# Inhibition Effect of Substituted Thiadiazoles on Corrosion Activity of N80 Steel in HCl Solution

Authors: M. Yadav; Sumit Kumar; Debasis Behera
Journal: Journal of Metallurgy
 (2013)

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## Abstract

The inhibition effect of some prepared compounds, namely, thiadiazole derivatives, on N80 steel corrosion in 15% HCl solutions has been studied by using the weight loss, electrochemical polarization, and electrochemical impedance spectroscopy techniques. It was found that the inhibition efficiency of the thiadiazole derivatives, namely, 2-amino-5-(4-methoxyphenyl)-1,3,4-thiazole (AMPT), 2-amino-5-phenyl-1,3,4-thiazole (APT), and 2-amino-5-(4-chlorophenyl)-1,3,4-thiazole (ACPT), increases with the increase in concentration. Inhibition efficiency follows the order AMPT > APT > ACPT. The effect of temperature on the corrosion was investigated by the weight loss method, and some thermodynamic parameters were calculated. The inhibitive action may be attributed to the adsorption of inhibitor molecules on the active sites of the metal surface following Langmuir adsorption isotherm. Polarization measurements indicated that thiadiazole derivatives act as mixed-type corrosion inhibitor. The adsorption of thiadiazole derivatives on N80 surface exposed to inhibitor-containing solutions was confirmed using SEM and FT-IR spectra.


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## Body

## 1. Introduction

N80 carbon steel has been generally used as the main construction material for down hole tubular, flow lines, and transmission pipelines in petroleum industry. In most industrial processes, acidic solutions are commonly used for pickling, industrial acid cleaning, acid descaling, oil well acidifying, and so forth [1–5]. It is commonly noticed that 15–28% of hydrochloric acid is used for the acidization of petroleum oil wells [6, 7]. Because of their aggressiveness, iron and its alloys get corrodes during these acidic applications particularly with the use of hydrochloric acid and sulphuric acid, which results in terrible waste of both resources and money [8]. Addition of inhibitor remains the necessary procedure to secure the metal against acid attack. Therefore, corrosion inhibitors for HCl solutions have attracted more attention. Most of the well-known corrosion inhibitors are organic compounds containing polar groups having nitrogen, sulphur, and/or oxygen atoms and heterocyclic compounds with polar functional groups and conjugated double bonds [9, 10]. These compounds can adsorb on the metal surface and partially blocking the active sites on the surface, thereby reducing the corrosion rate. Among different nitrogen- and sulphur-containing compounds, thiosemicarbazide has been reported to be a potential inhibitor for different metals [11, 12]. Most of the investigation is related to the application of common inhibitors like various derivatives of the compound of aminothiazoles [13], benzotriazoles [14–16], thioimidazole [17], mercapto-5-triazole [18], and thiadiazole [19, 20] which have been used as potential inhibitors for this system.Keeping in the view of the above excellent behavior of the organic compounds containing nitrogen and sulphur as corrosion inhibitors, some compounds of substituted thiadiazoles, namely, 2-amino-5-(4-methoxyphenyl)-1, 3,4-thiadiazole (AMPT), 2-amino-5-phenyl-1,3,4-thiadiazole (APT), and 2-amino-5-(4-chlorophenyl)-1,3,4-thiadiazole (ACPT), have been synthesized, and their corrosion inhibition property was studied by the weight loss technique and electrochemical techniques.


## 2. Experimental Section

### 2.1. Materials

#### 2.1.1. Composition of N80 Sample

The working electrode and specimens for weight loss experiments were prepared from oil well N80 steel having the percentage composition (wt.%) as shown in Table1. The specimens were mechanically abraded with 320, 400, 600, 800, 1000, and 1200 emery paper, washed in acetone and double-distilled water, then dried, and put into the cell.Table 1
Composition of specimens.
C
Mn
Si
P
S
Cr
Fe
0.31
0.92
0.19
0.01
0.008
0.20
Remainder


#### 2.1.2. Solutions

The aggressive solutions were made up of 37% HCl (Rankem). The concentration of the used inhibitors ranged from 20 ppm to 200 ppm in 15% HCl. All solutions were prepared in double-distilled water.


#### 2.1.3. Synthesis of Corrosion Inhibitor

The inhibitors AMPT, APT, and ACPT were synthesized in the laboratory by a reported method [21] as shown in Scheme 1. An equimolar mixture of substituted benzoic acid (0.05 mol), thiosemicarbazide (0.05 mol), and phosphorous oxychloride (15 mL) was refluxed gently for half an hour. After cooling, water was added (45 mL) and the mixture was refluxed for 4 h and filtered. The solution was neutralized with saturated solution of potassium hydroxide. The precipitate was filtered and recrystallized from ethanol. Their purity was confirmed by TLC. The structure of inhibitors is shown in Table 2.Table 2
Name and structural formulae of compounds thiadiazole derivatives.
Compound
Structure
Name and abbreviation
1
2-Amino-5-(4-methoxyphenyl)-1,3,4-thiadiazole (AMPT)
2
2-Amino-5-phenyl-1,3,4-thiadiazole (APT)
3
2-Amino-5-(4-chlorophenyl)-1,3,4-thiadiazole (ACPT)Scheme 1


### 2.2. Methods

#### 2.2.1. Weight Loss Method

The N80 specimens were cut in the dimension of 3 cm × 3 cm × 0.1 cm with a small hole of 2 mm at the upper edge of specimen for weight loss studies, and the size of the electrodes was 1 cm × 1 cm with 4 cm long tag for electrochemical studies with an exposed area of 1 cm2 towards aggressive solution. The specimens were mechanically abraded successively with 1/0, 2/0, 3/0, and 4/0 grade emery papers. The surface was thoroughly washed with soap, running tap water, and distilled water and finally was degreased with acetone. The samples were dried and stored in vacuum desiccators before immersion in the test solution. Weight loss measurements were performed at 303 K for 6 h by immersing the N80 steel coupons into 250 mL of 15% hydrochloric acid in the absence and the presence of various amounts of inhibitors. After the elapsed time, the specimens were taken out, washed, dried, and weighed accurately. The experimental studies were performed at temperature range of 303 K to 333 K. The inhibition efficiency (IE%) and surface coverage (θ) were determined by using following equation [22]:
(1)CR(mmpy)=87.6ΔWDAT,
where ΔW = weight loss (mg), D = density of specimen (g/cm3), A = area of specimen (cm2), and T = exposure time (hours). The degree of surface coverage (θ) by inhibitors can be expressed in terms of corrosion rate as
(2)θ=CR0-CRinhCR0,IE%=CR0-CRinhCR0×100,
where CR0 = corrosion rate in the absence of inhibitor and CRinh = corrosion rate in the presence of inhibitor.


#### 2.2.2. Electrochemical Method

The electrochemical experiments were carried out in a three-necked glass assembly containing 250 mL of the electrolyte with different concentrations of inhibitors (from 20 ppm to 200 ppm by weight) dissolved in it. The electrochemical studies were carried out with N80 steel strips having an exposed area of 1 cm2. The rest of the samples were isolated from the working environment by a cold mounting process, for which epoxy resin had been used. A conventional three-electrode cell consisting of N80 steel as a working electrode, platinum as counter electrode, and a saturated calomel electrode as reference electrode was used. Polarisation studies were carried out using an AMEL Potentiostat 2053, Italy, connected to a personal computer. Prior to electrochemical measurements, the working electrode was first immersed into the test solution for 30 min to establish a steady-state open circuit potential. After the establishment of the open circuit potential, dynamic polarization curves were obtained with a scan rate of 1 mV/s in the potential range from −470 to −390 mV with scan rate of 10 m Vs−1. All potentials were measured against SCE. All experiments were performed at 30 ± 0.2°C in an electronically controlled air thermostat. The inhibition efficiency (IE %) was calculated using the following equation:
(3)%IE=[(I0-Iinh)I0]×100,
where I0 = corrosion current in the absence of inhibitor and Iihn = corrosion current in the presence of inhibitor.AC-impedance studies were carried out in three-electrode cell assembly containing N80 steel as the working electrode, platinum as counter electrode, and saturated calomel as reference electrode. The impedance measurements were carried out at an open circuit potential (OCP), after 30 min immersion of the N80 steel electrode in the corrosive medium. The impedance data were acquired in the frequency range of 100 KHz–50 mHz with an AC voltage amplitude of 10 mV. 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:
(4)%IE=[Rct(inh)-RctRct(inh)]×100,
where Rct = charge transfer resistance in absence of inhibitor; Rct(Inh) = charge transfer resistance in presence of inhibitor.


