High solids organic-inorganic hybrid coatings based on silicone-epoxy-silica coating with improved anticorrosion performance for AA2024 protection

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High solids organic-inorganic hybrid coatings based on silicone-epoxy-silica coating with improved anticorrosion performance for AA2024 protection X. Chen*, S.F. Wen**, T. Feng, X. Yuan Northwestern Polytechnical University, Xi’an 710129, China

ARTICLE INFO

ABSTRACT

Keywords: Hybrid organic-inorganic coating Sol-gel Corrosion protection Impact resistance

A hybrid organic-inorganic coating was prepared with silicone-epoxy coating and prehydrolyzed TEOS (HTEOS) through the sol-gel process. This study aimed to study the effect of HTEOS on the corrosion protection performance of hybrid coatings on AA2024 alloy. The obtained hybrid coatings were characterized by Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and water contact angle tests. The impact resistance of the coatings was estimated by the cupping test. Electrochemical impedance spectroscopy (EIS) and scanning vibration electrochemical technology (SVET) tests were used to evaluate the corrosion resistance of the hybrid coating. With the presence of 4 wt% HTEOS, the hybrid coating showed better corrosion resistance and impact resistance. The superior protection of the hybrid coating with 4 wt% HTEOS could be attributed to the enhanced SieOeSi network and higher proportion of the inorganic phase.

1. Introduction Organic-inorganic hybrid materials have been extensively used to protect metals from corrosion [1–3]. A variety of organic-inorganic coatings based on alkoxysilane [4], silicon derivatives and polysiloxanes have been synthesized via the sol-gel method [5]. The precursor used in the sol-gel method can be regarded as X(CH2)nSi(OR)m, where X represents an organofunctional group and (OR) is a hydrolysable alkoxy group. During the hydrolytic process, Si(OR)m hydrolyzes to silanol groups (SieOH), which can react with metal hydroxides (MeOH). The formation of SieOeM promotes coating bonding to the metal substrate. In addition, SieOH groups condense with themselves to form siloxane bonds (SieOeSi), which facilitates the formation of more cross-links and a denser film on the metal surface. Thus, the hybrid coatings provide promising protection against corrosion by creating a chemical barrier between the metal and an aggressive environment. However, hybrid coatings based only on alkoxysilanes, silicon or polysiloxanes are often short in offering adequate long-term corrosion protection due to their tendency to form cracks and microforms during the condensation process. These cracks and microforms are associated with poor thermal and mechanical properties of the coating. Through these defective areas, the aggressive ions can diffuse into the coating/metal interface; thus, corrosion occurs.

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It is appropriate to consider some polymeric materials to prevent cracks and pores from occurring [6]. Many thermoset polymers can be used for the synthesis of organic-inorganic nanocomposites to hinder crack propagation and further make up the deficiency of these sol-gel silane coatings. Among them, epoxy resin is famous for its excellent adhesion and superior chemical and corrosion resistance. It has been widely used in a variety of fields, such as adhesives, coatings, and sealants. As epoxy resin has inherent brittleness and poor impact strength, tetraethoxysilane (TEOS) is incorporated into the resin as an inorganic phase to achieve epoxy-silica hybrid materials [7–9]. In terms of the epoxy-silica hybrid coating, the inherent weakness of polymeric materials is overcome by introducing inorganic fillers, whereas crack propagation of the sol-gel coating is hindered in the presence of the polymer. The epoxy hybrid coating presents a promising alternative to conventional coating systems because it shows improved corrosion protection when compared to coating materials based on organic, inorganic and organic-inorganic hybrids, which has attracted the attention of many researchers and engineers [10–12]. Torrico et al. [13] synthesized epoxy-siloxane-silica hybrid nanocomposites by varying the molar ratio between the coupling agent (GPTMS) and the inorganic precursor (TEOS). The study showed that a high impedance modulus of up to 1 GΩ cm2 was achieved for a 1.5% TEOS addition,

Corresponding author. Co-corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected] (S.F. Wen).

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https://doi.org/10.1016/j.porgcoat.2019.105374 Received 28 March 2019; Received in revised form 19 September 2019; Accepted 25 September 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: X. Chen, et al., Progress in Organic Coatings, https://doi.org/10.1016/j.porgcoat.2019.105374

