International Journal of Scientific & Engineering Research, Volume 4, Issue 12, December-2013 1986

ISSN 2229-5518

Determination of the Thermodynamic Parameters, Characterizing the Adsorption and Inhibitive Properties of some 3-phenyl-2-

thioxo- 4-thiazolidinone Derivatives

S.A. Abd El-Meksoud1, A.M. El-Desoky2, A.Z. El-Sonbati3,*, A.A.M. Belal1, R.A. El-Boz1

1Chemistry Department, Faculty of Science, Port Said University, Egypt.

2 Engineering Chemistry Department, High Institute of Engineering & Technolog, Damietta, Egypt.

3Chemistry Department, Faculty of Science, Damietta University, Damietta 34517, Egypt.

AbstractThe corrosion inhibition of C-steel in 1 M H2 SO4 solution by five newly synthesized 3-phenyl-2-thioxo-4- thiazolidinone derivatives has been investigated using (weight loss, potentiodynamic polarization and electrochemical imped- ance spectroscopy , EIS) techniques. Potentiodynamic polarization showed that these derivatives are mixed-type inhibitors. The percentage inhibition efficiency was found to increase with increasing the concentration of inhibitor and with decreasing temper- ature. The Frumkin isotherm was found to provide an accurate description of adsorption behavior of the 3-phenyl-2-thioxo-4- thiazolidinone derivatives. Some thermodynamic parameters were computed and discussed. The variation in inhibition efficien- cy depends on the type of functional group substituted in benzene ring. It was found that the presence of donating group (such as OCH3 ) better facilitates the adsorption of molecules on the surface than in the case with withdrawing groups (such as Cl and NO2 ). The data obtained from the three different methods are in good agreement.

Keywords —Thermodynamic parameters, Corrosion inhibition, EIS, 4-thiazolidinone derivatives.

*Corresponding Author: E-mIail: elsJonbatisch@ySahoo.com; Tel.: +E2 01060081581; FaRx: +2 0572403868

1 INTRODUCTION

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arbon steel is the major structural material in industry, the protection of steel against corrosion has attracted. As
most steels are generally stable in neutral and alkaline media,
acidic environments are the major concern [1]. Acid solutions are generally used for the removal of undesirable scale and rust in several Petrolum processes. Inhibitors are used in this process to control metal dissolution. Most of the well-known acid inhibitors are organic compounds containing O, S, and
/or N atoms [2-5]. Although there are several studies on the corrosion inhibition effects of organic compounds in acidic solutions [6-15]. The inhibitive action is connected with several factors including the structure and the charge distribution on the molecule, the number and the types of adsorption sites, and the nature of interaction between the molecule and the metal surface [16]. Corrosion inhibition occurs via adsorption of the organic molecule on the corroding metal surface follow- ing some known adsorption isotherms with the polar groups acting as active centers in the molecules. The resulting adsorp- tion film acts as a barrier that isolates the metal from the cor- roding and efficiency of inhibition depends on the mechanical, structural and chemical characteristics of the adsorption layer formed under particular conditions [17,18].
The aim of this work is to study the effect of some 3- phenyl-2-thioxo-4-thiazolidinone derivatives as inhibitors
for the corrosion of C- steel in 1 M H2 SO4 solutions using weight loss, potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) tech- niques. These derivatives were selected because: availability, with large molecular size and containing three N and two O atoms.

2 EXPERIMENTAL

2.1. COMPOSITION OF MATERIAL SAMPLES

Table (1): Chemical composition (wt %) of the carbon steel.

Element

C

Mn

P

Si

Fe

Weight (%)

0.200

0.350

0.024

0.003

Rest

2.2. CHEMICALS

A- SULPHORIC ACID. (BDH GRADE)

B- ORGANIC ADDITIVES

3-phenyl-2-thioxo-4-thiazolidinone derivatives were pre-
pared as reported [19]. Table (2) shows the name and the mo- lecular structures of these compounds. The acid solutions were made from AR grade H2 SO4 . An appropriate concentra- tion of acid was prepared using bidistilled water. 10-3 M stock solutions from the investigated compounds were prepared by dissolving the appropriate weights of the used chemically
pure solid compounds in absolute ethanol.

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Table (2): Molecular structures of 3-phenyl-2-thioxo-4-


thiazolidinone derivatives.

