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MAO coating on magnesium alloy
Applied Surface Science 463 (2019) 535–544
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Full Length Article
Effect of alloy cations on corrosion resistance of LDH/MAO coating on magnesium alloy
T
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Junfeng Chena,e, , Wenxin Lina,d, Shiyan Lianga, Linchi Zoub,d, Chen Wanga, Binshu Wanga, ⁎ Mufu Yanc, Xiping Cuic, a
School of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350116, China School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118, China c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China d Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China e State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute (LSMRI), Qingdao 266101 b
A R T I C LE I N FO
A B S T R A C T
Keywords: Alloy cations Magnesium alloys Layered double hydroxide Corrosion resistance
Micro-arc oxidation (MAO) can improve corrosion resistance of magnesium alloys, while the further promotion of corrosion resistance is significantly restricted due to the existence of pore and micro crack. In this study, three types of LDH/MAO coating were prepared via covering layered double hydroxide (LDH) on MAO pre-treated AZ31 alloy, and the effects of these three types of LDH on the corrosion resistance of AZ31 alloy were investigated for the first time. Detailed morphology and composition of the LDH/MAO coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Besides, the corrosion resistance of AZ31 alloy with different LDH/MAO coatings was also tested by electrochemical work station. The results from polarization curves and electrochemical impedance spectroscopy indicate that AZ31 alloy coated with Ni-LDH/MAO has the higher impedance modulus and lower corrosion current density than Zn-LDH/MAO and Al-LDH/MAO. The anti-corrosion property of AZ31 alloy covering Ni-LDH/MAO coating is remarkably enhanced during long-term immersion in 1 M NaCl solution, since Ni-LDH film growing on the original MAO coating can hinder the penetration of chloride ions and effectively adsorb a large number of NO3− to exchange Cl− in corrosion solution.
1. Introduction Owing to the excellent properties such as low density, high strength and high thermal conductivity, magnesium alloys are commonly used in different industries [1,2] {Bowen, 2014 #101}. Nevertheless, Mg alloy exist poor corrosion resistance due to their low standard potential, resulting in restrict its industrial applications [3,4]. Recently, surface modification as an effective strategy enhancing corrosion resistance of magnesium alloy attracts abundant researches, including ultrasonicassisted electroless plating coatings [5], stannate conversion coatings [6] and lanthanum conversion coatings [7]. These coatings are either harmful or high cost, which not suitable for large-scale promotion [8]. Micro-arc oxidation(MAO), commonly known as plasma electrolytic oxidation(PEO), is a novel and environmentally surface modification technique, which leads to produce a highly adherent oxide ceramic coatings on key engineering metals such as Al [9–11], Ti [12–14], Mg [15,16]. When the applied voltage exceeds the critical voltage during
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micro-arc oxidation process, creating the formation of instantaneous high temperature and high pressure on alloy surface, resulting in the oxide coating on the substrate surface is punctured and a large number of micro-arc discharges appear, which form a discharge channel. The complex electrochemical reactions are carried out in the channels, which leads to the formation of ceramic layer with substrate oxide as the main component. However, the inherent defects such as micropores and micro-cracks as the result of discharge breakdowns and gas evolution during the rapid solidification of molten oxides limit the corrosion protection of MAO in marine environment, since the chloride ion could easily reach the substrate through these defects [17].Cui et al. [18] proposed the low porosity of coating played a significant role in improving the corrosion resistance of MAO coating. Therefore, decreasing defects of MAO coating is benefit to its anti-corrosion. Combining MAO and other film techniques is an important method to reduce the inherent defects and enhance corrosion resistance of MAO [19,20]. The layered double hydroxide (LDH), which has outstanding
Corresponding authors at: School of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350116, China (J. Chen). E-mail address: [email protected] (J. Chen).
https://doi.org/10.1016/j.apsusc.2018.08.242 Received 22 March 2018; Received in revised form 12 July 2018; Accepted 28 August 2018 Available online 28 August 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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coating were cleaned using deionized water and dried in warm air.
adsorption and high corrosion resistance due to its unique structure and chemical composition, is consisted of positively charged brucite-like layers, interlayer anions and water molecules. In recent years, many researchers have studied LDH in depth by density functional theory [21,22]. The distinctive structure of LDH can absorb anions and release inhibitors in the corrosive environment, which can remarkably improve corrosion resistance of materials, but In general, the chemical formula 3+ x+ y− of LDH is [M2+ Ax/y·nH2O, where M2+ represent bivalent 1-x Mx (OH)2] 2+ cation, for instance Mg , Zn2+or Co2+, and M3+ is trivalent cation, such as Al3+ or Fe3+, and Ay− is an interlayer exchangeable anion [23]. So it is interesting to combine MAO with LDH to form coating, which can considerably increase the anti-corrosion. At present, many studies focus on the direct preparation of LDH on metals, but there are few reports about LDH/MAO coating. Dou et al. [24] found that Ni-AlLDH nano-flakes synthesized on the MAO of 6061 Al alloy by in situ method had high resistance to corrosion, which restrained corrosive agent to the metal substrate. Serdechnova et al. [25] reported that the Zn-Al-LDH/MAO coating containing corrosion inhibiting anions on surface of 2024 aluminum alloy exhibited a superior barrier properties as compared with the single MAO coating. The LDH synthesized in reaction solution with different alloy cations has diverse performance. However, at present researchers focused on the effect of single LDH film on the corrosion resistance of MAO, hardly any work about the influence of different types LDH on the morphology and corrosion properties of LDH/MAO coating on magnesium alloy has been published. Furthermore, the effect of different alloy cations on LDH/MAO coating and the interaction between LDH and MAO have not been reported. In this study, three types of alloy cations fabricating LDH film were prepared followed MAO pre-treatment on Mg alloy. The microstructure and phase composition of MAO/LDH coatings, and the influence mechanism of LDH/MAO coating produced by different cations on the corrosion resistance of Mg alloys was investigated.
