Controlling the rate of degradation of Mg using magnesium fluoride and magnesium fluoride-magnesium phosphate duplex coatings

Mohn Sthyrj P, Ph.D, Rvichndrn K, Ph.D,?, Snkr Nrynn TSN, Ph.D,b

a Department of Analytical Chemistry, University of Madras, Chennai-600 025, India

b Department of Mechanical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Eonyang-eup, Ulju-gun, Ulsan,689-798, Republic of Korea

Abstract The thin and porous Fluoride Conversion Coating FCC with many cracks could not offer a significant improvement in corrosion resistance for Mg.Magnesium phosphate coating improves the corrosion resistance of Mg, good bioactivity, promotes cell viability and cyto-compatibility and exhibits antibacterial activity.However, rapid dissolution in Mg in acidic magnesium phosphate containing solutions leads to the development of an inhomogeneous coating.The present study attempts to prevent the excessive dissolution of Mg by forming a fluoride conversion coating as a pre-treatment in the first stage followed by deposition of magnesium phosphate coating in the second stage to develop magnesium fluoride-magnesium phosphate duplex coatings.The morphological features, structural characteristics, nature of functional groups, corrosion behavior in Hanks’ balanced salt solution and bioactivity in simulated body fluid are assessed to ascertain the suitability of the magnesium fluoride-magnesium phosphate duplex coating in controlling the rate of degradation of Mg and improving its bioactivity using uncoated Mg and fluoride conversion coated Mg as reference.The findings of the study reveal that the magnesium fluoride-magnesium phosphate duplex coating could offer an excellent corrosion resistance and improve the bioactivity of Mg.

Keywords: Fluoride conversion coating; Magnesium phosphate coating; Duplex coating; Corrosion resistance; Bioactivity.

1.Introductio n

Magnesium and its alloys are considered as the green material of the 21stcentury and received considerable attention in many fields including automotive, aerospace and biomedical [1].The rapid rate of degradation of Mg and its alloys,accumulation of hydrogen evolution and local alkalinization of the medium are the major limitations in using Mg and its alloys for biomedical applications.Surface modification is a viable approach to control the rate of degradation of Mg and its alloys.Among the various methods, chemical conversion coating is simple, cost-effective and versatile and has the ability to treat even components with complex designs such as implants and stents.The chemical conversion coatings are uniform and highly adherent.The ability of chemical conversion coatings to offer corrosion protection for Mg and its alloys is highlighted by Chen et al.[2]Besides the inorganic type conversion coatings such as chromate, phosphate, etc.,organic conversion coatings and rare earth-based conversion coatings have also been developed for Mg and its alloys[3,4].The challenges and opportunities of various types of conversion coatings on Zn are addressed by Guo et al.[5]

Formation of fluoride conversion coatings (FCC) on Mg and its alloys is well-established.FCC formed on Mg and its alloys reduced the corrosion rate, controlled localized increase in pH and accumulation of hydrogen gas, exhibited good biocompatibility, facilitated cell adhesion and proliferation, promoted formation of calcium apatites and bone healing, restricted biofilm formation and exhibit no cytotoxic effect [6-15].In spite of these advantages, FCC has not beenwidely accepted as a suitable coating to modify the surface of Mg and its alloys for biomedical applications due to the following reasons: FCC is thin (?1 to 3 μm), porous and consists of many cracks in the coated layer.Hence, FCC could not offer a significant improvement in corrosion resistance for Mg in HBSS or SBF.A two-step alkali-fluoride treatment, which involves formation of a thick Mg(OH)2layer on Mg and its alloys during the first step and subsequently converting the Mg(OH)2to MgF2in the second step has been proposed as an alternative way to increase the thickness of the MgF2coating [16].Some modifications in the methodology of deposition of FCC such as galvanically coupling Mg with more electropositive metal like Pt, agitation of electrolyte and addition of potassium carbonate are explored to increase the kinetics of deposition and to tailor the chemical composition of FCC [17].Nevertheless, the coating thickness could not be increased to a large extent.It has been shown that the FCC actually consists of both MgF2and Mg(OH)2-xFxand complete conversion of the later compound to MgF2takes more time [6-15,17].Hence, in spite of an increase in treatment time even up to 168 h, the thickness of the FCC is not increased.The slow reaction kinetics could be due to the passivation ability of MgF2as well as due to the slow conversion of Mg(OH)2-xFxto MgF2.However, FCC has been used as a pretreatment for subsequent deposition of bioactive coatings[18,19].

Magnesium phosphate coating could improve the corrosion resistance of Mg and its alloys.The corrosion current density of the magnesium phosphate coated AZ31 Mg alloy is decreased by more than two orders of magnitude when compared to the uncoated alloy [20-23].Phuong et al.[24]have reported that magnesium phosphate coated Mg offers a better corrosion resistance than zinc phosphate conversion coated Mg.The in situ growth of magnesium phosphate coating is considered responsible for its excellent adhesion with the base metal[15].Due to this attribute magnesium phosphate coating is used as a pretreatment for polyetherimide (PEI) layer [25].The magnesium phosphate coating facilitated the attachment between the PEI layer and Mg.It has been shown that even after the PEI layer breaks down during immersion in SBF,the underlying magnesium phosphate coating could limit the extent of corrosion of Mg.The magnesium phosphate coating exhibits good bioactivity, which is evidenced by the formation of apatite crystals on its surface within 72 h of immersion in SBF [25].Due to its ability to limit the extent of corrosion and accumulation of Mg2+ions, the magnesium phosphate coating promotes cell viability and cytocompatibility [23,25,26].The biocompatibility of magnesium phosphate in physiological media has already been established [27].The ability of magnesium phosphate (newberyite) coatings to exhibit antibacterial activity has also been established [28].

