BMPD-Assisted Enhancement of Corrosion Resistance of Carbon Steel: Experimental and First-Principle DFTB Insights

Abstract

Green corrosion inhibitors are gaining recognition for their sustainable, cost-effective, and environmentally friendly nature, along with their impressive water solubility and high corrosion inhibition efficiency. They offer a promising solution to combat corrosion issues that plague various industries. However, to harness the full potential of these eco-friendly corrosion inhibitors, a profound understanding of their development and underlying mechanisms is essential. This knowledge is the key to paving the way for the next generation of corrosion protection materials. Herein, a comprehensive study was conducted to understand the adsorption, corrosion inhibition efficiency, and stability of 3-benzoyl-4-hydroxy-2,6-bis(4-methoxyphenyl)-4-phenylcyclohexane-1,1-dicarbonitrile (BMPD). This study investigated the performance of BMPD applied to carbon steel (CS) in 1 M hydrochloric acid (HCl) solution. The corrosion inhibition effect was examined using weight loss, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and theoretical studies. The surface morphology was also characterized and Tafel polarization analysis shows that BMPD is a mixed inhibitor. The results obtained by electrochemical impedance spectroscopy indicate that the inhibitory effect increases with increasing inhibitor concentration. The adsorption of BMPD on a CS surface obeyed the Langmuir adsorption isotherm. Thermodynamic parameters were calculated and discussed. Furthermore, this study involved a comprehensive computational analysis of the BMPD compound. Using quantum chemical calculations and first-principle simulations, we delved into the structural and electronic properties of BMPD as well as the interfacial adsorption mechanisms between the studied molecule and the iron surface.

1. Introduction

Corrosion is a naturally occurring phenomenon that affects our lives and causes degradation to household products, airplanes, automobiles, public roads, bridges, and distribution systems [1]. Carbon steel (CS), a foundational material deeply ingrained in various industries, notably within oil and gas refining, has played a pivotal role since its advent in the eighteenth century. Recognized for its wide availability, cost-effectiveness, and robust mechanical properties, CS serves as a linchpin for infrastructure and equipment across diverse applications, from structural components to critical pipelines, owing to its blend of strength and affordability. However, its susceptibility to corrosion, particularly in demanding environments, presents a noteworthy challenge. This juxtaposition of exceptional properties and inherent vulnerability accentuates the ongoing necessity for cutting-edge research and advancements in corrosion protection strategies, ensuring the enduring reliability of CS within the intricate fabric of industrial processes [2,3,4,5]. Acidic media such as hydrochloric acid are used in many fields such as pickling of metals, cleaning of boilers, acidizing of oil wells, and acid descaling [6,7,8,9,10]. In addition, HCl is one of the acids that are widely used in improving the productivity of wells in oil and gas production. Since hydrochloric acid solutions are highly corrosive, their access to CS surfaces results in corrosion problems. In the ongoing efforts against corrosion phenomena, diverse preventive methods have emerged as vital safeguards for infrastructure and assets. These methods encompass cathodic protection, corrosion-resistant alloys, protective coatings, optimized concrete chemistry, and corrosion inhibitors. Cathodic protection manipulates the electrical potential of metals to thwart electrochemical corrosion, while corrosion-resistant alloys provide intrinsic protection against corrosive environments. Protective coatings create a physical barrier between metal surfaces and corrosive elements, optimizing concrete chemistry and enhancing resistance in concrete-based structures. However, one of the most significant practical methods used is utilizing corrosion inhibitors in the industrial field as well as in academic studies [11,12,13,14,15,16,17,18].

In order to identify compounds that have the potential to serve as corrosion inhibitors, one crucial structural feature that stands out is the presence of Lewis basic sites. These sites can be found in both inorganic and organic compounds, and each has its own distinct set of attributes. In the past, inorganic corrosion inhibitors were favored for their high oxidative power, which made them effective in battling corrosion [8]. However, these inorganic compounds come with a significant drawback because they are highly toxic to both human health and the environment. This toxicity raises considerable concerns and limits their practicality in many applications. In contrast, organic corrosion inhibitors have been gaining prominence for several compelling reasons. First, they are more cost-effective, making them an attractive option for various industries. Moreover, these organic compounds are known for their reduced toxicity, which is a significant advantage when it comes to safeguarding human health and the environment [18]. Their environmentally friendly nature has made them increasingly appealing, as we strive to adopt more sustainable and responsible practices in our material protection strategies.

