Corrosion resistance of glucose-induced hydrothermal calcium phosphate coating on pure magnesium

Accepted Manuscript Full Length Article Corrosion resistance of glucose-induced hydrothermal calcium phosphate coating on pure magnesium Ling-Yu Li, Lan-Yue Cui, Bin-Liu, Rong-Chang Zeng, Xiao-Bo Chen, ShuoQi Li, Zhen-Lin Wang, En-Hou Han PII: DOI: Reference:

S0169-4332(18)32629-1 https://doi.org/10.1016/j.apsusc.2018.09.203 APSUSC 40507

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

21 March 2018 10 September 2018 24 September 2018

Please cite this article as: L-Y. Li, L-Y. Cui, Bin-Liu, R-C. Zeng, X-B. Chen, S-Q. Li, Z-L. Wang, E-H. Han, Corrosion resistance of glucose-induced hydrothermal calcium phosphate coating on pure magnesium, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.09.203

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Corrosion resistance of glucose-induced hydrothermal calcium phosphate coating on pure magnesium Ling-Yu Li1, Lan-Yue Cui1, Bin-Liu1, Rong-Chang Zeng1, *, Xiao-Bo Chen2, Shuo-Qi Li1, Zhen-Lin Wang3, En-Hou Han4 1

College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China 2 School of Engineering, RMIT University, Carlton 3053, VIC, Australia 3 School of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China 4 National Engineering Centre for Corrosion Control, Institute of Metals Research, Chinese Academy of Sciences, Shenyang, 110016, China

Abstract: Glucose-induced composite coatings containing crystalline calcium phosphate and Mg(OH)2 interlayer were prepared on pure Mg substrate through hydrothermal deposition from alkaline solution. Surface composition, morphology and corrosion resistance of the coatings were characterized through XRD, FTIR, SEM, XPS, electrochemical and hydrogen evolution measurements. Results reveal that calcium phosphate coatings were composed of dicalcium phosphate anhydrous, calcium-deficient hydroxyapatite and hydroxyapatite. Corrosion resistance of pure Mg specimens was improved by the formation of such a calcium phosphate coating. The findings provide a novel strategy to design calcium phosphate conversion coatings with satisfactory corrosion resistance for biodegradable Mg implants.

Keywords: Magnesium, Glucose, Ca-P coating, Biomaterial, Corrosion resistance

1

1.

Introduction Biodegradable orthopaedic implants are expected to assist injured tissue

reconstruction through providing temporary mechanical support and then completely dissolving after full recovery of the bone fractures in the physiological environment [1-3]. Magnesium (Mg) and its alloys are emerging material candidates for biodegradable implants owing to their excellent biocompatibility [4, 5], mechanical compatibility to bone [6, 7], low density and high biodegradability [8-10]. However, the high degradation kinetics of Mg alloys exceeds the threshold that the physiological system can tolerate with. Release of a massive quantity of hydrogen gas, Mg2+ ions and a sharp increase in pH will deteriorate mechanical integrity of the load-bearing implants [11-13], and incur inflammatory and cytotoxic reactions at the interface between living tissues and implants. Such severe concerns over the applicability and safety of Mg alloys for biomedical applications must be mitigated through feasible strategies prior to commercial and clinical trials [14]. To date, considerable effort has been devoted to controlling the degradation or corrosion behaviour of Mg based implant materials. In general, alloying bulk Mg with noble and biocompatible elements, such as zinc (Zn) [15], and microstructural optimization through severe plastic deformation and amorphization [16] have been extensively attempted for yielding Mg alloys with a low degradation rate. Issues associated with alloying approaches include high production cost, complicated manufacturing procedures, a small number of options of alloying elements, and more importantly, limited efficacy in controlling the degradation rate of Mg [17]. Surface modification and coatings are a key and feasible technology to provide strong protection to Mg against corrosion in corrosive media, such as human body fluid. Implementation of an appropriate coating can not only retard the initial degradation of Mg alloys, but also improve biocompatibility of the established protective coatings [18-21]. Calcium phosphate (Ca-P) based coatings outperform the peers owing to their superior biocompatibility, high osteoconductivity, and low toxicity in the physiological environment [22-24]. Several methods have been developed for producing functional Ca-P coating on 2

Mg alloys for biomedical employment, such as physical dipping [25], plasma spraying [26], electrodeposition [27], microarc oxidation (MAO) [28], chemical treatment [29] and hydrothermal preparation [30]. Indeed, hydrothermal synthesis is a simple and cost-effective technique for preparation of Ca-P coating on Mg alloys. In the human body, growth of Ca-P, one of the main components of bone, is regulated by a series of biological reagents, such as, glucose, proteins [31], peptide [32] and vitamins [33]. For instance, interactions between proteins and hydroxyapatite (HA) are crucial considerations for designing biomaterials for bone regeneration. Morphology of HA evidently affects the interaction strength between protein/peptide and HA substrate [31]. Some research has pointed out that skeleton is a site of significant glucose uptake and as a major contributor to the regulations over organismal metabolisms [34]. Particularly, Wang et al. [35] concluded that high glucose levels could inhibit the development of cranial neural crest cells by affecting cell apoptosis. Thus, glucose plays an important role in human bone metabolism. However, few studies have focused on this respect of glucose in regulating the growth of apatite in human body fluid. Glucose, a simple type of sugar, is an important and basic energy resource for metabolic activities [36]. Recently, studies of the impact of glucose on bone growth attract much attention. Maycas et al. [37] found that a high glucose concentration in the human body exerts a disparate effect on osteocyte mechanotransduction. High glucose-containing media allow the formation of osteoclast-like cells but can’t resorb HA; whilst fluid flow conditioned media suppress osteoclastogenesis even under high glucose conditions. In addition, high glucose contents exhibit a slight influence on basal RANKL or IL-6 secretion or their inhibition induced by fluid flow in MLO-Y4 cells [37]. Our previous study [38] unveils that the presence of glucose accelerates corrosion kinetics of pure Mg in regular saline solutions, whilst suppresses it in Hank’s solution. In normal saline solutions, glucose transforms into gluconic acid, which attacks metallic oxides, decreases the pH of the solution, and consequently promotes the absorption of chloride ions on Mg surface and accelerates corrosion. In contrast, better corrosion resistance in Hank’s solution is attributed to the coordination 3

of glucose with Ca2+ ions, which facilitates the formation of Ca-P compounds on Mg substrates. We postulate that the presence of glucose in Hank’s solution may contribute to the growth of Ca-P based coating on Mg and mitigating the degradation kinetics. It is apparent that glucose imposes a crucial influence on both Mg degradation and growth of Ca-P in the physiological environments, which is not fully studied yet. It is hypothesized herein that it could be a sound strategy to utilize glucose as “catalyst” for boosting the formation of Ca-P barrier coating on Mg alloys to address their corrosion challenges. This study aims to validate such a hypothesis through preparation of Ca-P coatings on Mg in a simple solution consisting of Ca2+, PO43- and glucose in stainless steel autoclaves at 120 °C for a certain time length. Physical and chemical features of the resulting coatings were characterized through electron microscopy, X-ray diffraction,

Fourier

transform

infrared

spectroscopy,

X-ray

photoelectron

spectroscopy and nanoscratch tests. Biodegradation behaviour is examined through potentiodynamic polarization curves, electrochemical impedance spectroscopy and immersion tests. Results demonstrate that glucose molecules coordinate with Ca2+ ions and contribute greatly to the nucleation of Ca-P coating on pure Mg. The yielded Ca-P coatings reduce anodic kinetics of Mg substrate in Hank’s solution to a great degree. The transformation of glucose into gluconic acid under hydrothermal conditions is the key point to facilitate the formation of corrosion resistant Ca-P coating on Mg alloys.

