Summary of Chemical Doctoral thesis: Synthesis and characterization of Silica/Polypyrrole Nanocomposite oriented for use in organic corrosion protection coating

The thesis aims to investigate and select suitable conditions for synthesis of Nanocompozit Silica / Polypyrol by In-situ method. Nature and study of corrosion inhibition ability of Silica / Polypyrol carbon steel. Select suitable conditions for synthesis of silica / Polypyrol anionic doped for In-situ method. Corrosion inhibitory ability for Nanocompozit Silica / Polypyrol-Anion Carbon steel for polybutyral coating. Corrosion protection for carbon steel of epoxy coating containing SiO2 / polypyrol-anion composite.

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY

GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY

ORGANIC CORROSION PROTECTION COATING

Major: Theoretical chemistry and Physical chemistry

Code: 9440119

SUMMARY OF CHEMICAL DOCTORAL THESIS

Hanoi – 2018

The thesis was completed at: Graduate University of Science and Technology - Vietnam Academy of Science and Technology

1 Assoc Prof Dr., To Thi Xuan Hang

2 Assoc Prof Dr Dinh Thi Mai Thanh

The thesis can be found at:

- Library of Graduate University of Science and Technology

- National library of Vietnam

A INTRODUCTION

1 The necessity of the research

Nanocomposite material has a wide range of applications in various areas including metal corrosion protection There are many methods of corrosion protection, but the simple, low cost and easy to apply method is organic protection coating Chromat is a highly effective corrosion inhibitor pigment in organic coatings, however it is highly toxic, which causes cancer, so countries around the world have gradually eliminated chromates and research into environmental friendly - corrosion inhibitors Corrosion inhibiting and metal protection properties of conductive polymers were first investigated by Mengoli in 1981 and DeBery in 1985, respectively Studies have shown that polymer films formed on metal surfaces have high adhesion and good protection, however, this method has limitations on the size of the material to be protected Therefore, recent studies have focused on the use of conductive polymers as corrosion inhibitors in organic coatings

This coating shows the advantages of conducting polymer overcomes the difficulties of film forming These studies focus on two of the most popular and important conductive polymers: polypyrrole (PPy) and polyaniline for corrosion protection of iron / steel Compared to polyaniline, PPy shows high electrical conductivity in both acidic and neutral environments, so it can be widely used in various fields such as energy storage devices, bio-sensors, materials photoelectric, anti-corrosion coating In addition, the synthesis of PPy films on metal substrates is easier due to the low oxidation potential of PPy Moreover, PPy is able to stabilize better than polyaniline However, PPy has low dispersibility, so the combination with nano additives to form nanocomposite is very interested in research Silica nanoparticles (SiO2) have high surface area, good dispersion, ease of preparation so the use nanosilica can improve the expansion; sound insulation; flexural strength; tensile strength; and corrosion protection performance The PPy's conductivity as well as the ability

of the ion-selective redox reaction greatly depends on the nature of the polymer and the synthesis conditions In addition, when corrosion occurs, PPy is capable of exchanging anions, so that the counter ions in the polymer also play an important role in the anticorrosion ability Counter anions, which is small in size and highly flexible, will easily be released from the polymer network While larger size anions can reduce bond length, leading to the increase of conductivity and solubility

Therefore, synthesis of silica/polypyrol nanocomposite and silica/polypyrol-counter anions

is a promising topic, using the advantages of PPy, silica as well as anionic component There are some studies subjecting the use of of PPy, PPy-anion, PPy/inorganic oxide However, there is no study about silica/polypyrrole nanocomposite as well as silica/polypyrrole exchanged counter

anions and its application in organic coatings for anticorrosion Therefore, the thesis "Synthesis

and characterization of silica/polypyrrole nanocomposite oriented for use in organic corrosion protection coatings" is needed, contributing to the synthesis and application of silica/polypyrrole

nanocomposite in the field of corrosion protection

2 The main contents and objectives of the thesis

- Investigation of the synthesis parameters of silica/polypyrrole and silica/polypyrrole-doped anions nanocomposite by in-situ method

- Characterization and corrosion inhibitor abilities of silica/polypyrrole nanocomposites for carbon steel

- Evaluation of corrosion protection for carbon steel of polyvinylbutyral and epoxy coatings

