Search for articles:
Chang-Yang Li
, Lan-Yue Cui
Corresponding authors:
Received: 2018-09-26
Revised: 2018-11-8
Accepted: 2018-11-26
Online: 2019-06-20
Copyright: 2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology
More
Abstract
One of the major obstacles for the clinical use of biodegradable magnesium (Mg)-based materials is their high corrosion rate. Micro-arc oxidation (MAO) coatings on Mg alloys provide mild corrosion protection owing to their porous structure. Hence, in this study a dense Mg(OH)2 film was fabricated on MAO-coated Mg alloy AZ31 in an alkaline electrolyte containing ethylenediamine tetraacetic acid disodium (EDTA-2Na) to reinforce the protection. Surface morphology, chemical composition and growth process of the MAO/Mg(OH)2 hybrid coating were examined using field-emission scanning electron microscopy, energy dispersive X-ray spectrometer, X-ray diffraction, X-ray photoelectron spectroscopy and Fourier transform infrared spectrophotometer. Corrosion resistance of the coatings was evaluated via potentiodynamic polarization curves and hydrogen evolution tests. Results manifested that the Mg(OH)2 coating possesses a porous nano-sized structure and completely seals the micro-pores and micro-cracks of the MAO coating. The intermetallic compound of AlMn phase in the substrate plays a key role in the growth of Mg(OH)2 film. The current density of Mg(OH)2-MAO composite coating decreases three orders of magnitude in comparison with that of bare substrate, indicating excellent corrosion resistance. The Mg(OH)2-MAO composite coating is beneficial to the formation of calcium phosphate corrosion products on the surface of Mg alloy AZ31, demonstrating a great promise for orthopaedic applications.
Keywords：
Magnesium (Mg) and its alloys have become a material of choice for temporary-bone implants owing to their low density, comparative elastic modulus to that of natural bone and excellent biocompatibility [1], [2], [3]. However, electrochemically active Mg alloys are vulnerable to corrosion in chlorine-ion-bearing solutions, such as human body fluid, which thus limits their clinical applications [4], [5], [6], [7], [8]. A large variety of feasible surface-modification strategies, such as chemical conversion coatings [9], [10], [11], micro-arc oxidation (MAO) [12], [13], [14], [15], layered double hydroxides (LDH) [16,17] and layer-by-layer self-assembly [18], [19], [20], [21], have been extensively developed to improve the corrosion resistance of Mg alloys. In this regard, MAO coatings outperform their competitors, attributed to their high hardness, highly metallurgical adhesion to substrate, and superior corrosion protection over a short time period. However, such protection degrades greatly with time owing to the porous nature of MAO coatings as a result of the electric sparks generated in the MAO processing. Particularly, open pores in MAO coatings lead to accumulation of corrosive media, and provide pathways for the ingress of corrosive ingredients into the interface of MAO/substrate, which makes a short in-service life of the MAO-coated Mg components [22,23].
In addition, sound pore-sealing techniques have been attempted over the past decades to address the Achilles heel of MAO coating systems. The mainstream procedures can be categorized into single or multiple steps. Time- and cost-effective one-step options generally involve a self-sealing procedure of MAO with ceramic oxide particles (e.g. SiO2 and TiO2) in electrolyte [24], [25], [26] with low sealing efficiency and lack of generic procedures applicable for a broad range of MAO coatings. For instance, Song et al. [26] have prepared a self-sealing MAO coating on Mg alloy AM60 in a fluorotitanate electrolyte, and found that fluoride ions have a great influence on the film compactness.