2.4. Microstructure characterization The morphology and composition of MAO and LDH/MAO coating were characterized by scanning electron microscopy (ZERISS SUPRA 55) and X-ray diffraction (RIGAKU Miniflex600). X-ray photoelectron spectroscopy (VG ESCALAB 250) was used to measure the surface chemical composition of the coatings. 2.5. Electrochemical tests In order to evaluate the corrosion resistance of the MAO coating with or without LDH film, the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) of all coatings were test in 3.5%NaCl solution. The samples served as worked electrode with an exposed area of 100 mm2, saturated calomel electrode (SEC) as the reference, and platinum network as the counter electrode. Besides, EIS test was not performed until immersed in 3.5%NaCl solution for 2 days 3. Results 3.1. Phase composition of MAO and LDH/MAO coating Fig. 1 shows the X-ray diffraction patterns of MAO and LDH/MAO coating. The two diffraction peaks appear corresponding to the characteristic reflections of (0 0 3) and (0 0 6) after hydrothermal reaction, suggesting the formation of LDH film. It can be found that the diffraction peak intensity of LDH/MAO coating with various alloy cations exist difference. Meanwhile, the diffraction peaks of LDH in Zn-LDH/ MAO are not obvious, which indicates only a little Zn-LDH production compared with other LDH/MAO coating. The chemical states of surface elements on LDH/MAO coating were measured using XPS. As shown in Fig. 2, high-resolution spectra of Al 2p and Mg 2p can be obviously observed in Al-LDH/MAO coating. Fig. 2(c) shows that two spin-orbit doubles characteristic of Ni2+,
2. Experimental methods 2.1. Materials In this paper, the commercial AZ31 alloy sheets(mean composition 3.08 wt%Al, 0.75 wt%Zn and 0.15 wt%Mn) with a size of 60 mm × 15 mm × 3 mm sheet were ground and polished using 600, 800, 1000, 1500#SiC abrasive paper. They were cleaned by alcohol, rinsed in deionized water, and then dried in warm air for MAO treatment. 2.2. Pre-treatment the MAO coating on AZ31 Mg alloy The pretreated AZ31 alloy sheets and stainless steel as the anode and cathode respectively, were immersed in the electrolyte, which consists of 20 g/L−1 Na2SiO3, 10 g/L−1 Na3PO4, 2 g/L−1 NaOH and deionized water. Electrical parameters of MAO are the current density of 2 A/dm2, frequency of 800 Hz and duty cycle of 10%. The AZ31 alloy sheets were rinsed in deionized water and dried in warm air after MAO treatment. 2.3. Synthesis of LDH/MAO coating Three types of LDH were in situ grown on the surface of MAO coating in three types of solution adjusted the pH to 7 using NH3·H2O, respectively. The three types of solution are Al(NO3)3·9H2O(0.01 mol) and NH4NO3(0.06 mol)in 100 mL deionized water, Ni(NO3)2·H2O (0.001 mol) and NH4NO3(0.006 mol)in 100 mL deionized water, Zn (NO3)2·H2O(0.01 mol) and NH4NO3(0.06 mol)in 100 mL deionized water, respectively. The samples with MAO coating were immersed in the above three types of solution at 95 °C for 1 h, so that samples with different coatings obtained originally designed Al-LDH/MAO, Ni-LDH/ MAO and Zn-LDH/MAO. Subsequently, the samples with LDH/MAO
Fig. 1. XRD pattern of samples with different coatings. 536
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Fig. 2. XPS spectra of LDH/MAO coating with different cations. (a) Al2p of Al-LDH/MAO; (b) Mg1s of Al-LDH/MAO; (c) Ni2p of Ni-LDH/MAO (d) Mg1s of Ni-LDH/ MAO; (e) Zn2p of Zn-LDH/MAO; (f) Al2p of Zn-LDH/MAO.
Ni3+and two shakeup satellites [26,27], Ni3+participates in the formation of LDH as a trivalent cation. However, no obvious Zn 2p and Al 2p spectra were detected in the Zn-LDH/MAO, which reveals only little LDH grown on MAO coating, leading to the extremely low content of Zn and Al elements. The results from XRD and XPS indicate that LDH is successfully synthesized on MAO coating. Furthermore, the results clearly suggest that different alloy cations supplied by solution play different important role in LDH/MAO coating under the same conditions.
3.2. Corrosion state after long time immersion Fig. 3 depicts the macroscopic images of MAO and LDH/MAO coated onAZ31 Mg alloy samples after immersed in 1 M NaCl solution for different days. The surface of Al-LDH/MAO coating is seriously damaged, resulting in the exposure of bald substrate after 4 days immersion, as shown in Fig. 3(e), which suggest that has the worst corrosion resistance. After 5 days immersion, the local exfoliation state of Zn-LDH/MAO coating is similar as that of MAO coatings, but in detail the exfoliation area of Zn-LDH/MAO coating was slightly smaller than 537
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Fig. 3. Photographs of samples with different coatings immersed in 1 M NaCl solution for different time. (a–c) morphology of the sample only with MAO coating (a) 0d, (b) 4d, (c) 5d; (d–f) morphology of the sample with Al-LDH/MAO coating (d) 0d, (e) 4d, (f) 5d; (g–i) morphology of the sample with Zn-LDH/MAO coating (g) 0d, (h) 4d, (i) 5d; (j–m) morphology of the sample with Ni-LDH/MAO coating: (j) 0d, (k) 4d, (l) 5d, (m) 7d.
Fig. 4. SEM images of MAO coating before and after LDH treatment by different cations. (a) MAO coating, (b) Al-LDH/MAO coating, (c) Ni-LDH/MAO coating, (d) Zn-LDH/MAO coating.
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Fig. 5. the cross-section images of the coating, (a–c) MAO, (d–f)Al-LDH/MAO.
Fig. 6. High-resolution SEM image of MAO coating before and after LDH treatment by different cations. (a) MAO coating, (b) Al-LDH/MAO, (c) Ni-LDH/MAO and (d) Zn-LDH/MAO.
In general, a large number of similar volcano pores distribute on the surface of each sample. Interesting, the morphology of Zn-LDH/MAO and Ni-LDH/MAO coating is similar to MAO coating, but the morphology of Al-LDH/MAO coating exhibits obviously different (Fig. 4). Comparing with the other coatings, there are more cracks on the surface of sample with Al-LDH/MAO coating. These cracks significantly degrade the corrosion resistance of Al-LDH/MAO coating, because corrosion medium is easier to penetrate the substrate through these cracks. It may be that aluminum is amphoteric metal, the aluminum ions are acidic under experimental conditions and react with MgO to destroy the original MAO coating. The cross-section image can further verify this opinion that aluminum ions destroy the original MAO coating, and
that of MAO. While a little local exfoliation doesn’t appears on the surface of Ni-LDH/MAO coating until immered for 7 days. Above results suggest that the order of corrosion resistance of samples with different coatings is sample with Ni-LDH/MAO coating > sample with Zn-LDH/ MAO coating ≈ sample with MAO coating > sample with Al-LDH/ MAO coating.
