The Role of Some Cationic Surfactants Based on Thiazine as Corrosion Inhibitors in Petroleum Applications: Experimental and Theoretical Approach.

Two cationic surfactants based on thiazine, dodecyl thiazin bromide (DTB) and hexyl thiazin bromide (HTB), were synthesized, characterized, and investigated as corrosion inhibitors for API X-65 type steel in oil wells' formation water under an H2S environment. Various spectroscopic techniques such as FTIR and 1H NMR were used to confirm the DTB and HTB chemical structures. The corrosion inhibition efficiency of the selected compounds was investigated using both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. The innovation of the current study is the existence of a long chain in the inhibitor molecule, which leads to an increase in the performance of the surfactant as a corrosion inhibitor, due to the increase in the surface area per molecule. It was found that these surfactants act as mixed-type inhibitors, leading to suppression of both the cathodic and the anodic processes by its adsorption on the electrode surface according to the Langmuir adsorption isotherm. Carbon steel's inhibitory mechanism was studied using an analogous circuit. The scanning electron microscope technique was used as a suitable analysis tool to show the nature of the layer designed on carbon steel. Quantum chemical calculations and Monte Carlo simulation techniques were used to support the obtained experimental results. Finally, a suitable mechanism for the inhibition process was proposed and discussed.

Introduction

Carbon steel is commonly used in petroleum processing facilities for refining and marine applications, due to its mechanical properties and also from an economical point of view.[1,2] It is well-known that the metallic surfaces are suffering from corrosion, and this is leading to significant economic losses in addition to a bad environmental impact.[3,4] It was reported in the literature that organic compounds with hetero atoms such as nitrogen, oxygen, sulfur, and phosphorus show promising efficiency in mitigating the aggressive attack of corrosive species on a metal surface.[5,6] These types of organic compounds act at the interface between the metal and the aggressive solution through the adsorption on metal surfaces, forming a protective layer that isolates the metallic surfaces from the corrosive environment.[7] Accordingly, the selection of this type of cationic surfactants in the present work is attributed to the presence of incorporated hetero-organic moieties, which offer more protection potential against the corrosion of steel, along with the presence of a long chain that increases the minimum area per molecule.[8−13] Cationic surfactants are a category of surface-active agents that dissociate in water to create surface-active cations. Because of the beneficial characteristics of these cationic surfactants, they have numerous applications in many sectors. It was reported that they are adsorbed on metal surfaces by both physical and chemical methods.[14−18] The novelty of the present work appears in using cationic surfactants containing a long chain and heteroatoms as nitrogen and sulfur, besides for the high solubility in oil wells’ formation water contaminated with H2S gas. The present work is aimed to prepare two novel cationic surfactants, DTB and HTB, and evaluate their performance as corrosion inhibitors for carbon steel in formation water under an H2S environment using different techniques. Also, our work is extended to carry out some theoretical studies to support the obtained experimental results. This Article is regarded as an onset of a series of works currently under investigation in our lab.

Results and Discussion

Potentiodynamic Polarization Measurements

Iron dissolution results in ferrous sulfide as the primary corrosion product when H2S is present; however, the protective layer is dissolved when the H2S gas concentration is high. The electrochemical parameters including the corrosion current density (icorr), the corrosion potential (Ecorr), the anodic and cathodic Tafel slopes (βa and βc), the degree of surface coverage, and the percentage inhibition efficiency were calculated[19−26] and are tabulated in Table .

