Fabrication and characterization of Mg-M layered double hydroxide films on anodized magnesium alloy AZ31

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Fabrication and characterization of Mg-M layered double hydroxide films on anodized magnesium alloy AZ31 Liang Wu a,b,∗ , Danni Yang c , Gen Zhang a,∗ , Zhi Zhang a , Sheng Zhang a , Aitao Tang a,b , Fusheng Pan a,b,∗ a

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Engineering Research Center for Magnesium alloys, Chongqing University, Chongqing 400044, China c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, Heilongjiang, China b

a r t i c l e

i n f o

Article history: Received 28 February 2017 Received in revised form 16 June 2017 Accepted 23 June 2017 Available online xxx Keywords: In-situ growth Sealing The LDH films Corrosion resistance Magnesium alloy

a b s t r a c t Highly oriented Mg-M layered double hydroxide (LDH) films were firstly fabricated by a facile in-situ growth method on anodized magnesium alloy AZ31. It is proved to be an effective method to seal the porous anodic oxide film. M presents metal cations, such as Al3+ , Cr3+ and Fe3+ . The characteristic structure, morphology and composition of the LDH films were investigated via field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), fourier transform infrared spectrometer (FT-IR) and energy dispersive spectrometer (EDS) respectively. Besides, the corrosion resistance of the LDH films on magnesium alloy was investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy. The result demonstrates that anodic oxide film can be sealed by the formation of LDHs with nano-container structures. The corrosion resistance of anodized magnesium alloy is remarkably improved by Mg-M LDHs, especially by Mg-Al LDHs. Finally, the protection mechanism of Mg-M LDHs was proposed. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The poor corrosion resistance of the magnesium alloy limits the viability of the increasing magnesium widespread application [1,3–7]. Whereas, magnesium alloys are of particular demand for the automobile, transport applications [4], mobile phones, aerospace and biomedicine [2,3] because of their low density, adequate mechanical properties [8–17]. At present, the main effective approach of enhancing the corrosion resistance of magnesium alloys is the surface treatment technology, among which the anodic oxidation is widely used [11,12], because of mature and facile process relatively. Nevertheless, on account of loose and porous structures of anodic films [18], the magnesium alloy substrate cannot be protected effectively once the film has contact with corrosion solutions. However, commercial application of Mg anodization to improve the corrosion resistance is still not well documented [19]. Thus, it is extremely urgent to find an effective sealing method of the anodic oxide film [20].

∗ Corresponding authors at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. E-mail addresses: [email protected] (L. Wu), [email protected] (G. Zhang), [email protected] (F. Pan).

Layered double hydroxides (LDHs) is considered to be an active corrosion system. LDHs is a promising class of typical twodimensional laminar nanomaterial [1,21], the general formula of which is [M2+ 1-x M3+ (OH)2 ]x+ (An− )x/n ·mH2 O, where the cations M2+ and M3+ occupy the octahedral holes in a brucite-like layer, and the metal anion An− is located in the hydrated interlayer galleries [22]. Such inorganic nano-container is widely used in the study of corrosion protection due to the advantage of small size, high loadings and easy modification. Moreover, the distinct characteristic of ion-exchange, that is, releasing the interlayer anion and adsorb Cl− at the same time when it comes to corrosive ions perceived in the environment. Consequently, the LDHs film growing on the anodized magnesium surface cannot only enhance the thickness of the protective film but also seal the porous anodic oxide layer effectively. Therefore, such structure possesses a synergistic effect to improve the corrosion resistance of the Mg alloy. Up to now, a two-step method is widely used, firstly, the LDH power precursor is synthesized by the co-precipitation method and then the film is obtained using a certain process. In recent studies, Zeng [23] adopted such method to prepare Zn-Al LDHs film on the surface of AZ31 by controlling the pH and temperature of the reaction solution, and demonstrated that the resulting film had the excellent adhesion with the magnesium substrate. In

