electrodeposition NaHAp coatings on CoNiCrMo

ELECTRODEPOSITION OF NaHAp COATINGS ON CoNiCrMo ALLOYS (NaHAp/CoNiCrMo) AND THEIR ELECTROCHEMICAL BEHAVIOR IN SIMULATED BODY FLUIDS SOLUTION Dinh Thi Mai Thanh1, Nguyen Thi Thom1, Pham Thi Nam1, Nguyen Thu Phuong1, Didier Devilliers2, David Grossin3, Ghislaine Bertrand3, Christophe Drouet3 Institute for Tropical Technology, Vietnam Academy of Science and Technology,18 Hoang Quoc Viet Street, Cau Giay, Hanoi, Vietnam Sorbonne Universités, UPMC, UMR CNRS PHENIX 8234, Paris, France CIRIMAT Carnot Institute, UMR CNRS/UPS/INPT 5085, University of Toulouse, ENSIACET, allée Emile Monso, 31030 Toulouse cedex 4, France Email: dmthanh@itt.vast.vn Received: , 2015; Accepted for publication: , 2015 ABSTRACT Sodium-hydroxyapatite coatings (NaHAp) were electrodeposited on the surface of CoNiCrMo alloys (NaHAp/CoNiCrMo) from an electrolyte solution containing Ca(NO3)2, NH4H2PO4 and NaNO3 The phase structure, composition and morphology of the coatings were studied by X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDX) and Scanning electron microscopy (SEM) The results showed that the obtained coatings were single phase crystals of HAp, with plate shape and that the sodium acts as a doping element in their composition (0.73%) The deposition temperature affected the thickness as well as the phase structure of the coatings When the deposition temperature increased, the thickness of the coating increased The XRD results showed that the coating obtained at 50°C was composed of single phase crystals of HAp with the thickness 6.34m The in vitro test with CoNiCrMo and NaHAp/CoNiCrMo materials in SBF solution was realized with different immersion times The results showed that the pH of SBF decreased and the mass of materials increased SEM images prove the formation of apatite on the surface of NaHAp/CoNiCrMo leading to the decrease of the corrosion current density during immersion process in SBF solution Keywords: CoNiCrMo, Electrodeposition, NaHAp, SBF INTRODUCTION Cobalt-based alloys have been widely used as a prosthetic implant in orthopedic surgery due to their superior stiffness, corrosion resistance, and in particular, high surface hardness [1,2,3] However, their slow osteointegration (about 3–6 months) and lack of bioactivity are not satisfactory for long-term implantation in orthopedic surgery and these disadvantages can create pain and discomfort of patients Therefore surface modifications were often carried out to make metallic implants bioactive Among all the surface treatments, hydroxyapatite (HAp) coatings deposited on the surface of Cobalt-based alloys to enhance their biocompatibility have been the subject of many researches Hydroxyapatite (Ca10(PO4)6(OH)2) has a similar chemical composition, crystal structure and high biocompatibility as the natural bone tissue [4,5] HAp is used as a popular material for bone and tooth implants in the biomaterial field HAp coating can be synthesized by electrophoretic deposition [6], plasma spraying [7], sol–gel [8], biomimetic deposition [9] and electrochemical deposition [4, 9] In the above methods, electrochemical deposition is one of the most promising methods to synthesize thin HAp coatings The applicability and bio-compatibillity of HAp coating on the surface of metals and alloys are attracting the attention of many scientists because they are an important factor to decide the applicability of implant materials Many articles were published on this topic In vitro bioactivity of some implant materials: titanium, alloy of titanium (Ti6Al4V), ceramic-glass scaffolds in the physiological environment such as the simulated body fluid medium, phosphate buffer, Ringer’s solution were evaluated [10-12] To improve several properties as reduced solubility, increase bioactivity and mechanical properties of HAp coatings deposited on biomedical metals and alloys , some ions such as Na +, Mg2 +, Zn2+, Al3+ have been substituted into HAp structure [13-15] In this work, we synthesized sodium doped hydroxyapatite (NaHAp) coatings on the surface of CoNiCrMo alloys by electrodeposition and we investigated their electrochemical behaviors in simulated body fluid (SBF) solution MATERIALS AND METHODS 2.1 