2001 complete

Monatshefte fuÈr Chemie 132, 245±258 (2001)

Inhibition Effect of Hydantoin Compoundson the Corrosion of Iron in Nitricand 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 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 overallcosts by protection and maintenance of materials used. Because iron is the back-bone of industrial constructions, the inhibition of iron corrosion in acidic solutionshas been studied in considerable detail. Many N-heterocyclic compounds with polargroups 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 bondbetween 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 theinhibition 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 theinhibitor, and the inhibitors were of a mixed type, i.e. both the anodic and cathodicpolarization processes were affected. No research work is available in the literature

� Corresponding author

to date about the application of hydantoin compounds as inhibitors for surface metalcorrosion. Accordingly, the objective of this work is to study the hydantoin com-pounds effects toward corrosion process of iron in nitric and sulfuric acid solutionsand 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 ofcompound 3 as an example (Fig. 1). Upon increasing the concentration of theadditive, the time required to reach Tmax increases. This indicates that the inhibitorretards 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

246 L. H. Madkour et al.

adsorption onto the surface of the metal. The extent of retardation depends on thedegree of coverage of the metal surface with the adsorbate. The temperature vs. timecurves provide a means of differentiating between weak and strong adsorption [4].Strong adsorption is noted in both acidic solutions, since a simultaneous increase int 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 revealthat the inhibition ef®ciency of the additive, as determined from the percentagereduction in RN, increases with increasing concentration of additives. Figure 2shows the relation between % RN and the molar concentration of different additives.The curves obtained are invariably sigmoid in nature, substantiating the idea that thepresent inhibitors retard the corrosion rate by adsorption according to the Temkinisotherm [5] (Eq. (1)).

� � c1 � ln�c2 � c� �1�

Fig. 2. Effect of additives concentration on % reduction in reaction number (% RN) of iron corrosion

in 2M HNO3

Hydantoin Derivatives as Corrosion Inhibitors 247

In Eq. (1), c is the concentration of the additive in the bulk of the solution, � is thedegree 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 thehydantion 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 MH2SO4 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 � �i=�C �max=

�C t/min �t/min log � RN=�C� % Red �

dmÿ3 ��t=min� minÿ1 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

Fig. 3. Weight loss vs. time curves of iron corrosion in 2M H2SO4 in the absence and in the presence

of different concentrations of 3 at 303 K


weight loss of iron depends on type and concentration of the additive in a similarway as thermometric measurements do. The ef®ciency of the inhibitors underinvestigation increases in the order 3 > 2 > 4 > 1 > 6 > 5 > 7. The inhibition effect maybe explained by considering the adsorption of the hydantoin molecules (with highnegative charge density at the hetero atom) on the metal surface [6, 7] consisting ofiron 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 potentiostaticconditions in 2 M HNO3 and 2 M H2SO4 in the absence as well as in the presence ofdifferent concentrations of inhibitor at 303 K. Figure 4 shows the polarization curvesof iron in 2 M nitric acid solution at different concentrations of 3; the resultsobtained for the other compounds were quite similar. The inhibition ef®ciencydepends 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-atomprobably act as the centers of adsorption, their basicity being affected by thecharacter of the substituents in �-position. The results show that the inhibitivepower 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 ofthe carbethoxy group which increases the electron density on the molecule andprovides an active adsorption center (oxygen atom) in addition to the two nitrogencenters already present.

Fig. 4. Potentiostatic polarization curves of iron in 2M HNO3 in the absence and in the presence of

different concentrations of 3


On the other hand, 1,3-dimethyl-5-phenylazo-2-thiohydantoin (6) has the lowestinhibition ef®ciency owing to the formation of an iron complex (see formula)

which is obviously less adsorbed in acidic solutions. This interpretation issupported 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 calculatedfrom Eq. (2), where u0 and u are the dissolution rates of iron in the absence and inthe presence of hydantoins, respectively.

� � �1ÿ u=u0� �2�From Table 2 it can be seen that the three different techniques afford the sameresults 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 303 K


Effect of temperature

The effect of temperature on the rate of corrosion of iron in 2 M HNO3 and 2 MH2SO4 containing 1� 10ÿ5 M hydantoin was studied in the temperature range of303±323 K. The corrosion rate increases with increasing temperature in the absenceas well as in the presence of the inhibitors. The increase of icorr is due to the absenceof a protective layer at the iron surface. Thus, the increase of temperature enhancesboth the iron dissolution and the additive desorption processes without leading toFe(II)-hydantoin complex formation. The protective layer decreases as thetemperature increases.

The Arrhenius parameters as purely empirical quantities enable us to discuss thevariation of rate constants with temperature. It was found experimentally that a plotof lnk against 1=T gives a straight line according to Eq. (3) [5].

lnk � lnAÿ 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), thefree energy change of activation (�Gz) is obtained from the Eyring equation [12]:(Eq. (5)). Another convenient form of Eq. (5) is Eq. (6).

