Enhanced Corrosion Protection of Epoxy/ZnO-NiO Nanocomposite Coatings on Steel

ZnO-NiO nanocomposite with epoxy coating on mild steel has been fabricated by the sol–gel assisted method. The synthesized sample was used to study corrosion protection. The analysis was performed by electrochemical impedance spectroscopy in 3.5% NaCl solution. The structural and morphological characterization of the metal oxide nanocomposite was carried out using XRD and SEM with Energy Dispersive Absorption X-ray (EDAX) analysis. XRD reveals the ZnO-NiO (hexagonal and cubic) structure with an average ZnO-NiO crystallite size of 26 nm. SEM/EDAX analysis of the ZnO-NiO nanocomposite confirms that the chemical composition of the samples consists of: Zn (8.96 ± 0.11 wt.%), Ni (10.53 ± 0.19 wt.%) and O (80.51 ± 3.12 wt.%). Electrochemical Impedance Spectroscopy (EIS) authenticated that the corrosion resistance has improved for the nanocomposites of ZnO-NiO coated along with epoxy on steel in comparison to that of the pure epoxy-coated steel.


Introduction
At the moment stainless steel can play an imperative role in a human lifetime by being used in applications like cars and chemical factories. Mild steel has been chosen because it has good mechanical properties but at the same time, can prevent the corrosion process. Corrosion can be the main reason for industrial accidents. Corrosion attacks the material by interaction with the environment [1]. Corrosive elements such as water, ions, and oxygen are competent at exceeding all coatings of a polymer. Epoxy resin is employed to protect the steel in most of the common polymers owing to its adhesion, chemical resistivity, mechanical and dielectric properties. Because of wear and abrasion of surface, metals with epoxy coatings are easily degraded. Pure epoxy coated metals exhibit extremely feeble corrosion resistance due to the convoluted cross-linked structure. Hence, various nanostructured fillers were employed to boost the defending properties of epoxy coatings.
Nanostructured materials are recognized for their exceptional physical and mechanical properties owing to their tremendously fine grain size and huge grain boundary volume. In past few years, bi and tri-metal oxide nanocomposites have been an area of vigorous integrative research, owing to their extensive technological purposes. Metal oxide nanocomposites provide the rewards of unique physical and chemical properties. The adherence of a composite coating was established to be imperative in the anticipation of corrosion and transfer in open circuit potential to the anodic site. The outstanding enhancement in the recital of these coatings has been linked with the enhancement of the barrier to dissemination, preclusion of charge transport by metal oxide nanomaterials [2].

Preparation of ZnO/NiO Nanocomposite
The ZnO and NiO nanoparticles were prepared by the sol-gel assisted method. A representative sol-gel assisted synthesis process was approved for the fabrication of a ZnO/NiO nanocomposite. An amount of 0.5 M of zinc acetate and 1 M of sodium hydroxide were dissolved in double distilled water (DD) to form solution A. A total of 0.5 M of nickel nitrate and 1 M of sodium hydroxide were dissolved in DD to form solution B. Then, solution B was included dropwise into the solution A. In order to achieve a homogenous solution, the prepared mixed solution was vigorously stirred for 5 h and made into a blackish gel. The resulting blackish gel was mixed with DD and centrifuged at 7000 rpm for 30 min followed by washing with DD and ethanol three times to remove the impurities. The black-colored product was kept on a hot plate for ignition and heated at 150 • C (six h). To acquire nanocrystalline powder, this was sintered at 400 • C for 4 h. A fine black colored powder was attained, and this was carefully collected for additional characterization purposes.

Preparation of Pure Epoxy
Epoxy resin (EM 9500) and curing agent (EM 9520) were mixed in the ratio of 3:1; this mixture was degassed in a bath sonicator. It was then coated into a steel substrate and permitted to dry for 24 h at room temperature.

