1. Monatshefte fuÈr Chemie 132, 245±258 (2001)
Inhibition Effect of Hydantoin Compounds
on the Corrosion of Iron in Nitric
and Sulfuric Acid Solutions
Loutfy H. Madkour, Amera M. Hassanein, Mohamed M. GhoneimÃ,
and Safwat A. Eid
Chemistry Department, Faculty of Science, Tanta University, 31527 Tanta, Egypt
Summary. The inhibition of corrosion of iron in 2 M nitric acid and 2 M sulfuric acid solutions by
substituted phenylhydantoin, thiohydantoin, and dithiohydantoin compounds was measured using
thermometric, weight loss, and polarization methods. The three methods gave consistent results. The
polarization curves indicated that the hydantoin compounds act as mixed-type inhibitors. The
adsorption of the inhibitors were found to obey the Temkin adsorption isotherm. The higher inhibition
ef®ciency of the additives in nitric with respect to sulfuric acid solution may be attributed to the
reduced formation of soluble quaternary nitrogen salts in nitric acid medium, favouring adsorption of
the parent additive on the metal surface. The obtained results indicate that the corrosion rate of iron in
both acids increases with increasing temperature, both in absence and presence of the tested
inhibitors. Kinetic-thermodynamic model functions and Temkin isotherm data are compared and
discussed. The synergistic effect of halide anions on the inhibition ef®ciency of the hydantoin
compounds was also investigated.
Keywords. Acid corrosion; Inhibition; Iron; Hydantoins; Synergistic effect.
Introduction
One of today's most important considerations in industry is the reduction of overall
costs by protection and maintenance of materials used. Because iron is the back-
bone of industrial constructions, the inhibition of iron corrosion in acidic solutions
has been studied in considerable detail. Many N-heterocyclic compounds with polar
groups and/or %-electrons are ef®cient inhibitors for iron corrosion in acidic media
[1]. These organic molecules can adsorb on the metal surface, forming a bond
between the electron pair of the nitrogen and/or the %-electron cloud and the metal,
thereby reducing the corrosive attack [1, 2]. It has also been reported that the
inhibition ef®ciency of sulfur-containing compounds is superior to that of nitrogen-
containing ones [3]. In nearly all cases there was evidence of chemisorption of the
inhibitor, and the inhibitors were of a mixed type, i.e. both the anodic and cathodic
polarization processes were affected. No research work is available in the literature
à Corresponding author

2. to date about the application of hydantoin compounds as inhibitors for surface metal
corrosion. Accordingly, the objective of this work is to study the hydantoin com-
pounds effects toward corrosion process of iron in nitric and sulfuric acid solutions
and also to determine the adsorption isotherms and to compare them with kinetic-
thermodynamic models of corrosion inhibition.
Results and Discussion
Thermometric measurements
The temperature change of the system involving iron in 2 M HNO3 or 2 M H2SO4
was followed in the absence as well as in the presence of different concentrations of
compound 3 as an example (Fig. 1). Upon increasing the concentration of the
additive, the time required to reach Tmax increases. This indicates that the inhibitor
retards the dissolution of iron in both corrosive acidic media, presumably by
Fig. 1. Temperature vs. time curves of iron corrosion in 2 M HNO3 in the absence and in the
presence of different concentrations of 3
246 L. H. Madkour et al.

3. adsorption onto the surface of the metal. The extent of retardation depends on the
degree of coverage of the metal surface with the adsorbate. The temperature vs. time
curves provide a means of differentiating between weak and strong adsorption [4].
Strong adsorption is noted in both acidic solutions, since a simultaneous increase in
t and a diminution in Tmax takes place, and both factors cause a large decrease in RN
(reduction in reaction number) of the system. The results reported in Table 1 reveal
that the inhibition ef®ciency of the additive, as determined from the percentage
reduction in RN, increases with increasing concentration of additives. Figure 2
shows the relation between % RN and the molar concentration of different additives.
The curves obtained are invariably sigmoid in nature, substantiating the idea that the
present inhibitors retard the corrosion rate by adsorption according to the Temkin
isotherm [5] (Eq. (1)).
ˆ c1 Á ln…c2 Á c† …1†
Fig. 2. Effect of additives concentration on % reduction in reaction number (% RN) of iron corrosion
in 2 M HNO3
Hydantoin Derivatives as Corrosion Inhibitors 247