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with a 25 °C increase in decomposition temperature. After 1 day of immersion, its impedance decreased to 7 × 108 Ω cm2. Bakhshandeh et al. [14] reported anti-corrosion epoxy-silica hybrid coatings based on the diglycidyl ether of bisphenol A (DGEBA), 3-aminopropyl triethoxysilane (APTES) and hydrolyzed-tetraethoxysilane (TEOS). It demonstrated that the morphology, mechanical and thermal properties of hybrid coatings are highly dependent on the concentration of APTES and TEOS. The corrosion resistance of the hybrid coatings is improved by increasing the inorganic phase content from 7.5 to 17.5 wt%, while the decomposition temperature increases by 5–22 ℃ compared with that of the pure epoxy sample. After 15 and 45 days of immersion in 3.5% NaCl solution, the impedance decreased from 109 Ω cm2 to 107–108 Ω cm2 and 105–106 Ω cm2, respectively. Piscitelli [15] prepared epoxy-based organic/inorganic hybrid materials with metaxylenediamine (MXDA) as the hardener and TEOS as the inorganic phase. The large amount of SiOH originates from TEOS, and r-glycidoxypropyltrimethoxysilane (GOTMS) improves the interaction with the organic components. The actual SiO2 content varies from 3.5 to 21.8%, while the hybrid displays a reduction in Tg with increasing siloxane content. Canosa [16] synthesized an efficient high solids coating based on the diglycidyl ether of 1,4-butanediol aliphatic epoxy resin cured with aminopropyl methyldiethoxysilane and chemically modified with alkoxysilane. The hybrid coating displays an improved corrosion performance and a decreased blistering resistance due to the enhanced reaction of alkoxides and the chemical interaction with the substrate. Bera et al. [17] utilized a water-based epoxy polymer with different organosilane compound mixtures to modify the chemical structure of the crosslinked silane network and achieved crack-free films on galvanized steel, which effectively protected the substrate. They reported that the 3% addition of amino grafted epoxy coating exhibited an improved corrosion resistance of 2.53 × 106 Ω cm2 and that after 360h, there was an order of magnitude decline. Although numerous studies have focused on developing anti-corrosion hybrid coatings, only a few research groups have succeeded in obtaining high-performance hybrid barrier layers with long-term corrosion resistance. In addition, the hybrid coatings suffer from poor impact resistance due to their relatively low proportion (less than 25%) of the inorganic phase. The aim of this research is to design high anticorrosion performance coatings with excellent impact resistance and long-term durability along with a high proportion of the inorganic phase. The silicone-epoxy resin was employed as the polymer matrix, and the prehydrolyzed TEOS, without an additional coupling agent, was the inorganic phase to obtain high performance hybrid organicinorganic coatings via the in situ sol-gel method. According to the previous study [18,19], the silicone-epoxy coating is highly resistant to chemicals and exhibits excellent weathering resistance. The corrosion resistance of the pure silicone-epoxy coating before immersion can be 108 Ω.cm2. Nevertheless, the effect of in situ filler formation on the improvement of corrosion resistance and durability of this novelty organic-inorganic hybrid coating, without further functionalization of TEOS with a coupling agent, has not yet been studied. Compared with reported epoxy-silica hybrid coatings, the silicone-epoxy hybrid coating is easy to produce, and superior properties can be achieved at low filler loading levels. A previous study obtained hybrid coatings via various coupling agents (such as APTES and GPTMS) along with TEOS. However, the silicone-epoxy coating already contains a certain number of hydroxyl groups originating from silicon derivatives that promote the interaction between the organic and inorganic components. Additionally, the silicone-epoxy hybrid coatings are high solids, low VOC coatings and are free from toxic materials, which satisfies the regulation requirements of safety and being health and environmentally friendly. In this paper, different concentrations of prehydrolyzed TEOS were added to the silicone-epoxy resin through the sol-gel process. The hybrid coatings were characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). In addition, the adhesion of the coating to the aluminum surface was analyzed by pull-

off testing, and the thermal stability of the hybrid coatings was evaluated by thermogravimetric analysis (TGA). Eventually, electrochemical impedance spectroscopy (EIS) and scanning vibration electrode technique (SVET) tests were used to evaluate the corrosion resistance properties of the hybrid coatings. 2. Experimental 2.1. Materials Silicone epoxy resin (EF, 99%, molar mass: 450 g/mol) and 3-aminopropyl-triethoxysilane (APTES, equivalent weight: 110 g/mol) were obtained from EVONIK, Germany. Tetraethoxysilane (TEOS, 99%), ethanol (95%), ammonia (25%), and hydrochloric acid (37% in water) were all purchased from Fuchen Chemical Reagent Co., Ltd. (China, Tianjin). All chemicals were used as received. AA2024 sheets purchased from Weida Instruments Co., Ltd. (China, Guangdong) were used as the substrate, having a nominal composition (wt%) of Cu = 4.36%, Mg = 1.49%, Fe = 0.35%, Si = 0.14%, Zn = 0.07%, Ti = 0.01%, Crmax = 0.01%, with the balance consisting of Al. All the coatings were applied to the alloy substrate by the exact same spraying procedure. Before the coating application, the alloys were ultrasonically degreased with acetone. 2.2. Fabrication of coatings 2.2.1. Preparation of prehydrolyzed TEOS EF resin was used as the organic component, while the prehydrolyzed TEOS was used as the inorganic part of the hybrid coating system. First, 1 mol of TEOS was dissolved in 3.84 mol of ethanol, and then 0.5 mol of water was added to the mixture. After stirring for 20 min, 0.001 mol of hydrochloric acid were added. The solution was kept for 24 h at 60 °C with magnetic stirring. The next step was to remove the solvent at 1 mmHg and 50 °C under a vacuum oven. The obtained solution was the prehydrolyzed TEOS, and named after as HTEOS. 2.2.2. Preparation of hybrid coatings The hybrid coatings were prepared by the addition of 2, 4 and 6 wt % HTEOS to the EF under mechanical mixing (600 rpm). Stoichiometric amounts of the curing agent APTES were added to the mixture under mechanical stirring for 15 min. The hybrid coatings were applied on the surface of AA2024 Al alloy by spin-coating at 500 rpm for 30 s. The samples were left for 8 h at room temperature and cured in oven at 60 ℃ for 6 h to ensure complete curing. For the comparison purpose, the varnish coating without the addition of HTEOS was also prepared as the control sample following the above-mentioned steps. The cured neat coating was named as SE coating. The structural representation of the silicone-epoxy hybrid coating is shown in Fig. 1(a), together with the image of the hybrid coating on the AA2024 substrate before the test (see Fig. 1(b)). 2.3. Characterization The thicknesses of the dry coatings were measured with a QNix 4500 digital thickness gauge, and the chemical structure of the coatings was investigated by a Nicolet IS10 Fourier Transformed Infrared Spectrometer. The spectra were obtained through attenuated total reflection (ATR) in the range of 600-4000 cm−1. The morphology of the samples were observed using a VEGA 3 LMH scanning electron microscopes (SEM). The contact angle of water droplets on the surface of the hybrid coating was measured by a DSA 100 in the sessile drop method. A drop of high-purity distilled water (5 μL) was located in three different areas of each sample. The thermal stability of the hybrid coatings was evaluated by thermogravimetric analysis (TGA) using an STA449F TA instrument. The conditions were as follows: nitrogen with 2

