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Vol. 8. Issue 6.
Pages 6336-6353 (November - December 2019)
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Vol. 8. Issue 6.
Pages 6336-6353 (November - December 2019)
DOI: 10.1016/j.jmrt.2019.09.051
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Studies of Inhibition effect “E & Z’’ Configurations of hydrazine Derivatives on Mild Steel Surface in phosphoiric acid
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M.E. Belghitia,b,
Corresponding author
elbelghiti10@gmail.com

Corresponding author.
, M. Mihita, A. Mahsounec, A. Elmeloukyd, R. Mghaiouinie, A. Barhoumif, A. Dafalib, M. Bakasseg, M.A. El Mhammedih, M. Abdennourih
a Laboratory of Nernest Technology, 163 Willington Street, Sherbrook, J1H5C7, Quebec, Canada
b Laboratory of Applied Analytical Chemistry, Materials & Environment (URAC 18), Faculty of Sciences, University M1er, B.P. 4808, Oujda, Morocco
c Equipe of Molecular Modelling and Spectroscopy, Sciences Faculty, University of Chouai Doukkali, BP20, 24000 El Jadida, Morocco
d Laboratory Physics of Condensed Matter, University Chouaib Doukkali, El-Jadida, Morocco
e Department of chemistry, physical chemistry laboratory applied materials, Faculty of Sciences-Ben M’sik, Hassan II University, Morocco
f Laboratoire de Chimie Organique, Bioorganique et Environnement, Département de Chimie, Faculté des Sciences, Université Chouaïb Doukkali, BP 20, 24000 El Jadida, Morocco
g Laboratory of Organic Chemistry, Biorganic & Environment (LCOBE), Faculty of Sciences, Chouaib Doukkali University, PO Box 20, M-24000 El-Jadida, Morocco
h University Soultan Moulay Slimane, Laboratory of Chemistry, Modeling and Environmental Sciences, Polydisciplinary Faculty, Khouribga, Morocco
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Figures (18)
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Tables (11)
Table 1. Impedance parameters obtained by fitted using the equivalent circuit.
Table 2. Parameter obtained of conductivity.
Table 3. Gravimetric results of mild steel in (C=2M/H3PO4) without and with addition of HZi at 308 (±0.5) K.
Table 4. Adsorption parameters of HZi on mild steel in phosphoric acid (2M) at 308K.
Table 5. Temperature influence on the weight loss parameters for mild steel in (2M/H3PO4) without and with 10−3 M of HZi.
Table 6. Activation descriptors Ea, ΔHa and ΔSa of the dissolution of steel in (C=2.0M/H3PO4) at C=10-3M of HZi.
Table 7. Simulated and Experimental 1H&13C Chemical Shifts for the Trans-Form of HZi.
Table 8. A Comparrison of Theoretically computed XRD for Cis and Trans HZi with Experimental Data.
Table 9. Quantum reactivity descriptors for the geometric Cis and Trans-Form of HZi
Table 10. Energy parameters (Kj/mol) for (Fe(111)/ Z & E-(HZi)) complexes
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Abstract

Inhibition effect of ‘Cis and Trans’ conformations of three families of azines namely: [1.2-bis(pyrrole-2-ylidenemethyl) hydrazine (HZ1), 1.2-bis(thiophene-2-idenemethyl) hydrazine (HZ2) and 1,2-Bis(furyl-2-lidenemethyl) hydrazine (HZ3)] on mild steel corrosion in 2.0M H3PO4, were investigated through electrochemical impedance spectroscopy, Weight loss measurements and X-ray diffraction. A compact HZi inhibitor film was fabricated on the steel surface, and the film showed high inhibition efficiency, also, a reduction of the inhibition efficiency IEexp(%) as the solution temperature. The isomers of Cis and Trans- HZi were studied on the basis of their degree of planarity, their local and global electronic properties as well as their deformation capacity to adhere to the Fe-surface, using DFT and molecular dynamic simulations. A comparative study by standard deviation (SD) of Cis- and Trans-HZi with DFT method shows the higher correlation between X-ray diffraction, 1H &13C NMR Chemical Shifts and Trans geometric form. The adsorption behaviour of the both forms (Cis & Trans)-HZi onto the Fe (111) face were investigated by Molecular Dynamics simulations in vacuo to verify their anti-corrosive efficiency. The results indicate that the adsorption energies, deformation energies and rigid adsorption energies of Trans-HZi was greater than Cis-HZi, which agree with the trends of the experimental inhibition efficiencies.

Keywords:
Azines
Corrosion inhibition
Trans & Cis-forms
FT-IR
X-Ray
NMR
DFT
Molecular Dynamics simulations.
Full Text
1Introduction

The use of mild steel or carbon steel is foremost in several construction works, possibly due to its superb mechanical properties and comparatively cheaper cost. Like other metals, however, its degradation, when in contact with corrosion agents like acids, is a considerable issue for its extensive use [1]. For instance, essential processes like pickling, descaling and oil well acidizing employ acids such as hydrochloric (HCl), sulfuric (H2SO4) and phosphoric (H3PO4) acid mediums, with the disadvantage of corroding metal components [2]. NACE (National Association of Corrosion Engineers) reportedly projected the global cost of corrosion to be US$2.5 trillion to emphasis the cost management of the corrosion effects, not limited to environmental damage and safety, in global society. Meanwhile, proper implementation of corrosion control measures can save about $875 billion annually [1,3]. This can be well achieved with the use of effective corrosion inhibitors. Thus, research attention is geared towards both exploring existing materials and developing new materials that are capable of inhibiting metal corrosion in presence of acid.

The Family of heterocyclic (HZi) Azines is an important class with broad applications in organic synthesis [4]. The anti-corrosion efficiency of organic heterocyclic compounds containing different donor atoms facilitates the adsorption on the Fe-surface obeys the following order: -S- > =N-> -O- [5]. The Azines, (〉N−−N〈), have achieved great significance in organic synthesis of heterocyclic compounds [6,7]. The Azine bond (=N-N=) play an important role as a corrosion inhibitor in heating systems for the isolation, the water or metal-surface treatment, the strong reduction ability and pharmacological activity [8–12]. The inhibitory effects are reinforced by the existence of the X-heteroatoms. The Azines has formally a single (=N-N=) bond and can adopt nearly planar; Cis, Trans or twisted conformations, depending upon the co-ligands bonded to the Fe2+ and their preference for different coordination geometries [13]. Several theoretical analyzes have been carried out for the two isomeric forms of Hetero-cyclic Azines [14,15].

In resumption of previous works on the acid corrosion inhibitors [16,17], the family of azine compounds: [1,2-bis (pyrrole-2-ylidenemethyl) hydrazine (HZ1), 1,2-bis (thiophen) 2-idenemethyl) hydrazine (HZ2) and 1,2-bis (furyl-2-lidemethyl) hydrazine (HZ3) were studied as corrosion inhibitors in (C=2.0M/H3PO4). The choice of ortho-phosphoric acid is justified by its wide use in the industry for example ‘Phosphate coatings’, very few publications have been published on corrosion inhibitors for mild steel/H3PO4. In addition, no studies on the relationship between structural parameters (Cis & Trans) and anti-corrosion efficiency of these molecules have been reported.

In order to continue the works already realized on the classification of anti-corrosion efficiency via quantum modeling and Molecular Dynamics simulations [16,17], we propose a new family of Azines (HZ1, HZ2 &HZ3). Fig. 1 shows their respective molecular structures that were determined by spectroscopic analyses and confirmed by theoretical modeling. Thus, the present work focused on theoretical investigation of geometric Cis- & Trans- HZi on mild steel surface in ortho-phosphoric acid by analyzing the interaction between these inhibitors and crystal (111) face.

Fig. 1.

