Organic nanoparticles of acetohydrazides as novel inhibitors for mild steel corrosion

Article abstract of DOI:10.1039/c8nj00602d

Novel organic nanoparticles (ONPs) of (E)-2-(4-(hydrazonomethyl)phenoxyl) acetohydrazide (H2) and N′((E)-benzylidene)-2-(4-((E)hydrazonomethyl)phenoxyl)acetohydrazide (H3) were prepared by the re-precipitation method. The ONPs were characterized using MS,

Full text of DOI:10.1039/c8nj00602d

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Organic nanoparticles of acetohydrazides as novel inhibitors for mild
steel corrosion
Howida S. Mandour1, Ahmed Abdel Nazeer1,2,*, Entesar AlꢀHetlani2, Metwally
Madkour2 and Yasser K. AbdelꢀMonem3
1Electrochemistry Laboratory, Physical Chemistry Department, National Research
Centre, Dokki, P.O. 12622, Giza, Egypt
2Chemistry Department, Faculty of Science, Kuwait University, P.O. Box: 5969, Safat
13060, Kuwait.
3Chemistry Department, Faculty of Science, Menoufia University, 32511 Shebin Elꢀ
Kom, Egypt
Abstract
Novel organic nanoparticles (ONPs) of (E)ꢀ2ꢀ(4ꢀ(hydrazonomethyl)phenoxyl)
acetohydrazide (H2) and N’((E)ꢀbenzylidene)ꢀ2ꢀ(4ꢀ((E) hydrazonomethyl)phenoxyl)
acetohydrazide (H3) were prepared by reꢀprecipitation method. The ONPs were
1
characterized by MS, HꢀNMR, 13CꢀNMR, FTꢀIR, DLS, TEM and XRD. Both
nanoparticles revealed a semispherical shape with a particle size of 3.0 nm and 2.0 nm
for H2 and H3 nanoparticles, respectively. The protective effect of ONPs on Mild steel
in 1.0 M HCl was monitored by polarization, electrochemical impedance spectroscopy
(EIS), scanning electron microscope (SEM) and atomic force microscopy (AFM). The
obtained results revealed that the inhibition efficiency increased with the inhibitor
concentration increment. Maximum inhibition efficiencies at a constant concentration
of 1x10ꢀ3 M were 88.2% and 96.7% for H2 and H3, respectively. The inhibition
mechanism depended on the physical adsorption of nanoparticles on steel surface
which was confirmed by SEM, AFM and impedance results. These novel
nanomaterials showed great antiꢀcorrosion behavior and thus can be potentially used
in industrial applications such as cooling water systems, pipes, and oil production
units.
Key words: Organic nanoparticles; Corrosion inhibition; Mild steel; EIS; SEM/AFM.
* Corresponding Author Eꢀmail: anazeer_nrc@yahoo.com
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Introduction
Despite their high fabrication cost, metals are commonly used in various industrial
applications due to their unique properties. However, the major limitation of using
such materials is that they undergo corrosion when exposed to harsh environments
which significantly reduces their life span. Corrosion is an electrochemical
phenomenon that converts metals to a more stable states upon exposure to some
corrosive media1
Mild steel is highly abundant and has prime mechanical properties, therefore, it
is used as a building material in many chemical and petrochemical applications2.
However, mild steel has a limited resistance to corrosion under drastic conditions,
including exposure to hydrochloric acid (HCl) which is used in many processes and
causes serious corrosion on mild steel3. Recently, the use of organic inhibitors is
considered an effective method for protecting steel from corrosion4ꢀ6. Organic
compounds are utilized as corrosion retardants for mild steel and iron because of their
ability to minimize corrosion in different corrosive media7, 8. Generally, theses organic
compounds (heterocyclic compounds) have been utilized as efficient corrosion
inhibitors due to their high adsorption on metallic surfaces through the presence of πꢀ
and nonꢀbonding electrons, aromatic rings and polar functional moieties, which
behave as adsorption centers9, 10
.
