inorganic green inhibitive pigment

Accepted Manuscript Active corrosion protection of mild steel by an epoxy ester coating reinforced with hybrid organic/inorganic green inhibitive pigment Zahra Sanaei, Ghasem Bahlakeh, Bahram Ramezanzadeh PII:

S0925-8388(17)33133-X

DOI:

10.1016/j.jallcom.2017.09.095

Reference:

JALCOM 43151

To appear in:

Journal of Alloys and Compounds

Received Date: 6 June 2017 Revised Date:

4 September 2017

Accepted Date: 9 September 2017

Please cite this article as: Z. Sanaei, G. Bahlakeh, B. Ramezanzadeh, Active corrosion protection of mild steel by an epoxy ester coating reinforced with hybrid organic/inorganic green inhibitive pigment, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.095. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Active corrosion protection of mild steel by an epoxy ester coating reinforced with hybrid organic/inorganic green inhibitive pigment

a

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Zahra Sanaeia, Ghasem Bahlakehb*1, Bahram Ramezanzadeha**2 Department of Surface Coatings and Corrosion, Institute for Color Science and Technology, P.O. Box 16765-654,

Tehran, Iran b

Department of Engineering and Technology, Golestan University, Aliabad Katool, Iran

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Abstract: In this work, a green hybrid organic-inorganic pigment based on zinc cationsCichoriumc intybus L leaf extract was studied from experimental and computational views. The

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hybrid pigment was characterized by thermal gravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The influence of hybrid pigment on the corrosion inhibition properties of the epoxy ester coating was studied by electrochemical impedance spectroscopy (EIS). Results showed that both barrier and active corrosion inhibition performance of the epoxy

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ester coating were remarkably improved by inclusion of hybrid pigment. Moreover, the results form molecular dynamics (MD) and quantum mechanics (QM) studies of different caffeic acidZn-chicoric acid complexes proved the organic-inorganic inhibitors adhesion onto steel surface.

1. Introduction

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Keywords: Green inhibitive pigment; Active corrosion inhibition; epoxy ester; EIS, MD; QM.

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Organic coatings have been widely used for corrosion protection of metal substrates due to their barrier role. However, the coating deterioration during its service time results in its barrier performance decline. Various types of fillers and pigments are added to the coatings to enhance their service life. Chromate based pigments are the most well-known inhibitive pigments that To whom correspondence should be addressed: 1*Dr.

Ghasem Bahlakeh: Tel.: +981734266235; Fax, +981734266235; e-mail, [email protected] . Ramezanzadeh: Tel.: 2122969771, e-mail, [email protected], [email protected].

2**Dr.Bahram

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have been added to the coatings formulations to enhance their self-repairing capability in the case of failure and damage creation. Due to their slight solubility, chromate pigments release inhibitive species, which give rise to metal surface passivation. However, the use of this

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invaluable inhibitive pigment in the organic coatings has been restricted because of the environmental rules and toxic nature of the pigment. Therefore, more demands are being made for finding other alternative inorganic fillers [1-10].

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In this regard, zinc phosphate pigments have been introduced as less toxic alternative corrosion inhibitive pigment [11-14]. However, low solubility of conventional zinc phosphate resulted in

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the poor inhibition action. To overcome this weakness, second and third generations of zinc phosphate and other alternatives have been developed through physical and/or chemical modifications on the anions and cations of the pigments. Iron zinc phosphate, potassium zinc phosphate, aluminum zinc phosphate, strontium aluminum polyphosphate (SAPP), zinc

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aluminum polyphosphate and lithium zinc phosphate are some examples of these modified phosphate pigments [14-20]. Due to higher solubility and better inhibition action, these types of pigments could provide higher degree of corrosion inhibition action. However, these pigments

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suffer from poor inhibition properties at long service life. The inorganic species like zinc cations and phosphate anions could not form stable, dense and barrier film on the active corrosion sites.

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As another approach, synergistic inhibition of mild steel in the presence of organic and inorganic inhibitors has been previously reported [21-25]. However, addition of organic corrosion inhibitors into the organic coating often negatively affects its curing behavior through interaction with functional groups of resin. This can results in the coating cross-linking density and therefore barrier properties decline [26, 27].

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Recently, the hybrid organic/inorganic pigments have been developed as fourth generation of corrosion inhibitive pigments. Combination of organic compounds and inorganic species results in the creation of a new generation of inhibitive pigment with higher solubility, greater inhibition

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efficiency and long term protection. There are numerous reports in the corrosion literature on the use of hybrid pigments in organic coatings [28]. In our recent works hybrid pigments based on zinc acetate (ZA)/benzotriazole (BTA) and potassium zinc phosphate (PZP)/benzotriazole (BTA)

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were analyzed [29-30]. Salehi et al. revealed the effective corrosion inhibition effect of zinc acetate/Urtica Dioica [31]. The corrosion inhibition of carbon steel by combination of

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dicarboxylic acids and zinc cations [32-33], phosphonated Glycine and Zn2+ [34], Zn2+ cations and benzimidazole [35], gluconates and Zn2+ cations [36] have been recently reported. However, apart from good inhibition properties of these inhibitors most of the organic synthetic inhibitors are toxic and expensive, providing serious problems to both human and environment.

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The use of green corrosion inhibitors extracted from natural products and renewable sources has been become an interesting research area in recent years. Plant extracts are non-toxic, biodegradable, cheap and effective source of corrosion inhibitors [37-40]. Therefore, in this

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study a green corrosion inhibitor was used for fabrication of hybrid organic/inorganic pigment. In the present study, a new generation of hybrid pigment was synthesized through chelation and

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complex formation between zinc cations and organic compounds extracted from Cichoriumc intybus L leaf extract. The chelation between zinc cations and chicoric and caffeic acids resulted in the creation of hybrid pigment. The hybrid pigment was characterized by FT-IR and TGA analyses. Then, it was introduced into an epoxy ester coating and its inhibition action in the coatings with and without artificial defect was investigated by EIS and salt spray tests. In addition, in order to gen detailed insights into the corrosion protection properties of synthesized

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inhibitors, computational methods of molecular dynamics (MD) simulations and ab initio quantum mechanics (QM) were applied over the considered complex inhibitors between caffeic

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and chicoric acids and inorganic zinc cations.

