Investigation of Sulfonium-Iodide-Based Ionic Liquids to Inhibit Corrosion of API 5L X52 Steel in Different Flow Regimes in Acid Medium

Abstract

The present work deals with the corrosion inhibition mechanism of API 5L X52 steel in 1 M H₂SO₄ employing the ionic liquid (IL) decyl(dimethyl)sulfonium iodide [DDMS⁺I^–]. Such a mechanism was elicited by the polarization resistance (R_p), potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) techniques, both in stationary and dynamic states. The electrochemical results indicated that the corrosion inhibition was controlled by a charge transfer process and that the IL behaved as a mixed-type corrosion inhibitor (CI) with anodic preference. The experimental results revealed maximal inhibition efficiency (IE) rates up to 93% at 150 ppm in the stationary state, whereas in turbulent flow, the IE fell to 65% due to the formation of microvortexes that promoted higher desorption of IL molecules from the surface. The Gibbs free energy of adsorption (ΔG°_ads) value of −34.89 kJ mol^–1, obtained through the Langmuir isotherm, indicated the formation of an IL monolayer on the metal surface by combining physisorption and chemisorption. The surface analysis techniques confirmed the presence of Fe_xO_y, FeOOH, and IL on the surface and showed that corrosion damage diminished in the presence of IL. Furthermore, the quantum chemistry calculations (DFT) indicated that the iodide anion hosted most of the highest occupied molecular orbital (HOMO), which eased its adsorption on the anodic sites, preventing the deposition of sulfate ions on the electrode surface.

1. Introduction

In the oil industry, steel corrosion is regarded as one of the most serious problems to be dealt with due to the exposure of this metallic material to different conditions throughout a variety of industrial operations. In particular, during the extraction and transport of oil, the formation of acid media and flow conditions increase the corrosion rate, which provokes the degradation of metallic structures, and therefore, pipeline failures and damage of equipment and installations, which are part of different processes as a result of the shortening of the useful life of steel pieces.^1,2 Diverse methods conceived to control corrosion at the industrial level have been put into practice; among these strategies, corrosion inhibitors (CIs) have been employed, which have been accepted willingly for their low cost and easy application with different alloys and corrosive media.^3,4 CIs control the corrosion reactions that take place on the metal by an adsorption process that is affected by different factors such as the nature and surface charge of the metal, electronic structure, steric factors, aromaticity, functional groups with double and triple bonds, and high-density heteroatoms (N, O, S, and P) present in the inhibiting organic molecules.^5,6

Inorganic compounds like chromates, nitrates, molybdates, phosphates, silicates, and arsenic have been used as CIs, which when combined with the oxide layer provide the metal surface with passivation protection.⁷ Notwithstanding, these compounds represent potential danger for the health, for they are highly toxic chemicals.⁸ Currently, environmental regulations concerning the use of chemical substances have limited the application of toxic compounds, which has led to the synthesis of low-toxicity CIs that are easy to handle and biodegradable, i.e., more environmentally friendly like the compounds known as ILs.^9,10 These molecules are salts consisting of organic or inorganic anions and organic cations that display, in general, melting points below 100 °C, i.e., these are liquids at ambient temperature and possess unique features that stem from the arrangement and chemical distribution of their ions; in addition, the ILs have negligible vapor pressure, thermal stability, nonflammability, high ionic conductivity, and wide electrochemical stability spectrum.¹¹⁻¹³

In the literature, the inhibiting properties in acid medium of ILs with imidazolium,^14,15 pyridinium,^16,17 ammonium,¹⁸⁻²⁰ pyridazinium,^21,22 and sulfonium²³⁻²⁵ cations have been reported. These last compounds have been studied in sulfamic,²³ phosphoric,²⁴ and hydrochloric and sulfuric²⁵ acids, reporting IE values from 84 to 98%. Their inhibition behavior has been attributed to the nature of the substituent groups: the presence of π electrons that are capable of interacting with the negative charge of the metal, aliphatic chains that promote hydrophobic properties, but mainly due to the capacity of S to accept electrons.

The iodide anion effect has been the subject matter of several studies, and it has been reported that the IE values of ILs with iodide anion have been higher than those displayed by other anions, as shown in Table 1. In this context, in 2008, Verma et al. analyzed ILs with the cation 2-hydroxyethyl-trimethyl-ammonium and three different anions, finding that the IL with iodide anion had higher IE values than those with acetate and chloride anions.²⁶ The anion influence on the inhibition process was studied by Mashuga et al. comparing ILs based on 1-hexyl-3-methylimidazolium with four different anions. It was reported that the IE obtained with the IL featuring the iodide anion (79%) or trifluoromethanesulfonate (81%) was slightly higher than those values given by fluoro-substituted anions such as tetrafluoroborate (78%) or hexafluorophosphate (73%).²⁷ In 2018, Azeez et al. suggested that the presence of the I^– anion as an electron donor increased the electron density of the cation heteroatoms (−C=N−) in ILs based on 1-alkyl-3-methylimidazolium iodide.²⁸

Table 1. State of the Art of Sulfonium and Iodide Compounds Used as CIs.

In addition, iodide has also been employed in acid corrosion as synergistic agent in inhibition processes of organic compounds. Cao et al. and Guo et al. showed that the IE of 1,1′[1,4-phenylenebis(methylene)] bis[3(carboxymethyl)imidazolium] chloride and l-tryptophane, respectively, increased significantly in the presence of KI as a synergistic agent.^29,30 Cao et al. indicated that the addition of KI could provide better IE by decreasing the IL concentration.²⁹ Moreover, Guo et al. suggested that iodide adsorbed on a metal surface tended to form joining bridges between organic cations and the positively charged iron surface, thus stabilizing the CI adsorption process;³⁰ similar conclusions were reported by Feng et al.³² Additionally, Farag et al.³¹ indicated that iodide could also promote a change in potential of the metal surface toward more negative values, facilitating the adsorption of positively charged species. According to the aforementioned, the inhibition properties that distinguish iodide from the halide group are its higher capacity to donate electrons, easy polarization, high hydrophobicity, greater atomic radius, and low electronegativity.³³

In addition to the effect exerted by the chemical structure on the corrosion inhibition mechanism, the medium conditions are essential factors that affect the performance of the CIs. Due to the multiple transfer phenomena of corrosive species and CI molecules in the metal–CI interphase, the proposal of a corrosion inhibition mechanism in the stationary state is complex. In addition, in the presence of flow, the adsorption mechanism is affected by factors such as sheer stress (τ_RDE) and formations of microvortexes that modify the adsorption and desorption of CI molecules and corrosive species. In contrast, the chemical structure of CIs is fundamental for the formation of stable films under flow conditions. For this reason, in the present work, a novel IL was synthesized, characterized, and evaluated in its inhibiting properties: decyl(dimethyl)sulfonium iodide [DDMS⁺I^–]. The IL was evaluated as CI of API 5L X52 steel under stationary and flow conditions [Reynolds numbers (N_RE) from 1000 to 5000] in 1 M H₂SO₄ solution. To analyze the inhibiting effect exerted by [DDMS⁺I^–], the R_p, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) electrochemical techniques were used. Afterward, the obtained data were processed by different adsorption isotherm models. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and X-ray photoelectron spectroscopy (XPS) surface analyses complemented the electrochemical results. Finally, to describe the behavior of the [DDMS⁺I^–] molecule, molecular simulation at DFT B3LYP/MIDIx theory level was carried out.

