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An Exploration about the Interaction of Mild Steel with Hydrochloric Acid in the Presence of N-(Benzo[d] Thiazole-2-yl)-1-Phenylethan-1-Imines

August 2019
The Journal of Physical Chemistry C 123(37)

August 2019
123(37)

DOI:10.1021/acs.jpcc.9b03994

Authors:

Bhawna Chugh

Netaji Subhas Institute of Technology

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Cost effective inhibition of mild steel corrosion employing eco-friendly materials is a high priority subject for the industries, these days. Therefore, in this work, a series of N-(benzo[d]thiazole-2-yl)-1-phenylethan-1-imines(BTPEI, BTCPEI, BTTEI and, BTPIA)for mild steel protection in 1 M HCl were explored by adopting methods like gravimetric, adsorption isotherms, and the electrochemical methods, for example, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation. Different spectral characterization such as FTIR and 1H NMR were used to confirm the synthesized derivatives. The experimental results affirmed that all the inhibitors are proven to be very efficient for combating corrosion for mild steel in acidic media. The determined thermodynamic and activation parameters were used to provide a further understanding of the mechanism of inhibitive activity. The computed Gibbs free energy values suggested strong chemical interaction of inhibitors with the mild steel surface, thereby supporting chemisorption. Potentiodynamic polarization data explained the mixed type nature of all the inhibitors. Impedance measurements point out that protective film deposits on the mild steel in the presence of the inhibitors. Studies like scanning electron microscope (SEM) with electron dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) further supported the adsorption inhibitive mechanism. The theoretical findings such as density functional theory (DFT), Fukui indices and molecular dynamics (MD) provided good agreement with different experimental techniques.

MD parameters for neutral and protonated compounds adsorbed on the Fe (110) surface in presence of solvent species

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

An Exploration about the Interaction of Mild Steel with Hydrochloric Acid

in the Presence of N-(Benzo[d] Thiazole-2-yl)-1-Phenylethan-1-Imines

Bhawna Chugh, Ashish Kumar Singh, Sanjeeve Thakur, Balaram Pani,

Ajit Kumar Pandey, Hassane Lgaz, Ill-Min Chung, and Eno E. Ebenso

J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 19 Aug 2019

Downloaded from pubs.acs.org on August 19, 2019

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An Exploration about the Interaction of Mild Steel with Hydrochloric Acid in

the Presence of N-(Benzo[d] Thiazole-2-yl)-1-Phenylethan-1-Imines

Bhawna Chugha, †, Ashish Kumar Singhb, *, †, Sanjeeve Thakura, Balaram Panic, Ajit Kumar

Pandeya, Hassane Lgazd, *, Ill-Min Chungd, *, Eno E. Ebensoe

aDepartment of Chemistry, Netaji Subhas University of Technology (formerly Netaji Subhas

Institute of Technology), New Delhi-110078, India

bDepartment of Applied Science, Bharati Vidyapeeth College of Engineering, New Delhi-110063,

India

cDepartment of Chemistry, Bhaskaracharya College of Applied Science, University of Delhi, New

Delhi-110078, India

dDepartment of Crop Science, College of Sanghur Life Science, Konkuk University, Seoul 05029,

South Korea

eMaterial Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and

Agricultural Sciences, North-West University, Private Bag X2046, Mmabatho 2735, South Africa

† First author

*Corresponding author

Email: ashish.singh.rs.apc@itbhu.ac.in

hlgaz@konkuk.ac.kr

imcim@konkuk.ac.kr

Tel: +91-9560285447

Abstract

Cost effective inhibition of mild steel corrosion employing eco-friendly materials is a

high priority subject for the industries, these days. Therefore, in this work, a series of N-

(benzo[d]thiazole-2-yl)-1-phenylethan-1-imines(BTPEI, BTCPEI, BTTEI and, BTPIA)for

mild steel protection in 1M HCl were explored by adopting methods like gravimetric,

adsorption isotherms, and the electrochemical methods, for example, electrochemical

impedance spectroscopy (EIS) and potentiodynamic polarisation. Different spectral

characterization such as FTIR and 1H NMR were used to confirm the synthesized

derivatives. The experimental results affirmed that all the inhibitors are proven to be very

efficient for combating corrosion for mild steel in acidic media. The determined

thermodynamic and activation parameters were used to provide a further understanding of the

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mechanism of inhibitive activity. The computed Gibbs free energy values suggested strong

chemical interaction of inhibitors with the mild steel surface, thereby supporting

chemisorption. Potentiodynamic polarization data explained the mixed type nature of all the

inhibitors. Impedance measurements point out that protective film deposits on the mild steel

in the presence of the inhibitors. Studies like scanning electron microscope (SEM) with

electron dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM) and x-ray

photoelectron spectroscopy (XPS) further supported the adsorption inhibitive mechanism.

The theoretical findings such as density functional theory (DFT), Fukui indices and

molecular dynamics (MD) provided good agreement with different experimental techniques.

1. Introduction

The economic loss and safety concern due to the destruction in metallic structure led

corrosion scientists to study corrosion and its protection measures that contribute to the

reduction of corrosion rate. Various acids like hydrochloric acid, sulphuric acid, and

phosphoric acid are used for pickling and acidization processes by the pickling industry.

These acids used for such pickling processes may cause corrosion and damage to the material

used1, 2.

Mild steel, an alloy of iron having a very low amount of carbon, is so versatile, easy

to use and cost-effective material. The low carbon content provides good mechanical strength

and high resistance, thus it has gained huge importance in the construction industry3, 4.

However, one of the major problems with mild steel is that it is easily susceptible to

corrosion on exposure to various aggressive environments. Thus, in view of all these, there

has been a demand to search such methods that help in solving the big problem of corrosion.

From the viewpoint of cost-effectiveness and ease, the development of corrosion inhibitors is

considered primarily the major and desirable approach nowadays5, 6. A study of the literature

affirms that numerous organic inhibitors have been studied to explain the corrosion inhibitive

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action of mild steel in acidic media. Also, the development of nontoxic as well as eco-

friendly inhibitor is predominantly favoured those are less harmful to the environment. Thus,

the use of organic inhibitors especially the green inhibitors as one of the promising materials

in different types of corrosion for protecting the mild steel surface in the acidic medium has

been reported by many researchers7–9. The anti-corrosion action of organic inhibitors is due to

its ability to inhibit cathodic and/or anodic reactions taking place over the metal electrode

surface that lowers metal dissolution10. The structure of an inhibitor also provides significant

importance in the inhibition mechanism11. Presence of heteroatoms viz. O, N, S and, P

facilitates the adsorption phenomenon of the inhibitor over the metallic surface and thereby

leaving fewer surfaces for aggressive species to attack and cause corrosion12, 13. Organic

moieties like benzothiazoles have been widely known for its extensive pharmacological and

biological properties viz. anti-tubercular 14,15, anti-microbial16,17, anti-malarial18, anti-

convulsant19, analgesic20,21, anti-inﬂammatory22, anti-diabetic23 ,anti-tumour 24etc. The

different properties, as well as the widespread use of benzothiazoles, promote itself for

designing and synthesizing its new derivatives.

Thus, it led us to investigate the corrosion inhibition property of compounds derived from

2-amino benzothiazole. This study reports synthesis and investigation of corrosion inhibiting

action of four 2-amino benzothiazole derivatives, N-(benzo[d]thiazole-2-yl)-1-phenylethan-1-

imine (BTPEI), N-(benzo[d]thiazole-2-yl)-1-(3-chlorophenyl) ethan-1-imine (BTCPEI), N-

(benzo[d]thiazole-2-yl)-1-(m-tolyl) ethan-1-imine (BTTEI) and, N-(benzo[d]thiazole-2-

ylimino) ethyl) aniline (BTPIA), obtained from the condensation of 2-amino benzothiazole

with acetophenone, 3-chloroacetophenone, 3-methyl acetophenone and 3-amino

acetophenone, respectively. The comparison of the anti-corrosion performance of different

synthesized inhibitors was tested using various methods viz. gravimetric, electrochemical

impedance spectroscopy (EIS) and potentiodynamic polarization method. Different surface

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characterizations e.g. scanning Electron Microscopy with energy dispersive x-ray (EDX),

atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) were employed

to explicate the adsorption process on the metal surface. In addition, the theoretical analyses

were also done that comprise density functional theory (DFT), Fukui indices and molecular

dynamics (MD) and it gave the similar trend of results as achieved from various experimental

methods.

2. Experimental

2.1 Materials & solutions

Mild steel(MS) strips with chemical composition(% by weight) of C(0.05),Si(0.009),

Mn(0.20), S(<0.01), P(0.012), Ni(0.0025), Cr(0.001) and Fe(remainder) were employed for

all the corrosion studies. Before any experiment, mild steel strip was primarily abraded with

emery paper of varied grades, cleaned using distilled water and degreased in acetone and then

eventually stored in desiccators for using those for further studies. The MS having

dimensions 2.5 × 2.0 × 0.064 cm3wereutilised for performing weight loss experiments and

morphological studies while the MS having 7.5 cm length and 1 × 1 cm2 exposed surface area

was utilized for undergoing electrochemical experiments.

The desired acid concentration was achieved by diluting 37% HCl (Merck, AR grade)

using distilled water. Other chemicals required like 2-aminobenzothiazole, substituted

acetophenone compounds were procured from Sigma-Aldrich and Spectrochem (India). The

stock solution was prepared in the 9:1ratio (1M HCl-ethanol) to ensure the solubility of

inhibitors and subsequently diluted to prepare different concentrations of inhibitors in the

solution of 1M HCl.

2.2 Synthesis of inhibitors

Benzothiazole derivatives viz. BTPEI, BTCPEI, BTTEI and, BTPIA were synthesized

by dissolving 1.50 g (0.01 M) of 2-amino benzothiazole in 20 ml ethanol in a round bottom

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flask and mixed with 1.20 g (0.01 M) of acetophenone, 1.35 g (0.01 M) of o-

aminoacetophenone, 1.34 g (0.01 M) of o-methylacetophenone, 1.54 g (0.01 M) of o-chloro

acetophenone prepared in ethanol (20 ml). The solution was allowed to reflux for 6 h at high

temperature using glacial acetic acid as a catalyst. Then, the final solution was left

undisturbed overnight and the precipitate was filtered out and kept for drying. Warm ethanol

was used for recrystallization of inhibitors to obtain highly purified products.

2.3 Gravimetric analysis

Gravimetric studies were conducted by weighing completely dried MS strips initially

without immersing into the solution and after immersion in 1M HCl solution with and

without varied amount ranging from 1 to 6mgL-1 of BTPEI, BTCPEI, BTTEI and, BTPIA at

temperatures ranging 308 to 338 K for 3h. The temperature was controlled using digital

thermostat (Macwin India Ltd). The Inhibition efficiency, surface coverage, and corrosion

rate were estimated by following formulas as:

(1)

o i

% 100

w w



 

(2)

o i

100

w w





 

(3)

87.6

(mmy ) w

CAtD



where, wi is weight loss with inhibitor while w0 is the weight loss without inhibitors.

2.4 Electrochemical studies

The CHI electrochemical workstation (Model: CHI-760) was employed for carrying

out all the electrochemical measurements using a corrosion cell comprising of three electrode

assembly. Mild steel can be used as a working electrode, platinum as a counter electrode and

saturated calomel electrode (SCE) as a reference electrode. In order to maintain stable open

circuit potential (OCP), the system was left undisturbed for 30 minutes before each

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experiment. The temperature during the experiment was maintained at 308 K. The frequency

range used in EIS measurements lies within100 kHz to 0.01 Hz and has an amplitude value of

10 mV peak to peak with an AC signal at Ecorr. CHI 760c software was utilized to interpret all

the obtained data and to find out all the required parameters.

(4)

i o

p p

EIS i

% 100

R R



 

where, and represents polarization resistance with and without inhibitors.

Potentiodynamic polarization plots can be obtained with the potential ranging from -

250 to +250 mV (SCE) and having a scan rate of 1mVs-1. The inhibiting potential of various

inhibitors was calculated from equation (5) as follows:

(5)

o i

corr corr

PDP o

corr

% 100

i i



 

where, and represents the corrosion current density in absence of any inhibitor and

corr

presence of inhibitors respectively.

