Materials Sciences and Applicatio ns, 2011, 2, 1268-1278
doi:10.4236/msa.2011.29171 Published Online September 2011 (http://www.SciRP.org/journal/msa)
Electrochemical and Quantum Chemical Studies of
5-Substituted Tetrazoles as Corrosion Inhibitors
for Copper in Aerated 0.5 M H2SO4 Solution
Pengju Liu1, Xia Fang2, Yongming Tang1, Chunning Sun1, Cheng Yao1
1College of Science, Nanjing University of Technology, Nanjing, China; 2Department of Chemistry, Jiangsu Institute of Education,
Nanjing, China.
Received May 16th, 2011; revised June 2nd, 2011; accepted June 9th, 2011.
ABSTRACT
Two 5-substituted tetrazoles, 5-(2-thienyl)-1,2,3,4-tetrazole (2-THTT) and 5-(4-pyridyl)-1,2,3,4-tetrazole (4-PYTT), as
copper corrosion inhibitors in aerated 0.5 M H2SO4 solution were studied by using potentiodynamic polarization, elec-
trochemical impedance spectroscopy (EIS) and quantum chemical calculation. Polarization curves indicate that inhibi-
tion efficiencies of tetrazoles increase with increasing inhibitors concentration. The electrochemical results show that
the inhibition efficiency of 2-THTT is higher than that of 4-PYTT. Inhibition efficiency of 2-THTT reached 98.9% at
very low concentrations (0.25 mM ) by EIS, which makes 2-THTT an efficient inhibitor in aerated 0.5 M H2SO4 solution.
The adsorption of 5-substituted tetrazoles on copper surface obeys the Langmuir isotherm. All the computed quantum
chemical parameters are found to correlate well with experimental inhibition efficiencies of inhibitors.
Keywords: Copper, Tetrazole, H2SO4, Corrosion, Inhibition
1. Introduction
As is known to all, using inhibitors is an important me-
thod of protecting materials from corrosion. Many hete-
rocyclic compounds containing sulfur, phosphorus, ni-
trogen, and oxygen atoms are efficient inhibitors of cor-
rosion for copper in acid media [1-7]. And there has been
much interest in the development of tetrazole compounds
as corrosion inhibitors. The inhibition efficiencies of
1,5-disubstitued 1,2,3,4-tetrazoles, in which the substitu-
ted groups include 1-phenyl-5-mercapto [8-13], 5-amino
[8,10,13], 1-phenyl [8,10,13], 1-methyl-5-mercapto [8],
5-(3-Aminophenyl) [14], have been studied for copper in
acidic media.
Recently, investigations of the relation between the ef-
ficiency and structural properties of inhibitor molecules
have been taken by many experimental techniques and
theoretical methods. Quantum chemistry calculations
have been widely used to study the reaction mechanisms
and to interpret the experimental results as well as to
solve chemical ambiguities [15,16]. This is a powerful
approach to investigate the mechanisms of reaction in the
molecule, and its structure and electronic parameters can
be obtained by quantum chemistry calculations [17,18].
Some quantum mechanical studies have successfully in-
terpreted the relation between the corrosion efficiency
and structural properties of inhibitor molecules [19-23].
The present work reports the inhibition of copper cor-
rosion in aerated 0.5 M H2SO4 solution by 5-(2-thienyl)-
1,2,3,4-tetrazole (2-THTT) and 5-(4-pyridyl)-1,2,3,4-te-
trazole (4-PYTT) (Figure 1). Potentiodynamic polariza-
tion, electrochemical impedance spectroscopy (EIS) and
quantum chemical calculation were used. A perusal of
literature reveals that 2-THTT and 4-PYTT have not
been reported as corrosion inhibitor. Tetrazoles studied
are expected to be good inhibitor for copper corrosion in
H2SO4 solution because they are heterocyclic compound
containing the tetrazole ring, in addition to the thienyl
and pyridyl group.
2. Experimental Method
2.1. Electrochemical Measurements
The aggressive solution were prepared by dilution of
concentrated H2SO4 (reagent-grade) with ultrapure water.
Ultrapure water was produced by New Human UP 900

