Article pubs.acs.org/IECR
Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Waterborne Epoxy Resin/Polydopamine Modified Zirconium Phosphate Nanocomposite for Anticorrosive Coating Xinxin Sheng,† Ruibin Mo,†,‡ Yue Ma,† Xinya Zhang,‡ Li Zhang,*,† and Hua Wu*,§ †
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Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China ‡ School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China § Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, 8093 Zurich, Switzerland S Supporting Information *
ABSTRACT: Replacing solvent-based coatings with waterborne resin ones has become an overall trend in the coating industrial to reduce pollution caused by volatile organic compounds. It is known that introduction of flexibly designable two-dimensional nanoplatelets or zirconium phosphate (ZrP) can improve the mechanical and barrier properties of a film from solvent-based coatings. In this work, we have designed a methodology to prepare nanocomposites for waterborne coating, which consist of waterborne epoxy resin (WER) and polydopamine modified ZrP (PDAZrP). It is found that PDA-ZrP has excellent compatibility with WER, and consequently, the film defects of pure WER are fixed. The PDA-ZrP/WER coating exhibits an extremely low corrosion current (1.09 × 10−9 A cm−2) at 1.0% PDA-ZrP loading. It is about 500 times lower than that of the pure WER coating (5.82 × 10−7 A cm−2), and consequently, the same coating has a much smaller rusted area after a salt spray test of 310 h. It should also be noted that all materials we used are eco-friendly, and all experimental procedures are conducted under mild conditions. organic coating films. Hitherto, various studies have proven that nanofillers such as graphene oxide (GO),17,18 montmorillonoid,19,20 hexagonal boron nitride (h-BN),21,22 and molybdenum disulfide (MoS2)23 can improve the anticorrosion performance of a coating film. It has been recently demonstrated for organic solvent-based coatings that another plate-like nanofiller, zirconium phosphate (ZrP), is effective in enhancing corrosion resistance.24,25 Compared with the other nanofillers mentioned above, ZrP has the advantages of good exfoliability26 and controllable size,27 and it can be easily designed for various applications such as fire retardance28 and catalysis;29 thus, it has great potential as a multifunctional filler. However, as an inorganic compound, ZrP usually suffers poor compatibility with polymer materials, causing low interface strength or even micro cracks at the interface. Thus, to make the best use of ZrP, appropriate surface modifications become necessary. Dopamine, a kind of hormone, can be easily deposited on the surface of a substrate via self-polymerization to form a polydopamine (PDA) layer, and it has excellent adhesion to
1. INTRODUCTION As a common and widespread problem, corrosion of metal causes enormous economic loss and safety problems worldwide. Extensive research on anticorrosion has been conducted to develop effective strategies against the metal corrosion such as using corrosion inhibiter,1,2 surface treatment,3,4 and organic (surface) coating.5−9 Among them, the most widely used approach is protecting metal with organic coating because of its ease of operation, excellent protection results,10,11 and costefficiency.12,13 To this aim, epoxy resin is one of the most commonly used coating materials, because of its outstanding mechanical and electrical insulating properties and chemical resistance. However, traditional coatings with epoxy resin are organic solvent based, involving large amounts of volatile organic compounds, which either lead to severe environmental pollution or require a high-cost recovery system. Therefore, it is highly desired to replace solvent based epoxy resins with waterborne ones for coating applications.14,15 However, coatings with waterborne epoxy resins (WER) suffer unsatisfactory barrier properties against permeation of water, oxygen, and corrosion ions, because certain amounts of hydrophilic groups and surfactants remain in the film structure.16 Incorporation of two-dimensional plate-like nanofillers can effectively promote the mechanical and barrier properties of © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
May 9, 2019 August 6, 2019 August 19, 2019 August 19, 2019 DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
