Effects of poly(o-phenylenediamine) functionalized SiC on the corrosion protection ability of neat polyurethane coating system in the marine environment

Abstract A novel nanocomposite consisting of polyurethane (PU), poly(o-phenylenediamine) (PoPD), and silicon carbide (SiC) nanoparticles was investigated for its application in marine environment through electrochemical techniques. The PoPD/SiC nanofillers were characterized by TGA, XRD, SEM/EDX, and TEM analyses. The anticorrosion and mechanical properties of different coating formulation in marine environment were evaluated by electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM). It was also found that the coating resistance of PU-PoPD/SiC nanocomposite was over 41 times higher than that of the PU coating. The PU-PoPD/SiC coatings on the brass showed low current of 1.9 I/nA due to copper dissolution and 6.8 I/nA due to zinc dissolution because of the well distribution of PoPD/SiC nanofiller in PU coating. The analyses of the resultant degradation products by SEM/EDX and XRD techniques confirmed the presence of Si which has a major role in protecting the brass surface against corrosion. Results showed that the PU composite with 2 wt.% PoPD/SiC hybrid nanofillers had outstanding coating performance. This nanocomposite demonstrated improved corrosion protection. As a result, the developed PU-PoPD/SiC nanocomposite has exceptional adhesion strength and anticorrosion properties and might be exploited to develop next-generation anticorrosive coatings.


Introduction
Copper and its alloys are extensively used in the aggressive environments because of their superior corrosion resistance, enhanced mechanical workability, and excellent thermal and electrical properties. Brass, as the heat exchanger tubes, is used in desalination and power plants. The formation of a protective cuprous oxide layer (Cu 2 O) provides the enhanced corrosion protection properties of copper and its alloys. [1] However, they are susceptible to corrosion, due to the formation of biofilms in the marine environment. [2] The corrosion and fouling are the main problems related to the heat exchangers and cooling water system. The formation of biofilm can induce the electrochemical corrosion at the metal/solution interface and deteriorates the metal surface. To prevent the metal surface from corrosive substances, polymer coating is one of the effective methods, which can act as a barrier layer in order to prevent direct contact of the metal surface with aggressive electrolytes. Hence, polymeric coatings on the metallic substrates have PU-SiC, and PoPD-SiC-PU were studied by the TGA and SEM/EDX analysis, respectively. The electrochemical properties of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass were investigated by electrochemical techniques, such as electrochemical impedance spectroscopy (EIS), potentiodynamic polarization studies and scanning electrochemical microscopy (SECM). The mechanical behaviors of PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass was analyzed by the adhesion, tensile and hardness test. Effectiveness of the nanocomposite coatings was analyzed in the presence of air and water by oxygen and water permeability test.

Chemical synthesis of polyorthophenylene diamine (PoPD) and polyurethane (PU)
Polyorthophenylenediamne was chemically synthesized following the same route reported. [41] Distilled water was used to dilute 0.25 mM o-phenylenediamine to 10 mL. To this solution, an aqueous solution of APS (2.5 mL of 0.1 M) was added with constant stirring. Then, without agitation, the reaction was proceeded at room temperature for 24 h. The resultant products were centrifuged with distilled water several times. Finally, the products were dried at room temperature in a vacuum. PU was synthesized according to the reported literature. [42] In a three-necked, round-bottomed flask with a condenser and stirrer that was sealed off from moisture, 25.02 g of methylene bis(4-phenyl isocyanate) and 40 mL of 4-methylpentanone-2 were added. 6.20 g of ethylene glycol dissolved in 40 ml of dimethyl sulfoxide was added to this quickly stirred solution. For 1.5 h, the reaction was heated at 115 C. The PU was subsequently precipitated using the clear, viscous solution. The hard, white polymer was broken up in a household blender, cleaned with water, and dried in a vacuum oven at 90 C. A 0.05% solution in N,N-dimethylformamide has an intrinsic viscosity of 1.01 at 30 C.

