Dendrimers: A new class of corrosion inhibitors for mild steel in 1?M HCl: Experimental and quantum chemical studies

Article abstract of DOI:10.1016/j.molliq.2016.10.117

The inhibition properties of two ammonia cored dendrimers have been investigated for mild steel corrosion in 1?M hydrochloric acid solution using chemical, electrochemical, surface and quantum chemical calculation methods. It was observed that inhibition efficiency increases with increasing the concentration of both the dendrimers and maximum efficiency was obtained at 50?ppm (50?mgL^??1) concentration. Potentiodynamic study suggested that investigated dendrimers behaved as mixed inhibitors and their adsorption on mild steel surface obeyed the Langmuir adsorption isotherm. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) examinations of mild steel surface confirmed the inhibition behavior of investigated inhibitor molecules. Atomic force microscopic (AFM) study revealed that studied compounds increases the surface smoothness of AFM micrographs by adsorbing on the metallic surface. Several quantum chemical calculations parameters were calculated and the results obtained were found to be consistent with the experimental findings.

Full text of DOI:10.1016/j.molliq.2016.10.117

Dendrimers: A new class of corrosion inhibitors for mild steel in 1 M HCl:
Experimental and quantum chemical studies
Chandrabhan Verma, Eno E. Ebenso, Yeddu Vishal, M.A.Quraishi
PII:
DOI:
Reference:
S0167-7322(16)32541-7
doi: 10.1016/j.molliq.2016.10.117
MOLLIQ 6521
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
1 September 2016
22 October 2016
24 October 2016
Please cite this article as: Chandrabhan Verma, Eno E. Ebenso, Yeddu Vishal,
M.A.Quraishi, Dendrimers: A new class of corrosion inhibitors for mild steel in 1 M
HCl: Experimental and quantum chemical studies, Journal of Molecular Liquids (2016),
doi: 10.1016/j.molliq.2016.10.117
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

ACCEPTED MANUSCRIPT
Dendrimers: A new class of corrosion inhibitors for mild steel in
1M HCl: Experimental and Quantum Chemical Studies
Chandrabhan Verma1, Eno E. Ebenso*,2, Yeddu Vishal3, M.A.Quraishi1
1. Department of Chemistry, Indian Institute of Technology, Banaras Hindu University,
Varanasi-221005
2. Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of
Agriculture, Science and Technology, North-West University (Mafikeng Campus),
Private Bag X2046, Mmabatho 2735, South Africa
3. Department of Metallurgical Engineering, Indian Institute of Technology, Banaras Hindu
University, Varanasi-221005
4. *Corresponding Author: E-mail: Eno.Ebenso@nwu.ac.za; Fax: +27183892052; Tel: +27
183892050, +27 183892051; Material Science Innovation & Modelling (MaSIM)
Research Focus Area, Faculty of Agriculture, Science and Technology, North-West
University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa
Abstract:
The inhibition properties of two ammonia cored dendrimers have been investigated for mild steel
corrosion in 1M hydrochloric acid solution using chemical, electrochemical, surface and
quantum chemical calculation methods. It was observed that inhibition efficiency increases with
increasing the concentration of both the dendrimers and maximum efficiency was obtained at 50
ppm (50 mgL-1) concentration. Potentiodynamic study suggested that investigated dendrimers
behaved as mixed inhibitors and their adsorption on mild steel surface obeyed the Langmuir
adsorption isotherm. Scanning electron microscopy (SEM) and energy dispersive X-ray
spectroscopy (EDX) examinations of mild steel surface confirmed the inhibition behavior of
investigated inhibitor molecules. Atomic force microscopic (AFM) study revealed that studied
compounds increases the surface smoothness of AFM micrographs by adsorbing on the metallic
surface. Several quantum chemical calculations parameters were calculated and the results
obtained were found to be consistent with the experimental findings.
Keywords: Dendrimer, Polarization, Corrosion inhibition, SEM/EDX/AFM, Quantum Chemical
calculation
1

ACCEPTED MANUSCRIPT
1. Introduction
Mild steel is a major construction material in various industries including food,
petroleum, and power production, chemical and electrochemical industries. The acidic solutions
are widely used to remove the rust and scales from the corroded mild steel for further uses such
as rolling, galvanizing etc. This process of rust removal is pronounced as acid pickling and the
acid pickling requires use of corrosion inhibitors in order to protect metals from aggressive
dissolution. Due to their ease synthesis and high effectiveness, organic compounds containing
heteroatoms particularly oxygen (O), nitrogen (N) and sulfur (S) have been reported as good
corrosion inhibitors for mild steel in acidic media [1-7]. Literature survey reveals that a large
number of organic compounds having heteroatoms have been utilized as effective corrosion
inhibitors for mild steel in acidic media. The nanometer scaled materials have gained much
attention due to the novel properties such as their high surface-to-volume ratio [8]. In wives of
this, the corrosion inhibition property of two ammonia cored dendrimers (nano material) namely,
Generation-G0 and Generation-G1 on mild steel corrosion in 1 M HCl was investigated.
A large number of synthetic methods have been developed to synthesis different types of
dendrimers having various chemical, biological and medicinal applications [9-11]. However,
literature survey reveals that only few of the dendrimers have been employed previously as
corrosion inhibitors. Khaled et al. [12] investigated the inhibition efficiency of polyamidoamine
dendrimer (PAMAM) on steel corrosion in 1M HCl solution using electrochemical and
computational analyses. These authors were found that investigated PAMAM dendrimer acts as
cathodic type inhibitors. Molecular dynamics simulation study revealed that the PAMAM adsorb
over the metallic surface through unshared electron pair of nitrogen as well as pi-electrons of
aromatic rings and amide groups. Verma et al. [13] studied the inhibition performance of three
ethylene diamine cored dendrimers, synthesized by condensation of ethylene diamine and methyl
acrylate, on mild steel corrosion in 1M hydrochloric acid solution using weight loss,
electrochemical and surface analyses methods. These authors were found that adsorption of these
dendrimers on metallic surface obeyed the Langmuir adsorption isotherm. Results also showed
that investigated dendrimers act as mixed type inhibitors. These authors were observed that
dendrimers acted as effective corrosion inhibitors and exhibited highest inhibition efficiency of
96.95% just only 25 mg/L (ppm) concentration. In view of this, in our present investigation we
2

