Journal of Alloys and Compounds 793 (2019) 202e211
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Effect of alkali/acid pretreatment on the topography and corrosion resistance of as-deposited CaP coating on magnesium alloys Song Jiang a, Shu Cai a, *, Yishu Lin a, Xiaogang Bao b, Rui Ling a, Dongli Xie a, Jiayue Sun a, Jieling Wei a, Guohua Xu b, ** a b
Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, People's Republic of China Department of Orthopedic Surgery, The Spine Surgical Center, Changzheng Hospital, Shanghai 200003, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2019 Received in revised form 15 April 2019 Accepted 19 April 2019 Available online 20 April 2019
The development of corrosion resistance biological coatings on magnesium and its alloy that can be used for controlling the degradation rate and corresponding biological properties from the surface of implants could have a significant impact in the field of bone tissue regeneration. This work reported the fabrication of quickly deposited biphasic calcium phosphate (CaP) coatings on alkali (sodium hydroxide, NaOH) and acid (hydrofluoric, HF) pretreated magnesium alloy (AZ31) substrates using microwave assisted method. The results indicated that the pretreatment layers (formed on NaOH or HF pretreated AZ31 substrates) could protect substrates from etching of Hþ during the deposition process of CaP coating. Meanwhile, different pretreatment layers have been shown to greatly influence the phase component, morphology, thickness and corrosion resistance of the resultant CaP coatings. The CaP coating based on the acid pretreatment layer was mainly composed of compact chrysanthemum-like Fdoped hydroxyapatite (HA) with obvious c-axis growth orientation and exhibited excellent corrosion resistance in the simulated body fluid (SBF), which could provide more than 10 weeks of protection for magnesium alloy substrate. © 2019 Elsevier B.V. All rights reserved.
Keywords: Magnesium alloy Pretreatment Microwave assisted treatment Calcium phosphate coating Corrosion resistance
1. Introduction Compared with the common metallic biomaterials such as stainless steels and Ti based alloys, magnesium (Mg) and its alloys supply superior mechanical properties matched to the natural bone which are beneficial to reduce the stress shielding effects, and present good biocompatibility in physiological environment [1,2]. Most importantly, Mg can be degraded and absorbed by human body that avoids the second surgery to take out the implants [3]. Therefore, Mg and its alloys are considered as potential candidates for biodegradable orthopedic implant materials. According to zheng's research, the biodegradable material should maintain its mechanical support for over 10 weeks [4] and most of researchers proposed that the corrosion rate in simulated body fluid (SBF) for Mg-based materials needed to be less than 0.5 mm/y, especially in the early stage of degradation [5]. However, due to the
* Corresponding author. ** Co-corresponding author. E-mail addresses:
[email protected] (S. Cai),
[email protected] (G. Xu). https://doi.org/10.1016/j.jallcom.2019.04.198 0925-8388/© 2019 Elsevier B.V. All rights reserved.
electrochemical activity of Mg, the corrosion rate of Mg and its alloys during the first 24 h immersion period in the SBF was in the range from 3 to 8 mm/y [4]. Thus, the naked Mg-based substrate would fully degrade within 2 weeks after the immersion that might cause the premature mechanical failure of implants and lead to inflammation of surrounding tissues, which hinders the further clinical application of Mg based implants [6,7]. To date, substantial methods have been proposed to tailor the corrosion rate of Mg. Among these, surface modification is widely reckoned as an effective method to reduce the corrosion rate of Mg with few changes on its mechanical and degradable features [8]. Calcium phosphate (CaP) coatings have been extensively investigated as the protective coating because of their great osteoinductive activity. Among these phases, brushite (CaHPO4$2H2O, DCPD), its anhydrous form monetite (CaHPO4, DCPA) and hydroxyapatite (Ca10(PO4)6(OH)2, HA) are some of the common CaP coatings to modify the surface of metallic implants, due to their outstanding degradability and biocompatibility [9e11]. Additionally, the structure of CaP coatings derived from solution are proved to be close to that of minerals in natural bone. Since Mg can react with water to release Mg2þ and hydrogen gas that may disturb the synthesis and
