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Hydrothermal synthesis and corrosion behavior of the protective coating on Mg-2Zn-Mn-Ca-Ce alloy
Progress in Natural Science: Materials International (xxxx) xxxx–xxxx
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Original Research
Hydrothermal synthesis and corrosion behavior of the protective coating on Mg-2Zn-Mn-Ca-Ce alloy☆ ⁎
Dan Songa,b,c, Guanghui Guoa, Jinghua Jianga, , Liwen Zhangb, Aibin Maa,d, Xiaolong Mab, Jianqing Chena, Zhaojun Chenga a
College of Mechanics and Materials, Hohai University, Nanjing 210098, China Department of Material Science and Engineering, North Carolina State University, Raleigh 27606, USA c School of Materials Science and Engineering, Southeast University, Nanjing 211100, China d Jiangsu Collaborative Innovation Center of Advanced Micro/Nano Materials & Equipment, Nanjing University of Science & Technology, Nanjing 210094, China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Protective coating Mg alloy Hydrothermal synthesis Microstructure Corrosion resistance
Protective coatings were synthesized on the Mg-2Zn-Mn-Ca-Ce Mg alloy through the hydrothermal method with de-ionized water as the reagent. The coatings were composed of Mg hydroxide, generally uniform and compact. Hydrogen evolution tests and electrochemical tests in the Hanks’ solution demonstrated that the Mg(OH)2 coatings effectively decreased the bio-degradation rate of the Mg alloy substrate. Microstructure observation showed that the coating formation on the secondary phases was more difficult than that on the α-Mg matrix, which led to micro cracks and pores on the secondary phases after drying. Over synthesizing time, the coating layer on secondary phases gradually becomes more compact and uniform. Meanwhile, owing to the thicker and more compact coatings, the corrosion resistance and protective efficiency were significantly improved with longer synthesizing time as well.
1. Introduction As the lightest structural metallic material，magnesium (Mg) and its alloys are widely used in aeronautical engineering, automobile industry and 3C (computer, communication and consumer) product fields due to their outstanding physical and mechanical properties, such as low densities, high strength, good machinability and potentially high strength/weight ratios [1–3]. Mg alloys also hold the great potential for applications in biomaterials because of their similar densities and elastic modulus with human bone, as well as the excellent biocompatibility in vivo experiments. Mg and its alloys have been studied as implant materials for bone screw, bone plates and vascular scaffold to replace traditional stainless steels, titanium alloy and biological ceramic materials [4]. However, the main limitation to their application in medical aspect is the fast bio-degradation rate in the body environment due to their poor corrosion resistance. Most of the Mg implants suffer severe degradation prior to the recovery of the injured tissues. Thus, it is of great practical significance to reduce the bio-degradation rate of Mg implants in order to prolong their service life. Various methods, such as alloying [5–7], heat treatment [8,9] and
surface treatment [10,11] have been employed to improve corrosion resistance of Mg implants. Progress has been witnessed in Mg-Zn-MnY alloy, Mg-Nd-Zn alloy and etc. [12,13] while further efforts are still imperative to further decrease biodegradation rate of the biomedical Mg alloys. Coating on Mg alloys can effectively provide a barrier layer between the metal and the corrosion medium, which provides an alternative to approach the issue. So far there are lots of coating technologies available for Mg alloy, such as micro-arc oxidation [14,15], electrochemical deposited coating [16], and chemical conversion coating [17]. Hydrothermal synthesis of protective coating is also one of the most promising methods because it is simple, efficient and cost-effective [18]. In addition, hydrothermal crystallization occurs on a 3-dimensional structure, making it easy for commercial scale-up. Last but not least, many literatures pointed out that protective coating synthesized by this method show excellent corrosion-inhibition property [19,20]. Up to now, most of the hydrothermal synthesized coatings on Mg and its alloys were fabricated in alkaline solutions for sufficient OH− ions [21,22]. The coatings were uniform and compact with good adhesion strength and better corrosion resistance. Recently, it was reported that the de-ionized water can also be used as the reagent to
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (J. Jiang). http://dx.doi.org/10.1016/j.pnsc.2016.11.002 Received 23 March 2016; Received in revised form 4 November 2016; Accepted 9 November 2016 1002-0071/ © 2016 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Song, D., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.11.002
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The material to be coated was cut from a casting bio-medical Mg alloy, which was designed and fabricated in our previous research [25]. The chemical compositions of this alloy were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) (Iris Advantage 1000, USA), as listed in Table 1. The specimens with the size of 10 mm×10 mm×5 mm were cut by an electric discharging machine, polished with SiC papers up to 1800 grades, ultrasonically cleaned in acetone and ethanol for 5 min each, and dried in air. The reactor used in this experiment is a stainless steel autoclave (100 mL) with a Teflon liner without electrical heating facility and pressure gauge. The de-ionized water was poured into the reactor to 70% volume as the reaction solution. Three parallel samples were treated in one reactor simultaneously. The reactor was heated via an electric furnace and the hydrothermal-heating temperature was set at 160 ± 0.1 °C. The hydrothermal-synthesizing time was counted after the furnace temperature reached the set temperature. In this work, three heating times of 3 h, 4.5 h and 6 h were set to study the effect of hydrothermal-synthesizing time on the microstructure and corrosion resistance of the protective coatings. The samples coated at different times were named 3h-coated sample, 4.5h-coated sample and 6hcoated sample, respectively.
and electrochemical tests. Hanks’ solution was selected as the simulated body fluid, which has been widely used in in-vitro tests of many implant materials. Table 2 presents the chemical composition of the Hanks’ solution used in this study. All the in-vitro tests were conducted at the temperature of 37 °C. Before the in-vitro tests, the coated and uncoated samples were molded in the epoxy with a squared exposure of 1 cm2. Regarding uncoated sample, the exposed surfaces were mechanically polished, then cleaned by acetone and ethanol. For the coated sample, the exposed surfaces were only cleaned by acetone and ethanol. The Hanks’ solution was renewed every single day to keep the corrosion environment consistent. After different immersion times, the evolved hydrogen was collected by an upside down funnel above a specimen during immersion test in Hanks’ solution, then went into a burette and gradually displaced the test solution in the burette. By this way, the volume of the evolved hydrogen can be easily measured by monitoring the height of the test solution in the burette. In this study, the hydrogen evolution rate was calculated in mL m−2 day−1, and the coated samples was measured for 108 h in Hanks’ solution. After in-vitro immersing tests, the corrosion morphologies of the samples were observed using a digital microscope (Hirox, KH-7700, USA). Electrochemical tests were adopted to investigate electrochemical corrosion behaviors of the coated and uncoated Mg alloy samples in the Hanks’ solution via a Parstat 2273 advanced potentiostat. A threeelectrode cell was employed, which was composed of the tested sample as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a large-area platinum sheet as the counter electrode. The samples for electrochemical testing were prepared similarly to that for the in-vitro immersion test, including the copper wire connection to the tested sample. Three electrochemical measurements, including open circuit potential (OCP) test, potentiodynamic polarization (PDP) test and electrochemical impendence spectroscopy (EIS) test, were systematically conducted. Because of the relatively poor corrosion resistance of Mg alloy, the immersion time of all OCP tests was set for 3600 s, and the PDP tests were performed at a common scan rate of 1 mV s−1. The frequency range of EIS tests were from 10 kHz to 10 mHz. To get enough feedback signals, the amplitude of sinusoidal potential used in coated and uncoated samples were 20 mV.
2.2. Microstructure characterization
3. Results and discussion
Before and after hydrothermal-synthesizing treatment, the macro morphologies of the samples were observed by a digital microscope (Hirox, KH-7700, USA). The surface and cross-sectional micromorphologies of the coatings and Mg alloy substrate were examined by scanning electron microscope (SEM，Hitachi, S3400N, Japan). Before the SEM observation, all the samples were conductively coated by gold. The element distribution of the substrate and coating was characterized by the energy dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) analysis of the coated sample was performed using a Bruker D8 Advance diffractometer (Bruker AXS, Germany) with Cu Kα1 radiation. The θ-2θ diffraction patterns were scanned from 5° to 90° with a scanning rate of 2°/min.
