Corrosion inhibition effect of spiropyrimidinethiones on mild steel in 15% HCl solution: Insight from electrochemical and quantum studies

Article abstract of DOI:10.1039/c5ra14406j

The effect of synthesized spiropyrimidinethiones, namely, 6′-(4-methoxyphenyl)-1′-phenyl-2′-thioxo-2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyrimidine]-2-one (MPTS) and 6′-(4-methoxyphenyl)-1′-phenyl-2′-thioxo-2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyrimidine

Full text of DOI:10.1039/c5ra14406j

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Corrosion inhibition effect of spiropyrimidinethiones on mild steel in 15%
HCl solution insight from: electrochemical and quantum studies
M. Yadav*, R.R. Sinha, Sumit Kumar, T. K. Sarkar
Department of Applied Chemistry, Indian school of Mines, Dhanbad 826004, India
Phone: +91ꢀ3262235428
Fax: +91ꢀ326ꢀ2296563
Email: yadav_drmahendra@yahoo.co.in
Abstract
The effect of synthesized spiropyrimidinethiones, namely, 6
thioxoꢀ2,3ꢀdihydroꢀ1Hꢀspiro[indolineꢀ3,4ꢀpyrimidine]ꢀ2ꢀone (MPTS) and 6
ꢀphenylꢀ2ꢀthioxoꢀ2,3ꢀdihydroꢀ1Hꢀspiro[indolineꢀ3,4ꢀpyrimidine]ꢀ2ꢀone (CPTS) on corrosion of
̕
̕ꢀ phenylꢀ2 ̕ꢀ
̕
̕
̕
̕
̕
1̕
̕
̕
̕
̕
̕
mild steel in 15% HCl solution was investigated by using weight loss and electrochemical
methods. Both inhibitors act as mixed inhibitor and their adsorption on mild steel obeyed
Langmuir’s adsorption isotherm. The potential of zero charge (EPZC) for the mild steel was
determined by electrochemical impedance spectroscopy method. Scanning electron microscopic,
Energy dispersion Xꢀray spectroscopic, FTIR, Xꢀray photoelectron spectroscopy (XPS) and
Atomic Force Microscopy were used to characterize the surface morphology of uninhibited and
inhibited mild steel specimens. The Density Functional Theory (DFT) was employed for
theoretical calculations.
Key words: Mild steel; Corrosion inhibition; EIS; Polarization; DFT calculations
1
. Introduction
Mild steel is extensively used as constructional material in various industries due to its excellent
mechanical properties and low cost, but its poor corrosion resistance in acids, restrains its utility.
Hydrochloric acid solutions are commonly used for pickling, industrial acid cleaning, acid deꢀ
scaling, and oil well acidifying processes. Acidization of petroleum oil wells for enhancing oil
production is commonly brought about by forcing 15% HCl solution through steel tubing into the
well to open up nearꢀbore channels in the formation and, hence, to increase the flow of oil.
Because of the aggressiveness of the acid solutions, mild steel corrodes severely during these
1

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processes, particularly with the use of hydrochloric acid, which results in terrible waste of both
resources and money. A corrosion inhibitor is often added to acid solutions for minimizing the
effect of corrosion on metal during these processes. The selection of appropriate inhibitors
mainly depends on its economic availability, environmental side effects, concentration,
temperature, type of acid, the presence of dissolved inorganic and/or organic substances even in
1
minor amounts and, of course, on the type of metallic material supposed to be protected .
Heterocyclic organic compounds containing sulfur, phosphorus, oxygen, nitrogen and aromatic
rings are reported in literature as most effective corrosion inhibitor for the metals in acidic
2
ꢀ9
10
medium . Some spiro compounds have been reported as good corrosion inhibitor for Al in
NaCl solution, and exhibit different inhibition performance with the difference in substituent
groups and substituent positions on the aromatic rings. The inhibitor, MPTS and CPTS contains
indoline and pyrimidine rings as two anchoring sites suitable for bonding with the metal surface.
So, it was expected that MPTS and CPTS would work as good corrosion inhibitor for mild steel
in 15% hydrochloric acid solution.
Although, the protection mechanism of organic corrosion inhibitors has not been clearly
1
1
understood , it is generally accepted that the organic inhibitors decrease the corrosion rate by
adsorbing on the metal/solution interface and blocking the active sites by displacing water
1
2ꢀ14
molecules and forming a compact barrier film on the metal surface
. The adsorption process
depends upon the nature and surface charge of the metal, the type of aggressive media, the
1
5
structure of the inhibitor and the nature of its interaction with the metal surface . Research
activities in present situations are geared towards developing the cheap, nonꢀtoxic and
environment friendly corrosion inhibitors. Theoretical calculations using Density Functional
16–18
Theory (DFT) are being used to explain the mechanism of corrosion inhibition
.
The objective of the present work was to evaluate the corrosion parameters of two selected
inhibitors (MPTS and CPTS) in mild steel/inhibitor/15% HCl solution system by means of
weight loss measurement, potentiodynamic polarization, and electrochemical impedance
spectroscopy (EIS) techniques. The choice of these compounds is based on molecular structure
considerations, i.e., the number of active centers and type of substituent’s present in these
compounds. Both the compounds have the same number of active centers but they only differ in
the substituent type (OCH in MPTS and Cl in CPTS) at the para position of phenyl group and
3
hence if a difference in the inhibition efficiency is observed, it should be predominately
2

