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INHIBITION OF MILD STEEL CORROSION IN SULFURICACID MEDIUM BY HYDROXYETHYL CELLULOSEI. O. Arukalam a , I. C. Madufor a , O. Ogbobe a & E. E. Oguzie ba Department of Polymer and Textile Engineering , Federal University of Technology ,Owerri , Nigeriab Electrochemistry and Materials Science Research Laboratory, Department of Chemistry ,Federal University of Technology , Owerri , NigeriaAccepted author version posted online: 29 May 2014.Published online: 21 Aug 2014.

To cite this article: I. O. Arukalam , I. C. Madufor , O. Ogbobe & E. E. Oguzie (2015) INHIBITION OF MILD STEEL CORROSIONIN SULFURIC ACID MEDIUM BY HYDROXYETHYL CELLULOSE, Chemical Engineering Communications, 202:1, 112-122, DOI:10.1080/00986445.2013.838158

To link to this article: http://dx.doi.org/10.1080/00986445.2013.838158

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Inhibition of Mild Steel Corrosion in Sulfuric AcidMedium by Hydroxyethyl Cellulose

I. O. ARUKALAM1, I. C. MADUFOR1, O. OGBOBE1, and E. E. OGUZIE2

1Department of Polymer and Textile Engineering, Federal University of Technology, Owerri, Nigeria2Electrochemistry and Materials Science Research Laboratory, Department of Chemistry,Federal University of Technology, Owerri, Nigeria

The inhibitive effect of hydroxyethyl cellulose (HEC) on mild steel corrosion in aerated 0.5M H2SO4 solution was studied usinggravimetric and electrochemical techniques. The effect of temperature on corrosion and inhibition was also investigated. The resultsshow that hydroxyethyl cellulose functioned as a good inhibitor in the studied environment and inhibition efficiency increased withconcentration of inhibitor. Potentiodynamic polarization measurements revealed that HEC inhibited both the cathodic and anodicpartial reactions of the corrosion processes. Impedance results clearly show that HEC inhibited the corrosion reaction by adsorp-tion onto the metal=solution interface by significantly decreasing the double layer capacitance (Cdl). This result was greatly pro-nounced in the presence of the inhibitor system (HECþKI) that contains halide additive. Temperature studies revealed anincrease in inhibition efficiency with rise in temperature. The adsorption behavior was found to obey the Freundlich isotherm.The values of activation energy, heat of adsorption, and standard free energy suggest that there was transition from physical tochemical adsorption mechanism of HEC on the mild steel surface. Quantum chemical calculations using the density functionaltheory (DFT) was employed to determine the relationship between molecular structure and inhibition efficiency.

Keywords: Adsorption; Chemisorption; Corrosion; Corrosion inhibition; Electrochemistry; Polymers

Introduction

The corrosion of steel and its alloys has been a subject ofutmost concern to many corrosion scientists and engineersdue to the widespread industrial applications of these mate-rials (Oguzie et al., 2011a,b). During service, these metalscome in contact with aggressive environments such as sulfu-ric acid. Sulfuric acid is used in numerous industrial pro-cesses as well as in the leaching of many metals from theirores (Banerjee et al., 2011a). The ability to take part in manydifferent chemical reactions makes sulfuric acid one of themost important and widely used commodities in the chemi-cal industry (Muthukrishnan et al., 2013; Rajeev et al.,2012). Its principal uses, among others, are in petroleumrefining to wash impurities out of gasoline and other refineryproducts. Large amounts of sulfuric acid are also used in theiron- and steel-making industry to remove oxidation, rust,and scale from rolled sheet and billets prior to sale. In thecourse of these important roles, the reactions of the acid, ifnot checked, negatively influence the stability and durabilityof the metals in contact with it by way of corrosion.

To control corrosion, efforts have been made to reducethe degradation of metals by protecting them from theattack of corrosive environments. One efficient strategy isto effectively isolate the metal surface from the corrosiveagents, which could best be achieved by use of corrosioninhibitors. Among the corrosion inhibitors in use, organicinhibitors have proved to be more effective, owing to thepresence of polar groups such as nitrogen, sulfur, and oxy-gen as well as cyclic molecules (Umoren et al., 2007). Thisinhibitive effect is attributable to the ability of thegroups to adsorb on the metal=corrosive solution interface(Anuradha et al., 2008; Migahed and Al-Sabagh, 2009),where they form an impervious or semi-impervious barrierto molecular movement.

The use of polymers as organic corrosion inhibitors hasattracted considerable attention recently due to the presenceof multiple adsorption sites along their macromolecules,ability to form complexes with metal ions, and stability withmetals in acid media (Rajendran et al., 2005; Umoren et al.,2010). Polymers can donate electrons to the unoccupied dorbitals of the metal surface to form coordinate covalentbonds and can also accept free electrons from the metal sur-face by using their antibond orbitals to form feedback bonds(Lesar and Milosev, 2009). By this they form complexes withthe metal surface that occupy a large surface area and thusconstitute excellent corrosion inhibitors (Rajendran et al.,2005; Satapathy et al., 2009).

