Electrochemical Characterization of CO₂ Corrosion Inhibition of API X100 by a Gemini Surfactant Under Static and Dynamic Conditions

Abstract

In this research work, the electrochemical evaluation of a non-ionic gemini surfactant as a green corrosion inhibitor for X100 pipeline steel in CO₂-saturated brine solution was carried out by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves (PPC). The corrosion inhibition performance of the gemini surfactant was studied in static and hydrodynamic conditions at room temperature and 60 °C. Electrochemical measurements showed that the inhibitor’s performance was enhanced with increasing inhibitor concentration and with increasing exposure time at room temperature, reaching the highest inhibition efficiency (η) at 100 ppm. With increasing temperature, the inhibitor efficiency decreased, with similar behavior at all concentrations. The analysis of the cathodic polarization curves at different rotation speeds showed the strong influence of mass transport on the cathodic process in the absence and the presence of the inhibitor. Under hydrodynamic conditions, PPC and EIS results indicated that the best inhibitor performance was with a concentration of 50 ppm, achieving a maximum inhibition efficiency of 91%. The adsorption of the inhibitor molecules on the surface obeyed the Langmuir isotherm, and the type of adsorption was mixed in all the study conditions. Surface characterization by scanning electron microscopy (SEM) revealed the formation of a protective corrosion inhibitor film.

1. Introduction

Corrosion is one of the greatest challenges facing the oil and gas industry, as it affects safety and operating costs. According to a NACE survey in 2018, the global annual corrosion cost to the oil and gas industry was estimated to be over $1.370 billion [1]. This electrochemical phenomenon occurs at all stages of the supply chain, from exploration and extraction (upstream) to transportation and storage of hydrocarbons (downstream) [2,3]. Several factors influence the mechanism and kinetics of metal corrosion in the oil field, such as operating pressure and temperature conditions, the presence of corrosive gases, and fluid flow dynamics [4].

Corrosion in the oil and natural gas industry manifests itself in several ways, the most common being CO₂ corrosion (sweet corrosion) in produced fluids [5]. This gas dissolves in the aqueous phase and acidifies the environment, accelerating the corrosion process. Approximately 60% of metal component failures in the oil field are due to CO₂ corrosion [6]. Although the mechanism of CO₂ corrosion has been extensively investigated and characterized in the literature [7,8], there are still aspects that are not fully understood, primarily the role of corrosion product/corrosion inhibitor films, as well as hydrodynamic conditions [9].

Carbon steel is the main material used for the transportation of crude oil and gas, because it is economically accessible and has good mechanical performance [10]. However, it is very susceptible to uniform corrosion and other forms of localized corrosion in CO₂ environments [11].

One of the most common methods of prevention and control of internal corrosion of carbon steel in the oil industry is the use of corrosion inhibitors, which are chemical compounds added in small quantities to the aqueous medium that reduce the corrosion rate [12]. For a chemical compound to be considered a viable candidate as a corrosion inhibitor, it is not enough to have excellent performance in reducing corrosion rates; it must also have minimal environmental impact. For this reason, the scientific community has focused on the study of compounds that may have corrosion inhibition activity and are environmentally friendly [13]. Among the environmentally friendly corrosion inhibitors evaluated in CO₂ environments is a type of surfactant known as gemini surfactant [14,15]. This organic compound is composed of two conventional surfactant molecules chemically linked by a “spacer” near their hydrophilic groups.

Regarding studies on the synthesis and assessment of gemini surfactants as corrosion inhibitors in CO₂ environments, Zhao & Li [16] synthesized four gemini surfactants (identified as n-3OH-n, where n = 12, 14, 16, and 18) and evaluated their corrosion inhibitory action on carbon steel in a CO₂- and H₂S-saturated brine solution. They found that the compound 14-3OH-14 obtained the highest inhibition efficiency of 97.3% at a concentration of 35 ppm. Wang et al. [17] investigated the corrosion inhibition performance of an asymmetric gemini surfactant for N80 steel in a CO₂-saturated brine solution. Their PPC results revealed that the surfactant acted as an anodic corrosion inhibitor, reaching its maximum inhibition efficiency (88.2%) at 80 ppm. In addition, Sanchez-Salazar et al. [18] studied a gemini surfactant whose hydrophobic portion was obtained from mango seed oil for X120 steel in a CO₂-saturated brine solution. Their results indicated that the surfactant had its best inhibition performance at a concentration of 100 ppm. In previous work, Carmona et al. [19] synthesized an imidazole-based gemini surfactant derived from palm oil. They evaluated the corrosion inhibition performance of this surfactant for supermartensitic stainless steel (SMSS) in a sour environment, achieving inhibition efficiencies above 95% using a concentration of 25 ppm. It is noteworthy that SMSS is not typically protected by corrosion inhibitors in the oil and gas industry, despite its high susceptibility to sour corrosion [20]. Therefore, the aim of this work was to study the potential application of this gemini surfactant in inhibiting sweet corrosion of X100 steel in a CO₂ environment under static and hydrodynamic conditions using electrochemical measurements and surface characterization.

