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Hydrothermal synthesis of protective coating on magnesium alloy using de-ionized water
Surface & Coatings Technology 206 (2012) 2961–2966
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Hydrothermal synthesis of protective coating on magnesium alloy using de-ionized water Yanying Zhu a, Qing Zhao b,⁎, Yun-Hong Zhang a, Guangming Wu c a b c
School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China School of Physics, Beijing Institute of Technology, Beijing 100081, PR China Beijing Institute of Petrochemical Technology, Beijing 102617, PR China
a r t i c l e
i n f o
Article history: Received 17 August 2011 Accepted in revised form 20 December 2011 Available online 28 December 2011 Keywords: Magnesium alloy Corrosion Hydrothermal method Coating
a b s t r a c t High corrosion rate is the “Achilles' heel” of magnesium and its alloys, which has severely limited their applications. Coating on these kinds of materials is an effective way to overcome this weakness. Protective coatings were successfully synthesized on AZ31 magnesium alloy by the hydrothermal method with de-ionized water as mineralizer in this paper. The structure, morphology, and composition of the coatings were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energydispersive spectroscopy (EDS), respectively. Potentiodynamic polarization tests and immersion tests for 7 and 14 days in 3.5 wt.% NaCl aqueous solution at room temperature were conducted to evaluate anticorrosion abilities of coatings. The influences of hydrothermal temperature and time on the thickness of the coating and corrosion resistance were investigated. The results show that the coatings are uniform and compact, composed of hexagonal magnesium hydroxide (Mg(OH)2) and a spot of monoclinic aluminum magnesium hydroxide (Mg2Al(OH)7). The thickness of coating varied from 2.2 μm to 27.2 μm, increasing with the hydrothermal temperature and time. Polarization curves and results of immersion tests of coated and uncoated AZ31 substrates demonstrate that coatings can improve the corrosion resistance effectively and the corrosion resistances are mainly increased with the thicknesses of the coatings and increased with hydrothermal temperature if the thicknesses are very close to each other. The static water contact angles of the coatings are all less than 13.5°, whereas that of the substrate is 40.5°, indicating that the coatings are highly hydrophilic. Tape test further verifies that there is a strong adhesion between the coating and the substrate. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are widely used in aeronautical and astronautical engineering, automobile industry and 3C product fields for their outstanding physical and mechanical properties, such as low specific weight, high strength, good machinability, high dimensional stability, etc. [1–3]. Especially they are potential degradable implant biomaterials as a consequence of their densities and elastic modulus extremely close to that of human bone, biodegradable and excellent biocompatibility in vivo experiments [4–6]. However, magnesium is quite chemically active, the corrosion resistance of magnesium alloys is usually very poor, particularly in the environment containing chloride ions [7,8], which restricts their prevalent use in the foregoing applications. Thus, it is crucial to reduce the corrosion rate of the magnesium alloys without compromising on the wonderful natural characteristics. Coating on the magnesium alloys can effectively
⁎ Corresponding author. Tel./fax: + 86 1068918710. E-mail address: [email protected] (Q. Zhao). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.12.029
prevent corrosion and provide a barrier layer between the metal and the corrosion medium. Up to now, there are lots of coating technologies available for magnesium alloys, which include electrochemical plating, anodic oxidation, conversion coatings, organic coatings, physical vapor deposition, and chemical vapor deposition [9,10]. In the early days, hydrothermal technology for film preparation was used to synthesize functional films, for example, BaTiO3 [11–13], PbTiO3 [14], PZT [15], and HAP [16] films. Recently, some researchers used this technology to generate a protection film on the metal substrate to prevent corrosion. Zhang et al. [17] fabricated zinc–aluminum layered double hydroxide (ZnAl-LDH) bilayer films on aluminum substrates by a one-step hydrothermal crystallization method (HCM). Derek E. Beving et al. [18] synthesized high-silicazeolite (HSZ) MFI coatings by in-situ HCM on a series of aluminum alloys. These films are uniform, compact and have strong adhesion to substrates, and can effectively protect substrates from corrosion. There are lots of advantages to prepare films by this technology, such as lower treatment temperature (typically 100–200 °C), high purity, large thicknesses, well-built adhesion, and simple process procedures. Meanwhile, hydrothermal crystallization occurs on a 3-
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dimensional structure, so that this technology can also be used to treat complex 3-dimensional objects. Magnesium hydroxide has been widely used in many fields. It is an efficient flame retardant which is broadly used in polymer materials [19]. It is also used as a catalyst [20], carbon dioxide absorbent [21], and an additive in paint and ceramics as well as a precursor for the synthesis of magnesium oxide [22]. Versatile morphologies and structures of magnesium hydroxide materials such as needle-like crystal, lamellar-like crystal, nanoplate, nanoflower, nanorod, and nanotube, have been synthesized via different methods [22–26]. Moreover, Ying Lv [27] and Takahiro Ishizaki [28] prepared magnesium hydroxide thin film by an electro-deposition and hydrothermal approach, respectively. In our previous study, we obtained protective coatings on the surface of AZ31 magnesium alloy, which consist of large quantities of Mg(OH)2 fine grains by the hydrothermal method using the 5.66 wt.% NaOH aqueous solution as the mineralizer. The analyzed results showed that the coatings successfully improved the corrosion resistance of AZ31 magnesium alloy in Hank's solution [29]. In order to further simplify the process of hydrothermal coating, in this study, we attempt to use de-ionized water to synthesize protective coatings on the AZ31 substrates which act as both the substrates and sole source of magnesium in the de-ionized water by the hydrothermal method. Effects of the hydrothermal temperature and time on the thickness and anti-corrosion ability of the coating grown on AZ31 substrate were investigated. However, the heating rate is also one of the important factors influencing the growth of the hydrothermal coatings [30], and we will do more studies about this issue in the future. The surface wettabilities of the synthesized coatings and the adhesion of the coatings to the substrates were studied, too. 2. Experimental procedures The specimens of wrought magnesium alloy (AZ31, about 3% Al and 1% Zn) were cut into small disks with 2 mm in thickness and 10 mm in diameter. Each substrate was polished with SiC papers successively up to 2000 grit, ultrasonically cleaned in de-ionized water, acetone, and ethanol for 10 min respectively. Then they were dried in the air. The de-ionized water was poured into an autoclave with a capacity of 135 ml, and was filled to 75% of its capacity. The autoclave used in this experiment is a very simple device which is a stainless steel autoclave with a Teflon liner without electrical heating facility and pressure gauge. The autoclave can be charged with reagents, and closed. Under external heating condition, the contents will be raised to higher temperatures and pressures than in an unsealed container. Subsequently, AZ31 substrates were put into the autoclave, heated to the hydrothermal temperature by an electric oven, and then kept the autoclave at this temperature for different hydrothermal times. The hydrothermal time was counted from 20 min after the time that the temperature reached hydrothermal temperature. We assumed that the temperature in the autoclave reached hydrothermal temperature during the additional time. The coatings were therefore synthesized under hydrothermal conditions. The hydrothermal conditions used in the experiment were 120 °C 3 h, 160 °C 1 h, 160 °C 2 h, 160 °C 3 h, 160 °C 4 h and 200 °C 3 h. We designated samples by the hydrothermal conditions hereinafter, for example, 160°C3h representing samples which were treated at 160 °C for 3 h. The microstructure of coating was investigated by X-ray diffraction (XRD), which was performed on a SHIMADZU XRD-7000 X-ray diffraction using Cu Kα radiation (λ = 0.154 nm), at 40 kV, 30 mA, and a scan speed of 2 °/min. The morphologies of surface and cross section were observed by field emitting scanning electron microscope (FESEM) using a Hitachi S4800 field emission scanning electron microscope. And the thicknesses of the coatings were measured through the cross-sectional FESEM micrographs of samples. The applied
