Tribological behavior of stainless steel in sulfuric acid in the presence of Thymus zygis subsp. gracilis essential oil: experimental and quantum chemical studies

2.1 Essential oil analysis

The TZ essential oil was extracted by hydrodistillation using Clevenger-type apparatus according to the method recommended in the European Pharmacopoeia (1997). The extracted essential oil was analyzed using a PerkinElmer Turbo Mass quadrupole-detector, coupled to a Perkin-Elmer 88 Auto system X, using Helium as carrier gas (1 mL/min). The ion source temperature was 150 °C; the temperature of the oven was programmed from 60 to 230 °C at 2 °C/min and then held isothermally at 230 °C (35 min). The injector temperature was fixed at 280 °C, the energy ionization at 70 eV, electron ionization mass spectra were acquired over the mass range 35–350 uma, split: 1/8, injection volume: 0.2 μL of pure oil.

2.2 Preparation of materials

The coupons studied (AISI 304L) with a surface of 30 × 30 mm² and thickness of 1 mm, that its chemical composition is shown in Table 1, was polished with several grades of alumina pastes down to 1 µm to obtain a mirror surface (Ra = 0.01 µm), using Buehler Motopol 2000 grinder/polisher machine. Then, rinsed with bidistilled water, degreased in acetone into an ultrasonic bath for 5 min, rinsed with bidistilled water and dried before its uses. The 0.5 M H₂SO₄ medium was prepared by diluting the H₂SO₄ analytical grade 95% with bidistilled water. The concentration range of TZ essential oil was chosen upon the maximum solubility that was 3 g·L⁻¹.

2.3 Tribological tests

Before all tests, the alumina balls Al₂O₃ with a diameter of 10 mm should be rinsed with ethanol. The tribocorrosion tests were conducted at 23 °C. The wear in a dry environment was carried out in ambient air with a relative humidity of 50% HR. For the environment of 0.5 M H₂SO₄ with and without the addition of 3 g·L⁻¹ of TZ essential oil, the tribocorrosion tests are conducted at room temperature using electrolyte volume of 20 mL with pH 0.2. The AISI 304L surface of 1 cm² was used as a working electrode (WE). The Ag/AgCl (3 M KCl) represents the reference electrode (RE) and the Pt electrode is used as a counter electrode.

The experimental system used during tribocorrosion tests is schematized in Figure 1. The current measurements and voltage are measured at a resolution of 1 pA and 1 µV, using potentiostat Solartron with electrochemical interface model 1287. After 10 min of coupon immersion into the test solution, the potential results were obtained for 1000 s at 1 Hz. At the end of the tribocorrosion tests, the counterbody was immediately removed from the WE. The electrochemical parameters obtained are compared to ASTM conventions (1994).

Figure 1:

Schematic drawing of electrochemical measurement technique connected to the fretting machine.

The tribocorrosion sliding tests were recorded at a normal force of 2 N at 1 Hz, and displacement amplitude of 1 mm, the equipment test used was reported earlier by Mohrbacher et al. (1995). The tribocorrosion tests were evaluated at 23 °C for 10,000 cycles at equally spaced time increments over the whole test duration, the number of cycles, tangential force, normal force, amplitude displacement, and the friction coefficient were logged.

The weight loss measurements were recorded before and after the tribological tests using an analytical balance (accuracy ± 0.0001 mg).

The measure of weight loss is computed according to Equation (1):

with, m₀ and m_f are the coupon weights before and after tribological tests, respectively. The inhibition efficiency IE(%) is determined according to Equation (2):

With, Δm0.5 M H2SO4 and Δminh represent the weight loss in 0.5 M H₂SO₄ without and with essential oils, respectively.

The electrochemical measurements were recorded using an EG&G Model 273A potentiostat controlled by the CorrWarre software, at a sweep rate of 1 mV/s and the measurement sweep interval was −800 to −200 mV/ESC.

3.1 Scanning electron microscopy and energy dispersive X-rays (SEM-EDX)

The wear tracks morphology was analyzed after the sliding tests, using a Philips scanning electron microscope (XL 30 FEG) connected to an electron dispersive X-ray (EDX) analysis device. Volumetric wear was measured by noncontact white light interferometry analysis using Wyco NT3300 optics profilometer with Vision software (version 2.210).