Address correspondence to E. E. Oguzie, Electrochemistryand Materials Science Research Laboratory, Department ofChemistry, Federal University of Technology, P.M.B., 1526Owerri, Nigeria. E-mail: oguziemeka@yahoo.com

Color versions of one or more of the figures in the articlecan be found online at www.tandfonline.com/gcec.

Chemical Engineering Communications, 202:112–122, 2015

Copyright # Taylor & Francis Group, LLC

ISSN: 0098-6445 print/1563-5201 online

DOI: 10.1080/00986445.2013.838158

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Numerous articles have been published on the use ofpolymers as corrosion inhibitors for mild steel corrosion inacid media. Polyethylene glycol methyl ether (PEGME)has been reported to inhibit mild steel in sulfuric acidsolution (Dubey and Singh, 2007). Polyethylene glycol(PEG) and polyvinyl alcohol (PVA) have also been reportedto inhibit mild steel corrosion in sulfuric acid medium(Umoren et al., 2006). The corrosion inhibition of mild steelin 1M H2SO4 in the presence of polyvinylpyrolidone (PVP)and polyacrylamide (PA) as inhibitors was also reported(Umoren and Obot, 2008). To the best of our knowledge,there has been no reported work on the corrosion inhibitionof mild steel in sulfuric acid solution by hydroxyethylcellulose (HEC). This, however, informed our decision toinvestigate the capability of HEC as a corrosion inhibitor.

Inhibition efficiency of corrosion inhibitors has beenfound to depend on physico-chemical properties suchas functional groups, steric effects, electronic density at thedonor atom, and p orbital character, as well as the natureof the corrosive environment (Avci, 2008; Issa et al., 2008).Quantum chemical calculation and molecular dynamic simu-lation have become an effective way to study the correlationbetween molecular structure and inhibition properties on themicroscopic level (Gece, 2008; Jamalizadeh et al., 2008).Quantum chemical calculations have proved to be a verypowerful tool for studying the corrosion inhibition mech-anism of inhibitors using the quantitative structure-activityrelationship (QSAR) approach (Gomez et al., 2005). Mole-cular dynamic simulation is used as a beneficial supplementto quantum chemical computation, which is applied to studythe interaction between corrosion inhibitors and metalsurface. It also solves the problem that the quantum chemicalmethod is usually used for small systems containing between10 and 100 atoms (Feng et al., 2007). Due to the expensiveand time-consuming nature of experimental methods, itcan be inferred that these two theoretical methods will playa more and more important role in studying corrosion

inhibitors with the continuous development of computerhardware and software technology.

The purpose of this work is to investigate the inhibitingeffect of hydroxyethyl cellulose (HEC; Figure 1) as corrosioninhibitor for mild steel in 0.5M H2SO4 solution using weightloss, potentiodynamic polarization, and electrochemicalimpedance spectroscopy (EIS) techniques. We also carriedout quantum chemical computations to optimize the inhibi-tor’s molecular structure and molecular dynamics simulationto predict the adsorption structures at a molecular level.

Experimental Section

Materials Preparation

Hydroxyethyl cellulose (Sigma-Aldrich, Germany) was usedas obtained, and the sulfuric acid used was BDH grade.Tests were performed on mild steel specimens with weightpercentage composition as follows: C, 0.04; Mn, 0.04; Cu,0.06; Si, 0.02; Cr, 0.05; and the balance Fe. The blank cor-rodent was 0.5M H2SO4 solution. For weight loss measure-ments, test solutions of hydroxyethyl cellulose were preparedin the concentration range 500–2000mg=L in 0.5M H2SO4.Extreme concentrations of inhibitor solution (500 and2000mg=L) were chosen for the electrochemical measure-ments and temperature studies. The effect of iodide ion oninhibition efficiency was studied by combining 500mg=LKI with 2000mg=L HEC.

Gravimetric (Weight Loss) Experiment

Gravimetric experiments were conducted on test coupons ofdimensions 2� 4� 0.2 cm. These were used as cut withoutfurther polishing but were degreased in absolute ethanol,dried in acetone, and weighed. The pre-cleaned and weighedcoupons were suspended in beakers containing the test solu-tions using glass hooks and rods. Tests were conducted undertotal immersion conditions in 250mL of the aerated andunstirred test solutions, at room temperature of 29� � 1�C.To determine weight loss with respect to time, the couponswere retrieved from test solutions at 24-h intervals progress-ively for five days, appropriately cleaned, dried, andreweighed. The weight loss was taken to be the differencebetween the weight of the coupons at a given time and theinitial weight. All tests were run in triplicate, and the datashowed good reproducibility. Average values for each experi-ment were obtained and used in subsequent calculations.

