Volume 17 Preprint 39
Quantum mechanical studies of a new inhibitor for the corrosion of mild steel in 2M sulphuric acid
S.Karthikeyan , K.Raja, P.A.Jeeva, M.Paramasivam
Keywords: Corrosion inhibitor, Thio compounds, Impedance measurements, Adsorption
The inhibitive action of 1-cyclohexyl-3-cyclopenta-1,3 dienyl-1,3 diphenyl thiourea (CCDDTU) on corrosion of mild steel in 2M H2SO4 has been studied using weight loss, gasometric measurements , potentiodynamic polarization and impedance studies. The studies clearly indicated that inhibitor reduced the dissolution of mild steel in sulphuric acid by behaving as cathodic inhibitor. The adsorption of this organic molecule on mild steel surface obeyed Temkinâ€™s adsorption isotherm. The quantum mechanical analysis proved the inhibition efficiency of the compound calculated from chemical and electrochemical measurements.
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Quantum mechanical studies of a new inhibitor for the corrosion of
mild steel in 2M sulphuric acid
S.Karthikeyan 1*, K.Raja2, P.A.Jeeva 2, M.Paramasivam3
Centre for Nanobiotechnology, VIT University, Vellore- 632 014, India.
School of Mechanical & Building Sciences, VIT University, Vellore- 632 014, India.
CSIR-Central Electrochemical Research Institute, Karaikudi-632014,India
corresponding author (firstname.lastname@example.org)
The inhibitive action
of 1-cyclohexyl-3-cyclopenta-1,3 dienyl-1,3 diphenyl thiourea
(CCDDTU) on corrosion of mild steel in 2M H2SO4 has been studied using weight loss,
gasometric measurements , potentiodynamic polarization and impedance studies. The
studies clearly indicated that inhibitor reduced the dissolution of mild steel in sulphuric
acid by behaving as cathodic inhibitor. The adsorption of this organic molecule on mild
steel surface obeyed Temkin’s adsorption isotherm. The quantum mechanical analysis
the inhibition efficiency of the compound calculated from chemical and
Keywords : Corrosion inhibitor, Thio compounds, Impedance measurements, Adsorption
Mild steel is a main class of materials due to their extensive industrial applications. It is
used in many industries due to its excellent mechanical properties. These are employed in
industries as pipelines for petroleum industries, storage tanks, acid transporation pumps
and valves , shipment vessels and chemical batteries1 in seashore. Due to their aggresive
nature of sulphate ion , acidic solution will create damage to the steel parts . Numerous
methods are used to reduce the corrosion of steel in acidic media. Among them, the use
of inhibitors is most commonly adapted
reported as corrosion inhibitors
. Several substituted thioureas have been
in acidic media. Recently the function of thiourea
derivatives as potential corrosion inhibitors for steel in acidic media was studied by
Karthikeyan et al [7-8] . The corrosion inhibiting property of these compounds is a
structural dependent. The lone pair of electrons decides
the adsorption of inhibitor on
the metal surface. The present paper describes a study of corrosion protection effect of 1cyclohexyl-3-cyclopenta-1,3 dienyl-1,3 diphenyl thiourea on corrosion of mild steel in
2M H2SO4 using weight loss, gasometric measurements and various electrochemical
techniques. Quantum mechanical studies have been performed to validate the role of
the inhibitor through adsorption phenomena on steel surface.
But studies on the influence of 1-cyclohexyl-3-cyclopenta-1,3 dienyl-1,3 diphenyl
thiourea on hydrogen permeation through mild steel during pickling are very scant. A
good inhibitor should have the following two main prerequisites: (1) it should exhibit
very good inhibition efficiency and (2) it should lessen the hydrogen permeation current
to a considerable extent. Some organic compounds give very high values of inhibition
efficiency, but they have a negligible effect in reducing the hydrogen permeation current
and vice versa. Compounds which fall
under this class are prone to
embrittlement in a later stage by the combination of permeated atomic hydrogen. This
delayed failure creates cracking, pitting, breakage, etc., on the surface of the metal.
