J. Basic. Appl. Chem., 5(1)1-15, 2015

© 2015, TextRoad Publication

ISSN 2090-424X

Journal of Basic and

Applied Chemistry

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*Corresponding Author: E. M. ATTIA, Chemistry Department, Faculty of Science (Girls), AL-Azhar University, Cairo,

Egypt

Expired Farcolin Drug as Corrosion Inhibitor for Carbon Steel in

1MHCl Solution

E. M. ATTIA

Chemistry Department, Faculty of Science (Girls), AL-Azhar University, Cairo, Egypt

Received: June 9, 2014 Accepted: August 7, 2014

ABSTRACT

The effect of expired Farcolin drug (EFD) on the corrosion inhibition of carbon steel (CS) electrode in 1M HCl

solution at 293 and 303 K was investigated. Weight loss, open circuit potential, potentiodynamic and

potentiostaticpolarization techniques were applied to study the corrosion behavior of CS in the absence and

presence of different concentrations (1, 2, 10 and 20%) of EFD. The polarization curves showed that EFD

behaved mainly as a mixed-type inhibitor. The inhibition efficiency increased with increasing EFD

concentration, but decreased with increasing solution temperature. It was also found that the adsorption as well

as the inhibition process followed first-order kinetics and obeyed Langmuir’s adsorption isotherm. Results

obtained from weight loss, open circuit and polarization measurements were in good agreement and confirmed

that EFD is a good corrosion inhibitor for CS in 1M HCl solutions.

KEYWORDS: Carbon steel, Expired drugs, Corrosion, Weight loss, Polarization.

1. INTRODUCTION

Low carbon steel is one of the most common types of steel used for general purposes because it is often

less expensive than other types of steel. The corrosion behavior of carbon steel in acidic solution is of

pronounced practical importance because of its widespread applications. Acid solutions are widely used in

industries such as pickling, cleaning, descaling etc., which generally lead to serious metallic corrosion. Organic

compounds usually serve as inhibitors, and generally protect the metal from corrosion by forming a protective

film on the metal surface (Ashassi-Sorkhabi et al., 2006; Amin et al., 2007; Deyab et al., 2009 and Samide et

al., 2011). Organic compounds containing nitrogen, sulphur and oxygen have been reported as excellent

inhibitors (Attia and El-Shafiey, 2009; El-Hajjaji et al., 2014; Singh et al., 2014 and Zaafarany, 2014). Most of

these compounds are unfortunately very expensive. However, great numbers of cheap drugs are known to

possess most of these qualities (Quraishi et al., 2012; Magaji et al., 2012; AL-Sawaad, 2013; Abdel Nazeer et

al., 2013 and Junaedi et al., 2013). When most medications pass their expiry date, they became unsuitable for

use. This often comes about as a result of the degradation of the active ingredients of the medication with

exposure to physical, chemical or microbiological variables like temperature, pressure, humidity, light, bacteria

as well as other components of the product known as excipients.

Unfortunately, there is a scanty of data on the use of the expired drugs as corrosion inhibitors. Therefore,

the aim of the present work is to study the possibility of using the expired Farcolin drug (which lost its activity

for human body) as a corrosion inhibitor for carbon steel.

2. EXPERIMENTAL DETAILS

2.1. Electrode preparation

Cylindrical specimen was taken from a rod of carbon steel (CS) with the chemical composition (wt%): 0.265

C; 0.107 Si; 0.513 Mn; 0.010 P; 0.050 S; 0.032 Cr; 0.0008 Mo; 0.008 Ni; 0.012 Al; 0.001 Co; 0.025 Cu; 0.0005

Nb; 0.0002 Ti; 0.0015 V; 0.004 W; 0.001 Pb; 0.0082 As; 0.0096 B and the remainder was Fe. The rod was

mounted in a glass tube of appropriate diameter using epoxy resin leaving a specified circular surface area (0.502

cm2) to contact the electrolyte, and used as working electrode. The surface of the working electrode was ground

with emery papers to 1200 grit, cleaned in distilled water, degreased in ethanol in sequence and finally dried.

2.2. Solutions and inhibitor

Farcolin (Syrup) is an Egyptian drug produced by Pharco Pharmaceuticals Company. It is a

bronchodilator, antiasthmatic and expectorant. In the present study, it was tested as a corrosion inhibitor at

1

ATTIA, 2015

different concentrations. The concentration of the solution is expressed in terms of % (v/v). Each 5 ml of Farcolin Syrup contains 2 mg of Salbutamol (the active ingredient of Farcolin Syrup) and 100 mg of ammonium chloride. The empirical formula of Salbutamol is C13H21NO3, with molecular weight 239.31

gmol-1 and melting point being 157-158 °C. The chemical name of Salbutamol is 4-[2-(tert-butylamino)-1-

hydroxyethyl]-2-(hydroxymethyl) phenol and its molecular structure is represented in Figure 1. Different

concentrations (1, 2, 10 and 20%) of expired Farcolin drug (EFD) were prepared by diluting a known volume of

drug in 1M HCl solution. The corrosive 1M HCl solution was prepared using reagent grade concentrated acid

and bi-distilled water.

