Research on Nano-Titanium Modiﬁed Phenolic Resin Coating and Corrosion Resistance

: Nano-titanium can be used in the ﬁeld of anticorrosive coatings due to its excellent corrosion resistance. In this paper, phenolic resin was modiﬁed by nano-titanium using a physicochemical method. The nano-titanium-modiﬁed phenolic resin was used as a matrix to prepare the anticorrosive coating. The microstructures of the coatings were analyzed by Scanning Electron Microscope (SEM), X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). Raman and UV spectrum adhesion of the coating was tested by a scratching method. The corrosion behavior was studied by electrochemical workstation and salt spray test. The results showed that the corrosion resistance of pure phenolic resin coating was signiﬁcantly improved by the nano-titanium-modiﬁed phenolic resin. The coating containing 4% titanium nanoparticles exhibited the best corrosion resistance, with the highest impedance and the smallest corrosion current. The coating remained intact after 480 h of salt spray, showing the best salt spray resistance performance.


Introduction
Corrosion is one of the most prevalent hazardous problems in all types of industries, with economic losses of USD 2.5 trillion annually [1].Corrosion of materials can be effectively reduced through appropriate corrosion control techniques.Currently, one of the most effective corrosion control techniques is to spray anti-corrosion coatings on metal surfaces [2,3].
The most widely used anti-corrosion coatings are organic coatings [4][5][6][7].However, some corrosive mediums can reach the substrate along the pores in organic polymers, causing localized corrosion [8,9].In the last decade or so, nanomaterials have often been filled into organic polymers to improve their corrosion resistance [10][11][12], such as graphene [13], silicon dioxide (SiO 2 ) [14], titanium (Ti) [15] and TiO 2 [16].Nanomaterials can form a "labyrinth effect" in the coating with their lamellar structure and large specific surface area.The pores in the coating can be filled largely, and then the path for corrosive mediums to reach the substrate can be lengthened [17,18].
The latest research of nano-titanium-modified polymers focused on the relationship between coating formulations, organizational structure, properties such as corrosion resistance and weathering resistance, and the development of high-performance nano-titanium titanium coatings.Chen et al. [19] have found that a titanium enamel phenol polymer (UTP) can alter the interlayer spacing of montmorillonite (OMMT), which enhanced the compatibility of UTPOMMT with epoxy resin (EP).Meanwhile, UTPOMMT possessed a zigzagging and complex pathway, which resulted in a coating with excellent densification and corrosion resistance.Guo et al. [20] used the in situ growth method to prepare KGO@LM fillers with good dispersion and compatibility in EP coatings so that EP coatings have excellent long-term corrosion resistance.
Phenolic resin (PF) is the first synthetic polymer material, which is formed by the polycondensation of phenols and aldehydes in the presence of a catalyst [21,22].Due to its excellent corrosion resistance and mechanical properties, it is widely used in the field of anti-corrosion coatings [23,24].Phenolic resin has become an important polymer material in the fields of aviation, shipping, power generation and construction.With the demand of science and technology and social development, high anti-corrosion performance phenolic resin has become a new development direction.In recent years, researchers have grafted various metals or metal oxides into phenolic resins to improve their corrosion resistance, such as magnesium, zinc and aluminum oxide [25,26].
Titanium has good corrosion resistance because it can easily form a dense titanium dioxide film in the air [27,28].The study aims to develop a novel waterborne anti-corrosion coating using nano-titanium and phenolic resin.In doing so, a nano-titanium-modified phenolic resin was prepared by a physicochemical method.The corrosion resistance of the coating with nano-titanium-modified phenolic resin was studied using an electrochemical workstation and salt spray test.The microstructures of the coating were analyzed by SEM, EDS, FTIR, and Raman and UV spectrum.

Experimental 2.1. Materials
The titanium nanopowder used in the test was purchased from Wuxi Dinglong Mining Co., Ltd.(Wuxi, China).The phenolic resin, NL curing agent and silane coupling agent KH550 were provided by Henan Huanshan Industry Co., Ltd.(Zhengzhou, China).The anhydrous ethanol solution and acetone solution were purchased from Taicang Xintian Alcohol Co. (Taicang, China).The anhydrous ethanol and acetone solution are analytically pure, and the rest of the above materials are industrial grade.Deionized water was used during the test.

Preparation of Nano-Titanium Modified Phenolic Resin
Nano-titanium-modified phenolic resins with 3 wt.%, 4 wt.% and 5 wt.% titanium nanopowder were prepared according to the following steps.
The nano-titanium-modified phenolic resin was prepared using a cylindrical flask at room temperature.Firstly, deionized water was added to the phenolic resin and magnetic stirred at 500-600 r/min for 5 min.Then, silane coupling agent KH550 and a certain amount of titanium powder were added and stirred at 300 r/min for 10 min to obtain a dark green slurry-like modified product.In this process, the molecular chain of the phenolic resin was broken, and the titanium was grafted onto the phenolic resin based on the principle of mechanochemistry.The formulation of nano-titanium-modified phenolic resin is shown in Table 1.

