Anticorrosion Effect of Ethoxylate Sulfanilamide Compounds on Carbon Steel in 1 M Hydrochloric Acid: Electrochemical and Theoretical Studies.

Metal corrosion is an important economic problem globally. One of the best ways to protect metal surfaces from corrosion is by the use of corrosion inhibitors, especially surfactants. This study assesses anticorrosion properties of three inhibitor compounds (S1, S2, and S3) of ethoxylate sulfanilamide containing 2, 10, and 20 units of ethylene oxide on carbon steel in 1 M HCl solution. The anticorrosive performance of S1, S2, and S3 was studied using potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), adsorption isotherm, surface tests (scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray diffraction (XRD) analysis), and computational studies (density functional theory (DFT) and molecular dynamics (MD) simulations) within the concentration range of 10-6 to 10-2 M at 30 ± 2 °C. The results of the methods used indicate that increasing the concentration of the inhibitor compounds improves the effectiveness of inhibition (from 50.9 to 98%), whereas the inhibition efficiency order for ethoxylated sulfanilamide compounds is S2 > S3 > S1 with the highest inhibiting efficiency, respectively, of 98.0, 95.0, and 90.0% for 10-2 M. Also, PDP indicated that S1, S2, and S3 inhibitors act as mixed-type inhibitors and their adsorption obeys the Langmuir adsorption isotherm model. Surface tests show that the studied compounds can significantly inhibit acid attack via chemical adsorption on the metal. Furthermore, all of the chemical descriptors derived from DFT indicate that the three inhibitors are quite well adsorbed by the adhesion centers on the CS surface. The three compounds' molecular geometries and electronic structures were calculated using quantum chemical calculations. Using theoretical computations, the energy difference between the highest occupied molecular orbital and the lowest occupied molecular orbital has been determined to represent chemical reactivity and kinetic stability of a composition.

Introduction

Metal corrosion, which leads to an increased cost of manufacture, is one of the main problems in the industry. Carbon steel is utilized in a wide range of application areas, particularly in the petroleum industry, from production to refinery, and it is a remarkably economical and appealing material for engineering applications due to its low cost, widespread accessibility, and good mechanical properties.[1,2] Carbon steel has become one of the primary and widely used materials playing a major role as a raw material in the metallurgical industry. Corrosion can be considered to be a chemical process resulting in the conversion of metal surfaces into the respective ionic species in the presence of moisture (H2O) and other reactive surrounding chemicals. Manufacturing machinery is exposed to corrosion using products such as hydrochloric acid or sulfuric acid for removal of mill scale, by acid washing, well-acidifying oil, and removal of localized deposits after processes have ended. It is generally agreed that carbon steel alloys start with HCl as a medium.[3,4] There are different defensive methods to prevent carbon steel from corrosion in severe conditions.[5,6] Organic inhibitors have been developed and consumed steadily since they are one of the most inexpensive and realistic forms of protecting metals against acid corrosion.[7,8] Strong adsorption of inhibitors to the metal surface through functional groups such as nitrogen, sulfur, phosphorus, oxygen, phenyl ring, or double bond is needed for metal corrosion inhibition.[9,10] The inhibition of corrosion of a metal occurs through physisorption or chemisorption on the metal surfacing of the inhibitor. The charged hydrophilic groups and the charged active centers attract each other electrostatically, causing physisorption on a metal surface. In the petroleum industry, corrosion inhibitors, mostly surfactants, are commonly used to protect iron and steel equipment used in hydrocarbon exploration, processing, transport, and refining.[11,12] In fact, the introduction of ethylene oxides (i.e., ethoxylation) into surfactant molecules enhances the inhibition effect of surfactants;[13] the nature of these groups also improves the solubility of surfactants. The benefits of using surfactants as corrosion inhibitors are due to the high efficiency of inhibition yield, low toxicity, and low price.[14] Chemisorption or electrostatic physical adsorption to the metal surface may be caused by the inhibitory activity of surfactant molecules in an aqueous solution.[15] A trend that is gaining attention from various industries is the production of surfactants based on natural renewable resources. Sulfonamides are antibiotics that are often used to treat infections caused by Gram-positive bacteria, fungi, and protozoa. Despite the fact that the advent of antibiotics has decreased the efficacy of sulfonamides, they nevertheless play a small but important role in physicians’ therapeutic services. Sulfonamides are a significant class of corrosion inhibitors owing to the presence of several adsorption centers. Their solubility in an acidic medium is due to the highly basic nature of aryl sulfonamidomethyl phosphonates.[16] Therefore, there are many chemical and electrochemical studies to determine the corrosion inhibition properties of sulfanilamide derivatives for mild steel in acidic media.[17] Sulfanilamide Schiff bases were synthesized and tested against mild steel corrosion in 1 M H2SO4 using electrochemical and weight loss techniques. The inhibition efficiencies (IEs) of the investigated sulfanilamide Schiff bases were 95, 91, and 89% for weight loss, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS), respectively, at 7.5 mM inhibitor concentration.[18,19] The efficiency of a nonionic gemini surfactant based on ethoxylated sulfonamide as a corrosion inhibitor was studied for carbon steel in 0.5 M H2SO4 using weight loss, PDP, and EIS techniques. Further, 92.5, 94.2, 94.6% was the inhibition efficiency for weight loss, PDP, and EIS, respectively, at 10–3 M concentration of the studied inhibitor.[20,21]

In this article, the corrosion inhibitor properties of ethoxylated sulfanilamides were investigated. In aqueous 1 M HCl solutions on carbon steel, the impact of the presence of an inhibitor on the corrosion rate was investigated. To determine their efficacy as corrosion inhibitors, electrochemical techniques and quantum calculations were used.

