Corrosion inhibition of carbon steel in hydrochloric acid by cationic arylthiophenes as new eco-friendly inhibitors:Experimental and quantum chemical study

Abd El-Aziz S.Fouda *,Mohamed A.Ismail ,Abdulraqeb A.Al-Khamri2 ,Ashraf S.Abousalem3

1 Department of Chemistry,Faculty of Science,El-Mansoura University,El-Mansoura 35516,Egypt

2 Department of Chemistry,Faculty of Education,Arts and Sciences,University of Saba Region,Marib,Yemen

3 Quality Control Laboratory,Operations Department,Jotun,Egypt

Keywords:Carbon steel Corrosion inhibition Adsorption Arylthiophene derivatives Polarization Electrochemical frequency modulation

ABSTRACT This study describes the adsorption behavior of three arylthiophene derivatives namely:2-(4-amidino-3-fluorophenyl)-5-[4-methoxy phenyl]thiophene dihydrochloride salt(MA-1217),2-(4-amidinophenyl)-5-[4-chlorophenyl] thiophene dihydrochloride salt (MA-1316) and 2-(4-amidino-3-fluorophenyl)-5-[4-ch lorophenyl]thiophene dihydrochloride salt(MA-1312)at C-steel in 1.0 mol·L-1 HCl interface using experimental and theoretical studies.Electrochemical and mass loss measurements showed that the inhibition efficiency (IE) of the arylthiophene derivatives increases with increasing concentrations and exhibited maximum efficiency 89%at 21×10-6 mol·L-1(MA-1217)by mass loss method.The investigated arylthiophene derivatives obey the Langmuir adsorption isotherm.From polarization studies the arylthiophene derivatives act as mixed-type inhibitors.Surface analysis were carried out and discussed.The mode of orientation and adsorption of inhibitor molecules on C-steel surface was studied using molecular dynamics (MD) simulations.Quantum chemical parameters as well as the radial distribution function indices and binding energies confirm the experimental results.

1.Introduction

Corrosion is a natural phenomenon transform metals and alloys to more stable form such as sulfides,oxides,and hydroxides through direct interaction with the surrounding environment [1].Corrosion has undesirable significances on human safety and on many industries,especially oil and gas industry.Thus,corrosion has become an important research subject[2].Carbon steel is commonly used in various industrial applications owing to its good mechanical properties,simple manufacturing process and low cost[3].Despite the advantageous properties of C-steel alloys,they easily corrode in corrosive environments.For example,during the pickling process in a hydrochloric acid medium which is widely used in different industrial processes,such as chemical cleaning,pickling of iron,descaling of boilers and oil well acidification.In comparison with other acids,hydrochloric acid is highly preferred because the salts formed during the process are soluble in water[4-8].Among the several methods available,the use of inhibitors is the most effective approach for corrosion control of metals in acid solutions [9].The addition of corrosion inhibitors to the aggressive solution is one of the most used method to prevent corrosion of C-steel [10].Organic inhibitors are often used to protect metals from corrosion,particularly molecules containing heteroatoms such as sulfur,oxygen,nitrogen,aromatic rings and π electrons [11].These atoms are the active sites of the adsorption process on the surface of the metal.Therefore,compounds that contain both sulfur(S),oxygen(O)and nitrogen(N)in their structural unit show greater inhibition efficiency than those that have only one of these heteroatoms[12].The inhibiting action of organic inhibitors is by adsorbing on the surface of metal by chemical or physical adsorption or both[13].These inhibitors form a protective film on C-steel surface and blocks the active sites,which reduces the harmful attack of the corrosive environment [14-20].Adsorption depends on the chemical structure of the inhibitor,the type of corrosive environment and the nature and condition of the metal surface [21].However,there is a need to develop some ecofriendly corrosion inhibitors due to the growing awareness of the importance of green chemistry in the field of corrosion science and technology all over the world [1].

In the present work,the effect of eco-friendly arylthiophene derivatives against corrosion of carbon steel in acidic solutions was investigated.Arylthiophene derivatives were selected in our study for their low cost,easy prepare,solubility in acidic media and contain heteroatoms such as oxygen,sulfur and nitrogen as active sites [22].In addition,our compounds have antiproliferation and anti-cancer activities as reported recently [23].Like other organic compounds,the presence of non-bonding electrons of heteroatoms(N,O,and S),as well as π-electrons and polar functional groups such as amino (—NH2),methoxy (—OCH3) in arylthiophene derivatives enhance the bonding between metal surfaces and the inhibitors and hence,adsorption of their molecules on the surface of the metal.These functional groups do not only enhance interactions with metallic atoms but also increase the solubility of inhibitor molecules in polar electrolytes such as water and acid solution[24-27].Foudaet al[28]studied the inhibition action of methoxy-substituted phenylthienyl benzamidines on the corrosion of C-steel in HCl and reached to inhibition efficiency of 95% at 21 × 10-6mol·L-1.Some arylthiophene derivatives were used as inhibitors for C-steel corrosion in HCl and gave 93.3%-91.1%inhibition efficiency[29].Foudaet al.[30] studied the experimental and computational chemical studies on cationic furanylnicotinamidines as novel corrosion inhibitors in HCl solution and gave 95% inhibition efficiency.

