Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium

Front. Mater. Sci. 2014, 8(3): 244 255 DOI 10.1007/s11706-014-0251-y RESEARCH ARTICLE Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium Cheng-Long LIU 1, Yi ZHANG 1, Chun-Yan ZHANG 1, Wei WANG 1, Wei-Jiu HUANG ( ) 1, and Paul K. CHU 2 1 School of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400050, China 2 Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Higher Education Press and Springer-Verlag Berlin Heidelberg 2014 ABSTRACT: In this work, we report on synergistic effect of chloride ion and albumin on the corrosion of pure magnesium through corrosion tests. We show that the adsorption of albumin mainly affects the anodic polarization behavior of pure magnesium in NaCl solution. Low concentration of albumin enhances the reaction reactivity of pure magnesium and the initial evolvement of hydrogen at the initial immersion time. Addition of 1 g/l albumin provides limited corrosion control for pure magnesium in NaCl solution. In comparison with low concentration albumin, addition of 10 g/l albumin can effectively inhibit the further dissolution of pure magnesium in test solutions with NaCl concentration of 0.2 0.8 wt.%, but this effect lowers gradually with increasing the concentration of chloride ion. KEYWORDS: magnesium; corrosion; albumin; biomaterial Contents 1 Introduction 2 Materials and methods 3 Results and discussion 4 Conclusions Abbreviations Acknowledgements References 1 Introduction Received May 3, 2014; accepted June 7, 2014 E-mail: huangweijiu@cqut.edu.cn Biodegradable or bioadsorbable implant materials, such as polymers and ceramics, have exhibited great advantages in cardiovascular or orthopedic applications. More recently magnesium and its alloys have, once again, been generating interest as possible replacement for some of the metallic alloys currently used, such as stainless steel, titanium based alloy, etc. due to good mechanical properties and biocompatibility [1 2]. The self-dissolving nature of magnesium alloys is different from that of polymers and ceramics, which is achieved through the in vivo corrosion in physiological environments. Therefore, it is significant to understand their corrosion process and mechanism clearly in human body. Up to now, a great number of studies have showed that the corrosion of magnesium alloys depends on not only their chemical composition, microstructure and production process, but also the test environments [3 5]. As for body fluid, it contains different types of organic compounds such as proteins and cells besides inorganic species including Cl, HCO 3, HPO 2 4, etc. [6], which may absorb on the implant surface and thereby influence the corrosion behavior. Vogt found that the corroded surface of LAE442 magnesium alloy was covered with corrosion products including Ca, Mg, P, O

Cheng-Long LIU et al. Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium 245 and a adsorbed protein layer about 20 μm thick, suggesting that the existence of protein layer could affect the corrosion process [7]. To better elucidate this effect, experiments using single protein or serum additions to different types of simulated body fluids have been carried out. Different results (little influence, increase or decrease) have been reported. Mueller found little influence of albumin on the corrosion of AZ31 after albumin addition [8]. Yamamoto et al. reported that amino acids accelerate dissolution of Mg through the chelating effect between Mg ions and amino molecules [1,8]. On the contrary, a number of investigations have shown the corrosion resistance of magnesium alloys to be improved in the presence of proteins [9 13]. The adsorbed protein layer could act as a diffusion barrier between the metal surface and the surrounding environment, hence inhibiting the infiltration of aggressive ions [4,9 13]. Although the aforementioned studies indicate clearly that the existence of proteins affects the corrosion process of magnesium alloys through blocking or chelating effects, the effect and mechanism are not uniform, which is expected to be composition-, production-, or environmentdependent. Some studies point out that the corrosion of magnesium alloys is generally controlled by the aggressive ions in body fluids, such as Cl, HCO 3, HPO 2 4, etc. Especially for Cl ions, they can transform the quasi protective MgO/ Mg(OH) 2 into soluble MgCl 2, thus accelerating magnesium dissolution [3 4,14 15]. Since the protein addition has an influence on the corrosion of magnesium alloys through blocking of chelating effects, there must be a competitive interaction between aggressive ions and protein molecules, which might cause the variation of corrosion process and mechanism for magnesium alloys. Thereby, if the competitive interaction can be elucidated, it is beneficial to explain the exact nature of this influence. However, at present, there is a lack of detailed discussion about this synergistic effect between aggressive ions and protein. The present work aims to characterize the synergistic effect of chloride ions and albumin molecules on the corrosion behavior of pure Mg, using in-situ corrosion observation and electrochemical methods. 