Fabrication of polyurethane nanocomposite coatings
About 1 g of PoPD and 0.4 g of SiC nanoparticles were dissolved in 50 mL of chloroform and it was diluted with 50 mL of double distilled water. Then, the solution was sonicated for 45 min to get uniform dispersion. The resultant PoPD/SiC particles were magnetically stirred for 5 h at 78 C. The resultant PoPD/SiC product was washed with double distilled water. The resultant solid was dried in room temperature and finely ground by using mortar and pestle.
The blending of PoPD/SiC with PU and polyisocyanate hardener was done with a ratio of 1:1 by means of magnetic stirring (3000 rpm) for 2 h, which resulted in the perfect coating materials. The synthesized nanocomposite was coated on brass by using spin coater. Similarly, different formulations such as PU, PU-SiC, and PU-PoPD were also coated on brass. The resultant coated brass was investigated for their anticorrosion and mechanical properties by electrochemical techniques and mechanical testing.

Characterization
The thermal properties of nano SiC, PU-SiC, PoPD-SiC, and PU-PoPD/SiC were evaluated by TGA (Model NETZSCH STA 449 F3 JUPITER). The crystallinity of nano SiC and SiC-PoPD particles were studied with X-ray diffractometer (Bruker model D8, Berlin, Germany) using Cu Ka radiation k-1.5406 Å. The 2h angles from 10 to 80 were used with the sweeping rate of 1 . The morphology of nanocomposite powders was analyzed by scanning electron microscope (SEM) and quantitative analysis of the elements present in the nanocomposite powders was evaluated by energy dispersive X-ray (EDX) analysis (Model FEI-Quanta FEG 200 F). All the images acquired using an operating voltage of 25 kilovolts (kV). The surface morphology of nano SiC and PoPD/ SiC particles were analyzed by a field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F30 S-TWIN) with the operating voltage of 200 kV.

Electrochemical analysis
The protection performance of PU, PoPD-SiC, PU-SiC, and PU-PoPD/SiC coated brass exposed to seawater against corrosion was evaluated by electrochemical work station (Biologic SP-240) using three electrode systems consisting of the coated specimens (working electrode), silver/silver chloride (Ag/AgCl) (reference electrode) and platinum (counter electrode). The corrosion behavior of the as prepared coated brass was investigated with the help of EIS and potentiodynamic polarization studies. The area of the specimen exposed to the solution was 1 cm 2 in the flat corrosion cell. The frequency range of 100 kHz-10 mHz was used to analyze the EIS studies. The polarization studies were carried out a scan rate of 10 mV/min. Triplicate analyses on the specimens were carried out in order to get the reproducibility of the results. The SECM measurements were carried out for PU, PoPD-SiC, PU-SiC, and PU-PoPD/SiC coated brass exposed to seawater for 1, 15, 30, and 60 d. This technique was carried out using ultra microelectrode tip (diameter of 20 mm), placed inside the cell. All measurements were done using Ag/AgCl as reference electrode and Pt strip as counter electrode. SECM scans were carried out by applying appropriate tip potentials. The copper ions were detected by applying the tip potential of þ0.34 V. At this parameter, the presence of cuprous (Cu þ ) ion can be detected through their oxidation to cupric ions (Cu 2þ ). The presence of zinc ions was detected by applying the tip potential of À1.00 V.

Analysis of corrosion products
The analyses of corrosion products observed in PU, PU-SiC. PU-PoPD, and PU-PoPD/SiC coated brass after 60 d of exposure to seawater were investigated by SEM/EDX techniques (Model FEI-Quanta FEG 200 F). The nature of corrosion products formed on PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass immersed in seawater after 60 d immersion was studied by XRD (Bruker model D8, Germany) in the range of 2h angles from 10 to 80 (Cu Ka radiation k-1.5406 Å).

Analyses of permeability and mechanical properties
The penetration of water in the pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass without and with the exposure to seawater for 1, 15, 30, and 60 d as per ASTMD3985. The permeability of oxygen in the pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass without and with the exposure to seawater for 1, 15, 30, and 60 d (MOCON Ox-Tran 2/21 instrument; Modern Controls Inc., New Castle, DE) as per the ASTM D3985. The bonding strength of PU, PoPD-SiC, PU-SiC, and PoPD-SiC-PU coated brass were analyzed by attaching it through epoxy glue and carried out the pull off adhesion test. Micro hardness test for PU, PoPD-SiC, PU-SiC, and PoPD-SiC-PU coated brass without and with the exposure to seawater for 1, 15, 30, and 60 d (HM113 Vicker's hardness tester) by applying the force of 100 g force (gf) in 5 s. Hardness measurements of the coated specimens were conducted for more than 5 times and the average values were taken.

