corrosion mechanism of micro arc oxidation treated biocompatible az31 magnesium alloy in simulated body fluid

Available online at www.sciencedirect.com HOSTED BY Progress in Natural Science Materials International Progress in Natural Science: Materials International 24 (2014) 516–522 www.elsevier.com/locate/pnsmi www.sciencedirect.com Original Research Corrosion mechanism of micro-arc oxidation treated biocompatible AZ31 magnesium alloy in simulated body fluid Ying Lia,g,1, Fang Luf,1, Honglong Lia,1, Wenjun Zhub,1, Haobo Panc, Guoxin Tand,nnn, Yonghua Laoa, Chengyun Ninga,g,n, Guoxin Nie,nn a School of Materials Science and Engineering, South China University of Technology, Guangzhou, China Department of Prosthodontics, Guanghua School of Stomatology, Sun Yat-sen University, Guangzhou, China c Center for Human Tissues and Organs Degeneration, Shenzhen Institute of Advanced Technology, Chinese Academy of Science, Shenzhen, China d Institute of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, China e Department of Orthopeadics and Traumatology, Nanfang Hospital, Southern Medical University, China f School of Chinese Materia Medica, Guangzhou University of Chinese Medicine, Guangzhou, China g Guangdong Province Key Laboratory of Biomedical Engineering, South China University of Technology, Guangzhou, China b Received July 2014; accepted September 2014 Available online 29 October 2014 Abstract The rapid degradation of magnesium (Mg) based alloys has prevented their further use in orthopedic trauma fixation and vascular intervention, and therefore it is essential to investigate the corrosion mechanism for improving the corrosion resistance of these alloys In this work, the effect of applied voltage on the surface morphology and the corrosion behavior of micro-arc oxidation (MAO) with different voltages were carried out to obtain biocompatible ceramic coatings on AZ31 Mg alloy The effects of applied voltage on the surface morphology and the corrosion behavior of MAO samples in the simulated body fluid (SBF) were studied systematically Scanning electron microscope (SEM) and X-ray diffractometer (XRD) were employed to characterize the morphologies and phase compositions of coating before and after corrosion The results showed that corrosion resistance of the MAO coating obtained at 250 V was better than the others in SBF The dense layer of MAO coating and the corrosion precipitation were the key factors for corrosion behavior The corrosion of precipitation Mg(OH)2 and the calcium phosphate (Ca–P) minerals on the surface of MAO coatings could enhance their corrosion resistance effectively In addition, the mechanism of MAO coated Mg alloys was proposed & 2014 Chinese Materials Research Society Production and hosting by Elsevier B.V All rights reserved Keywords: Biodegradable; Mechanism; Micro-arc oxidation; AZ31 magnesium alloy; Corrosion resistance Introduction Mg and its alloys have been increasingly used in orthopedic trauma fixation and vascular intervention due to their excellent biodegradability, good biocompatibility and outstanding mechanical properties [1–5] However, the rapid degradation rate will n Corresponding author Tel.: ỵ86 20 22236059 Corresponding author Tel.: ỵ86 20 61641744 nnn Corresponding author Tel.: ỵ 86 13631419254 E-mail addresses: tanguoxin@126.com (G Tan), imcyning@scut.edu.cn (C Ning), fgxni@graduate.hku.hk (G Ni) These authors contributed equally to this work and should be considered co-first author Peer review under responsibility of Chinese Materials Research Society nn weaken their mechanical properties before the blood vessel sufficiently heals [4,6] Even worse, high local alkaline environment may lead to inflammation at the implanted site Hence, it is essential to control the degradation rate of Mg alloys Besides alloying [7–12], surface modification are the main methods to control the degradation rate of Mg alloys, and myriads of surface techniques have been developed, such as MAO [22,23], ion implantation [13,14], plasma anodisation [15], chemical conversion films [16–19], and electrochemical deposition [20,21] Among these methods, MAO technique has recently becoming one of the most attractive ways which may enhance the hardness, wear resistance, corrosion resistance by forming remarkably dense and hard coating on substrates Meanwhile, the porous microstructure of MAO coating is beneficial for the rapid adhesion and growth of http://dx.doi.org/10.1016/j.pnsc.2014.08.007 1002-0071/& 2014 Chinese Materials Research Society Production and hosting by Elsevier B.V All rights reserved Y Li et