One of the most important sources which cause a breakdown of the coating is Ultraviolet light. Miklecˇic´ et al. [1] and Asmatulu et al. [2] revealed that high adversity of UV-radiation is the reason for causing photochemical decomposition of the coatings which causing highly Reactive Free Radicals (RFR) on the surface of the coating that exposure to UV light. These RFR are groups of atoms or molecules that contain extra electrons that have the affinity to connect with the molecules present in the structure of the coating. The process of pairing the RFR leading to break the covalent bonds into minor bonds and initiates the oxidation through cross-linking reaction, however, the degradation process usually happens when the molecules of RFR absorb energy higher than the bond energy. Thus, UV light causing the dissolution of the coatings by impairment of the physical, chemical and physicochemical characteristics of the coating [3].

Mostly, coatings are the most greatly used and cost-effective method for protection, with organic coatings being the most generally utilized for the protection of metals [3].

The main purpose for coating structural parts can be reviewed as follows [4]:

• Increased wear resistance to increase equipment life

• Enabling the use of high operating temperatures, which allows improving the functional performance and mechanical properties such as creep and fatigue

• The possibility of repairing and replacing equipment

• Improving the function of the coating to reduce the cost of equipment used

The small size of Nanoparticles (NPs) makes them useful for many applications as they are invisible to the human eye. There are two main criteria for the use of nanoparticles in coatings [5]:

• High transparency in the visible range of the electromagnetic spectrum

• It can absorb infrared radiation in the electromagnetic spectrum

A composite nanocoatings is a material consist of two phases, isolated by interface region from each other. The material must involve the nanometer size in one dimension where the main component is called the matrix through which the fillers are dispersed [6].

ZnO is the most widely used semiconductors, it has certain properties and good physical properties elevated electrochemical stability and excellent electronic characteristics. It is known that when the particle sizes of the semiconductors reduce to the nanometer scale, these materials often show quantum size actions, showing various electric and optical characteristics from bulk materials [7].

Nanocomposite materials which based on silica are very significant and they rated as the most challenging composite systems for the quantum confinement of semiconductor. Nanocomposites ZnO/SiO2 films or powders have been extensively explored for different applications, such as photonic crystals, photocatalysis, gas sensors, white light emission and vacuum fluorescent displays [7].

Different research revealed that the coatings of silicate have good resistance to corrosion for the surface of metals. Alkali silicate is solutions consist of two fractions: (a) alkali ions, hydroxide ions and silicate ions (SiO4_), (b) colloidal fractions [8].

Due to the high absorbance of ZnO NPs to the UV radiation from the light, it has been widely utilized as an outstanding material for absorbing UV radiation. As compared with the organic UV absorbers, ZnO has various advantages like physical and chemical stability under UV light, low toxicity, transparency due to their ability to neglect light scattering power [9].

In this research ZnO in the nano-scale at different contents were dispersed in K₂SiO₃ as a matrix to produce composite nanocoatings thin films. The film was deposited on a low carbon steel substrate of the heat exchanger by the spray coating method to protect the surface of the tube from UV radiation. The optical properties of the prepared film were obtained from many methods such as the UV-visible test which including (absorbance and reflectivity) and the FTIR test.

Materials: (1) The used ZnO NPs were purchased from Sigma Aldrich made in the USA with the following properties: purity equal to 99.9%, white to pale yellow color, Melting point of 1975°C, true density of 5.6 g/cm³, the molecular weight of 81.39 g/mol and the particle size is in the range of 80-200 nm (2) Potassium hydroxide (KOH) was purchased from Sigma Aldrich with a molecular weight of 65.11 g/mol which was made in India at Thomas Baker Company, (3) The silica used in this work has a white color in the powdered form was made in India at Central Drug House (CDH) company with M.W of 60.08 g/mol and mesh size of 230-400, (4) potassium silicate K₂SiO₃ was prepared using potassium hydroxide and silica gel and (5) Low carbon steel tube was provided by Alkufa cement factory in Kufa/Najaf/Iraq.

Methods 

Prepare and Cutting the Low Carbon Steel: In this study, low carbon steel was used. The sample was cut into small (25×15×6 mm) specimens and then polished using a Buehler metaserv 250 grinder polisher device with 180, 220, 320, 400, 600, 1000 and 1200 silica carbide abrasive papers underwater as illustrated in Figure 1.

Ceramic Nanocoatings Preparation

To calculate the weight of the NPs that are added to the potassium silica matrix, the following equation was used [10]:

Where:

• W_m: Eight of the matrix
• W_p: The weight of particles
• ρ_m: Density of the matrix 
• ρ_p: Density of the particles
• V_f: Volume fraction of NP

The nanocomposites were prepared by the addition of ZnO NPs in (0.005, 0.01and 0.02) wt% into K₂SiO₃ solution as illustrated in Table1.

