Surface & Coatings Technology

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1 Surface & Coatings Technology 204 (2010) Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: Comparison of calcium phosphate coatings on Mg Al and Mg Ca alloys and their corrosion behavior in Hank's solution Zhang Chun-Yan a,b,, Zeng Rong-Chang a, Liu Cheng-Long a, Gao Jia-Cheng b a School of Materials Science & Engineering, Chongqing University of Technology, Chongqing, , China b College of Materials Science & Engineering, Chongqing University, Chongqing, , China article info abstract Article history: Received 29 January 2010 Accepted in revised form 14 April 2010 Available online 18 April 2010 Keywords: Calcium phosphate coating Magnesium alloys Corrosion Hank's solution Calcium phosphate coatings were prepared on AZ31 and Mg 1.0Ca magnesium alloys by electrochemical deposition. Hydrogen evolution testing and potentiodynamic electrochemical technique were employed to investigate the corrosion behavior of the coated alloys in Hank's solution. The results show that the deposited coatings mainly consist of flaky brushite (DCPD, CaHPO 4 2H 2 O) crystallites. The obvious increase in free corrosion potential (open circuit potential) and decrease in corrosion current density of the coated samples indicate that the coating significantly enhanced the corrosion resistance of the substrates. The hydrogen evolution rates of the coated samples were much lower than that of the substrates during initial immersion in Hank's solution. However, the corrosion rate of the coated samples increased rapidly once the samples suffered pit corrosion. Furthermore, the corrosion properties of the coated samples have much to do with the corrosion properties of their substrates Elsevier B.V. All rights reserved. 1. Introduction Traditional biomedical metal materials such as stainless steels (316L), titanium alloys (Ti6Al4V) and cobalt chromium-based alloys have been widely used as orthopedic implants. However, as permanent implants in the human body, they will always release cytotoxic elements (such as cobalt, chromium, nickel and vanadium), which are considered potentially harmful to tissues of the human body [1,2]. Since these metal implants should be removed after the first operation, another surgical intervention is necessary with all its personal, medical, social and economical consequences and costs. In comparison, magnesium and its alloys are biodegradable in human body fluids, and the corrosion product of magnesium is likely to be physiologically beneficial [3,4]. Moreover the mechanical properties of magnesium are similar to those of natural bone [5]. In recent years, magnesium alloys with less impurity by new casting techniques have gained increasing interest as biodegradable metal implant in orthopedic surgery. However, magnesium and its alloys are characterized by low corrosion resistance in the high chloride environment of physiological systems, evolving hydrogen gas in the corrosion process at a rate too fast to be dealt with by the tissue, causing subcutaneous gas pockets, hence limits their use [5,6]. One solution to delay the degradation rate of magnesium alloys is to coat the base material. Calcium phosphates, including hydroxyapatite (HA), octacalcium phosphate (OCP) and brushite (DCPD), have Corresponding author: School of Materials Science & Engineering, Chongqing University of Technology, Chongqing, , China. Tel.: address: zhangchunyan@cqut.edu.cn (Z. Chun-Yan). been widely used as coatings of bone implants because of their excellent biocompatibility, non-toxicity, bioactivity and osseoconductive [7]. In addition, it was reported the magnesium alloy promotes or accelerates the deposition of calcium and phosphate [8]. Recently, calcium phosphate coating has been used to decrease the degradation rates of magnesium alloys. The results indicate calcium phosphate coatings can provide effective corrosion protection for magnesium alloys [9,10]. The present authors also studied the effects of the pre-calcification and anodized oxidation treatment of AZ31 alloy on the formation of calcium phosphate coating [11]. But to the best of our knowledge, no report on the study of the influence of the calcium phosphate coating on the hydrogen evolution rate of magnesium alloys can be found in the literatures [9,10]. This study aims to prepare calcium phosphate coatings on AZ31 and Mg 1.0Ca alloys by way of electrochemical deposition and compare their different bio-degradation behavior by means of potentiodynamic polarization test and hydrogen evolution test in Hank's solutions. 2. Experimental 2.1. Sample preparation Two hot extruded magnesium alloys, AZ31 (3.0 wt.% Al; 1.0 wt.% Zn; 0.20 wt.% Mn; Mg. Bal) and Mg 1.0Ca (1.0 wt.% Ca; Mg. Bal) sheets with a size of mm 3 were used as substrates for electro-deposition. The sample surfaces were ground with SiC abrasive papers up to 1000 grits, then ultrasonically cleaned in acetone for 15 min, rinsed in distilled water and finally dried at room temperature /$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.surfcoat