Electrochemical Measurements

Metal samples (working electrodes) for electrochemicalexperiments were machined into rectangular specimens andfixed in polytetrafluoroethylene (PTFE) rods by epoxy resinin such a way that only one square surface area of 1.0 cm2

was left uncovered and immersed in the test solution. Theexposed surface was polished with silicon carbide abrasivepaper (from grade #200 to #1000), degreased in acetone,rinsed with distilled water, and dried in warm air. Then2000mg=L concentration of HEC was chosen to study thekinetics and mechanism of the electrochemical processthat occurs at the metal=solution interface. Again, 500mg=LFig. 1. Structure of hydroxyethyl cellulose.

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KI was added to the inhibitor solution to assess thesynergistic or antagonistic effect on the corrosion inhibitionof HEC. Electrochemical experiments were conducted byusing a VersaSTAT 3 model electrochemical cell, with V3Studio software. A graphite rod was used as a counterelectrode and a saturated calomel electrode (SCE) as a refer-ence electrode. The electrodes were connected by a Luggincapillary to the electrochemical cell. Measurements wereperformed in aerated and unstirred solutions at the endof 1 h of immersion at 303K after they reached the opencircuit potential (OCP). Impedance measurements weremade at corrosion potentials (Ecorr) over a frequency rangeof 100 kHz–10mHz, with a signal amplitude perturbationof 5mV. Potentiodynamic polarization studies were carriedout in the potential range� 250mV versus corrosion poten-tial. The scan rate was 0.333mV s�1. Each test was run intriplicate to verify the reproducibility of the system.

Computational Details

All theoretical computations were performed within theframework of density functional theory (DFT) usingMaterials Studio Modeling 4.0 software (Accelrys Inc.).The electronic structures of HEC and the Fe surface weremodeled by means of the DFT electronic structure programDMol3 using a Mulliken population analysis. The Fe surfacewas chosen to represent a mild steel surface since it is themajor constituent in the alloy. Electronic parameters forthe simulation include restricted spin polarization using theDNP basis set and the Perdew-Wang (PW) local correlationdensity functional. The distribution of frontier molecularorbitals and Fukui indices were assessed for the inhibitormolecule, with a view to establishing the active sites as well aslocal reactivity of the molecules. Molecular dynamics (MD)simulation of the non-covalent interaction between the inhibi-tor molecules and the Fe surface was performed using Forcitequench molecular dynamics to sample many different lowenergy configurations and identify the low energy minima.

Calculations were carried out, using the COMPASS forcefield and the Smart algorithm, in a simulation box 30 A�25 A� 29 A with periodic boundary conditions to modela representative part of the interface, devoid of arbitraryboundary effects. The box was comprised of the Fe slab,cleaved along the (1 1 0) plane and a vacuum layer of 20 Aheight. The geometry of the bottom layer of the slab wasconstrained to the bulk positions, whereas other degrees offreedom were relaxed before optimizing the Fe (1 1 0)surface, which was subsequently enlarged into a 10� 8supercell. The inhibitor molecule was adsorbed on one sideof the slab. The temperature was fixed at 303K, with NVE(microcanonical) ensemble, with a time step of 1 fs and simu-lation time of 5 ps. The system was quenched every 250 steps.

Results and Discussion

Gravimetric (Weight Loss) Measurements

The corrosion rates of mild steel in blank solution of0.5M H2SO4 and different concentrations of inhibitor

solution were calculated from the expression (Kumpawatet al., 2009):

Corrosion Rate ðmm=yÞ ¼ 87; 600W

qAtð1Þ

where W is the weight loss in grams (g), q the density of themild steel coupon (g=cm3), A the exposed surface area of thecoupon (cm2), and t the immersion time (h). The values ofcorrosion rate for mild steel dissolution in 0.5M H2SO4 withand without different concentrations of HEC as a functionof immersion time (in days) and concentration of inhibitorare presented in Figure 2. The results show that corrosionrate decreased gradually in the inhibited solution as theHEC concentration increased, as well as with prolongedimmersion time. This characteristic behavior of HEC indi-cates inhibition of the corrosion process. The decrease inthe corrosion rate with immersion time could be due toincreasing stability of the interfacial adsorption layer onthe metal surface. However, at 48 h, the corrosion rate ofmild steel in the uninhibited acid solution increased. Thiscould be due to catalytic increase of the corrosion reactionarising possibly from metallic impurities in the alloy. Asthe impurities are used up in the reaction, the reaction ratethen goes down and continues decreasing with time as thecorrosive acid molecules are consumed. Quantitative charac-terization of the protective effect of HEC on mild steelcorrosion in 0.5M H2SO4 involved calculation of the inhi-bition efficiency (I.E.%) and surface coverage (h) as follows(Khaled, 2009):

I :E:% ¼ 1� CRinh

CRblank

� �� 100 ð2Þ

h ¼ 1� CRinh

CRblank

� �ð3Þ

Fig. 2. Corrosion rates against time for mild steel corrosionin 0.5M H2SO4, in the absence and presence of differentconcentrations of HEC.

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CRinh and CRblank represent the corrosion rates in thepresence and absence of inhibitor respectively. The inhibitionefficiency of HEC for mild steel corrosion in 0.5M H2SO4

as a function of HEC concentration is shown in Figure 3.Inhibition efficiency can be seen to increase with increasingconcentration of HEC, with maximum efficiency observedto be 93.99% at 2000mg=L. Again, inhibition efficiencycan be seen to maintain almost steady values at t> 48h,suggesting stability of the inhibitor action.