4. Discussion It is well known that the corrosion performance of samples with different LDH/MAO coating is close with morphology of coating. The morphology of MAO and LDH/MAO coatings are shown in Fig. 4 and 6. 539
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Fig. 7. Mapping of elements for (a) Al-LDH/MAO, (b) Ni-LDH/MAO and (c) Zn-LDH/MAO.
The surface morphology of sample with Ni-LDH/MAO coating and its corresponding elements mapping of Mg, Ni, O and Si are shown in Fig. 6(c) and 7(b), respectively. Flake-like structure is not only observed in the smooth region, but also at the inherent defects of original MAO coating. Its morphology is similar to that of the reported LDH layer [24]. Ni-LDH film, which didn’t destroy the original MAO coating, is formed on the MAO coating, and then covers the pores and cracks effectively. This confirms that Ni-LDH/MAO coating exited abundant NO3−which exchange aggressive medium in corrosive environment effectively improves the corrosion resistance of the sample. Fig. 6(d) depicts the morphology of Zn-LDH/MAO coating, the local area of the coating forms the flake accumulation, while the other areas are basically the same as those of MAO. Combining the mapping of elements, Fig. 7(c) shows that Zn element almost clusters on the flakes, which also suggests that Zn cation only produces LDH film in the local region. The reason for this phenomenon is Zn and Mg element can only be divalent, which hard to form trivalent cation. But Mg alloy contains a small amount of Al elements, which form LDH with Zn through the discharge channel to the surface during MAO process. The effect of LDH including different cations on the original MAO coating was further verified by
Fig. 5 shows the cross-section morphology of MAO and Al-LDH/MAO coating. Comparing with MAO coating, the cross-section of Al-LDH/ MAO coating exists the phenomenon of obvious destruction and defect, which revealing the original MAO coating is destroyed after hydrothermal reaction containing Al cation. Hence, Al-LDH/MAO coating exhibits low corrosion resistance, which is consistent with the observation of corrosion state after long time immersion. Moreover, based on the high magnification images (Fig. 6), there is obvious difference between MAO and LDH/MAO coating, and the surface morphology of LDH film formed by different alloy cations is also apparently different. The granular products, which uniformly distributed on the surface of sample, can be observed on the surface of Al-LDH/MAO coating after hydrothermal reaction containing Al cation. Energy dispersive spectrometer (EDS) is used to analysis the elements distribution of Al-LDH/ MAO coating. As shown in Fig. 7(a), Mg, O, Si and Al element distribute on the surface of sample with Al-LDH/MAO coating. Mg and O mainly come from MgO, which produced in the MAO process, while Si element primarily originates from electrolyte. The uniform distribution of Al should be attributed to Al element participates in hydrothermal reaction to form the LDH film, which is consistent with the previous XRD. 540
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Fig. 8. the cross-sectional image of the coating, (a) MAO coating, (b) Al-LDH/MAO, (c) Ni-LDH/MAO, (d) Zn-LDH/MAO.
the rest of coatings, and its corrosion current density is approximately one order of magnitude higher than MAO coating. Because abundant cracks are produced during the Al-LDH treatment process as shown in Fig. 4(b), which leads to destroy of the original MAO coating and decrease of the corrosion protection. Impressively, the corrosion potential and corrosion current density of Zn-LDH/MAO coating is almost the same as that of MAO in accordance with observation of the Fig. 4. It is noteworthy that Ni-LDH/MAO coating has the lowest corrosion current density in all coatings, which indicates Ni-LDH coating obviously improve the corrosion resistance of Mg alloy with MAO coating. In this paper, Al-LDH/MAO coating has the worst anti-corrosion, the corrosion resistance of Zn-LDH/MAO is similar with the original MAO coating, while Ni-LDH/MAO coating performs excellent corrosion resistance. Therefore, the corrosion resistance of three kinds of composite coating just needs to compared Ni-LDH/MAO coating with other MAO coating on AZ31 Mg alloy. Not only the current density of Ni-LDH/MAO coated on AZ31 Mg alloy (3.2 × 10-7A) is smaller than other MAO coating on AZ31 Mg alloy, but also the anti-corrosion performs better in immersed corrosion test [19,28,29]. EIS curves can effectively reflect MAO and LDH/MAO coating corrosion performance. The Bode plots for MAO and LDH/MAO coating after immersing in 3.5 wt%NaCl for 2 days are shown in Fig. 10. Meanwhile, Fig. 10 also describes the fitting curves based on the corresponding equivalent circuits. It is found that two time constants appear in the Bode plots. The time constants of high and intermediate frequency correspond to the electrochemical response of the porous layer and the barrier layer of coating, respectively. The phase angle of Ni-LDH/MAO coating in high frequency bigger than the other coating, which indicates the porous layer of Ni-LDH/MAO coating effectively prevent the penetration of corrosive medium to the substrate. While the phase angle of Al-LDH/MAO is close to 0°, suggesting that porous layer has been completely penetrated by the corrosion medium after immersion for 2 days, and the corresponding electrochemical response of the barrier layer also decreases. The EIS curve of Zn-LDH/MAO coating
Fig. 9. Potentiodynamic polarization curves of the samples with different coatings in 3.5% NaCl solution.
cross-sectional observation. The cross-section of MAO and MAO/LDH coating are characterized. The thickness of the three kinds of MAO/LDH coating are similar to original MAO coating as shown in Fig. 8, which suggests that the ion has no considerably influence on the thickness of original coating in this work. Additionally, LDH is a extremely thin film growing on the original MAO coating. Hence, the relationship between the thickness and the anti-corrosion properties is not studied in this paper. On the other hand, in order to further detect the corrosion performance of LDH/MAO coating, potentiodynamic polarization test and EIS have been employed to evaluate the electrochemical properties of different coatings. The potentiodynamic polarization curves of the MAO coating with and without LDH treatment were depicted in Fig. 9. It can be seen that corrosion potential of Al-LDH/MAO coating is lower than 541
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Fig. 10. Bode diagram of the samples with different coatings after immersing in 3.5% NaCl solution for 2 days.
