Volume 20 Preprint 64
Studies on the green inhibition of steel corrosion in acid medium
S.Karthikeyan , P.A.Jeeva , K.Raja
Keywords: Corrosion, quantum, hydrogen permeation, impedance, inhibition
Corrosion behaviour of mild steel in 2M Phosphoric acid with Tinidazole as corrosion inhibitor has been studied by using weight loss, Potentiodynamic polarization, electrochemical impedance spectroscopy, Hydrogen permeation and diffuse reflectance spectroscopic studies. All these techniques reveal that inhibition efficiency increases with the increase in the concentration of inhibitor. Polarization studies indicated that inhibitor behaved as cathodic inhibitor. Diffuse reflectance spectroscopy confirmed the adsorption of inhibitor on the mild steel surface obeying Langmuir adsorption isotherm. A Gaussian simulation technique was used to track the quantum mechanical analysis and recognized correlations between different types of descriptors and measured corrosion inhibition efficiency for inhibitor. The quantum chemical analysis demonstrates the inhibition efficiencies of the compound determined by electrochemical methods.
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Studies on the green inhibition of steel corrosion in acid
S.Karthikeyan *, P.A.Jeeva , K.Raja
Centre for Innovative Manufacturing Research, VIT University, Vellore, Tamilnadu, India.
(*Corresponding author: firstname.lastname@example.org)
Corrosion behaviour of mild steel in 2M Phosphoric acid with Tinidazole as corrosion
electrochemical impedance spectroscopy, Hydrogen permeation and diffuse reflectance
spectroscopic studies. All these techniques reveal that inhibition efficiency increases with
the increase in the concentration of inhibitor. Polarization studies indicated that inhibitor
behaved as cathodic inhibitor. Diffuse reflectance spectroscopy confirmed the adsorption of
inhibitor on the mild steel surface obeying Langmuir adsorption isotherm. A Gaussian
simulation technique was used to track the quantum mechanical analysis and recognized
correlations between different types of descriptors and measured corrosion inhibition
efficiency for inhibitor. The quantum chemical analysis demonstrates the inhibition
efficiencies of the compound determined by electrochemical methods.
Keywords: Mild Steel, Corrosion Inhibition, Tinidazole, Adsorption Isotherm, Quantum
Mild steel is an important category of metals due to its excellent mechanical
properties. It is extensively used under different conditions in chemical and allied industries
in handling acidic, alkaline and salt solutions. Mild is used in industries as pipelines for
petroleum industries, storage tanks, reaction vessel and chemical batteries . Acid
solutions are widely used in many industrial processes. Acids are used for acid cleaning,
pickling and descaling due to their chemical properties [2-5]. Acids cause damage to the
substrate, because of their corrosive nature. Several methods were used to decrease the
corrosion of metals in acidic medium, but the use of inhibitors is most commonly used
Organic compounds are widely used as corrosion inhibitors for mild steel in acidic
media [11-16]. The rate of corrosion decreases by adsorption of organic inhibitors on the
metal surface. The inhibitors block the active sites by displacing water molecules and form
a compact barrier film on the metal surface. The most of the organic inhibitors are toxic,
highly expensive and non environment friendly. Research activities in recent times are
geared towards developing the cheap, non-toxic drugs as environment friendly corrosion
The aim of this work is to investigate the corrosion protection efficiency of Tinidazole
for mild steel corrosion in 2M H3PO4. We came to know that exceedingly few reports are
available by using this compound as corrosion inhibitor in 0.1M H2SO4 [22-24]. No concrete
report is available for the use these compounds as corrosion inhibitors in 1M H3PO4. From
the literature the higher concentration of H3PO4 acts as pickling solution for mild steel for
electroplating, battery electrodes using sulphur containing organic compounds. Use of this
inhibitor in 2M H3PO4 will reduce the metal loss in acid medium. The compound is large
enough and sufficiently planar to block more surface area on the mild steel. The inhibition
efficiency was calculated using weight loss measurement, potentiodynamic polarization
studies, impedance techniques, hydrogen permeation studies and diffuse reflectance
methods. A definite correlation exists
between different types of descriptors and
measured corrosion inhibition efficiency for Tinidazole using chemical and electrochemical
2. Experimental Details
Mild steel specimens of size 1x4 cm2 were used for weight loss and electrochemical
studies. The aggressive solution of 2M H3PO4 (AR Grade) was used for all the studies. The
antibiotic namely Tinidazole was purchased from the corresponding manufacturing
company. The structure of the antibiotic is given in the figure 1. Electrochemical
experiments were performed using a three electrode cell assembly with mild steel samples
as working electrode, 4cm2 area of platinum as counter electrode and Hg/Hg2Cl2/KCl as the
reference electrode. The surfaces of corroded and corrosion inhibited mild steel specimens
were examined by diffuse reflectance studies in the region 200-700 nm using U-3400
spectrometer (UV-VIS-NIR Spectrometer, Hitachi, Japan).