2. Results and Discussion

2.1. Electrochemical Tests

2.1.1. E_OCP as a Function of Time

Figure 1 shows the open-circuit potential (E_OCP) values for API 5L X52 steel in 1 M H₂SO₄ in the absence and presence of different concentrations of [DDMS⁺I^–]. It can be observed that the E_OCP values were displaced toward more positive values with increasing IL concentration with respect to the blank. These E_OCP displacements because of IL addition indicate both the adsorption of IL molecules on the metal surface and the formation of an inhibiting film. Furthermore, it was found that the E_OCP stability started from 800 s.

Figure 1.

E_OCP plot as a function of time of API 5L X52 steel in 1 M H₂SO₄ with and without IL.

2.1.2. Polarization Resistance

Figure 2 shows the R_p behavior as a function of: (a) the [DDMS⁺I^–] concentrations and (b) different N_RE. In general, it is observed that the slope of the current density (i) vs overpotential (η) diminished with the CI concentration (C), indicating that the presence of IL increased the R_p of the metal–electrolyte interphase due to the adsorption of inhibiting molecules from the formation of a layer on the steel surface in 1 M H₂SO₄. This inhibiting film produced resistance to the flow of electrons participating in the redox reactions, generating changes in the kinetics of the electrochemical reactions involved in the corrosion process. Furthermore, the slope increase at different N_RE in the absence and presence of IL can be observed, which indicates that the diffusive processes under flow conditions modified the IL adsorption and interfacial properties of the metal–electrolyte system.

Figure 2.

R_p of API 5L X52 steel in 1 M H₂SO₄ with (a) different [DDMS⁺I^–] concentrations at the stationary state and (b) with 150 ppm of [DDMS⁺I^–] under flow conditions at 298 K.

Table 2 displays the R_p results and it can be seen that the maximal values were obtained at C above 100 ppm of CI and at the stationary state. The IE_Rp was calculated from eq 1 ³⁴⁻³⁷

1

where the superindexes In h and 0 refer to the experiments in the presence and absence of CI, respectively.

Table 2. Electrochemical Parameters Obtained by the R_p Technique of API 5L X52 Steel in 1 M H₂SO₄ with Different Concentrations of [DDMS⁺I^–].

system	NRE	C(ppm mM–1)	–Ecorr (mV)	Rp(Ω cm2)	IERp (%)
blank	0	0/0	466 ± 0	50 ± 1
	1000	0/0	437 ± 0	44 ± 0
	2000	0/0	444 ± 0	38 ± 2
	3000	0/0	436 ± 0	23 ± 1
	4000	0/0	439 ± 0	16 ± 0
	5000	0/0	439 ± 0	16 ± 0
[DDMS+I–]	0	25/0.076	444 ± 3	95 ± 2	47.14 ± 1.78
	0	50/0.151	426 ± 4	371 ± 7	86.43 ± 0.45
	0	75/0.227	429 ± 5	473 ± 24	89.30 ± 0.34
	0	100/0.303	418 ± 6	699 ± 35	92.51 ± 0.22
	0	125/0.378	420 ± 4	673 ± 18	92.42 ± 0.23
	0	150/0.454	419 ± 3	674 ± 17	92.56 ± 0.22
	1000	150/0.454	394 ± 1	317 ± 0	85.98 ± 0.01
	2000	150/0.454	381 ± 1	278 ± 1	86.28 ± 0.06
	3000	150/0.454	376 ± 0	151 ± 1	84.49 ± 0.13
	4000	150/0.454	380 ± 4	87 ± 2	81.14 ± 0.48
	5000	150/0.454	388 ± 3	48 ± 1	67.36 ± 0.79

2.1.3. Potentiodynamic Polarization

Figure 3a,b presents the PDP behavior as a function of C and different N_RE. It is observed that the increase in C caused the displacement of the potentiodynamic curves toward low current density values with respect to the blank. Under flow conditions, the N_RE increase intensified the corrosion process in the absence and presence of IL, displacing the potentiodynamic curves toward higher current density values, which indicates more serious surface damage in the presence of IL.

Figure 3.

Potentiodynamic polarization of API 5L X52 steel in 1 M H₂SO₄ with (a) different [DDMS⁺I^–] concentrations at the stationary state and (b) with 150 ppm of [DDMS⁺I^–] under flow conditions at 298 K.

At the stationary state (Table 2), lower current density values in the presence of CI are reported; this fact can be attributed to the growing blocking of active sites as a function of the [DDMS⁺I^–] concentration, which replaced water molecules and aggressive ions such as H⁺, H₃O⁺, O^2–, and SO₄^2– on the metallic substrate. The optimal adsorption process of the CI molecules on the metal surface occurred at a concentration of 100 ppm; in contrast, higher i values were obtained under flow conditions due to the wall shear stress (τ_RDE) increase. This behavior pattern was originated by the flow regime change, which provoked higher desorption rate of the CI film as N_RE was increased and then, the progressive diminution of IE. The IE_PDP was calculated by eq 2 ^34,35

2

where the superindexes Inh and 0 refer to the experiments in the presence and absence of CI, respectively.

At the stationary state, the reported corrosion potential (E_corr) displacement values with respect to the blank fall within the interval ranging from 43 to 65 mV, i.e., within the ±85 mV interval, which indicates that [DDMS⁺I^–] matches the behavior pattern of a mixed-type CI, retarding both the cathodic and anodic reactions occurring during the corrosion process^38,39 with anodic preference due to the displacement toward positive potentials (E_corr⁰ < E_corr^Inh).⁴⁰ This phenomenon is related to the preferential adsorption of [I^–] and the contribution of the [DDMS⁺] cations on the anodic and cathodic sites, respectively, which diminished the attack of water molecules and aggressive ions in the medium. Under flow conditions, the IL anodic preference was not affected sensibly. Notwithstanding, the β_c values under flow conditions, reported in Table 3, are slightly higher than those obtained at the stationary state. This increase suggests that the IL modified the kinetics of the reduction reactions, thus retarding the hydrogen evolution by CI adsorption. Then, it can be concluded that the contribution by the IL anions and cations to the inhibition process is the same.

