Experimental and theoretical study on corrosion inhibition of o-phenanthroline for aluminum in HCl solution

Journal of the Taiwan Institute of Chemical Engineers xxx (2013) xxx–xxx

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JTICE-774; No. of Pages 11

Experimental and theoretical study on corrosion inhibition ofo-phenanthroline for aluminum in HCl solution

Xianghong Li a,*, Shuduan Deng b, Xiaoguang Xie c

a Faculty of Science, Southwest Forestry University, Kunming 650224, PR Chinab Faculty of Materials Engineering, Southwest Forestry University, Kunming 650224, PR Chinac School of Chemical Science and Technology, Yunnan University, Kunming 650091, PR China

A R T I C L E I N F O

Article history:

Received 3 June 2013

Received in revised form 13 October 2013

Accepted 20 October 2013

Available online xxx

Keywords:

Aluminum

EIS

Polarization

SEM

Quantum chemical calculation

Corrosion inhibition

A B S T R A C T

The inhibition effect of o-phenanthroline on the corrosion of aluminum in HCl solution was investigated

by weight loss, open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance

spectroscopy (EIS) and scanning electron microscopy (SEM) methods. The neural and protonated

molecular structures of o-phenanthroline were studied by quantum chemical calculation. The results

show that o-phenanthroline is a good inhibitor, and the adsorption obeys Langmuir isotherm. From

polarization curves, o-phenanthroline acts as a cathodic inhibitor. EIS spectra exhibit a large capacitive

loop at high frequencies followed by a large inductive one at low frequencies. The inhibition action is

confirmed by SEM images.

� 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

HCl is often used as industrial acid cleaning, chemical orelectrochemical etching and acid pickling of aluminum, but itshows strong corrosiveness on aluminum. Therefore, it isnecessary to add inhibitors to retard the corrosion of aluminumin HCl. Most well-known inhibitors are organic compoundscontaining nitrogen, sulfur, and oxygen atoms. Up to now, variousorganic compounds are reported as good corrosion inhibitors foraluminum in HCl solution, such as aliphatic amines [1,2], aromaticamines [3], aromatic aldehyde [4], aromatic acids [5], carbonylcompounds [6], phenol [7], thiosemicarbazide derivatives [8],hydrazine derivatives [9,10], amino acids [11], antibacterial drugs[12,13], Schiff bases [14–18], alkylimidazolium ionic liquids [19],organic dye [20], anionic surfactants [21–23], cationic surfactants[24,25], nonionic surfactants [26,27], natural polymer [28] andthiourea derivatives [29].

Besides the above-mentioned organic inhibitors, N-heterocycliccompounds contain many aromatic systems and electronegative Natoms; this makes they are good potential inhibitors. In previousinvestigations, pyridine derivatives [30], benzotriazole derivatives

* Corresponding author. Tel.: +86 871 3861218; fax: +86 871 3863150.

E-mail address: [email protected] (X. Li).

Please cite this article in press as: Li X, et al. Experimental and thaluminum in HCl solution. J Taiwan Inst Chem Eng (2013), http://dx

1876-1070/$ – see front matter � 2013 Taiwan Institute of Chemical Engineers. Publis

http://dx.doi.org/10.1016/j.jtice.2013.10.007

[31], triazoline derivatives [32] and tetrazole derivatives [33] havebeen used as the corrosion inhibitors for aluminum in HCl media.These results indicate that N-heterocyclic compounds exert theirinhibition via adsorption on aluminum surface through Nheteroatom, as well as those with conjugated double bonds oraromatic rings in their molecular structures. Furthermore, theadsorption of inhibitor on metal surface is influenced by thechemical structure of inhibitor, the nature and charged surface ofmetal, the distribution of charge over the whole inhibitor moleculeand the type of aggressive media. As an ordinary N-heterocycliccompound, o-phenanthroline that contains two N-heterocyclicrings with abundant p-electrons and lone electron pairs hasfavorable adsorption characteristics. In 1989, Banerjee and Misra[34] reported the inhibitive effect of o-phenanthroline on thecorrosion of steel in H2SO4 solution. Afterwards, the synergisticinhibition effect of o-phenanthroline and chloride ion on corrosionof steel in H2SO4 [35] and H3PO4 [36] solutions was reported.However, to the best of our knowledge, the literature available todate about o-phenanthroline as the corrosion inhibitor foraluminum in HCl solution is almost scant.

It is well known that the inhibition performance mainlydepends on some physicochemical and electronic properties of theinhibitor molecule which relate to its functional groups, stericeffects, electronic densities and orbital characters. Using quantumchemical calculation, the structural parameters of inhibitor

eoretical study on corrosion inhibition of o-phenanthroline for.doi.org/10.1016/j.jtice.2013.10.007

hed by Elsevier B.V. All rights reserved.