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addition, the similar method was employed in manufacturing ZnAl LDH film on the AZ91 substrate [24]. As a result, the magnesium alloy was endowed with a better corrosion resistance according to the electrochemical test. However, it cannot be ignored that the co-precipitation method is rather cumbersome as well as uncontrollable. The in-situ growth LDH film can be obtained easily, and only one kind of metallic ion is needed. So it proves significant potential. Besides, Fuente [25] synthesized the Al-Zn-vanadate hydrotalcite on aluminum alloys using the co-precipitation and air-spraying method. Chen et al. [26–28] developed a new two-step technique to fabricate Mg-Al LDH on magnesium alloy AZ31. A precursor film with network cracks is first formed in the pretreatment solution and then this film is transformed into Mg-Al LDH after the post treatment. Furthermore, they optimized the experimental conditions and studied the growth mechanism. Moreover, Li [29,30] adopted the in-situ growth method to prepare LDH film on the anodic alumina substrate. In his study, the anodic alumina substrate was sealed by boiling water before and after respectively, and then the Zn-Al LDH film added vanadate corrosion inhibitor ions was fabricated. It was concluded that the LDH film with the corrosion inhibitor enhanced the corrosion resistance of the aluminum alloy. Kuznetsov [31] also adopted this similar novel approach to fabricated Zn-Al LDH on anodized aluminum alloy successfully. The anodized aluminum alloy was sealed by the formation of LDHs. Conceptually, the sealing of anodic layer using in situ growth LDH looks promising to provide an enhanced combined passive/active corrosion protection. A LDH treatment may seal the pores in the anodized layer (barrier effect), imparting at the same time active corrosion protection via release of corrosion inhibitor when aggressive species reach the pores [31]. By that analogy, the magnesium alloy, having the similar property with the aluminum alloy, also can be treated by such method to enhance the corrosion resistance and expand the usable range. However, there are few publications reporting on the formation of LDHs on anodized magnesium alloys. In this work, we developed a LDH-based sealing method for the anodized magnesium alloy AZ31. Three kinds of LDH films were fabricated directly using the magnesia of anodic films, and only one type of the M3+ cation(Al3+ , Cr3+ or Fe3+ ) was added in the reaction solution. Such one-step in-situ method is quite simple as well as convenient, also the technology proves stable. Besides, the characteristic of structure, morphology and composition of the LDH layers were investigated via FE-SEM, XRD, FT-IR and EDS. Also potentiodynamic polarization and EIS were used to study the corrosion resistance of the LDH layers on magnesium alloys. 2. Experimental 2.1. Materials The substrate material used in this study is AZ31 magnesium alloys (the nominal compositions in wt.%: Al 2.5–3.5, Zn 0.6–1.3, Mn 0.2–1, Ca 0.04, Si 0.1, Cu 0.05, and balanced Mg). Before the preparation of the LDH films, the samples were mechanically ground with SiC paper up to 2000 grit to ensure the same surface roughness, ultrasonically cleaned in ethyl alcohol, and then dried in cold air. After that above, the samples were anodized in an electrolyte containing 7.14 g/L NaOH and 4 g/L NaAlO2 for 30 min with applied voltage of 20 V to form anodic films. 2.2. Synthesis of Mg-M LDH The Mg-M LDH films were prepared by immersing the anodized samples in 0.05 M M(NO3 )3 (M = Al, Cr and Fe) and 0.3 M NH4 NO3 mixture solution with a pH value among the alkaline range by

adding the diluted ammonia. And the synthesis was carried out at 125 ◦ C for 12 h with the reaction solutions putting into the Teflonlined autoclave. After that, the filmed samples were rinsed with deionized water, ultrasonically cleaned in ethyl alcohol and dried under ambient conditions.

