Journal of Materials Science & Technology, 2020, 52(0): 83-88 DOI: 10.1016/j.jmst.2020.04.014

Research Article

In vitro crevice corrosion of biodegradable magnesium in different solutions

Bowei Chena,1, Hongliu Wua,1, Ruibang Yib, Wenhui Wanga, Haidong Xuc, Shaoxiang Zhang,c, Hongzhou Penga, Junwei Maa, Haomiao Jianga, Rui Zana, Shuang Qiaoa, Yu Suna, Peng Houd, Pei Han,d, Jiahua Ni,e, Xiaonong Zhang,a

State Key Laboratory of Metal Matrix Composites, School of Material Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

School of Materials Science and Engineering, Central South University, Changsha 410083, China

Suzhou Origin Medical Technology Co. Ltd., Suzhou 215513, China

Orthopaedic Department, Shanghai Jiao Tong University Affiliated Sixth People’s Hospital, Shanghai 200233, China

Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

Corresponding authors: *. E-mail addresses:sxzhang@originmedtech.com(S. Zhang),hanpeicn@163.com(P. Han),jiahua.ni@sjtu.edu.cn(J. Ni),xnzhang@sjtu.edu.cn(X. Zhang).1The authors equally contributed to this work.

Abstract

Magnesium (Mg) is a promising biomedical metal because of its biodegradability. The crevice between tissue and Mg implant can not be neglected in some implantation sites due to inducing crevice corrosion of Mg. In this paper, a new single mold was designed to build the in vitro experimental setup and four kinds of solutions, i.e. the deionized water (DW), the 0.9 wt.% sodium chloride solution (NaCl), the phosphate buffer saline (PBS) and the modified simulated body fluid (m-SBF) were used to explore necessary factors of crevice corrosion in Mg. It was observed that crevice corrosion in Mg sheets would occur in NaCl and PBS solution under 0.2, 0.5 and 0.8 mm crevice thickness. And it was found that there were two necessary factors, i.e. chloride ion and crevice dimension, in crevice corrosion. For the high-purity Mg cannulated screws, crevice corrosion could occur inside tunnel when immersed in PBS.

Keywords: Mg ; Crevice corrosion ; Cannulated screw ; Chloride ion ; Crevice size

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Cite this article

Bowei Chen, Hongliu Wu, Ruibang Yi, Wenhui Wang, Haidong Xu, Shaoxiang Zhang, Hongzhou Peng, Junwei Ma, Haomiao Jiang, Rui Zan, Shuang Qiao, Yu Sun, Peng Hou, Pei Han, Jiahua Ni, Xiaonong Zhang. In vitro crevice corrosion of biodegradable magnesium in different solutions. Journal of Materials Science & Technology[J], 2020, 52(0): 83-88 DOI:10.1016/j.jmst.2020.04.014

1. Introduction

Mg and its alloys are used as medical metal materials for several advantages, such as biodegradability, good biocompatibility and mechanical compatibility [[1], [2], [3], [4], [5], [6]]. However, Mg have poor corrosion resistance and active chemical properties, especially in the body environment containing large amount of chloride ions which could accelerate its corrosion rate. The rapid degradation leads to the mechanical integrity loss of Mg implants before completing medical treatment, large hydrogen evolution and severe alkalinization around the local environment. In addition, some in vivo environment factors, e.g. the crevice between tissue and Mg implant, would affect the corrosion rate of Mg implants.

As it is well known, common crevice corrosion of steel, aluminum and other structural metals is caused by the difference in oxygen concentration between inside and outside of crevice [[7], [8], [9]]. While the corrosion of Mg is not sensitive to the oxygen concentration, it is theoretically believed that Mg generally does not experience crevice corrosion. On the contrary, Ghali et al [10] proposed that Mg was possible to undergo crevice corrosion in their review of Mg’s corrosion, and they believed that crevice corrosion occurring in Mg was caused by the hydrolysis reaction of some ions in the crevice other than oxygen concentration difference. Shi et al [11] conducted electrochemical experiments of Mg and its alloys. They found that the boundary between the embedded samples and the resin was severely corroded and had many corrosion pits, which were considered to be crevice corrosion. But they did not give the specific mechanism of crevice corrosion. In the biomedical Mg implants, Denkena et al [12] studied the degradation of the LAE442 bone screws and plate system as an internal fixation in the New Zealand white rabbit, and they found that the contact area between screws and plate suffered the most serious corrosion. These corrosion phenomena of Mg and its alloys in the crevice structure lead to new thinking about the crevice corrosion in Mg and its alloys.