Results and Discussion

Open Circuit Potential (OCP)

Figure shows the open circuit potential (OCP) obtained over a broad potential spectrum in various S1, S2, and S3 concentrations (1 × 10–6 to 1 × 10–2 M). They are compared to the blank curve, which was obtained under similar conditions in a 1 M HCl solution. It can be seen clearly from Figure that the OCP is stable after 30 min, and the system appears to have reached steady conditions. Besides, the OCP values of carbon steel are in a positive direction after adding S1. This is because S1 highly affected the cathodic reduction reaction than the anodic one. Figure S2,S3 shows that the open potential values of carbon steel were in the negative direction after adding S2 and S3, which confirms that the damping effect of the compounds S1 and S2 is greater on the anodic reaction than on the cathodic reaction.[22,23]

Figure 1

Open circuit potential plots for the carbon steel electrode in 1 M HCl solution without (blank) and with various concentrations of (a) S1, (b) S2, and (c) S3 compounds.

Potentiodynamic Polarization (PDP)

Figure provides effective potentiodynamic polarization diagrams for the metallic substrate at 25 °C in tested environments with varying concentrations of sulfanilamide derivatives (S1, S2, and S3). When S1, S2, and S3 are present in the acid solution, the polarization curves shift to lower current density values of 0.054, 0.013, and 0.028 mA·cm2, respectively, than when S1, S2, and S3 are absent (0.544 mA·cm2). This simply means that the presence of the formulated molecules reduces the rate of the anodic and cathodic reactions in a violent acidic medium, lowering the corrosion rate of carbon steel.

Figure 2

Potentiodynamic polarization curves for carbon steel in 1 M HCl at different concentrations of (a) S1, (b) S2, and (c) S3 inhibitor compounds at 25 °C ± 1.

The electrochemical variables obtained from the profiles of polarization of 1 M hydrochloric acid for carbon steel are presented in Table . The obtained parameters include the potential for corrosion (Ecorr), the slopes (ba and bc) of anodic and cathodic Tafel, and the density of corrosion current (icorr). The values of icorr have been obtained by linear extrapolation of ba and bc values.[24] It could also be inferred by analyzing the values included in Table that βa and βc are slightly changed upon the addition of S1, S2, and S3 compared to the blank. This implies that both cathodic and anodic sites are blocked by the inhibitor molecules, resulting in the inhibition of cathodic and anodic reactions. In addition, the obtained lower icorr values at higher inhibitor concentrations, indicating a lower rate of corrosion because of a large number of adsorbed particles of S1, S2, and S3 on the active sites of the metal, thus leads to an increase in the covered area.

Table 1

Polarization Parameters for Carbon Steel in 1.0 M HCl with Different Concentrations of the Corrosion Inhibitor at Room Temperaturea

concn × 10–5		E (mV)	icorr (mA·cm2)	βa (mV)	Bc (mV)	CR (mpy) × 105	θ	EFicorr (%)	EFCR (%)
	1 M HCl	–511	0.544	145	–137	643
S1	0.1	–511	0.238	273	–188	281	0.56	56	56
	1	–516	0.155	131	–86	183	0.72	72	72
	10	–524	0.118	294	–167	140	0.78	78	78
	100	–529	0.069	182	–129	82	0.87	87	87
	1000	–540	0.054	276	–166	64	0.90	90	90
S2	0.1	–513	0.122	116	–118	144	0.78	78	78
	1	–530	0.049	189	–138	58	0.91	91	91
	10	–521	0.042	157	–130	50	0.92	92	92
	100	–514	0.014	135	–121	17	0.97	97	97
	1000	–536	0.013	180	–126	15	0.98	98	98
S3	0.1	–529	0.176	219	–130	208	0.68	68	68
	1	–530	0.068	186	–133	80	0.88	88	88
	10	–529	0.046	178	–128	54	0.92	92	92
	100	–530	0.039	191	–127	46	0.93	93	93
	1000	–515	0.028	192	–122	33	0.95	95	95

mpy, milliinch per year.

The corrosion potential displacements (Ecorr) created by the presence of S1, S2, and S3 were measured according to eq where Ecorrinh and Ecorro are the corrosion potentials with and without the inhibitor for a sample, respectively. Various studies have shown that the corrosion resistance behavior can be recognized from the value of Ecorr as follows.[25]

Inhibition is ascribed to a simple geometric blockage of anticorrosive particles on the base metal if the Ecorr values are just about negligible or zero, and the IE is directly related to the concentration of the tested inhibitors.[26]

S1, S2, and S3 are a mixed type if the displacement values are lower than 85 mV.

If the displacement values are above +85 mV or below −85 mV, the molecules under study may also be anodic or cathodic, respectively.[27,28]

For the present work, S1, S2, and S3 may classify as mixed-type inhibitors to ΔEcorr values. The decrease in icorr and the increase in inhibition efficiency (% IE) with increased additive concentrations indicate that S1, S2, and S3 act as carbon steel anticorrosive compounds in 1 M HCl. The efficiency (ηp %) was computed using eq where the corrosion current densities without and with the inhibitor are icorro and icorr, respectively. The efficiency (ηp %) of S1, S2, and S3 has been measured and presented in Table , showing that ηp % varies directly as the molarity of the inhibitor increases to 1 × 10–2 M/L. The order of inhibition efficiency decreased as follows: S2 > S3 > S1.[29,30]

