Electrochemical and nonelectrochemical analyses of cardo polyesters at the metal/0.5?M H2SO4 interface for corrosion protection

Article abstract of DOI:10.1007/s11164-019-03910-4

With the aim of decreasing the corrosion of metal specimens, two polyesters, namely 4-(1-(4-methoxyphenyl)cyclohexyl)phenyl 3-oxobutanoate (MPOB) and 4-(1-(4-methoxyphenyl)cyclohexyl)phenyl 10-oxoundecanoate (MPOU), were synthesized and utilized as corrosion inhibitors. The synthesized polyesters were characterized by Fourier-transform infrared (FT-IR) and nuclear magnetic resonance spectral analyses, followed by thermogravimetric and differential scanning calorimetry analyses. The protective effect of the polyesters on mild-steel specimens in 0.5?M H₂SO₄ medium was evaluated by nonelectrochemical and electrochemical methods. Gravimetric measurements revealed a decreased corrosion rate with increasing concentration of the inhibitors, reaching a maximum inhibition efficiency of 79.88% for MPOB and 92.98% for MPOU at 1000?ppm concentration at room temperature. The obtained experimental data were best fit by the Langmuir adsorption isotherm, suggesting monolayer adsorption. Thermodynamic parameters supported a physisorption mechanism. Electrochemical impedance spectroscopy showed increased charge-transfer resistance (R_ct), in turn decreasing the double-layer capacitance and thereby favoring good inhibition of corrosion of mild steel. Mixed-type inhibition was revealed by potentiodynamic polarization analysis, suppressing anodic metal dissolution and cathodic hydrogen evolution. The mode of adsorption of the inhibitors on the mild-steel surface was additionally evaluated by morphological study using FT-IR and atomic force microscopy (AFM) analyses.

Full text of DOI:10.1007/s11164-019-03910-4

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03910-4
Electrochemical and nonelectrochemical analyses of cardo
polyesters at the metal/0.5 M H SO interface for corrosion
2
4
protection
1
1
1
Chan Basha Nusrath Unnisa · Subramanian Chitra · Gowraraju Nirmala Devi ·
1
2
1
Ayyasamy Kiruthika · Selvaraj Mohana Roopan · Venkatesan Hemapriya ·
Ill‑Min Chung · Seung‑Hyun Kim · Mayakrishnan Prabakaran
3
3
3
Received: 17 November 2018 / Accepted: 20 June 2019
©
Springer Nature B.V. 2019
Abstract
With the aim of decreasing the corrosion of metal specimens, two polyesters,
namely 4-(1-(4-methoxyphenyl)cyclohexyl)phenyl 3-oxobutanoate (MPOB) and
4
-(1-(4-methoxyphenyl)cyclohexyl)phenyl 10-oxoundecanoate (MPOU), were syn-
thesized and utilized as corrosion inhibitors. The synthesized polyesters were char-
acterized by Fourier-transform infrared (FT-IR) and nuclear magnetic resonance
spectral analyses, followed by thermogravimetric and diꢀerential scanning calorime-
try analyses. The protective eꢀect of the polyesters on mild-steel specimens in 0.5 M
H SO medium was evaluated by nonelectrochemical and electrochemical methods.
2
4
Gravimetric measurements revealed a decreased corrosion rate with increasing con-
centration of the inhibitors, reaching a maximum inhibition eꢁciency of 79.88%
for MPOB and 92.98% for MPOU at 1000 ppm concentration at room temperature.
The obtained experimental data were best ꢂt by the Langmuir adsorption isotherm,
suggesting monolayer adsorption. Thermodynamic parameters supported a phys-
isorption mechanism. Electrochemical impedance spectroscopy showed increased
charge-transfer resistance (R ), in turn decreasing the double-layer capacitance and
ct
thereby favoring good inhibition of corrosion of mild steel. Mixed-type inhibition
was revealed by potentiodynamic polarization analysis, suppressing anodic metal
dissolution and cathodic hydrogen evolution. The mode of adsorption of the inhibi-
tors on the mild-steel surface was additionally evaluated by morphological study
using FT-IR and atomic force microscopy (AFM) analyses.
Keywords Mild steel · Corrosion · Adsorption isotherm · Corrosion rate · Acid
medium
Chan Basha Nusrath Unnisa and Ill-Min Chung contributed equally to this work.
*
Mayakrishnan Prabakaran
prabakarannitt@gmail.com; prabakaran@konkuk.ac.kr
Extended author information available on the last page of the article
Vol.:(012
1
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C. B. N. Unnisa et al.
Introduction
Mild steel has become a prominent raw material for use in various sectors due to
its interesting properties such as high mechanical strength, hardness, and ductil-
ity. Its low cost facilitates wide usage, resulting in adverse eꢀects nowadays [1]
as its properties can be deteriorated when subjected to industrial practices such as
acid pickling, descaling, and cleaning, due to the process commonly referred to
as corrosion [2]. With the aim of minimizing such corrosion processes and asso-
ciated economical losses, enormous work has been reported by many research-
ers, utilizing organic and inorganic compounds as suitable inhibitors [3–6]. Due
to the limited range of inorganic inhibitors, considerable attention has been
attracted to organic inhibitors, whose maximum solubility, adsorbing nature, and
ꢂlm-forming ability cause them to behave as eꢁcient corrosion retarders. Gen-
erally, organic compounds with heteroatoms such as N, O, or S carrying a lone
pair of electrons, aromatic rings, and π-conjugated systems behave as eꢁcient
corrosion inhibitors [7–15] for protection of metal specimens after either physi-
cal [16] or chemical adsorption [17]. However, their tedious synthetic methodol-
ogy and exorbitant price have led to severe criticism, paving the way towards the
use of polymeric compounds. Attention towards polymers for minimizing metal
dissolution is increasing nowadays due to the extensive delocalization of their π
electrons, which makes them behave as better inhibitors compared with organic
compounds [18]. To mention a few studies, Dudukcu et al. [19] used poly-5-ami-
noindole on stainless-steel medium, while sulfonated chitosan (S-CTS)-doped
polyaniline (PANI) was reported by Yi et al. [20], Ahmed et al. [21] adopted
electrophoretic deposition of chitosan on mild-steel substrates, Yabuki et al. [22]
applied cellulose nanoꢂbers as a corrosion inhibitor, Babaladimath et al. inves-
tigated the eꢀect of xanthan gum-graft-polyaniline in HCl medium [23], Qiu
et al. [24] functionalized a metal surface with polyaniline-graphene oxide, and
Yadav et al. [25] inhibited corrosion using pyranopyrazole derivatives. From the
literature, it is clear that limited work has been reported on use of polyesters as
corrosion inhibitors. Thus, to identify less hazardous replacements that still ful-
ꢂll the basic requirements such as ecofriendly nature, safe handling, availability,
and low cost, speciꢂc polyesters were chosen for the present study. Among the
various existing polymers, polyesters oꢀer excellent properties such as recycla-
bility, comfort, abrasion resistance, and biodegradability, enabling their use in
many applications. However their poor physical and mechanical properties as
well as steric hindrance due to the curved structure [26] have led to the use of
aromatic polyesters with higher mechanical and thermal stability [27] along with
the advantages of high functionality, low viscosity, and good solubility. As a tar-
get for designing polymers with satisfactory properties containing both aliphatic
1
3

