Comparative Study of Corrosion Inhibition Effect for Ordinary Steel in HCl 5.0 M

Abstract

In the present work the influence of the two organic inhibitors, namely : phenyl (2- (3,4,5 trimethoxyphenyl) -1H-benzo [d] imidazol-1-yl) methanone (P3) and ( 2- (4 -methoxyphenyl) -1H-benzo [d] imidazol-1-yl) (phenyl) methanone (P4) on the ordinary steel corrosion in 5.0 M HCl solution was investigated by using electrochemical measurements, scanning electron spectroscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDX), UV-visible spectroscopy and theoretical study. The obtained results showed that these compounds act as mixed inhibitors. It is obtained that the inhibition efficiency increases with their concentrations and reaches 97.0% and 91.3% at 10⁻³ M of P3 and P4, respectively. These findings were confirmed by the electrochemical impedance spectroscopy measurements, where the transfer resistance values increase with the concentrations. In addition, the effect of temperature range from 298 K to 328 K was investigated and indicated that P3 and P4 keep their performance at height temperature. Moreover, density functional theory explained the relationship between the molecular structure of imidazole derivatives and inhibition performance obtained experimentally. Additionally, the morphological surface and EDX analyses indicated that these compounds act by the formation of a protective layer on the ordinary steel surface. In order to confirm the possibility of the formation of P3-Fe and P4-Fe complexes, the UV–Visible absorption spectra was used. The obtained results indicated the formation of a complex between Fe²⁺ and molecules inhibitors.

Keywords:
Imidazole derivatives; Ordinary steel; Corrosion inhibition; 5 M HCl; DFT calculations; SEM/EDX analyse; UV–visible spectroscopy

1. Introduction

The importance of protection of iron materials against corrosion in acidic solutions has been increased by the fact that they are more susceptible to be attacked in aggressive industrial environments. Thus, the corrosion is a major industrial problem (estimated at 3.4% of world gross product in 2013), which cover all the means of combating corrosion, the replacement of corroded parts or structures and the direct and indirect consequences of accidents due to corrosion¹. In addition, hydrochloric acid, for example, was often used as a pickling acid for iron and its alloys to remove undesirable corrosion products². This acid will cause metal corrosion upon the already cleaned surface after the corrosion products elimination. So, in order to reduce the undesirable destructive effect and prevent metal dissolution, the corrosion inhibitors were used³. The inhibition efficiency of the inhibitors is largely depending on their adsorption on the metal surface³. Most well-known inhibitors are organic compounds containing hetero-atoms such as nitrogen, sulphur, and/or oxygen atoms^4,5. In addition, many N-heterocyclic compounds have been demonstrated as effective inhibitors against metals corrosion in acidic solutions^6-8. The use of these organic compounds as inhibitors is one of the most used methods for protecting materials from corrosion^9,10. Recently, many researches have reoriented their attention to the development of new organic corrosion inhibitors such as imidazopyridine¹¹, imidazole^12,13, triazepine carboxylate¹⁴, and tetrazole^15,16. So, the majority of these organic compounds act via adsorption on the metal surface; where their adsorption depend mainly on certain physicochemical properties of the inhibitor molecule such as functional groups, steric factors, aromaticity, electron density at the donor atoms and π orbital character of donating electrons^17,18, the electronic molecular structure^19,20, and so on.

In this study, the inhibition efficiency effect of two new imidazole derivatives, namely, phenyl (2- (3,4,5-trimethoxyphenyl) -1H-benzo [d] imidazol-1-yl) methanone called (P3) and ( 2- (4 -methoxyphenyl) -1H-benzo [d] imidazol-1-yl) (phenyl) methanone called (P4) on ordinary steel in 5 M hydrochloric acid solution was investigated by using electrochemical measurements, theoretical study, SEM/EDX analyses and UV-Vis spectroscopy. Also, the adsorption behaviour of these series and the effect of temperature solution on their performance were examined.

2. Experimental Procedure

2.1. Synthesis of inhibitors

The 0.01 mol of O-phenylenediamine and 0.01 mol of benzoic acid were heated in a sealed tube with 10 mL of 20% hydrochloric acid for 4 hours at 418-423 K. After this, the neutralization of the reaction mixture gave a small amount, which 6% raw product recrystallization from aqueous ethanol to give colorless products. Their molecular structures are presented in Figure 1.