To modify α-ZrP with PDA, 500 mg of α-ZrP and 250 mg of dopamine hydrochloride were dispersed into 500 mL of 10 mM Tris-HCl buffer solution (pH 8.5) with probe sonication for 30 min in an ice bath (500 W output power, 0.5 s work time, 0.5 s pause time). Then, the dispersion was stirred for 24 h at room temperature under a N2 atmosphere. After that, the PDA-modified α-ZrP (PDA-ZrP) was separated by centrifugation and washed with deionized water several times to remove unreacted dopamine. Finally, the obtained PDA-ZrP was redispersed in water and stored for further experiments. 2.3. Preparation of PDA-ZrP/WER Nanocomposite Coatings. Nanocomposite coatings containing different concentrations of PDA-ZrP were achieved by the following procedures. First, the as-prepared PDA-ZrP dispersion was mixed with an amine curing agent by bath sonication for 10 min to obtain a homogeneous dispersion. Then, WER was added into the dispersion with probe sonication for 10 min to form a uniform binary colloid. Note that the mass ratio of the epoxy resin to the amine curing agent that was added in the PDA-ZrP dispersion was 3:1 and was based on the supplier’s suggestion. The final coatings were designed with four levels of PDA-ZrP concentration: 0, 0.20, 0.50, and 1.0 wt %. Subsequently, all the prepared binary colloidal dispersions were cast on the sandblasted mild carbon steel substrates (Q235) by using an applicator. The samples were cured at room temperature for 4 days and postcured at 105 °C for 2 h. It should be noted that in the above introduction of PDAZrP into the WER film, we used a binary colloidal approach. In the literature, the nanofillers were typically introduced into polymer matrices by direct addition of the nanofiller powder into the polymer dispersion.35−37 The advantage of using powder is that the nanofillers can be introduced to the matrix in a large amount, but the dispersion of the nanofillers inside the matrix is typically very poor, in the form of agglomerates. Instead, with our colloidal approach, we can warrant that PDAZrP can disperse in the polymer matrix at the nanoscale, with negligible agglomeration. On the other hand, because of the limitation for PDA-ZrP that can be dispersed in water, we cannot introduce substantially large amounts of the filler in the WER dispersion. Thus, we have investigated a range of the PDA-ZrP nanoplatelets up to 1.0 wt %. Further increases in the PDA-ZrP amount would lead to dilution instability of the WER dispersion, and then we would have to add more surfactants, which would further reduce the performance of the film. 2.4. Characterization. The chemical structures of ZrP and PDA-ZrP were investigated by FT-IR. Freeze-dried ZrP and PDA-ZrP powders were used for the characterization, and the spectra were collected by an FT-IR spectrometer (Bruker, TENSOR 27) in the scanning range of 4000−400 cm−1. X-ray diffraction (XRD) was performed on a Rigaku Ultima IV using a Cu Kα radiation source, at 40 kV, 30 mA, and 120 kW, with a scanning rate of 3°/min and a step of 0.02. The sizes of ZrP and PDA-ZrP for XRD were smaller than 300 μm and obtained through sieving. X-ray photoelectron spectroscopy (XPS) analysis was carried out by a Thermo ESCALAB 250XI spectrometer. ZrP and PDA-ZrP aqueous dispersions were investigated by UV−vis absorption at a concentration of 100 ppm. The absorption spectra were recorded with a UV−vis spectrophotometer (T9, Persee) in a scanning wavelength range from 900 to 200 nm.
almost any surface.30,31 It has been previously applied for surface modifications of GO32 and MOFs33 to improve the compatibility between the fillers and the epoxy resins and to improve the anticorrosion performance of the composite. However, no studies can be found in the open literature on the applications of PDA to modify ZrP surface for WER coatings. Therefore, in this work, we have employed PDA to modify a ZrP surface, leading to a very stable aqueous dispersion of PDA-ZrP. This dispersion is then mixed with WER to form a binary colloidal dispersion for the nanocomposite coating, as sketched in Figure 1. The chemical structure, crystallinity,
Figure 1. Schematic diagram for the preparation of the PDA-ZrP/ WER nanocomposite coating and how PDA-ZrP improves the barrier properties of the coating.
thermo-stability, and morphology of ZrP and PDA-ZrP are investigated by FT-IR, XRD, XPS, UV−vis, TGA, AFM, SEM, and TEM. The cross section of the nanocomposite film is observed by SEM and TEM, and the anticorrosion performance of the films with different amounts of ZrP has been investigated by EIS, potentiodynamic polarization, and salt spray tests.