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors 1269
for Copper in Aerated 0.5 M H2SO4 Solution
2-THTT 4-PYTT
Figure 1. Structures of 5-substituted tetrazoles.
(Human Corporation) Purified Water Systems. 2-THTT
and 4-PYTT were synthesized by the method reported in
the literature referenced [24].
A traditional three-electrode cell was used for electro-
chemical measurements. A platinum sheet electrode was
used for the auxiliary electrode, and the reference elec-
trode was a saturated calomel electrode (SCE) with a
Luggin capillary. All potentials were measured with re-
spect to the SCE. The working electrode was the “Gaoss
Union” (a company) customized commercial products.
And it, in the form of a rod cut from pure copper
(99.99% purity, Φ = 6 mm) sheet, had a geometric area
of 0.2827 cm2. Prior to each experiment, the electrode
was ground with various grades of grit emery paper (up
to 2000 grit), cleaned with absolute ethanol, thoroughly
washed with ultrapure water and finally dried in dry air.
All experiments were repeated three times at the desired
temperature ±1˚C.
Electrochemical experiments were carried out using a
CHI 660C electrochemical workstation. Potentiodynamic
polarization curves were recorded at a sweep rate of 1
mV/s. Electrochemical impedance spectroscopy (EIS)
measurement were carried out at open-circuit potential
(OCP). The ac frequency range extends from 100 kHz to
10 mHz with a 5 mV peak-to-peak sine wave as the ex-
citation signal. Electrochemical data were obtained after
1 h of immersion with the working electrode at the rest
potential, and all tests have been performed in non-de-
aerated solution under unstirred conditions. Electro-
chemical data were analyzed and fitted with ZSimpWin
ver. 3.51.
2.2. Computation of Quantum Chemical
Parameters
Quantum chemistry calculations were carried out by semi-
empirical PM3 method from the program package MO-
PAC2007. A full optimization of all geometrical vari-
ables without any symmetry constraint was performed at
the Restricted-Hartree-Fock (RHF) level. Molecular
structures were optimized to the gradient <0.01 in the
vacuum phase.
3. Results and Discussion
3.1. Polarization Curves
Polarization curves of the copper electrode in aerated 0.5
M H2SO4 solution at 30˚C in the absence and presence of
5-substituted tetrazoles are shown in Figure 2. Electro-
chemical parameters, i.e., corrosion potential (Ecorr), ca-
thodic and anodic Tafel slope (bc and ba) and corrosion
current density (Icorr), obtained by extrapolation of the
Tafel lines, are presented in Table 1. The inhibition effi-
ciencies (E%) of the tetrazole compounds in 0.5 M
H2SO4 are also given in Table 1, and it (E%) was calcu-
lated according to the relation [25] :

0
0
% 100
corr corr
corr
II
EI

 (1)
where 0
corr
I
and Icorr are the corrosion current densities
obtained from uninhibited and inhibited solution, respect-
tively.
As can been seen from Figure 2, both anodic and ca-
thodic reactions of corrosion process were significantly
inhibited while the tetrazole derivatives were added to
the acid solution. The addition of the compounds studied
decreased the current densities in a large domain anodic
and cathodic of potential. In literature, it has been re-
ported that [26,27] if the displacement in Ecorr
1) is >85
mV with respect to Ecorr, the inhibitor can be seen as a
cathodic or anodic type and 2) if the displacement in Ecorr
is <85 mV, the inhibitor can be seen as mixed type. In
our study, the maximum displacement in Ecorr value is 17
mV towards anodic region, which indicates that all tetra-
zoles studied are mixed-type inhibitors [28]. In naturally
aerated acidic medium, copper corrosion involves a si-
multaneous anodic dissolution and cathodic oxygen re-
duction reaction (ORR) process [29,30]. The increase in
inhibition efficiency with increasing inhibitor concentra-
tion may be attributed to the formation of a barrier film,
which prevents the attack of acid on the metal surface
[31]. From Figure 2, it also can be seen that the anodic
curves in Figure 2(a) and Figure 2(b) look different.
Those in Figure 2(a) show a step in the anodic curve
between about 0 and 0.08 V (vs SCE), and this in Figure
2(b) do not show this. The two inhibitors have a different
effect on the anodic reaction. It may be caused by a
stronger effect of adsorption of 2-THTT on copper sur-
face than 4-PYTT. This show that 2-THTT may show
better corrosion inhibition than 4-PYTT in the aerated
0.5M H2SO4 solution.
It can be observed from Table 1 that the Icorr values
decrease considerably in the presence of 2-THTT and
4-PYTT and the Icorr values have a trend to decrease
with the increasing inhibitor concentration. Correspond-
ingly, inhibition efficiencies (E%) values increase with
the increasing inhibitor concentration and then reach a
maximum value at 0.25 mM in the both cases of inhibit-
tors. Additionally, it is also evident that 2-THTT presents