Figure 1
Molecular structure of phenyl (2-(3,4,5-trimethoxyphenyl)-1H-benzo[d]imidazol-1-yl) methanone (P3) and (2-(4-methoxyphenyl)-1H-benzo[d]imidazol-1-yl)( phenyl) methanone (P4).

2.2. Materials

The corrosion tests were performed on ordinary steel; where its chemical composition is presented in Table 1 (the rest is ordinary steel balance).

Table 1
Chemical composition of ordinary steel sample.

Elements	C	Si	Mn	Cr	Mo	Ni	Al	Cu	Co	V	W
Percentage (wt. %)	0.11	0.24	0.47	0.12	0.02	0.10	0.03	0.14	< 0.0012	< 0.003	0.06

For the electrochemical measurements and surface analyses, the used ordinary steel specimens were prepared by polishing with emery paper at different grit sizes (from 60 to 1200), rinsing with distilled water, degreasing in ethanol, and drying at hot air. The aggressive solution of 5.0 M HCl was prepared by dilution of analytical grade 35% HCl with distilled water.

2.3. Electrochemical cell and methods

2.3.1. Potentiodynamic polarization measurement

For this study, the working electrode was immersed in corrosive solution during 30 minutes to obtain a steady state of open circuit potential (Eocp). After this, the potentiodynamic polarization curve was recorded by polarization from the negative direction, under potentiodynamic conditions corresponding to 1 mV/s (sweep rate) and under air atmosphere, to positive direction using a VoltaLab PGZ 100, which was controlled by a personal computer.

The corrosion kinetic parameters were determined by a fitting using Stern -Geary equation²¹; and the inhibition efficiency was calculated by the relationship:

$${\eta }_{\text{PP}}=\frac{i_{\text{corr}}^{\text{0}}-i_{\text{corr}}}{i_{\text{corr}}^{\text{0}}}\times 100$$ (1)

Where i°_corr and i_corr are the corrosion current densities values without and with inhibitor, respectively.

2.3.2. Electrochemical impedance spectroscopy measurements (EIS)

The electrochemical impedance spectroscopy measurements were carried out using a transfer function analyzer (VoltaLab PGZ 100), with a small amplitude ac. Signal (10 mV.rms), over a frequency domain from 100 KHz to 10 mHz at 298 K and an air atmosphere. The results were then analyzed in terms of equivalent electrical circuit using Z-View software like as previously²²; and the inhibition efficiency was determined using the relationship:

$${\eta }_{\text{EIS}}=\frac{R_{\text{ct}}-R_{\text{ct}}^{\text{0}}}{R_{\text{ct}}}\times 100$$ (2)

Where R°_ct and R_ct are the transfer resistance values in the absence and the presence of inhibitor, respectively.

2.4. Quantum chemical calculations

In order to establish the relationship between the molecular structure and the obtained inhibition efficiency and to highlight the descriptors which are the best to describe these corrosion inhibition character, the quantum study have been adopted by the DFT (B3LYP) method using the 6-31G basis (d, p), where the calculation is done using a Gaussian software 09W^23-25. The geometric optimization of P3 and P4 was carried out in the gaseous phase.

The following equations express the various molecular quantum parameters such as electronegativity (χ), electronic affinity (A), ionization potential (I) and gap energy (ΔE_gap)^26-29:

$\Delta E_{gap}=E_{LUMO}-E_{HOMO}$ (3)

$I=-E_{HOMO}$ (4)

$A=-E_{LUMO}$ (5)

$$\chi =\frac{I+A}{2}$$ (6)

Where E_HOMO is the energy of the highest occupied molecular orbital and E_LUMO is the energy of the lowest vacant molecular orbital.

