Adsorption Behavior of Glucosamine-Based, Pyrimidine-Fused

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Adsorption Behavior of Glucosamine Based Pyrimidinefused Heterocycles as Green Corrosion Inhibitors for Mild Steel: Experimental and Theoretical Studies Chandrabhan Verma, Eno E. Ebenso, Lukman O. Olasunkanmi, Mumtaz Ahmad Quraishi, and Ime Bassey Obot J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04429 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 15, 2016

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Adsorption Behavior of Glucosamine Based Pyrimidine-Fused Heterocycles as Green Corrosion Inhibitors for Mild Steel: Experimental and Theoretical Studies Chandrabhan Vermaa, Lukman O. Olasunkanmib, c, Eno E. Ebensob, c, M.A. Quraishi a,* and I.B. Obotd a

Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi

221005,India b

Department of Chemistry, School of Mathematical & Physical Sciences, Faculty of Agriculture,

Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa c

Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of

Agriculture, Science and Technology, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa d

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

Petroleum and Minerals, Dhahran 31261, Saudi Arabia *Corresponding author: Ph.no. +91-9307025126; Fax: +91- 542- 2368428 E-mail: [email protected]; [email protected]

Abstract: Effects of electron donating (-CH3 and-OH) and electron withdrawing (-NO2) substituents on the corrosion inhibition efficiency of four glucosamine based substituted pyrimidine-fused heterocycles (CARBs) on mild steel corrosion in 1 M HCl have been investigated using gravimetric, electrochemical, surface morphology (SEM, AFM and EDX) and computational techniques. Gravimetric studies showed that protection performances of the compounds increase with increase in concentration. Both electron withdrawing (-NO2) and electron donating (-CH3 and -OH) groups were found to enhance the inhibition efficiency but the effect is more pronounced with electron donating substituents. The compounds were found to be


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cathodic type inhibitors as inferred from the results of potentiodynamic polarization studies. EIS studies suggested that the studied compounds inhibit metallic corrosion by adsorbing on metallic surface. The adsorption of the inhibitor molecules on steel surface was further supported by SEM, AFM and EDX analyses. Adsorption of CARBs on mild steel surface obeyed the Langmuir adsorption isotherm. Theoretical studies using quantum chemical calculations and molecular dynamics simulations provided additional insights into the roles of the –OH, –CH3, and -NO2 substituents on the corrosion inhibition performances of the studied inhibitors.

1. Introduction Mild steel has been widely used as a main construction material for piping works in various industries. It has found applications in downhole casing or tubing, flow lines and transmission or distribution pipelines in oil and gas industries.1-3 Petroleum oil well acidization is an essential technique that is routinely used in oil and gas industries for the purpose of stimulating oil-well to ensure enhanced oil production.4,5 This process however endangers the life of steel gadgets as a result of acid driven corrosion. In order to prevent this undesirable reaction, corrosion inhibitors are often added to the acid solution during acidification process.6-8 These compounds inhibit corrosion by adsorbing on metallic surface using heteroatoms (e.g. N, O, S), polar functional groups (e.g. -OH, -NH2, -NO2, -CN etc.), pi-electrons and aromatic rings as adsorption centers.9-11 Inhibitors retard metal corrosion by adsorbing on metallic surface and the process is influenced by some factors, which include molecular size of inhibitor, nature of substituents, inhibitor concentration, solution temperature and nature of test solution.8,9,11 Multicomponent reactions (MCRs) have emerged as a powerful technique towards “green compliant” synthesis. MCRs have advantages of satisfying both economic and environmental provisos of the green chemistry approaches. In addition, it ensures high chemical selectivity, high yield, short reaction time, mild reaction condition and operational simplicity.12,13 Being a one-step reactions in which three or more reactants are combined to yield product, MCRs are able to satisfy quite a number of other prerequisites of green chemistry synthesis including small number of steps, facile automation, minimum waste generation due to reduced work-up steps and simple purification procedure, which enhances the synthetic efficiency.14-16 The increasing ecological awareness, strict environmental regulations and the need to reduce environmental pollution together with its side effects on human health have necessitated the recent efforts in



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ensuring that corrosion inhibition studies are “green” chemistry compliant. In this regard, efficient chemical transformation of naturally occurring materials such as carbohydrates, fatty acids and lipids into organic compounds particularly by using MCRs has attracted considerable attentions.17,18 Khan et al.19 in a recent review gave a detailed account of modern advances in the use of carbohydrates in MCRs. Carbohydrate based MCRs appear to be very versatile as quite a number of different carbohydrates have been explored in combination with several other organic molecules especially acids and carbonyls.19,20 MCRs involving D-glucosamine, barbituric acid and aldehydes have also been reported.20 Due to their natural availability in large quantities, biosynthesis using greenhouse (CO2) gas, biocompatibility, biodegradability and high solubility in water, hexoses and their derivatives are well established “green” chemicals that have been used for variety of chemical transformations.21-24 Compounds derived from carbohydrates often show biological activities such as antibiotics, antiviral medicines, protein glycosylation and glycosylation inhibitor activities.25-28 Some compounds derived from carbohydrates are also widely used in cancer metastasis, apoptosis, neuronal proliferation, cosmetic, detergent, food, cloths, sweetening agent, lumber, paper, and so on.24,26-28 Furthermore, oxidation of glucose gives rise to biodegradable gluconic acid, which is used in the pharmaceutical and food industry as complexing and/or acidifying agent. Literature survey revealed that in spite of their environmental friendliness, only few reports are available on the use of carbohydrates and their derivatives as metallic corrosion inhibitors or as precursors for the synthesis of corrosion inhibitors.29,30 In view of this, the present study is carried out on the synthesis of four glucosamine derivatives, namely, 5-phenyl-10-((3R,5S,6R)-2,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro2H-pyran-3-yl)-9,10-dihydropyrido[2,3-d:6,5-d']dipyrimidine-2,4,6,8(1H,3H,5H,7H)-tetraone (CARB-1),

5-(4-nitrophenyl)-10-((3R,5S,6R)-2,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-

2H-pyran-3-yl)-9,10-dihydropyrido[2,3-d:6,5-d']dipyrimidine-2,4,6,8(1H,3H,5H,7H)-tetraone (CARB-2), 5-(p-tolyl)-10-((3R,5S,6R)-2,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran3-yl)-9,10-dihydropyrido[2,3-d:6,5-d']dipyrimidine-2,4,6,8(1H,3H,5H,7H)-tetraone and

(CARB-3)

5-(4-hydroxyphenyl)-10-((3R,5S,6R)-2,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-

pyran-3-yl)-9,10-dihydropyrido[2,3-d:6,5-d']dipyrimidine-2,4,6,8(1H,3H,5H,7H)-tetraone (CARB-4) and investigation of their protection efficiency for mild steel to mitigate its corrosion


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in 1 M HCl. To the best of our knowledge this is the first time these compounds are being tested as inhibitors of mild steel corrosion in 1 M HCl. It is important to mention that these compounds, especially CARB-2 and CARB-3 are among the series of compounds recently reported by Panahi and co-workers20 as MCR products of D-glucosamine, barbituric acid and aromatic aldehydes, however, their inhibition potentials for mild steel corrosion in 1 M HCl medium have not been investigated. The main objective of this study is to assess the effects of electron donating (-OH and –CH3) and electron withdrawing groups on the inhibition efficiency of these carbohydrate based compounds. Corrosion inhibition performances of the compounds were investigated using gravimetric, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning electron microscopy (SEM), atomic force microscopy (AFM), electron dispersive Xray spectroscopy (EDX), quantum chemical calculations and molecular dynamics simulation methods.