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deposition of CaP coating [8,12,13], thus destroying the compactness and homogeneity of as-deposited coatings. Therefore, it is essential to pretreat Mg alloy substrates to obtain a temporary protective layer. Anawati et al. [14] proved that the alkali pretreatment of NaOH could markedly enlarge the surface activity for the deposition of apatite and induce a uniform apatite coating on Mg(OH)2 layer. Zhao et al. [15] also demonstrated that the alkali pretreatment could decrease the surface roughness of Mg alloy and might enhance the corrosion resistance of the followed bioglass coating. In addition, hydrofluoric acid (HF) conversion treatment is another promising pretreatment on Mg alloy. Both Ren [16] and Lin [17] used MgF2 layer as a chemical inert interlayer to obtain a firm CaP coating by sol-gel dip coating. Meanwhile, Rad et al. [18] reported that nanoplate like DCPD and nanoneedle like HA coating could precipitate on the surface of MgF2 by electrochemical deposition. Both the alkali and acid pretreatment could facilely change the surface properties such as component, microstructure and layer thickness of Mg substrate even with complex shape, which might play an important role on the protective properties of as-deposited coating based on the research from Zhang [19]. Nevertheless, the trend of current study about surface modification mainly focus on the influences of coating preparation methods and few articles revealed the effects of different alkali and acid pretreatment of Mg alloy substrate on the coating preparation and corrosion resistance. Recently, a novel microwave assisted coating has been proposed as an effective method for synthesis CaP coatings on Mg alloys due to its advantages, such as lessen cost and lower coating temperature, rapid coating formation and high efficiency with energy saving [20e23]. Incorporating with the microwave energy, the deposition driving force was remarkably accelerated, making the whole deposition process completed within a few minutes [20,22,23]. In literatures, many parameters of the method which affected the microstructure and properties of the deposition coatings, such as the temperature [22], Ca/P molar ratio of the solution [24] and microwave time [25], have been discussed in detail. Nevertheless, relatively few researches about the influences of different pretreatments of Mg alloys on the microwave assisted deposition of CaP coatings were reported. In our previous work [24], different pretreatments of alkali and heating on Ti6Al4V alloy substrates were proved to change the surface phase composition and eventually induced apatite coating deposition with different structure under microwave energy. Moreover, the effect of pretreatment on coating structure necessarily lead to variety of properties such as corrosion resistance, degradation behavior and bioactivity, which were essential for clinical application. Therefore, the different influences of alkali (NaOH) and acid (HF) pretreatment of Mg alloy on composition and topography of the as-deposited CaP coating and the electrochemical corrosion resistance, long-time degradation behavior of the coated substrates in SBF were studied in this work. 2. Experiment 2.1. Sample pretreatment and preparation Mg alloy substrates were machined from commercial AZ31 alloy plates (Al 3%, Zn 1%, Mn 0.2%, Fe < 0.005%, Mg in balance, all in wt.%) with an approximate dimension of 10 mm 10 mm 2 mm. Each sample was progressively polished with SiC papers from 800 to 2000 grit, then cleaned ultrasonically in ethanol and distilled water respectively for 10 min and dried at room temperature for use. Based on our previous works and relevant reports, the pretreatment processes were optimized as shown in the follow. For alkali pretreatment, the prepared samples were immersed in 1 M NaOH solution at 80 C for 2 h. These samples were noted as AP. As for the
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acid pretreatment, the naked substrates were immersed in 20% HF solution at 37 C for 2 h, which were noted as HP. The samples after pretreatment were rinsed with distilled water and dried in the warm air. The CaP coatings were prepared on the surfaces of sample AP and sample HP by microwave assisted deposition method. The process parameters were optimized by adjusting pH value, concentration of the coating solution, the reacting time and temperature. In the present study, only the optimal condition was referred. In brief, the coating solution was prepared by successively dissolving 0.1 mol/L Ca(NO3)2$4H2O and 0.06 mol/L NH4H2PO4 in 100 mL distilled water under magnetic stirring to make sure the solution was clear and transparent. The pH value of the solution was approximate 4.0 without any adjustment. 