2 Z. Chun-Yan et al. / Surface & Coatings Technology 204 (2010) The electro-deposition of calcium phosphate was performed at 40 C for 4 h in a mixed solution of M Ca(NO 3 ) 2 4H 2 O and M NH 4 H 2 PO 4 at ph 5.0. A conventional cell was fitted with a saturated calomel electrode and a cathodic potential of 3.0 V applied. The AZ31 and Mg 1.0Ca sheets were used as the cathodes for electro-deposition Corrosion testing Electrochemical measurement Electrochemical tests were carried out by an EG G 273 Model electrochemical potentiostat using a classical three electrodes cell with platinum as counter electrode, saturated calomel electrode SCE as reference electrode and the uncoated or coated samples as working electrode with an exposed surface area of 2.84 cm 2. The potentiodynamic polarization curves of the coated or uncoated samples in Hank's solution were obtained at a scan rate of 0.5 mv s 1.ThepHofHank's solution is 7.4.The chemical composition of Hank's solution includes: NaCl:8 g/l, KCl: 0.4 g/l, CaCl 2 :0.14 g/l, NaHCO 3 :0.35g/l,C 6 H6O 6 :1.0g/l MgCl 2 6H 2 O: 0.1 g/l, MgSO 4 7H 2 O: 0.06 g/l, KH 2 PO 4 : 0.06 g/l, Na 2 HPO 4 12H 2 O: 0.06 g/l Immersion tests The volume of hydrogen evolution during the immersion is related to the dissolution of magnesium. The corrosion products do not influence the relationship between hydrogen evolution and magnesium dissolution [12]. The degradation rate of magnesium can thus be monitored by the evolved hydrogen volume. The testing equipment is shown in Fig.1. The samples with and without coating were soaked in a beaker containing Hank's solution at an ambient temperature (10 14 C), respectively. A funnel was placed over the specimen, which ensured the collection of hydrogen gas from the specimen surface. A burette was mounted over the funnel and filled with Hank's solution. The volume of evolved hydrogen was measured every 12 h. The total surface area exposed to the solutions was 10 cm Results and discussion 3.1. Morphology and chemical composition of the coatings At the preliminary stage of electro-deposition, some visible bubbles were observed on the surface of both AZ31 and Mg 1.0Ca alloys. After 4 h deposition, white homogeneous coatings were obtained on the substrates. Figs.1 and 2 show the SEM micrographs of the coatings on the surfaces of AZ31 and Mg 1.0Ca alloys, respectively. The morphology of the coating on the AZ31 is similar to that of the coating on Mg 1.0Ca. Both coatings consist of irregularly arranged lamellate crystallites layer. It was reported that the implant materials with pore sizes above 75 µm are propitious to bone cell ingrowth [13] and thus to accelerate the healing of the damaged bone [14]. Therefore, the loose structure of the coatings formed on magnesium alloys may benefit the healing of the damaged bone. The results of the EDS are summarized in Table 1. It was found that the coatings on both substrates are rich in calcium, phosphorus and oxygen; the atomic ratios of Ca/P of the coatings were about 1.0. The coatings formed on the AZ31 and Mg 1.0Ca alloys were thick enough that no magnesium peaks were detected by EDS analysis. The similarity of the morphology and chemical composition of the two coated samples indicates that the variation of the substrate had little influence on the morphologies and chemical compositions of the coatings XRD analysis Figs. 4 and 5 show the XRD patterns of the AZ31 and Mg 1.0Ca samples, respectively before and after 4 h electro-deposition. As opposed to the uncoated samples, besides the peaks of the substrates, 2.3. Surface characterization The structure of the deposit was analyzed by virtue of X-ray diffraction (XRD), and the morphology was analyzed by scanning electron microscopy (SEM, JSM-64660LV). An attached energy dispersive X-ray spectroscopy system (EDS, Oxford Isis) was used to examine the chemical composition of the deposit. Fig. 1. Schematic drawing of a hydrogen evolution collection set-up. Fig. 2. SEM micrographs of the coating on AZ31 alloy with different magnification after electro-deposition ( 与文章表达不同 ).

