Synthesis and evaluation of some new hydrazones as corrosion inhibitors for mild steel in acidic media

Article abstract of DOI:10.1007/s11164-018-03730-y

Mild steel corrosion in HCl solution is an example of corrosion in acidic mediums. The ongoing research efforts to develop novel environmentally friendly corrosion inhibitors raise questions regarding their ability to effectively protect steel from corros

Full text of DOI:10.1007/s11164-018-03730-y

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-03730-y
Synthesis and evaluation of some new hydrazones
as corrosion inhibitors for mild steel in acidic media
Hassane Lgaz1 · Abdelkarim Chaouiki2 · Mustafa R. Albayati3 ·
Rachid Salghi2 · Yasmina El Aoufr4 · Ismat H. Ali5 · Mohammad I. Khan6 ·
Shaaban K. Mohamed7,8 · Ill‑Min Chung1
Received: 26 July 2018 / Accepted: 29 December 2018
© Springer Nature B.V. 2019
Abstract
Mild steel corrosion in HCl solution is an example of corrosion in acidic mediums.
The ongoing research eforts to develop novel environmentally friendly corrosion
inhibitors raise questions regarding their ability to efectively protect steel from
corrosion. Herein, a series of experimental studies were conducted to explain the
scientifc mechanism of adsorption of four hydrazone derivatives (HDZs) namely,
2-((2,3-dimethylphenyl)amino)-N′-((1E,2E)-3-phenylallylidene)benzohydrazide
(HDZ1)
(E)-2-((2,3-dimethylphenyl)amino)-N′-(4-hydroxybenzylidene)benzohy-
drazide (HDZ2) (E)-2-((2,3-dimethylphenyl)amino)-N′-(1-phenylethylidene)benzo-
hydrazide (HDZ3) and N′-cyclohexylidene-2-((2,3-dimethylphenyl)amino)benzohy-
drazide (HDZ4) on mild steel (MS) in 1.0 M HCl using chemical, electrochemical
and surface characterization techniques. All results show that the inhibitor mol-
ecules form a stable layer on steel surface through chemical and physical interac-
tions. HDZs adsorption onto the steel surface was found to follow Langmuir model.
Furthermore, electrochemical results demonstrated that our developed inhibitors act
as mixed-type inhibitors, with HDZ1 showing the highest polarization resistance and
lowest corrosion current density. X-ray difraction and scanning electron microscope
were used to study corrosion products phases and surface morphology of MS sam-
ples. Our fndings provide deeper insights into understanding the interaction mecha-
nisms of HDZs with a steel surface and can be helpful to explore novel approaches
to mitigate the steel dissolution.
Keywords Mild steel · Corrosion inhibition · HCl · Hydrazone derivative ·
Mefenamic acid
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-018-03730-y) contains supplementary material, which is available to authorized users.
*
Ill-Min Chung
imcim@konkuk.ac.kr
Extended author information available on the last page of the article
Vol.:(0123456789)
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H. Lgaz et al.
Introduction
Petroleum refning and related industries which produce multiple fuels and products
from petroleum are very important and essential to the function of society. Much of this
infrastructure, especially, oil and gas pipelines are made from mild steel. Acidic condi-
tions are widely used in industry for cleaning, descaling, pickling and oil well acidizing.
In fact, the corrosion phenomenon for various metals is especially prominent in these
industries processes [1–3]. Therefore, the search for chemical substances that can slow
down the corrosion rate is a never-ending task. Over several years, much attention has
been devoted to the design, synthesis and evaluation of a large number of compounds
containing heterocyclic units for potential use as corrosion inhibitors [4–6]. However,
very little research has been done so far using hydrazone derivatives on the inhibition
of corrosion. Hydrazones are important compounds possessing a wide spectrum of bio-
logical and pharmacological properties [7]. Owing to their unique physical and chemi-
cal properties, they have been reported to possess potent antimicrobial, analgesic, anti-
fungal, anti-infammatory and antitumoral activities [8]. On the other hand, Mefenamic
acid (MA), a non-steroidal Anti-Infammatory Drug (NSAID), is well known as one of
the most common drugs to the treatment of mild to moderate pain along with other dis-
eases [9, 10]. Functionalization of the free carboxylic group in MA like other NSAIDs
could control the most efective side efect of these drugs such as gastrointestinal toxic-
ity [11]. One of the most efective functionalization of carboxylic group is the conver-
sion into hydrazone derivatives [12]. In addition to the advantage of having a MA, the
hydrazones are known to exhibit both hard and soft base characters which can facilitate
donor–acceptor interactions with electron defcient metal centres. Thus, this synthetic
approach can be very useful in the development of efective compounds and satisfy the
ever-increasing requirements of the environmental regulation.
This research work is an attempt to develop a better insight and understanding of
mechanisms related to corrosion inhibition process of mild steel in 1.0 M HCl using
four hydrazone derivatives incorporated Mefenamic acid sub-structure, namely,
2-((2,3-dimethylphenyl)amino)-N′-((1E,2E)-3-phenylallylidene)benzohydrazide
(HDZ1),
(E)-2-((2,3-dimethylphenyl)amino)-N′-(4-hydroxybenzylidene)benzohy-
drazide (HDZ2), (E)-2-((2,3-dimethylphenyl)amino)-N′-(1-phenylethylidene)benzo-
hydrazide (HDZ3) and N′-cyclohexylidene-2-((2,3-dimethylphenyl)amino)benzohy-
drazide (HDZ4) which may provide a platform for further investigations of structurally
similar compounds. To the best of our knowledge, no such research has been conducted
and this rarity prompted to report the case. We probed the interaction between hydra-
zone derivatives and the steel surface in 1.0 M HCl using chemical, electrochemical,
X-ray difraction (XRD) and scanning electron microscope (SEM) studies.
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Synthesis and evaluation of some new hydrazones as corrosion…
Materials and methods
Synthesis
Melting points were recorded using Stuart melting point apparatus. The IR spectra
1
were measured using FTIR spectrometer (Shimadzu, Japan). The H NMR and 13C
NMR spectra were recorded with Bruker (Bruker 400 MHz; Bruker, Billerica, USA)
in DMSO. Scheme 1 represents the general procedure for synthesis of hydrazones
(3a–d).
Synthesis and characterization of methyl 2‑[(2,3‑dimethylphenyl)amino]benzoate
(1) A mixture of (2.0 gm, 8.28 mmol) Mefenamic acid and (2.0 gm, 16.6 mmol)
anhydrous potassium carbonate was added to dry acetone (80 ml) and stirred for 1 h.
Excess of Methyl Iodide (3.6 gm, 24.8 mmol) then added. The resulting mixture was
refuxed for 18 h. The reaction mixture was fltered when hot and the resultant cakes
were washed with dry acetone (6×2 ml). The fltrate was evaporated under reduced
pressure to aford the ester as an oily material solidifed after 4 days at room tem-
perature with m.p=100–102 °C (reported [13] 102–103 °C). IR (KBR): (C=O ester
1725 cm−1), (NH3 332 cm−1), (C–H aliphatic 2999–2943 cm−1), (=C–H aromatic
3012 cm−1).
Synthesis and characterization of 2‑[(2,3‑dimethylphenyl)amino]benzohydrazide
(2) A mixture of Mefenamic acid methyl ester (1) (1 mmol) and hydrazine hydrate
(85%, 2.5 mmol) was irradiated, in closed vessel, under microwave irradiation at
300 W and 2500 °C for 7 min. The reaction mixture was cooled, the separated solid
Scheme 1 Hydrazones synthesis scheme
1 3

