Research on fabrication and characteristic properties of zirconium oxide film combination with silane on steel substrate as pretreatment for organic coating

would allow a simpler process, at the same time, propose a mechanism for zirconia and silane to be together formed film on the substrate. Factors directly related to films forming by the method of immersion in hexaflorozirconic acid solution could be mentioned as: temperature, pH, concentration and dipping time. Several studies have shown that when the temperature of the solution was increased, the corrosion performance and properties of the zirconia film were reduced. Solution pH and concentration are two directly correlated parameters. However, a low concentration hexaflorozirconic acid solution (with pH of about 3 to 4) usually leaded to a zirconia film with better pretreatment effect, and when the pH was changed, it would affect the forming film decisively. 3 Based on the above discusses, with the desire to manufacture a steel surface pretreatment film which is effective as phosphate and chromate, the topic of the thesis was chosen: "Research on fabrication and characteristic properties of zirconium oxide film combination with silane on steel substrate as pretreatment for organic coating”. Goals of the thesis - Preparing of zirconia/silane combined films on steel subtrate for organic coatings to replace phosphate and chromate pretreatment; - Proposing the mechanism of film formation process and assessing characteristics of morphology, composition, electro- chemical properties and bonding of the zirconia/silane film. Main contents of the thesis - Research on preparation of zirconium oxide film on steel and selecting initial conditions on solution pH and dipping time as basis parameters for manufacturing zirconia/silane film; - Research on manufacturing zirconia/silane films on steel substrates; explain the process of formation and their characterization of morphology, composition, electrochemistry and bonding; - Study the role of zirconia/silane pretreatment film for powder coating. Scientific and application of the thesis On the scientific side, the thesis has contributed new points in the research of steel surface treatment films for coatings to replace phosphates and chromate. In practice, the results of the thesis are the basis for the development of technology for manufacturing steel surface treatment films for environmentally friendly coatings in Vietnam. 4 Detail goals of the thesis - Preparing of zirconium oxide and zirconium oxide/silane films on steel subtrate by chemical immersion method. - Interpreting the mechanism of forming ZrO2/silane film and describing their characterization of morphology, composition, electrochemistry and bonding. - Identifying some basic factors affecting the film forming process; selecting suitable conditions to form film. - Manufacturing of zirconium oxide/silane film with high corrosion resistance, improved adhesion and long-term protection performance of powder coating compared to zinc phosphate. CHAPTER 1. OVERVIEW 1.1. The traditional method of steel surface treatment. Overview of mechanical treatment methods. Concept, development history, formation mechanism, properties and technological diagrams of chemical treatment methods: phosphate, chromate. 1.2. Zirconia-based treatment method: Mechanism of formation, pretreatment efficiency, characterization and influencing factors; 1.3. Silane-based treatment method: Mechanism of formation, pretreatment efficiency, characterization and influencing factors; 1.4. Zirconia and silane combined treatment method: Presentation of several methods were applied to combine between zirconia and silane; The advantages of these combining methods compared to an individual method; CHAPTER 2. EXPERIMENTAL AND METHODS 2.1. Research scheme 5 2.2. Main materials and chemicals - Carbon steel samples (Quoc Viet Company) were abraded with SiC polishing paper, degreased, rusted and rinsed with distilled water and stored in a dehumidifier (bare samples). - ZrF4 crystal, 99,99% purity, white (Sigma), Silane A-1100: γ- APS (China, 99% purity). 