2. EXPERIMENTAL SECTION 2.1. Materials. Zirconyl chloride octahydrate (ZrOCl2· 8H2O, 99%), phosphoric acid (H3PO4, 85%), dopamine hydrochloride (DA, 98%), and Tris (hydroxymethyl) aminomethane hydrochloride (Tris-HCl, 99%) were purchased from Aladdin Reagent Company Ltd. Waterborne epoxy resin (WER; BC2060, epoxide equivalent solid content of 60%) and water-based amine curing agent (solid content of 60%) were provided by Banco Chemicals Company Ltd. 2.2. Synthesis of PDA-Modified α-ZrP. The synthesis of α-ZrP was based on the procedure reported in the literature.34 First, 6.0 g of ZrOCl2·8H2O was added into 400 mL of 6 M H3PO4 in a round-bottom flask, with refluxing at 96 °C for 48 h. Afterward, the product was centrifuged at 9000 rpm and washed with deionized water several times. Then, the obtained α-ZrP was freeze-dried for 24 h. B
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Thermogravimetric analysis (TGA) was performed on a Netzsch TG (model 204) from 30 to 800 °C under a N2 atmosphere (100 mL/min) with a heating rate of 10 °C/min. The morphology of ZrP and PDA-ZrP were investigated with a Hitachi SU-8220 field emission scanning electron microscope (SEM). The pictures of the fracture surface of the nanocomposite coatings were taken by a Hitachi SU-8010 SEM, and an energy dispersive X-ray spectrometer (EDS) linked to the SEM was used to examine the distributions of the elements in the nanocomposite coatings. Transmission electron microscopy (TEM) images of ZrP, PDA-ZrP, and PDA-ZrP/WER were obtained using a field emission transmission electron microscope (TEM 2100, JEOL) at an accelerating voltage of 80 kV. Atomic force microscopy was also used to characterize ZrP and PDA-ZrP (AFM, XE-100, Park Systems) at 25 °C and 50% relative humidity in tapping mode. Differential scanning calorimetry (DSC) measurements were performed using a DSC3 instrument (Mettler-Toledo) with nitrogen as the purge gas. A DSC trace of 0 to 100 °C was obtained at a heating rate of 10 °C/min. Open circuit potential and electrochemical impedance spectra were measured on a Chenhua Chi660E electrochemical workstation after immersion of the nanocomposite films in 3.5 wt % NaCl solution for 20 days. EIS was carried out with a standard three-electrode system: a saturated Ag/AgCl electrode as the reference electrode, a graphite electrode as the counter electrode, and the specimens as the working electrodes. The area of each working electrode was 7.07 cm2. The measurements were done under open circuit potential (OCP) with an amplitude of 10 mV over a frequency range from 0.1 Hz to 100 kHz. The Tafel curve was recorded by potentiodynamic polarization scanning from −1 to 0 V, with a scan rate of 1 mV/s. The salt spray test was conducted on the coatings, each having an artificial scratch, as an accelerated corrosion evaluation for mild steel covered by pure epoxy resin and nanocomposite coatings according to GB/T 1771-2007/ ISO 7253:1996.
Figure 2. FT-IR spectra of ZrP and PDA-ZrP.