2 Materials and methods Pure magnesium (3N, Huachang Magnesium Limited Company, Shanxi, China) was machined into 10 mm 10 mm 4 mm disks for surface characterization, immersion experiments, and electrochemical tests. The samples used in the corrosion tests were prepared by the standard technique of grinding with SiC abrasive paper up to 1200 grit, followed by ultrasonic cleaning in acetone, absolute ethanol, and distilled water sequentially. Four types of solutions with different concentration of NaCl (0.2, 0.4, 0.6, and 0.8 wt.%) were chosen as the original testing solutions. The solutions with bovine serum albumin (BSA) had albumin concentrations of 1 and 10 g/l. The immersion test was carried out at (371) C. The samples were grinded, polished, and cleaned prior to the test. The corrosion morphology of each alloy immersed in the electrolytes in a special electrolytic cell was observed and recorded at 20 magnification on a Remote Measurement System. The surface morphology of the corroded alloy sample was recorded and studied simultaneously. Insitu observation was found to be very important because as soon as the samples were taken out from the solution and exposed to air, the corrosion products turned white and the morphology subsequently changed. During the experiment, the sample was vertically immersed in the solution in the cell with the working surface of the sample parallel to the window on the cell side wall. The digital camera and microscopic head were perpendicular to the outside surface of the window. The morphology was observed and recorded every 5 min until 60 min. The electrochemical corrosion behavior was investigated using a Zahner Zennium electrochemical instrumentation system. A three-electrode cell with the sample as the working electrode, saturated calomel electrode as the reference electrode, and platinum plate as the counter electrode was employed. The open-circuit potential (OCP) were measured for 3600 s. All the potentiodynamic polarization tests were carried out using a scan rate of 0.1 mv/s. The experiments were conducted at (371) C in triplicate without aeration with fresh solutions and newly polished electrodes. The electrochemical impedance spectra were measured from a solution volume of 250 ml. A frequency range from 10 mhz to 10 khz was selected. Before the impedance measurement, the sample was kept floating at the OCP and for each impedance spectrum, the potential was set to the actual value of the OCP. The disturbing voltage amplitude chosen was 10 mv. The period of one measurement series was 10, 30 and 60 min. The electrochemical impedance spectroscopy (EIS) spectra were fitted with the ZSimpWin software.

246 Front. Mater. Sci. 2014, 8(3): 244 255 3 Results and discussion The dependence of the OCP for pure magnesium is shown in Figs. 1(a) 1(c) in 0.2 0.8 wt.% NaCl solutions in the absence and presence of BSA. In all NaCl solutions, the OCP values fluctuate rapidly before 20 min immersion. Because the OCP is determined by both the anodic and cathodic reaction, according to the electro-neutrality theory, the decreased anodic current will move the OCP gradually in the positive direction so that the cathodic current can become small enough to counteract the reduced anodic current which depends on the corrosion resistance of the surface film [16]. Hara et al. have examined the growth process of oxidation film of pure magnesium alloy in NaCl solution by in situ ellipsometry, and found that the surface film, composed mainly of Mg(OH) 2, grew rapidly after immersion, however, this hydroxide film on magnesium was quasi-passive [17]. Chloride ions are involved in the intermediate step of magnesium dissolution, and make the hydroxide film more active or increase the film-free area [18]. Therefore, the great fluctuation of the OCP values in NaCl solution can be ascribed to the variation of surface film atop pure magnesium. With increasing the immersion time, the changes of the OCP values gradually slow reaching a relatively stable value, which may be attributed to the approximate balance between the formation and dissolution of hydroxide film. Compared with the variation of the OCP values in NaCl solution, the initial fluctuation of pure magnesium in NaCl solution including bovine serum albumin is also great, but soon becomes stable. In a biological milieu, the initial response to a material surface being placed is for water molecules to reach the surface and create a water shell around the material on a nanosecond time scale, which promotes the formation of hydroxide film. In the second stage from seconds to hours, the material is covered by an adsorbed layer of proteins initially present in the extracellular matrix [19]. In NaCl solution including albumin, similar adsorption process could happen. Some studies reported that Ca 2+,Mg 2+,orH 2 PO 4 ion bridges can be formed between the oxide layer and albumin molecules [20 21]. The initial fluctuation of pure magnesium in NaCl