Corrosion resistance enhancement of magnesium alloy by N-doped graphene quantum dots and polymethyltrimethoxysilane composite coating

Journal Pre-proof Corrosion resistance enhancement of magnesium alloy by N-doped graphene quantum dots and polymethyltrimethoxysilane composite coating B.K. Jiang, A.Y. Chen, J.F. Gu, J.T. Fan, Y. Liu, P. Wang, H.J. Li, H. Sun, J.H. Yang, X.Y. Wang PII:

S0008-6223(19)30912-1

DOI:

https://doi.org/10.1016/j.carbon.2019.09.013

Reference:

CARBON 14583

To appear in:

Carbon

Received Date: 26 April 2019 Revised Date:

27 August 2019

Accepted Date: 3 September 2019

Please cite this article as: B.K. Jiang, A.Y. Chen, J.F. Gu, J.T. Fan, Y. Liu, P. Wang, H.J. Li, H. Sun, J.H. Yang, X.Y. Wang, Corrosion resistance enhancement of magnesium alloy by N-doped graphene quantum dots and polymethyltrimethoxysilane composite coating, Carbon (2019), doi: https:// doi.org/10.1016/j.carbon.2019.09.013. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.

Graphical Abstract

Corrosion resistance enhancement of magnesium alloy by N-doped graphene quantum dots and polymethyltrimethoxysilane composite coating B. K. Jiang a, A. Y. Chen a, *, J. F. Gu b, J. T. Fan c, Y. Liu d, P. Wang a, H. J. Li a, H. Sun a

a

, J. H. Yang a, X. Y. Wang a, *

School of Material Science & Engineering, University of Shanghai for Science and Technology, Shanghai, 200093, China b

School of Material Science & Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China

c

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing, 100081, China

d

Key Laboratory of Near Net Forming of Jiangxi Province, Nanchang University, Nanchang, 330031, China

Abstract A

composite

coating

of

N-doped

graphene

quantum

dots

(N-GQDs)/polymethyltrimethoxysilane (PMTMS) is prepared on the surface of AZ91D magnesium alloy via electrodeposition and subsequent silane treatment. The microstructure of the N-GQDs/PMTMS composite coating is characterized by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The corrosion resistance performance is investigated by electrochemical impedance spectroscopy (EIS) and immersion test Corresponding author. Tel.: +86-21-55270632; Fax: +86-21-55270632; E-mail address: [email protected] (AY Chen); [email protected] (XY Wang).

in NaCl solution (3.5 wt.%). The N-GQDs/PMTMS composite coating exhibits a significant enhancement of corrosion resistance due to the chemical bonding of N-GQDs coating with Mg substrate and PMTMS coating. The formation mechanisms and nature of the corrosion resistance of the N-GQDs/PMTMS coating are discussed. Keywords: Magnesium, N-doped graphene quantum dots, Coating, Electrochemical deposition, Corrosion resistance 1. Introduction Magnesium alloys are known as green-engineering materials with great developments in automobile, electronics, and aerospace manufacturing because of their low density, high specific strength and stiffness, and excellent electromagnetic shielding performance [1-3]. However, the poor corrosion resistance of magnesium alloys severely limits the service life. In the past decades, many strategies are proposed to improve the corrosion resistance of magnesium alloys, such as ion implantation [4], anodization [5], electrodeposition [6], and surface coating [7-10]. Among all these methods, surface coating is an effective way to improve the corrosion resistance by isolating the magnesium alloy from corrosion medium, including graphene coating [11], layered double hydroxides [12], polymeric coating [13], silane coating [14], as well as their composite coatings [15]. Cui et al. developed a polymethyltrimethoxysilane (PMTMS) coating on magnesium AZ31 alloy by combining micro-arc oxidation (MAO) pre-treatment [16]. The MAO/PMTMS composite coating exhibits a better corrosion resistance than the MAO coating because the PMTMS is more stable and acts as a physical barrier to prevent the attack 2

from corrosive solution. However, the PMTMS coating is susceptible to be swelling and peeling off. Graphene oxide (GO) has been widely examined in the field of metal corrosion protection because of its chemical stability, thermal stability, and excellent barrier property. As a physical barrier, the GO coating with low defect density can improve the corrosion resistance, while the isolated GO sheets can serve as the cathode sites, resulting in galvanic corrosion behavior [11,17,18]. Therefore, the GO nanosheets are usually hybridized with other polymers or inorganic compounds to enhance the corrosion resistance [19-21]. For example, Ramezanzadeh et al. found that the addition of GO-polyaniline-cerium oxide (GO-PAni-CeO2) particles to the epoxy matrix could significantly enhance the barrier protection property, and they thought PAni-CeO2 compounds effectively inhibited the active corrosion performance of the GO nanosheets [19]. Recently, Zhao et al. found that the incorporation of GO in the plasma electrolytic oxidation (PEO) coating could effectively decrease the number of micro-pores within the coating, and the corrosive electrolyte diffused into the Mg surface could be blocked by GO composite coating [22]. However, there are many pores and pore bands in the interface between the coating and substrate. The poor interface bonding results in a limit improvement in corrosion resistance. Furthermore, the graphene coating exhibits poorly adhesion with the metal interface due to the physical interaction of GO nanosheets with metal substrate, and falls off easily. Therefore, surface-pretreatment steps are performed to increase the adhesion force between the coating and substrate [22-24]. However, long-term reliability is 3

