Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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JIEC-2260; No. of Pages 7

Enhanced corrosion inhibition effect of tannic acid in the presenceof gallic acid at mild steel/HCl acid solution interface

I.B. Obot *, A. Madhankumar

Centre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran, 31261 Saudi Arabia

A R T I C L E I N F O

Article history:

Received 11 June 2014

Received in revised form 27 September 2014

Accepted 18 October 2014

Available online xxx

Keywords:

Mild steel

EIS

Tannic acid

Gallic acid

HCl

A B S T R A C T

The corrosion inhibition behaviour of tannic acid (TA), gallic acid (GA) and their blends on mild steel in

1 M HCl solution was studied via electrochemical methods, namely, polarization (Tafel plots) and

electrochemical impedance spectroscopy (EIS). The surface morphology of mild steel in 1 M HCl, TA

alone, and in combination with GA was elucidated using scanning electron microscopy (SEM). The

results show that TA can protect mild steel from acid corrosion better than GA. However, inhibition effect

of TA can be enhanced by its combination with GA. The adsorption behaviour of TA against steel

corrosion obeyed Langmuir adsorption isotherm. Furthermore, polarization results suggest that TA and

GA blends behaves as mixed-type inhibitor of predominant anodic effect. EIS results reveal that the

charge transfer resistance value of the steel inhibited by TA in combination with GA is significantly

higher than that of steel inhibited by TA alone. The SEM images confirmed the enhanced film formation

on the steel surface in the presence of TA and GA blended in the ratio 9:1 when compared with TA alone.

Quantum chemical calculations have been used to provide insights into the mechanism of interaction of

TA and GA on mild steel.

� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

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jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

Introduction

Due to its excellent mechanical properties and low cost, mildsteel is the constructional material of choice in many chemical andpetrochemical industries. The major drawback of mild steel is its lowcorrosion resistance. Hydrochloric acid solutions are widely used inseveral industrial processes such as oil well acidizing, acid pickling,acid cleaning, and acid descaling. Because of the aggressiveness ofacid solutions, mild steel corrodes severely during these processes[1,2]. The use of corrosion inhibitors is of great practical importancein these processes in order to minimize metallic waste and reducecost. Most of the well-known acid inhibitors are organic compoundsespecially those containing N, O, S and P in addition to severalmultiple bonds in their molecular structure. Nevertheless, most ofthese compounds are not only expensive but also toxic to humansand the environments. Growing environmental concern andenforcement of strict environmental regulations have compelledresearchers to focus on the development of eco-friendly andbiodegradable corrosion inhibitors [3–9].

* Corresponding author. Tel.: +966 3 860 8283; fax: +966 3 860 3996.

E-mail address: obot@kfupm.edu.sa (I.B. Obot).

Please cite this article in press as: I.B. Obot, A. Madhankumar, J. Ind

http://dx.doi.org/10.1016/j.jiec.2014.10.019

1226-086X/� 2014 The Korean Society of Industrial and Engineering Chemistry. Publis

The use of tannins in corrosion protection has been disclosedsince 1936 in Great Britain patent no. GB450547. Development andindustrial uses of tannins as green corrosion inhibitors in theformulations of pigments, chemical cleaning agents for removingiron-based deposits and oxygen scavengers for boiler watertreatment system has been reviewed [10]. Tannic acid (TA)(Fig. 1), which belongs to the class of natural, non- toxic andenvironmentally friendly compounds has been reported recentlyas corrosion inhibitor for steel, Cu and Zn in atmospheric condition[11]. The results obtained indicate that steel was the leastprotected while Cu was the best protected. The corrosion of mildsteel in seawater wet/dry cyclic condition is inhibited effectivelyby tannic acid [12]. The value of the inhibition efficiency obtainedfrom the polarisation curves was 86.05%. Tannic acid acts as acathodic-type corrosion inhibitor, retarding the cathodic process ofthe corrosion reactions by forming ferric tannates on the mild steelsurface according to the authors. Also recently, tannic acid at 4.2 g/lwas found to provide low protection (49.5%) in 2 M HCl and 38.5%at 1 M H2SO4 to steel [13].

In order to enhance the performance of tannic acid as mild steelcorrosion inhibitor, synergistic addition of gallic acid (GA) (Fig. 1)to TA to boast its inhibition efficiency has been carried out inthe present study. Synergistic inhibition, which is an improved

. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.10.019

hed by Elsevier B.V. All rights reserved.

Fig. 1. 2D molecular structures of (a) gallic acid (GA) and (b) tannic acid (TA).

