Adsorption Mechanism, Kinetics, Thermodynamics, and Anticorrosion Performance of a New Thiophene Derivative for C-Steel in a 1.0 M HCl: Experimental and Computational Approaches

Abstract

Metal surfaces can become damaged by corrosion when they interact with their surroundings, leading to huge financial losses. The use of corrosion inhibitors is one of the most crucial ways to combat the risky and hazardous effects of corrosion. In the present research, electrochemical techniques and surface analysis have been used to characterize the inhibition properties of (3-hydroxy-4-((4-nitrophenyl)diazenyl)-5-(phenylamino)thiophen-2-yl)(phenyl)methanone for the corrosion of carbon steel in an aerated 1.0 M HCl solution. Steel’s corrosion resistance was discovered to be improved by the presence of the examined inhibitor in 1.0 M HCl medium through the adsorption of the inhibitor species to create a barrier layer. The findings showed that when inhibitor concentrations increased and solution temperatures decreased, the inhibition performance (%IE) of the compound under study enhanced. In the light of inhibitor probe’s chemical makeup and theoretical analysis, the mechanism of the inhibition process was addressed. In a 1.0 M HCl solution containing 5 × 10⁻⁵ M of the inhibitor, the inhibition performance, at room temperature, was found to be almost 97%. The electrochemical results revealed that the examined compound successfully prevented carbon steel corrosion as a mixed-type inhibitor. The Langmuir and Freundlich isotherms are pursued by the adsorption of the examined inhibitor. Additionally, using Arrhenius and transition state equations, the activation thermodynamic parameters ΔE_a, ΔH*, and ΔS* were determined and explained. The adsorption process was illustrated using DFT computation and MC simulations. The experimental findings and theoretical simulations concurred surprisingly well. Finally, the paper presents a discussion of the inhibitory mechanism.

1. Introduction

In both daily life and industrial operations, metal corrosion is a common and well-established occurrence. It can decrease a metal or alloy’s functionality, reduce its lifespan, and increase maintenance costs. Additionally, engineering quality, public safety, and considerable financial loss are all at serious risk from corrosion [1,2]. Because of its beneficial characteristics, such as superior mechanical pliability, superior cost-effective performance, and strong thermal and electrical conductivity, the petrochemical sector, pipeline transportation industry, power plants, the navy, and the military commonly use carbon steel. Numerous circumstances, including an acidic solution, a microbiological environment, an oxygen-rich medium, and a medium with high conductivity, can cause steel to corrode [3,4,5,6,7,8]. One example is pickling with HCl, which is frequently used in industrial processes including chemical cleaning, alloy pickling, the descaling of boilers, and the acidification of oil wells [9]. In comparison with other acids, HCl acid is favored considerably since the salts produced through the method are soluble in water [10,11,12,13,14]. Although corrosion cannot be completely eliminated, its rate can be reduced by implementing efficient protective measures. It is predicted that if proper preventative measures are taken, 20–30% of corrosion losses could be prevented [15,16]. One of the most useful and economical approaches now taken in business is the usage of corrosion inhibitors. The adsorption of corrosion inhibitors onto metal surfaces creates a barrier that protects them from corrosive media. Therefore, there is always a significant demand for the discovery and development of new inhibitors with high inhibition capacities [17,18,19,20,21,22]. Corrosion inhibitors are often organic substances with heteroatoms (O, N, S, and/or P) and aromatic rings or long alkyl chains [23,24,25,26,27,28]. In acidic settings, several N-heterocyclic compounds act as powerful corrosion inhibitors because they contain polar groups and/or π electrons. An inhibitor can stop the corrosion of metals by physically or chemically adhering to the surface of the alloy [29]. In terms of application, heteroatoms and other compounds with functional groups that may donate a single pair of electrons are advantageous as inhibitors of metal corrosion [30].

