Hydrophobic epoxy resin coating with ionic liquid conversion pretreatment on magnesium alloy for promoting corrosion resistance

Journal Pre-proof Hydrophobic epoxy resin coating with ionic liquid conversion pretreatment on magnesium alloy for promoting corrosion resistance Liting Guo, Changdong Gu, Jie Feng, Yongbin Guo, Yuan Jin, Jiangping Tu

PII:

S1005-0302(19)30314-7

DOI:

https://doi.org/10.1016/j.jmst.2019.06.024

Reference:

JMST 1719

To appear in: Received Date:

19 May 2019

Revised Date:

24 June 2019

Accepted Date:

28 June 2019

Please cite this article as: Guo L, Gu C, Feng J, Guo Y, Jin Y, Tu J, Hydrophobic epoxy resin coating with ionic liquid conversion pretreatment on magnesium alloy for promoting corrosion resistance, Journal of Materials Science and amp; Technology (2019), doi: https://doi.org/10.1016/j.jmst.2019.06.024

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.

Research Article

Hydrophobic epoxy resin coating with ionic liquid conversion pretreatment on magnesium alloy for promoting corrosion resistance

Liting Guo1,2, Changdong Gu1,2,*, Jie Feng3, Yongbin Guo4, Yuan Jin4, Jiangping Tu1,2

1

School of Materials Science and Engineering, State Key Laboratory of Silicon Materials,

2

ro of

Zhejiang University, Hangzhou 310027, China

Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou 310027, China

College of Materials Science & Engineering, Zhejiang University of Technology, Hangzhou 310014, China Zotye Automobile Co. Ltd, Yongkang 321300, China

re

4

-p

3

lP

[Received 19 May 2019; Received in revised form 24 June 2019; Accepted 28 June 2019]

* Corresponding author. Ph.D.; Tel.: +86 571 87952573. E-mail address: [email protected]

na

Gu).

(C.D.

ur

Abstract

Jo

A hydrophobic epoxy resin coating with an environmental-friendly deep eutectic solvent (DES)-based conversion pretreatment was proposed to enhance the corrosion resistance of magnesium alloys. The hydrophobic epoxy resin coatings on the AZ31B magnesium alloy with and without the DES-based conversion pretreatment were thoroughly compared. It is found that the DES-based conversion film on the AZ31B magnesium alloy is mainly composed of MgH2, MgO and MgCO3. Furthermore, the conversion film possesses porous structure, which provides more anchor points for the following epoxy resin coating. However, without the DES-conversion 1

pretreatment, the epoxy resin is difficult to be attached on the substrate during the dip-coating process. The double layered hybrid coating system promotes the corrosion resistance of the magnesium alloys significantly, which can be ascribed to the unique architecture and component including the hydrophobicity of the surface layer, the dense and interlocked epoxy resin, and the corrosion resistant DES-based conversion pretreatment.

ro of

Keywords: Mg alloy; Deep eutectic solvent; Corrosion resistance; Double layered hybrid coating

1. Introduction

Magnesium and its alloys, having low density, high strength-to-weight ratio and easy

recyclability, are highly appropriate engineering materials in some fields such as aerospace,

-p

automobile industry and infrastructure[1-7]. However, magnesium exhibits almost no corrosion

re

resistance in aqueous environments, limiting its wide applications[8-13]. Researchers have developed several surface treatments to improve the corrosion resistance of the magnesium alloys, including

lP

electro- or electroless metal plating[14-16], anodizing[17-20], and chemical conversion coatings[21-24]. Chemical conversion coating process is widely used for corrosion protection due to its multifunction and facile operation. The chromate conversion coatings[25] involving toxic hexavalent chromium

na

have been highly regulated. Therefore, non-hexavalent chromium-based conversion coatings are becoming more attractive and readily available at a commercial level, such as stannate coatings[26-28],

ur

rare earth coatings[29, 30], phosphate/permanganate coatings[31] and so on. However, most of conversion processes are limited in the aqueous solvents, which may lead to the hydrogen evolution

Jo

corrosion of the magnesium alloy substrate. It would be more promising to manipulate the conversion processes in non-aqueous media, such as ionic liquids (ILs)[32-35]. Deep eutectic solvents (DESs) are a new type of ionic liquid[36]. DESs have several advantages

over traditional ILs such as lower toxicity, easy availability and preparation[37-39]. We have demonstrated that conversion films can be formed on the magnesium alloys from the DESs via ionothermal strategy[40] and anodic method[41], respectively. However, the DES-based conversion film is very thin and porous, which only provides short-term corrosion protection for substrates in 2

NaCl aqueous solutions[42]. Therefore, in order to improve the corrosion resistance of the DES-based conversion films, the following organic coatings seems to be indispensable[43]. Most of coatings applying organic compound as solvents are not environment-friendly. Therefore, waterborne epoxy resin coatings have attracted much attention due to an excellent adhesive strength with substrates and a high surface hardness[44-50]. However, due to the native properties of waterborne resins, e.g., the presence of hydrophilic groups or surfactants, coatings based on such resins generally suffer from poor water resistance, resulting in poor corrosion resistance. Therefore, a hydrophobic organic coating using a waterborne epoxy resin emulsion should be a good candidate to cover the DES-based

ro of

conversion films on the magnesium alloys. Herein, a double layered hybrid coating system with a hydrophobic organic top layer and a porous DES-based inorganic conversion film as the bottom layer was proposed on the magnesium alloy substrate to improve the corrosion resistance of the substrate. The DES-based conversion film

