Synthesis and Enhanced Corrosion Protection Performance of

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Synthesis and Enhanced Corrosion Protection Performance of Reduced Graphene Oxide Nanosheet/ZnAl-layered Double Hydroxides Composite Film by Hydrothermal Continuous Flow Method Xiaohu Luo, Song Yuan, Xinyu Pan, Caixia Zhang, Shuo Du, and Yali Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Apr 2017 Downloaded from http://pubs.acs.org on April 27, 2017

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ACS Applied Materials & Interfaces

Synthesis and Enhanced Corrosion Protection Performance of Reduced Graphene Oxide Nanosheet/ZnAl-layered Double Hydroxide Composite Films by Hydrothermal Continuous Flow Method Xiaohu Luo,a,b Song Yuan,a Xinyu Pan,a Caixia Zhang,a Shuo Du,a and Yali Liua*1 a

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of

Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, P. R. China b

School of Chemistry and Chemical engineering, Qiannan Normal University for

Nationalities, Duyun, Guizhou 558000, P. R. China

* E-mail address: [email protected]

ACS Paragon Plus Environment


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Abstract Prevention of water, oxygen, and chloride ions contained in hydrotalcite interlayers from diffusing through the layered double hydroxides (LDH) is of crucial importance in corrosion protection. In this work, a hybrid composed of reduced graphene

oxide

(RGO)

nanosheets

/Zn2+/Al3+

layered

double

hydroxide

(RGO/ZnAl-LDH) composite films on the surface of 6N01 aluminum (Al) alloy was successfully synthesized by a novel and facile hydrothermal continuous flow method, which

enabled direct growth of the composite on the surface of the Al alloy substrate.

The structure and morphology of the RGO/ZnAl-LDH composite films were fully characterized. Based on electrochemical measurements in a NaCl solution, the RGO/ZnAl-LDH composite film significantly enhanced the corrosion protection, as compared with the ZnAl-LDH film. The RGO/ZnAl-LDH composite film could maintain an outstanding corrosion resistance after 7-days immersion in a high concentration of NaCl solution (i.e., 5.0 wt%). The enhanced corrosion resistance was attributed to the barrier effect on diffusion of water, oxygen, and chloride ions by the RGO contained in the RGO/ZnAl-LDH composite films.

Keywords: reduced graphene oxide nanosheet, layered double hydroxides, corrosion protection, hydrothermal continuous flow method, barrier property

1. Introduction Nowadays, aluminum (Al) alloys have attracted considerable attention due to their extensive applications in automotive, power tools, aerospace and recreational equipment. However, Al alloys is prone to corrosion in aqueous environments, restricting their service durability. In the past decades, various surface treatment


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techniques have been developed to enhance the corrosion resistance of Al alloys, such as chemical conversion films,1,2 polymer coatings,3 silane films4, plasma electrolytic oxidation5, etc. Particularly, chemical conversion films have been paid much attention due to its operational simplicity, high efficiency and low cost. Generally, preparation of the conversion films includes multi-steps, and the post-treatments (e.g., chemical etching or thermal heat-treatment) are usually involved. It is imperative to develop facile, cost-effective and scalable approaches to prepare conversion films to meet the practical requirements for Al alloys. Layered double hydroxide (LDH) anionic or hydrotalcite clays are well-known protective films for Al alloys. The general chemical formula of LDH can be written as [M1-x2+Mx3+(OH)2]X+(An-)x/n·mH2O, where M2+ is divalent and M3+ is trivalent cations, and An- represents the majority of organic and inorganic anions. The LDHs have been investigated in terms of its corrosion protection for metals/alloys including Al alloys,6 magnesium alloys,7 and steels8 by virtues of their unique nanocontainer structure, high corrosion resistance and ion-exchange capacity. The LDH films are prepared primarily by two types of method, i.e., in-situ or the two-step methods. For the two-step method, LDH powder precursors are prepared by the coprecipitation method, and then, the film is treated by hydrothermal processes. Uan et al. used in-situ method to prepare Mg4.38Zn0.22Al2(OH)3.19CO3·mH2O conversion film with a good crystallite structure on AZ91D Mg alloys in aqueous HCO3-/CO32- medium.9 Guo et al. fabricated ZnAl-LDH films on Al substrate by one-step hydrothermal crystallization method. The bilayer film exhibited a low corrosion current density of 10-8 A/cm2 and a large impedance of



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16 MΩ.10 The corrosion resistance of the films during long-term immersion in the aqueous solutions was not studied. Chen et al. prepared a compact and uniform Mg/Al-LDH conversion film on AZ31 Mg alloys by the two-step in-situ growth method.After 48 h of immersion in 0.1 M NaCl solution, the film produced corrosion pits due to diffusion of chloride ions.11 Zeng et al. synthesized a molybdate intercalated hydrotalcite (MgAl-LDH-MoO42-) film on AZ31 Mg alloy using a combined coprecipitation and hydrothermal process.12 The film possessed the ion-exchange and self-healing ability, and improved the corrosion protection compared to the pure MgAl-LDH film. However, the film was damaged by chloride ions contained in the hydrotalcite interlayer by ionic exchange after 144 h of immersion in 3.5 wt% NaCl solution. There exist two major problems during preparation of the LDH films by either in-situ method or the two-step method. On the one hand, the chloride ions contained in the hydrotalcite interlayer by ionic exchange can damage the film and generate crevice and pits with the increasing immersion time. On the other hand, the adsorption of water on the film surface forms a diffusion pathway for oxygen and chloride ions, further facilitating the cathodic reaction. Therefore, prevention of aggressive media from diffusion through the LDH layer and blocking of water from adsorption on the film surface to form the diffusion pathway for oxygen and chloride ions are prerequisites for further enhancement of the corrosion protection of the LDH film. Graphene as a 2D layer of sp2-hydridized carbon atoms has found its extensive applications in nanocomposites,13,14 catalyst support,15 hydrogen storage,16 luricant17


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and metal protection18-20. Recently, the liquid-phase exfoliation from natural graphite has been used to prepare graphene at a large scale.21 Studies has also been conducted to investigate the potential application of graphene as a corrosion barrier because of its higher aspect ratio and lower density. Yeh et al. prepared the polyaniline (PANI)/graphene composites by using the PANI chains grafted on the surface of graphene for corrosion protection of steels. The as-obtained film exhibited an outstanding corrosion protection performance compared with the pure PANI.22 Yu et al. used the chemical bonding of vinyl-grafted GO and styrene by in situ polymerization to prepare Polystyrene (PS)/GO nanocomposites with 2 wt% GO, which showed an excellent corrosion protection compared with neat PS. The corrosion protection efficiency was increased to 99.5%.23 These methods were generally complicated, and it was difficult to conduct in practice. Moreover, lots of toxic agents were used in the synthetic procedure. Moreover, there has been no report on the graphene/LDH composite films as the corrosion protection material for metal/alloys. In this work, we reported a newly developed and facile hydrothermal continuous flow method to prepare reduced graphene oxide (RGO)/ZnAl-LDH composite film on the surface of a 6N01 Al alloy (Figure S1), enabling a direct growth on the alloy substrate. Initially, the exfoliated GO nanosheet was prepared by the modified Hummers method24,25, and reduced to the reduced graphene nanosheet (RGO) by the chemical reduction method using the environmentally friendly vitamin as the reductant. Subsequently, the RGO/ZnAl-LDH film on the surface of 6N01 Al alloy was synthesized through the facile hydrothermal continuous flow method. RGO was



