Synthesis of hydrotalcites bearing corrosion inhibitors and fabrication of nanocpmposite coatings 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 420 - 670 423 - 630 Narrow Weak δZn-O, δAl-O, 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 Fig. 3.6: UV-VIS spectra of 100 times diluted solution of HTBA after reaction with HNO3 Fig. 3.7: UV-VIS spectra of 100 times diluted solution of HTBAS 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 No. Samples Concentration BTSA (M) Sample mass Content BTSA (%) 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 HTBA HTBAS 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. 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. Hydrolysis Condensation Hydrogen bond Hydrotalcite surface Hydrotalcite surface Hydrotalcite surface Link formation 9 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. Hydroxide layer Corrosion inhibitor The hydrotalcite surface is silanized with APS Hydroxide layer 10 Table 3.6: RP value and inhibition efficiencies of hydrotalcite samples Solution Rp (cm2) inhibition efficiency (%) 0.1 M NaCl solution without corrosion inhibitor 200 0.1 M NaCl solution with 3 g/L HTBA 5890 96.6 % 0.1 M NaCl solution with 3 g/L HTBAS 5700 96.5 % 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 No. Sample Modified hydrotalcite content in solvent-borne epoxy coatings (%) 1 EP 0 2 EP-HTBA 3 3 EP-HTBAS 3 3.1.3.1. Structure of epoxy coatings co