1

Inhibition of mild steel corrosion in acidic medium by Vanillin

(biodegradable) cationic surfactants

Ismail Aiad a

, Samy M. Shabana, Mohamed M. El-Sukkary

a, E. A. Soliman

b

& Moshira Y. El-Awady a,

A Petrochemical Department, Egyptian Petroleum Research Institute,

B Faculty of Science, Ain Shams University, Cairo, Egypt

Abstract

Three cationic surfactants based on Schiff base were laboratory prepared. These compounds are

N-(3-((4-hydroxy-3-methoxybenzylidene)amino)propyl)-N,N-dimethyldecan-1- ammonium

bromide (I), N-(3-((4-hydroxy-3-methoxybenzylidene)amino)propyl)-N,N-dimethyldodecan-1-

ammonium bromide (II) and N-(3-((4-hydroxy-3-methoxybenzylidene)amino)propyl)-N,N-

dimethylhexadecan-1- ammonium bromide (III). The chemical structure was confirmed by

different techniques, FTIR, 1HNMR and mass spectroscopy. Three techniques were used for the

corrosion inhibition evaluation, mainly; the weight loss, Tafl polarization and electrochemical

impedance. The effect of these compounds on the sulfate reducing bacteria was determined by

the serial dilution method. The results showed that the prepared compounds have good

antimicrobial activities against gram positive and gram negative bacteria and fungi as well as

they have higher efficiency as corrosion inhibitors for carbon steel in 1.0 M HCl .

Keywords: Steel, Weight loss, Potentiodynamic polarization, Impedance, Acid solution,

Adsorption isotherm.

1. Introduction

Cationic surfactants is a class of surfactants which mainly used as carbon steel corrosion

inhibitors, especially in acidic medium, due to their high adsorptive character to metal surface

resulting from surfactants amphibathic structure [1-2] beside positive charge, electronegative

atoms, unsaturated bonds and plane conjugated systems including all kinds of aromatic cycles [3-

7].

Both hydrophilic and hydrophobic parts of surfactants enhance the inhibition on the metal

surface in a corrosive medium in addition cationic surfactants have many advantages such as low

price, low toxicity, and easy production [8-9].

2

Schiff base compounds are effective inhibitors in acid media, due to the presence of the ـــ CH=N

.group [10-12]ـــ

This work aimed to modify dimethylaminopropylamine (DMAPA) which contains one

primary and one tertiary amine group to corrosion inhibitors based on Schiff base and compare

their abilities to protect steel from 1.0 M HCl corrosive actions at elevated temperatures using

three different techniques, weight loss, polarization and electrochemical impedance. The

thermodynamic parameters of inhibitor adsorption on the metal surface and metal dissolution

were studied. In addition, investigation the adsorption mechanism of these inhibitors on a carbon

steel surface.

2. Materials and experimental techniques

2.1. Chemical inhibitors:

The used cationic surfactants inhibitors were prepared according to reference [13]. The

chemical structures were shown in Scheme (1).

Scheme 1: Scheme preparation of cationic surfactants inhibitors.

Solution:

I

II

III

3

The aggressive solutions (1.0 M HCl) were prepared by dilution of analytical grade, 37% HCl

with distilled water. The concentration of the prepared ompounds in corrosive solution was

ranging from 1*10-5

to 1*10-2

M.

2.2. Corrosion Measurements:

The corrosion inhibition of carbon steel in 1.0 M HCl in the presence and absence of

inhibitors has been employed using three different techniques. These techniques are weight loss

method, potentiodynamic polarization method and electrochemical impedance spectroscopy

(EIS).

2.2.1. Weight Loss Method:

The weight loss technique was used to measure the inhibition of prepared cationic surfactants

as corrosion inhibitors for mild steel specimens having a composition of: 0.21 C, 0.035 Si, 0.51

Mn, 0.82 P (weight %), and the remainder is Fe. Each specimen was sequentially machined into

regular shapes of 3 cm x 6.0 cm x 0.6 cm with a total surface area of 46.8 cm2. The specimens

were sequentially abraded with different emery papers, degreased with acetone, washed with

distilled water and dried. The coupons initial weight using an analytic balance (precision: ±0.1

mg) was recorded before immersion in 150 ml closed beakers containing 100 ml of corrosive

solution without and with different concentrations of the synthesized cationic compounds [14].

The specimens were taken out, washed, then dried and reweighed accurately. The corrosion rates

of mild steel have been determined for the 24 h immersion period at 25, 40, 55 and 70 ±1°C. All

experiments were carried out in triplicate and the average weight loss values were obtained [15].