Preparation of Coatings
A certain amount of phenolic resin was first dissolved with deionized water.Then, nano-titanium-modified phenolic resin was added and magnetically stirred at 500 r/min to disperse homogeneously.Finally, a certain amount of curing agent was added and dispersed homogeneously with a magnetic stirrer to obtain the nano-titanium-modified phenolic resin anticorrosive coatings.The reference formula is shown in Table 2.The substrate was Q235 steel panels with dimensions of 30 mm × 20 mm × 0.2 mm.The surface of the steel panels was first polished with 200# sandpaper and then cleaned with acetone solution and ethanol solution to remove the rust or dirt on the surface.Finally, the steel panels were put into the vacuum drying oven for drying.The coating was uniformly applied to the substrate with a special brush and cured at room temperature (25 ± 2 • C) for 168 h to complete the coating production.The Ti-PF coating preparation process is illustrated in Figure 1.
Coatings 2023, 13, x FOR PEER REVIEW 3 of 15 to disperse homogeneously.Finally, a certain amount of curing agent was added and dispersed homogeneously with a magnetic stirrer to obtain the nano-titanium-modified phenolic resin anticorrosive coatings.The reference formula is shown in Table 2.The substrate was Q235 steel panels with dimensions of 30 mm × 20 mm × 0.2 mm.The surface of the steel panels was first polished with 200# sandpaper and then cleaned with acetone solution and ethanol solution to remove the rust or dirt on the surface.Finally, the steel panels were put into the vacuum drying oven for drying.The coating was uniformly applied to the substrate with a special brush and cured at room temperature (25 ± 2 °C) for 168 h to complete the coating production.The Ti-PF coating preparation process is illustrated in Figure 1.

Structural Characterizations and Performance Testing
The morphology of the nano-titanium-modified phenolic resin coating was analyzed using an SEM equipped with an EDS that scanned a 1 cm × 1 cm surface of the coating after gold spraying.
FTIR was used to characterize the synthesized sample coatings by taking a point in the coating with absorption peaks ranging from 4000 to 500 cm −1 with a resolution of 4.0 cm −1 .
A UV-visible spectrophotometer was used to determine the UV absorption of the coating.
Raman spectroscopy tests were conducted using a laser Raman spectrometer (WiTech alpha 300R, Barberà del Vallès, Spain) using a wavelength of 532 nm under the ambient atmosphere.The spectral acquisition times were 5 scans accumulated with 10 s/scan.Each time, 10 spots distributed over the surface of the specimens were tested to ensure representativeness of the results.

Structural Characterizations and Performance Testing
The morphology of the nano-titanium-modified phenolic resin coating was analyzed using an SEM equipped with an EDS that scanned a 1 cm × 1 cm surface of the coating after gold spraying.
FTIR was used to characterize the synthesized sample coatings by taking a point in the coating with absorption peaks ranging from 4000 to 500 cm −1 with a resolution of 4.0 cm −1 .
A UV-visible spectrophotometer was used to determine the UV absorption of the coating.
Raman spectroscopy tests were conducted using a laser Raman spectrometer (WiTech alpha 300R, Barberà del Vallès, Spain) using a wavelength of 532 nm under the ambient atmosphere.The spectral acquisition times were 5 scans accumulated with 10 s/scan.Each time, 10 spots distributed over the surface of the specimens were tested to ensure representativeness of the results.
The adhesion of coatings was tested using the cross-cut referring to ISO 2409:2020 [29].
The corrosion behavior of the coatings was tested by a natural salt spray test referring to ISO 9227:2017 [30].
Electrochemical measurements of coatings were performed using an electrochemical workstation (CHI-760E China).A three-electrode system was formed by the sample, platinum sheet, and calomel electrode with saturated KCI solution.Electrochemical impedance spectroscopy was tested at 10 −2 -10 5 Hz range and used 10 mV amplitude.During testing, the sample was tested with a 1 cm 2 bare leakage area.

Results and Discussion
3.1.Scanning Electron Microscope and X-ray Spectroscopy Analysis SEM images of phenolic resin coatings with different mass fractions of nano-titanium are shown in Figure 2. The bright-colored dots are titanium nanopowders in Figure 2. It can be seen that the nano-titanium has not agglomerated in the form of spheres.As shown in Figure 2a,b, there were some holes and a large number of defects in the coating with 3% titanium nanopowder.Some of the titanium nanopowder was not dispersed uniformly, and agglomerates were formed, which attenuates the small size effect of the nanomaterials.As shown in Figure 2c,d, there were holes and a small number of defects in the coating with 4% nano-titanium powder.The addition of nano-titanium powder increased the roughness of the coating.There is no obvious agglomeration, although the distribution of nano-titanium powder is more scattered.As shown in Figure 2e,f, there were holes and a large number of defects in the coating with 5% nano-titanium powder.The agglomeration of nano-titanium powder was the most serious phenomenon, which may be due to the high mass fraction of nano-titanium powder relative to the phenolic resin.According to the above surface morphology, it can be inferred that the coating containing 4% nano-titanium powder has fewer holes and defects, and the dispersion degree of the nano-titanium powder and the phenolic resin is the largest.The compatibility of the interaction between them is good so that the nanoparticles can give full play to their roles.
The adhesion of coatings was tested using the cross-cut referring to ISO 2409:2020 [29].
The corrosion behavior of the coatings was tested by a natural salt spray test referring to ISO9227:2017 [30].
Electrochemical measurements of coatings were performed using an electrochemical workstation (CHI-760E China).A three-electrode system was formed by the sample, platinum sheet, and calomel electrode with saturated KCI solution.Electrochemical impedance spectroscopy was tested at 10 −2 -10 5 Hz range and used 10 mV amplitude.During testing, the sample was tested with a 1 cm 2 bare leakage area.

