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HUE UNIVERSITY UNIVERSITY OF EDUCATION ---oOo--- DINH QUY HUONG A RESEARCH ON ANTIOXIDANTS, METAL CORROSION INHIBITORS BY QUANTUM CHEMICAL CALCULATIONS AND EXPERIMENTAL METHODS Major

HUE UNIVERSITY UNIVERSITY OF EDUCATION

-oOo -

DINH QUY HUONG

A RESEARCH ON ANTIOXIDANTS, METAL CORROSION INHIBITORS BY QUANTUM CHEMICAL CALCULATIONS

AND EXPERIMENTAL METHODS

Major: INORGANIC CHEMISTRY

Code: 9440113

SUMMARY OF CHEMISTRY THESIS

HUE, 2020

The work was completed at Department of Chemistry, University of Education, Hue University

Supervisors:

1 Assoc Prof Dr Tran Duong

2 Assoc Prof Dr Pham Cam Nam

INTRODUCTION

Materials oxidation and metal corrosion are problems that cause serious harm, affect to the national economy In the corrosion field, the study of measures to prevent metal corrosion is an urgent task, requiring scientists to focus on research There are many methods to prevent metal corrosion, in which the use of metal corrosion inhibitors is a low-cost way with a high efficiency Compounds containing sulfur or nitrogen are known to be highly effective corrosion inhibitors for steel in acid solutions In addition, the presence of benzene ring is also an important factor, it increases the electrostatic interaction between inhibitors and metal surfaces, enhances the ability to inhibit metal corrosion for a long time Therefore, compounds containing heteroatoms and benzene rings will be one of the options to be considered when studying corrosion inhibitors

In terms of antioxidants, the use of antioxidants in the fields

of food, medicine and industry are also issues to be investigated Antioxidants are usually compounds with low dissociation energy of N-H, O-H and S-H bonds Antioxidant capacity of a given compound can be evaluated by three mechanisms involving hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SETPT) and sequential proton loss electron transfer (SPLET)… Experimentally, 2.2-diphenyl-1-picrylhydrazyl and 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonate) methods are widely used to quantitatively determine the antioxidant activity of the compound These methods are not only rapid, simple and inexpensive but also provide first-hand information on the overall antioxidant capacity of the test system

The search for compounds that have the ability to inhibit metal corrosion and the antioxidant ability, plays an important role in life However, up to now, no studies have mentioned this issue Moreover, a combination of experimental methods and theoretical calculations is necessary in scientific research This is also one of the research orientations that attracts the attention of all scientists all over the world

From the above scientific analysis, "A research on antioxidants, metal corrosion inhibitors by quantum chemical calculations and experimental methods" is selected as the research topic in this thesis

New contributions of the thesis

The thesis obtained some new findings as follows:

- 1-phenyl-2-thiourea has a better ability to inhibit mild steel corrosion than 1,3-diisopropyl-2-thiourea in 1.0 M HCl solution with efficiencies of 92.00 % at 20 oC, 94.05 % at 30 °C, 96.95 % at 45 °C and 98.96 % at 60 °C at the concentration of 5.103 M

- The mild steel corrosion inhibition ability of 1-phenyl-2-thiourea is better than urotropine in both 1.0 M HCl and 3.5 % wt NaCl solution in the same concentration and temperature conditions

- The mild steel corrosion inhibition ability of 1-phenyl-2-thiourea in acidic environment is better than in salt environment

- 1-phenyl-2-thiourea has a higher ability to capture DPPH• and ABTS•+ than 1,3-diisopropyl-2-thiourea in ethanol These results are also consistent with quantum chemical calculations

- phenyl-2-selenourea exhibits better antioxidant capacity than phenyl-2-thiourea In selenourea derivatives, compounds containing electron donating groups give better antioxidant capacity than compounds containing electron accepting groups

1 11 (41 methoxyphenyl)1 21 selenourea is selected as a potential steel corrosion inhibitor and a good antioxidant in research compounds

CHAPTER 1 OVERVIEW

The thesis has conducted an overview of theoretical issues as follows:

1.1 OVERVIEW OF METAL CORROSION

1.1.1 Concept of metal corrosion

1.1.2 Classification of metal corrosion process

1.1.3 Harm of metal corrosion

1.1.4 Steel corrosion

1.1.5 Methods to prevent metal corrosion

1.1.6 Corrosion inhibitors

1.1.7 Activity mechanism of metal corrosion inhibitors

1.1.8 Requirements for metal corrosion inhibitors

1.1.9 Range of using corrosion inhibitors

1.1.10 Research situation on the metal corrosion inhibition ability 1.2 OVERVIEW OF ANTIOXIDANTS

1.2.1 Introduction about antioxidants

1.2.2 Anti-oxidation mechanism

1.2.3 Research situation of antioxidants

1.3 OVERVIEW OF RESEARCH METHODS

1.3.1 Experimental methods of metal corrosion inhibition research 1.3.2 Experimental methods of antioxidant research

1.3.3 Quantum chemical calculation methods

1.3.4 Quantum chemical calculation softwares

CHAPTER 2 RESEARCH CONTENTS AND METHODS 2.1 RESEARCH CONTENTS

The thesis focuses on five main contents as follows:

- Compare the steel corrosion inhibition of 1-phenyl-2-thiourea with 1,3-diisopropyl-2-thiourea

- Compare the mild steel corrosion inhibition ability of thiourea with urotropine

1-phenyl-2 Research on the antioxidant capacity of 11-phenyl-2 phenyl1-phenyl-2 21-phenyl-2 thiourea and 1,3-diisopropyl-2-thiourea

- Research on the antioxidant capacity of 1-phenyl-2-selenourea and its derivatives by quantum chemical calculations

- Propose a potential compound that can inhibit steel corrosion and oxidation capacity by quantum chemical calculations

2.2 EXPERIMENTAL METHODS

2.2.1 Chemical compounds

2.2.2 Potentiodynamic polarization measurements (PDP)

In 1.0 M HCl solution, potentiodynamic polarization curves were measured by scanning the potential from −0.55 V to 0.00 V with a sensitivity of 7 In 3.5 % wt NaCl solution, the potential was scanned from −1.30 V to −0.80 V with a sensitivity of 5 The scanning rate was 1 mV.s−1 in both solutions The working electrode was low carbon steel with a surface area of 0.196 cm2, the rest was surrounded by an epoxy layer, leaving only the work surface in contact with the solution

2.2.3 Electrochemical impedance spectroscopy (EIS)

EIS was performed at open-circuit potential with an alternating current amplitude of 10 mV using a frequency region of

10 mHz to 100 Hz The total number of points was 30

2.2.4 Scanning electron microscope analysis (SEM)

Specimens of mild steel were immersed in 1.0 M HCl and

3.5 % wt NaCl solution with and without PTU (5.10−3 M) for 24 hours at room temperature Surface analysis of the steel was then carried out using a JSM-6010PLUS/LV scanning electron microscope with an energy dispersive Xray analyzer attached

2.2.5 2,2-diphenyl-1-picrylhydrazyl (DPPH • ) assay

DPPH• was diluted in ethanol at a concentration of 6.7.105

M The antioxidant with various concentrations was added into DPPH• with the volume scale of 3:1 The reaction mixture was stirred and kept in the dark for 30 minutes The absorbance of the resulting solutions was measured at 517 nm The DPPH• radical

scavenging activity was determined via the IC50DPPH value

2.2.6 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS •+ ) assay

ABTS•+ radical cation was produced by the reaction of 7 mM 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium solution and 140 mM K2S2O8 The final concentration of K2S2O8 is 2.45 mM After 16 hours, ABTS•+ solution was diluted with ethanol

to an absorbance of 0.70 ± 0.05 at 734 nm After addition of 1.0 mL

of antioxidant to 3 mL of diluted ABTS•+, the solution was kept for 6 minutes at room temperature

1-phenyl-2-thiourea (PTU) and 1,3-diisopropyl-2-thiourea (ITU) were chosen to investigate mild steel corrosion inhibition ability

3.1.1 Investigation of mild steel corrosion inhibition ability of PTU and ITU in 1.0 M HCl

3.1.1.1 Effect of inhibitor concentration on the mild steel corrosion inhibition ability

Figure 3.1 showed that both cathode and anode current densities decreased in the presence of PTU and ITU The steel

corrosion density depended on the concentration of inhibitors At 30

oC, inhibition efficiencies of PTU reached of 90.54 %; 91.11 %; 93.88 % and 94.95 % corresponding to the concentrations of 104, 5.104, 103 and 5.103 M (Table 3.1) However, inhibition efficiency

of ITU only changed much at the concentration of 5.103 M The steel corrosion inhibition efficiencies of ITU reached 75.56 %; 76.67

%; 77.78 % and 83.33 % at concentrations of 104, 5.104, 103 and 5.103 M, respectively

Figure 3.1 Polarization curves of mild steel in 1.0 M HCl with

various concentrations of (a) PTU and (b) ITU in 1 hour at 30 °C

Table 3.1 Polarization parameters of mild steel in 1.0 M HCl at

various concentrations of PTU and ITU at 20, 30, 45, and 60 °C

Inhibitors Tempera ture( o C) C (M)

Ecorr (V)

103 0.39 33.40 22.40 0.03 88.00 (1.22) 5.103 0.40 31.20 19.30 0.02 92.00 (1.30)