accelerating voltage was 5 kV. All samples were sputtered with gold. The element distribution of the sample cross section along the line perpendicular to surface was measured by the energy dispersive Xray spectrometers (EDS) attached to Hitachi Field-Emission S4800. The anticorrosion abilities of the coatings were evaluated by potentiodynamic polarization tests and immersion tests in 7 and 14 days in 3.5 wt.% NaCl aqueous solution at room temperature with naked AZ31 substrates as control. The potentiodynamic polarization tests were carried out on a Zahner IM6e impedance analyzer using a three-electrode system composed of a sample to be tested as the working electrode, a platinum plate electrode as the counter electrode, and a saturated calomel electrode as the reference electrode. The immersion tests were performed according to ASTM-G31-72 [31]. Before immersing into the solution, the dimensions and masses of the samples were measured by a vernier caliper and an electric balance, respectively. Three parallel specimens were used in the test and the ratio of solution volume to specimen area was 0.40 ml/mm 2. The samples were taken out after 7 and 14 days of immersion test and rinsed with de-ionized water and ethanol, and dried at room temperature before weighing. Finally, the masses of the dried samples were measured. The corrosion rate is calculated as follows: v¼
m0 −m1 St
ð1Þ
where v is the corrosion rate, m0 is the sample mass before immersing experiment, m1 is the mass after immersing experiment, S is the surface area of the sample, and t is the corrosion time. The surface wettability of the synthesized coating on AZ31 was studied by a JC2000C1 static drop contact angle/interfacial tension measuring machine (Shanghai Zhongchen Digital Technology Apparatus Co. Ltd). In order to further characterize the quality of the grown coating on AZ31 substrate, a tape test was employed to evaluate the adhesion of the coating to the substrate according to ASTM D 3359-02 method B [32], which is used to establish whether the adhesion of a coating to a metallic substrate is at a generally adequate level in the laboratory. We made the cross-cut pattern at 90° angles through the coating first. Since thicknesses of coatings assessed in the experiment are less than 50 μm, the spacing of parallel cuts is 10 mm. The coating was brushed lightly with a soft brush after each cut to remove excess debris from the surface. A soft brush was used to remove any detached flakes or ribbons of films after making the required cuts. A piece of 3M Scotch 610 tape was placed over the grid, and rubbed firmly with the eraser on the end of a pencil to ensure good contact with the coating. After 120 s, the tape was removed rapidly from the free end at an angle of about 180°. Finally, the grid area was inspected for removal of coating from the substrate using the illuminated magnifier, and the rating of the adhesion was evaluated by comparison with descriptions and illustrations in ASTM standard. 3. Results Coatings synthesized at 120 °C 3 h, 160 °C 1 h, 160 °C 2 h, 160 °C 3 h and 160 °C 4 h were smooth and tan-yellow with vitreous luster, while the coating synthesized at 200 °C 3 h was almost entirely peeled off. Fig. 1 shows the X-ray diffraction pattern of sample 160°C3h, together with the Mg(OH)2 powder for comparison. In addition to the characteristic peaks of the AZ31 substrate, the XRD pattern of the sample can be assigned to diffractions of hexagonal structure for Mg(OH)2 according to JCPDS 75-1527. The two peaks below 25° can be assigned to the [003] and [006] diffractions of Mg2Al(OH)7 according to JCPDS 48-0601. The XRD pattern of the coating on AZ31 substrate displays obviously increasing of relative peak intensities corresponding to the [101] and [110] diffractions of the Mg(OH)2 phase, while the [001] diffraction is very weak, which is significantly
Y. Zhu et al. / Surface & Coatings Technology 206 (2012) 2961–2966
Fig. 1. XRD patterns of the coating of 160 °C 3 h as well as the Mg(OH)2 powder.
different from the pattern of the Mg(OH)2 powder random oriented. It reveals a good orientation of Mg(OH)2 crystallites in the coating with their ab face perpendicular to the substrate (c-axis parallel to the substrate). The peak broadening of Mg(OH)2 indicates a quite small grain size of the Mg(OH)2 particles in the coating. Fig. 2 shows the surface morphology of the coating of 160°C3h which was observed by FESEM. These images demonstrate that the
Fig. 2. FESEM images for (a) surface morphology of the coating of 160°C3h; (b) magnification of region “1” in image (a); (c) magnification of region “2” in image (a).
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coating surface is composed of big hexagonal flakes and small irregular shaped platelets. The big hexagonal flakes are mostly around 400 nm–600 nm in diameter and almost parallel to the substrate; however, the small platelets are typically around 60 nm in diameter and nearly perpendicular to the substrate. The FESEM image of the cross-section of the coating of 160°C3h is shown in Fig. 3. The coating is obviously uniform and dense with a thickness of 18.5 μm. The element distributions along the line perpendicular to the surface are given in Fig. 3, and element content analysis of the region marked “A” is shown in Table 1. According to the element distributions across the Mg(OH)2 and AZ31 substrate, a transition layer in thickness of 6 μm is formed between the coating and the substrate. The analysis result of element content in region “A” indicates that the atomic ratio of Mg/O is close to that of the stoichiometric ratio (1:2) in Mg(OH)2. These results suggest that the coating mainly consists of magnesium hydroxide nanoparticles. Fig. 4 shows the relationship between the coating thickness and the hydrothermal time at 160 °C. We can conclude that the thickness of coating increases with hydrothermal time, and the coating deposition rate is a linear function of hydrothermal time. And the thickness of coating of 120°C3h is 2.7 μm, which is much smaller than that of 160°C3h, reveals that the thickness of coating increases with hydrothermal temperature. Polarization curves of naked AZ31 substrate and AZ31 with coatings of 120°C3h, 160°C1h, 160°C2h, 160°C3h and 160°C4h in the 3.5 wt.% NaCl aqueous solution at room temperature are shown in Fig. 5. The corrosion potentials (Ecorr) and the corrosion current densities (icorr) were derived directly from the potentiodynamic polarization curves by the Tafel extrapolation method, and the results are summarized in Table 2. It can be noticed that the corrosion current densities (icorr) of the AZ31 with coatings are reduced compared with the naked AZ31 substrate and the extent of the reduction increases with the thickness of coating. AZ31 with coating of 160°C3h affords a reduction in icorr to 5% of that of the naked AZ31 substrate, whereas the value of icorr is reduced to 0.08% of that of the naked substrate for the sample 160°C4h. However, the corrosion current densities (icorr) of the sample 160°C1h are about 11% of that of sample 120°C3h, through they have the similar values of coating thicknesses. The possible reason is that the structure of coating on the sample 160°C1h is more compact than that on sample 120°C3h. To our knowledge, the lower the corrosion current density is, the higher the corrosion resistance is obtained. Therefore, the corrosion rate of AZ31 magnesium alloy is effectively decreased by the Mg(OH)2 coatings. The result of the immersion test in 3.5 wt.% NaCl solution for 7 and 14 days at room temperature is shown in Fig. 6. A significant number of hydrogen bubbles were released from the surface of naked AZ31 substrates as soon as they were immersed into the NaCl solution, while the bubbles appeared from only the edge of AZ31 with coating of 160°C3h until 5 days later. We can generally estimate that large hydrogen bubbles are seen from the samples when the substrates under the coatings are corroded. The reason is that hydrogen is produced in the corrosion process of AZ31 in NaCl solution. The average corrosion rates of AZ31 substrate and AZ31 with coatings for 7 and 14 days reveal considerable differences according to Fig. 6. The corrosion rate of sample with coating decreases with the thickness of coating except that of sample 120°C3h which has a close corrosion rate to the AZ31 substrate. This is highly consistent with the result of potentiodynamic polarization test. The morphologies of samples after the corrosion test are displayed in Fig. 7, and the samples with coatings of 160°C2h, 160°C3h and 160°C4h are nearly intact, only with a tiny local corrosion at the edge of the coating. However, the edge of naked AZ31 substrate and samples 120°C3h and 160°C1h suffered severe damage. The results of potentiodynamic polarization test and immersion test demonstrate that the coatings can improve the corrosion resistance of AZ31 alloy efficiently. It can be attributed to the thick,
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Fig. 3. Cross section of the coating of 160°C3h and the element distribution along the line perpendicular to the surface.