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attributed to a difference in the electronic effect of the substituent type. Quantum chemical
calculations were performed in order to determine the correlation between the corrosion
inhibition properties of the investigated spiropyrimidinethiones and their molecular structures.
2
. Material and methods
2.1. Synthesis of inhibitors
The studied inhibitors, namely, 6′ꢀ(4ꢀmethoxyphenyl)ꢀ1′ꢀphenylꢀ2′ꢀthioxoꢀ2′,3′ꢀdihydroꢀ1′Hꢀ
spiro[indolineꢀ3,4′ꢀpyrimidine]ꢀ2ꢀone (MPTS) and 6′ꢀ(4ꢀmethoxyphenyl)ꢀ1′ꢀphenylꢀ2′ꢀthioxoꢀ
2
′,3′ꢀdihydroꢀ1′Hꢀspiro[indolineꢀ3,4′ꢀpyrimidine]ꢀ2ꢀone (CPTS) were synthesized according to
1
9
a previously reported experimental procedure as shown in Scheme 1. The purity of the
synthesized compounds was checked by thin layer chromatography (TLC).The structure of the
1
MPTS and CPTS was confirmed by elemental analysis, FTIR, and H NMR spectroscopy.
Scheme 1. Synthetic route and structure of MPTS and CPTS.
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MPTS
Yield: 58%; M.P. 187; Analytical data calculated for C H N SO ; C, 69.73; H, 4.60; N, 10.17;
2
4
19
3
2
S, 7.75 Found: C, 69.12; H, 4.58 %; N, 10.05; S, 7.48.
1
H NMR (300 MHz, DMSOꢀd6) δ ppm: δ 8.12‐8.28 (m, 13H, Ar‐H), 6.64 (s, 1H, CH), 9.85 (s,
H, 2NH), 4.68 (s, 3H, OCH3).
2
−
1
IR (KBr) λmax (cm ): 1510 (C=S), 1660 (C=O), 3240 (NH), 1160 (ꢀOCH ).
3
CPTS
Yield: 62%; M.P. 242; Analytical data calculated for C H N SOCl; C, 66.19; H, 3.84; N,
23
16
3
10.47; S, 7.67 Found: C, 65.96; H, 3.82 %; N, 10.36; S, 7.57.
1
H NMR (300 MHz, DMSOꢀd6) δ ppm: δ 8.24‐8.46 (m, 13H, Ar‐H), 6.68 (s, 1H, CH), 9.78 (s,
H, 2NH).
IR (KBr) λmax (cm ): 1500 (C=S), 1620 (C=O), 3250 (NH).
.2. Mild steel specimens
2
−
1
2
Mild steel specimens of size 6.0 cm × 2.5 cm × 0.1 cm and 1.0 cm × 1.0 cm × 0.1 cm with
composition (w%) C, 0.12; Mn, 0.11; Cu, 0.01; Si, 0.02; Sn, 0.01; P, 0.02; Ni, 0.02 and
remainder iron were employed for the weight loss and electrochemical studies, respectively. The
specimens were abraded using emery papers of different grit sizes up to 1200 grit, polished with
Al O (1 ꢁm and then 0.3 ꢁm particle size), washed with tap water followed by distilled water,
2
3
degreased with acetone, dried and stored in desiccator.
.3. Test solution
2
The test solution (15% HCl solution) was prepared by dilution of AR grade HCl with double
distilled water. The volume of test solution was 250 mL for weight loss study 150 mL for
electrochemical study. The concentration range of inhibitors employed was 25 to 150 ppm (mg
ꢀ
1
L ).
2.4 Weight Loss Measurements
Weight loss measurements were performed in the absence and presence of different
concentrations (25 to 150 ppm) of inhibitors at different temperatures (303Kꢀ 333K) according to
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2
0
the standard methods . The corrosion rate (CR), inhibition efficiency (η%) and surface coverage
2
1
(
θ) were determined by following equations :
4
8
.76×10 ×W
−
1
CR mmy
=
(1)
(
)
D× A×t
2
where, W = weight loss (g), A = area of specimen (cm ) exposed in acidic solution,
ꢀ3
t = exposure time (h), and D = density of mild steel ( g cm ).
C R − C R
0
i
θ
=
(2)
C R0
CR − CRi
0
(3)
η
(%) =
×100
CR0
where, CR and CR are corrosion rate in absence and presence of inhibitors.
0
i
2.5 Electrochemical methods
The electrochemical studies were conducted in a conventional threeꢀelectrode cell consisting of
2
mild steel sample of 1 cm exposed area as working electrode, a platinum counter electrode and a
saturated calomel electrode (SCE) as reference electrode, using CH electrochemical workstation
(Model No: CHI 760D, manufactured by CH Instruments, Austin, USA) at 303 K.
ꢀ1
Potentiodynamic polarization curves were obtained at a scan rate of 1 mVs in the potential
range ꢀ250 to +250 mV vs. SCE at open circuit potential. The linear Tafel segments of anodic
and cathodic curves were extrapolated to obtain corrosion current densities (i ). The percentage
corr
inhibition efficiency (η%), was calculated using the equation
0
corr
i
− icorr
0
corr
η
(%) =
×100
(4)
i
0
where, i
and icorr are the values of corrosion current density in the absence and presence of
corr
inhibitor, respectively.
Electrochemical impedance measurements were carried out using AC signals of
amplitude 10 mV peak to peak at the open circuit potential in the frequency range of 100 kHz to
0 mHz. The experimental impedance data were fitted to appropriate equivalent circuit using
1
ZSimpWin.3.21 software. The inhibition efficiency (η%) was calculated from charge transfer
resistance values obtained from impedance measurements using the following relation
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Rct (inh) − Rct
η
(%) =
×100
(5)
Rct (inh)
where Rct(inh) and Rct are charge transfer resistance in presence and absence of inhibitor,
respectively. The values of double layer capacitance (C ) were calculated from charge transfer
dl
2
2
resistance and CPE parameters (Y and n) using the expression
0
1
ꢀn 1/n
C = (Y R
ct
)
(6)
dl
0
where Y is CPE constant and n is CPE exponent . The value of n represents the deviation from
0
the ideal behavior and it lies between 0 and 1.
2.6. FTIR spectrum Analysis
The FTIR spectrum of the pure compound and film formed on the surface of N80 steel samples
was recorded on Perkin Elmer FTIR, (Spectrum – 2000) spectrophotometer.
2.7. Scanning electron microscopic and energy dispersive X-ray spectroscopy analysis
For surface morphological study of the uninhibited and inhibited mild steel samples, SEM and
EDX images were recorded using the instrument HITACHI S3400N.
2.8. Atomic Force Microscopy
The AFM images of polished, uninhibited and inhibited mild steel specimens were carried out
using a Nanosurf Easyscan2 instrument, Model: NTꢀMDT, Russia; Solver Proꢀ47.
2
.9. XPS ananysis
The chemical composition of adsorbed film of inhibitors on mild steel was detected by XPS
VSW Spectrometer) employing Al Ka (1486.6 eV) as the incident radiation source and the
(
binding energy of C 1s (284.6 eV) was used as a reference. XPS analysis was performed without
Argon sputtering of the specimens.
2
.10. Quantum chemical study
Complete geometrical optimization of the investigated inhibitors were performed using Density
Functional Theory (DFT) with the Becke’s three parameter exchange functional along with the
Lee–Yang–Parr nonlocal correlation functional (B3LYP) with 6ꢀ31G (d, p) basis set
2
3, 24
implemented in Gaussian 03 program package
the highest occupied and lowest unoccupied molecular orbital (EHOMO and ELUMO), energy gap
ꢂE), dipole moment (ꢀ) absolute electronegativity (χ), global hardness (γ) and softness (σ), and
. Theoretical parameters such as the energy of
(
fraction of electrons transferred (ꢂN) were determined.
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3
. Results and discussion
3.1. Weight loss measurements
3.1.1. Effect of inhibitor concentration and temperature
Corrosion parameters for mild steel in 15% HCl solution were determined from weight loss
measurement data in the absence and presence of different concentrations (25ꢀ250 ppm) of the
studied inhibitors at different temperatures (303ꢀ333 K) and listed in Table 1. Inspection of the
data in Table 1, reveals that inhibition efficiency increases with increasing the concentration of
each inhibitor up to 150 ppm concentration and after 150 ppm concentration remains almost
constant therefore 150 ppm was taken as optimum concentration and the concentration range was
taken as 25 ppm to 150 ppm for other studies. The inhibition efficiency of both inhibitors
decreases with an increase in temperature. Such behavior can be interpreted on the basis that the
inhibitors exert their action by adsorbing themselves on the mild steel surface and an increase in
temperature resulted in desorption of some adsorbed inhibitor molecules leading to a decrease in
2
5
the inhibition efficiency . It is evident from Table 1 that both inhibitors are good inhibitors
even at the concentration as low as 25 ppm. The inhibition efficiency of MPTS and CPTS at 150
ppm concentration was found to be 94.8% and 93.4% respectively, while it was 67.4% and
64.7% respectively at 25 ppm concentration at 303 K (Table 1). The inhibition efficiency of
MPTS was found to be better than CPTS at all concentrations and temperatures. The inhibitors
MPTS and CPTS have nearly same size and number of active centers, the difference between the
two inhibitors is that MPTS contains ꢀOCH substituent on phenyl ring whereas CPTS contains ꢀ
3
Cl substituent on phenyl ring present in these inhibitors. Thus, the difference in corrosion
inhibition efficiency between MPTS and CPTS is due to different nature of these substituent’s (ꢀ
OCH and ꢀCl). The delocalized πꢀelectron density at phenyl ring in case of MPTS increased due
3
to electron donating nature of methoxy (−OCH ) substituent whereas the electron density on
3
phenyl ring in case of CPTS decreased due to electron withdrawing nature of chloro (−Cl)
substituent. The higher delocalized πꢀelectron density at phenyl ring in case of MPTS as
compared to CPTS facilitate greater adsorption of MPTS on mild steel surface than CPTS,
leading to higher inhibition efficiency of MPTS than CPTS. The same effect of substituent has
2
6
been reported in literature by some researchers .
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3.1.2. Effect of exposure period
The variation of the inhibition efficiency for MPTS and CPTS at optimum concentration (150
ppm) with immersion time is shown in Figure 1. The corrosion parameters such as corrosion rate
and corrosion inhibition efficiencies obtained for MPTS and CPTS at optimum concentration
(
150 ppm) and different exposure periods (6ꢀ120 h) are shown in Table 2. It is clear from the
Fig. 1 that inhibition efficiency of MPTS and CPTS decreases with increase in exposure time (6ꢀ
20 h) at 303 K due to the partial desorption of adsorbed inhibitors on increasing the exposure
1
27
time .
.1.3. Thermodynamic and activation parameters
3
The apparent activation energy (E ) for dissolution of mild steel in 15% HCl was calculated by
a
using the Arrhenius equation:
−
Ea
(
7)
log CR =
+ log A
2
.303RT
ꢀ1
ꢀ1
where R is the molar gas constant (8.314 J K mol ), T is the absolute temperature (K) and A is
the Arrhenius preꢀexponential factor. Figure 2 (a, b) presents the Arrhenius plot of log CR
against 1/T for the corrosion of mild steel in 15% HCl solution in the absence and presence of
inhibitors (MPTS and CPTS) at concentrations ranging from 25 to 150 ppm. From Figure 1 (a,
b), the activation energy was calculated using the expression E = ꢀ(slope) × 2.303R. The
a
calculated values of E are summarized in Table 3. It is evident from Table 3 that the values of
a
the apparent activation energy for the inhibited solutions were higher than that for the
uninhibited solution and continued to increase on increasing the concentration of inhibitor. This
explains that the energy barrier of corrosion reaction increases with the concentration of inhibitor
2
8
.
From the thermodynamic and kinetic point of view, the unimolecular reactions is characterized
by following equation :
Ea−ꢀH* = RT
(8)
As observed from Table 3, for both inhibitors Ea > ꢀH* by a value which approximately equal to
RT. Hence, mild steel sample corrodes in 15% HCl solutions either in absence or presence of
different concentrations of the studied inhibitors by a unimolecular reaction.
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The values of standard enthalpy of activation (ꢂH*) and standard entropy of activation (ꢂS*) for
the formation of the activation complex in the transition state was calculated by using the
transition state equation:
*
*
RT
Nh
 ꢀS 
 ꢀH 
CR =
exp
exp −
(9)