Results and discussion
Analysis of functionalized nano SiC

SEM/EDX
The surface morphology of SiC, PoPD-SiC, and PU-SiC nanoparticles are presented in Figure  1(a,c,e), respectively. It is observed from Figure 1(a) that the granular structures of SiC nanoparticles are uniformly distributed with quasi-spherical in shape and the grain size of 30-100 nm. In Figure 1(c,e), it is observed that, there is a nucleation of SiC nanoparticles on the polymers. It is evident that the polymers are covered with SiC nanoparticles and form more composite nanostructures. The elemental analyses of SiC, PU-SiC, and PoPD-SiC nanoparticles are given in Figure 1(b,d,f), respectively. Pure SiC nanoparticles are composed of only silicon (Si) and carbon (C) elements. PoPD-SiC contains the elements, such as C, N, Si, and C, whereas in PU-SiC nanoparticles, C, N, O, Si, and C elements are present.

TEM
The morphology and the size of pure SiC and functionalized SiC nanoparticles were determined from the TEM images, as shown in Figure 2(a,b), respectively. All the nanoparticles are nearly spherical. In Figure 2(a), agglomerated nano SiC particles are displayed. In Figure 2(b), agglomeration of the SiC nanoparticles is reduced due to its strong interaction with polymers. It is also seen that the surface of the polymers is covered with SiC nanoparticles. Uniform dispersion of the functionalized SiC nanoparticles is also observed due to functionalization SiC nanoparticles.

TGA
The thermal stability of nano SiC and functionalized SiC nanoparticles is presented in Figure 3. The addition of SiC nanoparticles to the polymers could enhance its thermal stability. The decomposition of pure nano SiC particles occurs with two sudden changes. The removal of moisture attached to SiC is started at 100 C and completely decomposed at 200 C with 60% weight loss. Addition of PoPD with SiC shows initial decomposition around 250 C and 50% weight loss is occurred at 380 C. Final decomposition is showed at around 700 C. Weight loss percentage is slightly reduced when conducting polymers are added to the SiC nanoparticles. PU-SiC and PoPD-SiC also show the weight loss of 40% at 250 C and 30% weight loss at 380 C. A minor weight loss percentage is obtained for PoPD-SiC nanoparticles, which show higher stability even at higher temperature. It is because of modification of PoPD with SiC nanoparticles and its interaction with PU gives a strong bonding formation.

