Summary of Chemical doctoral thesis: Synthesis of hydrotalcite bearing benzothiazolylthiosuccinic acid (BTS) modified by silane and applied in solventborne epoxy coating for corrosion protection of carbon steel

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Synthesis and structural analysis of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by silane; study on corrosion inhibiting ability for steel of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by silane.

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Nội dung Text: Summary of Chemical doctoral thesis: Synthesis of hydrotalcite bearing benzothiazolylthiosuccinic acid (BTS) modified by silane and applied in solventborne epoxy coating for corrosion protection of carbon steel

VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY NGUYEN TUAN ANH Project name: SYNTHESIS OF HYDROTALCITES BEARING CORROSION INHIBITORS AND FABRICATION OF NANOCPMPOSITE COATINGS FOR CORROSION PROTECTION OF CARBON STEEL Major: Organic chemistry Code: 9.44.01.14 SUMMARY OF CHEMICAL DOCTORAL THESIS Hanoi – 2018
The thesis was completed at: Graduate University of Science and Technology - Vietnam Academy of Science and Technology. Scientific Supervisors: 1. Assoc. Prof. Dr. To Thi Xuan Hang, Institute for Tropical Technology - Vietnam Academy of Science and Technology. 2. Assoc. Prof. Dr. Trinh Anh Truc, Institute for Tropical Technology - Vietnam Academy of Science and Technology.
A. INTRODUCTION 1. The urgency of thesis Corrosion of metal causes great damage to the economy of countries in the world as well as in Vietnam, so the corrosion protection of metals is very necessary. Organic coatings are widely used for corrosion protection of metal structures. Pigment inhibits corrosion in paint film plays an important role in ensuring the anti-corrosion protection of coatings. Chromates are the best inhibitive pigments, but due to their high toxicity and unfriendly to the environment, it is increasingly limited in their use. There have been many research to study the replacement of chromates in organic coatings by non- toxic pigments and additives. One of atractive researchs is the fabrication of inhibitive pigments based on hydrotalcite. The application of hydrotalcites is based on their ability to absorb and exchange anion, and flexibility of anions between the layers. The coatings containing hydrotalcites bearing organic anions such as benzotriazolate and oxalate have also been studied. In addition, hydrotalcites containing decavanadate, vanadate have been studied and applied in the anti-corrosion protection coating for aluminum and magnesium alloys. However, these coatings are not as protective as the coatings containing chromates. The protective properties of organic coatings containing hydrotalcites depend on the dispersion of hydrotalcite in the polymer matrix. To improve the dispersion of hydrotalcite in the polymer matrix, silane compounds are used to modify the hydrotalcite surface. In addition, the presence of silane improves the adhesion between film containing hydrotalcite bearing corrosion inhibitor and metal surfaces. Therefore, the title of thesis is “Synthesis of hydrotalcites bearing corrosion inhibitors and fabrication of nanocomposite coatings for corrosion protection of carbon steel”. This work contributes to the development of metal anti-corrosion protection coatings 2. The main contents and objectives of the thesis - Synthesis of hydrotalcite bearing benzothiazolylthiosuccinic acid (BTS) modified by silane and applied in solventborne epoxy coating for corrosion protection of carbon steel: + Synthesis and structural analysis of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by silane. + Study on corrosion inhibiting ability for steel of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by silane. 1