Organic inhibitors can adsorb onto the metal/solution interface via four distinct
mechanisms: (a) electrostatic attraction between charged inhibitor and the metal; (b)
interaction between uncharged electron pairs in the inhibitor and the metal; (c)
interaction between πꢀelectrons and the metal and (d) a combination of mechanism (a)
and (c)11, 12. Such organic inhibitors usually contains heteroatoms with electronegative
functional groups such as oxygen, nitrogen, sulfur, phosphorus, unsaturated bonds and
conjugated aromatic rings which serve as adsorption centers13. Generally, the
tendency to form stronger coordination bonds and, as a result, the inhibition efficiency
increases according to the following trend: O < N < S < P14.
ONPs are thought to be more advantageous than inorganic and metallic
nanoparticles, because of their infinite synthetic routes15. ONPs are mainly prepared
by one of two main methods: emulsification or reꢀprecipitation methods16. The former
is a twoꢀstep method; the first step includes the preparation of an emulsification
system to create nanodroplets of certain sizes wherein organic compounds were
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previously solubilized. The nanoparticles are formed in the second step of the process
by numerous mechanisms such as gelation, precipitation, or polymerization17. The reꢀ
precipitation method is of a great interest, it is a oneꢀstep method based on the addition
of organic material solution into a relatively large volume of another solvent that is
poorly miscible with its solution18. This rapid alteration in the medium persuades the
precipitation of organic molecules19.
The main purpose of this study is the investigation of the inhibition ability of
the novel acetohydrazides ONPs towards mild steel corrosion in 1.0 M HCl solution.
To the extent of our knowledge, we believe that this is the first report utilizing ONPs
as corrosion inhibitors instead of the widely used inorganics. After a through
characterization of the freshly prepared ONPs, different techniques were used to
monitor their inhibition performance. Techniques such as potentiodynamic
polarization (PDP), electrochemical impedance spectroscopy (EIS) complemented by
SEM and AFM were used to study the surface morphology of steel alloy after
exposure to the HCl solution with and without inhibitors.
2. Experimental part
2.1. Materials and instruments
The corrosion measurements were carried out on Mild steel specimens with a
composition in wt.% as follow (C: 0.076, P: 0.012, Mn: 0.192, Si: 0.026, Cr: 0.050,
Al: 0.023 and the rest Fe). All chemicals (pꢀhydroxy benzaldehyde, anhydrous
potassium carbonate, ethylchloroacetate, benzaldehyde, hydrazine hydrate,
hydrochloric acid, glacial acetic acid) and solvents (ethanol, acetone) were of
analytical grade (Sigma ꢀ Aldrich) and were used as received without any further
purification. TLC was performed using Polygram SIL G/UV 254 TLC plates, and
visualization was performed under ultraviolet light at 254 nm and 350 nm. Anhydrous
1
solvents were either supplied by SigmaꢀAldrich. H and 13C NMR spectra were
recorded using a Bruker DPX 400 at 400 MHz and 100 MHz, respectively, and IR
spectra were obtained with a Jasco 6300 FTIR. EIMS and FABꢀMS spectra were
measured using a highꢀresolution GCꢀMS (DFS) thermo spectrometer with EI (70
EV). Xꢀray powder diffraction (XRD) was conducted using D8 Advance
diffractometer with a copper target and nickel filter with CuKα radiation (λ =
0.154056 nm). The shape of the nanoparticles was investigated using transmission
electron microscopy (TEM) using a Joel JEM 1230 operating at 120 kV. Zeta potential
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(ζ) measurements for isoelectric point (IEP) determinations were measured by Zeta
sizer Nano ZS (Malvern Instruments Ltd, Malvern, UK). The determined
electrophoretic mobilities were converted to ζꢀpotentials by using Dispersion
Technology Software, version 5.03, Malvern Instruments Ltd., UK. Also the surface
morphology of the specimens was examined using scanning electron microscope
(SEM) using (JSMꢀ6300 JEOL). AFM analysis was carried out using a model
VEECOꢀNano scope IV multiple AFM/SPM.