uniform, and compact coatings formed by the hydrothermal reaction. The transportation of species (e.g. water and chloride ions) in the solution is blocked by the coating, which decreases the corrosion rate. The water wettabilities of the samples were studied by measuring the static contact angles of water on sample surfaces and the static
Table 1 The element content analysis of region “A” at the cross section of the coating in Fig. 3. Element
Wt.%
At.%
Mg O Al C
39.8 55.5 0.9 3.8
30.0 63.6 0.6 5.9
contact angle of water on naked AZ31 substrate is 40.5°, which is close to the static water contact angle on AZ31 reported by Ref. [32], whereas that on AZ31 with coatings is much lower: 11° for 160°C1h, 8.0° for 160°C2h, 7.5° for 160°C3h, 6.5° for 160°C4h and 13.5° for 120°C3h. These results suggest that the coating surfaces are more hydrophilic than the naked substrate. The adhesion strength of the coating to the substrate is a very important factor to determine the coating quality. Strong adhesion of the coating to the substrate is essential in the practical application. Fig. 8 shows the appearance of the cross-cut surface of sample 160°C3h after tape test according to ASTM D 3359-02 method B. In the process of adhesion test, no delamination or peeling off occurred on the cross-cutting surface of coating, and no significant peeling of either material after cross cutting through the Mg(OH)2 coating according to Fig. 8, indicate physically powerful adhesion between
Y. Zhu et al. / Surface & Coatings Technology 206 (2012) 2961–2966
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Table 2 Corrosion potential (Ecorr) and corrosion current density (icorr) of the naked AZ31 and samples with coatings prepared under different hydrothermal conditions. Sample
Ecorr (V)
icorr (μA/cm2)
AZ31 120 °C 160 °C 160 °C 160 °C 160 °C
− 1.356 − 1.328 − 1.291 − 1.337 − 1.392 − 1.324
6763 6072 654.2 114.8 330.2 5.421
3h 1h 2h 3h 4h
þ
2H ðaqÞ þ 2e→H2 ðgÞ
Fig. 4. Effect of hydrothermal time on the thickness of coating at 160 °C.
2þ
the coating and the AZ31 substrate (classification 4B according to ASTM D 3359-02 method B). The strong adhesion of coating to the AZ31 substrate might be attributed to the formation of the interpenetrating transition layer, which can enhance bonding between the coating and AZ31 substrate.
4. Discussion The crystal of magnesium hydroxide belongs to the hexagonal system and has a layered structure that consists of sheets of Mg octahedral stacked parallel to (001) plane with hydrogen bonding between layers. The ideal morphology of magnesium hydroxide is the hexagon shaped plate according to chemical bonding theory of single crystal, and the top/bottom faces of the plate correspond to the (001) planes [34]. We can obtain other morphologies by modifying crystallization conditions. In this paper, magnesium hydroxide nanocrystals were formed and grew on AZ31 magnesium alloy under the hydrothermal conditions with de-ionized water as mineralizer. The reaction of magnesium with water can be described as follows: þ
−
H2 OðlÞ→H ðaqÞ þ OH ðaqÞ
2þ
MgðsÞ→Mg ðaqÞ þ 2e
−
Mg ðaqÞ þ 2OH ðaqÞ→MgðOHÞ2 ðsÞ
ð2Þ
ð3Þ
Fig. 5. Potentiodynamic polarization curves of samples in 3.5 wt.% NaCl aqueous solution at room temperature.
ð4Þ
ð5Þ
Under the hydrothermal condition, the pressure of water is high, and the ionization constant of the water increases with the temperature and pressure, therefore the reaction of magnesium with water is exacerbated in the process, and a thick Mg(OH)2 coating can be synthesized in short time. The reaction between the solution and the AZ31 substrate occurred at the solid/liquid interface. Initially, a large number of fine Mg(OH)2 crystalline grains were generated at the interface under the hydrothermal conditions. Due to their high surface energy and inherent characteristic of polarity, these fine crystalline particles aggregated and covered the surface of the substrate. Subsequently, water permeated the initial Mg(OH)2 covering by passing through its interstices or pores and reacted with the AZ31 substrate below. A great number of fine Mg(OH)2 crystals were synthesized at the new solid/liquid interface and then aggregated. By repeating this process, the Mg(OH)2 coating was finally prepared on the AZ31 substrate. During this process, the prior generated crystal particles were modified by water via the process of dissolution recrystallization and closer to the ideal crystal gradually. Therefore, we observed big hexagonal flakes on the coating surface as shown in Fig. 2, and the pH value of de-ionized water in our study increased from the initial neutral 7 to 8 after the hydrothermal experiment. The XRD result with (101) preferred orientation in Fig. 1 suggests that irregular nanoplates with the edges preferentially directed out of the AZ31 substrate are the main composition of the coating. Ying Lv et al. [27] reported that the electrodeposited Mg(OH)2 thin film with (101) preferred orientation exhibited continuous porous network of nanosheets, whereas the film with (001) preferred orientation was composed of dense nanoplates. They explained this result by using
Fig. 6. Corrosion rates of samples after7 and 14 days immersion in 3.5% NaCl solution at room temperature.
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5. Conclusions
Fig. 7. Corrosion morphologies of samples after 7 and 14 days immersion in 3.5% NaCl solution at room temperature.