Scanning Electron Microscope and X-ray Spectroscopy Analysis
SEM images of phenolic resin coatings with different mass fractions of nano-titanium are shown in Figure 2. The bright-colored dots are titanium nanopowders in Figure 2. It can be seen that the nano-titanium has not agglomerated in the form of spheres.As shown in Figure 2a,b, there were some holes and a large number of defects in the coating with 3% titanium nanopowder.Some of the titanium nanopowder was not dispersed uniformly, and agglomerates were formed, which attenuates the small size effect of the nanomaterials.As shown in Figure 2c,d, there were holes and a small number of defects in the coating with 4% nano-titanium powder.The addition of nano-titanium powder increased the roughness of the coating.There is no obvious agglomeration, although the distribution of nano-titanium powder is more scattered.As shown in Figure 2e,f, there were holes and a large number of defects in the coating with 5% nano-titanium powder.The agglomeration of nano-titanium powder was the most serious phenomenon, which may be due to the high mass fraction of nano-titanium powder relative to the phenolic resin.According to the above surface morphology, it can be inferred that the coating containing 4% nanotitanium powder has fewer holes and defects, and the dispersion degree of the nano-titanium powder and the phenolic resin is the largest.The compatibility of the interaction between them is good so that the nanoparticles can give full play to their roles.EDS analysis was used to understand the distribution of titanium nanoparticles in Ti-PF coatings.As shown in Figure 3, the black part was PF and the bright part was titanium nanoparticles.As shown in the red areas of Figure 3b,d,f, it can be seen that the 3% Ti-PF coating had very little titanium nanoparticles and obvious agglomeration.The 4% Ti-PF coating had high titanium nanoparticle content and a more uniform distribution, with only a small portion of aggregation.The 5% Ti-PF coating had an uneven distribution of titanium nanoparticles and an obvious agglomeration phenomenon.The agglomeration phenomenon was attributed to the different chemical properties of PF and titanium nanoparticles, which caused titanium nanoparticles to self-polymerize.Overall, the distribution of titanium nanoparticles in PF was more uniform when the content of titanium nanoparticles was 4%.The anti-corrosion effect was the best.EDS analysis was used to understand the distribution of titanium nanoparticles in Ti-PF coatings.As shown in Figure 3, the black part was PF and the bright part was titanium nanoparticles.As shown in the red areas of Figure 3b,d,f, it can be seen that the 3% Ti-PF coating had very little titanium nanoparticles and obvious agglomeration.The 4% Ti-PF coating had high titanium nanoparticle content and a more uniform distribution, with only a small portion of aggregation.The 5% Ti-PF coating had an uneven distribution of titanium nanoparticles and an obvious agglomeration phenomenon.The agglomeration phenomenon was attributed to the different chemical properties of PF and titanium nanoparticles, which caused titanium nanoparticles to self-polymerize.Overall, the distribution of titanium nanoparticles in PF was more uniform when the content of titanium nanoparticles was 4%.The anti-corrosion effect was the best.EDS analysis was used to understand the distribution of titanium nanoparticles in Ti-PF coatings.As shown in Figure 3, the black part was PF and the bright part was titanium nanoparticles.As shown in the red areas of Figure 3b,d,f, it can be seen that the 3% Ti-PF coating had very little titanium nanoparticles and obvious agglomeration.The 4% Ti-PF coating had high titanium nanoparticle content and a more uniform distribution, with only a small portion of aggregation.The 5% Ti-PF coating had an uneven distribution of titanium nanoparticles and an obvious agglomeration phenomenon.The agglomeration phenomenon was attributed to the different chemical properties of PF and titanium nanoparticles, which caused titanium nanoparticles to self-polymerize.Overall, the distribution of titanium nanoparticles in PF was more uniform when the content of titanium nanoparticles was 4%.The anti-corrosion effect was the best.

