Cyanoacetamide derivatives as corrosion inhibitors; Synthesis, Characterization, DFT and Monte Carlo simulation

Corrosion behavior of X65-type carbon steel exposed to 1M HCl aqueous solution was studied in the absence and presence of various concentrations of new synthesized 3-(4-chlorophenyl)-2-cyano-N(4-hydroxyphenyl) acrylamide, abbreviated as (P2), and 6-amino-4-(4-chlorophenyl)-1-(4hydroxyphenyl)-2-oxo-1,2-dihydropyridine-3,5-dicarbonitrile, abbreviated as (P3), at 25C. Potentiodynamic polarization data indicated that the synthesized Cyanoacetamide derivatives suppress both anodic and cathodic reactions via adsorption on the carbon steel surface and blocking the active sites. The adsorption of the inhibitor molecules forms a protective film which decreases the surface heterogeneity. Electrochemical impedance spectroscopy (EIS) measurements reveal that as the inhibitor concentration is increased, both the inhibition efficiency (η%) and the charge transfer resistance (Rt) are increased while the electrochemical double layer capacity (Cdl) is decreased. The experimental impedance data were analyzed according to a proposed equivalent circuit model for the electrode/electrolyte interface. Chemical descriptors are calculated through the density functional theory (DFT), also adsorption of inhibitors on the metal surface investigated through Monte Carlo simulation. The mechanism of corrosion inhibition was discussed given the obtained results of surface analysis and the molecular structure of the additive obtained from quantum chemical calculations.


Introduction
Cyanoacetamide-N-derivatives are privileged structures and considered one of the most important precursors for heterocyclic synthesis [1][2][3]. The simplest and most convergent preparation of this class of compounds has been described elsewhere [4,5]. They are extensively utilized as reactants or reaction intermediates since the carbonyl group, the active hydrogen on C2, and the cyano function of these compounds are suitably positioned to enable reactions with common reagents to form a variety of heterocyclic compounds with pharmacological applications, for example, antitumor [6,7], anti-inflammatory [8,9], and antibacterial and antifungal [10][11][12][13] activity, in addition to other medicinal applications [14,15]. Recently many derivatives of cyanoacetamide were reported as a new class of corrosion inhibitors [16,17]. Computational and theoretical methods are a considerable tool to calculate chemical and quantum descriptors based on theoretical assumptions [18][19][20][21]. Molecular dynamics (MD) simulation used currently to investigate the orientation of corrosion inhibitors over the metal surface [22][23][24]. Furthermore, density functional theory (DFT) approximates the inhibition efficiency of the inhibitors through their electronic molecular structure [25]. In the current work, the inhibition efficiency of two synthesized inhibitors as displayed in Scheme 1 was evaluated on the metal surface. Chemical hardness, electronegativity, chemical potential, electrophilicity, and nucleophilicity as widely used chemical descriptors have been investigated to assess the accomplishment between experimental results and theoretical approaches [26,27]. Based on the molecular structure of the inhibitors, these descriptors will assess the inhibition efficiency in terms of chemical reactivity [28]. Also, a Monte Carlo simulation was conducted to calculate the inhibitor's adsorption energy on the iron (Fe) crystal surface [29].

Chemical composition of the investigated carbon steel alloy
Carbon steel specimens used in this investigation were cut from unused petroleum pipelines having the chemical composition of carbon steel alloy is listed in table 1. Page: 35 www.raftpubs.com

Corrosive medium
The corrosive medium employed in this study is a 1M HCl aqueous solution. The selection of this corrosive medium was made to simulate the oil wells acidizing jobs that are frequently executed to stimulate oil production. During the acidizing process, the oil well, tubing is susceptible to acidic corrosion on the internal surface of the production tube.

Synthesis and characterization of 2-cyano-N-(4-hydroxyphenyl) acetamide [P1]
An equimolar amount of p-aminophenol and ethyl cyanoacetate were fused in an oil bath for 30 minutes then left to cool.