2.2. Methods used for coIrrosionJmeasuremeSnts

2.3. Electrochemical measurements

The experiments were carried out potentiodynamically in a thermostated three electrode cell. Platinum foil was used as counter electrode and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. The working electrode was in the form of a square cut from C- steel under investigation and was embedded in a Teflon rod with an exposed area of 1 cm2. This electrode was immersed in
100 ml of a test solution for 30 min until a steady state open- circuit potential (Eocp ) was attained. Potentiodynamic polari- zation was conducted in an electrochemical system (Gamry framework instruments version 3.20) which comprises a PCI/
300 potentiostat, controlled by a computer recorded and stored the data. The potentiodynamic curves were recorded by changing the electrode potential from -1.0 to 0.0 V versus SCE with scan rate of 5 mV/s. All experiments were carried out in freshly prepared solution at constant temperature (30 ±1 oC) using a thermostat. %IE and the degree of surface coverage (θ) were defined as:

where icorr and iicorr,(inh) are the uninhibited and inhibited corro- sion current density values, respectively, determined by ex-

2.2. Weight loss tests

C- steel sheets of 20 mm x 20 mm x 2 mm were

2.2. Weight loss tests

C- steel sheets of 20 mm x 20 mm x 2 mm were abraded
with different grades of emery paper up to 1200 grit and then washed with bidistilled water and acetone. After weighing accurately, the specimens were immersed in 100 ml H2 SO4 solution with and without addition of different concentrations of inhibitors. After 3 hrs, the specimens were taken out, washed, dried, and weighed accurately. The average weight loss of the three parallel C- steel sheets could be obtained at required temperature. The inhibition efficiency (%IE) and the degree of surface coverage (θ) of the investigated inhibitors on the corrosion of C-steel were calculated as follows [20]:

where Wo and W are the values of the average weight loss in the absence and presence of the inhibitor, respectively.
trapolation of Tafel lines.
The electrochemical impedance spectroscopy (EIS) spectra
were recorded at open circuit potential (OCP) after immersion the electrode for 15 min in the test solution. The ac signal was
5 mV peak to peak and the frequency range studied was be- tween 100 kHz and 0.2 Hz. All Electrochemical impedance experiments were carried out using Potentiostat/Galvanostat/ Zra analyzer (Gamry PCI 300/4). A personal computer with EIS300 software and Echem Analyst 5.21 was used for data fitting and calculating.
The inhibition efficiency (%IE) and the surface coverage (θ)

of the used inhibitors obtained from the impedance measure- ments were calculated by applying the following relations:
where Roct and Rct are the charge transfer resistance in the ab- sence and presence of inhibitor, respectively.

3. Results and discussion

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3.1. Weight loss measurements

Fig. (1) represents the weight loss-time curves in the ab-
sence and presence of different concentrations of compound (I). Similar curves were obtained for other inhibitors (not shown). Table (3) collects the values of surface coverage, inhi- bition efficiency and corrosion rate obtained from weight loss measurements in 1 M H2 SO4 at 30 ± 0.1 °C. The results of this Table show that the presences of inhibitors reduce the corro- sion rate of C- steel in H2 SO4 and hence, increase the inhibi- tion efficiency. The inhibition achieved by these compounds decreases in the following order:
Compound (I) > Compound (II) > Compound (III) > Com-
pound (IV) > Compound (V).

Table (3): Inhibition efficiency of all inhibitors at different concentrations of inhibitors as determined from weight loss method for C-steel at 30 oC.

3.1.1. Adsorption isotherm

One of the most convenient ways of expressing adsorption

quantitatively is by deriving the adsorption isotherm that characterizes the metal/inhibitor/environment system [21]. The surface coverage (θ) values were tested graphically to al- low fitting of a suitable adsorption isotherm. The plot of θ versus log C (Fig. (2)) yielded a straight line clearly proving that the adsorption of this inhibitor from 1 M HR2RSOR4R solution on C-steel surface obeys Frumkin adsorption isotherm, where:



Plotting of θ versus log C the data gives S-shape curve.

Fig. (1): Weigh loss-time curves for C-steel dissolution in 1 M HR2RSOR4R in absence and presence of different concentrations of inhibitor (I) at 30 °C.

Fig. (2): θ- log c curves for C-steel dissolution in 1 M HR2RSOR4 Rof different inhibitors from weigh loss measurement at 30 oC.

