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Article

Enhanced Corrosion Resistance and Surface Wettability of PVDF/ZnO and PVDF/TiO2 Composite Coatings: A Comparative Study

by
Adel M. A. Mohamed
1,*,
Abdulrahman I. Alateyah
2,*,
Hosam Hasan
1,
Penchal Reddy Matli
3,
Mohamed M. El-Sayed Seleman
1,
Essam Ahmed
1,
Waleed H. El-Garaihy
2,4,* and
Teresa D. Golden
5
1
Department of Metallurgical and Materials Engineering, Faculty of Petroleum and Mining Engineering, Suez University, Suez 43512, Egypt
2
Department of Mechanical Engineering, College of Engineering, Qassim University, Unaizah 56452, Saudi Arabia
3
Department of Physics, Sri Krishnadevaraya University, Anantapur 515003, India
4
Mechanical Engineering Department, Faculty of Engineering, Suez Canal University, Ismailia 41522, Egypt
5
Department of Chemistry, University of North Texas, 1155 Union Circle #305070, Denton, TX 76203, USA
*
Authors to whom correspondence should be addressed.
Coatings 2023, 13(10), 1729; https://doi.org/10.3390/coatings13101729
Submission received: 24 August 2023 / Revised: 27 September 2023 / Accepted: 2 October 2023 / Published: 4 October 2023
(This article belongs to the Section Corrosion, Wear and Erosion)
Browse Figures
Versions Notes

Abstract

:
This study aims to enhance the practical performance of PVDF/ZnO and PVDF/TiO2 composite coatings known for their distinctive properties. The coatings, applied through spray coating with PVDF and ZnO or TiO2 nanoparticles on glass, steel, and aluminum substrates, underwent a comprehensive evaluation. Surface wetting properties and morphology were respectively evaluated using a technique involving liquid droplets and an imaging method using high-energy electrons. Potentiodynamic polarization was used to compare corrosion resistance between coated and bare substrates. Nanoindentation was used to assess coating hardness, and bonding strength was subsequently quantified. The results revealed that PVDF/ZnO composite coatings had higher water contact angles (161 ± 5° to 138 ± 2°) and lower contact angle hysteresis (7 ± 2° to 2 ± 1°) compared to PVDF/TiO2 and PVDF coatings. Moreover, corrosion tests demonstrated superior protection for steel and aluminum surfaces coated with superhydrophobic PVDF/ZnO. Nanoindentation indicated enhanced mechanical properties with TiO2 nanoparticles, with adhesion results favoring TiO2 over ZnO nanoparticles.

1. Introduction

Over the past few years, the focus on creating materials with combined superhydrophobic and corrosion-resistant attributes has garnered substantial attention, driven by their potential utility in a wide range of sectors, including biomedical, electronics, and energy industries [1,2,3]. Among the different materials, polymer-based nanocomposites have shown great potential for achieving both superhydrophobicity and other desirable properties such as corrosion resistance [4,5].
Superhydrophobic polymer composite materials have been the subject of extensive research due to their unique properties and potential applications in various fields such as self-cleaning surfaces, anti-icing, and drag reduction [6,7]. These materials possess high water repellency, with water droplets easily rolling off their surfaces, leaving no trace [8]. The superhydrophobicity of these materials is attributed to their hierarchical structures, which are typically composed of micro- and nanoscale features [9].
The fabrication of superhydrophobic polymer composite materials involves incorporating hydrophobic additives such as nanoparticles, polymers, or fluorinated compounds into the polymer matrix [10]. Research in this field has focused on developing new synthesis methods and improving the properties of existing materials. For instance, incorporating zinc oxide nanoparticles into nanocosmposite coatings has been shown to improve their corrosion resistance [11]. In addition, various methods such as electrospinning, layer-by-layer assembly, chemical etching, electrospinning, plasma treatment, and sol–gel techniques have been utilized to fabricate superhydrophobic polymer composites with unique properties and structures [1,2,3,12]. There are various types of polymer-based nanocomposites that can be used to achieve superhydrophobic and corrosion-resistant properties, such as polyurethane [13], polystyrene [14], polyethylene [15], and poly(methyl methacrylate) [16]. While these materials have shown promising results in achieving superhydrophobicity and corrosion resistance, another polymer, polyvinylidene fluoride (PVDF), is also a good candidate as a polymer-based composite. PVDF is a semi-crystalline thermoplastic fluoropolymer that is relatively easy to include in coating processes. PVDF-based nanocomposites have been shown to exhibit excellent superhydrophobicity, high mechanical strength, thermal stability, and chemical resistance, making them ideal for various applications including self-cleaning surfaces, anti-icing coatings, and marine applications [17,18]. The incorporation of inorganic nanoparticles such as titanium dioxide (TiO2), silica (SiO2), cerium dioxide (CeO2), graphene oxide (GO), zinc oxide (ZnO), and alumina (Al2O3) into the PVDF matrix can enhance its superhydrophobicity and corrosion resistance properties [11,19,20,21,22,23].
TiO2 and ZnO are well-known metal oxide nanoparticles with excellent mechanical, electrical, and optical properties [24]. They are also biocompatible and environmentally friendly, making them attractive materials for various applications. In addition to their unique properties, the large surface area and high aspect ratio of these nanoparticles can provide roughness to the surface of the composite, which is a key factor for achieving superhydrophobicity [25,26,27,28].
The aim of this study is to investigate the effect of TiO2 and ZnO nanoparticles on the superhydrophobic and corrosion resistance properties of PVDF-TiO2 and PVDF-ZnO nanocomposites through analyzing the surface morphology, contact angle, and corrosion resistance.