4

2.

Materials and methods

2.1. Raw materials Pure Mg cast ingots (99.97% purity, Fe≤35 ppm), provided by Guangling Magnesium Industry Science and Technology Co. Ltd., China, were utilized for the study. As-cast ingot was cut into square plates with dimensions of 20 mm × 20 mm × 3 mm. Sample surface was mechanically ground with silicon carbide papers progressively to a 2500 grit finish, then rinsed with acetone and ethanol for 5 min at room temperature, and finally dried with warm air.

2.2. Preparation of the glucose-induced Ca-P composite coating

Ca-P composite coatings were fabricated on pure Mg substrate through hydrothermal treatment, as schematically illustrated in Fig. 1. Solution A was prepared by dissolving Ca(NO3)24H2O (250 mmol/L) and glucose (500 mmol/L) in a certain volume of deionized water. Dropwise addition of 2 mol/L NaOH solution into Solution A under magnetic stirring at room temperature to achieve Solution B with a pH value of 10.0. A further addition of 250 mmol/L KH2PO4 into Solution B under stirring for 30 min yielded Solution C as the final coating solution. Subsequently, the as-ground Mg specimens and Solution C with a constant ratio of 1: 6 (cm2/mL) were transferred into a Teflon-lined stainless steel pressure vessel, heated up to 120 °C, and held for 24 h in electric oven. After naturally cooling down to room temperature, the treated Mg specimens were retracted, rinsed with deionized water and dried with warm air. In addition, Mg samples were prepared with similarly procedure in Solution A (glucose-free) as controls.

2.3. Surface characterizations

Surface morphologies of the films were examined via field-emission scanning 5

electron microscopy (FE-SEM, Nova Nano SEM 450, USA). Crystallographic structures were identified using X-ray diffraction (XRD, Rigaku D/MAX 2500 PC, Japan). Fourier transform infrared spectroscopy (FTIR, Nicolet 380, Thermo electron, USA) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermoelectron, USA) were employed to examine the chemical bonds of the coating.

2.4. Corrosion evaluation

2.4.1. Electrochemical tests Electrochemical measurements were performed on bare and coated pure Mg in Hank’s solution at ambient temperature. A typical three-electrode system (PAR Model 2273, Princeton, USA) with saturated calomel electrode (SCE) as reference electrode, platinum mesh as counter electrode, and Mg samples with exposed area of 1 cm2 as working electrode was utilized. Open circuit potential (OCP) was conducted for 600 s prior to the electrochemical impedance spectra (EIS) measurements [20]. EIS tests were carried out at a disturbing potential of 10 mV over a frequency range from 100 kHz to 0.01 Hz at OCP. Then, potentiodynamic polarization (PDP) was measured from -2000 mVSCE to -1000 mVSCE at a scan rate of 1 mV s-1. The key electrochemical parameters (i.e. free corrosion potential (Ecorr) and corrosion current density (icorr)) were fitted using the PowerSuite software. EIS plots were analysed via ZSimpWin software (version 3.10 USA) to obtain the proper equivalent circuit (EC) models. In addition, polarization resistance (Rp) was calculated by the simplified Stern-Geary equation [39]:

Rp = βa • βc / 2.303 • icorr ( βa + βc )

(1)

Where a and c represent the Tafel slopes of anode and cathode, respectively.

2.4.2. Hydrogen evolution tests Hydrogen evolution was carried out by placing the samples under an inverted funnel connected to a graduated burette in Hank’s solution at 37 ± 0.5 °C for 336 h. 6

Additionally, a constant ratio of solution volume to sample surface area was set as 50 mL/1cm2. Five parallel measurements were performed for reproducibility. Hydrogen evolution rate VH (mL·cm-2·h-1) can be expressed as: (2)

VH = V / st

where V is the hydrogen volume (mL), s is the exposed area of the sample to solution (cm2), and t is immersion time (h).

2.5. Scratch tests

In order to measure the bonding strength between coating to substrate, nanoscratch tests were performed (MicroMaterials Ltd, Nanotest system), using Rockwell diamond probe, with a 25 μm tip diameter. A sliding speed of 1mm/min during the scan was employed and the applied load linearly increased to 24 N until a 2-mm long scratch was produced. Furthermore, the tests were performed in triplicate, and an in situ optical microscopy system was used to photograph the coating surface.

7

3.

Results

3.1. Surface analysis

Fig. 2 depicts SEM morphologies of the Ca-P composite coating induced by glucose (Fig. 2a and 2b) and the counterpart produced from glucose-free solution (Fig. 2c and 2d). EDS analysis of the samples is presented in Table 1. All the coatings display block-like particles with varying sizes. It is striking that the coatings induced by glucose exhibit denser and more uniform features than the glucose-free samples (controls), though some small cracks can be observed. Note that, the control group samples have more protrusion on the relatively small block-like crystals and the protrusion regions are composed of a bulk part or some groups. EDS analysis (Table 1) reveals that the coatings mainly consist of C, O, Ca and P, and trace amounts of Mg. The atomic ratio of Ca/P is around 1.0, attributing to the inevitable corrosion of Mg-based materials in aqueous solutions, which is a result of the high ionization tendency of Mg, and the readily substitute of the released Mg2+ ions for Ca2+ ions in Ca-P coatings [40, 41]. The appearance of Ca-deficient Ca-P coating can be attributed to the following aspects. Firstly, a trace of Mg2+ ions can catalyze the heterogeneous nucleation and then promote the growth of Ca-P coating [42]. Secondly, the substitutes of Mg2+ ions can increase the nucleation rate of Ca-P kinetically [43]. Lastly, the substitutes of Ca2+ by Mg2+ ions are able to reduce the grain size and crystallinity [44]. It is worth noting that the Ca/P ratio of spectrum 5 can reach 1.75, which may be attributed to the combination of CaP compound, like CaHPO4 (DCPA), Ca10(PO4)6(OH)2 (HA) and Ca10-X(HPO4)X(PO4)6-X(OH)2-X (CDHA). The results can be further confirmed by XRD patterns in Fig. 6. Cross-sectional images of the glucose-free sample and Ca-P composite coating were displayed in Fig. 3 (a and b). It is obvious that both coatings show a double layered strcuture: an inner Mg(OH)2 layer and an outer Ca-P layer. As depicted, the glucose-free sample displays relatively large blocks but with more cracks and pores (Fig. 3a); whereas the Ca-P and Mg(OH)2 composite coating induced by glucose 8