2.2. Synthesis of (E)ꢀ2ꢀ(4ꢀ(hydrazonomethyl)phenoxyl) acetohydrazide (H2)
nanoparticles
Scheme 1 Preparation of (E)ꢀ2ꢀ(4ꢀ(hydrazonomethyl)phenoxyl)acetohydrazide
To a stirred solution of pꢀhydroxybenzaldehyde (1) (0.61 g, 5 mmol) and K2CO3 (0.67
g, 5 mmol) in dry acetone (10 mL) the ethyl chloroacetae (0.61 g, 5 mmol) was added
dropwise. The reaction mixture was refluxed for 6 h. The solid product was collected
by the filtration, washed and recrystallized from ethanol to afford colorless crystals of
ester (2). A mixture of ethoxycarbonylmethylꢀpꢀoxobenzaldehyde (2) (2.08 g, 0.1
mole), hydrazine hydrate (1.5 g, 0.3 mole) and ethanol (30 ml) was refluxed for 5h.
The reaction mixture was cooled to room temperature, collected by filtration and
recrystallized from ethanol. Bright yellow powder, yield 1.87 g (90%); mp. 145ꢀ147
°C. m/z (EI) = 208.0 [M]+ (100%).
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2.3. Synthesis of N’((E)ꢀbenzylidene)ꢀ2ꢀ(4ꢀ((E) hydrazonomethyl)phenoxyl)
acetohydrazide (H3) nanoparticles
Scheme 2 Preparation of N’((E)ꢀbenzylidene)ꢀ2ꢀ(4ꢀ((E)hydrazonomethyl)
phenoxyl)acetohydrazide
A mixture of H2 (2.08 g, 0.1 mole), benzaldehyde (1.06 g, 0.1 mole) in ethanol (30
ml) was refluxed for 5h. The reaction mixture was cooled to room temperature,
collected by filtration and recrystallized from ethanol. Bright yellow powder, yield
2.67 g (90%); mp. 145ꢀ147 °C. m/z (EI) = 296.2 [M]+ (100%).
2.4. Electrochemical measurements
The polarization experiments were carried out in a conventional threeꢀelectrode cell
with 1 cm2 platinum foil as the counter electrode, a saturated calomel electrode as a
reference electrode and Mild steel sheet (1 cm x 1 cm) embedded in epoxy resin of
polytetrafluoroethylene as the working electrode. Before polarization scanning,
working electrode was immersed in the test solution for 30 minute until steady state
potential was attained. All potentiodynamic measurements were carried out with a
Gamry framework instruments (version 3.20), controlled by a computer which also
recorded and stored the data. Measurements were performed in the 1M HCl solution
containing different concentrations of the tested ONPs by changing the electrode
potential automatically from ꢀ 750 to ꢀ 200 mV vs. SCE at scan rate of 0.5 mV sꢀ1.
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EIS tests were performed at 25 ±1 oC in a three electrode assembly as above. The EIS
spectra were recorded at EOC after immersion of the electrode for 30 minutes in the test
solution in the frequency range from 100 kHz and 0.5 Hz and the AC signal was 5 mV
peak to peak. A personal computer with EIS300 software and Echem analyst 5.21 was
used for data fitting.