2. Experimental Procedure

2.1. Starting Materials

The materials used in the study included polyvinylidene fluoride (PVDF) and ZnO and TiO2 nanoparticles (100–200 nm particle size) provided by Sigma-Aldrich (Hamburg, Germany). The solvents used in the study included N, N-dimethylformamide (DMF), hexane, and stearic acid, which were supplied by the Al-SAFWA Center, Egypt.

2.2. Samples Preparation

The steel, aluminum, and glass substrates underwent an initial cleaning process, involving ultrasonic treatment in acetone for 8 min followed by a similar treatment in distilled water for 8 min. Subsequently, they were dried using a stream of air before their utilization. To achieve a uniform mixture, 5 g of PVDF was dissolved in DMF at 50 °C and agitated at 600 rpm for a duration of 120 min. In each experiment, varying concentrations of TiO2 or ZnO nanoparticles (1%, 1.5%, or 2%) were individually introduced into a hexane solution that included stearic acid. The mixture was agitated at 500 rpm for a duration of 120 min to achieve an even dispersion of nanoparticles within the solution. After obtaining the homogeneous solution of PVDF, the dispersion solution of either ZnO or TiO2 nanoparticles was introduced into the PVDF solution to produce the composite coating. The intricate solutions were thoroughly blended using a magnetic stirrer and gently heated to produce the ultimate solutions. These solutions were then applied to the prepared surfaces using a spray coating method, resulting in the creation of a uniform coating. Subsequently, the coated substrates were exposed to a drying treatment within a furnace set at 80 °C for a period of 20 min. Table 1 provides a summary of the experimentation conditions for both ZnO and TiO2, while Table 2 presents the chemical compositions of the various steel, aluminum, and glass substrates.

2.3. Characterization Techniques

The study used various techniques to evaluate the properties of the coatings on steel, aluminum, and glass substrates. The contact angle (CA) and contact angle hysteresis (CAH) were determined at room temperature using the Attension Biolin device (Model: Theta Optical Tensiometers, manufactured in Helsinki, Finland) with distilled water droplets on the coated substrates. Measurements were collected from at least five separate locations, and the average value was then calculated for each sample. The corrosion performance of both untreated and coated substrates was assessed by analyzing potentiodynamic polarization curves using a Potentiostat/Galvanostat (Model: VersaSTAT 3, AMETEK GmbH, Hadamar-Steinbach, Weiterstadt, Germany) in 3.5 wt% NaCl solutions at room temperature. To evaluate the adhesion strength between the coating and substrates, the DeFelsko Digital Pull-off Adhesion tester was utilized (Model: PosiTest AT-M, manufactured by DeFelsko Corporation, Shandong, Beijing, China). The prepared samples were analyzed for their morphology and chemical composition using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) conducted with a JEOL JSM-IT200 instrument from Tokyo, Japan. To prepare the samples for SEM and EDS analysis, they underwent vacuum drying and were subsequently sputter-coated with a layer of gold (Au). The indentation hardness was determined by utilizing the nanoindentation method (Nano indenter model: G200, KLA Corporation, Milpitas, CA, USA) with a pyramidal-shaped Berkovich indenter, applying a force of 0.15 mN for PVDF, PVDF/ZnO, and PVDF/TiO2 coatings.