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successfully embedded in the LDH layers, offering an outstanding barrier against diffusion of water, oxygen, and chlorides ions through the RGO/ZnAl-LDH film. The structure of the prepared films were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), Raman spectra, X-ray photoelecton spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The corrosion resistance of the RGO/ZnAl-LDH composite film was measured by potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS).The relationship between the microstructure of graphene and its outstanding corrosion protection performance was discussed. To the best of our knowledge, it is the first time that the RGO/ZnAl-LDH composite film was synthesized and used as a corrosion protection material.

2. EXPERIMENTAL SECXTION 2.1. Hydrothermal continuous flow reactor (HCFR) setup. The hydrothermal continuous flow reactor was comprised of five major components (Figure S1), which were the precursor solution reservoir, the high performance liquid chromatography (HPLC) pump, the Teflon-lined 316 stainless steel reaction column, the home-built tube furnace with temperature control, and the pressure regulator with the pressure between 100 to 1000 psi. The 316 stainless steel tubing was used to connect to each other. Before the reaction, the system should be washed by continuously flushing with the ultrapure water. The reaction column should be preloaded with the carbon paper and


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the precursor solution should be to its maximum volume, and sealed with the 316 stainless steel caps. The reaction column was placed in the tube furnace and connected with the pressure regulator and the HPLC pump. 2.2. Raw material. Natural graphite was purchased from Sigma Aldrich Co., Ltd (Shanghai, China). All reagents were of analytical grade and were supplied by Sigma Aldrich Co., Ltd (Shanghai, China). They were used without further purification. 2.3. Preparation of GO and RGO. GO was obtained from the natural graphite by a modified Hummers method.24,25 Graphite powder (3 g, 500 mesh) was added into an 60 ℃ solution of concentrated H2SO4 (5 mL), K2S2O8 (2.5 g) and P2O5 (2.5 g), then the mixture was kept at 60 ℃ for 6 h on a hotplate. Successively, the solution was cooled to 25 ℃ and washed with deionized water until the pH=7.0. The product was dried at 60 ℃ for 12 h. As-obtained pre-oxidized graphite was carried out to oxidation by Hummers’ method. Pre-oxidized graphite powder (3.0 g) was put into 0 ℃ solution of concentrated H2SO4 (69 mL), then KMnO4 (9.0 g) was added gradually with mechanical stirring and the reaction temperature was kept to be below 20 ℃. After the addition of KMnO4 (9.0 g), the solution was kept at 35 ℃ for 2 h, and then diluted with deionized water (138 mL). Successively, the mixture was kept at the room temperature for 2 h with stirring, and then 500 mL deionized water was added into the mixture. After that, 15 mL of 30% H2O2 was added to the suspension with mechanical stirring, during this process, the suspension became brilliant yellow together with bubbling. Further, mixture was filtered by centrifugation and washed with 500 mL hot HCl (1.0 M) solution to remove metal ions, then washed with



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deionized water until pH=7.0. Finally, the humid GO was obtained. Reduced graphene oxide (RGO) was prepared by chemical reduction from GO. Firstly, GO powder (1.0 g) was ultrasonicated in 250 mL deionized water, then resulting suspension was titrated with the ammonia (0.5 M) solution with mechanical stirring at 25 ℃ until pH=10.0, accompanied with aging at 98 ℃ for 24 h. The yellow-brown solution become a dark precipitate gradually in the reduction process. After the reduction, the black precipitate was filtered by centrifugation, washed with deionized water several times until pH=7.0, and dried in a vacuum oven at 45 ℃ for 24 h to obtain pure RGO. The as-obtained RGO (0.002 g) was redispersed in 100 mL ultrapure (20.2 MΩ cm) water for further use. 2.4.

Precursor

solution

preparation.

Certain

amounts

of

0.060

mol

Zn(NO3)2·6H2O, 0.142 mol NH4NO3 and 0.010 mol Al(NO3)3·9H2O were dissolved in 160 mL ultrapure water (20.2 MΩ cm), then aqueous ammonia solution (1%, w/w) was added into the mixture until pH=about 5.6. Then 10 mL of above RGO dispersion (0.001 mg/mL) was added into the above mixture with stirring. After that, the mixture was transferred to the reservoir, and used as the feeding precursor solution for continuous flow reaction. 2.5. Substrate preparation. 6N01 Al alloys substrate with 5 cm in width and 5 cm in length were used in all experiments. Before the synthesis of the RGO/ZnAl-LDH composite film, the surfaces of 6N01 aluminum alloy were mechanically ground with SiC papers up to 3000 grit to obtain the same surface roughness, then ultrasonically cleaned in absolute ethyl alcohol and dried in a flow of nitrogen gas. Then the


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aluminum alloy was pushed into the center of the hydrothermal continuous flow reactor before the sealing of reaction column and the loading of

precursor solution.

2.6. Hydrothermal continuous flow reaction. Firstly, the reaction column was preloaded with the precursor solution (50 mL), then was placed through the tube furnace and connected with the pressure regulator and the high performance liquid chromatography pump. After that, the furnace was turned on to heat the reaction solution and kept at 130 ℃ for 45 min to allow the seed growth before the precursor solution was flowed at a speed of 0.5 mL min-1. The pressure was set to about 90 psi and kept the constant through the reaction. Afterward, the temperature was set to 80 ℃ and would stay constant. After 2 h reaction, the heating was stopped and nitrogen gas flow was used to cool down the reaction column. Subsequently, the pressure was released by opening the pressure regulator, and then aluminum alloy substrate was removed from the reaction column, washed with deionized water and ethanol. Finally, the resulting substrate was dried in the flowing of nitrogen gas. The product was labeled as RGO/ZnAl-LDH-t2 film. Other RGO/ZnAl-LDH-t1 film and RGO/ZnAl-LDH-t3 film samples were prepared in the same manner, but with a different concentrations of RGO dispersion (0.0005 mg/mL and 0.002 mg/mL, respectively). Additionally, for comparison, the pure ZnAl-LDH film without RGO dispersion on the 6N01 Al alloy surface was synthesized under the same condition, the product was labeled as ZnAl-LDH film.

