Enhanced corrosion inhibitive effect of p-methoxybenzylidene-4,4′-dimorpholine assembled on nickel oxide nanoparticles for mild steel in acid medium

Article abstract of DOI:10.1039/c4ra13390k

The corrosion inhibition efficiency of p-methoxybenzylidene-4,4′-dimorpholine (p-MBDM) and p-MBDM assembled on nickel oxide nanoparticles (NiONPs) was investigated using three techniques: weight loss, electrochemical impedance spectroscopy (EIS), and galv

Full text of DOI:10.1039/c4ra13390k

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Enhanced corrosion inhibitive effect of p-
methoxybenzylidene-4,40-dimorpholine assembled
on nickel oxide nanoparticles for mild steel in acid
medium†
Cite this: RSC Adv., 2015, 5, 7098
a
a
b
a
*
*
Poonam M. Wadhwani, Divya G. Ladha, Vikram K. Panchal and Nisha K. Shah
The corrosion inhibition efficiency of p-methoxybenzylidene-4,40-dimorpholine (p-MBDM) and p-MBDM
assembled on nickel oxide nanoparticles (NiONPs) was investigated using three techniques: weight loss,
electrochemical impedance spectroscopy (EIS), and galvanostatic polarization. For the first time, p-
MBDM and p-MBDM assembled on NiONPs were used as efficient inhibitors for mild steel (MS) in 2 M
hydrochloric acid (HCl) solution. From weight loss measurements, the results show that the inhibition
efficiency increases with an increase in inhibitor concentration. Moreover, with increasing temperature,
the inhibition efficiency increases for p-MBDM assembled on NiONPs, whereas it decreases for p-MBDM
only. EIS spectra demonstrate that the charge transfer resistance in the case of p-MBDM assembled on
NiONPs is comparatively more than that of p-MBDM. Analysis of polarization data indicates that both
inhibitors act as mixed type inhibitors. The present study reveals that p-MBDM assembled on NiONPs is
more efficient than p-MBDM alone. Additionally, the characterization of synthesized products was
performed by proton nuclear magnetic resonance (1H NMR), transmission electron microscopy (TEM),
dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectroscopy, powder X-ray diffraction
(PXRD), and electrospray ionisation mass spectrometry (ESI-MS). The surface morphology of the MS was
further carried out by scanning electron microscopy (SEM) and atomic force microscopy (AFM).
Received 29th October 2014
Accepted 8th December 2014
DOI: 10.1039/c4ra13390k
www.rsc.org/advances
order to enhance the efficiency of various inhibitors for the
protection of metals.13–23
Introduction
Among various metals and their alloys, mild steel is one of
the most common types of high volume steel in production
because it is inexpensive and it possesses material properties
that are acceptable for many applications. Steel is frequently
used to manufacture structural steel, oil and gas pipes, petro-
leum, armour, knives, bullets, nuts, bolts, chains, hinges, cars,
domestic goods, etc. Hydrochloric acid solution easily causes
damage to mild steel when extensively used for cleaning, oil well
acidizing, and descaling and pickling, which results in an
enhanced corrosion rate.24 Therefore, the acid solution has to
be inhibited in order to overcome this problem. Inhibitors,
when added to acid solutions, protect mild steel from corrosion
via adsorption at the metal/solution interface by forming a
compact barrier lm and dislocating water molecules on the
surface.25 The presence of heteroatoms such as N, O, P, and S as
well as those containing multiple bonds are potent corrosion
inhibitors. A literature survey revealed that a variety of organic
inhibitors such as heterocyclic compounds and Schiff bases
have been synthesized to prevent the corrosion of mild steel in
HCl solution.26–31 However, previous research reports indicate
that the inhibitive properties of organic compounds such as
Nanoparticles are considered to be a discovery of modern
science because of their higher surface-to-volume ratio with
decreasing particle size. Various transition metal oxide nano-
particles have attracted attention in material science in many
areas, ranging from bioengineering to automotive engineering.
Many properties of nanometer-sized material have proved to be
superior as compared to those of bulk materials.1,2 Nano-
particles draw much attention due to their broad range of high
technology applications such as in smart windows, electro-
chemical supercapacitors, as a transparent p-type semicon-
ducting layer with a wide band gap, and as an antiferromagnetic
thin lm.3–5 They have been extensively used as sensors, anodes
for ion batteries, electrochromic displays, drugs, and cata-
lysts.6–12 Recently, nanoparticles such as zinc oxide, magnetite,
hematite, silica, cerium, zirconium oxide, titanium dioxide,
ingot iron, halloysite, and silver have been of great interest in
aDepartment of Chemistry, School of Sciences, Gujarat University,
Ahmedabad-380009, Gujarat, India
bR. G. Shah Science College, Vasna, Ahmedabad-382170, Gujarat, India. E-mail:
vikram60panchal@gmail.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra13390k
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surfactants and polymeric thiols have been increased by the 8H). ESI-MS (m/z): 293 (M + 1). Elemental analysis: anal. calcd
incorporation of nanoparticles.32–34
for C16H24N2O3: C, 65.73; H, 8.27; N, 9.58 found: C, 65.79; H,
For the rst time, the application of NiONPs was extended to 8.21; N, 9.49.