Materials The hydroxyapatite coatings (NaHAp) were electrodeposited on the CoNiCrMo (M64BC) biometallic alloy with the elemental composition given in Table A coupon of CoNiCrMo (1.5x1x0.2 cm3) was used as a cathode (working electrode) for the experiments Prior to electrodeposition the cathode was polished with SiC papers (ranging from P320 to P1200 grit), followed by ultrasonic rinsing in distilled water for 45 minutes and then dried at room temperature Candle gel was used to cover the substrate and limit the working area to 1cm2 The following chemicals were used for the experiments: Calcium nitrate tetrahydrate (Ca(NO3)2.4H2O, M = 236.15 g/mol, 99% pure), ammonium dihydrogen phosphate (NH4H2PO4, M = 115.03 g/mol, pure 99%) and sodium nitrate (NaNO3, M = 84.99 g/mol, 99% pure) were imported from China Table Elemental content of CoNiCrMo alloys Element Co Cr Mo Ni C Content (%) 64 28 < 1.00 < 0.14 2.2 Deposition equipment and procedure The electrolyte solution contained Ca(NO3)2·4H2O 3x10-2 M and NH4H2PO4 1.8x10-2 M with a Ca/P ratio being 1.67 dissolved in distilled water NaNO3 0.06 M was also added in order to improve the conductivity of the electrolyte solution and the electrochemical reduction of NO 3− ions also contributed to generate OH− [13] The pH of the electrolyte solution was 4.4 The electrodeposition equipment consisted of a beaker containing the above electrolyte solution held to the required temperature in a water bath during the course of an experiment The working electrode was a CoNiCrMo sheet A Pt foil was used as counter electrode (anode) and an Hg/Hg2Cl2/KCl (SCE) electrode was used as reference The electrodeposition was carried out on an AUTOLAB with different synthesis conditions: scanning potential ranges: to -1.6; to -1.7; to -1.8 and to -1.9 V/SCE; scanning rates: 1, 3, and mV/s (1, 3, and cycles, respectively) The reaction temperature was kept at 35, 50 and 70oC by a thermostat (VELP, Italia) 2.3 The determination of NaHAp coating thickness The NaHAp coating deposited on the surface of CoNiCrMo alloys was determined by weighing the CoNiCrMo samples before and after synthesis by a Precisa analytical balance (XR 205SM-PR, Swiss) We can calculate the coatings thickness by the following equation m  m  V h D.S  V  S h  D (1) D = 3.13 g/cm3 is the specific mass of NaHAp (approximated with the specific mass of HAp) [15]; the mass (m) and volume (V) of the NaHAp coating; h is the thickness of NaHAp coating; S = 1cm2 is the working area of CoNiCrMo cathode 2.4 Coating characterization The chemical structure of the NaHAp coatings was determined by Fourier transform infrared spectroscopy (FT-IR 6700, Nicolet) using KBr pellet technique at room temperature, in the range from 400 to 4000 cm-1 with cm-1 resolution and eight scans signal average The crystal structure was characterized by thin film X-ray diffraction (XRD) (SIEMENS D5005 Bruker-Germany, CuK radiation ( = 1.5406 Å)), operated at 40kV and 30mA, with step angle of 0.030o/s and in a 2 degree range of 10°-90o The composition of elements in NaHAp coatings was determined by energy-dispersive X-ray spectroscopy (EDS) on a JSM 6490-JED 1300 Jeol, Japan Surface morphology of the NaHAp coatings before and after immersion in the SBF solution was examined using a Hitachi S-4800 Scanning Electron Microscopy (SEM) 2.5 Preparation of simulated body fluid (SBF) and in vitro test A SBF solution was used to perform the electrochemical behavior studies of NaHAp/CoCrNiMo The chemical composition of the SBF solution is shown in Table The pH of the SBF solution was adjusted to 7.4 and the temperature was maintained at 37◦C [11, 17, 18] Table 2: Chemical composition of the SBF solution Compound Content (g/l) NaCl 8.00 KCl 0.40 CaCl2 0.18 NaHCO3 0.35 Na2HPO4·2H2O 0.48 MgCl2·6H2O 0.10 KH2PO4 0.06 MgSO4·7H2O 0.10 Glucose 1.00 CoNiCrMo alloys and NaHAp/CoNiCrMo samples limited to cm2 of active area were used as working electrodes and immersed in a three electrodes cell containing 40 ml of the SBF solution with a saturated calomel electrode (SCE) as a reference electrode and Pt foil as a counter electrode The electrochemical cell was incubated at 37oC in a water bath for 1, 3, 7, 17, 24 and 28 days The working