solution including albumin may be ascribed to the following process. Once pure magnesium is dipped into the test solution, the spontaneous formation of hydroxide film causes the decrease of its anodic dissolution current, which shifts the OCP in positive direction, and then the partial dissolution of the hydroxide layer caused by chloride ions leads to the decrease of corrosion resistance. Because the succedent adsorption of albumin molecules needs a mass of Mg 2+ as ionic bridges, this effect results in further negative shift of the OCP. The adsorption of albumin has been testified atop the Mg Ca surface by Fourier transform infrared spectroscopy (FTIR) [10]. However, once the triangular albumin molecules arrive at the surface, they transform to a flattened triangular shape with marked size, which can easily inhibit the transfer of aggressive ions from the solution to the meal surface [22]. Therefore, the diffusion barrier may result in the slow negative shift of the OCP in NaCl solution including bovine serum albumin, which can also cause the variation of corrosion resistance of surface film. We will elucidate it using the following EIS tests. It can be seen that the OCP becomes stable gradually with increasing the immersion time in either NaCl solution or NaCl solution with albumin in Figs. 1(a) (c). The OCP values after 3600-s immersion in all test solutions are shown in Fig. 2. With increasing NaCl concentration, the OCP becomes more and more negative, which is in agreement with the previous studies [18]. Comparing the NaCl solutions and NaCl solution including albumin, two interesting phenomena are visible. One is that albumin addition shifts the OCP to more negative value in all NaCl solution, which is similar to Retting s studies [12]. The other is that albumin addition can change the influence of chloride ions on the dissolution of pure magnesium. The main conclusion from the time-dependent curves of the open circuit potential show that albumin addition indeed changes the surface reaction of pure magnesium in NaCl solution. In order to characterize the early corrosion stages of pure magnesium in all used solutions, the corrosion morphology and hydrogen evolution process were monitored in-situ by an optical microscope with a digital camera. Figure 3 provides selected illustrations from the in-situ observations of the details of the corrosion morphologies for pure magnesium in 0.8 wt.% NaCl solution with and without albumin at 10, 30 and 60 min immersion, respectively. Comparing 0.8 wt.% NaCl and 0.8 wt.% NaCl with albumin, two main differences can be observed from the corrosion morphologies. One is that addition of 10 g/l BSA results in an increase of the amount and density of hydrogen bubbles compared with 0.8 wt.% NaCl at the same exposure time. Hydrogen evolution took place uniformly over the whole sample surface. Otherwise, hydrogen evolution was concentrated at the corroding sites.

Cheng-Long LIU et al. Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium 247 Fig. 2 OCP values of pure magnesium in NaCl solution with and without BSA after 3600-s immersion. composed of a larger number of small hydrogen bubbles and subsequently the formation of corrosion pits. Moreover, the size of the hydrogen bubbles increased very slowly with increasing exposure time. It is known to us that magnesium dissolution in aqueous environment generally proceeds by an electrochemical reaction with water to produce magnesium hydroxide and hydrogen gas. The overall reaction may be expressed as the sum of the following partial reactions [18]: MgðsÞ!Mg 2þ ðaqþþ2e ðanodic reactionþ (1) 2H 2 O þ 2e! H 2 þ 2OH ðaqþðcathodic reactionþ (2) Mg 2þ ðaqþþ2oh ðaqþ! MgðOHÞ 2 ðproduct formationþ (3) Fig. 1 Variations of the OCP of pure magnesium as a function of immersion time in NaCl solution with and without BSA. The other is that corrosion became visible on the surface in the form of pitting corrosion in three solutions, as shown in Fig. 4, suggesting that addition of albumin can not change the initial corrosion type of pure magnesium in NaCl solution. Also visible were a number of vertical streams It is well established that the isoelectric ph of serum albumin is 4.7 4.9. At higher ph values, albumin molecules undergo a neutral acidic transition and become negatively charged [21]. In the present study, the ph value of NaCl solution is near neutral, hence the Mg 2+ cations may bridge the negatively charged albumin molecule and magnesium surface. At the beginning of immersion in NaCl solution including albumin, the albumin adsorption may result in the rapid consumption of Mg 2+ cations, enhancing the anodic reaction rate (1). The increase of the number of electrons will promote the cathodic reaction (2). Therefore, it can be seen that the amount and density of hydrogen bubbles increase in NaCl solution including 10 g/l albumin in Fig. 3. The formation of quasi-protective magnesium hydroxide may be inhibited because of the rapid consumption of Mg 2+ cations, which may be reason for the initial decrease gradually of the OCP in Fig. 1(c).