Electrochemical Impedance Spectroscopy Results

Electrochemical impedance spectroscopy is a well-established and powerful tool in the study of materialscorrosion behavior. It provides information on the metalsurface properties and kinetics of the electrochemical pro-cesses at the metal=solution interface and gives evidence ofinhibitor adsorption (i.e., mechanistic information) (Oguzieet al., 2011b). The corrosion of mild steel in 0.5M H2SO4

solution in the presence and absence of HEC was studiedby EIS measurements at room temperature after an exposureperiod of 30min. Figure 4 shows the Nyquist plot obtainedfrom impedance data and depicts depressed non-exact semi-circles. The high-frequency intercept with the real axis in theNyquist plots is assigned to the solution resistance (Rs) andthe low-frequency intercept with the real axis is ascribed tothe charge transfer resistance (Rct). To obtain the numericalvalues of the above impedance parameters, the impedancespectra were analyzed by fitting to the equivalent circuitmodel Rs(CdlRct). This electrically equivalent circuit is gener-ally used to model electrochemical behavior and to calculateparameters of interest such as electrolyte resistance (Rs),charge transfer resistance (Rct), and double layer capacitance(Cdl) (Khaled, 2009). In this equivalent circuit (Figure 5), thesolution resistance is shorted by a constant phase element(CPE) that is placed in parallel to the charge transfer resist-ance. The CPE is used in place of a capacitor to compensate

for deviations from ideal dielectric behavior arising fromthe inhomogeneous nature of the electrode surfaces. Theimpedance of the CPE is given by

ZCPE ¼ Q�1ðjxÞ�n ð4Þ

where Q and n represent the magnitude and exponent of theCPE respectively, j is an imaginary number, and x is theangular frequency in rad s�1. The corresponding electro-chemical parameters are presented in Table I and reveal thatthe magnitude of Rct value increased in inhibited systems,with corresponding decrease in the double-layer capacitance(Cdl). The double-layer capacitance values were calculatedusing the expression:

Cdl ¼1

2pfmaxRctð5Þ

where fmax is the maximum frequency. The obtained valuesof Cdl are presented in Table I.

Lower double-layer capacitance suggests reduced electriccharge stored, which is a consequence of increased adsorp-tion layer that acted as a dielectric constant. The increasein Rct values in inhibited systems, which corresponds to anincrease in the diameter of the Nyquist semicircle, confirmsthe corrosion inhibiting effect of HEC and HECþKI andis much more pronounced in the latter system, implying thatKI synergistically enhanced the corrosion-inhibiting effect of

Fig. 3. Inhibition efficiency against time for mild steel corrosionin 0.5M H2SO4, in the absence and presence of differentconcentrations of HEC.

Fig. 4. Nyquist impedance plots of mild steel corrosion in 0.5MH2SO4 for HEC in the absence and presence of KI.

Fig. 5. Equivalent circuit used to model the impedance results.

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HEC. In other words, lower Cdl values correspond toreduced double-layer capacitance, which, according to theHelmholtz model (Equation (6)) results from a decrease inthe dielectric constant (e) or an increase in the interfaciallayer thickness (d).

Cdl ¼eeoAd

ð6Þ

where e is the dielectric constant of the medium, eo thevacuum permittivity, A the electrode area, and d the thick-ness of the interfacial layer.

Since adsorption of an organic inhibitor on a metal sur-face involves the replacement of adsorbed water moleculeson the surface, the smaller dielectric constant of the organicmolecule than the water molecule as well as the increasedthickness of the interfacial layer due to inhibitor adsorptionact simultaneously to reduce the double-layer capacitance.This provides experimental evidence of adsorption of HECon Fe surface. The significantly lower Cdl value of the HECþKI system supports the assertion that the iodide ion signifi-cantly enhances adsorption of HEC on the metal=solutioninterface.

The inhibition efficiency from electrochemical measure-ments could be calculated from the results obtained by usingthe following equation:

I :E:% ¼ 1� Rct

Rctinh

� �� 100 ð7Þ

where Rct and Rctinh are the charge transfer resistancewithout and with addition of inhibitor respectively. Thecalculated vales are given in Table I.

