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An insight into the effect of buffer layer on the electrochemical performance of MgF2 coated magnesium alloy ZK60
Accepted Manuscript An insight into the effect of buffer layer on the electrochemical performance of MgF2 coated magnesium alloy ZK60
Usman Riaz, Zia ur Rahman, Hassnain Asgar, Umair Shah, Ishraq Shabib, Waseem Haider PII: DOI: Reference:
S0257-8972(18)30332-3 doi:10.1016/j.surfcoat.2018.03.081 SCT 23261
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
7 February 2018 23 March 2018 26 March 2018
Please cite this article as: Usman Riaz, Zia ur Rahman, Hassnain Asgar, Umair Shah, Ishraq Shabib, Waseem Haider , An insight into the effect of buffer layer on the electrochemical performance of MgF2 coated magnesium alloy ZK60. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.03.081
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ACCEPTED MANUSCRIPT An insight into the effect of buffer layer on the electrochemical performance of MgF2 coated Magnesium alloy ZK60 Usman Riaza, Zia ur Rahmanb, Hassnain Asgara, Umair Shaha, Ishraq Shabiba,b, Waseem Haidera,b a
School of Engineering and Technology, Central Michigan University, Mount Pleasant 48859, USA b
Science of Advanced Materials, Central Michigan University, Mount Pleasant 48859, USA
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Corresponding Author: [email protected]
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Abstract
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Magnesium (Mg) has emerged as potential implant material owing to its property of biodegradation. The roadblock to the commercial use of Mg as implant material is its fast
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degradation in body fluids. The degradation of the Mg and its alloys can be retarded by surface
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coatings. In this work, the potential of MgF2 coating on the surface of Mg alloy ZK60 (Mg6.9Zn-0.8Zr) was evaluated for its corrosion properties. Two-step chemical conversion process
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was used to coat MgF2 on the surface of ZK60 alloy. In the first step, a secondary layer of
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Mg(OH)2 was introduced by boiling the samples in NaOH solution. In the second step, these samples were immersed in hydrofluoric acid to obtain MgF2 coating. SEM, IR Spectroscopy, and
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XRD were employed to confirm the formation of Mg(OH)2 and MgF2. The wettability tests
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showed an increase in surface hydrophobicity as a result of conversion treatment. The potentiodynamic polarization tests exhibited an improvement in the corrosion potential from -
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1.52 V vs. SCE to -1.49 V vs. SCE after two-step conversion treatment. Moreover, coated sample witnessed a noticeable drop in hydrogen evolution compared to untreated ZK60. For a better insight, the results were compared to the MgF2 coatings achieved on the surface of ZK60 without any buffer layer. The coating of MgF2 with a buffer layer of Mg(OH)2 on the surface of ZK60 exhibited a noble corrosion potential, controlled degradation, and nominal hydrogen evolution compared to the untreated ZK60.
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ACCEPTED MANUSCRIPT Keywords: Magnesium fluoride; Chemical conversion; Degradation; Hydrogen evolution; Potentiodynamic polarization.
1. Introduction The potential of magnesium (Mg) and its alloys as biodegradable implant material has been an
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attraction for the researchers looking for an optimized biocompatible and biodegradable material
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for implant applications. The density of Mg is 1.74 g/cm3 making it the lightest of all the engineering metals [1,2]. Mg has better strength/density ratio than stainless steel and titanium
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alloys that are commonly used implant materials [3] The elastic modulus of Mg (45 GPa) is slightly higher than the elastic modulus of bone (3-20 GPa)[4], significantly reducing the
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possibility of stress shielding of bone[4]. On the other hand, the elastic modulus of iron (Fe) is
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211.4 GPa making it and its alloys much stiffer than bone and vulnerable to stress shielding [4].
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The property of Mg that set it apart from other implant materials is biodegradation [4]. Mg implants possess the unique property of biodegradation in the physiological conditions reducing
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the risk of secondary surgery to remove the implant and long-term undesirable interactions between the implant and tissues [5,6]. Mg has been under investigation for the cardiovascular
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applications in the form of stents [7–11] and orthopedic applications in the form of rods, plates,
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pins, and screws [12–15]. The issue associated with the Mg implants is the fast degradation in physiological conditions and subsequent loss of mechanical strength leading to implant failure [4,16–18]. Apart from losing mechanical strength, another issue related to the fast degradation of Mg is the evolution of Hydrogen (H2) gas [19,20].The evolution of H2 with the degradation in an aqueous solution is governed by the given reaction. Mg + 2H2O → Mg(OH)2 + H2 The anodic and cathodic reactions are following: 2
ACCEPTED MANUSCRIPT Mg → Mg2+ + 2e2H2O + 2e- → H2 + 2OHMg2+ + 2OH- → Mg(OH)2 The H2 gas accumulated in the gas pockets can severely damage the tissues close to the implant [20]. In addition to the tissue damages, the gas bubbles can block the regular flow of blood that
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addressed before the commercial use of Mg in implant applications.
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put the life of the patient at risk [20]. The rapid degradation of Mg and H2 evolution needs to be
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The surface treatment and alloy formation are common methods to improve the properties of Mg
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alloys. The surface treatment of Mg and its alloys is a relatively simple method as compared to alloying to control the rapid degradation. In surface treatment, the aim is to achieve a protective
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coating on the surface of Mg to slow down the degradation process and increase the corrosion resistance. Several coating methods are currently in practice to improve the corrosion resistance
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of magnesium. These methods include chemical conversion [21–23], micro arc oxidation [24,25],
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anodization [26,27], ion implantation [28], electrochemical deposition [17] and physical vapor
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deposition [29].
In chemical conversion, the specimens to be coated are immersed in a treating solution. The
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surface of the specimen reacts with the species solution and form Mg compounds on the surface.
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The layer of Mg compounds acts as a protective film to decelerate corrosion. The quality of protective film depends on multiple factors including the composition of treating solution, alloy type, and experimental parameters such as temperature, pre-treatments, and post-treatment [30,31]. Calcium Phosphate (Ca-P) [22,23,32,33] and fluoride [34–39] based surfaces are the most common coatings achieved by this method. Magnesium fluoride (MgF2) coatings are formed on the surface of Mg by immersing the Mg specimen in hydrofluoric acid (HF) [36]. The resistive nature of MgF2 coating has been proved in 3
ACCEPTED MANUSCRIPT the literature [37,40]. Additionally, the daily recommended intake of fluoride in the body is 2-5 mg [40] reducing the risk of the adverse effects of degradation of the MgF2 layer[40]. The biocompatibility of MgF2 coating on Mg-Nd-Zn-Zr was studied by the cytotoxicity evaluation of human umbilical vein endothelial cells (HUVEC) in endothelial basal medium (EBM). The MgF2 treated sample extracts exhibited better cell viability than untreated samples[41]. The corrosion
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resistance, non-toxic nature and a simple method of treating are the main attractions of MgF2
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coatings by the conversion process.