3.2 Characterization of surface topography and roughness

The surface topography and roughness of wear track on coupons were analyzed by confocal microscopy (NanoFocus µsurf^®), comprising a light source (LED), a rotating multi-pinhole disc, an objective lens with a piezo drive, and a CCD camera.

4.1 DFT and ESP calculations

Gaussian 09 program, Revision-D.01 (Frisch et al. 2009) was used to obtain the optimized geometries of TZ essential oil main components such as thymol, p-cymene, γ-terpinene, and borneol using the DFT method (Gece 2008; Obot et al. 2015; Verma et al. 2016) combined to the basis 6-311G++(d, p) (Hariharan and Pople 1973) and the functional B3LYP (Hehre et al. 1972). For these compounds, the distribution densities of HOMO (occupied molecular orbital) and low unoccupied molecular orbital (LUMO) were produced. In addition, we are interested in determining the quantum parameters (QPs) such as the energy of HOMO (E_HOMO), the energy of LUMO (E_LUMO), the energy gap ΔE_inh between E_HOMO and E_LUMO energies (ΔE_inh = E_LUMO – E_HOMO) (Geerlings et al. 2003; Hsissou et al. 2019; Pearson 1988), the global chemical potential μ_inh (μ_inh = 0.5 [E_HOMO – E_LUMO]) (Benallou et al. 2019; Singh et al. 2018; Vanasundari et al. 2017) and the hardness η_inh (η_inh = ΔE_inh) (Benallou et al. 2019; Singh et al. 2018; Vanasundari et al. 2017). The fraction of electrons transferred ΔN from each compound as an inhibitor to bulk iron surface (110), is calculated according to Pearson relationship as follows:

where φ_Fe(110) and η_Fe(110) are known as the working function and the absolute hardness of bulk iron surface (110), respectively, and then η_inh and μ_inh are the global chemical potential and the global hardness of inhibitor, respectively (Lukovits et al. 2001; Martinez 2003; Sastri and Perumareddi 1997).

The values of φ_Fe(110) and η_Fe(110) are 4.82 and 0 eV, respectively (Kokalj 2012; Parr and Pearson 1983; Tang et al. 2013). Furthermore, the electron back donating character of inhibitor (ΔE_bd-inh) is another descriptor that is used to describe the reactivity between metal surface and adsorbent; this parameter is expressed as −η_inh/4 (Olasunkanmi et al. 2016).

Additionally, the reactive centers of the concerned compounds are estimated from three approaches, the spreading analysis of HOMO and LUMO densities (DFMO), the ESP analysis in which: the red color reflects the strong negative electrostatic potential (EP); yellow for a moderately negative EP; blue reflects the strong positive EP and green for the moderately positive EP (Goulart et al. 2013). And, from the computation of electrophilic attacks P⁺ and nucleophilic attacks P⁻ indexes in terms of natural bond orbitals analysis (Domingo et al. 2013).

4.2 Metropolis Monte Carlo (MC) simulations

The adsorption simulation of each inhibitor on the iron surface (110) into corrosive medium (200 of H₂O, 20 H₃O⁺, and 10 SO4²⁻) was explored using the adsorption locator module and the COMPASS II force field integrated into the Materials studio version 8 program package (Sun 1998). The single-crystal Fe (110) surface is built using sketching tools and optimized with the smart minimizer using DMolˆ3 at the DNP + basis and B3LYP functional. The single crystal plane (110) was extended to the multi-surface area of Fe (110), using the supercell containing 10 layers with a vacuum layer of 30 Å along the C-axis. The system inhibitor/solution/Fe (110) was simulated in a simulation box of 17.89 × 17.89 × 38.34 Å with periodic boundary conditions. Concerning the crystallographic surface Fe (110) plane, it was chosen for its densely-packed surface (Vanasundari et al. 2017).

5.1 Essential oil chemical composition

To further our research on the activities of TZ essential oil, we are interested to assess the tribocorrosion behavior of this essential oil. Our previous study showed that thymol (42.5%), p-cymene (23%), γ-terpinene (8.9%), and borneol (4.8%) are the main compounds of TZ essential oil (Figure 2) (Ouknin et al. 2020b).

Figure 2:

Molecular structures of the main components of TZ essential oil.

5.2 Weight loss measurement

TZ essential oil’s effect on the tribocorrosion behavior of AISI 304L in 0.5 M H₂SO₄ was studied by weight loss measurements. The weight loss values (Δm) and the inhibition efficiency E (%) are pooled in Table 2.