R
RT

where, h is Planck’s constant and N is the Avogadro number, respectively.
A plot of log (CR/T) against 1/T (Figure 3 a, b) gave straight lines with a slope of −ꢂH*/2.303R
and an intercept of [log(R/Nh) +(ꢂS*/2.303R)], from which the activation thermodynamic
parameters ꢂH* and ꢂS* were calculated, as listed in Table 3. The negative value of ꢂS* for
both inhibitors indicates that the formation of the activated complex in the rate determining step
represents an association rather than a dissociation step, meaning that a decrease in disorder
2
9
takes place during the course of the transition from reactants to activated complex .
.1.4. Adsorption isotherm
3
The surface coverage (θ) for different concentrations of both studied inhibitors in 15%
hydrochloric acid obtained from weight loss method was tested graphically for fitting a suitable
adsorption isotherm. Plotting C /θ vs. Cinh yielded a straight line [Fig. 4] with a correlation
inh
2
coefficient (R ) and slope values as given in Table 4 at different temperatures. The correlation
coefficient and slope values for both inhibitors as shown in Table 3 are near to unity indicating
that the adsorption of these inhibitors obey the Langmuir adsorption isotherm represented by the
following equation.
Cinh
1
=
+ Cinh
θ
K ads
(10)
where, Cinh is the inhibitor concentration and Kads is the equilibrium constant for adsorptionꢀ
desorption process. From the intercept of Figure 3, the values of Kads were calculated and
represented in Table 4. Large values of Kads obtained for both studied inhibitors imply more
efficient adsorption and hence better corrosion inhibition efficiency. Using the values of K , the
ads
◦
values of ꢂG was calculated by using the equation:
ads
0
ads
ꢀ
G = −RT ln
(
55.5Kads
)
(11)
where R is the gas constant and T is the absolute temperature (K). The value of 55.5 is the
ꢀ1
concentration of water in solution in mol L . Calculated values of ꢁG˚ are listed in Table 4.
ads
◦
ꢀ1
Normally, a value of ꢂG around ꢀ20 kJ mol or less negative is assumed for electrostatic
ads
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interactions between inhibitor molecules and the charged metal surface (physisorption), and a
ꢀ1
value around ꢀ40 kJ mol or more negative is indicative of charge sharing or transferring from
30
organic species to the metal surface to form a coordinate bond (chemisorption) . Thus, the
◦
negative values of ꢂG in the range of ꢀ40 kJmolꢀ1 to ꢀ30 kJ molꢀ1, indicate that the adsorption
ads
mechanism of inhibitor on mild is a combination of both physisorption and chemisorptions. The
◦
calculated ꢂG
values for MPTS and CPTS were found in the range of ꢀ36.2 to –39.0 and ꢀ
ads
ꢀ1
3
6.1 to –38.8 kJmol , respectively, at different temperatures (303ꢀ333 K). These values are
between the threshold values for physical adsorption and chemical adsorption, indicating that the
adsorption process of these inhibitors at mild steel surface involve both the physical as well as
◦
chemical adsorption. More negative value of ꢂG for MPTS indicates a stronger interaction
ads
between the inhibitor molecules and the metal surface. It is apparent from the Table 3 that, the
0
negative value of ꢂG decreased on increasing the temperatures, suggesting that the adsorption
ads
of inhibitor molecules on mild steel surface is not favorable with increasing experimental
temperature, indicating that physisorption has the major contribution while chemisorption has the
3
1
minor contribution in the adsorption process. Similar conclusion was also reported by Ozcan ,
who studied the use of cystine as a corrosion inhibitor on mild steel in sulfuric acid.
The enthalpy and entropy changes for the adsorption of inhibitors (MPTS and CPTS) on mild
3
2
steel surface were determined from the basic thermodynamic equation :
0
0
0
ꢀG
= ꢀH ꢀ TꢀS
(12)
ads
ads
ads
0
0
where ꢀH
and ꢀS
are the standard enthalpy and entropy changes of adsorption process,
ads
ads
0
0
respectively. The plot of ꢀG ads versus T was linear (Fig. 5) with the slope equal to ꢀ ꢀS
and
ads
0
0
0
intercept of ꢀH ads. The calculated values of ꢀH
and ꢀS
are listed in Table 4. The ꢂHads
ads
ads
values are negative for both studied inhibitors, suggests that the adsorption of inhibitor’s
molecules on mild steel surface is an exothermic process. It has been reported in literature that an
endothermic adsorption process (ꢂHads > 0) is due to chemisorption while an exothermic
adsorption process (ꢂHads < 0) may be attributed to physisorption, chemisorption or a mixture of
33
both . In an exothermic process, physisorption is distinguished from chemisorption by
−
1
considering the absolute value of ꢂH , it is lower than 40 kJ mol for the physisorption process
ads
−1
whereas for chemisorption process approaches 100 kJ mol . In the present study, H values for
ads
−1
−1
MPTS and CPTS are ꢀ67.83 kJ mol and ꢀ66.41 kJ mol respectively, which are larger than the
common physical adsorption heat, but smaller than the common chemical adsorption heat,
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emphasizing that mixed type (both physical and chemical) adsorption take place. The similar
3
4–36
results were also reported by other authors
.
0
The negative value of ꢀS
obtained for adsorption of both the inhibitors as shown in Table 4
ads
suggested that before the adsorption of inhibitor’s molecules on the mild steel surface, inhibitor
molecules might freely move in the bulk solution, but with the progress in the adsorption,
inhibitor molecules were orderly adsorbed on the mild steel surface, as a result a decrease in
3
7
entropy is observed . From the thermodynamic principles, since the adsorption is an exothermic
3
8
process, it must be accompanied by a decrease of entropy .
3.2. Electrochemical Studies
3.2.1. Polarization studies
The potentiodynamic polarization curves of mild steel in 15% HCl solution in the absence and
presence of various concentrations of MPTS and CPTS at 303 K are shown in Fig. 6(a) and (b),
respectively. It is clear from the Fig. 6(a) and (b), that both, the anodic metal dissolution and
cathodic hydrogen evolution reactions are inhibited after the addition of inhibitors to the15%
HCl solution. The inhibition of these reactions are more pronounced with the increasing inhibitor
concentrations, results increase in inhibition efficiency. The inhibitor molecules are first
3
9
adsorbed on the mild steel surface and blocking the available reaction sites . The surface
coverage increases with increase in inhibitor concentrations. The corrosion current densities were
calculated by extrapolation of linear parts of anodic and cathodic curves to the point of
intersection. The electrochemical corrosion parameters such as corrosion potential (Ecorr), anodic
Tafel slope (β ), cathodic Tafel slope (β ), corrosion current density (icorr) and percentage
a
c
inhibition efficiency (η %) obtained from these curves are given in Table 5. It is clear from Table
that both the anodic and cathodic current density values are considerably decreased in inhibited
4
solutions when compared with the uninhibited solution. The observed phenomenon may be the
result of covering of adsorbed inhibitor molecules on the mild steel surface and decreasing
dissolution of mild steel. The minor shift in Ecorr value (15 mV) towards negative direction in
presence of inhibitor as compared to the Ecorr value in absence of inhibitor, indicating that MPTS
4
0
and CPTS acts as mixed type inhibitor with predominant control of cathodic reaction .
.2.2. EIS studies
3
Electrochemical impedance measurements were undertaken to provide information on the
kinetics of the electrochemical processes at the mild steel/acid interface. Nyquist plots for mild
1
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steel corrosion in 15% HCl solution in the absence and presence of different concentrations (25ꢀ
50 ppm) of the inhibitors MPTS and CPTS are shown in Figure 7 a, b, respectively. The
1
Nyquist plots for both inhibitors show single semicircles over the frequency range studied,
corresponding to one time constant. The capacitive loops are not perfect semicircles, because of
4
1
nonꢀhomogeneity and roughness of the mild steel surface . The impedance spectra were
analyzed by fitting the experimental data to the equivalent circuit model shown in Figure 8,
which is a parallel combination of the charge transfer resistance (R ) and the constant phase
ct
element (CPE), both in series with the solution resistance (R ). This type of electrochemical
s
4
2
equivalent circuit was reported previously to model the iron/acid interface . The CPE is used in
place of a capacitor to compensate deviations from ideal dielectric behavior arising from the
inhomogeneous nature of the electrode surfaces. The impedance of the CPE is given by
ꢀ1
ꢀn
ZCPE = Y (jω)
(13)
0
1
/2
where Y and n stand for the CPE constant and exponent, respectively, j = (−1) is an imaginary
0
ꢀ1
number, and ω is the angular frequency in rad s (ω = 2πf), where f is the frequency in Hz. The
electrochemical parameters such as solution resistance (R ), charge transfer resistance (R ) and
s
ct
CPE constants (Y and n) obtained from fitting the experimental data of Nyquist plots in the
0
equivalent circuit shown in Figure 8 are presented in Table 6. The data shown in Table 5 reveal
that the value of R increases with addition of inhibitors as compared to the blank solution, the
ct
increase in R value is attributed to the formation of a protective film at the metal/solution
ct
interface. The C value decreases on increasing the concentration of both the inhibitors,
dl
indicating the decrease in local dielectric constant and/or to an increase in the thickness of the
electrical double layer, suggesting that the inhibitor molecules are adsorbed at the metal/solution
4
3
interface .
The Bode phase angle plots (Fig. 9 a, b) show single maximum (one time constant) at
intermediate frequencies, broadening of this maximum in presence of inhibitors accounts for the
formation of a protective layer on the electrode surface. Figure 9 (a, b) shows that the impedance
value in the presence of both inhibitors is larger than in absence of inhibitors and the value of
impedance increases on increasing the concentration of both studied inhibitors. These mean that
the corrosion rate is reduced in presence of the inhibitors and continued to decreasing on
increasing the concentration of inhibitors.
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Electrochemical results (η %) are in good agreement with the results (η %) obtained by weight
loss experiments.
3
.2.3. Potential of zero charge (EPZC)
The phenomenon of adsorption is influenced by the chemical structure of the inhibitor, the
charge or dipole moment of the inhibitor molecules and the charge of the metal surface. The
potential of zero charge (EPZC) of mild steel in 15% HCl solution in presence of inhibitors was
determined by using electrochemical impedance spectroscopy method. The minimum on
the double layer capacitance (C ) versus applied voltage (E) plots (Fig. 10) is considered as the
dl
value of EPZC of the mild steel. The surface charge of the metal can be defined by the position of
the open circuit potential (E ) with respect to the EPZC . The net surface charge of the mild steel
ocp
at the open circuit potential was evaluated according to the equation:
E = E
r
− EPZC
(14)
oc p
4
4
where E is the Antropov’s “rational” corrosion potential . The positive and negative values of
r
Er suggesting the positive and negative net charge on the mild steel surface, respectively, at open
circuit potential. Table 7 shows the values of E , EPZC and E for both inhibitors at 150 ppm
ocp
r
concentration. The positive value of E for both the inhibitors with respect to EPZC , as shown in
r
Table 7, suggesting that mild steel surface is positively charged at open circuit potential in
presence of 150 ppm of these inhibitors. Since mild steel surface is positively charged therefore
−
anions (Cl ions) in aqueous hydrochloric acid solution gets adsorbed on the mild steel surface.
−
After the adsorption of the Cl ions, the mild steel surface becomes negatively charged. Hence,
+
+
the protonated positively charged form of inhibitor (MPTS and MPTS ) adsorbed on the surface
−
of mild steel via electrostatic interactions with the Cl ions already adsorbed on mild steel
surface. Other sides, the inhibitor molecules may also adsorbed on the mild steel surface via the
unshared pair of electrons present on N, O and S atoms.
3.3. Analysis of FTIR spectra
The FTIR spectra of pure inhibitors and reflectance absorption FTIR spectra of the inhibited mild
steel samples were recorded and represented in Figure 11 (a, b). The pure MPTS shows the IR
ꢀ1
bands around 3240, 1660 and 1510 cm due to presence of –NH, C=O and C=S, respectively,
ꢀ1
whereas pure CPTS shows the IR band around 3250, 1620 and 1500 cm due to presence of –
NH, C=O and C=S group, respectively. The reflectance absorption FTIR spectra of the exposed
ꢀ1
specimens in MPTS and CPTS show bands around 3180, 1640, 1460 and 3250, 1620, 1500 cm
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3