The effects of MgF2 protective layers on Mg-Zn-Zr alloys have been of great interest in recent
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years [36,38,40]. Mg-3.2Zn-0.8Zr screws coated with MgF2 were evaluated for their corrosion
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resistance, mechanical properties, and biocompatibility. The MgF2 coated screws experienced an improvement in corrosion rate at the initial stage, higher yield and ultimate tensile strengths as
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compared to uncoated samples without producing any adverse biological effect [42]. Similarly,
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MgF2 coated Mg-3Zn-0.5Zr alloy was evaluated for its biological activity and the results suggested an improvement in the osteoblastic activity and bone formation [36]. In both these
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methods, Mg alloys were coated with MgF2 without any pre-treatment or buffer layer. Recently
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Mg-0.5Zn-0.45Zr (commonly known as ZK60) coated with HF acid with an etching pretreatment was studied and the coated specimens exhibited lower current densities and hydrogen
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evolution than that of untreated ZK60 [38]. In this work, the MgF2 layer was formed on the surface of Mg alloy ZK60 in a dual treatment process by first introducing a buffer layer of Mg(OH)2. To compare the results, MgF2 coating without a buffer layer was also investigated in this study. Although, the pretreatment by boiling Mg alloy samples in sodium hydroxide (NaOH) solution to achieve magnesium hydroxide (Mg(OH)2) layer has been widely studied in the literature but the effects of the intermediate layer of Mg(OH)2 on the surface properties of ZK60 haven't been investigated in the literature. The aim 4
ACCEPTED MANUSCRIPT of this work is to not only study the properties of the final MgF2 layer but also evaluate the buffer Mg(OH)2 layer. The coatings were characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) , and X-ray diffraction (XRD). The evolution of hydrogen gas was tested in phosphate buffer saline (PBS). The biodegradation was evaluated by the electrochemical testing in the PBS at 37˚ C and 5 % CO2 . The aim of the study is to evaluate
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the effect of the buffer layer of Mg(OH)2 on the corrosion performance of MgF2 coating on the
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surface of Mg alloy ZK60. 2. Materials and Methods
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2.1. Sample Preparation
ZK60 (Mg-6.9Zn-0.8Zr) discs of diameter 25.4 mm and a thickness of 5 mm were cut from the
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commercially available ZK60 rod (Source One Metals ®, MI, USA). The surface of the discs
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was grinded subsequently with 180, 320, 400, 600, 800 and 1200 grit size SiC emery papers to
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get a smooth surface. To avoid any surface contaminations, the samples were cleaned ultrasonically in acetone and deionized water for 2 minutes. Finally, the discs were air-dried.
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The untreated samples are called as ZK60-UT in the following discussion.
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2.2 Coating procedure
The grinded discs were boiled in NaOH solution having a concentration of 200g/L [43–45] for 4
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hours. After the NaOH treatment, the samples were removed from the solution, ultrasonically cleaned in deionized water for 2 minutes and dried in air. These samples are termed as ZK60-NT in the following discussion. The ZK60-NT samples were finally immersed in HF acid (48 %) contained in a polypropylene beaker for 96 hours. The concentrated acid was used to ensure uniform and dense coatings [46]. Afterwards, the samples were removed and washed ultrasonically in deionized water for 2 minutes and air dried. These samples are labeled as ZK60-DT (Dual Treatment). 5
ACCEPTED MANUSCRIPT For comparative analysis, ZK60 samples were also treated with HF acid for 96 hours but without any buffer layer of Mg(OH)2. Afterwards, samples were ultrasonically washed and air dried. The MgF2 coated samples without buffer layer are named as ZK60-HT. 2.3 Surface Characterization The surface morphology of the formed films was characterized using scanning electron
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microscopy (SEM, Hitachi 3400). The film thickness and elemental content mapping were also
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measured using SEM. The film thickness was measured at 5 different points to find an average thickness value. The elemental composition at the surface of samples was evaluated by the
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energy dispersive spectroscopy (EDS) coupled with SEM. All the EDS measurements were repeated at least 3 times to ensure repeatability. The average values are presented in the
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discussion. The phase composition of the coating was identified using X-ray diffraction with the
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Cu-Kα radiation having a wavelength of 0.15406 nm. The samples were scanned from 10˚- 90˚ at
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a scan rate of 0.5˚/min. FTIR-ATR spectrometer (NicoletTMiSTM 50) was used to obtain the Infra-red (IR) spectra of the samples under study in the attenuated total reflection (ATR) mode.
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Wettability of the surfaces was evaluated by measuring the contact angle using a contact angle
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goniometer (Attension Theta- DSC Q 2000) with the monochromatic light source by pouring a drop of deionized water on the surface of samples. The contact angles at 3 different spots of the
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sample surface were measured and the average value of all the readings was used. A still camera was used to capture the image of the water droplet. 2.4 Electrochemical Testing The electrochemical behavior of the samples was studied using a three-electrode cell setup with Mg samples as working electrodes, saturated calomel electrode (SCE) as a reference, and graphite rod as a counter electrode. The Gamry-Potentiostat (Ref-3000) was used for the electrochemical studies. The electrolyte for the electrochemical tests was phosphate buffer saline 6
ACCEPTED MANUSCRIPT (PBS) with a pH value of 7.4. PBS tablets (Sigma-Aldrich®) were dissolved in deionized water to prepare an electrolyte. The concentration of the PBS was 1 tablet/200 ml of deionized water. The composition of the PBS is given in Table I [5]. All the electrochemical tests were performed inside a humidified incubator maintained at 37 ˚C and 5 % CO2. The exposed surface area was 1 cm2. Before each test, the open circuit potential was stabilized for 10 hours. The potentiodynamic
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scans were conducted by polarizing the surface over the potential range of -1.5 to 1.5 V vs OCP
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at the scan rate 5 mV/sec. 2.5 Hydrogen Evolution
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The hydrogen evolution experiments were performed in the PBS at the temperature of 37 ˚C. The temperature was maintained by setting the evolution setup in an incubator. Three samples of each
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type were used in the study. The samples were immersed in a beaker containing the PBS solution.
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An inverted funnel was placed in each beaker to collect the hydrogen gas. Graduated cylinders
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filled with PBS were placed above the funnel to observe the amount of released hydrogen. The released hydrogen gas lowers the level of PBS in the tube by displacing the PBS. The difference
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in the initial level and level at observation time gives the value of evolved hydrogen gas. The values were measured at every 24 hours for 7 days. The pH was measured at the end of day 7 to
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calculate the effect of degradation on the pH of PBS. The initial pH of the PBS was 7.4.
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3. Results and Discussion 3.1. Characterization of Coatings The surface morphologies and elemental composition at the surface of Mg samples are presented in the Fig. 1, and Table II, respectively. The change in the surface morphologies in all samples is evident from the SEM images in Fig. 1. Fig. 1a represents the surface image of untreated Mg alloy ZK60-UT and a smooth grinded surface with few grinding marks can be observed. Fig. 1b demonstrates the NaOH treated ZK60 alloy. A dense layer can be observed on the surface of 7
ACCEPTED MANUSCRIPT ZK60-NT. The new surface is uniform in nature and the grinding marks are no more visible. The protective film is predominant with oxides and hydroxides. The oxygen content on the surface increased from 2.66 ± 0.85 % to 55.80 ± 0.72 % after the NaOH treatment. The newly formed layer as a result of this treatment on the surface of ZK60 is Mg(OH)2 [43,47]. This formation of
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Mg(OH)2 was confirmed by characterizing the surface with XRD and FTIR.
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The surface morphology of ZK60-HT is represented in Fig. 1c. The grinding marks are still
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visible indicating the formation of very thin coating on the surface. The appearance of the film was blackish in color. The elemental composition of the new surface confirmed the presence of
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fluorine on the surface of the sample with a percentage of 27.69 ± 1.65 %. The resulting layer is
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MgF2 and the chemical reaction governing the conversion process is given below[48]. Mg + 2HF → MgF2 + H2
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Fig. 1d represents the surface morphology of ZK60-DT. The cracks are clearly visible on the
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surface of the sample. The new layer is predominately composed of fluorine with a percentage of 55.56 ± 1.65 %. The percentage of fluorine in ZK60-DT was almost twice to that of fluorine
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percentage in ZK60-HT. The conversion reaction of Mg(OH)2 layer in the HF acid is governed
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by the following reaction [43].