groups in PDA-ZrP. However, the presence of the hydroxyl and amino groups on PDA-ZrP is important for the preparation of the nanocomposite, because they have strong chemical or physical interactions with the epoxy resin matrix. Therefore, we will discuss the existence of hydroxyl and amino groups later with XPS. XRD. To further investigate the functionalization of PDA on the surface of α-ZrP, we have performed XRD to see the crystalline feature of the ZrP and PDA-ZrP surface. In Figure 3, the reflection angles (2θ) of 11.6, 19.7, 25.0, and 34.1°
3. RESULTS AND DISCUSSION Before discussion of the results from the PDA-ZrP/WER nanocomposite coatings, it should be mentioned that we have also tried to mix the surface-unmodified ZrP dispersion with WER. It is found that in this method, there is severe agglomeration of ZrP nanoplatelets, leading to gelation of the entire system and thus making it impossible to perform the coating. Therefore, in the following, we discuss only the results using the PDA-modified ZrP (PDA-ZrP). 3.1. Chemical and Physical Properties of PDA-ZrP. FTIR Analysis. FT-IR spectroscopy is used to investigate the existence of PDA on the surface of α-ZrP because changes of functional groups on the surface can be easily detected by FTIR. Figure 2 shows the characteristic absorption bands of ZrP at 515, 1040, 3140, and 3598 cm−1, which are, respectively, from the stretching vibrations of Zr−O, the orthophosphate groups, P−O−H with hydrogen bonding, and P−O−H without hydrogen bonding.38,39 After the PDA modification, the absorption bands of C−H at 2950 cm−1 and CC at 1530 cm−1 are easily found in the spectrum of PDA-ZrP, indicating that PDA has been effectively deposited on the ZrP surface. Hydroxyl and amino groups of PDA-ZrP can hardly be found, because of the strong P−O−H absorption band of ZrP itself, which can overlap the absorption of hydroxyl and amino
Figure 3. XRD patterns of ZrP and PDA-ZrP.
represent the ZrP interlayers of (002), (110), (112), and (020), respectively.24 Most of the peaks for PDA-ZrP remain the same as those for ZrP, but the intensities of all the peaks are lower for PDA-ZrP than for ZrP. This indicates that the surface crystallinity is significantly lower for PDA-ZrP because of the coverage of the amorphous PDA layer. Moreover, the peak of interlayer (002) was shifted to 2θ = 10.4°, indicating the possible occurrence of intercalation of PDA into the C
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. XPS spectra of (a) O 1s for ZrP and of (b) O 1s and (c) N 1s for PDA-ZrP.
solution at a controlled pH (8.5) according to Linert.43 The polymerization reaction of dopamine and the reaction between dopamine and ZrP are likely to be competitive. Thus, we conducted the modification of ZrP in a nitrogen atmosphere and used only a little O2 to initiate the process. As shown in Figure 5, PDA-ZrP prepared at 25 °C has a higher UV
interlayer of the ZrP nanoplatelets during the PDA modification, leading to increases in interlayer spacing. XPS Analysis. Active amino groups on PDA-ZrP can interact with the epoxy resin, and they play an essential role in the cocuring process. The N−H bonds on PDA-ZrP can react with the epoxy resin to form covalent bonds, and the O−H bonds on PDA-ZrP can go through hydrogen bonding with the epoxy resin. The XPS results are shown in Figure 4, where the black solid curves are the observed data, and the colored dashed and solid curves are the fitting data. The bonding energy and assigned chemical groups for each peak are listed in Table 1, Table 1. XPS Bonding Energy Assignments functional group
bonding energy (eV)
OP O−P O−C R−NH2 R1−NH−R2 R1−NR2
533.3 531.6 534.3 403.8 401.9 400.4
and these results are consistent with the literature reports.40,41 The change in the O 1s bonding energy reveals that a C−O bonds exist in PDA-ZrP, and the hydroxyl groups remain in the bonds between the carbon and oxygen atoms (i.e., C−O instead of CO).42 Different C−N bonding situations are also confirmed by Figure 4c. Most of the amino groups in PDA-ZrP are secondary amines. These hydroxyl and amino groups in PDA-ZrP undergo chemical reactions or strong physical interactions with the epoxy resin and the curing agent, effectively promoting the compatibility and interface strength between PDA-ZrP and the resin matrix, enhancing the mechanical properties of the film. UV−Vis Absorption Spectroscopy. The polymerization of dopamine is initiated by an autoxidation process in aqueous
Figure 5. UV−vis spectroscopy of ZrP and PDA-ZrP prepared at 25 °C and of PDA-ZrP prepared at 60 °C.
