On the transformation textures influenced by deformation in electrical steels, high manganese steels and pure titanium sheets

Abstract

Transformation texture is normally different to deformation and recrystallization textures, thus influencing materials properties differently. As deformation and recrystallization are often inseparable to transformation in materials which shows a variety in types such as diffusional or non-diffusional transformations, different phenomena or rules of strengthening transformation textures occur. This paper summarizes the complicated phenomena and rules by comparison of a lot of authors’ published and unpublished data collected from mainly electrical steels, high manganese steels and pure titanium sheets. Three kinds of influencing deformation are identified, namely the dynamic transformation with concurrent deformation and transformation, the transformation preceded by deformation and recrystallization and the surface effect induced transformation, and the textures related with them develop in different mechanisms. It is stressed that surface effect induced transformation is particularly effective to enhance transformation texture. It is also shown that the materials properties are also improved by controlled transformation textures, in particular in electrical steels. It is hoped that these phenomena and processing techniques are beneficial to the establishment of transformation texture theory and property improvement in practice.

Graphical abstract

Keywords

electrical steel / high manganese steel / recrystallization / transformation / titanium / deformation

1 Introduction

In many areas of industrial production, scaling and precipitation of calcium carbonate (CaCO₃) occur frequently [1]. Scaling may seem insignificant, but according to statistics, the annual economic losses caused by scaling account for approximately 0.25% of the global gross domestic product. The deposition and adherence of CaCO₃ to equipment or pipeline surfaces can result in significant consequences such as heightened thermal resistance, diminished heat transfer efficiency, reduced pipeline flow cross-section, and increased flow resistance [2]. Consequently, scaling poses a substantial threat to industrial production. Surface coating technology stands as a prevalent and effective method for inhibiting scale formation.

Coatings can effectively isolate metals from media, thereby reducing the potential for scale deposition and adhesion on metal surfaces [3]. The use of superhydrophobic coatings with a water contact angle (WCA) greater than 150° and a sliding angle (SA) less than 10° has garnered attention in various fields such as self-cleaning, anti-scaling, anti-corrosion, and drag reduction in recent years [4]. Polyvinylidene fluoride (PVDF), a thermoplastic fluoropolymer known for its excellent thermal stability, mechanical properties, and chemical resistance, finds wide-ranging applications in ultrafiltration, batteries, pyroelectric materials, aviation, and other industries [5]. Incorporating fillers such as nano zinc oxide (nano-ZnO) [6], zeolite [7], and carbon nanotubes (CNTs) [8] can improve the anti-scaling performance of coatings [9–10]. However, it is important to note that the rough structure of superhydrophobic coatings is susceptible to damage which may compromise their superhydrophobic properties and limit practical applications [8]. Nano titanium dioxide (nano-TiO₂) exhibits stable chemical properties, excellent acid and alkali resistance, low cost, and non-toxicity. In recent years, nano-TiO₂ has found applications in plastics, functional fibers, coatings, and various other fields. The addition of nano-TiO₂ to coatings can improve their light resistance, weather resistance, and heat resistance. Furthermore, TiO₂ whiskers demonstrate stable properties and excellent temperature resistance, leading to widespread utilization across multiple industries [11–12]. Incorporating TiO₂ whiskers into coatings can significantly enhance the scale inhibition performance of metal materials [8,13–14]. However, due to the high surface free energy of nano-TiO₂ particles leading to aggregation, it is difficult to achieve uniform dispersion in the coating, which impacts the protective and mechanical properties of the coating. Treatment of nano-TiO₂ with polyethylene glycol (PEG) notably improves the anti-fouling performance of PVDF/TiO₂ composite film [15]. Additionally, nano-TiO₂ particles modified with 3-(trimethoxysilyl) propyl methacrylate silane can disperse regularly in water-based acrylic coating, thus improving the wear resistance of the coating [16]. Given the poor bonding stability of nano-TiO₂ particles on the coating surface, maintaining its long-term functionality is a challenge. Obviously, surface modification is crucial for the improvement of its application performance in coatings.