6 pretreated samples were immersed in the coating solution in a beaker with a temperature sensor to monitor the solution temperature, which was placed into a microwave chemical reactor with 2.45 GHz microwave frequency (Tangshan nano source microwave thermal instrument, China). According to Shen's report [25], 10 min microwave radiation was sufficient to induce CaP coatings complete deposition and longer radiation time may lead to by-product, such as NaNO3, which was detrimental to properties of CaP coatings. Consequently, the working procedure was set as 5 min to heat from room temperature to 100 C and then maintained at 100 C for 10 min. The coating process of different samples (alkali pretreated and acid pretreated sample) was exactly the same and these samples after the same microwave time (10 min) were referred to as APM10 and HPM10, respectively. And the naked Mg alloy substrates were also treated under the same coating process as control group, which were named as NPM10. Additionally, to figure out the deposition process of CaP coatings, the acid pretreatment samples radiated with different microwave times were correspondingly noted as HPM0 (100 C for 0 min), HPM5 (100 C for 5 min). After microwave heating, the samples were rinsed with ethanol and distilled water three time, and then dried in warm air. 2.2. Characterization The surface morphologies of pretreated and CaP coated samples, and the cross-section micrographs of sample APM10 and HPM10 were investigated by field-emission scanning electron microscope (FE-SEM, Hitachi S4800, Japan). The corresponding element compositions of samples were analyzed by energy dispersive spectrum (EDS, 7401 Oxford) attached to FE-SEM. The low-angle (1 ) X-ray diffraction (XRD, Rigaku, Japan) with Cu Ka radiation was used to characterize the phase compositions, which collected data over an angle range of 10e70 (2q values). 2.3. Electrochemical tests The electrochemical tests containing potentiodynamic polarization tests and electrochemical impedance spectra (EIS) measurements were conducted via an electrochemical workstation (CHI600E, China) in SBF (t-SBF, pH ¼ 7.40) at 37 C. The SBF solution used in this work was prepared as reported before [13,20]. To carry out these tests, three-electrode cell with saturated calomel reference electrode, platinum foil as the counter electrode and the sample with surface area of 1.0 cm2 as the working electrode was dipped into the SBF. The three-electrode cell was contacted with the SBF solution for 10 min to reach a suitably stable open circuit potential (OCP) before the tests. The potentiodynamic polarization curves were scanned with a rate of 1 mV/s from OCP - 0.5 V to OCP þ0.5 V and the EIS test was conducted over a frequency range from 105 Hz to 102 Hz. At least 5 parallel samples were tested to ensure repeatability. The polarization resistance (Rp) was calculated
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according to eq. (1), in which the bc and ba were the cathodic and anodic Tafel slop from the polarization curves, separately. The relevant corrosion rate (Pi) was obtained through the eq. (2).
Rp ¼
bcba 2:3ðbc þ baÞIcorr
Pi ¼ 22:85 Icorr
(1) (2)
2.4. Immersion test To evaluate the long-time degradation behavior of the pretreated and CaP coated samples, the in vitro immersion test was performed with immersing samples in the SBF (pH ¼ 7.40) at 37 ± 0.5 C by using the WE-3 immersion oscillator, and the volume of SBF was calculated based on a volume-to-sample area of 20 mL/ cm2, according to ASTM G31-72. During the whole immersion test, the SBF was refreshed every 2 days. After different time points, the sample was picked out from the SBF. The corrosion behavior of coated sample with different immersion time was evaluated by EIS and potentiodynamic polarization tests that were mentioned above.
the chemical conversion time was only 2 h, making the products difficult to be detected by XRD and the Mg substrate exhibited apparent signal in the spectrum. The EIS and potentiodynamic polarization tests were used to evaluate the protective effect of the pretreatment layer (Fig. 2). The Nyquist plots of three samples were shown in Fig. 2(a) and the dimension of the semicircle in plots could visually reflect the barrier effect of layers [20,27]. Sample HP exhibited the largest semicircle diameter compared with other two samples and its curve only contained one capacitive loop without other clear capacitive or inductive loop that related to the metal dissolution and pitting corrosion. These results suggested the superior barrier effect of the acid pretreated layer. Moreover, the corrosion potential (Ecorr) and the corrosion current densities (Icorr) were extracted from the polarization curves (Fig. 2(b)) by the Tafel extrapolation method. These relevant parameters were summarized in the Table 1. Among the three kind of samples, sample HP showed the most positive corrosion potential (1.59 V) with the minimal corrosion current density (3.68 mA/cm2) and the biggest polarization resistance (11.82 kU cm2), while the electrochemical anti-corrosion properties of sample AP was also enhanced to some extent compared with that of the naked Mg alloy. From the above results, it could be concluded that the pretreatment layer improved the corrosion resistance of Mg alloys and the acid pretreatment could provide more effective protection to the substrate.