Table 3. Electrochemical Parameters Obtained by PDP Curves of API 5L X52 steel in 1 M H₂SO₄ with Different Concentrations of [DDMS⁺I^–].

system	NRE	C(ppm mM–1)	–Ecorr (mV)	βc(mV dec–1)	icorr(μA cm–2)	Vcorr(mm year–1)	IEPDP (%)
blank	0	0/0	466 ± 1	108 ± 1	300.18 ± 4.23	3.48 ± 0.04
	1000	0/0	437 ± 0	164 ± 31	155.6 ± 3.05	6.25 ± 0.12
	2000	0/0	444 ± 0	141 ± 12	173.76 ± 19.31	6.98 ± 0.15
	3000	0/0	436 ± 0	126 ± 10	322.32 ± 3.63	12.94 ± 0.15
	4000	0/0	439 ± 0	108 ± 3	334.14 ± 17.77	14.35 ± 0.10
	5000	0/0	439 ± 0	121 ± 13	482.82 ± 10.78	19.39 ± 0.43
[DDMS+I–]	0	25/0.076	444 ± 3	128 ± 8	170.99 ± 4.71	1.98 ± 0.05	43.03 ± 1.76
	0	50/0.151	426 ± 4	126 ± 2	64.42 ± 1.23	0.74 ± 0.01	78.53 ± 0.51
	0	75/0.227	429 ± 5	104 ± 3	48.27 ± 2.80	0.56 ± 0.03	83.92 ± 0.96
	0	100/0.303	418 ± 6	80 ± 6	27.11 ± 1.07	0.31 ± 0.01	90.97 ± 0.38
	0	125/0.378	420 ± 4	78 ± 7	24.67 ± 0.54	0.28 ± 0.01	91.78 ± 0.21
	0	150/0.454	419 ± 3	92 ± 10	23.89 ± 0.12	0.27 ± 0.01	92.04 ± 0.12
	1000	150/0.454	394 ± 1	207 ± 15	24.47 ± 0.17	0.98 ± 0.01	84.27 ± 0.11
	2000	150/0.454	381 ± 1	172 ± 21	26.25 ± 0.59	1.05 ± 0.01	84.89 ± 0.34
	3000	150/0.454	376 ± 0	176 ± 4	50.91 ± 0.08	2.04 ± 0.01	84.20 ± 0.03
	4000	150/0.454	380 ± 4	224 ± 1	74.56 ± 0.01	2.99 ± 0.01	77.69 ± 0.01
	5000	150/0.454	388 ± 3	216 ± 14	117.06 ± 8.63	4.70 ± 0.14	75.76 ± 3.58

2.1.4. Electrochemical Impedance Spectroscopy

Figure 4 shows the Nyquist impedance curves at stationary and dynamic states and in the absence and presence of IL, where the formation of depreciated semicircles can be observed, which is attributed to the surface roughness and heterogeneity. In contrast, in the presence of [DDMS⁺I^–], an increase in the semicircle diameter as a function of C can be observed, which indicates that the corrosion process was controlled by the charge transfer resistance through the interface;⁴¹ this behavior pattern occurred at both stationary and dynamic states. In contrast, the diameter decrease in the semicircles as N_RE increased indicates that the flow made the charge transfer and desorption rate of the species adsorbed on the metallic surface grow.⁴²

Figure 4.

Nyquist impedance curves recorded for API 5L X52 steel in 1 M H₂SO₄ solution with (a) different [DDMS⁺I^–] concentrations at the stationary state and (b) with 150 ppm of [DDMS⁺I^–] under flow conditions at 298 K.

In the absence of IL and the stationary state, the formation of an inductive loop in the low-frequency region can be observed, which can be attributed to intermediate reactions related to the adsorption of water, aggressive ions (H⁺, H₃O⁺, O^2–, and SO₄^2–) and/or insoluble solution corrosion products.^43,44 This behavior pattern was also observed at the dynamic state in the absence and presence of IL, which was caused by diffusive processes of the electrolyte in the metal/IL interface. Nevertheless, in the presence of [DDMS⁺I^–] at the stationary state, the inductive loop disappeared, which suggests the stabilization of the charge transfer processes because of the formation of a more homogeneous and stable inhibiting film than the one produced at the dynamic state.

Figure 5 shows the Bode plots at stationary and dynamic states. In the presence of IL, a higher displacement of the |Z| values and phase angle than those obtained with the blank can be observed, which suggests a reduction of the adsorption of aggressive ions, thus stabilizing the surface. Furthermore, the diagrams show a maximal point for the phase angle at intermediate frequencies, which indicates the existence of a relaxation time constant (τ_dl) associated with the charge transfer resistance (R_ct) and capacitance of the electric double layer (C_dl).⁴³ At the stationary state, the presence of another relaxation time constant is also observed, which is related to a capacitive process due to the accumulation of corrosion products/CI, despite the presence of the IL, the corrosive medium ions are adsorbed on the surface causing their damage.

Figure 5.

Bode plots recorded for API 5L X52 steel in 1 M H₂SO₄ solution with (a) different [DDMS⁺I^–] concentrations at the stationary state and (b) with 150 ppm of [DDMS⁺I^–] under flow conditions at 298 K.

The equivalent electrical circuits (EECs) employed to fit the experimental data from the impedance spectra are shown in Figure 6.⁴⁵⁻⁴⁸ What follows is the description of the physical interpretation of the EEC elements: R_s represents the solution resistance; R_ct is the resistance to the charge transfer on the surface, which depends on the charge transfer between the electronic conduction region (metal) and the ionic conduction region (solution); CPE_dl is a constant phase element (CPE) related to the C_dl, which suggests the accumulation of charge in that interphase; R_L is an inductive resistance and L is an inductor and both elements represent the relaxation process of the intermediates involved in the oxidation reactions (H⁺, H₃O⁺, O^2–, and SO₄^2–); and CPE_f and R_f are the capacitance and resistance related to the formation of corrosion products and/or the IL film on the metal surface.

Figure 6.

Equivalent electrical circuits of the steel corrosion process in 1 M H₂SO₄ solution: (a) under stationary and flow conditions, (b) at different [DDMS⁺I^–] concentrations at the stationary state, and (c) with 150 ppm of [DDMS⁺I^–] under flow conditions.

The constant phase elements (CPEs) in Figure 6 represent the nonideal capacitances of the EIS spectra, which were calculated with eq 3

3

where Y₀ is the proportional factor and n is an empirical exponent between 0 and 1, which is related to the surface heterogeneity, where n values close to 1 are characteristic of homogeneous surfaces.⁴⁹

Tables 4 and 5 show the EIS parameters at stationary and dynamic conditions, respectively. It is observed that the R_s values do not present variation (ΔR_s ∼ 1 Ω·cm²) at different [DDMS⁺I^–] concentrations and flow conditions. Likewise, at the stationary state, the R_f and C_f values do not show significant changes in the inhibitor film–solution interphase with increasing [DDMS⁺I^–] concentration.

Table 4. EIS Parameters Obtained for API 5L X52 Steel in 1 M H₂SO₄ without and with Different Concentrations of [DDMS⁺I^–] at the Stationary State.