2. Materials and Methods

2.1. Materials and Test Solution

Cubic specimens of API X100 steel were machined to dimensions of 10 × 10 × 10 mm, welded with copper wire, and encapsulated in epoxy resin, leaving an exposed area of 1 cm². The chemical composition (wt.%) of the X100 pipeline steel is given in

Table 1. Chemical composition of API 5L X100 (wt.%).

Element (wt.%)
C	Mn	Si	Cr	Ni	Mo	Nb/V	Cu	Ti	Fe
0.05	1.75	0.36	0.36	0.29	0.2	0.03	0.51	0.04	Bal.

. The test solution was a CO₂-saturated brine solution (3.5 wt.% NaCl) to study the corrosion process under sweet conditions. The reagents used were of analytical grade. The organic, film-forming corrosion inhibitor is a novel gemini surfactant derived from palm oil (Figure 1) whose synthesis and chemical composition were reported in a previous work [19]. The corrosion inhibitor was dissolved in 5% by weight isopropanol and added to the solution at concentrations of 25, 50, and 100 ppm.

2.2. Electrochemical Measurements

Electrochemical measurements were performed using an electrochemical workstation with a conventional three-electrode configuration cell: the X100 steel was the working electrode (WE), an Ag/AgCl (sat., KCl) electrode was the reference electrode (RE), and a graphite rod was the auxiliary electrode. Before each electrochemical test, the surface of the WE was polished with 600-grade SiC abrasive paper, rinsed with distilled water, degreased with acetone, and dried in air. The test solution was bubbled with CO₂ gas for 1 h; then, the open-circuit potential (E_ocp) of the WE was measured for 30 min to ensure steady-state conditions prior to applying electrochemical measurements. The CO₂ bubbling continued throughout the test. For the EIS measurements, an AC perturbation signal of 10 mV vs. E_ocp of amplitude with a frequency range of 10,000 to 0.1 Hz was imposed, recording 10 lectures per frequency decade. The PPC measurements were carried out applying an overpotential of ±500 mV vs. E_ocp, using a potential scan rate of 1 mV/s. Electrochemical measurements in static conditions were conducted at 25 and 60 °C. The temperature of the experiment was maintained using a water bath temperature control system.

Turbulent flow conditions were simulated using a rotating cylinder electrode (RCE). The rotation rate was fixed at 1000 RPM, with a Reynolds number (Re) of 16,084. At Re > 200, the turbulent flow regime is reached [21]. For this case, the WEs were cylindrical electrodes of X100 steel with an exposed area of 4.14 cm².

2.3. Surface Analysis by SEM-EDS

For surface characterization, the samples were exposed for 24 h in the inhibited (50 and 100 ppm) and uninhibited (0 ppm) solutions. The corrosion morphology of the samples was examined using a focused ion beam scanning electron microscope (FIB-SEM, FEI Nova 200 NanoLab), coupled to an energy-dispersive spectrometer (EDS, EDAX Si-Li detector). The EDS was used to identify the elemental composition of the steel surface and corrosion products in the inhibited and uninhibited samples.

3. Results and Discussion

3.1. Corrosion Inhibitor Performance Under Static Conditions

3.1.1. Open Circuit Potential (E_ocp)

Figure 2 shows the effect of the addition of the corrosion inhibitor on the open circuit potential value of the X100 steel in the CO₂-saturated brine solution at different concentrations of gemini surfactant at 25 and 60 °C. As shown in Figure 3, the electrochemical potential at which the kinetics of the anodic and cathodic reactions reach quasi-equilibrium was similar in the absence of corrosion inhibitor at both temperatures (E_OCP around −675 mV vs. Ag/AgCl). After adding corrosion inhibitor, the E_ocp value shifted towards more positive values (anodic direction) as the inhibitor concentration increased. However, this shift (∆E) was not enough to classify the surfactant as an anodic inhibitor, but rather as a mixed-type corrosion inhibitor [22].

3.1.2. EIS Measurements

Figure 3 shows the EIS spectra of the X100 steel immersed in CO₂-saturated brine solution at different concentrations of corrosion inhibitor after 0 h and 24 h of exposure. In Nyquist curves, a larger semicircle diameter reflects greater resistance to charge transfer and therefore greater corrosion resistance. As depicted in Figure 4a, the diameter of the semicircle increased as the inhibitor concentration increased, except at 25 ppm at the beginning of the test, where the diameter of the semicircle was smaller than the diameter of the blank solution. Moreover, the Nyquist curves, in the presence of the inhibitor, apparently included a capacitive loop followed by a straight line at low frequencies. In Figure 3b, the Bode ϕ plots showed a single peak, indicating a single time constant (τ) associated with the charger transfer process of the electrochemical reactions.