2D structures of the series of Azines compounds HZi (HZ1, HZ2 & HZ3).

(0.06MB).
2Computational methodology2.1Quantum chemical descriptors and parameters definition

DFT simulations were performed using Lee–Yang–Parr correlation functional (B3LYP) and 6-311++G(2d,2p) basis set by means of the Gaussian 09W package [18]. The structural geometry of each of the of the heterocyclic Azines was firstly minimized, followed by the vibrational frequency analysis, prior to other quantum chemical calculations whose outputs were used to estimate some useful derived parameters such as EA(electron affinity), IP (ionization potential), η (global hardness), χ (electronegativity), ω (global electrophilicity index), ΔN (quantity of electron transferred to Fe atom from inhibitor molecule), μ(Dipole moment) and TE(Total Energy) were used to explain the electron transfer mechanism between the neutral form of the both geometric ‘Cis & Trans’ isomerism inhibitor molecules (HZi) and the Fe-surface in vacuo [19–21].Visual inspections were performed using the GaussView program (version 5.0.8) [22] and Chemcraft program version 1.8 (build 489) [23].

IE  Ionization energy (eV)  IA=−EHOMO  (1) 
EA  Electron affinity (eV)  IE=−ELUMO  (2) 
ΔEg  Energy gap (eV)  ΔEg=ELUMO−EHOMO  (3) 
χ  Absolute electronegativity(eV)  χ=12ΔEg  (4) 
η  Global hardness(eV)  η=−12ΔEg  (5) 
S  Global softness(eV)-1  S=−2ΔEg  (6) 
Ω  Global electrophilicity index(eV)  ω=−(EHOMO+E)LUMO2  (7) 
ΔEb−d  Back donation(eV)  ΔEb−d=−ΔEg8  (8) 
ΔN  Electron fraction transferred from HZi to the Fe-surface  ΔN=−χFe−χinh2(ηFe−ηinh)  (9) 

2.2Molecular Dynamics simulation (MDs)

The MDs were conducted out to further gain insight into the interaction between the adsorbate and the iron sorbent. Metropolis Molecular Dynamics simulation methodology [24] using’ Forcite’ module in Biovia Material Studio v 8.0 software[25], was used to model the surface―inhibitor molecules interactions. The ‘Forcite geometry minimization of two-possible conformational Cis(Z) and Trans(E) isomers of HZi in neutral form before putting them on the Fe-face were performed by B3LYP/DNP+basis sets. Herein, the interaction between the Fe (111) crystal face and Z-HZi and E-HZi conformational isomers is executed in a 3D periodic simulation box (Lx=Ly=0.35, Lz=0.40nm) with periodic boundary conditions. The crystal (111) face was chosen for this simulation because it is among the thermodynamically stable miller indices faces as reported in the literature [26]. The simulation system was carried out with a slab thickness of 0.05nm, a supercell of (8×8) and a vacuum of 0.3nm along the C-direction (Oz-axis) with periodic boundary conditions in order to simulate the representative part of an interface devoid of any arbitrary boundary effects. The COMPASS force field is an ab initio force field that provides correct predictions of gas phase properties like conformational, structural, vibrational as well as the condensed phase properties such as cohesive energies and interaction energies for a wide range of organic molecules, inorganic molecules and metals [27]. MDs were run in microcanonical (NVT) ensemble at 308K. Interaction energy (ΔEint) or binding energy (ΔEbind)between the Fe substrate and inhibitor molecule could be calculated by:

Where Esubs is the energy of the Fe-substrate, Einh is the energy of the free inhibitor, and ETotal is the total energy of the system.

3Molecular proprieties

The geometries and electronic structures for Cis and Trans isomers of HZi in vacuo were calculated by the minimization of equilibrium geometries and are gathered in Fig. 2.

Fig. 2.

Minimized geometry for Cis & Trans- HZi.

(0.23MB).
3.1Synthesis procedure of Azines HZi

A series of symmetrical Azines (HZi) has been synthesized in one-step, yields. Indeed, to a solution of each heterocycle-2-carbaxaldehyde (31.24mmol) in dry ether (20ml), Azines was added (0.5g, 15.62mmol) with a few drops of glacial acetic acid as a catalyst. The mixture was stirred at room temperature for 72h, the formed product was filtered and washed with dry ether. (FT-IR,1H-NMR and 13C-NMR, XRD and MS) identified structures of HZi.

3.2Spectral measurements of Azines HZi

HZ1 : Yellow powder. Yield 85%. Mp=113°C. Rf=0,33 (silica/CH2Cl2) 1H NMR (300MHz, CDCl3) δppm: 8,59 (s, 2H, Himine); 7,62 (d, 2H, Hᾳ); 7,03 (d, 2H, Hγ); 6,63 (m, 2H, Hβ). 13C NMR (75MHz, CDCl3) δppm: 150,76 (2C, Cimine); 148,14 (2C, furan-Cδ); 146,56 (2C, furanC); 0118,20 (2C, furan-Cϒ); 112,58 (2C, furan-Cβ). m/z (M+):189. IR (KBr, cm–1): ν(N-H)=3119, ν (CH=N, imine)=1630, ν(N-N)=1503, ν(C-C)=1540, ν (C=O, furan)=1309, ν(C-H)=1071, 1008, 966, 869, 847.

HZ2: Yellow powder. Yield 64%.Mp=167°C. Rf=0,76 (silica/CH2Cl2) 1H NMR (300MHz, DMSO) δ ppm: 8,82 (s, 2H, Himine); 7,76 (d, 2H, Hᾳ), 7,61 (d, 2H, Hγ); 7,18 (t, 2H, Hβ). 13C NMR (75MHz, DMSO) δ ppm : 156,26 (2C, Cimine) ; 138,87 (2C, thiophen-Cδ) ; 134,25 (2C, thiophen-C) ; 131,45 (2C, thiophen-Cβ); 128,75 (2C, thiophen-Cγ). m/z (M+): 121,02. IR (KBr, cm-1): ν(N-H)=3295, ν (CH=N, imine)=1609, ν (C=S, thiophene)=1321, ν(C=C)=1540, ν(C-H)=1040.

HZ3: Yellow powder. Yield 62%.Mp=186°C. Rf=0,32 (silica/CH2Cl2). 1H NMR (300MHz, DMSO) δ ppm: 11,52 (s, 1H, pyrrole-NH); 8,36 (s, 2H, Himine); 6,96 (d, 2H, Hᾳ); 6,59 (s, 2H, Hγ); 6,16 (m, 2H, Hβ). 13C NMR (75MHz, DMSO) δ ppm: 151,03 (2C, Cimine); 127,81 (2C, pyrrole-Cδ) ; 123,71 (2C, pyrrole-Cγ) ; 115,25 (2C, pyrrole-Cβ) ; 110,14 (2C, pyrrole-C). m/z (M+):187,08. IR (KBr, cm-1): ʋ(N-H)=3212, ʋ (CH=N, pyrrol)=1616, ν(C=C)=1540, ν(N-N)=1443, 1407, 1294, 1132, ν(C-H)=1028, 953, 881, 810.

4Experimental method4.1Weight loss(WL) measurements

Before all measurements, the steel samples (0.09% -P, 0.01% -Al, 0.38% -Si, 0.05% -Mn, 0.21% -C, 0.05% -S and remainder iron) were abraded with a series of emery boards papers from 400 to 1200 grids. The samples were thoroughly washed ultrasonically with ultrapure water and acetone, and finally dried under cold air flow. Gravimetric experiments were performed according to standard methods [28–32]. The WL measurements were performed in a double-walled glass cell. The volume of the solution was 100cm3. The temperature of the solution was 308 (± 0.5) K controlled by thermostatically. The WL of steel in (C=2.0M) aggressive solutions without and with addition of HZi inhibitors was determined after immersion in H3PO4 for 4h. The steel specimens used for these examinations had a rectangular shape (1.5cm×1.5cm×0.2cm).

