Theoretical and Experimental Studies for Inhibition Potentials of Imidazolidine 4-One and Oxazolidine 5-One Derivatives for the Corrosion of Carbon Steel in Sea Water

Two derivatives of Iimidazolidin 4-one (IMID4) and Oxazolidin 5-one (OXAZ5), were investigated as corrosion inhibitors of corrosion carbon steel in sea water by employing the theoretical and experimental methods. The results revealed that they inhibit the corrosion process and their %IE followed the order: IMID4 (89.093%)  OXAZ5 (80.179%). The %IE obtained via theoretical and experimental methods were in a good agreement with each other. The thermodynamic parameters obtained by potentiometric polarization measurements have supported a physical adsorption mechanism which followed Langmuir adsorption isotherm. Quantum mechanical method of Density Functional Theory (DFT) of B3LYP with a level of 6-311++G (2d, 2p) were used to calculate the geometrical structure, physical properties and inhibition efficiency parameters, in vacuum and two solvents (DMSO and H2O), all calculated at the equilibrium geometry, and correlated with the experimental %IE. The local reactivity has been studied through Mulliken charges population analysis. The morphology of the surface changes of carbon steel were studied using SEM and AFM techniques.


Introduction
Corrosion of metals is a serious material deterioration problem from both structural and economic integrity standpoint, but it can be largely controlled by the suitable strategies. Carbon steel (C.S) is being widely used as engineering alloy, however, corrosion of carbon steel occurs in almost all practical environments [1]. Metallic corrosion is a spontaneous process (∆G< 0) that causes damage in almost all sections of human activity. Among the most influenced structures are the pipes for oil transportation [2]. The biggest anxiety of the corrosion scientists is efficient protection of the metals without disturbing the environmental peace. Hence, the study of corrosion processes and their inhibition turn out to be the main goal of many researchers now [3]. Basically, corrosion inhibitor is a substance added in a small amount of the corrosive environment to reduce the corrosion rate of the metal or alloys [4]. Most inhibitors utilized in manufacturing are organic compounds. These organic inhibitors containing donor atoms such as O, S and N. Inhibitors including triple or double bonds had an important role in simplifying the adsorption process of these compounds [5]. The type of mechanism that inhibitors applied was adsorption mechanism, these mechanism summarized in adhere the inhibitor molecule on the metal surface to form a protective barrier against corrosive agents in the environment. The adsorbed molecule of inhibitors can affect the corrosion reaction either by physically blocking the active sites presents on the metal surface, or by changing the activation barriers of the anodic and cathodic partial reactions of the corrosion process, or may be both [6]. The study of inhibition efficiency also involved quantum calculations to support the experimental results. In density function theory (DFT) the researcher has to test huge number of organic compounds to select a potential corrosion inhibitor. This type of search costs money, effort and time. However, the development of hardware and software technology in this field makes it capable to choose a good corrosion inhibitor from organic compounds with reduced cost, effort and time [7].
The aim of this work is to study the inhibition efficiency of two organic compounds of (IMID4) and (OXAZ5) [8], see Figure-1. The inhibition of (IMID4) and (OXAZ5) were studied experimentally in saline solution of 3.5% NaCl using potentiostatic polarization meaurements, and theoretically, using DFT method of (6-311/ B3LYP++G (2d, 2p)) level using Gaussian 09 program. These calculations were done for obtaining the parameters of the corrosion efficiency in three media (vacuum, DMSO, and water). (IMID4 and OXAZ5 were also recently proved to have a wide range of biological and pharmacological activities, anti-tumor and anti oxidant activities [8]).

. Preparation of carbon steel samples
Carbon steel's rod was 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) [9]. The rod mechanically cutting into pieces forming a cyclic specimen of carbon steel 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) grades, then washed with tap water, distilled water and degreased with acetone, washed again with distilled water, and finally held in a desiccators after it is dried in room temperature.

Preparation of solutions 2.2.1. Salt blank solution
35 gm of sodium chloride (NaCl) was dissolved in (100 ml) distilled water; then transferred into 1liter volumetric flask which contained 6ml of dimethyl sulfoxide (DMSO) solvent. The solution volume was then completed to (1L) with distilled water. We used 3.5% NaCl in this study in order to avoid some problems related to ohmic drop.