248 Front. Mater. Sci. 2014, 8(3): 244 255 Fig. 3 Typical in situ corrosion morphology for pure magnesium in different electrolytes after different duration of time (10, 30, and 60 min): (a) 0.8 wt.% NaCl; (b) 0.8 wt.% NaCl + 1 g/l albumin; (c) 0.8 wt.% NaCl + 10 g/l albumin. When the albumin absorption cover the whole magnesium surface, the cooperative effect from albumin absorption and magnesium hydroxide layer will stabilize the variation of the OCP. In Fig. 5, the variation of hydrogen bubbles is depicted in 0.8 wt.% NaCl + 10 g/l albumin. In the initial immersion, the changes of the amount and diameter of hydrogen bubbles are great, and meanwhile the OCP fluctuates. After 10-min immersion, the amount of hydrogen bubbles stops to increase, while the diameter increases continuously. At 60 min, the hydrogen bubbles almost cover the whole surface. The hydrogen bubbles begin to break after 30 min immersion, and some black points are shown in Fig 3(a), suggesting the formation of corroded pits [23]. Figure 4 shows that pure magnesium suffers more severe corrosion attack in 0.8 wt.% NaCl solution. The polarization behaviors of pure magnesium in the used solutions after 30-min immersion are displayed in Figs. 6 and 7. In different solutions, all samples show similar cathodic and anodic polarization behaviors, which are not symmetrical. The cathodic branch is much steeper than the anodic branch. In this study, it could be considered that the anodic process is much more sensitive than the cathodic process to Cl. In NaCl solution, the increase of Cl concentration results in an improvement of the corrosion current density fitted using Tafel extrapolation, as shown in Fig. 8. This means that the increasing Cl concentration destroys the integrity of magnesium hydroxide film and provides a larger area of pure magnesium exposure to the solution for anodic dissolution [24]. Figures 7(a) and 7(b) show the cathodic and anodic polarization curves of pure magnesium in NaCl solution with 1 and 10 g/l albumin, respectively. The corrosion potentials (E corr ) shift to negative direction with increasing NaCl concentration in the presence of 1 and 10 g/l albumin. The cathodic branches are shifted to more positive potential direction and anodic branches are shifted to negative potential direction with increasing NaCl

Cheng-Long LIU et al. Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium 249 Fig. 4 Corrosion morphology of pure magnesium in (a) 0.8 wt.% NaCl and (b) 0.8 wt.% NaCl + 10 g/l albumin after 60-min immersion. concentration. The movements of the anodic and cathodic branches are similar for the solutions in the presence of 1 or 10 g/l albumin. Using Tafel extrapolation, the anodic and cathodic current density was obtained, as shown in Fig. 8. It can be seen that the influence of albumin on the cathodic polarization of pure magnesium is limited. In contrast, the anodic polarization is more sensitive to the addition of albumin. Padilla found that the fractional coverage of the adsorbed albumin on the electrode usually increased with increasing potential [22]. Therefore, the positive shift of anodic potential may result in more adsorption of albumin. Comparing the solutions of NaCl solution with 1 and 10 g/l albumin, the anodic current density values in the presence of 10 g/l albumin increasing from (0.090.004) to (0.3040.013) ma/cm 2 while those in the presence of 1 g/l albumin increase from (0.1240.014) to (0.3380.008) ma/cm 2 with increasing NaCl concentration. Compared with the anodic current density at each same concentration of NaCl, these values are lower, suggesting albumin addition inhibits the anodic polarization. It is known that the corrosion current is proportional to the corrosion resistance, and the decrease of anodic current density can be ascribed to the variation of surface state of pure magnesium. Generally speaking, in the early stages of the corrosion process, countless electrochemical cells begin to form on the surface of pure magnesium. Mg 2+ cations will transfer to the cathodic areas, and meanwhile, some anoions, e.g. Cl and OH, will migrate to the dissolution sites [25]. Herein, the negative albumin molecule in near neutral NaCl solution will adsorb onto the magnesium surface through the Mg 2+ bridge [20], which could impede the further migration of Mg 2+ cations to the cathodic sites. Besides, the adsorbed albumin molecules turn into a flattened shape with increasing size, and retard Fig. 5 Comparative observation of the development of hydrogen bubbles and OCP.

