Synthesis, characterization and corrosion inhibition studies of N-phenyl-benzamides on the acidic corrosion of mild steel: Experimental and computational studies

Accepted Manuscript Synthesis, characterization and corrosion inhibition studies of Nphenyl-benzamides on the acidic corrosion of mild steel: Experimental and computational studies

Ankush Mishra, Chandrabhan Verma, H. Lgaz, Vandana Srivastava, M.A. Quraishi, Eno E. Ebenso PII: DOI: Reference:

S0167-7322(17)35189-9 doi:10.1016/j.molliq.2017.12.011 MOLLIQ 8307

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

31 October 2017 1 December 2017 4 December 2017

Please cite this article as: Ankush Mishra, Chandrabhan Verma, H. Lgaz, Vandana Srivastava, M.A. Quraishi, Eno E. Ebenso , Synthesis, characterization and corrosion inhibition studies of N-phenyl-benzamides on the acidic corrosion of mild steel: Experimental and computational studies. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2017.12.011

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ACCEPTED MANUSCRIPT Synthesis, characterization and corrosion inhibition studies of N-Phenylbenzamides on the acidic corrosion of mild steel: Experimental and computational studies Ankush Mishraa, Chandrabhan Vermab,c*, H. Lgaz,d,e Vandana Srivastavaa, M. A. Quraishia,f, and Eno E Ebensob,c** Department of Chemistry, Indian Institute of Technology (Banaras Hindu University),

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a

b

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Varanasi-221005,

Department of Chemistry, School of Chemical and Physical Sciences, Faculty of Natural and

c

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Agricultural Sciences, North-West University, Private BagX2046, Mmabatho 2735, South Africa Material Science Innovation &Modelling (MaSIM) Research Focus Area, Faculty of Natural

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and Agricultural Sciences, North-West University, Private Bag X2046, Mmabatho 2735, South Africa

Laboratory of Applied Chemistry and Environment, ENSA, Universite Ibn Zohr, PO Box 1136,

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d

80000 Agadir,Morocco.

Laboratory of separation methods, Faculty of Science, Ibn Tofail University PO Box 242,

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e

Kenitra, Morocco

Center of Research Excellence in Corrosion, Research Institute, King Fahd University of

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f

Petroleum & Minerals, Dhahran 31261, Saudi Arabia.

Eno. Ebenso ([email protected])

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Abstract:

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Corresponding Author’s Emails: C. Verma ([email protected]);

Present study aims to demonstrate the effect of electron withdrawing nitro (-NO2) and

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electron releasing methoxy (-OCH3) substituents on the inhibition behavior of N-Phenylbenzamide derivatives (BNAs), namely N-(4-nitrophenyl) benzamide (BNA-1; -NO2), Nphenylbenzamide (BNA-2; -H) and N-(4-methoxyphenyl)benzamide (BNA-3; -OCH3) for mild steel acidic (1M HCl) corrosion. Results of the computational and experimental studies showed that methoxy (-OCH3) substituent enhances the inhibition efficiency whereas nitro (-NO2) decreases the inhibition efficiency. Electrochemical impedance spectroscope (EIS) study showed that BNAs acted as interface corrosion inhibitors and polarization study shows they acted as cathodic type corrosion inhibitors. They showed maximum efficiencies of 89.56%, 93.91% and 96.52% for BNA-1, BNA-2 and BNA-3, respectively. The BNAs strongly (high Kads values) and

ACCEPTED MANUSCRIPT spontaneously (negative ∆G0 values) adsorbed at metal/ electrolyte interfaces and their mode of adsorption obeyed the Langmuir adsorption isotherm. Surface investigation of the mild steel surfaces using AFM and SEM analyses revealed that BNAs adsorb on the surface and increase the energy barrier for corrosive dissolution which is also supported by the higher values of Ea (activation energy). DFT study carried out using 3-21G, 6-31G and 6-311G basis sets provides significant support to the experimental efficiency order. DFT indices like EHOMO, ELUMO,

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electronegativity (χ), softness (σ), hardness (η), dipole moment (μ), energy band gap (∆E) and

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fraction of electron transfer (∆N) were derived for all basis sets. Experimental and DFT study

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was further supported by molecular dynamic (MD) simulations study.

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Keywords: Acid inhibitors, Phenyl-benzamides, Langmuir isotherm, MD simulations, computational (DFT) study. 1. Introduction

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Acidic cleaning, acid descaling, oil well acidifications are some common industrial processes those are frequently being used for cleaning of the impure metals and alloys. However,

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these processes result into the loss of metallic components by their chemical and electrochemical

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reactions with the aggressive cleaning (acidic) solutions. The dissolution of metallic materials by the chemical and electrochemical reactions is generally termed as corrosion. Therefore, for

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effective and economic cleaning of metals and alloys the corrosive losses should be avoided or prevented. In view of this, several methods of corrosion protection are being used among them

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application of synthetic compounds are most economic, effective and widely used representatives for the retardation of metallic corrosion in aggressive solutions [1-3]. Generally, these compounds possess affinity to adsorb strongly on the metallic surfaces through their

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electron rich centers. The electron rich centers comprise of their multiple bonds of aromatic and aliphatic rings and their side chain substituents as well as non-bonding and π-electrons of functional groups such as -OCH3, -OH, -COOC2H5, -O-, -NO2, -CONH2, -CN, -SO3H and COOH [3-6]. The polar substituents exert significant effect on the physiochemical properties of these compounds such as their solubility, mode of adsorption/ planarity of the metallic surface and electron density on the donor sites which ultimately determine their effectiveness towards inhibition of corrosion [7,8]. It has been reported that electron releasing (+I and +R) groups (OH, -NH2, -OCH3) enhances electron density at the donor sites/ adsorption centers and thereby

ACCEPTED MANUSCRIPT enhances the protection tendency of the inhibitor molecules through their existence [8-12]. Conversely, electron withdrawing (-I and –R) substituents (-CN, -NO2, -COOC2H5) decreases electron density at the donor sites/ adsorption centers therefore decrease the inhibition efficiency of the inhibitors through their manifestation [8-12]. Previously, our research group demonstrated the influence of hydroxyl(s) (–OH), methyl (-CH3), methoxy (-OCH3), methoxy and hydroxyl (OCH3+-OH), nitrile (-CN), nitro (-NO2) and several other substituents on the inhibition

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efficiency of synthetic inhibitors for metallic corrosion in several electrolytic media [13-15].

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In the perpetuation of our reports [13-15], we herein investigated the inhibition efficiency of three newly synthesized N-Phenyl-benzamides, namely 4-Nitro-N-phenyl-benzamide (BNA-1), N-Phenyl-benzamide (BNA-2), and 4-Methoxy-N-phenyl-benzamide (BNA-3) on mild steel

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dissolution in aggressive acidic solution using experimental and theoretical studies. Weight loos, electrochemical (PDP and EIS) and surface (AFM) experimental methods was selected to study

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the inhibition property of BNA molecules. Benzamide and its derivatives represent a broad range of organic compounds that have been used for several industrially and biologically useful

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applications [16-18]. Literature survey reveals that several benzamide derivatives possess significant anti-inflammatory, anti-microbial, anti-cancerous, anti-analgesic and other biological

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activities [16-18]. The increasing demand of benzamide derivatives forces the scientist working in the synthetic chemistry to develop new such compounds having applications other that

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biological activities. Benzamide derivatives are also being employed as inhibitors for metallic corrosion in different electrolytic media [19,20]. Presence of amide functional group (-CONH2)

Experimental sections

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2.