Electrochemical measurements Potentiostatic polarization study
The potentiostat set up has included the following: a host computer with Mat lab software, magnetic stirrer, thermostat, potentiostat and galvanostat (Germany, 2000), The main part in 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 which is represented the working electrode. This is used to determine the working electrode potential due to another electrode namely as reference electrode; located close to working electrode. A reference electrode was silver-silver chloride (Ag/AgCl, 3.0M KCl). The third electrode is a platinum auxiliary electrode with (10cm) length. The starting step was represented in immersing the working electrode in the test solution for fifteen minutes (15 min), 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 solution were done at temperatures of (293, 303, 313 and 323) K.

Results and discussion Quantum chemical calculations
The quantum electronic parameters are used to investigate the efficiency of corrosion inhibition such as: the highest occupied molecular orbital (E HOMO ), the energy of the lowest unoccupied molecular orbital (E LUMO ), the energy gap (ΔE HOMO-LUMO ), electro-negativity (χ), dipole moment (μ), electron affinity (EA), ionization energy (IP), softness (S), global hardness (η), global electro-philicity (ω), the fraction of transferred electrons (ΔN) and the total energy (E tot ) [10].

Molecular structures calculations
The organic inhibitors compounds were built using Chem. Draw of Mopac program, see Figure-1. Gaussian 09 packages were used for calculating the fully optimize structure [8], see Figure-2, using quantum mechanical method of DFT of Becke's three-parameter of Lee, Yang and Parr (B3LYP) with 6-311++G (2d, 2p) level of the theory [11]. In addition to vacuum, the equilibrium geometry was calculated in two solvents of DMSO and H 2 O.   Figure-4 shows the geometrical optimization of the studied inhibitors in vacuum including HOMO and LUMO density distributions. For (IMID4) inhibitor the HOMO is mainly located on (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-nitrophenyl) moiety (which is the most planar moiety in the molecule). For (OXAZ5) inhibitor, the HOMO is mainly located on the imidazo[1,2-a] pyridine-3-yl]-3-(4-methoxy-phenyl)-oxazolidin-5-one moiety, and LUMO is located on the imidazo[1,2-a] pyridine-3-yl] moiety only.

Global molecular reactivity
To study the influence of molecular geometry on the mechanism and efficiency of inhibition, the chemical quantum calculations for inhibition efficiency were performed. The quantum chemical parameters, such as: the energy of the highest occupied molecular orbital (E HOMO ), the energy of the lowest unoccupied molecular orbital (E LUMO ), energy gap (ΔE HOMO-LUMO ), the dipole moment (μ), hardness (η), electro-negativity (χ), global softness (S), global electro-philicity index ( ) and electron transferred (ΔN), are all shown in Tables-(3a, 3b), (4a, 4b) for (IMID4) inhibitor and (OXAZ5) inhibitor, respectively.
Frontier orbital theory was used in predicting the adsorption centers of the inhibitor responsible of the reaction metal surface/molecule [12]. According to this theory, the formation of a transition state is due to an interaction between the Frontier orbital's (HOMO and LUMO) of the reactants. Parr et al. has introduced the index of global electro-philicity (ω) which is also related to the electron affinity (EA) and ionization potential (I.P) [4]. When interaction occurs between the inhibitor and the metal surface, flow of electrons takes place from the lower electronegativity molecule to the higher electronegativity metal, this transfer of electrons continues until the chemical potential becomes equal [4]. The electron transferred (ΔN) was calculated using theoretical (χ Fe ) and (η Fe ) values for mild steel of (7.0eV mol -1 ) and (0.0eV mol -1 ). The following Equations (1)(2)(3)(4)(5)(6)(7)(8)

The active sites of the two inhibitors:
The inhibition of the studied inhibitors was determined via DFT Mulliken charges population analysis; which gave an indication of the reactive centers of the molecules (electrophilic 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 by 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 present. The electrophilic attack site was controlled by the negative charge value.
The nucleophilic and electrophilic electronic charge values of the two inhibitors are found to be greater in the solutions of DMSO and H 2 O than in vacuum, this is shown in Tables-(5, and 6) for IMID4 and OXAZ5, respectively.
The orders of the nucleophihic reactive sites of IMID4 inhibitor were found to be as: C14 C12 C15 C1 C2 N9 N20 C8 C7 and the electrophihic reactive sites order were found to be as: C5 C13 C16 C10 C22 C19 For OXAZ5 inhibitor, the orders of the nucleophihic reactive sites of OXAZ5 inhibitor were found to be as: C24 O27 O20 C14 C1 N9 C12 C7 C2 and the electrophihic reactive sites order are: C5 C19 C16.