2.5 Surface characterisation

2.5.1 SEM

Surface morphological images of MS strips with and without various inhibitors in 1M

HCl were achieved by using a table top Scanning electron microscope (Model-Hitachi TM

3000), where the voltage applied was around 15 kV. For SEM analysis, MS strips were

immersed in various solutions with 6 mg L-1of all inhibitors separately for time 3 h. After an

interval of 3 h, these MS strips were taken out from the solution and further scanned.

2.5.2 EDX

The detection of elemental distribution in the film, obtained due to adsorbed

inhibitors, was investigated using electron dispersive spectroscopy (OXFORD SWIFT ED

3000) by giving voltage 15 KV.



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2.5.3 AFM

The effect of the addition of benzothiazole compounds in the solution having MS was

well understood from atomic force micrographs. Prior to the experiment, the MS samples

were prepared by dipping in 1 M HCl solutions with 6 mg L-1 of various inhibitors for time

period 3 h. This study was performed using an atomic force microscope; Model Bruker

(Dimension ICON with scan Asyst) by conducting tapping mode. Also, all the outcomes

were analyzed using Nanoscope Analysis software 8.2.

2.5.4 XPS

The X-Ray Photoelectron spectroscopy is one of the informative tools for detecting

the elemental distribution of the compounds. Multiprobe Surface Analysis instrument

(Scienta Omicron, Germany) by applying pressure of 5 × 10 -11Torr was used for detection.

The survey scan results were captured for the surface of MS coupon having dimension 1 × 1

cm2comprising of6 mg L-1BTPIA which shows maximum inhibition efficiency among all the

investigated inhibitors and further analyzed with individual reading counts vs. energy.

2.6 Theoretical study

2.6.1 Quantum chemical calculations

Quantum chemical parameters and Fukui functions were computed with module namely

DMol3 (quantum mechanical code in DFT approximation using Materials Studio Software)

25, 26. Geometry optimization and Fukui function estimations were conducted on the basis of

first-principle generalized gradient approximation (GGA) method of DFT in the method

suggested by Perdew, Burke, and Ernzerhof (PBE) 27. The COSMO implicit solvent model

was implied for performing all calculations28. Within the framework of Koopmans' theorem,

chemical reactivity descriptors can be estimated by following equations29:

(6)

HOMO

I E 

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(7)

LUMO

A E 

Where, I and A denotes the ionization potential and the electron affinity respectively, while

EHOMO and ELUMO signify highest occupied and the lowest unoccupied molecular orbitals

energies. The following equations were employed to determine the absolute electronegativity

( ) and the absolute hardness ( ):





I A







(9)

I A







The fraction of transferred electrons (ΔN) was calculated from following equation1:

inh

2( )

 



 

  

(10)

The work function (ϕ) of the Fe (110) was generally known to be 4.82 eV while the absolute

hardness of iron was estimated as 0 since I = A for bulk metals31, 32.

Yang and Mortier2 introduced the finite difference approximations from which Fukui

functions for nucleophilic and electrophilic attacks can be estimated. The dual descriptor

which is a local reactivity descriptor (LRD) proposed by Morell et al.3 were also calculated to

get more informative insights about the active sites that are favorable for nucleophilic and

electrophilic attacks. The condensed Fukui functions and the dual descriptor were estimated

on the basis of Hirschfeld Population Analysis (HPA) as follows4:

k k k

( +1) ( )f q N q N 

For nucleophilic attack (11)

k k k

( ) ( 1)f q N q N  

For electrophilic attack (12)

k k

(k)f f f 

  

Dual descriptor (13)

where,

signifies electronic population of an atomic site within a molecule in its neutral (N),

anionic (N+1) or cationic (N-1) state.

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2.6.2 Molecular dynamic simulations

Molecular dynamics simulation (MDS) is one of the very popular probing technique

to understand the interactions on an atomic scale5. Herein, MDS was performed using the

Forcite module in the Materials studio package6. The Fe (110) plane was used as the iron

substrate layer due to its high stabilization energy and its highly packed structure7. The

solvent layer that contains water molecules (491), chlorine and hydronium ions (9) along with

an inhibitor molecule was collected with the iron layer in one simulation box

(24.82×24.82×35.69 Å3) and optimized by steepest descent and conjugated gradient

algorithms8. The COMPASS force field 9and the NVT canonical ensemble were used for all

simulations. The simulations were completed in a time step of 1 fs and simulation time of

2000 ps at 303 K, which is controlled using the Andersen algorithm10.When the system

reaches the equilibrium state, the values of interaction and the binding energies (

Binding interaction

E E 

)can be computed using subsequent equation11:

interaction total surface+solution inhibitor

( + )E E E E 

(14)

Where,

surface+solution

signifies the total energy possessed by Fe(110) and solution without

inhibitor molecule,

inhibitor

refers to the total energy of an inhibitor molecule alone while

total

denotes the overall energy of the full system.

3 Results & Discussion

3.1 Synthesis &Spectral analysis

The inhibitors were synthesized in reasonably good yield according to the synthetic scheme

shown in Fig. 1. The synthesized inhibitors were further characterized by different

spectroscopic techniques viz. FTIR, 1H NMR, 13C NMR and elemental analysis (CHN). The

1H NMR and 13C NMR spectra of the synthesized inhibitors are given as Fig S1& S2 in the

supporting information. The spectral data of the synthesized inhibitors are presented as:

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BTPEI

(Chemical Formula-C15H12N2S, Mol.wt-252.33); yield = 77%, M. P. = 108 oC

FT-IR (KBr, cm−1): 1641 (N=C), 1432 (NH, benzothiazole), 3394(NH stretching) and

2923(CH stretching).

1H NMR: δ 2.45 (3H, s, -CH3), 6.94-6.98 (2H, dd, J = 7.7, 7.5, 1.4 Hz, phenyl), 7.14-7.18

(2H, dd, J = 7.8, 7.4, 1.5, 0.4 Hz, benzothiazole), 7.28 (1H, dd, J = 7.4, 1.7, 1.5 Hz, phenyl),

7.30 (1H, dd, J = 8.0, 1.4, 0.5 Hz, benzothiazole), 7.44 (1H, dd, J = 7.5, 1.9, 0.5 Hz,

benzothiazole), 7.61 (2H, dd, J = 7.8, 1.6, 1.5, 0.4 Hz, phenyl).

13C- NMR (DMSO); δ (ppm):166.9(C=N); 153.2 (C-N); 131.4, 125.9, 121.3, 118.1(aromatic

and benzothiazole), 28.6 (CH3).

CHN: C (71.5%), H (4.8%) and N (11.1%)

BTPIA

(Chemical Formula-C15H13N3S, Mol.wt-267.35); yield = 81%, M. P. = 100 oC

FT-IR (KBr, cm−1): 1640 (N=C), 1448 (NH, benzothiazole), 3343 (NH stretching) and

2920(CH stretching).

1H NMR: δ 1.04(3H, s, CH3), 6.47 (1H, dd, J = 8.0, 1.2, 0.5 Hz, phenyl), 6.69-6.71 (2H,

dd, J = 7.7, 7.5, 1.2 Hz, phenyl), 6.96-6.97 (3H, 7.47 dd, J = 7.6, 1.4, 0.5 Hz, benzothiazole),

6.98 (1H, dd, J = 637.7, 1.2, 0.5 Hz, phenyl), 7.28-7.30 (1H, 7.46dd, J = 8.0, 7.5, 1.6 Hz,

benzothiazole), 7.0 (2H, dd, J = 7.7, 1.6, 0.5 Hz,-NH2).

13C- NMR (DMSO); δ (ppm): 167.0(C=N); 153.1 (C-N); 151.4, 134.6, 132.6, 131.3,

125.9, 121.3, 118.2, 117.3, 114.8(aromatic and benzothiazole), 28.3 (CH3)

CHN: C (67.4%), H (4.9%) and N (15.7%)

BTTEI

(Chemical Formula-C16H14N2S, Mol.wt-266.36); yield = 79% M.P. = 106 oC

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FTIR (KBr, cm−1): 1642 (N=C), 1443(NH, benzothiazole), 3395(NH stretching) and

2920(CH stretching).

1H NMR: δ 1.00-1.02(6H, m, 2-CH3), 6.94 (1H, dd, J = 7.4, 1.5, 0.4 Hz, phenyl), 6.94-7.18

(3H,dd, J = 8.0, 7.8, 1.6 Hz, benzothiazole), 7.28-7.30 (1H, td, J = 7.4, 1.6 Hz,

benzothiazole), 7.42-7.52 (2H, 7.89 dd, J = 8.0, 1.9, 0.5 Hz, phenyl), 8.12 (1H, dd, J = 8.0,

1.6, 0.5 Hz, phenyl).

13C- NMR (DMSO); δ (ppm): 167.0(C=N); 153.2 (C-N); 131.4, 125.9, 121.3, 118.2

(aromatic and benzothiazole), 32.2 (CH3; CH3-C=N), 20.4 (CH3 attached to benzene ring)

CHN: C (72.1%), H (5.3%) and N (10.5%)

BTCPEI

(Chemical Formula-C15H11ClN2S, Mol.wt-286.78); yield = 84%, M.P. = 105 oC

FTIR (KBr, cm−1): 1642 (N=C), 1443(NH, benzothiazole), 3394(NH stretching) and 3055

(CH stretching).

1H NMR: δ 1.02 (3H, s, -CH3), 6.95 (2H, dd, J = 7.7, 7.6, 1.6 Hz, 2-CH, phenyl), 6.97 (1H,

dd, J = 8.0, 7.4, 1.6 Hz, -CH, phenyl), 7.15-7.19 (2H, dd, J = 8.0, 1.1, 0.5 Hz, 2-

CH,benzothiazole), 7.28-7.30 (2H, dd, J = 8.0, 1.6, 0.5 Hz, 2-CH,benzothiazole), 7.69 (1H,

dd, J = 7.7, 1.6, 0.5 Hz, -CH, phenyl).

13C- NMR (DMSO); δ (ppm): 167.0(C=N); 152.8 (C-N); 131.2, 126.0, 121.4, 118.1(aromatic

and benzothiazole); 31.1 (CH3)

CHN: C (62.8%), H (3.9%) and N (9.76%)

3.2 Weight loss study

3.2.1 Effect of Inhibitor concentration

A gradual decrease in the mild steel weight loss was noticed on increasing the amount

of inhibitors at 308 K. Subsequently, inhibition efficiency is increased due to more adsorption

and surface coverage12. This interpreted that the inhibitors BTPEI, BTCPEI, BTTEI and,

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BTPIA offer potential anticorrosion effect for mild steel in 1 M HCl at concentration 6 mgL-

1. The best performance of BTPIA could be explained on the basis of inclusion of the electron

releasing free amino group. The corrosion rate trend attained was in the order of

BTCPEI>BTPEI > BTTEI > BTPIA as depicted in Fig.2a.

3.2.2 Comparative findings for potential of corrosion inhibition

The corrosion mitigating potential of the inhibitors, BTPEI, BTCPEI, BTTEI and,

BTPIA were further compared with anti-corrosion potential of some earlier studied thiazole

derivatives in different medium43-46.The comparative data has been provided in Table 1. The

concentrations of the different inhibitors used in this study were in the range of 1-6 mg L-1

while other thiazole derivatives studied earlier were used in the range of 50-455.53 mg L-1.

The concentrations used in this study were significantly low than that of reported thiazole

derivatives. The maximum corrosion inhibiting potential associated with BTPIA could be

related to the presence of electron releasing –NH2 group attached to benzene nucleus making

protonation stable and facilitated the charge transfer process.