2. Experimental details 2.1. Synthesis of the corrosion inhibitors (CARBs) CARBs were synthesized according to the method reported by Nourisefat et al.20 The overall synthetic scheme is given in Fig. 1, while other relevant information about the compounds including their molecular structures, IUPAC nomenclature, and spectroscopic characterization data are listed in Table 1. Additional spectroscopic and analytical data for the compounds are shown in Fig. S1 of the supporting information.

2.2. Mild steel sample and test solution The mild steel sheets utilized for the corrosion tests composed of about 99.26 % of Fe by weight. Other minor elemental compositions are the same as reported elsewhere.31 Surface pretreatment of the mild steel samples was carried out as previously reported.31 Test solution of 1 M HCl was prepared by appropriately diluting the stock solution (37 % HCl obtained from MERCK).



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2.3. Weight loss and electrochemical measurements Weight-loss studies were carried out by immersing freshly pre-treated mild steel specimens of the dimension 2.5 × 2.0 × 0.025 cm3 in aggressive solutions of 1 M HCl containing different concentrations of CARBs. The samples, which were accurately weighed before immersion in the test solutions were retrieved after 3 h. The mild steel samples were then washed with distilled water, and then acetone, before finally dried in a desiccator. The samples were reweighed, and differences in weights before and after immersion were recorded as the weightloss. The experiments were carried out in triplicate to ensure reproducible results and average weight-loss values were utilized to calculate the inhibition efficiency of each CARB at various concentrations using the equation:32,33

η% =

wo − wi × 100 wo

(1)

where w0 and wi are respectively the mean or average weight-loss values without and with various concentrations of CARBs. For the electrochemical measurements, mild steel specimens with one-sided exposed surface area of 1 cm2 were used as the working electrodes (WE). A three-electrode electrochemical glass-cell system comprising the WE, counter electrode (platinum rod), and reference electrode (saturated calomel electrode, SCE) was used for the experiments. Potentiodynamic polarization curves were recorded by sweeping the electrode potential ±0.25 away from the operating corrosion potential (Ecorr) at 1.0 mVs-1 scan rate. Tafel extrapolations were used to obtain corrosion current density (icorr) values and other relevant electrochemical data. The inhibition efficiency was determined by using the equation:32,33

η% =

0 i icorr − icorr ×100 0 icorr

(2)

where, i°corr and icorr are respective corrosion current densities without and with different concentrations of CARBs. EIS spectra were recorded at open circuit potential in the frequency range of 100 kHz to 0.01 Hz using 10 mV peak to peak amplitude AC signals. Accurate fitting and simulation


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analyses of the spectra were carried out and corresponding inhibition efficiencies were calculated as:32,33

Rcti − Rct0 ×100 Rcti

η% =

(3)

where, Rct0 and Rcti are charge transfer resistances without and with different concentrations of CARBs respectively. All electrochemical experiments were performed with the aid of Gamry Potentiostat/Galvanostat (Model G-300) instrument equipped with Gamry Echem Analyst 5.0 software, which was used for fitting and simulations. 2.4. SEM and AFM surface morphology studies SEM and AFM surface morphology studies were carried out as reported elsewhere.31 Freshly pre-treated mild steel coupons were immersed in 1 M HCl solutions without and with different concentrations of the inhibitors. The specimens were retrieved after 3 h, washed with distilled water and dried before subjected to SEM and AFM analyses. 2.5. Quantum chemical studies Molecular quantum chemical studies were carried out on CARBs as described in our earlier studies.31,35 Optimized geometries of the compounds were obtained using the B3LYP/631+G(d,p) functional and basis set combination.35-37 All the calculations were carried out with the aid of Gaussian 09 software suite.38 Selected quantum chemical parameters including the frontier molecular orbital (FMO) energies and other derivative quantities were computed for the optimized molecules. Molecular quantum chemical parameters such as the energy gap (∆E) between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), global electronegativity (χ), and fraction of electrons transfer (∆N) were derived using the respective equations:39-42 ΔE = E LUMO − E HOMO

χ =−

1 (E LUMO + E HOMO ) 2

ΔN =

χ Fe − χ inh 2(η Fe + η inh )

(4) (5)

(6)



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where EHOMO and ELUMO are the HOMO and LUMO energies respectively. In calculating ∆N, the electronegativity of atomic Fe ( χ Fe ) was equated to 7.0 eV based on the Pearson’s electronegativity scale,43 while the hardness of bulk Fe was approximated to 0 eV/mol. Prospective local sites for nucleophilic and electrophilic attacks were predicted by calculating the Fukui indices as:39,42,45,47

f k+ = qk ( N +1) (r ) − qk ( N ) (r )

(7)

f k− = qk ( N ) (r ) − qk ( N −1) (r )

(8)

where f k+ and f k− are the Fukui indices for nucleophilic and electrophilic attacks respectively,

q k ( N +1) , q k ( N ) and q k ( N −1) are electron densities on the kth atom in systems with (N+1), (N) and (N1) electrons. Atomic electron densities were approximated to their corresponding Mulliken atomic charges.46 Graphical electron density isosurfaces of the Fukui functions were obtained with the aid of Multiwfn software.47,48 2.6. Monte Carlo simulation Theoretical description of adsorption of CARBs on mild steel surface was simulated using Monte Carlo approach. CARBs were made to interact with Fe(110) surface by using the adsorption locator module in the Materials Studio 7.0 software suite. The inhibitor molecules were first optimized using Forcite module. Fe(110) crystal was built and relaxed to a minimum energy using COMPASS force field. The optimized inhibitor molecule was made to adsorb on the refined Fe(110) surface with the aid of adsorption locator module to obtain equilibrium configuration of inhibitor/Fe(110) system. Adsorption energy of the most stable configuration of inhibitor/Fe(110) system was obtained for each inhibitor (CARB).49-52

3. Results and discussion 3.1. Weight loss experiments 3.1.1. Effect of concentration The effect of different concentration of CARBs on the inhibition of mild steel corrosion in 1 M HCl was studied using gravimetric method due to its simplicity and good reliability. The observed weight-loss values of triplicate measurements are highly reproducible giving standard deviations ranging from 0.00025 to 0.0003. The inhibition efficiency (η%) and other parameters