explore their inhibitive properties in protecting metal from
Synthesis of nickel oxide (NiO) nanoparticles. The synthesis
corrosion. The main objective of the present work was to of NiO nanoparticles was performed in two steps. The rst step
compare the corrosion inhibition properties of newly synthe- consisted of the homogeneous precipitation method, in which
sized p-methoxybenzylidene-4,40-dimorpholine (p-MBDM) and the precursor of the nano-NiO was synthesized. In this method,
p-methoxybenzylidene-4,40-dimorpholine assembled on nickel 1 g of nickel nitrate hexahydrate [Ni(NO3)2$6H2O] was accu-
oxide nanoparticles. During the synthesis procedure, the rately weighed and dissolved in 40 ml of distilled water, and 4 g
nanoparticles adsorb the organic compound and then easily of sodium hydrogen carbonate (NaHCO3) was dissolved in 40 ml
liberate it when they come in contact with aqueous hydrochloric of distilled water in a separate container. The solution of
acid. Different techniques such as the weight loss method, NaHCO3 was added drop by drop to the Ni(NO3)2$6H2O solu-
electrochemical impedance spectroscopy (EIS), galvanostatic tion, which was stirred with a magnetic stirrer at room
polarization, scanning electron microscopy (SEM), and atomic temperature until a homogeneous solution was obtained. The
force microscopy (AFM) were used to calculate the inhibition mixture was then transferred to a round bottom ask, sealed,
efficiency of p-MBDM and p-MBDM assembled on NiONPs for and the temperature was maintained at 80 ꢂC for 1 h in a
mild steel in 2 M HCl.
heating mantle. Then, the solution was ltered and thoroughly
washed with distilled water to remove the possibly adsorbed
ions and chemicals. In the second step, the collected product
was dried at 110 ꢂC and was further heated at 500 ꢂC for 2 h to
obtain the product, which was dark in color (i.e., NiO
nanoparticles).
Experimental section
Materials preparation
Mild steel specimens with the following chemical composition
(weight percentage) were used in the experiments: C ¼ 0.17%;
Mn ¼ 0.46%; Si ¼ 0.026%; Cr ¼ 0.05%; P ¼ 0.012%; Cu ¼
0.135%; Al ¼ 0.023%; Fe ¼ 99.12%. Specimens were cut into 3
cm ꢀ 3 cm ꢀ 0.12 cm pieces that were mechanically abraded
with 150, 300, and P150 (518 Wetordry)™ grade emery papers.
They were abraded to a mirror nish using jeweler's rouge and
then used for weight loss measurements. They were further
rinsed with acetone, followed by double-distilled water, dried in
warm air, and then stored in moisture-free desiccators before
Synthesis of the nanostructure p-methoxybenzylidene-4,40-
dimorpholine (p-MBDM) assembled on nickel oxide nano-
particles. The nickel oxide nanoparticles were mixed with p-
MBDM using 20 ml of acetone and stirred continuously for 2 h.
The solution was ltered, and the residue was collected and
used as a corrosion inhibitor, which is represented in Fig. 2.
Characterization of the synthesized p-MBDM, NiONPs, and
p-MBDM assembled on NiONPs. Proton nuclear magnetic
resonance (1H NMR) analysis of p-MBDM was performed using
a Bruker instrument at the National Facility for Drug Discovery
(NFDD) Centre to conrm the structure; CDCl3 was used as a
immersion in
a corrosive medium. For electrochemical
impedance measurements and galvanostatic polarization,
metal specimens of circular design with an apparent surface
area of 6.156 cm2 were used as the working electrode. The
handle and the back of the specimen were coated with Perspex
transparent thermoplastic, leaving only the circular portion of
the specimen exposed to corrosive medium. An aggressive 2 M
HCl solution was prepared by dilution of 35% HCl (Finar). The
concentration of the used inhibitors ranged from 0.01 g Lꢁ1 to
1
solvent and TMS was the internal standard with a H resonant
frequency of 400 MHz. The electrospray ionisation mass spectra
(ESI-MS) of p-MBDM were recorded using a Waters Acquity QDa-
Mass Detector instrument at room temperature. Transmission
electron microscopy (TEM) was performed to understand the
grain size and morphologies of NiONPs and p-MBDM assem-
bled on NiONPs using a JEOL JEM-2100F microscope with 200
kV accelerating voltage (at Jawaharlal Nehru University (JNU),
New Delhi). The average diameter range of the NiONPs and p-
MBDM assembled on NiONPs was obtained by dynamic light
scattering (DLS) with a Nanotrac™ 252 ultrane particle
3.00 g Lꢁ1
.
Synthesis
Synthesis of p-methoxybenzylidene-4,40-dimorpholine (p- analyzer that performed size analysis while the particles were in
MBDM). p-methoxybenzylidene-4,40-dimorpholine used in the acetone solvent. Fourier transform infrared (FTIR) spectra were
present work was synthesized using
2 g of p-methox- obtained using a Bruker Tensor 27 instrument with a frequency
ybenzaldehyde (p-MB) and 4 g of morpholine. The reaction was range from 4000 to 400 cmꢁ1 for p-MBDM and p-MBDM
carried out with mechanical stirring in a round bottom ask assembled on NiONPs in a KBr matrix. X-ray diffraction patterns
that was in an ice salt-bath, through a condensation reaction in of p-MBDM and p-MBDM assembled on NiONPs were recorded
ethanol media. The reaction mixture was stirred for 4–5 h at with a SEIFERT-FPM (XRD7) diffractometer, using Cu Ka X-ray
˚
room temperature and then refrigerated overnight. The pure lines at 1.5406 A as the radiation source at 40 kV and 30 mA
product was ltered and air-dried.35 Fig. 1 shows the synthetic power. The diffractograms were recorded in the 2q range of
pathway for p-methoxybenzylidene-4,40-dimorpholine. p-MBDM 10–90.