3 3638 Z. Chun-Yan et al. / Surface & Coatings Technology 204 (2010) Table 1 Chemical composition of the areas marked in Figs. 2 and 3, at.%. Spectrum O P Ca Ca/P some new peaks appear at 2θ=11.7, 20.9, 23.4, 29.3 and 35.4 in the coated sample, which correspond to the (020), (021), (040), (041) and (060) interplanar spacings of brushite (DCPD, CaHPO 4 2H 2 O). The narrow line widths and high intensities of the diffraction peaks indicate that the crystallinity of DCPD is relatively high. The formation process of calcium phosphate coatings can be described by a combination of several reactions including acid-base reaction, precipitation reaction and crystallization process [14,15]. The reaction processes can be expressed as: The water at the cathode surface is decomposed into hydrogen gas: H 2 O þ 2e H 2 þ 2OH The hydroxyl ions interact with phosphate species as follows: ð1þ Fig. 4. XRD patterns of AZ31 samples before and after electro-deposition. Ca 2þ þ HPO 2 4 þ 2H 2 O CaHPO 4 2H 2 O ðdcpdþ ð4þ 3Ca 2þ þ 2PO 3 4 þ nh 2 O Ca 3 ðpo 4 Þ 2 nh 2 O ðtcpþ ð5þ H 2 PO 4 H 2 PO 4 þ OH HPO 2 4 þ H 2 O ð2þ þ 2OH PO 3 4 þ 2H 2 O ð3þ 8Ca þ 4PO 3 4 þ 2HPO 2 4 þ 5H 2 Ca 8 H 2 ðpo 4 Þ 6 5H 2 O ðocpþ ð6þ 10Ca 2þ þ 6PO 3 4 þ 2OH Ca 10 ðpo 4 Þ 6 ðohþ 2 ðhaþ ð7þ The formation of DCPD among calcium phosphate phases is closely dependent upon the supersaturation and the ph values of the solutions. At lower ph, DCPD may be involved as a precursor phase [16,17]. In a physiological environment at ph 7.4, DCPD will transform into hydroxyl apatite (HA) [18]. It is reported that magnesium ion markedly inhibits HA crystal growth in a solution supersaturated only with respect to this phase, and has a modest influence on the kinetics of OCP growth, but has no effect on DCPD crystallization [17]. Therefore, in the case of ph 5.0 of the solutions, the deposited coatings on AZ31 and Mg 1.0Ca alloys predominately consisted of DCPD (CaHPO 4 2H 2 O) crystallites Electrochemical behaviour The polarization curves for the uncoated and Ca P coated alloy samples in Hank's solution are shown in Fig. 6. The Tafel fit is Fig. 3. SEM micrographs of the coating on Mg 1.0Ca alloy with different magnification after electro-deposition. Fig. 5. XRD patterns of Mg 1.0Ca samples before and after electro-deposition.

4 Z. Chun-Yan et al. / Surface & Coatings Technology 204 (2010) The measured hydrogen volume can be converted into the number of moles of dissolved Mg using the ideal gas law: PV ¼ nrt ð9þ Fig. 6. Polarization curves of AZ31 and Mg 1.0Ca alloys with and without coatings. employed to analyze the polarization curves [10]. The electrochemical parameters of the substrates with and without the coatings in Hank's solution are listed in Table 2. The open circuit potentials (OCP) of the coated samples are much higher than that of the substrates. The OCP values of the AZ31 and Mg 1.0Ca alloys are 1.62 V and 1.65 V, respectively; the coated samples are 1.50 V and 1.53 V respectively. The great difference indicates that the Ca P coating can provide effective protection for magnesium alloys. Furthermore, the corrosion current densities of coated samples are 100 times lower than those of the substrates, which indicate that the Ca P coatings improved corrosion resistance of the magnesium alloys. Moreover, the Mg 1.0Ca alloy showes lower corrosion resistance than that of AZ31alloy, and the corrosion resistance of the coated AZ31 alloy is higher than that of the coated Mg 1.0Ca alloy. Furthermore, a passivation region ranging from 1.58 V to 1.45 V is displayed in the polarization curve of AZ31, indicating that a passive film is formed on the surface of AZ31 magnesium in Hank's solution. At breakdown potential (E bk ), there is a steep increase