Organic inhibitors containing heteroatoms (sulfur, phosphorous, nitrogen, and oxygen) and π-bonds in their molecules are extensively used in the protection of metals and alloys [19,20,21,22,23,24,25,26,27] and are considered effective corrosion inhibitors in a variety of acidic solutions, both practically and theoretically [28,29,30,31,32,33]. Although many synthetic compounds showed good anticorrosive properties, most of them are highly toxic to both human beings and the environment [34]. To enhance the effectiveness and reduce the toxicity of corrosion inhibitors, a strategic approach involves the design and synthesis of green organic compounds. These compounds often serve as mixed inhibitors, acting on both the anodic and cathodic corrosion reactions, and they have the ability to create a protective hydrophobic film on the metal surface. This protective film forms due to the presence of heteroatoms and functional groups, facilitating the adsorption process. Organic compounds can act as physical barriers, particularly against corrosive agents like chlorides, or provide electrostatic repulsion to compounds with negatively charged chain ends [32]. Additionally, they enable other inhibition mechanisms, such as the chelating mechanism. A wide range of organic materials, including organic core–shell structures, polyacrylamide, and dimethyl-ethanol-amine, have found applications as corrosion inhibitors for steel rebar, offering a versatile and eco-friendly solution to combat the ever-persistent issue of corrosion. Building on the experience of our prior investigations into green corrosion inhibitors and recognizing the pressing demand for highly effective and environmentally friendly corrosion prevention solutions, we are delighted to introduce a novel compound named 3-benzoyl-4-hydroxy-2,6-bis(4-methoxyphenyl)-4-phenylcyclohexane-1,1-dicarbonitrile. Figure 1 represents the chemical structure of the BMPD corrosion inhibitor. In this study, a deep study was undertaken to unravel the effect of this newly synthesized molecule as a corrosion inhibitor for CS in 1 M HCl solution. The corrosion inhibition effect was examined through a range of techniques, including weight loss measurements, PDP, EIS, and SEM/EDS analyses. Employing quantum chemical calculations and first-principle simulations uncovered the compound’s structural and electronic properties, as well as the interfacial adsorption mechanisms when it comes into contact with the iron surface, in a bid to understand the bonding mechanisms of BMPD-Fe(110) interactions.

2. Experimental

2.1. Corrosive Electrolyte and Materials

In this investigation, a corrosive solution consisting of 1 M HCl was employed. The material under study is carbon steel (CS) characterized by a chemical composition of 0.38 wt % C, 0.23 wt % Si, 0.68 wt % Mn, 0.16 wt % Cu, 0.016 wt % S, and 0.09 wt % Co, with the remaining balance being Fe. For weight loss (WL) measurements, CS coupons were prepared with dimensions of 4 × 1 × 0.3 cm³. Prior to conducting the experiments, pre-treatment of the CS samples was carried out through grinding using emery paper with SiC abrasives at different grades, namely, 400, 800, 1200, and 2000. The study involved a range of concentrations for the BMPD, spanning from 10⁻⁶ M to 10⁻³ M. These concentrations were utilized to evaluate the corrosion inhibition properties of BMPD on CS in the presence of the 1 M HCl solution. Weight loss measurements of the CS sheets provided insights into the corrosion behavior under varying BMPD concentrations.

2.2. Weight Loss Method

Conforming to the approved method “ASTM G 31-72” [35], WL measurements were carried out. CS sheets were diligently prepared through mechanical polishing, cleaning, drying, and precise weighing before immersion into the corrosive medium. The specimens were kept in 50 mL of 1 M HCl, both with and without BMPD inhibitor at 298 K for 24 h of immersion. Thereafter, the samples were removed from the solution, rinsed with distilled water, degreased in acetone, dried, and then reweighed. Triplicate experiments were performed in each case and the mean value of WL was calculated. The obtained data in the absence and presence of BMPD were used to calculate the corrosion rate (W_corr) and inhibition efficiency (E_WL) using the following equations:

$$W_{corr}=\frac{\mathsf{\Delta }m}{St}$$

(1)

where $\mathsf{\Delta }m$ is the typical mass loss expressed in g, S represents the exposed area in cm², and t is the required immersion time of the specimens inside the corrosive solution (h).

$$E_{WL}\left(\%\right)= \frac{{W_{corr}-W}_{corr}^{\prime }}{W_{corr}}\times 100$$

(2)

where W_corr and W′_corr, respectively, represent the corrosion rates of the CS after immersion in the absence and presence of the inhibitor.

