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Thermodynamic study on corrosion inhibition of [Fe.sub.78][B.sub.13][Si.sub.9] metallic glass alloy in [Na.sub.2]S[O.sub.4] Solution at different temperatures.

Abstract

The corrosion inhibition characteristics of benzaldehyde thiosemicarbazone and it's p-substituted derivatives on corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] metallic glass alloy in 0.2M [Na.sub.2]S[O.sub.4] were investigated at different temperature (20, 30, 40, 50, 60[degrees]C). Presence of chloride ion in inhibitor free test solution has accelerated effect on alloy corrosion while opposite behavior is observed by bromide ion. Electrochemical results indicated that all the investigated compounds were acted as mixed type inhibitors at elevated temperatures. The trend of inhibition efficiency with temperature to be suggested physical adsorption of these compounds on the corroding amorphous surface. The observed inhibition action for NaCl or NaBr with benzaldehyde thiosemicarbazone compounds was explained to be due to a joint adsorption of both the inhibitors and halide ion on amorphous surface and tow suggested schemes were drown. Thermodynamic functions [DELTA][E.sub.app], [DELTA][H.sup.*] and [DELTA][S.sup.*] has been calculated and are discussed in absence and presence of halide ions.

Keywords: metallic glasses, iron--based alloy, corrosion inhibition, thiosemicarbazone, temperature effect, halide ion, polarization, impedance.

Introduction

Metallic Glasses, in general, have good corrosion resistance in aqueous solutions, both to general and to localized attack [1,2].

The corrosion of Fe- base glassy alloys has been studied in different media [3-5]. The glassy alloys were found suitable for use in distribution, power transformers and in motors because of extremely low core loss. It combines high induction and superb magnetic properties at frequencies and at induction and operating temperatures of these devices. Fe- base glassy alloys can be used in inductors, current transformers, and other devices requiring high permeability and low core loss at low frequencies [6].

[Fe.sub.78][B.sub.13][Si.sub.9] metallic alloy often do not form passive films in aggressive environments and therefore it is essential to control the corrosion of this alloy by some means. The use of inhibitors is one of these ways [7-11]. Organic compounds containing N and S atoms have been reported as superior corrosion inhibitors than those containing N or S atoms alone [12,13], among these thiosemicarbazone (thioSCAzn).

Thiosemicarbazone (thioSCAzn) and their derivatives have continued to be the subject of extensive investigation in chemistry and biology owing to their broad spectrum of anti tumor [14].

Temperatures have great effect on the rate of metals dissolution. The inhibitor efficiency dependence on the temperature and the comparison of the values of effective activation energy ([DELTA][[E.sub.app]) of the corrosion process in absence and in presence of inhibitors leads to some conclusions concerning the mechanism of the inhibiting action. Temperature increase leads usually to a decrease in inhibition efficiency with the resulting variation of the effective activation energy value often interpreted an indication of the formation of an adsorptive film of a physical (electrostatic) character. The opposite dependence demonstrates that a chemisorptive bond between the organic molecule and the metal surface is probable [15].

In continuation of our work [16] on the effect of temperature on the values of the electrochemical parameters and corrosion inhibition of [Fe.sub.78][B.sub.13][Si.sub.9] metallic alloy by benzaldehyde thiosemicarbazone and it's p-substituted derivatives in absence and presence of chloride or bromide ions by electrochemical methods: the impedance and polarization.

Experimental

Experiments were carried out in 0.2 M [Na.sub.2]S[O.sub.4] solution in absence and presence of different thiosemicarbazone derivatives [Fig.1]. The chemicals ([Na.sub.2]S[O.sub.4], NaCl and NaBr) used were of BDH and methanol were of Hyman. Solutions were prepared by using bi-distilled water. All tests were performed in range (20-60) [degrees]C [+ or -]1.

[Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy used was supplied by Good Fellow. The specimens were used without any mechanical polishing with working area (30 [mm.sup.2]) in all experiments. The electrochemical measurements were made on the bright face of the working electrode. The electrode was degreased with alcohol and rinsed several times with bi-distilled water and finally cleaned in an ultrasonic bath.

Electrochemical measurements have been achieved by connecting the electrochemical cell to ACM Gill AC and to a Samsung computer. Potentiodynamic polarization curves were performed with scan rate of 1 mV/s. Electrode potentials were measured with respect to a silver / silver chloride reference electrode with a Luggin capillary bridge and a platinum wire counter electrode.

