decrease in double-layer capacitance C_dl leads to a reduction in local dielectric constant and/or an increase in the thickness of the double layer (Lei et al., 2015). This might be due to adsorption of molecules at the metal/electrolyte interface (Verma et al., 2014). These findings indicate that the action of inhibitor molecules acts by pure geometric blocking of the electrode surface (Znini et al., 2012). Similarly, the polarization data also confirm the EIS results.

Figure 7:

Nyquist plots of steel immersed in 1 m HCl with and without inhibitors at 293 K.

Figure 8:

Bode modulus and phase-angle plots for steel in 1 m HCl with and without inhibitors at 293 K.

3.3 Thermodynamic parameters

The effect of temperature on the corrosion rate was examined in order to investigate the adsorption of inhibitor compounds on the metal surface (Figure 9). The temperature modifies the mechanism of inhibitor adsorption. In addition, the active compounds may undergo decomposition and/or rearrangement (Noor & Al-Moubaraki, 2008). The inhibition efficiency of different fractions of essential oil of Atlas cedar was studied by the potentiodynamic measurements in the temperature range of 293–323 K. The electrochemical parameters of polarization curves are listed in Table 5. The results show that the corrosion current density is significantly reduced when temperature increased. Therefore, the inhibition efficiency increases strongly. This suggests higher adsorption of the inhibitor molecules onto the metal surface, which block the active corrosion sites (Bothi Raja and Sethuraman, 2008; Anawe et al., 2015).

Figure 9:

Polarization curves for steel in 1 m HCl with different temperatures of inhibitors.

The corrosion reaction can be represented by Arrhenius-type process by the following relation:

where i_corr is the corrosion current density of steel, E_a is the apparent activation energy, A is the frequency factor, R is the universal gas constant (R=8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature.

The apparent activation energy was determined from the slope of ln i_corr versus 1/T depicted in Figure 10. On the other hand, the enthalpy (ΔH a*) and entropies (ΔSa*) of activation for a different part of essential oil can be obtained from the following equation (Verma et al., 2014):

Figure 10:

Arrhenius plots of ln i_corr versus 1/T (K−1) for steel in 1 m HCl with and without 1 g/l of different parts of essential oil.

where i_corr is the corrosion current density, h is Planck’s constant, N is Avogadro’s number, ΔSa* is the entropy of activation, ΔHa* is the enthalpy of activation, T is the absolute temperature, and R is the universal gas constant. Figure 11 indicates the variation of ln i_corr/T versus 1/T, the slope is equal to −ΔHa*/R and the intercept (lnRhN+ΔSR), where the values of ΔSa* and ΔHa* are determined.

Figure 11:

Transition-state plots of ln i_corr/T versus 1/T (K−1) in 1 m HCl with and without 1 g/l of different fractions of essential oil.

From Table 6, it becomes apparent that the activation energy decreases in the presence of different fractions of essential oil with respect to the blank. This trend is an indicator of better inhibition efficiency with temperature, which is attributed as the chemisorption of inhibitor molecules (Znini et al., 2012; Lei et al., 2015).

It is interesting that the ΔHa∗ value is positive (8.96 and 21.65 kJ/mol) for essential oil and atlantones, respectively, further confirming the endothermic nature of the process. On the other hand, the alcohols part shows the exothermic nature of the process with the negative enthalpy of activation (−30.18 kJ/mol). The decrease in value of ΔSa∗ reveals the decrease in disorder, which is attributed to the adsorption of the organic inhibitor accompanied by desorption of water molecules on the steel surface/solution interface (Wang et al., 2015). Furthermore, this negative value of ΔSa*  indicates the degree of randomness at the interface metal/solution during the adsorption (Adewuyi et al., 2014).

3.4 Adsorption isotherms

In order to understand the mechanism of interaction between inhibitor molecules and the metal surface, various adsorption isotherms were tested such as Langmuir, Temkin, and Freundlich in an attempt to fit the experimental data. It is found that the adsorption mechanism of the inhibitor on steel surface follows Langmuir adsorption isotherm, as shown in Figure 12.

Figure 12:

Langmuir adsorption plots for steel in 1 m HCl solution containing different concentrations of essential oil at 293 K.

The Langmuir isotherm expected that the adsorbed molecules occupied only one site and there was no interaction with other molecules adsorbed on the metal surface (Boumhara et al., 2015).

The surface coverage (θ) is calculated using the following relation:

The Langmuir isotherm is given by the following relation (Boumhara et al., 2015):

where θ is the surface coverage, C_inh is the inhibitor concentration, and K_ads is the equilibrium constant of standard adsorption. Figure 12 shows the linear evolution plots of C_inh/θ against C_inh. The experimental points and the calculated lines reveal a very good fit with the regression coefficient up to 0.99, which suggests that the experimental data are in full agreement with the Langmuir isotherm. This indicates the adsorption of inhibitor molecules on the metallic surface to form a film that isolates metal from the aggressive environment (Flores et al., 2011). From the intercepts of the straight lines, the value of equilibrium constant of standard adsorption was calculated; K_ads=7.23. K_ads is related to the standard free energy of adsorption, ΔGads∘ by the following relation:

where CH2O is the mass concentration of water in the solution (1000 g/l), R is the universal gas equilibrium constant, and temperature T is 293 K. The negative value of ΔGads∘ implies that the adsorption of essential oil on the steel surface is spontaneous. The calculated ΔGads∘ value is −21.64 kJ mol⁻¹. Generally, the energy values of ΔGads∘ around −20 kJ mol⁻¹ or less negative are associated with an electrostatic interaction between charged inhibitor molecules and charged metal surface (physisorption); those around −40 kJ mol⁻¹ or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type bond (chemisorptions) (Boumhara et al., 2015; Idouhli et al., 2016). The ΔGads∘ is around −20 kJ mol⁻¹; this proves that the adsorption of essential oil onto the steel surface belongs to the physisorption and the adsorptive film has an electrostatic character (Deng & Li, 2012; Li et al., 2012).

3.5 SEM observation and EDX measurements

3.5.1 Scanning electron microscopy

Surface analysis of steel was examined using SEM micrographs after 2-h immersion, as shown in Figure 13. The morphology of steel after immersion in hydrochloric acid 1 m solution is very rough and damaged due to corrosion. However, it is obvious that the addition of 1 g/l of inhibitors leads to less damage in the surface compared to the blank, suggesting the formation of a protective layer on the steel surface. The morphology of steel specimen with alcohols part is smoother than in the presence of essential oil and atlantones, indicating its good inhibition efficiency.

Figure 13:

SEM images of steel (A) in 1 m HCl medium after 2 h of immersion at 298 K, (B) with essential oil after 2 h of immersion at 298 K, (C) in 1 m HCl medium with Atlantones after 2 h of immersion at 298 K, (D) Himachaloles after 2 h of immersion at 298 K.

Close examination of the SEM images shows an enhancement of surface coverage and formation of a protective film on the steel surface (Bousskri et al., 2016). This could be generally attributed to the decrease in contact between the steel of surface and the aggressive medium.