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and π-electrons of the aromatic ring(s) serves as centers for meta-inhibitor interactions [19,20].

2.1. Materials and chemicals All reactions were performed in round bottom flasks under open air condition. Solvents and chemicals were purchased from commercial suppliers and used without further purification. Thin layer chromatographic analysis, silica gel plates with UV254 fluorescence indicator (MERCK Kieselgel 60 F254, 0.25 mm thick) were used and visualized by exposure to ultraviolet light (UV), then further analyzed by putting in I2 chamber. The synthetic scheme for BNA molecules are shown in Fig. 1. Melting points of the investigated BNA molecules were determined using open capillary melting point apparatus. For IR spectral measurements of the studied BNA

ACCEPTED MANUSCRIPT molecules Perkin–Elmer Spectrum 100 FT–IR spectrophotometer was employed whereas 1H and 13

C NMR spectral analyses were carried out using Bruker Avence spectrometers (1H NMR at 13

500 MHz,

C NMR at 126 MHz) at 298 K in CDCl3 and DMSO–d6 solvents with

tetramethylsilane as internal standard. Mass spectra were measured on water’s Quattro Micro V 4.1. To an oven dried round bottom flask, 1 equivalent of p-substituted aniline in DCM and 1 equivalent of DMAP was added. Reaction temperature was maintained to 0 °C and 1 equivalent

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of benzoyl chloride was added with dropping funnel. . The reaction temperature of 0 °C was

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maintained using ice bath with occasional use of sodium chloride salt. After the complete

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addition, reaction mixture was allowed to stir at room temperature for 2 hrs. Progress of reaction was monitored with TLC. After completion of reaction, washed with dil. HCl and dried over

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sodium sulfate and recrystallized with DCM. The synthesized BNAs derivatives tested as effective inhibitors for mild steel with wt.% composition of Cr (0.05%), Al (0.023%), Mn

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(0.192%), Si (0.026%), C (0.076%), P (0.012%) and Fe (99.621%) in 1M HCl manuscript from dilution of 37% HCl (MERCK) in double ionized water. The testing material surfaces were

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abraded and cleaned with 600-1200 mesh size emery papers followed by their washing with water (distilled) and acetone. The metallic surface becomes relatively smoother after abrasive

2.2. Methods

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2.2.1. Experimental methods

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cleaning with the emery papers of high mesh size like 1000 and 1200.

2.2.1.1. Weight loss methods

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First of all, the effect of concentration of synthesized BNA molecules on mild steel corrosion have been demonstrated using weight loss experiments as discussed previous methods [13-15].

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For more reproducibility of the weight loss data, weight loss experiments at each concentration were triply performed and mean values have been reported. The effect of BNAs concentrations for mild steel corrosion using weight loss and electrochemical studies on their optimum concentrations were carried out at 308K temperature. The weight loss indices were determined using following equations [13-15]: CR 

87.6  w AtD

% 

CR  CR(i) CR

(1) 100

(2)

ACCEPTED MANUSCRIPT 

CR  CR(i)

(3)

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In the above, D, A and w represent the density of test (mild steel), surface area (2.5 cm× 2.0 cm× 0.025 cm) and weight loss after time t (3 hrs), respectively. CR(i) and CR denotes the values of corrosion rates inhibited and non-inhibited situations. 2.2.1.2. Electrochemical studies

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In order demonstrate the inhibitive mechanism of BNAs action, electrochemical analysis was performed after weight loss measurement employing Gamry apparatus (Potentiostat), model G-

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300 which have Gamry Echem Analyst 5.0 software for interpretation of data. The specimen’s preparation was exactly same as described in our previous literature reports [13-15].

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Electrochemical studies at each concentration performed for the two times and best value was chosen for each set of experiments. Saturated calomel, platinum foil and mild steel specimens

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were selected as reference, counter and working electrodes, respectively. After 30 minutes dipping time for stabilization of OCP i.e. open circuit potential, EIS measurements was carried

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out. The EIS study was performed using AC signal that have amplitude of 10 mV and frequency range of 0.01 Hz to 100 kHz. Fitting of EIS (Nyquist plots) data into a suitable equivalent circuit

  

 %  1 

Rp   100 Rp (i ) 

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calculated as follows [13-15]:

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results into derivation of polarization resistance (Rp) using that inhibition efficiency was

(4)

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In the above, equation Rp(i) and Rp denote the values of polarization resistance values under inhibited and non-inhibited situations, respectively. PDP study was carried out by changing the

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mild steel (working electrode) potential from -250 mV - +250 mV against the corrosion potential (Ecorr). Corrosion current density (icorr) values were computed through extrapolating the straight parts of the Tafel curves using which η% was derived as follows [13-15]:

% 

0 i icorr  icorr 100 0 icorr

(5)

where, i0corr and iicorr denotes current densities of non-inhibited and inhibited working electrodes respectively. 2.2.1.3. Surface studies

ACCEPTED MANUSCRIPT Ziess-Evo 50 XVP instrument was employed for SEM analysis. The magnification for SEM study was 2000×. NT-MDT multimode AFM instrument consisting of Russia 111 under solvers scanning probe microscope manager with lonely beam cantilever having frequency 240-255 kHz that has a fixed spring constant of 11.5 N/m in semi contact mode was undertaken for in the present study. For interpretation of AFM images, the NOVA programme was undertaken. The 5 mm×5 mm area was selected for AFM study.

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3. Results and discussions

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3.1. Inhibitors characterization methods. Their 1H and

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The newly synthesized N-Phenyl-benzamide derivatives characterized using their spectral C NMR spectra are given in Fig. 2 and their characterization data are

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listed in Table 1. Their characterization data are as follows: 4-Nitro-N-phenyl-benzamide (BNA1): Mol. Formula: C13H10N2O3; Mol. Wt. 242.23; Yellow solid; Yield = 94%; FT-IR (KBr, cm-1):

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3067, 2358, 1744, 1653, 1595, 1499, 1401, 1336, 1302, 1243, 1170, 1104, 998, 899, 844; 1H NMR (500 MHz, DMSO-d6) δ 10.82 (s, 1 H), 8.27 (d, 2 H), 8.06 (d, 2 H), 7.97 (d, 2 H), 7.60 (m, 13

C NMR (126 MHz, DMSO-d6) δ 166.39, 145.45, 142.60, 134.24, 132.35, 128.67,

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3 H) ppm;

128.01, 124.92, 119.92 ppm; EM m / z (%): 243 (M+1), 105 (100); N-Phenyl-benzamide (BNA-