electrode was polarized in the potential range ±10 mV around its open circuit potential with a scanning rate potential v = 1mV/s The polarization resistance Rp and the corrosion current Icorr were calculated from equations (2) and (3) with B = 0.023 for NaHAp/CoNiCrMo samples and 0.025 for CoNiCrMo samples (values determined from the Tafel slopes of the I-E curves from equations (4)) Rp = Icorr = ΔE (2) Δi B Rp (3) (4) With |bc| is the absolute value of the cathodic Tafel slope ba is the value of the anodic Tafel slope RESULTS AND DISCUSSION 3.1 Electrodeposition of NaHAp coatings on CoNiCrMo 3.1.1 Effect of the scanning potential range Figure shows the cathodic polarization curve of CoNiCrMo substrate in the electrolyte at the scanning potential range from to - 1.9 V/SCE with a scanning rate of mV/s, at 50oC The cathodic polarization curve is representative of the electrochemical reactions occurring at the electrode surface during the deposition process of NaHAp coatings: When the voltage was less negative than -1 V/SCE, the current density was mainly due to the reduction of oxygen on CoNiCrMo [18] O2 + 2H2O + 4e-  4OH- (5) In the potential range from -1 to - 1.6 V/SCE, the current density was almost constant, it characterizes the mass transfer process of NaHAp coatings on the surface of CoNiCrMo More negative potentials induce other ions being involved in the reduction process Some reactions can be proposed: H2PO4- + e- HPO42- + ½ H2 (6) HPO4 + e  PO4 + ½ H2 (7) 2H2O + 2e  H2 + 2OH (8) 2- - 3- - - 0.01 1E-3 i (mA/cm ) 1E-4 1E-5 1E-6 1E-7 1E-8 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 E (V/SCE) -1.4 -1.6 -1.8 -2.0 Figure The cathodic polarization curve of CoNiCrMo substrate in the electrolyte solution The generated hydroxide and phosphate ions react with Ca2+ and Na+ ions to form NaHAp coatings on the CoNiCrMo cathode according the chemical reaction (9) Based on the analysis of the cathodic polarization curves, we performed NaHAp deposition on electrodes in the above electrolyte solution with the different scanning potential ranges: from to -1.6; to -1.7; to -1.8 and to - 1.9 V/SCE during cycles with a scanning rate of mV/s, at 50oC The mass of NaHAp coatings are displayed in Figure and Table The mass of NaHAp coatings increases with the scanning potential range from (0 -1.6 V/SCE) to (0 -1.7 V/SCE) and the maximum mass is 1.42 g/cm2 (4.537 m thick) However, when the scanning potential range is increased, the mass of NaHAp coatings on the electrode surface decreases It can be explained as follow: with the negative scanning potential range, the formation of ions on the electrode surface is larger, leading to the diffusion of ions from the surrounding fluid of the working electrode into solution and forming NaHAp Furthermore when more negative potentials are achieved, the reactions (6), (7), (8) are enhanced, more H2 gas is generated on the electrode surface It could probably be the cause of the NaHAp coating delamination leading to chips that fall into the electrolyte solution Therefore, the scanning potential range from to -1.7V/SCE was chosen to synthesize NaHAp coatings on CoNiCrMo alloys (10-x)Ca2+ + xHPO42- + (6-x)PO43- + (2-x)OH- → Ca10-x(HPO4)x(PO4)6-xOH2-x that could be substituted with CO32- et Na+ (9) 1.5 to -1.7 1.4 1.3 NaHAp mass (mg) 1.2 1.1 1.0 0.9 0.8 to -1.8 0.7 0.6 0.5 to -1.6 0.4 to -1.9 0.3 0.2 0.1 0.0 Figure The variation of the NaHAp mass for different scanning potential ranges Table The mass and the thickness of the NaHAp coatings at the different potential ranges Potential range (V/SCE) to -1.6 to -1.7 to -1.8 to -1.9 NaHAp mass (mg/cm2) 0.508 ± 0.013 1.421 ± 0.020 0.658 ± 0.026 0.233 ± 0.065 Thickness (m) 1.621 ± 0.040 4.537 ± 0.057 2.102 ± 0.072 0.743 ± 0.04 FTIR spectra presented in Figure provide useful information about the nature of apatite All of the spectra show the characteristic bands due to PO43- ions (asymetric streching vibration of P-O bond at1040 cm-1; asymetric O-P-O bending mode at 560 and 610 cm-1) The stretching vibration of hydroxyl group for the standard HAp should be confirmed by the absorption at 3572 cm-1 and 632 cm-1bands but are not distinguishable on the FTIR spectra This could be explained by the