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Applied Surface Science 356 (2015) 341–347

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

orrosion mitigation of J55 steel in 3.5% NaCl solution by aacrocyclic inhibitor

mbrish Singha,b, Yuanhua Lina,∗, I.B. Obotc, Eno E. Ebensod,e, K.R. Ansari f,.A. Quraishi f

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Chengdu, Sichuan 610500, ChinaDepartment of Chemistry, LFTS, Lovely Professional University, Phagwara, Punjab 144402, IndiaCentre of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi ArabiaDepartment of Chemistry, School of Mathematical & Physical Sciences, North-West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735,outh AfricaMaterial Science Innovation & Modelling (MaSIM) Focus Area, Faculty of Agriculture, Science and Technology, North-West University (Mafikeng Campus),rivate Bag X2046, Mmabatho 2735, South AfricaDepartment of Applied Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India

r t i c l e i n f o

rticle history:eceived 15 July 2015eceived in revised form 11 August 2015ccepted 11 August 2015vailable online 14 August 2015

a b s t r a c t

1,2,4,7,9,10-Hexaazacyclo-pentadeca-10,15-dien-3,5,6,8-tetraone (HPT) a macrocyclic compound hasbeen studied using electrochemical methods and scanning electrochemical microscopy (SECM) tech-niques. The Rct values increased and Cdl values decreased with the increase in concentration of theinhibitor. The corrosion inhibition of J55 steel in 3.5 wt.% NaCl solution saturated with CO2 by polar-ization studies revealed that HPT acted as a mixed type inhibitor. The adsorption of HPT on the J55 steelsurface obeyed the Langmuir adsorption isotherm. The thermodynamic parameters (K , �G◦ ) were

eywords:55 steelISnhibitor

acrocyclicECM

ads ads

also computed and discussed.© 2015 Elsevier B.V. All rights reserved.

orrosion

. Introduction

Macrocyclic compounds have planar structure and four pyr-ole subunits with the nitrogen atoms as the binding sites. Theetal is at the center bonded to four nitrogen atoms of the pyr-

ole subunits [1]. Recently macrocyclic compounds have emergeds new and potential class of inhibitors to mitigate corrosion. Thevailable information suggests that, despite the high ability of theacrocyclic compounds to interact strongly with metal surface, lit-

le attention have been focused on these compounds as corrosionnhibitors [2]. The corrosion inhibition occurs by the adsorption of

hese macrocyclic compounds on to the metal surface which form

protective layer [3–5].Carbon dioxide corrosion of steel pipelines is a complex process

hat causes serious problems due to localized corrosion. J55 steel

∗ Corresponding author.E-mail addresses: yhlin28@163.com, drambrishsingh@gmail.com (Y. Lin).

ttp://dx.doi.org/10.1016/j.apsusc.2015.08.094169-4332/© 2015 Elsevier B.V. All rights reserved.

can be used for CO2 situation which causes worse corrosion cir-cumstances in the oilfields situated in the northeast part of SichuanProvince, China.

The mitigation of corrosion on the metal surface is done usinginhibitors as one of the common methods [6]. In the present studythe inhibiting effect of 1,2,4,7,9,10-hexaazacyclo-pentadeca-10,15-dien-3,5,6,8-tetraone (HPT) was investigated on the corrosion ofJ55 steel using Tafel and electrochemical impedance spectroscopy(EIS) methods.

2. Experimental

2.1. Synthesis of the macrocyclic compound (HPT)

The macrocyclic compound was synthesized in the lab accordingto the procedure described elsewhere [7,8]. Fig. 1 represents the

molecular structure of macrocyclic compound (HPT). J55 steel of(wt.%) [9]: C 0.24; Si 0.22; Mn 1.1; P 0.103; S 0.004; Cr 0.5; Ni 0.28;Mo 0.021; Cu 0.019; Fe remainder was mechanically abraded withSiC papers to a mirror finish.

342 A. Singh et al. / Applied Surface Sc

N

N

NH

NH

NH

O

O

ONH O

Ft

2

mt1ss

s

2

pGyceswt

ig. 1. Structure of 1,2,4,7,9,10-hexaazacyclo-pentadeca-10,15-dien-3,5,6,8-etraone (HPT).