The inhibition efficiency can be calculated on the basis of the data from the electrochemical experiments from Eq.1:

[Inh..sub.p]% = ([i.sup.[??].sub.corr]- [i.sub.corr] / [i.sup.[??].sub.corr]) X 100 (1)

where [i.sup.[??].sub.corr] and [i.sub.corr] denote corrosion current densities in absence and presence of 10-[sup.4]M of the inhibitors at each studied temperature respectively, in absence and presence of halide ions, determined by extrapolation of Tafel lines to the corrosion potential .

Impedance data was obtained in the frequency range 10 KHz - 0.5 Hz. The inhibition percentage efficiencies (Inh.[R.sub.ct]%) of the corrosion rate in presence of studied thiosemicarbazone compounds calculated as follows Eq.2:

Inh.[R.sub.ct]% = ([R.sup.-1.sub.cto] - [R.sup.-1.sub.ct] / [R.sup.-1.sub.cto]) x 100 (2)

where [R.sup.-1.sub.cto] and [R.sup.-1.sub.ct] are the reciprocals of charge transfer resistance of glassy alloy in absence and the presence of [10.sup.-4]M of the inhibitors at each studied temperature respectively, at absent and presence of halide ions.

[FIGURE 1 OMITTED]

Results and Discussion

Effect of temperature on corrosion inhibition of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in absence and presence of fixed concentration of the inhibitors:

Effect of temperature in the domain (20-60[degrees]C) on corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in 0.2 M [Na.sub.2]S[O.sub.4] solution are shown in Figs. 2 (a&b). The kinetic parameters ([i.sub.corr]), 1/[R.sub.ct] and [C.sub.dl] values are directly proportional to temperature. At the lower temperatures (20-40[degrees]C) the anodic polarization curves show two transient passivity region, first in the rang from -700 to -780mV and anther at range from -550 to 680 mV. The Nyquist curves show approximately open-end semicircle type expect at 20[degrees]C. This behavior at employed temperatures indicates that the charge transfer process mainly controls the corrosion of Fe78B13Si9 glassy alloy. The diameter of semicircles decrease with increasing temperature.

[FIGURE 2 OMITTED]

In presence of [10.sup.-4]M of benzaldehyde thiosemicarbazone and it's p-substituted derivatives indicating higher dissolution of the alloy. Polarization measurements show that nearly all studied inhibitors at the different temperatures act as mixed inhibitors as shown in Fig.3a in presence of inhibitor (BrBndthioSCAzn) for example.

In the presence of inhibitors Nyquist spectra at 20[degrees]C of the studied inhibitors was found to have along tail at low frequency (LF) region as shown in Fig. 3b in presence of inhibitor (BrBndthioSCAzn). This indicates the formation of coating film on the amorphous surface where ions diffusion will take place after the addition of the inhibitors [17]. All the remain Nyquist diagrams show approximately a semicircle indicating that the charge transfer process control the alloy corrosion and the rise temperature does not change the mechanism of the amorphous alloy dissolution. The depression of semicircle connected with certain increase in heterogeneity could be resulting from surface metal roughening. The latter may be caused by enhanced dissolution of the metal, which takes place at high temperatures [15].

[FIGURE 3 OMITTED]

The effect of temperature on inhibition efficiency was calculated in 0.2 M [Na.sub.2]S[O.sub.4] solution containing 10% MeOH in absence and presence [10.sup.-4]M of studied compounds at different temperatures and mentioned in Table 1.

The dependence of inhibition efficiency, [Inh..sub.p] % and Inh.[R.sub.ct] %, with temperature variation depicted in Fig. 4, indicates good agreements between polarization and impedance studies.

[FIGURE 4 OMITTED]

Inhibitor (MEBndthioSCAzn) gives good inhibiting properties at low temperatures (20 and 30oC) then it's efficiency decreases suddenly, whereas (BndthioSCAzn), (EtBndthioSCAzn) and (BrBndthioSCAzn) compounds have less reduce inhibition efficiency with temperature rise. The decrease in the inhibition efficiency of these inhibitors with the rise of temperature advocated the physical nature of adsorption mechanism or columbic type of adsorption on the amorphous surface [16,18,19].