After 24 h (Figure 3c), the EIS spectra underwent a significant modification and exhibited a significant improvement in corrosion inhibition performance at concentrations above 25 ppm. This can also be seen in the impedance modulus |Z| values at low frequencies in the Bode plots (Figure 3c,d), where the |Z| values increased as inhibitor concentration increased. Furthermore, the presence of a τ was more clearly observed in the Bode ϕ plot in the frequency range of 10⁴–10² Hz at concentrations of 50 and 100 ppm, which can be attributed to the presence of the corrosion inhibitor film adsorbed on the metal surface [23].

At a temperature of 60 °C, the Nyquist curves (Figure 4a) showed the same behavior as at room temperature but manifested a decrease in impedance values, reflecting an increase in the kinetics of the corrosion reactions and a reduction in the corrosion inhibition effect of the surfactant. At 0 h of exposure, the diameter of the semicircles increased as inhibitor concentration increased, whereas at the end of the test (Figure 4c), the anticorrosive properties of the surfactant improved over time, making the inhibition performance similar at all concentrations. This behavior can also be observed in the value of |Z| at low frequencies (Figure 4a,b). The Bode ϕ plot also exhibited the same τ constants as those presented at room temperature.

It is noteworthy that the Nyquist curves usually presented a diffusive behavior at lower frequencies at both temperatures at 0 h in the solution in the presence of the inhibitor, suggesting mass transport limitations in the charge transfer process (mixed control) [24]. After 24 h of exposure, the straight line at low frequencies was not present, meaning that the inhibitor adsorption blocked the active sites on the surface, causing the charge transfer process to dominate in the measured frequency range.

The quantitative analysis of the EIS spectra for the X100 steel in CO₂-containing solution with and without the corrosion inhibitor was performed by fitting with electrical equivalent circuits (EEC), which were proposed to describe the metal–solution interface (Figure 5). The electrical parameters of the EECs with their physical meanings are as follows: R_s is the solution resistance, R_ct stands for the charge transfer resistance, and Q_dl is the non-ideal capacitance represented by a constant phase element (CPE). The CPE is a parameter derived from fractional calculus, whose physical interpretation has been attributed to surface heterogeneity, which causes a non-uniform distribution of interfacial capacitances along the metal surface [25]. The real capacitance (C_dl) of the electrode was estimated from the CPE parameters (the admittance Y₀ and the exponent n) using the Brug et al. [26] Equation (1):

$$C_{dl}={Y_0}^{{\displaystyle \frac{1}{n}}}{({R_s}^{-1}+{R_{ct}}^{-1})}^{{\displaystyle \frac{n-1}{n}}}$$

(1)

Likewise, the parameters Q_f and R_f involve the presence of the corrosion inhibitor film interface. Finally, W is the Warburg element, which stands for the semi-infinite diffusion process of electroactive species in solution. The expression for the impedance of the element, W (Z_w), is given by (2) [27]:

$$Z_W={\displaystyle \frac{\sigma }{\sqrt{\omega }}}\left(1-j\right)$$

(2)

where ω is the angular frequency, j is the imaginary number, and σ is the Warburg coefficient.

The use of different EECs for fitting the EIS spectra indicates an evolution of the metal–solution interface as a function of time. The simplified Randles circuit (Figure 5a) and the EEC of Figure 5c were used for the blank solution, whereas the EEC of Figure 5b,c were used in the presence of corrosion inhibitor. Some authors [28,29] have also proposed these electrical equivalent circuits for carbon steel in CO₂-saturated solutions in the absence and presence of corrosion inhibitors.

Table 2. Electrochemical parameters derived from the fitting of EIS data of API X100 steel in CO₂-saturated NaCl 3.5% solution at 25 °C at 0 and 24 h of exposure time.