Fourier Transform Infrared Spectroscopy and Raman Spectrum Characterization
The FTIR spectra of the modified phenolic resin coatings with different titanium nanoparticle content are shown in Figure 4.By comparison, it was found that the Ti-PF coatings showed IR absorption peaks specific to phenolic resin.The stretching vibration peaks of benzene ring C-H bond at 3448 cm −1 and aldehyde group -CH2 at 2971 and 2859 cm −1 were found.A new absorption peak appeared in the KH550 modification, which was different from the pure PF coatings.The C-O-C asymmetric vibration peak at 1050 cm −1 and the Si-O-Ti asymmetric vibration peak at 642 cm −1 were found in the Ti-PF coatings, indicating a silane coupler in the phenolic resin.Si-O-Ti, with a characteristic absorption peak at 642 cm −1 , indicated that silane coupling agent KH550 was grafted with titanium nanopowder [31], and nano-titanium-modified phenolic resin was prepared.The 4% Ti-PF coating with a characteristic absorption peak at 642 cm −1 had the highest absorption intensity, which indicated that the surface grafting was maximally accomplished with 4% nanotitanium content.When the nano-titanium content was 3%, less grafting and coating were carried out with phenolic resin due to the small amount of nano-titanium content.When the nano-titanium content was 5%, agglomeration of nano-titanium and phenolic resin took place, and only a small portion of the grafting and coating was accomplished.

Fourier Transform Infrared Spectroscopy and Raman Spectrum Characterization
The FTIR spectra of the modified phenolic resin coatings with different titanium nanoparticle content are shown in Figure 4.By comparison, it was found that the Ti-PF coatings showed IR absorption peaks specific to phenolic resin.The stretching vibration peaks of benzene ring C-H bond at 3448 cm −1 and aldehyde group -CH 2 at 2971 and 2859 cm −1 were found.A new absorption peak appeared in the KH550 modification, which was different from the pure PF coatings.The C-O-C asymmetric vibration peak at 1050 cm −1 and the Si-O-Ti asymmetric vibration peak at 642 cm −1 were found in the Ti-PF coatings, indicating a silane coupler in the phenolic resin.Si-O-Ti, with a characteristic absorption peak at 642 cm −1 , indicated that silane coupling agent KH550 was grafted with titanium nanopowder [31], and nano-titanium-modified phenolic resin was prepared.The 4% Ti-PF coating with a characteristic absorption peak at 642 cm −1 had the highest absorption intensity, which indicated that the surface grafting was maximally accomplished with 4% nano-titanium content.When the nano-titanium content was 3%, less grafting and coating were carried out with phenolic resin due to the small amount of nano-titanium content.When the nano-titanium content was 5%, agglomeration of nano-titanium and phenolic resin took place, and only a small portion of the grafting and coating was accomplished.

Fourier Transform Infrared Spectroscopy and Raman Spectrum Characterization
The FTIR spectra of the modified phenolic resin coatings with different titanium nanoparticle content are shown in Figure 4.By comparison, it was found that the Ti-PF coatings showed IR absorption peaks specific to phenolic resin.The stretching vibration peaks of benzene ring C-H bond at 3448 cm −1 and aldehyde group -CH2 at 2971 and 2859 cm −1 were found.A new absorption peak appeared in the KH550 modification, which was different from the pure PF coatings.The C-O-C asymmetric vibration peak at 1050 cm −1 and the Si-O-Ti asymmetric vibration peak at 642 cm −1 were found in the Ti-PF coatings, indicating a silane coupler in the phenolic resin.Si-O-Ti, with a characteristic absorption peak at 642 cm −1 , indicated that silane coupling agent KH550 was grafted with titanium nanopowder [31], and nano-titanium-modified phenolic resin was prepared.The 4% Ti-PF coating with a characteristic absorption peak at 642 cm −1 had the highest absorption intensity, which indicated that the surface grafting was maximally accomplished with 4% nanotitanium content.When the nano-titanium content was 3%, less grafting and coating were carried out with phenolic resin due to the small amount of nano-titanium content.When the nano-titanium content was 5%, agglomeration of nano-titanium and phenolic resin took place, and only a small portion of the grafting and coating was accomplished.peaks appeared for 3%, 4% and 5% nano-titanium-modified phenolic resin coatings.The nano-titanium and phenolic resin completed the grafting, but no obvious characteristic peaks appeared, which may be due to the agglomeration of nano-titanium.Only a small amount of nano-titanium and phenolic resin completed the grafting, and a large amount of phenolic resin still existed.When the content of nano-titanium is 4%, the wave peak is the highest, and the amount of nano-titanium grafted with phenolic resin is the largest.This result is consistent with the FTIR spectrum results.
Coatings 2023, 13, x FOR PEER REVIEW 7 of 15 peaks appeared for 3%, 4% and 5% nano-titanium-modified phenolic resin coatings.The nano-titanium and phenolic resin completed the grafting, but no obvious characteristic peaks appeared, which may be due to the agglomeration of nano-titanium.Only a small amount of nano-titanium and phenolic resin completed the grafting, and a large amount of phenolic resin still existed.When the content of nano-titanium is 4%, the wave peak is the highest, and the amount of nano-titanium grafted with phenolic resin is the largest.This result is consistent with the FTIR spectrum results.