+ Influence of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by silane on corrosion protection performance of solventborne epoxy coating. - Synthesis of hydrotalcite bearing molydate modified by silane and applied in waterborne epoxy coating for corrosion protection of carbon steel: + Synthesis and structural analysis of hydrotalcite bearing molydate modified by silane. + Study on corrosion inhibiting ability for steel of hydrotalcite bearing molydate modified by silane. + Influence of hydrotalcite bearing molydate modified by silane on corrosion protection performance of waterborne epoxy coating. 3. Scientific significance, practice and new contributions of the thesis - Successful synthesis of hydrotalcites containing corrosion inhibitors (benzothiazolylthiosuccinic acid and molybdate) and application of modified hydrotalcites in organic coatings for corrosion protection of carbon steel. Hydrotacite containing benzothiazolylthiosuccinic acid with surface modified by silane has corrosion inhibition efficiency of 96% at 3 g/L concentration. Hydrotakcite containing molydate with surface modified by silane has has corrosion inhibition efficiency of 95% at 3 g/L The result is also a premise to open up a research direction is application of hydrotalcite bearing corrosion inhibitor with silane modified surface in corrosion protection of carbon steel. - Preparation of the epoxy coating containing hydrotalcite intercalated with corrosion inhibitors for corrosion protection of carbon steel. The modification by silane has improved the dispersion of hydrotalictes in epoxy, thus enhancing the inhibition effect of hydrotalcite in epoxy coatings 4. Structure of the thesis The thesis includes 127 pages. Introduction: 2 pages; Chapter 1. Background Overview: 36 pages; Chapter 2. Experiment: 16 pages; Chapter 3. Results and discussions: 59 pages; Conclusion: 2 pages; New contributions of the thesis: 1 page; List of author’s reports published: 1 page; 25 tables, 73 figures and 87 references. B. CONTENT OF THE THESIS Chapter 1: OVERVIEW The thesis gives the bibliography of organic coatings, corrosion inhibitors, organic modified hydrotalcite and application of hydrotalcite in organic coatings. Chapter 2: EXPERIMENTAL AND RESEARCH METHODS 2.1. Chemicals, materials and instruments 2.1.1. Chemicals and materials 2
a) Chemicals: Al(NO3)3.9H2O, Zn(NO3)2.6H2O, Na2MoO4..2H2O (sodium molybdate inhibitor), C11H9O4S2N (benzothiazolylthiosuccinic acid inhibitor, C8H22O3N2Si (N-(2-aminoethyl)-3-aminopropyltrimethoxisilan) , C9H20O5Si (3-glycidoxipropyltrimethoxi silan), NaCl, C2H5OH, C8H10 (xylen), NaOH, YD-011X75 epoxy (Kudo), EPON 828 epoxy (Hexion), Polyamin 307D-60 hardener (Kudo), EPIKURE 8537-WY-60 hardener (Hexion) . b) Materials - Hydrotalcite, hydrotalcite bearing corrosion inhibitor and hydrotalcite bearing corrosion inhibitor modified by silane powder - Carbon steel composition: Fe = 98%; C = 0.14 – 0.22%; Si = 0.05 – 0.17%; Mn = 0.4 – 0.65%; Ni ≤ 0.3%; S ≤ 0.05%; P ≤ 0.04%; Cr ≤ 0.3%; Cu ≤ 0.3%; and As ≤ 0.08%. The working surface area is 1 cm2 soaked in 0.1 M NaCl solution, 0.1 M NaCl solution containing modified hydrotalcite. - The carbon steel sheets with a size of 10 × 15 × 0.2 cm are coated a solventborne epoxy coating containing modified hydrotalcite and a waterborne epoxy coating containing modified hydrotalcite. 2.1.2. Instruments Glass cups of 200 mL, 500 mL, and 1000 mL; Globe bottle with flat bottom and 3 neck of 250 mL and 500 mL; Hopper drip; convection tube; glass chopstick; Stove with magnetic stirrer; Vacuum cabinet; pH meter; SiC papers, from P400 to P1200 grit (Japan); spin-coater machine. 