4. In Eq. (1), c is the concentration of the additive in the bulk of the solution, is the
degree of coverage of the investigated metal surface by the adsorbed molecules,
and c1 and c2 are constants. The order of increasing the inhibition ef®ciency of the
hydantion compounds as determined by % RN is 3 2 4 1 6 5 7.
Weight loss measurements
Figure 3 shows the effect of the time of immersion on the corrosion of iron in 2 M
H2SO4 solution in the absence as well as in the presence of different amounts of 3.
The curves obtained in the presence of additives fall below that of the free acid. The
Table 1. Effect of concentration 3 on the thermometric parameters of Fe in 2 M HNO3
c/mol Á ia
C maxa
C t/min Át/min log RNa
CÁ % Red Á
dmÀ3
…Átamin† minÀ1
in RN
0 35.0 59.0 60 0.400
5 Â 10À7
35.0 45.9 75 15 1.18 0.686 0.150 62.5
1 Â 10À6
35.0 44.0 75 15 1.18 0.749 0.120 70.0
5 Â 10À6
34.8 44.0 90 30 1.48 0.791 0.100 75.0
1 Â 10À5
35.1 42.2 90 30 1.48 0.835 0.078 80.5
5 Â 10À5
34.9 39.8 105 45 1.65 0.899 0.048 88.0
1 Â 10À4
35.0 40.1 120 60 1.78 0.921 0.041 89.8
Fig. 3. Weight loss vs. time curves of iron corrosion in 2 M H2SO4 in the absence and in the presence
of different concentrations of 3 at 303 K
248 L. H. Madkour et al.

5. weight loss of iron depends on type and concentration of the additive in a similar
way as thermometric measurements do. The ef®ciency of the inhibitors under
investigation increases in the order 3 2 4 1 6 5 7. The inhibition effect may
be explained by considering the adsorption of the hydantoin molecules (with high
negative charge density at the hetero atom) on the metal surface [6, 7] consisting of
iron atoms with incomplete d shells [8]. Also, formation of a metal-inhibitor com-
plex on the corroding iron surface (surface chelation) may play a role [9].
Polarization measurements
Anodic and cathodic polarization of iron was carried out under potentiostatic
conditions in 2 M HNO3 and 2 M H2SO4 in the absence as well as in the presence of
different concentrations of inhibitor at 303 K. Figure 4 shows the polarization curves
of iron in 2 M nitric acid solution at different concentrations of 3; the results
obtained for the other compounds were quite similar. The inhibition ef®ciency
depends on many factors including number of adsorption sites or functional groups,
basicity, and molecular size. In the present case, the N-atom and the O-atom
probably act as the centers of adsorption, their basicity being affected by the
character of the substituents in -position. The results show that the inhibitive
power increases with increasing chain length: 3-carbethoxy-1-phenylhydantoin (3)
is found to be the most ef®cient inhibitor. This may be attributed to the presence of
the carbethoxy group which increases the electron density on the molecule and
provides an active adsorption center (oxygen atom) in addition to the two nitrogen
centers already present.
Fig. 4. Potentiostatic polarization curves of iron in 2 M HNO3 in the absence and in the presence of
different concentrations of 3
Hydantoin Derivatives as Corrosion Inhibitors 249