#### 2.2.3. Scanning Electron Microscopic (SEM) Analysis

The N80 specimens of size 1.0 cm × 1.0 cm × 0.06 cm  were abraded with a series of emery papers (grades 320, 500, 800, and 1200) and then washed with distilled water and acetone. After immersion in 15% HCl in the absence and the presence of optimum concentration of inhibitor (AMPT)  at 30°C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then the SEM images were recorded using JEOL JSM-6380 LA analytical scanning electron microscope.


#### 2.2.4. FT-IR Spectroscopy

For the FTIR studies the N80 steel coupons were immersed in 15% HCl containing optimum concentration of inhibitors (AMPT, APT, and ACPT) for 24 h. The test coupons were then removed from the solution, washed thoroughly with distilled water and dried. The film formed on the N80 surface in the presence of the inhibitor was collected by nonmetallic scrapper from the surface of the steel for spectral analysis with Fourier transform infrared spectroscopy (FT-IR). FT-IR spectra were recorded between 4000 cm−1 and 400 cm−1, and the scraped films were recorded using a Perkin Elmer Model 2000, by KBr pellet method.


## 2.1. Materials

### 2.1.1. Composition of N80 Sample

The working electrode and specimens for weight loss experiments were prepared from oil well N80 steel having the percentage composition (wt.%) as shown in Table1. The specimens were mechanically abraded with 320, 400, 600, 800, 1000, and 1200 emery paper, washed in acetone and double-distilled water, then dried, and put into the cell.Table 1
Composition of specimens.
C
Mn
Si
P
S
Cr
Fe
0.31
0.92
0.19
0.01
0.008
0.20
Remainder


### 2.1.2. Solutions

The aggressive solutions were made up of 37% HCl (Rankem). The concentration of the used inhibitors ranged from 20 ppm to 200 ppm in 15% HCl. All solutions were prepared in double-distilled water.


### 2.1.3. Synthesis of Corrosion Inhibitor

The inhibitors AMPT, APT, and ACPT were synthesized in the laboratory by a reported method [21] as shown in Scheme 1. An equimolar mixture of substituted benzoic acid (0.05 mol), thiosemicarbazide (0.05 mol), and phosphorous oxychloride (15 mL) was refluxed gently for half an hour. After cooling, water was added (45 mL) and the mixture was refluxed for 4 h and filtered. The solution was neutralized with saturated solution of potassium hydroxide. The precipitate was filtered and recrystallized from ethanol. Their purity was confirmed by TLC. The structure of inhibitors is shown in Table 2.Table 2
Name and structural formulae of compounds thiadiazole derivatives.
Compound
Structure
Name and abbreviation
1
2-Amino-5-(4-methoxyphenyl)-1,3,4-thiadiazole (AMPT)
2
2-Amino-5-phenyl-1,3,4-thiadiazole (APT)
3
2-Amino-5-(4-chlorophenyl)-1,3,4-thiadiazole (ACPT)Scheme 1


## 2.1.1. Composition of N80 Sample

The working electrode and specimens for weight loss experiments were prepared from oil well N80 steel having the percentage composition (wt.%) as shown in Table1. The specimens were mechanically abraded with 320, 400, 600, 800, 1000, and 1200 emery paper, washed in acetone and double-distilled water, then dried, and put into the cell.Table 1
Composition of specimens.
C
Mn
Si
P
S
Cr
Fe
0.31
0.92
0.19
0.01
0.008
0.20
Remainder


## 2.1.2. Solutions

The aggressive solutions were made up of 37% HCl (Rankem). The concentration of the used inhibitors ranged from 20 ppm to 200 ppm in 15% HCl. All solutions were prepared in double-distilled water.


## 2.1.3. Synthesis of Corrosion Inhibitor

The inhibitors AMPT, APT, and ACPT were synthesized in the laboratory by a reported method [21] as shown in Scheme 1. An equimolar mixture of substituted benzoic acid (0.05 mol), thiosemicarbazide (0.05 mol), and phosphorous oxychloride (15 mL) was refluxed gently for half an hour. After cooling, water was added (45 mL) and the mixture was refluxed for 4 h and filtered. The solution was neutralized with saturated solution of potassium hydroxide. The precipitate was filtered and recrystallized from ethanol. Their purity was confirmed by TLC. The structure of inhibitors is shown in Table 2.Table 2
Name and structural formulae of compounds thiadiazole derivatives.
Compound
Structure
Name and abbreviation
1
2-Amino-5-(4-methoxyphenyl)-1,3,4-thiadiazole (AMPT)
2
2-Amino-5-phenyl-1,3,4-thiadiazole (APT)
3
2-Amino-5-(4-chlorophenyl)-1,3,4-thiadiazole (ACPT)Scheme 1


## 2.2. Methods

### 2.2.1. Weight Loss Method

The N80 specimens were cut in the dimension of 3 cm × 3 cm × 0.1 cm with a small hole of 2 mm at the upper edge of specimen for weight loss studies, and the size of the electrodes was 1 cm × 1 cm with 4 cm long tag for electrochemical studies with an exposed area of 1 cm2 towards aggressive solution. The specimens were mechanically abraded successively with 1/0, 2/0, 3/0, and 4/0 grade emery papers. The surface was thoroughly washed with soap, running tap water, and distilled water and finally was degreased with acetone. The samples were dried and stored in vacuum desiccators before immersion in the test solution. Weight loss measurements were performed at 303 K for 6 h by immersing the N80 steel coupons into 250 mL of 15% hydrochloric acid in the absence and the presence of various amounts of inhibitors. After the elapsed time, the specimens were taken out, washed, dried, and weighed accurately. The experimental studies were performed at temperature range of 303 K to 333 K. The inhibition efficiency (IE%) and surface coverage (θ) were determined by using following equation [22]:
(1)CR(mmpy)=87.6ΔWDAT,
where ΔW = weight loss (mg), D = density of specimen (g/cm3), A = area of specimen (cm2), and T = exposure time (hours). The degree of surface coverage (θ) by inhibitors can be expressed in terms of corrosion rate as
(2)θ=CR0-CRinhCR0,IE%=CR0-CRinhCR0×100,
where CR0 = corrosion rate in the absence of inhibitor and CRinh = corrosion rate in the presence of inhibitor.


### 2.2.2. Electrochemical Method

The electrochemical experiments were carried out in a three-necked glass assembly containing 250 mL of the electrolyte with different concentrations of inhibitors (from 20 ppm to 200 ppm by weight) dissolved in it. The electrochemical studies were carried out with N80 steel strips having an exposed area of 1 cm2. The rest of the samples were isolated from the working environment by a cold mounting process, for which epoxy resin had been used. A conventional three-electrode cell consisting of N80 steel as a working electrode, platinum as counter electrode, and a saturated calomel electrode as reference electrode was used. Polarisation studies were carried out using an AMEL Potentiostat 2053, Italy, connected to a personal computer. Prior to electrochemical measurements, the working electrode was first immersed into the test solution for 30 min to establish a steady-state open circuit potential. After the establishment of the open circuit potential, dynamic polarization curves were obtained with a scan rate of 1 mV/s in the potential range from −470 to −390 mV with scan rate of 10 m Vs−1. All potentials were measured against SCE. All experiments were performed at 30 ± 0.2°C in an electronically controlled air thermostat. The inhibition efficiency (IE %) was calculated using the following equation:
(3)%IE=[(I0-Iinh)I0]×100,
where I0 = corrosion current in the absence of inhibitor and Iihn = corrosion current in the presence of inhibitor.AC-impedance studies were carried out in three-electrode cell assembly containing N80 steel as the working electrode, platinum as counter electrode, and saturated calomel as reference electrode. The impedance measurements were carried out at an open circuit potential (OCP), after 30 min immersion of the N80 steel electrode in the corrosive medium. The impedance data were acquired in the frequency range of 100 KHz–50 mHz with an AC voltage amplitude of 10 mV. 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:
(4)%IE=[Rct(inh)-RctRct(inh)]×100,
where Rct = charge transfer resistance in absence of inhibitor; Rct(Inh) = charge transfer resistance in presence of inhibitor.