Table 1

Polarization Parameters of the CS at Various Concentrations of DTB and HTB in Formation Water and the Corresponding Inhibition Efficiencies

inhibitor	concn (ppm)	βa, (mV dec–1)	–βc, (mV dec–1)	–Ecorr (mV vs SCE)	Icorr, (μA cm–2)	θ	IE%
blank	0	193.7 ± 1.2	150.9 ± 1.3	828.3 ± 7.2	9.31 ± 0.02
DTB	50	112.8 ± 1.3	155.3 ± 1.4	800.1 ± 5.6	3.71 ± 0.01	0.6015	60.15
	100	108.3 ± 1.7	142.5 ± 1.5	814.2 ± 4.3	2.92 ± 0.01	0.6863	68.63
	150	102.7 ± 1.9	139.7 ± 1.6	764.5 ± 3.1	1.85 ± 0.01	0.8012	80.12
	200	99.1 ± 2.1	136.1 ± 1.9	780.4 ± 2.6	1.37 ± 0.01	0.8528	85.28
	250	95.6 ± 2.3	135.4 ± 2.1	716.2 ± 2.5	0.97 ± 0.01	0.92058	92.058
HTB	50	109.2 ± 1.5	166.7 ± 1.5	776.4 ± 6.8	3.97 ± 0.01	0.5735	57.35
	100	107.6 ± 1.9	145.2 ± 1.7	814.1 ± 3.9	3.35 ± 0.01	0.6401	64.01
	150	103.8 ± 2.3	140.8 ± 1.9	788.5 ± 2.8	2.15 ± 0.01	0.7690	76.90
	200	101.5 ± 2.4	138.3 ± 2.1	740.2 ± 5.4	1.76 ± 0.01	0.8109	81.09
	250	100.8 ± 1.9	136.1 ± 2.3	722.6 ± 1.9	1.24 ± 0.01	0.8668	86.68

Figure a,b shows the polarization curves of the two compounds DTB and HTB with respect to the blank curve. By careful inspection of the polarization curves, it is clear that the values of the corrosion current density (icorr) were significantly reduced by increasing the dose of chemical injected. The values of inhibition efficiency (η%) and the degree of surface coverage (θ) were calculated according to the following equations:where i and i0 represent the values of the corrosion current densities in the presence and absence of various doses of the inhibitor, respectively.

Figure 1

Potential–log current density curves of carbon steel electrode immersed in formation water in the absence and presence of different doses of (a) DTB and (b) HTB inhibitors.

A careful inspection of the polarization curves indicated that the Tafel lines of the two compounds (DHB and HTB) are slightly shifted to more positive potentials with respect to the blank. This fact means that the organic compounds under consideration work as a mixed type; that is, they retard both anodic dissolution of carbon steel in addition to the cathodic discharge reaction. Also, it is clear that the values of both βa and βc are nearly constant and do not depend on the change in inhibitor concentrations. This behavior confirms that inhibitors under investigation retard the corrosion process with a change in the mechanism of occurrence. The corrosion current densities (icorr) decreased with increasing concentration.[27] The obtained results in Table indicated that the corrosion current density values are significantly lower in the presence of the corrosion inhibitors as compared to the uninhibited solution.

The results show that compound DTB has a better percentage inhibition efficiency than that of compound HTB. The p–d bonds can be formed between inhibitor molecules and the empty orbital of iron, which may explain this.[28−30] This could be attributed to the increase leading to an increase in the surface area per molecule that covers the surface of carbon steel and isolates it from the aggressive environment.

The long-term behavior of the undertaken compounds (DTB and HTB) as corrosion inhibitors was studied by plotting the corrosion current density (icorr) against the immersion time (t) as shown in Figure . It is clear from the change in icorr with time that the undertaken compounds (DTB and HTB) were adsorbed on the carbon steel surface to form a stable protective film for a long period of time.

Figure 2

The variation of corrosion current density with the immersion time for carbon steel in formation water in the absence and presence of 50 ppm of the inhibitors (DTB and HTB).

Electrochemical Impedance Spectroscopy (EIS)

Nyquist and Bode plots of carbon steel in oil field formation water in the absence and presence of various doses of the compounds DTB and HTB are shown in Figures and 4, respectively.

Figure 3

EIS data (Nyquist plots) for carbon steel immersed in oil field formation water in the presence and absence of various doses of (a) DTB and (b) HTB.

Figure 4

EIS data (Bode plots) for carbon steel immersed in oil field formation water in the presence and absence of various doses of (a) DTB and (b) HTB.