absorption at 284 nm than that at 60 °C. This arises because at 25 °C, more dopamine or polydopamine with an unsaturated chemical structure is successfully grafted onto the ZrP surface, whereas a higher reaction temperature (60 °C) benefits the oxidative polymerization of dopamine instead of its reaction with ZrP. As a result, more free PDA was generated, which was removed during the centrifugal purification of PDA-ZrP. D
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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the malleable PDA layer on the ZrP nanoplatelet surface with much lower contract can be clearly seen. A better observation of the PDA modification to ZrP nanoplatelets can be obtained from the AFM and TEM images. Figure S1 in the Supporting Information (SI) shows the AFM images of the ZrP nanoplatelets before and after the PDA modification. The average thickness of the ZrP nanoplatelets is about 30 nm, whereas after the PDA modification, it increases to ∼37 nm. This confirms the successful modification. From the general consensus in the literature,31 the PDA modification should generate a layer of PDA on the surface of the nanofillers. However, from a comparison of the TEM images of the ZrP nanoplatelets before and after the PDA modification (Figure S2 in the SI), we can see that the pristine ZrP nanoplatelets have a round disk shape with a diameter in the range of 164 to 262 nm, and one cannot distinguish the layers inside the ZrP nanoplatelets. When the ZrP nanoplatelets are modified with PDA, as shown in Figure S2c and particularly in Figure S2d, the edge of PDA-ZrP displays a staircase-like structure, indicating that PDA-ZrP may experience certain exfoliation and intercalation during the PDA modification. This can be better observed from the TEM images of the frozen cross-section of the PDA-ZrP/WER film, which will be discussed in the following section. 3.2. Nanocomposite Coating and Its Performance. SEM and TEM Images of the Coating Film. The SEM pictures of the cross-sections for the films formed with WER in the absence and presence of PDA-ZrP are shown in Figure 8a,b,
Therefore, ambient temperature is more suitable for the ZrP modification with PDA, having the advantage of saving energy and making the source material green. TGA Analysis. TGA analysis was performed for both ZrP and PDA-ZrP, and the results are shown in Figure 6. ZrP went
Figure 6. TG-DTG curves of ZrP and PDA-ZrP from 30 to 800 °C.
through a double-dehydration process and then condensed to pyrophosphate. The first stage of dehydration of ZrP is at about 160 °C and makes up 4.2% of the total weight loss; this is attributed to crystalline water in ZrP. The second dehydration of ZrP occurred at 403 °C, resulting in a loss of about 4% of the weight, because of the escape of water located in the interior of the layers. Zirconium phosphate starts to lose water further at 511 °C, because of condensation and its becoming zirconium pyrophosphate.44 The final residual weight percentage of ZrP at 800 °C is 88%. For PDA-ZrP, 2.5% of weight loss happens before 100 °C, and this is probably attributable to desorption of carbon dioxide. As mentioned in the XPS analysis, PDA contains a certain amount of secondary amine groups, which can capture CO2 in the atmosphere to become carbonate, resulting in its gaining weight.45 The weight loss of ZrP mostly overlapped with the weight loss from the decomposition of PDA. Moreover, the decomposition of PDA starts at about 190 °C and goes to about 615 °C, and the final residual weight percentage of PDAZrP at 800 °C is 59.5%. Therefore, the PDA mass fraction in PDA-ZrP can be roughly estimated from the difference between the final residual weight percentages of ZrP and PDA-ZrP at 800 °C. The result shows that the synthesized PDA-ZrP contains nearly 28.5 wt % PDA. SEM, TEM, and AFM Images. Figure 7a shows the SEM picture of the ZrP nanoplatelets without the PDA modification. These platelets have an overall diameter (width) of 100 to 300 nm. The SEM picture of PDA-ZrP is shown in Figure 7b, and
Figure 8. Cross sections of (a) pure WER film and (b) PDA-ZrP/ WER film.
