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Materials Chemistry and Physics 156 (2015) 95e104

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

Adsorption and corrosion inhibiting effect of riboflavin on Q235 mildsteel corrosion in acidic environments

Maduabuchi A. Chidiebere a, b, Emeka E. Oguzie b, Li Liu a, Ying Li a, *, Fuhui Wang a

a Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Rd, Shenyang 110016, Chinab Electrochemistry and Materials Science Research Laboratory, Department of Chemistry, Federal University of Technology Owerri, PMB 1526 Owerri,Nigeria

h i g h l i g h t s

* Corresponding author.E-mail address: liying@imr.ac.cn (Y. Li).

http://dx.doi.org/10.1016/j.matchemphys.2015.02.0310254-0584/© 2015 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� The inhibitory mechanism wasinfluenced by the nature of acidanions.

� RF has reasonable inhibition effectespecially in 1 M HCl solution.

� Polarization studies showed that RFfunctioned as a mixed type inhibitor.

� Improved surface morphology wasobserved in the presence of RF.

a r t i c l e i n f o

Article history:Received 31 March 2014Received in revised form29 January 2015Accepted 21 February 2015Available online 28 February 2015

Keywords:MetalsCorrosionElectrochemical techniquesMolecular dynamics

a b s t r a c t

The inhibiting effect of Riboflavin (RF) on Q235 mild steel corrosion in 1 M HCl and 0.5 M H2SO4 at 30 �Ctemperature was investigated using electrochemical techniques (electrochemical impedance spectros-copy and potentiodynamic polarization). The obtained results revealed that RF inhibited the corrosionreaction in both acidic solutions. Maximum inhibition efficiency values in 1 M HCl and 0.5 M H2SO4 were83.9% and 71.4%, respectively, obtained for 0.0012 M RF. Polarization data showed RF to be a mixed-typeinhibitor, while EIS results revealed that the RF species adsorbed on the metal surface. The adsorption ofRF followed Langmuir adsorption isotherm. Atomic force microscopy (AFM), Fourier transform infraredspectroscopy (FTIR) and scanning electron microscopy (SEM) studies confirmed the formation of aprotective layer adsorbed on the steel surface. Quantum chemical calculations were used to correlate theinhibition ability of RF with its electronic structural parameters.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

The control of mild steel corrosion has become an issue of sig-nificant concern for materials technologists and corrosion scien-tists. Despite the fact that mild steel finds wide range oftechnological applications such as petroleum production and

refining, chemical processing, marine applications and construc-tion [1e6], its poor corrosion resistance especially in acid solutions[7,8] curtails its utility. Acid solutions are widely used as descaling,pickling and cleaning agents in industries, and thus generate cor-rosive effects on mild steel and other materials made of iron [9,10].Among the various methods used to control the corrosion process[11], application of inhibitors is the most practical and efficient wayto achieve this objective.

Organic compounds, especially those containing polar functionswith oxygen, nitrogen and/or sulfur in a conjugated system are

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e10496

usually employed as inhibitors against the corrosion of mild steel inaggressive environments [12e18]. They function by adsorption onmetal surfaces, and thus form protective layers that hinder thecorrosion process. In terms of adsorption strength of inhibitors,certain factors are of significant consideration, including: the na-ture and surface charge of the metal, composition of the electrolyteand the structure of the inhibitor.

A number of these inhibitors are really hazardous to health andthe environment as well, and have come under increasing ecolog-ical scrutiny and stringent environmental regulations. Currentresearch efforts now focus on the development of non-toxic,inexpensive and environmentally friendly corrosion inhibitors asalternatives. The requisite electronic structural characteristics ofcorrosion inhibitors such as the presence of heteroatoms, extensiveconjugation, and substituted heterocycles are readily found inbiochemical compounds used as drugs [19]. Interestingly, somevitamins have been shown to be useful in this regard, which is notvery surprising, since they contain electronic features which help toinhibit the corrosion of metals in various aggressive solutions[20e23]. Sekine et al. [24], studied the inhibitive effect of ascorbicacid (AA) and folic acid (FA) on mild steel corrosion in 0.3% NaClenvironment and observed chemisorption of both compounds asthe primary mechanism of corrosion inhibition. Efficiency of inhi-bition diminished at high concentrations due to the formation ofsoluble iron (II) chelates. AA has as well been investigated as acorrosion inhibitor in diverse systems, with varying results[25e29].