XRD patterns
The XRD patterns observed for nano SiC and PoPD-SiC nanoparticles are shown in  FTIR analysis FTIR spectra were used to characterize the chemical structures of nano SiC, PoPD, and PoPD/ SiC, and the findings are shown in Figure S1(a-c), respectively. As illustrated in Figure S1(a), FTIR studies of SiC nanoparticles reveal a peak at 831 cm À1 due to Si-C stretching. The typical FTIR spectra of PoPD are shown in Figure 1(b). The N-H stretching vibrations of the -NHand -NH 2 groups have peaks at 3375 and 3191 cm À1 , respectively. The C¼C stretching vibrations of quinoid and benzoid rings are centered at 1623 and 1528 cm À1 , respectively, while the C-N stretching vibrations of the quinoid and benzoid imine units are centered at 1367 and 1235 cm À1 . Furthermore, the 1, 2, 4-trisubstituted benzene rings are suggested by the distinctive bands of C-H out of plane deformation at 843, 755, and 597 cm À1 . The FTIR spectra of PoPD/SiC are shown in Figure S1(c). With modest shifts and without changing the PoPD structure, all of the key peaks associated with the PoPD are also found in the PoPD functionalized SiC (PoPD/SiC). This demonstrates that PoPD and SiC nanoparticles interact. These findings imply that PoPD successfully functionalizes SiC nanoparticles.
SECM analysis SECM images of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass at þ0.34 V for Cu and À1.00 V for Zn immersed in seawater for 1 and 60 d are shown in Figures 5 and 6, respectively. A high current of 9.0 and 15.0 I/nA for PU-coated brass of 1 and 60 d, respectively, are shown in Figure 6. As the immersion days are increased, the oxidation of Cu to Cu 2þ occurs. It is because of non-uniformity of PU coatings on the specimen. In case of PU-SiC, the coatings were comparatively uniform on the specimen, because of a strong bonding formation of SiC nanoparticles with PU. The agglomerated nano SiC particles cause higher current. Hence, the measured current is lowered to 4.9 I/nA (1d) and 9.1 I/nA (60 d) as compared to PU coatings.  The PU-PoPD coatings on the brass showed current of 2.2 I/nA at 1 d and 3.5 I/nA at 60 d when the tip was scanned on the scratched surface. This confirms very less dissolution of Cu to Cu 2þ ions at 1 d and very high dissolution of copper at 60 d. The corrosion resistance of the coatings was still enhanced by modifying PoPD with SiC nanoparticles and blended with PU. This type of  The corrosion products formed are well controlled even at prolonged immersion. The PU-PoPD/ SiC coating shows an enhanced corrosion protection to brass and prolong life of metal/alloys. SECM images of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass in seawater at À1.00 V for Zn immersed for 1 and 60 d are shown in Figure 6. When the PU coating surface was scanned, higher dissolution of Zinc into Zn 2þ ions was occurred and resulted in high current (14.1 I/nA for 1 d and 24.3 I/nA for 60 d). The porous coating of PU caused higher dissolution of Zn into Zn 2þ ions. The PU-SiC coated brass showed low current as compared to PU coatings for the immersion period of 1 d (6.1 I/nA) and 60 d (12.2 I/nA). When SiC nanoparticles are added to PU, it reduces the porosity and enhances the mechanical strength by means of strong interaction of nano SiC with PU. Hence, the agglomeration caused higher current. The PU-PoPD coated brass showed less current for 1 d immersion (4.2 I/nA) and 60 d immersion (9.1 I/nA). The blending of two polymers makes strong interaction between them and increases its mechanical properties. However, the PU-PoPD/SiC-coated brass showed very less current. The measured current is found to be 3.2 I/nA at 1 d and 6.8 I/nA at 60 d. Therefore, the SECM results confirmed that the PU-PoPD/SiC coated brass showed less current and resulted in the less dissolution of Cu to Cu 2þ ions and Zn to Zn 2þ ions. Hence, the PU-PoPD/SiC-coated brass could be used as the best barrier coatings to the alloy surface.