2.2. Synthesis of hydrotalcite, hydrotalcite bearing corrosion inhibitor and hydrotalcite bearing corrosion inhibitor modified by silane. 2.2.1. Synthesis of hydrotalcite Hydrotalcite is synthesized in globe bottle with flat bottom and 3 neck (500 mL) as follows: 90 mL solution containing 0.03 M Zn(NO3)2, and 0.015 M Al(NO3)3 is added drop into 145 mL solution of 0.0313 M NaOH during 1 hour. The reaction was conducted in N2 gas, stirred and refluxed at 65 °C. pH solution is adjusted at 8-10 by using the concentrated 1 M NaOH solution. After 24 hours of reaction, the precipitate obtained is filtered and washed several times with distilled water (water removed CO2). The precipitate was dried 24 hours at 50 0C under vacuum and obtained 7 g hydrotalcite. The experiment was repeated three times. 2.2.2. Synthesis of hydrotalcite bearing benzothiazolylthiosuccinic acid Hydrotalcite bearing benzothiazolylthiosuccinic acid (HTBA) is synthesized in globe bottle with flat bottom and 3 neck (500 mL) as follows: 90 mL solution containing 0.03 M Zn(NO3)2, and 0.015 M Al(NO3)3 is added drop into 145 mL solution containing 0.06 M benzothiazolylthiosuccinic acid and 0.0313 M NaOH during 1 hour. The reaction was conducted in N2 gas, stirred and refluxed at 65 °C. pH solution is adjusted at 8-10 by using the concentrated 1 M NaOH solution. 3
After 24 hours of reaction, the precipitate obtained is filtered and washed several times with ethanol/ distilled water. The precipitate was dried 24 hours at 50 0C under vacuum and obtained 7.5 g hydrotalcite bearing benzothiazolylthiosuccinic acid. The experiment was repeated three times. 2.2.3. Synthesis of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by N - (2-aminoethyl) -3-aminopropyltrimethoxisilane Hydrotalcite bearing benzothiazolylthiosuccinic acid modified by N - (2-aminoethyl) -3-aminopropyltrimethoxisilane (HTBAS) is synthesized in globe bottle with flat bottom and 3 neck (250 mL) as follows: Hydrotalcite bearing benzothiazolylthiosuccinic acid (HTBA) is dispersed in ethanol. The ethanol solution containing HTBA is added drop into 20 mL solution containing N - (2-aminoethyl) -3-aminopropyltrimethoxisilane during 30 min (Silane content is 3% compared to HTBA). The reaction mixture is stirred at 60 °C for 6 hours, then filtered and washed with ethanol. The precipitate was dried 24 hours at 50 0C under vacuum and obtained HTBAS with content of 3% silane compared to HTBA. The experiment was repeated three times. 2.2.4. Synthesis of hydrotalcite bearing molydate Hydrotalcite bearing molydate is synthesized in globe bottle with flat bottom and 3 neck (500 mL) as follows: 90 mL solution containing 0.03 M Zn(NO3)2, and 0.015 M Al(NO3)3 is added drop into 145 mL solution containing 0.0313 M molydate and 0.0313 M NaOH during 1 hour. The reaction was conducted in N2 gas, stirred and refluxed at 65 °C. pH solution is adjusted at 8-10 by using the concentrated 1 M NaOH solution. After 24 hours of reaction, the precipitate obtained is filtered and washed several times with distilled water (water removed CO2). The precipitate was dried 24 hours at 50 0C under vacuum and obtained 6.5 g hydrotalcite bearing molydate. The experiment was repeated three times. 2.2.5. Synthesis of hydrotalcite bearing molydate modified by N - (2- aminoethyl) -3-aminopropyltrimethoxisilane Hydrotalcite bearing molydate modified by N - (2-aminoethyl) -3- aminopropyltrimethoxisilane (HTMS) is synthesized in globe bottle with flat bottom and 3 neck (250 mL) as follows: Hydrotalcite bearing molydate (HTM) is dispersed in ethanol. The ethanol solution containing HTM is added drop into 20 mL solution containing N - (2-aminoethyl) -3- aminopropyltrimethoxisilane during 30 min (Silane content is 3% compared to HTM). The reaction mixture is stirred at 60 °C for 6 hours, then filtered and washed with ethanol. The precipitate was dried 24 hours at 50 0C under vacuum and obtained HTMS with content of 3% silane compared to HTM. The experiment was repeated three times. 2.2.6. Synthesis of hydrotalcite bearing molydate modified by 3- glycidoxipropyltrimethoxisilane 4