in the corrosion current density. With increasing anodic polarization potential, the coating is attacked at a potential E bk ; the coated substrate suffered pitting corrosion. The E bk2 ( 1.35 V) of the coated sample is higher than E bk1 ( 1.45 V) of the uncoated sample, while the passivation current density is ten times lower than that of the coated samples. No apparent passivation region could be observed on the samples of coated and uncoated Mg 1.0Ca alloy. Thus, they are more susceptible to corrosion. The reason for this may be that there are Al and Zn elements in the AZ31 alloy. Hydrogen evolution test is reliable, easy to implement and not prone to errors inherent in the weight loss method [13]. In addition, this technique has the advantage that variations in the degradation rates can be monitored by the hydrogen evolution rates, allowing the study of the degradation rate variation vs. exposure time [19]. The hydrogen evolution vs. immersion time curves of the coated and uncoated AZ31 samples are shown in Fig. 7. At the initial stage of immersion, some bubbles can be seen on the surface of the AZ31 substrate, while no bubbles were observed on the surface of the coating sample. After a 10 day immersion, the hydrogen volume of the coated sample was 6.2 ml, that of the AZ31 substrate was 28 ml, indicating that the hydrogen evolution rate of the uncoated sample is much higher than that of coated sample and that a Ca P coating can significantly inhibit the degradation of AZ31 magnesium. Similarly, Fig. 8 shows the volume of the hydrogen evolution of coated and uncoated Mg 1.0Ca samples in Hank's solution. As soon as the uncoated Mg 1.0Ca specimen was immersed into the solution, gas bubbles rose from the specimen surface, but no bubbles were observed on the surface of the coated Mg 1.0Ca sample during the initial stage of immersion. The hydrogen volume of the coated Mg 1.0Ca sample was 0.6 ml, and that of the Mg 1.0Ca substrate was 13.2 ml after 12 h immersion. The results indicate that Ca P coating significantly inhibited the degradation rates of Mg 1.0Ca alloy. The hydrogen evolution rate of the coated sample is with about 2.5 ml/h somewhat lower than that of the uncoated Mg 1.0Ca, which is approximately 4 ml/h. The result shows that the Ca P coating can decrease the degradation rate of Mg 1.0Ca in Hank's solutions, especially at the initial 12 h immersion. The hydrogen adsorption rate that could be tolerated by the human body is 2.25 ml/cm 2 /day [20]. The hydrogen evolution rate of pure magnesium at 37 C in Hank's solution is 40 ml/cm 2 /day [20]. The rapidly generated hydrogen bubbles will lead to significant subcutaneous gas pockets, which may delay the healing of surgical region, result in necrosis of tissues and cause discomfort [21]. On the other hand, the hydrogen evolution rate of the AZ31 alloy is ml/cm 2 /day. The hydrogen evolution rate of the coated AZ31 sample is as low as ml/cm 2 /day during 10 day immersion. Both of the hydrogen evolution rates are significantly lower than the tolerance level in the human body. However, after 12 h immersion the hydrogen evolution rates of the coated and uncoated Mg 1.0Ca alloys are as high as 7 10 ml/cm 2 /day which is far beyond the tolerance limits in the human body Hydrogen evolution Mg dissolution in aqueous environments generally proceeds by its electrochemical reaction with water to produce Mg(OH) 2 and hydrogen [19] Mg þ 2H 2 O Mg 2þ þ 2OH þ H 2 ð8þ Table 2 Parameters obtained from the polarization curves in Hank's solution. Samples E corr /V vs. SCE i corr /A/cm 2 E bk /V vs. SCE Mg1.0Ca substrate Mg1.0Ca with coating AZ31 substrate AZ31 with coating Fig. 7. Hydrogen evolution volume of coated and uncoated AZ31 in Hank's solution.