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors
for Copper in Aerated 0.5 M H2SO4 Solution
1270
Table 1. Electrochemical parameters for the corrosion of copper in 0.5 M H2SO4 containing different concentrations of in-
hibitors at 30˚C.
Inhabitor Cinh (mM) Ecorr (mV) Icorr (μA·cm−2) bc (mV·dec−1) ba (mV·dec−1) IE (%)
Blank - −24 69.14 −614.25 49.75 −
0.05 −9 9.46 −456.62 52.65 86.3
0.10 −10 8.18 −474.16 75.35 88.2
0.15 −7 5.98 −352.11 66.13 91.4
0.20 −7 5.64 −378.64 72.20 91.9
2-THTT
0.25 −10 4.80 −343.05 66.53 93.1
0.05 −17 12.70 −401.61 41.16 81.6
0.10 −20 10.78 −425.71 44.13 84.4
0.15 −21 9.02 −392.93 45.10 87.0
0.20 −29 6.67 −394.48 44.77 90.4
4-PYTT
0.25 −29 5.11 −368.87 45.84 92.6
Figure 2. Potentiodynamic polarization curves for copper in the absence and presence of (a) 2-THTT and (b) 4-PYTT.
slightly better performance than 4-PYTT, which can be
correlated to the difference in the heteroatom of the sub-
stituted heterocyclic between the two molecules.
3.2. Effect of Temperature
Some information about adsorption mechanism of the
inhibitor can be obtained from the activation energy val-
ues, and the activation energy of corrosion process can
be obtained by investigating the influence of temperature
on corrosion inhibition. Effect of temperature was stud-
ied by polarization tests in the absence and presence of
0.1 mM of 2-THTT and 4-PYTT, as shown in Figure 3.
Corresponding electrochemical data are given in Table 2.
As seen from Figure 3 and Table 2, the corrosion cur-
rent density increases with the increasing temperature,
both in uninhibited and inhibited solutions. In the solu-
tion containing 2-THTT, slight changes in values of in-
hibition efficiencies are observed in the range of tem-
perature studied. Thus the compound can be regarded as
the temperature-independent inhibitors. However, from
Table 2, it is clear that the efficiency of 4-PYTT de-
creases with the increasing temperature. Therefore, 4-
PYTT should be assigned as temperature-dependent in-
hibitor.
In order to calculate the activation energy of the corro-
sion process, Arrhenius equation was used [32]:
exp a
corr
E
Ik RT