�Hz � Ea ÿ RT �4�

k � kBT

heÿ�Gz=RT �5�

�Gz � RT � lnkBT

hÿ lnk

� ��6�

From �Hz and �Gz, the entropy change of activation (�Sz) can be obtainedaccording to Eq. (7).

�Sz � ��Hz ÿ�Gz�=T �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� 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


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� 10ÿ5 M hydantoin inhibitors


From the thermodynamic parameters in Table 3 it can be seen that Ea increases asthe inhibition ef®ciency of the additives increases. This suggests that the process iscontrolled by a surface reaction, since the energy of activation for the corrosionprocess is above 20 kJ �molÿ1 [13]. The Ea value for iron dissolution in 5 M H2SO4

has been reported as 20:2 kJ �molÿ1 [12] and 51.4 kJ molÿ1 [14±16]. For iron in 3 MHCl and 1 M HNO3, an Ea values of 11.8 kJ �molÿ1 has been reported [17], whereasin 2 M HNO3 20.06 kJ �molÿ1 have been found [18].

Generally, one can say that the nature and the concentration of the electrolytegreatly affect the activation energy of the corrosion process. The presence of theinhibitor causes a change in the value of the apparent activation energy. Thus, itindicates no change in the rate-determining step brought about by the presence ofthe hydantoin inhibitor.

Kinetic-thermodynamic model of corrosion inhibition

To evaluate the kinetic parameters and correlate them to the corrosion inhibitionmechanism, it is of value to analyze the kinetic data obtained in the presence ofhydantoin inhibitors from the standpoint of the generalized mechanistic schemeproposed by El-Awady et al. [19, 20]. Table 4 comprises the values of 1=y whichgive the number of active sites occupied by a single organic molecule; K is thebinding constant [21]. The values of B (equilibrium constant) and f (lateralinteraction parameter, 1/c1) are also reported in Table 4. The large negative valuesof �G� indicate that the reaction proceeds spontaneously and is accompanied by ahigh ef®cient adsorption. Large values of K and B (c2) point to better inhibitionef®ciency of the tested hydantoin compounds, i.e. stronger electrical interactionbetween the double layer at the phase boundary and the adsorbing molecules. Ingeneral, the equilibrium constant of the adsorption process was found to rise withincreasing inhibition ef®ciency.

Table 3. Activation energy (Ea), enthalpy change (�Hz), free energy change (�Gz), and entropy change (�Sz) for the

dissolution of Fe in 2 M acid in the presence of 1� 10ÿ5 M inhibitor at 313 K

InhibitorEa

kJ �molÿ1

�Hz

kJ �molÿ1

�Gz

kJ �molÿ1

ÿ�Sz

J � Kÿ1 �molÿ1

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


Synergistic effects of halide anions

The effects of Iÿ, Brÿ, and Clÿ on the polarization curves of iron at 298 K in theabsence and presence of hydantoin inhibitors in 0:5 M H2SO4 solution were studied.The in¯uence on the inhibition ef®ciency was observed in presence of each anionalone 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 havea higher inductive effect than Brÿ and Clÿ [22], they are less attached to the metalsurface and, consequently, easily displaced by the inhibitor molecules. The netincrement of the inhibition ef®ciency is de®ned in Eq. (8), where Px and Pinh are theprotection ef®ciency of the anion and inhibitor, respectively, and Ptot is the totalprotection ef®ciency of the corrosive medium containing the anion and inhibitortogether.

�P � Ptot ÿ �Px � Pinh� �8�The correlation of �P vs. logc for the applied synergistic anions in acidic solutionwas studied. Since the halide anions interact strongly with the iron surface due tochemisorption [23, 24], the inhibitory effect is strengthened due to the coadsorptionof the anions. As a result, the surface coverage area ��P� and, consequently, theinhibition ef®ciency both increase. KI is the most effective among the investigatedsalts. 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 inTable 5.

Table 4. Curve ®tting of data to the kinetic-thermodynamic model (r � 0:94) and the Temkin isotherm for hydantoin

inhibitors in 2 M acid at 303 K

Kinetic model Temkin isotherm

Medium 1=y Kÿ�G�

kJ �molÿ1

1

C1

C2 � 104 ÿ�G�

kJ �molÿ1

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


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� 10� 0:1 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 � �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.

Table 5. Electrochemical parameters of Fe in the presence of 10ÿ3 M KI and different concentrations of 3 in 0:5 M H2SO4

at 298 K

logC

mol � dmÿ3

Ecorr

mV vs: SCE

icorr

mA=cm2

Rcorr

mpy

�c

V � decadeÿ1

�c

V � decadeÿ1% lnh

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


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 303:0� 1:0 K 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 � Wt: loss �pure�ÿWt: loss �inh�Wt: loss �pure� � 100.


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 � 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� 1:0 K in an ultra-thermostat.

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Received June 5, 2000. Accepted (revised) September 13, 2000

258 L. H. Madkour et al.: Hydantoin Derivatives as Corrosion Inhibitors