Figure 1: Molecular structure of Salbutamol.

2.3. Weight loss measurements

Cylindrical carbon steel specimens of 8 mm diameter and 3 mm thickness were used as samples for

weight loss tests. Samples were successively polished with emery papers, degreased in ethanol, washed in

doubly distilled water, dried and finally weighted. Triplicate samples were immersed in 15ml of the test

solution in open wide test tubes to check reproducibility of results. A small hole of about 2 mm diameter near

the edge of the specimens was made to help holding them with plastic string and suspend them into the

corrosive medium. The test tubes were covered with aluminum foil and inserted into a water bath maintained

at 293K. After passing the specified immersion time, each specimen was withdrawn from the test solution,

dried in moisture-free desiccator and re-weighted. The experiment was repeated at 303K. In each case, the

difference in weight was taken as the weight loss. The corrosion rate was calculated according to the

following Equation 1 (Ansari et al., 2014): �� � ∆�/�� (1)

Where �� is the corrosion rate (gcm-2 min-1), ∆� (g) is the difference in the specimen weight before and

after immersion in the tested solution, � is the surface area of the carbon steel specimen (cm2) and �is the

exposure time (min). The inhibition efficiency �% was calculated according to Equation 2:

�% � � 100 (2)

The degree of surface coverage � was calculated using Equation

3:

� � 1 ���

�₀

(3)

Where �� and �₀ are the values of weight losses of carbon steel in inhibited and uninhibited solutions,

respectively.

2.4. Open circuit potential measurements

Open-circuit potential measurements were carried out in conventional glass cell of 50 ml solution.

Potential was measured with respect to an external saturated calomel electrode (SCE) interfaced to the test

solution via a salt bridge. The working electrode was immersed vertically in the test solution and the potential

was recorded as a function of time till steady state values were established. The potential measurements were

carried out with the aid of digital multimeter (Keithley, Model 175, USA).

2.5. potentiodynamic and Potentiostaticpolarization

Potentiodynamic polarization measurements were performed with the aid of a double walled glass cell

(50 ml solution) containing three openings for electrodes. A saturated calomel electrode (SCE) provided with a

Luggin capillary probe and a platinum sheet (4cm2) were used as the reference and the counter electrodes,

respectively. The E‒log i curves for all solutions were swept from -0.8 V(SCE) to +0.4 V(SCE) at scan rate of

2.7 mVs-1. The potential range was chosen to obtain the reduction and oxidation processes of the metal during

2

J. Basic. Appl. Chem., 5(1)1-15, 2015

the potential sweeps. The potentiodynamic curves were recorded after keeping the electrode for 300 sec at the

initial potential.

In potentiostatic polarization measurements, a constant potential of -200, -100, +2, +10 and +30

mV(SCE) was applied to the CS electrode in 1M HCl free and containing 10% of inhibitor. Also, a constant

potential of +10 m V(SCE) was applied to 1M HCl solution containing different concentrations (1, 2, 10 and

20%) of inhibitor at 293 and 303K. Potentiodynamic and potentiostatic polarization measurements were

generated using a Wenking Electronic Potentioscan (Model 73).

3. RESULTS AND DISCUSSION

3.1. Weight loss measurements

The iron content of carbon steel is predominantly affected during the corrosion of steel; the anodic

reaction involves the formation of ���� (Equation 4) while the cathodic reaction (in acidic environment)

involves the evolution of hydrogen gas (Equation 5). (Eddy et al., 2008 and Amin et al., 2009),

�� ⇌ ����

+ 2�� (4)

2��

+ 2��

⇌ �� (5)

Thus the overall reaction is as follows,

�� + 2��

⇌ ����

+ �� (6)

Equation6illustrates that as the corrosion proceeds, Fe is consumed as Fe2+, and H2 are evolved. This

indicates that the rate of corrosion could be monitored by knowing the amount of Fe that is consumed or the

amount of Fe2+that is formed. Figure 2 illustrated the weight loss (gcm−2) of carbon steel in 1M HCl at various

time intervals, in the absence and presence of different concentrations of the EFD at 293K and 303K.The

present results indicated that weight loss of CS in presence of various concentrations of expired drug varied

linearly with time and was much lower than that obtained in free solution. The linearity obtained indicated the

absence of insoluble surface film during corrosion and that the inhibitors were first adsorbed onto the metal

surface and, therefore, impede the corrosion process (Lokesh et al. 2014).The increase in the inhibitor

concentration was accompanied by a decrease in weight-loss at both temperatures. These results led to the

conclusion that EFD is fairly efficient as inhibitor for carbon steel dissolution in 1M HCl solution.

Figure 2: Variation of weight loss with exposure time for corrosion of CS in 1M HCl solution in absence

and presence of different concentrations of EFD:

(a) 293 K (b) 303 K.