250 Front. Mater. Sci. 2014, 8(3): 244 255 Fig. 6 Polarization curves for pure magnesium in solutions including different concentrations of NaCl. the filtration of Cl, which could be beneficial to the integrity of magnesium hydroxide layer. Therefore, the formation of corrosion products and adsorbed albumin will reduce the electrochemical corrosion somewhat. According to the variation of anodic current density at the same albumin concentration, the influence of Cl concentration on the protective effect of albumin exists, which may be ascribed to the influence of ionic concentration on the protein surface interaction [26]. The polarization results indicate that addition of BSA decreases the anodic corrosion current density, suggesting the increase of corrosion resistance. Herein, the variation of corrosion resistance was measured by EIS. In Fig. 9, the Nyquist plots of pure magnesium after 10, 30, and 60 min immersion in the used solutions are depicted. These EIS Fig. 7 Polarization curves of pure magnesium in different test solutions: (a) 0.8 wt.% NaCl + 1 g/l albumin; (b) 0.8 wt.% NaCl + 10 g/l albumin.

Cheng-Long LIU et al. Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium 251 Fig. 8 Cathodic and anodic corrosion current density obtained from the polarization curves of pure magnesium in all test solutions: the solid symbols for cathodic current density and the open symbols for anodic current density. spectra are similar except for the difference in the diameters of the loops, suggesting the corrosion mechanisms of pure magnesium in different test solutions are the same, but their corrosion rates could be different [27 28]. After immersion for 10 min, the plots in three types of solutions consist of one high-frequency capacitance loop and one medium frequency capacitance loop. The diameter of the highfrequency capacitance of pure magnesium in 0.2 wt.% NaCl is greatest, followed by that in 0.2 wt.% NaCl with 10 and 1 g/l albumin. In contrast, 10 g/l albumin addition results in the greatest diameter of the high frequency capacitance in 0.4 and 0.8 wt.% NaCl. With increasing NaCl concentration, the effect of 1 g/l albumin may change. At 0.8 wt.% NaCl solution, the diameter of the high frequency capacitance in the solution including 1 g/l albumin is greater than that in 0.8 wt.% NaCl. When the immersion time reaches 30 min, one low-frequency inductance loop appears and the diameter of high and low capacitance loops increase. The variations of the highfrequency capacitance are similar to those at 10-min immersion with the changes of NaCl concentration. After 60-min immersion, some changes are shown in the Nyquist plots, especially in NaCl solution including 10 g/l albumin. Compared with the plots in NaCl solution with and without 1 g/l albumin, only one high-frequency capacitance loop appeared in NaCl with 10 g/l albumin, and moreover, the diameter of which is greater than those in the other solutions. The variation of the diameter of high frequency suggests the changes of pure magnesium/ solution interface and processes that occur on the surface in the presence of chloride ions and albumin. Our understanding of electrochemical kinetics of pure magnesium in different solutions is developed primarily from the studies of Gu et al. [11,28]. The Nyquist plots with two time constants are fitted with the equivalent circuit (R s (Q dl R ct )(Q f R f )), which are attributed to the presence of electric double layer and surface film, respectively. R s is related to the resistance of the electrode between working and reference electrodes. R ct (a charge transfer resistance) is parallel to the electric double layer constant phase element (CPE) at the vulnerable regions, Q dl, and a CPE assigned to the film layer, Q f, is parallel to R f (the film layer resistance). Contu suggested and