After adding the necessary amount of nanomaterials to the solution, the solution is placed on the stirrer (Jenway 1000 Series) for 10 minutes to mix the nanoparticles with the solution and then the solution is placed in the ultrasonic mixing device (1200W Ultrasonic processor from MTI Corporation company in the USA) to dispersion the NPs in the solution.

Table 1: Illustrate the Details of Preparation K₂SiO₃/ZnO NPs

Figure 1: LCS (A) Sample of LCS Tube and (B) the Tube after Cutting, Grinding and Polishing

Figure 2: (a) Airbrush Device and (b) Set Up of Spray Coating

Spray Coating Procedure

This technique was achieved using the setup system which consists of an atomizer that contains a container for suspension solution and nozzle for spraying which is directed onto the specimen surface about (5 cm) above specimens as shown in Figure 2b. The spraying of suspension achieved by F75 stainless steel airbrush spray gun with 1.5 mm nozzle (India MART company) as shown in Figure 2a.

Physical Characterization of ZnO/K₂SiO₃ Nanocomposite Solution

Viscosity: The viscosity of the nanocomposite was investigated by a rotational viscometer model from the Fungilab S.A device and the obtained results are shown in Table 2. ZnO in the nano-size has various defects such as the small size, instability and ability to agglomeration, therefore different materials are used as stabilizers. The silica is a very attractive candidate because of its great surface area and the ability of high adsorption and hydrophobic and hydrophilic properties to reserve the clustering of ZnO NP and reduce the growth of particles [7].

Josue Lopez-Rodriguez et al. [11] revealed that the viscosity of ZnO NPs in the K₂SiO₃ is increased as the concentration is raising. Figure 3 illustrates that the viscosity of K₂SiO₃ solution increased with increased weight percent of ZnO NPs.

Nearly, all the research illustrated that the presence of NP at a low volume fraction in the matrix solution leading to a great increase in the viscosity. Prasher et al. [12] showed that the viscosity change because of the change in the concentration of NPs.

Thermal Conductivity

The thermal conductivity test was performed using KD2 Pro thermal properties analyzer from Decagon Devices, Inc. device. The dispersed NPs in DW greatly increase thermal conductivity as illustrated in Table 3. It is known that the thermal conductivity of NPS depends on the volume fraction and properties of NPs. From Figure 4 it can be noticed that the thermal conductivity increases as the volume fraction of the NPs increases, which agrees well with the reported results [13].

In the system with low content of NPs, a small amount of NPs contribute to arrange conductive chains [14]. 

Figure 3: The Relationship between Viscosity and Concentration of K₂SiO₃/ZnO

Table 2: Characterization of K₂SiO₃/ZnO NPs

Therefore, the contribution of NPS into the thermal conductivity of the nanocomposite appears to be less than that of the matrix, so that the composites show low thermal conductivities. With the increasing ZnO content, many ZnO particles touch each other to begin to form ZnO conductive chains, which greatly contribute to the thermal conductivities of nanocomposites [14].

Optical Characterization

T90+ Double Beam Spectrophotometer by PG Instruments Ltd device was used to investigate the optical properties. The concentration with 0.005 wt% ZnO was neglected because it did not show a noticeable change in the optical properties as compared with the original specimen.

Absorbance of ZnO/K₂SiO₃ Nanocomposite

The results revealed that the ZnO/K₂SiO₃ solution is more photoactive. Also, the photocatalytic efficiencies of ZnO/K₂SiO₃ solutions are increased with increasing ZnO content from 0.01 to 0.02 wt% ZnO. Ali et al. [7] reported a correlation between the photonic efficiencies and the ZnO/K₂SiO₃ solutions at different ZnO content.

Figure 3: The Relationship between Viscosity and Concentration of K₂SiO₃/ZnO

Figure 4: The Relationship between Thermal Conductivity and Concentration of K₂SiO₃/ZnO

Table: Characterization of K₂SiO₃/ZnO NPs

They reveal that the photonic efficiency increases with increasing ZnO content up to 0.02 wt% ZnO. It can be drawn that the blue shift of the bandgap of ZnO/K₂SiO₃, 354 nm toward 357 nm for the ZnO/K₂SiO₃ samples, confirms the presence of ZnO nanoparticles on the surface of K₂SiO₃ as shown in Figure 5 [15].

Absorbance of ZnO/K₂SiO₃ Composite Nanocoating

The absorbance spectrum for multilayers of the nanocomposite coating shows that the absorbance decrease with increasing the number of layers. Figure 6 show that the absorption peak for one layer can be located at 355.50 nm wavelength while the two layers of coating showed higher absorbance at a wavelength of 356.57 nm. The addition of nanoparticles increases the absorbance of the coating.