3. RESULTS AND DISCUSSION



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3.1. Fabrication and Characterization of GO, RGO and RGO/ZnAl-LDH-ts film. Figure 1a shows the XRD patterns of natural graphite, GO and RGO. The characteristic diffraction peak of exfoliated GO appears at 2θ of around 10.8º (001) with a basal spacing of 0.82 nm,26,27 which is attributed to the formation of some oxygen-containing functional groups on the surface of graphite. After GO is reduced to RGO by the vitamin, most of oxygen-containing functional groups are removed. As for the spectrum of RGO, it is seen that there is a weak and broad diffraction peak (002) at around 25.7º, which is indicative of the reduction of GO and the exfoliation of the layered RGO.28,29 The interlayer spacing of RGO (d=0.39 nm) is a little larger than that of natural graphite (d=0.34), which is due to the formation of agglomerates between the RGO nanosheets via the π-π stacking interaction. Figure 1b shows the XRD spectra of ZnAl-LDH film-coated and RGO/ZnAl-LDH-ts (s=1, 2, 3) film-coated 6N01 Al alloys as well as Al alloy substrate. The diffraction peaks at 2θ=45.5º and 65.7º arise from the Al substrate. The ZnAl-LDH film-coated Al alloy shows the intensive characteristic diffraction peaks, which is in agreement with hydrotalacite-like family, i.e. (003), (006), (012) and (010).30 The RGO/ZnAl-LDH-ts film-coated Al alloyshows all the characteristic diffraction peaks of ZnAl-LDH. The isolated graphene carbon diffraction peak (e.g., (002) peak at 25.7º) is not observed in the RGO/ZnAl-LDH-ts film. The RGO of the RGO/ZnAl-LDH-ts film is confirmed by the following characterization of FT-IR spectra, Raman spectra and XPS spectra. Compared to the ZnAl-LDH film, the hydrotalcite-like characteristic diffraction peaks of (003), (006), (012) and (010) of RGO/ZnAl-LDH film are more symmetric and sharper with the


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increase of the RGO loading ratios, indicating that the increasing content of RGO can improve the crystallinity of the hybrid material. The residual oxygenated functional groups on the RGO surface possess a strong electrostatic repulsion and hydrophilicity, which can stabilize the dispersion of RGO. This is consistent with the result of zetal potential of RGO ( Figure S2 ). The zetal potential of the RGO is about -60.54 mV due to the presence of the residual oxygen functionalities on the RGO surface. In principle, the zetal potential with the absolute value is larger than 30 mV, meaning a stable dispersion. This indicates that the dispersion of RGO is stable because of the strong electrostatic repulsion. The FT-IR spectra of GO, RGO, ZnAl-LDH film and RGO/ZnAl-LDH-ts film are shown in Figure 2. For GO (Figure 2a), several characteristic peaks associated with different carbon-oxygen functional groups are included in the spectrum.The absorption peak at about 1738 cm-1 is attributed to the C=O stretching vibration of COOH group, and the absorption peak at about 1621 cm-1 is associated with the C=C/C-C stretching vibration arising from carbon backbone.The absorption peak at about 1396 cm-1 corresponds to C-OH stretching vibration of carboxyl.The others at about 1229 and 1063 cm-1 correspond to the epoxy (C-O-C) and alkoxy (C-O) stretching vibrations, respectively.31,32 Moreover, a strong and broad absorption band at 3435 cm-1 is assigned to the stretching vibration of hydroxide groups (νO-H) resulting from water molecule and hydroxyl groups of COOH groups. It is noted that the peaks in the RGO spectrum is not as obvious as those for GO. There is a low absorption peak at about 1622 cm-1, which is associated with the skeletal aromatic vibration. The lower relative



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intensity of peak at 1064 cm-1 is due to alkoxy groups stretching vibrations. For the spectra of RGO/ZnAl-LDH-ts composite (Figure 2b), it is seen that the C=O and C-O stretching

vibrations

completely

disappear.

Compared

to

ZnAl-LDH,

RGO/ZnAl-LDH-ts has an obvious peak at round 1622 cm-1, which is caused by C=C/C-C stretching vibration arising from the RGO carbon backbone. In addition, for the RGO/ZnAl-LDH-ts composite film, the intensity peak at 1622 cm-1 increases with the increasing RGO loading. There are some absorption peaks at the low-frequency region, which are due to the metal-oxygen and metal-hydroxyl stretching vibrations in the lattice of LDH phases. Based on the spectral analysis, it is concluded that the RGO and LDH phases are included in the RGO/ZnAl-LDH-ts composite film. Figure 3 shows the Raman spectra of GO, RGO, ZnAl-LDH film and RGO/ZnAl-LDH-ts film, respectively. It is noted that, in Figure 3a,the D-band and the G-band shift from 1353.7 cm-1 and 1578.4 cm-1 for GO to 1352.4 cm-1 and 1577.6 cm-1 for RGO, respectively, indicating the formation of the graphene structure resulting from the reduction of GO.33-35 Generally, the intensity ratio of the D-band and the G-band (ID/IG) provides direct evidences of the degree of lattice distortion and the surface defect of a graphite layer within carbon materials. The value of ID/IG of GO increases gradually from 0.93 to 1.53 for RGO. Compared with that of GO, the higher value of ID/IG for RGO is attributed to the decrease in the size of the in plane sp2 domain, the unrepaired defects present on the graphene structure, and the removal of oxygen functional groups in the GO structure.35,36 It is seen from Figure 3b that a broad D band and G band are at 1346.8 cm-1 and 1576.4 cm-1, respectively, in the


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spectrum of RGO/ZnAl-LDH-ts composite film compared with ZnAl-LDH film. In the spectrum of the RGO/ZnAl-LDH-ts composite film, in addition to the G band and D band, two higher relative intensity of band at 598.1 cm-1 and 1068.3 cm-1 are obtained, which are correlated with the typical Raman peaks of ZnAl-LDH phases. The XPS spectra of GO, RGO, ZnAl-LDH film and RGO/ZnAl-LDH-ts composite film are shown in Figures 4a and 4b. The binding energy is corrected by referencing the C 1s peak to 284.3 eV in the XPS measurements. The C 1s XPS of GO, RGO and RGO/ZnAl-LDH-t2 composite films are shown in Figures 4c and 4d. Evidently, the XPS spectrum of RGO/ZnAl-LDH-ts composite film exhibits a low relative intensity of C 1s peak and O1s peak, and two peaks at 1021.8 eV and 1044.8 eV, which are due to Zn2p3/2 and Zn2p1/2 respectively. This also confirms that the inclusion of ZnAl-LDH phases in the RGO/ZnAl-LDH-ts composite film. The composition of the oxygen functional groups in GO is different from that of RGO and RGO/ZnAl-LDH-ts. For GO, the C 1s spectrum is split into four characteristic peaks for carbon atoms associated with different oxygen functional groups, which are in agreement with the previous reports.37 The dominant peak at around 284.3 eV is attributed to the sp2 graphitized carbon. The other peaks at around 285.6, 286.5, 288.3, and 290.1 eV are assigned to -C-OH, C-O-C, C=O and O-C=O groups, respectively.38 It is found that the peak intensities of -C-OH, C-O-C, and C=O groups for the RGO and RGO/ZnAl-LDH-ts decrease markedly compared with the GO. The area of the C 1s components of GO shows that the nonoxygenated ring C is about 44.5%, while that of the RGO is around 73.6%. The results suggest the successful removal of the oxygnated