H. Lgaz et al.
was washed with water, fltered, dried and recrystallized from ethanol to aford the
hydrazide (2) with m.p=119–121 °C (reported [14] 118–120 °C). IR(KBR): (C=O
amide 1654 cm−1), (NH 3290 cm−1)(NH2,3290–3309 cm−1), (C–H aliphatic 2949),
1
(=C–H aromatic 3018 cm−1). H NMR (400 MHz, DMSO): δ=2.1(s,3H,CH3),
2.2(s,3H,CH3), 9.4(s,1H,NH), 9.8(br.s,1H,NH amide), 3.5(br.s,2H,-NH2),
6.7–7.9(m,7H,Ar–H). 13C-NMR δ=13.5(CH3), 20.2(CH3), 113.9, 115.9, 116.9,
119.6, 125.0, 125.8, 128.2,129.3,131.8, 137.7,139.3,145.9 (Aromatic carbons),168.4
(C=O amide).
General procedure for synthesis of hydrazones (3a–d) [11] An equimolar mixture
of appropriate aldehydes or ketones (0.01 mmol) and mefenamic acid hydrazide 2
(0.01 mmol) with few drops of glacial acetic acid in 25 ml of absolute ethanol was
refuxed for 4–5 h. The reaction was concentrated and cooled to half and the solid
that precipitated was collected and recrystallized from ethanol to aford 3a–d.
2‑[(2,3‑dimethylphenyl)amino]‑N′‑[(1E)‑1‑phenylethylidene] benzohydrazide (3a,
C23H23N3O) m.p=185–188 °C; IR(KBR): (C=O amide 1660 cm−1), (C–H ali-
phatic 2830 cm−1), (C–H aromatic 3029 cm−1), (NH 3260 cm−1)(NH 3193 cm−1),
(C=N 1604 cm−1). 1H NMR (400 MHz, DMSO): δ=2.1(s,3H,CH3), 2.2(s,3H,CH3),
2.3(s,3H,CH3), 9.2(s,1H,NH), 11.8(s,1H,NH amide), 6.7–8.3(m,12H,aromatic).
N′‑cyclohexylidene‑2‑[(2,3‑dimethylphenyl)amino]benzohydrazide (3b, C21H25N3O)
m.p=201–203 °C; IR(KBR): (C=O amide 1666 cm−1), (C–H aliphatic 2867–
2952 cm−1), (C–H aromatic 3035 cm−1), (NH 3178 cm−1)(NH 3123 cm−1), (C=N
1
1607 cm−1). H NMR (400 MHz, DMSO): δ=2.1(s,3H,CH3), 2.2(s,3H,CH3), 1.1–
2.4(m, 10H, aliphatic), 9.2(s,1H,NH),10.3(s,1H,NH amide),6.6–7.6(m,7H,aromatic).
N′‑[(1E,2E)‑3‑phenylprop‑2‑en‑1‑ylidene]‑2‑(2,3‑dimethylanilino) benzohydrazide
(3c, C24H23N3O) m.p=206–210 °C; IR(KBR): (C=O amide 1650 cm−1),(NH
1
3256 cm−1)(NH 3125 cm−1), (FC=N 1620 cm−1). H NMR (400 MHz, DMSO):
δ=2.1(s,3H,CH3), 2.2(s,3H,CH3), 9.2(s,1H,NH), 10.4(s,1H,NH amide),
6.7–7.9(m,14H,aromatic and –CH=CH– two protons), 7.8(s,1H, –CH=N–). 13C-
NMR δ=13(CH3), 20(CH3), 111–147 (18 Aromatic carbons and –CH=CH– two
carbons), 165(C=O amide), 162(C=N).
2‑[(2,3‑dimethylphenyl)amino]‑N′‑[(E)‑(4‑hydroxyphenyl)methylidene]
benzohy‑
drazide (3d, C22H21N3O2) m.p=188–191 °C; IR(KBR): (C=O amide 1655 cm−1),
(NH 3115 cm−1)(NH 3290 cm−1), (C–H aliphatic 2939 cm−1), (C=N 1613 cm−1).
1H-NMR (400 MHz, DMSO): δ=2.07 (s,3H,CH3), 2.2(s, 3H,CH3), 9.2(s,1H,NH),
10.2(s,1H,NH), 11.9(sb,1H,OH), 6.7–7.5(m,11H,aromatic), 7.8(s,1H, –CH=N–).
13C-NMR δ=13(CH3), 20.08(CH3), 111–147(18 aromatic carbons), 162(C=N),
165(C=O amide).
1 3