The present investigation continues to focus on the applicationof vitamins for metallic corrosion control and reports on theinhibiting efficacy of riboflavin (vitamin B2) on mild steel corrosionin acidic environments. Inspection of the structure of riboflavin (RF)in Fig. 1 reveals the presence of heteroatoms and aromatic ringswhich plays a significant role in corrosion inhibition of metals. Theinhibiting effect was evaluated by potentiodynamic polarizationand electrochemical impedance spectroscopy techniques. The sur-face of mild steel in the absence and presence of RF has beenvisualized by SEM and AFM, while FTIRwas used to characterize thespectroscopic features of the adsorbed inhibitor. Quantum chemi-cal calculations were employed to correlate the inhibition ability ofRF with its electronic structural parameters [30e33].

2. Experimental

Q235 mild steel specimens of the composition (weight %) C e

0.30, Si e 0.30, Mn e 0.30, P e 0.045, S e 0.050, Cr e 0.064, Cu e

0.040, Ti e 0.04 and the balance Fe were used for the studies. Thesewere wet-polished with silicon carbide abrasive paper (from grade#240 e #1000), degreased in acetone, rinsed with distilled waterand dried in warm air. All the chemicals used for preparation of the

Fig. 1. Chemical structure of riboflavin.

test solutions were of analytical grade (Sinopharm Chemical Re-agent Co., Ltd). Double distilled water was used in the solutionpreparations. For experiments containing riboflavin, the powderwas added to blank solutions to reach final concentrations of0.00012 M, 0.00024 M, 0.00036 M and 0.0012 M, respectively.Experiments were performed at 30 ± 1 �C and non-deaeratedsolutions.

Electrochemical experiments were carried out in a standardthree-electrode glass cell of 500 ml capacity using a PARC Parstart-2273 Advanced Electrochemical System workstation. A platinumfoil was used as counter electrode and, a saturated calomel elec-trode (SCE) was used reference electrodes. A mild steel specimen of1 cm2 dimension was used as working electrode. Electrochemicalmeasurements were carried out, using standard procedures, inaerated solutions at the end of 1800 s of immersion, which allowedthe OCP values to attain steady state. Temperature was fixed at30 ± 1 �C. EIS measurements were carried out at the corrosionpotentials (Ecorr) over a frequency range of 100 kHze10mHz, with asignal amplitude of 5 mV. Zsimpwin 3.0 software was used toanalyze the impedance data.

The Potentiodynamic polarization (PDP) experiments wereconducted at a scan rate of 0.333 mV/s. The potential range was±250 mV versus corrosion potential. The Powersuite software wasused to analyze the polarization data [24].

The XL-30FEG scanning electron microscope was used toobserve the morphological changes of the mild steel surface afterimmersion in 1 M HCl and 0.5 M H2SO4, in the absence and pres-ence of the inhibitor.

Fourier transform infrared (FTIR) spectra (KBr) were recordedusing a Nicolet Magna-IR 560 FTIR spectrophotometer. The fre-quency ranges from4000 to 400 cm�1. The spectra for RF powder aswell as the protective film formed on the steel surface after 24 himmersion in both acidic environments; containing 0.0012 M, RFwas recorded by scrapping the film, mixing it with KBr, and makingthe pellets.

Quantum chemical calculations and molecular dynamics simu-lations were performed using the density functional theory (DFT)electronic structure programs Forcite and DMol3 as contained inthe Materials Studio 4.0 software (Accelrys, Inc.) [18].

The morphology of the mild steel surface was studied by atomicforce microscopy. For in-situ AFM observation (Picoplus 2500),mild steel specimenwith dimensions 15�15� 2mmwere fixed ina Teflon electrochemical cell with an O e ring to hinder the testsolution from leaking. The study was carried out in the absence andpresence of 0.0012 M RF at 30 ± 1 �C.