		C(ppm mM–1)
parameter	blank	25/0.076	50/0.151	75/0.227	100/0.303	125/0.378	150/0.454
χ2	0.158 ± 0.004	0.193 ± 0.010	0.214 ± 0.012	0.216 ± 0.012	0.188 ± 0.035	0.187 ± 0.018	0.186 ± 0.030
Rs(Ω·cm2)	0.68 ± 0.25	1.00 ± 0.09	1.17 ± 0.10	1.13 ± 0.14	1.27 ± 0.10	1.26 ± 0.23	1.16 ± 0.26
Rf(Ω·cm2)	1.47 ± 0.31	1.73 ± 0.10	1.91 ± 0.11	1.88 ± 0.01	1.98 ± 0.10	1.98 ± 0.18	1.87 ± 0.23
Cf(μF cm–2)	0.09 ± 0.03	0.31 ± 0.03	0.25 ± 0.03	0.26 ± 0.03	0.21 ± 0.02	0.18 ± 0.06	0.14 ± 0.04
RL(Ω·cm2)	277.07 ± 46.04
L(H·cm2)	29.72 ± 2.84
Rct(Ω·cm2)	47.86 ± 3.61	278.21 ± 9.04	353.77 ± 28.29	620.84 ± 23.96	841.84 ± 27.11	734.82 ± 17.05	759.31 ± 24.14
Y0·10–6 (S·sn cm–2)	281.05 ± 21.39	70.84 ± 2.80	80.22 ± 5.83	49.61 ± 6.51	39.16 ± 6.98	46.45 ± 7.49	62.55 ± 16.46
n	0.86 ± 0.01	0.88 ± 0.01	0.88 ± 0.01	0.89 ± 0.01	0.89 ± 0.01	0.89 ± 0.01	0.89 ± 0.01
Cdl(μF cm–2)	140.46 ± 4.08	41.42 ± 1.28	50.04 ± 3.58	31.87 ± 3.02	25.61 ± 3.13	29.87 ± 3.78	42.06 ± 7.89
τdl (ms)	6.72 ± 0.54	11.52 ± 0.52	17.70 ± 1.90	19.79 ± 2.02	21.56 ± 2.72	21.95 ± 2.82	31.94 ± 6.08
Rp(Ω·cm2)	42.96 ± 2.83	280.94 ± 9.04	356.85 ± 28.29	623.85 ± 23.96	845.09 ± 27.11	738.06 ± 17.05	762.34 ± 24.14
IEEIS (%)		84.70 ± 1.12	87.96 ± 1.24	93.11 ± 0.52	94.91 ± 0.37	94.18 ± 0.40	94.36 ± 0.41

Table 5. EIS Parameters Obtained for API 5L X52 Steel in 1 M H₂SO₄ with 150 ppm of [DDMS⁺I^–] in Flow Conditions.

regimen	laminar				transitory		turbulent
NRE	1000		2000		3000		4000		5000
C(ppm mM–1)	0	150/0.454	0	150/0.454	0	150/0.454	0	150/0.454	0	150/0.454
χ2	0.158 ± 0.005	0.143 ± 0.005	0.196 ± 0.028	0.139 ± 0.014	0.121 ± 0.014	0.148 ± 0.006	0.075 ± 0.001	0.123 ± 0.004	0.056 ± 0.008	0.081 ± 0.002
Rs(Ω·cm2)	1.15 ± 0.08	0.55 ± 0.05	1.11 ± 0.18	0.91 ± 0.30	1.56 ± 0.02	0.78 ± 0.44	2.09 ± 0.51	1.59 ± 0.22	1.75 ± 0.14	1.03 ± 0.18
Rf(Ω·cm2)	1.91 ± 0.07		1.87 ± 0.16		2.28 ± 0.01		3.15 ± 0.40		3.93 ± 1.75
Cf(μF cm–2)	0.38 ± 0.01		0.42 ± 0.10		0.37 ± 0.01		0.149 ± 0.05		0.149 ± 0.07
RL(Ω·cm2)	130.77 ± 24.77	987.63 ± 73.62	108.86 ± 23.50	962.36 ± 88.12	91.09 ± 11.54	587.13 ± 55.43	90.24 ± 13.93	294.38 ± 39.74	74.28 ± 10.35	123.07 ± 18.89
L(H·cm2)	41.03 ± 0.71	266.48 ± 11.81	27.89 ± 5.23	230.37 ± 11.76	15.66 ± 1.92	124.71 ± 18.83	16.28 ± 2.94	57.79 ± 16.245	10.78 ± 1.56	40.05 ± 9.31
Rct(Ω·cm2)	28.71 ± 4.35	261.47 ± 4.83	26.78 ± 3.79	225.64 ± 20.8	22.02 ± 1.67	170.15 ± 7.57	22.69 ± 3.8	174.65 ± 6.79	17.37 ± 2.60	101.49 ± 7.49
Y0·10–6 (S·sn cm–2)	296.13 ± 1.11	55.23 ± 0.15	351.00 ± 13.57	73.55 ± 4.47	377.76 ± 21.09	88.52 ± 4.56	391.95 ± 6.06	137.23 ± 8.18	406.45 ± 3.69	158.92 ± 5.43
n	0.86 ± 0.01	0.83 ± 0.01	0.83 ± 0.01	0.807 ± 0.01	0.839 ± 0.01	0.819 ± 0.01	0.833 ± 0.01	0.815 ± 0.02	0.847 ± 0.01	0.83 ± 0.02
Cdl(μF cm–2)	136.40 ± 1.83	23.53 ± 1.67	137.5 ± 0.72	27.63 ± 0.34	150.65 ± 7.04	34.98 ± 4.66	152.17 ± 11.19	59.00 ± 2.33	165.94 ± 10.42	68.13 ± 6.78
τdl (ms)	3.92 ± 0.60	6.15 ± 0.45	3.68 ± 0.52	6.23 ± 0.58	3.32 ± 0.30	5.95 ± 0.84	3.45 ± 0.63	10.30 ± 0.57	2.88 ± 0.47	6.91 ± 0.86
Rp(Ω·cm2)	26.60 ± 3.03	207.28 ± 4.41	24.48 ± 2.62	183.69 ± 14.02	21.57 ± 1.16	132.70 ± 7.58	23.37 ± 2.57	111.21 ± 13.85	19.75 ± 2.47	56.65 ± 4.47
IEEIS (%)		87.17 ± 1.49		86.67 ± 1.75		83.74 ± 1.10		78.99 ± 2.59		65.14 ± 5.17