Fig.1: Structure of Tinidazole
2.2. Weight loss studies
The concentrations of inhibitor used for weight loss and electrochemical study were
from 2x10-4M to 20x10-4M. Mild steel specimens of size 1x4 cm2 were abraded with
different emery papers and washed with acetone. The cleaned samples were then washed
with double distilled water and finally dried and kept in the desiccator. The weight loss
study was carried out at room temperature for three hours in 2M H3PO4. The inhibition
efficiency (IE %) was determined by the following equation
Inhibition Efficiency (IE %) =
(W0 –Wi /W0) X 100
Where W0 & Wi are the weight loss values in the absence and presence of the
2.3. Electrochemical studies
Potentiodynamic polarization measurements were carried out in a conventional three
electrode cylindrical glass cell, using CH electrochemical analyzer. The solution was
deaerated for 15 minutes before carryout the polarization studies. The working electrode
was maintained at its corrosion potential for 10 min. until a steady state was obtained. The
mild steel surface was exposed to various concentrations of inhibitors in 100mL of
1M H3PO4 at room temperature. The inhibition efficiency (IE %) was calculated using the
Inhibition Efficiency (IE %) =
(I0 –I /I0) X 100
Where I0 and I are the corrosion current density without and with the inhibitor respectively.
The potentiodynamic current-potential curves were recorded by changing the
electrode potential automatically from -750mV to +150mV versus the open circuit
potential. The corresponding corrosion current (I
was recorded. Tafel plots were
constructed by plotting E versus log I. Corrosion Potential (Ecorr), corrosion current density
(Icorr) and cathodic and anodic slopes (βc and βa) were calculated according to known
Impedance measurements were carried out in the frequency range from 0.1 to
10000 Hz using amplitude of 20 mV and 10 mV peak to peak with an AC signal at the
open-circuit potential. The impedance diagrams were plotted in the nyquist representation.
Charge transfer resistence (Rct) and double layer capacitance (Cdl) values were obtained from
nyquist plot [25, 26]. The percentage inhibition efficiency was calculated from the equation
(Rct - R’ct / Rct) x 100
Inhibition Efficiency (IE %) =
Where R’ct and Rct are the corrosion current of mild steel with and without inhibitor
2.4. Hydrogen permeation studies
The hydrogen permeation study was carried out using an adaptation of modified
Devanathan and Stachurski’s , two compartment cell as described elsewhere . Hydrogen
permeation current was recorded in the absence and presence of inhibitors.
Diffuse reflectance spectroscopy
The surfaces of corroded and corrosion inhibited mild steel specimens were
examined by diffuse reflectance studies in the region 200- 700 nm using U-3400
spectrometer [UV-VIS-NIR Spectrometer, Hitachi, Japan].
2.6. Theoretical calculations
Quantum calculations were carried using MOPAC 2000 program of CS Chemoffice
packet program. The energy of highest occupied molecular orbital (HOMO), lowest
unoccupied molecular orbital (LUMO), Dipole moment (), hardness, absolute softness and
total energy of the molecule were calculated with the above given software package.
3. Results and discussion
3.1. Weight loss studies
The values of inhibition efficiency (IE %), corrosion rate (CR) and surface coverage (θ)
calculated for Tinidazole in 2M H3PO4 at different concentrations from the weight loss data
are summarized in the table-1. It is obvious that inhibition efficiency enhances with
increase in the inhibitor concentration. In addition the rate of corrosion has reduced with
increase in inhibitor concentration. Maximum inhibition efficiency is obtained at 45x10-3 M
concentrations of the inhibitor.
Table 1. Weight loss parameters for the corrosion of mild steel immersed in 2M phosphoric
acid in presence of different concentrations of Tinidazole
Inhibitor Conc. (M)
3.2. Potentiodynamic polarization studies
Polarization curves for mild steel in 2M H3PO4 containing different concentrations of
inhibitor are given in figure-2. The values of corrosion potential (Ecorr) , corrosion current
densities (Icorr), anodic tafel slope (βa) ,cathodic tafel slope (βc) surface coverage(θ) and
inhibition efficiency (IE%) calculated using polarization curves are summarized in table-2.
Fig 2. Polarization curves of mild steel recorded in 2M H3PO4 in presence of different
concentrations of Tinidazole
According to the results, corrosion current (Icorr) value decreases with increase in the
concentration of the inhibitor. The inhibition efficiency (IE %) and surface coverage (θ)
increases with increase in inhibitor concentration. The maximum inhibition efficiency is
achieved at 45x10-3 M concentration. Both βa and βc are reduced, but the values of βc are
decreased to a greater extent. This indicates that the compound behave as cathodic
2 Potentiodynamic polarization parameters for mild steel immersed in 2M H3PO4 in
the presence of different concentrations of Tinidazole.