Hydrotalcite bearing molydate modified by 3- glycidoxipropyltrimethoxisilane (HTMGS) is synthesized in globe bottle with flat bottom and 3 neck (250 mL) as follows: Hydrotalcite bearing molydate (HTM) is dispersed in ethanol. The ethanol solution containing HTM is added drop into 20 mL solution containing 3- glycidoxipropyltrimethoxisilane during 30 min (Silane content is 3% compared to HTM). The reaction mixture is stirred at 60 °C for 6 hours, then filtered and washed with ethanol. The precipitate was dried 24 hours at 50 0C under vacuum and obtained HTMGS with content of 3% silane compared to HTM. The experiment was repeated three times. 2.3. Preparation of epoxy coating containing modified hydrotalcite 2.3.1. Preparation of steel samples The carbon steel with size 10×15×0.2 cm was cleaned of surface rust, washed with distilled water, ethanol and then dried. 2.3.2. Preparation of solventborne epoxy coating containing modified hydrotalcite The epoxy coatings containing HTBA 3% (EP-HTBA), HTBAS 3% (EP-HTBA), HTM 3% (EW-HTM), HTMS 3% (EW-HTMS), and HTMGS 3% (EW-HTMGS) are prepared by a spin-coater machine. After drying, the thickness of the coating is 30 μm. 2.4. The analytical methods IR and UV-vis spectra were measured at Institute for Tropical Technology. XRD diagrams and FESEM images were realized at Institute of Material Science. AAS analysis were realized at Institute of Chemistry. 2.5. Electrochemical methods Polarization curves and electrochemical impedance spectra were carried out on AUTOLAB equipment at Institute for Tropical Technology. 2.6. Mechanical properties Adhesion (ASTM D4541-2010) and impact resistance (ISO D- 58675) of coatings were measured at the Institute for Tropical Technology 2.7. Salt spray test The samples were tested in salt spray chamber according to ASTM B- 117 standard at Institute for Tropical Technology. Chapter 3. RESULTS AND DISCUSSTION 3.1. Synthesis of hydrotalcite bearing benzothiazolylthiosuccinic acid (BTS) modified by silane and applied in solventborne epoxy coating for anti-corrosion protection of carbon steel 3.1.1. Synthesis and structural analysis of hydrotalcite bearing benzothiazolylthiosuccinic acid modified by N - (2-aminoethyl) -3- aminopropyltrimethoxisilane Table 3.1: The physical state of the samples 5
No. Samples The physical state 1 HT Precipitation with fine powder, white 2 HTBA Precipitation with fine powder, light yellow 3 HTBAS Precipitation with fine powder, light yellow 3.1.1.1. Structural analysis by IR spectra * IR spectra of BTSA, HT, HTBA The IR spectra and the characteristic bands of BTSA, HT, HTBA are shown in Figure 3.1 and Table 3.2. Fig. 3.1: IR spectra of BTSA (a), HT (b) and HTBA (c) Table 3.2: IR spectra analysis of BTSA, HT, HTBA Wavenumber (cm-1) Shape Intensity Vibration BTSA HT HTBA 420 - 670 423 - 630 Narrow Weak δZn-O, δAl-O, δAl-O-Zn. 995 990 Narrow Weak δC-H (Aromatic) 1367 1363 Narrow Strong NO2 (-O-NO2) 1634 1595 Narrow Strong δOH (H2O) 1721 Narrow Strong C=O (-COOH) 1423 Narrow Strong C=C (Aromatic) 1520 Narrow Weak C=O (-COO-) 3421 3434 3445 Broad Strong O-H IR results showed that BTSA was inserted into the structure of hydrotalcite. In the structure of HTBA, BTSA is in the carboxylate form. + IR spectra of N - (2-aminoethyl) -3-aminopropyltrimethoxisilane (APS), HTBA and HTBAS The IR spectra and the characteristic bands of of APS, HTBA, and HTBAS samples are shown in Figure 3.2 and Table 3.3. 6