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Fig. 1. (a) Schematic representation of silicone-epoxy-silica hybrids, (b) Silicone-epoxy- silica coating coated AA2024.

a continuous flux of 50 ml/min, 5 mg of sample, Al2O3 crucible, temperature range from 25 to 800 ℃, and a heating rate of 10 °C/min. The pull-off adhesion test was conducted utilizing a DeFelsko PosiTest AT in accordance with ASTM-D4541. Before testing, the coated samples and dollies were all cleaned with ethanol. The dollies had a 20-mm diameter and were glued to the coated samples with the two-component methyl methacrylate adhesive. Two days later, the pull-off testing equipment was attached to the loading fixture. The force applied to the loading fixture was gradually increased at a rate of 15 mm/min until the plug of the coating material was detached from the metal. All tests were carried out on five samples, and the average value was reported. The impact resistance test was performed according to ASTM D6905. A steel punch with a hemispherical head and a diameter of 15.9 mm (2 pounds) was dropped down from 50 cm and impacted at the back of the painted samples. The impacted areas were examined for cracking using a magnifier. Five specimens were prepared and tested.

the enhanced cross-linking and interphase compatibility. Compared with the other coatings, some significantly larger aggregations and cracks appear in the hybrid coating having a 6 wt% HTEOS. The morphology reveals the phase separation for high silica concentrations. Consequently, 4% HTEOS in SE coating yields a more homogeneous hybrid structure with enhanced interphase compatibility and crosslinking density. FTIR spectra of the hybrid coatings were compared to analyze the structure of the coating. Fig. 3(a) shows the FTIR spectra of the asreceived TEOS and prehydrolyzed TEOS (HTEOS). From the spectrum of TEOS, the bands at 2976, 2929 and 2890 cm−1 correspond to the symmetric and asymmetric stretching vibrations of the CH group (CH2 and CH3). These bands at approximately 1290–1490 cm−1 also correspond to the angular deformation of the bands CH2 and CH3. The absorption bands at 1169, 1107, 1084, 965 and 792 cm−1 refer to the stretching vibrations of SieO in the SieOeC [21]. After hydrolysis for 24 h, the FTIR spectrum of HTEOS shows a new band at 3357 and 900 cm−1, corresponding to the axial deformation of OeH bond. The appearance of the OeH bond indicates the hydrolysis of SieOeC. The SieOeC bond hydrolyzed to SieOH and then condensed to SieOeSi The bond between 1000 and 1100 cm−1 associate with the formation of the Si-O-Si bonds, which confirms the condensation of the SieOH groups. As noted in some investigations, HTEOS exhibits a cyclic ring and linear SieOeSi structure. Furthermore, the small band at 800 cm−1 is assigned to a rocking vibration of SieO2 [22]. In addition, the absence of bands from 2800 to 3000 cm−1 and a decrease in the intensity of bands from 1300–1400 cm−1 is also noticeable. Water and alcohol are always formed during the hydrolysis of TEOS. During the vacuum treatment, the volatile organic compounds are removed from HTEOS. Fig. 3(b) shows the spectra of the EF resin, APTES and cured neat SE coating. The bands at 3295 cm−1 and 3361 cm−1 of APTES correspond to the symmetrical stretching and antisymmetric stretching vibration of NH2, respectively. The absorption peak at 910 cm−1 of EF resin is related to the epoxide groups [23]. In the spectrum of the SE coating, the bands at 3295, 3361 and 910 cm−1 that are associated with NH2 and epoxide are absent. The spectrum for the SE coating reveals the occurrence of the curing reaction between the amine groups of APTES and the epoxide groups of EF resin. The appearance of the peak of the SE coating at approximately 1430 cm−1 is related to the CeN stretching band, which also implies the curing process of the hybrid coating. Obviously, the cured SE coating contains peaks at 1023 and 1131 cm−1, originating from the hydrolyzed SieOH in APTES and siloxane in the EF resin. The FTIR spectra of the hybrid coatings with different HTEOS contents are presented in Fig. 3(c). It is obvious that the addition of HTEOS affects the formation of SieOeSi crosslinks, as shown in Fig. 1(a). The main reaction in the hybrid coating can be mainly characterized as the condensation of eSieOH. The SieOH groups in HTEOS will be consumed by the eSieOH from EF resin and APTES, forming a denser SieOeSi structure. Normally, the stronger the SieOeSi vibration, the higher the cross-linking density. By inclusion of 4 wt% HTEOS, the SieOeSi vibration strengths increase, which indicates an enhanced cross-linking density of coating [24]. The ratio of