(2)
where k is the pre-exponential factor, Ea is the apparent

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors 1271
for Copper in Aerated 0.5 M H2SO4 Solution
Table 2. Electrochemical parameters and activation parameters of the corrosion of coppr in 0.5 M H2SO4 solution in the ab-
sence and presence of 0.1 mM inhibitor.
Inhibitor Tmp (˚C) Ecorr (mV) Icorr (μA·cm−2) IE (%) Ea (kJ·mol−1)
25 −28 59.42 -
30 −24 69.14 -
35 −21 97.44 -
Blank
40 −23 119.22 -
37.73
25 −13 5.98 89.9
30 −10 8.18 88.2
35 −7 11.10 88.6
2-THTT
40 −12 14.60 87.8
46.32
25 −23 7.23 87.8
30 −20 10.78 84.4
35 −16 17.01 82.5
4-PYTT
40 −14 25.78 78.4
66.28
Figure 3. Effect of temperature on the cathodic and anodic responses for copper in 0.5 M H2SO4 + 0.1 mM of (a) 2-THTT and
(b) 4-PYTT.
activation energy of the corrosion process, R is the gas
constant and T is the absolute temperature. Figure 4
shows Arrhenius plots of the logarithm of the current
density vs 1/T for copper in the corrosive medium with
and without addition of 0.1 mM of 2-THTT and 4-PYTT.
Straight lines are obtained with a slope of (−Ea/R). Acti-
vation parameters obtained from this graph are given in
Table 2.
It is well known that unchanged or lower values of Ea
in inhibited systems compared to the blank indicate the
chemisorption mechanism, while higher values of Ea
suggest the physical adsorption mechanism [33-37].
From Table 2, it is observed that the values of Ea ob-
tained in presence of 2-THTT and 4-PYTT increase sig-
nificantly compared to the values of Ea in inhibitor-free
solution. This indicates a physisorption mechanism of the
two 5-substituted tetrazoles on copper surface that occurs
in the first stage [38-40]. The increase in activation en-
ergy can be attributed to an appreciable decrease in the
adsorption of the inhibitor on the copper surface with the
increase in temperature [39,41-43].
3.3. Electrochemical Impedance Spectroscope
(EIS)
The corrosion of copper in aerated 0.5 M H2SO4 solution
in the presence 2-THTT and 4-PYTT was investigated by

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors
1272
for Copper in Aerated 0.5 M H2SO4 Solution
Figure 4. Arrhenius plots for copper in 0.5 M H2SO4 and
0.5 M H2SO4 + 0.1 mM inhibitors.
EIS at 30˚C after 1 h of immersion. Nyquist plots of
copper in the absence and presence of the tetrazole com-
pounds are presented in Figure 5. Obviously, the pre-
sence of the tetrazoles leads to changes of the impedance
diagrams both in shape and size. A depressed high-fre-
quency capacitive loop, followed by an obvious low-
frequency diffusion behavior which indicates the diffu-
sion of O2, is observed in Figure 5 in the blank solution;
otherwise, the depressed capacitive loop seems to be
coupled with low-frequency diffusion behavior in Figure
5. By comparing the Nyquist spectra obtained in the
presence or absence of the tetrazoles, it is found that the
Warburg impedance disappeared in the solution contain-
ing 2-THTT. However, the Warburg impedance still ex-
isted in the solution containing 4-PYTT.
The cathodic reaction that occurs on the copper sur-
face in 0.5 M H2SO4 is the oxygen reduction, since Cu is
nobler than H+ in the electromotive series, a cathodic
reaction other than the displacement of H+ can account
for metal dissolution. In addition, the appearance of the
Waburg impedance is due to the diffusion of oxygen fol-
lowed by its reduction reaction in aerated 0.5 M H2SO4.
This is readily available in terms of O2 reduction from
solution [44],
2
O4H4e2HO


2

, (3)
While, the anodic reaction occurs through the dissolu-
tion of copper on two steps as follows [45],
CuCu e

 (fast) (4)
2
CuCu e

 (slow) (5)
From Figure 5, it may be indicated that the corrosion
of the copper in the presence of 2-THTT is mainly con-
Figure 5. Nyquist diagrams for copper in 0.5 M H2SO4 in
the absence and presence of (a) 2-THTT and (b) 4-PYTT.
trolled by a charge transfer process [46,47]. The presence
of 2-THTT inhibits significantly the diffusion of O2 from
bulk solution to the interface of copper/solution. In addi-
tion, from the Nyquist spectra it can be concluded that
the corrosion of the copper in the blank solution and in
the solution containing 4-PYTT are mixed control pro-
cesses by activation and diffusion. And the addition of
4-PYTT don’t effectively prevent the diffusion of O2
from bulk solution to the interface of copper/solution.
This may be one interpretation that 2-THTT has a higher
inhibition efficiency than 4-PYTT.
The impedance spectrum displays a high frequency
capacitive loop and the Warburg impedance in the low
frequency can be analyzed with the equivalent circuit in
Figure 6(a) [48], in which Rt represents the charge
transfer resistance, W the Warburg impedance and Rs the
solution resistance. A constant phase element (CPE) is
substituted for the capacitive element to give a more ac-
curate fit [49], as most capacitive loops are depressed
semi-circles rather than regular semi-circles. The imped-