questionable due to the incomplete and uncontrollable coverage. Compared with GO nanosheets, graphene quantum dots (GQDs) exhibit many unusual properties resulting from their distinct structure features [25]. First, GQDs are highly stable dispersion in water and exhibit good film-forming property, which avoid the insoluble problem of GO in the coating [23]. Second, GQDs have a large number of edge structures due to the very small sizes of several nanometers. These edge groups, such as hydroxyl, epoxy and carbonyl groups, can provide bonding capability with the other coating materials or the substrate, and also are susceptible to be functionalized with other additive materials [25,26]. Most importantly, the third point is that the chemical structures of GQDs can be fine-tuned by element-doping according to the performance requirements, such as the N-doping, S-doping, and N,S-doping [27,28]. However, the GQDs are not widely employed in the corrosion protection largely for the zero-dimensional structure. In the present work, we develop an N-doped GQDs (N-GQDs)/PMTMS composite coating onto the surface of AZ91D Mg alloy. The preparation process of N-GQDs/PMTMS coating is illustrated in Fig. 1, where the N-GQDs are electrodeposited on the surface of Mg alloy, and then PMTMS is coated on the N-GQDs surface by silylation reaction of methyltrimethoxysilane (MTMS). The N-GQDs as an intermediate layer chemically bonds the Mg substrate and PMTMS coating through the functional groups of N-GQDs. The microstructure and corrosion protection performance of the N-GQDs/PMTMS composite coating are investigated, and the reasons of corrosion resistance are discussed. 4

Fig. 1. Preparation process of the N-GQDs/PMTMS composite coating. 2. Experimental 2.1. Materials and chemicals As-casted AZ91D Mg alloy (chemical composition: 9.3 wt.% Al, 0.62 wt.% Zn, 0.37 wt.% Mn) was supplied by Shanghai Light Metal Co., Ltd., China. The MTMS (CH3Si(CH3O)3, MW=136.22, 99%) was purchased from Shanghai Makclin Biochemical Co., Ltd. (China). The AZ91D Mg alloy specimen with a dimension of 20 mm ×15 mm ×10 mm was ground with SiC sand papers up to a 1500 grit, then cleaned by deionized water and alcohol solution, and dried in warm air. 2.2. Synthesis of N-GQDs N-GQDs were synthesized via hydrothermal method proposed by Li et al. [29]. Specifically, 1.68 g citric acid and 1.44 g urea were dissolved in 40 ml de-ionized (DI) water. After fully dissolution, the resulting solution was transferred into 50 ml Teflon-lined autoclave, and then heated up to 180

for 8 h. The N-GQDs were

obtained by adding ethanol into the solution and ultrasonic cleaned for 3 times after staying for 3 weeks, then centrifuged at 10,000 rpm for 5 min, and the final product was dried at 80

in an oven.

2.3. Preparation of the N-GQDs/PMTMS composite coating 5

The N-GQDs coating was electrodeposited on the Mg alloy surface using a CHI660B electrochemical workstation with a three-electrode cell of a platinum electrode as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the AZ91D Mg alloy sample as the working electrode. The electrolyte solution was 100 ml N-GQDs with a concentration of 10 µM after ultrasonic dispersion for 30 min. After optimizing process parameters, the electrodeposition was carried out at a constant current density of 100 mA cm-2 for 8 min. The Mg alloy specimen after electrodeposition of N-GQDs coating was dipped into MTMS solution (MTMS: ethanol: DI water = 3: 10: 20) at 50

for 2 h, then PMTMS

coating was obtained on the N-GQDs surface after the hydrolysis of MTMS. Final, the N-GQDs/PMTMS composite coating was heated at 60

for 2 h.