T (°C)	t (h)	Cinh (ppm)	Rs (Ωcm2)	Rf (Ωcm2)	Qf-Y0 (sn/Ωcm2)	Qf-n	Rct (Ωcm2)	CPEdl-Y0 (sn/Ωcm2)	Qdl-n	σ (Ωcm2/s0.5)	Cdl (μF/cm2)	χ2 × 104	η (%)
25	0	0	7.20				119.9	8.54×10−4	0.75		248.39	0.35
		25	4.89	3.43	4.39×10−3	0.21	102.3	4.78×10−4	0.82	5.75	160.14	0.48	-
		50	6.79	21.28	1.10×10−3	0.99	157.3	1.06×10−3	0.70	8.04	223.07	1.22	23.78
		100	7.63	11.41	5.02×10−4	0.74	168.50	3.14×10−5	1.00	14.32	31.39	1.38	28.84
	24	0	7.07				136.6	1.13×10−3	0.84		621.90	1.50
		25	4.78	9.35	1.25×10−3	0.75	229.5	6.23×10−5	1.00		62.29	1.94	40.48
		50	6.37	8.24	1.75×10−4	0.77	524.13	4.03×10−4	0.80		127.68	2.95	73.94
		100	8.33	16.49	1.85×10−4	0.79	608.06	3.02×10−4	0.84		130.95	9.05	77.54
60	0	0	3.53				32.1	1.44×10−3	0.77		288.17	18.50
		25	3.40	1.98	6.36×10−4	0.56	71.57	6.90×10−4	0.80		150.10	6.80	55.15
		50	5.07	2.87	8.72×10−4	0.65	102	6.23×10−5	1.00	9.85	62.28	10.01	68.53
		100	1.97	4.58	7.77×10−4	0.74	116.50	4.34×10−5	1.00	5.81	43.39	12.80	72.45
	24	0	3.64	9.81	4.94×10−3	0.79	61.29	4.96×10−4	1.00	15.02	496.30	4.00
		25	5.54	3.39	3.51×10−4	0.85	262.3	7.15×10−4	0.79		230.00	3.28	76.48
		50	8.54	3.07	7.22×10−5	0.83	254.3	4.26×10−4	0.83		188.87	3.91	75.90
		100	3.78	6.48	4.92×10−4	0.78	305.20	1.14×10−4	0.92		64.28	1.51	79.92

summarizes the electrochemical parameter values ascertained from fitting the EIS data using the EECs in Figure 5. At both temperatures, the R_ct values were very close for all concentrations at 0 h. In contrast, the R_ct values augmented significantly after 24 h as the inhibitor concentration increased. This indicates that the inhibitor molecules require a long time to adsorb to the metal surface. Moreover, the C_dl value usually decreased as the inhibitor concentration increased, as a result of the adsorption of inhibitor molecules, which produced a decrease in the local dielectric constant and/or an increase in the thickness of the electrochemical double layer by replacing the water molecules previously adsorbed at the interface [30]. The corrosion inhibition efficiency (η) was calculated from the R_ct values using the following equation [31]:

$$\eta ={\displaystyle \frac{R_{ct\left(inh\right)}-R_{ct\left(0\right)}}{R_{ct\left(inh\right)}}}\times 100$$

(3)

where the subscripts ₀ and _inh correspond to the solution without and with the corrosion inhibitor. As shown in Table 2, although the effectiveness of the surfactant in suppressing the corrosion rate decreased with increasing temperature, the values of η remained around 75–80% at 100 ppm at room temperature and all concentrations of inhibitor at 60 °C, showing good inhibition performance in the CO₂ environment.

3.1.3. Polarization Curve Results

Figure 6 displays the polarization curves of X100 steel in the CO₂-saturated brine solution at different concentrations of corrosion inhibitor at 25 and 60 °C. At room temperature (Figure 6a), the E_corr shifted towards more positive values in the presence of the inhibitor, while at 60 °C (Figure 6b), the E_corr value was similar with and without corrosion inhibitor, except for the concentration of 25 ppm. It should be pointed out that this shift was less than ± 85 mV compared to the E_corr of the uninhibited solution. Furthermore, similar behavior of the anodic and cathodic branches was obtained at both temperatures, since the anodic and cathodic current densities shifted to lower values in the presence of the corrosion inhibitor, with the inhibition effect in the cathodic branch at 60 °C the most noticeable. These results revealed that the gemini surfactant acted as a mixed-type corrosion inhibitor, with a predominant effect on the anodic reactions at 25 °C and the cathodic reactions at 60 °C.

The kinetics parameters extracted from polarization curves using the Tafel extrapolation method are listed in

Table 3. Kinetic parameters obtained from the PPC under static conditions at 25 and 60 °C.