exhibit a more dense and uniform structure (Fig. 3b), implying that the coating induced by glucose may have a better corrosion resistance. The corrosponding elemental mapping images (Fig. 3c-3g) demonstrate that the coating contains O, Mg, Ca and P. The existance of elemental Mg and O as well as the alkaline environment indicate that the bottom film could be Mg(OH)2 coating. Noticeablely, the surface of Ca-P coating induced by glucose is uneven, so the coating thickness is changeable. From line scanning image (Fig. 3h), the maximum thickness of the Ca-P coating is 25 μm. In order to further make sure the growth process of Ca-P crystal structure induced by glucose, especially for glucose induced, different reaction time towards initial growth were investigated. Surface morphologies of the coating of various reaction time have been revealed in Fig. 4. The glucose induced Ca-P composite coating displayed multilateral flaky surface with a reaction of 20 min (Fig. 4a and 4b), which were the most commonly morphology of the Ca-P coating. Specially speaking, the bottom layer exhibited center divergent sheet, the protruding flake consists of flower-like structure. With a reaction of 1 h (Fig. 4c and 4d), block-like particles occured. The protruding flower-like structure appeared to be suitable for nucleation. The bottom layer transformed to rod-like particles. When reaction time reached to 2 h (Fig. 4e and 4f), the sample surface showed dense block-like particles on the whole scale with diverse crystal shape and size. Moreover, the corresponding EDS elemental mapping of the samples are shown in Fig. 5. Ca and P elements cover on the sample surface in the initial 20 min, which indicates that the Ca-P coating can deposite quickly on the substrate. Small amout of Mg and C can also be observed. As for 1 h and 2 h, Ca-P coating was continuously crystallizing on the sample to form the uniform and dense film. The phase analysis was applied using XRD measurements in Fig. 6. The composite coating mainly composed of DCPA (CaHPO4). Beisdes, the Ca-P coatings also consisted of HA (Ca10(PO4)6(OH)2), and CDHA (Ca10-X(HPO4)X(PO4)6-X(OH)2-X). In particular, the intensity of Ca-P compounds induced by glucose are higher than the contrast sample. Fig. 7 demonstrates the FTIR spectra of Ca-P composite coating 9

glucose-free (a) and induced by glucose (b). The sharp but weak peak at around 3742 cm-1 can be ascribed to the O-H stretching in the crystal structure of Mg(OH)2 [45]. The band at around 1650 cm-1 originated from H2O bending vibration and the bands ataround 2362 cm-1 result from CO2 in the air [46]. The peaks at 1392 cm-1 is assigned to –OH [47]. The broad strong peak at 1131, 1069, 994 and 886 cm-1 represent H2PO4and 563 cm-1 is corresponded to the PO43- group [48], which well suggested the formation of calcium phosphate compound on the sample surface. Typical XPS analysis of the Ca-P composite coating on pure Mg substrate are presented in Fig. 8. Fig. 8a shows the whole range of the binding energy survey on the composite coating. Obviously, the surface chemical compositions, in good agreement with the XRD and FTIR results, containing Ca, P, C and O elements. The elements of Ca, P and O originated from the composite coating, and most of C derived from the glucose. In order to figure out the glucose function on the crystalline of the coating, detailed information of the high-resolution XPS data for Ca and C were collected. Similarly, the glucose-free sample of Ca 2p spectra was also measured. Fig. 8b and c designate the curve fits of the C 1s, Ca 2p spectra of the glucose induced Ca-P composite coating. The C 1s spectra can be split into three peaks, the C 1s signal at 285.6 eV could be attributed to a hydrocarbon species (C-H/C-C). The other component at 285.8 eV may be connected with the presence of C-O bonding. while the special peak at 288.3 eV, corresponds to the O-C=O group [49]. It is rather remarkable that the presence of the C-H/C-C and C-O groups were extremely strong, demonstrating the large proportion of this group came from glucose and the glucose adsorption on the sample surface was rapid [38]. The Ca 2p spectra has been divided into three peak, (-COO)2Ca at 347.4 eV and Ca-P chemical bonds at 346.8, 350.5 eV. The presence of (-COO)2Ca disclosed the formation of –COOH during the transformation of glucose, contributing the attack of the Ca-P conpound and improving the formation of (-COO)2Ca. As comparable, Ca 2p of the glucose-free sample was showed in Fig. 8d. The peaks at 347.0 eV and 350.5 eV could be attributed to Ca-P compound [50, 51]. In a word, the XPS analysis demonstrates that the formation of –COOH from glucose plays a dominant role for the increased 10

corrosion of the composite coating. In addition, the aldehyde group from glucose is very active and can be transformed to carboxyl group, which attacks the Ca2+ promptly on the sample surface and then forms the Ca-P coating.

3.2. Electrochemical measurements

Fig. 9(a-c) shows the EIS measurements of (I) Pure Mg substrate, (II) glucose-free coating and (III) Ca-P composite coating induced by glucose in Hank’s solution. Fig. 9b are the enlarged Fig. 9a. Besides, the Nyquist plots were fitted with the corresponding EC, which were also displayed in Fig. 9d,e. For the EIS of the substrate (Fig. 9a), Nyquist plots consist of capacitive loops at high and middle frequencies, followed by an inductive loop in the low-frequency range. Capactive loop in the high-frequency range corresponds to the oxidation layer (Rc1) formed in air and interface diffusion constant phrase element (CPE1) in Fig. 9d. The constant phrase element (CPE, ZCPE = [Y0(j)n]-1, [52]) acts as a pure resistor when n = 0 and as an ideal capactior when n = 1. Where Y0 is constant of the CPE, j is the variable for simusoidal pertubatons with  = 2, n varies between 0 and 1. The middle capacitance loop is correlated to charge transfer resistance (Rct) and interface diffusion constant phrase element (CPE2). At low frequencies, inductive loop corresponding to an inductor L and a resistor RL, which attributed to the absorption and peeling of the corrosion products such as Mg(OH)2 [53]. It is worth noting that the glucose induced Ca-P coating with two capactive loops (Fig. 9a) shows the largest diameters among the three samples. The absence of inductive loop implys the composite coating can effectively protect the Mg substrate from corrosion. For the glucose-free sample (Fig. 9b), a similar loop to the glucose induced coating was observed, indicating a close corrosion mechanism is in effect. But a lower corrosion resistance due to the smaller dimensions of the capactive loop. Both of the coatings in high frequency region correspond to the film of crystalline Ca-P coating. The larger second loop corresponds to the Mg(OH) 2 layer. Besides, Ca-P coating cannot change the electrochemical corrosion behaviors of the composite 11

