Green synthesis and corrosion inhibition study of 2-amino-: N ′-((thiophen-2-yl)methylene)benzohydrazide

Article abstract of DOI:10.1039/c7nj04162d

2-Amino-N′-((thiophen-2-yl)methylene)benzohydrazide (ATMBH) was synthesized by a conventional method, as well as by microwave (MW) irradiation. The MW assisted synthesis of ATMBH was found to have good yield and was more eco-friendly compared to the conve

Full text of DOI:10.1039/c7nj04162d

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Green Synthesis and Corrosion Inhibition Study of 2ꢀaminoꢀN'ꢀ(thiophenꢀ2ꢀyl)
methylene) benzohydrazide
A. K. Singh1, *, S. Thakur2, , B. Pani3, G. Singh4
1Department of Applied Science, Bharati Vidyapeeth’s College of Engineering, New
Delhi 110063
2Department of Chemistry, NSIT, University of Delhi, New Delhi, INDIA
3 Department of Chemistry, Bhaskaracharya College of Applied Science, University of
Delhi, INDIA
4 Department of Chemistry, University of Delhi, INDIAꢀ110007
*Corresponding author
Tel. No. +919560285447
Email: ashish.singh.rs.apc@itbhu.ac.in
Abstract
2ꢀaminoꢀN'ꢀ(thiophenꢀ2ꢀyl) methylene) benzohydrazide (ATMBH) was
synthesized by conventional method as well as by microwave (MW) irradiation. The MW
assisted synthesis of ATMBH was found to have good yield and more ecoꢀfriendly
compared to conventional method of synthesis. The synthesized compound ATMBH was
characterized by FTIR, 1H NMR, 13C NMR and CHN analysis. The corrosion inhibition
performance of ATMBH in 0.5 M H2SO4 was analyzed by gravimetric and
electrochemical methods. The potentiodynamic polarization study revealed that ATMBH
acted as mixed type of inhibitor. The results obtained from these methods are in good
agreement and are well supported by contact angle measurements.
Key Words: Acid Solutions; Mild steel; EIS; AFM; Kinetic parameters; SEM; NMR
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1. Introduction
The importance and application of Schiff bases has ever attracted enormous
research interest and created huge scope for future research. A variety of Schiff bases
have been studied as antiꢀmicrobial, antiꢀbacterial, antiꢀfungal, antiꢀtumor, antiꢀcancer,
and as antiꢀmalarial [1ꢀ3]. Schiff bases having thiophene nucleus have been very
important for study because these compounds can be used as a structural unit to
synthesize charge transporting molecules which can be used in transistors, light emitting
diodes and organic solar cells [4ꢀ7]. Thiophene nucleus has also been established as a
potential unit in the fast growing chemical world of heterocyclic compounds having very
promising characteristics. Due to larger size of Sꢀatom as compared to C, N and O atom,
its lone pair can be more easily delocalized in heterocyclic ring. A variety of Schiff’s
bases with thiophene moiety have been synthesized which are very versatile especially
for making variety of biologically active complexes [8ꢀ11].
Acid hydrazide and hydrazones have shown number of applications in biological
as well as analytical chemistry [12ꢀ16]. Since a variety of Schiff bases have been studied
as corrosion inhibitors recently; Thus in view of importance of hydrazide and hydrazones
we have synthesized hydrazone with thiophene moiety and studied its effect on corrosion
mitigation of mild steel in acidic medium.
It has been known for a long time that molecules undergo excitation under
electromagnetic radiation. This effect is utilized in house hold microwave oven to heat up
food. However, Chemists have only been using microwave as a reaction methodology for
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a few years. So, microwave assisted organic synthesis is one of the fastest growing area
in the field of organic synthesis.
Conventional methods for various chemical syntheses is very well documented
and practiced. The methods for synthesis of organic compounds have continuously
modified. In recent years, Microwave assisted synthesis has been developed as an
efficient, clean and economically viable method compared to traditional heating. In many
cases reactions that normally require many hours at reflux temperature under classical
conditions can be completed within a few minutes or even seconds in a microwave oven.