2.2.2. Potentiodynamic polarization:

Electrochemical experiments were carried out using a Voltalab 40 Potentiostat PGZ 301 in a

conventional electrolytic cell with three-electrode arrangement: saturated calomel reference

electrode (SCE), a platinum rod as a counter electrode and the working electrode (WE) had the

form of rod from carbon steel. Prior to each experiment, the specimen was treated as in weight

loss experiment. The electrode potential was allowed to stabilize 60 min before starting the

measurements. The exposed electrode area to the corrosive solution is 0.7 cm2. All experiments

were conducted at 25oC. Potentiodynamic polarization curves were obtained by changing the

4

electrode potential automatically (from -1000 to -200 mV vs. SCE) at open circuit potential with

a scan rate of 2 mVs-1

.

2.2.3. Electrochemical Impedance Spectroscopy (EIS):

The same C-steel electrode used in potentiodynamic polarization was used. Measurements were

obtained using a Voltalab 40 for all EIS measurements with a frequency range of 100 kHz to 50

mHz with a 4 mV sine wave as the excitation signal at open circuit potential. If the real part is

plotted on the X-axis and the imaginary part is plotted on the Y-axis of a chart, we get a Nyquist

Plot, The charge transfer resistance values (Rct) was calculated from the difference in impedance

at lower and higher frequencies.

3. RESULTS AND DISCUSSION:

3.1. Corrosion Measurements:

3.1.1. Gravimetrical Evaluation of the Synthesized Inhibitors:

I. Effect of Inhibitor Concentrations:

The inhibition effect of prepared cationic surfactants on the corrosion of mild carbon steel

in 1.0M hydrochloric acid were calculated, the immersion time was 24 hours in the presence and

absence of different concentrations of prepared compounds.

The inhibition efficiency (η %) of an inhibitor was calculated from the following equation:

η = ((CR – CR\)/ CR)*100 (1)

The CR and CR\ are the corrosion rates of carbon steel in the absence and presence of

inhibitors at a given inhibitor concentration and temperature. The corrosion rate was calculated

from the following equation:

CR = W / St (2)

Where W is the weight loss in mg, S is the surface area of the mild steel species in Cm2 and t

is the time of immersion in hours.

The weight loss data was listed in Tables (1-3), from which, increasing the inhibitor

concentrations, the corrosion rates decrease as a result of decreasing metal loss, due to increasing

inhibitors adsorption on the metal surface which leads to increasing coverage metal area (θ). This

5

adsorption forms a protection layer on the metal surface, which decreases the contact between

the metal surface and the aggressive medium, so decreasing the fatal effect of aggressive

medium on the metal surface. This adsorption can be explained by an electrostatic interaction

between positive center in cationic surfactants and cathodic sites on the metallic surface [16].

II. Effect of Temperature:

The effect of solution temperatures on the corrosion inhibition of C-steel in 1.0M HCl

solution investigated and the data was listed in Tables (1-3) from which, the metal corrosion rate

increases rapidly in the absence and presence of different concentrations of inhibitors by

increasing the temperature. Because increasing the temperature considered as an accelerating

factor in these electrochemical reactions, where increasing the temperature increases the

activation energy of the reacted species and hence the reaction is occurring much faster. Data

reveals that increasing the temperature of testing solution from 25 to 70o

C, leading to increasing

the corrosion rate from 0.0603 to 0.268mg Cm-2

h-1

of the inhibited solution for concentration of

1x10-2

M (compound I), meanwhile the corrosion rate of the uninhibited solution increase from

0.37 to 3.87 mg Cm-2

h-1

. Therefore, the inhibition efficiency increases from 83.75to 93.07% for

this inhibitor (compound I). Meanwhile, it increases from 87.73 to 94.9 % and from 89.01 to

96.18 % for compounds II and III, Respectively.

It is reported that the temperature increase of the tested solution leads to strong adsorption

(Chemical adsorption) of the inhibitor molecules on the metal surface where some chemical

changes occur in the inhibitor molecules, leading to an increase in the electron densities at the

adsorption centers of the molecule, causing an improvement in inhibitor efficiency [17,18]. In

addition, rising temperature of cationic surfactants leading to increasing free energy of

adsorption on the metal surface; i.e., Increasing rate of adsorption on the metal surface [19].

III. Effect of Hydrophobic Chain Length:

The data in Tables (1-3) reveal that increasing the hydrophobic chain length has an

enhancing effect on the increasing of the corrosion inhibition. This is due to the presence of these

chains form successive protective layers on the metal surface, keeping of the corrosive species

away from the metal surface and decrease the corrosion process. In addition, the repulsion

between the polar corrosive medium and these nonpolar chains decreases the interaction and

6

consequently decreases the corrosion processes occurred for these surfaces. Increasing the

hydrophobic chain length, accompanied by increasing rate of adsorption at a metal surface [19],

so inhibition efficiency increase.