Electrochemical and nonelectrochemical analyses of cardo…
and aromatic units, linear long-chain aromatic polyesters containing cardo moie-
ties and methylene units were synthesized, as their presence enhances the solu-
bility as well as the interaction required for the corrosion inhibition eꢀect [28].
To address emerging applications, the performance of two synthesized polyesters,
namely 4-(1-(4-methoxyphenyl) cyclohexyl)phenyl 3-oxobutanoate (MPOB) and
4
-(1-(4-methoxyphenyl)cyclohexyl)phenyl 10-oxoundecanoate (MPOU), for pro-
tection of metal in aggressive 0.5 M H SO medium was evaluated and compared
2
4
for the ꢂrst time in this work.
Experimental
Materials and methods
Metal coupons of appropriate dimensions were obtained mechanically followed
by successive polishing using various grades of silicon carbide sheets. Samples in
2
rod form embedded in Teꢃon with exposed area of 0.785 cm were used in electro-
chemical measurements. Analar H SO was used to prepare the corrodent at 0.5 M
2
4
concentration.
Synthesis of polymers
Step (i): Synthesis of diacid chloride (monomer I) Reꢃux of the respective dicarboxylic
acids with excess thionyl chloride in presence of small amount of pyridine as cata-
lyst resulted in formation of diacid chlorides, which were subsequently recrystal-
lized using n-hexane.
Step (ii): Synthesis of 4,4′‑(cyclohexane‑1,1‑diyl) diphenol (monomer II) The reac-
tion was initiated by charging a 500-ml round-bottomed ꢃask with cyclohexanone
(
0.1 M), phenol (1 M), and 3-mercaptopropionic acid (3-MPA, 0.01 M) followed
by a mixture of conc. HCl and acetic acid in ratio of 2:1 with gentle stirring. The
desired monomer was obtained by dissolving the resulting product in ethyl acetate
and subsequently washing with sodium bicarbonate [29].
Step (iii): Synthesis of polymers (i) 4‑(1‑(4‑methoxyphenyl)cyclohexyl)phenyl 3‑oxobu-
tanoate (MPOB) and (ii) 4‑(1‑(4‑methoxyphenyl)cyclohexyl)phenyl 10‑oxoundecanoate
(
MPOU) The polymers were synthesized following a similar procedure to that
reported [30], replacing citramide as phase-transfer catalyst with sodium lauryl sul-
fate (SLS). Thus, the cardo polyesters were synthesized by following the framework
shown in the scheme using interfacial polycondensation (IPC)
1
3

C. B. N. Unnisa et al.
+
ClOC
R'
COCl
HO
R
OH
IPC / PTC
CHCl / H O
3
2
O
C
O
O
R
O
R'
C
where
R′=–(CH )– for MPOB or –(CH ) – for MPOU.
2
2 8
Spectral characterization of polymers
FT‑IR and NMR spectral characterization
The structure of the synthesized polymers was characterized by FT-IR spectra in
−1
the range from 4000 to 400 cm using an ATR-FTIR-IR Shimadzu spectrometer,
followed by detailed NMR spectral determination (Bruker) with dimethyl sulfoxide
(
DMSO) as solvent.
Thermal analysis
The thermal response of the polyesters was studied using thermogravimetry analysis
(
TGA) and diꢀerential scanning calorimetry (DSC) by placing minimum amount of
samples in an aluminum pan and heating in the temperature range from 30 to 600 °C
at a rate of 10 °C/min.
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Evaluation of anticorrosive nature
Gravimetric measurements (inꢀuence of concentration)
For preliminary evaluation of mild-steel dissolution, gravimetric measurements
were carried out in presence of 100 ml 0.5 M H SO test solution containing vari-
2
4
ous concentrations of the additives (10, 50, 100, 500, and 1000 ppm). Prior to
each run, the mild-steel surface was polished with silicon carbide sheets and com-
pletely immersed in triplicate with the assistance of glass hooks, then extracted
after 3 h. The performance of the added additives was also examined at higher
temperatures in the range of 303–333 K for 1 h, after which they were washed,
dried, and reweighed by microanalytical balance. Using the mean values, the fol-
lowing parameters were evaluated from the expressions [31]
CR − CR
0
inh
IE (%) =
× 100
(1)
CR0
CR − CR
0
inh
Surface coverage (ꢀ) =
,
(2)
CR0
where the corrosion rate of uninhibited and inhibited medium is represented as CR
0
and CR .
inh
Electrochemical measurements
To evaluate the kinetics of the corrosion inhibition process as well as the cathodic
and anodic partial reactions taking place, a three-electrode cell was assembled
2
using mild steel with 0.785 cm exposed area as working electrode, and plati-
num and calomel as counterelectrode and reference electrode in stagnant aerated
test solution of 0.5 M H SO with net capacity of 100 ml. The electrochemical
2
4
data were computed using IVIUM CompactStat software, including both imped-
ance and polarization parameters. Measurements were carried out at steady state
open-circuit potential (OCP) in the frequency range from 10 kHz to 0.01 Hz at
peak-to-peak amplitude of 10 mV. The electrochemical parameters deduced from
the Nyquist plot were used in the expression below to evaluate the inhibition
eꢁciency:
Rct(inh) − Rct(blank)
IE (%) =
× 100,
(3)
Rct(inh)
where R
and R
refer to the charge-transfer resistance in presence and
ct(inh)
ct(blank)
absence of inhibitor. The double-layer capacitance (C ) can be determined from the
dl
maximum frequency on the imaginary impedance axis using the expression
ꢀ
ꢁ
�
max
�
f −Z
= 1∕2ꢀ R C .
ct dl
(4)
1
3

