2. 462 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
(η), softness (σ), electronegativity (χ), proton affinity, global elec-
trophilicity (ω), global nucleophilicity (ε) and total energy (sum
of electronic and zero-point energies). Previously, some work has
been done in our laboratory on using mono– and bis–azo dye com-
pounds as inhibitors on aluminum in HCl and NaOH solutions [26].
The aim of the present study was to investigate and compare
the inhibition of corrosion of iron in 2.0 M HNO3 and 2.0 M NaOH
solutions by six synthesized mono-azo dye derivatives shown in
Fig. 1 at 303 K. We have determined the inhibition efficiencies of
these compounds using weight loss, thermometric, spectroscopy
measurements and polarization curves method. Theoretical stud-
ies on electronic and molecular structures of substituted mono-azo
dyes were carried out with the help of quantum chemical calcula-
tions and molecular dynamics simulations (MDS) approach to de-
termine the most effective corrosion inhibitor among them.
R=
α-Naphthyl; the compound namely mono-α-naphthyl amine
(MAD_1)
β- Naphthyl; the compound namely mono -β -naphthyl amine
(MAD_2)
C6H4OMe-p; the compound namely mono -p-anisidine (MAD_3)
C6H4Me-p; the compound namely mono -p-toluidine (MAD_4)
C6H4Me-o; the compound namely mono -o-toluidine (MAD_5)
C6H4Me-m; the compound namely mono -m-toluidine (MAD_6)
2. Experimental details
2.1. Synthesis of the mono-azo dye compounds
The investigated mono-azo dye (MAD_1–6) derivatives were
synthesized by diazotization of primary aromatic amines and cou-
pling with the corresponding naphthol derivatives in the ratio
1:1. The compounds are purified and characterized by elemen-
tal analysis, IR, UV–visible spectroscopic investigation; mass spec-
troscopy and 1Hnmr spectra spectroscopy techniques. The inhibitor
solutions were prepared by dissolving the appropriate amount in
10 cm3 Analar ethanol. The desired volume of the free inhibitor
was added to the electrolyte solution. The ratio of ethanol was kept
constant for each test. This stock solution was used for all exper-
imental purposes. The concentration range of azo dye inhibitors
employed was 5×10−7 M – 10−4 M at 303 K. The chemical struc-
ture and IUPAC name of synthesized azo dye compounds are given
in Fig. 1. The corrosion tests were performed on iron specimens
of following composition (wt. %): C=0.16, Mn=0.37, Si=0.05,
S=0.015 and remainder Fe. Iron specimens of size 2.0×2.0×0.1
and 10×1 × 0.l cm were used for weight loss and thermomet-
ric measurements, respectively. Solution of 2.0 M HNO3 and 2.0 M
NaOH were prepared by dilution of Analar analytical grade using
double distilled water.
2.2. Measurements
2.2.1. Weight loss measurements
Weight loss experiments were done according to the stan-
dard methods as reported in literature [27]. The corrosion rates,
OH
HO N N
Mono-α-naphthyl amine (MAD_1)
OH
HO N N
Mono-β -naphthyl amine (MAD_2)
OH
HO N N OMe
Mono-p-anisidine (MAD_3)
OH
HO N N Me
Mono-p-toluidine (MAD_4)
OH
HO N N
Me
Mono-o-toluidine (MAD_5)
OH
HO N N
Me
Mono-m-toluidine (MAD_6)
Fig. 1. Chemical molecular structures of synthesized mono-azo dye (MAD_1-6)
derivatives.
3. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 463
CR (mg cm−2 h−1) were calculated according to the following equa-
tion [28,29]:
CR = (Wb − Wa)/At (1)
where Wb and Wa are coupon weights measured before and after
immersion in the test solutions, A is the exposed area and t is the
exposure time (5h).
The inhibition efficiency IE (%) was calculated according to the
following equation [30]:
IE(%) =
CR − CR(inh)
CR
× 100 (2)
where CR and CR (inh) are the values of corrosion rate (mg cm−2
h−1) of iron in uninhibited and inhibited solutions, respectively.
2.2.2. Thermometric measurements
The reaction vessel used was basically the same as that de-
scribed by Mylius [31]. An iron piece (10×1 × 0.l cm) was im-
mersed in 30 cm3 of either 2.0 M HNO3 and/or 2.0 M NaOH in the
absence and presence of additives, and the temperature of the sys-
tem was followed as a function of time. The procedure for the
determination of the metal dissolution rate by the thermometric
method has been described previously [31,32]. The reaction num-
ber (RN) is defined [33] as:
RN =
(Tmax − Ti)
t
(3)
where Tmax and Ti, are the maximum and initial temperatures, re-
spectively, and t is the time (in minutes) required to reach the
maximum temperature. The percent reduction in RN [34,35] is
then given as:
%reduction in RN =
(RN free − RN inh)
RN free
× 100 (4)
2.2.3. Electrochemical measurements
A conventional three – electrode cell was used with a 1.0 cm2 Pt
sheet as the counter electrode which was separated from the main
cell compartment by a glass sinter. The potentials of the work-
ing electrode were referred to a saturated calomel electrode (SCE).
In order to avoid contamination, the reference electrode was con-
nected to the working- electrode through a salt bridge filled with
the test solution. The tip of the bridge was pressed against the
working electrode in order to compensate the ohmic drop. Prior
to each experimental measurement, the solution under investiga-
tion (25 cm3) was freed of oxygen by passing prewashed pure ni-
trogen through it for a sufficient time. Measurements were per-
formed on a planar disk electrode (A = 1 cm2). The iron electrodes
were carefully degreased, and the edges were masked by appropri-
ate resins (Duracryle, Spofa–Dental, and Praha). The surface of the
iron electrodes were prepared by mechanical grinding and polish-
ing as given elsewhere [2–5,8–10,35]. The electrodes were rinsed in
an ultrasonic bath containing bid stilled water and finally washed
with bid stilled water immediately before being immersed in the
cell. The pretreatment procedure was repeated before each ex-
periment. Anodic and cathodic potentiostatic polarization of iron
electrodes were measured with a (Wenking Potentioscan model
POS 73). Potentials and currents were determined by digital multi
meters. Corrosion current densities (Icorr) were determined by ex-
trapolation of the anodic and cathodic Tafel lines to the free corro-
sion potential value (Ecorr). Each experiment was conducted with
a freshly prepared solution and with newly polished electrodes.
The cell temperature was kept constant at 303.0 ± l.0 K in an ultra-
thermostat.
The inhibition efficiencies at different inhibitor concentrations
were calculated using the following equation:
IE(%) =
Icorr − Icorr(inh)
Icorr
× 100 (5)
Fig. 2. Absorption spectra of Fe ions containing 10−4
M (MAD_1) and (MAD_5) in
2.0 M HNO3; and (MAD_6) in 2.0 M NaOH: (a) without inhibitors (b) with inhibitors
at 303 K.
where Icorr and Icorr(inh) are the corrosion current densities for un-
inhibited and inhibited solutions, respectively.
2.2.4. Spectrophotometric measurements
UV–visible absorption spectrophotometric method was applied
on the corrosive solutions produced from the corrosion of iron
samples, either without or with (MAD_1) and (MAD_5) in 2.0 M
HNO3; and (MAD_6) in 2.0 M NaOH, respectively. All the spectra
measurements were carried out using a Perkin–Elmer UV–Visible
Lambda 2 spectrophotometer, as shown in Fig. 2.
4. 464 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
3. Computational details
3.1. Quantum chemical calculations
Density Functional Theory (DFT) is one of the most important
theories that have been presented to predict the reactivity or sta-
bility of chemical species. Nowadays, theoretical methods based on
DFT have been very popular [36]. In the present study, DFT cal-
culations were carried out using Gaussian 9.0 Program [37]. The
input files of studied molecules were prepared with Gauss View
5.0.8 [38]. A full optimization was performed up to a higher basis
set denoted by 6-31G++ (d, p) because this basis set gives more
accurate results in terms of the determination of geometries and
electronic properties for a wide range of organic compounds. The
calculations in both gas and aqueous phases were also made us-
ing other levels of theory such as HF and DFT/B3LYP methods with
SDD, 6-31++G (d, p) and 6-31 G basis sets. In parallel with devel-
opments in both quantum chemistry and DFT, based on the ion-
ization energy and electron affinities of chemical species (atom,
ion or molecule), quantum chemical description such as chemical
hardness (η), chemical potential (μ) and electronegativity (χ) are
defined as follows [39–41].
η =
I − A
2
(6)
χ = −μ =
I + A
2
(7)
Pearson who introduced the chemical hardness concept de-
scribed as the multiplicative inverse of chemical hardness the soft-
ness (σ) [42,43].
σ =
1
η
(8)
One of the theorems that provide great facilities to computa-
tional chemists has been proposed by Koopmans [44]. This theo-
rem presents an alternative method to predict the ionization en-
ergies and electron affinities of chemical compounds. According to
the theorem, the negative of the highest occupied molecular or-
bital energy and the negative of the lowest unoccupied molecu-
lar orbital energy corresponds to ionization energy and electron
affinity, respectively (-EHOMO= I and –ELUMO= A). If so, within the
framework of aforementioned theorem, one can write the follow-
ing mathematical formulas for chemical hardness, electronegativity
and chemical potential [45].
η =
ELUMO − EHOMO
2
(9)
μ = −χ =
ELUMO + EHOMO
2
(10)
Proton affinity (PA) is one of the most important indicators of
electron donating abilities [46] of molecules because there is a re-
markable correlation gas phase basicity and proton affinity. Proton
affinities of molecules can be compared with the help of via fol-
lowing equations.
PA = E(pro) − (E(non−pro) + EH+ ) (11)
where, Enon-pro and Epro are the energies of the non-protonated and
protonated inhibitors, respectively. EH
+ is the energy of H+ ion and
was calculated as:
EH+ = E(H3O+ ) − E(H2O) (12)
According to global electrophilicity index (ω) proposed by Parr
[47], the electrophilicity of any chemical species is associated with
its electronegativity and hardness and is defined mathematically as
given as follows. It is important to note that nucleophilicity (ε) is
known as the multiplicative inverse of the electrophilicity [48].
ω =
μ2
2η
=
χ2
2η
(13)
ε = 1/ω (14)
3.2. Molecular dynamics simulation
Molecular dynamics simulations (MDS) is very popular for
the investigation regarding the interaction between the inhibitor
molecule and the concerned metal surface. The interaction be-
tween mono-azodye inhibitors and the iron surface was simulated
using Forcite module of Materials Studio 6.0 program developed
by Accelrys Inc. [49,50]. Herein, we had chosen the Fe (110) sur-
face, which is a density packed surface and was the most stable
[51] to simulate the adsorption process. Five layers of iron atoms
were used to ensure that the depth of the surface was greater than
the non-bond cutoff radius used in the calculation. The MD simu-
lation was performed at 303 K controlled by the Andersen thermo-
stat, NVT ensemble, with a time step of 1.0 fs and simulation time
of 1000 ps, using the COMPASS [52] force field. Non-bond Inter-
actions, Van der Waals and electrostatic, were set as atom-based
summation method and Ewald summation method, respectively,
with a cutoff radius of 1.55 nm. Details of simulation process can
be referred to some previous literature [53].The interaction energy
between the inhibitor molecules and the Fe (110) surface is calcu-
lated by Eq. (15)
Einteraction = Etotal − (Esurface + Einhibitor) (15)
Herein, the total energy of the surface and inhibitor molecule
is designated as Etotal, Esurface is the surface energy without the in-
hibitor and Einhibitor is the energy of the adsorbed inhibitor on the
surface. The binding energy of the inhibitor molecule is expressed
as Ebinding =−Einteraction.
4. Results and discussion
The inhibition efficiencies of the six synthesized mono-azo dye
(MAD_1–6) derivatives on the corrosion of iron in 2.0 M HNO3
and 2.0 M NaOH solutions using chemical (gravimetric, thermo-
metric, UV–visible spectrophotometric) and electrochemical poten-
tiostatic polarization measurements were investigated. Quantum
chemical calculations and molecular dynamics simulation (MDS)
studies were applied, discussed and correlated with the experi-
mental methods. The calculated binding energies of the azodye
molecules on (110) Fe surface demonstrated that these molecules
are very effective inhibitors against the corrosion of iron in HNO3
and NaOH media. The obtained results in the study are given in
detail below.
4.1. Gravimetric measurements
The gravimetric method (weight loss) is probably the most
widely used method of inhibition assessment [54–60]. Corrobora-
tive results between weight loss and other techniques have been
reported [61,62]. It is the most accurate and precise method for
determining metal corrosion rate because the experiment is easy
to replicate and, although long exposure times may be involved,
the relatively simple procedure reduces the propensity to introduce
systematic errors. The effect of addition of different (MAD_1–6)
derivatives at various concentrations on the iron corrosion in 2.0 M
HNO3 and 2.0 M NaOH solutions was studied by weight loss mea-
surements at 303 K after 5–6 h immersion. The values of inhibition
efficiency IE (%) and surface coverage (θ) obtained from weight loss
5. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 465
Table 1
Corrosion parameters obtained from weight loss measurements for iron in 2.0 M HNO3 containing various concentrations of the synthesized
mono-azo dye (MAD_1–6) inhibitors at 303 K.
Inhibitor type, Conc. (M) (MAD_1) (MAD_2) (MAD_3) (MAD_4) (MAD_5) (MAD_6)
θ IE (%) θ IE (%) θ IE (%) θ IE (%) θ IE (%) θ IE (%)
5×10−7
0.647 64.7 0.647 64.7 0.643 64.3 0.613 61.3 0.610 61.0 0.610 61.0
1×10−6
0.671 67.1 0.656 65.6 0.644 64.4 0.615 61.5 0.611 61.1 0.616 61.6
5×10−6
0.676 67.6 0.659 65.9 0.646 64.6 0.624 62.4 0.613 61.3 0.626 62.6
1×10−5
0.685 68.5 0.672 67.2 0.657 65.7 0.654 65.4 0.645 64.5 0.644 64.4
5×10−5
0.746 74.6 0.733 73.3 0.729 72.9 0.720 72.0 0.711 71.1 0.711 71.1
1×10−4
0.785 78.5 0.776 77.6 0.764 76.4 0.750 75.0 0.740 74.0 0.735 73.5
measurements are listed in Table 1. The corrosion rate values (mg
cm−2 h−1) of iron in 2.0 M HNO3 and 2.0 M NaOH solutions, re-
spectively, decrease as the concentration of inhibitor increase. The
results show that the IE (%) and (θ) values increase as the con-
centration of the inhibitor increases from 5×10−7 – 1×10−4 M.
