# A Characteristic  Investigation of Aminocyclohexane-N′-methylurea as a Corrosion Inhibitor for Mild Steel in 1 NH2SO4

Authors: P. Thiraviyam; K. Kannan; P. Premkumar
Journal: Journal of Chemistry
 (2013)

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## Abstract

The newly synthesized aminocyclohexane-N′-methylurea Mannich base was characterised using FT-IR, H1NMR, and C13NMR spectra and also it was tested as corrosion inhibitor for the mild steel in 1 N sulphuric acid. Inhibitive study of this compound was carried out by weight loss method over the temperature range of 303–333 K, potentiodynamic polarisation, and AC impedance methods. The inhibition efficiency was increased with increasing concentration of inhibitor whereas it was decreased with increasing temperature. Potentiodynamic polarisation study showed that ACMU is a mixed type inhibitor. AC Impedance study reveals that the corrosion of steel was mainly controlled by a charge transfer process. Surface analysis was carried out using SEM technique. The adsorption of inhibitor follows the Tempkin and Langmuir adsorption isotherms. The activation energy (Ea), free energy change (ΔGads), enthalpy change (ΔHads) and entropy change (ΔSads) were calculated to understand the corrosion inhibition mechanism.


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## Body

## 1. Introduction

Mild steel is one of the well-known materials used in chemical and allied industries for handling of acids, alkali, and salt solutions. But its susceptibility to corrosion in acid medium is the major obstacles for its larger scale application [1]. Sulphuric acid is mostly employed in chemical industry particularly in pickling process to remove oxide scale from metal surface at elevated temperature [2]. Generally organic compounds are added into the corrosive environment to control the dissolution of metal in acid medium. The corrosion inhibition is mainly decided by the formation of donor-acceptor surface complex between free or π electrons of an inhibitor and vacant d orbitals of the metal atom. A survey of literature reveals that the selection of inhibitor is mostly based upon the type and number of hetero atoms like N, O, and S present in the organic compounds. Many of the available and synthesised organic compounds were used as corrosion inhibitors for mild steel corrosion [1–8], particularly very few newly synthesised Mannich bases were also used as corrosion inhibitor for mild steel corrosion [9].In the present investigation newly synthesised Mannich base aminocyclohexane-N′-methylurea is tried as a corrosion inhibitor on mild steel corrosion in aqueous sulphuric acid solutions. This compound has one amine group and two amide group which is very essential for an organic compound to behave as an effective corrosion inhibitor. The inhibition efficiency of this compound was studied by weight loss, potentiodynamic polarisation, and AC impedance methods.


## 2. Experimental Procedure

### 2.1. Preparation of Specimens

Mild steel specimens of composition Fe = 99.686, Ni = 0.013, Mo = 0.015, Cr = 0.043, S = 0.014,P=0.009, Si = 0.007, Mn = 0.196, and C = 0.017 were cut to size of 5 cm × 1 cm. The surface of specimens were polished with emery papers ranging from 1/0 to 4/0 grit grades and decreased with trichloroethylene, washed with triply distilled water, and finally dried. Dried specimens were stored in vacuum desiccator containing silica gel. Weight loss measurements were performed as per ASTM method described previously [10].


### 2.2. Synthesis of ACMU

Aminocyclohexane-N′-methylurea was synthesised by the mannich base reaction of cyclohexylamine, formaldehyde, and urea. Equimolar concentration of cyclohexylamine, formaldehyde, and urea were mixed in the water medium and are stirred vigorously for about three hours. The product was precipitated out as white solid with 80 percent yield. The crude product was purified by column chromatography using chloroform (70%) and methanol (30%) solvent system and then recrystallised using ethanol. The purity of the compound was confirmed by TLC using chloroform: methanol (7 : 3) eluant. Structure of this compound was confirmed with FT-IR and NMR spectral analysis. The Mannich base reaction is shown in Scheme1.Scheme 1


### 2.3. Preparation of Inhibitor Solutions

0.050 N ACMU in sulphuric acid solution was prepared by dissolving 8.55 g of ACMU in 1000 mL of 1 N sulphuric acid then it was diluted to 0.020, 0.015, 0.010, 0.005, 0.001 N ACMU using 1 N sulphuric acid. These diluted solutions were used as corrosion inhibitor for further studies.


### 2.4. Structural Elucidation (FT-IR and NMR)

The FT-IR spectra were recorded on a JASCO FT-IR 430 spectrophotometer in KBr pellet. The H1 NMR and C13 NMR spectra were recorded on a Bruker AC 300F (300 MHz) NMR spectrometer using CDCl3 as solvent and TMS as an internal standard.


### 2.5. Weight Loss Studies

Weight loss measurements were carried out by weighing the mild steel specimens before and after immersion in the glass vessel containing 100 mL test solution for different time intervals (1 hr, 3 hrs, and 5 hrs) with and without inhibitor. The same procedure was repeated for different temperatures (303 K, 318 K, and 333 K) with one hour immersion period. From the weight loss studies, corrosion rate, inhibition efficiency and surface coverage were calculated using the following equations (1), (2), and (3) [11]:(1)Corrosionratek=8.76×10000WATD,
where T—time of exposure in hrs; W—weight loss of test specimen in g; A—area of test specimen in cm2, D—density of material in gcm−3, k—corrosion rate in mmpy,
(2)InhibitionefficiencyIE%=WU-WIWU×100,(3)Surfacecoverageθ=WU-WIWU,WI and WU are the corrosion rate of inhibited and uninhibited mild steel in 1 N sulphuric acid.