Synthesis and characterization of 3-(4chlorophenyl)-2-cyano-N-(4hydroxyphenyl) acrylamide [P2]
To an ethanolic solution of the 2-cyano-N-(4hydroxyphenyl) acetamide (0. 35 Figures S1-S3). The surface examination was carried out using a scanning electron microscope (JEOL JSM-5410, Japan). The energy of the acceleration beam employed was 20KV. All micrographs were taken at a magnification power (100-500X). EDX system attached with a JEOL JSM-5410 scanning electron microscope was used for the elemental analysis or chemical characterization of the surface of carbon steel before and after applying the synthesized P2 and P3 inhibitor.

Potentiodynamic polarization measurements
A conventional three-electrode glass cell, consisting of the carbon steel working electrode (WE), a platinum counter electrode (CE), and a saturated calomel electrode (SCE) as a reference electrode, was used for electrochemical measurements as illustrated in igure 1. The electrochemical measurements were carried out using Volta lab80 (Tacusselradiometer PGZ402) controlled by the Tacussel corrosion analysis software model (Volta master 4). All the measurements were carried out in air-saturated solutions and at ambient temperature (25 o C). The working electrode was first immersed in the formation water for 1.0 hours to establish a steady-state open circuit potential (Eocp). The open-circuit potential for the working electrode was recorded over time for 3 hours under the testing solution at 25 o C with and without different concentrations of the synthesized P2 and P3 inhibitors. The test period was maintained to ensure achieving a stable potential value. After determining the open circuit potential, potentiodynamic polarization curves were obtained by changing the applied electrode potential automatically from -900 mV to -30 mV with a scan rate of 1 mV/s in both cathodic and anodic potentials to investigate the polarization behavior.

Electrochemical impedance spectroscopy (EIS)
Impedance spectra were obtained in the frequency range between 100 kHz and 50 MHz using 20 steps per frequency decade at open circuit potential after 3 hours of immersion time. AC signal with a 20-mV amplitude peak to peak was used to perturb the system. EIS diagrams are given in Nyquist representation.

Results and Discussion
Potentiodynamic polarization measurements inhibitor. Electrochemical parameters such as corrosion potential (Ecorr.), corrosion current density (icorr.), cathodic and anodic Tafel slopes (bc and ba), and polarization resistance (Rp) were calculated. From the obtained polarization curves, the corrosion current densities (icorr.) were decreased with increasing inhibitor concentration concerning the blank (inhibitor-free solution). These results indicate the production of a good protective layer on the surface of carbon steel. The degree of surface coverage (θ) and the percentage inhibition efficiency (IE %) was calculated using the following equations [17]. Page: 37 www.raftpubs.com where io and i are the corrosion current densities in the absence and presence of the inhibitor, respectively. The values of polarization resistance (Rp) were calculated from the wellknown Stern -Geary equation.
From the obtained data, the Tafel lines are shifted to more negative and more positive potentials for the anodic and cathodic processes, respectively relative to the blank curve. This means that the selected compound acts as a mixed type inhibitor, i.e., promoting retardation of both anodic and cathodic discharge reactions. Also, the slopes of the cathodic and anodic Tafel lines are approximately constant and independent of the inhibitor concentration. This means that the selected inhibitor does not affect the metal dissolution mechanism. Complete data obtained from polarization measurements for inhibitors P2 and P3 are summarized in tables 2 and 3, respectively.  Page: 38 www.raftpubs.com    Page: 39 www.raftpubs.com