Figure 5: UV-Visible Absorbance Spectrum of ZnO/K₂SiO₃ Nanocomposite Solutions

Consequently, for 0.02 wt% ZnO NPs, the absorbance peak for one layer was located at 358.23 nm wavelength while the two layers show an absorbance peak at 359.65 nm wavelength.

Reflectivity of ZnO/K₂SiO₃ Composite Nanocoating

Figure 7 shows the reflectance spectra of the investigated thin film samples. We found that the reflectance of ZnO/K₂SiO₃ one layer coating is 6.1% and for the two layers coating the reflectance was 4.2% for 0.01wt% ZnO NPs. The presence of SiO2 reduces reflectance to 4.2% for one layer coating and 2.4% for two layers of coating for 0.02wt% ZnO NPs [16].

The height of the reflectance peak is low and the fractional bandwidth is large for the K₂SiO₃/ZnO composite photonic crystals film prepared with a short deposition time. This is because during short deposition times the ZnO particles adhere randomly on the surfaces of the SiO2 spheres, producing a disordered structure. As the deposition time increased, the height of the reflectance peak becomes higher and the fractional bandwidth becomes smaller because interstitial spaces of the SiO2 photonic crystal template are filled with ZnO, resulting in a more ordered structure [17].

As expected, the absorption of the films increases when the number of layer increase. It is seen that the absorbance is limited only by the surface reflectance in the visible region [18].

All the film thicknesses were measured to be 132 µm to 283.8 µm. The coating was nearly transparent for 250–700 nm wavelength of light. In contrast, the nanocomposite films could block the UV light below 360 nm. When the ZnO/K₂SiO₃ concentration increased, the UV-shielding performance of the films increased and their absorbance was still very high in the visible region. Further increasing the ZnO/K₂SiO₃ content would cause a little loss of reflectance. However, these films could not block the UV light within the entire 200-360 nm range at the thickness of 132 µm to 283.8µm. According to Bouguer–Lambert-Beer law, the absorbance of a film is directly proportional to the film thickness. That is to say, increasing the film thickness will be an efficient way to attain good UV-shielding performance [19].

Figure 6: Absorbance Spectrum of: (a) 0.01 wt% ZnO NPs/K₂SiO₃ and (b) 0.02 wt% ZnO NPs/ K₂SiO₃ Multilayers Coating

Figure 7: Reflectance Spectrum of:  (a) 0.01 wt% ZnO NPs– K₂SiO₃ and (b) 0.02 wt% ZnO/K₂SiO₃ Multilayer Coating

Figure 8: FTIR Spectra of ZnO NPs Powder

Figure 9: (a) FTIR Spectra of (0.01wt %) ZnO/K₂SiO₃ Coating (b) FTIR Spectra of (0.02wt %) ZnO/K₂SiO₃ Coating

FTIR Spectrum for ZnO NPs

Zinc oxide nanoparticles were subjected to FT-IR analysis to detect the various characteristic functional group associated with the nanoparticles as shown in Figure 8. 

The peak located at 3456.44 cm^-1 is attributed to the stretching vibration of the hydroxyl group [20]. The peak centered at 2924.09 cm^-1 is due to stretching of the C-H alkaline group [21]. The peak at 1637.56 cm^-1 is due to the asymmetrical and symmetrical stretching of zinc carboxylate [21]. The peaks located at 1618.28 cm^-1 and1514.12 cm^-1 corresponds to C=O symmetric stretching vibration [22]. The peak is located at 1455.26 cm^-1 due to the stretching and bending modes of the hydroxyl O-H group of H and O [23]. The peak centered at 1384.89 cm^-1 can be attributed to the stretching of the C-N bond of the primary amine or stretching of the C-O bond of the primary alcohol [20]. The peak located at 987.55 cm^-1 is due to the stretching of C-O in amino acids [24]. The peak located at 885.33 cm^-1 and 840.98 cm^-1 are due to the formation of tetrahedral coordination of ZnO [22]. The peak centered at563.21 cm^-1 is corresponding to metal-oxygen vibration mode [20]. The peak at 543.21cm⁻¹ and 520.78 cm⁻¹ are the characteristic absorptions of the Zn-O bond [24]. The peak located at 466.77 cm^-1 is the signature of the ZnO bond [23].