2.3. Characterization The surface microstructure, chemical composition as well as thickness of the films were observed via a field-emission scanning electronic microscope (FE-SEM, JSM-7800F, JEOL, Japan)equipped with an energy dispersive X-ray spectrometer (EDS). The accelerating voltage of SEM is 10 Kv. The whole samples used for the SEM observation were sputtered with platinum to guarantee excellent conductivity. In addition, crystallographic structures of the films were determined by an X-ray diffractometer(XRD, Rigaku D/Max 2500X, Rigaku, Japan) with a Cu target (␭ = 0.154 nm). The accelerating voltage of 40Kv and the current of 150 mA, and the patterns gained at a glancing angle of 2◦ . Fourier transform infrared (FT-IR, iS5 FT-IR, Nicolet, American) spectra were obtained in the range of 4000–400 cm−1 . Moreover, the corrosion resistance was investigated in 3.5 wt.% NaCl solution by potentiodynamic polarization curves and electrochemical impedance spectra (CIMPS-2 Zahner, Germany). A conventional three-electrode system was applied, and it is consisted of a saturated calomel reference electrode (SCE), a platinum foil as counter electrode and the samples as working electrode with tested area of 1 cm2 . Meanwhile, the samples ought to be immerged in the solution for 20 min before the electrochemical tests. And the amplitude of the sinusoidal perturbation was 5 mV(vs.Eocp ), the polarization curves were recorded with a sweep rate of 2 mV/s. EIS measurements were acquired at the frequency range of 100 KHz to 10 mHz. Besides, three parallel samples were tested at any rate for each kind of condition to ensure the repeatability of results. Gravimetric measurements were performed by immersing in 3.5 wt.% NaCl solution at room temperature, and the size of the samples were 2 cm × 2 cm. Prior to the immersion tests, specimens were measured and weighed. After soaking in the solution for 336 h, the samples were extracted, rinsed with ethyl alcohol, dried in the air, and then weighed again in order to calculate the mass loss per unit surface area.

3. Results and discussion 3.1. Characterization of Mg-M LDH films Fig. 1 shows scanning electron microscope (SEM) images at lower and higher magnifications of the LDH film, where M presents the metal cations of Fe3+ , Cr3+ and Al3+ , from which the blade-like surface can be seen obviously [32,33], and it proves the result of the combination between metal hydroxide and the inner ions. At the same time, the SEM images indicate that the types of metal cations have greatly influence on the morphology of the samples. Mg-Al LDH “blade” is flat relatively, while Mg-Fe LDH and Mg-Cr LDH are rather close to curly petals. Besides, the Mg-Cr LDH nanosheet is small, which shows that the crystallinity of Mg-Al LDH is higher than that of Mg-Cr LDH. Furthermore, compared with the images at lower and higher magnification, the porous structures of anodic oxide films were sealed effectively by the formation of LDH layer as shown in Fig. 1. And the energy dispersive spectrometer (EDS) results shown in Fig. 2 demonstrate that the metal cations Fe3+ , Cr3+ and Al3+ take part in the formation of the film indeed. Also, Fig. 3 shows the cross-sectional view of the films, and the thickness of the anodic oxide film is almost 14 ␮m. After the growth

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Fig 1. SEM surface morphology images at low and higher magnifications of the anodized and Mg-M LDHs, (a) and (b) the anodized magnesium alloy, (c) and (d) the Mg-Fe LDHs, (e) and (f) the Mg-Cr LDHs, (g) and (h) the Mg-Al LDHs.

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Fig. 2. The EDS spectra of the films, (a)the anodized magnesium alloy, (b) the Mg-Fe LDHs, (c) the Mg-Cr LDHs, (d) the Mg-Al LDHs.

of LDHs, the thicknesses of the films almost remain unchanged or increase a bit. The glancing angle X-ray diffraction (GAXRD) patterns of the samples are depicted in Fig. 4. According to the XRD patterns of the Mg-Fe, Mg −Cr and Mg-Al LDH films on anodized magnesium alloys, a typical layered structure characteristic of the Mg-M LDH film with identical peaks are in accordance with the (003) and (006) reflections of the LDH phase, while none of obvious peaks were found from the patterns of the magnesium alloy or anodized magnesium alloy when the 2␪ is below 20◦ . Therefore, it indicates that the LDHs are indeed obtained by means of the method above. Also, the sharp Mg-M LDH film typical crystal plane diffraction peaks of (003) and (006) demonstrate that the obtained LDHs possess integrated layered structure and excellent crystal structure. This result is consistent well with the published values for Mg-M LDH films [34]. Fig. 5 shows the FT-IR spectra of the anodized AZ31 and Mg-M LDH with different metal cations. As observed, bands corresponding to the H O H stretching vibration are at approximately 3695 cm−1 , a strong and broad absorption band centered on 3384 cm−1 can be ascribed to the hydroxyl stretching band(-OH), arising from metal hydroxyl groups and hydrogen bonded interlayer water molecules, so the result can prove that the LDH films are fabricated on the surface of the substrate. Another absorption band recorded at about 1641 cm−1 is attributed to the hydroxyl deformation mode of water (H2 O) [35]. The band at 1366 cm−1 is assigned to the stretching vibration of interlayer NO3− , but the band around 1370 cm−1 for the anodized substrate is CO3 2−[3] , because of CO2 in the air during preparing the samples. In addition, the lower wave number bands at 400–700 cm−1 are mainly due to the M-O and M-O-M and O-M-O vibration modes [36]. Consequently,