4.2Electrochemical measurements (EIS)

Electrochemical measurement(EIS) was conducted on a Tacussel electrochemical workstation (Tacussel-Radiometre PGZ-100) equipped with a standard three-electrode cell system under non stirred condition. The steel specimen was served as WE (working electrode), a 4cm2 platinum sheet was utilized as CE(counter electrode), and a saturated calomel electrode used as RE (reference electrode). All the potentials were in reference to the RE. The exposed surface area of disk S=(0.5)2×πcm2 was fixed. All the tests were carried out in a temperature-controlled water bath at 308(±0.5) K.

EIS analysis was then performed on stable EOCP at a disturbance sinusoidal signal of 10mV amplitude within the frequency range (100kHz ―1Hz). The EIS data were analysed using ZViewer v. 0.0.2.2. The inhibition efficiencies IEimp(%) obtained by the EIS test were calculated as follows:

where Rct and Rct0 are the charge transfer resistant without and with different concentrations of HZi.

5Results and discussion5.1Impedance measurement5.1.1Nyquist and Bode plots

EIS is widely used to explore the adsorption of corrosion inhibitors because it provides a convenient and rapid method for evaluation of the surface properties of metal materials. Fig. 3 illustrates the relevant Nyquist diagrams for the mild steel electrode in (2.0M /H3PO4) solution without and with the addition of HZi concentration at 308 (± 0.5) K.

Fig. 3.

Impedance spectra of mild steel in H3PO4 (2M) of HZi at 308(±0.5K).

(0.21MB).

The capacitive loops in the Nyquist diagrams (Fig. 3) presents a depressed semi-circle at high frequencies. These depressed circles might be due to the frequency dispersion of the interfacial impedance and the in-homogeneity of the HZi on mild steel/H3PO4, while the tailed line indicates steel dissolution controlled by diffusion mechanism and oxygen reduction [33,34]. Introducing HZi sharply increased the diameter of the capacitive loop, and the diameter continuously increased with increasing HZi concentration. This finding implies the formation of an adsorbed film and the protective ability for mild steel. Moreover, the shapes of the curves for the inhibited samples are the same as the uninhibited ones, indicating the addition of HZi increased the impedance but did not alter the other electrochemical characteristics of this system. The different values obtained Table 1 show the representative parameter values of the best fit to experimental data and allow describing the overall impedance through Equ (11).

Table 1.

Impedance parameters obtained by fitted using the equivalent circuit.

InhibitorsConcentrationRct(Ohm.cm2fmax(Hz)  Cdl(μF/cm2IEimp(%) 
Blank  2M H3PO4  04.18  250.00  152.30 
HZ11×10-512.01  200.00  65.77  65.19 
5×10-516.26  158.23  61.86  74.29 
1×10-417.00  158.23  59.67  75.41 
5×10-419.88  158.23  50.60  78.97 
1×10-325.44  158.23  49.21  83.57 
HZ21×10-518.88  125.00  67.44  77.86 
5×10-520.02  125.00  63.60  79.12 
1×10-421.80  125.00  58.40  80.82 
5×10-426.90  125.00  47.33  84.46 
1×10-333.74  125.00  44.93  87.61 
HZ31×10-511.38  250.00  55.94  63.26 
5×10-515.50  125.00  82.14  73.03 
1×10-417.74  125.00  71.77  76.43 
5×10-424.06  100.00  64.80  82.62 
1×10-327.15  100.00  58.62  84.60 

Table 1 shows the obtained EIS data. As seen that the Cdl value decreased with the addition of HZi inhibitors. These values continuously decreased with increasing inhibitor concentration(from10-6 to 10-3M), which could be attributed to a decrease in local dielectric constant and the exposed steel surface and/or an increase in the electrical double-layer thickness [31]. Accordingly, it could be inferred that the HZi interacted with mild steel surface by adsorption action, and thus the reduction in the Cdl value was due to the gradual displacement of H2O molecules by HZi on the mild steel/solution interface, leading to decreased extent of the mild steel dissolution [30]. The presence of HZi increased the Rct values, and this effect was enhanced with increasing HZi concentration. This finding suggests the formation of a HZi -adsorption film on the mild steel substrate, which retarded the charge transfer. Following these trend, IEimp(%) values increased with increasing HZi inhibitors concentration.The classification of HZi according to its IEimp(%) is: HZ2(87.57%)〉HZ3(84.6%)〉HZ1(83.57%). The superior inhibition performance indicates that HZi confer effective protection against mild steel corrosion in (C=2.0M/ H3PO4).

The actual part of the impedance conduction to the grain boundaries. The chemical composition of the intermetallic compounds is identical at low frequency because one conduction DC is dominated. Only their size varies according to their location in the material: Micronics for surface apparent phases and nanometric for grain boundary phases such that water molecules and H+ protons present in the medium.

In order to progress in the quantification of phenomena shows in Fig. 4, we first propose to decompose the cooperative electrical and chemical phenomena, indicated by the representation of the imaginary part of the complex impedance.

Fig. 4.

Variation real part Zre of impedance as function of frequency.

(0.27MB).
5.1.2Study of the imaginary part of the impedance

Fig. 5 illustrations the variation of the imaginary part(Zim) of the impedance as a function of the frequency, in order to show the existence of the dielectric relaxation which does not appear in the representation of the dielectric permittivity. At very low frequencies, the polarization follows the alternating field, so that its contribution to the dielectric constant is maximal and the losses do not appear. At very high frequencies, the field alternates too fast for the polarization to increase and there is no contribution to the dielectric constant - no energy is lost in the medium [33]. But somewhere between these two extremes, polarization begins to lag behind the electric field from which dissipation of energy at the same time the relaxation time. So, we can say at the lowest frequencies.

Fig. 5.

Imaginary part Zim of impedance variation as function of frequency.

(0.29MB).

It is found that the relaxation frequency changes for some concentrations for HZ1 and HZ3. The relaxation frequency does not change for inhibitor HZ2. This shows that the relaxation frequency for HZ2 due to a specific behavior of the HZ2 molecule.

5.1.3Equivalent electrical circuit and modeling of mechanism of conduction

The EIS data were fitted using an classical equivalent circuit (Fig. 6) composed of solution resistance (Rs), charge-transfer resistance (Rct), and constant-phase angle element CPEdl, which are related to electrical double-layer capacitance (Cdl). During curve fitting, ideal capacitors (Cdl) were replaced by CPEdl because of the non-ideal capacitive behaviour of the inhomogeneous electrode. The impedance function of the CPE can be described as follows :

Fig. 6.

Corresponding equivalent circuit used to fit the EIS experimental data.

(0.13MB).
5.1.4Comparative study by Nyquist diagrams

This comparison is made to compare with that found by Nyquiste diagram. We found that HZ1 has a very high ionic conductivity compared to other inhibitors such as HZ2 and HZ3. The relaxation frequency extracted from the imaginary conductivity measurement as a function of frequency has a lower frequency of HZ2 (125Hz) equivalent to a relaxation frequency in the Bode representation of value (125Hz). in frequency results in a majority contribution to the level of the ionic conductivity at high frequency thanks to the mesomeric effects of positions of the active sites.

Based on Fig. 7, the increase in resistance can be detected significantly with increasing effect of the mesomeric inhibitor. Greater mesomeric effect at HZ1, then HZ3 and finally HZ2 in descending order of the mesomeric effect

Fig. 7.

Nyquist diagram of HZi(i=1,2,3) for 10-3 M at 308(±0.5K).