3. Results and discussion

3.1. Electrochemical measurements

It is well known that corrosion reaction is an electrochemicalprocess and as such electrochemical measurements are most suit-able for obtaining detailed mechanistic insights into corrosionprocess. Experiments were conducted to determine the effect ofvarious concentrations of RF on the electrochemical corrosionbehavior of Q235 mild steel in 1 M HCl and 0.5 M H2SO4,respectively.

3.1.1. Potentiodynamic polarization measurementsPolarization measurements were carried out to investigate the

effect of RF on the anodic and cathodic partial reactions of thecorrosion process [34]. Parts a and b of Fig. 2 show typical poten-tiodynamic polarization curves for mild steel specimens in 1 M HCland 0.5 M H2SO4, in the absence and presence of RF. The mild steelspecimen in both acidic environments displayed active dissolution

Fig. 2. Potentiodynamic polarization curves of mild steel in: (a) 1 M HCl and (b) 0.5 MH2SO4 solution in the absence and presence of RF, respectively.

Table 1Polarization parameters for mild steel in 1 M HCl and 0.5 M H2SO4 in the absenceand presence of RF.

System Ecorr(mVvs. SCE)

Icorr(mA cm�2)

bc(mV dec�1)

ba(mV dec�1)

IE (%) Surfacecoverage(q)

1 M HClBlank �466.4 183.1 122.7 100.50.00012 M RF �454.5 64.7 104.6 89.6 64.6 0.6460.00036 M RF �455.1 46.9 101.5 92.9 74.7 0.7470.0012 M RF �448.2 29.4 113.4 84.3 83.9 0.8390.5 M H2SO4

Blank �465.2 1340.1 135.7 204.30.00012 M RF �461.7 925.1 127.3 190.9 30.9 0.3090.00036 M RF �465.3 519.8 92.3 148.4 61.2 0.6120.0012 M RF �461.7 383.6 60.2 140.8 71.4 0.714

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104 97

with no distinctive transition to passivation within the studiedpotential range. The corresponding electrochemical parameters,namely, corrosion potential (Ecorr), corrosion current density (Icorr),anodic (ba) and cathodic (bc) Tafel slopes were obtained and theirvalues are shown in Table 1. The data presented therein shows thatthe Icorr decreased in the presence of RF compared to the blanksolution and the trend continued with an increase in the concen-tration of the inhibitor. Introduction of RF into 1 M HCl environ-ment slightly shifts Ecorr values towards the anodic direction andthe effect increases with an increase in the concentration of RF,whereas in 0.5 M H2SO4 Ecorr shift was quite negligible.

Accordingly, it can be concluded that RF retarded the anodicmetal dissolution process and cathodic Hþ ion reduction in bothacidic solutions [35,36]. It has been suggested [36] that if thedisplacement in Ecorr on addition of inhibitor is greater than 85 mVthe inhibitormay then be classified as cathodic or anodic type and ifthe displacement is less than 85 mV, then the inhibitor may beregarded as mixed-type. Accordingly, our obtained results implythat RF behaved as a mixed type inhibitor. However at over voltageshigher than circa �300 mV (SCE), it was observed that steeldissolution dominates the RF adsorption, this effect was more sig-nificant in 1 M HCl environment.

The values of the corrosion current density in the absence and

presence of the inhibitor were used to calculate the inhibition ef-ficiency from polarization data as follows:

IE% ¼ IcorrðblÞ � IcorrðinhÞ

IcorrðblÞ

!� 100 (1)

The values obtained are also presented in Table 1 and reveal thatefficiency of inhibition increased steadily with RF concentration inboth environments. RF was more effective in 1 M HCl compared to0.5 M H2SO4.