The microwaveꢀassisted chemical transformations have become important because of
several advantages over the conventional thermal reactions. Important advantage of this
method includes reduction of reaction time, improved yield of organic compound as well
as its quality. Due to its ability to couple directly with the reaction molecule and by
passing thermal conductivity leading to a rapid rise in the temperature, microwave
irradiation has been used to improve many organic syntheses.
A general assumption with regard to organic reactions is that they are performed
in a solvent medium. The reason behind this concept is simple: uniform heating or
cooling of the reaction can be carried out easily in solution phase. Solventꢀfree synthesis
is gaining importance as a tool for the synthesis of a wide variety of useful and important
compounds, with the number of reactions conducted under these conditions increasing.
Solventꢀfree processes are not only environmentally benign but also economical [17].
Because a solvent is not required, toxic wastes can be minimized or eliminated, and so
the cost of solvent and waste treatment is reduced. Recent advances in this area include,
for example, aldol and related reactions [18, 19].
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Corrosion has always been a major problem of metallic structure because
economic cost due to corrosion failure is so high; also it leads to public risk so, it is
needed to develop some novel substance which can retard corrosion of metallic structure
especially in acidic medium. The use of organic compound is one of the most promising
methods to check corrosion. Organic compounds having hetero atoms (N, S, O, and P)
show good inhibition efficiency as it facilitates adsorption of compound on the metal
surface to mitigate further corrosion [20]. Further, inspection of literature reveals that
presence of C=N bond [21, 22] makes Schiff’s bases good corrosion inhibitor. A variety
of Schiff bases [23ꢀ29] (given in Table 1) have been studied and were found as good
inhibitor against corrosion of mild steel in acidic medium. Though Schiff bases are
efficient corrosion inhibitor in general but especially those Schiff bases which have Sꢀ
atoms exhibit superior inhibiting action against corrosion; the comparative data of some
Schiff bases is presented in Table 1. Examination of Table 1 reveals that Sꢀcontaining
Schiff bases exhibit better efficiency than others against corrosion [23ꢀ29]. Our earlier
study [29] on anticorrosion behavior of thiophene based hydrazone in hydrochloric acid
medium proved the efficiency of thiophene based Schiff base against mild steel
corrosion. Thus, this fact led the authors to study this ATMBH molecule as corrosion
inhibitor in 0.5 M H2SO4.
This article reports microwave assisted solvent free synthesis of 2ꢀaminoꢀN'ꢀ
((thiophenꢀ2ꢀyl) methylene) benzohydrazide (ATMBH). This compound was earlier
synthesized by some other researchers by conventional method [30ꢀ33]. Magyar et. al
[34] reported green synthesis of this compound ATMBH using ball milling technique.
This compound ATMBH was studied as an antiꢀmicrobial agent by Alsaeedi et al [31]. In
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view of importance of green chemistry and cleaner environment, we have achieved
microwave assisted synthesis of 2ꢀaminoꢀN'ꢀ((thiophenꢀ2ꢀyl) methylene) benzohydrazide
(ATMBH) under solvent free condition in presence of montmorillonite clay and study its
effect on corrosion of mild steel in acidic medium. Montmorillonite clay with its large
surface area and high Bronsted and Lewis acidity has emerged as an environmentally
benign solid acid catalyst, replacing many hazardous acid catalysts in organic synthesis
and reactions, both in laboratory and industry.