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such as corrosion rate (CR) and surface coverage (θ) at various concentration of the inhibitors are given in Table 2. Graphical representations of the variation of inhibition efficiency with inhibitor concentration are shown in Fig. S2a (supporting information). Careful examination of the results showed that protection efficiencies of the studied inhibitors increase with increasing concentrations. Maximum values of (inhibition efficiency) 88.69% for CARB-1, 90.86% for CARB-2, 93.47% for CARB-3, and 96.52% for CARB-4 were obtained at 7.41 x 10-5 mol/L. It has been reported that at lower concentrations, inhibitors preferably adsorb by flat orientation such that as the concentration increases, surface coverage and consequently inhibition efficiency increases.53,54 However, if the concentration of the inhibitor is increased beyond certain (optimum) value, the inhibitor molecules adsorb perpendicularly onto the metallic surface due to electrostatic repulsion between the molecules at higher concentration.55,56 Therefore, after the optimum concentration of the inhibitor, the inhibition performance does not change significantly. The order of inhibition efficiencies in Table 2 is CARB-4 > CARB-3 > CARB-2 > CARB-1. The high inhibition efficiency of CARB-2 compared to CARB-1 is due to the presence of additional nitrogen (N) and oxygen (O) atoms of the nitro group in spite of its high electron withdrawing nature. Unlike earlier reports,57-59 the results in the present study revealed that the presence of nitro group increases the inhibition performance. This may be attributed in part to the relatively large molecular size of CARB-2 molecule, or the presence of –NO2, whose electron withdrawing ability may decrease the electron density at the adsorption sites and consequently facilitate the adsorption of the inhibitor molecule on metallic surface via retro-donation. The inhibitor molecules that contain electron releasing methoxy and hydroxyl groups showed comparatively higher inhibition efficiency. The higher inhibition efficiency of CARB-4 than CARB-3 is attributed to the higher electron releasing tendency of -OH group in CARB-4 compared to the -CH3 group in CARB-3. The greater electron releasing ability of –OH ensures higher electron densities at the adsorption sites of CARB-4 and thereby enhances its ability to donate charges to the metal during the adsorption process. 3.1.2. Temperature effect The effects of temperature on corrosion rate (CR) of mild steel in 1 M HCl in the absence and presence of maximum concentrations of the studied compounds were recorded between 308 and 338 K inclusive, and the results are listed in Table 3. Graphical representations of the



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variation of inhibition efficiency with temperature are shown in Fig. S2b of the supporting information. The results showed that CR increases with increasing temperature of the solution. The increase in CR with increase in solution temperature can be attributed to desorption of initially adsorbed inhibitor molecules, which results in greater surface area of mild steel being exposed to the aggressive acidic ions in the corrosive medium.60,61 The decrease in the inhibition efficiencies at high temperatures can also be associated with rapid etching, desorption, and decomposition and/or rearrangement of inhibitor molecules.60,61 The variation of CR with temperature can be expressed in the Arrhenius equation:62,63

(9) where CR is the corrosion rate in mgcm-2 h-1, A is the Arrhenius pre-exponential factor, Ea is the activation energy, R is the gas constant and T is absolute temperature. The Arrhenius plots for the corrosion of mild steel in the studied corrosive media are shown in Fig. 2. The values of Ea were derived from the slopes ( −ΔEa 2.303R ) and listed in Table 4. The trend of the values of Ea is CARB-4 (78.86 kJ/mol) > CARB-3 (63.56 kJ/mol) > CARB-2 (63.38 kJ/mol) > CARRB1(60.53 kJ/mol) > 1 M HCl blank (28.48 kJ/mol). This clearly reveals that the inhibitors increase the energy barrier associated with corrosion reaction and therefore reduce the corrosion rate. More so, the trend of Ea for the inhibitors correlates with the order of inhibition efficiency of the compounds. The higher values of Ea in the presence of inhibitors might be as a result of the inhibitor molecules forming protective film on the steel surface,64 which consequently reduces the corrosion rate. 3.1.3. Adsorption isotherms Generally, corrosion inhibitors mitigate metallic corrosion in acid solution through adsorption onto the surface. The investigation of adsorption characteristics of inhibitor molecules on metallic surface is an important aspect of corrosion inhibition study because it provides structural information about the double layer along with the thermodynamic information. The adsorption of the studied CARB molecules on mild steel surface was investigated by fitting the experimental surface coverage data into various adsorption isotherms, which include Langmuir, Temkin and Frumkin models. Langmuir adsorption isotherm gave the best fits judging from near


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unity values of the observed regression coefficient (R2). The values of the slopes, intercepts, and regression coefficient for Langmuir and Temkin adsorption isotherms for all the studied compounds are listed in Table S1 of the supporting information. The Langmuir, Temkin and Freundluich adsorption isotherm plots are shown in Fig. 3. Fig. 3 and Table S1 clearly reveal the best fits obtained from Langmuir isotherm. Although, the values of R2 are near unity for the Langmuir isotherm plots, but the slopes of the lines deviate appreciably from unity (Table S1). Such deviation has been associated with intermolecular interactions between the adsorbed inhibitor molecules. These intermolecular interactions result in mutual repulsion or attraction between the molecules, and they are not accounted for in the Langmuir adsorption isotherm.65,66 Furthermore, as the surface coverage increases by increasing inhibitors concentrations, the surface energy changes, leading to deviation of the slope value from the unity.66,67 The Langmuir isotherm can be represented as:68

K ads C =

θ 1−θ

(10)

where Kads is the equilibrium constant for the adsorption process, C is inhibitor concentration, and θ is surface coverage. The values of Kads were obtained at different temperatures from the intercept of the plots. The standard free energy of adsorption (ΔG0ads) was obtained using: ο ΔGads = − RT ln(55.5K ads )

(11)

where, R is universal gas constant, T is absolute temperature, and 55.5 is the molar concentration of water in the bulk solution. The values of Kads and ΔG0ads at various temperatures are listed in Table 5. Values of ΔG0ads are always used to classify adsorption process as physisorption (when ΔG0ads = -20 kJ mol-1, or less negative) or chemisorption (for ΔG0ads = -40 kJ mol-1 or more negative).69,70 In the present study, high values of Kads (1.69 x 103- 33.29 x 103 M-1) and large negative values of ΔG0ads (32.19-36.96 kJ mol-1) suggest that the inhibitor molecules adsorbed trongly and spontaneously onto steel surface.71,72 The values of ΔG0ads in-between the threshold values -20 and -40 kJ mol-1 suggest that the adsorption of the inhibitor molecules involves a mixed-mode physisorption and chemisorption mechanism.72,73



