Inhibition of aluminum corrosion by phthalazinone and synergistic effect of halide ion in 1.0 M HCl

Current Applied Physics 12 (2012) 325e330

Contents lists available at ScienceDirect

Current Applied Physics journal homepage: www.elsevier.com/locate/cap

Inhibition of aluminum corrosion by phthalazinone and synergistic effect of halide ion in 1.0 M HCl Ahmed Y. Musa*, Abdul Amir H. Kadhum, Abu Bakar Mohamad, Mohd Sobri Takriff, Eng Pei Chee Department of Chemical and Process Engineering, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 5 May 2011 Received in revised form 2 July 2011 Accepted 4 July 2011 Available online 8 July 2011

The inhibitive effects of 1-(2H)-phthalazinone (PTO) for aluminum alloy (2024) corrosion in 1.0 M HCl solution and the synergistic effect of KI on the corrosion inhibition efficiency were assessed using electrochemical measurements. Results showed that the inhibition efficiency increased with an increase in concentration of the PTO and synergistically increased with addition of KI. Adsorption characteristic of PTO molecules in absence and presence of KI was approximated by Freundlich and Langmuir adsorption isotherm models, respectively. The synergistic effect is found to decrease with increase in the concentration of PTO and a competitive inhibition mechanism exists between KI and PTO cations. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Acid corrosion Electrochemical measurements Corrosion inhibitor Synergistic effects

1. Introduction Aluminum and its alloys are widely used in many industries such as reaction vessels, pipes, machinery and chemical batteries because of their advantages. Hydrochloric acid (HCl) solution is used for pickling, chemical and electrochemical etching of aluminum. Aluminum generally exhibits passive behavior in aqueous solution due to the formation of strong and compact adherent passive oxide film on its surface which influences its corrosion susceptibility. The adhesive passivating surface oxide film is amphoteric and consequently the metal dissolves readily when exposed to aggressive acidic and alkaline solutions. In fact, the corrosion of metallic materials in acidic solution causes considerable costs [1]. One of the most practical methods for protection against corrosion in acidic media is use of inhibitors [2e6]. Most of the effective and efficient organic inhibitors are the compounds which contain hetero-atoms such as oxygen, nitrogen, sulfur, and phosphorus which allowed adsorption on the metal surface [7]. To be effective, an inhibitor must also displace water from the metal surface, interact with anodic or cathodic reaction sites to retard the oxidation and reduction corrosion reaction, and prevent transportation of water and corrosion active species on the surface [8].

* Corresponding author. E-mail addresses: [email protected], [email protected] (A.Y. Musa). 1567-1739/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2011.07.001

Numerous organic and inorganic compounds including polymers have been proved to be effective corrosion inhibitors for metals in acidic medium [1]. Synergism has become one of the most important effects in inhibition processes and serves as the basis for all modern corrosion inhibitor formulations. Synergistic effect is the upgrading of inhibition efficiency of organic compounds in the presence of some anions, particularly halide ions [9]. It is thought that the anions are able to improve adsorption of the organic cations in solution by forming intermediate bridges between the metal surface and the positive end of the organic inhibitor. Corrosion inhibition synergism results from increased surface coverage arising from ion-pair interactions between the organic cations and the anions [9]. Many investigations concerning synergistic inhibition effects have been carried out and are being carried out. Murakawa et al. [10] have investigated the synergistic and antagonistic effects of organic or inorganic anions with alkyl or cyclic amines on the corrosion inhibition of Fe in HClO4. Rudresh and Mayanna [11] have studied the effect of halide ions on the spontaneous dissolution and the anodic and cathodic polarization of zinc in perchloric acid solution with and without n-decylamine. Aramaki et al. [12] have reported the synergistic effect of various anions and the tetra-n-butylammonium cation on the inhibition of iron corrosion in 1 M HClO4 and their joint adsorption on iron. Pavithra et al. [13] have studied the synergistic effect of iodide ions and benzisothiozole-3piperizine hydrochloride (BITP) on corrosion inhibition of mild steel in 0.5 M H2SO4. Eduok et al. [14] have investigated the