3.2.3 Adsorption isotherm

The adsorption of inhibitors could be better understood by different adsorption

isotherms, i.e. Frumkin, Freundlich, Temkin, Langmuir isotherm137. Each one of these

isotherms was tried and it was affirmed that among all, Langmuir adsorption isotherm was

best fitted. The condition of the Langmuir isotherm is given as follows:

(15)

inh inh

ads



 

where, θ denotes surface coverage, Cinh as the concentration of inhibitor and Kads for the

equilibrium constant of adsorption-desorption process. The graph obtained by plotting Cinh/θ

vs. Cinh is shown in Fig. 2b. The intercept obtained from this graph can be used from the

determination of values of Gibb’s free energy according to the equation described below:

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(16)

ads ads

ln(55.5 )G RT K  

where, 55.5 is basically the concentration of water and describes the adsorption constant.

ads

Also, Table 1 presents the estimation of value the of adsorption constant and free energy of

adsorption that predicts the spontaneity of the adsorption process. The values of

ads

G

ads

G

< 20 kJ mol-1 imply complete electrostatic association while those with > 40 kJ mol-1

represents chemical interaction48-50. So, the present investigation involves the estimations of

which were found in the range - 40.9 to – 43.3 kJ mol-1which recommended the

ads

G

association of all the inhibitors with the mild steel surface via chemisorption.

The dimensionless separation factor, (An important parameter to discuss the nature

of adsorption) could be calculated using the value of adsorption constant ( ) as per

ads

equation (17) as:

(17)

ads inh

RK C



The value of decides adsorption process. The values of RL in the range 0 <RL< 1, RL>1

and RL=0 are the criteria for favourable, unfavourable and irreversible adsorption process,

respectively51.The values of RL for all the studied compounds were evaluated and found

below than 1 (listed in Table1 ) which indicated favourable adsorption of inhibitors at all

concentrations on the mild steel surface. Moreover, reduction in the RL values with

enhancement in inhibitor’s amount might have resulted from chemisorption, which is also

obvious from the evaluated values.

ads

G

3.2.3 Thermodynamic activation parameters

The corrosion of mild steel is affected by variation in temperature explained by

applying Arrhenius and transition state relations, i.e. 18 and 19 as described below:

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(18)

log log

2.303RT





 

(19)

* *

RT exp exp

Nh R RT

S H

C   

 

   

   

where, is the Activation energy, is pre-exponential factor, is universal gas constant, T



is absolute temperature, CR indicates corrosion rate, entropy of activation and is

S

H

enthalpy of activation, is Avogadro number and is Planck’s constant

All the thermodynamic and activation parameters for MS coupon in 1M HCl in the

absence and presence of various inhibitors were determined by plotting log CR vs. 1/T and

logCR/T vs. 1/T as shown in Fig.2c-d. All the desired parameters obtained from the curves are

given in Table 2. The activation energy value, is lower for inhibited solutions with

different inhibitors i.e. 30.1kJ mol-1 for BTPEI, 22.4kJ mol-1 for BTPIA, 25.5 kJ mol-1

BTTEI, 27.1 kJ mol-1 for BTCPEI than that for uninhibited solution (32.8 kJ mol-1). The

lower corrosion rate is favoured by higher activation energy and lower Arrhenius pre-

exponential factor (λ). In this study, the values of λ are lower for inhibited solutions as

compared to that of uninhibited solution, so, decrease in corrosion rate is determined by

Arrhenius pre-exponential factor52. The estimations of enthalpy of activation ( ) suggests

H

positive value for MS in presence and absence of inhibitors which refers the endothermic

nature for the formation of activated complex 14.On correlating the values of entropy of

activation ( ) for inhibited and uninhibited solutions, as recorded in Table 2, it could be

S

observed that the entropy of activation was decreased for inhibited solution. This might be as

a result of the decrease in disordering on going from reactant to activated complex15.

3.3 FT-IR Studies

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Recrystallized pure benzothiazole derivatives were characterized using FT-IR and

ATR mode. For ATR characterization, spectra were obtained by immersing coupons with all

inhibitors with 6 mgL-1 concentration in 1M HCl for 3h. Fig. 3 shows the IR spectra of pure

benzothiazole derivatives, i.e. BTPEI, BTCPEI, BTTEI and, BTPIA and the ATR spectra of

the MS coupons with and without different inhibitors. It could be easily visualized that some

peaks got disappeared in case of ATR mode. The (C=N) azomethine group stretching

frequency for pure BTPEI, BTCPEI, BTTEI and, BTPIA appeared at 1641, 1640, 1642, 1642

cm-1 respectively which confirmed the formation of inhibitors. On the other hand, IR-ATR

spectra show that all the peaks are shifted to lower frequency range. These observations

clearly indicated that the azomethine group of benzothiazole derivatives in BTPEI, BTCPEI,

and BTTEI and, BTPIA might be adsorbed on the metallic surface53. The peaks ascribing to

pure inhibitors are also prominent as described in ATR spectra of mild steel with adsorbed

inhibitors as shown in Fig.3 but with a slight decrease in frequency which might be due to

redistribution of electrons between metal d orbitals and inhibitor moiety.

3.4 Electrochemical measurements

3.4.1 Open circuit potential vs. time

The open circuit potential (OCP) of a material predicts its thermodynamic tendency to

be electrochemically oxidized in a corrosive medium54.The mild steel working electrode was

immersed in different aggressive HCl solutions with different concentrations of inhibitors for

30 minutes and its OCP was measured as a function of time up to 30 min before each EIS and

Tafel run. After this time, a steady-state OCP, corresponding to corrosion potential (Ecorr) of

working electrode, was obtained. Fig. 4 represents the OCP curves of MS samples in HCl

solution with different concentrations of inhibitors. Open circuit potential (OCP) changes

often helpful to predict which reaction, cathodic or anodic, is more affected. Fig. 4 shows that

the OCP of the mild steel shifted towards more negative value by the presence of inhibitors

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which confirms that the inhibitors influenced the cathodic reaction under open circuit

condition.

3.4.2 Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy is considered a very useful strategy in the

corrosion field which is used to determine the corrosion rates occurring in steel/solution

interface. The fundamental of inhibition is well understood from the capacitive and resistive

behaviour of the metal surface. The Nyquist graphs of mild steel in 1 M HCl with and

without all the four inhibitors can be represented in Fig. 5a-d. In the Nyquist plots, an

imaginary impedance component (Z″) at y-axis is drawn with respect to the real impedance

component (Z′) at x-axis. Corrosion inhibition by an inhibitor could be explained using

Nyquist plots by the increase in the diameter of the semicircle, due to up gradation in

resistivity16.All the obtained EIS parameters are represented in Table 3.

The Nyquist plots attained in bare HCl solution as well as with lower concentration of

all the inhibitors consist of the high-frequency capacitive loop followed by a low-frequency

inductive loop. The EIS data obtained for the blank acid solution as well as with initial two

concentrations of studied inhibitors fit with the equivalent circuit presented as Fig 6a. The

time constant of the charge transfer process and double layer capacitance accounted for high-

frequency capacitive loop while relaxation process due to adsorbed of species viz. and

ads

on the working electrode surface contributed for the formation of a low-frequency

ads

inductive loop. The shape of the capacitive loop looks depressed semicircle55 instead of a

perfect semicircle and this depression of the capacitive loop is due to inhomogeneity of

electrode surface. The regular increase in the value of n which measures surface

heterogeneity suggests the increase in homogeneity of the surface due to adsorption of

inhibitors.

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The equivalent circuit presented as Fig 6a consists of double layer capacitance (Cdl)

and charge transfer resistance (Rct) arranged to parallel with each other which is subsequently

connected in series with a parallel combination of inductance (L) and resistance due to

inductance (RL). The presence of inductance (L) with initial two concentrations, i.e. 1 & 2 mg

L-1of the inhibitors confirms the continuous dissolution of mild steel via charge transfer

phenomenon which is confirmed from the achieved inhibition efficiency values (given in

Table 3) of the inhibitors at their lower concentrations. Also, an enhancement in charge

transfer resistance values is seen with the addition of inhibitor, thus suggesting effective

resistance for corrosion.

The relation of CPE magnitude (Y0) is related to impedance as given in following equation:

(20)

CPE n

( )

ZY j





where, ω denotes angular frequency and j and n are imaginary number and phase shift that

describes deviation from ideal capacitance behaviour due to roughness and inhomogeneity of

the metal surface. CPE explains different components in the circuit on the basis of n values.

i.e., resistance (n = 0, A = R), capacitance (n=1, A = C), inductance (n = -1, A = L) or

Warburg impedance (n = 0.5, A).More value of exponent n implies smoothness and high

surface homogeneity, whereas smaller value of n suggests surface porosity that permits ions

or solvent17.

Double layer capacitance values are computed by using the followingequation:

(21)

1 n n

dl 0 ct

( )C Y R 



The values of Rct and Cdl obtained for different concentrations of inhibitors trend opposite to

each other which is associated with the fact that inhibitors adsorbed more effectively with

their increasing concentrations. Similar results were obtained by other researchers57.

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However, the EIS data obtained with higher concentrations of inhibitors (4 & 6 mg L-

1) were not accurately fitted with the equivalent circuit represented in Fig 6a. In order to

attain more appropriate fit for all these experimental data, another equivalent circuit (given as

Fig. 6b) was used. This circuit comprises of charge transfer resistance (Rct) and double layer

capacitance, (capacitance at higher frequency) which are arranged in series to a parallel

combination of Rf (resistance due to adsorbed inhibitor’s film) and Ca(capacitance due to

adsorption of inhibitor’s film). The ideal fitting of EIS data with the proposed equivalent

circuits is shown in Fig. 6c-d (Nyquist plot of mild steel with 1 mg L-1 and 6 mg L-1 of

BTPIA). Inhibition efficiency trend was same as that obtained from gravimetric

measurements i.e. BTCPEI< BTPEI < BTTEI < BTPIA. The maximum inhibition efficiency

of BTPIA could be attributed due to free amino group.

Fig. 7a-ddepictsBode-phase graphs for both uninhibited as well as inhibited solutions.

Bode plots are the graphs plotted between impedance │Z│and log of frequency while phase

vs. log of frequency. These are frequency dependent plots which provide information related

to behaviour of the protective film formed on the electrode as a result of inhibitor adsorption.

The deviation of maxima of phase angle from ideal 90o could be explained on the basis of

frequency dispersion or inhomogeneous surface that might be as a result of surface roughness

and porous layer obtained because of adsorption of the inhibitors. Therefore, it could be seen

that the more impedance value describes the difficulty in the electrode reaction while the

more negative value of phase angle suggested a smoother surface that implies effective

adsorption of inhibitors over the surface of mild steel18.

3.4.3 Potentiodynamic Polarisation study

Fig. 8a-d illustrates the potentiodynamic polarisation curves for the mild steel in absence

and presence of varying concentration of all inhibitors. All the related corrosion parameters

were determined using the extrapolation of Tafel plots such as corrosion current density

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(icorr), corrosion potential (Ecorr), cathodic slope (



c) and anodic slopes ((



a) and

corresponding inhibition efficiency (EPDP) values obtained using equation 5 and are tabulated

in Table 4.It could be seen from the data that corrosion current density (icorr) values are

decreased for inhibited system and subsequently with the increase in the inhibitor

concentration for each inhibitor. It suggested that the inhibitor results in reduction of anodic

dissolution of mild steel and also it promotes the retardation of the hydrogen evolution

mechanism.

This result is due to the adsorption of inhibitor molecules on the active sites of metal

surface19.Moreover, there is no significant change in values of inhibited system with

corr

reference to blank and maximum displacement in value was< 85 mV suggesting these

corr

benzothiazole compounds as mixed type inhibitors20,21.The Table 4 shows that the inhibitors,

BTCPEI, BTPEI, BTTEI and BTPIA markedly affected both cathodic as well as anodic

reactions as more pronounced change was observed in the values of βc compared to that of βa.

This finding suggested that all the inhibitors are mixed type, predominantly cathodic

inhibitors61.

All the cathodic polarization curves resulted in to parallel Tafel lines which confirmed

the activation controlled hydrogen evolution reaction. Anodic polarization curves with 6 mg

L-1 BTCPEI, 4 & 6 mg L-1 of BTPEI, 1 & 6 mg L-1 of BTTEI and 1 & 6 mg L-1 of BTPIA

showed abruptly reduction of corrosion current beyond 336, 356, 340 and 330 mV corrosion

potential. This is due to passivation of mild steel surface with that particular concentration of

inhibitor at respective corrosion potential.