-p

as the bottom layer in the coating system was fabricated by immersing the substrate in the choline chloride-urea DES heated at about 160 °C. Then the organic top layer was obtained by a facile

re

dip-coating process from a waterborne epoxy resin emulsion with an additive of silane coupling agent. The microstructure and corrosion resistance of the hybrid coatings were investigated in

lP

details.

2.1. Materials

na

2. Experimental

ur

AZ31B magnesium alloys with a chemical composition (95.7% Mg, ~2.9% Al, ~0.8% Zn, ~0.6% Mn) was used as substrates. The choline chloride (ChCl, [HOC2H4N(CH3)3+Cl-], AR, ≥98.0%) and

Jo

urea ([CO(NH2)2], AR, ≥99.0%) were purchased from Aladdin. The DES was prepared by stirring the mixture of ChCl and urea in a mole ratio of 1:2 (ChCl:urea) at 80 °C until a homogeneous colorless liquid was formed. Waterborne epoxy emulsion (EP-20) and its curing agent (HGA-50), were all purchased from Zhejiang Anbang New Material Ltd. 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FAS-17) was purchased from Sicong New Material Ltd (Quanzhou, China). 3

2.2. Preparation of double layered hybrid coating The preparation of the double layered hybrid coating on magnesium alloys is sketched in Fig. 1, which involves a DES-based conversion pretreatment and the following hydrophobic epoxy resin coating process. The AZ31B magnesium substrates were cut to the size of 30 mm × 17 mm × 1 mm. The samples were abraded using SiC abrasive paper up to 1000 grit, and then washed in an acetone, then dried in air. The DES-based conversion film was obtained by immersing the Mg plates in the beaker (100 ml) containing DES (15 ml) at 160 °C for various time (10, 20, 30 and 60 min). The

ro of

conversion samples were rinsed in methanol and de-ionized water, and then dried in flowing nitrogen. A simple and efficient dip-coating process was proposed to fabricate a hydrophobic epoxy resin coating as top layer. Firstly, 1 ml FAS-17 was first dispersed in a mixture of 60 mL deionized water and 10 mL ethanol by magnetic stirring for 120 min, and then epoxy resin (7 g) and curing agent (2 g)

-p

were added into the mixture liquid and stirred for 10 min. The conversion samples were withdrawn from the above coating solution at a constant rate of 40 cm/min for several cycles. Finally, the coated

re

samples were heated at 120 °C for 1 h. A control sample was also fabricated by dip-coating the hydrophobic epoxy resin coating on the bare AZ31B magnesium alloy without DES-based

lP

conversion pretreatment. Hereafter, the hydrophobic epoxy resin coating on the bare AZ31B magnesium alloy with and without DES-based conversion pretreatment were denoted as double

2.3. Characterization

na

layered hybrid coating and simple epoxy resin coating, respectively.

ur

Morphology and chemical composition of the DES-based conversion film and the hybrid coating were characterized by a field emission scanning electron microscope (FE-SEM, Hitachi

Jo

SU-70) with energy dispersive X-ray spectrometer (EDS) attachment. The cross section of sample was abraded using SiC abrasive paper up to 2000 grit, and then polished with diamond paste. Finally, the sample was washed in acetone and dried in air. X-ray diffraction (XRD, XPert Pro-MPD with CuKα radiation, λ=0.15406 nm) and X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD) using monochromatic AlKα radiation with E=1486.6 eV after Ar ion sputtering were detected to analyze chemical component. Fourier transform infrared spectroscopy (FT-IR, NicoletiS50) was acquired using KBr pellets. The wettability of samples was characterized by measuring the contact 4

angle (CA). The CA value was determined by a contact angle meter (OCA 20, Dataphysics) based on a sessile drop measuring method with a water droplet volume of 4 μL. An electronic analytical balance (0.1 mg) was used to measure the mass change of samples after the treatment. 2.4 Electrochemical characterization Potentiodynamic polarization measurement was carried out in the 3.5 wt% NaCl (analytical grade reagent, purity ~99.5%) aqueous solution to evaluate the performance of the samples. Polarization curves were recorded by the three-electrode cell (Chenhua Instruments Inc., China) with

ro of

Pt as counter electrode, Ag/AgCl (saturated KCl) as reference electrode and samples (1 cm2 exposed area) as working electrode. Each specimen was immersed in the solution for at least 15 min to

establish the stable OCP before test. Then the experiments were conducted at a scan rate of 1 mV s-1.