3.1. Surface and cross-sectional morphologies of the coatings
coat pure Mg and Mg alloys with sufficient corrosion resistance [23,24]. Clearly, the de-ionized water is better than alkaline solution considering biocompatibility. Previously, a novel kind of Mg-2Zn-Mn-Ca-Ce alloy was designed and fabricated [25]. It exhibits improved corrosion resistance, but its in-vitro bio-degradation rate was still away from medical standards. To further reduce the bio-degradation rate of the Mg-2Zn-Mn-Ca-Ce alloy, the hydrothermal synthesis of the protective coating in de-ionized water was conducted in this study. Meanwhile, the effects of hydrothermal-synthesizing time, as well as the alloy's secondary phases, on the microstructure and corrosion resistance of the protective coatings were also systematically studied. 2. Experimental 2.1. Synthesizing process of the protective coating on Mg-2Zn-MnCa-Ce alloy
Fig. 1a is a typical SEM photograph of the substrate alloy, which clearly shows two major micro-constituents: α-Mg phase and secondary phases. The secondary phases has been identified as Ca2Mg6Zn3, Mg2Ca and Mg12CeZn phase in previous work, which were net-like and distributed along the grain boundaries of the α-Mg phase [25]. In Fig. 1b and c, after hydrothermal synthesis for 3 h, a macroscopically uniform and compact coating had been formed on the Mg alloy substrate, including some of flower-shaped stacking clusters. However, careful examinations of SEM photograph reveal some micro defects, including cracks and pores. It's also worth to note that those micro defects always exist in the vicinity of the secondary phases of the Mg alloy substrate. In contrast, the coating layer on the α-Mg matrix is more compact, and free of micro crack and pores under the set magnification. With the increase of hydrothermal-synthesizing time, as shown in 4.5h coated sample (Fig. 1d and e), the stacking clusters increased and also became larger. Meanwhile, the presence of those micro defects gradually reduced on the surface. Finally in 6h-coated sample (Fig. 1f and g), with sufficient reaction time, micro cracks and pores nearly disappeared on the surface. In addition to the observed improvement of the coating compactness and uniformity, the structure units of the coating layer also change in shape with the increase of synthesizing time. The well-known hexagonal flake structural units
2.3. Corrosion tests Corrosion behaviors of the coated and uncoated samples were studied by in-vitro tests, including hydrogen-evolution immersion test Table 1 Chemical composition of Mg-2Zn-Mn-Ca-Ce alloy (wt%). Zn
Mn
Ca
Ce
Mg
2.00
0.50
1.02
1.35
Balance
2
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Table 2 Chemical composition of Hanks’ solution. Solution
Hanks'
Chemical composition (mmol L−1) NaCl
CaCl2
MgSO4
KCl
KH2PO4
Na2HPO4
D-Glucose
NaHCO3
137
1.261
0.814
5.33
0.44
0.338
5.56
4.17
mole hydrogen. Therefore, in theory, measuring the volume of hydrogen evolution is equivalent to measuring the weight-loss of Mg dissolved. Basically, the hydrogen evolution is caused by the biodegradation in the Mg alloy substrate. The surface coating will provide protection against the penetration of corrosive medium into the Mg alloy substrate while, in turn, the generated hydrogen in the substrate will also destroy the integrity of the coating. Therefore, one can envision that the lower hydrogen-evolution rate of the sample will indicate the slower degradation and the better protection of the coatings. Clearly, the hydrogen-evolution rate of the uncoated sample is times larger than that of the coated samples during the whole immersion testing, which indicate the significant protective effect of the coating. With the increase of immersion time, the line slope (can be regarded as hydrogen-evolution rate) of the uncoated sample increased, indicating the continuous increased bio-degradation rate. As the hydrogen-evolution rate of the uncoated sample was extremely larger, the calculation of its hydrogen evolution rate was stopped after immersion for 3 days since its difference to coated samples becomes remarkable. In contrast to the uncoated sample, there's an incubation period, when detectable hydrogen evolution has not been generated, in all three coated samples. As marked in Fig. 5, this incubation period was about 39h in the 3h-coated sample, 48h in the 4.h-coated sample and 81h in the 6h-coated sample, respectively. The incubation period is a crucial parameter to characterize the protective efficiency of the coating: the longer incubation period，the better protection of the coating. The incubation period is mainly decided by the penetration processing of the corrosive medium through the coating into the substrate. Enhanced compactness and larger thickness of the coating will provide stronger barrier against the penetration of corrosive medium, which made the incubation period of the coated sample much longer. The increased compactness and thickness of the coating with the increased synthesizing time has been verified in the previous SEM observation and thereby explicitly explains the enhanced incubation period of the coatings. After incubation period, all the coated samples experienced the increased hydrogen evolution. This phenomenon should be caused by the mutual promotion of accelerated biodegradation of the substrate and the breakdown of the coating caused by hydrogen evolution. After immersion for 108 h, the totally hydrogen evolution of the 3h-coated sample, 4.5h-coated sample and 6h-coated sample are about 5.05 mL cm−2, 3.35 mL cm−2 and 2.35 mL cm−2, respectively. Fig. 6 presents the optical macro-morphologies of the coated and uncoated samples after hydrogen-evolution immersion in Hanks’ solution for 5 days. It turned out that the uncoated sample (Fig. 6a) has been completely corroded, whereas all the coated samples (Fig. 6b– d) showed little change compared to the original coatings illustrated in Fig. 2. The uncoated sample surface has been covered with thick and stacked corrosion products, presenting grey surface with local and severe corrosion zones. According to our previous research [25], the corrosion products were mainly composed of Mg(OH)2. Relatively, the coated samples suffered minor corrosion damage, with limited macro corrosion spots on the coating surface. Given the number and size of the corrosion spots, the corrosion damage of the 4.5h and 6h synthesized coatings were relatively less than that of the 3h-synthesized coating.
[23,24] of the coatings gradually become obvious and larger, especially in stacking clusters. Note that the hexagonal flake structural units can be found all over the coatings, covering both the α-Mg phase and the secondary phase of the substrate. Even it can also be found in the 3hcoated sample surface in the form of extremely small size. Fig. 2 shows the cross-sectional morphologies of the coated samples. All the coatings were well bonded to substrate, free of micro-cracks at the substrate/coating interface. There are two remarkable differences between the three coated samples. First, the average coating thickness of the 3.5h-coated sample is about 14 µm, while those of the 4.5h-coated and 6h-coated sample are about 18 µm and 20 µm, respectively. The coating thickening with the increase of hydrothermal-synthesizing time is reasonable. The other crucial difference is that the coatings of the 4.5h-coated and the 6h-coated sample are more uniform and compact than that of the 3h-coated sample in the cross-sectional view. Pores and loose connections can be easily found inside of the coating of the 3h-coated sample, particularly above the secondary phases of the substrate alloy, which were marked by the white arrow in the Fig. 2a. 3.2. Phase and chemical composition analysis of the coatings XRD identification was employed to analyze the phases of the coated samples. As shown in Fig. 3, the typical Mg patterns and Mg(OH)2 patterns are identified, corresponding to the α-Mg phase of the Mg alloy substrate and the coating layer, respectively. The absence of the secondary phases in XRD patterns is probably attributed to its minor volume fraction and the sensitivity of the X-ray instrument under the current settings. Moreover, no other phase was detected in the XRD pattern, which means the coatings on the samples may be mainly composed of Mg(OH)2. Beside the similarity of the three samples, some difference can be still found in the XRD. Apparently, the peak intensity of the Mg(OH)2 was significantly increased for longer synthesizing time，and its intensity ratio to α-Mg phase qualitatively indicate the comparisons of their volumes. Given the same scanning depth, the relatively intensified Mg(OH)2 peak is an apparent evidence to elucidate the thickening of the coating layer. This phenomenon completely agrees with the cross-sectional SEM observation of coating layer. Fig. 4 is the EDS mapping analysis of the cross section of the 6hcoated sample's coating and the substrate alloy. As a result, the coating elements are mainly composed by Mg and O, which is consistent with XRD analysis. In the substrate, the Zn, Ca and Ce alloying elements are primarily distributed in the secondary phase, and the Mn element is distributed uniformly in the α-Mg matrix. This result agrees well with our previous research [25], which reported that the secondary phases were Ca2Mg6Zn3, Mg2Ca and Mg12CeZn phase. 3.3. Hydrogen-evolution immersion testing The obtained hydrogen-evolution rates in immersion tests are shown in Fig. 5. The overall corrosion reaction of Mg at its free corrosion potential can be expressed as follows [26,27]. Mg+H2O→Mg2++2OH−+H2
(1)
This means that the dissolution of one mole Mg atom generates one 3
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Fig. 1. SEM microstructure and surface morphologies of the coated and uncoated sample (a) Microstructure of the uncoated Mg alloy, (b) and (c), (d) and (e), (f) and (g) are the surface morphologies of the 3h-coated, 4.5h-coated and 6h-coated samples observed at low and high magnification, respectively.