(0.01MB).
5.1.5Modeling Analysis

Fig. 8 shows the variation of the electrical hopping conductivity σho. The mesomeric effect arises due to the substituents of the heteroatoms(-O-,-S- & =N-) in the both heterocyclic rings attached to azine bond(=N-N=). This effect favors a better conduction by jumping under the influence of the electric field applied to the sample. For example, Fig. 8 shows that the best σho is that of HZ1 for the concentration C=10-3 M at 308(±0.5K). This behavior has been discussed by Wang et al. [34], which reflects the aggregation of inhibitors particles due to compression of the electrical double layer, which favors the release of adsorbed water.

Fig. 8.

Variation of conductivity σac as function of frequency at 308(±0.5K) for 10-3 M.

(0.18MB).

In order to determine the mechanisms responsible for the very high conductivity, impedance spectroscopic investigations have been carried out. The electrical behaviour has been summarized in this figure at different HZi inhibitors.

The values of the fitted equivalent electrical circuit, modeled of the conductivity ac of all inhibitor molecules are listed in the Table 2.

Table 2.

Parameter obtained of conductivity.

Inhibitors  σHF (S/cm²)  σBF (S/cm²)  (CPE) (F.Sn)  n coefficient dispersion 
HZ3  0.2044±0.29  0.0443±0.29  0.0001491±1.93  0.8834±0.29 
HZ1  0.2041±0.26  0.0492±0.28  0.00020917±1.70  0.87212±0.28 
HZ2  0.1933±0.26  0.0354±0.26  0.00012619±1.62  0.85819±0.25 
6Gravimetric measurements6.1Effect of HZi concentration

The effect of addition of Azines HZi at different concentrations in the range 10-3-10-6M on the corrosion of steel in (C=2.0M /H3PO4) was studied by weight loss method at 308(±0.5) K after half hours of immersion period. From the weight loss results, the corrosion (Wcorr) rate, the inhibition efficiency IEW(%) of Azines HZi and the degree of surface coverage (θ) were calculated by means of the following Eqs (12&13):

Where Wcorr and Wcorr0 are the corrosion rate for (mild steel/ H3PO4) without and with HZi.

Table 3, summarizes the obtained values of Wcorr and IEW(%). It is obvious from these results that this series of Azines HZi inhibits the corrosion of mild steel at all concentrations used in this study. From the Table 6, it can be observed that the Wcorr of mild steel decreases while the protection efficiency increases as the inhibitors concentration increases in (C=2M/H3PO4). This effect is hugely marked at higher concentration of inhibitors. The classification of these inhibitor molecules according to its IEW(%) is: HZ2〉HZ3〉HZ1. It is predictable from the molecular structure of HZi that the inhibitor HZ2 will have the highest anti-corrosion performance (IEW(%)). It is due to the high electronegativity (electron donating groups) of both heterocyclic rings (thiophene (HZ2) than pyrrol (HZ3) and furan (HZ1)) linked to bond azine(=N1-N1’=) can give and share electrons to the empty orbital of iron on the steel surface, substituting aggressive substances on the Fe-surface and forming coordination links. The corrosion inhibition can be attributed to the interaction for (HZi/mild steel/H3PO4) interfaces [34].

Table 3.

Gravimetric results of mild steel in (C=2M/H3PO4) without and with addition of HZi at 308 (±0.5) K.

Inhs  HZ1HZ2HZ3
C (M)  Wcorrmg/cm2.h  IEW%  Wcorrmg/cm2.h  IEW%  Wcorrmg/cm2.h  IEW% 
Blank  4.1322±0.024  ------  4.1322±0.024  -------  4.1322±0.024  ------- 
1×10-5  1.5508±0.055  62.47  1.3752±0.016  66.72  1.8240±0.008  55.86 
5×10-5  1.3136±0.001  68.21  1.1876±0.008  71.26  1.5826±0.016  61.70 
1×10-4  1.1764±0.013  71.53  1.0492±0.001  74.61  1.2483±0.011  69.79 
5×10-4  1.1037±0.003  73.29  0.8194±0.003  80.17  0.8702±0.005  78.94 
1×10-3  0.9340±0.008  77.40  0.7235±0.002  82.49  0.7698±0.003  81.37 
6.1.1Adsosption Isotherm

The adsorption phenomenon of the organic inhibitors on the surface of metal is considered one of the most important factor of the inhibitor action in acidic media [16]. It is always possible to trace them back to adsorption mechanisms while determining the surface coverage (θ). In this study, the adsorption isotherms known in this scientific area were evaluated. Based on results obtained through gravimetric method, it has been found that the Langmuir isotherm is most suitable for the three compounds HZi and the corresponding equation is as follows [41]:

Kads is the constant of adsorption; the Kads value was calculated from the intercepts of the straight lines C– axis and related to the standard free energy of adsorption (ΔG°ads) according this equation:

Where R is the gas constant (8.314J/KmoL), T is the absolute temperature (K), and the value

55.55 is the concentration of water in the solution expressed in M. The calculated values of ΔG°ads, Kads and R2 from gravimetric data for the HZi are reported in Table 4.

Table 4.

Adsorption parameters of HZi on mild steel in phosphoric acid (2M) at 308K.

Inhibitors HZi    Kads (104 M-1)  ΔG°ads (kJ/mol) 
HZ1  0.999  8.59  −39.38 
HZ2  0.999  9.77  −39.71 
HZ3  5.84  −38.39 

Fig. 9 shows the plots of C– axis against C. A very good fit is observed with the regression coefficients up to 1 and the slopes of the obtained lines are near unity (that of HZ1 is close to 1.2), meaning that each inhibitor molecules occupies one active site on the metal surface and that the experimental data are well described by Langmuir isotherm and exhibit single layer adsorption characteristic [16]. In fact, the obtained Kads values are considered as a measure of the adsorption strengths at the interface inhibitor/ metal [19,41]. From Table 4, we remark that the HZ2, which have a highest efficiency, give a most high value of Kads leading in the strongest interaction between the double layer existing at the phase boundary and the adsorbed molecules. The calculated ΔGads° values for HZ1,HZ2andHZ3 are -39.38, -39.71 and -38.39kJ/mol, respectively. The negative values of ΔGads° indicates that the spontaneous adsorption of HZi and the stability of the adsorbed layer on the metal surface; HZ2 have always a highest value of ΔGads° (in absolute value). According to the literature, the ΔGads° values of -20 Kj/mol or less negative are associated to physical adsorption; those of -40 Kj/moL or more negative involves chemical adsorption. [16,41]. The obtained values of ΔGads°, values close to −40kJ/moL, indicating that the adsorption mechanism of the HZi molecules on mild steel in phosphoric acid solution (2M) is more chemical than physical adsorption (a chemisorption).

Fig. 9.

Langmuir adsorption isotherm for mild steel in phosphoric acid (2M) containing HZ1, HZ2 & HZ3 at 308K.

(0.07MB).
6.1.2Influence of temperature

In Table 5, we remind the effect of the temperature increase on the anti-corrosion property of HZi. The gravimetric experiments were conducted in the range of 308–338(±0.5) K, without and with C=10-3M of HZi. We also note that the inhibition efficiency decresed with increase in temperature from 308 to 338(±0.5) K, indicates that high temperature dissolution of steel predominates over adsorption at the iron surface. This can be explicated by the decrease of the strength of the adsorption process at high temperature, and can suggesting that physiorption occurs.

Table 5.

Temperature influence on the weight loss parameters for mild steel in (2M/H3PO4) without and with 10−3 M of HZi.