3.1.2. Electrochemical impedance spectroscopy measurementsImpedancemeasurements provide insight on the characteristics

and kinetics of the electrochemical processes taking place at themetal/solution interface. Figs. 3 and 4 represent the Nyquist plotsfor mild steel in 1 M HCl and 0.5 M H2SO4 in the absence andpresence of different concentrations of RF. In 1 M HCl environment,the Nyquist plots show single semicircles for all systems over thefrequency range studied, relating to one time constant in the Bodeplots. The high frequency intercept with the real axis in the Nyquistplots is assigned to the solution resistance (Rs) and the low fre-quency intercept with the real axis is ascribed to the charge transferresistance (Rct). In a 0.5 M H2SO4 environment, the Nyquist plots inthe presence of RF are characterized by a large capacitive loop athigh-to-medium frequency, and an inductive arc at low frequency.The capacitive loop at high frequencies shows the phenomenoncorrelated with the double electric layer. This arises from the timeconstant of the electrical double layer and charge transfer in thecorrosion process [34]. The inductive arc at the low frequency (LF)range could be attributed to the relaxation of adsorption speciessuch as SO4

2�ads and Hþ

ads on the metal surface, displaying anegative change in the surface coverage with the potential on theelectrode surface [32,37]. It is evident in Figs. 3 and 4, that additionof RF to both acidic solutions result in an increase in the size of theNyquist semicircle (charge transfer resistance) and the bode phaseangle, which all points towards inhibition of the corrosion process.

In order to determine the numerical values of the variousimpedance parameters presented in Table 2, the impedance spectrain 1 M HCl and 0.5 M H2SO4 were analyzed by fitting to theequivalent circuit models Rs (QdlRct) and Rs (QdlRct (LRL)) respec-tively, (see Fig. 5) which have been used previously to model themild steel/acid interface [32,38,39]. A CPE is used in place of acapacitor to compensate for deviations from ideal dielectricbehavior arising from the inhomogeneous nature of the electrodesurfaces. The impedance of the CPE is given by;

ZCPE ¼ Q�1(ju)�n (2)

Fig. 3. Electrochemical impedance spectra of mild steel in 1 M HCl solution in theabsence and presence of RF: (a) Nyquist and (b) Bode phase angle plots.

Fig. 4. Electrochemical impedance spectra of mild steel in 0.5 M H2SO4 solution in theabsence and presence of RF: (a) Nyquist and (b) Bode phase angle plots.

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e10498

where Q and n stand for the CPE constant and exponent, respec-tively, j2 ¼ �1 is an imaginary number, and u is the angular fre-quency in rad s�1 (u ¼ 2pf when f is the frequency in Hz), CPE canrepresent resistance (ZCPE ¼ R, n ¼ 0), capacitance (ZCPE ¼ C, n ¼ 1),Warburg impedance (ZCPE ¼ W, n ¼ 0.5), or inductance (ZCPE ¼ L,n ¼ �1). From the data presented in Table 2, it is clear that RFincreased the Rct values at all concentrations in 1 M HCl and 0.5 MH2SO4.

Modification of the interface by adsorbed inhibitor lowered thevalues of Cdl, according to Helmholtz model [18]:

Cdl ¼εεoAd

(3)

ε represents the dielectric constant of the medium, εo is thevacuum permittivity, A is the electrode area, and d is the thicknessof the interfacial layer. The decrease in Qdl value thus results from adecrease in the dielectric constant and/or an increase in the doublelayer thickness, due to the adsorption of RF on the corroding steelsurface. This involves the replacement of water molecules by the RFspecies which has smaller dielectric constants. The increase inresistance with inhibitor concentration, suggests enhancedadsorption of RF molecules on the mild steel surface and efficientblocking of the steel surface [40,41].

The inhibition efficiency (IE%) from impedance data was calcu-lated by comparing the values of the charge transfer resistance in

the absence and presence of RF as follows:

IE% ¼ RctðinhÞ � Rct

RctðinhÞ

!� 100 (4)

where Rct and Rct (Inh) denotes charge transfer resistance in theabsence and presence of the inhibitor. The obtained inhibition ef-ficiency values are presented in Table 2.