Magnesium phosphate coatings fabricated by the simple immersion method has encountered many problems [20,29].The phosphating bath is generally acidic and operated at relatively higher temperatures [30,31].Such experimental conditions deemed to be necessary since the Mg/Mg alloy should serve both as the source of Mg2+ions as well as to thesupport the formation of magnesium phosphate coating.The rapid and heterogeneous dissolution of Mg/Mg alloy has led to an inhomogeneous coating.Reducing the acidity and/or the temperature increased the processing time to more than 72 h,which limits the practical application of this method[20].The introduction of Mg(OH)2as a source of Mg though reduced the processing time, has led to the formation of magnesium phosphate coating with many cracks, which lowers its corrosion protective ability [24].To prevent excessive dissolution of Mg and to eliminate the formation of an inhomogeneous coating, the Mg can be pretreated using FCC.A combination of FCC and magnesium phosphate coating might work well to reduce the rate of degradation of Mg as well to increase its bioactivity and cell growth.In this perspective,the present paper aims at to prepare magnesium fluoride-magnesium phosphate duplex coatings on magnesium and to compare its ability to decrease the rate of corrosion of Mg and to promote bioactivity with that of the magnesium fluoride coated and uncoated Mg.

2.Materials and methods

2.1.Materials used and methodology of deposition

Magnesium (Mg) ingot (commercially pure grade)was cut to small samples with a dimension of 20 mm × 20 mm × 3 mm, polished using SiC coated abrasive papers, starting with a grit size of 600 followed by 800, 1200 and 1500, cleaned using acetone by ultrasonication and dried.The chemical composition of the electrolytes and operating conditions used for the formation of MgF2coating on Mg by chemical conversion method(CCM) and formation of MgHPO4coating over the surface of MgF2coated Mg is performed in two stages, are given in Table 1.The first stage treatment involves the formation of MgF2coating on Mg by CCM.Accordingly, the Mg sample was immersed in 100 ml of 48% HF at 27 ± 1°C for 24 h.The MgF2coated Mg sample was thoroughlyrinsed with deionized water to remove any acid residues and dried.The MgF2coated Mg sample prepared in stage I was used for the formation of magnesium phosphate coating by CCM in stage II.Magnesium chloride hexahydrate (20 g/L)and diammonium hydrogen orthophosphate (14 g/L) were prepared separately.The MgF2coated Mg sample was immersed in the magnesium chloride solution followed by the addition of the diammonium hydrogen orthophosphate solution.The initial pH and temperature of the solution mixture was 4.20 ± 0.10 and 27 ± 1 °C, respectively.The formation of magnesium phosphate was allowed to continue for different periods of time from 0.5, 1.0, 1.5 and 2.0 h.The magnesium fluoride-magnesium phosphate duplex coated Mg samples were rinsed with deionized water and dried.Deposition of magnesium phosphate coating on Mg in the first stage followed by fluoride conversion coating in the next stage has led to dissolution of the magnesium phosphate coating, resulting in the formation of fluoride conversion coating only.The schematic representation of the formation of MgF2coating and magnesium fluoride-magnesium phosphate duplex coating is shown in Fig.S1 (please refer supplementary file).

Table 1Chemical composition and operating conditions used for the deposition of MgF2 and MgF2?MgHPO4 duplex coating in two stages by chemical conversion method (CCM).

2.2.Evaluation of morphological features, phase contents and nature of functional groups

The morphological features of fluoride conversion coatings and magnesium fluoride-magnesium phosphate duplex coatings formed on Mg were examined using scanning electron microscopy (SEM) (Quattro FEGESEM with EDS, Thermo Fisher scientific, Singapore).The elemental composition of the tops surface layer was determined by energy X-ray analysis (EDS) attached with the SEM.The phases present in the fluoride conversion coatings and magnesium fluoridemagnesium phosphate duplex coatings formed on Mg were assessed by X-ray diffraction (XRD) measurement (Smart-Lab X-Ray Diffractometer, Rigaku Corporation, Japan) using Cu Kαradiation at 50 kV and 100 mA.Fourier-transform infrared (FT-IR) spectroscopy (Agilent CARY 630) was employed to ascertain the functional groups present in the magnesium fluoride-magnesium phosphate duplex coating.The details of sample preparation for recording the FT-IR spectrum are already addressed in our earlier paper [32].

2.3.Evaluation of corrosion behavior

The corrosion behavior of fluoride conversion coated Mg and magnesium fluoride-magnesium phosphate duplex coated Mg was evaluated by electrochemical studies as well as by static immersion test in comparison with that of the uncoated one.

(i)Electrochemical studies

A potentiostat/galvanostat/frequency response analyzer(CH Instruments; Model No: CHI 760D) was used to record the potentiodynamic polarization curves as well as the Nyquist and Bode plots by electrochemical impedance spectroscopy (EIS).A flat cell consisting of the uncoated/coated Mg sample as the working electrode, a graphite rod as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode and HBSS as the electrolyte maintained at 27 ± 1 °C was used for all the electrochemical measurements.The polarization curves were recorded at a scan rate of 100 mV min?1from ?0.25 V vs.SCE in the cathodic direction to +0.250 V vs.SCE in the anodic direction, from the open circuit potential (OCP).The electrochemical parameters were obtained from the polarization curves using Tafel extrapolation method and the corrosion rate is calculated using the corrosion current density values.The details of calculation of corrosion rate are already explained in our earlier paper [32].EIS studies were performed at their respective OCP in the frequency range between 10,000 Hz and 0.01 Hz with a perturbation voltage of ±10 mV (root mean square).ZSimpWin 3.21 software was used to analyze the Nyquist data and to propose an equivalent electrical circuit model.The electrochemical studies were repeated at least three times to ensure reproducibility of the test results.

(ii)Static immersion test

The uncoated Mg,fluoride conversion coated Mg and magnesium fluoride-magnesium phosphate duplex coated Mg samples were subjected to immersion in HBSS at 37 ± 1°C under static conditions, open to air, for 240 h.The volume-to-area ratio of the HBSS to the uncoated/coated Mg samples was maintained at 60 ml cm?2according to ASTM G31-72.After immersion for 240 h, the samples were carefully removed, rinsed in deionized water and dried.The change in morphological feature and elemental composition of the surface layer after immersion was determined by SEM and EDS, respectively.The corrosion products and remnant coatings were removed using a solution mixture of 200 gL?1CrO3and 10 gL?1AgNO3at 27 ± 1 °C for 15 min under ultrasonication to examine the extent of corrosion attack on uncoated as well as coated Mg.