In this study, octadecyltrimethoxysilane (OTMS) was employed to functionalize TiO₂ whiskers, resulting in the improved uniformity of their dispersion in the superhydrophobic coating. The coating was prepared through air spraying and incorporation of above-mentioned silane-modified TiO₂ whiskers into PVDF and fluorinated ethylene propylene (FEP) resins, which exhibited outstanding scale inhibition performance. The synergistic interplay between hydrophobic properties of this silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating and spatial constraints imposed by silane-modified TiO₂ whiskers effectively impedes the ingress of scaling media (such as Ca²⁺ and CO₃²⁻). We posit that this study presents a highly promising approach for the fabrication of superhydrophobic coatings with resistance to scaling.

2 Experimental

2.1 Materials and reagents

6061 Aluminum plate (20 mm × 80 mm × 1 mm) was provided by Wuxi Baojing Aluminum Co., Ltd. PVDF powders (Shanghai 3F Co., Ltd., China), FEP (DuPont USA), TiO₂ whiskers (≥ 99.9 wt.%, Kegong Metallurgical Materials Co., Ltd.), epoxy resin (Nanjing Huntsman Advanced Materials Co., Ltd.), and low-molecular-weight (LMW) polyamide resin (Beijing Xiangshan Co., Ltd.) were used in this study. Reagents used here included OTMS (90%, Shanghai McLean Biochemical Technology Co., Ltd.), absolute ethanol (C₂H₅OH; Tianjin Yongsheng Fine Chemical Co., Ltd.), ethyl acetate (Tianjin Fuyu Fine Chemical Co., Ltd.), sodium bicarbonate (NaHCO₃; Tianjin Zhiyuan Chemical Reagent Co., Ltd.), and calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O; Tianjin Hengxing Chemical Reagent Manufacturing Co., Ltd.).

2.2 Treatment of substrate

The 6061 aluminum plate is polished using 600, 800, and 1200 mesh silicon carbide sandpapers to eliminate the oxide film on its surface. The polished aluminum plate was first cleaned with deionized water, and then immersed in an absolute ethanol solution to be further cleaned with ultrasound for 5 min aiming at the removal of dirt and grease attached to its surface. Afterwards, the aluminum plate was carefully wiped clean with filter papers in preparation for the subsequent use.

2.3 Preparation of PVDF‒FEP and TiO₂‒PVDF‒FEP coatings

Epoxy resin, known for its exceptional bonding performance with the aluminum plate, was selected as the coating substrate for this experiment. Initially, 2 g of epoxy resin and 10 g of ethyl acetate were carefully measured and combined in a 50 mL beaker. The mixture was then subjected to ultrasonic dispersion for 10 min using an ultrasound instrument. Subsequently, 1 g of LMW polyamide resin was weighed and added to the aforementioned solution, followed by the dispersion for an additional 10 min. The resulted coating solution was subjected to room-temperature spraying at a pressure of 0.6 MPa. Following this step, the prepared sample underwent curing in an electric heating blast drying oven at a temperature of 150 °C for 1.5 h, resulting in the formation of an epoxy resin coating.

Afterwards, 0.3 g of TiO₂ whiskers, 0.7 g of PVDF, 0.3 g of FEP, and 10 g of anhydrous ethanol were separately weighed, and combined in a 50 mL beaker for ultrasonic dispersion treatment lasting 30 min. The above-dispersed solution was then applied onto the epoxy resin coating substrate via spraying. Subsequently, the sample underwent oven drying at 180 °C for 1.5 h, thus designated as the TiO₂‒PVDF‒FEP coating. Similarly, a PVDF‒FEP coating was prepared using an identical procedure but omitting the addition of TiO₂ whiskers.

