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In vitro corrosion of micro-arc oxidation coating on Mg-1Li-1Ca alloy — The influence of intermetallic compound Mg2Ca
Journal of Alloys and Compounds 764 (2018) 250e260
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
In vitro corrosion of micro-arc oxidation coating on Mg-1Li-1Ca alloy d The influence of intermetallic compound Mg2Ca Zi-You Ding a, Lan-Yue Cui a, Xiao-Bo Chen b, Rong-Chang Zeng a, *, Shao-Kang Guan c, Shuo-Qi Li a, Fen Zhang a, Yu-Hong Zou d, Qing-Yun Liu d, ** a
College of Material Science and Engineering, Shandong University of Science and Technology, Qingdao, 266590, China School of Engineering, RMIT University, Carlton, 3053, VIC, Australia School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450002, China d College of Chemicals and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2018 Received in revised form 6 June 2018 Accepted 7 June 2018
The presence of second phase Mg2Ca has remarkable impacts on the formation and degradation of MAO coating. Microstructure, formation and degradation behaviors of substrate and MAO coating were performed. Results revealed that pitting and filiform corrosion occurred on bare Mg-1Li-1Ca alloy. MAO coating significantly improved the corrosion resistance of alloy. The roles of Mg2Ca on the formation and degradation mechanisms of MAO coating were discussed. Micro-arc oxidation for the Mg-Li-Ca alloy initiated at a-Mg phase with depleted Ca content. The growth rate of coating on a-Mg phase is faster than that of GBs/Mg2Ca. The MAO coating began to degrade from intermetallic compound Mg2Ca, then it underwent the attack from the solution and stress concentration caused by corrosion products. © 2018 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Micro-arc oxidation Coating Intermetallic compounds Degradation
1. Introduction To date, great attention has been paid on magnesium (Mg) and its alloys as revolutionary biomaterials due to their remarkable physical and mechanical properties, such as low density, high specific strength, and excellent bio-compatibility [1]. Considerable studies have demonstrated that Mg and its alloys hold a promise as degradable orthopedic implants [2,3]. However, the equilibrium electrode potential of Mg is 2.37 V vs. standard hydrogen electrode (SHE), much lower than that of most metals [4,5]. Hence, Mg alloys are highly susceptible to corrosion in aggressive media [6,7]. Generally, to enhance the corrosion resistance of Mg alloys, following measures have been taken: purifying [8], alloying with noble elements [9,10] and surface passivation [11]. Basically, physical and chemical properties of pure Mg can be greatly changed by alloying with Al, Zn, Mn, RE, Ca, Li, etc. [12,13]. The formation of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q.-Y. Liu).
(R.-C.
https://doi.org/10.1016/j.jallcom.2018.06.073 0925-8388/© 2018 Elsevier B.V. All rights reserved.
Zeng),
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intermetallic compounds, also called second phases, such as AlMn in AZ31, and Mg2Ca in Mg-Ca alloys, is common when alloying elements exceed their solid solubility limit with respect to Mg. Of a large number of binary Mg alloys [14,15], Mg-Ca and Mg-Li alloys attract great attention [16]. Ca is a vital component of human bones [9]. Li can transform crystalline structure of Mg from hexagonal closed-packed (HCP) into body-centered cubic (BCC) when appropriate quantity of Li is added into the bulk [17,18]. With increasing concentration of Li in Mg-Li binary alloy, microstructure is characterized by a (-Mg) phase, aþb (-Li) phase and b phase [19,20]. In vivo study performed by Witte et al. [3,21] discloses that Mg-Li alloy (LAE442) exhibits superior corrosion resistance than Mg alloy AZ91D. A further investigation shows Mg-Li-Ca alloy possesses a four-layered oxide film and good biocompatibility [22,23]. Thereby, surface modifications [24], i.e. micro-arc oxidation (MAO) [25], chemical conversion coating [26,27], and layer-by-layer assembled coating [28e30], are used to improve corrosion resistance of Mg alloys. MAO or plasma electrolytic oxidation (PEO) is a high voltage plasma-assisted anodic oxidation process, accompanied by visible sparking at anode/electrolyte interface [31,32]. MAO coating is a ceramic coating in situ grown on valve metals (i.e. Al, Mg and Ti).
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Fig. 1. Three-dimensional microstructure: (a) optical micrograph, (b) SEM image and (c) EDS point ID results of extruded Mg-1Li-1Ca alloy.
Mg can form an adherent and electrically insulating anodic oxide layer. This anodic oxidation layer acts as barrier to the flow of ions and electrons, reducing the rate of further oxidation to highly low values in aqueous electrolytes [33]. For MAO coating, major limitations are the presence of micro-pores and cracks on the surface [34]. The penetration of aggressive solutions into interface of MAO coating/substrate along micro-pores and cracks leads to a premature failure of coating for long-term immersion tests. As such, it is a pressing need to understand the reason for the defect formation of MAO coatings on Mg alloys and the role of intermetallic compounds in the formation of MAO coating. So far, a great deal of research has been focused on the preparation of coatings. It is noted that several studies involving the impacts of intermetallic compounds on formation of chemical conversion coatings [35]. Our previous study shows that microstructure and chemical compositions of Mg alloys exerted a significant influence on the formation mechanism and corrosion resistance of their chemical conversion Ca-P and Zn-Ca-P coatings [36,37]. These results are attributable to the dissolution of Mg substrates and released species, i.e. engagement of Mg2þ ions into coating formation. And simultaneously the alkalization of processing bath leads to deposition of coatings. Usually, second phases
play as micro-cathodes in the corrosion process of multicomponent Mg alloys [38]. Zhou et al. [39] suggested that galvanic effect between a-Mg phase and b phase in AZ91 alloy resulted in flower-like phosphate nucleus preferentially depositing on b phase; whilst ball-like phosphate nucleus predominately deposited in a-Mg phase interior due to the micro-galvanic effect within a phase. Since MAO coating is also a type of chemical conversion films, both MAO and chemical phosphate coatings share a common feature. Namely, substrate participates the formation of coatings through chemical reactions between species in solutions and metallic ions released from substrates. Unfortunately, little literature reports the effects of secondary phase on MAO coating formation for Mg alloys [40,41]. Wang et al. [42] demonstrated that sparking discharge preferentially occurs on a-Mg phase instead of Mg17Al12 (b) phase at initial stage of MAO formation; selective growth of coating, nevertheless, diminishes gradually over oxidation time and finally b phase will not further prevent the formation of coating at prolonged stage. That is, Mg17Al12 phase leads to disintegration and discontinuity of MAO coating on Mg alloy AZ91D. A further study by Chen et al. [43] revealed that both AZ91 substrate and its MAO coating exhibit increased corrosion resistance due to the presence of finer and
Fig. 2. (a) SEM images of MAO coating and its (b) EDS result, (c) cross-sectional view of coating and its (d) EDS scan mapping.
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protection of a mixed SF6 (1% in volume fraction) and CO2 (99% in volume fraction) atmosphere. Ingots with dimensions of 600 mm 200 mm 200 mm were prepared by pouring the melt into a preheated steel mold. They were homogenized at 450 C for 10 h following air-cooling. Then, ingots were extruded into 4 mm thick sheets through an 800-ton extruder (Yuan Hang) of Magnesium Industry in Chongqing Science and Technology Company, Ltd. at an extrusion ratio of 20.4: 1 and an extrusion rate of 1 m/min. The mold and extrusion temperature were set at 350 C and 300 C, respectively. The extruded sheets were cut into a size of 20 mm 20 mm 4 mm, then ground with water by continuous grades of SiC abrasive paper from 150 to 2500 grit, and polished with 1.0 mm diamond paste into a mirror-like surface. 2.2. MAO preparation
Fig. 3. XRD patterns of substrate and MAO coating of Mg-1Li-1Ca alloy.
MAO coating was fabricated on an MAO power supply (MAOI50C, Chengdu Pulsetech Electrical Co., Ltd) under a constant current control mode in an alkaline solution of 10 g/L NaOH and 18 g/L Na2SiO3. The processing was conducted at following electric parameters: current density 40 mA/cm2, frequency 2000 Hz and duty cycle 35% as well as duration 300 s. The bath temperature was kept at 20e40 C by a stirring and cooling system.
more evenly distributed b phase precipitations. However, MAO coating on the as-cast one shows chemical and morphological discontinuity on a and b phases with coarse and imperfect regions on phase boundaries. MAO coating on aged Mg alloy AZ91 with nano-sized b phase precipitations shows no marked differences on two-phase boundaries and is more homogeneous with fewer pores and defects. Conversely, coating in absence of b phase is characterized by most inhomogeneous microstructure and lowest corrosion resistance. This scenario is ascribed to the absence of protective aluminum-containing oxides in coating and more severe localized oxidation behavior adjacent to second phase AlMn. As for ternary Mg-Li-Ca alloys, inadequate literature on corrosion investigation is available [15]. Therefore, fundamental study on the corrosion characteristics of Mg-Li-Ca alloy is necessary. In present work, the emphasis is placed on the influences of second phase Mg2Ca on formation of MAO coating and taking further insights into degradation mechanism of MAO coating.
The etching solution include two solutions: first contains 3 mL acetic acid, 2 mL nitric acid, 1 g oxalic acid and 100 mL H2O; second solution is 3 mL nitric acid and 100 mL H2O. Samples were firstly immersed in first solution for 5e15 s, and second solution for 4e10 s. Microstructure and surface morphology of substrate and its MAO coating were observed via an optical microscope (OM, Olympus-GX41) and scanning electron microscope (FE-SEM, Nova Nano SEM450), respectively. Average grain size was acquired by counting the number of grains intercepted by three concentric circles sufficiently long to traverse at least 50 intercepts. Optical graphs and SEM images would be edited and grouped up by a graphics transaction.
2. Experimental
2.4. Corrosion behavior
2.1. Material preparation
Degradation rate was characterized using electrochemical tests and hydrogen evolution tests.
Mg-1Li-1Ca ingots, cast at the Institute of Metals Research, Chinese Academy of Sciences, are composed of 1.0 wt. % Li, 0.5 wt. % Ca and Mg balance. They were fabricated by melting under the
2.3. Microstructural observation
2.4.1. Electrochemical tests Electrochemical tests were performed on an electrochemical
Fig. 4. (a) Hydrogen evolution rate of bare substrate and its MAO coating, and (b) curves of pH value vs. immersion time: substrate and MAO coating of Mg-1Li-1Ca alloy in Hanks' solution.