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3.2. Electrochemical measurements 3.2.1. Potentiodynamic polarization study Tafel polarization curves for mild steel in 1 M HCl without and with maximum concentration of each of the studied inhibitors are shown in Fig. 4. Potentiodynamic polarization parameters, such as Ecorr, icorr, anodic and cathodic Tafel slopes (βa, βc respectively) were derived by extrapolating the linear portions of the Tafel curves at both cathodic and anodic sides to the Ecorr. The electrochemical parameters obtained from the Tafel analyses and the resulting corrosion inhibition efficiency values are listed in Table 6. It was observed that addition of CARBs (inhibitors) affects both cathodic and anodic Tafel slopes. In other words, the inhibitors affect both the anodic mild steel dissolution and cathodic hydrogen evolution reactions. This implies that CARBs are mixed-type corrosion inhibitors.74 The magnitudes of the shift in Ecorr are 44 mV, 46 mV, 72 mV and 78 mV for CARB-1, CARB-2, CARB-3 and CARB-4, respectively, which further corroborate the mixed-type inhibitive behaviour of CARBs.74-76 However, the results further revealed that the shift in the cathodic curves is more prominent than that of anodic curves, indicating that these compounds predominantly act as cathodic type inhibitors. The inhibitors cause significant decrease in corrosion current density (icorr), and the values of icorr at optimum concentration of the inhibitors are in the order: CAR-1 (132.0 mV/dec) > CARB-2 (86.4 mV/dec) > CARB-3 (58.7 mV/dec) > CARB-4 (36.2 mV/dec) which is just converse to the order of inhibition efficiency. 3.2.2. Electrochemical impedance spectroscopy (EIS) EIS study was undertaken to gain insight into the kinetics and characteristics of the electrochemical processes which occur at the mild steel / 1 M HCl interface in presence of the studied inhibitors. Fig. 5a represents the Nyquist plots for mild steel dissolution in 1 M HCl in absence and presence of optimum concentration of the inhibitors. It can be seen that impedance spectra in the presence of optimum concentration of inhibitors furnished signal capacitive loop suggesting that investigated inhibitor molecules act primarily as interface inhibitors.76,77 This finding also suggests that inhibition of metallic corrosion takes place by simple surface coverage mechanism.77 The electrochemical impedance data was analyzed using the simple equivalent circuit of the form in Fig. 5b, which comprises the solution resistance (Rs), charge transfer resistance (Rct) and constant phase element (CPE). The use of CPE for acid based


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electrochemical corrosion of metals often yields better approximation.78 The impedance of the CPE can be represented as:

(12) where, Y0 is CPE constant, ω is angular frequency, j is imaginary number (j2 = -1), and n is phase shift (exponent). A low value of n represents high surface inhomogeneity and vice versa. Moreover, the value of n can also be used to represent the nature of CPE, such that CPE characterizes a resistance when n = 0, (Y0 = R); capacitance when n = 1 (Y0 = C); inductance when n = -1, (Y0 = 1/L); or Warburg impedance when n = 0.5, (Y0 = W). The values of the EIS parameters obtained after the spectra fitting are listed in Table 7. The results in Table 7 show that n varies from 0.844 to 0.875 in the presence of the inhibitors, which is comparatively higher than that of the blank, suggesting that surface inhomogeneity decreases in the presence of inhibitors due to formation of protective film on the steel surface. The increased surface smoothness is further supported by increased values of phase angle in the presence of inhibitors as reflected in the Bode plots (Fig. 5c) and Table S2. The increased values of the slopes of the linear portion of the Bode impedance modulus plots at intermediate frequencies (Table S2) further support the formation of protective film of inhibitor molecules on the steel surface. More so, the closeness of the values of n to unity suggests that the CPE of the mild steel/electrolyte interface in the present study behaves as a pseudo-capacitor. The imperfect semicircle of the Nyquist plots has been related to the deviation of n from unity (surface inhomogeneity), i.e. the pure capacitive behavior could not be achieved due to surface inhomogeneity caused by interfacial and structure origin. The double layer capacitance was derived using the equation:79 Cdl = Y0 (ωmax) n-1

(13)



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where, ωmax (rad s-1) is the angular frequency corresponding to the maximum value of imaginary impedance. Other EIS parameters such as Rs, Rct, Cdl, and the corresponding inhibition efficiency (η%) are given in Table 7. The higher values of Rct and lower values of Cdl in presence of inhibitors are attributed to increased surface coverage and decreased dielectric constant as well as thickness of the electric double layer, respectively.78,80 3.3. Surface morphology studies 3.3.1. SEM/EDX analyses SEM/EDX is one of the most commonly used techniques for examining and analyzing the surface and elemental component of corrosion related specimens. The interaction of metals with the corrosive environment, particularly with regard to the morphology of the metal surface and accumulation of corrosion products can be examined by SEM analysis. Fig. 6 shows the SEM images of mild steel samples immersed in 1 M HCl without and with maximum concentration of the studied inhibitors after 3 h immersion time. The surface of mild steel specimen retrieved from the non-inhibitor containing acid blank (Fig. 6a) is highly corroded and damaged, showing mountain-like appearances resulted from direct attack of the steel by acidic ions. However, SEM images of mild steel specimens in the presence of optimum concentration of CARBs (Fig. 6b-e) show comparative smoother surfaces, which is attributable to adsorption of the inhibitor molecules on metallic surface. The adsorbed inhibitor molecules isolate the metallic surface from the corrosive medium and thereby ensuring less corroded and smoother surface. The corresponding EDX spectra of mild steel surfaces are shown in Fig. S3. The presence of oxygen signal in the EDX spectrum of the mild steel retrieved from blank acid medium is attributed to formation of oxide film by slow oxidation of surface Fe atoms. In the presence of the inhibitors, the intensity of oxygen signals in the EDX spectra of steel surfaces increases, which suggests the adsorption of the inhibitors on metallic surface. The presence of nitrogen signals in the EDX spectra of mild steel retrieved from the inhibitor containing solutions further suggests the adsorption of inhibitors on the metallic surface.


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3.3.2. AFM measurements AFM surface analyses were performed on steel surfaces after 3 h immersion in acidic solutions without and with maximum concentration of the inhibitors. The AFM micrographs are shown in Fig. 7. It is observed from Fig. 7a that the surface of mild steel immersed in the blank acid medium was highly corroded, revealing mountain-like appearance. This is associated with direct exposure of mild steel surface to corrosive acidic ions. The calculated average surface roughness for the steel in blank acid medium was 388 nm. However, the AFM micrographs of the mild steel surfaces in the presence of optimum concentration of the inhibitors (Fig. 7b-e) show significant improvement in the smoothness of the steel surface. This is due to the adsorption of CARB molecules on the steel surface, which ensures protection of the surface from the corrosive environment, and thereby inhibits mild steel corrosion. The calculated average surface roughnesses (in nm) are 171, 143, 117, and 84, in presence of CARB-1, CARB-2, CARB-3 and CARB-4, respectively. 3.4. Computational theoretical studies 3.4.1. Quantum chemical studies Fig. 8 shows the optimized molecular structures of CARBs. The structure corresponding to the lowest energy configuration in each case is the one that has both the phenyl and pyrantriol rings twisted out-of-plane of the dihydropyridodipyrimidinetetraone ring. Both the phenyl and pyrantriol rings are twisted towards the opposite sides of the molecular plane of the dihydropyridodipyrimidinetetraone ring. Electron density isosurfaces of HOMO and LUMO of CARBs are shown in Fig. 9. The HOMO electron density surface reveals the electron rich sites of the molecule from which electrons could be donated to suitable vacant orbitals of accepting specie (the metal). The LUMO electron density surface indicates the electron deficient regions of the molecule.40,42,81 These sites have higher chances of accepting electrons from appropriate occupied orbitals of donor specie (the metal) during retro-bonding interactions between inhibitor and metal. The HOMO electron densities of CARB-1 and CARB-2 are essentially distributed over the entire dihydropyridodipyrimidinetetraone ring. The atoms of both the phenyl and pyrantriol rings are not considerably involved in this distribution. This is due to their electron donating effects to the dihydropyridodipyrimidinetetraone ring. The implication of this is that the molecules of