Figure 3 illustrated the effect of concentration of EFD on the weight losses at 293 and 303K. It has been

observed that the weight losses of CS increased with increasing temperature but decreased as the concentration

of the inhibitor increases. Temperature had a great effect on the rate of metal corrosion as it is considered the

accelerating factor in most of chemical reactions. It increases the energy of the reacted species, as a result,

chemical reaction get much faster. The corrosion of iron is a chemical reaction in which the Fe atoms at the

metal surface react with the negatively charged anions. Hence, increasing temperature of the environment

increases the activation energy of the Fe atoms at the metal surface and accelerates the corrosion process of

carbon steel in the acidic media. Table 1 indicated that the corrosion rate of CS increased with increasing both

of the periods of contact and solution temperature but decreased with increasing inhibitor concentration.

0.00

0.02

0.04

0.06

0.08

0.10

0 20 40 60 80 100 120

Wt.

Lo

ss,

gcm

-2

Time, min

(a)0%

1%

2%

10%

20% 0.00

0.02

0.04

0.06

0.08

0.10

0 20 40 60 80 100 120

Wt.

lo

ss ,

gcm

-2

Time, min

(b)

0%

1%

2%

10%

20%

3

ATTIA, 2015

Figure 3: Variation of weight loss of CS with concentration of EFD after immersion period of

120 min at 293 and 303K.

Table 1: Corrosion rates (gcm-2min-1) of CS in 1M HCl in absence and presence of different concentrations of EFD at 293 and 303 K after 60 and 120 minutes of immersion

Time Temp. Concentrations of EFD in % (v/v)

0% 1% 2% 10% 20%

60 min 293K 1.012616 0.532869 0.365206 0.149402 0.029880

303K 1.118858 0.581009 0.476428 0.348606 0.119522

120 min 293K 1.328021 0.624170 0.415007 0.166003 0.039841

303K 1.560425 0.637450 0.604250 0.365206 0.121182

The effect of exposure time on the inhibition efficiency at different inhibitor concentrations is shown in Figure 4. The inhibition efficiency for all concentrations studied at 293 K increased with increasing

immersion time (Figure 4(a)). This suggested increasing adsorption of the inhibitor molecules onto metal

surface with increasing exposure time (Singh et al., 2010). Whereas, with increasing temperature to 303 K,

the inhibition efficiency decreases with time at the first 60 minutes followed by increasing its values with

increasing immersion time (Figure 4(b)).The first decrease in inhibition efficiency may be due to desorption of

Salbutamol, the active ingredient of inhibitor molecules and preferential adsorption of chloride ions on the steel

surface. The ammonium chloride (in drug structure) and corrosive hydrochloric acid represent the source of

chloride ions in solution. After a period of 60 minutes, the inhibitor molecules could distribute and fit itself on

the steel surface and compete with the adsorption of chloride ions, therefore its inhibition efficiency increases.

Figure4: Variation of inhibition efficiency of EFD in 1M HCl with exposure time on CS electrode: (a) (a) 293 K (b) 303 K.

The variation of inhibition efficiency with inhibitor concentrations at 293 and 303 K after

immersion time of 60 and 120 minutes is shown in Figure 5. It was observed that EFD inhibited the corrosion of

CS in 1M HCl at all concentrations used in the study. The corrosion inhibition efficiency increased with a

corresponding increase in the concentration of the EFD at a fixed immersion time. This may be due to

sufficient adsorption onto the metal surface and wider coverage by the inhibitor molecules (Abdel Motaal

0.00

0.01

0.02

0.03

0.04

0 5 10 15 20

Wt.

Lo

ss,

gcm

-2Cinh.

, % (V/V)

303K

293K

0

20

40

60

80

100

0 20 40 60 80 100 120

IE %

Time, min

(a)

1%

2%

10%

20%

0

20

40

60

80

100

0 20 40 60 80 100 120

IE %

Time, min

(b)

1%

2%

10%

20%

4

J. Basic. Appl. Chem., 5(1)1-15, 2015

and AL- Qasmi, 2010). Maximum inhibition efficiency was shown at 20% concentration of the inhibitor which

reached to 91% and 98% at 303 and 293 K, respectively. This indicated that the degree of inhibition depends

upon the concentration of inhibitor, the immersion period and the solution temperature.

Figure 5: Variation of inhibition efficiency of EFD in 1M HCl with inhibitor concentration on CS

electrode after immersion time of 60 and 120 min at 293 and 303K.

Kinetics of Corrosion Inhibition

Chemical kinetics treatment of the data was necessary in order to obtain information about the order of the

reaction. The kinetics of the system may be proposed by estimating the concentration of the corroding CS in terms of weight loss per volume (g/l) of the corrodent, and later converted to molar concentrations via mass of

metal-molar mass of iron relation. According to Sharma and Sharma (2004), assuming a (mol/l) is the initial concentration of the carbon steel (��) and x (mol/l) is the final concentration of CS had decomposed into

corrosion products after time t. Therefore, the corrodent concentration of CS at time t is a – x (mol/l).The

presented finding demonstrated that a plot of log(a − x) or log[CS] on Y-axis against time on X-axis gives

straight line graphs with regression coefficient (R2) values are almost equal to unity, confirming a first order

kinetics (Figure 6).

Figure 6: kinetic plots for the corrosion of CS in the absence and presence of various concentrations of

EFD at: (a) 293 K (b) 303 K.