Preparation of Epoxy/ZnO/NiO Nanocomposites
Amounts of 1 and 1.8%-ZnO/NiO nanopowder was dispersed in 5 mL chloroform using an ultrasonicator for 30 min. Then the solution was mixed with Epoxy resin and stirred overnight. The homogenous mixture was placed over the hot plate at 80 • C until the chloroform evaporated. After this, the mixture was degassed for 10 min and hardener was included in the mixture and mixed well. The adopted epoxy: the Hardener-mixing ratio was 3:1. Then the final mixture was coated on a steel substrate and dried at room temperature. The above synthesis steps were used to prepare Epoxy/ZnO-NiO composite samples with different concentrations of ZnO/NiO (1 and 2.5%).

Different Steps for the Preparation of Epoxy with Metal Oxide Composites
An amount of 0.06 g of ZnO-NiO powder was added to 5 mL of chloroform. The mixed solution was sonicated for 30 min. An amount of 3 mL of Resin was added to the sonicated solution under constant stirring for 24 h. After 24 h, 1 mL of Hardener was added in the above solution and stirred again for 1 h.

Corrosion Behavior of Steel Before and After the Coating
Corrosion is the interface between the metal and the environment, which will damage the properties of the metal itself. Interaction between the metal and oxygen can cause the formation of oxide layers. Electrochemical instruments can tell us if the materials that we coat in the metal can prevent corrosion or not. We tested the steel with a nanocomposite epoxy polymer in an electrochemical instrument with NaCl solution to test for electrical conductivity. The principle behind this being, the lower the current the current the better the protection from corrosion. We checked coated steel, and it showed that the epoxy prevented corrosion well. In addition, the nanocomposite played a good role in the prevention of corrosion.

Characterization
The fabricated ZnO-NiO nanocomposite was inspected via morphological and structural analysis. The crystallinity, crystal structure, physical parameters (lattice constant, crystallite size, and dislocation density) of the synthesized ZnO-NiO nanocomposite were analyzed by using X-ray diffraction analysis. The XRD pattern was examined by the X'PERT-Pro MPD, PANalytical Co., Almelo, Netherlands diffractometer equipped with CuKα radiation (1.5404 Å), using a step scanning mode and a tube voltage of 40 kV and a current of 15 mA. The scanning range varies between 10 • and 80 • , with the scanning rate (2 • /min) and sampling rate (0.04 • /min), varying correspondingly. Morphological studies were conducted by scanning electron microscopy (SEM). In the SEM analysis, the accelerating voltage was 5 kV and the working distance was attuned to approximately 6 mm. The contrast and brightness of the images were set to the most favorable values so that particles could be effortlessly distinguished from the background. Energy Dispersive Absorption X-ray (EDAX) analysis is a technique of elemental analysis connected to electron microscopy supported by the creation of characteristic X-rays that discloses the occurrence of elements present in the specimens. The SEM used has an in built EDAX (SEM, Hitachi S-4800, Hitachi, Tokyo, Japan).

Electrochemical Experiments
Initially the samples were coated on well-polished steel sheets using single-layer smart coatings (SLSCs). The electrochemical measurements were executed in a double-jacketed cell with the three-electrode system. Epoxy with ZnO-NiO nanocomposite was utilized as a working electrode. Ag/AgCl and graphite rods were employed as counter and reference electrode, respectively. All electrochemical experiments were carried out in 3.5 wt.% NaCl saturated solution with diverse concentrations of the prepared composites using (reference 3000, Gamry Co., Warminster, PA, USA). Steel was dunked into the 3.5% NaCl solution for 30 min to accomplish the balanced state for the metal with the solution before each testing. The EIS analyses were performed in a frequency of 0.1 Hz to 100 kHz with AC amplitude of 10 mV using a reference 3000, Gamry Co., USA. The potentiodynamic polarization plots were accomplished versus the reference electrode potential.