### 2.6. Electrochemical Measurements

The electrochemical measurements were carried out with the conventional three-electrode system. The working electrode was the mild steel of 1 cm2 area and the rest portions are covered with araldite. This working electrode was polished with various grades of emery papers, washed with triply distilled water and degreased with acetone. A rectangular platinum foil of 1 cm2 was used as the counter electrode and saturated calomel electrode (SCE) as reference electrode. All the three electrodes were immersed in 1 N sulphuric acid solution with and without inhibitor. Measurements were performed using CH electrochemical analyser Model CHI 608B instrument.  The polarisation measurements were carried out at a scan rate of 2 mV/s and the impedance measurements were carried out in the frequency range of 10 kHz to 0.01 Hz at the open circuit potential. Electrochemical measurements were initiated about 30 min after the working electrode was immersed in solution to stabilize the steady state potential.


### 2.7. Scanning Electron Microscope (SEM)

The mild steel specimens were immersed in 1 N sulphuric acid solution with and without inhibitor for about 24 hours. After the immersion, the specimens were washed with triply distilled water and dried at room temperature. The morphology of treated mild steel surface was examined by using scanning electron microscope.


## 2.1. Preparation of Specimens

Mild steel specimens of composition Fe = 99.686, Ni = 0.013, Mo = 0.015, Cr = 0.043, S = 0.014,P=0.009, Si = 0.007, Mn = 0.196, and C = 0.017 were cut to size of 5 cm × 1 cm. The surface of specimens were polished with emery papers ranging from 1/0 to 4/0 grit grades and decreased with trichloroethylene, washed with triply distilled water, and finally dried. Dried specimens were stored in vacuum desiccator containing silica gel. Weight loss measurements were performed as per ASTM method described previously [10].


## 2.2. Synthesis of ACMU

Aminocyclohexane-N′-methylurea was synthesised by the mannich base reaction of cyclohexylamine, formaldehyde, and urea. Equimolar concentration of cyclohexylamine, formaldehyde, and urea were mixed in the water medium and are stirred vigorously for about three hours. The product was precipitated out as white solid with 80 percent yield. The crude product was purified by column chromatography using chloroform (70%) and methanol (30%) solvent system and then recrystallised using ethanol. The purity of the compound was confirmed by TLC using chloroform: methanol (7 : 3) eluant. Structure of this compound was confirmed with FT-IR and NMR spectral analysis. The Mannich base reaction is shown in Scheme1.Scheme 1


## 2.3. Preparation of Inhibitor Solutions

0.050 N ACMU in sulphuric acid solution was prepared by dissolving 8.55 g of ACMU in 1000 mL of 1 N sulphuric acid then it was diluted to 0.020, 0.015, 0.010, 0.005, 0.001 N ACMU using 1 N sulphuric acid. These diluted solutions were used as corrosion inhibitor for further studies.


## 2.4. Structural Elucidation (FT-IR and NMR)

The FT-IR spectra were recorded on a JASCO FT-IR 430 spectrophotometer in KBr pellet. The H1 NMR and C13 NMR spectra were recorded on a Bruker AC 300F (300 MHz) NMR spectrometer using CDCl3 as solvent and TMS as an internal standard.


## 2.5. Weight Loss Studies

Weight loss measurements were carried out by weighing the mild steel specimens before and after immersion in the glass vessel containing 100 mL test solution for different time intervals (1 hr, 3 hrs, and 5 hrs) with and without inhibitor. The same procedure was repeated for different temperatures (303 K, 318 K, and 333 K) with one hour immersion period. From the weight loss studies, corrosion rate, inhibition efficiency and surface coverage were calculated using the following equations (1), (2), and (3) [11]:(1)Corrosionratek=8.76×10000WATD,
where T—time of exposure in hrs; W—weight loss of test specimen in g; A—area of test specimen in cm2, D—density of material in gcm−3, k—corrosion rate in mmpy,
(2)InhibitionefficiencyIE%=WU-WIWU×100,(3)Surfacecoverageθ=WU-WIWU,WI and WU are the corrosion rate of inhibited and uninhibited mild steel in 1 N sulphuric acid.


## 2.6. Electrochemical Measurements

The electrochemical measurements were carried out with the conventional three-electrode system. The working electrode was the mild steel of 1 cm2 area and the rest portions are covered with araldite. This working electrode was polished with various grades of emery papers, washed with triply distilled water and degreased with acetone. A rectangular platinum foil of 1 cm2 was used as the counter electrode and saturated calomel electrode (SCE) as reference electrode. All the three electrodes were immersed in 1 N sulphuric acid solution with and without inhibitor. Measurements were performed using CH electrochemical analyser Model CHI 608B instrument.  The polarisation measurements were carried out at a scan rate of 2 mV/s and the impedance measurements were carried out in the frequency range of 10 kHz to 0.01 Hz at the open circuit potential. Electrochemical measurements were initiated about 30 min after the working electrode was immersed in solution to stabilize the steady state potential.