3. Results and Discussion
3.1 Characterization of the ONPs
3.1.1 Structural elucidation
Schemes (1 and 2) displayed preparation of both H2 and H3 nanoparticles. The
molecular weights of the synthesized corrosion inhibitors are 208.221 and 296.33 for
H2 and H3, respectively, which were calculated based on their proposed molecular
formulas (C9H12N4O2) and (C16H16N4O2) and confirmed via mass spectrometry
measurements. The FTꢀIR spectra of the ONPs showed absorption bands at 3250 and
3125 cm−1 for hydrazide NHꢀNH2 and 2819.0 cm−1 for (−CH2− aliphatic), while
carbonyl stretching at 1647 cm−1. The bands at 1510 cm−1 and 1250 cm−1 are for N=C
1
and CꢀO bonds, respectively. The HꢀNMR spectrum of H2 shows a singlet at δ 9.39
ppm which is attributed to the NHꢀNH2 protons and at δ 4.35 (s, 2H, for −CH2) and δ
1
4.47 ppm (brs, 2H, NH2). While the HꢀNMR spectrum of H3 exhibits a singlet at δ
9.39 ppm due to the NHꢀNH2 protons and at δ 8.03 ppm for phenyl group and δ 5.21
13
ppm (brs, 2H, NH2). From CꢀNMR for H2 compound, a band at 166.63 ppm due to
the carbonyl group, and bands at 115.14, 124.99, 127.24 and 130.25 ppm are from the
carbon atoms in the aromatic ring. While 13CꢀNMR for H3 compound, a band
appeared at 168.78 ppm for the carbonyl group, and the bands at 114.98, 125.14,
126.80, 127.17, 128.85, 130.25 and 133.79 ppm were from carbon atoms in the
aromatic rings.
3.1.2 Morphological and particle size characterization
3.1.2.1 Transmission Electron Microscopy
TEM images for both ONPs are shown in Fig. 1. Both nanoparticles revealed a
semispherical shape with a particle size of 3.0 nm and 2.0 nm for H2 and H3 organic
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nanoparticles respectively. Furthermore, as shown from the TEM images, the
nanoparticles are monodispersed without any notable aggregation.
Figure 1 Transmission electron micrographs for H2 (A) and H3 (B).
3.1.2.2 XꢀRay Diffraction (XRD)
XRD patterns of H2 and H3 ONPs showed broad peaks which confirm its presence in
the nanoscale as depicted in Fig. 2. Crystallite size measurements were determined
from the fullꢀwidth at half maximum (FWHM) of the strongest peak at 2θ = 16.3° and
26.2° for H2 and H3 nanoparticles respectively, using Scherrer’s equation D = Kλ/(β
cosθ). Where, D is the average crystal diameter, β is the full width at halfꢀmaximum,
K is a constant (K = 0.90), λ is the wavelength (0.154 nm), and θ is the diffraction
angle. The determined crystallite sizes were found to be 5.0 nm and 2.3 nm for H2 and
H3 nanoparticles respectively.
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Figure 2 XRD patterns for H2 and H3 ONPs.
3.1.3 Determination of the Isoelectric Point (IEP)
The isoelectric point for both H2 and H3 NPs were measured using dynamic light
scattering (DLS). The data for H2 NPs exhibited two ionization sites and subsequently
two isoelectric points at pH values of 2.3 and 3.6, respectively as shown in Fig. 3A.
H2 NPs contained two functionalities: amine group and amide group. The amine is
protonated in the acidic medium (HCl), however, its conjugate acid will be uncharged
in the strong acidic medium which could explain the zero charge at such an acidic pH
value20. On the other hand, the amide group in an acidic medium can undergo
hydrolysis and form a carboxyl group which is unprotonated under acidic conditions,
thus this could explain the second isoelectric point,21 the same can be explained for H3
NPs as shown in Fig. 3B.
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Figure 3 Isoelectric point of H2 (A) and H3 (B) ONPs.
3.2 Electrochemical studies
3.2.1 Potentiodynamic polarization study
Tafel curves of bare Mild steel alone and with various cᴏncentrations of H2
and H3 NPs were recorded as shown in Fig. 4. The electrochemical parameters
including corrosion potential (Ecorr.), corrosion current density (icorr.), Tafel cathodic
and anodic slopes (βc, βa) and efficiency of inhibition (IE%) were obtained from this
Figure by extrapolation and the results are presented in Table 1. The values of (IE%)
were calculated by the fᴏllowing equation22, 23
:
`
ꢅꢁ
IE% ꢀ ꢁ
ꢂꢃꢄꢄ. ꢆ 100
(1)
ꢂꢃꢄꢄ.