The poorer efficiency gets with temperature increase may be explained due to the time between the process of adsorption and desorption of inhibitor molecules over metal surface (Eq.3), which is become shorter with increase in temperature. Hence, the metal surface remains exposed to the aggressive environment for a longer period thereby increasing the rate of corrosion with increase in temperature and therefore inhibition efficiency falls for all studied inhibitors [20].

[Inh..sub.(sol)] + X [H.sub.2][O.sub.(ads)] [??] [Inh..sub.(ads)] + X [H.sub.2][O.sub.(sol)] (3)

(X is the number of water molecules replaced by one molecule of organic adsorbate.)

As can be seen from Fig.4, (MEBndthioSCAzn) is more strongly adsorbed on amorphous surface than any other inhibitors. The adsorbability decrease in the sequence:

(MEBndthioSCAzn) > (EtBndthioSCAzn) > (BndthioSCAzn) > (BrBndthioSCAzn)

The performance of (MEBndthio-SCAzn) as corrosion inhibitor compared with (EtBndthioSCAzn) and (BndthioSCAzn) at [10.sup.-4]M of each of them can be attributed to the presence of the effective electron releasing group (MeO), hence the increased electron density around the active site (C = S) will lead to greater adsorption of methoxy substituted thiosemicarbazone (MEBndthioSCAzn) on the iron base alloy surface than ethyl substituted derivative (EtBndthioSCAzn), and the unsubstantiated compound (BndthioSCAzn).

The lower value of Inh.% for (BrBndthioSCAzn) as compared with the unsubstantiated compound (BndthioSCAzn) may be attributed to the effect of the bromide atom (electron with drawing inductive effect) which decrease the electron availability on the reaction site and cause less adsorption of bromo derivative on the alloy surface, this trend has been reported by Quraishi etal. [21].

The heat of adsorption (Q) has been calculated from log [THETA]/1-[THETA] vs [10.sup3]/T plots [Langmuir isotherm, Eq.4]. The parameter [THETA] is the part of the surface covered by inhibitor molecule were calculated using: [[THETA]=1-([i.sub.corr/[i.sup.o.sub.corr])] for each inhibitor.

[[THETA]/1-[THETA]] = AC exp (-Q/RT) (4)

Figure 5 shows such plots for different inhibitors at [10.sup.-4]M. The average heat of adsorption obtained from the slope of the above plots are negative and less than 40 [kJmol..sup.-1][Table 2], reflecting the exothermic behavior of the thiosemicarbazone adsorption on the amorphous surface [22].

The apparent activation energy, enthalpy and entropy ([DELTA][E.sub.app], [DELTA][H.sup.*] and [DELTA][S.sup.*]) for the corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in 0.2 M [Na.sub.2]S[O.sub.4] solution containing 10% MeOH were calculated from Arrhenius type Equation:

[FIGURE 5 OMITTED]

[i.sub.corr] = A exp (-[DELTA][E.sub.app]/RT) (5)

1/[R.sub.ct] = A exp (-[DELTA][E.sub.app]/RT) (6)

The plot of log [i.sub.corr] or log 1/[R.sub.ct] against [10.sup.3]/T from polarization and impedance studies, respectively, gave a straight line with a slope of (-[DELTA][E.sub.app]/2.303R), and from transition--state Equation:

([i.sub.corr]/T) = (R/Nh) exp([DELTA][S.sup.*]/R) exp(-[DELTA][H.sup.*]/RT) (7)

(1/[R.sub.ct]/T) =(R/Nh) exp ([DELTA][S.sup.*]/R) exp (-[DELTA][H.sup.*]/RT) (8)

where h is Plank's constant, N is Avogadro's number, a plot of log ([i.sub.corr]/T) or log (1/[R.sub.ct]/T) against [10.sup.3]/T, gave straight line of slope (-[DELTA][H.sup.*]/2.303R), and the intercept is [log (R/Nh) + [DELTA][S.sup.*]/2.303R], from which the values of [DELTA][H.sup.*] and [DELTA][S.sup.*] were deduced. The calculated thermodynamic parameters from both methods are given in Table 2, and are in good agreements.

It is clear that the values of [DELTA][E.sub.app] and [DELTA][H.sup.*] of the corrosion processes of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy are nearly the same and higher in the presence of inhibitors than in free 0.2M [Na.sub.2]S[O.sub.4] solution, indicating that the energy barrier of corrosion reaction increase in the presence of inhibitors without changing the mechanism of dissolution [12]. However, the highest values of activation energy are associated with the most efficient inhibitor. The degree of increasing the activation energy in present work is in good agreements with the above-mentioned sequence of this inhibitors.

