The adsorption and corrosion inhibition of 8-hydroxy-7-quinolinecarboxaldehyde derivatives on C-steel surface in hydrochloric acid

A.M.Eldesoky ,S.G.Nozha *

1 Engineering Chemistry Department,High Institute of Engineering&Technology,New Damietta,Egypt

2 Al-Qunfudah Center for Scienti fic Research(QCSR),Al-Qunfudah University College,Umm Al-Qura University,Saudi Arabia

3 Ministry of Health,Domyat Laboratory,Damietta,Egypt

1.Introduction

Carbon steel has been extensively used under different conditions in petroleum industries[1].Aqueous solutions of acids are among the most corrosive media and are widely used in industries for pickling,acid cleaning of boilers and oil well[2–6].The main problem concerning C-steel applications is its relatively low corrosion resistance in acidic solution.Several methods were used currently to reduce corrosion ofC-steel.One ofsuch methodsisthe use oforganic inhibitors[7–11].The effect of aniline compounds,on the corrosion of mild steel in HCl has been studied[12].Effective inhibitors are heterocyclic compounds that have л bonds,heteroatoms such as sulfur,oxygen ornitrogen and the sites ofthese elements are considered as chemisorption center[13].These elements possesshigherelectron density and are called as electroactive groups.The electron density on these centers is the direct measure of IE of the inhibitor[14].These molecules depend mainly on certain physical properties of the inhibitor molecules such as functional groups,steric factors,electron density at the donor atom and electronic structure of the molecules[15,16].Regarding the adsorption of the inhibitor on the metal surface,two types of interactions are responsible.One is physical adsorption which involves electrostatic force between ionic charges or dipoles of the adsorbed species and electric charge atmetal/solution interface.Otherischemical adsorption,which involves charge sharing or charge transfer from inhibitor molecules to the metal surface to form coordinated types of bonds[17].The selection of appropriate inhibitors mainly depends on the type of acid,its concentration,and temperature.The in fluence of such quinolines and their derivatives(N-heterocyclic compounds)on the corrosion of mild steel in acidic solution has been mentioned by many authors[18–22].Although some Schiff base quinoline derivatives have been reported as corrosion inhibitors for steel in acidic medium[20–22],no work to the best of our knowledge has been documented on the corrosion inhibition potentials of azo quinoline derivatives.

The objective of the present investigation is to study the corrosion inhibition of C-steel in 2 mol·L?1HCl using 8-hydroxy-7-quinolinecarboxaldehyde derivatives and to propose a suitable mechanism for the inhibition process.It was also the purpose of the present work to discuss the relationship between quantum chemical calculations and experimental protection efficiencies of the tested 8-hydroxy-7-quinolinecarboxaldehyde derivatives by determining various quantum chemical parameters.These parameters include the highest occupied molecular orbital(EHOMO)and the lowest unoccupied molecular orbital(ELUMO),the energy difference(ΔE)betweenEHOMOandELUMO.

2.Experimental

2.1.Composition of material samples

Table 1Chemical composition of the C-steel in weight%

2.2.Chemicals and solutions

Hydrochloric acid(37%),ethyl alcohol was purchased from Al-gomhoria Company.Bidistilled water was used throughout all the experiments.

The organic inhibitors used in this study were some 8-hydroxy-7-quinolinecarboxaldehyde derivatives,listed in Table 2.

8-Hydroxy-7-quinolinecarboxaldehyde was prepared according to El-Sonbati and El-Bindary[23].8-Hydroxy-7-quinolinecarboxaldehyde(HLn)was prepared according to El-sonbatiet al.[24,25].As following,azo dye ligands(HLn)were prepared from aniline or itsp-substituted derivatives(10 mmol)were dissolved in hydrochloric acid(20 mmol/25 ml distilled H2O).The hydrochloric compound was diazotized below 0–5 °C with a solution of sodium nitrite(0.8 g,10 mmol,30 ml distilled H2O).The diazonium chloride was coupled with an alkaline solution of oxine(1.7 g,10 mmol)in 20 ml of pyridine.The crude dye was collected by filtration and was crystallized from DMF,then dried in a vacuum desiccator over P2O5.