C. B. N. Unnisa et al.
Potentiodynamic polarization parameters were predicted by sweeping the
potential from −200 to +200 mV at a scan rate of 1 mV/s. From the intercept of
the extrapolated cathodic and anodic Tafel lines, E
and I
were determined,
corr
corr
followed by evaluation of the inhibition eꢁciency (IE) as
Icorr(blank) − Icorr(inh)
IE (%) =
× 100,
(5)
Icorr(blank)
where I
and Icorr(inh) represent the corrosion current in absence and presence
corr(blank)
of inhibitor.
Morphological examination
FT‑IR spectra of metal specimens
To evaluate the possibility of various sites involved in adsorption, the surface ꢂlm
formed on the mild-steel specimen after immersion in 1000 ppm of respective poly-
mers was evaluated by ATR-FT-IR spectrometry (Shimadzu).
Atomic force microscopy (AFM) analysis
Detailed inspection of the surface ꢂlm formed on the mild-steel surface was carried
out by AFM analysis using a multimode scanning probe microscope (NT-MDT),
followed by projection of the results to give roughness values.
Results and discussion
Spectral characterization of polymers
FT‑IR and NMR spectral characterization
−
1
The IR spectra (Fig. 1a, b) showed a characteristic band at 1713.83 cm and
−
1
1
719.61 cm pertaining to >C=O linkages of MPOB and MPOU, followed by
−1
−1
–
C–O–C– of ether linkages at 1216.17 cm and 1224.85 cm owing to the con-
ꢂrmation of ester groups. The stretching frequency of intervening methylene units
−1
−1
was observed at 2917.46 cm for MPOB and 2926.14 cm for MPOU. The bands
−
1
corresponding to –C=C– stretching of aromatic ring were elicited at 1515.15 cm
−1
1
and 1625.10 cm for both MPOB and MPOU. The results of H NMR spectroscopy
δ, ppm) are shown in Fig. 2a, b, revealing peaks in the range of 6.61–8.11 ppm
(
pertaining to aromatic protons of MPOB, and at 6.55–8.21 ppm for MPOU. The
–
1
CH protons of cyclohexane rings were elicited from the multiplets in the range of
.1–2.1 ppm. Multiplets at 2.2–2.6 ppm represented –CH protons attached with
2
carbonyl moieties [32].
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Fig. 1 FT-IR spectra of a MPOB and b MPOU
Thermal analysis
Close inspection of the thermogravimetric curves in Fig. 3a, b reveals that MPOB
and MPOU are stable up to 150 °C and 190 °C, followed by single-step degrada-
tion. Total char yield of 55% and 40% was elicited for MPOB and MPOU due to the
increase in alkyl spacers [33] facilitating oxidation. The DSC thermograms (Fig. 4a,
b) predicted melting endotherms at 52 °C of MPOB and 122 °C for MPOU, indi-
cating amorphous state, while crystallinity was not evident due to the absence of
1
3

C. B. N. Unnisa et al.
Fig. 2 1H NMR spectra of a MPOB and b MPOU
exothermic peaks. Increased alkyl spacers along with bulky aromatic substituents
are the main factors enhancing the glass-transition temperature (T ) [34].
g
Evaluation of anticorrosive performance
Gravimetric measurements
Inꢀuence of concentration Table 1 summarizes the eꢀect of the polymeric additives
on the mild-steel surface, revealing a decreased corrosion rate [5] with increasing
concentration (10, 50, 100, 500, and 1000 ppm). It is observed that the adsorption of
the inhibitors onto the metal surface is triggered by their functional groups, as is clear
from the surface coverage (θ) values [35] calculated using Eq. 2. The increased inhi-
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Fig. 3 TGA thermograms of a MPOB and b MPOU
bition eꢁciency of MPOU can be correlated with its high surface coverage of 0.9298,
approaching unity, which favors maximum coverage of the surface compared with
MPOB, whose θ value was 0.7988 at concentration of 1000 ppm [35]. The corrosion
inhibition action of the polymers might be attributed due to the presence of oxygen
atoms that favor interaction between nonbonding electron pairs and vacant orbitals of
the metal surface [36–38]. The inhibition eꢁciency calculated from Eq. 1 showed a
maximum of 92.98% for MPOU, which can be explained by the aromatic anchoring
sites and increased length of the alkyl chain, behaving as an electron donor due tothe
1
3