UV Spectrum Analysis
Figure 6 shows the UV spectra of pure PF coatings and modified phenolic resin coatings with different titanium nanoparticle content.It can be seen that both PF coatings with and without modification have similar absorbance in the range of ≤185 nm.This may be due to the fact that phenolic resins have an aromatic ring containing unbonded electrons of oxygen and π-bonds formed by C=O.In the UV region of 185-400 nm, the absorption of nano-titanium-modified coatings is higher than that of pure PF coating.This indicates that the addition of modified titanium nanoparticles is beneficial to increase the absorption of the coatings.

UV Spectrum Analysis
Figure 6 shows the UV spectra of pure PF coatings and modified phenolic resin coatings with different titanium nanoparticle content.It can be seen that both PF coatings with and without modification have similar absorbance in the range of ≤185 nm.This may be due to the fact that phenolic resins have an aromatic ring containing unbonded electrons of oxygen and π-bonds formed by C=O.In the UV region of 185-400 nm, the absorption of nano-titanium-modified coatings is higher than that of pure PF coating.This indicates that the addition of modified titanium nanoparticles is beneficial to increase the absorption of the coatings.
Coatings 2023, 13, x FOR PEER REVIEW 7 of 15 peaks appeared for 3%, 4% and 5% nano-titanium-modified phenolic resin coatings.The nano-titanium and phenolic resin completed the grafting, but no obvious characteristic peaks appeared, which may be due to the agglomeration of nano-titanium.Only a small amount of nano-titanium and phenolic resin completed the grafting, and a large amount of phenolic resin still existed.When the content of nano-titanium is 4%, the wave peak is the highest, and the amount of nano-titanium grafted with phenolic resin is the largest.This result is consistent with the FTIR spectrum results.

UV Spectrum Analysis
Figure 6 shows the UV spectra of pure PF coatings and modified phenolic resin coatings with different titanium nanoparticle content.It can be seen that both PF coatings with and without modification have similar absorbance in the range of ≤185 nm.This may be due to the fact that phenolic resins have an aromatic ring containing unbonded electrons of oxygen and π-bonds formed by C=O.In the UV region of 185-400 nm, the absorption of nano-titanium-modified coatings is higher than that of pure PF coating.This indicates that the addition of modified titanium nanoparticles is beneficial to increase the absorption of the coatings.

Electrochemical Performance of Coatings
Figure 7 shows the electrochemical Tafel curves, the self-corrosion potentials and the self-corrosion currents of different coatings.There was a very small difference in the self-corrosion potentials among pure PF, 3%, 4% and 5% Ti-PF coatings.However, the difference in the self-corrosion currents was large.Among them, the 4% Ti-PF coating had the lowest self-corrosion current, indicating the best corrosion resistance.Figure 8 shows the impedance diagrams of pure PF coatings and modified phenolic resin coatings with different titanium nanoparticle content.It can be seen that the pure PF, 3%, 4% and 5% Ti-PF coatings have semicircular shaped impedance arcs, but the impedance arc diameter of the Ti-PF coating was significantly larger than that of the pure PF coating, and the impedance arc diameter of the 4% Ti-PF coating was much larger than that of the pure PF coating.This was because the higher the impedance value of the coating, the larger the arc diameter of the coating was.The 4% Ti-PF coating had the highest impedance value and, thus, the largest arc diameter.The impedance data were fitted by the equivalent circuit, and the fitting parameters for the impedance spectra of coatings are shown in Table 3.The equivalent circuit consisted of the solution resistance (R s ), the polarization resistance (R 1 ) and the non-ideal capacitor (CPE 1 ).Polarization resistance (R 1 ) was used to measure the corrosion resistance of the coating [32].It can be seen that the addition of nano-titanium increases the polarization resistance value of the coatings.One of the 4% Ti-PF coatings had the largest polarization resistance value, indicating the best corrosion resistance.The results are consistent with the impedance plots.Figure 9 shows the bode plots of nano-titanium composite coatings with different contents.It can be seen that the impedance modulus in the low-frequency region (|Z| 0.01 Hz ) of all the Ti-PF coatings with different contents is significantly higher than that of the pure PF coating [33].The 4% Ti-PF coating has the largest impedance modulus, which is about 1.08 × 10 4 Ω cm 2 .Normally, the impedance modulus at low frequency (|Z| 0.01 Hz ) is an important parameter for evaluating the corrosion resistance of the organic coatings.It is shown that the incorporation of nanotitanium significantly enhances the corrosion resistance of the pure PF coating.Similarly, the phase angle In the high-frequency region (|Z| 10 3 -10 4 Hz ) also reflects the corrosion resistance of the coatings well, which decreases gradually for the pure PF coatings but hardly changes for the Ti-PF coatings [34].The 4% Ti-PF coating showed the slowest decreasing trend, which indicates the best corrosion protection.