nonstoichiometry of the apatite phase Moreover, the presence of contributions in the regions around 1385 cm-1 and 880 cm-1 respectively can be assigned to the vibration modes of CO32- The occurrence of CO32-may be due to the reaction of atmospheric CO2 with OH− ions, hence the content of CO32- ions is rather low [16, 17] These results are in good agreement with the reported data in literature [19, 20] NaHAp samples that were prepared at the different potential ranges did not have significant discrepancies in FTIR spectra to -1.8 V/SCE 4000 3500 PO3- 2- CO3 PO3- H2O H2O to -1.6 V/SCE 2- to -1.7 V/SCE CO3 Transmittance to -1.9 V/SCE 3000 2500 2000 1500 -1 1000 500 Wave number (cm ) Figure FTIR spectra of the NaHAp coatings synthesized at different scanning potential ranges 3.1.2 Effect of scanning rate The Figure show the cathodic polarization curves of NaHAp/CoNiCrMo formation in the electrolyte solution with the different scanning rates The scanning rate was varied from to mV/s With this parameter increase, the cathodic current density increases The reduction rate was stronger; NaHAp was created more and more The mass of NaHAp coatings was also determined by the mass of CoNiCrMo samples before and after electrodeposition (Figure 5) The mass increases with the scanning rate and reaches a maximum value at 3mV/s (1.923 mg/cm2) At higher scanning rate, the mass of NaHAp on the electrode surface decreases due to the formation NaHAp in the solution.The thickness of the coatings can be estimated by the formula (1) and is given in Table The thickest NaHAp coating is around 6.142 m with electrodeposition conditions: potential scanning range to -1.7 V/SCE, scanning rate mV/s at 50oC 0,000 -0,002 mV/s i (mA/cm ) -0,004 -0,006 mV/s -0,008 mV/s -0,010 mV/s -0,012 mV/s 0,0 -0,2 -0,4 -0,6 -0,8 -1,0 -1,2 -1,4 -1,6 -1,8 -2,0 E (V/SCE) Figure Cathodic polarization curves of CoNiCrMo in the electrolyte solution with the different scanning rates 2.0 1.8 NaHAp mass (mg) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Scanning rate (mV/s) Figure The variation of NaHAp mass for different scanning rates Table The variation of the mass and the thickness of NaHAp coatings with the scanning rates Scanning rate (mV/s) NaHAp mass (mg/cm ) 1.053 ± 0.013 1.705 ± 0.06 1.923 ± 0.014 1.430 ± 0.01 0.540 ± 0.05 Thickness (m) 3.367 ± 0.535 5.447 ± 0.078 6.142 ± 0.044 4.580 ± 0.036 1.725 ± 0.160 3.1.3 Effect of electrodeposition temperature The effect of the temperature on the mass of NaHAp coatings is shown in Fig and Table NaHAp mass increases with the temperature from 35 to 70°C NaHAp mass is maximum, 2.42 mg/cm2 (the coating thickness: 7.732 m, respectively) at 70°C but NaHAp coatings become more porous (image is not shown) The increase of the mass as well as of the thickness of the coatings can be explained as following: the deposition temperature may change the reaction rate and the diffusion rate of ions The high temperature can either promote the formation of NaHAp coatings on the electrode surface or the bulk precipitation due to high diffusion rate; the rise of the temperature can lower hydrogen bubble attachment on the substrate surface, allowing the growing of NaHAp films The coating is less damaged and more adherent, accordingly In summary, by adjusting the temperature in the deposition process, the mass and the thickness of NaHAp coatings can be controlled 2.4 2.2 NaHAp mass (mg) 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 35 50 o Temperature ( C) 70 Figure 6: The mass of NaHAp coatings synthesized for different deposition temperatures Table 5: Mass and thickness of NaHAp coatings for different temperatures Deposition temperature (oC) 35 50 70 NaHAp mass (mg/cm ) 1.923 ± 0.014 2.055 ± 0.075 2.42 ± 0.08 Thickness (m) 6.142 ± 0.044 6.566 ± 0.240 7.732 ± 0.256 The XRD diffraction data of NaHAp coatings synthesized at the different temperatures are recorded in the 2 of 10-90o The obtained XRD patterns are shown in Figure The XRD pattern of NaHAp synthesized at 50oC exhibits the hydroxyapatite phase The typical peaks are found at 2 of 31.99o (211) and 26.05o (002).The XRD pattern of NaHAp coatings synthesized at 70oC presents the characteristic peaks of HAp with higher intensity