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CARB-1 and CARB-2 have high tendencies of donating electrons to the vacant orbitals of the metal using the electron rich dihydropyridodipyrimidinetetraone ring. The HOMOs of CARB-3 and CARB-4 on the other hand are mainly pi-orbitals and the electron densities are distributed over the entire phenyl ring. The HOMOs of these molecules extend to the C=C pi-electron centres of the dihydropyridodipyrimidinetetraone ring, and also to the electron donating –CH3 and –OH substituents on the phenyl rings of CARB-3 and CARB-4 respectively. This suggests that CARB-3 and CARB-4 are most likely to utilize their phenyl moieties in donating electrons to vacant metallic atomic orbitals. The LUMOs of all the compounds except CARB-2 are considerably distributed over the entire atoms of the dihydropyridodipyrimidinetetraone ring and extend slightly to the phenyl ring. The reverse is the case for CARB-2 as the LUMO electron density is mainly found on the hydroxyphenyl ring. The electron density distributions of the FMOs suggest that the pyrantriol ring is not likely to participate in the charge-sharing relationships between the inhibitor molecules and the metal. Table 8 contains the list of selected quantum chemical parameters of the molecules. A higher value EHOMO suggests a better chance of electron donating ability of an inhibitor molecule to the metal. A lower value of ELUMO advises a better chance of electron accepting ability of an inhibitor during backward donation from the metal.42,45,48,82 Generally, higher value of the energy gap, ∆E suggests higher reactivity and higher inhibition efficiency for a molecule. A molecule with higher electronegativity (χ) may show higher inhibition efficiency, if its protection performance depends on electron accepting ability and vice versa. An inhibitor molecule may also have higher inhibition efficiency based on higher value of ∆N, which informs larger fraction of electrons in forward donation to the metal.42,43,81 The values of quantum chemical parameters for the studied compounds did not give a completely uniform trend in this regard. However, the results show that CARB-4 with the highest observed protection efficiency has the highest EHOMO. A closer inspection of the results would reveal that comparisons of these parameters are better made between CARB-1 and CARB-2 as a group, and also CARB-3 and CARB-4 separately. CARB-1 and CARB-2 exhibits similar electron density distributions of the HOMOs, while the FMOs of CARB-3 and CARB-4 look alike. In this respect, the values of the ELUMO is such that CARB-2 < CARB-1, which suggests that CARB-2 has higher propensity to receive electrons from appropriate filled orbitals of the metal than CARB-1. This might inform its higher inhibition performance. CARB-4 has a


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higher EHOMO than CARB-3, which suggests higher ability to donate electrons to vacant orbitals of the metal and hence the observed higher inhibition efficiency. A lower value of ∆E, higher value of χ, and lower value of ∆N for CARB-2 compared to CARB-1 are all in support of its higher inhibition efficiency based on better tendency to accept electrons from the filled orbitals of metallic atoms for back-bonding formation. Conversely, a higher value of EHOMO, lower value of ELUMO, lower value of ∆E, lower value of χ, and higher value of ∆N for CARB-4 are all in support of its higher inhibition performance than CARB-3 based on better ability to donate electrons to the metal. Electron density isosurfaces of the Fukui indices, f k+ and f k− are shown in Fig. 10. In the inhibitor molecules, the atomic sites that are most vulnerable to nucleophilic and electrophilic attacks are indicated by f k+ and f k− respectively. At 0.0025 isosurface value, it was observed that the sites with high values of f k+ are mainly found on the dihydropyridodipyrimidinetetraone rings for CARB-1, CARB-3 and CARB-4. These sites include the carbonyl functional groups and the immediate carbon atoms. The two carbon atoms adjacent to the N-atom of the dihydropyridine ring are also susceptible to nucleophilic attacks. The sites of nucleophilic attacks in CARB-2 are mainly found on the nitro group attached to the phenyl ring and the two carbon atoms adjacent to the N-atom of the dihydropyridine ring. For all the compounds except CARB-4, the f k− electron densities are principally concentrated on the carbon (C) and nitrogen (N) atoms of the dihydropyridine ring and the carbonyl oxygen atoms of the dihydropyridodipyrimidinetetraone ring. These atoms represent the sites that are most disposed to electrophilic attacks in these molecules. The hydroxyphenyl group appears to be the major site of electrophilic attacks in CARB-4. The f k− electron densities in CARB-4 also extend to the C=C pi-electron region and carbonyl oxygen atoms in dihydropyridodipyrimidinetetraone ring, as well as the N-atom of the dihydropyridine ring.

4.

Monte Carlo simulation Recently, molecular dynamic simulation has appeared to be an effective method for

theoretical description of the adsorption tendency of inhibitor molecule on metallic surface. In



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the present study, Monte Carlo simulation was carried out to describe the adsorption behavior of the studied inhibitor molecules on the Fe (110) surface. The side and top views equilibrium configurations of the inhibitors/Fe(110) systems after simulations is represented in Fig. 11. As shown in Fig. 11, molecules of CARBs adsorbed on Fe(110) cleaved surface in near planar orientations. The calculated energy parameters of the equilibrium configurations of CARBs/Fe(110) systems are recorded in Table 9. The large negative values of Eads for all studied inhibitors suggest that these molecules strongly and spontaneously adsorbed onto Fe(110) surface.31,34,82,83 The magnitudes of adsorption energy, Eads (kJ/mol) for the tested inhibitors follow the order CARB-4 (234.8) > CARB-3 (222.6) > CARB-2 (213.3) > CARB-1 (210.2). This trend agrees with the order of inhibition efficiency obtained experimentally.31,34,82,83 However, the values of rigid adsorption energy and deformation energy do not follow any regular trends.

5.

Conclusion Corrosion inhibition characteristics of some glucosamine based substituted pyrimidine-

fused heterocycles (CARBs) have been investigated on acid corrosion of mild steel in 1 M HCl medium. The following conclusions were drawn from the results: 1. The glucosamine based substituted pyrimidine-fused heterocycles (CARBs) acted as efficient corrosion inhibitors for mild steel in 1 M HCl and their inhibition efficiency increases with increasing concentrations of the inhibitor. 2. The adsorption of CARBs on mild steel obeyed the Langmuir adsorption isotherm and occurred via both physisorption and chemisorption mechanisms. 3. The adsorption of the studied compounds onto the mild steel surface was supported by SEM, EDX and AFM analyses. 4. CARBs are mixed type inhibitors with predominantly cathodic inhibitive effects. 5. Quantum chemical calculations provided more detailed accounts of how different electron donating and accepting substituents tuned the extent and mode of the donor-acceptor interactions between the metal and the inhibitor molecules. The relative inhibitive effects of CARB-1 and CARB-2 (CARB-2 > CARB-1) was related to the degree of retrodonation that the molecules can afford, while the relative inhibitive effects of CARB-3 and CARB-4 (CARB-4 > CARB-3) related well with the extent of charge donation by the molecules.


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6. Monte Carlo simulations study reveals that CARBs adsorbed on the mild steel surface in near parallel orientations and the predicted adsorption energy values correlate with experimental inhibition efficiency. 7. Both experimental and theoretical results support increased inhibition efficiency in the presence of electron donating (-CH3, -OH) and electron withdrawing (-NO2) substituents. However, the CARB molecules with electron donating substituents showed higher protection efficiency for the metal than the one with electron withdrawing group.