respectively. It is evident that without PDA-ZrP, WER suffers an imperfect filming process as a result of imperfect coalescence of epoxy resin emulsion droplets.14 As a result, a large amount of pores can be observed on the cross-section of the film formed from pure WER, allowing the permeation of water, oxygen and corrosion media. Instead, when PDA-ZrP is introduced, Figure 8b does not show the presence of pores. The interface between PDA-ZrP and WER is also indistinct, indicating good compatibility and strong interfacial affinity between them. EDS mapping is employed to show the element distributions on the cross-section of the nanocomposite film. As shown in Figure 9, the element zirconium is evenly distributed in the observed area, indicating that PDA-ZrP has been homogeneously distributed in the WER matrix. The TEM images of the frozen cross-section of the PDAZrP/WER film are given in Figure S3 in the SI. It can be clearly seen that after the PDA modification, the multilayers of ZrP nanoplatelets have been separated to some extent. Together with the XRD patterns in Figure 3, which show an increase in the interlayer spacing in the PDA-ZrP nanoplatelets, we can conclude that during the in situ polymerization, the formed PDA chains can intercalate between the
Figure 7. SEM pictures of (a) ZrP and (b) PDA-ZrP nanoplatelets. E
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Table 2. Open Circuit of Nanocomposite Films with Different Amounts of PDA-ZrP
Figure 9. EDS mapping of C and Zr elements in the cross-section of the nanocomposite film.
PDA-ZrP content (wt %)
open circuit potential (V)
0 0.2 0.5 1.0
−0.588 −0.535 −0.380 −0.361
positive value, which means that the barrier properties of the film were improved by the incorporation of PDA-ZrP. The EIS for a time shorter than 15 days could not be successfully collected because the barrier properties of the film were still good enough that the current through the electrode was too low to be detected by the used measurement equipment. Then, EIS measurement at 20 days became feasible, and the results are shown in Figure 10. The low frequency bode modulus (|Z|0.1Hz in Figure 10b) is a straight indicator for anticorrosion properties of film. It is clear that more PDA-ZrP in the nanocomposite film results in better overall anticorrosion properties. The film with 1.0% PDA-ZrP has a |Z|0.1Hz value of 5.07 × 106 Ω cm2, which is 1 order of magnitude larger than that of the pure WER coating (1.79 × 105 Ω cm2). As already noticed above, the substantially improved performance results from three aspects: First, with the introduction of PDA-ZrP, as shown in Figure 8b, the film presents negligible pores, and the interface between PDA-ZrP and the epoxy resin is indistinct, indicating good compatibility and strong interfacial affinity between them. Second, the PDAZrP nanoplatelets are dispersed homogeneously and randomly in the polymer matrix with negligible agglomeration. The random distribution of PDA-ZrP leads to torturous paths for the permeation of corrosive mediums, and the absence of agglomeration results in reduced permeable cross-sections and prolonged torturous paths. Both of these factors lead to enhanced corrosion resistance of the nanocomposite. Third, the degree of cross-linking in the film has been increased by the introduction of PDA-ZrP, which can improve the barrier properties of the film, thus enhancing the anticorrosion performance. The bode angle plot in Figure 10c for the film with 0% PDAZrP exhibits two time constants. Even if only one time constant is present for most of the films containing PDA-ZrP, two time constants occur also for the film with 0.5% PDA-ZrP, and the time constants overlap enough to hide the true nature of the system.32 Therefore, the equivalent circuits in Figure 11 are used to fit the EIS results. The circuit contains the solution resistance (Rs), the coating constant phase element (CPEc), the polarization resistance (Rp), the double layer constant phase element (CPEdl), and the charge transfer resistance (Rct). In the case with 0% PDA-ZrP, there is still a Warburg impedance element (Zw). Rp stands for electric resistance of the coating against the charge transfer happening in the pores and thus also represents the porosity of the coating. Furthermore, the capacitor element (CPEc) is an indicator of the barrier properties of the coating film. CPEdl and Rct are directly related to the corrosion surface between the coating film and the substrate, standing for the factors in a real corrosion interface. The Tafel curve (Figure 12) is another electrochemical analysis tool for evaluating the anticorrosion performance of the nanocomposite film. All the derived electrochemical parameters are presented in Table 3. The corrosion current