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Fig. 5. (a) Curve of open circuit potential vs. immersion time, (b) potentiodynamic polarization curves of MAO coating and its substrate in Hanks' solution.
systems (Princeton VersaSTAT 4) in Hanks' solution in a water bath of 37 ± 0.5 C. Electrolyte, Hanks' solution, contains the following components: 8.0 g/L NaCl, 0.4 g/L KCl, 0.14 g/L CaCl2, 0.35 g/L NaHCO3, 1.0 g/L glucose (C6H6O6), 0.1 g/L MgCl2$6H2O, 0.1 g/L MgSO4$7H2O, 0.06 g/L KH2PO4, 0.126 g/L Na2HPO4$12H2O, and 1 L distilled water at 37 ± 0.5 C. Chemical reagents are supplied by Qingdao Jingke Instrument Reagent Co., Ltd. A typical threeelectrode system was applied with samples as working electrode with an exposed area of 1 cm2, a saturated calomel electrode (SCE, sat'd KCl) as reference electrode and a platinum foil electrode as counter electrode. Stabilization time is 4000 s at a scan rate of 1 mV/s for OCP test. Electrochemical Impedance Spectroscopy (EIS) test is done after the end of OCP test. The scan frequency of EIS range from 100 kHz to 10 mHz with a perturbation amplitude of 10 mV RMS. The EIS were fitted by the ZSimpWin software. During polarization test, the start potential is 0.4 V vs. OCP and final potential is þ0.5 V vs. OCP with 2 mV as step height and 1 s as step time. 2.4.2. Hydrogen evolution All hydrogen evolution tests were conducted at a water bath of 37 ± 0.5 C. The electrochemical measurements were conducted in Hanks' solutions. 2.4.3. pH measurement Variation in pH values of Hanks' solution was monitored through a digital pH meter (PHS-3C) at a water bath of 37 ± 0.5 C. 2.4.4. Surface characterization Surface morphology of MAO coating was observed via SEM. Chemical compositions and components of samples were analyzed using an energy-dispersive X-ray spectrum (EDS, Oxford X-MaxN). Crystallographic structure of samples was determined using an Xray diffractometer (XRD, Rigaku D/MAX 2500PC, Japan) with Cu Ka radiation. Scan range is 20 e85 with 8 /min as scan rate. Jade 5.0 is used to calibrate the diffraction peak for XRD, and the results are confronted with those reported in most literature. 3. Results 3.1. Microstructural observation Three-dimensional (3D) optical and SEM images (Fig. 1) reveal that microstructure of extruded Mg-1Li-1Ca alloy is characterized by equiaxed grains with a (-Mg) phases and intermetallic compound Mg2Ca particles, which agrees with our previous investigation [19]. The average grain size is approximately 20.2 mm (Fig. 1a),
but smaller size ones (about 7 mm) also exists, for which dynamic recrystallization occurred during hot extrusion. Mg2Ca particles, detected by EDS in Fig. 1c, are predominantly distributed at grain boundaries and presented a strip distribution along extrusion direction (Fig. 1b). Moreover, mechanical twin crystals were also identified. 3.2. Surface morphology and chemical composition Surface morphology of the prepared MAO coating of Mg-1Li-1Ca alloy is shown in Fig. 2a. A large amount of micro-pores are present, which is a typical morphology of MAO coating of Mg alloys [44,45]. Most of them are less than 1 mm. Fig. 2b reveals the presence of Mg, Si, O and traces of Ca in MAO coating. Fig. 3 shows XRD patterns of substrate and MAO coating. The presence of MgO demonstrates that coating forms during MAO treatment bathed in an electrolyte solution of NaOH and Na2SiO3. Also, MgSiO3 and CaSiO3 were detected. Interfacial morphology (Fig. 2c) of the MAO coating discloses that it has an averaged thickness of about 3.46 mm. Furthermore, EDS mapping (Fig. 2d) of the cross-sectional view unveils that MAO coating predominantly contains Mg, O and Si, which is in line with the previous results in Fig. 2b. 3.3. Corrosion behavior Mg and its alloys are prone to corrosion in an aggressive solution, and following reactions occur [46,47]. Anodic reaction: Mg / Mg2þ þ 2e
(1)
Cathodic reaction: 2H2O þ 2e / H2[ þ 2OH
(2)
Total reaction: Mg þ 2H2O / Mg(OH)2 þ H2[
(3)
Table 1 Electrochemical parameters of polarization curves of Mg-1Li-1Ca alloy substrate and its MAO coating. Sample Substrate MAO coating
Ecorr/VSCE 1.592 1.551
ba/103 mV 40.33 114.21
bc/103 mV 342.83 285.42
icorr/A$cm2 5
2.087 10 4.730 107
Rp/U$cm2 750.78 74881.64
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Fig. 6. (a) Nyquist plots with fitted curves, (b) Bode plots of bare Mg-1Li-1Ca substrate and its MAO coating, equivalent circuit of EIS: (c) substrate, (d) MAO coating in Hanks' solution.
According to corrosion reactions, hydrogen gas (H2) and Mg(OH)2 are generated in experiments from total reaction (3). The reaction of Mg is accompanied by generation of H2 and OH on basis of cathodic reaction (2), so the characteristics can be described by hydrogen evolution rate and pH value of corrosion solution.
3.3.1. Hydrogen evolution and pH value measurements Hydrogen evolution rate (HER) as a function of immersion time duration of Mg-1Li-1Ca alloy and its MAO coating in Hanks' solution for 144 h was plotted in Fig. 4a. HER varies with immersion time. In term of bare substrate, HER slows down due to the formation of corrosion products film covering the entire surface of substrate, which provides mild protection to the substrate. For MAO coating, three stages of corrosion progress are exhibited. After initial of immersion tests (Stage I), no significant changes in hydrogen evolution rate are evident at Stage II, which is attributed to the highly protective function of MAO coating to bare substrate. Finally, hydrogen evolution rate increases after 120 h (Stage III), which may be incurred by physical ruptures of MAO coatings, resulting from massive pitting and filiform corrosion [48]. After immersion for 144 h, average hydrogen rates of bare and MAOcoated Mg-1Li-1Ca specimens were 2.082 103 mL cm-2 h-1 and 4.038 104 mL cm-2 h-1, respectively, demonstrating MAO coating effectively improved corrosion resistance of this alloy. Fig. 4b indicates that pH value of solution increases due to degradation reactions. Note that, pH values of substrate are close to that of MAO coating in the early stage of immersion (t < 160 min). After initial stage, pH value for substrate is higher than MAO
coating, indicating more OH ions and Mg(OH)2 were generated from Eq. (2) and Eq. (3). In summary, corrosion rate of MAO coated specimens is much smaller than that of the bare substrate, which is consistent with above analysis.
3.3.2. Electrochemical tests Initial delay time duration is 4000 s to stabilize open circuit potential (OCP) in Fig. 5a. When OCP difference is greater than 50 mV, there is a risk of increased corrosion rate due to galvanic coupling. Unfortunately, galvanic corrosion may occur between substrate and MAO coating of Mg-1Li-1Ca alloy, because initial difference of OCPs is 164 mV. In addition, the difference of OCPs slowly decreased from 164 mV to 98 mV with immersion time, indicating a continual galvanic corrosion between substrate and MAO coating. Fig. 5b shows polarization curves of bare Mg-1Li-1Ca substrate and its MAO coating. Values of corrosion current density (icorr) and corrosion potential (Ecorr) were estimated through Tafel extrapolation fitting, as shown in Table 1. The MAO coating with lower two order of magnitudes of icorr than substrate, reveals the effective protection for the substrate. Ecorr of bare Mg-1Li-1Ca alloy (1.592 VSCE) is more negative than that of MAO coated specimen (1.551 VSCE), indicating a higher thermodynamically tendency to corrosion of substrate. Additional, the anodic polarization branch for MAO coating is significantly shifted towards higher icorr, indicating severe micro-cracks on coating surface. The A, B, C and D, sites on anodic branch of MAO coating, reveal the breakdown and repassivation of MAO coating and passive film. Rp is the polarization resistance that is inversely proportional to
Table 2 Fitting results of EIS of substrate and MAO coating in Hanks' solution. Sample
Rs/U$cm2 CPE/U1$cm2$s1 n
Substrate 83.14 MAO coating 92.7
5
2.971 10 3.09 106
Rct/U$cm2 Cf/F$cm2
0.6751 223.60 0.6577 878.10
6.631 10 e
CPEf/U1$cm2$s1 nf 6
e 7.202 106
L/H$cm2
Rf/U$cm2 3
3
RL/U$cm2 3
Chi-square/103
e 1.83 10 5.46 10 8.44 10 1.56 0.7228 3.908 104 1.321 104 9.578 104 0.9977
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Fig. 7. Macroscopic corrosion morphology: substrate of immersion for (a) 0 h, (b) 144 h, MAO coating of immersion for (c) 0 h, (d) 144 h in Hanks' solution.
corrosion rate. It can be calculated by the simplified Stern-Geary equation [49,50]:
Rp ¼
ba $bc 2:303icorr ðba þ bc Þ
(4)
where ba and bc are the anodic and cathodic Tafel slopes, respectively, and icorr is the corrosion density. Moreover, the corresponding electrochemical parameters, ba, bc and Rp, were listed in Table 1. Rp was improved from 758.78 U cm2 to 74881.64 U cm2, revealing high corrosion resistance of MAO coating than that of substrate. According to Nyquist plots and Bode plots (Fig. 6), MAO coating leads to a highly improvement in the corrosion resistance in comparison to substrate. The Nyquist plots show similar components: a capacitive reactance arc in high frequency (HF) range, a smaller capacitive reactance arc following in the middle frequency (MF), and an inductance reactance arc in the low frequency (LF) range, which is in accordance with our previous work [49]. For substrate, capacitive reactance arc in the HF region corresponds to electrical double layer at the interface of substrate surface and solution; and capacitive reactance arc in the MF region corresponds to oxidation layer formed in air; and the inductive reactance arc in the LF implies pitting and absorption of products. For MAO coating, capacitive reactance arc in the first HF region corresponds to the porous outer layer and the second arc corresponds to the denser barrier layer. Impedance reactance arc in LF region corresponds to generation of products. Fig. 6c and d shows the equivalent circuit of EIS. Rs refers to solution resistance, and Rct represents the charge transfer resistance. In contrast to substrate, Rct of MAO coating increases from 223.60 U cm2 to 878.10 U cm2, which indicates lower dissolution
rate of the film than that of substrate. The Rf and Cf designate the second capacitive reactance arc in MF region. Constant phase element (CPE) is used to replace capacitance Cf in MAO coating. CPEf indicates the deviation of Cf to theoretical value due to the considerably porous structure of MAO coating. A dimensionless coefficient, n was introduced to fix this deviation. The second capacitive reactance arc is related to the denser sub-layer of the coating. The fitting results of EIS are listed in Table 2. 4. Discussion 4.1. Influence of Mg2Ca on corrosion of substrate For further evaluation of corrosion features of bare substrate and its MAO coating, macroscopic morphologies of as-ground Mg-1Li1Ca alloy specimens and ultrasonically cleaned ones after being immersed in Hanks' solution were taken by a digital camera (Fig. 7). Surface of the as-ground Mg-1Li-1Ca alloy exhibits a smooth finish (Fig. 7a), while traces of pitting can be observed clearly after an immersion for 144 h (Fig. 7b, marked by arrows). With increasing immersion time, corrosion pits expend inwards the bulk substrate. Localized corrosion is mainly corrosion type of Mg-1Li1Ca alloy in Hanks' solution and its corrosion mechanism is discussed in the following part. The corrosion pits initiated at a-Mg matrix neighboring intermetallic compounds - Mg2Ca particles. Micro-galvanic corrosion between them leads to pitting corrosion. Via micro-electrochemical testing, Nicholas et al. [51] believed Mg2Ca with rapid anodic kinetics compared with Mg matrix. So, second phase would also impact the degradation of MAO coating. The coating surface is uniform and smooth with some defects before immersion (marked by circles) due to the second phase, Mg2Ca. After immersion, slight traces of corrosion occur on the
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Fig. 8. Morphology of MAO coating obtained (a) before and after appearance of arc discharges for (b) (c) 5 s, (d) 10 s and EDS spectrum.