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JIEC-2260; No. of Pages 7

performance of a mixture of inhibitors compared with theindividual inhibitors as a corrosion prevention technology formetals and alloys has proven to be an effective method. Thismethod has been widely reported in the literature and also formsthe basis for modern inhibitor formulations for industrialapplications [14].

The aim of the present work is to investigate the inhibitionperformance of TA and GA in 1 M HCl and to determine if there isany synergistic effect between the blends of TA and GA towardssteel corrosion protection in 1 M HCl. The corrosion protectionproperties of TA and GA were examined by polarization andelectrochemical impedance spectroscopy techniques. Surfacemorphology of the steel surface in the absent and present of TAand TA–GA blends was investigated using scanning electronmicroscopy (SEM). Quantum chemical calculations were furtheremployed to provide insights into the reactive behaviour of TAand GA on steel surface.

Experimentals

Test specimens were cut from a typical mild steel pipeline withchemical composition shown in Table 1. The tested coupon is inconformance with API X60 material specifications and the cross-sectional surface was ground with different grade emery papers(400, 600, 800 and 1200), rinsed with acetone, washed thoroughlywith doubly distilled water and used as the working electrode.

Table 1Chemical composition of the tested coupon.

C Si Mn P S

0.125 0.52 1.83 0.0028 0.0044

Co Ti Nb V W

0.0023 0.0058 0.053 0.078 0.134

As Bi Ta Ca Ce

0.0045 <0.0015 0.0100 0.0059 0.0021

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Tannic acid (TA) and Gallic acid (GA) were obtained from SigmaAldrich, USA and used without any further purification.

Potentiodynamic polarization and electrochemical impedancemeasurements were done using a conventional three-electrodesystem (model: Gamry Instrument potentiostat/galvanostat/ZRA(Reference 3000), USA) consisting of a mild steel working electrode(WE) with an exposed area of 0.7855 cm2, platinum as the counterelectrode and a saturated calomel electrode (SCE) as the reference.Tafel curves were obtained by changing the electrode potentialautomatically from �250 to +250 mV versus the open-circuitpotential (Ecorr) at a scan rate of 1 mV/s. Before polarizationmeasurements, the WE was kept in the test solution for 1 h foropen circuit potential (OCP) to be reached. Corrosion currentdensity (icorr) was determined from the intercept of extrapolatedcathodic and anodic Tafel lines at the corrosion potential (Ecorr).

Electrochemical impedance measurements were performed inthe frequency range 10 mHz to 100 kHz with an a.c. amplitude of10 mV (rms) at the rest potential. All the experiments have beencarried out at 30 8C.

Morphological studies of the carbon steel surface wereundertaken by SEM examinations of the surfaces exposed todifferent test solutions after 24 h using a JSM-5800 LV scanningelectron microscope.

Quantum chemical calculations were conducted with Gaussian03 commercial software [15]. The electronic properties calculationof TA and GA molecules were accomplished by semi empirical PM3method because of the size of TA.

Cr Ni Mo Al Cu

0.121 0.091 0.079 0.043 0.296

Pb B Sb Sn Zn

>0.030 0.0012 <0.0010 0.081 >0.032

Zr La Se N Fe

<0.0015 0.029 <0.0020 >0.019 <96.2

. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.10.019

Table 2Kinetic parameters obtained from Tafel plots for carbon steel in the absence and

presence of different concentrations of TA at 30 8C.

Conc. (g/l) Ecorr (mV) icorr

(mA/cm2)

ba

(mV/dec)

bc

(mV/dec)

h (%)

Blank �494 437.0 128 136 –

0.1 �488 144.2 106 113 67.0

0.5 �478 79.5 93 108 81.8

1.0 �477 38.1 96 111 91.2

Table 3Kinetic parameters obtained from Tafel plots for carbon steel in the absence and

presence of TA and GA and with TA–GA blends at 30 8C.

Conc. (g/l) Ecorr (mV) icorr

(mA/cm2)

ba

(mV/dec)

bc

(mV/dec)

h (%)

Blank �494 437.0 128 136 –

1.0 TA �477 38.1 96 111 91.2

0.9TA + 0.1 GA �492 29.6 75 99 93.2

0.5 TA + 0.5 GA �488 177.0 105 130 59.4

1.0 GA �481 111.0 133 118 74.5

10-7

10-6

10-5

10-4

10-3

10-2

10-1

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.21.0g/L GA

0.5g/L TA + 0.5g/L GA

0.9g/L TA + 0.1g/L GA

1.0g/L TA

Pote

nti

al,

V (

SC

E)

Log (Curre nt Density), icorr A cm -2

Blank

Fig. 2. Polarization measurement for low carbon steel in the absence and in the

presence of different concentrations of TA in 1 M HCl at 30 8C.

