graphene coating on AA-2024 alloy

Accepted Manuscript Title: Preparation and characterization of a GPTMS/graphene coating on AA-2024 alloy Authors: Yuchao Dun, Yu Zuo PII: DOI: Reference:

S0169-4332(17)31133-9 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.116 APSUSC 35797

To appear in:

APSUSC

Received date: Revised date: Accepted date:

6-12-2016 9-3-2017 16-4-2017

Please cite this article as: Yuchao Dun, Yu Zuo, Preparation and characterization of a GPTMS/graphene coating on AA-2024 alloy, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.116 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Preparation and characterization of a GPTMS / graphene coating on AA-2024 alloy Yuchao Dun, Yu Zuo* Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, China Corresponding author, E-mail: [email protected]

Highlights 

A KH560/graphene coating was prepared on 2024 aluminum alloy by immersing and curing.



The thickness of the composite coating increased greatly compared with silane coating.



The covalent metallic-siloxane bonds improved the adhesion force greatly.



The laminate structure increased the hardness and declined brittleness of the coating.



The composite coating showed much higher corrosion resistance in NaCl solution.

Abstract: A -(2,3-epoxypropoxy) propyltrimethoxysilane/graphene (GPTMS/rGO) coating on AA-2024 aluminum alloy was prepared by immersing the aluminum alloy sample in a silane/graphene oxide solution and curing in oven at 180 ℃. Silanol groups were grafted onto graphene oxide sheets during hydrolysis. The graphene oxide was stacked layer by layer through silanol groups. The synthesized coating was characterized with Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Raman spectra and scanning electron microscopy. The thickness of the composite coating increased greatly compared with that of silane coating, due to the mutual riveting effect. 1

The covalent metallic-siloxane bonds (AlOSi) improved the adhesion force greatly. The laminate structure of graphene increased the hardness and declined the brittleness over 200 ℃. The GPTMS/rGO coating showed good corrosion resistance. In 3.5% NaCl solution the anodic current density of the aluminum alloy sample with GPTMS/rGO coating was reduced by several orders of magnitude compared with those of bare aluminum alloy or the sample with graphene film.

Key words: Graphene; Silane; Coating; Structure; Corrosion; Adhesion

1. Introduction Aluminum alloys are widely used in many industries due to the good properties including high strength to weight ratio, excellent electrical and thermal conductivity, and good ductility. However, aluminum alloys frequently experience localized corrosion such as pitting and intergranular corrosion when exposed to chloride-containing environment [1-3]. Various surface techniques such as anodizing, microarc oxidation, chemical conversion films and organic coatings are used to protect aluminum alloys from corrosion. Recently, graphene is paid much attention because it is considered as a kind of ideal anti-corrosion materials due to its unique chemical inertness, thermal stability and two-dimensional structure. Graphene can not only be used as protective barrier alone but also be combined with other materials to create composites [4]. The growth of monolayer and multi-layer graphene on metal surface, which can increase the corrosion resistance of the substrate, may be achieved by chemical vapor deposition (CVD) method. Merisalu [5] prepared a graphene-polypyrrole thin hybrid coating on copper by CVD. The hybrid coating protected copper from corrosion in a certain extent, but intense galvanic corrosion of copper appeared at the defect sites of graphene. Nam [6] studied the graphene layer against hydrogen embrittlement of copper. Graphene can interrupt the penetration of hydrogen by forming C-H sp3 bonds, which induced a distortion of graphene structure and increased the defects in the graphene. Pu [7] deposited nickel on SUS304 stainless steel and then deposited graphene on nickel by CVD. The deposited nickel reduced the formation of metal carbide and catalyzed 2

graphene precipitation at high temperatures. The ultra-thin graphene layer not only protected the substrate but maintained excellent low interfacial contact resistance. Stoot [8] also deposited a nickel seed layer on stainless steel and a multi-layer of graphene on the seed layer. The multi-layer graphene film has fewer defects than monolayer graphene, with good anti-corrosion performance. Monolayer and multi-layer graphene could protect the substrate. However, defects in graphene will cause galvanic corrosion and accelerate local corrosion. In order to avoid defects in graphene, graphene and graphene oxide (GO) were combined with some composites to get a hybrid coating. Li [9] found that the addition of graphene oxide into hyaluronic acid-hydroxyapatite could increase the deposition rate and inhibit the creation and propagation of cracks in the coatings. Gao [10] proposed that GO promotes nucleation and crystallization for rapid hydroxyapatite growth, forming uniform and dense HA/GO hybrid coating which dramatically improves the corrosion resistance of Mg alloys in simulated body fluid. Due to the unique flake-like structure, graphene can be used as fillers to increase cross-linking of epoxy resin. Yu [11] added GO loaded with nano-TiO2 into epoxy resin. The corrosion resistance was significantly enhanced due to the laminated TiO2-GO hybrids which could obstruct permeation of the electrolyte into the micro-pores. Ramezanzadeh [12] modified the surface of graphene oxide with polyisocyanate resin. Addition of 0.1 wt.% grafted graphene in polyurethane prolonged the lifetime in salt spray test. Graphene in resin extends the permeation path of aggressive ions and water molecules. The molecule structure for silane coupling agent contains two different functional groups and the general structure is Y(CH2)nSiX3, where X represents a silicon ester which can transform by hydrolysis reaction to a silanol group, and Y represents an organic functional group such as chorine, amines or vinyl. Typically, the value of n is 3. The organic functional groups could improve the compatibility of silane with polymers. The silanol groups (-SiOH), hydrolysate from silane, could form covalent metallo-siloxane bonds with the metal oxide surface. Therefore, silane coupling agents are usually used as a surface treatment agent to improve the interface bonding strength [13]. Siliane surface treatment was used to substitute the phosphating and chromating 3

processes of coating pretreatment. During the curing process, silinol groups cross-linked to form a stable Si-O-Si network structure which can protect the substrate from corrosion. Ooij [14] found that the thickness of silane films almost kept unchanged

after dipping in the solution for 1 minute. The thickness increased with the solution concentration, from 5 to 400 nm when the concentration is 0.1-5% [15]. The greatest thickness ever reported for silane film was prepared by using a 5% bis-amino silane solution and the thickness was 798.5 nm [16]. Besides, thicker silane films are usually more brittle [17]. GPTMS is a kind of coupling agent with epoxy, which is widely used in sizing agents, sealants and thermoplastic resin. Graphene could be functionalized by GPTMS. Ding [18] used graphene grafted with GPTMS as reinforcement of cyanate ester/bismaleimide copolymers. The functionalized graphene increased the thermal stability and impact strengths of the co-polymers. GPTMS was usually used as coupling agent to improve the dispersion of graphene in different resins [19, 20], while graphene grafted with GPTMS as a corrosion barrier alone was seldom studied. In this work, a GPTMS/graphene (GPTMS/rGO) hybrid coating was prepared on AA-2024 aluminum alloy substrate. The aluminum alloy sample was immersed in a silane hydrolysis solution with graphene oxide, and a gel film was deposited on the surface. After curing, a GPTMS/graphene coating with more than 10 µm thickness was obtained. The GPTMS/rGO coating showed quite good corrosion resistance, excellent adhesion force, relatively high micro-hardness and good thermal shock resistance.