IV. Effect of Hydrophilic Group (head group):

The adsorption of the inhibitor molecules on the mild steel surface, explained on the base

of the donor acceptor interaction between π electrons of donor groups and aromatic rings of the

inhibitors and the vacant d orbitals of iron surface atoms [20, 21]. With an increase in the

electron density at the reaction center, the chemisorption bonds between the inhibitor and the

metal are strengthened [22, 23].

3.1.2. Potentiostatic Evaluation of the Synthesized Inhibitors:

The polarization data of mild steel in 1.0 M HCl in the absence and presence of different

concentrations of inhibitors under this study were given in Figs. (1-3). Various electrochemical

corrosion parameters were determined. These parameters are corrosion potential (Ecorr),

corrosion current density (icorr), anodic tafel line slopes (βa) and cathodic tafel line slopes (βc).

These parameters were obtained by extrapolation of tafel line and illustrated in Table (4) in

addition surface coverage area (θ) and inhibition efficiency (η).

The degree of surface coverage (θ) and the percentage inhibition efficiency (η %) were

calculated using the following equations:

θ =1- (i ⁄ io) (3)

η = (1-(i ⁄ io ))*100 (4)

Where (i₀), and (i), are the corrosion current densities in the absence and presence of the

inhibitor, respectively.

Data in Table (4) revealed that the corrosion current density Icorr values decreased

considerably in the presence of inhibitor, as the inhibitor concentration increase the corrosion

current density decrease, correspondingly η increase. Because of increasing the blocked fraction

on the electrode surface by inhibitors adsorption, the surface coverage area (θ) increase.

7

The current density of uninhibited solution decreases from 0.21mAcm-2

to 0.025, 0.017 and

0.015 mAcm-2

for compounds I, II and III at maximum concentration (1x10-2

), respectively.

Meanwhile the maximum efficiency is 88, 92 and 92.7% for compounds I, II and III,

respectively.

The slight change of (βa) and (βc) of prepared cationic surfactants indicate that these

inhibitors effect on both anodic and cathodic reactions [24], the slight change upon increasing

inhibitor concentrations indicate that the prepared cationic surfactants don't change the reaction

inhibition mechanism [25].

The addition of inhibitors shifts the Ecorr values towards the positive Tables (4). Since the

largest displacement does not exceed 80 mV at 25oC, it may be concluded that theses inhibitors

act as a mixed-type (anodic/cathodic) inhibitors with a predominantly anodic reaction, mean

inhibitors reduces the anodic dissolution of mild steel and also retards the cathodic hydrogen

evolution reaction but the effect on the anodic dissolution reactions is more than on the cathodic

hydrogen evolution reactions [26].

3.1.3. Electrochemical impedance spectroscopy (EIS):

Figs. (4-6), represented a typical set of Nyquist plots, which are regarded as one part of a

semi-circle in 1.0 M HCl solution with and without inhibitors at 25oC after an exposure period of

1 hours. The impedance diagrams are not perfect semicircles, which related to the frequency

dispersion because of the roughness and inhomogeneity of electrode surface [27].

The impedance spectra of different Nyquist plots were analyzed by fitting the experimental

data to a simple equivalent circuit model which shown in Fig. (7), which includes the solution

resistant Rs and the double layer capacitance Cdl, which is placed in parallel to the charge transfer

resistance Rct [28].

The charge transfer resistance values (Rct) were calculated from the difference between

impedance values at lower and higher frequencies as suggested by Haruyama et al. [29]. The

double layer capacitance (Cdl) was obtained from the following equation [30]:

f (-Z″img ) = 1⁄ (2πCdl Rct) (5)

8

Where Z″img is the frequency of maximum imaginary components of the impedance and Rct is

the charge transfer resistances. The inhibition efficiency percentage (η) of corrosion of carbon

steel is calculated from Rct as follows [31]:

η = ((Roct -Rct) / R

oct)*100 (6)

Ro

ct and Rct are charge transfer resistance values with and without inhibitors, respectively.

From the electrochemical impedance parameters and the inhibition efficiency (η) which

listed in Table (5), it is clear that the impedance spectra exhibit one single capacitive loop,

indicating that the charge transfer takes place at the electrode / solution interface, and the transfer

process controls corrosion reaction and the presence of inhibitor does not change the mechanism

of steel dissolution [32].