As far as we know no concrete report has been published so for CCDDTU in 2M
H2SO4 with use of potentiodynamic polarization, impedance measurements The structure
of the CCDDTU is given in the figure.1. Different concentrations of inhibitor were
prepared and their inhibition efficiencies in 2M H2SO4 water were investigated.
Mild steel specimens of compositions, C = 0.08%, P = 0.07%, Si = O%, S = O%,
Mn = 0.41% and Fe remainder, and of size 4 x 1 x 0.020 cm were taken for weight loss
and gasometric studies. The weight loss study  was done at room temperature for two
hours in 2M H2SO4. The inhibition efficiency (IE %) was determined by the following
equation, I.E (%)
= (W0 –Wi /W0) X 100
Where W0 & Wi are the weight loss values in the absence and presence of the compound.
A mild steel cylindrical rod of the same composition as above and embedded in araldite
resin with an exposed area of 0.283 cm2 was used for potentiodynamic polarisation and
AC impedance measurements.
H2SO4potentiodynamically (1 mV s-1) using Sinsil –Electrochemical workstation,USA.
A platinum foil and Hg/Hg2SO4/2M H2SO4 were used as auxiliary and reference
electrodes respectively. Double layer capacitance (Cdl) and charge transfer resistance
values (Rct,) were studied
using AC impedance measurements [10-13]. The hydrogen
permeation study was carried out using the methodology modified Devanathan and
Stachurski’s two compartment cell, as reported earlier.. Quantum mechanical
calculations were carried using Gaussian software . The energy of highest occupied
molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and dipole
moment (µ), were evalaueted with the above given software package.
Results and Discussion
Weight loss and Gasometric measurements
Table 1 gives the results of inhibition efficiency for different concentrations of 1cyclohexyl-3-cyclopenta-1,3 dienyl-1,3 diphenyl thiourea for the corrosion of mild steel
in 2M H2SO4 obtained from weight loss and gasometric measurements. It is observed
that the inhibitor retards the corrosion of mild steel effectively in salt water. The
inhibition of corrosion of brought about by CCDDTU can be due to the following
1.The interaction between the lone pairs of electrons of the sulfur atom of the
inhibitor and the positively charged mild steel surface .
2.The affinity of lone pairs of electrons of the nitrogen atoms and the positively
charged metal surface .
3.The presence of one cyclo hexyl and three benezene moieties in the inhibitor
which exerts inductive (+I) effect and increase the electro density on the sulfur atom that
leads to effective adsorption of CCDDTU than the unsubstituted thiourea  .
A good conformity between the values of inhibition efficiency obtained by weight loss
and gasometric methods is found.
Potentiodynamic polarization studies
The corrosion kinetic parameters such as Tafel slopes ( ba and bc ),corrosion current
(Icorr ) and corrosion potential (Ecorr ) and percentage of inhibition measured from tafel
plots for mild steel in 2M H2SO4 containing different concentrations of inhibitor are
presented in table 2.
The values of ba, bc and Icorr are agreeing well with earlier studies using thiourea
derivatives [11-13]. Also, increasing concentrations of CCDDTU enhances the Tafel
slopes values,but the values of cathodic Tafel slope bc are enhanced more. So the
inhibition of corrosion of mild steel cathodic control in the present acid media.. Values
of Ecorr are shifted to positive direction in comparision with uninhibited condition in the
presence of various
concentrations of the inhibitor. This is due to the formation of
closely adherent adsorbed film of CCDDTU on the metal surface.
Corrosion inhibition of mild steel in 2M H2SO4 solution in the presence and
absence of inhibitor was monitored by impedance spectroscopy measurements and the
results are given in table.3. At all concentrations range of inhbitor , large capacitive loops
at higher frequency range followed by small capacitive loops at lower frequency range
were visualized [14-18].. Also the values of Rct are found to increase with an increase in
concentrations of compound in 2M H2SO4
. It was noticed that values of Cdl have
lessen by increasing concentrations of compound in the acid medium.