2.3.Methods

2.3.1.Mass loss measurements

Rectangularspecimens ofC-steelwith dimensions 2.1 cm×2.0 cm×0.2 cm were abraded with different grades of emery paper,degreased with acetone,rinsed with bidistilled water and dried between filter papers.After weighting accurately,the specimens were immersed in 100 ml of 2 mol·L?1HCl with and without different concentrations of inhibitors at 30°C.After different immersion periods,the C-steel samples were taken out,washed with bidistilled water,dried and weighted again.The mass loss values are used to calculate the corrosion rate(R)in mm·a?1by Eq.(1):

whereDis the Fe density in g·cm?3,Ais the exposed area in cm2;Tis exposure time in hour.The inhibition efficiency(IE)and the degree of surface coverage(θ)were calculated from Eq.(2):

whereR*andRare the corrosion rates of C-steel in the absence and in the presence of inhibitor,respectively.

2.3.2.Electrochemical measurements

Electrochemical measurements were conducted in a conventional three electrodes thermostated cell assembly using a Gamry potentiostat/galvanostat/ZRA(model PCI300/4).A platinum foil and saturated calomel electrode(SCE)were used as counter and reference electrodes,respectively.The C-steel electrodes were 1 cm×1 cm and were welded from one side to a copper wire used for electrical connection.The electrodes were abraded,degreased and rinsed as described in weight loss measurements.All experiments were carried out at temperature(30 ± 1)°C.The potentiodynamic curves were recorded from ?500 to 500 mV at a scan rate 1 mV·s?1after the steady state is reached(30 min)and the open circuit potential(OCP)was noted.The IE and degree of surface coverage were calculated from Eq.(3):

Table 2Chemical structure,names,molecular weights and molecular formula of inhibitors

wherei0corrandicorrare the corrosion current densities of uninhibited and inhibited solution,respectively.

Electrochemical impedance spectroscopy(EIS)and electrochemical frequency modulation(EFM)experiments were carried out using the same instrument as before with a Gamry framework system based on ESA400.Gamry applications include software EIS300 for EIS measurements and EFM140 for EFM measurements;computer was used for collecting data.Echem Analyst 5.5 Software was used for plotting,graphing and fitting data.EIS measurements were carried out in a frequency range of 100 kHz to 10 mHz with amplitude of 5 mV peakto-peak using ac signals at respective corrosion potential.EFM carried out using two frequencies 2 and 5 Hz.The base frequency was 1 Hz.In this study,we use a perturbation signal with amplitude of 10 mV for both perturbation frequencies of 2 and 5 Hz.

2.3.3.Theoretical study

The molecular structures of the investigated compounds were optimized initially with PM3 semiempirical method so as to speed up the calculations.The resulting optimized structures were fully reoptimized using an initio Hartree–Fock(HF)with 6-31G basis set.The molecules were built with the Gauss View 3.09 and optimized using Gaussian 03 W program[25].

3.Results and Discussion

3.1.Mass loss measurements

Fig.1 shows the mass loss–time curves for the corrosion of C-steelin 2 mol·L?1HCl in the absence and presence of different concentrations of compound(HL1).Similar curves for other compounds were obtained(not shown).The data of Table 3 show that,the inhibition efficiency increases with increase in inhibitor concentration from 1× 10?6to 11×10?6mol·L?1.The maximuminhibition efficiency wasachieved at 11 × 10?6mol·L?1;therefore IE tends to decrease in the following order:

Fig.1.Weightloss-time curves for C-steel dissolution in 2 mol·L?1 HClin the absence and presence of different concentrations of compound(HL1)at(30± 1)°C.