2.3. Preparation of surface treatment solution Preparation of H2ZrF6 solution: ZrF4 was completely dissolved in HF solution and then distilled water was added into H2ZrF6 acid solution obtained Zr4+ = 50 ppm. Preparation of H2ZrF6/silane solution: Silane A-1100 with different concentration added to H2ZrF6 solution to form H2ZrF6/silane solution. Fabrication of zirconia/silane films Fabrication of zirconium oxide films Characterization of morphology, composition, electrochemistry and bonding Preparation of chemicals, samples and H2ZrF6 solution Adhesion and corrosion resistance under paint, long-term protection performance of coating Discussion, conclusion Determining the appropriate conditions of pH and dipping time according to corrosion resistance and adhesion Mechanism of film formation process The influence of dipping time and silane concentration on characteristic properties Preparation of chemicals, samples and solution of H2ZrF6/silane 6 2.4. Methods for substrate treatment of samples 2.4.1. Surface treatment in H2ZrF6 solution To form zirconia film, the bare samples were immersed in H2ZrF6 solution in a combination of pH varying from 1 to 6, the division was 1 and the time varying from 1 to 6 minutes, the division was 0,5. 2.4.2. Surface treatment in H2ZrF6 solution combined with silane To form zirconia/silane film, the bare samples were immersed in H2ZrF6/silane solution in a combination of silane concentration varying from 0 %  0,05 % (v/v), the division was 0,0125 %, the time varying from 1 to 6 minutes, the division was 0,5. 2.4.3. Surface treatment with a two steps by immersion The bare samples were treated in H2ZrF6 solution to form zirconia film then in silane solution to form silane film (two steps). After surface treatments, the samples were dried with a dry air stream (70 ± 3 °C) for about 15 minutes in the laboratory. 2.5. Methods, equipments and technique EIS and DC were conducted using PGSTAT204N with 3- electrode cell in 3,5% NaCl. Frequency: 100kHz10mHz; ± 100 mV. Scan rate of 1mV/s, step of 1 mV. The OCP were performed during film formation for 6 minutes. Surface morphology was investigated by FE-SEM on Jeol 7401F (Japan). Components and bonds in the film were studied by FT-IR on Bruker Alpha (Germany) in wave number of 3000500 cm-1, EDS was investigated by Jeol 7401F and XRD patterns in the following mode: 2: 20  80o; speed: 0,05o/giây; Cu (Kα) = 1,5406 Å. To quickly assess the decrease in adhesion and the degree of corrosion under the incision, the samples were immersed in 3,5 % NaCl with different exposure time, according to ASTM D 1654-5. 7 Adhesion was assessed by ASTM D3359 (X-shaped incision) and ASTM D4541 (PosiTest AT-M). The salt spray testing (JIS 8502:1999) was conducted on Q-FOG CCT600 (USA): pH: 6,5 ÷ 7,2; NaCl: 5%; pressure: 1,0Atm, temperature: 35 ÷ 37 oC; saturation temperature: 47 ÷ 49 oC; speed: 2 mL/h. Natural testing was conducted in accordance with ISO 4628: 2016 (Part 8) at the Marine research and testing station, Vietnam - Russia Tropical Center, Hon Tre Islands, Nha Trang city, Khanh Hoa province. CHAPTER 3. RESULTS AND DISCUSSION 3.1. Research on manufacturing zirconia film 3.1.1. Effect of pH of hexaflorozirconic acid solution 3.1.1.1. Effect of pH on corrosion resistance of samples EIS spectrum, polarization curve were at different pHs (Figure 3.1, 3.3) had similar shape but different radians. Figure 3.1, 3.3. The EIS spectrum and PD curve at different pHs. Nova 2.0 software, equivalent diagram, capacitance formula: and Tafel extrapolation were used to identify the typical parameters (Table 3.1, 3.2). The zirconia film formation has increased the corrosion resistance. Rp value was higher (Jcorr was lower) when the pH was between 3 and 5 and reached the largest value at pH = 4. When pH <3, the acidity was high, Fe was dissolved quickly and the Zr film if formed was easy also dissolved. When 8 pH> 5 alkalinity increased, the anode reaction decreased, the cathode reaction slowed, so the pH at the surface-solution interface increased not enough to form Zr oxide. Table 3.1. Electrochemical parameters of the film at different pHs. Parameter Bare pH of H2ZrF6 solution 2 3 4 5 6 Rs (Ω.cm2) 72,63 ± 0,34 72,86 ± 0,31 72,38 ± 0,64 74,63 ± 0,26 73,40 ± 0,49 73,59 ± 0,38 Rp (Ω.cm2) 664,29 ±14,88 947,04 ± 16,65 2177,68 ± 37,23 3198,74 ± 46,38 2438,82 ± 41,46 1372,37± 30,31 C (µF.cm-2) 970 660 310 280 306 536 Y0 (±%) 0,003643 ± 1,903 0,002487 ± 1,546 0,000875 ± 1,650 0,000993 ± 0,952 0,001122 ± 1,249 0,001959 ± 1,700 n (±%) 0,8145 ± 0,881 0,8202 ± 0,663 0,7855 ± 0,596 0,7886 ± 0,356 0,8016 ± 0,486 0,8250 ± 0,708 χ² 0,02798 0,02239 0,02854 0,01311 0,03318 0,03287 Table 3.2. Tafel extrapolation results of samples at different pHs. Parameter Bare pH of H2ZrF6 solution 2 3 4 5 6 E (- mV/SCE) 560,8 633,2 676,4 690,2 683,9 637,6 Jcorr (µA/cm2) 137 38,8 7,7 7,2 8,0 74 3.1.1.2. Effect of pH on the adhesion of powder coating Figure 3.4. Adhesion of powder coating at different pHs. Base sample pH=2 pH=3 pH=4 pH=5 pH=6 9 Flaking degrees (Figure 3.4) showed that the samples were treated at solution pH 3 or 4, achieving the best results, the incisions were almost unchanged (level 5). The rests appeared certain flaking marks, showing lower levels of adhesion testing. 3.1.2. Effect of immersion time in H2ZrF6 solution 3.1.2.1. Effect of immersion time on corrosion resistance of samples EIS spectrum, polarization curve of the samples with different time (figures 3.5, 3.6), datas archieved from EIS, PC (tables 3.3, 3.4). Figure 3.5, 3.6. EIS spectrum and PD curve with different time. Table 3.3. Electrochemical parameters of films with different time. Parameter Bare Immersion time 2 mins 3 mins 4 mins 5 mins 6 mins Rs (Ω.cm2) 72,63 ± 0,34 73,59 ± 0,35 73,87 ± 0,38 74,63 ± 0,26 73,66 ± 0,35 75,74 ± 0,41 Rp (Ω.cm2) 664,29 ± 14,88 1151,83 ± 25,23 2381,55 ± 40,25 3198,74 ± 46,38 1953,97 ± 49,44 1000,94 ± 28,83 C (µF.cm-2) 970 711 679 280 731 1160 Y0 (±%) 0,003643 ± 1,903 0,002549 ± 1,576 0,002139 ± 1,6326 0,000993 ± 0,952 0,002350 ± 1,2641 0,003797 ± 1,744 n (±%) 0,8145 ± 0,881 0,8268 ± 0,699 0,8027 ± 0,754 0,7886 ± 0,356 0,7915 ± 0,667 0,7528 ± 0,861 χ² 0,02798 0,02740 0,02564 0,01311 0,03512 0,02704 10 Table 3.4. Polarizing resistance and capacitance of the films. Parameter Bare The immersion time 2 mins 3 mins 4 mins 5 mins 6 mins E (- mV/SCE) 560,8 630,5 649,3 690,2 650,7 634,7 Jcorr (µA/cm2) 137 16,1 12,5 7,2 16,1 18,1 The formation of zirconia films increased corrosion resistance of the substrate from 2 to 5 times. Jcorr of treated steel samples were greatly reduced from 7,5 to 19 times, compared to the bare sample. As the immersion time increased, the increased Rp value (Jcorr decreased) because of completing of film formation. But , immersion time was too much leading to decrease Rp value (Jcorr increased) because of film was too thick, heterogeneous, cracking due to heat drying, dehydration. 3.1.2.2. Effect of immersion time on the adhesion of powder coating Assessment of flaking (Figure 3.7) showed that samples treated with immersion time from 3 to 5 minutes achieved good results, the incision almost unchanged (level 5), better than the samples treated with time of 6 or 2 minutes. Figure 3.7. Adhesion of powder coating with different immersion time. 3.2. Fabrication and characterization of zirconia/silane film Bare 2 mins 3 mins 4 mins 5 mins 6 mins t 11 3.2.1. Process dynamics and film composition The trend of OCP (Figure 3.8) showed that the steel electrode was gradually moving toward the positive side during film formation. The film was quickly formed within the first 2 minutes, slowed down until 4 minutes and stabilized to 6 minutes. Figure 3.8. Trend of OCP value of bare sample in H2ZrF6/silane. Initially, when