Electrochemical impedance spectroscopic studies
The electrochemical performance of the uncoated brass, pure PU, and PU with different weight percentages of PoPD/SiC coated brass immersed in seawater is depicted in Figure S1. It is seen that the maximum resistance is obtained for PU with 2.0 wt.% PoPD/SiC coated brass as compared to other weight percentages. It is because of the fact that the increased weight percentage of PoPD/SiC upto 2.0 wt.% could enhance its bonding with PU. Hence, the PU-PoPD/SiC coated brass with 2.0 wt.% of PoPD/SiC showed better adhesion to the surfaces of metal/alloy so that the interaction of corrosive electrolytes with the specimen is reduced. Thus, PU with 2.0 wt.% PoPD/ SiC coated brass is considered as an optimum percentage to be used in all electrochemical analyses.
The Nyquist plots of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass in seawater for 1, 15, 30, and 60 d are presented in Figure 7(a-d), respectively. Maximum resistance is observed for all the coated specimens in 1 d immersion. As the immersion days are increased, the decrease in resistance is observed because of the interactions of corrosive ions with the brass surface. Figure 8 displays the equivalent circuit model which is used to fit the resultant Nyquist plots. The EIS spectra are fitted with model (R s (R c Q)(R ct C dl )) to characterize the pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass. The EIS spectra are fitted with model (R s (R ct C dl )) to characterize the uncoated brass. R s , R ct, and C dl are the circuit component denoted for the uncoated brass. R s , R coat , R ct , Q, and C dl are representing the resistances of solution, charge transfer, and coating, constant phase element (CPE), and capacitance due to double layer, respectively. The CPE is obtained in the equivalent circuit because of protective coatings on brass. The equation for CPE impedance is as follows: [43] The angular frequency, CPE, and its exponent are represented, respectively, by x, Q, and n.
The corresponding parameters of the fitted graphs are shown in Table 1    the polymers, which lead to strong bonding on the brass surface. Therefore, the PU-PoPD/SiC nanocomposite could act as superior corrosion protection for the brass specimen.
Potentiodynamic polarization studies Figure 9 presents the potentiodynamic polarization curves observed for pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass immersed in seawater for 1, 15, 30, and 60 d. The polarization curves are fitted and the fitted parameters are displayed in Table 2. In case of uncoated brass, very low corrosion potential (E corr ) and very high corrosion current density (i corr ) values are obtained for 1 d (401 mV; 0.745 mA/cm 2 ) and 60 d (1040 mV; 13.75 mA/cm 2 ) immersion. The corrosive ions could easily penetrate into the metal specimens and forms more corrosion products as compared to coated specimens. Whereas the E corr and i corr values of pure PU-coated brass at 1 d in seawater are found to be À445 mV and 6.20 mA/cm 2 . However, the E corr and i corr values of PU-coated brass for 60 d are found to be À705 mV and 9.55 mA/cm 2 . These values confirm the non-uniform coating of PU on brass due to its porous nature. Due to this porosity, the bonding strength of PU coating on the brass specimen is reduced drastically. Hence, as the immersion period increases, the corrosive ions easily interact with the brass specimen through the porous coatings and form the corrosion products. The E corr and i corr values of PU-SiC coated brass for 1 d in seawater are found to be À201 mV and 3.17 mA/cm 2 . The E corr values and i corr values confirm that the PU-SiC coatings alone are not acting as corrosion protection performance to the metal specimen. This is due to the fact that non-uniform coating results from the agglomerated nano SiC in the PU matrix. The E corr (À144 mV) and i corr (2.22 mA/cm 2 ) values obtained for PU-PoPD coated brass are very low as compared to PU and PU-SiC coatings. Though blending of PU and PoPD polymers produce synergistic effect which results in the stronger adhesive property, the bonding strength between the polymers could be enhanced by the addition of nanoparticles. Very low i corr (1.34 mA/cm 2 ) is observed for PU-PoPD/SiC coated brass immersed in seawater for 1 d immersion. As the immersion period is increased to 60 d, the i corr value is increased to 3.98 mA/cm 2 . However, these parameters do not enhance too high as compared to 1 d immersion. This is because of a strong bonding formation between the polymers due to the addition of SiC nanoparticles and its synergistic effect on the brass surface. Thus, the lower values of i corr for PU-PoPD/SiC coated brass indicate the better bonding strength on the brass surface. Thus, PU-PoPD/SiC-coated brass shows maximum corrosion-resistant behavior even for a prolonged period of immersion. The electrochemical performance of certain comparable coating materials is presented in Table 3. The electrochemical performance of the PU-PoPD/SiC nanocomposite was found to be better than that of other nanocomposites. [44][45][46][47][48][49][50][51][52][53][54][55][56] SEM/EDX analysis of the coated brass The SEM analyses of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass after 60 d in seawater are shown in Figure 10(a,c,e,g), respectively. It is observed from Figure 10(a), pure PU coated brass show non-uniform coatings with pores/defects. The pores in the PU coating cause reduced bonding strength with the metal/alloy interface. This helps to increase the movement of corrosive ions into the metal specimen and causes dissolution of Zinc and copper. In Figure  10(c), PU-SiC coatings show non-uniform distribution on the brass surface, with agglomeration of SiC nanoparticles. This weakens the strength of the bonding of PU with SiC as well as with the brass surface. When these coated specimens immersed in seawater, negative ions easily interact with the brass interface. The coatings are uniform in PU-PoPD coated brass but crack is observed on the surface in Figure 10(e). This is due to the less bonding strength between the polymers and improper blending at few places in its structure. It is observed from the figure, the PU-PoPD coatings are strongly adhered to the brass interface as compared to the PU-SiC coatings, because of its synergistic effects. In case of the PU-PoPD/SiC coated brass (Figure 10(g)), uniform coatings without pores and cracks are observed. This is due to the well functionalization of SiC nanoparticles with PoPD and its uniform dispersion in PU leads to strong binding This work between the polymers and the brass interface. This also helps to enhance the superior adhesion strength of the nanocomposite with the brass surface. The elemental analyses of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass immersed in seawater are displayed respectively in Figure 10(b,d,f,h). The detection of C, O, and N proves the PU-coated brass as shown in Figure 10(b). The pores/defects in the PU coating permit the movement of aggressive ions into the brass/coating interface. The increased peaks of Cu and Zn confirm the greater exposure of the coated brass with the electrolytes. It is confirmed from the figures that the peak obtained for Zn is high as compared to Cu, which clearly indicates that dissolution of Zn is higher because of its higher reactivity. Hence, the corrosion products are formed in a shorter period of exposure to the electrolytes. The PU-SiC coated brass ( Figure  10(d)) confirms that the C, N, O, Si, Cu, and Zn elements are present. Here, the Cu and Zn peaks are reduced in comparison to pure PU coat. This is due to the bonding of SiC nanoparticles with PU. From Figure 10(f), it is observed that PU-PoPD coatings on brass show that the C, O, N, Cu, and Zn elements are present. The peaks obtained for Cu and Zn are reduced as compared to PU-SiC coating, which confirms less interactions of corrosive ions with the brass interface due to its strong bonding to the brass interface. Therefore, less dissolution of Cu and Zn is occurred. In case of PU-PoPD/SiC coated brass, the C, O, N, Si, Cu, and Zn elements are present, confirming the presence of polymers and SiC nanoparticles in the coatings ( Figure  10(h)). The Cu and Zn peaks are considerably shortened, which confirm very less interaction of anions to the brass interface. This proves the uniform distribution of functionalized PoPD in the PU coatings on the brass specimen. Hence, PU-PoPD/SiC coated brass showed enhanced protective performance with less dissolution of Cu and Zn in brass.  [57] In general, when the brass is exposed to seawater, Cu is changed to Cu 2 O initially. Then, it interacts with chlorides in the electrolytes to form CuCl, which produces end-degradation materials consisting of Cu 2 (OH) 3 Cl. In Figure 11(a), pure PU displays maximum dissolution of Cu-Zn and minimum dissolution of Zn to form the degradation products (Zn 5 (OH) 8 Cl 2 and Zn 12 (OH) 15 Cl 3 (SO 4 ) 3 5H 2 O). However, the degradation products are formed quickly when the coated brass is exposed to pronged period (60 d) in seawater (Figure 11(b)) as compared to shorter period (1 d). This is because of the pore/defects in the PU coating due to non-uniformity. However, for the PU-SiC coated brass, the peaks are slightly reduced for the degradation products without significant changes. This is because of the reduction in the strength of the bonding due to agglomerated nano SiC in the PU. This leads to the penetration of aggressive chloride ions into the interface of coating/alloy which causes the formation of degradation products. The PU-PoPD coated brass shows reduced peak intensity for Cu-Zn phase due to their synergistic effects. The PU-PoPD/SiC coated brass displays drastic reduction of all the peaks in 1 d exposure of seawater. It proves the efficient blending of PoPD/SiC with PU. Therefore, less corrosion products are observed even after a long exposure to seawater (60 d). The peak intensity for corrosion products is considerably reduced in 60 d immersion in comparison with PU-PoPD, and PU-SiC coatings. Hence, PU-PoPD/SiC coatings showed high corrosion protection to the brass surface even if it is exposed to seawater for a longer period. Figure 12(a,b) displays the permeability of the oxygen and water curves for pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass exposed to seawater for 1, 15, 30, and 60 d. The permeability of oxygen gas and water molecules in the coated specimen was measured at 25 C under pressure of 10 bar. It is shown that the permeability of oxygen and water in the brass surface is very high for PU coatings, because of its non-uniformity and porosity. As the immersion period increases, the oxygen and water permeability are also very high, which induce the corrosion process on the alloy specimen. The oxygen and water permeability in the PU-SiC coatings is somehow reduced because of the addition of nano SiC particles to the PU matrix, but the agglomerated nano SiC particles reduce the bonding strength toward the surface of the brass. For the PU-PoPD coated brass, the permeability of oxygen gas and water in the alloy surface are reduced because of its synergistic effects, which lead to strong adhesion strength to the brass surface. The addition of SiC nanoparticles to PoPD and its dispersion in PU coating results in the reduction of oxygen and water permeability in the PU-PoPD/SiC nanocomposite. It is proved that the PU-PoPD/SiC coatings block the path of the O 2 and water molecules to move through the coating, because of strong bonding formation between the polymers because of the surface modification of nano SiC particles by PoPD. The permeability of oxygen and water toward the brass surface is well controlled when the PU-PoPD/SiC coated samples are immersed in seawater for long period up to 60 d. Thus, the best O 2 gas barrier performance and less water permeability were obtained for the PU-PoPD/SiC coatings compared to other investigated coatings.    Figure 13(b,c) shows the values obtained by hardness and tensile tests of pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass without and with exposure to seawater for 1, 15, 30, and 60 d. The hardness value (410-100 MPa) and tensile strength  of pure PU resin are decreased completely as the exposure to seawater is increased from 1 to 60 d. It confirms that PU coating has poor mechanical properties and allows the corrosion to occur rapidly on the metal surface through the exposure of corrosive ions. Hence, it reduces the bonding capacity of the coatings with brass surface. The PU-SiC coated brass shows the decreased values of hardness from 820 MPa (1 d) to 520 MPa (60 d) and the tensile capacity values are decreased from 80 MPa (1 d) to 50 MPa (60 d). The hardness and tensile strength value are slightly varied from 1 to 60 d immersion, because of agglomerated nano SiC particles in PU, which generally supports the nonuniformity of the coatings. The PU-PoPD coated brass displays the increased value of hardness of 1140 MPa at 1 d and its value is decreased to 900 MPa at 60 d. On the other hand, it shows increased value of tensile strength of 102 MPa at 1 d and its value is decreased to 78 MPa at 60 d. However, the PU-PoPD/SiC coated brass shows an enhanced hardness and tensile strength values as compared to PU, PU-PoPD, and PU-SiC coatings. There is only a slight variation of hardness values (1330 MPa at 1 d to 1150 MPa at 60 d) and tensile strength (135 MPa at 1 d to 120 MPa at 60 d). This is because of the perfect surface modification of nano SiC particles with PoPD and its uniform distribution in the PU, which makes strong adhesion between the polymers and the brass interface. Hence, the PU-PoPD/SiC coated brass could act as superior protective coatings to the brass surface even for a prolonged exposure to seawater. Figure S3(a-d) shows the macroscopic morphologies of (a) pure PU, (b) PU-SiC, (c) PU-PoPD, and (d) PU-PoPD/SiC nanocomposite-coated brass after 60 d of exposure to natural seawater. Polymeric coatings on metal surfaces improved corrosion resistance in general. [58] On the sample's scribed surface, the extent of corrosion formation was investigated. The accumulation of corrosion products grows as exposure time increases, and the corrosion products become darker. The corrosion products for the PU coatings are piled up after 60 d, as shown in Figure S3(a). Due to the non-uniform PU coatings on the brass surface, salt sprays flowed into the scribed surface due to gravity, causing further corrosion through deep negative ion interactions in the brass interface. On the PU-SiC and PU-PoPD coatings, the corrosion product is slightly regulated in a few areas ( Figure S3(b,c)). The production of corrosion products on the brass interface is well controlled with the PU-PoPD/SiC coating. After being sprayed in seawater for 60 d, a little amount of corrosion products formed at the corner of the PU-PoPD/SiC coating ( Figure S3(d)). This demonstrates that the incorporation PoPD functionalized SiC in the PU coatings on brass have improved corrosion protection.