Potentiodynamic Polarization Results

The kinetics of the anodic and cathodic reactions occurringon mild steel electrodes in 0.5M H2SO4 solutions in theabsence and presence of inhibitor was studied using polari-zation measurements. Figure 6 presents typical anodic andcathodic Tafel polarization curves of mild steel corrosionin 0.5M H2SO4 in the presence of HEC and HECþKI.The plots indicate that the anodic and cathodic reactionsin all systems follow Tafel’s law. The inhibited solution con-taining HEC produced a pronounced anodic effect shiftingthe Ecorr in the anodic direction, and a very high anodic inhi-biting effect was observed. However, the corrosion currentdensity was lower than that of the blank corrodent, indicat-ing retardation of diffusion of corrosive agents to the metal

substrate. For inhibited solution containing HECþKI, Ecorr

was slightly displaced towards the cathodic end, while thecathodic and anodic branches were shifted to lower valuesof corrosion current density. These results indicate thatHEC and HECþKI exhibited cathodic and anodic inhibit-ing effects and could, therefore, be classified as mixed-typeinhibitors. Electrochemical parameters collected from Tafelplots are presented in Table I. The polarization resistance,Rp was determined from the slope of the polarization curves,obtained from linear polarization curves, and in turn deter-mined from linear polarization measurements, in the poten-tial range from the corrosion potential. The inhibitionefficiency was estimated using the relation:

I :E:% ¼ 1� Rp

R inhp

!� 100 ð8Þ

where R inhp and Rp are polarization resistances for inhibited

and uninhibited samples respectively. The calculated valuesare shown in Table I.

Adsorption Considerations

Adsorption of organic inhibitor molecules on a corrodingmetal surface and subsequent metal-inhibitor interactionmay be regarded as a displacement reaction process, whereinthe inhibitor molecules displace water molecules pre-adsorbed on the mild steel surface. This behavior could best

Table I. Impedance and polarization parameters for mild steel in 0.5M H2SO4 in the presence and absence of HEC

System

Impedance data Polarization data

Rct n I.E.% Cdl (uF cm�2)� 10�3 Ecorr (mV(SCE)) Rp (X cm2) I.E.%

Blank 9.434 0.8949 — 16.9 �463.54 12.702 —HEC 35.02 0.8631 73.061 3.21 �478.72 54.834 76.836HECþKI 129.8 0.8154 92.732 1.23 �479.10 175.01 92.742

Fig. 6. Polarization curves for mild steel corrosion in 0.5MH2SO4 using HEC in the absence and presence of KI.

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be described by adsorption isotherms (Hoseini et al., 2003).The adsorption of HEC on mild steel surface was found tofollow the Freundlich adsorption isotherm given by (Pintoet al., 2011):

log h ¼ log kads þ a logC ð9Þ

where 0< a< 1, h is the surface coverage, kads the adsorptionequilibrium constant, and C the inhibitor concentration. Thecorresponding plots of surface coverage, h versus inhibitorconcentration, C are shown in Figure 7.

Figure 7 reveals a linear plot with slope 0.48 and coef-ficient of fit R2¼ 0.9020. From the slope the value of awas calculated; it shows that 0< a< 1, which confirms theapplicability of the Freundlich adsorption isotherm. Thedeviation of the slope from unity is attributed to the rateof interaction between the adsorbed species on the metalsurface. The interaction between the adsorbed species isnot taken into account during derivation of the Freundlichadsorption isotherm, while the interaction between theorganic species with functional groups on the anodic andcathodic sites of the metal surfaces plays a very significantrole. This interaction may be either mutual attraction orrepulsion. The value of kads was calculated from the logar-ithm of the intercept of the isotherm line. The positive valueof the adsorption equilibrium constant (kads¼ 3.66) reflectsthe adsorption ability of this inhibitor on mild steel surface.

The standard Gibbs free energy of adsorption of aninhibitor (DGo

ads) on mild steel surface can be evaluated withthe following equation (John et al., 2010):

DGoads ¼ �RT ln

55:5hCð1� hÞ

� �ð10Þ

where C is the concentration of inhibitor in g=L. FromEquation (10), the DGo

ads values for 500mg=L HEC werecalculated to be �38.13, �35.70, �33.12, and �45.92kJmol�1

at temperatures of 303, 313, 323, and 333K respectively.

The DGoads values obtained for 2000mg=L HEC are �7.25,

�8.02, �10.13, and �11.55 kJmol�1 for the four tempera-tures studied. The negative values of standard Gibbs freeenergy of adsorption indicate strong interaction betweeninhibitor molecules and the mild steel surface. Generally,the standard Gibbs free energy values of �20KJmol�1 orless negative are associated with an electrostatic interactionbetween charged molecules and charged metal surface(physical adsorption); those of �40KJmol�1 or more nega-tive involve charge sharing or transfer from the inhibitormolecules to the metal surface to form a coordinate covalentbond (chemical adsorption) (Hussin and Kassim, 2011). Thecalculated standard Gibbs free energy of adsorption valuesfor 500mg=L HEC are more negative and therefore suggestchemical adsorption, while those for 2000mg=L HEC showless negative values, which is associated with physicaladsorption. Therefore, it can be concluded that adsorptionis physical at 2000mg=L and chemical at 500mg=L HEC.This could be explained on the basis that at lower concen-tration, there was increased inhibitor mobility, whichenhanced its adsorption on the mild steel surface formingelastic adsorption film, which is characteristic of polymers.Physical adsorption is observed with higher inhibitorconcentration due to reduced molecular mobility arisingfrom increased viscosity of the system. The dependenceof DGo

ads on temperature can be explained by two cases asfollows (Obi-Egbedi et al., 2012):

i. DGoads may increase (become less negative) with the

increase of temperature, which indicates the occurrenceof an exothermic process.

ii. DGoads may decrease (become more negative) with

increasing temperature, indicating the occurrence of anendothermic process.