2.4 Preparation of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating

Initially, 0.3 g of TiO₂ whiskers and 10 g of anhydrous ethanol were separately measured and subsequently placed in a 50 mL beaker for ultrasonic dispersion treatment lasting 2 h. Following this, 0.2 mL of OTMS was introduced into the aforementioned solution and subjected to further sonication for additional 1 h yielding a silane-modified TiO₂ suspension. Subsequently, 0.7 g of PVDF and 0.3 g of FEP were incorporated into the silane-modified TiO₂ suspension, which was then sonicated for another 30 min. Finally, under controlled pressure conditions, the resulted coating solution was atomized onto an epoxy resin matrix and cured in a hot air-drying oven at 180 °C for 1.5 h to obtain the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating.

2.5 Scaling tests of different coatings

In this study, scaling tests were performed on coatings within a supersaturated CaCO₃ solution. To expedite the scaling process, the experimental temperature was maintained at 60 °C. The supersaturated CaCO₃ solution was prepared through the reaction between Ca(NO₃)₂·4H₂O and NaHCO₃ expressed by Eq. (1) as follows [14]:

(1)$\begin{array}{c}{\mathrm{C}\mathrm{a}(\mathrm{N}\mathrm{O}}_3{)}_2\cdot {4\mathrm{H}}_2{\mathrm{O}+2\mathrm{N}\mathrm{a}\mathrm{H}\mathrm{C}\mathrm{O}}_3\to {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3\downarrow +\\ {2\mathrm{N}\mathrm{a}\mathrm{N}\mathrm{O}}_3{+5\mathrm{H}}_2{\mathrm{O}+\mathrm{C}\mathrm{O}}_2\uparrow \end{array}$

At first, 1 L of 14.20 g·L⁻¹ Ca(NO₃)₂·4H₂O and 1 L of 10.08 g·L⁻¹ NaHCO₃ solutions were prepared, which were then placed in a water bath at a maintained temperature of 60 °C for 0.5 h.

Afterwards, 125 mL of each solution was measured using a measuring cylinder and transferred into individual 250 mL glass bottles, which were heated in the same water bath at 60 °C. The coating samples were vertically suspended inside glass bottles filled with the supersaturated CaCO₃ solution and subjected to continuous heating. At specific intervals, the samples were removed from the solution, gently rinsed with deionized water, and dried at 105 °C until the weight was no longer changed followed by the determination in the weight gain of the coating. During the scaling test process, an inductively coupled plasma (ICP) spectrometer (2100 DV, Perkin Elmer, USA) was employed to analyze the concentration of Ca²⁺ ions within the system. Additional amounts of Ca(NO₃)₂·4H₂O and NaHCO₃ were added as necessary to maintain consistent concentrations of Ca²⁺ and CO₃²⁻ ions in this system.

2.6 Characterization

The coating samples were examined using scanning electron microscopy (SEM) on an EVO18 microscope to observe their microscopic morphology, and element contents and distributions of typical coating areas were analyzed through energy dispersive X-ray spectroscopy (EDS). Fourier transform infrared spectroscopy (FTIR) was conducted to determine the chemical compositions of the coating samples. The WCA of each coating sample was measured in at least five different locations for the calculation of the mean value. Following the scaling experiment, X-ray powder diffraction (XRD) analysis was conducted to examine the crystal structure of scaling on the coating surface, while FTIR was performed to identify its functional groups.