.2. Fourier transform infrared spectroscopy (FTIR)

Nicolet-6700 spectrophotometer was used in transmittanceode to record the FTIR spectra (KBr). The peaks of various func-

ional group as shown in Fig. 2 are 950.87 cm−1 OH group;067.02 cm−1 C O stretching; 1133.39 cm−1 C C (aromatic)/C Otretching; 1394.71 cm−1 C C (aromatic); 1614.55 cm−1 C Otretching; 2896.28 cm−1 CH stretching vibration; 2970.94 cm−1

CH stretching/OH group; 3232.26 cm−1; 3414.77 cm−1 N Htretching.

.3. Electrochemical measurements

The impedance spectroscopy and Tafel polarization waserformed through Autolab Potentiostat/Galvanostat (ModelSTAT302N) incorporated with FRC and GPES softwares for anal-ses. The four neck cells were used for electrochemical studiesonsisting of the working electrode, auxiliary electrode, reference

lectrode, and the fourth one was used to saturate CO2 in theolution after which it was sealed. All the electrochemical testsere started after the electrodes were immersed in 3.5% NaCl solu-

ion with and without HPT for 30 min to attain a stable potential.

Fig. 2. FTIR analy

ience 356 (2015) 341–347

The linear and smooth open circuit potential (OCP) plot of J55 steelwith and without HPT versus time (200 s) was obtained confirminga stable potential as shown in Fig. 3 [10]. Measurement of open cir-cuit potential provides the potential distribution on the corrodingsurface and information on the free corrosion potential as the start-ing point for the application of electrochemical protection methods[11,12]. Less fluctuation of potential for the solution with and with-out inhibitor confirms that the stabilization time of 30 min wasenough to attain a stable value of Ecorr [47].

Tafel curves were obtained from −300 to +300 mV versus OCP ata scan rate of 1 mV s−1. Impedance measurements were carried outon OCP in a frequency range from 100 kHz to 0.01 Hz with ampli-tude of 10 mV peak-to-peak [13]. The inhibition efficiency wasevaluated from the measured Icorr values using the relationship:

�% = Ibcorr − Ii

corr

Ibcorr

× 100 (1)

where Ibcorr is the corrosion current for 3.5% NaCl solution and Ii

corr isthe corrosion current of HPT, respectively. The following equationwas used to calculate the inhibition efficiency from the impedancevalues:

�% = Rict − Rb

ct

Rict

× 100 (2)

where Rict is the charge transfer resistance for HPT and Rb

ct is thecharge transfer resistance of 3.5% NaCl solution, respectively.

2.4. Scanning electrochemical microscopy (SECM)

The SECM is a technique in which current flows through a micro-electrode immersed in an electrolytic solution and situated close toa metal surface. The metal can either be a conductive, semiconduc-

tive or insulating material [14]. An electrochemical work station ofCHI900C model was used with J55 steel as working electrode, a ref-erence electrode, and a platinum counter electrode for all the tests[15].

sis of HPT.

A. Singh et al. / Applied Surface Science 356 (2015) 341–347 343

Fig. 3. Open circuit potential plots of J55 steel in the absence and presence of HPT in 3.5% NaCl solution saturated with CO2.

F encyN

3

3

p

ig. 4. Electrochemical impedance parameters (a) Nyquist, (b) Bode, (c) theta-frequaCl solution in the absence and presence of HPT.

. Results and discussion

.1. EIS measurement

Fig. 4a–c shows the Nyquist plots, Bode-modulus plots and thehase angle-frequency plots with and without HPT. The Nyquist

plots and (d) equivalent circuit used at an amplitude of 10 mV for J55 steel in 3.5%

diagram (Fig. 4a) contains depressed semicircles with the centerunder the real axis with one capacitive loop in the high frequency

(HF) zone, and one inductive loop in the lower frequency (LF) zone.The LF inductive loop (50 ppm and 100 ppm) may be a consequenceof the layer stabilization by-products of the corrosion reactionat the electrode involving inhibitor molecules and their reactive

344 A. Singh et al. / Applied Surface Science 356 (2015) 341–347

Table 1Electrochemical impedance parameters at an amplitude of 10 mV for J55 steel in 3.5% NaCl solution in the absence and presence of HPT.