Structural and Morphological Analysis of the ZnO/NiO Composite
The XRD diffractogram of the ZnO, NiO, and ZnO-NiO nanocomposite is depicted in Figure 1 Table 1. There were no contamination peaks detected in the XRD pattern, which proves the superior clarity in the synthesized ZnO-NiO nanocomposite. The average crystallite size (D) of the composite was computed through the Debye Scherrer formulae followed by estimation of the dislocation density (δ) [22].  The SEM micrographs of the ZnO-NiO nanocomposite with different magnifications are shown in Figure 2a,b. The surface of the nanocomposite was not smooth. At 10KX magnification, the average grain size of the powder was revealed to be around 100 nm. Figure 2a illustrates the SEM image at a higher enlargement and it can be observed that the particles were detained collectively owing to feeble physical forces. Here, particles were produced with sizes in the micron range. Figure 2b shows the SEM images at a higher enlargement, and it can be observed that particles of sizes less than 100 nm were produced. It also provides an understandable idea about the particle separation, which can be seen from the fact that the particles are divided effortlessly and not extremely influenced by agglomeration. The composite with Ni was not demonstrated to have a remarkable outcome on the particle size of ZnO. Since Ni ions were isolated onto the surface of ZnO, they might have control over the growth to minute grains [23]. As seen in Figure 2a   The SEM micrographs of the ZnO-NiO nanocomposite with different magnifications are shown in Figure 2a,b. The surface of the nanocomposite was not smooth. At 10KX magnification, the average grain size of the powder was revealed to be around 100 nm. Figure 2a illustrates the SEM image at a higher enlargement and it can be observed that the particles were detained collectively owing to feeble physical forces. Here, particles were produced with sizes in the micron range. Figure 2b shows the SEM images at a higher enlargement, and it can be observed that particles of sizes less than 100 nm were produced. It also provides an understandable idea about the particle separation, which can be seen from the fact that the particles are divided effortlessly and not extremely influenced by agglomeration. The composite with Ni was not demonstrated to have a remarkable outcome on the particle size of ZnO. Since Ni ions were isolated onto the surface of ZnO, they might have control over the growth to minute grains [23]. As seen in Figure 2a,b, particles with different sizes were present in the nanocomposite powder.
The EDAX spectrum of the ZnO-NiO nanocomposite (Figure 2c) confirms that the chemical composition of the samples consists of Zn (8.96 ± 0.11 wt.%), Ni (10.53 ± 0.19 wt.%) and O (80.51 ± 3.12 wt.%). It was found that the chemical composition corresponded to the requirement and EDAX validates the effective incorporation of Ni into the ZnO nanoparticles [23]. No contamination peaks were present in the spectra representing the chemical purity of the sample.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 15 3.12 wt.%). It was found that the chemical composition corresponded to the requirement and EDAX validates the effective incorporation of Ni into the ZnO nanoparticles [23]. No contamination peaks were present in the spectra representing the chemical purity of the sample.

Potentiodynamic Polarization (PP) Characterization
The corrosion fortification performances of the steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings were estimated by EIS and Potentiodynamic polarization measurements. The Tafel plots of the steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings subjected to 3.5% NaCl solution are illustrated in Figure 3. It can be seen from the figures that there was change in Tafel slope plots, and this was owing to the passive layer arrangement on steel. It can be seen that the EP/1.8 ZnO-NiO nanocomposite coating demonstrated superior resistance to that of other coated samples. The epoxy customized ZnO-NiO coated steel was not active in the direction of the corrosion reaction as can be observed from Figure 3. The corrosion parameters computed from Tafel plots are illustrated in Table 2. The Tafel plots demonstrate that the EP/1.8 ZnO-NiO had the lowest corrosion current and rate. As a result, it had the uppermost corrosion protection in contrast to the other tested coatings. Furthermore, the corrosion potential of the EP/1.8 ZnO-NiO altered in a positive direction, which represents greater protection behavior. This is owing to the constraints placed on the progress of corrosive ions to the

