Theoretical and Experimental Study for Corrosion Inhibition of Carbon Steel in Salty and Acidic Media by A New Derivative of Imidazolidine 4- One

A new imidazolidine 4-one derivative, of namly 2-[2-(4-Bromo-phenyl)-imidazo [1,2-a] pyridine-3-yl]-3-(4-nitro-phenyl)-imidazolidine-4-one (BPIPNP) was investigated as corrosion inhibitor for carbon steel in salty (3.5% NaCl) and acidic (0.5M HCl) solutions using potentiometric polarization measurements. The results revealed that the percentage inhibition efficiencies (%IE) in the salty solution (90.67%) are greater than that in the acidic solution (83.52%). Experimentally, the thermodynamic parameters obtained have supported a physical adsorption mechanism and which followed Langmuir adsorption isotherm. Density Functional Theory (DFT) of quantum mechanical method with B3LYP 6-311++G (2d, 2p) level was used to calculate geometrical structure, physical properties and inhibition efficiency parameters, in vacuum and two solvents (DMSO and H2O), all at the equilibrium geometry. The surface changes of carbon steel were studied using Scanning Electron Microscopy SEM and Atomic Force Microscopy (AFM) techniques.


Introduction
Corrosion is an undesirable phenomena occur because chemical or electrochemical reactions between a metal and its environment. It is a spontaneous process including decrease in Gibb's free energy [1]. In spite of that corrosion process is not completely avoid but there are many methods to inhibit it. One of these methods are using inhibitors, inhibitor is a substance added in a small concentration to corrosive media causes decrease in corrosion rate of the area that exposed to that environment [2]. Many of inhibitors that used in industry are organic heterocyclic compounds [3]. Organic heterocyclic inhibitors usually have hetero atoms. The type of mechanism that inhibitors applied was adsorption mechanism, these inhibitors forming a preventative film on the metal surface. Quantum chemical calculations were used to study the reaction mechanism and to solve chemical ambiguities. The structural and electronic parameters of the inhibitors molecules can be obtained by theoretical calculations using computational methodologies of quantum chemistry [4]. In this research we focused on the Imidazo[1,2-a]pyridine derivative (BPIPNP), a heterocyclic entities and pharmacologically important molecule, Figure-1. Several procedures for their synthesis have been studied [5].
The aim of this work is to study the inhibition efficiency of the organic inhibitor (BPIPNP) which was prepared by Naeemah Al-Lami et. al [6]; experimentally, in salty (3.5% NaCl) and acidic (0.5M HCl) solutions using potentiostatic method, and theoretically, the calculations were done in three media (vacuum, DMSO, and water) depending on quantum mechanical parameters using DFT method with [6-311/ B3LYP++G (2d, 2p)] level using Gaussian 09 program.

Experimental details 2.1. Preparation of carbon steel samples
Carbon steel's rod is symbolized as (C45) with the following percentage of metallic materials in composition (wt %): (0.122% C, 0.206% Si, 0.641% Mn, 0.016% P, 0.031% S, 0.118% Cr, 0.02% Mo, 0.105% Ni, and 0.451% Cu) [7]. The rod mechanically cutting in to pieces forming a disk specimen of carbon steel a with 1.6 cm diameter and 3 mm thickness, each of these specimen was refined with emery paper (silicon carbide SiC) in different grades (80, 150, 220, 320, 400, 1000, 1200 and 2000), then washed with tap water, distilled water and degreased with acetone, washed again with deionizer water, and finally held in a desiccators after it is dried at room temperature.

Preparation of solutions 2.2.1. Blank of the salt solution
35 gm of (NaCl salt) was dissolved in (100 ml) distal water; transferred the formative solution in to (1000ml) volumetric flask, which contained (6ml) of dimethyl sulfoxide (DMSO) solvent. The volume of the solution was completed to (1L) by adding distal water.

Acid blank solution
40 ml (0.5M) of HCl was diluted by distilled water to (1liter) in a volumetric flask, after adding (6ml) of the solvent of dimethyl sulfoxide (DMSO).