Mg(OH)2 3D structural models and hydrated states of Mg 2 + in solution with different interfacial pH value. However, the Mg(OH)2 coating prepared in this paper is (101) preferred orientation as well as high dense. The exact reason for this phenomenon is still unclear at present, and further studies are required. Generally, it is believed that a hydrophobic surface can reduce the electrolyte to penetrate through the surface and thus enhances the corrosion resistance of the coating system [35], but the synthesized coatings show highly hydrophilic. On the other hand, the surface energy is directly related to the surface wettability. According to adsorption theory, if the surface energy of the substrate is higher than the surface tension of the liquid, the substrate surface will be wetted. The (001) plane has the lowest surface energy among all crystal planes of Mg(OH)2 [34,36], and the (001) surface is electrostatically neutral and hydrophilic according to the result of molecular dynamics computer simulation [37]. Since the surface energies of other crystal planes are higher than that of (001) plane, the planes are all hydrophilic. Hence, the crystal of magnesium hydroxide is hydrophilic. It is reasonable that the coating composed of Mg(OH)2 nanocrystals is hydrophilic. However, the coating can offer much higher corrosion resistance whereas the surface is highly hydrophilic. In fact, the highly hydrophilic and high performance anticorrosive coating has been reported by Takahiro Ishizaki et al. [33]. The simulation results in Ref. [37] also indicate that the components of the H2O self-diffusion coefficient's tensors parallel to both the hydrophobic surface and hydrophilic surface (XX and YY components) are much larger than the ZZ component perpendicular to it, because diffusion perpendicular to the surfaces is greatly restricted by the film thickness. Therefore, the hydrophobicity isn't necessary for anticorrosive films, and the integrity and density of films may have more influence on the corrosion resistance ability. Moreover, the hydrophilic surface is usually beneficial to the cell adhesion, and the improvement of surface wettability may develop the ability of cell adhesion [38]. It shows that this hydrothermal method is much suitable for surface modification of biomedical magnesium alloy implants.
Fig. 8. SEM image of the coating of 160 °C 3 h tested for adhesion.
Coatings with a thickness from 2.2 μm to 27.2 μm have been successfully fabricated on AZ31 substrates by an extremely simple hydrothermal process only using de-ionized water as the reagent. The coatings were uniform and compact, composed of hexagonal magnesium hydroxide (Mg(OH)2) and a spot of monoclinic aluminum magnesium hydroxide (Mg2Al(OH)7). We investigated the influence of hydrothermal temperature and time on the thickness and corrosion resistance of the coating, and discussed the mechanism of the coating formation. The coatings with enough thickness can prevent the AZ31 substrates corrosion effectively according to the potentiodynamic polarization test and the immersion test. The coatings showed strong adhesion to the substrate. This treatment process is simple and environmentally friendly (only use water) and can treat the work piece with complicated shapes. Furthermore, as Mg(OH)2 is nontoxic and hydrophilic, this treatment method is very promising for surface modification of biomedical magnesium alloys. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2009IM033000 and 2011AA120101). Additional support was provided by the National Natural Science Foundation of China (11075077 and 50935001). References [1] Z. Yang, J.P. Li, J.X. Zhang, G.W. Lorimer, J. Robson, Acta Metall. Sin. (English Letters) 21 (2008) 313. [2] K.U. Kainer, Magnesium-Alloys and Technologies, Wiley-VCH Verlag, Weinheim, 2003. [3] Y. Kojima, Mater. Sci. Forum 350–351 (2000) 3. [4] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728. [5] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid State Mater. Sci. 12 (2008) 63. [6] F. Witte, Acta Biomater. 6 (2010) 1680. [7] B.L. Mordike, T. Ebert, Mater. Sci. Eng., A 302 (2001) 37. [8] D. Eliezer, E. Aghion, F.H. (Sam) Froes, Adv. Perform. Mater. 5 (1998) 201. [9] R.W. Revie, H.H. Uhlig, in: R.W. Revie, H.H. Uhlig (Eds.), Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, fourth ed., John Wiley & Sons, Inc., Hoboken New Jersey, 2008, p. 399. [10] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [11] E. Shi, C.R. Cho, M.S. Jang, S.Y. Jeong, H.J. Kim, J. Mater. Res. 9 (1994) 2914. [12] J.O. Eckert Jr., C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, J. Am. Ceram. Soc. 79 (1996) 2929. [13] C.R. Cho, M.S. Jang, S.Y. Jeong, S.J. Lee, B.M. Lim, Mater. Lett. 23 (1995) 203. [14] A.T. Chien, J. Sachleben, J.H. Kim, J.S. Speck, F.F. Lange, J. Mater. Res. 14 (1999) 3303. [15] T. Morita, T. Kanda, M. Kurosawa, T. Higuchi, Jpn. J. Appl. Phys. 36 (1997) 2998. [16] H. Ishizawa, M. Makoto, J. Biomed. Mater. Res. 29 (1995) 1071. [17] X.X. Guo, S.L. Xu, L.L. Zhao, W. Lu, F.Z. Zhang, D.G. Evans, X. Duan, Langmuir 25 (2009) 9894. [18] D.E. Beving, A.M.P. McDonnell, W.S. Yang, Y.S. Yan, J. Electrochem. Soc. 153 (2006) B325. [19] J. Formosa, J.M. Chimenos, A.M. Lacasta, L. Haurie, Thermochim. Acta 515 (2011) 43. [20] E. Suzuki, Y. Nomoto, M. Okamoto, Y. Ono, Catal. Lett. 48 (1997) 75. [21] R.V. Siriwardane, R.W. Stevens Jr., Ind. Eng. Chem. Res. 48 (2009) 2135. [22] J.C. Yu, A.W. Xu, L.Z. Zhang, R.Q. Song, L. Wu, J. Phys. Chem. B 108 (2004) 64. [23] J.P. Lv, L.Z. Qiu, B.J. Qu, J. Cryst. Growth 267 (2004) 676. [24] Y. Ding, G.T. Zhang, H. Wu, B. Hai, L.B. Wang, Y.T. Qian, Chem. Mater. 13 (2001) 435. [25] C.L. Yan, D.F. Xue, L.J. Zou, X.X. Yan, W. Wang, J. Cryst. Growth 282 (2005) 448. [26] M. Dinamani, P.V. Kamath, J. Appl. Electrochem. 34 (2004) 899. [27] Y. Lv, Z. Zhang, Y.Q. Lai, J. Li, Y.X. Liu, CrystEngComm (13) (2011) 3848. [28] T. Ishizaki, S.P. Cho, N. Saito, CrystEngComm (11) (2009) 2338. [29] Y.Y. Zhu, G.M. Wu, Y.H. Zhang, Q. Zhao, Appl. Surf. Sci. 257 (2011) 6129. [30] M.J.M. Mies, E.V. Rebrov, J.C. Jansen, M.H.J.M. de Croon, J.C. Schouten, Micropor. Mesopor. Mat. 106 (2007) 95. [31] ASTM-G31-72: Standard Practice for Laboratory Immersion Corrosion Testing of Metals, ASTM, Philadelphia, PA, USA, 2004. [32] ASTM D 3359-02: Standard Test Methods for Measuring Adhesion by Tape Test, ASTM, Philadelphia, PA, USA, 2002. [33] T. Ishizaki, I. Shigematsu, N. Saito, Surf. Coat. Technol. 203 (2009) 2288. [34] D.F. Xu, X.X. Yan, L. Wang, Powder Technol. 191 (2009) 98. [35] J. Zhang, Y.F. Chan, Q.S. Yu, Prog. Org. Coat. 61 (2008) 28. [36] P. Fellner, J. Híveš, V. Khandl, M. Králik, J. Jurišová, T. Liptaj, L. Pach, Chem. Pap. 65 (2011) 454. [37] J.W. Wang, A.G. Kalinichev, R.J. Kirkpatrick, Geochim. Cosmochim. Acta 70 (2006) 562. [38] J. Takebe, S. Itoh, J. Okada, K. Ishibashi, J. Biomed. Mater. Res. A 51 (2000) 398.