2.4. Microstructure characterization The UV-Vis spectrum was recorded on a Lambda 25 UV-Vis spectrophotometer (PerKinElmer, USA). The photoluminescence spectroscopy (PL) was performed using a Shimadzu RF-5301 Fluorescence spectrophotometer. Raman scattering spectrum was obtained using Horiba Jobin Yvon HR800 spectrometer with 532 nm laser. Transmission electron microscopy (TEM, Tecnai G2 F20) was employed to analyze the microstructure of the as-prepared N-GQDs. The morphology of the N-GQDs was observed by atomic force microscope (AFM, Shimadzu SPM9700) operating at the semi-tapping mode. The surface morphologies before and after corrosion tests were observed by scanning electron microscopy (SEM, FEI Quanta 6

450) equipped with energy-dispersive X-ray (EDX) spectroscopy. X-ray diffraction (XRD) patterns were recorded using a Bruker D8 Advance with Cu Ka radiation in the range of 20 - 80 °. The surface chemical states of the coatings were analyzed by X-ray photoelectron spectroscopy (XPS) using a ThermoFisher-250XI X-ray electron spectrometer with focused monochromatized Al Kα radiation. 2.5. Corrosion test 2.5.1. Electrochemical measurement Electrochemical measurement was performed with a three electrode cell of SCE as the reference electrode, a platinum sheet as the counter electrode, and the sample as the working electrode by using CHI660B electrochemical workstation in 3.5 wt.% NaCl solution. A stable open-circuit potential (OCP) was established within 1500 s. Electrochemical impedance spectra (EIS) measurement was carried out in a frequency range of 0.01 - 105 Hz at a disturbing potential of 10 mV after OCP measurement. The impedance data were fitted with equivalent circuits and values of the circuit elements were extracted using ZSimDemo software. 2.5.2. Salt solution immersion test In order to determine the long-term corrosion resistance of the coatings, immersion tests were carried out in 30 ml 3.5 wt.% NaCl solution at room temperature. The as-prepared samples were sealed with paraffin except the upper surface. The surface morphologies before and after immersion were observed by FE-SEM. 3. Results 3.1. Microstructure of the starting N-GQDs 7

Fig. 2a shows the UV-Vis and PL spectra of the N-GQDs. UV-Vis spectrum exhibits two absorption peaks at 343 and 478 nm, corresponding to the π→π* transition of aromatic sp2 domain and n→π* transition of C-N or C=O, respectively [28]. The maximum PL emission wavelength is at 445 nm, emitting blue fluorescence light. Fig. 2b shows the Raman spectrum of the N-GQDs, where two peaks of the D and G bands are resolved at 1337 and 1556 cm-1, respectively. The intensity ratio of the D band to G band (ID/IG) is about 0.92. The relatively low value of ID/IG indicates a high crystallinity of N-GQDs induced by the N doping [30,31]. The granular particles are observed in the AFM image (Fig. 2c), where the topographic heights are in the range of 0.8 to 3.3 nm, corresponding to 2-10 graphene layers (inset in Fig. 2c). The TEM observation also exhibits spherical or nearly spherical particles of N-GQDs (Fig. 2d). The high-resolution (HRTEM) image clearly displays the hexagonal graphene networks with highly nanocrystalline, as circled in Fig. 2e. The statistic results from TEM and HRTEM images indicate that particle sizes range from 2 to 10 nm with an average size of 6 nm (Fig. 2f). 3.2. Microstructure of the N-GQDs coating The Mg alloy substrates were electrochemically treated in distilled water and N-GQDs solution with the same deposition parameters, respectively. The deposition process and surface morphologies are provided in Supplementary Movie 1. After electrodeposition, the Mg alloy obtained in distilled water has a light brown surface (Supplementary Movie 1), and the SEM images present a flat and smooth surface (Fig. 3a and the inset). However, the N-GQDs coating exhibits a black surface 8

(Supplementary Movie 1) with many rough islands, as indicated by arrows in Fig. 3b. The magnification of the flat zone in Fig. 3b shows some granular particles on the surface (inset in Fig. 3b). This result suggests that the growth of islands originates from the inhomogeneous stacking. The cross-sectional SEM images display that the layer thickness of the N-GQDs coating is about 5 - 9 µm, and no obvious boundaries or cracks are found between the Mg matrix and N-GQDs coating, as shown in Fig. 3c and the magnification of Fig. 3d. Element mappings of the N-GQDs coating indicate that N-GQDs layer is composed of not only C, O and N elements (Fig. 3e-g), but also Mg and Al elements (Fig. 3h and i), revealing that Mg and Al elements diffuse into the N-GQDs coating to form new compounds.

Fig. 2. Microstructure of the starting N-GQDs. (a) UV-Vis and PL spectra; (b) Raman spectrum; (c) AFM image; (d) TEM image; (e) HRTEM image; (f) Particle size distribution.

9

Fig. 3. FE-SEM images of the N-GQDs coating. (a, b) Surface morphologies of the Mg alloys obtained in distilled water and N-GQDs solution after electrodeposition, respectively; (c, d) Cross-sectional morphologies of the N-GQDs coating; (e-i) Element mappings of C, O, N, Mg, and Al elements of (d). The insets in (a) and (b) are the corresponding magnifications of the rectangular zones. The detailed microstructure of the N-GQDs coating is observed by TEM after peeling off the N-GQDs coating from the Mg substrate, as shown in Fig. 4. The N-GQDs particles are observed in the film after electrodeposition, as indicated by circles in Fig. 4a. The nanoparticles are randomly oriented, as indicated by discontinuous diffraction rings of the selected-area election diffraction (SAED) pattern of Fig. 4b. The crystal constants, calculated from the diffraction rings of the A, B, and C positions, are 0.140, 0.206, and 0.278 nm. The corresponding bond lengths of GQDs are shown in the inset of Fig. 4b [27,28]. HRTEM image also displays the hexagonal graphene network (marked by circles in Fig. 4c). The EDX of the N-GQDs film shows that Mg and Al elements are co-existed with the C and O elements, as 10

given in Fig. 4d.