is insoluble in water but soluble in ethanol and acetone. Yield:
85%, m.p. 115 C. H NMR (400 MHz, CDCl3) d ppm: 5.016 (s, performed in 100 ml of 2 M HCl solution with the addition of
1H), 6.90 (d, 2H), 7.03 (d, 2H), 3.60 (m, 8H), 3.76 (s, 3H), 2.34 (m, inhibitors (p-MBDM and p-MBDM assembled on NiONPs) at
Weight loss measurements. Weight loss measurements were
1
ꢂ
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Fig. 1 Synthetic pathway for p-methoxybenzylidene-4,40-dimorpholine (p-MBDM).
concentrations of 0.01, 0.1, 0.5, 1.0, 1.5, 2.0, and 3.0 g Lꢁ1 and
without inhibitors for 4 h. The temperature effect was also
studied at 303 K, 313 K, 323 K, and 333 K for the exposure period
of 4 h using a water-circulated thermostat (Equitron). The
specimens were immersed in different inhibitor concentra-
tions, and aer 4 h, they were withdrawn, rinsed with distilled
water, and accurately weighed. The experiments were carried
out in triplicate, and the average values were reported.
Electrochemical measurements. Electrochemical impedance
spectroscopy and galvanostatic polarization studies were per-
formed to elucidate the electrochemical behavior of the inhib-
itor. Both types of measurements were carried out using a
standard three-electrode cell in which mild steel acted as a
working electrode along with a Ag/AgCl reference electrode and
a platinum counter electrode. All measurements were accom-
plished using 2 M HCl without stirring under atmospheric
conditions. A stabilization period of 30 min was allowed prior to
the electrochemical measurement, which proved to be suffi-
cient to attain a stable open circuit potential (OCP). To verify the
data, each measurement was obtained in triplicate.
Electrochemical impedance spectroscopy (EIS) was per-
formed using AUTOLAB. Alternating current impedance
measurements were carried out in open circuit potential mode
with an amplitude of 5 mV and frequency ranging from 10 kHz
to 1 Hz. Experimental impedance spectroscopy data were tted
using a frequency response analyzer (FRA).
Fig. 3 TEM images of (a) NiONPs and (b) p-MBDM assembled on
NiONPs.
In the galvanostatic polarization study, the current density
was varied in the range of 2 ꢀ 10ꢁ4 to 3.25 ꢀ 10ꢁ2 A cmꢁ2. The
potential of the working electrode (mild steel) was measured
against a saturated calomel electrode (SCE). The potential (E in
mV) and log of the current density (log I in A cmꢁ2) were plotted
to obtain various corrosion parameters. From the intersection
of the linear cathodic and anodic Tafel plots of the polarization
curves, various corrosion parameters such as Tafel slope (ba, bc),
Fig. 2 Synthetic pathway for p-methoxybenzylidene-4,40-dimorpholine (p-MBDM) assembled on nickel oxide nanoparticles (NiONPs).
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corrosion current densities (Icorr), and corrosion potentials respectively. Fig. 3a indicates that the NiONPs possess a rod-like
(Ecorr) were evaluated. shape. The shape of p-MBDM assembled on NiONPs is hexag-
Surface morphology. The surface micrographs of the mild onal, indicating that p-MBDM self-assembled on the nickel
steel specimens in different test solutions were obtained by oxide nanoparticles.36 Additionally, the TEM images reveal the
SEM and AFM. The size of the mild steel specimen for SEM and stabilization of the NiONPs due to the interaction with inhibitor
AFM was 3 cm ꢀ 3 cm ꢀ 0.12 cm, as described in Section 2.1. molecules.
The precleaned specimens were immersed for 4 h in the blank
Dynamic light scattering (DLS) measurements. To evaluate
solutions (2 M HCl) and in an inhibited solution of p-MBDM the hydrodynamic size of p-MBDM assembled on NiONPs, DLS
and p-MBDM assembled on NiONPs. Aer that, the mild steel analysis was performed. The dynamic light scattering (DLS)
specimens were taken out, washed with distilled water, dried in technique illustrates that before surface modication, the NiO
air, and submitted for SEM and AFM surface examination. SEM had an average diameter of approximately 35 nm. Aer the
observations were carried out at 100ꢀ magnication on a Carl addition of p-MBDM, the average hydrodynamic diameter
Zeiss Evo instrument at an accelerating voltage of 15 kV. AFM increased to approximately 50 nm, maintaining a size and
measurements were obtained using a Nanoscan (Nanosurf Easy stability that is close to the average hydrodynamic diameter of
Scan Controller 2) instrument.
NiONPs. Fig. 4a and b illustrates the hydrodynamic size of
NiONPs and p-MBDM assembled on NiONPs, respectively. The
polydispersity values were 0.38 and 0.98 for NiONPs and p-
MBDM assembled on NiONPs, respectively, which suggests that
the prepared products are nearly monodispersed.
Results and discussion
Effect of p-MBDM on the size of nanoparticles
Transmission electron microscope (TEM) analysis. The
morphology and size of NiONPs and p-MBDM assembled on
NiONPs were investigated by TEM. Fig. 3a and b represents the
Interaction of p-MBDM with NiONPs
Fourier transform infrared (FTIR) spectroscopy analysis. The
TEM images of NiONPs and p-MBDM assembled on NiONPs, binding mechanism of p-MBDM assembled on NiONPs was
Fig. 4 Histograms of the particle size distribution for (a) NiONPs and (b) p-MBDM assembled on NiONPs.