Electrochemical Performance of Coatings
Figure 7 shows the electrochemical Tafel curves, the self-corrosion potentials and the self-corrosion currents of different coatings.There was a very small difference in the selfcorrosion potentials among pure PF, 3%, 4% and 5% Ti-PF coatings.However, the difference in the self-corrosion currents was large.Among them, the 4% Ti-PF coating had the lowest self-corrosion current, indicating the best corrosion resistance.Figure 8 shows the impedance diagrams of pure PF coatings and modified phenolic resin coatings with different titanium nanoparticle content.It can be seen that the pure PF, 3%, 4% and 5% Ti-PF coatings have semicircular shaped impedance arcs, but the impedance arc diameter of the Ti-PF coating was significantly larger than that of the pure PF coating, and the impedance arc diameter of the 4% Ti-PF coating was much larger than that of the pure PF coating.This was because the higher the impedance value of the coating, the larger the arc diameter of the coating was.The 4% Ti-PF coating had the highest impedance value and, thus, the largest arc diameter.The impedance data were fitted by the equivalent circuit, and the fitting parameters for the impedance spectra of coatings are shown in Table 3.The equivalent circuit consisted of the solution resistance (Rs), the polarization resistance (R1) and the non-ideal capacitor (CPE1).Polarization resistance (R1) was used to measure the corrosion resistance of the coating [32].It can be seen that the addition of nano-titanium increases the polarization resistance value of the coatings.One of the 4% Ti-PF coatings had the largest polarization resistance value, indicating the best corrosion resistance.The results are consistent with the impedance plots.Figure 9 shows the bode plots of nano-titanium composite coatings with different contents.It can be seen that the impedance modulus in the low-frequency region (|Z|0.01HZ) of all the Ti-PF coatings with different contents is significantly higher than that of the pure PF coating [33].The 4% Ti-PF coating has the largest impedance modulus, which is about 1.08 × 10 4 Ω cm 2 .Normally, the impedance modulus at low frequency (|Z|0.01HZ) is an important parameter for evaluating the corrosion resistance of the organic coatings.It is shown that the incorporation of nano-titanium significantly enhances the corrosion resistance of the pure PF coating.Similarly, the phase angle In the high-frequency region (|Z|10 3 -10 4 HZ) also reflects the corrosion resistance of the coatings well, which decreases gradually for the pure PF coatings but hardly changes for the Ti-PF coatings [34].The 4% Ti-PF coating showed the slowest decreasing trend, which indicates the best corrosion protection.

Salt Spray Resistance Test
The results of the salt spray test are shown in Figure 10.After 480 h of exposure, the pure PF coatings showed severe corrosion damage at the scratches and blistering, rusting and pitting on the surface, which indicated that the pure PF coatings had poor corrosion resistance.After the addition of titanium nanoparticles, the corrosion damage of the Ti-PF coatings with different contents was greatly reduced after 480 h of exposure, which indicated that the salt spray resistance of the pure PF coatings could be significantly improved by the addition of nano-titanium nanoparticles.Among them, the surface of 4% Ti-PF coating again showed only a small amount of pitting corrosion, no blistering or corrosion diffusion, almost intact.This phenomenon was consistent with the results of the EIS test,

Salt Spray Resistance Test
The results of the salt spray test are shown in Figure 10.After 480 h of exposure, the pure PF coatings showed severe corrosion damage at the scratches and blistering, rusting and pitting on the surface, which indicated that the pure PF coatings had poor corrosion resistance.After the addition of titanium nanoparticles, the corrosion damage of the Ti-PF coatings with different contents was greatly reduced after 480 h of exposure, which indi-

Salt Spray Resistance Test
The results of the salt spray test are shown in Figure 10.After 480 h of exposure, the pure PF coatings showed severe corrosion damage at the scratches and blistering, rusting and pitting on the surface, which indicated that the pure PF coatings had poor corrosion resistance.After the addition of titanium nanoparticles, the corrosion damage of the Ti-PF coatings with different contents was greatly reduced after 480 h of exposure, which indicated that the salt spray resistance of the pure PF coatings could be significantly improved by the addition of nano-titanium nanoparticles.Among them, the surface of 4% Ti-PF coating again showed only a small amount of pitting corrosion, no blistering or corrosion diffusion, almost intact.This phenomenon was consistent with the results of the EIS test, indicating that the addition of nano-titanium nanoparticles can significantly improve the salt spray resistance of pure PF coatings.When the addition of titanium nanoparticles was 4%, the Ti-PF coating had the best salt spray resistance performance.indicating that the addition of nano-titanium nanoparticles can significantly improve the salt spray resistance of pure PF coatings.When the addition of titanium nanoparticles was 4%, the Ti-PF coating had the best salt spray resistance performance.In a further study of its corrosion resistance, 4% nano-titanium-modified phenolic resin coating was subjected to a salt spray resistance test for 768 h.The specific experimental process parameters are shown in Table 4.As shown in Figure 11, it can be seen that after 768 h of continuous spraying, the 4% Ti-PF coating showed crack-like corrosion, and the coating was completely destroyed in the corrosive medium.The corrosion resistance of the phenolic resin coating can be significantly improved by adding titanium nanoparticles.The nano-titanium-modified phenolic resin coatings first started to corrode from the periphery, which was due to the fact that the coating samples were not completely enclosed around the samples, and the sample substrates were exposed to the salt spray environment.The loss of solvents or volatiles in the coating, as well as the degradation and aging of the coating molecular chains, lead to changes in the surface state of the coating, which reduces the gloss of the coating.In a further study of its corrosion resistance, 4% nano-titanium-modified phenolic resin coating was subjected to a salt spray resistance test for 768 h.The specific experimental process parameters are shown in Table 4.As shown in Figure 11, it can be seen that after 768 h of continuous spraying, the 4% Ti-PF coating showed crack-like corrosion, and the coating was completely destroyed in the corrosive medium.The corrosion resistance of the phenolic resin coating can be significantly improved by adding titanium nanoparticles.The nano-titanium-modified phenolic resin coatings first started to corrode from the periphery, which was due to the fact that the coating samples were not completely enclosed around the samples, and the sample substrates were exposed to the salt spray environment.The loss of solvents or volatiles in the coating, as well as the degradation and aging of the coating molecular chains, lead to changes in the surface state of the coating, which reduces the gloss of the coating.