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Fig. 3. X-ray diffraction patterns of the coated samples..
uncoated sample may be attributed to the formation of partialprotective surface layer (unstable thin Mg(OH)2 film with porous structure). In contrast, the OCP values remained level during the whole test in all coated samples and their difference is trivial. The little fluctuation in OCP values may be caused by the exchange of the aggressive medium, and the relatively stable OCP values in coated samples also indicate the efficient barrier against the penetration of mass aggressive medium into the substrate. PDP test was conducted to study the coatings’ positive effect against the corrosion propagation in the substrate under the situation of strong polarization. All the samples were immersed in the solution for 1 h before polarization tests to achieve their stable OCP values. The corrosion potentials (Ecorr) and the corrosion current densities (Icorr) were derived directly from the PDP curves (as seen in Fig. 8) by the Tafel extrapolation method, and are summarized in Table 3. Note that these samples have similar polarization curve shapes in the tested solution. The coated sample had nobler Ecorr values than the uncoated sample, and their values slightly increased with hydrothermal-synthesizing time. The nobler Ecorr values of the coated samples should be caused by the appearance of the Mg(OH)2 coating. In addition, the Icorr of the coated sample drastically decreased. The Icorr value of the 6h-coated (3.53×10−7 A cm−2) decreased by two orders of magnitude compared with the uncoated sample (5.85×10−5 A cm−2). To our knowledge, Icorr presents a kinetic characteristic of a given metal-electrolyte system, which can characterize the corrosion rate directly. The lower Icorr is, the less corrosion rate can be obtained. In our study, the corrosion current was created from the corrosion damage in Mg alloy substrate. The less Icorr means less corrosion damage in the substrate of the coated sample, evidencing the efficient kinetic barrier of the coatings. Again, this protective effect was enhanced with increasing hydrothermal-synthesizing time. EIS was further conducted to study the stability of the coating in the Hanks’ solution. Fig. 9 shows EIS Nyquist plots of the coated and uncoated samples after in-vitro immersion for one hour. All Nyquist plots were composed of capacitive arcs and inductive arcs. The inductive arcs demonstrate that all the samples have already been corroded to some extent. The capacitive arc diameter is associated with charge-transfer resistance. The larger the diameter is, the better corrosion resistance the experimental sample has [29,30]. Generally, the capacitive arcs of the coated samples are of greater diameters than that of the uncoated substrate sample, indicating their higher corrosion resistances. This phenomenon becomes even noticeable when the synthesizing-time is longer. Moreover, the coated samples have two capacitive arcs while the uncoated substrate has only one. Two capacitive arcs are the typical Nyqusit plot of many coated samples
Fig. 2. Cross section morphologies of the coated samples. (a) 3h-coated sample, (b) 4.5h-coated sample and (c) 6h-coated sample.
3.4. Electrochemical bio-degradation behavior Electrochemical tests were performed to study the electrochemical in-vitro bio-degradation behavior of the coated and uncoated samples in the Hanks’ solution. Fig. 7 presents the OCP curves of the coated and uncoated samples immersed in Hanks’ solution during the initial 3600 s. Generally, the coated samples have nobler OCP values compared to the uncoated Mg alloy substrate. Note that the OCP values of the coated samples were actually determined by the mixed electrode potential of both the substrate and the coating layer. Therefore, one can infer that the coating layer of Mg(OH)2 would have much nobler potential in the Hanks’ solution. During the whole OCP testing, the uncoated sample underwent two stages, including the fast-increasing stages for about 1000 s followed by a stable stage with less vibration in OCP values. According to the Ref. [28], the increasing OCP value of the 5
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Fig. 4. EDS mapping analysis of the cross section of the 6h-coated sample.
used. The Rs(CdlRt(RLL)) equivalent circuit corresponds to the uncoated sample, which presents one conductive loop and one inductive loop. The Rs(Cf(Rp(CdlRt(RLL)))) equivalent circuit corresponds to the coated samples, which presents two conductive loops and one inductive loop. In those models, Rs is the electrolyte solution resistance, Rp and Cf represent the microporous resistance and capacitance of the Mg(OH)2 coating, Cdl and Rt represent the double layer capacitance and the charge transfer resistance of the substrate, RL and L represent the inductive loop [32]. The fitted EIS parameters of the coated and
because the high/low-frequency arcs are related to EIS signal responded from the substrate/coating, respectively [31]. This is also the case in our study. It is noteworthy that both capacitive arcs of the coating and substrate increased with the synthesizing time. The increased coating arc suggests the enhanced corrosion resistance of the coating, and the increased substrate arc suggests the less corrosion damage in the substrate. Equivalent circuit was widely used for the accurate simulation of the EIS data. According to the EIS plots, two equivalent circuits were 6
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Fig. 7. Open circuit potential curves of the coated and uncoated samples in Hanks’ solution for the initial 3600 s.
Fig. 5. Hydrogen-evolution rates of the coated and uncoated samples immersed in Hanks’ solution.
Fig. 6. Optical macro-morphologies of the coated and uncoated samples after hydrogen-evolution immersion in Hanks’ solution for 5 day. (a) uncoated sample; (b) 3h-coated sample; (c) 4.5h-coated sample and (d) 6h-coated sample.
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resistance named after pore or ionic conducting defect resistance. In the substrate/coating/medium corrosion system, the Rp can be regarded as the corrosion resistance of the coating, and the Cf reflects the compactness of the coating. As seen in Table 4, the increase in Rp values and decrease in Cf values indicated that the gradually increased corrosion resistance and better compactness of the coatings occurred with increasing hydrothermal-synthesizing time. Based on the discussions on EIS results, one can find that the hydrothermal-synthesized coatings can provide sufficient protection to the substrate, and their protective efficiency will be improved gradually with the synthesizing time. 3.5. Effect of alloys’ secondary phase and hydrothermal-synthesizing time on the coating formation It is generally believed that a hydrophobic surface can reduced the penetration of electrolyte through the surface and can enhance the corrosion resistance of the coating system [33]. As a corrosion barrier, the characters of the protective coating, such as the thickness, compactness and uniformity, should play great roles in the protective efficiency. According to the microstructure observation and corrosion tests, it can be confirmed that the Mg(OH)2 protective coatings can be synthesized on the surface of Mg-2Zn-Mn-Ca-Ce biomedical Mg alloy by the hydrothermal synthesis method, and its protective efficiency gradually increases with increasing the hydrothermal synthesizing time due to the to the thicker and more compact coatings. The synthesizing reaction of the hydrothermal protective coating on magnesium alloy using de-ionized water has been reported in the Ref. [23]. The coating-forming reaction of Mg with water can be described as follows:
Fig. 8. Polarization curves of the samples obtained after immersion in Hanks’ solution for 1 h. Table 3 Corrosion potential (Ecorr) and corrosion current density (Icorr) of the uncoated samples. Sample
Ecorr (v)
Icorr (A cm−2)
Uncoated 3h-coated 4.5h-coated 6h-coated
−1.486 −1.383 −1.371 −1.327
5.85×10−5 4.42×10−6 6.78×10−7 3.53×10−7
H2O(l)→H++OH−(aq)
(2)
Mg(s)→Mg2+(aq)+2e
(3)
2H+(aq)+2e→H2(g)
(4)
Mg2+(aq)+2OH−(aq)→Mg(OH)2(s)
(5)
According to Zhu's research [23], longer synthesizing time promoted the thickening of coatings due to more generation and deposition of Mg(OH)2 on the substrate surface. The similar phenomenon was also found in this study. In addition to synthesizing parameters, the alloying elements and the microstructure of the substrate alloy should also have important impacts on the formation and performance of the coatings. In our study, the Mg-2Zn-Mn-Ca-Ce biomedical Mg alloy has the typical multiple-phase structure (as shown in Fig. 1a), including α-Mg phase and secondary phases (Ca2Mg6Zn3, Mg2Ca and Mg12CeZn phase). As demonstrated in previous SEM analysis, the coating on the α-Mg phase was much thicker and compacter than that on the secondary phases in the early synthesis stage (say 3h-coating in this work). Over time, the coatings on multiple phases become less different, as shown in the 6hcoating samples. A possible mechanism to elucidate this phenomenon is proposed as below. The coating formation on multiple-phase Mg alloy can be reasonably divided into three stages: nucleation, thickening and densification,
Fig. 9. EIS Nyquist plots of the samples obtained after immersion in Hanks’ solution for 1 h.