Table 1 The corrosion potential (Ecorr ), corrosion current density (Icorr ) of all the samples. Treatment process

Ecorr (V/SCE)

Icorr (␮A/cm2 )

ba (V/dec)

-bc (V/dec)

AZ31 substrate anodized AZ31 Mg-Al LDH coating Mg-Fe LDH coating Mg-Cr LDH coating

−1.51 ± 0.24 −1.48 ± 0.36 −1.34 ± 0.23 −1.44 ± 0.14 −1.47 ± 0.36

32.68 ± 1.36 4.698 ± 0.56 0.1178 ± 0.36 1.087 ± 0.59 2.162 ± 0.42

0.25 ± 0.03 0.26 ± 0.03 0.33 ± 0.05 0.45 ± 0.01 0.36 ± 0.07

0.16 ± 0.04 0.18 ± 0.03 0.16 ± 0.01 0.16 ± 0.01 0.16 ± 0.06

on the basis of above analysis of the FT-IR spectrum, such in-situ growth method where the metal cations M2+ are provided by the anodized magnesium could successfully generate Mg-M LDH films on magnesium alloy AZ31. 3.2. Corrosion resistance of the LDH films Potentiodynamic polarization curves were applied to investigate the corrosion resistance of the LDH film, the anodized magnesium as well as the magnesium alloy substrate. The results of that are shown in Fig. 6. At the same time, Table 1 lists the corrosion potential (Ecorr), the corrosion current density (Icorr) and open circuit potential (OCP) of Mg-M LDH films prepared with different type of the cations M3+ . Ecorr values of the bare magnesium alloy and the anodized magnesium alloy are −1.51 V vs. SCE and −1.48 V vs. SCE, respectively. And the samples that of the Mg-Cr, Mg-Fe and Mg-Al LDH films exhibit high values ranging from −1.47 V vs. SCE, −1.44 V vs. SCE to −1.34 V vs. SCE, which proves more positive than that of both the substrate and the anodized magnesium alloy. In addition, as for Icorr, the magnesium alloy and the anodized one keep 3.27 × 10−5 A/cm−2 and 4.698 × 10−6 A/ cm−2 respectively. Mean-

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Fig. 3. The the cross-sectional view of the film, (a) the anodized magnesium alloy, (b) the Mg-Fe LDHs, (c) the Mg-Cr LDHs, (d) the Mg-Al LDHs.

Fig. 5. The FT-IR spectra of the anodized magnesium alloy and Mg-M LDHs. Fig. 4. The GAXRD patterns of the magnesium alloy substrate, the anodized one, Mg-Al LDHs, Mg-Cr LDHs and Mg-Fe LDHs.