3.3. Electrochemical impedance studies
The Nyquist representations of impedance performance of mild steel in 2M H3PO4
with and without addition of different concentrations of Tinidazole are shown in the
figure-3. A large capacitive circle at higher frequency range is observed at all
concentrations of the inhibitor. The higher frequency capacitive loop is due to the
adsorption of inhibitor molecule .
Fig. 3: Nyquist plot for mild steel immersed in 2M H3PO4 containing different concentrations
Values of charge transfer resistance (Rct) and double layer capacitance (Cdl) derived
from Nyquist plots are shown in table 3. The values of Rct are found to increase with
increase in concentration of inhibitor in 2M H3PO4. It is found that values of Cdl are fetched
down by increasing concentrations of inhibitor in the acid. This can be ascribed to the wellbuilt adsorption of the compounds on the metal surface.
3 Electrochemical impedance parameters for mild steel immersed in 2M H3PO4 in the
presence and absence of different concentrations of Tinidazole.
3.4. UV spectral reflectance studies
The reflectance curves for polished specimen, specimen dipped in 2M H3PO4 and
various concentrations of inhibitor are given in the figure.4. The percentage of reflectance is
highest for polished mild steel and it steadily reduces for the specimen dipped in 2M H3PO4
solution. This observation discloses that the change in surface feature is due to the
corrosion of mild steel in acid. The reflectance percentage of steel in the presence of
inhibitor is higher than steel as immersed in blank. This validates that the surface property
of steel are not altered further due to the formation of film on the metal. The reflectance
percentage decreases with increase in thickness of the inhibitor film formed on metal
surface. Similar observation has been made by Madhavan et al .
Fig. 4: UV Reflectance curves of mild steel in 2M H3PO4 solution with 20x10-4M
concentration of the inhibitor.
3.5. Adsorption isotherm and thermodynamic parameters
The inhibitive action of inhibitor in highly aggressive media is due to its adsorption
on the metal surface. The degree of surface Coverage (θ) for different concentrations of
Electrochemical Impedance studies. The acquired data was tested graphically for fitting
suitable isotherm [30-32]. Almost a straight line was obtained by plotting log (C/θ) Vs
log C as shown in Figure-5, which proves that the adsorption of these compounds on steel
surface obeys Langmuir adsorption isotherm.
Fig. 5: Langmuir’s adsorption isotherm plots for the adsorption Tinidazole in 2M H3PO4 on
the surface of mild steel.
The Langmuir isotherm for the adsorbed layers is given by the equation ,
Cinh/θ =1/Kads + Cinh
Where Kads is the equilibrium constant of the adsorption/desorption process.
Adsorption equilibrium constant [Kads] and free energy of adsorption [∆G0ads] were
calculated using the equation 
Kads= 1/Cinh x θ/1-θ
∆G0ads = -2.303RT log [55.5Kads]
Where 55.5 is the molar concentration of water in solution . R is the gas constant,
T is the temperature. The values of adsorption equilibrium constant [Kads] and free energy of
adsorption [∆G0ads] are given in table-4. The negative values of [∆G0ads] pointed out that
adsorption of inhibitors is spontaneous process. It is reported that values of [∆G0ads] is of
or higher involve charge sharing or transfer from the inhibitor to the metal
chemisorptions [36-38]. The values of free energy of adsorption
in our experiment lies in the range -28 to -32 kJmol-1, demonstrating that the
adsorption is not a simple physisorption, but it may involve some other interactions .
Table 4: Gibbs free energy parameters and adsorption equilibrium constant [K] of inhibitor
at various temperatures evaluated by weight loss method.
3.6. Hydrogen permeation measurements
Hydrogen permeation currents are recorded in H3PO4 in the absence and presence of
inhibitor. This study has been taken up with a plan of selecting the inhibitor with a view to
their efficacy on the reduction of hydrogen uptake . The values of permeation current
with respect to time are given in table-5.
Figure 6 shows the variation of permeation
current vs time for mild steel in 2M H3PO4 in the presence of Tinidazole.
Table 5: Values of permeation current for mild steel in 2M H3PO4 and in presence of
inhibitors with respect to change in time
Permeation Current (A
The Tinidazole reduces
the permeation current to the extent of 53%. The corrosion
inhibition efficiency of the compound in 2M H3PO4
definite correlation is noticed
follows the same fashion
. Thus a
between the corrosion inhibition efficiency and the extent of
reduction in the permeation current of the compound. It is a recognized fact that higher βc
value for an inhibiting compound, the lesser is the corrosion and hydrogen ingress on the
metal. An increase in the βc value, favours
to increase in the energy barrier for proton
discharge and reduction in the evolution of hydrogen. This in turn leads to lower
permeation of hydrogen through the mild steel.