Therefore, the decrease in DGoads at high concentration of

inhibitor shows that there was desorption of the adsorbedinhibitor molecules due to instability arising from meremechanical screening of the steel surface.

Effect of Temperature

To study the effect of temperature on corrosion andcorrosion inhibition processes, gravimetric (weight loss)experiments were performed at 10K intervals in the tempera-ture range 303–333K in uninhibited acid (0.5M H2SO4) andin inhibited solutions containing 500 and 2000mg=L HEC(representing low and high inhibitor concentrations). Theresults obtained for a 6-h immersion period are presented inTable II. Corrosion rate in uninhibited acid increased withrise in temperature, whereas that in the presence of HECdecreased with temperature increase. Accordingly, inhibitionefficiency increased with rise in temperature as depicted inFigure 8. Such increase in inhibition efficiency with rise intemperature implies that the adsorbed inhibitor is chemi-sorbed on the metal surface (Banerjee et al., 2011b; De Souzaand Spinelli, 2009). We point out here that the obtained valueof inhibition efficiency at maximum HEC concentration fora 6-h immersion period at 333K (70.35%) is less thanthat obtained after 24h of immersion at room temperature

Fig. 7. Freundlich isotherms for HEC adsorption on mild steelin 0.5M H2SO4 solution.

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(80.6%), confirming increased molecular action at elevatedtemperature. This is substantiated by the fact that polymersolubility and its diffusion is a slow process, especially atroom temperature. The reason is due to the macromolecularnature of polymers.

The relationship between the corrosion rate (CR) of mildsteel in acidic medium and temperature (T) is oftenexpressed by the Arrhenius equation (Oguzie et al., 2011a):

CR ¼ A exp�Ea

RT

� �ð11Þ

Ea is the activation energy, A the pre-exponential, factor andR the universal gas constant. The variation of logarithm ofcorrosion rate with reciprocal of absolute temperature isshown in Figure 9 for 0.5M H2SO4 in the absence and pres-ence of HEC. The calculated values of Ea are presented inTable II. Addition of HEC is seen to decrease the apparentactivation energy for the corrosion reaction in 0.5M H2SO4,implying that the compound is more effective at hightemperatures, in agreement with the trend of inhibitionefficiency with temperature (Dehri and Ozcan, 2006; DeSouza and Spinelli, 2009; Oguzie et al., 2008). The highervalue of Ea for 2000mg=L HEC than for 500mg=L HECis a confirmation of the tendency towards physisorption,and this corroborates the results of DGo

ads stated above. Also,

the results obtained could be explained on the basis thatHEC is chemically adsorbed on the mild steel at lower con-centration and physically adsorbed at higher concentration.

The values of the heat of adsorption were evaluated fromthe kinetic thermodynamic model (Oguzie et al., 2011a):

½h=1� h� ¼ A:C expð�Qads=RTÞ ð12Þ

where A is a constant, C is the inhibitor concentration, h isthe occupied and (1� h) the vacant sites not occupied byinhibitor. Plots of ln[h1� h] against 1=T for two concentra-tions of HEC (500 and 2000mg=L) are shown in Figure 10.The corresponding values of Qads are given in Table II. Ahigh value of Qads is related to high adsorption of HECon the metal surface. Lower value of Qads observed with2000mg=L HEC is attributable to desorption of the inhibi-tor molecules due to instability on the metal surface possiblyarising from increase in shear rate, which leads to decreasein viscosity (pseudoplasticity). Such behavior has beenobserved for polymeric materials and, according to Katchy(2000), often occurs in the presence of highly solvatedspecies, wherein solvated layers could be sheared away withincreasing shear rate, thereby reducing molecular inter-actions. In summary, it could be concluded that lowerconcentration of HEC favored chemisorption mechanism,whereas at the higher concentration of 2000mg=L, theadsorption-desorption transition possibly exists. Thissuggests movement from chemical to physical adsorption.

Fig. 8. Inhibition efficiency against temperature for mild steelcorrosion in 0.5M H2SO4 using different concentrations ofHEC.

Table II. Calculated values of corrosion rate, inhibition efficiency, apparent Ea, and Qads for mild steel corrosion in 0.5M H2SO4

Temp.Conc.

Corrosion rate (mm=y) Inhibition efficiency (I.E.%)

Ea (kJ=mol) Qads (kJ=mol)303K 313K 323K 333K 303K 313K 323K 333K

Blank 15.41 16.85 19.91 30.53 — — — — 18.37 —500mg=L HEC 11.95 11.17 10.45 12.72 23.12 27.05 47.04 58.04 2.16 45.392000mg=L HEC 9.57 9.18 7.94 8.83 38.69 43.96 60.74 70.35 3.24 38.16

Fig. 9. Arrhenius plots for mild steel corrosion in 0.5M H2SO4

in the absence and presence of HEC.