Our previous study [13] confirmed crevice corrosion could occur in Mg under in vitro and in vivo condition. However, we only used phosphate buffer saline (PBS) as in vitro solution and made four crevice thicknesses in one mold. In order to figure out necessary factors of crevice corrosion, we designed a new crevice mold to separate different crevice thicknesses and use four different kinds of solutions to immerse the experimental setup in this study. After that, we proposed a new mechanism of crevice corrosion in Mg.

2. Materials and methods

2.1. Materials preparation

The sheets of high-purity Mg (HP-Mg, 99.98 wt.%, chemical compositions listed in Table 1) with 20 mm length, 5 mm width and 1 mm thickness, as shown in Fig. 1(a), were manufactured from the as-rolled HP-Mg rods which were processed from the as-cast HP-Mg as described in the reference [14]. All specimens were ground with SiC paper up to 1200 grit and followed by ultrasonic cleaning in deionized water and ethanol for 5 min, respectively.

Table 1   Chemical compositions (in parts per million (ppm)) of HP-Mg by weight.

AlSiMnFeZnTiNiMg
1020202020<10<5Balance

New window| CSV


Fig. 1.

Fig. 1.   Pictures of (a) Mg sheet, (b) Mg cannulated screw, (c) schematic diagram of single crevice mold and (d) immersion setup.


HP-Mg screws made from the same as-rolled HP-Mg rods were machined to hollow structure in Suzhou Origin Medical Technology Co., Ltd. As shown in Fig. 1(b), the cannulated screws with 1.5 mm diameter middle hole totally had 30 mm length and were followed by ultrasonic cleaning in deionized water and ethanol for 5 min.

Fig. 1(d) showed the whole immersion setup, which was consisted of crevice mold, HP-Mg sheet and elastic cord. The crevice mold, as shown in Fig. 1(c), had 10 mm length, 3 mm width and five different kinds of crevice thicknesses, as 0.2 mm, 0.5 mm, 0.8 mm, 1.0 mm and 1.5 mm. The error bar for crevice thickness was 0.01 mm. The elastic cord was used to bundle Mg sheets and crevice molds to make sure the crevice thickness between Mg sheet and mold.

2.2. In vitro immersion experiment

Four kinds of immersion solutions were used in the degradation experiments, including the deionized water (DW), the 0.9 wt.% sodium chloride solution (NaCl), the PBS and the m-SBF. The PBS contained 8 g NaCl, 0.2 g KCl, 1.44 g Na2HPO4, 0.24 g KH2PO4, in 1 L deionized water [15]. The m-SBF was prepared by adding the following reagents in the sequence of 5.403 g NaCl, 0.504 g NaHCO3, 0.426 g Na2CO3, 0.225 g KCl, 0.230 g K2HPO4·3H2O, 0.311 g MgCl2·6H2O, 17.892 g HEPES (2-(4-(2-hydroxyethyl)-1-piperazinyl) ethanesulfonic acid) previously dissolved in 100 mL 0.2 M NaOH aqueous solution, 0.293 g CaCl2, 0.072 g Na2SO4 and 15 mL 1.0 M NaOH aqueous solution into deionized water to make totally 1 L solution [16].

There were three duplicate immersion setups in every crevice thickness and three HP-Mg sheets without bundled with the crevice mold to be the control group, named as 0 mm group. All groups were placed into 150 mL solution for immersion test at 37 °C water bath. The pH values were measured per 24 h by a pH meter (FE20, Mettler Toledo) during immersion test. After immersion for 96 h, all Mg sheets were taken out and ultrasonically cleaned by 180 g/L chromic acid with 10 g/L AgNO3 to remove corrosion products and then washed with the deionized water and ethanol, and then dried in the air.

The formula to calculate the corrosion rate of Mg is:

$\text{CR}=\frac{W}{tS\rho }\times 87.6\times 1000$

where W is the weight loss (g), t is the immersion time (h), S is the sample surface area (cm2), and ρ is the density of Mg (1.74 g/cm3).