3.1.2. Effect of tempertature

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To investigate the mechanism of inhibition and to de- termine the activation energy of corrosion process, weight loss curves of C- steel in 1M H2 SO4 were studied at various tem- peratures (30–60 oC) in the absence and presence of different concentrations of investigated compounds. These inhibitors retard the corrosion process at lower temperatures [22] where- as the inhibitions are considerably decreased at elevated tem- peratures. The increasing of the corrosion rate with increasing the temperature is suggestive of physical adsorption of the investigated inhibitors on C-steel surface. Fig. (3) represents the Arrhenius plots of natural logarithm of corrosion rate ver- sus 1/T, for C- steel in 1 M H2 SO4 solution, in absent and presence of different concentration of inhibitor (I ). The val- ues of slopes of these straight lines permit the calculation of
the activation energy, E*a, according to:

where K is the corrosion rate, A is the pre-exponential factor, Ea * is the apparent activation energy, R is the universal gas constant and T is the absolute temperature.
The values of Ea * are given in Table (4). The results of
Table (4) revealed that, the values of Ea were increased by

Fig. (3): log K (corrosion rate)-1/T curves for C-steel dissolution in 1M

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increasing the concentration of the investigated inhibitors in-

dicating the dissolution of C-steel under these conditions is activation controlled and also, indicates the energy barrier of the corrosion reaction increases in the presence of these addi- tives. Similar results were obtained by other authors [23-25]. The higher values of Ea * are good evidence for the strong ad- sorption of compound (I) on C-steel surface. Also, Free energy of activation (ΔG*) were calculated by applying the transition state equation [26]:
where, h is Planck’s constant and K is Boltzmann’s constant. The enthalpy of activation (ΔH*) and the entropy of activation (ΔS*) were calculated by applying the following equations [27]:

The values of (ΔH*) are positive and higher in the presence of the inhibitors than in its absence. This implies that energy barrier of the corrosion reaction in the presence of the investigated compounds increases and indicates the endothermic behavior of the corrosion process. On the other hand ΔS* values are lower and have negative values in presence of the additives, this means that addition of these compounds cause a decrease in the disordering in going from
reactants to the activated complexes [28,29].

HR2RSOR4R in absence and presence of different concentrations of inhibitor (I).

Fig. (4): log (corrosion rate /T)- (1/T) curves for C-steel dissolu tion in 1M HR2RSOR4R in absence and presence of different concentration of inhibitor (I).

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Table (4): Activation parameters for the dissolution of C-steel

in presenc and absence of different concentration of inhibitors in 1 M HR2RSOR4R.

Also, the results of θ and %IE where calculated using iRcorrR. values. The percentage inhibition efficiencies (%IE) calcu-

lated from iRcorrR and RRpR of the investigated compounds are giv- en in Table (7). An inspection of the results obtained from this Table reveals that, the presence of different concentrations of the additives reduces the anodic and cathodic current densi- ties and the polarization resistance. This indicates that the in- hibiting effects of the investigated compounds. The order of decreasing inhibition efficiency from iRcorrR and RRpR is:

Compound (I) > Compound (II) > Compound (III) > Com- pound (IV) > Compound (V).
The values of %IE determined from the two methods are

very close to each other indicating the validity of the obtained results.

3.2. Potentiodynamic polarization

Fig. (5) shows the potentiodynamic polarization curves for C- steel without and with different concentrations of com- pound (I) at 30 oC. Similar curves were obtained for other compounds. The obtained electrochemical parameters; cathod- ic RcR) and anodic RaR) Tafel slopes, corrosion potential (ERcorrR), corrosion current density (iRcorrR), and polarization resistance (RRpR) were obtained and listed in Table. (5). Table (5) shows that iRcorrR decreases by adding the additives and by increasing their concentration. In addition, ERcorrR does not change obviously. Also βRaR and βRcR do not change markedly, which indicates that the mechanism of the corrosion reaction of C-steel does not change. Fig. (4) clearly shows that both anodic and cathodic reactions are inhibited, which indicates that investigated com- pounds act as mixed-type inhibitors [30,31]. The inhibition achieved by these compounds decreases in the following or-

der:
Compound (I) > Compound (II) > Compound (III) > Compound (IV) > Compound (V).
The degree of surface coverage, θ at constant potential

is given by the following relation from the polarization re- sistance

where RRp(free)R and RRp(inh)R are the polarization resistance of unin- hibited and inhibited solutions, respectively. The percentage of inhibition efficiency (%IE) at each concentration was calcu-

lated using the equation:

Fig. (5): Potentiodynamic polarization curves for C-steel dissolution in 1

M HR2RSOR4R in absence and presence of different concentrations of compound (I) at 30 oC.

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Table (5): Electrochemical kinetic parameters obtained from potentiodynamic polarization technique for the corrosion of for C- steel in 1 M HR2RSOR4R at different concentrations of inves- tigated inhibitors at 30 °C.