Fig. 11. Equivalent circuit of the samples with different coatings immersion in 3.5% NaCl solution.
the corrosion resistance of the sample. The impedance modulus of NiLDH/MAO coating at low frequency, which is one order of magnitude larger than that of Al-LDH/MAO coating, is highest compared with that of the other coating. In contrast, the Al-LDH/MAO coating has the lowest impedance modulus compared with the rest of coating. Additionally, in the sample with Zn-LDH/MAO coating, Zn-LDH formed just at local area, which improve the corrosion resistance is extremely limited. In order to evaluate the influent of alloy cations on corrosion resistance, the equivalent circuit was employed to fit the EIS dates. Equivalent circuit of all coatings after immersing in 3.5% NaCl for 2 days is shown in Fig. 11. MAO coating is usually divides to porous layer and barrier layer in the equivalent physical model [24]. Red LDH
Table 1 Fitting results of EIS plots of the samples with different coatings after immersing in 3.5% NaCl solution for 2 days. Sample
RpΩ cm2
RbΩ cm2
RctΩ cm2
χ2
MAO Al-LDH/MAO
11.99
1.33× 10 4
4828
1.52× 10−4
0.15
3.85× 103
1542
4.67× 10−4
Ni-LDH/MAO
59.31
4.65× 10 4
7273
3.80× 10−4
Zn-LDH/MAO
11.40
1.44× 10 4
4579
1.96× 10−4
is almost the same as that of MAO coating, which implies the corrosion performance of these two coatings is close to each other. Besides, it is well known that the higher Z modulus at lower frequencies, the better 542
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Fig. 12. the SEM images of the coating tested for adhesion, (a) low magnification and (b) high magnification of Al-LDH/MAO, (c) low magnification and (d) high magnification of Ni-LDH/MAO, (e) low magnification and (f) high magnification of Zn-LDH/MAO.
film covers the defects of porous layer. In the equivalent circuit, Rs represents solution resistance, and Rp represents porous layer resistance, and Rb represents barrier layer resistance, and Rct represents charge transfer resistance. Table 1 gives the results of fitting the electrochemical parameters of the equivalent circuit. Form Table 1,compared with MAO coating, the Rp and Rb values of Ni-LDH/ MAO obviously increased. Specially, Rp of Ni-LDH/MAO coating is 5 times that of MAO coating, while the increase of Rb is not obvious. It is because that due to special structure of LDH and anion-exchange property of LDH, Cl− in corrosion environment can be replaced by NO3− into the LDH film. Thereby, reducing the concentration of Cl− can effectively weaken the penetration and damage on MAO coating from active Cl− in NaCl solution. The surface is completely covered by Ni-LDH film in the sample with Ni-LDH/MAO coating as shown in Fig. 6(c), so abundant NO3− ions are superseded by Cl−, which results
in improving the corrosion resistance of the sample with Ni-LDH/MAO coating. However, both Rp and Rb of Al-LDH/MAO coating are lower than Rp and Rb of MAO coating, due to the destruction of original MAO coating, and them are 0.15 and 3852 respectively. The Rb and Rp of ZnLDH/MAO are basically the same as that of MAO coating. The above results suggest that anti-corrosion of MAO coating can be improved by forming Ni-LDH film, and Zn-LDH can’t effectively enhance corrosion resistance due to only growing in local surface area, but the corrosion resistance of the sample with Al-LDH/MAO coating decrease. The anticorrosion mechanism of LDH/MAO coating can be concluded. Firstly, LDH film forms on the MAO coating to seal its defects such as crack and pores, which can hinder the penetration of chloride ions into the matrix to improve protection of coating. Secondly, the increasing of corrosion resistance is attributed to the adsorption of chloride ions. Nitrate ions of LDH can be replaced chloride ions in the environment via the ability of
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ion exchange. LDH film containing nitrate synthesized on MAO of aluminum alloy improve effectively the barrier property because of the unique anion-exchange of LDH [24,30,31]. Strong adhesive of LDH to the MAO coating, which is beneficial to improve the corrosion resistance, is an important property that cannot be ignored. The adhesion strength of LDH was studied using Herein scratch adhesion test [24,32]. The SEM images of the coatings after testing show that LDH film of Al-LDH/MAO and Ni-LDH/MAO exhibit no obvious peeling off, as shown in Fig. 12, which clear suggests that LDH exist strong adherence. Although the Zn-LDH film isn’t directly scratched due to LDH film only forming at some local area, LDH film is also not peeled near the scratched area where still high stress exists.
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5. Conclusions In this study, the influence mechanism of LDH/MAO coating produced by different alloy cations on the corrosion resistance of Mg alloys was investigated, through microstructure, phase composition and electrochemical characterization. Alloy cations fabricating LDH play important role in the corrosion property of LDH/MAO coating on Mg alloy, since the different alloy cations result in different interaction between LDH and MAO. (1) Granular structure of Al-LDH was synthesized on the MAO film by hydrothermal reaction. However, the original MAO coating was destroyed, which results in the decrease of anti-corrosion resistance. (2) The Ni-LDH film was grown on MAO coating and numerous flakes structure completely covered the porous layer of MAO, so that Ni-LDH/MAO coating improves the corrosion performance of AZ31 alloy. (3) Zn-LDH film just grows at some local areas of the original MAO coating, and a part of defects of original MAO coating are repaired by Zn-LDH film. Therefore, the corrosion resistance of ZnLDH/MAO coating on AZ31 Mg alloy exhibits a little improvement. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 51501040, 51771064, U1537201 and 51871057), and Science Foundation of Education Department of Fujian Province (Grant No. JK2015030 and JA15341), and Science Foundation of Fujian Province (Grant No. STHJ-KF1702). References [1] P.K. Bowen, J. Drelich, J. Goldman, Magnesium in the murine artery: probing the products of corrosion, Acta Biomaterialia 10 (2014) 1475–1483. [2] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, Y.C. Xin, Z.Q. Zhang, Y. Liu, X.H. Chen, What is going on in magnesium alloys? J. Mater. Sci. Technol. (2017). [3] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G.E. Thompson, Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings, Corros. Sci. 50 (2008) 1744–1752. [4] S. Pommiers, J. Frayret, A. Castetbon, M. Potin-Gautier, Alternative conversion coatings to chromate for the protection of magnesium alloys, Corros. Sci. 84 (2014) 135–146. [5] Z.W. Zhang, Ultrasonic-assisted electroless Ni-P plating on dual phase Mg-Li alloy, J. Electrochem. Soc. 162 (2015) c64. [6] L. Yang, M. Zhang, J. Li, X. Yu, Z. Niu, Stannate conversion coatings on Mg-8Li alloy, J. Alloy. Compd. 471 (2009) 197–200. [7] D. Song, X. Jing, J. Wang, S. Lu, P. Yang, Y. Wang, M. Zhang, Microwave-assisted synthesis of lanthanum conversion coating on Mg-Li alloy and its corrosion resistance, Corros. Sci. 53 (2011) 3651–3656.