However, the overall hardness (η) and the chemical softness (σ) are given as follows³⁰:

$$\eta =\frac{\Delta E_{gap}}{2}$$ (7)

$$\sigma =\frac{1}{\eta }$$ (8)

The global electrophile index (ω) was introduced by Parr³¹ and is given by:

$$\omega =\frac{{\mu }^2}{2\eta }$$ (9)

Where μ is the electronic chemical potential, and obtained as follows: $\mu =-{\chi }^2$_. This index measures the propensity of chemical species to accept electrons. Thus, a good more reactive nucleophile was characterized by a lower value of μ and ω.

On the other hand, the transferred electrons number (ΔN) was calculated as follows³²:

$$\Delta N=\frac{{\chi }_{Fe}-{\chi }_{inh}}{2\left({\eta }_{Fe}+{\eta }_{inh}\right)}$$ (10)

Where the χ_Fe = 7.0 eV and η_Fe = 0 eV represent respectively the absolute electronegativity of iron and the molecule inhibitor³³. The η_Fe and η_inh denote respectively the absolute hardness of iron and the inhibitor molecule.

2.5. UV-visible spectrophotometer

In order to characterize the free solution and inhibited after six days of immersion at 298 K, the spectro-photometric method was carried out in 10^-3 M of each inhibitor with 5.0 M HCl solution without and with ordinary steel sample by using a Beckman DU640 UV/Vis spectrophotometer.

2.6. Surface analysis

The morphology and the chemical composition the ordinary steel surface were obtained by the scanning electron microscopy (SEM) instrument coupled with Energy Dispersive Spectroscopy (EDX) (SEM, JOEL JSM-5500) at 10 keV with magnification (800×) after 24 hours of immersion in 5.0 M HCl solution at 298 K without and with 10^-3 M of each inhibitor.

3. Results and Discussion

3.1. Potentiodynamic polarization measurement

Figure 2 shows the Tafel polarization curves for ordinary steel in 5.0 M HCl in the absence and presence of different concentrations of P3 or P4 at 298 K. Their extracted electrochemical parameters, such as the corrosion potential E_corr, the cathodic and anodic Tafel slopes ß_c and ß_a, the corrosion current density i_corr, and the inhibition efficiency values η_pp (%) are determined from polarization curves and presented in Table 2. It is clear that i_corr values decrease sharply with the presence of P3 or P4. Correspondingly, the η_pp(%) values increase with inhibitor concentration to reach a maximum of 97.0% and 91.7% at 10^-3 M for P3 and P4, respectively. These values suggested that two compounds act as effective inhibitors for ordinary steel corrosion in 5.0 M HCl solution. In addition, it is noted that these compounds act as mixed type inhibitors. On the other hand, it is remarked that the cathodic Tafel slopes generally unchanged with an increase of P3 and P4 concentrations; indicating that the inhibitors addition does not alter the hydrogen reaction mechanism³⁴. For the same, the obtained anodic Tafel slopes indicated that the P3 and P4 addition changes the anodic dissolution of ordinary steel.

Figure 2
Tafel polarization curves for ordinary mild steel obtained in 5.0 M HCl solution containing different concentrations at 298 K of (a) P3 or (b) P4.

Table 2
Electrochemical parameters and inhibition efficiency values for ordinary steel in 5.0 M HCl solution without and with P3 or P4 addition at different concentrations.

Compounds	C	Ecorr mV/SCE	icorr	βχ	βα	Ɵ	η PP
	M		µA cm-2	mV dec-1	mV dec-1		%
5.0 M HCl	0	-494	4429	-202	92	-	-
P 3	10-3	-472	130	-169	127	0.97	97.0
	10-4	-516	520	-174	117	0.882	88.2
	10-5	-449	1578	-180	118	0.643	64.3
	10-6	-483	2731	-188	111	0.383	38.3
P 4	10-3	-625	385	-159	120	0.913	91.3
	10-4	-450	671	-177	114	0.848	84.8
	10-5	-580	685	-139	115	0.845	84.5
	10-6	-525	1277	-155	124	0.711	71.1