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groups of GO through the chemical reduction, which agrees with the above FT-IR and Raman analysis. From Figure 4b, it is seen that the relative intensity of C 1s peak of the RGO/ZnAl-LDH-ts slightly increases with the increasing RGO loading ratio, indicating that more RGO nanosheets are incorporated into the RGO/ZnAl-LDH-ts composite film as the RGO loading increases. This variation is also consistent with the FT-IR results. The morphology of the prepared RGO is characterized by AFM, and shown in Figure S3. The average thickness of the RGO calculated from the height profile of the AFM image is 0.8-1.0 nm. Compared to the theoretical value of 0.34 nm for one-atom-thick graphene, the as-prepared RGO should be three-atom-thick graphene. The ideal three-atom-thick RGO embedded in the ZnAl-LDH material provides a promising barrier effect. The morphology of the prepared materials are shown in Figure 5. For RGO, the SEM and TEM images (Figures 5a and 5b) show that the edge of RGO possesses a corrugated and scrolled sheet-like structure, indicating that the RGO agglomerates slightly with each other by van der Waals interactions. From the HRTEM image (Figure 5c), the space of the hexagonal lattice of the RGO in the crystalline area is approximately 0.35 nm, which is attributed to the restored sp2 carbon by chemical reduction. For the ZnAl-LDH composite film (Figure 5d) after the thermal treatment, the film grown on the substrate is loose and porous lamellar. For RGO/ZnAl-LDH-ts films (Figures 5e-5g), the growth of the RGO/ZnAl-LDH on Al substrate is more compact with the increase of the RGO loading compared with the ZnAl-LDH film.


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The cross-sectional views (Figures 5e-1-5g-1) of the RGO/ZnAl-LDH-ts film demonstrate that the films are quite compact and thick. The film thickness is approximately 1.8 µm. Moreover, the RGO is embedded in the ZnAl-LDH layer. To further find RGO embedded in the RGO/ZnAl-LDH composite film, the RGO/ZnAl-LDH composite (Figure 5h) scraped from the RGO/ZnAl-LDH-t2 film exhibits ZnAl-LDH layered stacks, where the tiny ZnAl-LDH nanoplatelets are firmly coated on the RGO surface. The well-crystalline ZnAl-LDH nanostructure exposes (012) lattices with the space of 0.26 nm (Figure 5h-1). Since most of the RGO base intercalated the LDH layer is covered with the LDH platelets, it is difficult to detect the graphene lattices. However, in the edged area, the extended graphene lattices with the spaces of 0.35 nm (Figure 5h-1) is identified. The results provides direct evidences that the RGO is embedded in the RGO/ZnAl-LDH composite film. Furthermore, the adhesion testing of RGO/ZnAl-LDH film on 6N01 Al alloy demonstrates that the RGO loading does not reduce the adhesion of the RGO/ZnAl-LDH composite film compared with the ZnAl-LDH film (Figure S4). The film exhibits a high adhesion, ranging from 19.63 to 20.03 MPa (Table S1). 3.2. Corrosion resistance of the RGO/ZnAl-films. Potentiodynamic polarization measurements were carried out to quantitatively analyze the corrosion resistance of bare 6N01 Al alloy, ZnAl-LDH film and RGO/ZnAl-LDH-ts (s=1, 2, 3) film in 3.5 wt% NaCl solution for different days. Prior to the measurement, the samples were soaked in the solution until the system was in a steady-state.39 Polarization curves were recorded from -500 mV to 500 mV with



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respect to corrosion potential at a scan rate of 0.5 mV s-1. Figure 6 shows the polarization curves of bare Al alloy, ZnAl-LDH film and RGO/ZnAl-LDH-ts film in the solution as a function of immersion time. It is seen that the polarization curves show a good linear Tafel region in both anodic and cathodic branches.The corrosion kinetic parameters including corrosion current density (icorr) and corrosion potential (Ecorr) can be derived using the nonlinear least-square fitting method of Levenberg/Marquardt via a software,40 and the results are listed in Table 1. It is seen that the Ecorr of ZnAl-LDH film after 0.5 day of immersion in 3.5 wt% NaCl solution is about -186 mV (SCE), which is more positive than that of bare 6N01 Al alloy (-261 mV, SCE), while the icorr of ZnAl-LDH is 0.0535 µA/cm2 , which is much lower than that of bare 6N01 Al alloy (2.513 µA/cm2). Thus, the 6N01 Al alloy is effectively protected by the ZnAl-LDH film. This is attributed to the facts that

the ZnAl-LDH

film acts as a physical barrier to block the aggressive mediums from attacking the underlying substrate. For RGO/ZnAl-LDH-ts film after 0.5 day of immersion in 3.5 wt% NaCl solution, the values of Ecorr and η increase, and Rcorr and icorr decrease with the increasing RGO loading ratio compared to the ZnAl-LDH film (Table 1 and Figure 8a). For the RGO/ZnAl-LDH-t3 film, the Ecorr value is up to -172 mV (SCE), and the icorr is only 0.0403 µA/cm2, indicating that the corrosion protection is improved compared to the ZnAl-LDH film. As the immersion time increasing to 7 days, the ZnAl-LDH film has the Ecorr of -234 mV (SCE) and the icorr increasing to 0.531 µA/cm2, while the protection efficiency drops from 97.87% to 78.87%. This implies that the ZnAl-LDH film is degraded, and


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can not effectively protect the underlying Al substrate. As shown in Figure S5a, the hydrotalcite structure is damaged and

pitting corrosion occur on the ZnAl-LDH film

surface. With the increase of the immersion time, chloride ions contained in the LDH layer can diffuse into the film/Al interface via the diffusion pathway formed by adsorption of water, gradually degrading the LDH protective film For the RGO/ZnAl-LDH-ts film after 7 days of immersion in 3.5 wt% NaCl solution, the Ecorr and η increase and icorr and Rcorr decrease with the increase of the RGO loading, suggesting that the RGO/ZnAl-LDH film provides corrosion protection to the Al alloy. The icorr values of RGO/ZnAl-LDH-t1 film and RGO/ZnAl-LDH-t3 film are approximately 0.0463 µA/cm2 and 0.0432 µA/cm2, respectively, slight increase compared to the ZnAl-LDH film. The Rcorr of the RGO/ZnAl-LDH-t1 film shows 11 times of decrease and the RGO/ZnAl-LDH-t3 film has about 12 times of decrease compared to the ZnAl-LDH film. At the same time, the RGO/ZnAl-LDH-ts film exhibits a remarkable improvement of η. The η value of RGO/ZnAl-LDH-ts film increases at least 98.16%, whereas that of ZnAl-LDH film increases 78.87% only. Moreover, as shown in Figure S5(b-d), the RGO/ZnAl-ts film remains compact and intact. Thus, after 7 days of immersion in 3.5 wt% NaCl solution, the corrosion resistance of the RGO/ZnAl-LDH-ts film is much better than that of the ZnAl-LDH film. This may be attributed to the presence of RGO in the RGO/ZnAl-LDH-ts composite film. Corrosion rate of metals generally increases with the increasing concentration of corrosive species such as Cl-, SO42-, PO42- , SO32- and HSO32-.41 Chloride ions, as one