3.5 Morphological studies

3.5.1SEM-EDX

The effect of the presence of inhibitors to the acid solution on morphological

appearance of mild steel can be seen by scanning electron micrographs of different mild steel

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samples immersed in acid solution in absence and presence of inhibitors. Fig. 9a (MS before

immersion in acid solution) reflects a bright and clear surface of mild steel due to corrosion

free surface. The EDX analysis however confirms the formation of oxide layer when MS gets

immersed in acid solution (Fig. 9b). The time interval between sample preparation and their

analysis is attributed to formation of oxide layer because MS samples get exposed to the

atmosphere in that time interval. Fig 9b represents surface morphology of MS coupon in 1M

HCl which has a highly corroded surface with splits and cavity because of the corrosion

process, therefore it elucidates that the MS surface is highly damaged by the corrosion

occurred due to highly corrosive medium. The interaction of mild steel with the Cl- ions

results in to formation of iron-chloride and subsequently formation of iron-oxides. However,

presence of the inhibitors prevented the MS surface from corrosion as shown by much

smoother surface of mild steel presented as Fig 9c-f.This demonstrates the formation of

defensive film of the inhibitor on the surface which prevents corrosion phenomenon.

Application of inhibitors, BTPEI, BTCPEI, BTTEI and, BTPIA, cover the MS surface

efficiently and thereby retarded the oxidation of Fe and consequently formation of iron-

chlorides and iron-oxides (Fig. 9c-f). The increased oxygen content on the inhibited mild

surface compared to bare surface is attributed to the fact that all the inhibitor molecules were

rich in oxygen. In the ,the peaks of Nitrogen and sulphur as constituents of

Fig.9c-f

benzothiazole derivatives are seen. This confirms the coordination bond of the inhibitors

BTPEI, BTCPEI, BTTEI and, BTPIA with the MS surface, thus helping in the mitigation of

corrosion.

3.5.2Atomic force microscopy (AFM)

The 3Das well as 2D images of mild steel surface obtained by atomic force

microscopy (AFM) of MS coupons in the absence and presence of the inhibitors BTPEI,

BTCPEI, BTTEI and, BTPIA is depicted in Fig 10a-f. Fig. 10ashows the surface image of

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mild steel coupon before immersing in acidic solution having average surface roughness (Rq)

value of 1158 nm while Fig. 10b presents the surface image of mild steel directly exposed to

acid solution with average surface roughness (Rq) of 32.5 nm. The effect of the addition of

inhibitors in acids on the surface morphology of mild steel surface can be seen from the

surface images of mild steel immersed in acid solution in presence of different inhibitors,

BTTEI, BTCEI, BTPEI and BTPIA (Fig. 10c-f respectively). The presence of inhibitors

prevents the mild steel surface from corrosion as the average surface roughness was found

reduced from 1158 nm of bare acid solution to 494, 303, 226 and 213 nm with BTTEI,

BTCEI, BTPEI and BTPIA respectively. The decreased surface roughness in the presence of

inhibitors attributed to their adsorption on MS surface.

3.5.1 X-ray photoelectron microscopy (XPS)

XPS analysis was used to justify the formation of the adsorbed layer of inhibitor

molecules on the mild steel surface. Due to superiority of BTPIAamong all investigated

inhibitors; XPS study was conducted to explain the adsorption of BTPIA. Fig. 11a

demonstrates the XPS survey scan spectrum which shows all the elements present on the

surface and thus, the elements C, O, N and Fe were recognized, which were inherent of the

inhibitor molecule. This provided a validation for the adsorption of inhibitor by inhibiting the

corrosion phenomenon.

The deconvoluted C 1s spectrum resulted in to three peaks as represented in Fig. 11b.

The primary peak situated at 285.1 eV could be ascribed to the C– C, C=C and C– H bonds

while the peak at 286.3 eV might be related to C=N bond in the inhibitor molecule62.The

peak around 288.6 eV might be given to C=N+, occurring most likely due to the protonation

of the =N–bond15. In Fig. 11c, the N 1s spectrum comprises of three peaks situated at 398.8

eV, 400.8 eV and 402.5 eV. The first peak at 398.8eVbasically corresponds to the

unprotonated nitrogen atoms (=N– structure) while the second peak is due to formation of N–

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Fe bond that is attributed due to the association of nitrogen in BTPIA inhibitor with the Fe in

steel surface22. The last peak at 402.5 eV is likely to be of nitrogen in the oxidized form

occurring due to protonation of nitrogen, which prompts a positive polarization of the

nitrogen atom. The deconvoluted O1s spectrum was fitted into two peaks as described in Fig.

11d. The peak located at 530.4 eV can be ascribed to O2− ion, bonded with Fe3+ in the iron

oxide i.e.Fe2O3.The peak at 532.4 eV might be attributed to OH− ion in FeOOH63.The S 2p

XPS spectra represented three peaks (Fig 11e) at 164.4 eV and 169.2 eV given as S-C bond

and S-Fe bond respectively63.The Cl 2p is deconvoluted into two peaks situated at 198.9 eV

for Cl 2p3/2 and 201.5 eV for Cl 2p1/2 which might be due to Cl- ions as a result of

aggressive environment(Fig. 11f) 64.The Fe 2p3/2 shows three peaks as illustrated in Fig. 11g.

The primary peak at 710.9 eV is ascribed to ferric oxide, for example, Fe2O3 (i.e., Fe3+ oxide)

and FeOOH (i.e., oxyhydroxide). The other peak situated at around 713.4 eV demonstrates

the presence of FeCl3 on the steel surface due to the hydrochloric corrosive medium, while

that at 725 and 727.3 eV might be due to the Fe 2p1/2 that confirms the presence of α-Fe2O3,

Fe3O4 and FeOOH64.

The results achieved from XPS investigation confirmed the adsorption of inhibitor on mild

steel surface. Particularly, the presence of the nitrogen& sulphur species on the steel surface

indicates that the benzothiazole derivative, BTPIAis effectively adsorbed on the mild steel by

physisorption as well as chemisorption, and supports other results.

3.6 Theoretical calculations

3.6.1 DFT calculations

The electronic properties of an organic compound and molecular information into its

interaction with a metal surface are important for understanding its corrosion inhibition

function and to design highly efficient corrosion inhibitors65. In this regard, DFT calculations

are performed to investigate global reactivity descriptors, which provide information

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regarding the most reactive regions in an inhibitor molecule. DFT calculations can also be

conducted to get more detailed insights into the reactivity of atomic sites in investigated

compounds by mean of Fukui functions and dual descriptors.

The optimized geometry and the distribution of the LUMO and HOMO orbitals of

four corrosion inhibitors are shown in Fig. 12 and Fig. 13. Table 5 lists all the obtained

quantum chemical parameters. It can be argued that as much as electronic properties have

their importance, there are other factors that can strongly affect the inhibitor molecule's

reactivity. There is a general consensus that an inhibitor molecule with a relatively flat

orientation can be a good corrosion inhibitor due to its ability to cover a large surface area

when it gets in contact with a metal surface66. It seems clear from Fig. 12 that BTPEI and

BTTEI have more planar molecular structures compared to other molecules i.e. BTCPEI and

BTPIA. The introduction of chlorine and –NH2 groups alter the geometry of these inhibitor

molecules.

We could investigate whether these functional groups have the same effect on the electronic

properties of inhibitors by analyzing the distribution of the HOMO and LUMO orbitals. The

isodensity in HOMO and LUMO orbitals of all compounds is dispersed over the entire

molecular structure as shown in Fig.13. The benzo-thiazole, amino-phenyl and methyl-phenyl

as well as Schiff base moieties of these compounds are all expected to be involved in donor-

acceptor interactions with the metal surface due to their high-dispersed density. An exception

can be found for the results of compound BTCPEI where its chloro-phenyl moiety seems to

be less reactive. Another interesting result that emerged from the orbital distribution is that

the amino-phenyl moiety could have a strong effect on the inhibitor reactivity since it has a

highly populated electron density. Overall, it can be assumed that the entire molecular

structure of inhibitors will be responsible for electron acceptance (the isodensity in LUMO

orbital) and the electron donation (the isodensity in HOMO orbital).

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The HOMO energy (EHOMO) corresponds to the ionization potential thus represents

the propensity of an inhibitor molecule to share an electron67. Higher HOMO energy

potentially leads to a higher electron donating ability. The LUMO energy (ELUMO), on the

other hand, corresponds to the electron affinity and it is a measure of the electron transport

level68. Lower LUMO energy means a higher electron-accepting capability of an inhibitor

molecule. In addition to HOMO and LUMO energies, the energy difference between EHOMO

and ELUMO is another important quantum chemical parameter. As per many published

studies68, 69, it is considered a crucial parameter in determining the adsorption behavior of an

inhibitor molecule. The lower ∆E indicates that the compound has better reactivity, thus a

higher adsorption capability.

From the data in Table 5, we can see that the electron donation capability of

inhibitors, demonstrated by their EHOMO values, increases remarkably in the order of

BTCPEI< BTPEI < BTTEI < BTPIA. This trend can also be observed if we look at the ΔN

values of inhibitors. All these data are in good relevance with the experimental outcomes.

However, if we look at the values of other parameters i.e. ELUMO and ΔE values, it can be

seen that they do not match the same trend. However, the difference is not significant, and it

might not alter the experimental performances. This similarity appears very clear when

comparing the electron density distribution that appears in LUMO and HOMO orbitals of

inhibitors.

In acidic medium, the presence of heteroatoms with a number of lone pairs in

inhibitor molecules suggests a high propensity for protonation at the heteroatom centers.

Therefore, correlations between global reactivity descriptors in protonated forms and

inhibitor performances could help explain the nature of inhibitor-iron interactions. The most

favorable active centre for protonation was chosen based on Mulliken atomic charges70. It

was found that, in all inhibitor molecules, the nitrogen atom N (10) has the highest negative

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charge, thus it is the most probable one for protonation. This was also confirmed by XPS

analysis. Quantum chemical descriptors for protonated forms of inhibitor molecules are listed

in Table 5while the optimized molecular structure and HOMO and LUMO orbitals

distribution are given as Fig S6 & S7 in supporting information. Interestingly, similar trends

were observed in HOMO energy and ΔN values. This indicates that the electron donating

power of protonated inhibitor molecules is also playing an important role in the adsorption

process. Further inspection of data in Table 5 revealed that the electron donating capacity of

protonated inhibitor molecules is significantly higher than that of neutral molecules. Notably,

ΔN values are positive, like in the case of neutral forms. These results suggest that the

electron donating capacity of tested molecules would increase the interaction between

inhibitor molecules and iron atoms leading to effective protection of the iron surface. It is

important to note that the protonation of inhibitor molecules makes the LUMO energy values

higher, thus protonated forms of investigated molecules would have a less electron accepting

capacity. Like in the case of neutral forms, no correlation was found between experimental

inhibition efficiency on one side and LUMO energy and E values on the other side. The

results discussed here strengthen the idea that the chemisorption of inhibitors onto the steel

surface was the predominant process.

It seems from these results that the reactivity of inhibitor molecules increases as the

electron donating power increases. Functional groups with high electron donating effect can

significantly alter the dual interactions between the lone pair-electron/π-electrons of

heteroatoms and vacant d-orbitals/unpaired electrons of iron atoms. It is important to note

here that despite the well-known importance of these theoretical data, in some cases, they are

insufficient to provide accurate insights about the inhibitor's performance and fail to provide

a precise correlation when very small changes in the inhibitor molecules are present. To get

more information about reactivity of inhibitors, we performed a local reactivity analysis of

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inhibitor molecules and also the simulation of inhibitor-metal interactions by mean of

molecular dynamics simulations aiming to get profound insights on the reactivity of present

compounds.