  308(±0.5) K318(±0.5) K328(±0.5) K338(±0.5) K
Inhs  Wcorrmg/cm2.h  IEW(%)  Wcorrmg/cm2.h  IEW(%)  Wcorrmg/cm2.h  IEW(%)  Wcorrmg/cm2.h  IEW(%) 
Blank  4.132  ****  9.077  ****  15.392  ****  22.792  **** 
HZ1  0.934  77.4  1.680  70.79  2.909  65.1  10.170  55.37 
HZ2  0.723  82.49  3.579  81.49  7.702  81.1  12.170  46.60 
HZ3  0.769  81.37  3.140  65.41  6.338  58.8  11.209  50.82 

CPE : Pseudo capacitance

The thermodynamic parameter descriptors of Fe-HZi complexes on can provide valuable information about the mechanism of corrosion inhibition. In order to determine these activation thermodynamic descriptors, the Arrhenius equation Eq. (16) and its alternative formulation called transition state equation Eq. (17) were used [33]:

Where Ea is the apparent activation energy, N is the Avogadro’s number, R is the universal gas constant and T(K) is the absolute temperature, h is the Planck’s constant, ΔHa the enthalpy of activation and ΔSa entropy of activation (Fig. 10).

Fig. 10.

Arrhenius plots for (mild steel/2M/H3PO4) without and with 10-3 M of HZi.

(0.11MB).

The calculated Ea without HZi in acidic medium (+55.36kJ/mol) is approximately in the same order of magnitude as that previously described [34]. The obtained Ea values with inhibitors HZ1,HZ2&HZ3 are +47.75, +99.73 and +88.88kJ/mol, respectively (Fig. 11).

Fig. 11.

Transition state plots for (mild steel/2M/H3PO4) without and with 10-3 M of HZi.

(0.12MB).

The Ea values in the presence of the HZ2&HZ3 inhibitors are higher than those of the un-inhibited acidic solution. This increase in Ea value may be interpreted as physisorption [35]. Moreover, Szauer et al. [36] explained that the increase in Ea value can be attributed to an appreciable decrease in the adsorption of the inhibitor on the Fe-surface with increase in temperature. The Ea value in the presence of HZ1 is lower than that of the uninhibited acid solution, indicating that chemisorption may be the type of adsorption of the inhibitor on the Fe-surface [37]. On the other hand, the thermodynamic parameter descriptors, ΔHa & ΔSa, were calculated and depicted in Table 6. The ΔHa values are positive in the absence and presence of Azines HZi, and the maximum ΔHa value was noted for HZ2 best inhibitors of this series. The positive ΔHa value reflect   the endothermic nature of mild steel dissolution process suggesting that its dissolution is slow in the presence of these compounds [38]. The high ΔHa value in the case of HZ2 indicated that this last is more strongly adsorbed onto the Fe-surface. One can notice that Ea and ΔHa values vary in the same way permitting to verify the known thermodynamic equation between the Ea and ΔHa: Ea−ΔHa=RT as exposed in Table 6[39].

Table 6.

Activation descriptors Ea, ΔHa and ΔSa of the dissolution of steel in (C=2.0M/H3PO4) at C=10-3M of HZi.

  Ea  ΔHa  ΔSa  Ea−ΔHa 
Blank  +55.362  +52.718  −62.091  +2.644 
HZ1  +47.750  +45.107  −99.465  +2.644 
HZ2  +99.732  +97.086  +68.190  +2.645 
HZ3  +88.882  +86.236  +33.321  +2.645 

The positive ΔSa value of HZ2 than other inhibitors reflects the fact that the adsorption process is accompanied by an increase of the entropy, which is the driving force for the adsorption of the inhibitor onto the Fe-surface [40]. The increase of ΔSa value is generally interpreted as an increase in disorder as the reactants are converted to the activated complexes. The large negative ΔSa value of HZ1 implies that the activated complex is the rate determining step, rather than the dissociation step.

7MD simulation7.1Simulated and Experimental 1H &13C NMR spectral analysis

The 1H & 13C NMR chemical shifts were also determined for HZi using the B3LYP/6-311++G(2d,2p) level of theory and SCRF with chloroform as solvent. The experimental 1H &13C chemical shifts are closer to the theoretical values and a linear relationships between experimental and theoretical 1H &13C values was found (Table 7). The higher correlation between 1H &13C NMR chemical shifts values of Trans-form between DFT method and experimental NMR are provided by standard deviation (SD). In fact, SD’s between the Trans-conformer and experimental NMR of 1H &13C chemical shift values are:(0.0102 & 0.2159ppm), (0.86 & 0.1038ppm) and (0.02756 & 0.20265ppm) for HZ1, HZ2 and HZ3, respectively. The 1H &13C NMR values of geometric Trans-forms of HZi are listed in Table 7.

Table 7.

Simulated and Experimental 1H&13C Chemical Shifts for the Trans-Form of HZi.

1H&13C Chemical shift  HZ1HZ2HZ3
  Exp*  DFT  Exp*  DFT  Exp*  DFT 
H(4) and H(4’)  8.59  8.62  8.82  8.88  11.52  11.66 
H(5) and H(5’)  7.62  7.71  7.76  7.78  8.36  8.44 
H(6) and H(6’)  7.03  7.18  7.61  7.63  6.96  7.11 
H(7) and H(7’)  6.63  6.72  7.18  7.22  6.59  6.66 
C(5) and C(5’)  150.76  150.80  156.26  156.29  151.03  152.09 
C(4) and C(4’)  148.14  148.79  138.87  139.01  127.81  128.79 
C(3) and C(3’)  146.56  146.88  134.25  134.88  123.71  129.88 
C(2) and C(2’)  118.20  118.13  131.45  132.13  115.25  115.13 
C(1) and C(1’)  112.58  122.53  128.75  128.53  110.14  110.53 
*

Exp in CDCl3.

7.2Frontier molecular orbitals

The employed DFT(B3LYP) method is suitable for rationalizing the frontier molecular orbitals for the assessment of inhibitor-metal interaction [42]. The EHOMO and ELUMO are directly obtainable, alongside with the ΔEg (EHOMO−ELUMO) from the computational output. Quantum reactivity descriptors are accomplished to study the influence of structural parameters on the inhibition efficiency of HZi and to unravel their adsorption mechanisms on the Fe-surface. Zhang et al [43], reported a convincing argument against the accuracy of quantitative values for EHOMO & ΔEg without necessary correction factors, especially for small molecules. Therein, depending on the DFT functional, equations were given to obtain corrected values from uncorrected values, with experimental validations [43]. With respect to the functional employed in this study, Eqs (18 & 19) were used to obtain corrected values for both EHOMO & ΔEg.

Frontier molecular orbital diagrams of Geometric ‘Cis and Trans’ Isomerism of HZi are calculated by DFT at [B3LYP/6-311++G(2d,2p), Gas] level and are summarized in Figs. 12 and 13.

Fig. 12.

HOMO-LUMO plots for Cis- HZi.

(0.2MB).
Fig. 13.

HOMOs-LUMOs plots for Trans- HZi.

(0.19MB).

From the charge density distribution of the frontier molecular orbitals for Cis/Trans geometric forms of Azines HZi as presented in Figs. 12 and 13, it could be seen that the Cis conformation has high HOMOs and low density LUMOs distributions than Trans which were mainly located around to the=N—N=and —N=C— moiety in the HZi. The HOMOs orbital is mainly derived from P2Z orbitals thanks to the delocalized character of the electrons due to the presence of the heterocyclic rings together with several (π-π)-electrons in the entire inhibitor molecule. Conversely, the formation of density LUMOs orbital does not involve the participation of the former P2Z orbitals. Thus; unoccupied (3d)-orbitals of mild steel can accept electrons from the inhibitors and forming a feed-back bonds between these inhibitor molecules and Fe2+ ions.

7.3Calculated geometric parameters

The link lengths, link angles and torsional angles of the conformations of HZi were determined theoretically, at the [B3LYP/6-311++G(2d,2p), gas] level, and experimentally (X-ray diffraction) and gathered in Table 8.