uncoated samples were illustrated in Table 4. The coated sample has larger Rt values and lower Cdl values compared to the uncoated sample, which indicates less corrosion damage and better surface integrity of the substrate of the coated samples. Meanwhile, with the increase of the synthesizing time, the coated sample showed gradually increased Rt values and decreased Cdl values. Those two kinds of parameters indicated the gradually alleviated corrosion damage in the substrate of the coated samples. Rp (polarization resistance) is the relevant Table 4 Fitted EIS parameters of the samples. Sample
Rs (Ω cm2)
Cf (10−6 F cm−2)
Rp (Ω cm2)
Cdl (10−6 F cm−2)
Rt (Ω cm2)
Uncoated 3h-coated 4.5h-coated 6h-coated
26.67 14.96 17.89 12.56
/ 31.1 21.2 12.3
/ 1907 2485 3816
138.4 71.8 40.5 30.2
1147 1604 1748 2451
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Fig. 10. Schematic diagrams of the coating formation mechanism (a) and illustration of surface morphologies of the coated and uncoated sample (b).
boundaries and producing the stacking cluster with large-size hexagonal flake structural units. Eventually, the coating layers over both phases became more compact and uniform as a whole over extended period of hydrothermal synthesizing. Fig. 10b is the schematic illustration of resulted surface morphologies over the time of hydrothermal synthesis followed by drying process, where the initial pattern is simulated from the SEM observation. As discussed in Fig. 10a, the coatings on the secondary phases were relatively thinner over a short time. Micro defects of cracks and pores were likely to be generated at sites adjacent to the secondary phases because of the shrinking stress during the drying processing after hydrothermal synthesis (as shown in Fig. 10b-2). In the long-time synthesized sample, the coating became more compact and uniform. Therefore, chances are less for the appearance of cracks and pores during the drying processing (as shown in Fig. 10b-3). The existence of cracks and pores is definitely harmful to the barrier efficiency of Mg(OH)2 coating due to the easy penetration of electrolyte through the cracks and pores in the corrosive environment. Overall, the illustration in Fig. 10 explains the poor corrosion resistance of the short-term synthesized specimen (3h-coated sample) in comparison to the one after long-time synthesis. Through this research, it can be concluded that the secondary phases of the Mg-2Zn-Mn-Ca-Ce alloy will greatly affect the formation and weaken the performance of its hydrothermal synthesized coating. In other words, homogenization via solid-solution treatment to reduce the second phase in substrate alloy is effective to improve the microstructure and performance of the hydrothermal synthesized coating. Meanwhile, increasing synthesizing time will benefit the thickness, uniformity and compactness of the coating, leading to the enhanced protective efficiency.
as schematically shown in Fig. 10a. First, the coating layer is nucleated from the initial deposition and crystallization of Mg(OH)2 crystals on the substrate surface (Fig. 10a-1). The required Mg2+ and OH- are generated from the original substrate/electrolyte interfacial reactions (2), (3), and therefore, their reaction rates are crucial to the coating nucleation rate. The α-Mg phase has larger Mg content than the secondary phases, providing more possible reaction sites. In addition, the secondary phases are more stable than the α-Mg phase [25], which makes it more difficult to react with water. Therefore, the coating nucleation rate on the secondary phases will be slower, which leads to the weaker and locally more defective coatings on the secondary phase. Next, during coating thickening (Fig. 10a-2), the coating on both the αMg phase and the secondary phases was not ideally compact, providing some corridors within the initial coating layer. The electrolyte can thereby penetrate to the substrate/coating interface along those corridors and continue the reactions, transforming the top substrate into new Mg(OH)2 layers. On the other hand, part of the generated Mg2+ and OH- will also diffuse to the coating surface through those corridors and coarsen the as-nucleated Mg(OH)2 crystals, i.e. thickening the coating. Both the two scenarios above are conceived to contribute to the coating thickening and make this process bi-directional. One direction is from the inner coating to the substrate (innerthickening, marked in white arrow) while the other direction is from the outer coating to the electrolyte (outer-thickening, marked in black arrow). For a similar reason in the nucleation stage, the thickening rate is assumed faster for α-Mg phase than that for the secondary phases until the coatings on α-Mg phase become sufficiently compact and uniform. Thereafter, densification stage starts (Fig. 10a-3). The thickening on the α-Mg phase will be largely retarded because the as-coated layer become thick and compact enough against the penetration of electrolyte. However, inner-thickening will also process on the secondary phase. Furthermore, phase boundaries between the α-Mg phase and the secondary phase still provide shortcut to the penetration of electrolyte, leading to continuous and local corrosion reaction along
4. Conclusion The protective coatings were fabricated on the Mg-2Zn-Mn-Ca-Ce 9
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[8] W. Zhou, T. Shen, N.N. Aung, Corros. Sci. 52 (2010) 1035–1041. [9] M.H. Tsai, M.S. Chen, L.H. Lin, M.H. Lin, C.Z. Wu, K.L. Ou, C.H. Yu, J. Alloy. Compd. 509 (2011) 813–819. [10] J. Gray, B. Luan, J. Alloy. Compd. 336 (2002) 88–113. [11] Y.H. Gu, S. Bandopadhyay, C.F. Chen, C.Y. Ning, Y.J. Guo, Mater. Des. 46 (2013) 66–75. [12] H. Hornberger, S. Virtanen, A.R. Boccaccini, Acta Biomater. 8 (2012) 2442–2455. [13] J.L. Li, L.L. Tan, P. Wan, X.M. Yu, K. Yang, Mater. Sci. Eng. C 49 (2015) 422–429. [14] K. Yang, X.L. Lin, Surface modification of magnesium and its alloys for biomedical applications. Volume 1: biological interactions, Mech. Prop. Test. (2015) 231–260. [15] X.J. Cui, X.Z. Lin, C.H. Liu, R.S. Yang, X.W. Zheng, M. Gong, Corros. Sci. 90 (2015) 402–412. [16] M. Razavi, M. Fathi, O. Savabi, B.H. Beni, D. Vashaee, L. Tayebi, Ceram. Int. 40 (2014) 9473–9484. [17] X.J. Jiang, R.G. Guo, S.Q. Jiang, Appl. Surf. Sci. 341 (2015) 166–174. [18] L.D. Wang, K.Y. Zhang, W. Sun, T.T. Wu, H.R. He, G.L. Liu, Mater. Lett. 106 (2013) 111–114. [19] H. Jeong, Y. Yoo, Surf. Coat. Technol. 284 (2015) 26–30. [20] Y. Zhu, G. Wu, Y.H. Zhang, Q. Zhao, Appl. Surf. Sci. 257 (2011) 6129–6137. [21] M. Tomozawa, S. Hiromoto, Appl. Surf. Sci. 257 (2011) 8253–8257. [22] M. Tomozawa, S. Hiromoto, Acta Mater. 59 (2011) 355–363. [23] Y.Y. Zhu, Q. Zhao, Y.H. Zhang, G.M. Wu, Surf. Coat. Technol. 206 (2012) 2961–2966. [24] R.K. Gupta, K. Mensah-Darkwa, D. Kumar, J. Mater. Sci. Technol. 30 (2014) 47. [25] F. Zhang, A.B. Ma, D. Song, J.H. Jiang, F.M. Lu, L.Y. Zhang, D.H. Yang, J.Q. Chen, J. Rare Earth 33 (2015) 93–101. [26] W. Zhou, D. Shan, E.H. Han, W. Ke, Corros. Sci. 50 (2008) 329–337. [27] F.M. Lu, A.B. Ma, J.H. Jiang, Y. Guo, D.H. Yang, D. Song, J.Q. Chen, Corros. Sci. 94 (2015) 171–178. [28] N. Hara, Y. Kobayashi, D. Kagaya, N. Akao, Corros. Sci. 49 (2007) 166–175. [29] D. Song, A.B. Ma, J.H. Jiang, P.H. Lin, D.H. Yang, J.F. Fan, Corros. Sci. 52 (2010) 481–490. [30] C.Z. Yao, H.B. Lv, T.P. Zhu, W.G. Zheng, X.D. Yuan, W. Gao, J. Alloy. Compd. 670 (2016) 239–248. [31] M. Mosiałek, G. Mordarski, P. Nowak, W. Simka, G. Nawrat, M. Hanke, R.P. Socha, J. Michalska Surf. Coat. Technol. 206 (2011) 51–62. [32] M. Razavi, M. Fathi, O. Savabi, D. Vashaee, L. Tayebi, Mater. Sci. Eng. C 41 (2014) 168–177. [33] K.Y. Chiu, M.H. Wong, F.T. Cheng, H.C. Man, Surf. Coat. Technol. 202 (2007) 590–598.