The entropy of activation, [DELTA][S.sup.*], in the absence and presence the inhibitors is negative. This implies that, the activated complex in the rate determining step respects association rather than dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex [18]. The less negative values of [DELTA][S.sup.*], in the rpesence of inhibitors implies that the presence of inhibitors leads the corrosion system to be close to the equilibrium state.

Effect of temperature on corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in 0.2 M [Na.sub.2]S[O.sub.4] containing 0.01M [Cl.sup.-]:

The effect of temperature in the range (20-60[degrees]C) on polarization and impedance measurements in 0.2 M [Na.sub.2]S[O.sub.4] solution containing 10% MeOH and 0.01M [Cl.sup.-] in absence and presence [10.sup.-4]M of benzaldehyde thiosemicarbazone and it's p-substituted derivatives has been performed. As expected, in chloride solution the corrosion current density ([i.sub.corr]) and double layer capacitance ([C.sub.dl]) increase with increasing temperature. The opposite behavior was performed for the charge transfer resistance ([R.sub.ct]). The negative sign of inhibition efficiency in Table(1) indicates the accelerating effect of [Cl.sup.-] which increases with temperature rise from 20 to 60[degrees]C [16].

Corrosion potential values in presence of thiosemicarbazone compounds show irregular shift, suggesting that these compounds are mixed inhibitors Fig.6 (a&b). Increase in the corrosion rate with the increase in temperature is related to the desorption process of adsorbed organic molecules at higher temperatures. The presence of transient passivity region in polarization curves, Fig.6a, in presence of inhibitors may due to joint adsorption [16, 23] of both the inhibitor and [Cl.sup.-] on amorphous surface (scheme 1).

[ILLUSTRATION OMITTED]

[FIGURE 6 OMITTED]

In the case of inhibitors MEBndthioSCAzn (not shown) and BrBndthioSCAzn Fig.6b, Nyquist spectrum at 20[degrees]C has a semicircular shape with long tail at LF region, and at 30[degrees]C in the case of inhibitor MEBndthioSCAzn, indicating the coating of the alloy surface by a film that formed by the adsorption of both the inhibitor and [Cl.sup.-], so the diffusion process takes place on the amorphous surface after the addition of both[17]. The remaining spectra of these inhibitors and other inhibitors have approximately a semicircular--type behavior (one time constant) at the temperature employed indicating that corrosion of the amorphous alloy in the test solution in presence of both ,the inhibitor and 0.01M [Cl.sup.-] is controlled mainly by one process which is the charge transfer process.

Table 1 represents the dependence of inhibition efficiency calculated from both the polarization and impedance measurements in test solutions containing 0.01M [Cl.sup.-] in absence and presence of [10.sup.-4]M of benzaldehyde thiosemicarbazone and it's psubstituted derivatives on temperature. Good agreement between the values of inhibition efficiency calculated from both measurements appear, also showing that the inhibitor efficiency in all cases decreases with increasing temperature. The values of inhibition efficiency confirm that the studied compounds act as efficient corrosion inhibitors in the range of the studied temperatures, especially compounds MEBndthioSCAzn and EtBndthioSCAzn in which the inhibition efficiency decreases after 50[degrees]C.

The reduction of the apparent activation energy [DELTA][E.sub.app] in presence 0.01M Cl- (19.91 [kJmol.sup.-1]) as compaired to the blank solution (20.30 kJmol-1) [Table 2], may be explained as a reduction in the energy barrier of the corrosion process, which means increase in the corrosion rate, as observed from the electrochemical measurements.

Comparison of [DELTA][E.sub.app] values with the value of [DELTA][E.sub.app] in the blank solution or in only [Cl.sup.-] solution it was found that the addition of [Cl.sup.-] to the corrosive medium containing MEBndthioSCAzn and BrBndthioSCAzn were increased. This can be interpreted to be due to strong adsorption (improved joint adsorption) process on amorphous surface leading to increase in the energy barrier for the corrosion reaction compared with that in absence of [Cl.sup.-] in the test solution, while the decrease in activation energy in presence of the inhibitor BndthioSCAzn and EtBndthioSCAzn means a reduction in energy barrier.