## 2.7. Scanning Electron Microscope (SEM)

The mild steel specimens were immersed in 1 N sulphuric acid solution with and without inhibitor for about 24 hours. After the immersion, the specimens were washed with triply distilled water and dried at room temperature. The morphology of treated mild steel surface was examined by using scanning electron microscope.


## 3. Results and Discussion

### 3.1. Characterisation of ACMU

Newly synthesised aminocyclohexane-N′-methylurea was characterised with data obtained from the FT-IR and NMR spectrum.


### 3.2. FT-IR Spectral Analysis

The FT-IR spectrum of ACMU is shown in Figure1. A sharp peak at 3400.54 cm−1 is assigned to secondary amine stretching. A double sharp peak at 1550–1650 cm−1 is assigned to the amide group. The sharp peak at 1039 cm−1 is attributed to the cyclohexane ring vibrations.Figure 1
FT-IR spectrum of ACMU.


### 3.3. NMR Spectral Analysis

#### 3.3.1.1H-NMR (300 MHz): Solvent CDCl3

The H1 NMR spectrum of ACMU is shown in Figure 2. The multiplet at 1.014–1.285 ppm is assigned to six hydrogen atoms labelled as 1 and 2, multiplet at 1.488–1.905 ppm is assigned to four hydrogen atoms labelled as 3, and multiplet at 2.849 ppm is assigned to one hydrogen atom labelled as 4. The multiplet at 1.81 ppm is assigned to one hydrogen atom labelled as 7, singlet at 1.870 ppm is assigned to two hydrogen atoms labelled as 5, and singlet at 6.184 ppm is assigned for the three hydrogen atoms labelled as 8 and 9.Figure 2
H1 NMR spectrum of ACMU.


#### 3.3.2.13C-NMR (300 MHz): Solvent CDCl3

The C13 NMR spectrum of ACMU is shown in Figure 3. The singlet at 25.70 ppm is assigned to two carbon atom labelled as 2, singlet at 26.03 ppm is attributed to one carbon atom labelled as 1, singlet at 29.94 ppm is referred to two carbon atoms labelled as 3, singlet 58.46 ppm is assigned to one carbon atom labelled as 5, singlet at 68.29 ppm is assigned to one carbon atom labelled as 4, and singlet at 150.6 ppm is assigned to one carbon atom labelled as 6.Figure 3
C13 NMR spectrum of ACMU.The proposed ACMU structure based upon the FT-IR,1H-NMR, and 13C-NMR spectrum is shown in Figure 4.Figure 4
Structure of ACMU.