the bare sample was immersed in H2ZrF6/silane, Fe was oxidized into the solution by anode reaction (Fe-2e→Fe2+). The Fe2+ ion would combine with ZrF6-2 to release Zr4+ into the solution (Fe2+ + ZrF6-2 → Zr4+ + FeF6-4). H+ ions were reduced by local cathode reaction on the surface, releasing H2 (2H+ + 2e → H2↑). The local pH result on the sample surface increased, resulting in precipitation of hydrated zirconium oxide. The crystal is germinated and then spread to the entire surface to form a zirconia film according to the equation (Zr4++ 3H2O→ ZrO2·H2O +4H+). Siloxane network formation reactions could also occur: In silane solution, ethoxy groups switch to silanol group (– Si(OC2H5)3+3H2O→–Si(OH)3+3C2H5OH). The silanol group was adsorbed by (Fe-OH) through hydrogen bonding to metal-siloxane (Si-O-Fe) bonds according to (–Si(OH)3+Fe-OH→H2N(CH2)3 Si(OH)2-O-Fe). Si-OH groups also formed stable siloxane (Si-O-Si) network by the equation (SiOH+SiOH→Si-O-Si+H2O). Immersion time (mins) 12 The presence of Zr (fig. 3.9b) and Zr, Si (fig. 3.9c) from EDS spectra proved the phase formation of Zr and Si in the film. The presence of O also indicated the presence of oxide or hydroxide of zirconium and the siloxane network. Other peaks may come from the substrate due to the very thin film. Figure 3.9. EDS spectrum of bare sample (a), sample treated after 4 minutes in H2ZrF6 (b) and H2ZrF6/silane (c). Table 3.5. The percentage of atoms in the zirconia/silane film was determined from the EDS spectrum. Percentage Fe O C Zr Si Al Cu By atoms 77,34 12,65 6,99 2,05 0,76 0,12 0,09 By mass 90,09 3,95 1,59 3,85 0,35 0,06 0,11 The ratio of Zr and Si in the film composition may represent the formation of zirconia and silane phases (Figure 3.10). Initially, ZrO2 formation speed was very fast, silane film formation speed was very slow. This result was due to the rapid electrochemical reaction to form ZrO2 at the beginning, which inhibited the covalent reaction to form a) b) c) 13 silane film on the sample surface. By the time, the ZrO2 film gradually completed covering the surface, the electrochemical reaction slowed down and the reaction of form covalent bonds between silane and metal became easier. Between 1 and 4 minutes, both Zr and Si concentrations increased, indicating that the two films were formed in parallel. This meant that silane both competed with Zr in forming film on steel substrate and created bonds around the newly formed ZrO2. Figure 3.10. Variations of Zr and Si ratio in ZrO2/silane film. 3.2.2. Surface morphology of zirconium oxide/silane film. The FE-SEM images of the samples (Figure 3.11) showed that zirconia film was morphologically arranged with spherical or elliptical particle structure, tens of nanoscale and irregularly shaped particles groups randomly distributed on the surface (Figure 3.11b). Figure 3.11c showed that zirconia/silane film had a finer, more tightly sealed characteristic. Figure 3.11. FE-SEM image of the untreated substrate (a), treated in H2ZrF6 (b) and H2ZrF6/silane (c). b) a) c) 14 3.2.3. Bonding in zirconium oxide/silane film In FT-IR spectra (Figure 3.12), peak at in 500-600 cm-1 indicated O- Zr-O bonds. It was reported that Si-O-Zr bonds were usually at 964 cm-1 wavenumber. Affected by the high positive charge of ZrO2, this bond could be elevated at 1050 cm-1. The peak in the range of 1000-1130 cm-1 was created by the Si-O-Si asymmetric bond, the long vibration and the separation of this peak showed the laminated structure and confirmed the bridging role of O between Zr and Si together. Figure 3.12. FT-IR spectra in ZrO2/silane film. The peak around the wavenumber of 1400 and 2900 cm-1 could represent deformation and asymmetric fluctuations of the –CH group (-CH2, -CH3). A peak of about 1600 cm-1 was typical for the valence of the N-H group in silane. The X-ray pattern (Figure 3.13) showed that one main peak at 2θ = 44,38 and the second peak at 2θ = 64,70 on both the bare sample and the ZrO2/silane film, which were from steel. The pic at 2θ = 35,26 in Figure 3.13b was typical for ZrO2 in the film. Only one peak of ZrO2 was obtained because of either the film was too thin, the dominance of the measurement belonged to Fe. From the Scherrer equation, the average particle size of ZrO2 was 81,27 nm. This result was quite similar to the particle parameters achieved by FE-SEM image. Wavenumber (cm-1) T ra n sm it ta n ce ( % ) 15 Figure 3.13. XRD pattetn (a-bare, b- ZrO2/silane film). 