while, the values of the Mg-Cr, Mg-Fe and Mg-Al LDH samples are 2.16 × 10−6 A/cm−2 , 1.09 × 10−6 A/cm−2 and 1.18 × 10−7 A/cm−2 respectively. Icorr values for the LDH-coated samples (Mg-Al) decrease by two orders of magnitude compared with that of the magnesium alloy substrate. A reasonable explanation for above result, on one hand, is that the LDH films enable corrosion ions to be blocked due to that the holes of anodic oxide films are sealed totally. On the other hand, this LDH-based film is the storage and release of

inhibitor on demand as a result of anion-exchange between inhibiting species and corrosion-relevant anions such as chloride and/or hydroxyl anions. EIS measurements were employed to further characterize the corrosion inhibition effect of the Mg-M LDH films on the anodized AZ31. Fig. 7a, b display the typical Bode diagrams, and Fig. 7c presents the equivalent circuit used for LDH films. It can be seen that the Mg-M LDH films have two time constants, the same with the anodized magnesium alloys. It can be explained that the whole film

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capacitor and resistor of inner layer of anodic oxide film, respectively. As shown the fitting results in Table 2, the LDHs grown on the anodized magnesium alloy own a higher values of Rout and Rinn , compared with that of the anodized magnesium alloy. The increases of Rout and Rinn can attributed to LDH-sealing effect and the inner andodic layer with more compact structure. Hence, these results indicate that the corrosion resistance of anodized magnesium alloy is improved effectively by the in-situ growth of LDH films. The LDH film on the anodized magnesium alloy possesses a higher value of Rout , which shows the growth of LDHs not only occurs in the surface but also in the pores of the anodic oxide film. Thus, these pores can be sealed gradually during the growth of LDHs, and the LDH crystal will grow throughout the outer layer of anodic oxide film and reach a steady state at last. Also, the similar phenomenon have been found before [35].

3.3. Protection ability of the LDH film Fig. 6. Tafel polarization curves in 3.5 wt.% NaCl solution of the bare magnesium alloy, the anodized magnesium alloy, the Mg-M LDH films on magnesium alloy.

is consisted of the inner layer of the thin anodic film and the outer layer of the LDH film. Generally speaking, a higher frequency corresponds to the outer layer, and the lower frequency corresponds to the inner layer. As we all known, the material with a higher Z modulus at lower frequencies exhibit better corrosion resistance on the metal substrate [30,32]. Thus, the samples coated LDH possesses better corrosion resistance, which are consistent with the result of the Tafel polarization. Fig. 7c is the equivalent circuit of the samples coated LDH to analyze in depth the contribution of the layers in terms of corrosion resistance. And Rsol presents the resistance of solution; Cout and Rout represent the capacitor and resistor of the outer LDH layer or porous layer of anodic oxide film; meanwhile, Cinn and Rinn present

The coated samples were immersed in 3.5 wt.% NaCl solution after 336 h. Fig. 8 presents the SEM morphologies of the initial MgM LDH film as well as the immersed sample, where the majority of the region of the sample coated LDH still keep the structure of nano-plates on different levels. The nanostructures of Mg-Al LDH film are the best preserved during the immersion for 366 h. Nevertheless, the size of the nano-plates reduced after immersion, and it is probably due to the dissolution of the nano-plates. Moreover, it can be tested that the initial LDH film is mainly composed of Mg, M(M = Fe3+ , Cr3+ and Al3+ ), and O elements without Cl signals detected as shown in Fig. 2, while Cl signals were observed on the same sample after immersion as shown in Fig. 9. Therefore, the above results illustrate that Cl− from NaCl in the solution can be captured on account of the ion-exchange ability of the LDH films, meanwhile, Cl− can be stored in the interlayer of the LDHs.

Fig. 7. The typical Bode diagrams (a, b) and the equivalent circuit for LDH film(c).

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Fig. 8. The SEM morphologies of the Mg-M LDH film before and after immersion for 366 h:(a) and (b) the anodized magnesium alloy, (c) and (d) the Mg-Fe LDHs, (e) and (f) the Mg-Cr LDHs, (g) and (h) the Mg-Al LDHs.

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Table 2 The corresponding main fitted parameters of all the samples.