Table 8.

A Comparrison of Theoretically computed XRD for Cis and Trans HZi with Experimental Data.

Link length (Å)  X-ray  Trans  Cis  link length(Å)  X-ray  Trans  Cis  link length(Å)  X-ray  Trans  Cis 
C1—C2  1.322  1.360  1.361  C1—C2  1.361  1.367  1.368  C1—C2  1.361  1.382  1.384 
C1—O1  1.359  1.356  1.351  C1—S1  1.693  1.726  1.724  C1—N2  1.361  1.363  1.359 
C4—O1  1.366  1.366  1.365  C4—S1  1.729  1.743  1.744  C4—N2  1,377  1.376  1.376 
C4—C5  1.427  1.433  1.441  C4—C5  1.435  1.437  1.443  C4—C5  1.435  1.432  1.436 
C2—C3  1.397  1.422  1.419  C2—C3  1.429  1.367  1.412  C2—C3  1.397  1.412  1.409 
C5—N1  1.272  1.285  1.288  C5—N1  1.280  1.285  1.289  C5—N1  1.308  1.288  1.292 
C3—C4  1.336  1.371  1.375  C3—C4  1.364  1.377  1.381  C3—C4  1,376  1.389  1.393 
N1—N1’  1.408  1.388  1.389  N1—N1’  1.395  1.384  1.386  N1—N1’  1.395  1.384  1.396 
Link angles        Link angles        Link angles       
C2-C1-O1  +111.39  +110.92  +111.06  C2-C1-S1  +112.10  +112.29  +112.47  C2-C1-N2  +109.1  +108.12  +108.18 
C3-C4-O1  +109.66  +109.47  +109.15  C3-C4-S1  110,45  +110.7  +110.2  C3-C4-N2  +106.9  +107.08  +106.90 
N1-C5-C4  +123.10  +123.02  +133.43  N1-C5-C4  +121.50  +122.17  +131.51  N1-C5-C4  +123.9  +122.52  +131.76 
C4-C3-C2  +107.45  +106.68  +106.91  C4-C3-C2  +112.85  +113.29  +113.77  C4-C3-C2  +107.7  +107.64  +107.70 
C5-N1-N1’  +111.29  +112.05  +115.48  C5-N1-N1’  +112.55  +112.85  +114.19  C5-N1-N1’  +110.9  +113.32  +116.15 
C4-C5=N1-N1’  +179.64  +180.0  C4-C5-N1-N1’  +178.9  180  C4-C5=N1-N1’  +179.64  +179.99  +0.6281 
Torsional angles        Torsional angles        Torsional angles       
O1-C4-C5=N1  −0.4  −0.001  −0.01  S1-C4-C5-N1  −0.4  −0.001  N2-C4-C5=N1  +0.4  −0.011  +0.5816 

Comparison of bond lengths listed in Table 8, for Cis & Trans isomers shows that, for any given substrate, the=N1—N1’ and C5—C4 bond length increases in the order Cis>Trans while the -C1—X distance decreases in the order Trans>Cis. The strength of the intermolecular interaction increases with the shortening of the -C—C-&-C—N=bond. As well for all inhibitors, the minimized geometric forms easily interconvert from Cis conformation to a Trans conformation (stable form) and reveal rotation around the=N1—N1’= and -C5—C4- bond. Overall, the Cis →Trans isomerization energy barriers are found to be influenced by the size, electronic character of substituted halogens and temperature [44].

Survey of Table 8 shown that, the Cis & Trans- HZi fully planar by means of X-ray and our DFT calculations at the B3LYP/6-311++G(2d,2p) level in vacuo. We have already used the standard deviation (SD) as part of comparative studies between the data of X-ray and those derived from the quantum mechanics calculations [41]. In the present work, a comparative study of conformations of the Azine substituents with DFT method shows a good correlation between Trans-Form and X-ray diffraction data. Indeed, the standard deviation (SD) between the Trans conformers (calculated by DFT) and X-ray values of HZi are:(0.06Å and 0.116 °), (0.22Å and 0.016°) and (0.05Å and 0.13°) for bond lengths and bond angles of HZ1, HZ2 and HZ3, respectively. Conversely, the standard deviation (SD) between the Trans conformers (calculated by DFT) and X-ray values of HZi are: (0.07Å and 0.05°), (0.0011Å and 0.012 °) and (0.003Å and 0.114°) for bond lengths and bond angles of HZ1, HZ2 and HZ3, respectively. Hence, the crystal geometry of HZi agrees more with the Trans than with the Cis geometric conformer.

7.4Molecular electrostatic potential surface (MEPs)

3D-distribution of MEPs is highly useful in predicting the reactive sites behavior of the symmetrical Azines. The MEP surface of the geometric ‘Cis & Trans’ conformers of all HZi are on overlaying of the electrostatic potential on to the isoelectronic density surfaces. This is a valuable tool for describing overall molecule charge distribution as well as anticipating sites of electrophilic addition. In the link Azine (N1-N1’) region of negative charges (red color) is seen around the electronegative nitrogen=N1 and=N1’ are susceptible for electrophilic attack. Blue color represents strongly positive region (electrophile region) and the predominant green region in the MEP surfaces corresponds to a potential halfway between the two extremes red and blue region in (thiophene, furan and pyrrole) heterocyclic rings. The MEP surface picture of HZi for Cis /Trans-forms are showed in Fig. 14.

Fig. 14.

MEPs of different geometries Cis and Trans conformers of HZi.

(0.13MB).
7.5Mulliken atomic charges (MC)

By extension, DFT Mulliken population analysis can be used to determine the active sites of single inhibitor molecules and its function entails using local descriptors to theoretically justify the HSAB principle [42]. This gives information about the most probable site for electrophilic or nucleophilic attack on the molecule.It is confirmed that the more negatively charged heteroatom is, the more is its ability to adsorb on the Fe-surface through a donor-acceptor type reaction [43]. The DFT-Mulliken charge distributions for the structures Cis- and Trans-forms of HZi are presented in Fig. 15.

Fig. 15.

DFT Mulliken charge distributions for Cis- and Trans- HZi.

(0.47MB).

The Fig. 15 representing the effective atomic charges from Mulliken populations of HZi at different conformers (Cis &Trans), shows that the Trans- HZi have high negative charge densities trough to the active’s sites specially the nitrogen atoms. This is due to the existence of possible rotations around the links -C5-C4- and=N1-N1’= inducing a big difference in dipole moment, which would lead to very different intermolecular forces [44,45].

When HZi adsorbed on the mild steel in aqueous solution of ortho-phosphoric acid, the Cis-form geometric (highest-energy structure) change to a planar geometry (lowest-energy structure) Trans form; thus, the theoretical study predicts the favored configuration as Trans only [46]. Experimentally, the Trans geometric conformer is synthesized and stable in water (not Cis) [47,48]. The molecules of HZi -Trans form have more potency charges than Cis form. This is due to the tautomeric effect between the two heterocyclic rings and bond Azine (Fig. 16). This delocalization character of electrons yields to a more stable planar structure of HZ2. Thus, the minimized structure is in accordance with the fact that corrosion inhibitors efficiency.

Fig. 16.

Schematic Representation of possible tautomeric forms ofTrans-HZi conformers.

(0.12MB).
7.6General Quantum descriptors

The reactivity of Cis &Trans-forms of Azines, on mild steel corrosion in aqueous solutions of Phosphoric acid has been explained based on electron-donor properties related to the structure and the mode of adsorption. preliminary study of inhibitor as isolated molecule, found that the solvent does not affect the molecular activity. So, this is an important argument to study the species reactivity only in vacuo [49].