Padilla et al. reaffirmed that the effective capacitance was a mixture of the characteristics of the oxide layer and adsorption of albumin [22,29]. Hence, we explain the CPE Q f as mixed characteristics of the corrosion product and albumin adsorption layers through the capacitances: Q f ¼ðC cp Þ 1 þðc ad Þ 1 : The constant phase element models a non-ideal capacitor in all the cases. The CPE is found to be necessary due to the heterogeneous nature of the electrode surface, expressed through the exponent n. In addition, some Nyquist plots show an inductive loop at low frequency, which is normally related to relaxation of adsorbed species. The Nyquist plots with three time constants are fitted with the equivalent circuit (R s (Q dl R ct )(Q f R f (R l L))). R l and L indicate the existence of metastable Mg + during the dissolution of pure magnesium substrate [27 28]. However, these inductive loops are not reproducible. Therefore, we will not discuss the variation of R l and L parameters later. The chi-square values on the order of 10 4 indicate excellent agreement between the experimental and modeled values. The fitted parameters are listed in Table 1. In the solutions with different NaCl concentration, the R f values increase with increasing immersion time, which can be attributed to corrosion products layer. When pure magnesium is dipped into the solution, magnesium surface suffer electrochemical corrosion through micro-electrochemical cells. Although magnesium hydroxide can be formed with the corrosion process very soon, it is difficult for magnesium hydroxide to precipitate in microcathode area from which H 2 evolves rapidly, shown in the in-situ corrosion observation. However, OH with faster diffusion velocity results in the thermodynamical precipitation of magnesium hydroxide principally in the vicinity of microanode [17,29]. As a result, the area percentage of active region reduces, the corrosion products layer thickens and its corrosion protection enhances progressively with exposure time.

252 Front. Mater. Sci. 2014, 8(3): 244 255 Fig. 9 Nyquist plots of pure magnesium as a function of immersion time in NaCl solution in the absence and presence of BSA. Comparing the R f values in the solutions with and without albumin, it can be seen that both NaCl concentration and albumin concentration have an obvious influence on the R f values. When the NaCl concentration is 0.2 and 0.4 wt.%, addition of 1 g/l bovine serum albumin leads to the decrease of R f values at three immersion time point. On the contrary, addition of 10 g/l bovine serum albumin causes its increase. In 0.8 wt.% NaCl solution, the R f values

Cheng-Long LIU et al. Synergistic effect of chloride ion and albumin on the corrosion of pure magnesium 253 Table 1 EIS fitted results of pure magnesium in NaCl solutions in absence and presence of bovine serum albumin Solution Time /min R s /(Ω cm 2 ) Q dl /(F cm 2 ) n 1 R ct /(Ω cm 2 ) Q f /(F cm 2 ) n 2 R f /(Ω cm 2 ) 10 324 2.64E 05 0.871 1380 1.57E 03 0.812 704 0.2 wt.% NaCl 30 344 1.19E 03 0.902 1639 2.17E 05 0.895 1754 60 349 6.96E 04 0.9 2314 1.85E 05 0.889 4759 10 390 2.59E 05 0.87 1162 1.19E 03 0.742 638 0.2 wt.% NaCl + 30 411 2.33E 05 0.872 1660 1.10E 03 0.832 1184 1 g/l albumin 60 420 1.12E 03 0.915 1527 2.07E 05 0.87 2668 10 432 7.39E 04 0.731 1100 1.40E 05 0.911 1483 0.2 wt.% NaCl + 30 435 1.20E 05 0.915 3374 6.40E 04 0.794 2114 10 g/l albumin 60 439 1.43E 05 0.888 4282 9.88E 04 0.989 6918 10 205 1.18E 03 0.750 952 2.18E 05 0.89 1192 0.4 wt.% NaCl 30 207 1.94E 05 0.892 1922 1.08E 03 0.805 1353 60 210 1.72E 05 0.9 2934 8.59E 04 0.805 2068 10 231 2.09E 04 0.901 887 1.33E 03 0.723 684 0.4 wt.% NaCl + 30 238 9.83E 04 0.754 1384 1.71E 05 0.912 1708 1 g/l albumin 60 242 1.47E 05 0.919 3040 6.79E 04 0.783 1939 10 239 1.13E 05 0.911 928 6.81E 04 0.759 1935 0.4 wt.