In contrast, the time-consuming two-step methods include both MAO and post-sealing process. So far, a number of two-step approaches have been employed for sealing pores in MAO coatings, such as polymer coatings (i.e., PLLA, chitosan and silane) [27], [28], [29], [30], inorganic fillers [31,32], and boiling water or alkaline treatment [33], [34], [35], [36]. Our previous investigations reveal that for MAO/PLLA composite coatings on Mg-Li-Ca-(Y) alloys, the outer PLLA coating peels off due to its physical swelling and preferential corrosion of the substrate alloy [27,37]. Compared with the mechanical or physical combination between organic coating and MAO coating, in-situ growth of Mg or titanium hydroxide coatings on top of MAO coatings with high adhesion to Mg or Ti alloys can avoid the risks of the coating fracture. The most common post-treatment method is conducted with boiling water or alkaline treatment. Cui et al. [28] attempted an alkaline post-treatment using 1 M NaOH followed by surface salinization to effectively seal the pores of the MAO coating on Mg alloy AZ31. Han et al. [36] have employed alkali treated in 5 M NaOH aqueous solution at 60 °C for 24 h. And the specimens were heated at 600 °C for 1 h and then furnace cooled. Consequently, a porous network structure Mg(OH)2 was prepared on Ti-24Nb-4Zr-8Sn alloy. Narayanan et al. [38] fabricated a Mg(OH)2 coating with a fine spherical or globular feature via an alkaline post-treatment using 3 M NaOH at 60 °C for 1 h. Although alkali treatments in a single alkaline solution for above-mentioned processes have been used to prepare Mg(OH)2 coatings on MAO surfaces, only a portion of micro-pores or micro-cracks were covered. Nevertheless, addition of a complexing agent, e.g., ethylenediamine tetraacetic acid disodium salt (Na-EDTA), into an alkaline solution to promote the growth of Mg(OH)2 coating on MAO coating has scarcely been reported.
Na-EDTA [39,40] is a hexadentate ("six-teeth") ligand and chelating agent with a strong ability to sequester metal ions such as Ca2+ and Mg2+. Tang et al. [41] fabricated a hydroxyapatite (HA, Ca10(PO4)6(OH)2) micro-flower coating on MAO surface in a solution (300 mmol/L Ca-EDTA and 200 mmol/L KH2PO4) at 80 °C for 6 h. The HA micro-flowers aggregate on the MAO surface with a size of 15-20 μm and a thickness of approximately 15 μm. But the corrosion current density (icorr) is only decreased one order of magnitude from 8.09 × 10-6 A/cm2 of bare substrate to 4.21 × 10-7 A/cm2. Kim et al. [34] prepared a Mg(OH)2 and HA hydride coating on MAO coating via hydrothermal treatment using 0.1 M Ca-EDTA and 0.5 M NaOH solution at 90 °C for different time frames (6, 12, 24, and 48 h). The surface has a multiple layer of a complex network structure. The coating thickness increases with increasing treatment time. And the coating treated after 24 h has the best corrosion resistance. The corrosion current density for composite coating decreases two orders of magnitude to 1.95 × 10-7 A/cm2. The results show that alkaline treatment time has a direct impact on corrosion resistance.
Moreover, the compactness and roughness of chemical conversion coatings and MAO coatings on Mg alloys is severely affected by intermetallic compounds or second phases (e.g. AlMn and Mg2Ca) [42]. This study aims to fabricate a biocompatible nano-structured Mg(OH)2 coating on MAO coating with improved corrosion resistance through an environmentally-friendly treatment, i.e. low treatment temperature, and short treatment time, and to understand the formation mechanism of Mg(OH)2-MAO composite coating. Especially, attention is paid on the influence of intermetallic compounds AlMn and EDTA as well as pore-sealing process of the Mg(OH)2 coatings on MAO coating.
Mg alloy AZ31 specimens with dimensions of 20 mm × 20 mm × 5 mm were used as substrate material. The substrates were ground with SiC sand papers progressively to 1500-grit finish and cleaned with de-ionized (DI) water, ethanol for 5 min at room temperature and then dried with warm air for preparation of MAO coatings. The pre-treated specimens were anodized using a house-made MAO equipment. The device is consisted of a power supply unit controlled by a single chip micyoco, a stainless-steel barrel as cathode, and a stirring and cooling system. MAO coatings were fabricated with 1 kW AC power supply with a duty cycle of 50%, a constant current of 0.3 A and a frequency of 100 Hz for 5 min. The electrolyte contained 10 g L-1 of NaOH and 8 g L-1 of phytic acid (Solution A) [43]. After coating preparation, samples were cleaned with DI water and dried using warm air.