Temp (°C)	Cinh (ppm)	Ecorr (mV)	ba (mV/dec)	bc (mV/dec)	icorr (mA/cm2)	η (%)
25	0	−711.64	86.52	−214.20	0.0344	-
	25	−618.43	55.11	−391.55	0.0197	42.59
	50	−626.50	59.51	−356.51	0.0137	60.11
	100	−635.13	58.46	−286.13	0.0068	80.13
60	0	−645.00	28.80	−320.10	0.1350	-
	25	−683.16	65.03	−296.22	0.0239	82.30
	50	−639.09	55.74	−385.93	0.0248	81.63
	100	−636.75	60.12	−380.07	0.0364	73.01

. The anodic (ba) and cathodic (bc) Tafel slopes provide information on the regime that controls the kinetics of corrosion reactions. The values of ba indicate that anodic dissolution was controlled by charge transfer, whereas the values of bc show that the cathodic reactions were strongly influenced by mass transfer [7]. As shown in Table 3, the addition of a corrosion inhibitor slightly modified the slope of the cathodic curve, increasing the bc values. However, the overall analysis of the polarization curves showed that the inhibitor acted as a mixed-type inhibitor at both temperatures.

The inhibitor’s performance using PPC was determined using the corrosion current density (i_corr), according to the following equation:

$$\eta ={\displaystyle \frac{i_{corr\left(0\right)}-i_{corr\left(inh\right)}}{i_{corr\left(0\right)}}} \times 100$$

(4)

It can be noted that the trend obtained regarding inhibitor efficiency as a function of concentration was very similar to the results obtained with EIS after 24 h of exposure at both temperatures. The highest η achieved from the PPC was 81.63% at 25 ppm at 60 °C.

3.2. Corrosion Inhibitor Performance Under Hydrodynamic Conditions

3.2.1. Open Circuit Potential (E_ocp)

Figure 7 shows the variation in the E_ocp values of the X100 steel in CO₂-saturated brine solution at different concentrations of corrosion inhibitor under turbulent flow conditions. By adding the inhibitor in the first few minutes, the E_ocp value shifted to approximately 50 mV in the anodic direction, which could be caused by preferential adsorption of inhibitor molecules at anodic sites. Likewise, the E_ocp value showed more positive values compared to the static condition. Some studies attributed this effect to increased corrosion product formation resulting from enhanced flow-induced anodic dissolution [32].

3.2.2. EIS Results

Figure 8 depicts the EIS spectra of the X100 steel in the CO₂-saturated brine solution at different concentrations of corrosion inhibitor under turbulent flow conditions at 0 h and 24 h of exposure. The Nyquist curve in the absence of the corrosion inhibitor at 0 h (Figure 9a) apparently consisted of a capacitive loop at high and intermediate frequencies and an inductive loop at low frequencies. This inductive loop has been attributed to the relaxation process of intermediate species, such as (FeOH)_ads, adsorbed on the steel surface in a solution at pH 4 [33]. Similar behavior was reported by Belarbi et al. [34]. After 24 h, this inductive loop did not appear, and diffusive behavior was present at low frequencies. Under turbulent flow conditions, flow patterns known as vortices or eddy currents can enhance mass transport [35].

In the presence of the corrosion inhibitor at both exposure times (Figure 8a,b), the diameter of the semicircle increased, reaching a maximum impedance at 50 ppm. A further increase in concentration decreased the semicircle’s diameter. Moreover, the dimensions of the Nyquist curves decreased significantly after 24 h, suggesting a deterioration in the adsorbed film of surfactant molecules due to the shear stress produced by the flow.

Bode ϕ plots (Figure 8b,d) exhibited two τ for the uninhibited and inhibited solutions, which can be associated with the processes and interfaces described in the Nyquist plots. As shown in Figure 8d, a sharp decrease was observed in the values of |Z| and ϕ after 24 h. This is a clear sign of the desorption of surfactant molecules and the decrease in the corrosion inhibition performance.

The fitting of the EIS spectra was performed in the absence of the inhibitor under turbulent flow conditions using the EECs in Figure 9. At 0 h, the EEC included a parallel RL arrangement that contemplates the inductive loop at low frequencies in the Nyquist plots, in which R_ad and L_ad are the resistance and inductance of adsorption of intermediate species. At 24 h, the Randles circuit, including the W element, was employed. Finally, the EEC of Figure 5b was also used for EIS spectra in the presence of the corrosion inhibitor.

Table 4. Electrochemical parameters derived from the fitting of EIS data of API X100 steel in CO₂-saturated NaCl 3.5% solution at 0 and 24 h of exposure time under turbulent flow conditions.