(

30

 0.24 36.40 24.80 0.90

104 0.33 48.00 21.70 0.09 90.54 (1.13) 5.104 0.35 33.10 20.60 0.08 91.11 (1.20)

-0.5 -0.4 -0.3 -0.2 -0.1 0.0 -6

-5 -4 -3 -2 -1 0 1 2

E corr (V)

HCl 1,0 M ITU 10 -4 M ITU 5.10 -4 M ITU 10 -3 M ITU 5.10 -3 M

1.0 M HCl

10 -4 M PTU 5.10 -4 M PTU

10 -3 M PTU 5.10 -3 M PTU

1.0 M HCl

10 -4 M ITU 5.10 -4 M ITU

10 -3 M ITU 5.10 -3 M ITU

5.103 0.38 33.00 20.80 0.05 94.95 (1.05)

45

 0.25 35.70 30.00 2.69

104 0.27 36.40 32.10 0.14 94.80 (1.09) 5.104 0.33 40.40 21.20 0.13 95.17 (1.11)

103 0.38 41.90 22.90 0.11 95.91 (1.32) 5.103 0.32 21.60 32.60 0.09 96.65 (1.23)

60

 0.24 37.10 30.40 7.35

104 0.30 30.50 30.40 0.29 96.10 (1.19) 5.104 0.33 37.90 22.40 0.13 98.17 (1.24)

103 0.30 40.10 22.30 0.08 98.95 (1.20) 5.103 0.32 39.60 26.50 0.08 98.96 (1.15)

ITU

20

 0.35 32.50 20.90 0.25

104 0.44 26.30 16.50 0.12 53.20 (1.30) 5.104 0.45 30.10 17.20 0.09 62.40 (1.21)

103 0.47 30.30 16.40 0.07 71.60 (1.01) 5.103 0.49 30.20 16.30 0.05 80.80 (1.20)

30

 0.24 36.40 24.80 0.90

104 0.38 28.60 17.70 0.22 75.56 (1.27) 5.104 0.39 34.10 19.60 0.21 76.67 (1.25)

103 0.39 34.60 17.20 0.20 77.78 (1.31) 5.103 0.42 34.50 17.20 0.15 83.33 (1.02)

45

 0.34 35.70 30.00 2.69

104 0.35 18.40 24.00 0.43 84.01 (1.40) 5.104 0.36 20.60 17.30 0.37 86.25 (1.35)

103 0.39 25.60 20.30 0.32 88.10 (1.22) 5.103 0.39 20.50 17.90 0.26 90.33 (1.30)

60

 0.35 37.10 30.40 7.35

104 0.37 38.40 37.70 0.81 88.98 (1.10) 5.104 0.36 37.40 28.50 0.73 90.07 (1.10)

103 0.39 40.20 41.50 0.60 91.84 (1.23) 5.103 0.37 34.00 32.00 0.54 92.65 (1.21)

Values in parenthesis in the last column of this table are the mean absolute deviation

Nyquist diagrams for steel in 1.0 M HCl with the presence of

PTU and ITU were displayed in Figure 3.2 All the impedance

spectra exhibited one single capacitive semicircle This showed that

the charge transfer process of the corrosion process and double layer

behavior mainly controlled the corrosion of carbon steel

(a) PTU (b) ITU

Figure 3.2 Nyquist plots of the corrosion of mild steel in 1.0 M HCl

with different concentrations of (a) PTU and (b) ITU at 30 °C

In general, in the presence of inhibitors in solution, Rct values

increased, and Cdl values decreased (Table 3.2) These might suggest

that the inhibitors formed a protective layer on the electrode surface

This layer made a barrier for mass and charge transfer Moreover, Rct

values of PTU were higher than that of ITU at the same

concentration, which proved that PTU could inhibit better than ITU

The best inhibition efficiency of PTU and ITU were 93.33 and 82.63

% according to the EIS method

Table 3.2 EIS parameters for the corrosion of mild steel in

1.0 M HCl in the absence and presence of inhibitors at 30 °C

10 -3 M PTU 5.10 -3 M PTU

HCl 1,0 M ITU 10 -4 M ITU 5.10 -4 M ITU 10 -3 M ITU 5.10 -3 M

1.0 M HCl

10 -4 M ITU 5.10 -4 M ITU

10 -3 M ITU 5.10 -3 M ITU

3.1.1.2 Effect of temperature on mild steel corrosion inhibition efficiency

PTU exhibited an effective inhibitor with high inhibition performances of 92.00 % at 20 °C, 94.95 % at 30 °C, 96.65 % at 45