3. Results and Discussion

3.1. Analysis of the Wettability Properties of Nanoparticle-Coated Surfaces

Contact angles serve as crucial indicators of solid surface energy, representing distinct coefficients of liquid/solid systems. In this regard, the WCA values were measured for different substrates coated with ZnO or TiO2 nanoparticles in PVDF polymer. The study also evaluated contact angle hysteresis (CAH) under optimal conditions. CAH is a measure of the disparity between the advancing and receding contact angles of a liquid droplet on a solid surface. It offers insights into the surface’s capacity to either retain or repel liquid, where lower CAH values signify a more stable and repellent surface. The application of fluorinated methyl groups as a coating on flat solid surfaces leads to a WCA of merely 120°, indicating a marginal level of superhydrophobicity [29]. However, the application of ZnO and TiO2 nanoparticles to the solid surfaces creates superhydrophobicity, as evidenced by the significantly high WCA values obtained in this study.
Table 3 displays the outcomes of WCA and WCAH measurements for diverse samples containing various nanoceramic particles within PVDF applied onto steel, aluminum, and glass substrates. Surface wettability is a measure of how easily a liquid spreads or beads up on it, and the WCA and WCAH values respectively provide information about the surface energy and its heterogeneity.
Samples 1–3 represent the bare steel, aluminum, and glass substrates, respectively. The WCA values for these samples range from 51° to 71°, indicating moderate to low wettability. The pinned WCAH values suggest that the surfaces are relatively homogeneous, with minimal variations in surface energy. Samples 4–6 represent the PVDF polymer alone coated on the same substrates as samples 1–3. The addition of the polymer increases the WCA values to above 90°, indicating a high degree of hydrophobicity. The corresponding WCAH values for these samples range from 25° to 38°, indicating that the polymer coating creates a more heterogeneous surface energy distribution. Samples 7–15 correspond to the incorporation of ZnO nanoparticles into the PVDF polymer. As the amount of ZnO increases, the WCA values also increase, with the highest WCA value of 161° observed for sample 15. The WCAH values decrease as the amount of ZnO increases, indicating a more homogeneous surface energy distribution. These results suggest that the addition of ZnO nanoparticles to PVDF enhances its hydrophobicity and creates a more homogeneous surface. Samples 25–33 signify the introduction of TiO2 nanoparticles into the PVDF polymer. The WCA values for these samples range from 126° to 142°, with the highest value observed for sample 32 on Al substrates. The WCAH values for these samples range from 2° to 7°, with the highest value observed for sample 32. Similar to the addition of ZnO particles, incorporating TiO2 nanoparticles into PVDF enhances its hydrophobicity and results in a more uniform distribution of surface energy.
Based on the results presented in Table 3 and shown in Figure 1, Figure 2 and Figure 3, ZnO nanoparticles show better wettability properties compared to TiO2 nanoparticles. For example, the maximum WCA achieved with ZnO coatings was 154 ± 1° for steel, 160 ± 1° for aluminum, and 161 ± 1° for glass, while the maximum WCA achieved with TiO2 coatings was 140 ± 1° for steel, 142 ± 1° for aluminum, and 141 ± 2° for glass. Furthermore, the WCAH values for ZnO coatings on all substrates exhibited a range of 4 ± 1° to 6 ± 1°, whereas the WCAH values attained with TiO2 coatings on all substrates were within the range of 1° to 2 ± 1°. This suggests that ZnO coatings have a better self-cleaning ability, as a higher WCAH value indicates a lower tendency for liquid droplets to stick to the surface. Furthermore, previous studies have also reported similar findings. The presence of minuscule surface irregularities at the nanoscale increased in terms of quantity as the concentration of ZnO nanoparticles rose. This resulted in heightened surface roughness of the film. These minute surface irregularities acted as air entrapments, giving rise to small air pockets situated between the water droplet and the gaps amidst these surface features. Therefore, this constrained the region of interaction between the water droplet and the solid surface, ultimately diminishing the contact between the liquid and solid, as discussed in [30]. This outcome is somewhat connected to the well-established Cassie–Baxter theory [31].
In accordance with the Cassie–Baxter theory, the Young–Laplace pressure at the interfaces acts as an impediment, obstructing full contact between the liquid droplet and the solid surface. Consequently, minute surface irregularities capture air, creating a stable three-phase interface involving solid, air, and liquid. To quantify the proportions of solid surface (f1) and air (f2) in contact with the liquid on both smooth and rough surfaces [32,33], we employed a formula correlating water contact angles (WCA) on these surfaces (θ and θ′, respectively). Within the intricate micro- and nanostructures of ZnO on the rough surface, the presence of trapped air played a pivotal role in achieving superhydrophobic properties in PVDF nanocomposites. This resulted in a higher proportion of trapped air and, consequently, an increased WCA. Comparing the WCAs of the flat PVDF film (∼90°) and the PVDF nanocomposite (∼154°) on steel, it was found that f2 equaled 0.90. Similarly, for Al substrates exhibiting a WCA of (∼160°), f2 was calculated to be 0.94.
Likewise, for TiO2, trapped air within the rough micro- and nanostructures primarily led to the superhydrophobic properties of PVDF nanocomposites. As a result, a larger volume of trapped air was achieved, leading to an increased WCA. Comparing the WCAs of the smooth PVDF film (∼90°) and the PVDF nanocomposite (∼140°) on steel, the value of f2 was determined to be 0.77.
There are several reasons why there is a difference in the superhydrophobicity of ZnO and TiO2. First, the surface morphology of the two materials is different. ZnO has a hierarchical micro/nanostructure, while TiO2 has a rough surface at the nanoscale. This disparity in morphology results in variations in the entrapment of air pockets between the water droplet and the surface. The irregularities on the ZnO surface are more efficient at capturing air, resulting in a greater proportion of trapped air and, consequently, a higher WCA. Second, the chemical composition of the materials also plays a role. ZnO has a higher surface energy than TiO2, which can lead to a stronger interaction between the solid surface and the liquid droplet. This interaction can result in an elevation of the contact angle, consequently enhancing superhydrophobicity. Third, the crystalline structure of the materials can also contribute to the difference in superhydrophobicity. ZnO has a wurtzite structure, while TiO2 has a rutile or anatase structure. The difference in structure can affect the surface energy and morphology of the materials, leading to differences in superhydrophobicity. Overall, the results suggest that the addition of ZnO and TiO2 nanoparticles to PVDF polymer enhances its hydrophobicity and creates a more homogeneous surface energy distribution. ZnO nanoparticles offer better wettability properties compared to TiO2 nanoparticles. These attributes have practical advantages in applications like self-cleaning surfaces, anti-fog coatings, and corrosion prevention. The results also demonstrate the importance of understanding the surface properties of coatings in optimizing their performance.