A.Y. Musa et al. / Current Applied Physics 12 (2012) 325e330

synergistic action caused by iodide ions on the corrosion inhibition of mild steel in 1 M H2SO4 by leaves and stem extracts of Sidaacuta. Abdel Rehim et al. [15] have studied the synergistic inhibition between adenine and iodide ion for steel corrosion in sulfuric. Amar et al. [16] have investigated the synergistic inhibition offered by Zn2þ and piperidin-1-yl-phosphonic acid (PPA) to the corrosion of Armco iron in 3% chloride solution. Li et al. [17] have reported the synergistic inhibition effect of rare earth cerium(IV) ion and 3,4dihydroxybenzaldehye (DHBA) on corrosion of cold rolled steel (CRS) in H2SO4 solution. Umoren et al. [18] have studied the synergistic inhibition effect of iodide ion and polyacrylamide (PA) on corrosion of pure iron in 0.5 M H2SO4 solution. The aim of the present work is to study the synergistic corrosion inhibition action of potassium iodide (KI) with 1-(2H)-phthalazinone (PTO) on aluminum alloy (Al2024) in 1.0 M HCl using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements.

20 a b 15 c -Zim (ohm cm2)

326

d 10

e

5

0

-5 0

5

10

15

20

25

Zre (ohm cm2 )

2. Experimental works The working electrode used in this research was aluminum alloy (Al2024) with the cylindrical shape and active surface area of 4.5 cm2. The composition of Al2024 was 3.8 wt.% Cu, 0.1 wt.% Cr, 0.5 wt.% Fe, 1.2 wt.% Mg, 0.3 wt.% Mn, 0.5 wt.% Si, 0.15 wt.% Ti, 0.25 wt.% Zn and balanced with the working solution was 1.0 M HCl. The specimens were cleaned according to ASTM standard G103 [19]. The inhibitor used in this research was 1-(2H)-phthalazinone (PTO). PTO was purchased from SigmaeAldrich and was used without further purification. The concentrations of PTO used for this research were 0.25, 0.5, 0.75 and 1 mM. The synergistic inhibition action study was carried out with 0.1% KI as halide ions. Electrochemical measurements were conducted with a Gamry water jacketed glass cell. The cell contained three electrodes, the working, counter and reference electrodes, comprised of mild steel, a graphite bar and a saturated calomel electrode (SCE), respectively. Measurements were performed using a Gamry Instrument Potentiostat/Galvanostat/ZRA ref 600 model. The potentiodynamic currentepotential curves were recorded by changing the electrode potential automatically from 200 to 200 mV versus SCE at a scan rate of 0.5 mV s1. EIS measurements were performed at corrosion potentials (Ecorr) over a frequency range of 10 kHz to 0.1 Hz, with a signal amplitude perturbation of 10 mV. Before performing the electrochemical measurements, the electrochemical cell was let to stabilize the steady state potential. Thermostat was used to maintain the temperature of corrosion cell.

Fig. 1. Nyquist plots for Al2024 in 1.0 M HCl containing various concentrations of PTO at 30  C: (a) blank, (b) 0.25, (c) 0.5, (d) 0.75 and (e) 1 mM.

connected in series with constant phase element, CPE in parallel with charge transfer resistance, Rct and inductor, L which is in series with layer resistance, RL. CPE is represented as

ZðuÞ ¼ Y0 $jua ;

(1)

where Y0 is CPE constant, u is angular frequency (rad/s), j2 ¼ 1 which is an imaginary number, and a is CPE exponent [21]. The fitted impedance parameters are listed in Table 1. The double layer capacitance (Cdl) values were calculated as follows:

Cdl ¼



1 a a: Y0 R1 ct

(2)

The inhibition efficiencies (IE%) from the charge transfer resistance were calculated by the equation shown below:

IE ð%Þ ¼

R0ct  Rct  100; R0ct

(3)