C. B. N. Unnisa et al.
Fig. 4 DSC curves of a MPOB and b MPOU
positiveinductive eꢀect [39], whose absence would result in moderate eꢁciency as
reported for polypropylene glycol [37].
Thermodynamic considerations To determine the response of the aromatic poly-
esters at high temperature, gravimetric measurements were also carried out in the
temperature range of 303–333 K for 1 h in a thermostat water bath for the selected
concentration of the inhibitors. It can be observed that the shorter time gap between
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Table 1 Inhibition eꢁciency of
Inhibitor
Conc. (ppm)
CR (mpy)
Surface cov- IE (%)
various concentrations of the
inhibitors of corrosion of mild
steel in 0.5 M H SO obtained
erage (θ)
2
4
Blank
MPOB
–
10
19.97
10.02
9.42
8.75
4.94
4.02
6.18
5.31
4.75
1.99
0.76
–
–
from mass loss measurements at
room temperature
0.4983
0.5284
0.5619
0.7526
0.7988
0.6904
0.734
49.83
52.84
56.19
75.26
79.88
69.04
73.4
5
1
5
1
0
00
00
000
MPOU
10
5
1
5
1
0
00
00
000
0.7619
0.9001
0.9298
76.19
90.01
92.98
Table 2 Inhibition eꢁciency of selected concentrations of inhibitors for corrosion of mild steel in 0.5 M
H SO obtained by weight loss measurements at higher temperature
2
4
Inhibitor Conc.
ppm)
303 K
313 K
323 K
333 K
(
IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy) IE (%) CR (mpy)
Blank
MPOB
–
10
1
1
–
17.39
–
80.13
–
102.01
69.42
57.86
45.49
46.69
40.26
15.04
–
122.34
84.93
72.29
60.76
57.86
52.23
20.00
38.80 10.64
34.26 52.67
46.65 42.75
62.19 30.30
60.57 31.59
63.36 29.36
31.95
43.28
55.41
54.23
60.53
85.26
30.58
40.91
50.34
52.71
57.31
83.65
00
000
48.55
71.79
63.08
65.98
91.11
8.95
4.91
6.42
5.92
1.55
MPOU 10
1
1
00
000
88.82
8.96
adsorption and desorption of the polyesters on the metal surface predominantly
enhanced the exposure to the acidic environment, leading to greater metal dissolution
at higher temperature and obviously lower inhibition eꢁciency [38], as presented in
Table 2. The relationship between the corrosion rate of mild steel and the temperature
of the environment can be considered in detail using thermodynamic parameters such
as the activation energy (E ), entropy of activation (ΔS), and enthalpy (ΔH) with the
a
aid of the Arrhenius equation or its alternate representation as a transition-state equa-
tion expressed as
[
(−E )∕RT]
a
Corrosion rate (CR) = A exp
,
(6)
(7)
◦ ◦
Corrosion rate (CR) = RT∕Nh exp (ΔS ∕R) exp (−ΔH ∕RT),
where R, T, E , ΔH°, and ΔS° are the universal gas constant, absolute tempera-
a
ture, energy of activation, enthalpy, and entropy. Based on the detailed Arrhenius
and transition-state plot in Fig. 5a, b, the above-mentioned parameters were eval-
uated and are presented in Table 3, providing additional mechanistic information
1
3

C. B. N. Unnisa et al.
Fig. 5 a Arrhenius and b transition plot of corrosion of mild steel in 0.5 M H SO medium in absence
2
4
and presence of inhibitors
regarding the adsorption of the additives onto the mild-steel surface. Inspection of
these results reveals that the increase in the activation energy from 51.81 kJ/mol
for the uninhibited medium to 67.53 kJ/mol and 69.55 kJ/mol after inhibition by
MPOB and MPOU corresponds to a decrease in the adsorption of additives on the
metal surface, favoring physisorption [39]. Electron transfer was less facilitated in
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Table 3 Activation parameters calculated by Arrhenius, transition state, and basic thermodynamic equa-
tions for mild-steel corrosion in 0.5 M H SO
2
4
Inhibitor Ea (kJ/mol) ΔH° (kJ/mol) ΔS° (kJ/mol/K) Heat of adsorp-
tion (Q)
ΔG° (kJ/mol)
03 313
3
323
333
Blank
MPOB 67.53
MPOU 69.55
51.81
49.17
64.89
66.91
− 55.99
− 14.23
− 17.49
–
17.02 17.57 18.13 18.69
4.39 4.53 4.68 4.82
5.36 5.54 5.72 5.89
− 25.4657
− 20.1045
the present oxidation–reduction process due to the increased energy barrier, leading
to an electrostatic interaction between the metal specimen and inhibitor, suggest-
ing physisorption, as is evident from the E values ranging from 40 to 80 kJ/mol
a
as reported earlier [40]. The endothermic nature corresponding to diꢁcult dissolu-
tion of metal is noted from the positive values of ΔH° for both the uninhibited and
inhibited medium, whose successive increase modiꢂes certain attachment sites that
are responsible for adherence onto the metallic surface, thereby reducing its stability
and shifting the inhibitor–metal equilibrium towards desorption of inhibitors [41].
The large negative value of ΔS° observed for the uninhibited solution of − 55.99 kJ/
mol/K is due to a more orderly arrangement in the rate-determining step. Thus, the
+
discharge of H ions in the rate-determining step is minimized by addition of the
inhibitors, providing a passage from a less to more orderly arrangement that favors a
low negative value of ΔS° for inhibited medium. In addition, the heat of adsorption
(
Q) evaluated from the kinetic thermodynamic expression below provides further
details on the adsorption mechanism:
ꢀ
ꢁ
Qads
ꢀ
log
= log A + log C −
,
(8)
1
− ꢀ
2.303RT
where A is a constant, C is the inhibitor concentration, θ is the occupied sites, and
1
1
−θ represents the vacant sites. Figure 6 shows a linear plot of log θ/1−θ versus
/T, whose slope (−Q/2.303) can be used to estimate the heat of adsorption (Q ).
ads
The negative values of Q shown in Table 3 support the proposed physisorption
ads
mechanism [42], indicating an inverse relationship between temperature and inhibi-
tion eꢁciency.
Adsorption isotherm One of the most critical eꢀects determining the performance
of an inhibitor on a metal surface is the adsorption, which should be strengthened
between the metal and inhibitor rather than the metal and H O molecules. To evalu-
2
ate this interaction, the calculated surface coverage (θ) values were ꢂt using various
adsorption isotherms, namely the Langmuir, Temkin, Flory–Huggins, and El-Awady
models, with the best ꢂt being observed for the Langmuir adsorption isotherm, rep-
resented as
C
1
=
+ C.
(9)
ꢀ
Kads
1
3