MAO samples were immersed in a solution consisting of 40 g L-1 of NaOH and 50 g L-1 of EDTA-2Na (Solution B) with pH 13.6 at 60 °C for 1 h. Then, the samples were cleaned with DI water and dried at 80 °C for 30 min. The preparation process is schematically illustrated in Fig. 1.
Fig. 1. Flow chart of the preparation process of Mg(OH)2-MAO composite coating.
Surface morphology of the coatings was observed using field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450, USA); and elemental composition was detected with equipped energy dispersive X-ray spectrometer (EDS). The chemical bonds existing in the coatings were characterized via a Fourier transform infrared spectrophotometer (FTIR, Nicolet 380, Thermo Electron Corporation, USA). Crystallographic microstructure of the alloy and the coatings was analyzed by means of an X-ray diffractometer (XRD, Rigaku D/MAX2500PC, Japan) with a Cu target (λ = 0.154 nm) at a scanning rate of 8°/min in the 2θ range from 5° to 80°.
Corrosion resistance of the Mg alloy substrate and various coatings was evaluated by electrochemical polarization and hydrogen evolution measurements. Potentiodynamic polarization curves were recorded through an electrochemical analyzer (PAR Model 2273, Princeton, USA). A three-electrode cell set-up was applied, consisting of the prepared sample as working electrode and a platinum sheet as counter and a saturated calomel electrode (SCE) as reference electrode. All the electrochemical experiments were performed in Hank’s balanced salt solution (HBSS), containing 8.0 g/L NaCl, 1.0 g/L C6H6O6 (Glucose), 0.35 g/L NaHCO3, 0.4 g/L KCl, 0.14 g/L CaCl2, 0.1 g/L MgCl2·6H2O, 0.06 g/L MgSO4·7H2O, 0.06 g/L KH2PO4 and 0.06 g/L Na2HPO4·12H2O at room temperature. Prior to potentiodynamic polarization tests, a stable open-circuit potential (OCP) was established within 600 s. The polarization curves were recorded with a sweep rate of 2 mV/s. The hydrogen evolution was tested by placing the samples with full surfaces exposed into in HBSS at 37.5 ± 0.1 °C under an inverted funnel connected to a graduated burette and measuring the solution level in the burette intermittently for 156 h [18,44].
Fig. 2 shows SEM images and EDS spectra of the MAO coating and Mg(OH)2-MAO composite coating. The MAO coating (Fig. 2(a)) exhibits a typical morphology with a variety of micro-pores and micro-cracks. Fig. 2(b) and (c) illustrates that the Mg(OH)2-MAO composite coating has a rose-like porous network structure with a size of less than 1 μm. As mentioned above [28,36,38], the preparation of Mg(OH)2 in a single alkaline solution does not completely seal the pores of MAO surface. So it is confirmed that EDTA-2Na promoted the growth of Mg(OH)2, and increased the content of Mg(OH)2 in the study.
Fig. 2. SEM images of (a) MAO coating, (b, c) Mg(OH)2-MAO composite coating, (d) element compositions and the cross-section of (e) MAO coating, (f) Mg(OH)2-MAO composite coating.
EDS analysis (Fig. 2(d)) shows that the MAO coating predominantly contains O, Mg and P. Particularly, P element was originated from the electrolyte, indicating that phytic acid was involved in the MAO process. It is noted that the composite coating includes merely O and Mg elements. The absence of P element indicates that the MAO coating was completely covered by a layer of Mg(OH)2.
Cross-sectional morphology of MAO coating and composite coating (Fig. 2(e) and (f)) reveals that the thickness of both coatings is approximately 3 μm. And no obvious Mg(OH)2 layer was found in Fig. 2(f), indicating that the Mg(OH)2 layer is extremely thin and interlocked with the MAO coating. Differing from the work of Tang et al. [41] and Kim et al. [34], the surface in this study exhibits a more refined structure in a nano-sized scale and Mg(OH)2 coating is ultrathin.