surface (marked by circles in Fig. 7d), but the surface is still smooth. Moreover, the number of pits of MAO coating is less than that of substrate. Thus, compared with Mg-1Li-1Ca substrate, MAO coating possesses better corrosion resistance. 4.2. Influence of Mg2Ca on the formation and corrosion of MAO coating In Fig. 8, surface morphology of MAO coating was obtained before and after appearance of sparks, and a mechanism illustration was shown in Fig. 9. EDS spectrum in Fig. 8 and Table 3 shows chemical compositions of specific points in Fig. 8. A thin and dense barrier layer was formed on the surface of sample at beginning of a voltage applied (before appearance of sparks, in Figs. 8a and 9a). There are no sparks and only some tiny oxygen bubbles can be seen on the sample surface at this stage, corresponding to traditional anodizing stage [44]. The surface was fully covered with coating with less micro-cracks. Once sparks occurred for 5 s, few strip-shape oxides began to form (Fig. 8b). That is to say, once the voltage exceeds the breakdown voltage, small and dense sparks appear and oxygen evolution also becomes
vigorous, which is reported by Zhao et al. [52]. The bright areas in a phase can be easily identified, and dark areas consists of GBs and second phase, marked by arrows, which also was reported by Zhang et al. [44]. Element C originated from air; Mg and Ca were from the alloy; and Si and O came from the electrolyte. It is noteworthy that (Table 3) there was a big difference in Ca content between the different phases: the presence of Ca for spectrums #1~#3 and the absence of Ca for spectrums #4 and #5. Spectrum #2 was detected higher Ca content than others, indicating the presence of Mg2Ca. For spectrum #4 in absence of Ca, a strip-shape protuberances (in Fig. 8c) could be compounds of MgO and MgSiO3 etc. In Fig. 8b, the coating was preferentially formed at a phase than GBs/Mg2Ca, due to differences of valve metal characteristics [53]. As marked by arrows in Fig. 8c, coating grew from a-Mg into GBs/Mg2Ca, indicating growth rate of anodized films on a-Mg phase was faster than that of GBs/Mg2Ca. That is, micro-arc oxidation for the Mg-Li-Ca alloy easily initiated at a-Mg phase with lower or depleted Ca content. It was consistent with what Khaselev et al. [54] reported, the growth of anodic coatings start on a phase and continued on second phase. With time increasing, bright areas almost covers the surface of
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Fig. 9. Formation mechanism illustration during micro-arc oxidation preparation: (a) before and (b) after appearance of arc discharges (c) 5 s, (d) 10 s.
a-Mg and GBs/Mg2Ca, and micro-pores and cracks (Fig. 8c) appeared on the interface of strip-like oxides and their neighboring areas. The micro-pores or cracks provided a path for solution and sparks to connect substrate, as a result, volcano-shape pores were formed (Fig. 8d). Fig. 9 illustrates the formation mechanism of MAO coating on Mg-1Li-1Ca alloy. The anodized process followed four steps: (1) the formation of a dense and thin barrier (Fig. 9a); (2) the initiation of oxides in the depleted Ca a-Mg matrix after the presence of sparks; (3) the horizontal growth of the oxide film (Fig. 10c); (4) and finally the thickening of the MAO coating (Fig. 10d). This is similar to previous report [52]. 4.3. Influence of Mg2Ca on degradation mechanism The surface morphologies of bare substrate and MAO coating after immersion in Hanks' solution are shown in Fig. 10. The alloy suffers evident damages after immersion, with obvious corrosion morphology: pitting and filiform corrosion in Fig. 10a and its insets. The galvanic corrosion occurs between mainly composed of two phases, Mg2Ca and a-Mg that act as anode and cathode of galvanic corrosion, respectively [23]. There are a lot of corrosion pits on the surface, even some have penetrated into the interior of the substrate (insets of Fig. 10a, marked by arrows). The filiform corrosion is a shallow membrane etch that develops rapidly when it occurs,
which results in dense mesh pattern spreading over the alloy surface. The filiform corrosion, whose head is anode and tail is cathode, usually occurs under protective coatings or anodic oxide layers [55,56]. Although no protective coating is applied to Mg-1Li-1Ca alloy, it is a natural passivity metal that can easily form a corrosion product film on its surface. In contrast with Fig. 10a, MAO coating underwent the attack with some micro-cracks (Fig. 10c and d), even more coating failure (Fig. 10b, marked by circles), can be seen after immersion for 144 h. According to our previous work [57,58], the situation of second phase is where MAO coating initially destroys. And in Fig. 10d, there is a certain orientation in the direction of micro-cracks, which may result from Mg2Ca with a strip distribution. A high magnification picture (insets of Fig. 10c and d) was taken for further observation of micro-cracks. Interestingly, most of micro-cracks passed through or originated from micro-pores, which might be caused by stress concentration due to the volume increase of corrosion products. In summary, MAO coating effectively slows down corrosion of substrate, but second phase promotes failure of MAO coating. Our previous study [57,58] have proved that corrosion products (precipitations and hydrogen gas) could promote MAO coating break and corrosion. Based on above-mentioned results of this study, degradation mechanism of MAO coating on Mg-1Li-1Ca alloy is described in Fig. 11, and degradation procedures can be concluded into some steps as following:
Table 3 EDS results of the samples in Fig. 8 for specific points in at. % (in wt. %). Element
Spectrum #1
Spectrum #2
Spectrum #3
Spectrum #4
Spectrum #5
O
33.88 (26.03) 7.37 (4.26) 54.36 (63.59) 4.03 (5.43) 0.36 (0.69)
30.40 (22.89) 6.55 (3.70) 59.30 (67.84) 2.62 (3.44) 1.13 (2.13)
40.46 (32.38) 10.07 (6.05) 43.45 (52.84) 5.53 (7.76) 0.49 (0.97)
52.76 (45.16) 12.67 (8.15) 25.95 (33.75) 8.62 (12.94) e e
33.76 (25.96) 7.08 (4.09) 54.52 (63.70) 4.64 (6.26) e e
C Mg Si Ca
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Fig. 10. Corrosion morphologies of: Mg-1Li-1Ca (a) substrate and (bed) MAO coating after an immersion of 144 h.
(1) The defects including micro-cracks or through pores on MAO coating provided paths for corrosive media (Hanks' solution) into the interface of MAO/substrate. The intermetallic particles (Mg2Ca) with higher potential relative to a-Mg matrix,
systematically enhanced dissolution kinetics, and sustained dissolution rate about an order magnitude greater than a-Mg [23]. Intermetallic compounds are predominantly distributed at grain boundaries and presented a strip distribution
Fig. 11. Degradation mechanism illustration for (a) (b) (c) surface and (d) (e) (f) sectional view of Mg-1Li-1Ca alloy coated MAO coating in Hanks' solution.
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along extrusion direction (Fig. 1b), so similar distribution and shape of corrosion pits will appear. These pits (Fig. 11d) will promote degradation rate of MAO coating. At the same time, hydrogen and corrosion precipitations, i.e. Mg(OH)2 and Ca(OH)2, formed and deposited in the top of corrosion pits on the interface of MAO/substrate, as shown in Fig. 11e. Additional, corrosion precipitations also cover the corrosion pits with a weak protection for substrate. In Fig. 5b, the point A, B, C and D on anodic branch of MAO coating, reveal the breakdown and repassivation of passive film of corrosion precipitations. (2) After a period of corrosion, MAO coating would be subject to stress from the substrate pointing to coating (Fig. 11a, d). These enhanced stresses have two sources: the volume expansion in formation of corrosion precipitations and additional stress of hydrogen bubble evolution. Then, some micro-cracks expanded and gradually merged (Fig. 11b). As a result, more solution penetrated into the interior of MAO coating and degradation rate increased, which was consistent with above results of hydrogen evolution measurement. In addition, MAO coating (MgO layer) may peeled off from the substrate due to the chemical dissolution. MgO þ H2O / Mg(OH)2
(5)
(3) Finally, partial MAO coating delaminated and fresh bare substrate was exposed to solution and hence the corrosion accelerated, as shown in Fig. 11c, f.
5. Conclusions The presence of second phase Mg2Ca has remarkable impacts on the formation and degradation of MAO coating. The microstructure, physical and corrosion properties of Mg-1Li-1Ca alloy and its MAO coating were examined, and following conclusions are drawn: (1). Filiform corrosion and pitting corrosion are main corrosion types for Mg-1Li-1Ca alloy, resulting from the micro-galvanic corrosion between second phase (Mg2Ca) and a-Mg matrix. (2). The presence of Mg2Ca particles has remarkable impact on the formation of MAO coating. Micro-arc oxidation on the Mg-Li-Ca alloy initiated at a-Mg phase with lower or depleted Ca content. The growth rate of coating on a-Mg phase was faster than that of GBs/Mg2Ca, so coating grew from a-Mg into GBs/Mg2Ca. (3). The degradation mechanism of MAO coating was proposed. The MAO coating initially degraded from intermetallic compound Mg2Ca, then it underwent the attack from the solution and stress concentration caused by corrosion products. As a result, partial delamination of MAO coating occurred.
Acknowledgement This work was supported by the National Natural Science Foundation of China (51571134, 51601108), the Scientific Research Foundation of Shandong University of Science and Technology (SDUST) for Recruited Talents (2013RCJJ006), and the SDUST Research Fund (2014TDJH104). Thanks also go to Miss Fei Xing Chongqing University of Technology for help on corrosion tests.