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evidenced by FTIR spectroscopy. Fig. 5a and b shows the FTIR Fig. 6b, it was noted that there are some additional peaks
for p-MBDM and p-MBDM assembled on NiONPs. The most observed in p-MBDM assembled on NiONPs as compared with
notable mode from p-MBDM is the CH3 stretching band p-MBDM. This conrms the formation of the nanoshell of the
centered at 3246 cmꢁ1 with several modes with a C–O stretching synthesized p-MBDM assembled on NiONPs. Moreover, the new
component in the 850–1180 cmꢁ1 region, C–C stretching at peak indicates that there is more crystallinity as compared to
1359 cmꢁ1, and aromatic ring C–H bending peaks observed at the amorphous structure of the individual p-MBDM.
1514 cmꢁ1 and 1606 cmꢁ1. The spectra of p-MBDM assembled
on NiONPs show a shiing of the –CH3 stretching mode from
3556 cmꢁ1 of the p-MBDM to 3246 cmꢁ1. Due to aggregation,
–CH3 of the stretching mode of p-MBDM bound to NiONPs.
These peak broadenings are evidence of the binding of NiONPs
with p-MBDM.
Powder X-ray diffraction (PXRD) measurements. The PXRD
diffraction patterns of p-MBDM and p-MBDM assembled on
Gravimetric data
Effect of the inhibitor concentration on the inhibition effi-
ciency and corrosion rate. To examine the inhibition potential
of inhibitors, the weight loss method is a useful, reliable, and
widely used technique. The effect of the addition of p-MBDM
and p-MBDM assembled on NiONPs with various inhibitor
NiONPs are represented in Fig. 6a and b, respectively. The
concentrations at 303 K aer 4 h immersion was investigated by
diffraction peaks for p-MBDM and p-MBDM assembled on
the weight loss technique. Corrosion parameters such as
NiONPs were found to be at 2q values of 10.4ꢂ, 18.1ꢂ, 20.5ꢂ and
surface coverage (q), inhibition efficiency (I.E.%), and corrosion
10.3ꢂ, 18.0ꢂ, 20.3ꢂ, 37.8ꢂ, 43.9ꢂ, and 63.6ꢂ, respectively. From
rate (CR) with concentration range from 0.01 g Lꢁ1 to 3.0 g Lꢁ1
Fig. 5 FTIR spectra of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.
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Fig. 6 PXRD spectra of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.
are compiled in Table 1. The corrosion rate (CR) was calculated
The surface coverage (q) and inhibition efficiency (I.E.%)
from the following equation:
were determined using the following equations:
W0 ꢁ W1
87:6W
q ¼
(2)
(3)
CR
¼
mmpy
(1)
W0
Atd
where the unit mmpy represents millimeters per year, W ¼
weight loss in milligrams, A ¼ area of the specimen in cm2
exposed in acidic solution, t ¼ immersion time in h, and d ¼
density of the material used (7.86 g cmꢁ3).
W0 ꢁ W1
I:E:ð%Þ ¼
ꢀ 100
W0
where W0 and W1 are the weight loss values in the absence and
presence of inhibitors, respectively.
Table 1 Corrosion parameters for mild steel surface in 2 M HCl in the presence of different concentrations of p-MBDM and p-MBDM assembled
on NiONPs from weight loss measurements at 308 K
Inhibitor conc. (g Lꢁ1
Blank
)
Weight loss (mg)
250.0
Surface coverage (q)
Inhibition efficiency (I.E.)
Corrosion rate (mg cmꢁ2 hꢁ1
35.6
)
p-Methoxybenzylidine-4,40-dimorpholine
0.01
0.10
0.50
1.00
1.50
2.00
3.00
149
73
27
12
6
0.244
0.404
0.492
0.620
0.700
0.756
0.828
24.4
40.4
49.2
62.0
70.0
75.6
82.8
26.9
21.2
18.1
13.5
10.7
8.7
5
4
6.1
p-Methoxybenzylidine-4,40-dimorpholine assembled on NiONPs
0.01
0.10
0.50
1.00
1.50
2.00
3.00
189
149
127
95
75
61
0.404
0.708
0.892
0.952
0.976
0.980
0.984
40.4
70.8
89.2
95.2
97.6
98.0
98.4
21.2
10.4
3.9
1.7
0.9
0.7
0.6
43
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Table 2 Corrosion rate and inhibition efficiencies at different temperatures for mild steel surface in the presence of different concentrations of
p-MBDM and p-MBDM assembled on NiONPs obtained through the weight loss method
308 K
318 K
328 K
338 K
Inhibitor
Corrosion rate
Inhibition
Corrosion rate Inhibition
Corrosion rate Inhibition
Corrosion rate Inhibition
conc. (g Lꢁ1
)
(mg mꢁ2 hꢁ1
)
efficiency (I.E.) (mg cmꢁ2 hꢁ1) efficiency (I.E.) (mg cmꢁ2 hꢁ1) efficiency (I.E.) (mg cmꢁ2 hꢁ1) efficiency (I.E.)
