The inhibition of corrosion of iron in 2M nitric acid and 2M 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.

1. Monatshefte fuÈ r 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. Ghoneim
,
and Safwat A. Eid
Chemistry Department, Faculty of Science, Tanta University, 31527 Tanta, Egypt
Summary. The inhibition of corrosion of iron in 2M nitric acid and 2M 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
Corresponding author

2. 246 L. H. Madkour et al.
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 2M HNO3 or 2M 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 2M HNO3 in the absence and in the
presence of different concentrations of 3

3. Hydantoin Derivatives as Corrosion Inhibitors 247
Fig. 2. Effect of additives concentration on % reduction in reaction number (% RN) of iron corrosion
in 2M HNO3
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 ln…c2 c† …1†

4. 248 L. H. Madkour et al.
Table 1. Effect of concentration 3 on the thermometric parameters of Fe in 2M HNO3
c/mol
i=
C max=
C t/min t/min log RN=
C % Red
dmÿ3 …t=min† min
ÿ1 in RN
0 35.0 59.0 60 0.400
5 10ÿ7 35.0 45.9 75 15 1.18 0.686 0.150 62.5
1 10ÿ6 35.0 44.0 75 15 1.18 0.749 0.120 70.0
5 10ÿ6 34.8 44.0 90 30 1.48 0.791 0.100 75.0
1 10ÿ5 35.1 42.2 90 30 1.48 0.835 0.078 80.5
5 10ÿ5 34.9 39.8 105 45 1.65 0.899 0.048 88.0
1 10ÿ4 35.0 40.1 120 60 1.78 0.921 0.041 89.8
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 2M
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
Fig. 3. Weight loss vs. time curves of iron corrosion in 2MH2SO4 in the absence and in the presence
of different concentrations of 3 at 303K

5. Hydantoin Derivatives as Corrosion Inhibitors 249
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 2M HNO3 and 2M 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 2M 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 2M HNO3 in the absence and in the presence of
different concentrations of 3

6. 250 L. H. Madkour et al.
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 ÿ u=u0† …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 2M HNO3 at 303K

7. Hydantoin Derivatives as Corrosion Inhibitors 251
Table 2. Comparison between the inhibition ef®ciency of 1±7 in 2M acid solutions as determined by
thermometric, weight loss, and polarization methods (1 10ÿ4 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
Effect of temperature
The effect of temperature on the rate of corrosion of iron in 2M HNO3 and 2M
H2SO4 containing 1 10ÿ5 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 1=T gives a straight line according to Eq. (3) [5].
lnk ˆ lnA ÿ Ea=RT …3†
The activation energies calculated from the slopes of lnIcorr vs. 1=T plots (Fig. 6)
are reported in Table 3.
The enthalpy change of activation (Hz) can be calculated from Eq. (4), the
free energy change of activation (Gz) is obtained from the Eyring equation [12]:
(Eq. (5)). Another convenient form of Eq. (5) is Eq. (6).
H
z ˆ Ea ÿ RT …4†
k ˆ kBT
h
ÿGz=RT …5†
e
z ˆ RT lnkBT
G
h
ÿ lnk
…6†
From Hz and Gz, the entropy change of activation (Sz) can be obtained
according to Eq. (7).
z ˆ …H
S
z ÿ G
z†=T …7†

8. 252 L. H. Madkour et al.
Fig. 6. Arrhenius plot of the current corrosion rate constant (icorr) vs. 1=T of iron in (a) 2M HNO3
and (b) 2M H2SO4 in the absence and in the presence of 1 10ÿ5M hydantoin inhibitors

9. Hydantoin Derivatives as Corrosion Inhibitors 253
Table 3. Activation energy (Ea), enthalpy change (Hz), free energy change (Gz), and entropy change (Sz) for the
dissolution of Fe in 2M acid in the presence of 1 10ÿ5 M inhibitor at 313K
ÿ1
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 molÿ1 [13]. The Ea value for iron dissolution in 5M H2SO4
ÿhas been reported as 20:2 kJ mol
1 [12] and 51.4 kJ molÿ1 [14±16]. For iron in 3M
HCl and 1M HNO3, an Ea values of 11.8 kJ molÿ1 has been reported [17], whereas
in 2M HNO3 20.06 kJ molÿ1 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 1=y 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 G 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.
Inhibitor
Ea
kJ mol
ÿ1
Hz
kJ mol
ÿ1
Gz
kJ mol
ÿ1
ÿSz
J K
ÿ1 mol
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