2.4.Evaluation of in vitro bioactivity

The ability of the surface to promote apatite formation when comes is commonly referred as bioactivity.To evaluate the bioactivity, the uncoated Mg, fluoride conversion coated Mg and magnesium fluoride-magnesium phosphate duplex coated Mg samples were immersed in Simulated Body Fluid (SBF) with an ionic concentration almost equal to that in human blood plasma under a biometric condition at 37±1°C for 240 h.The SBF solution was refreshed every 48 h of immersion to maintain a proper balance of ionic concentration of the constituents.After 240 h of immersion, the coated Mg samples were removed, washed with deionized water and dried.The change in morphological features of the uncoated and coated Mg samples before and after immersion in SBF for 240 h was assessed using SEM.The change in elemental composition at the top surface layer was acquired usingEDS to understand the chemical reactions that could have occurred during immersion in SBF.To support and validate the changes that could have happened, the nature of functional groups present on the top surface layer was determined by FT-IR spectroscopy.

Fig.1.(a-d) Surface morphology, (c) EDS spectra and elemental composition acquired at the surface of MgF2 coating formed on Mg by CCM using 48%HF at 27 °C for 24 h.

3.Results and discussion

3.1.Surface morphology of MgF2 coating formed on Mg by CCM

The surface morphology of MgF2coating formed on magnesium by CCM using 48% HF at 27 °C for 24 h is shown in Fig.1.The coating is relatively thin, which is evident from the polishing marks underneath the coating (marked by arrow marks in (Figs.1(a) and 1(b)).The coating formation followed the polishing marks.In spite of the uniform surface coverage there are plenty of cracks on the coated layer.In addition, some fine pores (marked by a circle) are also evident (Fig.1(b)).Formation of cracks and pores in FCC has also been observed by other researchers [1-10,17,33].Higher magnification images reveal formation of clusters of particles (Fig.1(c)) and agglomeration of nano-sized particles(Fig.1(d)).

3.2.Elemental composition of MgF2 coating formed on Mg by CCM

The EDS spectrum and elemental composition acquired at the surface of MgF2coating formed on Mg by CCM using 48% HF at 27 °C for 24 h is shown in the inset of Fig.1(c).The EDS spectrum indicates the presence of F, O and Mg as the major elements.The presence of 6.48 at.% O points out that the FCC is not purely MgF2but it also contains othercompounds involving oxygen.The formation of Mg(OH)2is one of the possibilities.TheΔG of formation of MgF2and Mg(OH)2are -476.6 kJ/mol and -359.3 kJ/mol, respectively.A comparison of theΔG of formation of MgF2and Mg(OH)2suggest that both could form simultaneously.Since,a 48%HF is used to form the FCC, Mg(OH)2cannot be stable in such acidic medium.Hence, it can be inferred that FCC formed on Mg could consists of MgF2as well as Mg(OH)2-xFx[6-10,34].The Mg(OH)2-xFxfurther reacts with the 48% HF in which the OH groups are slowly replaced by F and the coating becomes completely MgF2when the x becomes 2.

3.3.Amount of magnesium phosphate coating formed over MgF2 coated Mg

The variation in coating weight of magnesium phosphate coating formed over MgF2coated Mg using 0.1 M MgCl2+ 0.1 M (NH4)2HPO4at 27 °C as a function of time is shown in Fig.2(a).A linear increase in coating weight is observed up to 1.5 h and a slight decrease in the rate of coating formation is noticed beyond 1.5 h.During the first 1.5 h,nucleation and growth of the magnesium phosphate occurs at a linear fashion.Being a conversion coating process, the rate of formation is likely to decrease following an increase in surface coverage with magnesium phosphate coating.In traditional conversion coating process, the Mg2+ions have to be generated through dissolution of the base metal.Hence,with an increase in treatment time, the availability of Mg2+ions become limited and this decreased the rate of increase in coating weight.In the present study, the Mg samples are pretreated with FCC, which would limit the extent of dissolution of Mg.However, the electrolyte solution used (0.1 M MgCl2)ensured a constant supply of Mg2+ions, which accounts for the linear increase in coating weight up to 1.5 h.The slight increase in coating weight observed between 1.5 h and 2.0 h is due to the increased surface coverage as well as etching of already formed magnesium phosphate by the mild acidic electrolyte solution (pH: 4.50).

3.4.Surface morphology magnesium phosphate coating formed over MgF2 coated Mg

The surface morphology of magnesium phosphate coatings formed over MgF2coated Mg at 27 °C for 0.5, 1, 1.5 and 2 h are shown in Figs.2(b-f).The morphological features indicate that the magnesium phosphate coating is composed of coarse sharp-edged polyhedral crystals.Ren et al.[26]and Jayaraj et al.[21]have also observed a similar morphological feature for newberyite and newberyite-struvite composite coatings formed on AZ31 Mg alloy.Boistelle and Abbona[35]have reported that the crystal habit and size of the magnesium phosphate crystals depends on the pH of the medium and growth time.Coatings formed at 0.5 h exhibit a random coverage with lack of coverage of phosphate coating (marked by a white arrow mark in Fig.2(b)) through which the underlying MgF2coating with a cracked morphology is visible.However, the uniformity of the coating is increased with an increase in treatment time from 0.5 to 2 h.In addition, the size of polyhedral crystals is decreased with an increase in treatment time from 0.5 to 2 h.A decrease in size of the newberyite crystals with an increase in treatment time was also observed earlier by Ren et al.[26]Coatings formed at 0.5 h show the formation of uneven crystal size and shape while those formed at 1 h exhibit the presence of rod-shaped(marked by black arrow marks in Fig.2(c)) as well as big polyhedral crystals.When the treatment time is increased to 1.5 h, both smaller and bigger crystals are formed while the uniformity is increased at 2 h.A higher treatment time is considered as beneficial to achieve a good surface coverage,new nuclei formation and its growth over a longer period of time [31].Coatings formed for 0.5, 1 and 1.5 h exhibit etching of the crystal faces (marked by a rectangle in Figs.2(b),2(c), and 2(d)) and the extent of etching is decreased with an increase in treatment time.On the contrary, no etching of the crystals could be observed for coatings formed for 2 h(Figs.2(e) and 2(f)).The etching of the crystal faces is believed to be due to the mildly acidic nature of the electrolyte(pH: 4.50).Ren et al.[26]have correlated dissolution of newberyite crystals as the main reason for shrinkage of crystal size upon prolonged treatment.

