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Surface & Coatings Technology

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An insight into the effect of buffer layer on the electrochemical performanceof 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, USAb Science of Advanced Materials, Central Michigan University, Mount Pleasant 48859, USA

A R T I C L E I N F O

Keywords:Magnesium fluorideChemical conversionDegradationHydrogen evolutionPotentiodynamic polarization

A B S T R A C T

Magnesium (Mg) has emerged as potential implant material owing to its property of biodegradation. Theroadblock to the commercial use of Mg as implant material is its fast degradation in body fluids. The degradationof the Mg and its alloys can be retarded by surface coatings. In this work, the potential of MgF2 coating on thesurface of Mg alloy ZK60 (Mg-6.9Zn-0.8Zr) was evaluated for its corrosion properties. Two-step chemical con-version process was used to coat MgF2 on the surface of ZK60 alloy. In the first step, a secondary layer of Mg(OH)2 was introduced by boiling the samples in NaOH solution. In the second step, these samples were immersedin hydrofluoric acid to obtain MgF2 coating. SEM, IR Spectroscopy, and XRD were employed to confirm theformation of Mg(OH)2 and MgF2. The wettability tests showed an increase in surface hydrophobicity as a resultof conversion treatment. The potentiodynamic polarization tests exhibited an improvement in the corrosionpotential from −1.52 V vs. SCE to −1.49 V vs. SCE after two-step conversion treatment. Moreover, coatedsample witnessed a noticeable drop in hydrogen evolution compared to untreated ZK60. For a better insight, theresults were compared to the MgF2 coatings achieved on the surface of ZK60 without any buffer layer. Thecoating 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.

1. Introduction

The potential of magnesium (Mg) and its alloys as biodegradableimplant material has been an attraction for the researchers looking foran optimized biocompatible and biodegradable material for implantapplications. The density of Mg is 1.74 g/cm3 making it the lightest ofall the engineering metals [1,2]. Mg has better strength/density ratiothan stainless steel and titanium alloys that are commonly used implantmaterials [3] The elastic modulus of Mg (45 GPa) is slightly higher thanthe elastic modulus of bone (3–20 GPa) [4], significantly reducing thepossibility of stress shielding of bone [4]. On the other hand, the elasticmodulus of iron (Fe) is 211.4 GPa making it and its alloys much stifferthan bone and vulnerable to stress shielding [4]. The property of Mgthat set it apart from other implant materials is biodegradation [4]. Mgimplants possess the unique property of biodegradation in the physio-logical conditions reducing the risk of secondary surgery to remove theimplant and long-term undesirable interactions between the implantand tissues [5,6]. Mg has been under investigation for the cardiovas-cular applications in the form of stents [7–11] and orthopedic

applications in the form of rods, plates, pins, and screws [12–15]. Theissue associated with the Mg implants is the fast degradation in phy-siological conditions and subsequent loss of mechanical strengthleading to implant failure [4,16–18]. Apart from losing mechanicalstrength, another issue related to the fast degradation of Mg is theevolution of Hydrogen (H2) gas [19,20].The evolution of H2 with thedegradation in an aqueous solution is governed by the given reaction.

+ → +Mg 2H O Mg(OH) H2 2 2

The anodic and cathodic reactions are following:

→ ++ −Mg Mg 2e2

+ → +− −2H O 2e H 2OH2 2

+ →+ −Mg 2OH Mg(OH)2

2

The H2 gas accumulated in the gas pockets can severely damage thetissues close to the implant [20]. In addition to the tissue damages, thegas bubbles can block the regular flow of blood that put the life of thepatient at risk [20]. The rapid degradation of Mg and H2 evolution

https://doi.org/10.1016/j.surfcoat.2018.03.081Received 7 February 2018; Received in revised form 23 March 2018; Accepted 26 March 2018

⁎ Corresponding author at: School of Engineering and Technology, Central Michigan University, Mount Pleasant 48859, USA.E-mail address: haide1w@cmich.edu (W. Haider).

Surface & Coatings Technology 344 (2018) 514–521

Available online 27 March 20180257-8972/ © 2018 Elsevier B.V. All rights reserved.