Blank
35.6
74.1
138.2
227.9
p-Methoxybenzylidine-4,40-dimorpholine
0.01
0.10
0.50
1.00
1.50
2.00
3.00
26.9
21.2
18.1
13.5
10.7
8.7
24.4
40.4
49.2
62.0
70.0
75.6
82.8
58.4
51.8
40.2
35.2
28.4
23.5
20.1
21.1
30.1
45.7
52.5
61.7
68.3
72.8
116.9
107.4
85.5
75.9
67.3
53.3
44.5
15.4
22.3
38.1
45.1
51.3
61.4
67.8
202.6
187.4
159.1
137.9
118.7
103.0
90.9
11.1
17.8
30.2
39.5
47.9
54.8
60.1
6.1
p-Methoxybenzylidine-4,40-dimorpholine assembled on NiONPs
0.01
0.10
0.50
1.00
1.50
2.00
3.00
21.2
10.4
3.9
1.7
0.9
40.4
70.8
89.2
95.2
97.6
98.0
98.4
45.2
25.9
8.3
4.1
1.9
39.0
65.0
88.8
94.5
97.5
98.3
98.7
86.8
49.1
17.0
11.4
3.8
37.2
64.4
87.7
91.8
97.2
98.6
99.1
151.1
84.0
30.9
20.8
10.4
6.4
33.7
63.1
86.4
90.9
95.4
97.2
99.4
0.7
0.6
1.3
1.0
2.0
1.3
1.4
Table 3 Thermodynamic parameters for adsorption of p-MBDM and
p-MBDM assembled on the surface of mild steel in 2 M HCl solution
Ea
DH*
DS*
Inhibitor conc. (g Lꢁ1
Blank
)
(kJ molꢁ1
)
(kJ molꢁ1
)
(J Kꢁ1 molꢁ1
)
53.70
51.02
ꢁ49.56
p-Methoxybenzylidine-4,40-dimorpholine
0.01
0.10
0.50
1.00
1.50
2.00
3.00
58.44
62.88
63.05
67.20
70.11
71.35
77.29
55.75
60.20
60.37
64.41
67.43
68.67
74.60
ꢁ36.55
ꢁ23.84
ꢁ25.00
ꢁ13.91
ꢁ6.04
ꢁ3.79
12.91
p-Methoxybenzylidine-4,40-dimorpholine assembled on NiONPs
0.01
0.10
0.50
1.00
1.50
2.00
3.00
56.71
56.57
60.37
73.91
71.06
60.55
26.20
54.03
53.89
57.68
71.21
68.38
57.86
23.52
ꢁ44.16
ꢁ49.36
ꢁ46.60
ꢁ9.47
ꢁ25.03
ꢁ61.14
ꢁ172.63
p-MBDM and p-MBDM assembled on NiONPs showed a
maximum inhibition efficiency of 82.8% and 98.4%, respec-
tively, at 3.0 g Lꢁ1 concentration and 303 K temperature. At a
lower concentration of inhibitor, i.e., 0.01 g Lꢁ1, the inhibition
efficiency was found to be 24.4% and 40.4% for p-MBDM and p-
MBDM assembled on NiONPs, respectively, which was almost
double. However, the weight loss and corrosion rate were found
Fig. 7 Arrhenius plots for mild steel corrosion rates (CR) in the absence
and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.
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absence and presence of p-MBDM and p-MBDM assembled on
NiONPs in 2 M HCl aer 4 h of immersion in the range of 308–
338 K using a thermostat. The inhibition efficiency values and
the corrosion rates obtained from weight loss measurements at
various temperatures (308–338 K) are summarized in Table 2.
The corrosion inhibition efficiency of p-MBDM and p-MBDM
assembled on NiONPs (up to the concentration of 1.5 g Lꢁ1) was
found to decrease, and an increase in the corrosion rate was
observed with increasing temperature. The inhibition efficiency
decreases and corrosion rate increases because of the desorp-
tion of adsorbed molecules and greater surface area of mild
steel that comes in contact with the acidic medium.39 The
inhibition efficiency for p-MBDM assembled on NiONPs having
concentrations 2.0 and 3.0 g Lꢁ1 was found to increase when the
temperature was increased from 303–308 K, which suggests that
at this concentration, the molecules effectively protect the metal
surface even at higher temperature. Moreover, p-MBDM
assembled on NiONPs was stable even at high temperature
because there was a negligible effect of temperature in HCl
solution, and the mild steel surface was protected. The activa-
tion parameters for the corrosion process were calculated from
the Arrhenius equation:40
ꢀ
ꢁ
ꢁEa
CR ¼ A exp
(4)
RT
where A represents the frequency factor, Ea is the apparent
activation energy, R is the gas constant (8.314 J Kꢁ1 molꢁ1), and
T is the absolute temperature. A plot of log CR vs. 1000/T is given
in Fig. 7a and b for p-MBDM and p-MBDM assembled on
NiONPs, respectively. The values of Ea in 2 M HCl in the absence
and presence of inhibitors were determined from the slope
values, which are listed in Table 3. The enthalpy and entropy of
activation can be calculated from an alternative form of the
Arrhenius equation:
Fig. 8 Transition-state plots for mild steel corrosion rates (CR) in the
absence and presence of (a) p-MBDM and (b) p-MBDM assembled on
NiONPs.
to decrease with an increase in the inhibitor concentration. The
corrosion rate values decreased from 35.61 to 6.125 and 0.57 mg
cmꢁ2 hꢁ1 for p-MBDM and p-MBDM assembled on NiONPs,
respectively, as the inhibitor concentration increased. This
behavior occurs because the extent of adsorption and surface
coverage due to the inhibitor on the mild steel surface increases
with the inhibitor concentration.37
This outcome supports the hypothesis that the mild steel
surface can be effectively covered by p-MBDM assembled on
NiONP molecules. The difference in the inhibition efficiency
might be due to the substituents and molecular sizes of the
inhibitors.38 Accordingly, for p-MBDM assembled on NiONPs, a
higher inhibition efficiency was attained because of the avail-
ability of an aromatic ring, the presence of electron donors (N
and O), and additional binding with NiONPs or it might be due
to the higher molecular mass of p-MBDM assembled on NiONPs
compared to p-MBDM. In addition, the surface area of NiONPs
is very large due to the nanoparticles, and thus p-MBDM
assembled on it increased the inhibition efficiency. It was
observed that p-MBDM possessed good inhibition efficiencies
against the corrosion of mild steel, but inhibition efficiencies
decrease for p-MBDM and increase for p-MBDM assembled on
NiONPs.