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Table 1 Inhibition efficiency values of ATMBH showing supremacy with some other studied inhibitors
Name of Schiff base
Conc. Of Inhibitor
(M)
Corrosive medium
H2SO4
Inhibition efficiency†
HCl
HCl
H2SO4
2ꢀaminoꢀN'ꢀ((thiophenꢀ2ꢀyl) methylene) benzohydrazide 4.07 ×10ꢀ4
(ATMBH)
ꢀ
+ (0.5 M)
ꢀ
96.91
‒
‒
‒
2ꢀaminobenzylimino derivatives [23]
(A)
(B)
(C)
9.38 ×10ꢀ4
6.74 ×10ꢀ4
5.94 ×10ꢀ4
+ (1 N)
+ (1 N)
+ (1 N)
59.9
66.1
77.3
ꢀ
ꢀ
ꢀ
‒
‒
‒
Benzylideneꢀpyridineꢀ2ꢀylꢀamine [24]
2ꢀamino phenol derivatives [25]
(A)
(B)
(C)
2.0 ×10ꢀ4
2.0 ×10ꢀ4
2.0 ×10ꢀ4
5.0 ×10ꢀ3
5.0 ×10ꢀ3
5.0 ×10ꢀ3
+ (1 N)
+ (1 N)
+ (1 N)
59.0
57.6
67.3
ꢀ
ꢀ
ꢀ
‒
‒
‒
(A)
(B)
(C)
+ (1 N)
+ (1 N)
+ (1 N)
65.0
85.0
88.0
ꢀ
ꢀ
ꢀ
5ꢀmercaptoꢀtriazole derivatives [26]
MSBT
MBBT
MBCT
1.8 ×10ꢀ3
1.9 ×10ꢀ3
1.7 ×10ꢀ3
+ (1 N)
+ (1 N)
+ (1 N)
+ (0.5 M)
+ (0.5 M)
+ (0.5 M)
88.0
93.0
96.0
97.0
98.0
99.0
‒
‒
‒
Schiff base containing disulfide bond[27]
(Sꢀcontaining)
I
II
III
5.0 ×10ꢀ2
5.0 ×10ꢀ2
5.0 ×10ꢀ2
+ (2 N)
+ (2 N)
+ (2 N)
90.5
77.2
86,9
ꢀ
ꢀ
ꢀ
Thiophene derivatives [28]
Thiophene hydrazone [29]
L
Hatbh
5.0 ×10ꢀ3
+ (1 N)
+ (1 N)
+ (1 M)
91.4
81.5
92.9
ꢀ
‒
1.22 ×10ꢀ4
† calculated from EIS data
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2. Experimental Procedure
2.1 2ꢀaminoꢀN'ꢀ((thiophenꢀ2ꢀyl) methylene) benzohydrazide (ATMBH)
In the present study, thiopheneꢀ2ꢀcarboxaldehyde is allowed to react with orthoꢀ
amino benzoylhydrazide conventionally [35] as well as in presence of catalytic amount of
acid clay under microwave radiation to get benzoyl hydrazone in reasonably good yield.
The synthesized compound displayed spectral data (IR, 1H NMR, 13CNMR and CHN
analysis) consistent with its assigned structure. The physical data and yield of synthesized
compound (of both methods; conventional as well as microwave assisted) are given in
Table 2.
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Table 2 Yield and Physical data of the product ATMBH
Product Time
Yield%b
Con.
M.P. (oC) IR
(ν cmꢀ1, KBr)
1HNMR
(DMSOꢀd6;δ ppm)
13C NMR
(DMSOꢀd6;δ ppm)
CHN (%)
Con.
(h)
MWa
(min)
MW
ATMBH
5
4
64
91
174
ν(NH) 3340 m;
11.567 (s,1H, NH)
6.35ꢀ6.75 (m, ArꢀH)
7.71 (2H, NH2)
7.43 (d,2H,
thiophene moiety)
7.37 (t, 1H,
161.78 (C=O)
149.11 (CꢀNH2)
C = 58.70
H = 4.48
N = 17.10
ν(C=O) 1670 m;
ν(C=N) 1585m;
ν(N–N) 990w
133.33 (C14); 116.83 (C13)
117.74 (C11); 119.62 (C15)
127.57 (C=N); 129.01 (C16)
130.43 (C5 of thiophene moiety)
128.24 (C2 of thiophene moiety)
129.01 (C1 of thiophene moiety )
143.02 (C4 of thiophene moiety)
thiophene moiety)
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Initially we attempted the reaction of thiopheneꢀ2ꢀcarboxaldehyde with orthoꢀ
amino benzoylhydrazide in presence of dry montmorillonite (K10) but found it sluggish.
In order to overcome this problem, we utilized the clay washed with mineral acid which
increases the acidity of clay significantly. Indeed no. of researchers has previously used
acid clay. This acid clay enhanced the reaction rate significantly.
In conventional method, thiopheneꢀ2ꢀcarboxaldehyde is refluxed with orthoꢀ
amino benzoylhydrazide in ethanolic solution in presence of glacial acetic acid as a
catalyst in a round bottom flask. After 5 h of refluxing followed by cooling at room
temperature, yellow color product was obtained.