Corrosion inhibition measurement Potentiodynamic Polarization Measurements.
Tables- (7, and 8) show the electrochemical corrosion parameters, such as: corrosion current density (I corr. ), corrosion potential (E corr. ) and Tafel slopes (ba and/or bc) for the two inhibitors [14]. Figure-3 presents potentiodynamic polarization curves for carbon steel in salt media containing different conditions of the two inhibitors. Corrosion efficiency (IE%) and the surface coverge (Ө) were measured using Equations (9, 10): Where I corr (in) is the inhibited corrosion current densities, I corr (un) is the uninhibited current densities.

Ө
The addition of the two derivatives cause a reduce in the corrosion rate (CR), i.e. shifts the cathodic and anodic curves to lower values of (I corr. ), and both cathodic and anodic reactions of carbon steel electrode corrosion inhibited by the two organic compounds in saline media. Figure-5 shows the polarisation curve for the corrosion of carbon steel in the salt solution, with and without the addition of IMID4 inhibitor at various concentrations, and at the optimum conditions of (20ppm) with temperature of (293K). Figure-6 shows the polarisation curve for the corrosion of carbon steel in the salt solution with and without the addition of OXAZ5 inhibitor at various concentrations, and with the optimum conditions of (20ppm) and at temperature of (293K).
Tables- (7, and 8) show that the increase in temperature led to increase (I corr. ), while the efficiencies IE% enhance with the increase the inhibitor concentration. The optimum conditions for IMID4 in the salt solution were observed at (293K and 20ppm); which corresponded to the lowest I corr. (14.52 μA.cm -2 ) and maximum IE% (89.093%, while OXAZ5 inhibitor have the optimum conditions at 293K and 20ppm too, this corresponded to lowest I corr. (25.33 μA.cm -2 ) and maximum IE% (80.973 %). The values of iron CR have decreased with increasing the concentration of the inhibitors and the addition of inhibitor to the blank solutions have led to increase the cathodic and anodic I corr without shifting the E corr . So, the two inhibitors can be described as mixed-type inhibitors. Inhibition occurred by adsorption and the inhibition effect results from the reduction corrosion reaction on the carbon steel surface area [15]. Table 7-Electrochemical data of C.S corrosion in sea water at different concentrations for IMID4 compound.

Kinetic and thermodynamic activation parameters for corrosion processes
Figures- (7, and 9) shown the straight line plots between log I corr and 1/T for (IMID4) and (OXAZ5) inhibitors respectively. The Arrhenius law is presented as a straight line of the logarithm of the corrosion rate. The activation parameters were calculated with and without inhibitors at different concentrations. The activation energy of the corrosion process (E a ), and the pre-exponential factor (A), were calculating from Equations 11. A plot of log (CR/ T) against (1/T) or log (I corr /T) against (1/T), Equation (12) gave a linear relationship with a slope of (−ΔH*/ 2.303R) and an intercept of [log(R/ Nh)+ (ΔS*/ 2.303R)]. This is shown in Figures-(8, and 10) for (IMID4) and (OXAZ5) inhibitors, respectively.
Log (Icorr) = Log A -Ea/ 2.303RT (11) Log (Icorr/ T)= log (CR/ T) = Log (R/ N h) + ΔS*/ 2.303R -ΔH*/ 2.303RT (12) Where (I corr ) is the corrosion current density which is equal to the corrosion rate (CR), (R) is the universal gas constant (8.314 J mol -1 K -1 ), (T) is the absolute temperature in K, (h) is Planck's constant (6.626 x 10 -34 J s), (N) is Avogadro's number (6.022 x 10 23 mol -1 ), ΔH* is the enthalpy of activation and ΔS*, is the entropy of activation. Accordingly, the activation thermodynamic parameters (ΔH* and ΔS*) were calculated for (IMID4) and (OXAZ5) inhibitors, respectively, as shown in Tables-(9, and 10). (ΔH*) values for the corrosion reaction in 3.5% NaCl at the temperature range of (293-323) K and different concentration were found to be positive values for both inhibitors; which may give an indication of an endothermic nature for this reaction [16]. Negative values of (ΔS*) for the corrosion reaction indicate a decrease in the degree of freedom and a consequent inhibition action [17]. The values of ΔG* for corrosion reaction were calculated from equation 13. The positive values of ΔG* indicating that the transition state of the adsorption process is not spontaneous, (see Tables-9, 10). ΔG* = ΔH*-T ΔS* (13) Table 9-Corrosion kinetic parameters for carbon steel in sea water (3.5% NaCl) for blank with various concentrations of (IMID4) inhibitor.