Fig. 3.2: IR spectra of APS (a), HTBA (b) and HTBAS (c) Table 3.3: IR spectra analysis of APS, HTBA, HTBAS Wavenumber (cm-1) Shape Intensity Vibration APS HTBA HTBAS Narrow Weak δZn-O, δAl-O, 420 - 670 423 - 630 δAl-O-Zn 990 990 Narrow Weak δCH (Aromatic) 1363 1363 Narrow Strong NO2 (-O-NO2) 1520 1520 Narrow Weak C=O (-COO-) 1595 1595 Narrow Strong δOH (H2O) 1640 1650 Narrow Medium δNH(-NH2) 2940, 2840 Narrow Medium CH2, CH3 3410 3445 3440 Broad Strong O-H, N-H Results of the spectrum analysis of APS, HTBA and HTBAS showed that APS was inserted into the structure of HTBAS. 3.1.1.2. Structural analysis by XRD pattern Fig. 3.3: XRD pattern of HT (a), HTBA (b) and HTBAS (c) XRD analysis (Fig. 3.3) showed that the distance between layers of HTBA or HTBAS are higher than that of HT, which suggests that the BTSA is inserted into hydrotalcite and increases the layer distance of hydrotalcite. 7
3.1.1.3. Mophology analysis by SEM Fig. 3.4: SEM images of HTBA Fig. 3.5: SEM images of HTBAS SEM images show that HTBA (Fig. 3.4) and HTBAS (Fig. 3.5) have plates shape with size about 50-200 nm. HTBAs are relatively clustered, while HTBASs are separated and have smaller particle sizes. The size reduction and separation may be explained by the silane reaction with the OH- group on the HT surface which reduces the bonding of HT particles. 3.1.1.4. Content of benzothiazolylthiosuccinic acid in HTBA and HTBAS HTBA HTBAS Fig. 3.6: UV-VIS spectra of 100 Fig. 3.7: UV-VIS spectra of 100 times diluted solution of HTBA after times diluted solution of HTBAS reaction with HNO3 after reaction with HNO3 Table 3.4: Absorption intensity of solutions No. Samples Absorption intensity 1 HTBA 0.141 2 HTBAS 0.151 Table 3.5: BTSA concentration and content of solutions Concentration No. Samples Sample mass Content BTSA (%) BTSA (M) 1 HTBA 0.00151 0.0309 34.6 2 HTBAS 0.00147 0.0309 33.69 Analysis results show that the content of BTSA in HTBA and HTBAS are not much different. Thus, surface modification by silane does not affect the content of BTSA present in HTBAS. 3.1.1.5. Analysis of silanization reaction of hydrotalcite bearing benzothiazolylthiosuccinic acid corrosion inhibitor On the surface of hydrotalcite, the major component is hydroxyl groups (-OH). According to the mechanism of silanization reaction, the 8
silanization of hydrotalcite bearing BTSA corrosion inhibitor by N-(2- aminoethyl)-3-aminopropyltrimethoxisilane is performed as follows: The first reaction is the hydrolysis of three methoxyl groups which produce silanol containing components (Si-OH); The second reaction is the condensation of silanol which produces the oligomer. These oligomers form hydrogen bonds with the -OH groups on the surface of hydrotalcite bearing BTSA corrosion inhibitor; finally, it is the drying process. A covalent bond is formed and comed with dehydration. The mechanism of surface modification of hydrotalcite by APS is shown in Figure 3.8. Hydrolysis Condensation Hydrotalcite surface Hydrogen bond Hydrotalcite surface Link formation Hydrotalcite surface Fig. 3.8: The stages occurring during the surface modification of hydrotalcite by N-(2-aminoethyl)-3- aminopropyltrimethoxisilane The silanization reaction of hydrotalcite bearing BTSA corrosion inhibitor by N-(2-aminoethyl)-3-aminopropyltrimethoxisilane is shown in Figure 3.9. 9