As the inhibitor concentration increased, the Rct values increased and the Cdl values tended

to decrease due to a decrease in local dielectric constant and/or an increase in the thickness of the

electrical double layer, suggesting that the inhibitor molecules acted by adsorption at the

metal/solution interface. Addition of synthesized inhibitors provided lower Cdl values, probably

because of replacement of water molecules by inhibitor molecule at the electrode surface. In

addition, the inhibitor molecules may reduce the capacitance by increasing the double layer

thickness according to the Helmholtz model [33]:

δorg = (εoεA)/Cdl (7)

Where ε is the dielectric constant of the medium, εo is the vacuum permittivity, A is the

electrode surface area and δorg is the thickness of the protective layer.

The smaller Cdl value in the presence of the inhibitor may be resulted from the effective

adsorption of the synthesized inhibitors on the electrode surface.

The results obtained from EIS measurements are in good agreement with that obtained from both

potentiodynamic polarization and weight loss measurements.

3.1.4. Activation Thermodynamic Parameters:

The apparent activation energy, Ea, were determined using the Arrhenius equation:

9

ln K = (-Ea/RT) + ln A (8)

Where, k is the corrosion rate, A is the pre-exponential factor (Arrhenuis constant), R is the gas

constant and T is the absolute temperature.

Arrhenius plots of ln k vs. 1/T in the absence and presence of different concentrations of

prepared compounds were shown graphically in Fig. (8) giving straight lines with linear

regression coefficients are very close to 1, indicating that the corrosion of steel in 1.0M HCl

without and with inhibitor follows the Arrhenius equation having a slope of (-Ea/R). Activation

energies were calculated and given in Table (6).

By inspection data in Table (6), the values of activation energy in the presence of inhibitors

were found to be lower than uninhabited solution. Both values of apparent activation energy (Ea)

and Arrhenius constant (A) decrease with increasing inhibitor concentration. The lower value of

the activation energy in the presence of inhibitors is attributed to chemisorption of these

compounds on the steel surface [34-39]. This conception is supported by the big size of the

prepared cationic surfactant molecule [40].

The change in enthalpy and entropy of activation values (ΔH*, ΔS*) was calculated from the

transition state theory:

ln (K/T) = ln (R/(NAh))+(ΔS*/R) ـــ (ΔH

*/RT) (9)

Where, h is the Plank constant, NA is the Avogadros number, and R is the ideal gas constant.

A plot of log (k/T) versus 1/T, gave straight lines as shown in Fig. (9) with a slope of ΔH*/R

and an intercept of log (R/Nh) + ΔS*/R. The values of ΔH* and ΔS* are calculated and listed in

Table (6). Inspection of these data reveals that the activation parameters (ΔH* and ΔS*) of the

dissolution reaction of carbon steel in 1.0M HCl in the presence of the inhibitors are less than

that of in the absence of inhibitors. The positive signs of the change in enthalpies (ΔH*) reflect

the endothermic nature of the steel dissolution process and means that the dissolution of steel is

difficult in the presence of inhibitors [41]. The change in entropy of activation in the presence

and absence of the inhibitors is negative. This implies that the activated complex in the rate-

determining step represents an association rather than dissociation, indicating that more order

takes place, going from reactant to activate complex [42].

10

3.1.5. Adsorption Isotherm:

The adsorption isotherm experiments were performed to have more insights into the

mechanism of corrosion inhibition, since it describes the molecular interaction of the inhibitor

molecule with the active sites on metal surface [43]. The degree of surface coverage (θ) was

determined from the weight loss measurements. We made a fit of surface coverage (θ) values in

different adsorption isotherm models and the correlation coefficient (R2) was used to choose the

isotherm that best fit experimental data and the best results were obtained for Langmuir

adsorption isotherm:

Langmuir isotherm C ⁄ θ = C + (1/Kads) (10)

Where Kads is the equilibrium constant of the adsorption process and C is the inhibitor

concentration. The linear relationships of C/θ versus C, as shown in Fig. (10), suggest that the

adsorption of prepared inhibitors on mild steel surface obeyed the Langmuir adsorption isotherm.

The correlation (R2) of the Langmuir adsorption nearly equal 1 for prepared inhibitors as

depicted in Tables (7). This isotherm postulates that there is no interaction between the adsorbed

molecules and the energy of adsorption is independent on the surface coverage (θ) and assumes

that the solid surface contains a fixed number of adsorption sites and each holds one adsorbed

species [44]. The slopes of the straight lines obtained from the plots of the Langmuir isotherm

for prepared inhibitors are more than unity. Therefore, it could be concluded that each inhibitor

unit occupies more than one adsorption site, also there are interactions between adsorbed species

on the metal surface as well as changes in the adsorption heat with increasing surface coverage

[45], factors which were not taken into consideration in the derivation of the isotherm.

Therefore, we can say that the adsorption of prepared cationic surfactant on the mild steel surface

in testing temperature can be more appropriately represented by a modified Langmuir equation.