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Results and discussion

Polarization measurements

Fig. 2 shows the Tafel polarization curves of TA for mild steelin 1 M HCl solution alone and in the presence of 0.1, 0.5 and 1.0 g/lTA at 30 8C. A serial of concentration ratio tests of TA and GA at atotal of 1 g/l were prepared for polarization measurements in orderto elucidate the possible synergistic inhibition effect of the twocompounds on mild steel. A typical plot of the blends is depicted inFig. 3. Values of the electrochemical corrosion parameters, such asthe corrosion potential (Ecorr), cathodic Tafel slope (bc), anodicTafel slope (ba), corrosion current density (icorr) and inhibitionefficiency h% for different concentrations of TA and both the TA–GAmixed systems are given in Tables 2 and 3, respectively. The

10-7

10-6

10-5

10-4

10-3

10-2

10-1

-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

1.0g/l TA

0.5g/l TA

0.1g/l TA

Blank

Pote

nti

al,

V (

SC

E)

Log (Current Density), icorr A cm -2

Fig. 3. Tafel polarization curves for low carbon steel in 1 M HCl with 1 g/l of TA and

GA and with TA/GA blends at a total of 1 g/l at 30 8C.

Please cite this article in press as: I.B. Obot, A. Madhankumar, J. Ind

inhibition efficiency (h%), was calculated as follows:

h% ¼ iocorr � icorr

iocorr

� 100 (1)

where iocorr and icorr are the values of corrosion current density inthe absence and presence of inhibitors, respectively.

It is clear from Fig. 2, that both anodic and cathodic regions areaffected with the introduction of TA to the corrosive systemindicating that TA is a mixed-type inhibitor. The higher the TAconcentration, the better the protection of mild steel. This isevident in Table 2 because the corrosion current densitydiminishes with increased in TA concentration. It is obvious alsothat the Ecorr shifts towards more positive values indicating that TAplays a stronger role in suppressing the anodic reaction. It isinstructive to note that similar results were obtained with TA–GAblends (Fig. 3); that is the Ecorr shifts towards more positive valuesand both anodic and cathodic regions were affected. Table 3 showsthat the corrosion current density (icorr) was lower for TA–GAblends consisting of 0.9 g/l + 0.1 g/l when compared to TA and GAalone at 1 g/l, and the GA–TA blend of 0.5 g/l + 0.5 g/l. This is anindication that the enhanced protection of TA is due to thesynergistic effect of GA addition. For example the 0.9 g/l + 0.1 g/lTA–GA blend produced inhibition efficiency of 93.2% whencompared to 91.2% and 74.5% for TA and GA alone (1 g/l).

Electrochemical impedance spectroscopy (EIS) study

Figs. 4 and 5 demonstrate changes in Nyquist plot for mildsteel in 1 M HCl and with the addition of 1 g/l of TA or GA alone andTA–GA blends at 30 8C. The calculated electrochemical impedanceparameters are given in Tables 4 and 5. The equivalent circuit usedfor the study is a Randle circuit as shown in Fig. 6. In this circuit, Rct

is the charge transfer resistance, CPE is the constant phaseelement, and Rs is the solution resistance. The inhibition efficiencyis calculated from the Rct values using the following formula:

h% ¼ Roct � Rct

Roct

� 100 (2)

where Roct and Rct are the charge-transfer resistances in the absence

and presence of the inhibitors, respectively.Fig. 4 shows an increase in the diameter of the semicircle

with increase in TA concentration, indicating that the inhibitionefficiency of TA increased with increase in TA concentration.

. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.10.019

Table 5Electrochemical parameters obtained from EIS plots for carbon steel in the absence

and presence of TA and GA and with TA/GA blends at 30 8C.

Conc. (g/l) Rs (V cm2) Rct (V cm2) Cdl (mF/cm2) n h (%)

Blank 8.61 48.8 544 0.91 –

TA 2.98 294.4 79 0.89 83.4

0.9TA + 0.1 GA 2.80 451.6 62 0.89 89.2

0.5 TA + 0.5 GA 4.50 119.5 128 0.88 59.1

1.0 GA 4.04 160.2 114 0.90 69.5

Table 4Electrochemical parameters obtained from EIS plots for carbon steel in the absence

and presence of different concentrations of TA at 30 8C.