ꢀ
ꢁ
ꢀ
ꢁ
RT
Nh
DS*
DH*
CR
¼
exp
exp ꢁ
(5)
R
RT
where N is Avogadro's number (6.02252 ꢀ 1023 molꢁ1), h is
Planck's constant (6.626176 ꢀ 10ꢁ34 J s), DS* is the entropy of
activation, and DH* is the enthalpy of activation.
A plot of log(CR/T) against 1000/T is shown in Fig. 8a and b
for p-MBDM and p-MBDM assembled on NiONPs, respectively,
which gives a straight line with a slope of (ꢁDH*/2.303R) and an
intercept of [log(R/Nh) + (DS*/2.303R)] from which the values of
DH* and DS* were calculated and are compiled in Table 3.
The values of Ea and DH* for inhibitor-free solution are 53.70
kJ molꢁ1 and 51.02 kJ molꢁ1, respectively. The higher values of
apparent activation energy for p-MBDM and p-MBDM assem-
bled on NiONPs suggest the creation of an energy barrier for
corrosion reaction in the presence of inhibitor. The increase in
the Ea may be interpreted as physical adsorption.41,42 It is
reasonable to assume that in this case, the electrostatic cation
adsorption is responsible for the good protective properties of
the compound. However, at higher concentrations of p-MBDM
assembled on NiONPs (i.e., 3.0 g Lꢁ1), the Ea value decreases,
which signies chemisorption.43 The lone pairs of electrons on
nitrogen and oxygen are responsible for the coordination type of
Effect of temperature on the inhibition efficiency and
corrosion rate. The impact of temperature was calculated in the
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Fig. 9 Nyquist plots for mild steel in the absence and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.
(more ordered behavior) with increasing inhibition efficiency.
This suggests that p-MBDM assembled on NiONP species may
be involved in the activated complex of the corrosion reaction,
leading to a more ordered system.46
Electrochemical measurements
Fig. 10 Equivalent circuit used to fit the EIS data.
Electrochemical impedance spectroscopy data. EIS experi-
ments were carried out in order to understand the kinetics and
characteristics of the electrochemical processes on mild steel.
The analysis was done in 2 M HCl solutions and also in different
concentrations of inhibitors for p-MBDM and p-MBDM assem-
bled on NiONPs. From the shape of the Nyquist plots repre-
sented in Fig. 9a and b for p-MBDM and p-MBSM assembled on
NiONPs, respectively, mechanistic information of the reaction
at the surface of mild steel was obtained. This proposes that the
increase in the diameter of the capacitive loop in the presence of
an inhibitor is bigger than that in its absence, and its magni-
tude is a function of the inhibitor concentration. To elucidate
the EIS data, an equivalent circuit known as Randle's circuit was
used, which is represented in Fig. 10. In the equivalent circuit,
Rs represents the solution resistance, Rct represents the charge
transfer resistance, and CPE represents the constant phase
element for the mild steel/acid interface model. The values of
adsorption of inhibitors onto the mild steel surface (chemi-
sorption), and aromatic rings are responsible for weak physical
forces between inhibitors and the mild steel surface (phys-
isorption).44 Therefore, p-MBDM assembled on NiONPs shows
mixed inhibitor characteristics, i.e., physisorption and chemi-
sorption, while p-MBDM exhibits physisorption.
The positive values of DH* indicate that the dissolution
process of mild steel is difficult and endothermic in nature.45
The lower value of DH* at 3.0 g Lꢁ1 concentration indicates high
inhibition efficiency at elevated temperature. The values of DS*
are lower for uninhibited solution than that for the inhibited
solution. This reveals that an increase in randomness occurred
upon going from reactants to the activated complex. However,
the values of DS* for p-MBDM assembled on NiONPs at 2.0 and
3.0 g Lꢁ1 inhibitor concentration shi to more negative values
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Table 4 Electrochemical impedance parameters for the corrosion of mild steel in the absence and presence of p-MBDM and p-MBDM
assembled on NiONPs
Inhibitor conc. (g Lꢁ1
Blank
)
Rs (ohm cm2)
Rct (ohm cm2)
Cdl (mF cmꢁ2
)
N
Inhibition efficiency (I.E.)
0.502
2.51
1235
0.88
p-Methoxybenzylidine-4,40-dimorpholine
0.01
0.10
0.50
1.00
1.50
2.00
3.00
0.962
0.579
0.578
0.579
0.801
0.695
0.734
3.24
4.09
4.84
6.31
8.00
9.64
13.85
1209
1181
971.7
898.4
843.0
723.4
687.1
0.89
0.81
0.86
0.97
0.77
0.98
0.83
22.6
38.4
48.2
60.3
68.6
73.9
81.8
p-Methoxybenzylidine-4,40-dimorpholine assembled on NiONPs
0.01
0.10
0.50
1.00
1.50
2.00
3.00
0.580
0.719
0.619
0.787
0.633
0.510
0.549
4.080
8.220
21.49
41.40
50.80
87.50
94.50
1171
0.81
0.80
0.93
0.85
0.87
0.80
0.84
38.6
69.5
88.8
93.9
95.1
97.1
97.3
813.0
623.2
347.4
308.8
192.1
82.13
Fig. 11 Galvanostatic polarization curves for mild steel in the absence and presence of (a) p-MBDM and (b) p-MBDM assembled on NiONPs.