3.1.3. Effect of immersion timeFigs. 6 and 7, represent the impedance response obtained over a

period of 120 h in the presence of 0.00012 M RF in both acidicenvironments (1 M HCl and 0.5 M H2SO4). Close scrutiny of theimpedance spectra, show that immersion time has a pronouncedeffect on the corrosion performance of RF. The obtained data arepresented in Tables 3 and 4, respectively. In 1 M HCl solution, it isapparent from the Nyquist (6a) and Bode phase angle (6b) plotsthat corrosion resistance increased considerably, going from 0.5 h(Rct ¼ 441.4 U-cm2) to 24 h of immersion (Rct ¼ 876.6 U-cm2),decreased steadily thereafter. The Bode phase angle was obviouslyhigher in the low frequency region going from 0.5 h to 24 h butdecreased thereafter, showing that the strength of the film layer

Table 2Electrochemical impedance parameters of mild steel in 1 M HCl and 0.5 M H2SO4 in the absence and presence of RF.

System Rs (u cm2) Rct (Ucm2)/Chi square n CPE (U�1Sncm�2) L (u cm2) RL (u cm2) IE (%)

1 M HClBlank 1.65 161.1 (0.0031) 0.86 8.58E-50.00012 M RF 1.77 441.4 (0.0027) 0.87 6.47E-5 63.50.00036 M RF 1.75 807.8 (0.0027) 0.88 5.81E-5 80.10.0012 M RF 1.79 943.7 (0.0021) 0.89 5.09E-5 83.90.5 M H2SO4

Blank 2.91 28.1 (0.0081) 0.89 1.35E-3 22.3 104.80.00012 M RF 3.39 44.0 (0.0095) 0.89 1.93E-4 65.8 198.2 36.10.00036 M RF 3.29 60.1 (0.0016) 0.91 1.47E-4 130.4 292.2 53.20.0012 M RF 3.41 83.0 (0.0078) 0.92 1.21E-4 386.6 455.5 66.1

Fig. 5. Equivalent circuit models with their respective fitting diagrams.

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104 99

formed on the surface of the metal becomes weakened withincreasing immersion time.

On the other hand, corrosion performance of RF in 0.5 M H2SO4solution decreased continuously from 40.8 U-cm2 (0.5 h) to 8.3 U-cm2 (120 h) as shown in Fig. 7. As expected, the trend of corre-sponding Qdl values in 1 M HCl solution suggests that processesoccurring at the metal/solution interface initially increased thecompactness (or thickness) of the inhibitor film within the initial48 h, whereas increasing Qdl values in 0.5 M H2SO4 suggest reducedcompactness of the adsorbed inhibitor layer, which permitsincreased penetration of the aggressive species, hence reducedcorrosion resistance.

3.2. Scanning electron microscopy

The surface morphologies of Q235 mild steel immersed in thetest acid solutions in absence and presence of 0.0012 M RF arepresented in Fig. 8. The images show that the surface of mild steelin uninhibited acid is rough due to active dissolution of the metal.However, the roughness reduced drastically on addition of RF to theacidic environments. The result shows that the presence of RFmolecules retarded the rapid dissolution of mild steel by formationof a protective layer on its surface.

Examination of the images in Fig. 8a and b reveal that the metalsurface was more protected in 1 M HCl compared to 0.5 M H2SO4

Fig. 6. Electrochemical impedance spectra of mild steel in 1 M HCl showing the effectof time in 0.00012 M RF for 120 h: (a) Nyquist and (b) Bode phase angle plots. Fig. 7. Electrochemical impedance spectra of mild steel in 0.5 M H2SO4 showing the

effect of time in 0.00012 M RF for 120 h: (a) Nyquist and (b) Bode phase angle plots.

Table 3Effect of immersion time on the impedance response of mild steel in 1 M HClcontaining 0.00012 M RF.