Eddy et al. (2008) applied log (weight loss) and used the following rate equation:

− log�����ℎ� �� = �� � 2.303⁄ (7)

Where �� is the first order rate constant and � is the time in minutes. Also, the half-life of a first order reaction is related to the rate constant according to equation 8 (Atkins, 2002):

��.�

= 0.963/�� (8)

Values of the rate constants and half- life calculated from the slopes of the kinetic plots are recorded in

Table 2. The results revealed that the first order corrosion rate constants (k1) decreased with increasing inhibitor

concentration and that the half- life of corrosion rates of CS in the presence of EFD were higher than the half-

life obtained for the free HCl solution indicating that EFD increased the half-life of corrosion rate of CS in 1M

HCl solution.

30

50

70

90

0 5 10 15 20

IE

%

Cinh., %(V/V)

120 min immersion at 293K

60 min immersion at 293K

120 minimmersion at 303K

60 min immersion at 303K

13

14

15

16

17

18

0 20 40 60 80 100 120

log

[C

S],

mo

l/L

Time, min

(a)

0%

1%

2%

10%

20%

13

14

15

16

17

18

0 20 40 60 80 100 120

log

[C

S],

mo

l/L

Time, min

(b)

0%

1%

2%

10%

20%

5

ATTIA, 2015

Table2: Kinetic parameters for the corrosion of CS in absence and presence of various concentrations of

EFD at 293 and 303K Inh. Conc.

% (v/v)

293K 303K

k1, min-1 ��.�

, min R2 k1, min-1 ��.�

, min R2

0 1.20E-02 5.79E+01 0.9769 7.37E-03 9.40E+01 0.9666

1 3.68E-03 1.88E+02 0.9812 3.92E-03 1.77E+02 0.9881

2 2.53E-03 2.74E+02 0.9757 2.99E-03 2.31E+02 0.9753

10 9.21E-04 7.52E+02 0.9761 2.07E-03 3.34E+02 0.9913

20 4.61E-05 1.50E+04 0.9250 4.61E-04 1.50E+03 0.9374

3.2. Open circuit potential measurements

Figure 7 represented the potential-time curves of CS corrosion in 1M HCl free and containing different

concentrations of EFD at 293 and 303K. The curves illustrated that, the potential of CS electrode in free 1M

HCl tended towards more negative potential values. It represented the breakdown of the pre-immersion air

formed oxide presented on the surface. Addition of EFD shifted the potential to positive direction. The potential

was getting more positive by increasing the concentration of EFD.

Figure 7: Potential- time curves of CS electrode in 1MHCl in absence and presence of different

concentrations of EFD: (a) 293 K (b) 303 K.

The attainment of the steady state after dissolution of the oxide film in 1MHCl containing different concentrations of EFD at 293 and 303 K indicates that dissolution – precipitation mechanism is followed.

This might be attributed to the interaction between inhibitor molecules and oxide film. This interaction led

to a greater area of metal being exposed to corrosive medium.

The immersions as well as the steady state potential values for CS electrode in all concentrations seemed

to coincide with that predicted by the Pourbaix diagrams (Pourbaix,1974) assuming passivity is caused by passive films containing ����� and/or �����. The electrochemical reactions involved in the formation of the

two oxides are given by equations 9 and 10:

2�� + 3��� = ������������ + 6� + 6�� (9)

2�� + 4��� = ����� + 8� + 8�� (10)

The general shift to noble direction points toward oxidation of ����� to �����, is represented by

the following equations:

4����� + 2��� = 6����� + 4� + 4�� (11)

4�� + �� + 2��� = 4��� (12)

4����� + �� = 6����� (13)

The high positive shift indicated the high amount of ����� presented in the oxide layer (Ahmed et

al., 2008).The general shift of open-circuit potential to more active values indicated the attack of the base

metal through the pores presented in the oxide.

3.3. Potentiodynamic polarization measurements

Polarization measurements were carried out in order to gain knowledge concerning the kinetics of the

cathodic and anodic reactions. Potentiodynamicpolarization curves of CS in 1M HCl in the absence and

-350

-330

-310

-290

-270

-250

-230

0 20 40 60 80 100 120

E,

mV

(SC

E)

Time, min

(a)

-350

-330

-310

-290

-270

-250

-230

0 20 40 60 80 100 120

E,

mV

(SC

E)

Time, min

(b)

6

J. Basic. Appl. Chem., 5(1)1-15, 2015

presence of various concentrations of EFD at 293 and 303Kare illustrated in Figure 8. The important corrosion

parameters derived from these curves are presented in Table 3. The linear Tafel segment of anodic and cathodic

curves are extrapolated to corrosion potential to obtain the corrosion current densities (Icorr). The corrosion

inhibition efficiency (IE%) was calculated from the measured Icorr values using equation 14 (Ansari et al.,

2014):

��% = �1 − �����

�����

°� × 100 (14)

Where �����° and ����� are the corrosion current densities obtained in uninhibited and inhibited solutions. The

corrosion current density is converted to a corrosion rate by using equation15 (Attia and El-Shafiey, 2009): �� = 0.13 ����� � ρ⁄ (15)

Where �� is the corrosion rate in mm/y, ����� is the corrosion current density in mA/cm2, e is the

equivalent weight of metal in g/mol, and ρ is the density of metal in g/cm3. The obtained results are the mean of

four experiments.