Supporting Information: Detail of the characterization results of the synthesized inhibitor molecules (Figure S1). Inhibition efficiency values versus inhibitors concentration and temperature (Figure S2 a-b). EDX spectra and corresponding elemental composition (Figure S3 a-e). Slopes, intercepts, and regression coefficients (R2) for Langmuir and Temkin adsorption isotherms (Table S1). Values of slope, intercept, correlation coefficient and phase angle calculated form Bode plots of the in absence and presence of studied compounds (Table S2). ACKNOWLEDGMENTS: Chandrabhan Verma, gratefully acknowledged Ministry of Human Resource Development (MHRD), New Delhi (India) for support.



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(68) Ghareba, S.; Omanovic, S. Interaction of 12-aminododecanoic acid with a carbon steel surface: Towards the development of ‘green’ corrosion inhibitors. Corros. Sci. 2010, 52, 2104–2113. (69) Goulart, C. M.; Esteves-Souza, A.; Martinez-Huitle, C.A.; Rodrigues, C.J.F.; Maciel, M.A.M.; Echevarria, A. Experimental and theoretical evaluation of semicarbazones and thiosemicarbazones as organic corrosion inhibitors. Corros. Sci. 2013, 67, 281–291. (70) Amin, M.A.; Ibrahim, M.M. Corrosion and corrosion control of mild steel in concentrated H2SO4 solutions by a newly synthesized glycine derivative. Corros. Sci. 2011, 53, 873– 885. (71) Issaadi, S.; Douadi, T.; Zouaoui, A.; Chafaa, S.; Khan, M. A.; Bouet, G. Novel thiophene symmetrical Schiff base compounds as corrosion inhibitor for mild steel in acidic media. Corros. Sci. 2011, 53, 1484-1488. (72) Amin, M.A.; Ahmed, M.A.; Arida, H.A.; Arslan, T.; Saraçoglu, M.; Kandemirli, F. Monitoring corrosion and corrosion control of iron in HCl by non-ionic surfactants of the TRITON-X series – Part II. Temperature effect, activation energies and thermodynamics of adsorption. Corros. Sci. 2011, 53, 540–548. (73) Bentiss, F.; Lebrini, M.; Lagrenee, M. Thermodynamic characterization of metal dissolution and inhibitor adsorption processes in mild steel/2,5-bis(n-thienyl)-1,3,4thiadiazoles/hydrochloric acid system. Corros. Sci. 2005, 47, 2915–2931. (74) Yıldız, R.; Dogan, T.; Dehri, I. Evaluation of corrosion inhibition of mild steel in 0.1 M HCl by 4-amino-3-hydroxynaphthalene-1-sulphonic acid. Corros. Sci. 2014, 85, 215–221. (75) Mourya, P.; Banerjee, S.; Singh, M.M. Corrosion inhibition of mild steel in acidic solution by Tagetes erecta (Marigold flower) extract as a green inhibitor. Corros. Sci. 2014, 85, 352–363. (76) Solmaz, R. Investigation of corrosion inhibition mechanism and stability of Vitamin B1 on mild steel in 0.5 M HCl solution. Corros. Sci. 2014, 81, 75–84. (77) Chevalier, M.; Robert, F.; Amusant, N.; Traisnel, M.; Roos, C., Lebrini, M. Enhanced corrosion resistance of mild steel in 1 M hydrochloric acid solution by alkaloids extract from Aniba rosaeodora plant: Electrochemical, phytochemical and XPS studies. Electrochimica Acta. 2014, 131, 96–105.


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(78) Roy, P.; Karfa, P.; Adhikari, U.; Sukul, D. Corrosion inhibition of mild steel in acidic medium by polyacrylamide grafted Guar gum with various grafting percentage: Effect of intramolecular synergism. Corros. Sci. 2014, 88, 246-253. (79) Yousefi, A.; Javadian, S.; Dalir, N.; Kakemam, J.; Akbari, J. Imidazolium-based ionic liquids as modulators of corrosion inhibition of SDS on mild steel in hydrochloric acid solutions: experimental and theoretical studies. RSC Adv., 2015, 5, 11697–11713. (80) Verma, C.; Singh, A.; Pallikonda, G.; Chakravarty, M.; Quraishi, M.A.; Bahadur, I.; Ebenso, E.E. Aryl sulfonamidomethylphosphonates as new class of green corrosion inhibitors for mild steel in 1M HCl: Electrochemical, surface and quantum chemical investigation. J. Mol. Liq. 2015, 209, 306–319. (81) Sasikumar, Y.; Adekunle, A.S.; Olasunkanmi, L.O.; Bahadur, I.; Baskar, R.; Kabanda, M.M.; Obot, I.B.; Ebenso, E.E. Experimental, quantum chemical and Monte Carlo simulation studies on the corrosion inhibition of some alkyl imidazolium ionic liquids containing tetrafluoroborate anion on mild steel in acidic medium. J. Mol. Liq. 2015, 211, 105–118. (82) Kayaa, S.; Tüzüna, B.; Kayaa, C.; Obot, I. B. Determination of corrosion inhibition effects of amino acids: Quantum chemical and molecular dynamic simulation study. J. Taiwan Ins. Chem. Eng. 2016, 58, 528–535. (83) Verma, C.; Olasunkanmi, L. O.; Obot, I. B.; Ebenso, Eno E.; Quraishi, M. A. 5Arylpyrimido-[4, 5-b] quinoline-diones as new and sustainable corrosion inhibitors for mild steel in 1 M HCl: a combined experimental and theoretical approach. RSC Adv. 2016, 6, 15639–15654.



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Figure caption: Fig. 1: Synthetic scheme for the investigated inhibitors (CARBs) Fig. 2: Arrhenius plots for the corrosion of mild steel in 1 M HCl without and with the inhibitors. Fig.3: (a) Temkin, (b) Freundluich, and (c) Langmuir adsorption isotherm plots for the adsorption of CARBs on mild steel surface in 1 M HCl Fig. 4: Potentiodynamic polarization curves for mild in the absence and presence of optimum concentrations of the studied inhibitors (CARBs) Fig.5a-c: (a): Nyquist plot for mild steel in 1 M HCl in the absence and presence of optimum of concentrations of CARBs (b): Equivalent circuit used for the analysis of the EIS spectra (c): Bode plots for mild steel in 1 M HCl in the absence and presence of optimum of concentrations of CARBs Fig.6: SEM images of mild steel surfaces: abraded (a), in 1 M HCl in the absence of CARBs (b), and in 1 M HCl in the presence of optimum concentration of CARB-1 (c), CARB-2 (d), CARB-3 (e) and CARB-4 (f) Fig. 7: AFM images of mild steel: (a) in 1 M HCl in the absence of CARBs, and in the presence of optimum concentration of (b) CARB-1, (c) CARB-2, (d) CARB-3, and (e) CARB-4 Fig. 8: Optimized molecular structures of (a) CARB-1, (b) CARB-2, (c) CARB-3 and (d) CARB-4 at B3LYP/6-31+(d,p) level of theory Fig. 9: The frontier molecular orbital (left-hand side: HOMO; and right-hand side: LUMO) of the studied CARBs (a) CARB-1 (b) CARB-2, (c) CARB-3, and (d) CARB-4 +