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X. Li et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2013) xxx–xxx2

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molecule can be obtained. Theoretically speaking, the inhibitivemechanism can be directly accounted for the chemical reactivity ofthe compound under study [37]. Up to now, some work appear tohave been done on quantum chemical studies in the literatureconcerning organic compounds as corrosion inhibitors for alumi-num in HCl [3,7,18,29,30,33], H3PO4 [38] and HNO3 [39]. Troughthese studies, the inhibition activity of a given inhibitor is mainlycorrelated with the theoretical parameters including the highestoccupied molecular orbital energy (EHOMO), the lowest unoccupiedmolecular orbital (ELUMO), dipole moment (m), Mulliken atomiccharge, etc. However, many quantum parameters about organicinhibitors are calculated in gas phase. In fact, acid inhibitors areused in acidic water solutions, and they could be protonated. Thus,in the field of theoretical calculation of acid inhibitors, the solventeffect and the protonated inhibitor molecule should be taken intoaccount.

In the present work, the inhibition effect of o-phenanthroline onthe corrosion of aluminum in 1.0 M HCl solution was firstlyinvestigated by weight loss, open circuit potential (OCP),potentiodynamic polarization curves, electrochemical impedancespectroscopy (EIS) and scanning electron microscope (SEM)methods. The adsorption isotherm and adsorption free energyDG8 were obtained and discussed. Effects of inhibitor concentra-tion, temperature, immersion time and acid concentration on thecorrosion inhibition were fully investigated. Quantum chemicalcalculation of DFT including the solvent effect is applied to studythe difference in theoretical parameters between neutral inhibitormolecule and protonated inhibitor molecule. It is expected toaccumulate useful information on the inhibition effect of o-phenanthroline on aluminum corrosion in HCl solution.

2. Experimental

2.1. Materials

Tests were performed with aluminum specimens (Al 1060)with the following composition (wt.%): 0.25% Si, 0.35% Fe, 0.05% Cu,0.03% Mn, 0.03% Mg, 0.05% Zn, 0.03% Ti, 0.05% V and bal. Al. Bothhydrochloric acid (37% HCl) and o-phenanthroline (C12H8N2) are ofanalytical grade. Fig. 1 shows the molecular structure of o-phenanthroline.

2.2. Weight loss measurements

Weight loss tests were conducted under total immersion in250 ml of non-deaerated 1.0 M HCl media at a certain temperaturecontrolled by a water thermostat. Three parallel aluminum sheetsof 2.5 cm � 2.0 cm � 0.20 cm were abraded by a series of emerypaper (grade 320-500-800) and then washed with distilled waterand acetone. After weighing by digital balance with sensitivity of�0.1 mg, the specimens were suspended in a beaker containing testsolution using glass hooks and rods. After immersion for 2 h, thespecimens were taken out, washed with bristle brush under runningwater in order to remove the corrosion product, dried with a hot airstream, and re-weighed accurately. The mean weight loss of three

Fig. 1. Chemical molecular structure of o-phenanthroline.


parallel aluminum sheets was obtained. The corrosion rate (v) andinhibition efficiency (hw) were calculated [25].

2.3. Electrochemical measurements

Electrochemical experiments were carried out in the conven-tional three-electrode cell with a platinum counter electrode (CE),a saturated calomel electrode (SCE) coupled to a fine Luggincapillary as the reference electrode and the working electrode(WE) which was in the form of square aluminum embedded inpolyvinyl chloride (PVC) holder using epoxy resin so that the flatsurface was the only exposed surface in the electrode. In order tominimize ohmic contribution, the tip of Luggin capillary was keptclose to WE. The test surface area of WE was 1.0 cm � 1.0 cm, andprepared as described above (Section 2.2). All electrochemicalmeasurements were carried out using a PARSTAT 2273 advancedelectrochemical system (Princeton Applied Research).

Before measurements of polarization curves and electrochemi-cal impedance spectroscopy (EIS), the electrode was immersed intest solution at open circuit potential (OCP) for 2 h at 20 8C to besufficient to attain a stable state. The potential of potentiodynamicpolarization curves was started from a potential of �250 mV to+250 mV versus OCP at a sweep rate of 0.5 mV/s. Inhibitionefficiency (hp) is calculated using the corrosion current densityvalues (icorr) [25].

Electrochemical impedance spectroscopy (EIS) was carried outat OCP in the frequency range of 10 mHz to 100 kHz using a 10 mVpeak-to-peak voltage excitation. The total number of points is 30.

2.4. Scanning electron microscope (SEM)

Samples of dimension 2.5 cm � 2.0 cm � 0.20 cm were pre-pared as described above (Section 2.2). After immersion in 1.0 MHCl solutions without and with addition of 2.0 mM o-phenanthro-line at 20 8C for 2 h, the specimens were cleaned with distilledwater, dried with a cold air blaster, and then examined by S-3000Nscanning electron microscope (SEM) (Hitachi High-Tech ScienceSystems Corporation, Japan).