ACCEPTED MANUSCRIPT
have reported the corrosion inhibition performance of two ammonia cored dendrimers using
weight loss, electrochemical impedance, potentiodynamic polarization, atomic force microscopic
(AFM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and
quantum chemical calculations methods. The novelty of the present work is that in our previous
investigation we have employed ethylene diamine to condense with methyl acrylate, while in our
present investigation we have employed cost effective ammonia to condense with methyl
acrylate. Therefore, unlike to previous analyses, dendrimers used in the present study are cost
effective and thereby green corrosion inhibitors for mild steel in hydrochloric acid solution.
Several quantum chemical calculation parameters such as energies of highest occupied (EHOMO
)
and lowest unoccupied molecular orbitals (ELUMO), energy band gap (ELUMO-EHOMO; ΔE), global
hardness (ρ) and softness (σ) were calculated for neutral and protonated form of dendrimers in
order to corroborate experimental results. The choice of these inhibitors as corrosion inhibitors is
based on following considerations:
1. The dendrimers can be easily synthesized from commercially available starting materials
2. The dendrimers contain –CO-NH2 group and electronegative heteroatoms (O, N).
3. These compounds are highly soluble in the tested acidic media due to presence of polar – NH2
groups
4. The dendrimers cover large surface area due to their nano size nature [14-17].
2. Experimental section
2.1 Synthesis of Corrosion Inhibitors:
The dendrimers used in present investigation were synthesized by condensation of
ammonia and methyl acrylate in methanol as described earlier in literature [18] as shown in Fig.
1. Ammonia and 100 equivalent of methyl acrylate were mixed in methanol. The ammonia
solution was added drop wise in methanolic solution of methyl acrylate. After mixing, reaction
mixture was stirred for 48 h at 37ᴼC. After completion of the reaction, the residual solvent was
removed by rotary evaporator and product was collected. Similar step was adopted for synthesis
of other generation dendrimer synthesis. The characterization data of investigated dendrimers
are: (1) 3,3',3''-nitrilotripropanamide (DENG-G0): IR spectrum (KBr cm-1): 3456, 3348, 2854,
3

ACCEPTED MANUSCRIPT
1680, 1530, 1484, 1236, 1012; 856; 1H NMR (300 MHz, DMSO) δ (ppm): 2.56, 2.83, 3.24, 8.52,
(2) N1, N1, N7, N7-tetrakis(3- amino-3-oxopropyl)-4-(3-( bis( 3-amino- 3-oxopropyl) amino)-3-
oxopropyl)heptanediamide (DENG-G1): IR spectrum (KBr cm-1): 3532, 3446, 3342, 2972, 2848,
1653, 1575, 1456, 1226, 1145; 968; 1H NMR (300 MHz, DMSO) δ (ppm): 2.64, 2.86, 3.42, 3.58,
8.69.
Generation-G0
(DEND-G0)
Generation- G1
(DEND-G1)
Fig. 1: Synthesis of investigated inhibitors
2.2 Materials:
Mild steel with chemical composition (wt %) Fe 99.30%, C 0.076%, Si 0.026%, Mn
0.192%, P 0.012%, Cr 0.050%, Ni 0.050%, Al 0.023%, and Cu 0.135%, was used for weight
loss, surface (SEM/EDX, FM) and electrochemical (potentiodynamic polarization and EIS)
measurements. The mild steel specimens were abraded and cleaned by SiC emery of different
grade ranging from 600-1200 mesh size, washed with double distilled water, degreased with
acetone in ultrasonic bath and finally allowed to dry under hot air blower. The gravimetric
experiments were performed on mild steel specimens having dimension of 2.5 cm × 2.0 cm ×
0.025 cm and mild steel samples with an expose area of 1 cm2 were used as working electrode
for all electrochemical measurements. All the gravimetric and electrochemical experiments were
performed in 1M HCl solution, prepared from AR-grade HCl (35%, MERCK) and double
distilled water.
4

ACCEPTED MANUSCRIPT
2.3 Gravimetric measurements:
The weight loss experiments were performed on mild steel specimens immersed in 1 M
HCl solution in the absence and presence of different concentrations of both studied dendrimers.
The volume of the tested solution was 100 ml 1M HCl and immersion time was 3hrs. The weight
loss experiments were triply performed to obtain better reproducibility of the data and mean
value was reported. Corrosion rate CR (mg cm-2 h-1) was calculated using following relation:
W
CR 
(1)
At
Where, W is the average weight loss of three parallel mild steel strips, A the exposed area of
mild strip and t is the immersion time (3hrs). From this calculated corrosion rate (CR), the
inhibition efficiency η% was calculated using the following relationship:
CR C
% 
R(i) 100
CR
(2)
The surface coverage (θ) was determined using the following equation:
CR CR(i)
CR
 