The cannulated Mg screws were placed flatwise on the bottom of the beaker with 80 ml PBS. After immersion for 336 h, the screws were taken out and washed following the same procedure in previous paragraph.

The surface morphologies of Mg sheets and screws without corrosion products were characterized by using an optical camera and a field-emission scanning electron microscope (FE-SEM, Sirion 200, FEI).

3. Results and discussion

3.1. In vitro immersion experiment

As shown in Fig. 2(a), the corrosion rates of all DW groups were almost the same, about 0.25 ± 0.02 mm/a, and those in all m-SBF groups showed similar tendency too, about 1.56 ± 0.09 mm/a. The result suggested that the crevice corrosion would not occur in the deionized water and the m-SBF solution. However, the corrosion rates of NaCl and PBS groups showed different behaviors. The groups in 0.2 mm, 0.5 mm and 0.8 mm thickness were much higher than other groups. This meant when the crevice thickness was 0.2 mm, 0.5 mm or 0.8 mm, the crevice corrosion would happen in NaCl and PBS in agreement with our previous study [13]. The only difference between the present study in PBS and the previous work was the corrosion rate in high crevice thickness of 1.0 mm and 1.5 mm. The corrosion rate in 1.5 mm was 0.38 ± 0.08 mm/a, almost equaled to 0.39 ± 0.11 mm/a in 0 mm in this study. This difference would ascribe to different crevice dimension. In the previous study, the length of crevice is 20 mm. But in this research, the length of crevice is 10 mm, which was shorter than that in the previous study. So exchanging of solution inside and outside the crevice in this study would be easier than that in the previous study under the 1.0 mm and 1.5 mm thickness of crevice. Meanwhile, it was also proved that the crevice dimension which hindered the free exchange of ions in solution inside and outside crevice was a necessary condition for the occurrence of crevice corrosion in Mg.

Fig. 2.

Fig. 2.   (a) Corrosion rates and pH values of Mg sheets immersed in (b) deionized water, (c) 0.9 wt.% NaCl solution, (d) PBS and (e) m-SBF.


As shown in Fig. 2(b) and (c), the final pH values after 96 h in the DW and the NaCl groups raised rapidly to 9∼10 in the first day, and gradually maintained stable at pH = 10 because of the low solubility of Mg(OH)2. So based on the fact that the NaCl groups suffered crevice corrosion, it was suggested that chloride ions could play an important role in the crevice corrosion. In two buffer solutions of PBS and m-SBF, the pH values were lower than that of the DW and the NaCl groups because of the buffer objects. In the PBS group, the final pH values were about 9 in higher corrosion rate groups with 0.2 mm, 0.5 mm and 0.8 mm thickness. The pH was higher than that of other groups with lower corrosion rate. And the final pH values in m-SBF were the smallest than that of any other groups, only 8.33 ± 0.06 due to the presence of HEPES buffering reagent in m-SBF solution [[17], [18], [19]]. It was suggested that the stronger buffer reagent could suppress the crevice corrosion of Mg.

3.2. Surface characterization

Fig. 3(a) showed the macroscopic appearance of Mg sheets from all groups. All groups outside the crevice had almost uniform corrosion appearance. All the DW and m-SBF groups showed almost uniform corrosion appearance in accordance with the corrosion rate in Fig. 2(a). The groups with the most severe corrosion inside the crevice in the NaCl and the PBS with thickness of 0.2 mm, 0.5 mm and 0.8 mm groups were the same ones with the highest corrosion rate in Fig. 2(a). And they were enclosed by the red dots rectangle in Fig. 3(a). There were different sizes of disk-like corrosion pits clearly on the surface. And the corrosion morphologies in larger magnification from SEM were similar as the previous study [13]. Only a specific corrosion pit which already penetrated Mg sheet in the NaCl-0.2 mm group was shown in Fig. 3(b) and (c). The corrosion morphology of this corrosion pit was river-like and there were some small pits inside the big pit. The corrosion pits were induced inside crevice, continuously propagating to penetrate the Mg sheet and laterally propagating to form a river-like corrosion morphology.

Fig. 3.