It is quite clear from the obtained results that in the presence of TZ essential oil, weight loss decreases and the inhibition efficiency is 84.38%. However, this can be attributed to the adsorption of TZ essential oil on the stainless steel surface, thereby blocking the reaction sites and protecting the surface against wear and corrosion (Boumezzourh et al. 2020).

5.3 Friction coefficient results (COF)

Figure 3 provides the AISI 304L friction coefficient (µ) behaviors, as a function of cycle numbers and the environment nature, at frequency of 1 Hz, a cycle numbers of 10,000 cycles, and linear displacement amplitude of 1 mm. The three environments tested are, the first in dry conditions, the second in 0.5 M H₂SO₄, and the third in 0.5 M H₂SO₄ with the addition of TZ essential oil (3 g·L⁻¹).

Figure 3:

Evolution of the friction coefficient of the tribological torque AISI 304/Al₂O₃ in the presence of three environments: dry conditions, 0.5 M H₂SO₄ and 0.5 M H₂SO₄ in the presence of TZ essential oil (3 g·L⁻¹) at 1 Hz, 1 mm, and 10,000 cycles for the normal force (2 N).

The report of the tangential effort F_t (N) on the normal effort F_n (N) allows us to define the friction coefficient value (µ) according to Equation (4).

According to Figure 3, it is clear that the evolution of friction coefficient in the function of cycle numbers present two domains. The first domain represents the “running-in” regime, it concerns about 1000 cycles, and this domain is associated with the surface adaptation that is characterized by a low number of mechanical stresses and the surface roughness. In this domain, the friction coefficient value increases significantly as will the cycle numbers. It is also clear that the friction coefficient evolution is relatively sloping in the third environment with essential oil (3 g·L⁻¹), for those in the first and second environment (dry and 0.5 M H₂SO₄), they follow the same order as the third environment (3 g·L⁻¹ of TZ oil). The second domain is presented between 1000 and 10,000 cycles, this one provides information about the friction coefficient evolution, is named: ‘usage’. In this domain, it is quite clear that the evolution of the friction coefficient value is less fast than the one of the preceding domains. In addition, the evolution of the friction coefficient value is the lowest in the environment of 0.5 M H₂SO₄ in the presence of TZ essential oil (3 g·L⁻¹) and appreciably of the same nature in the two other environments (0.5 M H₂SO₄ and in dry condition (HR = 50%)).

The lowest friction coefficient value obtained in the environment (0.5 M H₂SO₄ in the presence of TZ essential oil (3 g·L⁻¹)) can be associated with the lubricant effect related to the addition of TZ essential oil (3 g·L⁻¹) to the sulfuric medium 0.5 M. It is also interesting to note that the values of the identified friction coefficient, for the tribological torque AISI 304/Al₂O₃ in the dry environment (HR = 50%), are similar to those previously reported (Ouknin et al. 2019a, 2020a).

5.4 Dissipated energy evolution during friction

The evolution of dissipated energy E_d (µJ) in the function of the cycle number and the nature of the environment, during the friction test, are presented in Figure 4. The values of dissipated energy for a cycle under total imposed slip condition are obtained by processing the tangential force F_t (N) as a function of the displacement δ. The curves obtained have generally quadratic form, a parallelogram. The area of the parallelogram obtained presents the dissipated energy in the friction test (Benea et al. 2014; Vincent 1994).

Figure 4:

Evolution of the dissipated energy in the contact of the tribological torque (AISI 304/Al₂O₃) in the course of friction and in the presence of three environments: dry conditions, 0.5 M H₂SO₄ and 0.5 M H₂SO₄ in the presence of TZ essential oil (3 g·L⁻¹) at 1 Hz, 1 mm, 10,000 cycles for the normal force (2 N).

The pace of the three curves shows that the energy dissipated in the contact is higher in domain 1, concerning 1000 cycles corresponding to the surface adaptation. In the domain 2 range of 1000–10,000 cycles, a slight decay of slope is observed in the environment (0.5 M H₂SO₄ with 3 g·L⁻¹ of TZ essential oil). However, the slope obtained in dry (HR = 50%) and 0.5 M H₂SO₄ environments are more accentuated and seem to be approximately in the same order of magnitude.

The pace of the three curves indicates a decreasing development of dissipated energy in the function of the number cycles, that reflects the facilitation of sliding by increasing the cycle number, regardless of the environment concerned.