30 a

3. Results and discussion

25 b

3.1. Electrochemical impedance spectroscopy (EIS) measurements -Zim (ohm cm2)

Nyquist plots for Al2024 in 1.0 M HCl for different concentrations of PTO in absence and presence of KI are shown in Figs. 1 and 2, respectively. In the recorded Nyquist plots, it is clearly seen that the impedance increases with increasing concentration of PTO for both absence and presence of KI. In addition, the recorded impedance in presence of KI exhibited larger value due to the synergistic effect of KI. From the inspection of Figs. 1 and 2, the impedance spectra consist of a large capacitive loop at high frequencies followed by a small inductive loop at low frequency. The similar behaviors have been reported for aluminum in HCl solution [20e23]. The capacitive loop is usually related to the charge transfer of the corrosion process and the inductive loop is attributed to the relaxation processes in the oxide film covering electrode surface [20]. The equivalent circuit model used to fit the experimental results in absence and presence of KI is shown in Fig. 4. The circuit consists of solution resistances, Rs

20

c d

15 e 10

5

0

-5 0

5

10

15

20

25

30

35

Zre (ohm cm2) Fig. 2. Nyquist plots for Al2024 in 1.0 M HCl containing 0.1% KI with various concentrations of PTO 30  C: (a) blank, (b) 0.25, (c) 0.5, (d) 0.75 and (e) 1 mM.

A.Y. Musa et al. / Current Applied Physics 12 (2012) 325e330 Table 1 EIS parameters for Al2024 in 1.0 M HCl at different concentrations of PTO in presence and absence of 0.1% KI. KI conc. (%)

Rct (ohm cm2)

a

Cdl (mF cm2)

Blank 0.25 0.25 0.5 0.5 0.75 0.75 1 1

e e 0.1 e 0.1 e 0.1 e 0.1

5.90 8.55 22.52 10.35 24.54 10.80 25.10 18.63 27.27

0.89 0.91 0.93 0.92 0.95 0.97 0.98 0.99 0.99

1.05 0.95 0.77 1.81 0.63 3.24 0.62 1.00 0.63

q

IE%

0.31 0.74 0.43 0.76 0.45 0.77 0.68 0.78

31 74 43 76 45 77 68 78

- 0.65

- 0.70

- 0.75

- 0.80 1E-6

0

where Rct and Rct are the values of charge transfer resistances with and without inhibitor, respectively. Table 1 gives the values of the charge transfer resistance Rct, double layer capacitance Cdl and inhibition efficiency obtained from Fig. 3. From the impedance data given in Table 1, it is clearly seen that the presence of PTO enhances the values of Rct and reduces the Cdl values. The decrease in Cdl is due to the adsorption of PTO to form an adherent film on the metal surface and suggests that the coverage of the metal surface with this film decreases the double layer thickness. The addition of KI further enhances Rct values and reduces Cdl values. This can be attributed to the enhanced adsorption of PTO in the presence of KI because of the synergistic effect of iodide ions. The IE% increased as the PTO concentration increased and it’s further increased in combination of KI with PTO.

3.2. Polarization measurements Tafel polarization curves for Al2024 in 1.0 M HCl for different concentrations of PTO are shown in Fig. 4. The related electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), Tafel anodic constant (ba), Tafel cathodic constant (bc) and inhibition efficiency (IE%) were obtained and listed in Table 2. These values were calculated from the Tafel fit routine provided by Gamry Echem. Analyst software; this routine uses a non-linear chi-squared minimization to fit the data to the SterneGeary equation. The values of inhibition efficiencies were calculated from polarization measurements according to the equation below:

IE ð%Þ ¼

- 0.60

-E vs SC E (mV)

PTO conc. (mM)

icorr  i0corr  100; icorr

(4)

0

where icorr and icorr are the uninhibited and inhibited corrosion current densities, respectively. From Fig. 4, the anodic and cathodic parts were both changed upon the addition of PTO to the acid

327

a

b

d

e

1E- 5

c

1E- 4

1E- 3

1E- 2

1E-1

1E+ 0

icorr (A cm-2) Fig. 4. Potentiodynamic polarization curve for aluminum in 1.0 M HCl containing various concentrations of phthalazinone at 30  C: (a) blank, (b) 0.25, (c) 0.5, (d) 0.75 and (e) 1 mM.