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Full Length Article
Effect of alloy cations on corrosion resistance of LDH/MAO coating on magnesium alloy
T
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Junfeng Chena,e, , Wenxin Lina,d, Shiyan Lianga, Linchi Zoub,d, Chen Wanga, Binshu Wanga, ⁎ Mufu Yanc, Xiping Cuic, a
School of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350116, China School of Materials Science and Engineering, Fujian University of Technology, Fuzhou 350118, China c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China d Key Laboratory of Eco-materials Advanced Technology (Fuzhou University), Fujian Province University, China e State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute (LSMRI), Qingdao 266101 b
A R T I C LE I N FO
A B S T R A C T
Keywords: Alloy cations Magnesium alloys Layered double hydroxide Corrosion resistance
Micro-arc oxidation (MAO) can improve corrosion resistance of magnesium alloys, while the further promotion of corrosion resistance is significantly restricted due to the existence of pore and micro crack. In this study, three types of LDH/MAO coating were prepared via covering layered double hydroxide (LDH) on MAO pre-treated AZ31 alloy, and the effects of these three types of LDH on the corrosion resistance of AZ31 alloy were investigated for the first time. Detailed morphology and composition of the LDH/MAO coatings were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Besides, the corrosion resistance of AZ31 alloy with different LDH/MAO coatings was also tested by electrochemical work station. The results from polarization curves and electrochemical impedance spectroscopy indicate that AZ31 alloy coated with Ni-LDH/MAO has the higher impedance modulus and lower corrosion current density than Zn-LDH/MAO and Al-LDH/MAO. The anti-corrosion property of AZ31 alloy covering Ni-LDH/MAO coating is remarkably enhanced during long-term immersion in 1 M NaCl solution, since Ni-LDH film growing on the original MAO coating can hinder the penetration of chloride ions and effectively adsorb a large number of NO3− to exchange Cl− in corrosion solution.
1. Introduction Owing to the excellent properties such as low density, high strength and high thermal conductivity, magnesium alloys are commonly used in different industries [1,2] {Bowen, 2014 #101}. Nevertheless, Mg alloy exist poor corrosion resistance due to their low standard potential, resulting in restrict its industrial applications [3,4]. Recently, surface modification as an effective strategy enhancing corrosion resistance of magnesium alloy attracts abundant researches, including ultrasonicassisted electroless plating coatings [5], stannate conversion coatings [6] and lanthanum conversion coatings [7]. These coatings are either harmful or high cost, which not suitable for large-scale promotion [8]. Micro-arc oxidation(MAO), commonly known as plasma electrolytic oxidation(PEO), is a novel and environmentally surface modification technique, which leads to produce a highly adherent oxide ceramic coatings on key engineering metals such as Al [9–11], Ti [12–14], Mg [15,16]. When the applied voltage exceeds the critical voltage during
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micro-arc oxidation process, creating the formation of instantaneous high temperature and high pressure on alloy surface, resulting in the oxide coating on the substrate surface is punctured and a large number of micro-arc discharges appear, which form a discharge channel. The complex electrochemical reactions are carried out in the channels, which leads to the formation of ceramic layer with substrate oxide as the main component. However, the inherent defects such as micropores and micro-cracks as the result of discharge breakdowns and gas evolution during the rapid solidification of molten oxides limit the corrosion protection of MAO in marine environment, since the chloride ion could easily reach the substrate through these defects [17].Cui et al. [18] proposed the low porosity of coating played a significant role in improving the corrosion resistance of MAO coating. Therefore, decreasing defects of MAO coating is benefit to its anti-corrosion. Combining MAO and other film techniques is an important method to reduce the inherent defects and enhance corrosion resistance of MAO [19,20]. The layered double hydroxide (LDH), which has outstanding
Corresponding authors at: School of Materials Science and Engineering, Fuzhou University, New Campus, Minhou, Fujian Province 350116, China (J. Chen). E-mail address: [email protected] (J. Chen).
https://doi.org/10.1016/j.apsusc.2018.08.242 Received 22 March 2018; Received in revised form 12 July 2018; Accepted 28 August 2018 Available online 28 August 2018 0169-4332/ © 2018 Published by Elsevier B.V.
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coating were cleaned using deionized water and dried in warm air.
adsorption and high corrosion resistance due to its unique structure and chemical composition, is consisted of positively charged brucite-like layers, interlayer anions and water molecules. In recent years, many researchers have studied LDH in depth by density functional theory [21,22]. The distinctive structure of LDH can absorb anions and release inhibitors in the corrosive environment, which can remarkably improve corrosion resistance of materials, but In general, the chemical formula 3+ x+ y− of LDH is [M2+ Ax/y·nH2O, where M2+ represent bivalent 1-x Mx (OH)2] 2+ cation, for instance Mg , Zn2+or Co2+, and M3+ is trivalent cation, such as Al3+ or Fe3+, and Ay− is an interlayer exchangeable anion [23]. So it is interesting to combine MAO with LDH to form coating, which can considerably increase the anti-corrosion. At present, many studies focus on the direct preparation of LDH on metals, but there are few reports about LDH/MAO coating. Dou et al. [24] found that Ni-AlLDH nano-flakes synthesized on the MAO of 6061 Al alloy by in situ method had high resistance to corrosion, which restrained corrosive agent to the metal substrate. Serdechnova et al. [25] reported that the Zn-Al-LDH/MAO coating containing corrosion inhibiting anions on surface of 2024 aluminum alloy exhibited a superior barrier properties as compared with the single MAO coating. The LDH synthesized in reaction solution with different alloy cations has diverse performance. However, at present researchers focused on the effect of single LDH film on the corrosion resistance of MAO, hardly any work about the influence of different types LDH on the morphology and corrosion properties of LDH/MAO coating on magnesium alloy has been published. Furthermore, the effect of different alloy cations on LDH/MAO coating and the interaction between LDH and MAO have not been reported. In this study, three types of alloy cations fabricating LDH film were prepared followed MAO pre-treatment on Mg alloy. The microstructure and phase composition of MAO/LDH coatings, and the influence mechanism of LDH/MAO coating produced by different cations on the corrosion resistance of Mg alloys was investigated.