Numerous studies have examined the impact of synthetic organic compounds, polymers, drugs, and plant extracts on the inhibition of metals in hydrochloric acid corrosion. This trait has prompted researchers to explore the potential use of drugs as corrosion inhibitors. D-penicillamine, a penicillin breakdown product, and L-cysteine, an amino acid frequently found in living organisms, were tested for their ability to suppress mild steel corrosion in a 1.0 M HCl solution using electrochemical techniques. These tests were conducted using a range of concentrations, immersion periods, and temperatures. Through a polarization analysis, the maximal %IE value of 5 mM L-cysteine in 1.0 M HCl solutions at 4 h immersion durations were calculated to be 91% [31]. Espinoza-Vázquez et al. [32] investigated the effect of fluconazole and its fragments as corrosion inhibitors of API 5L X52 steel in 1.0 M HCl solution, where it appeared to be an active chemical at its low concentrations. According to Samide et al. [33], the corrosion of carbon steel in 1.0 M HCl solution was inhibited by sulfacetamide and attained an efficiency of 84.7% at a concentration of 10 mM. There are four main chemical components in a magnolia grandiflora leaf: 3,7-dimethyl-2,6-octadien-1-ol (DTO), santamarine (STA), lanuginosine (LGS), and anonaine (ANI). In a study by Chen et al. [34], magnolia grandiflora leaf extract was made using the water extraction method, and the extract’s effectiveness at preventing Q235 steel corrosion in 1.0 M HCl was assessed using several techniques. The outcomes demonstrated that the extract acted as a mixed-type corrosion inhibitor, and that at 298 K and 500 mg/L, where LGS contributed the most to corrosion protection, its corrosion inhibition efficiency exceeded 85%. Wang et al. [35] also demonstrated the potential of Ficus tikoua leaf extract as a corrosion inhibitor for carbon steel in 1.0 M HCl solution. By using quantum chemical calculations, the authors determined that the 5-methoxypsoralen (5-MOP) components of the extract performed the most significant impact in the suppression of steel corrosion. This extract demonstrated outstanding protective effectiveness at 200 mg/L in the temperature range of 298 K to 318 K, with respective values of 95.8% (298 K), 95.7% (308 K), and 95.0% (318 K). A new organic epoxy resin, N, N, 1-tri(oxiran-2-ylmethoxy)-5-((oxiran-2-ylmethoxy)thio)-1H-1,2,4-triazol-3-amine, was synthesized by Damej et al. in recent studies [36,37] and was employed as a corrosion inhibitor for carbon steel C38 in 1.0 M HCl solution. Unpredictably, through stationary and transiency measurements, the inhibition efficiency of this new epoxy resin reached a value of 92% at a concentration of only 1 mM. Additionally, it was noted that the surface morphology of C38 steel in the presence of the inhibitor remained clean after immersion in 1.0 M HCl solution, demonstrating that this epoxy resin effectively protected the C38 steel against corrosion. Also, according to earlier investigations, thiophene derivatives effectively suppress corrosion [38,39,40,41,42]. Thiophene and its replaced derivatives are widely utilized in medicinal chemistry as anti-microbial, antioxidant, anti-corrosion, and anti-cancer compounds due to their safety [43,44].

This study examines the safe chemical (3-hydroxy-4-((4-nitrophenyl)diazenyl)-5-(phenylamino)thiophen-2-yl)(phenyl)methanone as an inhibitor to control the corrosion of carbon steel in aerated 1.0 M HCl solution. The choice of this molecule depends on its chemical composition, as it contains many effective functional groups and heteroatoms such as OH, CO, -N=N-, NO₂, S and N, which help enhance its adsorption strength on the alloy surface and ensure its safety. Researchers have examined the effects of temperature and concentration on the efficiency of inhibition and the pace of corrosion. Additionally, kinetic and thermodynamic characteristics have been calculated. To comprehend the inhibitory mechanism, DFT computations and MC simulations are used for theoretical foretelling.

2. Experimental Details

2.1. Preparation of the Testing Media and the Investigated Surface

The molecule used as a corrosion inhibitor in this work (Figure 1) was synthesized as described elsewhere [45]. The investigated molecule was immersed in 1.0 M HCl solution at its concentration range of 1 × 10⁻⁶ to 5 × 10⁻⁵ M. The inhibitor was then dissolved in 3 mL of ethanol before the acid was added. The tested corrosive medium was a 1.0 M HCl solution, which is made by diluting 37% hydrochloric acid in bi-distilled water. The solution was standardized using a 1 M Na₂CO₃ standard solution. This solution can be made without an inhibitor or with varied amounts of an inhibitor. The alloy used in this study was carbon steel, which has a conformation (wt%) of Si (0.002%), P (0.007%), Mn (0.910%), C (0.200%), and leftover iron. Following the 120-, 240-, 400-, 600-, 800-, and 1200-grit emery paper abrasion procedure, each coupon was washed with distilled water and acetone before being allowed to air dry in accordance with ASTM guidelines before the experiments began [46].

2.2. Electrochemical Analysis

The electrochemical studies were carried out using a Gamry device of the Potentiostat/Galvanostat/ZRA (REF 600) type (Gamry, Warminster, PA, USA). A circular carbon steel specimen with an exposed area of 1 cm² served as the working electrode for the measurements, while a saturated calomel electrode and a platinum rod (also with an approximate 1 cm² surface area) served as the reference and counter electrodes (SCE). In order to achieve a steady-state open-circuit potential (OCP) and ensure that the working electrode’s surface had acquired a steady-state potential, the carbon steel samples were submerged in a corrosive environment for 30 min prior to the measurement. The potentiodynamic measurements assess the corrosion current potential and inhibition effectiveness of carbon steel that has been exposed to acidic environments both in the presence and absence of an inhibitor within a range of −850 to −100 mV vs. open-circuit potential (OCP) and at scan rate of 0.1 mV s⁻¹. The polarization cell received a predetermined amount (100 mL) of the test fluid. The %η (percent inhibition efficiency calculated by corrosion current density values) can be calculated using Equation (1).

$$\%\eta =(I\_corr^(\left(0\right))-I\_corr^(\left(i\right)\left)\right)/(I\_corr^(\left(0\right)\left)\right)\times 100$$

(1)

In the absence and presence of an inhibitor, the corrosion current densities, individually, are $I\_corr^\left(\right(0\left)\right)$ and $I\_corr^\left(\right(i\left)\right)$.

