Evaluation of corrosion inhibition and adsorption behavior of Thymuszygis subsp. gracilis volatile compounds on mild steel surface in 1 m HCl

2.1 Plant material

The aerial parts of TZ were gathered in May 2016 (full flowering) in Khenifra (Morocco). A sample voucher has been deposited in the herbarium of the Faculty of Sciences of Marrakech (Morocco). The plant materials were dried at ambient temperature before isolation of the volatile compounds.

2.2 Essential oil isolation

The dried plant material (100 g) was distilled in water (3 h) using a Clevenger apparatus according to the method recommended by the European Pharmacopoeia Convention Inc (1997). The return on volatile compounds was 1.4%. The essential oil distillates were dried on anhydrous sodium sulfate, filtered, and stored at 4°C until analysis.

2.3 Gas chromatography (GC) and GC-mass spectrometry (MS)

TZ essential oils were analyzed using a Perkin-Elmer Turbo mass detector as described previously (Ouknin et al., 2018a).

2.4 Preparation of materials

The samples used in this study were cylindrical discs cut from an MS rod with the following composition: 0.1 wt.% P, 0.38 wt.% Si, 0.02 wt.% Al, 0.05 wt.% Mn, 0.25 wt.% C, 0.07 wt.% S, and the remaining Fe used for weight loss measurements. The disc was fitted by silicon on a plastic tube as a holder, leaving the surface area exposed to the corrosive electrolyte. The surface of the MS was prepared with emery paper by increasing the grain size (800, 1200, and 2000), rinsed with bidistilled water, degreased with acetone in an ultrasonic bath for 5 min, washed again with bidistilled water, and then dried with warm air before use. The aggressive solutions of HCl (1 m) were prepared by diluting 37% analytical HCl with bidistilled water. The concentration range of TZ volatile compounds was 0.5–3 g/l. This concentration range was chosen based on the maximum solubility of TZ oil.

2.5 Weight loss measurements

In the gravimetric experiment, weight loss measurements were conducted in 1 m HCl with and without the presence of different concentrations of TZ volatile compounds ranging from 0.5 to 3 g/l at 298 K. After 6 h immersion in 1 m HCl, the samples were weighed using an analytical balance (accuracy±0.0001 mg). The weight loss enabled us to calculate the average corrosion rate in mg/cm²/h. The corrosion rate (W) and inhibition efficiency (E%) were calculated from Equations (1) and (2), respectively:

where Δm (mg) is the sample weight before and after immersion in the tested solution; W₀ and W_inh are the values of rate corrosion (mg/cm²/h) of MS in uninhibited and inhibited solutions, respectively; S is the sample area (cm²), and t is the exposure time (h). The surface coverage degree was calculated using Equation (3):

where θ is the surface coverage and W_inh and W₀ are the corrosion rate for steel in the presence and absence of an inhibitor, respectively.

2.6 Electrochemical studies

Electrochemical measurements were made in a single-compartment electrochemical cell designed for the assembly of different types of flat samples for electrochemical tests (298 K) with a three-electrode system. The working electrode (WE), in the form of a 2.8 cm diameter disc, was embedded in polytetrafluoroethylene. A saturated calomel electrode and a platinum disc electrode were used as reference and auxiliary electrodes, respectively. The temperature was thermostatically controlled at 298 K.

2.7 Potentiodynamic polarization curves

Polarization curve studies were worked out using the EG&G Instruments potentiostat-galvanostat (model 273A) at 298 K with and without the addition of various concentrations of TZ (0.5–3 g/l) in 1 m HCl at a scanning frequency of 0.5 mV/s. Before recording the cathodic polarization curves, the MS electrode was polarized at −800 mV for 10 min. For anodic curves, the electrode potential was scanned from its corrosion potential after 30 min to the free corrosion potential at more positive values. The test solution was deaerated with pure nitrogen. The open circuit potential (OCP) was able to reach a stable state that was reached after 1800 s. Previously, electrochemical measurements were performed in the absence and presence of the inhibitor. In the case of polarization method, inhibition efficiency (E_I%) was determined using the following relation:

where I₀ and I_inh are the corrosion current density without and with the inhibitor, respectively.

2.8 EIS

Electrochemical measurements OCP recording over time and EIS were worked out using an EG&G Instruments potentiostat (model 273A) controlled by CorrWarre software. The circular MS exposure surface (2.8 cm diameter) to the solution was used as a WE. After the steady-state current at a given potential was determined, the sine wave voltage (10 mV) peak to peak, at frequencies between 10⁻² and 10,000 Hz, was superimposed on the remaining potential. Computer programs were used to check the measurements made at rest potentials after 30, 60, and 90 min exposure.