Solution Rs

(� cm2)Rct

(� cm2)n Y0

(�−1 sn/cm2)� (s) Cdl

(�F cm2)L(H cm2)

Chi-square � (%) Surfacecoverage (�)

3.5% NaCl 2.5 137 0.829 292 465.5 80.6 – 0.0003 – –HPT 50 ppm 2.7 737 0.832 139 154.1 50.4 128 0.0009 81 0.81HPT 100 ppm 3.7 881 0.851 104 124.8 42.6 104 0.0007 84 0.84HPT 200 ppm 2.8 1319 0.855 99 52.9 45.8 – 0.0017 90 0.90HPT 400 ppm 1.5 1907 0.871 74 19.9 42.0 – 0.0015 93 0.93

Table 2The slopes of the Bode impedance magnitude plots at intermediate frequencies (S)and the maximum phase angles (˛) for J55 steel in CO2 saturated 3.5% NaCl solutionin the absence and presence of HPT.

C (ppm) −S −˛◦

3.5% NaCl 0.491 32.7HPT 50 ppm 0.512 58.5HPT 100 ppm 0.634 67.7

p2fp

s(Tebs

ce

C

wc

�

toffc[4A−alT5aael

3

t

-0.3-0.4-0.5-0.6-0.7-0.8-0.9-1.0-1.1

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

log I (A

cm

-2)

E ( V/ SCE)

3.5% Na Cl 50 pp m HPT 100 pp m HPT 200 pp m HPT 400 pp m HPT

tion/desorption process. This equation can be rearranged to

HPT 200 ppm 0.645 70.7HPT 400 ppm 0.733 72.9

roducts [16]. The LF inductive loop was absent for 100 ppm and00 ppm inhibitor concentration which may be attributed to theormation of a protective film causing hindrance in the dissolutionrocess [17].

The model of equivalent circuit used to fit the curves consists ofolution resistance (Rs), charge transfer resistance (Rct), inductanceL) and constant phase element (CPE) as shown in Fig. 4d [18,19].he presence of inductance in the impedance spectra in the pres-nce of investigated inhibitors indicated that iron is still dissolvedy the direct charge transfer at the inhibitor adsorbed electrodeurface [20].

In Table 1, the double-layer capacitance (Cdl) values can be cal-ulated from CPE parameter values Y and n using the followingquation [21]:

dl = Yωn−1

sin(n(�/2))(3)

here ω is the angular frequency, i.e., 2�fmax. The relaxation timeonstants (�) can be given according to dielectric theory:

= 12�fmax

(4)

Table 1 shows the calculated values of � and Chi-square valuehat supports the quality of the equivalent circuit used. At 400 ppmf inhibitor concentration, the interface parameter (�) decreasedrom 465.5 s to 19.9 s while the capacitance (Cdl) value decreasingrom 80.6 to 42.0 �F cm−2 signifies that the charge and dis-harge rates to the metal–solution interfaces are greatly decreased22–24,3]. The best inhibition efficiency of 93% was observed at00 ppm of HPT with Rct value of 1907 � cm2 in 3.5% NaCl solution.

linear relationship between log |Z| against log f, with slope near1 and the phase angle −90◦, can be observed in the intermedi-te frequency region. The analyses of linear relationship betweenog |Z| versus log f give a slope value between 0.491 and 0.733 [25].he phase angle for 3.5% NaCl (−32.7◦), HPT 25 ppm (−58.5◦), HPT0 ppm (−67.7◦), HPT 100 ppm (−70.7◦), and HPT 400 ppm (−72.9◦)pproaching −80◦ has been observed as shown in Table 2. The phasengle plots show the plots approaching toward −90◦ in the pres-nce of HPT that could be attributed to the formation of a protectiveayer on electrode surface [20,26–28].

.2. Polarization measurements

It is clear from Fig. 5 that the presence of inhibitor decreaseshe corrosion rate. It is observed that HPT inhibits both the

Fig. 5. Polarization curves for J55 steel in 3.5% NaCl at a scan rate of 1 mV s−1 in theabsence and presence of HPT.

cathodic hydrogen evolution and anodic metal dissolution reac-tions [29–31]. Potentiodynamic parameters of corrosion currentdensity (Icorr), corrosion potential (Ecorr), cathodic and anodic Tafelslopes (bc, ba) and inhibition efficiency (�%) are given in Table 3.