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2): Mol. Formula: C13H11NO; Mol. Wt. 197.23; White solid; Yield = 95%; FT-IR (KBr, cm-1): 3340, 1650, 1575, 1260, 750, 692; 1H NMR (500 MHz, CDCl3) δ 7.93 – 7.79 (m, 3H), 7.64 (d, 2 13

C NMR (126 MHz, CDCl3) δ

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H), 7.55 (t, 1 H), 7.48 (t, 2 H), 7.37 (t, 2 H), 7.15 (t, 1 H) ppm;

165.90, 138.06, 135.16, 131.99, 129.25, 128.94, 127.16, 124.73, 120.35 ppm; EM m/z (%):198

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(M+1), 105 (100); 4-Methoxy-N-phenyl-benzamide (BNA-3): Mol. Formula: C14H13NO2; Mol. Wt. 227.09; Yield = 92%; FT-IR (KBr, cm-1): 3323, 1640, 1509, 1452, 1399, 1321, 1229, 1107,

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1022, 817; 1H NMR (500 MHz, CDCl3) δ 7.85 (d, 3 H), 7.60 – 7.35 (m, 5 H), 6.89 (d, 2 H), 3.81 (s, 3 H) ppm;

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C NMR (126 MHz, CDCl3) δ 165.82, 156.76, 135.15, 131.82, 131.13, 128.86,

127.12, 122.28, 114.36, 55.63 ppm; EM m / z (%): 228 (M+1), 105 (100).

3.2. Corrosion inhibition study 3.2.1.Gravimetric measurements 3.2.1.1. Effect of BNAs concentration The influence of BNAs concentrations on the dissolution of mild steel is shown in Fig. S1 and Table 2. Inspection of these results manifest that the values of protection abilities and surface

ACCEPTED MANUSCRIPT coverage’s increase while corrosion rates values decrease with the BNAs concentrations ranging from 2.54×10-5 to 12.68×10-5 mol/L. The increased inhibition ability of BNA molecules at their higher concentration is attributed to increased surface coverage values. Further, BNA-3, BNA-2 and BNA-1 showed the maximum efficiencies of 96.52%, 91.93% and 89.56% at 12.68×10-5 mol/L concentration. However, from Table 2 it can be seen that increase in the BNAs concentration from 10.14×10-5 to 12.68×10-5 mol/L didn’t cause any significant enhancement in

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their protection ability that suggests that 10.14×10-5 mol/L is optimum concentration. The order

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of BNAs inhibition efficiency can be explain with the help of Hammett substituent constant (σ)

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values of the substituents attached to the aromatic ring of the aldehyde moiety. BNA-3 contains – OCH3 (methoxy) substituent that have maximum (in negative) value of σ i.e. -0.22 which is an

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indicative that methoxy group is an electron donor substituent and enhances the electron density on the sites responsible for metal-inhibitor interactions that ultimately results into highest

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inhibition efficiency of BNA-3 [7-9]. In contrast, the lowest inhibition efficiency of the BNA-1 is attributed to the nitro (-NO2) substituent that has highest positive value of Hammett substituent

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constant i.e. +0.78 which indicates that presence of nitro group decreases the electron density from the sites responsible for metal-inhibitor interaction and thereby decreases the inhibition

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efficiency as compared to non-substituted inhibitor molecule (BNA-1) [7-9].

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3.2.1.2. Activation parameter (effect of temperature) In addition to the effect of BNAs concentrations, effect of temperature has also been studied on

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the protection power of BNA molecules towards mild steel corrosion and presented in Fig. S2 and Table 3. Examination of these results reveals that inhibition power of all three BNA

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molecules decreases with increase in the surrounding temperature. This type of effect is generally attributed to the enhanced kinetic energy of the BNA molecules at high solution temperature that ultimately decreases the attractive interactions between inhibitor and metal surfaces [7-9,13-15]. Additionally in acidic solution (1M HCl), increased temperature can cause molecular rearrangement, fragmentation and possibly rapid etching which are also exert negative impact for metal-inhibitor interactions. The Arrhenius equation (equation 12) is most repeatedly being employing in order to define the impact of temperature over the protection capability of inhibitors [13-15]:

ACCEPTED MANUSCRIPT log CR 

 Ea  log A 2.303RT

(12)

In equation (10), CR, A, Ea, R and T represent corrosion rate (in mgcm-2h-1), Arrhenius preexponential factor, activation energy (kJ/mol), gas constant and absolute temperature, respectively. Arrhenius plots (log CR vs. 1/T) for mild steel dissolution with and without BNA

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molecules are presented in Fig. 3. The values of Ea derived from Arrhenius slope are 79.16

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kJmol-1, 69.38 kJmol-1 and 56.88 kJmol-1 for BNA-3, BNA-2 and BNA-1. The higher values of Ea for inhibited conditions as compared to inhibited condition (Ea= 28.48 kJmol-1) indicate the

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mild steel dissolution under inhibited situations acquired higher energy barrier as compared to under uninhibited condition [13-15, 21-23]. It is documented in literature that the increase in Ea

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value in inhibited situation is also attributed to the electrostatic interactions between oppositely

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charged metallic surfaces and inhibitor molecules [21-23].

3.2.1.3. Thermodynamic parameters (adsorption isotherm)

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The mechanism and effectiveness BNA molecules adsorption on the metallic surface can be derived from their adsorption isotherm plots which is a plotted between log of concentration and

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surface coverage on that concentration. To find best adsorption isotherm, surface coverage values of BNAs and log of their concentrations were tailored in several isotherms such as

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Temkin, Langmuir and Freundluich. The Langmuir adsorption isotherm is presented in Fig. 4 while Temkin and Freundluich adsorption isotherms are presented as Fig. S3. The values

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intercept, slope and regression coefficients for different tested isotherm are presented in Table S1. The Langmuir adsorption isotherm that can be accessible as follows [13-15]:



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K ads C 

1

(13)

In Langmuir equation, θ denotes the surface coverage by BNA molecules at the concentration C, and Kads denotes the constant for the adsorption-desorption phenomenon. Obviously, magnitude of Kads is a measure for the effectiveness of the metal-inhibitor interactions. Its high value demonstrates the strong interaction between metallic surface and adsorption centers of the inhibitor (BNA) molecules. The values of Kads also serve as digits for the calculations of standard

ACCEPTED MANUSCRIPT Gibb’s free energy (∆Gads) for BNA (inhibitor) molecules adsorption as per the following relationship [13-15]:

Gads   RT ln(55.5Kads )

(14)

In above relationship, all symbols have their usual meaning and significance. The value 55.5

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represents the aqueous concentration (molL-1) in acid solution. Table 4 denotes the ∆Gads and

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Kads values for BNA molecules at different investigated temperatures. Examination of the results revealed decrease in the values of Kads with increase in temperature which is ultimate results of

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decrease effectiveness of the interaction occurring among inhibitors and metallic surface. The high values of Kads for BNA-1 (1.19×104), BNA-2 (1.50×104) and BNA-3 (3.32×104) indicate the