3.3. Electrochemical Impedance Spectroscopy (EIS)

The corrosion of C- steel in 1 M HR2RSOR4R in the presence of the investigated compounds was investigated by EIS method at 30 oC after 20 min immersion. Nyquist plots in the absence and presence of investigated compound (I) are presented in Fig. (6). Similar curves were obtained for other inhibitors. It is apparent that all Nyquist plots show a single capacitive loop, both in uninhibited and inhibited solutions. The impedance data of C- steel in 1 M HR2RSOR4 Rare analyzed in terms of an equivalent circuit model Fig. (7) which includes the solution resistance RRsR and the double layer capacitance Cdl which is placed in parallel to the charge transfer resistance Rct [32] due to the charge transfer reaction. For the Nyquist plots it is ob- vious that low frequency data are on the right side of the plot and higher frequency data are on the left. This is true for EIS data where impedance usually falls as frequency rises (this is not true for all circuits). The capacity of double layer (CRdlR) can

be calculated from the following equation:

where fRmax Ris maximum frequency. The parameters obtained from impedance measurements are given in Table (8). It can see from Table (6) that the values of charge transfer resistance Rct increase with inhibitor concentration [33]. In the case of impedance studies, %IE increases with inhibitor concentration in the presence of investigated inhibitors and the %IE of these investigated inhibitors is as follows:

Compound (I) > Compound (II) > Compound (III) > Compound (IV) > Compound (V).

The impedance study confirms the inhibiting charac- ters of these compounds obtained from potentiodynamic po- larization and weight loss methods. It is also noted that the (CRdlR) values tend to decrease when the concentration of these compounds increases. This decrease in (CRdlR ), which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that these compounds molecules function by adsorption at the metal/solution interface [34]. The inhibiting effect of these compounds can be attributed to their parallel adsorption at the metal solution interface. The parallel adsorption is owing to

the presence of one or more active center for adsorption

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from EIS technique for the corrosion of C- steel in 1 M HR2RSOR4R

at different concentrations of investigated inhibitors at 30 °C.

Fig. (6): The Nyquist plot for corrosion of C-steel in 1 M HR2RSOR4 Rin absence and presence of different concentrations of compound (I) at 30 °C.

4- Chemical structure and corrosion inhibition

Inhibition efficiency of the carbon steel corrosion in 1 M HR2RSOR4R solution by 3-phenyl-2-thioxo-4-thiazolidinone deriva- tives using the above techniques was found to depend on the number of adsorption active sites in the molecule and their charge density, molecular size and stability of these deriva-

tives in acidic solutions [35].

Fig. (7): Equivalent circuit model used to fit the impedance spectra.

Table (6): Electrochemical kinetic parameters obtained

The order of increased inhibition efficiency for 2- thioxothiazolidin-4-one derivatives is:
Compound (I) > Compound (II) > Compound (III) > Compound (IV) > Compound (V) as indicated from the differ- ent methods.
Compound (I) has the highest percentage inhibition effi- ciency. This due to the presence of p-OCHR3R group which is an electron repelling group with negative Hammett constants (σ
= -0.27). This group will increase the electron charge density
on the molecule. Compound (II) comes after compound (I), this is due to the presence of p-CHR3 Rgroup which is an electron donating group with negative Hammett constants (σ = -0.17), Also this group will increase the electron charge density on

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the molecule but with lesser amount than p-OCH3 group in compound (I). Compound (III) with Hammett constants (σ =
0.0) comes after compound (II) in percentage inhibition effi- ciency because H- atom in p-position has no effect on the charge density on the molecules. Compound (IV) and (V) come after compound (III) in percentage inhibition efficien- cies. This is due to both p-Cl and p-NO2 groups are electron withdrawing groups with positive Hammett constants (σCl =
+0.23, σNO2 = + 0.78) and their order of inhibition depends on
the magnitude of their withdrawing character [36].
Comparative analysis for potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) study of ligands (HLn ) is shown in Fig. (8) It is observed that the HL1 is more potentiodynamic polarization measure- ments and electrochemical impedance spectroscopy (EIS) than the other ligands at concentration = 1 × 10-5 Molar.
3-phenyl-2-thioxo-4-thiazolidinone derivatives show good corrosion inhibition property against C-steel corrosion in
1M H2 SO4 solution. Inhibition efficiencies are related to con-
centration, temperature and chemical structure of the investi- gated isoindoline derivatives. Generally, 3-phenyl-2-thioxo-4- thiazolidinone derivatives inhibition efficiencies increase when concentration increases and temperature decreases. Presence of aromatic ring and hetero atom such as nitrogen atom on the 3-phenyl-2-thioxo-4-thiazolidinone structure causes a significant increase in inhibition efficiency. All inves- tigated compounds affected both anodic and cathodic reac- tions, so they are classified as mixed type inhibitors. Adsorp- tion of these derivatives on the C-steel surface obeys Frumkin adsorption isotherm. EIS measurements clarified that the cor- rosion process was mainly charge transfer controlled and no charge in the corrosion mechanism occurred due to the inhibi- tor addition to acidic solutions.

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