2.3. Electrochemical Measurements

Electrochemical measurements have emerged as powerful techniques in studying and evaluating the corrosion behavior of CS. All electrochemical tests were conducted in the corrosion cell. The cell consisted of three electrodes, working (CS sample) with an exposure area of 1 cm², a reference electrode (saturated calomel electrode), and a counter electrode (platinum). The measurements were performed using Wuhan Corrtest Instruments Corp., Ltd., (Wuhan, China), controlled with a computer, and the electrochemical data were acquired using an electrochemical analyzer software (CS Studio6). Before each test, the working electrode was dipped for 30 min in the solution to allow the open circuit potential (OCP) to achieve a steady state. For the potentiodynamic polarization study, the potential sweep ranges from −800 to −200 mV versus SCE with a scan rate of 1 mV s⁻¹ at a temperature of 298 K. The EIS tests were performed at OCP with a frequency range between 100 KHz and 10 mHz at 10 points per decade based on a 10 mV perturbation. Electrochemical parameters such as corrosion current densities (I_corr), corrosion potential (E_corr), and Tafel slopes (β_a, β_c) can be derived by extrapolation of the linear Tafel segments of the polarization curves.

2.4. Surface Morphology

A scanning electron microscope (SEM, JEOL-Model JSM-IT 100, Peabody, MA, USA) coupled with energy-dispersive X-ray analysis (EDX) was used to monitor surface morphological changes. The analysis by SEM/EDX was carried out on the surface of CS samples before and after immersion for 24 h in 1 M HCl acid solutions with and without the optimum concentration of 10⁻³ of BMPD. The specimens were cleaned with distilled water and used for the analysis.

2.5. Density Functional Theory Calculations

The elucidation of the intricate molecular mechanisms underpinning corrosion inhibition can be readily achieved through an exploration of the quantum chemical (QC) properties and the interfacial dynamics at the BMPD–metal interface. This approach offers a valuable tool for gaining deeper insights into the adsorption processes and the consequential electronic properties. Leveraging the cutting-edge techniques of theoretical chemistry, we employ a spectrum of theoretical frameworks, including density functional theory (DFT) and density functional based tight-binding (DFTB). These advanced methods allow us to not only dissect the structural attributes of the BMPD molecule but also correlate them with its adsorption behavior, with the aim of having a profound understanding of the molecular intricacies at play in the field of corrosion inhibition. In this work, we employed Gaussian 16 W software (Rev: C.01) is a powerful tool to model, generate optimized configurations, and delve into the electronic stability of the studied molecule. This exploration was conducted within the context of the aqueous phase, as defined by the SMD model, a crucial consideration when studying the behavior of molecules in aqueous environments [30]. To facilitate the calculations, the functional B3LYP/6-311G++(d,p) was utilized, providing a robust foundation for our computational analyses. Ground state computations were integrated into the DFT parameters, ensuring a comprehensive examination of the BMPD’s characteristics and behavior. The detailed information concerning the computational techniques employed in this research paper draws upon the methodologies established in our prior research works [21,22,23].

3. Results and Discussion

3.1. Gravimetric Experiments

The corrosion rates of the CS and the inhibition efficiency obtained from the mass loss measurements in 1 M HCl containing various inhibitor concentrations for 24 h of immersion at 303 K are exhibited in Figure 2. As can be seen from this Figure, the percentage inhibition performance improved with increasing inhibitor concentration and reached a maximum value of 95.73% at 10⁻³ M. Moreover, the corrosion rate is decreased by increasing the concentration of BMPD in the blank solution. This is an obvious indicator that BMPD molecules have the ability to control or reduce the dissolution of CS in the corrosive solution, leading to a full blockage of the active sites on the surface exposed to the electrolyte [36,37,38].