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strong and effective interactions between BNAs and mild steel surface. For BNA molecules values of ∆Gads vary from 33.36 kJmol-1 to 36.96 kJmol-1 which suggest that BNA molecules

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spontaneously adsorbed on metallic surface through their several electron rich adsorption. Careful examination of the results revealed that values of ∆Gads for BNAs approach more

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towards chemisorption region (-40 kJmol-1 or more negative) as compared to the physisorption region (-20 kJmol-1 or less negative) [13-15,24,25]. On the basic of this observation, it is

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concluded that adsorption of BNA molecules obeyed the physiochemisorption mechanism with

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slight dominance of chemisorption mode [13-15,24,25]. 3.3. Electrochemical analyses

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3.3.1. OCP analysis

The variation of open circuit potential (EOCP) which is a potential generated on working (mild

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steel) electrode against the SCE potential, with time (40 minutes) for BNA molecules is shown in Fig. 5. The EOCP vs. time curves represent straight lines with and without of BNA molecules which is an indicative of stabilization of steady state potential in both situations. It is further observed from the Fig. 5 that EOCP under inhibited situations are shifted towards nobler (negative) direction which indicates the complete dissolution/ removal of surface oxide layer and adsorption of BNA molecules on the metallic surface [21, 26]. 3.3.2. Polarization analysis

ACCEPTED MANUSCRIPT Fig. 6 denotes the Tafel polarization curves for working electrode dissolution in the absence and presence of 10.14 ×10-5 molL-1 concentration of BNA molecules. Polarization parameters under inhibited and uninhibited situations are given in Table 5. Results (Table 5 and Fig.6) showed the BNA molecules affect the mechanism of anodic and cathodic mild steel dissolution reactions and shift current density towards lower lesser value without influencing the common characteristics of Tafel curves. This observation is an indication that BNA molecules retard mild steel corrosion

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by blocking the active sites located over the surface [13-15,27,28]. The blocking of active sites

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mainly attributed due to adsorption of BNAs through their electron rich adsorption centers.

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Careful observation shows that as compared to the Tafel slopes of uninhabited situation, in the inhibited conditions (in the presence of BNAs) values of cathodic Tafel slopes (βc) are more

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influenced as compared to the anodic Tafel slopes (βa) which indicates that BNAs mainly behaved as cathodic type inhibitors [13-15]. Shift in the corrosion potential (Ecorr) in the presence

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of BNA molecules than in their absence gives information about the type of inhibitors. The shift in Ecorr value more than -85 mV suggests that inhibitor has some specific inhibitive behavior

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either for anodic or cathodic reactions. However, if the shift is less than -85 mV then inhibitor can be defined as mixed type. The BNA-1, BNA-2 and BNA-3 showed the maximum shift in the

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value of Ecorr of -72 mV, 77 mV and 81 mV, respectively. The shift in Ecorr values towards negative directions indicates that BNA molecules act as mixed type inhibitors with little bit

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3.3.3. EIS analysis

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cathodic predominance behavior [13-15,27,28].

Fig. 7 a-b represents the Bode and Nyquist plots under inhibited (in the presence of 10.14×10-5

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molL-1 BNAs concentration) and uninhibited situations for mild steel dissolution in 1M HCl. Single imperfect semicircular Nyquist plots in inhibited by BNAs and uninhibited condition suggests that mild steel dissolution takes place through single charge transfer mechanism [1315,27,28]. Development of single maxima in the Bode plots at around intermittent frequency regions. Imperfect semicircuit of Nyquist plots under studied conditions is attributed to the surface inhomogeneity which is a consequence of corrosion phenomenon. Nyquist plots under inhibited situations show much higher diameters as compared to the uninhibited situation which is also clear from the increased values of polarization resistances (Rp) in the presence of BNA molecules (inhibited situations) as presented in Table 6. The increased Rp values in the inhibited

ACCEPTED MANUSCRIPT conditions revealed that process of iron oxidation or charges (electrons) transfer from the metal to electrolytic solution acquired substantial resistance due to blockage of surface active sites by BNA molecules. EIS parameters were computed with the aid of equivalent circuit (Fig. 7c) that contains two resistances; namely polarization and solution resistances (Rp and Rs, respectively) and a constant phase element (CPE). For metals dissolution process in aggressive solutions, replacement of Rct by Rp is gives better approximation as Rp contains several associated

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resistances along with charge transfer resistance (Rct), such as film, pore and diffusion resistances

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(Rf, Rpr and Rd, respectively) i.e. Rp = Rct+Rf+Rpr+Rd etc. For acidic metal-inhibitor interactions,

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employment of the CPE gives closer and accurate approximation. The impedance of CPE (ZCPE)

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(15)

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is generally signified as follows:

Where, n, ω, j and Y0 denote the phase shift, angular frequency, imaginary number and CPE

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constant, respectively. The n value is being used to measure for surface characteristics like surface homogeneity and nature of the CPE. The CPE behaves like an inductance for n = −1 (Y0

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= 1/L); resistance for n = 0 (Y0 = R); capacitance for n = 1 (Y0 = C); and Warburg impedance for n = 0.5 (Y0 = W) situations [22,27,28]. In our present case magnitudes of n vary from 0.827 to

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0.853 which indicates that CPE acts as pseudo-capacitor. The increased values of n in Table 6 and phase angle values of inhibited Bode plots indicate that metallic surfaces become smoother

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under same condition which is resulted due to adsorption of BNA molecules on the surface.

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Equation (12) was employed the derived double layer capacitance (Cdl) [13-15,22,27,28]: Cdl = Y0 (ωmax) n-1

(16)

In the above equation, ωmax denotes the angular frequency that corresponds to maximum value of imaginary impedance. Table 6 reveals that presence of BNA molecules in the corrosive environment (1M HCl) causes significant enhancement in the Rp and decrease in Cdl values revealed that increase in surface coverage and decrease in electric double layer values in the presence of BNA molecules [13-15,27-30]. 3.4. Surface (SEM-AFM) analyses

ACCEPTED MANUSCRIPT Figs. 8 and 9 show the SEM and AFM micrographs of mild steel surface corroded with and without BNA molecules for 3 hrs. Fig. 8a is the SEM micrograph of surface corroded in the absence of BNA molecules which showed a highly corroded surface. This outcome suggests that in the absence of BAN molecules, corrosive attack of aggressive acidic solution causes substantial damage of metallic surface which comes out in the form of highly corroded surface. However, the SEM images (Fig. 8b-c) of the surfaces inhibited by BNA molecules show

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smoothness in the morphology which is a consequence of adsorption of BNA molecules on the

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metal-electrolyte interfaces. Similar results were observed by AFM analysis of mild steel surface

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corroded with and without BNA molecules. Aggressive acidic corrosion mild steel surface in the absence of BNA molecules results into very high average surface roughness of 392 nm as

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depicted in Fig. 9a. However, in the presence of optimum concentration of BNAs resulted significant improvement in the morphology of the AFM micrographs (Fig. 9b-d). The computed

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average roughnesses were 169nm, 146nm and 105 nm for BNA-1, BNA-2 and BNA-3, respectively. The SEM and AFM analyses supported the assumption that BNA molecules inhibit

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surface dissolution through the formation of protecting covering.