solution. This indicates that PTO affects both anodic dissolution and hydrogen evolution reactions. These results implied that PTO acts as a mixed-type inhibitor [24,25]. It can be seen form Fig. 4 that the increasing in PTO concentrations led to irregularities in the anodic polarization current (overlapping). Fig. 4 also reveals that the cathodic polarization curves followed abnormal trends when the concentrations of PTO increased. The irregularity observed in the anodic region was probably due to variation effects in the adsorbed PTO on the surface. The most probable variation effect of the PTO inhibitor, which is responsible for overlapping in the anodic current, was the protonation process. The protonation of PTO resulted in the attachment of a proton to the highly negative Oatom [26]. Hence, both unprotonated and protonated species of PTO could exist in acid solution. Due to electrostatic attraction, the cationic species adsorbed onto the cathodic sites of mild steel and reduced the evolution of hydrogen, thereby protecting the cathodic sites of steel. The adsorption of PTO at anodic sites could be attributed to the presence of unprotonated molecular species. An increase in the inhibitor concentration probably increased the number of protonated species on the surface and thus decreased the cathodic currents and limited the decrease in the anodic currents (with overlapping behavior). This could explain the strange PTO anodic behavior observed at different concentrations. Additional p-electrons in PTO, which increased the delocalization, may have also contributed to this behavior. Due to increased delocalization, the PTO p-electrons were easily translated to Fe atoms. When this phenomenon is more prevalent than protonation, there is a decrease in anodic current.

Table 2 PDP parameters for Al2024 in 1.0 M HCl at different concentrations of PTO in presence and absence of 0.1% KI.

CPE

Rs

Ecorr icorr bc q PTO conc. KI conc. ba (V dec1) (V dec1) (mV vs SCE) (mA cm2) (mM) (%)

Rct

L

RL

Fig. 3. Equivalent circuit model used to fit the impedance data.

Blank 0.25 0.25 0.5 0.5 0.75 0.75 1 1

e e 0.1 e 0.1 e 0.1 e 0.1

0.15 0.07 0.54 0.09 0.39 0.01 0.37 0.11 0.34

0.09 0.08 0.11 0.09 0.10 0.06 0.11 0.09 0.11

712 659 755 669 747 628 751 641 751

3.70 2.78 1.65 1.88 1.13 1.69 1.07 1.11 0.89

0.25 0.56 0.49 0.69 0.54 0.71 0.7 0.76

IE%

25 56 49 69 54 71 70 76

328

A.Y. Musa et al. / Current Applied Physics 12 (2012) 325e330

Table 2 revealed that the icorr values decreased in presence of PTO and the addition of 0.1% KI further reduces icorr values. The Ecorr values were shifted positively but the values shifted back to negative side in the presence of 0.1% KI. This indicates that the anodic reaction is drastically inhibited in presence of KI [20]. The cathodic Tafel slopes (bc) remain almost unchanged in absence and presence of PTO, indicted that the inhibitive action of PTO is due to adsorption of inhibitor cations on the cathodic active sites. In the other word, the hydrogen evolution reaction (cathodic reaction) was suppressed without changing its mechanism (i.e. simple blocking mechanism). The anodic Tafel slopes remain almost unchanged in the presence of PTO but with values lesser than that for the absence of PTO. The anodic polarization of aluminum in 1.0 M HCl is described by dissolution of aluminum metal. The mechanism described as below [27] Al þ H2O 4 AlOHads þ Hþ þ e

(5)

AlOHads þ 5H2O þ Hþ 4 Al3þ $ 6H2O þ 2e

(6)

Al3þ þ H2O 4 [AlOH]2þ þ Hþ

(7)

[AlOH]2þ þ Cl 4 [AlOHCl]þ In the present study the inhibited solutions contain Cl ions; hence the dissolution process (anodic process) may be preceded by the formation of AlOHCl leading to the observed decrease in ba value. The values of inhibition efficiency increase with increase in PTO concentration reaching a maximum value of 69.91% while the addition of KI improved slightly the inhibition efficiency of PTO.