2. Experimental 2.1. Materials

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The steel (CK10) specimen, with dimension of 10 cm ×8 cm × 0.2 cm, was prepared from Foolad Mobarakeh Co. (Iran). The chemical composition of CK10 was as follow: Fe 99.18, Mn

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0.3-0.6, S ≤0.05, P ≤0.04, C 0.08-0.13. Zinc nitrate, Zn(NO3)2 and sodium chloride, NaCl, salts were purchased from Merck Co. (Germany) and Dr.Mojalali Co. (Iran), respectively. The Cichorium intybus L leaf was prepared from north coast of Iran, dried and powdered away from sunlight. The epoxy ester resin (60% solid content and 5 mgKOH/g), EE-430CS, was prepared

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from Resitan Co. (Iran). Calcium (40%, 0.5% metal content), Cobalt (60%, 0.1% metal content), and Lead (73%, 0.32% metal content) were prepared from Iran Color producer Co and used as

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dryer for epoxy ester resin. Xylene and butyl acetate solvents were prepared from Saba shimi Co.

2.2. Zn-CIL.L hybrid pigment synthesis procedure

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The extract of Cichorium intybus L leaf was prepared and dried. For this purpose, 50 g Cichorium intybus L leaf powder was added to 1000 mL deionized water and stirred for 3 h at 70 °C. The mixture was filtered to get a clear extract of CIL.L. The extract was dried at 60 °C for 24 h. The hybrid pigment was synthesized through addition of 4 g zinc nitrate to 100 mL distilled water containing 0.48 g Cichorium intybus L extract. The mixture was stirred for 24 h at 60 °C. The hybrid pigment was obtained through centrifugation of the resultant mixture, and washing

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the precipitates with distilled water for three times and finally drying the obtained product at 60

2.3. Epoxy ester/hybrid pigment coating preparation procedure

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°C for 3 h.

2 g hybrid pigment was added to 100 g epoxy ester resin and mixed with a mixer (2000 rpm) for 1 h and then homogenized (10000 rpm) for 10 min to reach proper dispersion of pigment in resin

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and get particle sizes less than 12 µm. Then, proper amounts of solvents, and driers including lead (0.5 wt.%), cobalt (0.2 wt.%), and calcium (1.2 wt.%), were added to the mixture of

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pigment and resin. The steel sheets were abraded by emery papers of 600, 800 and 1200 grades, degreased by acetone and coated by epoxy ester coatings with and without hybrid pigment. The test samples were prepared through application of coatings on the steel sheets by a film applicator, curing at room temperature for 1 week and post-curing at 60 °C for 3 h. The dry film

2.4. Characterization

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thickness was 65±5 µm.

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The chemical composition of hybrid pigment was characterized by a Perkin Elmer Spectrum FTIR instrument. The test was carried out by a KBr pellet method in the wavenumber range of 400-

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4000 cm-1. The thermal stability of the hybrid pigment was evaluated by a STAR SW model TGA, under air atmosphere, and the test was carried out in the temperature range of 25-700 °C by a heating rate of 5 °C/min. The morphology and composition of the film precipitated over the steel surface were characterized by a Phenom ProX model SEM, equipped with an EDS. EIS analysis was performed to characterize the corrosion protection performance of the epoxy ester

coatings

with

and

without

hybrid

pigment.

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For

this

purpose

an

Autolab

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Potentiostat/Galvanostat model EIS instrument was employed. The EIS test was performed in a conventional three-electrode cell including reference electrode (saturated calomel electrode (SCE)), counter electrode (platinum) and working electrode (coated steel specimens with an area

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of 1 cm2). The EIS measurements were done at open circuit potential (OCPSCE), in the frequency range of 10 kHz to 10 mHz, by applying an amplitude sinusoidal voltage of ±10 mV and at different immersion times. The EIS experiment was done in 3.5 wt.% NaCl solution for various

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times on the steel samples coated with epoxy ester coatings with and without hybrid pigment, before and after creation of an artificial scratch (25 mm in length). The coated samples were

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exposed to salt spray test chamber for 300 h and evaluated according to ASTM B117 standard.

3. Computational details

3.1. Construction of organic-inorganic inhibitors

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Figure 1 depicts the molecular structure of chicoric acid and caffeic acid corrosion inhibition agents. To build organic-inorganic inhibitors of chicoric acid-zinc-caffeic acid, these compounds were bonded to a zinc cation (Zn+2) from their hydroxyl oxygen atoms. As exhibited, in the

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structure of chicoric acid and caffeic acid six and three hydroxyl groups are present, respectively, which are able bond to zinc atom. As a result, different complex inhibitors based on these acidic

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compounds and zinc cation could be taken into considered. However, in order to reduce the number of constructed complexes and reliably simulate their possible interactions with steel substrate, the hydroxyl groups from different sites of the chicoric acid and caffeic acid were chosen for covalent bonding to Zn atom. These active moieties include the -OH connected to aromatic benzene cycle and that in carboxylic acid (-COOH) group in each of the acidic inhibitors, as shown in Figure 1. When these hydroxyls were linked to zinc cation, their H atoms

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were removed. With the use of two -OH functionalities in both acids, four different kinds of organic-inorganic chicoric acid-zinc-caffeic acid (designated as Chi-Zn-Caf) were built for subsequent examinations. In order to comprehensively explore the adsorption features and

were conducted on all the constructed organic-inorganic inhibitors. Figure 1

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3.2. Ab initio QM calculations

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electronic properties of complex inhibitors, both MD simulation and QM calculation studies

Ab initio QM calculations were carried out to find the lowest energy structure of all above-

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mentioned organic-inorganic complex corrosion inhibitors. To find these structures, the inhibitor geometries were subjected to optimization process performed in two successive steps. First, the geometries of all complex inhibitors were refined utilizing Hartree-Fock (HF) theory, which was executed by means of 6-31G** basis set for light atoms (i.e., C, N, O and H) and effective core

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potential (ECP) based on Lanl2dz for single Zn atom. In the next step, the inhibitors were geometry optimized through density functional theory (DFT) method [41, 42]. The B3LYP hybrid functional was applied to describe the exchange-correlation interactions first with 6-

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31G** basis function and then with 6-311G** one for light atoms and Lanl2dz for Zn atom [4345]. As the metal corrosion inhibition happens in aqueous phase, these stepwise QM calculations

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were conducted in solution. The effects linked to solvent were implicitly applied with the theory of self-consistent reaction field (SCRF) based on Tomasi’s polarized continuum model (PCM) [46] using the integral equation formalism [47, 48]. The aqueous phase in this theory is treated as a continuum containing uniform dielectric constant, while the solute molecule is positioned inside a cavity defined within the continuum [49]. The inhibitors with optimized geometries were then analyzed so as to compute their electronic properties including highest occupied molecular

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orbital (HOMO) as well as lowest unoccupied molecular orbital (LUMO), the HOMO and LUMO energies (EHOMO, ELUMO) and their energy difference (∆EL-H = ELUMO- EHOMO), and average distribution of electrons. The natural bond orbital (NBO) theory (version 3.1) was