coatings due to Ca-P coating contains porosity [54]. The EC for the two compoite coatings are showed in Fig. 9e. Fitting data are summaried in Table 2. In this EC model, Rs represents the solution resistance, Rct represents the charge transfer resistance, and Rc is the coating resistance paralleled with CPE, Specifically, for the both composite coatings, Rc1 represents the Ca-P coating. Rc2 represents Mg(OH)2 film, located at the interface between the Ca-P coating and the Mg substrate. Generally, higher Rct value implies the lower dissolution rate. The composite coatings wihout glucose improved Rct value obviously from 409.5 to 815.2 Ω·cm2, while the glucose induced composite coatings notably gave rise to 2390 Ω·cm2, demostrating a remarkable improvement in corrosion resistance. Moreover, From the IZI Bode plot (Fig. 9c), it can be seen that the total impedance of the glucose induced Ca-P coating was higher than that of the comparable sample and the substrate, indicating that the Ca-P composite coating significantliy improved the corrosion resistance of the pure Mg substrate. The PDP curves and corresponding parameters of the substrate and coated samples in Hank’s solution were displayed in Fig. 10 and Table 3. The a decreaed, but the c increased of the coated samples. The significant reduction of a reflects a more effective modification of the Ca-P coating. It can be seen that the Rp of the samples follows this consequence: the substrate (3.01 × 106 Ω·cm2) glucose-free coating(4.03 × 106 Ω·cm2) glucose-induced coating (9.56 × 106 Ω·cm2), revealing that the coated samples exhibited a lower thermodynamic trend. The Ecorr of the glucose induced coatings increased from -1780 to -1495 mv/SCE, what’s more, the icorr of glucose induced composite coatings (6.79 × 10-6 A·cm-2) was reduced by an order of magnitude from that of the substrate (2.36 × 10-5 A·cm-2) and the glucose-free samples (1.39 × 10-5 A·cm-2), which suggested that the corrosion resistance is effectively enhanced. 3.3. Nanoscratch tests Fig.11 illustrates the bonding force between the Ca-P composite coating and the substrate via the nanoscratch tests. It can be found that the two Ca-P composite 12

coatings, measured by the critical load during the adhesive failure, exhibite a relatively good bonding strength. As exhibited in Fig. 11a, the critical load of the glucose-free samples was 9.5 N; whereas that of the Ca-P composite coatings induced by glucose (Fig. 11b) was 11.7 N. This means that the bonding strength of the coating induced by glucose is slightly improved.

3.4. Immersion tests The hydrogen evolution rate (HER) curves of the coated and uncoated samples in Hank’s solution are displayed in Fig.12. Obviously, the Ca-P composite coating significantly decreased the HER of the pure Mg substrate. What’s more, the HER of the Ca-P composite coating induced by glucose is the lowest (0.07 mL cm-2 h-1), which is much lower than that of the comparable coating (0.10 mL cm-2 h-1) and the substrate (0.18 mL cm-2 h-1). A similar flucating trend was observed in test conditions. The HERs of pure Mg substrate and glucose-free sample climbed quickly at initial stage, and then slighty decreased. Furthermore, the HERs of glucose-free sample were much higher than the substrate in the first 20 h, which can be ascribed to the outer porous band of the coating. Specifically, An initial increase of the glucose-free sample caused by the deterioration of the coating, and subsequent slight decrease owing to the formation of corrosion production like phosphate precipitates that partially protected the sample [55]. For Ca-P composite coating induced by glucose, the HER keeps a relatively lower level owing the protection of the Ca-P coating. Fig. 13 shows the SEM morphologies of the pure Mg substrate, Ca-P composite coating induced by glucose and glucose-free sample after immersion in Hank’s solution for 336 h. For glucose-induced Ca-P composite coating (Fig. 13a and 13b), after 336 h of immersion, the surface is still present on the appearance of block-like crystal particles. In addition, small cracks and corrosion products can still be observed, indicating that Ca-P composite coating induced by glucose suffered slightly damage. As for its counterpart (Fig. 13c and 13d), the coating prepared in absence of glucose undergoes extremely delamination. It can be seen a heart-shaped pits in the middle of 13

the picture and more corroison products were coverd on the sample surface, which revealed that the glucose played a significant role in the Ca-P composite coating for corrosion protection of the samples. The pure Mg substrate was subjected to severe corrosion. Deep cracks and irregular corrosion products occurred on the sample surface (Fig. 13 e and 13f). Fig. 14 displays the corresponding EDS analysis of the samples (Fig.13 spectrum 1-6) after an immersion in Hank’s solution. The predominant components formed on the sample surface were C, O, Mg, Ca and P elements. For glucose-induced composite coating, the Ca and P were derived from the Ca-P coating and remained a relatively high level (Fig.14, spectra 1 and 2). While for glucose-free coating (Fig.14, spectra 3 and 4), the contents of the Ca and P were much lower than the glucose induced. Moreover, spectrum 4 shows the lowest content of Ca and P and higher percent of Mg, resulting from severe corrosion. Spectra 5 and 6 confirm that the formed corrosion products on the Mg substrate were possible not only Mg(OH)2 but also Ca-P compounds. The results were confirmed in the following FTIR and XRD tests. Namely, after a 336 h immersion in Hank’s solution, the glucose-induced Ca-P composite coating performed good corrosion resistance. The FTIR spectra of the substrate, glucose-free coating and Ca-P composite coating induced by glucose samples immersed in Hank’s solution for 336 h are shown in Fig. 15. Clearly, the FTIR spectra of the two composite coatings are similar (Fig. 15b nad 15c). The sharp peak at 3697 cm−1 indicated the formation of Mg(OH)2 corrosion products. The band at 1653 cm−1 arose from H2O. The absorption peak observed at 1422 cm−1 arose from O-H groups. The peak at 566 cm−1 can be ascribed to PO43−. The presence of sharp peak at 1128 and 882 cm−1 are considered to be the H2PO4− groups, confirming that the two composite coatings keep its integrity to some extent. In addition, for Ca-P coating induced by glucose (Fig. 15c), this Mg-OH peak is rather shallow compared to glucose-free coating (Fig. 15b) and the substrate (Fig. 15a), which implys less formation of corrosion product on the sample surface. Fig. 16 depicts the XRD patterns of the substrate and two composite coatings after a 336 h immersion in Hank’s solution. For pure Mg substrate, the main corrosion 14

products are Mg(OH)2 and with little Ca-P compounds such as HA and DCPA (Fig. 16a). Mg(OH)2 formed on the surface of glucose-free samples and the DCPA peaks were remarkably decreased (Fig. 16b), indicating that this coating suffered a great damage in corrosive medium. The significant characteristic peaks of Ca-P composite coating induced by glucose were DCPA and relative lower peaks of Mg(OH)2, which proved that the glucose played a crucial role in keeping the composite coating intensity. Certainly, the XRD results are in good agreement with the FTIR results. 4.