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needs to be addressed before the commercial use of Mg in implantapplications.

The surface treatment and alloy formation are common methods toimprove the properties of Mg alloys. The surface treatment of Mg andits alloys is a relatively simple method as compared to alloying tocontrol the rapid degradation. In surface treatment, the aim is toachieve a protective coating on the surface of Mg to slow down thedegradation process and increase the corrosion resistance. Severalcoating methods are currently in practice to improve the corrosion re-sistance of magnesium. These methods include chemical conversion[21–23], micro arc oxidation [24,25], anodization [26,27], ion im-plantation [28], electrochemical deposition [17] and physical vapordeposition [29].

In chemical conversion, the specimens to be coated are immersed ina treating solution. The surface of the specimen reacts with the speciessolution and form Mg compounds on the surface. The layer of Mgcompounds acts as a protective film to decelerate corrosion. The qualityof protective film depends on multiple factors including the composi-tion of treating solution, alloy type, and experimental parameters suchas temperature, pre-treatments, and post-treatment [30,31]. CalciumPhosphate (Ca-P) [22,23,32,33] and fluoride [34–39] based surfacesare the most common coatings achieved by this method.

Magnesium fluoride (MgF2) coatings are formed on the surface ofMg by immersing the Mg specimen in hydrofluoric acid (HF) [36]. Theresistive nature of MgF2 coating has been proved in the literature[37,40]. Additionally, the daily recommended intake of fluoride in thebody is 2–5mg [40] reducing the risk of the adverse effects of de-gradation of the MgF2 layer [40]. The biocompatibility of MgF2 coatingon Mg-Nd-Zn-Zr was studied by the cytotoxicity evaluation of humanumbilical vein endothelial cells (HUVEC) in endothelial basal medium(EBM). The MgF2 treated sample extracts exhibited better cell viabilitythan untreated samples [41]. The corrosion resistance, non-toxic natureand a simple method of treating are the main attractions of MgF2coatings by the conversion process.

The effects of MgF2 protective layers on Mg-Zn-Zr alloys have beenof great interest in recent years [36,38,40]. Mg-3.2Zn-0.8Zr screwscoated with MgF2 were evaluated for their corrosion resistance, me-chanical properties, and biocompatibility. The MgF2 coated screwsexperienced an improvement in corrosion rate at the initial stage,higher yield and ultimate tensile strengths as compared to uncoatedsamples without producing any adverse biological effect [42]. Simi-larly, MgF2 coated Mg-3Zn-0.5Zr alloy was evaluated for its biologicalactivity and the results suggested an improvement in the osteoblasticactivity and bone formation [36]. In both these methods, Mg alloyswere coated with MgF2 without any pre-treatment or buffer layer. Re-cently Mg-0.5Zn-0.45Zr (commonly known as ZK60) coated with HFacid with an etching pre-treatment was studied and the coated speci-mens exhibited lower current densities and hydrogen evolution thanthat of untreated ZK60 [38].

In this work, the MgF2 layer was formed on the surface of Mg alloyZK60 in a dual treatment process by first introducing a buffer layer ofMg(OH)2. To compare the results, MgF2 coating without a buffer layerwas also investigated in this study. Although, the pretreatment byboiling Mg alloy samples in sodium hydroxide (NaOH) solution toachieve magnesium hydroxide (Mg(OH)2) layer has been widely stu-died 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 theliterature. The aim of this work is to not only study the properties of thefinal MgF2 layer but also evaluate the buffer Mg(OH)2 layer. Thecoatings 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 buffersaline (PBS). The biodegradation was evaluated by the electrochemicaltesting in the PBS at 37 °C and 5% CO2. The aim of the study is toevaluate the effect of the buffer layer of Mg(OH)2 on the corrosionperformance of MgF2 coating on the surface of Mg alloy ZK60.