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of the most aggressive species, can degrade metals/alloys and destroy the protective surface layer.42 To investigate the corrosion protection of the ZnAl-LDH film and RGO/ZnAl-LDH-ts film in chloride environments, the specimens were soaked in 5.0 wt% NaCl solution for 0.5 day and 7 days, respectively. The polarization curves are shown in Figure 6 (c, d) and the corrosion kinetic parameters are listed in Table 1. After 7 days of immersion in 5.0 wt% NaCl solution, the Ecorr and icorr values of the ZnAl-LDH film are -423 mV (SCE) and 6.174 µA/cm2, respectively. This is consistent with the decrease of η from 97.82% to 70.13%. For RGO/ZnAl-LDH-ts film, after 7 days of immersion in 5.0 wt% NaCl solution, the Ecorr and η increase, and the icorr and Rcorr decrease with the increasing RGO loading ratio. The corrosion rate of the RGO/ZnAl-LDH-t1 film exhibits a 14 times of reduction compared to the ZnAl-LDH film, and the RGO/ZnAl-LDH-t3 film exhibits a 16 times of reduction. The RGO/ZnAl-LDH-ts film provides a substantial improvement over η. The RGO/ZnAl-LDH-t1 film exhibits an increase of η by 98.03%, compared to that of bare Al alloy, whereas the ZnAl-LDH film causes an increase of 70.13% only. For the RGO/ZnAl-LDH-t3, the η increases by approximately 98.25%. The changes of Ecorr and icorr also provide direct evidences that the RGO/ZnAl-LDH-ts exhibits the outstanding corrosion protection property. Moreover, the corrosion protection performance of the film is better than the literature data (Table S2). To better understand the corrosion resistance of the RGO/ZnAl-LDH-ts film, EIS was measured, and the Nyquist plots are shown in Figure 7. The electrical equivalent circuit shown in Figure S6 is employed to fit the EIS data, where Rs, CPEc and Rf


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represent the electrolyte resistance, the film capacitance and the polarization resistance which refers to the degree of difficulty for corrosion to occur on the specimen.43 The Rf is equivalent to the sum of charge transfer resistance (Rct) and film resistance (Rp), i.e., Rf= Rct + Rp. A CPE (constant phase element) is used in order to obtain a better fitting, since the metal/solution interface does not present like a real capacitor. The CPE is thus used to replace a double layer capacitor Cdl.43 The impedance of the CPE is express as:

[

Z CPE = f 0 ( jw) n

]

−1

where f0 is a proportionality coefficient, w is the angular frequency and j is the imaginary number. Because of the normal time-constant distribution, the effective film capacitance (Qc) is calculated from the CPE parameters by: QC

1 n

(Y × R ) = f

0

Rf

where Y0 and n are CPE admittance and CPE exponent, respectively. Generally, the corrosion rate decreases with the increase of Rf. The fitting impedance parameters for the samples immersed in 3.5 wt% NaCl solution for 7 days are summarized in Table 2. Diffusion of electrolyte into the film usually results in the increase of Qc because of the large relative permittivity of the electrolyte. As the Al alloy corrosion progresses, the size of the Nyquist plot increases with an increase in the charge transfer resistance. It is seen from Figure 7 that the diameter of the semicircles increases in the sequence of bare Al alloy < ZnAl-LDH film < RGO/ZnAl-LDH-t1 film < RGO/ZnAl-LDH-t2 film < RGO/ZnAl-LDH-t3 film, which indicates that the charge transfer resistance



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increases as the RGO loading is increased. The Rf values of bare Al alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1film, RGO/ZnAl-LDH-t2 film, and RGO/ZnAl-LDH-t3 film are 8.91×102, 1.94×104, 2.14×106, 2.23×106, and 2.43×106 Ω cm2, respectively. The EIS plots of RGO/ZnAl-LDH-ts film show nearly ideal semicircles due to the diffusion effect, indicative of the poor conductivity of the film layer and the solution, which is consistent with their Qc values. The RGO/ZnAl-LDH-t1, RGO/ZnAl-LDH-t2, and RGO/ZnAl-LDH-t3 films produce the Qc values of 0.521±0.035, 0.465±0.042, and 0.401 ±0.21 µF/cm2, respectively, which are much lower than that of the ZnAl-LDH film (i.e., 73.42±0.55 µF/cm2 ). As-obtained Bode plots of the samples in 3.5 wt% NaCl solution at various immersion times are shown in Figure S7. The changes in impedance at the low frequency limit (i.e., 10 mHz) and the breakpoint frequency (fb, frequency at -45° phase angle) of the samples are shown in Figure S8. From Figures S7 and S8a, it is noted that the phase angle and the low frequency impedance (|Z|0.01Hz) for the samples except for the bare Al alloy are higher at the beginning of the immersion (i.e., 0.5 day), indicating a good corrosion protection performance at the initial immersion stage. It is also seen that the increase of the RGO loading ratio causes the substantial increase of the impedance at the low frequency compared to the ZnAl-LDH film, indicative of the improvement of the corrosion protection. With the increase of the immersion time, the low frequency impedance of the ZnAl-LDH film declines, and the |Z|0.01Hz value of the ZnAl-LDH film drops from 3.51×106 Ω cm2 to about 6.73×104 Ω cm2 after 7 days of immersion, which suggests that the protective performance decreases with time. As a