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employed to determine the average electronic distribution centered on inhibitor atoms [50]. Furthermore, the intrinsic electronegativity (χ) and global hardness (η) properties of all complex

according to the following relations [51]:

I = − E HOMO

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(1)

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inhibitors were computed, which depend on their electron affinity (A) and ionization potential (I)

A = −E LUMO

(2)

I +A 2 I −A η= 2

χ=

(3)

(4)

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By using these electronic parameters the fracture of electrons (∆N) which has transferred from inhibitor compounds to steel (iron) atoms was examined through following equation:

χ Fe − χ inhibitor 2(ηFe + ηinhibitor )

(5)

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∆N =

A value of 7 eV/mol was used for iron electronegativity and its global hardness was supposed to

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be 0 eV/mol [52]. All these first-principles ab initio QM calculations were done by using Gaussian 09 program package [53]. The electronic characteristics of all organic-inorganic corrosion inhibitors were further examined

r

by analysis of Fukui indices (FI) [54]. The Fukui function of f ( r ) was evaluated by taking first

r

derivative of electron density ( ρ ( r ) ) with respect to the number of electrons ( N ) at constant

r

potential (ν ( r ) ):

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r r  ∂ρ (r )  f (r ) =    ∂N ν ( rr )

(6)

By employing approximation based on finite difference (FD) and Hirshfeld population analysis

r

+

r ( r ) ) and

r ( r ) ) behavior of inhibitors were assessed using the following relations:

r r r f + ( r ) = ρ N +1 (r ) − ρ N ( r )

(7)

r r r f − ( r ) = ρ N ( r ) − ρ N −1 ( r )

(8)

r

r

r

ρ N (r ) and ρ N −1 (r ) are respectively the electronic

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In these equations, the quantities ρ N +1 ( r ) ,

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nucleophilic ( f

−

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(HPA) [55], the atom-condensed f ( r ) functions relevant for electrophilic ( f

density of anionic, neutral and cationic species. The computations of the Fukui functions were performed by making use of DMol3 DFT code [56]. The generalized-gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) scheme [57] was utilized to take into consider the

3.3. Building iron substrate

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interactions related to exchange-correlation with double numeric polarization (DNP) basis set.

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The adsorption and corrosion protective features of hybrid inhibitors over carbon steel substrate were done using classical MD simulations. In the framework of such simulations, the carbon

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steel substrate was represented by pure iron metal as also applied in earlier works [49, 52, 61]. In order to build an iron surface for inhibitors adhesion, the crystalline unit cell of iron was initially cleaved along its (110) plane using surface builder module in Materials Studio software. This crystallographic Fe (110) surface has been previously adopted in MD simulations of corrosion inhibitors interactions with carbon steel substrate [49, 52, 58-61]. The thickness of cleaved surface was set to be 15 Å, which is equivalent to eight rows of Fe atoms. Afterwards, the surface was replicated in x and y axes with periodic boundary conditions so as to increase its 9

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surface area and to realistically model the inhibitors-iron substrate interactions. Finally, a vacuum region of thickness 3 nm was positioned on top of the constructed Fe (110) surface to remove the periodic boundary conditions in direction normal to surface (i.e., z axis). The

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dimension of the created Fe (110) substrate was approximately 3.5×3.5×4.5 (nm)3.

3.4. Molecular dynamics simulations

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Similar to QM calculations, molecular dynamics simulations were also done under humid conditions. For the case of MD simulations, all organic-inorganic chicoric acid-Zn-caffeic acid

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inhibitors with lowest energy structures resulted from QM calculations based on DFT study at B3LYP/6-311G** theory level were laid over the Fe (110) substrate. Then, using Amorphous cell module in Materials Studio software [56] a simulation box containing 600 water molecules was built at a density of 1 g.cm-3. Upon constructing water cell, its length and width was chosen

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equal to surface area of Fe (110) cell (i.e., 3.5 nm ×3.5 nm) such that it can be used as a layer above iron cell. The constructed water cell was then optimized for 1000 steps by Smart minimizer algorithm. The optimization was performed using COMPASS force field and atom-

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based cutoff for van der Waals interactions and Ewald summation method for electrostatic interactions [62, 63]. Subsequently, this optimized water cell was placed above the highest Fe

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layer by the use of Layer builder module in Materials Studio software package [56]. It is worth to mention that a0.00002 kcal.mol-1 for energy, 0.001 kcal.mol-1.Å-1 for force and 0.00001 Å for displacement. 0.8 nm vacuum space was placed above the layered structure. The dimension of the constructed simulation cell was about 3.5×3.5×5.2 (nm)3. Before MD simulations, the prepared simulation cells consisting of solvated inhibitor adsorbate and Fe (110) adsorbent were optimized in two successive steps applying Smart algorithm

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available in Materials Studio software [56]. First, an optimization was carried out for 20000 steps using accuracy level of medium with convergence criteria of 0.001 kcal.mol-1 for energy, 0.5 kcal.mol-1.Å-1 for force and 0.015 Å for displacement. Then, the optimized cells were further

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optimized by accuracy level of ultra-fine with convergence tolerances of 0.00002 kcal.mol-1 for energy, 0.001 kcal.mol-1.Å-1 for force and 0.00001 Å for displacement. Afterwards, all minimum energy cells were subjected to 2000 ps (2 ns) MD simulations without any restrictions. These

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dynamics simulations were conducted in NVT ensemble at 298 K by the use of Forcite module. All bonded and non-bonded parameters required for potential energy calculation were adopted

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from COMPASS force field [62, 63], except the inhibitors atomic charges which were calculated through aforementioned QM computations using NBO approach. The non-bonded van der Waals (vdW) interactions were computed with atom-based cutoff, and the electrostatic ones were modeled with Ewald summation technique. To solve the Newton’s motion equation, the velocity

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Verlet integrator was applied with a time step of 1 fs (10-15 s) [64]. Within MD simulations, to monitor temperature at desired value the Andersen thermostat was used. In addition, the position

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of all metallic atoms was kept fixed at their bulk values during dynamics simulations.