Mg(OH)2 + 2HF → MgF2 + 2H2O
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The film thickness of all the coated surfaces is presented in Fig. 2. A very thin film with a thickness of 0.62 ± 0.19 μm was identified at the surface of ZK60-HT as shown in Fig. 2b. Given the thin nature of the film, few grinding marks in Fig. 1c were clearly visible even after the coating. The boiling of samples in NaOH resulted in the formation of a relatively thick layer of Mg(OH)2 with a thickness of 25.35 ± 0.80 μm. The reaction of Mg(OH)2 intermediate surface with the HF acid led to a new layer of MgF2 with a thickness of 6.32 ± 1.22 μm and can be seen
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ACCEPTED MANUSCRIPT in Fig. 2c. The two-step conversion process formed a MgF2 layer with a greater thickness than formed by one step conversion procedure. The elemental distribution mapping of samples is presented in Fig. 3. The elemental distribution of ZK60-NT in Fig. 3b confirms the high content of oxygen on the surface. The Mg and O are
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homogeneously distributed on the surface. The elemental mapping of ZK60-HT in Fig. 3c
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represents the uniformly distributed fluorine on the surface of the sample. The mapping confirms
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the uniform nature of MgF2 film in case of ZK60-HT. Fig. 3d represents the elemental distribution on the surface of ZK60-DT. The fluorine is distributed all over the surface
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solidifying the idea the uniform layer. Although the MgF2 coating in two-step conversion is not
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as uniform as in ZK60-HT, it is almost 10 times thicker than the MgF2 in ZK60-HT. The uniformity of MgF2 protective coating in two-step conversion procedure highly depends on the
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intermediate Mg(OH)2 surface. The NaOH solution should be stirred continuously in coating
uniform conversion of MgF2.
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procedure to ensure uniform Mg(OH)2 layer. The uniform Mg(OH)2 will assist in the more
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The chemical composition of the surface was further evaluated by employing the IR
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spectroscopy. Fig. 4 presents the IR spectra of the samples. The peaks in the range 1800-2250 cm-1 appeared because of the diamond crystal used in the ATR mode [49]. Furthermore, no
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obvious peaks are observed in case of ZK60-UT while after treatment with hydroxide enrich solution (ZK60-NT) a sharp peak of Mg(OH)2 was appeared around 3688 cm-1 [50,51] because of the OH bond stretch. For ZK60-DT (Fig. 4d), the peak around 440 and 1650 cm-1 was observed owing to the stretching vibrations between Mg and F in MgF2 bond [52,53]. Additionally, a broad peak centering around 3350 cm-1 appeared which could be referred to the residual Mg(OH)2 content. A strong peak of MgF2 around 575 cm-1 appeared in case of ZK60-HT
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ACCEPTED MANUSCRIPT (Fig. 4c) [52]. It is evident from the figures 4c and 4d that MgF2 is formed on the surface as a result of the chemical conversion process. The XRD pattern of ZK60-UT, ZK60-NT, ZK60-HT, and ZK60-DT are presented in the Fig. 5. The Mg planes of (100), (002), (101), (110), (200) and (004) were identified at 2Ѳ = 33˚, 35˚,
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37˚, 57˚, 68˚ and 72˚respectively in case of ZK60-UT. The peaks of Mg were in accordance with
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the literature [54,55]. The XRD pattern of ZK60-NT sample confirmed the presence of a
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hexagonal structure of Mg(OH)2. The Mg(OH)2 peaks at 2Ѳ = 19º and 52º for (001) and (102) planes, respectively, were well aligned with the literature[56–58]. The XRD pattern of ZK60-HT
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affirms the presence of monoclinic MgF2 on the surface of ZK60 after HF treatment. The MgF2
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peaks at 2Ѳ = 37˚ and 41˚ for the (111) and (120) planes were observed. Apart from the above mentioned two MgF2 peaks, an additional MgF2 peak at 2Ѳ = 28˚ was identified in case of ZK60-
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DT. The presence of an additional MgF2 peak in ZK60-DT confirms that the MgF2 layer in
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ZK60-UT is more identifiable and well defined as compared to that of ZK60-HT. The peaks of MgF2 were validated from the literature[54,59] for both the cases (ZK60-HT (Fig. 5c) and ZK60-
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DT (Fig. 5d)).
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The contact angles were measured at 3 different spots on the surfaces of samples to evaluate the wettability of formed layers and the effect of coatings on the hydrophilicity of the ZK60-UT
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surface. The results presented in the Fig. 6 are the averages of the three readings for each individual sample. The ZK60-UT exhibited the lowest contact angle of 82.49 ± 1.66˚ among the four samples. The treatment of NaOH had a negligible impact on the contact angle of untreated surface and its value only increased 1.95˚ with the new value was 84.44 ± 1.22˚. The treatment of sample without a buffer layer demonstrated an increase in the contact angle with a value of 91.21 ± 2.01˚. The ZK60-DT has exhibited the highest contact angle of all four with a value of 91.83 ±
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ACCEPTED MANUSCRIPT 0.69˚ indicating the increase in the hydrophobicity with the formation of the MgF2 layer. The summary of contact angle values is presented in Table III. 3.2 Electrochemical Results The effect of NaOH, HF and NaOH-HF treatment on the degradation behavior of ZK60 alloy was studied by potentiodynamic polarization. Fig. 7 presents the polarization curves of the samples.
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The cathodic portion of the curve demonstrates the hydrogen evolution while that of the anodic
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presents the active dissolution reactions [60]. The anodic region of polarization curve of ZK60UT demonstrating a rapid increase in corrosion current density with a slight increase in the
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corrosion potential. At the potential -1.10V vs. SCE, the activity slowed down as afterward there was a minor increase in the current densities indicating the passivation. The treatment of ZK60-
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UT samples with NaOH exhibited a minor increase in corrosion potential. The value of potential
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increased from -1.52 V vs. SCE to -1.50 V vs. SCE. Moreover, the current densities shifted from
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3.32 × 10-5 A/cm2 to a slightly higher value of 3.44 × 10-5 A/cm2. The anodic region of the ZK60NT exhibited a small passivation patch from potential of -1.37 V vs. SCE to -1.30 V vs. SCE.
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After this potential the sample again showed rapid activity and finally stabilized at a potential of 0.90 V vs. SCE. This phenomenon indicates the formation of a passive layer that didn’t last long
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making sample again susceptible to corrosion. The treatment of ZK60-UT with HF witnessed an
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increase in the corrosion potential and reduction in corrosion current density. The corrosion potential was shifted from -1.52 V vs. SCE to -1.42 V vs. SCE and a corrosion current density of 4.52 × 10-6 A/cm2 was achieved with HF treatment. The anodic polarization region of the curve indicated the passivation at a potential of -0.51V vs. SCE. Moreover, initially the corrosion current density of ZK60-HT is much lower than ZK60-UT. The rapid increase in the corrosion current density indicates a non-stable film on the surface that draws more current once the degradation starts. The extremely thin layer of ZK60-HT along with insufficient adherence are 11
ACCEPTED MANUSCRIPT the main reasons for this instability. The rationale behind using the intermediate Mg(OH) 2 is to coat MgF2 layer with higher thickness to ensure stability of the surface. The potentiodynamic curve of ZK60-DT exhibited the effectiveness of buffer layer of Mg(OH)2 in achieving lowest corrosion current density. The anodic region of the potentiodynamic curve of ZK60-DT suggests a formation of passive film at a potential of -1.42 V vs. SCE. This passive film diminishes at -
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1.32 V vs. SCE and degradation activity started again. The shift in the anodic curve towards the
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left indicates the activity of the ZK60-DT samples at lower current densities. The corrosion potential of ZK60-DT improved from -1.52 V vs. SCE to -1.49 V vs. SCE. The corrosion current
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density of ZK60-DT is 3.71 × 10-6 A/cm2, suggesting a reduction in the value as compared to ZK60-UT after the NaOH-HF treatment. The formation of this MgF2 would act as a barrier layer
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to the corrosion attacks, significantly reducing the degradation. The summary of Ecorr and Icorr
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values is presented in Table IV.