Electrochemical impedance spectroscopy (EIS)
The corrosion behavior of carbon steel in deep oil wells produced water in the absence and presence of various concentrations of the synthesized inhibitor was investigated by the EIS technique. Nyquist and Bode's plots are shown in figures 3-5, respectively. It is clear from the plots that the impedance response of carbon steel in produced water was significantly changed after the addition of the inhibitor molecules. Various parameters such as the charge transfer resistance (Rt), double layer capacitance (Cdl), and percentage inhibition efficiency IE % were calculated according to the following equations and listed in tables 4 and 5, respectively. The values of Rt were given by subtracting the high-frequency impedance from the low-frequency one as follows [30].
The values of Cdl were obtained at the frequency fmax, at which the imaginary component of the impedance is maximal -Zmax using the following equation.
The percentage inhibition efficiency IE % was calculated from the values of Rt using the following equation.
( ) Where Rt and Rt (inh) are the charge transfer resistance values in the absence and presence of inhibitor, respectively. Increasing the value of charge transfer resistance (Rt) and decreasing the value of double-layer capacitance (Cdl) by increasing the inhibitor concentration indicates that the surfactant molecules inhibit the corrosion rate of carbon steel in deep oil wells produced water by adsorption mechanism [31].   Page: 40 www.raftpubs.com     Figure 6a shows an SEM image of a polished carbon steel surface. The micrograph shows a characteristic inclusion, which was probably an oxide inclusion [32]. Figure 6b shows SEM of the surface of the carbon steel specimen after immersion in produced water for 30 days in absence of an inhibitor, while figure 6c shows SEM of the surface of another carbon steel specimen after immersion in produced water for the same time interval in presence of 150 ppm of the synthesized inhibitor. The resulting scanning electron micrographs reveal that the surface was strongly damaged in absence of the inhibitor, but in presence of 150 ppm of the inhibitor, there is less damage to the surface. This confirms the observed high inhibition efficiency of the inhibitor at this concentration.

Energy Dispersive Analysis of X-Rays (EDX)
EDX spectrum in Figure 6a shows the characteristic peaks of some of the elements constituting the polished carbon steel surface. The spectrum of the polished carbon steel surface after immersion in the produced water in the absence and presence of the inhibitor for 30 days, are shown in figure 6b& c, respectively. The spectra of Figure 6c show that the Fe peak is considerably decreased relative to the samples in Figure 6a & b. This decrease of the Fe band is indicated that the strongly adherent protective film of the inhibitor formed on the polished carbon steel surface, which leads to a high degree of inhibition efficiency [33]. The oxygen signal apparent in Figure 6b is due to the carbon steel surface exposed to the produced water in absence of the inhibitor. Therefore, EDX and SEM examinations of the carbon steel surface support the results obtained from electrochemical methods that the synthesized surfactant inhibitor is a good inhibitor for the carbon steel in the oil wells produced water.

Monte Carlo simulation
Monte Carlo simulation was conducted to explore the alignment and the lowest configuration adsorption energies of the two inhibitors on the metallic iron surface in the aqueous phase. Inhibitor molecules were geometry optimized using the for cite module applying the COMPASSII force field with Gasteiger charges, and Quasi-Newton algorithms at a fine quality [29]. The electrostatic and Vander Waals forces calculated using atom and group-based, respectively. annealing simulation conducted between the inhibitor molecules and Fe crystal built with a surface cleavage of (-111) and vacuum slab thickness of 30.00 Å in a simulation box (4.05 °A× 4.05 °A× 30.82 °A) with periodic boundary conditions alongside with 5 water molecules [34]. The adsorbed inhibitors were translated and rotated around the metal surface to screen the minimum adsorption energy sites for attachment [29]. A Page: 42 www.raftpubs.com sequence of NVT (canonical ensemble) calculations were performed at 300 K and 1 atm with a timestep of 0.5 fs [17]. Figure 7 displays the configuration outputs from the annealing simulation. The chemical descriptors outputs from the MC simulation are listed in table 5,6. The adsorption energy is an indicator of inhibitor efficiency. The more negative value of adsorption energies reveals the higher inhibition efficiency [22,26,29,35,36]. The obtained results in table 6 indicate that the inhibition efficiency of P2 inhibitors is higher than that of the P3 inhibitor.     Page: 46 www.raftpubs.com