4. Results and discussions

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4.1. Characterization of chicoric acid-zinc-caffeic acid compounds The chelation and complex formation between the organic compounds existed in the Cichoriumc intybus L leaf extract, i.e., chicoric and caffeic acids, and zinc cations was studied by FT-IR and TGA analysis. The chicoric and caffeic acids contain large number of oxygen containing groups like OH, C=O and COOH [65]. These groups highly tend to make complex with zinc cations. The electron rich oxygen groups can donate the lone pairs with empty orbital of zinc cations [65-

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68], casing a chelation between chicoric acid-zinc-caffeic acid. The chelation between zinc cations and chicoric and caffeic acids was studied by FT-IR analysis. The FT-IR spectra of the Cichoriumc intybus L leaf extract and hybrid pigment obtained through chelation between

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chicoric acid-zinc-caffeic acid are depicted in Figure 2. It can be seen from Figure 2 that the FT-IR spectrum of Cichoriumc intybus L leaf extract includes five main absorption peaks at 1050 cm-1, C-O-C, 1390 cm-1, COO-, 1590 cm-1, C=C of aromatic ring, 2910 cm-1, -C-H (CH3),

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and an intense broad peak at 3400 cm-1, -O-H [69-73]. These observations clearly indicate that the organic compounds existed in the Cichoriumc intybus L leaf extract, i.e., chicoric and caffeic

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acids, include plenty of oxygen containing groups like hydroxyl, carbonyl and carboxylic [66]. On the other hand the FT-IR spectrum of hybrid pigment show absorption peaks similar to Cichoriumc intybus L leaf extract. Also, some new peaks appeared in the FT-IR spectrum of hybrid pigment that cannot be seen in the FT-IR spectrum of Cichoriumc intybus L leaf extract.

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The new absorption band appeared in the range of 440-540 cm-1 is attributed to the Zn-O bond vibration, indicating the chelation between the oxygen containing groups and zinc cations [7173,75, 76]. The chelation between zinc cations and chicoric and caffeic acids results in the

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creation of complex structures as shown in Figure 1. The complex formation between the COOH group and zinc cations resulted in the appearance of a second absorption band at 1310

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cm-1, showing a hypsochromic shift with regard to the main absorption peak of this group observed at 1590 cm-1. Also, the inter-molecular hydrogen bonding between the organic compounds exited in the hybrid pigment resulted in the observation of multiple resolved absorption peaks for O-H vibration in the range of 3100-4000 cm-1. All of these results confirm the complex formation between chicoric acid-zinc-caffeic acid, resulting in the creation of a hybrid pigment.

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TGA analysis was carried out to investigate the thermal stability of the hybrid pigment. The TGA/DTG thermograms of CIL.L powder and hybrid pigment are shown in Figure 3. Four weight loss steps can be seen in the DTG plot of the CIL.L powder. The weight losses occurred

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in the ranges of 25-100 °C and 100-200 °C are attributed to the evaporation of water molecules that are physically and chemically attached to the CIL.L, respectively [25, 77]. The weight loss occurred in the temperature range of and 250-350 °C is attributed to decomposition and

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deterioration of oxygen containing functional groups i.e., hydroxyl, carbonyl and carboxylic. The weight loss occurred at temperatures higher than 400 °C is negligible; indicating the stability of

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aromatic rings existed in the CIL.L powder. For the hybrid pigment, Figure 3 (a2), the weight loss occurred in the same temperature ranges but the significant shift of the weight loss peak related to deterioration of oxygen containing functional groups to higher temperatures can be clearly seen. Observation of a sharp distinctive peak at 325-450 °C indicates that the chelation

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between zinc cations and CIL.L extract could remarkably enhance its thermal stability. These observations clearly show that the thermal deterioration of the oxygen functional groups of CIL.L compounds bonded to zinc cations, i.e., chicoric acid-zinc and caffeic acid-zinc, occurs at

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higher temperatures. In fact, the chelation between zinc cations and CIL.L provides complexes with higher thermal stability than CIL.L. These observations are in good agreement with FT-IR

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results.

Figure 3

4.2. Corrosion measurements 4.2.1. EIS measurement of the epoxy ester-hybrid pigment with artificial defect The inhibition action of hybrid pigment was evaluated by EIS technique in the epoxy ester coating with an artificial defect. The Bode diagrams of these samples at various immersion times

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are depicted in Figures 4 and 5. According to these figures two resolved time constants are present in the Bode diagrams of the steel panels coated with neat epoxy ester coating up to 6 h immersion. This indicates that the coating is intact and the electrolyte did not diffuse to the

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coating/metal interface. The coating provides good barrier against electrolyte diffusion and the only way of the corrosive agents access to the metal surface is through defect site. As the immersion time elapsed a new time constant appeared at high frequency range, indicating

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development of a surface film inside defect site. Appearance of a second time constant at high frequency in the Bode plots of the neat epoxy ester coating is attributed to the creation of slightly increased up to 6 h

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corrosion products inside the scratch. As a result the |Z|10

mHz

immersion (Figure 6). The corrosion products could slightly decrease the access of corrosive agents to the metal surface. It can be seen that further increase of immersion time resulted in the weakening of the time constant observed at high frequency and the maximum phase angle at

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high frequency significantly decreased. These indicate that as the immersion time elapses the film of corrosion products, which was not intact and dense, noticeably deteriorated. Also, the corrosive electrolyte diffused beneath the coating through diffusion into the coating matrix. This

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resulted in the significant decrease in |Z|10 mHz as shown in Figure 6. Results show that the

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phase angle plot of neat epoxy ester coating became broader and the maximum phase angle related to the time constant appeared at low frequency range significantly decreased. The decrease in phase angle at low frequency and |Z|10 mHz is attributed to the corrosive electrolyte diffusion beneath the coating, resulting in the coating delamination and corrosion products development beneath the coating. Figure 5 shows the shift of breakpoint frequency (fb) to higher values for the neat epoxy ester coating, indicating coating delamination [78-80]. The loss of adhesion is attributed to the increase of local pH at cathodic sites as a result of cathodic reaction 14

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(2H2O+O2+4e→4OH-). The Na+ cations migrate to cathodic regions present at coating/metal interface through defect site and form NaOH, providing high alkalinity. At this condition the coating/metal interfacial adhesion bonds can be strongly deteriorated, leading to the coating

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delamination. It is clear from Figure 5 that at all immersion times, except 6 h immersion, the Bode phase plot include two resolved time constants. Unlike neat epoxy sample the increase of immersion time did not result in the significant change in Bode phase plot shape. The maximum

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phase angle did not decrease and no shift of fb can be seen in Figure 4a1. All of these observations reveal that the coating remained intact and the coating delamination did not occur.

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Appearance of a new time constant at high frequency and the increase of phase angle at low frequency ranges after 6 h immersion for this sample clearly show deposition of a protective film inside scratch and over the steel surface at coating/metal interface. The increase of |Z|10 mHz up to 24 h can be seen for the epoxy ester coating containing hybrid pigment. From Figure 6 it is

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evident that at all immersion time the |Z|10 mHz of the coating loaded with hybrid pigment is remarkably greater than the neat epoxy sample. These observations reveal that the hybrid pigment could enhance the coating protection degree through providing active inhibition

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behavior. No significant shift of fb and decrease in the maximum phase angle at low frequency

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can be seen for this coating during immersion. This indicates that the coating delamination is not significant and the hybrid pigment could significantly reduce the rate of electrolyte diffusion beneath the coating.