3. Results 3.1. Characterization of pretreated AZ31 alloy
3.2. Characterization of as-deposited CaP coating on pretreated AZ31 substrates
It was well known that both NaOH and HF could react with Mg to generate the chemical conversion layer which may alter the surface properties and further affect the formation process of subsequent coatings. Therefore, to compare different influences of alkali and acid pretreatments on the deposition process and corrosion resistance of the as-deposited CaP coating, the surface microstructure, chemical composition and electrochemical corrosion resistance of the pretreated Mg alloy substrates were characterized. The results of surface morphology, XRD pattern and elemental composition of the pretreated substrates were investigated and displayed in Fig. 1. For samples of alkali or acid pretreated, it could be observed that obvious scratches existed on the surface of sample AP and sample HP (Fig. 1(a) and (b)), which were formed during the early polish. Additionally, as shown in the inset of Fig. 1(a), massive flakes overlapped in confusion on the surface of sample AP, presenting a typical morphology of Mg(OH)2 under the similar reaction condition [25]. However, except some micropores with irregular size, it seemed that there were few differences between the surface of naked Mg alloy and sample HP. Actually, from the enlarged image (inset of Fig. 1(b)), the surface of sample HP was composed of compact nanoparticles and most of the surface was denser than that of sample AP. From the XRD pattern of sample AP (Fig. 1(c)), it can be determined that the main phase of alkali pretreatment was Mg(OH)2, which was consistent with our previous studies [15,25]. However, only the diffraction peaks of Mg (JCPDS No. 35-0821) appeared in the XRD pattern of sample HP (Fig. 1(c)), suggesting that no new phase was emerged on Mg alloy substrate after acid pretreatment, while the results of EDS (Fig. 1(d)) indicated that the surface contained high concentration of fluorine and magnesium elements with little oxygen element. Yan et al. [26] reported that after 48 h chemical conversion in the 50 wt% HF solution, the layer on Mg mainly composed MgF2 with little MgO. So, it can be speculated that the surface composition of sample HP was the nanocrystalline or amorphous products of MgF2 and MgO. Moreover, in this study, the acid pretreatment layer was thin since
The CaP coating was prepared on the naked, alkali and acid pretreated Mg alloy substrates through the microwave assisted method at 100 C for 10 min, respectively (labeled as sample NPM10, APM10 and HPM10) and Fig. 3 showed their surface morphologies. It could be found that the as-deposited CaP coatings on the three samples presented totally different microstructure, though all of them contained two forms of depositions. The naked Mg alloy substrate (Fig. 3(a) and (b)) was covered by a porous network coating and spherical depositions with diameter of 25 mm randomly scattered on its surface. The bottom network coating was composed by large quantities of tape-like precipitates, which interlaced each other leading to the formation of holes with various size. In terms of sample APM10, the prism-like crystals with 10e30 mm in width were distributed on the bottom flake-like coatings as shown in the insert of Fig. 3(c). And from the high magnification image in Fig. 3(d), there were some obvious scratches beneath the bottom flake-like coating, in which some inconspicuous cracks could be observed (as indicated by black arrows), indicating that the flake-like coating was thin and incomplete and the integrity of pretreated Mg(OH)2 layer beneath the flake-like coating might be destroyed during the microwave process. The defects on the surface of sample APM10 would lead to poor protection of the as-deposited CaP coating. However, compared with other samples, the bottom coating of sample HPM10 (as shown in Fig. 3(e) and (f)) was more compact and intact without any distinct pores or microcrack. Although some similar prism-like crystals embedded in its surface, the bottom coating was stacked by chrysanthemum-like clusters. Enlarging this cluster (Fig. 3(f)), quantities of nanoneedles densely composed as the petals of these chrysanthemum-like structure. In addition, the marginal nanoneedles between different clusters interweaved each other avoiding the formation of obvious gap, which isolated the substrate from aggressive surroundings. Moreover, it has been reported that this chrysanthemum-like surface structure with nanoneedles could effectively enhance the specific surface area and surface roughness, which were beneficial for osteoblast differentiation and promoted
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Fig. 1. Surface morphology of samples after (a) alkali (AP) and (b) acid pretreatment (HP); (c) XRD patterns of pretreated samples (AP and HP); (d) EDS spectrum of area A in (b).
Fig. 2. Nyquist plots (a) and polarization curves (b) of pretreated samples and naked Mg alloy substrates in the SBF.
relevant genes expression [28]. Based on the previous studies, the differences in the structure of CaP coatings might match diverse phase composition and influence their corrosion resistance properties. The prism-like crystal was the
typical structure for triclinic DCPA and the nanoneedle composed clusters structure was similar to that of HA oriented along with the c-axis [29,30]. Thus, to figure out the composition of as-deposited CaP coatings with different pretreatment, the X-ray diffraction
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Table 1 Corrosion properties of the pretreated samples and naked Mg alloy substrates. Samples
Ecorr (V/SCE)
Icorr (mA/cm2)
Rp (kU$cm2)
Pi (mm/y)
Naked Mg alloy Sample AP Sample HP
1.65 1.67 1.59
45.61 10.80 3.68
0.93 3.80 11.82
1.04 0.25 0.08
was used to characterize the phase compositions of three different coated samples and the results were shown in Fig. 4. For sample NPM10, except for the intense (020) diffraction peak of DCPD, only some main diffraction peaks of DCPD and DCPA with weak intensity were observed, indicating that the porous coating might contained DCPD and DCPA with specific orientation and low crystallizability. And the phase composition of sample APM10 was similar to that of NPM10, while the intensities of its diffraction peaks were in consent with the standard cards of brushite (JCPDS No. 72-0713) and monetite (JCPDS No. 71-1759). However, in the XRD pattern of sample HPM10, despite the peaks of DCPA changed little, the weak diffraction peaks of HA (JCPDS No. 09-0432) appeared instead of
Fig. 4. XRD patterns of three different coated samples.