2. Experimental Natural graphite (325 mesh) was purchased from Qingdao Ruisheng Graphite Co., Ltd. GPTMS was purchased from Sinopharm Chemical Reagent Co., Ltd. All the other chemicals were purchased from Beijing Chemical Works with analytical grade. The substrate material was 2024 aluminum alloy (AA-2024) and the composition of the alloy is shown in Table 1. 2.1 Preparation of graphene oxide Graphene oxide was synthesized by modified Hummers method [21] (The details may be seen in the Supporting Information (SI)). After exfoliation by ultra-sonication, 4

single layered graphene oxide was prepared (SI Fig. 1). 2.2 Preparation of GPTMS/rGO and graphene films GO-GPTMS solution was prepared by mixing GO solution into the silane solution and the concentration of GO, GPTMS, ethanol and water were 0.05 wt.%, 10 wt.%, 10 wt.%, 79.95 wt.%, respectively. The pH of the solution was kept about 5 by adding glacial acetic. The size of AA-2024 aluminum alloy sample was 20 mm×10 mm×3 mm. The sample surface was abraded with emery papers from 240# to 1000#, then the sample was etched in 40 g/L NaOH solution for 2 min which could introduce hydroxyl to the surface. GPTMS/rGO gel coating could be obtained after the sample was immersed in the GO-GPTMS solution for 12 h and cured in an oven at 180 ℃ for 3 h. The film was mainly affected by immersion time and the contents of GPTMS and graphene oxide (details in SI). For comparison, graphene and graphene oxide film were prepared by dipping the AA-2024 sample into 0.1 wt.% GO solution without GPTMS for 6 h. Then the graphene oxide film was obtained by curing the sample in an oven at 60 ℃ while the graphene film was obtained by curing at 180 ℃. The GPTMS film for hardness test was prepared by evaporating the 10 wt.% silane solution without GO. Artificial aging experiment was carried out on UV weathering resistance test chamber (B-UV-Ⅱ). The aging process was 0.63 W/m2 with a UVA-340 lamp for 4 h at 60 ℃ followed with cooling by spraying for 4 h at 50 ℃. For comparison, anodic film was prepared on the AA-2024 sample (AAO). Prior to anodizing, the abraded samples were subjected to the following pre-treatments: alkaline etching in 40 g/L NaOH solution for 2 min, and desmutting in 200 g/L HNO3 at room temperature for 2 min. The samples were anodized using a mixed electrolyte (200 g/L H2SO4, 20 g/L oxalic acid and 15 g/L glycerol) for 60 min under a current density of 2.0 A/dm2 [22]. The temperature of the electrolyte was maintained at 25 ± 2 °C with stirring. The anodized samples were then sealed in boiled deionized water for 30 min. 2.2 Characterization Thickness of the coating was measured by TT230 coating thickness gauge. Raman spectroscopy was measured with a Renishaw inVia Raman microscope, using a laser excitation wavelength of 532 nm and a scan range of 1000-3000 cm-1. The 5

micro-hardness of the film was measured with a Fischer HM2000 micro-hardness tester. The load and the dwell time used in this study were 200 mN and 20 s, respectively. The adhesion force measurement of the film was performed according to ASTM 3359-2002. The Fourier transform spectrometer spectroscopy (FTIR) was obtained using a model TENSOR27 Fourier transform infrared spectrometer with a scan range 600-4000 cm-1. The surface composition of the samples was analyzed with an ESCALAB 250 X-ray photoelectron spectrometers (XPS). The spectrum of samples was referenced to the main C 1s peak in the narrow scan, which was taken at 284.8 eV. Peak fitting was performed by using Thermo Avantage version 5.52. Smart type backgrounds were used for all the fits. The surface morphology of the GPTMS/G coating was observed with a Hitachi S4700 scanning electron microscope (SEM). The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/MAX-2500 diffractometer with Cu Kα radiation (λ = 1.5406 Å). A PARSTAT2273 electrochemical working station (Princeton) was used for electrochemical impedance measurement. A 10 mV perturbation was applied and the scanning frequency range was 100 kHz-10 mHz. The surface area of working electrode was about 10 cm2. The solution was 3.5 wt % NaCl and the impedance spectra were measured at the open circuit potential after immersion for 24 h at room temperature. Potentiodynamic polarization curves were measured in 3.5 wt % NaCl solution using a CS350 electrochemical workstation after monitoring the open circuit potential for 24 h. The surface area of the working electrode was 0.8 cm2. A platinum plate and a saturated calomel electrode were used as the counter and reference electrode, respectively. The scanning rate was 0.5 mV/s.

3. Results and discussion 3.1 Thickness and appearance of GPTMS/rGO film The commonly used silane solution for surface treatment was mixture of deionized water and ethanol. As shown in Fig. 1, the obtained films are uniform with very small deviations in thickness values, and the thickness increased with the increase of 6