Table 3Variation ofinhibition efficiency(IE)ofdifferentcompounds with their molar concentrations from mass loss measurements at 120 min immersion in 2 mol·L?1 HCl at(30 ± 1)°C

3.1.1.Adsorption isotherm

The adsorption isotherms are considered to describe the interactions of the inhibitor molecule with the active sites on the metal surface[26].Attempts were made to fit ? values to various isotherms including Frumkin,Langmuir,Temkin,and Freundlich.The results were best fitted by far by the Langmuir adsorption isotherm which has the following equation[27,28]:

whereCis the concentration of inhibitor;Kadsis the adsorptive equilibrium constant;and ? is the surface coverage of 8-hydroxy-7-quinolinecarboxaldehyde derivatives on carbon steel,which can be calculated by the ratio of IE/100 for different inhibitors concentrations[29].Fig.2 show the straight lines ofC/?vs.Cat(30 °C).The linear correlation coefficients(0.926),(0.85851)and(0.7017)at 30°C are almost equal to 1 and the slopes(1.0018),(1.016)and(1.042)at 30°C are close to 1,which con firms the assumption that the adsorption of 8-hydroxy-7-quinolinecarboxaldehyde derivative compounds on the carbon steel surface obeys Langmuir adsorption isotherm.The adsorptive equilibrium constants atT(30°C)namelyKare(4.39× 105),(2.53 × 105)and(1.71 × 105)L·mol?1,respectively.

The standard adsorption free energy(ΔG?ads)can be obtained according to the following equation[30,31].

Fig.2.Curve fitting of corrosion data for C-steel surface in hydrochloric acid in presence of different concentrations of organic additives to the Langmuir adsorption isotherm at(30 ± 1)°C.

The negative values of ΔG?ads(Table 4)suggest that the adsorption of 8-hydroxy-7-quinolinecarboxaldehyde derivatives on the carbon steel surface is spontaneous.Generally,the values of ΔG?adsaround or less than ?20 kJ·mol?1are associated with the electrostatic interaction between charged molecules and the charged metal surface(physisorption);while those around or higher than ?40 kJ·mol?1mean charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of metal bond(chemisorption)[32].The values of ΔG?adsobtained were approximately equal to(42.85–40.48)kJ·mol?1,indicating that the adsorption mechanism of these investigated 8-hydroxy-7-quinolinecarboxaldehyde derivatives on C-steel in 2 mol·L?1HCl solution involves both electrostatic adsorption and chemisorptions[33,34].The thermodynamic parameters point towards both physisorption(minor contributor)and chemisorptions(major contributor)of the inhibitors onto the metal surface.TheKadsfollows the same trend in the sense that large values ofKadsimply better more efficient adsorption and hence better inhibition efficiency.

Table 4Binding constant(K)and free energy of adsorption(ΔG?ads)ofinhibitors with their molar concentrations at(30 ± 1)°C from mass loss measurements at 120 min immersion in 2 mol·L?1 HCl

3.1.2.Effect of temperature

The activation energies(E*a)for the corrosion of C-steel in the absence and presence of different concentrations of 8-hydroxy-7-quinolinecarboxaldehyde derivatives were calculated using Arrheniustype Eq.[35]:

whereAis the pre-exponential factor,kis the rate constant,E*ais the apparent activation energy of the corrosion process,Ris the universal gas constant andTis the absolute temperature.Arrhenius plots of logk vs.1/Tfor C-steel in 2 mol·L?1HCl in the absence and presence of different concentrations of inhibitor(HL1)are shown graphically in Fig.3.The variation of logk vs.1/Tis a linear one and the values ofE*awere calculated from the slope of these lines and given in Table 5.The increase inE*awith the addition of concentration of inhibitors(HL1–HL3)indicating that the energy barrier for the corrosion reaction increases.It is also indicated that the whole process is controlled by surface reaction,since the activation energy of the corrosion process is larger than 20 kJ·mol?1.

Fig.3.Arrhenius plots(lg k vs.1/T)for corrosion of C-steel surface in hydrochloric acid in the absence and presence of different concentrations of compound(HL1).

Enthalpy and entropy of activation(ΔH*,ΔS*)for the corrosion of C-steel in 2 mol·L?1HCl were obtained by applying the transition state Eq.(7):

Table 5Thermodynamic activation parameters for the dissolution of C-steel surface in hydrochloric acid in the absence and presence ofdifferentconcentrations ofinvestigated compounds

Fig.4.Plots of(lg k/T)vs.1/T for corrosion of C-steel surface in hydrochloric acid in the absence and presence of different concentrations of compound(HL1).