ꢁ
ꢂꢃꢄꢄ.
where icorr. and i`corr. are the corrosiᴏn current in the absence and presence of inhibitors,
respectively. The maximum corrosion inhibition efficiencies (IE%) recorded at
concentration of 1x10ꢀ3 M were 88.2 and 96.7% for H2 and H3, respectively. From the
results, icorr. was reduced in the presence of the inhibitors especially as their
concentrations increased. This was probably due to the inhibitor adsorption ᴏn the
metallic surface which blocked the active sites on steel surface and protected it from
corrosive media24, 25. Depending on the values of Ecorr., these synthetic ONPs can be
classified as cathodic, anodic or mixedꢀtype inhibitors. In the present work, the values
of Ecorr. showed irregular relationship, however, the results listed in Table 1 illustrated
that the presence of the inhibitors changed the cathodic and anodic Tafel slopes values
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but it was clear that βc values were greatly affected compared with the values of βa
indicating that the ONPs can be considered as mixed type inhibitors predominately
cathodic type inhibitor26, 27. Ultimately affecting both the anodic and cathodic
reactions related to metal dissolution and hydrogen evolution, respectively.
A
B
Figure 4 Polarization curves for blank Mildꢀsteel in 1M HCl solution and in presence
various cᴏncentrations of H2 (A) and H3 (B) at 25 ºC.
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Table 1 Electrochemical data measured using polarization technique for bare mild
steel alloy in 1 M HCl alone and containing various concentrations of H2 and H3
ONPs at 25oC and scan rate 20 mV/min.
icorr.,
ꢀEcorr.,
ꢀβc,
βa,
corrosion rate
( CR) mm/year
1610.0
Conc., ppm
%IE
θ
µA cmꢀ2
mV mV dec.ꢀ1 mV dec.ꢀ1
Blank
H2
3517.0 461.5
1093.0 508.9
691.0 507.9
498.3 515.6
444.5 497.8
413.5 507.8
730.2 477.9
316.6 475.8
244.3 476.0
158.1 488.1
116.7 485.8
304
230
210
233
196
207
287
199
189
219
163
165
160
191
173
142
106
110
84
ꢀꢀ
ꢀꢀ
1 x 10ꢀ5
5 x 10ꢀ5
1 x 10ꢀ4
5 x 10ꢀ4
1 x 10ꢀ3
1 x 10ꢀ5
5 x 10ꢀ5
1 x 10ꢀ4
5 x 10ꢀ4
1 x 10ꢀ3
0.689 68.9
0.804 80.4
0.858 85.8
0.874 87.4
0.882 88.2
0.792 79.2
0.910 91.0
0.931 93.1
0.955 95.5
0.967 96.7
500.1
316.3
228.1
203.5
189.3
331.5
168.2
140.1
122.4
63.7
H3
94
106
88
3.2.2 Electrochemical impedance spectroscopy (EIS)
EIS measures electrochemical changes in situ in which enables monitoring the
physical processes occurring at the metal/electrolyte interface28, 29. Fig. 5 shows
Nyquist plots recorded for Mildꢀsteel corrosion in the absence and presence of various
cᴏncentrations of the ONPs inhibitors. The data recorded exhibited a capacitive loop
which indicated that the ONPs inhibited the corrosion process by adsorbing at the
metal/electrolyte interface30, 31. The addition of ONPs inhibitors to the metal surface
resulted in an expansion of the diameter of the semicircles showing a better inhibitive
capacity32. The EIS data was fitted using the circuit shown in Fig. 6 which involved Rs
(solution resistance of electrolyte), Rct (charge transfer resistance), where Rct refers to
the capability of the covering film of the ONPs adsorbed on the metallic surface to
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hinder the transfer of charges across the metal/solution interface, and CPE (constant
phase element) is defined as follows33, 34
:
ꢋ
ꢇꢈꢉꢊ ꢀ ꢌ (ꢎꢏ)
(2)
ꢐ
ꢍ
where Y0 is the ratio of the constant phase element, j is an imaginary number (j2 =
−1),