This modified is named Villamil isotherm, which suggested by Villamil, taking into

consideration the interactions between adsorbate species as well as changes in the heat of

adsorption with changing surface coverage as follows [46]:

C ⁄ θ = nC + (n/Kads) (11)

11

Where n is slope value obtained by the above plot, and referee to number of displacement

adsorbed water molecule from metal surface, and the intercept permit the calculation of

equilibrium constant Kads for all prepared cationic surfactant at different temperatures and listed

in Table (7), the adsorptive equilibrium constant (Kads) increases with temperature increasing,

which indicates that the inhibitor is easily and strongly adsorbed onto the carbon steel surface at

relatively higher temperature. This is due to formation of a coordinated bond between the

prepared Cationic surfactants and the d-orbital of iron on the surface of steel through a lone pair

of electron of N, and O atoms.

Thermodynamic parameters of adsorption play an important role in understanding the

mechanism of adsorption process of the metal surface. The adsorption heat (∆Hoads) calculated

using van’t Hoff equation:

ln Kads= ـــ ∆Hoads ⁄ (RT) +Constant (12)

Where (-∆Hoads/T) is the slope of the straight-line lnKadsvs 1/T, R is the gas constant and T is

absolute temperature.

The standard adsorption free energy (∆Goads) and standard adsorption entropy (∆S

oads) obtained

according to the following equation:

∆Goads = ـــ RT Ln (55.5Kads) (13)

The value of 55.5 is the molar concentration of water in the solution expressed in molarity units

(mol L-1

).

∆Soads = (∆H

oads -∆G

oads) ⁄ T (14)

All thermodynamic parameters ∆Goads, ∆H

oads and ∆S

oads were calculated from the above

equations and depicted in Tables (7)

The positive values of ∆Hoads show that, the adsorption of the inhibitors is an endothermic

process [47], which indicates that why the inhibition efficiency increases with the temperature

increasing. Such behavior can be interpreted increasing in temperature resulted in sorption of

inhibitor molecules in the metal surface so inhibition efficiency increases.

12

The negative values of ∆Goads suggest that the adsorption of inhibitor onto the carbon steel

surface is a spontaneous process; inspection data in Table (7), ∆Goads were ranged from -35.6 to -

43.5 Kjmol-1

that indicate that the adsorption process of inhibitor on metal surfaces is mixed

physical and chemical adsorption [48].

Values of ∆Soads listed in Table (7) have a positive sign, as was expected, since the endothermic

adsorption process always accompanied by an increase of entropy, which is the driving force of

the adsorption of inhibitor onto CS surface.

4- Conclusion:

From the obtained results the following conclusion can be drowned:

1- The prepared compounds have good inhibition effect on the C-steel in the 1.0M HCl.

2- The obtained results from electrochemical techniques and weight loss measurements are

in good agreement.

3- The synthesized cationic surfactants can be used as corrosion inhibitors for carbon steel

in 1.0M HCl. Their inhibiting properties increase with increasing both inhibitors

concentrations and hydrophobic chain length.

4- The prepared compounds act as a mixed-type inhibitor in 1.0 M HCl.

5- Double-layer capacitances decrease with respect to the blank solution when these

inhibitors were added. This fact may be explained on the basis of adsorption of these

inhibitors on the steel surface.

6- The apparent activation energies of corrosion in the presence of any of the three tested

surfactants are smaller than that in bare HCl solution.

7- The adsorption of prepared inhibitors on C-Steel surface obeys Villamil adsorption

isotherm. The adsorption process is an endothermic process accompanied by an increase

in entropy.

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16

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Chem. 19 (2013) 2004–2009.

17

Table (1): Corrosion rate of carbon steel, surface coverage and percentage inhibition efficiency

for carbon steel in 1.0 HCl of compound (I) at different temperatures

Temp. °C

Conc. of inhibitor, M

Weight loss (mg)