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The conversion method is frequently used to coat Mg alloys with phosphate coatings [22,61–65]. The results of MgF2 coatings in our work exhibited improvements in terms of potential increase
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and reduction in corrosion current density when compared to calcium phosphate, barium
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phosphate and magnesium phosphate coatings on Mg-Al alloys [63–65]. The electrochemical results indicated the formation of a MgF2 layer with the lowest corrosion
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current density when pre-treated with NaOH. The formation of Mg(OH)2 buffer layer lead to more corrosion resistant MgF2 final layer. Although the ZK60-UT and ZK60-NT samples were treated in exactly same conditions to get ZK60-HT and ZK60-DT respectively, the effectiveness of pre-treatment with NaOH is evident from the results. 3.3 Hydrogen Evolution Fig. 8 presents the cumulative hydrogen evolved from the surfaces of samples vs. the number of days. The evolution of hydrogen gas is governed by the following reaction [66]. 12
ACCEPTED MANUSCRIPT Mg + 2H2O → Mg2+ + 2OH- + H2 At the initial stage, the hydrogen gas evolved was maximum from the surface of ZK60-NT. The ZK60-HT and ZK60-DT samples showed the small volume of evolved hydrogen suggesting a barrier coating of MgF2. The evolution of H2 from the surface of ZK60-NT is associated to the Mg(OH)2, generating the higher H2 gas. The H2 generation on the ZK60-UT and ZK60-NT
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samples kept on decreasing with the number of days and on the 7th day, the evolved value from
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the ZK60-UT was slightly higher than ZK60-DT and H2 evolution from the ZK60-NT was
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comparable to the evolution from ZK60-HT and ZK60-DT. The hydrogen gas evolved from the
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surfaces of ZK60-HT and ZK60-DT was almost uniform for 7 days with a negligible increase over the time. However, the evolved hydrogen for ZK60-HT was slightly higher than ZK60-DT.
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The protective layer of ZK60-DT was still intact on the 7th day with no visible signs of
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degradation. There were some points only along the thickness of ZK60-DT discs that showed small corroded regions. The cumulative hydrogen evolution of ZK60-UT and ZK60-NT was 6.86
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± 1.81 ml/cm2 and 9.25 ± 2.41 ml/cm2 respectively. The amount of evolved hydrogen from
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ZK60-HT was 3.86 ± 0.54 ml/cm2 lower than that of ZK60-UT and ZK670-NT samples. The hydrogen evolution of ZK60-DT was much more consistent with a small standard deviation of
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0.26. The value of hydrogen evolution from ZK60-DT was 3.01± 0.26 ml/cm2 which is much less
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than ZK60-UT and ZK60-NT and slightly less as compared to ZK60-HT. The pH of the PBS solution containing ZK60-HT and ZK60-DT samples changed to 9.4 and 9.7 respectively after 7 days. Whereas the pH of solutions with ZK60-UT and ZK60-NT were 10.14 and 10.09 respectively after 1 week. The change in pH was lowest in the case of ZK60-HT with a pH increase of 2.0 after 7 days. 4.0 Conclusion
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ACCEPTED MANUSCRIPT MgF2 coatings formed via chemical conversion process on the surface of ZK60 alloy were investigated for the corrosion properties. The MgF2 coating was formed on the surface of ZK60 (Mg-6.9Zn-0.8Zr) with a buffer layer of Mg(OH)2 by the conversion treatment in sodium hydroxide (NaOH) solution. The results were compared to the ZK60 samples coated with MgF2 without a buffer layer. SEM images of the samples under study showed the formation of a dense
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layer of Mg(OH)2 and a more uniform final layer of MgF2. The elemental composition exhibited
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the presence of high percentage of oxygen after NaOH treatment and fluorine after HF treatment further solidifying the idea of formation of Mg(OH)2 and MgF2. The XRD and IR spectroscopy
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also confirmed the formation of Mg(OH)2 and MgF2 coatings. The potentiodynamic polarization curves exhibited a shift of potential towards the positive side for ZK60-HT and ZK60-DT.
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Similarly, the corrosion current densities in ZK60-HT and ZK60-DT shifted towards lower
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values with the lowest value exhibited by ZK60-DT. The MgF2 coated surfaces ZK60-DT and ZK60-HT showed reduced H2 evolution as compared to the untreated samples. The aim of the
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work was to study the effect of Mg(OH)2 in achieving more noble potential of ZK60 along with
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lower current densities and minimum hydrogen evolution, and the results proved the effectiveness of the intermediate layer in the formation of more thick and dense MgF2 film with
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Table I Chemical Composition of PBS Solution (g/l)
KCl 0.40
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NaHCO3 0.35
KH2PO4 0.06
MgSiO4 0.20
CaCl2 0.14
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Na2HPO4 0.06
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NaCl 8.00
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Table II EDS Analysis of the Samples, n=3
O (wt.%)
Zn (wt.%)
Zr (wt.%)
F (wt.%)
ZK60-UT
90.23 ± 0.63
2.66 ± 0.85
6.53 ± 0.72
0.60 ± 0.20
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ZK60-NT
43.75 ± 0.55
55.80 ± 0.72
0.30 ± 0.17
0.70 ± 0.11
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ZK60-HT
64.80 ± 1.65
3.43 ± 0.60
3.46 ± 0.68
0.30 ± 0.00
27.69 ± 1.65
33.70 ± 0.34
8.56 ± 0.05
2.13 ± 0.51
0.06 ± 0.05
55.56 ± 0.28
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ZK60-DT
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Mg (wt.%)
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Table III
Summary of Contact angles with their standard deviation, n=3
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Contact Angle (Degree) 82.49 84.44 91.21 91.83
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ZK60-UT ZK60-NT ZK60-HT ZK60-DT
Standard Deviation (Degrees) 1.66 1.22 2.01 0.69
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Table IV
Summary of Ecorr and Icorr values after Tafel fitting
Ecorr (V vs SCE) -1.52 -1.50 -1.42 -1.49
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ZK60-UT ZK60-NT ZK60-HT ZK60-DT
Icorr(A/cm2) 3.32 × 10-5 3.44 × 10-5 4.52 × 10-6 3.71 × 10-6
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Figure 1- SEM Images of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT & (d) ZK60-DT Figure 2- Film thickness of (a) ZK60-HT, (b) ZK60-NT, (c) ZK60-DT
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Figure 3- EDS elemental mapping of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT, (d) ZK60-DT Figure 4- ATR Spectra of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT & (d) ZK60-DT
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Figure 5- XRD Patterns of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT, (d) ZK60-DT Figure 6- Contact Angle of ZK60-UT, ZK60-NT, ZK60-HT & ZK60-DT
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Figure 7- Potentiodynamic Polarization curves of ZK60-UT, ZK60-NT, ZK60-HT & ZK60-DT
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Figure 8-Hydrogen evolution of ZK60 samples over a period of 7 days
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Figure 4
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Figure 7
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Figure 8
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ACCEPTED MANUSCRIPT Highlights
MgF2 with a buffer layer of Mg(OH)2 was coated on ZK60 alloy via two step chemical conversion
The presence of MgF2 increased the contact angle of ZK60 alloy
A significant decrease in Icorr value as compared to untreated was observed with two step
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conversion
Hydrogen evolution was significantly reduced in the presence of MgF2 coating
The presence of buffer layer improved the corrosion resistance of MgF2 coating
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Usman Riaz, Zia ur Rahman, Hassnain Asgar, Umair Shah, Ishraq Shabib, Waseem Haider PII: DOI: Reference:
S0257-8972(18)30332-3 doi:10.1016/j.surfcoat.2018.03.081 SCT 23261
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
7 February 2018 23 March 2018 26 March 2018
Please cite this article as: Usman Riaz, Zia ur Rahman, Hassnain Asgar, Umair Shah, Ishraq Shabib, Waseem Haider , An insight into the effect of buffer layer on the electrochemical performance of MgF2 coated magnesium alloy ZK60. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sct(2017), doi:10.1016/j.surfcoat.2018.03.081
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ACCEPTED MANUSCRIPT An insight into the effect of buffer layer on the electrochemical performance of MgF2 coated Magnesium alloy ZK60 Usman Riaza, Zia ur Rahmanb, Hassnain Asgara, Umair Shaha, Ishraq Shabiba,b, Waseem Haidera,b a
School of Engineering and Technology, Central Michigan University, Mount Pleasant 48859, USA b
Science of Advanced Materials, Central Michigan University, Mount Pleasant 48859, USA
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Corresponding Author: [email protected]
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Abstract
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Magnesium (Mg) has emerged as potential implant material owing to its property of biodegradation. The roadblock to the commercial use of Mg as implant material is its fast
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degradation in body fluids. The degradation of the Mg and its alloys can be retarded by surface
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coatings. In this work, the potential of MgF2 coating on the surface of Mg alloy ZK60 (Mg6.9Zn-0.8Zr) was evaluated for its corrosion properties. Two-step chemical conversion process
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was used to coat MgF2 on the surface of ZK60 alloy. In the first step, a secondary layer of
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Mg(OH)2 was introduced by boiling the samples in NaOH solution. In the second step, these samples were immersed in hydrofluoric acid to obtain MgF2 coating. SEM, IR Spectroscopy, and
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XRD were employed to confirm the formation of Mg(OH)2 and MgF2. The wettability tests
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showed an increase in surface hydrophobicity as a result of conversion treatment. The potentiodynamic polarization tests exhibited an improvement in the corrosion potential from -
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1.52 V vs. SCE to -1.49 V vs. SCE after two-step conversion treatment. Moreover, coated sample witnessed a noticeable drop in hydrogen evolution compared to untreated ZK60. For a better insight, the results were compared to the MgF2 coatings achieved on the surface of ZK60 without any buffer layer. The coating of MgF2 with a buffer layer of Mg(OH)2 on the surface of ZK60 exhibited a noble corrosion potential, controlled degradation, and nominal hydrogen evolution compared to the untreated ZK60.