al / Progress in Natural Science: Materials International 24 (2014) 516–522 cells, resulting in a significantly stronger bond to the parent tissue Gu et al showed that MAO might be a promising method for Mg– Ca alloys, which significantly enhanced the corrosion resistance and biocompatibility of these alloys [7] It has been reported that MAO coatings on Mg alloys can improve the corrosion resistance in SBF However, most of studies focus on the corrosion behavior of MAO coatings on AZ91 Mg alloy or pure Mg in SBF, few studies focus on MAO coatings on AZ31 Mg alloy [24,25] In addition, the effects of applied voltage on corrosion behavior of MAO coating on AZ31 Mg alloy in SBF was also rarely investigated Generally, corrosion behavior of MAO coated on Mg alloys in physiological conditions was greatly different from those of Mg alloys treated by other techniques Hence, in order to develop better Mg-based biomaterials, it is essential to understand the role of MAO coatings during the corrosion process of Mg alloys under physiological conditions In this paper, AZ31 Mg alloy was treated by the MAO technique under different voltages, and the effects of voltages on the corrosion behavior of MAO coated alloys were investigated To develop a better understanding of corrosion mechanisms in physiological conditions, the role of immersion time on the corrosion behaviors of MAO coated AZ31 was also studied Experimental procedure 2.1 MAO coatings preparation Commercially available AZ31 Mg alloy with a size of 40 mm  10 mm  mm (mass fraction: Al 3.0%, Mn 0.2%, Zn 0.1%, Si 0.05%, Cu 0.01%, balance Mg) was used as starting materials Specimens were mechanically grinded by waterproof abrasive paper progressively up to 3000 grint, and then ultrasonic rinsed for 10 with acetone, ethanol and deionized water, respectively Prior to undergoing the MAO process, the based electrolyte of alkaline silicate was prepared from the solution of Na3 (PO4)2 (20.0 g/L) in distilled water The coatings were obtained using a 50 kW capacity MAO equipment (MAO 20, Chengdu PULSETECH Electrical CO., China) under a constantly pulse frequency of 100 Hz for at DC voltages of 250 V, 300 V and 350 V, respectively, During this course, the pulse ratio is 0.3 After treatment, the obtained samples were washed with distilled water and dried in the air 517 the counter electrode, saturated calomel electrode as the reference electrode, and specimen as the working electrode All experiments were carried out at the open circuit potential (OCP) with an equilibrium time to receive OCP of The polarization curves were scanned from À 2.0 V at 3.0 mV/s All potentiodynamic polarization data were analyzed by Gamry Echem Analyst software 2.3 Surface characterization Scanning electron microscope (SEM) was used to observe the microstructures of the samples before and after immersion in SBF The compositions of the corrosion products were characterized by X-ray diffraction (XRD, D8 Advance Bruker Co., Germany), using Cu Kα (λ¼ 1.5406 Å) radiation in stepscan mode (2θ ¼ 0.02 per step) Results and discussion 3.1 Effects of voltage on the surface morphologies Fig illustrated the surface morphologies of MAO coatings made from the Na3PO4 solutions at different voltages It can be seen clearly that the coatings displayed porous microstructure with some volcano top-like micropores Meanwhile, as the applied voltage increasing, the size of microspores on MAO coating increased When the applied voltage was 250 V, a porous oxide coating with a pore diameter of 1.6 μm was formed on the surface As the voltage was increased to 300 V, the size of microspores increased to approximately 2.0 μm However, when the voltage reached 350 V, some cracks appeared on the coating with the pore size increasing to 2.5 μm slightly The microstructure evolution with applied voltage can be attributed to the dielectric breakdown of MAO layers When the applied voltage was 250 V, a mild sparking was observed and uniformly distributed on the substrate surface, resulting in fine micro-pores With the applied voltage increasing, the discharge intensity of single spark was enhanced so that the pore size of the layer increased at higher applied voltage Especially, the discharge spark was serious which brought about the thermal stress in the coating at 350 V, which induced some cracks on the surface of coatings after the solidification 3.2 Potentiodynamic polarization tests 2.2 Electrochemical tests To evaluate the corrosion behavior of MAO coatings, potentiodynamic polarization tests were employed using a computer controlled Corrosion Cell