2. Materials and methods

2.1. Sample preparation

ZK60 (Mg-6.9Zn-0.8Zr) discs of diameter 25.4mm and a thicknessof 5mm were cut from the commercially available ZK60 rod (SourceOne Metals ®, MI, USA). The surface of the discs was grinded subse-quently with 180, 320, 400, 600, 800 and 1200 grit size SiC emerypapers to get a smooth surface. To avoid any surface contaminations,the samples were cleaned ultrasonically in acetone and deionized waterfor 2min. Finally, the discs were air-dried. The untreated samples arecalled as ZK60-UT in the following discussion.

2.2. Coating procedure

The grinded discs were boiled in NaOH solution having a con-centration of 200 g/L [43–45] for 4 h. After the NaOH treatment, thesamples were removed from the solution, ultrasonically cleaned indeionized water for 2min and dried in air. These samples are termed asZK60-NT in the following discussion.

The ZK60-NT samples were finally immersed in HF acid (48%)contained in a polypropylene beaker for 96 h. The concentrated acidwas used to ensure uniform and dense coatings [46]. Afterwards, thesamples were removed and washed ultrasonically in deionized waterfor 2min and air dried. These samples are labeled as ZK60-DT (DualTreatment).

For comparative analysis, ZK60 samples were also treated with HFacid for 96 h but without any buffer layer of Mg(OH)2. Afterwards,samples were ultrasonically washed and air dried. The MgF2 coatedsamples without buffer layer are named as ZK60-HT.

2.3. Surface characterization

The surface morphology of the formed films was characterized usingscanning electron microscopy (SEM, Hitachi 3400). The film thicknessand elemental content mapping were also measured using SEM. Thefilm thickness was measured at 5 different points to find an averagethickness value. The elemental composition at the surface of sampleswas evaluated by the energy dispersive spectroscopy (EDS) coupledwith SEM. All the EDS measurements were repeated at least 3 times toensure repeatability. The average values are presented in the discus-sion. The phase composition of the coating was identified using X-raydiffraction with the Cu-Kα radiation having a wavelength of0.15406 nm. The samples were scanned from 10°- 90° at a scan rate of0.5°/min. FTIR-ATR spectrometer (NicoletTMiSTM 50) was used toobtain the Infra-red (IR) spectra of the samples under study in the at-tenuated total reflection (ATR) mode.

Wettability of the surfaces was evaluated by measuring the contactangle using a contact angle goniometer (Attension Theta- DSC Q 2000)with the monochromatic light source by pouring a drop of deionizedwater on the surface of samples. The contact angles at 3 different spotsof the sample surface were measured and the average value of all thereadings was used. A still camera was used to capture the image of thewater droplet.

2.4. Electrochemical testing

The electrochemical behavior of the samples was studied using athree-electrode cell setup with Mg samples as working electrodes, sa-turated calomel electrode (SCE) as a reference, and graphite rod as acounter electrode. The Gamry-Potentiostat (Ref-3000) was used for theelectrochemical studies. The electrolyte for the electrochemical testswas phosphate buffer saline (PBS) with a pH value of 7.4. PBS tablets(Sigma-Aldrich®) were dissolved in deionized water to prepare anelectrolyte. The concentration of the PBS was 1 tablet/200mL ofdeionized water. The composition of the PBS is given in Table 1 [5]. All

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the electrochemical tests were performed inside a humidified incubatormaintained at 37 °C and 5% CO2. The exposed surface area was 1 cm2.Before each test, the open circuit potential was stabilized for 10 h. Thepotentiodynamic scans were conducted by polarizing the surface overthe potential range of −1.5 to 1.5 V vs. OCP at the scan rate 5mV/s.

2.5. Hydrogen evolution

The hydrogen evolution experiments were performed in the PBS atthe temperature of 37 °C. The temperature was maintained by settingthe evolution setup in an incubator. Three samples of each type wereused in the study. The samples were immersed in a beaker containingthe PBS solution. An inverted funnel was placed in each beaker tocollect the hydrogen gas. Graduated cylinders filled with PBS wereplaced above the funnel to observe the amount of released hydrogen.The released hydrogen gas lowers the level of PBS in the tube by dis-placing the PBS. The difference in the initial level and level at ob-servation time gives the value of evolved hydrogen gas. The values weremeasured at every 24 h for 7 days. The pH was measured at the end ofday 7 to calculate the effect of degradation on the pH of PBS. The initialpH of the PBS was 7.4.