Melting point measured in open capillary and is uncorrected. IR spectra were
recorded on a Jasco FT/IRꢀ5300 spectrophotometer. 1H NMR spectrum run on a Jeol AL
300 FTNMR spectrometer and the chemical shifts is expressed as δ ppm using TMS as
internal reference. Elemental analysis performed on Exeter Analytical Inc. ‘‘Model CEꢀ
440 CHN analyzer. All commercially available chemicals were purchased from Sigma
Aldrich.
The use of MW radiation appreciably increases the yield of the product and
decreases the reaction time. So, it provides a fast, clean methodology for the synthesis of
studied hydrazone as compared to conventional method.
The structure of the studied compound 2ꢀaminoꢀN'ꢀ((thiophenꢀ2ꢀyl) methylene)
benzohydrazide (ATMBH) is presented as Fig.1.
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Fig. 1 Structure of studied compound ATMBH
2.2 Corrosion Inhibition Study
2.2.1 Gravimetric analysis
The aggressive solution of acid were made from 98% H2SO4 supplied from E
Merck. The stock solution of studied inhibitor was made and the weight loss
measurements were performed in absence and different concentrations of ATMBH
ranged from 0.81 × 10ꢀ4ꢀ 4.07 × 10ꢀ4 M according to ASTM Gꢀ31[36]. The mild steel
coupons of size 2.5 × 2.0 × 0.025 cm3 having composition by weight: C, 0.17; Mn, 0.46;
Si, 0.26; S. 0.017; P, 0.019; balance Fe were used. All the mild steel coupons were
abraded by emery paper of grade 200ꢀ1200 prior to gravimetric analysis.
The inhibition efficiency of the studied compound ATMBH based on weight loss method
can be calculated using the Eqn [37]:
w0 − w
i
η
WL % =
×100
(1)
w0
where, w0 and wi are the weight loss values in absence and presence of inhibitor
respectively.
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2.2.2 Electrochemical Study
Electrochemical measurements were performed at the steadyꢀstate corrosion
potential using a Gamry waterꢀjacketed glass cell. A threeꢀelectrode cell consisting of
mild steel working electrode (WE), a platinum counter electrode (CE), and saturated
calomel electrode (SCE) as a reference electrode, was used for electrochemical
measurements at 30 oC. All experiments were performed in non stirring condition. The
measurements were performed using the Gamry Instrument
Potentiostat/Galvanostat/ZRA (REF 300) model (Gamry, Warminster, PA, USA). DC105
and EIS300 software by Gamry were used to perform the corrosion potential,
potentiodynamic polarization and electrochemical impedance spectroscopy (EIS).
The EIS measurements were carried out in a frequency range from 100 kHz to 0.00001
kHz under potentiostatic conditions, with amplitude of 10 mV peakꢀto peak, using the
AC signal at Ecorr. All EIS data were fit to appropriate equivalent circuit using the Gamry
Echem Analyst software. The potentiodynamic polarization curves were recorded in the
potential range of −250 to +250 mV (SCE) with a scan rate of 1 mV s−1.The
electrochemical experiments started to be collected approximately 180 min after
immersing working electrode in the solution which was proved to be sufficient to achieve
a stable value of Ecorr
.
2.3 Morphological Study
2.3.1 Atomic Force Microscopy (AFM)
The surface morphology of mild steel specimens immersed in 0.5 M H2SO4 in
absence and presence of 4.07 × 10ꢀ4 M conc. of ATMBH investigated by AFM analysis.
AFM measurements were performed using NTꢀMDT multimode atomic force
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microscope with frequency range 2.4−2.5 × 105 Hz and spring constant of 11.5 N m−1
with NOVA program used for image rendering [38]. To record the topographical images,
MS sample were prepared as in gravimetric analysis. These MS sample were immersed in
0.5 H2SO4 in absence and presence of 4.07 × 10ꢀ4 M ATMBH for 3 h; then they are taken
out, washed and dried to record the AFM images.
2.3.2 Scanning Electron Microscopy (SEM)
The effect of acid solution in absence and presence of ATMBH was also
investigated using scanning electron microscopy (SEM). The mild steel coupon was first
abraded with 200ꢀ1200 emery paper, immersed in acid solution in absence and presence
of 4.07 × 10ꢀ4 M ATMBH. After 3 h of immersion period, the MS coupons were taken
out, washed with distilled water and then SEM images were scanned with Jeol JSMꢀ5400
scanning electron microscope.