Potentiodynamic Polarization (PP) Characterization
The corrosion fortification performances of the steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings were estimated by EIS and Potentiodynamic polarization measurements. The Tafel plots of the steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings subjected to 3.5% NaCl solution are illustrated in Figure 3. It can be seen from the figures that there was change in Tafel slope plots, and this was owing to the passive layer arrangement on steel. It can be seen that the EP/1.8 ZnO-NiO nanocomposite coating demonstrated superior resistance to that of other coated samples. The epoxy customized ZnO-NiO coated steel was not active in the direction of the corrosion reaction as can be observed from Figure 3. The corrosion parameters computed from Tafel plots are illustrated in Table 2. The Tafel plots demonstrate that the EP/1.8 ZnO-NiO had the lowest Coatings 2020, 10, 783 7 of 14 corrosion current and rate. As a result, it had the uppermost corrosion protection in contrast to the other tested coatings. Furthermore, the corrosion potential of the EP/1.8 ZnO-NiO altered in a positive direction, which represents greater protection behavior. This is owing to the constraints placed on the progress of corrosive ions to the substrate by the deposition of boosted cross-linked chemical structures and improved dispersion on surface modified using the ZnO-NiO composite [24][25][26][27][28]. This acted as a strong physical barrier. This behavior develops the corrosion resistance rate (CR). As a result, the CR of epoxy tailored ZnO-NiO nanocomposite coatings was established to be very low compared to that of the samples without surface modification coating. The outcome verifies that pure epoxy and epoxy-based nanocomposites (different ratio) diminished the CR [29][30][31][32].
Coatings 2020, 10, x FOR PEER REVIEW 7 of 15 substrate by the deposition of boosted cross-linked chemical structures and improved dispersion on surface modified using the ZnO-NiO composite [24][25][26][27][28]. This acted as a strong physical barrier. This behavior develops the corrosion resistance rate (CR). As a result, the CR of epoxy tailored ZnO-NiO nanocomposite coatings was established to be very low compared to that of the samples without surface modification coating. The outcome verifies that pure epoxy and epoxy-based nanocomposites (different ratio) diminished the CR [29][30][31][32].

EIS Characterization
The corrosion resistance of the nanocomposites coatings was premeditated by EIS analysis. Figure 4 reveals the Nyquist plots of steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings in 3.5% NaCl solution. The curve appears as a two-time constant semicircle because of the permeation of a corrosive medium through the defects or pores, coming out in under-film corrosion and coating delamination. The low-frequency impedance displays the elevated stability of the coatings through immersion.

EIS Characterization
The corrosion resistance of the nanocomposites coatings was premeditated by EIS analysis. Figure 4 reveals the Nyquist plots of steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings in 3.5% NaCl solution. The curve appears as a two-time constant semicircle because of the permeation of a corrosive medium through the defects or pores, coming out in under-film corrosion and coating delamination. The low-frequency impedance displays the elevated stability of the coatings through immersion.
The corrosion performance of the bare steel was considered the baseline position. The EP/ZnO-NiO composite illustrated excellent corrosion inhibition compared to that of steel as the Epoxy layer nearly enclosed the steel surface and formed a physical barrier that obstructs the transmission of the ions from the salt solution. In EP/1.8 ZnO-NiO, the impedance value at small frequency was improved when compared to that of Epoxy. This is accredited to the superior interfacial contact of ZnO-NiO with epoxy via the surface functionalities, good ZnO-NiO dispersion, and the long convoluted corridor for the stabbing molecules [33][34][35][36][37]. The corrosion performance of the bare steel was considered the baseline position. The EP/ZnO-NiO composite illustrated excellent corrosion inhibition compared to that of steel as the Epoxy layer nearly enclosed the steel surface and formed a physical barrier that obstructs the transmission of the ions from the salt solution. In EP/1.8 ZnO-NiO, the impedance value at small frequency was improved when compared to that of Epoxy. This is accredited to the superior interfacial contact of ZnO-NiO with epoxy via the surface functionalities, good ZnO-NiO dispersion, and the long convoluted corridor for the stabbing molecules [33][34][35][36][37]. Figure 5 illustrates Bode and phase angle plots for steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings. It is evident that the EP/1.8 ZnO-NiO nanocomposite coating  Figure 5 illustrates Bode and phase angle plots for steel, epoxy, EP/1 ZnO-NiO, and EP/1.8 ZnO-NiO nanocomposite coatings. It is evident that the EP/1.8 ZnO-NiO nanocomposite coating demonstrates the utmost maximum phase angle and impedance change from the low to the high-frequency region compared to the pure epoxy and EP/1 ZnO-NiO coated samples. The EP/1.8 ZnO-NiO nanocomposite coated mild steel had superior charge transfer resistance compared to pure epoxy and EP/1 ZnO-NiO coated samples. It can be observed that the EP/1.8 ZnO-NiO coating displayed an enlarged time constant in the series of the high frequencies that is accredited to its improved barrier properties [38][39][40]. As can be seen from Table 3, the EP/1.8 ZnO-NiO nanocomposite coatings exhibited superior anticorrosive performance when compared to other reported nanomaterials and thin film coating systems.
NiO nanocomposite coated mild steel had superior charge transfer resistance compared to pure epoxy and EP/1 ZnO-NiO coated samples. It can be observed that the EP/1.8 ZnO-NiO coating displayed an enlarged time constant in the series of the high frequencies that is accredited to its improved barrier properties [38][39][40]. As can be seen from Table 3, the EP/1.8 ZnO-NiO nanocomposite coatings exhibited superior anticorrosive performance when compared to other reported nanomaterials and thin film coating systems.