3.2. Electrochemical impedance spectroscopy

The obtained Nyquist diagrams for ordinary steel in uninhibited and inhibited 5.0 M HCl solution are shown in Figure 3. The electrochemical parameters derived from these diagrams and the calculated inhibition efficiency values are presented in Table 3. It is noted that all impedance spectra have a single capacitive loop, which suggests that the ordinary steel corrosion process in 5.0 M HCl with and without inhibitors is mainly controlled by a charge transfer process³⁵. Moreover, these diagrams are mainly similar for all tested concentrations, indicating that there is virtually no change in the corrosion mechanism³⁶. In addition, these Nyquist diagrams are not perfect semicircles, which can be attributed to the frequency dispersion of the interfacial impedance³⁷, and/or due to the surface roughness, the chemical heterogeneity of the surface, and the adsorption-desorption process of molecules inhibitors on ordinary steel surface³⁸. On the other hand, it is observed that the semicircles diameter increase with the presence of P3 or P4, which may be related to the increase in surface coverage of the ordinary steel by the molecules inhibitors. In order to extract the EIS data the proposed equivalent circuit shown in Figure 4 was used. Thus, the CPE element was used to explain the depression of the capacitance semicircle, corresponding to many proprieties like the above. This element is represented by the following expression^39,40:

Figure 3
Nyquist impedance diagrams for ordinary mild steel in 5.0 M HCl solution at 298 K containing different concentrations of (a) P3 or (b) P4.

Table 3
Electrochemical impedance parameters and inhibition efficiency values for ordinary steel in 5.0 M HCl solution without and with P3 or P4 addition at different concentrations.

	C	Rs	Rct	Qdl	ndl	Cdl	η EIS
	M	Ω cm2	Ω cm2	µF Sn-1		µFcm-2	%
5.0 M HCl	00	0.55	5	359	0.858	127	-
P3	10-3	0.88	173.4	60.5	0.852	27.5	97.1
	10-4	1.32	41.17	149.5	0.841	47.02	87.8
	10-5	0.87	14.47	215.4	0.834	84.38	65.4
	10-6	1.13	8.10	218.1	0.901	109.1	38.3
P4	10-3	0.87	57	104	0.892	52.8	91.2
	10-4	0.95	32.7	137	0.885	73.2	84.7
	10-5	0.92	32.2	219	0.862	99.6	84.4
	10-6	0.87	16.9	360	0.845	142	70.4

Figure 4
Proposal electrical equivalent circuit used for metal/solution interface.

$Z_{CPE}=\frac{1}{Q{(jw)}^{ndl}}$ (11)

3.3. Effect of solution temperature

To calculate the activation and thermodynamic parameters of corrosion process and study the mechanism action of inhibitors, the effect of solution temperature on the inhibition efficiency of P3 and P4 for ordinary steel 5.0 M HCl was studied at the temperature range from 298 K to 328 K. The obtained potentiodynamic polarization curves for ordinary steel in 5.0 M HCl in the absence and the presence of 10^-3 M of P3 or P4, are presented in Figures 5 and 6. Their extracted electrochemical parameters and the calculated inhibition efficiency are illustrated in Table 4. It is clear that the current density values increase and the η_pp % inhibition efficiency values decrease slightly with the increase of the temperature solution. These finding were attributed to the increase of the kinetic energy and/or the decrease in the adsorption capacity of the organic compounds at high temperature⁴¹.

Figure 5
Potentiodynamic polarization curves for ordinary mild steel 5.0 M HCl solution in the absence of inhibitors at different temperatures range.

Figure 6
Potentiodynamic polarization curves for ordinary mild steel in 5.0 M HCl with the presence of 10-3 M of (a) P3 or (b) P4 at different temperatures range.

Table 4
Electrochemical parameters and inhibition efficiency values obtained for ordinary steel in 5.0 M HCl without and with 10^-3 M of P3 and P4.

	T	Ecorr mV/SCE	icorr µA/cm2	βc	βa	ηPP
	K			mV/dec	mV/dec	%
Blank solution	298	-494	4429	-202	92	-
	308	-499	6213	-189	99	-
	318	-497	9179	-175	122	-
	328	-496	16923	-156	132	-
10-3 M of P3	298	-472	130	-169	127	97.0
	308	-477	325	-185	117	94.7
	318	-458	764	-188	111	91.6
	328	-428	2016	-194	105	88.0
10-3 M of P4	298	-625	385	-159	120	91.3
	308	-464	675	-155	116	89.1
	318	-460	1198	-158	124	86.9
	328	-434	2543	-159	127	84.9