From Table 3, it is clear that the values of corrosion current density (Icorr) of uninhibited solution decreased

from 5.8 to 0.45 mA/cm2 which resulted in 92.2% inhibition efficiency at 303K and from 1.8 to 0.09 mA/cm2

and increased the inhibition efficiency up to 95.0 % at 293K in the presence of 20% EFD in 1M HCl solution.

Generally, addition of EFD did not cause any significant change in the value of Ecorr at 303K. The maximum

displacement in Ecorr value was 10 mV only for 2, 10 and 20% inhibitor concentrations and 30mV for 1%

inhibitor concentration. On the other hand, at 293K, anodic behaviors can be observed for 2, 10 and 20%

inhibitor concentration, while cathodic behavior was observed at 1% inhibitor concentration. This meant that

the hydrogen evolution controlling process (Equation 5) is reduced and the dissolution of the anode (Equation

4) can be controlled at 293K.The classification of a compound as an anodic or cathodic inhibitor is feasible

when the corrosion potential displacement is at least 85mV in relation to that one measured for the blank

solution (Junaedi et al., 2013). Therefore, EFD could be classified as a mixed type inhibitor, but under

prominent anodic control.

Figure 8: Potentiodynamic polarization curves of CS electrode in 1M HCl in absence and presence of

different concentrations of EFD: (a) 293K and (b) 303K.

The anodic Tafel slopes (βa) were observed to change with addition of inhibitors suggesting that

the inhibitor was first adsorbed onto the metal surface and impeded the passage of metal ions from the oxide-

free metal surface into the solution, by merely blocking the reaction sites of the metal surface thus affecting the

anodic reaction mechanism. The cathodic Tafel slopes (βc) were also change in the presence of inhibitor which ensured that EFD is a mixed type inhibitor.

From Table 3, it was clear that as the concentration of EFD increased, the corrosion rates decreased and

the inhibition efficiency increased. These results revealed that, at the two studied temperatures, the adsorption

of inhibitor as well as the degree of surface coverage on CS electrode increased as the inhibitor concentration

increased. On the other hand, it is pronounced that inhibition efficiency decreased with increasing temperature,

which indicated desorption of inhibitor molecules at higher temperature.

-3

-2

-1

0

1

2

3

-800 -600 -400 -200 0 200 400

log

i,

mA

cm-2

E, mV (SCE)

(a)

0%

1%

2%

10%

20%-3

-2

-1

0

1

2

3

-800 -600 -400 -200 0 200 400

log

i,

mA

cm-2

E, mV (SCE)

(b)

0%

1%

2%

10%

20%

7

ATTIA, 2015

Table 3: Potentiodynamic polarization parameters of CS electrode in 1M HCl containing different

concentrations of EFD at 293 and 303 K Temp. Cinh.

% (v/v)

Ecorr

(mV/SCE)

Icorr

(mA/cm2)

βa (mV/dec.)

- βc

(mV/dec.)

CR

mmpy

IE

%

293K

0 -380 1.80 173 165 1.662 -

1 -470 0.36 107 93

0.332 81.53

2 -320 0.22 123 133 0.203 87.70

10 -325 0.18 112 105 0.166 90.00

20 -310 0.09 119 122 0.083 95.00

303K

0 -310 5.80 115 145 5.356 -

1 -280 1.80 120 104 1.662 68.96

2 -320 -320

1.60 141 110 1.477 72.41

10 -320 0.96 142 144 0.886 83.44

20 -320 0.45 175 133 0.415 92.24

Thermodynamic considerations

In order to study the effect of temperature on the corrosion rate of CS in the presence of EFD as an

inhibitor, Arrhenius equation,(Equation 16), is used (Ebenso et al., 2009):

log �������� =

���.���� × � ��� −

���� (16)

Where � is the activation energy of the corrosion reaction, � is the universal gas constant, 8.314 J/mol

K, � is the absolute temperature. ��� and ���, are the values of corrosion rates at 293 and 303 K respectively.

The values of � for the inhibited corrosion reaction of CS electrode were calculated and tabulated in

Table 4. Generally, the higher values of activation energy (�) in the presence of inhibitor than in its absence

were attributed to its physical adsorption, while the chemisorption is pronounced in the opposite case

(Mohammed et al., 2012). In the present study, values of � of CS in1M HCl solution containing

different concentrations (1-20%) of EFD are higher than that in free HCl solution. These � values

indicated that the corrosion reaction of CS was retarded by EFD. It also supported the phenomenon of physical

adsorption. Szauer and Brandt (1981) explained that the increment in � can be attributed to an

appreciable decrease in the adsorption of the inhibitor on the metal surface with increase in temperature and

a corresponding increase in corrosion rates occurs due to the fact that greater area of metal is exposed to acid

environment. Values of heat of adsorption (�) were calculated using equation 17 (Ebenso et al., 2009):

� = 2.303� log�

1 − �� − log�

1 − ��� ����

�� − ��� (17)

Where, Ө is the surface coverage degree. Calculated values of Qads were negative indicating that the

adsorption of EFD on CS surface is exothermic (Table 4).