### 2.2.3. Scanning Electron Microscopic (SEM) Analysis

The N80 specimens of size 1.0 cm × 1.0 cm × 0.06 cm  were abraded with a series of emery papers (grades 320, 500, 800, and 1200) and then washed with distilled water and acetone. After immersion in 15% HCl in the absence and the presence of optimum concentration of inhibitor (AMPT)  at 30°C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then the SEM images were recorded using JEOL JSM-6380 LA analytical scanning electron microscope.


### 2.2.4. FT-IR Spectroscopy

For the FTIR studies the N80 steel coupons were immersed in 15% HCl containing optimum concentration of inhibitors (AMPT, APT, and ACPT) for 24 h. The test coupons were then removed from the solution, washed thoroughly with distilled water and dried. The film formed on the N80 surface in the presence of the inhibitor was collected by nonmetallic scrapper from the surface of the steel for spectral analysis with Fourier transform infrared spectroscopy (FT-IR). FT-IR spectra were recorded between 4000 cm−1 and 400 cm−1, and the scraped films were recorded using a Perkin Elmer Model 2000, by KBr pellet method.


## 2.2.1. Weight Loss Method

The N80 specimens were cut in the dimension of 3 cm × 3 cm × 0.1 cm with a small hole of 2 mm at the upper edge of specimen for weight loss studies, and the size of the electrodes was 1 cm × 1 cm with 4 cm long tag for electrochemical studies with an exposed area of 1 cm2 towards aggressive solution. The specimens were mechanically abraded successively with 1/0, 2/0, 3/0, and 4/0 grade emery papers. The surface was thoroughly washed with soap, running tap water, and distilled water and finally was degreased with acetone. The samples were dried and stored in vacuum desiccators before immersion in the test solution. Weight loss measurements were performed at 303 K for 6 h by immersing the N80 steel coupons into 250 mL of 15% hydrochloric acid in the absence and the presence of various amounts of inhibitors. After the elapsed time, the specimens were taken out, washed, dried, and weighed accurately. The experimental studies were performed at temperature range of 303 K to 333 K. The inhibition efficiency (IE%) and surface coverage (θ) were determined by using following equation [22]:
(1)CR(mmpy)=87.6ΔWDAT,
where ΔW = weight loss (mg), D = density of specimen (g/cm3), A = area of specimen (cm2), and T = exposure time (hours). The degree of surface coverage (θ) by inhibitors can be expressed in terms of corrosion rate as
(2)θ=CR0-CRinhCR0,IE%=CR0-CRinhCR0×100,
where CR0 = corrosion rate in the absence of inhibitor and CRinh = corrosion rate in the presence of inhibitor.


## 2.2.2. Electrochemical Method

The electrochemical experiments were carried out in a three-necked glass assembly containing 250 mL of the electrolyte with different concentrations of inhibitors (from 20 ppm to 200 ppm by weight) dissolved in it. The electrochemical studies were carried out with N80 steel strips having an exposed area of 1 cm2. The rest of the samples were isolated from the working environment by a cold mounting process, for which epoxy resin had been used. A conventional three-electrode cell consisting of N80 steel as a working electrode, platinum as counter electrode, and a saturated calomel electrode as reference electrode was used. Polarisation studies were carried out using an AMEL Potentiostat 2053, Italy, connected to a personal computer. Prior to electrochemical measurements, the working electrode was first immersed into the test solution for 30 min to establish a steady-state open circuit potential. After the establishment of the open circuit potential, dynamic polarization curves were obtained with a scan rate of 1 mV/s in the potential range from −470 to −390 mV with scan rate of 10 m Vs−1. All potentials were measured against SCE. All experiments were performed at 30 ± 0.2°C in an electronically controlled air thermostat. The inhibition efficiency (IE %) was calculated using the following equation:
(3)%IE=[(I0-Iinh)I0]×100,
where I0 = corrosion current in the absence of inhibitor and Iihn = corrosion current in the presence of inhibitor.AC-impedance studies were carried out in three-electrode cell assembly containing N80 steel as the working electrode, platinum as counter electrode, and saturated calomel as reference electrode. The impedance measurements were carried out at an open circuit potential (OCP), after 30 min immersion of the N80 steel electrode in the corrosive medium. The impedance data were acquired in the frequency range of 100 KHz–50 mHz with an AC voltage amplitude of 10 mV. 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:
(4)%IE=[Rct(inh)-RctRct(inh)]×100,
where Rct = charge transfer resistance in absence of inhibitor; Rct(Inh) = charge transfer resistance in presence of inhibitor.


## 2.2.3. Scanning Electron Microscopic (SEM) Analysis

The N80 specimens of size 1.0 cm × 1.0 cm × 0.06 cm  were abraded with a series of emery papers (grades 320, 500, 800, and 1200) and then washed with distilled water and acetone. After immersion in 15% HCl in the absence and the presence of optimum concentration of inhibitor (AMPT)  at 30°C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then the SEM images were recorded using JEOL JSM-6380 LA analytical scanning electron microscope.


## 2.2.4. FT-IR Spectroscopy

For the FTIR studies the N80 steel coupons were immersed in 15% HCl containing optimum concentration of inhibitors (AMPT, APT, and ACPT) for 24 h. The test coupons were then removed from the solution, washed thoroughly with distilled water and dried. The film formed on the N80 surface in the presence of the inhibitor was collected by nonmetallic scrapper from the surface of the steel for spectral analysis with Fourier transform infrared spectroscopy (FT-IR). FT-IR spectra were recorded between 4000 cm−1 and 400 cm−1, and the scraped films were recorded using a Perkin Elmer Model 2000, by KBr pellet method.


## 3. Results and Discussion

### 3.1. Weight Loss Measurement

Corrosion inhibition efficiency (IE%) offered by inhibitors (AMPT, APT, and ACPT) has been evaluated by weight loss technique after 6 h of immersion at 303 K which is given in Table3. From this table it is apparent that inhibition efficiency (IE%) was increased with the increase in the concentration of the inhibitor [23].  The maximum of 82.3% inhibition efficiency was obtained at the 200 ppm inhibitor (AMPT) concentration. No considerable effect on the inhibition efficiency (IE%) was observed by a further increase in the inhibitor concentration in the acid solution. From this table, the order of the inhibition efficiency follows the order AMPT > APT > ACPT. Corrosion inhibition studies were also carried out at different temperatures. Corrosion parameters, namely, corrosion rate (CR), surface coverage (θ), and inhibition efficiency (IE %) of  N80 steel in 15% HCl in the presence and absence of inhibitor at different temperature, obtained from weight loss measurements, are shown in Table 4. As shown from this table, by increasing the concentration of the inhibitor, corrosion rate decreases; as a result, the inhibition efficiency was increased with rise in temperature [24]. For all concentrations of AMPT, the systems at 333 K gave the highest efficiencies. The maximum efficiency of 92% was obtained for 200 ppm AMPT at 333 K. The variation in corrosion rate with different concentration of thiadiazole derivatives in 15% HCl at different temperature is shown in Figure 1. It is evident from Figure 1 that the inhibitor (AMPT, APT, and ACPT) has shown remarkable corrosion inhibition at a higher temperature. The variation of inhibition efficiency with the temperature is shown in Figure 2. From this figure, order of the inhibition efficiency of thiadiazole derivatives is as follows: AMPT > APT > ACPT. High efficiency of these compounds was attributed to the presence of extensively delocalized π electrons of the phenyl rings, planarity, and the presence of lone pair of electrons on N and S atoms which favored a greater adsorption of inhibitors on the metal surface [25].Table 3
The variation of percentage inhibition efficiency obtained from weight loss studies.
Concentration (ppm)
(IE percent) inhibition efficiency
AMPT
APT
ACPT
20
63.27
60.32
57.32
50
67.72
64.51
60.65
100
74.84
70.45
67.47
150
80.27
77.58
73.51
200
82.32
79.03
75.71Table 4
Corrosion parameters, namely, corrosion rate (CR), surface coverage (θ), and inhibition efficiency IE (%) of  N80 steel in 15% HCl in the presence and the absence of the inhibitor at different temperatures, obtained from weight loss measurements.
Inhibitor
Conc (ppm)
303 K
313 K
323 K
333 K
CR (mmpy)
θ
IE %
CR (mmpy)
θ
IE %
CR (mmpy)
θ
IE %
CR (mmpy)
θ
IE %
Blank
—
2.672
—
—
9.084
—
—
17.943
—
—
60.348
—
—
20
0.983
0.632
63.2
2.906
0.680
68.0
4.719
0.737
73.7
14.00
0.768
76.8
50
0.863
0.677
67.7
2.416
0.734
73.4
3.893
0.783
78.3
10.00
0.833
83.3
AMPT
100
0.673
0.748
74.8
1.607
0.823
82.3
2.440
0.864
86.4
6.155
0.898
89.8
150
0.529
0.802
80.2
1.317
0.855
85.5
1.920
0.893
89.3
5.129
0.915
91.5
200
0.446
0.823
82.3
1.090
0.880
88.0
1.596
0.911
91.1
4.586
0.924
92.4
20
1.060
0.603
60.3
3.148
0.653
65.3
5.678
0.683
68.3
16.66
0.724
72.4
50
0.948
0.645
64.5
2.831
0.688
68.8
4.670
0.739
73.9
12.69
0.789
78.9
APT
100
0.790
0.704
70.4
2.128
0.766
76.6
3.583
0.800
80.0
9.975
0.839
83.9
150
0.600
0.775
77.5
1.769
0.805
80.5
3.019
0.832
83.2
8.189
0.864
86.4
200
0.552
0.793
79.3
1.349
0.851
85.1
2.486
0.861
86.1
6.914
0.894
89.4
20
1.140
0.573
57.3
3.455
0.619
61.9
6.263
0.651
65.1
19.60
0.675
67.5
50
1.053
0.606
60.6
3.332
0.633
63.3
5.363
0.701
70.1
14.83
0.754
75.4
ACPT
100
0.869
0.674
67.4
2.503
0.725
72.5
4.419
0.754
75.4
12.40
0.794
79.4
150
0.708
0.735
73.5
2.082
0.770
77.0
3.676
0.795
79.5
10.72
0.822
82.2
200
0.649
0.757
75.7
1.637
0.819
81.9
3.136
0.825
82.5
8.762
0.855
85.5A plot of corrosion rate versus inhibitor’s concentrations at different temperatures. (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)Figure 2
Variation of inhibition efficiency of the studied compounds with the temperature containing optimum concentration of inhibitor.