3 Results and discussion

3.1 Microscopic morphology and elemental analyses of different coatings

According to Fig.1, the PVDF‒FEP coating exhibits a rough surface structure consisting of PVDF and FEP (Fig.1(a1) and 1(a2)). The static WCA measurement for the PVDF‒FEP coating yielded a value of 110.6°. Upon incorporation of TiO₂ whiskers into the coating, the rough structure of the TiO₂‒PVDF‒FEP coating increased significantly. SEM images revealed the presence of TiO₂ whiskers within the coating matrix (Fig.1(b1)), although their distribution was non-uniform with localized agglomeration observed in certain areas (Fig.1(b2)). The static WCA of the TiO₂‒PVDF‒FEP coating was increased to 127.4°, demonstrating a significant improvement compared to that of the PVDF‒FEP coating. This enhancement can be attributed to the introduction of TiO₂ whiskers, which enhance the roughness of the coating and consequently increase its hydrophobicity in comparison with the PVDF‒FEP coating. Furthermore, modification with OTMS resulted in a more uniform distribution of TiO₂ whiskers within the TiO₂‒PVDF‒FEP coating (Fig.1(c1) and 1(c2)). This may be due to the modification treatment connecting the surface of TiO₂ whiskers with the active group −Si(OH)₃ in OTMS, making it easier to disperse in solvents. As a result of this modification, the hydrophobicity of the silane-modified TiO₂‒PVDF‒FEP coating has been significantly improved, with a WCA of 151.2°, demonstrating its superhydrophobicity.

Fig.1 SEM images and contact angles of (a1)(a2) the PVDF–FEP coating, (b1)(b2) the TiO2–PVDF–FEP coating, and (c1)(c2) the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating.

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EDS was utilized for the acquisition of images and analysis of elemental compositions of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating, as depicted in Fig.2. The microstructure of the silane-modified TiO₂‒PVDF‒FEP superhydrophobic coating reveals a uniform distribution of needle-shaped TiO₂ whiskers within the coating (Fig.2(a)). The composition of the coating primarily consists of six elements, C, O, F, Si, Ti, and Al (Fig.2(b)). Specifically, the F element originates from PVDF and FEP, while Si, Ti, and Al are associated with OTMS, TiO₂ whiskers, and substrates, respectively. According to the distribution of the Ti element (Fig.2(c) and 2(d)) as well as the microstructure of the coating (Fig.1 and Fig.2(a)), it is evident that TiO₂ whiskers modified with OTMS can be uniformly distributed in the coating, filling the rough structural gaps and coating pores formed by PVDF and FEP. This ultimately enhances the hydrophobicity of the coating to a certain extent (Fig.1(c)).

Fig.2 (a) Microscopic morphology (obtained through EDS), (b) element distribution, (c) Ti element distribution, and (d) EDS element mapping results of the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating.

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3.2 Structure characterization of coating

FTIR analysis of TiO₂ whiskers before and after the modification was performed, and the results are illustrated in Fig.3. The characteristic peaks at 950, 865, and 778 cm⁻¹ are attributed to the vibration of Ti−O in unmodified TiO₂ whiskers (Curve a in Fig.3). The sharp peaks observed at 2918 and 2845 cm⁻¹ are caused by the symmetric stretching vibration of CH₂. Furthermore, the peak at 1468 cm⁻¹ can be assigned to the symmetric bending vibration of CH₃, while the peaks at 1090 and 822 cm⁻¹ belong to the vibrations of Si−O−Si and Si−C, respectively (Curve b in Fig.3). Upon comparison with the unmodified TiO₂ whiskers (Curve a in Fig.3) and OTMS (Curve b in Fig.3), it is observed that the modified TiO₂ whiskers exhibit new characteristic peaks at 1700 and 507 cm⁻¹, corresponding to Si−O−Si and Ti−O−Si bond vibrations, respectively. This observation suggests a chemical reaction between TiO₂ whiskers and OTMS has taken place, indicating successful modification of the TiO₂ whiskers by OTMS.

Fig.3 FTIR results of unmodified TiO2 whiskers (a), OTMS (b), and modified TiO2 whiskers (c).