The hydrotalcite surface is silanized with APS Hydroxide layer Corrosion inhibitor Hydroxide layer Fig. 3.9: Schematic diagram of silanization reaction of hydrotalcite bearing BTSA by N-(2-aminoethyl)-3-aminopropyltrimethoxisilane 3.1.2. Study on corrosion inhibitor ability for steel of HTBA and HTBAS Fig 3.10: The polarization curves of steel after 2h immersion in ethanol/water solution containing 0.1 M NaCl without corrosion inhibitor (), with 3 g/L HTBA (■) and with 3 g/L HTBAS (●) The results of the polarization curves (Fig. 3.10) showed that HTBAs and HTBAS were anodic inhibitors. 10
Table 3.6: RP value and inhibition efficiencies of hydrotalcite samples Solution Rp (cm2) inhibition efficiency (%) 0.1 M NaCl solution without corrosion 200 inhibitor 0.1 M NaCl solution 5890 96.6 % with 3 g/L HTBA 0.1 M NaCl solution 5700 96.5 % with 3 g/L HTBAS Fig. 3.11: The Nyquist plot of steel after 2h immersion in ethanol/water solution containing 0.1 M NaCl without corrosion inhibitor (a), with 3 g/L HTBA (b) and with 3 g/L HTBAS (c) The results in Table 3.6 show that the inhibition efficiencies of HTBA and HTBAS are very close, which are very high and reache over 96%. 3.1.3. The effects of HTBA and HTBAS on anticorrosion protection of solvent-borne epoxy coatings Table 3.7: Composition of solvent-borne epoxy coatings Modified hydrotalcite content in No. Sample solvent-borne epoxy coatings (%) 1 EP 0 2 EP-HTBA 3 3 EP-HTBAS 3 3.1.3.1. Structure of epoxy coatings containing HTBA và HTBAS + IR spectra The IR spectra and the characteristic bands of solventborne epoxy coatings containing HTBA and HTBAS are shown in Figure 3.12 and Table 3.8. 11
Fig. 3.12: IR spectra of EP0 (a), EP-HTBA (b)and EP-HTBAS (c) Table 3.8: IR spectra analysis of EP0, EP-HTBA, EP-HTBAS Wavenumber (cm-1) EP- EP- Shape Intensity Vibration EP0 HTBA HTBAS Narrow Weak δZn-O, δAl-O, 420 423 δAl-O-Zn. 1040, 1035, 1035, 1250 Narrow Weak C-O-C 1250 1250 (epoxy) 2850 - 2850, Narrow CH3, 2850, 2930 Medium 2920 2930 CH2 1640 1600 1600 Narrow Weak δNH 3420 3400 3410 Broad Strong N-HO-H The IR spectra analysis of solventborne epoxy coatings containing HTBA and HTBAS showed that the epoxy coatings with the presence of hydrotalcite still exhibited characteristic peaks of epoxy. The structrure of epoxy coatings does not change in the presence of hydrotalcite. This demonstrates that the coatings retains the properties of the epoxy coating. + SEM images Fig. 3.13: SEM images of epoxy Fig. 3.14: SEM images of epoxy coatings containing 3 % HTBA coatings containing 3 % HTBAS The SEM images (Figure 3.13, Figure 3.14) showed that both HTBA and HTBAS were very small in size (100-500 nm), dispersed very well in epoxy coatings. However, HTBAS was dispersed better than HTBA in epoxy. The impovement of hydrotalcite dispersion increases protection performance of the epoxy coatings. The results of SEM analysis have explained the efficiency of increasing the dispersion in epoxy network of HTBAS with surface modification by silane. 12
+ Determination of basic distance of HTBA and HTBAS in epoxy coating by XRD diffraction Fig. 3.15: XRD pattenr of HTBA (a), epoxy coatings with HTBA (b), HTBAS (c) và màng epoxy chứa HTBAS (d) The analysis results of XRD pattent (Figure 3.15) showed good dispersion of hydrotalcite in the epoxy coatings. The surface modification by silane increased the dispersibility of hydrotalcite. These results were in agreement with the results of the above SEM analysis. 3.1.3.2. Evaluation of anticorrosion protection of epoxy containing containing HTBA and HTBAS by electrochemical impedance The impedance plost of coatings after after 1h, 14 days and 28 days immersion in 3% NaCl solution are presented in Fig. 3.16, 3.17 and 3.18. is shown in Figure 3.17 Fig. 3.16: The Nyquist plots of EP0 (a), EP-HTBA (b) and EP- HTBAS (c) after 1h immersion in 3% NaCl solution Fig. 3.17: The Nyquist plots of EP0 (a), EP-HTBA (b) and EP- HTBAS (c) after 14 days immersion in 3% NaCl solution 13