t (h)	Cinh (ppm)	Rs (Ωcm2)	Rf (Ωcm2)	Qf-Y0 (sn/Ωcm2)	Qf-n	Rct (Ωcm2)	CPEdl-Y0 (sn/Ωcm2)	Qdl-n	Rad (Ωcm2)	Lad (Hcm2)	σ (Ωcm2/s0.5)	Cdl (μF/cm2)	χ2 × 104	η (%)
0	0	12.82				245.3	3.27×10−4	0.72	119.60	26.27		37.08	22.8
	25	18.07	47.37	1.03×10−4	0.80	1340.4	3.45×10−5	0.76				3.51	2.79	81.70
	50	17.17	80.73	6.85×10−5	0.80	1653.4	5.90×10−5	0.76				6.89	7.44	85.16
	100	17.72	27.61	1.68×10−4	0.83	871.76	6.63×10−5	0.81				13.37	1.37	71.86
24	0	14.17				115	2.16×10−3	0.74			8.85	617.20	9.25
	25	27.92	21.36	6.99×10−4	0.67	566.44	9.32×10−5	0.87				39.48	3.88	79.70
	50	23.25	122.77	8.09×10−4	0.64	639.66	2.60×10−4	0.78				61.45	2.87	82.02
	100	19.61	26.93	5.39×10−5	0.73	445.82	2.10×10−4	0.70				20.28	3.54	74.20

lists the electrochemical parameter values derived from the fitting of the EIS spectra under hydrodynamic conditions. The values of R_ct and η indicated that the best performance of the gemini surfactant under turbulent flow conditions was achieved by adding a concentration of 50 ppm, while the highest concentration evaluated (100 ppm) produced the lowest R_ct and η values. In addition, the corrosion inhibition effect of surfactant caused a decrease in the order of magnitude of the values of C_dl compared to those obtained in the blank solution, due to the adsorption of inhibitor molecules at the metal–solution interface.

3.2.3. Polarization Curve Results

Figure 10 shows the polarization curves of the X100 steel in the CO₂-saturated brine solution at different concentrations of the corrosion inhibitor under turbulent flow conditions, and

Table 5. Kinetic parameters obtained from the PPC under turbulent flow conditions.

Cinh (ppm)	Ecorr (mV)	ba (mV/dec)	bc (mV/dec)	icorr (mA/cm2)	η (%)
0	−495.37	85.37	--	0.5179	--
25	−535.78	105.56	--	0.3052	41.07
50	−560.75	79.81	−392.75	0.044	91.05
100	−544.8	115.47	--	0.2894	44.1

summarizes the kinetic parameters derived from Tafel extrapolation. As depicted in Figure 10 and Table 5, the E_corr value shifted toward more negative values but was less than −85 mV relative to the E_corr value of the blank solution, indicating that the surfactant acted as a mixed-type inhibitor.

Except for the concentration at 50 ppm, the cathodic branch manifested a diffusion-controlled process and a well-defined limiting current density (i_lim). Electroactive species such as H⁺ and H₂CO₃ can diffuse from the bulk solution and be reduced at the metal–solution interface [24]. Meanwhile, the anodic dissolution presented activation control. This was confirmed by the Tafel slope values shown in Table 5, where bc = ∞ and ba was around 120 mV/dec. Unlike static conditions, the predominant inhibition effect at 1000 RPMM was mainly on cathodic reactions. This fact was evident in the behavior of the cathodic curve at 50 ppm, where no limiting current was observed.

Finally, the η values revealed that the best performance of the corrosion inhibitor at 1000 RPM was at 50 ppm, agreeing with the results of EIS.

3.2.4. Cathodic Kinetics in the CO₂-Saturated Brine Solution with and Without the Corrosion Inhibitor

For a further understanding of the inhibition action of gemini surfactant on cathodic kinetics, Figure 11 shows the cathodic polarization curves of X100 steel in a CO₂ environment at different rotation rates with and without the addition of 50 ppm of corrosion inhibitor. At 0 ppm (Figure 11a), it can be noted that the concentration polarization region and the i_lim value increased as the rotation rate increased. In contrast, the cathodic polarization curves exhibited a less defined concentration polarization region, especially at rotation rates below 3000 RPM. This effect suggests that adsorption of inhibitor molecules in cathodic active sites affects the cathodic reaction mechanism. Dominguez et al. [36] claimed that organic corrosion inhibitors only modify the charge transfer mechanism, whereas the i_lim remains unchanged. This statement can be confirmed by comparing the cathodic curves with and without inhibitor at rotation rates below 3000 RPM, as shown in Figure S1 of the Supplementary Materials.

The dependence between rotation rate expressed as peripheral velocity (u_RCE) and i_lim in a RCE is given for the empirical linear relationship found by Eisenberg (Equation (5)) [37]:

$$i_{lim, i}=0.0791nF{C}_{b, i}{d}_{RCE}^{-0.3}{v}^{-0.344}{D}_i^{0.644}{u}_{RCE}^{-0.7}$$

(5)

where n is the transferred electrons, d is the external diameter of the cylindrical electrode, F is the Faraday constant, C is the concentration of the cathodic species (i) in the bulk solution, ν is the kinematic viscosity of the solution, and D_i is the diffusion coefficient of the cathodic species (i). The i_lim value was ascertained by averaging cathodic current density (i_c) values between −850 and −900 mV vs. Ag/AgCl [38].