-p

Electrochemical impedance spectroscopy measurements were performed at room temperature in potentiostatic mode with the frequency ranges between 100 kHz and 10 mHz. Samples were

re

immersed in the NaCl solution for 15 min before test. The EIS plots were analyzed using ZSimpWin software based on the appropriate equivalent circuit model. For exploring the long-term corrosion

lP

resistance of the samples, a total immersion test was carried out in a 3.5 wt% NaCl aqueous solution at room temperature for a certain time. The corroded surface was visually inspected and recorded by

na

camera as a function of the immersion time (0 min, 10 min, 2 h, 4 h, 32 h, and 120 h). 3. Results and discussion

ur

3.1. Microstructure of the DES-based conversion film

Jo

Fig. 2 displays the pretreatment time dependent surface morphology of the AZ31B substrates in the DES-based conversion film process. As shown in Fig. 2, the surface morphology of the substrate is highly related to the pretreatment time. At the beginning of the reaction, the substrate becomes rough and some holes are formed on the surface. The density of the holes on the conversion film increases with the pretreatment time. The size of the holes is about 0.1-0.2 μm at the pretreatment time of 30 min. Then the size becomes larger with a range of 0.2-0.5 μm at the reaction time of 60 min. The coral-like morphology of the conversion films should be resulted from the substantial 5

reactions between substrates and DES. Upon heating the DES can react with the AZ31B magnesium alloy surface to form the porous conversion film[40]. A typical cross-sectional SEM image merged with EDS linear analysis of the DES-based conversion film with reaction time of 60 min is given in Fig. 3. As can be seen from the Fig. 3(a), it is indicated that the film thickness is about 2 μm, which can also be confirmed by the element profile as given in Fig. 3(b). The contents of C, O and Mg fluctuate significantly in the region of conversion film (Fig. 3(b)). The composition of C and O increases slightly in the porous layer. From the analysis

ro of

of the scanning line, the DES-based conversion film is mainly composed of Mg, Al, O, and C. 3.2. Crystallography and surface chemical component of the conversion film

XRD patterns corresponding to the bare AZ31B magnesium alloy substrate and the DES-based

-p

conversion film with a reaction time of 60 min are presented in Fig. 4, respectively. The XRD pattern of the bare AZ31B magnesium alloy substrate is similar with the pure Mg, which might be attributed

re

the low content of second phase in the alloy. For the DES-based conversion film, the peaks corresponding to the substrate can be clearly observed, which confirms that the conversion film is

lP

very thin as given in Fig. 3. From the XRD patterns as shown in Fig. 4, it can be inferred that the film is composed of MgH2 (JCPDS 12-0697), which is in accordance with the previous study[42]. The XPS analysis was carried out to investigate the surface chemistry of the DES-based

na

conversion film with a reaction time of 60 min. The XPS survey data including the high resolution XPS spectra for Mg 2p, C 1s, O 1s, Al 2p and Zn 2p is shown in Fig. 5. A deconvolution procedure

ur

including Gaussian profile and a Shirley background is utilized to understand the bonding state of Mg 2p, O 1s and C 1s. The Mg 2p spectrum for conversion film could be resolved into two spectra

Jo

having peaks at 49.7 and 49 eV, which has been attributed to the formation of MgCO3 and MgO, respectively. The C 1s spectrum shows three peaks at 289, 286.1 and 284.6 eV[51], which is corresponding to CO32-, C-N/C-O, C-C components, respectively. It indicates that some decomposition products of DES could react with the substrate to form the conversion films. Similarly, the O 1s spectrum is also consisted of two peaks at 531.8 and 530.8 eV[52], which confirms the presence of MgCO3 and MgO in the conversion film. The Al 2p and Zn 2p spectrums indicate the existence of Al2O3 and Zn[53], which should be originated from the substrate. With the analysis of 6

XRD and XPS, it suggests that the conversion film should be mainly composed of MgH2, MgO and MgCO3. According to the XRD and XPS analysis, the DES-based conversion coating should be a composite. The surface of the coating may be mainly composed of amorphous MgO and MgCO3. The Mg hydride should be embedded in the coatings. The reaction between DES with AZ31B magnesium alloy is complicated[37, 40]. Combining with the previous study[42], we propose that the Mg alloy reacts with the decomposition products of urea, thus leading to the formation of MgH2, MgO and MgCO3 on the substrates.

ro of

3.3. Microstructure of the hydrophobic epoxy resin coating Fig. 6 demonstrates the mass change of the AZ31B magnesium alloy sample with and without DES-pretreatment during the dip-coating cycles. The mass of the bare substrate after two cycles of

-p

dip-coating process is only increased about 0.79%. However, the mass augment of the DES-based conversion film after two cycles of dip-coating process is about 2.04%, which indicates that rough

re

film could absorb more resin due to syphonage effect of nano-porous structure in the conversion film. After eight dip-coating cycles, the mass of the conversion film is increased 6.37%, which is about

lP

twice as the AZ-EP sample.