The comparable values of [DELTA][E.sub.app] and [DELTA][H.sup.*] of the corrosion process [Table 2], indicates that no change has happened in the mechanism of corrosion [12]. Also the corrosion system passes from a random state to an equilibrium state (less negative values of [DELTA][S.sup.*]).

Effect of temperature on corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in 0.2 M [Na.sub.2]S[O.sub.4] containing 0.01M [Br.sup.-]:

The corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in 0.2M [Na.sub.2]S[O.sub.4] solution containing 10% MeOH and 0.01M [Br.sup.-] in absence and presence of [10.sup.-4]M of benzaldehyde thiosemicarbazone and it's p-substituted derivatives has been studied over the temperature range between 20 and 60[degrees]C. In bromide solution, [i.sub.corr] values increased progressively with increase in the bulk solution temperature. Transient passivity regions present at anodic polarization curves at low temperatures (20-40[degrees]C), may be due to adsorption of [Br.sup.-] on the alloy surface. The impedance diagrams approximately have semicircle-type behavior at all temperatures indicating that the corrosion process is controlled by charge transfer process [17]. The positive value of Inh.% show the inhibiting effect of [Br.sup.-] ion on the alloy corrosion.

Rise in temperature from 20 to 60[degrees]C is associated with an increase in the corrosion current in bromide and inhibitor solutions as shown in Fig.7(a&b) in presence of BrBndthioSCAzn (as example). The desorption process at the higher temperatures may be responsible for the increase of corrosion rate since the surface alloy becomes less covered. Anodic polarization carves, show some transient passive region at low temperatures almost at 20 and 30[degrees]C, this may be due to a physical adsorption of both the inhibitor and [Br.sup.-] on the alloy surface.

[FIGURE 7 OMITTED]

The corresponding Nyquist spectra in presence of 0.01 [Br.sup.-] with benzaldehyde thiosemicarbazone and it's derivatives are semicircular with long tails at LF region at low temperatures as shown in Fig.7b for BrBndthio-SCAzn. This behavior is shown only when the amorphous surface has been coated by a film which is formed by joint adsorption [16,23] of both the halide ion and inhibitor molecules, assuming that this joint adsorption of the inhibitor molecules and [Br.sup.-] is ionic in nature and of the overlap or multi adsorption type [scheme 2].

The diffusion tail is also observed at Nyquist diagrams in presence of inhibitor MEBndthioSCAzn and BrBndthioSCAzn in the range (40-60[degrees]C) where the [b.sub.f] values (which is defined as [b.sub.f] = - [sigma] [[omega].sup.-1/2], and qualitatively represented as the Warburg impedance) become larger as the temperature decreases. The larger [b.sub.f] might indicate more difficulty by the ions to diffuse through the pores within the inhibitor films, which mean that the inhibitor films are less porous or have pores with smaller equivalent diameters [17].

[ILLUSTRATION OMITTED]

Inhibition efficiency calculated from both methods show good agreement as shown in Table 1. The decrease of inhibition efficiency with temperature rise confirms the physical adsorption of [Br.sup.-] and inhibitor molecules on amorphous alloy surface as suggested previously.

The high value of [DELTA][E.sub.app] in presence of [Br.sup.-] (32.93 [kJmol.sup.-1]) [Table 2] supported the inhibition behavior of [Br.sup.-] in the tested solutions. Values of [DELTA][E.sub.app] calculated for the solution containing 0.01M [Br.sup.-] and [10.sup.-4]M of each inhibitor is relatively higher than that obtained in the blank solution. This result can be explained with the prediction that the protection of amorphous alloy in presence of Br- with inhibition includes the physical adsorption of both at the amorphous surface. The low values of [DELTA][E.sub.app] in presence of 0.01M [Br.sup.-] and inhibitors BndthioSCAzn and MEBndthioSCAzn compared with values in the free solution indicates the increase of the corrosion rate while the increase of [DELTA][E.sub.app] in presence of both [Br.sup.-] and inhibitors EtBndthioSCAzn and BrBndthioSCAzn indicates the increase of the inhibition efficiency.

The good agreement between [DELTA][E.sub.app] and [DELTA][H.sup.*] values indicates that the addition of Br-to the test solution does not change the mechanism of corrosion [12]. The less negative values of [DELTA][S.sup.*] in presence of [Br.sup.-] in [Na.sub.2]S[O.sub.4] solution confirm the stabilization of corrosion system, the corrosion reaction will proceed very slow.