3.2. Surface Morphology Analysis

The surface characteristics of the generated coatings were examined by SEM at multiple magnification levels, revealing several significant attributes. At high magnification, the surface microstructure of the hydrophobic coating, depicted in Figure 4 and Figure 5, has sponge-like structures that suggest the presence of PVDF polymer as the primary constituent of the coating film [34,35]. The PVDF sample shows a uniform surface structure with minor roughness, which is easily distinguishable. Furthermore, the surface of the coating contains several relatively large microvoids. EDS measurements revealed that the PVDF polymer mainly consists of two peaks of fluorine (F) and carbon (C). The presence of these elements within the coating serves as confirmation that the film is composed of PVDF polymer. These findings are crucial for understanding the properties and behavior of the coating. The sponge-like structure found in the hydrophobic PVDF coating is known to enhance the WCA on the surface, which in turn increases its hydrophobicity. The presence of microvoids can also affect the physicochemical properties of the coating by influencing its porosity, adhesion, and durability. Therefore, a thorough understanding of the morphology and composition of the coating is necessary for designing and optimizing its performance in various applications.
Furthermore, the study investigated the impact of adding ZnO nanoparticles on the morphology of the composite coatings. Figure 4a–c demonstrate a substantial morphological change resulting from the incorporation of 1.5% ZnO nanoparticles, as evident from the bright areas. The composite coatings containing ZnO particles were observed to have micro/nanoscale structured roughness on surfaces, composed of ZnO particles with various morphologies, predominantly agglomerates due to their smaller size. Figure 4e provides a visual representation of how the nano-sized structures generated by the ZnO particles effectively trap air, resulting in the formation of air pockets between the water droplet and the gaps within the grooves. This structural arrangement restricts the degree of contact between the water droplet and the solid surface, consequently reducing the area of interaction between the liquid and the solid. This micro/nanoscale configuration plays a crucial role in enhancing the superhydrophobic properties of the films, as previously discussed in the WCA analysis. The films exhibited a static WCA of 154 ± 1° and a WCAH of 4 ± 1°, and these properties can be attributed to the micro/nanoscale structure. Furthermore, the EDS analysis of the coating material, which contains PVDF + 1.5% ZnO (point 1 in Figure 4c), confirmed the presence of carbon and fluorine within the PVDF structure, as well as zinc (Zn) and oxygen (O) from the incorporated ZnO. Table 4 provides a summary of the elemental analysis of the coated surfaces using EDS.
Conversely, as depicted in Figure 5, the introduction of TiO2 nanoparticles also induced alterations in the morphology of the PVDF coatings. The composite coatings comprised aggregates of TiO2 particles and surfaces with small-scale structured roughness created using TiO2 particles that exhibited various shapes, although primarily forming agglomerates. However, the size of many particles was found to be more than 100 nm, which could explain the relatively low WCA of such surfaces. The large size of these nanoparticles formed larger aggregates, which increased the solid fraction contacting the water droplet and decreased the air pocket fraction that should carry the droplet to cause superhydrophobicity. To reiterate, this micro-scale structure enhances the films’ hydrophobic properties, as demonstrated by their static WCA of 141 ± 2° and a minimal WCAH of 2 ± 1°. The EDS analysis, shown in Table 4, showed the presence of carbon and fluorine in PVDF and titanium (Ti) and oxygen (O) in the integrated TiO2.
In summary, the findings in this section suggest that incorporating ZnO and TiO2 nanoparticles into the PVDF polymer alters the coating’s morphology and contributes to the attainment of superhydrophobicity. The ZnO particles mainly form agglomerates, whereas the TiO2 particles mostly form aggregates. The smaller size of ZnO particles compared to TiO2 particles leads to the creation of additional air pockets between the water droplet and the surface gaps. As a result, the contact area between the liquid and solid is reduced, facilitating the achievement of superhydrophobicity in the films. These films exhibit a high static WCA and a low WCAH. The differences in the structure of composite coatings containing PVDF + 1.5% ZnO and PVDF + 2% TiO2 can be attributed to variations in the size and shape of the nanoparticles.

