Volume 18 Preprint 8
Studies on the reduction of mild steel corrosion and hydrogen permeation in acidic media
S.Karthikeyan , P.A.Jeeva, K.Raja, M.Paramasivam
Keywords: Corrosion inhibitor, Carbenicillin( CBN) , hydrogen permeation
The reduction of corrosion and hydrogen permeation through mild steel in 0.5M H2SO4 and 1M HCl has been studied using weight loss measurements and various electrochemical techniques in the presence and absence of a green inhibitor viz., Carbenicillin( CBN) . The inhibitor is found to be more inhibitive in H2SO4 than in HCl. Potentiodynamic polarization studies clearly justify the fact that this compound acted as a mixed inhibitor; but predominantly as a cathodic inhibitor. Hydrogen permeation studies and AC impedance measurements also confirm the improved performance of the compound in H2SO4. The adsorption of this compound on the mild steel surface follows Temkinâ€™s adsorption isotherm.
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Studies on the reduction of mild steel corrosion and hydrogen permeation in
S.Karthikeyan* , P.A.Jeeva, K.Raja, M.Paramasivam#
Manufacturing Division, School of Mech.and Building Sciences, VIT University,
CP-BED research lab , Batteries Division,CSIR-CECRI,Karaikudi-630006,India
The reduction of
corrosion and hydrogen permeation through mild steel in
0.5M H2SO4 and 1M HCl has been studied using weight loss measurements and
various electrochemical techniques in the presence and absence of a green
inhibitor viz., Carbenicillin( CBN) . The inhibitor
is found to be more inhibitive
in H2SO4 than in HCl. Potentiodynamic polarization studies clearly justify the
fact that this compound acted as a mixed inhibitor; but predominantly as a
measurements also confirm the
improved performance of the compound in
H2SO4. The adsorption of this compound on the mild steel surface follows
Temkin’s adsorption isotherm.
Keywords : Corrosion inhibitor, Carbenicillin( CBN) , hydrogen permeation
* corresponding author (firstname.lastname@example.org)
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 [6-10].
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 Carbenicillin( CBN)
for mild steel corrosion in 1M H2SO4. 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 H2SO4. From the literature the
higher concentration of H2SO4 acts as pickling solution for mild steel for
compounds. Use of this inhibitor in 1M H2SO4 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,
permeation studies and diffuse reflectance methods. A definite correlation
between different types of descriptors and measured corrosion
inhibition efficiency for Carbenicillin( CBN)
using chemical and electrochemical
Mild steel specimens of compositions, C = 0.07%, P = 0.08%, Si = O%, S = O%,
Mn = 0.34% and Fe remainder, and of size 5 x 2 x 0.025 cm were used for
weight loss and hydrogen permeation studies. 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 galavaostatic polarisation and AC impedance
All the inhibitors were preliminarily screened by a weight loss method described
earlier.  Both cathodic and anodic polarisation curves were recorded
galvanostatically (1 mA s-1) using corrosion measurement system BAS Model:
1OOA computerised electrochemical analyser (made in West Lafayette, Indiana)
and PL-10 digital plotter (DMP-40 series, Houston Instruments Division).
A platinum foil, Hg/Hg2Cl2/1 M HCl and Hg/HgtS04/0.5 M H2SO4 were used as
auxiliary and reference electrodes, respectively. The hydrogen permeation study
was carried out using an adaptation of the modified Devanathan and
Stachurski’s two compartment cell, as described earlier.“.‘4 Double layer
capacitance (Cdl) and charge transfer resistance values (R,) were obtained using
AC impedance measurements as described in an earlier publication.”
3. RESULTS AND DISCUSSION
3.1 Weight loss and Gasometric measurements
Table 1 gives the values of inhibition efficiency for different concentrations of
Carbenicillin( CBN) for the corrosion of mild steel in 1M HCl and 0.5 M H2SO4
obtained from weight loss and gasometric measurements.It is found that the
compound inhibits the corrosion of mild steel in both acids, but more effectively
in H2SO4 . This can be attributed to the lesser adsorption of sulphate ions on the
metal surface, thereby leaving more space for the organic molecules to get
in the of HCl the stronger adsorption of the chloride ions on the metal
surface leaves less space for organic molecules to get adsorbed. So in H2SO4
,the coverage of the metal surface by the organic molecules is significantly
more,giving rise to higher values of inhibition efficiency for all concentrations of
the compound used [9-12].
It is found that there is very good conformity between the values of inhibition
efficiency obtained by weight loss and gasometric methods.
3.2 Galvanostatic polarization studies
Table 2(a) and 2(b) give values of corrosion kinetic parameters such as Tafel
( ba and
bc ),corrosion current (I
) and corrosion potential (E
inhibition efficiency obtained from galvanostatic polarization curves for mild
steel in 1M HCl and 0.5 M H2SO4
containing different concentrations of
Carbenicillin( CBN) . It can be seen from this table that values of Tafel slopes
are very much similar to those reported earlier [12,13].
It can also be seen from these
increasing concentrations of
Carbenicillin( CBN) enhances the values of both ba and bc ,but the values of bc
are enhanced to greater extent. So the inhibition of corrosion of mild steel in
both acids is under cathodic control. Values of Ecorr is shifted to less negative
values in the presence of different concentrations of compound. This can be
ascribed to the formation of closely adherent adsorbed film on the metal
surface. The presence of increasing concentrations of Carbenicillin( CBN) brings
values in both the acids. It can also be seen that most of the values of
inhibition efficiency obtained by weight loss measurements and galvanostatic
polarization studies agree very well.