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ACCEPTED MANUSCRIPT Keywords: Magnesium fluoride; Chemical conversion; Degradation; Hydrogen evolution; Potentiodynamic polarization.
1. Introduction The potential of magnesium (Mg) and its alloys as biodegradable implant material has been an
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attraction for the researchers looking for an optimized biocompatible and biodegradable material
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for implant applications. The density of Mg is 1.74 g/cm3 making it the lightest of all the engineering metals [1,2]. Mg has better strength/density ratio than stainless steel and titanium
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alloys that are commonly used implant materials [3] The elastic modulus of Mg (45 GPa) is slightly higher than the elastic modulus of bone (3-20 GPa)[4], significantly reducing the
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possibility of stress shielding of bone[4]. On the other hand, the elastic modulus of iron (Fe) is
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211.4 GPa making it and its alloys much stiffer than bone and vulnerable to stress shielding [4].
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The property of Mg that set it apart from other implant materials is biodegradation [4]. Mg implants possess the unique property of biodegradation in the physiological conditions reducing
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the risk of secondary surgery to remove the implant and long-term undesirable interactions between the implant and tissues [5,6]. Mg has been under investigation for the cardiovascular
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applications in the form of stents [7–11] and orthopedic applications in the form of rods, plates,
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pins, and screws [12–15]. The issue associated with the Mg implants is the fast degradation in physiological conditions and subsequent loss of mechanical strength leading to implant failure [4,16–18]. Apart from losing mechanical strength, another issue related to the fast degradation of Mg is the evolution of Hydrogen (H2) gas [19,20].The evolution of H2 with the degradation in an aqueous solution is governed by the given reaction. Mg + 2H2O → Mg(OH)2 + H2 The anodic and cathodic reactions are following: 2
ACCEPTED MANUSCRIPT Mg → Mg2+ + 2e2H2O + 2e- → H2 + 2OHMg2+ + 2OH- → Mg(OH)2 The H2 gas accumulated in the gas pockets can severely damage the tissues close to the implant [20]. In addition to the tissue damages, the gas bubbles can block the regular flow of blood that
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addressed before the commercial use of Mg in implant applications.
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put the life of the patient at risk [20]. The rapid degradation of Mg and H2 evolution needs to be
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The surface treatment and alloy formation are common methods to improve the properties of Mg
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alloys. The surface treatment of Mg and its alloys is a relatively simple method as compared to alloying to control the rapid degradation. In surface treatment, the aim is to achieve a protective
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coating on the surface of Mg to slow down the degradation process and increase the corrosion resistance. Several coating methods are currently in practice to improve the corrosion resistance
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of magnesium. These methods include chemical conversion [21–23], micro arc oxidation [24,25],
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anodization [26,27], ion implantation [28], electrochemical deposition [17] and physical vapor
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deposition [29].
In chemical conversion, the specimens to be coated are immersed in a treating solution. The
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surface of the specimen reacts with the species solution and form Mg compounds on the surface.
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The layer of Mg compounds acts as a protective film to decelerate corrosion. The quality of protective film depends on multiple factors including the composition of treating solution, alloy type, and experimental parameters such as temperature, pre-treatments, and post-treatment [30,31]. Calcium Phosphate (Ca-P) [22,23,32,33] and fluoride [34–39] based surfaces are the most common coatings achieved by this method. Magnesium fluoride (MgF2) coatings are formed on the surface of Mg by immersing the Mg specimen in hydrofluoric acid (HF) [36]. The resistive nature of MgF2 coating has been proved in 3
ACCEPTED MANUSCRIPT the literature [37,40]. Additionally, the daily recommended intake of fluoride in the body is 2-5 mg [40] reducing the risk of the adverse effects of degradation of the MgF2 layer[40]. The biocompatibility of MgF2 coating on Mg-Nd-Zn-Zr was studied by the cytotoxicity evaluation of human umbilical vein endothelial cells (HUVEC) in endothelial basal medium (EBM). The MgF2 treated sample extracts exhibited better cell viability than untreated samples[41]. The corrosion
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resistance, non-toxic nature and a simple method of treating are the main attractions of MgF2
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coatings by the conversion process.
The effects of MgF2 protective layers on Mg-Zn-Zr alloys have been of great interest in recent
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years [36,38,40]. Mg-3.2Zn-0.8Zr screws coated with MgF2 were evaluated for their corrosion
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resistance, mechanical properties, and biocompatibility. The MgF2 coated screws experienced an improvement in corrosion rate at the initial stage, higher yield and ultimate tensile strengths as
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compared to uncoated samples without producing any adverse biological effect [42]. Similarly,
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MgF2 coated Mg-3Zn-0.5Zr alloy was evaluated for its biological activity and the results suggested an improvement in the osteoblastic activity and bone formation [36]. In both these
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methods, Mg alloys were coated with MgF2 without any pre-treatment or buffer layer. Recently
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Mg-0.5Zn-0.45Zr (commonly known as ZK60) coated with HF acid with an etching pretreatment was studied and the coated specimens exhibited lower current densities and hydrogen
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evolution than that of untreated ZK60 [38]. In this work, the MgF2 layer was formed on the surface of Mg alloy ZK60 in a dual treatment process by first introducing a buffer layer of Mg(OH)2. To compare the results, MgF2 coating without a buffer layer was also investigated in this study. Although, the pretreatment by boiling Mg alloy samples in sodium hydroxide (NaOH) solution to achieve magnesium hydroxide (Mg(OH)2) layer has been widely studied in the literature but the effects of the intermediate layer of Mg(OH)2 on the surface properties of ZK60 haven't been investigated in the literature. The aim 4
ACCEPTED MANUSCRIPT of this work is to not only study the properties of the final MgF2 layer but also evaluate the buffer Mg(OH)2 layer. The coatings were characterized by scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) , and X-ray diffraction (XRD). The evolution of hydrogen gas was tested in phosphate buffer saline (PBS). The biodegradation was evaluated by the electrochemical testing in the PBS at 37˚ C and 5 % CO2 . The aim of the study is to evaluate
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the effect of the buffer layer of Mg(OH)2 on the corrosion performance of MgF2 coating on the
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surface of Mg alloy ZK60. 2. Materials and Methods
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2.1. Sample Preparation
ZK60 (Mg-6.9Zn-0.8Zr) discs of diameter 25.4 mm and a thickness of 5 mm were cut from the
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commercially available ZK60 rod (Source One Metals ®, MI, USA). The surface of the discs
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was grinded subsequently with 180, 320, 400, 600, 800 and 1200 grit size SiC emery papers to
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get a smooth surface. To avoid any surface contaminations, the samples were cleaned ultrasonically in acetone and deionized water for 2 minutes. Finally, the discs were air-dried.