3 The scientific significance, practicality and new contributions of the thesis

- Silica/polypyrrole nanocomposites were synthesized by in-situ method in the presence of doped anions, such as: dodecyl sulfate, benzoate and oxalate The synthesize nanocomposites have spherical structure, diameter in the range of 50-150 nm Nanocomposite contains the oxalate anion showed the best inhibitor ability in polivinylbutyral coatings

- The potential application of silica/polypyrrole-doped oxalate nanocomposite in epoxy coatings has been evaluated for corrosion protection The results were obtained by electrochemical methods showed that silica/polypyrrole-doped oxalate nanocomposite significantly improved corrosion resistance of epoxy coating The results open up the prospect of using silica/polypyrrole-doped oxalate nanocomposite as a corrosion inhibitor in organic coatings

4 Structure of the thesis

The thesis includes 127 pages: introduction (3 pages), the overview (35 pages), experimental (13 pages), results and discussions (60 pages), conclusion (1 page) , new contributions of the thesis (1 page), list of published scientific works (1 page), 9 tables, 63 images and graphs, 141 references

B CONTENT OF THE THESIS CHAPTER I OVERVIEW

The thesis has summarized literature over the world about synthesis of silica, polypyrrole, silica/polypyrrole composites and its application, special in anticorrosion

CHAPTER II EXPERIMENTAL

2.1 Materials

- Pyrrole, C4H5N, (97 %, Germany); TEOS, Si(OC2H5)4, (South Korea); PVB, (C8H14O2)n, (Japan)

- HCl, FeCl3, Na2C2O4, CH3(CH2)11OSO3Na, NaC6H5CO2, C3H6O, CH4O (China)

- Epoxy bisphenol A, Epotec YD011-X75 and Polyamide 307D-60 (South Korea)

2.2 Synthesis of silica/polypyrrole nanocomposites

2.2.1 Silica

TEOS was dropped slowly into 140 ml HCl solution with pH = 1 The mixture was stirred for 24 hours at room temperature, and then was heated at 80oC during 24 hours The precipitate was washed with distilled water to pH = 7 and dried at 80oC for 24 hours in a vacuum oven

2.2.2 Silica/polypyrrole nanocomposites

Prepared three solutions:

- Solution 1: SiO2 were dispersed in 40 ml H2O or C2H5OH by ultra-sonic in 30 minutes

- Solution 2: 1 mmol pyrrole were dispersed in 20 ml H2O

- Solution 3: 0.05 mol FeCl3.6H2O were dissolved in 40 ml H2O or C2H5OH

Solution 2 was dropped slowly into solution 1, stirred for 1 hour Then solution 3 was dropped slowly into above mixture, stirred for 24 hours The mixture was filtered and washed 5 times with distilled water and once with mixture of methanol and acetone to remove unwanted products The precipitate was dried at 80oC in 24 hours in vacuum oven

To synthesis SiO2/PPy-doped anions, follow the same process, only additional of 2.5 mmol NaC2O4 (Ox) or NaC12H25SO4 (DoS) or C7H5NaO2 (Bz) in solution 2

2.2.3 Synthesis polyvinylbutyral coatings containing SiO 2 /PPy nanocomposites

Step 1: Carbon steel sheets were used as substrate (10×5×0.2 cm) The sheets were cleaned with soap, distilled water and ethanol, dried and marked Then the sheets were polished with abrasive paper 600 grades, washed by distilled water, ethanol and dried

Step 2: PVB solution was prepared by dissolving 10 wt% of PVB in mixture of propanol and ethanol (ratio 1:1) The SiO2/PPy nanocomposites were dispered into PVB solution by continuous magnetic stirring and sonication for 4 hours

Step 3: The liquid paints were deposited on the bare steel using a spin-coater at rotating speeds up to 600 rpm Finally, all samples were dried at ambient temperature for 7 days The dry films thickness is about 11±2 µm (measured by Minitest 600 Erichen)

2.2.4 Synthesis epoxy coatings containing SiO 2 /PPy nanocomposites

The synthesize process is similar, epoxy solutions were prepared by dispersed 5 wt% SiO2/PPy nanocomposites into epoxy and xylene by magnetic stirring The epoxy coatings containing SP, SPO1, SPO2 and SPO3 were labeled as ESP, ESPO1, ESPO2 and ESPO3, respectively The rotating speed is 1000 rpm due to the high viscosity of epoxy solutions The dry films thickness is about 25±2 µm (measured by Minitest 600 Erichen)

- XPS were measured at Future Industry Institute using X-radiations with Al at 15 kV- 15 mA