3.3 Hydrogen permeation measurements
Hydrogen permeation measurements results for the corrosion of mild steel in
the presence and absence of the inhibitor are presented in Table 3.Hydrogen
permeation current for mild steel in 1M HCl is more, because of the aggressive
nature of chloride ions.It can be seen from the table that the presence of
Carbenicillin( CBN) in both acids enhances the permeation current.
The enhancement in permeation current can be attributed to the decomposition
of the compound on the mild steel surface .In all the mechanisms suggested
so far, invariable the product of decomposition of Carbenicillin( CBN)
which is evolved on the metal surface. Its formation can be detected by
radiometric measurements,if labeled thiourea 35 s or its derivatives are used
. The whole process takes place in two stages.In the first stage,
molecules are adsorbed on the metal surface by virtue of
the interaction of lone pairs of electrons of nitrogen sulfur.In the second stage,
the adsorbed molecules of the compound slowly undergoes chemical changes.
The molecule usually decomposes with the formation of H2S by the action of
hydrogen evolved on the metal.
Hough et al  investigated that the enhanced permeation of hydrogen ions
through the metal surface in acidic solutions in the presence of thiourea and
derivatives may be due to the presence of increased concentration of surface
This can be attributed to the inhibition of the recombination of hydrogen atoms
to form hydrogen molecules. Trabanelli and Zucchi  reported that sulfur of
hydrogen sulfide can 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,if the concentration of Carbenicillin(
is more. A similar observation has been made by Lahiri etal  that
hydrogen permeation current increases with increase in the concentration of diortho tolyl thiourea.
3.4 Impedance studies
Values of charge transfer resistance (Rt )and double layer capacitance (Cdl)
derived from Nyquist plots are shown in table 4.It can be seen in table that the
values of Rt is found to increase with increase in concentration of Carbenicillin(
in both the acids. Values of double layer capacitance are found to be
more for 1M HCl than for 0.5M H2SO4 .It is found that values of Cdl are brought
down by increasing concentrations of Carbenicillin( CBN) in both the acids.
This can be attributed to increasing adsorption of the compound on the metal
surface with increase in its concentration.
A plot of surface coverage (ø ) versus log C gives a straight line showing that the
adsorption of CBN on the mild steel surface from both acids obeys Temkin’s
adsorption isotherm. This points to corrosion inhibition by this compound,
being a result of its adsorption on the metal surface.
3.5.Quantum chemical calculations
Quantum chemical calculations were carried out to investigate the
adsorption and inhibition mechanism of the inhibitors. Figure 1 shows the
optimized structure of Carbenicillin. The values of calculated quantum chemical
parameters i.e. EHOMO (highest occupied molecular orbital), ELUMO (lowest
unoccupied molecular orbital), ∆E (energy gap), µ (dipole moment), (softness)
etc. are summarized in table-5.
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 πelectrons of heterocyclic atoms and the vacant d-orbitals of the metal surface
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 (ELUMOEHOMO) 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 µ, will favours a strong interaction
of inhibitor molecule with the metal surface .
Other indicators are absolute electro negativity (), absolute hardness
(ɳ). 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 is the reciprocal of the hardness. , ɳ, 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
= I + A/ 2, ɳ = I – A / 2, = 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 Carbenicillin with low electro negativity.
The higher value of dipole moment and lower total energy for
Carbenicillin( CBN) indicates the strong interaction of inhibitor with metal that
leading to improved adsorption. The halogen atom exerts +M effect on
Carbenicillin, leads to enhanced corrosion inhibition.
From figure 2 it can be observed that the energy highly occupied molecular
orbital’s (HOMO) are localized on hetero atoms for Carbenicillin. From Figure 3
it is observed that lowest unoccupied molecular orbital’s (LUMO) of CBN, which
is responsible for the better adsorption.
1. Carbenicillin( CBN)
inhibits the corrosion of mild steel in both acids, but
shows a better performance in 0.5M H2SO4.
2. The inhibition of corrosion of mild steel in both the acids, by the compound
is under cathodic control.
3. The presence of CBN in both the acids is found to enhance the extent of
ingress of hydrogen through mild steel.
4. R t and Cdl values obtained from impedance measurements confirm the better
performance of the compound.
5. The adsorption of the compound on mild steel surface obeys Temkin’s
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Table 1. Values of inhibition efficiency for the corrosion of mild steel in 1M HCl
and 0.5 M H2SO4 in the presence of different concentrations of Carbenicillin(
CBN) obtained from weight loss and gasometric measurements.
Inhibition efficiency (%)
Table 2.a Corrosion kinetic parameters of mild steel in 1M HCl in the presence
of different concentrations of Carbenicillin( CBN)
obtained from galvanostatic
Concentration Ecorr (mV)
Tafel slopes in mV in
Table 2.b Corrosion kinetic parameters of mild steel in 1M H 2SO4 in the
presence of different concentrations of Carbenicillin( CBN)
galavanostatic polarization studies.
Concentration Ecorr (mV)
Tafel slopes in mV in
Table 3 Values of permeation current for the corrosion of mild steel in 1M HCl
and 0.5 M H2SO4 in the presence of different concentrations of Carbenicillin(
Steady state permeation current (µA)
Table 4.Impedance parameters for the corrosion of mild steel in 1M HCl and 0.5
M H2SO4 in the presence of different concentrations of Carbenicillin( CBN) .
Fig 1.Optimized structure of Carbenicillin
Fig 2.HOMO of Carbenicillin
Fig 3. LUMO of Carbenicillin