Discussion In the hydrothermal system, the transformation of glucose into gluconic acid can

attract Ca2+ ions onto Mg surface, which could induce Ca-P composite coating and protect the underlying Mg. Fig. 17 presented the detailed shematic illustrations of the mechanism of glucose-induced Ca-P coating. Mg is highly active in the aqueous precursor solutions, which can be dissolved rapidly and released a large number of Mg2+ and OH- ions, so that Mg(OH)2 precipiatates quickly formed on the sample surface, in particular in such an alkaline solution. as the same time, H2PO4- ions in the alkaline solution C are transformed into PO43-ions. The chemical reactions as follows: M g (s) → M g2 + (aq) + 2e _

2H 2 O (aq) + 2e

_

(3) _

→ 2OH (aq) + H 2 ↑

(4)

_

M g2 + (aq) + 2OH (aq) → M g(OH)2 -

H 2 PO 4 + OH- → HPO4 HPO4

2-

+ OH- → PO 4

3-

2-

(5)

+ 2H 2 O

(6)

+ H 2O

(7)

Remarkably, in the second stage, the glucose (CH2OH(CHOH)4CHO) can change into gluconic acid (CH2OH(CHOH)4COOH), as confirmed by XPS analysis [39]. Then, glucose acid is able to react with Mg(OH)2 coating, slightly destroying the protective film. The adsorption of glucose on the sample surface is beneficial to the 15

accumulation of Ca2+ ions on Mg surface owing to the chelation of glucose with Ca2+ ions. The following reactions are related to the addition of glucose:

CH 2 OH(CHOH)4 CHO → RCOOH

(8)

2RCOOH + M g(OH)2 → (RCOO)2 M g + 2H 2 O

(9)

2RCOOH + Ca 2 + → (RCOO)2 Ca

(10)

-

Where R is the CH 2OH(CHOH)4 group from CH2OH(CHOH)4CHO. In the third stage, Ca-P starts to nucleate, and the absorbed Ca2+ ions reacts with HPO42- and PO43- ions in precursor solution to form the Ca-P coating (DCPA, HA and CDHA). Chemical reactions indicate as follows:

Ca 2 + + HPO4

2-

→ CaHPO4

10Ca 2 + + 8OH- + 6HPO4 (10 - x)Ca 2 + + xHPO 4

2-

2-

(11) → Ca10 (PO4 ) 6 (OH)2 + 6H 2 O

+ (10 - x)PO 4

3-

(12)

+ (2 - x)OH - →

Ca10 - x (HPO4 ) x (PO4 ) 6 - x (OH)2 - x

(13)

In addtion, the coatings are more dense and compact compared to that glucose-free samples. Glucose belongs to a sort of polyol aldehyde, which can coordinate with metal ions in aqueous solutions using its polyhydroxy units. Herein, glucose molecular intensively coordinating with Ca2+ ions in the precursor solutions were attracted onto Mg surface. In particular, glucose is transformed into gluconic acid with ionization groups (carboxyl), which induces the Mg surface negatively charged. Ca2+ ions would compete with other ions like Mg2+ ions and coordinate with the negatively charged group (carboxyl), which reduces the net charges from the molecular of glucose acid. Consequently, the negative charges will coordinate with Ca2+ ions for charge neutralization. The more Ca2+ ions absorbed on the sample surface, the more benifical for formation of Ca-P coating. Ca2+ ions react with the 16

HPO42- and PO43- ions in alkaline environment and form DCPA, HA and CDHA. It should be pointed that the corrosion resistance of our Ca-P and Mg(OH)2 composite coating is not worse than that of the coating produced by other traditional hydrothermal methods. Xia et al. [56] fabricated Ca-P coating on Mg alloy ZK60 using hydrothermal deposition. They mainly investigated the effect of Ca/P ratio on the structural and corrosion properties of Ca-P coating. Nevertheless, pH value of the treating solution was regulated to 4.0 by using HCl solution, leading to a rapid dissolution of the substrate. The thickness of the coating was about 20 μm, similar to that of the coating reported herein but not dense compared to the glucose induced Ca-P coating. It is well known that Ca-P coating displays excellent biocompatibility and improve corrosion resistance of pure Mg in Hank’s solution [57]. Based on the aforementioned discussion, the dense and thick Ca-P and Mg(OH)2 composite coating were successfully formed on the pure Mg surface, which enhanced the degradation resistance of pure Mg. These demonstrated the glucose was significant in forming such a Ca-P coating system. 5.

Conclusions (1) Ca-P composite coating was fabricated with an aid of glucose as “catalyst” through a hydrothermal treatment. The yielded coatings exhibit a highly compact morphology with numerous individual block-shape paricles as buliding unit. (2) Compared to the coatings produced in a glucose-free solution, glucose induced Ca-P composite coating significantliy improved the corrosion resistance, increasing the Ecorr from -1780 mVSCE to -1495 mVSCE, decreasing the icorr one order of magnitude and enhancing the Rct from 815.2 Ω·cm2 to 2390 Ω·cm2, In addtion, HER values also decreased from 0.10 mL·cm-2·h-1 to 0.07 mL·cm-2·h-1. (3) The formation of the Ca-P composite coating was inccured by the addition of glucose, which is favourable to the accumulation of Ca2+ ions, contributes to nucleation of Ca-P coating, and a gradual transformation from multilateral flaky pieces into block-like particles. 17

(4) The findings could provide new insight for design and development biocompatible conversion coatings to mitigate the corrosion progress of Mg implants in biological environments. Further in-depth studies using an in vivo animal model need to be investigated.

Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51571134), the Shandong University of Science and Technology (SDUST) for Research Fund (2014TDJH104) and Science and Technology Innovation Fund of SDUST for graduate students (SDKDYC180371).