XRD patterns of Mg alloy AZ31 substrate, MAO and Mg(OH)2-MAO composite coatings are illustrated in Fig. 3. Both the MAO layer and the composite coating consists of α-Mg matrix and MgO phase. Although there is a certain amount of P element in the MAO coating (Fig. 2(d)), it is noticed that no peaks of crystal phase associated with phosphate are seen in Fig. 3. The peaks of Mg(OH)2 (Fig. 3(c)) indicates the growth of Mg(OH)2 coating.
Fig. 3. XRD patterns of (a) Mg AZ31, (b) MAO coating and (c) Mg(OH)2-MAO composite coating.
Fig. 4 shows FTIR spectra of (a) MAO and (b) Mg(OH)2-MAO composite coatings. The Mg-O peak (Fig. 4(a)) at 451 cm-1 proves the formation of MgO in the MAO coating. And the adsorption peak at 3421 cm-1 in Fig. 4(b) is -OH groups, confirming the existence of Mg(OH)2 in the composite coating. This finding is in agreement with the results of XRD pattern in Fig. 3.
Fig. 4. FT-IR spectra of (a) MAO and (b) Mg(OH)2-MAO composite coatings.
Fig. 5(a) illustrates potentiodynamic polarization curves of bare Mg alloy AZ31 substrate, MAO coating and Mg(OH)2-MAO composite coatings in HBSS and the corresponding parameters (Fig. 5(b) and (c)). Corrosion potential (Ecorr) mainly describes the thermodynamic property of metals; and corrosion resistance is evaluated by corrosion current density (icorr). Compared to MAO coating, Ecorr of the Mg(OH)2-MAO composite coating increases from -1.64 to -1.36 VSCE, demonstrating that the thermal stability of the composite coating is improved. And Ecorr of the MAO coating is lower than that of the substrate. This result will be discussed in detail in the discussion section.
Fig. 5. (a) Polarization curves, and corresponding (b) icorr and (c) Ecorr of (I) AZ31 substrate, (II) MAO coating and (III) Mg(OH)2-MAO composite coating in HBSS.
Corrosion rate, icorr of the MAO coating decreases two orders of magnitude from 5.97 × 10-5 A/cm2 of bare substrate to 3.72 × 10-7 A/cm2. Moreover, icorr of the Mg(OH)2-MAO composite coating decreases one order of magnitude from 3.72 × 10-7 to 5.69 × 10-8 A/cm2 in comparison to the MAO coating, implying that the Mg(OH)2 coating further improves corrosion resistance of the MAO coating. Compared with the results from Tang et al. [41] and Kim et al. [34], a better or similar corrosion resistant ultrathin coating was yielded herein in a shorter treatment time frame at a lower treatment temperature.
Hydrogen evolution rate (HER) of (a) bare Mg alloy AZ31 substrate, (b) MAO and (c) Mg(OH)2-MAO composite coatings in HBSS for 156 h is shown in Fig. 6. During the first (I) stage, HER increases firstly and then decreases, suggesting the formation of corrosion products Mg(OH)2 on the alloy surface. During the second (II) period, HER starts to increase, showing the dissolution and delamination of the corrosion products film resulting in exposing a fresh surface. And HER of the composite coating is higher due to the fact that the Mg(OH)2 coating dissolved more easily than MgO in HBSS due to the transformation of Mg(OH)2 into MgCl2 [45,46]. During the third (III) stage, HER of the substrate alloy tended to decrease slowly, indicating the formation of a new corrosion products (Mg(OH)2) film. And HER of the Mg(OH)2-MAO composite coating is the smallest, indicating that more corrosion products (Ca-P compounds) were formed. A subsequent discussion will disclose the chemical composition of the corrosion products. In general, HER of the composite coating is lower than that of the MAO coating. In other words, corrosion resistance of the composite coating was better than that of its MAO coating. This finding is in accordance with the results of the potentiodynamic polarization curves.
Fig. 6. Hydrogen evolution rates of (a) the AZ31 substrate, (b) MAO coating and (c) Mg(OH)2-MAO composite coating in HBSS for 156 h.
In our previous studies, the presence of the second phases or intermetallic compounds in the substrate alloy was found to have an important effect on the formation of MAO and chemical conversion coatings. For example, Mg2Ca phases shift the formation and corrosion mechanisms of MAO coating on Mg-Li-Ca alloy [48]. And the AlMn phase in Mg alloys AZ31 and AM30 acts as initiator for the formation of a Ca-doped zinc phosphate coating [42,47]. Size and distribution of the AlMn particles affect the surface morphology, roughness and corrosion resistance.
