# Corrosion Inhibition of Tubing Steel during Acidization of Oil and Gas Wells

Authors: M. Yadav; Sumit Kumar; P. N. Yadav
Journal: Journal of Petroleum Engineering
 (2013)

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## Abstract

Acidization is an oil reservoir stimulation technique for increasing oil well productivity. Hydrochloric acid is used in oil and gas production to stimulate the formation. The acid treatment occurs through N80 steel tubes. The process requires a high degree of corrosion inhibition of tubing material (N80 steel). In the present investigation effect of synthesized amino acid compounds, namely,  acetamidoleucine (AAL) and benzamidoleucine (BAL) as corrosion inhibitors for N80 steel in 15% HCl solution was studied by polarization, AC impedance (EIS), and weight loss measurements. It was found that both the inhibitors were effective inhibitors and their inhibition efficiency was significantly increased with increasing concentration of inhibitors. Polarization curves revealed that the studied inhibitors represent mixed type inhibitors. AC impedance studies revealed that charge transfer resistance increases and double layer capacitance decreases in presence of inhibitors. Adsorption of inhibitors at the surface of N80 steel was found to obey Langmuir isotherm.


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## Body

## 1. Introduction

Acidization of a petroleum oil well is one of the important stimulation techniques for enhancing oil production. It is commonly brought about by forcing a solution of 15% to 28% hydrochloric acid into the well through N80 tubing to open up near bore channels in the formation and hence to increase the flow of oil. To reduce the aggressive attack of the acid on tubing and casing materials (N80 steel), inhibitors are added to the acid solution during the acidifying process. The effective acidizing inhibitors that are usually found in commercial formulations are acetylenic alcohols, alkenyl phenones, aromatic aldehydes, nitrogen-containing heterocyclics, quaternary salts, and condensation products of carbonyls and amines [1–6]. However, these inhibitors are effective only at high concentrations, and they are harmful to the environment due to their toxicity, so it is important to search for new nontoxic and effective organic corrosion inhibitors for N80 steel—15% HCl system. In this regard, amino acids have a promising alternative for the design of ecofriendly corrosion inhibitors [7–10] which satisfy the environmental requirements.In the present investigation acetamidoleucine (AAL) and benzamidoleucine (BAL) were synthesized, and their corrosion inhibiting properties on N80 (oil-well tubular steel) steel was studied in 15% HCl by using weight loss, polarization, and AC impedance techniques. Influence of temperature (from 298 to 333 K) on the inhibition behavior was studied. Both the activation and thermodynamic parameters governing the corrosion process were calculated and discussed.


## 2. Material and Methods

### 2.1. Materials

The working electrode and specimens for weight loss experiments were prepared from N80 steel sheets (supplied by ONGC) having the following percentage by weight (%wt) composition: C: 0.31, Mn: 0.92, Si: 0.19, P: 0.01, S: 0.008, Cr: 0.20, and Fe: remainder.


### 2.2. Weight Measurements

The specimens for the weight loss experiments were of the size 3 cm × 3 cm × .1 cm, and for electrochemical studies the exposed surface area of the working electrode was 1 cm2. For weight loss experiments 300 mL of 15% HCl (v/v) was taken in 500 mL glass beakers. The inhibition efficiencies (IE) were evaluated after a preoptimized time interval of 6 h using 10, 20, 50, 100, and 150 ppm by weight of inhibitors. The specimens were removed from the electrolyte, washed thoroughly with distilled water, dried, and weighed. The inhibition efficiencies (IE) were calculated using the following equation:
(1)%IE=[CR0-CRiCR0]×100,
where CR0 is the weight loss in absence of inhibitor, and CRi is the weight loss in presence of inhibitor.Corrosion rate (CR) for the specimen was calculated with the help of the following equation:(2)CR(mpy)=534WDAT,
where W=weight loss (mg); D=density of specimen (g/cm3); A=area of specimen (cm2); T=exposure time (hours).


### 2.3. Adsorption Isotherms

The information about the electrochemical interaction of inhibitors with metal surface can be provided by the nature of adsorption isotherm. Different adsorption isotherm such as Langmuir, Temkin, and Frumkin, were tested graphically by ploting surface coverages(θ) and concentration of studied (AAL and BAL) on N80 steel in HCl solution. The surface coverage was calculated using the following equation:
(3)θ=[CR0-CRiCR0]×100,
where CR0 is the weight loss in absence of inhibitor, and CRi is the weight loss in presence of inhibitor.


### 2.4. Electrochemical Polarization Studies

The electrochemical studies were performed in a three necked glass assembly containing     150 mL of the electrolyte with different concentrations of inhibitors (10, 50, and 150 ppm by weight) dissolved in it. The potentiodynamic polarization studies were carried out with N80 steel strips having an exposed area of 1 cm2. A conventional three electrode cell consisting of N80 steel as working electrode, platinum as counter electrode, and a saturated calomel electrode as reference electrode were used. Polarisation studies were carried out using VoltaLab 10 electrochemical analyser, and data was analysed using Voltamaster 4.0 software. The potential sweep rate was 10 mVs−1. All experiments were performed at 25±0.2°C in an electronically controlled air thermostat. For calculating inhibition efficiency by electrochemical polarization method, the following formula was used:
(4)%IE=[I0-IinhI0]×100,
where I0 is corrosion current in absence of inhibitor, and Iinh is the corrosion current in presence of inhibitor.


### 2.5. AC Impedance Studies

AC impedance studies were carried out in a three electrode cell assembly using computer-controlled VoltaLab 10 electrochemical analyzer, as well as N80 steel as the working electrode, platinum as counter electrode, and saturated calomel as reference electrode. The data were analysed using Voltamaster 4.0 software. The electrochemical impedance spectra (EIS) were taken in the frequency range from 10 kHz to 1 mHz at the rest potential by applying 5 mV sine wave AC voltage. The charge transfer resistance(Rct) and double layer capacitance (Cdl) were determined from Nyquist plots. The inhibition efficiencies were calculated from charge transfer resistance values by using the following formula:
(5)%IE=[Rct(inh)-RctRct(inh)]×100,
where Rct is the charge transfer resistance in absence of inhibitor, and Rct(inh) is the charge transfer resistance in presence of inhibitor.