6. On the other hand, 1,3-dimethyl-5-phenylazo-2-thiohydantoin (6) has the lowest
inhibition ef®ciency owing to the formation of an iron complex (see formula)
which is obviously less adsorbed in acidic solutions. This interpretation is
supported experimentally by spectroscopic analysis (mainly ultraviolet spectra).
The Temkin adsorption isotherm is found to be ideally obeyed in acidic solutions
(Fig. 5), indicating that the main inhibition process takes place through adsorption
[10, 11].
The degree of surface coverage () by the adsorped molecules was calculated
from Eq. (2), where u0 and u are the dissolution rates of iron in the absence and in
the presence of hydantoins, respectively.
ˆ …1 À uau0† …2†
From Table 2 it can be seen that the three different techniques afford the same
results for the inhibition of corrosion of iron in both acidic media.
Fig. 5. Variation of iron surface coverage () with the logarithmic concentration of different additives
in 2 M HNO3 at 303 K
250 L. H. Madkour et al.

7. Effect of temperature
The effect of temperature on the rate of corrosion of iron in 2 M HNO3 and 2 M
H2SO4 containing 1 Â 10À5
M hydantoin was studied in the temperature range of
303±323 K. The corrosion rate increases with increasing temperature in the absence
as well as in the presence of the inhibitors. The increase of icorr is due to the absence
of a protective layer at the iron surface. Thus, the increase of temperature enhances
both the iron dissolution and the additive desorption processes without leading to
Fe(II)-hydantoin complex formation. The protective layer decreases as the
temperature increases.
The Arrhenius parameters as purely empirical quantities enable us to discuss the
variation of rate constants with temperature. It was found experimentally that a plot
of lnk against 1aT gives a straight line according to Eq. (3) [5].
lnk ˆ lnA À EaaRT …3†
The activation energies calculated from the slopes of lnIcorr vs. 1aT plots (Fig. 6)
are reported in Table 3.
The enthalpy change of activation (ÁHz
) can be calculated from Eq. (4), the
free energy change of activation (ÁGz
) is obtained from the Eyring equation [12]:
(Eq. (5)). Another convenient form of Eq. (5) is Eq. (6).
ÁHz
ˆ Ea À RT …4†
k ˆ
kBT
h
eÀÁGz
aRT
…5†
ÁGz
ˆ RT Á
lnkBT
h
À lnk
…6†
From ÁHz
and ÁGz
, the entropy change of activation (ÁSz
) can be obtained
according to Eq. (7).
ÁSz
ˆ …ÁHz
À ÁGz
†aT …7†
Table 2. Comparison between the inhibition ef®ciency of 1±7 in 2 M acid solutions as determined by
thermometric, weight loss, and polarization methods (1 Â 10À4
M inhibitor, 303 K)
% Inhibition
Inhibitor Thermometric Weight loss Polarization
HNO3 H2SO4 HNO3 H2SO4 HNO3 H2SO4
1 71.4 74.4 78 73 75.7 88.2
2 85.5 80.9 87 82 90.9 90
3 92.1 88.3 90 85 93.8 91
4 77.3 77.1 85 76 84.8 90
5 62.5 60.1 74 66 69.6 87.3
6 64.9 64.9 76 63 74.1 88.1
7 53.8 53.1 69 59 68.3 81.4
Hydantoin Derivatives as Corrosion Inhibitors 251

8. Fig. 6. Arrhenius plot of the current corrosion rate constant (icorr) vs. 1aT of iron in (a) 2 M HNO3
and (b) 2 M H2SO4 in the absence and in the presence of 1 Â 10À5
M hydantoin inhibitors
252 L. H. Madkour et al.