C. B. N. Unnisa et al.
Fig. 6 Plot of log(θ/1−θ) versus 1000/T for mild steel in 0.5 M H SO medium in absence and presence
2
4
of inhibitors
Fig. 7 Langmuir adsorption isotherm parameters for mild steel in 0.5 M H SO containing various con-
2
4
centrations of inhibitors at room temperature
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Table 4 Langmuir adsorption isotherm parameters for mild steel in 0.5 M H SO containing various
2
4
concentrations of inhibitors at room temperature
Inhibitor
Slope
K
R2
◦
−ΔGads (kJ mol−1
)
MPOB
MPOU
1.22
1.03
34.21
19.51
0.9981
0.9985
36.42
35.01
Table 5 Thermodynamic parameters for mild-steel corrosion in 0.5 M H SO at diꢀerent temperatures
2
4
calculated from van’t Hoꢀ and Gibbs–Helmholtz equations
◦
ΔGads (kJ mol−1
Inhibitor van’t Hoꢀ equation
Gibbs–Helm-
holtz equation
◦
)
ΔHads (kJ mol−1) ΔSads (kJ mol−1) ΔHads (kJ mol−1) 303 K
◦
◦
313 K 323 K 333 K
MPOB − 25.64
MPOU − 20.23
0.0135
0.0433
− 25.64
− 20.23
− 29.88 − 29.73 − 29.92 − 30.29
− 33.39 − 33.83 − 34.05 − 34.77
The plot of C/θ versus C in Fig. 7 shows a linear response with a regression
2
coeꢁcient R close to unity, an important parameter that validates the isothermal
approach, suggesting monolayer adsorption [43, 44] of the additives on the metal
surface. The adsorption–desorption (K ) coeꢁcient obtained from the intercept
ads
can be correlated with the Gibb’s free energy of adsorption (ΔG°) using the follow-
ing expression, and the data are summarized in Table 4:
◦
ꢀ
ꢁ
ΔGads = −RT ln K × A ,
ads
(10)
where R is the universal gas constant, and T and A represent the temperature and
concentration of water.
The ΔG° values for MPOB and MPOU in Table 5 of − 36.42 kJ/mol and −
3
5.01 kJ/mol predict spontaneous adsorption, as values around − 40 kJ/mol indicate
chemisorption while values up to − 20 kJ/mol indicate physisorption. The observed
values of ΔG° indicate both physisorption and chemisorption, but predominantly
physisorption, where the electrostatic interaction between the metal and inhibitor is
induced by the nonbonding electron pair of oxygen atom and aromatic π-electrons of
the added polymers [40].
Electrochemical impedance spectroscopy (EIS)
The EIS technique allows the evaluation of the mode of inhibition occurring at
the metal–electrolyte interface in the presence of selected concentrations of poly-
mers, i.e., 10, 100, and 1000 ppm. After attaining steady-state open-circuit potential
(
OCP), the impedance values were recorded; their values are presented in Table 6.
Figure 8a, b shows the impedance response of mild steel in the form of Nyquist
plots, exhibiting single capacitive semicircles that emphasize corrosion via a charge-
transfer process, whereas the mild depressed loops are due the dispersion in the
applied frequency and microroughness of the electrode surface [45]. The inhibition
1
3

C. B. N. Unnisa et al.
Table 6 Alternating-current (AC) impedance parameters for corrosion of mild steel with selected con-
centrations of inhibitors in 0.5 M H SO
2
4
Inhibitor
Conc. (ppm)
Rct (ohm cm2)
Cdl (µF/cm2)
χ2
IE (%)
Blank
MPOB
–
10
1
1
10
1
1
15.8
24.93
27.01
41.79
35.67
49.95
101.86
21.5
41.1
32.1
27.1
36.1
23.1
19.2
1.05×10−4
1.21×10−
1.14×10−
–
4
36.62
41.50
62.19
55.71
68.37
84.49
4
00
000
−
4
4
4
1.17×10
MPOU
1.19×10−
1.85×10−
00
000
1.52×10−4
eꢁciency calculated from Eq. 3 using the data obtained by ꢂtting the Randles equiv-
alent circuit reveals that, with increasing concentration, the adsorption of the inhibi-
tor on the mild-steel surface increased, revealing a pronounced inhibitory eꢀect
[
46], represented by the successive increase in the diameter of the semicircles as
well as the R values accompanied by a slowly corroding system. On investigating
ct
the polymers MPOU and MPOB at any selected concentration, the electron cloud
on the molecules (primarily due to benzene ring) enhances its adsorption, leading to
2
less metal dissolution [47]. The decrease in C from 21.5 µF/cm for the uninhibited
dl
medium to various inhibited media favors increased surface coverage and thickness
of the adsorbed layer [48], thereby hindering penetration of ions and favoring good
inhibition performance. On the other hand, the electrical capacitance alternatively
termed C , generated between the charged mild steel and the corrodent, minimizes
dl
its electrical tendency, depending on the extent of displacement of already adsorbed
water molecules and other ions [41]. The chemical thermodynamics of mild steel
(
containing a major proportion of iron) suggests its corrosion in acid medium with
the displacement of hydrogen from the acid solution [49], in contrast to the results
obtained in presence of the polymeric additives.
The Bode modulus enables a representation similar to a Nyquist plot and can
be applied to evaluate EIS data in terms of the angular frequency and phase angle
shift. The dependence of the impedance on frequency is clearly shown in Fig. 9a, b,
whereas this behavior is implicit in the complex Nyquist plot. It can be clearly seen
from these ꢂgures that the phase angle exhibited by the uninhibited medium was
−
22°, suggesting inductive behavior, whereas on addition of inhibitors, the phase
angle increased successively up to − 60°, approaching capacitive behavior, which
implies the formation of a protective barrier [50]. Generally, a corroding system
exhibits a low phase angle, which is increased by addition of inhibitors, leading to
a decreased roughness of the electrode surface and resulting in a better inhibitory
action, as predicted by Prasanna et al. [51–53].
Potentiodynamic polarization technique
This technique has widely contributed to electrochemical analysis, being based on
measurements of the current developed when varying the electrode potential over a
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Fig. 8 Nyquist plot for mild steel in 0.5 M H SO for selected concentrations of the inhibitor a MPOB
2
4
and b MPOU
selected range, enabling evaluation of electrochemical parameters such as the cor-
rosion current density (I ), corrosion potential (E ), anodic Tafel slope (b ), and
corr
corr
a
cathodic Tafel slope (b ). From the polarization curves shown in Fig. 10a, b, various
c
parameters were deduced and are presented in Table 7, from which the inhibition eꢁ-
ciency can be calculated using Eq. 5. The cathodic and anodic current branches show
linear regions that intersect at the OCP, thereby yielding the total current density (I ).
corr
1
3