High correlation coefficients between IEimp(%) and some local quantum reactivity descriptors (μ, Δ Eg, σ & ΔEb-d) were found for both forms Trans and Cis (Table 9). Indeed, as electronic properties, these parameters play a main role in the corrosion inhibition mechanism [16]. The effect of the structural parameters on the inhibition efficiency of the both geometric forms, their relative stability and their adsorption mechanisms on the Fe-surface.The quantum reactivity descriptors of Geometric ‘Cis & Trans’ forms and inhibition efficiency IEimp(%) of  HZi are exposed in Table 9 where R² is stands for the correlation coefficient between IEimp(%) and the considered local quantum reactivity indices.

Table 9.

Quantum reactivity descriptors for the geometric Cis and Trans-Form of HZi

(Cis)(Trans)  HZ3(Cis)  HZ3(Trans)  HZ2(Cis)  HZ2(Trans)  HZ1(Cis)  HZ1(Trans)   
+0.39+0.39  −606.22  −605.226  −1291.9  −1291.904  −645.92  −645.930  T.E (au) 
+0.001+0.02  −5.6239  −5.4642  −6.0125  −5.9529  −5.878  −5.8718  EH (eV) 
+0.06+0.39  −1.9358  −1.8155  −2.3641  −2.3333  −2.136  −2.1695  EL (eV) 
+0.90  +0.903.0230  0.000003  1.3877  0.0000015  1.367  0.000008  μ (debye) 
+0.92+0.97  +0.542  +0.548  +0.548  +0.5530  +0.5350  +0.5400  σ (eV)-1 
+0.003+0.02  +5.6239  +5.4642  +6.0125  +5.9529  +5.8784  +5.8718  I (eV) 
+0.02+0.005  +3.7798  +3.6398  +4.1883  +4.1431  +4.0075  +4.0206  χ (eV) 
+0.06+0.0004  +1.9358  +1.8155  +2.3641  +2.3333  +2.1366  +2.1695  A (eV) 
+0.95+0.97  +1.8441  +1.8243  +1.8242  +1.8098  +1.8709  +1.8511  η (eV) 
+0.07+0.0007  +3.8738  +3.6311  .8081  +4.7424  +4.2921  +4.3664  ω (eV) 
+0.95+0.98  −0.4610  −0.4561  −0.4561  −0.4524  −0.4677  −0.4628  Δb-d(eV) 
+0.001+0.04  +0.8730  +0.9210  +0.7710  +0.789  +0.800  +0.805  ΔN 
+0.98+0.99  +3.6882  +3.6487  +3.6484  +3.6196  +3.7418  +3.7023  ΔEg (ev) 
    84.60    87.61    83.57  IEimp(%) 

The difference in the stability between geometric Cis-& trans-Forms can be estimated from the corresponding total energy (TE) reported in Table 9. In Trans-Form, the two heterocyclic moieties are opposite to each, whereas, in Cis the two heterocyclic moieties are on the same sides. The possibility existing in the conformation Cis-Form of all HZi is ruled out because severe steric crowding exists between the two heterocyclic, the geometric Trans-form was found to be more stable than the Cis-form [49]. Thus, the Azines adopt a ‘Trans’ configuration slightly more stable than the geometric Cis conformers.

In the totally symmetric structures where μ(debey) values tend to→ 0, there is no μ(debey) to interact with water and hence, we have a lower energy of solvation. Conversely, the structures of the Cis isomer are slightly deviated out of the plane due to the steric hindrance between the heterocyclic rings (thiophen, furan and pyrrol) attached to the chain linked in hydrazine.

The energy gap(ΔEg) is an important parameter as a function of reactivity of the inhibitor molecule towards an understanding the adsorption process on the Fe-surface. High chemical reactivity and low kinetic stability are general features of a molecule with low ΔEg[50]. Reportedly, excellent corrosion inhibitors are usually organic compounds, which not only offer electrons to unoccupied orbital of the metal but also accept free electrons from the metal [51]. A molecule with a low ΔEg is more polarizable, is generally associated with the high chemical activity and low kinetic stability, and is termed soft molecule [52]. The inhibition efficiency of Trans conformer increases when ΔEg decrease. The low value ΔEg( +3.6196eV) of Trans-HZ2 indicate the high (IEimp) than the same geometry conformers HZ3 & HZ1. This means that the molecule HZ2 of the geometric ‘Trans-form’ could have better performance as a corrosion inhibitor (there is excellent linearity between ΔEg values and experimental data with a correlation coefficient R² which tend to→ 1)).

It is clear from Table 9, that HZ1, HZ2 & HZ3, have higher IP (ionization potential) with low AP(electron affinity). In fact, under DFT minimization, the vertical electron affinities show positive values indicating that these inhibitors are extremely stable and may not undergo any reaction easily. The low IP of geometric ‘Trans-form’ indicates the high inhibition efficiency than geometric Cis-form [55]. The calculated ionization potential (I) follows the order HZ2 > HZ1 > HZ3 which does not support the order obtained for the inhibition efficiencies (value of R² tends to →0).

The both descriptors η(hardness) and S(softness) are important properties to measure the molecular reactivity and stability. It is apparent that the parametre η signifies the resistance towards the polarization or deformation of the electron cloud of the ions, atoms or inhibitor molecules under small perturbation of chemical reaction. A hard molecule has a large ΔEg value and a soft molecule has a small ΔEg value [56]. Table 9 shown that the geometric ‘Trans’ conformers are energetically more stable than the geometric ‘Cis’ conformers, whereas, in contrast, the parameter η of the Cis isomer is greater than the Trans isomer. In the present study, the Trans-HZ2 has the lowest η (+1.8098eV) and the lowest ΔEg (+3.6196eV) when compared to the other Trans-conformers of HZ3 & HZ1. Normally, the inhibitors with the least value of η (hence the highest value of global softness) are expected to have the highest inhibition efficiency [57]. For the simplest transfer of electron, adsorption could occur at the part of the molecule where S, which is a local property, has a highest value [58]. Trans-form of HZ2 with the S value of +0.553eV has the highest IEimp. The high performance of geometric Trans-HZ2 is attributed to the size of the molecule covering the surface and thereby inhibits corrosion of metal. Hence, we have a good linear correlation between geometric proprieties and IEimp, on the one hand, the η (absolute hardness) and S (softness) values and IEimp(%) values, on the other hand with good regression coefficients (with a correlation coefficient R² of +0.97).

The μ (Dipole moment) is another crucial descriptor that is well correlated with inhibition efficiency of a molecule in the literature [59]. Since the distance between the charges is larger in the Trans-forms than in the Cis-forms, the μ of the Cis-forms are expected to be higher than that on the Trans-forms for a given system. Indeed, the calculated μ are greater for the Cis-forms than for the Trans-forms for all inhibitors. A good linear correlation between μ values of geometric Trans-form and IEimp(%) is obtained (R²=+0.904). This increases the contact area between the molecule and the Fe-surface increases the corrosion inhibition ability of HZi.The geometric Trans-HZ2 may be adsorbed onto the Fe-surface by vertical or horizontal forms, involving the displacement of water molecules form on the metal surface.

From the Table 9, it was found that the ΔEb−d values are favored for the geometric Trans-form than Cis-form. In Table 9, the order followed for Trans-form is HZ2> HZ3> HZ1, which indicates that back-donation, is favored for the molecule Trans-HZ2which is the best inhibitor. The increase in inhibition efficiency of Trans-HZ2 is due to the transfer of electron from HZi to Fe2+ in mild steel or vice versa.This result, in turn, corroborate with the growth inhibition observed IEimp(%) experimentally (R² =+0.96)).