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comparison, the RGO/ZnAl-LDH-ts film exhibits a minimal decrease in the low frequency impedance as evidenced by the |Z|0.01Hz values of the RGO/ZnAl-LDH-t1 and RGO/ZnAl-LDH-t3 films changed to about 2.41×106 Ω cm2 and 6.87×106 Ω cm2, respectively. The results show that the RGO loading ratio causes the significant improvement of the corrosion protect property of the RGO/ZnAl-LDH film, which may be due to the ability of RGO in the RGO/ZnAl-LDH composite film to act as a physical barrier to effectively decrease diffusion of water, oxygen, chloride ions and corrosive species. Furthermore, the breakpoint frequency (fb) is a proper parameter to determine the corrosion protection performance of the film. It is seen from Figures S7 and S8b that the fb shifts to higher frequencies with an increase in the immersion time. The increase of fb is evident for the ZnAl-LDH film. The RGO/ZnAl-LDH-ts exhibits a less increase in fb with the increase in RGO loading ratio compared to the ZnAl-LDH film. For example, the fb of the film increases in the sequence of RGO/ZnAl-LDH-t3 < RGO/ZnAl-LDH-t2 < RGO/ZnAl-LDH-t2 < ZnAl-LDH. The results suggests that the RGO/ZnAl-LDH-ts composite film enhances the corrosion protection performance relative to the ZnAl-LDH film. This is attributed to the RGO in RGO/ZnAl-LDH-ts composite film, which acts as a barrier for the corrosive electrolyte diffuse towards the the film/metal interface. Figure 8 shows the XRD spectra of RGO/ZnAl-LDH-ts film-coated and ZnAl-LDH film-coated Al alloy specimens before and after 7 days of immersion in 3.5 wt% NaCl solution. It is seen that both (003) and (006) diffraction peaks of the ZnAl-LDH film



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shift to a higher degree, indicative of the intercalation of chloride ions in the galleries of ZnAl-LDH crystal. Previous study reported that chloride ions are easier to be intercalated into the gallery of the LDH crystal compared to nitrate ions because of a larger binding energy resulting from the interlayer ions and the metal hydroxide layers.44 However, the shift of both (003) and (006) diffraction peaks of the RGO/ZnAl-LDH-ts film after 7-day immersion to the higher degree is negligible. When the as-prepared ZnAl-NO3--LDH film is soaked in the 3.5 wt% NaCl solution, the nitrate ions are replaced by chloride ions due to the large amount of chloride ions in the solution. With the increasing immersion time, chloride ions in the ZnAl-LDH crystal gradually diffuse into the film/Al alloy interface. This is understood from the distribution of chloride ions in the ZnAl-LDH film (Figure 9a). It is seen from the EDX spectra of the three different positions in the cross section of ZnAl-LDH film that the relatively intensity of chloride peaks does not change, indicating that

chloride

ions diffuse into the film/Al alloy interface. In contrast, it is apparent from Figure 9b that the relatively intensity of chloride peaks markedly decreases with the increase of the thickness of the RGO/ZnAl-LDH-t2 film. For example, the chloride peaks are not detected at the film/Al alloy interface. These observations indicate that the RGO in the RGO/ZnAl-LDH-ts film act as the barrier and prevents chloride ions from diffusing through the film, resulting in improvement of the corrosion protection. To further investigate the barrier property of the RGO/ZnAl-LDH-ts for chloride ions, the rejection performance of samples were carried out using the sodium chloride (NaCl) as a model, and the results areshown in Figure 10. The rejection of the samples


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decreases as the increase of the immersion time, but the RGO/ZnAl-LDH-ts shows the minimal decrease as evidenced by that the rejection values of the RGO/ZnAl-LDH-t1, RGO/ZnAl-LDH-t2, and RGO/ZnAl-LDH-t3 are approximately 97.89%, 98.86%, and 99.16%, respectively. The rejection of the RGO/ZnAl-LDH increases as the RGO loading ratio increases in comparison with the ZnAl-LDH. The high rejection of the RGO/ZnAl-LDH-ts for chloride ions compared with that of ZnAl-LDH is attributed to the presence of RGO in the RGO/ZnAl-LDH-ts composite.

3.3. Mechanism of Superior Corrosion Protection Performance of the RGO/ZnAl-LDH composite film. Compared to the ZnAl-LDH film, the RGO/ZnAl-LDH-ts film exhibits a significant improvement in the corrosion protection performance, which is mainly attributed to the synergistic effect between RGO and the LDH layer. Previous studies confirmed that graphene shows a high impermeable property and outstanding barrier effect against oxygen and water.45,46 According to the SEM images of the prepared RGO/ZnAl-LDH-ts films (Figure 5(e-1-g-1)), the RGO are disordered arranged without a certain orientation on the alloy surface. Oxygen is difficult to pass through the RGO/ZnAl-LDH layer. As a result, the cathodic reaction at the alloy/solution interface is effectively alleviated. As reported, the RGO is hydrophobic in nature with a hierarchical roughness and high porosity, coupled with a low surface energy (Figure S9). Air is easily entrapped in the nanopores which act as another type of diffusion barrier to water due to the minimal contact between the graphene surface and water droplets. Thus, water is



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difficult to diffuse through the RGO/ZnAl-LDH layer due to the low diffusion coefficient. Additionally, the RGO in the RGO/ZnAl-LDH nanocomposites can increase the tortuosity of the diffusion pathway for oxygen and waters. Chloride ion rejection is another important factor for corrosion resistance of the RGO/ZnAl-LDH composite film. Diffusion of chloride ions usually causes crevices and pits on the film surface. Chloride ion rejection is attributed to the electrostatic repulsion between the negatively charged chloride ions and negatively charged oxygen-containing functional groups at the edges of RGO. The Donnan exclusion mechanism is appropriate to explain the rejection performance of charged nanosheets.48,49 According to the Donnan exclusion, the chloride ion concentration contained in the RGO/ZnAl-LDH nanocomposites is much lower than that in the electrolyte. When the electrolyte passes through the films, chloride ions are rejected due to the Donnan potential. Therefore, the chloride ion concentration can keep at minimum at the Al alloy/film interface, and the RGO/ZnAl-LDH film can be intact. The crevices and pitting corrosion are effectively prevented. Based on the above experimental results and analysis, a mechanistic model for enhanced corrosion resistance of the RGO/ZnAl-LDH film is proposed in Figure 11, wherethe role of RGO in the RGO/ZnAl-LDH film for corrosion resistance can be clearly illustrated. In conclusion, the outstanding anti-corrosion mechanism for RGO in the RGO/ZnAl-LDH film includes four contributions, i.e., anti-penetration, hydrophobicity, increase of the tortuosity of the diffusion path way, and electrostatic repulsion.


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CONCLUSIONS The the RGO/ZnAl-LDH composite as a corrosion protection film is fabricated by a facile and newly developed hydrothermal continuous flow method. The structure and surface morphology of the RGO/ZnAl-LDH composite film are characterized by XRD, FT-IR, Raman, XPS, and SEM. It is confirmed that the RGO/ZnAl-LDH composite film exhibits a superior corrosion protection performance as compared with the ZnAl-LDH film upon immersion in 3.5 wt% and 5.0 wt% NaCl solutions. The corrosion resistance is attributed to the barrier property of RGO in the RGO/ZnAl-LDH composite film. The ability of RGO in the RGO/ZnAl-LDH composite film to prevent water from forming ionic conducting path for oxygen and chloride ion diffusion is of crucial importance for its anti-corrosion. The electrostatic repulsion between negatively charged chloride ions and negatively charged oxygen-containing functional groups on the RGO prevented chloride ions from diffusing through the film. This work represents a breakthrough in the development of RGO-LDH composite films as corrosion protection materials, and may provide a new pathway for generating application in other metallic and/or alloy against corrosion.