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors
for Copper in Aerated 0.5 M H2SO4 Solution
1273
the charge transfer resistance as follows:

0
% 100
tt
t
RR
ER


(8)
Figure 6. (a) The equivalent circuit to fit the EIS for copper
displaying a Warburg impedance ; (b) the equivalent circuit
to fit the EIS for copper displaying one capacitive loop.
where and
Rt are the charge transfer resistance val-
ues without and with inhibitor, respectively. The same
trend about inhibition efficiencies was obtained from the
two methods, Polarization curves and EIS results. The
different values of inhibition efficiency were obtained
from two methods and it may be attributed to the differ-
ent surface status of the electrode in two measurements
[42]. EIS measurements were performed at the rest po-
tential, while in polarization measurements the electrode
potential was polarized to high overpotential, non-uni-
form current distributions, solution conductivity, counter
and reference electrode placement, etc., all this will lead
to the difference between the electrode area actually un-
dergoing polarization and the total area [51].
0
t
R
ance spectra displaying one capacitive loop may be ana-
lyzed with the electrical circuit in Figure 6(b) [48]. The
physical meaning of Rs, Rt and CPE is the same with
what they express in Figure 6(a).
The CPE is a special element whose impedance value
is a function of the angular frequency, ω, and whose
phase is independent of the frequency. The impedance of
a constant phase element is described by the expression:

1
0
n
CPE
ZYj 


 (6)
where Y0 is a proportional factor, ω the angular frequency
and n the meaning of a phase shift. For n = 0, CPE
represents a resistance, for n = 1 a capacitance, for n =
0.5 a Warburg element and for n = −1 an inductance.
According to C.H. Hsu, F and Mansfeld [50], the values
of the double-layer capacitance (Cdl) can be obtained
from the equation:
As it can be seen from Table 3, the Cdl tends to de-
crease with the increase of the concentration. Rt tends to
increase with the increase of the concentration. Com-
pared with 4-PYTT, the more pronounced effect and
higher charge transfer resistance have been found in the
case of 2-THTT. This phenomenon may indicate that, for
5-substituted tetrazoles compounds, the inhibition ability
depends on the position of the heteroatom and the types
of the heterocycle on the substituent.

1
0
n
dl m
CY 



 (7)
where m

 is the angular frequency at which the imagi-
nary part of the impedance has a maximum.
The decrease in the Cdl, which can result from the de-
crease in local dielectric constant and/or the increase in
the thickness of the electrical double layer, suggests that
the tetrazoles molecules take the place of water mole-
cules, and then function by adsorption at the metal/solu-
Charge transfer resistance values (Rt) and double layer
capacitance values (Cdl) were obtained and shown in Ta-
ble 3. The inhibition efficiency (E%) was calculated by
Table 3. Parameters of fitted the EIS for copper in 0.5 M H2SO4 containing different concentrations of inhibitors at 30˚C.
Inhibitor Cinh (mM) Rs (Ωcm2) Rt (Ωcm2) Cdl (μF·cm−2) W (mΩ·cm2) E (%)
Blank - 0.84 123.3 133.70 28.14 -
0.05 0.80 2905 25.59 - 95.8
0.10 0.77 5658 15.00 - 97.8
0.15 0.82 5873 14.41 - 97.9
0.20 0.75 8091 9.52 - 98.5
2-THTT
0.25 0.73 10860 7.49 - 98.9
0.05 0.71 516.1 67.45 17.49 76.1
0.10 0.80 544.9 67.23 12.08 77.4
0.15 0.82 774.5 61.80 8.01 84.1
0.20 0.82 1084 55.24 4.54 88.6
4-PYTT
0.25 0.83 1318 40.08 3.11 90.6