#### 3.1.1. Thermodynamic and Activation Parameters

Thermodynamic and activation parameters play important role in understanding the inhibitive mechanism. The weight loss measurements were done in the temperature range of 303–333 K in the absence and the presence of different concentrations of inhibitors (AMPT, APT, and ACPT) in 15% HCl for N80 steel. The apparent activation energy (Ea) for dissolution of N80 steel in 15% HCl can be expressed using the Arrhenius equation,
(5)logCR=-Ea2.303RT+logA,
where CR is the corrosion rate, Ea is the apparent activation energy, R is the molar gas constant (8.314 J K−1 mol−1), T is the absolute temperature, and A is the Arrhenius pre-exponential factor. Figure 3 represents the Arrhenius plot of logCR against 1/T for the corrosion of N80 steel in 15% HCl solution with or without the presence of AMPT, APT, and ACPT at concentrations ranging from 20 ppm to 200 ppm. From Figure 3, the slope (-Ea/R) of each individual line was determined, and the activation energy has been calculated by using (Ea = (slope) × 2.303 × R) and the values of Ea were summarized in Table 5. It is evident from Table 5 that the decrease in activation energy Ea indicates the retardation in corrosion rate which could have occurred because of the adsorption of the inhibitors at the surface of the metal [26].Table 5
Activation parameter for N80 in 15% HCl solution in the absence and the presence of the inhibitor obtained from weight loss measurements.
Inhibitor
Concentration      (ppm)
E
a  (kJ mol−1)
Δ
H
* (kJ mol−1)
Δ
S
* (Jmol−1 K−1)
Blank
—
85.06
84.14
40.99
20
72.93
66.09
-
26.78
50
66.67
64.75
-
32.29
AMPT
100
60.32
62.80
-
40.62
150
62.32
59.13
-
54.68
200
63.78
49.40
-
88.48
20
75.40
71.61
-
8.63
50
70.50
66.87
-
24.38
APT
100
70.15
65.50
-
30.71
150
72.25
67.63
-
25.73
200
69.41
66.00
-
32.34
20
78.53
71.73
-
6.60
50
72.64
68.45
-
18.37
ACPT
100
72.66
68.72
-
19.13
150
74.13
69.77
-
17.30
200
71.15
69.44
-
20.50Arrhenius plots of log CR versus 1000/T for N80 steel corrosion in 15% HCl (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)The value of enthalpy of activation of theΔH* and the entropy of activation ΔS* can be calculated by using the following formula:
(6)CR=RTNhexp(ΔS*R)exp(-ΔH*RT),
where CR is the corrosion rate, H* is the enthalpy of activation, S* is the entropy of activation, h is Planck’s constant, and N is the Avogadro number, respectively.A plot of log (CR/T) against 1/T(Figure 4) gave straight lines with slope of (-ΔH*/2.303R) and intercept of (logR/Nh+ΔS*/2.303R) from which the activation thermodynamic parameters (ΔH* and ΔS*) are estimated and listed in Table 5. The results which are presented in Table 5 show that the enthalpy of activation is all positive. The positive sign of the enthalpy reflects the endothermic nature of the N80 steel dissolution process [27]. The negative value of ΔS* (Table 5) for all the three inhibitors indicates that activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disorder takes place during the course of transition from the reactant to the activated complex, and the positive value of ΔS* refers to an increase in the disorder [28].Transition state plot of log CR/T versus 1000/T for  N80 steel in 15% HCl at different concentrations of (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)


### 3.2. Potentiodynamic Polarization Measurements

The anodic and cathodic polarisation curves for the corrosion of N80 steel in 15% HCl in the presence and the absence of varying concentrations of inhibitors (AMPT, APT, and ACPT) at 303 K are shown in Figure6. The linear Tafel segments of the cathodic and anodic curves were extrapolated to the point of intersection to obtain the corrosion potential (Ecorr) and corrosion current density (icorr). The electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), anodic (ba) and cathodic (bc) Tafel slope, and percentage inhibition efficiency (IE%) determined from polarisation curve are summarized in Table 6. The data in Table 6 clearly show that the current density decreases with the presence of thiadiazole inhibitors (AMPT, APT, and ACPT); this indicated that inhibitors adsorbed on the metal surface, and hence the inhibition efficiency increases with the increase in the inhibitor concentration up to an optimum value. Thereafter, the increase in the inhibitor concentration resulted in negligible increase in inhibition efficiency and the presence of inhibitor caused small changes in (Ecorr) value. This implies that the inhibitor acts as a mixed-type inhibitor, affecting both anodic and cathodic reactions [29]. According to Li et al. [30], if the displacement in Ecorr is more than ±85 mV/SCE relating to E (corrosion potential of the blank), the inhibitor can be considered as a cathodic or anodic type. If the change in Ecorr is less than 85 mV, the corrosion inhibitor may be regarded as a mixed type. But the maximum displacement in the present case is less than 20 mV/SCE, which indicates that AMPT, APT, and ACPT act as mixed-type inhibitors. It is concluded that the inhibitor’s molecules retard the corrosion process without changing the mechanism of corrosion process in the medium of investigation. However, the shift of Ecorr values towards positive direction on increasing the concentration of thiadiazole derivatives suggests anodic control over the reaction.Table 6
Electrochemical parameter and percentage inhibition efficiency obtained from polarisation studies for N80 steel in 15% HCl solution in the presence or the absence of the inhibitors at 303 K.
Inhibitor
Concentration      (ppm)
E
corr (mVSCE)
b
a (mV dec−1)
b
c (mV dec−1)
I
corr (μA cm−2)
% IE
Blank
—
-
427
95
135
573
20
-
434
55
104
178
68.87
50
-
437
64
82
160
72.02
AMPT
100
-
438
77
93
134
76.54
150
-
445
157
142
95
83.37
200
-
447
111
128
80
86.51
20
-
428
74
147
203
64.52
50
-
432
61
73
182
68.21
APT
100
-
433
53
84
154
73.21
150
-
436
92
98
106
81.58
200
-
438
108
44
103
82.03
20
-
430
68
102
229
60.02
50
-
431
75
91
205
64.05
ACPT
100
-
432
88
79
177
69.07
150
-
434
97
108
133
76.81
200
-
436
118
156
114
80.07