biomedical Mg alloy through the hydrothermal synthesizing method with de-ionized water as the reagent. The fabricated coatings were primarily composed of Mg hydroxide. The coatings were generally uniform and compact, and effectively reduced the bio-degradation of the Mg alloy substrate. Due to the lower reaction and deposition rate, the coating was more difficult to be formed on the secondary phases than on the α-Mg matrix, which led to the micro cracks and pores of the short-time synthesized coating on the secondary phases after drying. With the longer synthesizing time, the coatings with enhanced compactness and uniformity were obtained over the whole substrate. Consequently, the corrosion resistance and protective efficiency of the coatings were significantly improved with increasing synthesizing time. Acknowledgements This work was supported by Natural Science Foundation of China (Grant No. 51308111), Joint Innovation Fund Project of Jiangsu Province of China (Grant No. BY2015002-02), the Fundamental Research Funds For The Central Universities (Grant Nos. 2015B18614 and 2016B0314) and Qing Lan Project of Jiangsu Province of China. References [1] B.L. Mordike, Mater. Sci. Eng. A 302 (2001) 37–45. [2] Z. Yang, J. Li, J. Zhang, G. Lorimer, J. Robson, Acta Metall. Sin. (Engl. Lett.) 21 (2008) 313–328. [3] H.Y. Ha, H.J. Kim, S.M. Baek, B. Kim, S.D. Sohn, H.J. Shin, H.Y. Jeong, S.H. Park, C.D. Yim, B.S. You, J.G. Lee, S.S. Park, Scr. Mater. 109 (2015) 38–43. [4] A. Atrens, M. Liu, N.I. Zainal Abidin, Mater. Sci. Eng. B 176 (2011) 1609–1636. [5] Y. Sun, B.P. Zhang, Y. Wang, L. Geng, X.H. Jiao, Mater. Des. 34 (2012) 58–64. [6] W.J. Zhang, M.H. Li, Q. Chen, W.Y. Hu, W.M. Zhang, W. Xin, Mater. Des. 39 (2012) 379–383. [7] X. Gu, Y. Zheng, Y. Cheng, S. Zhong, T. Xi, Biomaterials 30 (2009) 484–498.
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Original Research
Hydrothermal synthesis and corrosion behavior of the protective coating on Mg-2Zn-Mn-Ca-Ce alloy☆ ⁎
Dan Songa,b,c, Guanghui Guoa, Jinghua Jianga, , Liwen Zhangb, Aibin Maa,d, Xiaolong Mab, Jianqing Chena, Zhaojun Chenga a
College of Mechanics and Materials, Hohai University, Nanjing 210098, China Department of Material Science and Engineering, North Carolina State University, Raleigh 27606, USA c School of Materials Science and Engineering, Southeast University, Nanjing 211100, China d Jiangsu Collaborative Innovation Center of Advanced Micro/Nano Materials & Equipment, Nanjing University of Science & Technology, Nanjing 210094, China b
A R T I C L E I N F O
A BS T RAC T
Keywords: Protective coating Mg alloy Hydrothermal synthesis Microstructure Corrosion resistance
Protective coatings were synthesized on the Mg-2Zn-Mn-Ca-Ce Mg alloy through the hydrothermal method with de-ionized water as the reagent. The coatings were composed of Mg hydroxide, generally uniform and compact. Hydrogen evolution tests and electrochemical tests in the Hanks’ solution demonstrated that the Mg(OH)2 coatings effectively decreased the bio-degradation rate of the Mg alloy substrate. Microstructure observation showed that the coating formation on the secondary phases was more difficult than that on the α-Mg matrix, which led to micro cracks and pores on the secondary phases after drying. Over synthesizing time, the coating layer on secondary phases gradually becomes more compact and uniform. Meanwhile, owing to the thicker and more compact coatings, the corrosion resistance and protective efficiency were significantly improved with longer synthesizing time as well.
1. Introduction As the lightest structural metallic material，magnesium (Mg) and its alloys are widely used in aeronautical engineering, automobile industry and 3C (computer, communication and consumer) product fields due to their outstanding physical and mechanical properties, such as low densities, high strength, good machinability and potentially high strength/weight ratios [1–3]. Mg alloys also hold the great potential for applications in biomaterials because of their similar densities and elastic modulus with human bone, as well as the excellent biocompatibility in vivo experiments. Mg and its alloys have been studied as implant materials for bone screw, bone plates and vascular scaffold to replace traditional stainless steels, titanium alloy and biological ceramic materials [4]. However, the main limitation to their application in medical aspect is the fast bio-degradation rate in the body environment due to their poor corrosion resistance. Most of the Mg implants suffer severe degradation prior to the recovery of the injured tissues. Thus, it is of great practical significance to reduce the bio-degradation rate of Mg implants in order to prolong their service life. Various methods, such as alloying [5–7], heat treatment [8,9] and
surface treatment [10,11] have been employed to improve corrosion resistance of Mg implants. Progress has been witnessed in Mg-Zn-MnY alloy, Mg-Nd-Zn alloy and etc. [12,13] while further efforts are still imperative to further decrease biodegradation rate of the biomedical Mg alloys. Coating on Mg alloys can effectively provide a barrier layer between the metal and the corrosion medium, which provides an alternative to approach the issue. So far there are lots of coating technologies available for Mg alloy, such as micro-arc oxidation [14,15], electrochemical deposited coating [16], and chemical conversion coating [17]. Hydrothermal synthesis of protective coating is also one of the most promising methods because it is simple, efficient and cost-effective [18]. In addition, hydrothermal crystallization occurs on a 3-dimensional structure, making it easy for commercial scale-up. Last but not least, many literatures pointed out that protective coating synthesized by this method show excellent corrosion-inhibition property [19,20]. Up to now, most of the hydrothermal synthesized coatings on Mg and its alloys were fabricated in alkaline solutions for sufficient OH− ions [21,22]. The coatings were uniform and compact with good adhesion strength and better corrosion resistance. Recently, it was reported that the de-ionized water can also be used as the reagent to
Peer review under responsibility of Chinese Materials Research Society. ⁎ Corresponding author. E-mail address: [email protected] (J. Jiang). http://dx.doi.org/10.1016/j.pnsc.2016.11.002 Received 23 March 2016; Received in revised form 4 November 2016; Accepted 9 November 2016 1002-0071/ © 2016 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Song, D., Progress in Natural Science: Materials International (2016), http://dx.doi.org/10.1016/j.pnsc.2016.11.002
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The material to be coated was cut from a casting bio-medical Mg alloy, which was designed and fabricated in our previous research [25]. The chemical compositions of this alloy were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES) (Iris Advantage 1000, USA), as listed in Table 1. The specimens with the size of 10 mm×10 mm×5 mm were cut by an electric discharging machine, polished with SiC papers up to 1800 grades, ultrasonically cleaned in acetone and ethanol for 5 min each, and dried in air. The reactor used in this experiment is a stainless steel autoclave (100 mL) with a Teflon liner without electrical heating facility and pressure gauge. The de-ionized water was poured into the reactor to 70% volume as the reaction solution. Three parallel samples were treated in one reactor simultaneously. The reactor was heated via an electric furnace and the hydrothermal-heating temperature was set at 160 ± 0.1 °C. The hydrothermal-synthesizing time was counted after the furnace temperature reached the set temperature. In this work, three heating times of 3 h, 4.5 h and 6 h were set to study the effect of hydrothermal-synthesizing time on the microstructure and corrosion resistance of the protective coatings. The samples coated at different times were named 3h-coated sample, 4.5h-coated sample and 6hcoated sample, respectively.
and electrochemical tests. Hanks’ solution was selected as the simulated body fluid, which has been widely used in in-vitro tests of many implant materials. Table 2 presents the chemical composition of the Hanks’ solution used in this study. All the in-vitro tests were conducted at the temperature of 37 °C. Before the in-vitro tests, the coated and uncoated samples were molded in the epoxy with a squared exposure of 1 cm2. Regarding uncoated sample, the exposed surfaces were mechanically polished, then cleaned by acetone and ethanol. For the coated sample, the exposed surfaces were only cleaned by acetone and ethanol. The Hanks’ solution was renewed every single day to keep the corrosion environment consistent. After different immersion times, the evolved hydrogen was collected by an upside down funnel above a specimen during immersion test in Hanks’ solution, then went into a burette and gradually displaced the test solution in the burette. By this way, the volume of the evolved hydrogen can be easily measured by monitoring the height of the test solution in the burette. In this study, the hydrogen evolution rate was calculated in mL m−2 day−1, and the coated samples was measured for 108 h in Hanks’ solution. After in-vitro immersing tests, the corrosion morphologies of the samples were observed using a digital microscope (Hirox, KH-7700, USA). Electrochemical tests were adopted to investigate electrochemical corrosion behaviors of the coated and uncoated Mg alloy samples in the Hanks’ solution via a Parstat 2273 advanced potentiostat. A threeelectrode cell was employed, which was composed of the tested sample as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a large-area platinum sheet as the counter electrode. The samples for electrochemical testing were prepared similarly to that for the in-vitro immersion test, including the copper wire connection to the tested sample. Three electrochemical measurements, including open circuit potential (OCP) test, potentiodynamic polarization (PDP) test and electrochemical impendence spectroscopy (EIS) test, were systematically conducted. Because of the relatively poor corrosion resistance of Mg alloy, the immersion time of all OCP tests was set for 3600 s, and the PDP tests were performed at a common scan rate of 1 mV s−1. The frequency range of EIS tests were from 10 kHz to 10 mHz. To get enough feedback signals, the amplitude of sinusoidal potential used in coated and uncoated samples were 20 mV.