Table 4: Activation energy and heat of adsorption of EFD on CS surface Cinh.

% (v/v) ��

kJ/mol

���� kJ/mol

0

1

2

10

20

86.42

118.87 146.58

123.66

118.89

……..

-50.70 -73.79

-42.83

-34.63

Adsorption isotherms Adsorption isotherms are very important in understanding the mechanism of inhibition of corrosion

reaction of metals and alloys. In the present study, Langmuir adsorption isotherm is found to be the best fit. A

famous form of the Langmuir model was illustrated in equation18 (Attia and El-Shafiey, 2009):

�� �.

�⁄ = 1 ���⁄ + �� �.

��⁄ (18)

8

J. Basic. Appl. Chem., 5(1)1-15, 2015

Where Cinh., is the inhibitor concentration, Ө, is the surface coverage degree, Bs, is the sorbent binding capacity

and K, is the binding constant, that is related to the adsorption/ desorption energy, and defined as follows

(Mokhtari et al., 2014):

� = 1

������ � ��� −∆���� � (19)

Where ������ �, is the molar concentration of the solvent, which in case of water is 55.5 mol/l. �, is the

universal gas constant, � is the absolute temperature and ∆��, is the adsorption free energy.

A straight line with correlation coefficient nearly equal to 1.0 was obtained on plotting �� �.

�⁄ against

�� �.

, suggesting adsorption of the EFD on the CS surface following Langmuir adsorption isotherm. The

isotherms at 293 and 303 K for different concentrations of EFD in 1M HCl were illustrated in Figure 9 and

Table 5.

Figure 9: Langmuir adsorption isotherm for EFD adsorbed on CS surface in 1M HCl solution:

(a) 293 K (b) 303 K.

The values of ∆�� were negative indicating that the adsorption reaction proceeded spontaneously and

occurred via physical adsorption mechanism. Generally, values of ∆��

up to -20 kJ/mol or lower as

obtained in this study were consistent with electrostatic interaction between the charged metal and

molecules, which signifies physical adsorption (Eddy et al., 2010; Abdel Hameed, 2011; Naqvi et al., 2011

and Bhat and Alva, 2011).The sorbent binding capacity, ��, reached its maximum sorption upon complete

saturation of adsorbent surface at 293 K. The slopes of �� �.

�⁄ versus �� �.

plots showed negligible deviation

from unity, which means expected and ideal simulating of the Langmuir adsorption isotherm.

Attempts were made to fit � values to kinetic thermodynamic model illustrated in equation 20 (Deyab,

2007):

log �1 − �� = log�` + � log�� �.

(20)

Where y is the number of inhibitor species occupying one active site, and k` is related to the binding

constant K by Equation 21(Abdel Nazeer et al., 2013):

�`����

= � (21)

A Plot of log � ����� against log�� �.

for different concentrations was shown in Figure 10 and the

adsorption parameters were represented in Table 5. It was observed that the value of binding constant (�) as obtained from the kinetic-thermodynamic model was high compared with that obtained from Langmuir

isotherm. The value of �� indicated that EFD was mainly adsorbed at two active sites of the metallic surface at

the two studied temperatures. The degree of linearity as measured by values of R2 was best applicable at 303

K. This confirmed that the adsorption behavior of the inhibitor was strongly influenced by temperature. On

the other hand, in Langmuir adsorption isotherm, R2 ≈ 1 which meant that the adsorption of EFD at the

electrode surface conformed to Langmuir model more than that of kinetic-thermodynamic model.

0

10

20

30

0 5 10 15 20

Cinh./Ө

Cinh.

, % (v/v)

(a)

0

10

20

30

0 5 10 15 20

Cinh./Ө

Cinh.

, % (v/v)

(b)

9

ATTIA, 2015

Figure 10: Kinetic- Thermodynamic adsorption isotherm of EFD on CS surface at 293 and 303K.

Table 5: Langmuir and kinetic-thermodynamic adsorption parameters of EFD on the CS surface in 1M

HCl at 293 and 303K Absolute temperature 293 K 303 K

Langmuir parameters

R2 0.9993 0.9977

K M-1 3.73 1.56

slope 1.04 1.06

BS 0.95 0.93

∆Gads kJ/mol -12.99 -11.24

Kinetic-thermodynamic model

R2 0.8777 0.9174

y 0.41 0.53

1/y 2.43 1.90

K M-1 41.84 3.63

∆Gads kJ/mol -18.88 -13.36

3.4. Potentiostatic polarization measurements

3.4.1. Effect of applied potential

Figure 11 showed the variation of current density with time at different applied potentials for CS in free

1M HCl at both 293 and 303 K. Applying positive voltages (2, 10, 30 and 100 mV(SCE)) resulted in increasing

the current densities smoothly until stability. However, using negative voltages (-100 and -200 mV(SCE)),

showed no current change with time. This was observed for the two studied temperatures. Generally, as the

applied potentials increased, the instantaneous (ii) as well as the stabilized (is) current densities also increased.