Synthesis and evaluation of some new hydrazones as corrosion…
Mild steel samples and corrosive solutions
The chemical composition in mass percent (wt%) of metal substrates used in the
research was: 0.36 C, 0.66 Mn, 0.27 Si, 0.02 S, 0.015 P, 0.21 Cr, 0.02 Mo, 0.22
Cu, 0.06 Al, and balance Fe. The mild steel samples were mechanically abraded
with a series of emery papers (400–1600 grades) and cleaned thoroughly. The
stock solutions were prepared by diluting 37% analytical grade HCl with double
distilled water.
Weight loss measurements
All the experimental procedures including the shape and size of mild steel speci-
mens were based on the ASTM standard [15]. In essence, mild steel samples were
immersed in uninhibited and inhibited solutions for 24 h at 303 K. The samples
were prepared as mentioned previously and weighted using a precision balance
with a sensitivity of 0.1 mg. After each test, MS specimens were taken out and
rinsed thoroughly with distilled water and acetone, dried and weighted accurately
again. The experiments were carried out in triplicates and only the average values
of weight losses were reported.
Electrochemical measurements
The electrochemical measurements were performed in a conventional three-elec-
trode cell kit using Volta Lab (Tacussel-Radiometer PGZ 100) potentiostat. The
three-electrode system consisted of a working electrode (mild steel) with the sur-
face area of 1 cm2 in contact with the electrolyte, Pt as an auxiliary electrode
alongside a saturated calomel reference electrode. Before electrochemical meas-
urements, the working electrode was frst immersed in the test solution for 30 min
in order to be in equilibrium with the environment. Measurement of the imped-
ance spectra was obtained in the frequency range of 10 mHz to 100 kHz and an
AC amplitude of 5 mV. The Tafel polarization measurements were recorded at a
scanning rate of 1 mV s−1 in the range from − 800 to − 200 mV versus SCE rela-
tive to the corrosion potential. All electrochemical measurements were performed
in a cell containing 80 mL of the electrolyte at 303 K.
SEM and XRD studies
The surface morphology and corrosion products phases of mild steel samples
were characterized with scanning electron microscopy and XRD in the absence
and presence of 5 × 10−3 M of inhibitors. SEM imaging and XRD analyses were
performed on samples after a 24 h immersion time. A Hitachi TM-1000 SEM
1 3