% NaCl + 30 254 9.69E 05 0.924 2562 6.36E 04 0.758 2305 10 g/l albumin 60 269 6.69E 04 0.775 4695 9.03E 05 0.927 6356 10 98 1.50E 04 0.922 911 1.18E 03 0.765 608 0.8 wt.% NaCl 30 100 1.18E 03 0.797 681 1.51E 05 0.92 1100 60 103 1.18E 03 0.771 889 1.49E 05 0.92 1432 10 117 1.41E 03 0.657 626 2.02E 05 0.905 1156 0.8 wt.% NaCl + 30 122 9.65E 04 0.747 1826 1.86E 05 0.906 1818 1 g/l albumin 60 128 9.07E 04 0.811 2328 1.63E 05 0.91 2072 10 143 1.07E 03 0.726 889 2.16E 05 0.891 1004 0.8 wt.% NaCl + 30 145 1.03E 03 0.797 1415 1.81E 05 0.9 1719 10 g/l albumin 60 149 8.09E 04 0.8 1854 1.58E 05 0.915 3328 are improved due to the albumin addition. The above difference may be ascribed to the variation of surface reactions. In the near neutral NaCl solution, the negative albumin molecule will adsorb onto the magnesium surface through Mg 2+ bridge [20], which is produced through the anodic reaction (1), and meanwhile, Mg 2+ ions are also needed to form magnesium hydroxide. In NaCl solution, because the mobility of Cl ions is lower that that of OH ions, Cl ions will arrive at the Mg(OH) 2 layer and start to exchange the OH ions, which cause the dissolution of Mg(OH) 2. Herein, driving force is the strong concentration gradient as Cl concentration is more than 3 powers higher than OH concentration [13]. When the NaCl concentration is 0.2 or 0.4 wt.%, low Cl concentration may result in the decrease of the amount of Mg 2+ ions. Therefore, the formation of Mg(OH) 2 layer and adsorption of albumin can be impeded due to the shortage of Mg 2+ ions. Besides, the adsorbed albumin layer may be incomplete, which can not effectively hinder the diffusion of Cl ions or H 2 O and slow down the corrosion process. The shortage of Mg 2+ ions could improve the anodic reaction (1) and the reaction reactivity of pure magnesium surface. The negative shift of the OCP in 0.2 and 0.4 wt.% NaCl solutions including 1 g/l albumin proves this phenomenon, as shown in Fig. 1. The bigger the charge transfer resistance (R ct ), the smaller the area fraction of film-vulnerable region [29 30]. According to the R ct values in Table 1 for pure magnesium in 0.2 or 0.4 wt.% NaCl solutions including 1 g/l albumin, these values are lower that those in 0.2 or 0.4 wt.% NaCl, respectively, suggesting more film-vulnerable regions. In contrast, when the albumin concentration is 10 g/l, the higher concentration of albumin may be beneficial to the integrity of the adsorbed albumin layer, which can effectively inhibit the further corrosion process [11]. With increasing the NaCl concentration to 0.8 wt.%, the higher concentration of Cl ions can lead to the higher corrosion rate of pure magnesium at the initial immersion, and more Mg 2+ ions can be produced. It may be beneficial to the formation of one homogenous and compact albumin adsorption layer, the presence of which could hinder the

254 Front. Mater. Sci. 2014, 8(3): 244 255 succedent diffusion of Mg 2+ and decrease the corrosion process including hydrogen evolution. The in-situ corrosion morphology in Fig. 5 indicates that the amount of hydrogen bubbles remains unchanged basically, but the diameter increases slowly. The increase of the R ct values in the solutions including 10 g/l albumin also suggests less film-vulnerable region. 4 Conclusions In the present study, corrosion behavior of pure magnesium in NaCl solutions with different concentration concentrations (0.2, 0.4, 0.6 and 0.8 wt.