3. Results and discussion

3.1. Characterization of coatings

The surface morphologies and elemental composition at the surfaceof Mg samples are presented in the Fig. 1, and Table 2, respectively. Thechange in the surface morphologies in all samples is evident from theSEM images in Fig. 1. Fig. 1a represents the surface image of untreatedMg alloy ZK60-UT and a smooth grinded surface with few grindingmarks can be observed. Fig. 1b demonstrates the NaOH treated ZK60alloy. A dense layer can be observed on the surface of ZK60-NT. Thenew surface is uniform in nature and the grinding marks are no morevisible. The protective film is predominant with oxides and hydroxides.The oxygen content on the surface increased from 2.66 ± 0.85% to55.80 ± 0.72% after the NaOH treatment. The newly formed layer as aresult of this treatment on the surface of ZK60 is Mg(OH)2 [43,47]. Thisformation of Mg(OH)2 was confirmed by characterizing the surfacewith XRD and FTIR.

The surface morphology of ZK60-HT is represented in Fig. 1c. Thegrinding marks are still visible indicating the formation of very thincoating on the surface. The appearance of the film was blackish in color.The elemental composition of the new surface confirmed the presenceof fluorine on the surface of the sample with a percentage of27.69 ± 1.65%. The resulting layer is MgF2 and the chemical reactiongoverning the conversion process is given below [48].

+ → +Mg 2HF MgF H2 2

Fig. 1d represents the surface morphology of ZK60-DT. The cracksare clearly visible on the surface of the sample. The new layer is pre-dominately composed of fluorine with a percentage of 55.56 ± 1.65%.The percentage of fluorine in ZK60-DT was almost twice to that offluorine percentage in ZK60-HT. The conversion reaction of Mg(OH)2layer in the HF acid is governed by the following reaction [43].

+ → +Mg(OH) 2HF MgF 2H O2 2 2

The film thickness of all the coated surfaces is presented in Fig. 2. Avery 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 thefilm, few grinding marks in Fig. 1c were clearly visible even after thecoating. The boiling of samples in NaOH resulted in the formation of arelatively 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 anew layer of MgF2 with a thickness of 6.32 ± 1.22 μm and can be seenin Fig. 2c. The two-step conversion process formed a MgF2 layer with agreater thickness than formed by one step conversion procedure.

The elemental distribution mapping of samples is presented inFig. 3. The elemental distribution of ZK60-NT in Fig. 3b confirms thehigh content of oxygen on the surface. The Mg and O are homo-geneously distributed on the surface. The elemental mapping of ZK60-HT in Fig. 3c represents the uniformly distributed fluorine on the sur-face of the sample. The mapping confirms the uniform nature of MgF2film in case of ZK60-HT. Fig. 3d represents the elemental distributionon the surface of ZK60-DT. The fluorine is distributed all over thesurface solidifying the idea the uniform layer. Although the MgF2coating in two-step conversion is not as uniform as in ZK60-HT, it isalmost 10 times thicker than the MgF2 in ZK60-HT. The uniformity ofMgF2 protective coating in two-step conversion procedure highly de-pends on the intermediate Mg(OH)2 surface. The NaOH solution shouldbe stirred continuously in coating procedure to ensure uniform Mg(OH)2 layer. The uniform Mg(OH)2 will assist in the more uniformconversion of MgF2.

The chemical composition of the surface was further evaluated byemploying the IR spectroscopy. Fig. 4 presents the IR spectra of thesamples. The peaks in the range 1800–2250 cm−1 appeared because ofthe diamond crystal used in the ATR mode [49]. Furthermore, no ob-vious peaks are observed in case of ZK60-UT while after treatment withhydroxide enrich solution (ZK60-NT) a sharp peak of Mg(OH)2 wasappeared 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 ob-served owing to the stretching vibrations between Mg and F in MgF2bond [52,53]. Additionally, a broad peak centering around 3350 cm−1

appeared which could be referred to the residual Mg(OH)2 content. Astrong peak of MgF2 around 575 cm−1 appeared in case of ZK60-HT(Fig. 4c) [52]. It is evident from the fig. 4c and d that MgF2 is formed onthe surface as a result of the chemical conversion process.