The maximum inhibition efficiency was about 78.5% at 1×10−4 M
for (MAD_1). From Table 1, it is clear that the order of inhibi-
tion efficiency of (MAD_1–6) derivatives is as follows: (MAD_1)>
(MAD_2)> (MAD_3)> (MAD_4)> (MAD_5)> (MAD_6).The adsorp-
tion isotherm experiments were performed to have more insights
into the mechanism of corrosion inhibition, since it describes the
molecular interaction of the inhibitor molecules with the active
sites on the iron surface [63]. The surface coverage, θ, was calcu-
lated according to the following equation:
θ =
CR0 − CR
CR0 − CRm
(16)
Where, CR0 and CR are the corrosion rates of iron for uninhibited
and inhibited solutions, respectively. CRm is the smallest corrosion
rate. The surface coverage values (θ) for different inhibitor concen-
tration were tested by fitting to various isotherms and the models
considered were [64]:
Temkin isotherm exp (f.θ) = kads.C (17)
Langmuir isotherm(θ/1 − θ) = kads.C (18)
Frumkin isotherm(θ/1 − θ)exp (−2 f.θ) = kads.C (19)
Freundluich isotherm θ = kads.C (20)
Where kads is the equilibrium constant for adsorption process, C is
the concentration of inhibitor and f is the energetic inhomogene-
ity. Attempts were made to fit the θ values to various isotherms
including Langmuir, Temkin, Frumkin and Freundluich. By far the
best fit is obtained with the Frumkin adsorption isotherm [64].
The plot of (θ) vs. log C gave S-shaped curves, suggest that the
adsorption of the investigated molecules on Fe surface obeyed the
Frumkin adsorption isotherm, as shown in Fig. 3. Stabilizing effect
[65] that comes from the complex compound formed indicates re-
arrangement of the charge density inside the molecule, thus shows
its corrosion inhibition. This is supported by U.V. spectrophotome-
ter analysis (Fig. 2), and also by conductivity measurements.
4.2. Thermometric measurements
Thermometric curves of the iron electrode in 2.0 M HNO3 with-
out and with addition of (MAD_1–6) derivatives at different con-
centrations are investigated and given in Fig. 4 for (MAD_1), (as
example) for the studied derivatives. The values of thermometric
parameters associated with thermometric measurements are listed
in Table 2. It is evident that, the dissolution of iron in 2.0 M HNO3
starts from the moment of immersion. On increasing the concen-
tration of the inhibitor from (5×10−7 – 1×10−4 M) the value of
Fig. 3. Variation of iron surface coverage (θ) with the logarithmic concentrations of
different substituted mono-azo dye derivatives in 2.0 M HNO3 at 303 K.
Tmax decreases, whereas the time (t) required reaching Tmax in-
creases, and both factors cause a large decrease in (RN) and in-
creasing of (% red RN) of the system [33], as shown in Table 2.
This indicates that the studied synthesized mono-azo dye additives
retard the dissolution presumably by strongly adsorption onto the
iron surface. The extent of inhibition depends on the degree of the
surface coverage (θ) of the metal surface with the adsorbate. Iron,
as an active element, always carries an air formed oxide, which
specifically and very strongly adsorbs H+ and OH− ions.
The dissolution of iron reactions takes place along the incu-
bation period. The heat evolved from these reactions accelerates
further dissolution of the oxide and activates the dissolution of
the iron metal exposed to the aggressive medium. The relation
between RN, time delay (࢞t) and/or log (࢞t) versus molar con-
centration of the additives confirms a two-step adsorption process
[66], at first a monolayer of the adsorbed is formed on the iron
electrode surface, and then it is followed by the adsorption of a
second adsorbed layer or a chemical reaction leading to the de-
position of the (azo dye-Fe complex) on the metal surface. The
plot of t and/or log ( t) as a function of log CIn yields a lin-
ear relation shape for the first region of the curve then a region
of constancy; this reveals the completion of the adsorbed mono-
layer of the inhibitor. In thermometric measurements (% red RNIn)
values are taken as the measure for the corrosion inhibition effi-
ciency (% In). Plots of % red RN versus molar concentration (CIn) of
the additives for iron corrosion in 2.0 M HNO3 are invariably sig-
moidal in nature as shown in Fig. 5. The inhibition efficiency of the
6. 466 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Table 2
Effect of different concentrations of (MAD_1) on the thermometric parameters of iron in 2.0 M HNO3.
Conc. (M) Log C(M) Ti °C Tmax °C t min. t min. Log t ϴ R.N. % Red in R.N.
2.0 M HNO3 19.5 50.8 68 – – – 0.460 –
5×10−7
−6.3 18.5 46.5 122 54 1.732 0.502 0.229 50.2
1×10−6
−6.0 18 46.0 130 62 1.792 0.532 0.215 53.2
5×10−6
−5.3 18 40.9 143 75 1.875 0.652 0.160 65.2
1×10−5
−5.0 18 39.9 145 77 1.886 0.671 0.151 67.2
5×10−5
−4.3 18 39.0 180 112 2.049 0.746 0.116 74.7
1×10−4
−4.0 18 37.4 196 128 2.107 0.786 0.098 78.7
Fig. 4. Temperature vs. time curves of iron corrosion in 2.0 M HNO3 in presence of
different concentrations of mono-α-naphthyl amine (MAD_1).
Fig. 5. Effect of substituted (MAD_1-6) derivatives on percentage reduction in re-
action number (% red. in RN) for iron corrosion in 2.0 M HNO3.
studied (MAD_1–6) derivatives depends on many factors, includ-
ing the molecular size, heat of hydrogenation, mode of interaction
with iron electrode surface, formation of metallic complexes and
the charge density on the adsorption sites. Adsorption is expected
to take place primarily through functional groups, essentially OH
and OCH3 would depend on its charge density as reported [67].
The thermometric technique cannot be applied for the iron corro-
sion in alkaline media because of the formation of oxide films on
the iron electrode surface, which formed only in near neutral and
slightly alkaline solutions.
4.3. Potentiostatic polarization measurements
Polarization curves of the iron electrode in 2.0 M HNO3 and
2.0 M NaOH solutions, respectively, without and with addition of
(MAD_1) derivative (as example) at different concentrations are
shown in Fig. 6. The values of electrochemical parameters asso-
ciated with polarization measurements, such as corrosion poten-
tial (Ecorr), corrosion current densities (Icorr) and Tafel slopes (βa,
βc) are listed in Table 3. The inhibition efficiencies were calculated
from Icorr values (Table 3) obtained from extrapolating Tafel lines to
the corrosion potential according to Eq. (5). The values of corrosion
current density (Icorr) decreased in presence of (MAD_1–6) deriva-
tives which suggests that the rate of electrochemical reaction was
reduced due to the formation of a barrier layer over the iron sur-
face by the inhibitor molecule. The parallel cathodic Tafel lines sug-
gested that the addition of inhibitors to the 2.0 M HNO3 solution
do not modify the hydrogen evolution mechanism and the reduc-
tion of H+ ions at the iron surface which occurs mainly through
a charge-transfere mechanism. The shift in the anodic Tafel slope
(βa) values may be due to the adsorption of nitrate ions/or in-
hibitor molecules onto the iron surface [68]. It is also clear that
there is a shift towards cathodic region in the values of corrosion
potential (Ecorr), from the fact that βc > βa. The extent of adsorp-
tion of inhibitor molecules onto the metal surface in term of the
surface coverage (θ) was calculated using Eq. (21) [69]:
θ =
ICorr(uninh) − ICorr(inh)
ICorr(uninh)
(21)
where ICorr (uninh) and ICorr (inh)are the corrosion current densities in
the absence and presence of the inhibitors, respectively.
From Table 3, it is also clear that the values of cathodic and an-
odic Tafel slope constant are slightly change and independent on
the inhibitors concentrations, indicating that the inhibition role of
these inhibitors is not through the interference on the reactions
of metal dissolution and reduction of protons. . It is clear from
the polarization curves (Fig. 6) that, the increase of the inhibitor
concentrations decreases the corrosion current (Icorr) which con-
sequently increases the surface coverage values; and consequently
increases the retardation of the iron dissolution in the acidic and
alkaline media. The results show that (MAD_1) at 1×10−4 M pro-
duce the lowest Icorr (7.713 mA cm−2) and the maximum IE (%) ob-
tained was 77.8% (Table 3).
7. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 467
Table 3
Potentiodynamic polarization corrosion parameters of Fe dissolution reaction in 2.0 M HNO3 and 2.0 M NaOH solutions in absence
and presence of 10−4
M mono-(MAD_1-6) substituted azodye inhibitors at 303 ± 1 K.
Inhibitor type Corrosive solution -Ecorr (mV(SCE)) Icorr (mA cm−2
) -βc (V dec−1
) βa (V dec−1
) θ IE (%)
Blank 2.0 M HNO3 340 34.670 0.960 0.879 – –
Blank 2.0 M NaOH 775 6.762 0.350 – – –
MAD_ 1 2.0 M HNO3 300 7.713 0.844 0.761 0.778 77.8
2.0 M NaOH 653 2.805 0.221 – 0.5851 58.5
MAD_ 2 2.0 M HNO3 305 8.184 0.844 0.752 0.763 76.3
2.0 M NaOH 646 2.612 0.251 – 0.5341 53.4
MAD_ 3 2.0 M HNO3 316 8.346 0.833 0.767 0.759 75.9
2.0 M NaOH 683 3.344 0.221 – 0.5054 50.5
MAD_ 4 2.0 M HNO3 295 9.690 0.830 0.780 0.721 72.1
2.0 M NaOH 650 3.712 0.224 – 0.4510 45.1
MAD_ 5 2.0 M HNO3 298 10.593 0.836 0.777 0.694 69.4
2.0 M NaOH 668 3.914 0.231 – 0.4211 42.1
MAD_ 6 2.0 M HNO3 320 10.656 0.841 0.761 0.692 69.2
2.0 M NaOH 700 4.185 0.267 – 0.3811 38.1
Fig. 6. Potentiostatic polarization curves of iron in (a) 2.0 M HNO3 and (b) 2.0 M
NaOH solutions with the (MAD_1) in different concentrations at 303 K.
Thus, the studied mono-azo derivatives act as adsorptive in-
hibitors, i.e. they reduce anodic dissolution and also retard the hy-
drogen evolution reaction [70] via blocking the active reaction sites
on the iron surface, or even can screen the covered part of the
electrode; and therefore protect it from the action of the corrosion
medium [71]. Mono-azo dyes were first adsorbed onto the metal
surface and impeded by merely blocking the reaction sites of the
metal surface without affecting the anodic and cathodic reaction
[72], which suggests the retardation of iron corrosion in inhibited
solution with respect to uninhibited.
According to Ferreira and others [73,74] if the displacement
in (Ecorr) values (i) >85 mV in inhibited system with respect to
uninhibited, the inhibitor could be recognized as cathodic or an-
odic type and (ii) if displacement in Ecorr is <85 mV, it could be
recognized as mixed-type. For studied (MAD_1–6) inhibitors, the
maximum displacement range was 45 mV towards cathodic region,
which indicates that all studied compounds are mixed- type (an-
odic/cathodic) inhibitors [75–77] in 2.0 M HNO3, cause anodic and
cathodic over potential and inhibit both the hydrogen evolution
and anodic dissolution processes. The inhibitors act mainly as ca-
thodic type in 2.0 M NaOH as seen in Fig. 6, whereas the maxi-
mum displacement in Ecorr value was 129 mV towards cathodic re-
gion. The cathodic reduction of the passive film on iron in 2.0 M
NaOH results in the formation of a non-reducible porous layer. The
surface layer was found to grow also during cathodic polarization;
actually the cathodically polarized surface can be covered with Fe
(OH)2, as given in Eqs. (24)–(26). Results indicate that, the cathodic
reaction is the rate-determining step and all the investigated ad-
ditives predominantly under cathodic control and act mainly as
cathodic inhibitors from the blocking adsorption type. The mag-
nitude of the displacement of Tafel plots is proportional to the
inhibitor concentration. The inhibition efficiency strongly depends
on the structure and chemical properties of the layers formed at
the electrode surface under prevailing experimental conditions. In-
crease in inhibition efficiencies with the increase of concentrations
of the azodye derivatives are due to its adsorption on iron surface
[78,79]. The higher IE (%) values in acidic solution with respect to
alkaline one, is due to the less negative potential of Fe in acidic
medium, favoring adsorption of the additive on the electrode sur-
face.
The anodic dissolution of iron and cathodic reactions in acidic
and/or alkaline media has been reported to proceed as follows
[80]:
Fe→Fe2+
+2e−
(22)
2H+
+2e−
→2Hads →H2 (23)
Fe+OH−
Fe OHads +H+
+e−
(rds) (24)
Fe OHads →FeOH+
+e−
(25)
Fe OH+
+H+
Fe2+
+H2O (26)
8. 468 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Table 4
Comparison between the inhibition efficiency IE (%) of (MAD_1–6) derivatives in 2.0 M HNO3 and 2.0 M NaOH
solutions as determined by weight loss, thermometric and polarization methods at (1×10−4
M inhibitor concen-
tration) at 303 K.
Inhibitor type Corrosion Inhibition Efficiency IE (%)
Weight loss Thermometric Potentiodynamic polarization
2.0 M HNO3 2.0 M NaOH 2.0 M HNO3 2.0 M NaOH 2.0 M HNO3 2.0 M NaOH
MAD_ 1 79 45 78.7 – 77.8 58.5
MAD_ 2 78 51 77.7 – 76.4 61.4
MAD_ 3 76 43 74.8 – 75.9 50.5
MAD_ 4 75 40 74.2 – 72.1 45.1
MAD_ 5 74 38 74.0 – 69.4 42.1
MAD_ 6 73 34 69.6 – 69.3 38.1
Where ’rds’ stands for rate-determining step. As a result of these
reactions, including the high solubility of the corrosion products,
it is evident that the weight loss of iron increases with time. It
may also suggest that the iron corrosion by HNO3 and NaOH is a
heterogeneous process involving several steps.
