Polypyrrole Functionalized Graphene Oxide Accelerated Zinc Phosphate Coating Under Low-Temperature

Qingsong Zhu,^1,2 Jingguang Liu,³ Xin Wang,^1,4 Yuxiang Huang,² Ying Ren,² Weiqiang Song,² Chenzhong Mu,⁵ Xianhu Liu,^1* Fengchun Wei,^2**  and Chuntai Liu^1***

 

1 The Key Laboratory of Material Processing and Mold of Ministry of Education, National Engineering Research Center for Advanced Polymer Processing Technology, Zhengzhou University, Zhengzhou 450002, PR China

2 College of Materials Science and Engineering, Henan University of Technology, Zhengzhou 450001, PR China

3 Jier Machine-Tool Group Co., LTD, Jinan250000, PR China.

4 Institute of Polymer Materials, Friedrich-Alexander University Erlangen-Nuremberg, Martensstr. 7, 91058 Erlangen, Germany

^{5 State Key Laboratory of Special Functional Waterproof Materials, Beijing Oriental Yuhong Waterproof Technology Co., Ltd, Beijing 100123, China}

^{* E-mail: xianhu.liu@zzu.edu.cn (X. Liu), ctliu@zzu.edu.cn (C. Liu) and fengchun_wei@haut.edu.cn}

^{Table of Content}

This paper introduces an environmentally friendly and fast accelerator for phosphate coating via polypyrrole functionalized graphene oxide.

 

Abstract:

Zinc phosphate coating, as an effective and fast anticorrosion technique for the metals, have been developed rapidly in recent years. However, it is still challenge to synthesize a low energy, environmentally friendly and efficient accelerator through a simple, facile method. Herein, as a new accelerator, polypyrrole functionalized graphene oxide (GO-PPy) nanocomposites were prepared by in-situ process to grow polypyrrole (PPy) film on graphene oxide (GO) surface, Incorporation of GO-PPy into phosphate baths accelerated the phosphating process of phosphate coating and promoted the nucleation and growth of phosphate crystals, achieving stronger corrosion resistance, which were confirmed by electrochemical measures and morphologies characteristic of the phosphate coating. Additionally, when the concentration of GO-PPy in the phosphate baths reached up 1.2 g/L, the phosphate coating possessed the most compact and uniform phosphate crystals and the best corrosion protection performance. Finally, the special mechanism of the phosphate process was discussed. This work introduces a new, low-energy, facile, environmentally friendly and alternative accelerator for the preparation of phosphate coatings.

Keywords: phosphate coating; corrosion protection; accelerate; phosphate crystals.

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1. Introduction

The extremely high environmental and economic impact on corrosion of metal substrate arouses large scientific interest to overcome this problem and require an extremely high environmental and economic transdisciplinary method.^[1,2] One of the easiest and most common approaches for preventing metals from corrosion is the application of various coatings.^[3-4] Zinc phosphate coating, for example, is commonly applied during drawing and extrusion of various metals because of its low cost and barrier function.^[5] Additionally, zinc phosphate coating can not only prevent metal corrosion but also improve organic coatings’ adhesion ability on the metal substrate, which are of great benefit to its practical industrial application.^[6] Unfortunately, traditional phosphating process needs to be carried out at high temperature such as 90-98°C, and it is extremely time consuming, accounting for enormous energy consumption.^[7-9]

   Thus, appropriate temperature for phosphating process has been of importance as a result of low energy consumption. However, phosphating process at low temperature is extremely slow and needed to be promoted by specialized materials such as nitrides, nitrates, and chlorates. Thus, these traditional accelerators validly reduce the phosphating time but are extremely unhealthy to human health and environment.^[10,11] Therefore, the environmental protection agency had considered those accelerators as harmful to the human. Many researchers have attempted to develop new accelerators.^[12,13] Environmental-friendly and effective accelerators are still urgently needed to upgrade traditional accelerators.

   Graphene oxide (GO) produces stable dispersion in many polar and nonpolar solvents including water owing to various oxygenated functional groups, and is thus the most important and widely used medium for many industrial applications.^[14,15] Researches confirmed that GO could prominently enhance the resistance of coatings because of its excellent impermeability to corrosive media.^[16,17] Zhang et al. incorporated GO into phosphating bath, and investigated its effect on corrosion resistance and microstructure of phosphate coating. This paper showed that GO, as a new accelerator, effectively speeded up phosphating process, and enhanced the corrosion protection performance of phosphate coatings.^[18] However, the corresponding impedance modulus of zinc phosphate coatings was very low. In addition, polypyrrole (PPy) has become the most studied conducting polymers owing to its easy synthesis, nontoxicity and good stability. Wang et al. researched the addition of PPy intercalated graphene into epoxy coating and showed a synergistic protection on the metal substrate.^[19] Moreover, hydrophilic PPy functionalized GO could improve the dispersion ability of the agglomerated GO in many polar and nonpolar solvents owing to the increase of interlayer spacing, as a result of more uniform nanosheets absorbed on the surface of mild steel compared to GO nanosheets. In view of those, using GO-PPy as an accelerator to speed up phosphating process and then adding GO-PPy into the phosphate bath have been hardly studied.