H. Lgaz et al.
at an accelerating voltage of 15 kV and Bruker D8 powder difractometer with
CuKα radiation (λCu = 1.5407 Å) were used for SEM and XRD analyses.
Results and discussion
Weight loss measurements
We performed gravimetric tests to explore the efects of diferent amounts of hydra-
zone derivatives on the corrosion rate of mild steel in 1.0 M HCl. The following
Eq. (1) was used to determine the corrosion rate in millimeters per year (mm year−1)
[16]:
K × W
CRW
=
(1)
A × t × ꢀ
here K=8.76×104 was used as constant. W, t and A are the mass loss in gram, the
time of exposure in hours and the exposed area in cm2 [17], respectively. The den-
sity of carbon steel is 7.86 g cm−3. The values of the inhibition efciency, ꢀWL(%),
are calculated using the following equation [16]:
◦
CR − CR
ꢀWL(%) =
× 100
(2)
CR
◦
where CR and CR are corrosion rates before and after the addition of hydrazone
derivatives, respectively.
The results derived from gravimetric tests are summed up in Table S1 (Supple-
mentary information (SI)) and Fig. 1. The results in Fig. 1 indicate that for all hydra-
zone derivatives, the addition of small amounts of inhibitors in 1.0 M HCl did not
Fig. 1 Variation of inhibition efciency and corrosion rate with inhibitor concentration at 303 K
1 3

Synthesis and evaluation of some new hydrazones as corrosion…
signifcantly decrease the corrosion rate. In contrast, the inhibitive efect was more
pronounced at a higher concentration. As the concentration of inhibitor increased,
more inhibitive activity could be gained in a concentration-dependent manner. The
most likely explanation of these results is that at higher concentrations, inhibitor mol-
ecules may have a strong ability to interact with iron atoms, and by increasing con-
centrations, more and more inhibitor molecules enter double layer level by replacing
preexisting water molecules [18–21]. The hydrazone derivatives were found to attain
the maximum ꢀWL(%) at 5×10−3 M whereas the efciency of adsorption was at the
order HDZ1>HDZ2>HDZ3>HDZ4. These results are addressed in more detail in the
next sections.
Electrochemical studies
Electrochemical studies provide unique valuable information about the response of
mild steel samples exposed to corrosion conditions and the possible infuence of inhibi-
tors on corrosion resistance. The polarization measurements (Tafel plots) collected dur-
ing the experiments are graphically represented in Fig. 2. The main electrochemical
parameters such as corrosion current density (icorr) corrosion potential (Ecorr), cathodic
and anodic Tafel slopes (ꢀc, ꢀa) along with the corresponding inhibition efciency were
determined and tabulated in Table 1.
Note that only results of optimum concentration are reported, and full electrochemi-
cal results are available in SI Figures S1-S3 and Tables S2 and S3. The inhibition ef-
ciency (ꢀPDP(%)) is calculated using [16]:
[
]
icorr
ꢀPDP (%) = 1 −
× 100
(3)
◦
icorr
Fig. 2 Polarization curves for mild steel in 1.0 M HCl and in presence of hydrazone derivatives at 303 K
1 3