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References [1] X.N. Gu, Y.F. Zheng, Y. Cheng, S.P. Zhong, T.F. Xi, In vitro corrosion and biocompatibility of binary magnesium alloys, Biomaterials 30 (2009) 484e498. [2] M. Staiger, A. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: A review, Biomaterials 27 (2006) 1728e1734. [3] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyerlindenberg, C.J. Wirth, H. Windhagen, In vivo corrosion of four magnesium alloys and the associated bone response, Biomaterials 26 (2005) 3557e3563. [4] F. Witte, The history of biodegradable magnesium implants: a review, Acta Biomater. 6 (2010) 1680e1692. [5] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, Y.C. Xin, Z.Q. Zhang, Y. Liu, X.H. Chen, What is going on in magnesium alloys? J. Mater. Sci. Technol. 34 (2018) 245e247. [6] S. Thomas, N.V. Medhekar, G.S. Frankel, N. Birbilis, Corrosion mechanism and hydrogen evolution on Mg, Curr. Opin. Solid St. M 19 (2015) 85e94. [7] G.S. Frankel, A. Samaniego, N. Birbilis, Evolution of hydrogen at dissolving magnesium surfaces, Corros. Sci. 70 (2013) 104e111. [8] A. Prasad, P.J. Uggowitzer, Z. Shi, A. Atrens, Production of high purity magnesium alloys by melt purification with Zr, Adv. Eng. Mater. 14 (2012) 477e490. [9] C.L. Liu, Y.J. Wang, R.C. Zeng, X.M. Zhang, W.J. Huang, P.K. Chu, In vitro corrosion degradation behaviour of MgeCa alloy in the presence of albumin, Corros. Sci. 52 (2010) 3341e3347. [10] A. Imandoust, C.D. Barrett, T. Al-Samman, K.A. Inal, H.E. Kadiri, A review on the effect of rare-earth elements on texture evolution during processing of magnesium alloys, J. Mater. Sci. 52 (2017) 1e29. [11] H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys - A review, Acta Biomater. 8 (2012) 2442e2455. [12] R.C. Zeng, W.C. Qi, H.Z. Cui, F. Zhang, S.Q. Li, E.H. Han, In vitro corrosion of asextruded MgeCa alloysdthe influence of Ca concentration, Corros. Sci. 96 (2015) 23e31. [13] M.B. Kannan, R.K. Raman, In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid, Biomaterials 29 (2008) 2306e2314. [14] F. Witte, J. Fischer, J. Nellesen, H.A. Crostack, V. Kaese, A. Pisch, F. Beckmann, H. Windhagen, In vitro and in vivo corrosion measurements of magnesium alloys, Biomaterials 27 (2006) 1013e1018. [15] G.S. Song, M.V. Kral, Characterization of cast MgeLieCa alloys, Mater. Char. 54 (2005) 279e286. [16] Z. Li, X. Gu, S. Lou, Y. Zheng, The development of binary MgeCa alloys for use as biodegradable materials within bone, Biomaterials 29 (2008) 1329e1344. [17] R. Wu, Y. Yan, G. Wang, L.E. Murr, W. Han, Z. Zhang, M. Zhang, Recent progress in magnesiumelithium alloys, Int. Mater. Rev. 60 (2015) 65e100. [18] X.B. Chen, C.Q. Li, D.K. Xu, Biodegradation of Mg-14Li alloy in simulated body fluid: a proof-of-concept study, Bioact. Mater. 3 (2017) 110e117. [19] R.C. Zeng, L. Sun, Y.F. Zheng, H.Z. Cui, E.H. Han, Corrosion and characterisation of dual phase MgeLieCa alloy in Hanks' solution: the influence of microstructural features, Corros. Sci. 79 (2014) 69e82. [20] Y.Y. Zhou, L.P. Bian, G. Chen, L.P. Wang, W. Liang, Influence of Ca addition on microstructural evolution and mechanical properties of near-eutectic Mg-Li alloys by copper-mold suction casting, J. Alloys Compd. 664 (2016) 85e91. [21] F. Witte, J. Fischer, J. Nellesen, C. Vogt, J. Vogt, T. Donath, F. Beckmann, In vivo corrosion and corrosion protection of magnesium alloy LAE442, Acta Biomater. 6 (2010) 1792e1799. [22] L.Y. Cui, L. Sun, R.C. Zeng, Y.F. Zheng, S.Q. Li, In vitro degradation and biocompatibility of Mg-Li-Ca alloysdthe influence of Li content, Sci. China Mater 61 (2017) 1e12. [23] S.S. Nene, B.P. Kashyap, N. Prabhu, Y. Estrin, T. Al-Samman, Biocorrosion and biodegradation behavior of ultralight Mge4Lie1Ca (LC41) alloy in simulated body fluid for degradable implant applications, J. Mater. Sci. 50 (2015) 3041e3050. [24] W. Peng, L. Tan, K. Yang, Surface modification on biodegradable magnesium alloys as orthopedic implant materials to improve the bio-adaptability: A review, J. Mater. Sci. Technol. 32 (2016) 827e834. [25] L.Y. Cui, R.C. Zeng, S.K. Guan, W.C. Qi, F. Zhang, S.Q. Li, E.H. Han, Degradation mechanism of micro-arc oxidation coatings on biodegradable Mg-Ca alloys: the influence of porosity, J. Alloys Compd. 695 (2016) 2464e2476. [26] X.B. Chen, N. Birbilis, T.B. Abbott, Review of corrosion-resistant conversion coatings for magnesium and its alloys, Corrosion -Houston Tx- 67 (2011) 35001e35005. [27] E. Rocca, C. Juers, J. Steinmetz, Corrosion behaviour of chemical conversion treatments on as-cast MgeAl alloys: electrochemical and non-electrochemical methods, Corros. Sci. 52 (2010) 2172e2178. [28] J.J. Richardson, M. Bjornmalm, F. Caruso, Multilayer assembly. Technologydriven layer-by-layer assembly of nanofilms, Science 348 (2015) aaa2491. [29] L.Y. Cui, R.C. Zeng, X.X. Zhu, T.T. Pang, S.Q. Li, F. Zhang, Corrosion resistance of biodegradable polymeric layer-by-layer coatings on magnesium alloy AZ31, Front. Mater. Sci. 10 (2016) 1e13. [30] L.Y. Cui, R.C. Zeng, S.Q. Li, F. Zhang, E.H. Han, Corrosion resistance of layer-bylayer assembled polyvinylpyrrolidone/polyacrylic acid and amorphous silica films on AZ31 magnesium alloys, RSC Adv. 6 (2016) 63107e63116. [31] G.P. Wirtz, S.D. Brown, W.M. Kriven, Ceramic coatings by anodic spark deposition, Adv. Manuf. Processes 6 (1991) 87e115.
260
Z.-Y. Ding et al. / Journal of Alloys and Compounds 764 (2018) 250e260
[32] C. Blawert, W. Dietzel, E. Ghali, G. Song, Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Adv. Eng. Mater. 8 (2006) 511e533. [33] M.M. Lohrengel, Thin anodic oxide layers on aluminium and other valve metals: high field regime, Mat. Sci. Eng. R 11 (1993) 243e294. [34] T.S.N.S. Narayanan, I.S. Park, H.L. Min, Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: prospects and challenges, Prog. Mater. Sci. 60 (2014) 1e71. [35] C. Ke, Y. Wu, Y. Qiu, J. Duan, N. Birbilis, X.B. Chen, Influence of surface chemistry on the formation of crystalline hydroxide coatings on Mg alloys in liquid water and steam systems, Corros. Sci. 113 (2016) 145e159. [36] R.C. Zeng, F. Zhang, Z.D. Lan, H.Z. Cui, E.H. Han, Corrosion resistance of calcium-modified zinc phosphate conversion coatings on magnesiumealuminium alloys, Corros. Sci. 88 (2014) 452e459. [37] R. Zeng, Z. Lan, L. Kong, Y. Huang, H. Cui, Characterization of calciummodified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy, Surf. Coating. Technol. 205 (2011) 3347e3355. [38] W.C. Kim, J.G. Kim, J.Y. Lee, H.K. Seok, Influence of Ca on the corrosion properties of magnesium for biomaterials, Mater. Lett. 62 (2008) 4146e4148. [39] W. Zhou, D. Shan, E.H. Han, W. Ke, Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy, Corros. Sci. 50 (2008) 329e337. [40] R.F. Zhang, Film formation in the second step of micro-arc oxidation on magnesium alloys, Corros. Sci. 52 (2010) 1285e1290. [41] R.F. Zhang, G.Y. Xiong, C.Y. Hu, Comparison of coating properties obtained by MAO on magnesium alloys in silicate and phytic acid electrolytes, Curr. Appl. Phys. 10 (2010) 255e259. [42] Y.Q. Wang, X.J. Wang, T. Zhang, K. Wu, F.H. Wang, Role of b phase during microarc oxidation of Mg alloy AZ91D and corrosion resistance of the oxidation coating, J. Mater. Sci. Technol. 29 (2013) 1129e1133. [43] Y. Chen, Y. Yang, W. Zhang, T. Zhang, F. Wang, Influence of second phase on corrosion performance and formation mechanism of PEO coating on AZ91 Mg alloy, J. Alloys Compd. 718 (2017) 92e103. [44] R.F. Zhang, S.F. Zhang, Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys, Corros. Sci. 51 (2009) 2820e2825. [45] M.S. Joni, A. Fattah-Alhosseini, Effect of KOH concentration on the electrochemical behavior of coatings formed by pulsed DC micro-arc oxidation (MAO) on AZ31B Mg alloy, J. Alloys Compd. 661 (2016) 237e244.
[46] R.C. Zeng, W.C. Qi, Y.W. Song, Q.K. He, H.Z. Cui, E.H. Han, In vitro degradation of MAO/PLA coating on Mg-1.21Li-1.12Ca-1.0Y alloy, Front. Mater. Sci. 8 (2014) 343e353. [47] H. Hermawan, D. Dube, D. Mantovani, Developments in metallic biodegradable stents, Acta Biomater. 6 (2010) 1693e1697. [48] L. Gawel, L. Nieuzyla, G. Nawrat, K. Darowicki, P. Slepski, Impedance monitoring of corrosion degradation of plasma electrolytic oxidation coatings (PEO) on magnesium alloy, J. Alloys Compd. 722 (2017) 406e413. [49] C. Liu, Q. Bi, A. Leyland, A. Matthews, An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II.: EIS interpretation of corrosion behavior, Corros. Sci. 45 (2003) 1257e1273. [50] C. Wang, B. Jiang, M. Liu, Y. Ge, Corrosion characterization of micro-arc oxidation composite electrophoretic coating on AZ31B magnesium alloy, J. Alloys Compd. 621 (2015) 53e61. [51] N.T. Kirkland, N. Birbilis, J. Waiker, T. Woodfield, G.J. Dias, M.P. Staiger, In-vitro dissolution of magnesium-calcium binary alloys: clarifying the role of calcium additions in bioresorbable magnesium implant alloys, J. Biomed. Mater. Res. B 95 (2010) 91e100. [52] L.C. Zhao, C.X. Cui, Q.Z. Wang, S.J. Bu, Growth characteristics and corrosion resistance of micro-arc oxidation coating on pure magnesium for biomedical applications, Corros. Sci. 52 (2010) 2228e2234. [53] Y.Q. Wang, K. Wu, F. Wang, Effects of second phases on microarc oxidation process of magnesium base materials, Acta Metall. Sin. 52 (2016) 689e697. [54] O. Khaselev, D. Weiss, J. Yahalom, Structure and composition of anodic films formed on binary MgeAl alloys in KOHealuminate solutions under continuous sparking, Corros. Sci. 43 (2001) 1295e1307. [55] C.F. Glover, G. Williams, Inhibition of corrosion-driven organic coating delamination and filiform corrosion on iron by phenyl phosphonic acid, Prog. Org. Coating 102 (2015) 44e52. [56] G. Williams, R. Grace, Chloride-induced filiform corrosion of organic-coated magnesium, Electrochim. Acta 56 (2011) 1894e1903. [57] R.C. Zeng, L.Y. Cui, K. Jiang, R. Liu, B.D. Zhao, Y.F. Zheng, In vitro corrosion and cytocompatibility of a microarc oxidation coating and poly(L-lactic acid) composite coating on Mg-1Li-1Ca alloy for orthopaedic implants, ACS Appl. Mater. Interfaces 8 (2016) 10014e10028. [58] C. Yu, L.Y. Cui, Y.F. Zhou, Z.Z. Han, X.B. Chen, R.C. Zeng, Y.H. Zou, S.Q. Li, F. Zhang, E.H. Han, S.K. Guan, Self-degradation of micro-arc oxidation/chitosan composite coating on Mg-4Li-1Ca alloy, Surf. Coating. Technol. 344 (2018) 1e11.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
In vitro corrosion of micro-arc oxidation coating on Mg-1Li-1Ca alloy d The influence of intermetallic compound Mg2Ca Zi-You Ding a, Lan-Yue Cui a, Xiao-Bo Chen b, Rong-Chang Zeng a, *, Shao-Kang Guan c, Shuo-Qi Li a, Fen Zhang a, Yu-Hong Zou d, Qing-Yun Liu d, ** a
College of Material Science and Engineering, Shandong University of Science and Technology, Qingdao, 266590, China School of Engineering, RMIT University, Carlton, 3053, VIC, Australia School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 450002, China d College of Chemicals and Environmental Engineering, Shandong University of Science and Technology, Qingdao, 266590, China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 February 2018 Received in revised form 6 June 2018 Accepted 7 June 2018
The presence of second phase Mg2Ca has remarkable impacts on the formation and degradation of MAO coating. Microstructure, formation and degradation behaviors of substrate and MAO coating were performed. Results revealed that pitting and filiform corrosion occurred on bare Mg-1Li-1Ca alloy. MAO coating significantly improved the corrosion resistance of alloy. The roles of Mg2Ca on the formation and degradation mechanisms of MAO coating were discussed. Micro-arc oxidation for the Mg-Li-Ca alloy initiated at a-Mg phase with depleted Ca content. The growth rate of coating on a-Mg phase is faster than that of GBs/Mg2Ca. The MAO coating began to degrade from intermetallic compound Mg2Ca, then it underwent the attack from the solution and stress concentration caused by corrosion products. © 2018 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Micro-arc oxidation Coating Intermetallic compounds Degradation
1. Introduction To date, great attention has been paid on magnesium (Mg) and its alloys as revolutionary biomaterials due to their remarkable physical and mechanical properties, such as low density, high specific strength, and excellent bio-compatibility [1]. Considerable studies have demonstrated that Mg and its alloys hold a promise as degradable orthopedic implants [2,3]. However, the equilibrium electrode potential of Mg is 2.37 V vs. standard hydrogen electrode (SHE), much lower than that of most metals [4,5]. Hence, Mg alloys are highly susceptible to corrosion in aggressive media [6,7]. Generally, to enhance the corrosion resistance of Mg alloys, following measures have been taken: purifying [8], alloying with noble elements [9,10] and surface passivation [11]. Basically, physical and chemical properties of pure Mg can be greatly changed by alloying with Al, Zn, Mn, RE, Ca, Li, etc. [12,13]. The formation of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q.-Y. Liu).