3.3. Corrosion Analysis

The corrosion performance of PVDF and PVDF + ZnO or TiO2 coatings was examined using Tafel polarization curves. Figure 6 and Figure 7 present Tafel polarization plots that illustrate the corrosion performance of steel and aluminum surfaces under different coatings (bare, PVDF-coated, and composite-coated) when immersed in a 3.5% sodium chloride (NaCl) solution. The scanning rate used was 2 millivolts per second (mVs−1) for both steel and aluminum substrates. Furthermore, Table 5 furnishes a comprehensive Tafel analysis of the polarization curves, providing valuable insights into the corrosion behavior of the diverse coatings and substrates within the specified environment.
The corrosion current density for steel exhibited a significant reduction, decreasing from 37.75 µA/cm² in bare steel to 3.34 µA/cm² when coated with PVDF, further decreasing to 1.02 µA/cm² in the case of (PVDF + 1.5% ZnO)-coated steel. Similarly, for aluminum, the corrosion current density notably decreased from 8.26 µA/cm² for untreated aluminum to 1.3 µA/cm² with a PVDF coating, further decreasing to 1.04 µA/cm² when coated with (PVDF + 1.5% ZnO). The protective efficiency of the steel coating increased from 91% with PVDF alone to 97% with the improved (PVDF + 1.5% ZnO) coating. Similarly, the protective efficiency of aluminum rose from 84% with solely a PVDF coating to 87% with the (PVDF + 1.5% ZnO) coating. Moreover, the coatings’ ability to protect the substrates was quantified by measuring polarization resistance (Rp), which is a measure of the coating’s ability to resist ion transfer through the coating. The Rp of the (PVDF + 1.5% ZnO)-coated steel exhibited approximately sevenfold improvement compared to the PVDF-coated steel.
Similarly, in the case of aluminum substrates, the Rp of aluminum coated with (PVDF + 1.5% ZnO) was roughly three times higher than that of aluminum coated exclusively with PVDF. These results suggest that both steel and aluminum, when shielded by the (PVDF + 1.5% ZnO) nanocomposite coating, display significantly reduced vulnerability to corrosion in comparison to when they are coated solely with PVDF. The incorporation of ZnO nanoparticles into the PVDF polymer resulted in the filling of gaps on the polymer surface, effectively obstructing the penetration of aggressive ions into the nanocomposite coating. As a result, aggressive ions, such as Cl-, were unable to reach the substrate surface. This contrasts with coatings that used PVDF alone, which had wider pores with lower pore resistance and allowed ion ingress.
On the other hand, in the case of steel substrates, the corrosion current density was significantly reduced when coated with PVDF alone or PVDF modified with 2% TiO2 (from 37.75 µA/cm² in bare steel to 3.34 and 2.91 µA/cm², respectively). This represents an improvement in corrosion resistance of over 91% with coatings containing PVDF alone and 92.3% with the modified nanocomposite coating (PVDF + 2% TiO2). For aluminum substrates, corrosion current density dropped from 8.26 µA/cm² in bare aluminum to 1.3 µA/cm² with the PVDF coating and 0.66 µA/cm² with the (PVDF + 2% TiO2) coating. This represents an improvement in corrosion resistance of 84% with the coating containing PVDF alone and 92% with the modified nanocomposite coating (PVDF + 2% TiO2). The Rp for the PVDF + 2% TiO2-coated steel is about five times greater than that for the steel coated with PVDF alone. Likewise, in the case of aluminum substrates, the Rp of the PVDF + 2% TiO2-coated aluminum is three times higher than that of the aluminum coated with PVDF alone. This suggests that the superhydrophobic PVDF + 2% TiO2 nanocomposite coating significantly improves the corrosion resistance of both steel and aluminum substrates, making them less susceptible to corrosion. Overall, the addition of the PVDF and PVDF modified with 2% TiO2 coatings to steel and aluminum substrates significantly improves their corrosion resistance, as demonstrated by the reduction in the corrosion current density and the increase in polarization resistance. The reasons for the improved corrosion resistance of PVDF + ZnO and PVDF + TiO2 coatings can be attributed to the ability of the nanoparticles to act as a barrier and inhibit the corrosion process, as well as their ability to enhance the adhesion and mechanical properties of the coating. The presence of the nanoparticles can also increase the density and decrease the porosity of the coating, which can improve its corrosion resistance. The modified nanocomposite coating (PVDF + 2% TiO2) shows superior performance compared to PVDF alone, indicating the potential for the development of new and improved coating technologies for corrosion protection.

3.4. Nanohardness Analysis

The mechanical properties of the coatings were evaluated by performing nanoindentation tests on coatings containing PVDF alone and those containing 1.5% ZnO or 2% TiO2 nanoparticles. The results demonstrated a noteworthy enhancement in the hardness and elastic modulus of the coatings upon the incorporation of both ZnO and TiO2 nanoparticles. Figure 8 visually shows the reduced penetration depth of the indenter in PVDF coatings after adding ZnO and TiO2 nanoparticles, highlighting their impact.
Table 6 provides an overview of the mechanical properties of the coatings, highlighting the substantial impact of incorporating 1.5% ZnO nanoparticles. This addition notably increased the elastic modulus, raising it from 44.4 MPa in PVDF-only coatings to 919.4 MPa. Furthermore, the hardness of PVDF coatings alone exhibited a significant boost, surging from 10.1 MPa to 137.5 MPa. The notable hardness increase can be credited to the substantial hardness of ZnO nanoparticles, significantly improving the mechanical properties of PVDF coatings. Conversely, the incorporation of 2% TiO2 nanoparticles also led to a significant rise in the elastic modulus, elevating it from 44.4 MPa in PVDF-only coatings to 1.036 GPa. Likewise, the hardness of coatings containing PVDF alone experienced a substantial increase, escalating from 10.1 MPa to 264 MPa. This indicates that TiO2 nanoparticles can also enhance the mechanical properties of PVDF coatings to a greater extent than ZnO nanoparticles.
The reason for this is that TiO2 nanoparticles may have a higher surface energy than ZnO nanoparticles, which allows them to form stronger chemical bonds with the PVDF matrix. This, in turn, leads to a more uniform distribution of nanoparticles throughout the PVDF matrix, resulting in a more homogeneous composite material with superior mechanical properties. In addition, the crystal structure of TiO2 nanoparticles allows for stronger interparticle interactions, which also contributes to the improvement in the mechanical properties of the composite coatings. Overall, the results of the nanoindentation tests show that the addition of ZnO and TiO2 nanoparticles can significantly improve the mechanical properties of PVDF coatings, with TiO2 nanoparticles having a greater effect than ZnO nanoparticles due to their higher surface energy and crystal structure.