Figure 4 Figure 5 Figure 6

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4.2.2. SEM/EDS analysis The morphology and composition of the film precipitated inside defect and at coating/metal interface of the epoxy ester coatings with and without hybrid pigment exposed to salt spray test

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for 200 h were investigated by SEM/EDS analysis. It can be seen from Figures 7 and 8 that corrosion products in large amount precipitated inside the scratch part of the coating without hybrid pigment. The corrosion products development beneath the coating can be also seen after

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200 h salt spray test. Detection of high concentration of oxygen inside the scratch clearly depicts deposition of corrosion products. It can be seen from the results that the precipitated film inside

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the scratch of the coating loaded with hybrid pigment includes high content of zinc and carbon, indicating the presence of both organic and inorganic parts of hybrid pigment. This means that the hybrid pigment could release inhibitive compounds inside scratch, leading to a protective film precipitation. The organic, i.e., chicoric acid and caffeic acid, and inorganic parts, zinc

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cations, of hybrid pigment can be partially dissolved in corrosive electrolyte and form insoluble complexes on the active corrosion sites of metal surface. Figure 7

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Figure 8

4.2.3. EIS measurement of the epoxy ester-hybrid pigment without artificial defect

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The corrosion protection performance of the defect free epoxy ester coatings (1 cm2) with and without hybrid pigment was studied by EIS during different immersion times. The Bode diagrams, the values of |Z|10

mHz

and fb are shown in Figure 9. It can be seen from Bode

diagrams that after 7 days immersion the coating containing hybrid pigment showed capacitive behavior in a wide frequency range. The phase angle close to -90° can be seen for this sample in wide frequency range and the |Z|10 mHz is greater than 1010 ohm cm2. These results confirm that

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the coating shows good barrier properties and restricted the electrolyte diffusion to the coating/metal interface. As the immersion time elapsed the |Z|10 mHz decreased for both neat

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epoxy ester coating and the one containing hybrid pigment. It is evident that at all immersion times the |Z|10 mHz of the hybrid pigment containing sample is much greater that the neat epoxy sample. The coating deterioration, the electrolyte diffusion into the coating/metal interface and the adhesion bonds destruction are the main reasons for the coating impedance value decline,

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indicating the loss of coating protection degree during the time. However, incorporation of

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hybrid pigment could reduce the electrolyte diffusion into the coating matrix, indicating their effect on the coating barrier performance enhancement. The fb significantly increased after 22 days immersion for the neat epoxy ester coating, revealing the significant coating delamination but the fb of the pigment loaded sample did not significantly change. These results reveal that not only the active inhibition effect but also the barrier role of hybrid pigment are responsible for the

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coating protection performance enhancement.

Figure 9

4.2.4. Corrosion inhibition mechanism of hybrid pigment

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In the case of epoxy ester coating applied on the steel surface the corrosive electrolyte diffuses

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into the coating/metal interface through coating porosities and defects. As a result the corrosion reactions, including anodic dissolution of iron (Fe→Fe2++2e) and cathodic oxygen reduction (2H2O+O2+4e→4OH-), take place at the coating/metal interface. The Na+ cations interact with OH- anions, resulting in the creation of strong NaOH alkaline agent at the coating/metal interface. The increase of pH at cathodic sites results in the coating/metal interfacial adhesion bonds deterioration. The adhesion bonds between the epoxy ester coating and steel surface is mostly in the form of physical and electromagnetic bond (hydrogen bonding). These are not

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strong and stable bonds in alkaline condition. The electrolyte diffusion into the coating/metal interface results in the coating delamination from the substrate. So, an active corrosion inhibition effect is required for the coating delamination prevention. The hybrid pigment is composed of

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organic-inorganic compounds like zinc bonded chicoric and caffeic acid compounds. According to Figure 10 the pigment can be partially dissociated in the electrolyte diffused into the epoxy ester coating, releasing inhibitive compounds like zinc cations and organic compounds like

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chicoric and caffeic acids. The zinc cations adsorb on the cathodic regions through reaction with zinc cations, forming insoluble zinc hydroxide (Zn2++2OH-→ Zn(OH)2). The cathodic reaction

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rate can be noticeably decreased and the pH rise cannot occur, resulting in the coating delamination rate reduction. In addition, the organic parts of hybrid pigment can interact with Fe2+ cations generated on the anodic regions, resulting in the complex formation and film precipitation. The oxygen groups of organic inhibitors, i.e., chicoric and caffeic acids, can donate

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the lone pair with empty 3d orbital of Fe2+ cations, leading to chelation between these compounds (Figure 10) [66-68, 81-83]. So the anodic reaction can be also retarded in this way. In fact, the hybrid pigment can show mixed anodic and cathodic inhibition of mild steel in

EP

chloride solution. Also, the barrier properties of the coating can be enhanced by incorporation of hybrid pigment. This means that the hybrid pigment could effectively fills the coating porosities

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and cavities, increasing the electrolyte diffusion pathways length. These mean that the hybrid pigment enhances both active inhibition properties and barrier role of the coating simultaneously. Figure 10

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4.3. Computational results 4.3.1. Molecular dynamics simulation The atomic-scale molecular dynamics simulations were conducted to obtain a molecular level

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understanding about the adsorption strength and mechanism of organic-inorganic chicoric acidZn-caffeic acid inhibitors over the carbon steel substrate. Figure 11 demonstrates the side and top views for equilibrated snapshots of all four types of complex inhibitors elucidated from the

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last step of 2 ns MD simulations. From these snapshots it is visible that complex type inhibitors of chicoric acid and caffeic acid localized in vicinity of the topmost layer of Fe (110) substrate.