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respectively due to presence of –NH, C=O and C=S group, respectively. The shift in peak
position of –NH, C=O and C=S in reflectance absorption FTIR spectra of both the inhibitors as
compared to their pure compounds spectra, indicating the involvement of these groups in
adsorption process of inhibitors with the mild steel surface.
3.4. Scanning electron microscopy
SEM images of mild steel specimen before and after immersion in 15% HCl solution as well as
in presence of 150 ppm of MPTS and CPTS are shown in Figure 12 (aꢀd). The morphology of
the mild steel specimen before immersion (Fig. 12 a) is very smooth and shows no corrosion
while mild steel specimen immersed in 15% HCl solution in the absence of inhibitor (Fig. 12 b)
is very rough and the surface is badly damaged due to metal dissolution. However, the presence
of 150 ppm of MPTS and CPTS suppresses the rate of corrosion and surface damage has been
diminished considerably (Figs. 12 c, d) as compared to the blank solution (Fig. 12 b), suggesting
formation of a protective inhibitor film at the mild steel surface.
3.5. Energy dispersive X-ray spectroscopy
Energy dispersive Xꢀray analysis (EDX) technique was employed in order to get information
about the composition of the surface of the mild steel sample in the absence and presence of
inhibitors in 15% HCl solution. The results of EDX spectra are shown in Figure 13 (aꢀd). The
EDX spectra of uninhibited mild steel specimen contains the peaks corresponding to the element
present in mild steel whereas inhibited mild steel contains the peaks corresponding to all the
elements present in the inhibitor molecules, indicating the adsorption of inhibitor molecules at
the surface of mild steel. The percentage atomic content of various elements on the surface of
mild steel specimen before immersion, after immersion and after inhibition was determined by
EDX and summarized in Table 8. The percentage atomic content of Fe for mild steel after
immersion in 15% HCl solution is 83.12%, and those for mild steel immersed in presence of 150
ppm of MPTS and CPTS are 72.48 % and 76.38%, respectively. The suppressed percentage
atomic content of Fe in inhibited specimen as compared with the uninhibited mild steel specimen
is due to the inhibitory film formed on the mild steel surface.
3.6. Atomic force microscopy
The threeꢀdimensional AFM images of polished (before immersion), uninhibited and inhibited
mild steel specimen are shown in Figure 14 (aꢀd). The average roughness of polished mild steel
specimen (Fig. 14 a) and mild steel specimen in 15% HCl solution without inhibitor (Fig. 14 b)
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were found as 25 and 450 nm. It is clearly shown in Figure 14(b) that mild steel sample is badly
damaged due to the acid attack on surface. However, in presence of optimum concentration (150
ppm) of MPTS and CPTS as shown in Figure 14 (c, d), the average roughness were reduced to
1
30 and 180 nm, respectively. The lower value of roughness for MPTS than CPTS reveals that
MPTS protects the mild steel surface more efficiently than CPTS in 15% HCl solution.
.7. XPS analysis
3
XPS analysis was carried out in order to investigate the composition of the adsorbed films on the
steel surfaces in the presence of inhibitors. The XPS spectra were obtained for the mild steel
surfaces taken out after 24 h immersion in 15% HCl solution containing 150 ppm of MPTS and
CPTS separately. The nature of both the XPS spectra were almost similar therefore here we are
discussing only the XPS spectra of MPTS protected mild steel surface. The XPS spectra,
obtained for MPTS (C 1s, Fe 2p, O 1s, S 2p, N 1s,) is shown in Figure 15 (a–e). These spectra
show complex forms which have been assigned to the corresponding species through a
deconvolution fitting procedure using the XPS PeakꢀFit 4.1 software.
The C 1s spectrum of protected mild steel in presence of MPTS are fitted into three peaks (284.6,
287.2 and 288.6 eV) (Fig. 15 a). The peak with characteristic binding energy of 284.6 eV can be
attributed to the C–C and C–H aromatic bonds. The peak at 287.2 eV may be assigned to the
carbon atoms bonded to nitrogen. The peak at 288.6 eV may be ascribed to the carbon atom of
45
the C–O and CꢀS bond .
The Fe 2p spectrum of MPTS adsorbed films (Fig. 15 b) exhibit two peaks at 711.4 and 715.2 eV
for Fe 2p3/2 and Fe 2p1/2, respectively which are attributed to ferric compound such as Fe O3
2
and FeOOH [40]. The formation of a stable and insoluble layer of Fe O and FeOOH reduces
2
3
ionic diffusion and thus improves the corrosion resistance of the mild steel in hydrochloric acid
4
5
solution. The peak, observed at 715.4 eV is characteristic of Fe (II) species .
The deconvolution of the O 1s spectrum of MPTS may be fitted into three main peaks shown in
2−
3+
Fig. 15 (c). The peak at 530.4 eV can be assigned to O , bonded with Fe in the Fe O oxides
2
3
[
48]. The peak located at 532.4 eV is ascribed to OH−, as in FeOOH. The small peak at 533.6 eV
4
5
may be assigned to the CꢀO bond and of adsorbed water with mild steel .
The S 2p XPS spectra for MPTS protected mild steel surface displayed three peaks (Fig 15 d) at
161.4 eV, 164.4 eV and 169.2 eV assigned as sulfide species, SꢀC bond and and SꢀFe bound
4
5
respectively .
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The N 1s XPS spectra for MPTS protected mild steel surface displayed three peaks (Fig 15 e).
The first peak located at 399.6 is attributed to C–N bond. The second peak at 401.2 eV is
assigned to nitrogen atoms of indolineand/or pyrimidine ring coordinated with the steel surface
(N–Fe). The last peak at 401.6 eV is attributed to positively charged nitrogen, and could be
4
5
related to protonated nitrogen atoms in indoline and/or pyrimidine ring .
From these observations it is concluded that the XPS results support the presence of adsorbed
MPTS on the mild steel surface.
3.8. Quantum chemical analysis
In order to study the effect of molecular structure on the inhibition efficiency, quantum chemical
calculations were performed by using DFT and all the calculations were carried out with the help
of complete geometry optimization. Optimized structure, EHOMO and ELUMO of MPTS and CPTS
are shown in Figure 16 (a, b). For the calculations of quantum chemical parameters the following
4
6
equations were used :
ELUMO + EHOMO
χ
γ
= −
(15)
16)
2
ELUMO − EHOMO
(
=
2
The inverse of the global hardness is designated as the softness, σ as follows:
1
σ
=
(17)
γ
where, hardness and softness measures the stability and reactivity of a molecule. Soft molecules
are considered to be more reactive than hard ones because they can offer electron to acceptors
easily. For the simplest transfer of electrons, adsorption could occur at the part of the molecule
4
5
where σ which is a local property, has the highest value .Thus, the inhibitor having maximum
value of softness adsorbed strongly at the surface of mild steel and shows maximum inhibition
efficiency.
46
The fraction of electrons transferred (ꢂN) was calculated by using the equation :
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χFe
−
χinh
γinh
ꢀN =
(18)
2
(
γFe
+
)
4
6
where a theoretical value of χ ≈ 4.06 eV is taken for iron and γ = 0 is taken assuming that
Fe
Fe
4
7
I=A for bulk metals .
The quantum chemical parameters such as the energy of the highest occupied molecular orbital