2.5. Quantum chemical calculations

Quantum chemical calculations were performed with DMol3

numerical based density function theory (DFT) in Materials Studio4.1 software from Accelrys Inc. [40]. Geometrical optimizationsand frequency calculations were carried out with the generalizedgradient approximation (GGA) functional of Becke exchange plusLee–Yang–Parr correlation (BLYP) [41] in conjunction with doublenumerical plus d-functions (DND) basis set [42]. Fine convergencecriteria and global orbital cutoffs were employed on basis setdefinitions. Considering the solvent effects, all the geometries werere-optimized at the BLYP/DND level by using COSMO (conductor-like screening model) [43] and defining water as the solvent.Through the frequency analysis, it is ensured that all optimizedspecies have no imaginary frequencies.

3. Results and discussion

3.1. Weight loss measurements

The weight loss method of monitoring the corrosion inhibitionis useful owing to its simple application and good reliability[44,45]. In the present study, the reproducibility of both corrosionrate and inhibition efficiency for triplicate determination is good(the relative standard deviation values of three parallel experi-ments are lower than 5%).



η

Fig. 2. Relationship between corrosion rate (v) and inhibition efficiency (hw)

obtained by weight loss method with the concentration of o-phenanthroline (c) in

1.0 M HCl at 20 8C (immersion time is 2 h). (The error bar is determined by the value

of standard deviation).

θ

Fig. 3. Langmuir isotherm adsorption mode of o-phenanthroline on the aluminum

surface in 1.0 M HCl at 20 8C from weight loss measurement.

η

Fig. 4. Effect of temperature on inhibition efficiency (hw) in 1.0 M HCl solution at

20 8C (weight loss method, immersion time is 2 h).

X. Li et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2013) xxx–xxx 3

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Fig. 2 shows the corrosion rate (v) and inhibition efficiency (hw)values with different concentrations of o-phenanthroline in 1.0 MHCl solution at 20 8C. The corrosion rate decreases noticeably withthe increase of o-phenanthroline concentration. In the absence ofinhibitor, the corrosion rate is higher than 62 g/(m2 h), but itdecreases to 2.65 g/(m2 h) in the presence of 2.0 mM o-phenan-throline. In other words, the inhibition performance enhances withthe inhibitor concentration. This behavior is due to the fact that theadsorption coverage of inhibitor on aluminum surface increaseswith the inhibitor concentration.

Also, it can be seen from Fig. 2 that when the concentration of o-phenanthroline is less than 0.8 mM, hw increases sharply with anincrease in concentration. hw values are 36.5–84.4% at respective0.2 and 0.8 mM o-phenanthroline, while a further increase causesno appreciable change in performance. The maximum hw is 95.7%at 2.0 mM and the inhibition is estimated to be 86.4% at 1.0 mM,and its protection is higher than 90% at 1.6 mM, which indicatesthat o-phenanthroline acts as a good inhibitor for aluminum in1.0 M HCl solution.

It is generally accepted that the inhibition is caused by theadsorption of inhibitor on the aluminum surface. Basic informationon the interaction between inhibitor and metal surface can beprovided by adsorption isotherm. Attempts were made to fit tovarious isotherms including Frumkin, Langmuir, Temkin, Freun-dlich, Bockris–Swinkels and Flory–Huggins isotherms. In thepresent system, the adsorption of inhibitor obeys Langmuirisotherm [25]:

c

u¼ 1

Kþ c (1)

where c is the concentration of inhibitor, K the adsorptiveequilibrium constant and u is the surface coverage and can becalculated by the following equation [46]:

u ¼ v0 � vv0 � vm

(2)

where vm is the smallest average corrosion rate.The straight line of c/u versus c is shown in Fig. 3, and the linear

correlation coefficient (r) is almost equal to 1 (r = 0.9962), whichconfirms the adsorption of inhibitor obeys Langmuir adsorptionisotherm. Noticeably, the slope (slope = 0.84) deviates from 1,which implies the interaction between the adsorbed inhibitorspecies cannot be neglected [47]. The adsorptive equilibriumconstant (K) is 3.24 � 103 M�1. It is related to the standard free


energy of adsorption (DG8) as represented the following equation[48]:

K ¼ 1

55:5exp

�DG0

RT

!(3)

where R is the gas constant (8.314 J/(K mol)), T is the absolutetemperature (K), the value 55.5 is the concentration of water insolution expressed in M [48]. The obtained DG8 is �29.5 kJ/mol.Generally, values of DG8 up to �20 kJ/mol are consistent with theelectrostatic interaction between the charged molecules and thecharged metal (physisorption) while those more negative than�40 kJ/mol involve sharing or transfer of electrons from theinhibitor molecules to the metal surface to form a co-ordinate typeof bond (chemisorption) [49]. In the present study, the value of DG8is within the range from �20 to �40 kJ/mol, which probably meansthat the adsorption involves both physical and chemical adsorp-tion.

Temperature is an important factor that modifies the corrosioninhibition of aluminum in acid. In order to study the effect oftemperature on the inhibition characteristic of o-phenanthroline,experiments are conducted from 20 to 50 8C at an interval of 5 8C inthe presence of 2.0 mM o-phenanthroline, and the results are



η

Fig. 5. Effect of immersion time (t) on inhibition efficiency (hw) in 1.0 M HCl solution

at 20 8C (weight loss method).