However, the activation and transition parameters of the corrosion process were calculated from an Arrhenius and transition curves according to the following equations⁴²:

$i_{corr}=A\exp \left(-\frac{E_a}{RT}\right)$ (12)

$i_{corr}=\frac{RT}{Nh}\exp \left(\frac{\Delta S_a}{R}\right)\exp \left(\frac{\Delta H_a}{RT}\right)$ (13)

Where E_a is the apparent activation energy, A is the Arrhenius factor, h is the plank constant, N is the Avogadro number, ΔH_a and ΔS_a are the enthalpy and entropy energies, and R is the gas constant.

On the other hand, the apparent activation energy was determined from the Ln(i_corr) = f(1/T) curves (Figure 7), and the ΔH_a and ΔS_a values were determined from Ln(i_corr/T) = f(1/T) (Figure 8). These parameters were listed in Table 5. It is observed that all the obtained values in the presence of P3 or P4 are higher than their absence. In fact, the value of ΔH_a for the dissolution reaction of ordinary steel in 5.0 M HCl in the presence of P3 is higher than in the presence of P4 and in the blank solution. These results reflect the endothermic nature of the ordinary steel dissolution process, which also due to the positive sign of enthalpy^43,44.

Figure 7
Arrhenius plots of ordinary mild steel in 5.0 M HCl without and with 10-3 M of (a) P3 or (b) P4.

Figure 8
Transition Arrhenius plots of ordinary mild steel in 5.0 M HCl without and with 10-3 M of (a) P3 or (b) P4.

Table 5
Values of activation and thermodynamic parameters for ordinary steel in 5.0 M HCl without and with 10^-3 M of P3 or P4.

	Ea (KJ/mol)	ΔHa (KJ/mol)	ΔSa (KJ/mol K)
Blank solution	35.68	33.09	-64.64
10-3 M of P3	73.70	71.10	33.91
10-3 M of P4	50.55	47.95	-34.94

However, a higher energy barrier for the corrosion process in the presence of inhibitors was associated with physical adsorption or low chemical bond between the inhibitors molecules and the ordinary steel surface^45,46, whereas unchanged or lower activation energy in the presence of inhibitors suggests chemisorption⁴⁷. So, in this study, it is found that the E_a value increases with the presence of inhibitors, which can be interpreted as physical adsorption. In addition, this increase can be attributed to the decrease of the inhibitor molecules adsorption on the metallic surface with the rise of temperature⁴⁸. In this context, the increase in temperature led to an increase in the electron density of the adsorption centers, which improved the inhibition efficiency⁴⁹.

Beyond that, Table 5 showed that the ΔS_a value in the presence of inhibitors is lower compared to blank solution. This phenomenon suggested that a decrease in randomness occurred between the transitions of reagents to the activated complex. Additionally, the obtained negative values of ΔS_a for P4 indicated that the activated complex in the velocity determination step is an association rather than dissociation step, which means that a decrease in disorder occurs, when substituting reactants to the activated complex^50,51.

3.4. Adsorption isotherm

The adsorption isotherm describing the adsorption behaviour of organic inhibitors is important for knowing the corrosion inhibition mechanism. They can provide basic information about the interactions between inhibitor molecules and the metal surface. Thus, several adsorption isotherms have been tested to adapt the surface coverage degree (θ) to adsorption isotherms, including Frumkin, Temkin, Freundlich, and Langmuir isotherms⁵². It is found that the plot C_inh/θ against C_inh (Figure 9) gives a straight line with a correlation coefficient of 0.999 for both inhibitors, providing that their adsorption on the ordinary steel surface obeys the Langmiur adsorption isotherm. This isotherm can be represented by⁵³:

$$\frac{C_{inh}}{\theta }=\frac{1}{K_{ads}}+C_{inh}$$ (14)

Where C_inh is the concentration of the used inhibitor (mol L^-1) and K_ads is the equilibrium constant for the adsorption and the desorption process, which is linked to the free energy of the adsorption (ΔG_ads)⁵⁴:

$$K_{ads}=\frac{1}{55.55}\exp \left(\frac{-\Delta G_{ads}^{}}{RT}\right)$$ (15)

Where R is the constant of universal gas, 55.55 is the water concentration in solution (mol L^-1), and T is the absolute temperature (K).