Conclusion

It can be concluded that:

i. The corrosion rate of [Fe.sub.78][B.sub.13] [Si.sub.9] metallic glass is directly proportional to temperature rise in 0.2 M [Na.sub.2]S[O.sub.4] solution.

ii. Benzaldehyde thiosemicarbazone and it's p-substituted derivatives perform well as inhibitors of the corrosion of [Fe.sub.78][B.sub.13][Si.sub.9] glassy alloy in 0.2M [Na.sub.2]S[O.sub.4] solution, but better performance is observed with MEBndthioSCAzn.

iii. The acceleration effect of chloride was found to increase with temperature rise from [20.sup.[??]] to [60.sup.[??]]C. The opposite behavior was observed by [Br.sup.-] ion.

iv. The presence of halide ions in the corrosive medium containing benzaldehyd thiosemicarbazone compounds inhibited the corrosion of iron base glassy alloy. Although inhibition efficiency decreases with temperature rise, their good performance continued.

v. On the basis of heat of adsorption, activation energy and the experimentally observed increase in inhibition at low temperatures, a physisorption process is proposed for both benzaldehyde thiosemicarbasozone compounds and halide ions.

References

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S.T. Arab (1) and K. M. Emran (2)

(1) Department of Chemistry, Girls' College of Education, P.O(55002), Jeddah 21413 Kingdom Saudi Arabia E-mail: s.t.arab@hotmail.com

(2) Dppartment of Chemistry, Girls' College of Education, Al-Madinah Al-Monawarah, Kingdom Saudi Arabia. E-mail: k_imran2000sa@yahoo.co.uk
Table 1: Inhibition percentages for [Fe.sub.78][B.sub.13][Si.sub.9]
metallic glass corrosion in 0.2M [Na.sub.2]S[O.sub.4] solution
containing 10% MeOH in absence and presence of [10.sup.-4] M of
studied inhibitors and 0.01M of [Cl.sup.-] or [Br.sup.-] at
different temperatures.

 BndthioSCAzn
Temp.
[degrees]C Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 - - 84.03 79.02
30 - - 79.20 77.08
40 - - 74.28 75.06
50 - - 71.27 74.06
60 - - 64.22 60.70

 MEBndthioSCAzn EtBndthioSCAzn
Temp.
[degrees]C Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 83.98 90.00 84.78 82.71
30 83.82 89.65 81.41 80.44
40 58.07 60.79 76.36 78.83
50 57.89 55.46 73.44 71.31
60 50.49 47.17 59.27 59.50

 BrBndthioSCAzn 0.01M CT
Temp.
[degrees]C Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 77.16 74.30 -21.64 -24.61
30 76.26 73.85 -23.84 -24.80
40 69.49 73.68 -35.22 -26.54
50 68.38 70.43 -45.95 -82.29
60 67.84 63.38 -69.85 -

Temp. BndthioSCAzn MEBndthioSCAzn
[degrees]C +0.01M [Cl.sup.-] +0.01M [Cl.sup.-]

 Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 88.09 85.77 90.75 89.54
30 84.03 85.63 88.87 89.48
40 8,283 85.05 78.83 80.04
50 80.10 80.53 75.69 76.63
60 79.71 79.96 57.00 45.00

Temp. EtBndthioSCAzn BrBndthioSCAzn
[degrees]C +0.01M [Cl.sup.-] +0.01M [Cl.sup.-]

 Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 76.27 81.04 89.70 88.68
30 72.06 79.71 89.18 88.24
40 7,133 79.67 8,442 87.60
50 68.16 75.97 78.58 77.49
60 53.07 48.74 75.27 77.21

Temp. 0.01M [Br.sup.-] BndthioSCAzn
[degrees]C +0.01M [Br.sup.-]

 Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 56.87 61.24 84.03 82.96
30 56.20 57.24 83.93 82.33
40 51.92 43.53 82.83 77.63
50 31.98 20.39 76.41 76.72
60 24.26 19.79 75.74 74.04

Temp. MEBndthioSCAzn EtBndthioSCAzn
[degrees]C +0.01M [Br.sup.-] +0.01M [Br.sup.-]

 Inh.[R.sub.ct] Inh.[R.sub.ct]
 Inh.p % % Inh.p % %

20 91.49 89.09 87.71 83.08
30 90.34 88.30 83.30 82.91
40 79.39 82.25 65.18 69.39
50 74.24 76.33 63.31 68.75
60 73.83 74.57 63.09 67.69