Mechanism
Pure PU coatings' corrosion resistance is mostly determined by their cross-link density and protective performance enhancements. Long-term hydrolytic interaction of PU coatings in an electrolyte, on the other hand, generates holes and cracks in their surface, allowing corrosive substances like H 2 O, O 2 , and Cl À ions to easily flow to the surface of the coated metal substrate, as illustrated in Figure S4. As a result, oxidation and reduction reactions occur at the coating/brass interface. The film's pore resistance changes as the immersion time in an aggressively corrosive solution increases. The electrolyte enters the coating in two ways, according to this. The corrosive solution can penetrate the capillaries first, causing coating defects; this corrosive solution is known as the capillary phase. Second, the high adhesive strength of the coating and the brass prevents the electrolytes from accessing the brass surface, slowing the deterioration process. [59] As a result, bonding strength is one of the most crucial factors impacting the corrosion protection efficacy of coatings. Redox reactions (Cu þ to Cu 2þ and Zn to Zn 2þ ) are promptly triggered when harsh ions permeate the brass substrate. After that, Cu 2þ and Zn 2þ react with electrolyte (OH À ion) to generate corrosion products, such as Zn 5 (OH) 8 Cl 2 , Zn 12 (OH) 15 Cl 3 (SO 4 ) 3 5H 2 O, Cu 2 SO 4 (OH)6H 2 O, Cu 2 Cl (OH) 3 , and Cu-Zn on pure PU, PU-SiC, PU-PoPD, and PU-PoPD/SiC coated brass. The inclusion of PoPD/SiC increases the barrier properties of PU coatings by restricting the flow of ions to the bare brass substrate.
The PU-SiC-coated brass produced fewer intermediates than the PU-coated brass. The barrier properties of the PU-SiC coating have improved. In contrast, SiC nanoparticles do not mix well with PU and aggregate together, resulting in poor dispersion and adhesion on brass surfaces. Because PU-SiC coatings are not homogeneous due to agglomeration, they have a greater barrier performance. To improve good distribution and interaction with the PU matrix, SiC nanoparticles were treated with PoPD. PoPD suppresses aggregation and resulting in a uniform PU dispersion when added to SiC nanoparticles. The amino groups of the PoPD molecule serve as active sites for coupling modifications while also inhibiting oxidation. When aggressive chloride ions are kept from penetrating the bare brass surface by these composite coatings immersed in an electrolyte, corrosion is prevented. As a result, PoPD/SiC nanoparticles have been found to improve PU coating barrier characteristics.