### 3.4. Weight Loss Studies

Each weight loss studies was repeated three times and also its standard deviation was calculated. The standard deviation was various between 0.5 and 2.1. Table1 shows the value of inhibition efficiency, surface coverage, and corrosion rate for various concentration of inhibitor in 1 N Sulphuric acid for the period of 1 hr, 3 hrs and 5 hrs. The addition of inhibitor to acid decreases the corrosion rate due to the adsorption of inhibitor on mild steel surface. The inhibition efficiency was found to be increased with increasing time and concentration of inhibitor. The maximum inhibition efficiency was found to be 92.75% in 1 N sulphuric acid for 0.020 N ACMU at 5 hrs.Table 1
Inhibition efficiency of ACMU on mild steel corrosion in 1 N H2SO4 at 303 K for various time intervals.
Concentration of inhibitor (N)
1 hr
3 hrs
5 hrs
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency(%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency(%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency(%)
Blank
20.061
—
—
21.770
—
—
22.736
—
—
0.001
8.470
0.5778
57.78
5.944
0.7270
72.70
4.636
0.7961
79.61
0.005
6.687
0.6667
66.67
5.052
0.7679
76.79
3.789
0.8333
83.33
0.010
5.795
0.7111
71.11
4.235
0.8055
80.55
3.210
0.8588
85.88
0.015
4.235
0.7889
78.89
3.641
0.8328
83.28
2.407
0.8941
89.41
0.020
3.121
0.8444
84.44
2.898
0.8669
86.69
1.649
0.9275
92.75Table2 shows the value of inhibition efficiency, surface coverage, and corrosion rate for various concentration of inhibitor in 1 N sulphuric acid for a period of 1 hr at various temperatures. The inhibition efficiency was found to be increased with increasing concentration of inhibitor and decreased with increasing temperature. This can be explained as follows: the inhibitor adsorbed on to the metal surface, and an increase in temperature resulted in desorption of some adsorbed molecules, leading to a decrease in inhibition efficiency [11]. The maximum inhibition efficiency was found to be 84.44% in 1 N sulphuric acid for 0.020 N ACMU at 300 K.Table 2
Inhibition efficiency of ACMU on mild steel corrosion in 1 N H2SO4 at various temperatures.
Concentration of inhibitor (N)
303 K
318 K
333 K
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency (%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency (%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency (%)
Blank
20.061
—
—
67.539
—
—
390.968
—
—
0.001
8.470
0.5778
57.78
33.658
0.5017
50.17
218.666
0.4407
44.07
0.005
6.687
0.6667
66.67
26.748
0.6040
60.40
160.266
0.5901
59.01
0.010
5.795
0.7111
71.11
21.176
0.6865
68.65
135.078
0.6545
65.45
0.015
4.235
0.7889
78.89
17.832
0.7360
73.60
119.029
0.6956
69.56
0.020
3.121
0.8444
84.44
12.928
0.8086
80.86
107.884
0.7241
72.41Thermodynamic/kinetic consideration.Adsorption behaviour of ACMU on mild steel surface was very much essential to understand the corrosion inhibition mechanism. Tempkin and Langmuir adsorption isotherms were tested by plotting log Cinh versus surface coverage (θ) and Cinh/θ versus Cinh respectively. Both plots gives straight line confirming inhibition of ACMU on mild steel corrosion in 1 N  sulphuric acid obeys Tempkin and Langmuir adsorption isotherm which is shown in Figures 5 and 6. This result reveals that the adsorption of ACMU on mild steel is mono layered. Langmuir adsorption isotherm is given in (4):
(4)LangmuiradsorptionisothermCinhθ=1kads+Cinh,
where Cinh is the inhibitor concentration and kads is the equilibrium constant and θ is the degree of surface coverage of the inhibitor.Figure 5
Tempkin adsorption isotherm plot for mild steel in 1 N H2SO4 with ACMU.Figure 6
Langmuir adsorption isotherm plot for mild steel in 1 N H2SO4 with ACMU.k
ads was calculated from the intercept of the straight line obtained in Langmuir adsorption isotherm. The Energy of activation and free energy of adsorption were calculated using (5):
(5)log⁡p2P1=Ea2.303R[1T1-1T2],ΔGads=-RTln⁡(55.5k).Equilibrium constantkads for various concentration of inhibitor was calculated by using (6). 								(6)kads=θC(1-θ),
where P1 and P2 are the corrosion rate at temperature T1 and T2, respectively, θ is the surface coverage, C is the concentration of inhibitor, and kads is the equilibrium constant.Calculated values of activation energy(Ea) and free energy of absorption (ΔGads) were given in Tables 3 and 4. Greater values of activation energy for the inhibited system over the uninhibited system suggested that the adsorption is physisorption. The negative value of ΔGads indicated the spontaneous adsorption of ACMU inhibitor on the mild steel surface and also there was strong interaction between inhibitor molecule and the mild steel surface [12]. Generally the values of ΔGads up to −20 KJ/mole are consistent with physisorption, while those around −40 KJ/mole or higher are associated with chemisorptions [13]. The calculated ΔGads values were around −20 KJ/mole indicated that the adsorption mechanism of ACMU on mild steel in 1 N sulphuric acid at the studied temperature was physisorption [14].Table 3
Calculated values of activation energy(Ea) and free energy of adsorption (ΔGads) for mild steel in 1 N H2SO4 with ACMU.
Concentration of ACMU (N)
Energy of activation(Ea) KJ/mole
Free energy change (ΔGads) KJ/mole
303–318 K
318–333 K
303–333 K
303 K
318 K
333 K
Blank
64.84
103.08
83.06
28.31
28.90
29.58
0.001
73.70
109.85
90.92
25.21
25.74
26.80
0.005
74.05
105.10
88.84
23.99
24.87
25.64
0.010
69.21
108.78
88.06
24.02
24.43
25.03
0.015
76.79
111.44
93.30
24.23
24.77
24.62
0.020
75.92
124.55
99.09
28.31
28.90
29.58Table 4
Data obtained from Langmuir adsorption isotherm for mild steel in 1 N sulphuric acid with ACMU in the temperature range of 303–333 K.
Temperature K
Free energy change (ΔGads) KJ/mol
Entropy of adsorption (ΔSads) J/mol/K
Enthalpy of adsorption (ΔHads) KJ/mol
303
−26.34
−13.79
−30.51
318
−27.64
−9.04
−30.51
333
−28.94
−4.71
−30.51ThermodynamicallyΔGads was related with entropy of adsorption and enthalpy of adsorption process via (7):
(7)ΔGads=ΔHads-TΔSads.The Langmuir adsorption isotherm however can be expressed by (8) [12]:
(8)log⁡θ1-θ=log⁡⁡A+log⁡⁡Cinh-Qads2.303RT,
where A is the constant, Qads is the heat of adsorption equal to enthalpy of adsorption ΔHads. If log (θ/1-θ) is plotted against 1000/T at various concentrations, the slope of the straight line was −ΔHads/2.303R. The negative value of ΔHads is indicated that the adsorption of ACMU onto the mild steel surface was exothermic. In the exothermic process physisorption was distinguished from chemisorption by considering the absolute value of ΔHads. For physisorption, absolute value ΔHads is lower than 40 KJ/mol, while for chemisorptions, it is around 100 KJ/mol [11]. Calculated values of ΔHads are lower than 40 KJ/mol therefore adsorption of ACMU on mild steel in 1 N sulphuric acid is physisorption.The negative value ofΔSads indicated that inhibitor molecules freely moving in the bulk solution and adsorbed in an orderly fashion onto the mild steel surface [15]. In addition, as the adsorption was an exothermic, it should have been accompanied by a decrease in entropy [16].