Fig. 3. Surface morphologies of the CaP coatings deposited on different substrates: (a) and (b) the naked AZ31 alloy; (c) and (d) the alkali pretreated sample; (e) and (f) the acid pretreated sample.
S. Jiang et al. / Journal of Alloys and Compounds 793 (2019) 202e211 Table 2 Element composition of the area B in Fig. 3(e). Element
C
O
F
Mg
Ca
P
Ca/P
Area B
15.81
54.18
2.47
0.77
16.84
9.93
1.69
the DCPD. Compared with the HA, the high intensity of peak on DCPA testified its higher crystallizability with larger crystal size. Interestingly, there were intense diffraction peaks of Mg in the XRD pattern of sample APM10, while those of sample HPM10 and sample NPM10 were weak, indicating the CaP coating on Mg(OH)2 pretreatment layer of sample APM10 was thinner than others. Combined with the differences in the morphologies between sample APM10 and HPM10, the phase of DCPA was corresponding to the prism-like crystal. Meanwhile, the DCPD and HA were corresponding to the bottom flake-like crystal and chrysanthemumlike cluster, respectively. Considering the inconspicuous peaks of HA in sample HPM10, the semi-quantitative analysis of EDS was conducted to the bottom area B in Fig. 3(e), and the results (Table 2) showed that the Ca/P ratio of chrysanthemum-like crystal was 1.69, which was close to that of the stoichiometric HA (1.67), indicating the existence of nanoneedle HA crystal. In addition, a little component of F element indicated some F entered the lattice of HA from MgF2 in the acid pretreatment layer and substituted the OH, which was believed to lead the growth orientation along caxis [25,31e33]. Meanwhile, the trace of Mg also reflected that the thickness of the as-deposited CaP coating on sample HPM10 was quite enough to shield the substrate from the environment. For the purpose of clarifying the effect of pretreatment on the thickness of as-deposited CaP coatings, the cross-section micrographs of sample APM10 and HPM10 were depicted in Fig. 5. The total thickness of the coating on sample APM10 was 6.7 mm while the coating of sample HPM10 reached to more than 100 mm and the pretreated layer was difficult to observe. The difference in the thickness of CaP coatings between sample APM10 and HPM10 was obvious and conformed to the results of morphology and XRD analysis, indicating that the pretreatment of Mg alloy substrate by alkali or acid solution might not only affect the surface structure but also change the deposition rate of the followed CaP coating. Moreover, as shown in Fig. 5(b), the dense spherical cluster was corresponding to the chrysanthemum-like HA in bottom coating and there were no distinct microcracks in the coating or the gap between the substrate and coating, demonstrating that the coating of sample HPM10 was effectively integrated with the Mg substrate. It was well known that a coating with proper thickness could provide an effective protection for the substrate and the intact structure ensured the coating away from peeling off from the substrate once it was implanted in body to repair the bone defects
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[34]. From the views of structure about surface and interface, compared with the alkali pretreatment, the acid pretreatment was more conducive to the deposition of CaP coating on Mg alloy through microwave assisted method.
3.3. Electrochemical corrosion resistance of as-deposited CaP coatings To evaluate the corrosion resistance of CaP coatings in the SBF, EIS and potentiodynamic polarization tests for sample NPM10, APM10 and HPM10 were performed and the results were shown in Fig. 6(a)e(c). It has been reported that the corrosion resistance of the coating had positive correlation with the size of the capacitive semicircle. And for simple comparison, the real impedance in Nyquist plot, which the imaginary part disappeared for the capacitive part, might be seen as the charge transfer resistance (Rt) to reflect the anticorrosion capability of the coating [16,20]. The electrochemical parameters of three kinds of samples were summarized and listed in Table 3. Compared with the naked Mg alloy and samples after pretreated (sample AP and HP) respectively, due to the existence of the followed CaP coating synthesized via microwave assisted method, the electrochemical properties of CaP coated samples were enhanced in different degree, even though the porous sample NPM10. However, the coating with different structure presented obvious difference in the extent of the promotion. The Rt of the coated sample HPM10 greatly increased to 181.90 kU cm2 from 15.00 kU cm2 of the HF pretreated sample (sample HP) and the Ecorr shifted to more positive side with the Icorr decreased to 0.15 mA/cm2 (1/20 to that of sample HP and almost 1/ 300 to that of the naked Mg alloy), while the coated sample APM10 exhibited slightly enhanced electrochemical properties within one order of magnitude. As for the Mg-based biomaterial, the coated sample with higher value of Rt and lower value of Icorr presented remarkable corrosion resistance. And compared with the typical values of the CaP coatings in the reported woks [17,18,25,35,36] (impedances of CaP coatings were in the range from 1 to 10 kU cm2 and the values of corrosion current density were always higher than 1 mA/cm2), the relevant electrochemical properties of coated sample HPM10 had been greatly improved, reaching the level rarely reported in the literature. The above results indicated that the surface structure could greatly influence the corrosion resistance of the coating, and the dense chrysanthemum-like coating of sample HPM10 could provide the most efficient anticorrosion properties. In our previous researches, it has been proved that the naked AZ31 alloy was vulnerable to corrosion and would fully degrade within 2 weeks after immersion in the SBF [25]. To further evaluate the long-time corrosion resistance of the CaP coatings on sample HPM10, the coated specimens were immersed in the SBF solution
Fig. 5. Cross-section micrographs of CaP coating based on (a) alkali pretreated sample; (b) acid pretreated sample.