w is the angular frequency, n is a deviation parameter related to a phase shift. The
values of corrosion inhibitor efficiency for Mildꢀsteel in 1.0 M HCl solution are
calculated using the following equation35:
ꢅ ꢓ
ꢑꢒ% ꢀ ꢓ
ꢖ 100
(3)
ꢔꢕ
ꢔꢕ.ꢍ
ꢓ
ꢔꢕ
where Rct and Rct,o are the resistance of charge transfer for Mildꢀsteel in 1.0 M HCl
solution in the presence and absence of the ONPs, respectively. The parameters
deduced from the EIS data are displayed in Table 2, they are an accurate examination
of the results detected the increase in Rct value with the addition of the ONPs which is
assigned to the thin film formed by the inhibitor adsorption on the steel surface36, 37
.
Also Cdl (double layer capacitance) can be calculated as follow38:
nꢀ1
Cdl = Yo(wmax
)
(4)
where, wmax to refers the frequency at which the imaginary quantum of impedance has
acquired the maximum (rad s_1) value. Decreasing the tendency of Cdl values with
increasing the ONPs concentrations can be elucidated by the Helmholtz type as
equation (5)39 which can be attributed to the displacement of water molecules slowly
by the ONPs, lowering the local dielectric constant and rustic electrical double layer40:
ꢍ
ꢗꢘꢙ ꢀ ɛ ɛ
ꢚ
(5)
ꢘ
where d is the electric doubleꢀlayer thickness, S is the surface area of steel electrode, ɛ
is the local dielectric constant and εo is the permittivity of the air. Consequently, the
decrease in Cdl values lead to constricting the exposed electrode surface area in
truculent solution and thus efficiently minimized corrosion of steel41. Accordingly, the
inhibition efficiencies increased with increasing inhibitors concentrations and reached
88.2% for H2 and 96.7% for H3 at 1x10ꢀ3M. These results of EIS studies were in
perfect conformity with the polarization results.
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A
B
Figure 5 Nyquest plots for corrosion of blank Mildꢀsteel in 1M HCl solution alone
and with various concentrations of H2 (A) and H3 (B) at 25 ºC.
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Figure 6 Circuit model used to fit the EIS Nyquist plots.
Table 2 EIS data of Mild steel corrosion in 1 M HCl with and without various
concentrations of H2 and H3 ONPs at 25oC.
Rs
ꢁ cm2
Rct
ꢁ cm2
Cdl
µFcm−2
Conc., M
n
IE%
θ
Blank
H2
1.34
1.25
1.13
1.08
1.46
1.53
1.19
1.12
0.97
1.29
1.41
15.4
0.829
0.877
0.848
0.899
0.891
0.901
0.834
0.858
0.846
0.876
0.891
105.1
70.4
61.2
55.3
49.7
47.6
60.3
51.6
43.4
38.7
34.2
ꢀꢀ
ꢀꢀ
1 x 10ꢀ5
5 x 10ꢀ5
1 x 10ꢀ4
5 x 10ꢀ4
1 x 10ꢀ3
1 x 10ꢀ5
5 x 10ꢀ5
1 x 10ꢀ4
5 x 10ꢀ4
1 x 10ꢀ3
52.9
0.709
0.801
0.845
0.884
0.902
0.802
0.932
0.943
0.952
0.959
70.9
80.1
84.5
88.4
90.2
80.2
93.2
94.3
95.2
95.9
77.2
99.4
132.3
156.8
77.8
225.1
272.2
322.3
378.2
H3
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3.2.3 Adsorption isotherm
The adsorption of the ONPs on the metallic surface depended on the
interactions at the interface between metal and solution where both the adsorbent and
adsorbate were present. In this work, the Langmuir adsorption isotherm exhibited the
best one that matched with the obtained data and based on the values of the correlation
coefficient (R2) which was almost near to unity. The Langmuir adsorption isotherm
can be represented as the following42:
ꢈ
ꢛ
ꢋ
ꢀ ꢜ + ꢗ
(6)
ꢝꢞꢟ
From the previous equation, C is the inhibitor concentration, θ is the coverage
area of surface and Kads represents the adsᴏrptionꢀdesorption constant. The plots of C/θ
against C for the studied ONPs inhibitors at certain temperature are shown in Fig. 7.