3.3. Corrosion kinetic parameters Activation energy can be obtained by Arrhenius equation as follows

 Ea ; ¼ A exp  RT 

icorr

(8)

where icorr is the corrosion current (A cm2), A is electrochemical constant, Ea is activation energy (J mol1), R is gas constant (8.314 J mol1 K1) and T is temperature (K). Taking the logarithm of the Arrhenius equation yields:

Table 3 Corrosion kinetic parameters for Al2024 in 1.0 M HCl in absence and presence of PTO. Condition

Ea (kJ mol1)

DHa (kJ mol1)

ΔSa (J mol1 K1)

Blank 1 mM PTO

148.16 188.40

145.50 185.73

300 427.67

 ln icorr ¼

(9)

 icorr ¼

     DHa D Sa RT exp ; exp  RT R Nh

(10)

where ΔHa, ΔSa are the enthalpy and the entropy of activation, respectively, N is Avogadro number (6.022  1023 molecule mol1) and h is the Plank’s constant (6.626  1034 J s mol1). The straight line equation can be obtained after rearranged eq. (11). The rearranged equation is:

    DSa DHa icorr R ¼ ln þ ln  : T R RT Nh

(11)

The values of enthalpy and entropy of activation for Al2024 corrosion in 1.0 M HCl in absence and presence of PTO can be evaluated from the slope and intercept of the curve of ln(icorr/T) versus 1/T, respectively as shown in Fig. 6. The values of enthalpy and entropy of activation for Al2024 corrosion are tabulated in Table 3. It can be seen from Table 3 that the obtained ΔHa for Al2024 in blank solution is lower than in inhibited solution, indicating that the corrosion reaction for Al2024 in blank solution needs lower

10

ln (icorr / T ) (A c m-2 K-1)

12

ln (icorr) (A cm2)

  1 þ ln A: T

PDP measurements were utilized to obtain the icorr values of Al2024 in the absence and presence of 1 mM of PTO at different temperatures of 30, 40, 50 and 60  C. These values were plotted as shown in Fig. 5. The values of activation energy of corrosion were determined from the slope of log(icorr) versus 1/T plots [28]. The Ea values for Al2024 in the absence and presence of 1 mM of PTO were calculated and listed in Table 3. It is clearly seen from the results obtained that the value of Ea is higher in the presence of PTO. This may be attributed to an appreciable decrease in adsorption process of the inhibitors on the metal surface with rise in temperature [1]. A transition state complex is decays to products after forming the high energy [29]. The mathematical form of transition state theory is shown as below:

16

8 Blank

4

Ea R

1mM

8

6

4

2

blank 1mM

0 2.95

3.00

3.05

3.10

3.15

3.20

3.25

3.30

3.35

1000/T(K-1) Fig. 5. Arrhenius plots for aluminum in 1.0 M HCl: (B) Blank and (,) 1 mM phthalazinone.

0 2.9

3.0

3.1

3.2

3.3

3.4

1000/T (K-1) Fig. 6. Transition state plots for aluminum 1 M HCl: (B) Blank and (,) 1 mM phthalazinone.

A.Y. Musa et al. / Current Applied Physics 12 (2012) 325e330

3.4. Adsorption isotherm

1.4

1.0

The nature of processes between inhibitor and metal surface can be provided by the adsorption isotherm [2]. The degrees of surface coverage (q) of different concentrations of PTO in 1.0 M HCl in presence and absence of KI were calculated from PDP measurements as shown elsewhere [2]. Attempts were made to fit q values to various isotherms including Frumkin, Freundlich, Langmuir and Temkin. By far, the best fit was obtained with Freundlich’s isotherm for the adsorption of PTO from 1.0 M HCl in absence of KI which is given below

1 log q ¼ log Kads þ log Cinh ; n

(12)

where Cinh is the inhibitor concentration, Kads is the equilibrium constant of the adsorption/desorption process, and n is the adsorption intensity. The parameter Kads can be obtained from the graph, Fig. 7, of the linear relationship between q versus log Cinh, known as the Freundlich’s plot [31]. The value of Kads is calculated from the intercept as 84.39 mol dm3. In case of the adsorption of PTO from 1.0 M HCl in presence of KI, the system follows Langmuir adsorption isotherm, which is given by