No cracks could be observed on the surface of magnesium phosphate coatings formed over MgF2coated Mg at 27°C for 0.5, 1, 1.5 and 2 h (Figs.2(b-f)).According to Zhao et al.[31]and Jayaraj et al.[21], cracks could be formed on magnesium phosphate coatings either due to the presence of Mg(OH)2or due to hydrogen evolution during the coating process.Formation of Mg(OH)2is most likely during conversion coating on Mg.In the present investigation, the Mg samples are pre-treated with a fluoride conversion coating.The presence of a MgF2coating could have limited the extent of dissolution of Mg and the formation of Mg(OH)2during the coating process.Since the electrolyte solution is mildly acidic(pH:4.50),the extent of hydrogen evolution will be limited.The voids between the polyhedral shaped crystals could have provided pathways for H2evolution, which could have avoided the formation of cracks on the coated surface.The pores presented on the coated surface(marked by a circle in Fig.2(c) and 2(d)) are due to the evolution of H2during the coating process.These pores could deleteriously influence the corrosion resistance of the coating.Zhao et al.[31]have also attributed the lack of cracks on the magnesium phosphate coating on AZ31 Mg alloy due to mild acidic nature of the electrolyte and due to the presence of voids in the flower-like crystals, which provides channels for H2evolution.

3.5.Elemental composition of magnesium phosphate coating formed overMgF2 coated Mg

The EDS spectrum and elemental composition acquired at the surface of magnesium phosphate coatings formed over MgF2coated Mg at 27 °C for 2.0 h are shown in the insert of Fig.2(f).The EDS spectrum indicates the presence of Mg,O, P and F as major elements.The presence of N could not be identified with confidence due to its low atomic number.Based on the elemental composition and atomic ratios of Mg, P and O, it is clear that the coating should be rich in newberyite (MgHPO4?3H2O).The possibility of formation of struvite cannot be ruled out completely.Boistelle and Abbona[35]have reported that newberyite is likely to form at relatively lower pH of the order of 4.20 while the formation of struvite requires a relatively higher pH.Jayaraj et al.[21]have reported that the struvite content is increased with an increase in pH of the electrolyte from 4.50 to 7.50.Since the pH the electrolyte used to prepare the magnesium phosphate coatings of the present study is 4.20 ± 0.10, the resultant coating should be rich in newberyite phase along with moderate

amounts of struvite.The presence of F can be traced back to the MgF2coating, which is formed in the first stage as a pretreatment.

Fig.2.(a) Variation in coating weight; and (b-f) surface morphology of MgHPO4 coatings formed on MgF2 coated Mg by CCM at 27 °C as a function of time (0.5 to 2 h): (b) MgHPO4 (0.5 h); (c) MgHPO4 (1.0 h); (d) MgHPO4 (1.5 h); and (e and f) MgHPO4 (2 h); (f) lower magnification image of ‘e’ along with the elemental composition (given in the inset of ‘f’).

3.6.Structural characteristics of magnesium phosphate coatings formed over MgF2 coated Mg

The XRD pattern of magnesium phosphate coatings formed over MgF2coated Mg (2 h), are shown in Fig.3(a).The XRD pattern reveals the presence of peaks pertaining to magnesium phosphate (newberyite,MgHPO4?3H2O) and magnesium ammonium phosphate (struvite, NH4MgPO4?6H2O).In addition, peaks pertaining to MgF2and Mg are also present.Jayaraj et al.[21]have observed the formation of both newberyite and struvite phases in the magnesium phosphate coatings deposited on AZ31 Mg alloy.According to Boistelle and Abbona [35], when the pH of the medium is＜5.80, only the newberyite (MgHPO4·3H2O)phase could occur.Ren et al.[26]have reported that for magnesium phosphate coatings prepared at lower temperatures,only the newberyite appears as a single phase.The magnesium phosphate coatings of the present study are prepared at 27 °C using an electrolyte with a pH of 4.20 ± 0.10.Hence, newberyite is expected to be the predominant phase in the coating.The magnesium phosphate coatings of the present study will hereafter be represented as MgHPO4.Jayaraj et al.[21]have also confirmed that the volume fraction of the struvite phase is increased with an increase in pH of the electrolyte from 4.50 to 7.50.The sharpness of the diffraction peaks suggests that the magnesium phosphate coating is highly crystalline, which substantiates the coarse sharp-edged polyhedral crystals observed from the morphological features of the magnesium phosphate coating(Figs.2(b-f)).It has been reported earlier that magnesium phosphate coatings formed under acidic conditions (pH: 5.60) are highly crystalline [26].

3.7.Nature of functional groups present in MgHPO4 coatings

The FT-IR spectra of MgHPO4coatings formed at 27 °C for 2 h is shown in Fig.3(b).The IR bands at 1015, 1070 and 1166 cm?1can be attributed to the triply degeneratedν3P-O asymmetric stretching vibrations of the PO43?group.The IR bands at 890 cm?1is due to the P-O-H out-of-plane bending while those observed at 1240 and 2480 cm?1can be correlated to P-O-H in-plane bending of HPO42-group.The O-H stretching vibrations of the HPO42-group are represented by the IR bands at 2330 and 2360 cm?1.The IR band at 690 cm?1can be related to theν4asymmetric bending vibration of CO32?group.The IR band at 1650 cm?1can be attributed to H-O-H bending vibration of H2O while those observed at 3270, 3480 and 3520 cm?1can be correlated to the O-H stretching vibration of H2O.During the formation of magnesium phosphate coatings on Mg using diammonium hydrogen phosphate, the possibility of struvite cannot be ruled out.The IR band at 2930 and 2400 cm?1could be attributed to the N-H stretching vibrations while the one observed at 1450 cm?1could be due to the N-H bending vibration [25,36,37].The N-H stretching vibration of the free NH groups is represented by the IR band at 3385 cm?1.The presence of characteristic bands pertaining to OH?, PO43?,HPO42-and N-H groups indicates that the magnesium phosphate coatings formed over MgF2coated Mg is likely to consist of newberyite and struvite phases.