immersion time and the content of GPTMS and GO. The GPTMS/rGO coating was not uniform when the content GO was 0.025%. The thickness almost kept unchanged when the immersion time is longer than 36 h. 3.2 Structure of GPTMS/rGO coating ATR-FTIR results of GO, rGO and the GPTMS/rGO coating are shown in Fig. 2. The broad and intense peak centered at 3400 cm-1 is due to the characteristic vibration of -OH groups. The characteristic absorption peaks at 1624 cm-1, 1411 cm-1 and 1066 cm-1 are attributed to C=O, C–O, and C–O–C respectively, revealing the existence of hydroxyl, carboxyl and epoxide groups in GO. The peak at 1731 cm-1 was assigned to C=C, suggesting the remaining un-oxidized graphitic domains [23-26]. Compared with GO, the hydroxyl peak of graphene spectra at 3400 cm-1 disappeared and the intensity of C-O peak at 1411 cm-1 decreased, indicating that hydroxyl and C-O were partially removed by high temperature. The band peaks at 1070 cm-1, 819 cm-1 and 776 cm-1 are due to bending vibrations and symmetric stretching vibrations of Si–O–Si bonds GPTMS. The peaks at 2925 cm-1 and 2855 cm-1 for GPTMS/rGO sample are assigned to the alkyl chains of GPTMS. The broad peak of GPTMS/rGO at 1010 cm-1 may be due to Si–O–C/Si–O–Si which confirms the reaction between GPTMS and graphene. The peaks at 2925 cm-1, 2855 cm-1, 1624 cm-1 and 1731 cm-1 also confirm that the surface of GPTMS/rGO coating is graphene grafted with silanol groups [27-30]. Fig. 3 is the XPS spectra of graphite, graphene oxide, GPTMS and GPTMS/rGO. Compared with the single peak of natural graphite, the C1s peak of GO indicates a significant degree of oxidation which can be deconvolved into four peaks: C-C, C-O, C=O and O=C-O, respectively [9, 19, 23] according to the FTIR results (Fig.3c). Uniqueness plots was used for the peak fitting as shown in Fig. 3b [31,32]. The details of parameters for peak fitting was shown in Table 2. As commonly prepared by Hummers method, the C/O ratio of graphene oxide was 2.14 according to the XPS elements analysis where a large number of epoxy and hydroxyl groups covers graphene oxide basal plane and carboxylic acid moieties line up along edges [33-36]. The AA-2024 samples were immersed in the GO-silane solution for 2 h and 10 h respectively, then the samples were cured in an oven at 60 ℃. The composition of the 7

sample surface was analyzed with XPS. After immersion in GO silane solution for 2 h, a colorless liquid film adsorbed on the sample surface. Fig. 3e shows the C 1s and Si 2p spectra of the sample. The Si 2p spectrum confirms silanol groups contained in the film. The C1s spectrum in Fig. 3e can be deconvoluted into three peaks, the same with GPTMS in Fig. 3d, in which C-C associated with the GPTMS alkyl chain and C-O assigned to C-O-C and C-O-Si group of GPTMS. The surface was covered with a golden brown gel after 10 h immersion. Graphene oxide adsorbed on the surface obviously. The C1s spectrum mainly consists of two overlapped peaks which are almost the same with the C1s spectrum of GO. The C1s spectrum also confirms that the surface of GPTMS/rGO coating is graphene oxide grafted with silanol groups. Fig. 3g-3i show the XPS results of GPTMS/rGO with different thickness by abrading a film that was originally 22 m in thickness. The C1s spectrum at the coating/alloy interface (0 µm) shows a broad peak which can be deconvoluted like the C 1s spectrum of GO. The C1s spectrum of the coating surface (22 µm) become narrower and the intensity increased greatly indicating the decrease of C-O and C=O groups. The intensity of O1s and Si2p peaks decreases from the bottom of the coating to the surface. Graphene oxide was reduced during the curing process of GPTMS/rGO coating. Table 3 shows the surface compositions at different depths. The highest atomic content of silicon appears at the interface due to the preferential adsorption of silanol groups on the substrate surface. The oxygen content decreased because many C-O and C=O groups were removed during heating. Therefore, the XPS results indicate that silanol groups adsorbed preferentially on the surface, and the grafted GO condensed with the silanol groups. Meanwhile, the adsorbed graphene oxide could provide sites for further adsorption of silanol groups. The surface of the GPTMS/rGO coating is graphene grafted with silanol groups and graphene on the surface was reduced more thoroughly. Raman spectroscopy, which is often used to study the defects and reduction of graphene oxide, was employed. In the Raman spectrum of GO (Fig. 4a), D band (1337 cm-1) and G band (1602 cm-1) could be distinguished easily. The G band represents the in-plane bond-stretching motion of the pairs of C sp2 atoms (the E2g phonons) and the D band is associated with the edges and defects. The weak D band and narrow G band of 8

natural graphite indicates the dominant sp2 graphitic crystal structure while the wide D band and G band suggest the sp3-hybridized carbon atom in GO. After oxidation, the graphitic crystal structure was destroyed and the numbers of edge and defect increase, which makes the graphite exfoliates into graphene oxide possible [37-39]. The degree of functionalization can be monitored by the intensity ratio of D band and G band (ID/IG) [40, 41]. The ID/IG value for graphite is 1.16 (Fig. 4b), but for graphene the value decreases to 0.96 (Fig. 4c), indicating that the graphene oxide was reduced and the structure of graphene was recovered partially under high temperature. ID/IG of GPTMS/rGO is 1.1, calculated from Fig. 4d, which means that graphene was functionalized by silanol groups. Functionalized graphene could condense with silanol groups of the liquid film [42-45]. Detailed structural information of GPTMS/rGO was obtained using XRD (Fig.5), which reveals the changes of GO interlayer distance, d002. The GPTMS has a single broad peak centered at 21.3° while the GPTMS/rGO has another peak around 9.5° and interlayer distance (d002) was 1.075 nm, larger than that of GO. GPTMS inserted into GO layers and enlarged the interlayer distance d002, which means there was a GPTMS layer between GO nano-sheets. The GO nano-sheets were grafted with GPTMS and the results were in agreement with FTIR and Raman results. The peak of GPTMS/rGO around 9.5° almost kept unchanged with the increase of thickness after different immersion time, indicating that the GPTMS/rGO coating was formed layer-by-layer on the substrate [46]. The formation process of GPTMS/rGO coating could be deduced from the results of ATR-FTIR, XPS and Raman. In the GPTMS solution, the silanes are hydrolyzed and silanol groups are formed according to Equation (1). Fig. 6a shows that silanol groups were grafted to GO nano-sheets forming SiO-GO according to Equation (2) [47]. When the aluminum alloy sample is immersed in the silane solution, excess SiOH groups and AlOOH from the substrate could form covalent metallic-siloxane bonds (AlOSi) [48] according to Equation (3). Silanol groups adsorb on the surface preferentially and form a liquid layer as shown in Fig. 6b. Excess SiOH groups would also condense among themselves and GO to form SiO-GO-OSi according to Equation (4) after longer 9

immersion time. The GO is condensed onto the liquid layer by silanol groups. Condensed GO with silanol groups could condense with extra silanol groups and GO again. The GO is stacked layer by layer through silanol groups as shown in Fig. 6c. During curing in the oven, linear siloxane and cyclic siloxane are produced during condensation. Hydroxyl and carboxyl groups are removed and Si-O-C groups increased according to FTIR results during condensation. Finally, the golden brown film turns into black as shown in Fig. 6d. KH-560 +3H2O= R-Si(OH)3+3CH3OH