(3)
where CR and CR(i) are the corrosion rate values in absence and presence of dendrimers
respectively.
2.4. Electrochemical measurements:
The conventional three electrode assembly, consisting of the mild steel with exposed
area 1 cm2 as working electrode, highly pure platinum foil with exposed area 1 cm2 as counter
electrode and saturated calomel electrode (SCE) as reference electrode, was used for all
electrochemical experiments. All the electrochemical experiments were performed with Gamry
Potentiostat / Galvanostat (Modal G-300) in the absence and presence of optimum concentration
of dendrimers in 1M HCl solution after 30 minutes immersion time. Echem analyst 5.0 software
was used to collect and analyses all the electrochemical data.
The electrochemical impedance measurements were performed in frequency range of 105
to 10-2 Hz under potentiodynamic condition having amplitude 10 mV peak to peak using AC
signal at open circuit potential. From the diameter of Nyquist plots, the charge transfer
5

ACCEPTED MANUSCRIPT
resistances (Rct) were calculated and the percentage inhibition efficiency was calculated using the
following equation:




Rct
%  1
100



Rct(i)

(4)
where Rct(i) and Rct are the charge transfer resistance in presence and absence of dendrimer,
respectively.
The studied potentiodynamic polarization were performed from -0.25V vs SCE (cathodic
potential) to +0.25 vs SCE (anodic potential) with respect to open circuit potential at a sweep
rate of 1.5 mVs-1. The corrosion values of current density (icorr) were obtained by extrapolating
the linear segment of Tafel cathodic and anodic curves from which the percentage inhibition
efficiency was calculated using the following equation:
ic0orr ii
% 
corr 100
ic0orr
(5)
Where, i0corr and icorr are the corrosion current density in the absence and presence of dendrimer,
respectively.
2.5. Surface characterization:
The mild steel samples were immersed in 100 ml HCl solution in absence and presence
of optimum concentration of dendrimers for 3hrs immersion time. After elapsed time, mild steel
specimens were withdrawn and their surfaces were examined by scanning electron microscopy
(SEM) using SEM modal, Ziess Evo 50 XVP instrument at an accelerating voltage of 5 kV at
100 magnifications. For surface morphology measurement by atomic force microscopy (AFM)
method the NT-MDT multimode AFM, Russia, controlled by Solver scanning probe microscope
controller having single beam cantilever having resonance frequency in the range of 240–255
kHz in Semi-contact mode with corresponding spring constant of 11.5 Nm-1 with NOVA
programme was used.
6

ACCEPTED MANUSCRIPT
2.7. Theoretical study:
The quantum chemical calculations based on density function theory (DFT) were used in
order to perform quantum chemical calculations. Several quantum chemical indices of the
molecules have been derived and discussed in order to understand the interaction between
metallic surface and inhibitor molecules.
3. Results and discussions
3.1 Weight loss(Gravimetric) experiments:
3.1.1 Effect of concentration and temperature:
The corrosion inhibition efficiency of the dendrimers on mild steel corrosion in 1M HCl
solution was investigated after 3 hrs immersion time in the absence and presence of different
concentrations of the dendrimers. The variation of inhibition efficiency with inhibitors
concentrations is depicted in Fig. 2 (a), and weight loss parameters such as corrosion rate (CR),
surface coverage (θ) and percentage inhibition efficiencies(η%) were calculated and given Table
1. It can be seen form the results that the inhibition efficiency increases with increasing the
concentrations of the dendrimers for both inhibitors. The maximum values of inhibition
efficiency of 94.34% and 96.95% were obtained for DEND-G0 and DENG-G1 (Fig. 2),
respectively at 50 mg/L concentration. The order of inhibition efficiency follows the order:
DEND-G1> DEND-G0.
7

ACCEPTED MANUSCRIPT
(a) (b)
Fig. 2: (a) Variation of inhibition efficiency with concentration
(b) Variation of inhibition efficiency with Temperature
Moreover, from results depicted in Fig. 2(a) and Table 1 it can be seen that further increase in
dendrimer concentrations did not cause any appreciable change in the corrosion inhibition
performance of the studied inhibitors indicating that 50 ppm is the optimum concentration in the
present investigation. The higher inhibition efficiency of the DENG-G1 as compared to the
DENG-G0 can be attributed to its high molecular size and presence of addition heteroatoms (O
and N) which enhances the bonding between the iron on metal surface and inhibitor molecules.
The weight loss experiments were also performed in order to the effect of temperature on
inhibition efficiency, in temperature range of 308 to 338 K. Moreover, the variation of inhibition
efficiency with solution temperature is shown in Fig. 2 (b). It can be observed from the figure
that inhibition efficiency decreases with increasing temperature. This decrease in inhibition
performance on increasing solution temperature is attributed due to desorption of adsorbed
inhibitors molecules which in turn resulted due to increased kinetic energy of the inhibitor
molecules which decreases the interaction between inhibitor and metallic surface [19]. Further,
rapid etching, molecular rearrangement and/ or molecular decomposition may also decrease the
inhibition efficiency at high solution temperature.
8

ACCEPTED MANUSCRIPT
Table 1: Weight loss parameters and corresponding inhibition efficiency of mild steel in 1M
HCl solution.
Inhibitor
Conc.
(mg/L)
----
10
CR
(mgcm-2h-1)
7.66
η %
θ
------
63.47
71.73
82.60
89.56
94.34
94.77
66.08
73.06
83.47
89.56
96.95
97.38
-----
Blank
DEND-G0
2.80
2.16
1.33
0.80
0.43
0.40
2.60
2.06
1.26
0.60
0.23
0.20
0.634
0.717
0.826
0.895
0.943
0.947
0.660
0.730
0.834
0.895
0.969
0.973
20
30
40
50
60
10
20
30
DEND-G1
40
50
60
3.1.2. Adsorption isotherm and free energy of adsorption:
The adsorption isotherm gives mechanistic information about the interaction between
inhibitor molecules and metal surface. In this study, several adsorption isotherms including
Langmuir, Temkin, Flory–Huggins and Frumkin adsorption isotherms were tested. Langmuir
isotherm gave the best fit with the values of regression coefficient very close to unity (Fig. 3 a).
The adsorption of the dendrimers in acidic solution can be considered as a quasi-substitution
process between the dendrimers in the aqueous phase [DENDs(sol)] and adsorbed water molecules
on metal surface [H2O(ads)] as represented by the following equation [20]:
DENDs(sol) +x H2O(ads)
DENDs(ads) + x H2O(sol)
where, x is the number of water molecules replaced by one dendrimer molecule. Thus the
adsorption of dendrimers accompanied by removal of adsorbed water molecules from the mild
steel surface. The adsorption of dendrimers on the mild steel surface in 1 M HCl solution
involves two types of interactions, one is due to electrostatic interaction between charged
inhibitor molecules and metal surface (physical adsorption) and other one is due to sharing of
charges between inhibitor molecules and vacant d- orbitals of the surface iron atoms (chemical
adsorption).
9