### 3.5. Potentiodynamic Polarisation Studies

Polarisation curves of mild steel electrode in 1 N sulphuric acid with and without ACMU at 303 K are shown in Figure7. The anodic Tafel slope and the cathodic Tafel slope were changed with concentration, which indicated that the inhibitor controls both the reactions [17].Figure 7
Potentiodynamic polaraisation curves of ACMU for mild steel in 1 N  H2SO4.It can be seen from the Table5 that the Icorr values were decreased with increasing concentration of inhibitor. A compound could be classified as cathodic or anodic type inhibitor when the change of Ecorr is larger than 85 mV [18]. Since the largest displacement shown by the ACMU was 14 mV, it could be concluded that the molecule is mixed type inhibitor [18]. The maximum inhibition efficiency of the ACMU inhibitor was found to be 89.03% in 1 N sulphuric acid for 0.020 N ACMU. Inhibition efficiency obtained from the polarisation method was in good agreement with weight loss studies.Table 5
Corrosion parameters of mild steel in 1 N H2SO4 with ACMU by potentiodynamic polarisation method.
S. No.
Concentration of ACMU (N)
E
corr (V)
Tafel slopes (mV/dec)
I
corr (μA/cm2)
Inhibition efficiency (%)
ba
bc
1
Blank
−0.459
74.6
64.8
1805.0
—
2
0.001
−0.462
100.2
77.9
277.0
84.65
3
0.005
−0.468
94.9
75.0
257.3
85.75
4
0.010
−0.467
99.9
79.8
220.7
87.77
5
0.015
−0.473
102.7
77.7
210.3
88.37
6
0.020
−0.470
110.6
75.7
198.0
89.03


### 3.6. AC Impedance Studies

Figure8 shows the impedance diagram of mild steel in 1 N sulphuric acid with and without inhibitor. Impedance parameters derived from Nyquist plots were given in Table 6. It was observed that the value of charge transfer resistance (Rct) increased with increase in concentration of inhibitor. On the other hand, the value of double layer capacitance (Cdl) decreased with increase in inhibitor concentration. This is due to the increase in surface coverage of the inhibitor. The inhibition efficiency of the inhibitor is increased with increase in concentration of inhibitor. The maximum inhibition efficiency was found to be 95.03% in 1 N sulphuric acid for 0.02 N ACMU. As impedance diagram for solutions examined have almost a semicircular appearance, it indicates that the corrosion of mild steel is mainly controlled by a charge transfer process [18]. Inhibition efficiency obtained from AC impedance method was good agreement with polarisation and weight loss studies.Table 6
AC-Impedance parameters for corrosion of mild steel in 1 N H2SO4 with ACMU.
Concentration of ACMU (N)
R
ct (Ohm cm2)
C
dl (μF/cm2)
Inhibition efficiency (%)
Blank
7.5
67.94
—
0.001
96
368.60
92.19
0.005
122
348.80
93.85
0.100
130
327.34
94.23
0.015
148
287.52
94.93
0.020
151
281.81
95.03Figure 8
AC Impedance curves of ACMU for mild steel in 1 N  H2SO4.


### 3.7. Scanning Electron Micrograph

Surface analysis was carried out by SEM technique in order to evaluate the surface conditions of the mild steel in contact with 1 N sulphuric acid solution. The micrographs of the mild steel surface in the presence of 1 N sulphuric acid with and without ACMU inhibitor are shown in Figures9(a) and 9(b). In the absence of inhibitor the surface was covered with a high density of pits. In the presence of the inhibitor the micrograph shows no evidence of pitting but shows a smooth surface. This result is due to the adsorption of inhibitor molecules around the pits. This passive film blocks the active sites present on the mild steel surface [19].(a) SEM micrographs of mild steel surface in 1 N  H2SO4. (b) SEM micrographs of mild steel surface in 1 N  H2SO4 with ACMU.
(a)
(b)


## 3.1. Characterisation of ACMU

Newly synthesised aminocyclohexane-N′-methylurea was characterised with data obtained from the FT-IR and NMR spectrum.


## 3.2. FT-IR Spectral Analysis

The FT-IR spectrum of ACMU is shown in Figure1. A sharp peak at 3400.54 cm−1 is assigned to secondary amine stretching. A double sharp peak at 1550–1650 cm−1 is assigned to the amide group. The sharp peak at 1039 cm−1 is attributed to the cyclohexane ring vibrations.Figure 1
FT-IR spectrum of ACMU.


## 3.3. NMR Spectral Analysis

### 3.3.1.1H-NMR (300 MHz): Solvent CDCl3

The H1 NMR spectrum of ACMU is shown in Figure 2. The multiplet at 1.014–1.285 ppm is assigned to six hydrogen atoms labelled as 1 and 2, multiplet at 1.488–1.905 ppm is assigned to four hydrogen atoms labelled as 3, and multiplet at 2.849 ppm is assigned to one hydrogen atom labelled as 4. The multiplet at 1.81 ppm is assigned to one hydrogen atom labelled as 7, singlet at 1.870 ppm is assigned to two hydrogen atoms labelled as 5, and singlet at 6.184 ppm is assigned for the three hydrogen atoms labelled as 8 and 9.Figure 2
H1 NMR spectrum of ACMU.