- The conductivities were measured by cyclic voltammetry method through the two-point-electrode without electrolyte with sample thickness is 1 cm and sample area is 1 cm2

- Open circuit potential and electrochemical impedance spectra were measured at Institute for Tropical technology

- Salt spray test was carried out followed by ASTM B117 standard at Institute for Frontier Materials

CHAPTER III RESULTS AND DISCUSSIONS

3.1 Synthesis and characterization of SiO 2 /PPy nanocomposties

3.1.1 Effect of synthesis solution

Synthesis solution plays an important role in dispersive ability, morphology and characterization of SiO2/PPy nanocomposites There were some studies reported that the presence

of ancol can improve the dispersion and modify surface characteristic of silica Therefore, SiO2/PPy nanocomposites were synthesized in solution containing water, mixture of ethanol: water = 2:3 and mixture of ethanol : water = 4:1, labeled as SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E, respectively

IR spectra (Figure 3.1) of SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW showed similar trend, included characteristic bands of SiO2 (~471, 794 and 1080 cm-1) and PPy (~1530, 1450, 1405 and 1050 cm-1)

EDX results of SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW are shown in figure 3.2 The

from polypyrrole Weight percentages of silicon increase from 20.18 to 21.07 and 22.08% with SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW, respectively

Figure 3.1 FT-IR spectra of SiO2, PPy

and SiO2/PPy nanocomposites

Figure 3.2 EDX diagrams of SiO2, PPy and SiO2/PPy nanocomposites

SEM photographs of synthesized SiO2 and SiO2/PPy nanocomposites are shown in figure 3.3 The synthesized nanocomposites have similar morphology with spherical shape Diameter of nanocomposites is higher than silica It can be explained by the deposition of pyrrole on the silica surface, the polymerization of pyrrole in the presence of oxidation agent

Figure 3.3 SEM photographs of SiO2 (a), SiO2/PPy-W (b), SiO2/PPy-EW (c) and SiO2/PPy-E (d)

Figure 3.5 shows UV-Vis spectra of SiO2, PPy SiO2/PPy-W, SiO2/PPy-E and SiO2/PPy-EW Characteristic peak of silica is observed at 300 nm In the case of PPy, there are two main peak, at 400-450 nm and broad peak at 900-1100 nm The first peak at low wavelength is presented for band gap of π-π* bond In the other hand, this peak also confirms the bipolarons state of PPy Peak at higher wavelength is characterized for conductive electron In comparison between spectra of PPy and nanocompositess, there is the change of peak position to higher wavelength zone This result indicated the longer conjugated bond, corresponding with the higher conductivity

Figure 3.5 UV-Vis spectra of samples Figure 3.6 CV diagram of samples

The electrical conductivities of samples were determined through CV-diagrams from figure 3.6 PPy has the highest conductivity, 0.432 S.cm-1 The conductivities of nanocomposites synthesized in water, ethanol:water = 2:3 and ethanol:water = 4:1 is 0.19, 0.14 and 0.11 S.cm-1, respectively It can be explained by the insulation of silica

Figure 3.7 showed the survey scans of PPy, SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E PPy spectra showed characteristic peak of carbon C1s, nitrogen N1s and clo Cl2p, in agreement with EDX results In comparison with PPy, XPS spectra of nanocomposites have two more peak, at 101.9 eV and 531.5 eV, represented for silicon Si2p and oxygen O1s These results indicated the presence of silica in nanocomposites

With PPy, the high resolution spectra included four components (figure 3.8) At the lowest bonding energy and highest intensity, the main peak at 285.1 eV, represented for C-C bond between Cα and Cβ in pyrrole ring Peak at 286.2 eV; 287.8 eV and 290.4 eV indicated PPy

at doped state Peak presented for C=N and =C-NH•+ (polaron) bond is observed at 286.2 eV The peak at 287.8 eV is assigned to –C=N+ bond of bipolaron PPy

Figure 3.7 XPS spectra of PPy, SiO2/PPy-W, SiO2/PPy-EW and SiO2/PPy-E

With N (1s) high resolution spectrum showed three components (figure 3.9) The signal at 399.6 eV was assigned to the –NH group of pyrrole ring At higher bonding energy, there were two

peak which assigned to pyrrole at doped state Peaks at 400.5 eV and 402.4 eV were assigned to