By careful inspection of Figure , one can conclude that the size of the obtained capacitive loop along the x-axis was increased with increasing concentrations of the two inhibitors under investigation, that is, DTB and HTB. From this behavior, it is evident that both the corrosion and the inhibition processes were controlled by the value of polarization resistance Rp according to the following equation:Figure shows in detail the equivalent circuit that fit well the obtained data where the solution resistance is Rs, film resistance is Rf, charge transfer resistance is Rct, and constant phase elements are CPEF and CPEdl. The acquired impedance data can be described by the equation below, as derived by the EIS analyzer program.where Y0 represents the admittance, j = −1, and w is the angular frequency.

Figure 5

EIS data for CS in the used formation water modeled using an equivalent circuit.

Table clearly shows that as the DTB and HTB concentrations rise, both Cdl and Cf drop. Molecular inhibitor adsorption to the necessary protective layer on carbon steel surfaces is indicated by this finding. According to the following two equations:[31]∑0 is the electrode surface exposed to the aggressive solution with a permittivity of 0, and this is determined by the adsorbed layer thickness (d), ∑ is the local dielectric constant, and F is the Faraday constant. Also, it is clear from the Bode plots in Figure that both phase angle and absolute impedance are increased by increasing the inhibitor concentrations. Such behavior supports well the inhibition effect of the selected compounds (DTB and HTB).[32−35]

Table 2

Impedance Parameters Obtained from EIS curves for the DTB and HTB Inhibitors

inhibitor	concn (ppm)	n1	Rf	QF	n2	Qdl (μF/cm2)	Rct	θ	IE%
DTB	0	0.87 ± 0.02	52.6 ± 2.1	132.1 ± 1.1	0.83 ± 0.05	538.1 ± 1.1	1.659 ± 0.10
	50	0.90 ± 0.03	12.3 ± 4.3	83.9 ± 0.8	0.87 ± 0.04	138.7 ± 3.5	2.81 ± 0.19	0.4132	41.32
	100	0.92 ± 0.02	2.77 ± 6.5	57.4 ± 0.6	0.89 ± 0.02	117.2 ± 3.9	3.73 ± 0.23	0.5581	55.81
	150	0.94 ± 0.01	3.9 ± 8.3	48.5 ± 0.5	0.91 ± 0.01	116.5 ± 4.3	4.51 ± 0.27	0.6347	63.47
	200	0.95 ± 0.01	527 ± 9.1	39.2 ± 0.4	0.92 ± 0.08	104.3 ± 5.7	8.66 ± 0.33	0.8095	80.95
	250	0.96 ± 0.01	544 ± 4.5	34.7 ± 0.6	0.93 ± 0.07	89.4 ± 6.2	13.31 ± 0.42	0.8761	87.61
HTB	50	0.89 ± 0.03	118 ± 3.5	79.3 ± 0.9	0.85 ± 0.2	132.6 ± 3.3	2.67 ± 0.17	0.3821	38.21
	100	0.91 ± 0.04	253 ± 6.3	62.8 ± 0.7	0.87 ± 0.01	113.4 ± 3.8	3.28 ± 0.21	0.4983	49.83
	150	0.92 ± 0.06	297 ± 7.1	38.5 ± 0.6	0.88 ± 0.02	97.3 ± 4.1	4.29 ± 0.25	0.6158	61.58
	200	0.93 ± 0.07	463 ± 8.3	29.1 ± 0.5	0.89 ± 0.03	93.2 ± 5.3	7.71 ± 0.31	0.7862	78.62
	250	0.94 ± 0.06	492 ± 9.1	19.2 ± 0.4	0.91 ± 0.03	85.1 ± 5.7	10.96 ± 0.39	0.8411	84.11

SEM Examination

SEM photomicrograph analysis is considered a powerful tool to observe the change in surface morphology of the investigated alloy, that is, X-65 type, dipped in the oil well water after and before adding inhibitor for 15 days. Figure a shows the SEM image for a specimen of carbon steel immersed in the oil formation water without inhibitor (blank). It is clear that the surface is thoroughly damaged due to the presence of aggressive ions that attack on the surface, leading to Sevier corrosion. Figure b shows the SEM image for a specimen of carbon steel immersed in the oil formation water in the presence of 250 ppm of the inhibitor DTB, while Figure c shows the SEM image for a specimen of carbon steel immersed in the oil formation water in the presence of 250 ppm of the inhibitor HTB. The above features confirm the anticorrosion behavior of the prepared compounds for X-65 steel in the formation water, which well agrees with other electrochemical results.