(1)

SiOH(solution) + GO(solution)= SiO-GO(solution) + H2O

(2)

SiOH(solution) + AlOOH (aluminum surface) = SiOAl(interface) + H2O

(3)

SiOH(solution)+ SiO-GO(solution) +SiOH(solution) = SiO-GO- OSi(GO film) + H2O

(4)

3.3 Mechanical properties and morphology of GPTMS/rGO coating The measured micro-hardness values of GPTMS/rGO coating, GPTMS coating and AA-2024 alloy are shown in Fig. 7. The deviations in the measured microhardness values are small, showing good reproducibility of the films. The hardness of GPTMS/rGO is almost 6 times that of the GPTMS film, close to the value of AA-2024 aluminum alloy, indicating good mechanical strength. Table 4 shows the adhesion measurement results for graphene oxide, graphene and GPTMS/rGO coatings on aluminum alloy. The adhesion of graphene and graphene oxide on AA-2024 surface was very poor and the film can be peeled off by 3M600 type tape. Graphene oxide adsorbs on the surface by hydrogen bonding which is weakened after curing in the oven. While the adhesion classification of GPTMS/rGO is 4B and less than 1% area was damaged during the test. The covalent metallic-siloxane bonds (AlOSi) effectively improve the adhesion of GPTMS/rGO. Thicker silane film are usually very brittle and micro-cracks appear when the curing temperature is more than 200 ℃ [49,50]. A thermal shock test was conducted on the GPTMS/rGO coating. The sample with GPTMS/rGO coating was immersed in water (20 ℃) immediately after heating in an oven at 250 ℃ for 1 h. No visible damage was found on the surface. Si-O-Si bond is unstable which makes silane layers very brittle. 10

Silanol groups were grafted with hydroxyl and carboxyl on the GO surface forming Si-O-C bond which is much more stable than the Si-O-Si bond. As depicted in Fig. 8a, there were some protrusion and ridges on the surface of rGO film. The protrusions might be caused by the evaporation of the adsorbed water and the ridges might be the stacked edges of GO nano-sheets. From Fig. 8b and 8c, we can find that some stacked GO nano-sheets and the surface was smooth. The condensation of GPTMS and evaporation of water led to the wrinkle morphology of GPTMS as shown in Fig. 8d. Some tiny particles could be found on the surface of GPTMS/rGO in Fig. 8e and the particles might be the condensed GPTMS. Fig. 8f shows the cross-section of GPTMS/rGO coating, which is compact and adhered well to the substrate, with the thickness of 16.7 µm. The laminate structure of graphene could rivet silanol groups and inhibit the generation and expansion of cracks which may decline the brittleness. 3.4 Corrosion resistance of GPTMS/rGO coating Fig. 9 shows the potentiodynamic polarization curves of AA-2024 aluminum alloy with or without coatings. AA-2024 aluminum alloy exhibits active corrosion in 3.5% NaCl solution with the open circuit potential about -0.8 V vs. SCE and the corrosion curent density is 0.51x10-6 A cm-2 by Tafel extrapolation as shown in Table 5. The AA-2024 sample with rGO coating shows lower corrosion rate about 1.1x10-7 A cm-2. The open circuit potential of the sample with GPTMS/rGO coating is about 450 mV more positive than those of the other samples and the corrosion current density is about 1.17x10-8 A cm-2, which is about two orders of magnitude lower than that of the bare AA-2024, very close to that of the sample with anodic film (60 m). On the other hand, in the anodic branches of the polarization curves, the anodic current density of the sample with rGO coating is two orders of magnitude lower than that of AA-2024 aluminum alloy, while the sample with GPTMS/rGO coating is also close to that of the sample with anodic film, almost five orders of magnitude lower compared with that of the bare aluminum alloy. The above results show that the hybrid GPTMS/rGO coating provided good protection to the substrate and effectively decreased the corrosion rate of aluminum alloy. 11

Fig. 10a shows the Nyquist plots of bare AA-2024 sample in 3.5% NaCl solution. At high frequency range there is an obvious capacitive arc, which could be considered as capacitance of faradic reaction process between the substrate and electrolyte, and at low frequency the impedance character of semi-infinite boundary diffusion process occurs clearly. The corrosion rate of bare aluminum alloy was mainly controlled by diffusion process. Fig. 10c shows the equivalent circuit model for bare Al alloy. Here, Rs is the solution resistance, Rct is the charge transfer resistance and Qdl the double layer capacitance. W is the Warburg impedance which is considered containing information of the coating, the metal substrate and diffusion [51]. The Nyquist plots of AA-2024 with graphene film shows two capacitive loops following a Warburg tail which may be attributed to penetration process in the film at high frequency and faradic reaction at the interface and diffusion impedance at low frequency. There are two semicircles in the Nyquist plots of GPTMS/rGO. The diameter of capacitive loops at high frequency for GPTMS/rGO is much bigger than that for graphene film [52, 53]. As shown in Fig. 10b, the impedance of GPTMS/rGO coating is much higher than those of the bare AA-2024 sample and graphene film coated sample, suggesting much better barrier performance. The equivalent circuit models for graphene and GPTMS/rGO are illustrated in Fig. 10f and Fig. 10g, respectively. Table 6 is the fitting parameters of EIS circuits. Qc and Rpo are attributed to the capacitance and resistance of graphene film or GPTMS/rGO coating. The Rct of AA-2024 with graphene film increased by three orders of magnitude than that of bare aluminum while the Rct of GPTMS/rGO increased by five orders of magnitude, indicating that both graphene film and GPTMS/rGO coating could act as barriers for AA-2024 aluminum alloy. The Rpo of GPTMS/rGO is much bigger than that of graphene film, indicating better anti-corrosion performance. The graphene film may prevent the aggressive ions contacting the substrate directly. The Warburg tail of graphene film indicates that the corrosive ions could penetrate the graphene film and initiate corrosion of the substrate. The GPTMS/rGO coating shows good resistance after 11 d immersion while the rGO film exfoliated from substrate after 2 d immersion. Fig. 10d shows the image of GPTMS/rGO samples after aging. No obvious cracks and defects could be found on the working surface while the tapes used to fix the sample 12

degraded into fragments. Hence the GPTMS/rGO coating was stable when exposed to ultraviolet light and moisture. The thicker and denser GPTMS/rGO layer could obviously prolong the penetration path for water and aggressive ions, effectively protecting the substrate alloy from corrosion.

