Effects of addition of polyvinyl pyrrolidone (PVP) as a corrosion inhibitor to Zr/V-based conversion coating on aluminum alloy

Polyvinyl pyrrolidone was selected as a green additive for zirconium-based conversion treatment on aluminum alloy substrate. Scanning electron microscope, energy dispersive spectroscopy and X-ray photoelectron spectroscopy were used to investigate the effect of polyvinyl pyrrolidone on the morphology and composition of the conversion coating. Neutral salt spray, electrochemical impedance spectroscopy, and Tafel polarization tests were used to characterize the corrosion resistance of the conversion coating. The results show that the conversion coating was mainly composed of metal oxide (ZrO2, VO2, V2O5, etc.), metal fluoride (ZrF4, Na3AlF6) and polyvinyl pyrrolidone. The density of Na3AlF6 crystal is increased by adding polyvinyl pyrrolidone into the conversion bath. This is due to the adsorption of polyvinyl pyrrolidone molecules on the surface of the substrate, which trap metal ions and thus facilitate the coating formation reaction. The corrosion resistance of the substrate treated by conversion coating containing polyvinyl pyrrolidone was significantly improved. When the amount of polyvinyl pyrrolidone introduced into the conversion bath is 1.5 g/L, the conversion coating can provide excellent corrosion resistance to the substrate. In addition, the non-polar groups in the polyvinyl pyrrolidone molecule can effectively improve the adhesion between the conversion coating and the electrophoretic coating.


Introduction
Aluminum alloy has the characteristics of low density, highspecific strength and ease of processing (Sheng et al. 2010;Odeshi et al. 2013;Holroyd NJH and Scamans GM 2016). It is widely used in transportation, packaging containers, aerospace, household appliances, machinery manufacturing, construction profiles, electronic communications and other fields (Sekularac and Milosev 2020;Sheng et al. 2010). Aluminum alloy has become one of the most widely used nonferrous metals. Inevitably, aluminum alloys also have some shortcomings, one of which is their weak resistance to corrosion (Burokas et al. 2009;Lan et al. 2012). When exposed to corrosive media, the aluminum alloy will be corroded in various forms such as pitting, crevice and filiform corrosion etc. (Zhong et al. 2013;Li et al. 2013;Guan et al. 2020). As a result, aluminum alloy products are at a high risk of corrosion during use. Due to the above reasons, aluminum alloy devices often need some effective anti-corrosion pretreatment before coating treatment.
There are many anti-corrosion methods for aluminum alloy, such as anodic oxidation, chemical conversion, electroplating and chemical plating (Li et al. 2016;Zhang et al. 2008;Din et al. 2015;Ezuber et al. 2008;Elsayed et al. 2016). Among them, titanium/zirconium-based (Ti/Zr-based) conversion treatment has been widely used in industry due to its advantages of convenient operation and low cost (Cerezo et al. 2014;Wong et al. 2022). In addition to improving the corrosion resistance of aluminum alloy, Ti/Zr-based conversion coating can also be used as an intermediate layer to improve the adhesion of organic coating. There are still many problems to be solved in order to perfectly apply Ti/Zr-based conversion treatment in industrial production. Among them, the effective improvement of the adhesion between the conversion coating and the organic coating is still an important research direction. One of the practical ways to overcome this problem is the addition of some organic additives to the composition of the conversion bath. These organics can provide special functional groups to bond with the organic coating, and the bonding force provided by this bonding method is much greater than the hydrogen bonding connection between the conversion coating and the organic coating (Dalmoro et al. 2013;Roland et al. 2016;Liu et al. 2022). It should be noted that these organic additives must meet the requirement of having good compatibility with organic coatings and also ensure that these organics are environmentally friendly.
Based on the above reasons, the polyvinyl pyrrolidone (PVP) was selected as a conversion bath additive. PVP is a green and water-soluble synthetic polymer with good emulsification and adhesion properties, low toxicity and good physiological compatibility (Ramezanzadeh B, et al. 2015). In addition, some excellent results have been obtained in the study of its application as a metal corrosion inhibitor (Yang et al. 2022;Khamis et al. 2018;Sheng et al 2010).