ACCEPTED MANUSCRIPT
(a)
(b)
(c)
Fig. 3: (a) Langmuir adsorption isotherm (b) Arrhenius plot (c) Transition state plots for mild
steel corrosion in 1M HCl solution
10

ACCEPTED MANUSCRIPT
The Langmuir isotherms can be represented by the following equation:
C(inh)
1

C(inh)

K(ads)
(6)
where, C(inh) is inhibitor concentration and Kads is equilibrium constant for adsorption-desorption
processes. Generally, the value of Kads increases with increasing the concentration of inhibitors
and decreases with temperature.
This temperature dependency of Kads can be best represented by following equation:
Gaοds  RT ln(55.5Kads )
(7)
where, the
55.5 represents the molar concentration of water in acid solution and the T is temperature (K).
is the free energy of adsorption, R (J mol-1 K-1) is the gas constant, the value
The values of
and Kads were calculated at studied temperatures and are given in Table 2.
Inspection of the results depicted in the Table reveals that the values of Kads decreases with
increasing the temperature (K) which is attributed to decreased extent of adsorption at elevated
temperatures. In this study the value of
that dendrimers adsorbed on mild steel surface by both physiosorption and chemisorption
ranges from -34.28 to -37.18 kJ mol-1 suggesting
methods and predominantly by chemisorption method as values of
kJ mol-1 [21].
are more close to -40
Table2: Value of free energy of adsorption (-ΔGads) and adsorption constant (Kads) at various
temperature
DEND-G0
DEND-G1
Kads
Kads
Temperature
(k J
mol-1)
(103M-1)
(104M-1)
(k J mol-1)
-35.52
-34.62
-34.32
-34.28
19.00
7.79
5.86
5.34
-37.18
-35.71
-35.36
-35.24
36.31
11.57
8.74
308
318
328
338
7.13
11

ACCEPTED MANUSCRIPT
3.1.3. Kinetic and thermodynamic consideration:
The weight loss experiments were also performed at different temperature ranging from
308 to 338 K, in order to derive the kinetic and thermodynamic parameters. It is generally
reported that the value of corrosion rate (CR) increases with increasing temperature and the
temperature dependency of corrosion rate (CR) can be best represented by Arrhenius and
transition state equations [22]:
Ea
2.303RT
log(CR )
log
(8)



* 





RT
ΔS
ΔH*
CR 
exp
exp 

Nh
R
RT


(9)
where Ea is the apparent effective activation energy, R is the general gas constant, T is the
absolute temperature, λ is the Arrhenius pre-exponential factor, h is the Plank’s constant, N is the
Avogadro’s number, ΔS* is the entropy of activation and ΔH* is the enthalpy of activation. In
Fig. 3 (b), linear plots between log CR vs 1/T are obtained with values of regression coefficient
(R2) close to unity. From the slopes (Ea 2.303R ) the values of Ea in the absence and presence
of optimum concentrations of inhibitor molecules were calculated and are given given in Table
3.
The transition state plots give a straight line between log CR/T vs 1/T [Fig. 3 (c)] with the
values of regression coefficient close to unity. From the slope ( H* 2.303R ,) and intercept (
*


log(R Nh)(S 2.303R) ) of which the values of ΔH* and ΔS* were calculated and given in


Table 3. Inspection of the Tabulated data reveals that the values of Ea in the presence of
dendrimers are higher as compared to that in their absence. The increased values of Ea in the
presence of inhibitors is resulted either due to physical adsorption that occurs in first stage [23,
24] or due to decrease in the adsorption of inhibitor molecules on the mild steel surface with
increase in solution temperature [25]. The increased values of ΔH* in presence of both inhibitor
molecules as compared to that in their absence, suggest that the dissolution of mild steel in
presence of dendrimers is difficult as compare to in their absence [26]. Inspection of the Table 3,
12

ACCEPTED MANUSCRIPT
further suggests that values of the ΔS* increases in presence of dendrimers which clearly
indicates that disorderness going to increase from reactant to product site due to increases in
entropy of the solution during adsorption process [27].
Table 3: Values of Ea, -ΔH*, ΔS* in absence and presence of optimum concentration of
dendrimers
Inhibitor
Ea
(kJmol-1)
28.48
ΔH*
(kJmol-1)
26.04
ΔS*
(Jmol−1 K−1)
-148.9
Blank
55.47
65.42
52.79
62.74
-58.77
-31.15
DEND-G0
DEND-G1
3.2 Electrochemical measurements:
3.2.1. Electrochemical impedance measurements:
Fig. 4(a) represents the Nyquist plot in absence and presence of optimum concentration
of the dendrimers in 1M HCl solution. The Bode phase angle and equivalent circuit used for
evaluation of electrochemical data are shown in Fig. 4(b) and (c), respectively. It can be
observed that the impedance nature of mild steel changes significantly in the presence of the
dendrimers without affecting the common features of the Nyquist plots. This finding suggests
that dendrimers inhibits mild steel corrosion through adsorption process without affecting the
mode of mild steel dissolution [28].
The Nyquist plots show a depressed capacitive loop in the high frequency (HF) range.
The formation of HF capacitive loop can be attributed to the charge transfer reaction and time
constant of the electric double layer and to the surface inhomogeneity of structural or interfacial
origin, such as those found in adsorption processes [29]. Several parameters including solution
resistance (Rs), charge transfer resistance (Rct), pseudo capacitance (Cdl) and η% calculated from
Nyquist plots are given in Table 4.The double layer capacitance (Cdl) values in the absence and
presence of inhibitors are calculated using the following equation [30]:
Yn1
Cdl 
sin n( / 2)