2.2. Microstructure characterization
3. Results and discussion
Before and after hydrothermal-synthesizing treatment, the macro morphologies of the samples were observed by a digital microscope (Hirox, KH-7700, USA). The surface and cross-sectional micromorphologies of the coatings and Mg alloy substrate were examined by scanning electron microscope (SEM，Hitachi, S3400N, Japan). Before the SEM observation, all the samples were conductively coated by gold. The element distribution of the substrate and coating was characterized by the energy dispersive X-ray spectrometer (EDS). X-ray diffraction (XRD) analysis of the coated sample was performed using a Bruker D8 Advance diffractometer (Bruker AXS, Germany) with Cu Kα1 radiation. The θ-2θ diffraction patterns were scanned from 5° to 90° with a scanning rate of 2°/min.
3.1. Surface and cross-sectional morphologies of the coatings
coat pure Mg and Mg alloys with sufficient corrosion resistance [23,24]. Clearly, the de-ionized water is better than alkaline solution considering biocompatibility. Previously, a novel kind of Mg-2Zn-Mn-Ca-Ce alloy was designed and fabricated [25]. It exhibits improved corrosion resistance, but its in-vitro bio-degradation rate was still away from medical standards. To further reduce the bio-degradation rate of the Mg-2Zn-Mn-Ca-Ce alloy, the hydrothermal synthesis of the protective coating in de-ionized water was conducted in this study. Meanwhile, the effects of hydrothermal-synthesizing time, as well as the alloy's secondary phases, on the microstructure and corrosion resistance of the protective coatings were also systematically studied. 2. Experimental 2.1. Synthesizing process of the protective coating on Mg-2Zn-MnCa-Ce alloy
Fig. 1a is a typical SEM photograph of the substrate alloy, which clearly shows two major micro-constituents: α-Mg phase and secondary phases. The secondary phases has been identified as Ca2Mg6Zn3, Mg2Ca and Mg12CeZn phase in previous work, which were net-like and distributed along the grain boundaries of the α-Mg phase [25]. In Fig. 1b and c, after hydrothermal synthesis for 3 h, a macroscopically uniform and compact coating had been formed on the Mg alloy substrate, including some of flower-shaped stacking clusters. However, careful examinations of SEM photograph reveal some micro defects, including cracks and pores. It's also worth to note that those micro defects always exist in the vicinity of the secondary phases of the Mg alloy substrate. In contrast, the coating layer on the α-Mg matrix is more compact, and free of micro crack and pores under the set magnification. With the increase of hydrothermal-synthesizing time, as shown in 4.5h coated sample (Fig. 1d and e), the stacking clusters increased and also became larger. Meanwhile, the presence of those micro defects gradually reduced on the surface. Finally in 6h-coated sample (Fig. 1f and g), with sufficient reaction time, micro cracks and pores nearly disappeared on the surface. In addition to the observed improvement of the coating compactness and uniformity, the structure units of the coating layer also change in shape with the increase of synthesizing time. The well-known hexagonal flake structural units
2.3. Corrosion tests Corrosion behaviors of the coated and uncoated samples were studied by in-vitro tests, including hydrogen-evolution immersion test Table 1 Chemical composition of Mg-2Zn-Mn-Ca-Ce alloy (wt%). Zn
Mn
Ca
Ce
Mg
2.00
0.50
1.02
1.35
Balance
2
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Table 2 Chemical composition of Hanks’ solution. Solution
Hanks'
Chemical composition (mmol L−1) NaCl
CaCl2
MgSO4
KCl
KH2PO4
Na2HPO4
D-Glucose
NaHCO3
137
1.261
0.814
5.33
0.44
0.338
5.56
4.17
mole hydrogen. Therefore, in theory, measuring the volume of hydrogen evolution is equivalent to measuring the weight-loss of Mg dissolved. Basically, the hydrogen evolution is caused by the biodegradation in the Mg alloy substrate. The surface coating will provide protection against the penetration of corrosive medium into the Mg alloy substrate while, in turn, the generated hydrogen in the substrate will also destroy the integrity of the coating. Therefore, one can envision that the lower hydrogen-evolution rate of the sample will indicate the slower degradation and the better protection of the coatings. Clearly, the hydrogen-evolution rate of the uncoated sample is times larger than that of the coated samples during the whole immersion testing, which indicate the significant protective effect of the coating. With the increase of immersion time, the line slope (can be regarded as hydrogen-evolution rate) of the uncoated sample increased, indicating the continuous increased bio-degradation rate. As the hydrogen-evolution rate of the uncoated sample was extremely larger, the calculation of its hydrogen evolution rate was stopped after immersion for 3 days since its difference to coated samples becomes remarkable. In contrast to the uncoated sample, there's an incubation period, when detectable hydrogen evolution has not been generated, in all three coated samples. As marked in Fig. 5, this incubation period was about 39h in the 3h-coated sample, 48h in the 4.h-coated sample and 81h in the 6h-coated sample, respectively. The incubation period is a crucial parameter to characterize the protective efficiency of the coating: the longer incubation period，the better protection of the coating. The incubation period is mainly decided by the penetration processing of the corrosive medium through the coating into the substrate. Enhanced compactness and larger thickness of the coating will provide stronger barrier against the penetration of corrosive medium, which made the incubation period of the coated sample much longer. The increased compactness and thickness of the coating with the increased synthesizing time has been verified in the previous SEM observation and thereby explicitly explains the enhanced incubation period of the coatings. After incubation period, all the coated samples experienced the increased hydrogen evolution. This phenomenon should be caused by the mutual promotion of accelerated biodegradation of the substrate and the breakdown of the coating caused by hydrogen evolution. After immersion for 108 h, the totally hydrogen evolution of the 3h-coated sample, 4.5h-coated sample and 6h-coated sample are about 5.05 mL cm−2, 3.35 mL cm−2 and 2.35 mL cm−2, respectively. Fig. 6 presents the optical macro-morphologies of the coated and uncoated samples after hydrogen-evolution immersion in Hanks’ solution for 5 days. It turned out that the uncoated sample (Fig. 6a) has been completely corroded, whereas all the coated samples (Fig. 6b– d) showed little change compared to the original coatings illustrated in Fig. 2. The uncoated sample surface has been covered with thick and stacked corrosion products, presenting grey surface with local and severe corrosion zones. According to our previous research [25], the corrosion products were mainly composed of Mg(OH)2. Relatively, the coated samples suffered minor corrosion damage, with limited macro corrosion spots on the coating surface. Given the number and size of the corrosion spots, the corrosion damage of the 4.5h and 6h synthesized coatings were relatively less than that of the 3h-synthesized coating.