Conc. (g/l) Rs (V cm2) Rct (V cm2) Cdl (mF/cm2) n h (%)

Blank 8.61 48.8 544 0.91 –

0.1 3.66 130.0 258 0.91 62.0

0.5 3.83 187.4 165 0.90 74.0

1.0 2.98 294.4 79 0.89 83.4

0 50 10 0 15 0 20 0 25 0 30 0

0

50

100

150

200

250

300

Blank

1.0g/L TA

0.5g/L TA

0.1g /L TA

- Z

'' (

Ωcm

2)

Z' (Ω cm2)

Fig. 4. Nyquist plots for low carbon steel in 1 M HCl in the absence and in the

presence of different concentrations of TA at 30 8C.

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The Nyquist plots in Fig. 5 also contain the typical depressedsemicircles which is usually attributed to the dispersing phenom-enon which is caused by roughness and inhomogeneities of theelectrode surface [16]. It is clear from Fig. 5 that with TA–GA blendof 0.9 g/l + 0.1 g/l, the diameter of semicircle was the largest whencompared with TA or GA alone (1 g/l) and with the blank. This is anindication of a higher charge transfer resistance Rct, and thereforehigh inhibitive effect of the blend. It can be concluded that TA–GAblends exhibits the highest inhibition efficiency for steel in 1 M HClwhen compared to TA and GA alone. In Table 4, the double layercapacitance,Cdl, values decreased with increase in the concentra-tion of TA. The least value of Cdl was obtained when TA and GA wereblended (0.9 g/l + 0.1 g/l) (Table 5). This may be due to the increasein the surface coverage on the carbon steel by the addition of GA toTA, which led to the increase in inhibition efficiency. The decreasein Cdl indicates an increase in the thickness of the double layerwhen inhibitor molecule is adsorbed on the electrode surface. Thechanges in Cdl values are believed to be due to the replacement of

0 10 0 200 300 400 500

0

100

200

300

400

500

1.0g/L GA

0.5g/L TA + 0.5g/L GA

0.9g/L TA + 0.1g/L GA

1.0g/L TA

Blank

- Z

'' (

Ωcm

2)

Z' (Ω cm2)

Fig. 5. Nyquist plots for low carbon steel in 1 M HCl with 1 g/l of TA and GA and with

TA–GA blends at a total of 1 g/l at 30 8C.

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water molecules from the electrode surface. The Cdl is related to thethickness of the protective layer (d). This is in accordance with theHelmholtz model and is given by the following equation:

Cdl ¼eeoA

d(3)

where e is the dielectric constant of the medium, eo is thepermittivity of the free space (8.854 � 10�12 F/m), and A is theeffective surface area of the electrode.

The double layer behaviour was approximated by a constant-phase element (CPE) rather than a pure capacitor (Fig. 6). The CPE isusually substituted for the capacitor to fit the Nyquist depressedsemicircles more accurately. The admittance, YCPE, and impedance,ZCPE of a CPE can be expressed as follows [17]:

YCPE ¼ Yo jvð Þn (4)

and

ZCPE ¼1

Yo

� �j � 2p f maxð Þn

� ��1(5)

where Yo is the amplitute comparable to a capacitance, j is thesquare root of �1, fmax is the AC frequency at maximum and n, thephase shift (�1 � n � 1), when n = 0, the CPE represents pureresistor, if n = +1, the CPE represents pure capacitor, and if n = �1,the CPE represents inductor.

Adsorption isotherm

Data obtained from the experimental results were fitted intodifferent adsorption models in order to obtained basic informationregarding the interaction of TA and the low carbon steel. Thesurface coverage (u = h%/100), at different concentrations of TA in

Fig. 6. Equivalent circuit diagram used to fit the impedance data in the absence and

presence of the inhibitors.

. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.10.019

Fig. 7. Langmuir adsorption plot for low carbon steel in 1 M HCl solution containing

different concentrations of TA at 30 8C.

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1 M HCl solution was evaluated using potentiodynamic measure-ments at 30 8C. Langmuir adsorption model provided the best fitfor the experimental data with the correlation coefficient of 0.99(Fig. 7). The slope of the linear line was approximately equals tounity (1.04) which is in accordance with the Langmuir isothermequation expressed as follows [18,19]:

C

u¼ 1

Kadsþ C (6)

where C is the inhibitor concentration in the solution and Kads

is the equilibrium constant for the adsorption/desorptionprocess.

The free energy of adsorption DGoads of the inhibitors on carbon

steel surface was determined using the following equation:

DGoads ¼ �RT ln Kads � 55:5ð Þ (7)

where R is the molar gas constant, T is the absolute temperatureand the value of 55.5 is the concentration of water in solutionexpressed in mol/l.