(R.-C.
https://doi.org/10.1016/j.jallcom.2018.06.073 0925-8388/© 2018 Elsevier B.V. All rights reserved.
Zeng),
[email protected]
intermetallic compounds, also called second phases, such as AlMn in AZ31, and Mg2Ca in Mg-Ca alloys, is common when alloying elements exceed their solid solubility limit with respect to Mg. Of a large number of binary Mg alloys [14,15], Mg-Ca and Mg-Li alloys attract great attention [16]. Ca is a vital component of human bones [9]. Li can transform crystalline structure of Mg from hexagonal closed-packed (HCP) into body-centered cubic (BCC) when appropriate quantity of Li is added into the bulk [17,18]. With increasing concentration of Li in Mg-Li binary alloy, microstructure is characterized by a (-Mg) phase, aþb (-Li) phase and b phase [19,20]. In vivo study performed by Witte et al. [3,21] discloses that Mg-Li alloy (LAE442) exhibits superior corrosion resistance than Mg alloy AZ91D. A further investigation shows Mg-Li-Ca alloy possesses a four-layered oxide film and good biocompatibility [22,23]. Thereby, surface modifications [24], i.e. micro-arc oxidation (MAO) [25], chemical conversion coating [26,27], and layer-by-layer assembled coating [28e30], are used to improve corrosion resistance of Mg alloys. MAO or plasma electrolytic oxidation (PEO) is a high voltage plasma-assisted anodic oxidation process, accompanied by visible sparking at anode/electrolyte interface [31,32]. MAO coating is a ceramic coating in situ grown on valve metals (i.e. Al, Mg and Ti).
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Fig. 1. Three-dimensional microstructure: (a) optical micrograph, (b) SEM image and (c) EDS point ID results of extruded Mg-1Li-1Ca alloy.
Mg can form an adherent and electrically insulating anodic oxide layer. This anodic oxidation layer acts as barrier to the flow of ions and electrons, reducing the rate of further oxidation to highly low values in aqueous electrolytes [33]. For MAO coating, major limitations are the presence of micro-pores and cracks on the surface [34]. The penetration of aggressive solutions into interface of MAO coating/substrate along micro-pores and cracks leads to a premature failure of coating for long-term immersion tests. As such, it is a pressing need to understand the reason for the defect formation of MAO coatings on Mg alloys and the role of intermetallic compounds in the formation of MAO coating. So far, a great deal of research has been focused on the preparation of coatings. It is noted that several studies involving the impacts of intermetallic compounds on formation of chemical conversion coatings [35]. Our previous study shows that microstructure and chemical compositions of Mg alloys exerted a significant influence on the formation mechanism and corrosion resistance of their chemical conversion Ca-P and Zn-Ca-P coatings [36,37]. These results are attributable to the dissolution of Mg substrates and released species, i.e. engagement of Mg2þ ions into coating formation. And simultaneously the alkalization of processing bath leads to deposition of coatings. Usually, second phases
play as micro-cathodes in the corrosion process of multicomponent Mg alloys [38]. Zhou et al. [39] suggested that galvanic effect between a-Mg phase and b phase in AZ91 alloy resulted in flower-like phosphate nucleus preferentially depositing on b phase; whilst ball-like phosphate nucleus predominately deposited in a-Mg phase interior due to the micro-galvanic effect within a phase. Since MAO coating is also a type of chemical conversion films, both MAO and chemical phosphate coatings share a common feature. Namely, substrate participates the formation of coatings through chemical reactions between species in solutions and metallic ions released from substrates. Unfortunately, little literature reports the effects of secondary phase on MAO coating formation for Mg alloys [40,41]. Wang et al. [42] demonstrated that sparking discharge preferentially occurs on a-Mg phase instead of Mg17Al12 (b) phase at initial stage of MAO formation; selective growth of coating, nevertheless, diminishes gradually over oxidation time and finally b phase will not further prevent the formation of coating at prolonged stage. That is, Mg17Al12 phase leads to disintegration and discontinuity of MAO coating on Mg alloy AZ91D. A further study by Chen et al. [43] revealed that both AZ91 substrate and its MAO coating exhibit increased corrosion resistance due to the presence of finer and
Fig. 2. (a) SEM images of MAO coating and its (b) EDS result, (c) cross-sectional view of coating and its (d) EDS scan mapping.
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protection of a mixed SF6 (1% in volume fraction) and CO2 (99% in volume fraction) atmosphere. Ingots with dimensions of 600 mm 200 mm 200 mm were prepared by pouring the melt into a preheated steel mold. They were homogenized at 450 C for 10 h following air-cooling. Then, ingots were extruded into 4 mm thick sheets through an 800-ton extruder (Yuan Hang) of Magnesium Industry in Chongqing Science and Technology Company, Ltd. at an extrusion ratio of 20.4: 1 and an extrusion rate of 1 m/min. The mold and extrusion temperature were set at 350 C and 300 C, respectively. The extruded sheets were cut into a size of 20 mm 20 mm 4 mm, then ground with water by continuous grades of SiC abrasive paper from 150 to 2500 grit, and polished with 1.0 mm diamond paste into a mirror-like surface. 2.2. MAO preparation
Fig. 3. XRD patterns of substrate and MAO coating of Mg-1Li-1Ca alloy.
MAO coating was fabricated on an MAO power supply (MAOI50C, Chengdu Pulsetech Electrical Co., Ltd) under a constant current control mode in an alkaline solution of 10 g/L NaOH and 18 g/L Na2SiO3. The processing was conducted at following electric parameters: current density 40 mA/cm2, frequency 2000 Hz and duty cycle 35% as well as duration 300 s. The bath temperature was kept at 20e40 C by a stirring and cooling system.
more evenly distributed b phase precipitations. However, MAO coating on the as-cast one shows chemical and morphological discontinuity on a and b phases with coarse and imperfect regions on phase boundaries. MAO coating on aged Mg alloy AZ91 with nano-sized b phase precipitations shows no marked differences on two-phase boundaries and is more homogeneous with fewer pores and defects. Conversely, coating in absence of b phase is characterized by most inhomogeneous microstructure and lowest corrosion resistance. This scenario is ascribed to the absence of protective aluminum-containing oxides in coating and more severe localized oxidation behavior adjacent to second phase AlMn. As for ternary Mg-Li-Ca alloys, inadequate literature on corrosion investigation is available [15]. Therefore, fundamental study on the corrosion characteristics of Mg-Li-Ca alloy is necessary. In present work, the emphasis is placed on the influences of second phase Mg2Ca on formation of MAO coating and taking further insights into degradation mechanism of MAO coating.
The etching solution include two solutions: first contains 3 mL acetic acid, 2 mL nitric acid, 1 g oxalic acid and 100 mL H2O; second solution is 3 mL nitric acid and 100 mL H2O. Samples were firstly immersed in first solution for 5e15 s, and second solution for 4e10 s. Microstructure and surface morphology of substrate and its MAO coating were observed via an optical microscope (OM, Olympus-GX41) and scanning electron microscope (FE-SEM, Nova Nano SEM450), respectively. Average grain size was acquired by counting the number of grains intercepted by three concentric circles sufficiently long to traverse at least 50 intercepts. Optical graphs and SEM images would be edited and grouped up by a graphics transaction.
2. Experimental
2.4. Corrosion behavior
2.1. Material preparation
Degradation rate was characterized using electrochemical tests and hydrogen evolution tests.
Mg-1Li-1Ca ingots, cast at the Institute of Metals Research, Chinese Academy of Sciences, are composed of 1.0 wt. % Li, 0.5 wt. % Ca and Mg balance. They were fabricated by melting under the
2.3. Microstructural observation
2.4.1. Electrochemical tests Electrochemical tests were performed on an electrochemical
Fig. 4. (a) Hydrogen evolution rate of bare substrate and its MAO coating, and (b) curves of pH value vs. immersion time: substrate and MAO coating of Mg-1Li-1Ca alloy in Hanks' solution.