2.3. Methodologies for Surface Characterization

2.3.1. Atomic Force Microscopy (AFM) Analysis

Atomic force microscopy (AFM) was used to determine the morphological properties of the carbon steel surface. In the absence of the inhibitor and in the case of the maximum concentration of this inhibitor (5 × 10⁻⁵ M), one molar of hydrochloric acid was used for the analysis. A silicon nitride probe (MLCT type; Bruker) was used, and the AFM analysis was carried out in contact mode. The Proscan software, version 1.8, was used to monitor the scanning settings, and picture processing was carried out using IP version 2.1. The carbon steel sheet was subjected to three different tests: one where the object was polished, another that involved immersing it in 1 molar hydrochloric acid, and a third in which it was submerged in a solution containing 1.0 M acid and 5 × 10⁻⁵ M of the compound. Next, the results of the three tests were contrasted.

2.3.2. Scanning Electron Microscopy (SEM)

In order to compare the surfaces of the carbon steel coupons with and without 5 × 10⁻⁵ M of the compound, the testers were allowed to interact with the tested solutions for one day. With the aid of a JEOL scanning electron microscope model T-200 (×500), an expansion of the morphology was investigated.

2.4. Molecular Dynamic and DFT Computations

Utilizing the Dmol3 and adsorption locator components of the Materials Studio V.7.0 program from Accelrys Inc., San Diego, CA, USA, DFT computations and molecular dynamic simulations were performed.

2.4.1. DFT Calculations

The inhibitor compound was improved using the GGA/BLYP foundation that was established in the aqueous phase [47]. The following equations were used to compute the ionization potential (I), electron affinity (A), electronegativity (χ), global hardness (η), and global softness (σ) [48].

I = −E_HOMO

(2)

A = −E_LUMO

(3)

$$\chi =(I+A)/2$$

(4)

$$\eta =(I-A)/2$$

(5)

σ = 1/η

(6)

The fraction of electrons that were adsorption-transferred from the inhibitor molecules to the metallic surface (ΔN) can be calculated using the equation below.

ΔN = (χ_Fe − χ_inh)/2(η_Fe + η_inh)

(7)

χ_Fe, χ_inh, η_Fe, and η_inh are the values for the electronegativity and hardness of the inhibitor molecules and Fe, respectively. χ_Fe is worth 7.00 eV, while η_Fe is worthless [49].

2.4.2. Simulation of Molecular Dynamics (MD)

The adsorption finder determines the potential adsorption configurations for the molecules in the (3-hydroxy-4-((4-nitrophenyl)diazenyl)-5-(phenylamino)thiophen-2-yl)(phenyl)methanone compound for the purpose of determining the effectiveness of the inhibition using molecular dynamic simulation on the Fe (1 1 0) surface [50]. In a simulated box (32.27 32.27 50.18) with periodic boundary conditions, the interactions between the (3-hydroxy-4-((4-nitrophenyl)diazenyl)-5-(phenylamino)thiophen-2-yl)(phenyl)methanone compound and the iron surface (110) were investigated. The optimization of the examined compound molecular energy was carried out using the traditional simulation engine Forcite [51]. Utilizing the layer creator, the corrosion system in the aqueous environment was developed, which is made up of molecules of the perfect inhibitor substance, water, and Fe (110) surface. The inhibitor’s ability to bind to the iron surface (110) was modeled using the COMPASS emulation research through force field [52].

3. Results and Analysis

3.1. Potentiodynamic Polarization (PDP) Investigation

Potentiodynamic polarization experiments were carried out in 1.0 M HCl solution at 25 °C in the absence and presence of various concentrations of the substance (5 × 10^–7–5 × 10^–5 M) shown in Figure 2. The I_corr (corrosion current density) and the corrosion potential (E_corr) of carbon steel in 1.0 M HCl solution without and with the inhibitor, as well as the inhibition efficiencies of the inhibitor (%η), are among the data derived from the PDP experiments that are included in

Table 1. PDP characteristics of carbon steel at different temperatures, in 1.0 M HCl solution, and with various inhibitor doses.