Figure 6

SEM analysis for the X-65 carbon steel surface. Sample (a) after immersion in test solution, (b) after immersion in test solution containing 250 ppm of DTB, and (c) after immersion in test solution containing 250 ppm of HTB.

Surface Tension Measurements

The CMC values of the synthesized surfactants were determined at various temperatures from the change in the slope of the plotted data of surface tension (γ) versus the natural logarithm of the solute molar concentration, ln C, as shown in Figure . The surface active properties obtained from the surface tension measurements are summarized and listed in Table .

Figure 7

Surface tension versus ln C of the compounds DTB and HTB.

Table 3

Surface Active Properties of the Synthesized Compounds DTB and HTB

inhibitor concn	CMC, mol/dm3	γcmc, mN/m	Γmax × 10–7, mol/m2	Amin,n, m2	ΠCMC	ΔGmic°, kJ mol–1	ΔGads°, kJ mol–1
DTB	2.49 × 10–5	35	9.72 × 10–11	170	37.3	–26.70	–30.35
HTB	1.84 × 10–4	33	6.65 × 10–10	191	39.3	–21.66	–26.20

The properties of the surfactant molecules are due to two parts: the organic hydrophobic part and the polar hydrophilic part. So even when the cationic surfactants are dissolved in water, the hydrophilic part is directed to the water phase and the other hydrophobic part of the surfactant is located on the surface to decrease the force generated from the water phase. Increasing the surfactants’ concentration reduces the water surface tension values. The intercept of the two regions at low concentrations and at higher surfactant concentrations gave the critical micelle concentration (CMC) values.

Table summarizes the values of the standard free energy of micellization (ΔGmic°) and the standard free energy of adsorption (ΔGads°). The obtained results indicated that compound DTB has a stronger adsorption ability on the carbon steel surface and a lower micellization ability than does compound HTB, leading to high corrosion protection.

Table 4

Standard Free Energy of Micellization (ΔGmic°) and Standard Free Energy of Adsorption (ΔGads°) as Obtained from Surface Tension Measurements and the Langmuir Adsorption Isotherm Model, Respectively

inhibitor	free energy of micellization, ΔGmic° (kJ mol–1)	free energy of adsorption, ΔGads° (kJ mol–1)
DTB	–26.70	–30.35
HTB	–21.66	–26.20

The standard free energy of micellization (ΔGmic°) for the investigated surfactants was calculated according to the following equation:

The free standard energy standard of adsorption (ΔGads°) for the synthesized surfactants was calculated from the equation:where Kads represents the value of the adsorption equilibrium constant. The obtained results indicated that compound DTB with a longer alkyl chain possesses a stronger adsorption affinity onto the carbon steel surface than does compound HTB with a shorter alkyl chain, and this is leading to a higher inhibition efficiency.[36]

It is clear that the obtained data listed in Table are in good agreement with the results obtained from both the potentiodynamic polarization and the electrochemical impedance spectroscopy techniques.

Molecular Modeling

The undertaken surfactants have a high adsorption affinity on the CS surface because of the presence of hetero atoms (N and S) in their chemical structures. DFT analysis can show the distribution of electron density over the molecular structure of the compounds.