wherehis the Planck's constant,Nis the Avogadro's number.A plot of logk/T vs1/Talso gave straight lines as shown in Fig.4 for C-steel dissolution in 2 mol·L?1HCl in the absence and presence of different concentrations of inhibitor(HL1).The slopes of these lines equal?ΔH*/2.303Rand the intercept equals logRT/Nh+(ΔS*/2.303R)from which the value of ΔH*and ΔS*was calculated and tabulated in Table 5.From these results,it is clear that the presence of the tested compounds increased the activation energy values and consequently decreased the corrosion rate of the C-steel.These results indicate that these tested compounds acted as inhibitors through increasing activation energy of C-steel dissolution by making a barrier to mass and charge transfer by their adsorption on C-steel surface.Positive sign of the enthalpies re flects the endothermic nature of the C-steel dissolution process.

All values ofE*aare larger than the analogous values of ΔH*indicating that the corrosion process must involved a gaseous reaction,simply the hydrogen evolution reaction,associated with a decrease in the total reaction volume[36].The values of ΔS*in the absence and presence of the tested compounds are large and negative;this indicates that the activated complex in the rate-determining step represents an association rather than dissociation step,meaning that a decrease in disordering takes place on going from reactants to the activated complex[37,38].

3.2.Potentiodynamic polarization studies

The cathodic and anodic polarization curves for C-steelin 2 mol·L?1HCl solution in absence and presence of various concentrations of the inhibitor(HL1)at(30 ± 1)°C are shown in Fig.5.Similar curves were obtained for other inhibitors(not shown).The various electrochemical parameters calculated from Tafel plots are given in Table 6.The lower corrosion current density(icorr)values in the presence of the inhibitors without causing significant changes in corrosion potential(Ecorr)suggest that the investigated compounds are mixed type inhibitors and are adsorbed on the surface thereby blocking the corrosion reaction.

Fig.5.Potentiodynamic polarization curves for the corrosion of C-steel surface in hydrochloric acid in the absence and presence of various concentrations of compound(HL1)at(30 ± 1)°C.

The results also show that the slopes of the anodic and the cathodic Tafel slopes(βaand βc)were slightly changed on increasing the concentration of the tested compounds.This indicates that there is no change in the mechanism of inhibition in the presence and absence of inhibitors.The values of βcare slightly higher than the values of βasuggesting a cathodic action of the inhibitor.The higher values of Tafel slope can be attributed to surface kinetic process rather the diffusioncontrolled process[39,40].

Fig.6.EIS Nyquist plots(a)and Bode plots(b)for C-steel surface in hydrochloric acid in the absence and presence of different concentrations of compound(HL1)at(30 ± 1)°C.

Table 6The effectofconcentration ofthe investigated compounds on the free corrosion potential(E corr),corrosion currentdensity(i corr),Tafelslopes(βa&βc),inhibition efficiency(IE)and degree of surface coverage for the corrosion of C-steel surface in hydrochloric acid at(30 ± 1)°C

3.3.Electrochemical impedance spectroscopy

The EIS provides important mechanistic and kinetic information for an electrochemicalsystem under investigation.Fig.6 shows the Nyquist(a)and Bode(b)plots obtained at open-circuit potential both in the absence and presence of increasing concentrations of investigated compound at(30± 1)°C.Nyquist impedance plots exhibits a single semi-circle shifted along the real impedance(Zr).The Nyquist plots of compound(HL1)do not yield perfect semicircles as expected from the theory of EIS,the impedance loops measured are depressed semicircles with their centers below the real axis,where the kind of phenomenon is known asthe “dispersing effect”asa resultoffrequency dispersion[41]and mass transport resistant[15]as well as electrode surface heterogeneity resulting from surface roughness,impurities,dislocations,grain boundaries,adsorption of inhibitors,formation of porous layers[42–44],etc.so one constant phase element(CPE)is substituted for the capacitive element,to explain the depression of the capacitance semi-circle,to give a more accurate fit.Impedance data are analyzed using the circuit in Fig.7;in whichRsrepresents the electrolyte resistance,Rctrepresents the charge-transfer resistance and the constant phase element(CPE).According to Hsu and Mansfeld[45]the correction of capacity to its real values is calculated from Eq.(8):

whereYοis the CPE coefficient,ωmaxis the frequency at which the imaginary part of impedance(?Zi)has a maximum andnis the CPE exponent(phase shift).