2.4. Corrosion resistance The corrosion behavior of the four samples was examined through EIS measurements, which were carried out by a CS350 electrochemical system at the open circuit potential (OCP). The experiments were conducted in a 3.5% NaCl solution with a frequency of 10−2 -105 Hz and a perturbation of 10 mV. The test was performed in a three-electrode cell in which the coated sample was used as the working electrode with an exposed area of 19.6 cm2, the reference electrode was a saturated KCl electrode and the auxiliary electrode was a platinum electrode. The impedance data was analyzed with a Zview software. The scanning vibration electrode technique (SVET) measurements were obtained using the VersaSCAN micro scanning electrochemical workstation (AMETEK, USA). An artificial scratch (1 mm long) was made with a surgical knife and was exposed to a 3.5% NaCl solution. The vibration frequency was 80 Hz with a 30 μm amplitude, and the scan area was 2 × 2 mm2 with a lateral step of 100 μm. 3. Results and discussion 3.1. Surface characterization Fig. 2 shows the surface morphologies of the hybrid coatings with various HTEOS concentrations. Fig. 2(a) shows the typical micrograph of the control SE coating. There are some discontinuities in the neat coating and agglomerations are observed in some places, which can be attributed to the heterogeneity of the coating. The heterogeneity may be attributed to the nonuniform curing [20]. A large number of smaller discontinued particles are detected upon the incorporation of HTEOS. Upon the addition of 2 wt% HTEOS, the discontinuity apparently decreases in the coating, showing the increased cross-linking effect. Fig. 2(b) shows an inhomogeneous micro dispersion of SiO2 particles inside the epoxy matrix. By increasing the HTEOS concentration to 4 wt %, a homogeneous surface and uniform distribution of particles is observed because of the solubilizing effect of SieOH chains. The homogenous appearance and the disappearance of discontinuities may imply 3

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Fig. 2. Micrographs of hybrid coatings with different HTEOS concentrations(a) 2%, (b) 2%, (c) 4%, (d) 6%.

Fig. 3. FTIR spectrum of (a) as received TEOS and HTEOS, (b) EF resin, APTES curing agent and cured SE coating, (c) 4 samples with different HTEOS addition, (d) the spectrum from 600–1200 cm−1 of above (c). 4

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Table 1 Thickness of four coatings with different HTEOS addition prepared on AA2024. Coating

Thickness (μm)

0% 2% 4% 6%

15.7 ± 0.4 14.8 ± 0.2 13.5 ± 0.8 16.2 ± 0.5

Table 2 The TG and DSC data of SE coating, 2%, 4% and 6% addition of HTEOS to the hybrid coatings. Sample

0 2% 4% 6%

peak height of the stretching vibrations (υOH/υSi-O-Si) is 0.1002, 0.1053, 0.0953, and 0.1190 for neat coating, 2%, 4% and 6% additive, respectively. The peak ratio can also show an evidence of the extent of curing and hydrophilicity of the coating. The result indicates that the addition of 6% HTEOS has a negative impact on the curing reaction and that the coating with 4% HTEOS has better condensation than other coatings. The thickness of the neat epoxy coating and coating with HTEOS addition is shown in Table 1. The average thickness for the coating with 2% and 4% HTEOS (14.8 and 13.5 μm) is slightly lower than the neat SE coating (15.7 μm), while the 6% HTEOS containing (16.2 μm) coating is slightly higher than that. The incorporation of HTEOS facilitates a more cross-linked network structure, resulting in a denser coating with decreased thickness. In contrast, 6% HTEOS has a negative effect on the curing reaction, which is in accordance with the FTIR and SEM results. To evaluate the cross-linking effect, DSC curves and TG curves obtained from TGA test were analyzed. TGA analysis was also employed to investigate the thermal stability of the hybrid coatings. The obtained results are shown in Fig. 4, while some important features are shown in Table 2. The glass transition temperature (Tg) of the neat SE coating is 68 ℃. It is obvious that the addition of HTEOS increases the Tg. As for the 4% HTEOS addition, 75.2 ℃ was the highest. It is known that the curing density of an organic coating is linearly correlated with Tg. Thus, the addition of 2% and 4% HTEOS increases the curing density of the hybrid coating, while 6% addition slows further curing. The TG curves show a two-step degradation mechanism. The same patterns of SE coating and hybrid coatings confirm that the addition of HTEOS did not produce any change in the degradation mechanism. The weight loss below 200 ℃ is the evaporation of the volatile components in the hybrid system. It is obvious that the change in weight loss of coating with 6% HTEOS is markedly. In the present study, this can be attributed to the residual silanol groups resulting from incomplete condensation. The next weight loss from 200 to 800 ℃ can be ascribed to the decomposition of polymer chains in the hybrid coatings. The decomposition temperature (Td) of the hybrid coating is approximately 20 ℃ higher than that of the pure SE coating, indicating a remarkably enhanced thermal stability. The residual masses of the four samples were compared to evaluate the inorganic silica domains. The results show that the inorganic content is 39–48 wt%. With increasing HTEOS content, the residual mass increased in the range of 0.75–8.01 wt.%.