3.8.Mechanism of formation of MgF2 and MgHPO4 coatings

When the Mg substrate is immersed in HF, a spontaneous reaction occurs between them, resulting in the formation of MgF2(Eq.(1)) and Mg(OH)2(Eq.(2)) on the surface of Mg.

After the MgF2coated sample is immersed in 0.1 M MgCl2?6H2O and 0.1 M (NH4)2solution at pH 4.30 ± 0.10,initially Mg2+ions could came out to the solution through the voids and cracks,leading to an increase in pH at the metal solution interface, which facilitates the formation of newberyite(MgHPO4) and struvite (MgNH4PO4?6H2O) over the surface of MgF2coated Mg sample.

3.9.Adhesive strength of MgF2 coating and MgF2?MgHPO4 duplex coating formed on Mg

The tensile-extension plots and interface bonding strength of MgF2coating and MgF2?MgHPO4duplex coating formed on Mg (2.0 h) are shown in Fig.4.The bonding strength of MgF2?MgHPO4duplex coating (13.46 MPa) is higher than the MgF2coating formed on Mg.The bonding strength of 13.46 MPa is slightly lower than the specified minimum bonding strength value of 15 MPa recommended for biomedical applications as per ISO 13,779-2 standard [38].During the initial stages of formation of conversion coatings, Mg(OH)2has been observed as one of constituents of the coating.Being a conversion coating, the Mg(OH)2layer is likely to promote adhesion of subsequently deposited layers [39,40].In this study, pretreatment of Mg by fluoride conversion coatings eliminates the formation of the Mg(OH)2layer.

Fig.3.(a) XRD pattern; and (b) FT-IR spectra of magnesium phosphate coatings formed over MgF2 coated Mg at 27 °C for 2 h.

Fig.4.Tensile-extension plots and bond strength of MgF2 coating and MgF2?MgHPO4 duplex coating formed on Mg.

3.10.Corrosion behavior of uncoated Mg, MgF2 coated Mg and MgF2?MgHPO4 duplex coated Mg in HBSS

3.10.1.Electrochemical studies

(i)Potentiodynamic polarization studies

The potentiodynamic polarization curves of uncoated Mg,MgF2coated Mg and MgF2?MgHPO4duplex coated Mg(2.0 h), in HBSS, are shown in Fig.5.The electrochemical corrosion parameters such as corrosion potential (Ecorr) and corrosion current density (icorr) are derived from the polarization curves by Tafel extrapolation method [41].The corrosion rate estimated using the icorrvalue is compiled and presented as a part of Fig.5.The formation of MgF2coating has led to a decrease in corrosion rate of Mg, which is evidenced by the shift in Ecorrfrom -1.79 ± 0.01 V to ?1.64 ± 0.01 V vs.SCE and a decrease in the icorrfrom 52.20 ± 5.16 μA/cm2to 14.20 ± 2.72 μA/cm2.The corrosion resistance offered by the MgF2?MgHPO4(2 h) duplex coating is much better than those provided by MgF2coating, which is substantiated by the shift in Ecorrfrom -1.79 ± 0.01 V to ?1.54 ± 0.01 V vs.SCE and a decrease in icorrfrom 52.20 ± 5.16 μA/cm2to 1.03 ± 0.42 μA/cm2.The large decrease in icorrvalue validates the ability of MgF2?MgHPO4(2 h) duplex coating to serve as a barrier layer, thus preventing the permeation of highly aggressive Cl?ions present in HBSS.

Coating weight/thickness, uniformity, porosity/defects,chemical composition and chemical stability of the coating exerts significant influence on the corrosion behavior of coated materials.Since the MgF2coating is very thin of the order of ?1 μm and consists of cracks and pores (Fig.1),it could not offer a better corrosion protective ability.The pores present in the MgF2coating also allows permeation of Cl?ions, leading to corrosion of Mg and delamination of the coated layer [17].The elemental composition of MgF2coating (Fig.1(c)) indicates the presence of Mg, F and O,which points out that the coating is indeed MgF2-x(OH)x.It has been established that the higher F/O ratio, the higher the corrosion [17].The coating weight of magnesium phosphate coating after 2 h of deposition is ?32 g/m2(Fig.2(a)),which could provide a good surface coverage over the MgF2coated Mg.Phuong et al.[42]have reported that magnesium phosphate coating offered an enhanced corrosion resistance for AZ31 Mg alloy.However, the coarse sharp-edged polyhedral crystals of magnesium phosphate as well the pores evolved during coating formation due to hydrogen evolution could provide channels for the permeation of aggressive Cl?ions through them (Figs.2(e-f)).Fortunately, no cracking of the coated layer or loose deposits could be observed for the MgF2?MgHPO4duplex coatings (Figs.2(e-f)).A thin inner layer of MgF2coupled with a thicker outer layer of MgHPO4proved to be a good combination in improving the corrosion resistance of Mg in HBSS.

(ii)Electrochemical Impedance Spectroscopic (EIS) studiesThe Nyquist plot of uncoated Mg in HBSS is shown in Fig.6(a).A capacity loop is observed in the high frequency region, which is due to the formation of a partially protective surface film.Mg(OH)2is identified as the most dominant cor-rosion product layer on the surface of uncoated Mg.Hence,the capacity loop at the high frequency region could be due to the formation of Mg(OH)2on the surface of Mg.The intermediate frequency region usually represents the charge transfer resistance and the double-layer capacitance at the metalsolution interface.Hence, the capacity loop that appears at the intermediate frequency region is due to the charge transfer resistance and the double-layer capacitance between Mg and HBSS.The inductive loop located at the low frequency region indicates the involvement of adsorbed intermediate corrosive species [32,43-54].It has been reported that the formation of an inducting loop during corrosion of Mg is a good indicator of the occurrence of localized corrosion of Mg [50-58].The capacitance loop at the high frequency region,the capacitance loop at the medium frequency region and an inductive loop at the low frequency region suggest the involvement of at least three time constants.

Electrochemical parameters derived from the potentiodynamic polarization curves of uncoated Mg, MgF2 coated Mg and MgF2?MgHPO4 duplex coated Mg in HBSS.