References [1] S. Shen, S. Cai, Y. Li, R. Ling, F. Zhang, G. Xu, F. Wang, Microwave aqueous synthesis of hydroxyapatite bilayer coating on magnesium alloy for orthopedic application, Chem. Eng. J., 309 (2017) 278-287. [2] L.Y. Li, L.Y. Cui, R.C. Zeng, S.Q. Li, X.B. Chen, Y. Zheng, M.B. Kannan, Advances in functionalized polymer coatings on biodegradable magnesium alloys - A review, Acta Biomater. (2018), in press https://doi.org/10.1016/j.actbio.2018.08.030. [3] G.L. Song, N.J. Dudney, J. Li, R.L. Sacci, J.K. Thomson, The possibility of forming a sacrificial anode coating for Mg, Corros. Sci. 87 (2014) 11-14. [4] L.Y. Cui, X.H. Fang, W. Cao, R.C. Zeng, S.Q. Li, X.B. Chen, Y.H. Zou, S.K. Guan, E.H. Han, In vitro corrosion resistance of a layer-by-layer assembled DNA coating on magnesium alloy, Appl. Surf. Sci. 457 (2018) 49-58. [5] Y.F. Zheng, X.N. Gu, F. Witte, Biodegradable metals, Mater. Sci. Eng., R 77 (2014) 1-34. [6] H. Hornberger, S. Virtanen, A.R. Boccaccini, Biomedical coatings on magnesium alloys - A review, Acta Biomater. 8 (2012) 2442-2455. [7] Q. Chen, G.A. Thouas, Metallic implant biomaterials, Mater. Sci. Eng., R 87 (2015) 1-57. [8] X.M. Liu, Q.Y. Yang, Z.Y. Li, W. Yuan, Y.F. Zheng, Z.D. Cui, X.J. Yang, K.W.K. Yeung, S.L. Wu, A combined coating strategy based on atomic layer deposition for enhancement of corrosion resistance of AZ31 magnesium alloy, Appl. Surf. Sci. 434 (2018) 1101-1111. [9] C.Q. Li, D.K. Xu, Z.R. Zeng, B.J. Wang, L.Y. Sheng, X.B. Chen, E.H. Han, Effect of volume fraction of LPSO phases on corrosion and mechanical properties of Mg-Zn-Y alloys, Mater. Des. 121 (2017) 430-441. [10] R.C. Zeng, L.Y. Cui, W. Ke, Biomedical magnesium alloys: composition, microstructure and corrosion, Acta Metall. Sin. 54 (2018) 1215-1235. [11] J. Sun, Y. Zhu, L. Meng, P. Chen, T. Shi, X. Liu, Y. Zheng, Electrophoretic deposition of colloidal particles on Mg with cytocompatibility, antibacterial performance, and corrosion resistance, Acta Biomater. 45 (2016) 387-398. 18

[12] L. Guo, W. Wu, Y. Zhou, F. Zhang, R. Zeng, J. Zeng, Layered double hydroxide coatings on magnesium alloys: A review, J. Mater. Sci. Technol. 34 (2018) 1455-1466. [13] L.Y. Cui, Y. Hu, R.C. Zeng, Y.X. Yang, D.D. Sun, S.Q. Li, F. Zhang, E.H. Han, New insights into the effect of Tris-HCl and Tris on corrosion of magnesium alloy in presence of bicarbonate, sulfate, hydrogen phosphate and dihydrogen phosphate ions, J. Mater. Sci. Technol. 33 (2017) 971-986. [14] N. Ostrowski, B. Lee, N. Enick, B. Carlson, S. Kunjukunju, A. Roy, P.N. Kumta, Corrosion protection and improved cytocompatibility of biodegradable polymeric layer-by-layer coatings on AZ31 magnesium alloys, Acta Biomater. 9 (2013) 8704-8713. [15] J.L. Wang, S. Mukherjee, D.R. Nisbet, N. Birbilis, X.B. Chen, In vitro evaluation of biodegradable magnesium alloys containing micro-alloying additions of strontium, with and without zinc, J. Mater. Chem. B. 3 (2015) 8874-8883. [16] L.Y. Cui, L. Sun, R.C.Zeng, Y.F. Zheng, S.Q. Li, In vitro degradation and biocompatibility of Mg-Li-Ca alloys - the influence of Li content, Sci. China Mater. 61 (2018) 607-618. [17] 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, G. Chen, K.K. Deng, H.Y. Wang, What is going on in magnesium alloys?, J. Mater. Sci. Technol. 34 (2018) 245-247. [18] Z.Y. Ding, L.Y. Cui, X.B. Chen, R.C. Zeng, S.K. Guan, S.Q. Li, F. Zhang, Y.H. Zou, Q.Y. Liu, In vitro corrosion of micro-arc oxidation coating on Mg-1Li-1Ca alloy - The influence of intermetallic compound Mg2Ca, J. Alloys Compd. 764 (2018) 250-260. [19] Q.S. Yao, F. Zhang, L. Song, R.C. Zeng, L.Y. Cui, S.Q. Li, Z.L. Wang, E.H. Han, Corrosion resistance of a ceria/polymethyltrimethoxysilane modified Mg-Al-layered double hydroxide on AZ31 magnesium alloy, J. Alloys Compd. 764 (2018) 913-928. [20] J.Y. Chen, X.B. Chen, J.L. Li, B. Tang, N. Birbilis, X. Wang, Electrosprayed PLGA smart containers for active anti-corrosion coating on magnesium alloy AMlite, J. Mater. Chem. A 2 (2014) 5738–5794. [21] Y.B. Zhao, H.P. Liu, C.Y. Li, Y. Chen, S.Q. Li, R.C. Zeng, Z.L. Wang, Corrosion resistance and adhesion strength of a spin-assisted layer-by-layer assembled coating on AZ31 magnesium alloy, Appl. Surf. Sci. 434 (2018) 787-795. [22] X.B. Chen, N. Birbilis, T.B. Abbott, Effect of [Ca 2+] and [PO43-] levels on the formation of calcium phosphate conversion coatings on die-cast magnesium alloy AZ91D, Corros. Sci. 55 (2012) 226–232. [23] J. Li, Y. Song, S. Zhang, C. Zhao, F. Zhang, X. Zhang, L. Cao, Q. Fan, T. Tang, In vitro responses of human bone marrow stromal cells to a fluoridated hydroxyapatite coated biodegradable Mg-Zn alloy, Biomaterials 31 (2010) 5782-5788. [24] K. Ronan, M.B. Kannan, Novel Sustainable Route for Synthesis of Hydroxyapatite Biomaterial from Biowastes, ACS Sustain. Chem. Eng. 5 (2017) 2237-2245. [25] X. Zhang, X.W. Li, J.G. Li, X.D. Sun, Preparation and characterizations of bioglass ceramic cement/Ca-P coating on pure magnesium for biomedical applications, ACS Appl. Mater. Interfaces. 6 (2014) 513-525. [26] N. Metoki, K. Sadman, K. Shull, N. Eliaz, D. Mandler, Electro-Assisted Deposition of Calcium Phosphate on Self-Assembled Monolayers, Electrochim. Acta. 206 (2016) 400-408. [27] P.P. Wu, Z.Z. Zhang, F.J. Xu, K.K. Deng, K.B. Nie, R. Gao, Effect of duty cycle on preparation and corrosion behavior of electrodeposited calcium phosphate coatings on AZ91, Appl. Surf. Sci. 426 (2017) 418-426. [28] H. Tang, Y. Han, T. Wu, W. Tao, X. Jian, Y.F. Wu, F.J. Xu, Synthesis and properties of 19