The samples The anodized AZ31 Mg-Fe LDH film Mg-Cr LDH film Mg-Al LDH film

Rsol  cm2 8.21 18.72 8.87 15.23

CPEout Cout/␮F cm−2 ␣out −5

1.17 × 10 4.80 × 10−7 2.61 × 10−7 4.38 × 10−7

0.77 0.59 0.61 0.54

Rout cm2 4610 2.13 × 104 6.66 × 103 2.87 × 104

CPEinn Cout /␮Fcm−2 ␣inn −4

1.10 × 10 4.81 × 10−5 1.32 × 10−6 6.97 × 10−5

0.12 0.76 0.54 0.84

Rinn  cm2 1639 3.20 × 104 2.17 × 104 5.68 × 104

Fig. 9. The EDS spectra of the films after immersion for 366 h, (a)the anodized magnesium alloy, (b) the Mg-Fe LDHs, (c) the Mg-Cr LDHs, (d) the Mg-Al LDHs.

Fig. 10. The corrosion weight loss of the films after immersion for 366 h, (a)the anodized magnesium alloy, (b) the Mg-Fe LDHs, (c) the Mg-Cr LDHs, (d) the Mg-Al LDHs.

Moreover, as shown in Fig. 10, the specimens were immersed in the 3.5 wt.% NaCl solution for 336 h to illustrate the long-term protectiveness of the surface film. It is clear that the specimens lost weight to some extent, and the anodized substrate, Mg-Fe, Mg-Cr and Mg-Al LDH films were about 11 mg, 7 mg, 3 mg and 1 mg respectively. Therefore, on one hand, it demonstrates that the Mg-Al LDH films have the best anticorrosion capacity among the samples, which is in accordance with the above results; on the other hand, it can be illustrated that the LDH films can provide a long time protection for sealing the loose and porous structures of anodic films. According to the result of the immersion experiment, the protection mechanisms can be concluded. First of all, the improvement in the corrosion resistance of LDH film can mostly be ascribed to the adsorption and retention of Cl− and the release of NO3 − because of the ability of ion exchange. Tedim et al. [36] reported that nitratecontaining LDH is effective chloride nano-traps so that it can delay the initiation of corrosion and the degradation of film. Secondly, based on the ion-exchange process, the released NO3 − ions concentrated on the surface of film, leading to the formation of a diffusion boundary layer containing high concentrations of NO3 − ions. The dissolved Mg2+ could form Mg(OH)2 under alkaline con-

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Fig. 11. The proposed corrosion protection mechanism model of the LDH film.

ditions. The formation of Mg(OH)2 can inhibit the expansion and spread of the pitting corrosion [1]. Further, the LDH film seals the porous anodic oxide film excellently and exhibits a strong chemical stability. Thus, the LDH film and anodic oxide film can maintain their integrity to protect the substrate from corrosion ions effectively even after long time immersion. Though charge transfer between the metal and the electrolyte solution occurs along with metal corrosion, LDH film and anodic oxide film could act as “fences” between the substrate and corrosion solution. So the composite films play a role of hindering the transfer of charge to inhibit the occurrence of corrosion. Thus, a corrosion protection mechanism model of the LDH is proposed, and the model is applied to illustrate the mechanisms of ion-exchange, synergistic inhibition and compactness. In the model (Fig. 11), four layers from top to bottom are the diffusion boundary layer, the LDH film, the anodic oxide layer and the AZ31 substrate. In conclusion, the anti-corrosion ability of the LDH film perhaps is due to the mechanism of ion-exchange, deposition of Mg(OH)2 and synergistic protection with anodic oxide film. 4. Conclusion In this study, three kinds of evenly and compact Mg-M LDH films have been successfully fabricated on the anodized magnesium alloys via a facial new in-situ growth method. First, the microstructure of films proves compact with nano-plates oriented nearly perpendicular to the substrate surface, and the anodic oxide film with loose and porous structures on the magnesium alloy is completely sealed. Besides, the types of metal cations applied to fabricate the films exhibit great effect on the morphologies of the LDH. Also, the prepared Mg-M LDH revealed that it markedly protects the magnesium alloy substrate against corrosion. The anticorrosion ability of the LDH film perhaps is due to the mechanism of ion-exchange, deposition of Mg(OH)2 and synergistic protection with anodic oxide film. Acknowledgements We acknowledge the financial support from the Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0388), the Fundamental Research Funds for the Central Universities (No. 106112017CDJXY130002), the National Key Research and Development Program of China (No.2016YFB0301101), the National Natural Science Foundation

of China (Project 51531002, 51474043), and Chongqing Municipal Government (CSTC2013JCYJC60001, CEC project, Two River Scholar Project and The Chief Scientist Studio Project).