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Fig. 6. Electrochemical properties of three different coated samples: (a) the Nyquist spectra, (b) the local amplification of the dotted portion in (a) and (c) potentiodynamic polarization curves; Electrochemical properties of the coated sample HPM10 immersed in SBF for different times: (d), (e) the Nyquist spectra and (f) the potentiodynamic polarization curves.
for different times. At the same time, the EIS and potentiodynamic polarization were performed for samples soaked after various periods to understand their degradation behavior, the results were presented in Fig. 6(d) and (e) and 6(f). As shown in Fig. 6(d) and (e), the sizes of semicircles in the Nyquist plots were decreased with
the immersion time prolonged to 4 weeks. Especially in the first two weeks, the Rt values of the coated sample sharply declined from 181.90 kU cm2 to 11.37 kU cm2 and the minimum Rt value was 5.17 kU cm2 after 4 weeks immersion. And then as the immersion continued, the sizes of the semicircle gradually increased to
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4. Discussion
Table 3 Corrosion properties of three different coated samples. Samples
Rt (kU$cm2)
Ecorr (V/SCE)
Icorr (mA/cm2)
NPM10 APM10 HPM10
3.33 4.98 181.90
1.61 1.55 1.49
11.55 7.09 0.15
13.84 kU cm2 and the Rt value was stabilized around 9.09 kU cm2 after 10 weeks immersion. Although the Rt of the coated sample underwent an obvious variation process as mentioned, its value was significantly higher than that of the naked Mg alloy (only 0.62 kU cm2 in the inset of Fig. 2(a)) during the whole immersion test, which demonstrated that the CaP coating on sample HPM10 could provide an effective long-term (at least 10 weeks) protection for Mg substrate. Meanwhile, as shown in Fig. 6(f), the polarization curves also indicated that the coating on sample HPM10 could relive the degradation of substrate and prolong its protection period. Over the whole process of soaking, the Ecorr floated between 1.45 V and 1.59 V while the maximum of the Icorr was only 4.646 mA/cm2 at the 28th day of immersion which was even lower than that of the coated sample APM10 before immersion (7.09 mA/cm2 in the Table 3). To compare the results more conveniently and better reflect the variation tendency of the degradation behavior of samples during the immersion, samples corresponding polarization resistance (Rp) and corrosion rate (Pi) were calculated and gathered in Fig. 7, where the two curves exhibited an apparent opposite trend. The Rp value of samples after immersion underwent a sharp drop at the initial 4 weeks and then slowly rose with fluctuation within a narrow range. Correspondingly, the Pi continually increased to the peak at 28th day immersion when the corrosion resistance of the coating decreased due to the aggressive solution and then the overall trend was downward with the immersion kept carrying out to 10 weeks. However, even at the maximum point, the value of Pi was 0.11 mm/y that satisfied the corrosion requirements for degradable biomaterials as bone fixture (less than 0.5 mm/y in the SBF at 37 C) [5]. In conclusion, the electrochemical tests proved that the pretreatment was a vital factor to influence the corrosion resistance and degradation behavior of the followed CaP coating synthesized by microwave assisted method. From the electrochemical point of view, the acid pretreatment was superior to alkali pretreatment in enhancing the coating anticorrosion capacity.
Fig. 7. The variation of polarization resistance and corrosion rate of the coated sample HPM10 during the 10 weeks immersion in the SBF.