Density function theory (DFT)
DFT calculations are widely implemented in computational chemistry due to the reliability and compatibility of their predictions with the experimental data [37]. Moreover, B3LYP has been assigned for systems comprising transition metal atoms during corrosion inhibition study, since the inhibitor adsorption on the metal surface depends on donor-acceptor interaction between the conjugated π-electrons of the heterocyclic atom and the vacant d-orbitals of the metal atoms [37]. Density functional theory (DFT) was conducted at the Becke's threeparameter exchange functional (B3) along with the Lee-Yang-Parr (LYP) gradient corrected nonlocal correlation functional (B3LYP) at medium quality [22,25,26,34,38]. The relation of the quantum chemical descriptors to the corrosion inhibition behavior of the reported inhibitors was investigated [38]. The definition and formula of these chemical descriptors are reported elsewhere [22,26,28,34,[38][39][40]. Table  7 summarizes the chemical descriptors calculated by DFT-theory. The optimized geometries of the reported inhibitors as well as Frontier molecule orbital (HOMO & LUMO) delocalization, and electron density map are shown in figure 8.  [26]. Red colors are designated for negative electrostatic potential, while blue regions are related to positive ones [25]. Ö and N atoms contain lone pairs of electrons, so displayed as red regions possessing the most negative charges. Ö and N atoms are the negatively charged sites that give electrons to the Fe atoms to form a coordinate bond [29,41]. The -OH and Clhave more positive charges and display dark blue regions. The white regions were assigned for zero electrostatic potential of the phenyl rings [25,26,42]. The stabilization of the HOMO and LUMO orbitals for the reported inhibitors reveals their electron-donating ability to the metallic surface [25]. By analyzing the chemical descriptors as displayed in Page: 47 www.raftpubs.com assumption is based on the Lewis acid-base theorem which states that the metal (Fe) surface will interact with softer molecules faster than the harder molecules [25]. According to Sanderson's electronegativity equalization principle [45,46], the lower electronegativity value of both P2 & P3 inhibitors ascertains its higher inhibition efficiency [44]. The results of electrophilicity indicate that reactive, and efficient nucleophile is characterized by a low value of electronic chemical potential (μ) and electrophilicity index () [38]. The fraction of electron transferred (ΔN) value confirms that the P2 inhibitor has a higher value of electron transfer (59.1218) so it exerts high inhibition efficiency attributed to the higher electrondonating ability to the metallic surface according to Lukovits's study [38,47]. The electron flows from the molecule orbitals to the Fe surface until the chemical equilibrium is attained [25]. ΔE Back-donation was calculated relevant to the global hardness and revealed that back-donation is favored for P2 and P3 as a good inhibitor [44]. Since ΔE Back-donation < 0, so the electron-back-donation process is energetically favored, and the adsorption of the inhibitor molecule on the Fe surface is strengthened [25]. The efficiency of P2 and P3 for corrosion inhibition resort to the presence of oxygen and nitrogen heteroatoms with abundant π-electrons so adsorb efficiently on the metallic surface and withstand the corrosion [22,25]. The polar heteroatoms energize the inhibitor-metal cation adsorption interaction [25]. As a result, active binding sites on the Fe surface are overcrowded, so, the anodic/cathodic interactions diminished during the electrochemical process [25].

Conclusion
The corrosion inhibition performance of the synthesized cyanoacetamide derivatives was investigated in 1 mol HCl for mild steel through conducting experimental and computational methodologies. Based on the obtained results from Potentiodynamic polarization and electrochemical impedance measurement, the following aspects can be highlighted; 1. The investigated nonionic surfactant is an effective inhibitor for corrosion of carbon steel in oil well produced water. 2. The adsorption of the inhibitor molecules obeyed the Langmuir adsorption isotherm. 3. The values of the percentage inhibition efficiency obtained from potentiodynamic polarization and electrochemical impedance spectroscopy are in good agreement. 4. The potentiodynamic polarization curves indicated that the inhibitor molecules inhibit both anodic metal dissolution and also cathodic oxygen reduction so that the undertaken surfactant is classified as a mixed-type inhibitor. 5. The inhibition mechanism is attributed to the strong adsorption ability of the selected surfactant on the carbon steel surface, forming a good protective layer, which isolates the surface from the aggressive environment. 6. The produced good protective film on the carbon steel surface was confirmed using SEM and EDX techniques. 7. The data obtained from the experimental techniques are confirmed by theoretical data obtained from quantum chemical calculations. 8. The Monte Carlo simulation and DFT approach indicate the higher inhibition efficiency of P2 inhibitor rather than P3 inhibitor.