Figure 9
Langmuir isotherm adsorption curves for (a) P3 and (b) P4.

It is found that the K_ads values are about 17.45 104 L/mol, and 32.09 104 L/mol for P3 and P4, respectively (Table 6). These high values reflect the high adsorption capacity of inhibitors on ordinary steel surface⁵⁵.

Table 6
Calculated parameters of the adsorption of P3 and P4 on ordinary steel surface.

Interface	R2	Kads (L/mol)	ΔGads(KJ/mol)
ordinary steel/5.0 M HCl/P3	0.99997	17.45 104	-39.85
ordinary steel/5.0 M HCl/P4	0.99997	32.09 104	-41.36

Additionally, it is obtained that the ΔG_ads values are - 39.85 kJ mol^-1 and - 41.36 kJ mol^-1 for P3 and P4, respectively. The negative values of these parameters indicate the spontaneity of the adsorption process and the stability of the adsorbed layer on the ordinary steel surface. In addition, it is well known that when the ΔG_ads value is about - 20 kJ mol^-1 or less indicates physisorption; those of the order of - 40 kJ mol^-1 or more involves chemisorption^56,57. On the other hand, the adsorption phenomenon of an organic molecule is not considered solely as a purely physical or chemical adsorption phenomenon^58,59. In our case, the obtained values of ΔG_ads may suggest chemisorption and physisorption modes for both compounds P3 and P4.

3.5. UV-visible spectroscopy

In order to confirm the possibility of formation of P3-Fe and P4-Fe complexes, the UV –Visible absorption analyses was used for ordinary steel in 5.0 M HCl solution containing 10^-3 M of P3 or P4 before and after 6 days of immersion. The obtained spectra are shown in Figure 10. The obtained figures show a difference between absorbance values and their intensities with the presence of inhibitors. It is noted that the absorbance increases after immersion of ordinary steel in the solution containing P3 or P4. This finding reveals the formation of a complex between Fe²⁺ ions and inhibitor molecules. The formation of this complex may be responsible for the observed deviation of absorbance and its intensity value⁶⁰. The electron absorption spectrum of P3 and P4 solutions prior to ordinary steel immersion has a main visible absorption band between 200 nm and 350 nm, with a corresponding absorbance of 3.0; and between 200 nm and 380 nm with an absorbance of 3.0. This band can be assigned to the π−π* transition, involving the entire electronic structure system of compounds with charge transfer character⁶¹. After six days of ordinary steel immersion, the absorption band (λ_max) of P3 and P4 underwent a red shift of 350-400 nm and 380-420 nm, suggesting an interaction between inhibitors molecules and Fe²⁺ ions in solution^62,63. These experimental findings are strong evidence of the possibility of the formation of iron-inhibitor complex. It should also be noted that there are no significant difference of the spectrum for P3 before and after immersion of ordinary steel, showing a possibility of weak interaction between it and P3 molecules (physisorption)⁶⁴, confirming the obtained value of free energy calculated from adsorption isotherm.

Figure 10
UV-visible spectra obtained for 5.0 M HCl solution containing 10-3 M of (a) P3 or (b) P4 before (Black) and after (Red) 6 days of immersion of ordinary mild steel.

3.6. SEM micrographs and EDX characterization

The SEM micrographs of the ordinary mild steel surface in 5.0 M HCl medium after 24 hours of immersion time and at 298 K in the absence and presence of 10-3 M of P3 or P4 were presented in Figure 11. The observation of the micrograph of ordinary mild steel in the acidic solution without inhibitors showed that the state surface was intensely damaged, due to quick corrosion degradation with the presence of the pitted areas (Figure 11a). This last form is typical to pitting corrosion⁶⁵. On the other hand, in the presence of 10-3 M of P3 (Figure 11b) or P4 (Figure 11c), it is observed that the surface is smooth and covered with the presence of the traces in the form of a plate, indicating the presence of the organic products. This remark showed that the corrosion protection is due to the formation of a deposit by the formation of a film of the molecule inhibitors on ordinary mild steel surface⁶⁶. In addition, it is observed that that the pitting areas disappear and the ordinary mild steel surface is almost free of corrosion products with the presence of P3 or P4.