Fig. 4. TEM images of N-GQDs coating. (a) Bright-field TEM image; (b) SAED pattern; (c) HRTEM image; (d) EDX pattern. The inset in (b) is an illustration of bond lengths of GQDs. The surface chemical structure of the N-GQDs coating is further analyzed by XPS spectra, as shown in Fig. 5. In survey spectrum of Fig. 5a, the chemical elements of C 1s, N 1s and O 1s at 284.5, 401.4 and 533.0 eV are from N-GQDs, while Al 2p and Mg 1s at 87.0 and 1395.3 eV originate from the diffusion of Al and Mg atoms into the N-GQDs coating during electrodeposition, as confirmed by the EDX results (Fig. 3h, 3i, and Fig. 4d). The contents of Al and Mg atoms are 3.2% (at.%) and 3.6% (at.%), respectively, suggesting that the Al atoms are more active than the Mg atoms due to the much higher Mg content in the Mg substrate. C 1s spectrum can be fitted into three peaks, representing the chemical groups of C−C/C−N/C−O, C=O and COO- [32], 11

as shown in Fig. 5b. The N element can be resolved into three chemical groups of pyridinic N (398.4 eV), pyrrodic N (399.9 eV) and graphitic N (402.3 eV) structures, as given in Fig. 5c. The total N content is 2.2% (at.%), and the pyrrodic N structure has the highest content (1.5%). The devolution analysis of O 1s spectrum shows more complex constituents, involving the MgO/Al2O3 oxides at 531.4 eV, Mg(OH)2 at 531.8 eV, Al(OH)3 at 532.5 eV, and O=C/O-C at 533.2 eV, as shown in Fig. 5d [33,34]. These results verify the formation of oxides and hydroxides of Mg and Al elements in the N-GQDs coating. Accordingly, the devolution results of Mg 1s and Al 2p spectra also prove the formation of MgO, Mg(OH)2 and Al2O3, Al(OH)3 in the N-GQDs coating, as given in Fig. 5e and f, respectively. The findings of oxides and hydroxides of Mg and Al elements in the N-GQDs coating suggest that interface between N-GQDs and Mg alloy substrate is chemically bonded. 3.3. Microstructure of the N-GQDs/PMTMS composite coating After polysiloxane treatment, the surface of N-GQDs/PMTMS composite coating is composed of granular pits (marked by A) and flat zone (marked by B), as given in Fig. 6a, where the walls of the granular pits are observed to be spherical particles with sizes of 100 - 800 nm (Fig. 6b). The granular pits might be induced by the ununiformed dehydration reaction during solidification. The cross-sectional SEM image of the N-GQDs/PMTMS composite coating exhibits that the thickness of PMTMS coating is in the range of 10 - 14 µm, as shown in Fig. 6c. More importantly, a crack-free interface is observed in the entire cross-section. The element mappings of Fig. 6d are given in Fig. 6e - i, where C, O and Si elements (Fig. 6e - g) uniformly 12

distribute in the PMTMS coating, while Mg and Al elements (Fig. 6h and i) are confined in the N-GQDs coating.

Fig. 5. XPS spectra of the N-GQDs coating. (a) Survey spectrum; (b) C 1s; (c) N 1s; (d) O 1s; (e) Mg 1s; (f) Al 2p;

13

Fig. 6. FE-SEM images of the N-GQDs/PMTMS composite coating. (a) Surface morphology; (b) Magnification of the rectangular zone in (a); (c, d) Cross-sectional morphologies; (e - i) Mappings of C, O, Si, Mg, and Al elements of (d). The XPS survey spectrum of the N-GQDs/PMTMS composite coating shows the C 1s, O 1s, Si 2s and 2p peaks (Fig. 7a). The chemical bonds of C-H/C-Si/C-O (Fig. 7b) and Si-O-Si (Fig. 7c and d) are originated from the PMTMS. Notably, the individual PMTMS coating on Mg alloy surface (inset in Fig. 7d) has only one chemical bond of Si-O-Si from the Si 2p spectrum. Hence, the chemical bond of Si-O-N/Si-O-C of the N-GQDs/PMTMS composite coating is newly formed, which might be produced by the chemical bonding between N-GQDs and PMTMS. Fig. 8 shows XRD patterns of the bare Mg alloy, PMTMS, N-GQDs, and N-GQDs/PMTMS composite coating. Except the diffraction peaks of the Mg substrate, two new diffraction peaks at 21.3 ° and 23.7 ° occur in the N-GQDs coating and N-GQDs/PMTMS composite coating. The compound is identified to be -(CH2)n- (n=7 - 8) according to the standard match (JCPDS NO: 40-1995), which might originate from the self-polymerization of 14

N-GQDs to form alkane structure during electrodeposition.