Impedance Investigations

To obtain further evidence of the resistance of corrosion, EIS investigations have been reported for C steel in 1 M HCl conditions with and without different amounts of S1, S2, and S3. Figure shows a typical set of Nyquist plots at 25 °C. Several parameters have been derived according to the given equations and are presented in Table where Rcto and Rct are the values of charge transfer resistance in the absence and the presence of the inhibitor compound, respectively. The Nyquist plot in Figure was supposed to be a semicircle with the base on the x-axis. The observed plot, on the other hand, was an arc of a circle with the middle some distance below the x-axis. These depressed semicircles have been clarified by the fact that some physical property of the system was not homogeneous or that the value of some physical property of the system was distributed (dispersion). From Figure , the degree of the semicircle and Cdl values for three compounds, S1, S2, and S3, are different depending on the n value presented in Table . The order of the depression of the semicircle for the three compounds is as follows: S2 < S3 < S1; these phenomena indicate that the corrosion rate decreases with an increase in the n value and a decrease in the depression of the semicircle.

Figure 3

Nyquist plots for carbon steel in 1 M HCl without and with various concentrations of (a) S1, (b) S2, and (c) S3 at 25 °C.

Table 2

Data of Constant Phase Element (CPE) Matching for Mild Steel in 1.0 M HCl with Different Concentrations of the Corrosion Inhibitor at 30 °Ca

	concn × 10–5	Rs (ohm·cm2)	Q (S·sn/cm2) × 10–5	n	Rct (ohm·cm2)	Cdl (mF cm–2)	Chi squared	τ (s)	EF (%)
	1 M HCl	2.9 ± 0.02	78.8 ± 2.6	0.8 ± 0.01	131 ± 3.22	0.447	1.6 × 10–2	0.059
S1	0.1	4 ± 0.11	50.09 ± 3	0.8 ± 0.01	267 ± 6.5	0.303	1.5 × 10–2	0.081	50.9
	1	7 ± 0.13	41.7 ± 2.97	0.71 ± 0.01	289 ± 7.9	0.176	1.5 × 10–2	0.051	54.8
	10	6 ± 0.13	40.08 ± 2.8	0.68 ± 0.01	305 ± 7.8	0.149	1.4 × 10–2	0.045	57
	100	5 ± 0.14	38.15 ± 2.5	0.66 ± 0.01	319 ± 8.1	0.129	1.4 × 10–2	0.041	59
	1000	4 ± 0.16	35.78 ± 2.8	0.64 ± 0.01	361 ± 10	0.113	1.7 × 10–2	0.041	64
S2	0.1	12 ± 0.14	41.04 ± 2.5	0.68 ± 0.01	319 ± 8.7	0.157	1.3 × 10–2	0.050	59
	1	9 ± 0.17	4.19 ± 0.22	0.81 ± 0.007	1931 ± 4.8	0.023	1.3 × 10–2	0.044	93.2
	10	16 ± 0.18	3.68 ± 2.7	0.8 ± 0.008	1947 ± 4.6	0.019	1.8 × 10–2	0.037	93.4
	100	11 ± 0.17	3.43 ± 0.14	0.76 ± 0.006	2515 ± 5.1	0.016	2 × 10–2	0.036	94.8
	1000	8 ± 0.21	3.01 ± 0.20	0.75 ± 0.007	2570 ± 6.5	0.013	1.8 × 10–2	0.033	95
S3	0.1	4 ± 0.06	14.8 ± 0.59	0.82 ± 0.006	1061 ± 2.5	0.099	1.3 × 10–2	0.0105	88
	1	6 ± 0.07	13.5 ± 0.6	0.78 ± 0.005	1192 ± 3.0	0.081	1.3 × 10–4	0.097	89
	10	5 ± 0.07	12.2 ± 0.63	0.78 ± 0.006	1206 ± 3.2	0.071	1.4 × 10–2	0.086	89.2
	100	4 ± 0.08	11.2 ± 0.76	0.75 ± 0.006	1293 ± 3.6	0.059	1.5 × 10–2	0.076	89.8
	1000	4 ± 0.02	10.3 ± 0.12	0.74 ± 0.002	1345 ± 4.2	0.051	1.3 × 10–4	0.069	90.3

Rs, solution resistance; Rct, charge transfer of resistance; Cdl, double-layer charge; and EF, inhibition efficiency.

The relaxation time (τ) is the amount of time required to return to the equilibrium state. The relaxation time was calculated using the formula In 1 M HCl, the obtained value of τ is 0.059 s. However, as the concentrations of S1 and S2 increased, the values of τ gradually decreased and reached 0.041 and 0.033 s at an optimum concentration (1 × 10–2 M), respectively, while in the case of S3, the value of τ increased and reached 0.069 s at an optimum concentration (1 × 10–2 M).

As a result, adding S1 and S2 reduces the value of the relaxation time constant (τ) and Cdl (Table ), implying that discharging and charging rates at the steel/solution interface are greatly reduced. As a result, the volume of the charge stored as capacitance and the discharge velocity as the relaxation time constant (τ) at the interface agree well with one another.[31]

For the analysis of EIS, the corresponding circuit (EC) was obtained using an EIS analyzer, as seen in Figure . Then, Qcdl is the double-layer capacitance, Rs is the solution resistance, and Rct is the charge transfer resistance. Table lists the values of the obtained impedance parameters, such as solution resistance (Rs), charge transfer resistance (Rct), phase shift (n), CPE constant phase elements (Y0), and double-layer capacitance (Cdl). The data obtained are consistent with the formation of a strong protective layer of the molecules examined on the metallic surface as shown by the following.[32]

Figure 4

Equivalent circuits compatible with the experimental impedance data in Figure for corrosion of the carbon steel electrode at different inhibitor concentrations (S1, S2, and S3).