The main objective of this work is to investigate the effect of PVP as a green conversion bath additive on the corrosion resistance of conversion coatings and adhesion to electrophoretic coating. The surface morphology of the samples was characterized by scanning electron Microscope (SEM). The composition of the conversion coating was measured by X-ray energy dispersive spectroscopy (XPS). The electrochemical behavior and corrosion resistance of the conversion coating in the corrosive medium were characterized by the AC impedance test (EIS), Tafel polarization test and neutral salt spray test (NSS). The binding ability of electrophoretic coating and conversion coating was evaluated by pull-off test and accelerated corrosion test.

Materials and sample preparation
The matrix used in chemical conversion treatment was aluminum alloy with a dimension of 10 × 10 × 2 mm. The composition of aluminum alloy was shown in Table 1.
The matrices were polished with sand paper up to 2000 grade. The aluminum alloy was degreased with 10.0% NaOH aqueous solution, the treatment time was 1 min, and the treatment temperature was set at 313 K. After the degreasing treatment was completed, the aluminum alloy surface was completely cleaned with deionized water and dried at room temperature. Then, the aluminum alloy substrates were immersed in Zr/V conversion baths prepared with H 2 ZrF 6 (0.01 mol/L), NaVO 3 (5.0 g/L), NaNO 3 (2.0 g/L), NaF (0.5 g/L) and deionized water. The pH value of the conversion bath was adjusted to 4 with NaOH. All the chemical reagents were of analytical grade purity without further purification, and the water used in the progress of experiment was deionized water. The temperature of chemical conversion treatment was set at 308 K for 3 min. Finally, the as-prepared samples were rinsed with deionized water and dried in a vacuum oven for 1 h.

Characterization of Zr/V/PVP conversion coating
The surface morphology of the samples coated with Zr/V conversion coating (Zr/V) and Zr/V conversion coating containing PVA (Zr/V/PVP) was observed by scanning electron microscope (SEM) of TESCAN 3 model. The scanning electron microscope equipped with a fieldemission gun, operated at 20 kV. The chemical composition of Zr/V conversion coating was measured by X-ray photoelectron spectroscopy (XPS). The XPS analysis was performed on the ESCAL AB250 with a radiation source of Al Kα at a power of 300 W and a vacuum degree of 1.0 × 10 −5 Pa. The binding energies of XPS signals were corrected assuming the C1s peak at 284.6 eV. The test results were analyzed with XPSPEAK software.
The PARSTAT 2273 electrochemical workstation was used to perform Tafel polarization and electrochemical impedance spectroscopy (EIS) measurements. A conventional three-electrode cell system was used for all the electrochemical tests. An aluminum alloy plate with a cathode area of 1 cm 2 and a Pt plate (with a cathode area of 4 cm 2 ) were used as the counter and working electrodes. Saturated calomel electrode, model 232, made by INESA Scientific Instrument Co., Ltd (Shanghai China) was used as reference electrode. The electrolyte used in the electrochemical test is 3.5% NaCl solution.
Tafel polarization measurements were measured under the scanning rate of 1 mV/s. The EIS test was performed in the frequency range of 10 5 -0.01 Hz and signal amplitude of 2 mV. Before the EIS test was conducted, the working electrode was placed in 3.5% NaCl solution for a while until the open-circuit potential became stable.
Neutral spray salt (NSS) tests were conducted using 5% NaCl solution with the pH of 6.5-7.0 at 308 K. The samples were placed perpendicularly with an angle of 30°. In a spray cycle, the samples were continuously sprayed for 8 h and then kept in NSS chamber for 16 h.

Adhesion measurement
The electrophoretic coating was applied to an aluminum alloy substrate coated with a conversion coating using CathoGuard 800 electrophoretic paint. This process was completed by cathodic electrophoresis. Electrophoresis tank was made of polyvinyl chloride plates, with a size of 20 cm × 10 cm × 10 cm. Aluminum alloy substrate with and without conversion coating was used as cathode and anode respectively for electrophoretic deposition. During cathodic electrophoresis, the voltage of DC power supply was 280 V. The temperature was 303 K and the treatment time was 240 s. The samples after cathodic electrophoresis were dried at 423 K for 15 min. The samples were placed at room temperature for 48 h. Then, a standard pull test was carried out with a Posi adhesion tester. To ensure the accuracy of the results, all pull-off tests were repeated three times.
To evaluate the matching ability of conversion coating and electrophoretic coating, accelerated corrosion test was carried out on the sample after cathodic electrophoresis.