Figure 11
SEM micrograph for ordinary mild steel surface after 24 h of immersion in 5.0 M HCl solution (a) blank solution, (b) with the presence of 10-3 M of P3 and (c) with the presence of 10-3 M of P4.

After studying the morphology of ordinary mild steel surface, its chemical composition was examined by EDX analyses. These EDX analyses at different zones reveal the presence of nitrogen, molybdenum, carbon, oxygen and chlorine atoms (Figure 12). For the blank solution, the presence of oxygen and iron atoms suggests the presence of corrosion products composed of iron hydroxide and/or oxide (Figure 12a). On the other hand, the presence of nitrogen and carbon testifies to the adsorption of inhibitors molecules on the ordinary mild steel surface. It is noted that the oxygen peak is much larger in the presence of P3 (Figure 12b) or P4 (Figure 12c), which is certainly due to their adsorption on the metallic surface. The presence of Cl atom suggested that the formed film by inhibitors blocks the attack by chloride ions, which accumulate on the film surface. In addition, in the construction of the inhibitory film, the presence of iron on the EDX spectrum can also be explained by the complex formation between the P3 or P4 molecules and the Fe2+ ions resulting from the iron dissolution.

Figure 12
EDX spectra for ordinary mild steel surface after 24 h of immersion in 5.0 M HCl solution (a) blank solution, (b) with the presence of 10-3 M of P3, and (c) with the presence of 10-3 M of P4.

3.7. Chemical quantum calculations

To be able to study the effect of the molecular structure on the inhibition efficiency of P3 and P4, quantum calculations have been made using the DFT. The optimized geometry, HOMO and LUMO orbitals for P3 and P4 in gaseous media, are shown in Figures 13 and 14. The quantum chemical parameters calculated at the B3LYP / 6-31 (d, p) level for the neutral and protonated forms of P3 and P4 compounds are illustrated in Table 7. In general, the increase of EHOMO value and the decrease of ELUMO value are related to the increase of the electron donation and acceptance capacity of the inhibitor to the vacant d-orbital of the metal and from the filled orbitals, respectively. In addition, the lower ΔE value designates the easier release of electrons, the stronger adsorption, the higher chemical activity, and kinetic stability^67,68. Thus, to become a good corrosion inhibitor, it is necessary to easily give electrons and easily accept them in vacant orbital of metal⁶⁹. It is found that the values of EHOMO follows the order: P4 > P3 for both neutral and protonated forms, suggesting the high inhibition efficiency observed for P4 due to its better potential to give electrons in its extreme molecular orbit an appropriate orbital in the metal atom compared to P3. In the same, the low ELUMO values tend to easily accept electrons⁷⁰. In fact, ELUMO for P3 is greater than P4 in protonated form, while it is almost equal in neutral form, suggesting that P3 is more willing to accept charges from metallic atomic orbitals. This parameter trend does not reflect the observed order of inhibition efficiency. In addition, the energy gap (∆E) is often used to characterize the chemical reactivity and kinetic stability of molecules⁷¹. The obtained trend of ∆E follows the order: P4 <P3, implying that P4 is the most reactive inhibitor.

Figure 13
Optimized structures of neutral and protonated forms of P3 and P4 calculated by DFT with B3LYP / 6-31g (d, p).

Figure 14
HOMO and LUMO boundary molecular orbitals for the neutral and protonated forms of P3 and P4.

Table 7
Quantum chemical parameters calculated at the B3LYP/6-31(d, p) level for the neutral and protonated forms of P3 and P4 compounds in gaseous and aqueous phases.