The most important indices in the calculation of the important parameters according to eqs –11 are the energy of the highest full-field molecular orbital (EH) and the energy of the lowest empty molecular orbital (EL).The absolute electronegativity and hardness (Fe) of iron metal were chosen to be 0 eV and 0.[37] The Fe (110) plan work function is 4.82 eV (Fe). The 3d orbital of iron, as well as the donor–acceptor interaction between DTB and HTB, are responsible for facilitating adsorption. With increasing EH values, electron donation from DTB and HTB into the vacant 3d iron orbital is enhanced.[38] DTB and HTB are more capable than EL of acquiring electrons from the full-field 3d iron orbital. As shown in Figure , the electron density for HOMO in the gas phase is distributed over quaternary nitrogen and halogen atoms, whereas the electron cloud in the solution phase is distributed over the hetero and aromatic moieties and the halogenated groups. LUMO electrons are distributed over the same HOMO centers in both the gas and the solvated phases. Both HOMO’s and LUMO’s are thought to be hubs of donor–acceptor interactions.[39]Figure depicts their alkyl chains using an electrostatic potential map that depicts the electron cloud distribution. This demonstrates that DTB and HTB have a proclivity to form a denser and stickier adsorption film.

Figure 8

Optimized molecular structures, HOMO, LUMO, total electron density distribution, and MAP, of DTB and HTB in the gas phase.

Table shows that the gas-phase N values are less than 3.6. DTB and HTB are electron donors that move electrons from metal surface to metal surface. The back-donation energy (Eback-donat) in Table indicates that electrons from full-filled 3d or 4S orbitals of Fe are transferred to the LUMO of DTB and HTB. The donation and back-donation processes confirm the compound adsorption activity.[40]

Table 5

Quantum Chemical Parameters of the Investigated Inhibitor

	EH (eV)	EL (eV)	ΔEgap	A	I	χ	η	ΔN
DTB	–8.386	–1.606	6.78	1.606	8.386	4.996	3.39	0.295575221
HTB	–7.908	–1.089	6.819	1.089	7.908	4.4985	3.4095	0.366842646

Monte Carlo Simulation (MCs)

DTB and HTB molecules are aligned in these images (Figure ) in a way that is best suited to the most stable plane of Fe crystals (110). The MCs process can be used to mimic the adsorption properties of DTB and HTB on a CS surface. Because of the flat and planar orientation positions of the DTB and HTB optimized structures in the gas and solvent phases over Fe (110),[41] the adsorption process (donor–acceptor interactions) is efficiently carried out, as shown in the side and top views of Figure . The data in Table show that DTB and HTB have a strong spontaneous adsorption affinity over the CS surface, resulting in large negative values of adsorption energy (Eads) in the gas or solvated phases. It is worth noting that the value of Eads in the solvent phase is greater than the equivalent in the gas phase. This is due to the formation of H-bonds between water solvent molecules and the nitrogen or/and oxygen atoms of DTB and HTB, which synergistically adsorb DTB and HTB to the CS surface. This is because the Eads value of DTB and HTB is greater than the value of water. DTB and HTB have been shown to be capable of forming a protective adsorption barrier film on the CS surface.[42] Laboratory data (gravimetric and electrochemical) agree with the interpretations of the DFT and MCs output indices.

Figure 9

Side and top views of the adsorption mode of DTB and HTB on Fe (110) substrate.

Table 7

Data of Physical Investigation and Chemical Analysis of Formation Water Used in This Work

Physical Investigation
total dissolved solids (T.D.S.)	9650 mg/L	density at 60 F	1.06 g/mL
salinity (as NaCl)	95556 mg/L	specific gravity	1.06
alkalinity (as CaCO3)	320 mg/L	pH at 25 °C	6.8
total hardness (as CaCO3)	14455 mg/L	conductivity	12.02 × 10–2 Ω/cm at 21.6 °C
		resistivity	0.0832 Ω m at 21.6 °C

The data in Table show that DTB and HTB have a strong spontaneous adsorption affinity over the CS surface, resulting in large negative values of adsorption energy (Eads) in the gas or solvated phases. It is worth noting that the value of Eads in the solvent phase is greater than the equivalent in the gas phase. The H-bond formed between the water solvent molecules and the nitrogen or/and oxygen atoms in the compound facilitates DTB adsorption on the CS surface. This demonstrates that compounds are capable of replacing water molecules adsorbing onto the CS surface and forming a protective adsorption barrier film.