Conclusions
This work is reported about the fabrication of PU-PoPD/SiC nanocomposite as superior corrosion protection and adhesion strength of the coating. Functionalized PoPD/SiC nanoparticles effectively increased the barrier and mechanical properties of PU and thereby reduced the corrosion process on brass when coated specimens are exposed to seawater even for a longer period. The PU-PoPD/SiC nanocomposite provided protection against corrosion and exhibited good mechanical properties. The well distribution of PoPD/SiC hybrid nanofiller (2.0 wt.%) in the PU resin led to the highest micro/nanoroughness, surface heterogeneity, and corrosion protection for the brass surface. The electrochemical studies were carried out through surface exposure to three natural seawater to evaluate the corrosion barrier properties of the nanocomposites. SECM, Tafel polarization, and EIS measurements showed that the corrosion protection of the coating was improved by the nanofiller addition. The highest impedance value was recorded for the PU-PoPD/SiC nanocomposite coating. The results of EIS revealed enhanced resistances of R coat as well as R ct and decreased values of capacitances for PU-PoPD/SiC coated brass. The mechanical tests confirmed the high adhesion as well as tensile strengths and also hardness for PU-PoPD/SiC coated brass. It is because of blending of two polymers with the inclusion of SiC nanoparticles, which showed uniform coatings with strong adhesion to brass. This outstanding corrosion protection of PU-PoPD/SiC coating was owed to the excellent distribution of PoPD/SiC nanofiller in the PU matrix. The developed PU-PoPD/SiC nanocomposite has the potential to be an affordable, long-lasting anticorrosion coating for a sustainable future.
The experimental details related to the article can be found in the Supporting information file. FTIR analysis of (a) SiC, (b) PoPD, and (c) PoPD/SiC and Nyquist plots obtained for bare Cu-Zn alloy, and the brass coated with PU, PU with various % of PoPD/SiC nanoparticles in the seawater are provided. The macroscopic morphologies of nanocomposite-coated brass and graphics for protection mechanism are included in the Supporting information file.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Data availability statement
The data that support the findings of this study are available on request from the corresponding author.