### 2.6. Synthesis of Inhibitors

One gram of leucine was dissolved in 25 mL of 5% NaOH solution in a conical flask. To this mixture benzoyl chloride/acetyl chloride (2.25 mL) was added in five portions in (0.49 mL increments) and shaken vigorously until all the chloride has reacted. Acidified with diluted hydrochloric acid the crude product was washed with cold ether. Finally, the desired product was recrystallized from ethanol [11]. The purity of the compounds was checked by TLC. The structure of the inhibitors are given in Scheme 1.Scheme 1


## 2.1. Materials

The working electrode and specimens for weight loss experiments were prepared from N80 steel sheets (supplied by ONGC) having the following percentage by weight (%wt) composition: C: 0.31, Mn: 0.92, Si: 0.19, P: 0.01, S: 0.008, Cr: 0.20, and Fe: remainder.


## 2.2. Weight Measurements

The specimens for the weight loss experiments were of the size 3 cm × 3 cm × .1 cm, and for electrochemical studies the exposed surface area of the working electrode was 1 cm2. For weight loss experiments 300 mL of 15% HCl (v/v) was taken in 500 mL glass beakers. The inhibition efficiencies (IE) were evaluated after a preoptimized time interval of 6 h using 10, 20, 50, 100, and 150 ppm by weight of inhibitors. The specimens were removed from the electrolyte, washed thoroughly with distilled water, dried, and weighed. The inhibition efficiencies (IE) were calculated using the following equation:
(1)%IE=[CR0-CRiCR0]×100,
where CR0 is the weight loss in absence of inhibitor, and CRi is the weight loss in presence of inhibitor.Corrosion rate (CR) for the specimen was calculated with the help of the following equation:(2)CR(mpy)=534WDAT,
where W=weight loss (mg); D=density of specimen (g/cm3); A=area of specimen (cm2); T=exposure time (hours).


## 2.3. Adsorption Isotherms

The information about the electrochemical interaction of inhibitors with metal surface can be provided by the nature of adsorption isotherm. Different adsorption isotherm such as Langmuir, Temkin, and Frumkin, were tested graphically by ploting surface coverages(θ) and concentration of studied (AAL and BAL) on N80 steel in HCl solution. The surface coverage was calculated using the following equation:
(3)θ=[CR0-CRiCR0]×100,
where CR0 is the weight loss in absence of inhibitor, and CRi is the weight loss in presence of inhibitor.


## 2.4. Electrochemical Polarization Studies

The electrochemical studies were performed in a three necked glass assembly containing     150 mL of the electrolyte with different concentrations of inhibitors (10, 50, and 150 ppm by weight) dissolved in it. The potentiodynamic polarization studies were carried out with N80 steel strips having an exposed area of 1 cm2. A conventional three electrode cell consisting of N80 steel as working electrode, platinum as counter electrode, and a saturated calomel electrode as reference electrode were used. Polarisation studies were carried out using VoltaLab 10 electrochemical analyser, and data was analysed using Voltamaster 4.0 software. The potential sweep rate was 10 mVs−1. All experiments were performed at 25±0.2°C in an electronically controlled air thermostat. For calculating inhibition efficiency by electrochemical polarization method, the following formula was used:
(4)%IE=[I0-IinhI0]×100,
where I0 is corrosion current in absence of inhibitor, and Iinh is the corrosion current in presence of inhibitor.


## 2.5. AC Impedance Studies

AC impedance studies were carried out in a three electrode cell assembly using computer-controlled VoltaLab 10 electrochemical analyzer, as well as N80 steel as the working electrode, platinum as counter electrode, and saturated calomel as reference electrode. The data were analysed using Voltamaster 4.0 software. The electrochemical impedance spectra (EIS) were taken in the frequency range from 10 kHz to 1 mHz at the rest potential by applying 5 mV sine wave AC voltage. The charge transfer resistance(Rct) and double layer capacitance (Cdl) were determined from Nyquist plots. The inhibition efficiencies were calculated from charge transfer resistance values by using the following formula:
(5)%IE=[Rct(inh)-RctRct(inh)]×100,
where Rct is the charge transfer resistance in absence of inhibitor, and Rct(inh) is the charge transfer resistance in presence of inhibitor.


## 2.6. Synthesis of Inhibitors

One gram of leucine was dissolved in 25 mL of 5% NaOH solution in a conical flask. To this mixture benzoyl chloride/acetyl chloride (2.25 mL) was added in five portions in (0.49 mL increments) and shaken vigorously until all the chloride has reacted. Acidified with diluted hydrochloric acid the crude product was washed with cold ether. Finally, the desired product was recrystallized from ethanol [11]. The purity of the compounds was checked by TLC. The structure of the inhibitors are given in Scheme 1.Scheme 1


## 3. Results

### 3.1. Weight-Loss Study

Weight-loss studies were performed in accordance with ASTM method. Test was conducted in 15% HCl (v/v) solution for 6 h at different temperatures (from 298 to 333 K) containing different concentration of inhibitors (10–150 ppm).


#### 3.1.1. Effect of Concentration

The percentage inhibition efficiencies (%IEs) in presence of 10, 20, 50, 100, and 150 ppm of AAL and BAL have been evaluated by weight-loss technique at 25°C, and the results are summarized in Table1. It is evident from these values that the inhibitor is significantly effective even at low concentrations like 10 ppm and there is a linear increase in %IE in the whole range of concentrations studied. The corrosion rate decreases with an increase in concentration of each inhibitor and reached the maximum value in the range of 150 ppm concentration (Figure 1).Table 1
Corrosion inhibition of N80 steel in 15% HCl in presence and absence of inhibitors AAL and BAL at different concentration.
Conc.(ppm)
AAL
BAL
CR (mpy)
%IE
CR (mpy)
%IE
0
9.55
—
9.55
—
10
3.98
58.2
3.41
64.2
20
3.39
64.4
2.66
72.1
50
2.55
73.3
1.78
81.3
100
1.88
80.3
1.10
88.4
150
1.70
82.2
0.93
90.2Figure 1
Variation of inhibition efficiency with concentration in presence of AAL and BAL.