Before the test, it is necessary to scratch two lines along the diagonal of the sample with a blade at a constant speed. It should be noted that these two scratches must penetrate the electrophoretic painting and the chemical conversion coating. The accelerated corrosion test was conducted in 5% NaCl, the temperature was controlled at 328 K, and the test duration was 10 days. After the test, 3 M adhesive tape is used to remove the electrophoretic coating that cannot effectively adhere to the substrate near the scratch.

Results and discussion
The surface morphology of the aluminum alloy substrate covered with Zr/V conversion coating was characterized by SEM as shown in Fig. 1. It can be seen that the conversion coating on the substrate surface includes two forms: amorphous and crystalline. The amorphous conversion coating is thin and uniform, which is the conventional morphology of Zr/V conversion coating. The crystalline part of the Fig. 1 a Surface morphology of Zr/V/PVP conversion coating and EDS spectra of (b) position 1 and c position 2 conversion coating is composed of small cubes with a side length of 2 μm. The crystalline and amorphous components of the conversion coating were characterized by energy dispersive spectroscopy (EDS). The main elements of the crystalline portion of the conversion coating are Na, Al and F, and the main elements of the amorphous portion are V and Zr. Figure 2 shows the surface morphology of samples coated with Zr/V and Zr/V/PVP conversion coating. It can be seen that the concentration of PVP has a great influence on the density of crystalline particles on the surface of the conversion coating. When no PVP was introduced into the conversion bath, the surface of as-prepared sample was sparsely distributed with small grains. Obviously, these sparsely distributed grains are difficult to provide effective protection for aluminum alloy substrate. With the increase of PVP content in the conversion bath, the grain coverage on the substrate surface increases gradually. When the PVP content in the conversion bath reaches 1.5 g/L, these grains are uniform and dense and can perfectly cover the entire substrate surface. However, when the PVP content exceeds 2.0 g/L, the formation of these grains is inhibited, and the coverage rate of these grains on the surface of the aluminum alloy substrate decreases.
The XPS patterns of the Zr/V/PVP conversion coatings are displayed in Fig. 3. The conversion coating contains mainly Na, V, Al, Zr, O, F and C elements. The results are consistent with those obtained by EDS analysis. The peak at 400.2 eV originates from the N 1 s peak of PVP, indicating that a certain amount of PVP is adsorbed on the substrate surface.
The binding energies of the two peaks in the high-resolution diagram of Zr 3d were 184.9 eV and 182.6 eV, which correspond to Zr 3d3/2 and Zr 3d5/2 of ZrF 4 and ZrO 2 respectively (Cerezo et al. 2013). The C 1 s spectrum can be fitted into three main peaks. The first peak (at about 288.7 eV) of C 1 s spectrum is derived from carbonyl (C=O) of PVP. The second peak of C 1 s spectrum correspond to the contributions of hydrocarbon (Sheng et al. 2010). The valence state of vanadium is relatively complex. After peak fitting, the V 2p peak can be divided into 517.5 eV and 516.5 eV. It can be seen from the spectrum analysis and literature that 516.5 eV corresponds to V 2p3/2 (516.3 eV) in VO 2 (V 2 O 4 ), and the spectrum peak at 517.5 eV is consistent with V 2p3/2 (517.6 eV) peak of V 2 O 5 (Zou et al. 2011). The vanadium oxide in the coating is composed of the tetravalent compound VO 2 and the pentavalent compound V 2 O 5 . In addition, sodium mainly exists in the form of Na-Al-F. Fluorine mainly exists in the form of Na-Al-F and Al-F.
Combined with the results of SEM, EDS and XPS, the composition of the conversion coating can be determined in principle. The main components of the amorphous conversion coating are VO 2 , V 2 O 5 , ZrO 2 and ZrF 4 . The main component of crystalline conversion coating is Na 3 AlF 6 . In addition, PVP molecules are also adsorbed on the surface of the aluminum alloy substrate.