System Rs (Ucm2) Rct (U cm2) n CPE (U�1Sncm�2) Chi square

0.5 h0.00012 M RF 1.77 441.4 0.8 6.47E-5 0.0027424 h0.00012 M RF 1.61 876.6 0.77 1.91E-4 0.0024648 h0.00012 M RF 1.62 543.7 0.78 1.97E-4 0.0041672 h0.00012 M RF 1.56 367.6 0.79 2.06E-4 0.0045996 h0.00012 M RF 1.59 305.4 0.78 2.17E-4 0.00508120 h0.00012 M RF 1.57 267.7 0.78 2.26E-4 0.00903

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104100

environment. This suggests the presence of a more compact/stronger protective layer formed on mild steel surface in thisenvironment; in agreement with the higher inhibition efficiencyobtained in 1 MHCl.

3.3. Atomic force microscopy

In-situ AFM analysis was carried out to investigate the surfacemorphology, orientation of the adsorbed inhibitor molecules andalso, the effect of RF on the progress of corrosion on the metalsurface. Fig. 9a and b depict the three dimensional (3D) in-situ AFMmorphologies of mild steel in 1 M HCl in the absence and presenceof 0.0012 M RF. Close scrutiny of the images in Fig. 9a shows thatthe surface is rough due to aggressive corrosion attack in theabsence of the inhibitor.

However, in the presence of the inhibitor (Fig. 9b) a new/smoothsurface morphology was observed with cylindrical shaped likeparticles that adsorbed on the metal surface. This could be attrib-uted to the adsorbed RF molecules on the mild steel surface.

3.4. Infrared spectroscopy

A comparison of the FTIR spectra of RF powder and that of theprotective layer formed on the mild steel surface are presented in

parts a, b of Fig. 10. Close scrutiny of the spectra reveals the pres-ence of some peaks; which could be ascribed to the existence offunctional groups. Almost all the peaks in RF powder were alsoobserved in the protective layer adsorbed on the metal surface,confirming the existence of these active groups in the inhibitor film.Some of the peaks were modified, while some vanished. In 1 M HCl

Table 4Effect of immersion time on the impedance response of mild steel in 0.5 M H2SO4

containing 0.00012 M RF.

System Rs (Ucm2) Rct (U cm2) n CPE (U�1Sncm�2) Chi square

0.5 h0.00012 M RF

3.4140.82 0.91 1.93E-4 0.00957

24 h0.00012 M RF

3.2722.33 0.94 1.26E-3 0.00294

48 h0.00012 M RF

3.1211.49 0.93 4.51E-3 0.00177

72 h0.00012 M RF

3.167.946 0.94 7.36E-3 0.00171

96 h0.00012 M RF

3.298.343 0.94 1.11E-2 0.00664

120 h0.00012 M RF

3.368.305 0.93 1.55E-2 0.00697

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104 101

there is a shift from 1723 to 1696 cm�1 due to the C]O stretching[42,43], the bands associated with OeH/NeH [39,44] at 3209 shiftsto 3320 cm�1, the bands associated with CeN at 1002 vanished. Theshift of C]C and CeO stretching frequencies from 1650 to1636 cm�1 and from 1243 to 1083 cm�1, respectively [42], suggeststhe adsorption of RF molecules on the corroding metal surface. In0.5 M H2SO4 the shift due to OeH/NeH and C]O stretching fre-quencies from 3209 to 3386 cm�1 and from 1723 to 1703 cm�1 alsoconfirm the adsorption of the inhibitor species on the metalsurface.

Fig. 8. SEM images of the mild steel surface after 24 h immersion at 30 �C in (a) 1 M HCl and

3.5. Adsorption isotherms

The adsorption of RF on the metal surface is important and canbe further understood from the adsorption isotherms [45]. Ac-cording to the Langmuir adsorption isotherm;

CðinhÞq

¼ 1Kads

þ CðinhÞ (5)

where C(inh) represents the inhibitor concentration, q is the degreeof surface coverage on the metal surface defined as q¼ IE/100, asevaluated from polarization measurements [46] and Kads is theequilibrium constant for the adsorptionedesorption process. Fig. 11shows the plot of C/q against C to be linear in both acid environ-ments, with slopes of 1.37 (1 M HCl) and 1.14 (0.5 M H2SO4),respectively, suggesting that adsorption of RF molecules onto amild steel surface obeys Langmuir adsorption isotherm. The slopesof the linear plots deviated from unity; this phenomenon can beattributed to the interactions between adsorbate species on themetal surface as well as changes in the adsorption heat withincreasing surface coverage [18]. The obtained values for Kads in 1MHCl and 0.5 M H2SO4 are 17391.3 and 4515.9. The high value of Kadsobtained in 1 M HCl environment shows strong adsorption abilityof RF molecules on the metal surface [47] due to the presence ofchloride ions, which is able to form intermediate bridges betweenthe positively charged metal surface and protonated RF species.