C. B. N. Unnisa et al.
Fig. 9 Bode plot for mild steel in 0.5 M H SO for selected concentrations of the inhibitors a MPOB and
2
4
b MPOU
The higher values of corrosion current density (Icorr) observed for the uninhibited
medium facilitate transfer of a large number of electrons, thereby enhancing the cor-
rosion rate compared with the inhibited media, for which less electron transfer occurs.
The observed E values do not exhibit a predominant change on successive increase
corr
of the concentration of the polymers, indicating no interference with the anodic disso-
lution or cathodic hydrogen evolution independently but rather having a mixed nature
[
52]. This mixed-type inhibition is conꢂrmed by the displacement of the E val-
corr
ues<85 mV [53], being 12.1 mV in the present evaluation. These results conꢂrm that
1
3

Electrochemical and nonelectrochemical analyses of cardo…
Fig. 10 Polarization curves for mild steel in 0.5 M H SO for selected concentrations of the inhibitors a
2
4
MPOB and b MPOU
the adsorption is favored by π-electrons of aromatic ring coexisting with the lone pair
of electrons of oxygen atom, favoring the maximum inhibition eꢁciency of 64.08%
for MPOB and 82.26% for MPOU, in good agreement with other measurements. The
enhanced eꢀect on addition of the polymers is evident from the suppression of metal
dissolution, thereby hindering passage of metal ions into solution (or) hydrogen evolu-
tion by getting adsorbed at the Fe/acid interface, helping to reduce the corrosion rate
by either blocking the reactive sites or modifying the activation energy [54].
1
3

C. B. N. Unnisa et al.
Table 7 Potentiodynamic polarization parameters for corrosion of mild steel with selected concentrations
of the inhibitors in 0.5 M H SO
2
4
Inhibitor
Conc. (ppm)
Tafel slope (mV/
dec)
−Ecorr (mV)
Icorr (μA/cm2)
IE (%)
ba
bc
Blank
MPOB
–
10
63
56
58
45
54
52
50
153
166
172
173
164
159
167
476.9
468.3
465.9
464.2
476.3
475.7
474.7
896.52
552.70
451.76
322.03
360.31
295.49
159.04
–
38.35
49.61
64.08
59.81
67.04
82.26
1
1
00
000
MPOU
10
1
00
1
000
Fig. 11 FT-IR spectra of inhibited mild-steel specimens
Morphological examination
FT‑IR spectra of metal specimens
To provide insight into the functional groups involved in adsorption, FT-IR spectra
were recorded for the protective layer formed on the metal surface [42]. Analysis
−
1
of the spectra in Fig. 11 reveals a >C=O shift from 1713.83 and 1719.61 cm to
−
1
1
737.94 and 1738.90 cm for MPOB and MPOU, followed by a –C–O–C– shift
−1
−1
from 1216.17 to 1243.21 cm and 1224.85 to 1249.81 cm for MPOB and MPOU.
−
1
Also a prominent shift is observed for –CH units from 2917.46 and 2926.14 cm to
2
1
3

Electrochemical and nonelectrochemical analyses of cardo…
2
957.97 and 2962.79 cm−1 in the case of both polyesters, indicating the possibility
of methylene units favoring adsorption on the metal surface. The –C=C– stretching
−
1
of aromatic ring shifted from 1515.15 to 1565.79 cm for MPOU and from 1625.10
−1
to 1569.33 cm for MPOB, revealing the involvement of aromatic π-electrons [45].
All these observed shifts conꢂrm the involvement of adsorption in enhancing the
bond formation.
Atomic force microscopy (AFM)
AFM is an emerging morphological analysis technique for use at the nano to micro-
scale, providing values of the root-mean-square roughness (S ), average roughness
q
(
S ), and peak to valley (P–V) height. The high values of S , S , and P–V height of
a
q
a
3
12.40 nm, 244.27 nm, and 2855.15 nm recorded for the uninhibited medium imply
a rough, corrosive, bumpy structure, as shown by the two-dimensional (2D) and
three-dimensional (3D) images in Fig. 12a. Comparing the appearance and rough-
ness data (S , S , and P–V height) of the inhibited specimens with uninhibited ones,
q
a
lower values of 243.71 nm, 217.26 nm, and 2355.98 nm and 172.37 nm, 139.18 nm,
and 1741 nm are observed for the specimens inhibited by MPOB and MPOU, sug-
2
+
gesting the possibility of formation of Fe –polymer complexes owing to the rel-
ative smoother surface of MPOU compared with MPOB, as seen from the AFM
images in Fig. 12b, c [55–57].
Fig. 12 AFM images of a uninhibited, b MPOB inhibited, and c MPOU inhibited medium
1
3

C. B. N. Unnisa et al.
SO42-
SO42-
-
-
O
O
C
2
Fe
(
C H )
C
O
(
Polymer-H)+
Repulsion
-
-
-
Fe
Fe
Fe
SO42-
SO42-
O
-
Fig. 13 Schematic illustration of proposed mechanism
Mechanism
It is certain that iron metal reacts chemically in the atmosphere to form Fe O ,
2
3
which remains undisturbed until the pickling process begins [54], resulting in hydro-
gen evolution.
Fe + Atmosphere → Fe O
2
3
The polymers added to minimize the hydrogen evolution adopt their protonated
+
form [polymer-H] and hinder metal dissolution by being adsorbed either electro-
2
−
statically by already adsorbed SO or via interaction of vacant d-orbitals of the
4
metal with π-electrons or the lone pair of electrons of oxygen atoms [58]. Despite
this attraction, an electrostatic repulsion arises between the metal bearing a positive
charge and the protonated polymer, thereby enhancing the dissolution of the metal.
However, the excess negative charge generated in the vicinity minimizes the repul-
sion, resulting in the formation of Fe-complexes [45]. Although both polymers pos-
sess a similar backbone, diꢀering only in the length of the alkyl spacers, a predomi-
nant factor determining their surface coverage is the +I eꢀect, which is additionally
credited with forming aromatic anchoring sites for better protection, as illustrated by
the scheme in Fig. 13, which is in good agreement with the surface morphological
analysis by FT-IR spectroscopy, revealing characteristic absorption shifts.
Conclusions
•
Based on the discussion above, it is clear that the synthesized polyesters could be
utilized as good inhibitors for mild steel in 0.5 M H SO medium.
2
4
1
3