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FTIR analysis of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating was carried out, and the results are shown in Fig.4. The characteristic peak at 3440 cm⁻¹ belongs to the vibration of −OH in TiO₂ whiskers, but may not be observed due to interference with the adsorbed water [17]. The peaks at 2934 and 1403 cm⁻¹ are related to the bending vibrations of −CH₃ and −CH₂, respectively. The peak at 1210 cm⁻¹ is attributed to the antisymmetric stretching vibration of CF₂ in PVDF and FEP. In addition, the peak at 489 cm⁻¹ is assigned to the vibration of the PVDF crystalline phase [18]. Upon comparison with the PVDF‒FEP coating (Curve a in Fig.4), the incorporation of unmodified TiO₂ whiskers led to the emergence of corresponding Ti−O bond vibration peaks at 950 and 778 cm⁻¹ in the TiO₂‒PVDF‒FEP coating, thereby indicating the successful integration of TiO₂ whiskers into the coating (Curve b in Fig.4). After the modification of TiO₂ whiskers with OTMS, a new absorption peak (Curve c in Fig.4) was observed in the infrared (IR) spectrum of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating, which corresponds to the Ti−O−Si peak at 920 cm⁻¹ [17]. This is attributed to the reactivity of hydroxyl groups present on the surface of TiO₂ whiskers, which facilitates the formation of Ti−O−Si bonds through crosslinking with siloxane groups in OTMS. This observation corroborates the successful modification of TiO₂ whiskers with OTMS and their incorporation into the coating, as supported from results of the EDS element content analysis (Fig.2).

Fig.4 FTIR results of the PVDF‒FEP coating (a), the TiO2‒PVDF‒FEP coating (b), and the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating (c).

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3.3 Scaling behaviors on surfaces of different coatings

Scaling behaviors of the coating surfaces were studied in a supersaturated CaCO₃ solution at 60 °C. As shown in Fig.5, with prolonging the soaking time of all coating samples in the solution, the masses of scaling on surfaces of the PVDF‒FEP coating, the TiO₂‒PVDF‒FEP coating, and the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating gradually increased. When the soaking time reached 360 h, the scaling masses of CaCO₃ on surfaces of above three coatings were 3.02, 2.28, and 1.90 mg·cm⁻², respectively. Among them, the surface scaling mass for the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating is the lowest, while that for the PVDF‒FEP coating exhibits the highest. In comparison with those on the PVDF‒FEP coating and the TiO₂‒PVDF‒FEP coating, the scale formation on the surface of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating decreased by 37.1% and 16.7%, respectively. Furthermore, the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating was evaluated in comparison to previously reported coatings. For instance, in comparison to the superhydrophobic PVDF/FEP/SiO₂/CNT-EDTA (PFSC-EDTA) coating which incorporates a scale inhibitor, the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating in this work exhibited only 76.3% of the scale formation observed on the PFSC-EDTA coating after scaling for 288 h [19]. All results indicate that the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating has excellent scale inhibition performance.

Fig.5 Relationships between immersion time and scale masses of CaCO3 on surfaces of the PVDF‒FEP coating (a), the TiO2‒PVDF‒FEP coating (b), and the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating (c).

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CaCO₃ is a common inorganic compound with distinct IR spectral characteristics. Therefore, the molecular composition and structure of the scale on the coating surface can be detected by FTIR. As shown in Fig.6, compared with the IR spectrum of the non-scaling coating (Fig.3), there was a significant change in the IR spectrum of the coating after 24 h of scaling. The peaks appearing at 2970 and 2840 cm⁻¹ correspond to the antisymmetric stretching vibration of −CH₂. The peaks at 2924 and 2870 cm⁻¹ are caused by the symmetric stretching vibration of −CH₂. The absorption peak at 960 cm⁻¹ belongs to the vibration of Ti−O (Curves b and c in Fig.6). The characteristic peak at 1796 cm⁻¹ originates from the vibration of C=O in CaCO₃. The peaks at 1420 and 1066 cm⁻¹ correspond to the antisymmetric vibration of C−O in CaCO₃ [20‒21]. In addition, the peaks appearing at 871 and 712 cm⁻¹ are derived from the out-of-plane bending vibration and in-plane bending vibration of CO₃²⁻, respectively [22]. Specifically, the peak at 2512 cm⁻¹ is the combined frequency of peaks at 1796 and 712 cm⁻¹ in CaCO₃ [20]. Compared with the IR spectra of the PVDF‒FEP coating (Curve a in Fig.6) and the TiO₂‒PVDF‒FEP coating (Curve b in Fig.6), the characteristic peak intensity of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating (Curve c in Fig.6) is weaker at 1420, 871, and 712 cm⁻¹.