CR,

mgcm-2h-1

Ө η,

%

25

0.00 416.6 0.3709 --- --- 1x10-5 140.7 0.12527 0.663 66.23

5x10-5 124.7 0.11102 0.7007 70.07

1x10-4 107.2 0.09544 0.7427 74.27

5x10-4 95.4 0.08494 0.771 77.1

1x10-3 87.1 0.07755 0.7909 79.09

5x10-3 75.9 0.06757 0.8178 81.78

1x10-2 67.7 0.06027 0.8375 83.75

40

0.00 1023.2 0.9110 --- --- 1x10-5 289.1 0.25739 0.7174 71.74

5x10-5 232.9 0.20735 0.7723 77.23

1x10-4 210.5 0.18741 0.7943 79.43

5x10-4 157.3 0.14005 0.8463 84.63

1x10-3 142.7 0.12705 0.8605 86.05

5x10-3 125.6 0.11182 0.8773 87.73

1x10-2 109.3 0.09731

0.8931 89.32

55

0.00 2260 2.0121 ---- ---- 1x10-5 520.9 0.46376 0.7695 76.95

5x10-5 416.1 0.37046 0.8159 81.59

1x10-4 338.4 0.30128 0.8503 85.03

5x10-4 297.8 0.26514 0.8682 86.82

1x10-3 251.4 0.22383 0.8888 88.88

5x10-3 214.8 0.19124 0.9049 90.49

1x10-2 183.8 0.16364

0.9187 91.87

70

0.00 4350 3.8729 ---- ---- 1x10-5 931.6 0.82942 0.7858 78.58

5x10-5 772.4 0.68768 0.8224 82.24

1x10-4 613.6 0.54630 0.8589 85.89

5x10-4 491.6 0.43768 0.8869 88.69

1x10-3 423.8 0.37732 0.9025 90.25

5x10-3 350.4 0.31197 0.9194 91.94

1x10-2 301.3 0.26825 0.9307 93.07

18

Table (2): Corrosion rate of carbon steel, surface coverage and percentage inhibition efficiency

for carbon steel in 1.0M HCl of compound (II) at different temperatures

Temp.

°C

Conc. of

inhibitor, M

Weight

loss (mg)

CR,

mgcm-2h-1

Ө η,

%

25

0.00 416.6 0.3709 --- --- 1x10-5 106.5 0.09482 0.7444 74.44

5x10-5 91.1 0.08111 0.7813 78.13

1x10-4 82.4 0.07336 0.8022 80.22

5x10-4 73.7 0.06562 0.8231 82.31

1x10-3 66.6 0.05929 0.8401 84.01

5x10-3 59.3 0.05280 0.8577 85.77

1x10-2 51.1 0.04550 0.8773 87.73

40

0.00 1023.2 0.9110 --- --- 1x10-5 232.9 0.20735 0.7724 77.24

5x10-5 188.7 0.16800 0.8156 81.56

1x10-4 163 0.14512 0.8407 84.07

5x10-4 133.8 0.11912 0.8692 86.92

1x10-3 116.8 0.10399 0.8858 88.58

5x10-3 104.4 0.09295 0.8980 89.80

1x10-2 91.2 0.08120 0.9109 91.09

55

0.00 2260 2.0121 ---- ---- 1x10-5 401.5 0.35746 0.8223 82.23

5x10-5 330.9 0.29461 0.8536 85.36

1x10-4 263.9 0.23495 0.8832 88.32

5x10-4 216.4 0.19266 0.9042 90.42

1x10-3 181.5 0.16159 0.9197 91.97

5x10-3 159.1 0.14165 0.9296 92.96

1x10-2 130.8 0.11645 0.9421 94.21

70

0.00 4350 3.8729 ---- ---- 1x10-5 685.4 0.61022 0.8424 84.24

5x10-5 545.1 0.48531 0.8747 87.47

1x10-4 445 0.39619 0.8977 89.77

5x10-4 378.3 0.33681 0.9130 91.30

1x10-3 317.6 0.28276 0.9270 92.70

5x10-3 267.9 0.23852 0.9384 93.84

1x10-2 221.9 0.19756 0.9490 94.90

19

Table (3): Corrosion rate of carbon steel, surface coverage and percentage inhibition efficiency

for carbon steel in 1M HCl of compound (III) at different temperatures

Temp.

°C

Conc. of

inhibitor, M

Weight

loss (mg)