It is proposed that AlMn phase also has a critical influence on the formation of Mg(OH)2 coating. In order to understand the formation process of Mg(OH)2 coating, Mg(OH)2-MAO composite coating was fabricated in Solution B for different time lengths. Fig. 7 shows SEM images of Mg(OH)2-MAO composite coatings after various soaking time. Fig. 8, Fig. 9 display atomic ratios and mapping images of corresponding elements: O, Mg, P, Al and Mn. The presence of higher concentration of Al and Mn elements designates AlMn phase (i.e., Al8Mn5 and AlMn) in the MAO coating [42,47]. It is noteworthy noting from Fig. 9 that more Mg(OH)2 precipitates around the AlMn phases.
Fig. 7. SEM images of Mg(OH)2-MAO composite coatings at various soaking times: 10 s (a-c), 30 s (d-f), 1 min (g-i), 5 min (j-l), 10 min (m-o), 20 min (p-r) and 30 min (s-u).
Fig. 8. Element compositions of Mg(OH)2-MAO composite coatings at various soaking time.
Fig. 9. EDS mapping images of Mg(OH)2-MAO composite coatings at various soaking times: 10 s (a), 30 s (b), 1 min (c), 5 min (d), 10 min (e), 20 min (f) and 30 min (g).
The defects or through-pores between the interfaces of AlMn/MAO coating offer a larger possibility of ingress of the solution into the interfaces of MAO/Mg substrate. In the initial immersion stage for 10 s (Fig. 7(a)-(c)), small Mg(OH)2 crystals preferentially formed around the AlMn phases. AlMn phase with a higher potential acts as the cathode; and α-Mg with a lower potential acts as the anode [27,42]. This result led to the dissolution of α-Mg and the formation of Mg(OH)2.
And the anode reaction followed as:
Mg→Mg2++2e- (1)
Cathodic reaction:
2H2O+2e-→2OH-+H2↑ (2)
Overall reaction:
Mg+2H2O→Mg(OH)2+H2↑(3)
With increasing time, the Mg(OH)2 precipitates around the AlMn phase gradually grew like a seed germination. And Fig. 7(j)-(l) shows that Mg(OH)2 already has a three-dimensional porous network structure. It is worth noting that Mg(OH)2 is centered on the AlMn phase and gradually diffuses outward. It indicates that the Mg2+ ions near the anode moved outwards onto the surface through the through pores of MAO coating, trapped by EDTA, and concentrated on the AlMn phase. The results promoted the growth of Mg(OH)2 above and around the AlMn phase.
Contemporary, MAO coating was also gradually changed into Mg(OH)2. The reaction is as follows:
MgO+H2O→Mg(OH)2 (4)
As seen in Fig. 2(b), the entire MAO coating is completely covered by porous network layers of Mg(OH)2. Based on the previous discussion, a model is proposed to explain the formation of Mg(OH)2-MAO composite coating (Fig. 10). Briefly, the porous network layer of Mg(OH)2 experienced three steps of growth: (a) dissolution of Mg, movement of Mg2+, and absorption of EDTA; (b) nucleation of Mg(OH)2 and transformation of MgO in MAO coating into Mg(OH)2; and (c) growth of Mg(OH)2. It can also be seen that Mg(OH)2 is also formed at the micro-pores and micro-cracks on MAO surface. Hence, it is speculated that the Mg(OH)2 in these sites can grow faster. This is similar to the case of cross-linking of polymer, to some degree.
Fig. 10. Schematic diagrams of the formation of Mg(OH)2 coating.