Table 6

Output Energies Calculated by Monte Carlo Simulation for DTB and HTB in the Gas Phase on Fe (110)

parameters	total energy	adsorption energy	rigid adsorption energy	deformation energy	3D atomistic
DTB	–157.11	–434.47	–137.99	–296.47	–434.47
HTB	–229.70	–552.68	–198.41	–354.27	–552.68

Mechanism of the Corrosion Inhibition Process

The adsorption process of organic inhibitor molecules depends on many physical and chemical properties such as electron density, chemical structure, metal nature, charges at the metal/solution interface, and the type of aggressive medium (pH and/or electrode potential).

These properties affect the mode in which the molecules interact on the metal surface. Adsorption of organic molecules on solid surfaces cannot be considered purely physical or chemical; a combination of both processes can occur in adsorption.[43−45] Physical interaction is accepted as the first step for the adsorption of molecules on the metal surface, and then chemical adsorption may occur via different charge-sharing processes.[46] In a solution of oil field formation water, the inhibitor molecules are adsorbed on the metal surface through the following interactions: (i) the electrostatic interaction between the positively protonated inhibitor and the chloride ions adsorbed on the carbon steel surface (physisorption process); (ii) the chemical interaction between the lone-pair electrons on the heteroatoms (N, S, and O) and the unoccupied d-orbital on the Fe surface (chemical adsorption process); and (iii) the donor–acceptor interaction between the π-electrons of the aromatic ring and the vacant d-orbitals on the metal surface. The presence of electron donation increases the electron density of the neighboring aromatic ring and makes the π-electrons more available for interaction with the C-steel surface, and thus strengthens the adsorption of the alkyl chain on the steel surface. A skeletal representative of the mode of adsorption of compound DTB is shown in Figure .

Figure 10

Mechanism of the corrosion inhibition of DTB.

It is well-known that as the alkyl chain length increases, the hydrophobicity of the compound increases, in addition to the increase in the minimum area per molecule Amin. This leads to an increase in the strength of the protective layer that isolates the surface from the environment and increases the inhibition efficiencies as well as the adsorption process.[47]

Experimental Section

Components of API5L X-65 Type Carbon Steel Alloy

Carbon steel samples were cut from an unused oil pipeline for this study (Belayim Petroleum Co., Egypt). The chemical composition (wt %) of carbon steel is C, 0.09; Si, 0.22; Mn, 1.52; P, 0.01; S, 0.05; Ni, 0.04; Cr, 0.02; Mo, 0.004; V, 0.002; Cu, 0.02; Al, 0.04, and the reset is Fe.

Deep Oil Wells’ Formation Water

Deep oil wells’ formation water naturally exists in the reservoir rocks before drilling. The chemical composition and physical features of the oil well formation water studied are shown in Table .

Testing Solution

The testing solution of this study is oil well formation water with the above-mentioned chemical composition. The reaction of sodium sulfide (3.53 mg L–1) with acetic acid (1.7 mg L–1) generates H2S gas.

Synthesis of the Inhibitors

Scheme shows the synthesis of the benzothiazole-based cationic surfactant DTB. To perform this procedure, the quaternization reaction was used. In a 250 mL round flask were charged 50 mM benzothiazole and 50 mM 1-bromooctadecane separately, in the presence of acetone (100 mL) as a solvent. The reaction mixture was refluxed under stirring for 18 h and cooled to room temperature. The obtained brown precipitate was filtered, washed twice with diethyl ether, and then recrystallized from acetone to afford the white crystal products of the cationic surfactants. The yields of the brown crystal product ranged between 78% and 86%.

Scheme 1

Synthesis of Cationic Surfactants (DTB and HTB)

Hexyl bromide-based cationic surfactant HTB was produced as depicted in Scheme . The quaternization reaction was used to perform this task using the same procedures as previously mentioned. The yield of the brown crystal products ranged between 80% and 90%.