The sequence of IE (%) values obtained from the polarization
measurements are in good agreement with those obtained from
weight loss and thermometric measurements as given in Table 4.
This agreement confirms the validity of the present chemical and
electrochemical measurements also support the explanation given
for the effect of chemical composition on the inhibitive action of
the investigated (MAD_1–6) inhibitors. Nevertheless, they showed
small differences in their absolute IE (%) values, could be attributed
to the different experimental conditions under which each tech-
nique was carry out.
4.4. Quantum chemical calculations
In the theoretical section of this study, the inhibition ef-
ficiencies of mono-azo dye (MAD_1-6) derivatives given in
Fig. 1 on corrosion of Fe were investigated by quantum chemical
and molecular dynamics simulation studies. The results obtained
in this study showed that these mono-azo dye compounds are
extremely effective corrosion inhibitors against the corrosion of
Fe. The discussions related to both quantum chemical parameters
calculated using Gaussian Program and binding and interaction
energies between inhibitors and metal surface determined by
applying molecular dynamics simulation (MDS) approach are
given below in detail. Quantum chemical parameters such as
EHOMO, ELUMO, ࢞E (HOMO–LUMO energy gap), global hardness
(η), softness (σ), dipole moment (D), electronegativity(χ), proton
affinity, global electrophilicity (ω), nucleophilicity (ε) and total en-
ergy are chemical reactivity descriptors that provide considerable
hints about electron donating or electron accepting abilities of
molecules. In this study, quantum chemical parameters calculated
with the help of HF/SDD, HF/6-311 G, HF/6-31++G, B3LYP/SDD,
B3LYP/6-311 G and B3LYP/6-31++G methods [81–83] for non-
protonated and protonated forms of studied molecules in both gas
and aqueous phase are provided in Tables 5–8.
The frontier orbital energies are useful parameters [84] in the
predicting of chemical reactivity of chemical compounds. In gen-
eral, it is assumed that EHOMO is related to electron donating abil-
ity. Therefore, the ability to donate electron of a molecule having
high EHOMO value will be more. Another useful parameter to com-
pare the electron donating or accepting abilities of molecules is
LUMO energy level. LUMO energy level is an indicator of electron
accepting abilities of molecules. It can be said that the energy of
LUMO is an indicator of the ability to accept electrons from metal
surface. It is concluded from Tables 5 and 6 that the inhibition effi-
ciencies according to frontier orbital energies of studied molecules
obey the order: MAD_ 1> MAD_ 2> MAD_ 3> MAD_ 4> MAD_ 5>
MAD_ 6.
The HOMO-LUMO energy gap (࢞E) is one of the parameters
widely used to describe the chemical reactivity. This quantity ex-
hibits the reactivity of molecules towards the metal surface. In fact,
the roles on corrosion inhibition efficiencies of molecules of chem-
ical hardness, softness and ࢞E can be discussed in the same para-
graph because these quantum chemical parameters are closely as-
sociated with each other. Chemical hardness is a measure of the
resistance towards electron cloud polarization or deformation of
chemical species. Pearson [85] who introduced the chemical hard-
ness concept in 1960 s stated that "hard molecules have a large
HOMO-LUMO energy gap and soft molecules have a small HOMO-
LUMO gap". In other words, a small energy gap implies high po-
larizability and a large energy gap implies low polarizability. Ac-
cording to Maximum Hardness Principle [86,87] based on chem-
ical hardness concept, “a chemical system tends to arrange itself
so as to achieve maximum hardness and chemical hardness can
be considered as a measure of stability.” With the framework of
these information given, it can be said that soft molecules (small
energy gap) acts good corrosion inhibitor and hard molecules
(stable=large energy gap) are not good corrosion inhibitors. On the
basis of the calculated chemical hardness, softness and energy gap
given in the related tables, the corrosion inhibition efficiency rank-
ing of studied mono-azo dye derivatives can be written as: MAD_
1> MAD_ 2> MAD_ 3> MAD_ 5> MAD_ 4> MAD_ 6.
Electronegativity can be described as the power of a chem-
ical species to attract to electrons to itself and this quantity is
widely used to estimate the inhibitive properties of molecules. To
determine the fraction of electron transferred from the inhibitor
molecules to metal surface, we used the Pearson method given by
Eq. (11) [88]. It is seen from this equation that the fraction of elec-
tron transferred increases as the differences in electronegativity’s
between metal and inhibitor molecules. According to Sanderson’s
[89,90] electronegativity equalization principle, the electron trans-
fer between metal and inhibitor continues until their electronega-
tivity values become equal with each other.
N =
χFe − χinh
2(ηFe + ηinh)
(27)
where χFe and χinh are electronegativity of Fe metal and elec-
tronegativity of inhibitor, respectively. ηFe and ηinh represent
chemical hardness value of Fe metal and chemical hardness value
of inhibitor, respectively. For Fe, the theoretical values of χFe and
ηFe are 7 eV and 0 eV. Elnga [91] and co-workers noted that the in-
hibition efficiency increases with increasing of ࢞N value. Consid-
ering the electronegativity’s calculated for studied mono-azo dye
derivatives in Tables 5 and 6 and the fraction of electron trans-
ferred values given in Table 9, the inhibition efficiency ranking of
aforementioned molecules can be given as: MAD_ 1> MAD_ 3>
MAD_ 2≈ MAD_ 4> MAD_ 5> MAD_ 6.
9. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 469
Table 5
Calculated quantum chemical parameters for neutral forms of studied mono-azo dye derivatives in gas phase (eV).
EHOMO ELUMO I A E η σ χ PA ω ε D (Debye) Energy
HF/SDD Level
MAD_1 −7.42169 1.36684 7.42169 −1.36684 8.78853 4.39427 0.22757 3.02743 −2.51920 1.04287 0.95889 0.8505 −27852.27331
MAD_2 −7.46006 1.43432 7.46006 −1.43432 8.89438 4.44719 0.22486 3.01287 −0.83722 1.02057 0.97984 0.5733 −27852.35944
MAD_3 −7.42713 1.68304 7.42713 −1.68304 9.11017 4.55509 0.21953 2.87205 −1.41495 0.90543 1.10444 1.6245 −26798.05968
MAD_4 −7.46850 1.63732 7.46850 −1.63732 9.10582 4.55291 0.21964 2.91559 −0.85926 0.93354 1.07119 0.2040 −24762.11131
MAD_5 −7.51884 1.55405 7.51884 −1.55405 9.07289 4.53645 0.22044 2.98239 −0.83170 0.98036 1.02004 0.8333 −24762.04208
MAD_6 −7.51639 1.59079 7.51639 −1.59079 9.10718 4.55359 0.21961 2.96280 −15.6291 0.96387 1.03748 0.4267 −24762.10353
HF/6-311 G Level
MAD_1 −7.36373 1.46643 7.36373 −1.46643 8.83016 4.41508 0.22650 2.94865 −2.54875 0.98464 1.01560 0.9012 −27854.66763
MAD_2 −7.39421 1.51759 7.39421 −1.51759 8.91180 4.45590 0.22442 2.93831 −0.86114 0.96879 1.03222 0.6664 −27854.75115
MAD_3 −7.35883 1.76440 7.35883 −1.76440 9.12323 4.56162 0.21922 2.79722 −1.41919 0.85764 1.16599 1.5103 −26800.26935
MAD_4 −7.40591 1.71052 7.40591 −1.71052 9.11643 4.55822 0.21938 2.84769 −0.88637 0.88953 1.12419 0.2232 −24764.21550
MAD_5 −7.45816 1.63841 7.45816 −1.63841 9.09657 4.54828 0.21986 2.90987 −0.85630 0.93083 1.07431 0.8916 −24764.14116
MAD_6 −7.45543 1.67379 7.45543 −1.67379 9.12922 4.56461 0.21908 2.89082 −0.85973 0.91540 1.09242 0.5101 −24764.21517
HF/6-31++G Level
MAD_1 −7.40945 0.98805 7.40945 −0.98805 8.39750 4.19875 0.23817 3.21070 −2.46527 1.22758 0.81461 0.7923 −27849.92824
MAD_2 −7.45026 1.00356 7.45026 −1.00356 8.45383 4.22691 0.23658 3.22335 −0.79243 1.22903 0.81365 0.5289 −27850.01679
MAD_3 −7.41707 1.01935 7.41707 −1.01935 8.43641 4.21821 0.23707 3.19886 −1.40739 1.21292 0.82446 1.5569 −26795.38592
MAD_4 −7.46033 1.02533 7.46033 −1.02533 8.48567 4.24283 0.23569 3.21750 −0.81523 1.21998 0.81969 0.2180 −24759.87966
MAD_5 −7.50931 0.99268 7.50931 −0.99268 8.50199 4.25100 0.23524 3.25832 −0.78674 1.24872 0.80082 0.8109 −24759.81563
MAD_6 −7.50714 1.01445 7.50714 −1.01445 8.52158 4.26079 0.23470 3.24634 −0.80756 1.23671 0.80860 0.3920 −24759.86986
B3LYP/SDD Level
MAD_1 −5.36858 −2.41802 5.36858 2.41802 2.95055 1.47528 0.67784 3.89330 −2.94384 5.13727 0.19466 1.1595 -28029.75580
MAD_2 −5.40776 −2.34047 5.40776 2.34047 3.06729 1.53365 0.65204 3.87412 −0.82906 4.89317 0.20437 0.8779 −28029.81839
MAD_3 −5.28068 −2.15625 5.28068 2.15625 3.12444 1.56222 0.64012 3.71847 −1.06126 4.42543 0.22597 1.5062 −26965.93713
MAD_4 −5.36449 −2.21666 5.36449 2.21666 3.14784 1.57392 0.63536 3.79058 −0.85510 4.56455 0.21908 0.3520 −24919.81657
MAD_5 −5.40558 −2.28850 5.40558 2.28850 3.11709 1.55854 0.64162 3.84704 −0.80756 4.74793 0.21062 1.1343 −24919.75834
MAD_6 −5.40558 −2.25067 5.40558 2.25067 3.15491 1.57746 0.63393 3.82813 −0.80976 4.64500 0.21529 0.6700 −24919.80098
B3LYP/6-311 G Level
MAD_1 −5.41320 −2.41095 5.41320 2.41095 3.00226 1.50113 0.66617 3.91208 −2.88244 5.09761 0.19617 1.1831 −28032.96506
MAD_2 −5.46164 −2.34700 5.46164 2.34700 3.11464 1.55732 0.64213 3.90432 −0.80813 4.89422 0.20432 0.9859 −28033.02299
MAD_3 −5.33320 −2.15815 5.33320 2.15815 3.17505 1.58752 0.62991 3.74568 −1.07710 4.41886 0.22630 1.3346 −26968.82349
MAD_4 −5.42300 −2.22945 5.42300 2.22945 3.19355 1.59678 0.62626 3.82622 −0.85159 4.58423 0.21814 0.4767 −24922.63003
MAD_5 −5.45538 −2.29911 5.45538 2.29911 3.15627 1.57814 0.63366 3.87724 −0.78468 4.76290 0.20996 1.2052 −24922.57305
MAD_6 −5.68450 −2.26373 5.68450 2.26373 3.42077 1.71039 0.58466 3.97412 −0.77736 4.61698 0.21659 0.8021 −24922.61599
B3LYP/6-31++G Level
MAD_1 −5.47307 −2.49612 5.47307 2.49612 2.97695 1.48847 0.67183 3.98459 −2.79229 5.33331 0.18750 1.0181 −28027.69518
MAD_2 −5.52613 −2.43353 5.52613 2.43353 3.09260 1.54630 0.64671 3.97983 −0.70190 5.12161 0.19525 0.8408 −28027.76721
MAD_3 −5.40477 −2.25013 5.40477 2.25013 3.15464 1.57732 0.63399 3.82745 −0.97883 4.64375 0.21534 1.4107 −26963.50983
MAD_4 −5.48667 −2.31244 5.48667 2.31244 3.17423 1.58712 0.63007 3.89956 −0.71447 4.79062 0.20874 0.3539 −24917.86945
MAD_5 −5.52504 −2.39054 5.52504 2.39054 3.13450 1.56725 0.63806 3.95779 −0.68590 4.99732 0.20011 1.0898 −24917.80588
MAD_6 −5.53293 −2.35271 5.53293 2.35271 3.18022 1.59011 0.62889 3.94282 −0.68337 4.88830 0.20457 0.6385 −24917.85633
To determine the active sites of inhibitor molecules, properties
such as neutral atomic charge, Fukui indices [92] and distribution
of frontier orbital can be considered. In the predicting of atomic
charges, Mulliken [93] population analysis is used. The atoms with
the highest negative charge represent the high tendency on the
metal surface. Molecules containing heteroatoms such as N, O, S
exhibit high tendency for protonation in acidic medium. For this
reason, the analysis of protonated forms of studied mono-azo dye
derivatives. In the protonation process of molecules, we consid-
ered the electron density maps given in Fig. 7 for the molecules.
Fig. 7 represents repartition of HOMO, LUMO densities, electro-
static potential structures of non-protonated and optimized struc-
tures of mono-azo dye (MAD_ 1–6) derivatives obtained from DFT
at the B3LYP/6-31++G (d,p) basis set. In Tables 7 and 8, quan-
tum chemical parameters calculated for protonated forms of stud-
ied molecules are presented. As stated above, proton affinity is a
measure of electron donating abilities of molecules. From the light
of data given in corresponding tables, the corrosion inhibition effi-
ciency ranking in terms of proton affinities of the molecules obeys
the order: MAD_ 1> MAD_ 3> MAD_ 2> MAD_ 4> MAD_ 5>
MAD_ 6.