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shown in Fig. 4. Clearly, hw decreases with the experimentaltemperature, which can be attributed to that the highertemperatures might cause the desorption of inhibitor moleculefrom aluminum surface.

The inhibition kinetics of the inhibition efficiency functioningwith the immersion time is another important factor in assessingthe stability of inhibitive behavior. Fig. 5 shows effect of immersiontime (0.5–6 h) on inhibition efficiency (hw) of 2.0 mM o-phenanthroline in 1.0 M HCl solution at 20 8C. Clearly, hw relieson the immersion time. hw increases firstly with immersion timefrom 0.5 to 2 h, and decreases slightly from 2 to 3 h, then decreasesgradually within the immersion time from 3 to 6 h. The optimumimmersion time is from 2 to 3 h, the inhibition efficiency is higherthan 90%. When the immersion time is postponed to 6 h, hw

reaches the minimum value of 44.7%. The reasons could be theadsorptive film of inhibitor that is dependent on the immersiontime [50]. Initially, the adsorptive film becomes more compact anduniform with the increase of immersion time (0.5–2 h), and theadsorption amount reaches saturated from 2 to 3 h. However,along with prolonging the immersion time, the adsorptive film isnot more uniform and instability [51] or desorption of theadsorbed inhibitor from metal surface [52] owing to the continual

η

Fig. 6. Effect of acid concentration (C) on inhibition efficiency (hw) in HCl solution at

20 8C (weight loss method, immersion time is 2 h).


aggressive corrosion of acid media for rather longer immersiontime.

Fig. 6 shows the effect of acid concentration (0.5–3.0 M) on theinhibition efficiency of 2.0 mM o-phenanthroline at 20 8C (immer-sion time is 2 h). It is of interest to note that the changed degree ofhw with acid concentration can be divided into two stages. Namely,hw increases slightly with the acid concentration in 0.5–1.0 M, thenhw decreases with the acid concentration in 1.0–3.0 M. When theacid concentration is 3.0 M, the minimum hw is 32.4%. The increasein hw with acid concentrations (0.5–1.0 M) may be due to the moreprotective film by the interaction of the inhibitor molecules withcorrosion products on the aluminum surface. On the other hand,the decrease in hw observed for the studied inhibitor at acidconcentrations above the inflection point (1.0 M) may beattributed to the increase tendency of the metal to react withthe acid and liberate hydrogen vigorously [53]. The hydrogenevolution from the electrode surface can be observed only if thealunminium was immersed in concentrated HCl solution. This maynot allow the establishment of the adsorption process thusresulting in a decline of inhibition performance.

3.2. Open circuit potential (EOCP) curves

The variation of open circuit potential (EOCP) of Al electrodewith time in 1.0 M HCl solution in the absence and presence of2.0 mM o-phenanthroline at 20 8C is shown in Fig. 7. It can be seenthat both curves have similar shapes. At initial 250 s, EOCP shiftsnegative direction quickly, which indicates that the performedoxide layer on the electrode surface dissolves and is not capable toimpart passivity to the metal electrode [54]. After 250 s, EOCP

exhibit increase in potential toward positive value with an increaseof immersion time and reaches steady state value after about 1.5 h.Also, EOCP would attain a stable state when the electrode wasimmersed in test solution for 2 h, so 2 h was chosen as theimmersion time in electrochemical studies for polarization curvesand EIS. Comparing with the curve for blank solution (withoutinhibitor), the curve shifts to the negative in presence of 2.0 mM o-phenanthroline, which indicates the cathodic corrosion is affectedprominently by o-phenanthroline.

3.3. Polarization curves

Potentiodynamic polarization curves for aluminum in 1.0 MHCl containing different concentrations of o-phenanthroline at

Fig. 7. EOCP-time curves for aluminum in 1.0 M HCl without and with 2.0 mM o-

phenanthroline at 20 8C.



Fig. 8. Potentiodynamic polarization curves for aluminum in 1.0 M HCl without and

with different concentrations of o-phenanthroline at 20 8C (immersion time is 2 h).

Fig. 9. Nyquist plots of the corrosion of aluminum in 1.0 M HCl without and with

different concentrations of o-phenanthroline at 20 8C (immersion time is 2 h).


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20 8C (immersion time is 2 h) are shown in Fig. 8. The presence ofinhibitor causes a prominent decrease in the corrosion rate i.e.,shifts the cathodic curves to lower values of current densities tomore extent. On the other hand, the anodic reaction of thecorrosion process is slightly inhibited. This means that o-phenanthroline mainly acts as a cathodic inhibitor.

As can be seen from Fig. 8, the cathodic polarization curves giverise to Tafel lines, indicating that the hydrogen evolution reactionis activation controlled. In addition, the presence of o-phenanthro-line does not affect the mechanism of hydrogen evolution, owing tothe parallel cathodic polarization curves.