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The untreated samples are called as ZK60-UT in the following discussion.
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2.2 Coating procedure
The grinded discs were boiled in NaOH solution having a concentration of 200g/L [43–45] for 4
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hours. After the NaOH treatment, the samples were removed from the solution, ultrasonically cleaned in deionized water for 2 minutes and dried in air. These samples are termed as ZK60-NT in the following discussion. The ZK60-NT samples were finally immersed in HF acid (48 %) contained in a polypropylene beaker for 96 hours. The concentrated acid was used to ensure uniform and dense coatings [46]. Afterwards, the samples were removed and washed ultrasonically in deionized water for 2 minutes and air dried. These samples are labeled as ZK60-DT (Dual Treatment). 5
ACCEPTED MANUSCRIPT For comparative analysis, ZK60 samples were also treated with HF acid for 96 hours but without any buffer layer of Mg(OH)2. Afterwards, samples were ultrasonically washed and air dried. The MgF2 coated samples without buffer layer are named as ZK60-HT. 2.3 Surface Characterization The surface morphology of the formed films was characterized using scanning electron
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microscopy (SEM, Hitachi 3400). The film thickness and elemental content mapping were also
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measured using SEM. The film thickness was measured at 5 different points to find an average thickness value. The elemental composition at the surface of samples was evaluated by the
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energy dispersive spectroscopy (EDS) coupled with SEM. All the EDS measurements were repeated at least 3 times to ensure repeatability. The average values are presented in the
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discussion. The phase composition of the coating was identified using X-ray diffraction with the
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Cu-Kα radiation having a wavelength of 0.15406 nm. The samples were scanned from 10˚- 90˚ at
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a scan rate of 0.5˚/min. FTIR-ATR spectrometer (NicoletTMiSTM 50) was used to obtain the Infra-red (IR) spectra of the samples under study in the attenuated total reflection (ATR) mode.
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Wettability of the surfaces was evaluated by measuring the contact angle using a contact angle
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goniometer (Attension Theta- DSC Q 2000) with the monochromatic light source by pouring a drop of deionized water on the surface of samples. The contact angles at 3 different spots of the
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sample surface were measured and the average value of all the readings was used. A still camera was used to capture the image of the water droplet. 2.4 Electrochemical Testing The electrochemical behavior of the samples was studied using a three-electrode cell setup with Mg samples as working electrodes, saturated calomel electrode (SCE) as a reference, and graphite rod as a counter electrode. The Gamry-Potentiostat (Ref-3000) was used for the electrochemical studies. The electrolyte for the electrochemical tests was phosphate buffer saline 6
ACCEPTED MANUSCRIPT (PBS) with a pH value of 7.4. PBS tablets (Sigma-Aldrich®) were dissolved in deionized water to prepare an electrolyte. The concentration of the PBS was 1 tablet/200 ml of deionized water. The composition of the PBS is given in Table I [5]. All the electrochemical tests were performed inside a humidified incubator maintained at 37 ˚C and 5 % CO2. The exposed surface area was 1 cm2. Before each test, the open circuit potential was stabilized for 10 hours. The potentiodynamic
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scans were conducted by polarizing the surface over the potential range of -1.5 to 1.5 V vs OCP
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at the scan rate 5 mV/sec. 2.5 Hydrogen Evolution
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The hydrogen evolution experiments were performed in the PBS at the temperature of 37 ˚C. The temperature was maintained by setting the evolution setup in an incubator. Three samples of each
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type were used in the study. The samples were immersed in a beaker containing the PBS solution.
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An inverted funnel was placed in each beaker to collect the hydrogen gas. Graduated cylinders
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filled with PBS were placed above the funnel to observe the amount of released hydrogen. The released hydrogen gas lowers the level of PBS in the tube by displacing the PBS. The difference
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in the initial level and level at observation time gives the value of evolved hydrogen gas. The values were measured at every 24 hours for 7 days. The pH was measured at the end of day 7 to
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calculate the effect of degradation on the pH of PBS. The initial pH of the PBS was 7.4.
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3. Results and Discussion 3.1. Characterization of Coatings The surface morphologies and elemental composition at the surface of Mg samples are presented in the Fig. 1, and Table II, respectively. The change in the surface morphologies in all samples is evident from the SEM images in Fig. 1. Fig. 1a represents the surface image of untreated Mg alloy ZK60-UT and a smooth grinded surface with few grinding marks can be observed. Fig. 1b demonstrates the NaOH treated ZK60 alloy. A dense layer can be observed on the surface of 7
ACCEPTED MANUSCRIPT ZK60-NT. The new surface is uniform in nature and the grinding marks are no more visible. The protective film is predominant with oxides and hydroxides. The oxygen content on the surface increased from 2.66 ± 0.85 % to 55.80 ± 0.72 % after the NaOH treatment. The newly formed layer as a result of this treatment on the surface of ZK60 is Mg(OH)2 [43,47]. This formation of
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Mg(OH)2 was confirmed by characterizing the surface with XRD and FTIR.
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The surface morphology of ZK60-HT is represented in Fig. 1c. The grinding marks are still
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visible indicating the formation of very thin coating on the surface. The appearance of the film was blackish in color. The elemental composition of the new surface confirmed the presence of
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fluorine on the surface of the sample with a percentage of 27.69 ± 1.65 %. The resulting layer is
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MgF2 and the chemical reaction governing the conversion process is given below[48]. Mg + 2HF → MgF2 + H2
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Fig. 1d represents the surface morphology of ZK60-DT. The cracks are clearly visible on the
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surface of the sample. The new layer is predominately composed of fluorine with a percentage of 55.56 ± 1.65 %. The percentage of fluorine in ZK60-DT was almost twice to that of fluorine
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percentage in ZK60-HT. The conversion reaction of Mg(OH)2 layer in the HF acid is governed
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by the following reaction [43].
Mg(OH)2 + 2HF → MgF2 + 2H2O
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The film thickness of all the coated surfaces is presented in Fig. 2. A very thin film with a thickness of 0.62 ± 0.19 μm was identified at the surface of ZK60-HT as shown in Fig. 2b. Given the thin nature of the film, few grinding marks in Fig. 1c were clearly visible even after the coating. The boiling of samples in NaOH resulted in the formation of a relatively thick layer of Mg(OH)2 with a thickness of 25.35 ± 0.80 μm. The reaction of Mg(OH)2 intermediate surface with the HF acid led to a new layer of MgF2 with a thickness of 6.32 ± 1.22 μm and can be seen
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ACCEPTED MANUSCRIPT in Fig. 2c. The two-step conversion process formed a MgF2 layer with a greater thickness than formed by one step conversion procedure. The elemental distribution mapping of samples is presented in Fig. 3. The elemental distribution of ZK60-NT in Fig. 3b confirms the high content of oxygen on the surface. The Mg and O are
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homogeneously distributed on the surface. The elemental mapping of ZK60-HT in Fig. 3c
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represents the uniformly distributed fluorine on the surface of the sample. The mapping confirms
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the uniform nature of MgF2 film in case of ZK60-HT. Fig. 3d represents the elemental distribution on the surface of ZK60-DT. The fluorine is distributed all over the surface
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solidifying the idea the uniform layer. Although the MgF2 coating in two-step conversion is not
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as uniform as in ZK60-HT, it is almost 10 times thicker than the MgF2 in ZK60-HT. The uniformity of MgF2 protective coating in two-step conversion procedure highly depends on the
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intermediate Mg(OH)2 surface. The NaOH solution should be stirred continuously in coating
uniform conversion of MgF2.