2.4. Microstructure characterization The morphology and composition of MAO and LDH/MAO coating were characterized by scanning electron microscopy (ZERISS SUPRA 55) and X-ray diffraction (RIGAKU Miniflex600). X-ray photoelectron spectroscopy (VG ESCALAB 250) was used to measure the surface chemical composition of the coatings. 2.5. Electrochemical tests In order to evaluate the corrosion resistance of the MAO coating with or without LDH film, the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) of all coatings were test in 3.5%NaCl solution. The samples served as worked electrode with an exposed area of 100 mm2, saturated calomel electrode (SEC) as the reference, and platinum network as the counter electrode. Besides, EIS test was not performed until immersed in 3.5%NaCl solution for 2 days 3. Results 3.1. Phase composition of MAO and LDH/MAO coating Fig. 1 shows the X-ray diffraction patterns of MAO and LDH/MAO coating. The two diffraction peaks appear corresponding to the characteristic reflections of (0 0 3) and (0 0 6) after hydrothermal reaction, suggesting the formation of LDH film. It can be found that the diffraction peak intensity of LDH/MAO coating with various alloy cations exist difference. Meanwhile, the diffraction peaks of LDH in Zn-LDH/ MAO are not obvious, which indicates only a little Zn-LDH production compared with other LDH/MAO coating. The chemical states of surface elements on LDH/MAO coating were measured using XPS. As shown in Fig. 2, high-resolution spectra of Al 2p and Mg 2p can be obviously observed in Al-LDH/MAO coating. Fig. 2(c) shows that two spin-orbit doubles characteristic of Ni2+,
2. Experimental methods 2.1. Materials In this paper, the commercial AZ31 alloy sheets(mean composition 3.08 wt%Al, 0.75 wt%Zn and 0.15 wt%Mn) with a size of 60 mm × 15 mm × 3 mm sheet were ground and polished using 600, 800, 1000, 1500#SiC abrasive paper. They were cleaned by alcohol, rinsed in deionized water, and then dried in warm air for MAO treatment. 2.2. Pre-treatment the MAO coating on AZ31 Mg alloy The pretreated AZ31 alloy sheets and stainless steel as the anode and cathode respectively, were immersed in the electrolyte, which consists of 20 g/L−1 Na2SiO3, 10 g/L−1 Na3PO4, 2 g/L−1 NaOH and deionized water. Electrical parameters of MAO are the current density of 2 A/dm2, frequency of 800 Hz and duty cycle of 10%. The AZ31 alloy sheets were rinsed in deionized water and dried in warm air after MAO treatment. 2.3. Synthesis of LDH/MAO coating Three types of LDH were in situ grown on the surface of MAO coating in three types of solution adjusted the pH to 7 using NH3·H2O, respectively. The three types of solution are Al(NO3)3·9H2O(0.01 mol) and NH4NO3(0.06 mol)in 100 mL deionized water, Ni(NO3)2·H2O (0.001 mol) and NH4NO3(0.006 mol)in 100 mL deionized water, Zn (NO3)2·H2O(0.01 mol) and NH4NO3(0.06 mol)in 100 mL deionized water, respectively. The samples with MAO coating were immersed in the above three types of solution at 95 °C for 1 h, so that samples with different coatings obtained originally designed Al-LDH/MAO, Ni-LDH/ MAO and Zn-LDH/MAO. Subsequently, the samples with LDH/MAO
Fig. 1. XRD pattern of samples with different coatings. 536
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Fig. 2. XPS spectra of LDH/MAO coating with different cations. (a) Al2p of Al-LDH/MAO; (b) Mg1s of Al-LDH/MAO; (c) Ni2p of Ni-LDH/MAO (d) Mg1s of Ni-LDH/ MAO; (e) Zn2p of Zn-LDH/MAO; (f) Al2p of Zn-LDH/MAO.
Ni3+and two shakeup satellites [26,27], Ni3+participates in the formation of LDH as a trivalent cation. However, no obvious Zn 2p and Al 2p spectra were detected in the Zn-LDH/MAO, which reveals only little LDH grown on MAO coating, leading to the extremely low content of Zn and Al elements. The results from XRD and XPS indicate that LDH is successfully synthesized on MAO coating. Furthermore, the results clearly suggest that different alloy cations supplied by solution play different important role in LDH/MAO coating under the same conditions.
3.2. Corrosion state after long time immersion Fig. 3 depicts the macroscopic images of MAO and LDH/MAO coated onAZ31 Mg alloy samples after immersed in 1 M NaCl solution for different days. The surface of Al-LDH/MAO coating is seriously damaged, resulting in the exposure of bald substrate after 4 days immersion, as shown in Fig. 3(e), which suggest that has the worst corrosion resistance. After 5 days immersion, the local exfoliation state of Zn-LDH/MAO coating is similar as that of MAO coatings, but in detail the exfoliation area of Zn-LDH/MAO coating was slightly smaller than 537
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Fig. 3. Photographs of samples with different coatings immersed in 1 M NaCl solution for different time. (a–c) morphology of the sample only with MAO coating (a) 0d, (b) 4d, (c) 5d; (d–f) morphology of the sample with Al-LDH/MAO coating (d) 0d, (e) 4d, (f) 5d; (g–i) morphology of the sample with Zn-LDH/MAO coating (g) 0d, (h) 4d, (i) 5d; (j–m) morphology of the sample with Ni-LDH/MAO coating: (j) 0d, (k) 4d, (l) 5d, (m) 7d.
Fig. 4. SEM images of MAO coating before and after LDH treatment by different cations. (a) MAO coating, (b) Al-LDH/MAO coating, (c) Ni-LDH/MAO coating, (d) Zn-LDH/MAO coating.
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Fig. 5. the cross-section images of the coating, (a–c) MAO, (d–f)Al-LDH/MAO.
Fig. 6. High-resolution SEM image of MAO coating before and after LDH treatment by different cations. (a) MAO coating, (b) Al-LDH/MAO, (c) Ni-LDH/MAO and (d) Zn-LDH/MAO.
In general, a large number of similar volcano pores distribute on the surface of each sample. Interesting, the morphology of Zn-LDH/MAO and Ni-LDH/MAO coating is similar to MAO coating, but the morphology of Al-LDH/MAO coating exhibits obviously different (Fig. 4). Comparing with the other coatings, there are more cracks on the surface of sample with Al-LDH/MAO coating. These cracks significantly degrade the corrosion resistance of Al-LDH/MAO coating, because corrosion medium is easier to penetrate the substrate through these cracks. It may be that aluminum is amphoteric metal, the aluminum ions are acidic under experimental conditions and react with MgO to destroy the original MAO coating. The cross-section image can further verify this opinion that aluminum ions destroy the original MAO coating, and
that of MAO. While a little local exfoliation doesn’t appears on the surface of Ni-LDH/MAO coating until immered for 7 days. Above results suggest that the order of corrosion resistance of samples with different coatings is sample with Ni-LDH/MAO coating > sample with Zn-LDH/ MAO coating ≈ sample with MAO coating > sample with Al-LDH/ MAO coating.