### 3.3.2.13C-NMR (300 MHz): Solvent CDCl3

The C13 NMR spectrum of ACMU is shown in Figure 3. The singlet at 25.70 ppm is assigned to two carbon atom labelled as 2, singlet at 26.03 ppm is attributed to one carbon atom labelled as 1, singlet at 29.94 ppm is referred to two carbon atoms labelled as 3, singlet 58.46 ppm is assigned to one carbon atom labelled as 5, singlet at 68.29 ppm is assigned to one carbon atom labelled as 4, and singlet at 150.6 ppm is assigned to one carbon atom labelled as 6.Figure 3
C13 NMR spectrum of ACMU.The proposed ACMU structure based upon the FT-IR,1H-NMR, and 13C-NMR spectrum is shown in Figure 4.Figure 4
Structure of ACMU.


## 3.3.1.1H-NMR (300 MHz): Solvent CDCl3

The H1 NMR spectrum of ACMU is shown in Figure 2. The multiplet at 1.014–1.285 ppm is assigned to six hydrogen atoms labelled as 1 and 2, multiplet at 1.488–1.905 ppm is assigned to four hydrogen atoms labelled as 3, and multiplet at 2.849 ppm is assigned to one hydrogen atom labelled as 4. The multiplet at 1.81 ppm is assigned to one hydrogen atom labelled as 7, singlet at 1.870 ppm is assigned to two hydrogen atoms labelled as 5, and singlet at 6.184 ppm is assigned for the three hydrogen atoms labelled as 8 and 9.Figure 2
H1 NMR spectrum of ACMU.


## 3.3.2.13C-NMR (300 MHz): Solvent CDCl3

The C13 NMR spectrum of ACMU is shown in Figure 3. The singlet at 25.70 ppm is assigned to two carbon atom labelled as 2, singlet at 26.03 ppm is attributed to one carbon atom labelled as 1, singlet at 29.94 ppm is referred to two carbon atoms labelled as 3, singlet 58.46 ppm is assigned to one carbon atom labelled as 5, singlet at 68.29 ppm is assigned to one carbon atom labelled as 4, and singlet at 150.6 ppm is assigned to one carbon atom labelled as 6.Figure 3
C13 NMR spectrum of ACMU.The proposed ACMU structure based upon the FT-IR,1H-NMR, and 13C-NMR spectrum is shown in Figure 4.Figure 4
Structure of ACMU.