[23,24] of the coatings gradually become obvious and larger, especially in stacking clusters. Note that the hexagonal flake structural units can be found all over the coatings, covering both the α-Mg phase and the secondary phase of the substrate. Even it can also be found in the 3hcoated sample surface in the form of extremely small size. Fig. 2 shows the cross-sectional morphologies of the coated samples. All the coatings were well bonded to substrate, free of micro-cracks at the substrate/coating interface. There are two remarkable differences between the three coated samples. First, the average coating thickness of the 3.5h-coated sample is about 14 µm, while those of the 4.5h-coated and 6h-coated sample are about 18 µm and 20 µm, respectively. The coating thickening with the increase of hydrothermal-synthesizing time is reasonable. The other crucial difference is that the coatings of the 4.5h-coated and the 6h-coated sample are more uniform and compact than that of the 3h-coated sample in the cross-sectional view. Pores and loose connections can be easily found inside of the coating of the 3h-coated sample, particularly above the secondary phases of the substrate alloy, which were marked by the white arrow in the Fig. 2a. 3.2. Phase and chemical composition analysis of the coatings XRD identification was employed to analyze the phases of the coated samples. As shown in Fig. 3, the typical Mg patterns and Mg(OH)2 patterns are identified, corresponding to the α-Mg phase of the Mg alloy substrate and the coating layer, respectively. The absence of the secondary phases in XRD patterns is probably attributed to its minor volume fraction and the sensitivity of the X-ray instrument under the current settings. Moreover, no other phase was detected in the XRD pattern, which means the coatings on the samples may be mainly composed of Mg(OH)2. Beside the similarity of the three samples, some difference can be still found in the XRD. Apparently, the peak intensity of the Mg(OH)2 was significantly increased for longer synthesizing time，and its intensity ratio to α-Mg phase qualitatively indicate the comparisons of their volumes. Given the same scanning depth, the relatively intensified Mg(OH)2 peak is an apparent evidence to elucidate the thickening of the coating layer. This phenomenon completely agrees with the cross-sectional SEM observation of coating layer. Fig. 4 is the EDS mapping analysis of the cross section of the 6hcoated sample's coating and the substrate alloy. As a result, the coating elements are mainly composed by Mg and O, which is consistent with XRD analysis. In the substrate, the Zn, Ca and Ce alloying elements are primarily distributed in the secondary phase, and the Mn element is distributed uniformly in the α-Mg matrix. This result agrees well with our previous research [25], which reported that the secondary phases were Ca2Mg6Zn3, Mg2Ca and Mg12CeZn phase. 3.3. Hydrogen-evolution immersion testing The obtained hydrogen-evolution rates in immersion tests are shown in Fig. 5. The overall corrosion reaction of Mg at its free corrosion potential can be expressed as follows [26,27]. Mg+H2O→Mg2++2OH−+H2
(1)
This means that the dissolution of one mole Mg atom generates one 3
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Fig. 1. SEM microstructure and surface morphologies of the coated and uncoated sample (a) Microstructure of the uncoated Mg alloy, (b) and (c), (d) and (e), (f) and (g) are the surface morphologies of the 3h-coated, 4.5h-coated and 6h-coated samples observed at low and high magnification, respectively.
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Fig. 3. X-ray diffraction patterns of the coated samples..
uncoated sample may be attributed to the formation of partialprotective surface layer (unstable thin Mg(OH)2 film with porous structure). In contrast, the OCP values remained level during the whole test in all coated samples and their difference is trivial. The little fluctuation in OCP values may be caused by the exchange of the aggressive medium, and the relatively stable OCP values in coated samples also indicate the efficient barrier against the penetration of mass aggressive medium into the substrate. PDP test was conducted to study the coatings’ positive effect against the corrosion propagation in the substrate under the situation of strong polarization. All the samples were immersed in the solution for 1 h before polarization tests to achieve their stable OCP values. The corrosion potentials (Ecorr) and the corrosion current densities (Icorr) were derived directly from the PDP curves (as seen in Fig. 8) by the Tafel extrapolation method, and are summarized in Table 3. Note that these samples have similar polarization curve shapes in the tested solution. The coated sample had nobler Ecorr values than the uncoated sample, and their values slightly increased with hydrothermal-synthesizing time. The nobler Ecorr values of the coated samples should be caused by the appearance of the Mg(OH)2 coating. In addition, the Icorr of the coated sample drastically decreased. The Icorr value of the 6h-coated (3.53×10−7 A cm−2) decreased by two orders of magnitude compared with the uncoated sample (5.85×10−5 A cm−2). To our knowledge, Icorr presents a kinetic characteristic of a given metal-electrolyte system, which can characterize the corrosion rate directly. The lower Icorr is, the less corrosion rate can be obtained. In our study, the corrosion current was created from the corrosion damage in Mg alloy substrate. The less Icorr means less corrosion damage in the substrate of the coated sample, evidencing the efficient kinetic barrier of the coatings. Again, this protective effect was enhanced with increasing hydrothermal-synthesizing time. EIS was further conducted to study the stability of the coating in the Hanks’ solution. Fig. 9 shows EIS Nyquist plots of the coated and uncoated samples after in-vitro immersion for one hour. All Nyquist plots were composed of capacitive arcs and inductive arcs. The inductive arcs demonstrate that all the samples have already been corroded to some extent. The capacitive arc diameter is associated with charge-transfer resistance. The larger the diameter is, the better corrosion resistance the experimental sample has [29,30]. Generally, the capacitive arcs of the coated samples are of greater diameters than that of the uncoated substrate sample, indicating their higher corrosion resistances. This phenomenon becomes even noticeable when the synthesizing-time is longer. Moreover, the coated samples have two capacitive arcs while the uncoated substrate has only one. Two capacitive arcs are the typical Nyqusit plot of many coated samples
Fig. 2. Cross section morphologies of the coated samples. (a) 3h-coated sample, (b) 4.5h-coated sample and (c) 6h-coated sample.
3.4. Electrochemical bio-degradation behavior Electrochemical tests were performed to study the electrochemical in-vitro bio-degradation behavior of the coated and uncoated samples in the Hanks’ solution. Fig. 7 presents the OCP curves of the coated and uncoated samples immersed in Hanks’ solution during the initial 3600 s. Generally, the coated samples have nobler OCP values compared to the uncoated Mg alloy substrate. Note that the OCP values of the coated samples were actually determined by the mixed electrode potential of both the substrate and the coating layer. Therefore, one can infer that the coating layer of Mg(OH)2 would have much nobler potential in the Hanks’ solution. During the whole OCP testing, the uncoated sample underwent two stages, including the fast-increasing stages for about 1000 s followed by a stable stage with less vibration in OCP values. According to the Ref. [28], the increasing OCP value of the 5
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Fig. 4. EDS mapping analysis of the cross section of the 6h-coated sample.
used. The Rs(CdlRt(RLL)) equivalent circuit corresponds to the uncoated sample, which presents one conductive loop and one inductive loop. The Rs(Cf(Rp(CdlRt(RLL)))) equivalent circuit corresponds to the coated samples, which presents two conductive loops and one inductive loop. In those models, Rs is the electrolyte solution resistance, Rp and Cf represent the microporous resistance and capacitance of the Mg(OH)2 coating, Cdl and Rt represent the double layer capacitance and the charge transfer resistance of the substrate, RL and L represent the inductive loop [32]. The fitted EIS parameters of the coated and
because the high/low-frequency arcs are related to EIS signal responded from the substrate/coating, respectively [31]. This is also the case in our study. It is noteworthy that both capacitive arcs of the coating and substrate increased with the synthesizing time. The increased coating arc suggests the enhanced corrosion resistance of the coating, and the increased substrate arc suggests the less corrosion damage in the substrate. Equivalent circuit was widely used for the accurate simulation of the EIS data. According to the EIS plots, two equivalent circuits were 6
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Fig. 7. Open circuit potential curves of the coated and uncoated samples in Hanks’ solution for the initial 3600 s.
Fig. 5. Hydrogen-evolution rates of the coated and uncoated samples immersed in Hanks’ solution.
Fig. 6. Optical macro-morphologies of the coated and uncoated samples after hydrogen-evolution immersion in Hanks’ solution for 5 day. (a) uncoated sample; (b) 3h-coated sample; (c) 4.5h-coated sample and (d) 6h-coated sample.