Electrochemical measurements Potentiostatic polarization study
The potentiostat set up has included the following: a host computer with Mat lab software (Germany, 2000) magnetic stirrer, thermostat. The main part of the apparatus is the corrosion cell; which was made out of Pyrex with 1L capacity. This cell consisted of two bowls external and internal. Three electrodes are mainly present in the electrochemical corrosion cell. Carbon steel specimen with (1cm 2 ) surface area represented the working electrode. This is used to determine the working electrode potential due to another electrode namely as reference electrode, located closed to working electrode. A reference electrode was silver-silver chloride (Ag/AgCl, 3.0M KCl). The auxiliary electrode is a platinum rode electrode with (10cm) length. The starting step was represented in immersing the working electrode in the test solution for fifteen minutes (15min), to establish a steady state open circuit potential (E ocp ). This potential was noted for starting the electrochemical measurements in the range of (±200 mV). All tests solutions were conducted at temperatures of (293, 303, 313 and 323) K.

Results and discussion Quantum chemical calculations
Quantum chemical methods are an important ways in electrochemistry studies, it's represented as the fastest ways for studying the structural nature of organic compounds specially those used as inhibitors and described how these inhibitors inhibited corrosion. The efficiencies of corrosion inhibitors are investigated by theoretically parameters of corrosion inhibition such as the energy of the Highest Occupied Molecular Orbital (E HOMO ), the energy of the Lowest Unoccupied Molecular Orbital (E LUMO ), the energy gap (ΔE HOMO-LUMO ) between E HOMO and E LUMO , electro negativity (χ), dipole moment (μ) electron affinity (A), ionization energy (IP), softness (s), global hardness (η), global electrophilicity (ω), and the fraction of transferred electrons (ΔN) [8].

Molecular geometry
The organic inhibitor compound was built using Chem. Draw of MOPAC program, see Figure-(2a). Gaussian 09 packages were used for calculating the fully optimize structure [9], see Figure-(2b), using DFT (Density Functional Theory) method of Becke's three-parameter of Lee, Yang and Parr (B3LYP) with a 6-311++G (2d, 2p) level of theory [10]. In addition to vacuum, the equilibrium geometry was calculated in two solvents of (DMSO and H 2 O).  proven that the compound is not planar with point group of C 1 [the cis dihedral angles aren't 0.0 degree and all of the trans dihedral angles are more or less than 180.0 degree]. Figure-  Figure-4 shows the geometrical optimization of the studied inhibitor in vacuum including HOMO and LUMO distributions. The HOMO is mainly located on the (2-(2-Biphenyl-4-yl-imidazo [1,2-a] pyridine-3-yl)) moiety. This indicates that the preferred actives sites for an electrophilic attack are located within the region around the nitrogen atoms. Moreover, the electronic density of LUMO was distributed at the aromatic ring and around the ring of (4-nitro-phenyl) moiety (the most planar region in the molecule).