Cinh

q

¼

1 Kads

þ Cinh :

(13)

The plot of Cinh/q versus Cinh, Fig. 8, for PTO in presence of KI yielded a straight line provided that the adsorption of PTO in presence of KI from 1.0 M HCl on the Al2024 surface obeys Langmuir adsorption isotherm. The value of Kads is calculated from the reciprocal of the intercept of isotherm line as 23,529.41 mol dm3. The high value of Kads shows that PTO is strongly adsorbed on the Al2024 surface in

-0.1

0.8 0.6 0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Cinh (mM) Fig. 8. Langmuir’s adsorption isotherms for PTO in presence of 0.1% KI in 1.0 M HCl.

presence of KI [2]. The value of Kads in absence of KI is lower than the value of Kads in presence of KI, this indicates that the PTO molecules were strongly adsorbed on the Al2024 surface in presence of KI; this was due to the synergistic effect of KI. The adsorption free energy (ΔG0ads) of the PTO on Al2024 surface can be calculated using eq. (14) as below:

DG0ads ¼ RT lnð55:5Kads Þ;

(14) 1

1

where R is the gas constant (8.314 J K mol ), T is the absolute temperature (K), and the value, 55.5, in the above equation is the concentration of water in solution in M [32]. The calculated ΔG0ads values were 21.291 and 35.475 kJ mol1 for the adsorption of PTO in absence and presence of KI, respectively. The obtained values of ΔG0ads are negative. The negative values ensure the spontaneity of the adsorption process and the stability of adsorbed layer on the Al2024 surface. The adsorption process of PTO in absence of KI is physical adsorption as the value of ΔG0ads was around 20 kJ mol1 while in the presence of KI, the adsorption process of PTO is chemical adsorption as the value of ΔGoads was around 40 kJ mol1 [2].

y = 0.7193x + 2.0093 R² = 0.9587

-0.2

3.5. Synergistic effect of KI Corrosion parameters obtained from EIS and PDP measurements for Al2024 in 1.0 M HCl containing 0.1% KI with different concentrations of PTO are shown in Tables 1 and 2. It can be seen from these tables that the inhibition efficiency increases with increase in the concentration of inhibitor. The addition of KI to the inhibitor solution enhances the value of inhibition efficiency. The synergism parameters (SI and Sq) were calculated using the relationship given by Aramaki and Hackerman [33]

-0.3

log

y = 1.2433x + 0.0425 R² = 0.9995

1.2

Cinh/ (mM)

energy to occur compared to the inhibited solution. The positive values of ΔHa indicate that the dissolution of Al2024 is endothermic process. The ΔSa for Al2024 in blank solution is also lower than ΔSa in inhibited solution. The positive sign of ΔSa shows a decrease in the systems order [1]. In presence of PTO, the system represents less orderly arrangement relative to initial state while in blank solution, the value of ΔSa decreased but it is still positive, which due to the higher order of the activated complex [30].

329

-0.4

-0.5

-0.6

SI ¼ -0.7 -4

-3.5

-3

-2.5

log Cinh (mM) Fig. 7. Freundlich’s adsorption isotherms for PTO in absence of 0.1% KI in 1.0 M HCl.

1  I1þ2 ; 0 1  I1þ2

I1þ2 ¼ ðI1 þ I2 Þ  ðI1 I2 Þ;

(15)

(16)

where I1 is inhibition efficiency of KI, I2 is inhibition efficiency of 0 PTO and I1þ2 is inhibition efficiency for combination of KI and PTO.