10. 254 L. H. Madkour et al.
Table 4. Curve ®tting of data to the kinetic-thermodynamic model (r ˆ 0:94) and the Temkin isotherm for hydantoin
inhibitors in 2M acid at 303K
Kinetic model Temkin isotherm
Medium 1=y K
ÿG
kJ mol
ÿ1
1
C1
C2 104
ÿG
kJ mol
Synergistic effects of halide anions
The effects of Iÿ, Brÿ, and Clÿ on the polarization curves of iron at 298K in the
absence and presence of hydantoin inhibitors in 0:5M 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 Iÿ Brÿ Clÿ. Since the iodide ions have
a higher inductive effect than Brÿ and Clÿ [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.
P ˆ Ptot ÿ …Px ‡ Pinh† …8†
The correlation of P 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 …P† and, consequently, the
inhibition ef®ciency both increase. KI is the most effective among the investigated
salts. Addition of 10ÿ3 M KI in the presence of a very low concentration
…1 10ÿ5 M† of 3 rises the inhibition ef®ciency from 48 to 93% as shown in
Table 5.
ÿ1
1 HNO3 12.50 1:30 109 41.96 7:20 10ÿ4 133.10 24.46
H2SO4 6.67 5:47 108 39.82 1:51 10ÿ3 16.72 19.44
2 HNO3 10.77 1:63 1010 53.93 1:32 10ÿ2 254.61 26.03
H2SO4 5.88 9:63 1010 52.62 2:56 10ÿ3 141.25 24.60
3 HNO3 8.43 1:01 1012 58.44 6:43 10ÿ2 258.67 26.09
H2SO4 8.24 5:02 1011 56.71 3:35 10ÿ2 243.60 25.93
4 HNO3 9.10 1:90 109 42.90 1:12 10ÿ3 167.49 25.02
H2SO4 7.14 5:57 109 45.57 1:85 10ÿ3 32.69 21.06
5 HNO3 4.79 9:50 108 41.19 2:16 10ÿ4 87.70 23.45
H2SO4 10.75 1:15 108 35.96 6:56 10ÿ4 2.72 15.04
6 HNO3 8.33 1:20 109 41.77 4:90 10ÿ4 118.85 24.19
H2SO4 5.88 3:59 108 38.78 1:26 10ÿ3 13.55 18.93
7 HNO3 3.96 2:30 108 37.64 9:36 10ÿ5 68.07 22.84
H2SO4 5.56 1:69 107 31.21 1:76 10ÿ4 2.30 14.63

11. Hydantoin Derivatives as Corrosion Inhibitors 255
Table 5. Electrochemical parameters of Fe in the presence of 10ÿ3 M KI and different concentrations of 3 in 0:5M H2SO4
at 298K
Experimental
Iron specimens and electrolytes
ÿ1
ÿ1 % lnh
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]. 2M
nitric and 2M 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 10 0:1 cm† was immersed in 30 cm3 of either 2M HNO3 or 2M 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 ÿ Ti†=t …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 ÿ RNinh†=RNfree† 100.
logC
mol dm
ÿ3
Ecorr
mV vs: SCE
icorr
mA=cm2
Rcorr
mpy

12. c
V decade

13. c
V decade
Free acid ÿ506 10.72 4.92 0.123 0.049 ±
1 10ÿ3 M KI ÿ471 9.57 4.39 0.113 0.042 10.7
1 10ÿ5 M 3 ÿ482 5.60 2.57 0.113 0.046 47.8
1 10ÿ3 M KI ‡ 1 10ÿ3 M 3 ÿ438 0.77 0.35 0.111 0.041 92.8
1 10ÿ4 M 3 ÿ438 0.77 0.35 0.111 0.041 92.8
1 10ÿ3 M KI ‡ 1 10ÿ4 M 3 ÿ431 0.75 0.34 0.115 0.032 93.0
1 10ÿ3 M 3 ÿ445 0.73 0.34 0.107 0.025 93.2
1 10ÿ3 M KI ‡ 1 10ÿ3 M 3 ÿ420 0.72 0.33 0.109 0.035 93.3
1 10ÿ2 M 3 ÿ395 0.63 0.28 0.108 0.026 94.2
1 10ÿ3 M KI ‡ 1 10ÿ2 M 3 ÿ440 0.41 0.19 0.102 0.022 96.2

14. 256 L. H. Madkour et al.
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 250cm3. 100cm3 of the test solution at 303:0 1:0K were employed in each experiment.
The iron pieces (2 2 0:1 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 %In ˆ Wt: loss …pure†ÿWt: loss …inh†
Wt: loss …pure† 100.

15. Hydantoin Derivatives as Corrosion Inhibitors 257
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 303:0 1:0K in an ultra-thermostat.
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Received June 5, 2000. Accepted (revised) September 13, 2000