Herein,Experimental techniques such as weight loss method,electrochemical measurements,and surface examination of carbon steel samples by AFM,SEM and EDX techniques.The kinetic parameters and standard thermodynamic were calculated and discussed.In addition,the adsorption of cationic arylthiophene derivatives on the carbon steel was studied at longer immersion times in order to understand the interactions between the arylthiophene derivatives and the surface of carbon steel.The theoretical calculations were performed,including a molecular dynamics(MD)simulation and quantum chemical calculations for the tested arylthiophenes to explain the mechanism of corrosion inhibition.In the Monte carol simulation,the adsorption of inhibitors was studied under various conditions;acid solutions and vacuum[27].

2.Experimental

2.1.Materials

2.1.1.Preparation of arylthiophene compounds

Scheme 1. Synthetic route of the investigated cationic arylthiophenes 2a-c,reagents and conditions:(i) Lithium bis-(trimethylsilyl)amide,(ii) ethanol/ hydrogen chloride.

Table 1 Chemical structure of the tested compounds

Synthesis of the studied cationic arylthiophene inhibitors was done according to the methodology reported by Ismailet al.[23]as depicted in Scheme 1.The same prepared batch of compounds was used as received in the present investigation.The chemical structure and molecular formulas were listed in Table 1.Investigated inhibitors MA-1312,MA-1316 and MA-1217 contain the same counter ion Cl-which is assumed to have little effect compared to the cationic part in the prevailing acid environment of 1.0 mol·L-1HCl.The study was carried out in the existence and nonexistence of the investigated inhibitors at various concentrations;(1 × 10-6,5 × 10-6,9 × 10-6,13 × 10-6,17 × 10-6and 21 × 10-6mol·L-1).All experiments were carried out in thermostatic conditions.

2.1.2.Carbon steel samples

The corrosion inhibition measurements were performed on Csteel with the following chemical composition (% (mass)):(0.07%C,0.3% Mn,0.05% Si,0.022% P,0.010% S,0.001% Ti,0.030% Al and Fe balance).

2.1.3.Solutions

The aggressive media (1.0 mol·L-1HCl) were freshly prepared from AR grade 37% HCl solution.The arylthiophene compounds were prepared by dissolving them in 10 ml of dimethylsulfoxide(DMSO)and 90 ml of ethanol to give a stock solution of concentration inhibitors (1 × 10-3mol·L-1).

2.2.Mass loss method

The mass loss (ML) measurements were performed on equalsize C-steel specimens with dimension (2 cm × 2 cm × 0.2 cm)at different temperatures (298,308,318 and 328 °C).The surface of C-steel specimens was prepared using different grades of polishing sandpaper (320-2000),then the specimens are washed with distilled water,dried in air at room temperature and weighed before placing in the test solution by analytical balance measuring to 0.0001 mg.The specimen mass of the sample was determined before and after immersion in 100 ml of 1 mol·L-1HCl without and with various concentrations of inhibitors each 30 min for 6 h.The inhibition efficiency (IE),corrosion rate (CR) and surface coverage(θ)were calculated by the following Eqs.(1)-(3)[28,29]:

whereWandaare mass loss(mg)and area (cm2) of the specimen,respectively,CRandCR(i)are corrosion rate(mg·cm-2·h-1)of carbon steel in the absence and presence of inhibitors,respectively andtis the exposure time (h).

2.3.Electrochemical measurements

Three electrochemical techniques,namely Electrochemical Frequency Modulation (EFM),Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP),were used for investigation the corrosion behavior of C-steel in 1.0 mol·L-1HCl without and with different concentrations of inhibitors at room temperature (25 °C).All electrochemical experiments were performed in a conventional electrochemical cell in a three-electrode glass vessel.These three electrodes consisted of a saturated calomel electrode as a reference electrode,a platinum wire as an auxiliary electrode and C-steel as a working electrode with a surface area of 1.0 cm2.The working electrode (C-steel)was fabricated from the same type of C-steel used for the mass loss methods[30].Prior to electrochemical tests,the working electrode was prepared following the same preparation procedure,then the C-steel electrode is immersed in the test solution for 30 min to attain a stable value of open circuit potential (OCP).The Polarization measurements were performed at -250 to +250 mVvs.OCP with a scan rate of 0.5 mV·s-1.The EIS measurements were performed at OCP with an amplitude signal of 10 mV in the frequency range from 10 kHz down to 10 mHz.The PDP corrosion values including anodic Tafel(βa)and cathodic Tafel(βc)slopes,corrosion potential(Ecorr) and corrosion current density(icorr)were obtained using Tafel extrapolation method.All experiments were performed at least three times to check reproducibility.