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Effect of KI Addition

Addition of halide ion further increased the inhibitionefficiency value as evidenced in the Nyquist spectra of impe-dance results (Figure 4) where HECþKI inhibitor systemshowed larger adsorption spectrum. The cooperative effectof KI with HEC is also evidenced in the polarization result(Figure 6), where the corrosion current density is signifi-cantly reduced, thereby retarding the movement of corrosionagents to the metal surface. This may be attributed to thestabilization of adsorbed halide ion by means of electrostaticattraction with the inhibitor, which leads to greater surfacecoverage and higher inhibition (Oguzie et al., 2004). Thesynergistic effect between HEC and KI is further explainedon the basis that iodide ions are strongly chemisorbedon the metal surface and HECþ ions are then adsorbed bycoulombic attraction on the metal surface where iodide ionsare already chemisorbed (Harek and Larabi, 2004). Thestabilization of the adsorption of inhibitor on the mild steelsurface caused by the interaction between HECþ and I�

leads to more surface coverage and, hence, greater corrosioninhibition. This result suggests that the iodide ions played animportant role in stabilizing the adsorbed HEC moleculesand the two species (I� and HEC) seem to compete for activesites of adsorption. The process is similar to the phenom-enon of anion-induced adsorption (Oguzie et al., 2004) andmay be represented as follows:

Is ! Iads ð13aÞ

Orgs þ Iads ! OrgIads ð13bÞ

where Is and Orgs are the halide ion and organic inhibitorrespectively in the bulk solution, and Iads and OrgIads referto the halide ion and ion-pair respectively in the adsorbatestate. This ion-pair interaction consequently increases thesurface coverage thereby reducing metal dissolution.

Computational Details

Quantum Chemical Calculations

In order to explore the molecular interactions in anexhaustive and rigorous manner, two complementary com-putational approaches have been adopted that involveassessment of the electron distribution in the interactingspecies in this study (HEC, Fe) as well as modeling theadsorption structures of HEC on the Fe surface. Certainelectronic structure parameters have been correlated withthe effectiveness of adsorption-type inhibitors. These includethe energy of the highest occupied molecular orbital(HOMO), which is associated with the capacity of a mol-ecule to donate electrons, the lowest unoccupied molecularorbital (LUMO) energy, corresponding to a tendency forelectron acceptance, and the HOMO-LUMO energy gap(Awad et al., 2010).

The electronic structures of HEC are presented inFigure 11; these include the geometry optimized structureof HEC, the total electron density, and HOMO and LUMOorbitals, as well as Fukui functions. The correspondingquantum chemical parameters are shown in Table III. Thereactive ability of an inhibitor is considered to be closelyrelated to the frontier molecular orbitals, the HOMO andLUMO. Higher HOMO energy (EHOMO) of the moleculemeans a higher electron-donating ability to appropriateacceptor molecules with low energy empty molecular orbital.

Another parameter that is used as a reactivity descriptorin chemical reactions is absolute hardness. Absolutehardness is an important property that measures both thestability and reactivity of a molecule (Geerlings and DeProft, 2000; Obot and Obi-Egbedi, 2010). In molecularorbital theory, it is found approximately as (Lesar andMilosev, 2009):

g ¼ ELUMO � EHOMOð Þ=2ðg in eVÞ ð14Þ

Equation (14) was used to calculate hardness of the HECmolecule, and the values are presented in Table IV. A hardmolecule has a large energy gap, and a soft molecule hasa small gap. Soft molecules are more reactive than hardmolecules, in general (Lesar and Milosev, 2009). Moreover,the gap between the HOMO and LUMO energy levels ofthe molecules is another important factor that should beconsidered. Excellent corrosion inhibitors are usually thoseorganic compounds that not only offer electrons to unoccu-pied orbitals of the metal but also accept free electrons fromthe metal (Gece and Bilgic, 2010). The energy gap betweenHOMO and LUMO indicates that the smaller energy gapresults in high corrosion inhibition, implying a soft-softinteraction.

The local reactivity of the HEC molecule was analyzed bymeans of Fukui indices and is presented in Table IV. Theatomic sites of the molecule that possesses the largestcondensed Fukui functions favor higher reactivity. Thus,the molecular sites with the maximum value of F� are thepreferred sites to which the inhibitor molecule will donatecharge when attacked by an electrophilic reagent. On theother hand, a large value of Fþ is assigned to the sites where

Fig. 10. Plots of ln[h=1� h] against 1=T at different concentrationsof HEC.

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the inhibitor molecule will receive charge when attacked by anucleophilic reagent. The oxygen atom, O12 (0.165), is themost favorable site on hydroxyethyl cellulose for an electro-philic attack and the O13, O16, and O17 positions show onlyslightly lower local reactivity. This indicates that all thesecenters are equivalent when they undergo an electrophilicattack. Turning to nucleophilic attack, the most reactive siteof hydroxyethyl cellulose is on O12 (0.015). O13 and O17have moderate values of Fþ (Table IV) and therefore partici-pate in electron transfer from metal to inhibitor molecules.