Temp. K	Inh. Conc. M	Ecorr mV(SCE)	Icorr μA/cm2	−βc mVdec−1	βa mVdec−1	θ	% IE	C.R (mm/Year)
25 °C	0	−441	9370	164 ± 4	161 ± 2	---	---	108.654
	5 × 10−7	−421	2445.76	158 ± 2	113 ± 2	0.739	73.9	28.361
	1 × 10−6	−404	2411.46	152 ± 2	163 ± 3	0.743	74.26	27.963
	5 × 10−6	−372	2171.59	137 ± 3	146 ± 3	0.768	76.82	25.182
	1 × 10−5	−395	1850.74	135 ± 1	133 ± 2	0.803	80.25	22.781
	5 × 10−5	−441	273.79	86 ± 0.7	97 ± 0.9	0.971	97.10	3.175
35 °C	0	−483	39,700	162 ± 3	164 ± 3	---	---	460.359
	5 × 10−7	−426	20,448.28	156 ± 3	155 ± 3	0.485	48.49	237.117
	1 × 10−6	−408	19,620.14	152 ± 4	159 ± 2	0.506	50.58	227.514
	5 × 10−6	−376	17,528.74	147 ± 2	162 ± 3	0.558	55.85	199.182
	1 × 10−5	−392	15,911.76	135 ± 3	153 ± 2	0.599	59.92	184.512
	5 × 10−5	−446	6000.32	123 ± 2	120 ± 2	0.849	84.89	12.779
45 °C	0	−475	47,900	174 ± 4	162 ± 3	---	---	555.446
	5 × 10−7	−450	25,185.34	163 ± 2	167 ± 2	0.474	47.42	292.048
	1 × 10−6	−430	23,836.96	144 ± 3	162 ± 3	0.502	50.24	276.412
	5 × 10−6	−413	22,127.88	136 ± 2	157 ± 4	0.538	53.80	267.182
	1 × 10−5	−380	19,914.90	125 ± 1	154 ± 2	0.584	58.42	230.932
	5 × 10−5	−398	8076.90	111 ± 3	134 ± 2	0.831	83.14	93.659
55 °C	0	−462	60,600	166 ± 2	145 ± 2	---	---	702.715
	5 × 10−7	−453	35,812.78	162 ± 3	123 ± 3	0.409	40.90	415.284
	1 × 10−6	−433	34,562.60	143 ± 3	163 ± 4	0.430	42.97	400.787
	5 × 10−6	−416	31,770.76	123 ± 2	144 ± 3	0.476	47.57	367.182
	1 × 10−5	−383	28,395.34	119 ± 2	163 ± 3	0.531	53.14	329.271
	5 × 10−5	−401	15,984.57	116 ± 2	152 ± 2	0.736	73.62	102.793

. The difference between the E_corr values of the utilized inhibitor and the blank solution was found to be less than 85 mV, demonstrating the mixed type of the inhibitors and lowering of the values of both the anodic and cathodic Tafel slope [53]. Figure 2 shows that the anodic metal dissolution and cathodic H₂ reduction processes were controlled by adding the inhibitor to 1.0 M hydrochloric acid solution. Additionally, the significance of this inhibition increased with higher inhibitor dosages. According to the calculations, when the cathodic curves produce almost parallel lines, the hydrogen discharge process becomes slower. The presence of the inhibitor in an aggressive medium controls the activation of this reaction [54]. The results for βa and βc presented in Table 1 demonstrate that the dissolving mechanism remained constant despite a modest variation in the reactions of cathodic hydrogen reduction and anodic alloy dissolution [55,56]. By increasing the compound’s concentration, as depicted in Figure 3, its adsorption increased, improving the polarization resistance on the surface of the carbon steel [57]. The repeatability of an experiment is established by conducting it three times.

3.2. Adsorption Studies

In order to protect the alloy surface from acidic attack, corrosion inhibitors adsorb protective layers onto it. This process can provide precise descriptions of the nature of the interaction between the molecules of the examined inhibitor and the steel surface [58]. For our new inhibitor, the experimental findings from the Tafel extrapolation method were fitted to various isotherms [59]. The data that the thermal model most closely resembles are given by Langmuir (Figure 4) and Henry’s (Figure 5) equations:

$$\complement /\theta =1/K\_(ads.)+\complement \left(\mathrm{Langmuir}\right)$$

(8)

$$\theta =K\_\left(ads\right)·\complement \left(\mathrm{Henry}\right)$$

(9)

The adsorption constant, surface coverage, and inhibitor concentration are all individually represented by the letters K_ads, θ, and C. The inhibitor’s adsorption constant (K_ads) was calculated using the slope and intercept of the straight lines (Henry and Langmuir isotherms, separately) that intersect the x-axis. The values are shown in Table 2. The standard free energy values (ΔG_ads) were derived using Equation (10), which illustrates how the values of K_ads affects ΔG_ads. In this formula, T (K) denotes temperature, R (J mol/K) denotes the universal gas constant, and water has a 55.5 molar content [60].