2.3.3 Contact angle measurement
In order to evaluate the affection of MS surface for bare acid as well as inhibited
acid solution, contact angle were measured by using the static sessile drop method with a
Rame–Hart goniometer (Netcong, USA). Before going to measure contact angle, the
surface of mild steel was properly cleaned to assure the mild steel surface free of grease,
dust or organic traces which can influence the contact angle measurement.
3. Results and Discussion
3.1 Synthesis
To synthesize 2ꢀaminoꢀN'ꢀ((thiophenꢀ2ꢀyl) methylene) benzohydrazide
(ATMBH) investigated as corrosion inhibitor, the reaction sequence outlined as Fig. 2
was followed. The starting material thiopheneꢀ2ꢀcarboaxaldehyde (A) was allowed to
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react with 2ꢀamino benzoyl hydrazine (B) by two methods: a & b. In method (a),
thiopheneꢀ2ꢀcarboaxaldehyde (A) was refluxed with 2ꢀamino benzoyl hydrazine (B) in a
round bottom flask in presence of glacial acetic acid as a catalyst for 5 h resulted in to
production of ATMBH whereas same product ATMBH can also be produced by method
b which involved microwave irradiation of A with B in presence of acid clay. The
method B considerably reduces the reaction time as well as greatly enhanced the yield of
product.
Fig. 2 Route of synthesis of compound ATMBH by (a) conventional method and (b) MW
assisted method
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The FTIR of this compound ATMBH shows absorption bands at 3340, 1667,
1585 and 990 cmꢀ1 (as presented in Table 2) which are assigned to ν(NH), ν(C=O),
ν(C=N) and ν(N–N).
The 1H NMR spectrum of ATMBH shows resonance signals between 6.35 and
6.75 ppm which are assigned to phenyl ring protons. ATMBH shows a low field signal at
11.567 ppm for –NH proton and is thus confirmed the presence of keto form of ATMBH;
as if enolization would have taken place then –NH proton would have been disappeared.
A doublet signal at 7.43 ppm is due to protons attached C3 and C5 of thiophene nucleus
while a triplet signal at 7.37 ppm is due to proton attached to C4 of thiophene moiety. A
signal at 7.71 ppm is observed which is assigned to ꢀNH2 proton.
The 13C NMR spectrum of ATMBH shows a group of peaks between 116.83 and
143.02 ppm attributed to the aromatic carbons of thiophene and phenyl rings. A peak at
161.78 ppm is observed due to carbonyl carbon while carbon attached to amino group
shows a peak at 149.11 ppm. There are individual signal for each thiophene nucleus
carbon as given in Table 2.
3.2 Gravimetric analysis
In order to study the effect of concentration of ATMBH on corrosion of mild steel
in 0.5 M H2SO4, gravimetric analysis have been performed at different concentration of
ATMBH and it was found that the studied compound has increasing efficiency with
increasing concentration (Fig. 3a). The result of gravimetric analysis presented as Fig. 3b
clearly indicated that the rate of corrosion mitigated considerably with an increasing
amount of studied compound ATMBH. This result supports the view that at higher
concentration there will be more and ATMBH molecule to be adsorbed on the MS
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surface and thereby lowered the corrosion rate which takes place initially due to
2ꢀ
adsorption of anion of acid used i.e. SO4 .
Fig. 3 Variation of (a) inhibition efficiency of ATMBH and (b) corrosion rate of
ATMBH against corrosion of mild steel in 0.5 M H2SO4 acid solutions in absence and
presence of different concentrations of ATMBH
3.3 Electrochemical Study
3.3.1 Electrochemical Impedance Spectroscopy (EIS)
The open circuit potential is a parameter which decides the thermodynamically
tendency of a material to be electrochemically oxidized in a corrosive medium [39, 40].
After a certain period of immersion it stabilizes around a stationary value. This potential
may vary with time because of the changed surface of the electrode. Before each EIS and
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Tafel experiments, the electrode was allowed to corrode freely in H2SO4 acid solutions in
absence and presence of different concentration of ATMBH and its open circuit potential
(OCP) was recorded as a function of time up to 180 min. After this time a steadyꢀstate
OCP, corresponding to the corrosion potential (Ecorr) of the working electrode, was
obtained.