The electrophilicity index (ω) is an important parameter that
indicates the tendency of the inhibitor molecule to accept the elec-
trons. Nucleophilicity (ε) is physically the inverse of electrophilicity
(1/ω). For this reason, it should be stated that a molecule that have
large electrophilicity value is ineffective against corrosion while a
molecule that have large nucleophilicity value is a good corrosion
inhibitor. Thus, for studied molecules, we can write the inhibition
efficiency ranking as: MAD_ 3> MAD_ 4> MAD_ 6> MAD_ 5>
MAD_ 2> MAD_ 1.
Another important electronic parameter considered in corrosion
studies is dipole moment (D). The dipole moment arises from non-
uniform distribution of charges on the various atoms in a molecule
and is used in the estimation of strength of intermolecular interac-
tions. Some authors [80,94] reported that, the inhibition efficiency
increases with increasing value of the dipole moment. On the other
hand some authors [95,96] also reported that irregularities can be
observed in the correlation between dipole moment with inhibi-
tion efficiency According to dipole moment values given in Tables
5 and 6, the inhibition efficiencies of mentioned compounds fol-
low the order: MAD_ 3> MAD_ 1> MAD_ 5> MAD_ 2> MAD_ 6>
MAD_ 4.
4.5. Molecular dynamics simulation (MDS)
The use of the molecular dynamics simulations is a useful and
modern tool [97,98] to investigate the interaction between in-
hibitors and metal surface. Thus in this study, molecular dynamics
10. 470 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Fig. 7. Repartition of HOMO, LUMO densities, ESP structures of non-protonated and optimized structures of mono-azo dye derivatives obtained from DFT at the B3LYP/6-
31++G (d,p) basis set.
11. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 471
Table 6
Calculated quantum chemical parameters for neutral forms of studied mono-azo dye derivatives in aqueous phase (eV).
EHOMO ELUMO I A E η σ χ PA ω ε D (Debye) Energy
HF/SDD Level
MAD_1 -7.68592 1.10397 7.68592 −1.10397 8.78989 4.39495 0.22753 3.29097 −4.52573 1.23215 0.81159 0.6161 −27852.27331
MAD_2 −7.72837 1.17989 7.72837 −1.17989 8.90826 4.45413 0.22451 3.27424 −3.52915 1.20345 0.83095 0.1969 −27852.35944
MAD_3 −7.73435 1.39596 7.73435 −1.39596 9.13031 4.56515 0.21905 3.16920 −3.87447 1.10005 0.90905 2.2876 −26798.05968
MAD_4 −7.75857 1.35269 7.75857 −1.35269 9.11126 4.55563 0.21951 3.20294 −3.48341 1.12595 0.88814 0.5423 −24762.11131
MAD_5 −7.77245 1.28820 7.77245 −1.28820 9.06065 4.53032 0.22073 3.24213 −3.46674 1.16011 0.86198 0.7104 −24762.04208
MAD_6 −7.78714 1.29201 7.78714 −1.29201 9.07915 4.53958 0.22028 3.24757 −3.49946 1.16164 0.86085 0.4615 −24762.10353
HF/6-311 G Level
MAD_1 −7.64020 1.16466 7.64020 −1.16466 8.80486 4.40243 0.22715 3.23777 −4.53656 1.19061 0.83990 0.6803 −27854.66763
MAD_2 −7.66197 1.26616 7.66197 −1.26616 8.92813 4.46406 0.22401 3.19791 −3.50039 1.14544 0.87303 0.2618 −27854.75115
MAD_3 −7.66714 1.49011 7.66714 −1.49011 9.15725 4.57862 0.21841 3.08852 −3.87003 1.04168 0.95999 2.1642 −26800.26935
MAD_4 −7.69598 1.44167 7.69598 −1.44167 9.13766 4.56883 0.21887 3.12716 −3.46132 1.07020 0.93441 0.4033 −24764.21550
MAD_5 −7.71394 1.38017 7.71394 −1.38017 9.09412 4.54706 0.21992 3.16689 −3.45650 1.10282 0.90677 0.8127 −24764.14116
MAD_6 −7.72537 1.38670 7.72537 −1.38670 9.11208 4.55604 0.21949 3.16934 −3.41122 1.10235 0.90715 0.4800 −24764.21517
HF/6-31++G Level
MAD_1 −7.40945 0.98805 7.40945 −0.98805 8.39750 4.19875 0.23817 3.21070 −4.48350 1.22758 0.81461 0.7923 −27849.92824
MAD_2 −7.70388 1.12847 7.70388 −1.12847 8.83234 4.41617 0.22644 3.28771 −3.45250 1.22380 0.81713 0.1783 −27850.01679
MAD_3 −7.71367 1.12139 7.71367 −1.12139 8.83506 4.41753 0.22637 3.29614 −3.85725 1.22971 0.81320 2.2556 −26795.38592
MAD_4 −7.73898 1.12928 7.73898 −1.12928 8.86826 4.43413 0.22552 3.30485 −3.41008 1.23159 0.81196 0.6000 −24759.87966
MAD_5 −7.75531 1.12466 7.75531 −1.12466 8.87996 4.43998 0.22523 3.31533 −3.41076 1.23777 0.80790 0.7376 −24759.81563
MAD_6 −7.76728 1.13744 7.76728 −1.13744 8.90472 4.45236 0.22460 3.31492 −3.42336 1.23403 0.81035 0.4154 −24759.86986
B3LYP/SDD Level
MAD_1 −5.60777 −2.75626 5.60777 2.75626 2.85150 1.42575 0.70138 4.18201 −4.82579 6.13334 0.16304 1.4742 −28029.75580
MAD_2 −5.63307 −2.66075 5.63307 2.66075 2.97232 1.48616 0.67287 4.14691 −3.32627 5.78567 0.17284 0.7150 −28029.81839
MAD_3 −5.56668 −2.51626 5.56668 2.51626 3.05042 1.52521 0.65565 4.04147 −3.51626 5.35449 0.18676 2.1846 −26965.93713
MAD_4 −5.62110 −2.56116 5.62110 2.56116 3.05994 1.52997 0.65361 4.09113 −3.29816 5.46981 0.18282 0.4879 −24919.81657
MAD_5 −5.63144 −2.61313 5.63144 2.61313 3.01831 1.50916 0.66262 4.12229 −3.29397 5.63005 0.17762 1.3813 −24919.75834
MAD_6 −5.65397 −2.59952 5.65397 2.59952 3.05445 1.52722 0.65478 4.12675 −3.32205 5.57549 0.17936 0.8146 −24919.80098
B3LYP/6-311 G Level
MAD_1 −5.41320 −2.41095 5.41320 2.41095 3.00226 1.50113 0.66617 3.91208 −4.73191 5.09761 0.19617 1.1831 −28032.96506
MAD_2 −5.66627 −2.65095 5.66627 2.65095 3.01532 1.50766 0.66328 4.15861 −3.27212 5.73540 0.17436 0.8175 −28033.02299
MAD_3 −5.59879 −2.50592 5.59879 2.50592 3.09287 1.54644 0.64665 4.05235 −3.53988 5.30949 0.18834 2.0066 −26968.82349
MAD_4 −5.65784 −2.55843 5.65784 2.55843 3.09940 1.54970 0.64529 4.10814 −3.24570 5.44517 0.18365 0.4617 −24922.63003
MAD_5 −5.67035 −2.61068 5.67035 2.61068 3.05967 1.52984 0.65366 4.14052 −3.24570 5.60318 0.17847 1.5091 −24922.57305
MAD_6 −5.69103 −2.59354 5.69103 2.59354 3.09750 1.54875 0.64568 4.14229 −3.25979 5.53948 0.18052 0.9308 −24922.61599
B3LYP/6-31++G Level
MAD_1 −5.67988 −2.76742 5.67988 2.76742 2.91246 1.45623 0.68671 4.22365 −4.67288 6.12514 0.16326 1.1488 −28027.69518
MAD_2 −5.70464 −2.68987 5.70464 2.68987 3.01477 1.50739 0.66340 4.19725 −3.17497 5.84354 0.17113 0.4636 −28027.76721
MAD_3 −5.65267 −2.56850 5.65267 2.56850 3.08416 1.54208 0.64847 4.11058 −3.50733 5.47860 0.18253 2.1182 −26963.50983
MAD_4 −5.70491 −2.61313 5.70491 2.61313 3.09178 1.54589 0.64688 4.15902 −3.15513 5.59466 0.17874 0.5661 −24917.86945
MAD_5 −5.71171 −2.65993 5.71171 2.65993 3.05178 1.52589 0.65536 4.18582 −3.15418 5.74128 0.17418 1.2929 −24917.80588
MAD_6 −5.73784 −2.65014 5.73784 2.65014 3.08770 1.54385 0.64773 4.19399 −3.13883 5.69665 0.17554 0.6888 −24917.85633
simulation studies were performed to calculate the binding ener-
gies of these mono-azo dye derivatives on iron surface and to in-
vestigate whether there is a remarkable correlation between exper-
imental inhibition efficiencies and binding energies for molecules
considered in this study. The binding energies between Fe (110)
surface and the six mono-azo dye (MAD_ 1–6) derivatives were
obtained using Eq. (15). The close contacts between these com-
pounds and Fe (110) metal surface as well as the best equilib-
rium adsorption configuration for the compounds considered are
depicted in Fig. 8. The calculated binding energies are presented in
Table 10. The obtained results given in Table 9 show that the bind-
ing energies calculated for the interactions between inhibitors and
metal surface are very high. It is important to note that high bind-
ing energy leads to a more stable inhibitor/surface interaction [99].
The calculated binding energies are increased in the order arrange-
ment: MAD_ 2> MAD_ 1> MAD_ 3> MAD_ 5> MAD_ 6> MAD_ 4.
4.6. Kinetic, adsorption isotherm and thermodynamics calculations
Correlation between kinetic-thermodynamic model and Frumkin
isotherm of the corrosion inhibition describe the behavior of the
inhibitor molecules and provide information about the interaction
of the inhibitor molecules with the electrode surface [100–102].
The adsorption of inhibitors at the metal-solution interface is rep-
resented as a substitution adsorption process between the inhibitor
molecules (Inh(sol)) and the water molecules on metallic surface
(H2Oads):
Inh(sol) +x H2Oads →Inh(ads) +H2Osol (28)
Where Inh(sol) and Inh(ads) are the inhibitor species dissolved
in the aqueous solution and adsorbed onto the metallic surface, re-
spectively. H2O(ads) is the water molecules adsorbed on the metal
surface and x is the ratio which represents the number of water
molecules replaced by a single inhibitor molecule. Fitting of the
gravimetric measurement data describes the mode of interaction
occurred between the inhibitor molecules and the metal surface.
Adsorption is a separation process involving two phases between
which certain components can be described by two main types
of interaction [100]: (1) physisorption which involves electrostatic
forces between ionic charges at the metal/solution interface. The
heat of adsorption is low and therefore this type of adsorption
is stable only at relatively low temperatures and; (2) chemisorp-
tions which involves charge sharing or charge transfer from the
inhibitor molecules to the metal surface to form a coordinate type
bond. In fact electron transfer is typically for transition metals
having vacant low-energy electron orbital. Chemisorptions is typ-
ified by much stronger adsorption energy than physical adsorp-
12. 472 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Table 7
Calculated quantum chemical parameters for protonated forms of studied mono-azo dye derivatives in gas phase (eV).
EHOMO ELUMO I A E η σ χ ω ε Energy
HF/SDD Level
MAD_1 −10.65770 −3.80391 10.65770 3.80391 6.85379 3.42689 0.29181 7.23080 7.62856 0.13109 −27862.15251
MAD_2 −9.92734 −1.95679 9.92734 1.95679 7.97055 3.98527 0.25092 5.94206 4.42982 0.22574 −27860.55666
MAD_3 −9.67291 −1.72413 9.67291 1.72413 7.94878 3.97439 0.25161 5.69852 4.08529 0.24478 −26806.83463
MAD_4 −10.81062 −1.93148 10.81062 1.93148 8.87915 4.43957 0.22525 6.37105 4.57142 0.21875 −24770.33057
MAD_5 −10.95457 −1.96713 10.95457 1.96713 8.98745 4.49372 0.22253 6.46085 4.64454 0.21531 −24770.23378
MAD_6 −10.93063 −1.95406 10.93063 1.95406 8.97656 4.48828 0.22280 6.44235 4.62358 0.21628 −24785.09265
HF/6-311 G Level
MAD_1 −10.65661 −3.66921 10.65661 3.66921 6.98739 3.49370 0.28623 7.16291 7.34283 0.13619 −27864.57638
MAD_2 −9.91999 −1.91025 9.91999 1.91025 8.00973 4.00487 0.24970 5.91512 4.36827 0.22892 −27862.97229
MAD_3 −9.63155 −1.61474 9.63155 1.61474 8.01681 4.00840 0.24948 5.62314 3.94418 0.25354 −26809.04854
MAD_4 −10.77525 −1.88440 10.77525 1.88440 8.89085 4.44542 0.22495 6.32983 4.50651 0.22190 −24772.46187
MAD_5 −10.92763 −1.91161 10.92763 1.91161 9.01602 4.50801 0.22183 6.41962 4.57093 0.21877 −24772.35746
MAD_6 −10.88627 −1.90508 10.88627 1.90508 8.98119 4.49059 0.22269 6.39568 4.55449 0.21956 −24772.43490
HF/6-31++G Level
MAD_1 −10.63565 −3.76364 10.63565 3.76364 6.87202 3.43601 0.29104 7.19965 7.54289 0.13258 −27859.75351
MAD_2 −9.92489 −2.96416 9.92489 2.96416 6.96073 3.48036 0.28733 6.44452 5.96660 0.16760 −27858.16922
MAD_3 −9.66937 −2.42864 9.66937 2.42864 7.24073 3.62037 0.27622 6.04900 5.05341 0.19789 −26804.15331
MAD_4 −10.81661 −2.96117 10.81661 2.96117 7.85544 3.92772 0.25460 6.88889 6.04126 0.16553 −24768.05489
MAD_5 −10.96002 −2.98756 10.96002 2.98756 7.97245 3.98623 0.25086 6.97379 6.10022 0.16393 −24767.96237
MAD_6 −10.93634 −2.98021 10.93634 2.98021 7.95613 3.97806 0.25138 6.95828 6.08558 0.16432 −24768.03742
B3LYP/SDD Level
MAD_1 −8.81411 −6.86739 8.81411 6.86739 1.94672 0.97336 1.02737 7.84075 31.58001 0.03167 −28040.05964
MAD_2 −8.20811 −5.44286 8.20811 5.44286 2.76524 1.38262 0.72326 6.82549 16.84744 0.05936 −28038.00745
MAD_3 −7.79503 −5.17401 7.79503 5.17401 2.62102 1.31051 0.76306 6.48452 16.04300 0.06233 −26974.35839
MAD_4 −8.65764 −5.45701 8.65764 5.45701 3.20063 1.60031 0.62488 7.05733 15.56128 0.06426 −24928.03167
MAD_5 −8.68404 −5.50110 8.68404 5.50110 3.18294 1.59147 0.62835 7.09257 15.80441 0.06327 −24927.92590
MAD_6 −8.70554 −5.48776 8.70554 5.48776 3.21777 1.60889 0.62155 7.09665 15.65134 0.06389 −24927.97074
B3LYP/6-311 G Level
MAD_1 −8.88622 −6.87501 8.88622 6.87501 2.01121 1.00560 0.99443 7.88062 30.87898 0.03238 −28043.20750
MAD_2 −8.29355 −5.48831 8.29355 5.48831 2.80524 1.40262 0.71295 6.89093 16.92720 0.05908 −28041.19112
MAD_3 −7.84565 −5.17673 7.84565 5.17673 2.66891 1.33446 0.74937 6.51119 15.88497 0.06295 −26977.26059
MAD_4 −8.73800 −5.50463 8.73800 5.50463 3.23336 1.61668 0.61855 7.12132 15.68433 0.06376 −24930.84162
MAD_5 −8.76703 −5.54545 8.76703 5.54545 3.22158 1.61079 0.62081 7.15624 15.89648 0.06291 −24930.71773
MAD_6 −8.78962 −5.53484 8.78962 5.53484 3.25478 1.62739 0.61448 7.16223 15.76068 0.06345 −24930.75335
B3LYP/6-31++G Level
MAD_1 −8.90908 −6.89297 8.90908 6.89297 2.01611 1.00805 0.99201 7.90102 30.96372 0.03230 −28037.84747
MAD_2 −8.29736 −5.67797 8.29736 5.67797 2.61939 1.30969 0.76354 6.98767 18.64080 0.05365 −28035.82911
MAD_3 −7.89708 −5.23225 7.89708 5.23225 2.66483 1.33242 0.75052 6.56466 16.17167 0.06184 −26971.84866
MAD_4 −8.77574 −5.69974 8.77574 5.69974 3.07600 1.53800 0.65020 7.23774 17.03021 0.05872 −24925.94392
MAD_5 −8.80295 −5.73512 8.80295 5.73512 3.06784 1.53392 0.65193 7.26903 17.22350 0.05806 −24925.85178
MAD_6 −8.83261 −5.72804 8.83261 5.72804 3.10457 1.55229 0.64421 7.28033 17.07262 0.05857 −24925.89970
tion. Such a bond is therefore more stable at higher temperatures.