3.5. Adhesion Analysis

Figure 9 and Figure 10 present the adhesion force results of PVDF coatings with different concentrations of ZnO and TiO2 nanoparticles on various substrates, including steel and aluminum (Al). For PVDF coatings without any nanoparticles (PVDF alone), the adhesion force values are 105 MPa for steel and 95 MPa for Al. The strong adhesion in the coating results from PVDF macromolecules forming cross-links through C–C or C=C functional groups during drying [36]. However, the addition of ZnO and TiO2 nanoparticles results in reduced adhesion compared to coatings with PVDF alone. For steel (Figure 9a), there is an initial 9% decrease in adhesion when adding 1% ZnO to PVDF and an overall 34% decrease for 2% ZnO compared to the pure PVDF. TiO2 addition (Figure 9b) shows the same trend, with a 26% decrease in adhesion for 1% TiO2 and a 52% overall decrease for the 2% TiO2 compared to the pure PVDF alone. Adhesion on aluminum substrates also decreased with the addition of nanoparticles to PVDF. The addition of ZnO to PVDF (Figure 10a) resulted in an overall decrease of 29% for 2% ZnO and 48% for 2% TiO2 coatings (Figure 10b). The addition of TiO2 nanoparticles to PVDF provided poorer adhesion compared to ZnO addition on both substrates.
These results suggest that the addition of TiO2 nanoparticles tends to have a more significant impact on the adhesion force of PVDF coatings compared to ZnO nanoparticles. As also shown in this study, PVDF/ZnO coatings did have better wettability and corrosion protection than the PVDF/TiO2 coatings as well. Furthermore, the adhesion force results may also be influenced by the interaction between the nanoparticles and the substrate, as well as the polymer matrix. For example, the type of substrate could affect the adhesion force values due to differences in surface properties and interactions with nanoparticles. Similarly, the type of nanoparticle (ZnO, TiO2) and its concentration could also affect the adhesion force values due to differences in size, shape, and surface chemistry, which in turn influence the interfacial interactions with the polymer matrix.
Overall, the adhesion force results suggest the addition of ZnO and TiO2 nanoparticles to PVDF coatings can influence the adhesion force values, with TiO2 nanoparticles generally showing a more significant impact compared to ZnO nanoparticles. However, further investigation is required for a comprehensive understanding of the underlying mechanisms and optimization of nanoparticle concentration to achieve the desired adhesion properties in PVDF coatings. Evidence from previous published work, as mentioned above, supports these findings and provides insights into the effects of nanoparticles on the adhesion force of polymer coatings.

4. Conclusions

In light of the earlier discussion, the following conclusions can be made:
  • Adding ZnO and TiO2 nanoparticles to PVDF polymer increases its hydrophobicity and surface energy distribution. ZnO has better wettability properties than TiO2 and is useful for self-cleaning surfaces, anti-fog coatings, and corrosion protection.
  • Adding ZnO and TiO2 nanoparticles changes the morphology of PVDF coatings and helps achieve superhydrophobicity. The difference in morphology is due to the size and shape of the nanoparticles.
  • ZnO nanoparticles fill gaps at the surface of PVDF coatings, which hinders the entry of aggressive ions and improves corrosion resistance. Nanoparticles act as a barrier and enhance the adhesion, mechanical properties, density, and porosity of coatings.
  • The addition of both ZnO and TiO2 nanoparticles significantly enhances the mechanical properties of PVDF coatings. Nanoindentation tests showed that the coatings containing 1.5% ZnO or 2% TiO2 exhibited increased hardness and elastic modulus compared to coatings with PVDF alone.
  • Adding TiO2 nanoparticles has a more significant impact on the adhesion force of PVDF coatings than ZnO nanoparticles. Changes in surface properties, such as surface roughness and energy, cause a decrease in adhesion force with increasing nanoparticle concentration.

Author Contributions

Conceptualization, A.M.A.M. and T.D.G.; methodology, H.H., A.M.A.M. and E.A.; software, M.M.E.-S.S. and P.R.M.; validation, A.I.A., W.H.E.-G. and E.A.; formal analysis M.M.E.-S.S. and P.R.M.; investigation, A.M.A.M., W.H.E.-G. and T.D.G.; resources, E.A. and A.M.A.M.; data curation, H.H. and A.I.A.; writing—original draft preparation, H.H.; writing—review and editing, A.M.A.M. and T.D.G.; visualization, H.H. and E.A.; supervision, A.M.A.M.; project administration, E.A.; funding acquisition, A.I.A. and W.H.E.-G. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All of the raw data supporting the conclusion of this paper were provided by the authors.