It is important to note that in anodic domain, it is difficult torecognize the linear Tafel regions. Accordingly, the corrosioncurrent density values are estimated accurately by extrapolatingthe cathodic linear region back to the corrosion potential. Similarfitting method has been used for aluminum in HCl [16,25] andH3PO4 [55] solutions in the presence of organic inhibitors. Table 1lists the electrochemical corrosion parameters of corrosion currentdensities (icorr), corrosion potential (Ecorr), cathodic Tafel slope (bc)and the inhibition efficiency (hp). Apparently, icorr decreases withthe inhibitor concentration. Correspondingly, hp increases with theinhibitor concentration, which is due to the increase in the blockedfraction of the electrode surface by adsorption. hp of 2.0 mM o-phenanthroline reaches up to a maximum of 98.2%, whichindicates that o-phenanthroline is a good inhibitor for aluminumin 1.0 M HCl. The presence of o-phenanthroline shifts Ecorr tonegative, which indicates that o-phenanthroline can be arrangedas a cathodic-type inhibitor [56]. Tafel slope of bc changes slightlyupon addition of o-phenanthroline, which means that thehydrogen evolution mechanism does not change in the presenceof inhibitor.

Table 1Potentiodynamic polarization curves parameters for the corrosion of aluminum in

1.0 M HCl containing different concentrations of o-phenanthroline at 20 8C.

c (mM) Ecorr (mV vs. SCE) icorr (mA/cm2) �bc (mV/dec1) hp (%)

0 �785.1 2470.8 146 –

0.2 �830.0 1247.8 161 49.5

0.6 �835.9 365.4 172 85.2

1.0 �842.0 232.3 176 90.6

1.6 �831.7 69.9 145 97.2

2.0 �834.5 44.2 144 98.2


3.4. Electrochemical impedance spectroscopy (EIS)

Fig. 9 represents the Nyquist diagrams for aluminum in 1.0 MHCl at 20 8C without and with various concentrations of o-phenanthroline. The impedance spectra consist of a large capaci-tive loop at high frequencies (HF) followed by a large inductive oneat low frequencies (LF). Generally, the inductive loop at LF is alwaysobserved for aluminum in hydrochloric acid [15,20,33], and thediameter of inductive loop is usually smaller to more extent thanthat of capacitive loop at HF. However, in the present system, thesize of inductive loop is almost equal to that of capacitive loop. Inother words, the whole diagram appears an elliptic shape, which isin accordance with our recent work about aluminum in HCl[25,57,58]. Similar shape is also found for aluminum in acidsolution inhibited by 8-hydroxy-quinoline [59] and Al–Cu alloy inhydrochloric acid [60]. After adding o-phenanthroline, the shape isnot changed throughout all tested concentrations, indicating thatthe corrosion mechanism does not change due to the inhibitoraddition [61].

According to Bessone et al. [62] and Brett [63], the capacitiveloop at HF could be attributed to the oxide layer on Al. In theexperiments, it is reasonable to assume that the electrode surfacehas been covered with an oxide layer because of the ex situ

pretreatment of the electrode. In fact, it is very difficult to producean oxide free Al surface. Even after producing such a surface, it isrepassivated very fast by O2 [64]. It can be correlated withdielectric properties of a surface layer; i.e. [metal-oxide-hydrox-ide-inhibitor]ads complex [65]. In 1.0 M HCl, the oxide layer can bedissolved in acidic medium (pH < 4).

On the other hand, the origin of the large inductive loop at LF isstill unclear. Adsorbed charged intermediates may result in aninductive loop [55]. This is more pronounced when the inter-mediates are strongly adsorbed. Lenderink et al. [66] haveattributed the phenomenon to the relaxation of adsorbed species

Fig. 10. Equivalent circuit used to fit the EIS.



Fig. 11. The representative figure of using the equivalent circuit to fit the

experimental data for 1.0 M HCl without inhibitor.


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like Hads+. Others suggested relaxation adsorbed intermediates

include Cl� [60], oxygen ion [67] or inhibitor species [68] on theelectrode surface. It might be also attributed to the re-dissolutionof the oxide layer surface [24] or Al-dissolution [60] at lowfrequencies. Inductive behavior is also observed for the pittedactive state and attributed to the surface area modulation or saltfilm property modulation [69].

The sizes of both capacitive and inductive loops increasesignificantly with an increase in inhibitor concentration. Thissuggests that the impedance of inhibited substrate increases witho-phenanthroline concentration. The EIS results are simulated bythe equivalent circuits shown in Fig. 10. Rs, Rt and RL are thesolution resistance, charge transfer resistance and inductiveresistance, respectively. CPE is constant phase element. L is theinductance, which is intimately associated with the inductive loop.The polarization resistance (Rp) can be calculated from thefollowing equation [57,58,66]:

R p ¼Rt � RL

Rt þ RL(4)

Then the inhibition efficiency (hR) is calculated from Rp usingthe following relation:

hR ¼R pðinhÞ � R pð0Þ

R pðinhÞ� 100% (5)

where Rp(0) and Rp(inh) are polarization resistance in the absenceand presence of inhibitor, respectively.