Gibb’s free energy of adsorption (
following equation:
ꢀ
Goads) for H2 and H3 were determined using the
ꢀ
Goads = ꢀRT ln (55.5 Kads)
In the above equation, R is the gas constant, T is the absolute temperature, and
55.5 is the molar concentration of water. The Kads and
Goads values are given in Table
3 for the investigated ONPs. The type of adsorption of inhibitor molecules on metallic
surface can be explained by using the values of
Go which is considered one of the
most successful techniques for this purpose43. Generally, if the value of
(7)
ꢀ
ꢀ
ads
ꢀ
Go ꢀ20
ads
KJmolꢀ1 or less negative is counted to be physical adsorption which is attributed to
electrostatic interaction between the adsorbent and adsorbate44, while if the value of
ꢀ
Go is ꢀ40 KJ molꢀ1 or more negative; it is considered to be chemical adsorption which
associated with sharing of charge45. From the results shown in Table 3, the values of
∆Go suggested the type of interaction between the ONPs and Mildꢀsteel surface
included both physical and chemical adsorption43.
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Figure 7 Langmuir adsᴏrption isotherm for corrosion inhibition of Mildꢀsteel in 1M
HCl solution in presence of H2 and H3 ONPs.
Table 3 Equilibrium constant (K), free energy of binding (ꢁGads) of H2 and H3 for
corrosion of Mild steel in 1 M HCl at 25oC
Langmuir isotherm
Inhibitor
R2
K (Mꢀ1)
2.2 x 105
2.6 x 105
ꢀꢁGads (kJ molꢀ1)
H2
H3
0.99998
0.99997
40.4
42.2
3.3 Characterization of steel surface
3.3.1 Scanning Electron Microscopy analysis
SEM micrographs of steel samples after the exposure to 1 M HCl solution for 24 h in
the absence and presence of the optimal concentration of the ONPs (1x10ꢀ3M) are
depicted in Fig. 8 (AꢀC). From these micrographs, it was obvious that the steel surface
subjected to the blank solution (Fig. 8 A) appeared porous and rough with random
scratches due to loss in the resistance against corrosion in the aggressive media.
However, the addition of the inhibitor ONPs to the acidic media protected the steel
surface from corrosion and a smooth surface was observed Fig. 8 (B, C). The
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smoother surface was more pronounced in the presence of H3 compared with H2.
Thus, it can be concluded that the presence of H3 and H2 ONPs decreased the
aggressive attack of the acidic solution, confirmed by the high efficiency of the
protective film formed by the ONPs on the surface of mild steel.
3.3.2 Atomic force microscopy analysis
AFM is considered an important tool in studying the morphology of surfaces which
helps in explaining the corrosion process at the interface between metal and solution at
the nanoscale and microscale. The threeꢀdimensional AFM graphs of mildꢀsteel
surface in 1 M HCl solution with and without the studied compounds H3 and H2 (1 x
10ꢀ3M) are depicted in Fig. 8 (AꢀC). Clearly, the AFM pattern of steel surface exposed
to 1 M HCl solution displayed a very porous surface with large and deep random pores
as a result of the aggressive media exposure (Fig. 8 A). On the other hand, in the
presence of the ONPs, relatively smooth and flat surfaces were obtained as shown in
Fig. 8 (B, C). These findings confirmed that the dissolution rate of steel was decreased
and controlled. The smoother surface obtained can be attributed to the protective thin
film formed by adsorption of ONPs on the steel surface.