The XRD pattern of ZK60-UT, ZK60-NT, ZK60-HT, and ZK60-DT arepresented in the Fig. 5. The Mg planes of (100), (002), (101), (110),(200) and (004) were identified at 2Ѳ=33°, 35°, 37°, 57°, 68° and72°respectively in case of ZK60-UT. The peaks of Mg were in ac-cordance with the literature [54,55]. The XRD pattern of ZK60-NTsample confirmed the presence of a hexagonal structure of Mg(OH)2.The Mg(OH)2 peaks at 2Ѳ=19° and 52° for (001) and (102) planes,respectively, were well aligned with the literature [56–58]. The XRDpattern of ZK60-HT affirms the presence of monoclinic MgF2 on thesurface of ZK60 after HF treatment. The MgF2 peaks at 2Ѳ=37° and41° for the (111) and (120) planes were observed. Apart from the abovementioned two MgF2 peaks, an additional MgF2 peak at 2Ѳ=28° wasidentified in case of ZK60-DT. The presence of an additional MgF2 peakin ZK60-DT confirms that the MgF2 layer in ZK60-UT is more identifi-able and well defined as compared to that of ZK60-HT. The peaks ofMgF2 were validated from the literature [54,59] for both the cases(ZK60-HT (Fig. 5c) and ZK60-DT (Fig. 5d)).

The contact angles were measured at 3 different spots on the sur-faces of samples to evaluate the wettability of formed layers and theeffect of coatings on the hydrophilicity of the ZK60-UT surface. Theresults presented in the Fig. 6 are the averages of the three readings foreach individual sample. The ZK60-UT exhibited the lowest contactangle of 82.49 ± 1.66° among the four samples. The treatment ofNaOH had a negligible impact on the contact angle of untreated surfaceand its value only increased 1.95° with the new value was84.44 ± 1.22°. The treatment of sample without a buffer layer de-monstrated an increase in the contact angle with a value of91.21 ± 2.01°. The ZK60-DT has exhibited the highest contact angle of

Table 1Chemical composition of PBS solution (g/L).

NaCl Na2HPO4 NaHCO3 KCl KH2PO4 MgSiO4 CaCl2

8.00 0.06 0.35 0.40 0.06 0.20 0.14

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all four with a value of 91.83 ± 0.69° indicating the increase in thehydrophobicity with the formation of the MgF2 layer. The summary ofcontact angle values is presented in Table 3.

3.2. Electrochemical results

The effect of NaOH, HF and NaOH-HF treatment on the degradationbehavior of ZK60 alloy was studied by potentiodynamic polarization.Fig. 7 presents the polarization curves of the samples. The cathodicportion of the curve demonstrates the hydrogen evolution while that ofthe anodic presents the active dissolution reactions [60]. The anodicregion of polarization curve of ZK60-UT demonstrating a rapid increasein corrosion current density with a slight increase in the corrosion po-tential. At the potential −1.10 V vs. SCE, the activity slowed down asafterward there was a minor increase in the current densities indicatingthe passivation. The treatment of ZK60-UT samples with NaOH ex-hibited a minor increase in corrosion potential. The value of potentialincreased from −1.52 V vs. SCE to −1.50 V vs. SCE. Moreover, thecurrent densities shifted from 3.32×10−5 A/cm2 to a slightly highervalue of 3.44× 10−5 A/cm2. The anodic region of the ZK60-NT ex-hibited a small passivation patch from potential of −1.37 V vs. SCE to−1.30 V vs. SCE. After this potential the sample again showed rapid