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Table 5 Galvanostatic polarization parameters for the corrosion of mild steel in the absence and presence of p-MBDM and p-MBDM assembled
on NiONPs
Inhibitor conc. (g Lꢁ1
Blank
)
ꢁEcorr (mV)
Icorr (mA cm2)
ba (mV decꢁ1
)
ꢁbc (mV decꢁ1
)
Inhibition efficiency (I.E.)
502
1000
329.0
362.96
—
p-Methoxybenzylidine-4,40-dimorpholine
0.01
0.10
0.50
1.00
1.50
2.00
3.00
507
510
517
515
522
523
525
776.2
616.6
524.8
398.1
316.2
251.2
186.2
297.2
289.5
352.6
328.6
348.5
323.7
312.5
357.7
346.2
369.4
425.0
390.0
452.9
468.8
22.4
38.3
47.5
60.2
68.4
74.9
81.4
p-Methoxybenzylidine-4,40-dimorpholine assembled on NiONPs
0.01
0.10
0.50
1.00
1.50
2.00
3.00
517
524
527
532
546
553
564
602.5
301.9
123.0
50.1
38.0
25.1
316.2
375.8
385
315.4
309.7
278.2
228
377.3
572.7
585.7
420.0
452.9
432.4
344.0
39.7
69.8
87.7
94.9
96.2
97.5
98.1
19.5
Rs, Rct, Cdl, n, and the inhibition efficiency are presented in strongly adsorbed on the surface of mild steel. This is due to an
Table 4. The corrosion inhibition efficiency (I.E.%) of the increase in the thickness of the electrical double layer or
inhibitors was calculated from the charge transfer resistance decrease in the local dielectric constant. These data indicate
values using the following equation:
that p-MBDM assembled on NiONPs is a more effective inhib-
itor than p-MBDM.
Rct ꢁ R0ct
I:E:ð%Þ ¼
Rct
ꢀ 100
(6)
Galvanostatic polarization data. The galvanostatic polariza-
tion curves in 2 M HCl solution for mild steel in the absence and
presence of different concentrations of p-MBDM and p-MBDM
assembled on NiONPs are shown in Fig. 11a and b, respectively.
This method includes the monitoring of the working electrode
potential by varying the current. The kinetic parameters of
electrochemical corrosion, namely, Ecorr, Tafel slopes (ba, bc),
Icorr, and I.E.% are listed in Table 5. The corrosion inhibition
efficiency (I.E.) was evaluated from the obtained Icorr values
using the following relationship:
where R0ct and Rct are the charge transfer resistance in the
absence and presence of inhibitor.
The double layer capacitance (Cdl) can be calculated by the
following equation:
1ꢁn 1/n
Cdl ¼ (QRct
)
(7)
where Q is the CPE constant and n is the CPE exponent that
provides information regarding the degree of surface inhomo-
geneity ensuing from inhibitor adsorption, surface roughness,
porous layer formation, etc. In the current work, the value of n
varies from 0.77 to 0.93, with the change in the concentration of
inhibitor due to the development of heterogeneities by the
attack of chloride ions on the surface of the mild steel. As a
result of the roughness and heterogeneity of the electrode
surface, these capacitive loops in all cases are not perfect
semicircles.
The inhibition efficiency was calculated using eqn (6). The
maximum inhibition efficiencies of 81.8% and 97.3% were
achieved from p-MBDM and p-MBDM assembled on NiONPs,
respectively, at an inhibitor concentration of 3.0 g Lꢁ1. This is
due to the increase in the value of Rct, which suggests the
formation of a protective layer at the mild steel-solution inter-
face.47 The value of Rct is much higher for p-MBDM assembled
on NiONPs than p-MBDM. Furthermore, the value of Cdl
decreases for both p-MBDM and p-MBDM assembled on
NiONPs, which denotes that the inhibitor molecules are
Icoorr ꢁ Icorr
I:E:ð%Þ ¼
ꢀ 100
(8)
Icoorr
where Iocorr and Icorr are the corrosion current densities in the
absence and presence of various concentrations of the
inhibitor.
As observed from the polarization graph, the anodic curves
shi to more positive potential and the cathodic curves shi to
more negative potential. It is observed from the literature that if
the shi of Ecorr is <85 mV, the inhibitor is considered to be a
mixed type, and if the Ecorr shi is >85, the inhibitor can act as a
cathodic or anodic type.48 In the present work, the shi in the
Ecorr values is <85 mV for both, i.e., p-MBDM and p-MBDM
assembled on NiONPs, suggesting that they have performed as a
mixed inhibitor.
Table 5 shows that the value of Icorr decreases with an
increase in the inhibitor concentration. The corrosion current
value of p-MBDM assembled on NiONPs superiorly decreases as
compared to p-MBDM, which reveals the enhanced power of p-
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Fig. 13 Scanning electron micrography of mild steel immersed for 4 h
(a) in 2 M HCl, (b) in 3.0 g Lꢁ1 of p-MBDM and (c) in 3.0 g Lꢁ1 of p-
MBDM assembled on NiONPs.
Fig. 12 Langmuir adsorption isotherm plot of mild steel in 2 M HCl for
(a) p-MBDM and (b) p-MBDM assembled on NiONPs.
p-MBDM and p-MBDM assembled on NiONPs mainly depends
on their ability to adsorb on the mild steel surface.
According to the displacement reaction, the adsorption of p-
MBDM/p-MBDM NiONPs at the mild steel/solution interface
can be written as:
MBDM assembled on NiONP molecules to inhibit mild steel
corrosion in 2 M HCl solution, as compared with p-MBDM.