3.2. Electrochemical Behavior by BMPD Inhibitor

3.2.1. EIS Behavior

EIS studies provide information on the mechanism of charge transfer, diffusion, and the kinetics of electrode operations as well as the surface properties of the systems being studied. EIS responses offer a specific understanding of the charge transfer processes and their rates in both uninhibited and inhibited solutions, allowing for meaningful comparisons between the two conditions. Nyquist diagrams of the CS in the 1 M HCl solution at 298 K and 30 min of immersion in the existence and absence of different concentrations of BMPD are presented in Figure 3. It is noted that all the Nyquist curves obtained show similar shapes, including one capacitive loop, and their diameters are affected by the change in the inhibitor concentration [39]. The radius of the capacitive loop increases as the concentration of BMPD increases, which explains how the co-existence of organic substances may facilitate the development of a protective layer on the electrode surface. The behavior of the impedance can be illustrated by adapting an electrochemical equivalent circuit (Figure 4), including a parallel combination of the charge-transfer resistance (R_ct) and constant phase element (CPE), both in series with the solution resistance (R_S). The inclusion of the CPE in place of a pure capacitive element was essential to account for the heterogeneity of the CS electrode. When considering the practical corrosion process in a real-world environment, we found the contribution of capacitance to be negligible, given the absence of an external source of electrical current in typical corrosion scenarios. As a result, our emphasis on characterizing the corrosion properties of the protective layer was solely placed on the resistance parameter. Thus, the adoption of this proposed circuit model proved more suitable for decoupling the various electrochemical processes occurring at the electrode interface, taking into account the physical and chemical states of the system. The electrochemical parameters are listed in

Table 1. The EIS parameters of CS in 1 M HCl in the absence and in the presence of various concentrations of BMPD at 298 K.

Inhibitor	C (mol/L)	Rct (Ω·cm2)	n	Q × 10 ̶ 4 (sn·Ω−1·cm−2)	Cdl (μF·cm−2)	ERct (%)
Blank	-	18	0.79	2.09	47	-
BMPD	10 ̶ 3	415	0.60	1.23	16	92.92
	10 ̶ 4	215	0.61	1.55	17	89.30
	10 ̶ 5	127	0.68	1.78	29	85.82
	10 ̶ 6	65	0.75	1.85	42	81.53

. Based on the following equation, the double-layer electrical capacity (C_dl) for each inhibitor concentration is calculated [40]:

$$C_{dl}=\sqrt[n]{Q\times R_{ct}^{1-n}}$$

(3)

where n is the phase shift, which allows calculating the surface heterogeneity of the CS, and Q is the CPE constant [41,42].

The inhibition efficiency was evaluated using the following expression:

$$E_{R_{ct}}\left(\%\right)=\left(\frac{{R_{ct}-R}_{ct}^0}{R_{ct}}\right)\times 100$$

(4)

where $R_{ct}^0$ and $R_{ct}$ signify the present the charge transfer resistance without and with the BMPD inhibitor.

According to Table 1, the R_ct value increased as the concentration of corrosion inhibitor increased, indicating that the adsorption of inhibitor molecules on the surface of the CS increases, which means that the exposed surface area of the CS decreases. The decline in the C_dl value could be explained by a lower local dielectric constant and/or an increase in the thickness of the electrical double layer on the CS substrate in the presence of BMPD [43,44].

3.2.2. Polarization Behavior

The polarization curves without and with different concentrations of BMPD in the 1 M HCl solution at 298 K are shown in Figure 5. The electrochemical parameters are reported in

Table 2. Polarization data and the corresponding inhibition efficiencies of CS in HCl solution with and without various concentrations of BMPD at 298 K.