#### 3.1.2. Effect of Temperature

The effect of temperature on the performance of AAL and BAL as corrosion inhibitors for N80 steel in 15% HCl was investigated by weight loss measurements in the temperature range 298–333 K in absence and presence of both the inhibitors at optimum concentration (150 ppm) in a thermostat. The variation of inhibition efficiency and corrosion rate with temperature are shown in Table2. The observation depicts that rate of corrosion increases with increase in temperature. This may be due to the fact that at higher temperature the metal organic complex layer dissociates leaving a porous diffused film, which is responsible for corrosion [12].Table 2
Corrosion parameters in absence and in presence of AAL and BAL at different temperatures.
Temperature (K)
Blank
AAL
BAL
CR (mpy)
IE%
CR (mpy)
IE%
CR (mpy)
IE%
298
9.55
—
1.70
82.2
0.93
90.2
303
12.09
—
2.63
78.2
1.79
85.1
313
19.27
—
4.87
74.7
4.21
78.1
323
30.42
—
10.51
65.4
9.01
70.3
333
48.94
—
20.09
58.9
18.31
62.5


#### 3.1.3. Adsorption Isotherms

To determine the adsorption mode, various isotherms were tested, and Langmuir adsorption isotherm was found to be best, which gives a straight line graph for the plot oflog(θ/1-θ)  verses logarithmic concentration of inhibitors  (Figure 2).Figure 2
Langmuir adsorption isotherm in presence of AAL and BAL.


#### 3.1.4. Kinetic Study

The apparent activation energy(Ea) was calculated  by using Arrhenius equation:
(6)logk=-Ea2.303RT+logA,
where k is rate of corrosion, Ea is the apparent activation energy, R is the universal gas constant, T is absolute temperature, and A is the Arrhenius preexponential factor. By plotting logk against 1/T the values of activation energy (Ea) have been calculated (Ea=-(Slope)×2.303×R) (Figure 3). Activation energy for N80 steel in 15% HCl increases in presence of inhibitors. The Ea values for AAL and BAL were found to be 57.6 kJ/mol, 68.7 kJ/mol, respectively, (Table 3), which are higher than the Ea value for blank solution (38.4 kJ/mol).Table 3
Thermodynamic parameter in absence and in presence of AAL and BAL.
Inhibitor
Δ
H
* (kJ/mol)
E
a (kJ/mol)
Δ
G
ads (kJ/mol)
Δ
S
* (J/mol)
Blank
—
38.4
—
—
AAL
−62.1
57.6
−32.2
−32.5
BAL
−69.4
68.7
−36.7
−22.5Figure 3
Arrhenius plot for the corrosion of N80 steel in 15% HCI in presence and in absence of AAL and BAL.The values of entropy of activation(ΔS*) and enthalpy of activation (ΔH*) were calculated by using the formula:
(7)k=(RTNh)exp(ΔS*R)exp(ΔH*RT),
where k is rate of corrosion, h is Planks constant, N is Avogadro,s number, ΔS* is the entropy of activation, and ΔH*  is the enthalpy of activation. A plot of log(k/T) verses 1/T (Figure 4) should give a straight line, with a slope of (-ΔH*/2.303R) and a intercept of [log(R/Nh)+ΔS*/2.303R], from which the values of ΔS* and ΔH* can be calculated (Table 3). The ΔH* values for AAL and BAL were found to be as −62.1 kJ/mol and −69.4 kJ/mol, respectively, that revealed the exothermic nature of corrosion reaction. ΔS* values were found to be −32.5 J/mol and −22.5 J/mol for AAL and BAL, respectively (Table 3).Figure 4
A plot oflogk/T versus 1/T in presence of  AAL and BAL.The average value for free energy of adsorption(ΔGads) were calculated using the following equation:
(8)K=155.5exp(-ΔGadsRT),
where θ is degree of coverage on metal surface, C is concentration of inhibitors in mol l−1, R is molar gas constant in J K−1 moL−1, and T is temperature. The value of 55.5 in the above equation is the concentration of water in the solution in mole/litre. The equilibrium constant (K) has been replaced by the equation
(9)K=θ(1-θ)C.By plottinglogK against 1/T the value of ΔGads can be calculated (ΔGads=-2.303×R×Slope) from the slope of the straight line obtained (Figure 5). The values of free energy of adsorption (ΔGads) for AAL and BAL were found to be −32.2 kJ/mol and −36.7 kJ/mol, respectively (Table 3). Values of ΔGads around −20 kJ/mol or lower are consistent with the electrostatic interaction between charged organic molecules and the charged metal surface (physisorption); those around −40 kJ/mol or higher involve charge sharing or transfer from the organic molecules to the metal surface to form a coordinate type of bond (chemisorptions). The value of ΔGads for AAL and BAL that was found to be −32.2 kJ/mol and −36.7 kJ/mol which is less than −40 kJ/mol indicated that in addition to electrostatic interaction, there may be some other interactions [13]. Lebrini et al. [14] studied some triazole derivatives as corrosion inhibitors of mild steel in 1 M HClO4. The Gibbs free energy of adsorption of these molecules were reported to be around −34 kJ/mol. They concluded that the adsorption mechanism of these molecules on steel involved two types of interactions, chemisorptions, and physisorption. Similar conclusion was found by Ozcan [15], who studied the use of cystine as a corrosion inhibitor on mild steel in sulfuric acid. Thus, adsorption of studied inhibitors at the surface of N80 steel is not pure physisorption but it is combination of physisorption as well as chemisorptions.Figure 5
A plot oflogK  versus 1/T in presence of AAL and BAL.