CR,

mgcm-2h-1

Ө η,

% 0.3709 --- ---

25

0.00 416.6 1x10-5 100.3 0.08930 0.7592 75.92

5x10-5 88.2 0.07853 0.7883 78.83

1x10-4 79 0.07033 0.8104 81.04

5x10-4 70.5 0.06277 0.8308 83.08

1x10-3 61.5 0.05475 0.8524 85.24

5x10-3 52.4 0.04665 0.8742 87.42

1x10-2 45.8 0.04078 0.8901 89.01

40

0.00 1023.2 0.9110 --- --- 1x10-5 207.9 0.18510 0.7968 79.68

5x10-5 167.7 0.14931 0.8361 83.61

1x10-4 135.4 0.12055 0.8677 86.77

5x10-4 124.4 0.11076 0.8784 87.84

1x10-3 98.8 0.08796 0.9034 90.34

5x10-3 90.8 0.08084 0.9113 91.13

1x10-2 78.7 0.07007 0.9231 92.31

55

0.00 2260 2.0121 ---- ---- 1x10-5 353.1 0.31437 0.8438 84.38

5x10-5 284.5 0.25329 0.8741 87.41

1x10-4 232.2 0.20673 0.8973 89.73

5x10-4 186.7 0.16622 0.9174 91.74

1x10-3 151.9 0.13524 0.9328 93.28

5x10-3 132.9 0.11832 0.9412 94.12

1x10-2 107.4 0.09562 0.9525 95.25

70

0.00 4350 3.8729 ---- ---- 1x10-5 621.1 0.55297 0.8572 85.72

5x10-5 479.7 0.42708 0.8897 88.97

1x10-4 383.8 0.34170 0.9118 91.18

5x10-4 334.5 0.29781 0.9231 92.31

1x10-3 260 0.23148 0.9402 94.02

5x10-3 196.8 0.17521 0.9548 95.48

1x10-2 166.1 0.14788 0.9618 96.18

20

Table (4): Potentiodynamic polarization parameters for corrosion of carbon steel in 1.0M HCl of

synthesized cationic surfactants I, II and III at 25oC at scanning rate 2 mV s-1

Inhibitor name

Conc. of inhibitor

(M)

Ecorr

mV

(SCE)

Icorr

mA cm-2

βa

mV dec-1

βc

mV dec-1

Ө

ηp

%

0.00 -550.3 0.2096 229.8 -168.6 ----- -----

I

1x10-5 -540.7 0.0671 92.8 -180.4 0.679 67.9 5x10-5 -546.8 0.0636 106.1 -148.3 0.697 69.7 1x10-4 -517.3 0.0465 91.3 -179.8 0.778 77.8 5x10-4 -548.9 0.0412 111.5 -175.4 0.803 80.3 1x10-3 -540.3 0.0385 107.9 -180.9 0.816 81.6 5x10-3 -520.1 0.0309 94.2 -176.8 0.853 85.3 1x10-2 -536.9 0.0249 100.6 -167.8 0.881 88.1

II

1x10-5 -542.5 0.0501 113.8 -152.9 0.761 76.1 5x10-5 -522.6 0.0444 95.65 -195.4 0.788 78.8 1x10-4 -541 0.0376 108.2 -191.9 0.821 82.1 5x10-4 -537.2 0.0300 115.4 -163.4 0.857 85.7 1x10-3 -516.6 0.0260 91.2 -178.4 0.876 87.6 5x10-3 -497.7 0.0246 98.2 -201.8 0.883 88.3 1x10-2 -547.5 0.0167 101.7 -150.3 0.920 92.0

III

1x10-5 -546 0.0471 112.8 -180.8 0.775 77.5

5x10-5 -517.2 0.0409 95.53 -167.2 0.805 80.5

1x10-4 -511 0.0346 124.9 -172.2 0.835 83.5

5x10-4 -527.2 0.0281 105.3 -190.8 0.866 86.6

1x10-3 -484.6 0.0242 96.2 -188.7 0.885 88.5

5x10-3 -523.3 0.0211 94.9 -183.5 0.899 89.9

1x10-2 -486.9 0.0151 103.8 -193.5 0.927 92.7

21

Table (5): EIS parameters for corrosion of carbon steel in 1.0M HCl of synthesized cationic

surfactants I, II and III at 25oC

Inhibitor name

Conc. of inhibitor

(M)

Rs

ohm cm2

Rct

ohm cm2

Cdl

µF cm-2

Ө ηZ

%

0.00 2.5 164. 85 241.27 ------ ------

I

1x10-5 2.85 501.12 35.56 0.6710 67.10

5x10-5 2.94 583.62 34.35 0.7175 71.75

1x10-4 2.98 740.21 30.09 0.7773 77.73

5x10-4 2.74 796.73 25.16 0.7931 79.31

1x10-3 3.11 831.07 24.12 0.8016 80.16

5x10-3 3.11 1082.78 20.76 0.8478 84.78 1x10-2

2.89 1215.71 20.68 0.8644 86.44

II

1x10-5 23.19 723.98 31.72 0.7723 77.23

5x10-5 3.14 794.49 31.64 0.7925 79.25

1x10-4 2.58 949.203 29.83 0.8263 82.63

5x10-4 2.93 1014.86 24.77 0.8376 83.76

1x10-3 3.18 1156.55 21.73 0.8575 85.74

5x10-3 2.75 1344.68 18.69 0.8774 87.74 1x10-2

3.41 1600.67 17.69 0.8970 89.70

III

1x10-5 4.05 708.38 31.44 0.7673 76.73

5x10-5 2.86 880.27 28.56 0.8127 81.27

1x10-4 2.69 947.09 26.54 0.8259 82.59

5x10-4 4.26 1059.92 23.72 0.8445 84.45

1x10-3 3.06 1208.00 20.81 0.8635 86.35

5x10-3 3.43 1525.17 18.57 0.8919 89.19

1x10-2 3.67 1890.78 16.83 0.9128 91.28

22

Table (6): Activation parameters values for carbon steel in 1.0M HCl of different concentrations

of the synthesized compounds I, II and III

Inhibitor name

Conc. of inhibitor

(M)