9. From the thermodynamic parameters in Table 3 it can be seen that Ea increases as
the inhibition ef®ciency of the additives increases. This suggests that the process is
controlled by a surface reaction, since the energy of activation for the corrosion
process is above 20 kJ Á molÀ1
[13]. The Ea value for iron dissolution in 5 M H2SO4
has been reported as 20X2 kJ Á molÀ1
[12] and 51.4 kJ molÀ1
[14±16]. For iron in 3 M
HCl and 1 M HNO3, an Ea values of 11.8 kJ Á molÀ1
has been reported [17], whereas
in 2 M HNO3 20.06 kJ Á molÀ1
have been found [18].
Generally, one can say that the nature and the concentration of the electrolyte
greatly affect the activation energy of the corrosion process. The presence of the
inhibitor causes a change in the value of the apparent activation energy. Thus, it
indicates no change in the rate-determining step brought about by the presence of
the hydantoin inhibitor.
Kinetic-thermodynamic model of corrosion inhibition
To evaluate the kinetic parameters and correlate them to the corrosion inhibition
mechanism, it is of value to analyze the kinetic data obtained in the presence of
hydantoin inhibitors from the standpoint of the generalized mechanistic scheme
proposed by El-Awady et al. [19, 20]. Table 4 comprises the values of 1ay which
give the number of active sites occupied by a single organic molecule; K is the
binding constant [21]. The values of B (equilibrium constant) and f (lateral
interaction parameter, 1/c1) are also reported in Table 4. The large negative values
of ÁG
indicate that the reaction proceeds spontaneously and is accompanied by a
high ef®cient adsorption. Large values of K and B (c2) point to better inhibition
ef®ciency of the tested hydantoin compounds, i.e. stronger electrical interaction
between the double layer at the phase boundary and the adsorbing molecules. In
general, the equilibrium constant of the adsorption process was found to rise with
increasing inhibition ef®ciency.
Table 3. Activation energy (Ea), enthalpy change (ÁHz
), free energy change (ÁGz
), and entropy change (ÁSz
) for the
dissolution of Fe in 2 M acid in the presence of 1 Â 10À5
M inhibitor at 313 K
Inhibitor
Ea
kJ Á molÀ1
ÁHz
kJ Á molÀ1
ÁGz
kJ Á molÀ1
ÀÁSz
J Á KÀ1
Á molÀ1
HNO3 H2SO4 HNO3 H2SO4 HNO3 H2SO4 HNO3 H2SO4
No 8.98 7.27 6.38 4.67 71.54 71.68 208.18 214.09
1 40.15 23.84 37.55 21.24 75.55 72.82 121.40 164.79
2 38.97 30.15 36.37 27.55 76.35 73.38 127.73 146.42
3 38.40 32.07 35.80 29.47 76.65 73.63 130.51 141.09
4 43.29 27.56 40.69 24.96 75.81 73.14 112.20 153.93
5 30.71 20.90 28.11 18.30 74.77 72.50 149.07 173.16
6 29.47 22.35 26.87 19.75 75.26 72.56 154.60 168.72
7 23.92 20.31 21.32 17.71 74.45 72.36 169.74 174.60
Hydantoin Derivatives as Corrosion Inhibitors 253