Fig. 7 displays the surface morphology of the AZ31B magnesium alloy sample with and without DES-pretreatment by one cycle of dip-coating process. In Fig. 7(a), abrasive tracks can be observed

na

on the surface of the bare AZ31B magnesium alloy substrate. After one dip-coating cycle, abrasive tracks can also be observed on the surface, as shown in Fig. 7(b). Clearly, the epoxy resin is difficult

ur

to be adhered to the surface of bare AZ31B magnesium alloy substrate. In contrast, as shown in Fig. 7(c) and (d), the surface of DES-based conversion film is uniformly covered by the epoxy resin after

Jo

one dip-coating cycle. The original porous structure in the DES-conversion film disappears after one dip-coating cycle of epoxy resin, which indicates that the conversion pretreatment process benefit for the adhesion of organic coatings. 3.4. Wettability of the hydrophobic epoxy resin coating Fig. 8 shows the geometry of about 4 μL water droplet on the hydrophobic epoxy resin coating. As can be seen in Fig. 8a and b, both the bare and the DES-pretreatment AZ31B magnesium alloy 7

are lyophilic to the water with CA values smaller than 90°. After dip-coating process, the CA values of both samples exceed 100°, which indicate the hydrophobic epoxy coatings had been formed successfully on both substrates. As shown in Fig. 8(c), the CA value for simple epoxy resin coating is gradually increased with the dip-coating cycles, which is lowest after dip-coating one cycle because the surface of bare AZ31B magnesium alloy is not completely covered by the epoxy resin. However, in Fig. 8(d), the CA values for double layered hybrid coating seem to be independent on the dip-coating cycle, which should be attributed to formation of the smooth organic coating after one dip-coating cycle. After one cycles, all samples are all hydrophobic to the water, which is essential

ro of

for improving the corrosion resistance. With increasing the dip-coating cycles, the CA value for hydrophobic epoxy resin coating is almost constant in the region of 100°-110°.

-p

3.5. Surface chemical component for the hydrophobic epoxy resin coating

Fourier transform infrared (FTIR) spectra was used to analyse the chemical composition of

re

samples. Fig. 9 shows the FTIR spectra for the bare AZ31B magnesium alloy substrate, the DES-based conversion film, and the double layered hybrid coating. The sharp band of 3685 may be

lP

ascribed to N-H group and the broad band between 3478 and 3275 cm-1 corresponds to stretching vibrations of -OH groups[49, 54]. The absorption bands at ~586, ~1005 and ~1120 cm-1 are undoubtedly assigned to MgH2, MgO and MgCO3[40], which is corresponding to previous analysis.

na

The peaks at ~1605 and ~1500 cm-1 characteristically belong to skeleton vibrations of benzene ring in epoxy resin main chain[49]. The band of epoxy groups is visible in the range of 950 cm-1 to 810

ur

cm-1, which belongs to the characteristic bands of epoxy radical[55]. The characteristic peak of Si–O–Si is also at ~1110 cm-1[56], and peaks of C-F is ~1290 cm-1[49]. From Fig. 9, it can be

Jo

concluded that epoxy resin has been successfully deposited on the DES-based conversion film. 3.6. Corrosion resistance measurements Fig. 10 shows the potentiodynamic polarization curves for various samples in a 3.5 wt% NaCl aqueous solution. The corrosion parameters of the samples are summarized in Table 1. As shown in Fig. 10(a), the comparison of potentiodynamic polarization curves of the DES-based conversion film with different reaction time is presented. It is clear that conversion films show better corrosion 8

resistance than the bare AZ31B magnesium alloy substrate. Moreover, the corrosion resistance of conversion films is highly depended on the pretreatment reaction time. The DES-based conversion film with reaction time of 60 min shows the best corrosion resistance with the corrosion current density (icorr= 92 μA/cm2). In Fig. 10b, it is found that the corrosion resistance of the double layered hybrid coating (icorr= 0.19 μA/cm2) is much higher than the epoxy resin coating without the DES-pretreatment process (icorr= 1.14 μA/cm2), which indicates that the DES-based conversion pretreatment process is beneficial for the improvement of corrosion resistance of the organic coating. The Nyquist plots in Fig. 11(a) show that the bare AZ31 magnesium alloy substrate is

ro of

characterized by a capacitive loop in the medium frequency range, and an inductive loop in the low frequency range. The capacitive loop can be attributed to the charge transfer process, whereas the inductive loop is related to the dissolution of Mg[43]. The low frequency inductive loop of the substrates could be attributed to the initiation of localized corrosion[57]. For the DES-based

-p

conversion film, a similar evolution process was observed, which is be related to the thickness of the film. However, a larger diameter of the capacitive loop is detected, which means a lower corrosion

re

rate. The capacitive loop of epoxy resin coating samples is larger than the bare AZ31 magnesium alloy substrate and DES-based conversion film. The double layered hybrid coating shows the largest

lP

capacitive loop, reached values approximately 104 Ω cm2, which means the lowest corrosion rate. In Fig. 11(b), it can be seen that the values of |Z| of the epoxy resin coating samples are higher than the

na

bare and DES-based conversion film. In Bode plot (Fig. 11(c)), the phase angle of the conversion film is approaching -62°, instead the double layered hybrid coating is approaching -80°, which suggests that a more stable and compact composite coating has been formed on the substrate[58].