### 3.2. Potentiodynamic Polarization Study

Figures6 and 7 show the polarization curves of N80 steel in 15% HCl solution in absence and presence of 10, 50, and 150 ppm of inhibitors AAL and BAL, respectively. The nature of the polarization curves remains almost same in absence and presence of both the inhibitors but in presence of inhibitors the curves shifted towards the lower current density as compared to the blank. The shift in current density towards lower current density in presence of inhibitors increases on increasing the concentration of the inhibitors. The electrochemical corrosion parameters including corrosion current densities (Icorr), corrosion potential (Ecorr), cathodic Tafel slope (βc), anodic Tafel slope (βa), and inhibition efficiency obtained from the polarization curves of AAL and BAL are presented in  Table 4. It is clear from the Figures 6 and 7 that with the increase of inhibitors concentration both anodic and cathodic currents were inhibited.Table 4
Electrochemical corrosion parameters in absence and in presence of AAL  and BAL at different concentration.
Inhibitors
Conc of inhibitors (ppm)
Tafel slopes
I
corr (μA/cm2)
E
corr (mV)
%IE
%IE from wt. loss
β
a (mV dec−1)
β
c (mV dec−1)
Blank
—
109
153
471
−468
—
—
AAL
10
113
162
193
460
59.0
58.2
50
118
170
108
−462
77.0
73.3
150
121
178
74
−478
84.1
82.2
BAL
10
116
171
172
−476
63.3
64.2
50
119
180
84
−480
82.1
81.3
150
125
187
35
−484
92.5
90.2Figure 6
Potentiodynamic polarization curves for N80 steel in 15% HCl in absence and in presence of AAL.Figure 7
Potentiodynamic polarization curves for N80 steel in 15% HCl in absence and in presence of BAL.It is apparent from the Table4 that Icorr decreases considerably in the presence of both the inhibitors, and %IE increases with the increase in the inhibitor concentration, due to the increase in the blocked fraction of the electrode surface by adsorption. The value of anodic and cathodic Tafel slopes increases in presence of both the inhibitors as compared to the blank solution. A minor shift in Ecorr values towards the negative direction was obtained in presence of both the inhibitors, indicating mixed nature of the inhibitors.


### 3.3. Electrochemical Impedance Spectroscopy

Nyquist plots of N80 steel in 15% HCl in the presence and absence of different concentrations (10, 50, and 150 ppm) of AAL and BAL at25±1°C are shown in the Figures 8 and 9. All the Nyquist plots obtained were semicircle in nature, the diameter of the semicircles increases with increase in inhibitor concentration, and shape is maintained throughout the tested concentration, indicating that almost no change in the corrosion mechanism occurs due to inhibitor action. The electrochemical parameters (Rct,Cdl) calculated from the Nyquist plots by using the equivalent circuit (Figure 10) in presence and absence of inhibitors are presented in Table 5. Inspection of Table 5 reveals that Rct values increase prominently while Cdl values tend to decrease with the increasing concentration of inhibitors, which suggest that inhibitors molecule function by adsorption on metal/solution interface. The values of electrochemical double layer capacitance (Cdl) were calculated at the frequency fmax, at which the imaginary component of the impedance is maximal (-Zi) by the following equation:
(10)Cdl=12πfmaxRct.Table 5
Electrochemical impedance parameters in absence and in presence of AAL and BAL.
Inhibitors
Conc. (ppm)
R
ct (Ω cm2)
C
dl  (μF cm−2)
IE%
Blank
—
176
662
—
AAL
10
439
369
59.9
50
745
173
76.3
150
1006
145
82.5
BAL
10
505
286
65.1
50
998
165
82.3
150
2301
55
92.3Figure 8
Nyquist plots of the corrosion of N80 steel in 15% HCl without and with different concentration of BAL.Figure 9
Nyquist plots of the corrosion of N80 steel in 15% HCl without and with different concentration of AAL.Figure 10
Equivalent circuit diagram.


### 3.4. SEM Study of Metal Surface Product

SEM microphotographs (Figures11(a), 11(b), 11(c), and 11(d)) of the film formed on the metal surface at 1000x magnifications were studied to analyze the change in the morphology of the metal surface after corrosion tests in presence and absence of the inhibitors. Figure 11(a) shows the SEM of polished sample (magnification 1000x). Figure 11(b) shows the SEM microphotographs of N80 steel (magnification 1000x) when exposed to 15% HCl solution at room temperature in absence of inhibitors. Figures 11(c) and 11(d) are the microphotographs of the metal surface when exposed to the acid medium in presence of BAL and AAL, respectively, at the same magnifications.Figure 11
SEM of (a) polished sample; (b) sample in presence of 15% hydrochloric acid; (c) sample in presence of 150 ppm of BAL; (d) sample in presence of 500 ppm of AAL.


## 3.1. Weight-Loss Study

Weight-loss studies were performed in accordance with ASTM method. Test was conducted in 15% HCl (v/v) solution for 6 h at different temperatures (from 298 to 333 K) containing different concentration of inhibitors (10–150 ppm).


### 3.1.1. Effect of Concentration

The percentage inhibition efficiencies (%IEs) in presence of 10, 20, 50, 100, and 150 ppm of AAL and BAL have been evaluated by weight-loss technique at 25°C, and the results are summarized in Table1. It is evident from these values that the inhibitor is significantly effective even at low concentrations like 10 ppm and there is a linear increase in %IE in the whole range of concentrations studied. The corrosion rate decreases with an increase in concentration of each inhibitor and reached the maximum value in the range of 150 ppm concentration (Figure 1).Table 1
Corrosion inhibition of N80 steel in 15% HCl in presence and absence of inhibitors AAL and BAL at different concentration.
Conc.(ppm)
AAL
BAL
CR (mpy)
%IE
CR (mpy)
%IE
0
9.55
—
9.55
—
10
3.98
58.2
3.41
64.2
20
3.39
64.4
2.66
72.1
50
2.55
73.3
1.78
81.3
100
1.88
80.3
1.10
88.4
150
1.70
82.2
0.93
90.2Figure 1
Variation of inhibition efficiency with concentration in presence of AAL and BAL.