3.2.4. Effect of silan concentration on morphology, composition and corrosion resistance of zirconia/silane film 3.2.4.1. Effect of silane concentration on surface morphology Figure 3.14. FE-SEM image at different silane concentration FE-SEM image showed that the presence of silane significantly improved surface smoothness compared to the case without silane. 3.2.4.2. Effect of silane concentration on film component The atom ratio of Si/Zr was shown in Figure 3.15. Si/Zr ratio in the case of 2 steps (0,025%) was approximately equal to case of 1-solution (0,0125%) and lower than 1-solution (0,025%) 0,0125 Fe Fe Fe Fe ZrO2 0,0 0,025 silane 2 steps 0,05 16 proved that combined film formed simultaneously during film formation. This ratio increased as the concentration of silane in the solution increased, reflecting the competition in forming the link between Zr, Si and steel substrate. 3.2.4.3. The effect of silan concentration on corrosion resistance The EIS spectra and electrochemical parameters of the film at different silane concentrations (Figure 3.16 and Table 3.9) showed that silane doped H2ZrF6 solution at different concentrations leading to an increase in both polarization resistance and capacitance of combined film. Table 3.6. Electrochemical parameters of samples at silane concentration. Paramete r Silane concentration (v/v) 2 steps 0 0,0125 0,025 0,05 Rp (Ω.cm2) 3198,74 ± 46,38 5279,68 ± 111,40 9116,37 ± 279,57 7830,15 ± 219,24 7152,84 ± 305,43 C (µF.cm-2) 280 422 327 413 321 3.2.5. Effect of immersion time on morphology, composition and corrosion resistance of zirconia/silan film 3.2.5.1. Effect of immersion time on surface morphology FE-SEM images of the samples with different time (Figure 3.17) showed that, from 2 to 3 minutes, the film was mainly zirconia, the surface was still porous and characterized by ZrO2 film. Figure 3.16. Nyquist spectra at different silane concentration. 17 Figure 3.17. FE-SEM image of samples with different time:: 2 mins (a), 3 mins (b), 4 mins (c), 5 mins (d), 6 mins (e), silane only (f). After 4 minutes the ZrO2/silane film was almost complete, however, the silane could still be formed, resulting surface morphology had silane characterization (Figure 3.17de). 3.2.5.2. Effect of immersion time on film components Figure 3.18 showed that the Si/Zr ratio in the forming combined film increased by immersion time. At 2 minutes, this ratio was very low (about 23/100), increased rapidly over time and reached about 36/100 and 38/100 respectively with 3 and 4 minutes. In the range of 2 to 4 minutes, this rate increased less, confirming the mutual competition during film formation. 3.2.5.2. Effect of immersion time on corrosion resistance The impedance spectrum and electrochemical parameters of the samples (Figure 3.19 and Table 3.7) showed that the arc's magnitude corresponded to the polarizing resistance of the film, which increased with immersion time from 2 to 4 minutes. a Figure 3.18. Ratio of Si/Zr with different immersion time. b c f e d 18 Figure 3.19. EIS spectra with different immersion time. Table 3.7. Electrochemical parameters of samples according to immersion time. Parameter Immersion time 2 mins 3 mins 4 mins 5 mins 6 mins Rp (Ω.cm2) 3198,74 ± 46,38 6851,81 ± 144,57 9116,37 ± 279,57 9080,63 ± 277,43 9078,74 ± 286,89 C (µF.cm-2) 312 374 327 413 486 3.3. The protection performance of fully painted samples 3.3.1. Adhesion measurement 3.3.1.1. Dry adhesion