### 3.3. Electrochemical Impedance Spectroscopy (EIS)

Nyquist plots of N80 steel in 15% HCl solution in the absence and the presence of different concentrations of AMPT, APT, and ACPT at 303 K are shown in Figures7(a), 7(b), and 7(c). It follows from Figures 7(a), 7(b), and 7(c) that a high frequency (HF) depressed charge-transfer semicircle was observed. The experimental data obtained from these plots are fitted by the equivalent electrical circuit shown in Figure 8. Such an equivalent circuit was also discussed by several researchers, who obtained similar depressed semicircles with single time constant due to surface heterogeneity of corrosion product covering the surface at random sites [31–33]. The charge-transfer resistance (Rct) values were calculated from the difference in impedance at low and high frequencies. 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:
(7)Cdl=12πfmaxRct.The constant phase element of double-layer (CPE) is mathematically expressed as(8)ZCPE=Y0-1(i)-n,
where Y0 is a proportionality factor and “n” has the meaning of phase shift. The value of “n” represents the deviation from the ideal behavior and it lies between 0 and 1.The charge transfer resistance (Rct), double-layer capacitance (Cdl) obtained from the Nyquist plots, and the calculated inhibition efficiency values (IE%) are shown in Table 7. It is clear that the value of Rct increases with increasing the concentration of the inhibitor, indicating that the corrosion rate decreases in the presence of the inhibitor [34]. It is also clear that the value of  Cdl decreases on the addition of inhibitors, indicating a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules function by the formation of the protective layer at the metal surface [35]. So, the changes in Rct and Cdl values were caused by the steady replacement of the water molecules by the adsorption of inhibitor on mild steel surface, reducing the extent of dissolution [36]. The maximum inhibition efficiency was obtained for AMPT at 200 ppm concentration. Both electrochemical methods (polarization measurement and EIS study) offered nearly the same trend.Table 7
Electrochemical impedance parameters and percentage inhibition efficiency for N80 steel in 15% HCl in the absence and the presence of the inhibitors at different concentrations at 303 K.
Inhibitor
Concentration (ppm)
R
ct (Ω cm2)
C
dl (μF cm2)
% IE
Blank
—
25
252
20
74
158
66.32
50
85
122
70.58
AMPT
100
110
87
77.25
150
142
73
82.37
200
178
67
85.97
20
66
82
62.42
50
75
66
67.71
APT
100
102
53
75.47
150
126
32
80.13
200
150
26
83.31
20
63
71
60.52
50
74
58
66.12
ACPT
100
80
41
68.58
150
107
24
76.53
200
134
20
81.03


### 3.4. Adsorption Isotherm

Basic information on the interaction between the organic inhibitors and the N80 steel surface is obtained from various isotherms. Adsorption of these organic inhibitors has displaced the water molecule from the metal surface. The most commonly used adsorption isotherms are Langmuir, Temkin, and Frumkin. By far, the best fit was obtained with the Langmuir adsorption isotherm. The degree of surface coverage (θ) for different concentrations of inhibitor in 15% hydrochloric acid has been evaluated by the weight loss value. According to the Langmuir adsorption isotherm, the surface coverage (θ) is related to the inhibition concentration by the following equation:
(9)Cinhθ=1Kads+Cinh,
where Cinh is the inhibitor concentration and Kads is the equilibrium constant for adsorption-desorption process.A plot of (Cinh/θ) versus Cinh (Figure 5) gives a straight line with an average correlation coefficient (R2), (R2=0.998, 0.997, and 0.999) for AMPT, APT, and ACPT, respectively, at 303 K which suggests that the adsorption obeys Langmuir adsorption isotherm. From the intercept, Kads is calculated, and by using the value of Kads the value of ΔGads0 was calculate by the following equation:
(10)ΔGads0=-RTln(55.5Kads),
where R is the gas constant and Tis the absolute temperature (K). The value of 55.5 is the molar concentration of water in solution in mol L−1. In the present study a large value of Kads was found for all the three studied inhibitors indicating the strong adsorption of inhibitor molecules at the surface of N80 steel. The values of ΔGads0 were −37.5, −35.2, and −32.1 kJ/mol for AMPT, APT, and ACPT, respectively. The negative values of ΔGads0 indicate a spontaneous adsorption process and stability of the adsorbed film of (AMPT, APT, and ACPT) on N80 surface [37]. It is generally accepted that for the values of ΔGads0 up to −20 kJ/mol, the types of adsorption were regarded as physisorption; the inhibition acts due to the electrostatic interactions between the charged molecules and the charged metallic surfaces, while the values around −40 kJ/mol or smaller were seen as chemisorption, which is due to the charge sharing or a transfer from the inhibitor’s molecules to the metal surface to form a covalent bond [38, 39]. The values of ΔGads0 in our measurements range from −30 to −40 kJ·mol−1; it is suggested that the adsorption of these molecules involves chemisorption.Langmuir plots of (Cinh/θ) versus Cinh for (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)Potentiodynamic polarization curves for N80 steel in 15% HCl in the presence and the absence of inhibitors at 303 K. (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)Nyquist plot for N80 steel in 15% HCl acid containing various concentrations of (a) AMPT, (b) APT, and (c) ACPT.(1) 0 ppm (2) 50 ppm (3) 100 ppm (4) 150 ppm, and (5) 200 ppm  at 303 K.
(a)
(b)
(c)Figure 8
The electrochemical equivalent circuit used to fit the impedance measurements.The inhibition efficiency afforded by all the three inhibitors may be attributed to the presence of electron rich (N, S) atoms and delocalizedπ electrons. The possible coordinating centres are unshared electron pair of nitrogen of –NH2 group, nitrogen of thiadiazole ring, C=N group, and π-electrons of phenyl and thiadiazole rings. The participation of phenyl ring during the adsorption of the inhibitors may be shown by higher % IE value in case of methoxy substituents having +I effect and lower values for chlorosubstituent with −I effect.


### 3.5. Scanning Electron Microscopy (SEM)

The surface morphology of mild steel specimens exposed to 15% HCl solutions in the absence and presence of 200 ppm of AMPT are shown in Figures9(a), 9(b), and 9(c). Figure 9(a) shows SEM image of polished bare N80 steel surface. Figure 9(b) reveals that the surface is severely corroded, and there is formation of different forms of corrosion products (iron oxides) on the surface in the absence of the inhibitor, and the entire surface is covered by a scale-like black corrosion product. The morphological feature of the inhibited surface with the addition of 200 ppm of AMPT is shown in Figure 9(c). On comparing these micrographs, it appears, that in the presence of inhibitors, the surface of the test material has improved remarkably with respect to its smoothness. The smoothening of the surface would have been caused by the adsorption of inhibitor molecules on it. But the closer look at such sites reveals that the inhomogeneities are due to the structural defects of the metal substrate and that these sites are also covered by the inhibitor film. This observation also accounts for the high inhibition efficiency values obtained during the weight loss studies of the inhibitor system.Figure 9
SEM image of N80 steel in 4 N HCl solution after 6 h immersion at 303 K (a) before immersion (polished) and (b) after immersion without inhibitor and (c) with inhibitor (AMPT).


### 3.6. FTIR Spectroscopic Analysis of Corrosion Product

The FTIR spectra of pure inhibitors and the surface films formed on N80 steel coupons immersed in 15% HCl containing optimum concentration of inhibitors (AMPT, APT, and ACPT)  for 6 h are presented in Figures10(a), 10(b), and 10(c). The FTIR spectra of pure AMPT show band at 3290 cm−1, 3135 cm−1, APT at 3355, 3160 cm−1 and ACPT at 3280, 3120 cm−1, which indicate the asymmetrical and symmetrical N–H stretching vibrations for the amino group. Bands at 3280 cm−1, 3125 cm−1 for AMPT, 3345, 3148 cm−1 for APT, and 3272, 3113 cm−1 for ACPT, respectively indicate the asymmetrical and symmetrical N–H stretching vibration for amino group. Strong peaks observed at 1640, 1630, and 1635 cm−1 are attributed to the N–H in plane bending vibrations of the amino group in pure AMPT, APT, and ACPT, respectively. Strong peaks observed at 1626, 1622, and 1620 cm−1 are attributed to the N–H in plane bending vibrations of the amino group in AMPT, APT, and ACPT, respectively. Strong bands at 1530, 1520, and 1525 cm−1 are attributed to the stretching vibration of the C=N group in the ring in pure AMPT, APT, and ACPT, respectively. Strong bands at 1523, 1512, and 1514 cm−1 are attributed to the stretching vibration of the C=N group in the ring in surface product of AMPT, APT, ACPT respectively. Peaks observed at 695, 690, and 680 cm−1 are attributed to the ring C–S vibration for pur AMPT, APT, and ACPT, respectively. Peaks observed at 690, 684, and 670 cm−1 are attributed to the ring C–S vibration for surface product of AMPT, APT, ACPT respectively. The presence of all the functional groups present in pure AMPT, APT, and ACPT in the spectrum of the surface product with a negative shift indicates the adsorption of inhibitors AMPT, APT, and ACPT at the surface of N80 steel.FTIR spectrum of the inhibitor (below), FT-IR spectrum of inhibitor adsorbed on metal surface (above). (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)