ΔG_ads = −RT ln (55.5 K_ads)

(10)

The enthalpy (ΔH_ads) value for the adsorption process was obtained by plotting log K_ads vs. 1/T, Equation (10), Figure 6. Following the application of Equation (11), employing the enthalpy and standard free energy values, the entropy values (ΔS_ads) were intended [61,62].

log K_ads = (−ΔH_ads/2.303 RT) + Constant

(11)

ΔG_ads = ΔH_ads − TΔS_ads

(12)

Table 2 gives the predicted thermodynamic characteristics of the adsorption process. This includes the enthalpy (ΔH_ads), adsorption equilibrium constant (K_ads), entropy (ΔS_ads), and Gibbs free energy (ΔG_ads). Based on the highest (Kads) value discovered at 25 °C, it is thought that the best inhibitor adsorption on the surface of carbon steel takes place at this temperature, creating the best barrier that protects the metal surface from the aggressive acidic medium [57]. The nature and spontaneity of the adsorption process can be identified. Both K_ads and ΔG_ads values affect these parameters, with negative ΔG_ads data showing the spontaneous nature of the compound adsorption process on the substrate’s surface. The physisorption (electrostatic interaction between the charged substrate surface and charged inhibitor molecules) is regarded to be present when the ΔG_ads data is less than −20 kJ mol⁻¹. In addition, a result higher than −40 kJ mol⁻¹ is considered to be a sign of chemisorption (charge allocation and/or electron transfer between the orbitals of the chemical molecules and those of the alloy to establish coordinate connection) [63,64]. Given this, the study’s findings regarding the values of ΔG_ads show that the compound molecules are adsorbed onto the substrate surface by both chemisorption and physisorption methods, with chemisorption dominating (mixed-type adsorption). The negative ΔH_ads value, which demonstrates that the chemical molecules are adsorbed on the alloy surface through an exothermic adsorption mechanism, provides an explanation for why the adsorption decreases at higher temperatures [65]. According to the declining values of ΔS_ads [66], the molecules of the inhibitor become more ordered as the adsorption process progresses.

Table 2. Data on adsorption taken from Figure 7 and Figure 8.

Isotherm	Temp. (K)	Log Kads	R2	−ΔG°ads (KJ/mol)	−ΔH0ads (KJ/mol)	ΔS°ads (J/mol)
Langmuir	298	4.77	0.998	37.16	33.85	11.12
	308	4.624	0.987	37.55		12.01
	318	4.585	0.990	38.54		14.74
	328	4.474	0.980	39.05		15.8
Henry	298	3.95	0.997	32.49	23.58	29.899
	308	3.94	0.993	33.52		32.27
	318	3.833	0.982	33.96		32.64
	328	3.764	0.991	34.59		33.56

Table 2. Data on adsorption taken from Figure 7 and Figure 8.

Isotherm	Temp. (K)	Log Kads	R2	−ΔG°ads (KJ/mol)	−ΔH0ads (KJ/mol)	ΔS°ads (J/mol)
Langmuir	298	4.77	0.998	37.16	33.85	11.12
	308	4.624	0.987	37.55		12.01
	318	4.585	0.990	38.54		14.74
	328	4.474	0.980	39.05		15.8
Henry	298	3.95	0.997	32.49	23.58	29.899
	308	3.94	0.993	33.52		32.27
	318	3.833	0.982	33.96		32.64
	328	3.764	0.991	34.59		33.56

3.3. Effect of Temperature

With the use of the potentiodynamic polarization method, comparing the degrees of alloy corrosion in 1.0 M hydrochloric acid and with the compound at various temperatures was possible. The temperature range was between 25 and 55 °C. Table 1 and Figure 7 and Figure 8 demonstrate how temperature affects the values for inhibitory efficiency and corrosion rate. Figure 9 shows the corrosion degree (log C.R) vs. time (1/T) for the alloy at 1.0 M HCl solution according to the presence or absence of the different quantities of the inhibitor. Given that all linear degradation coefficients are quite close to 1, the kinetic model may be able to adequately explain the carbon steel metal corrosion in 1.0 M HCl solution. This picture shows how the Arrhenius equation is applied to reach intersection A and how the slopes of the straight lines are sloped: $(-E\_a^\ast )/2.303R$ [37].

$$\complement .R=A〖exp〗^\left(\right(〖-E〗\_a^\ast )/2.303R)$$

(13)

The letters A, E*a, and C.R stand for a constant that depends on the kind of metal, electrolyte, and apparent activation energy. The association between log (C.R/T) vs. 1/T for carbon steel in 1.0 M hydrochloric acid solution and at different dosages of the substance is shown in Figure 10. The transition condition is shown to be associated with the slope (−ΔH/2.303R) and the intercept (log (R/Nh) + ΔS/2.303R) [67].

$$\complement .R=(RT/Nh)\mathrm{e}\mathrm{x}\mathrm{p}\left(\right(∆S^\ast )/R)\mathrm{e}\mathrm{x}\mathrm{p}\left(\right(-∆H^\ast )/RT)$$

(14)

The Planck constant is h, the entropy of activation is ΔS*, the enthalpy of activation is ΔH*, the Avogadro number is N, and the entropy of activation is ΔS*. In

Table 3. Effect of inhibitor concentration on the thermodynamic activation parameters of carbon steel corrosion in 1.0 M HCl solution.