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Fig. 5. (a) Curve of open circuit potential vs. immersion time, (b) potentiodynamic polarization curves of MAO coating and its substrate in Hanks' solution.
systems (Princeton VersaSTAT 4) in Hanks' solution in a water bath of 37 ± 0.5 C. Electrolyte, Hanks' solution, contains the following components: 8.0 g/L NaCl, 0.4 g/L KCl, 0.14 g/L CaCl2, 0.35 g/L NaHCO3, 1.0 g/L glucose (C6H6O6), 0.1 g/L MgCl2$6H2O, 0.1 g/L MgSO4$7H2O, 0.06 g/L KH2PO4, 0.126 g/L Na2HPO4$12H2O, and 1 L distilled water at 37 ± 0.5 C. Chemical reagents are supplied by Qingdao Jingke Instrument Reagent Co., Ltd. A typical threeelectrode system was applied with samples as working electrode with an exposed area of 1 cm2, a saturated calomel electrode (SCE, sat'd KCl) as reference electrode and a platinum foil electrode as counter electrode. Stabilization time is 4000 s at a scan rate of 1 mV/s for OCP test. Electrochemical Impedance Spectroscopy (EIS) test is done after the end of OCP test. The scan frequency of EIS range from 100 kHz to 10 mHz with a perturbation amplitude of 10 mV RMS. The EIS were fitted by the ZSimpWin software. During polarization test, the start potential is 0.4 V vs. OCP and final potential is þ0.5 V vs. OCP with 2 mV as step height and 1 s as step time. 2.4.2. Hydrogen evolution All hydrogen evolution tests were conducted at a water bath of 37 ± 0.5 C. The electrochemical measurements were conducted in Hanks' solutions. 2.4.3. pH measurement Variation in pH values of Hanks' solution was monitored through a digital pH meter (PHS-3C) at a water bath of 37 ± 0.5 C. 2.4.4. Surface characterization Surface morphology of MAO coating was observed via SEM. Chemical compositions and components of samples were analyzed using an energy-dispersive X-ray spectrum (EDS, Oxford X-MaxN). Crystallographic structure of samples was determined using an Xray diffractometer (XRD, Rigaku D/MAX 2500PC, Japan) with Cu Ka radiation. Scan range is 20 e85 with 8 /min as scan rate. Jade 5.0 is used to calibrate the diffraction peak for XRD, and the results are confronted with those reported in most literature. 3. Results 3.1. Microstructural observation Three-dimensional (3D) optical and SEM images (Fig. 1) reveal that microstructure of extruded Mg-1Li-1Ca alloy is characterized by equiaxed grains with a (-Mg) phases and intermetallic compound Mg2Ca particles, which agrees with our previous investigation [19]. The average grain size is approximately 20.2 mm (Fig. 1a),
but smaller size ones (about 7 mm) also exists, for which dynamic recrystallization occurred during hot extrusion. Mg2Ca particles, detected by EDS in Fig. 1c, are predominantly distributed at grain boundaries and presented a strip distribution along extrusion direction (Fig. 1b). Moreover, mechanical twin crystals were also identified. 3.2. Surface morphology and chemical composition Surface morphology of the prepared MAO coating of Mg-1Li-1Ca alloy is shown in Fig. 2a. A large amount of micro-pores are present, which is a typical morphology of MAO coating of Mg alloys [44,45]. Most of them are less than 1 mm. Fig. 2b reveals the presence of Mg, Si, O and traces of Ca in MAO coating. Fig. 3 shows XRD patterns of substrate and MAO coating. The presence of MgO demonstrates that coating forms during MAO treatment bathed in an electrolyte solution of NaOH and Na2SiO3. Also, MgSiO3 and CaSiO3 were detected. Interfacial morphology (Fig. 2c) of the MAO coating discloses that it has an averaged thickness of about 3.46 mm. Furthermore, EDS mapping (Fig. 2d) of the cross-sectional view unveils that MAO coating predominantly contains Mg, O and Si, which is in line with the previous results in Fig. 2b. 3.3. Corrosion behavior Mg and its alloys are prone to corrosion in an aggressive solution, and following reactions occur [46,47]. Anodic reaction: Mg / Mg2þ þ 2e
(1)
Cathodic reaction: 2H2O þ 2e / H2[ þ 2OH
(2)
Total reaction: Mg þ 2H2O / Mg(OH)2 þ H2[
(3)
Table 1 Electrochemical parameters of polarization curves of Mg-1Li-1Ca alloy substrate and its MAO coating. Sample Substrate MAO coating
Ecorr/VSCE 1.592 1.551
ba/103 mV 40.33 114.21
bc/103 mV 342.83 285.42
icorr/A$cm2 5
2.087 10 4.730 107
Rp/U$cm2 750.78 74881.64
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Fig. 6. (a) Nyquist plots with fitted curves, (b) Bode plots of bare Mg-1Li-1Ca substrate and its MAO coating, equivalent circuit of EIS: (c) substrate, (d) MAO coating in Hanks' solution.
According to corrosion reactions, hydrogen gas (H2) and Mg(OH)2 are generated in experiments from total reaction (3). The reaction of Mg is accompanied by generation of H2 and OH on basis of cathodic reaction (2), so the characteristics can be described by hydrogen evolution rate and pH value of corrosion solution.
3.3.1. Hydrogen evolution and pH value measurements Hydrogen evolution rate (HER) as a function of immersion time duration of Mg-1Li-1Ca alloy and its MAO coating in Hanks' solution for 144 h was plotted in Fig. 4a. HER varies with immersion time. In term of bare substrate, HER slows down due to the formation of corrosion products film covering the entire surface of substrate, which provides mild protection to the substrate. For MAO coating, three stages of corrosion progress are exhibited. After initial of immersion tests (Stage I), no significant changes in hydrogen evolution rate are evident at Stage II, which is attributed to the highly protective function of MAO coating to bare substrate. Finally, hydrogen evolution rate increases after 120 h (Stage III), which may be incurred by physical ruptures of MAO coatings, resulting from massive pitting and filiform corrosion [48]. After immersion for 144 h, average hydrogen rates of bare and MAOcoated Mg-1Li-1Ca specimens were 2.082 103 mL cm-2 h-1 and 4.038 104 mL cm-2 h-1, respectively, demonstrating MAO coating effectively improved corrosion resistance of this alloy. Fig. 4b indicates that pH value of solution increases due to degradation reactions. Note that, pH values of substrate are close to that of MAO coating in the early stage of immersion (t < 160 min). After initial stage, pH value for substrate is higher than MAO
coating, indicating more OH ions and Mg(OH)2 were generated from Eq. (2) and Eq. (3). In summary, corrosion rate of MAO coated specimens is much smaller than that of the bare substrate, which is consistent with above analysis.
3.3.2. Electrochemical tests Initial delay time duration is 4000 s to stabilize open circuit potential (OCP) in Fig. 5a. When OCP difference is greater than 50 mV, there is a risk of increased corrosion rate due to galvanic coupling. Unfortunately, galvanic corrosion may occur between substrate and MAO coating of Mg-1Li-1Ca alloy, because initial difference of OCPs is 164 mV. In addition, the difference of OCPs slowly decreased from 164 mV to 98 mV with immersion time, indicating a continual galvanic corrosion between substrate and MAO coating. Fig. 5b shows polarization curves of bare Mg-1Li-1Ca substrate and its MAO coating. Values of corrosion current density (icorr) and corrosion potential (Ecorr) were estimated through Tafel extrapolation fitting, as shown in Table 1. The MAO coating with lower two order of magnitudes of icorr than substrate, reveals the effective protection for the substrate. Ecorr of bare Mg-1Li-1Ca alloy (1.592 VSCE) is more negative than that of MAO coated specimen (1.551 VSCE), indicating a higher thermodynamically tendency to corrosion of substrate. Additional, the anodic polarization branch for MAO coating is significantly shifted towards higher icorr, indicating severe micro-cracks on coating surface. The A, B, C and D, sites on anodic branch of MAO coating, reveal the breakdown and repassivation of MAO coating and passive film. Rp is the polarization resistance that is inversely proportional to
Table 2 Fitting results of EIS of substrate and MAO coating in Hanks' solution. Sample
Rs/U$cm2 CPE/U1$cm2$s1 n
Substrate 83.14 MAO coating 92.7
5
2.971 10 3.09 106
Rct/U$cm2 Cf/F$cm2
0.6751 223.60 0.6577 878.10
6.631 10 e
CPEf/U1$cm2$s1 nf 6
e 7.202 106
L/H$cm2
Rf/U$cm2 3
3
RL/U$cm2 3
Chi-square/103
e 1.83 10 5.46 10 8.44 10 1.56 0.7228 3.908 104 1.321 104 9.578 104 0.9977
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Fig. 7. Macroscopic corrosion morphology: substrate of immersion for (a) 0 h, (b) 144 h, MAO coating of immersion for (c) 0 h, (d) 144 h in Hanks' solution.
corrosion rate. It can be calculated by the simplified Stern-Geary equation [49,50]:
Rp ¼
ba $bc 2:303icorr ðba þ bc Þ
(4)
where ba and bc are the anodic and cathodic Tafel slopes, respectively, and icorr is the corrosion density. Moreover, the corresponding electrochemical parameters, ba, bc and Rp, were listed in Table 1. Rp was improved from 758.78 U cm2 to 74881.64 U cm2, revealing high corrosion resistance of MAO coating than that of substrate. According to Nyquist plots and Bode plots (Fig. 6), MAO coating leads to a highly improvement in the corrosion resistance in comparison to substrate. The Nyquist plots show similar components: a capacitive reactance arc in high frequency (HF) range, a smaller capacitive reactance arc following in the middle frequency (MF), and an inductance reactance arc in the low frequency (LF) range, which is in accordance with our previous work [49]. For substrate, capacitive reactance arc in the HF region corresponds to electrical double layer at the interface of substrate surface and solution; and capacitive reactance arc in the MF region corresponds to oxidation layer formed in air; and the inductive reactance arc in the LF implies pitting and absorption of products. For MAO coating, capacitive reactance arc in the first HF region corresponds to the porous outer layer and the second arc corresponds to the denser barrier layer. Impedance reactance arc in LF region corresponds to generation of products. Fig. 6c and d shows the equivalent circuit of EIS. Rs refers to solution resistance, and Rct represents the charge transfer resistance. In contrast to substrate, Rct of MAO coating increases from 223.60 U cm2 to 878.10 U cm2, which indicates lower dissolution
rate of the film than that of substrate. The Rf and Cf designate the second capacitive reactance arc in MF region. Constant phase element (CPE) is used to replace capacitance Cf in MAO coating. CPEf indicates the deviation of Cf to theoretical value due to the considerably porous structure of MAO coating. A dimensionless coefficient, n was introduced to fix this deviation. The second capacitive reactance arc is related to the denser sub-layer of the coating. The fitting results of EIS are listed in Table 2. 4. Discussion 4.1. Influence of Mg2Ca on corrosion of substrate For further evaluation of corrosion features of bare substrate and its MAO coating, macroscopic morphologies of as-ground Mg-1Li1Ca alloy specimens and ultrasonically cleaned ones after being immersed in Hanks' solution were taken by a digital camera (Fig. 7). Surface of the as-ground Mg-1Li-1Ca alloy exhibits a smooth finish (Fig. 7a), while traces of pitting can be observed clearly after an immersion for 144 h (Fig. 7b, marked by arrows). With increasing immersion time, corrosion pits expend inwards the bulk substrate. Localized corrosion is mainly corrosion type of Mg-1Li1Ca alloy in Hanks' solution and its corrosion mechanism is discussed in the following part. The corrosion pits initiated at a-Mg matrix neighboring intermetallic compounds - Mg2Ca particles. Micro-galvanic corrosion between them leads to pitting corrosion. Via micro-electrochemical testing, Nicholas et al. [51] believed Mg2Ca with rapid anodic kinetics compared with Mg matrix. So, second phase would also impact the degradation of MAO coating. The coating surface is uniform and smooth with some defects before immersion (marked by circles) due to the second phase, Mg2Ca. After immersion, slight traces of corrosion occur on the
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Fig. 8. Morphology of MAO coating obtained (a) before and after appearance of arc discharges for (b) (c) 5 s, (d) 10 s and EDS spectrum.