Kit (Gamry Instruments, Inc.) Samples with an exposed area of 4.18 cm2 were immersed in SBF for days at 36.570.5 1C The pH value of SBF solution was buffered to 7.25 with tris (hydroxymethyl), minomethane ((CH2OH)3CNH2) and 1.0 mol/L HCl And the ion concentration in SBF were nearly equal to those in human blood plasma [26, Table 1] The corresponding composition of the SBF was listed in [27, Table 2] Meanwhile, a conventional three electrodes electrochemical cell was used with platinum as 3.2.1 Effects of voltage on corrosion resistance of MAO coatings Potentiodynamic polarization curves of MAO coated samples obtained at different voltages in SBF were shown in Fig The important parameter Icorr was generated directly from the potentiodynamic polarization curves by analyzed using the Gamry Echem Analyst software and experimental data were summarized in Fig It could be observed that the corrosion resistance of MAO coated samples decreased with the increasing of the voltage irrespective of immersion time And the 250 V sample showed the highest corrosion resistance, which was indicated by the lowest Icorr than those of the other samples (Fig 3) With the increasing 518 Y Li et al / Progress in Natural Science: Materials International 24 (2014) 516–522 Fig Effect of the voltage on MAO coating morphology: (a) 250 V (b) 300 V and (c) 350 V voltage, the discharge spark became sharper, which brought about larger pores and even some microcracks on the surface of coating The pores and microcracks of the MAO coating can provide channels for the corrosive liquid to enter into the interior coating As a result, the corrosion resistance was gradually weakened 3.2.2 Effects of immersion time on the corrosion resistance of MAO coatings Fig showed the corrosion behaviors of MAO coatings evaluated by potentiodynamic polarization tests in SBF for different immersion time According to Fig and Fig 4, it could be clearly observed that the corrosion resistance of the samples changed accompanying with the immersion time in SBF The samples treated at 250 V (Mg-250) exhibited the lowest Icorr during the whole immersion time The Icorr values of the three kinds of samples displayed the same trend, and exhibited the lowest values of 1.67, 2.46 and 9.95 μA/cm2 after immersion in SBF for days at 250 V, 300 V, 350 V, respectively After days immersion, this value reached the lowest point of 1.8, 2.89 and 8.77 μA/cm2 again seen that a thin layer with lots of cracks appeared on the substrate, which could be ascribed to the dehydration, inducing the formation of shrank when the samples were dried in air According to EDS result (Fig 5b), the chemical compositions includes O, P, Ca, Mg and Al elements The elements of Mg and Al originated from the AZ31 substrate, and Ca and P originated from the new products deposited on the AZ31 surface Fig showed the XRD patterns obtained from Mg250 and its corrosion products after socking in SBF for days The corroded surface was composed of Mg, brucite-Mg(OH)2, hydroxyapatite(HA)-Ca10(PO4)6(OH)2 and Ca2P2O7 The Mg came from the substrate Brucite, hydroxyapatite and Ca2P2O7 were the corrosion products The formation of hydroxyapatite was due to HPO24 and Ca2 ỵ ions in SBF The dissolution of metal Mg was accompanied with the hydrogen evolution reaction, resulting in the increase of pH value in SBF With the immersion time increasing and the increase in [OH À ], the calcium phosphate (Ca–P) minerals appeared on the surface of Mg alloy, which attributed to the following precipitation reaction: 3.3 Characterization of corroded samples 5Ca2 þ þ 3PO34 À þ OH À -Ca5 ðPO4 Þ3 OH Fig exhibited the surface morphologies and compositions of Mg-250 after socking in SBF for days It could be clearly So, the thin layer could be confirmed to be the calcium phosphate (Ca–P) minerals ð1Þ Y Li et al / Progress in Natural Science: Materials International 24 (2014) 516–522 519 Fig Tafel curves of MAO coatings produced at different voltages after immersion in the SBF for the same days: (a) day (b) days ( c) days and (d) days on the AZ31 Mg alloy was composed of two layers: a porous outer layer and a dense inner layer which could be seen from Fig 7a The porous outer layer has several large-sized and deep pores In contrast, the inner layer was denser with smaller pores The MAO coated samples corroded as following reactions after immersion in SBF: Anodic reactions Mg-Mg2 ỵ ỵ 2e À ð2Þ Cathodic reactions Fig Results of potentiodynamic polarization behavior of three samples after immersion in the SBF for different times 3.4 Corrosion mechanism model of MAO