Comp. conc. (M)	E*a KJ mol−1	−ΔH* KJ mol−1	−ΔS*J mol−1 k−1	log A
Blank	37.52	35.77	980.39	9.57
5 × 10−7	83.48	83.98	856.83	12.98
1 × 10−6	84.14	84.67	851.09	14.18
5 × 10−6	91.33	91.87	843.43	14.79
1 × 10−5	91.9	92.2	833.86	15.78
5 × 10−5	94.77	94.99	810.88	17.78

, the computed activation energy (E*a), activation enthalpy (ΔH*), and activation entropy (ΔS*) values are given. According to the data and our research, these additions raise the activation energy, activation entropy, and activation enthalpy of reactions, all of which inhibit corrosion development. The amount of inhibitor added to the acid solution boosts the active enthalpy (ΔH*), indicating that the energy barrier in the corrosion reaction is somewhat controlled by the inhibitor’s presence and is increased by doing so. The blank sample and the inhibitory solution’s negative and considerable entropy activation demonstrate that the activated molecule differentiates itself through the binding procedure as opposed to the dissociation stage. Compared to their initial state, the activated molecules had a strong organizational structure [68].

3.4. Surface Investigations

3.4.1. SEM Analysis

To evaluate the corrosive effects on the alloy surface both with and without the tested inhibitor (5 × 10⁻⁵ M), before and after being immersed in the acidic solution for 24 h at 298 K, the test specimens were captured in images using a scanning electron microscope (SEM). Figure 11a illustrates how the carbon steel sample appears smooth before immersion and how some surface scratches indicate the metal’s smooth surface. This demonstrates that no corrosion-related byproducts or inhibitor complexes developed on the carbon steel surface. After being exposed to the corrosive fluid, the sample was photographed to reveal the extent to which corrosion had damaged the unconstrained specimen’s surface. Additionally, the morphology of the corrosion products is quite irregular and cube-shaped. The fact that the damage was greatly lessened after the inhibitor was introduced to the mixture demonstrates how effectively this chemical inhibits corrosion and shields the alloy surface from corrosive agents (Figure 11). When exposed to the aggressive solution containing the inhibitor, the carbon steel surface behaved differently than in the absence of inhibitor, appearing flat and tightly packed rather than uneven and potholed. This suggests that an adsorption layer formed on the surface, shielding the metal from aggressive corrosion.

3.4.2. Analysis with Atomic Force Microscopy (AFM)

Atomic force microscopy (AFM) has been found to be one of the most precise techniques in this inquiry for verifying previously obtained results. This scan is supplemented through a continued resolution that is 1000 times the optical diffraction limit for organizing nanoscale fractions [69].

Table 4. Data on roughness for carbon steel after one day of immersion in a 1 M HCl solution and with a 1 × 10⁻⁵ M compound.

Models	Sq(nm)	Sa(nm)	Maximum Peak- to-Valley Height (nm)
Alloy free	24.791	21.54	133.05
Substrate in 1 M HCl	592.05	486.76	3842.0
Substrate + 1MHCl + 5 × 10−5 M inhibitor	213.2	187.40	3652.8

displays the findings of this survey. Sq stands for root-mean-square roughness, and it represents the height deviations’ average measured both inside and outside of the assessment length and measured from the mean line. The average roughness and the maximum single peak-to-valley height in the neighboring sampling heights, respectively, are signified by the symbols P-V and S_a. Sa also shows the average deviation of the roughness profiles of all of the spots across the evaluation time from a mean line. Figure 12 displays three examples of an alloy. The first model is a pure alloy piece, the second one is an alloy that has spent 24 h in a 1.0 M HCl solution, and the third one is a piece of carbon steel that has spent 24 h in a 1.0 M HCl solution in which the inhibitor is present at the highest concentration possible (5 × 10⁻⁵ M). The small imperfection on the polished alloy surface is the result of atmospheric corrosion. The material that was immersed in 1 molar of hydrochloric acid is missing from Figure 12, exposing the corroded alloy surface. The Sq, Sa, and P-V alloy surfaces have heights of 592.05 nm, 486.76 nm, and 3842.0 nm, respectively. These findings demonstrate that the refined alloy’s surface is smoother than the surface of the alloy that was dipped in 1.0 M HCl. The average surface roughness of the carbon steel decreased while immersed in 1 M HCl, going from 486.76 to 187.40 nm, when 5 × 10⁻⁵ M of the inhibitor was present. Because the inhibitor molecules adsorbed onto the substrate surface and reduced the interaction between the alloy and hydrochloric acid, the alloy surface was prevented from uniformly corroding because the highest concentration smoothed the surface (Figure 12 and Table 4). This outcome was consistent with the value of ΔS*, which demonstrated that species ordering increased in the presence of the compound. Similar findings were seen in other research as well [67,68].

3.5. Theoretical Investigations

3.5.1. Calculations of DFT

In order to understand the nature of the interaction between the inhibitor molecules’ adsorption centers and the alloy surface, DFT analyses were conducted. The ideal inhibitor molecules’ neutral and protonated structures are shown in Figure 13 along with their HOMO and LUMO distributions, and their quantum chemical characteristics are listed in

Table 5. Inhibitor structure in the aqueous phase and its quantum chemical parameters.