Inhibitor	C (mol/L)	−Ecorr (mV/SCE)	−βc (mV·dec ̶ 1)	βa (mV·dec−1)	Icorr (μA·cm−2)	EI (%)
Blank	-	463	168	129	636	-
BMPD	10−3	505	149	102	37	94.18
	10−4	519	144	109	68	89.30
	10−5	520	142	106	112	82.38
	10−6	472	150	103	184	71.06

. As seen from Figure 5, the presence of BMPD considerably lowered the density of corrosion currents. A decrease in the cathodic current densities can be explained by the reduction in the exposed surface area caused by the adsorption of inhibitor molecules, which leads to a reduction in hydrogen evolution reaction, while the decrease in anodic current density is caused by the reduced dissolution reaction of CS. It is essential to highlight that at a potential value above ≈ −0.32 V, a significant alteration is observed in the anodic branch of the Tafel curves. This phenomenon, commonly recognized as the desorption potential, manifests as a notable rise in anodic current. The observed effect is predominantly ascribed to a desorption potential that brings about substantial modifications in the inhibiting film. In fact, the polarization of the electrode is influenced by various factors. With the addition of a corrosion inhibitor to the solution, the corrosion inhibitor undergoes adsorption onto the surface of the CS surface, resulting in the formation of a protective film. This adsorption process, in turn, modifies the properties of the interface between the solution and the CS. The alterations introduced by these modifications impact the polarization of the electrode by influencing both the electrode reaction and the diffusion within the medium. Consequently, the addition of the corrosion inhibitor leads to changes in the Tafel coefficients. Table 2 shows the electrochemical polarization results from the BMPD inhibitor, including the potential of corrosion (E_corr), corrosion current density (I_corr), anodic and cathodic Tafel slopes (β_a and β_c), and inhibition efficiency (E_I %). The E_I is calculated using the following equation [45]:

$$E_I\left(\%\right)=\left(1-\frac{I_{corr}^i}{I_{corr}}\right)\times 100$$

(5)

where $I_{corr}$ and $I_{corr}^i$ are the corrosion current densities for the CS electrode in the uninhibited and inhibited solutions, respectively.

Furthermore, it can be seen from Table 2 that I_corr decreases with increasing inhibitor concentrations and the inhibition efficiency values reach 94.18% at 10⁻³ of inhibitor. In addition, if the shift in E_corr is greater than 85 mV compared to E_corr in an uninhibited solution, the inhibitor can be identified as either cathodic or anodic in nature; otherwise, it can be regarded as a mixed inhibitor [46,47,48,49]. In this study, the displacement in E_corr is less than 85 mV compared to the blank solution, indicating that BMPD acts as a mixed-type inhibitor.

3.3. Adsorption Isotherm

The adsorption isotherm can be used to describe the interactions between the inhibitor molecules and the CS surface. Different adsorption isotherms, such as Langmuir, Frumkin, Freundlich, and Temkin, were studied in the adsorption mode. It was determined that the adsorption performance of BMPD molecules on CS obeys the Langmuir adsorption principle [50,51,52,53].

According to the Langmuir isotherm, Ɵ is related to the inhibitor concentration C_inh by the following Equation (6):

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

(6)

where C_inh signifies the inhibitor concentration, K_ads means the constant of adsorption equilibrium, and θ denotes the coverage area. The plot of the Langmuir adsorption isotherm of BMPD using the data from the gravimetric studies is shown in Figure 6, which presents a linear regression with a slope approximately equal to 1 and a correlation coefficient value near unity (R² > 0.999). The values of the adsorption equilibrium constant K_ads could be determined from the intercepts of lines on the C_inh/θ axis, and K_ads was related to the standard free adsorption energies $\Delta G_{ads}^0$ by the following equality [54,55]:

$$K_{ads}=\frac{1}{55.5} exp\left(\frac{{-∆G}_{ads}^0}{RT}\right)$$

(7)

where the value 55.5 represents the molar concentration of water, T is the absolute temperature, and R is the universal gas constant.

According to

Table 3. Langmuir parameters of BMPD for CS in 1 M HCl obtained from the fitted line of Figure 6.

Inhibitor	Slope	Kads (L·mol−1)	R2	${∆\mathit{G}}_{\mathit{a}\mathit{d}\mathit{s}}^0$ (kJ/mol)
BMPD	1.04	49.441	0.999	−42.41

, the negative value of ${∆G}_{ads}^0$ suggests that the BMPD adsorption progression on the CS surface is spontaneous [56,57]. As we all know, if the value of ${∆G}_{ads}^0$ is equal to or less than −40 kJ/mol, it shows the chemical adsorption through the interaction between lone pair electrons in inhibitor molecules and the metal surface, while the value is near −20 kJ·mol⁻¹ or more positive, suggests the physical adsorption. [58,59]. In this work, the quantity of free energy of adsorption is equal to −42.41 kJ·mol⁻¹, indicating that the inhibitor molecules are adsorbed on the CS surface by strong chemical bonds. The Gibbs free energy of adsorption is equal to –42.41 kJ/mol and the negative value of ${∆G}_{ads}^0$ indicates that the BMPD inhibitor is spontaneously adsorbed on the CS surface in the form of a neutral molecule by chemisorption [52].