## 3.4. Weight Loss Studies

Each weight loss studies was repeated three times and also its standard deviation was calculated. The standard deviation was various between 0.5 and 2.1. Table1 shows the value of inhibition efficiency, surface coverage, and corrosion rate for various concentration of inhibitor in 1 N Sulphuric acid for the period of 1 hr, 3 hrs and 5 hrs. The addition of inhibitor to acid decreases the corrosion rate due to the adsorption of inhibitor on mild steel surface. The inhibition efficiency was found to be increased with increasing time and concentration of inhibitor. The maximum inhibition efficiency was found to be 92.75% in 1 N sulphuric acid for 0.020 N ACMU at 5 hrs.Table 1
Inhibition efficiency of ACMU on mild steel corrosion in 1 N H2SO4 at 303 K for various time intervals.
Concentration of inhibitor (N)
1 hr
3 hrs
5 hrs
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency(%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency(%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency(%)
Blank
20.061
—
—
21.770
—
—
22.736
—
—
0.001
8.470
0.5778
57.78
5.944
0.7270
72.70
4.636
0.7961
79.61
0.005
6.687
0.6667
66.67
5.052
0.7679
76.79
3.789
0.8333
83.33
0.010
5.795
0.7111
71.11
4.235
0.8055
80.55
3.210
0.8588
85.88
0.015
4.235
0.7889
78.89
3.641
0.8328
83.28
2.407
0.8941
89.41
0.020
3.121
0.8444
84.44
2.898
0.8669
86.69
1.649
0.9275
92.75Table2 shows the value of inhibition efficiency, surface coverage, and corrosion rate for various concentration of inhibitor in 1 N sulphuric acid for a period of 1 hr at various temperatures. The inhibition efficiency was found to be increased with increasing concentration of inhibitor and decreased with increasing temperature. This can be explained as follows: the inhibitor adsorbed on to the metal surface, and an increase in temperature resulted in desorption of some adsorbed molecules, leading to a decrease in inhibition efficiency [11]. The maximum inhibition efficiency was found to be 84.44% in 1 N sulphuric acid for 0.020 N ACMU at 300 K.Table 2
Inhibition efficiency of ACMU on mild steel corrosion in 1 N H2SO4 at various temperatures.
Concentration of inhibitor (N)
303 K
318 K
333 K
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency (%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency (%)
Corrosion rate(mmpy)
Surface coverage(θ)
Inhibition efficiency (%)
Blank
20.061
—
—
67.539
—
—
390.968
—
—
0.001
8.470
0.5778
57.78
33.658
0.5017
50.17
218.666
0.4407
44.07
0.005
6.687
0.6667
66.67
26.748
0.6040
60.40
160.266
0.5901
59.01
0.010
5.795
0.7111
71.11
21.176
0.6865
68.65
135.078
0.6545
65.45
0.015
4.235
0.7889
78.89
17.832
0.7360
73.60
119.029
0.6956
69.56
0.020
3.121
0.8444
84.44
12.928
0.8086
80.86
107.884
0.7241
72.41Thermodynamic/kinetic consideration.Adsorption behaviour of ACMU on mild steel surface was very much essential to understand the corrosion inhibition mechanism. Tempkin and Langmuir adsorption isotherms were tested by plotting log Cinh versus surface coverage (θ) and Cinh/θ versus Cinh respectively. Both plots gives straight line confirming inhibition of ACMU on mild steel corrosion in 1 N  sulphuric acid obeys Tempkin and Langmuir adsorption isotherm which is shown in Figures 5 and 6. This result reveals that the adsorption of ACMU on mild steel is mono layered. Langmuir adsorption isotherm is given in (4):
(4)LangmuiradsorptionisothermCinhθ=1kads+Cinh,
where Cinh is the inhibitor concentration and kads is the equilibrium constant and θ is the degree of surface coverage of the inhibitor.Figure 5
Tempkin adsorption isotherm plot for mild steel in 1 N H2SO4 with ACMU.Figure 6
Langmuir adsorption isotherm plot for mild steel in 1 N H2SO4 with ACMU.k
ads was calculated from the intercept of the straight line obtained in Langmuir adsorption isotherm. The Energy of activation and free energy of adsorption were calculated using (5):
(5)log⁡p2P1=Ea2.303R[1T1-1T2],ΔGads=-RTln⁡(55.5k).Equilibrium constantkads for various concentration of inhibitor was calculated by using (6). 								(6)kads=θC(1-θ),
where P1 and P2 are the corrosion rate at temperature T1 and T2, respectively, θ is the surface coverage, C is the concentration of inhibitor, and kads is the equilibrium constant.Calculated values of activation energy(Ea) and free energy of absorption (ΔGads) were given in Tables 3 and 4. Greater values of activation energy for the inhibited system over the uninhibited system suggested that the adsorption is physisorption. The negative value of ΔGads indicated the spontaneous adsorption of ACMU inhibitor on the mild steel surface and also there was strong interaction between inhibitor molecule and the mild steel surface [12]. Generally the values of ΔGads up to −20 KJ/mole are consistent with physisorption, while those around −40 KJ/mole or higher are associated with chemisorptions [13]. The calculated ΔGads values were around −20 KJ/mole indicated that the adsorption mechanism of ACMU on mild steel in 1 N sulphuric acid at the studied temperature was physisorption [14].Table 3
Calculated values of activation energy(Ea) and free energy of adsorption (ΔGads) for mild steel in 1 N H2SO4 with ACMU.
Concentration of ACMU (N)
Energy of activation(Ea) KJ/mole
Free energy change (ΔGads) KJ/mole
303–318 K
318–333 K
303–333 K
303 K
318 K
333 K
Blank
64.84
103.08
83.06
28.31
28.90
29.58
0.001
73.70
109.85
90.92
25.21
25.74
26.80
0.005
74.05
105.10
88.84
23.99
24.87
25.64
0.010
69.21
108.78
88.06
24.02
24.43
25.03
0.015
76.79
111.44
93.30
24.23
24.77
24.62
0.020
75.92
124.55
99.09
28.31
28.90
29.58Table 4
Data obtained from Langmuir adsorption isotherm for mild steel in 1 N sulphuric acid with ACMU in the temperature range of 303–333 K.
Temperature K
Free energy change (ΔGads) KJ/mol
Entropy of adsorption (ΔSads) J/mol/K
Enthalpy of adsorption (ΔHads) KJ/mol
303
−26.34
−13.79
−30.51
318
−27.64
−9.04
−30.51
333
−28.94
−4.71
−30.51ThermodynamicallyΔGads was related with entropy of adsorption and enthalpy of adsorption process via (7):
(7)ΔGads=ΔHads-TΔSads.The Langmuir adsorption isotherm however can be expressed by (8) [12]:
(8)log⁡θ1-θ=log⁡⁡A+log⁡⁡Cinh-Qads2.303RT,
where A is the constant, Qads is the heat of adsorption equal to enthalpy of adsorption ΔHads. If log (θ/1-θ) is plotted against 1000/T at various concentrations, the slope of the straight line was −ΔHads/2.303R. The negative value of ΔHads is indicated that the adsorption of ACMU onto the mild steel surface was exothermic. In the exothermic process physisorption was distinguished from chemisorption by considering the absolute value of ΔHads. For physisorption, absolute value ΔHads is lower than 40 KJ/mol, while for chemisorptions, it is around 100 KJ/mol [11]. Calculated values of ΔHads are lower than 40 KJ/mol therefore adsorption of ACMU on mild steel in 1 N sulphuric acid is physisorption.The negative value ofΔSads indicated that inhibitor molecules freely moving in the bulk solution and adsorbed in an orderly fashion onto the mild steel surface [15]. In addition, as the adsorption was an exothermic, it should have been accompanied by a decrease in entropy [16].