The direct proportion of Rct values to S1, S2, and S3 concentrations indicates that the corrosion resistance of the metallic substrate is regulated primarily by the charge transfer process.

The inverse proportion between the values of Cdl and the concentrations of S1, S2, and S3 is related to the increased replacement of H2O particles by adsorbed inhibitors in the double layer, thus lowering the corrosion rate on the surface of C steel by an adsorption mechanism.[33]

ηI % is directly proportional to the concentrations of S1, S2, and S3 up to 1 × 10–2 M/L. This may be attributed to the high surface coverage (θ) of the inhibitor.

The evaluation of S1, S2, and S3 inhibitor efficiency obtained from EIS and PDP measurements can be compared (Table S1), and higher efficiency than those reported in the literature refs (15, 40, 42, 44) for steel in acidic medium is observed.

Adsorption Calculations

If the activity of the inhibitors is related to their adsorption on the metallic substrate as the electrochemical findings verify, the use of an adsorption isotherm is probable.[34] Adsorption isotherms generally describe the manner in which inhibitor molecules interact with active metal surface locations.[35]

Electrochemical tests are used to determine the surface coverage (θ) as a function of the inhibitor concentration (Cinh) by matching it to different isotherms to select the appropriate fit describing the behavior of the investigated molecules. The isotherm of Langmuir has provided the best overview of the adsorption and is represented aswhere Kads is the adsorptive equilibrium constant. Figure represents the linear relationships of Cinh/θ versus C, simply indicating that now the adsorption of S1, S2, and S3 on the metal in HCl obeys the Langmuir isotherm.[36]

Figure 5

Langmuir adsorption isotherms for inhibition of carbon steel corrosion in 1.0 M HCl by S1, S2, and S3 at different concentrations.

The Langmuir statement suggests a set percentage of active sites on a hard surface and that each position contains a single adsorbed species.[37] From Table , the ΔGadso values are around – 40 kJ mol–1 (−38, −42, and −39 for S1, S2, and S3, respectively). Hence, physical and chemical adsorption processes coexist, but chemical adsorption highly predominates by charge transfer or sharing between surfactants and the steel surface.[16]

Table 3

Different Thermodynamic Parameters of the Linear Regression Cinh/θ and Cinh in 1 M HCl for the Corrosion Inhibitors at Room Temperature

compound	slope	R2	Kads	ΔGads (kJ mol–1)
S1	1.11	1	71 429	–38
S2	1.02	1	434 782	–42
S3	1.05	1	142 857	–39

Scanning Electron Microscopy (SEM)

Figure presents SEM micrographs of the surface of a CS specimen after 24 h of treatment in 1 M HCl without an inhibitor and after application in corrosive solution for the same period with 1 × 10–2 M/L of S1, S2, and S3. The subsequent SEM micrographs indicate that without an inhibitor, the surface was seriously damaged, but the surfaces with inhibitors S1, S2, and S3 were less damaged, confirming the efficiency of these compounds to prevent corrosion.

Figure 6

SEM micrographs for the surface of carbon steel specimens after 24 h of immersion in 1 M HCl solution: (a) without a corrosion inhibitor and with 1 × 10–2 mol L–1 inhibitor compounds (b) S1, (c) S2, and (d) S3.

Energy-Dispersive X-ray (EDX) Spectroscopy

After treatment with and without inhibitors (S1, S2, and S3), the EDX spectrum of the polished metal surface for 1 day is shown in Figure (1 M HCl, S1, S2, and S3), indicating that the iron peak is massively reduced compared to the samples in Figure a. This drop in the iron band demonstrates that an attached resistant inhibitor film has been deposited on the surface of polished C steel, resulting in good IE.[38] The surface evaluations of the CS (EDX and SEM) thus confirmed that the formulated molecules (S1, S2, and S3) in the tested medium represent strong inhibitors (in line with the results obtained from electrochemical methods).

Figure 7

EDX elemental analysis spectra for carbon steel immersed in 1.0 M HCl (a) without a corrosion inhibitor and with 1 × 10–2 mol L–1 inhibitor compounds (b) S1, (c) S2, and (d) S3.

X-ray Diffraction (XRD) Technique

The XRD technique has also been used to provide methodological confirmation of the inhibition properties of the prepared ethoxylated sulfanilamides (S1, S2, and S3) for carbon steel. Figure presents XRD graphs for uninhibited and inhibited carbon steel after immersion in the acidic medium for 24 h. When the graphs were properly analyzed, it was clear that the carbon steel sample was exposed to corrosion in the absence of an inhibitor. Iron oxide formation is clearly shown by the appearance of magnetite peaks (Fe3O4 and FeOOH) at 2θ = 14.73, 26.97, 37.12, 53.54, and 61.34°. The extreme peak of iron arises at 2θ = 45.6°.[39] Magnetite was not found by XRD on any of the specimens treated with S1, S2, and S3. As mentioned by Momber, magnetite is the final state of an oxide film on carbon steel after conversion from esmeraldite (γ-FeO(OH)) to goethite (α-FeO(OH)) and, finally, magnetite.

Figure 8

XRD patterns of corrosion products formed on the surface of carbon steel in 1 M HCl (a) without a corrosion inhibitor and with 1 × 10–2 mol L–1 inhibitor compounds (b) S1, (c) S2, and (d) S3.