3.6.2 Local reactivity: Fukui Functions

Fukui functions are certainly most extensively employed for the prophecy of local

chemical reactivity of corrosion inhibitors71. Hence, it has been an important subject of

research since this knowledge provides precise information on the corresponding

electrophilic and nucleophilic behavior of inhibitor molecules72. Atomic sites with maximum

values of and represent susceptible sites for nucleophilic and electrophilic attack,

f

f

respectively71. The dual descriptor is an important way to develop more informative

descriptor that can explain the nucleophilic and electrophilic attack well. An atomic site with

a positive dual descriptor means that it is prone for nucleophilic attack while the negative

dual descriptor indicates the propensity for electrophilic attack. The condensed Fukui

functions and the dual descriptor of four compounds are calculated and given in Table 6. The

benzothiazole and the Schiff base moieties that have a several number of potential adsorption

sites are expected to be the primary contributors to donor-acceptor interactions. However, to

further evaluate the reactive sites distribution, it would be important to investigate the

reactivity of these compounds taking into account the impact of functional groups. The data

in Table 6 indicate that BTPEI and BTTEI have a similar distribution of reactive sites. In

addition, all the reactive sites in these compounds have almost an equal strength in term of

electrophilic and nucleophilic power. Another fact that arises from Fukui function results is

that the benzo-thiophene and the Schiff base are the most susceptible moieties for

electrophilic attack. We can also see that except amino-phenyl in BTPIA, the substituted

phenyl in other compounds has a nucleophilic character. This impact results in a widespread

distribution of reactive sites throughout the whole molecular skeleton of BTPIA, which is

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probably the main origin of the higher inhibition efficiency of BTPIA. Further inspection of

the outcomes illustrated that the nitrogen atom of the Schiff base moiety in BTPEI compound

has an electrophilic character whereas it has a nucleophilic character in other compounds,

which is probably the reason why its electron donation power is higher than BTCPEI. On the

other hand, it seems that the BTCPEI compound has a significantly higher adsorption

capacity whereas its experimental performance is less than other compounds. A possible

explanation of this result is that its geometry when it gets in contact with the iron surface

could have a non-planar orientation, which is unfavorable for corrosion inhibition.

3.6.3 MD simulations

As demonstrated above, although quantum chemical calculations are useful in

investigating electronic properties of an inhibitor molecule, it is difficult to provide a good

assessment of inhibitor adsorption on metal surface since the metal surface and the solvent

are not considered in these calculations. In this perspective, it is critical to understand how

inhibitor molecules interact with the metal surface in presence of all the active species (like

H2O, H3O+, Cl−). Such insights can be obtained from MDS, which were emerged as a

powerful modeling approach to explain the plausible mechanism of corrosion inhibition73, 74.

The main purpose of adding the corrosive species is to mimic the real corrosion process as

accurately as possible. Fig. 14 and Fig. 15 represent, respectively, the obtained optimized

adsorption configurations of neutral and protonated inhibitor molecules on Fe (110) surface.

Interestingly, in both cases, i.e. neutral and protonated forms, BTPEI, BTTEI and BTPIA

molecules have a similar adsorption profile with varying adsorption intensity based on the

electronic properties of each molecule. A flat, almost parallel adsorption of these molecules

on iron surface was obtained. In contrast, the compound BTCPEI has a different adsorption

profile. Its benzothiazole moiety adsorbed parallel on Fe(110) surface while the substituted

phenyl oriented towards the solvent layer.

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In the case of organic corrosion inhibitors, parallel or flat orientation is the perfect

adsorption profile that could facilitate the donor-acceptor interactions between the lone pair-

electron/π-electrons of reactive sites in inhibitors and vacant d-orbitals electrons of iron

atoms. Such interactions lead to the development of a compact adsorption film. This is not

the case in the case of BTCPEI where its molecule has not a full flat adsorption, which

decreases its inhibition performance. This might explain well why the BTCPEI compound

has a lowest corrosion inhibition value despite its good electronic properties. Thus, it is clear

that electronic properties of an inhibitor molecule are not sufficient to get a full understanding

of its adsorption on metal surface and can lead to a wrong interpretation of experimental

results.

The presence of heteroatoms and functional groups increases adsorption substantially and

lead to competitive adsorption. The adsorption capability can be estimated from the

interaction energy of investigated inhibitors when they get in contact with the metal surface.

Interaction energies of neutral and protonated inhibitor molecules derived from MDS are

listed in Table 7. More interaction energy may infer a better adsorption between an inhibitor

molecule and an iron surface75. The same conclusions can be obtained from the binding

energy values. Inhibitors having higher binding energy exhibited significantly higher

adsorption ability76. Interestingly, all inhibitor molecules have strong interaction energy and

its increasing trend follows the experimental results. On the other hand, by comparing the

absolute value of interaction energies of neutral and protonated molecules, it appears that

there are few differences between both forms. The small decrease in interaction energy values

of protonated molecules was the result of their strong solvation energy in solvent phase,

which reduces the adsorption77.

The presence of an electron-rich moiety like benzothiazole causes much stronger

adsorption of all inhibitor molecules owing to a higher attraction between its reactive sites

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and vacant iron orbitals (or d-band holes). The observed difference in adsorption capabilities

could be explained by the electron donation power as well as by the orientation of inhibitor

molecules when they get in contact with the metal surface. It is apparent that the outcomes

from the MD simulations were found to be in good relevance with those obtained from

experimental tests. The addition of chlorine in the phenyl moiety of BTPEI, provoked

noticeable changes in its inhibitive performance whereas other functional groups i.e. methyl

and amino groups improved its corrosion inhibition efficacy.

4. Mechanism of inhibition

Experimental and theoretical studies give an overall picture of the corrosion inhibition

mechanism, and therefore the nature of inhibitor-Fe interactions. XPS results confirm the

presence of physical and chemical interactions of inhibitor molecules with the metal

surface. The presence of the Fe-N complex bond on the metal surface is an indication of

the formation of a very stable and corrosion-resistant layer, thus blocking active corrosion

sites77. The performance of tested compounds is shown to be very dependent on the

molecular structure of inhibitor molecules. Heteroatoms and functional groups possess a

large number of lone pair of electrons, which could play an important role in binding

between inhibitor molecules and iron atoms78. Furthermore, in the presence of HCl

solution, inhibitor molecules were susceptible to protonation, which is confirmed by XPS

and DFT studies. This suggests that the pre-adsorbed Cl- ions will attract the protonated

inhibitor molecules as an initial step on adsorption process79. However, in this study, the

initial step is relatively fast and chemical interactions may be the more predominant

mechanism as demonstrated by DFT and isotherms studies. Looking deeper, it is evident

that chemisorption is the predominant adsorption mechanism, but it cannot be ruled out

that weak physisorption can take place. Chemisorption occurs through the lone pairs of

electrons of heteroatoms and conjugated double bonds. This inhibition mechanism

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involves two types of interaction, i.e. electron transfer from inhibitor’s active sites to the

vacant d orbital of the metal and back donation of electrons to the electron-deficient C=C

bonds79.

4 Conclusions

(1) All the synthesized benzothiazolecompounds; BTPEI, BTCPEI, BTTEI and, BTPIA

have shown strong anticorrosion effect on mild steel in 1M HCl media. The trend for

inhibition efficiency obeys the order BTCPEI< BTPEI < BTTEI < BTPIA obtained

from all the experimental calculations.

(2) The Gibbs free energy values suggested strong chemical interaction of inhibitors with

the MS surface, thereby supported chemisorption.

(3) Results of potentiodynamic polarization curves indicated mixed type nature of all the

inhibitors.

(4) Theoretical Quantum chemical calculations of synthesized inhibitors found strong

relevance with experimental outcomes.

(5) The XPS analysis of BTCPEI further affirmed the adsorption of inhibitor on the mild

steel surface.

Supporting Information

Figures S1-S7

Author’s Information

1. Bhawna Chugh

Email: chugh.bhawna2011@gmail.com

2. Ashish Kumar Singh

Email: ashish.singh.rs.apc@itbhu.ac.in

Orchid: 0000-0001-8076-0816

3. Sanjeeve Thakur

Email: sanjeevethakur63@yahoo.co.in

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4. Balaram Pani

Email: Balarampani63@gmail.com

5. Ajit Kumar Pandey

Email: ajitpandeyhmr@gmail.com

6. Hassane Lgaz

Email: hlgaz@konkuk.ac.kr

Orcid: 0000-0001-8506-5759

7. Ill-Min Chung

Email: imcim@konkuk.ac.kr

8. Eno E Ebenso

Email: Eno.Ebeno@nwu.ac.za

Orcid: 0000-0002-0411-9258

Acknowledgement

Author AKS is grateful to Bharati Vidyapeeth’s College of Engineering to provide

platform to carry out research work. Authors BC and AKP are thankful to NSUT for

providing facilities to carry out research work. We are also thankful to our colleagues from

respective Institution who assisted directly or indirectly for this research work.

Conflict of Interest

No conflict of interest to declare.

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Caption of Figures

Fig. 1 Scheme of Synthesis and structure of studied compounds

Fig. 2 (a) Variation of corrosion rate of mild steel in 1 M HCl solution against concentration

of different inhibitors, (b) Langmuir’s adsorption isotherm for adsorption of inhibitors, (c)

plot of log CRvs. 1/T and (d) plot of log CR/Tvs. 1/T

Fig 3 (a), (b), (c), (d) IR spectra of synthesized inhibitors BTPEI, BTCPEI, BTTEI and

BTTEI; (a̍), (b̍), (c̍) and (d̍) IR spectra of the film deposited on the MS surface due to

adsorption of BTPEI, BTCPEI, BTTEI and BTPIA

Fig. 4 The change in OCP as a function of time for mild steel in 1 M HCl in absence and

presence of different concentrations of (a) BTCPEI, (BTPEI), BTTEI and (d) BTPIA

Fig. 5 Nyquist plots of mild steel in 1 M HCl in absence and presence of different inhibitors

(a) BTCPEI, (b) BTPEI, (c) BTTEI and (d) BTPIA

Fig. 6 (a) & (b) equivalent circuits used to fit the experimental data; (c) & (d) fitting of

experimental data obtained with 1 mg L-1 and 6 mg L-1 of BTPIA with the equivalent circuits

proposed in Fig. 5 (a) & (b)

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Fig. 7 Bode-Phase angle plots of mild steel in 1 M HCl obtained with different concentrations

of inhibitors (a) BTCPEI, (b) BTPEI, (c) BTTEI and (d) BTPIA

Fig. 8 Potentiodynamic polarization curves obtained with different concentrations of

inhibitors (a) BTCPEI, (b) BTPEI, (c) BTTEI and (d) BTPIA

Fig. 9 SEM image and EDX spectra of mild steel (a) & (a̍) before immersion in HCl solution,

(b) & (b̍) after immersion in bare acid solution, (c) & (c̍) after immersion in HCl with BTPEI,

(d) & (d̍) after immersion in HCl with BTCPEI, (e) & (e̍) after immersion in HCl with BTTEI

and (f) & (f̍) after immersion in HCl with BTPIA

Fig. 10 3d atomic force micrographs and height profile images of mild steel (a) before

immersion in HCl solution, (b) after immersion in bare HCl solution, (c) after immersion in

HCl with BTTEI, (d) after immersion in HCl solution with BTPEI, (e) after immersion in

HCl solution with BTCPEI and (f) after immersion in HCl solution with BTPIA

Fig. 11 XPS spectra of mild steel surface removed after immersion in 1 M HCl solution with

6 mg L-1 of BTCPEI: (a) survey scan, (b) narrow scan spectra of C, (c) N, (d) O, (e) S, (f) Cl

and (g) Fe

Fig. 12 Optimized molecular structure of investigated compounds obtained from DFT

calculations

Fig 13 Frontier molecule orbital density distributions for investigated compounds obtained

from DFT calculations

Fig. 14 Side and top views of the final adsorption of neutralinhibitor molecules on the Fe

(110) surface in presence of solvent species

Figure 15 Side and top views of the final adsorption of protonated inhibitor molecules on the

Fe (110) surface in presence of solvent species.