4. Conclusions (1) Graphene oxide was functionalized by grafting with GPTMS. GPTMS/rGO coating on AA-2024 aluminum alloy was obtained by immersing the sample in a GO/silane solution and curing in oven at 180℃. Silanol groups was grafted onto GO sheets during hydrolysis. The graphene oxide was stacked layer by layer through silanol groups. The graphene was functionalized and reduced during the curing process. (2) The thickness of GPTMS/rGO coating increased greatly compared with that of GPTMS coating, due to the mutual riveting effect. The covalent metallic-siloxane bonds (AlOSi) improved the adhesion force greatly. The laminate structure of graphene could increase the hardness and decline the brittleness at high temperature. As the result, the GPTMS/rGO coating showed good mechanical properties. (3) The GPTMS/rGO coating showed excellent anti-corrosion performance. EIS results confirmed a significant improvement of barrier property. In 3.5% NaCl solution the anodic current density of the aluminum alloy sample with GPTMS/rGO coating was reduced by several orders of magnitude compared with those of bare aluminum alloy sample or the sample with graphene film.

13

References [1] W.L. Zhang, G.S Frankel, Transitions between pitting and intergranular corrosion in AA2024, Electrochim. Acta. 48 (2003) 1193-1210. [2] M.L. Zheludkevich, K.A. Yasakau, S.K. Poznyak, M.G.S. Ferreira, Triazole and thiazole derivatives as corrosion inhibitors for AA2024 aluminium alloy, Corros. Sci. 47 (2005) 3368-3383. [3] D. Zhu, W. J. V. Ooij, Corrosion protection of AA 2024-T3 by bis-[3-(triethoxysilyl)propyl] tetrasulfide in neutral sodium chloride solution. Part 1: corrosion of AA 2024-T3, Corros. Sci. 45 (2003) 2163-2175. [4] I. Wlasny, P. Dabrowski, M. Rogala, I. Pasternak, W. Strupinski, J.M. Baranowski, Z. Klusek, Impact of electrolyte intercalation on the corrosion of graphene-coated copper, Corros. Sci. 92 (2015) 69-75. [5] M. Merisalu, T. Kahro, J. Kozlova, A. Niilisk, A. Nikolajev, M. Marandi, A. Floren, H. Alles, V. Sammelselg, Graphene-polypyrrole thin hybrid corrosion resistant coatings for copper, Synth. Met. 200 (2015) 16-23. [6] T.H. Nam, J. H. Lee, S. R. Choi, J. B. Yoo, J. G. Kim, Graphene coating as a protective barrier against hydrogen embrittlement, Int. J. Hydrogen Energ. 39 (2014) 11810-11817. [7] N.W. Pu, G.N. Shi, Y.M. Liu, X.L. Sun, J.K. Chang, C.L. Sun, M.D. Ger, C.Y Chen, P.C. Wang, Y.Y. Peng, C.H. Wu, S. Lawes, Graphene grown on stainless steel as a high-performance and ecofriendly anti-corrosion coating for polymer electrolyte membrane fuel cell bipolar plates, J. Power Sources 282 (2015) 248-256. [8] A.C. Stoot, L. Camilli, S.A. Spiegelhauer, F. Yu, P. Bøggild, Multilayer graphene for long-term corrosion protection of stainless steel bipolar plates for polymer electrolyte membrane fuel cell, J. Power Sources 293 (2015) 846-851. [9] M. Li, Q. Liu, Z.J. Jia, X.C. Xu, Y.Y. Shi, Y. Cheng, Y.F. Zheng, T.F. Xi, S.C. Wei, Electrophoretic deposition and electrochemical behavior of novel graphene oxide-hyaluronic acid-hydroxyapatite nanocomposite coatings, Appl. Surf. Sci. 284 (2013) 804-810. [10] F. Gao, C.Y. Xu, H. Hu, Q. Wang, Y.Y. Gao, H. Chen, Q.N. Guo, D.Z. Chen, D. Eder, Biomimetic synthesis and characterization of hydroxyapatite/graphene oxide hybrid coating on Mg alloy with enhanced corrosion resistance, Mater. Lett. 138 (2015) 25-28. [11] Y Z.X. Yu, H.H. Di, Y. Ma, Y. He, L. Liang, L. Lv, X. Ran, Y. Pan, Z. Luo, Preparation of graphene oxide modified by titanium dioxide to enhance the anti-corrosion performance of epoxy coatings, Surf. Coat. Technol. 276 (2015) 471-478. [12] B. Ramezanzadeh, E. Ghasemi, M. Mahdavian, E. Changizi, M. H. Mohamadzadeh Moghadam, Covalently-grafted graphene oxide nanosheets to improve barrier and corrosion protection properties of polyurethane coatings, Carbon 93 (2015) 555-573. [13] W.J.V. Ooij, D. Zhu, M. Stacy, A. Seth, T. Mugada, J. Gandhi, P. Puomi, Corrosion protection properties of organofunctional silanes - An overview, Tsinghua Sci. Technol. 10 (2005) 639-664. [14] Y W.Yuan, W.J.V. Ooij, Characterization of organofunctionalsilane films on Zinc substrates, J. 14