Tg (°C)

68 72.6 75.2 68.8

Td (°C)

309.3 319.2 317.8 313.9

Temperature for mass loss (°C) 50%

Maximal rate

477.2 535.8 599.1 483.7

321.1 333.3 357.3 327.9

Residual Mass (%)

39.33 44.29 47.34 40.08

The increased incorporation of silica causes the increase in Td. This can be attributed to the stronger interaction between the polymer chains and the in situ silica. The high bond energy and thermal stability of SieO and SieC cause the hybrid coating to be difficult to decompose. Moreover, the higher temperature for 50% mass loss and maximal rate illustrate a higher thermal stability of hybrid coatings compared with the neat SE coating. The result claims that the thermal stability of hybrid coatings is improved due to the stronger polymeric-inorganic chain [25]. It can be inferred that the 4% addition of HTEOS into the SE coating enhances the cross-linking density and promotes the formation of more silica filler. On the other hand, the superfluous HTEOS slows further cross-linking, which results in a lack of ability to incorporate more fillers than the 2% and 4% addition of HTEOS. The contact angle of the hybrid coatings is shown in Fig. 5(a). The unmodified SE coating shows water contact angles of 75.3 ± 0.8°. It is obvious that the contact angle increases with the inclusion of HTEOS. In particular, the contact angle presents 90.4 ± 0.9° when the HTEOS concentration increases to 4 wt%. However, at 6 wt% loading of HTEOS, the contact angle is the lowest. Incorporation of appropriate HTEOS in the SE coating decreases the hydrophilicity, while at higher loadings, it increases the hydrophilicity. The result confirms the stronger OH peak of 6% HTEOS discussed from the FTIR spectrum, making the coating surface more hydrophilic. In contrast, the less hydrophilic 4% HTEOS coating surface means that water hardly spreads on the coating, which implies that the 4% HTEOS addition enhances the water resistance. To evaluate the adhesion of the coatings, pull-off tests were carried out on the coatings containing different HTEOS. The results of the pulloff test are given in Fig. 5(b). The adhesion strength is enhanced by increasing the concentration of HTEOS from 2 to 4 wt%, however, the adhesion strength follows a descending trend at a concentration of 6 wt %. The increased adhesion can be attributed to the increasing SieOeAl between silanol (SieOH) and Al substrate (AleOH). In contrast, the addition of more HTEOS (6 wt%) cannot promote the adhesion of the coating, which can be attributed to the excess Si-OH structure in the coating. The redundant SieOH causes a decrease in the coating adhesion, especially after immersion for 24 h. The fracture analysis of coatings containing 0 and 4% HTEOS is

Fig. 4. TG curves (a) and DSC curves (b) of SE coating, 2%, 4% and 6% addition of HTEOS hybrid coatings. 5

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Fig. 5. (a) The contact angle of the hybrid coatings, (b)the results of the pull-off test.

Fig. 6. The fracture micrographs of coatings containing 0 and 4% HTEOS.

shown in Fig. 6. The fracture surface of the neat epoxy exhibits oriented brittle fracture patterns, and the area between the oriented cracks is smooth due to a rapid crack propagation. Compared with the neat coating, the micropores formed by solvent evaporation cannot be detected in the fracture structure of the coating with 4% HTEOS, which indicates an improved barrier property by the addition of HTEOS. By the addition of HTEOS, the cross-section of the hybrid coating is coarse, and the fracture crack propagation is disturbed. A more stable SieOeSi network is introduced into the epoxy matrix. The irregular section indicates that the chemical connection between the organic and inorganic phase is raised due to the enhanced SieOeSi structure. The microphase separation was restrained by the stronger chemical interaction. The impact resistance is the ability of the coating to resist rapid deformation. Fig. 7 shows the pictures of the four samples after the impact test from a height of 50 cm with a 2-pound dart. The neat SE coating exhibits complete coating deterioration due to the mechanical stress spreading through voids, pinholes or inhomogeneously cured areas. Upon the addition of HTEOS, a clear enhancement in the impact resistance can be easily observed. It is obvious that the sample with 4 wt % HTEOS presents no cracks and does not peel off compared with the