Fig. 7. XPS spectra of the N-GQDs/PMTMS coating. (a) Survey spectrum; (b) C 1s; (c) O 1s; (d) Si 2p; The inset in (d) is the Si 2p of the individual PMTMS coating.

Fig. 8. XRD patterns of the bare Mg alloy, PMTMS coating, N-GQDs coating and N-GQDs/PMTMS composite coating. 3.3. Corrosion protection performance 3.3.1. Electrochemical corrosion 15

Fig. 9 shows the OCP curves of the bare Mg alloy, PMTMS coating, N-GQDs coating, and N-GQDs/PMTMS composite coating in 3.5 wt.% NaCl solution. The OCP of the samples are ranked in decreasing order as follows: N-GQDs/PMTMS composite coating (-1.38 V) > N-GQDs coating (-1.53 V) > PMTMS coating (-1.58 V) > Mg substrate (-1.59 V). This result implies that the N-GQDs coating and the N-GQDs/PMTMS composite coating can improve the corrosion resistance of Mg alloy.

Fig. 9. OCP curves of the bare Mg alloy, N-GQDs coating, PMTMS coating and N-GQDs/PMTMS composite coating in 3.5 wt.% NaCl. The corrosion behaviors are further studied via EIS measurements in the 3.5 wt.% NaCl solution, as shown in Fig. 10a. The N-GQDs/PMTMS coating displays the largest diameter of the capacitive loop among the four samples. Fig. 10b is the magnification of the rectangular zone in Fig. 10a, where the bare Mg alloy is characterized by a capacitive loop in the medium to high frequency range and an inductive loop in the low frequency range, similar to the previously reported result [16]. The capacitive loop is related to the charge transfer process, whereas the inductive loop is due to the dissolution and pitting corrosion of Mg alloy. The Nyquist 16

plot of the N-GQDs coating is similar to the Mg alloy, suggesting a similar corrosion behavior to the AZ91D Mg alloy. However, the corrosion resistance is clearly enhanced due to the larger diameter of the capacitive loop of the N-GQDs coating. Compared with the N-GQDs coating, the PMTMS coating shows a relative larger capacitance arc diameter (Fig. 10a), which might originate from the surface hydrophobicity of the PMTSM coating [6,8,16].

Fig. 10. Nyquist (a, b) and Bode (c, d) plots of the different samples. (b) Magnification of the rectangular zone in (a); The insets in (a) and (b) are the EC models of the N-GQDs/PMTMS coating, and of both bare Mg alloy and N-GQDs coating, respectively. The dotted lines in (a) and (b) are the EC fitted curves. 17

The inset in Fig. 10a is the fitted equivalent circuit (EC) model of the N-GQDs/PMTMS, and that in Fig. 10b is the EC model of the bare Mg alloy and N-GQDs coating. The Rs represents the solution resistance, R1 and Rct are the charge transfer resistance from the pitting corrosion zone and loose corrosion product, respectively [35, 36]. A constant phase element (CPE) is used instead of a pure capacitance for the coating capacitance [37]. For the bare Mg alloy and the N-GQDs coating (inset in Fig. 10b), the high-frequency capacitance loop is described by CPE1 and Rct to characterized loose corrosion product layer, and the low-frequency inductive loop is depicted by R1 and L, indicating the onset of pitting corrosion. However, no inductive character is determined in the N-GQDs/PMTMS coating (inset in Fig. 10a), suggesting that the pitting corrosion is weakened in the composite coating. From the fitting results shown in Table 1, the N-GQDs/PMTMS composite coating has the largest Rs, more than 6 times that of bare Mg alloy, indicating that chemical reaction between the metallic surface and the ions in the electrolyte solution is effectively inhibited. CPE is related to corrosion reaction area, i.e. low CPE implies relatively smaller corroded area. The CPE1 values exhibit a decrease in the following order: bare Mg substrate (1.0×10-5 Ω-1·sn·cm-2) > N-GQDs coating (4.5×10-6 Ω-1·sn·cm-2) > N-GQDs/PMTMS (4.3×10-9 Ω-1·sn·cm-2). The values of n for all the coatings are close to 1, indicating a capacitance behavior [38]. The Rct presents in ascending order: bare Mg substrate (78.3 Ω·cm2) < N-GQDs coating (185.4 Ω·cm2) < N-GQDs/PMTMS coating (1.7 × 104 Ω·cm2). Bode plots, characterized by log |Z| vs. log f and -phase angel vs. log f, are given in Fig. 10c and d, respectively. The larger 18

low-frequency impedance modulus, |Z|, indicates a better corrosion protection performance, and the ascending order is: bare Mg alloy (21 Ω cm2) < N-GQDs coating (95 Ω cm2) < N-GQDs/PMTMS composite coating (1.3 ×10