Colloid Interface Sci. 185 (1997) 197-209. [15] V. Palanivel, D. Zhu, W. J. V. Ooij, Nanoparticle-filled silane films as chromate replacements for aluminum alloys, Prog. Org. Coat. 47(2003) 384-392. [16] D.Q. Zhu, W.J.V. Ooij, Enhanced corrosion resistance of AA 2024-T3 and hot-dip galvanized steel using a mixture of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl] amine, Electrochim. Acta 49 (2004) 1113-1125. [17] P.H. Suegama, H.G.D. Melo, A.A.C. Recco, A.P. Tschiptschin, I.V. Aoki, Corrosion behavior of carbon steel protected with single and bi-layer of silane films filled with silica nanoparticles, Surf. Coat. Technol. 202 (2008) 2850-2858. [18] J. Ding，Y. Huang，T. Han，Y. Wang, Synthesis of functionalized graphene oxide nanoflakes using silane coupling agents: Reinforcement of cyanate ester/bismaleimide nanocomposites, High Perform. Polym. 28 (2015). [19] P. Ming, Y.Y. Zhang, J. Bao, G. Liu, Z. Li, L. Jiang, Q.F. Cheng, Bioinspired highly electrically conductive graphene-epoxy layered composites, RSC Adv. 5 (2015) 22283-22288. [20] E.D. Grayfer, L.S. Kibis, A.I. Stadnichenko, O.Y. Vilkovd, A.I. Boronin, E.M. Slavinskaya, O.A. Stonkus, V.E. Fedorov, Ultradisperse Pt nanoparticles anchored on defect sites in oxygen-free few-layer graphene and their catalytic properties in CO oxidation, Carbon, 89 (2015) 290-299. [21] O.C. Compton, B. Jain, D.A. Dikin, A. Abouimrane, K. Amine, S.T. Nguyen, Chemically active reduced graphene oxide with tunable C/O ratios. Acs Nano, 5 (2011) 4380-91. [22] J.M. Zhao, S.L Chen, X.H Zhao, Y. Zuo, The sealing effect of Cerium salt on anodic film of LY12 aluminum alloy under electric filed. China Surf. Eng, 1 (2008) 28-32. [23] C.C. Teng, C.C.M. Ma, C.H. Lu, S.Y. Yang, S.H. Lee, M.C. Hsiao, M.Y. Yen, K.C.C hiou, T.M. Lee, Thermal conductivity and structure of non-covalent functionalized graphene/epoxy composites. Carbon, 49 (2011) 5107-5116. [24] J.I. Paredes, S. Villar-Rodil, J.M.D. Tasco´n, A. Martı´nez-Alonso, Graphene oxide dispersions in organic solvents. Langmuir, 24 (2008) 10560-10564. [25] G.X.Wang, B. Wang, J. Park, J. Yang, X.P. Shen, J. Yao, Synthesis of enhanced hydrophilic and hydrophobic graphene oxide nanosheets by a solvothermal method, Carbon 47 (2009) 68-72. [26] Z.X. Yu, H.H. Di, Y. Ma, L. Lv, Y. Pan, H.L. Zhang, Y. He, Fabrication of graphene oxide–alumina hybrids to reinforce the anti-corrosion performance of composite epoxy coatings, Appl. Surf. Sci. 351 (2015) 986-996. [27] C. Xu, R.S.Yuan, X. Wang, Selective reduction of graphene oxide, New Carbon Mater. 29 (2014) 62-66. [28] W.H. Luo, L.H. Zhao. The influence of GPTMS on the performance of silica-ionic liquid hybrid membrane, Solid State Ionics, 268 (2014) 94-101. [29] G. Lakshminarayana, M. Nogami, I.V. Kityk, Novel hybrid proton exchange membrane electrolytes for medium temperature non-humidified fuel cells, J. Alloy. Compd. 509 (2011) 2238-224. [30] D.Q. Zhu, W. J. V. Ooij, Structural characterization of bis-[triethoxysilylpropyl]tetrasulfide and 15

bis-[trimethoxysilylpropyl]amine silanes by Fourier-transform infrared spectroscopy and electrochemical impedance, spectroscopy, J. Adhes. Sci. Technol. 16 (2002) 1235-1260. [31] B. Singh, A. Diwan, V. Jain, A. Herrera-Gomez, J. Terry, M. R. Linford, Uniqueness Plots: A Simple Graphical Tool for Identifying Poor Peak Fits in X-ray Photoelectron Spectroscopy, Applied Surface Science, 387 (2016) 155-162. [32] H. Ganegoda, D.S. Jensen, D. Olive, L Cheng, C.U. Segre, M.R. Linford, J. Terry, Photoemission studies of fluorine functionalized porous graphitic carbon, J. Appl. Phys. 111 (2012) 053705. [33] L.M. Zhao, X. Feng, X.J. Mi, Y.F. Li, H.F. Xie, X.Q. Yin, Mechanical reinforcement and shape memory effect of graphite nanoplatelet-reinforced epoxy composites, J. Intell. Mater. Syst. Struct. 26 (2014) 1491-1497. [34] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, JACS 80 (1958) 1339-1339. [35] B.J. Hong, O.C. Compton, Z. An, I. Eryazici, S.B.T. Nguyen, Successful Stabilization of Graphene Oxide in Electrolyte Solutions: Enhancement of Bio-functionalization and Cellular Uptake, Acs Nano, 6 (2012) 63-73. [36] O.C. Compton, B. Jain, D.A. Dikin, A. Abouimrane, K. Amine, S.T. Nguyen, Chemically active reduced graphene oxide with tunable C/O ratios, ACS Nano 5 (2011) 4380-91. [37] A.Q. Mao, D.H. Zhang, X. Jin, X.L. Gu, X.Q. Wei, G.J. Yang, X.H. Liu, Synthesis of graphene oxide sheets decorated by silver nanoparticles in organic phase and their catalytic activity, J. Phys. Chem. Solids 73 (2012) 982-986. [38] G.X. Wang, B. Wang, X.L. Wang, J. Park, S.X. Dou, H. Ahn, K. Kim, Sn/graphene nanocomposite with 3D architecture for enhanced reversible lithium storage in lithium ion batteries, J. Mater. Chem. 19 (2009) 8378-8384. [39] F. Li, Y. Liu, C.B. Qu, H.M. Xiao, Y. Hua, G. X. Sui, S.Y. Fu, Enhanced mechanical properties of short carbon fiber reinforced polyethersulfone composites by graphene oxide coating, Polymer, 59 (2015) 155–165. [40] Y.Chen, C.J. Zhao, Z.T. Wang, J.C. Song, H. Zhang, S.C. Wen, Erbium-doped fiber laser passively mode-locked by a position-adjustable graphene saturable absorber, Opt. Eng. 51 (2012) 527-529. [41] C.M. Willemse, K. Tlhomelang, N. Jahed, P.G. Baker, E.I. Iwuoha, Metallo-graphene nanocomposite electrocatalytic platform for the determination of toxic metal ions, Sensors (Basel) 11 (2011) 3970-3987. [42] Y. Jun, Z.J. Fan, T. Wei, W.Z. Qian, M.L. Zhang, F. Wei, Fast and reversible surface redox reaction of graphene-MnO2 composites as supercapacitor electrodes, Carbon 48 (2010) 3825-3833. [43] S.G. Kim, O.K. Park, J.H. Lee, B.C. Ku, Layer-by-layer assembled graphene oxide films and barrier properties of thermally reduced graphene oxide membranes, Carbon lett. 14 (2013) 247-250. [44] C. Chen, M.C. Long, M. Xia, C.H. Zhang, W.M. Cai, Reduction of graphene oxide by an in-situ photoelectrochemical method in a dye-sensitized solar cell assembly, Nanoscale Res. Lett. 7 (2012) 1-5. 16