results of those containing either greater or smaller amounts of HTEOS. This can be attributed to the enhancement in the interface interactions between SiO2/siloxane and the silicone-epoxy matrix. The improved bonding results in enhanced adhesion of the internal coating and coating/substrate interface, restricting the chain mobility. In addition, the denser coating decreases the microvoids and the mechanical stress, which reduces the chance of crack propagation and adhesion loss. Scheme 8(a) and 8(b) illustrates the formation of the branched polyepoxy chains and siloxane/silica nanostructures, respectively. The NH2 from the hardener (APTES) reacts with the epoxy groups from silicone-epoxy resin (EF resin), resulting in the extension of a long chain structure of cured SE coating. HTEOS promotes the polymerization of organic and inorganic phases, resulting in a denser network. Moderate inorganic domains yield a homogeneous hybrid structure as is shown in the morphology. The extent of polymerization is dependent on the HTEOS concentration in the sol. The silica gel formation is limited when HTEOS is at a lower concentration (2%). When HTEOS is at 4% concentration, both condensation and polymerization of inorganic and organic reactions are dominant. When HTEOS is present at higher concentrations (6%), the polymerization of the inorganic network is

Fig. 7. Pictures of four coatings after the impact test (a) neat SE coating, (b) 2% HTEOS coating, (c) 4% HTEOS coating, (d) 6% HTEOS coating. 6

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Fig. 8. Schematic description of the silicone-epoxy-silica hybrid with (a) 0% HTEOS (b) 4% HTEOS.

favored, leading to the destabilization of the SieOeSi structure and the production of cracks in the hybrid. The reason consists in the high amount of eOH that has not taken part in the condensation yet. The enhanced interphase interaction and decrease in free volume could be the reason of the improved thermal stability and impact resistance (Fig. 8).

AleOeSi and SieOeSi, which may offers a further protection [28]. The further cross-linking of the hybrid organic coating via the reaction between SieOH groups in the matrix further promotes the coating corrosion resistance. However, the impedance of the coating with 6% HTEOS decreases significantly, which can be attributed to the hydrophilic and cracked nature of the hybrid coating. The coating with 6% HTEOS contains more nonreactive SieOH and a cracked surface, which absorbs much more water, causing the impedance to drop rapidly. The diffusion of corrosive electrolyte also results in a decrease of the 2% and 4% HTEOS coatings from 332 h to 550 h. The impedance value of 6% HTEOS increases prominently at 550 h, which can be attributed to the corrosion products and filling the pores of the coating. However, after 628 h of immersion, an ascending trend for the coating with 6% HTEOS can be observed again. During the entire immersion period, the impedance of the coating with 4% HTEOS remains high and stable, confirming a restricted coating degradation in the presence of 4% HTEOS. The coating delamination and electrochemical active surface area beneath the coating can be studied by fb [29,30]. The delaminated areas of organic coatings could be analyzed with the following equation:

3.2. Corrosion resistance 3.2.1. Electrochemical characteristics EIS technique was used to evaluate the effect of HTEOS on the corrosion performance of silicone-epoxy coatings. Before the EIS test, samples were immersed in 3.5% NaCl solutions for 1 h to achieve a stable open circuit electrode. The variation of |Z|0.01 and breakpoint frequency (frequency values for a 45-phase angle- fb) are depicted in Fig. 9. The impedance at low frequency is a good estimation of the whole performance of the coated samples [26,27]. The impedance value of the coating with 4% HTEOS is the highest (2 × 109 Ω.cm2) at 180 h, which is one order of magnitude higher than that of the 6% HTEOS coating (1 × 109 Ω cm2) and half an order of magnitude higher than that of the coating without HTEOS (5 × 108 Ω cm2). It is clear that after 628 h of immersion, the low frequency impedance for the coating with 4% HTEOS is the highest, and the lowest impedance is for the neat coating, indicating the advantages of the corrosion protection properties of HTEOS. For the coating with 0%, 2% and 4% HTEOS, the impedance values increased from 180 h to 332 h. The slight increase in impedance is related to the formation of

fb = K

At A0

(1)

whereK = (1 2) w 0 , At is the delaminated area, A0 is the total area of the tested sample, ρ is the resistivity of the coating, εw is the dielectric constant of water, and ε0 is the vacuum permittivity. With the corrosive medium penetrating into the coating, ρ declines and εw increases. Thus, K can be approximately regarded as a constant. Therefore, the

Fig. 9. (a) The variation of |Z|0.01 of four samples during immersion from 180 h to 628 h, (b) the breakpoint frequency (frequency values for a 45-phase angle- fb ) obtained from the Bode plot. 7

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Fig. 10. EIS results of four hybrid coatings at (a) 180 h, (b) 332 h, (c) 550 h, (d) 628 h.