Fig. 3.18: The Nyquist plots of EP0 (a), EP-HTBA (b) and EP- HTBAS (c) after 28 days immersion in 3% NaCl solution - After 1h immersion in 3% NaCl solution, the impedance of the EP- HTBAS and EP-HTBA samples was higher than that of EP0. - After 14 days immersion in 3% NaCl solution, with epoxy coatings containing hydrotalcites, the electrolyte does not penetrate the coatings to the metal surface. - After 14 days immersion in 3% NaCl solution, with epoxy coatings containing hydrotalcites, the electrolyte is not completely penetrated the coatings to the metal surface. Therefore, metal corrosion has not occurred. Fig. 3.19: The Rf variation of EP0 Fig. 3.20: The Z10mHz variation of (♦), EP-HTBA(■) and EP-HTBAS EP0 (♦), EP-HTBA(■) and EP- (●) vs. immersion time in 3% NaCl HTBAS (●) vs. immersion time in 3% solution NaCl solution The analysis results of the Rf value and the impedance modulus value at 10 mHz frequency of the sample according to immersion time (Fig. 3.19, Fig. 3.20) show that the presence of HTBA and HTBAS increased the barrier properties and corrosion resistance of the epoxy coatings and the modification by silane enhances the effect of HTBA. 14
3.1.3.3. Mechanical properties of epoxy coatings with HTBA and HTBAS Table 3.9: Adhesion and impact resistance of epoxy coatings with HTBA and HTBAS Samples Adhesion (N/mm2) Impact resistance (kg.cm) EP0 1.5 180 EP-HTBA 2.0 180 EP-HTBAS 2.2 180 The results showed that HTBA with silnae modified surface increased adhesion of epoxy coatings 3.1.3.4. Salt spray test (a) (b) (c) Fig. 3.21: Photographs of steel coated EP0 (a), EP- HTBA (b) and EP-HTBAS after 96 h salt spray test with 3% NaCl solution The results of the salt spray test (Fig. 3.21) of the samples showed that modification with silane increased the protection of the epoxy coatings. This result is in agreement with the results of impedance measurement and adhesion. 3.1.3.5. Corrosion protection mechanism of solventborne epoxy coatings with HTBA and HTBAS Hydrotalcite bearing BTSA with surface modified by silane (APS), thus enhanced the dispersion of hydrotalcite bearing corrosion inhibitor in epoxy coating. This may be explained that the silane activates at the interface between the inorganic compound (hydrotalcite) and the organic compound (solventborne epoxy coating) to bond or pair up these two incompatible materials (the result of HTBAS dispersion in epoxy coatings was demonstrated by SEM images, X-ray diffraction and IR spectra in section 3.1.3.1.). The bond between the modified hydrotalcite and the solventborne epoxy coatings is simulated in Figure 3.22 Fig. 3.22: Simulate the interconnection between modified hydrotalcite and epoxy coatings 15
On the other hand, the presence of HTBAS will significantly increase the adhesion of the epoxy coatings ( the result in section 3.1.3.3). This can be explained that the modified hydrotalcite has the ability to adsorb onto the surface of the metal, thus increasing the bonding capacity between the epoxy and the metal surface. Thus, the presence of HTBA and HTBAS in the solventborne epoxy coatings increases barrier properties and corrosion resistance of the coatings. Especially when the paint film is defected, under the effect of aggressive environment (Cl- anion), hydrotalcite bearing BTSA corrosion inhibitor in the paint film will occur ion exchange reactions. As a result, the BTSA will be released from hydrotalcite to protect the carbon