In contrast, the R_ct values diminished as the N_RE increased as shown by the diameter in the Nyquist spectra. However, in the presence of [DDMS⁺I^–] and independent of the N_RE, it is observed that the R_ct values grew significantly with the IL concentration, suggesting that the corrosion process was controlled by the charge transfer resistance through the interface by the IL adsorption.⁴¹ The electrical double layer of the steel–electrolyte system behaves electrically as a capacitor, i.e., there is an electric charge and discharge process that leads to the corrosion process by electron transfer. This process is represented with the C_dl electric element. At the stationary state, the C_dl values diminished with respect to the blank, indicating the formation of an IL homogeneous film that blocked the reaction sites distributed uniformly in the interphase.⁴⁶ Notwithstanding, with the N_RE increase, a gradual growth of the C_dl values is observed, which suggests the modification of the metal–solution interphase due to the reduction of the double layer thickness and/or to the increase of the dielectric constant,⁵⁰ i.e., to the formation of a thinner and more heterogeneous CI film than at the stationary state, which can be attributed to the electrolyte diffusive processes in the metal/IL interphase as a consequence of the surface damage caused by the partial hydromechanical removal of the film.⁵¹

The impedance parameter (τ_dl) was calculated employing eq 4 and it is reported in Table 4

4

It is observed that the τ_dl values increased with the CI concentration. This parameter measures the time required by the metal–solution interphase to return to an equilibrium state. This fact indicates that a higher number of IL molecules adsorbed on the surface makes the electric charge and discharge process slower, which favors the reduction of the corrosion rate.⁵² In contrast, the process dynamic state provokes the diminution of τ_dl, i.e., the acceleration of electron transfer (Table 5).

Likewise, it is observed that the R_L and L values diminished with increasing N_RE. This behavior pattern is observed at low frequencies in the Nyquist plots (Figure 3) and is associated with an increase in the desorption rate of the intermediate products of redox reactions (H⁺, H₃O⁺, O^2–, and SO₄^2–) and adsorbed [DDMS⁺I^–] molecules.

At the stationary state (Table 4), the increase in CI concentration raised the n values, i.e., the surface homogeneity; this fact confirmed the diminution of the corrosion surface damage in the presence of an inhibitor. In contrast, under flow conditions, n values were diminished (Table 5), which indicated the increase in the corrosion rate by the desorption of inhibitor molecules from the surface as a consequence of the generated sheer stress.

The R_p values were calculated by the sum of the resistive elements corresponding to each EEC. It can be observed that the R_p values were increased by the IL concentration, improving the IE_EIS calculated with eq 3. However, at the dynamic state, the reduction of the R_p values as a function of N_RE is revealed, which exerted a negative effect on the IE_EIS falling to 22% with the flow regime change.

2.1.5. Inhibition Efficiency

As it can be observed in Figure 7, [DDMS⁺I^–] presented IEs within the interval ranging from 40 to 94% at the stationary state, which can be associated with the competition of chemical species to occupy available surface active sites and suggests that on the cathodic and anodic sites occurred the preferential adsorption of [DDMS⁺] and [I^–] with respect to H⁺, H₃O⁺, O^2–, and SO₄^2–. In addition, it is revealed that from the optimal CI concentration of 100 ppm, the maximal IE of 93% was reached.

Figure 7.

IE for API 5L X52 steel in 1 M H₂SO₄ (a) at different [DDMS⁺I^–] concentrations under the stationary state and (b) with 150 ppm of [DDMS⁺I^–] under flow conditions.

According to the obtained results, the IL adsorption diminished slightly in laminar regime (N_RE = 1000–2000), which can be attributed to the increase in the transport of CI molecules toward the metal surface that kept the IL adsorption and formation of complexes,^53,54 which in turn reduced the τ_RDE negative effect and IE in 8% with respect to the stationary state. The IE fell to 86% from the transitory regime (N_RE ≥ 3000) due to the formation of microvortexes that promoted higher desorption of IL molecules from the surface, making it difficult to form a continuous CI film.⁵⁵ This negative effect exerted by the increasing electrode rotation speed was also reported by Ismail et al. and Benmahammed et al. who observed the diminution of the IE values of organic CIs from 47 and 89% up to 3 and 60%, respectively.^42,56

Meanwhile, in turbulent regime, this phenomenon was intensified, provoking the reduction of the IE down to values around 65%. The hydromechanical removal and formation of a nonuniform inhibitor film made the adsorption and orientation of IL molecules difficult, which also prompted the desorption of metal/IL/medium complexes and the adsorption of aggressive ions on newly available active sites, thus increasing the surface instability.⁵⁷

The results confirm the influence of iodide on the inhibition process, which played a major role in the obtained IE of [DDMS⁺I^–]. Also, the presence of π orbitals in S with high capacity to donate electrons and the hydrophobic properties of the decyl chain contributed to the adsorption process as reported in the literature.⁵⁸

2.2. Adsorption Isotherms

To theoretically describe the nature of the interactions between the IL and the steel surface during the inhibition process, the experimental data were fitted employing adsorption isotherm models such as Langmuir, Freundlich, Temkin, and Frumkin,^49,59,60 where θ is the surface fraction covered by the CI (θ = IE/100), which was obtained by the average of the IE values by the R_p and PDP techniques, C is the IL concentration in mM, and K_ads is the adsorption equilibrium constant.

The Langmuir isotherm of the H₂SO₄ – [DDMS⁺I^–] system is shown in Figure 8. The additional isotherm models are presented in Figure S1. The linear fitting of the experimental data produced a correlation coefficient (R²) of 0.988, a slope of 0.99, and an intercept of 0.035. This empirical model suggests the formation of a [DDMS⁺I^–] monolayer, where the adsorption occurred only on a finite number of defined sites, energetically identical and equivalent with neither lateral interactions nor steric hindrance among the CI molecules, where just one of them can occupy an active site on the surface.⁶¹ The K_ads value was equal to 2.82 × 10⁴ mM^–1 and was calculated by the reciprocal of the y-intercept (K_ads = 1/b) of the linear regression in Figure 8. This parameter allows the computation of the Gibbs free energy of adsorption (ΔG°_ads) from eq 5 (62,63)

5

where R is the universal gas constant (8.314 kJ mol^–1·K^–1), T is the system absolute temperature (K), and 55.5 is the water concentration in the solution. The ΔG°_ads of [DDMS⁺I^–] was equal to −35.36 kJ mol^–1, which suggests that the IL adsorption mechanism involved the combination of physisorption (electrostatic forces) and chemisorption (coordinate bonds),^37,64 which implied a higher interaction between the IL and the steel surface. The adsorption process can be mainly related to the chemical properties of [I^–] due to its high ionic radius and electronegativity lower than other halides, and the boosting inhibition effect by forming intermediate bridges between the metallic substrate and the [DDMS⁺] cation, which promoted the production of a CI monolayer on the surface with a hydrophobic region (cation alkyl chains) toward the solution core.⁶⁵⁻⁶⁷ Furthermore, iron presents affinity to sulfur heteroatoms, which promoted the formation of [DDMS⁺]–Fe complexes that were adsorbed directly on the metallic surface.⁶⁸

Figure 8.

Langmuir isotherm for the API 5L X52 steel electrode in 1 M H₂SO₄ solution with [DDMS⁺I^–] at 298 K.