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and CPEdl are the constant phase element of the coating, the constant phase element of the oxide layer and the constant phase element of the double layer, respectively. It is significant that the coating with 4% HTEOS has the highest capacitance arc at all times. During the entire immersion time, there is no obvious change in the electrochemical behavior. In Fig. 10, it is noticed that during 180 h of immersion, all the coatings are characterized with two-time constants. After 332 h of immersion, the tail of the 2% HTEOS hybrid coating transforms to a larger semicircle, which is related to the oxide reinforcement at the metallic interface. After 550 h of immersion, coatings with 2% and 4% HTEOS exhibit a third-time constant. The time constant at low frequencies is related to the response of the coating/metal interface, while the high frequency is related to the coating on the outer part of the coating/ electrolyte interface, and the intermediate frequency is related to the inner coating of the coating/substrate interface. The appearance of the third-time constant indicates corrosion at the coating/substrate interface. With a longer immersion time, Rct decreases and CPEdl increases. Water uptake and coating delamination results in a higher CPEdl. Poor adhesion and more hydrophilicity of the neat coating and 6% HTEOS lead to a weak corrosion resistance. As a function of immersion time up to a period of 532 h, it can be seen that the impedance at low frequency decreases along with the maximum phase angle shifting to a higher frequency. Electrolyte diffusion into the coating and progressive damage of the coating can be responsible for the decrease in the impedance and narrowing the capacitance region. After 628 h, the maximum phase angle of the pure coating shifts to a higher frequency than that of the HTEOS-modified coating, suggesting the degradation of the pure coating. In addition, the impedance of the SE coating is the lowest, suggesting a loss of barrier performance. As shown in Table 3, the R ct of the 2% HTEOS coating is higher than that of the 6%-HTEOS coating and the CPEdl of the 2% HTEOS coating is much smaller than that of the 6% HTEOS coating. This is due to the reaction between the interface of the substrate and the aggressive medium. It is obvious that the interfacial corrosion reaction of the 6% HTEOS coating is stronger than that of the 2% HTEOS coating. This is consistent with the conclusion that water penetrates quickly in the 6% HTEOS coating. During the entire immersion process, the 4% HTEOS coating shows the highest Rc values, and the phase angle is approximately 90° at high frequencies, proving that the coating with 4% HTEOS has the best corrosion resistance in 3.5% NaCl solution. The enhanced corrosion resistance of the 4% HTEOS coating can be related to the denser SieOeSi network and reduced hydrophilic properties of

Fig. 11. Equivalent circuits used to fit the EIS results.

delaminated area of the coating was proportional to fb. In other words, the higher fb was, the more delaminated area at the interface of the coating/metal was. The change of fb from 180 h to 628 h in the neat coating is relatively larger than in the hybrid coatings. Among the three hybrid coatings, 4% HTEOS samples showed the lowest fb values. The results clearly demonstrate the coating delamination from the substrate. It can be concluded that coating with 4% HTEOS behaves more effectively, resulting in a lower electrolyte diffusion to the coating/metal interface. In the case of poor adhesion, which causes an increase in the electrochemical active area, the addition of 4% HTEOS in the epoxy coating has a better enhancement than the others owing to the higher adhesion and excellent resisting properties. This outcome is consistent with the adhesion test, in which the delaminated area of the coating with 4% HTEOS is smaller than that of the other coatings. Fig. 10 depicts the Bode and Nyquist plots of the Al alloy substrates coated with the hybrid coatings and the neat coating after immersion in 3.5 wt% NaCl solutions from 180 h to 628 h. To obtain better insight into the corrosion behavior of the coating/alloy system, equivalent circuits (shown in Fig. 11) were used to fit the EIS curves, where Rs, Rc, Rct and Rox are the solution resistance, coating resistance, charge transfer resistance and resistance of the intermediate oxide-based layer between the substrate and sol-gel coating, respectively, and CPEc, CPEox Table 3 Fitting results of the EIS data of the four coatings immersed in 3.5% NaCl solution. Sample

Rc (Ohm cm2)

CPEc −1

Y0(ohm

−2

cm

n

s )

nc

Rox (Ohm cm2)

CPEox −1

Y0(nohm

−2

cm

n

s )

nox

Rct (Ohm cm2)

CPEdl Y0(nohm−1 cm−2 sn)