Adsorption isotherm
Adsorption isotherms are necessary to elucidate the corrosion inhibition mechanism since they express the interaction between the inhibitor molecules and the active sites on the carbon steel surface. In this study, the results were based on the Langmuir isotherm; see Figures- (12,14) for (IMID4) inhibitor and (OXAZ5) inhibitor, respectively. Langmuir adsorption isotherm can be expressed by Equation (14) as follows [18]: C/ Ө = (1/ K ads ) + C… (14) Whereas C is the inhibitor concentration in 3.5% NaCl and K ads is the adsorption/ desorption equilibrium constant. A plot of C/θ versus C in the salt media, could be used to determine the equilibrium adsorption constant K ads . Further the standard free energy change ΔG°a ds values for the adsorption are calculated using Equation 15.
K ads = (1/55.55) exp (-ΔG°a ds / RT) (15) Where K is the equilibrium constant, R is the universal gas constant and T is the absolute temperature and 55.5 is the concentration of water in solution in mol/dm 3 . The negative values of ΔG°a ds ensured the spontaneity of the adsorption process and stability of the adsorbed layer on the carbon steel surface. The enthalpy and entropy for the adsorption of (IMID4 and (OXAZ5) on mild steel are deduced by using the thermodynamic Equation of (16).
The negative values of ΔG°a ds reflect the spontaneous adsorption. Generally, the values of ΔG°a ds around -20 kJ mol -1 or more positive are consistent with physisorption, while those around -40 kJ mol -1 or more negative with chemisorptions [19]. The calculated values of ΔG°a ds were found to be in the range of (-13.640 to -8.221kJ mol -1 ) and (-11.575 to -11.155kJ mol -1 ) at temperatures of (293 to 323K) for (IMID4 and OXAZ5) inhibitors respectively. The entropy ΔS°a ds value was positive confirming that the increase in disordering takes place on going from the reactant to the adsorbed species [15]. The negative sign of ΔH°a ds in salt media indicated that the adsorption of inhibitor molecules is an exothermic and physisorption process. For IMID4 inhibitor, ΔHº ads was found to be (-68.393kJ mol -1 ), and it was found to be (-5.692kJ mol -1 ) for (OXAZ5).       Figures-(15a, 16a), show the damaged surface of carbon steel obtained when the metal was remained immersed in saline water without (IMID4) and (OXAZ5) inhibitors, respectively. However, Figures-(15b, 16b) show the smoothness and regularity on the surface of carbon steel in the presence of (IMID4) and (OXAZ5) inhibitors, respectively, in saline water when compared to Figures-(15a, 16a); which indicated the reduction of the surface corrosion. This improvement in the surface morphology is due to the formation of protective films of inhibitors (IMID4) and (OXAZ5) on the carbon steel surface, and hence, indicated the inhibition of the corrosion [20]. a b Figure 15-SEM images of C.S in a 3.5% NaCl saline solution at 293K (a) without (IMID4), (b) In the presence of 20 ppm of the organic compound (IMID4). a b Figure 16-SEM images of carbon steel in a 3.5% NaCl saline solution at 293K (a); without (OXAZ5) and (b); In the presence of 20 ppm of the organic compound of (OXAZ5).

Atomic Force Microscopy (AFM)
The surface morphology of carbon steel samples in a saline solution of 3.5% NaCl in absence and presence of the optimum concentration (20 ppm) of the two inhibitors were investigated by AFM. The results are shown in Figures-(17(a-f)). The average roughness is shown in Figures-(17(a, b))), indicated that the C.S samples surface is badly damaged due to 3.5% NaCl salt attack. The average roughness (S a ) for the carbon steel surface is 3.97nm in salt solution without the presence of any inhibitor [21]. (S a ) was reduced to 2.53nm in the presence of the optimum concentration (20 ppm) of compound (IMID4) Figures-(17 (c,d)), and it was reduced to 2.26nm in the presence of the optimum concentration (20 ppm) of (OXAZ5) inhibitor Figures-(17 (e,f)).