330

A.Y. Musa et al. / Current Applied Physics 12 (2012) 325e330

Table 4 Synergism parameter of Al2024 in 1.0 M HCl. PTO conc. (mM)

0.25 0.5 0.75 1

EIS measurements

PDP measurements

SI

Sq

SI

Sq

0.67 0.61 0.62 0.39

0.69 0.68 0.65 0.52

0.69 0.68 0.65 0.52

0.69 0.68 0.65 0.52

effect is found to decrease with increase in the concentration of PTO and a competitive inhibition mechanism exists between iodide anion and PTO cations. Acknowledgment The authors gratefully acknowledge Universiti Kebangsaan Malaysia for the support of this project under Grant no. UKMGGPM-NBT-037-2011. References

1  q1þ2 Sq ¼ ; 0 1  q1þ2

(17)

q1þ2 ¼ ðq1 þ q2 Þ  ðq1 q2 Þ;

(18)

where q1 is surface coverage by KI, q2 is surface coverage by PTO and 0 q1þ2 is surface coverage by both KI and PTO. The calculated synergism parameters are listed in Table 4. Generally, if the values of Sq < 1 imply that antagonistic behavior prevails which may lead to competitive adsorption, whereas Sq > 1 indicates a synergistic effect. The iodide ions enhance the stability of inhibitor on metal surface by co-adsorption mechanism. Co-adsorption mechanism may be either competitive or co-operative. For competitive adsorption, the anion and the inhibitor cation are adsorbed at different sites on the metal surface. In co-operative adsorption, the anion is chemisorbed on the metal surface and the cation is adsorbed on a layer of the anion. Competitive and co-operative mechanism may occur simultaneously in some cases [34]. The result shows that the values of Sq are less than one. This indicates that there are competitive mechanisms exist between iodide anion and PTO cations. The values of Sq were found to decrease with increasing of inhibitor concentration. This revealed that PTO can act as an effective inhibitor in 1.0 M HCl at smaller concentration in presence of KI. The Al2014 surface has positively charged if refer to mechanism from eq. (6). The protonated PTO molecules are difficult to approach on positively charge of Al2024 surface due to electrostatic repulsion. In presence of iodide ions, the I− will be adsorbed on the Al2024 surface and made the protonated PTO molecules easily reached the surface of Al2024 [35]. 4. Conclusion The synergistic inhibition for Al2024 corrosion in 1.0 M HCl was achieved using KI as halide ion and PTO as a corrosion inhibitor. Results showed that the inhibition efficiency increases with the concentration of PTO and increases further with the presence of KI. PDP measurement revealed that the PTO acted as a mixed-type inhibitor. The addition of PTO to 1.0 M HCl increases the activation energy for Al2024 corrosion process. The adsorption of PTO in absence of KI follows the Freundlich’s isotherm while in presence of KI; the adsorption of PTO follows the Langmuir adsorption isotherm. Based on the adsorption equilibrium constant values, the PTO molecules were strongly adsorbed on the Al2024 surface in presence of KI. The adsorption process of PTO molecules from the 1.0 M HCl on the Al2024 surface in absence and presence of KI was occurred physically and chemically, respectively. The synergistic