Fig.1. ML-time curve for C-steel dissolution in 1.0 mol·L-1 HCl without and with various concentrations of inhibitors (a) MA-1217 (b) MA-1316 (c) MA-1312 at 25 °C.

2.4.Surface analysis

2.4.1.SEM/EDX

The surface morphology of polished C-steel and the inhibited and uninhibited samples were investigated by scanning electron microscopy (SEM) Model (Quanta 250 FEG,FEI Company,Netherlands) and energy dispersive X-ray (EDX) to obtain information on surface morphology and the elemental composition of the surfaces of polished and the corroded C-steel with and without arylthiophene inhibitors.Prior to the immersion of C-steel in acidic solutions without and with inhibitors,the C-steel surface was polished and washed with distilled water and dried in room temperature.

2.4.2.AFM

The AFM (model:FlexAFM3) technique was used to capture micrographs and calculate the surface roughness of carbon steel samples in the absence and presence optimal concentration of arylthiophene inhibitors.The AFM was operated in contact mode using a nonconductive silicon probe using nano surf C300(version 3.5.0.31) software.

2.4.3.Antimicrobial activity

The antimicrobial activity of tested inhibitors for the growth of nitrifying bacteria strains has been tested by seal analytical colony counter.The bacteria were provided from water samples isolated from cooling towers of Fertilizer plant in Egypt (El-Delta towers for Fertilizers).The bacteria were cultured on petri dishes before preparing a spore suspension in absence and presence of nitrifying bacteria [27].

2.5.Theoretical calculations quantum

2.5.1.DFT computations

The corrosion inhibition mechanism of organic compounds has been widely explored by quantum chemical calculations using DFT approach for the reliability of the results in less time.The geometry optimizations for the studied arylthiophene derivatives were performed at DFT level with functional B3LYP and basis set 6-31 (d,p).The quantum chemical parameters energy calculated were very useful to understand the mechanism of reactions,predict the physical and chemical properties of compounds,study of the effect of molecular structures on the inhibition action.The quantum chemical parameters such as total hardness (η),transferred electron fraction (ΔN),global softness (s),electronegativity (χ),dipole moment (μ),EHOMO,ELUMOand energy gap (ΔELUMO-HOMO) were obtained to study the adsorption of inhibitors on carbon steel surface.The electronic affinity (A) and the ionization potential (I) are directly related to the energies of LUMO and HOMO respectively,according to the relation:

DFT has been very successful in providing theoretical bases for common chemical concepts such as hardness and electronegativity:

The number of transferring electrons (ΔN) can be obtained by:

Table 2 ML corrosion parameters of C-steel in 1.0 mol·L-1 HCl solution in the absence and presence of different concentrations of inhibitors at various temperatures

Fig.2. Arrhenius plots for dissolution of C-steel in the 1 mol·L-1 HCl without and with various concentrations of inhibitors (a) MA-1217 (b) MA-1316 (c) MA-1312.

where ηFeand ηinhare the absolute hardness of iron and inhibitor,respectively,and χFeand χinhare absolute electronegativities of iron and inhibitor,respectively.The theoretical values of ηFeandχFewere used in this investigation to calculate the number of electrons transferred.Global softness (σ) is the ability of an atom or a group of atoms to receive electrons and was calculated from following Eq.(8):

Table 3 Activation parameters obtained from mass loss method for C-steel in 1.0 mol·L-1 HCl without and with various concentrations of the inhibitors

2.5.2.Monte Carlo simulations

The interaction between the studied compounds and surface of Fe (110) plane was performed using Monte Carlo simulations.Since the inhibitors studied have reduced the corrosion of carbon steel in acidic solutions,it is also important to study the adsorption of neutral and protonated inhibitor molecules in the existence of water molecules,competing for active sites on the surface of Fe(110).For this aim,100 water molecules were inserted as coadsorbates.Each of the water molecules and inhibitor molecules was first subjected to geometry optimization using the citing module.

3.Results and Discussion

3.1.Mass loss method

3.1.1.Effect of concentration

The effect of concentration on the corrosion of C-steel in 1.0 mol·L-1HCl with and without different inhibitor concentrations was investigated by mass loss method as shown in Fig.1.The values of the corrosion parameters obtained after 6 hours immersion in the test solutions are given in Table 2.From Table 2,with increasing inhibitor concentration (1 × 10-6to 21 × 10-6mol·L-1),the corrosion rate of C-steel decreases significantly and the corrosion inhibition efficiency increases.The increase in inhibitor concentration leads to more surface area of C-steel was covered by inhibitor molecules and forming of protective film on the metal surface which reduces the corrosive attack,or in other words,better inhibition efficiency is observed at higher concentration[31,32].The maximum IE for MA-1217,MA-1316 and MA-1312 are 89.1%,85.5% and 73.5% respectively at 21 × 10-6mol·L-1,and no change observed in IE above this concentration,thus it was selected as the optimum concentration[33].The effectiveness of the inhibition of the compounds could be attributed to the presence of S,O and N atoms in their molecule;these atoms increase the chemical adsorption on the metal through the free electrons that play a role important in the inhibition process of metal corrosion due to increasing the interaction of the inhibitor with C-steel surface [7].