NH•+ of polaron PPy and =NH+ of bipolaron PPy, respectively

Figure 3.8 High resolution C1s and N1s of

From the analysis of XPS spectra, the weight percentages of each element and oxidation state of nitrogen were listed in table 3.3 The results indicated that when change the synthesis solution, weight percentage of element insignificant changed Percentages of nitrogen at neutralize and polaron state were higher than that of nanocomposites It showed the higher oxidative ability Therfore, the percentages of nitrogen at bipolaron state are higher, the lower conductivities

Figure 3.10 High resolution C1s and N1s of

SiO2/PPy-EW

Figure 3.11 High resolution C1s and N1s of

SiO2/PPy-E

Bảng 3.3 Analysis parameter from XPS spectra

3.1.1 Effect of pyrrole/silica ratio

The quantities of silica showed important affect to the formation of nanocomposites Therefore, in this study, SiO2/PPy nanocomposites were synthesized at constant quantity of PPy and silica changed from 2,5 mmol (SP1); 5 mmol (SP2); 7,5 mmol (SP3) to 10 mmol (SP4) The ratio of pyrrole/silica changed from 0.4, 0.2, 0.13 to 0.1, respectively

Figure 3.12 IR spectra of SiO2, PPy, SP1,

EDX results showed four main elements in nanocomposites: carbon, nitrogen, oxygen and silicon (figure 3.13) When the quantity of silica in synthesized solution increase, the weight percentage of silicon in nanocomposites increase, from 20.48 to 21.19, 25.03 and 28.14%, in SP1,SP2, SP3 and SP4, respectively

SEM photographs of silica, SP1, SP2, SP3 and SP4 were shown in figure 3.14 All the samples had spherical shapes When forming nanocomposites, diameter of sample was increase Moreover, when the quantity of silica increased, the particles sizes also increased It might due to the polymerization of PPy, cover silica shell

Figure 3.14 SEM photographs of SiO2 (a), SP1 (b), SP2 (c), SP3 (d) and SP4 (e)

Figure 3.15 TGA diagrams of PPy, SP1, SP2, SP3 and SP4

Figure 3.15 showed TGA diagrams of samples With SP1, SP2, SP3 and SP4, TGA diagrams had same trend, the weight loss was 48.5, 42.2, 38.1 and 32%, respectively TGA diagrams consisted of two stages: an initial weight loss at less than 100oC due to the loss of water absorption in the surface The second loss from 100-650oC might due to the degradation of polypyrrole blackbone and the decomposition of oxidation agent However, the total weight loss of nanocomposites was lower than that of PPy It can be explained by the high thermal resistance of silica Calculated from TGA results, weight percentage of silica in SP1, SP2, SP3 and SP4 is 51, 57,

61 and 67%

3.1.2 Electrochemical characteristic of SiO2 /PPy nanocomposites

3.1.3.1 Inhibitive ability in NaCl 3% solution

Figure 3.16 showed the open circuit potential of carbon steel immerse in NaCl 3% include and not include 3 g/L SP1, SP2, SP3 and SP4 nanocomposites after 36 hours Initially, for bare steel, OCP value reached -0.6 VSCE, then decreased over time After 20 hours of immersion, OCP value is -0.7 VSCE, and kept stably The decrease of OCP can be explained by the erosion formation After 36 hours of immersion, the OCP value of bare steel was -0.7 VSCE, reaching the corrosion potential of steel

In the case of SP1, SP2, SP3 and SP4, OCP varied with the same trend In the beginning, it reached -0.32, -0.32, -0.37 and -0.40 VSCE, respectively These results showed that SiO2/PPy nanocomposites can shift the OCP of steel to passive region, which is demonstrated the role of anodic inhibitor Over time, the value of OCP dropped toward negative value, however, always positive than that of bare steel Therefore, SiO2/PPy showed good inhibitive ability, but it decreased overtime due to the erosion of corrosive agents After 36 hours of immersion, the OCP of SP1, SP2, SP3 and SP4 were -0.63, -0.64, -0.68 and -0.68 VSCE, respectively

Figure 3.16 OCP variation over time of carbon steel in 3%NaCl solution include and

not include 3g/L SP1 (b), SP2 (c), SP3 (d), SP4 (e) after 36 hours of immersion

3.1.3.2 Corrosion protection for carbon steel of PVB film containing SiO2 /PPy nanocomposites

PVB is an organic coating, which is easy for synthesis, non-toxic and short tested time Therefore, PVB film was used to investigate the inhibitive ability of SiO2/PPy nanocomposites