Parameters	Phases	Neutral forms		Protonated forms
		P3	P4	P3H+	P4H+
EHOMO (eV)	G	-5.7490	-5.5947	-6.0214	-9.2172
	A	-6.0318	-5.8100	-6.4824	-6.5327
ELUMO(eV)	G	-1.9132	-1.9081	-2.2033	-5.3204
	A	-2.0360	-2.0106	-2.6488	-2.6156
∆EeV)	G	3.8358	3.6866	3.8181	3.8967
	A	3.9958	3.7993	3.8336	3.9171
μ(D)	G	4.1935	3.7081	9.8922	4.8470
	A	5.0431	3.6683	9.9686	6.9952
χ(eV)	G	3.8311	3.7514	4.1124	7.2688
	A	4.0339	3.9103	4.5656	4.5742
η(eV)	G	1.9179	1.8433	1.9090	1.9483
	A	1.9979	1.8996	1.9168	1.9585
σ (eV-1)	G	0.5214	0.5424	0.5238	0.5132
	A	0.5005	0.5264	0.5216	0.51057
∆N	G	0.8261	0.8811	0.7562	-0.06899
	A	0.7423	0.8132	0.6350	0.61927

However, the overall chemical hardness (η) and flexibility (σ) have all been used as molecular descriptors of reactivity and selectivity⁷². In this concept, a hard molecule would have a large ΔE value, while a soft molecule would have a small ΔE value (based on Lewis's theory of acid and bases and Pearson's concept of hard and soft acids and bases). The order between structures based on σ values is P4 < P3 in protonated form and the inverse in the neutral form (adsorption usually occurs at the region of the molecule, where σ has the highest value)⁷³. Additionally, the electronegativity (χ) is often described as the negative of the potential of global electronic chemistry⁷⁴. The lowest χ value for P3H+ suggests that it tends to release its loosely bound electrons more easily than P4H+. This improves the best adsorption and inhibition activity⁷⁵. On the other hand, there is an opinion that a low dipole moment promotes high inhibition efficiency, and the accumulation of inhibitor molecules on the metal surface⁷⁶. However, no general trend of relationship exists between the dipole moments of P3 and P4 with the experimental inhibition efficency. So, even though P4 with the least dipole moment had the lowest inhibition efficiency, while P3 with significantly high inhibition efficiency was found to have a higher dipole moment. This observation is somewhat in agreement with the hypothesis that a high moment improves the dipole-dipole interactions between the inhibitor molecules and the charged metal surface; and thus promotes the performance inhibition^77,78.

4. Conclusion

In this study, two imidazole derivatives, namely : phenyl (2- (3,4,5 trimethoxyphenyl) -1H-benzo [d] imidazol-1-yl) methanone (P3) and ( 2- (4 -methoxyphenyl) -1H-benzo [d] imidazol-1-yl) (phenyl) methanone (P4) on the ordinary steel corrosion in 5.0 M HCl solution was investigated by using electrochemical measurements, scanning electron spectroscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDX), UV-visible spectroscopy and theoretical study; and the principal important conclusions are:

• P3 and P4 compounds retard ordinary steel dissolution in 5.0 HCl solution, where their inhibition efficiency depends on the type of methoxy group in their structures; and therefore they improve the corrosion resistance of ordinary steel in a pickling solution with HCl.

• The potentiodynamic polarization curves indicated that all compounds react as mixed-type inhibitor, and their η_PP % depend on their concentrations, which confirmed by EIS results.

• The obtained results showed that P3 had better corrosion inhibition efficiency for ordinary steel in 5.0 M HCl than that of P4. This higher inhibition efficiency of P3 was attributed the great number of –OCH₃ groups (three methoxy groups) presented in P3 structure.

• The inhibition efficiency of P3 and P4 decreases slightly with temperature, and their adsorption on ordinary steel surface was spontaneous, and labeled by electrostatic interactions.

• The adsorption of P3 and P4 molecules on ordinary steel surface obeys to the Langmuir adsorption isotherm.

• SEM analysis of the surface indicated the formation of a protective layer on ordinary steel surface and the UV-visible spectroscopy of the solution demonstrated the formation inhibitor-complex in solution.

• DFT calculation studies indicated that the electrons of aromatic rings, oxygen, and nitrogen heteroatoms were the major adsorption centers for strong donor/acceptor interactions with the unoccupied d-orbital of ordinary steel surface.

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Publication Dates

• Publication in this collection
  03 June 2022
• Date of issue
  2022

History

• Received
  19 Oct 2021
• Reviewed
  10 Mar 2022
• Accepted
  31 Mar 2022