Consistently, I.E. increases with the inhibitor concentration,
and the maximum values obtained for p-MBDM and p-MBDM
assembled on NiONPs were observed to be 81.4% and 98.1%,
respectively. This indicates that the deposition of corrosion
products on the mild steel occurred.49 In general, the inhibition
efficiencies obtained from Tafel polarization curves and elec-
trochemical impedance spectroscopy are lower than ones ach-
ieved from weight loss measurements.
Adsorption considerations. Adsorption depends mainly on
(i) the charge and the nature of the metal surface, (ii) the elec-
tronic characteristics of the metal surface, (iii) the adsorption of
the solvent and other ionic species, and (iv) the electrochemical
potential at the solution interface.50 The inhibition efficiency of
p-MBDM/p-MBDM NiONPs(soln) + nH2O(ads)
/
p-MBDM/p-MBDM NiONPs(ads) + nH2O(soln)
(9)
where n is the number of water molecules removed from the
mild steel surface for each molecule of inhibitor adsorbed. The
interaction between the mild steel surface and inhibitor can be
deduced from the adsorption isotherm. The mode of adsorp-
tion of the inhibitor on a mild steel surface in 2 M HCl at 308 K,
318 K, 328 K, and 338 K was made by tting the experimental
data with various isotherms such as the Langmuir, Temkin,
Freundlich, and El-awady isotherms. The best t was obtained
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interaction or adsorption of p-MBDM assembled on NiONPs on
mild steel was observed, which is regarded as a characteristic of
a good corrosion inhibitor for mild steel in 2 M HCl. The result
obtained from SEM analysis is consistent with the data
obtained from our electrochemistry experiments.
Atomic force microscopy (AFM) analysis. The surface
morphology at the nano to micro scale was further inspected by
atomic force microscopy, which examines the nature of the
protective layer formed on the mild steel surface from the three-
dimensional topography.51 Fig. 14a–c illustrates the 3D images of
the mild steel immersed in 2 M HCl solution, p-MBDM, and p-
MBDM assembled on NiONPs, respectively. Fig. 14a shows that the
surface is severely scratched by the aggressive HCl solution,
whereas later, Fig. 14b and c illustrates that the surface of mild
steel is protected from HCl attack by p-MBDM assembled on
NiONPs as compared to p-MBDM alone. The average roughness of
the mild steel in 2 M HCl was found to be 1000 nm, while in the
presence of p-MBDM and p-MBDM assembled on NiONPs, the
values are 327.4 nm and 197.2 nm, respectively. The reduced
roughness value for p-MBDM assembled on NiONPs in compar-
ison to p-MBDM indicates that a protective layer forms that is
composed of p-MBDM assembled on NiONPs.
Fig. 14 AFM 3D images of mild steel immersed (a) in 2 M HCl, (b) in 3.0
g Lꢁ1 of p-MBDM and (c) in 3.0 g Lꢁ1 of p-MBDM assembled on
NiONPs.
Conclusions
A novel p-MBDM assembled on nickel oxide nanoparticles
showed a strong binding mechanism by FTIR, PXRD, and mass
spectrometry, and the hydrodynamic size of NiONPs with p-
MBDM was conrmed by DLS and TEM. Weight loss measure-
ments revealed that with an increase in inhibitor concentration,
the inhibition efficiency increases drastically for p-MBDM
assembled on NiONPs as compared to p-MBDM. The effect of
temperature was used to obtain the inhibition efficiency, which
decreases for p-MBDM, whereas for p-MBDM assembled on
NiONPs, the inhibition efficiency increases at higher concen-
tration. The adsorption model conrmed the Langmuir
adsorption isotherm. EIS spectra indicate that the value of Cdl
decreases and Rct increases for both inhibitors. Galvanostatic
polarization data proved that the current density sharply
decreases for p-MBDM assembled on NiONPs as compared with
p-MBDM, and the inhibitors have been noted as mixed corro-
sion inhibitors. The adsorbed layer over the surface of mild steel
has been additionally conrmed by SEM and AFM analysis.
when a plot of C/q vs. C gave a straight line for both inhibitors.
This suggests that the adsorption of p-MBDM and p-MBDM
assembled on NiONPs on a metal surface follows the Langmuir
adsorption isotherm. The Langmuir adsorption is given by the
following equation:
Cinh
1
¼
þ Cinh
(10)
q
Kads
where Cinh is the inhibitor concentration, q is the surface
coverage, and Kads is the adsorption equilibrium constant.
The plot in Fig. 12a and b shows that the regression coeffi-
cients are very close to unity, which signies that the adsorption
of active S.B./S.B. NiONPs molecules on a mild steel surface
obeys the Langmuir adsorption isotherm. On the basis of this
data, it may be considered that a monolayer of inhibitor is
formed on the mild steel surface and therefore reduces the
corrosion process.
Surface examinations
Acknowledgements
Scanning electron microscopy (SEM) analysis. Scanning
electron microscopy was applied to further conrm the corro- The authors acknowledge Dr Utpal Joshi and his research students
sion inhibition ability of p-MBDM and p-MBDM assembled on for their kind help in the AFM analysis of the samples. One of the
NiONPs. Fig. 13a shows a micrograph of the mild steel authors (Poonam Wadhwani, JRF) is greatly thankful to the
immersed in 2 M HCl solution, which illustrates the pits that INSPIRE Programme (DST), New Delhi, for the nancial support.
have developed due to the corrosive attack in the absence of the
inhibitor. Fig. 13b and c shows the micrograph of mild steel in
the presence of p-MBDM and p-MBDM assembled on NiONPs
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