    Herein, this is the first report of concerning the possibility of GO-PPy as a fast, available accelerator to speed up the phosphating progress of phosphate coating. GO-PPy was synthesized by in-situ process to grow PPy film on the surface of GO. Thus, the GO-PPy was added into conventional phosphate bath to fabricate zinc phosphate coating on the surface of the mild steel. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were employed to assess the corrosion protection property of phosphate coating. X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) were used to characterize the chemical composition and morphological characteristic of phosphate coating. Finally, the phosphating mechanism of phosphate coating with GO-PPy in phosphating bath was discussed based on the above experimental results.

2.  Experimental

2.1. Synthesis of GO-PPy

GO-PPy composites were prepared according to our previous reports.^[20,21] Briefly, these composites were prepared by in-situ polymerization of Py on the surface of GO.

2.2. Fabrication of phosphate coating

Mild steel pans with the dimension of 120 mm×50 mm×0.28 mm were used as the substrate of zinc phosphate coating, and these chemical composition (wt%) were ≤0.22 C, ≤0.34 Si, ≤0.045 P, ≤0.05 S, ≤0.70 Mn and blance Fe, mild steel pans were first grinded with 200, 600, 1000 grit SiC sand paper to remove oxide layer, respectively. Then, these mild steel pans were immersed in 10 wt% NaOH solution at 40 ℃ for 10 minutes to remove all alkali. These degreasing mild steel pans were rinsed with ethanol solution and dried to eliminate excess NaOH. After that, mild steel pans were exposed to 150 mL of phosphating bath at 40 ℃ for 20 min. The chemical composition of phosphate baths is depicted in Table 1. After the completing phosphating, the mild steel pans were rinsed with ethanol and dried. The concentration of GO in phosphating bath was 1.2 g/L, and the resultant specimen was named as GO-1.2. While the concentrations of GO-PPy nanocomposites in phosphating solution were 0, 0.8, 1.0, 1.2, and 1.6 g/L, respectively. The resultant specimens were named as BP, GP-0.8, GP-1.0, GP-1.2, and GP-1.6, respectively.

Table 1. Chemical composition of the phosphating baths.

	Zn(H2PO4)2	Zn(NO3)2	H3PO4	GO	GO-PPy
BP	55 g/L	60 g/L	8 g/L
GO1.2	55 g/L	60 g/L	8 g/L	1.2
GP-0.8	55 g/L	60 g/L	8 g/L	g/L	0.8 g/L
GP-1.0	55 g/L	60 g/L	8 g/L		1.0 g/L
GP-1.2	55 g/L	60 g/L	8 g/L		1.2 g/L
GP-1.6	55 g/L	60 g/L	8 g/L		1.6 g/L

 

2.3. Characterization

Morphologies of phosphate coatings were characterized by FESEM (an Inspect F50 Scanning Electron Microscope). The crystalline structure and composition of phosphate coating were analyzed by XRD (Bruker D8 Advance). The corrosion protection property of phosphate coatings was evaluated by electrochemical test. All measurements were conducted by the electrochemical workstation (RST5200F, Suzhou Ruisitai Instrument Technology Co., Ltd.). The geometric area of the working electrode was 12 cm², which was connected with the electrolytic solution (3.5 wt% NaCl solution). The polarization plots were collected via a 1 mV/s scan rate. EIS tests were conducted with the frequency range of 100 kHz-0.01 Hz using 5 mV amplitude with alternating current signal.

3. Results and discussion

3.1. Potentiodynamic polarization characterization

The potentiodynamic polarization curves are shown in Fig. 1 and polarization parameters are listed in Table 2, respectively. In general, a material with a higher corrosion potential (E_corr) and a lower i_corr (corrosion current density) is held to possess a lower tendency of oxidation and corrosion and therefore higher corrosion resistance performance.^[18] From Fig. 1 and Table 2, E_corr for BP and GO-1.2 were -623 and -590 mV, and that for GP-0.8, GP-1.0, GP-1.2 and GP-1.6 were -561, 574, 564 and -608 mV, respectively. This indicated that E_corr for GO-1.2 raised 33 mV compared to that of BP, and E_corr for GP-0.8 raised 62 mV compared to that of BP. Meanwhile, i_corr for BP and GO-1.2 were 5.72 and 5.16 μA/cm², and that for GP-0.8, GP-1.0, GP-1.2 and GP-1.6 were 2.94, 3.16, 3.79 and 3.91 μA/cm², respectively. This indicated that i_corr for GO-1.2 was decreased to 0.56 μA/cm², i_corr for GP-0.8 decreased to 1.81 μA/cm².Thus the addition of the accelerator GO or GO-PPy in phosphate baths could obviously improve E_corr and decrease i_corr. According to Table 2, GP-0.8 had the largest E_corr and the lowest i_corr compared with other specimens, which indicated its best corrosion resistance performance.