References [1] F. Zhang, Z.G. Liu, R.C. Zeng, et al., Corrosion resistance of Mg–Al-LDH coating on magnesium alloy AZ31, Surf. Coat. Technol. 258 (2014) 1152–1158. [2] M. Hasan, L. Begum, On numerical modeling of low-head direct chill ingot caster for magnesium alloy AZ31, J. Magnes. Alloys 2 (4) (2014) 275–286. [3] J.K. Lin, J.Y. Uan, C.P. Wu, et al., Direct growth of oriented Mg–Fe layered double hydroxide (LDH) on pure Mg substrates and in vitro corrosion and cell adhesion testing of LDH-coated Mg samples, J. Mater. Chem. 21 (13) (2011) 5011–5020. [4] M. Kiani, I. Gandikota, M. Rais-Rohani, et al., Design of lightweight magnesium car body structure under crash and vibration constraints, J. Magnes. Alloys 2 (2) (2014) 99–108. [5] A. Atrens, G.L. Song, F. Cao, et al., Advances in Mg corrosion and research suggestions, J. Magnes. Alloys 1 (3) (2013) 177–200. [6] F.S. Pan, X.H. Chen, T. Yan, et al., A novel approach to melt purification of magnesium alloys, J. Magnes. Alloys 4 (1) (2016) 8–14. [7] L. Yang, Q. Jiang, M. Zheng, et al., Corrosion behavior of Mg–8Li–3Zn–Al alloy in neutral 3. 5% NaCl solution, J. Magnes. Alloys 4 (1) (2016) 22–26. [8] G.L. Song, A. Atrens, Corrosion mechanisms of magnesium alloys, Adv. Eng. Mater. 1 (1) (1999) 11–33. [9] A. Atrens, M. Liu, N.I.Z. Abidin, Corrosion mechanism applicable to biodegradable magnesium implants, Mater. Sci. Eng.: B 176 (20) (2011) 1609–1636. [10] C. Taltavull, Z. Shi, B. Torres, et al., Influence of the chloride ion concentration on the corrosion of high-purity Mg, ZE41 and AZ91 in buffered Hank’s solution, J. Mater. Sci. 25 (2) (2014) 329–345. [11] A. Atrens, G.L. Song, M. Liu, et al., Review of recent developments in the field of magnesium corrosion, Adv. Eng. Mater. 17 (4) (2015) 400–453. [12] H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys–a review, Acta Biomater. 8 (7) (2012) 2442–2455. [13] N. Hort, Y. Huang, D. Fechner, et al., Magnesium alloys as implant materials–principles of property design for Mg–RE alloys, Acta Biomater. 6 (5) (2010) 1714–1725. [14] J. Kuhlmann, I. Bartsch, E. Willbold, et al., Fast escape of hydrogen from gas cavities around corroding magnesium implants, Acta Biomater. 9 (10) (2013) 8714–8721 (13). [15] H. Waizy, A. Weizbauer, M. Maibaum, et al., Biomechanical characterisation of a degradable magnesium-based (MgCa0. 8) screw, J. Mater. Sci. 23 (3) (2012) 649–655. [16] Z. Abidin, N. Ishida, D. Martin, et al., Magnesium corrosion in different solutions, Mater. Sci. Forum 690 (2011) 369–372. [17] X.B. Chen, N. Birbilis, T.B. Abbott, Review of corrosion-resistant conversion coatings for magnesium and its alloys, Corrosion 67 (3) (2011) 1–16. [18] C. Blawert, W. Dietzel, E. Ghali, et al., Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Adv. Eng. Mater. 8 (6) (2006) 511–533. [19] C. Blawert, W. Dietzel, E. Ghali, et al., Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Adv. Eng. Mater. 8 (6) (2006) 511–533.