Electrochemical and nonelectrochemical analyses of cardo…
•
The increased alkyl length chain along with the aromatic sites of MPOU helped
it achieve the highest inhibition eꢁciency of 92.98%, while the lesser number of
alkyl moieties in MPOB resulted in lower inhibition eꢁciency of 79.88% at con-
centration of 1000 ppm.
•
•
•
At higher temperature, lower inhibition eꢁciency was found, due to desorption
of inhibitors, favouring the physisorption mechanism.
The adsorption of inhibitors on the mild-steel surface followed the Langmuir
adsorption isotherm, suggesting a combined mode of adsorption.
Electrochemical measurements revealed an increase in the charge-transfer resist-
ance and decrease in the double-layer capacitance on increasing the concentra-
tion of the inhibitors, indicating the formation of a protective barrier on the mild-
steel surface.
•
•
The E
values obtained using the polarization technique indicating a mixed
corr
type of inhibition.
Morphological studies carried out on mild-steel specimens by FT-IR and AFM
analyses also supported the formation of a ꢂlm on the metal surface.
Acknowledgements This paper was supported by the KU Research Professor Program of Konkuk
University.
References
1
2
.
.
M.R. Noor El-Din, R.K. Farag, O.E. Elazbawy, Int. J. Electrochem. Sci. 11, 815 (2016)
T. Douadi, H. Hamani, D. Daoud, M. Al-Noaimi, S. Chafaa, J. Taiwan Inst. Chem. Eng. 71, 388
(
2017)
3
.
J.N. Asegbeloyin, P.M. Ejikeme, L.O. Olasunkanmi, A.S. Adekunle, E.E. Ebenso, Materials 8, 2918
(
2015)
4
5
6
7
.
.
.
.
M. Yadav, R.R. Sinha, S. Kumar, T.K. Sarkar, RSC Adv. 5, 70832 (2015)
H. Lgaz, R. Salghi, K.S. Bhat, A. Chaouiki, Shubhalaxmi, S. Jodeh, J. Mol. Liq. 244, 154 (2017)
S. Saravanamoorthy, S. Velmathi, Prog. Org. Coat. 76, 1527 (2013)
H. Zarrok, H. Oudda, A.E. Midaoui, A. Zarrouk, B. Hammouti, M.E. Touhami, A. Attayibat, S.
Radi, R. Touzani, Res. Chem. Intermed. 38, 2051 (2012)
8
9
.
.
P. Mourya, P. Singh, R.B. Rastogi, M.M. Singh, Appl. Surf. Sci. 380, 141 (2016)
M. Bouanis, M. Tourabi, A. Nyassi, A. Zarrouk, C. Jama, F. Bentiss, Appl. Surf. Sci. 389, 952
(
2016)
1
1
1
0. X. Ma, X. Jiang, S. Xia, M. Shan, X. Li, L. Yu, Q. Tang, Appl. Surf. Sci. 371, 248 (2016)
1. M. Gopiraman, N. Selvakumaran, D. Kesavan, R. Karvembu, Prog. Org. Coat. 73, 104 (2012)
2. H. Zarrok, A. Zarrok, R. Salghi, M.E. Touhami, H. Oudda, B. Hammouti, R. Touir, F. Bentiss, S.S.
Al-Deyab, Int. J. Electrochem. Sci. 8, 6014 (2013)
1
1
3. E.B. Ituen, O. Akaranta, S.A. Umoren, J. Mol. Liq. 246, 112 (2017)
4. M. Gopiraman, N. Selvakumaran, D. Kesavan, I.S. Kim, R. Karvembu, Ind. Eng. Chem. Res. 51,
7
910 (2012)
1
1
1
1
5. A. Dutta, S.S. Panja, M.M. Nandi, D. Sukul, J. Chem. Sci. 127, 921 (2015)
6. R. Solmaz, E.A. Sahin, A. Doner, G. Kardas, Corros. Sci. 53, 3231 (2011)
7. O.K. Abiola, N.C. Oforka, Mater. Chem. Phys. 83, 315 (2004)
8. C. Jeyaprabha, S. Sathiyanarayanan, K.L.N. Phani, G. Venkatachari, Appl. Surf. Sci. 252, 966
(
2005)
1
2
2
9. M. Dudukcu, G. Avcı, Prog. Org. Coat. 97, 110 (2016)
0. Y. Yi, G. Liu, Z. Jin, D. Feng, Int. J. Electrochem. Sci. 8, 3540 (2013)
1. A. Rasha, A. Ahmed, R.A. Farghali, A.M. Fekry, Int. J. Electrochem. Sci. 7, 7270 (2012)
1
3