EDTA molecules play a vital role in the nucleation and growth of Mg(OH)2 [28,36,38], attributed to the excellent complexing ability to Mg2+. EDTA is a hexadentate ("six-toothed") ligand and chelating agent, i.e., its ability to sequester metal ions such as Mg2+. Usually, EDTA has seven different forms: H6Y2+, H5Y+, H4Y, H3Y-, H2Y2-, HY3- and Y4- in aqueous solutions with different pH values [39,40]. The anion Y4- is the ligand species with the strongest complexability. The greater the concentration of Y4- ions, the more stable the complex. When the pH value of solution is greater than 10, the presence of species Y4- will predominate [39]. In this experiment, the pH value of solution B is about 13.6. Thus, EDTA is possible present in the form of HY3- and Y4-. If EDTA is complexed with Mg2+, it must occur in the early stages of dissolution of α-Mg, because Mg substrate surface and solution have more Mg2+ at this time.
To further understand the role of EDTA, N element was detected for samples immersed in Solution B for 10 s and 1 h through X-ray photoelectron spectroscopy (XPS). The spectra of the samples immersed for 10 s and 1 h are shown in Fig. 11(a) and (b) depicts the survey scan and high resolution spectra of N 1s for the samples after immersion in Solution B for different time, respectively. After the sample of immersing in Solution B for 10 s, N 1s spectra can be resolved into two peaks: main peak at 400.1 eV attributed to N—C group; and a peak at 397.3 eV that may be related to the presence of N—H group. The results reveal that EDTA is absorbed and chelated with Mg2+ ions on the surface in the initial stage, and form the inner layer of Mg(OH)2. The specific reaction is as follows:
Mg2++ Y4-→ MgY2- (5)
Fig. 11. XPS spectra of the samples immersed in solution B for 10 s and 1 h: (a) survey and (b) high dissolution curves of N 1s.
However, after immersing in Solution B for 1 h, the peak of N 1s diminished. This result indicates that the Mg(OH)2 coating covered the inner layer of MgY2-. And EDTA loses its effect in the late stage of coating growth. Therefore, only in the initial stage of coating growth, EDTA plays an important role in promoting the nucleation of Mg(OH)2.
When the open circuit potential (OCP) between two metals exceeds 50 mV/SCE, a micro-couple coupling effect is generated to increase the probability of galvanic corrosion [49], [50], [51]. The metal with lower OCP is subject to corrosion; whereas the other metal deserves a protection. Fig. 12 displays that the initial potential difference between the MAO coating and the AZ31 substrate is 240 mVSCE, far exceeding 50 mVSCE, which may lead to galvanic corrosion in the interface of MAO/Mg substrate via through pores or micro-cracks of the MAO coating. It is not surprised that the OCP of the Mg substrate slowly increased due to the formation of Mg(OH)2; whilst the OCP of the MAO coating gradually decreased from the initial 240 to 10 mVSCE, indicating that the degradation of MAO coating. The increase in OCP for Mg(OH)2-MAO composite coating implies the formation of Ca-P precipitates (Fig. 13).
Fig. 12. Open circuit potential as a function of immersion time of (a) AZ31 substrate, (b) MAO coating and (c) Mg(OH)2-MAO composite coating.
Fig. 13. SEM images of (a) AZ31 substrate, (b) MAO coating and (c) Mg(OH)2-MAO composite coating and (d) the corresponding element compositions in HBSS after a 156 h immersion.
Fig. 13 illustrates the SEM images of the (a) bare Mg alloy AZ31 substrate, (b) MAO coating and (c) composite coating and (d) the corresponding element compositions in HBSS after an immersion of 156 h. It is clear that a dry riverbed corrosion morphology can be observed on the surfaces of the bare AZ31 substrate (Fig. 13(a)), MAO coating (Fig. 13(b)) and Mg(OH)2-MAO composite coating (Fig. 13(c)). From the inset images, whether it is an MAO coating or a composite coating, the surface micro-pores were filled with corrosion products (i.e. Ca-P compounds), effectively providing secondary protection. Moreover, it was found that a part of the Mg(OH)2 was dissolved, which corresponds to the third (III) stages of immersion (Fig. 6). The corresponding element compositions in HBSS after 156 h immersion are presented in Fig. 13(d). From the image, the content of Ca and P on the surface of Mg(OH)2-MAO composite coating is by far the highest in comparison with that of the bare AZ31 substrate and MAO coating. However, the atomic ratio of calcium to phosphorus is only 0.83, which is far less than 1.67 for HA due to the presence of P in the MAO coating.