Figure 12 shows the comparison of i_lim values obtained without and with the addition of the inhibitor as a function of u_RCE. It can be seen that the i_lim values had good linear correlation with the u_RCE^0.7. Furthermore, the i_lim values were lower, and the slope of the straight line decreased in the presence of the inhibitor. Based on the Eisenberg expression (Equation (5)), the decrease in slope could be attributed to the fact that the adsorption of inhibitor molecules on the metal surface further limits mass transport at the interface, thus decreasing the diffusion coefficient of the electroactive species.

3.3. Adsorption Isotherms

To ascertain the mechanism of adsorption of the inhibitor molecules on the steel surface, linear regression analysis was conducted using different adsorption isotherm models, such as Temkin, Freundlich, and Langmuir. The corrosion inhibition efficiencies obtained from the EIS results at 24 h were related to the surface coverage (θ) of the adsorbed inhibitor molecules as follows [39]:

$$\theta ={\displaystyle \frac{\eta }{100}}$$

(6)

The experimental data were fitted with different adsorption models, revealing that the Langmuir isotherm presented the best fit in all conditions. Figure 13 shows the linear behavior of the C_inh/θ vs. C_inh plot at room temperature, at 60 °C, and under hydrodynamic conditions. The mathematical expression of the Langmuir isotherm model is given by Equation (7).

$${\displaystyle \frac{C_{inh}}{\theta }}={\displaystyle \frac{1}{K_{ads}}}+C_{inh}$$

(7)

where C_inh (in M) is the concentration of gemini surfactant, and K_ads (in L/mol) is the adsorption equilibrium constant. This model establishes that molecules occupy independent sites on the surface, and there are no interactions of attraction or repulsion between adjacent molecules.

The most important thermodynamic parameter of adsorption, the standard Gibbs free energy (°∆G_ads, in kJ/mol), was calculated from K_ads using Equation (8):

$${∆G}_{ads}=-RT\mathit{ln}(55.5K_{ads})$$

(8)

Table 6. Thermodynamic adsorption parameters derived from the Langmuir adsorption isotherm.

Condition	Kads (L/mol)	∆Gads (kJ/mol)
25 °C	26,456	−35.16
60 °C	268,442	−40.90
1000 RPM 25 °C	148,527	−39.43

shows the thermodynamic parameters derived from the adjustment of experimental data with the Langmuir adsorption isotherm model. The ΔG_ads values ranged between −35 and −40 kJ/mol. The negative value of ΔG_ads indicates that the adsorption of the inhibitor on the steel surface is a spontaneous process. According to the literature [31,40], ΔG_ads values in the range of −40 kJ/mol imply a charge transfer from the inhibitor molecules to the metal surface, forming a coordinate covalent bond (chemisorption). Therefore, ΔG_ad values were found for the three experimental conditions in the range of −35 to −40 kJ/mol, which correspond to chemical adsorption.

3.4. Surface Analysis by SEM-EDS

SEM images of the steel surface after 24 h of exposure to the test solution are displayed in Figure 14. As shown in Figure 14a, the sample prior to exhibition in the environment showed a freshly polished surface. In contrast, the steel surface at 0 ppm (Figure 14b) exhibited an extensive formation of porous, cracked, and poorly protective corrosion products throughout the entire surface. With the addition of corrosion inhibitor (Figure 14c), the steel surface showed a smooth and compact layer, with little evidence of corrosion products. The EDS results (Figure 14a1–c1) support the anticorrosive properties of the corrosion inhibitor film, since the highest amount of O and Cl was identified in the sample exposed to the uninhibited solution, whereas the amount of C, O, and Cl decreased, and the amount of the matrix, Fe, increased by adding the corrosion inhibitor.

3.5. Corrosion Inhibition Mechanism of Gemini Surfactant in CO₂ Environment

In a CO₂ environment, the gaseous CO₂ dissolves in the aqueous phase, generating carbonic acid (H₂CO₃), a weak acid that dissociates to hydrogen protons (H⁺), acidifying the solution (pH 3.5–4.5). The electrochemical reactions that occur in a CO₂-saturated brine solution are the following:

For anodic reactions, the dissolution of Fe takes place according to [41]:

$$Fe+H_{2}O\leftrightarrow {FeOH}_{ads}+H^{+}+e^{-}$$

(9)

$${FeOH}_{ads}\leftrightarrow {FeOH}_{sol}^{+}+e^{-}$$

(10)

$${FeOH}_{sol}^{+}+H^{+}\leftrightarrow {Fe}_{sol}^{2+}+H_{2}O$$

(11)

Moreover, the chloride ion present in the solution can participate in the anodic dissolution through a competition process at the adsorption sites with the hydroxyl intermediates [24]:

$$Fe.H_2{O}_{ads}+{Cl}^{-}\to {FeCl}_{ads}^{-}+H_{2}O$$

(12)

$${FeCl}_{ads}^{-}+{OH}^{-}\to {FeOH}^{+}+{Cl}^{-}+{2e}^{-}$$

(13)

Das Chagas Almeida et al. [33] reported that the inductive loop associated with the adsorption of intermediate species also occurs in CO₂-free solutions, suggesting that CO₂ does not play an additional role in the anodic dissolution of Fe.