EHOMO), the energy of the lowest unoccupied molecular orbital (ELUMO), energy gap ꢂE (ꢂE =
ELUMOꢀ EHOMO), ꢂE [ꢂE = E – EHOMO(Fe)], ꢂE [ꢂE = E – EHOMO(inhibitor)] ,
(
1
1
LUMO(inhibitor)
2
2
LUMO(Fe)
dipole moment (µ), absolute electronegativity (χ), global hardness (γ) and softness (σ), and
fraction of electrons transferred (ꢂN) were calculated and summarized in Table 9.
The reactivity of a chemical species can be defined in terms of frontier orbitals: the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [44].
According to the frontier molecular orbital (FMO) theory of chemical reactivity, the formation of
a transition state is due to interaction between HOMO and LUMO of reacting species. The
smaller the orbital energy gap (ꢂE) between the participating HOMO and LUMO, the stronger is
4
8
the interactions between two reacting species . The results listed in Table 9 show that the
interaction between the HOMO of the inhibitor and the LUMO of the Fe atom, as represented by
ꢂE [ꢂE = E – EHOMO(inhibitor)] , is stronger than that between the HOMO of the Fe atom
LUMO(Fe)
2
2
and the LUMO of the inhibitor ꢂE [ꢂE = E – EHOMO(Fe)]. In principle, the
LUMO(inhibitor)
1
1
interaction between the HOMO of the inhibitor and the LUMO of the Fe atom, ꢂE , should be
2
dominated by the interaction between the HOMO of the Fe atom and the LUMO of the inhibitor,
4
9
ꢂE , for good inhibitive property . Adsorption of the inhibitor on the mild steel surface can
1
occur on the basis of donorꢀacceptor interactions between the loneꢀpair electron of the
heteroatom present in the inhibitors and the vacant d orbital of the mild steel surface Fe atoms.
The inhibitors with high values of EHOMO have a tendency to donate electrons to appropriate
50, 51
acceptors with lowꢀenergy, empty molecular orbital
. The calculated value of EHOMO for
MPTS (−4.884 eV) and CPTS (−4.998 eV) being higher than that for Fe (−5.075 eV) indicates
that both inhibitors have a tendency to donate electrons to vacant d orbital of Fe. On the other
hand, a lower value of ELUMO (−1.747 eV) for Fe than that for MPTS (−1.176 eV) and CPTS
(
−1.146 eV) favors Fe to accept electrons. The lower values of ꢂE as compared to ꢂE1 for both
2
the inhibitors suggesting that more donation of electrons occur from inhibitor to metal.The
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higher value of EHOMO and lower value of ꢂE for MPTS as compared to CPTS, indicating that
2
MPTS is better inhibitor than CPTS.
The higher value of dipole moment of MPTS and CPTS as shown in Table 9, clearly suggest that
both inhibitors are polar compounds and can easily, donate electrons to form strong dπꢀpπ bond
5
2
.
According to HSAB theory hard acids prefer to coꢀordinate to hard bases and soft acid to soft
bases. Fe is considered as soft acid and will coꢀordinate strongly to molecule having maximum
softness (σ) value. The inhibitor MPTS, which has the highest value of σ (0.278 eV), than CPTS
has the higher inhibition efficiency than CPTS (Table 8). Generally, ꢂN shows inhibition
efficiency resulting from electrons transferred from the inhibitor molecule to the iron atom.
53
According to Lukovits et al. , if the value of ꢂN is less than 3.6, the inhibition efficiency
increases with increasing electronꢀdonating ability of the inhibitor at the metal surface. An
improvement in electronꢀreleasing power was shown by replacing one hydrogen atom of phenyl
ring by electronꢀdonating substituent (−OCH group) as in case of MPTS, which improved the
3
inhibition efficiency but at the other hand a diminishing effect has been observed by electronꢀ
withdrawing substituent (−Cl) as in case of CPTS.
Fig. 16 (a, b), reveal that the HOMO location in both inhibitors is mostly distributed in vicinity
of the pyrimidine rings and –C=S, C=O groups. The LUMO location in both inhibitors is mostly
distributed in vicinity of the pyrimidine, indoline and phenyl rings and –C=S, C=O group. These
indicate the reactive sites of the inhibitors for interaction between inhibitor molecules and mild
steel surface.
3.9. Mechanism of inhibition
Corrosion inhibition of mild steel in hydrochloric acid solution by MPTS and CPTS can be
explained on the basis of molecular adsorption. These compounds inhibit corrosion by
controlling both anodic as well as cathodic reactions. In acidic solutions these inhibitors exist as
protonated species. In both inhibitors nitrogen atoms present in the molecules can be easily
protonated in acidic solution and convert into quaternary compounds. These protonated species
adsorbed on the cathodic sites of the mild steel and decrease the evolution of hydrogen. The
adsorption on anodic site occurs through πꢀelectrons of aromatic rings and lone pair of electrons
of nitrogen, sulphur and oxygen atoms which decrease the anodic dissolution of mild steel. The
inhibitors MPTS and CPTS are expected to get adsorbed on surface of mild steel through the
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lone pairs of electrons on N, S and O atoms present in these inhibitors. The schematic illustration
of different modes of adsorption on metal/acid interface is shown in Fig. 17.
4
. CONCLUSIONS
(
(
(
1) Both the synthesized spiro compounds are good corrosion inhibitor for the corrosion of mild
steel in 15% HCl solutions and inhibition efficiency of MPTS is better than CPTS.
2) The potentodynamic polarization studies showed that both the studied inhibitors are mixed type
inhibitor.
3) EIS measurements showed that charge transfer resistance (R ) increases and double layer
ct
capacitance (C ) decreases in presence of inhibitors, suggested the adsorption of the inhibitor
dl
molecules on the surface of mild steel.
(
4) The results obtained from SEM, AFM and Langmuir adsorption isotherm suggested that the
mechanism of corrosion inhibition is occurring mainly through adsorption process.
5) Quantum chemical results of MPTS and CPTS showed good correlation with the experimental
results.
(
5
. ACKNOWLEDGMENTS
The authors acknowledge the financial supports of the CSIR New Delhi and Indian School of
Mines, Dhanbad, India
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Table 1. Corrosion parameters of mild steel in 15% HCl solution in the presence and absence of inhibitors
at different temperatures, obtained from weight loss measurements.
Conc.
ppm)
303 K
313 K
323 K
333 K
(
CR
mmy )
θ
η %
CR
θ
η %
CR
mmy )
θ
η %
CR
(mmy )
θ
η %
ꢀ
1
ꢀ1
(
ꢀ
1
ꢀ1
(
(mmy )
Blank
MPTS
20.20
ꢀ
ꢀ
34.55
ꢀ
ꢀ
55.47
ꢀ
ꢀ
92.69
ꢀ
ꢀ
2
5
5
0
6.58
3.97
0.674
0.803
67.4
80.3
11.91 0.655 65.5
20.87
13.12
0.623
0.763
62.3 38.16
76.3 24.96
0.588 58.8
0.730 73.0
7.34
4.58
0.787 78.7
0.867 86.7
7
5
2.41
0.880
88.0
8.60
0.844
84.4 17.24
0.814 81.4
1
1
00
50
1.66
1.05
0.917
0.948
91.7
94.8
3.20
2.29
0.907 90.7
0.933 93.3
6.27
4.71
0.886
0.915
88.6 13.22
91.4 10.32
0.857 85.7
0.888 88.8
2
2
00
50
0.97
0.91
0.952
0.955
95.2
95.5
2.28
0.934 93.4
0.936 93.6
4.16
4.05
0.915
0.917
92.5 10.01
92.7 9.82
0.892 89.2
0.894 89.4
2.21
CPTS
2
5
7
5
0
5
7.11
4.34
2.75
1.95
1.31
0.647
0.784
0.863
0.903
0.934
0.946
0.948
64.7
78.4
86.3
90.3
93.4
94.6
94.8
12.84
8.04
5.17
3.74
2.75
0.628 62.8
0.767 76.7
0.850 85.0
0.891 89.1
0.920 92.0
0.934 93.4
0.936 93.6
22.23
14.40
9.67
0.599
0.740
0.825
0.870
0.900
0.903
0.905
59.9 40.54
74.0 27.18
82.5 18.99
87.0 14.54
90.0 11.57
90.3
0.562 56.2
0.706 70.6
0.795 79.5
0.843 84.3
0.875 87.5
0.877 87.7
0.879 87.9
1
1
00
50
7.19
5.35
2
2
00
50
90.5
2
3