 

 

 

 

 

 

 

Fig. 1 Tafel curves of specimens obtained from phosphating bath.

     When the concentration of GO-PPy in the phosphating bath raised up to 1.6 g/L, E_corr for GP-1.6 decreased and i_corr for GP-1.6 increased evidently, which indicated that excessive GO-PPy in the phosphating bath was not conducive to increase E_corr or decrease i_corr of the specimens, and its corrosion resistance ability became worse. This excellent anticorrosion property was also evident by the minimum corrosion rate. According to Table 2, the corrosion rate of GP-0.8 was 3.46×10^-2 mm/year, which was smaller than that of BP, GO-1.2 and GP-1.6, indicating its best corrosion resistance performance.

Fig. 2 Nyquist (a) and Bode (b-c) plots of all phosphate coatings immersed in 3.5 wt % NaCl solution.

Fig. 3 Equivalent circuit model of the phosphate coatings.

 

Furthermore, R_ct for phosphate coatings first increased with increasing the concentration of GO-PPy in phosphating bath, and reached the maximum at the concentration of 1.2 g/L, and then decreased again. Nevertheless, the R_ct for GP-1.6 was still around two times higher than that of BP. The largest R_ct for GP-1.2 thereby revealed that it showed the best corrosion resistance performance than other specimens, which was consistent with the results of EIS measures.

Table 2. Polarization parameters obtained from potentiodynamic polarization curves.

Sample	Ecorr (mV vs. Ag/AgCl)	icorr (μA/cm2)	ba (mV/dec)	bc(mV/dec)	Corrosion rate (mm/year)
BP	-623	5.72	51	110	6.73×10-2
GO-1.2	-590	5.16	58	60	6.08×10-2
GP-0.8	-561	2.94	54	74	3.46×10-2
GP-1.0	-574	3.16	55	70	3.73×10-2
GP-1.2	-564	3.79	53	100	4.46×10-2
GP-1.6	-608	3.91	56	79	4.61×10-2

 

Fig. 5. FESEM images of phosphate coatings with various concentrations of GO-PPy and different phosphating time.

The GO-PPy in the phosphating bath had an obvious impact on the decrease in the size of phosphate crystals which were covered on specimen and raised the amount of micro-cathode points which could accelerate the reaction of hydrogen evolution.^[26] This rapidly consumed free phosphoric acid in phosphating bath and increased the pH values at interfaces between the specimen and phosphating bath. The soluble primary phosphate was transformed into insoluble tertiary phosphate because of the increase of pH value, and then deposited on mild steel substrate surface.^[27]

   Throughout the phosphating process, the morphologies of phosphate crystals had changed from plate-like to slab-like as GO-PPy was added into phosphating bath, phosphate crystals covered the mild steel surface showing the most denser and uniform at the concentration of 1.2 g/L GO-PPy. Unfortunately, as the concentration of GO-PPy reached 1.6 g/L, excessive GO-PPy nanosheets would overlap the steel substrate surface and phosphate crystals were no longer uniform and compact, and large crystals appeared, which reduced the crystal coverage degree on the surface of mild steel.

3.6. Mechanism of phosphating process

The phosphating mechanism of traditional phosphating bath had been widely researched.^[23,28] When exposed to phosphate bath, the mild steel was first transformed into iron ions and H₂ was released at the initial stage of phosphating process as shown in Equation (1). 

                      (1)

The pH at the phosphating bath/mild steel interface raised because of the transition from hydrogen ions to H₂, which accelerated the ionization of phosphoric acid as depict in (2) reaction, and thus resulted in PO₄^3-. As (1) and (2) continued forward, the content of PO₄³⁺ and Fe²⁺ reached saturated. Then, Fe²⁺ reacted with PO₄³⁺ on the surface of mild steel to form insoluble ferrous phosphate as follows.

       (2)

 

             (3)

As the reaction continued, the deposited insoluble phosphate could be crystallized to form phosphate coating. Moreover, phosphate crystals including Zn₃(PO₄)₂·4H₂O and Zn₂Fe(PO₄)₂·4H₂O covered the specimen surface, which was confirmed by Fig. 4.