The adhesion of the paint film according to the different options (Table 3.8) proves that all the treated samples have a higher adhesion than steel. The result is that the zirconia membrane is tightly bound to the substrate while the silane outside plays a good role as a double bonding agent. Table 3.8. The result determines the adhesion of powder coating. Parameters Bond strength values (MPa) Bare sample 3,04 ± 0,23 Treated in H2ZrF6 after 4 mins 4,20 ± 0,50 19 Treated in H2ZrF6/silane 0,0125 % after 4 mins 4,85 ± 0,63 Treated in H2ZrF6/silane 0,025 % after 4 mins 6,04 ± 0,59 Treated in H2ZrF6/silane 0,05 % after 4 mins 5,68 ± 0,51 Treated in Two steps of treatment 5,83 ± 0,47 Treated in H2ZrF6/silane 0,025 % after 2 mins 3,66 ± 0,34 Treated in H2ZrF6/silane 0,025 % after 3 mins 4,72 ± 0,48 Treated in H2ZrF6/silane 0,025 % after 5 mins 5,92 ± 0,53 Treated in H2ZrF6/silane 0,025 % after 6 mins 5,85 ± 0,49 Silane APS 0,025 % after 4 mins 4,35 ± 0,23 Zn-Phosphate 5,87 ± 0,57 Adhesion of samples, were treated in H2ZrF6/silane solution, improved compared to individual methods. 3.3.1.2. Wet adhesion The degree of wet adhesion reduction indicated that the coating was pervaded by the electrolyte solution (ions, water ...). Samples which had higher dry adhesion would maintain better wet adhesion (Figure 3.20). Figure 3.20. Impaired wet adhesion of different samples. 3.3.2. Corrosion protection performance 3.3.2.1. EIS results The EIS spectrum of the samples were shown in Figures 3.24 and 3.25. The best results are obtained with zirconia/silane and phosphate samples, which are higher than that of 2 steps and individual 20 methods. After 60 days, the treated samples were well protected. Magnitude of EIS of ZrO2/silane was at least declined. The bare sample was corrosive Figure 3.21, 3.22. EIS of samples after 1 and 60 days in NaCl 3,5 %. Significant attenuation of |Z|10mHz (Figure 3.23) were the highest with untreated samples and the least with ZrO2/silane and zinc phosphate samples. 3.3.2.2. Test results in 3,5% NaCl solution Figure 3.24 and table 3.9 showed that rust creep from scribe of the ZrO2/silane was 1,7 mm (rate number 7), lower than the rest and much lower than the untreated sample at 3,2 mm (5). Figure 3.24. Sample images after 1 month of immersion in NaCl 3,5%. Bare ZrO2 ZrO2/silane 2 steps Silane 0,025% Zn-phosphate Figure 3.23. |Z|10mHz trend after 90 days of in NaCl 3,5 %. 21 Table 3.9. Rust creep from scribe after 1 month of immersion Parameter Rust creep from scribe Rate number The coating without treatment 3,2 mm 5 The coating with ZrO2 treatment 2,3 mm 6 The coating with ZrO2/silane treatment 1,7 mm 7 The coating with 2 steps treatment 2,1 mm 6 The coating with silane treatment 2,2 mm 6 The coating with phosphate treatment 2,5 mm 6 3.3.2.3. Satl spray method. After 400 hours of salt spray test (Figure 3.25, 3.26), the ZrO2/silane sample was as effective as Zn-phosphate (level 8). The rests were at level 7 and the worst sample with out treatment (level 6). Figure 3.25. Images of coating samples after 400h of salt spray test. Figure 3.26. Rust creep from scribe after different exposure. 3.3.2.4. Natural test results By the time, changes in humidity, temperature, rain, sunshine, chlorine sedimentation, etc. will destroy the paint. The group of Bare ZrO2 ZrO2/silane 2 steps Silane 0,025% Zn-phosphate 22 samples treated with zirconia/silane and phosphate showed similar results and less corrosion. Before testing 12 months 24 months Mẫu nền ZrO2 ZrO2/ silane 2 steps Silane 0,025 % Zn- phosphate Figure 3.27. Images of coating samples of natural test. Figure 3.28. Rust creep from scribe after different exposure. 23 CHAPTER 4. CONCLUSION ZrO2/silane combined pretreatment film for coating has been fabricated chemically (dipping in solution). The mechanism of film forming was based on electrochemical reaction and covalent bond between solution and steel surface. Initially, the zirconium oxide film was fo