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Fig. 1 Scheme of Synthesis and structure of studied compounds

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Fig. 2 (a) Variation of corrosion rate of mild steel in 1 M HCl solution against concentration of different inhibitors, (b) Langmuir’s adsorption

isotherm for adsorption of inhibitors, (c) plot of log CRvs. 1/T and (d) plot of log CR/Tvs. 1/T

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Fig 3 (a), (b), (c), (d) IR spectra of synthesized inhibitors BTPEI, BTCPEI, BTTEI and BTTEI; (a̍), (b̍), (c̍) and (d̍) IR spectra of the film

deposited on the MS surface due to adsorption of BTPEI, BTCPEI, BTTEI and BTPIA

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Fig. 4 The change in OCP as a function of time for mild steel in 1 M HCl in absence and presence of different concentrations of (a) BTCPEI,

(BTPEI), BTTEI and (d) BTPIA

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Fig. 5 Nyquist plots of mild steel in 1 M HCl in absence and presence of different inhibitors (a) BTCPEI, (b) BTPEI, (c) BTTEI and (d) BTPIA

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Fig. 6 (a) & (b) equivalent circuits used to fit the experimental data; (c) & (d) fitting of experimental data obtained with 1 mg L-1 and 6 mg L-1 of

BTPIA with the equivalent circuits proposed in Fig. 6 (a) & (b)

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Fig. 7 Bode-Phase angle plots of mild steel in 1 M HCl obtained with different concentrations of inhibitors (a) BTCPEI, (b) BTPEI, (c) BTTEI

and (d) BTPIA

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Fig. 8 Potentiodynamic polarization curves obtained with different concentrations of inhibitors (a) BTCPEI, (b) BTPEI, (c) BTTEI and (d)

BTPIA

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Fig. 9 SEM image and EDX spectra of mild steel (a) & (a̍) before immersion in HCl solution, (b) & (b̍) after immersion in bare acid solution, (c)

& (c̍) after immersion in HCl with BTPEI, (d) & (d̍) after immersion in HCl with BTCPEI, (e) & (e̍) after immersion in HCl with BTTEI and (f)

& (f̍) after immersion in HCl with BTPIA

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Fig. 10 3d atomic force micrographs and height profile images of mild steel (a) before immersion in HCl solution, (b) after immersion in bare

HCl solution, (c) after immersion in HCl with BTTEI, (d) after immersion in HCl solution with BTPEI, (e) after immersion in HCl solution with

BTCPEI and (f) after immersion in HCl solution with BTPIA

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Fig. 11 XPS spectra of mild steel surface removed after immersion in 1 M HCl solution with 6 mg L-1 of BTCPEI: (a) survey scan, (b) narrow

scan spectra of C, (c) N, (d) O, (e) S, (f) Cl and (g) Fe

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Fig. 12Optimized molecular structure of investigated compounds obtained from DFT calculations

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Fig 13 Frontier molecule orbital density distributions for investigated compounds obtained from DFT calculations.

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Fig. 14Side and top views of the final adsorption of neutral inhibitor molecules on the Fe (110) surface in presence of solvent species.

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Fig 15Side and top views of the final adsorption of protonated inhibitor molecules on the Fe (110) surface in presence of solvent species.

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Table 1 Inhibition efficiency comparison of the inhibitors, BTCPEI, BTPEI, BTTEI and BTPIA with some other earlier studied thiazole

derivatives

Inhibitor

Concentration of Inhibitor

(mg L-1)

Inhibition Efficiency

(%)

Reference

BTCPEI

BTPEI

BTTEI

BTPIA

425.50

455.53

64..2

ABT

300

50.2

MBT

334

67.2

BTA

67.4

MBTA

72.1

TBTA

76.4

2-(2-hydroxyphenyl)benzothiazole

92.1

2-(2, 5-dihydroxyphenyl)benzothiazole

51.6

(4-benzothiazole-2-yl-phenyl)-dimethyl-amine

96.8

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Table 2Activation and thermodynamic adsorption parameters for the adsorption of different inhibitors in 1 M HCl solution

Name of Inhibitor

(kJ mol-1)

λ (mg cm-2)

(kJ mol-1)

H

(J K-1mol-1)

S

(mol-1)

ads

(kJ mol-1)

ads

G

32.8

1.87 × 107

30.1

-114.7

BTCPEI

27.1

2.85 × 105

24.4

-149.5

1.57 × 105

40.9

0.20

BTPEI

30.1

1.30 × 106

27.5

-136.8

2.69 × 105

42.3

0.14

BTTEI

25.5

1.22 × 105

22.9

-156.5

3.24 × 105

42.7

0.11

BTPIA

22.4

2.94 × 104

19.7

-168.4

3.99 × 105

43.3

0.09

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Table 3 Electrochemical impedance parameters for mild steel in 1 M HCl in absence and presence of different concentrations of inhibitors

EIS parameters obtained after fitting the experimental data with equivalent circuit

of Fig 5a

EIS parameters obtained after fitting the experimental data with equivalent circuit of Fig

Inhibitor

Cinh (mg L-1)

Rs(cm2)

Rct(cm2)

Y0(10-6-1 cm-2)

RL (cm2)

L (H)

Cdl(F cm-2)

EEIS %

Goodness of

fit

Rs(cm2)

Rct(cm2)

Yd(10-6-1 cm-2)

Rf(cm2)

Y0(10-6-1 cm-2)

Ca(F cm-2)

Rp(cm2)

EEIS %

Goodness of

fit

0.8

26.5

242

0.795

4.2

18.8

65.8

0.114 × 10-3

0.7

44.2

212

0.798

8.5

10.9

64.8

40.0

0.111 × 10-3

1.1

63.3

190

0.800

10.9

8.1

63.0

58.1

0.118 × 10-3

0.9

91.2

160

0.805

7.6

178

0.745

16.7

96.8

72.6

0.121 × 10-3

BTCPEI

1.0

118.5

142

0.811

13.1

159

0.746

16.1

131.6

79.8

0.112 × 10-3

1.1

50.5

200

0.801

18.4

9.7

63.6

47.5

0.110 × 10-3

0.9

87.5

162

0.805

15.2

6.9

57.3

69.7

0.120 × 10-3

1.2

105.5

151

0.808

14.4

164

0.773

27.7

119.9

78.6

0.121 × 10-3

BTPEI

1.1

157.5

125

0.811

20.8

135

0.778

25.2

178.3

85.1

0.118 × 10-3

1.3

51.6

185

0.806

6.6

8.0

60.2

48.6

0.129 × 10-3

1.0

92.5

160

0.811

6.0

7.1

59.3

71.3

0.122 × 10-3

0.9

115.5

143

0.820

29.9

121

0.773

23.2

145.4

81.7

0.112 × 10-3

BTTEI

1.1

146.6

130

0.824

39.6

105

0.781

22.5

186.2

85.7

0.116 × 10-3

1.3

59.5

171

0.818

5.7

7.7

61.6

55.4

0.129 × 10-3

1.1

100.6

133

0.827

4.1

6.6

53.9

73.7

0.120 × 10-3

0.8

138.1

117

0.831

40.1

0.810

19.5

178.2

85.1

0.110 × 10-3

BTPIA

1.3

270.2

0.841

49.9

0.813

16.4

320.1

91.7

0.115 × 10-3

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Table 4 Potentiodynamic polarization parameters for mild steel in 1 M HCl in absence and presence of different concentrations of inhibitors

Name of Inhibitor

Inhibitor’s conc. (mg L-1)

(mV vs. SCE)

corr

E

(μA cm-2)

corr

(mV dec-1)



(mV dec-1)





PDP %E

458

563

195

468

356

148

36.7

467

314

212

44.2

458

213

187

61.8

BTCPEI

466

112

139

80.1

483

321

143

42.9

478

192

173

65.9

485

170

141

69.8

BTPEI

479

121

86.8

460

307

184

45.5

488

175

166

68.9

472

119

131

78.9

BTTEI

477

127

88.4

469

265

205

52.9

462

163

142

71.0

467

101

114

82.0

BTPIA

472

145

89.9

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Table 5Computed quantum chemical parameters for four inhibitor molecules using DFT method

Inhibitors

EHOMO

(eV)

ELUMO

(eV)

∆Egap

(eV)

∆N110

Neutral

-5.079

-2.707

2.372

0.39

BTPEI

Protonated

-2.980

-1.254

1.726

1.56

Neutral

-5.163

-2.314

2.849

0.37

BTCPEI

Protonated

-3.129

-1.323

1.806

1.43

Neutral

-5.055

-2.682

2.373

0.40

BTTEI

Protonated

-2.955

-1.252

1.703

1.59

Neutral

-4.866

-2.303

2.563

0.48

BTPIA

Protonated

-2.929

-1.22

1.709

1.60

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Table 6Fukui function and dual descriptor indices of four compounds calculated at DFT/m-GGA level

PIA

f

f

( )f k

BTTEI

f

f

( )f k

BTCPEI

f

f

( )f k

BTPEI

f

f

( )f k

C(1)

4.1

4.3

-0.2

C(1)

4.7

5.3

-0.6

C(1)

5.6

6.5

-0.9

C(1)

4.7

5.4

-0.7

C(2)

2.8

2.9

-0.1

C(2)

-1

C(2)

3.7

5.5

-1.8

C(2)

4.1

-1.1

C(3)

2.5

C(3)

2.7

3.2

-0.5

C(3)

3.2

3.6

-0.4

C(3)

2.7

3.1

-0.4

C(4)

2.3

2.2

0.1

C(4)

2.3

2.9

-0.6

C(4)

3.5

-0.5

C(4)

2.3

2.9

-0.6

C(5)

3.5

3.8

-0.3

C(5)

4.8

-0.8

C(5)

4.8

6.3

-1.5

C(5)

4.9

-0.9

C(6)

2.6

2.9

-0.3

C(6)

2.8

3.7

-0.9

C(6)

3.4

-1.6

C(6)

2.8

3.8

-1

C(7)

5.9

6.5

-0.6

C(7)

7.2

7.3

-0.1

C(7)

8.1

8.2

-0.1

C(7)

7.3

C(8)

4.0

3.1

0.9

C(8)

3.9

5.2

-1.3

C(8)

-1

C(8)

4.1

5.2

-1.1

S(9)

7.3

7.7

-0.4

S(9)

7.9

10.8

-2.9

S(9)

8.5

13.6

-5.1

S(9)

8.2

-2.8

N(10)

7.8

4.6

3.2

N(10)

6.3

6.2

0.1

N(10)

6.9

4.6

2.3

N(10)

6.3

6.4

-0.1

C(11)

9.5

3.4

6.1

C(11)

7.9

6.1

1.8

C(11)

9.4

5.3

4.1

C(11)

8.1

2.1

C(12)

2.5

1.3

1.2

C(12)

1.9

C(12)

2.5

1.6

0.9

C(12)

1.9

0.1

C(13)

1.6

1.7

-0.1

C(13)

2.8

0.8

C(13)

0.6

0.1

0.5

C(13)

2.7

0.7

C(14)

2.9

4.8

-1.9

C(14)

3.8

2.8

C(14)

1.7

1.2

0.5

C(14)

3.7

2.7

C(15)

2.6

3.1

-0.5

C(15)

2.8

2.2

0.6

C(15)

1.3

0.7

C(15)

2.7

2.1

0.6

C(16)

4.0

4.6

-0.6

C(16)

4.5

4.1

0.4

C(16)

2.2

1.5

0.7

C(16)

5.1

3.6

1.5

C(17)

2.3

3.5

-1.2

C(17)

2.2

0.2

C(17)

1.5

1.1

0.4

C(17)

2.8

2.2

0.6

C(18)

3.2

3.9

-0.7

C(18)

4.4

1.8

2.6

C(18)

1.7

1.1

0.6

C(18)

N(31)

3.1

7.6

-4.5

C(31)

0.8

0.6

0.2

Cl(31)

2.9

2.4

0.5

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Table 7 MD parameters for neutral and protonated compounds adsorbed on the Fe (110) surface in presence of solvent species

Neutral

Protonated

Simulation models

𝐸

interaction

(kJ/mol)

𝐸

interaction

(kJ/mol)

BTPEI

-666.04

-651.27

BTCPEI

-578.47

-569.03

BTTEI

-671.36

-663.79

BTPIA

-702.95

-695.38

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Relative Comparison of Adsorption and Corrosion Inhibition Studies of Expired Vitamin B12 and C Over Mild Steel in 0.5 M H2SO4

Article

Full-text available

Feb 2024

In comparison to organic corrosion inhibitors, green corrosion inhibitors are in great need for mitigating corrosion of metals and alloys ascribed to their benign toxicity in addition to absence of heavy metals and toxic substances. Contemporary research conducted in this field is more focused on implementing an efficient approach towards corrosion mitigation. The present work highlights the evaluation of anti-corrosive potential of expired Vitamin B and Vitamin C over mild steel in 0.5M H2SO4. Gravimetric and electrochemical studies have been utilized as affirmatory tool for corrosion retarding ability of the synthesized derivative. Concentration increase brought a profound rise in corrosion inhibition efficiency whereas variation in temperature from 308 K-328 K retarded the protection ability of respective inhibitors whereas electrochemical studies presented depressed semi-circles in Nyquist plots and potentiodynamic values demarcated both inhibitors as mixed type. Furthermore, a comparison among the two tested vitamins has been drawn. Contact angle measurements have been involved here to confirm the inhibitors adsorption over mild steel surface. The present experimental work will provide future insights for researchers working in field of corrosion.