### 3.1.2. Effect of Temperature

The effect of temperature on the performance of AAL and BAL as corrosion inhibitors for N80 steel in 15% HCl was investigated by weight loss measurements in the temperature range 298–333 K in absence and presence of both the inhibitors at optimum concentration (150 ppm) in a thermostat. The variation of inhibition efficiency and corrosion rate with temperature are shown in Table2. The observation depicts that rate of corrosion increases with increase in temperature. This may be due to the fact that at higher temperature the metal organic complex layer dissociates leaving a porous diffused film, which is responsible for corrosion [12].Table 2
Corrosion parameters in absence and in presence of AAL and BAL at different temperatures.
Temperature (K)
Blank
AAL
BAL
CR (mpy)
IE%
CR (mpy)
IE%
CR (mpy)
IE%
298
9.55
—
1.70
82.2
0.93
90.2
303
12.09
—
2.63
78.2
1.79
85.1
313
19.27
—
4.87
74.7
4.21
78.1
323
30.42
—
10.51
65.4
9.01
70.3
333
48.94
—
20.09
58.9
18.31
62.5


### 3.1.3. Adsorption Isotherms

To determine the adsorption mode, various isotherms were tested, and Langmuir adsorption isotherm was found to be best, which gives a straight line graph for the plot oflog(θ/1-θ)  verses logarithmic concentration of inhibitors  (Figure 2).Figure 2
Langmuir adsorption isotherm in presence of AAL and BAL.


### 3.1.4. Kinetic Study

The apparent activation energy(Ea) was calculated  by using Arrhenius equation:
(6)logk=-Ea2.303RT+logA,
where k is rate of corrosion, Ea is the apparent activation energy, R is the universal gas constant, T is absolute temperature, and A is the Arrhenius preexponential factor. By plotting logk against 1/T the values of activation energy (Ea) have been calculated (Ea=-(Slope)×2.303×R) (Figure 3). Activation energy for N80 steel in 15% HCl increases in presence of inhibitors. The Ea values for AAL and BAL were found to be 57.6 kJ/mol, 68.7 kJ/mol, respectively, (Table 3), which are higher than the Ea value for blank solution (38.4 kJ/mol).Table 3
Thermodynamic parameter in absence and in presence of AAL and BAL.
Inhibitor
Δ
H
* (kJ/mol)
E
a (kJ/mol)
Δ
G
ads (kJ/mol)
Δ
S
* (J/mol)
Blank
—
38.4
—
—
AAL
−62.1
57.6
−32.2
−32.5
BAL
−69.4
68.7
−36.7
−22.5Figure 3
Arrhenius plot for the corrosion of N80 steel in 15% HCI in presence and in absence of AAL and BAL.The values of entropy of activation(ΔS*) and enthalpy of activation (ΔH*) were calculated by using the formula:
(7)k=(RTNh)exp(ΔS*R)exp(ΔH*RT),
where k is rate of corrosion, h is Planks constant, N is Avogadro,s number, ΔS* is the entropy of activation, and ΔH*  is the enthalpy of activation. A plot of log(k/T) verses 1/T (Figure 4) should give a straight line, with a slope of (-ΔH*/2.303R) and a intercept of [log(R/Nh)+ΔS*/2.303R], from which the values of ΔS* and ΔH* can be calculated (Table 3). The ΔH* values for AAL and BAL were found to be as −62.1 kJ/mol and −69.4 kJ/mol, respectively, that revealed the exothermic nature of corrosion reaction. ΔS* values were found to be −32.5 J/mol and −22.5 J/mol for AAL and BAL, respectively (Table 3).Figure 4
A plot oflogk/T versus 1/T in presence of  AAL and BAL.The average value for free energy of adsorption(ΔGads) were calculated using the following equation:
(8)K=155.5exp(-ΔGadsRT),
where θ is degree of coverage on metal surface, C is concentration of inhibitors in mol l−1, R is molar gas constant in J K−1 moL−1, and T is temperature. The value of 55.5 in the above equation is the concentration of water in the solution in mole/litre. The equilibrium constant (K) has been replaced by the equation
(9)K=θ(1-θ)C.By plottinglogK against 1/T the value of ΔGads can be calculated (ΔGads=-2.303×R×Slope) from the slope of the straight line obtained (Figure 5). The values of free energy of adsorption (ΔGads) for AAL and BAL were found to be −32.2 kJ/mol and −36.7 kJ/mol, respectively (Table 3). Values of ΔGads around −20 kJ/mol or lower are consistent with the electrostatic interaction between charged organic molecules and the charged metal surface (physisorption); those around −40 kJ/mol or higher involve charge sharing or transfer from the organic molecules to the metal surface to form a coordinate type of bond (chemisorptions). The value of ΔGads for AAL and BAL that was found to be −32.2 kJ/mol and −36.7 kJ/mol which is less than −40 kJ/mol indicated that in addition to electrostatic interaction, there may be some other interactions [13]. Lebrini et al. [14] studied some triazole derivatives as corrosion inhibitors of mild steel in 1 M HClO4. The Gibbs free energy of adsorption of these molecules were reported to be around −34 kJ/mol. They concluded that the adsorption mechanism of these molecules on steel involved two types of interactions, chemisorptions, and physisorption. Similar conclusion was found by Ozcan [15], who studied the use of cystine as a corrosion inhibitor on mild steel in sulfuric acid. Thus, adsorption of studied inhibitors at the surface of N80 steel is not pure physisorption but it is combination of physisorption as well as chemisorptions.Figure 5
A plot oflogK  versus 1/T in presence of AAL and BAL.