3.5.1. Computational methods

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3.5. Computational analyses

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DFT analysis was carried out using Gaussian 09 (Version D.01) program containing correlation functional of B3LYP (Lee-Yang-Paar) in addition to Becke three-parameter hydride functional

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was chosen for DFT study for neutral, protonated as well as solvated forms of BNA molecules. During all DFT calculations 6-31+G(d, p) basis set was selected. The values of frontier

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molecular orbital energies (ELUMO and EHOMO) were computed from the most established conformations of BNA molecules. Other DFT parameters were computed using following relationship [31,32]: E  E LUMO  E HOMO



1 E LUMO  E HOMO  2

 

1 E LUMO  E HOMO  2

(6)

(7)

(8)

ACCEPTED MANUSCRIPT N 

 Fe   inh 2 Fe   inh 

(9)

Where, EHOMO, ELUMO, ∆E, χ, σ, η, μ and ∆N represent the energy of frontier molecular orbitals, energy gap, electronegativity, softness, hardness, dipole moment and fraction of electron transfer. In present investigation the selected values of χFe and ηFe, were 4.26 eV and 0 eV,

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respectively. In general, work functions for polycrystalline, (100), (110) and (111) forms of iron

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are 4.26 eV, 4.64 eV, 4.52 eV and 4.74 eV, respectively. In the study, selection of 4.26 eV value

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as work function for iron is attributed to the association with lowest energy and maximum stability of this form of iron [33-35].

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The interaction among BNA molecules and metallic surface was also carried out by MD simulations in which Materials Studio 6.0 was employed [36]. For the simulations, iron (Fe)

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crystal with 110 plane and slab of 5 Å was imported and cleaved. Selection of 110 plane in the present for the interaction of BNA molecules on iron surface is based on the fact that this crystal

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structure is associated with minimum energy because of its most packed crystal lattice. Energy of the undertaken iron surface i.e. 110 surface was diminished through smart minimizer technique.

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In order to obtained sufficient area for BNAs and Fe surface interactions, a high metallic surface area of 10×10 supercell was taken. The zero thickness vacuum slabs were built for simulations

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process. The size of supercell was a = b = 24.82 Å c = 25.14 Å was selected for the simulations that encompasses 9Cl-, 491 H2O, 9H3O+ and solitary molecule of BNA inhibitors. The MD

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simulations was performed in the simulation box of 24.82×24.82×35.69 Å3 area employing discover module through a time period of 1 fs and simulation period of 2000 ps at 298 K and

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NVT collective. The simulations were carried with the aid of COMPASS force field [37]. In interacting system, the information about BNAs and Fe (110) interactions was obtained in the form of interaction (Einteraction) and binding (Ebinding) energies those can be computed using following equations (10) and (11) [38,39]:

Einteraction  Etotal  ( Esurface+solution +Einhibitor )

(10)

EBinding   Einteraction

(11)

ACCEPTED MANUSCRIPT In above two equations, Etotal is the magnitude of entire energy related to the iron surface and solution (electrolyte) in the absence of BNA molecules whereas Einhibitor denotes the entire energy related to the BNA molecules. 3.5.2. Detailed description of results In order to further investigate the mechanism of BNAs molecules interaction on the metal

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surface DFT study was performed on the neutral as well as protonated forms of BNA molecules.

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The DFT calculations were carried out employing several B3LYP functional and three basis sets

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namely 3-21G, 6-31G and 6-311G among best correlation was observed in the case of 3-21G basis set. The optimized, HOMO and LUMO (frontier) molecular orbital pictures of the BNA molecules derived using 3-21G basis set are given in Fig. 10 and those derived using other basis

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sets (6-31G and 6-311G) are given in Fig. S4. Several DFT parameters obtained for three tested BNA molecules using all basis sets are given in Table 7. It is well documented that in acidic

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solution heteroatoms of organic inhibitors such as nitrogen, sulfur and oxygen easily protonate to exist in their cationic (protonated) forms. In our present we protonate the amidic nitrogen since it

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has maximum (negative) Mulliken charge. The Mulliken charge on each and every atoms of BNA molecules are presented in Table 8. Frontier molecular orbitals (HOMO and LUMO)

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represent the sites for electron (charge) donation and/ or acceptation. In general, HOMO represents the site of inhibitor molecules responsible for donation of charge and LUMO denotes

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the site for electron acceptation [13-15,32,38,39]. From the frontier molecular orbital pictures of the BNA molecules it is clear that HOMO and LUMO distributed almost on the whole part of the

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molecules which is an indication that almost entire part of the BNA molecules involve in the process of electron sharing (donation and acceptation). Careful observation of the Fig. 10 reveals

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that the presence of electron withdrawing nitro (-NO2) substituent in BNA-1 reduces the electron donating ability of the nitrophenyl ring as HOMO is mainly located over second phenyl ring. While, in remaining two BNA molecules HOMO is located over the entire BNA molecules. Inspection of the results (Table 7) revealed EHOMO values (in eV) for both forms of inhibitors (neutral and protonated) are increasing from going BNA-1 to BNA-3 that indicates that their electron donating ability enhances in the same order. In present case, neutral forms of BNA molecules ELUMO and ∆E values did not showed any regular trends. However, for their protonated forms, values of ∆E decreasing on going BNA-1 to BNA-3 which suggest that chemical reactivities of the BNA molecules are increasing according that comes in form of their

ACCEPTED MANUSCRIPT inhibition efficiency order. The lowest value (2.013642 eV) of ∆E for BNA-3 suggests that it is most reactive among the tested BNA molecules and therefore exhibited highest protection efficiency. Electronegativity (χ) is measure of a tendency to withdraw the electrons. Obviously, chemical reactivity, metal-inhibitor interactions or inhibition efficiency decrease with increasing the electronegativity of the inhibitor (BNA) molecules [13-15,36,31,38,39]. The BNA-1, BNA-2 and BNA-3 showed the electronegativities (protonated forms) values of 4.251815 eV, 3.016675

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eV and 1.219886 eV, respectively. The order of electronegativity values showed that BNA-1 has

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minimum tendency of charge transfer while BNA-3 has maximum. Chemical reactivity thereby

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protection ability of the organic compound decreases with increase in the value global hardness (η) and converse is also true for global softness (σ) [13-15,36,31,38,39]. For protonated forms of

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BNA molecules values of η are decreasing from going BNA-1 to BNA-2 and values of σ are increasing in from BNA-1 to BNA-3. These observations suggest that chemical reactivities of

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BNA molecules and thereby their protection ability increasing from going BNA-1 to BNA-3 [1315,33,31,35,36]. The BNA-3 shows maximum global softness among investigated BNA

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molecules which indicates BNA-3 is most reactive and best inhibitor for mild steel in 1M HCl. The relative interactions of BNA molecules on the mild steel surface was further studied by their

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values of fraction of electron transfer (∆N) which is measure of the transfer of net electron transfer occurring from inhibitors (BNAs) to metallic surface [13-15,36,31,38,39]. The BNA-3,

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BNA-2 and BNA-1 showed the ∆N values (in protonated forms) of 2.870477 eV, 0.664121 eV and 0.457563 eV, respectively which is in consistent to the order of their inhibition efficiency.