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resistance named after pore or ionic conducting defect resistance. In the substrate/coating/medium corrosion system, the Rp can be regarded as the corrosion resistance of the coating, and the Cf reflects the compactness of the coating. As seen in Table 4, the increase in Rp values and decrease in Cf values indicated that the gradually increased corrosion resistance and better compactness of the coatings occurred with increasing hydrothermal-synthesizing time. Based on the discussions on EIS results, one can find that the hydrothermal-synthesized coatings can provide sufficient protection to the substrate, and their protective efficiency will be improved gradually with the synthesizing time. 3.5. Effect of alloys’ secondary phase and hydrothermal-synthesizing time on the coating formation It is generally believed that a hydrophobic surface can reduced the penetration of electrolyte through the surface and can enhance the corrosion resistance of the coating system [33]. As a corrosion barrier, the characters of the protective coating, such as the thickness, compactness and uniformity, should play great roles in the protective efficiency. According to the microstructure observation and corrosion tests, it can be confirmed that the Mg(OH)2 protective coatings can be synthesized on the surface of Mg-2Zn-Mn-Ca-Ce biomedical Mg alloy by the hydrothermal synthesis method, and its protective efficiency gradually increases with increasing the hydrothermal synthesizing time due to the to the thicker and more compact coatings. The synthesizing reaction of the hydrothermal protective coating on magnesium alloy using de-ionized water has been reported in the Ref. [23]. The coating-forming reaction of Mg with water can be described as follows:
Fig. 8. Polarization curves of the samples obtained after immersion in Hanks’ solution for 1 h. Table 3 Corrosion potential (Ecorr) and corrosion current density (Icorr) of the uncoated samples. Sample
Ecorr (v)
Icorr (A cm−2)
Uncoated 3h-coated 4.5h-coated 6h-coated
−1.486 −1.383 −1.371 −1.327
5.85×10−5 4.42×10−6 6.78×10−7 3.53×10−7
H2O(l)→H++OH−(aq)
(2)
Mg(s)→Mg2+(aq)+2e
(3)
2H+(aq)+2e→H2(g)
(4)
Mg2+(aq)+2OH−(aq)→Mg(OH)2(s)
(5)
According to Zhu's research [23], longer synthesizing time promoted the thickening of coatings due to more generation and deposition of Mg(OH)2 on the substrate surface. The similar phenomenon was also found in this study. In addition to synthesizing parameters, the alloying elements and the microstructure of the substrate alloy should also have important impacts on the formation and performance of the coatings. In our study, the Mg-2Zn-Mn-Ca-Ce biomedical Mg alloy has the typical multiple-phase structure (as shown in Fig. 1a), including α-Mg phase and secondary phases (Ca2Mg6Zn3, Mg2Ca and Mg12CeZn phase). As demonstrated in previous SEM analysis, the coating on the α-Mg phase was much thicker and compacter than that on the secondary phases in the early synthesis stage (say 3h-coating in this work). Over time, the coatings on multiple phases become less different, as shown in the 6hcoating samples. A possible mechanism to elucidate this phenomenon is proposed as below. The coating formation on multiple-phase Mg alloy can be reasonably divided into three stages: nucleation, thickening and densification,
Fig. 9. EIS Nyquist plots of the samples obtained after immersion in Hanks’ solution for 1 h.
uncoated samples were illustrated in Table 4. The coated sample has larger Rt values and lower Cdl values compared to the uncoated sample, which indicates less corrosion damage and better surface integrity of the substrate of the coated samples. Meanwhile, with the increase of the synthesizing time, the coated sample showed gradually increased Rt values and decreased Cdl values. Those two kinds of parameters indicated the gradually alleviated corrosion damage in the substrate of the coated samples. Rp (polarization resistance) is the relevant Table 4 Fitted EIS parameters of the samples. Sample
Rs (Ω cm2)
Cf (10−6 F cm−2)
Rp (Ω cm2)
Cdl (10−6 F cm−2)
Rt (Ω cm2)
Uncoated 3h-coated 4.5h-coated 6h-coated
26.67 14.96 17.89 12.56
/ 31.1 21.2 12.3
/ 1907 2485 3816
138.4 71.8 40.5 30.2
1147 1604 1748 2451
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Fig. 10. Schematic diagrams of the coating formation mechanism (a) and illustration of surface morphologies of the coated and uncoated sample (b).
boundaries and producing the stacking cluster with large-size hexagonal flake structural units. Eventually, the coating layers over both phases became more compact and uniform as a whole over extended period of hydrothermal synthesizing. Fig. 10b is the schematic illustration of resulted surface morphologies over the time of hydrothermal synthesis followed by drying process, where the initial pattern is simulated from the SEM observation. As discussed in Fig. 10a, the coatings on the secondary phases were relatively thinner over a short time. Micro defects of cracks and pores were likely to be generated at sites adjacent to the secondary phases because of the shrinking stress during the drying processing after hydrothermal synthesis (as shown in Fig. 10b-2). In the long-time synthesized sample, the coating became more compact and uniform. Therefore, chances are less for the appearance of cracks and pores during the drying processing (as shown in Fig. 10b-3). The existence of cracks and pores is definitely harmful to the barrier efficiency of Mg(OH)2 coating due to the easy penetration of electrolyte through the cracks and pores in the corrosive environment. Overall, the illustration in Fig. 10 explains the poor corrosion resistance of the short-term synthesized specimen (3h-coated sample) in comparison to the one after long-time synthesis. Through this research, it can be concluded that the secondary phases of the Mg-2Zn-Mn-Ca-Ce alloy will greatly affect the formation and weaken the performance of its hydrothermal synthesized coating. In other words, homogenization via solid-solution treatment to reduce the second phase in substrate alloy is effective to improve the microstructure and performance of the hydrothermal synthesized coating. Meanwhile, increasing synthesizing time will benefit the thickness, uniformity and compactness of the coating, leading to the enhanced protective efficiency.
as schematically shown in Fig. 10a. First, the coating layer is nucleated from the initial deposition and crystallization of Mg(OH)2 crystals on the substrate surface (Fig. 10a-1). The required Mg2+ and OH- are generated from the original substrate/electrolyte interfacial reactions (2), (3), and therefore, their reaction rates are crucial to the coating nucleation rate. The α-Mg phase has larger Mg content than the secondary phases, providing more possible reaction sites. In addition, the secondary phases are more stable than the α-Mg phase [25], which makes it more difficult to react with water. Therefore, the coating nucleation rate on the secondary phases will be slower, which leads to the weaker and locally more defective coatings on the secondary phase. Next, during coating thickening (Fig. 10a-2), the coating on both the αMg phase and the secondary phases was not ideally compact, providing some corridors within the initial coating layer. The electrolyte can thereby penetrate to the substrate/coating interface along those corridors and continue the reactions, transforming the top substrate into new Mg(OH)2 layers. On the other hand, part of the generated Mg2+ and OH- will also diffuse to the coating surface through those corridors and coarsen the as-nucleated Mg(OH)2 crystals, i.e. thickening the coating. Both the two scenarios above are conceived to contribute to the coating thickening and make this process bi-directional. One direction is from the inner coating to the substrate (innerthickening, marked in white arrow) while the other direction is from the outer coating to the electrolyte (outer-thickening, marked in black arrow). For a similar reason in the nucleation stage, the thickening rate is assumed faster for α-Mg phase than that for the secondary phases until the coatings on α-Mg phase become sufficiently compact and uniform. Thereafter, densification stage starts (Fig. 10a-3). The thickening on the α-Mg phase will be largely retarded because the as-coated layer become thick and compact enough against the penetration of electrolyte. However, inner-thickening will also process on the secondary phase. Furthermore, phase boundaries between the α-Mg phase and the secondary phase still provide shortcut to the penetration of electrolyte, leading to continuous and local corrosion reaction along
4. Conclusion The protective coatings were fabricated on the Mg-2Zn-Mn-Ca-Ce 9
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biomedical Mg alloy through the hydrothermal synthesizing method with de-ionized water as the reagent. The fabricated coatings were primarily composed of Mg hydroxide. The coatings were generally uniform and compact, and effectively reduced the bio-degradation of the Mg alloy substrate. Due to the lower reaction and deposition rate, the coating was more difficult to be formed on the secondary phases than on the α-Mg matrix, which led to the micro cracks and pores of the short-time synthesized coating on the secondary phases after drying. With the longer synthesizing time, the coatings with enhanced compactness and uniformity were obtained over the whole substrate. Consequently, the corrosion resistance and protective efficiency of the coatings were significantly improved with increasing synthesizing time. Acknowledgements This work was supported by Natural Science Foundation of China (Grant No. 51308111), Joint Innovation Fund Project of Jiangsu Province of China (Grant No. BY2015002-02), the Fundamental Research Funds For The Central Universities (Grant Nos. 2015B18614 and 2016B0314) and Qing Lan Project of Jiangsu Province of China. References [1] B.L. Mordike, Mater. Sci. Eng. A 302 (2001) 37–45. [2] Z. Yang, J. Li, J. Zhang, G. Lorimer, J. Robson, Acta Metall. Sin. (Engl. Lett.) 21 (2008) 313–328. [3] H.Y. Ha, H.J. Kim, S.M. Baek, B. Kim, S.D. Sohn, H.J. Shin, H.Y. Jeong, S.H. Park, C.D. Yim, B.S. You, J.G. Lee, S.S. Park, Scr. Mater. 109 (2015) 38–43. [4] A. Atrens, M. Liu, N.I. Zainal Abidin, Mater. Sci. Eng. B 176 (2011) 1609–1636. [5] Y. Sun, B.P. Zhang, Y. Wang, L. Geng, X.H. Jiao, Mater. Des. 34 (2012) 58–64. [6] W.J. Zhang, M.H. Li, Q. Chen, W.Y. Hu, W.M. Zhang, W. Xin, Mater. Des. 39 (2012) 379–383. [7] X. Gu, Y. Zheng, Y. Cheng, S. Zhong, T. Xi, Biomaterials 30 (2009) 484–498.
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