The impedance diagrams are provided in the Nyquist representation. Charge transfer resistance (R_t) and double layer capacitance (C_dl) were obtained from Nyquist plots. R_t was measured from the impedance difference at low and high frequencies as suggested by Tsuru et al. (1978). The inhibition efficiency obtained from R_t was calculated according to the followed relationship (5):

R _t and R′t are R_t values without and with TZ oil, respectively.

C _dl and the frequency at which the imaginary component of the impedance is maximal (−Z_max) were determined by Equation (6):

where ω=2π.f_max. The impedance diagrams were obtained for frequency range 10,000–10⁻² Hz at the OCP for MS in 1 m HCl in the presence and absence of TZ oil.

3.1 SEM-EDX

MS samples were immersed in 1 m HCl in the absence and presence of TZ oil for 6 h immersion. SEM-EDX was carried out using an FEI Quanta 200, EDAX Metek New XL30 instrument at an accelerating voltage of 20 kV.

3.2 3D profilometry

MS micrographs surface before and after immersion in 1 m HCl with and without corrosion inhibitor were analyzed using the microtopographic method (3D), the characterization of the surface effected using the method with high-resolution optics, piloted by the software of profilometry “surface map”. This method is based on the utilization of a table XY, which moves on the surface to be characterized under a light pen; the last sends on the surface a beam of white light and analyzes the beam reflected by this surface by means of a confocal microscopy device provided with a chromatic lens, allowing a coding of the distances by the slowness of wave of the thoughtful light. The microtopographic analysis was carried out using the Mountain Map Universal software, which allows a wide variety of data processing.

3.3 FTIR analysis

The chemical composition of MS surfaces in 1 m HCl in the presence of TZ was assessed by FTIR analysis (Perkin Elmer, UATR Two). Infrared spectra were measured in the wavelength range 4000–400 cm⁻¹.

4.1 Chemical composition of the volatile compounds

The volatile compounds of TZ obtained by hydrodistillation were analyzed by GC and GC-MS. Six main components were identified, which accounted to 90.7% of the total amount (Table 1). TZ oil was dominated by oxygenated monoterpenes (54.4%) followed by hydrocarbon monoterpenes (36.3%). Among them, thymol (42.5%), p-cymene (23%), γ-terpinene (8.9%), and borneol (4.8%) were identified as major compounds (Table 1). According to the results, it was obvious that the studied volatile compounds belong to “thymol chemotype”. The molecular structures of the main components are illustrated in Figure 1. Recently, an analysis of TZ oil showed that the main compounds were thymol (20.06–32.46%), carvacrol (16.07–20.74%), p-cymene (15.32–20.74%), and γ-terpinene (2.68%; Yakoubi et al., 2014).

Figure 1:

Molecular structures of the major components of TZ oil.

4.2 Weight loss measurements

Experiments were performed with different concentrations of TZ oil at 298 K on MS corrosion in 1 m HCl was studied by weight loss measurements after 6 h immersion. E% and W are summarized in Table 2.

The gravimetric data obtained in the absence and presence of TZ oil at different concentrations are shown in Table 2. It is very clear that TZ inhibits MS corrosion in 1 m HCl at all concentrations (0.5–3 g/l), and W decreases continuously with increasing concentration at 298 K. Indeed, Figure 2 shows that W of MS decreases with increasing inhibitor concentration, whereas E% of TZ increases with increasing concentration to reach a maximum value of 80.4% at a concentration of 3 g/l. Chebli et al. (2019) reported that the essential oils of Thymus algeriensis show an inhibitory efficacy of 81.68% at 2 g/l. This behavior can be attributed to the increase of the surface covered θ (E%/100). This is due to the adsorption of TZ phytochemical components onto the MS surface resulting in the blocking of the reaction sites and as a consequence the protection of the MS surface from the attack of the corrosion active ions in the acid medium. Consequently, we can conclude that TZ is a good corrosion inhibitor for MS in 1 m HCl.

Figure 2:

W variation and E% of MS in 1 m HCl in the presence of TZ oil.

4.3 Adsorption isotherm and thermodynamic parameters

It is well recognized that the adsorption process of the inhibitor depends on its electronic characteristics, nature of the metal surface, temperature, steric effects, and different degrees of surface activity. In fact, H₂O solvent molecules could also be adsorbed at the metal/solution interface. In the aqueous solution, the adsorption of inhibiting molecules can be considered as a quasi-substitution process between the aqueous phase inhibitor Inh_sol and the water molecules on the electrodes surface H₂O_ads (Darriet et al., 2013).

where x is the size ratio, that is, the number of water molecules replaced by one organic inhibitor.