(10)
13

ACCEPTED MANUSCRIPT
where Y0 is the CPE coefficient, n is the CPE exponent (phase shift), ω is the angular frequency.
The ωmax represents the frequency at which the imaginary component reaches at maximum. It is
the frequency at which the real part (Zreal) is midway between the low and high frequency x-axis
intercepts. The phase shift provides a general idea about surface morphology of the metal/
electrolyte interfaces. In general, high value of phase shift associated with high surface
smoothness. The increased values of phase shift in presence of inhibitor molecules indicate that
surface smoothness increased owing to the formation of protective film by inhibitor molecules at
metal/ electrolyte interfaces [31].
From Table 4 it is seen that the Cdl values decrease in the presence of dendrimers which is
attributed to a decrease in local dielectric constant and/or an increase in the thickness of
electrical double layer. This finding suggests that dendrimers inhibit the mild steel corrosion by
adsorption at the metal/electrolyte interfaces [32].From Table 4 it is also clear that the values of
Rct increases in the presence of dendrimers. Generally, higher value of Rct associated with high
charge transfer resistance and therefore high inhibition efficiency [33].
The Bode plots of impedance magnitude and phase angles were recorded for working electrode
in 1 M HCl solution in the absence and presence of optimum concentration of dendrimers at its
open circuit potential. The phase angle having magnitude -900 with slope value unity is
associated with ideal capacitive behavior. The deviation from the ideal capacitive behavior of
mild steel in present study is due to rough electrode surface which is caused by aggressive attack
of corrosion [33]. However, from the Bode plots, it is clear that phase angle remarkably
increased in presence dendrimers suggesting that dendrimers retard the corrosion process by
adsorption on the metal surface.
14

ACCEPTED MANUSCRIPT
(a)
(b)
(c)
Fig. 4 (a-c)
(a) Nyquist plot in absence and presence of optimum concentration
(b) Bode plot in absence and presence of optimum concentration
(c) Equivalent circuit used for analysis of EIS data
Table 4: Electrochemical impedance parameters and corresponding inhibition efficiency
of dendrimers in 1M HCl solution
Inhibitor
Conc (ppm)
Rs
Rct
(Ω cm2)
9.58
n
Y0
Cdl
η%
(Ω cm2)
1.12
(μF cm−2)
106.2
.0
50
50
0.827
0.893
0.838
249.8
101.4
124.4
--
Blank
1.182
1.117
173.7
259.2
42.03
31.24
94.48
96.30
DEND-G0
DEND-G1
15

ACCEPTED MANUSCRIPT
3.2.2. Potentiodynamic polarization measurements:
The Tafel polarization studies were also performed on the mild steel surface in absence
and presence of the optimum concentration of inhibitors in 1M HCl solution. The
potentiodynamic polarization nature of mild steel in the absence and presence of optimum
concentration of dendrimers is shown in Fig. 5. From this Figure it is clear that both cathodic and
anodic reactions were affected in presence of dendrimers suggesting that dendrimers retard the
corrosion process by altering the mechanism of mild steel dissolution. The various
potentiodynamic polarization parameters were derived from Tafel slopes are given in Table 5.
From this Table, it is depicted that corrosion current densities (icorr) decreases in the presence of
dendrimers. This finding suggests that dendrimers inhibit the mild steel corrosion in 1M HCl
solution. The corrosion potential (Ecorr) does not change significantly in presence of dendrimers
suggesting that investigated dendrimers were mixed type inhibitors [34].
Fig. 5: Potentiodynamic polarization curves for mild steel corrosion in 1M HCl solution in
absence and presence of optimum concentration of dendrimers
16

ACCEPTED MANUSCRIPT
Table 5: Potentiodynamic polarization parameters and corresponding inhibition efficiency
Inhibitor
Blank
Conc
(mg/L)
0.0
icorr
(μA/cm2)
1150
Ecorr
βa
βc
η%
…..
(mV/SCE) (mV/dec) (mV/dec)
-445
-507
-489
70.5
88.40
76.7
104.7
115.2
147.2
….
50
50
71.35
28.4
0.9379
0.97.53
93.79
97.53
DEND-G0
DEND-G1
3.3 Surface studies
3.3.1 Scanning electron microscopic study:
The mild steel specimens were immersed in test solution for 3 hrs immersion time in the
absence and presence of optimum concentration of the investigated dendrimers. After the elapsed
time, the specimens were taken out and their surface morphology was determined. Fig. 6 (a)
represents the SEM micrograph of the mild steel specimen corroded in the acidic solution in
absence of the inhibitors which is characterized by highly corroded and damaged surface with
pits and cracks. However, in the presence of optimum concentrations of the studied dendrimers
[Fig. 6 (b-c] the surface morphologies of the specimens have been remarkably improved owing
to the formation of the protective film on the metallic surface. This finding further suggests that
dendrimers inhibit mild steel corrosion in acid solution by adsorption process.
17