ur

However, the phase angle of double layered hybrid coating is not -90o, which confirms the coating is not very compact. Therefore, the corrosion could happen on the some defective points of the surfaces

Jo

of coatings, which explains the inductive loop of the double layered hybrid coating. The equivalent circuits for fitting the EIS data are given in Fig. 11(d) and (e), which

corresponding to the DES-conversion film and the double layered hybrid coating, respectively. Herein, R1 and CPE1 represent the resistance and capacitance of charge transfer resistance and double layer capacitance. R2 and CPE2 are assigned to the resistance and capacitance of epoxy resin coating layer. The values of n which vary between 0 and 1 are account for the deviation from the ideal capacitive behavior due to roughness factors. RL and L are related to the inductive loops in the 9

EIS spectra[59]. The equivalent circuits shown in Fig. 11(d) and (e) were used for fitting EIS data (Fig. 11(a)-(c)). It is found that the proposed equivalent circuits are good fit for the experimental data for most of the frequency range, confirming the validity of the equivalent circuits. The fitting data obtained by Zview 3.1c software is demonstrated in Table 2. As discussed above, the value of n is related to the coating roughness. It suggests that the surface of the bare AZ31 magnesium alloy is the smoothest, and DES-based conversion film is the roughest. The double layered hybrid coating has both a low capacitance (1.97×10-7 Sn Ω-1 cm-2) and a high resistance (4.04×104 Ω cm2), which is higher than that of the bare AZ31B magnesium alloy. It suggests that double layered hybrid coating

ro of

is important for corrosion resistance improvement, which is agreement with the results of the potentiodynamic polarization testing.

The optical photographs of the various samples in a 3.5 wt% NaCl solution are summarized in Table 3, which shows the surface changes as a function of the immersion time. The bare AZ31B

-p

magnesium alloy substrate is covered by corrosion products after being immersed for 10 min, and many pits appear on its surface. With the immersion time increasing to about 4 h, the surface is

re

gradually corroded and completely covered by corrosion products. However, the DES-based conversion film has not been corroded until for 2 h, which indicates the conversion film has a certain

lP

corrosion protection. The epoxy resin coatings show better corrosion resistance no mater whether there is DES-conversion film on the magnesium alloys or not. Furthermore, the double layered

na

hybrid coating has not been corroded until 120 h, which indicates there is a significant improvement compared with conversion film. However, the epoxy resin coating without the DES-conversion pretreatment has a few corrosion pits for 32 h. It demonstrates the conversion film is helpful in the

ur

corrosion resistance improvement.

Jo

3.7. Corrosion mechanism of the double layered hybrid coating On the basis of the experimental results presented, the following corrosion mechanism is

proposed for the double layered hybrid coating. When the bare AZ31 magnesium alloy or DES-based conversion film is immersed in NaCl aqueous solution, Cl- can interact either directly with the bare AZ31 magnesium alloy, or penetrate porous layer of the conversion film with corrosion occurring[60]. Our results indicate that the double layered hybrid coating, i.e. the hydrophobic epoxy resin coating 10

with the DES-conversion pretreatment is a promising strategy for improving the corrosion resistance of magnesium alloy, which might be ascribed to the following considerations. First, hydrophobic epoxy resin coating is capable of trapping a large amount of air and prevents Cl- from contacting with the substrate, which provides first corrosion barrier for the substrate. Second, the epoxy resin coating is smooth and dense, as verified by a high value of resistance (R2), which can effectively prevent Cl- penetrating the coating once the hydrophobicity is collapsed in the corrosion medium. Lastly, the bottom layer of the DES-conversion film in the double layered hybrid coating is also corrosion resistant and the porous structure of the conversion film provides stronger adhesion and

ro of

more reservoir for the top layer of the hydrophobic epoxy resin. 4. Conclusion

-p

A hydrophobic epoxy resin coating with a deep eutectic solvent (DES)-based conversion

pretreatment was proposed to enhance the corrosion resistance of magnesium alloys. The surface morphology of the DES-conversion film was porous, which significantly relied on the reaction time.

re

The size of holes in the conversion film was in a range of 0.2-0.5 μm for a reaction time of 60 min.

lP

The thickness of the DES-based conversion film was about 2 μm. XRD, FTIR and XPS analysis suggested that the conversion film was mainly composed of MgH2, MgO and MgCO3. It was found that the porous conversion pretreatment was ready to trap more waterborne epoxy resins during the

na

dip-coating process and provide strong bonding interface. Corrosion testing indicated that the double layered hybrid coating system could promote the corrosion resistance of the magnesium alloy

ur

significantly, which should be attributed to the hydrophobicity of the surface layer, the dense and

Jo

adhesive epoxy resin, and the corrosion resistant DES-based conversion pretreatment.

Acknowledgments

This work was supported financially by the Natural Science Foundation of Zhejiang Province

(No. LY19B030008) and the National Key Research and Development Program of China (No. 2016YFF0204300).