Fig. 3.   (a) Camera photos of Mg sheets immersed in the deionized water, the 0.9 wt.% NaCl solution, the PBS solution and the m-SBF solution (two red lines in every groups showed the position of crevice area and the red dots rectangle emphasized the specific groups with severe local corrosion pits), (b) at low and (c) at high magnification of the corrosion morphology in the NaCl-0.2 mm group.


After 14 days immersion, severe corrosion perforation appeared in the central part of Mg cannulated screw (Fig. 4(a)). A large number of corrosion pits appeared around the big hole (Fig. 4(b)), and the obvious corrosion pits could be observed at larger magnification (Fig. 4(c)). However, the corrosion morphology far away from the corrosion perforation was relatively uniform, and no obvious hole was observed. After cutting through the whole screw to two half parts, the corrosion morphology inside the middle tunnel around the big hole was clearly observed. Inside the tunnel, severe local corrosion appearance with different sizes of pits was also observed near the hole (Fig. 4(e) and (f)). At larger magnification, the corrosion morphologies of corrosion pits (Fig. 4(g) and (h)) were similar to the crevice corrosion, as shown in Fig. 3 and in the previous study [13].

Fig. 4.

Fig. 4.   Surface morphologies of (a-c) external and (d-h) internal of Mg cannulated screw.


Based on the above results, it was suggested that the presence of chloride ions and crevice dimensions were two necessary factors to induce crevice corrosion in Mg. Additionally, the buffering reagent would suppress crevice corrosion of Mg. Based on the comparison of pH values and corrosion rates in the NaCl and DW groups, the presence of chloride ions was a necessary factor to induce crevice corrosion in Mg. That proved the hypothesis that chloride ion should play an important role in crevice corrosion [13]. A more detailed experiment was required to verify which specific chloride ion concentration could induce the crevice corrosion. Besides that, for the NaCl and PBS groups, severe crevice corrosion only occurred in the 0.2 mm, 0.5 mm, 0.8 mm thickness groups. This suggested that the crevice dimension was also one of important factors to affect the process of crevice corrosion. According to pH values and corrosion rates in the PBS and m-SBF groups, the stronger buffering reagent would make pH values of whole solution more stable to suppress the formation of abundant Mg(OH)2, which would block the entrance of crevice to induce the crevice corrosion.

To summarize the results and discussion, a novel mechanism was proposed, as shown in Fig. 5, to describe the crevice corrosion. When Mg sheet was immersed into the aqueous solution containing chloride ions, the reaction which consumed H2O would happen following this equation:

$\text{Mg}+2{{\text{H}}_{2}}\text{O}\to \text{M}{{\text{g}}^{2+}}+2\text{O}{{\text{H}}^{-}}+{{\text{H}}_{2}}$

Fig. 5.

Fig. 5.   Schematic mechanism of crevice corrosion occurred in biodegradable Mg.


If crevice dimension was too small for ions in solution inside and outside crevice to exchange, the concentration of chlorine ions inside crevice would rise due to the consumption of water and then became much higher than that outside crevice after some hours. And the corrosion potential of Mg surface inside crevice became lower than that of the surface outside crevice. According to the Ref. [20], when chloride ion concentration increased from 0.005 mol/dm3 to 2.5 mol/dm3, the Ecorr would drop from -1.45 V to -1.7 V. This difference of corrosion potential could make Mg surface inside crevice to be the anode and Mg surface outside crevice to be the cathode. Thus the galvanic corrosion between Mg surfaces inside crevice and outside crevice could accelerate the corrosion rate of Mg inside crevice. At the same time, the loose white corrosion precipitation generated during the corrosion process would accumulate inside crevice, which would affect the geometry structure of the crevice, and also caused the occlusion effect to aggravate crevice corrosion [21].

4. Conclusions

(1) Mg sheets immersed in the 0.9 wt.% NaCl and the PBS solution under the crevice dimension with 10 mm length, 3 mm width and 0.2, 0.5, or 0.8 mm thickness exhibited severe crevice corrosion.

(2) Mg cannulated screws immersed in the PBS solution had crevice corrosion, which perforated the whole screw wall from internal.

(3) Chloride ions and crevice dimensions were two necessary but not sufficient factors for crevice corrosion in Mg. The buffering reagent would suppress crevice corrosion of Mg.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (No. 51571142) and the National Key Research and Development Program of China (No. 2018YFC1106600).