8Molecular Dynamics simulation (MDs)

MDs was implemented to inspect the experimental data and elucidate the adsorption behavior and inhibition mechanism of the two-possible conformational Cis(Z) and Trans(E) isomers of HZi on the steel surface in vacuum. The experiments incorporating the theoretical calculation and MDs can provide an insight into the understanding of interactions between the adsorbate and substrate. The Atomistic MDs can reasonably predict the lowest-energy adsorption and most favorable configuration of the both forms Z-HZi and E-HZi conformations on Fe (111) surface in the gas phase. The Final and lowest energy configuration results of the E-Z configuration of HZi obtained by forcite module simulation in vacuo at 308K are depicted in Fig. 17.

Fig. 17.

Side and top views of most stable adsorption configurations for (Fe (111) / (E&Z)-HZi/Gas phase) systems at 308K.

(0.37MB).

The adsorption of the both conformers (Z-HZi & E-HZi) on Fe (111) surface takes nearly parallel to the surface so as to maximize its contact with the Fe-surface, as shown as the MDs (Fig. 17). Several outputs and descriptors derived Z-HZi & E-HZi configurational isomers in vacuo at 308K by ‘’forcite’’ module simulation is listed in Table 10.

Table 10.

Energy parameters (Kj/mol) for (Fe(111)/ Z & E-(HZi)) complexes

Complexes  ETotal  Eads  R.A.E  EDef  dEads/dNi Z & E-(HZi
HZ1-E  −54.696  −98.725  −71.721  −27.004  −98.725 
HZ1-Z  −74.849  −74.845  −74.849  −74.849  −74.849 
HZ2-E  −15.50  −234.70  −72.67  −162.03  −234.70 
HZ2-Z  −95.719  −163.263  −71.860  −91.407  −163.267 
HZ3-E  −150.070  −98.093  −77.7640  −20.3294  −98.093 
HZ3-Z  −52.695  −96.724  −69.755  −26.969  −96.724 

In this work, the Eads (adsorption energy) is defined as the sum of the R.A.E (rigid adsorption energy) and the EDef (deformation energy) for the complexe. The R.A.E reports the energy released (or required) when the unrelaxed adsorbate before the geometry optimization step are adsorbed on the Fe (111) face in the gas phase. The EDef reports the energy released when the adsorbed component inhibitor molecule is relaxed on the Fe- surface. Table 12, also shows (dEads/dNi), which defines the energy of the substrate–adsorbate configurations where one of the adsorbate components has been removed.

From the Table 10, it’s quite clear that the large negative values of adsorption energies for Z and E configurational isomers of HZi suggest that all inhibitor molecule conformation can be adsorbed onto the Fe (111) surface strongly. We can see also that the E conformations has higher interaction energy compared with E conformations of HZi. Base of the adsorption energy values obtained by ‘Forcite’ module simulations, the adsorption strength of two E conformations of inhibitors HZi on Fe-surface in vacuo can follow the order: HZ2-Trans > HZ1-Trans > HZ3-Trans. This same trend is observed with respect to quantum chemical parameters that are well correlated with corrosion inhibition performance. Altogether, results from this computational studies are in good agreement with the inhibition efficiencies reported experimentally.

The Radial Distribution gr∼r Functions of the two-possible conformational Z-HZi & E-HZi are obtained by “Forcite’’ module simulations (Fig. 18). As shown in Fig. 18, the distance between the active centers in both conformers (Cis and Trans)-HZi and Fe (111) surface in vacuo are in the range 2.91-3.20Å, less than 3.55Å, which designated that chemical bonds have formed between E & Z geometry corrosion inhibitors and Fe atoms and that outside 3.55 by Van der Waals and Coulomb interactions [60,61].

Fig. 18.

Radial Distribution gr∼r Functions for (Fe (111) / (E&Z)-HZi/Gas phase) systems at 308K.

(0.25MB).

The distance between Fe atom and heteroatoms of azines indicates the strength for the metal-inhibitor complex; a shorter distance indicates stronger interactions while a longer distance indicates feebler interactions. As can be seen in Fig. 18, that the trend follows the order: HZ2-Trans1-Trans ≤ HZ3-Cis < HZ3-Trans < HZ2-Cis< HZ1-Cis, which confirms that the E-conformers has the strongest interaction with the Fe-surface than Z-conformers. This is consistent with our observed experimental trend of inhibitory efficacy of E-conformers inhibitors.

9Conclusion

On the basis of the systematic experimental and theoretical investigation of HZi as a corrosion inhibitor of mild steel in this work, the following points can be drawn:

  • EIS results were in good agreement with those obtained from Weight loss measurements, and the calculate of inhibition efficiency (IEexp(%)) increased with increasing HZi concentrations in the order HZ2〉HZ3〉HZ1.

  • HZi showed superior inhibitive ability at relatively high temperatures, although the corrosion of mild steel /H3PO4 was accelerated by the increase in temperature. The thermodynamic parameter descriptors indicate that HZi is adsorbed on mild steel surface in 2M H3PO4 solution by an endothermic process and reveal that the adsorption mechanism of HZi is mainly chemisorption.

  • The Gaussian 09W/DFT/B3LYP/6−311++G** calculations on the HZi inhibitors were used to evaluate the conformational analysis of the two possible conformers(Cis and Trans)-HZi; to identify Trans- HZi conformation (the most stable conformation); and to determine the HOMO-LUMO energies, bond lengths, bond angles and torsional angles, MEPs surface and MC, which were theoretically derived. The Trend of The quantum chemical parameters and Molecular Dynamics simulations of Trans conformers is HZ2 > HZ1 > HZ3, giving the best accordance with the IEexp(%).

  • The comparative study of the crystal structures of HZi with DFT method shows a good correlation between geometric Trans-form and X-ray diffraction data. In fact, the standard deviation between the Trans conformers (calculated by DFT) and X-ray values of HZi are: (0.06Å and 0.116 °), (0.22Å and 0.016 °) and (0.05Å and 0.13 °) for bond lengths and bond angles of HZ1, HZ2 and HZ3, respectively.

  • The theoretical computed 1H and 13C NMR chemical shift for Trans-form of all inhibitors; in turn, compare well with the experimental assignment, thus validating our results. Hence, the Trans- HZi should exist in solution only under normal conditions.

This statement is to certify that all Authors have seen and approved the manuscript being submitted. We warrant that the article is the Authors' original work. We warrant that the article has not received prior publication and is not under consideration for publication elsewhere. On behalf of all Co-Authors, the corresponding Author shall bear full responsibility for the submission. This research has not been submitted for publication nor has it been published in whole or in part elsewhere. We attest to the fact that all Authors listed on the title page have contributed significantly to the work, have read the manuscript, attest to the validity and legitimacy of the data and its interpretation, and agree to its submission to the Journal of Materials Research and Technology.

All authors agree that author list is correct in its content and order and that no modification to the author list can be made without the formal approval of the Editor-in-Chief, and all authors accept that the Editor-in-Chief's decisions over acceptance or rejection or in the event of any breach of the Principles of Ethical Publishing in the Journal of Materials Research and Technology being discovered of retraction are final.

Acknowledgments

Dr: M.E. Belghiti, would like to thank, Atika Lahbal, for the invaluable support. She has always been there for me to encourage me to get the best out of me.

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Professor M. E. Belghiti is currently a member of the Laboratory of Nobel Technology, 163 Willington Street, Sherbrook, J1H5C7, Quebec, Canada. Born in 1977, he earned his Ph.D. in Materials Science and Corrosion at University M1er, B.P. 4808, Oujda, Morocco. His research activities focus on the electrochemical behavior of metals and alloys, corrosion and inhibition, the development of new surface modification processes and the fireproofing of materials. He has published more than 20 scientific research articles with h-index=10 and presented about 10 papers at symposia and national / international meetings. In addition, he is a critic in several international journals.

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Journal of Materials Research and Technology

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