3.4. Influence of the Temperature on Inhibition Performances

Temperature has a considerable effect on the corrosion rate and inhibiting efficacy, particularly in an acidic medium. The study of the temperature effect on the corrosion behavior of the CS using the EIS technique at various temperatures in the absence and the presence of BMPD at 10⁻³ M were studied and shown in Figure 7 and Figure 8 and the corresponding data are given in

Table 4. Thermodynamic parameters for the adsorption of BMPD in 1 M HCl on the CS at different temperatures.

Inhibitor	Temperature (K)	Rct (Ω·cm2)	n	Q × 10 ̶ 4 (sn·Ω ̶ 1 cm ̶ 2)	Cdl (μF·cm ̶ 2)	ERct (%)
Blank	298	18	0.79	2.09	47	-
	308	11	0.77	2.69	7	-
	318	8	0.80	2.37	5	-
	328	5	0.81	2.17	4	-
BMPD	298	415	0.60	1.23	16	95.66
	308	205	0.70	1.45	32	94.63
	318	140	0.72	1.63	34	94.28
	328	98	0.74	1.38	38	94.89

. In the studied temperature range, the values of R_ct decrease with increasing temperature in uninhibited and inhibited solutions. The decrease in the charge transfer resistance can be attributed to the decrease in the average kinetic energy of the interacting species. Nevertheless, BMPD remained an effective inhibitor as the inhibition performance had no effect with the increase in temperature. This may be attributed to the higher adsorption of inhibitor molecules onto the metal surface.

3.5. Kinetic-Thermodynamic Corrosion Parameters

It is important to highlight that a comprehensive understanding of the adsorption process at the steel–solution interface can be enriched by considering the thermodynamic characteristics of the inhibitors. In this section, examining the thermodynamic aspects provides valuable insights into the factors influencing inhibitor adsorption and contributes to a more nuanced comprehension of the overall process. For this purpose, the Arrhenius equations were used to determine the link between the activation energy and the corrosion rate in a corrosion process, and the linear form of the Arrhenius and transition-state equations can be given as follows (Equations (8) and (9)):

$$I_{corr}=A exp\left(-\frac{E_a}{R T}\right)$$

(8)

where A is the Arrhenius pre-exponential factor, R is the universal gas constant, and T is the absolute temperature. The values of the activation energy of the metal dissolution in the inhibitor-containing solution or inhibitor-free solution can be determined from the slope (−Ea/R) of ln I_corr vs. (1000/T) as shown in Figure 9.

The thermodynamic parameters such as entropy and enthalpy were described by the transition-state equation [60,61,62]:

$$I_{corr}=\frac{RT}{Nh}exp\left(\frac{{∆S}^{*}}{R}\right)exp\left(-\frac{{∆H}^{*}}{RT}\right)$$

(9)

where N, h, ΔH^*, and ΔS^* are Avogadro number, Planck’s constant, the enthalpy, and the entropy, respectively.

Utilizing data from polarization studies, Figure 10 illustrates the regression analysis of [lin I_corr/T] vs. [1000/T], both in the absence and presence of the optimal concentration of the BMPD inhibitor. The values of ΔH^* and ΔS^* were obtained with the slopes of (−ΔH^*/R) and intercepts of (ln(R/Nh) + ΔS^*/R) of the fitted line, and the obtained results are listed in

Table 5. Thermodynamic parameters for CS with and without the BMPD compound.

Inhibitor	Ea (kJ/mol)	∆H* (kJ/mol)	∆S* (J/mol·K)	Ea−ΔH*
Blank	36.38	33.79	−191.53	2.60
BMPD	41.02	38.42	−201.43	2.60

.