Electrochemical impedance spectroscopy is a recent and powerful tool in
corrosion and solidꢀstate laboratories. Electrochemical impedance spectroscopy was
performed in order to through some light into the correlation between electrochemical
measurements and corrosion rates. The electrochemical impedance spectroscopy was
performed in absence and presence of different concentrations of ATMBH in 0.5 H2SO4
medium after 180 min immersion of MS coupons. The EIS spectra for MS in absence and
presence of different concentration of ATMBH presented as Fig. 4. An equivalent circuit
to fit and analyze the impedance data is presented as Fig. 4d. Different impedance
parameters calculated by fitting the data to given equivalent circuit is given as Table 3.
The corrosion inhibition efficiency of inhibitor using EIS data can be calculated using the
Eqn:
Rpi − Rp0
η
EIS % =
×100
(1)
Rpi
Rpi
Rp0
polarization resistance in presence and absence of inhibitor,
where, and
respectively. This polarization resistance Rp can be calculated by adding Rads (adsorption
resistance) to the Rct (charge transfer resistance), i.e. Rp = Rct + Rads . An increase in Rp with
increasing concentration of ATMBH indicates charge transfer process control the
corrosion of mild steel.
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Fig. 4 (a) Nyquist plots, (b) Bode magnitude plot , (c) Phase angle plot for the corrosion of mild steel in 0.5 M H2SO4 in the absence and
presence of different concentrations of ATMBH and (d) Equivalent circuit proposed to fit the EIS experimental data
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Table 3 Impedance parameters for Mild Steel in 0.5 M H2SO4 in the absence and presence of different concentration studied inhibitor ATMBH
Conc. of ATMBH
(M × 10ꢀ4)
Rs
Rct
n
Y0
Cdl
Cads
Rads
Rp
ηEIS%
(ꢁ cm2)(ꢁ cm2)
(10ꢀ6 ꢁꢀ1 cmꢀ2) (ꢂF cmꢀ2)
(F cmꢀ2)
(ꢁ cm2)
(ꢁ cm2)
ꢀ
0.96
1.04
1.13
1.21
1.53
1.07
16.20
34.64
51.48
116.6
496.1
490.3
0.850 237.4
0.860 77.1
0.864 60.4
0.867 52.7
0.879 40.6
0.884 34.9
89.0
29.4
24.3
24.1
23.7
20.5
ꢀ10.30
ꢀ0.25
ꢀ0.14
ꢀ0.16
ꢀ0.06
ꢀ0.36
ꢀ2.0
14.20
30.83
46.25
91.30
424.53
459.60
ꢀ
0.81
1.63
2.44
3.26
4.07
ꢀ3.81
ꢀ5.23
ꢀ25.31
ꢀ71.57
ꢀ30.70
53.94
69.29
84.44
96.65
96.91
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The Nyquist and BodeꢀPhase plots of mild steel in absence and presence of
different concentrations of ATMBH are presented as Fig. 4aꢀc. Inspection of Nyquist plot
revealed that there are two different regions present; the first one is high frequency
capacitive loop and the second one is low frequency inductive loop. Increasing
concentration of ATMBH resulted into increased diameter of capacitive loop which is
indication of increased effectiveness of ATMBH against corrosion of mild steel. The
inductive loop may be associated with the adsorption of M2+ on the surface or it may be
attributed to relaxation process obtained by adsorption species like SO42ꢀ and Ha+ds [41].
The increment of impedance or decrease in Cdl with increasing ATMBH concentration
(double layer capacitance) as observed from Table 3 is due to adsorption of the same on
MS surface resulting in to increased thickness of double layer as Cdl is related to
thickness of protective layer by Eqn [42]:
ε0εr
Cdl
δorg
=
(2)
where, δorg is the thickness of protective layer,
ε
0 is dielectric constant and
ε
r is relative
dielectric constant.
Theoretically, ideal Nyquist plot should be semicircle having its centre on Xꢀaxis.