Basic information on the adsorption of inhibitor on metal sur-
faces can be provided by adsorption isotherm. Attempts were made
to fit experimental data to various isotherms including Frumkin,
Langmuir, Temkin, Freundlich, Bockris–Swinkels and Flory–Huggins
isotherms. All these isotherms are of the general form [103]:
f (θ, x)exp (−2αθ) = KadsC (29)
where f (θ, x) is the configurational factor which depends on the
physical mode and the assumptions underlying the derivation of
the isotherm, θ the degree of surface coverage, C the inhibitor con-
centration, x the size factor ratio, α the molecular interaction pa-
rameter, and Kads the equilibrium constant of the inhibitor adsorp-
tion process. In this study, correlation coefficient (R2) was used to
determine the best fit isotherm which was obtained from Frumkin
adsorption isotherm. According to this isotherm, θ is related to
the inhibitor concentration by the following equation [104]:
exp (−2αθ) = KadsC (30)
where the molecular interaction parameter α can have both pos-
itive and negative values. Positive values of α indicates attraction
forces between the adsorbed molecules while negative values indi-
cate repulsive forces between the adsorbed molecules [104]. Upon
rearrangement of Eq. (30), the following equation is obtained:
θ = [1/(−2α)] ln (KadsC) (31)
If the parameter f is defined as:
f = −2α (32)
where f is the heterogeneous factor of the metal surface describing
the molecular interactions in the adsorption layer and the hetero-
geneity of the metal surface. Eq. (32) clearly shows that the sign
between f and α is reverse, that is, if α < 0, then f > 0; if α >0,
then f < 0. Accordingly, if f > 0, mutual repulsion of molecules oc-
curs and if f < 0 attraction takes place. If Eq. (32) is substituted
into Eq. (31), then the Frumkin isotherm equation [105] has the
following form:
θ = (1/ f )ln (KadsC) (33)
(θ) could be calculated by the following relationship [106]:
θ = IE(%)/100 (34)
Eq. (33) can be transformed into:
θ = (1/ f )ln Kads + (1/ f )lnC (35)
Eq. (35) is a different form of the Frumkin isotherm. The plot of
θ versus log C (Fig. 3) gives an S-shaped graph, suggest that the
13. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 473
Table 8
Calculated quantum chemical parameters for protonated forms of studied mono-azo dye derivatives in aqueous phase (eV).
EHOMO ELUMO I A E η σ χ ω ε Energy
HF/SDD Level
MAD_1 −8.03395 0.81907 8.03395 −0.81907 8.85302 4.42651 0.22591 3.60744 1.46997 0.68029 −27864.15904
MAD_2 −8.27614 0.84546 8.27614 −0.84546 9.12160 4.56080 0.21926 3.71534 1.51330 0.66081 −27863.24859
MAD_3 −7.89490 0.90533 7.89490 −0.90533 8.80023 4.40012 0.22727 3.49479 1.38786 0.72053 −26809.29415
MAD_4 −8.50662 0.92655 8.50662 −0.92655 9.43317 4.71659 0.21202 3.79003 1.52275 0.65671 −24772.95472
MAD_5 −8.53193 0.90206 8.53193 −0.90206 9.43399 4.71699 0.21200 3.81493 1.54269 0.64822 −24772.86882
MAD_6 −8.55206 0.91921 8.55206 −0.91921 9.47127 4.73563 0.21116 3.81643 1.53782 0.65027 −24772.96299
HF/6-311 G Level
MAD_1 −8.00075 −0.70832 8.00075 0.70832 7.29244 3.64622 0.27426 4.35454 2.60023 0.38458 −27866.56419
MAD_2 −8.23477 0.88465 8.23477 −0.88465 9.11942 4.55971 0.21931 3.67506 1.48102 0.67521 −27865.61154
MAD_3 −7.84102 1.00166 7.84102 −1.00166 8.84268 4.42134 0.22618 3.41968 1.32247 0.75616 −26811.49938
MAD_4 −8.45192 0.98098 8.45192 −0.98098 9.43290 4.71645 0.21202 3.73547 1.47926 0.67601 −24775.03682
MAD_5 −8.48158 0.95921 8.48158 −0.95921 9.44079 4.72040 0.21185 3.76119 1.49845 0.66736 −24774.95766
MAD_6 −8.49682 0.95812 8.49682 −0.95812 9.45494 4.72747 0.21153 3.76935 1.50271 0.66547 −24774.98639
HF/6-31++G Level
MAD_1 −8.00375 −0.78288 8.00375 0.78288 7.22087 3.61044 0.27697 4.39331 2.67297 0.37412 −27861.77174
MAD_2 −8.24484 0.79431 8.24484 −0.79431 9.03915 4.51957 0.22126 3.72527 1.53528 0.65135 −27860.82929
MAD_3 −7.87585 0.86206 7.87585 −0.86206 8.73792 4.36896 0.22889 3.50689 1.40747 0.71050 −26806.60317
MAD_4 −8.47641 0.86696 8.47641 −0.86696 9.34337 4.67169 0.21406 3.80473 1.54933 0.64544 −24770.64974
MAD_5 −8.49927 0.84193 8.49927 −0.84193 9.34120 4.67060 0.21411 3.82867 1.56926 0.63724 −24770.58639
MAD_6 −8.52077 0.85798 8.52077 −0.85798 9.37875 4.68938 0.21325 3.83139 1.56520 0.63890 −24770.65322
B3LYP/SDD Level
MAD_1 −6.17839 −4.05943 6.17839 4.05943 2.11897 1.05948 0.94386 5.11891 12.36604 0.08087 −28041.94159
MAD_2 −6.23935 −3.00770 6.23935 3.00770 3.23165 1.61582 0.61888 4.62352 6.61488 0.15117 −28040.50466
MAD_3 −5.82383 −2.93940 5.82383 2.93940 2.88443 1.44221 0.69338 4.38161 6.65592 0.15024 −26976.81339
MAD_4 −6.26411 −2.94892 6.26411 2.94892 3.31519 1.65759 0.60328 4.60652 6.40084 0.15623 −24930.47473
MAD_5 −6.26329 −2.97967 6.26329 2.97967 3.28362 1.64181 0.60908 4.62148 6.50443 0.15374 −24930.41231
MAD_6 −6.28969 −2.99055 6.28969 2.99055 3.29913 1.64957 0.60622 4.64012 6.52618 0.15323 −24930.48303
B3LYP/6-311 G Level
MAD_1 −6.22302 −4.05317 6.22302 4.05317 2.16985 1.08493 0.92172 5.13809 12.16673 0.08219 −28045.05697
MAD_2 −6.28452 −3.01885 6.28452 3.01885 3.26566 1.63283 0.61243 4.65169 6.62597 0.15092 −28043.65511
MAD_3 −5.87607 −2.99518 5.87607 2.99518 2.88089 1.44045 0.69423 4.43563 6.82941 0.14643 −26979.72337
MAD_4 −6.31173 −2.96498 6.31173 2.96498 3.34675 1.67338 0.59759 4.63835 6.42841 0.15556 −24933.23573
MAD_5 −6.31364 −2.99436 6.31364 2.99436 3.31927 1.65964 0.60254 4.65400 6.52544 0.15325 −24933.17875
MAD_6 −6.33813 −3.00280 6.33813 3.00280 3.33533 1.66766 0.59964 4.67046 6.54006 0.15290 −24933.23578
B3LYP/6-31±+G Level
MAD_1 −6.22003 −4.07439 6.22003 4.07439 2.14563 1.07282 0.93213 5.14721 12.34776 0.08099 −28039.72806
MAD_2 −6.29921 −3.03872 6.29921 3.03872 3.26049 1.63025 0.61340 4.66897 6.68587 0.14957 −28038.30218
MAD_3 −5.94465 −3.08389 5.94465 3.08389 2.86076 1.43038 0.69912 4.51427 7.12351 0.14038 −26974.37716
MAD_4 −6.33377 −2.98511 6.33377 2.98511 3.34866 1.67433 0.59725 4.65944 6.48331 0.15424 −24928.38458
MAD_5 −6.32996 −3.01804 6.32996 3.01804 3.31192 1.65596 0.60388 4.67400 6.59625 0.15160 −24928.32006
MAD_6 −6.36479 −3.01749 6.36479 3.01749 3.34730 1.67365 0.59750 4.69114 6.57450 0.15210 −24928.35516
adsorption of the investigated (MAD_1-6) derivatives on the iron
obeyed the Frumkin adsorption isotherm. Straight lines of CInh/θ
versus CInh plots as shown in Fig. 9, indicate that the adsorption of
the inhibitor molecules on the metal surface obeyed Frumkin ad-
sorption model, this isotherm can be represented as:
CInh/θ = 1/Kads + CInh (36)
The strong correlation coefficients of the fitted curves are
around unity (r > 0.985). This reveals that the inhibition tendency
of the inhibitors is due to the adsorption of the molecules on the
Fe surface [107] (Table 11). The slopes of the CInh/θ versus CInh
plots are close to ≡ 1.3 which indicates the ideal simulating and
expected from Frumkin adsorption isotherm [107]. Kads values were
calculated from the intercepts of the straight lines on the CInh/θ
axis [108]. The relatively high values of the adsorption equilibrium
constant (Kads) as given in Table 11, reflect the high adsorption
ability of these molecules on iron surface. The value of Kads is re-
lated to the standard free energy of adsorption ( G°ads) by the fol-
lowing Eq. (37).
Kads = (1/55.5)exp(−DG◦
ads/RT ) (37)
where R is the universal molar gas constant (kJ mol−1K−1) and T is
the absolute temperature (K). The value of 55.5 is the molar con-
centration of water in solution expressed in mol L−1.The calculated
values of G°ads and Kads of the tested (MAD_1-6) inhibitors were
listed in Table 11.
Table 9
Calculated fractions of electron transferred from the inhibitor molecules to metallic surface.
Inhibitor EHOMO (eV) ELUMO (eV) I (eV) A (eV) χ (eV) η (eV) ࢞N
MAD_1 −5.47307 −2.49612 5.47307 2.49612 3.98459 1.48847 1.012
MAD_2 −5.52613 −2.43353 5.52613 2.43353 3.97983 1.54630 0.976
MAD_3 -5.40477 −2.25013 5.40477 2.25013 3.82745 1.57732 1.005
MAD_4 −5.48667 −2.31244 5.48667 2.31244 3.89956 1.58712 0.976
MAD_5 −5.52504 −2.39054 5.52504 2.39054 3.95779 1.56725 0.970
MAD_6 −5.53293 −2.35271 5.53293 2.35271 3.94282 1.59011 0.961
14. 474 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Fig. 8. Equilibrium adsorption configurations of the studied synthesized mono-azo dye (MAD_ 1-6) derivatives on Fe(110) surface.
Fig. 9. Relation between CInh and CInh/θ of the synthesized (MAD_1) and (MAD_6) for iron in 2.0 M HNO3 at 303 K.
Table 10
Interaction and binding energies of studied
inhibitors adsorbed on Fe(110) surface.