coating immersed in the SBF Based on the electrochemical performance tests, a schematic diagram for the corrosion process of the MAO coated AZ31 Mg alloy was presented in Fig The MAO coating formed O2 ỵ 2H2 O ỵ 4e -4OH 3ị 2H2 O ỵ 2e -2OH ỵ H2 ↑ ð4Þ Consequently the following reaction also occurs Mg2 þ þ 2OH À -MgðOHÞ2 ↓ ð5Þ Based on the above reactions, at the initial stage of immersion time, the metal Mg was corroded and transferred into Mg (OH)2 film with SBF rapidly penetrating into the porous outer layer which could be clearly shown in Fig 7b The phase of Mg(OH)2 was detected in the coating from the XRD pattern With immersion time increasing, the corrosion production of Mg(OH)2 increased and some pores were filled by these precipitates, which decreased the porosity of the coating Consequently, it would take a long time for SBF to go through 520 Y Li et al / Progress in Natural Science: Materials International 24 (2014) 516–522 Fig Tafel of MAO coatings produced at the same voltages after immersion in the SBF for different days: ( a) 250 V (b) 300 V and ( c) 350 V Fig SEM and EDS elemental composition of mineralization on the 250 V sample after socked in SBF for days the outer surface layer into the inner layer (Fig 7c) The results indicated that MAO coating had the protective function in the corrosion process in SBF However, the Mg(OH)2 film formed in SBF was porous, allowing SBF diffused into MAO coating With the extension of immersion time, Cl À contained in corrosive mediums adsorbed on the sample surface to dissolve the Mg(OH)2 film as the following reaction was MgOHị2 ỵ 2Cl -MgCl2 ỵ 2OH À ð6Þ As a result, Cl could accelerate the corrosion of MAO coating by forming MgCl2, and weaken the corrosion resistance of the MAO coating At the early stage, the pH value of SBF increased with the degradation of the MAO coated Mg alloy, which attributed to the precipitation of Mg(OH)2 during this course Meanwhile, Cl À ions could dissolve the Mg(OH)2 deposit, which acted as a barrier to prevent the degradation of Mg alloy After immersion in SBF for a long time, more calcium phosphate (Ca–P) minerals were accumulated on the surface to form a thin layer (Fig 7c and d), which could be determined by XRD, SEM and EDS (Figs and 6) The layer became relatively thicker (Fig 7e) and created a barrier that resist the corrosive SBF penetrating into the inner layer of MAO coating These result indicated that the degradation rate would Y Li et al / Progress in Natural Science: Materials International 24 (2014) 516–522 521 reduce significantly due to the barrier effect of both MAO coating and the precipitation layer when MAO coated magnesium alloys continues to be immersed in SBF Conclusions Fig XRD of on the 250 V sample surface and its corrosion product after socked in SBF for days: (a) surface phase (b) corrosion product phase in solution (1) Potentiodynamic polarization curves exhibited Mg-250 showed better corrosion resistance in the SBF physiological conditions (2) The corrosion resistance of the samples varied and a model for corrosion mechanism of MAO coated Mg alloy was proposed The dense layer played a crucial role in the corrosion process of MAO coating The corrosion product of Mg(OH)2 and the formed precipitation layer on MAO coating during corrosion process also effectively prevented Mg alloy from degrading Fig Schematic diagram of the corrosion process and mechanism of MAO coated AZ31 Mg alloy upon immersion in the SBF 522 Y Li et al / Progress in Natural Science: Materials International 24 (2014) 516–522 Acknowledgments This research work was financially sponsored by National Basic Research Program of China (Grant no 2012CB619100) and the National Natural Science Foundation of China (Grant nos 21105029, 51102097, 51232002) References [1] H.S Brar, M.O Platt, M Sarntinoranont, P.I Martin, M.V Manuel, Jom-us 61 (2009) 31–34 [2] F Witte, N Hort, C Vogt, S Cohen, et al., Curr Opin Solid State Mater Sci 12 (2008) 63–72 [3] M.P Staiger, A.M Pietak, J Huadmai, G Dias, Biomaterials 27 (2006) 1728–1734 [4] R Zeng, W Dietzel, F Witte, N Hort, C Blawert, Adv Eng Mater 10 (2008) B3–B14 [5] P Zartner, R Cesnjevar, H Singer, M Weyand, Catheter Cardio Interv 66 (2005) 590–594 [6] G.L Song, A Atrens, Adv Eng Mater (1999) 11–33 [7] X Gu, N Li, W Zhou, Y Zheng, et al., Acta Biomater (2011) 1880–1889 [8] X Gu, W Zhou, Y Zheng, Y Cheng, et 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MgCl2, and weaken the corrosion resistance of the MAO coating At the early stage, the pH value of SBF increased with the degradation of the MAO coated Mg alloy,