11

References [1] G.L. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11-33. [2] Q.H. Yuan, X.S. Zeng, Y. Liu, L. Luo, J.B. Wu, Y.C. Wang, G.H. Zhou, Carbon 96 (2016) 843-855. [3] W.Q. Xu, N. Birbilis, G. Sha, Y. Wang, J.E. Daniels, Y. Xiao, M. Ferry, Nat. Mater. 14 (2015) 1229-1235. [4] X.B. Chen, N. Birbilis, T.B. Abbott, Corrosion 67 (2011) 1-16. [5] L. Mao, L. Shen, J.H. Chen, Y. Wu, M. Kwak, Y. Lu, Q. Xue, J. Pei, L. Zhang, G.Y. Yuan, R. Fan, J.B. Ge, W.J. Ding, ACS Appl. Mater. Interf. 7 (2015) 5320-5330.

ro of

[6] G.S. Frankel, Nat. Mater. 14 (2015) 1189-1190.

[7] L.P. Wu, C.G. Wang, D.B. Pokharel, I.I.N. Etim, L. Zhao, J.H. Dong, W. Ke, N. Chen, J. Mater. Sci. Technol. 34 (2018) 2084-2090.

-p

[8] M. Esmaily, J.E. Svensson, S. Fajardo, N. Birbilis, G.S. Frankel, S. Virtanen, R. Arrabal, S. Thomas, L.G. Johansson, Prog. Mater. Sci. 89 (2017) 92-193.

re

[9] F.Y. Cao, G.L. Song, A. Atrens, Corros. Sci. 111 (2016) 835-845.

[10] S. Heise, S. Virtanen, A.R. Boccaccini, J. Biomed. Mater. Res. Part A 104 (2016) 2628-2641.

lP

[11] H.K. Singh, K.V. Yeole, S.T. Mhaske, Chem. Eng. J. 295 (2016) 414-426. [12] C.G. Wang, L.P. Wu, F. Xue, R.Y. Ma, I.I.N. Etim, X.H. Hao, J.H. Dong, W. Ke, J. Mater. Sci. Technol. 34 (2018) 1876-1884.

na

[13] X.P. Lu, Y. Li, P.F. Ju, Y. Chen, J.S. Yang, K. Qian, T. Zhang, F.H. Wang, Corros. Sci. 148 (2019) 264-271. [14] A. Bakkar, V. Neubert, Electrochem. Commun. 9 (2007) 2428-2435.

ur

[15] J. Zhang, Z.H. Xie, H. Chen, C. Hu, L.X. Li, B.N. Hu, Z.W. Song, D.L. Yan, G. Yu, Surf. Coat. Technol. 342 (2018) 178-189.

Jo

[16] L.P. Wu, Z.D. Yang, G.W. Qin, J. Alloys Compd. 694 (2017) 1133-1139. [17] G.L. Song, Z.M. Shi, Corros. Sci. 85 (2014) 126-140. [18] C. Blawert, W. Dietzel, E. Ghali, G. Song, Adv. Eng. Mater. 8 (2006) 511-533. [19] X.P. Lu, C. Blawert, D. Tolnai, T. Subroto, K.U. Kainer, T. Zhang, F.H. Wang, M.L. Zheludkevich, Corros. Sci. 139 (2018) 395-402. [20] X.P. Lu, C. Blawert, Y.D. Huang, H. Ovri, M.L. Zheludkevich, K.U. Kainer, Electrochim. Acta 187 (2016) 20-33. 12

[21] G.L. Song, Adv. Eng. Mater. 7 (2005) 563-586. [22] G. Song, A. Atrens, Adv. Eng. Mater. 9 (2007) 177-183. [23] M. Dabala, K. Brunelli, E. Napolitani, M. Magrini, Surf. Coat. Technol. 172 (2003) 227-232. [24] F. Zhang, C.L. Zhang, L. Song, R.C. Zeng, S.Q. Li, H.Z. Cui, J. Mater. Sci. Technol. 31 (2015) 1139-1143. [25] Q.J. Meng, G.S. Frankel, Surf. Interf. Anal. 36 (2004) 30-42. [26] L. Anicai, R. Masi, M. Santamaria, F. Di Quarto, Corros. Sci. 47 (2005) 2883-2900. [27] C.S. Lin, H.C. Lin, K.M. Lin, W.C. Lai, Corros. Sci., 48 (2006) 93-109. [28] H.H. Elsentriecy, K. Azumi, H. Konno, Electrochim. Acta 53 (2007) 1006-1012.

[30] X. Jiang, R.G. Guo, S.Q. Jiang, Appl. Surf. Sci. 341 (2015) 166-174.

ro of

[29] T. Takenaka, Y. Narazaki, N. Uesaka, M. Kawakami, Mater. Trans. 49 (2008) 1071-1076.

[31] M. Zhao, S.S. Wu, J.R. Luo, Y. Fukuda, H. Nakae, Surf. Coat. Technol. 200 (2006) 5407-5412.

[32] J.A. Latham, P.C. Howlett, D.R. MacFarlane, M. Forsyth, Electrochem. Commun. 19 (2012) 90-92.