[45] H.J. Shin, K.K. Kim, A. Benayad, S.M. Yoon, H.K. Park, I.S. Jung, M.H. Jin, H.K. Jeong, J.M. Kim, J.Y. Choi, Y.H. Lee, Efficient Reduction of Graphite Oxide by Sodium Borohydride and Its Effect on Electrical Conductance, Adv. Funct. Mater. 19 (2009) 1987-1992. [46] T.K. Kim, J.Y. Cheon, K. Yoo, J. W. Kim, S.M. Hyun, H.S. Shin, S.H. Joo, H.R. Moon, Three-dimensional pillared metallomacrocycle-graphene frameworks with tunable micro- and mesoporosity[J]. J. Mater. Chem. A, 2013, 1(29):8432-8437. [47] M.M. Zhang, H.X. Yan, C. Gong, F.F. Zhang, Hyperbranched polysiloxane functionalization of graphene oxide for improved mechanical properties of cyanate ester nanocomposites, J. Compos. Mater., 49 (2014) 939-948. [48] V. Palanivel, D.Q. Zhu, W.J.V. Ooij, Nanoparticle-filled silane films as chromate replacements for aluminum alloys, Prog. Org. Coat. 47 (2003) 384-392. [49] X. Wang, G.L. Li, A.J. Li, Z.G. Zhang, Influence of thermal curing on the fabrication and properties of thin organosilane films coated on low carbon steel substrates, J. Mater. Process. Technol. 186 (2007) 259-264. [50] G.L. Li, X.M. Wang, A.J. Li, W.Q. Wang, L. Zheng, Fabrication and adhesive properties of thin organosilane films coated on low carbon steel substrates, Surf. Coat. Technol. 201 (2007) 9571-9578. [51] Y. Chen, T. Hong, M. Gopal, W.P. Jepson, EIS studies of a corrosion inhibitor behavior under multiphase flow conditions, Corros. Sci. 42 (2000) 979-990. [52] J.G. Liu, G.P. Gong, C.W. Yan, EIS study of corrosion behaviour of organic coating/Dacromet composite systems, Electrochim. Acta 50 (2005) 3320-3332. [53] Z.Q. Bai, C.F. Chen, M.X. Lu, J.B. Li, Analysis of EIS characteristics of CO 2 corrosion of well tube steels with corrosion scales, Appl. Surf. Sci. 252 (2006) 7578-7584.

17

Figure captions

Fig. 1 (a) Relationship between the film thickness and the content of GO and GPTMS, (b) Relationship between the film thickness and immersion time in 10 % GPTMS+0.05 % GO solution Fig. 2 ATR-FTIR spectra of GO, rGO, GPTMS and GPTMS/rGO coatings Fig. 3 XPS spectra of GO and graphite: (a) Survey of graphite, GO, GPTMS and GPTMS/rGO (b) Uniqueness plots for the C 1s narrow scan of GO with all peak position, peak height and peak width fixed, FWHM of C-O floated, (c) C 1s narrow scan of graphite, (d) C 1s peak fitting of GPTMS, (e) and (f) C1s and Si2p spectra after immersion for 2 h and 10 h, (g)-(i) C1s, O 1s and Si 2p spectra of coatings with different thickness Fig.4 the Raman spectra of graphite, GO, rGO and GPTMS/rGO coatings Fig. 5 XRD patterns of GPTMS and GPTMS/rGO with different thickness Fig. 6 Preparation process of GPTMS/rGO coating: (a) GO grafted with silanol groups, (b) Adsorption of silanol groups, (c) Adsorption of GO grafted with silanol groups and (d) Curing process Fig. 7 The micro-hardness of GPTMS film and GPTMS/rGO coating Fig. 8 The morphology of rGO film and GPTMS/rGO coating: (a) The surface of rGO film by SEM, (b) and (c) The morphology of rGO film by AFM, The surface of GPTMS/rGO film by SEM, (e) The morphology of GPTMS/rGO coating by AFM, (f)The cross section of GPTMS/rGO coating Fig. 9 Polarization curves of AA-2024, AA-2024/rGO, AA-2024/GPTMS/rGO and AA-2024/AAO samples after 24 h OCP test in 3.5% NaCl solution, with the surface area of 0.8 cm2 Fig. 10 EIS and fitting circuits of AA-2024 aluminum alloy with and without coatings (scatters for measured data and lines for fitting results): (a) Nyquist plots, (b) and (c) Bode plots of AA-2024 with and without coatings, (d) Artificial aging experiment of GPTMS/rGO, (e) Fitting circuit of AA2024, (f) Fitting circuit of AA2024/G, (g) Fitting circuit of GPTMS/rGO

18

30

GO-0.025%

GO-0.05%

GO-0.075%

GO-0.1%

Thickness / m

25 20 15 10 5 0

10%

7.5%

5%

15%

Content of GPTMS / wt.

a

14

Thickness / m

12 10 8 6 4 2 0

12 h

24 h

36 h

Immersion time

b Fig.1

19

48 h

60h

1070

Absorbance / a.u.

GO rGO-180 C GPTMS GPTMS/rGO

1010

819 776

2925 2855

1624 1731

3400

4000

3500

3000 2500 2000 1500 -1 Wavenumber / cm

Fig. 2

20

1000

500

(c)

(b)

Graphite GO GPTMS 0m 10m 22 m

Graphite

Intensity / a.u.

Reduced Chi Squared

40

30 Intensity / a.u.

(a)

20

292

290

288

286

284

282

GO C-C (284.8)

C-O(286.6) C=O(287.4)

10

C(O)O(288.9)

0 1200

1000

800

600

400

200

1.0

0

1.2

1.4

1.6

(d)

106

104

102

100

98

GPTMS-C 1s C-C

Intensity / a.u.

Intensity / a.u.

2.0

2.2

2.4

2.6

2.8

292

290

288

108

(f)

106

104

102

Immersion 2h-C 1s

286

284

282

Binding Energy / eV

(e) Immersion 2h-Si 2p

GPTMS-Si 2p

108

1.8

C-C Peak Width / eV

Binding Energy / eV

100

Intensity / a.u.