Liquid	Neutral	Protonated
−EHOMO (eV)	−5.088	−4.047
−ELUMO (eV)	−3.631	−3.864
ΔE (eV)	1.457	0.183
η (eV)	0.729	0.092
σ (eV)−1	1.37	10.87
Pi (eV)	−4.36	−3.96
$x$ (eV)	4.36	3.96
dipole moment (debyes)	15.6051	30.6926
Molecular area (Å2)	432.11	438.855
∆Nmax (e)	−0.054	1.74

. The frontier orbital theory states that the HOMO and LUMO energies govern the interaction between the inhibitor molecule and the metal [65]. Chemistry uses the terms E_HOMO and E_LUMO to refer to a molecule’s ability to give and obtain electrons separately [70]. Smaller E_LUMO values indicate a compound’s capacity to receive electrons, whereas larger E_HOMO values indicate a compound’s capacity to donate electrons to the open d-orbital on the surface of carbon steel [71]. It was also suggested that an inhibitor molecule with a higher efficacy in preventing corrosion would have a smaller energy hole among the LUMO and HOMO energies (ΔE). Due to the fact that removing an electron from its outer shell orbital only necessitates a small amount of ionization energy, an electron from the E_HOMO state is provided to the E_LUMO state. The E_HOMO and E_LUMO values in Table 5 show that the inhibitor can provide or take electrons, which is significant. The proton form of the compound has a smaller energy gap (ΔE) than the neutral form, indicating that the inhibitor reacts with the steel surface more forcefully [72]. Small energy gap molecules are more flexible than large energy gap ones. Because the electrons travel to the acceptor so quickly, the molecule is more reactive than harder materials. A substance’s composition, stability, and reactivity can all be determined using the absolute hardness (η) and softness (σ) values. The only part of the inhibitor molecule that can adsorb is the one with the greatest value for simple electron transfer [73]. The inhibitor molecules are converted to Lewis bases in the corrosion system, while alloy molecules are converted to Lewis acids. For avoiding the soft acid corrosion of bulk metals, soft-base inhibitors are the most effective. Information about structural organization is disseminated and rationalized using the dipole moment, the third important property [74,75]. Another indicator that is frequently used to forecast the direction of corrosion inhibition is the dipole moment (μ). It measures the polarity of the connection and reveals the number of electrons scattered within it. Strong dipole–dipole interactions between the inhibitor and iron surface result in better inhibition and long-lasting adsorption on the surface of carbon steel [76]. An organic dipole’s electrostatic attraction to a charged alloy surface speeds up the actual adsorption of the organic retarder onto the metal surface. Therefore, large dipole moments are favorable for adsorption molecules. The dipole torque in protons altered dramatically, as seen by analyzing the dipole moments, proving that the inhibitor is actually physically adsorbed on the steel surface in the form of a proton can molecule [77]. However, the research does not provide any indication of a connection between inhibitory efficacy [78]. It is asserted that the electronic inhibitor on the metal surface will be more effective at suppressing the reaction if ΔN is lower than 3.6 [79]. An electron’s ability to bind inhibitors (χ) is referred to as its electronegativity. As the attraction of an electron to a carbon steel surface grows stronger, the value of inh increases (the protonated version has a high value for electronegativity). Since the inhibitory particles are in stronger contact with the more electroactive carbon steel surface, there is higher inhibition, according to previous studies [79,80,81,82]. According to Table 5, the examined compound is a suitable inhibitor. By increasing the region where chemical molecules are in contact with the surface of carbon steel, the inhibition is more effective.

3.5.2. Molecule Electrostatic Potential (MESP)

The effective 3D visual method known as MESP mappings can be used to differentiate between the total charge dispersion and the net electrostatic impact created over a molecule [83]. It is possible to discern the maximum electron density in Figure 14’s red hues, with MEP standing for the most significant negative (nucleophilic reaction). On the other side, the blue colours represent the electrophilic reaction [84]. Heteroatoms and double conjugate bonds are particularly prevalent in regions with high electron densities. Unfavorable positions are indicated by sulfur, nitrogen, and oxygen groups, which facilitate electrophilic attacks. The positive indications are the blue-colored hydrogen atoms, which are more vulnerable to nucleophilic assaults.