To depict the adsorption process of the inhibitor on the metal surface, adsorption isotherms were used. The Frumkin, Temkin, and Langmuir adsorption isotherms were used to describe the adsorption process of TZ oil on the metal surface.

The Langmuir adsorption isotherm is associated with the chemisorption or physisorption phenomenon, whereas the Temkin adsorption isotherm explains the heterogeneity formed on the metal surface (Wahyuningrum et al., 2008). In this case, the Langmuir, Frumkin, and Temkin adsorption isotherms were applied to explain the adsorption process of TZ oil on the MS surface:

where θ is the coverage surface, K is the adsorption-desorption equilibrium constant, C_inh is the inhibitor concentration, and g is the adsorbate parameter.

The dependence of the covered surface θ obtained by the ratio E%/100 as a function of TZ concentration (C_inh) was graphically tested for these various adsorption isotherms.

The linear regression parameters between C/θ and C are listed in Table 3, and the straight lines of C/θ with respect to C in 1 m HCl at 298 K are shown in Figure 3. It is evident that all linear correlation coefficients (R²) are almost equal to 1, and the slope values are also close to 1, indicating that the adsorption of TZ on the steel surface is governed by the Langmuir adsorption isotherm. This result shows that the adsorbed molecules occupy only one site and that there is no interaction with the other adsorbed species as demonstrated by Cheng et al. (2007).

Figure 3:

Langmuir adsorption isotherm of TZ oil on MS surface in 1 m HCl at 298 K.

4.4 OCP

OCP variation over time of MS in 1 m HCl with and without the addition of different concentrations of TZ is shown in Figure 4. The figure shows that stable OCP values are obtained after 1800 s immersion. There is an also observed introduction of TZ oil into the corrosive medium and a potential shift to noble values than those observed in the blank solution. The positive displacement of the corrosion potential depends on TZ concentration. This suggests the influence of TZ volatile compounds on anodic and cathodic polarization. Similar results were obtained by Boumezzourh et al. (2019) on the essential oils of Ammodaucus leucotrichus, which show that with the presence of essential oils the potential reaches noble values, suggesting the adsorption of compounds on the metal surface.

Figure 4:

OCP plots for MS in 1 m HCl in the absence and presence of TZ oil.

4.5 Potentiodynamic polarization curves

The polarization curves of MS specimens in 1 m HCl in the absence and presence of different concentrations of TZ essential oil at 298 K are grouped together in Figure 5. The respective kinetic parameters, including corrosion current density (I_corr), corrosion potential (E_corr), cathodic slopes (βc), and inhibition efficiency (IE%), are provided in Table 3.

Figure 5:

Polarization curves of MS in 1 m HCl in the presence and absence of TZ oil.

From the polarization curves analysis (Figure 5), it is revealed that the addition of essential oil shows an inhibition effect in both anodic and cathodic parts of polarization curves. This is due to the modification of the evolution of the cathodic hydrogen mechanism as well as the anodic dissolution of steel, which suggests that TZ strongly inhibits the corrosion process of MS, and its capacity as a corrosion inhibitor is improved as its concentration increases. In addition, the slope values of the cathodic Tafel βc, in the presence of TZ oil, change significantly with the concentration of the inhibitor, indicating the influence on cathodic reactions, which are consistent with the results demonstrated by Cao (1996).

Table 3 shows that I_corr decreases significantly with increasing inhibitor concentration. As a result, inhibition efficiency (IE%) increases with the concentration of the inhibitor to reach its high value (83.57%) at 3 g/l. This behavior suggests that the protective film of TZ oil adsorption formed on the surface of MS tends to be increasingly complete and stable. The maximum displacement value (E_corr) is 35 mV, which suggests that TZ oil acts as a mixed-type inhibitor (Boumezzourh et al., 2019).

4.6 EIS

EIS was performed to investigate the concentration effect on MS in 1 m HCl in the absence and presence of TZ oil. The last was investigated by EIS at 298 K after an exposure time of 30, 60, and 90 min in 1 m HCl. Nyquist plots for MS obtained on the interface in the absence and presence of TZ at different concentrations are given in Figures 6–8.

Figure 6:

Nyquist diagrams of MS in 1 m HCl with and without TZ oil after 30 min immersion.

Figure 7:

Nyquist diagrams of MS in 1 m HCl with and without TZ oil after 60 min immersion.

Figure 8:

Nyquist diagrams of MS in 1 m HCl with and without TZ oil after 90 min immersion.