ZrP nanoplatelets. It should be mentioned that the role played by PDA in the intercalation is different from that of typical exfoliating agents (TBAOH, TBA, Jeffamine, etc.).26 In the present cases, because the monomer (dopamine) can directly interact with the ZrP surface, the polymerization occurs directly on the ZrP surface. Although the formed PDA chains can intercalate among the ZrP nanoplatelets, they glue on all the surfaces of the nanoplatelets. Thus, it follows that we have mainly intercalation but not complete exfoliation. In addition, as the chain length increases, PDA fills the interspace among the nanoplatelets at the edges and hinders the diffusion of the monomer into the center of the ZrP nanoplatelets, thus leading to insufficient intercalation in the central parts. From Figure S3, the PDA-ZrP nanoplatelets are dispersed homogeneously and randomly in the polymer matrix with negligible agglomeration. This is in agreement with the observations in the literature46,47 that proper orientation of the ZrP nanoplatelets is mainly due to the excluded-volume effect of the neighboring nanoplatelets. When the volume fraction of ZrP in the polymer matrix is below 2.0%, the ZrP nanoplatelets are randomly dispersed.48−50 The random distribution of PDA-ZrP leads to torturous paths for the permeation of corrosive mediums, and the absence of agglomeration results in reduced permeable cross-sections and prolonged torturous paths. Both of the factors are helpful for enhancing the corrosion resistance of the nanocomposite. DSC Measurements of the Coating Films. The DSC measurements have been performed for all the coating films, and the DSC curves are displayed in Figure S4 in the SI, showing that the glass transition temperature (Tg) value increases progressively as the amount of PDA-ZrP in the coating film increases. In particular, it increases from 70.1 °C for the pure WER film to 74.7 °C for the PDA-ZrP/WER film with 1.0 wt % PDA-ZrP. Because the Tg value typically increases with the degree of cross-linking, the results confirm that the cross-link density in the PDA-ZrP/WER films increases with the amount of PDA-ZrP, which should also be helpful for enhancing anticorrosion performance.51 The improved cross-link density results from the amino groups remaining on the surface of the PDA-ZrP nanoplatelets, which can react with the epoxy groups of WER.33,52,53 Anticorrosion Performance. The anticorrosion performance of the nanocomposite film was evaluated by open circuit potential and electrochemical impedance spectroscopy. Table 2 shows the open circuit potentials of the films containing different masses of PDA-ZrP after immersion of the films in 3.5% NaCl solution for 20 days. Generally, an open circuit potential with a more negative value implies that the film has less resistance to the penetration of a corrosive medium.17 The results in Table 2 reveal that as the PDA-ZrP mass increases in the film, the open circuit potential shifts progressively toward a F
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 10. EIS of nanocomposite films with different masses of PDA-ZrP. (a) Nyquist plot, (b) Bode modulus plot, and (c) Bode angle plot.
Figure 11. Equivalent circuits for nanocomposite coatings with different masses of PDA-ZrP.
density (Icorr) and corrosion potential (Ecorr) were generated by CHI660e electrochemical workstation software via extrapolation of the linear parts on the anodic branch and the cathodic branch of the Tafel curves. Ka and Kc are the slopes of the extrapolation lines of the anodic and cathodic branches of the Tafel curves, respectively. With these parameters, the polarization resistance (Rp) can be calculated by the Stream−Geary equation:5 Rp =
Figure 12. Tafel curves of nanocomposite films with different masses of PDA-ZrP.