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[20] Y. Huang, W. Gan, K.U. Kainer, et al., Role of multi-microalloying by rare earth elements in ductilization of magnesium alloys, J. Magnes. Alloys 2 (1) (2014) 1–7. [21] D. Scarpellini, C. Falconi, P. Gaudio, et al., Morphology of Zn/Al layered double hydroxide nanosheets grown onto aluminum thin films, Microelectron. Eng. 126 (2014) 129–133. [22] Y. Dong, F. Wang, Q. Zhou, Protective behaviors of 2-mercaptobenzothiazole intercalated Zn–Al-layered double hydroxide coating, J. Coat. Technol. Res. 11 (5) (2014) 793–803. [23] R.C. Zeng, X.T. Li, Z.G. Liu, et al., Corrosion resistance of Zn–Al layered double hydroxide/poly (lactic acid) composite coating on magnesium alloy AZ31, Front. Mater. Sci. 9 (4) (2015) 355–365. [24] F. Wu, J. Liang, Z. Peng, et al., Electrochemical deposition and characterization of Zn-Al layered double hydroxides (LDHs) films on magnesium alloy, Appl. Surf. Sci. 313 (2014) 834–840. [25] J.M. Vega, N. Granizo, D. De La Fuente, et al., Corrosion inhibition of aluminum by coatings formulated with Al–Zn–vanadate hydrotalcite, Prog. Org. Coat. 70 (4) (2011) 213–219. [26] J. Chen, Y. Song, D. Shan, et al., In situ growth of Mg–Al hydrotalcite conversion film on AZ31 magnesium alloy, Corros. Sci. 53 (2011) 3281–3288. [27] J. Chen, Y. Song, D. Shan, et al., Study of the in situ growth mechanism of Mg–Al hydrotalcite conversion filmon AZ31 magnesium alloy, Corros. Sci. 63 (2012) 148–158. [28] J. Chen, Y. Song, D. Shan, et al., In situ growth process of Mg–Al hydrotalcite conversion film on AZ31 Mg alloy, J. Mater. Sci. Technol. 31 (2015) 384–390.

[29] Y.D. Li, S.M. Li, Y. Zhang, et al., Enhanced protective Zn–Al layered double hydroxide film fabricated on anodized 2198 aluminum alloy, J. Alloys Compd. 630 (2015) 29–36. [30] Y.D. Li, S.M. Li, Y. Zhang, et al., Fabrication of superhydrophobic layered double hydroxides films with different metal cations on anodized aluminum 2198 alloy, Mater. Lett. 142 (2015) 137–140. [31] B. Kuznetsov, M. Serdechnova, J. Tedim, et al., Sealing of tartaric sulfuric (TSA) anodized AA2024 with nanostructured LDH layers, RSC Adv. 6 (2016) 13942–13952. [32] X. Guo, S. Xu, L. Zhao, et al., One-step hydrothermal crystallization of a layered double hydroxide/alumina bilayer film on aluminum and its corrosion resistance properties, Langmuir 25 (17) (2009) 9894–9897. [33] A.J. Vreugdenhil, V.J. Gelling, M.E. Woods, et al., The role of crosslinkers in epoxy–amine crosslinked silicon sol–gel barrier protection coatings, Thin Solid Films 517 (2) (2008) 538–543. [34] M. Qian, A.M. Soutar, X.H. Tan, et al., Two-part epoxy-siloxane hybrid corrosion protection coatings for carbon steel, Thin Solid Films 517 (17) (2009) 5237–5242. [35] P. Ding, Z. Li, Q. Wang, et al., In situ growth of layered double hydroxide films under dynamic processes: Influence of metal cations, Mater. Lett. 77 (2012) 1–3. [36] J. Tedim, A. Kuznetsova, A.N. Salak, et al., Zn–Al layered double hydroxides as chloride nanotraps in active protective coatings, Corros. Sci. 55 (2012) 1–4.

Please cite this article in press as: L. Wu, et al., Fabrication and characterization of Mg-M layered double hydroxide films on anodized magnesium alloy AZ31, Appl. Surf. Sci. (2017), http://dx.doi.org/10.1016/j.apsusc.2017.06.244