ACCEPTED MANUSCRIPT
Fig. 6: SEM micrographs in absence (a) and presence of DEND-G0 (b) and DEND-G1 (c) after
3h immersion time
3.3.2 EDX analysis:
The EDX spectra of the mild steel after 3h immersion time in the absence and presence of
optimum concentration of the dendrimers are shown in Fig. 7(a-c). Fig. 7(a), represents the EDX
spectrum of mild steel corroded in free acid without any inhibitor which clearly shows the
signals for the presence of iron (Fe), carbon (C) and oxygen (O). Presence of oxygen in the EDX
spectrum of uninhibited mild steel specimen is attributed due to formation of ferrous and ferric
oxide due to atmospheric oxidation during SEM-EDX analyses. Moreover, signal for nitrogen is
absent in the EDX spectrum of uninhibited mild steel specimen. However, EDX spectra in
presence of dendrimers [Fig. 7(b-c)] shows the characteristics peaks for nitrogen and oxygen
which is might be due to formation of protective film by dendrimers on mild steel surface.
Further, careful examination of the EDX spectra of inhibited mild steel specimens revealed that
intensity for signal corresponding to oxygen have been significantly increased due to adsorption
of the dendrimers over metallic surface.
18

ACCEPTED MANUSCRIPT
Fig. 7: EDX spectra of mild steel surface of (a) 1M HCl solution (b) inhibited mild steel (1 M HCl + 50
mg/L DEND-G0), (1 M HCl + 50 mg/L DEND-G1)
3.3.3 Atomic force microscopy:
The three dimensional picture of mild steel surfaces in the absence and presence of optimum
concentration of the dendrimers after 3h immersion time is shown in Fig. 8. Fig. 8(a), represent the
surface of mild steel in free acid solution, showing characteristics pits and cracks with badly damaged
surface. The calculated average surface roughness was 392 nm in absence of dendrimers. However, in
presence of dendrimers surface morphologies remarkably improved due to formation of protective film
on mild steel surface. The average surface roughnesses were 119 nm and 84 nm in presence of DEND-
G0 and DEND-G1, respectively.
19

ACCEPTED MANUSCRIPT
(a)
(b)
(c)
Fig. 8: Atomic force micrographs of mild steel surface of (a) mild steel in 1 M HCl, (b) inhibited
mild steel (1 M HCl + 50 mg/L DEND-G0), (c) inhibited mild steel (1 M HCl + 50 mg/L
DEND-G1).
20

ACCEPTED MANUSCRIPT
3.4 Quantum Chemical calculation:
Figs. 9 and 10 represent the geometrically optimized structures and frontier electron
distribution of the neutral inhibitor molecules. Several quantum chemical parameters such as
such as energy of highest occupied molecular orbital (EHOMO), energy of lowest unoccupied
molecular orbital (ELUMO), EHOMO–ELUMO energy gap (ΔE), dipole moments (μ), global hardness
(ρ) and softness (σ) are summarized in the Table 6. Generally, EHOMO represents the electron
donating ability of the inhibitor molecule to the d-orbital of the surface metal atoms. An inhibitor
having larger value of the EHOMO associated with higher electron donating ability and lower
electron accepting ability of the inhibitor molecules. Moreover, the highest occupied molecular
frontier electron distribution represents the region susceptible for electron donation. From the
EHOMO electron distribution it can be seen that frontier electron density is mainly localized over
the center nitrogen atom, suggesting that this nitrogen atom mainly participating in the electron
donation. Conversably, ELUMO represents the electron accepting tendency of the inhibitor
molecule into its empty anti-bonding molecular orbitals. Generally, the inhibitors having larger
values of ELUMO associated with lesser electron donating ability and greater electron accepting
ability of the molecules [35]. From the frontiers molecular electron distribution it can be
observed that LUMO is also mainly localized over the center nitrogen atom and few adjacent
carbon atoms, indicating that this part mainly accepting electrons from d-orbital of the surface
iron atoms during retro-donation.
Fig. 9: Optimized structure of (a) DEND-G0 (b) DEND-G1
21

ACCEPTED MANUSCRIPT
Fig. 10: Frontier molecular orbitals of (a) DEND-G0 (b) DEND-G1
From the theoretical data depicted in Table 6, it can be observed that DEND-G1 associated with
larger value of EHOMO and smaller value of ELUMO as compared to DEND-G0 suggesting that
DEND-G1 possesses higher corrosion inhibition ability than DEND-G0. The energy band gap;
ΔE (ELUMO-EHOMO) is another important reactivity parameter that can be used to predict the
relative order of adsorption. In general, a lower value of ΔE is associated with higher chemical
reactivity and therefore high inhibition efficiency. In our present investigation values of ΔE
obeyed the order: DENG-G0 > DENG-G1 indicating that DENG-G1 has greater tendency of
adsorption [36]. The global hardness (ρ) and softness (σ) were also calculated from the values of
EHOMO and ELUMO using equations 11 and 12 [37]. A lower value of hardness and higher value of
softness is consisted with high chemical reactivity and therefore high inhibition efficiency. In our
22