Observation of Nyquist plots obtained revealed that they are depressed semicircle with its
centre below Xꢀaxis. This is due to inhomogeniety of electrode surface or that there is
some distribution (dispersion) of the value of some physical property of the system and
thus it is required to incorporate a new element, i.e. constant phase element (CPE) in
equivalent circuit instead of ideal capacitance [43].
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Mathematically, a CPE’s impedance is given by
1
n
= Y = Q0 ( j
ω
)
(3)
Z
where, Q0 has the numerical value of admittance (1/|Z|) at
ω
=1 rad secꢀ1.
The resultant of this equation is that phase angle of CPE impedance is independent of
frequency and has a value of ꢀ(90∗n) . When n is close to unity, it resembles capacitor but
the phase angle is not 90°.The factor n is known as roughness coefficient. Its value
generally decreases with the increase in surface roughness. The factor n is defined as
1− 2
180
α
n =
(4)
is the depression angle which is the measure of depression of Nyquist plot.
The decreasing phase angle with increasing concentration of ATMBH is
where,
α
attributed to decrease in capacitive behavior at the metal solution interface due to
decreased MS dissolution rate.
The value of value of n decides the behavior of CPE [44], if n = 0, Y0 = R, CPE
represents resistance), it represents capacitance if n = 1, Y0 = C, inductance if n = ꢀ1, Y0 =
L and Warburg impedance if n = 0.5, Y0 =W. The Cdl values can be calculated using CPE
parameters as
n−1 1/ n
ct
Cdl = (Y0R
)
(5)
The different EIS parameters obtained by fitting experimental data to the equivalent
circuit are presented as Table 3.
3.3.2 Potentiodynamic polarization
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The current–potential relationships (cathodic and anodic) for mild steel in 0.5 M
H2SO4 in the absence and presence of different concentrations of ATMBH is shown in
Fig.5. The Tafel parameters such as corrosion potential (Ecorr), corrosion current density
(icorr), cathodic & anodic slope (βc
& βa) and percentage efficiency (ηPDP%) are computed
and presented as Table 4. In this case inhibition efficiency can be calculated by using
relation:
ic0orr − iciorr
(6)
η
PDP % =
×100
ic0orr
Observation of Table 4 reveals that both cathodic and anodic current decreases in
presence of ATMBH resulted in to decreased corrosion rate. Also, examination of
cathodic and anodic Tafel slope, βc and βa indicated that anodic slope is relatively
constant while cathodic Tafel slope is increased as compared to bare acid solution. The
relatively constant value of anodic Tafel slope indicates that it is simply adsorbed on the
reaction sites of MS surface and thereby lowers the corrosion rate while increased values
of cathodic Tafel slope affects the cathodic reaction mechanism; indicated higher energy
will be required for evolution of H2 gas. The results obtained from gravimetric analysis,
electrochemical impedance spectroscopy (EIS) and Tafel polarization are in good
agreement.
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Fig. 5 Tafel polarization curves for mild steel in 0.5 M H2SO4 acid solution in absence and presence of different concentrations
of ATMBH
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New Journal of Chemistry
Table 4 Tafel polarization parameters for mild steel in absence and presence of different concentrations of ATMBH in 0.5 M H2SO4
Conc. of ATMBH
ꢀEcorr
(mV vs. SCE)
icorr
βa
βc
ηPDP%
(M × 10ꢀ4)
ꢀ
(ꢂA cmꢀ2)
731
310
218
89
(mV decꢀ1)
(mV decꢀ1)
127
469
465
477
472
468
456
73
61
71
74
65
77
ꢀ
0.81
1.63
2.44
3.26
4.07
152
165
143
165
57.6
70.2
87.9
94.1
95.6
43
33
175
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3.3.3 Adsorption isotherm
Corrosion inhibition is mainly due adsorption and therefore it is usually studied
through adsorption isotherm [45], i.e. relation between amounts of adsorbate adsorbed on
the surface of adsorbent. Depending on the forces acting between adsorbate and
adsorbent, adsorption is of two types: Physical adsorption or physisorption and chemical
adsorption or chemisorption. Physisorption involves simple wander wall’s forces or
electrostatic interaction between adsorbate and adsorbent while chemisorption involves
chemical forces between them [46]. In this study, several adsorption isotherms were
tested in order to fit the experimental data but Langmuir adsorption isotherm provides
best linear fit as its linear correlation coefficient close to unity.