%) in the absence and presence of 1 and 10 g/l BSA is systematically investigated. The concentration of NaCl solution affects the effect of albumin on the electrochemical behavior of pure magnesium. The variation of OCP shows that albumin addition with lower concentration enhances the reaction reactivity of pure magnesium. Addition of albumin improves the initial forming rate of hydrogen through insitu corrosion observation. Potentiodynamic polarization and EIS tests further indicate that the concentration of chloride ion affects the protective effect of albumin. In all used NaCl solutions, addition of 1 g/l albumin result in lower corrosion resistance and rapid corrosion process at initial immersion. In comparison with low concentration albumin, addition of 10 g/l albumin can effectively inhibit the further dissolution of pure magnesium in NaCl solution, but the inhibitive effect lowers with increasing the concentration of chloride ion. Abbreviations BSA CPE EIS FTIR OCP Acknowledgements The work was financially supported by the National Natural Science Foundation of China (Grant Nos. 31000430 and 51201192) and the Science and Technology Development Program of Chongqing Science and Technology Commission (cstc2012gg-yyjs0224). References bovine serum albumin constant phase element electrochemical impedance spectroscopy Fourier transform infrared spectroscopy open-circuit potential [1] Yamamoto A, Hiromoto S. Effect of inorganic salts amino acids and proteins on the degradation of pure magnesium in vitro. Materials Science and Engineering C, 2009, 29(5): 1559 1568 [2] Witte F, Hort N, Vogt C, et al. Degradable biomaterials based on magnesium corrosion. Current Opinion in Solid State and Materials Science, 2008, 12(5 6): 63 72 [3] Xin Y, Hu T, Chu P K. In vitro studies of biomedical magnesium alloys in a simulated physiological environment: a review. Acta Biomaterialia, 2011, 7(4): 1452 1459 [4] Virtanen S. Biodegradable Mg and Mg alloys: Corrosion and biocompatibility. Materials Science and Engineering B, 2011, 176 (20): 1600 1608 [5] Witte F. The history of biodegradable magnesium implants: a review. Acta Biomaterialia, 2010, 6(5): 1680 1692 [6] Balamurugan A, Rajeswari S, Balossier G, et al. Corrosion aspects of metallic implants An overview. Materials and Corrosion, 2008, 59(11): 855 869 [7] Vogt C, Bechstein K, Gruhl S, et al. Investigation of the degradation of biodegradable Mg implant alloys in vitro and in vivo by analytical methods. In: Kainer K U, Weimar D G M, eds. Proceeding of the 8th International Conference on Magnesium Alloys and Their Applications. Weinheim: Wiley-VCH, 2010, 1162 1174 [8] Mueller W-D, de Mele M F L, Nascimento M L, et al. Degradation of magnesium and its alloys: dependence on the composition of the synthetic biological media. Journal of Biomedical Materials Research Part A, 2009, 90(2): 487 495 [9] Liu C, Xin Y, Tian X, et al. Degradation susceptibility of surgical magnesium alloy in artificial biological fluid containing albumin. Journal of Materials Research, 2007, 22(7): 1806 1814 [10] Liu C L, Wang Y J, Zeng R C, et al. In vitro corrosion degradation behaviour of Mg Ca alloy in the presence of albumin. Corrosion Science, 2010, 52(10): 3341 3347 [11] Gu X N, Zheng Y F, Chen L J. Influence of artificial biological fluid composition on the biocorrosion of potential orthopedic Mg Ca, AZ31, AZ91 alloys. Biomedical Materials, 2009, 4(6): 065011 [12] Rettig R, Virtanen S. Time-dependent electrochemical characterization of the corrosion of a magnesium rare-earth alloy in simulated body fluids. Journal of Biomedical Materials Research Part A, 2008, 85(1): 167 175 [13] Hornberger H, Witte F, Hort N, et al. Effect of fetal calf serum on the corrosion behaviour of magnesium alloys. Materials Science and Engineering B, 2011, 176(20): 1746 1755 [14] Xin Y C, Hu T, Chu P K. Influence of test solutions on in vitro studies of biomedical magnesium alloys. Journal of the Electrochemical Society, 2010, 157(7): C238 C243 [15] Xin Y, Huo K, Tao H, et al. Influence of aggressive ions on the degradation behavior of biomedical magnesium alloy in physiological environment. Acta Biomaterialia, 2008, 4(6): 2008 2015

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