°C, and 98.96 % at 60 °C (Table 3.1) While inhibition performances

of ITU were only 80.80 % at 20 °C, 83.33 % at 30 °C, 90.33 % at 45

°C, and 92.65 % at 60 °C

3.1.1.3 Adsorption isotherms and thermodynamic parameters

(a) PTU (b) ITU

Figure 3.3 Temkin’s adsorption isotherms of (a) PTU and (b) ITU

on the surface of mild steel in 1.0 M HCl

According to Figure 3.3, the correlation coefficients of the

plots between lnC versus θ were considerably different from unit (except for R2 at 20, 45 °C) They proved that adsorption of PTU and ITU on the steel surface did not follow the Temkin isotherm

(a) PTU (b) ITU

Figure 3.4 Langmuir’s adsorption isotherms of (a) PTU and (b) ITU

in 1.0 M HCl

Next, Langmuir adsorption was applied for evaluation and

was shown in Figure 3.4 The straight lines between C and C/θ were

found with the correlation coefficients close to 1, and the slope

1.08 4.17.10 0.999

1.01 2.84.10 0.999

1.03 6.43.10 0.999

y x R





PTU-20 o C PTU-30 o C PTU-45 o C PTU-60 o C

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007

1.21 1.46.10 0.999

1.08 1.07.10 0.999

1.10 2.22.10 0.999

y x R

1.19 5.20.10 0.999

values in the Langmuir equation were approximately equal to 1 These results proved that the adsorption of PTU and ITU on the electrode surface obeyed the Langmuir adsorption isotherm, and each PTU or ITU molecule only accounted for one adsorption position

Table 3.3 Kads, , and values of the adsorption

processes for PTU and ITU in 1.0 M HCl

for PTU and ITU proved that their adsorption processes on mild steel surfaces were spontaneous The positive values of indicated that there was an increase in chaos between reactant molecules on the metal electrode surface

3.1.2 Investigation of inhibition corrosion of PTU in 3.5 % wt NaCl

Observing the results shown in Figure 3.5, the anode branches almost overlaped, while the cathode branches exhibited considerable variation This proved that PTU was an cathode inhibitor in sodium chloride solution The highest inhibition efficiency of PTU was

55.70 % in 3.5 % wt NaCl solution (Table 3.4)

Figure 3.5 Polarization curves of steel in 3.5% wt NaCl with

different concentrations of PTU at 30 oC

Table 3.4 Polarization parameters of mild steel in 3.5 % wt NaCl at

different concentration of PTU

(V)

icorr (iinh ) (mA.cm 2 )

H

(%)

3.5 % wt NaCl + 5.103 M PTU 1.21 0.22 55.70 3.5 % wt NaCl + 103 M PTU 1.18 0.26 49.08 3.5 % wt NaCl + 5.104 M PTU 1.13 0.28 44.84 3.5 % wt NaCl + 104 M PTU 1.11 0.29 43.50

3.1.3 Comparison of metal corrosion inhibition between PTU and urotropine in acid and salt environments

The inhibition efficiencies of urotropine were 65.11 % in acid solution and 46.56 % in salt solution (Table 3.5)

Table 3.5 Polarization parameters of mild steel in 1.0 M HCl and

3.5 % wt NaCl in the presence of 5.103 M Urotropine

(V)

icorr (iinh ) (mA.cm

2 )

H (%)

1.0 M HCl + 5.103 M urotropine 0.25 0.31 65.11 3.5 % wt NaCl + 5.103 Murotropine 1.12 0.27 46.56

-1.3 -1.2 -1.1 -1.0 -0.9 -0.8 -5

-4 -3 -2 -1 0 1

10 -3 M PTU 5.10 -3 M PTU

3.1.4 Effect of molecular structure and environment on the corrosion inhibition ability of thiourea derivatives

3.1.4.1 Effect of inhibitor molecular structure on mild steel

corrosion inhibition efficiency

PTU and ITU were derivatives of thiourea, they were only different regarding the molecular structures of the substituents nearby the nitrogen atom The rest of the inhibitor molecule affected the electron density at the functional group; therefore, it also influenced the adsorption on the metal surface In the PTU molecule,

an important structural factor was a benzene ring, it rose electrostatic interaction and gave the higher coverage between inhibitors and metal surface All these factors helped PTU to give the better inhibition performance than ITU

Compared with urotropine, inhibition efficiency of PTU was higher This could be explained that PTU had a flat structure, so it was easily adsorbed onto the iron surface, while urotropine had a cage structure with four nitrogen atoms located at 4 peaks, therefore, its adsorption was more difficult due to the bulky structure

3.1.4.2 Effect of environment on steel corrosion inhibition

efficiency

At the same concentration, the inhibition efficiency of PTU

in 1.0 M HCl acid solution was higher than in 3.5 % wt NaCl solution (Figure 3.6)

Figure 3.6 Comparison of inhibition efficiency of PTU in 1.0 M

HCl and 3.5 % wt NaCl solutions

0.000 0.001 0.002 0.003 0.004 0.005 0

20 40 60 80