In summary, Fukui analysis, together with the charge distri-bution, predicts that HEC species possess more than oneactive center (the multiple OH groups and the aromaticstructure), which enables multicenter adsorption of theinhibitor molecule on a metal surface, which is characteristicof polymer corrosion inhibitors. In comparison with theexperimental study, the reactivity of HEC as defined byquantum chemical computation confirms the inhibitingreaction of HEC demonstrated by gravimetric (weight loss)and electrochemical measurements.

Molecular Dynamics (MD) Simulations

The adsorption of HEC on the mild steel surface was under-studied using molecular dynamics simulations. By carefulexamination of Figure 12, it could be noticed that the inhi-bitor adsorbed nearly parallel to the iron surface wherea chemical bond could possibly occur through donation ofp electrons of the aromatic ring and those from the hydroxylfunction to the metal. In order to investigate the interactionbetween the inhibitor molecule and the metal surface, we

Table IV. Calculated values of quantum chemical properties forthe most stable conformation of HEC

Property Value

EHOMO (eV) �5.589ELUMO (eV) �0.139ELUMO-HOMO (eV) 5.450(ELUMO�EHOMO)=2 (eV) 2.725Adsorption energy (eV) �119.5Maximum Fþ (Mulliken) 0.015 O(12)Maximum F� (Mulliken) 0.165 O(12)

Fig. 11. Electronic properties of hydroxyethyl cellulose (HEC): (a) Optimized structure, (b) Total electron density, (c) HOMOOrbital,(d) LUMO Orbital, (e) Fukui function for Nuclephilic attack and (f) Fukui function for electronic attack.

Table III. Calculated Mulliken atomic charges and Fukuiindices for nucleophilic (Fþ) and electrophilic (F�) attacks ofHEC molecule

Atom Fþ Atom F�

C(1) �0.009 C(1) �0.001C(2) �0.009 C(2) �0.037C(3) �0.004 C(3) �0.019C(4) �0.028 C(4) �0.012O(5) �0.007 O(5) 0.080C(6) �0.023 C(6) �0.018C(7) �0.003 C(7) �0.009O(8) �0.001 O(8) 0.040C(9) �0.012 C(9) �0.020C(10) �0.012 C(10) �0.011O(11) �0.182 O(11) 0.001O(12) 0.015 O(12) 0.165O(13) 0.008 O(13) 0.055C(14) �0.014 C(14) �0.036C(15) �0.003 C(15) �0.015O(16) 0.000 O(16) 0.032O(17) 0.007 O(17) 0.060

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calculated the adsorption energy (Eads) from the relation(Fu et al., 2010):

Eads ¼ Ecomplex � Einh þ EFeð Þ ð15Þ

where EFE is the total energy of the iron surface and Einh

is the total energy of the inhibitor compound. When theadsorption occurs between the inhibitor and the iron, theenergy of the new system is expressed as Ecomplex (Fu et al.,2010). The obtained value of Eads (�119.5 kJ=mol) suggeststhat the adsorption could occur spontaneously. The highnegative value of Eads reflects the higher stability of theformed complex and accordingly increases its inhibitionefficiencies. The higher the value of adsorption energy, theeasier the inhibitor adsorbs on the metal surface and thehigher the inhibition efficiency. The image in Figure 12(a)depicts specific (close contact) interactions between theHEC molecule and the Fe slab, which should facilitate thechemisorptive interactions suggested by our experimentalresults.

Conclusions

1. Hydroxyethyl cellulose inhibited mild steel corrosion in0.5M H2SO4 solution. The inhibition efficiency wasfound to increase as the inhibitor concentration increasedas given by the weight loss results. Also, the inhibitionefficiency increased with rise in temperature. Potentio-dynamic polarization measurements showed that HECinhibited both the cathodic and anodic reaction processesand thus classified it as a mixed-type inhibitor. EISmeasurements carried out confirmed the adsorption ofthe inhibitor.

2. The inhibitive efficacy of HEC is probably due to electro-static bonding of the polar end of the inhibitor at higherconcentration and coordinate bonding at lower concen-tration of the inhibitor to the mild steel surface as wellas adsorption of the bulky cyclic structure of the inhibitorcovering a large surface area of the metal.

3. HEC molecule adsorption on the mild steel surfacefollows the Freundlich adsorption isotherm.

4. The values of activation energy, heat of adsorption, andstandard free energy suggest that there was transitionfrom physical to chemical adsorption mechanism ofHEC on the mild steel surface.

5. The inhibitor, HEC, was adsorbed on the corrodingmild steel surface through multicenter adsorption sites(multiple OH groups and the aromatic structure), asrevealed by the Fukui functions.

6. Synergistic effect between HEC and KI was observed.The adsorption of HEC was stabilized by the presenceof iodide ions in the solution.

Funding

The assistance from the Electrochemistry and MaterialScience Research Unit (EMRU), Department of Chemistry,Federal University of Technology, Owerri, Nigeria, is grate-fully acknowledged.

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