4

Ω cm2). The

other electrochemical tests are shown in Supplementary Figure S1 and S2, and Table S1 and S2). These results reveal that the N-GQDs/PMTMS composite coating enables a significant improvement of corrosion resistance. Table 1 Electrochemical data obtained via EC fitting of the EIS curves. Sample

Rs (Ω·cm2)

CPE1 (Ω-1·sn·cm-2)

n1

R1 (Ω·cm2)

CPE2 (Ω-1·sn·cm-2)

n2

Rct (Ω·cm2)

RL (Ω·cm2)

Mg alloy

12.7

1.0×10-5

0.99

114

-

-

78.3

20.4

N-GQDs

80.8

4.5×10-6

0.84

789

-

-

185.4

486.1

N-GQDs/PMTMS

85.0

4.3×10-9

1

1.66×10-8

0.99

1.7×104

1.8×104

-

3.3.2. Salt solution immersion test The surface morphologies of the bare Mg alloy, PMTMS coating, N-GQDs coating, and N-GQDs/PMTMS composite coating are show in Fig. 11 after immersion in 3.5 wt.% NaCl solution for different durations. Compared with the original surface morphology of Fig. 11a, the bare Mg alloy is completely corroded after immersing 8 h, and then occurs more serious corrosion with much more rougher corrosion surface when the immersing time increases to 26 h, as shown in Fig. 11e and i. As to the PMTMS coating and N-GQDs coating, the two samples start to be corroded at 8 h, where the corrosion area are indicated by arrows in Fig. 11f and g, respectively. When the immersing time increases to 26 h, the corrosion pits spread the entire surface of the PMTMS coating and half surface of the N-GQDs coating, as shown in Fig. 11j and k. Therefore, the N-GQDs coating is relative stable than the PMTMS coating in 19

the long-term corrosion, which illustrates the chemical bonding between the N-GQDs and Mg substrate is more effective in improving the bonding strength than the physical adsorption of PMTMS coating. This founding is also consistent with the OCP results (Fig. 9). However, the N-GQDs/PMTMS composite coating starts to be corroded at the immersing time of 194 h, where the corrosion pits are indicated by arrow in Fig. 11l. The magnifications of the corrosion pits (circle zones in Fig. 11i - l) of the four samples are shown in Supplementary Figure S3a - f. The surface of the bare Mg alloy and PMTMS coating are corroded very badly after immersing 26 h, and many deep cracks are observed on the surface (Supplementary Figure S3a and b). However, the N-GQDs coating after immersing 26 h and the N-GQDs/PMTMS coating after immersing 194 h exhibit relatively smooth surface, and some flat pits are observed on the surface, as shown in Supplementary Figure S3c-e. These results reveal that the N-GQDs/PMTMS specimen shows the best long-term stability compared with the other specimens.

Fig. 11. Surface morphologies of the samples with different coatings before and after 20

immersion tests in 3.5 wt.% NaCl. (a - d) Bare Mg alloy, PMTMS, N-GQDs, and N-GQDs/PMTMS specimens; (e - h) Corresponding specimens of (a - d) after immersing 8 h; (i - k) Bare Mg alloy, PMTMS, and N-GQDs specimens after immersing 26 h; (l) N-GQDs/PMTMS specimen after immersing 194 h; The EIS curves of the four specimens (bare Mg alloy, N-GQDs, PMTMS, and N-GQDs/PMTMS) after immersion tests are shown in Fig. 12, where all the EIS curves of 0.42 h represent the initial state after OCP tests (1500 s). The Nyquist plots of the bare Mg alloy (Fig. 12a) and PMTMS (Fig. 12e and the inset) specimens both exhibit a much smaller semicircle arc at the low frequency zone after 8 h immersion, indicating dissolution and negligible protection of the surface film. The Rct of PMTMS coating significantly decreases from 771.8 Ω cm2 (0.42 h) to 19.2 Ω cm2 (8 h) and 3.4 Ω cm2 (26 h), see supplementary Table S3. The accelerated corrosion of PMTMS coating can be attributed to the coating delamination by the diffusion of corrosive solution through the defect sites. Similarly, the Nyquist plots of the N-GQDs coating also show a gradual decrease of capacitance arc diameter with immersing time from 0.42 h to 26 h (Fig. 12c), and the Rct decreases from 185.4 Ω cm2 (0.42 h) to 41.4 Ω cm2 (8 h) and 26 Ω cm2 (26 h). However, the diameter of the capacitive loop of the N-GQDs/PMTMS composite coating after immersion for 8 h is obviously enlarged, as shown in Fig. 12g. The Rct of the N-GQDs/PMTMS composite coating increases from 1.7×104 Ω cm2 (0.42 h) to 3.4×1012 Ω cm2 (8 h), which originates from the self-healing of the PMTMS driven by water swelling and hydrolytic polycondensation [16]. With further prolonging of the immersion time, the 21