-

Fig. 10: Fukui indices (left-hand side: f ; and right-hand side: f ) of the studied APQDs (a) CARB-1 (b) CARB-2, (c) CARB-3, and (d) CARB-4 visualized at 0.0025 isosurface. Fig. 11: Side (left-hand side) and top (right-hand side) views of the equilibrium configurations obtained after Monte Carlo simulations for the adsorption of (a) CARB-1, (b) CARB-2, (c) CARB-3 and (d) CARB-4 on Fe (110) surface


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Table Caption: Table 1: IUPAC name, molecular structure, molecular formula, melting point and analytical data of studied compounds (CARBs) Table 2: Weight loss parameters obtained for mild steel in 1 M HCl without and with different concentrations of CARBs Table 3: Variation of CR and η % with temperature in the absence and presence of optimum concentration of CARBs. Table 4: Activation energies for mild steel dissolution in 1 M HCl in the absence and of optimum concentration of CARBs Table 5: The values of Kads and ∆G◦ads for mild steel in the absence and presence of optimum concentration of CARBs at different temperatures. Table 6: Tafel polarization parameters for mild steel in 1 M HCl in the absence and presence of optimum concentrations of CARBs Table 7: EIS parameters for mild steel in 1 M HCl in the absence and presence of different concentrations of CARBs Table 8: Quantum chemical parameters of the studied compounds obtained at the B3LYP/631+G(d,p) level of theory.

Table 9: Energy parameters obtained from Monte Carlo simulations for the adsorption of the studied inhibitors on Fe (110) surface (in kcal/mol).



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Fig. 1: Synthetic scheme for the investigated inhibitors (CARBs)

Fig. 2: Arrhenius plots for the corrosion of mild steel in 1 M HCl without and with the inhibitors.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig.3: (a) Temkin, (b) Freundluich, and (c) Langmuir adsorption isotherm plots for the adsorption of CARBs on mild steel surface in 1 M HCl


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 4: Potentiodynamic polarization curves for mild in the absence and presence of optimum concentrations of the studied inhibitors (CARBs)



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Fig.5a-c:

(a): Nyquist plot for mild steel in 1 M HCl in the absence and presence of optimum of concentrations of CARBs (b): Equivalent circuit used for the analysis of the EIS spectra

(c): Bode plots for mild steel in 1 M HCl in the absence and presence of optimum of concentrations of CARBs



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig.6: SEM images of mild steel surfaces: abraded (a), in 1 M HCl in the absence of CARBs (b), and in 1 M HCl in the presence of optimum concentration of CARB-1 (c), CARB-2 (d), CARB-3 (e) and CARB-4 (f)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fig. 7: AFM images of mild steel: (a) in 1 M HCl in the absence of CARBs, and in the presence of optimum concentration of (b) CARB-1, (c) CARB-2, (d) CARB-3, and (e) CARB-4


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CARB-1

CARB-2

CARB-3

CARB-4

Fig. 8: Optimized molecular structures of investigated inhibitor molecules at B3LYP/6-31+(d,p) level of theory



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CARB-1-HOMO

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CARB-2-HOMO

CARB-3-HOMO

CARB-4-HOMO

CARB-1-LUMO

CARB-2- LUMO

CARB-3- LUMO

CARB-4- LUMO

Fig. 9: The frontier molecular orbital (left-hand side: HOMO; and right-hand side: LUMO) of the studied CARBs (a) CARB-1 (b) CARB-2, (c) CARB-3, and (d) CARB-4


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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+

-

Fig. 10: Fukui indices (left-hand side: f ; and right-hand side: f ) of the studied APQDs (a) CARB-1 (b) CARB-2, (c) CARB-3, and (d) CARB-4 visualized at 0.0025 isosurface.


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a

b

c



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d

Fig. 11: Side (left-hand side) and top (right-hand side) views of the equilibrium configurations obtained after Monte Carlo simulations for the adsorption of (a) CARB-1, (b) CARB-2, (c) CARB-3 and (d) CARB-4 on Fe (110) surface


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1 2 3 Table 1: IUPAC name, molecular structure, molecular formula, melting point and analytical 4 data of studied compounds (CARBs) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 255-phenyl-105-(4-nitrophenyl)-105-(p-tolyl)-10-((3R,5S,6R)5-(4-hydroxyphenyl)-1026 ((3R,5S,6R)-2,4,5((3R,5S,6R)-2,4,52,4,5-trihydroxy-6((3R,5S,6R)-2,4,5-trihydroxy27 trihydroxy-6trihydroxy-6(hydroxymethyl)tetrahydro6-(hydroxymethyl)tetrahydro28 (hydroxymethyl)tetrahyd (hydroxymethyl)tetrahydr 2H-pyran-3-yl)-9,102H-pyran-3-yl)-9,1029 30ro-2H-pyran-3-yl)-9,10o-2H-pyran-3-yl)-9,10dihydropyrido[2,3-d:6,5dihydropyrido[2,3-d:6,531dihydropyrido[2,3-d:6,5- dihydropyrido[2,3-d:6,5d']dipyrimidined']dipyrimidine32 d']dipyrimidined']dipyrimidine2,4,6,8(1H,3H,5H,7H)2,4,6,8(1H,3H,5H,7H)33 2,4,6,8(1H,3H,5H,7H)2,4,6,8(1H,3H,5H,7H)tetraone (CARB-3): Mol. tetraone (CARB-4): Mol. 34 Formula: C22H23N5O9; Mol. Formula: C21H21N5O10; Mol. 35tetraone (CARB-1): Mol. tetraone (CARB-2): Mol. 36Formula: C21H21N5O9; Formula: C21H20N6O11; wt. 501.44; Yellow crystals; wt. 503.41; Yellow crystals; 37Mol. wt. 487.42; Yellow Mol. wt. 532.41; Yellow Yield = 79%; mp 2510C; FT- Yield = 78%; mp 208-2100C; 38crystals; Yield = 79%; crystals; Yield = 84%; mp IR (KBr, cm-1): 3778, 3456, FT-IR (KBr, cm-1): 3662, 39 0 0 C; FT-IR 198-200 C; FT-IR (KBr, 3284, 3224, 2958, 2854, 3458, 3286, 3059, 2986, 40mp 198-200 -1 -1 (KBr, cm ): 3682, 3476, cm ): 3648, 3456, 3286, 2364, 1776, 1688, 2867, 2324, 1992, 1746, 41 3256, 3052, 2986, 2859, 3028, 2984, 2806, 2568, 1548,1536, 1452, 1428, 1568, 1514, 1449, 1312, 42 432572, 2384, 1748, 1635, 2334, 1689, 1622, 1542, 1362, 1192, 1132, 1018, 953, 1276, 1196, 1132, 1017, 998, 441578, 1448, 1426, 1369, 1478, 1362, 1287, 1245, 846, 673; 1H-NMR 926, 882, 837, 742, 623; 1H45 1254, 1232, 1124, 1043, 1167, 1079, 1017, 884, (500MHz, DMSO-d6/TMS) NMR (500MHz, DMSO46 1008, 956, 876, 834, 722, 778, 743, 654; 1H-NMR δ (ppm): 1.146 (S, 1H), d6/TMS) δ (ppm): 1.14647 1.236 (S, 1H), 2.500 (S, 3H), 1.234 (d, 1H), 2.500 (S, 1H), 48646; 1H-NMR (500MHz, (500MHz, DMSO49DMSO-d6/TMS) δ d6/TMS) δ (ppm): 1.145 3.340 (t, 1H), 3.724-3.888 3.337 (t, 1H), 3.458 (t, 1H), 50(ppm): 2.500 (S, 3H), (S, 1h), 2.500 (S, 1H), (m, 4H), 6.917-6.980 (m, 6.785-6.956 (m, 5H), 7.062513.344 (t, 1H), 6.6103.350 (t, 1H), 6.582-6.968 3H), 7.054-7.115 (m, 3H), 7.170 (m, 5H), 11.221-11.207 52 6.840 (m, 5H), 7.067(m, 5H), 7.070-7.779 (m, 10.793-10.929 (d, 2H), (q, 1H), 11.177-11.114 (q, 53 5H), 7.779-7.956 (m, 5H), 11.114-11.300 (m, 1H), 1H) 547.110 (m, 5H), 7.3327.474 (m, 4H), 11.1168.007-8.637 (m, 1H), 11.314-11.459 (m, 1H) 55 5611.177 (q, 1H), 11.20211.004 (S, 1H), 11.1145711.281 (q, 1H) 11.333 (m, 1H). 58 59 60 ACS Paragon Plus Environment