Temp. BrBndthioSCAzn
[degrees]C +0.01M [Br.sup.-]

 Inh.[R.sub.ct]
 Inh.p % %

20 90.00 98.42
30 89.60 89.29
40 88.98 87.84
50 87.26 87.13
60 66.55 63.39

Table 2: Values of heat of adsorption (Q) and thermodynamic parameters
for [Fe.sub.78][B.sub.13][Si.sub.9] metallic glass corrosion in 0.2M
[Na.sub.2]S[0.sub.4] solution containing 10%MeOH in absence and
presence of [10.sup.-]4 M of studied inhibitors from Polarization and
Impedance measurements, and in absence and presence of 0.01M of
[Cl.sup.-] or [Br.sup.-]

Halide Heat of
Ion adsorption
 Q
 kJ[mol.sup.-1]

--- Blank -
 BndthioSCAzn -20.91
 MEBndthioSCAzn -37.37
 EtBndthioSCAzn -25.38
 BrBndthioSCAzn -11.04

[Cl.sup.-] Blank + 0.01 [Cl.sup.-] -
 BndthioSCAzn -
 MEBndthioSCAzn -
 EtBndthioSCAZn -
 BrBndthluSCAzn -

[Br.sup.-] Blank + 0.01 [Br.sup.-] -
 BndthioSCAzn -
 MEBndthioSCAzn -
 EtBndthioSCAzn -
 BrBndthioSCAzn -

 Polarization

 [DELTA][E.sub.app] [DELTA][H.sup.*]
 kJ[mol.sup.-1] kJ[mol.sup.-1]

Blank 20.30 17.71
BndthioSCAzn 36.07 33.43
MEBndthioSCAzn 43.56 40.99
EtBndthioSCAzn 39.09 36.46
BrBndthioSCAzn 28.20 25.62

Blank + 0.01 [Cl.sup.-] 19.91 24.61
BndthioSCAzn 23.93 21.27
MEBndthioSCAzn 51.52 48.84
EtBndthioSCAZn 32.36 29.67
BrBndthluSCAzn 40.02 37.41

Blank + 0.01 [Br.sup.-] 32.93 30.25
BndthioSCAzn 30.06 27.57
MEBndthioSCAzn 40.78 38.10
EtBndthioSCAzn 39.06 36.36
BrBndthioSCAzn 40.97 38.49

 Polarization Impedance

 [DELTA][S.sup.*] [DELTA][E.sub.app]
 J[mol.sup.-1] kJ[mol.sup.-1]

Blank -187.37 22.55
BndthioSCAzn -148.76 31.77
MEBndthioSCAzn -122.67 59.36
EtBndthioSCAzn -139.41 39.27
BrBndthioSCAzn -172.68 28.98

Blank + 0.01 [Cl.sup.-] -163.30 -
BndthioSCAzn -190.13 30.44
MEBndthioSCAzn -101.67 55.53
EtBndthioSCAZn -158.86 39.83
BrBndthluSCAzn -139.80 39.06

Blank + 0.01 [Br.sup.-] -152.20 39.25
BndthioSCAzn -169.81 31.59
MEBndthioSCAzn -136.31 40.93
EtBndthioSCAzn -138.61 38.10
BrBndthioSCAzn -137.65 31.59

 Impedance

 [DELTA][H.sup.*] [DELTA][S.sup.*]
 kJ[mol.sup.-1] J[mol.sup.-1]K

Blank 19.93 -233.98
BndthioSCAzn 30.96 -209.79
MEBndthioSCAzn 56.83 -127.31
EtBndthioSCAzn 36.68 -192.15
BrBndthioSCAzn 26.55 -172.68

Blank + 0.01 [Cl.sup.-] 29.83 -165.60
BndthioSCAzn 27.95 -223.43
MEBndthioSCAzn 52.85 -141.67
EtBndthioSCAZn 36.95 -190.49
BrBndthluSCAzn 39.38 -196.75

Blank + 0.01 [Br.sup.-] 36.76 -184.56
BndthioSCAzn 29.10 -217.68
MEBndthioSCAzn 39.29 -186.47
EtBndthioSCAzn 35.42 -195.99
BrBndthioSCAzn 29.10 -217.76
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Author:Arab, S.T.; Emran, K.M.
Publication:International Journal of Applied Chemistry
Geographic Code:7SAUD
Date:Jan 1, 2007
Words:5193
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