The Nyquist plots are shown in Fig. 4a. All curves present a single semicircle with different radii. The single time increases with the increase of PVP concentration in the conversion bath, indicating that PVP as an additive has effectively improved the corrosion resistance of the conversion coating. When the amount of PVP is higher than 1.5 g/L, the impedance of the conversion coating reaches Equivalent electrical circuit, as shown in Fig. 4a, is adopted to fit the electrochemical parameters of EIS measurement and investigate the electrochemical parameters of corrosion process. This equivalent circuit is suitable for analyzing the impedance data of traditional porous coated electrodes. In this equivalent circuit, R s represents solution resistance, R ct represents charge transfer resistance, R f represents film resistance, and C dl represents double-layer capacitance. CPE is a constant-phase element used as a substitute for pure capacitor element. In the actual electrochemical measurement, the frequency response characteristics of the electric double-layer capacitance at the interface between the solid electrode and the solution are not consistent with the 'pure capacitance'. There is a big or small deviation, which is shown as a semicircle distortion on the Nyquist diagram. Therefore, the electric double-layer capacitance is characterized by a constant phase angle element.
From the Bode impedance plots, it is not difficult to see that PVP has a large effect on the corrosion resistance of the electrode. The total impedance of the system measured at the low frequency end of the samples prepared under different conditions has a difference of about an order of magnitude. And there are certain rules that apply to this difference. As the concentration of PVP in the conversion bath increases, the curve continuously moves upward, and the electrochemical impedance value of the electrode continuously increases. When the concentration of PVP reaches 1.5 g/L, the |Z| 10 mHz value reached 1.79 × 10 5 Ω, and the corrosion resistance of the conversion coating is obviously superior to other conditions. When PVP in the solution exceeds 2.0 g/L, the impedance of the prepared sample decreases and the corrosion resistance of the sample deteriorates.
The parameters used in the fitting procedure are presented in Table 2. The order of magnitude of all Y values in the Table is 10 −6 . The conversion coating presents the characteristics of low capacitance and high resistance. It shows that the conversion coating has good corrosion resistance in 3.5 wt% NaCl solution. The charge transfer resistance (R ct ) and the film resistance (R f ) of the conversion coating presents a consistent trend of change. That is, with the increase of PVP content in the conversion bath, they initially show an increasing trend. And they reach the maximum when the PVP content in the conversion bath is 1.5 g/L. At this time, R f and R ct values are 461.3 Ω and 2.385 × 10 5 Ω respectively. With the further increase of PVP content, the R f and R ct values of the conversion coating decreased, indicating that  the corrosion resistance of the conversion coating did not increase with the increase of PVP concentration. Figure 5 illustrates the Tafel polarization curves of asprepared samples in 3.5% NaCl solution. It can be seen that the anode and cathode branches of the polarization curve of the prepared samples move in the direction of low current density. The conversion coating can inhibit both the anodic process and the cathodic process of the corrosion reaction, thus inhibiting the entire corrosion reaction. In addition, the concentration of PVP has a great influence on the corrosion resistance of the conversion coating. With the increase of PVP concentration, the corrosion resistance of the conversion coating is improved. When the PVP concentration is not higher than 1.5 g/L, the cathode branch and anode branch of the electrochemical curve move to the direction of low current density with the increase of PVP concentration. Once the PVP concentration exceeds 2.0 g/L, the cathode and anode branches move to the direction of increasing current density, which means that the corrosion resistance of the conversion coating gradually decreases. Table 3 shows the electrochemical parameters obtained by fitting the polarization curve with the processing software provided by the electrochemical workstation. Where E corr is the self-corrosion potential and J corr is the corrosion current density. From the above electrochemical parameters, it can be seen that the appropriate concentration of PVP additive can increase the polarization resistance and significantly reduce the corrosion current of the sample. The corrosion current density J corr of the as-prepared sample with PVP content of 1.5 g/L is one order of magnitude lower than that of the sample without additives.
It can be seen that PVP plays an important role in promoting the formation of conversion coating and improving its corrosion resistance. According to relevant research, the PVP molecule exhibits strong adsorption on the surface of the active electrode and can form a three-dimensional film on the surface of the electrode (Bashir et al. 2022;Sazou D and Deshpande P P. 2017). At the same time, the nitrogen element in the five-membered heterocyclic ring structure of PVP molecule and the oxygen element in the carbonyl group all have lone pair electrons, which leads to the strong tendency of PVP molecule to complex with metal ions with empty orbits. The main metal ions such as V 4+ /V 5+ and Zr 4+ in the conversion bath that constitute the conversion coating will gather on the surface of aluminum alloy substrate in the form of metal complexes, which provides necessary conditions for the formation of conversion coating. In the initial stage of conversion coating formation, the active site of the aluminum alloy is etched and dissolved. Without PVP molecules participating in the coating forming reaction, these dissolved Al 3+ will also react with the conversion bath to form Na 3 AlF 6 . However, these Al 3+ will inevitably diffuse into the conversion bath under the promotion of concentration gradient. At this time, the Na 3 AlF 6 crystalline grain is relatively sparse. With the introduction of PVP into the conversion bath, the diffusion of Al 3+ into the conversion bath is inhibited. More Al 3+ ions can participate in reactions Eqs.