Global molecular reactivity
To study the influence of molecular geometry on the mechanism and efficiency of inhibition, some of the chemical quantum calculations were performed such as, the energy of highest occupied molecular orbital (E HOMO ), the energy of the lowest unoccupied molecular orbital (E LUMO ), the energy gap (ΔE HOMO-LUMO ) and the dipole moment (μ). The other quantum chemical parameters are all shown in Tables-(2a, 2b).
The Frontier orbital theory was used in predicting the adsorption centers of the inhibitor responsible of the reaction metal surface/ molecule [11]. According to this theory, the formation of a transition state is due to an interaction between the Frontier orbital's (HOMO and LUMO) of reactants with the metal surface. The HOMO energy (E HOMO ) is often associated with the electron donating ability of the molecule, thus, inhibitors with high values of (E HOMO ) have a tendency to donate electrons to appropriate acceptor with low empty molecular orbital energy. Contrariwise, LUMO energy (E LUMO ) indicates the ability of molecule for electron-accepting, the lowest value of E LUMO , the higher the capability of accepting electrons. The energy gap between the Frontier orbital's (ΔE HOMO-LUMO ) is also an important factor in describing the molecular activity, so when the energy gap is decreased, the inhibitor efficiency increased [12]. Activation hardness has been also defined on the basis of the ΔE HOMO-LUMO energy gap. The qualitative definition of hardness is closely related to the polarizability, since any decrease in the energy gap usually leads to easier polarization of the molecule. All these parameters which are related to the efficiency of the inhibition of a molecule, values of E HOMO , E LUMO , ΔE HOMO-LUMO , electronegativity (χ), molecular dipole moment, softness (S), global hardness (η), the fraction of electron transferred (ΔN), were calculated using the density functional theory (DFT) and have been used to understand the properties and activity of the newly prepared organic compounds and to help in the explanation of the experimental data obtained for the corrosion process. The ionization potential (IP) and electron affinity (EA) of the inhibitors are calculated according to Koopman's theorem [13], using the following Equations [14]: (2) The electronegativity (χ) and the chemical hardness (η) according to Pearson, operational and approximate definitions can be evaluated using the following relations [14]: IP η= (IP-EA)/ 2 (4) Global chemical softness (S), which describes the capacity of an atom or group of atoms to receive electrons [9], was estimated by using Equation 5: The Global electrophilicity index ( ) introduced by Parr [15]. It was used for calculating the electronegativity and chemical hardness parameters, Equation 6: The dipole moment (μ in Debye) is an important electronic parameter that results from the nonuniform distribution of charges on the different atoms in the molecule. The increasing in the value of dipole moment increases the adsorption between a chemical compound and metal surface [16]. Dipole moment for BPIPNP inhibitor in vacuum is (8.8305 Debye), increased in DMSO and H 2 O as a result of increasing polarity of the solvent.
The ionization potential, IP can be approximated as the negative of the E HOMO [17]. Low values of IE increase the effectiveness of the inhibitor. The IP of BPIPNP inhibitor in the vacuum is (6.480eV), decrease in DMSO and H 2 O solvents.
EA is the amount of energy released when adding an electron to an atom or molecule [18]. A high value of EA indicates a less stable inhibitor (good corrosion inhibitor). The electron affinity of BPIPNP in the vacuum is (2.909eV), be a higher on using DMSO and H 2 O solvents.
Chemical Hardness (η) is a measure of the ability of atom or molecule to transfer the charge. Increasing (η) decreases the stability of molecule, so the inhibitor possessed a high value of (η) is considered to be a good inhibitor. (η) value for BPIPNP in the vacuum is (1.785eV), be a lower in DMSO and H 2 O solvents.
Chemical Softness (S) is a measure of the flexibility of an atom to receive electrons (S), Molecules having a high value of S are considered to be a good inhibitor. The values of (S) in the vacuum is (0.560 eV), increase in DMSO and H 2 O.
The electronegativity ( ) is the ability of an atom or a group to pull electrons, High electronegativity indicates a good inhibitor [19]. The calculated ( ) for BPIPNP in the vacuum was found to be (4.694eV), decreased in DMSO and H 2 O solvents.
Global electrophilicity index ( ) is the measure of the stability of an atom after gaining an electron [20], Low value of (ω) meaning the molecule has a good inhibition. In the vacuum is (6.171eV), increased in DMSO and H 2 O. ΔN (Difference in number of electrons transferred) is the fraction of electrons transferred from an inhibitor to carbon steel surface. BPIPNP has ΔN value up to (0.645) in the vacuum and increased in solvents leading to increase the ability of inhibition efficiency, when the two systems, Fe, and inhibitor, are brought together.

Active sites of the (BPIPNP) inhibitor
The inhibition of the studied inhibitors was determined using DFT Mulliken charges population analysis; which gave an indication of the reactive centers of the compounds (electrophilic centers and nucleophilic centers). For that, region that have a large electronic charge are chemically softer than the region that have a small electronic charge. Thus, the density of electron may play an important role in the chemical reactivity calculating. The chemical adsorption interactions are either by orbital interactions or electrostatic. The nucleophilic attack sites will be the place where the positive charge value is a maximum, and hence only the charges on the oxygen (O), nitrogen (N), and some carbon atoms would be presented. The electrophilic attack site was controlled by the negative charge value.
The nucleophilic and electrophilic electronic charge values of compounds are stronger in DMSO and H 2 O solutions than in vacuum. Table-3 shows Mulliken charges population (ecu) analysis for the (BPIPNP) compound in media (vacuum, DMSO, and H 2 O). According to this table, the order of the nucleophihic reactive sites of (BPIPNP) inhibitor is: O30 C14 C12 N9 C2 C1 C23 N20 C8 C15 C26, and the order of the electrophihic reactive sites order is: C5 C21 C10 C18.