Computational Studies

We can consider this work as the effect of 4-amino benzene sulfonamide, where hydrogen atoms are substituted in the two amino groups by ethylene oxide units, on the inhibition efficiency of CS in an acidic medium. Here, S1 results from substitution of one hydrogen atom of SO2NH2 by two ethylene oxide units, S2 is the product of replacing each hydrogen atom in the two amino groups by three ethylene oxide units, and S3 is obtained by substituting five ethylene oxide units for each hydrogen atom in the two amino groups. Accordingly, quantum calculations and molecular dynamics simulations have been conducted to investigate the influence of the molecular structure on the inhibition efficiency of ethoxylated sulfanilamides (S1, S2, and S3) and their adsorption orientation on the metallic surface.

Quantum Chemical Calculations

The bond lengths, bond angles, and dihedral angles of the investigated ethoxylated sulfanilamides were first geometrically and electronically optimized. The molecular structures obtained from the optimization process with minimum energies and highest occupied molecular orbitals (HOMOs), and lowest unoccupied molecular orbitals (LUMOs) are given in Figure .

Figure 9

Optimized, HOMO, and LUMO structures of S1, S2, and S3 obtained by the DFT method.

It is obvious from Figure that the HOMO electron density distributions in S1, S2, and S3 compounds are located across the 4-amino benzene sulfonamide moiety, while the ethylene oxide units are not involved. This suggests that this site is responsible for electron-donating ability among the three investigated ethoxylated sulfanilamides. Also, the electron density of LUMO is located across the 4-amino benzene sulfonamide moiety; therefore, this part of the ethoxylated sulfanilamide molecules is the best site for both electron-accepting and electron-donating interactions with the CS surface.[40] The obtained outcomes from the quantum calculation are collected in Table .

Table 4

Quantum Chemical Parameters of Gas and Aqueous Phases of S1, S2, and S3

molecule	EHOMO (eV)	ELUMO (eV)	ΔE (eV)	μ (D)	TE (keV)	MV (cm3 mol–1)	TNC (e)	η (eV)	ε (eV–1)	ω (eV)	X (eV)	ΔN (e)	IE* (%)
DFT/B3LYP/3-21G (Gas)
S1	–6.1066	–0.61477	5.491836	10.2958	–32.429	173.425	–5.6157	2.58984	0.4863	2.0565	3.243	0.3045	68.65
S2	–5.8137	–0.63434	5.179423	8.9048	–74.026	541.565	–13.536	2.58971	0.4983	2.0069	3.224	0.3081	95.31
S3	–5.8327	–0.65310	5.179695	9.1856	–107.304	794.356	–19.370	2.74591	0.4925	2.0304	3.361	0.2657	90.83
DFT/B3LYP/3-21G (Aqueous)
S1	–5.7689	–0.1819	5.5870	10.4506	–32.4177	151.113	–6.0315	2.7935	0.6311	1.5846	2.975	0.3302	68.65
S2	–6.0248	–1.2690	4.7558	7.0990	–74.0271	488.192	–13.6966	2.3779	0.3576	2.7965	3.646	0.2467	95.31
S3	–5.7665	–0.7793	4.9872	10.9718	–107.3056	794.356	–20.2095	2.4936	0.4656	2.1478	3.272	0.3102	90.83
DMol3 (Gas)
S1	–5.6930	–0.2912	5.4018		–32.389		–5.833	2.7009	0.6034	1.6574	2.9921	0.3384	68.65
S2	–5.3551	–0.2257	5.1294		–73.798		–13.655	2.5647	0.6588	1.5180	2.7904	0.3957	95.31
S3	–5.7667	–1.2140	4.5527		–106.922		–15.926	2.2763	0.3737	2.6759	3.4904	0.2921	90.83
S1-H+	–7.9240	–5.8722	2.0518		–32.397		–4.931	1.0259	0.0431	23.1917	6.8981	–1.0128	68.65
S2-H+	–7.1580	–5.2863	1.8718		–73.880		–12.499	0.9359	0.0483	20.6839	6.2222	–0.7491	95.31
S3-H+	–6.7747	–5.0138	1.7608		–107.068		–18.042	0.8804	0.0507	19.7306	5.8942	–0.6101	90.83
DMol3 (Aqueous)
S1	–6.0155	–0.7687	5.2469		–32.606		–6.156	2.6234	0.4560	2.1930	3.3921	0.2721	68.65
S2	–5.6726	–0.7151	4.9576		–73.798		–14.477	2.4788	0.4860	2.0576	3.1939	0.3280	95.31
S3	–5.9106	–2.0088	3.9018		–106.922		–17.366	1.9509	0.2489	4.0185	3.9597	0.2205	90.83
S1-H+	–6.3005	–2.7804	3.5200		–32.401		–5.246	1.7600	0.1707	5.8567	4.5405	0.0794	68.65
S2-H+	–6.0337	–2.9942	3.0396		–73.885		–13.421	1.5198	0.1492	6.7035	4.5139	0.1007	95.31
S3-H+	–5.9902	–3.3164	2.6739		–107.073		–19.453	1.3369	0.1235	8.0981	4.6533	0.0623	90.83