5.5 Potentiodynamic polarization measurements

Figure 5 presents the potentiodynamic polarization curves of AISI 304L coupons in 0.5 M H₂SO₄ with and without the addition of 3 g·L⁻¹ of TZ essential oil at 298 K. Table 3 gathered the kinetic parameters including corrosion current density (I_corr), the corrosion potential (E_corr), cathodic slopes (βc) also the inhibition efficiency (IE %).

Figure 5:

Polarization curves for AISI 304L in the absence and presence of TZ essential oil in 0.5 M H₂SO₄.

As shown by the polarization curves, the current density value decreases in the cathodic and anodic domains in the presence of 3 g·L⁻¹ TZ essential oil. Moreover, the presence of essential oil influences cathodic reactions and the hydrogen reaction mechanism, traduced by the modification of the cathodic Tafel slopes values (βc).

The displacement of E_corr gives an idea about the inhibitor type, a displacement higher than 85 mV indicates that the inhibitor is cathodic or anodic type. For the one less than 85, it indicates that the inhibitor acts as a mixed inhibitor type. In our case the displacement of E_corr is 52 mV, indicating that TZ essential oil acts as a mixed inhibitor type (Hsissou et al. 2020b,d; Ouknin et al. 2018, 2020b).

From the data in Table 3, it can be seen that in the presence of 3 g·L⁻¹ of TZ essential oil, the value of the inhibition efficiency (IE %) is 83.80% and that the corrosion current densities (Icorr) decrease considerably.

5.6 Electrochemical behavior during reciprocating sliding tests

The open-circuit potential behavior (OCP) before, during, and after the tribocorrosion test is presented in Figure 6. The studied test is evaluated in the frequency of 1 Hz with displacement amplitude of 1 mm, 10,000 cycles for the normal force (2 N) in 0.5 M H₂SO₄ without and with the addition of 3 g·L⁻¹ of TZ essential oil.

Figure 6:

Evolution of the open circuit potential recorded before, during, and after fretting tests of TZ oil in sulfuric middle at 1 Hz, 1 mm, and 10,000 cycles for the normal force (2 N).

From Figure 6 it is clear that the OCP move to the more cathodic values and is established during the friction test, regardless of the environment tested. On the other hand, the most cathodic values are measured in a 0.5 M H₂SO₄ environment without the addition of essential oil, the addition of 3 g·L⁻¹ TZ essential oil shows that the OCP range above the previous with 100 mV.

In the environment of 0.5 M H₂SO₄, the quasi-stationary values of the potential are caused by the surface layer elimination. At the end of the contact, a part of the surface area is totally or partially destroyed on the contact areas. At the end of the sliding test, it is interesting to note that there is a reformation of the surface layer, which is responsible for the value OCP increasing sharply from the one measured before the friction test.

For the environment 0.5 M H₂SO₄ with the addition of 3 g·L⁻¹ of TZ essential oil, the quasi-stationary value of OCP shows the same trend as the one observed in 0.5 M H₂SO₄ only. The gap of 100 mV observed in the OCP compared to the one of the 0.5 M H₂SO₄ environment indicates that the passage of corundum ball (Al₂O₃) on the contact area does not destroy the surface layer formed and/or the one of essential oil adsorbed on the surface. Concerning the oscillations of OCP during friction (Curve (2) in Figure 6) in the presence of TZ essential oil, they can be attributed to the adsorption or partial removal (depassivation/repassivation) of the essential oil components adsorbed on the metal surface.

5.7 Periodic depassivation and repassivation

The periodic fluctuation of the passive layer thickness during the intermittent sliding is reflected by the OCP cyclic evolution (Figure 7). The OCP drops during sliding and rises during nonsliding periods (t_off = 200 s) because it is the result of coupling the passive and depassivated areas.

Figure 7:

Evolution of the OCP of AISI 304L with time intermittent (t_off = 200 s) sliding tests.

It is clear from Figure 7 that the OCP move to the cathodic value during the sliding. At the end of the sliding test (5 s), it is interesting to indicate that there is the reformation of the surface layer, which is responsible for the raise of the OCP value compared to the one measured before the friction test. Probably, the load of 2 N did not completely destroy the passive film or there was still enough time for the repassivation of the surface, they can be attributed to the adsorption or partial removal of TZ essential oil components adsorbed on the metal surface (Bratu et al. 2007; Ouknin et al. 2020a).