RSC Advances
Page 24 of 52
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DOI: 10.1039/C5RA14406J
Table 2: Effect of time on corrosion rate and inhibition efficiency without and with 150 ppm of MPTS
and CPTS at 303 K.
Exposure
Period
Blank
MPTS
CPTS
CR
CR
η (%)
CR
η (%)
(
h)
ꢀ1
ꢀ1
ꢀ1
(
mmy )
(mmy )
1.05
(mmy )
1.31
6
20.2
94.8
92.0
90.4
88.7
86.6
84.2
93.5
91.2
89.5
86.6
84.2
82.3
2
4
8
2
6
109.4
157.9
182.6
202.4
222.6
8.75
9.62
4
15.15
20.63
27.12
35.17
16.57
24.46
31.97
39.40
7
9
1
20
Table 3. Activation parameter for mild steel in 15% HCl solution in the absence and presence of
inhibitors obtained from weight loss measurements.
ꢀ
1
*
*
ꢀ1 ꢀ1
Inhibitor
Concentration
E ( kJmol )
ꢁH (kJ/mol)
ꢁS (Jmol K )
a
(
ppm)
Blank
ꢀ
2
50
7
00
50
42.34
48.96
51.15
54.80
58.15
65.59
39.70
46.32
48.51
52.16
55.51
60.95
ꢀ89.20
ꢀ76.75
ꢀ73.31
ꢀ66.03
ꢀ58.18
ꢀ43.82
5
MPTS
5
1
1
CPTS
25
48.42
51.05
53.88
56.04
60.42
45.79
48.43
51.24
53.40
57.78
ꢀ77.90
ꢀ73.88
ꢀ67.94
ꢀ63.73
ꢀ52.44
5
7
0
5
1
1
00
50
2
4