The Nyquist plots of MgF2coated Mg and MgF2?MgHPO4duplex coated Mg (Fig.6(b)) consist of only a single semicircle without any inductive loop, which suggest their ability to offer an excellent corrosion resistance to Mg.It has been established that a higher charge transfer resistance and a lower capacitance signify a better corrosion resistance of Mg in simulated body fluids [50,59-61].A comparison of Figs.6(a) and 6(b) reveals that both MgF2coated Mg and MgF2?MgHPO4duplex coated Mg (2.0 h)offer a better corrosion resistance in HBSS than those offered by the uncoated Mg.The extent of corrosion protection offered by the MgF2?MgHPO4duplex coated Mg is much superior than the MgF2coated ones.

The Bode impedance plots of uncoated Mg, MgF2coated Mg and MgF2?MgHPO4duplex coated Mg(2.0 h),in HBSS,recorded at their respective OCP’s, are shown in supplementary results Fig.S2(a) while the corresponding Bode phase angle plots are shown in supplementary results Fig.S2(b).It has been established that the |Z| value at 0.01 Hz in the Bode impedance plot and the phase angle maximum in Bode phase angle plot signify the corrosion protection efficiency.Accordingly,the higher|Z|value and higher phase angle maximum exhibited by MgF2?MgHPO4duplex coated Mg indicates its ability to provide a good corrosion protection for Mg in HBSS, followed by MgF2coated Mg and uncoatedMg.The ranking in extent of corrosion protection offered by the coated Mg inferred from EIS studies supports those observed from potentiodynamic polarization studies.

Fig.6.Nyquist plots of (a) uncoated Mg; (b) MgF2 coated Mg and MgF2?MgHPO4 duplex coated Mg in HBSS, recorded at their respective open circuit potentials along with the corresponding equivalent circle models.

After analyzing the Nyquist data, two different equivalent electrical circuit models are suggested that could correlate the corrosion behavior of uncoated and coated Mg (Inset of Figs.6(a)and 6(b)).The validity of these equivalent electrical circuit models are ascertained on the basis ofa ＜5% error of non-linear least square fitting of the experimental data.The electrochemical parameters after fitting the Nyquist datawith the respective equivalent electrical circuit are compiled in Table 2.The equivalent electrical circuit model suggested for corrosion of uncoated Mg in HBSS (inset of Fig.6(a))is similar to the one proposed to account for the corrosion behavior of AZ31 Mg alloy and CP-Mg in SBF and HBSS[62-65].The equivalent electrical circuit model suggested for the corrosion of MgF2coated Mg and MgF2?MgHPO4duplex coated Mg (Inset of Fig.6(b)) is similar to the one used by Singhbabu et al.[66]to explain the corrosion behavior of cold-rolled steel in 3.5% NaCl and by Mohan Sathyaraj et al.[32]to account for the corrosion behavior of carbonate conversion-polycaprolactone duplex coating in HBSS.In both the equivalent electrical circuit models, Rs, CPE, RLand L represent the solution resistance, constant phase element, inductive resistance and inductance, respectively.Since depressed semicircles are appeared in the Nyquist plots of both uncoated and coated Mg, a CPE is used instead of Cdl.Moreover, the CPE could effectively account for deviations in non-homogenous surface reactions, varying thickness and roughness,current fluctuations during corrosion process at the interface [21,53].

Table 2Electrochemical parameters derived from the Nyquist plots of uncoated Mg, MgF2 coated Mg and MgF2?MgHPO4 duplex coated Mg in HBSS recorded at their respective open circuit potentials.

3.10.2.Static immersion test in HBSS

(i)Volume of H2 evolved and change inpHduring immersion in HBSS

The volume of H2evolved and change in pH of the corrosive medium, are good indicators to estimate the extent of corrosion attack of Mg and its alloys [50,59-61,67].The volume of H2evolved and change in pH of HBSS during immersion of uncoated Mg and MgF2?MgHPO4duplex coated Mg in HBSS at 37 ± 1 °C, measured as a function of immersion time is shown in Fig.7(a).During corrosion, dissolution of Mg (Eq.(6)) and evolution of H2occurs.For both uncoated Mg as well as for MgF2?MgHPO4duplex coated Mg, there observed to be a steady increase in the volume of H2evolved with an increase in immersion time up to 240 h (Fig.7(a)).However, the extent of increase in the volume of H2evolved is significantly lower for MgF2?MgHPO4duplex coated Mg.For uncoated Mg, the entire surface is susceptible for corrosion attack by HBSS.In contrast,for MgF2?MgHPO4duplex coated Mg, the duplex coating provides a uniform surface coverage and acts as a barrier layer delaying the corrosion attack.Though the formation of Mg(OH)2layer on uncoated Mg is expected to limit the volume of H2evolved, the porous nature of the layer allows permeation of HBSS and hence the evolution of H2is sustained throughout the immersion time.The total volume of H2evolved for uncoated Mg amounts to 6.32 ml/cm2after 240 h of immersion while it is limited to 2.91 ml/cm2for MgF2?MgHPO4duplex coated Mg.The reduction of water (Eq.(7)) is responsible for the observed increase in pH (Fig.7(b)).For uncoated Mg, a rapid increase in pH is observed during the first 50 h of immersion in HBSS.Between 50 h and 100 h of immersion,there is a considerable decrease in the rate of increase in pH due to the formation of Mg(OH)2(Eq.(8)), which is identified as the main corrosion product during corrosion of Mg in HBSS.Beyond 100 h and up to 240 h of immersion, there observed to a slight increase in pH.This is due to the porous nature of Mg(OH)2layer which allows permeation of HBSS through them, thus facilitating corrosion of Mg.Dissolution of Mg(OH)2by the Cl?ions present in HBSS leading to the formation of soluble MgCl2which sustains the increase in pH (Eq.(9)).The reactions that occur during corrosion of uncoated/coated Mg in HBSS are given as below:

Fig.7.Change in (a) volume of H2 evolved; and (b) pH of HBSS measured during immersion of uncoated Mg and MgF2?MgHPO4 duplex coated Mg in HBSS at 37 ± 1 °C, as a function of immersion time.