5.8 Wear mechanisms

Figure 8 represents the different features of wear obtained after 10,000 cycles in the three environments tested.

Figure 8:

SEM-EDX after reciprocating sliding tests performed of stainless steel (AISI 304): (a) in ambient air, (b) 0.5 M H₂SO₄ and (c) in 0.5 M H₂SO₄ with TZ essential oil (3 g·L⁻¹ TZ).

As shown in Figure 8a the friction in dry condition with relative humidity (HR = 50%), there is the presence of the debris (3rd body), reflecting the wear adhesive mechanism, the debris was retained in the contact area or it was ejected outside the contact zone. However, in a sulfuric environment without essential oil (Figure 8b), a physicochemical attack on the AISI 304L surface as shown in Figure 8b, reflecting the tribochemical wear mechanism, that indicates the difference between the two environments in term of the destroyed surface.

Furthermore, the wear mechanism in the presence of TZ essential oil (3 g·L⁻¹) is different than those observed in the dry and in sulfuric acid without the addition of essential oil. The last one is purely wear abrasive without 3rd body. On the other hand, Figure 8c indicates the presence of the parallel striations to the friction direction.

In the 0.5 M H₂SO₄ and dry (50% HR) environments, the observation of the features of wear shows that we are in the presence of a wear mechanism, this is confirmed by the high value of the friction coefficient and the decrease of dissipated energy (Figure 4). The high value of the friction coefficient is because of the adhesive type opposing to the movement after the connections rupture, the mechanism involves the 3rd body, facilitate the sliding is one of its consequence identified. In the presence of TZ essential oil, the lubricant aspect of the environment explains the low values of the friction coefficient and those of the dissipated energy. The EDX spectrum of the metal surface in three environments shows that in the presence of TZ essential oil, there is an oxygen bond corresponding to the presence of phenolic compounds adsorbed on the metal surface, as shown at a concentration of 3 g·L⁻¹ the surface is improved compared to that in the absence of the inhibitor.

5.9 Wear volume loss

From the equation of half-ellipse area, the worn area has been calculated using the following Equation (5):

R and r represent the large and small radius of the ellipse, respectively. The volume worn is obtained by multiplying the area of the ½ ellipse by the length of the trace rubbed either, δ = 1 mm.

The different depths of wear obtained after 10,000 cycles in the various environments studied are gathered in Figure 9.

Figure 9:

(a, b, and c): Wear volume obtained after 10,000 cycles in the three environments studied.

From Figure 9, we note that the maximum wear volume of 3754.14 ± 65.20 µm³ corresponds to the dry environment Air-HR = 50%, followed by that of 0.5 M H₂SO₄ with a volume worn of 2817.94 ± 47.80 µm³. However, in the presence of TZ essential oil in a sulfuric medium, the volume worn was approximately 1484 ± 21.13 µm³. This reduction of AISI 304L surface damage in the presence of essential oil can be because of the presence of oxygen atom in the functional groups of essential oil compounds, that are generally known by their anticorrosion activity. Those compounds could inhibit the chemical and mechanical effect on the AISI 304L surface, according to the two possible mechanisms. The first mechanism is named chemisorption; in this mechanism, the neutral molecules are adsorbed on the AISI 304L surface and sharing electrons between oxygen atom and iron. Concerning the second mechanism, the organic molecules of TZ essential oil are adhered to the metal/solution interface and decrease the surface area in which the anodic and cathodic reaction takes place, this mechanism can be because of the difficulty of the protonated molecules to approach the positive charge of stainless steel surface (Boumezzourh et al. 2019). The lubricating aspect of the essential oils allows the protection of the surface layer formed and/or the essential oil adsorbed on the surface during friction, which explains the reduction of wear volume in the presence of TZ essential oil (Bratu et al. 2007).

6.1 Density functional theory calculations

6.1.1 QP parameters of structures

The HOMO and LUMO orbitals are very important tools to describe the global reactivity of systems with unsaturated bonds and/or heteroatoms such as oxygen, nitrogen, and sulfur, and, the reactive functional groups, for example, –NH, –N=N–, –C=C–, C=O, –CN, –OH, =S, aromatic moiety, etc. In this respect, a molecule with a high value of E_HOMO, low value of E_LUMO, and low value of ΔE_inh is more polarizable and it is generally associated to a high chemical reactivity and low kinetic stability. Interestingly, if ΔN is positive and less than 3.6, the compound has a tendency to donate electrons (π and/or lone pair electrons) to the unoccupied orbital (3d) of the iron surface to form coordinate covalent bonds. Besides, the negative value of ΔE_bd-inh indicates that the compound is ready to accept free electrons (3d) from the metal, to form retro-donating bonds.