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procedure to ensure uniform Mg(OH)2 layer. The uniform Mg(OH)2 will assist in the more
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The chemical composition of the surface was further evaluated by employing the IR
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spectroscopy. Fig. 4 presents the IR spectra of the samples. The peaks in the range 1800-2250 cm-1 appeared because of the diamond crystal used in the ATR mode [49]. Furthermore, no
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obvious peaks are observed in case of ZK60-UT while after treatment with hydroxide enrich solution (ZK60-NT) a sharp peak of Mg(OH)2 was appeared around 3688 cm-1 [50,51] because of the OH bond stretch. For ZK60-DT (Fig. 4d), the peak around 440 and 1650 cm-1 was observed owing to the stretching vibrations between Mg and F in MgF2 bond [52,53]. Additionally, a broad peak centering around 3350 cm-1 appeared which could be referred to the residual Mg(OH)2 content. A strong peak of MgF2 around 575 cm-1 appeared in case of ZK60-HT
9
ACCEPTED MANUSCRIPT (Fig. 4c) [52]. It is evident from the figures 4c and 4d that MgF2 is formed on the surface as a result of the chemical conversion process. The XRD pattern of ZK60-UT, ZK60-NT, ZK60-HT, and ZK60-DT are presented in the Fig. 5. The Mg planes of (100), (002), (101), (110), (200) and (004) were identified at 2Ѳ = 33˚, 35˚,
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37˚, 57˚, 68˚ and 72˚respectively in case of ZK60-UT. The peaks of Mg were in accordance with
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the literature [54,55]. The XRD pattern of ZK60-NT sample confirmed the presence of a
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hexagonal structure of Mg(OH)2. The Mg(OH)2 peaks at 2Ѳ = 19º and 52º for (001) and (102) planes, respectively, were well aligned with the literature[56–58]. The XRD pattern of ZK60-HT
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affirms the presence of monoclinic MgF2 on the surface of ZK60 after HF treatment. The MgF2
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peaks at 2Ѳ = 37˚ and 41˚ for the (111) and (120) planes were observed. Apart from the above mentioned two MgF2 peaks, an additional MgF2 peak at 2Ѳ = 28˚ was identified in case of ZK60-
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DT. The presence of an additional MgF2 peak in ZK60-DT confirms that the MgF2 layer in
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ZK60-UT is more identifiable and well defined as compared to that of ZK60-HT. The peaks of MgF2 were validated from the literature[54,59] for both the cases (ZK60-HT (Fig. 5c) and ZK60-
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DT (Fig. 5d)).
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The contact angles were measured at 3 different spots on the surfaces of samples to evaluate the wettability of formed layers and the effect of coatings on the hydrophilicity of the ZK60-UT
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surface. The results presented in the Fig. 6 are the averages of the three readings for each individual sample. The ZK60-UT exhibited the lowest contact angle of 82.49 ± 1.66˚ among the four samples. The treatment of NaOH had a negligible impact on the contact angle of untreated surface and its value only increased 1.95˚ with the new value was 84.44 ± 1.22˚. The treatment of sample without a buffer layer demonstrated an increase in the contact angle with a value of 91.21 ± 2.01˚. The ZK60-DT has exhibited the highest contact angle of all four with a value of 91.83 ±
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ACCEPTED MANUSCRIPT 0.69˚ indicating the increase in the hydrophobicity with the formation of the MgF2 layer. The summary of contact angle values is presented in Table III. 3.2 Electrochemical Results The effect of NaOH, HF and NaOH-HF treatment on the degradation behavior of ZK60 alloy was studied by potentiodynamic polarization. Fig. 7 presents the polarization curves of the samples.
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The cathodic portion of the curve demonstrates the hydrogen evolution while that of the anodic
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presents the active dissolution reactions [60]. The anodic region of polarization curve of ZK60UT demonstrating a rapid increase in corrosion current density with a slight increase in the
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corrosion potential. At the potential -1.10V vs. SCE, the activity slowed down as afterward there was a minor increase in the current densities indicating the passivation. The treatment of ZK60-
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UT samples with NaOH exhibited a minor increase in corrosion potential. The value of potential
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increased from -1.52 V vs. SCE to -1.50 V vs. SCE. Moreover, the current densities shifted from
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3.32 × 10-5 A/cm2 to a slightly higher value of 3.44 × 10-5 A/cm2. The anodic region of the ZK60NT exhibited a small passivation patch from potential of -1.37 V vs. SCE to -1.30 V vs. SCE.
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After this potential the sample again showed rapid activity and finally stabilized at a potential of 0.90 V vs. SCE. This phenomenon indicates the formation of a passive layer that didn’t last long
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making sample again susceptible to corrosion. The treatment of ZK60-UT with HF witnessed an
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increase in the corrosion potential and reduction in corrosion current density. The corrosion potential was shifted from -1.52 V vs. SCE to -1.42 V vs. SCE and a corrosion current density of 4.52 × 10-6 A/cm2 was achieved with HF treatment. The anodic polarization region of the curve indicated the passivation at a potential of -0.51V vs. SCE. Moreover, initially the corrosion current density of ZK60-HT is much lower than ZK60-UT. The rapid increase in the corrosion current density indicates a non-stable film on the surface that draws more current once the degradation starts. The extremely thin layer of ZK60-HT along with insufficient adherence are 11
ACCEPTED MANUSCRIPT the main reasons for this instability. The rationale behind using the intermediate Mg(OH) 2 is to coat MgF2 layer with higher thickness to ensure stability of the surface. The potentiodynamic curve of ZK60-DT exhibited the effectiveness of buffer layer of Mg(OH)2 in achieving lowest corrosion current density. The anodic region of the potentiodynamic curve of ZK60-DT suggests a formation of passive film at a potential of -1.42 V vs. SCE. This passive film diminishes at -
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1.32 V vs. SCE and degradation activity started again. The shift in the anodic curve towards the
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left indicates the activity of the ZK60-DT samples at lower current densities. The corrosion potential of ZK60-DT improved from -1.52 V vs. SCE to -1.49 V vs. SCE. The corrosion current
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density of ZK60-DT is 3.71 × 10-6 A/cm2, suggesting a reduction in the value as compared to ZK60-UT after the NaOH-HF treatment. The formation of this MgF2 would act as a barrier layer
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to the corrosion attacks, significantly reducing the degradation. The summary of Ecorr and Icorr
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values is presented in Table IV.