The variation in the values of −bc and ba in the presence ofHPT indicates that both the anodic metal dissolution and cathodichydrogen evaluation processes are inhibited. The mitigation ofthese processes was better with increase in HPT concentration.In the presence of HPT, a displacement in the values of Ecorr

to 62 mV, suggests that HPT act as mixed type of inhibitor[32]. The value of Icorr decreases with increasing HPT concen-tration from 0.93 mA cm−2 to 0.07 mA cm−2 and the inhibitionefficiency increases up to 92%. Corrosion current density (Icorr)values decrease due to the adsorption of the inhibitor moleculesthereby blocking the active sites on the J55 steel surface. In thisway, the surface area available for H+ ions was decreased whilethe actual reaction mechanism remains unaffected [33]. The Tafelpolarization data also confirms the impedance results about theinhibitor action of HPT.

3.3. Adsorption characteristics of the inhibitor

In order to confirm the adsorption of HPT and to explainthe inhibitor/metal interactions on the J55 steel surface adsorp-tion isotherms were studied. The results obtained from both theTafel and impedance studies were fitted into different adsorptionisotherms. Adsorption of HPT was found to obey the Langmuiradsorption isotherm [34]. Degree of surface coverage (�) is relatedwith adsorbate in the bulk of electrolyte (Cinh) according to theequation given below:

� = KadsCinh1+KadsCinh

Langmuir isotherm (5)

where Kads is the equilibrium constant for the adsorp-

[35]:

Cinh

�= 1

Kads+ Cinh (6)

A. Singh et al. / Applied Surface Science 356 (2015) 341–347 345

Table 3Polarization parameters for J55 steel in 3.5% NaCl at a scan rate of 1 mV s−1 in the absence and presence of different concentrations of HPT.

Conc. (ppm) Tafel data

Ecorr (V vs. SCE) Icorr (mA cm−2) ba (mV d−1) −bc (mV d−1) � (%) Surfacecoverage (�)

3.5% NaCl −0.739 0.93 63 119 – –HPT 50 ppm −0.801 0.32 51 94 66 0.66HPT 100 ppm −0.751 0.26 38 98 72 0.72HPT 200 ppm −0.765 0.14 26 81 85 0.85HPT 400 ppm −0.744 0.07 22 76 92 0.92

(b)

0.00150.00120.00090.00060.00030.0000

0.0002

0.0004

0.0006

0.0008

0.0010

0.0012

0.0014

0.0016

0.0018

Cin

h(M) /

θθCinh (M)

Langmu ir Adso rption Is othe rm Linear Fit Resu lt

R2 Val ue = 0.99 936

(a)

0.00150.00120.00090.00060.00030.0000

0.000 2

0.000 4

0.000 6

0.000 8

0.001 0

0.001 2

0.001 4

0.001 6

0.001 8

Cin

h(M) /

θθ

Cinh (M)

La ngmu ir Adso rpti on Is other m Linear Fit Re sult

R2 V alu e = 0.9 9991

Fig. 6. Langmuir adsorption isotherm plots for (a) EIS

Table 4Thermodynamic parameters for the adsorption of HPT on J55 steel.

Concentration (M) EIS Tafel

Kads (M−1) �G◦ads

(kJ mol−1)Kads (M−1) �G◦

ads

(kJ mol−1)

tl0Lb

�

weTot−aw

3

o[spa[

3.5% NaCl – – – –400 ppm HPT 7705 −32.1 7705 −32.1

Langmuir adsorption isotherm plots were obtained by plottinghe values of Cinh/� versus values of Cinh (Fig. 6a and b). The obtainedinear-fit plots of HPT with correlation coefficient (R2) ranging.99991 for EIS and 0.99936 for Tafel polarization suggested thatangmuir adsorption was obeyed by the inhibitor [36,37]. Kads cane related to free energy of adsorption, �Gads as [38]:

G◦ads = −RT ln(55.5Kads) (7)

here the value 55.5 is the concentration of water in solutionxpressed in mol L−1. The values of Kads and �G◦

ads are given inable 4. The negative values of �G◦

ads ensure the spontaneityf the adsorption process and stability of the adsorbed film overhe steel surface [39]. The calculated value of �G◦

ads for HPT is32.1 kJ mol−1 (Table 4), which probably means that both physicaldsorption and chemical adsorption (comprehensive adsorption)ould take place [40].