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This visual finding clarifies the fact that organic-inorganic corrosion inhibitors based on chicoric and caffeic acids and zinc cation tend to adhere to iron surface and thereby establish a corrosionresistive film above surface. Additionally, the side view of visualized pictures clearly displays that all four constructed complex inhibitors stabilized near the metallic surface with completely

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planar alignment of their molecular skeleton with respect to surface. The top view of all ultimate cells discloses that the inhibitors distributed over the surface and their aromatic benzene rings were entirely parallel relative to Fe (110) adsorbent. Such type of adsorbed inhibitor

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configuration maximizes its surface coverage ability, which in turn gives rise to strengthened

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protective-film formation ability and an enhanced corrosion resistance effect. Figure 11

The inhibitors adsorption onto iron substrate was also quantitatively examined by assessing the adsorption energy parameter, ∆Eads, using the expression: ∆Eads = Einhibitor/Fe – (Einhibitor + EFe). In this formula, the Einhibitor/Fe denotes the potential energy of the simulation cell containing inhibitor and Fe (110) surface, while the Einhibitor and EFe are respectively indicative of the potential energies of the isolated inhibitor molecule and isolated metallic surface. The calculated ∆Eads values are also presented in Figure 11 for each snapshot. It can be seen that the adsorption 19

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energy for all hybrid inhibitors are negative, which quantitatively confirms the organic-inorganic inhibitors capability to establish a corrosion-resistive layer above carbon steel substrate. These

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outcomes are in close agreement with our experimental results.

4.3.2. Quantum mechanics calculation

The electronic-scale quantum chemical studies were done to shed light on the electronic

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characteristics of chicoric acid-zinc-caffeic acid inhibitors responsible for their surface binding. From the MD simulations it was found that corrosion inhibiting compounds attached to surface

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with a flat configuration. Previous reports suggested that that such an inhibitor interfacial alignment is linked to their donor-acceptor interactions with surface atoms, leading to formation of adsorbate-adsorbent coordination bond [51, 84, 85]. In donor-acceptor interactions, the electron-rich sites in an adsorbate molecule (e.g., O, N, S and P atoms, double and triple bonds,

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and aromatic rings) donate their electrons to vacant orbitals in adsorbent atoms. As a consequence, the ability of electron-rich sites for electron donation is a key parameter that strongly affects the surface binding of inhibitors through donor-acceptor adhesion mechanism.

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On the basis of frontier molecular orbital theory, the interfacial charge sharing and transfer depends on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular

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orbital (LUMO). The HOMO in adsorbate substances is reflective of the reactive groups which have the greatest ability for electron donation, while the LUMO regions are indicative of the adsorbate atoms which accept electrons from surface cations. The pictures for optimized molecular structure together with the HOMO and LUMO plots in four types of chosen organic-inorganic inhibitors are provided in Figure 12. It is evident from these results that HOMO and LUMO distribution in the constructed four chicoric acid-Zn-caffeic acid

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inhibitors differed from each other. In complexes of Chi-Zn-Caf1 and Chi-Zn-Caf3, where the chicoric acid is linked to zinc cation bonded to carboxyl oxygen atom of the caffeic acid, the HOMO distributed over the chicoric acid fragment of the complex inhibitors. As displayed, in

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these two inhibitors, the HOMO located almost over aromatic benzene ring, two -OH groups bonded to ring and C-C double bond. However, in Chi-Zn-Caf2 and Chi-Zn-Caf4 inhibitors in which the caffeic acid is covalently connected to Zn+2 through its hydroxyl O bonded to benzene

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cycle, the HOMO is noted to take place over the whole structure of caffeic acid. Consequently, it is concluded that depending on the type of caffeic acid covalent linkage to Zn cation the electron

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donation to unfilled orbitals of surface Fe cations could happen from both chicoric acid and caffeic acid fragments of organic-inorganic corrosion inhibitors. This proposes that both of the acidic compounds can serve as active sites for inhibitor adhesion onto metallic surface. On the other hand, the LUMO regions in all complexes of acidic agents and Zn cation occurred only

orbitals.

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upon the chicoric acid, and thus this acid can also accept electrons from surface atoms with filled

Figure 12

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From an electronic point of view, the energies of HOMO and LUMO (EHOMO and ELUMO), and their gap of ELUMO - EHOMO (∆EL-H) strongly influence the intensity of electronic charge transfer

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within donor-acceptor interactions. A HOMO which has higher energy level (i.e., less negative) is able to give more electrons to metallic surface, and a LUMO containing smaller energy (i.e., more negative ELUMO) is able to receive more easily surface electrons. Accordingly, a decreased ∆EL-H between LUMO and HOMO results in intensified surface binding caused by stronger donor-acceptor interactions. These quantum chemical parameters for all organic-inorganic inhibitor are listed in Table 1. The extent of inhibitors adhesion was also examined through QM

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factor of fracture of electrons transferred (∆N) which was determined from inhibitors electronegativity (χ) and hardness (η). These factors are also collected in Table 1. Based on the tabulated parameters, it is understood that the values for ∆EL-H and ∆N are almost the same for

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all four investigated types of complex inhibitors, suggesting their similar attachment to steel substrate. This observation accords well with identical adsorption energies of considered inhibitors found in MD simulations. The slightly lower ∆N in case of Chi-Zn-Caf1 is ascribed to

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its higher energy difference of ∆EL-H. Table 1

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To further investigate the reactive centers of organic-inorganic chicoric acid-Zn-caffeic acid inhibitors which contribute in coordination bond interactions with surface metal atoms, the Fukui indices were evaluated. Figure 13 presents the graphical distribution of atom-condensed Fukui -

functions relevant for electrophilic ( f ) and nucleophilic ( f

+

) behavior of corrosion inhibitors.

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From the depicted results it is seen that when the caffeic acid is bonded to zinc atom through its carboxyl O atom (i.e., Chi-Zn-Caf1 and Chi-Zn-Caf3), the

f - function took place only over the

chicoric acid active regions of benzene cycle and its neighboring O atoms as well as C-C double

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bond. In the other two cases of complex inhibitor (i.e., Chi-Zn-Caf2 and Chi-Zn-Caf4) the

for

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caffeic acid emerged as sites for Fukui function representing the electrophilic attack. Such results

f - function accord well with the HOMO distribution, and thus once again highlight the

crucial role of both acids in inhibitors adhesion via electrophilic behavior against surface atoms. Furthermore, analogous to LUMO results, the chicoric acid fragment in all selected organicinorganic inhibitors acted as the active zones for

f

+

function and thus is susceptible to

nucleophilic attacks at the interface. It can also be observed that the inorganic Zn atom behaved

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as nucleophilic site, which signifies that inorganic atoms could also intensify the inhibitor adsorption extent by involving in donor-acceptor interactions. Figure 13

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5. Conclusions

1- FT-IR analysis revealed the complex formation between zinc cations and organic compounds of CIL.L. TGA results showed the enhancement of thermal properties of

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hybrid pigment.

2- EIS analysis revealed that incorporation of hybrid pigment into the epoxy ester coating

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provided effective active corrosion inhibition properties and remarkably improved its barrier performance. The hybrid pigment is partially soluble in corrosive electrolyte and can release active inhibiting agents.