activity and finally stabilized at a potential of 0.90 V vs. SCE. Thisphenomenon indicates the formation of a passive layer that didn't lastlong making sample again susceptible to corrosion. The treatment ofZK60-UT with HF witnessed an increase in the corrosion potential andreduction in corrosion current density. The corrosion potential wasshifted from −1.52 V vs. SCE to −1.42 V vs. SCE and a corrosioncurrent density of 4.52× 10−6 A/cm2 was achieved with HF treatment.The anodic polarization region of the curve indicated the passivation ata potential of−0.51 V vs. SCE. Moreover, initially the corrosion currentdensity of ZK60-HT is much lower than ZK60-UT. The rapid increase inthe corrosion current density indicates a non-stable film on the surfacethat draws more current once the degradation starts. The extremely thinlayer of ZK60-HT along with insufficient adherence are the main rea-sons for this instability. The rationale behind using the intermediate Mg(OH)2 is to coat MgF2 layer with higher thickness to ensure stability ofthe surface. The potentiodynamic curve of ZK60-DT exhibited the ef-fectiveness of buffer layer of Mg(OH)2 in achieving lowest corrosioncurrent density. The anodic region of the potentiodynamic curve ofZK60-DT suggests a formation of passive film at a potential of −1.42 Vvs. SCE. This passive film diminishes at −1.32 V vs. SCE and de-gradation activity started again. The shift in the anodic curve towardsthe left indicates the activity of the ZK60-DT samples at lower current

Fig. 1. SEM Images of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT & (d) ZK60-DT.

Table 2EDS analysis of the samples, n= 3.

Mg(wt%)

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 –ZK60-NT 43.75 ± 0.55 55.80 ± 0.72 0.30 ± 0.17 0.70 ± 0.11 –ZK60-HT 64.80 ± 1.65 3.43 ± 0.60 3.46 ± 0.68 0.30 ± 0.00 27.69 ± 1.65ZK60-DT 33.70 ± 0.34 8.56 ± 0.05 2.13 ± 0.51 0.06 ± 0.05 55.56 ± 0.28

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densities. The corrosion potential of ZK60-DT improved from −1.52 Vvs. SCE to−1.49 V vs. SCE. The corrosion current density of ZK60-DT is3.71×10−6 A/cm2, suggesting a reduction in the value as compared toZK60-UT after the NaOH-HF treatment. The formation of this MgF2would act as a barrier layer to the corrosion attacks, significantly re-ducing the degradation. The summary of Ecorr and Icorr values is pre-sented in Table 4.

The conversion method is frequently used to coat Mg alloys withphosphate coatings [22,61–65]. The results of MgF2 coatings in ourwork exhibited improvements in terms of potential increase and re-duction in corrosion current density when compared to calcium phos-phate, barium phosphate and magnesium phosphate coatings on Mg-Alalloys [63–65].

The electrochemical results indicated the formation of a MgF2 layerwith the lowest corrosion current density when pre-treated with NaOH.The formation of Mg(OH)2 buffer layer lead to more corrosion resistantMgF2 final layer. Although the ZK60-UT and ZK60-NT samples weretreated in exactly same conditions to get ZK60-HT and ZK60-DT re-spectively, the effectiveness of pre-treatment with NaOH is evidentfrom the results.

Fig. 2. Film thickness of (a) ZK60-HT, (b) ZK60-NT, (c) ZK60-DT.

Fig. 3. EDS elemental mapping of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT, (d) ZK60-DT.

Fig. 4. ATR Spectra of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT & (d) ZK60-DT.

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3.3. Hydrogen evolution

Fig. 8 presents the cumulative hydrogen evolved from the surfacesof samples vs. the number of days. The evolution of hydrogen gas isgoverned by the following reaction [66–68].

+ → + ++ −Mg 2H O Mg 2OH H2

22

At the initial stage, the hydrogen gas evolved was maximum fromthe surface of ZK60-NT. The ZK60-HT and ZK60-DT samples showedthe small volume of evolved hydrogen suggesting a barrier coating ofMgF2. The evolution of H2 from the surface of ZK60-NT is associated tothe Mg(OH)2, generating the higher H2 gas. The H2 generation on the

Fig. 5. XRD Patterns of (a) ZK60-UT, (b) ZK60-NT, (c) ZK60-HT, (d) ZK60-DT.

Fig. 6. Contact Angle of ZK60-UT, ZK60-NT, ZK60-HT & ZK60-DT.