-p

[33] P.C. Howlett, J. Efthimiadis, P. Hale, G.A. van Riessen, D.R. MacFarlane, M. Forsyth, J. Electrochem. Soc. 157 (2010) C392-C398.

re

[34] P.C. Howlett, W. Neil, T. Khoo, J.Z. Sun, M. Forsyth, D.R. MacFarlane, Isr. J. Chem. 48 (2008) 313-318. [35] M. Forsyth, P.C. Howlett, S.K. Tan, D.R. MacFarlane, N. Birbilis, Electrochem. Solid State Lett. 9 (2006)

lP

B52-B55.

[36] E.L. Smith, A.P. Abbott, K.S. Ryder, Chem. Rev. 114 (2014) 11060-11082.

na

[37] X. Ge, C.D. Gu, X.L. Wang, J.P. Tu, J. Mater. Chem. A 5 (2017) 8209-8229. [38] D.E. Crawford, L.A. Wright, S.L. James, A.P. Abbott, Chem. Commun. 52 (2016) 4215-4218. [39] L.T. Guo, C. Gu, J. Tu, Surf. Technol. 48 (2019) 10-18.

ur

[40] C.D. Gu, W. Yan, J.L. Zhang, J.P. Tu, Corros. Sci. 106 (2016) 108-116. [41] C.D. Gu, X.J. Xu, J.P. Tu, J. Phys. Chem. C 114 (2010) 13614-13619.

Jo

[42] L.T. Guo, C. Gu, J. Zhang, X. Wang, K. Wang, Y. Jin, J. Tu, Surf. Coat. Technol. 357 (2019) 833-840. [43] X.J. Cui, X.Z. Lin, C.H. Liu, R.S. Yang, X.W. Zheng, M. Gong, Corros. Sci. 90 (2015) 402-412. [44] I.A. Kartsonakis, A.C. Balaskas, E.P. Koumoulos, C.A. Charitidis, G. Kordas, Prog. Org. Coat. 76 (2013) 459-470. [45] J.Y. Hu, Q. Li, X.K. Zhong, L. Zhang, B. Chen, Prog. Org. Coat. 66 (2009) 199-205. [46] A. Adhikari, R. Karpoormath, S. Radha, S.K. Singh, R. Mutthukannan, G. Bharate, M. Vijayan, High Perform. Polym. 30 (2018) 181-191. 13

[47] J.C. Cabanelas, S.G. Prolongo, B. Serrano, J. Bravo, J. Baselga, J. Mater. Process. Technol. 143 (2003) 311-315. [48] M. Shon, H. Kwon, Corros. Sci. 49 (2007) 4259-4275. [49] J. Zhang, W.C. Zhang, J.J. Lu, C.X. Zhu, W.Q. Lin, J. Feng, Prog. Org. Coat. 121 (2018) 201-208. [50] M.L. Yang, J.H. Wu, D.Q. Fang, B. Li, Y. Yang, J. Mater. Sci. Technol. 34 (2018) 2464-2471. [51] D.K. Aswal, K.P. Muthe, S. Tawde, S. Chodhury, N. Bagkar, A. Singh, S.K. Gupta, J.V. Yakhmi, J. Cryst. Growth 236 (2002) 661-666. [52] K. Nakamura, M. Tsunakawa, Y. Shimada, A. Serizawa, T. Ishizaki, Surf. Coat. Technol. 328 (2017) 436-443.

ro of

[53] X. Xiang, X.T. Zu, S. Zhu, C.F. Zhang, L.M. Wang, Nucl. Instrum. Methods Phys. Res. Sect. B 250 (2006) 192-195.

[54] W. Wysocka, A. Przybyl, G. Wojciechowski, B. Brzezinski, J. Mol. Struct. 516 (2000) 157-160. [55] T. Jin, Y.Q. Han, R.Q. Bai, X. Liu, J. Nanosci. Nanotechnol. 18 (2018) 4971-4981.

-p

[56] W.G. Ji, J.M. Hu, J.Q. Zhang, C.N. Cao, Corrosion Sci. 48 (2006) 3731-3739.

[57] C.N. Cao, J.Q. Zhang, An Introduction of Electrochemical Impedance Spectroscopy, Science Press, Beijing,

re

2002. (in Chinese)

[58] Z.B. Wang, H.X. Hu, C.B. Liu, Y.G. Zheng, Electrochim. Acta 135 (2014) 526-535.

lP

[59] M. Hatami, M. Yeganeh, A. Keyvani, M. Saremi, R. Naderi, J. Solid State Electrochem. 21 (2017) 777-785.

Jo

ur

na

[60] Z.W. Wang, Q. Li, Z.X. She, F.N. Chen, L.Q. Li, X.X. Zhang, P. Zhang, Appl. Surf. Sci. 271 (2013) 182-192.

14

ro of

Figure list:

Jo

ur

na

lP

re

-p

Fig. 1. Schematics of preparation of the double layered hybrid coating on magnesium alloy.

15

ro of -p re

lP

Fig. 2. Reaction time dependent surface morphologies of the AZ31B substrates during the DES-based conversion pretreatment: (a) DES-10 min; (b) DES-20 min; (c) DES-30 min; (d) DES-60

Jo

ur

na

min. Insets are the corresponding magnified images.