C. B. N. Unnisa et al.
2
2
2
2
2
2. A. Yabukia, T. Shiraiwaa, I.W. Fathona, Corros. Sci. 103, 117 (2016)
3. G. Babaladimath, V. Badalamoole, S.T. Nandibewoor, Mater. Chem. Phys. 205, 171 (2018)
4. C. Qiu, D. Liu, K. Jin, L. Fang, G. Xie, J. Robertson, Mater. Chem. Phys. 198, 90 (2017)
5. M. Yadav, L. Gope, N. Kumari, P. Yadav, J. Mol. Liq. 216, 78 (2016)
6. D. Thirumoolan, V.A. Katkar, G. Gunasekaran, T. Kanai, K.A. Basha, Prog. Org. Coat. 77, 1253
(
2014)
2
2
2
7. X. Ma, H. Niu, W. Cai, T. Xiao, C. Wang, X. Baia, Y. Zhang, W. Wang, React. Funct. Polym. 108,
3 (2016)
8. A.A. Al-Amiery, A.A.H. Kadhum, A. Kadihum, A.B. Mohamad, C.K. How, S. Junaedi, Materials 7,
87 (2014)
9. P.N. Honkhambe, N.S. Bhairamadgi, M.V. Biyani, P.P. Wadgaonkar, M.M. Salunkhe, Eur. Polym. J.
6
7
4
6, 709 (2010)
3
3
3
3
3
3
3
3
3
0. A. Muthusamy, S.C. Murugavel, Des. Monomers Polym. 6, 187 (2003)
1. I.O. Arukalam, Carbohydr. Polym. 112, 291 (2014)
2. K. Zakaria, A. Hamdy, M.A. Abbas, O.M. Abo-Elenien, J. Taiwan Inst. Chem. Eng. 65, 530 (2016)
3. G. Latha, M. Natarajan, K. Balaji, S.C. Murugavel, High Perform. Polym. 26, 125 (2014)
4. N.R. Jadhav, V.L. Gaikwad, K.J. Nair, H.M. Kadam, Asian. J. Pharm. 3, 82 (2009)
5. A.M. Alsabagh, M.A. Migahed, H.S. Awad, Corros. Sci. 48, 813 (2006)
6. H. Hamani, T. Douadi, D. Daoud, M. Al-Noaimi, S. Chafaa, Measurement 94, 837 (2016)
7. M.M. Solomon, S.A. Umoren, J. Environ. Chem. Eng. 3, 1 (2015)
8. T.K. Chaitra, K.N. Mohana, D.M. Gurudatt, H.C. Tandon, J. Taiwan Inst. Chem. Eng. 67, 521
(
2016)
3
4
4
4
4
9. M.A. Deyab, A. El-Rehim, Int. J. Electrochem. Sci. 8, 12613 (2013)
0. M.H. Hussin, A.A. Rahim, M. Nasir, M. Ibrahim, N. Brosse, Measurement 78, 90 (2016)
1. N. Anusuya, J. Saranya, P. Sounthari, A. Zarrouk, S. Chitra, J. Mol. Liq. 225, 406 (2017)
2. S.A. Umoren, U.M. Eduok, M.M. Solomon, A.P. Udoh, Arab. J. Chem. 9, 209 (2016)
3. M. Abdallah, A.M. El-Defrawy, I.A. Zaafarany, M. Sobhi, A.H.M. Elwahy, M.R. Shaaban, Int. J.
Electrochem. Sci. 9, 2186 (2014)
4
4
4
4
4. G.K.R. Solmaza, E. Altunbas, Mater. Chem. Phys. 125, 796 (2011)
5. Y. Sangeetha, S. Meenakshi, C.S. Sundaram, Carbohydr. Polym. 136, 38 (2016)
6. M.M. Solomon, S.A. Umoren, J. Colloid Interface Sci. 462, 29 (2016)
7. C.B.N. Unnisa, G. Nirmala Devi, V. Hemapriya, S. Chitra, I.M. Chung, S.H. Kim, M. Prabakaran,
Constr. Build. Mater. 165, 866 (2018)
4
4
8. A. Pal, S. Dey, D. Sukul, Res. Chem. Intermed. 42, 4531 (2016)
9. R. Anitha, S. Chitra, V. Hemapriya, I.M. Chung, S.H. Kim, M. Prabakaran, Constr. Build. Mater.
2
13, 246 (2019)
5
5
0. P. Singh, A. Singh, M.A. Quiraishi, Res. Chem. Intermed. 40, 595 (2014)
1. B.M. Prasanna, B.M. Praveen, N. Hebbar, T.V. Venkatesha, H.C. Tando, Int. J. Ind. Chem. 7, 9
(
2016)
5
5
5
2. A. Biswas, S. Pal, G. Udayabhanu, Appl. Surf. Sci. 353, 173 (2015)
3. A.A. Al-Amiery, A.A.H. Kadhum, A.B. Mohamad, S.A. Junaedi, Materials 6, 1420 (2013)
4. P.A. Lozada, O.O. Xomet, D.G. Lucero, V.N. Likhanova, A. Marco, D. Aguilar, I.V. Lijanova, E.A.
Estrada, Materials 7, 5711 (2014)
5
5
5
5
5. D. Mahalakshmi, C.B.N. Unnisa, V. Hemapriya, E.P. Subramaniam, S.M. Roopan, S. Chitra, I.M.
Chung, S.H. Kim, M. Prabakaran, Bulg. Chem. Commun. 51, 31 (2019)
6. V. Hemapriya, M. Prabakaran, K. Parameswari, S. Chitra, S.H. Kim, I.M. Chung, J. Ind. Eng.
Chem. 40, 106 (2016)
7. I.M. Chung, V. Hemapriya, K. Ponnusamy, N. Arunadevi, S. Chitra, C.H. Youn, S.H. Kim, M. Pra-
bakaran, J. Electrochem. Sci. Technol. 9(3), 238 (2018)
8. A. Jamal, M. Nasser, M.A. Sathiq, Arab. J. Chem. 9, 691 (2016)
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1
3

Electrochemical and nonelectrochemical analyses of cardo…
Aꢀliations
1
1
1
Chan Basha Nusrath Unnisa · Subramanian Chitra · Gowraraju Nirmala Devi ·
1
2
1
Ayyasamy Kiruthika · Selvaraj Mohana Roopan · Venkatesan Hemapriya ·
Ill‑Min Chung · Seung‑Hyun Kim · Mayakrishnan Prabakaran
3
3
3
1
Department of Chemistry, PSGR Krishnammal College for Women, Coimbatore, India
Department of Chemistry, VIT University, Vellore, India
2
3
Department of Crop Science, College of Sanghur Life Science, Konkuk University,
Seoul 05029, South Korea
1
3