diameter of the capacitive loop recovers back to the starting level (0.42 h), and the Rct drops to 7.6×105 Ω cm2 after 26 h immersion. Then, the semicircle arc at the low frequency zone reduces remarkably, and the Rct decreases to 54.2 Ω cm2 after 194 h immersion, as shown in the inset of Fig. 12g. These observations manifest that the intermediate N-GQDs layer and PMTMS coating exhibit obviously synergistic effect in the long-term protection. Meanwhile, Bode plots shown in Fig. 12b, d, f, and h also explain the same results as well. The EIS results are well agree with the observations of immersion tests.

22

Fig. 12. Nyquist and Bode plots of the bare Mg alloy (a, b), N-GQDs (c, d), PMTMS (e, f), and N-GQDs/PMTMS (g, h) specimens after immersing different times. The EC models of the different specimens are inserted in the corresponding Nyquist plots. The insets in (e, g) are the magnifications of the corresponding rectangular zones. 4. Discussion 4.1. Formation of N-GQDs/PMTMS composite coating Generally,

the following reactions

are supposed 23

to

occur during the

electrodeposition, as given in Eqs. 1 - 3 [3,16]: Anodic reaction: Mg → Mg

+ 2e and Al → Al

+ 3e

(1)

Cathodic reaction: 2H O + 2e → 2OH + H ↑

(2)

Total reaction: Mg + 2H O → Mg OH

+ H ↑ and 2Al + 6H O → 2Al OH

+ 3H ↑

(3)

However, these products of Mg(OH)2 and Al(OH)3 shall disperse loosely in the water and surface of Mg alloy [3]. Factually, in the counterpart experiment, there are no hydroxides and oxides formed on the surface of Mg alloy under the same electrodeposition condition without N-GQDs in electrolyte, see Fig. 3a, and the Supplementary Movie 1. However, the elemental mapping images show that Mg, Al, C, O and N elements co-exist in the N-GQDs coating, indicating that Mg and Al atoms in the Mg substrate diffuse into the N-GQDs coating (Fig. 3). Meanwhile, the XPS results show that the Mg(OH)2, MgO, Al(OH)3 and Al2O3 are resolved in the N-GQDs coating (Fig. 5). All those results reveal that the N-GQDs induce the formation of hydroxides and oxides on the surface of the Mg alloy. The chemical groups of N-GQDs, including -OH, -COOH, pyrrolic and pyridinic, can react with some Mg(OH)2 and Al(OH)3 by dehydration to form MgO and Al2O3 oxides, as illustrated in Fig. 13a and b. Notably, N-GQDs directly bond with Mg alloy substrate by the chemical connection of Mg-O-GQDs (or Al-O-GQDs), as marked by I zone in Fig. 13b, exhibiting a strong interface bonding. Meanwhile, the N-GQDs assemble with each other to form -(CH2)n- (II zone in Fig. 13b) through ring-opening reaction of pyrrolic and pyridinic structures and C-O-C (III zone in Fig. 13b) through the 24

dehydration reaction of hydroxyl and carboxyl groups. These chemical reactions are trigged by the quantum size and a large number of active groups at the edges. When coated MTMS onto the surface of the N-GQDs coating, the MTMS firstly occurs hydrolysis reaction to produce (HO)3-Si-CH3, as given in Eq. 4 as follows [16]: RO

− Si − CH + 3H O → HO

− Si − CH + 3ROH

(4)

Then, the (HO)3-Si-CH3 reacts with the hydroxyl groups on the N-GQDs coating via hydrolysis condensation reaction, finally, the compact PMTMS coating is formed, as shown in Fig. 13c. Noted that the N-GQDs react with hydroxyl groups of (HO)3-Si-CH3 to form C-O-Si linkages, as marked by IV zone in Fig. 13c. The chemical bonding between N-GQDs coating with PMTMS coating is verified by the XPS analysis in Fig. 7d, which results in a strong interface adhesion without structural defects.

25

Fig. 13. Schematic illustration of the formation of N-GQDs/PMTMS composite coating. (a) Formation of hydroxides of Mg and Al during electrodeposition; (b) Dehydration reaction between N-GQDs and hydroxides of Mg and Al (I zone) or the self-polymerization of N-GQDs (II and III zones); (c) Chemical bonding between N-GQDs and PMTMS (IV zone).