Systems Ebinding (kcal mol−1
)
Fe(110)+MAD_1 174.6
Fe(110)+MAD_2 177.7
Fe(110)+MAD_3 164.1
Fe(110)+MAD_4 156.4
Fe(110)+MAD_5 158.7
Fe(110)+MAD_6 157.1
The kinetic parameters calculated from Kinetic-thermodynamic
model proposed by El-Awady et al. [109,110] is given in Eqs.
(38) and (39) as following:
q/(1 − q) = K [I]
y
(38)
Or log (θ/1 − θ) = log K + y log [I] (39)
where y is the number of inhibitors molecules [I] occupying one
active site, and K’ is a constant, if relationship (39) is plotted and
applicable in Fig. 10. As seen, satisfactory linear relation is ob-
served for the studied (MAD_1-6) compounds. Hence, the sug-
gested model fits the obtained experimental data. The slope of
such lines is the number of inhibitor molecules occupying a sin-
gle active site, (y) and the intercept is the binding constant (log
K’). As mentioned, 1/y gives the number of active sites occupied
by a single organic molecule and K’y is the equilibrium constant
for the adsorption process. The binding constant (Kb) correspond-
ing to that obtained from the known adsorption isotherms curve
fitting is given by the following equation:
Kb = K (1/y)
(40)
Table 11 comprises the values of 1/y and Kb for the studied
mono-azodye inhibitors. This table show that the number of active
sites occupied by one molecule is (1/y ≡ 2 - 10).
Values of 1/y greater than unity implies the formation of multi-
layer of the inhibitor molecules on the metal surface, whereas, val-
ues of 1/y less than unity indicates that a given inhibitor molecule
will occupy more than one active site [64]. According to the pro-
posed kinetic-thermodynamic model, the adsorption takes place
via formation of multilayer of the inhibitor molecules on the iron
15. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 475
Table 11
Fitting parameters of the kinetic-thermodynamic model and the Frumkin adsorption isotherms of the synthesized
(MAD_1-6) inhibitors in 2.0 M HNO3 and 2.0 M NaOH solutions at 303 ± 1 K.
Inhibitor type Medium Kinetic model Frumkin isotherm
1/y Kb − G°ads kJ mol−1
−f Kads − G°ads kJ mol−1
MAD_ 1 2.0 M HNO3 8.57 2.16×108
38.21 23.42 36307 16.33
2.0 M NaOH 2.86 4707.10 11.18 40.46 8394 12.64
MAD_ 2 2.0 M HNO3 8.98 2.01×108
38.03 23.70 34593 16.20
2.0 M NaOH 2.71 11636.71 13.46 36.70 9772 13.03
MAD_ 3 2.0 M HNO3 9.50 1.92×108
37.91 24.08 32359 16.04
2.0 M NaOH 1.99 4042.06 10.80 41.44 7568 12.38
MAD_ 4 2.0 M HNO3 8.40 4.48×107
34.25 24.52 29991 15.81
2.0 M NaOH 3.45 1614.81 8.49 44.03 6745 12.09
MAD_ 5 2.0 M HNO3 8.86 4.43×107
34.22 24.85 28444 15.71
2.0 M NaOH 5.39 1164.87 7.67 45.86 6151 11.86
MAD_ 6 2.0 M HNO3 9.57 7.52×107
35.55 25.03 27669 15.64
2.0 M NaOH 4.44 980.97 7.23 50.73 5058 11.37
Fig. 10. Application of kinetic-thermodynamic model on (MAD_1-6) inhibitors of iron in (a) 2.0 M HNO3 acid and (b) 2.0 M Na OH at 303 K.
electrode surface. The slope values do not equal unity (gradi-
ent slopes <1), hence the adsorption of these synthesized azo
dye compounds on iron surface does not obey a Langmuir ad-
sorption isotherm [111,112]. Frumkin adsorption isotherm (Eq. (19))
represents best fit for experimental data obtained from applying
these (MAD_1-6) compounds as chemical inhibitors on iron in
2.0 M HNO3 and 2.0 M NaOH solutions.The values of Kads (equi-
librium constant of the inhibitor adsorption process) and (f) are
tabulated in Table 11. The lateral interaction parameter (f) has
negative values, this parameter is a measure of the degree of steep-
ness of the adsorption isotherm. The adsorption equilibrium con-
stant (Kads) calculated from Frumkin equation acquires lower val-
ues than those binding constant (Kb) obtained and calculated from
the kinetic-thermodynamic model. The lack of compatibility of the
calculated (Kb) and experimental (Kads) values may be attributed
to the fact that Frumkin adsorption isotherm is only applicable to
cases where one active site per inhibitor molecule is occupied. The
lateral interaction parameter was introduced to treat deviations
from Langmuir ideal behavior, whereas the kinetic-thermodynamic
model uses the size parameter. The values of the lateral inter-
action parameter (−f) were found to be negative and increase
from ∼= 23 to 5, this denotes that, an increase in the adsorption
16. 476 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
energy takes place with the increase in the surface coverage (θ).
Adsorption process is a displacement reaction involving removal
of adsorbed water molecules from the electrode metal surface and
their substitution by inhibitor molecules. Thus, during adsorption,
the adsorption equilibrium forms an important part in the over-
all free energy changes in the process of adsorption. It has been
shown [113] that, the free energy change (࢞Go
ads) increases with
increase of the solvating energy of adsorbing species, which in
turn increases with the size of hydrocarbon portion in the organic
molecule and the number of active sites. Hence, the increase of
the molecular size leads to decreased solubility, and increased ad-
sorb ability. The negative values of G°ads indicating the spon-
taneously adsorption [114] and strong interactions between in-
hibitor molecules and the metal surface [115]. Generally, values
of G°ads around −20 kJ mol−1 or lower values (obtained from
Frumkin isotherm) are consistent with the electrostatic interaction
between charged organic molecules and the charged metal surface
(physisorption); while those around −40 kJ mol−1 or higher (ob-
tained from Kinetic model) involve charge sharing or transfer from
the organic molecules to the metal surface to form a co-ordinate
type of bond (chemisorptions) [116–118]. The values of G°ads for
(MAD_1-6) compounds on iron in 2.0 M HNO3 and 2.0 M NaOH so-
lutions are tabulated in Table 11, indicate a physical adsorption,
i.e. physisorption mechanism. In addition to electrostatic interac-
tion, there may be some other interactions [119,120]. The high Kads
and G°ads values may be attributed to higher adsorption of the
inhibitor molecules at the metal-solution interface [121]. In ph-
ysisorption process, it is assumed that acid anions such as NO3
−
ions are specifically adsorbed on the metal surface, donating an
excess negative charge to the metal surface. In this way, potential
of zero charge becomes less negative which promotes the adsorp-
tion of inhibitors in cationic form [100]. The large negative values
of ( Go
ads), obtained for the compounds, indicate that the reaction
is proceeding spontaneously and accompanied with a high efficient
adsorption. Although, the obtained values of the binding constant
(Kb) from the kinetic model and the modified equilibrium constant
(Kads) from Frumkin equation are incompatible, generally have large
values (Table 11), mean better inhibition efficiency of the investi-
gated (MAD_1-6) compounds i.e. stronger electrical interaction be-
tween the double layer existing at the phase boundary and the ad-
sorbing molecules. In general, the equilibrium constant of adsorp-
tion (Kads) was found to become higher with increasing the inhibi-
tion efficiency of the inhibitor studied as obtained in Table 11.
4.7. Mechanism of corrosion inhibition
The mechanism of the inhibition processes of the corrosion in-
hibitors under consideration is mainly the adsorption one. The pro-
cess of adsorption is governed by different parameters almost de-
pend on the chemical structure of these inhibitors. The presence
of nitrogen and oxygen in the organic structures makes the forma-
tion of pπ-dπ bond resulting from overlap of 3d electrons from Fe
atom to the 2p half or non-completely filled orbital of the nitro-
gen and oxygen atoms possible, which enhances the adsorption of
the compounds on the metal surface. These molecules are able to
adsorb on the metal surface through N and O atoms, azo groups
and aromatic rings which are electron donating groups [63]. Ad-
sorption of the studied (MAD_1–6) molecules on iron surface in-
terferes with the adsorption of the anions NO3
− and OH− present
in acid and alkaline solutions, respectively. The inhibitor molecule,
also can undergo the formation of a chelate complex (Fe-inhibitor
complex) with Fe3+ ions resulting in the corrosive media, and will
be readily adsorbed or deposited on the metal surface. The inhi-
bition efficiencies increase with increasing the inhibitor concen-
tration, molecular weight and immersion time, whiles it decrease
with increasing temperature. It was found that, the mode of ad-
sorption depends on the affinity of the iron metal towards the π
– electron clouds of the ring system [66]. Fe has a greater affinity
towards aromatic moieties; hence it adsorbs benzene rings in a flat
orientation as discussed latter.
The adsorption process of (MAD_1–6) molecules on the metal
surface interfere with the adsorption of the anions [109] present
in the corrosive 2.0 M HNO3 acid. The anodic dissolution of iron
follows the steps [122]:
Fe+NO3
−
(FeNO3
−
) ads (41)
(FeNO3
−
) ads (FeNO3
−
) ads +e−
(42)
(FeNO3
−
) ads (FeNO3
+
)+e−
(43)
(FeNO3
+
) Fe2+
+NO3
−
(44)
Mono-azo dyes have basic character and expected to be proto-
nated in equilibrium with the corresponding neutral form in strong
acid solutions.
MAD+H+
MADH+
(45)
Because iron surface carried positive charge, NO3
− ions should
be first adsorbed onto the positively charged metal surface accord-
ing to reaction (41). Then the inhibitor molecules adsorb through
electrostatic interactions between the negatively charged metal
surfaces and positively charged mono-azo dye molecule (MADH+)
and form a protective (FeNO3
− MADH+) ads layer. In this way, the
oxidation reaction of (FeNO3
−) ads as shown by reaction steps from
(42) to (43) can be prevented [7,123]. During the corrosion of iron
in strong acid solution, the cathodic reaction is reduction of H+
ions to molecular hydrogen according to reaction mechanism given
below [7,122]:
Fe+H+ (FeH+
) ads (46)
(FeH+
) ads +e−
→(FeH) ads (47)
(FeH) ads +H+
+e−
→Fe+H2 (48)
Because the rate of hydrogen gas evolution is directly pro-
portional to the rate of corrosion, the measuring of hydrogen
gas evolved at cathodic sites as a function of reaction time can
give valuable information about starting and continuity of the
corrosion. The relative speed and effectiveness of the gasomet-
rical techniques as well as their suitability for in situ monitor-
ing, any perturbation by an inhibitor with respect to gas evolu-
tion in metal/solution system has been established in the liter-
ature [11,124]. The protonated mono-azo dye molecules are also
adsorbed at cathodic sites of metal in competition with hydro-
gen ions according to Eq. (47). The adsorption of protonated
(MADH+) molecules reduces the rate of hydrogen evolution re-
action [68,122,125]. In acidic solutions the inhibitor can exist as
cationic species (Eq. (49)) which may be adsorbed on the cathodic
sites of the iron and reduce the evolution of hydrogen:
MAD+2 H+
[MAD H] 2 +
(49)
The protonated MAD, however, could be attached to the iron
surface by means of electrostatic interaction between NO3
− and
protonated MAD since the iron surface has positive charges in
the acid medium [126]. This could further be explained based
on the assumption that in the presence of NO3
−, the negatively
charged NO3
− would attach to positively charged surface. When
MAD adsorbs on the iron surface, electrostatic interaction takes
17. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 477
OH
O
N
N
H
R
OH
O
N
N
R
H
OH
O
N
N
H
R
OH
O
N
N
R
H
Fe
+2
Fe
+3
OH
O
N
N R
OH
O
N
N
R
Fe
O
O
N
N R
O
O
N
N
R
Fe
-
- 2 Na+
Fig. 11. . Proposed structure of the complex compounds formed between (MAD_1-
6) inhibitors and Fe ions in acidic and alkaline corrosive solutions.
place by partial transference of electrons from the polar atoms
(N and O atoms and the delocalized π-electrons around the het-
erocyclic rings) of MAD to the metal surface. In addition to elec-
trostatic interaction (physisorption) of MAD molecules on the iron
surface, molecular adsorption may also play a role in the adsorp-
tion process. A close examination of the chemical structure of
(MAD_1–6), reveals that MAD molecules have structure character-
ized by the presence of chelation centers mainly located on nitro-
gen’s and oxygen. From theoretical and experimental results ob-
tained, N and O atoms are the likely sites of complexation of MAD
with the Fe ions (Fig. 11) in the corrosive acidic and alkaline media
which will result in the formation of a six-membered, redox active
chelate ring [127,128]. The UV–visible absorption spectra (Fig. 2)
of the solution containing the inhibitor after the immersion of the
iron specimen indicated the formation of a complex with the iron
surface allowing the formation of adhesive film. Such an adhesive
film covered the metal surface isolating the metal surface from the
corrosive media. Finally, it should also be emphasized that, the
large size and high molecular weight of mono-azo dye molecule
can also contribute the greater inhibition efficiency of (MAD_1–6)
[129].
In order to present more details, the molecular orbital den-
sity distributions for the six studied mono-azo dyes are shown.
Figs. 7 and 8 shows that all of investigated (MAD_1-6) have very
similar electronic density on their HOMO, so different inhibition
effectiveness observed in six molecules cannot be explained in
terms of EHOMO. However, it can be found in Figs. 7 and 8 that
the orbital density distributions on LUMO of (MAD_1–6) are sim-
ilar to each other and their only difference is related to orbital
density distributions of LUMO that is localized over the atoms N
and O and follows the following order: MAD_ 1> MAD_ 2> MAD_
3> MAD_ 4> MAD_ 5> MAD_ 6, the data obtained from quantum
chemical calculations and molecular dynamic simulations (MDS)
are tabulated in Tables 5–10). Finally, we remark that we do not
know the real structure of mono-azo dye films; instead these ar-
guments are used to demonstrate the differences in inhibition effi-
ciency of these molecules. In general, the adsorption of (MAD_1–6)
molecules at the iron electrode surface depends on the molecu-
lar size, charge distribution and deformability of the active center
as well as the charge on the metal surface undergoing corrosion.