4.1. The effects of pretreatment on the formation of the CaP coatings
Mg þ 2Hþ ¼ Mg2þ þ H2
(3)
Mg2þ þ 2OH ¼ Mg(OH)2
(4)
Mg þ 2HF ¼ MgF2 þ H2
(5)
For Mg-based implants, the component and structure of surface coatings determined their corrosion resistance and degradation period in biological environment [25,28]. In the present study, it has been proved that the conditions of pretreatment were effective for controlling the surface topography and phase composition. According to the electrochemical results and SEM observation (Figs. 2 and 3), the pretreatment layer would play an important role in the protection of Mg alloy substrate, which might affect the surface microenvironment during the deposition of the followed CaP coating. Considering the active chemical properties of Mg, the acid of the coating solution (pH ¼ 4.0) and the efficiency of microwave irradiation to accelerate reactions, when the naked Mg alloy was immersed in the coating solution, the reaction of eq. (3) would easily occur and led to massive Mg2þ and H2 formation surrounding the surface of substrate. It has been reported that excess Mg2þ had strong inhibitory effect on apatite crystal growth and favored amorphous or poor crystallized apatite formation [12]. In addition, the violent release of H2 from the surface disturbed deposition process for CaP coating [13]. These mentioned influences would lead to the formation of porous coating as shown in Fig. 3(a) and (b). However, once the Mg alloy substrate contact with NaOH or HF solution, the corresponding pretreatment layer would firstly form on the surface of AZ31alloy according to the eq. (3) and (4) and eq. (5), respectively. And the release of H2 gas was the main factor for the formation of some micropores in the acid pretreatment layer (Fig. 1) [26]. In the potentiodynamic polarization curve of the naked Mg alloy (Fig. 2(b)), its cathodic branch combined with the cathodic hydrogen evolution reaction [37]. As for the pretreated samples, the polarization current densities of sample AP and sample HP were lower than that of the naked sample, manifesting that the original hydrogen evolution reaction (eq. (3)) was effectively suppressed by the pretreatment layer. This protection effect guaranteed the compact structure and better crystallinity of CaP coatings on sample APM10 and HPM10, when compared with the sample NPM10. For the microwave assisted method, crystals would easily nucleate and grow up on the surface of the substrate and its surrounding solution, usually making a coating with bilayer structure [25]. Compared the structure of CaP coating on sample APM10 and HPM10 (Fig. 3(d) and (e)), the main difference about the structure existed on their bottom coating, which demonstrated that the pretreatment layer primarily affected the process of nucleation and growth for the CaP coating adjoined. And this phenomenon might be caused by the different microenvironment between the surfaces of the pretreatment layer. When immersed into the coating solution and heated from room temperature to 100 C, the pretreatment layer would partially dissolve to form different ions. It has been pointed out that the primary effects for Mg(OH)2 layer on coating formation process was the negative charged hydroxyl groups around its surface, which were beneficial for apatite to fast nucleation [38]. Moreover, the acid environment was suitable for DCPD or DCPA to deposit from solution, whose pH stability range in aqueous was 2.0e6.0 [39]. All these factors led to the formation of
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thin flake-like bottom coating on the Mg(OH)2 pretreatment layer. As for the acid pretreatment layer, few articles had reported its effects on the followed CaP coating and many only regarded it as a chemical passive layer [16e18]. However, some of the MgF2 nanoparticles in Fig. 1(b) would also dissolve in the coating solution once the temperature exceeded 90 C, releasing F ions to the liquid [30]. Additionally, the liquid could permeate the pretreatment layer through these micropores to react with the Mg alloy substrate that generated moderate Mg2þ and OH. As a result, the pH of the coating solution around the surface of the substrate increased. When the condition for nucleation were satisfied, the ions such as 2 Ca2þ, Mg2þ, PO3 4 , HPO4 , OH and F would react to generate iondoped HA. Differ from ions around the Mg(OH)2 pretreatment layer, the F derived from MgF2 in the acid pretreatment layer played a major role to decide the growth orientation of the HA. Some researchers have revealed that F would specifically replace OH in the negative charged (100) a-surface of the HA and caused a-axis contraction, causing the crystal presented an obvious c-axis oriented growth along. And trace of F (more than 0.9 ppm) could completely trigger this change of structure for apatite and finally get a nanoneedle-like topography with increased specific surface area [31e33]. Because of the increased driving force and decreased activation free energy (DG) caused by microwave heating, the decomposition of MgF2, nucleation and growth of F-doped HA could occur within the original 5 min from microwave starting (demonstrated in the surface morphologies and XRD pattern of CaP coatings prepared with different microwave heating time, Fig. S1 and Fig. S2). With the reaction time prolong, the number of chrysanthemum-like aggregation continually increased and accumulated each other to fully packed the whole surface. Once the chrysanthemum-like coating completed, it physically cut off the contact between the pretreatment layer and the coating solution. As a result, the decomposition of MgF2 and alkalization of vicinity surrounding the substrate would gradually weaken. The farther away from the surface was, the lower concentration of F in the solution. With the decrease of F and OH, the residual Ca2þ and phosphate ions tend to synthesize as DCPA from the solution, similar to that of the coating on sample APM10, and eventually randomly scattered on the bottom coating. 