Ea (kJ mol

-1)

Linear

regression coefficient

∆H* (kJ mol

-1)

∆S* (J mol

-1 K

-1)

0.00 44.47 0.9992 41.82 -112.7

I

1x10-5 35.52 0.9996 32.87 -151.84

5x10-5 34.27 0.9983 31.62 -157.31

1x10-4 32.37 0.9969 29.72 -164.68

5x10-4 31.47 0.9956 28.81 -169.08

1x10-3 30.07 0.9966 27.42 -174.50

5x10-3 29.03 0.9984 26.388 -179.02

1x10-2 28.3 0.9974 25.65 -182.48

II

1x10-5 34.85 0.9964 32.19 -156.16

5x10-5 33.43 0.994 31.04 -161.39

1x10-4 31.47 0.9977 28.81 -169.78

5x10-4 30.53 0.9981 27.88 -174.09

1x10-3 29.04 0.9963 26.38 -179.99

5x10-3 28.03 0.9972 25.38 -184.24

1x10-2 27.02 0.9934 24.36 -188.79

III

1x10-5 34.07 0.9982 31.41 -159.44

5x10-5 31.83 0.9996 29.18 -168.13

1x10-4 29.93 0.9992 27.27 -175.64

5x10-4 28.75 0.9934 26.09 -180.48

1x10-3 26.89 0.9939 24.24 -187.99

5x10-3 24.72 0.9969 22.07 -196.14

1x10-2 23.71 0.9928 21.05 -200.69

23

Table (7): Thermodynamic parameters from Villamil adsorption isotherm on carbon steel surface

in 1.0M HCl containing different concentrations of the synthesized compound I, II and III at

different temperatures

Inhibitor

name

Temp.

oC Slope R2

Kads x10-4

M-1

∆Gads

kJ mol-1

∆Hads

kJ mol-1

∆Sads

J mol-1 K-1

I

25 1.19 0.9999 3.057 -35.56

10.79

155.46

40 1.12 0.9999 4.283 -38.22 156.53

55 1.09 0.9999 4.889 -40.42 156.06

70 1.07 0.9999 5.486 -42.59 155.57

II

25 1.14 0.9999 3.835 -36.12

10.92

157.76

40 1.09 0.9999 5.513 -38.88 159.02

55 1.06 0.9999 6.254 -41.09 158.48

70 1.05 0.9999 6.935 -43.26 157.88

III

25 1.12 0.9999 3.992 -36.22

10.92

161.62

40 1.08 0.9999 6.18 -39.18 163.33

55 1.05 0.9999 7.026 -41.41 162.65

70 1.04 0.9999 7.652 -43.54 161.77

24

Fig. (1): Potentiodynamic polarization curves for the carbon steel in 1.0M HCl in the absence

and presence of different concentrations of (I) at scanning rate 2 mV s-1

.

Fig. (2): Potentiodynamic polarization curves for the carbon steel in 1.0M HCl in the absence

and presence of different concentrations of (II) at scanning rate 2 mV s-1

.

25

Fig. (3): Potentiodynamic polarization curves for the carbon steel in 1.0M HCl in the absence

and presence of different concentrations of (III) at scanning rate 2 mV s-1

.

26

Fig. (4): Nyquist plots for the carbon steel in 1.0M HCl in the absence and presence of different

concentrations of compound (I).

Fig. (5): Nyquist plots for the carbon steel in 1.0M HCl in the absence and presence of different

concentrations of compound (II).

27

Fig. (6): Nyquist plots for the carbon steel in 1.0M HCl in the absence and presence of different

concentrations of compound (III).

Fig. (7): Electrical equivalent circuit used for modeling the interface C-steel/1.0M HCl solution

in the absence and presence of the prepared cationic surfactants.

28

Fig. (8): Arrhenius plots for carbon steel dissolution in absence and presence of different

concentrations of prepared cationic surfactant in 1M HCl solution where A is (I), B is (II) and C

is (III).

29

Fig. (9): Relationship between Ln K/T and the reciprocal of the absolute temperature in absence

and presence of different concentrations of prepared cationic surfactant in 1M HCl solution

where A is (I), B is (II) and C is (III).

30

Fig. (10): Villamil adsorption isotherm model on the carbon steel surface of prepared compound

1.0M HCl at different temperatures where A is (I), B is (II) and C is (III).