The general form of the Langmuir equation is shown below,
Cinh
1
=
+ Cinh
(7)
θ
Kads
where, θ is the value of surface coverage on the mild steel, Cinh is ATMBH concentration
in the acid solution, and Kads is the equilibrium constant of the adsorptionꢀdesorption
process. The plot of Cinh/θ vs. Cinh is shown in Fig.6. The value of Kads can be calculated
from intercept of Langmuir adsorption isotherm plot which is related to Gibb’s free
energy of adsorption by Eqn [47];
ꢀGa0ds = −RT ln(55.5Kads )
(8)
where, 55.5 is the amount of water in acid solution.
The value of adsorption constant Kads and ꢀGa0ds are presented as Table 5. This
value of free energy ( ꢀGa0ds ) gives the idea about interaction between adsorbate and
adsorbent, i.e. ATMBH and the mild steel surface. In general, ꢀ20 kJ molꢀ1or less
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indicates physical interaction between adsorbate and adsorbent while ꢀ40 kJ molꢀ1 or
more indicates chemical interaction between them [48, 49]. In this study, the value of
ꢀGa0ds is 39.1 kJ molꢀ1 which indicates both type of adsorption, i.e. physisorption and
chemisorption is involved during interaction of ATMBH to MS surface. This is also
supported by lowering of activation energy at higher concentration of ATMBH. The high
value of Kads and ꢀGa0ds indicated strong interaction of ATMBH molecule with MS surface
and hence it high inhibition efficiency.
It is well known fact that corrosion inhibitors mitigate the corrosion of metal and
alloy by being adsorbed to that surface. Actually, it solvent, i.e. water molecule which
could also adsorbed on metal surface thus adsorption of inhibitor can be regarded as quasi
substitution process which depends on the composition of molecule, electrochemical
potential and physical environment, i.e. temperature of that solution.
The adsorption of molecule to the metal surface can be regarded as:
(9)
Org + xH2Oads ↔ xH2Oaq + Org ads
aq
where, x is the size ratio, i.e. number of water molecules displaced by per molecule of
inhibitor.
The charge of mild steel surface determined from the value of potential of zero
charge [50] in this study as well as in our earlier study based on thiophene
benzoylhydrazone in 1 N HCl [29] makes the surface of mild steel positively charged on
which acid anions are adsorbed. Thus, mild steel surface becomes negatively charged
which attracts protonated inhibitor molecule to be adsorbed on its surface. Thus, rate of
compared to Cl− expects
2−
corrosion is mitigated in both cases. The less adsorption of
SO4
lower inhibition efficiency of inhibitor molecule in hydrochloric acid. In addition to
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adsorption of 2− , ATMBH molecule interacted to the mild surface in a number of ways
SO4
to be adsorbed and therefore it’s excellent performance against mild steel corrosion is
justified.
The IR spectrum of passivating film of inhibitor molecule ATMBH is recorded
and is presented as Fig. 7. The peaks at 1590, 1645 and 3330 cmꢀ1 respectively assigned
to C=N, C=O and –NH are attributed to adsorption of ATMBH molecule at the mild steel
surface.
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New Journal of Chemistry
Table 5 Activation parameters for mild steel corrosion in 0.5 M H2SO4 in absence and presence of 4.07 × 10ꢀ4 M ATMBH
1
Inhibitor
ATMBH
Kads (105 × Mꢀ1)
−ꢀGa0ds (kj mol− )
Conc. of ATMBH
(M × 10ꢀ4)
E
a (kJ molꢀ1)
ꢀH* (kJ molꢀ1)
ꢀS* (J molꢀ1Kꢀ1)
1.09
ꢀ
26.74
43.82
47.98
58.71
74.91
79.12
24.01
41.09
45.25
55.98
72.18
76.39
ꢀ135.99
ꢀ88.66
ꢀ77.46
ꢀ48.86
ꢀ4.31
0.81
1.63
2.44
3.26
4.07
39.98
3.51
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Fig. 6 Langmuir adsorption isotherm plots for the adsorption of ATMBH on the surface of mild steel
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Fig. 7 IR spectrum of passivating film of the ATMBH molecule adsorbed on the mild steel surface
29