nct

0%, 0%, 0%, 0%,

180h 332h 550h 628h

6.18e8 6.36e6 2.14e8 4.60e6

1.94e-10 1.45e-10 1.91e-10 1.51e-10

0.98±0.01 0.99±0.01 0.98±0.04 0.98±0.01

6.37e8 1.32e9 2.23e8 7.08e6

1.35e-10 2.24e-10 5.68e-9 6.52e-10

0.82±0.02 0.89±0.04 0.72±0.02 0.82±0.02

7.67e8 2.48e8 8.94e6

6.23e-8 4.76e-9 2.72e-7

0.99±0.01 0.38±0.05 0.67±0.02

2%, 2%, 2%, 2%,

180h 332h 550h 628h

5.18e8 5.62e7 3.71e7 3.64e7

1.69e-10 1.64e-10 1.32e-10 1.44e-10

0.95±0.02 0.95±0.04 0.95±0.01 0.94±0.01

1.30e9 5.47e9 4.69e8 1.94e9

2.25e-8 1.63e-9 8.09e-8 2.77e-9

0.76±0.03 0.78±0.04 0.87±0.02 0.98±0.01

4.94e8 1.69e9

5.29e-9 6.53e-9

0.93±0.03 0.87±0.02

4%, 4%, 4%, 4%,

180h 332h 550h 628h

2.25e9 8.91e9 4.01e9 3.56e9

1.54e-10 1.39e-10 1.06e-10 1.10e-10

0.96±0.02 0.97±0.02 0.96±0.01 0.96±0.02

2.55e9 3.97e9 4.37e9 1.05e9

1.45e-10 1.38e-10 1.10e-10 2.77e-9

0.80±0.02 0.75±0.04 0.90±0.02 0.99±0.02

6%, 6%, 6%, 6%,

180h 332h 550h 628h

6.95e7 1.41e8 1.55e7 1.02e8

1.61e-10 1.39e-10 1.04e-10 1.09e-10

0.95±0.04 0.96±0.02 0.96±0.02 0.96±0.02

2.23e9 3.13e8 1.13e8 7.73e7

2.69e-9 1.28e-9 7.11e-10 2.42e-9

0.74±0.02 0.84±0.05 0.97±0.02 0.90±0.02

2.27e8 4.77e6

1.03e-8 4.16e-7

0.93±0.04 0.74±0.02

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Fig. 12. Digital images of the coated samples after exposure to the saline medium for 1440 h. (The area of circle is 19.6 cm2).

Fig. 13. SEM micrographs for neat coating and 4% HTEOS coating after 2000 h immersion.

the coating. The digital photographs of the four coated samples after exposure in the electrochemical cell for 1440 h are shown in Fig. 12. Pitting and crevice corrosion of neat SE coating and 6% HTEOS coating was visible after 1440 h of immersion. The coatings with the 2% and 4% HTEOS additions remain undamaged for 1440 h of immersion. The diffusion of electrolytes and corrosive agents from the coating surface to the substrate is faster due to the increased hydrophilicity of the neat coating and the 6% HTEOS coating. The SEM micrographs of the neat coating and 4% HTEOS coating after 2500 h of immersion are shown in Fig. 13. After 2500 h of immersion, the neat coating shows cracks and flaking compared with preimmersion (Fig. 1(b)). The cracking indicates the degradation of the coating. The appearance of the coating with the 4% addition of HTEOS is free of degradation, and the film has no cracks. This can be attributed to the flexibility and high adhesion of the 4% HTEOS hybrid coating, which slows down the delamination of the coating that results from the accumulation of products and the production of hydroxyl ions. Therefore, the tendency to form microcracks is reduced. On the one hand, the enhanced SieOeSi structure improves the flexibility of the polymer. The TGA results show that the inorganic content is 39–48 wt%. Several reports have stated that having a 30–50% inorganic phase is beneficial to impact the flexibility of the hybrid coatings [31–33]. The dense cross-linked network decreases the probability of crack propagation resulting from the external stress originating from corrosion attacks. On the other hand, the diffusion of the electrolyte through the alloy/ coating interface has the least resistance due to the high adhesion of the coating. Furthermore, the addition of 4% HTEOS decreases the hydrophilicity of the coating.

current density through Ohm’s law. The current density maps of the neat coating and coating with 4% HTEOS immersed in 3.5% NaCl solution are presented in Fig. 14. Noticeably, both the current density and anodic area of the neat coating are much larger than those of the hybrid coating. After immersion for 12 h, the current density of the neat coating is 0.06 μA/cm2, which is larger than that (0.01 μA/cm2) of the coating with 4% HTEOS. There is no obvious anodic current in the hybrid coating with 4% HTEOS. With the infiltration of water (72 h), the anodic current density and the area of the anodic scan of the neat coating are larger than those of the hybrid coating. After 120 h of immersion, the aggravated corrosion reaction was still observed for the neat coating with an enlarged corrosion area. The smaller anodic area indicates the enhanced corrosion resistance and adhesion property of the hybrid coating. 4. Conclusion Hybrid coatings based on a silicone-epoxy resin and prehydrolyzed TEOS (HTEOS) were prepared. Silica were incorporated into the hybrid coatings through an in situ polymerization method. The effect of HTEOS on the parent hybrid coating has been investigated, and three conclusions can be drawn: (1) The incorporation of HTEOS facilitated a denser SieOeSi structure and stronger chemical bonding between the organic and inorganic phases was enhanced. The coating with 4% HTEOS possessed a hybrid structure with less hydrophilic properties composed of homogeneous silica domains closely bonded to a cross-linked epoxy matrix. (2) The silicone epoxy coating with 4% HTEOS provided an improved protection to AA2024 and long durability compared with that of the neat coating. The hybrid coating exhibited a better adhesion to the substrate due to the presence of a sufficient silica phase at the

3.2.2. SVET studies The corrosion behavior of the hybrid coating was also investigated by a SVET test. The potential signals were converted into the local 10

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Fig. 14. SVET maps of the current density for the detected surface of (a) neat coating, (b) coating with 4% HTEOS after various immersion time in 3.5 NaCl solution.

coating/alloy interface and resulted in a less delamination upon immersion in the corrosive solution. The improved corrosion protection of the hybrid coating could be attributed to the enhanced SieOeSi network and a higher percentage of SiO2, resulting in better flexibility, enhanced impact resistance and adhesion of the hybrid coating. (3) In contrast, when HTEOS was present at a high concentration (6%), the polymerization of the inorganic network and cross-linking density was reduced. The residual OeH increased the hydrophilicity and decreased the corrosion resistance of the hybrid coating.

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