Similar results was observed by Harikumar  and others [14-18] for the corrosion of
mild steel in acidic media using Ampicilin drug and thio compounds as inhibitors.
Hydrogen permeation measurements
Hydrogen permeation measurements results for the dissolution of mild steel in the
inhibited and inhibited conditions are presented in Table 4.Hydrogen permeation current
for un inhibited mild steel in 2M H2SO4 is more, because of the aggressive nature of
anions of the acid and also the inhibitor enhances the permeation current . The
enhancement in permeation current can be attributed to the decomposition of the
CCDDTU molecules on the steel surface by forming H2S gas [ 15-16].. Trabanelli and
Zucchi  investigated that that sulfur of hydrogen sulfide act as negative catalyst for
the recombination of hydrogen atoms into molecular hydrogen. It can be seen from the
table that the enhancement of permeation current is more due to the formation of more
number of hydrogen sulphide gas , if the concentration of inhibitor is more .
Quantum mechanical studies:
The computed quantum chemical parameters like energy of highest occupied
molecular orbital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO),
LUMO- HOMO, energy gap (∆E), dipole moment (µ), are summarized in Table 5. The
HOMO and LUMO distribution on thiourea moiety (Figure 3-4) is greater than
cyclohexyl moieties and three benzene rings of the compound. According to Tang et
al., when a molecule possess similar frontier orbitals, its inhibition efficiency can be
correlated to the energy levels of HOMO and LUMO and the difference between them. It
has been greatly claimed that, higher the value of EHOMO, greater is the ease for an
inhibitor to donate electrons to unoccupied d orbital of metal atom and higher is the
inhibition efficiency. Further lower the ELUMO, easier is the acceptance of electrons from
metal atom to facilitate good adhesion of inhibitor. The gap between HOMO–LUMO
energy levels of molecules was another important parameter that needs to be considered.
Higher the value of ∆E of an inhibitor, higher is the inhibition efficiency of that inhibitor.
It has been reported that, large values of dipole moment will enhance corrosion inhibition
1. 1-cyclohexyl-3-cyclopenta-1,3 dienyl-1,3 diphenyl thiourea inhibits the
corrosion of mild steel effectively in high aggressive acid medium.
2. The inhibition of corrosion of mild steel in 2M H2SO4 , by the compound is
under cathodic control.
3. Rct and Cdl values obtained from impedance measurements established the
impressive performance of the compound.
4. The adsorption of the compound on mild steel surface follows Temkin’s
5. Quantum mechanical studies validate the performance of CCDDTU as
excellent corrosion inhibitor for mild steel in 2M H2SO4.
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Table 1. Values of inhibition efficiency for the corrosion of mild steel in 2M H2SO4
in the presence of different concentrations of CCDDTU obtained from weight loss
and gasometric measurements.
Inhibition efficiency (%)
Weight loss Studies
Table 2: Corrosion kinetic parameters of mild steel in in 2M H2SO4 in the presence
of different concentrations of CCDDTU obtained from potentiodynamic
(mV vs SCE)
Table 3.Impedance values for the corrosion of mild steel in 2M H2SO4 in the
presence of different concentrations of CCDDTU.
Sea water solution
Charge Transfer resistance Double layer capacitance (Cdl)
Table 4. Values of permeation current for the corrosion of mild steel in 2M
sulphuric acid in the presence of different concentrations of inhibitor .
Concentration of Inhibitor Steady state permeation current (µA)
Table 5: Quantum mechanical parameters for the inhibitor
Figure 1 Structure of inhibitor
Figure 2 Temkin’s adsorption isotherm for CCDDTU in 2M H2SO4.
Figure 3. Highly occupied molecular orbital (HOMO) for CCDDTU.
Figure 4. Lowest unoccupied molecular orbital (LUMO) for CCDDTU.