3.5.3. Fukui Indices and Mulliken Charges

The locations of the donor and acceptor of the molecule’s active centers can be determined using Mulliken atomic charges and Fukui indices. The results showed that the inhibitor neutral and protonated particles were distributed in various electrophilic and nucleophilic sites. The outcomes show that these species are more reactive to the carbon steel atoms due to the active sites (donor–acceptor) in the inhibitor. There are both negative and positive numbers in the Mulliken distribution. Due to this finding, all of the proton molecule locations significantly shifted in the discovery’s favor, showing that neutral molecules are flexible and that there was an increase in the amount of chemical interaction through the proton inhibitor. The inhibitor’s total negative charge (TNC) increased as it protonated (Figure 15), demonstrating similar behavior [85]. This demonstrates the increased frequency of structural interactions between protonated particles. The inhibitor has the ability to protonate at these sites in conditions with high acid (HCl) concentrations, creating molecules that are positively charged. This chemical contains a minimum of one heteroatom. As a result, they engage in interactions with the anions (Cl⁻), which are widely dispersed on the alloy’s surface. A concept will be used to investigate how protonation affects the nearby inhibitory substance centers. The atoms of the examined inhibitor are shown in Figure 16 with added markers (Fukui’s index (FI)) that have higher values of f_k⁺ and f_k⁻ in both protonated and non-protonated forms. This image’s comprehensive analysis demonstrates that for assault systems that employ both electrophilic and nucleophilic ions, every molecule has a unique atom composition. The highest Fukui function (f_k⁺) LUMO-connected atom in the compound’s molecules, which improves reactivity to the donor reagent, is the most sought-after target for nucleophilic assault. The atom in the compound with the maximum value of the Fukui function (f_k⁻) is chosen for electrophilic attack because it is related to the HOMO and provides the reactivity towards an acceptor reagent. After protonation, the f_k⁺ values of some atoms increased because they could now take electrons; however, the data for their f_k⁻ values fell because their donor characteristics were lost as their centers were blocked by H⁺ protons (Figure 16). The changed atoms within a same compound with changing f values further supports the hypothesis that the same compound molecules can take and give electrons in two different forms (the neutral state and protonated form).

3.5.4. Calculations Based on Molecular Dynamics

Through applying MD simulations and maintaining the adsorption locator module in situ, one may theoretically explain how the examined inhibitor chemicals interact with the alloy surface. Because of this, Figure 17 depicts the inhibitor molecules’ most effective adsorption configurations on the alloy surface, which are positioned in a flat or nearly parallel arrangement, exhibiting an expansion of the spectrum of adsorption and the extreme surface coverage [86,87,88,89]. In addition,

Table 6. Information and specifications obtained from a molecular dynamic simulation of the compound (neutral–protonated–vacuum) adsorption on alloy surface.

Factor	Neutral	Protonated	Vacuum
Total energy (kcal/mol)	−4934.314	−4939.977	−196.544
Adsorption energy (kcal/mol)	−4964.036	−4975.76	−226.242
Rigid adsorption energy (kcal/mol)	−5143.159	−5154.99	−233.159
Deformation energy (kcal/mol)	179.122	179.227	6.917
dEad/dNi (kcal/mol)	−234.58	−186.48	−226.242

includes the data of the molecular dynamic simulation; this contains the adsorption energy for restful adsorbate molecules when the process is neutral, protonated, or vacuum, the rigid adsorption energy for unrelaxed adsorbate molecules, and the deformation energy for relaxed adsorbate molecules [80]. Contrary to what was shown in earlier studies, Table 6 shows that the inhibitor has a higher adsorption energy for both neutral and protonated forms [38]. According to this study, the compound strongly adheres to the surface of the alloy, generating immobile adsorbed coats that protect the alloy from corrosion. Table 6’s results show that this inhibitor has a higher inhibitory efficiency than other substances that have been studied in the past [38], accounting for the higher adsorption energies of its relaxed and unrelaxed forms. This is true both before and after the geometry optimization process. dE_ads/dNi (kcal mol⁻¹) measures the amount of energy that is created when one of the adsorbate molecules splits from the formation of the adsorbent substrate [90,91,92,93]. The fact that water has low dEads/dNi values—around 11.67 kcal mol⁻¹ in a neutral system and 8.63 kcal mol⁻¹ in a protonated arrangement—indicates that the inhibitor’s molecules can take the place of water molecules. Research in theory and practice demonstrates that the inhibitor molecules are subsequently forcedly adsorbed on the carbon steel surface and produce a strong adsorbed protective coat, creating a corrosion barrier for the alloy surface in challenging circumstances. As the adsorption energy in an aqueous solution rises over the vacuum value, the particle’s efficacy is increased on the alloy surface (Table 6).

4. Conclusions

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The investigated substance was found to prevent carbon steel from corroding in 1.0 M HCl medium and adheres to the Langmuir and Henry isotherms for adsorption, where both isotherms denote both physical and chemical adsorption, but chemical adsorption is the predominant type.

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Electrochemical experiments revealed that while the inhibitory efficacy diminishes as the temperature rises, it increases when the concentration of the examined substance increases. This feature facilitates the molecules’ binding on the steel surface.

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Potentiodynamic polarization measurements show that the employed inhibitor operates as a mixed-type inhibitor.

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SEM and AFM studies substantially validate the electrochemical and theoretical findings, and they also verify the inhibitor’s adsorption of the inhibitor molecules on alloy surfaces.

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The investigation’s results and theoretical analysis provided evidence in favor of the claim that the compound is a potent corrosion inhibitor.

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According to a Fukui function analysis, the main reactive sites for corrosion inhibitors are phenyl carbon atoms, oxygen, and nitrogen.