As shown in Table 5, the value of E_a for the solution with BMPD is higher than that obtained for the uninhibited solution. In addition, the activation energy was augmented with the increased inhibitor concentration, indicating that adding BMPD to the corrosion medium causes a protective layer of the inhibitor to form on the CS surface, resulting in a larger energy barrier of the corrosion reaction compared to the uninhibited solution [63].

The positive sign of ΔH^* suggests that the dissolution process of CS in the studied solution is endothermic [64,65,66]. Moreover, the difference between Ea and ΔH^* is about 2.6 kJ/mol, which is the same as the value of the RT (2.63 kJ/mol) and showed that the corrosion process is a unimolecular reaction [67]. Negative values of entropies, obtained in the presence and absence of BMPD, imply that the adsorption process is always accompanied by the formation of an activated complex [68].

3.6. Morphological Observation of Protective Organic Layer

SEM is one of the most widely used techniques that utilizes a beam of electrons to scan the surface of a sample and produce high-resolution images and elemental analysis. The morphology of CS immersed in 1 M HCl acid solution without and with the presence of inhibitor for 24 h was studied, as shown in Figure 11. Based on the SEM image, it can be determined that the rough corrosion products have been formed on the surface of the sample exposed to the HCl solution without BMPD. On the other hand, the CS immersed in acid with 10⁻³ M of the BMPD shows that a protective layer is formed on the surface. The composition of the organic adsorption layer on the CS surface in 1 M HCl acid solution with corrosion inhibitors was performed by EDX analysis. The results demonstrate the presence of a large percentage of iron and a high peak associated with Cl Cl⁻ on the CS substrate before the addition of an inhibitor Figure 11. However, with the addition of an inhibitor, Figure 11 depicts a less damaged surface with small peaks of Cl⁻, and the distribution of the C element is confirmed when the inhibitor is added, suggesting the appearance of an inhibitor. These findings support that the BMPD compound inhibits the corrosion of the steel by forming a layer that limits the access of electrolytes to the surface.

3.7. Predicting Reactive Sites and Possible Adsorption Behavior Based on DFT/DFTB Calculations

Quantum chemistry is a powerful tool in the field of corrosion inhibition studies, enabling a profound understanding and predictive analysis of how different molecules perform in various systems. In this context, quantum chemical calculations serve as the foundation for exploring the correlation between the structure of the inhibitor molecule and its relationship to inhibition mechanisms. In our study, we employed the frontier molecular orbital (MO) theory to identify active sites and obtain additional quantitative insights into the donor–acceptor properties of the inhibitor molecule. Figure 12 shows the optimized geometry, the highest occupied, the lowest unoccupied MOs (HOMO, LUMO), the distributions of BMPD molecules, together with their electrostatic potential (ESP) distribution obtained by quantum chemical calculations. The quantum chemical parameters obtained, including E_HOMO, E_LUMO, and ΔE (energy gap), are also presented in Figure 12. Typically, the inhibitor molecule adheres to the metal surface through a donor–acceptor interaction. These interactions are made possible by the active sites within the inhibitor molecule. Hence, it is crucial to identify which active sites on the inhibitor molecule are responsible for facilitating this donor–acceptor interaction with the metal surface atoms. As shown in Figure 12b, the ESP map exhibits distinct regions of red and blue, signifying nucleophilic and electrophilic reactivity, respectively. Notably, the prominent red regions are primarily concentrated around the oxygen and nitrogen atoms, highlighting their significance as the primary active adsorption sites when binding to the Fe surface. Furthermore, the regions around aromatic rings present a local positive electrostatic potential. Clearly, the electron-rich regions with low electrostatic potential are potential nucleophilic centers for binding to the metal surface. Upon closer observation, it is evident that the BMPD exhibits electron-donating characteristics. This is exemplified by the uniform distribution of HOMO orbital across different molecular groups, as shown in Figure 12c. Conversely, the LUMO orbital displays a reduced distribution in the proximity of nitrogen and oxygen atoms, primarily due to the presence of lone electron pairs (Figure 12d). This suggests that these active adsorption sites have the capability to engage In electron sharing with metal atoms, resulting in the formation of covalent bonds. To quantitatively explain the chemical reactivity of the BMPD molecule, the energy of HOMO and LUMO orbitals are calculated. In general, a low value of E_LUMO indicates that the molecule is an electron acceptor, while a high E_HOMO value signifies its propensity to donate electrons to a suitable acceptor. The energy gap between the E_LUMO and E_HOMO energies is another pivotal parameter in describing molecular activity and pinpointing reactive sites. Herein, the calculated energy gap for BMPD in the aqueous phase is 4.31 eV, suggesting that the inhibitor molecule adsorbs more readily on the metal surface.