H. Lgaz et al.
Table 1 The parameters of PDP of the mild steel in uninhibited and inhibited solutions at 303 K
Inhibitors
Concentration
(M)
−Ecorr
(mV vs.
SCE)
−ꢀc (mV dec−1
)
ꢀa (mV dec−1
)
icorr (μA cm−2
)
ꢀPDP (%)
Uninhibited
HDZ1
–
496
493
480
483
478
150
174
144
154
140
92
59
63
69
70
597
28
48
76
94
–
5 × 10−3
5 × 10−3
5 × 10−3
5 × 10−3
95
92
87
84
HDZ2
HDZ3
HDZ4
◦
In this equation, icorr corresponds to the corrosion current density under unin-
hibited solutions, and icorr is the corrosion current density in presence of inhibitor
obtained by extrapolation of the Tafel lines.
After adding the diferent concentrations of hydrazone derivatives, we observed
decreased current densities in both anodic and cathodic branches as expected. This
efect became more pronounced as a function of increasing inhibitor concentration,
suggesting that many of the active surface sites were blocked due to the adsorption
of inhibitor molecules. Moreover, the results showed that the corrosion potential val-
ues remained nearly in the same range in the presence of all inhibitors concentra-
tions suggesting that HDZs are mixed type corrosion inhibitors [22]. We can also
observe from the Fig. 2 that all cathodic branches decreased markedly to lower val-
ues and give rise to parallel lines indicating that inhibitors molecules do not modify
the hydrogen evolution mechanism and this reaction is activation controlled [23].
In other words, the reaction mechanism remains unafected while the surface area
available for hydrogen evolution is signifcantly decreased and the access of H+ to
the mild steel surface became more difcult [24]. In contrast, we observed mark-
edly diferent efects of the inhibitor molecules on the metal dissolution reaction.
At low applied over potentials, a gradual decrease of the mild steel dissolution rate
was observed while at higher positive potentials, precisely over −250 mV vs. SCE,
a signifcant reduction in the corrosion resistance was observed. Typically, at low
over potentials, a protective layer of adsorbed molecules is formed and when the
electrode potential reaches desorption potential, inhibitor molecules adsorbed on the
steel surface commence to move away and no longer can sufciently protect the mild
steel at a higher potential than desorption potential [25].
There is no denial that a lower corrosion rate leads to an improved corrosion
resistance. Fortunately, useful information to explain performance diferences of the
diferent hydrazone derivatives can be obtained from direct electrochemical param-
eters. As can be seen from Table 1, the addition of inhibitor molecules signifcantly
reduces the corrosion current density values of the inhibited sample. Based on the
current data, we can indeed conclude that the order of increase of inhibitive efect of
tested hydrazones on steel corrosion is as follows: HDZ1 >HDZ2 >HDZ3 >HDZ4.
This result let us to assume that the relative performance of the diferent inhibitors is
determined by their structure, essentially by the number of heteroatoms, the nature
of the functional groups, the length of the π-electron system and etc. Polarization
1 3

Synthesis and evaluation of some new hydrazones as corrosion…
measurement results are consistently in good agreement with those from weight loss
tests.
Electrochemical impedance spectroscopy results are displayed as Nyquist and
Bode plots in Fig. 3. The examination of the results indicates that all Nyquist plots
show only one depressed capacitive semicircle and only one-time constant was
observed in Bode format. This implies that only one phenomenon occurred, and the
charge transfer process is the only efective pathway to control the dissolution of
mild steel at the metal/solution interface [26].
The results showed that the impedance response of the mild steel in 1.0 M HCl was
signifcantly afected by adding hydrazone derivatives. As expected, we found that the
Fig. 3 Impedance results for mild steel in 1.0 M HCl and in presence of hydrazone derivatives at 303 K a
Nyquist plots, b Bode and phase angle plots
1 3