The energy values of the frontier molecular orbital (FMO) either HOMO or LUMO play an important role in describing the chemical reactivity. The energy of the HOMO is a function of the ability of the organic compounds to donate electrons, contrary to LUMO, which is a function of the ability of the organic compounds to obtain electrons. The selection of good organic compounds as corrosion inhibitors is based on the highest values of HOMO and the lowest values of LUMO.[41] On inspecting the values of HOMO in Table , it is clear that S2, which poses the highest inhibition efficiency, has the highest EHOMO by the DFT/B3LYP/3-21G basis set calculated for the gas phase and by DMol[3] for both gas and aqueous phases. We can rank the three investigated ethoxylated sulfanilamides according to the HOMO as S2 > S3 > S1; EHOMO. This confirms the obtained experimental ranking. The DFT/B3LYP/3-21G calculations also show that S1 possesses the highest value of LUMO either in gas or aqueous phase; therefore, it has the lowest ability to interact with the CS surface. The difference in energy between the HOMO and the LUMO (ΔE) is an important factor in determination of different aspects of organic compounds, such as the chemical reactivity, kinetic stability, chemical hardness/softness, and optical polarizability.[42] The high electronic stability and the low reactivity are a result of the large values of ΔE; on the other hand, low values of ΔE indicate high reactivity as it is easy for electrons to get excited and transfer from the HOMO to the LUMO. Therefore, a good organic corrosion inhibitor is based on lower values of ΔE.[43] The DFT/B3LYP/3-21G calculations also show that S2 has the lowest value of ΔE either in gas or aqueous phase, which indicates the highest response along with other ethoxylated sulfanilamides. The efficiency of ethoxylated sulfanilamides increases on decreasing values of ΔE·ΔN, which is an electron transfer from the ethoxylated sulfanilamides to the CS surface that has also been calculated. The ΔN values of the ethoxylated sulfanilamides are all positive (ΔN > 0). This is an indication of the ability of these compounds to share their electrons with the CS surface.[44] The donating ability of the studied molecules was also confirmed by the nucleophilicity index (ε) and its reverse index, the electrophilicity index (ω), which are calculated by the formulasNucleophilicity can be related to the electron-donating power, while chemical potential and electrophilicity are related to the electron-accepting power.[45,46] S2 has the highest value of (ε) and the lowest value of (ω) in all of the calculated methods. The total negative charge (TNC) responsible for the electrostatic attraction between the ethoxylated sulfanilamides and the CS surface was also calculated, where S1 has the lowest value, and this reflects its low values of inhibition efficiency. The low values of inhibition efficiency of S1 could also be attributed to its low molecular volume. The low molecular volume contributes to a decrease in the region of interaction between both the S1 molecule and the CS surface, as well as less surface coverage.

Local Reactivity: Fukui Functions

The nucleophilic sites (f+), electrophilic sites (f–), and the dual descriptor (Δf(k)) were calculated using the Mulliken population analysis by the following equationswhere q is the atomic charge in its neutral (N), cationic (N – 1), or anionic (N + 1) state.

The outcome Fukui data for the ethoxylated sulfanilamides (S1, S2, and S3) are collected in Table .

Table 5

Condensed Fukui Functions and the Dual Descriptor of S1, S2, and S3 Obtained Using the DMol3 Method

	DBB				MBB				CBB
atom	f+	f-	Δf	atom	f+	f-	Δf	atom	f+	f-	Δf
C (1)	0.032	0.059	–0.027	C (1)	–0.006	0.047	–0.053	C (1)	0.016	0.051	–0.035
C (2)	0.035	0.012	0.023	C (2)	0.094	0.016	0.078	C (2)	0.001	0.03	–0.029
C (3)	0.054	0.08	–0.026	C (3)	0.052	0.057	–0.005	C (3)	0.024	0.027	–0.003
C (4)	0.089	0.014	0.075	C (4)	0.026	0.016	0.01	C (4)	0.011	0.016	–0.005
C (5)	–0.001	0.049	–0.05	C (5)	0.034	0.05	–0.016	C (5)	0.012	0.06	–0.048
C (6)	0.109	0.02	0.089	C (6)	0.104	0.015	0.089	C (6)	0.011	–0.007	0.018
N (11)	0.038	0.13	–0.092	S (11)	0.055	0.015	0.04	S (11)	0.019	0.035	–0.016
S (14)	0.029	0.035	–0.006	O (12)	0.049	0.041	0.008	O (12)	0.127	0.055	0.072
O (15)	0.053	0.054	–0.001	O (13)	0.041	0.045	–0.004	O (13)	0.145	0.043	0.102
O (16)	0.066	0.042	0.024	N (14)	0.019	0.111	–0.092	N (14)	0.021	0.109	–0.088
N (17)	0.014	0.001	0.013	C (15)	–0.021	–0.029	0.008	C (15)	–0.007	–0.041	0.034
C (19)	–0.027	–0.001	–0.026	C (16)	–0.02	–0.028	0.008	C (16)	–0.022	–0.025	0.003
C (22)	0.003	0.016	–0.013	O (27)	0.004	0.006	–0.002	C (17)	0.007	–0.001	0.008
O (25)	0.008	0.012	–0.004	C (28)	–0.012	–0.016	0.004	C (22)	0	0.005	–0.005
C (26)	–0.009	–0.012	0.003	C (29)	0.002	0.002	0	O (27)	0.003	0.01	–0.007
C (29)	–0.006	–0.004	–0.002	O (34)	0.001	0.001	0	C (28)	–0.007	–0.015	0.008
O (31)	0.01	0.008	0.002	C (35)	–0.007	–0.008	0.001	C (29)	0.003	0.002	0.001
				O (41)	0.002	0.003	–0.001	O (34)	–0.003	0.003	–0.006
				O (43)	0.005	0.008	–0.003	C (35)	0	–0.01	0.01
				C (44)	–0.012	–0.016	0.004	C (36)	0.001	0.003	–0.002
				C (45)	0.001	0.001	0	O (41)	–0.008	0.021	–0.029
				C (51)	–0.007	–0.008	0.001	C (42)	–0.002	–0.019	0.017
				O (57)	0.003	0.003	0	C (43)	0.016	–0.011	0.027
				N (59)	–0.012	–0.001	–0.011	O (48)	–0.008	0.008	–0.016
				C (60)	–0.01	–0.009	–0.001	C (49)	–0.004	–0.005	0.001
				C (61)	–0.012	–0.008	–0.004	C (50)	0.005	–0.001	0.006
				C (62)	0.002	0.002	0	N (55)	0.088	–0.019	0.107
				C (67)	0.007	0.005	0.002	C (56)	–0.097	0.053	–0.15
				O (72)	0.011	0.011	0	C (57)	–0.05	0.011	–0.061
								C (58)	0.052	–0.043	0.095
								C (63)	0.033	–0.024	0.057
								O (68)	0.01	0.011	–0.001
								C (69)	–0.015	–0.005	–0.01
								C (70)	0.003	0.002	0.001