Data of FTIR

The FTIR spectrum (Figure ) of the prepared inhibitors (DTB, HTB) shows two peaks at 3355 and 3412 cm–1 ascribed to N–H in both inhibitors, 2923 and 2851 cm–1 corresponding to CH3 and CH2, in addition to the peak at 1065 cm–1 and the fingerprint peak at 722 cm–1 that referred to the asymmetric and symmetric stretching quaternary nitrogen atom (N+–C) as shown in Figure a,b.

Figure 11

(a,b) FTIR spectrum of the synthesized inhibitors: (a) DTB and (b) HTB.

1H NMR Spectrum Spectroscopic Analysis

Figure a,b shows the substance shift at δ (0.96) for 1H proton (a) −CH3, the substance shift δ (4.13) for 1H proton (b), and the chemical shifts between δ (7.2) for 1H protons (g) and (h) of the benzene ring. All of these chemical reactions prove that compound DTB was properly synthesized. The chemical shifts that obviously appeared in the 1H NMR spectrum were regarded as strong evidence that compound HTB was successfully prepared. The obtained surfactant possesses a hydrogen proton distribution that is consistent with its predicted structure in Figure a,b.

Figure 12

Nuclear magnetic resonance spectroscopy of the inhibitor: (a) 1H NMR of DTB and (b) 1H NMR of HTB.

Electrochemical Measurements

Potentiodynamic Polarization Measurement

To determine potentiodynamic polarization, we used an e-glass cell with a working electrode of 1 cm2 surface area and two other electrodes: a platinum reference electrode and a SCE saturated with water. A potentiostat type (Tacussel-radiometer PGZ402) and corrosion analysis software model PGZ402 were used. The scan rate was 1 mV s–1 after immersion of the three electrodes into the test solution for 60 min to obtain the steady-state potential in the absence and presence of a certain inhibitor concentration.

Electrochemical Impedance Spectroscopy (EIS)

We performed EIS on the same electrochemical cell at an OCP amplitude of 10 mV between the frequencies of 100 kHz and 20 mHz.

Surface Tension Measurements

For varied concentrations of the examined surfactants, the surface tension was measured with a Kruss K6 tensiometer, a direct surface tension measurement utilizing the ring method.

Surface Examination

After 12 days in the test solution, the intended samples were withdrawn, cleaned, rinsed, and dried with distilled water before being analyzed using SEM.

Quantum Calculation Details

In this work, quantum chemical calculations were carried out by using Gaussian 09W software, with a neutral and cationic LP (LP, LPH+)+G(d, p) basis set at the DFT level. The corresponding quantum chemical parameters were obtained and discussed in detail. Moreover, a study on the adsorption mode and the corresponding bonding strength of the inhibitor molecules on a metal surface is indispensable to understand its inhibitive performance. The interaction between the inhibitor molecules and steel surface was highlighted using a molecular mechanics method as implemented in the Forcite module. A simulation box of 24.3 × 17.2 × 67.1 Å dimension containing 5 layers of Fe (110), 1 inhibitor molecule, 500 H2O molecules, and a 40 Å vacuum layer was established. The periodic boundary condition and the COMPASS force field were used in this system. A fine quality simulation was accomplished with a 500 ps simulation time and a 1 fs time step using the NVT canonical ensemble.

Conclusions

In deep oil well formation water, surfactants DTB and HTB are effective inhibitors of the breakdown of carbon steel. The structure of the surfactants was confirmed by FTIR and nuclear magnetic resonance 1H NMR spectroscopic techniques. The obtained polarization data confirmed that the selected cationic surfactants act as mixed-type inhibitors, but the anodic effect was predominant. As seen in SEM micrographs, the corrosion-resistant environment was effectively isolated from the surface. How the synthesized surfactants performed as corrosion inhibitors was increased with increasing concentration until the CMC was reached. Also, EHOMO and the ΔN parameters are further proof of the inhibition effect of the used surfactants. Finally, the Monte Carlo simulation approach confirmed the correlation between both the experimental and the theoretical results.