Adhesion Test
The adhesion of the modified phenolic resin coatings with different contents of nanotitanium is shown in Figures 12 and 13.It can be seen that the adhesion grade of the pure PF coating was grade 2, and the adhesion grade of the Ti-PF coatings was grade 1.The addition of titanium nanoparticles can improve the adhesion of the coating.The nanotitanium can form stable physical or chemical bonds with the substrate, which improves the cross-linking density of the coating, thus consolidating the adhesion and showing a better adhesion grade [35].

Adhesion Test
The adhesion of the modified phenolic resin coatings with different contents of nanotitanium is shown in Figures 12 and 13.It can be seen that the adhesion grade of the pure PF coating was grade 2, and the adhesion grade of the Ti-PF coatings was grade 1.The addition of titanium nanoparticles can improve the adhesion of the coating.The nanotitanium can form stable physical or chemical bonds with the substrate, which improves the cross-linking density of the coating, thus consolidating the adhesion and showing a better adhesion grade [35].

Anti-Corrosion Mechanism of Composite Coating
The addition of nanomaterials can precisely fill the pores in traditional coatings, which fundamentally blocks the penetration of external corrosive substances into the coating [36,37].Nano-titanium will form a dense oxide film at room temperature, which has excellent corrosion resistance.So, the introduction of nano-titanium in anti-corrosion coatings can enhance the corrosion resistance of the coating.
A nano-titanium-modified polymer was prepared by a physicochemical method in this study.Nano-titanium powder and phenolic resin were combined through a chemical bonding force to form a spatial mesh structure during magnetic stirring.The adhesion and hydrophobicity of the coating were enhanced by the modification.It was difficult for corrosive mediums to be adsorbed on the surface of the coating.The barrier ability of the coating was thus enhanced to the corrosive mediums [38].
The nano-titanium-modified phenolic resin can disperse nano-titanium uniformly in the system.A mesh structure was formed and effectively filled the defects in the coating.The infiltration of corrosive mediums was slowed down, and the "labyrinth effect" was increased [39].Therefore, the anti-corrosion effect of the coating was improved by the nano-titanium [40].

Anti-Corrosion Mechanism of Composite Coating
The addition of nanomaterials can precisely fill the pores in traditional coatings, which fundamentally blocks the penetration of external corrosive substances into the coating [36,37].Nano-titanium will form a dense oxide film at room temperature, which has excellent corrosion resistance.So, the introduction of nano-titanium in anti-corrosion coatings can enhance the corrosion resistance of the coating.
A nano-titanium-modified polymer was prepared by a physicochemical method in this study.Nano-titanium powder and phenolic resin were combined through a chemical bonding force to form a spatial mesh structure during magnetic stirring.The adhesion and hydrophobicity of the coating were enhanced by the modification.It was difficult for corrosive mediums to be adsorbed on the surface of the coating.The barrier ability of the coating was thus enhanced to the corrosive mediums [38].
The nano-titanium-modified phenolic resin can disperse nano-titanium uniformly in the system.A mesh structure was formed and effectively filled the defects in the coating.The infiltration of corrosive mediums was slowed down, and the "labyrinth effect" was increased [39].Therefore, the anti-corrosion effect of the coating was improved by the nano-titanium [40].

Anti-Corrosion Mechanism of Composite Coating
The addition of nanomaterials can precisely fill the pores in traditional coatings, which fundamentally blocks the penetration of external corrosive substances into the coating [36,37].Nano-titanium will form a dense oxide film at room temperature, which has excellent corrosion resistance.So, the introduction of nano-titanium in anti-corrosion coatings can enhance the corrosion resistance of the coating.
A nano-titanium-modified polymer was prepared by a physicochemical method in this study.Nano-titanium powder and phenolic resin were combined through a chemical bonding force to form a spatial mesh structure during magnetic stirring.The adhesion and hydrophobicity of the coating were enhanced by the modification.It was difficult for corrosive mediums to be adsorbed on the surface of the coating.The barrier ability of the coating was thus enhanced to the corrosive mediums [38].
The nano-titanium-modified phenolic resin can disperse nano-titanium uniformly in the system.A mesh structure was formed and effectively filled the defects in the coating.The infiltration of corrosive mediums was slowed down, and the "labyrinth effect" was increased [39].Therefore, the anti-corrosion effect of the coating was improved by the nano-titanium [40].