## 3.5. Potentiodynamic Polarisation Studies

Polarisation curves of mild steel electrode in 1 N sulphuric acid with and without ACMU at 303 K are shown in Figure7. The anodic Tafel slope and the cathodic Tafel slope were changed with concentration, which indicated that the inhibitor controls both the reactions [17].Figure 7
Potentiodynamic polaraisation curves of ACMU for mild steel in 1 N  H2SO4.It can be seen from the Table5 that the Icorr values were decreased with increasing concentration of inhibitor. A compound could be classified as cathodic or anodic type inhibitor when the change of Ecorr is larger than 85 mV [18]. Since the largest displacement shown by the ACMU was 14 mV, it could be concluded that the molecule is mixed type inhibitor [18]. The maximum inhibition efficiency of the ACMU inhibitor was found to be 89.03% in 1 N sulphuric acid for 0.020 N ACMU. Inhibition efficiency obtained from the polarisation method was in good agreement with weight loss studies.Table 5
Corrosion parameters of mild steel in 1 N H2SO4 with ACMU by potentiodynamic polarisation method.
S. No.
Concentration of ACMU (N)
E
corr (V)
Tafel slopes (mV/dec)
I
corr (μA/cm2)
Inhibition efficiency (%)
ba
bc
1
Blank
−0.459
74.6
64.8
1805.0
—
2
0.001
−0.462
100.2
77.9
277.0
84.65
3
0.005
−0.468
94.9
75.0
257.3
85.75
4
0.010
−0.467
99.9
79.8
220.7
87.77
5
0.015
−0.473
102.7
77.7
210.3
88.37
6
0.020
−0.470
110.6
75.7
198.0
89.03


## 3.6. AC Impedance Studies

Figure8 shows the impedance diagram of mild steel in 1 N sulphuric acid with and without inhibitor. Impedance parameters derived from Nyquist plots were given in Table 6. It was observed that the value of charge transfer resistance (Rct) increased with increase in concentration of inhibitor. On the other hand, the value of double layer capacitance (Cdl) decreased with increase in inhibitor concentration. This is due to the increase in surface coverage of the inhibitor. The inhibition efficiency of the inhibitor is increased with increase in concentration of inhibitor. The maximum inhibition efficiency was found to be 95.03% in 1 N sulphuric acid for 0.02 N ACMU. As impedance diagram for solutions examined have almost a semicircular appearance, it indicates that the corrosion of mild steel is mainly controlled by a charge transfer process [18]. Inhibition efficiency obtained from AC impedance method was good agreement with polarisation and weight loss studies.Table 6
AC-Impedance parameters for corrosion of mild steel in 1 N H2SO4 with ACMU.
Concentration of ACMU (N)
R
ct (Ohm cm2)
C
dl (μF/cm2)
Inhibition efficiency (%)
Blank
7.5
67.94
—
0.001
96
368.60
92.19
0.005
122
348.80
93.85
0.100
130
327.34
94.23
0.015
148
287.52
94.93
0.020
151
281.81
95.03Figure 8
AC Impedance curves of ACMU for mild steel in 1 N  H2SO4.


## 3.7. Scanning Electron Micrograph

Surface analysis was carried out by SEM technique in order to evaluate the surface conditions of the mild steel in contact with 1 N sulphuric acid solution. The micrographs of the mild steel surface in the presence of 1 N sulphuric acid with and without ACMU inhibitor are shown in Figures9(a) and 9(b). In the absence of inhibitor the surface was covered with a high density of pits. In the presence of the inhibitor the micrograph shows no evidence of pitting but shows a smooth surface. This result is due to the adsorption of inhibitor molecules around the pits. This passive film blocks the active sites present on the mild steel surface [19].(a) SEM micrographs of mild steel surface in 1 N  H2SO4. (b) SEM micrographs of mild steel surface in 1 N  H2SO4 with ACMU.
(a)
(b)


## 4. Conclusions

The following conclusions were made from the study.(i)
Inhibition efficiency of the newly synthesised ACMU was increased with increasing concentration of inhibitor and  time and decreased with temperature.(ii)
The maximum inhibition efficiency observed in weight loss method was 92.75% in 1 N sulphuric acid for 0.02 N ACMU at 303 K.(iii)
The negative value ofΔGads indicated that the adsorption is spontaneous.(iv)
The negative value ofΔHads indicated that the adsorption is exothermic.(v)
Free energy of adsorption and Enthalpy of adsorption values shows the adsorption process is physisorption.(vi)
ACMU follows Tempkin and Langmuir adsorption isotherm.(vii)
ACMU behaves as mixed type inhibitor.(viii)
The maximum inhibition efficiency obtained in polarisation method was 89.03% in 1 N sulphuric acid for 0.02 N ACMU at 303 K.(ix)
AC impedance experiments indicated that the corrosion of mild steel in 1 N sulphuric acid with ACMU was mainly controlled by charge transfer process.(x)
The maximum inhibition efficiency obtained in AC impedance method was 95.03% in 1 N sulphuric acid for 0.02 N ACMU at 303 K.(xi)
Inhibition efficiency obtained in AC impedance and potentiodynamic polarisation methods were in good agreement with conventional weight loss method.



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*Source: 370821-2012-09-13.xml*