2.3. Surface Analysis Techniques

2.3.1. SEM

Figure 9a displays the micrographs of the steel surface after its immersion in the acid medium without IL. Surface damage and the palpable mass loss due to steel oxidation and the presence of corrosion products can be observed. In contrast, the metallic samples protected with 150 ppm of [DDMS⁺I^–], Figure 9b, present a homogeneous morphology, which confirms the adsorption of the IL molecule, thus blocking the active sites, diminishing the surface damage and mass loss of the metallic samples caused by the medium attack.

Figure 9.

SEM/EDS of API 5L X52 steel after 4 h of immersion in 1 M H₂SO₄ solution: (a, c) absence and (b, d) presence of 150 ppm of [DDMS⁺I^–] at 298 K.

Figure 9c shows the EDS spectrum of API 5L X52 steel in the absence of IL. Signals corresponding to S and O can be observed, which are characteristic elements of the corrosion products that are common in steel–H₂SO₄ systems like Fe_xO_y, FeOOH, and FeSO₄. The low intensity of S and O peaks is related to the sample rinsing process after its retrieving from the corrosive medium, which favored the removal of excess corrosion products.

The EDS spectrum in the presence of IL, Figure 9d, displays the reduction of the O signal, which suggests the winding down of the production of corrosion products due to the formation of low-solubility complexes that limited the interaction between corrosive ions and the metallic surface. Likewise, a predominant Fe signal can be observed, which confirms the properties of [DDMS⁺I^–] as CI of steel in acid medium.

2.3.2. DRIFTS

Figure 10 shows the DRIFTS spectra of the steel surface attacked for 4 h in 1 M H₂SO₄ in the presence of 150 ppm of inhibitor. The sample has a characteristic absorption band of the Fe–O bond of Fe₃O₄ at 568 cm^–1.^69,70 The signal appeared at around 1130 cm^–1, which corresponds to the alkylsulfonium group. The bands at ∼1643 cm^–1 correspond to iron oxide and the alkylsulfonium group too, while the band at 1736 cm^–1 corresponds to the remaining H₂SO₄.^71,72 The O–H stretching bands from physisorbed molecular water can be observed at 3386 cm^–1, while the signals at 2957 and 2854 cm^–1 belong to the C–H stretching signal of the inhibitor aliphatic group and Fe₃O₄.^73,74 The C–H scissoring of the alkyl chain signals appears at 1459 cm^–1.

Figure 10.

DRIFTS spectra of the steel surface after immersion in 1 M H₂SO₄ for 4 h at 298 K in the presence of 150 ppm of inhibitor.

2.3.3. XPS

XPS technique was used to analyze the oxidation state of the elements present on the API 5L X52 steel surface attacked for 4 h in 1 M H₂SO₄ in the presence of 150 ppm of inhibitor. The general XPS spectra indicate the presence of Fe (2p), O (1s), C (1s) S (2p), and I (3d). Fe is attributed to the metal substrate. Oxygen and sulfur signals are related mainly to corrosion products and salts deposited on the metal surface due to the corrosive medium type. The high carbon signal (60.4%) is ascribed not only to adventitious carbon but also to the presence of CI on the metal surface. There is also an iodine signal, which can be associated with the corrosion inhibitor structure.

Figure 11a shows the high-resolution C 1s peak, which deconvoluted to three peaks. The main peak at 284.8 eV indicates the presence of C–C and C–H bonds attributed to the hydrocarbon alkyl chain bond,^75,76 whereas the signal at 286.4^75,77 is related to C–S bonds present in the molecule; the signal at 288.6 eV is attributed to the presence of COO stemming from carbon impurities and substrate carbon.^78,79

Figure 11.

XPS deconvoluted profiles of (a) C 1s, (b) S 2p, (c) O 1s, (d) Fe 2p, and (e) I 3d on the iron sample surface exposed to 1 M H₂SO₄ with 150 ppm of [DDMS⁺I^–] for 4 h at 298 K.

Figure 11b shows the high-resolution S 2p peak deconvoluted to four peaks. The S 2p_3/2 binding energies at 162.0 and S 2p₁ at 163.1 eV were associated with the S–C bond, while the peaks at 168.4 and 169.5 eV for S 2p_3/2 and S 2p₁, respectively, are attributed to sulfate species (SO₄^2–), showing a clear correlation with the sulfur oxidation degree.^75,80−83

In the O 1s region, Figure 11c, the main peak at 531.9 eV corresponds to oxygen in the hydroxide form, which is present in oxyhydroxides (FeOOH),^78,84 whereas the signal at 530.1 eV is associated with oxygen in oxidized form as ferrosoferric oxide (Fe₃O₄),^85,86 while the peak at 532.9 eV is ascribed to oxygen in the sulfate group (SO₄^2–).⁸⁷

The Fe 2p spectrum exhibits mainly three peaks (Figure 11d): (1) at 706.8 eV (Fe 2p_3/2) and 719.6 eV (Fe 2p_1/2), corresponding to metal Fe⁰; (2) at 710.4 eV (Fe 2p_3/2) and 723.7 eV (Fe 2p_1/2), related to ferrosoferric oxide (Fe₃O₄);⁸⁸ and (3) at 713.4 eV (Fe 2p_3/2) and 727.2 eV (Fe 2p_1/2), corresponding to oxyhydroxides (FeOOH).⁸⁹

The two main signals located at 619.3 and 630.8 eV (Figure 11e) can be ascribed to the photoemission peaks from I 3d₅ and I 3d₃, respectively, and correspond to the iodide ion.^90,91

2.4. Computer Simulation Analysis

2.4.1. Geometry

Figure 12 shows the optimal [DDMS⁺I^–] structure. The absence of imaginary frequencies confirms the system structural stability. To take the molecule to the ground state, neuter charge multiplicity 1 (singlet) was employed. In the optimal structure, the methyl and decyl groups were distributed outward to the plane with respect to sulfur (trigonal pyramidal shape). As for [I^–], it presented a preference to be located on the alkyl groups linked to sulfur, where the interaction with sulfur was minimized by the carbon and hydrogen atoms,⁹² being the most stable IL form. Regarding the decyl group, it displayed the characteristic linear conformation of a C ≥ 4 alkyl chain that has been reported in different ILs based on ammonium and imidazolium.⁹³⁻⁹⁵

Figure 12.

Optimized structure of [DDMS⁺I^–] in vacuum obtained at B3LYP/MIDIx level. All atoms are at their relaxed positions. Gray: carbon, white: hydrogen, yellow: sulfur, and purple: iodine.

Table 6 reports the lengths and bond angles for each group of atoms. It is observed that the C–C, S–C, and C–H bonds did not present changes in vacuum and aqueous medium [employing COSMO (COnductor-like Screening MOdel) as the solvation model, which uses a dielectric constant of 78.4 to simulate the medium].⁹⁶ In contrast, the space between sulfur and iodide shows slight distancing (∼0.33 Å), which increased the interaction capacity with other molecules. As for the bond angles of the different atom groups, they presented negligible variations when the medium was changed, suggesting that the distribution of the [DDMS⁺I^–] atoms was not affected.