4. Discussion It is well known that the corrosion performance of samples with different LDH/MAO coating is close with morphology of coating. The morphology of MAO and LDH/MAO coatings are shown in Fig. 4 and 6. 539
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Fig. 7. Mapping of elements for (a) Al-LDH/MAO, (b) Ni-LDH/MAO and (c) Zn-LDH/MAO.
The surface morphology of sample with Ni-LDH/MAO coating and its corresponding elements mapping of Mg, Ni, O and Si are shown in Fig. 6(c) and 7(b), respectively. Flake-like structure is not only observed in the smooth region, but also at the inherent defects of original MAO coating. Its morphology is similar to that of the reported LDH layer [24]. Ni-LDH film, which didn’t destroy the original MAO coating, is formed on the MAO coating, and then covers the pores and cracks effectively. This confirms that Ni-LDH/MAO coating exited abundant NO3−which exchange aggressive medium in corrosive environment effectively improves the corrosion resistance of the sample. Fig. 6(d) depicts the morphology of Zn-LDH/MAO coating, the local area of the coating forms the flake accumulation, while the other areas are basically the same as those of MAO. Combining the mapping of elements, Fig. 7(c) shows that Zn element almost clusters on the flakes, which also suggests that Zn cation only produces LDH film in the local region. The reason for this phenomenon is Zn and Mg element can only be divalent, which hard to form trivalent cation. But Mg alloy contains a small amount of Al elements, which form LDH with Zn through the discharge channel to the surface during MAO process. The effect of LDH including different cations on the original MAO coating was further verified by
Fig. 5 shows the cross-section morphology of MAO and Al-LDH/MAO coating. Comparing with MAO coating, the cross-section of Al-LDH/ MAO coating exists the phenomenon of obvious destruction and defect, which revealing the original MAO coating is destroyed after hydrothermal reaction containing Al cation. Hence, Al-LDH/MAO coating exhibits low corrosion resistance, which is consistent with the observation of corrosion state after long time immersion. Moreover, based on the high magnification images (Fig. 6), there is obvious difference between MAO and LDH/MAO coating, and the surface morphology of LDH film formed by different alloy cations is also apparently different. The granular products, which uniformly distributed on the surface of sample, can be observed on the surface of Al-LDH/MAO coating after hydrothermal reaction containing Al cation. Energy dispersive spectrometer (EDS) is used to analysis the elements distribution of Al-LDH/ MAO coating. As shown in Fig. 7(a), Mg, O, Si and Al element distribute on the surface of sample with Al-LDH/MAO coating. Mg and O mainly come from MgO, which produced in the MAO process, while Si element primarily originates from electrolyte. The uniform distribution of Al should be attributed to Al element participates in hydrothermal reaction to form the LDH film, which is consistent with the previous XRD. 540
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Fig. 8. the cross-sectional image of the coating, (a) MAO coating, (b) Al-LDH/MAO, (c) Ni-LDH/MAO, (d) Zn-LDH/MAO.
the rest of coatings, and its corrosion current density is approximately one order of magnitude higher than MAO coating. Because abundant cracks are produced during the Al-LDH treatment process as shown in Fig. 4(b), which leads to destroy of the original MAO coating and decrease of the corrosion protection. Impressively, the corrosion potential and corrosion current density of Zn-LDH/MAO coating is almost the same as that of MAO in accordance with observation of the Fig. 4. It is noteworthy that Ni-LDH/MAO coating has the lowest corrosion current density in all coatings, which indicates Ni-LDH coating obviously improve the corrosion resistance of Mg alloy with MAO coating. In this paper, Al-LDH/MAO coating has the worst anti-corrosion, the corrosion resistance of Zn-LDH/MAO is similar with the original MAO coating, while Ni-LDH/MAO coating performs excellent corrosion resistance. Therefore, the corrosion resistance of three kinds of composite coating just needs to compared Ni-LDH/MAO coating with other MAO coating on AZ31 Mg alloy. Not only the current density of Ni-LDH/MAO coated on AZ31 Mg alloy (3.2 × 10-7A) is smaller than other MAO coating on AZ31 Mg alloy, but also the anti-corrosion performs better in immersed corrosion test [19,28,29]. EIS curves can effectively reflect MAO and LDH/MAO coating corrosion performance. The Bode plots for MAO and LDH/MAO coating after immersing in 3.5 wt%NaCl for 2 days are shown in Fig. 10. Meanwhile, Fig. 10 also describes the fitting curves based on the corresponding equivalent circuits. It is found that two time constants appear in the Bode plots. The time constants of high and intermediate frequency correspond to the electrochemical response of the porous layer and the barrier layer of coating, respectively. The phase angle of Ni-LDH/MAO coating in high frequency bigger than the other coating, which indicates the porous layer of Ni-LDH/MAO coating effectively prevent the penetration of corrosive medium to the substrate. While the phase angle of Al-LDH/MAO is close to 0°, suggesting that porous layer has been completely penetrated by the corrosion medium after immersion for 2 days, and the corresponding electrochemical response of the barrier layer also decreases. The EIS curve of Zn-LDH/MAO coating
Fig. 9. Potentiodynamic polarization curves of the samples with different coatings in 3.5% NaCl solution.
cross-sectional observation. The cross-section of MAO and MAO/LDH coating are characterized. The thickness of the three kinds of MAO/LDH coating are similar to original MAO coating as shown in Fig. 8, which suggests that the ion has no considerably influence on the thickness of original coating in this work. Additionally, LDH is a extremely thin film growing on the original MAO coating. Hence, the relationship between the thickness and the anti-corrosion properties is not studied in this paper. On the other hand, in order to further detect the corrosion performance of LDH/MAO coating, potentiodynamic polarization test and EIS have been employed to evaluate the electrochemical properties of different coatings. The potentiodynamic polarization curves of the MAO coating with and without LDH treatment were depicted in Fig. 9. It can be seen that corrosion potential of Al-LDH/MAO coating is lower than 541
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Fig. 10. Bode diagram of the samples with different coatings after immersing in 3.5% NaCl solution for 2 days.