Corrosion Inhibition Mechanism
The projected illustration of the corrosion protection mechanism of the nanocomposites on the metal surface is shown in Figure 6. Since the corrosion procedure in an unbiased chloride solution can be performed with an anodic route of suspension of Fe and a decrease in the cathodic route of O 2 and H 2 O, OH − can be produced; Fe 2 O 3 is then created via a series of the chemical oxidation route. Diverse materials for corrosion protection were produced and found from the restrained nature of the corrosion effect, such as evading O 2 and H 2 O diffusion onto the metal composite surface and hindering metal oxidation. Significant study of the mechanism of corrosion protection with nanocomposite materials can be categorized as follows: (i) the high surface energy owing to nanomaterials system guides the creation of appropriate surface irregularity and hydrophobic surfaces at the top of the Epoxy/1.8 ZnO-NiO coating, and (ii) the formation of an adjacent interconnecting structure with tough adhesion at the coating-composite interface [54][55][56][57][58].

Conclusions
In this work, the sol-gel assisted ZnO-NiO nanocomposite and its structural, morphological, and compositional properties were investigated. XRD revealed a hexagonal ZnO -cubic NiO arrangement with an average crystallite size of 26 nm. SEM images confirmed the agglomerated particles with sizes of less than 100 nm of the prepared composite. EIS and PP experiment results established that the ZnO-NiO with Epoxy coating has the superior anticorrosion capability. EIS studies demonstrated higher charge and resistance for the epoxy/1.8-ZnO-NiO nanocomposite coated steel. The improved anticorrosion performance was exhibited by the EP/1.8 ZnO-NiO-epoxy coating compared to the pure epoxy, EP/1 ZnO-NiO coatings. This could be accredited to the surface alteration of the ZnO-NiO composite to speed up the probable chemical communications between the composite and epoxy matrix. The anticorrosion (Epoxy/1.8 ZnO-NiO) coating has outstanding physical barrier qualities, which can be employed in the corrosion protection field.

Conclusions
In this work, the sol-gel assisted ZnO-NiO nanocomposite and its structural, morphological, and compositional properties were investigated. XRD revealed a hexagonal ZnO -cubic NiO arrangement with an average crystallite size of 26 nm. SEM images confirmed the agglomerated particles with sizes of less than 100 nm of the prepared composite. EIS and PP experiment results established that the ZnO-NiO with Epoxy coating has the superior anticorrosion capability. EIS studies demonstrated higher charge and resistance for the epoxy/1.8-ZnO-NiO nanocomposite coated steel. The improved anticorrosion performance was exhibited by the EP/1.8 ZnO-NiO-epoxy coating compared to the pure epoxy, EP/1 ZnO-NiO coatings. This could be accredited to the surface alteration of the ZnO-NiO composite to speed up the probable chemical communications between the composite and epoxy matrix. The anticorrosion (Epoxy/1.8 ZnO-NiO) coating has outstanding physical barrier qualities, which can be employed in the corrosion protection field.