(1) and (2), resulting in the densification of Na 3 AlF 6 grains (Liu et al. 2016). PVP molecules can form a three-dimensional film on the surface of the substrate, which has been confirmed by relevant research (Yang et al. 2022). When the content of PVP on the aluminum alloy substrate and the solid/liquid interface between the substrate and the conversion bath is too high, the above coating formation reaction will inevitably be inhibited, and the corrosion resistance of the prepared sample will be poor. Figure 6 shows the average impedance modulus measured at 10 mHz as a function of NSS time for the as-prepared sample coated with Zr/V conversion coating and Zr/V/ PVP conversion coating. For the sample covered with Zr/V conversion coating, the corrosion resistance of the sample (1) Al + 6F − = AlF 3− 6 + 3e − (2) AlF 3− 6 + 3Na + = Na 3 AlF 6

Fig. 5
Polarization curves of as-prepared samples in 3.5% NaCl solution was improved after 1 day NSS test, and the |Z| 10 mHz value reached 3.18 × 10 5 Ω. The tetravalent vanadium in the coating is oxidized to pentavalent, which leads to the increase of |Z| 10 mHz value. The pentavalent vanadium exists in the form of V 2 O 5 , it presents a more stable electrochemical performance. With the increase of NSS time, the corrosion resistance of the conversion coating did not continue to improve and showed a trend of continuous decreased. The results reflect that the Zr/V conversion coating is self-repairing to a certain extent in the corrosive environment. However, this kind of self-repairing ability of the sample is gradually weakened with the increase of exposure time in the corrosive environment. The introduction of PVP additives can further improve the ability of samples to resist long-term corrosion. The PVP molecules adsorbed on the substrate surface and the Zr/V-based conversion coating can jointly inhibit the corrosive medium and the electron migration of the corrosion reaction. Therefore, the introduction of PVP can effectively improve the salt spray corrosion resistance property of as-prepared samples. The optical pictures of the samples after the NSS test at different times are shown in Fig. 7. For the sample without PVP addition, 80% of the surface brightness of the sample decreases after 1 day of NSS test. After 2~3 days of NSS test, the above dark areas did not increase significantly, but their color gradually deepened and turned to brown red. According to Zhong et al. the vanadium in the conversion coating has a strong self-healing ability. The V 2 O 5 produced by hydrolysis and oxidation became the main component of the conversion coating, which also led to the conversion coating changed into brownish-red color (Zhong et al. 2013). The Zr/V/PVP conversion coating can provide longer-lasting protection for aluminum alloys. After 2 days of NSS testing, there were localized locations where the gloss of the conversion coating decreased, and the area of these parts accounted for about 10%. When the NSS test time reached 6 days, the area of the corroded areas increased slightly, but its percentage could still be kept within 15%.
The adhesion between the electrophoretic coating and the aluminum alloy substrate was measured using a Posi adhesion tester, and the results are shown in Fig. 8. The conversion coating on the aluminum alloy surface provides excellent adhesion conditions for the electrophoretic coating. Compared with the bare aluminum alloy plate, the adhesion between the aluminum alloy covered with Zr/V-based conversion coating and the electrophoretic coating was significantly improved, with a value of 8.9 MPa. The introduction of PVP can further improve the adhesion between aluminum alloy and electrophoretic coating, which can reach 11.2 MPa.