On the other hand, cathodic reactions involve the reduction of H⁺ and H₂CO₃ as follows:

$$2H_{\left(aq\right)}^{+}+{2e}^{-}\to H_{2\left(g\right)}$$

(14)

$${2H}_2{CO}_3+{2e}^{-}\to H_{2\left(g\right)}+{2HCO}_3^{-}$$

(15)

Although the direct reduction of H₂CO₃ (15) has been considered within the CO₂ corrosion mechanism, recent studies [42,43] suggest that this reaction does not contribute to the charge transfer process due to a buffering effect.

It should be pointed out that these cathodic reactions were controlled by mass transfer under turbulent flow conditions. Some authors [24,44] have suggested that the total contribution of the i_lim is composed of the sum of a component related to the diffusion of the electroactive species (i_{lim dif}) and a component independent of the flow rate attributed to the hydration reaction of H₂CO₃ (i_{lim RH2CO3}).

The corrosion inhibition of X100 steel by gemini surfactant molecules depends on their adsorption and formation in mono- or multilayers on the surface, in addition to other factors such as the chemical structure of the surfactant, nature of the corrosive medium, and surface charge of the metal [45]. A schematic of the chemical and physical interactions between the surfactant molecule and the metal surface is illustrated in Figure 15. In acidic conditions, it was reported that the metal surface acquires a positive charge [46]. This causes electrostatic attraction and the adsorption of chloride ions on the surface. Furthermore, the surfactant molecules can become protonated, mainly at the nitrogen (N) atom. The proton of a weak acid in the solution can be easily removed to react with the N of the imidazole ring and the N of the amine group, which have a strong basic character [47]. In this way, electrostatic interactions between the surfactant molecules and the metal surface can be carried out. Moreover, gemini surfactant molecules can be chemically adsorbed at the metal–solution interface through an interaction between the π electrons of the imidazole ring or the lone pairs of N and O heteroatoms with the unfilled d orbitals of Fe atoms on the surface through coordinate bonding.

On the other hand, the adsorption of surfactant molecules can form monolayers or multilayers on the metal surface, depending on whether the surfactant concentration in the solution is greater than the critical micelle concentration (CMC). CMC is a crucial surface activity parameter to ascertain whether a surfactant has the potential to be a good corrosion inhibitor, since above the CMC, the metal surface is covered with a monolayer of surfactant molecules, and additional molecules form multilayers on the metal surface or micelles within the solution [48,49].

According to the experimental results, a significant increase in η values was noted at 50 ppm under static conditions, whereas under turbulent flow conditions, the optimal concentration was at 50 ppm. This suggests that the CMC of gemini surfactant in solution could be around 50 ppm at room temperature. Figure 15 illustrates the arrangement of surfactant molecules forming bilayers.

4. Conclusions

A gemini surfactant derived from palm oil was assessed as a corrosion inhibitor for X100 steel in CO₂-containing solution under static and hydrodynamic conditions. The main conclusions are listed:

• The PPC results indicated that the inhibitor acted as a mixed-type inhibitor, having a predominant effect on anodic dissolution under static conditions at room temperature, and a greater influence on the cathodic reactions under static conditions at 60 °C and under turbulent flow conditions.
• The results of electrochemical measurements showed that the corrosion inhibition performance of the surfactant at 25 °C enhanced as the concentration and exposure time increased, reaching a maximum η of 80% at 100 ppm. Meanwhile, η was independent of the concentration after 24 h of exposure at 60 °C, obtaining a maximum η value of 82.3%.
• Under hydrodynamic conditions, electrochemical measurements indicated that the surfactant achieved a maximum inhibition efficiency of 91% at 50 ppm. The effectiveness of the corrosion inhibitor in suppressing the corrosion rate decreased after 24 h of exposure.
• The analysis of the adsorption isotherms revealed that the surfactant adsorption on the surface obeyed the Langmuir isotherm, with a chemisorption mechanism for the three conditions studied.
• Surface characterization by SEM and EDS confirmed the formation of a protective corrosion inhibitor film on the surface of the X100 steel.