This could be illustrated by the fact that, by increasing the applied potential, the rate of chemical attack of the

oxide film increases. Moreover, the rate of anion penetration becomes pronounced (Attia, 2008).

Figure 12 illustrated the effect of different applied potential on potentiostatic behavior of CS in 10% EFD.

It was observed that the behavior of CS in presence of 10% EFD was similar to that free from inhibitor (Figure

11). The surface charging capacity � was calculated from the fundamental equation:

� = �/� (22)

Where � is the electric charge passed through the metal surface (In the case of constant current, � = It, �

= It / �). The reciprocal capacitance (� -1), was in proportion to the thickness of the oxide film according to

the expression of the layer capacitance presented in the Helmholtz model (Tao et al., 2009):

� = ��°��� (23)

Where �, is the thickness of the oxide film, �, is the dielectric constant of oxide and �°, is the

permittivity of free space (8.845x10-14 F/cm).

Values of initial current density (ii), stabilized current density (is), surface charging capacity (C), and

thickness of formed oxide film (d) for CS in 1M HCl with and without 10% EFD at different applied potentials

at 293 and 303K are summarized in Tables 6 and 7, respectively. Values of ii, is and C for CS in free 1M HCl

were always higher than those in the presence of 10% EFD inhibitor in all applied potentials at the two studied

temperatures. Values of oxide film thickness at the two temperatures are generally higher for inhibited solution

than that free from inhibitor, and generally have higher values at the lower temperature. This illustrated the

positive action of EFD on the corrosion inhibition of CS in 1M HCl at 293 and 303K and the negative action of

0.0

0.5

1.0

1.5

0.0 0.5 1.0 1.5 2.0

Log(θ/1-θ)

Log Cinh

293K

303K

10

J. Basic. Appl. Chem., 5(1)1-15, 2015

increasing temperature on the corrosion inhibition process. These results were in good agreement with those

obtained in potentiodynamic polarization measurements.

Figure 11: Potentiostatic polarization curves of CS in free 1M HCl at different applied potentials: (a)

293 K (b) 303 K.

Figure 12: Potentiostatic polarization curves of CS in 10% EFD at different applied potentials:

(a) 293 K (b) 303 K.

Table (6): Values of ii, is, C and d for CS in 1M HCl with and without 10 % EFD at different applied

potentials at 293 K Applied Potential

Free 1M HCl 1M HCl + 10% EFD ii is C d ii is C d

mV(SCE) mA/cm2 mA/cm2 mF/cm2 Ǻ mA/cm2 mA/cm2 mF/cm2 Ǻ -200 0.8 0.97 0.097 1.29 E-3 0.002 0.01 0.001 1.26 E-1 -100 4.8 8.00 6 2.09 E-5 2 2.70 1.4 9.33 E-5

+2 22.0 29.00 725 1.74 E-7 18 18.10 271 4.64 E-7

+10 27.0 38.00 228 5.53 E-7 17 21.10 158 7.96 E-7 +30 34.0 40.00 67 1.89 E-6 20 26.00 43 2.91 E-6

+100 49.0 53.00 21 6.06 E-6 42 48.80 15 8.61E-6

Table (7): Values of ii, is, C and d for CS in 1M HCl with and without 10% EFD at different applied

potentials at 303 K Applied Potential

Free 1M HCl 1M HCl + 10% EFD ii is C d ii is C d

mV(SCE) mA/cm2 mA/cm2 mF/cm2 Ǻ mA/cm2 mA/cm2 mF/cm2 Ǻ -200 2.3 3.2 1.4 8.72 E-5 1.9 3.20 1.2 1.04 E-4 -100 5.0 10.0 6.0 2.09 E-5 5.0 9.85 4.0 3.19 E-6

+2 22.0 36.0 324.0 3.87 E-8 18.0 24.00 120.0 1.05 E-6

+10 38.0 46.0 276.0 4.55 E-7 20.0 25.20 101.0 1.24 E-6 +30 43.0 49.4 123.0 1.01 E-6 22.0 28.00 47.0 2.69 E-6

+100 48.0 56.0 22.0 5.61 E-6 44.0 52.00 21.0 6.04 E-6

0

10

20

30

40

50

60

0 2 4 6 8 10

Cu

rre

nt

de

nsi

ty,

mA

/cm

2

Time, min

(a)

0

10

20

30

40

50

60

0 2 4 6 8 10

Cu

rre

nt

de

nsi

ty ,

mA

/cm

2

Time, min

(b)

0

10

20

30

40

50

60

0 2 4 6 8 10

curr

en

t d

en

sity

, m

A/c

m2

Time, min

(a)

0

10

20

30

40

50

60

0 2 4 6 8 10

Cu

rre

nt

de

nsi

ty,

mA

/cm

2

Time, min

(b)

11

ATTIA, 2015

3.4.2. Effect of inhibitor concentration

Figure 13 and Table 8 illustrated the effect of inhibitor concentration at constant applied voltage of 10

mV(SCE) on the potentiostatic behavior of CS at 293 and 303 K. It is reported earlier that ����� really contains more than the stoichiometric proportion of iron; evidently, part of this excess iron must be in the ferrous conditions. So, ferric oxide is containing vacant sites in the oxygen lattice. For the maintenance of the electrical neutrality, these oxygen defects must be associated with the ���� ions in the lattice. The rise of temperature

will allow lattice defects from the interior of the grains to move up to the surface and thus provide

quickly dissolving material (Evans, 1978). This was observed for CS in free HCl where the initial and stabilized

current densities increase from 27 and 38 mA/cm2 to 38 and 46 mA/cm2 when the temperature rose from 293

to 303 K, respectively (Table 8), indicating the conversion to quickly dissolving condition by increasing

temperature.