The impedance parameters are listed in Table 2. Therepresentative example of using the equivalent circuit to fit theexperimental data for HCl without inhibitor (blank solution) isshown in Fig. 11. The chi-squared (x2) is used to evaluate theprecision of the fitted data [70–72]. Table 2 reveals that x2 values

Table 2EIS parameters for the corrosion of aluminum in 1.0 M HCl containing o-phenanthrolin

c (mM) Rs (V cm2) Rt (V cm2) RL (V cm2) RP (V cm2)

0 0.70 4.57 0.34 0.32

0.2 0.75 7.51 0.51 0.48

0.6 0.74 21.16 2.05 1.87

1.0 1.04 36.73 3.70 3.36

1.6 0.40 73.83 7.28 6.63 41

2.0 0.35 103.4 8.56 7.91


are low, which confirms that the fitted data have good agreementwith the experimental data. It is observed that Rs values are verysmall, which again confirms that the IR drop could be small.Generally, a large Rt is associated with a slower corroding system.In contrast, better protection provided by an inhibitor can beassociated with a decrease in capacitance of the metal/solutioninterface. Rp increases with addition o-phenanthroline, whichindicates the electrode exhibits slower corrosion in the presence ofinhibitor. In the presence of o-phenanthroline, the decrease in CPEcomparing with that in blank solution (without inhibitor), whichcan result from a decrease in local dielectric constant and/or anincrease in the thickness of the electrical double layer, suggeststhat the inhibitor molecule functions by adsorption at the metal/solution interface [73,74].

From Table 2, hR increases with the concentration of o-phenanthroline, and the maximum hR is 96.0%. This result againconfirms that o-phenanthroline exhibits good inhibitive perfor-mance for aluminum in 1.0 M HCl solution. Inhibition efficienciesobtained from weight loss (hw), potentiodynamic polarizationcurves (hp) and EIS (hR) are in good reasonably agreement.

3.5. Scanning electron microscopy (SEM)

Fig. 12 shows the SEM images of aluminum surface. It can beseen from Fig. 12(a) that the aluminum samples before immersionseems smooth and appears some abrading scratches on thesurface. After immersion in uninhibited 1.0 M HCl solution for 2 h,the aluminum surface appears an aggressive attack of thecorroding medium as shown in Fig. 12(b). Furthermore, thecorrosion products appear very uneven and cube-shaped mor-phology, and the surface layer is rather rough. On the contrary,Fig. 12(c) shows that there is much less damage on the aluminumsurface in the presence of 2.0 mM o-phenanthroline, which furtherconfirms the inhibition ability.

3.6. Quantum chemical calculations

Quantum chemical calculations were done in order to discussthe adsorption mode through light on the inhibitor molecularstructure. It is well known that the N-heterocyclic compound couldbe protonated in the acid solution. According to some quantumchemical studies about protonated N-heterocyclic inhibitor in HClsolution [75], the proton affinity is clearly favored toward thehetero N atom of N-heterocyclic ring. Thus, the chemical structureof protonated o-phenanthroline molecule is shown in Fig. 13. Thecalculated value of protonated affiliation energy (PA) is as large as1144.4 kJ/mol, which confirms that o-phenanthroline is easilyprotonated by H+.

Fig. 14 shows the optimized molecular structures of neutral andprotonated o-phenanthroline. Clearly, two molecular structuresare in one plane. It is well known that organic inhibitor can formcoordination bonds between the unshared electron pairs of O, N orS atom and the empty p-orbitals of Al atom [33]. The largernegative charge of the atom, the better is the action as an electronicdonor. Mulliken charges of the atoms are listed in Table 3. Bycareful examination of the values of Mulliken charges, the larger

e at 20 8C.

CPE (mF/cm2) L (H cm2) x2 hR (%)

101 2.5 7.84 � 10�3 –

106 5.9 1.85 � 10�2 33.3

50 10.7 1.13 � 10�2 82.9

57 14.2 8.60 � 10�3 90.5

28.9 4.78 � 10�2 95.2

56 62.4 9.96 � 10�2 96.0



Fig. 12. SEM micrographs of aluminum surface: (a) before immersion; (b) after 2 h of immersion at 20 8C in 1.0 M HCl; (b) after 2 h of immersion at 20 8C in 2.0 mM o-

phenanthroline +1.0 M HCl.


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negative atoms are found in N10, N14, which are active adsorptivecenters. For protonated o-phenanthroline, the Mulliken charge ofN14 becomes more negative than N10. This result implies that ifthe inhibitor is protonated, N14 exhibits more active than N10.