## 3.1. Weight Loss Measurement

Corrosion inhibition efficiency (IE%) offered by inhibitors (AMPT, APT, and ACPT) has been evaluated by weight loss technique after 6 h of immersion at 303 K which is given in Table3. From this table it is apparent that inhibition efficiency (IE%) was increased with the increase in the concentration of the inhibitor [23].  The maximum of 82.3% inhibition efficiency was obtained at the 200 ppm inhibitor (AMPT) concentration. No considerable effect on the inhibition efficiency (IE%) was observed by a further increase in the inhibitor concentration in the acid solution. From this table, the order of the inhibition efficiency follows the order AMPT > APT > ACPT. Corrosion inhibition studies were also carried out at different temperatures. Corrosion parameters, namely, corrosion rate (CR), surface coverage (θ), and inhibition efficiency (IE %) of  N80 steel in 15% HCl in the presence and absence of inhibitor at different temperature, obtained from weight loss measurements, are shown in Table 4. As shown from this table, by increasing the concentration of the inhibitor, corrosion rate decreases; as a result, the inhibition efficiency was increased with rise in temperature [24]. For all concentrations of AMPT, the systems at 333 K gave the highest efficiencies. The maximum efficiency of 92% was obtained for 200 ppm AMPT at 333 K. The variation in corrosion rate with different concentration of thiadiazole derivatives in 15% HCl at different temperature is shown in Figure 1. It is evident from Figure 1 that the inhibitor (AMPT, APT, and ACPT) has shown remarkable corrosion inhibition at a higher temperature. The variation of inhibition efficiency with the temperature is shown in Figure 2. From this figure, order of the inhibition efficiency of thiadiazole derivatives is as follows: AMPT > APT > ACPT. High efficiency of these compounds was attributed to the presence of extensively delocalized π electrons of the phenyl rings, planarity, and the presence of lone pair of electrons on N and S atoms which favored a greater adsorption of inhibitors on the metal surface [25].Table 3
The variation of percentage inhibition efficiency obtained from weight loss studies.
Concentration (ppm)
(IE percent) inhibition efficiency
AMPT
APT
ACPT
20
63.27
60.32
57.32
50
67.72
64.51
60.65
100
74.84
70.45
67.47
150
80.27
77.58
73.51
200
82.32
79.03
75.71Table 4
Corrosion parameters, namely, corrosion rate (CR), surface coverage (θ), and inhibition efficiency IE (%) of  N80 steel in 15% HCl in the presence and the absence of the inhibitor at different temperatures, obtained from weight loss measurements.
Inhibitor
Conc (ppm)
303 K
313 K
323 K
333 K
CR (mmpy)
θ
IE %
CR (mmpy)
θ
IE %
CR (mmpy)
θ
IE %
CR (mmpy)
θ
IE %
Blank
—
2.672
—
—
9.084
—
—
17.943
—
—
60.348
—
—
20
0.983
0.632
63.2
2.906
0.680
68.0
4.719
0.737
73.7
14.00
0.768
76.8
50
0.863
0.677
67.7
2.416
0.734
73.4
3.893
0.783
78.3
10.00
0.833
83.3
AMPT
100
0.673
0.748
74.8
1.607
0.823
82.3
2.440
0.864
86.4
6.155
0.898
89.8
150
0.529
0.802
80.2
1.317
0.855
85.5
1.920
0.893
89.3
5.129
0.915
91.5
200
0.446
0.823
82.3
1.090
0.880
88.0
1.596
0.911
91.1
4.586
0.924
92.4
20
1.060
0.603
60.3
3.148
0.653
65.3
5.678
0.683
68.3
16.66
0.724
72.4
50
0.948
0.645
64.5
2.831
0.688
68.8
4.670
0.739
73.9
12.69
0.789
78.9
APT
100
0.790
0.704
70.4
2.128
0.766
76.6
3.583
0.800
80.0
9.975
0.839
83.9
150
0.600
0.775
77.5
1.769
0.805
80.5
3.019
0.832
83.2
8.189
0.864
86.4
200
0.552
0.793
79.3
1.349
0.851
85.1
2.486
0.861
86.1
6.914
0.894
89.4
20
1.140
0.573
57.3
3.455
0.619
61.9
6.263
0.651
65.1
19.60
0.675
67.5
50
1.053
0.606
60.6
3.332
0.633
63.3
5.363
0.701
70.1
14.83
0.754
75.4
ACPT
100
0.869
0.674
67.4
2.503
0.725
72.5
4.419
0.754
75.4
12.40
0.794
79.4
150
0.708
0.735
73.5
2.082
0.770
77.0
3.676
0.795
79.5
10.72
0.822
82.2
200
0.649
0.757
75.7
1.637
0.819
81.9
3.136
0.825
82.5
8.762
0.855
85.5A plot of corrosion rate versus inhibitor’s concentrations at different temperatures. (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)Figure 2
Variation of inhibition efficiency of the studied compounds with the temperature containing optimum concentration of inhibitor.


### 3.1.1. Thermodynamic and Activation Parameters

Thermodynamic and activation parameters play important role in understanding the inhibitive mechanism. The weight loss measurements were done in the temperature range of 303–333 K in the absence and the presence of different concentrations of inhibitors (AMPT, APT, and ACPT) in 15% HCl for N80 steel. The apparent activation energy (Ea) for dissolution of N80 steel in 15% HCl can be expressed using the Arrhenius equation,
(5)logCR=-Ea2.303RT+logA,
where CR is the corrosion rate, Ea is the apparent activation energy, R is the molar gas constant (8.314 J K−1 mol−1), T is the absolute temperature, and A is the Arrhenius pre-exponential factor. Figure 3 represents the Arrhenius plot of logCR against 1/T for the corrosion of N80 steel in 15% HCl solution with or without the presence of AMPT, APT, and ACPT at concentrations ranging from 20 ppm to 200 ppm. From Figure 3, the slope (-Ea/R) of each individual line was determined, and the activation energy has been calculated by using (Ea = (slope) × 2.303 × R) and the values of Ea were summarized in Table 5. It is evident from Table 5 that the decrease in activation energy Ea indicates the retardation in corrosion rate which could have occurred because of the adsorption of the inhibitors at the surface of the metal [26].Table 5
Activation parameter for N80 in 15% HCl solution in the absence and the presence of the inhibitor obtained from weight loss measurements.
Inhibitor
Concentration      (ppm)
E
a  (kJ mol−1)
Δ
H
* (kJ mol−1)
Δ
S
* (Jmol−1 K−1)
Blank
—
85.06
84.14
40.99
20
72.93
66.09
-
26.78
50
66.67
64.75
-
32.29
AMPT
100
60.32
62.80
-
40.62
150
62.32
59.13
-
54.68
200
63.78
49.40
-
88.48
20
75.40
71.61
-
8.63
50
70.50
66.87
-
24.38
APT
100
70.15
65.50
-
30.71
150
72.25
67.63
-
25.73
200
69.41
66.00
-
32.34
20
78.53
71.73
-
6.60
50
72.64
68.45
-
18.37
ACPT
100
72.66
68.72
-
19.13
150
74.13
69.77
-
17.30
200
71.15
69.44
-
20.50Arrhenius plots of log CR versus 1000/T for N80 steel corrosion in 15% HCl (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)The value of enthalpy of activation of theΔH* and the entropy of activation ΔS* can be calculated by using the following formula:
(6)CR=RTNhexp(ΔS*R)exp(-ΔH*RT),
where CR is the corrosion rate, H* is the enthalpy of activation, S* is the entropy of activation, h is Planck’s constant, and N is the Avogadro number, respectively.A plot of log (CR/T) against 1/T(Figure 4) gave straight lines with slope of (-ΔH*/2.303R) and intercept of (logR/Nh+ΔS*/2.303R) from which the activation thermodynamic parameters (ΔH* and ΔS*) are estimated and listed in Table 5. The results which are presented in Table 5 show that the enthalpy of activation is all positive. The positive sign of the enthalpy reflects the endothermic nature of the N80 steel dissolution process [27]. The negative value of ΔS* (Table 5) for all the three inhibitors indicates that activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disorder takes place during the course of transition from the reactant to the activated complex, and the positive value of ΔS* refers to an increase in the disorder [28].Transition state plot of log CR/T versus 1000/T for  N80 steel in 15% HCl at different concentrations of (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)