## 3.1.1. Effect of Concentration

The percentage inhibition efficiencies (%IEs) in presence of 10, 20, 50, 100, and 150 ppm of AAL and BAL have been evaluated by weight-loss technique at 25°C, and the results are summarized in Table1. It is evident from these values that the inhibitor is significantly effective even at low concentrations like 10 ppm and there is a linear increase in %IE in the whole range of concentrations studied. The corrosion rate decreases with an increase in concentration of each inhibitor and reached the maximum value in the range of 150 ppm concentration (Figure 1).Table 1
Corrosion inhibition of N80 steel in 15% HCl in presence and absence of inhibitors AAL and BAL at different concentration.
Conc.(ppm)
AAL
BAL
CR (mpy)
%IE
CR (mpy)
%IE
0
9.55
—
9.55
—
10
3.98
58.2
3.41
64.2
20
3.39
64.4
2.66
72.1
50
2.55
73.3
1.78
81.3
100
1.88
80.3
1.10
88.4
150
1.70
82.2
0.93
90.2Figure 1
Variation of inhibition efficiency with concentration in presence of AAL and BAL.


## 3.1.2. Effect of Temperature

The effect of temperature on the performance of AAL and BAL as corrosion inhibitors for N80 steel in 15% HCl was investigated by weight loss measurements in the temperature range 298–333 K in absence and presence of both the inhibitors at optimum concentration (150 ppm) in a thermostat. The variation of inhibition efficiency and corrosion rate with temperature are shown in Table2. The observation depicts that rate of corrosion increases with increase in temperature. This may be due to the fact that at higher temperature the metal organic complex layer dissociates leaving a porous diffused film, which is responsible for corrosion [12].Table 2
Corrosion parameters in absence and in presence of AAL and BAL at different temperatures.
Temperature (K)
Blank
AAL
BAL
CR (mpy)
IE%
CR (mpy)
IE%
CR (mpy)
IE%
298
9.55
—
1.70
82.2
0.93
90.2
303
12.09
—
2.63
78.2
1.79
85.1
313
19.27
—
4.87
74.7
4.21
78.1
323
30.42
—
10.51
65.4
9.01
70.3
333
48.94
—
20.09
58.9
18.31
62.5


## 3.1.3. Adsorption Isotherms

To determine the adsorption mode, various isotherms were tested, and Langmuir adsorption isotherm was found to be best, which gives a straight line graph for the plot oflog(θ/1-θ)  verses logarithmic concentration of inhibitors  (Figure 2).Figure 2
Langmuir adsorption isotherm in presence of AAL and BAL.


## 3.1.4. Kinetic Study

The apparent activation energy(Ea) was calculated  by using Arrhenius equation:
(6)logk=-Ea2.303RT+logA,
where k is rate of corrosion, Ea is the apparent activation energy, R is the universal gas constant, T is absolute temperature, and A is the Arrhenius preexponential factor. By plotting logk against 1/T the values of activation energy (Ea) have been calculated (Ea=-(Slope)×2.303×R) (Figure 3). Activation energy for N80 steel in 15% HCl increases in presence of inhibitors. The Ea values for AAL and BAL were found to be 57.6 kJ/mol, 68.7 kJ/mol, respectively, (Table 3), which are higher than the Ea value for blank solution (38.4 kJ/mol).Table 3
Thermodynamic parameter in absence and in presence of AAL and BAL.
Inhibitor
Δ
H
* (kJ/mol)
E
a (kJ/mol)
Δ
G
ads (kJ/mol)
Δ
S
* (J/mol)
Blank
—
38.4
—
—
AAL
−62.1
57.6
−32.2
−32.5
BAL
−69.4
68.7
−36.7
−22.5Figure 3
Arrhenius plot for the corrosion of N80 steel in 15% HCI in presence and in absence of AAL and BAL.The values of entropy of activation(ΔS*) and enthalpy of activation (ΔH*) were calculated by using the formula:
(7)k=(RTNh)exp(ΔS*R)exp(ΔH*RT),
where k is rate of corrosion, h is Planks constant, N is Avogadro,s number, ΔS* is the entropy of activation, and ΔH*  is the enthalpy of activation. A plot of log(k/T) verses 1/T (Figure 4) should give a straight line, with a slope of (-ΔH*/2.303R) and a intercept of [log(R/Nh)+ΔS*/2.303R], from which the values of ΔS* and ΔH* can be calculated (Table 3). The ΔH* values for AAL and BAL were found to be as −62.1 kJ/mol and −69.4 kJ/mol, respectively, that revealed the exothermic nature of corrosion reaction. ΔS* values were found to be −32.5 J/mol and −22.5 J/mol for AAL and BAL, respectively (Table 3).Figure 4
A plot oflogk/T versus 1/T in presence of  AAL and BAL.The average value for free energy of adsorption(ΔGads) were calculated using the following equation:
(8)K=155.5exp(-ΔGadsRT),
where θ is degree of coverage on metal surface, C is concentration of inhibitors in mol l−1, R is molar gas constant in J K−1 moL−1, and T is temperature. The value of 55.5 in the above equation is the concentration of water in the solution in mole/litre. The equilibrium constant (K) has been replaced by the equation
(9)K=θ(1-θ)C.By plottinglogK against 1/T the value of ΔGads can be calculated (ΔGads=-2.303×R×Slope) from the slope of the straight line obtained (Figure 5). The values of free energy of adsorption (ΔGads) for AAL and BAL were found to be −32.2 kJ/mol and −36.7 kJ/mol, respectively (Table 3). Values of ΔGads around −20 kJ/mol or lower are consistent with the electrostatic interaction between charged organic molecules and the charged metal surface (physisorption); those around −40 kJ/mol or higher involve charge sharing or transfer from the organic molecules to the metal surface to form a coordinate type of bond (chemisorptions). The value of ΔGads for AAL and BAL that was found to be −32.2 kJ/mol and −36.7 kJ/mol which is less than −40 kJ/mol indicated that in addition to electrostatic interaction, there may be some other interactions [13]. Lebrini et al. [14] studied some triazole derivatives as corrosion inhibitors of mild steel in 1 M HClO4. The Gibbs free energy of adsorption of these molecules were reported to be around −34 kJ/mol. They concluded that the adsorption mechanism of these molecules on steel involved two types of interactions, chemisorptions, and physisorption. Similar conclusion was found by Ozcan [15], who studied the use of cystine as a corrosion inhibitor on mild steel in sulfuric acid. Thus, adsorption of studied inhibitors at the surface of N80 steel is not pure physisorption but it is combination of physisorption as well as chemisorptions.Figure 5
A plot oflogK  versus 1/T in presence of AAL and BAL.