Figure 1 .
Figure 1.Schematic illustration of the preparation of Ti-PF coatings.

Figure 1 .
Figure 1.Schematic illustration of the preparation of Ti-PF coatings.

Figure 2 .
Figure 2. SEM images of (a) 3% Ti-PF and (b) the enlarged view of the red section in (a), SEM images of (c) 4% Ti-PF and (d) the enlarged view of the red section in (c), SEM images of (e) 5% Ti-PF and (f) the enlarged view of the red section in (e).

Figure 2 .
Figure 2. SEM images of (a) 3% Ti-PF and (b) the enlarged view of the red section in (a), SEM images of (c) 4% Ti-PF and (d) the enlarged view of the red section in (c), SEM images of (e) 5% Ti-PF and (f) the enlarged view of the red section in (e).

15 Figure 2 .
Figure 2. SEM images of (a) 3% Ti-PF and (b) the enlarged view of the red section in (a), SEM images of (c) 4% Ti-PF and (d) the enlarged view of the red section in (c), SEM images of (e) 5% Ti-PF and (f) the enlarged view of the red section in (e).

Figure 4 .
Figure 4. Infrared spectra of modified phenolic resin coatings with different titanium nanoparticle contents.The Raman spectra of the modified phenolic resin coatings with different nano-titanium contents are shown in Figure5.Compared with the pure PF coatings, absorption

Figure 4 .
Figure 4. Infrared spectra of modified phenolic resin coatings with different titanium nanoparticle contents.The Raman spectra of the modified phenolic resin coatings with different nano-titanium contents are shown in Figure5.Compared with the pure PF coatings, absorption

Figure 4 .
Figure 4. Infrared spectra of modified phenolic resin coatings with different titanium nanoparticle contents.The Raman spectra of the modified phenolic resin coatings with different nanotitanium contents are shown in Figure 5. Compared with the pure PF coatings, absorption

Figure 5 .
Figure 5. Raman spectra of modified phenolic resin coatings with different titanium nanoparticle contents.

Figure 6 .
Figure 6.UV spectra of nano-titanium composite coatings with different contents.

Figure 5 .
Figure 5. Raman spectra of modified phenolic resin coatings with different titanium nanoparticle contents.

Figure 5 .
Figure 5. Raman spectra of modified phenolic resin coatings with different titanium nanoparticle contents.

Figure 6 .
Figure 6.UV spectra of nano-titanium composite coatings with different contents.Figure UV spectra of nano-titanium composite coatings with different contents.

Figure
Figure 6.UV spectra of nano-titanium composite coatings with different contents.Figure UV spectra of nano-titanium composite coatings with different contents.

Figure 7 .
Figure 7. Tafel curves and related data for nano-titanium composite coatings with different contents.Figure 7. Tafel curves and related data for nano-titanium composite coatings with different contents.

Figure 7 .
Figure 7. Tafel curves and related data for nano-titanium composite coatings with different contents.Figure 7. Tafel curves and related data for nano-titanium composite coatings with different contents.

Figure 8 .
Figure 8. Impedance plot of nano-titanium composite coatings with different contents.

Figure 8 .
Figure 8. Impedance plot of nano-titanium composite coatings with different contents.

Figure 8 .
Figure 8. Impedance plot of nano-titanium composite coatings with different contents.

Figure 9 .
Figure 9. Bode plot of nano-titanium composite coatings with different contents.

Figure 13 .
Figure 13.Adhesion test of nano-titanium composite coatings with different contents.

Figure 13 .
Figure 13.Adhesion test of nano-titanium composite coatings with different contents.

Figure 13 .
Figure 13.Adhesion test of nano-titanium composite coatings with different contents.

Table 2 .
Reference formulations for nano-titanium phenolic resin coatings.

Table 2 .
Reference formulations for nano-titanium phenolic resin coatings.

Table 3 .
Equivalent circuit fitting parameters for impedance spectra of coatings.

Table 4 .
Process parameters for salt spray resistance test of coatings.Discoloration of the coating and rust spots on the surface of the boards 480 Discoloration of the coating and rust spots on the surface of the boards 576 Discoloration of the coating and rust spots on the surface of the boards Light rusting at scratches, rusting on the lower side of boards 672 Discoloration of the coating and rust spots on the surface of the boards Rust at scratches, rust spreading on the lower side of the panel and also on the left side

Table 4 .
Process parameters for salt spray resistance test of coatings.
480Discoloration of the coating and rust spots on the surface of the boards 576 Discoloration of the coating and rust spots on the surface of the boards Light rusting at scratches, rusting on the lower side of boards

h Nano-Titanium-Modified Phenolic Resin Coating 672
Discoloration of the coating and rust spots on the surface of the boards Rust at scratches, rust spreading on the lower side of the panel and also on the left side 768 Discoloration of coating, spreading of rust at scratches Cracked rust on panel