H. Lgaz et al.
diameters of the semicircles increased with the increasing of the HDZs concentration.
It is important to note that by increasing the inhibitor concentration, more inhibitor
molecules adsorbed on the steel surface, so that a non-conducting protective layer can
be formed by an efective replacement of the pre-adsorbed water molecules [27].
Furthermore, we address the evolution of phase angle by measuring their values at
arbitrarily chosen frequency (1 kHz) as shown in Fig. 4 (only HDZ1). Clearly, the phase
angle value increased considerably as the inhibitor concentration increased. This indi-
cates increased adsorption of inhibitor molecules which, in turn, permits the formation
of a more protective surface layer. The results were also supported by the increase of
the absolute impedance at low frequencies in the Bode plots. Furthermore, whatever
the concentration used, the angle phase values are higher than that of the blank, but
lower than −90°, which describes the non-ideal capacitor.
The Nyquist diagrams represented in Fig. 3 are depressed semicircles and not per-
fect as expected from the theory of EIS, which is a common phenomenon on solid elec-
trodes. Generally, a slight inclination of the impedance plots is frequently observed and
it is related to the surface heterogeneity attributed to roughness and inhomogeneities of
solid surfaces [28]. To account for this efect, we replace the capacitance of the electric
double-layer by a constant phase element (CPE) in the electrical circuit model used
to ft the EIS data (Fig. 5). The equivalent circuit model includes a circuit (Rp//CPE)
(polarization resistance in parallel with the CPE) in series with the resistance of the
solution.
The CPE formula most often used for adjustment is defned by the following equa-
tion [28].
1
ZCPE
=
(4)
Q(jꢀ)n
Fig. 4 Phase angle values at 1 kHz for mild steel in 1.0 M HCl in presence of HDZ1
1 3

Synthesis and evaluation of some new hydrazones as corrosion…
Fig. 5 The electrical circuit model used to ft the EIS data
where Q is the CPE magnitude, ω is the angular frequency, j2 =−1 is the imaginary
unit and n the exponential term, which can be associated with the roughness of the
metal surface [29]. When n is close to 1, 0 or −1, the CPE represents a capacitor, a
resistance or inductance, respectively. In addition, if n=0.5 the result is equivalent
to Warburg difusion. The CPE is extremely fexible for adjusting impedance dia-
grams. The double layer capacitance value (Cdl) is defned by the following equation
[30]:
ꢀ
n
Cdl = Q × R1p−n
(5)
All electrochemical impedance parameters along with the inhibition efciency
ꢀEIS(%) are listed in Table 2 and S3 (SI). The protection efciencies were calculated
according to Eq. (6):
[
]
Rp(inh) − Rp
ꢀEIS(%) =
× 100
(6)
Rp(inh)
here Rp and Rp(inh) represent the polarization resistances of metal coupons in 1.0 M
HCl before and after the addition of the inhibitor, respectively.
From Table 2, it is clearly seen that the values of the polarisation resistance in
presence of hydrazone derivatives are much higher than that in blank solution which
Table 2 Impedance parameters for corrosion of mild steel in uninhibited and inhibited solutions at 303 K
Q × 10−4
(Sn Ω−1 cm−2
Inhibitors
Concentration/M
Rs (Ω cm2) Rp (Ω cm2)
n
Cdl (µF cm−2
)
ꢀEIS (%)
)
Uninhibited
HDZ1
HDZ2
HDZ3
HDZ4
–
0.08
2.50
22.3
853.8
0.79 35.340
182
6
–
97
5 × 10−3
5 × 10−3
5 × 10−3
5 × 10−3
0.86
0.81
0.78
0.82
0.1331
0.2924
0.4872
0.7802
2.80
2.50
3.10
419.0
293.0
166.4
10
14
30
94
92
86
1 3

H. Lgaz et al.
is indicative of decreased metal dissolution reaction. Meanwhile, a marked decrease
was observed in the double layer capacitance in presence of inhibitor molecules.
This led to changes in the interface indicative of the enhanced adsorption of inhibi-
tors on steel surface, and consequently there had been a large increase in the thick-
ness of the electric double layer. Then, the dissolution of steel efciently minimized.
As much as the substituent of diferent hydrazone derivatives difers so does dif-
fer the impacts on steel/electrolyte interface. Like potentiodynamic polarization
and weight loss results, the inhibition performances of tested compounds follow the
order of HDZ1 >HDZ2 >HDZ3 >HDZ4.
Adsorption isotherm
Several studies dealing with the interaction between organic compounds and steel
surface have revealed that in most instances the inhibitor molecules establish their
inhibition via the adsorption onto the metal surface. This adsorption process of
inhibitor molecules can be through physical, chemical or physicochemical adsorp-
tion. Such information can be typically obtained from the use of adsorption isotherm
models [28]. In our study, the experimental data were subjected into various adsorp-
tion isotherms such as Langmuir, Temkin, and Frumkin. The Langmuir model was
found to provide the best description of inhibitor adsorption. The general equation
of this isotherm is [31]:
C
1
=
+ C
(7)
ꢀ
Kads
The terms shown in the above equation are defned as follow: Kads is the adsorp-
tion–desorption equilibrium constant and Cinh is the inhibitor concentration. The
ΔG◦ads values (free energy of adsorption) were calculated using the following equa-
tion [32]:
◦
ΔGads = − RT ln(Kads × 55.5)
here 55.5 is the concentration of H2O in solution in mol L−1, R and T are the molar
gas constant (8.3144598 J/K) and absolute temperature respectively.
(8)
The plot of C/θ versus C gave the best ft (Fig. 6) with R2 and slope values close
to 1 confrming that the adsorption of all hydrazone derivatives on the surface
of the mild steel in 1.0 M HCl obeys the Langmuir adsorption isotherm. Impor-
tantly, the data in Table 3 suggested that the magnitude of the adsorption free
energy and the adsorption–desorption equilibrium constant followed the order of
HDZ1 >HDZ2 >HDZ3 >HDZ4. These results indicate that the extent of adsorp-
tion of tested compounds varies widely with the corresponding diferences in their
molecular structures.
The results summarized in Table 3 also indicate that the adsorption free
energy values of all compounds are between − 40 and − 30 kJ mol−1, which indi-
cate the presence of a physical and chemical adsorption of hydrazone derivatives
on steel surface [33]. As we mentioned above, there are signifcant diferences
1 3