   (4)

 

       (5)

 

When GO-PPy sheets were introduced into phosphating bath, GO-PPy nanosheets were easy to be absorbed on the surface of mild steel because of its abundant groups and large specific surface area. the absorbed GO-PPy nanosheets activated mild steel surface and increased the number of micro-cathode points. This accelerated the dissociation process of H₃PO₄ and the dissolving reaction of mild steel,^[29] which promoted the following crystallization reactions. Furthermore, the absorbed GO-PPy on the substrate surface could capture dissolved Fe²⁺ and Zn²⁺, then the crystallization reactions including (3), (4) and (5) might be speeded up even at low concentrations of GO-PPy in phosphate baths. Additionally, the conductive PPy could increase the layer spacing owing to the interaction with π-π bonds, which indicated that more uniformly dispersed nanosheets might be absorbed at phosphating solution/steel interface. As a result, the above-mentioned role had been strengthened.

  GO-PPy nanosheets accelerated the phosphating process, and promoted the crystallization of phosphate crystals in the phosphating coating. This could explain many phosphate crystals could be observed and grew up for GP-1.2 at the initial stage of phosphating process. Meanwhile, phosphate crystals were scarcely discovered on the surface of BP. Additionally, the early formed crystals were regarded as new nucleation points to speed up the following crystallization of phosphate crystals and decreased the size of crystals. Then, the maximum number of phosphate crystals was obtained in the early phosphating process at the concentration of 1.2 g/L GO-PPy and its grain size was the smallest in the completed phosphating stage.

As exhibited in Fig. 5, when excessive GO-PPy was added into phosphate bath, excessive GO-PPy nanosheets were deposited and overlapped on the steel substrate surface. Then, the dissolution reaction (1) of the iron was suppressed and restricting the reaction (2&3) of phosphoric acid decomposition. Therefore, the insufficient Fe²⁺ and PO4^3- resulted in the crystallization process slowly. This can explain that fewer loaded phosphate crystals covered on the surface of GP-1.6 compared with GP-1.2. As a result, GP-1.2 obtained the most compact and thickest coating with the smallest size of phosphate crystals. Based on the above discussion, as GO-PPy was added, the (4) and (5) reactions of phosphate crystal crystallization were accelerated to form Zn₂Fe(PO₄)₂·4H₂O and Zn₃(PO₄)₂·4H₂O crystals in phosphating coating. Phosphating baths generated many Zn²⁺ ions as depicted in Table 1, which were conducive to form Zn₃(PO₄)₂·4H₂O crystals. Additionally, the added excessive GO-PPy nanosheets restricted (1) reaction and Fe²⁺ ions were inadequate, which led to the gradual decrease in the peak intensities of Zn₂Fe(PO₄)₂·4H₂O.

   In brief, the steel specimen in phosphating bath absorbed the added GO-PPy nanosheets and was regarded as the deposited bed to capture the generated metal ions (Zn²⁺ and Fe²⁺), which could accelerate the reaction of the crystallization reaction. However, excessive GO-PPy nanosheets would overlap the surface of steel substrate and surpassed the dissociation of phosphoric acid and dissolution of irons. Therefore, Fe²⁺ and PO4³⁻ ions were inadequate, which inhibited the following crystallization reactions. Moreover, hydrophilic PPy functionalized GO improved the dispersion ability in phosphating bath owing to the increase of interlayer spacing, resulting in more uniform nanosheets absorbed on the surface of mild steel compared to GO nanosheets. 

4.  Conclusions

In this paper, GO-PPy was prepared by in-situ process to grow PPy film on the surface of GO, and investigated the probability of using the functionalized GO as accelerator to accelerate phosphating process on mild steel. Results demonstrated that the addition of GO-PPy into the phosphating bath could accelerate the phosphating process and promote the crystallization of phosphate crystals, acquiring a better corrosion protection. The specific mechanism was that GO-PPy could be absorbed on the mild surface, then acted as sedimentary beds which captured metal ions and promoted the nucleation of phosphate crystals. Moreover, the functionalized GO had a better dispersion in phosphate baths compared to GO, which strengthened the above role. As the concentration of GO-PPy was optimized to be 1.2 g/L, the specimen possessed the most compact and uniform phosphate crystals, indicating its best corrosion protection performance. However, excessive GO-PPy nanosheets would overlap mild steel surface, which surpassed the dissociation process of phosphoric acid and dissolution process of irons. As a result, PO₄³⁻ and Fe²⁺ ions were inadequate, which inhibited the following crystallization reactions. But owing to the fact that the co-deposited GO-PPy sheets acted as additional barrier to block the permeation of corrosive medias, the phosphate coatings with high contents of GO-PPy showed stronger corrosion protection compared to pure phosphate coating. Therefore, GO could be used as a new accelerator to apply in the traditional phosphating industry.

Acknowledgments

This study was supported by Henan provincial science and technology project of 192102210036 and 132102210209.

Conflict of Interest

There is no conflict of interest.

Supporting Information

Not Applicable.

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