16

ro of

Fig. 3. Typical cross-sectional SEM image merged with EDS linear analysis of the DES-based

conversion film with a reaction time of 60 min: (a) cross-sectional SEM image; (b) EDS linear

Jo

ur

na

lP

re

-p

analysis.

17

Fig. 4. XRD patterns of (a) the bare AZ31B magnesium alloy substrate and (b) the DES-based conversion film with a reaction time of 60 min. Hexagonal magnesium phase (JCPDS 35-0821) and

Jo

ur

na

lP

re

-p

ro of

tetragonal MgH2 phase (JCPDS 12-0697) are detected in the DES-pretreatment sample.

Fig. 5. XPS core-level spectra of the DES-based conversion film with a reaction time of 60 min: (a) survey XPS spectra; (b) Mg 2p; (c) C 1s; (d) O 1s; (e) Al 2p; (f) Zn 2p. A deconvolution procedure 18

including Gaussian profile and a Shirley background is utilized to understand the bonding state of the

re

-p

ro of

elements.

Jo

ur

na

during the dip-coating cycles.

lP

Fig. 6. Mass change of the AZ31B magnesium alloy sample with and without DES-pretreatment

19

ro of -p re

lP

Fig. 7. Surface morphologies of the AZ31B magnesium alloy sample with and without DES-pretreatment by dip-coating method for one cycle: (a) the bare AZ31B magnesium alloy

na

substrate; (b) DES-based conversion film; (c) simple epoxy resin coating; (d) double layered hybrid

Jo

ur

coating.

20

ro of -p

re

Fig. 8. (a, b) Contact angles (CA) corresponding to the bare AZ31B magnesium alloy substrate and the DES-based conversion film, respectively and CA changes of the epoxy resin coatings on the

Jo

ur

na

function of dip-coating cycles.

lP

AZ31B magnesium alloy substrate with (c) and without (d) the DES-based conversion film as a

21

ro of -p

re

Fig. 9. FTIR spectra of the surfaces of the bare AZ31B magnesium alloy substrate, the DES-based

Jo

ur

na

lP

conversion film, and the double layered hybrid coating, respectively.

22

ro of

Fig. 10. Potentiodynamic polarization curves of samples measured in 3.5 wt% NaCl solution at room temperature: (a) the DES-based conversion films with different reaction times; (b) different coating systems in this study. Potentiodynamic polarization curve of the bare AZ31B magnesium alloy

Jo

ur

na

lP

re

-p

substrate was given in the figures for comparison

23

ro of -p re lP

na

Fig. 11. EIS and fitting results for the AZ31B magnesium alloy substrate, the DES-based conversion film, the epoxy resin coating with and without the DES-based conversion film: (a) Nyquist plots; (b)

ur

Bode plots of |Z| vs. frequency; (c) bode plots of phase angle vs. frequency; equivalent circuits of the EIS plots for (d) the bare AZ31B magnesium alloy substrate and the DES-based conversion film and

Jo

(e) epoxy resin coating samples.

24

Table list: Table 1 Corrosion parameters of various samples, which were derived from Fig. 10. Samples

Ecorr (V vs. Ag/AgCl)

icorr (μA/cm2)

Substrate

-1.52

725

DES-10 min

-1.50

219

βa: 205 βc: -64

DES-20 min

-1.48

192

βa: 189 βc: -90

DES-30 min

-1.50

147

βa: 176 βc: -125

DES-60 min (Conversion film)

-1.47

92

βa: 167 βc: -149

-1.45

1.14

βa: 131 βc: -100

-1.42

0.19

βa: 125 βc: -115

ro of

βa: 222 βc: -159

Jo

ur

na

lP

re

-p

Simple epoxy resin coating Double layered hybrid coating

Tafel slope (mV/dec)

25

Table 2 Fitting results of the various samples, which were derived from Fig. 11. Rs (Ω

CPE1 (Sn

cm2)

Ω-1 cm-2)

Substrate

9

1.35×10-5

Conversion film

4.9

Samples

R1

CPE2 (Sn

(Ω cm2)

Ω-1 cm-2)

0.99

191.2

-

1.85×10-5

0.94

255.3

3.8

8.26×10-8

0.81

1

5.57×10-8

0.80

Simple epoxy resin

n

R2

L (H

RL (Ω

Errors

(Ω cm2)

cm-2)

cm2)

(%)

-

-

107.7

68.01

<10

-

-

-

217.2

225

<6.47

329.6

2.17×10-7

0.949

1.60×104

1.39×104

1.64×104

<5.74

390.3

1.97×10-7

0.9197

4.04×104

3.07×104

3.48×104

<5.46

n

coating Double layered

Jo

ur

na

lP

re

-p

ro of

hybrid coating

26

Table 3 Summary of the corroded surfaces in a 3.5 wt% NaCl solution for various exposure times. Sample

0 min

10 min

2h

4h

32 h

120 h

Substrate

ro of

Conversion film

Simple epoxy resin coating

Jo

ur

na

lP

re

-p

Double layered hybrid coating

27