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Hydrothermal synthesis of protective coating on magnesium alloy using de-ionized water Yanying Zhu a, Qing Zhao b,⁎, Yun-Hong Zhang a, Guangming Wu c a b c
School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China School of Physics, Beijing Institute of Technology, Beijing 100081, PR China Beijing Institute of Petrochemical Technology, Beijing 102617, PR China
a r t i c l e
i n f o
Article history: Received 17 August 2011 Accepted in revised form 20 December 2011 Available online 28 December 2011 Keywords: Magnesium alloy Corrosion Hydrothermal method Coating
a b s t r a c t High corrosion rate is the “Achilles' heel” of magnesium and its alloys, which has severely limited their applications. Coating on these kinds of materials is an effective way to overcome this weakness. Protective coatings were successfully synthesized on AZ31 magnesium alloy by the hydrothermal method with de-ionized water as mineralizer in this paper. The structure, morphology, and composition of the coatings were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and energydispersive spectroscopy (EDS), respectively. Potentiodynamic polarization tests and immersion tests for 7 and 14 days in 3.5 wt.% NaCl aqueous solution at room temperature were conducted to evaluate anticorrosion abilities of coatings. The influences of hydrothermal temperature and time on the thickness of the coating and corrosion resistance were investigated. The results show that the coatings are uniform and compact, composed of hexagonal magnesium hydroxide (Mg(OH)2) and a spot of monoclinic aluminum magnesium hydroxide (Mg2Al(OH)7). The thickness of coating varied from 2.2 μm to 27.2 μm, increasing with the hydrothermal temperature and time. Polarization curves and results of immersion tests of coated and uncoated AZ31 substrates demonstrate that coatings can improve the corrosion resistance effectively and the corrosion resistances are mainly increased with the thicknesses of the coatings and increased with hydrothermal temperature if the thicknesses are very close to each other. The static water contact angles of the coatings are all less than 13.5°, whereas that of the substrate is 40.5°, indicating that the coatings are highly hydrophilic. Tape test further verifies that there is a strong adhesion between the coating and the substrate. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are widely used in aeronautical and astronautical engineering, automobile industry and 3C product fields for their outstanding physical and mechanical properties, such as low specific weight, high strength, good machinability, high dimensional stability, etc. [1–3]. Especially they are potential degradable implant biomaterials as a consequence of their densities and elastic modulus extremely close to that of human bone, biodegradable and excellent biocompatibility in vivo experiments [4–6]. However, magnesium is quite chemically active, the corrosion resistance of magnesium alloys is usually very poor, particularly in the environment containing chloride ions [7,8], which restricts their prevalent use in the foregoing applications. Thus, it is crucial to reduce the corrosion rate of the magnesium alloys without compromising on the wonderful natural characteristics. Coating on the magnesium alloys can effectively
⁎ Corresponding author. Tel./fax: + 86 1068918710. E-mail address: [email protected] (Q. Zhao). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.12.029
prevent corrosion and provide a barrier layer between the metal and the corrosion medium. Up to now, there are lots of coating technologies available for magnesium alloys, which include electrochemical plating, anodic oxidation, conversion coatings, organic coatings, physical vapor deposition, and chemical vapor deposition [9,10]. In the early days, hydrothermal technology for film preparation was used to synthesize functional films, for example, BaTiO3 [11–13], PbTiO3 [14], PZT [15], and HAP [16] films. Recently, some researchers used this technology to generate a protection film on the metal substrate to prevent corrosion. Zhang et al. [17] fabricated zinc–aluminum layered double hydroxide (ZnAl-LDH) bilayer films on aluminum substrates by a one-step hydrothermal crystallization method (HCM). Derek E. Beving et al. [18] synthesized high-silicazeolite (HSZ) MFI coatings by in-situ HCM on a series of aluminum alloys. These films are uniform, compact and have strong adhesion to substrates, and can effectively protect substrates from corrosion. There are lots of advantages to prepare films by this technology, such as lower treatment temperature (typically 100–200 °C), high purity, large thicknesses, well-built adhesion, and simple process procedures. Meanwhile, hydrothermal crystallization occurs on a 3-
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dimensional structure, so that this technology can also be used to treat complex 3-dimensional objects. Magnesium hydroxide has been widely used in many fields. It is an efficient flame retardant which is broadly used in polymer materials [19]. It is also used as a catalyst [20], carbon dioxide absorbent [21], and an additive in paint and ceramics as well as a precursor for the synthesis of magnesium oxide [22]. Versatile morphologies and structures of magnesium hydroxide materials such as needle-like crystal, lamellar-like crystal, nanoplate, nanoflower, nanorod, and nanotube, have been synthesized via different methods [22–26]. Moreover, Ying Lv [27] and Takahiro Ishizaki [28] prepared magnesium hydroxide thin film by an electro-deposition and hydrothermal approach, respectively. In our previous study, we obtained protective coatings on the surface of AZ31 magnesium alloy, which consist of large quantities of Mg(OH)2 fine grains by the hydrothermal method using the 5.66 wt.% NaOH aqueous solution as the mineralizer. The analyzed results showed that the coatings successfully improved the corrosion resistance of AZ31 magnesium alloy in Hank's solution [29]. In order to further simplify the process of hydrothermal coating, in this study, we attempt to use de-ionized water to synthesize protective coatings on the AZ31 substrates which act as both the substrates and sole source of magnesium in the de-ionized water by the hydrothermal method. Effects of the hydrothermal temperature and time on the thickness and anti-corrosion ability of the coating grown on AZ31 substrate were investigated. However, the heating rate is also one of the important factors influencing the growth of the hydrothermal coatings [30], and we will do more studies about this issue in the future. The surface wettabilities of the synthesized coatings and the adhesion of the coatings to the substrates were studied, too. 2. Experimental procedures The specimens of wrought magnesium alloy (AZ31, about 3% Al and 1% Zn) were cut into small disks with 2 mm in thickness and 10 mm in diameter. Each substrate was polished with SiC papers successively up to 2000 grit, ultrasonically cleaned in de-ionized water, acetone, and ethanol for 10 min respectively. Then they were dried in the air. The de-ionized water was poured into an autoclave with a capacity of 135 ml, and was filled to 75% of its capacity. The autoclave used in this experiment is a very simple device which is a stainless steel autoclave with a Teflon liner without electrical heating facility and pressure gauge. The autoclave can be charged with reagents, and closed. Under external heating condition, the contents will be raised to higher temperatures and pressures than in an unsealed container. Subsequently, AZ31 substrates were put into the autoclave, heated to the hydrothermal temperature by an electric oven, and then kept the autoclave at this temperature for different hydrothermal times. The hydrothermal time was counted from 20 min after the time that the temperature reached hydrothermal temperature. We assumed that the temperature in the autoclave reached hydrothermal temperature during the additional time. The coatings were therefore synthesized under hydrothermal conditions. The hydrothermal conditions used in the experiment were 120 °C 3 h, 160 °C 1 h, 160 °C 2 h, 160 °C 3 h, 160 °C 4 h and 200 °C 3 h. We designated samples by the hydrothermal conditions hereinafter, for example, 160°C3h representing samples which were treated at 160 °C for 3 h. The microstructure of coating was investigated by X-ray diffraction (XRD), which was performed on a SHIMADZU XRD-7000 X-ray diffraction using Cu Kα radiation (λ = 0.154 nm), at 40 kV, 30 mA, and a scan speed of 2 °/min. The morphologies of surface and cross section were observed by field emitting scanning electron microscope (FESEM) using a Hitachi S4800 field emission scanning electron microscope. And the thicknesses of the coatings were measured through the cross-sectional FESEM micrographs of samples. The applied