Table 6. Bond Lengths and Angles Calculated for [DDMS⁺I^–].

	medium (Å)
bond	vacuum	water
C–C	1.54	1.54
S–C	1.84	1.84
C–H	1.10	1.10
S···I	4.20	4.53
C–C–C	59.25	59.25
C–S–C	39.41	39.41
H–C–H	112.15	112.15
H–H–H	60.4	60.4
S–C–C	110.28	110.28
C–C–C	112.26	112.26
H–C–H	107.85	107.85
H–H–H	59.99	59.99

2.4.2. Energetic Analysis

As observed in Figure 13a, the highest occupied molecular orbital (HOMO) works as an electron donor and is located in the [I^–] ion, whereas (b) the lowest unoccupied molecular orbital (LUMO) functions as an electron acceptor, since it is the cation empty internal orbital and, in this case, it extended itself from [S⁺] to the methyl groups and decyl (C₅).

Figure 13.

(a) Highest occupied molecular orbital (HOMO) and (b) lowest unoccupied molecular orbital (LUMO) of [DDMS⁺I^–]. All are obtained at the optimized geometry in vacuum.

Regarding the molecular electrostatic potential (MEP) mapping, it is employed to determine reactive sites for electrophilic and nucleophilic attacks by visualizing the charge distribution, which is related to the molecule electron density. Figure 14 shows the MEP of [DDMS⁺I^–], where three different colors can be seen: the red color is associated with the IL most electronegative atom, the iodide anion, and electrophilic reactivity, i.e., with the most susceptible site to electrophilic attacks that cedes electrons to the steel surface to form coordinate bonds with the empty Fe orbital. The blue color located in the cation [S⁺] is related to the nucleophilic reactivity, which is the most susceptible site to nucleophilic attacks, i.e., the site that accepts electrons from other species. Finally, the green color indicates that both the methyl and decyl groups present neuter charge, i.e., their electron configurations are full.⁹⁷ This behavior pattern has been observed in works on similar compounds like the one by Haque et al. on N-methyl-N,N,N-trioctylammonium chloride, where the blue region is located on the nitrogen atom of the cationic part, whereas the red region on the chloride ion.⁹³

Figure 14.

MEP isosurface of [DDMS⁺I^–] obtained at the B3LYP/MIDIx level.

The MEP and MO analyses confirmed that the [I^–] anion worked as an electron donor and the [S⁺] cation as an electron acceptor, which represent sites associated with molecule adsorption processes.

2.4.3. Quantum Parameters

For the reactivity analysis of [DDMS⁺I^–], the HOMO (E_HOMO) and LUMO (E_LUMO) energy values were calculated as shown in Table 7. According to the theory, high E_HOMO values are associated with a higher tendency to donate electrons to appropriate receiving molecules with low-energy empty molecular orbitals. In contrast, low E_LUMO values suggest higher probability of accepting electrons on the metallic surface. The latter indicates that if a CI exhibits high E_HOMO and low E_LUMO, it will have better interaction with the metallic surface.^42,98 As observed in Table 7, E_HOMO is relatively higher and E_LUMO is relatively lower in both media, which suggests that [DDMS⁺I^–] presents a strong tendency to donate and accept electrons.

Table 7. Dipole Moment, HOMO and LUMO Energies, and Energy Gap (ΔE_L–H) of [DDMS⁺I^–].

system	–EHOMO (eV)	–ELUMO (eV)	ΔEL–H (eV)	μ (Debye)
vacuum	5.98	1.77	4.21	10.77
water	5.91	1.63	4.29	15.92

As for the formation of a transition state, it is due to the interaction between the MOs, i.e., to the energy gap between E_HOMO and E_LUMO (ΔE_L–H = E_LUMO – E_HOMO). This parameter is considered as a descriptor of the molecular activity, where low ΔE_L–H values indicate higher polarization and an increase in the metal surface reactivity, which is associated with better CI behavior. Table 7 shows that [DDMS⁺I^–] presented a significant difference regarding ΔE_L–H in both media, which facilitated the adsorption.^93,99 The μ values for [DDMS⁺I^–] were equal to 10.77 and 15.92 Debye in vacuum and water, respectively, which resulted to be higher than those reported for water (1.85 Debye), suggesting the preference to replace the adsorbed water molecules on the metallic surface by the IL.^93,100

2.5. Corrosion Inhibition Mechanism

As it is known, iodides exert an inhibitory effect on corrosion, which occurs mainly in the anodic sites of the metal surface, slowing down the dissolution of iron in the form of iron cations (Fe²⁺).^101,102 Based on theoretical calculations, the iodide anion hosts the most part of HOMO, which attracts it to the anode sites.

Fe⁺²(OH)_ads^– or Fe⁺³(OH)_ads^– are formed at the anodic sites after adsorption of water and hydroxide anions (OH^–), which in turn form corrosion products like magnetite (Fe₃O₄) or through the path of “green rust” to form iron oxyhydroxides (FeOOH), usually lepidocrocite; the presence of these corrosion products was confirmed by XPS analysis. In the presence of corrosion inhibitor, very few corrosion products typical for the sulfuric acid medium such as rozenite and melanterite were detected,¹⁰³ which means that I^– ions prevented the deposition of sulfate ions on the electrode surface, and hindered sulfate passivation, slowing down the formation on a larger scale of corrosion products that contain the sulfate anion in their structure.

However, since the CI is a mixed-type inhibitor, it acted not only on anodic sites, but also on cathodic sites (Figure 15). As the CI cation is a trialkyl sulfonium derivative, where LUMO is located, it accepted electrons. The sulfonium ion approached the cathode sites on the metal surface by competing with H₃O⁺ and displacing water. Since the CI cation is much larger than H₃O⁺, it removed the protons from the metal surface in the cathode zone and thus slowed down the hydrogen formation reaction. Based on the electrochemical analysis, the CI concentration of 50 ppm caused notable inhibition of the corrosive process (greater than 80%), while from the concentration of 75 ppm, there was no big change in the anticorrosive effect (>90%) in laminar and (>80%) transitory flows.

Figure 15.

Corrosion inhibition mechanism of API 5L X52 steel in 1 M H₂SO₄ containing [DDMS⁺I^–].

3. Conclusions

From the present work, the following conclusions were drawn.

• Electrochemical measurements showed that the charge transfer rate in the presence of CI was lower than the blank, which evidenced the formation of a protecting film.

• It was observed that the IE values increased with the IL concentration. Furthermore, under hydrodynamic conditions, the IE values diminished by the desorption of IL molecules due to sheer stress.

• ΔG°_ads values suggested that the IL adsorption process was a combination of physisorption and chemisorption.