steel and Cl- ions will be retained in the hydrotalcite structure. The corrosion protection mechanism of the epoxy coatings containing hydrotalcite bearing BTSA corrosion inhibitor is shown in Figure 3.23. Fig. 3.23: The corrosion protection mechanism of the epoxy coatings containing hydrotalcite bearing BTSA corrosion inhibitor Summary of Section 3.1 The hydrotalcite intercalated with benzothiazolylthiosuccinic acid corrosion inhibitor (HTBA) was successfully synthesized and its surface was modified by N-(2-aminoethyl)-3-aminopropyltrimethoxisilane (HTBAS). The HTBA and HTBAS have a particle size of 50-200 nm. Electrochemical measurements show that HTBA and HTBAS are anodic corrosion inhibitors, and inhibition efficiency achieve 96% at a concentration of 3 g/L in ethanol/water (2/8) solution contain 0.1 M NaCl. Their presence has significantly increased the protective performance of the solventborne epoxy coatings. The surface modification with silane has the effect of increasing dispersibility, thus enhancing the reinforcing effect of hydrotalcite bearing BTSA inhibitor in epoxy coatings. 3.2. Synthesis of hydrotalcite bearing molypdate modified by silane and applied in watertborne epoxy coatings for corrosion protection of carbon steel 3.2.1. Synthesis and structural analysis of hydrotalcite bearing molypdate modified by N - (2-aminoethyl) -3-aminopropyltrimethoxisilane and 3 - glycidoxipropyltrimethoxisilane 16
Table 3.10: The physical state of the samples No. Samples The physical state 1 HTM Precipitation with fine powder, white 2 HTMS Precipitation with fine powder, white 3 HTMGS Precipitation with fine powder, white 3.2.1.1. Structural analysis by IR spectra * IR spectra of natrimolypdate, HT, and HTM The IR spectra of Natrimolypdate, HT and HTM samples are shown in Figure 3.24 and Table 3.11. Fig. 3.24: IR spectra of natri molypdate (a), HT (b), and HTM(c) Table 3.11: IR spectra analysis of natrimolypdate, HT, HTM Wavenumber (cm-1) Shape Intensity Vibration Natrimolypdate HT HTM δZn-O, δAl- 420 - 670 423 - 630 Narrow Weak O, δAl-O-Zn Mo-O- 840 835 Narrow Medium Mo (MoO42-) Narrow Strong NO2 (-O- 1367 1367 NO2) 1640 1634 1635 Narrow Strong δOH (H2O) 3445 3441 3425 Broad Strong O-H Results of the spectrum analysis of molypdate, HT, and HTM showed that MoO42- was inserted into the structure of HTM. This result is consistent with previous publications. * IR spectra of APS, HTM, GS, HTMGS Fig. 3.25: IR spectra of APS (a), HTMS (b), GS (c) and HTMGS (d) 17
Table 3.12: IR spectra analysis of APS, HTMS, GS, HTMGS Wavenumber (cm-1) Shape Intensity Vibration APS HTMS GS HTMGS 428, 618 437, 620 Narrow Weak δZn-O, δAl-O 835 830 Narrow Medium Mo-O-Mo (MoO42-) 1083 1090 1094 Narrow Medium Si-O-Si 1385 1365 Narrow Strong NO2(-O-NO2) 1635 1640 Narrow Medium δNH(-NH2) 2940, 2944, 2942, Medium Narrow CH2, CH3 2840 2843 2844 3370 3428 3513 3428 Broad Strong O-H,N-H Results of the spectrum analysis of GS and HTMGS showed that GS has appeared on the surface of HTMGS. 3.2.1.2. Structural analysis by XRD pattern Fig. 3.26: XRD pattern of HTM (a), HTMGS (b) and HTMS (c) Results of the XRD pattern analysis (Fig. 3.26) showed that MoO42- was inserted into the structure of HTM and increased the layer distance of hydrotalcite. APS and GS mainly adhered to the hydrotalcite surface without inserting between the hydroxide layers. 3.2.1.3. Mophology analysis by SEM Fig. 3.27: SEM images of HTM (a), HTMGS (b) and HTMS (c) SEM images (Fig. 3.27) showed that HTM had plates shape with size about 50-200 nm, thickness of about 2-4 nm. HTMS and HTMGS had plates shape and similer. Compared to HTM, the HTMS and HTMGS had a smaller thickness and more separation. 18