## 3.2. Potentiodynamic Polarization Study

Figures6 and 7 show the polarization curves of N80 steel in 15% HCl solution in absence and presence of 10, 50, and 150 ppm of inhibitors AAL and BAL, respectively. The nature of the polarization curves remains almost same in absence and presence of both the inhibitors but in presence of inhibitors the curves shifted towards the lower current density as compared to the blank. The shift in current density towards lower current density in presence of inhibitors increases on increasing the concentration of the inhibitors. The electrochemical corrosion parameters including corrosion current densities (Icorr), corrosion potential (Ecorr), cathodic Tafel slope (βc), anodic Tafel slope (βa), and inhibition efficiency obtained from the polarization curves of AAL and BAL are presented in  Table 4. It is clear from the Figures 6 and 7 that with the increase of inhibitors concentration both anodic and cathodic currents were inhibited.Table 4
Electrochemical corrosion parameters in absence and in presence of AAL  and BAL at different concentration.
Inhibitors
Conc of inhibitors (ppm)
Tafel slopes
I
corr (μA/cm2)
E
corr (mV)
%IE
%IE from wt. loss
β
a (mV dec−1)
β
c (mV dec−1)
Blank
—
109
153
471
−468
—
—
AAL
10
113
162
193
460
59.0
58.2
50
118
170
108
−462
77.0
73.3
150
121
178
74
−478
84.1
82.2
BAL
10
116
171
172
−476
63.3
64.2
50
119
180
84
−480
82.1
81.3
150
125
187
35
−484
92.5
90.2Figure 6
Potentiodynamic polarization curves for N80 steel in 15% HCl in absence and in presence of AAL.Figure 7
Potentiodynamic polarization curves for N80 steel in 15% HCl in absence and in presence of BAL.It is apparent from the Table4 that Icorr decreases considerably in the presence of both the inhibitors, and %IE increases with the increase in the inhibitor concentration, due to the increase in the blocked fraction of the electrode surface by adsorption. The value of anodic and cathodic Tafel slopes increases in presence of both the inhibitors as compared to the blank solution. A minor shift in Ecorr values towards the negative direction was obtained in presence of both the inhibitors, indicating mixed nature of the inhibitors.


## 3.3. Electrochemical Impedance Spectroscopy

Nyquist plots of N80 steel in 15% HCl in the presence and absence of different concentrations (10, 50, and 150 ppm) of AAL and BAL at25±1°C are shown in the Figures 8 and 9. All the Nyquist plots obtained were semicircle in nature, the diameter of the semicircles increases with increase in inhibitor concentration, and shape is maintained throughout the tested concentration, indicating that almost no change in the corrosion mechanism occurs due to inhibitor action. The electrochemical parameters (Rct,Cdl) calculated from the Nyquist plots by using the equivalent circuit (Figure 10) in presence and absence of inhibitors are presented in Table 5. Inspection of Table 5 reveals that Rct values increase prominently while Cdl values tend to decrease with the increasing concentration of inhibitors, which suggest that inhibitors molecule function by adsorption on metal/solution interface. The values of electrochemical double layer capacitance (Cdl) were calculated at the frequency fmax, at which the imaginary component of the impedance is maximal (-Zi) by the following equation:
(10)Cdl=12πfmaxRct.Table 5
Electrochemical impedance parameters in absence and in presence of AAL and BAL.
Inhibitors
Conc. (ppm)
R
ct (Ω cm2)
C
dl  (μF cm−2)
IE%
Blank
—
176
662
—
AAL
10
439
369
59.9
50
745
173
76.3
150
1006
145
82.5
BAL
10
505
286
65.1
50
998
165
82.3
150
2301
55
92.3Figure 8
Nyquist plots of the corrosion of N80 steel in 15% HCl without and with different concentration of BAL.Figure 9
Nyquist plots of the corrosion of N80 steel in 15% HCl without and with different concentration of AAL.Figure 10
Equivalent circuit diagram.


## 3.4. SEM Study of Metal Surface Product

SEM microphotographs (Figures11(a), 11(b), 11(c), and 11(d)) of the film formed on the metal surface at 1000x magnifications were studied to analyze the change in the morphology of the metal surface after corrosion tests in presence and absence of the inhibitors. Figure 11(a) shows the SEM of polished sample (magnification 1000x). Figure 11(b) shows the SEM microphotographs of N80 steel (magnification 1000x) when exposed to 15% HCl solution at room temperature in absence of inhibitors. Figures 11(c) and 11(d) are the microphotographs of the metal surface when exposed to the acid medium in presence of BAL and AAL, respectively, at the same magnifications.Figure 11
SEM of (a) polished sample; (b) sample in presence of 15% hydrochloric acid; (c) sample in presence of 150 ppm of BAL; (d) sample in presence of 500 ppm of AAL.