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Dipole moment is another DFT based parameter which is frequently being utilized to determine the relative inhibition ability of two or more organic inhibitors. Although, both positive and

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negative trends of inhibition efficiencies have been tested in literature with the dipole moment value but it expected that an organic compound with high magnitude of dipole moment is more polarized and would cover larger metallic surface as compared to the inhibitors having lower value of dipole moment. The dipole moment values of the investigated inhibitor are increasing from BNA-1 to BNA-3. This order of dipole moments well supports the experimental inhibition efficiency order. It is expected that investigated corrosion inhibitors undergo solvation in the aqueous medium therefore DFT study on BNA molecules has also been performed in solvated phase The optimized, HOMO and LUMO orbitals of BNA-1, BNA-2 and BNA-3 molecules are presented in Fig. 10 and the DFT indices are presented in Table 7. The solvated BNA-1, BNA-2

ACCEPTED MANUSCRIPT and BNA-3 molecules are presented as BNA-1-Sol, BNA-2-Sol and BNA-3-Sol, respectively. Results showed that DFT indices derived for solvated form of BNA molecules are consisted with the theoretical (DFT) indices derived for their neutral and protonated forms. Values of EHOMO increasing on going BNA-1 to BNA-3 indicating that electron donating tendency is enhancing in the same sequence. Conversely, values of energy band gap (∆E) and electronegativity (χ) are decreasing from going BNA-1 to BNA-2 indicating that reactivity thereby protection tendency as

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well as electron donating ability increasing in the same trend. The values of hardness (η) are

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decreasing and values of softness (σ) are increasing from moving BNA-1 to BNA-3 which also

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validates the protection ability of the three investigated inhibitor molecules and sequence derived from experimental and other theoretical technique. Although, values of ELUMO and dipole

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moment do not showed any regular trends, however rest of the DFT indices derived for solvated forms of BNA molecules successfully established their trend of inhibition efficiencies.

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3.5.3. MD study

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MD simulations study provides significant relationship between molecular structure of inhibitor molecules and their inhibition efficiency or adsorption ability [40-42]. It is well established that

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in aqueous acidic solution inhibitor molecules are surrounded by water (solvent) molecules. In this situation, inhibitor molecules are far apart from metallic surface. However, they start to

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move on the surface as soon as simulation started. Figs. 11 and 12 represent the side and tope views for the adsorption of the BNA molecules on the Fe (110) surface. It can be seen from the

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figures that BNA molecules adsorbed over the Fe (110) surface by almost planer/ flat orientations thereby cover the larger metallic surface area responsible for the inhibition

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protection ability of the BNA molecules [40-44]. Values of the binding and interaction energies (Ebinding and Einteraction, respectively) are presented in Table 9. It can be observed that reciprocal of Ebinding is taken as Einteraction. Negative values of Einteraction for BNA molecules revealed that they have spontaneous tendency of adsorption. The effectiveness of the BNA molecules is increases with increasing the magnitude of the Einteraction [40-44]. In present study, BNA-1, BNA-2 and BNA-3 showed the Einteraction values of -631.011 kJmol-1, -789.730 kJmol-1 and -885.117 kJmol-1, respectively. The values of Einteraction are well consistent with the results of other analyses like weight loss, electrochemical, surface and DFT studies. 4. Conclusions

ACCEPTED MANUSCRIPT In the present study three N-Phenyl-benzamide derivatives (BNAs) are tested as effective inhibitors for mild steel corrosion in aggressive acidic solution of 1M HCl. Results showed that inhibition efficiency of BNAs increases with their concentrations and decreases with temperature. EIS study revealed that BNA molecules act at adsorbate at metal-electrolyte interfaces and enhances the polarization resistances through their adsorption. Polarization study revealed that BNAs molecules behaved as cathodic type corrosion inhibitors. Adsorption of

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BNA molecules on the interfaces obeyed Langmuir adsorption isotherm. Adsorption mechanism

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of BNAs on mild steel surface was supported by SEM and AFM studies. DFT study was carried

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out using B3LYP functional and different basis sets like 3-21G, 6-31G and 6-311G. It was observed that 3-21G basis set gives best representation and therefore use for DFT calculation in

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the protonated (BNA-1-H+, BNA-2-H+ and BNA-3-H+) and solvated (BNA-1-Sol, BNA-2-Sol and BNA-3-Sol) forms of BNA molecules. DFT results showed that frontier molecular electron

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distribution located over the almost entire part of the BNA molecules. A good consistency among the experimental and DFT results was observed. MD simulation studies provide good

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support to the experimental and DFT studies and showed that BNA molecules adsorbed by their flat orientation. Their values of Einteraction are consistent with the results derived from other

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studies.

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Acknowledgment

Ankush Mishra, gratefully acknowledges MHRD, New Delhi (India) for providing financial

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supports and CIFC-IIT (BHU) Varanasi for providing instrumental facilities. Dr. Verma, thankfully acknowledge the North-West University, Mafikeng Campus, South Africa for

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providing financial supports under the postdoctoral fellowship scheme. References [1] [2]

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ACCEPTED MANUSCRIPT Figure Caption

Fig. 1: Synthetic scheme for BNAs. Fig. 2: 1H and 13C NMR spectra of the synthesized BNA molecules. Fig. 3: log CR verses 1/T (Arrhenius) plots for corrosion of mild steel in 1M HCl under

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uninhibited and inhabited by BNA molecules.

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Fig.4: Langmuir adsorption isotherm plots for the adsorption of tested BNA molecules on

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metallic surface at 308K temperature.

Fig. 5: EOCP vs time curves for corrosion of mild steel in 1M hydrochloric acid solution in the

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absence and presence of BNA molecules at 308K temperature.

Fig. 6: potentiodynamic polarization (anodic and cathodic) curves for mild steel in 1M HCl in

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the absence and presence of BNA molecules at 308K temperature. Fig.7: Nyquist (a) and Bode (b) curves for mild steel corrosion in 1M hydrochloric acid solution in the absence and presence of BNA molecules at 308K temperature.

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(c): Equivalent circuit employed for interpretation of EIS data.

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Fig. 8: SEM images of mild steel surfaces at 2000x magnification corroded (a) without and with the presence of (b) BNA-1, (c) BNA-2 and (d) BNA-3, at 308K temperature.

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Fig. 9: AFM images of mild steel surfaces corroded (a) without and with the presence of (b) BNA-1, (c) BNA-2 and (d) BNA-3, at 308K temperature. Fig. 10: Optimized, HOMO and LUMO frontier molecular orbital pictures of neutral (ABN-1,

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ABN-2 and ABN-3), protonated (ABN-1-H+, ABN-2-H+ and ABN-3-H+) and solvated

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((ABN-1-Sol, ABN-2-Sol and ABN-3-Sol) forms of BNA molecules using B3LYP functional and 3-21G basis set. Fig. 11: Side views of the initial adsorption of tested BNA molecules on the iron surface (110) in electrolytic 1M HCl solution. Fig.12: Side and top views of the final adsorption of tested BNA molecules on the Fe (110) surface in the electrolytic solution of 1M HCl.