## 3.1.1. Thermodynamic and Activation Parameters

Thermodynamic and activation parameters play important role in understanding the inhibitive mechanism. The weight loss measurements were done in the temperature range of 303–333 K in the absence and the presence of different concentrations of inhibitors (AMPT, APT, and ACPT) in 15% HCl for N80 steel. The apparent activation energy (Ea) for dissolution of N80 steel in 15% HCl can be expressed using the Arrhenius equation,
(5)logCR=-Ea2.303RT+logA,
where CR is the corrosion rate, Ea is the apparent activation energy, R is the molar gas constant (8.314 J K−1 mol−1), T is the absolute temperature, and A is the Arrhenius pre-exponential factor. Figure 3 represents the Arrhenius plot of logCR against 1/T for the corrosion of N80 steel in 15% HCl solution with or without the presence of AMPT, APT, and ACPT at concentrations ranging from 20 ppm to 200 ppm. From Figure 3, the slope (-Ea/R) of each individual line was determined, and the activation energy has been calculated by using (Ea = (slope) × 2.303 × R) and the values of Ea were summarized in Table 5. It is evident from Table 5 that the decrease in activation energy Ea indicates the retardation in corrosion rate which could have occurred because of the adsorption of the inhibitors at the surface of the metal [26].Table 5
Activation parameter for N80 in 15% HCl solution in the absence and the presence of the inhibitor obtained from weight loss measurements.
Inhibitor
Concentration      (ppm)
E
a  (kJ mol−1)
Δ
H
* (kJ mol−1)
Δ
S
* (Jmol−1 K−1)
Blank
—
85.06
84.14
40.99
20
72.93
66.09
-
26.78
50
66.67
64.75
-
32.29
AMPT
100
60.32
62.80
-
40.62
150
62.32
59.13
-
54.68
200
63.78
49.40
-
88.48
20
75.40
71.61
-
8.63
50
70.50
66.87
-
24.38
APT
100
70.15
65.50
-
30.71
150
72.25
67.63
-
25.73
200
69.41
66.00
-
32.34
20
78.53
71.73
-
6.60
50
72.64
68.45
-
18.37
ACPT
100
72.66
68.72
-
19.13
150
74.13
69.77
-
17.30
200
71.15
69.44
-
20.50Arrhenius plots of log CR versus 1000/T for N80 steel corrosion in 15% HCl (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)The value of enthalpy of activation of theΔH* and the entropy of activation ΔS* can be calculated by using the following formula:
(6)CR=RTNhexp(ΔS*R)exp(-ΔH*RT),
where CR is the corrosion rate, H* is the enthalpy of activation, S* is the entropy of activation, h is Planck’s constant, and N is the Avogadro number, respectively.A plot of log (CR/T) against 1/T(Figure 4) gave straight lines with slope of (-ΔH*/2.303R) and intercept of (logR/Nh+ΔS*/2.303R) from which the activation thermodynamic parameters (ΔH* and ΔS*) are estimated and listed in Table 5. The results which are presented in Table 5 show that the enthalpy of activation is all positive. The positive sign of the enthalpy reflects the endothermic nature of the N80 steel dissolution process [27]. The negative value of ΔS* (Table 5) for all the three inhibitors indicates that activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disorder takes place during the course of transition from the reactant to the activated complex, and the positive value of ΔS* refers to an increase in the disorder [28].Transition state plot of log CR/T versus 1000/T for  N80 steel in 15% HCl at different concentrations of (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)


## 3.2. Potentiodynamic Polarization Measurements

The anodic and cathodic polarisation curves for the corrosion of N80 steel in 15% HCl in the presence and the absence of varying concentrations of inhibitors (AMPT, APT, and ACPT) at 303 K are shown in Figure6. The linear Tafel segments of the cathodic and anodic curves were extrapolated to the point of intersection to obtain the corrosion potential (Ecorr) and corrosion current density (icorr). The electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (icorr), anodic (ba) and cathodic (bc) Tafel slope, and percentage inhibition efficiency (IE%) determined from polarisation curve are summarized in Table 6. The data in Table 6 clearly show that the current density decreases with the presence of thiadiazole inhibitors (AMPT, APT, and ACPT); this indicated that inhibitors adsorbed on the metal surface, and hence the inhibition efficiency increases with the increase in the inhibitor concentration up to an optimum value. Thereafter, the increase in the inhibitor concentration resulted in negligible increase in inhibition efficiency and the presence of inhibitor caused small changes in (Ecorr) value. This implies that the inhibitor acts as a mixed-type inhibitor, affecting both anodic and cathodic reactions [29]. According to Li et al. [30], if the displacement in Ecorr is more than ±85 mV/SCE relating to E (corrosion potential of the blank), the inhibitor can be considered as a cathodic or anodic type. If the change in Ecorr is less than 85 mV, the corrosion inhibitor may be regarded as a mixed type. But the maximum displacement in the present case is less than 20 mV/SCE, which indicates that AMPT, APT, and ACPT act as mixed-type inhibitors. It is concluded that the inhibitor’s molecules retard the corrosion process without changing the mechanism of corrosion process in the medium of investigation. However, the shift of Ecorr values towards positive direction on increasing the concentration of thiadiazole derivatives suggests anodic control over the reaction.Table 6
Electrochemical parameter and percentage inhibition efficiency obtained from polarisation studies for N80 steel in 15% HCl solution in the presence or the absence of the inhibitors at 303 K.
Inhibitor
Concentration      (ppm)
E
corr (mVSCE)
b
a (mV dec−1)
b
c (mV dec−1)
I
corr (μA cm−2)
% IE
Blank
—
-
427
95
135
573
20
-
434
55
104
178
68.87
50
-
437
64
82
160
72.02
AMPT
100
-
438
77
93
134
76.54
150
-
445
157
142
95
83.37
200
-
447
111
128
80
86.51
20
-
428
74
147
203
64.52
50
-
432
61
73
182
68.21
APT
100
-
433
53
84
154
73.21
150
-
436
92
98
106
81.58
200
-
438
108
44
103
82.03
20
-
430
68
102
229
60.02
50
-
431
75
91
205
64.05
ACPT
100
-
432
88
79
177
69.07
150
-
434
97
108
133
76.81
200
-
436
118
156
114
80.07


## 3.3. Electrochemical Impedance Spectroscopy (EIS)

Nyquist plots of N80 steel in 15% HCl solution in the absence and the presence of different concentrations of AMPT, APT, and ACPT at 303 K are shown in Figures7(a), 7(b), and 7(c). It follows from Figures 7(a), 7(b), and 7(c) that a high frequency (HF) depressed charge-transfer semicircle was observed. The experimental data obtained from these plots are fitted by the equivalent electrical circuit shown in Figure 8. Such an equivalent circuit was also discussed by several researchers, who obtained similar depressed semicircles with single time constant due to surface heterogeneity of corrosion product covering the surface at random sites [31–33]. The charge-transfer resistance (Rct) values were calculated from the difference in impedance at low and high frequencies. 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:
(7)Cdl=12πfmaxRct.The constant phase element of double-layer (CPE) is mathematically expressed as(8)ZCPE=Y0-1(i)-n,
where Y0 is a proportionality factor and “n” has the meaning of phase shift. The value of “n” represents the deviation from the ideal behavior and it lies between 0 and 1.The charge transfer resistance (Rct), double-layer capacitance (Cdl) obtained from the Nyquist plots, and the calculated inhibition efficiency values (IE%) are shown in Table 7. It is clear that the value of Rct increases with increasing the concentration of the inhibitor, indicating that the corrosion rate decreases in the presence of the inhibitor [34]. It is also clear that the value of  Cdl decreases on the addition of inhibitors, indicating a decrease in the local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules function by the formation of the protective layer at the metal surface [35]. So, the changes in Rct and Cdl values were caused by the steady replacement of the water molecules by the adsorption of inhibitor on mild steel surface, reducing the extent of dissolution [36]. The maximum inhibition efficiency was obtained for AMPT at 200 ppm concentration. Both electrochemical methods (polarization measurement and EIS study) offered nearly the same trend.Table 7
Electrochemical impedance parameters and percentage inhibition efficiency for N80 steel in 15% HCl in the absence and the presence of the inhibitors at different concentrations at 303 K.
Inhibitor
Concentration (ppm)
R
ct (Ω cm2)
C
dl (μF cm2)
% IE
Blank
—
25
252
20
74
158
66.32
50
85
122
70.58
AMPT
100
110
87
77.25
150
142
73
82.37
200
178
67
85.97
20
66
82
62.42
50
75
66
67.71
APT
100
102
53
75.47
150
126
32
80.13
200
150
26
83.31
20
63
71
60.52
50
74
58
66.12
ACPT
100
80
41
68.58
150
107
24
76.53
200
134
20
81.03