Supporting Information. Characterization, Schematic of the hydrothermal continuous flow reactor, Zeta Potential of the RGO dispersed in water solution, AFM image of the RGO nanosheet, SEM images of the ZnAl-LDH film and RGO/ZnAl-LDH-ts (s=1-3) film after the cross cut tape test, SEM images of ZnAl-LDH film and RGO/ZnAl-LDH-ts (s=1-3) film after 7 days of immersion in 3.5 wt% NaCl solution, The equivalent electrical circuit for impedance measurement model, Bode plots and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Variations of impedance data for logZ (a) at 10 mHz and log fb (b) of bare aluminum alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film immersed in 3.5 wt% NaCl solution (pH=7.0) for different days, and the digital photo of the RGO nanosheet film. This is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors Y. Liu ([email protected]. Tel.:+86 135 0748 8663)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The research is financially supported by the Major Science and Technology Projects of Hunan Province, China (2015GK1004), the Research Foundation of Education Bureau of Guizhou Province, China (Guizhou [2015]402), and the Joint Foundation of Science and Technology Department of Guizhou Province, China (Guizhou “LH” [2014]7430).


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(b)♠ aluminum alloy substrate

00

2

(a)

♦ ZnAl-LDH ♦ ♦

♠ RGO/ZnAl-LDH-t3

♦

GO

15

20

25

30

2theta(deg)

35

40

0

RGO/ZnAl-LDH-t2 RGO/ZnAl-LDH-t1 ZnAl-LDH

Graphite 10

♦ ♠ 01

RGO

2

d=0.39

01

d=0.82

Intensity(a.u )

00

2

00

6

00

1

00

3

d=0.34

I ntensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6N01 aluminum substrate

10

20

30

40

2theta(deg)

50

60

70

Figure 1. XRD spectra (a) of Graphite, GO and RGO; XRD spectra (b) of 6N01 Al alloy substrate, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film.


νO-H

Page 28 of 41

νskeletal aromatic

(b) RGO/ZnAl-LDH-t3

1621.2 1738 1621 1396 1229 1063

GO

νskeletal aromatic

RGO/ZnAl-LDH-t2

1621.7 RGO/ZnAl-LDH-t1

1622.1 ZnAl-LDH

1064

1622

RGO

T ran sm ittan ce(% )

3435

C -O H C -O -C C -O

(a)

T ran sm ittan ce (% )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C=O


4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1)

500 4000

3500

3000

2500

2000

1500

Wavenumber(cm-1)

1000

500

Figure 2. FT-IR spectra (a) of GO and RGO; FT-IR spectra (b) of ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film.


Page 29 of 41

(a)

(b)

598.1

1068.3

D band:1346.8 G band:1576.4

D band:1352.4

G band:1577.6

RGO D band:1353.7

RGO/ZnAl-LDH-t3

Intensity(a.u)

In ten sity (a.u )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RGO/ZnAl-LDH-t2

RGO/ZnAl-LDH-t1

G band:1578.4

GO

500

1000 1500 Raman shift (cm-1)

2000

ZnAl-LDH 500

1000

1500

Raman shit(cm-1)

2000

Figure 3. Raman spectra (a) of GO and RGO; Raman spectra (b) of ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film.


RGO/ZnAl-LDH-t2

A l2p

Intensity(a.u)

Intensity(a.u)

ZnLM M ZnLM M

RGO/ZnAl-LDH-t1

RG0

Z n 3s

01s

C 1s

C1s

O 1s

(b)

Zn 2 p 1/2 Zn 2 p 3/2

(a)

G0 RGO/ZnAl-LDH-t3

1200

1000

800

600

400

200

Binding energy(eV)

0

1200

(c)

1000

800

600

400

Binding energy(eV)

200

0

(d)

286.5

285.6

284.3

Intensity(a.u)

Intensity(a.u)

284.3

288.3 290.1

282

284

286

288

290

292

(e)

285.6 286.5 288.3 290.1

282

284

286

288

290

292

Binding energy(eV)

Binding Energy(eV) 284.3

Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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OKLL


285.6 286.5 288.3 290.1

282

284

286

288

290

292

Binding energy(eV)

Figure 4. XPS spectra (a) of GO and RGO; XPS sspectra (b) of ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film; C 1s XPS spectra of GO (c), RGO (d) and RGO/ZnAl-LDH-t2 film (e) .


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-

Figure 5. SEM (a) , TEM (b) and HRTEM (c) images of the RGO; SEM images of the (d) ZnAl-LDH film, (e) RGO/ZnAl-LDH-t1 film, (f) RGO/ZnAl-LDH-t2 film, and (g) RGO/ZnAl-LDH-t3 film; Images in (d-1), (e-1), (f-1), (g-1) correspond to the cross-sectional views of the four films; SEM (h) and HRTEM (h-1) images of the RGO/ZnAl-LDH composite scraped from the RGO/ZnAl-LDH-t2 film.


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-3

(a) bare aluminum alloy

-4

-5

bare aluminum alloy

-5

-6 RGO/ZnAl-LDH-t1

-7

ZnAl-LDH RGO/ZnAl-LDH-t2

-8

RGO/ZnAl-LDH-t3

-9

-6 RGO/ZnAl-LDH-t1

-7 ZnAl-LDH

RGO/ZnAl-LDH-t2

-8 RGO/ZnAl-LDH-t3

-9

-10 -0.6

-0.4

-0.2

0.0

Potential(Vvs.SCE)

0.2

0.4

-0.6

-0.4

-0.2

0.0

Potential(Vvs.SCE)

0.2

0.4

-3

(c)

(d)

bare aluminum alloy

-4

-5

bare aluminum alloy

-5

-6 RGO/ZnAl-LDH-t1

-7

ZnAl-LDH

RGO/ZnAl-LDH-t2

-8

RGO/ZnAl-LDH-t3

-9

-10 -0.8

-10 -0.8

C u rren t d en sity log (i/A cm -2 )

-4

(b)

C u rren t d en sity lo g (i/A cm -2 )

C u rren t d en sity log(i/A cm -2 )

-4

C u rren t d en sity log(i/A cm -2 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-6

RGO/ZnAl-LDH-t1

-7

ZnAl-LDH

RGO/ZnAl-LDH-t2

-8

RGO/ZnAl-LDH-t3

-9

-10

-0.6

-0.4

-0.2

0.0

Potential(Vvs.SCE)

0.2

0.4

-0.8

-0.6

-0.4

-0.2

0.0

Potential(Vvs.SCE)