3- The computational observations from MD simulations and QM calculations affirmed that

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chosen caffeic acid-Zn-chicoric acid complex corrosion inhibitors were able to adsorb onto mild steel substrate through their electron-rich aromatic rings and oxygenated functionalities sites and also inorganic zinc cations.

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Figure captions: Figure 1- Chemical structure of chicoric acid (Chi), caffeic acid (Caf) and their different chicoric acid-zinc-caffeic acid (Chi-Zn-Caf) complexes. The hydroxyl (-OH) groups in inhibitors

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chosen for covalent bonding with zinc atom are shown in blue color.

Figure 2- The FT-IR spectra of (a1) CIL.L powder and (a2) hybrid Zn-CIL.L pigment

Figure 3- The (a1) TGA and (a2) DTG curves of the CIL.L powder and hybrid Zn-CIL.L

SC

pigment

Figure 4- Bode-modulus plots of the epoxy ester coatings with an artificial defect (2 cm in

M AN U

length) immersed in 3.5 wt.% NaCl solution for different immersion times; (a1) coating with 2 wt.% hybrid Zn-CIL.L pigment and (a2) without pigment.

Figure 5- Bode-phase plots of the epoxy ester coatings with an artificial defect (2 cm in length) immersed in 3.5 wt.% NaCl solution for different immersion times; (a1) coating with 2 wt.%

TE D

hybrid Zn-CIL.L pigment and (a2) without pigment.

Figure 6- The values of |Z|10 mHz for the epoxy ester coatings with an artificial defect (2 cm in length) immersed in 3.5 wt.% NaCl solution for different immersion times.

EP

Figure 7- (a1) The epoxy ester coating without hybrid pigment exposed to salt spray test for 300 h, (a2) SEM images and (a3) EDS results from scratch zone and steel/coating interface.

AC C

Figure 8- (a1) The epoxy ester coating with hybrid pigment exposed to salt spray test for 300 h, (a2) SEM images and (a3) EDS results from scratch zone and steel/coating interface. Figure 9- Bode diagrams of the steel panels coated with epoxy ester coatings (a1) without and (a2) with 2 wt.% hybrid pigment; (a3) the results of |Z|10 mHz and (a4) fb versus immersion time. Figure 10- Schematic presentation of the corrosion inhibition mechanism.

29

ACCEPTED MANUSCRIPT

Figure 11- The side and top views of the final snapshots of different organic-inorganic chicoric acid-zinc-caffeic acid (Chi-Zn-Caf) inhibitors over Fe (110) surface. The water molecules in side views were omitted for clarity.

inorganic chicoric acid-zinc-caffeic acid (Chi-Zn-Caf) inhibitors.

RI PT

Figure 12- The B3LYP/6-311G** optimized geometry, HOMO and LUMO of different organic-

Figure 13- The computed Fukui indices of different organic-inorganic chicoric acid-zinc-caffeic

AC C

EP

TE D

M AN U

SC

acid (Chi-Zn-Caf) inhibitors.

30

ACCEPTED MANUSCRIPT

Table 1- The calculated HOMO and LUMO energies (eV), ELUMO - EHOMO energy gap (∆EL-H) electron affinity (A), ionization potential (I), electronegativity (χ), hardness (η), and fracture of

311G** level. Inhibitor

EHOMO

ELUMO

∆E

A

Chi-Zn-Caf1

-6.017

-2.090

3.927

6.017 2.090 4.0535 1.9635 0.7503

Chi-Zn-Caf1

-5.591

-2.121

3.470

5.591 2.121 3.856

Chi-Zn-Caf1

-5.568

-2.133

3.435

5.568 2.133 3.8505 1.7175 0.9170

Chi-Zn-Caf1

-5.597

-2.126

3.471

5.597 2.126 3.8615 1.7355 0.9042

χ

M AN U TE D EP AC C

η

∆N

1.7350 0.9061

SC

I

RI PT

electrons transferred (∆N) for different organic-inorganic inhibitors determined at the B3LYP/6-

ACCEPTED MANUSCRIPT HO O HO

HO

HO

O

O O

Chicoric acid

O

OH

O OH

HO

OH

Caffeic acid

O

HO O

HO

Zn O

O

O

HO

O

O O

O HO

OH

HO

OH

O

Zn

Chi-Zn-Caf2

HO

O

O

O

O OH

M AN U

OH

SC

O

Chi-Zn-Caf1

OH

O

O

Zn

O

OH

O

O

O

HO

O O OH

O

OH

OH

O

OH

O

O

O

HO

Zn

O

O

OH

O

O HO

RI PT

HO HO

HO O

O

O

OH

OH

O OH

Chi-Zn-Caf3

HO

OH

O

OH

Chi-Zn-Caf4

Figure 1- Chemical structure of chicoric acid (Chi), caffeic acid (Caf) and their different

TE D

chicoric acid-zinc-caffeic acid (Chi-Zn-Caf) complexes. The hydroxyl (-OH) groups in

AC C

EP

inhibitors chosen for covalent bonding with zinc atom are shown in blue color.

ACCEPTED MANUSCRIPT

100 80

HO O

60

HO

HO O

O

C-H

40

O O OH

HO

O

20

RI PT

Transmillance (%)

(a1)

HO

(a2)

60

HO O HO

HO….HOOC

Zn O

O

O O

M AN U

40

SC

80

O

O HO

20

OH

OH

3440

COO-Zn

Zn-O

OH

COO-

Aromatic Ring

HO

0 3940

O

OH

O

2940

2440

1940

1440

EP

TE D

Wavenumbers (cm-1) Figure 2- The FT-IR spectra of (a1) CIL.L powder and (a2) hybrid Zn-CIL.L pigment

AC C

Transmillance (%)

100 0

940

440

ACCEPTED MANUSCRIPT

120

(a1) 80 60 40 20

RI PT

Weight loss (%)

100

CIL.L

Zn(NO3)2-Inh

0 0

100

200

300

400

(a2)

M AN U

0

-0.002 -0.004

700

Step 3 Step 1 Step 2

-0.008 0

100

TE D

-0.006

200

300

CIL.L

Step 4 400

Zn(NO3)2-Inh 500

600

EP

Temperature (ᵒC)

Figure 3- The (a1) TGA and (a2) DTG curves of the CIL.L powder and hybrid Zn-CIL.L pigment

AC C

Deriv . Weight (% .ᵒC)

0.002

600

SC

Temperature (ᵒC)

500

700

ACCEPTED MANUSCRIPT

100000

(a1)

|Z| (ohm cm2)

10000

100

1 0.1

1

10

f (Hz)

100

1000

M AN U

0.01

SC

10

100000

(a2) 10000

|Z| (ohm cm2)

3h

6h

24 h

48 h

72 h

RI PT

1000

30 min

100000

30 min

3h

6h

24 h

48 h

72 h

TE D

1000

10000

100

1 0.01

EP

10

0.1

1

10

f )Hz)

100

1000

10000

100000

AC C

Figure 4- Bode-modulus plots of the epoxy ester coatings with an artificial defect (2 cm in length) immersed in 3.5 wt.% NaCl solution for different immersion times; (a1) coating with 2 wt.% hybrid Zn-CIL.L pigment and (a2) without pigment.