accelerating voltage was 5 kV. All samples were sputtered with gold. The element distribution of the sample cross section along the line perpendicular to surface was measured by the energy dispersive Xray spectrometers (EDS) attached to Hitachi Field-Emission S4800. The anticorrosion abilities of the coatings were evaluated by potentiodynamic polarization tests and immersion tests in 7 and 14 days in 3.5 wt.% NaCl aqueous solution at room temperature with naked AZ31 substrates as control. The potentiodynamic polarization tests were carried out on a Zahner IM6e impedance analyzer using a three-electrode system composed of a sample to be tested as the working electrode, a platinum plate electrode as the counter electrode, and a saturated calomel electrode as the reference electrode. The immersion tests were performed according to ASTM-G31-72 [31]. Before immersing into the solution, the dimensions and masses of the samples were measured by a vernier caliper and an electric balance, respectively. Three parallel specimens were used in the test and the ratio of solution volume to specimen area was 0.40 ml/mm 2. The samples were taken out after 7 and 14 days of immersion test and rinsed with de-ionized water and ethanol, and dried at room temperature before weighing. Finally, the masses of the dried samples were measured. The corrosion rate is calculated as follows: v¼
m0 −m1 St
ð1Þ
where v is the corrosion rate, m0 is the sample mass before immersing experiment, m1 is the mass after immersing experiment, S is the surface area of the sample, and t is the corrosion time. The surface wettability of the synthesized coating on AZ31 was studied by a JC2000C1 static drop contact angle/interfacial tension measuring machine (Shanghai Zhongchen Digital Technology Apparatus Co. Ltd). In order to further characterize the quality of the grown coating on AZ31 substrate, a tape test was employed to evaluate the adhesion of the coating to the substrate according to ASTM D 3359-02 method B [32], which is used to establish whether the adhesion of a coating to a metallic substrate is at a generally adequate level in the laboratory. We made the cross-cut pattern at 90° angles through the coating first. Since thicknesses of coatings assessed in the experiment are less than 50 μm, the spacing of parallel cuts is 10 mm. The coating was brushed lightly with a soft brush after each cut to remove excess debris from the surface. A soft brush was used to remove any detached flakes or ribbons of films after making the required cuts. A piece of 3M Scotch 610 tape was placed over the grid, and rubbed firmly with the eraser on the end of a pencil to ensure good contact with the coating. After 120 s, the tape was removed rapidly from the free end at an angle of about 180°. Finally, the grid area was inspected for removal of coating from the substrate using the illuminated magnifier, and the rating of the adhesion was evaluated by comparison with descriptions and illustrations in ASTM standard. 3. Results Coatings synthesized at 120 °C 3 h, 160 °C 1 h, 160 °C 2 h, 160 °C 3 h and 160 °C 4 h were smooth and tan-yellow with vitreous luster, while the coating synthesized at 200 °C 3 h was almost entirely peeled off. Fig. 1 shows the X-ray diffraction pattern of sample 160°C3h, together with the Mg(OH)2 powder for comparison. In addition to the characteristic peaks of the AZ31 substrate, the XRD pattern of the sample can be assigned to diffractions of hexagonal structure for Mg(OH)2 according to JCPDS 75-1527. The two peaks below 25° can be assigned to the [003] and [006] diffractions of Mg2Al(OH)7 according to JCPDS 48-0601. The XRD pattern of the coating on AZ31 substrate displays obviously increasing of relative peak intensities corresponding to the [101] and [110] diffractions of the Mg(OH)2 phase, while the [001] diffraction is very weak, which is significantly
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Fig. 1. XRD patterns of the coating of 160 °C 3 h as well as the Mg(OH)2 powder.
different from the pattern of the Mg(OH)2 powder random oriented. It reveals a good orientation of Mg(OH)2 crystallites in the coating with their ab face perpendicular to the substrate (c-axis parallel to the substrate). The peak broadening of Mg(OH)2 indicates a quite small grain size of the Mg(OH)2 particles in the coating. Fig. 2 shows the surface morphology of the coating of 160°C3h which was observed by FESEM. These images demonstrate that the
Fig. 2. FESEM images for (a) surface morphology of the coating of 160°C3h; (b) magnification of region “1” in image (a); (c) magnification of region “2” in image (a).
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coating surface is composed of big hexagonal flakes and small irregular shaped platelets. The big hexagonal flakes are mostly around 400 nm–600 nm in diameter and almost parallel to the substrate; however, the small platelets are typically around 60 nm in diameter and nearly perpendicular to the substrate. The FESEM image of the cross-section of the coating of 160°C3h is shown in Fig. 3. The coating is obviously uniform and dense with a thickness of 18.5 μm. The element distributions along the line perpendicular to the surface are given in Fig. 3, and element content analysis of the region marked “A” is shown in Table 1. According to the element distributions across the Mg(OH)2 and AZ31 substrate, a transition layer in thickness of 6 μm is formed between the coating and the substrate. The analysis result of element content in region “A” indicates that the atomic ratio of Mg/O is close to that of the stoichiometric ratio (1:2) in Mg(OH)2. These results suggest that the coating mainly consists of magnesium hydroxide nanoparticles. Fig. 4 shows the relationship between the coating thickness and the hydrothermal time at 160 °C. We can conclude that the thickness of coating increases with hydrothermal time, and the coating deposition rate is a linear function of hydrothermal time. And the thickness of coating of 120°C3h is 2.7 μm, which is much smaller than that of 160°C3h, reveals that the thickness of coating increases with hydrothermal temperature. Polarization curves of naked AZ31 substrate and AZ31 with coatings of 120°C3h, 160°C1h, 160°C2h, 160°C3h and 160°C4h in the 3.5 wt.% NaCl aqueous solution at room temperature are shown in Fig. 5. The corrosion potentials (Ecorr) and the corrosion current densities (icorr) were derived directly from the potentiodynamic polarization curves by the Tafel extrapolation method, and the results are summarized in Table 2. It can be noticed that the corrosion current densities (icorr) of the AZ31 with coatings are reduced compared with the naked AZ31 substrate and the extent of the reduction increases with the thickness of coating. AZ31 with coating of 160°C3h affords a reduction in icorr to 5% of that of the naked AZ31 substrate, whereas the value of icorr is reduced to 0.08% of that of the naked substrate for the sample 160°C4h. However, the corrosion current densities (icorr) of the sample 160°C1h are about 11% of that of sample 120°C3h, through they have the similar values of coating thicknesses. The possible reason is that the structure of coating on the sample 160°C1h is more compact than that on sample 120°C3h. To our knowledge, the lower the corrosion current density is, the higher the corrosion resistance is obtained. Therefore, the corrosion rate of AZ31 magnesium alloy is effectively decreased by the Mg(OH)2 coatings. The result of the immersion test in 3.5 wt.% NaCl solution for 7 and 14 days at room temperature is shown in Fig. 6. A significant number of hydrogen bubbles were released from the surface of naked AZ31 substrates as soon as they were immersed into the NaCl solution, while the bubbles appeared from only the edge of AZ31 with coating of 160°C3h until 5 days later. We can generally estimate that large hydrogen bubbles are seen from the samples when the substrates under the coatings are corroded. The reason is that hydrogen is produced in the corrosion process of AZ31 in NaCl solution. The average corrosion rates of AZ31 substrate and AZ31 with coatings for 7 and 14 days reveal considerable differences according to Fig. 6. The corrosion rate of sample with coating decreases with the thickness of coating except that of sample 120°C3h which has a close corrosion rate to the AZ31 substrate. This is highly consistent with the result of potentiodynamic polarization test. The morphologies of samples after the corrosion test are displayed in Fig. 7, and the samples with coatings of 160°C2h, 160°C3h and 160°C4h are nearly intact, only with a tiny local corrosion at the edge of the coating. However, the edge of naked AZ31 substrate and samples 120°C3h and 160°C1h suffered severe damage. The results of potentiodynamic polarization test and immersion test demonstrate that the coatings can improve the corrosion resistance of AZ31 alloy efficiently. It can be attributed to the thick,
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Fig. 3. Cross section of the coating of 160°C3h and the element distribution along the line perpendicular to the surface.