hydroxyapatite-containing coating on AZ31 magnesium alloy by micro-arc oxidation, Appl. Surf. Sci. 400 (2017) 391-404. [29] S.T. Jiang, J. Zhang, S.Z. Shun, M.F. Chen, The formation of FHA coating on biodegradable Mg-Zn-Zr alloy using a two-step chemical treatment method, Appl. Surf. Sci. 388 (2016) 424-430. [30] L. Chang, L. Tian, W. Liu, X. Duan, Formation of dicalcium phosphate dihydrate on magnesium alloy by micro-arc oxidation coupled with hydrothermal treatment, Corros. Sci. 72 (2013) 118-124. [31] Q. Wang, M. Wang, X. Lu, K. Wang, L. Fang, F. Ren, G. Lu, Effects of atomic-level nano-structured hydroxyapatite on adsorption of bone morphogenetic protein-7 and its derived peptide by computer simulation, Sci Rep. 7 (2017) 15152. [32] S.K. Min, H.K. Kang, S.Y. Jung, D.H. Jang, B.M. Min, A vitronectin-derived peptide reverses ovariectomy-induced bone loss via regulation of osteoblast and osteoclast differentiation, Cell Death Differ. (2017). [33] Y. Nakamura, T. Suzuki, M. Kamimura, K. Murakami, S. Ikegami, S. Uchiyama, H. Kato, Vitamin D and calcium are required at the time of denosumab administration during osteoporosis treatment, Bone Res. 5 (2017) 17021. [34] M.L. Zoch, D.S. Abou, T.L. Clemens, D.L. Thorek, R.C. Riddle, In vivo radiometric analysis of glucose uptake and distribution in mouse bone, Bone Res. 4 (2016) 16004. [35] X.Y. Wang, S. Li, G. Wang, Z.L. Ma, M. Chuai, L. Cao, X. Yang, High glucose environment inhibits cranial neural crest survival by activating excessive autophagy in the chick embryo, Sci Rep. 5 (2015) 18321. [36] L.Y. Li, B. Liu, R.C. Zeng, S.Q. Li, F. Zhang, Y.H. Zou, H.G. Jiang, X.B. Chen, S.K. Guan, Q.Y. Liu, In vitro corrosion of magnesium alloy AZ31 - a synergetic influence of glucose and Tris, Front. Mater. Sci. 12 (2018) 184-197. [37] M. Maycas, M.T. Portolés, M.C. Matesanz, I. Buendía, J. Linares, M.J. Feito, D. Arcos, M. Valletregí, L. Plotkin, P. Esbrit, High glucose alters the secretome of mechanically stimulated osteocyte-like cells affecting osteoclast precursor recruitment and differentiation, J. Cell. Physiol. 232 (2017) 3611. [38] R.C. Zeng, X.T. Li, S.Q. Li, F. Zhang, E.H. Han, In vitro degradation of pure Mg in response to glucose, Sci Rep. 5 (2015) 13026. [39] Y. Wang, L.Y. Cui, R.C. Zeng, S.Q. Li, Y.H. Zou, E.H. Han, In vitro degradation of pure magnesium - The effects of glucose and/or amino ccid, Materials (Basel) 10 (2017) 725. [40] M.M. Monteiro, N.C.C.D. Rocha, A.M. Rossi, G.D.A. Soares, Dissolution properties of calcium phosphate granules with different compositions in simulated body fluid, J. Biomed. Mater. Res., Part A. 65 (2003) 299-305. [41] H. Wang, S. Zhu, L. Wang, Y. Feng, X. Ma, S. Guan, Formation mechanism of Ca-deficient hydroxyapatite coating on Mg-Zn-Ca alloy for orthopaedic implant, Appl. Surf. Sci. 307 (2014) 92-100. [42] J.E. Gray-Munro, M. Strong, The mechanism of deposition of calcium phosphate coatings from solution onto magnesium alloy AZ31, J. Biomed. Mater. Res., Part A. 90A (2009) 339-350. [43] X. Lu, Y. Leng, Theoretical analysis of calcium phosphate precipitation in simulated body fluid, Biomaterials 26 (2005) 1097-1108. [44] K.S. Tenhuisen, P.W. Brown, Effects of magnesium on the formation of calcium-deficient hydroxyapatite from CaHPO4·2H2O and Ca4(PO4)2O, J. Biomed. Mater. Res. 36 (1997) 306. [45] Y.Y. Zhu, G.M. Wu, Y.H. Zhang, Q. Zhao, Growth and characterization of Mg(OH)2 film on 20

magnesium alloy AZ31, Appl. Surf. Sci. 257 (2011) 6129-6137. [46] H. Zhang, M. Liu, H. Fan, X. Zhang, An efficient method to synthesize carbonated nano hydroxyapatite assisted by poly(ethylene glycol), Mater. Lett. 75 (2012) 26-28. [47] H.P. Teng, C.J. Yang, J.F. Lin, Y.H. Huang, F.H. Lu, A simple method to functionalize the surface of plasma electrolytic oxidation produced TiO2 coatings for growing hydroxyapatite, Electrochim. Acta. 193 (2016) 216-224. [48] R.C. Zeng, X.T. Li, L.J. Liu, S.Q. Li, F. Zhang, In vitro degradation of pure Mg for esophageal stent in artificial saliva, J. Mater. Sci. Technol. 32 (2016) 437-444. [49] J. Tong, X. Han, S. Wang, X. Jiang, Evaluation of structural characteristics of huadian oil shale kerogen using direct techniques (Solid-State13C NMR, XPS, FT-IR, and XRD), Energy Fuels. 25 (2011) 4006-4013. [50] Z.Q. Yao, Y. Ivanisenko, T. Diemant, A. Caron, A. Chuvilin, J.Z. Jiang, R.Z. Valiev, M. Qi, H.J. Fecht, Synthesis and properties of hydroxyapatite-containing porous titania coating on ultrafine-grained titanium by micro-arc oxidation, Acta Biomater. 6 (2010) 2816-2825. [51] Hongbo B. Lu, Charles T. Campbell, a. Daniel J. Graham, Buddy D. Ratner†, Surface characterization of hydroxyapatite and related calcium phosphates by XPS and TOF-SIMS, Anal. Chem. 72 (2000) 2886-2894. [52] P. Zoltowski, On the electrical capacitance of interfaces exhibiting constant phase element behaviour, J. Electroanal. Chem. 443 (1998) 149-154. [53] H.F. Nabavi, M. Aliofkhazraei, A.S. Rouhaghdam, Electrical characteristics and discharge properties of hybrid plasma electrolytic oxidation on titanium, J. Alloys Compd. 728 (2017) 464-475. [54] A. Zomorodian, M.P. Garcia, T. Moura e Silva, J.C. Fernandes, M.H. Fernandes, M.F. Montemor, Corrosion resistance of a composite polymeric coating applied on biodegradable AZ31 magnesium alloy, Acta Biomater 9 (2013) 8660-8670. [55] L.Y. Cui, R.C. Zeng, S.Q. Li, F. Zhang, E.H. Han, Corrosion resistance of layer-by-layer assembled polyvinylpyrrolidone/polyacrylic acid and amorphous silica films on AZ31 magnesium alloys, RSC Adv. 6 (2016) 63107-63116. [56] K. Xia, H. Pan, T. Wang, S. Ma, J. Niu, Z. Xiang, Y. Song, H. Yang, X. Tang, W. Lu, Effect of Ca/P ratio on the structural and corrosion properties of biomimetic CaP coatings on ZK60 magnesium alloy, Mater. Sci. Eng. C 72 (2017) 676-681. [57] L. Xu, F. Pan, G. Yu, L. Yang, E. Zhang, K. Yang, In vitro and in vivo evaluation of the surface bioactivity of a calcium phosphate coated magnesium alloy, Biomater. 30 (2009) 1512-1523.