Figure 8 SEM and AFM micrographs of Mild steel surface after immersion in 1M
HCl solution without (A) and with the optimum concentration of H2 (B) and H3 (C) at
25 ºC, scale bare in SEM is 10 µm.
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3.4. Suggested corrosion inhibition mechanism
It is essential to postulate the corrosion inhibition mechanism of the studied new
organic nanoparticles to steel surface in the acidic medium. According to the literature,
it is well known that the organic compounds can suppress or prevent metals
dissolution by the physical or chemical adsorption on their surfaces46. In the present
work the adsorption of the investigated ONPs was confirmed to be a physisorption
process due to the electrostatic interaction at the metal/solution interface based on the
Langmuir isotherm. Several factors are responsible for the adsorption of the inhibitors
on the metal surface such as the presence of unshared electron pairs on oxygen and
nitrogen atoms and/or the πꢀelectrons of the aromatic rings which can form a
donor/acceptor surface complex with the vacant dꢀorbitals of the surface metal47.
The adsorption of the investigated ONPs can be proved depending on the surface
coverage results (θ) which increased with increasing the inhibitors concentration as
shown in Table 1; this confirmed their effective protection of the steel surface from
HCl attacking. In aqueous acidic solutions, the H2 and H3 existed either as neutral or
positively charged particles as confirmed by the isoelectric point studies. However, as
previously reported that the steel surface bears a positive charge in an acidic media48,
so it is difficult for these molecules to interact with the positively charged steel surface
because of the electrostatic repulsion. Interpretation of this problem depended on the
presence of the negatively chloride ions with the smaller degree of hydration which is
responsible on the presence of excess of negative charges which acted as a mediator
between the adsorbed inhibitor molecules and metal surface.
Furthermore, the H3 compound showed higher inhibition efficiency compared with
H2 compound due to its higher molecular weight and the presence of one additional
aromatic ring which resulted in better stacking on the metal surface and hence
increased the protection from corrosion. The layer formed on the steel surface is
believed to suppress both the anodic and cathodic reactions on the steel surface as
demonstrated from Tafel slope. One of the important findings in this study is the
pronouncedly high inhibition efficiency which could be a result to the presence of the
nanoparticles that have high surface area which was responsible for blocking the
active sites on the steel surface making it withstand the dissolution in the aggressive
environments.
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4. Conclusion
New ONPs of acetohydrazides were successfully synthesized by reꢀprecipitation
method. Structural elucidation, morphological and particle size characterizations were
performed to confirm the structure, spherical and nanoscale particle size of both
nanoparticles. Subsequently, the synthesized ONPs were effectively used as corrosion
inhibitors for mild steel in acidic medium. The study showed that the investigated
ONPs can efficiently suppress the mild steel dissolution in 1 M HCl solution.
Polarization results confirmed controlled the anodic and cathodic reactions in the
presence of ONPs. Superior inhibition efficiencies of 88.2% and 96.7% were obtained
using small concentration (1x10ꢀ3 M) of H2 and H3, respectively. The inhibition
mechanism occurred through the adsorption of the ONPs on the metallic surface
blocking its active sites, which was confirmed by SEM, AFM and impedance results.
The values of the adsorption energy obtained supported the physical adsorption of
these molecules which further validated the results. The outstanding results obtained
may help in the opportunity for applying the investigated ONPs in the industrial
applications and these ONPs could be further chemically modified to obtain particles
of different chemical nature suitable for various applications.
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Novel organic nanoparticles were successfully prepared using re-precipetation
method showed superior protection for mild steel corrosion in the acidic media.