The data obtained from fitted spectra are listed in Table 7.The IE was calculated from Eq.(9):

whereRctandRct*are the charge-transfer resistances with and without the inhibitors,respectively.

Data in Table 7 show that theRsvalues are very small compared to theRctvalues.Also,theRctvalues increase and the calculatedCdlvalues decrease by increasing the inhibitor concentrations,which causes an increase of θ andYI.The highRctvalues are generally associated with slower corroding system[40,46].The decrease in theCdlsuggests that inhibitors function by adsorption at the metal/solution interface.

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

Table 7Electrochemicalkinetic parameters obtained from EIS technique for C-steelsurface in hydrochloric acid in the absence and presence ofdifferentconcentrations ofinvestigated compounds at(30 ± 1)°C

The inhibition efficiencies,calculated from EIS results,show the same trend as those obtained from polarization measurements.The difference of inhibition efficiency from two methods may be attributed to the different surface status of the electrode in two measurements.EIS was performed at the rest potential,while in polarization measurements the electrode potentialwas polarized to high over potential,nonuniform current distributions,resulted from cell geometry,solution conductivity,counter and reference electrode placement,etc.,will lead to the difference between the electrode area actually undergoing polarization and the total area[45].

Fig.8.EFM spectra for C-steel surface in hydrochloric acid in the absence and presence of different concentrations of compound HL1 at(30 ± 1)°C.

3.4.Electrochemical frequency modulation measurements

Boschet al.recently proposed electrochemical frequency modulation(EFM)as a new electrochemical technique for online corrosion monitoring[46–48].EFM is a rapid and nondestructive corrosion rate measurement technique that can directly give values of the corrosion current without prior knowledge of the Tafel constants.

In corrosion research,it is known that the corrosion process is non-linear in nature,a potential distortion by one or more sine waves willgenerate responses atmore frequencies than the frequencies ofapplied signal.Virtually,no attention has been given to the intermodulation or electrochemical frequency modulation.However,EFM showed that this non-linear response contains enough information about the corroding system so that the corrosion current can be calculated directly.The great strength of the EFM is the causality factors which serve as an internal check on the validity of the EFM measurement.With the causality factors the experimental EFM data can be verified.

Fig.8 shows the current response contains not only the input frequencies,but also contains frequency components which are the sum,difference,and multiples of the two input frequencies.

The electrochemical corrosion kinetic parameters at different concentrations of inhibitors in 2 mol·L?1HCl at 30 °C were shown in Table 8.The inhibition efficiency was found to increase with increasing the inhibitor concentrations.The causality factorsCF-2 andCF-3 in Table 8 are close to their theoretical values of 2.0 and 3.0,respectively indicating that the measured data are of good quality.

The calculated inhibition efficiency obtained from weight loss,Tafel polarization and EIS measurements are in good agreement with that obtained from EFM measurements,therefore IE tends to decrease in the following order:

3.5.Quantum chemical calculations

Theoretical calculations were performed for only the neutral forms,in order to give further insight into the experimental results.Values of quantum chemical indices such as energies of LUMO and HOMO(EHOMOandELUMO),and energy gap ΔE,are calculated by semiempirical AM1,MNDO and PM3 methods that have been given in Table 9.The reactive ability of the inhibitor is related toEHOMOandELUMO[49,50].HigherEHOMOof the adsorbent leads to higher electron donating ability[51].LowELUMOindicates thatthe acceptoraccepts electrons easily.The calculated quantumchemicalindices(EHOMO,ELUMO)of investigated compounds are shown in Table 9.The difference in ΔE=ELUMO?EHOMOis the energy required to move an electron from HOMO to LUMO.Low ΔEfacility adsorption of the molecule and thus will cause higher inhibition efficiency.