3.1.2.Effect of temperature

The effect of temperature on the IE was studied by the ML method in the temperature range of 298 to 328 K without and with various concentrations of arylthiophene derivatives during a 6 h immersion.The values listed in Table 2 show that the inhibition efficiency increases,and the corrosion rate decreases with increasing temperature.This is because the increase in temperature increases the bonding between the metal surface and the inhibitor molecules [19].Apparently,the results obtained assume that the inhibitor molecules act by adsorption on the carbon steel surface by blocking the active sites to form a protective film on the carbon steel surface.The effect of temperature on the corrosion rate(CR)is expressed using the Arrhenius equation [34]:

wherekcorris the corrosion rate obtained from ML experiments,Ris the gas constant,Tis the absolute temperature,E*ais the apparent activation energy andAis the Arrhenius pre-exponential factor.Fig.2 shows the Arrhenius plots of (lgkcorr)vs.(1/T) for corrosion of metal in acidic test solution absence and presence of various concentrations of the inhibitors at different temperatures (25-55 °C).As shown in the figure,straight lines were obtained with the slopeand intercept ofA.Table 3 shows lower values ofin the presence of inhibitors compared to their absence,indicating the mechanism of chemisorption in this solution and refers to a considerable increase in adsorption of the inhibitor on a metal surface with an increase in temperature [35,36].The enthalpy and entropy activation of the corrosion process can be calculated by using the transition state Eq.(10) [37,38]:

whereNis Avogadro’s number (6.022 × 1023mol-1),ΔH* and ΔS*are the enthalpy and entropy of activation andhis Planck’s constant(6.626 × 10-34J·s).The graphs the transition state of (lgkcorr/T)vs.(1/T) for the two inhibitors studied are shown in Fig.3.From the values ΔH* and ΔS*,the straight lines were obtained with a slope of (-ΔH*/R) and the intersection of (ln (R/Nh)+ΔS*/R).The data presented in Table 3 shows that ΔH*has a positive value that indicates the endothermic nature for the dissolution process of C-steel[25,39].Additionally,large and negative values of ΔS*indicating the formation of an activated complex in the rate determining step represents an association rather than dissociation[40,41].The entropy comparison of activation values in the absence and the presence of inhibitors shows that the ΔS* values are less negative for inhibited solution than for uninhibited solution,which is probably attributed to the increase in the entropy of the solvent (H2O) resulting from the desorption of H2O from the C-steel surface in the presence of inhibitors [42].

3.1.3.Adsorption isotherm study

To understand the mechanism of corrosion inhibition,it is necessary to know the adsorption behavior of the inhibitors on the carbon steel surface.Adsorption isotherms are valuable models for studying the interaction between the metal surface and inhibitor molecules.The linear regression coefficient (R2) close to unity was obtained upon plottingC/θvs.C,as given in Fig.4.This indicates that the adsorption inhibitors on the metal surface obey the Langmuir adsorption isotherm.The Langmuir adsorption isotherm was calculated using Eq.(11) [43,44]:

Fig.3. Kinetic transition state plots for C-steel corrosion in the 1.0 mol·L-1 HCl without and with various concentrations of inhibitors(a)MA-1217(b)MA-1316(c)MA-1312.

Fig.4. Langmuir isotherm plots for C-steel in the 1.0 mol·L-1 HCl in the presence of 21×10-6 mol·L-1 concentration of inhibitors(a)MA-1217(b)MA-1316(c)MA-1312 at various of temperature.

Table 4 Adsorption thermodynamic results of arylthiophene derivatives on C-steel in 1.0 mol·L-1 HCl at different temperatures

whereCis the corrosion inhibitor concentration in the solution,θ is the surface coverage andKadsis the adsorption equilibrium constant,which was calculated from the following Eq.(12) [43]:

whereTwas the thermodynamic temperature(K),Rwas the universal gas constant(8.314 J·mol-1·K-1),55.5 was the molar concentration of water andwas the free adsorption energy.Table 4 showsKadsvalues are relatively high,due to increase adsorption of inhibitors on the metal surface and hence increase inhibition efficiency.The value ofwas used to understand the adsorption type of inhibitors.As can be seen in Table 4,the value ofis large and negative,which means that the adsorption of inhibitor molecules on the surface of the metal is spontaneous[45,46].From literature,the adsorption of an inhibitor is described as physisorption ifvalues at around (-20 kJ·mol-1) or lower negative and chemisorption if thevalues are in order of(-40 kJ·mol-1)or higher negative.Thevalues for the inhibitors studied ranged from-42 to-51 kJ·mol-1,indicating that the adsorption mechanism of the inhibitors on metal surface in 1.0 mol·L-1HCl solution is distinctively chemisorption[47-49].In other words,these inhibitors can be absorbed on the surface of the metal by establishing strong bonds.The heat of adsorptioncan be calculated by using the van’t Hoff equation [13,50]:

The plots of lg (Kads)vs.1/Tfor both inhibitors are shown in Fig.5.The straight lines were obtained with a slope equal toand intercept equal to [R-lg(55.5)] [19].The standard adsorption entropyat various temperatures can be estimated from Gibbs-Helmholtz equation[51]:

3.2.Electrochemical technique

3.2.1.Potentiodynamic polarization (PDP) techniques

Polarization measurements were made to obtain important information on the kinetics of anodic and cathodic reactions.Potentiodynamic polarization curves for C-steel in 1.0 mol·L-1HCl solution with and without various concentrations of arylthiophene derivatives at 25°C are shown in Fig.6.Electrochemical corrosion parameters such as (icorr,Ecorr,βa,βcand IE) are calculated from the extrapolation of the polarization curves shown in Fig.6 and recorded in Table 5.The inhibition efficiency can be determined from (icorr) values by the following Eq.(15) [53]:

whereicorrandicorr(inh)are corrosion current densities of C-steel in acid solution in the absence and presence of inhibitors,respectively.Table 5 shows that corrosion current densities decreased and inhibition efficiency values increased with increasing inhibitor concentration,indicating that adsorption of the inhibitors over the metallic surface and blocking active sites [46].In literature,if the change inEcorrvalue in the existence of the inhibitor is higher than 85 mV as compared to theEcorrvalue of blank,the inhibitor is considered as anodic or cathodic type.However,if the change in theEcorrvalue is less than 85 mV,it is considered a mixed type inhibitor [54,55].In our study,the change in theEcorrvalue is 66 mV,which indicates that the inhibitors studied acts as a mixed type inhibitor with a slight cathodic dominance.The results of the PDP measurements are in good agreement with the results obtained from the mass loss method.

Fig.5. vant’s Hoff plots(lg Kads vs.1/T)for the adsorption of inhibitors(a)MA-1217(b) MA-1316 (c) MA-1312 at 25 °C on CS surface in 1.0 mol·L-1 HCl.

Table 5 PDP parameters of C-steel corrosion in 1.0 mol·L-1HCl without and with various concentrations of the arylthiophene inhibitors at 25 °C

3.2.2.Electrochemical impedance spectroscopy (EIS) techniques

The surface properties and kinetics of the electrode processes of the studied systems can be observed by electrochemical impedance spectroscopy.The equivalent circuit used in this system is shown in Fig.7.Nyquist and Bode graphs for the corrosive dissolution of carbon steel in a 1.0 mol·L-1HCl solution in the absence and presence of different concentrations of inhibitors are shown in Figs.8 and 9,respectively.The Nyquist graphs displayed that the diameter of the semicircular capacitance loop increases with increasing inhibitor concentration.This is attributed to the occurrence of a charge transfer in the solution [56,57].Electrochemical impedance parameters such as charge transfer resistance (Rct),double layer capacitance (Cdl) and inhibition efficiency (IE) were calculated with the help of the equivalent circuit and are given in Table 6 which shows that the values ofCdldecrease with increasing inhibitor concentration,which is most probably due to the decrease in local dielectric constant and/or increase in thickness of the electrical double layer This suggests that the inhibitors actviaadsorption of inhibitor molecules on the metal/acid interface[58,59]and the decrease in theCdlvalues is caused by the gradual replacement of water molecules by the adsorption of inhibitor molecules on the electrode surface,while the values of the charge transfer resistance (Rct) increases,due to the increase of the thickness of double layer.The value of double layer capacitance(Cdl) can be determined using maximum frequency from the following Eq.(16) [30]:

The inhibition efficiency was measured from the polarization resistance using the following Eq.(17) [60]:

whereRct(inh)andRctare the charge transfer resistance with and without inhibitors,respectively.The inhibition efficiency increases with increasing inhibitor concentrations due to increased coverage of inhibitor surface on the metal surface [44,61].The results of the EIS measurements agree with those obtained with the PDP techniques.