The adhesion measured by the pull-off method can only reflect the adhesion between the electrophoretic coating and the substrate under ideal conditions. To further evaluate the Fig. 6 Average impedance modulus at 10 mHz (|Z| 10 mHz ) as a function of NSS for: Non-PVP and PVP added conversion treated sample Fig. 7 Optical images of the conversion coatings before and after NSS test for different time effect of the conversion coating on adhesion, it is necessary to perform an accelerated corrosion test. After the accelerated corrosion test, the electrophoretic coating on the surface of the bare aluminum alloy was peeled off in a large area along the scratch. This means that the electrophoretic coating can only provide limited protection for the aluminum alloy. Once exposed to corrosive environment for an electron time, the electrophoretic coating can hardly provide effective protection for aluminum alloy. The adhesion between the aluminum alloy substrate covered with Zr/V-based conversion coating and the electrophoretic coating r is more stable. However, scattered bubbles and concentrated peeling points near scratches mean that the combination ability of the electrophoretic coating and the substrate still needs further improvement. This elevation of adhesion can be achieved by means of the introduction of PVP molecules. As shown in Fig. 9c, the aluminum alloy substrate with PVP molecules participating in the coating formation presents better affinity with the electrophoretic coating. Although the electrophoretic coating peels off on both sides of the scratch, the total width remains about 2.5 mm and there are no bubbles or other defects.
According to the related studies, the special structure and functional groups of PVP molecule result in strong adhesion and coating formation tendency on the surface of metal substrates (Yang et al. 2022). On the one hand, this layer of PVP film can inhibit the diffusion of Al 3+ ion generated in the etching process at the initial stage of coating formation into the conversion bath. The high Al 3+ ion concentration near the substrate/solution interface leads to the promotion of the reaction Eq. (2). The inhibition of Al 3+ ion diffusion becomes more obvious and the NaAlF 6 crystal in the conversion coating becomes denser with the increase of PVP introduction. On the other hand, the nitrogen in the five-membered rings and the oxygen in the carbonyl group of the PVP molecule both have lone pair electrons, leading to a strong tendency to complex with empty-orbit metal ions. On the surface of the aluminum alloy matrix, the main metal ions forming the conversion coating, such as V 4+ /V 5+ and Zr 4+ , are enriched in the form of metal complexes. The polarization of the concentration caused by the coating formation reaction will be inhibited, and the coating formation reaction will also be promoted. The reaction mechanism of Zr/V/PVP conversion coating formation process is shown in Fig. 10.
PVP plays an important role in promoting the compatibility between Zr/V/PVP conversion coating and electrophoretic coating. In the molecular structure of PVP, the lactam group has the affinity with the polar group, and its carbon chain and methylene on the five-membered ring are nonpolar groups. This structure can not only ensure its water solubility, but also be miscible with a variety of organic solvents. The non-polar groups of PVP molecules exposed on Fig. 8 The adhesion values obtained for the electrophoretic coating applied on the a bare aluminum alloy and b Zr/V and c Zr/V/PVP treated substrates Fig. 9 The optical pictures of electrophoretic coating prepared on the a bare aluminum alloy and b Zr/V and c Zr/V/PVP treated substrates after accelerated corrosion test the outer surface of the conversion coating can fully interact with the non-polar groups of the electrophoretic coating, and cross-link with the polymer resin through its own long molecular chain. This is the essential reason why PVP additives can improve the adhesion between electrophoretic coating and substrate.
In addition, PVP is an effective metal corrosion inhibitor. There is a strong adsorption between the polar groups of PVP molecules and the substrate. PVP molecules can form a three-dimensional film on the substrate. This film has an excellent effect in preventing the penetration of corrosive media due to the repellency of non-polar groups to corrosive substances (Yang et al. 2022).

Conclusions
In this paper, the effects of addition of PVP as a green corrosion inhibitor to the conversion bath on the morphological, corrosion resistance and affinity with electrophoretic coating of conversion coating were analyzed on aluminum alloy. The results obtained are as follows: 1. The as-prepared conversion coating is mainly composed of amorphous ZrO 2 , ZrF 4 , V 2 O 5 , VO 2 and crystalline Na 3 AlF 6 and PVP. 2. The introduction of PVP molecules plays an important role in modulating the formation of Na 3 AlF 6 grains. When the PVP content is about 1.5 g/L, the Na 3 AlF 6 crystal can uniformly cover the whole substrate, and the corrosion resistance of the conversion coating is significantly improved. 3. PVP molecules can be adsorbed on the active sites on the surface of the substrate to form self-assembled monolayers. This kind of self-assembled monolayers can cooperate with the Zr/V conversion coating on the substrate to prevent corrosive media from penetrating into the substrate and improve the corrosion resistance of the substrate. 4. The non-polar groups of PVP molecules can effectively interact and cross link with the electrophoretic coating, which provides a basis for the affinity between the electrophoretic coating and the substrate.