Corrosion inhibition measurement Potentiodynamic Polarization Measurements
The electrochemical kinetics of metallic corrosion process can be characterized by determining at least three polarization parameters, such as corrosion current density (I corr .), corrosion potential (E corr .) and Tafel slopes (ba and/or bc). The corrosion behaviour can be determined by a polarization curve (E versus log I). The evaluation of the polarization parameters leads to the determination of the corrosion rate (C.R). Using Tafel extrapolation method, it is possible to obtain the I corr at the E corr by the extrapolation of anodic and/or cathodic Tafel lines [21].
Measurements were performed in 3.5% NaCl solution and acidic solution containing different concentrations of the tested inhibitor (BPIPNP). The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential to obtain the corrosion current densities I corr and inhibition efficiency percentage IE%, Equation 8: Where I corr (in) is the inhibited corrosion current densities, I corr (un) is the uninhibited current densities. The values of polarization resistance Rp was calculated using Equation 9 [22]: The surface coverage ( ) of the carbon steel corrosion immersed in 3.5% NaCl solution and acidic solution containing different (BPIPNP) concentration (C) could be estimated, using Equation 10: While the corrosion rate (CR) was calculated by Equation 11: The addition of the imidazolidine 4-one derivative causing decrease in the corrosion rate, i.e. shifts of the catholic and anodic curves to lower values of current densities, and both cathodes and anodic reactions of carbon steel electrode corrosion are inhibited by the inhibitor in both 3.5% NaCl solution and acidic solution. Figure-5 shows potentiodynamic polarization curves for C.S (C45) in the salt solution, with and without the addition of (BPIPNP) inhibitor at various concentrations, and at the optimum conditions of (20ppm) with temperature of (293K), on the other hand, Figure-6 shows potentiodynamic polarization curves for C.S (C45) in the acidic solution with and without the addition of (BPIPNP) at various concentrations, and at the optimum conditions of (20ppm) with temperature of (293K).
Table-4, collects the values of corrosion rates of C.S and inhibition efficiency of inhibitor studied at various concentrations and different temperature in salt solution, while, Table-5, the values of corrosion rates of C.S and inhibition efficiency of inhibitor studied at various concentrations and different temperature in acidic solution. These tables showing that increasing temperature lead to increase the corrosion current densities I corr. , while the efficiencies IE% enhances with the increasing the inhibitor concentration. The optimum conditions for BPIPNP in the salt solution were observed at 293K and 20ppm corresponded to lowest I corr. (12.41 μA.cm -2 ) and maximum IE% (90.67%) and The optimum conditions for BPIPNP in the acidic solution were observed at 293K and 20ppm too, corresponded to lowest I corr. (32.38 μA. cm -2 ) and maximum IE% (83.52%). The values of iron corrosion rate CR were decreased with increasing the concentration of (BPIPNP) inhibitor and the addition of inhibitor to the blank solutions increased the cathodic and anodic current densities without shifting the corrosion potential, so (BPIPNP) inhibitor can be described as a mixed-type inhibitor. Its inhibition caused by adsorption and the inhibition effect results from the reduction of the reaction area on the surface of the carbon steel [23].

Corrosion kinetic and thermodynamic activation parameters
Arrhenius equation (Equation 12) was used to study the effect of temperature on the inhibited corrosion reaction carbon steel.
Log (I corr ) = Log A -Ea / 2.303RT ….(12) Where Ea is the energy activation of the corrosion reaction (kJ mol -1 ) and A is the pre-exponential factor (in molecules cm -2 s -1 ), R is the gas constant and T is the absolute temperature (Kelvin). Values of Ea were derived from the slopes of linear relationship between (log icorr) versus (1/T), and (A) obtained from the intercepts, see Figures-7 and 9 for salty and acidic media respectively.
The enthalpy changes values (ΔH*) of the corrosion reaction in 3.5% NaCl and acidic media are positive values that give an indication of endothermic nature for this reaction [24]. Negative values of the entropy (ΔS*) for corrosion process meaning a decrease in the degree of freedom and a consequent restriction of the corrosion process. The values of ΔG* for corrosion process were calculated from the following relation: ΔG* = ΔH*-T ΔS* …. (15)