Acknowledgments

The researchers would like to thank the Deanship of Scientific Research, Qassim University, for funding the publication of this project.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 13 01729 g001
Figure 1. Wettability (average WCA and WCAH) of bare, PVDF-coated, and (PVDF + 1, 1.5 and 2% ZnO/TiO2)-coated steel substrates (samples no. 1, 4, 7, 25, 10, 28, 13, and 31).
Figure 1. Wettability (average WCA and WCAH) of bare, PVDF-coated, and (PVDF + 1, 1.5 and 2% ZnO/TiO2)-coated steel substrates (samples no. 1, 4, 7, 25, 10, 28, 13, and 31).
Coatings 13 01729 g001
Coatings 13 01729 g002
Figure 2. Wettability (average WCA and WCAH) of bare, PVDF-coated, and (PVDF + 1, 1.5 and 2% ZnO/TiO2)-coated Al substrates (samples no. 2, 5, 8, 26, 11, 29, 14, and 32).
Figure 2. Wettability (average WCA and WCAH) of bare, PVDF-coated, and (PVDF + 1, 1.5 and 2% ZnO/TiO2)-coated Al substrates (samples no. 2, 5, 8, 26, 11, 29, 14, and 32).
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Coatings 13 01729 g003
Figure 3. Wettability (average WCA and WCAH) of bare, PVDF-coated, and (PVDF + 1, 1.5, and 2% ZnO/TiO2)-coated glass substrates (samples no. 3, 6, 9, 27, 12, 30, 15, and 33).
Figure 3. Wettability (average WCA and WCAH) of bare, PVDF-coated, and (PVDF + 1, 1.5, and 2% ZnO/TiO2)-coated glass substrates (samples no. 3, 6, 9, 27, 12, 30, 15, and 33).
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Coatings 13 01729 g004aCoatings 13 01729 g004b
Figure 4. (a–c) SEM images of PVDF composites with 1.5% ZnO at various magnifications, (d) EDS analysis at point 1 in (c), and (e) ZnO nanoparticle size observed in SEM.
Figure 4. (a–c) SEM images of PVDF composites with 1.5% ZnO at various magnifications, (d) EDS analysis at point 1 in (c), and (e) ZnO nanoparticle size observed in SEM.
Coatings 13 01729 g004aCoatings 13 01729 g004b
Coatings 13 01729 g005aCoatings 13 01729 g005bCoatings 13 01729 g005c
Figure 5. (a–c) SEM images of PVDF composites with 2% TiO2 at various magnifications, (d) EDS analysis at point 1 in (c), and (e) TiO2 nanoparticle size observed in SEM.
Figure 5. (a–c) SEM images of PVDF composites with 2% TiO2 at various magnifications, (d) EDS analysis at point 1 in (c), and (e) TiO2 nanoparticle size observed in SEM.
Coatings 13 01729 g005aCoatings 13 01729 g005bCoatings 13 01729 g005c
Coatings 13 01729 g006
Figure 6. Tafel plots of bare steel, PVDF-coated steel, and coated steel with coatings of different (a) ZnO contents (1, 1.5, and 2%), and (b) TiO2 contents (1, 1.5, and 2%) in 3.5% NaCl solution (Scan rate = 2 mVs−1).
Figure 6. Tafel plots of bare steel, PVDF-coated steel, and coated steel with coatings of different (a) ZnO contents (1, 1.5, and 2%), and (b) TiO2 contents (1, 1.5, and 2%) in 3.5% NaCl solution (Scan rate = 2 mVs−1).
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Coatings 13 01729 g007
Figure 7. Tafel plots of bare Al, PVDF-coated Al, and coated Al with coatings of different (a) ZnO contents (1, 1.5, and 2%), and (b) TiO2 contents (1, 1.5, and 2%) in 3.5% NaCl solution (Scan rate = 2 mVs−1).
Figure 7. Tafel plots of bare Al, PVDF-coated Al, and coated Al with coatings of different (a) ZnO contents (1, 1.5, and 2%), and (b) TiO2 contents (1, 1.5, and 2%) in 3.5% NaCl solution (Scan rate = 2 mVs−1).
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Figure 8. Nanoindentation loading and unloading curve for (a) PVDF alone-coating, (b) PVDF composites containing 1.5% ZnO, and (c) PVDF composites containing 2% TiO2.
Figure 8. Nanoindentation loading and unloading curve for (a) PVDF alone-coating, (b) PVDF composites containing 1.5% ZnO, and (c) PVDF composites containing 2% TiO2.
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Figure 9. Adhesion force between steel substrates and coatings containing PVDF alone, (a) coatings with PVDF + (1, 1.5, and 2%) ZnO, and (b) coatings with PVDF + (1, 1.5, and 2%) TiO2.
Figure 9. Adhesion force between steel substrates and coatings containing PVDF alone, (a) coatings with PVDF + (1, 1.5, and 2%) ZnO, and (b) coatings with PVDF + (1, 1.5, and 2%) TiO2.
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Figure 10. Adhesion force between Al substrates and coatings with PVDF alone, (a) coatings with PVDF + (1, 1.5, and 2%) ZnO, and (b) coatings with PVDF + (1, 1.5, and 2%) TiO2.
Figure 10. Adhesion force between Al substrates and coatings with PVDF alone, (a) coatings with PVDF + (1, 1.5, and 2%) ZnO, and (b) coatings with PVDF + (1, 1.5, and 2%) TiO2.
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Table 1. Experimental conditions for steel, aluminum, and glass substrates with ZnO or TiO2 nanoparticles.
Table 1. Experimental conditions for steel, aluminum, and glass substrates with ZnO or TiO2 nanoparticles.
SubstrateConditionZnO Nanoparticles
(g/100 mL)		TiO2 Nanoparticles
(g/100 mL)
SteelBare--
PVDF1.0/1.5/2.01.0/1.5/2.0
AluminumBare--
PVDF1.0/1.5/2.01.0/1.5/2.0
GlassBare--
PVDF1.0/1.5/2.01.0/1.5/2.0
Table 2. Chemical composition of steel, aluminum, and glass substrates.
Table 2. Chemical composition of steel, aluminum, and glass substrates.
SteelCMnCuSiWAlVNbPSFe
0.1050.780.0840.0420.0400.0380.0380.0360.0330.018Bal.
AlSiFeMgMnTiVCuCrAl
0.2280.2680.820.0770.0150.0110.0090.0005Bal.
GlassSiO2Al2O3TiO2Cr2O3Fe2O3CaOMgONa2OK2OSO3
72.241.440.0350.0020.0711.500.3213.640.350.21
Table 3. Surface wettability of nanoceramic coatings on different substrates (n = 5 readings).
Table 3. Surface wettability of nanoceramic coatings on different substrates (n = 5 readings).
Sample No.Nanoceramic
Particles		Amount (g)SubstrateWCA (deg.)WCAH (deg.)
1Bare-Steel54 ± 3Pinned
2Bare-Al71 ± 3Pinned
3Bare-Glass51 ± 3Pinned
4PVDF alone5Steel90 ± 238 ± 3
5PVDF alone5Al91 ± 235 ± 2
6PVDF alone5Glass93 ± 225 ± 2
7ZnO1Steel138 ± 22 ± 1
8ZnO1Al157 ± 24 ± 1
9ZnO1Glass144 ± 25 ± 2
10ZnO1.5Steel154 ± 14 ± 1
11ZnO1.5Al160 ± 15 ± 1
12ZnO1.5Glass156 ± 16 ± 1
13ZnO2Steel149 ± 16 ± 1
14ZnO2Al154 ± 35 ± 1
15ZnO2Glass161 ± 57 ± 2
25TiO21Steel131 ± 22 ± 1
26TiO21Al126 ± 13 ± 1
27TiO21Glass137 ± 24 ± 1
28TiO21.5Steel128 ± 13 ± 1
29TiO21.5Al126 ± 24 ± 1
30TiO21.5Glass127 ± 13 ± 1
31TiO22Steel140 ± 12 ± 1
32TiO22Al142 ± 17 ± 1
33TiO22Glass141 ± 23 ± 2
Table 4. Surface element composition analysis of PVDF Coatings with ZnO and TiO2 additives on Al substrate using EDS.
Table 4. Surface element composition analysis of PVDF Coatings with ZnO and TiO2 additives on Al substrate using EDS.
Coating TypeC
(Mass %)		F
(Mass %)		O
(Mass %)		Zn
(Mass %)		Ti
(Mass %)
PVDF alone47.6552.35---
PVDF + 1.5% ZnO39.3031.2211.7717.71-
PVDF + 2% TiO254.928.3030.95-5.83
Table 5. The corrosion performance of different coatings on steel and aluminum substrates.
Table 5. The corrosion performance of different coatings on steel and aluminum substrates.
SubstrateCoatingβa (mV)βc (mV)Rp (K Ω cm2)CR Rate (mpy)i_corr (µA/cm2)η (%)
SteelBare1171390.71737.75-
PVDF7319870.243.3491
PVDF + 1.5% ZnO156371470.0461.0297
PVDF + 2% TiO2401589360.212.9192.3
AlBare5412223.548.26-
PVDF93865280.081.384
PVDF + 1.5% ZnO432373830.071.0487
PVDF + 2% TiO2510163820.050.6692
Table 6. Mechanical properties of coatings containing PVDF alone and coatings containing PVDF with ZnO and TiO2 nanoparticles.
Table 6. Mechanical properties of coatings containing PVDF alone and coatings containing PVDF with ZnO and TiO2 nanoparticles.
SampleHardness
(MPa)		Elastic Modulus
(MPa)		Maximum Depth
(nm)
PVDF alone10.144.41360.6
PVDF + 1.5% ZnO137.5919.4314.8
PVDF + 2% TiO2264.01036.1305.4
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Mohamed, A.M.A.; Alateyah, A.I.; Hasan, H.; Matli, P.R.; El-Sayed Seleman, M.M.; Ahmed, E.; El-Garaihy, W.H.; Golden, T.D. Enhanced Corrosion Resistance and Surface Wettability of PVDF/ZnO and PVDF/TiO2 Composite Coatings: A Comparative Study. Coatings 2023, 13, 1729. https://doi.org/10.3390/coatings13101729