4.2. Improved corrosion resistance of acid pretreated AZ31 alloy The macroscopic appearances (Fig. S3) and electrochemical corrosion tests (Figs. 6 and 7) among different coated samples before and after long-term immersion in the SBF had demonstrated that the acid pretreatment was beneficial to enhance the corrosion resistance of the CaP coating synthesized by microwave assisted method and effectively prolonged the degradation period of the coated AZ31 alloy to more than 10 weeks. And it has been revealed that the ideal clinical Mg-based degradable implant should maintain its mechanical integrity for at least 10e12 weeks [1,5]. It was well known that the physical barrier effect of the anticorrosion coating was closely linked with the integrity and structure of the coating, since porous coating or microcracks (as shown in Fig. 3(b) and (d)) served as shortcuts for corrosive medium to permeate and led to the early failure of the coating. For the acid pretreatment, its excellent electrochemical properties were mainly attributed to the chrysanthemum-like structure in the bottom coating with maximum thickness (at least 100 mm as shown in Fig. 5). Once contact with the SBF, the compact coating effectively blocked the contact of solution with Mg alloy surface, avoiding etching of aggressive mediums such as Cl or Hþ. Additionally, as analyzed before, the bottom coating mainly contained F-doped HA, which was the most stable apatite compared with other CaP phase such as DCPD or DCPA, reducing the coating decomposition or
peeling from the substrate during the electrochemical tests. In a word, compared with the coating without pretreatment or underwent alkali pretreatment, it was coordination of the structure and component of the bottom coating that ensured the coated sample HPM10 possessed remarkable corrosion resistance in the origin of the immersion. With the extension of immersion time, the CaP coating usually underwent a series of dissolution and deposition to generate new mineralization layer and the long-term protection of the coating to sample HPM10 might relate to the solubility of the coating [25,28,35]. As shown in Fig. S4, XRD patterns indicated that the dissolution did not change the phase composition of the coating and the sediments might be amorphous apatite. However, from the view of structure, the dissolution of the F-doped HA destroyed the integrity and compact structure of the bottom coating, causing the anti-corrosion ability sharply decreased. Moreover, Dorozhkin [39] revealed that DCPA crystal scattered on the top of the coating was much more degradable than the bottom F-doped HA. Its degradation might lead to high ion concentration of Ca2þ and PO3 4 in the interface between the specimen and SBF, which was beneficial for the formation of mineralization layer to repair some microcracks caused during the immersion [20,25,35]. According to the variation tend of Rt and Pi, in the initial 4 weeks, the dissolution of CaP coating might be dominant, and the deposition of mineralization layer was carried out slowly. After that, as the mineralization layer formed, it could inhibit further degradation of the bottom coating. In the end, the synergistic effects between the mineralization layer and the original CaP coating might ensure the rate of dissolution and deposition process keep balance, avoiding the rapid failure of the coating during immersion. 5. Conclusions In this study, different pretreatment layers were formed through alkali and acid chemical conversion treatment of Mg alloy substrate prior to microwave assisted method. These layers were proved to have powerful effects on the phase component, surface morphology and thickness of the followed CaP coating that synthesized via microwave assisted method and finally influenced the anti-corrosion capacity of the coated AZ31 alloy substrates in the SBF. The following conclusions could be concluded: 1. During the deposition process of CaP coatings, the pretreatment layer was essential to relieve the corrosion of AZ31 alloy substrate caused by the acid coating solution. 2. After acid pretreatment of Mg alloy substrate, the formed MgF2 in the pretreatment layer induced compact chrysanthemumlike F-doped HA deposition with obvious c-axis growth orientation. 3. The CaP coating deposited on the acid pretreatment layer exhibited excellent corrosion resistance, whose charge transfer resistance was increased to 181.90 kU cm2 with the corrosion current density decreased from 45.61 mA/cm2 of naked Mg alloy to 0.15 mA/cm2. 4. The coating, synthesized through a combination of acid pretreatment and microwave assisted method, effectively prolonged the degradation period of Mg alloy substrate. And after 10 weeks immersion in the SBF, its corrosion rate kept below 0.5 mm/y, meeting the requirement as biodegradable implant materials. Acknowledgements Authors acknowledge the financial support by National Natural Science Foundation of China [Grant No. 51872197, 51572186 and
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81271954] and Shanghai Committee of Science and Technology, China [Grant No. 1541195100]. And we also acknowledge Mr. Chang Lin for his help in the experimental work via Tianjin e Hainan university innovation fund cooperation project [Grant No. 2018XZC-0105].
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