Synthesis and evaluation of some new hydrazones as corrosion…
Fig. 6 Langmuir adsorption isotherm for the studied inhibitors on mild steel in 1 M HCl at 303 K
Table 3 The adsorption
ΔG◦ads (kJ mol−1
)
Inhibitors
Slope
Kads (M−1
)
R2
parameters for the corrosion
of MS in inhibited solutions at
303 K
HDZ1
HDZ2
HDZ3
HDZ4
1.04
1.06
1.03
1.06
24569
21472
19663
6732
0.9997
0.9995
0.9998
0.9995
−35.5
−35.2
−35.0
−32.3
between the tested inhibitor molecules in terms of functional groups, which lead
to a diferent extent of inhibitor adsorption.
SEM study
SEM analysis is a useful tool to characterize the surface morphology of the mild
steel samples. The SEM images of the MS samples before and after their immer-
sion in 1.0 M HCl are given in Fig. 7. The results of the SEM imaging in Fig. 7
showed a highly corroded and damaged steel surface, largely due to rapid corro-
sion attack in acidic solution. Opposite to that, a smoother surface morphology
was observed after the addition of hydrazone derivatives, which is mainly attrib-
uted to the formation of an adsorption layer thereby decreasing the corrosion
efects. Moreover, there are still some traces of scratches and some polishing
lines resulting from the steel surface treatment.
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H. Lgaz et al.
Fig. 7 SEM pictures of mild steel in acid solution and in presence of 5×10−3 M HDZs after 24 h immer-
sion time at 303 K
X‑ray difraction analysis
The inhibitory efect of the hydrazone derivatives on the corroded steel surface was
displayed through SEM analysis. However, to further address this efect, the samples
of polished, corroded (blank) and inhibited mild steel were examined by X-ray dif-
fraction. Figure 8 presents XRD patterns of the three steel samples. In the polished
and corroded samples, the difraction peaks at about 44.6°, 64.6°, and 82.4° were the
characteristic peaks of Fe. In addition, the difraction peaks corresponding to 28.3°,
35.2° and 36.2° were observed in the corroded sample due to the formation of the
Fig. 8 X-ray difraction pattern of polished, corroded and inhibited mild steel samples
1 3