Therefore, the high value of ΔN and μ_inh and the lower value of ΔE_inh reflect that the compound exhibits a strong capacity to share their electrons with the iron surface (110), and consequently the compound is more adsorbed on the iron surface (110). Furthermore, the negative value of ΔE_bd-inh means that the compound exhibits a stronger ability to receive electrons from the metal. In this study, the calculated QPs aimed to evaluate global reactivity of nonprotonated forms (Ei) i.e., E1, E2, E3, and E4, and also their protonated forms (EiH+) i.e., E1H+, E2H+, E3H+, and E4H+; the QP parameters are exposed in Table 4 and in Figure 11. Accordingly, the protonation of the studied compounds leads to an increase of ΔN and μ_inh and a decrease of ΔE_inh. This result suggests that the transfer of electrons from the studied compounds to the iron surface is very easy when these compounds are in their protonated form. This indicates the prominent role of the protonation to ensure the formation of adsorbent bonds. It is also worth noting that the retro-donating character of these compounds becomes very important in their protonated forms. All these results point to the fact that the thymol is considered to be a more effective adsorbent onto the iron surface, because of its high value of ΔN (0.82 e), lower value of ΔE_inh (2.26 eV), and lower negative value of ΔE_bd-inh (−0.57 eV). From all QPs results of the studied protonated compounds, we conclude that the strength of adsorption can be ranged as follow: E4H+ > E1H+ > E2H+ > E3H+. Figure 11 shows that E4H + structure is different from that of the E1H+, by the presence of hydroxyl group (–OH). Consequently, the high adsorption of E4H + on the iron surface, with respect to the E1H + could be attributed to the number of lone pair electrons present on the oxygen atom of the hydroxyl group. Similarly, to investigate the role of aromaticity attached to E4H+ and E1H+, the QPs of E4H+ and E1H + without aromaticity (E4H + WA and E1H + WA) are calculated (Table 4 and Figure 11). The inspection of Table 4 and Figure 11 shows that the destruction of the aromaticity of E4H+ and E1H + causes a strong decrease of ΔN and a great increase of ΔE_inh, which indicate clearly the effect of aromaticity on the adsorption of E4H + toward Fe (110) surface.

Table 4:

Calculated QPs for the components as demonstrated in Figure 10.

	E HOMO (eV)	E LUMO (eV)	ΔEinh (eV)	μ inh (eV)	η inh (eV)	ΔN (e)	ΔEbd-inh (eV)
E1	−6.20	−0.30	5.90	−2.95	5.90	0.16	−1.48
E2	−6.46	−0.37	6.09	−3.05	6.09	0.15	−1.52
E3	−7.36	−0.38	6.98	−3.49	6.98	0.10	−1.75
E4	−6.14	−0.45	5.69	−2.85	5.69	0.17	−1.42
E1H+	−4.61	−1.59	3.01	−1.51	3.01	0.55	−0.75
E2H+	−4.74	−1.64	3.10	−1.55	3.10	0.53	−0.78
E3H+	−4.12	−0.88	3.24	−1.62	3.24	0.49	−0.81
E4H+	−3.26	−1.00	2.26	−1.13	2.26	0.82	−0.57
E1H + WA	−7.85	−0.21	7.64	−3.82	7.64	0.07	−1.91
E4H + WA	−7.44	−0.33	7.11	−3.56	7.11	0.09	−1.78

Figure 11:

Energy gap ΔE_inh, electron back donating energy ΔE_bd-inh and fraction of electrons transferred ΔN for all investigated structures.