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors
1274
for Copper in Aerated 0.5 M H2SO4 Solution
tion interface [52].
3.4. Adsorption Isotherms
Basic information on the interaction between the inhibit-
tor and the metal surface can be provided by the adsorp-
tion isotherm, which depends on the degree of electrode
surface coverage (θ). The EIS data were used to evaluate
the surface coverage values as follows:

 
, 0
01
dl dl
dl dl
CC
CC
 





 (9)
where

0dl and

1dl are the double layer capa-
citances of the inhibitor-free and entirely inhibitor-cove-
red surfaces, respectively, ,dl
C
 C  
C

is the composite total
double layer capacitance for any intermediate coverage
(θ). Because of the small permittivity of organic com-
pound,
 1dl approximates to zero. The surface co-
verage values (θ) were tested graphically to allow fitting
of a suitable adsorption isotherm including Langmuir,
Fumkin, Temkin, etc. The plots of Cinh/θ against Cinh for
2-THTT and 4-PYTT give straight lines with almost unit
slope (Figure 7 and Table 4). The correlation coeffi-
cients of 2-THTT and 4-PYTT are 0.9997 and 0.9666,
correspondingly. It is to say that their correlation coeffi-
cients are both
C
 
greater than 0.95. This indicates that the
tetrazole compounds obey the Langmuir adsorption iso-
therm on the copper in 0.5M H2SO4:
1
inh
inh
KC
K
C
  (10)
where K is the equilibrium constant of the adsorption
process. The free energy of adsorption can be
calculated from the equation: ads
G
1exp
55.5
ads
G
KRT






(11)
As it can be seen from Table 3, the negative ads
G

values of the tetrazole compounds are obtained, indicat-
ing that the adsorption process takes place spontaneously.
It is well known that values of ads of the order of
20 kJ·mol−1 or lower indicate a physisorption mechanism;
those of the order of 40 kJ·mol−1 or higher involve charge
sharing or a transfer from the inhibitor molecules to the
metal surface to form a co-ordinate type of bond [53].
G
The ads
G

values of 2-THTT and 4-PYTT are −38.7
kJ·mol−1 and −34.5 kJ·mol−1 respectively. This indicates
that the adsorption mechanism of 2-THTT and 4-PYTT
on copper in 0.5 M H2SO4 is not typical chemisorption.
Combined with the analysis to polarization curves, it may
be concluded that the adsorption of 2-THTT and 4-PYTT
is a combination of physisorption and chemisorption.
3.5. Quantum Chemical Calculation
In order to study the effect of molecular structure on the
inhibition efficiency, quantum chemical calculations
were performed to use semi-empirical PM3 method. The
optimized geometry of molecules is shown in Figure 8.
The computed quantum chemical properties such as en-
ergy of highest occupied molecular orbital (EHOMO), en-
ergy of lowest unoccupied molecular orbital (ELUMO), the
energy band gap (E

) and dipole moment (μ) are sum-
marized in the Table 5.
EHOMO often indicates the electron donating ability of
the molecule and the inhibition efficiency increases with
the increasing EHOMO values. High EHOMO values indicate
that the molecule has a tendency to donate electrons to
appropriate acceptor molecules with low energy empty
molecular orbitals. The increasing values of the EHOMO
facilitate adsorption by influencing on the transport pro-
cess through the adsorbed layer [54]. ELUMO indicates the
ability of the molecules to accept electrons. The lower
value of ELUMO, the more probable the molecule accepts
Figure 7. Langmuir adsorption plots of the inhibitors on
copper in 0.5 M H2SO4.
Table 4. The results of Langmuir isotherm for the inhibitors.
Inhibitors Fitting equations Correlation coefficient ΔGads (kJ·mol−1)
2-THTT Cinh/θ = 1.19 × 10−5 + 1.02 Cinh 0.9997 −38.7
4-PYTT Cinh/θ = 6.27 × 10−5 + 1.28 Cinh 0.9666 −34.5