10. Synergistic effects of halide anions
The effects of IÀ
, BrÀ
, and ClÀ
on the polarization curves of iron at 298 K in the
absence and presence of hydantoin inhibitors in 0X5 M H2SO4 solution were studied.
The in¯uence on the inhibition ef®ciency was observed in presence of each anion
alone or in the presence of any of the different additives together with the anion.
The extent of the effect follows the order IÀ
BrÀ
ClÀ
. Since the iodide ions have
a higher inductive effect than BrÀ
and ClÀ
[22], they are less attached to the metal
surface and, consequently, easily displaced by the inhibitor molecules. The net
increment of the inhibition ef®ciency is de®ned in Eq. (8), where Px and Pinh are the
protection ef®ciency of the anion and inhibitor, respectively, and Ptot is the total
protection ef®ciency of the corrosive medium containing the anion and inhibitor
together.
ÁP ˆ Ptot À …Px ‡ Pinh† …8†
The correlation of ÁP vs. logc for the applied synergistic anions in acidic solution
was studied. Since the halide anions interact strongly with the iron surface due to
chemisorption [23, 24], the inhibitory effect is strengthened due to the coadsorption
of the anions. As a result, the surface coverage area …ÁP† and, consequently, the
inhibition ef®ciency both increase. KI is the most effective among the investigated
salts. Addition of 10À3
M KI in the presence of a very low concentration
…1 Â 10À5
M† of 3 rises the inhibition ef®ciency from 48 to 93% as shown in
Table 5.
Table 4. Curve ®tting of data to the kinetic-thermodynamic model (r ˆ 0X94) and the Temkin isotherm for hydantoin
inhibitors in 2 M acid at 303 K
Kinetic model Temkin isotherm
Medium 1ay K
ÀÁG
kJ Á molÀ1
1
C1
C2 Â 104 ÀÁG
kJ Á molÀ1
1 HNO3 12.50 1X30 Â 109
41.96 7X20 Â 10À4
133.10 24.46
H2SO4 6.67 5X47 Â 108
39.82 1X51 Â 10À3
16.72 19.44
2 HNO3 10.77 1X63 Â 1010
53.93 1X32 Â 10À2
254.61 26.03
H2SO4 5.88 9X63 Â 1010
52.62 2X56 Â 10À3
141.25 24.60
3 HNO3 8.43 1X01 Â 1012
58.44 6X43 Â 10À2
258.67 26.09
H2SO4 8.24 5X02 Â 1011
56.71 3X35 Â 10À2
243.60 25.93
4 HNO3 9.10 1X90 Â 109
42.90 1X12 Â 10À3
167.49 25.02
H2SO4 7.14 5X57 Â 109
45.57 1X85 Â 10À3
32.69 21.06
5 HNO3 4.79 9X50 Â 108
41.19 2X16 Â 10À4
87.70 23.45
H2SO4 10.75 1X15 Â 108
35.96 6X56 Â 10À4
2.72 15.04
6 HNO3 8.33 1X20 Â 109
41.77 4X90 Â 10À4
118.85 24.19
H2SO4 5.88 3X59 Â 108
38.78 1X26 Â 10À3
13.55 18.93
7 HNO3 3.96 2X30 Â 108
37.64 9X36 Â 10À5
68.07 22.84
H2SO4 5.56 1X69 Â 107
31.21 1X76 Â 10À4
2.30 14.63
254 L. H. Madkour et al.

11. Experimental
Iron specimens and electrolytes
Iron specimens (0.16% C, 0.05% Si, 0.37% Mn, 0.015% S) were used in the present study. Prior to
each experiment, the electrodes were mechanically polished with successive grades of emery paper,
degreased in pure acetone, washed in running bidistilled water, dried, and weighed before being
inserted in the cell to remove any oxide layer or corrosion product from the surface [25, 26]. 2 M
nitric and 2 M sulfuric solutions were prepared by diluting Analar reagents by bidistilled water.
Additives
The structural formulae of the investigated hydantoin derivatives 1±7 are given below.
The compounds were prepared according to methods reported in the literature [27±29]. Their
purity was checked by melting point determinations and spectroscopy. The hydantoin solutions were
prepared by dissolving the appropriate amount of compound in 25 cm3
Analar EtOH. The desired
volume of the free inhibitor was added to the electrolyte solution. The solvent effect must be
considered by mixing a de®nite volume of EtOH to the free acid to reach a constant ratio of EtOH in
each test in the absence and presence of different concentrations of inhibitor.
Thermometric measurements
The reaction vessel used was basically the same as that described by Mylius [30]. An iron piece
…1 Â 10 Â 0X1 cm† was immersed in 30 cm3
of either 2 M HNO3 or 2 M H2SO4 in the absence and
presence of additives, and the temperature of the system 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 [4, 30]. The reaction number (RN) is de®ned as given in Eq. (9) [31].
RN ˆ …Tmax À Ti†at …9†
Tmax and Ti are the maximum and initial temperatures, respectively, and t is the time (in minutes)
required to reach the maximum temperature. The percent reduction in RN [32] is then given as
……RNfree À RNinh†aRNfree† Â 100.
Table 5. Electrochemical parameters of Fe in the presence of 10À3
M KI and different concentrations of 3 in 0X5 M H2SO4
at 298 K
logC
mol Á dmÀ3
Ecorr
mV vsX SCE
icorr
mAacm2
Rcorr
mpy
c
V Á decadeÀ1
c
V Á decadeÀ1
% lnh
Free acid À506 10.72 4.92 0.123 0.049 ±
1 Â 10À3
M KI À471 9.57 4.39 0.113 0.042 10.7
1 Â 10À5
M 3 À482 5.60 2.57 0.113 0.046 47.8
1 Â 10À3
M KI ‡ 1 Â 10À3
M 3 À438 0.77 0.35 0.111 0.041 92.8
1 Â 10À4
M 3 À438 0.77 0.35 0.111 0.041 92.8
1 Â 10À3
M KI ‡ 1 Â 10À4
M 3 À431 0.75 0.34 0.115 0.032 93.0
1 Â 10À3
M 3 À445 0.73 0.34 0.107 0.025 93.2
1 Â 10À3
M KI ‡ 1 Â 10À3
M 3 À420 0.72 0.33 0.109 0.035 93.3
1 Â 10À2
M 3 À395 0.63 0.28 0.108 0.026 94.2
1 Â 10À3
M KI ‡ 1 Â 10À2
M 3 À440 0.41 0.19 0.102 0.022 96.2
Hydantoin Derivatives as Corrosion Inhibitors 255