Thus, the increased formation of mono-azo dye-metal complexes
leads to the formation of an insoluble film of the complex on the
metal surface, which furnishes an additional inhibitive property
to that of the investigated (MAD_1–6). Proposed structure of the
MAD-Fe complex compounds formed between mono-azo dye in-
hibitors and Fe ions in acidic and alkaline corrosive solutions was
shown in Fig. 11.
Skeletal representation of the proposed mode of adsorption
of the investigated (MAD_1- 6) inhibitors is shown in Fig. 12,
which clearly indicates that, iron has greater affinity towards aro-
matic moieties and were found to adsorb benzene rings in a
flat orientation. The substituent (MAD_3) shows the best perfor-
mance (Fig. 13). This can be explained on the basis that compound
(MAD_3) can be chemisorbed as a tri dentate surface ligand. The
surface coordination is through the oxygen atoms from both the
OH and OCH3 groups which raises the possibility of transferring
the unshared electron of the molecule to iron in comparison to
other derivatives and therefore results in a better adsorption [130].
It was concluded that, the mode of adsorption depends on the
affinity of the iron metal towards the π-electron clouds of the
ring system [66]. From experimental measurements, the order of
increasing the corrosion inhibition efficiency IE (%) of (MAD_1–6)
inhibitors on iron in acidic and alkaline solutions was follows the
arrangement: MAD_ 1> MAD_ 2> MAD_ 3> MAD_ 4> MAD_ 5>
MAD_ 6.
(MAD_ 1) is the most efficient inhibitors of the investigated
(MAD_1-6). This seems to be adsorbed on the iron surface through
each of adsorption oxygen centers and π-electron system of the
benzene rings. It was found that, substituted phenyl rings in the α-
position of mono-azo derivative increases longitudinal polarization
of the π -electron clouds. Thus, the adsorbed species lie flat on the
iron surface causing a higher inhibitive effect value than the others
derivatives. When the phenyl rings lies in the β-position for com-
pound (MAD_ 2) this is because transverse polarization and conse-
quently their adsorption are relatively decreased on the metal sur-
face. The adsorption of (MAD_ 3) inhibitor depends on the three
oxygen adsorption sites. Methyl (CH3-) group is more basic than
the H-atom, so its presence within the azo dyes molecule causing
increasing the localization of the π-electron clouds on the Fe metal
surface depending on its position as follows: p > o > m-position.
Thus, compound (MAD_ 4) lie before (MAD_ 5) and the compound
(MAD_ 6) comes at the end of the investigated (MAD_1-6) deriva-
tives.
5. Conclusions
The six mono-azo dyes are effective inhibitors of corrosion of
iron exposed to 2.0 M HNO3 and 2.0 M NaOH solutions, respec-
tively, at 303 K. The polarization curves showed that the com-
pounds were mixed-type inhibitors in acidic and only cathodic
in alkaline solutions. Correlation between kinetic-thermodynamic
18. 478 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Fig. 12. Skeletal representation of the proposed mode of adsorption of six mono-azo dye (MAD_1–6) derivatives on the iron surface.
Fig. 13. Skeletal representation of the proposed mode of adsorption of mono-p-
anisidine (MAD_3) on the iron surface.
model and Frumkin adsorption isotherm of the corrosion inhibi-
tion is described. Data obtained from quantum chemical calcula-
tions using DFT at HF/SDD, HF/6-311 G, HF/6-31++G, B3LYP/SDD,
B3LYP/6-311 G and B3LYP/6-31++G methods were correlated to the
inhibitive effect of the compounds. Molecular dynamic simulations
(MDS) employing Monte Carlo sampling approach were performed
using Material Studio software program to search for the most sta-
ble configuration and adsorption energies for the interaction of the
(MAD_1–6) corrosion inhibitors on Fe (110) interface. Experimental
and theoretical calculations are in good agreement. Our approach
will be help full for quick prediction of a potential inhibitor from
a lot of similar inhibitors and subsequently in their rational design
and synthesis for corrosion inhibition application.
Acknowledgments
I gratefully acknowledge Faculties of Science, Departments
of Chemistry at Baljarashi, Al-Baha University, (Saudi Arabia),
Cumhuriyet University (Turkey), and Tongren University (China),
for the financial assistance and facilitation of our research.
References
[1] Prabhu RA, Venkatesha TV, Shanbhag AV, Praveen BM, Kulkarni GM,
Kalkhambkar RG. Quinol-2-thione compounds as corrosion inhibitors for mild
steel in acid solution. Mater Chem Phys 2008;108(2):283–9.
[2] Madkour LH, Elroby SK. Inhibitive properties, thermodynamic, kinetics and
quantum chemical calculations of polydentate Schiff base compounds as
corrosion inhibitors for iron in acidic and alkaline media. Int J Ind Chem
2015;6(3):165–84.
[3] Madkour LH, Elroby SK. J Corr Sci Eng (JCSE) 2014:17.
[4] Madkour LH, Zinhome UA. J Corr Sci Eng (JCSE) 2010:13.
[5] Madkour LH, Elroby SK. Stand Sci Res Essays 2014;2(13):680–704.
[6] Doner A, Solmaz R, Ozcan M, Kardas G. Corros Sci 2011;53:2902–13.
[7] Solmaz R, Kardas GC, Ulha M, Yazici B, Erbil M. Investigation of adsorption
and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hy-
drochloric acid media. Electrochim Acta 2008;53(20):5941–52.
[8] Madkour LH, Hassanein AM, Ghoneim MM, Eid SA. Inhibition Effect of hydan-
toin compounds on the corrosion of iron in nitric and sulfuric acid solutions.
Monatshefte fiir Chemie 2001;132:245–58.
[9] Madkour LH, Elmorsi MA, Ghoneim MM. Monatshefte fiir Chemie
1995;126:1087–95.
[10] Madkour LH, Ghoneim MM. Inhibition of the corrosion of 16/14 austenitic
stainless steel by oxygen and nitrogen containing compounds. Bull Elec-
trochem 1997;13(1):1–7.
[11] Solmaz R, Mert ME, Kardas G, Yazici B, Erbil M. Adsorption and cor-
rosion inhibition Effect of 1,10-Thiocarbonyldiimidazole on mild steel in
H2SO4 solution and synergistic effect of iodide ion. Acta Phys Chim Sinica
2008;24(7):1185–91.
[12] Altunba ¸s E, Solmaz R, Karda ¸s G. Mater Chem Phys 2010;121:354–8.
[13] Kaya S, Kaya C, Guob L, Kandemirli F, Tüzün B, U˘gurlu ˙I, et al. Quantum chem-
ical and molecular dynamics simulation studies on inhibition performances of
some thiazole and thiadiazole derivatives against corrosion of iron. J Mol Liq
2016;219:497–504.
[14] Solmaz R, ¸Sahin EA, Doner A, Karda ¸s G. Corros Sci 2011;53:3231–40.
[15] Al-Doori1 HH, Shihab MS. J Al-Nahrain Univ 2014;17(3):59–68.
19. L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480 479
[16] Hamani H, Douadi T, Al-Noaimi M, Issaadi S, Daoud D, Chafaa S. Corros Sci
2014;88:234–45.
[17] Abdul-Nabi AS, Jasim EQ. J Basrah Res (Sci) 2013;3(3):82–106 9.
[18] H. Shokry, R. Shaha, E.M. Mabrouk, J Adv Chem (2013) 5 (2) 701–718
[19] Nagiub AM, Mahross MH, Khalil HFY, Mahran BNA, Yehia MM, El-Sab-
bah MMB. Portugaliae Electrochimica Acta 2013;31(2):119–39.
[20] Abdallah M, Moustafa E. Inhibition of acidic corrosion of carbon steel by
some mono and bis azo dyes based on 1,5 dihydroxynaphihalene. Anal Chim
2004;94(601).
[21] Touhami F, Aouniti A, Abed Y, Hammouti B, Kertit S, Ramdani A, Elkacemi K.
Corrosion inhibition of armco iron in 1 M HCl media by new bipyrazolic
derivatives. Corros Sci 2000;42:929–40.
[22] E. Kraka, D. Cremer, Computer design of anticancer drugs, J Am Chem Soc 122
(2000) 8245–8264.
[23] Kraka E, Cremer D. Structure and stability of enediynes containing het-
ero atoms – A quantum chemical investigation. J Mol Struc-Theochem
2000;506:19–211.
[24] Madkour LH, Elshamy IH. Experimental and computational studies on the in-
hibition performances of benzimidazole and its derivatives for the corrosion
of copper in nitric acid. Int J Ind Chem 2016;7(2):195–221.
[25] Parac M, Grimme S. J Phys Chem A 2003;106:6844–50.
[26] Madkour LH, Issa RM, El-Ghrabawy IM. J Chem Res (S) 1999;408-409:1701–26
& (M).
[27] ASTM G 31-72. Standard Guide Practice for Laboratory Immersion Corrosion
Testing of Metals. Immersion corrosion evaluations using G31-72 provide a
straightforward and simple method of determining the rate of corrosion in
aqueous solutions. It is most appropriate for determine the corrosivity of liq-
uids in static applications.
[28] Li XH, Deng SD, Mu GN, Fu H, Yang FZ. Corros Sci 2008;50:420–30.
[29] Li XH, Deng SD, Fu H, Mu GN. Corros Sci 2008;50:2635–45.
[30] Vracar LM, Drazic DM. Corros Sci 2002;44:1669–80.
[31] Mylius FZ. Metallkunde 1922;14:233.
[32] Aziz K, Shams El Din AM. A simple method for thedetermination of the inhi-
bition efficiency of surfactants. Corros Sci 1965;5(7):489–501.
[33] Mylius FZ. Eine kritische Zusammenfassung Metallkunde 1924;16:81.
[34] Fouda AS, Elasmy AA. Efficiency of some phenylthiosemicarbazide derivatives
in retarding the dissolution of Al in NaOH solution. Monatshefte fu¨r Chemie
Chem Mon 1987;118(6–7):709–16.
[35] Fouda AS, Madkour LH, Elshafei AA, Elasklany AH. Mat.-wiss. u. Werkstofftech
1995;26:342–6.
[36] Adamo C, Jacquemin D. The calculations of excited-state properties with time
dependent density functional theory. Chem Soc Rev 2013;42:845–56.
[37] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-
man Jr., et al., Gaussian 03 W, Gaussian Inc., Wallingford, CT, 2004.
[38] R.D. Dennington, T.A. Keith, C.M. Millam GaussView 5.0 Wallingford, CT;
(2009).
[39] Chermette H. Chemical reactivity indexes in density functional theory. J Comp
Chem 1999;20:129–54.
[40] Parr RG, Chattaraj PK. Principle of maximum hardness. J Am Chem Soc
1991;113:1854–5.
[41] Iczkowski RP, Margrave JL. Electronegativity. J Am Chem Soc
1961;83:3547–51.
[42] Yang W, Lee C, Ghosh SK. Molecular softness as the average of atomic soft-
nesses: companion principle to the geometric mean principle for electroneg-
ativity equalization. J Phys Chem 1985;89:5412–14.
[43] Yang W, Parr RG. Hardness, softness and the Fukui function in the electronic
theory of metals and catalysis. Proc Natl Acad Sci 1985;82:6723–6.
[44] Koopmans T. Ordering of wave functions and eigen-energies to the individual
electrons of an atom. Physica 1933;1:104–13.
[45] Islam N, Ghosh DC. A new algorithm for the evaluation of the global hardness
of polyatomic molecules. Int J Quant Chem 2011;109:917–31.
[46] Kaya C. Inorganic Chemisry 1 and 2. Ankara: Palme Publishing; 2011.
[47] Parr RG, Sventpaly L, Liu S. Electrophilicity index. J Am Chem Soc
1999;121:1922–4.
[48] Chattaraj PK, Sarkar U, Roy DR. Electrophilicity index. Chem Rev
2006;106:2065–91.
[49] Frenkel D, Smit B. Understanding molecular simulation: From algorithms to
applications. 2nd ed. San Diego: Academic Press; 2002.
[50] Kirkpatrick S, Gelatt CD, Vecchi MP. Optimization by simulated annealing. Sci-
ence 1983;220:671–80.
[51] Guo L, Zhang ST, Lv TM, Feng WJ. Comparative theoretical study on the cor-
rosion inhibition properties of benzoxazole and benzothiazole. Res Chem In-
termed 2015;41:3729–42.
[52] Sun H. COMPASS: an ab initio force-field optimized for condensed-phase ap-
plications overview with details on alkane and benzene compounds. J Phys
Chem B 1998;102:7338–64.
[53] Kaya S, Tüzüna B, Kaya C, Obot IB. J Taiwan Inst Chem Eng 2015;000:1–8.
[54] Behpour M, Ghoreishi SM, Mohammadi N, Soltani N, Salavati-Niasari M. Cor-
ros Sci 2010;52:4046–57.
[55] Obi-Egbedi NO, Obot IB. Corros Sci 2011;53:263–75.
[56] Musa AY, Khadom AA, Kadhum AH, Mohamad AB, Takriff MS, Taiwan J. Inst
Chem Eng 2010;41:126–8.
[57] Khadom AA, Yaro AS, Kadum AH, Taiwan J. Inst Chem Eng 2010;41:122–5.
[58] Bouklah M, Hammouti B, Lagrenee M, Bentiss F. Corros Sci 2006;48:2831–42.
[59] Chitra S, Parameswari K, Sivakami C, Selvaraj A. Chem Eng Res Bull
2010;14:1–6.
[60] Zou Y, Wang J, Zheng YY. Corros Sci 2011;53:208–16.
[61] El-Naggar MM. Corros Sci 2007;49:2226–36.
[62] Umoren SA, Ebenso EE. Mater Chem Phys 2007;106:387–93.
[63] Emregul KC, Hayvalı M. Corros Sci 2006;48:797–812.
[64] Frumkin AN. Z Phys Chem 1925;116:466–84.
[65] Babic- Samardzija K, Khalid KF, Heckerman. J App Surf Sci 2005;240(I-4):327.
[66] Sankarapapavinasam S, Ahmed MF. J Appl Electrochem 1992;22:390.
[67] A. El-Sayed, D Kagaku, 66(2) (1998) 176.