3.2.3.Electrochemical frequency modulation techniques

Electrochemical frequency modulation (EFM) technique is a non-destructive electrochemical technique,can directly and fast calculate the value of the corrosion current without previous information of Tafel slopes and has an inside self-check in CF-2 and CF-3 causality factors to validate the accuracy of results [62].The EFM spectra of C-steel in 1.0 mol·L-1HCl solutions without and with the addition of different concentrations of arylthiophene derivatives are shown in Fig.10.The calculated electrochemical parameters (icorr,βc,βa,CF-2,CF-3 and IE) are recorded in Table 7.The corrosion inhibition efficiency was evaluated using the following Eq.(18) [30]:

whereicorrandicorr(inh)are the corrosion current densities in the absence and presence of inhibitors,respectively.The experimental values of CF-2 and CF-3 obtained (Table 7) are in agreement with the theoretical values of the causal factors designated respectively as 2 and 3,which suggests that the measured data are of high precision[63].A significant reduction in corrosion current density was observed,as well as the corrosion rate with the increase in the concentration of the inhibitors,which indicates the compounds investigated prevent the corrosion of carbon steel in acid solutions [64].

3.3.Surface analysis study

3.3.1.Scanning electron microscope (SEM)

Fig.6. PDP plots of C-steel corrosion in 1.0 mol·L-1 HCl without and with various concentrations of the arylthiophene inhibitors (a) MA-1217 (b) MA-1316 (c) MA-1312 at 25 °C.

Fig.7. Equivalent circuit used to fit experimental EIS data.

The SEM images of carbon steel samples immersed in the uninhibited and inhibited solutions and bare polished carbon steel sample are shown in Fig.11(a)-(e).it can be seen that the surface of the carbon steel sample before immersion (Fig.11(a)) is smoother and after immersion in the non-inhibiting solution(Fig.11(b)),it becomes a very rough surface with great corrosion and cracks distributed over the surface due to the aggressive attack of the acid solution[14,65].However,the damage has significantly decreased in the presence of arylthiophene inhibitors (Fig.11(c)—(e)) which exhibit a softer and smoother surface.This smoother surface morphology refers to the formation of a protective arylthiophene inhibitors film on the surface of the metal[57,66-68].

3.3.2.Energy dispersive X-ray (EDX)

EDX analysis was performed to study the nature of the form of a protective film on the carbon steel surface exposed to a 1.0 mol·L-1HCl solution without and with the optimal concentration of the inhibitors.Fig.12 shows EDX spectra before and after immersion in the inhibitors for 6 hours.Table 8 shows the percentage composition of C-steel elements determined by EDX.The characteristics iron peak intensity was higher for C-steel in inhibited solution and unimmersed carbon steel surface as compared to carbon steel in the uninhibited solution.When the carbon steel sample was immersed in the solution in the absence of inhibitor,the iron content of the surface of the polished carbon steel decreased from 90.59% to 57.73%with the high oxygen content of 33.82%,as shown in (Fig.12 and Table 8),which indicates the corrosive acid attack on the metal surface [37].However,for the carbon steel samples immersed in the inhibited solutions,the oxygen content decreased from 31%to 27%over that of the uninhibited solution.This indicates blocking active sites of the C-steel surface by a corrosive acid attack[51].From this,it can be said that the arylthiophene molecules adsorbed on the carbon steel surface and forming a protective film.

3.3.3.Atomic force microscopy (AFM)

AFM is a powerful tool for investigating surface morphology and provides additional evidence of the adsorption of inhibitor molecules on the surface of the metal.The three-dimensional AFM images of the carbon steel surface immersed in uninhibited and inhibited solution are shown in Fig.13(a)—(e).The average roughness of the carbon steel surface in the uninhibited solution is 993.76 mm(Fig.13(b))compared to the polished surface whose average roughness is 17 mm (Fig.13(a)) which due to the severe corrosion of the metal surface.However,in the inhibited solutions(Fig.13(c)—(e))at the optimum concentration(21×10-6mol·L-1),the average roughness decreases to 52.12 mm for (MA-1217),56.78 mm for (MA-1316) and 61.41 mm for (MA-1312).This confirms that the surface in the existence of inhibitors is smoother compared to the surface in the absence of inhibitors.The smoothness of the surface is attributed to the formation of a protective film adsorbed from the inhibitor molecules prevent corrosion of the carbon steel surface [69].

Fig.8. Nyquist plot for C-steel dissolution in the 1.0 mol·L-1 HCl without and with various concentrations of the inhibitors(a)MA-1217(b)MA-1316(c)MA-1312 at 25°C.

3.4.Biocidal analysis

It is very important to use inhibitors to prevent corrosion of metals in aqueous solutions which do not initiate Microbialinitiated corrosion.In the current work,the investigated inhibitors were evaluated against growth of nitrifying bacteria which commonly inhabits the cooling towers in most Fertilizers’ plants.The effect of the arylthiophene derivatives against the growth of this certain bacteria (number of colonies) is illustrated in Fig.14.Among the tested compounds,the fluorinated arylthiophene derivative MA-1312 has the highest activity against bacterial growth and its activity is superior to other compounds with the same concentration.The efficiency of the compounds in reducing the growth of bacteria was found in the following order:MA-13 12 ＜MA-1316 ＜MA-1217.MA-1312 contains Fluorine and Chlorine atoms have suppressing effect on the bacteria growth because these atoms bind to the cellular wall of bacteria and disrupt its metabolic process.The biocidal action of the arylthiophene derivatives was evaluated by using DMSO/ethanol as solvent since the arylthiophenes have a lower solubility in water.Consequently,a blank solution such as DMSO/ethanol without arylthiophene was used as a negative control for bacterial growth under the same conditions to eliminate the interference of the solvent with the arylthiophene influence.In addition,the concentrations used in our work to prevent bacterial growth (21 × 10-6)is safe and non-toxic and equals the optimal concentration of these inhibitors that prevent corrosion of the C-steel at 1.0 mol·L-1HCl [70-72].