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The conversion method is frequently used to coat Mg alloys with phosphate coatings [22,61–65]. The results of MgF2 coatings in our work exhibited improvements in terms of potential increase
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and reduction in corrosion current density when compared to calcium phosphate, barium
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phosphate and magnesium phosphate coatings on Mg-Al alloys [63–65]. The electrochemical results indicated the formation of a MgF2 layer with the lowest corrosion
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current density when pre-treated with NaOH. The formation of Mg(OH)2 buffer layer lead to more corrosion resistant MgF2 final layer. Although the ZK60-UT and ZK60-NT samples were treated in exactly same conditions to get ZK60-HT and ZK60-DT respectively, the effectiveness of pre-treatment with NaOH is evident from the results. 3.3 Hydrogen Evolution Fig. 8 presents the cumulative hydrogen evolved from the surfaces of samples vs. the number of days. The evolution of hydrogen gas is governed by the following reaction [66]. 12
ACCEPTED MANUSCRIPT Mg + 2H2O → Mg2+ + 2OH- + H2 At the initial stage, the hydrogen gas evolved was maximum from the surface of ZK60-NT. The ZK60-HT and ZK60-DT samples showed the small volume of evolved hydrogen suggesting a barrier coating of MgF2. The evolution of H2 from the surface of ZK60-NT is associated to the Mg(OH)2, generating the higher H2 gas. The H2 generation on the ZK60-UT and ZK60-NT
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samples kept on decreasing with the number of days and on the 7th day, the evolved value from
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the ZK60-UT was slightly higher than ZK60-DT and H2 evolution from the ZK60-NT was
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comparable to the evolution from ZK60-HT and ZK60-DT. The hydrogen gas evolved from the
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surfaces of ZK60-HT and ZK60-DT was almost uniform for 7 days with a negligible increase over the time. However, the evolved hydrogen for ZK60-HT was slightly higher than ZK60-DT.
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The protective layer of ZK60-DT was still intact on the 7th day with no visible signs of
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degradation. There were some points only along the thickness of ZK60-DT discs that showed small corroded regions. The cumulative hydrogen evolution of ZK60-UT and ZK60-NT was 6.86
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± 1.81 ml/cm2 and 9.25 ± 2.41 ml/cm2 respectively. The amount of evolved hydrogen from
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ZK60-HT was 3.86 ± 0.54 ml/cm2 lower than that of ZK60-UT and ZK670-NT samples. The hydrogen evolution of ZK60-DT was much more consistent with a small standard deviation of
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0.26. The value of hydrogen evolution from ZK60-DT was 3.01± 0.26 ml/cm2 which is much less
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than ZK60-UT and ZK60-NT and slightly less as compared to ZK60-HT. The pH of the PBS solution containing ZK60-HT and ZK60-DT samples changed to 9.4 and 9.7 respectively after 7 days. Whereas the pH of solutions with ZK60-UT and ZK60-NT were 10.14 and 10.09 respectively after 1 week. The change in pH was lowest in the case of ZK60-HT with a pH increase of 2.0 after 7 days. 4.0 Conclusion
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ACCEPTED MANUSCRIPT MgF2 coatings formed via chemical conversion process on the surface of ZK60 alloy were investigated for the corrosion properties. The MgF2 coating was formed on the surface of ZK60 (Mg-6.9Zn-0.8Zr) with a buffer layer of Mg(OH)2 by the conversion treatment in sodium hydroxide (NaOH) solution. The results were compared to the ZK60 samples coated with MgF2 without a buffer layer. SEM images of the samples under study showed the formation of a dense
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layer of Mg(OH)2 and a more uniform final layer of MgF2. The elemental composition exhibited
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the presence of high percentage of oxygen after NaOH treatment and fluorine after HF treatment further solidifying the idea of formation of Mg(OH)2 and MgF2. The XRD and IR spectroscopy
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also confirmed the formation of Mg(OH)2 and MgF2 coatings. The potentiodynamic polarization curves exhibited a shift of potential towards the positive side for ZK60-HT and ZK60-DT.
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Similarly, the corrosion current densities in ZK60-HT and ZK60-DT shifted towards lower
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values with the lowest value exhibited by ZK60-DT. The MgF2 coated surfaces ZK60-DT and ZK60-HT showed reduced H2 evolution as compared to the untreated samples. The aim of the
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work was to study the effect of Mg(OH)2 in achieving more noble potential of ZK60 along with
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lower current densities and minimum hydrogen evolution, and the results proved the effectiveness of the intermediate layer in the formation of more thick and dense MgF2 film with
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enhanced corrosion resistance and reduced hydrogen evolution.
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ACCEPTED MANUSCRIPT AZ31B Magnesium Alloy, J. Mater. Sci. Technol. 30 (2014) 666–674. doi:10.1016/j.jmst.2013.12.015.
[41] L. Mao, L. Shen, J. Chen, Y. Wu, M. Kwak, Y. Lu, Q. Xue, J. Pei, L. Zhang, G. Yuan, R. Fan, J. Ge, W. Ding, Enhanced Bioactivity of Mg–Nd–Zn–Zr Alloy Achieved with Nanoscale MgF2 Surface for Vascular Stent Application, ACS Appl. Mater. Interfaces. 7 (2015) 5320–5330. doi:10.1021/am5086885.
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[64] Y. Chen, B.L. Luan, G.-L. Song, Q. Yang, D.M. Kingston, F. Bensebaa, An investigation of new barium phosphate chemical conversion coating on AZ31 magnesium alloy, Surf. Coatings Technol. 210 (2012) 156–165. doi:10.1016/J.SURFCOAT.2012.09.009.
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Table I Chemical Composition of PBS Solution (g/l)
KCl 0.40
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NaHCO3 0.35
KH2PO4 0.06
MgSiO4 0.20
CaCl2 0.14
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Na2HPO4 0.06
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NaCl 8.00
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Table II EDS Analysis of the Samples, n=3
O (wt.%)
Zn (wt.%)
Zr (wt.%)
F (wt.%)
ZK60-UT
90.23 ± 0.63
2.66 ± 0.85
6.53 ± 0.72
0.60 ± 0.20
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ZK60-NT
43.75 ± 0.55
55.80 ± 0.72
0.30 ± 0.17
0.70 ± 0.11
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ZK60-HT
64.80 ± 1.65
3.43 ± 0.60
3.46 ± 0.68
0.30 ± 0.00
27.69 ± 1.65
33.70 ± 0.34
8.56 ± 0.05
2.13 ± 0.51
0.06 ± 0.05
55.56 ± 0.28
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Mg (wt.%)
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Table III
Summary of Contact angles with their standard deviation, n=3
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Contact Angle (Degree) 82.49 84.44 91.21 91.83
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ZK60-UT ZK60-NT ZK60-HT ZK60-DT
Standard Deviation (Degrees) 1.66 1.22 2.01 0.69
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Table IV
Summary of Ecorr and Icorr values after Tafel fitting
Ecorr (V vs SCE) -1.52 -1.50 -1.42 -1.49
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ZK60-UT ZK60-NT ZK60-HT ZK60-DT
Icorr(A/cm2) 3.32 × 10-5 3.44 × 10-5 4.52 × 10-6 3.71 × 10-6
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Figure 1- SEM Images of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT & (d) ZK60-DT Figure 2- Film thickness of (a) ZK60-HT, (b) ZK60-NT, (c) ZK60-DT
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Figure 3- EDS elemental mapping of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT, (d) ZK60-DT Figure 4- ATR Spectra of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT & (d) ZK60-DT
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Figure 5- XRD Patterns of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT, (d) ZK60-DT Figure 6- Contact Angle of ZK60-UT, ZK60-NT, ZK60-HT & ZK60-DT
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Figure 7- Potentiodynamic Polarization curves of ZK60-UT, ZK60-NT, ZK60-HT & ZK60-DT
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Figure 8-Hydrogen evolution of ZK60 samples over a period of 7 days
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Figure 2
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Figure 4
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Figure 7
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Figure 8
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ACCEPTED MANUSCRIPT Highlights
MgF2 with a buffer layer of Mg(OH)2 was coated on ZK60 alloy via two step chemical conversion
The presence of MgF2 increased the contact angle of ZK60 alloy
A significant decrease in Icorr value as compared to untreated was observed with two step
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Hydrogen evolution was significantly reduced in the presence of MgF2 coating
The presence of buffer layer improved the corrosion resistance of MgF2 coating
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