[1] S.A. Umoren, I.B. Obot, I.O. Igwe, Op. Corros. J. 2 (2009) 1e7. [2] A.Y. Musa, A.A.H. Kadhum, A.B. Mohamad, M.S. Takriff, A.R. Daud, S.K. Kamarudin, Corros. Sci. 52 (2010) 526e533. [3] A.Y. Musa, A.A.H. Kadhum, A.B. Muhamad, Int. J. Electrochem. Sci. 5 (2010) 1911e1921. [4] A.Y. Musa, A.A.H. Kadhum, M.S. Takriff, A.R. Daud, S.K. Kamarudin, N. Muhamad, Corros. Eng. Sci. Technol. 45 (2010) 163e168. [5] A.Y. Musa, A.A.H. Kadhum, A.B. Mohamad, A.A.B. Rahoma, H. Mesmari, J. Mol. Struct. 969 (2010) 233e237. [6] A.Y. Musa, A.A.H. Kadhum, A.B. Mohamad, A.R. Daud, M.S. Takriff, S.K. Kamarudin, N. Muhamad, Int. J. Electrochem. Sci. 4 (2009) 707e716. [7] I.B. Obot, S.A. Umoren, N.O. Obi-Egbedi, J. Mater. Environ. Sci. 2 (2011) 60e71. [8] R. Fuchs-Godec, V. Dolecek, Colloids Surf. A: Physicochem. Eng. Aspects 244 (2004) 73e76. [9] E.E. Benso, H. Alemu, S.A. Umoren, I.B. Obot, Int. J. Electrochem. Sci. 3 (2008) 1325e1339. [10] T. Murakawa, T. Kato, S. Nagaura, N. Hackerman, Corros. Sci. 8 (1968) 483e489. [11] H.B. Rudresh, S.M. Mayanna, Corros. Sci. 19 (1979) 361e370. [12] K. Aramaki, M. Hagiwara, H. Nishihara, Corros. Sci. 27 (1987) 487e497. [13] M.K. Pavithra, T.V. Venkatesha, K. Vathsala, K.O. Nayana, Corros. Sci. 52 (2010) 3811e3819. [14] U.M. Eduok, S.A. Umoren, A.P. Udoh, Arab. J. Chem. (2010). doi:10.1016/ j.arabjc.2010.09.006. [15] S.S. Abdel Rehim, O.A. Hazzazi, M.A. Amin, K.F. Khaled, Corros. Sci. 50 (2008) 2258e2271. [16] H. Amar, J. Benzakour, A. Derj, D. Villemin, B. Moreau, T. Braisaz, A. Tounsi, Corros. Sci. 50 (2008) 124e130. [17] X. Li, S. Deng, H. Fu, G. Mu, Corros. Sci. 51 (2009) 2639e2651. [18] S.A. Umoren, Y. Li, F.H. Wang, Corros. Sci. 52 (2010) 1777e1786. [19] ASTM G1-3, Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. [20] A.A. El Maghraby, Op. Corros. J. 2 (2009) 189e196. [21] A.Y. Musa, A.B. Mohamad, A.A.H. Kadhum, Y.B.A. Tabal, J. Mater. Eng. Perform. 20 (2011) 394e398. [22] A. Yurt, S. Ulutas, H. Dal, Appl. Surf. Sci. 253 (2006) 919e925. [23] H. Ashassi-sorkhabi, B. Shaabani, B. Aligholipour, D. Seifzadeh, Appl. Surf. Sci. 252 (2006) 4039e4047. [24] M. Bouklah, B. Hammouti, A. Aouniti, M. Benkaddour, A. Bouyanzer, Appl. Surf. Sci. 252 (2005) 6236e6242. [25] R. Solmaz, M.E. Mert, G. Kardas¸, B. Yazici, M. Erbıl, Acta Physico-Chim. Sin. 24 (2008) 1185e1191. [26] A.Y. Musa, A.A.H. Kadhum, A.B. Mohamad, A.R. Daud, M.S. Takriff, S.K. Kamarudin, Corros. Sci. 51 (2009) 2393e2399. [27] Q.B. Zhang, Y.X. Hua, Mater. Chem. Phys. 119 (2010) 57e64. [28] R. Sanchez-Tovar, M.T. Montanes, J. Garcia-Anton, Corros. Sci. 52 (2010) 722e733. [29] A.A. Khadom, A.S. Yaro, A.A.H. Kadhum, A.S. Al Taie, A.Y. Musa, Amer. J. Appl. Sci. 6 (2009) 1403e1409. [30] V.L. Kolev, K.D. Danov, P.A. Kralchevsky, G. Broze, A. Mehreteab, Langmuir 18 (2002) 9106e9109. [31] L.O.B. Benetoli, H. Santana, C.T.B.V. Zaia, D.A.M. Zaia, Monatsh Chem. 139 (2008) 753e761. [32] I.B. Obot, N.O. Obi-Egbedi, Corros. Sci. 52 (2010) 198e204. [33] K. Aramaki, N. Hackerman, J. Electrochem. Soc. 116 (1969) 568e574. [34] M.K. Pavithra, T.V. Venkatesha, K. Vathsala, K.O. Nayana, Corros. Sci. 52 (2010) 3811e3819. [35] A.S. El-Gaber, A.S. Fouda, A.M. El Desoky, Ciência Tecnol. Mater. 20 (2008) 71e77.