Electrochemical and Quantum Chemical Studies of 5-Substituted Tetrazoles as Corrosion Inhibitors 1275
for Copper in Aerated 0.5 M H2SO4 Solution
(a) (c)
(b) (d)
Figure 8. Optimized structures of the tetrazoles (a) is verti-
cal view of 2-THTT and (b) is plan view of 2-THTT; (c) is
vertical view of 4-PYTT and (d) is plan view of 4-PYTT).
Table 5. Calculated quantum chemical parameters of the
tetrazole compounds.
compounds EHOMO (eV) ELUMO(eV) ΔE (eV) μ (Debye)
2-THTT −9.89 −1.47 8.42 6.21
4-PYTT −10.42 −1.41 9.01 4.24
electrons [7]. The energy gap between LUMO and
HOMO (
L
UMOHOMO ) is a parameter that the
smaller value causes higher inhibition efficiencies of the
molecule [7,55]. For the dipole moment (μ), highervalues
will favorite enhancement of corrosion inhibition [52]. It
is reported that excellent corrosion inhibitors are usually
those organic compounds which not only offer electrons
to the unoccupied d orbital of the metal, but also accept
free electrons from the metal into anti-bonding orbital to
form a feedback bond [56,57].
EE E 
It is clear from Figure 8 that both of the two tetrazoles
studied are planar structures. The planar structure can
provide a larger coverage area and so it can contribute to
the adsorption of tetrazole on copper surface.
It is clear from Table 5 that 2-THTT has the higher
values of EHOMO, μ and the lower values of ELUMO, E

than 4-PYTT. These Quantum chemical parameters in-
dicate that 2-THTT has higher inhibition efficiency than
4-PYTT. The result is consistent with the discussion in
Subsection 3.1 and 3.3. The inhibition efficiency is asso-
ciated with the changes of the EHOMO and ELUMO, which
suggests that 2-THTT is perhaps the better acceptor of
the electron as well as the better donor of the electron
than 4-PYTT. Due to the stronger electron-withdrawing
effect of thienyl than pyridyl, the dipole moments of
2-THTT is higher than that of 4-PYTT. And because of
that, a correlation between the dipole moments of the
molecules and inhibition efficiencies is observed. The
energy gap between LUMO and HOMO () is also-
found to correlate well with inhibition efficiencies of
inhibitors. Thus we can conclude that energy of highest
occupied molecular orbital (EHOMO), energy of lowest
unoccupied molecular orbital (ELUMO), the energy band
gap () and dipole moment (μ) are found to correlate
well with experimental inhibition efficiencies of inhibit-
tors.
E
E
4. Conclusions
The following conclusions can be drawn from this study.
1) Both of the two 5-substituted tetrazoles show effi-
cient inhibition properties for the corrosion of copper in
0.5 aerated M H2SO4 solution, but 2-THTT shows better
performance. The inhibition efficiency increases with the
increasing concentration of the inhibitors and inhibition
efficiency of 2-THTT reaches 93.1% at 0.25 mM.
2) Polarization curves measurements show that both
the two 5-substituted tetrazoles are mixed-type inhibitors.
3) Results of EIS suggest that the corrosion of the
copper in the presence of 2-THTT is mainly controlled
by a charge transfer process. The corrosion of the copper
in the blank solution and in the solution containing
4-PYTT is a mixed control process by activation and
diffusion. In addition, the appearance of the Waburg im-
pedance is due to the diffusion of oxygen followed by its
reduction reaction.
4) The adsorption of 5-substituted tetrazoles on the
copper/solution interface obeys the Langmuir adsorption
isotherm model. The negative ads values indicate
that the adsorption of 5-substituted tetrazoles on the cop-
per surface in aerated 0.5 M H2SO4 solution is spontane-
ous.
G
5) The quantum chemical calculations show that en-
ergy of highest occupied molecular orbital (EHOMO), en-
ergy of lowest unoccupied molecular orbital (ELUMO), the
energy band gap (E

) and dipole moment (μ) are found
to correlate well with experimental inhibition efficiencies
of inhibitors.
5. Acknowledgements
This work is supported by the Open Fund of the State
Key Laboratory of Materials-Oriented Chemical Engi-
neering (KL09-9), the postgraduate practice innovation
fund of Jiangsu province, and the doctor thesis innova-
tion fund of Nanjing University of Technology.
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