ACCEPTED MANUSCRIPT Table Captions Table 1: IUPAC name, molecular structure, molecular formula, melting point, FT-IR, 1H and 13

C NMR and HRMS data.

Table 2: The weight loss parameters obtained for mild steel in 1 M HCl containing different concentrations of BNAs at 308K temperature.

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Table 3: Variation of CR and η % with temperature in absence and presence of optimum

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concentration of BNA molecules in 1M HCl at 308K-338K temperatures.

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Table 4: The values of Kads and ∆G◦ads for mild steel in absence and presence of optimum concentration of BNAs in 1M HCl at different studied temperatures (308-338K).

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Table 5: Potentiodynamic polarization parameters for mild steel corrosion in the absence and presence of optimum concentration of BNA molecules at 308K temperature.

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Table 6: EIS parameters derived for mild steel in the absence and presence of 10.14×10-5 molL-1 concentration of BNA molecules at 308K temperature.

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Table 7: DFT parameters for neutral as well as protonated forms of BNA molecules using B3LYP functional and commonly used basis sets.

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Table 8: Mulliken charges on the different atoms of the BNA molecules. Table 9: Selected energy parameters obtained from MD simulations for adsorption of BNA

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molecules on Fe (110) surface.

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Fig. 1: Synthetic scheme for BNAs.

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Fig. 2: 1H and 13C NMR spectra of the synthesized BNA molecules.

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Fig. 3: log CR verses 1/T (Arrhenius) plots for corrosion of mild steel in 1M HCl under uninhibited and inhabited by BNA molecules.

Fig.4: Langmuir adsorption isotherm plots for the adsorption of tested BNA molecules on metallic surface at 308K temperature.

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Fig. 5: EOCP vs time curves for corrosion of mild steel in 1M hydrochloric acid solution in the absence

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Fig. 6: potentiodynamic polarization (anodic and cathodic) curves for mild steel in 1M HCl in the absence and presence of BNA molecules at 308K temperature.

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Fig.7: Nyquist (a) and Bode (b) curves for mild steel corrosion in 1M hydrochloric acid solution in the absence and presence of BNA molecules at 308K temperature. (c): Equivalent circuit employed for interpretation of EIS data.

Fig. 8: SEM images of mild steel surfaces at 2000x magnification corroded (a) without and with the presence of (b) BNA-1, (c) BNA-2 and (d) BNA-3, at 308K temperature.

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Fig. 9: AFM images of mild steel surfaces corroded (a) without and with the presence of (b) BNA-1, (c) BNA-2 and (d) BNA-3, at 308K temperature.

ACCEPTED MANUSCRIPT Fig. 10: Optimized, HOMO and LUMO frontier molecular orbital pictures of neutral (ABN-1, ABN-2 and ABN-3), protonated (ABN-1-H+, ABN-2-H+ and ABN-3-H+) and solvated ((ABN-1Sol, ABN-2-Sol and ABN-3-Sol) forms of BNA molecules using B3LYP functional and 3-21G basis set. HOMO orbital

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Fig. 11: Side views of the initial adsorption of tested BNA molecules on the iron surface (110)

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Fig.12: Side and top views of the final adsorption of tested BNA molecules on the Fe (110) surface in the electrolytic solution of 1M HCl.

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N-(4nitrophenyl)benza mide (BNA-1)

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N-(4methoxyphenyl)b enzamide (BNA-3)

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Mol. Formula: C13H10N2O3; Mol. Wt. 242.23; Yellow solid; Yield = 94%; FT-IR (KBr, cm-1): 3067, 2358, 1744, 1653, 1595, 1499, 1401, 1336, 1302, 1243, 1170, 1104, 998, 899, 844; 1H NMR (500 MHz, DMSO-d6) δ 10.82 (s, 1 H), 8.27 (d, 2 H), 8.06 (d, H), 7.97 (d, 2 H), 7.60 (m, 3 H); 13C NMR (126 MHz, DMSO-d6) δ 166.39, 145.45, 142.60, 134.24, 132.35, 128.67, 128.01, 124.92, 119.92; EM m/z (%): 243 (M+1), 105 (100). Mol. Formula: C13H11NO; Mol. Wt. 197.23; White solid; Yield = 95%; FT-IR (KBr, cm-1): 3340, 1650, 1575, 1260, 750, 692; 1H NMR (500 MHz, CDCl3) δ 7.93 – 7.79 (m, 3H), 7.64 (d, 2 H), 7.55 (t, 1 H), 7.48 (t, 2 H), 7.37 (t, 2 H), 7.15 (t, 1 H) ppm; 13C NMR (126 MHz, CDCl3) δ 165.90, 138.06, 135.16, 131.99, 129.25, 128.94, 127.16, 124.73, 120.35 ppm; EM m/z (%):198 (M+1), 105 (100). Mol. Formula: C14H13NO2; Mol. Wt. 227.09; Green solid; Yield = 92%; FT-IR (KBr, cm-1): 3323, 1640, 1509, 1452, 1399, 1321, 1229, 1107, 1022, 817; 1H NMR (500 MHz, CDCl3) δ 7.85 (d, 3 H), 7.60 – 7.35 (m, 5 H), 6.89 (d, 2 H), 3.81 (s, 3 H); 13C NMR (126 MHz, CDCl3) δ 165.82, 156.76, 135.15, 131.82, 131.13, 128.86, 127.12, 122.28, 114.36, 55.63; EM m/z (%): 228 (M+1), 105 (100).

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Weight loss

−2

(mg cm h )

efficiency (η

coverage

%)

(θ)

54.78

0.5478

74.78

0.7478

1.300

83.04

0.8304

0.866

88.69

0.8869

3.466

89.56

0.8956

3.166

58.69

0.5869

1.633

78.69

0.7869

0.833

89.13

0.8913

15

0.500

93.47

0.9347

14

0.077

93.91

0.9391

84

2.800

63.47

0.6347

36

1.200

84.34

0.8434

7.61 × 10-5

17

0.566

92.6

0.926

10.14 × 10-5

9

0.300

96.08

0.9608

12.68× 10-5

8

0.044

96.52

0.9652

230

7.66

BNA-1

2.54 × 10-5

104

3.466

5.07 × 10-5

58

1.933

7.61 × 10-5

39

10.14 × 10-5

26

12.68× 10-5

24

2.54 × 10-5

95

5.07 × 10-5

49

7.61 × 10-5

25

10.14 × 10-5 -5

2.54 × 10-5

AC

CE

AN

M

PT

5.07 × 10-5

ED

12.68× 10

-----

CR

---

BNA-3

Surface

---

Blank

BNA-2

Inhibition

T

(mg)

−1

US

(M)