FTIR Spectrum for ZnO NPs/ K₂SiO₃ Coating

Figure 9a and b illustrate the analysis of FTIR to examine the interactions between SiO2-ZnO nanocomposites. The peal centered at 432.05 cm^-1 is the strong absorption peak for ZnO NPs [25]. Zn-O bond was located at the peak of 451.34 cm^-1 [26]. The peak located at 484.13 cm^-1 is due to the bending vibration of the Si-O-Si bond [26]. The peak 538.14 cm^-1 is associated with a double six membered ring which confirms the presence of microporous structure [27]. The peal located at 596.00 cm^-1 is associated with the presence of the Zn-OH bond [28]. The peak 1022.27 cm^-1 shows the identical Si-O-Si bond which shows asymmetric stretching vibration [28,29]. The peak centered at 118.71 cm^-1 is because of the bending vibration of the Si-O-Si group [30]. The peak at 1622.13 cm^-1 indicates bending and stretching vibrations of the O-H group [30]. The peak located at 1639.64 cm^-1 is due to the asymmetric stretching of the C-O group [28]. The peak at 1662.64 cm^-1 indicates the presence of the C-C group [29]. The hydroxyl group of ZnO can be shown in the peak 3419.79 cm^-1 [31]. Finally, the peak located at 3454.51 cm^-1 is due to the presence of Si-OH and O-H which physically adsorbed water and the asymmetric stretching of –OH bond [31].

FTIR Spectrum for ZnO NPs

Zinc oxide nanoparticles were subjected to FT-IR analysis to detect the various characteristic functional group associated with the nanoparticles as shown in Figure 8. 

The peak located at 3456.44 cm^-1 is attributed to the stretching vibration of the hydroxyl group [20]. The peak centered at 2924.09 cm^-1 is due to stretching of the C-H alkaline group [21]. The peak at 1637.56 cm^-1 is due to the asymmetrical and symmetrical stretching of zinc carboxylate [21]. The peaks located at 1618.28 cm^-1 and1514.12 cm^-1 corresponds to C=O symmetric stretching vibration [22]. The peak is located at 1455.26 cm^-1 due to the stretching and bending modes of the hydroxyl O-H group of H and O [23]. The peak centered at 1384.89 cm^-1 can be attributed to the stretching of the C-N bond of the primary amine or stretching of the C-O bond of the primary alcohol [20]. The peak located at 987.55 cm^-1 is due to the stretching of C-O in amino acids [24]. The peak located at 885.33 cm^-1 and 840.98 cm^-1 are due to the formation of tetrahedral coordination of ZnO [22]. The peak centered at 563.21 cm^-1 is corresponding to metal-oxygen vibration mode [20]. The peak at 543.21 cm⁻¹ and 520.78 cm⁻¹ are the characteristic absorptions of the Zn-O bond [24]. The peak located at 466.77 cm^-1 is the signature of the ZnO bond [23].

FTIR Spectrum for ZnO NPs/ K₂SiO₃ Coating

Figure 9a and b illustrate the analysis of FTIR to examine the interactions between SiO2-ZnO nanocomposites. The peal centered at 432.05 cm^-1 is the strong absorption peak for ZnO NPs [25]. Zn-O bond was located at the peak of 451.34 cm^-1 [26]. The peak located at 484.13 cm^-1 is due to the bending vibration of the Si-O-Si bond [26]. The peak 538.14 cm^-1 is associated with a double six membered ring which confirms the presence of microporous structure [27]. The peal located at 596.00 cm^-1 is associated with the presence of the Zn-OH bond [28]. The peak 1022.27 cm^-1 shows the identical Si-O-Si bond which shows asymmetric stretching vibration [28, 29]. The peak centered at 118.71 cm^-1 is because of the bending vibration of the Si-O-Si group [30]. The peak at 1622.13 cm^-1 indicates bending and stretching vibrations of the O-H group [30]. The peak located at 1639.64 cm^-1 is due to the asymmetric stretching of the C-O group [28]. The peak at 1662.64 cm^-1 indicates the presence of the C-C group [29]. The hydroxyl group of ZnO can be shown in the peak 3419.79 cm^-1 [31]. Finally, the peak located at 3454.51 cm^-1 is due to the presence of Si-OH and O-H which physically adsorbed water and the asymmetric stretching of –OH bond [31].

In conclusion, a composite nanocoating of ZnO/K₂SiO₃ was prepared by a spray coating method. The viscosity and thermal conductivity of the ZnO/K₂SiO₃ nanocomposite were increased as the concentration of the ZnO rose. The results of absorbance for the multilayer coating showed that the absorbance toward UV light increased as the number of the layer is increase as well as the increasing the concentration of the NPS. The result of FTIR revealed that a certain peak was formed at 3419.79 cm^-1 wavelengths and all the other peaks are shifted to the blue shift as the concentration of ZnO NPs increase. 

Acknowledgment 

The authors would like to express their thanks and gratitude for the assistance provided by the Nanotechnology Research Unit (NAMRU) at the Faculty of Engineering at the University of Kufa/IRAQ. The authors would like to express their appreciation to the University of Kufa/College of pharmacy for its support of this work.

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