The bond gap energy ΔEincreases from compounds(HL1to HL3).This fact explains the decreasing inhibition efficiency in this order((HL1)＞(HL2)＞(HL3)),as shown in Table 9,and Fig.9 shows the optimized structures of the three investigated compounds.So,the calculated energy gaps show reasonably good correlation with the efficiency of corrosion inhibition.Table 9 also indicates that compound(HL1)possesses the lowest total energy that means that compound(HL1)adsorption occurs easily and is favored by the highest softness.The HOMO and LUMO electronic density distributions of these molecules were plotted in Fig.9.For the HOMO of the studied compounds that the benzene rings,N-atoms and O-atom have a large electron density.The data presented in Table 9 show that the calculated dipole moment decreases from compound HL1＞compound HL2＞compound HL3.

3.6.Inhibition mechanism

Inhibition of the corrosion of C-steel in 2 mol·L?1HCl solution by some 8-hydroxy-7-quinolinecarboxaldehyde derivatives is determined by weight loss,potentiodynamic anodic polarization measurements,electrochemical impedance spectroscopy(EIS),and electrochemical frequency modulation method(EFM),it was found that the inhibition efficiency depends on concentration,nature of metal,the mode of adsorption of the inhibitors and surface conditions.

The observed corrosion data in the presence of these inhibitors,namely:

i)the decrease of corrosion rate and corrosion current with increase in concentration of the inhibitor.

ii)the linear variation of weight loss with time.

iii)the shift in Tafel lines to higher potential regions.

iv)the decrease in corrosion inhibition with increasing temperature indicates that desorption of the adsorbed inhibitor molecules takes place.

v)the inhibition efficiency was shown to depend on the number of adsorption active centers in the molecule and their charge density.

The corrosion inhibition is due to the adsorption of the inhibitors at the electrode/solution interface,the extent of adsorption of an inhibitor depends on the nature of the metal,the mode of adsorption of the inhibitor and the surface conditions.Adsorption on C-steel surface is assumed to take place mainly through the active centers attached tothe inhibitor and would depend on their charge density.The transfer of lone pairs of electrons on the oxygen atoms(CO/OH mode of the aldhyde and hydroxyl groups)[25]to the C-steel surface to form a coordinate type of linkage is favored by the presence of a vacant orbital in iron atom of low energy.Polar character of substituents in the changing part of the inhibitor molecule seems to have a prominent effect on the electron charge density of the molecule.

Table 8Electrochemical kinetic parameters obtained by EFMtechnique for C-steelsurface in hydrochloric acid withoutand with various concentrations of8-hydroxy-7-quinolinecarboxaldehyde derivatives(HL n)at(30 ± 1)°C

Table 9The calculated quantum chemical properties for investigated compounds

Itwas concluded thatthe mode ofadsorption depends on the affinity of the metal towards the π-electron clouds of the ring system.Metals such as Cu and Fe,which have a greater affinity towards aromatic moieties,were found to adsorb benzene rings in a flat orientation.The order of decreasing the percentage inhibition efficiency of the investigated inhibitors in the corrosive solution was as follow:

Compound HL1exhibits excellent inhibition power due to:(i)the presence ofp-CH3group which is an electron donating group with negative Hammett constant(σ=?0.17),also this group will increase the electron charge density on the molecule,and(ii)its largermolecular size that may facilitate better surface coverage.

Compound HL2comes after compound HL1in inhibition efficiency.This is due to the fact that it has a lesser molecular size and has no substituent inp-position(H-atom with σ=0.0)which contributes no charge density to the molecule.

Compound HL3comes after compound HL2in inhibition efficiency.This is due to the presence ofp-NO2which has a positive Hammettconstant(σ=+0.78)i.e.group which lowers the electron density on the molecule and hence,lower inhibition efficiency.

Fig.9.The Highest Occupied Molecular Orbital(HOMO)and the Lowest Unoccupied Molecular Orbital(LUMO)of the investigated compounds.

4.Conclusions

From the results of the study the following may be concluded:

1)Testing of azo 8-hydroxy-7-quinolinecarboxaldehyde derivatives establishes a very good inhibition for C-steel surface in hydrochloric acid solution.

2)The inhibition efficiency is in accordance to the order:compound HL1＞compound HL2＞compound HL3.

3)The inhibition efficiencies of the tested compounds increase with increasing of their concentrations.

4)The adsorption of these compounds on C-steel surface in HCl solution follows a Langmuir adsorption isotherm.

5)The values of inhibition efficiencies obtained from the different independenttechniques showed the validity of the obtained results.

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