Table 3Summary of Contact angles with their standard deviation, n= 3.

Contact angle(degree)

Standard deviation(degrees)

ZK60-UT 82.49 1.66ZK60-NT 84.44 1.22ZK60-HT 91.21 2.01ZK60-DT 91.83 0.69

Fig. 7. Potentiodynamic Polarization curves of ZK60-UT, ZK60-NT, ZK60-HT &ZK60-DT.

Table 4Summary of Ecorr and Icorr values after Tafel fitting.

Ecorr (V vs. SCE) Icorr(A/cm2)

ZK60-UT −1.52 3.32× 10−5

ZK60-NT −1.50 3.44× 10−5

ZK60-HT −1.42 4.52× 10−6

ZK60-DT −1.49 3.71× 10−6

Fig. 8. Hydrogen evolution of ZK60 samples over a period of 7 days.

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ZK60-UT and ZK60-NT samples kept on decreasing with the number ofdays and on the 7th day, the evolved value from the ZK60-UT wasslightly higher than ZK60-DT and H2 evolution from the ZK60-NT wascomparable to the evolution from ZK60-HT and ZK60-DT. The hy-drogen gas evolved from the surfaces of ZK60-HT and ZK60-DT wasalmost uniform for 7 days with a negligible increase over the time.However, the evolved hydrogen for ZK60-HT was slightly higher thanZK60-DT. The protective layer of ZK60-DT was still intact on the 7thday with no visible signs of degradation. There were some points onlyalong the thickness of ZK60-DT discs that showed small corroded re-gions. The cumulative hydrogen evolution of ZK60-UT and ZK60-NTwas 6.86 ± 1.81mL/cm2 and 9.25 ± 2.41mL/cm2 respectively. Theamount of evolved hydrogen from ZK60-HT was 3.86 ± 0.54mL/cm2

lower than that of ZK60-UT and ZK670-NT samples. The hydrogenevolution of ZK60-DT was much more consistent with a small standarddeviation of 0.26. The value of hydrogen evolution from ZK60-DT was3.01 ± 0.26mL/cm2 which is much less than ZK60-UT and ZK60-NTand slightly less as compared to ZK60-HT. The pH of the PBS solutioncontaining ZK60-HT and ZK60-DT samples changed to 9.4 and 9.7 re-spectively after 7 days. Whereas the pH of solutions with ZK60-UT andZK60-NT were 10.14 and 10.09 respectively after 1 week. The changein pH was lowest in the case of ZK60-HT with a pH increase of 2.0 after7 days.

4. Conclusion

MgF2 coatings formed via chemical conversion process on the sur-face of ZK60 alloy were investigated for the corrosion properties. TheMgF2 coating was formed on the surface of ZK60 (Mg-6.9Zn-0.8Zr) witha buffer layer of Mg(OH)2 by the conversion treatment in sodium hy-droxide (NaOH) solution. The results were compared to the ZK60samples coated with MgF2 without a buffer layer. SEM images of thesamples under study showed the formation of a dense layer of Mg(OH)2and a more uniform final layer of MgF2. The elemental compositionexhibited the presence of high percentage of oxygen after NaOHtreatment and fluorine after HF treatment further solidifying the idea offormation of Mg(OH)2 and MgF2. The XRD and IR spectroscopy alsoconfirmed the formation of Mg(OH)2 and MgF2 coatings. The po-tentiodynamic polarization curves exhibited a shift of potential towardsthe positive side for ZK60-HT and ZK60-DT. Similarly, the corrosioncurrent densities in ZK60-HT and ZK60-DT shifted towards lower valueswith the lowest value exhibited by ZK60-DT. The MgF2 coated surfacesZK60-DT and ZK60-HT showed reduced H2 evolution as compared tothe untreated samples. The aim of the work was to study the effect ofMg(OH)2 in achieving more noble potential of ZK60 along with lowercurrent densities and minimum hydrogen evolution, and the resultsproved the effectiveness of the intermediate layer in the formation ofmore thick and dense MgF2 film with enhanced corrosion resistance andreduced hydrogen evolution.

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