The Corrosion Inhibition Performance of Eco-Friendly bis-Schiff Bases on Carbon Steel in a Hydrochloric Solution

Article

Full-text available

Dec 2023

Corrosion inhibitors are widely used as an important tool for the prevention and remediation of different materials exposed to corrosive industrial processes. Corrosion inhibitors are usually added to acid pickling solutions to reduce the deterioration of metallic materials and particularly, corrosion due to hydrochloric acid. In this work, three bis-Schiff bases (BS2, BS4 and BS8) were synthesized and characterized using spectroscopic methods, and their anti-corrosive effects on AISI 1020 carbon steel in a hydrochloric acid solution were studied using gravimetric and electrochemical techniques and quantum chemical methods. The results showed that all substances act as potential corrosion inhibitors as BS8 exhibited the highest efficiency (98%) of all methods. The compounds adsorbed on the metal surface were as per the El-Awady adsorption isotherm. Morphological aspects of the metal were observed upon applying SEM, and the theoretical results acquired from the quantum chemical calculation for molecular properties and the Fe(110) surface adsorption proved to be compatible with the experimental results.

Study on the effect of anti-oxidation film formed by composite system on solder balls performance

Article

Apr 2024
MICROELECTRON RELIAB

toghan-et-al-2024-effect-of-adsorption-and-interactions-of-new-triazole-thione-schiff-bases-on-the-corrosion-rate-of

Article

Full-text available

Feb 2024

Ahmed Mohammed Eldesoky

Due to the unique properties of steel, including its hardness, durability, and superconductivity, which make it an essential material in many industries, it lacks corrosion resistance. Herewith, two novel triazole-thione Schiff bases, namely, (E)-5-methyl-4-((thiophen-2-ylmethylene)amino)-2,4-dihydro-3H-1,2,4-triazole-3-thione (TMAT) and (E)-4-(((5-(dimethylamino)thiophen-2-yl)methylene)amino)-5-methyl-2,4-dihydro-3H-1,2,4-triazole-3-thione (DMTMAT), were synthesized and characterized. The corrosion inhibition (CI) ability of these two molecules on carbon steel in an aqueous solution of 1 M HCl as well as their interaction with its surface was studied using a number of different techniques. The results confirmed that the CI capability of these organic molecules depends on their strong adsorption on the metal surface and the formation of a protective anticorrosion film. Weight loss tests revealed that the inhibition efficiencies of TMAT and DMTMAT were 91.1 and 94.0%, respectively, at 1 × 10–3 M concentrations. The results of electrochemical impedance spectroscopy (EIS) indicated that there was a direct relationship between the inhibitor concentration and the transfer resistance. Potentiodynamic polarization (PDP) experiments have proven to be mixed-type inhibitors of C-steel in aqueous hydrochloric acid solution and follow the Langmuir adsorption isotherm model. Several thermodynamic and kinetic parameters were calculated. The negative values of the adsorption-free energy are −36.7 and −38.5 kJ/mol for TMAT and DMTMAT, respectively, confirming the spontaneity of the adsorption process. The MD simulation study’s findings show that the inhibitor molecules are nearly parallel to the metal surface. The interaction energy calculated by the MD simulation and the inhibitory trend are the same. The practical implementation is consistent with what the computer models predicted.

Exploring corrosion protection for mild steel in HCl solution: An experimental and theoretical analysis of an antipyrine derivative as an anticorrosion agent

Article

Full-text available

Jan 2024

The corrosion of mild steel in HCl solution remains a critical issue in various industrial applications. In the quest for effective corrosion inhibitors, 4‐(2‐Hydroxy‐3‐Methoxybenzylideneamino) antipyrine (HMBA) has emerged as a promising candidate. This study investigates the inhibitory properties of HMBA on mild steel corrosion in HCl solution through weight loss measurements, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP) techniques. The experiments spanned over various time periods, including 1, 5, 10, 24, and 48 h. The results reveal that HMBA exhibits exceptional inhibition efficiency (IE), with an impressive 94.7% inhibition rate. This outstanding performance underscores its potential as a corrosion inhibitor for mild steel in aggressive HCl environments. To elucidate the adsorption behavior of HMBA on the mild steel surface, Langmuir isotherm modeling was employed, demonstrating a strong correlation between the experimental data and the Langmuir adsorption isotherm model. Furthermore, the study employs density functional theory (DFT) to gain insight into the mechanism of HMBA inhibition. DFT calculations suggest that both physisorption and chemisorption mechanisms are involved in the interaction between HMBA and the mild steel surface. The calculated Gibbs free energy of adsorption () is found to be approximately , indicating a spontaneous and energetically favorable adsorption process. In conclusion, HMBA emerges as a highly effective corrosion inhibitor for mild steel in HCl solution, offering impressive IE over various time intervals. The combination of experimental techniques, such as WL, EIS, and PDP, along with computational insights from DFT calculations, provides an understanding of the inhibitory properties of HMBA. These findings hold great promise for the development of environmentally friendly corrosion inhibitors in industrial applications.

Electrochemical and computational insights into the utilization of 2, 2- dithio bisbenzothiazole as a sustainable corrosion inhibitor for mild steel in low pH medium

Article

Nov 2023
ENVIRON RES

N-Methyl-2-(1-(5-methylthiophen-2-yl)ethylidene) hydrazinecarbothioamide as Corrosion Inhibitor for Mild Steel in HCl Solution: Weight Loss and DFT Investigations

Article

Jan 2023

For This study investigated the corrosion inhibition performance of N-methyl-2-(1-(5-methylthiophen-2yl)ethylidene)hydrazinecarbothioamide (MTET) on mild steel in hydrochloric acid (HCl) solution using weight loss and density functional theory (DFT) methods. The effects of temperature were also studied at the range of 303 to 333 K. The corrosion inhibition efficiency of MTET increased with increasing concentration and exposure time, while it decreased with increasing temperature. At the highest concentration of 0.0005 M, the inhibition efficiency reached 95.5% after 5 hours of immersion time. The Langmuir adsorption isotherm was found to describe the adsorption behavior of MTET on the mild steel surface, and the free energy value indicated that the inhibition process was both chemical and physical in nature. DFT calculations showed that the adsorption of MTET on the mild steel surface involved the formation of chemical bonds between the nitrogen and sulfur atoms of MTET and the iron atoms of mild steel. Overall, the results suggest that MTET is an effective inhibitor for mild steel corrosion in HCl solution, and the DFT calculations provide valuable insights into the underlying mechanisms of the inhibition process.

Understanding of remarkable corrosion combating action of N-(benzo[d]thiazol-2-yl)-1-(2-substituted phenyl) methanimines: Electrochemical, surface and computational approach

Article

Nov 2023
INORG CHEM COMMUN

The present study highlights the inspection of corrosion combating potential of four thiazole-derived Schiff bases; N-(6-methoxybenzo[d]thiazol-2-yl)-1-phenylmethanimine (MTPM), 1-(2-chlorophenyl)-N-(6-methoxybenzo[d]thiazol-2-yl) methanimine(CMTM), N-(6-methoxybenzo[d]thiazol-2-yl)-1-(2-methoxyphenyl) methanimine (MTMM) and N-(6-methoxybenzo[d]thiazol-2-yl)-1-(2-nitrophenyl)methanimine (MTNM) againstmild steel in 0.5 M H2SO4 with employment of gravimetric, electrochemical, and theoretical studies. The extreme inhibition efficacy of 96.3 % was recorded for MTMM at an ideal concentration of 10.50 × 10−4 M. Investigation delineates the concentration and substituent effect concerning inhibition efficiency. The inclusion of thiazole ring in synthesized structures is considered responsible for the compact protective coating formed as a result of mixed interactions during their adsorption over metal surfaces following Langmuir adsorption curves. Surface investigations; Atomic force microscopy (AFM) and Scanning electron microscopy (SEM) were involved to outline the comparative degradation of mild steel surface when subjected to synthesized inhibitors. X-ray photoelectron spectroscopy (XPS) and Electron dispersive spectroscopy (EDS) evaluated the possible presence of inhibitor’s film over mild steel surface. Theoretical examination; Density Functional Theory (DFT) and Molecular Dynamic Simulation (MD) have been acclimated additionally as supporting evidence for experimental results.

Anticorrosion Agents for Carbon Steel in Acidic Environments: Synthesis and Quantum Chemical Analysis of New Schiff Base Compounds with Benzylidene

Article

Full-text available

Oct 2023

Novel Schiff bases (SBs), namely, N1,N2-bis(2-(((E)-4-chlorobenzylidene)amino)ethyl)ethane-1,2-diamine (I), N1,N2-bis(2-(((E)-4-(dimethylamino)benzylidene)amino)ethyl)ethane-1,2-diamine (II), and N1,N′1-(ethane-1,2-diyl)bis(N2-((((Z)-4-dimethylamino)benzylidene) amino)methylethane-1,2-diamine) (III), were prepared and characterized by using elemental analysis, IR, and ¹H NMR spectroscopy. For assessing carbon steel in diverse settings, with and without inhibitors at varying concentrations, electrochemical frequency modulation (EFM), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PP) techniques were employed. The results showed that the synthesized inhibitors effectively decreased the corrosion rate of carbon steel in acidic media and the inhibition efficiency reached up to 93% for compound III at a concentration of 250 ppm. In addition, all prepared compounds were successful as anticorrosion agents, and the inhibition mechanism followed chemisorption from the Langmuir isotherm. The data obtained from the theoretical analysis show that the efficiency of the prepared compounds was in the order III < II < I. Furthermore, quantum chemical calculations were performed to gain insight into the electronic structure of the compounds. The analysis of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) showed that compound III had the highest surface coverage due to its specific molecular structure and spacer. This observation agreed well with the Langmuir adsorption data.

A combined experimental and theoretical study of a novel corrosion inhibitor derived from thiophen

Article

Full-text available

Oct 2023

In this study, we synthesized a novel corrosion inhibitor derived from thiophene and conducted a comprehensive evaluation of its inhibitory properties through both experimental and theoretical approaches. Our investigation encompassed experimental assessments employing Mass loss tests and electrochemical techniques. Additionally, we performed computational studies to delve into the electronic structure and bonding characteristics of the inhibitor, aiming to elucidate its inhibitory mechanism. Our findings revealed that the synthesized inhibitor displayed remarkable inhibitory efficiency, demonstrating its effectiveness in preventing the corrosion of mild steel. Specifically, the thiophene derivative exhibited an impressive inhibitory efficiency of 92.8%, underscoring its potential as a robust corrosion inhibitor for mild steel. Furthermore, this study delved into optimizing the conditions for employing the thiophene derivative as a corrosion inhibitor. Our investigation revealed that the most effective inhibition was achieved at a concentration of 0.5 mM and a temperature of 303 K. To elucidate the interaction between the inhibitor and the mild steel surface, we applied the Langmuir adsorption isotherm concept, shedding light on both the physical and chemical adsorption processes of the thiophene derivative on the metal's surface. Our investigations demonstrated that the addition of the inhibitor significantly reduced the corrosion rate of the metal. Our computational results further reinforced these experimental findings, indicating that the inhibitor formed stable adsorption complexes on the metal surface. This dual confirmation from experimental and computational approaches strengthens the confidence in the inhibitor's efficacy in mitigating corrosion.