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Table 4. Thermodynamic parameters of adsorption for studied inhibitors on mild steel in 15% HCl
solution at different temperatures.
2
Inhibitor Temperature
K)
303K
Kads
ꢂG˚ads
R
ꢂH˚ads
ꢂS˚ads
ꢀ1
ꢀ1
ꢀ1
ꢀ1 ꢀ1
(
(M )
2.4×10
2.7×10
(kJ mol )
ꢀ39.0
ꢀ38.2
ꢀ37.1
ꢀ36.2
ꢀ38.8
ꢀ38.0
ꢀ37.0
ꢀ36.1
(kJ mol )
(J mol K )
4
4
4
MPTS
0.999
0.996
0.997
0.994
0.993
0.996
0.997
0.998
313K
323K
333K
2.9×10
3.2×10
2.2×10
2.5×10
2.7×10
3.0×10
ꢀ67.83
ꢀ95
4
4
4
4
4
CPTS
303K
3
3
3
13K
23K
33K
ꢀ66.41
ꢀ91
Table 5. Electrochemical parameters determined from polarization measurements for mild steel
in 15% HCl solution in the presence and absence of different concentrations of inhibitors at
303K.
Conc.
Ecorr
βa
(mV dec )
ꢂβc
icorr
η %
ꢀ1
ꢀ1
ꢀ2
(
ppm)
(mV vs SCE)
(mV dec )
(µAcm )
Blank
ꢀ495
93
142
568
ꢀ
MPTS
2
5
7
5
0
5
ꢀ503
ꢀ498
ꢀ500
73
86
91
124
131
148
169.4
107.7
59.6
70.1
81.0
89.5
1
1
00
50
ꢀ505
ꢀ505
62
77
162
152
46.5
29.5
91.8
94.8
CPTS
2
5
7
5
0
5
ꢀ502
ꢀ504
ꢀ508
ꢀ510
ꢀ500
94
74
58
78
85
138
114
157
129
140
186.3
113.6
67.6
51.1
39.9
67.2
80.0
88.1
91.0
92.9
1
1
00
50
2
5