It is clearly observed that at constant applied potential of 10 mV(SCE), the presence of EFD decreased

the recorded current densities than that recorded in free HCl solution. Table 8 demonstrated that, at a specified

concentration, the initial current density, stabilized current density and surface charging capacity were always

higher at the high temperature. On the other hand, at a specified temperature, the increase in inhibitor

concentration led to decreasing the initial current density, stabilized current density and surface charging

capacity. On the contrary, the oxide thicknesses decreased with increasing temperature and increased with

increasing inhibitor concentrations. These results confirm the positive action of EFD on the corrosion inhibition

of CS in 1M HCl solution.

Figure 13: Potentiostatic polarization curves of CS in 1M HCl in absence and presence of different

concentrations of EFD at constant applied voltage of 10 mV(SCE): (a) 293 K (b) 303 K.

Table (8): Values of ii, is, � and � for CS in 1M HCl containing different concentrations of EFD at constant

applied voltage of 10 mV(SCE) at 293 and 303 K EFD

Concentration % (v/v)

293 K 303 K ii

mA/cm2 is

mA/cm2 C

mF/cm2 d

Ǻ * 10-7 ii

mA/cm2 is

mA/cm2 C

mF/cm2 d

Ǻ * 10-7 0 27 38.0 228 5.51 38 46.0 345 3.64

1 20 25.5 191 6.57 33 44.0 220 5.71

2 19 22.3 167 7.51 27 38.0 205 6.12

10 17 21.1 158 7.94 20 25.2 101 0.12

20 18 20.2 151 8.29 17 21.0 157 7.98

Values of 1/� for CS electrode were calculated at different time intervals for different

concentrations of EFD at 293 and 303 K. Figure 14 showed the decrease of film stability (1/ �) with time for all

concentrations of expired drug at the two temperatures which may be due to the increasing number of defects in

the film layer matrix.

15

20

25

30

35

40

45

50

0 2 4 6 8 10

Cu

rre

nt

de

nsi

ty,

mA

/cm

2

Time, min

(a)

15

20

25

30

35

40

45

50

0 2 4 6 8 10

curr

en

t d

en

sity

, m

A/c

m2

Time, min

(b)

12

J. Basic. Appl. Chem., 5(1)1-15, 2015

Figure14:Variation of reciprocal capacitance, 1/C, with time for different concentrations of EFD on CS

electrode: (a) 293K and (b) 303K.

Mechanism of inhibition

The inhibition efficiency of EFD against the corrosion of carbon steel in 1M HCl can be explained on the

basis of the number of adsorption sites, molecular size and mode of interaction with the metal surface (Singh

and Quraishi 2010). Physical adsorption requires presence of both electrically charged surface of the metal and

charged species in the bulk of the solution. In this study, iron in carbon steel represents the metal which having

vacant low-energy electron orbital, and EFD represents the inhibitor which containing molecules having

relatively loosely bound electrons. However, Salbutamol is a complex organic compound containing a

secondary amino group. It is therefore organic base which can be protonated in an acid medium. Thus, they

become cations, existing in equilibrium with the corresponding molecular form as the following:

Salb + xH ↔ [Salbx]x+ (24)

The protonated compound, however, could attach to the carbon steel surface by means of electrostatic

interaction between Cl- and protonated salbutamol since the carbon steel surface has positive charge in the HCl

medium (Singh and Quraishi 2009). Extra explanation based on the assumption that in the chloride media, the

negatively charged Cl- would attach to positively charged surface. When salbutamol adsorbs on the carbon steel

surface, electrostatic interaction takes place by partial transference of electrons from the polar atom (N atom

and delocalized π-electrons of the aromatic ring) of the studied inhibitor to the metal surface.

CONCLUSION The above mentioned results revealed that EFD is a good inhibitor for the corrosion of CS in 1M HCl and

inhibition efficiency is more pronounced with increase in the inhibitor concentration. The inhibition efficiency

of EFD decrease with temperature, leading to the conclusion that the protective film of this compound formed

on the CS surface is less stable at higher temperature. The potentiodynamic polarization curves imply that, EFD

acts as a mixed type inhibitor, but under prominent anodic control for corrosion of CS in 1M HCl. The values of

ΔG°ads indicate adsorption of the inhibitor by physical process, and weight loss indicates adsorption of the

inhibitor on carbon steel is a first order reaction. The adsorption process of the inhibitor on CS surface is

spontaneous and exothermic; it favors a physical adsorption mechanism and is best described by Langmuir

adsorption isotherm.

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