.4. Scanning electrochemical microscopy (SECM)

The SECM tests were performed in AC-Amperometry mode tobtain the color mapping and 3-D figures of the J55 metal surface41–44]. Fig. 7a–h shows the x-axis and y-axis images of the metal

urface as visualized by scanning electrochemical microscope. Arobe approach curve test was performed on both x-axis and y-xis to confirm that the tip was in the vicinity of the metal surface45–47].

and (b) Tafel at different concentrations of HPT.

The color mapping figure for 3.5% NaCl solution showed varia-tions in color due to rapid change in the current when the tip wasclose to the metal surface as shown in Fig. 7a and b. The color waschanged slightly in the case of HPT inhibitor on both x and y axesas shown in Fig. 7e and f. The color mapping figures hints that thesurface is less corroded in the presence of HPT while rough in itsabsence. A lower current is observed when the tip of the probeis brought near the metal surface with HPT (insulating surface).This may be ascribed to the insulating film that blocks the diffu-sion of oxygen toward the tip as shown in Fig. 7g and h [48–51]. Onthe other hand, the current increases when the tip of the probe isbrought near the metal surface without inhibitor (conducting sur-face). This may be accounted to the presence of redox mediator thatrevived at the surface as shown in Fig. 7c and d [52]. The J55 steelsurface remains conductive devoid of HPT inhibitor, and insulatingin existence of HPT inhibitor, that can be confirmed by the enhance-ment in the current (conducting) and by reduction in the current(insulating) [53].

4. Mechanism of corrosion mitigation

When CO2 gas is passed in NaCl solution, it reacts with watermolecules and forms carbonic acid. In this acidic solution steelsurface becomes positively charged (Fig. 8). So, initially Cl− ionsmay get adsorbed on positively charged J55 steel surface. Thisacidic environment causes protonation of inhibitor molecules.This protonated inhibitor then interact with the Cl− and getadsorb through electrostatic interactions (physical adsorption),by forming a protective layer (FeCl− inhibitor+)ads [54,55]. Alsothe inhibitor molecule contains lone pair of electrons on het-

eroatoms which are donated to vacant 3d-orbitals of iron atoms(chemical adsorption). In order to make a strong adsorption layerfilled orbitals of iron atoms gives their electrons to the inhibitormolecules via reterodonation.

346 A. Singh et al. / Applied Surface Science 356 (2015) 341–347

Fig. 7. SECM figures for (a) 3.5% NaCl x axis, (b) 3.5% NaCl y axis, (c) 3.5% NaCl x axis 3D, (d) 3.5% NaCl y axis 3D, (e) 400 ppm HPT x axis, (f) 400 ppm HPT y axis, (g) 400 ppmHPT x axis 3D, and (h) 400 ppm HPT y axis 3D.

A. Singh et al. / Applied Surface Sc

FN

5

1

2

3

4

A

S

R

[

[[

[[

[

[

[[[

[

[[

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[

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[[

[[

[

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[[[[

[[

[

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[[

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[

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ig. 8. Mechanism of corrosion mitigation in the presence of HPT inhibitor in 3.5%aCl solution saturated with CO2.

. Conclusions

. HPT shows an inhibition efficiency of 93% for electrochemicalimpedance spectroscopy and 92% for Tafel polarization.

. Addition of HPT increased the Rct values (137–1907 � cm2) anddecreased the Cdl values (80.6 �F cm2 to 42.0 �F cm2).

. Tafel polarization curves (62 mV) indicated that HPT acted asmixed type inhibitor.

. The SECM analyses showed that the J55 steel surface was pro-tected in the presence of HPT.

cknowledgement

Authors are thankful for the project fund from National Naturalcience Foundation of China (No. 51274170).

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