The adsorptionedesorption equilibrium constant is related tothe standard free energy of adsorption (DGo

ads) according to theequation: DGo

ads ¼ �RTIn (55.5Kads), where R is the universal gasconstant and T is the absolute temperature. The obtained DGo

adsvalues for RF were �31.3 kJ mol�1 (1 M HCl) and �34.7 kJ mol�1

(0.5 M H2SO4), respectively, which lie between the valuesof �20 kJ mol�1 and �40 kJ mol�1 anticipated for physical andchemical adsorption respectively [48e53].

(b) 0.5 M H2SO4: (i) in the absence of RF (ii) in the presence of 0.0012 M RF, respectively.

Fig. 9. The three dimensional (3D) in-situ AFM morphologies for mild steel in: (a) absence and (b) presence of 0.0012 M RF, respectively.

Fig. 10. FTIR spectra of: (a) riboflavin powder and (b) the surface film on mild steelspecimens immersed in 1 M HCl and 0.5 M H2SO4 solutions containing 0.0012 M RF.

Fig. 11. Langmuir adsorption isotherm plots for RF in 1 M HCl and 0.5 M H2SO4

solution.

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104102

3.6. Inhibition mechanism

The charge on the metal surface is due to the electric field thatbuilds up at the interface on immersion in the electrolyte. Ac-cording to Antropov et al., [54] it can be determined by comparingthe potential of zero charge (EPZC) and the open circuit potential(EOCP) of the metal in the test solution. It has been reported that themetal surface is positively charged in an acidic environment[34,35]. RF is expected to undergo protonation in the acid solutionsunder study, though the degree of protonationwould vary in either

medium. Hence we expect both protonated and non protonated RFin solution, therefore, the observed inhibiting effect of RF can beattributed to the participation of both protonated and molecularspecies in the adsorption process.

If protonated RF exert the predominant inhibiting effect, theninhibition efficiency should be higher in 1 M HCl; due to specificadsorption of chloride ions on the metal surface, which facilitatesadsorption of positively charged species. Otherwise, the inhibitingeffect should be the same in both environments, if non protonatedspecies exert a predominant effect. The higher inhibition efficiencyof RF in 1 M HCl suggests a significant contribution from the RFprotonated species. However, the effectiveness of RF in 0.5 MH2SO4is evidence of involvement of non protonated RF in the overallcorrosion inhibition process, in agreement with the mixed-typeinhibition mechanism suggested by the polarization results.

3.7. Theoretical considerations

The experimental studies carried out so far show that thecorrosion inhibiting effect of RF occurs via adsorption on thecorroding metal surface. Such interactions (metal-inhibitor) have

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104 103

been theoretically studied at the molecular level with the aid ofcomputer simulations in the framework of the DFT.We have carriedout such computations to model the electronic structure of the RF.The purpose of the computational investigation is no enough as toprovide detailed explanation of the adsorption of RF, but rather toprovide a theoretical framework which helps to identify theadsorption mechanisms as well as the adsorption strength.