A similar trend is also observed for MgF2?MgHPO4duplex coated Mg.However, the saturation point is quickly reached since most of the surface is already covered by the duplex coating.In addition, the rate of increase in pH is also decreased considerably as the duplex coating could serve as a barrier layer limiting the extent of dissolution of Mg in HBSS.The saturation in pH of HBSS is reached after 150 h of immersion for uncoated Mg, which is due to the accumulation of corrosion products (mostly Mg(OH)2).However, for MgF2?Mg phosphate duplex coated Mg, the saturation point is reached in ?75 h.This is due to the better surface coverage of the MgF2?Mg phosphate duplex coated Mg, which prevents the exposure of the base metal underneath the coating to HBSS.The corrosion occurs at the pores/defects in the coating and as soon as the corrosion products (Mg(OH)2)accumulates at these regions, a saturation point is reached.

(ii)Morphological features of the surface layer after immersion in HBSS

The change in morphological features of MgF2coated Mg and MgF2?MgHPO4coated Mg, after immersion in HBSS at 37 ± 1 °C for 10 days, are shown in Figs.8(a-d).When compared to the morphological features of the as-prepared MgF2coating on Mg (Fig.1) as well as MgF2?MgHPO4coating on Mg (Figs.2(e) and 2(f)), there is a considerable change in the surface morphology of these coatings after immersion in HBSS at 37 ± 1 °C for 10 days.The evolution of a cracked morphology along with accumulation of corrosion products all over the surface of MgF2coated Mg (Figs.8(a) and 8(b))reveals penetration of Cl?ions through the cracks and pores present in the MgF2coating.The dissolution of Mg and the following increase in pH of HBSS at the MgF2coated Mg-HBSS interface has led to the formation of Mg(OH)2, which precipitates on the entire surface of MgF2coated Mg.The rapid corrosion attack and accumulation of corrosion products point out the inability of MgF2coating to offer a better corrosion protection for Mg.The coarse sharp-edged polyhedral crystals of magnesium phosphate coating (Figs.8(c) and 8(d)) are completely changed to thin needle-like crystals.The surface also exhibits the formation of many cracks along with globular-like particles.The cracks could have originated due to the stress in the coated layer or they might have emerged during drying of the coating or exposure of the sample in the chamber during SEM analysis.However, when compared to the morphological features of MgF2coated Mg (Figs.8(a)and 8(b)), the extent of corrosion attack is rather limited on MgF2?MgHPO4duplex coated Mg (Figs.8(c) and 8(d)).(iii)Elemental composition acquired at the surface layer

The EDS spectra and elemental composition acquired at the surface of both MgF2coated Mg and MgF2?MgHPO4coated Mg after immersion in HBSS at 37 ± 1 °C for 10 days are shown in supplementary results Fig.S3(a).The EDS spectra of MgF2coated Mg indicates the presence of Mg, O and F as the major elements along with a smaller amount of C, Ca, P, Na and Cl.Elemental composition acquired at the surface of MgF2coated Mg after immersion in HBSS points out a large decrease in the F content from 62.99 at.%to 14.10 at.%, a moderate decrease in the Mg content from 29.53 at.%to 17.91 at.% and a large increase in the O content from 7.48 at.%to 57.21 at.%when compared to those before immersion.This inference suggests dissolution of a large portion of the MgF2coating in HBSS.The presence of smaller amounts of Ca (0.78 at.%) and P (1.78 at.%) implies the formation of calcium phosphates.The higher content of P than Ca suggests the formation of magnesium phosphates.

The EDS spectra of MgF2?MgHPO4duplex coated Mg after immersion in HBSS at 37 ± 1 °C for 10 days indicates the presence of Mg, O, Ca, P and F as the major elements along with a smaller amount of C, Na, and Cl (supplementary results Fig.S3(b)).Elemental composition acquired at the surface of MgF2?MgHPO4coated Mg after immersion in HBSS points out a large increase in F content from 2.11 at.%to 17.74 at.% and a moderate increase in Mg content from 11.78 at.% to 16.05 at.% when compared to those before immersion.The P content is slightly decreased from 9.28 at.%to 5.42 at.% while the O content exhibits a large decrease from 76.83 at.% to 53.39 at.%, after immersion in HBSS.These inferences suggest dissolution of a large portion of the magnesium phosphate coating in HBSS.The raise in interfacial pH following dissolution of magnesium phosphate coating has enabled precipitation of freshly precipitated calcium phosphate,which is evidenced by the slightly higher Ca (2.07 at.%) and P (5.42 at.%) contents.The presence of Na and Cl on the surface layer of both MgF2coated Mg and MgF2?MgHPO4coated Mg, after immersion in HBSS at 37 ± 1 °C for 10 days could have emerged from HBSS.The MgF2?MgHPO4coated Mg has not completely degraded and it offered a better corrosion protection for Mg when compared to MgF2coated Mg.

(iv)Extent of corrosion attack

The extent of corrosion attack of Mg after immersion in HBSS is a good indicator of the ability of the coating to limit the permeation of aggressive Cl?ions through them[68-70].The extent of corrosion attack of MgF2coated Mg and MgF2?MgHPO4coated Mg after immersion in HBSS at 37 ± 1 °C for 10 days followed by removal of corrosion products and remnant coatings is shown in Figs.8(e)and 8(f),respectively.Localized corrosion is found to be the predominant mechanism for MgF2coated Mg [71,72].The deep pitsformed on the surface of MgF2Mg(Fig.8(e))are likely to undermine its mechanical integrity,thus making it unsuitable for load bearing implant applications.For MgF2?MgHPO4duplex coated Mg, smaller pits spreading uniformly throughout the surface are observed with no obvious deep pits (Fig.8(f)).Hence, it is clear that MgF2coated Mg could offer a limited protection whereas the MgF2?MgHPO4coated Mg would provide sufficient protection towards the localized corrosion attack of Mg in HBSS.

Fig.8.Surface morphology of (a, b) MgF2 coated Mg: and (c, d) MgF2?MgHPO4 duplex coated Mg after immersion in HBSS at 37 ± 1 °C for 240 h.(e,f) Extent of localized corrosion attack on (e) MgF2 coated Mg; and (f) MgF2?MgHPO4 duplex coated Mg after removal of corrosion products and remnant coatings.