3.5.Theoretical details

3.5.1.DFT calculations

All quantum chemical properties were obtained by geometric optimization,based on Kohn-Sham approach,at DFT level,with respect to the whole nuclear.DFT calculations are very powerful approach to explore the electron properties of the studied molecules and find correlation to their inhibition performance.The quantum chemical descriptors are described by the energy of Frontier molecular orbitals FMOs,HOMO and LUMO at equilibrium for the geometrical optimized structures of the inhibitor molecules in the ground state.The most important parameters areEHOMO,ELUMOand the energy gap between LUMO and HOMO which used to study the inhibition effect of the compounds studied.The frontier orbital (HOMO and LUMO) of chemical species plays an important role in defining their reactivity.In general,the HOMO and LUMO orbitals depicts the molecular centers where electron donation and acceptance occur,respectively.HOMO is an the most outer orbital that contains electrons and has the highest level of energy so that it describes the electron donating ability of inhibitors.While,the LUMO is the internal orbital wherein electrons can transfer from a d-orbital of metal atom.Fig.15 shows that the HOMO of the inhibitors is localized in the aryl ring and near the N atoms,whereas LUMO was localized mainly in the thiophene rings.Fig.16 shows the optimized molecular structures,HOMO and LUMO orbitals of inhibitor molecules that use DFT model chemistry.The location of HOMO orbitals for inhibitors in two rings indicates why the reactivity of inhibitors is stronger with the metal surface.Thus,adsorption of inhibitors on the metal surface is controlled by the HOMO orbital.The higherEHOMOvalues of the molecules indicate a greater tendency to donate electrons to the vacant d-orbitals of metal,and this means increase the corrosion inhibition efficiency of metal in acid solutions [28].Table 9 shows that theEHOMOvalue for MA-1217 is higher than that of MA-1316 and MA-1312,indicating that MA-1217 has a greater capacity for electron donation.This may be due to the existence of the electron donating methoxy group attached to the phenyl ring.On the other hand,lowerELUMOvalues of the molecules indicate a tendency to accept electrons [73].The low values of the energy gap (ΔE),indicates to easier donate electrons for dorbital metal,due to that the energy needed to transport a HOMO electron from the molecule will be decreased,which means an increase of adsorption of inhibitor molecules on the metal surface[56].Further inspection of Table 8 shows that higher ΔNvalues indicate a greater tendency to donate electrons to the metal surface [74].Lower values of dipole moment (μ) indicates high adsorption of inhibitors on the metal surface.

Fig.9. Bode plot for C-steel dissolution in the 1.0 mol·L-1 HCl without and with various concentrations of the inhibitors (a) MA-1217 (b) MA-1316 (c) MA-1312 at 25 °C.

Table 6 EIS parameters of C-steel corrosion in 1.0 mol·L-1 HCl without and with various concentrations of the arylthiophene inhibitors

Fig.10. EFM spectra of C-steel dissolution in the 1.0 mol·L-1 HCl without and with various concentrations of the inhibitors (a) MA-1217 (b) MA-1316 (c) MA-1312 at 25 °C.

3.5.2.Monte Carlo simulation

Monte Carlo simulation was performed to understand further the interaction of the investigated molecules with the metal surface in a vacuum and in acidic conditions.Top and side views of the more stable configuration for adsorption of the neutral and protonated types of arylthiophene derivatives studied on Fe(110) cleaved surface is shown in Figs.16 and 17.In aqueous solutions,arylthiophene derivatives exist as neutral molecules or as cations (protonated arylthiophene).The arylthiophene molecules adsorbed in parallel orient to the Fe (110) surface,ensuring optimal interactions of the heteroatoms and πelectrons with the metallic surface,thus increasing the surface coverage [75].In addition,the inhibitor molecules remove water molecules from the adsorption sites,indicating that an increase in the number of inhibitor molecules in the test solution may desorb more water molecules [76].Table 10 shows values of(adsorption energy,total adsorption,deformation energies and rigid absorption) calculated using Monte Carlo simulation.It is clear from the table that the adsorption energy of MA-1217 at the surface of Fe (110) in the presence of water is greater than that of MA-1316 and MA-1312.This is attributed to the existence of the electron donor methoxy group (—OCH3) in the structure MA-1217,which acts as additional active centers.Compared to MA-1217,the existence of the Cl-group on the MA-1316 and MA-1312 structures eliminates the electron density of its molecular structure and decrease its interactions with the iron surface.