The computations were carried out using Mulliken populationanalysis in the DFT electronic structure program DMol3. Electronicparameters used for the simulation include restricted spin polari-zation using the DND basis set and the PerdeweWang (PW) local-correlation-density functional. The molecular structure of the RFwas subjected to geometry optimization using a COMPASS forcefield and the Smart minimization method by high-convergencecriteria. Thereafter, we modeled the electronic structure of RFmolecule, the distribution of frontier orbitals and Fukui indices inorder to corroborate the active/reactive sites and the local reactivityof RF. Fig. 12 presents the: (a) optimized structure, (b) HOMO, (c)LUMO, (d) electrophilic (F�) (e) nucleophilic (Fþ) and (f) electrondensity. The electron density is distributed all around RF molecule;therefore we expect a flat-lying adsorption position. The localreactivity of the molecule as determined from the Fukui indices (FI)reveal that the F� sites correlate with the HOMO locations, whereasthe Fþ sites correlate with the LUMO locations, showing the areasthrough which the molecule would interact with the metal surface[55]. Close scrutiny of Fig. 12b shows that the HOMO orbital islocated around the aromatic indole vicinity whereas the LUMO(Fig. 12c) density is saturated around the carbonyl group. Theadsorption of inhibitor molecules on the metal surface is due to thedonoreacceptor interaction between inhibitor molecules and themetal surface. An effective corrosion inhibitor will have a strongtendency to donate electrons, receive electrons or adsorb stronglyto the metal surface [56].

We calculated the ionization (I) and electron affinity (A) using

Fig. 12. Electronic properties of riboflavin: (a) Optimized structure, (b) HOMO, (c) LUMOwhite ¼ H; gray ¼ C; red ¼ O; blue ¼ N.

the Koopmans theorem [57] which derives its relationship betweenthe energies of the HOMO and LUMO, the electron affinity and theionization potential, respectively. I and A are related to EHOMO andELUMO according to the equations: I ¼ �EHOMO, A ¼ �ELUMO. Theabsolute electronegativity (c) and absolute hardness (h) of RFmolecules was calculated using the equation:

X ¼ I þ A2

(6)

h ¼ I � A2

(7)

s ¼ 1h

(8)

The reactivity parameters c, h and global softness (s) are veryuseful properties used to explain molecular reactivity and stability.Lower h value and higher s value will result to higher inhibitionefficiency [36]. Some of the calculated quantum chemical param-eters related to the molecular electronic structure of RF are pre-sented in Table 5. The ability of a molecule to accept electrons isassociated with ELUMO while the electron donating ability of themolecule is related to EHOMO. The energy gap between the LUMOand HOMO energy levels (DE) is of significant consideration todetermine the inhibition efficiency. The molecules having low en-ergy gap DE (ELUMO� EHOMO) values will render good inhibitionefficiencies hence the energy to excite an electron from the lastoccupied orbital will be less [18]. In the same way, high values ofEHOMO indicate the propensity of the molecule to donate electronsto an appropriate acceptor with vacant molecular orbitals [58e60].

4. Conclusions

The obtained results revealed that RF is an effective inhibitor for

, (d) Electrophilic f (�), (e) Nucleophilic f (þ) and (f) Electron density. Atom legend:

Table 5Calculated quantum chemical parameters related to the molecular electronicstructure of the RF.

EHOMO (eV) ELUMO (eV) Н (eV) Х (eV) DEL�H (eV) S (eV) MSA (sq. Å)

�5.850 �4.919 0.466 5.385 0.931 2.146 410.57

MSA ¼ molecular surface area.

M.A. Chidiebere et al. / Materials Chemistry and Physics 156 (2015) 95e104104

mild steel in both acidic environments. Inhibition efficiency,increased with concentration. Polarization measurements showthat RF functioned as a mixed-type inhibitor, which the impedanceresults indicate was achieved through adsorption of RF moleculeson the metal surface. The inhibition efficiencies obtained from theimpedance and polarization measurements are in considerableagreement. The adsorption process obeys the Langmuir adsorptionisotherm. The values of Kads and DGo

ads calculated from theadsorption isotherm suggest strong and spontaneous adsorption ofthe inhibitor on mild steel surface. SEM and AFM results confirmthe formation of a protective layer on the mild steel surface, whileFTIR studies revealed the functional groups present in the adsorbedprotective layer. The calculated quantum chemical parametersassociated with the molecular structure of RF confirm its inhibitingefficacy.

Acknowledgments

M. A. Chidiebere is grateful to the Chinese Academy of Sciencesand the Academy of Sciences for the Developing World (awardnumber: FR 3240267238) for the award of CAS-TWAS Fellowship.

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