ACCEPTED MANUSCRIPT

80

(a1) -phase angle (deg)

60

30 min

3h

6h

24 h

48 h

72 h

RI PT

40

20

0 1

10

f (Hz)

100

1000

M AN U

(a2)

60

40

20

0 0.01

0.1

TE D

-phase angle (deg)

80

0.1

10000

SC

0.01

1

10

f (Hz)

100

1000

100000

30 min

3h

6h

24 h

48 h

72 h

10000

100000

EP

Figure 5- Bode-phase plots of the epoxy ester coatings with an artificial defect (2 cm in

AC C

length) immersed in 3.5 wt.% NaCl solution for different immersion times; (a1) coating with 2 wt.% hybrid Zn-CIL.L pigment and (a2) without pigment.

ACCEPTED MANUSCRIPT

14000

RI PT

10000

SC

8000

4000

M AN U

6000

Epoxy ester coating/hybrid pigment Neat epoxy ester coating

2000 0

10

20

30

40

50

60

70

80

TE D

Immersion time (day)

Figure 6- The values of |Z|10 mHz for the epoxy ester coatings with an artificial defect (2 cm

EP

in length) immersed in 3.5 wt.% NaCl solution for different immersion times.

AC C

|Z|/ohm cm2)

12000

ACCEPTED MANUSCRIPT (a1)

15 mm (a2) Beneath the coating

SC

RI PT

Inside the scratch

(2)

M AN U

(1)

AC C

(1)

Beneath the coating

EP

TE D

(a3)

Inside the scratch (2) Figure 7- (a1) The epoxy ester coating without hybrid pigment exposed to salt spray test for 300 h, (a2) SEM images and (a3) EDS results from scratch zone and steel/coating interface.

(a1) ACCEPTED MANUSCRIPT

15 mm (a2) Beneath the coating

RI PT

Inside the scratch

SC

(2)

M AN U

(1)

EP

Beneath the coating

AC C

(1)

TE D

(a3)

Figure 8- (a1) The epoxy ester coating with hybrid pigment exposed to salt spray test for 300 h, (a2) SEM images and (a3) EDS results from scratch zone and steel/coating interface.

12

30 days

11

90

10

75

8

60 6 45 4

30

2

15

0

0 -2

-1

0

1

2

3

Neat epoxy ester coating

8 7 6

5 5

4

60

6

45

4

0

1

7 Days

15 days

22 days

30 days

2

3

4

(a4)

Neat epoxy ester coating

800

600

400

200 15 0

0 5

15

25

Immersion time (days)

EP

log (f/Hz)

TE D

30

-Phase angle (deg) fb (Hz)

75

8

-1

AC C

Figure 9- Bode diagrams of the steel panels coated with epoxy ester coatings (a1) without and (a2) with 2 wt.% hybrid pigment; (a3) the results of |Z|10 mHz and (a4) fb versus immersion time.

35

Zn-CIL.L pigment/Epoxy ester coating

1000

M AN U

10

-2

25

SC

90

0

15

Immersion time (days)

105

(a2)

2

(a3)

9

log (f/Hz) 12

log (|Z|/ohm cm2)

Zn-CIL.L pigment/Epoxy ester coating

RI PT

22 days

105

ACCEPTED (a1) MANUSCRIPT log (|Z|/ohm cm2)

15 days

-Phase angle (deg)

log (|Z|/ohm cm2)

10

7 days

35

ACCEPTED MANUSCRIPT HO O HO

HO

Zn2+

O

Zn2+

O O O

HO

O

2+

Zn

OH

OH

OH

HO

O

RI PT

HO

HO

O

2H2O+O2+4e→4OH-

HO

Zn

Fe→Fe2++2e O

O

O

O

O

HO

O

O

Fe

O

HO

HO

O

O

O

O OH

OH

O O O

O

O

O

OH

HO

O

O

O

OH

OH

HO

O

HO

O

O

HO

OH

O

OH

Cathodic region

HO

O

HO

OH

O

M AN U

OH

O

HO

O

O O

HO

HO

HO

O

O

O

SC

OH

O

O

O

HO

Anodic region

H2O, O2, Cl-, Na+

-

AC C

EP

TE D

H2O, O2, Cl , Na

+

O

OH

HO

Zn O

HO

O

Figure 10- Schematic presentation of the corrosion inhibition mechanism.

OH

O

ACCEPTED MANUSCRIPT

Chi-Zn-Caf1

Chi-Zn-Caf3

Chi-Zn-Caf4

∆Eads=-319.3

∆Eads=-333.9

∆Eads=-324.4

M AN U

SC

RI PT

∆Eads=-325.9

Chi-Zn-Caf2

Figure 11- The side and top views of the final snapshots of different organic-inorganic chicoric acid-zinc-caffeic acid (Chi-Zn-Caf) inhibitors over Fe (110) surface obtained at the

AC C

EP

TE D

end of 2 ns MD simulations. The water molecules in side views were omitted for clarity.

ACCEPTED MANUSCRIPT

HOMO

LUMO

Chi-Zn-Caf4

M AN U

Chi-Zn-Caf3

SC

Chi-Zn-Caf2

RI PT

Chi-Zn-Caf1

Optimized geometry

Figure 12- The B3LYP/6-311G** optimized geometry, HOMO and LUMO of different

AC C

EP

TE D

organic-inorganic chicoric acid-zinc-caffeic acid (Chi-Zn-Caf) inhibitors.

ACCEPTED MANUSCRIPT

-



SC

Chi-Zn-Caf2

TE D

M AN U

Chi-Zn-Caf3 Chi-Zn-Caf4

(FI for nucleophilic attack) f

RI PT

Chi-Zn-Caf1

(FI for electrophilic attack) f

Figure 13- The computed Fukui indices of different organic-inorganic chicoric acid-zinc-

AC C

EP

caffeic acid (Chi-Zn-Caf) inhibitors.

ACCEPTED MANUSCRIPT A hybrid organic-inorganic inhibitive pigment was synthesized and characterized

•

Hybrid pigment significantly improved corrosion inhibition performance of coating

•

Theoretical studies proved the inhibitors adhesion onto mild steel surface.

AC C

EP

TE D

M AN U

SC

RI PT

•