ACCEPTED MANUSCRIPT
present case, the values of softness follow the order: DENG-G0 < DENG-G1, which in
accordance to the order of inhibition efficiency obtained experimentally [38].
(11)
(12)
Lastly, the corrosion inhibition efficiency of both the studied dendrimers was correlated
with their dipole moment obtained by quantum chemical calculations. Generally, an organic
compound having higher value of dipole moment is more susceptible for polarization when come
to the metallic surface [37,38]. I our present study, results depicted in Table show that DENG-G1
is associated with high dipole moment indicating that it is more polarization and cover the larger
metallic surface area, and therefore acted as better inhibitor as compared to DENG-G0.
Organic compounds containing heteroatoms in form of polar group and/ or in cyclic ring
exist in protonated form due to presence of unshared electron pairs. Therefore, in our present
investigation, quantum chemical calculation was also performed on protonated form of the
inhibitor molecules. The fully optimized and frontier electron distribution pictures of the studied
dendrimers are shown in Figs. 11 and 12 and derived parameters are given in Table 6. Form the
frontier molecular electron distribution pictures it can be seen that unlike to neutral DENG-G0
HOMO is localized almost entire part of the molecule and therefore the inhibitor would act as
better inhibitor in protonated form. Moreover, careful examination of the Fig. 12 shows that for
DENG-G0, LUMO is mainly concentrated on the terminal amide groups, unlike to neutral form
of the inhibitor were LUMO was localized over central nitrogen atom only. In DENG-G1,
HOMO is localized around the central nitrogen atom similar to the neutral from of the inhibitor.
However, unlike to neutral form, LUMO is concentrated mainly on the ethyl group embedded
between central nitrogen and terminal amide groups. The quantum chemical calculation
parameters calculated for protonated form of dendrimers are given in Table 6. Inspection of the
results shown in Table 6 showed that EHOMO, ELUMO, ΔE, hardness (ρ), softness (σ) and dipole
moment (µ) calculated for protonated form of dendrimers obeyed the same order that obtained
for neutral form of dendrimers.
23

ACCEPTED MANUSCRIPT
Fig. 11: Optimized structure of protonated form of DEND-G0 (a), and DEND-G1 (b).
Fig. 12: Frontier molecular orbitals of protonated form of DEND-G0 (a) and DEND-G1 (b).
24

ACCEPTED MANUSCRIPT
Table 6: Quantum chemical calculation parameters obtained from geometrically optimized
molecules.
σ
Inhibitor
EHOMO
ELUMO
ΔE
µ
ρ
(Hartree)
(Hartree)
(Hartree)
(D)
(Hartree) (Hartree)
Neutral from of dendrimers
-0.246
-0.014
0.314
0.560
4.75
0.280
0.130
3.57
7.67
DEND-G0
DEND-G1
0.246
0.260
15.33
Protonated form of dendrimers
DEND-G0-H+
DEND-G1- H+
-0.25317
-0.23291
0.00082
0.00762
0.253
0.240
2.157
9.702
0.127
0.120
7.874
8.333
4. Mechanism of inhibition
The mild steel dissolution in acid solution can occur by two steps i.e. anodic mild steel
dissolution and cathodic hydrogen evolution. The anodic dissolution of the mild steel in acid
hydrochloric acid solution follows the steps given below [39,40]:
(13)
(14)
(15)
(16)
It is well known that organic inhibitors containing heteroatoms exist in their cationic
forms due to protonation of the heteroatoms and mild steel surface become negatively charged
due to adsorbed chloride ions. These two appositively charged species attracted each other by
electrostatic force of attraction. The cathodic hydrogen evolution cam be represented by
following steps [39,40]:
(17)
(18)
25

ACCEPTED MANUSCRIPT
(19)
The positively charged inhibitor adsorbed to the cathodic site of metal and thereby reduces the
rate of hydrogen evolution. An organic inhibitor can adsorb on mild steel by:
1. Electrostatic interaction between charged inhibitors and charged mild steel surface
(physisorption).
2. unshared electron pairs of heteroatoms and vacant d-orbital of Fe surface atoms
(chemisorption), or
3. interaction of d-electron of iron surface atom to the vacant orbital of inhibitor
molecule (retro donation) and
4. Combination of above processes [39-41].
During first step of adsorption, the protonated form of dendrimers in 1M HCl start
competing with H+ ions for electrons on metal surface. However, after evolution of H2 gas, the
positively charged dendrimers returns to their neutral form and heteroatoms (O, N) with free lone
pairs of electrons promote chemical adsorption [42]. The accumulation of extra negative charge
on mild steel surface render it’s to donate electrons to anti bonding molecular orbitals. It is well
stabilized that inhibitors can also accept the electrons into their anti-bonding molecular orbitals
from the surface iron atoms and thus strengthens the adsorption by synergism [43]. In our present
case the order of efficiency is: DEND-G1 > DEND-G0.
5. Conclusions
From above study this can be concluded that:
1. Dendrimers are good corrosion inhibitors for mild steel in 1M HCl solution.
2. The inhibition efficiency increases with the increase of the concentration of the inhibitors
and maximum was found at 50 ppm concentration.
3. The potentiodynamic polarization study reveals that dendrimers are mixed type inhibitors
4. The adsorption of the dendrimers follows the Langmuir adsorption isotherm.
5. The negative value of Gibb’s free energy suggests the spontaneous nature of adsorption.
6. SEM analysis suggests that dendrimers form protective film on the mild steel surface.
7. Theoretical calculation is in good agreement with the experimental data.
26