Fig.6: Hydrogen permeation Vs Time curves for mild steel immersed in 2M H3PO4 and
45x10-3 M concentration of inhibitor
3.7. Mechanism of corrosion inhibition
The adsorption of Tinidazole on the mild steel surface is found to be majorly physical
in nature. Physical adsorption is a process of electrostatic attraction between charged
species in the solution and the metal surface. If the metal surface is positively charged, the
adsorption of negatively charged species is facilitated. Positively charged species can also
adsorb on the positively charged metal surface with the help of negatively charged
intermediate, which adsorb first on the positively charged metal surface and allows
positively charged species to adsorb on it.
Thus the adsorption of Tinidazole may take place in two different ways as
(i) The sulphoxide group of Tinidazole may hinder the adsorption of inhibitor on
steel .However the delocalized electrons of nitrogen atoms may release electrons
which favours the adsorption of Tinidazole on steel surface.
(ii) The inhibitor may compete with acid anions for the adsorption sites at the water
covered surface on steel
and adsorb by donating electrons to the mild steel
surface [41, 42].
3.8. Quantum chemical calculations
Quantum chemical calculations were carried out to investigate the adsorption and
inhibition mechanism of the inhibitors. Figure 7 shows the optimized structure of
Tinidazole. The values of calculated quantum chemical parameters i.e. EHOMO (highest
occupied molecular orbital), ELUMO (lowest unoccupied molecular orbital), ∆E (energy gap),
(dipole moment), σ (softness) etc. are summarized in table-6.
Fig. 7: Optimized structure of Tinidazole
EHOMO is associated with the electron-donating ability of the molecule. Several
researchers have shown that the adsorption of an inhibitor on metal surface can occur on
the basis of donor-acceptor interactions between the π-electrons of heterocyclic atoms and
the vacant d-orbitals of the metal surface atoms [43-45]. A high value of EHOMO indicates a
tendency of a molecule to donate electrons to acceptor molecules with low energy empty
molecular orbital. Increasing values of EHOMO facilitates the adsorption and increases the
inhibition efficiency by influencing the transport process through the adsorbed layer .
ELUMO indicates the ability of the molecule to accept the electrons, hence these are acceptor
states. The lower the value of ELUMO, the more probable is that the molecule can accept
electrons and increase the inhibition efficiency. Regarding ∆E (ELUMO-EHOMO) lower values of
energy difference will cause higher inhibition efficiency because energy to release electron
from last occupied orbital will be low. When dipole moment is concerned higher values of ,
will favours a strong interaction of inhibitor molecule with the metal surface .
Other indicators are absolute electro negativity (χ), absolute hardness (ȃ). Absolute
electro negativity is a chemical property that describes the ability of a molecule to attract
electron towards itself in a covalent bond. Absolute hardness is measured by the energy
gap between EHOMO and ELUMO. Absolute softness σ is the reciprocal of the hardness. χ, ȃ, σ
are calculated using the energies of HOMO and LUMO orbital’s of the inhibitor molecules
are related to the ionization potential (I), electron affinity (A) by the following relations
χ = I + A/ 2, ȃ = I – A / 2, σ = 2 / I – A
Where I = -EHOMO, A = -ELUMO
The results deduced indicate that the electron flow will happen from the molecule with low
electro negativity towards that of higher value until the chemical potentials are same. In our
studies the best inhibition effect is shown by Tinidazole with low electro negativity.
Table 6: Quantum mechanical parameters for Tinidazole for the corrosion of mild steel
inphosphoric acid medium.
The higher value of dipole moment and lower total energy for Tinidazole indicates
the strong interaction of inhibitor with metal that leading to improved adsorption. The
nitrogen atom exerts +M effect on Tinidazole, leads to enhanced corrosion inhibition.
From figure 8 it can be observed that the energy highly occupied molecular orbital’s
(HOMO) are localized on hetero atoms for Tinidazole.
Fig.8: The Highest occupied molecular orbital of Tinidazole
From Figure 9 it is observed that lowest unoccupied molecular orbital’s (LUMO) of
Tinidazole, which is responsible for its better adsorption of the inhibitor on steel surface
than Ampicillin and its allied derivatives.
Fig. 9: The lowest unoccupied molecular orbital of Tinidazole
1. The use of antibiotic viz., Tinidazole as corrosion inhibitor in 2M H3PO4 was
studied using weight loss, potentiodynamic polarization, impedance
measurements and hydrogen permeation studies.
2. The adsorption of inhibitor on mild steel surface follows Langmuir adsorption
isotherm. The adsorption of inhibitor on steel surface is further justified by diffuse
reflectance spectra results.
3. The quantum mechanical studies validate the performance of antibiotic as potential
corrosion inhibitor for mild steel in 2M H3PO4.
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