## 3.4. Adsorption Isotherm

Basic information on the interaction between the organic inhibitors and the N80 steel surface is obtained from various isotherms. Adsorption of these organic inhibitors has displaced the water molecule from the metal surface. The most commonly used adsorption isotherms are Langmuir, Temkin, and Frumkin. By far, the best fit was obtained with the Langmuir adsorption isotherm. The degree of surface coverage (θ) for different concentrations of inhibitor in 15% hydrochloric acid has been evaluated by the weight loss value. According to the Langmuir adsorption isotherm, the surface coverage (θ) is related to the inhibition concentration by the following equation:
(9)Cinhθ=1Kads+Cinh,
where Cinh is the inhibitor concentration and Kads is the equilibrium constant for adsorption-desorption process.A plot of (Cinh/θ) versus Cinh (Figure 5) gives a straight line with an average correlation coefficient (R2), (R2=0.998, 0.997, and 0.999) for AMPT, APT, and ACPT, respectively, at 303 K which suggests that the adsorption obeys Langmuir adsorption isotherm. From the intercept, Kads is calculated, and by using the value of Kads the value of ΔGads0 was calculate by the following equation:
(10)ΔGads0=-RTln(55.5Kads),
where R is the gas constant and Tis the absolute temperature (K). The value of 55.5 is the molar concentration of water in solution in mol L−1. In the present study a large value of Kads was found for all the three studied inhibitors indicating the strong adsorption of inhibitor molecules at the surface of N80 steel. The values of ΔGads0 were −37.5, −35.2, and −32.1 kJ/mol for AMPT, APT, and ACPT, respectively. The negative values of ΔGads0 indicate a spontaneous adsorption process and stability of the adsorbed film of (AMPT, APT, and ACPT) on N80 surface [37]. It is generally accepted that for the values of ΔGads0 up to −20 kJ/mol, the types of adsorption were regarded as physisorption; the inhibition acts due to the electrostatic interactions between the charged molecules and the charged metallic surfaces, while the values around −40 kJ/mol or smaller were seen as chemisorption, which is due to the charge sharing or a transfer from the inhibitor’s molecules to the metal surface to form a covalent bond [38, 39]. The values of ΔGads0 in our measurements range from −30 to −40 kJ·mol−1; it is suggested that the adsorption of these molecules involves chemisorption.Langmuir plots of (Cinh/θ) versus Cinh for (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)Potentiodynamic polarization curves for N80 steel in 15% HCl in the presence and the absence of inhibitors at 303 K. (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)Nyquist plot for N80 steel in 15% HCl acid containing various concentrations of (a) AMPT, (b) APT, and (c) ACPT.(1) 0 ppm (2) 50 ppm (3) 100 ppm (4) 150 ppm, and (5) 200 ppm  at 303 K.
(a)
(b)
(c)Figure 8
The electrochemical equivalent circuit used to fit the impedance measurements.The inhibition efficiency afforded by all the three inhibitors may be attributed to the presence of electron rich (N, S) atoms and delocalizedπ electrons. The possible coordinating centres are unshared electron pair of nitrogen of –NH2 group, nitrogen of thiadiazole ring, C=N group, and π-electrons of phenyl and thiadiazole rings. The participation of phenyl ring during the adsorption of the inhibitors may be shown by higher % IE value in case of methoxy substituents having +I effect and lower values for chlorosubstituent with −I effect.


## 3.5. Scanning Electron Microscopy (SEM)

The surface morphology of mild steel specimens exposed to 15% HCl solutions in the absence and presence of 200 ppm of AMPT are shown in Figures9(a), 9(b), and 9(c). Figure 9(a) shows SEM image of polished bare N80 steel surface. Figure 9(b) reveals that the surface is severely corroded, and there is formation of different forms of corrosion products (iron oxides) on the surface in the absence of the inhibitor, and the entire surface is covered by a scale-like black corrosion product. The morphological feature of the inhibited surface with the addition of 200 ppm of AMPT is shown in Figure 9(c). On comparing these micrographs, it appears, that in the presence of inhibitors, the surface of the test material has improved remarkably with respect to its smoothness. The smoothening of the surface would have been caused by the adsorption of inhibitor molecules on it. But the closer look at such sites reveals that the inhomogeneities are due to the structural defects of the metal substrate and that these sites are also covered by the inhibitor film. This observation also accounts for the high inhibition efficiency values obtained during the weight loss studies of the inhibitor system.Figure 9
SEM image of N80 steel in 4 N HCl solution after 6 h immersion at 303 K (a) before immersion (polished) and (b) after immersion without inhibitor and (c) with inhibitor (AMPT).


## 3.6. FTIR Spectroscopic Analysis of Corrosion Product

The FTIR spectra of pure inhibitors and the surface films formed on N80 steel coupons immersed in 15% HCl containing optimum concentration of inhibitors (AMPT, APT, and ACPT)  for 6 h are presented in Figures10(a), 10(b), and 10(c). The FTIR spectra of pure AMPT show band at 3290 cm−1, 3135 cm−1, APT at 3355, 3160 cm−1 and ACPT at 3280, 3120 cm−1, which indicate the asymmetrical and symmetrical N–H stretching vibrations for the amino group. Bands at 3280 cm−1, 3125 cm−1 for AMPT, 3345, 3148 cm−1 for APT, and 3272, 3113 cm−1 for ACPT, respectively indicate the asymmetrical and symmetrical N–H stretching vibration for amino group. Strong peaks observed at 1640, 1630, and 1635 cm−1 are attributed to the N–H in plane bending vibrations of the amino group in pure AMPT, APT, and ACPT, respectively. Strong peaks observed at 1626, 1622, and 1620 cm−1 are attributed to the N–H in plane bending vibrations of the amino group in AMPT, APT, and ACPT, respectively. Strong bands at 1530, 1520, and 1525 cm−1 are attributed to the stretching vibration of the C=N group in the ring in pure AMPT, APT, and ACPT, respectively. Strong bands at 1523, 1512, and 1514 cm−1 are attributed to the stretching vibration of the C=N group in the ring in surface product of AMPT, APT, ACPT respectively. Peaks observed at 695, 690, and 680 cm−1 are attributed to the ring C–S vibration for pur AMPT, APT, and ACPT, respectively. Peaks observed at 690, 684, and 670 cm−1 are attributed to the ring C–S vibration for surface product of AMPT, APT, ACPT respectively. The presence of all the functional groups present in pure AMPT, APT, and ACPT in the spectrum of the surface product with a negative shift indicates the adsorption of inhibitors AMPT, APT, and ACPT at the surface of N80 steel.FTIR spectrum of the inhibitor (below), FT-IR spectrum of inhibitor adsorbed on metal surface (above). (a) AMPT, (b) APT, and (c) ACPT.
(a)
(b)
(c)


## 4. Conclusions

(1)
The synthesized thiadiazoles show good inhibition efficiencies for the corrosion of N80 steel in 15 wt% HCl solutions and the inhibition efficiency increases with increasing the concentration of these inhibitors and with the temperature. The increasing order of inhibiting performance of the inhibitors is in the order of: AMPT > APT > ACPT.
(2)
Thiadiazoles inhibitors were found to be mixed-type inhibitors from Tafel polarization measurements. While one equivalent structure model was selected to fit the experimental data of the impedance diagram (EIS). And the results are good in agreement with weight loss measurements.
(3)
It is suggested from the results obtained from Langmuir adsorption isotherm that the mechanism of corrosion inhibition is occurring mainly through adsorption process.
(4)
Surface morphological studies such as FTIR and SEM analysis showed that a film of inhibitor is formed on the electrode surface.



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*Source: 256403-2013-04-18.xml*