The atom that possesses the highest value of (f–) is the nitrogen atom attached directly to the aromatic ring with values of 0.13, 0.111, and 0.109 for S1, S2, and S3, respectively.[47] This indicates its high ability to donate electrons to the CS surface. The carbon atoms of the aromatic moiety also possess high values of (f–) and participate in the donating process. Using the dual descriptor (Δf(k)), we can obtain two indications for both donor sites (Δf(r) < 0) and acceptor sites (Δf(r) > 0). Our data indicate that for the donor sites, the N atom is the best with values of −0.092, −0.092, and −0.088 for S1, S2, and S3, respectively. On the other hand, C (6) is the best acceptor site for both S1 and S2 with a value of 0.089. The nitrogen of the sulfonamide group is considered the best acceptor site in the S3 compound with a value of 0.107.

Molecular Dynamics Simulations

In recent years, molecular dynamics simulation has been used as a theoretical tool in corrosion studies to investigate the interaction energy and thus the geometry of adsorption for inhibitors on metal surfaces. The interaction between the three investigated ethoxylated sulfanilamides (S1, S2, and S2) and the Fe(110) surface in the neutral/protonated forms and gas/aqueous phase is studied. The aqueous phase is approached by adding 300 H2O, 20 Cl–, and 20 H3O+/19 H3O+ besides the neutral/protonated sulfanilamides. The two views (top and side) of the best adsorption configurations of the ethoxylated sulfanilamides on the Fe(110) surface are displayed in Figure . As shown in Figure , all of the three ethoxylated sulfanilamides have the ability for parallel adsorption on the Fe(110) surface, especially through the 4-amino benzene sulfonamide moiety by the sites of HOMOs and LUMOs. The parallel orientation for the adsorption process enhances the coverage of more surface area of steel. In the case of S3, the side view shows that one of the ethylene oxide chains is perpendicular to the Fe(110) surface. Therefore, it is not involved in the surface coverage and reduces the inhibition effect of S3. The different forms of energies belonging to the adsorption process are recorded in Table . It is clear that S1 possesses the lowest binding energy on the Fe surface. This agrees with its lowest value of experimental adsorption energy. The protonated form is the preferred form of adsorption as the binding energy increased upon protonation.

Figure 10

Side and top views of the most appropriate configuration for adsorption of neutral molecules on the Fe(110) surface obtained by MD simulations in aqueous solution.

Table 6

Outputs and Descriptors Calculated by Molecular Dynamics Simulation for Adsorption of S1, S2, and S3 on Fe(110) (in kcal mol–1)

phase	inhibitor	total energy (kcal mol–1)	adsorption energy (kcal mol–1)	rigid adsorption energy (kcal mol–1)	deformation energy (kcal mol–1)	(dEads/dNi) (kcal mol–1)	binding energy (kcal mol–1)	IE* (%)
gas phase	S1	–153.736	–135.316	–138.821	3.504	–135.316	135.316	68.65
	S2	–299.593	–336.946	–350.350	13.405	–336.946	336.946	95.31
	S3	–400.561	–555.745	–462.557	–93.189	–555.745	555.745	90.83
	S1-H+	–133.380	–190.869	–127.629	–63.240	–190.869	190.869	68.65
	S2-H+	–181.670	–339.747	–275.612	–64.134	–339.747	339.747	95.31
	S3-H+	–334.265	–559.859	–446.577	–113.281	–559.859	559.859	90.83
aqueous phase	S1	–6878.283	–7151.238	–7112.928	–38.309	–105.978	7151.238	68.65
	S2	–7457.130	–7661.585	–7654.972	–6.613	–137.532	7661.585	95.31
	S3	–7485.031	–7745.259	–7740.950	–4.308	–222.886	7745.259	90.83
	S1-H+	–6896.883	–7342.368	–7208.811	–69.131	–284.724	7342.368	68.65
	S2-H+	–7011.698	–7389.666	–7305.540	–84.125	–307.238	7389.666	95.31
	S3-H+	–7187.612	–7464.993	–7395.862	–133.557	–520.093	7464.993	90.83

Adsorption of a Corrosion Inhibitor (Mechanism Foretell)

According to the theory, oxygen, nitrogen, and sulfur are the active centers where inhibitor adsorption on the metallic surface can occur based on the donor–acceptor reaction. Since the atom with the most negative charges can quickly donate electrons to atoms’ empty electronic orbitals, the ethoxylate sulfanilamide inhibitors form an electrostatic force with the metal atoms. Because oxygen atom is highly negatively charged, similar to nitrogen and sulfur, it has the ability to bond to metal surfaces and can also be adsorbed easily. The inhibition mechanism in this study is the formation of a protective layer above the iron atoms that protects them from acidic media by increasing the ethylene oxide units. Due to the increase in the ethylene oxide units (20 units), molecules coil around themselves, causing steric hindrance between the active centers (O, N, and S) and the positive iron molecules, as seen in Figure . Therefore, the order of the inhibitors according to the protective effect on the metal in acid media is S2 > S3 > S1.