To gain insight into the adsorption behavior of the BMPD inhibitor and its interfacial mechanism at the interface, DFTB calculations have been performed. The primary advantage of DFTB, as compared to conventional DFT methods, lies in its capacity to account for the interactions occurring between the inhibitors and the metal surface atoms. These calculations aimed to elucidate the coordination modes of BMPD and also shed light on the charge density difference (CDD) between the inhibitor molecule and metallic surface. The most adsorption configuration of BMPD on the Fe(110) surface obtained from DFTB simulations is shown in Figure 13. Before conducting any optimization, the BPMD inhibitor was initially positioned at specific locations, maintaining a distance of 4 Å above the surface, as clearly illustrated in Figure 13 (side view). This choice was driven by the intention to promote attractive forces between the inhibitor molecule and the facet, facilitating a more comprehensive quantification of the type and strength of these interactions. As depicted in Figure 13, the BMPD molecule forms its most stable adsorption arrangement by adhering parallel to the metallic surface. In this pattern, the nitrogen atom, along with the π-current of the aromatic rings, directly participates in chemisorption interactions with the metal surface. In the context of chemical adsorption, it is crucial to verify that the bond length between the inhibitor molecule and the metal surface is in harmony with the combined covalent radii of the atoms involved in the chemical bonding. Consequently, two covalent chemical interactions are established, involving the nitrogen atom from BMPD and two Fe atoms at distances of 1.55 Å and 1.69 Å. Furthermore, CDD is computed for the most stable structure, with the yellow and blue regions signifying areas of charge accumulation and depletion, respectively. Evidently, there is an electron deficiency observed on the active atoms within the BMPD molecule, while electron accumulation is noticeable on the bonded Fe atoms. This observation suggests that the adsorption of the BMPD molecule onto the Fe surface is primarily achieved through electron transfer from the bonded nitrogen atoms to the Fe surface. Remarkably, the interplay of donor–acceptor characteristics and electronic density between the aromatic rings of BMPD and the metal surface enhances the adsorption energy (E_ads = −5.99 eV), likely owing to surface-π interactions. Consequently, BMPD exhibits the most robust inter- and intra-molecular interactions, which could be the primary factor driving the formation of its parallel adsorption configuration. These interactions play a critical role in regulating and promoting the electron transfer between the inhibitor molecule and the metallic surface. As a result, they contribute to the formation of more robust chemical and physical bonds, ultimately bolstering the adsorption behavior and hybridization between the BMPD molecule and Fe(110) surface.

4. Conclusions

This research has effectively evaluated the effective inhibitory properties of 3-benzoyl-4-hydroxy-2,6-bis(4-methoxyphenyl)-4-phenylcyclohexane-1,1-dicarbonitrile (BMPD) in preventing the corrosion of CS in a solution containing 1 M HCl. The experimental findings demonstrated favorable anticorrosive efficacy at low concentrations and room temperature. Furthermore, the studied organic compound exhibited remarkable stability at elevated temperatures, substantiating its excellence as a corrosion inhibitor. The results indicate that the inhibitor is categorized as a mixed-type corrosion inhibitor. The adsorption behavior of the investigated compounds adhered to Langmuir’s model, and this adsorption process was determined to be primarily chemical in nature. The SEM/EDS results provide evidence that the CS surface is indeed coated with a protective film generated by the inhibitor, thereby confirming the existence of a highly effective protective layer that contributes to its notable effectiveness. Theoretical simulations conducted through quantum chemical calculations and first-principle simulations further substantiated the superior performance of BMPD and its exceptional adsorption properties. Furthermore, first-principles DFTB simulations suggested that the BMPD molecule can form covalent bonds with iron atoms, displaying bond-breaking behavior upon interaction with the metallic surface. Both experimental and theoretical simulations provided compelling evidence regarding the novel compound’s capability to effectively prevent the acid corrosion of CS.