1400

98

C-C

Immersion 10 h-Si 2p

108

106

104

102

Immersion 10h-C 1s

100

98

C-C C-O

C-O

C-O-C

C=O

C-O-Si

C(O)O

C-O-Si

292

290

288

286

284

282

280

292

290

288 286 284 Binding Energy / eV

Binding Energy / eV

(e)

292

290

288

286

284

Binding Energy / eV

282

280

290

288

286

284

282

280

Binding Energy / eV

Si 2p-0m Si 2p-10m Si 2p-22 m

(i)

O 1s-0m O 1s-5m O 1s-22 m

Intensity / a.u.

Intensity / a.u.

C 1s-0m C 1s-5m C 1s-22 m

292

280

Intensity / a.u.

(g)

282

540

538

536

534 532 530 Binding Energy / eV

Fig. 3

21

528

526

108

106

104

102

100

Binding Energy / eV

98

96

D

(b)

G

(c)

GO

G

Intensity / a.u.

Intensity / a.u.

(a)

graphite

2D

D

G

rGO

2D

(d)

D

G

GPTMS/rGO

D

2D

500

1000

1500

2000

2500

3000

500

1000

1500

2000

2500 -1

-1

Raman Shift / cm

Raman Shift / cm

Fig. 4

22

3000

Intensity / a.u.

GPTMS/rGO-12.5 m

GPTMS/rGO-9.5 m GPTMS/rGO-8 m GPTMS/rGO-6 m GPTMS

0

10

20

30

40

50

2theta / degree

Fig. 5

23

60

70

80

90

(a)

(b)

(d)

(c)

Fig. 6

24

100

Micro-hardness / HV

82.5 80

67.7 60

40

20

0

11.5

GPTMS

GPTMS/rGO

Fig. 7

25

AA-2024

Fig. 8

26

1.2 0.8

E/V

0.4 0.0 -0.4 -0.8

AA-2024-blank AA-2024-rGO AA-2024-GPTMS/rGO AA-2024-AAO

-1.2 -1.6 -10 10

10

-8

10

-6

10

-4

I / Amps.cm

Fig. 9

27

10 -2

-2

10

0

3M

(b) 10M

AA-2024-blank AA-2024-rGO AA-2024-GPTMS/rGO-1d AA-2024-GPTMS/rGO-5d AA-2024-GPTMS/rGO-11d

AA-2024-blank AA-2024-rGO AA-2024-GPTMS/rGO-1d AA-2024-GPTMS/rGO-5d AA-2024-GPTMS/rGO-11d

1M

300

100k

|Z| / .cm

-Z'' / .cm

2

2

2M

0 0

100

30.0k

0.0 0.0

0 0

(c)

1M

30.0k

0.0 60.0k 0.0

2

Z' / .cm

90

10

2.0k

2M

10m

3M

1

10

100

1k

10k

100k

(d)

GP TMS/rGO GP TMS/rGO-ageing 26 d

Absorbance / a.u.

70

100m

Frequency / Hz

AA-2024-blank AA-2024-rGO AA-2024-GPTMS/rGO-1d AA-2024-GPTMS/rGO-5d AA-2024-GPTMS/rGO-11d

80

60

Phase angle / 

10k

1k

2.0k

60.0k

1M

300

50 40 30 20 10 0 10m

100m

1

10

100

1k

10k

100k

4000

3500

(e)

3000

2500

2000

1500

(g)

(f)

500

Qc

Qc

Qdl Rs

Rs

1000

-1

Wavenumber / cm

Frequency / Hz

Qdl

Rs

Qdl

W

Rct

Rpo

W

Rct

Fig. 10

28

Rpo Rct

Table 1 The major compositions of AA 2024 Elements\

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

Al

Content (wt.%)

0.5

0.5

3.8-4.9

0.3-0.9

1.2-1.8

0.1

0.25

0.15

balance

29

Table 2 Parameters for peak fitting of GO C 1s Name

Peak BE

Height CPS

Height Ratio

Area CPS.eV

Area Ratio

FWHM fit param (eV)

L/G Mix (%) Sum

C-C C-O C=O O-C=O

284.8 286.6 287.4 288.9

29944.96 16624.92 15538.87 3279.02

1 0.56 0.52 0.11

62896.99 26041.04 25191.64 5359.72

1 0.41 0.4 0.09

1.93 1.47 1.52 1.54

6.23 0 0 0

30

Table 3 Surface composition at different thickness Atomic %

C

O

Si

0 μm 10 μm 22 μm

60.26 63.11 75.64

29.3 27.66 17.28

10.45 9.23 7.08

31

Table 4 Adhesion test of graphene oxide, graphene and GPTMS/rGO Film

GO

rGO

GPTMS/rGO

Classification (ASTM)

0

0

4B

Percent area removed

100%

100%

less than 1%

Photograph

32

Table 5 Corrosion potential(Ecorr), corrosion current density(icorr)

3.5%NaCl AA-2024-GPTMS/rGO AA-2024-rGO AA-2024 AA-2024-AAO

icorr (A/cm2) 1.17x10-8

Ecorr(V) -0.327 -0.752 -0.792 -0.742

1.1x10-7 0.51x10-6 1.3 x10-8

33

Ba(mV) 310 13 246 258

Bc(mV) 295 240 194 134

Table 6 Fitting parameters of EIS circuit Rel. Rs (Ω/cm2)

std.

Qdl

error

(Ω-1cm2)

n

(%) AA2024-blank AA2024-rGO AA-2024GPTMS/rGO-1d

17.69

3.47х10-5

112.4

7.08

1.01х10-6

--

214.3

17.69

2.80х10-7

0.80

11.73

--

Rel.

Rel.

Rel.

std.

std.

Rct

error

error

(Ω/cm2)

(%)

(%)

Rel.

std.

W

error

(Ω-1s1/2cm2)

error

Qc

(%)

(Ω-1cm2)

(%)

Rel.

std.

Rpo

std.

error

(Ω/cm2)

error

(%)

(%)

6.54

592.3

18.05

1.98х10-3

22.24

--

--

--

--

--

9.60

1.54х105

18.86

4.29х10-5

12.97

1.84х10-6

2.43

805

2.60

1.10

6.54

6.39х107

4.43

--

--

1.98х10-9

5.53

4.57х105

16.35

--

34