In addition the characteristic peaks of dicalcium phosphate dehydrate (DCDP, CaHPO4.2H2O) and tricalcium phosphate (TCP, Ca3(PO4)2) (denoted 2; in the Fig.7) are also observed on the patterns Specially, when the deposition temperature was 35oC, the intensity of the characteristic peaks of HAp are very low Therefore, the deposition temperature at 50oC was chosen to synthesize NaHAp/CoNiCrMo 1: HAp 2: DCDP 3: TCP 1 3 1 11 1 1 11 1 o 70 C Intensity 1 1 1 o 50 C 32 3 10 20 30 33 1 40 50 2 (degree) 60 o 35 C 70 80 90 Figure XRD patterns of NaHAp coatings synthesized for different temperatures: 35, 50 and 70oC The elemental composition of NaHAp coatings is determined by EDX analysis (Figure 8) The result show the presence of the main elements in HAp compounds The peaks related to O, P and Ca elements are clearly seen in the spectrum which corresponds to the composition of 46.11%, 17.2 and 31.27%, respectively (Table 6) Besides, the presence of sodium is identified in the coatings at an amount of 0.73wt.% This result shows that the sodium was substituted in the HAp composition of the coatings 4000 Ca 3000 Counts P 2000 O 1000 Ca C Cl Na 0 Energy (KeV) 10 Figure The EDX spectrum of NaHAp coating synthesized at 50oC Table Chemical composition of NaHAp coatings Elements % mass % atom O 46.11 62.63 Ca 31.27 16.96 P 17.20 12.07 Na 0.73 0.69 Cl 0.69 0.42 C 7.24 From the above results, we were able to choose the best suitable conditions to synthesize NaHAp coatings on the surface of CoNiCrMo alloys for studying the electrochemical behavior of NaHAp/CoNiCrMo in the simulated body fluid (SBF): potential scanning range: to -1.7 V/SCE; scanning rate: mV/s; deposition temperature: 50oC 3.2 The electrochemical behavior of NaHAp/CoNiCrMo in the simulated body fluid (SBF) The variation of the pH value Figure shows the pH values of the SBF solution containing CoNiCrMo alloys or NaHAp/CoNiCrMo at different immersion time, at 37oC The pH value of the SBF solution before soaking is 7.4 During the immersion time, the pH values of the SBF solutions decreased However, the pH of the SBF solution containing NaHAp/CoNiCrMo decreased more strongly It can be hypothesized that NaHAp crystals on the surface of CoNiCrMo behave as the starting point to promote the formation of apatite The formation of apatite was of the reason of the decrease of the pH.When NaHAp/CoNiCrMo is immersed into the SBF solution, the solution pH decreases strongly after immersion days This result shows that the formation of apatite was a lot After 28 immersion days into the SBF solution, the pH value of the solution containing NaHAp/CoNiCrMo was 6.14 7.6 7.4 (1) 7.2 pH 7.0 6.8 6.6 6.4 6.2 (2) 6.0 -2 10 12 14 16 18 20 22 24 26 28 30 Immersion time (day) Figure 9: The pH value of the SBF solution containing (1) CoNiCrMo and (2) NaHAp/CoNiCrMo materials vs immersion time at 37oC Variation of weight The weight of CoNiCrMo substrate after 28 immersion days in the SBF solution does not have changed Therefore the decrease of the solution pH was explained by the formation of apatite in the solution The variation of weight of NaHAp/CoNiCrMo samples during immersion time is displayed in Figure 10 All of time, the weight of NaHAp/CoNiCrMo samples increased but after immersion days it increased strongly (Δm = 0.97 mg) and then it increased continuously with 14, 21 and 28 immersion days It is clear that the formation of apatite crystals on the surface of NaHAp/CoNiCrMo increased significantly These results are confirmed by the SEM images (Figure 11) 3.5 3.0 mmg 2.5 2.0 1.5 1.0 0.5 0.0 10 14 Time (day) 21 28 Figure 10: Variation of the weight of NaHAp/CoNiCrMo vs immersion time in the SBF solution The SEM images Figure 11 displays SEM images of NaHAp/CoNiCrMo samples which were immersed in the SBF solution during 0, 7, 14 and 21 days The SEM image of the NaHAp/CoNiCrMo sample before immersion in the SBF solution shows that the NaHAp crystals have fairly uniformed plate shapes After immersion days in the SBF solution the nucleation of the apatite crystals are observed The plates of NaHAp crystals were fully covered by apatite crystals which have cactus form When the immersion time in the SBF solution is longer, the formation of apatite