In order to further determine the corrosion products, Fig. 14 displays (a) XRD patterns and (b) FTIR spectra of all samples after the hydrogen evolution tests in HBSS. It can be seen from Fig. 14a that all sample surfaces have weak diffraction peaks of Mg(OH)2 [44,19,52,53]. It is worth noting that Mg(OH)2-MAO composite coating has a distinct characteristic peak at 28° compared to other samples, which is ascribed to HA phase. It is noted that the Ca/P in HA is 1.67, which is higher than the EDS result (0.83). But it is indeed found that the content of Ca increased with increasing P concentration (Fig. 13(d)). Hence, it is proved that a porous Mg(OH)2 network structure is more favorable for inducing the formation of calcium and phosphorus products. And the adsorption peaks for FTIR spectra at 1056 cm-1 in Fig. 14(b) is PO43- groups, further confirming existence of the Ca-P precipitates or HA.
Fig. 14. (a) XRD patterns and (b) FTIR spectra of (I) AZ31 substrate, (II) MAO coating and (III) Mg(OH)2-MAO composite coating after the hydrogen evolution tests in HBSS.
Fig. 15 illustrates the corrosion mechanism of Mg(OH)2-MAO composite films in HBSS. The composite coatings immersed in HBSS solution containing Cl-, H2PO4- and Ca2+ ions, H2O and EDTA as well (Fig. 15(a)). On one hand, the Mg(OH)2-MAO coating gradually dissolved and cracks appeared slowly. On the other hand, H2O molecules and chloride ions (Fig. 15(b)) penetrated through Mg(OH)2 layer and pores or micro-cracks of MAO coating into the interface of MAO layer and the Mg substrate, leading to chemical corrosion of α-Mg substrate surrounding the AlMn phases. Basically, AlMn phase in Mg alloy AZ31 with a higher potential acts the cathode; whereas the neighboring α-Mg matrix becomes the anode, and is susceptible to corrosion [4,44]. Once H2O molecules reached the interface of the MAO/Mg substrate, the α-Mg matrix near AlMn phase was preferentially corroded and the reactions follow as Eqs. (1), (2), (3), (4). And Mg(OH)2 reacted with Cl- formed soluble MgCl2 (Fig. 15(c)), as shown in Eq. (6) [46].
Mg(OH)2+2Cl-→MgCl2+2OH- (6)
Fig. 15. Schematic representation of the corrosion mechanism of Mg(OH)2-MAO composite coating in HBSS.
At the same time, in the weakly alkaline HBSS, the dihydrogen phosphate (H2PO4-) ions converted into hydrogen phosphate (HPO42-), then into phosphate (PO43-) [54], [55], [56] (Fig. 15(d)). These reactions follow as:
H2
H
Finally, Ca2+ ions reacted with PO43- ions, resulted in the formation of HA, which was deposited on the coating surface, and provided protection for the substrate in HBSS solution. The reaction obeys:
10Ca2++6
(1) A nano-sized porous network structure ultra-thin Mg(OH)2 coating was fabricated on MAO surface via a water bath method and EDTA-2Na promoted the nucleation and growth of Mg(OH)2. The initiation sites of the Mg(OH)2 film neighbored at intermetallic compound AlMn phases.
(2) Corrosion current density of Mg(OH)2-MAO composite coating decreased one and three orders of magnitude from those of its MAO coating and substrate, respectively. And Mg(OH)2-MAO composite coating led to HA deposition and the lowest hydrogen evolution rate in HBSS after 156 h. Results indicate Mg(OH)2-MAO composite coating had better corrosion resistance.
(3) The formation and corrosion mechanism of Mg(OH)2-MAO composite coating was proposed. Mg(OH)2-MAO coating on Mg alloy AZ31 demonstrated a promise for clinical applications for degradable biomaterials.
This work was supported by the National Natural Science Foundation of China (Nos. 51571134 and 51601108) and the SDUST Research Fund (No. 2014TDJH104).
The authors have declared that no competing interests exist.
WeChat
/
| 〈 |
|
〉 |