Fukui function is necessary in understanding the local siteselectivity. The Fukui function ( f ð~rÞ) is defined as [76,77]:

f ð~rÞ ¼ @rð~rÞ@N

� �Vð~rÞ

(6)

The nucleophic attack Fukui function ( f ð~rÞþ) and electrophilicattack Fukui function ( f ð~rÞ�) can be calculated as [78]:

f ið~rÞþ ¼ qiðN þ 1Þ � qiðNÞ (7)

f ið~rÞ� ¼ qiðNÞ � qiðN � 1Þ (8)

where qi(N + 1), qi(N), qi(N � 1) are charge values of atom i forcation, neutral and anion, respectively. The values of f ð~rÞþ andf ð~rÞ� are also listed in Table 3. Generally, high values of f ð~rÞþ andf ð~rÞ� mean the high capacity of the atom to gain and lost electron,

respectively. For the nucleophic attack, the most reactive sites areN10 and N14 for o-phenanthroline; and C11 and C13 for

Fig. 13. Chemical molecular structure of protonated o-phenanthroline.

Fig. 14. Optimized molecular structures of the neutral and protonated o-

phenanthroline: (a) o-phenanthroline; (b) protonated o-phenanthroline.

Please cite this article in press as: Li X, et al. Experimental and theoretical study on corrosion inhibition of o-phenanthroline foraluminum in HCl solution. J Taiwan Inst Chem Eng (2013), http://dx.doi.org/10.1016/j.jtice.2013.10.007


Table 3Quantum chemical parameters of Mulliken charge, f ð~rÞþ and f ð~rÞ� for neutral and

protonated o-phenanthroline molecules.

Atom Mulliken charge f (r)+ f (r)�

Phena p-Phenb Phen p-Phen Phen p-Phen

C1 0.105 0.112 0.012 0.020 0.015 0.018

C2 0.163 0.184 0.059 0.040 0.052 0.049

C3 0.163 0.299 0.059 0.059 0.052 0.058

C4 0.105 0.122 0.012 0.008 0.015 0.008

C5 �0.231 �0.222 0.025 0.033 0.092 0.112

C6 �0.231 �0.211 0.025 0.032 0.092 0.090

C7 �0.200 �0.187 0.066 0.021 0.048 0.057

C8 �0.190 �0.174 0.070 0.042 0.023 0.025

C9 �0.064 �0.050 0.030 0.021 0.055 0.058

N10 �0.357 �0.367 0.078 0.031 0.042 0.049

C11 �0.200 �0.162 0.066 0.132 0.048 0.041

C12 �0.190 �0.171 0.070 0.035 0.023 0.024

C13 �0.064 0.032 0.030 0.105 0.055 0.040

N14 �0.357 �0.418 0.078 0.066 0.042 0.013

a Phen refers to o-phenanthroline.b p-Phen refers to protonated o-phenanthroline.

Fig. 15. The highest occupied molecular orbital (HOMO) of the neutral and

protonated o-phenanthroline: (a) o-phenanthroline; (b) protonated o-

phenanthroline.


G Model


protonated o-phenanthroline, which can accept electrons frommetal surface to form back-donating bond. On the other hand, thevalues of f ð~rÞ� indicate that it will happen on the C5 and C6 forboth neutral and protonated molecules, which can denoteelectrons to metal surface to form coordinate bond.

The dipole moment (m) is widely used the polarity of themolecule, and related to the inhibitive ability [79]. Inspection ofTable 4 reveals that the dipole moment (m) of either o-phenanthroline or protonated o-phenanthroline is higher thanthat of H2O (m = 1.84 Debye). The high value of dipole momentprobably increases the adsorption between chemical compoundand metal surface [80]. Accordingly, the adsorption of both o-phenanthroline and protonated o-phenanthroline molecules fromthe aqueous solution can be regarded as a quasi-substitutionprocess between the inhibitor compound in the aqueous phase[inhibitor(sol)] and water molecules at the electrode surface[H2O(ads)].

Besides the above-mentioned quantum chemical parameters,the global reactivity of a molecule depends on moleculardistributions. HOMO is often associated with the capacity of amolecule to donate electrons, whereas LUMO represents the abilityof the molecule to accept electrons. The electric/orbital densitydistributions of HOMO and LUMO for the neutral and protonatedmolecules are shown in Figs. 15 and 16, respectively. For both o-phenanthroline and protonated o-phenanthroline molecules, theelectron densities of both HOMO and LUMO are well proportionedin the whole molecule. That is, there is electron transferring in theinteraction between the inhibitor molecule and metal surface. Theadsorption of inhibitor on aluminum may be in a manner in whichthe planes of o-phenanthroline and protonated o-phenanthrolineare parallel to the aluminum surface.

The energy values of highest occupied molecular orbital(EHOMO), energy of lowest unoccupied molecular orbital (ELUMO)and the separation energy (ELUMO–EHOMO, DE) are also presented inTable 4. High value of EHOMO indicates a tendency of the moleculeto donate electrons to act with acceptor molecules with low-energy, empty molecular orbital [33]. Similarly, ELUMO represents

Table 4Quantum chemical parameters of m, EHOMO, ELUMO and DE for neutral and

protonated o-phenanthroline molecules.