Fig.9.Change in morphological features of (a, b) MgF2 coated Mg; and (c, d) MgF2?MgHPO4 duplex coated Mg after immersion in SBF at 37 ± 1 °C for 240 h.

3.11.In vitro bioactivity of MgF2 and MgF2?MgHPO4 duplex coated Mg in SBF

(i) Morphology of the surface layer

The surface morphology of MgF2coated Mg and MgF2?MgHPO4duplex coated Mg after immersion in SBF at 37 ± 1 °C for 10 days are shown in Fig 9.When compared to the morphological features of the as-prepared MgF2coating on Mg (Fig.1) and MgF2?MgHPO4duplex coating on Mg (Fig.2e, 2f), there is a considerable change in the surface morphology after immersion in SBF at 37 ± 1 °C for 10 days (Fig.9).The appearance of globular-like particles suggests the formation of newly formed calcium phosphate particles [65].The corrosion process of Mg in SBF was studied earlier by Wang et al.[73], Walter and Bobby Kannan[74]and Ascencio et al.[75].According to them, dissolution of Mg in SBF enables an increase in pH, which leads to the precipitation of Mg(OH)2along with the nucleation of globular-like apatite particles on the surface of Mg.The cracks in the surface layer (Fig.9(a)) as well as removal of the MgF2coating at certain regions (Fig.9(b)) suggest the inability of MgF2coating to offer sufficient corrosion protection for Mg in SBF.Many needle-like crystals are formed on the surface of MgF2?MgHPO4duplex coated Mg after immersion in SBF (Fig.9(c)).In addition, cracking of the coated layer along with accumulation of corrosion products could beobserved (Figs.9(d)).This is due to the partial dissolution of magnesium phosphate coating in SBF.

(ii) Elemental composition acquired at the surface layer

The EDS spectra after immersion in SBF indicates the presence of Mg, O, Ca, P and F as the major elements,moderate amount of C and a small amount of Na and Cl on the surface of both MgF2coated Mg (Refer supplementary results Figs.S4(a)) and MgF2?MgHPO4duplex coated Mg (Refer supplementary results Figs.S4(b)).The elemental composition acquired at the surface of MgF2coated Mg after immersion in SBF points out a large decrease in the F content from 62.99 at.% to 23.81 at.%, a moderate decrease in the Mg content from 29.53 at.% to 16.72 at.% and a large increase in O content from 7.48 at.% to 44.03 at.%.The presence of Ca (3.28 at.%) and P (4.47 at.%) indicates the formation of calcium phosphates on the surface of MgF2coated Mg after immersion in SBF.The higher amount of P than Ca suggests the formation of magnesium phosphates along with calcium phosphates or a mixture of calcium magnesium phosphates.The elemental composition acquired at the surface of MgF2?MgHPO4duplex coated Mg after immersion in SBF points out a large increase in F content from 2.11 at.% to 19.90 at.% and a moderate increase in the Mg content from 11.78 at.% to 16.89 at.%.There is a large decrease in the O content from 76.83 at.% to 50.81 at.% and P content from 9.28 at.% to 1.86 at.%.The presence of a lower content of Ca (1.28 at.%) and P (1.86 at.%) indicates the formation of calcium phosphates is relatively low on the surface of MgF2?MgHPO4coated Mg after immersion in SBF.The presence of Na and Cl in the surface layer of MgF2?MgHPO4coated Mg could have emerged from SBF and validates the penetration of the coated layer by the aggressive Cl?ions.

4.Conclusion

The findings of the study reveal that MgF2coating formed on Mg by chemical conversion coating method could serves as an effective base for the deposition of MgHPO4coating.The morphology of MgHPO4coating formed over MgF2coated Mg indicates development of coarse sharp-edged polyhedral crystals with no cracks on their surface.The uniformity of the MgHPO4coating is increased while the size of polyhedral crystals is decreased with an increase in treatment time from 0.5 to 2.0 h.XRD measurement confirms the formation of newberyite and struvite phases along with magnesium fluoride phase in the MgF2?MgHPO4duplex coated Mg.The IR bands point out the presence of peaks pertaining to OH?, PO43?, HPO42-and N-H groups, further supporting the inferences made in XRD pattern.The bonding strength of MgF2?MgHPO4duplex coated Mg is 13.46 MPa,which is slightly lower than recommended value of 15 MPa for biomedical applications as per ISO 13,779-2 standard.The nobler shift in Ecorr, decrease in icorr, higher |Z|, higher phase angle maximum and absence of an inductive loop at low frequency region signify the ability of MgF2?MgHPO4duplex coated Mg to offer a better corrosion resistance in HBSS when compared to MgF2coated Mg and uncoated Mg.Long-term immersion test further substantiates the ability of MgF2?MgHPO4duplex coated Mg to provide a higher corrosion resistance for Mg in HBSS.Analysis of the surface layer formed on Mg after long-term immersion in HBSS shows retention of a part of the MgF2?MgHPO4duplex coating along with the formation of newly formed calcium phosphates.XRD pattern confirms the formation of calcium phosphate and calcium magnesium phosphate phases, which is further supported by the presence of various stretching and bending vibrations of HPO42?, PO43-and OH?groups in the FT-IR spectra.Localized corrosion is found to be the predominant mode of corrosion attack irrespective of whether the Mg is coated or not.However, the nature of pits developed on coated and uncoated Mg differs a lot; uncoated Mg exhibits the generation of deeper pits whereas the pits are rather shallow on MgF2?MgHPO4duplex coated Mg.The appearance of globular-like particles and needle-shaped crystals after immersion in SBF at 37 ± 1 °C for 10 days as well as the identification of Ca and P in the EDS analysis signify the ability of MgF2?MgHPO4duplex coating to exhibit a better bioactivity.The study concludes that MgF2?MgHPO4duplex coating is a suitable approach to control the rate of degradation of Mg towards the development of Mg-based degradable implants.

Acknowledgements

One of the authors, Dr.P.Mohan Sathyaraj, expresses his sincere thanks to University Grand Commission (UGC)for providing a research fellowship to support this research program under the non-net category.The characterization facilities viz., scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction, extended by Vels University, Pallavaram, Chennai, is gratefully acknowledged.The authors thank Mr.K.Krishnakumar, Vels University for his timely help in measurements.

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jma.2021.06.005.