3.6.Mechanism of inhibition

The mechanism of inhibition could be attributed to the adsorption of arylthiophene molecules on the C-steel surface.Generally,two modes of adsorption of inhibitor molecules on the surface of carbon steel,the chemical mode (chemisorption)or the physical mode (physisorption) is considered.The neutral molecules of arylthiophene are adsorbed on the C-steel surface by a chemical adsorption mechanism through the π-electron metal interaction or by donating lone pairs of electrons from the heteroatoms to the empty d-orbital of iron that forms a coordinate covalent bond with the C-steel surface.This inhibits the dissolution of the anodic iron.The arylthiophene molecules can be adsorbed by the physisorption mechanism.The arylthiophene molecules are protonated in an acidic solution,and it has been established that the C-steel surface bears a positive charge in acid.It is therefore difficult for arylthiophene molecules to come directly to the surface of C-steel.In contrast,chloride ions adsorbed on the metal surface by hydration,creating negative charges on the metal,and protonated arylthiophene molecules are adsorbed by electrostatic interaction (physisorption) withchloride ions adsorbed on the surface of C-steel.This inhibits the cathodic evolution reaction of hydrogen.Therefore,it could be said that arylthiophene molecules inhibit carbon steel corrosion by controlling anodic and cathodic reactions.Scheme 2 is a visible representation of the adsorption mechanism of arylthiophene molecules on the C-steel surface in 1.0 mol·L-1HCl.

Table 7 EEM parameters of C-steel corrosion in 1.0 mol·L-1 HCl without and with various concentrations of the arylthiophene inhibitors

Fig.11. SEM images of C-steel smooth surface (a),after 6 hours immersion in 1.0 mol·L-1 HCl in the inexistence (b) and existence 21 × 10-6 mol·L-1 of inhibitors (c—e).

Fig.12. EDX image of smooth C-steel surface (a),after 6 hours immersion in 1.0 mol·L-1 HCl in the inexistence (b) and existence 21 × 10-6 mol·L-1 of inhibitors (c—e).

Table 8 Surface composition of C-steel in 1.0 mol·L-1 HCl inexistence and existence 21 × 10-6 mol·L-1 of the arylthiophene inhibitors after (6 hours) immersion

Fig.13. AFM images of smooth C-steel surface(a),after 6 hours immersion in 1.0 mol·L-1 HCl in the inexistence(b)and existence 21×10-6 mol·L-1 of inhibitors(c and e).

Fig.14. Colonies of bacteria grown in Petri dishes and counted by the Doc-it colony counter instrument.

Fig.15. Electron distribution on the frontier molecular orbitals of the investigated compounds:left HOMO and right LUMO.

Fig 16. Top and side views for the most stable adsorption position of the inhibitors on Fe (110) surface in vacuum.

Table 9 The quantum chemical parameters of the investigated inhibitors at the DFT method for the inhibitors in vacuum and solution

Table 10 Monte Carlo simulation parameters calculated for the most stable configuration of investigated compounds on Fe (110) surface in vacuum and acid solution conditions

Fig 17. Top and side views for the most stable adsorption position of the inhibitors on Fe (110) surface in acid.

Scheme 2. Schematic representation of the adsorption of arylthiophene molecules on C-steel surface in 1.0 mol·L-1 HCl.

4.Conclusions

In this study,the inhibition efficiencies of arylthiophene derivatives as inhibitors for C-steel corrosion in 1.0 mol·L-1HCl solution were evaluated and discussed using experimental tests,surface examination such as SEM,EDX,AFM and FT-IR analysis,and quantum chemical calculations and MD simulation.Taking into account all the results obtained,the following conclusion was drawn:

(1) In the mass loss method,the inhibition efficiency increases with increasing concentration and temperature.

(2) The investigated cationic arylthiophenes act as corrosion inhibitors and their adsorption obey the Langmuir adsorption isotherm.

(3) The positive values of ΔSadsand the positive values of ΔHadsindicate that the adsorption process of the inhibitor occurs by chemisorption and that the reaction is endothermic.

(4) The polarization study indicated that arylthiophene derivatives are mixed type corrosion inhibitors.

(5) The EIS study suggests that cationic arylthiophenes inhibit the corrosion of carbon steel by forming a protective film on the metal surface.

(6) SEM,EDX,AFM and FT-IR studies confirm that the molecules of the investigated arylthiophene adsorb effectively on the metal surface.

(7) Quantum chemical study revealed that the studied compounds have high affinity to adsorb onto the metal surface by the formation of chemical bonding.

(8) The results of DFT and molecular dynamics simulations corroborate the results obtained experimentally.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Authors gratefully acknowledge the financial support provided by the Ministry of Higher Education &Scientific Research of Yemen.