CR

IP

Inhibitors

ACCEPTED MANUSCRIPT Table 3: Variation of CR and η % with temperature in absence and presence of optimum concentration of BNA molecules in 1M HCl at 308K-338K temperatures. Corrosion rate (CR) (mgcm-2 h-1) and Inhibition efficiency (η%)

Temperature (K)

BNA-1

BNA-2

CR

η%

CR

η%

CR

308

7.66

---

0.86

88.69

0.50

318

11.0

---

1.6

85.45

1.13

328

14.3

---

3.2

77.672

338

18.6

---

6.2

66.78

BNA-3 CR

η%

T

Blank

η%

0.30

96.08

89.69

0.90

91.81

2.50

82.55

2.06

85.58

5.60

70.00

4.93

73.57

US

CR

IP

93.47

AN

Table 4: The values of Kads and ∆G◦ads for mild steel in absence and presence of optimum

Kads (104 M-1)

Inhibitor

328

BNA-1

1.19

0.72

BNA-2

1.50

0.84

BNA-3

3.32

1.60

338

308

318

328

338

0.44

0.25

34.33

34.12

33.84

33.36

0.50

0.29

34.92

34353

34.19

33.74

0.69

0.31

36.96

36.22

35.10

33.97

CE

ED

318

-∆G◦ads (k Jmol-1)

PT

308

M

concentration of BNAs in 1M HCl at different studied temperatures (308-338K).

AC

Table 5: Potentiodynamic polarization parameters for mild steel corrosion in the absence and presence of optimum concentration of BNA molecules at 308K temperature. Ecorr

icorr

βa

-βc

(mV/SCE)

(μA/cm2)

(mV/dec)

(mV/dec)

Blank

-445

1150

70.5

114.6

….

BNA-1

-517

99.8

77.0

162.5

91.32

BNA-2

-522

74.3

67.07

120.2

93.53

BNA-3

-526

56.4

86.5

130.5

95.09

Inhibitor

η%

ACCEPTED MANUSCRIPT

η%

θ

106.21

---

…..

0.824

34.79

252.6

0.835

31.62

320.9

0.853

31.04

Rs

Rp

N

(Ω cm2)

(Ω cm2)

Blank

1.12

9.58

0.827

BNA-1

0.80

185.5

BNA-2

0.81

BNA-3

0.90

Cdl (μF cm−2)

0.9483

IP

94.83

CR

Inhibitor

T

Table 6: EIS parameters derived for mild steel in the absence and presence of 10.14×10-5 molL-1 concentration of BNA molecules at 308K temperature.

96.20

0.9620

97.01

0.9701

BNA-1

EHOMO ELUMO (eV)

(eV)

-6.050

-

∆E

χ

η

σ

(eV)

(eV)

(eV)

(eV)

3.179

4.461

1.589

0.629

0.063

2.77

4.460

3.345

2.230

0.448

0.264

4.127

4.669

3.121

2.335

0.428

0.299

4.305

2.965

4.663

1.483

0.674

0.136

3.639

4.429

3.402

2.214

0.451

0.252

4.192

4.595

3.246

2.298

0.435

0.277

3.849

3.006

4.880

1.503

0.665

0.206

3.561

4.433

3.644

2.217

0.451

0.197

4.257

AN

3-21G

Inhibitor

M

Basis set

US

Table 7: DFT parameters for neutral as well as protonated forms of BNA molecules using B3LYP functional and commonly used basis sets. ∆N

μ (Debye)

BNA-2

ED

2.871 -5.574

-

1.115

6-31G

PT

-5.456

CE

BNA-3

BNA-1

AC

BNA-2

BNA-3

-6.146

-5.617

-

0.786 3.181 1.188

-5.544

0.948

6-311G

BNA-1

-6.383

3.377

BNA-2

-5.861

1.428

-5.787

-

4.609

3.482

2.304

0.434

0.225

3.878

6.006

4.252

3.003

0.333

0.001

7.231

5.998

3.017

2.999

0.333

0.250

6.003

2.014

1.220

1.007

T

BNA-3

1.639

6.022

3.490

3.389

1.745

0.573

0.249

3.547

3.001

2.665

1.500

0.666

0.532

3.651

2.893

US

ACCEPTED MANUSCRIPT

0.691

0.672

3.446

1.178 3-21G

BNA-1-H+

-7.255

1.249

BNA-2-H+

-6.016

0.018

-2.227

-

-5.134

BNA-2-Sol

-4.165

BNA-3-Sol

-3.761

1.643 1.165 0.868

2.314

1.447

AN

BNA-1-Sol

CR

0.213 3-21G

0.993

IP

BNA-3-H+

M

Table 8: Mulliken charges on the different atoms of the BNA molecules. BNA-2

Atom

Mulliken charge

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

C C C C C C C O N C C C C C C

0.854 -0.098 -0.175 -0.139 0.456 -0.150 -0.148 -0.547 -1.007 0.369 -0.196 -0.187 -0.176 -0.209 -0.184

Atom

Mulliken charge

Num ber

Atom

Mulliken charge

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

C C C C C C C O N C C C C C C

0.847 -0.115 -0.167 -0.177 -0.170 -0.181 -0.137 -0.551 -1.008 0.370 -0.198 -0.188 -0.177 -0.201 -0.184

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

C C C C C C C O N C C C C C C

0.848 -0.123 -0.008 0.035 0.335 -0.027 0.072 -0.557 -0.679 0.370 -0.047 -0.017 -0.014 -0.043 0.128

Num ber

AC

CE

PT

Number

BNA-3

ED

BNA-1

ACCEPTED MANUSCRIPT H H H H H H H H H H H

0.159 0.188 0.191 0.191 0.214 0.333 0.152 0.172 0.165 0.168 0.313

16 17 18 19 20 21 22 23 24 25 26 27

T

16 17 18 19 20 21 22 23 24 25 26

IP

-0.041 -0.319 -0.312 0.174 0.239 0.235 0.228 0.336 0.157 0.177 0.172 0.174 0.315

CR

N O O H H H H H H H H H H

O H H H H H H H H H H H

-0.269 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

M

AN

US

16 17 18 19 20 21 22 23 24 25 26 27 28

molecules on Fe (110) surface.

PT

System

ED

Table 9: Selected energy parameters obtained from MD simulations for adsorption of BNA

(kJ/mol)

(kJ/mol)

- 885.117

885.117

Fe(110)/BNA-2

- 789.730

789.730

Fe(110)/BNA-1

- 631.011

631.011

AC

CE

Fe(110)/BNA-3

ACCEPTED MANUSCRIPT

Highlights

AC

CE

PT

ED

M

AN

US

CR

IP

T

1. N-Phenyl-benzamides (BNAs) investigated as effective corrosion inhibitors for mild steel in aggressive acidic medium. 2. BNAs act as cathodic type inhibitors. 3. Their adsorption obeyed the Langmuir adsorption isotherm. 4. Experimental results were supported by computational (DFT and MD) results 5. Interactions of BNAs-mild steel surface involve donor-acceptor interactions.