## 4. Discussion

The experimental results in the (Table1) reveal that inhibition efficiency offered by AAL and BAL increases on increasing their concentration indicating that adsorption of inhibitors increases as concentration increases resulting in reduction of corrosion rate. It is clear for the Table 2 that the inhibition efficiency of both the inhibitors decreases on increasing the temperature. The decrease in efficiency at higher temperature may be due to desorption of inhibitors from the N80 steel surface at higher temperatures. The surface coverage data of AAL and BAL shows that both the inhibitors followed Langmuir adsorption isotherm indicating that adsorption is the root cause of corrosion inhibition.The values of apparent activation energy(Ea) in the inhibited solution were found to be higher as compared to that in the blank acid (Table 3). The increase in activation energy Ea indicates the retardation in corrosion rate which could have occurred because of adsorption of the inhibitors at the surface of the metal. The higher value of Ea in presence of both the inhibitors as compared to the blank solution also indicated that the dissolution of N80 steel was decreased by making a barrier by their adsorption on metal surface. The ΔH* values for AAL and BAL were found to be as −62.1 kJ/mol and −69.4 kJ/mol, respectively, that revealed the exothermic nature of corrosion reaction. ΔS* values were found to be −32.5 J/mol and −22.5 J/mol for AAL and BAL, respectively (Table 3).The value ofΔGads for AAL and BAL was found to be −32.2 kJ/mol and −36.7 kJ/mol, respectively (Table 3). The negative values of ΔGads are consistent with the spontaneity of the adsorption process. Generally, the energy values of −20 kJ/mol or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface, physisorption; those of −40 kJ/mol or more negative involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate covalent bond, chemisorptions [16]. In present study, the ΔGads value obtained for both inhibitors on N80 steel in 15% HCl solution was higher than −20 kJ/mol but less than −40 kJ/mol (Table 3); this indicates that the adsorption is neither typical physisorption nor typical hemisorptions but it is complex mixed type; that, is the adsorption of inhibitor molecules on the N80 steel surface in present study involves both physisorption and chemisorptions. The negative values of ΔS* obtained in the inhibited and uninhibited solutions suggest that the activated complex is the stage that determines the corrosion rate [17]. The negative values of the entropy change of activation (ΔS*) for AAL and BAL also imply that the activated complex in the rate determining step represents an association rather than a dissociation step, indicating that a decrease in disordering takes place on going from reactants to the activated complex [18]. The negative value of enthalpy change of activation (ΔH*) reflects the exothermic nature of activation of corrosion process.The variation in the values ofβa and βc in presence of both the inhibitors (Table 4) indicates that both the anodic and cathodic processes are controlled. The significant reduction in Icorr at higher concentration level (150 ppm) indicated better inhibition performance at higher concentration. In 15% HCl solution, the presence of AAL and BAL cause a minor shift in Ecorr, which indicates that inhibitor molecules are mixed type inhibitors. Generally, if the displacement in Ecorr is >85 mV with respect to Ecorr in uninhibited solution, the inhibitor can be seen as a cathodic or anodic type [19, 20]. In our study the maximum displacement is 12 mV in BAL and 10 mV in AAL, which indicates that both inhibitors can be arranged as a mixed type inhibitor.In EIS study an increase inRct value is observed with increasing the inhibitors concentration, suggesting that the charge transfer process is retarded due to decrease in the uncovered surface available for corrosion reaction. Due to the nonhomogeneity or roughness of the metal surface the observed semicircles of capacitive loops were depressed into the Zi which is often referred to as frequency dispersion [21]. It is worth mentioning that the double layer capacitance (Cdl) value is affected by imperfections of the surface [22]. The Cdl values were found to decrease with increase in concentration of inhibitors solutions. This behavior is generally seen for system where inhibition occurred due to the formation of a surface film by the adsorption of inhibitor on the metal surface [23]. Decrease in Cdl, which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the inhibitor molecules act by adsorption at the metal/solution interface [24].Figure11(b) shows the SEM microphotographs of N80 steel (magnification 1000x) when exposed to 15% HCl solution at room temperature in absence of inhibitors. Figure 11(b) shows that the steel surface appears to be very rough in absence of inhibitors. This is due to formation of uniform flake type corrosion products on the metal surface. No pitting and other separate phase are visible in microphotograph. Figures 11(c) and 11(d) are the microphotographs of the metal surface when exposed to the acid medium in presence of BAL and AAL, respectively, at the same magnifications. In comparison to the microphotograph of the steel surface without inhibitor the photographs of the exposed surface in presence of inhibitors are found to be covered with a semiglobular type protective film of compounds uniformly spread over the surface. The protective film is formed due to adsorption of the inhibitor molecules on the steel surface.The inhibition efficiency afforded by AAL and BAL may be attributed to the presence of electron-rich N atoms of amino group. The inhibition efficiency of BAL is higher as compared to AAL due to presence of delocalizedπ-electrons and its bigger size. In the aqueous acidic solutions AAL and BAL exist either as neutral molecules or in the form of protonated molecules (cations). These inhibitors may adsorb on the metal/acid solution interface by one and/or more of the following ways: (i)
electrostatic interaction of protonated inhibitors with already adsorbed chloride ions,(ii)
interaction between unshared electron pairs of heteroatoms and vacant d orbital of iron surface atoms.Generally, two modes of adsorption could be considered. In one mode, the neutral inhibitors may be adsorbed on the surface of N80 steel through the chemisorptions mechanism, involving the displacement of water molecules from the N80 steel surface and the sharing electrons between the heteroatoms and iron. In another mode, since it is well known that the steel surface bears positive charge in acid solution [25], so it is difficult for the protonated inhibitors to approach the positively charged mild steel surface (H3O+/metal interface) due to the electrostatic repulsion. Since chloride ions have a smaller degree of hydration, thus they could bring excess negative charges in the vicinity of the interface and favour more adsorption of the positively charged inhibitor molecules, and the protonated inhibitors adsorb through electrostatic interactions between the positively charged molecules and the negatively charged metal surface. Thus there is a synergism between adsorbed Cl- ions and protonated inhibitors. Experimental data reveals that %IE inhibitors are in the order BAL > AAL.


## 5. Conclusion

AAL and BAL both act as good corrosion inhibitors for corrosion of N80 steel in 15% HCl solution. The inhibition efficiency values increase with the increase in inhibitors concentration but decrease with increasing temperature for corrosion of N80 steel in 15% HCl. The adsorption of the AAL and BAL on N80 steel surface obeys Langmuir adsorption isotherm. Variation in the values ofβa and βc (Tafel slopes) and shift in the values of corrosion potential (Ecorr) indicate that both the tested inhibitors are mixed type inhibitors; EIS measurements show that charge transfer resistance (Rct) increases and double layer capacitance (Cdl) decreases in presence of inhibitors. The SEM of the inhibited metal surface suggested the adsorption of the inhibitor molecules on the surface of N80 steel.



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*Source: 354630-2012-12-20.xml*