Fig. 11. Equivalent circuit of the samples with different coatings immersion in 3.5% NaCl solution.
the corrosion resistance of the sample. The impedance modulus of NiLDH/MAO coating at low frequency, which is one order of magnitude larger than that of Al-LDH/MAO coating, is highest compared with that of the other coating. In contrast, the Al-LDH/MAO coating has the lowest impedance modulus compared with the rest of coating. Additionally, in the sample with Zn-LDH/MAO coating, Zn-LDH formed just at local area, which improve the corrosion resistance is extremely limited. In order to evaluate the influent of alloy cations on corrosion resistance, the equivalent circuit was employed to fit the EIS dates. Equivalent circuit of all coatings after immersing in 3.5% NaCl for 2 days is shown in Fig. 11. MAO coating is usually divides to porous layer and barrier layer in the equivalent physical model [24]. Red LDH
Table 1 Fitting results of EIS plots of the samples with different coatings after immersing in 3.5% NaCl solution for 2 days. Sample
RpΩ cm2
RbΩ cm2
RctΩ cm2
χ2
MAO Al-LDH/MAO
11.99
1.33× 10 4
4828
1.52× 10−4
0.15
3.85× 103
1542
4.67× 10−4
Ni-LDH/MAO
59.31
4.65× 10 4
7273
3.80× 10−4
Zn-LDH/MAO
11.40
1.44× 10 4
4579
1.96× 10−4
is almost the same as that of MAO coating, which implies the corrosion performance of these two coatings is close to each other. Besides, it is well known that the higher Z modulus at lower frequencies, the better 542
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Fig. 12. the SEM images of the coating tested for adhesion, (a) low magnification and (b) high magnification of Al-LDH/MAO, (c) low magnification and (d) high magnification of Ni-LDH/MAO, (e) low magnification and (f) high magnification of Zn-LDH/MAO.
film covers the defects of porous layer. In the equivalent circuit, Rs represents solution resistance, and Rp represents porous layer resistance, and Rb represents barrier layer resistance, and Rct represents charge transfer resistance. Table 1 gives the results of fitting the electrochemical parameters of the equivalent circuit. Form Table 1,compared with MAO coating, the Rp and Rb values of Ni-LDH/ MAO obviously increased. Specially, Rp of Ni-LDH/MAO coating is 5 times that of MAO coating, while the increase of Rb is not obvious. It is because that due to special structure of LDH and anion-exchange property of LDH, Cl− in corrosion environment can be replaced by NO3− into the LDH film. Thereby, reducing the concentration of Cl− can effectively weaken the penetration and damage on MAO coating from active Cl− in NaCl solution. The surface is completely covered by Ni-LDH film in the sample with Ni-LDH/MAO coating as shown in Fig. 6(c), so abundant NO3− ions are superseded by Cl−, which results
in improving the corrosion resistance of the sample with Ni-LDH/MAO coating. However, both Rp and Rb of Al-LDH/MAO coating are lower than Rp and Rb of MAO coating, due to the destruction of original MAO coating, and them are 0.15 and 3852 respectively. The Rb and Rp of ZnLDH/MAO are basically the same as that of MAO coating. The above results suggest that anti-corrosion of MAO coating can be improved by forming Ni-LDH film, and Zn-LDH can’t effectively enhance corrosion resistance due to only growing in local surface area, but the corrosion resistance of the sample with Al-LDH/MAO coating decrease. The anticorrosion mechanism of LDH/MAO coating can be concluded. Firstly, LDH film forms on the MAO coating to seal its defects such as crack and pores, which can hinder the penetration of chloride ions into the matrix to improve protection of coating. Secondly, the increasing of corrosion resistance is attributed to the adsorption of chloride ions. Nitrate ions of LDH can be replaced chloride ions in the environment via the ability of
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ion exchange. LDH film containing nitrate synthesized on MAO of aluminum alloy improve effectively the barrier property because of the unique anion-exchange of LDH [24,30,31]. Strong adhesive of LDH to the MAO coating, which is beneficial to improve the corrosion resistance, is an important property that cannot be ignored. The adhesion strength of LDH was studied using Herein scratch adhesion test [24,32]. The SEM images of the coatings after testing show that LDH film of Al-LDH/MAO and Ni-LDH/MAO exhibit no obvious peeling off, as shown in Fig. 12, which clear suggests that LDH exist strong adherence. Although the Zn-LDH film isn’t directly scratched due to LDH film only forming at some local area, LDH film is also not peeled near the scratched area where still high stress exists.
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5. Conclusions In this study, the influence mechanism of LDH/MAO coating produced by different alloy cations on the corrosion resistance of Mg alloys was investigated, through microstructure, phase composition and electrochemical characterization. Alloy cations fabricating LDH play important role in the corrosion property of LDH/MAO coating on Mg alloy, since the different alloy cations result in different interaction between LDH and MAO. (1) Granular structure of Al-LDH was synthesized on the MAO film by hydrothermal reaction. However, the original MAO coating was destroyed, which results in the decrease of anti-corrosion resistance. (2) The Ni-LDH film was grown on MAO coating and numerous flakes structure completely covered the porous layer of MAO, so that Ni-LDH/MAO coating improves the corrosion performance of AZ31 alloy. (3) Zn-LDH film just grows at some local areas of the original MAO coating, and a part of defects of original MAO coating are repaired by Zn-LDH film. Therefore, the corrosion resistance of ZnLDH/MAO coating on AZ31 Mg alloy exhibits a little improvement. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (Grant Nos. 51501040, 51771064, U1537201 and 51871057), and Science Foundation of Education Department of Fujian Province (Grant No. JK2015030 and JA15341), and Science Foundation of Fujian Province (Grant No. STHJ-KF1702). References [1] P.K. Bowen, J. Drelich, J. Goldman, Magnesium in the murine artery: probing the products of corrosion, Acta Biomaterialia 10 (2014) 1475–1483. [2] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, Y.C. Xin, Z.Q. Zhang, Y. Liu, X.H. Chen, What is going on in magnesium alloys? J. Mater. Sci. Technol. (2017). [3] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G.E. Thompson, Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings, Corros. Sci. 50 (2008) 1744–1752. [4] S. Pommiers, J. Frayret, A. Castetbon, M. Potin-Gautier, Alternative conversion coatings to chromate for the protection of magnesium alloys, Corros. Sci. 84 (2014) 135–146. [5] Z.W. Zhang, Ultrasonic-assisted electroless Ni-P plating on dual phase Mg-Li alloy, J. Electrochem. Soc. 162 (2015) c64. [6] L. Yang, M. Zhang, J. Li, X. Yu, Z. Niu, Stannate conversion coatings on Mg-8Li alloy, J. Alloy. Compd. 471 (2009) 197–200. [7] D. Song, X. Jing, J. Wang, S. Lu, P. Yang, Y. Wang, M. Zhang, Microwave-assisted synthesis of lanthanum conversion coating on Mg-Li alloy and its corrosion resistance, Corros. Sci. 53 (2011) 3651–3656.
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