As shown in Figures 6–8, in uninhibited and inhibited 1 m HCl, impedance spectra show a single capacitive loop, indicating that steel corrosion is mainly controlled by the charge transfer process (Bentiss et al., 2005). It should be noted that these capacitive loops in 1 m HCl are not perfect semicircles that can be attributed to the frequency dispersion effect due to the roughness and heterogeneity of the electrode surface (Bockris and Yang, 1991). In addition, the capacitive loop diameter in the presence of the inhibitor is larger than that of the control solution and increases with the concentration of the inhibitor. This suggests that the impedance of the inhibited substrate increases with the concentration of TZ oil and leads to good inhibition performance. Electrochemical parameters such as R_t, C_dl, and f_max (maximum frequency) derived from Nyquist diagrams and E_Rt inhibition efficiency (%) are calculated and presented in Table 4.

In line with the corrosion parameters (R_t, f_max, C_dl, and E_Rt) in Table 4, it appears that R_t increases with inhibitor concentration and that therefore E_Rt at 3 g/l is 83.3%, 82.25%, and 82% for 30, 60, and 90 min, respectively. Indeed, the presence of TZ oil is accompanied by an increase of R_t in acid solution, confirming a charge transfer process that mainly controls MS corrosion. The double layer capacity values are also reduced in the presence of an inhibitor and the decrease in C_dl values. The decrease in C_dl is due to the adsorption of TZ oil on the metal surface, which results in the formation of a film or complex from the acid solution. It should be noted that increasing the value of R_t with the concentration of the inhibitor leads to an increase in the effectiveness of corrosion inhibition. The work carried out by Chebli et al. (2019), on essential oils of Thymus broussonnetii Boiss subsp. broussonnetii, also showed that R_t and E_Rt increase with oil concentration. However, the double layer capacity values are also reduced in the presence of an inhibitor.

4.7 SEM-EDX

Figure 9A–C shows the SEM-EDX of MS samples after exposure to 1 m HCl and the changes that occurred during the corrosion process in the absence and presence of TZ oil. Severe corrosive attack is observed for the sample exposed to 1 m HCl (Figure 9A) and in the presence of TZ oil (Figure 9B and C), and the surface is considerably improved. This enhancement in surface morphology indicates the formation of a good protective film by the volatile compound of TZ, which is responsible for corrosion inhibition. The EDX spectrum of MS includes peaks corresponding to all the elements present in the inhibiting molecules, indicating their adsorption to the surface of the MS. Similar results were obtained by Ouknin et al. (2018b) on essential oils of Thymus munbyanus, which show that at a concentration of 3 g/l the surface is improved compared to that in the absence of the inhibitor.

Figure 9:

SEM-EDX of MS after 6 h immersion in (A) 1 m HCl, (B) 1 m HCl with 2 g/l TZ, and (C) 1 m HCl with 3 g/l TZ.

The TZ oil inhibition effect suggests a strong binding of oil components on the metal surface due to the presence of heteroatoms, blocking the active sites and thus reducing W. In addition, the inhibition of this oil can be assigned to the synergistic intermolecular effect of the various active components such as thymol and carvacrol (Raja and Sethuraman, 2009).

4.8 Roughness measurements

The roughness of the MS surface was assessed by microtopography (3D) before and after corrosion in the absence and presence of TZ at different concentrations (0.5–3 g l⁻¹).

It is clearly seen from Figures 10 and 11 that the steel sample shows a rough surface due to acid corrosion. However, the presence (2 and 3 g/l) of volatile compounds retarded the corrosion and the surface of the inhibited MS specimen is smoothened as shown in Figures 12 and 13.

Figure 10:

Microtopographic images of MS surface after polishing.

Figure 11:

Microtopographic images of MS after immersion tests in 1 m HCl without inhibitor.

Figure 12:

Microtopographic images of MS after immersion tests in 1 m HCl with 2 g/l TZ oil.

Figure 13:

Microtopographic images of MS after immersion tests in 1 m HCl with 3 g/l TZ oil.

The decrease in roughness can be very well understand to be due to the formation of adsorbed protective film of components from TZ on the metal steel surface by the process of physical adsorption.

4.9 FTIR analysis

The important IR absorption bands of inhibitors are given in Figure 14. The strong absorption band (Figure 14) at 3424 cm⁻¹ is attributed to O-H stretching vibration that at 2927 cm⁻¹ the absorption band is related to C-H stretching vibration. The strong band at 1623 cm⁻¹ is assigned to C=O stretching vibration. The C-H bending band in -CH₂ is found to be at 1407 cm⁻¹. Other absorption bands at 1138, 1102, and 1054 cm⁻¹ are due to C-O stretching vibration.

Figure 14:

FTIR spectrum of TZ on MS surface after immersion in 1 m HCl with 3 g/l TZ.

The absorption bands below 1000 cm⁻¹ correspond to the aliphatic C-H group (Shuduam & Xianghong, 2012; Ouknin et al., 2018b). This result confirms that TZ contains functional groups such as (O-H) and (C=O).