Molecule m (Debye) EHOMO (eV) ELUMO (eV) DE (eV)

o-phenanthroline 5.38 �5.784 �2.399 3.385

protonated o-phenanthroline 4.32 �6.465 �3.581 2.884


the ability of the molecule to accept electrons, and the lower valueof ELUMO suggests the molecule accepts electrons more probable[33]. Both EHOMO and ELUMO of protonated o-phenanthrolinedecrease compared with those of neutral o-phenanthroline. Thus,it could be deduced that the protonated o-phenanthrolinestrengthens accepting electrons from metal surface while weakensdonating electrons to metal surface.

The separation energy (energy gap) DE (ELUMO–EHOMO) is animportant parameter as a function of reactivity of the inhibitormolecule toward the adsorption on metallic surface. As DE

decreases, the reactivity of the molecule increases in visa, whichfacilitates adsorption and enhances the efficiency of inhibitor [33].DE value of protonated o-phenanthroline is lower than neutral o-phenanthroline, which again confirms that the protonatedmolecule exhibits better inhibitive performance than neutralmolecule.

3.7. Explanation for inhibition

It has been assumed that organic inhibitor molecule establishits inhibition action via the adsorption of the inhibitor on the metalsurface. The adsorption process is mainly influenced by thechemical structure of inhibitor and the nature of metal/solutioninterface. Generally, owing to the complex nature of adsorptionand inhibition of a given inhibitor, it is impossible for singleadsorption mode between inhibitor and metal surface.

According to the results of quantum chemical calculations ofSection 3.6, in aqueous acidic solutions, the o-phenanthroline



Fig. 16. The lowest unoccupied molecular orbital (LUMO) of the neutral and

protonated o-phenanthroline: (a) o-phenanthroline; (b) protonated o-

phenanthroline.


G Model


exists either as neutral molecule or in the form of protonated o-phenanthroline. Thus, three explanations of inhibition are listed asfollows:

(i) Due to the interaction between N atom and H+, the H+ at theelectrode surface is reduced, which could prevent thecorrosion of Al to some extent [30].

(ii) The mechanism for the corrosion of aluminum in HCl solutionwas proposed [31]. According to this mechanism, the anodicdissolution of aluminum follows the main steps:

Al þ Cl� $ AlClads� (9)

AlClads� þ Cl� ! AlCl2

þ þ 3e� (10)

The protonated o-phenanthroline cations can electrostati-cally interact with AlClads

� species which are formed in step(9), and then prevent oxidation reaction of AlClads

� to AlCl2+ as

shown by reaction (10).(iii) As mentioned above in Section 3.6, the molecular structure of

protonated o-phenanthroline remains unchanged with re-spect to its neutral form, two N atoms on the ring remainingstrongly blocked, so both neutral o-phenanthroline andprotonated o-phenanthroline may be adsorbed on the metalsurface via the chemisorption mechanism, involving thedisplacement of water molecules from the metal surfaceand the sharing electrons between the N atoms and the empty


p-orbitals of Al atom. Also, they can be adsorbed on the metalsurface on the basis of donor-acceptor interactions betweenp-electrons of the heterocycles and vacant p-orbitals of Al.

4. Conclusion

(1) o-Phenanthroline acts as a good inhibitor for the corrosion ofaluminum in 1.0 M HCl. Inhibition efficiency increases with theinhibitor concentration, while decreases with temperature. Theadsorption of o-phenanthroline on aluminum surface obeysLangmuir adsorption isotherm.

(2) hw increases firstly with immersion time from 0.5 to 2 h, andthen decreases gradually within the immersion time. hw

increases with the acid concentration in 0.5–1.0 M, thendecreases prominently with the acid concentration in 1.0–3.0 M.

(3) o-Phenanthroline acts as a cathodic-type inhibitor. EIS spectraexhibit a large capacitive loop at high frequencies followed by alarge inductive one at low frequency values. The impedance ofinhibited substrate increases with o-phenanthroline concen-tration.

(4) SEM clearly shows that the introduction of o-phenanthrolineinto 1.0 M HCl solution results in efficiently retarding thealuminum corrosion.

(5) o-Phenanthroline exists either as neutral molecule or in theform of protonated molecule. The chemical molecules of o-phenanthroline and protonated o-phenanthroline are in oneplane, and the electron densities of both HOMO and LUMO arewell proportioned in the whole molecule. The protonatedmolecule strengthens accepting electrons while weakensdonating electrons.

Acknowledgements

This work was carried out in the frame of research projectssupported by Chinese Natural Science Foundation (51161023) andApplied Fundamental Research Foundation of Yunnan Province(2009CD072). The electrochemical measurements were carriedout on PARSTAT 2273 advanced electrochemical system (PrincetonApplied Research) provided by Advanced Science InstrumentSharing Center of Southwest Forestry University.

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Experimental and theoretical study on corrosion inhibition of o-phenanthroline for aluminum in HCl solution