Synthesis and evaluation of some new hydrazones as corrosion…
corrosion products. In contrast, no apparent peaks were observed for the inhibited
sample and the corrosion products were mainly suppressed. These results confrm
again that the hydrazone derivatives greatly reduce the attack of HCl solution on the
steel surface via formation of a protective layer [34].
Anticorrosion mechanism of hydrazone derivatives
Despite the large number of published works on the biological properties of hydra-
zone derivatives, the studies on the efectiveness and mechanism of these com-
pounds in corrosion inhibition remain insufcient. Herein, this study was designed
to address the gaps in knowledge regarding the use of hydrazone derivatives pre-
pared via functionalization of the MA. The results showed that the hydrazone deriv-
atives are efective corrosion inhibitors with great electronic properties. Based on
the results we obtained in this study, we propose a mechanism for the interaction of
tested hydrazones with steel surface in Fig. 9 taking as example the HDZ1.
Due to the presence of heteroatoms in the molecular structure of the tested hydra-
zones, it is thought to have a greater proton afnity. This is also a common feature in
many corrosion inhibition studies under acidic environment [35]. In the meantime,
the chlorine ions play an important role in the adsorption process of the tested com-
pounds. According to several previous studies examining the corrosion inhibition
of mild steel in HCl medium [36–39], the surface of mild steel possesses a negative
charge in HCl solution in the presence of inhibitors. Thus, it is assumed that Cl− ion
frst get adsorbed onto the positively charged metal surface by columbic attraction
and then cationic form of inhibitor molecules can be adsorbed through electrostatic
interactions between the positively charged molecules and the negatively charged
metal surface. This process can be easily found in our case due to the presence of
nitrogen and oxygen atoms in all tested hydrazones. This physisorption process is
H3C
N
H
H
N
CH3
O
N
H
H
Cl-
Cl-
Cl-
Cl-
Fe
Fe
+
+
+
Fe
+
Fe
Fe
Fe
+
+
Fe
+
Electrostatic interaction
to
Physisorption
Donation
Electron tranfert
d
pi
n to
Electrons transfert
d
Chimisorption
to
Electron transfer d
Retrodonation
pi*
Fig. 9 Pictorial representation of adsorption of HDZ1 on MS surface in 1.0 M HCl
1 3

H. Lgaz et al.
the frst step and the base of chemical adsorption, then, the hydrazone derivatives
can be chemically adsorbed through acceptor–donor interactions [40]. The pres-
ence of π-electrons of the phenyl rings and unshared electron pairs in heteroatoms
can ensure the donation of the electron to the vacant d-orbitals of iron, which is
chemical adsorption. Another form of the chemical interaction is what′s known as
back-donation of electrons from flled d-orbitals of the iron to empty π-acceptors on
organic compounds. The greater amount of the reactive sites will lead to an extra
negative charge on steel surface due to the strong donation of electrons which will
be transferred to inhibitor molecules by means of back-donation interactions, there-
fore enhancing the adsorption on the metal surface.
Conclusion
In summary, this work has investigated the inhibition efect of four hydrazone
derivatives prepared through functionalization of MA on the mild steel corrosion
in 1.0 M HCl. An experimental approach has been done to probe the corrosion inhi-
bition mechanism underlying the efectiveness of these inhibitors. Among tested
compounds, the HDZ1 produces better inhibitive performances. The electrochemi-
cal results illustrate that the HDZs behave as mixed type inhibitors; their addition to
corrosive solutions leads to a high increase of the polarization resistance and corre-
spondingly lower double-layer capacitance. The information about adsorption mode
was extracted from Langmuir adsorption model. These insights seem very important
and encourage us to invest in the development of other corrosion inhibitors through
functionalization of others NSAID’s. A comprehensive theoretical overview of fac-
tors potentially afecting the performance of tested compounds will be reported in
future work.
Acknowledgements The authors extend their appreciation to the Deanship of Scientifc Research at
King Khalid University for funding this work through research groups program under Grant Number
R.G.P-1.21-38.
Compliance with ethical standards
Confict of interest The authors declare that they have no confict of interest.
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H. Lgaz et al.
Afliations
Hassane Lgaz1 · Abdelkarim Chaouiki2 · Mustafa R. Albayati3 ·
Rachid Salghi2 · Yasmina El Aoufr4 · Ismat H. Ali5 · Mohammad I. Khan6 ·
Shaaban K. Mohamed7,8 · Ill‑Min Chung1
1
Department of Crop Science, College of Sanghur Life Science, Konkuk University,
Seoul 05029, South Korea
2
Laboratory of Applied Chemistry and Environment, ENSA, University Ibn Zohr, PO Box 1136,
Agadir, Morocco
3
Department of Chemistry, College of Education, Kirkuk University, Kirkuk, Iraq
4
Materials, Nanotechnology and Environment Laboratory, Faculty of Sciences, Mohammed V
University Rabat, Rabat, Morocco
5
Department of Chemistry, College of Science, King Khalid University, P. O. Box 9004,
Abha 61413, Kingdom of Saudi Arabia
6
Chemical Engineering Department, College of Engineering, King Khalid University, Abha,
Kingdom of Saudi Arabia
7
Faculty of Science and Engineering, Health Care Division, Manchester Metropolitan University,
Manchester, England M1 5GD, UK
8
Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt
1 3