CR =
kMIcorr ρm
where k is a constant (3268.6 mol/A), M is the molecular weight (56 g/mol), and ρm is the density (7.85 g/cm3). The obtained results, presented in Table 3, show that with increases in the mass of PDA-ZrP in the film, the polarization resistance increases, and the corrosion rate decreases. Specifically, at 1.0% PDA-ZrP, the corrosion current is reduced to 1.09 × 10−9 A/ cm2, which is near 500 times lower than that of the pure WER film (5.82 × 10−7 A/cm2). These results further confirm that
K a × Kc 2.303 × (K a + Kc)Icorr
The corrosion rate (CR) is calculated by the following formula: G
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 3. Electrochemical Corrosion Parameters Derived by Tafel Curves group
Ecorr (V)
Icorr (A/cm2)
0% PDA-ZrP 0.2% PDA-ZrP 0.5% PDA-ZrP 1.0% PDA-ZrP
−0.572 −0.487 −0.388 −0.489
5.82 9.30 8.93 1.09
× × × ×
10−7 10−8 10−9 10−9
Ka
Kc
Rp (kΩ)
0.197 0.204 0.203 0.225
−0.2 −0.198 −0.202 −0.234
74.04 469.13 4922 45 695
the introduction of PDA-ZrP in the WER film indeed substantially promotes the anticorrosion performances. Visual observation of the performances of the films with 0, 0.2, 0.5, and 1.0% PDA-ZrP after exposure to a salt spray test for 310 h is given in Figure 13. The specimen (mild carbon
CR (mm/year) 1.36 2.17 2.08 2.54
× × × ×
10−2 10−3 10−4 10−5
of the coating are greatly improved after introduction of the PDA-ZrP nanoplatelets, leading to excellent anticorrosion performances.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02557. AFM image and its corresponding analysis chart used to measure the geometric parameters of ZrP and PDA-ZrP; TEM images of ZrP, PDA-ZrP, and PDA-ZrP waterborne epoxy coatings; and DSC curves of 0% PDA-ZrP, 0.2% PDA-ZrP, 0.5% PDA-ZrP, and 1.0% PDA-ZrP (PDF)
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Figure 13. Visual performances of (a) pure ER and of films containing (b) 0.2% PDA-ZrP, (c) 0.5% PDA-ZrP, and (d) 1.0% PDA-ZrP exposed to salt spray tests for 310 h.
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (L.Z.). *E-mail:
[email protected] (H.W.). ORCID
steel) covered by the film in the absence of PDA-ZrP is seriously rusted, because of the presence of micropores and filming defects in the WER without PDA-ZrP. It can be clearly seen that the rusted area becomes smaller and smaller when more PDA-ZrP is added in the nanocomposite film. For the specimen covered with the nanocomposite film of 1.0% PDAZrP, no obvious rust is found except for in the scratched region. This indicates that the barrier properties of the WER film have been greatly improved through integration with PDA-ZrP, and the corrosion medium cannot easily penetrate through the film to the substrate. Therefore, the visual performance of the salt spray test is in accordance with the results of electrochemical analysis, and the anticorrosion performance of the nanocomposite coating increases as the mass of PDA-ZrP in the film increases.
Xinya Zhang: 0000-0003-2485-8397 Hua Wu: 0000-0002-2805-4612 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. Dr. Pengcheng Lin for his useful suggestion on the synthesis of ZrP. REFERENCES
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4. CONCLUSIONS We have prepared ZrP, polydopamine modified ZrP (PDAZPA), and nanocomposite films consisting of WER and PDAZrP. The chemical groups, crystallinity, thermostability, and morphology of ZrP and PDA-ZrP have been investigated by FT-IR, XRD, XPS, UV−vis, TGA, TEM, and SEM. The nanocomposites have been used to coat mild steel in order to investigate their anticorrosion properties via electrochemical analysis and salt spray tests. The results show that PDA successfully modifies the ZrP surface, leading to decreases in the crystallinity and thermostability of ZrP and improvement of the compatibility with the WER matrix. It has also been observed that PDA can intercalate into the interspace between the ZrP layers. From the cross-section of the nanocomposite coating film, all the PDA-ZrP nanoplatelets are well distributed in the resin matrix, with negligible agglomeration. In addition, compared with the pure WER film, the PDA-ZrP/WER film contains negligible amount of pores and has higher crosslinking density. It is therefore found that the barrier properties H
DOI: 10.1021/acs.iecr.9b02557 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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