[68] Ahamad I, Prasad R, Quraishi MA. Thermodynamic, electrochemical and quan-
tum chemical investigation of some Schiff bases as corrosion inhibitors for
mild steel in hydrochloric acid solutions. Corros Sci 2010;52(3):933–42.
[69] Avci G. Colloid Surf. A: Physicochem Eng Aspec 2008;317:730–6.
[70] Khaled KF, Hackenman N. Electrochim Acta 2004;49:485–95.
[71] Abd El-Maksoud SA, Fouda AS. Mater Chem Phys 2005;93:84–90.
[72] Abdel-Rehim SS, Ibrahim MAM, Khaled KF. Mater Chem Phys
2001;70:268–73.
[73] Ferreira ES, Giancomelli C, Giacomelli FC, Spinelli A. Mater Chem Phys
2004;83:129–34.
[74] Li WH, He Q, Pei CL, Hou BR. J Appl Electrochem 2008;38:289–95.
[75] Quartarone G, Bonaldo L, Tortato C. Appl Surf Sci 2006;252:8251–7.
[76] Chauhan LR, Gunasekaran G. Corros Sci 2007;49:1143–61.
[77] Hegazy MA. Corros Sci 2009;51:2610–18.
[78] da Silva AB, D’Elia E, da Cunha Ponciano Gomes JA. Corros Sci
2010;52:788–93.
[79] Bayol E, Kayakirilmaz K, Erbil M. Mater Chem Phys 2007;104:74–9.
[80] Li X, Deng S, Fu H, Li T. Adsorption and inhibition effect of 6-ben-
zylaminopurine on cold rolled steel in 1.0 M HCl. Electrochim Acta
2009;54(16):4089–98.
[81] Becke AD. Phys Rev 1988;38:3098–100.
[82] Lee C, Yang W, Parr RG. Phys Rev 1988;37:785–9.
[83] Hehre WJ, Radom L, Schleye PvR, Pople JA. Ab initio molecular orbital theory.
New York: Wiley; 1989.
[84] Obot IB, Macdonald DD, Gasem ZM. Density functional theory (DFT) as a
powerful tool for designing new organic corrosion inhibitors. Part 1: An
overview. Corros Sci 2015.
[85] Pearson RG. Hard and soft acids and bases. J Am Chem Soc
1963;85(22):3533–9.
[86] Pearson GR. The principle of maximum hardness. Acc Chem Res
1993;26(5):250–5.
[87] Parr RG, Chattaraj PK. Principle of maximum hardness. J Am Chem Soc
1991;113(5):1854–5.
[88] Martinez S. Mater Chem Phys 2002;77:97–102.
[89] Sanderson RT. Chemical bond and bond energy. New York: Academic Press;
1976.
[90] Sanderson RT. Electronegativities in inorganic chemistry. J Chem Edu
1954;31:2–7.
[91] Awad MK, Mustafa MR, Elnga MMA. J Mol Struct (Theochem)
2010;959:66–74.
[92] Fukui K. Role of frontier orbitals in chemical reactions. Science
1982;218:747–54.
[93] Rodriguez-Valdez LM, Martínez-Villafañe A, Glossman-Mitnik D. Computa-
tional simulation of the molecular structure and properties of heterocyclic or-
ganic compounds with possible corrosion inhibition properties.. J Mol Struct
THEOCHEM 2005;713(1):65–70.
[94] Li X, Deng S, Fu H. Prog Org Coat 2010;67:420–6.
[95] Khaled KF, Babíc-Samard_zija K, Hackerman N. Electrochim Acta
2005;50:2515–20.
[96] Bereket G, Hür E, Ögretir C. Theochem. J Mol Struct 2002;79:578.
[97] Musa YAhmed, Ramzi Jalgham TT, Abu Bakar Mohamad M. Molecular dy-
namic and quantum chemical calculations for phthalazine derivatives as cor-
rosion inhibitors of mild steel in 1 M HCl.. Corros Sci 2012;56:176–83.
[98] BObot I, Obi-Egbedi NO, Ebenso EE, Afolabi AS, Oguzie EE. Experimen-
tal, quantum chemical calculations, and molecular dynamic simulations in-
sight into the corrosion inhibition properties of 2-(6-methylpyridin-2-yl)
oxazolo [5, 4-f][1, 10] phenanthroline on mild steel. Res Chem Intermed
2013;39(5):1927–48.
[99] John S, Joy J, Prajila M, Joseph A. Electrochemical, quantum chemical
and molecular dynamics studies on the interaction of 4-amino-4H,3,5-
di(methoxy)-1,2,4-triazole (ATD), BATD, and DBATD on copper metal in 1 N
H2SO4. Mater Corros 2011;62:1031–41.
[100] Noor EA, Al-Moubaraki AH. Mater Chem Phys 2008;110:145–54.
[101] Bentiss F, Jama C, Mernari B, El Attari H, El Kadi L, Lebrini M, et al. Corros Sci
2009;51:1628–35.
[102] Valcarce MB, Vazquez M. Mater Chem Phys 2009;115:313–21.
[103] Sahin M, Bilgic S, Yilmaz H. Appl Surf Sci 2002;195:1–7.
[104] Umoren SA, Ogbobe O, Igwe IO, Ebenso EE. Corros Sci 2008;50:1998–2006.
[105] Khaled KF. Appl Surf Sci 2004;230:307.
[106] Solomon MM, Umoren SA, Udosoro II, Udoh AP. Corros Sci
2010;52(4):1317–25.
[107] Badawy WA, Ismail KM, Fathi AM. Electrochim Acta 2006;51:4182–9.
[108] Abdallah M. Corros Sci 2002;44:717–28.
[109] El-Awady AA, Abd-El-Nabey BA, Aziz SG, Khalifa M, Al-Ghamedy HA. Interna-
tional J Chem 1990;1(4):169–79.
[110] El-Awady AA, Abd-El-Nabey BA, Aziz SG. J Electrochem Soc 1992;139:2149.
[111] Fouda AS, Mousa MNH, Taha FI, El-Neanaa AI. J Corrosion Sci 1986;26:719.
[112] Langmuir I. J Am Chem Soc 1947;39.
![Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers
journal homepage: www.elsevier.com/locate/jtice
Quantum chemical calculations, molecular dynamics simulation and
experimental studies of using some azo dyes as corrosion inhibitors
for iron. Part 1: Mono-azo dye derivatives
Loutfy H. Madkoura,∗
, Sava ¸s Kayab
, Cemal Kayab
, Lei Guoc
a
Chemistry Department, Faculty of Science and Arts, Baljarashi, Al-Baha University, P.O. Box 1988 Al-Baha, Saudi Arabia
b
Cumhuriyet University, Faculty of Science, Department of Chemistry, 58140 Sivas, Turkey
c
School of Material & Chemical Engieering, Tongren University, Tongren 554300, PR China
a r t i c l e i n f o
Article history:
Received 6 January 2016
Revised 14 August 2016
Accepted 10 September 2016
Available online 1 October 2016
Keywords:
Density functional theory
Molecular dynamics simulation
Corrosion
Mono-azo dyes
Iron
Adsorption
a b s t r a c t
This study consists of two parts. In the first part, the inhibitive performance of six mono-azo dye
(MAD_1–6) derivatives was investigated experimentally (gravimetric, thermometric, UV–visible spec-
trophotometric and electrochemical potentiostatic methods) and computationally against corrosion of Fe
metal in 2 M HNO3 and 2 M NaOH solutions. Density functional theory (DFT) calculations and molec-
ular dynamics simulation (MDS) approach were performed. Quantum chemical parameters such as the
highest occupied molecular orbital energy (EHOMO), lowest unoccupied molecular orbital energy (ELUMO),
the energy gap between ELUMO and EHOMO ( E), dipole moment (D), chemical hardness (η), softness (σ),
electronegativity (χ), proton affinity, global electrophilicity (ω), global nucleophilicity (ε) and total energy
(sum of electronic and zero-point energies) were calculated and discussed with the help of HF/SDD, HF/6-
311 G, HF/6-31++G, B3LYP/SDD, B3LYP/6-311 G and B3LYP/6-31++G methods. Polarization measurements
indicate that (MAD) compounds are of mixed-type inhibitor in acidic, act mainly as cathodic in alkaline
solution. Kinetic model involving binding constant (Kb), active sites (1/y) and standard free energy values
of adsorption (࢞Go
) were compared with the parameters of equilibrium constant (Kads), lateral interac-
tion (f) and (࢞Go
), that obtained from Frumkin adsorption isotherm model. Then, we calculated binding
energies on Fe (110) surface of the inhibitors. The theoretical data obtained are in good agreement with
the experimental inhibition efficiency results.
© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction
Metals and alloys used in many engineering applications are
susceptible to corrosion in aqueous media. Iron and its alloys, the
most widely used among them, is also highly susceptible to corro-
sion, especially in acidic and alkaline media [1–5]. One of the best
known methods for corrosion protection is the use of inhibitors
[6,7]. Different types of organic compounds have been reported to
act as inhibitors of corrosion [8–14]. Azo dyes as the most widely
used as inhibitors class is controlled by its economic availability,
its efficiency to inhibit the substrate material and its environmen-
tal side effects [15–19]; their application in various fields, such as
the dyeing of textiles, and fibbers [20]. The presence of –N=N–
group in azo dye molecules enhances their adsorption ability and
∗
Corresponding author. fax: +966 77247272.
E-mail addresses: loutfy_madkour@yahoo.com, lha.madkour@gmail.com,
loutfy.madkoor@science.tanta.edu.eg (L.H. Madkour).
corrosion inhibition efficiency. The planarity (π) and lone pair
of electrons present on the N atoms are the important structural
features that determine the adsorption of these molecules on
to the metal surface [1]. The inhibition effect was also found to
depend on some physicochemical and electronic properties of
the organic inhibitor which relate to its functional groups, steric
effects, electronic density on donor atoms, and orbital character of
donating electrons [21]. Quantum chemical methods have already
proven to be very useful in determining the molecular structure as
well as elucidating the electronic structure and reactivity [22] of
potent inhibitors [23]. Thus, it has become a common practice to
carry out quantum chemical calculations in corrosion inhibition
studies. The predicted properties of reasonable accuracy can be
obtained from density functional theory (DFT) calculations [24,25].
Some quantum chemical parameters, which influence the elec-
tronic interaction between surface atoms and inhibitors, are the
energy of highest occupied molecular orbital (EHOMO), the energy
of lowest unoccupied molecular orbital (ELUMO), the energy gap
EHOMO − ELUMO ( E) and dipole moment (D), chemical hardness
http://dx.doi.org/10.1016/j.jtice.2016.09.015
1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.](https://image.slidesharecdn.com/3ba4ba6f-337b-42c3-b00b-20f4713c7899-161110105708/85/taiwan-2016-1-320.jpg)
![462 L.H. Madkour et al. / Journal of the Taiwan Institute of Chemical Engineers 68 (2016) 461–480
(η), softness (σ), electronegativity (χ), proton affinity, global elec-
trophilicity (ω), global nucleophilicity (ε) and total energy (sum
of electronic and zero-point energies). Previously, some work has
been done in our laboratory on using mono– and bis–azo dye com-
pounds as inhibitors on aluminum in HCl and NaOH solutions [26].
The aim of the present study was to investigate and compare
the inhibition of corrosion of iron in 2.0 M HNO3 and 2.0 M NaOH
solutions by six synthesized mono-azo dye derivatives shown in
Fig. 1 at 303 K. We have determined the inhibition efficiencies of
these compounds using weight loss, thermometric, spectroscopy
measurements and polarization curves method. Theoretical stud-
ies on electronic and molecular structures of substituted mono-azo
dyes were carried out with the help of quantum chemical calcula-
tions and molecular dynamics simulations (MDS) approach to de-
termine the most effective corrosion inhibitor among them.
R=
α-Naphthyl; the compound namely mono-α-naphthyl amine
(MAD_1)
β- Naphthyl; the compound namely mono -β -naphthyl amine
(MAD_2)
C6H4OMe-p; the compound namely mono -p-anisidine (MAD_3)
C6H4Me-p; the compound namely mono -p-toluidine (MAD_4)
C6H4Me-o; the compound namely mono -o-toluidine (MAD_5)
C6H4Me-m; the compound namely mono -m-toluidine (MAD_6)
2. Experimental details
2.1. Synthesis of the mono-azo dye compounds
The investigated mono-azo dye (MAD_1–6) derivatives were
synthesized by diazotization of primary aromatic amines and cou-
pling with the corresponding naphthol derivatives in the ratio
1:1. The compounds are purified and characterized by elemen-
tal analysis, IR, UV–visible spectroscopic investigation; mass spec-
troscopy and 1Hnmr spectra spectroscopy techniques. The inhibitor
solutions were prepared by dissolving the appropriate amount in
10 cm3 Analar ethanol. The desired volume of the free inhibitor
was added to the electrolyte solution. The ratio of ethanol was kept
constant for each test. This stock solution was used for all exper-
imental purposes. The concentration range of azo dye inhibitors
employed was 5×10−7 M – 10−4 M at 303 K. The chemical struc-
ture and IUPAC name of synthesized azo dye compounds are given
in Fig. 1. The corrosion tests were performed on iron specimens
of following composition (wt. %): C=0.16, Mn=0.37, Si=0.05,
S=0.015 and remainder Fe. Iron specimens of size 2.0×2.0×0.1
and 10×1 × 0.l cm were used for weight loss and thermomet-
ric measurements, respectively. Solution of 2.0 M HNO3 and 2.0 M
NaOH were prepared by dilution of Analar analytical grade using
double distilled water.
2.2. Measurements
2.2.1. Weight loss measurements
Weight loss experiments were done according to the stan-
dard methods as reported in literature [27]. The corrosion rates,
OH
HO N N
Mono-α-naphthyl amine (MAD_1)
OH
HO N N
Mono-β -naphthyl amine (MAD_2)
OH
HO N N OMe
Mono-p-anisidine (MAD_3)
OH
HO N N Me
Mono-p-toluidine (MAD_4)
OH
HO N N
Me
Mono-o-toluidine (MAD_5)
OH
HO N N
Me
Mono-m-toluidine (MAD_6)
Fig. 1. Chemical molecular structures of synthesized mono-azo dye (MAD_1-6)
derivatives.](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)