Adsorption isotherm
The adsorption isotherms are useful to describe the reaction between the inhibitor molecules with carbon steel surface. Langmuir adsorption isotherm can be represented by the following Equation [14].
C/ Ө= (1/ K ads ) + C (14) C: is the concentration of the inhibitor in 3.5% NaCl solution and in 0.5M HCl solution. K ads : is the adsorption/ desorption equilibrium constant. A plot of C/ θ versus C in the salt and acidic media, could be used to determine the equilibrium adsorption constant K ads for both solutions, Figures-12 and 14. Figures-11 and 13, and Tables-8 and  9 show Langmuir isotherms functions of the adsorption process. The ΔG ads was calculated using Equation (15) [15].

Al-Joborry and Kubba
Iraqi Journal of Science, 2020, Vol. 61, No. 8, pp: 1842-1860 1855 ΔG ads = -2.303 RT Log (55.55K ads ) (15) Whereas R is the gas constant (J K -1 mol -1 ), T is the absolute temperature (K), and 55.5 is the molar concentration of water (mol L -1 ) in solution. By plotting K ads versus (1/ T) the ΔG°a ds was extracted from the slope. The entropy and enthalpy adsorption values were obtained by using Equations (15,16), as shown in Figure-12 for the salt medium and Figure-14 for the acidic medium.
ΔG°ads= −RT ln Kads (16) ΔG°ads= ΔH°ads− TΔS°ads (17) The negative values of the ΔG°a ds reflect the spontaneous adsorption. In general, values of ΔG°a ds higher than (-40 kJ mol -1 ) are compatible with physisorption and those lower than (-40 kJ mol -1 ) involve chemisorptions [16]. The calculated values for ΔG°a ds were found in the range of (-12.250 to -9.817 kJ mol -1 ) at different temperatures (293-323K) in the salt media, while, ΔG°a ds were found in the range of (-11.279 to -8.966 kJ mol -1 ) at different temperatures (293-323K) in the acidic medium. These values fall between the threshold values for the physisorption. The entropy ΔS°a ds value was positive confirming that the corrosion process is entropically favorable [25]. The negative value of ΔH°a ds in the salt and acidic media indicates the adsorption of inhibitory compounds on the C.S surface is an exothermic process. For compound (BPIPNP) ΔHº ads is equal to (-25.369 kJ mol -1 ) in the salt medium, while, the acidic medium have ΔHº ads is equal to (-20.506 kJ mol -1 ).    Table 9-Langmuir parameters for adsorption of (BPIPNP) compound on carbon steel surface in 0.5M HCl solution at different temperature/

Scanning Electron Microscopy (SEM)
In Figures-(15a, 16a) the badly damaged surface obtained when the metal was remained immersed in saline water and acidic solution respectively. However, Figures-(15b, 16b) shown C.S surface in the presence of inhibitor (BPIPNP) in saline water and in acidic solution had respected smoothness as compared to Figures-(15a, 16a), indicating reduction of the corrosion rate. This improvement in surface morphology is due to the formation of a protective film of compound (BPIPNP) inhibitor on the C.S surface which is represent for inhibition of corrosion [26].

Conclusions
-The new synthesized imidazolidine 4-one (BPIPNP) derivative was theoretically found to be a good organic corrosion inhibitor for carbon steel in both saline and acidic media.
-The inhibition efficiency were measured for (BPIPNP) derivative experimentally using potentiodynamic polarization measurements reflected that the studied inhibitor (BPIPNP) could be classified as a mixed inhibitor in both saline and acidic media. -The inhibition efficiency increased with increasing the inhibitor concentration and decrease with increase temperature (physisorption inhibition).
-The inhibition efficiency for corrosion of carbon steel by using (BPIPNP) inhibitor is higher in salty medium than in acidic medium.
-The adsorption of (BPIPNP) compound on C-steel follows the Langmuir adsorption isotherm model. -The high values of K ads indicate a good corrosion inhibition of carbon steel in salty and acidic media by BPIPNP.
-The (ΔG°a ds ) values indicate the inhibition process to be a physical adsorption process.