surface (marked by circles in Fig. 7d), but the surface is still smooth. Moreover, the number of pits of MAO coating is less than that of substrate. Thus, compared with Mg-1Li-1Ca substrate, MAO coating possesses better corrosion resistance. 4.2. Influence of Mg2Ca on the formation and corrosion of MAO coating In Fig. 8, surface morphology of MAO coating was obtained before and after appearance of sparks, and a mechanism illustration was shown in Fig. 9. EDS spectrum in Fig. 8 and Table 3 shows chemical compositions of specific points in Fig. 8. A thin and dense barrier layer was formed on the surface of sample at beginning of a voltage applied (before appearance of sparks, in Figs. 8a and 9a). There are no sparks and only some tiny oxygen bubbles can be seen on the sample surface at this stage, corresponding to traditional anodizing stage [44]. The surface was fully covered with coating with less micro-cracks. Once sparks occurred for 5 s, few strip-shape oxides began to form (Fig. 8b). That is to say, once the voltage exceeds the breakdown voltage, small and dense sparks appear and oxygen evolution also becomes
vigorous, which is reported by Zhao et al. [52]. The bright areas in a phase can be easily identified, and dark areas consists of GBs and second phase, marked by arrows, which also was reported by Zhang et al. [44]. Element C originated from air; Mg and Ca were from the alloy; and Si and O came from the electrolyte. It is noteworthy that (Table 3) there was a big difference in Ca content between the different phases: the presence of Ca for spectrums #1~#3 and the absence of Ca for spectrums #4 and #5. Spectrum #2 was detected higher Ca content than others, indicating the presence of Mg2Ca. For spectrum #4 in absence of Ca, a strip-shape protuberances (in Fig. 8c) could be compounds of MgO and MgSiO3 etc. In Fig. 8b, the coating was preferentially formed at a phase than GBs/Mg2Ca, due to differences of valve metal characteristics [53]. As marked by arrows in Fig. 8c, coating grew from a-Mg into GBs/Mg2Ca, indicating growth rate of anodized films on a-Mg phase was faster than that of GBs/Mg2Ca. That is, micro-arc oxidation for the Mg-Li-Ca alloy easily initiated at a-Mg phase with lower or depleted Ca content. It was consistent with what Khaselev et al. [54] reported, the growth of anodic coatings start on a phase and continued on second phase. With time increasing, bright areas almost covers the surface of
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Fig. 9. Formation mechanism illustration during micro-arc oxidation preparation: (a) before and (b) after appearance of arc discharges (c) 5 s, (d) 10 s.
a-Mg and GBs/Mg2Ca, and micro-pores and cracks (Fig. 8c) appeared on the interface of strip-like oxides and their neighboring areas. The micro-pores or cracks provided a path for solution and sparks to connect substrate, as a result, volcano-shape pores were formed (Fig. 8d). Fig. 9 illustrates the formation mechanism of MAO coating on Mg-1Li-1Ca alloy. The anodized process followed four steps: (1) the formation of a dense and thin barrier (Fig. 9a); (2) the initiation of oxides in the depleted Ca a-Mg matrix after the presence of sparks; (3) the horizontal growth of the oxide film (Fig. 10c); (4) and finally the thickening of the MAO coating (Fig. 10d). This is similar to previous report [52]. 4.3. Influence of Mg2Ca on degradation mechanism The surface morphologies of bare substrate and MAO coating after immersion in Hanks' solution are shown in Fig. 10. The alloy suffers evident damages after immersion, with obvious corrosion morphology: pitting and filiform corrosion in Fig. 10a and its insets. The galvanic corrosion occurs between mainly composed of two phases, Mg2Ca and a-Mg that act as anode and cathode of galvanic corrosion, respectively [23]. There are a lot of corrosion pits on the surface, even some have penetrated into the interior of the substrate (insets of Fig. 10a, marked by arrows). The filiform corrosion is a shallow membrane etch that develops rapidly when it occurs,
which results in dense mesh pattern spreading over the alloy surface. The filiform corrosion, whose head is anode and tail is cathode, usually occurs under protective coatings or anodic oxide layers [55,56]. Although no protective coating is applied to Mg-1Li-1Ca alloy, it is a natural passivity metal that can easily form a corrosion product film on its surface. In contrast with Fig. 10a, MAO coating underwent the attack with some micro-cracks (Fig. 10c and d), even more coating failure (Fig. 10b, marked by circles), can be seen after immersion for 144 h. According to our previous work [57,58], the situation of second phase is where MAO coating initially destroys. And in Fig. 10d, there is a certain orientation in the direction of micro-cracks, which may result from Mg2Ca with a strip distribution. A high magnification picture (insets of Fig. 10c and d) was taken for further observation of micro-cracks. Interestingly, most of micro-cracks passed through or originated from micro-pores, which might be caused by stress concentration due to the volume increase of corrosion products. In summary, MAO coating effectively slows down corrosion of substrate, but second phase promotes failure of MAO coating. Our previous study [57,58] have proved that corrosion products (precipitations and hydrogen gas) could promote MAO coating break and corrosion. Based on above-mentioned results of this study, degradation mechanism of MAO coating on Mg-1Li-1Ca alloy is described in Fig. 11, and degradation procedures can be concluded into some steps as following:
Table 3 EDS results of the samples in Fig. 8 for specific points in at. % (in wt. %). Element
Spectrum #1
Spectrum #2
Spectrum #3
Spectrum #4
Spectrum #5
O
33.88 (26.03) 7.37 (4.26) 54.36 (63.59) 4.03 (5.43) 0.36 (0.69)
30.40 (22.89) 6.55 (3.70) 59.30 (67.84) 2.62 (3.44) 1.13 (2.13)
40.46 (32.38) 10.07 (6.05) 43.45 (52.84) 5.53 (7.76) 0.49 (0.97)
52.76 (45.16) 12.67 (8.15) 25.95 (33.75) 8.62 (12.94) e e
33.76 (25.96) 7.08 (4.09) 54.52 (63.70) 4.64 (6.26) e e
C Mg Si Ca
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Fig. 10. Corrosion morphologies of: Mg-1Li-1Ca (a) substrate and (bed) MAO coating after an immersion of 144 h.
(1) The defects including micro-cracks or through pores on MAO coating provided paths for corrosive media (Hanks' solution) into the interface of MAO/substrate. The intermetallic particles (Mg2Ca) with higher potential relative to a-Mg matrix,
systematically enhanced dissolution kinetics, and sustained dissolution rate about an order magnitude greater than a-Mg [23]. Intermetallic compounds are predominantly distributed at grain boundaries and presented a strip distribution
Fig. 11. Degradation mechanism illustration for (a) (b) (c) surface and (d) (e) (f) sectional view of Mg-1Li-1Ca alloy coated MAO coating in Hanks' solution.
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along extrusion direction (Fig. 1b), so similar distribution and shape of corrosion pits will appear. These pits (Fig. 11d) will promote degradation rate of MAO coating. At the same time, hydrogen and corrosion precipitations, i.e. Mg(OH)2 and Ca(OH)2, formed and deposited in the top of corrosion pits on the interface of MAO/substrate, as shown in Fig. 11e. Additional, corrosion precipitations also cover the corrosion pits with a weak protection for substrate. In Fig. 5b, the point A, B, C and D on anodic branch of MAO coating, reveal the breakdown and repassivation of passive film of corrosion precipitations. (2) After a period of corrosion, MAO coating would be subject to stress from the substrate pointing to coating (Fig. 11a, d). These enhanced stresses have two sources: the volume expansion in formation of corrosion precipitations and additional stress of hydrogen bubble evolution. Then, some micro-cracks expanded and gradually merged (Fig. 11b). As a result, more solution penetrated into the interior of MAO coating and degradation rate increased, which was consistent with above results of hydrogen evolution measurement. In addition, MAO coating (MgO layer) may peeled off from the substrate due to the chemical dissolution. MgO þ H2O / Mg(OH)2
(5)
(3) Finally, partial MAO coating delaminated and fresh bare substrate was exposed to solution and hence the corrosion accelerated, as shown in Fig. 11c, f.
5. Conclusions The presence of second phase Mg2Ca has remarkable impacts on the formation and degradation of MAO coating. The microstructure, physical and corrosion properties of Mg-1Li-1Ca alloy and its MAO coating were examined, and following conclusions are drawn: (1). Filiform corrosion and pitting corrosion are main corrosion types for Mg-1Li-1Ca alloy, resulting from the micro-galvanic corrosion between second phase (Mg2Ca) and a-Mg matrix. (2). The presence of Mg2Ca particles has remarkable impact on the formation of MAO coating. Micro-arc oxidation on the Mg-Li-Ca alloy initiated at a-Mg phase with lower or depleted Ca content. The growth rate of coating on a-Mg phase was faster than that of GBs/Mg2Ca, so coating grew from a-Mg into GBs/Mg2Ca. (3). The degradation mechanism of MAO coating was proposed. The MAO coating initially degraded from intermetallic compound Mg2Ca, then it underwent the attack from the solution and stress concentration caused by corrosion products. As a result, partial delamination of MAO coating occurred.
Acknowledgement This work was supported by the National Natural Science Foundation of China (51571134, 51601108), the Scientific Research Foundation of Shandong University of Science and Technology (SDUST) for Recruited Talents (2013RCJJ006), and the SDUST Research Fund (2014TDJH104). Thanks also go to Miss Fei Xing Chongqing University of Technology for help on corrosion tests.