26

4.2. Effect of N-GQDs/PMTMS composite coating on the corrosion resistance The N-GQDs/PMTMS composite coating exhibits excellent corrosion resistance, which can be attributed to the chemical bonding of the N-GQDs coating and the physical barrier of PMTMS coating. The N-GQDs coating exhibits two superiorities. One is that the N-GQDs coating attaches both the Mg substrate and PMTMS coating by chemical bonding to form an interface-free intermediate layer, where the chemical bonding possesses a high surface coverage because of the quantum size and massive edge groups of the N-GQDs. The other is the planar graphene structure blending with hydroxides and oxides of Mg and Al by self-polymeration of N-GQDs during electrodeposition. This complex structures can protect the Mg alloy from the aggression of corrosive medium, as shown in Fig. 9 and Fig. 11. It should be pointed out that the thicker layers do not further increase the corrosion resistance, as verified by the N-GQDs coatings with longer electrodeposion time (10 and 15 min), as shown in Supplementary Figure S4a. The reason for this is that the formation of Mg (Al) oxides and hydroxides are inhibited by the thicker layer due to the diffusion difficulty of Mg and Al ions in the thicker N-GQDs coating. As reported in many works, the surface pre-treatments are performed to generate reactive substrate surface in order to enhance the interfacial interaction [5,7,16,22,23]. However, some micro-pores, or micro-cracks are subsequently produced, which conversely facilitate more severe corrosion and result in faster deterioration. Although the post-sealing process can improve the corrosion property, such treatment merely provides limited protection. Different from these surface layers, the N-GQDs coating is characterized by a 27

compact structure and a good interface bonding, and no surface treatment is required in this method. Compared with the N-GQDs coating, the N-GQDs/PMTMS composite coating exhibits much better corrosion resistance. The reason is that the product of Mg(OH)2 in the N-GQDs coating can be dissolved by the adsorbed Cl− on the coating surface, and changed into soluble MgCl2 [24], as shown in Eq. (5): + 2!" →

!" + 2

(5)

The dissolution of Mg(OH)2 shall destroy the compactness of the surface passivation layer [24]. However, the PMTMS coating can seal the Mg(OH)2 on the N-GQDs coating by the silicon-oxygen (O-Si-O) units, which endow the coating with more stable and long-term protection. This protection function is deteriorated by thicker PMTMS coating, as shown in Supplementary Figure S4b. The N-GQDs/PMTMS composite coating with immersion time of 3 h in MTMS solution shows a worse corrosion resistance than the one with immersion time of 2 h. The micro-cracks on the surface, which are produced by the ununiformed dehydration reaction during solidification, deteriorate the corrosion resistance, as indicated by arrows in the Supplementary Figure S4c. Commonly, the local exfoliation of the organic coating by the weak interface bonding leads to a serious pitting corrosion. Nevertheless, the silicon hydroxyl groups produced by the hydrolysis of MTMS react with the functional groups, such as hydroxyl, pyridinic, pyrrodic groups, of the N-GQDs to form C-Si-O or C-Si-N bonds by the dehydration reaction (Fig. 7d). These chemical bonds are strongly connected the N-GQDs coating with the PMTMS coating, which 28

reduces the probability of swelling and falling off the substrate. Thus, N-GQDs/PMTMS composite coating is more effective in corrosion protection. 5. Conclusions (1) An N-GQDs/PMTMS composite coating is developed onto the AZ91D Mg alloy surface combining electrodeposition of N-GQDs with silane treatment of MTMS, where the composite coating is compose of two layers of 7 µm-thick N-GQDs coating and 12 µm-thick PMTMS coating. (2) The N-GQDs coating is composed of N-GQDs, Mg(OH)2, MgO, Al(OH)3, and Al2O3, which serves as an intermediate layer to connect the Mg substrate through the chemical bonds of Mg-O-GQD and Al-O-GQD, and the PMTMS coating through the chemical bonds of Si-O-GQD. (3) Compared with the bare Mg alloy, the N-GQDs/PMTMS composite coating shows a noticeable enhancement of corrosion resistance performance, characterized by the largest Rs, more than 6 times that of bare Mg alloy. The CPE1 shows a decrease of orders of magnitude in the following order: bare Mg substrate (1.0×10-5 Ω-1·sn·cm-2) > N-GQDs coating (4.5×10-6 Ω-1·sn·cm-2) > N-GQDs/PMTMS (4.3×10-9 Ω-1·sn·cm-2). (4) The enhanced corrosion protection performance of the N-GQDs/PMTMS composite coating can be attributed to the strong chemical bonding of the N-GQDs coating and sealing layer of PMTMS coating. Acknowledgments We thank the National Natural Science Foundation of China (grants 51771121, 51572173, 51602197, and 51702212). Prof. Wang gratefully acknowledges the 29

financial supports from Innovation Program of Shanghai Municipal Education Commission (2019-01-07-00-07-E00015) and Shanghai Academic/Technology Research Leader Program (19XD1422900).

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