uniform, and compact coatings formed by the hydrothermal reaction. The transportation of species (e.g. water and chloride ions) in the solution is blocked by the coating, which decreases the corrosion rate. The water wettabilities of the samples were studied by measuring the static contact angles of water on sample surfaces and the static
Table 1 The element content analysis of region “A” at the cross section of the coating in Fig. 3. Element
Wt.%
At.%
Mg O Al C
39.8 55.5 0.9 3.8
30.0 63.6 0.6 5.9
contact angle of water on naked AZ31 substrate is 40.5°, which is close to the static water contact angle on AZ31 reported by Ref. [32], whereas that on AZ31 with coatings is much lower: 11° for 160°C1h, 8.0° for 160°C2h, 7.5° for 160°C3h, 6.5° for 160°C4h and 13.5° for 120°C3h. These results suggest that the coating surfaces are more hydrophilic than the naked substrate. The adhesion strength of the coating to the substrate is a very important factor to determine the coating quality. Strong adhesion of the coating to the substrate is essential in the practical application. Fig. 8 shows the appearance of the cross-cut surface of sample 160°C3h after tape test according to ASTM D 3359-02 method B. In the process of adhesion test, no delamination or peeling off occurred on the cross-cutting surface of coating, and no significant peeling of either material after cross cutting through the Mg(OH)2 coating according to Fig. 8, indicate physically powerful adhesion between
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Table 2 Corrosion potential (Ecorr) and corrosion current density (icorr) of the naked AZ31 and samples with coatings prepared under different hydrothermal conditions. Sample
Ecorr (V)
icorr (μA/cm2)
AZ31 120 °C 160 °C 160 °C 160 °C 160 °C
− 1.356 − 1.328 − 1.291 − 1.337 − 1.392 − 1.324
6763 6072 654.2 114.8 330.2 5.421
3h 1h 2h 3h 4h
þ
2H ðaqÞ þ 2e→H2 ðgÞ
Fig. 4. Effect of hydrothermal time on the thickness of coating at 160 °C.
2þ
the coating and the AZ31 substrate (classification 4B according to ASTM D 3359-02 method B). The strong adhesion of coating to the AZ31 substrate might be attributed to the formation of the interpenetrating transition layer, which can enhance bonding between the coating and AZ31 substrate.
4. Discussion The crystal of magnesium hydroxide belongs to the hexagonal system and has a layered structure that consists of sheets of Mg octahedral stacked parallel to (001) plane with hydrogen bonding between layers. The ideal morphology of magnesium hydroxide is the hexagon shaped plate according to chemical bonding theory of single crystal, and the top/bottom faces of the plate correspond to the (001) planes [34]. We can obtain other morphologies by modifying crystallization conditions. In this paper, magnesium hydroxide nanocrystals were formed and grew on AZ31 magnesium alloy under the hydrothermal conditions with de-ionized water as mineralizer. The reaction of magnesium with water can be described as follows: þ
−
H2 OðlÞ→H ðaqÞ þ OH ðaqÞ
2þ
MgðsÞ→Mg ðaqÞ þ 2e
−
Mg ðaqÞ þ 2OH ðaqÞ→MgðOHÞ2 ðsÞ
ð2Þ
ð3Þ
Fig. 5. Potentiodynamic polarization curves of samples in 3.5 wt.% NaCl aqueous solution at room temperature.
ð4Þ
ð5Þ
Under the hydrothermal condition, the pressure of water is high, and the ionization constant of the water increases with the temperature and pressure, therefore the reaction of magnesium with water is exacerbated in the process, and a thick Mg(OH)2 coating can be synthesized in short time. The reaction between the solution and the AZ31 substrate occurred at the solid/liquid interface. Initially, a large number of fine Mg(OH)2 crystalline grains were generated at the interface under the hydrothermal conditions. Due to their high surface energy and inherent characteristic of polarity, these fine crystalline particles aggregated and covered the surface of the substrate. Subsequently, water permeated the initial Mg(OH)2 covering by passing through its interstices or pores and reacted with the AZ31 substrate below. A great number of fine Mg(OH)2 crystals were synthesized at the new solid/liquid interface and then aggregated. By repeating this process, the Mg(OH)2 coating was finally prepared on the AZ31 substrate. During this process, the prior generated crystal particles were modified by water via the process of dissolution recrystallization and closer to the ideal crystal gradually. Therefore, we observed big hexagonal flakes on the coating surface as shown in Fig. 2, and the pH value of de-ionized water in our study increased from the initial neutral 7 to 8 after the hydrothermal experiment. The XRD result with (101) preferred orientation in Fig. 1 suggests that irregular nanoplates with the edges preferentially directed out of the AZ31 substrate are the main composition of the coating. Ying Lv et al. [27] reported that the electrodeposited Mg(OH)2 thin film with (101) preferred orientation exhibited continuous porous network of nanosheets, whereas the film with (001) preferred orientation was composed of dense nanoplates. They explained this result by using
Fig. 6. Corrosion rates of samples after7 and 14 days immersion in 3.5% NaCl solution at room temperature.
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5. Conclusions
Fig. 7. Corrosion morphologies of samples after 7 and 14 days immersion in 3.5% NaCl solution at room temperature.