Reference

M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias , Biomaterials, 27(2006), pp. 1728-1734.

DOI      URL     [Cited within: 1]

G.L. Song , Corros. Sci., 49(2007), pp. 1696-1701.

DOI      URL     [Cited within: 1]

Y.F. Zheng, B. Liu, X.N. Gu , Mater. Rev., 23(2009), pp. 1-6.(in Chinese)

DOI      URL     [Cited within: 1]

F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, Biomaterials, 26(2005), pp. 3557-3563.

DOI      URL     [Cited within: 1]

S.X. Zhang, X.N. Zhang, C.L. Zhao, J.N. Li, Y. Song, C.Y. Xie, H.R. Tao, Y. Zhang, Y.H. He, Y. Jiang, Y.J. Bian , Acta Biomater., 6(2010), pp. 626-640.

DOI      URL     [Cited within: 1]

Y.F. Zheng, X.N. Gu, F. Witte , Mater. Sci. Eng., R, 77(2014), pp. 1-34.

DOI      URL     [Cited within: 1]

W.H. Wang, Q.S. Wang, C.Y. Wang, J. Yi, J. Loss Prev. Process Ind., 29(2014), pp. 163-169.

DOI      URL     [Cited within: 1]

Q. Hu, G.A. Zhang, Y.B. Qiu, X.P. Guo , Corros. Sci., 53(2011), pp. 4065-4072.

DOI      URL     [Cited within: 1]

L.L. Machuca, S.I. Bailey, R. Gubner E.L.J. Watkin,M.P. Ginige,A.H. Kaksonen,K. Heidersbach, Corros. Sci., 67(2013), pp. 242-255.

DOI      URL     [Cited within: 1]

E. Ghali, W. Dietzel, K.U. Kainer, J. Mater. Eng. Perform., 13(2004), pp. 7-23.

DOI      URL     [Cited within: 1]

Z.M. Shi, A. Atrens , Corros. Sci., 53(2011), pp. 226-246.

DOI      URL     [Cited within: 1]

B. Denkena, J. Khler, J. Stieghorst, A. Turger, J. Seitz, D.R. Fau, L. Wolters, N. Angrisani, J. Reifenrath, P. Helmecke, Procedia CIRP, 5(2013), pp. 189-195.

DOI      URL     [Cited within: 1]

H.L. Wu, C.J. Zhang, T.F. Lou, B.W. Chen, R.B. Yi, W.H. Wang, R.P. Zhang, M.C. Zuo, H.D. Xu, P. Han, S.X. Zhang, J.H. Ni, X.N. Zhang , Acta Biomater., 98(2019), pp. 152-159.

DOI      URL     [Cited within: 5]

P. Han, P.F. Cheng, S.X. Zhang, C.L. Zhao, J.H. Ni, Y.Z. Zhang, W.R. Zhong, P. Hou, X.N. Zhang, Y.F. Zheng, Y.M. Cai , Biomaterials, 64(2015), pp. 57-69.

DOI      URL     [Cited within: 1]

A. Oyane, H.M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, J. Biomed. Mater. Res. Part A, 65(2003), pp. 188-195.

[Cited within: 1]

R. Dulbecco, M. Vogt, J. Exp. Med., 99(1954), pp. 167-182.

DOI      URL     [Cited within: 1]

N.I. Zainal Abidin, A.D. Atrens, D. Martin, A. Atrens, Corros. Sci., 53(2011), pp. 3542-3556.

DOI      URL     [Cited within: 1]

N.I. Zainal Abidin, B. Rolfe, H. Owen, J. Malisano, D. Martin, J. Hofstetter, P.J. Uggowitzer, A. Atrens, Corros. Sci., 75(2013), pp. 354-366.

DOI      URL     [Cited within: 1]

N.I. Zainal Abidin, D. Martin, A. Atrens, Corros. Sci., 53(2011), pp. 862-872.

DOI      URL     [Cited within: 1]

G. Williams, H. Ap Llwyd Dafydd, R. Subramanian, H.N. McMurray, Corrosion, 73(2017), pp. 471-481.

DOI      URL     [Cited within: 1]

B. Vuillemin, R. Oltra, R. Cottis, D. Crusset , Electrochim. Acta, 52(2007), pp. 7570-7576.

DOI      URL     [Cited within: 1]

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