0.2

0.4

Figure 6. Potentiodynamic polarization curves (a, b) of bare 6N01 Al alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film measured after 0.5 day and 7 days of immersion in 3.5 wt% NaCl solution (pH=7.0), respectively. Potentiodynamic polarization curves (c, d) of bare 6N01 Al alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film after 0.5 day and 7 days of immersion in 5.0 wt% NaCl solution (pH=7.0), respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 1. Electrochemical polarization parameters of bare 6N01 Al alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film after 0.5 day and 7 days of immersion in 3.5 wt% NaCl solution (pH=7.0) and 5.0 wt% NaCl solution (pH=7.0). 3.5 wt% NaCl Solution Samples

Immersion

Ecorr(mV vs

icorr(µA/

ba(mV

-bc(mV

Rcorr

period (days)

SCE)

cm2)

dec-1 )

dec-1 )

(mm/year)

Bare 6N01 Al alloy

0.5

-263

2.513

133

121

2.92×10-1

—

ZnAl-LDH film

0.5

-186

0.0535

114

108

6.21×10-3

97.87%

RGO/ZnAl-LDH-t1 film

0.5

-178

0.0427

108

98

4.95×10-3

98.30%


0.5

-175

0.0411

107

96

4.77×10-3

98.36%


0.5

-172

0.0403

103

95

4.67×10-3

98.40%

bare 6N01 Al alloy

7

-318

4.722

164

128

5.47×10-1

-87.90%

ZnAl-LDH film

7

-234

0.531

129

116

6.15×10-2

78.87%


7

-193

0.0463

113

105

5.37×10-3

98.16%


7

-179

0.0446

109

103

5.17×10-3

98.22%


7

-175

0.0432

108

102

5.01×10-3

98.28%

η (%)

5.0 wt% NaCl Solution Bare 6N01 Al alloy

0.5

-287

2.618

146

125

3.04×10-1

—

ZnAl-LDH film

0.5

-203

0.0572

123

116

6.64×10-3

97.82%


0.5

-184

0.0437

109

104

5.08×10-3

98.35%


0.5

-178

0.0416

106

105

4.83×10-3

98.41%


0.5

-173

0.0411

104

98

4.77×10-3

98.43%

bare 6N01 Al alloy

7

-423

6.174

173

143

7.16×10-1

-135.83%

ZnAl-LDH film

7

-326

0.782

153

121

9.06×10-2

70.13%


7

-212

0.0515

120

116

5.97×10-3

98.03%


7

-206

0.0483

115

112

5.60×10-3

98.15%


7

-202

0.0457

114

109

5.30×10-3

98.25%


Page 35 of 41

3000

/Z"/(Ω cm 2 )

2.0M

2000

1.6M

1000 0

/Z"/(Ω cm 2 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

1500

3000

4500

Z'(Ωcm2)

1.2M

800.0k

bare 6N01 aluminum alloy ZnAl-LDH RGO/ZnAl-LDH-t1 RGO/ZnAl-LDH-t2 RGO/ZnAl-LDH-t3

400.0k 0.0 0.0

500.0k

1.0M

1.5M

Z'(Ωcm2)

2.0M

2.5M

Figure 7. Nyquist diagrams of bare 6N01 Al alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film after 7 days of immersion in 3.5 wt% NaCl solution (pH=7.0) (The plots for bare 6N01 Al alloy and ZnAl-LDH film are shown in inset).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 41

Table 2. EIS parameters for bare 6N01 Al alloy, ZnAl-LDH film, RGO/ZnAl-LDH-t1 film, RGO/ZnAl-LDH-t2 film and RGO/ZnAl-LDH-t3 film after 7 days of immersion in 3.5 wt% NaCl solution (pH=7.0). CPE Samples

2

Rf (Ωcm )

2

Qc (µF/cm2)

Rs (Ωcm ) Y0 (µsn/Ωcm2)

n

bare 6N01 Al alloy

8.91 × 102

7.52

3198±22

0.99±0.01

3220±23

ZnAl-LDH film

1.94 × 104

8.33

72.12±0.46

0.99±0.01

73.42±0.55


2.14 × 106

6.91

0.511±0.045

0.98±0.01

0.521±0.035


2.23 × 106

6.54

0.441±0.037

0.98±0.01

0.465±0.042


2.43 × 106

5.89

0.397 ±0.31

0.99±0.01

0.401 ±0.21


Page 37 of 41

♠ aluminum alloy substrate♠ ♦ ZnAl-LDH ♦

Intensity(a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


♦

0

10

♦

♦ ♠

♦

20

30

40

50

60

f e d c b a 70

80

2theta(deg)

Figure 8. XRD spectra of 6N01 Al alloy substrate (a), as-synthesized ZnAl-LDH film-coated 6N01 Al alloy substrate before immersion in NaCl solution (b), ZnAl-LDH film (c), RGO/ZnAl-LDH-t1 film (d), RGO/ZnAl-LDH-t2 film (e) and RGO/ZnAl-LDH-t3 film (f)-coated 6N01 aluminum alloy substrate after 7 days of immersion in 3.5 wt% NaCl solution (pH=7.0).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Al

Al

(a-1)

Al

(a-3)

(a-2)

Cl

Cl

Cl Zn

1

2

3

KeV

4

5

6

0

1

Al

Cl

Zn

2

3

KeV

4

5

6

Al

(b-2)

(b-1)

1

2

3

KeV

4

5

6

5

6

Al

(b-3)

Zn

Zn

Cl

Cl Zn

0

Cl

Cl

Zn 0

Page 38 of 41

1

Cl

Cl

2

3

KeV

4

5

6

0

1

2

3

KeV

4

5

6

0

1

2

3

4

KeV

Figure 9. (a-1), (a-2) and (a-3) exhibit the corresponding EDX spectra of the three different positions (1-3) in the cross section of ZnAl-LDH film (a) after 7 days of immersion in 3.5 wt% NaCl solution; (b-1), (b-2) and (b-3) exhibit the corresponding EDX spectra of the three different positions (1-3) in the cross section of RGO/ZnAl-LDH-t3 film (b) after 7 days of immersion in 3.5 wt% NaCl solution.


Page 39 of 41

102 99 96 93

R(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90 87 84

RGO/ZnAl-LDH-t3 membrane RGO/ZnAl-LDH-t2 membrane RGO/ZnAl-LDH-t1 membrane ZnAl-LDH membrane Pure epoxy membrane

81 78 75 0

20

40

60

80

100 120 140 160 180

Time(days)

Figure 10. Variations of chloride ion rejection of pure epoxy membrane, ZnAl-LDH membrane, RGO/ZnAl-LDH-t1 membrane, RGO/ZnAl-LDH-t2 membrane and RGO/ZnAl-LDH-t3 membrane as a function of time.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11. The mechanistic model for corrosion protection offered by the RGO/ZnAl-LDH-ts (s=1, 2, 3) films.


Page 40 of 41

Page 41 of 41


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