12. Weight loss measurements
The reaction basin used in this method was a graduated glass vessel of 6 cm inner diameter and a total
volume of 250 cm3
. 100 cm3
of the test solution at 303X0 Æ 1X0 K were employed in each experiment.
The iron pieces (2 Â 2 Â 0X1 cm) were prepared as described before, weighed, and suspended under
the surface of the test solution by about 1 cm by suitable glass hooks. After speci®ed periods of time,
three pieces of iron were taken out of the test solution, rinsed with doubly distilled water, dried, and
re-weighed. The average weight loss at a certain time for each test of three samples was taken. The
percentage of inhibition (% In) of different concentrations of the inhibitors was calculated according
to 7 In ˆ WtX loss …pure†ÀWtX loss …inh†
WtX loss …pure† Â 100.
256 L. H. Madkour et al.

13. Polarization 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 working
electrode were referred to a saturated calomel electrode (SCE). In order to avoid contamination, the
reference electrode was connected to the working electrode through a salt bridge ®lled with 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 investigation (25 cm3
) was
freed of oxygen by passing prewashed pure nitrogen through it for a suf®cient time. Measurements
were performed on a planar disk electrode (A ˆ 1 cm2
). The iron electrodes were carefully degreased,
and the edges were masked by appropriate resins (Duracryle, Spofa-Dental, Praha). The surface of the
iron electrodes were prepared by mechanical grinding and polishing as given elsewhere [25, 26]. The
electrodes were rinsed in an ultrasonic bath containing bidistilled water and ®nally washed with
bidistilled water immediately before being immersed in the cell. The pretreatment procedure was
repeated before each experiment.
Anodic and cathodic potentiostatic polarization curves of iron electrodes were measured with a
Wenking Potentioscan Model POS 73. Potentials and currents were determined by digital
multimeters. Corrosion current densities (icorr) were determined by extrapolation of the anodic and
cathodic Tafel lines to the free corrosion potential value (Ecorr). Each experiment was conducted with
a freshly prepared solution and with newly polished electrodes. The cell temperature was kept
constant at 303X0 Æ 1X0 K in an ultra-thermostat.
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Hydantoin Derivatives as Corrosion Inhibitors 257

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Received June 5, 2000. Accepted (revised) September 13, 2000
258 L. H. Madkour et al.: Hydantoin Derivatives as Corrosion Inhibitors