Figure 11

Schematic mechanism of adsorption of inhibitor compounds S1, S2, and S3 on the carbon steel surface.

Materials and Investigational Methods

In this study, three sulfanilamide compounds S1, S2, and S3 with different amounts of ethylene oxide were used, and the chemical structures of these compounds are listed in Table . These three compounds have been prepared and studied for their biological activity and inhibition of corrosion primarily through weight loss in a previous study.[48]

Table 7

Designation, Molecular Weight, and HLB for Ethoxylated Sulfanilamide

Metal Sample Specifications

As a working electrode for electrochemical experiments, a carbon steel cylinder with the same shape as the epoxy resin and a surface area of 1 cm2 was used. Using an emery sheet (320:1200), the tested surface of the electrode was first polished, rinsed, degreased with acetone, and then dried.

Inhibitor Solutions

A dilution process of AR class 37% HCl was used for preparing vigorous 1.0 M HCl solutions using bidistilled water. For S1, S2, and S3, the used HCl concentration was between 1 × 10–2 and 1 × 10–6 M/L.

Electrochemical Measurements

A Voltalab master radiometer (PGZ 301 model) with Zsimpwin software program was used to perform polarization and impedance studies. A three-electrode system was used with carbon steel as the working electrode, a saturated calomel electrode as the reference electrode, and a counter electrode of platinum wire. Impedance detection was performed via an AC sine wave amplitude of 10 mV with an open circuit potential and a frequency range from 100 kHz to 50 MHz. Polarization measurements required potential (−800: −300 mV) to OCP at a 0.2 mV s–1 scanning speed. The temperature used for the electrochemical experiments was 30 ± 2 °C (ASTM G3-74 and G-87). Corrosion current densities (icorr) can be achieved by extrapolation of the linear Tafel part of the anodic and cathodic curves to the corrosion potential axes.

Surface Tests

Scanning Electron Microscopy

For surface inspection, a scanning electron microscope (JEOL JSM-53000, Japan) was used. The beam energy of acceleration used was 25 kV. At a magnification power of X 1000, all of the surface morphologies were obtained.

Energy-Dispersive X-ray (EDX) Spectroscopy

The EDX instrument connected to the JEOL JSM-53000 scanning electron microscope was used to study the structural components of the film formed on the metallic substrate and postapplication of S1, S2, and S3.

Computational Details

Complete geometric optimizations of ethoxylated sulfanilamides were performed using DFT (density functional theory) with the Yang–Parr nonlocal feature vector (B3LYP) and the 3-21G basis set.[49,50] Complete molecular structure simulation together with the vibration study of configured S1, S2, and S3 was carried out using the Gaussian 09 software kit.[51] To provide validation for the computational data, the geometry optimizations of the ethoxylated sulfanilamides were also studied using the DMol[3] model under DFT approximation implemented in Materials Studio computational software.[52] Frontier molecular orbital (FMO) either HOMO or LUMO plays an important role in predicting an inhibitor compound’s adsorption centers. From the values of EHOMO and ELUMO, we can calculate important electrochemical parameters, such as ionization potential (I), electron affinity (A), absolute electronegativity (X), absolute hardness (η), and the fraction of electron transferred (ΔN). These parameters could be used in the interpretation of the inhibition effect and the use of organic compounds as corrosion inhibitors,[53,54] as determined by the following relationshipswhere φ = 4.82 eV mol–1 and ηFe = 0 eV mol–1 for iron.

The adsorption of ethoxylated sulfanilamides on the CS surface was simulated using the Materials Studio software’s adsorption locator module where COMPASS is used for force field approximations.

Conclusions

In acidic conditions, S1, S2, and S3 proved to be effective inhibitors of CS in aggressive media (1.0 M HCl) using electrochemical analysis (PDP and EIS), adsorption isotherm, surface tests (SEM, EDX, and XRD), and computational studies (DFT and molecular dynamics (MD) simulations); however, the grades of inhibitor efficiency depended on the concentration of the inhibitor.

The overall results of the various tests show that the investigated inhibitors showed excellent inhibition behavior for the CS sample.

In addition, PDP and EIS analysis show that 10–2 M S2 in 1.0 M HCl has a significantly higher inhibition efficiency equal to 98 and 95%, respectively (the order of inhibition efficiency is S2 > S3 > S1).

Polarization results show that S1, S2, and S3 are mixed-type inhibitors in 1 M HCl medium, which the impedance data suggest was accomplished by adsorption of the inhibitors on the mild steel surface.

The Langmuir adsorption isotherm governs the adsorption mechanism. The inhibitor species were heavily adsorbed on the metal surface, as shown by the negative values obtained for ΔG.

The ΔGadso values are around −40 kJ/mol (−38, −42, and −39 for S1, S2, and S3, respectively). Hence, the physical and chemical adsorption processes coexist.

The development of a defensive film adsorbed on the steel surface was observed using SEM and EDX spectroscopy.

Quantum chemical calculations show that the inhibitor molecules’ heteroatoms, which serve as active sites, specifically adsorb on a mild steel surface.

Quantum chemical calculations based on DFT of parameters related to the electronic properties of various inhibitor components demonstrated the inhibitor’s ability to adsorb on a steel surface.