crystal increases After 14 and 21 immersion days, apatite crystals grew with higher density Specially, with the sample immersed during 21 days in the SBF solution, apatite crystals grew up to form a thicker block These results are suitable with pH decreasing and increasing sample weight in the immersing process Figure 11: SEM images of NaHAp/CoNiCrMo samples before and after different times of immersion in the SBF solution Polarization measurements Variation of parameters Ecorr, Icorr, Rp according to immersion time were calculated from polarization potential measurements results of CoCrNiMo and NaHAp/CoCrNiMo samples in the SBF solution (Table 7) With CoCrNiMo samples, the corrosion current density decreases insignificantly during immersion time and Rp hardly increases because the apatite film formation on the material surfaces is very thin In contrast, with NaHAp/CoCrNiMo sample the corrosion current density strongly decreases (about times) after immersion days due to the apatite formation on the surface At long immersion times, the corrosion current density increases because of the dissolution of the electrodeposited hydroxyapatite Generally, at the all immersion times the corrosion current density of CoCrNiMo i higher than the one of NaHAp/CoCrNiMo and the polarization resistance of CoCrNiMo is lower than the one of NaHAp/CoCrNiMo This could indicate that the NaHAp coatings protect the CoCrNiMo substrate Table 7: Variation of Ecorr, Icorr, Rp of CrCoNiMo and NaHAp/CrCoNiMo in the SBF solution vs the immersion time Time (day) CrCoNiMo NaHAp/CrCoNiMo Ecorr (V/SCE) Icorr (µA) Rp (k) Ecorr (V/SCE) Icorr (µA) Rp (k) -0.07 3.42 7.3 -0.142 1.77 13 -0.13 3.60 6.94 -0.170 0.43 54 -0.11 2.59 9.62 -0.127 1.05 22 14 -0.087 2.59 9.62 -0.117 1.15 20 21 -0.102 2.25 11.11 -0.142 1.15 20 CONCLUSION NaHAp coatings have been successfully synthesized on CoCrNiMo surface by the electrodeposition method via a simple technique The optimal conditions were determined as follows: the scanning potential range from to −1.7 V/SCE during cycles at scanning rate mV/s, temperature of 50°C, with the electrolyte composition of Ca(NO3)2·4H2O 3x10-2 M, NH4H2PO4 1.8x10-2 M, NaNO3 0.06 M The obtained coatings were single phase crystals of HAp, plate shape The results of the in vitro test with CoNiCrMo substrate and NaHAp/CoNiCrMo samples in the SBF solution were realized with different immersion times SEM images showed the formation of apatite on the surface of NaHAp/CoNiCrMo during the immersion time in the SBF solution The apatite crystals had a cactus-like shape and they fully covered the surface material after immersion days After 28 immersion days, the formation of apatite formed a thick block The electrochemical behavior of NaHAp/CoNiCrMo in the SBF solution showed the formation of apatite and HAp coatings behave as a protective layer for Co alloys substrate Acknowledgments This work was supported by Viet Nam Academy of Science and Technology (under grant no VAST HTQT FRANCE 03/15-16) and was conducted within the context of the International Associated Laboratory “Functional Composite Materials” (LIA-FOCOMAT) created between the ITT (Vietnam)-VAST and the CIRIMAT (France)-CNRS institutes REFERENCES K Grandfield, A Palmquist, S Gonỗalves, A Taylor, M Taylor, L Emanuelsson, P Thomsen, H Engqvist.- Free form fabricated features on CoCr implants with and without hydroxyapatite coating in vivo: a comparative study of bone contact and bone growth induction, J Mater Sci.: Mater Med 22 (2011) 899-906 T.M Devine, J Wulff.- Cast vs wrought cobalt-chromium surgical implant alloys, J Biomed Mater Res (1975) 151-167 3 S Hiromoto, E Onodera, A Chiba, K Asami, T Hanawa.- Microstructure and corrosion behaviour in biological environments of the new forged low-Ni 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SBF solution containing NaHAp/ CoNiCrMo decreased more strongly It can be hypothesized that NaHAp crystals on the surface of CoNiCrMo behave as the starting point to promote the formation of apatite... cathodic polarization curve of CoNiCrMo substrate in the electrolyte solution The generated hydroxide and phosphate ions react with Ca2+ and Na+ ions to form NaHAp coatings on the CoNiCrMo cathode