A, Open circuit potential

Figure 3.19 showed the OCP variation of carbon coated with PVB and PVB containing 10 wt% nanocomposites With carbon steel coated with PVB, in the beginning, OCP value was -0.4 VSCE This result confirmed the good barrier protection of PVB film However, overtime, the OCP decreased, which was explained by the diffusion of chloride ions across the film, attacking the steel surface After 36 hours of immersion, the OCP value dropped to -0.651 VSCE

OCP values of PVB-SP2 and PVB-SP3 had the same fluctuation trend Initially, the OCP were almost equivalent, reached -0.3 VSCE The potential maintained for 2 hours then decreased toward negative values It might due to the weakening of barrier protective ability After 5 hours of immersion, the OCP value fluctuated continuously It can be explained as follows: the addition of nanocomposites showed double effects in corrosion protection First, SiO2/PPy nanocomposites reinforced the barrier protection of the film Then, when corrosion occur due to the attack of corrosive agents, PPy acted as an oxidation agent on steel surface, creating a passive oxide layer, preventing further corrosion (Figure 3.20) After 36 hours, PVB-SP2 and PVB-SP3 reached -0.47 VSCE

Figure 3.17 The variation of OCP values over time of

carbon steel coated with PVB (a), PVB- SP1 (b),

PVB-SP2 (c), PVB-SP3 (d) and PVB-SP4 (e) 3%

NaCl solution

Figure 3.18 Protection mechanism of

SiO2/PPy film

The OCP value of steel coated with PVB-SP1 and PVB-SP4 tended to be more stable, reached -0.2 and 0.33 VSCE, respectively Then, OCP value dropped after 10 hours of immersion due to the diffusion of chloride ions After that, OCP values were stable and maintained up to 30 hours After 36 hours of immersion, OCP values of PVB-SP1 and PVB-SP4 reached -0.39 and -0.5 VSCE, respectively These results indicated that when the silica content increased, the protection mechanism did not change but the protection was significantly reduced This is explained as follows: PPy played an important role in the anodic inhibition, due to the strong oxidation capacity

of PPy, which acts as an oxidant, activate the substrate In addition, silica reinforced barrier protection With the high content of silica, the mechanical protection of film is improved, but the amount of PPy decreased, therefore reducing the inhibitive effects, resulting in the decrease of OCP values From the above results, it can be seen that the presence of nanocomposites in the film can increase the physical strength, preventing the penetration of chloride ions Moreover, nanocomposites can acts as inhibitor, and SP1 showed the highest inhibitive ability

B, Electrochemical impedance spectra

Figure 3.19 Bode diagrams of PVB-SP1, PVB-SP2, PVB-SP3 and PVB-SP4 after

immersion time in 3% NaCl solution: 10 minutes (□), 10 hours (○), 36 hours

(Δ)

Bode impedance spectra of steel coated with PVB and PVB containing 10 wt% nanocomposites were shown in figure 3.19 For PVB coating containing SiO2/PPy nanocomposites, modulus value at the intial time was higher than that of PVB, it might due to the presence of SiO2/PPy can increase the barrier protection Impedance decreased after 10 hours of immersion, however, up to 36 hours, impedence slightly decresed with 4 samples This results were explained by the dual protection of nanocomposites First, silica exhibited shielding capablities The presence of silica in the organic coating prevents the connection between polymer network, therefore causing the low diffusion rate of oxygen, hydrogen and corrosive ions

In the other words, erosive ions needed longer time to diffuse, attack to the interface between polymer coating and metal surface to create the corrosive reactions

3.2 Doped SiO2/PPy nanocomposites

Many studies have shown that PPy doped with anions such as molybdate, oxalate, dodecyl sulfate, benzoate have corrosion protection ability for metals in chloride solution PPy can passivate metals, however, after time, when the PPy is completely reduced, PPy will lose its protection ability At this point, doped anions can be released, which is capable of reintroducing metal With the small doped anions, such as oxalate, it is easy to be released from polymer, forming complexes with metals, filling the corrosive sites With large doped anions, such as benzoate or dodecyl sulfate, the flexibility is poorer but it can limit the penetration of chloride ions into the coating Thus, the effects of different sizes - doped anions on characterization and corrosion protection performance have been investigated

3.2.1 Characterization of SiO2 /PPy-doped nanocomposites

SiO2/PPy nanocomposites is synthesized in aqueous solution, 2.5 mmol SiO2, 1 mmol Py and

Figure 3.21 showed the infrared spectrum of SiO2, PPy SiO2/PPyDoS, SiO2/PPyOx and