6.1.2 Local reactivity study

6.1.2.1 FMO and ESP analysis

To determine the active regions of the studied compounds, the distribution of HOMO and LUMO densities (FMO) and the depiction of the ESP surfaces are calculated. The ESP analysis is widely used to provide more information about the overall molecular charge distribution and predicting reactive positions of nucleophilic and/or electrophilic attacks. The red color region represents the high negative charges of electrostatic potential (EP), the blue color region represents strongly positive EP, the green region in the ESP surfaces corresponds to a moderate positive charge EP, and the yellow region is moderate negative charge EP. Figure 12 shows the representations of FMO and ESP surfaces related to the compounds Ei and EiH+. From Figure 12, we can see that the protonation of Ei compounds influences the LUMO densities. In contrast, the HOMO densities of these compounds are almost unchanged. For example, for E4 both the HOMO of the benzene ring and HOMO of the hydroxyl group (–OH) are not influenced by the protonation, but it must be noted that the protonation effect generates a high density of LUMO on the HO–benzene ring bond, which is absent in the nonprotonated form of E4. This observation is confirmed by the ESP analysis of E4, in which the density of positive charge is mainly sited around the hydrogen atom of the hydroxyl group (–OH), the last becomes relatively important in its protonated form E4H+. Furthermore, we note that E4H + presents both a strong density of the negative charge and a moderate density of positive charge throughout carbon atoms of the benzene ring. Moreover, for E4H + we can observe a moderate density of negative charge on the oxygen atom of the hydroxyl group (–OH), this result indicates clearly that the protonated form of thymol (E4H+) have a strong active site when this compound is interacting with the iron surface by donor-acceptor interaction to form chemical coordination high bonds. Similarly, the ESP pictures of E2 and E2H+, given in Figure 11, show for both forms that, the region of high negative charges is observed around carbon atoms of the benzene ring, that suggesting the high ability of E1 to donate its electrons to the iron surface. The ESP picture of E3 shows a high negative charge around the two double bonds (C=C) in the cycle ring, which is approximately dispreaded in its protonation form of E3H+; suggesting the low reactivity of E3 toward the iron surface. In other words, the E2 and its protonated form E2H + even if they are not plane, present active sites for the nucleophilic and electrophilic attacks. This reflects evidently the more reactivity of E2 and E2H + compared to E3 and E3H+. Combining with the above results, we conclude that the strong interaction EiH+…Fe surface in this study is related to the protonated form of thymol (E4H+), resulting from its high ability to donate electrons (π and/or lone pair electrons) to the unoccupied orbital (3d) of iron surface, to form coordinate covalent bonds, and accept free electrons (3d) from the iron surface to form retro-donating bonds. Consequently, the thymol will be the appropriate tribocorrosion inhibitor among the compounds under study.

Figure 12:

Depictions of HOMO and LUMO densities and ESP surfaces for E1H+, E2H+, E3H+, and E4H + compounds. Note to ESP plots: (red) strong negative electrostatic potential (EP); (yellow) moderately negative (EP); (blue) strong positive (EP); (green) moderately positive (EP).

6.1.2.2 Parr functions analysis of E4 and E4H+

To identify the most nucleophilic and electrophilic sites of organic compounds, Parr functions P− and P+ are extensively used to give clear information about atomic sites responsible for the nucleophilic and electrophilic interactions, respectively. However, the high values of P− and P+ correspond to the most nucleophilic and electrophilic centers, respectively. Although, the atoms with negative values (or the values close to zero) of P− and P+ are considered as nonreactive atoms. The values of electrophilic P+ and the nucleophilic P− Parr functions are calculated for the significant atoms of the nonprotonated and protonated form of thymol and regrouped in Table 5. Then, the good nucleophilic and electrophilic centers are characterized by maximum values of P− and P+, respectively, and vice versa. As shown in Table 5, the protonation process modifies the nucleophilic P− and electrophilic P+ Parr functions. Surprisingly, we note an increase in the number of the nucleophilic centers around the benzene ring (Table 5). In other words, we particularly note that the protonation of thymol causes an increase of the nucleophilic character of oxygen atom O7 of hydroxyl group, and also causes a significant increase of the electrophilic character for such carbon atoms of the benzene ring (C2, C3, C4, C5, and C6). Which indicates that the nucleophilic and electrophilic behavior of thymol will be more favored in its protonated form than in the nonprotonated one. In the light of these results, we note that the protonation process increases the nucleophilic and electrophilic attacks throughout the benzene ring and hydroxyl group of thymol toward the iron surface, and ensuring high adsorption between the thymol and Fe (110) surface.

Table 5:

Significant nucleophilic P− and electrophilic P+ Parr functions of the thymol in its protonated (E4H⁺) and nonprotonated form (E4).