21

Captions of Figures

Fig. 1. Schematic illustrations of the preparation of Ca-P and Mg(OH)2 composite coating on pure Mg.

Fig. 2. SEM morphologies of (a, b) Ca-P and Mg(OH)2 composite coating and (c, d) glucose-free samples (controls)

Fig. 3. Cross-sectional morphologies of (a) glucose-free samples (controls), (b) Ca-P and Mg(OH)2 composite coating induced by glucose and its elements mapping images of (c) Mg, (d) C, (e) P, (f) Ca, (g) O and (h) line scanning image.

Fig. 4. SEM images of the Ca-P and Mg(OH)2 composite coating induced by glucose for different reaction time durations, (a, b) 20 min, (c, d) 1 h and (e, f) 2 h.

Fig. 5. Corresponding EDS mapping of the Fig 4. (a, c, e). Ca-P and Mg(OH)2 composite coating induced by glucose for different reaction time, (a-e) 20 min; (f-j) 1 h and (k-o) 2 h.

Fig. 6. XRD patterns of (a) pure Mg, (b) glucose-free samples (controls) and (c) Ca-P and Mg(OH)2 composite coating induced by glucose.

Fig. 7. FTIR spectra of (a) glucose-free samples (controls) and (b ) Ca-P and Mg(OH)2 composite coating induced by glucose.

Fig. 8. XPS analysis of the glucose induced Ca-P and Mg(OH)2 composite coating on Mg substrate. (a) Broad survey, high-resultion survey of (b) C 1s spectra, (c) Ca 2p spectra; and (d) Ca 2p spectra of the glucose-free samples (controls).

22

Fig. 9. EIS and the fitted results for (I) Pure Mg substrate, (II)glucose-free samples (controls) and (III) Ca-P and Mg(OH)2 composite coating induced by glucose. (a) Nyquist plot and (b) enlarged Nyquidt plots, (c) Bode plots of IZI vs. frequency in hank’s solution; equivlent circuits of (d) Mg substrate and (e) Ca-P and Mg(OH)2 composite coating for glucose-induced and glucose-free.

Fig. 10. Polarization curves of (a) the Mg substrate, (b) Ca-P and Mg(OH)2 composite coating induced by glucose and (c) glucose-free samples (controls) . Fig. 11. Nanoscratch tests results of the (a) glucose-free samples (controls) and (b) Ca-P and Mg(OH)2 composite coating induced by glucose. Fig. 12. Hydrogen evolution rates of the (a) pure Mg substrate, (b) glucose-free samples (controls)and (c) Ca-P and Mg(OH)2 composite coating induced by glucose. Fig. 13. SEM morphologies (a-f) of the samples after 336 h immersion; (a, b) the glucose induced coating, (c, d) glucose-free samples (controls) and (e, f) pure Mg substrate.

Fig. 14. EDS analysis of the samples after 336 h immersion. The number of 1-6 corresponding to Fig. 13. Fig. 15. FTIR spectra of (a) pure Mg substrate, (b) glucose-free samples (controls) and (c) glucose induced Ca-P and Mg(OH)2 composite coating in Hank’s sloution after 336 h of immersion.

Fig. 16. XRD patterns of (a) pure Mg substrate, (b) glucose-free samples (controls) and (c) glucose induced Ca-P and Mg(OH)2 composite coating immersed in Hank’s solution after 336 h.

Fig. 17. Schematic illustration of the glucose induced Ca-P and Mg(OH)2 composite coating via hydrothermal treatment on pure Mg. 23

Tables Table 1 EDS analysis data of Ca-P and Mg(OH)2 composite coatings in Fig. 1. point

C

O

Mg

Ca

P

Ca/P molar ration

wt%

at%

wt%

at%

wt%

at%

wt%

at%

wt%

at%

1

13.73

22.93

40.70

51.02

0.05

0.04

23.48

11.75

22.04

14.27

0.82

2

7.41

14.11

33.78

48.27

0.03

0.03

34.53

19.70

24.24

17.89

1.10

3

8.30

14.12

46.68

59.64

0.01

0.01

23.13

11.79

21.89

14.44

0.82

4

5.46

9.29

51.32

65.50

-

-

21.95

11.18

21.27

14.02

0.80

5

2.99

6.89

20.35

35.28

-

-

53.17

36.79

23.50

21.04

1.75

6

4.18

9.30

20.20

33.73

-

-

41.99

27.99

33.62

28.99

0.97

Table 2 Electrochemical data obtained by equivalent circuit fitting of the EIS curves Sampl

Rs(Ω

Q1(Ω-1·

n

Rc1(Ω

Q2(Ω-1·

n

Rc2(Ω

C(F·

Rct(Ω

Q3(Ω-1·

RL(Ω

L(H·

2

e

·cm2

cm-2·s-1

1

·cm2)

cm-2·s-1

2

·cm2)

cm-2

·cm2)

cm-2·s-1

·cm2)

cm2)

10-

)

)

74.6

1.9×10-

0

5

.

Substr ate(I)

) 39.8

)

2.0×10-

0

5

.

7 Gluco

59.3

se-fre

3.3×10-

0

6

.

e (II) Gluco se(III)

143.

9.2×10

2

7

0 .

-

409.5

8.0×10-

3643

84.3

5.9

5

8 1161

3.5×10-

0

5

.

5 -

-

4

)

1695

4.5×

815.2

-

-

-

2.5

2390

-

-

-

7.1

10-5

7 1486

3.6×10

-

5

6

0

5058

3.9× -5

.

10

5

Table 3 Electrochemical parameters of the polarization curves. Samples

Ecorr (mVSCE)

jcorr (A·cm-2)

a (mV·dec-1)

-c (mV·dec-1)

Rp (Ω·cm2)

Mg (a) Glucose-free (b) Glucose (c)

-1780 -1428 -1495

2.36×10-5 1.39×10-5 6.79×10-6

406.82 207.21 239.96

273.18 341.85 396.77

3.01×106 4.03×106 9.56×106

26

Graphical Abstract

Utilizing glucose as “catalyst” can boost the formation of Ca-P barrier coating on Mg alloys to address their corrosion challenges. The yielded coatings exhibit a highly compact morphology with numberous individual block-shape paricles as buliding unit.

24



A novel method for preparation of Ca-P coating on Mg via glucose is proposed.



An alkaline precursor solution instead of acidic solution for preparation of Ca-P coating is used.



The Ca-P coating exhibits a refined and compact morphology in presence of glucose.



The glucose-induced Ca-P coating on Mg surface shows better corrosion resistance.



The presence of glucose is favorable to the nucleation of Ca-P compounds on Mg.

25