5 3640 Z. Chun-Yan et al. / Surface & Coatings Technology 204 (2010) original Mg alloys in physiological environment. However, the brushite coatings are neither dense nor thick enough to provide corrosion protection for Mg alloy substrate effectively. A dense, finegrained, thick, well adhering CP might increase the degradation resistance of Mg alloys. The electrochemical synthesis factors, including solution composition, ph and deposition temperature, affect the purity, crystallinity, morphology and mechanical strength of the resulting coatings. So several different conditions have been explored for electro-deposition of coating with two layers: the inner layer is dense and fine-grained which may give better corrosion protection and the outer layer is loose and well-crystallized which may provide appropriate pore sizes for the bone cell ingrowth to the implants. Related experiments to improve the quality of the coating are undergoing. Fig. 8. Hydrogen evolution volume of coated and uncoated Mg 1.0Ca in Hank's solution. 4. Conclusions (1) Calcium phosphate coatings were successfully formed on AZ31 and Mg 1.0Ca magnesium alloys by electrochemical deposition. The deposited coatings consist of flaky brushite (DCPD, CaHPO 4 2H 2 O) crystallites. The chemical compositions of the substrates have little impact on the morphology, chemical composition and the crystal structure of the coatings. (2) The free corrosion potentials of the coated samples are higher than that of substrate. The corrosion current densities of coated samples are two orders lower than those of the substrates, indicating that the Ca P coating can improve the corrosion resistance of magnesium alloys. (3) The hydrogen evolution rates of the coated samples are much lower than those of their substrate in Hank's solution, indicating Ca P coating can inhibit the degradation rates of magnesium alloys. The hydrogen evolution rates of the coated and uncoated AZ31 alloys are significantly below the tolerance level of the human body. However, the hydrogen evolution rates of both of the coated and uncoated Mg 1.0Ca alloy are too high to be tolerated by human tissue. This consequence displays that the chemical compositions of the substrates have an important influence on the formation of Ca P coatings and their corrosion resistance. Based on these results, it might be concluded that the calcium phosphate coating can moderate decrease degradation rate of the Acknowledgements The work was financially supported by Key technologies R & D Program and Natural Science Foundation Project of CQ CSTC 2008BB4062, 2008BB0063, and 2009AB4008. References [1] D. Upadhyay, M.A. Panchal, R.S. Dubey, V.K. Srivas-Tava, Mater. Sci. & Eng. A. 432 (2006) 1. [2] P.A. Dearnley, Surf. & Coat. Technol. 198 (2005) 483. [3] N.L. Saris, E. Mervaala, H. Karppanen, J.A. Khawaja, Clin. Chim. Acta 294 (2000) 1. [4] D. Williams, Med. Device Technol. 17 (2006) 9. [5] P.S. Mark, M.P. Alexis, Hi Jerawala, D. George, Biomaterials 27 (2006) [6] F. Witte, J. Fischer, J. Nellesen, H. Crostack, H.A. Crostac, Biomaterials 27 (2006) [7] M. Osigo, Y. Yamashita, T. Matsumoto, J. Det. Res. 77 (1998) [8] L.P. Xu, G.N. Yu, E.L. Zhang, F. Pan, K. Yang, J. Biomed. Mater. Res. Part A. 4 (2007) 703. [9] F.Z. Cui, J.X. Yang, Y.P. Jiao, Q.S. Yin, Y. Zhang, I.S. Lee, Front. Mater. Sci. China 2 (2008) 143. [10] Y.W. Song, D.Y. Shan, E.H. Han, Mater. Lett. 62 (2008) [11] C.Y. Zhang, R.C. Zeng, J. Chen, H. Yang, Z.Q. Tan, Rare Met. Mater.& Eng. 38 (2009) [12] G.L. Song, A. Atrens, Adv. Eng. Mater. 12 (2003) 837. [13] E. Wintermantel, J. Mayer, J. Blum, K.-L. Eckert, P. Lüscher, M. Mathey, Biomaterials. 17 (1996) 83. [14] K.A. Hing, S.M. Best, K.E. Tanner, et al., J. Mater. Sci. Mater. Med. 10 (1999) 663. [15] Y. Han, K.W. Xu, Mater. Sci. Mater. Med. 10 (1999) 243. [16] S. Mats, A. Johnson, H. George, Crit. Rev. Oral Biol. Med. 3 (1992) 61. [17] X.F. Xiao, X.L. Tang, Y.J. Gao, Z. Xu, R.F. Liu, Cailiao Baohu. 38 (2005) 29. [18] A. Malsy, M. Bohner, Eur Cells Mater. 10 (2005) 28. [19] Y.C. Xin, K.F. Huo, H. Tao, G.Y. Tang, K. Paul, Acta Biomater. 4 (2008) [20] E. E. Aghion, A. Arnon, D. Atar, G. Segal. Biodegradable magnesium alloys and uses thereof, Patent, International Application No.: PCT/IL2007/ (China) [21] G. Song, S. Song, Adv. Eng. Mater. 9 (2007) 298.