• The surface analyses supported the [DDMS⁺I^–] inhibition process, exhibiting less damage of the metallic surface by IL adsorption.

• The HOMO of [DDMS⁺I^–] suggested that the iodide anion had the capacity to donate electrons, which eased its adsorption on the anodic sites, thus preventing the depositing of sulfate ions on the electrode surface.

4. Methods

4.1. Synthesis and Characterization of Decyl(dimethyl)sulfonium Iodide

The molecular structure of the synthesized IL is depicted in Table 8. Iodomethane (≥99%, Sigma-Aldrich), n-decyl methyl sulfide (97%, Alfa Aesar), and trifluoroacetic acid (99%, Sigma-Aldrich) were employed in the synthesis. The compound was characterized by NMR. The ¹H (300 MHz) spectrum was recorded on a JEOL Eclipse-300 equipment in DMSO-d₆, and chemical shifts were expressed in ppm relative to tetramethylsilane as the internal standard. Decyl(dimethyl)sulfonium iodide was synthesized according to a similar previously described procedure.¹⁰⁴

Table 8. Chemical Structure of the IL Evaluated as CI.

4.1.1. Decyl(dimethyl)sulfonium Iodide [DDMS⁺I^–]

In total, 3.17 g (22.3 mmol) of methyl iodide, 1.60 g (8.47 mmol) of n-decyl methyl sulfide, and 3.12 g (27 mmol) of trifluoroacetic acid were mixed in a 25 mL round-bottom flask, which was closed with a stopper. The reaction was performed at room temperature without stirring. After 30 min, diethyl ether (100 mL) was added to precipitate the product as a white solid. Yield: 2.50 g (89%). ¹H NMR (300 MHz, DMSO-d₆): 3.34 (s, 6H), 3.88 (m, 2H), 1.91 (s, 2H), 1.60 (m, 2H), 1.33–1.20 (m, 14H), 0.85 (t, J = 6 Hz, 3H).

4.2. Materials and Test Solutions

API 5L X52 steel coupons with the exposed surface area of 0.289 cm² were employed with the following chemical composition (wt %): C ≤ 0.28%, Mn ≤ 1.4%, P ≤ 0.030%, S 0.030%, V ≤ 0.15%, Nb 0.15%, Ti ≤ 0.15%, Cu 0.25%, Ni 0.25%, Cr 0.25%, Mo 0.15%, and Fe as the main element. Before each test, the metal samples were polished with SiC emery paper (from 400 to 1200). Afterward, they were degreased with acetone–ethanol and exposed to an ultrasonic bath to eliminate particles adhered to the surface. Finally, the polished samples were dried under nitrogen flow.¹⁰⁵ 1 M H₂SO₄ was the corrosive solution and it was prepared with analytic-grade acid and deionized water. [DDMS⁺I^–] was evaluated at concentrations ranging from 25 to 150 ppm.

4.3. Electrochemical Tests

Electrochemical tests were performed employing a Potentiostat/Galvanostat AutoLab apparatus model PGSTAT302N. The software NOVA 2.1.4 was used to obtain and analyze the experimental data. A glass electrochemical cell equipped with three electrodes was used: counter electrode (platinum 99%), working electrode (API 5L X52 steel), and reference electrode (Ag/AgCl). All of the tests were carried out at 25 ± 1 °C in aerated medium. In addition, the tests were run in triplicate and the reported results represent the average.

The working electrode was immersed in the electrolytic solution for 20 min until reaching the E_OCP. The R_p measurements were carried out within a potential interval of ± 25 mV vs E_OCP(106) whereas that of PDP was of ± 250 mV vs E_OCP;¹⁰⁷ both tests took place at a scanning rate of 0.1666 mV s^–1. EIS tests were developed within a frequency interval ranging from 100 kHz to 10 mHz using a sinusoidal wave with 5 mV of amplitude after stabilizing the E_OCP.¹⁰⁸

The flow tests were performed with a rotating disc Metrohom RDE II 309.¹⁰⁹ The N_RE was calculated from eq 6 ^110,111

6

where U is the cylinder peripheral speed (m s^–1), d is the electrode diameter (m), and v is the electrolyte kinematic viscosity (m² s^–1). The τ_RDE generated on the surface was calculated with eq 7 ^112,113

7

where ρ is the density (kg m^–3), r is the cylinder radius (m), and ω is the electrode angular velocity (rad s^–1). In Table 9, ω, N_RE, and τ_RDE values under laminar, transitory, and turbulent hydrodynamic regime conditions, respectively, can be observed.

Table 9. Angular Velocity, N_RE, and Shear Stress as Functions of the RDE Rotation Rate.

regimen	rotation rate (rpm)	ω (rad s–1)	NRE	τRDE (Pa)
laminar	343	35.9	1000	0.320
	686	71.84	2000	1.040
transitory	1029	107.76	3000	2.073
turbulent	1372	143.68	4000	3.381
	1715	179.6	5000	4.940

4.4. Surface Analysis

The samples employed for the surface analyses were prepared by following the methodology for electrochemical tests and polished with 1 μm alumina. The metal coupons were immersed in the corrosive medium in the absence and presence of 150 ppm of CI for 4 h at 25 °C. Afterward, the metal samples were retrieved from the medium and rinsed with deionized water and dried with nitrogen.¹⁰⁵ The surface of API 5L X52 steel was analyzed by SEM/EDS on a JEOL-JSM-6300 microscope. The study of the treated metal surfaces was carried out using DRIFTS; these measurements were performed in situ using a Thermo Scientific Nicolet 560 Spectrometer in a series of spectra recorded with identical resolution (4 cm^–1). The XPS analysis was performed with a K-Alpha Thermo Fisher Scientific spectrometer with monochromatic Al Kα (1486.6 eV) and vacuum pressure of 1 × 10^–9 Torr. The pass energy values for the study and high-resolution spectra were set at 160 and 20 eV, respectively. The obtained spectra were referred to adventitious carbon (284.8 eV) and the peak fitting was performed using the software Thermo Avantage v.5.9915.

4.5. Computational Details

The IL corrosion inhibition performance was supported with first-principles energy calculations. The [DDMS⁺] cation was optimized structurally considering different positions of the [I^–] anion without symmetry restriction and singlet state (Multiplicity 1). The computations were developed with the density functional theory (DFT) by the Gaussian 09W software¹¹⁴ with B3LYP/MIDIx theory level.^115,116 The entries were generated with the software Gauss View v6.0. The properties were obtained employing the lowest total energy configuration, whereas the CI theoretical performance was established through the analysis of the molecular orbitals and dipolar moment (μ),^42,98 which were calculated from the optimized structure under standard temperature and pressure conditions.¹¹⁷

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c05192.

• Other analyzed models of adsorption isotherms for API 5L X52 steel in 1 M H₂SO₄ solution with [DDMS⁺I^–] (PDF)

The authors declare no competing financial interest.