Generally it is widely accepted that DGoads values more

negative than �40 kJ/mol involve charge sharing or transferfrom the inhibitor molecules to the metal surface to form acoordinate type of bond (chemisorption) whereas thoseapproximately �20 kJ/mol or lower is attributed to physicaladsorption or electrostatic attraction between the charge metalsurface and the charged organic molecule in the solution [20].

Fig. 8. SEM images of low carbon steel after 24 h in (a) 1 M HCl (b)

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The computed DGoads value of TA is �17.24 kJ/mol according to

Eq. (5) indicating physical adsorption of TA molecules on mildsteel.

SEM analysis

Scanning electron microscopic (SEM) images were obtainedafter 24 h of immersion to study the micro-structural changes ofmild steel specimen in 1 M HCl solution and in the presence of theadditives. Fig. 8(a–c) shows the SEM image of mild steel surfaceafter immersion for 24 h in 1.0 M HCl, in 1 g/l TA and in TA–GAblend of 0.9 g/l + 0.1 g/l, respectively. The mild steel surfaceexposed in 1 M HCl acid solution in the absence of TA appearedto be strongly damaged with lots of pits and cavities. Thus it can beobserved that mild steel is severely corroded in aggressive acidmedia. The specimen exhibited some level of protection whenimmersed in the acid solution in the presence of TA. However,more uniform coverage and better protection was evident whenTA–GA blend was introduced.

Quantum chemical studies

Quantum chemical calculations were further employed toprovide insights into the mechanism of charge transfer andreactivity of the inhibitors with the mild steel surface. Fig. 9presents the frontier molecular orbitals (HOMO and LUMO) andMulliken charges of TA and GA. Obviously as seen from thefigure, the HOMO and LUMO are located on a small part ofTA while for GA they are localized on the entire molecule.The Mulliken charges are mainly located on the oxygen atomsof TA and GA (The red colour represents atoms with excessnegative charges). TA has more oxygen atom than GA. Theadsorption of TA onto mild steel surface is expected to bestronger than that of GA. This is confirmed experimentally dueto the fact that TA has a higher inhibition efficiency than GA formild steel corrosion in 1 M HCl.

The reactivity of a molecule is governed by the frontierorbitals [21–28]. These are the energy of the highest occupiedmolecular orbital (EHOMO), which indicates the ability of amolecule to donate electrons to an acceptor molecule and theenergy of the lowest unoccupied molecular orbital (ELUMO),representing the propensity of a molecule to accept electrons.The difference between the HOMO and the LUMO energies (DE)gives an indication of the chemical reactivity of a molecule[29]. When comparing two or more corrosion inhibitor, the onewith high values of EHOMO, lower values of ELUMO and a small DE

1 M HCl + 1 g/l TA and (c) I M HCl + 0.9 g/l TA/0.1 g/l GA blend.

. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.10.019

Fig. 9. Frontier molecular orbitals (HOMO and LUMO) and Mulliken charges of TA and GA.

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should exhibit a higher interaction with the metal surface[30,31]. Thus according to Table 6, TA should inhibit steelcorrosion better than GA. This conclusion is in accordance withexperimental results. For the dipole moment, TA has a highervalue than GA. This is expected due to the fact that it has moreoxygen atoms in its molecular structure. This high value of the

Table 6Quantum chemical parameters for GA and TA calculated using PM3 method.

Inhibitors EHOMO (eV) ELUMO (eV) DE (eV) Dipole

moment (D)

GA �9.336 �0.729 8.607 3.740

TA �9.279 �1.086 8.193 10.333

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dipole moment for TA increases the polarity of the molecule andits adsorption on mild steel surface than GA.

Conclusions

The following conclusions may be drawn from this study:

1. TA and GA inhibit mild steel in 1 M HCl but a better inhibitionwas obtained with TA–GA blend.

2. The experimental results obtained from polarisation curvesindicate that TA, GA and TA–GA blends are mixed-type inhibitor.

3. Charge transfer resistance was highest with TA–GA blendconsisting of 0.9 g/l + 0.1 g/l when compared to the value

. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.10.019

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obtained for 1 g/l TA and 1 g/l of GA indicating its betterprotection on steel.

4. SEM confirms the better protection of mild steel in 1 M HCl inthe present of TA–GA combination of 0.9 g/l + 0.1 g/l.

5. Quantum chemical calculations gave further insight on thereactivity of TA and GA towards mild steel.

Acknowledgements

The authors gratefully acknowledged the Center of ResearchExcellence in Corrosion (CORE-C) at King Fahd University ofPetroleum and Minerals (KFUPM), Dhahran, Saudi Arabia forfunding.

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