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References [1] X.N. Gu, Y.F. Zheng, Y. Cheng, S.P. Zhong, T.F. Xi, In vitro corrosion and biocompatibility of binary magnesium alloys, Biomaterials 30 (2009) 484e498. [2] M. Staiger, A. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: A review, Biomaterials 27 (2006) 1728e1734. [3] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyerlindenberg, C.J. Wirth, H. Windhagen, In vivo corrosion of four magnesium alloys and the associated bone response, Biomaterials 26 (2005) 3557e3563. [4] F. Witte, The history of biodegradable magnesium implants: a review, Acta Biomater. 6 (2010) 1680e1692. [5] X.J. Wang, D.K. Xu, R.Z. Wu, X.B. Chen, Q.M. Peng, L. Jin, Y.C. Xin, Z.Q. Zhang, Y. Liu, X.H. Chen, What is going on in magnesium alloys? J. Mater. Sci. Technol. 34 (2018) 245e247. [6] S. Thomas, N.V. Medhekar, G.S. Frankel, N. Birbilis, Corrosion mechanism and hydrogen evolution on Mg, Curr. Opin. Solid St. M 19 (2015) 85e94. [7] G.S. Frankel, A. Samaniego, N. Birbilis, Evolution of hydrogen at dissolving magnesium surfaces, Corros. Sci. 70 (2013) 104e111. [8] A. Prasad, P.J. Uggowitzer, Z. Shi, A. Atrens, Production of high purity magnesium alloys by melt purification with Zr, Adv. Eng. Mater. 14 (2012) 477e490. [9] C.L. Liu, Y.J. Wang, R.C. Zeng, X.M. Zhang, W.J. Huang, P.K. Chu, In vitro corrosion degradation behaviour of MgeCa alloy in the presence of albumin, Corros. Sci. 52 (2010) 3341e3347. [10] A. Imandoust, C.D. Barrett, T. Al-Samman, K.A. Inal, H.E. Kadiri, A review on the effect of rare-earth elements on texture evolution during processing of magnesium alloys, J. Mater. Sci. 52 (2017) 1e29. [11] H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys - A review, Acta Biomater. 8 (2012) 2442e2455. [12] R.C. Zeng, W.C. Qi, H.Z. Cui, F. Zhang, S.Q. Li, E.H. Han, In vitro corrosion of asextruded MgeCa alloysdthe influence of Ca concentration, Corros. Sci. 96 (2015) 23e31. [13] M.B. Kannan, R.K. Raman, In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid, Biomaterials 29 (2008) 2306e2314. [14] F. Witte, J. Fischer, J. Nellesen, H.A. Crostack, V. Kaese, A. Pisch, F. Beckmann, H. Windhagen, In vitro and in vivo corrosion measurements of magnesium alloys, Biomaterials 27 (2006) 1013e1018. [15] G.S. Song, M.V. Kral, Characterization of cast MgeLieCa alloys, Mater. Char. 54 (2005) 279e286. [16] Z. Li, X. Gu, S. Lou, Y. Zheng, The development of binary MgeCa alloys for use as biodegradable materials within bone, Biomaterials 29 (2008) 1329e1344. [17] R. Wu, Y. Yan, G. Wang, L.E. Murr, W. Han, Z. Zhang, M. Zhang, Recent progress in magnesiumelithium alloys, Int. Mater. Rev. 60 (2015) 65e100. [18] X.B. Chen, C.Q. Li, D.K. Xu, Biodegradation of Mg-14Li alloy in simulated body fluid: a proof-of-concept study, Bioact. Mater. 3 (2017) 110e117. [19] R.C. Zeng, L. Sun, Y.F. Zheng, H.Z. Cui, E.H. Han, Corrosion and characterisation of dual phase MgeLieCa alloy in Hanks' solution: the influence of microstructural features, Corros. Sci. 79 (2014) 69e82. [20] Y.Y. Zhou, L.P. Bian, G. Chen, L.P. Wang, W. Liang, Influence of Ca addition on microstructural evolution and mechanical properties of near-eutectic Mg-Li alloys by copper-mold suction casting, J. Alloys Compd. 664 (2016) 85e91. [21] F. Witte, J. Fischer, J. Nellesen, C. Vogt, J. Vogt, T. Donath, F. Beckmann, In vivo corrosion and corrosion protection of magnesium alloy LAE442, Acta Biomater. 6 (2010) 1792e1799. [22] L.Y. Cui, L. Sun, R.C. Zeng, Y.F. Zheng, S.Q. Li, In vitro degradation and biocompatibility of Mg-Li-Ca alloysdthe influence of Li content, Sci. China Mater 61 (2017) 1e12. [23] S.S. Nene, B.P. Kashyap, N. Prabhu, Y. Estrin, T. Al-Samman, Biocorrosion and biodegradation behavior of ultralight Mge4Lie1Ca (LC41) alloy in simulated body fluid for degradable implant applications, J. Mater. Sci. 50 (2015) 3041e3050. [24] W. Peng, L. Tan, K. Yang, Surface modification on biodegradable magnesium alloys as orthopedic implant materials to improve the bio-adaptability: A review, J. Mater. Sci. Technol. 32 (2016) 827e834. [25] L.Y. Cui, R.C. Zeng, S.K. Guan, W.C. Qi, F. Zhang, S.Q. Li, E.H. Han, Degradation mechanism of micro-arc oxidation coatings on biodegradable Mg-Ca alloys: the influence of porosity, J. Alloys Compd. 695 (2016) 2464e2476. [26] X.B. Chen, N. Birbilis, T.B. Abbott, Review of corrosion-resistant conversion coatings for magnesium and its alloys, Corrosion -Houston Tx- 67 (2011) 35001e35005. [27] E. Rocca, C. Juers, J. Steinmetz, Corrosion behaviour of chemical conversion treatments on as-cast MgeAl alloys: electrochemical and non-electrochemical methods, Corros. Sci. 52 (2010) 2172e2178. [28] J.J. Richardson, M. Bjornmalm, F. Caruso, Multilayer assembly. Technologydriven layer-by-layer assembly of nanofilms, Science 348 (2015) aaa2491. [29] L.Y. Cui, R.C. Zeng, X.X. Zhu, T.T. Pang, S.Q. Li, F. Zhang, Corrosion resistance of biodegradable polymeric layer-by-layer coatings on magnesium alloy AZ31, Front. Mater. Sci. 10 (2016) 1e13. [30] L.Y. Cui, R.C. Zeng, S.Q. Li, F. Zhang, E.H. Han, Corrosion resistance of layer-bylayer assembled polyvinylpyrrolidone/polyacrylic acid and amorphous silica films on AZ31 magnesium alloys, RSC Adv. 6 (2016) 63107e63116. [31] G.P. Wirtz, S.D. Brown, W.M. Kriven, Ceramic coatings by anodic spark deposition, Adv. Manuf. Processes 6 (1991) 87e115.
260
Z.-Y. Ding et al. / Journal of Alloys and Compounds 764 (2018) 250e260
[32] C. Blawert, W. Dietzel, E. Ghali, G. Song, Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Adv. Eng. Mater. 8 (2006) 511e533. [33] M.M. Lohrengel, Thin anodic oxide layers on aluminium and other valve metals: high field regime, Mat. Sci. Eng. R 11 (1993) 243e294. [34] T.S.N.S. Narayanan, I.S. Park, H.L. Min, Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: prospects and challenges, Prog. Mater. Sci. 60 (2014) 1e71. [35] C. Ke, Y. Wu, Y. Qiu, J. Duan, N. Birbilis, X.B. Chen, Influence of surface chemistry on the formation of crystalline hydroxide coatings on Mg alloys in liquid water and steam systems, Corros. Sci. 113 (2016) 145e159. [36] R.C. Zeng, F. Zhang, Z.D. Lan, H.Z. Cui, E.H. Han, Corrosion resistance of calcium-modified zinc phosphate conversion coatings on magnesiumealuminium alloys, Corros. Sci. 88 (2014) 452e459. [37] R. Zeng, Z. Lan, L. Kong, Y. Huang, H. Cui, Characterization of calciummodified zinc phosphate conversion coatings and their influences on corrosion resistance of AZ31 alloy, Surf. Coating. Technol. 205 (2011) 3347e3355. [38] W.C. Kim, J.G. Kim, J.Y. Lee, H.K. Seok, Influence of Ca on the corrosion properties of magnesium for biomaterials, Mater. Lett. 62 (2008) 4146e4148. [39] W. Zhou, D. Shan, E.H. Han, W. Ke, Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy, Corros. Sci. 50 (2008) 329e337. [40] R.F. Zhang, Film formation in the second step of micro-arc oxidation on magnesium alloys, Corros. Sci. 52 (2010) 1285e1290. [41] R.F. Zhang, G.Y. Xiong, C.Y. Hu, Comparison of coating properties obtained by MAO on magnesium alloys in silicate and phytic acid electrolytes, Curr. Appl. Phys. 10 (2010) 255e259. [42] Y.Q. Wang, X.J. Wang, T. Zhang, K. Wu, F.H. Wang, Role of b phase during microarc oxidation of Mg alloy AZ91D and corrosion resistance of the oxidation coating, J. Mater. Sci. Technol. 29 (2013) 1129e1133. [43] Y. Chen, Y. Yang, W. Zhang, T. Zhang, F. Wang, Influence of second phase on corrosion performance and formation mechanism of PEO coating on AZ91 Mg alloy, J. Alloys Compd. 718 (2017) 92e103. [44] R.F. Zhang, S.F. Zhang, Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys, Corros. Sci. 51 (2009) 2820e2825. [45] M.S. Joni, A. Fattah-Alhosseini, Effect of KOH concentration on the electrochemical behavior of coatings formed by pulsed DC micro-arc oxidation (MAO) on AZ31B Mg alloy, J. Alloys Compd. 661 (2016) 237e244.
[46] R.C. Zeng, W.C. Qi, Y.W. Song, Q.K. He, H.Z. Cui, E.H. Han, In vitro degradation of MAO/PLA coating on Mg-1.21Li-1.12Ca-1.0Y alloy, Front. Mater. Sci. 8 (2014) 343e353. [47] H. Hermawan, D. Dube, D. Mantovani, Developments in metallic biodegradable stents, Acta Biomater. 6 (2010) 1693e1697. [48] L. Gawel, L. Nieuzyla, G. Nawrat, K. Darowicki, P. Slepski, Impedance monitoring of corrosion degradation of plasma electrolytic oxidation coatings (PEO) on magnesium alloy, J. Alloys Compd. 722 (2017) 406e413. [49] C. Liu, Q. Bi, A. Leyland, A. Matthews, An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II.: EIS interpretation of corrosion behavior, Corros. Sci. 45 (2003) 1257e1273. [50] C. Wang, B. Jiang, M. Liu, Y. Ge, Corrosion characterization of micro-arc oxidation composite electrophoretic coating on AZ31B magnesium alloy, J. Alloys Compd. 621 (2015) 53e61. [51] N.T. Kirkland, N. Birbilis, J. Waiker, T. Woodfield, G.J. Dias, M.P. Staiger, In-vitro dissolution of magnesium-calcium binary alloys: clarifying the role of calcium additions in bioresorbable magnesium implant alloys, J. Biomed. Mater. Res. B 95 (2010) 91e100. [52] L.C. Zhao, C.X. Cui, Q.Z. Wang, S.J. Bu, Growth characteristics and corrosion resistance of micro-arc oxidation coating on pure magnesium for biomedical applications, Corros. Sci. 52 (2010) 2228e2234. [53] Y.Q. Wang, K. Wu, F. Wang, Effects of second phases on microarc oxidation process of magnesium base materials, Acta Metall. Sin. 52 (2016) 689e697. [54] O. Khaselev, D. Weiss, J. Yahalom, Structure and composition of anodic films formed on binary MgeAl alloys in KOHealuminate solutions under continuous sparking, Corros. Sci. 43 (2001) 1295e1307. [55] C.F. Glover, G. Williams, Inhibition of corrosion-driven organic coating delamination and filiform corrosion on iron by phenyl phosphonic acid, Prog. Org. Coating 102 (2015) 44e52. [56] G. Williams, R. Grace, Chloride-induced filiform corrosion of organic-coated magnesium, Electrochim. Acta 56 (2011) 1894e1903. [57] R.C. Zeng, L.Y. Cui, K. Jiang, R. Liu, B.D. Zhao, Y.F. Zheng, In vitro corrosion and cytocompatibility of a microarc oxidation coating and poly(L-lactic acid) composite coating on Mg-1Li-1Ca alloy for orthopaedic implants, ACS Appl. Mater. Interfaces 8 (2016) 10014e10028. [58] C. Yu, L.Y. Cui, Y.F. Zhou, Z.Z. Han, X.B. Chen, R.C. Zeng, Y.H. Zou, S.Q. Li, F. Zhang, E.H. Han, S.K. Guan, Self-degradation of micro-arc oxidation/chitosan composite coating on Mg-4Li-1Ca alloy, Surf. Coating. Technol. 344 (2018) 1e11.