ACCEPTED MANUSCRIPT
References
[1]
[2]
S.A. Ali, M.T. Saeed, S.V. Rahman, Corros. Sci., 45 (2003) 253-266.
M. Lagrenee, B. Mernari, M. Bouanis, M. Traisnel, F. Bentiss, Corros. Sci., 44 (2002)
573-588.
[3]
[4]
[5]
[6]
C. Verma, M.A. Quraishi, A. Singh, J. Mol. Liq., 212 (2015) 804–812
K. C. Emregul, A. A. Akay, O. Atakol, Mater. Chem. Phys., 93 (2005) 325-329.
M. Elayyachi, A. El Idrissi, B. Hammouti, Corros. Sci., 48 (2006) 2470-2479.
C. Verma, L. O. Olasunkanmi, E. E. Ebenso, M. A. Quraishi, I. B. Obot, J. Phys. Chem.
C, 120 (2016) 11598−11611.
[7]
M.A. Quraishi, I. Ahamad, A.K. Singh, S.K. Shukla, B. Lal, V. Singh, Mater. Chem.
Phys. 112 (2008) 1035-1039.
[8]
[9]
H.J. Shin, I.W. Hwang, Y.N. Hwang, J. Phys. Chem. B., 107 (2003) 4699-4704.
D. A. Tomalia, A. M. Naylor, W. A. Goddard, Angew. Chem., Int.Ed., 29 (1990) 138-
175.
[10] E. R.Gillies, J. M. J. Frechet, Drug. Disco. Very Today., 10 (2005) 35-43.
[11] U. Boas, P. M. H. Heegaard, Chem. Soc. Rev., 33 (2004) 43-63.
[12] K.F. Khaled, A. A. Atta, N. S. Abdel-Shafi, J. Mater. Environ. Sci., 5 (2014) 831-840.
[13] C. Verma, M. A. Quraishi, Anal. Bioanal. Electrochem., 8 (2016) 104-123.
[14] D. Boris, M. Rubinstein, Macromolecules, 29 (1996) 7251-7260.
[15] Chai, M. Y. Niu, W.J. Youngs and P.L. Rinaldi,. J. Am. Chem. Soc., 123 (2001) 4670-
4678.
[16] A. M. Caminade, R. Laurent and J.P. Majoral, Advan. Drug Delivery Reviews, 57 (2005)
2130-2146.
[17] V. Damme, F. Th, L. Brobeck, J. Control. Relea., 102 (2005) 23-38.
[18] Tae-il Kim, Jung-un Baek, Cheng Zhe Bai, and Jong-sang Park, Bull. Korean Chem.
Soc., 27 (2006) 1894-1896.
[19] N. K. Gupta, C. Verma, M.A. Quraishi, A. K. Mukherjee, J. Mol. Liq., 215 (2016) 47–57
[20] N.A. Negm, N.G. Kandile, E.A. Badr, M.A. Mohammed, Corros. Sci., 65 (2012) 94-103.
[21] C. Verma, P. Singha, I. Bahadur, E.E. Ebenso, M.A. Quraishi, J. Mol. Liq., 209 (2015)
767–778.
27

ACCEPTED MANUSCRIPT
[22] E.E. Ebenso, N.O. Eddy, A. O .O Odiongenyi, Portugaliae Electrochimica. Acta., 27
(2009) 13-22.
[23]
[24]
E.A. Noor, A.H. Al-Moubaraki, Mater. Chem. Phys., 110 (2008) 145-154.
L. Larabi, Y. Harek, O. Benali, S. Ghalem, Prog. Org. Coat., 54 (2005) 256-262.
[25] C. Verma, A. Singh, G. Pallikonda, M. Chakravarty, M.A. Quraishi, I. Bahadur, E.E.
Ebenso, J. Mol. Liq., 209 (2015) 306–319
[26] C. Verma, L. O. Olasunkanmi, E. E. Ebenso, I. B. Obot, M. A. Quraishi, RSC Adv., 6
(2016) 53933–53948.
[27] I. Ahamad, M.A. Quraishi, Corros. Sci., 51 (2009) 2006-2013.
[28] I. Ahamad, R. Prasad, M. A. Quraishi, Corros. Sci., 52 (2010) 1472-1481.
[29] C. Verma, E.E. Ebenso, I. Bahadur, I.B. Obot, M.A. Quraishi, J. Mol. Liq., 212 (2015)
209–218
[30] A. K. Singh, M. A. Quraishi, Corros. Sci., 52 (2010) 152-160.
[31] C. B. Verma, M.A. Quraishi, E.E. Ebenso, Int. J. Electrochem. Sci., 8 (2013) 10864-
10877.
[32] M.A. Quraishi, J. Rawat, Mater. Chem. Phys., 70 (2001) 95-99.
[33] E. Naderi, A. H. Jafari, M. Ehteshamzadeha, M. G. Hosseini, Mater. Chem. Phys., 115
(2009) 852-858.
[34] R. Solmaz, G. Kardas, B. Yazıcı, A M. Erbil, Colloids Surf., 312 (2008) 7-17.
[35] H. Keles, M. Keles, I. Dehri, O. Serindag, Mater. Chem. Phys., 112 (2008) 173-179.
[36] O. O. Xometl, N. V. Likhanova, M. A. D. Anguilar, E. Arce, H. Dorantes, P. A. Lozada,
Mater. Chem. Phys., 110 (2008) 344-351.
[37] V. S. Sastri, J. R. Perumareddi, Corrosion., 53 (1997) 617-622.
[38] I. Lukovits, E. Klaman, F. Zucchi, Corrosion., 57 (2001) 3-8.
[39] R. Solmaz, G. Kardas, B. Yazıcı, M. Erbil, Colloids Surf., A 312 (2008) 7−17.
[40] A. K.Singh, Ind. Eng. Chem. Res. 51 (2012) 3215−3223.
[41] D.P. Schweinsberg, G.A. George, A.K. Nanayakkara, D.A. Steiner, Corros. Sci. 28
(1988) 33-42.
[42] H. Shorky, M. Yuasa, I. Sekine, R.M. Issa, H.Y. El-Baradie, G.K. Gomma, Corros.Sci.,
40 (1998) 2173-2186.
28

ACCEPTED MANUSCRIPT
[43] C. Verma, M. A. Quraishi, L. O. Olasunkanmi, E. E. Ebenso, RSC Adv., 5 (2015) 85417–
85430.
[45] D. K. Yadav, M. A. Quraishi, Ind. Eng. Chem. Res., 51 (2012) 8194-8210.
[46] C. Verma, L. O. Olasunkanmi, I. B. Obot, Eno E. Ebenso, M. A. Quraishi, RSC Adv., 6
(2016) 15639–15654.
29