Understanding corrosion inhibition of mild steel in acid medium by new benzonitriles: Insights from experimental and computational studies

Article

Full-text available

Sep 2018
J MOL LIQ

Two benzonitrile derivatives, namely 4-(isopentylamino)-3-nitrobenzonitrile (PANB) and 3-amino-4-(isopentylamino)benzonitrile(APAB) have been synthesized and evaluated as corrosion inhibitors for mild steel (MS) in 1 M HCl solution at 303 K by gravimetric, potentiodynamic polarization (PDP) curves, and electrochemical impedance spectroscopy (EIS) methods, as well as Density Functional Theory (DFT) and molecular dynamics (MD) simulations. The results suggest that tested compounds are excellent corrosion inhibitors for mild steel with PANB showing superior performance. Polarization measurements revealed that PANB and APAB behaved as mixed type inhibitors. The polarization resistance, according to EIS studies, found to be dependent on the inhibitor's concentration. The adsorption of PANB and APAB on mild steel surface obeyed Langmuir's adsorption isotherm. On the one hand, DFT and MD simulations are being used to explain the effect of the molecular structure on the corrosion inhibition efficiency and on the other hand to simulate the adsorption of benzonitrile derivatives on mild steel surface. The protection of carbon steel in 1 M HCl was confirmed by using scanning electron microscope (SEM) and Atomic Force Microscopy (AFM). Electrochemical, DFT and MD simulations results are in good agreement.

Synthesis and investigation of pyran derivatives as acidizing corrosion inhibitors for N80 steel in hydrochloric acid: Theoretical and experimental approaches

Article

May 2018
J ALLOY COMPD

The corrosion inhibition performance of pyran derivatives (AP) on N80 steel in 15% HCl was investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, weight loss, contact angle and scanning electron microscopy (SEM) measurements, DFT and molecular dynamic simulation. The adsorption of APs on the surface of N80 steel obeyed Langmuir isotherm. Potentiodynamic polarization study confirmed that inhibitors are mixed type with cathodic predominance. Molecular dynamic simulation was applied to search for most stable configuration and adsorption energies for the interaction of the inhibitors with Fe (110) surface. The theoretical data obtained are in most cases in agreement with experimental results.

1-Ethyl 3-methylimidazolium thiocyanate ionic liquid as corrosion inhibitor of API 5L X52 steel in H2SO4 and HCl media

Article

Mar 2019
CORROS SCI

The corrosion inhibition behavior of 1-ethyl 3-methylimidazolium thiocyanate, (EMIM)+(SCN)− ionic liquid (IL), on API 5 L X52 steel immersed in 0.5 M H2SO4 and 0.5 M HCl aqueous solutions were studied. The kinetic corrosion parameters were determined using a gravimetrical method and electrochemical tests (polarization curves, electrochemical impedance spectroscopy). The (EMIM)+(SCN)− exhibited good inhibition efficiency, IE, (82.9% and 77.4% for H2SO4 (75 ppm) and HCl (100 ppm) solutions) properties in both solutions acting as a mixed-type inhibitor. In H2SO4 the IE increased with increasing IL concentration and temperature, while in HCl IE decreased with the temperature increase. The IL adsorption mechanism followed the Langmuir isotherm, presenting a competition between the physical and chemical interactions. Surface analysis techniques (energy dispersive spectroscopy and X-ray photoelectron spectroscopy) indicate that the inhibitor formed a protective film on the steel surface, evidencing the adsorption of the cation (EMIM)+ and the anion (SCN)− of the IL on the steel surface, which corroborates that both the (EMIM)+ and the (SCN)− of IL interact with the substrate. The evaluation of interactions of (EMIM)+(SCN)− molecules with H2SO4 and HCl in the presence of water on different surfaces of iron Fe and Fe2O3 (110) plane was performed, using molecular dynamics to determine the inhibitor adsorption energies in both acid media. The simulation results are in close agreement with the experimental observations that the inhibition efficiency is better in H2SO4 solution in comparison with that in HCl due to the higher adsorption energy values obtained in H2SO4 medium.

4-aminoazobenzene modified natural glucomannan as a green eco-friendly inhibitor for the mild steel in 0.5 M HCl solution

Article

Feb 2019
CORROS SCI

The corrosion inhibition effect of a green eco-friendly 4-aminoazobenzene modified natural glucomannan (GL-PA) on the corrosion protection of mild steel in 0.5 M HCl solution is investigated by electrochemical measurements and surface characterization. The results show that the GL-PA acts as a mixed-type inhibitor and can provide effective corrosion inhibition for mild steel. The corrosion inhibition is achieved by the chemical as well as physical adsorption of the GL-PA on the surface of the mild steel, and the adsorption fairly obeys the Langmuir isotherm.

2-Amino-4-(4-methoxyphenyl)-thiazole as a novel corrosion inhibitor for mild steel in acidic medium

Article

Jan 2019
PROG ORG COAT

The inhibitory effects of 2-amino-4-(4-methoxyphenyl)-thiazole (MPT) on mild steel corrosion were evaluated in 0.5 M H2SO4 and 1 M HCl solutions, respectively. From the analysis of potentiodynamic polarization curves and electrochemical impedance spectroscopy, MPT can effectively inhibit the corrosion of mild steel in acidic solutions. Under the optimal conditions, the inhibition efficiency (η) was as high as 95% in 0.5 M H2SO4 solution. The inhibition property of MPT was much better than that of the reported corrosion inhibitor 2-amino-4-phenylthiazole (APT). Polarization curves also demonstrated that MPT mainly inhibited the anodic corrosion of mild steel in HCl solution and the cathodic corrosion in H2SO4 solution, respectively. The adsorption of MPT on the surface of mild steel was found to obey Langmuir adsorption isotherm in both acid solutions. Quantum calculations suggest that the addition of electron donor group (−OCH3) endowed the molecule better adsorption ability on mild steel surface. The interaction between protonated MPT and anions on Fe (0 0 1) surface was firstly investigated using molecular dynamic stimulations, and the results showed that with the adsorption of sulfate ions, MPTH⁺ had a superior binding energy on the surface of Fe. The investigation of UV–vis spectrum demonstrated the direct correlation of the experimental data and the theoretical calculations.

A Review: Therapeutic and Biological Activity of Benzothiazole Derivatives

Article

Sep 2017

The Benzothiazole nucleus is present in compounds involved in research aimed at evaluating new products that possess interesting biological activities, such as antitumor, antimicrobial, anthelmintic, antileishmanial, anticonvulsant and anti-inflammatory. The present review focuses on the benzothiazoles with potential activities that are now in development. The synthesized benzothiazole derivatives could be considered as lead molecule for the development of therapeutic agents.

Halogen-Substituted Acridines as Highly Effective Corrosion Inhibitors for Mild Steel in Acid Medium

Article

Oct 2018

A series of acridines were designed and synthesized for the development of effective inhibitors for mild steel corrosion in 1 M HCl solution, in which the halogen-substituted acridines showed better inhibitive performance than the non-halogen-substituted acridines. The corrosion protection properties of the halogen-substituted acridines, including 2-chloro-9-phenylacridine (CPA), 2-chloro-9-(2-fluorophenyl)acridine (CFPA) and 2-bromo-9-(2-fluorophenyl)acridine (BFPA), were further investigated using weight loss test and electrochemical techniques. The results indicated the halogen-substituted acridines have excellent inhibitiion performance, and these acridines act as mixed type inhibitors with predominant cathodic effectiveness. Adsorption of acridines on a mild steel surface obeyed the Langmuir adsorption isotherm. The adsorption of the inhibitor molecules on steel surface was further supported by scanning electron microscope (SEM), scanning electrochemical microscope (SECM) and FTIR spectroscopy. The inhibition mechanism of the investigated halogen-substituted acridines was derived using DFT based quantum chemical calculations for their neutral as well as protonated forms. Both experimental and DFT studies suggested that the inhibition efficiency of three halogen-substituted acridines followed the order of η(BFPA) > η(CFPA) > η(CPA).

Atomistic Simulation: A Unique and Powerful Computational Tool for Corrosion Inhibition Research

Article

Oct 2018

It is difficult to understand the atomistic information on the interaction at the metal/corrosion inhibitor interface experimentally which is a key to understanding the mechanism by which inhibitors prevent the corrosion of metals. Atomistic simulations (molecular dynamics and Monte Carlo) are mostly performed in corrosion inhibition research to give deeper insights into the mechanism of inhibition of corrosion inhibitors on metal surfaces at the atomic and molecular time scales. A lot of works on the use of molecular dynamics and Monte Carlo simulation to investigate corrosion inhibition phenomenon have appeared in the literature in recent times. However, there is still a lack of comprehensive review on the understanding of corrosion inhibition mechanism using these atomistic simulation methodologies. In this review paper, we first of all introduce briefly some important molecular modeling simulations methods. Thereafter, the basic theories of molecular dynamics and Monte Carlo simulations are highlighted. Several studies on the use of atomistic simulations as a modern tool in corrosion inhibition research are presented. Some mechanistic and energetic information on how organic corrosion inhibitors interact with iron and copper metals are provided. This atomic and molecular level information could aid in the design, synthesis and development of new and novel corrosion inhibitors for industrial applications.

On the Understanding of the Adsorption of Fenugreek Gum on Mild Steel in an Acidic Medium: Insights from Experimental and Computational Studies

Article

Sep 2018
APPL SURF SCI

Nowadays, a properly designed corrosion protection system, which is essential for large-scale practical application remains a big challenge. Herein, we report a combined experimental and theoretical study of the adsorption of a galactomannan polysaccharide, namely, Fenugreek gum (FG) on the mild steel (MS) surface in 1.0 M HCl by using electrochemical techniques, X-ray photoelectron spectroscopy (XPS), DFT, molecular dynamic (MD) simulations, radial distribution function (RDF) and mean square displacement (MSD). XPS results show that the FG forms a layer on steel surface. FG adsorption onto the steel surface was found to follow Langmuir model. The electrochemical results demonstrated that FG acts as mixed-type inhibitor. Scanning electron microscope (SEM) was used to examine the surface morphology of the samples. Our findings provide deeper insights into understanding the interaction mechanisms of FG with MS and can be helpful to explore novel approaches to mitigate the steel dissolution occurring at the acidic environment.

Gravimetric, electrochemical and surface studies on the anticorrosive properties of 1-(2-pyridyl)-2-thiourea and 2-(imidazol-2-yl)-pyridine for mild steel in hydrochloric acid

Article

Jun 2018

The 1-(2-pyridyl)-2-thiourea (TP) and 2-(imidazol-2-yl)-pyridine (IP) are described here for the first time as inhibitors for mild steel in acid medium based on investigations with weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The experimental results revealed that 1-(2-pyridyl)-2-thiourea and 2-(imidazol-2-yl)-pyridine are effective corrosion inhibitors for mild steel in acid medium, and their maximum corrosion inhibition efficiency at 4 × 10-4 M are 93.57% and 96.66%, respectively. TP and IP are determined as mixed-type inhibitors based on polarization studies, and their adsorption on mild steel surface follows Langmuir adsorption isotherm and physical adsorption is dominant. The formation and characteristic of protective layer on the steel surface were verified from their spectra of scanning electrochemical microscope (SECM), UV–visible, FT-IR and X-ray photoelectron spectroscopy (XPS) methods. Besides, the correlation between inhibition efficiency and molecular structure of inhibitor was theoretically studied via quantum chemical calculations.

An Exploration about the Interaction of Mild Steel with Hydrochloric Acid in the Presence of N-(Benzo[d] Thiazole-2-yl)-1-Phenylethan-1-Imines

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