AMA Style

Mohamed AMA, Alateyah AI, Hasan H, Matli PR, El-Sayed Seleman MM, Ahmed E, El-Garaihy WH, Golden TD. Enhanced Corrosion Resistance and Surface Wettability of PVDF/ZnO and PVDF/TiO2 Composite Coatings: A Comparative Study. Coatings. 2023; 13(10):1729. https://doi.org/10.3390/coatings13101729

Chicago/Turabian Style

Mohamed, Adel M. A., Abdulrahman I. Alateyah, Hosam Hasan, Penchal Reddy Matli, Mohamed M. El-Sayed Seleman, Essam Ahmed, Waleed H. El-Garaihy, and Teresa D. Golden. 2023. "Enhanced Corrosion Resistance and Surface Wettability of PVDF/ZnO and PVDF/TiO2 Composite Coatings: A Comparative Study" Coatings 13, no. 10: 1729. https://doi.org/10.3390/coatings13101729

APA Style

Mohamed, A. M. A., Alateyah, A. I., Hasan, H., Matli, P. R., El-Sayed Seleman, M. M., Ahmed, E., El-Garaihy, W. H., & Golden, T. D. (2023). Enhanced Corrosion Resistance and Surface Wettability of PVDF/ZnO and PVDF/TiO2 Composite Coatings: A Comparative Study. Coatings, 13(10), 1729. https://doi.org/10.3390/coatings13101729

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