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Table 2: Weight loss parameters obtained for mild steel in 1 M HCl without and with different concentrations of CARBs inhibitor

Blank CARB-1

CARB-2

CARB-3

CARB-4

Conc

Weight loss

CR

Inhibition

Surface

(mol/L)

(mg)

(mg cm−2 h−1)

efficiency (η

coverage

%)

(θ)

0.00

230

7.66

---

---

1.88 x 10-5

103

3.43

55.21

0.5521

3.76 x 10-5

61

2.03

73.47

0.7347

5.63 x 10-5

40

1.33

82.6

0.826

7.41 x 10-5

26

0.86

88.69

0.8869

1.88 x 10-5

99

3.30

56.95

0.5695

3.76 x 10

-5

53

1.76

76.95

0.7695

5.63 x 10-5

33

1.10

85.65

0.8565

7.41 x 10-5

21

0.70

90.86

0.9086

1.88 x 10-5

94

3.13

59.13

0.5913

3.76 x 10-5

44

1.46

80.86

0.8086

5.63 x 10-5

26

0.86

88.69

0.8869

7.41 x 10

-5

15

0.50

93.47

0.9347

1.88 x 10-5

86

2.86

62.60

0.6260

3.76 x 10-5

32

1.06

86.08

0.8608

5.63 x 10-5

14

0.46

93.91

0.9391

7.41 x 10-5

8

0.26

96.52

0.9652


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Table 3: Variation of CR and η % with temperature in the absence and presence of optimum concentration of CARBs. Corrosion rate (CR) (mg cm-2 h-1) and Inhibition efficiency (η%)

Temperature (K)

Blank CR

η%

308

7.66

---

318

11.0

328 338

CARB-1 CR

CARB-2

η%

CR

CARB-3

CARB-4

η%

CR

η%

CR

0.866 88.69 0.700

90.86

0.500

93.478 0.266

96.52

---

2.033 81.51

1.533

86.06

1.066

90.303

0.733

93.33

14.3

---

4.266 70.23

2.966

79.30

2.300

83.953

1.766

87.67

18.6

---

7.733 58.57

5.766

69.10

4.466

76.071

4.133

77.85

Table 4: Activation energies for mild steel dissolution in 1 M HCl in the absence and of optimum concentration of CARBs

inhibitor

Ea (kJmol-1)

Blank

28.48

CARB-1

60.53

CARB-2

63.38


η%


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CARB-3

63.56

CARB-4

78.86

Page 50 of 53

Table 5: The values of Kads and ∆G◦ads for mild steel in the absence and presence of optimum concentration of CARBs at different temperatures. Kads (103 M-1)

inhibitor

-∆G◦ads (k Jmol-1)

308

318

328

338

308

318

328

338

CARB-1

9.41

5.29

2.83

1.69

33.72

33.29

32.63

32.19

CARB-2

11.93

7.41

4.59

2.68

34.33

34.18

33.96

33.48

CARB-3

17.18

11.17

6.27

3.81

35.26

35.27

34.81

34.47

CARB-4

33.29

16.79

8.53

4.21

36.96

36.35

35.64

34.75

Table 6: Tafel polarization parameters for mild steel in 1 M HCl in the absence and presence of optimum concentrations of CARBs

Inhibitor

Blank

Conc

Ecorr

βa

mol/L

(mV/SCE)

(µA/cm2)

---

-445

70.5

βc

icorr

η%

θ

----

----

(mV/dec) (mV/dec) 114.6

1150

CARB-1

7.41 x 10-5

-489

75.9

96.1

132.0

88.52

0.8852

CARB-2

7.41 x 10-5

-491

63.9

96.3

86.4

92.48

0.9248


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CARB-3

7.41 x 10-5

-517

69.4

71.2

58.7

94.89

0.9489

CARB-4

7.41 x 10-5

-523

64.9

154.2

36.2

96.85

0.9685

Table 7: EIS parameters for mild steel in 1 M HCl in the absence and presence of different concentrations of CARBs Inhibitor

Blank

Conc

Rs

Rct

mol/L

(Ω cm2)

(Ω cm2)

---

1.12

9.58

CARB-1

7.41 x 10-5

0.743

CARB-2

7.41 x 10-5

1.071

143.6

CARB-3

7.41 x 10-5

0.65

CARB-4

7.41 x 10-5

1.4

75.8

N

Cdl

η%

θ

----

----

(µF cm−2) 0.827 0.853

106.21 75.85

87.36

0.8736

0.870

49.01 93.32

0.9332

195.7

0.844

36.48 95.10

0.9510

288.8

0.875

27.83 96.68

0.9668



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Table 8: Quantum chemical parameters of the studied compounds obtained at the B3LYP/631+G(d,p) level of theory. Parameters→

EHOM

ELUMO

∆E

χ

Compounds↓

O

(eV)

(eV)

(eV)

∆N

(eV) CARB-1

-6.340

-1.707

4.633

4.023

0.643

CARB-2

-6.681

-2.567

4.114

4.624

0.578

CARB-3

-6.220

-1.669

4.550

3.944

0.672

CARB-4

-5.926

-1.686

4.241

3.806

0.753

Table 9: Energy parameters obtained from Monte Carlo simulations for the adsorption of the studied inhibitors on Fe (110) surface (in kcal/mol). Systems

Total energy

Deformation Ebinding energy

-597.810

Adsorption Rigid energy (Eads) adsorption energy -210.235 -242.597

Fe(110) + CARB-1

8.271

210.235

Fe(110) + CARB-2

-603.967

-213.372

-230.638

17.265

213.372

Fe(110) + CARB-3

-589.034

-222.651

-232.753

-12.225

222.651

Fe(110) + CARB-4

-571.106

-234.898

-240.352

-10.714

234.898


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Adsorption Behavior of Glucosamine-Based, Pyrimidine-Fused

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