Mg(OH)2 3D structural models and hydrated states of Mg 2 + in solution with different interfacial pH value. However, the Mg(OH)2 coating prepared in this paper is (101) preferred orientation as well as high dense. The exact reason for this phenomenon is still unclear at present, and further studies are required. Generally, it is believed that a hydrophobic surface can reduce the electrolyte to penetrate through the surface and thus enhances the corrosion resistance of the coating system [35], but the synthesized coatings show highly hydrophilic. On the other hand, the surface energy is directly related to the surface wettability. According to adsorption theory, if the surface energy of the substrate is higher than the surface tension of the liquid, the substrate surface will be wetted. The (001) plane has the lowest surface energy among all crystal planes of Mg(OH)2 [34,36], and the (001) surface is electrostatically neutral and hydrophilic according to the result of molecular dynamics computer simulation [37]. Since the surface energies of other crystal planes are higher than that of (001) plane, the planes are all hydrophilic. Hence, the crystal of magnesium hydroxide is hydrophilic. It is reasonable that the coating composed of Mg(OH)2 nanocrystals is hydrophilic. However, the coating can offer much higher corrosion resistance whereas the surface is highly hydrophilic. In fact, the highly hydrophilic and high performance anticorrosive coating has been reported by Takahiro Ishizaki et al. [33]. The simulation results in Ref. [37] also indicate that the components of the H2O self-diffusion coefficient's tensors parallel to both the hydrophobic surface and hydrophilic surface (XX and YY components) are much larger than the ZZ component perpendicular to it, because diffusion perpendicular to the surfaces is greatly restricted by the film thickness. Therefore, the hydrophobicity isn't necessary for anticorrosive films, and the integrity and density of films may have more influence on the corrosion resistance ability. Moreover, the hydrophilic surface is usually beneficial to the cell adhesion, and the improvement of surface wettability may develop the ability of cell adhesion [38]. It shows that this hydrothermal method is much suitable for surface modification of biomedical magnesium alloy implants.
Fig. 8. SEM image of the coating of 160 °C 3 h tested for adhesion.
Coatings with a thickness from 2.2 μm to 27.2 μm have been successfully fabricated on AZ31 substrates by an extremely simple hydrothermal process only using de-ionized water as the reagent. The coatings were uniform and compact, composed of hexagonal magnesium hydroxide (Mg(OH)2) and a spot of monoclinic aluminum magnesium hydroxide (Mg2Al(OH)7). We investigated the influence of hydrothermal temperature and time on the thickness and corrosion resistance of the coating, and discussed the mechanism of the coating formation. The coatings with enough thickness can prevent the AZ31 substrates corrosion effectively according to the potentiodynamic polarization test and the immersion test. The coatings showed strong adhesion to the substrate. This treatment process is simple and environmentally friendly (only use water) and can treat the work piece with complicated shapes. Furthermore, as Mg(OH)2 is nontoxic and hydrophilic, this treatment method is very promising for surface modification of biomedical magnesium alloys. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2009IM033000 and 2011AA120101). Additional support was provided by the National Natural Science Foundation of China (11075077 and 50935001). References [1] Z. Yang, J.P. Li, J.X. Zhang, G.W. Lorimer, J. Robson, Acta Metall. Sin. (English Letters) 21 (2008) 313. [2] K.U. Kainer, Magnesium-Alloys and Technologies, Wiley-VCH Verlag, Weinheim, 2003. [3] Y. Kojima, Mater. Sci. Forum 350–351 (2000) 3. [4] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Biomaterials 27 (2006) 1728. [5] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid State Mater. Sci. 12 (2008) 63. [6] F. Witte, Acta Biomater. 6 (2010) 1680. [7] B.L. Mordike, T. Ebert, Mater. Sci. Eng., A 302 (2001) 37. [8] D. Eliezer, E. Aghion, F.H. (Sam) Froes, Adv. Perform. Mater. 5 (1998) 201. [9] R.W. Revie, H.H. Uhlig, in: R.W. Revie, H.H. Uhlig (Eds.), Corrosion and Corrosion Control: An Introduction to Corrosion Science and Engineering, fourth ed., John Wiley & Sons, Inc., Hoboken New Jersey, 2008, p. 399. [10] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [11] E. Shi, C.R. Cho, M.S. Jang, S.Y. Jeong, H.J. Kim, J. Mater. Res. 9 (1994) 2914. [12] J.O. Eckert Jr., C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, J. Am. Ceram. Soc. 79 (1996) 2929. [13] C.R. Cho, M.S. Jang, S.Y. Jeong, S.J. Lee, B.M. Lim, Mater. Lett. 23 (1995) 203. [14] A.T. Chien, J. Sachleben, J.H. Kim, J.S. Speck, F.F. Lange, J. Mater. Res. 14 (1999) 3303. [15] T. Morita, T. Kanda, M. Kurosawa, T. Higuchi, Jpn. J. Appl. Phys. 36 (1997) 2998. [16] H. Ishizawa, M. Makoto, J. Biomed. Mater. Res. 29 (1995) 1071. [17] X.X. Guo, S.L. Xu, L.L. Zhao, W. Lu, F.Z. Zhang, D.G. Evans, X. Duan, Langmuir 25 (2009) 9894. [18] D.E. Beving, A.M.P. McDonnell, W.S. Yang, Y.S. Yan, J. Electrochem. Soc. 153 (2006) B325. [19] J. Formosa, J.M. Chimenos, A.M. Lacasta, L. Haurie, Thermochim. Acta 515 (2011) 43. [20] E. Suzuki, Y. Nomoto, M. Okamoto, Y. Ono, Catal. Lett. 48 (1997) 75. [21] R.V. Siriwardane, R.W. Stevens Jr., Ind. Eng. Chem. Res. 48 (2009) 2135. [22] J.C. Yu, A.W. Xu, L.Z. Zhang, R.Q. Song, L. Wu, J. Phys. Chem. B 108 (2004) 64. [23] J.P. Lv, L.Z. Qiu, B.J. Qu, J. Cryst. Growth 267 (2004) 676. [24] Y. Ding, G.T. Zhang, H. Wu, B. Hai, L.B. Wang, Y.T. Qian, Chem. Mater. 13 (2001) 435. [25] C.L. Yan, D.F. Xue, L.J. Zou, X.X. Yan, W. Wang, J. Cryst. Growth 282 (2005) 448. [26] M. Dinamani, P.V. Kamath, J. Appl. Electrochem. 34 (2004) 899. [27] Y. Lv, Z. Zhang, Y.Q. Lai, J. Li, Y.X. Liu, CrystEngComm (13) (2011) 3848. [28] T. Ishizaki, S.P. Cho, N. Saito, CrystEngComm (11) (2009) 2338. [29] Y.Y. Zhu, G.M. Wu, Y.H. Zhang, Q. Zhao, Appl. Surf. Sci. 257 (2011) 6129. [30] M.J.M. Mies, E.V. Rebrov, J.C. Jansen, M.H.J.M. de Croon, J.C. Schouten, Micropor. Mesopor. Mat. 106 (2007) 95. [31] ASTM-G31-72: Standard Practice for Laboratory Immersion Corrosion Testing of Metals, ASTM, Philadelphia, PA, USA, 2004. [32] ASTM D 3359-02: Standard Test Methods for Measuring Adhesion by Tape Test, ASTM, Philadelphia, PA, USA, 2002. [33] T. Ishizaki, I. Shigematsu, N. Saito, Surf. Coat. Technol. 203 (2009) 2288. [34] D.F. Xu, X.X. Yan, L. Wang, Powder Technol. 191 (2009) 98. [35] J. Zhang, Y.F. Chan, Q.S. Yu, Prog. Org. Coat. 61 (2008) 28. [36] P. Fellner, J. Híveš, V. Khandl, M. Králik, J. Jurišová, T. Liptaj, L. Pach, Chem. Pap. 65 (2011) 454. [37] J.W. Wang, A.G. Kalinichev, R.J. Kirkpatrick, Geochim. Cosmochim. Acta 70 (2006) 562. [38] J. Takebe, S. Itoh, J. Okada, K. Ishibashi, J. Biomed. Mater. Res. A 51 (2000) 398.