RSC Advances
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Table 6. Electrochemical parameters determined from electrochemical impedance measurements
for mild steel in 15% HCl solution in the presence and absence of different concentrations of
inhibitors at 303K.
Conc.
ppm)
Rs
Rct
Y0
n
Cdl
η %
2
2
ꢀ2
2
(
(ꢃ cm ) (ꢄ cm )
(µFcm )
(µF cm )
Blank
MPTS
0.93
20
573
0.845
252
ꢀ
25
50
75
0.76
0.54
0.69
66
112
161
237
157
122
0.865
0.872
0.878
123.8
86.7
70.6
69.7
82.1
87.5
1
1
00
50
0.73
0.82
230
327
91
58
0.893
0.923
57.2
41.6
91.3
93.8
CPTS
2
5
7
5
0
5
0.82
0.63
0.69
0.58
0.74
62
98
148
217
296
268
183
145
113
69
0.852
0.861
0.867
0.872
0.912
131.5
95.6
80.4
65.5
47.4
67.7
79.6
86.5
90.7
93.2
1
1
00
50
Table 7.Values of E , EPZC and E recorded for mild steel in 15% HCl solution in absence and
ocp
r
presence of 150 ppm of MPTS and CPTS at 303 °C.
Inhibitor Eocp (mV vs SCE) EPZC (mV vs SCE)
Er (mV vs SCE)
MPTS
CPTS
−508
−500
−520
−505
12
05
2
6

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Table 8. Percentage atomic contents of elements obtained from EDX spectra
Inhibitors
Mild Steel
Blank HCl
MPTS
Fe
C
S
ꢀ
Mn
0.46
0.28
0.16
0.12
Cl
ꢀ
N
ꢀ
O
ꢀ
85.26
83.12
72.48
76.38
12.46
15.68
19.26
19.84
ꢀ
2.29
0.34
0.64
ꢀ
6.36
15.24
14.12
1.84
1.72
5.36
5.48
CPTS
Table 9. Quantum chemical parameters for both inhibitors.
Inhibitor EHOMO
ELUMO
ꢂE
ꢂE1
ꢂE2
µ
γ
σ
χ
(eV)
ꢂ
ꢁN
ꢀ1
(
eV)
(eV)
(eV)
(eV)
(eV)
(D)
(eV )
(
eV)
ꢂ
Fe
MPTS
CPTS
ꢀ5.075
ꢀ4.884
ꢀ4.998
ꢀ1.747
ꢀ1.176
ꢀ
ꢂ
ꢂ
ꢂ
ꢂ
3.707
4.899
3.929
3.137
6.718
1.854 0.539 3.030
0.278
ꢀ1.146 3.852
3.251 6.034
1.926 0.519 3.072 0.256
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7

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Figures captions
Figure 1. Variation of inhibition efficiency with exposure period in presence of 150 ppm
concentration of MPTS and CPTS at 303 K.
Figure 2. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 15% HCl solution
(a) MPTS (b) CPTS.
Figure 3. Transition state plots of log CR/T versus 1000/T for mild steel in 15% HCl solution at
different concentrations (a) MPTS (b) CPTS.
Figure 4. Langmuir plots of (C /θ) versus Cinh for (a) MPTS (b) CPTS.
inh
0
Figure 5. The relationship between ꢀG and Temperature.
ads
Figure 6. Potentiodynamic polarization curves for mild steel in 15% HCl solution in the
presence and absence of inhibitor (a) MPTS (b) CPTS at 303 K.
Figure 7. Nyquist plot for mild steel in 15% HCl solution containing various concentrations of
(a) MPTS (b) CPTS at 303 K.
Figure 8. Equivalent circuit applied for fitting of the impedance spectra.
Figure 9. Bode (log f vs log|Z|) and phase angle (log f vs – phase) plots of impedance spectra
for mild steel in 15% HCl solution in the presence and absence of inhibitor (a) MPTS (b) CPTS
at 303 K.
Figure 10. The plot of C vs. applied potential for mild steel in 15% HCl solution containing
dl
150 ppm of MPTS and CPTS at 303 K.
Figure 11. FTIR spectrum of the pure inhibitors and film formed on the surface of N80 steel
specimen.
Figure 12. SEM images of mild steel in 15% HCl solution after 6 h immersion at 303K (a)
Before immersion (polished), (b) After immersion without inhibitor (c) After immersion with
150 ppm MPTS (d) After immersion with 150 ppm CPTS.
Figure 13. EDX spectra of mild steel specimens (a) Before immersion (b) After immersion
without inhibitor (c) After immersion with 150 ppm MPTS (d) After immersion with 150 ppm
CPTS.
Figure 14. AFM micrograph of mild steel surface (a) Before immersion (b) After immersion
without inhibitor (c) After immersion with 150 ppm MPTS (d) After immersion with 150 ppm
CPTS at 303 K.
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8

Page 29 of 52
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Figure 15. The XPS deconvoluted profiles for C 1s, Fe 2p, O 1s, S 2p and N 1s for mild steel
after 24 h immersion period in 15% HCl solution in the presence of 150 ppm of MPTS at 303 K.
Figure 16. The optimized structure (left), HOMO (center) and LUMO (right) distribution for
molecules (a) MPTS (b) CPTS. [Atom legend: white = H; Grey =C; blue = N; red = O Yellow=
S; Green = Cl].
Figure 17. The schematic illustration of different modes of adsorption on metal/acid interface.
2
9