E4			E4H+
Atoms	P−	P+	Atoms	P−	P+
C1	0.351	−2.576	C1	0.266	−1.261
C2	−0.163	0.291	C2	0.162	0.992
C3	0.326	−0.050	C3	0.213	0.322
C4	0.159	0.624	C4	0.253	6.927
C5	−0.083	2.028	C5	0.391	3.097
C6	0.210	0.298	C6	0.049	0.992
O7	0.158	0.099	O24	0.206	−0.004

1. For atom numbering see Figure 10.

6.2 Adsorption behavior analysis

For the studied compounds the planarity is an important factor that should be considered to understand the adsorption mode of these compounds on the Fe (110) surface. In this context, before starting this study, some significant dihedral angles of the protonated compounds are calculated and illustrated in Figure 13. According to this Figure, we note that the dihedral angles calculations around the full atoms of E1H+, E3H+, and E4H + are close to ±0 or 180° (except two methyls of isopropyl group), that signify high planarity of these compounds. However, we evidenced that despite these compounds having a planar area, the E4H+ is one among compounds adsorbing almost on flat orientation onto the iron surface (110) to maximize surface coverage and contact (see later in Figure 14). Thus, is probably because of the presence of hydroxyl group attached to the plane of the benzyl ring moiety of E4H+ (protonated thymol), in this case, the high inhibition efficiency of the protonated thymol could be attributed to the number of lone pair electrons present on the oxygen atom of the hydroxyl group. Otherwise, we note that the p-cymene molecule is relatively neighboring to the Fe (110) surface with respect to the γ-terpinene, which indicates the role of the aromatic ring (delocalization of π electrons) for stronger adsorption. In addition, it appears that the hydroxyl group of the borneol molecule is not adhered to the metal surface, outstanding to the presence enormously of the steric effect caused by the presence of the methyl group as well to the nonplanarity of this molecule, which provides certainly a weak interaction between the borneol and the iron surface.

Figure 13:

Dihedral angles of inhibitors given in degree.

Figure 14:

Side views of stable adsorption configurations of inhibitors in their protonated form on the Fe (110) surface in solution (100 molecules of H₂O, 10H₃O⁺, and 5SO₄²⁻) at 297.15 K.

The nature of inhibitor interactions and their orientation onto Fe surface (110) was studied by the MC method. This technique was performed to predicting the strength and behavior adsorptions of each inhibitor. Side views of stable adsorption configurations of the studied interfaces (inhibitor/100water/20H₃O⁺/10SO4²⁻/Fe (110)) using adsorp-tion Locator modules are given in Figure 14. From this figure, we suggest that the adsorption regions of all inhibitors on Fe (110) are unsaturated double bonds C=C and/or oxygen atom (–O–). To estimate the most adsorption between the inhibitor and iron surface such energies are calculated: the total energy (E_T) reports the global energy for the systems under study (inhibitor/solution/Fe(110)), this parameter is calculated by optimizing the whole system. Therefore, the adsorption energy (E_ads), which displays the energy, required when the relaxed inhibitors are adsorbed on the Fe surface (110), this parameter is commonly recognized to evaluate the strength and mechanism of adsorption. Then, the rigid adsorption energy (E_Rads) reflects the energy released, when the unrelaxed inhibitor molecule is adsorbed on the Fe surface (110) before its optimization step. The deformation energy (E_Def) is the energy released when the adsorbed inhibitor molecule is relaxed on the Fe (110) surface; another term of energy namely the desorption energy dE_ads/dNi, represents the required energy when one of the molecule inhibitors was removed from the system of inhibitor/solution/Fe (110) surface.

Table 6 shows that the system of (E4H+/solution/Fe(110)) is the most stable (low value of E_T) among the four systems under study. Further, the negative values of E_ads mean that the studied adsorption can be spontaneously (Gece 2008). Moreover, we observed that the system of E4H+/solution/Fe(110) shows the high negative value of E_ads (−825.45 kcal/mol) comparing to the other systems under study, which indicates a stronger interaction between the thymol inhibitor and the iron surface (110) in the acidic solution (100 H₂O, 20 H₃O⁺, 10 SO4²⁻), this is mainly because of the presence of electron-donating group (–OH) attached to the benzene ring characterized by delocalization π-electrons, that serves as supplementary reactive sites of the larger thymol adsorption on Fe (110) surface (Emregul and Atakol 2004). As consequence, we conclude that the thymol is the best tribocorrosion inhibitor among the four compounds under study. The obtained results are in accordance with the experimental results revealing that the adsorption nature of extract TZ essential oil on the stainless mild in 0.5 H₂SO₄ is a typical chemisorption process.