Fig.6 FTIR results of the PVDF‒FEP coating (a), the TiO2‒PVDF‒FEP coating (b), and the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating (c) after scaling for 24 h.

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Furthermore, XRD analysis of scale formed on the coating surface was performed to elucidate its crystalline phase compositions, as illustrated in Fig.7. The diffraction peaks at 44.4°, 65.2°, and 78.1° are characteristic of the aluminum substrate. After soaking in a supersaturated CaCO₃ solution for 24 h, the predominant crystal forms of CaCO₃ on surfaces of the PVDF‒FEP coating, the TiO₂‒PVDF‒FEP coating, and the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating were identified as calcite, aragonite, and vaterite, respectively. Specifically, the peaks at 2θ = 29.4° (1 0 4), 39.3° (2 0 5), 43.2° (2 0 2), 47.2° (0 2 4), and 48.5° (2 0 2) are assigned to the calcite structure, while those for aragonite and vaterite appear at angles of 38.3° (1 1 0) and 42.8° (0 0 8), respectively. Notably, for the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating, the intensities of characteristic peaks corresponding to calcite, aragonite, and vaterite were lower than those observed for coatings of both PVDF‒FEP and TiO₂‒PVDF‒FEP, consistent with the scaling trend of different coatings (Fig.5). This indicates that the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating exhibits superior scale inhibition performance that hinders the deposition of CaCO₃ on its surface.

Fig.7 XRD patterns of the PVDF‒FEP coating (a), the TiO2‒PVDF‒FEP coating (b), and the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating (c) after scaling for 24 h.

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The morphology of CaCO₃ crystals on surfaces of different coatings was analyzed by SEM, as shown in Fig.8. After the immersion in a saturated CaCO₃ solution for 24 h, a large number of cube-, flake-, and needle-shaped crystals were generated on the surface of the PVDF‒FEP coating, with a high crystal packing density. The diameters of the cube and sheet crystals are approximately 25‒30 and 10‒15 μm, respectively, while the maximum length of the needle-like crystal reaches about 20 μm (Fig.8(a)). Compared with that of the PVDF‒FEP coating, the stacking density of crystals on the surface of the TiO₂–PVDF–FEP coating is significantly diminished, and crystal shapes predominantly exhibit cubic and irregularly block forms (Fig.8(b)). However, it is detected that only a limited number of cubic crystals with the diameter ranging from 5 to 18 μm are distributed across the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating (Fig.8(c)). When compared with both the PVDF‒FEP coating and the TiO₂‒PVDF‒FEP coating, the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating demonstrates significantly fewer CaCO₃ crystals, exhibiting its superior scale inhibition performance. Combined with the results from FTIR and XRD analyses (Fig.6 and Fig.7) of the scale on the coating surface, it can be detected that cube- and flake-like crystals correspond to calcite, while the needle-like structure represents aragonite [23].

Fig.8 SEM images of (a) the PVDF‒FEP coating, (b) the TiO2‒PVDF‒FEP coating, and (c) the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating after scaling for 24 h.

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3.4 Mechanistic analysis of silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating

After modifying TiO₂ whiskers using ODTMS and introducing coatings, the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating prepared demonstrates a significant influence on the formation of CaCO₃ crystals. The excellent scale inhibition performance of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating can be primarily attributed to three aspects: Firstly, the presence of an air film on the surface of the superhydrophobic coating acts as a barrier layer to inhibit scaling. Compared with the TiO₂‒PVDF‒FEP coating (Fig.9(a)), TiO₂ whiskers in the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating exhibit a more uniform distribution. The formed micron-nano structure effectively encapsulates air, forming an air film that isolates scaling media from the metal substrate, and significantly delays or inhibits the deposition and adhesion of CaCO₃ on the coating surface (Fig.9(b)). Secondly, the dispersion of silane-modified TiO₂ whiskers within the superhydrophobic TiO₂‒PVDF‒FEP coating (Fig.1) creates a unique micro-nano structure that imposes spatial restrictions within the coating, limiting crystal growth direction and inhibiting scale formation (Fig.9(b)). In comparison to the TiO₂‒PVDF‒FEP coating, there is a notable reduction in the surface scaling amount for the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating (Fig.5), accompanied by lower characteristic diffraction peak intensities for calcite, aragonite, and vaterite (Fig.7). Finally, the superhydrophobicity of the coating enhances the difficulty in the crystallization of CaCO₃, inhibiting the formation of scale. According to the theory of crystal nucleation [24‒25], when the radius of the crystal embryo (r) is less than the critical radius r_c, the Gibbs free energy (∆G) increases with the rise of r. This means that the crystal embryo cannot grow, that is, it cannot form a stable crystal nucleus, which disappears immediately after formation. The critical Gibbs free energy (∆G_c^*) corresponding to r_c is the nucleation work of the critical nucleus, a higher value of which indicates an increased amount of work necessary to form the nucleus, thereby elevating the difficulty in nucleation, expressed by Eqs. (2)–(4) as follows [26]:

Fig.9 Schematic diagrams of the CaCO3 scaling formation on (a) the TiO2‒PVDF‒FEP coating and (b) the silane-modified superhydrophobic TiO2‒PVDF‒FEP coating.

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(2)$\mathrm{\Delta }{G}_{\text{c}}^{\ast }=\frac{16\text{π}{\gamma }^3}{3{(\mathrm{\Delta }{G}_{\mathrm{v}})}^2}f(\theta )$

(3)$r_{\mathrm{c}}=-\frac{2\gamma }{\mathrm{\Delta }{G}_{\mathrm{v}}}$

(4)$f(\theta )=\frac{(2+\cos \theta ){(1-\cos \theta )}^2}{4}$

where $\theta$ is the contact angle, $\mathrm{\Delta }{G}_{\mathrm{v}}$ is the change in free energy corresponding to the change in unit volume of crystal nucleus, and $\gamma$ is the interfacial energy between the liquid and the solid. The derivation of f(θ) in Eq. (4) shows that the function is monotonically increased [26], indicating that with the enhancement of the contact angle between the droplet and the substrate surface, the value of ∆G_c^* also rises accordingly. After the modification of TiO₂ whiskers with silane, the hydrophobicity of the silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating was significantly improved (Fig.1). The optimization of the superhydrophobic coating performance increases the energy barrier for CaCO₃ crystal nucleation, thereby diminishing the occurrence probability of the scaling formation on the coating surface. Therefore, the synergistic effect derived from air films, silane-modified TiO₂ whiskers, and superhydrophobic surfaces ensures excellent anti-scaling properties belonging to this superhydrophobic TiO₂‒PVDF‒FEP coating.

4 Conclusions

We have developed a novel silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating with excellent anti-scaling properties. The synergistic effect of silane-modified TiO₂ whiskers and hydrophobic surfaces on the anti-scaling performance has been thoroughly discussed. On the one hand, the micro-nano structure formed by silane-modified TiO₂ whiskers effectively restricts the growth direction of CaCO₃ crystals, thereby inhibiting the formation and growth of scale. On the other hand, air films on the surface of the superhydrophobic coating act as a barrier between scaling media and metal substrate, hindering deposition and adhesion of CaCO₃ on the coating surface. In a static scaling test for 360 h, the scale mass for the silane-modified TiO₂‒PVDF‒FEP coating is 1.90 mg·cm⁻², which is decreased by 37.1% and 16.7% compared with those for the PVDF‒FEP coating and the TiO₂‒PVDF‒FEP coating, respectively. The silane-modified superhydrophobic TiO₂‒PVDF‒FEP coating exhibits excellent scale inhibition performance and holds significant promise for advancement in the field of scaling mitigation.

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Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51771024).

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