Review: Degradable Magnesium Corrosion Control for Implant Applications

Abstract

Magnesium (Mg) alloys have received increasing interest in the past two decades as biomaterials due to their excellent biological compatibility. However, the corrosion resistance of Mg alloys is relativity low which limits their usage in degradable implant applications, and controlling the corrosion resistance is the key to solving this problem. This review discusses the relative corrosion mechanisms, including pitting, filiform, high temperature, stress corrosion, etc., of Mg alloys. Various approaches like purification (Fe, Ni, Cu, etc.), micro-alloying (adding Zn, Mn, Ca, RE elements, and so on), grain refinement (severe plastic deformation, SPD, etc.), and surface modifications (various coating methods) to control corrosion and biological performance are summarized. Moreover, the in vivo implantations of Mg alloy vascular stents and the issues that have emerged based on the reports in recent years are introduced. It is recommended that corrosion mechanisms should be further investigated as there is no method that can remove all the impurities and a new purification approach needs to be developed. The concentration of micro-alloy elements should be carefully controlled to avoid superfluous compounds. Developing new continuous SPD methods to achieve fine-grained Mg alloys with a large size scale is necessary. The development of a multifunctional coating could also be considered in controlling the Mg degradation rate. Moreover, the research trends and challenges in the future of Mg biomaterials are proposed.

Keywords: biodegradable Mg alloy; corrosion resistance control; implants; mechanisms.

Figure 17

(I) SEM top-view micrographs of the (a) AZ31B substrate; (b) Zn layer; (c) Al–Mn alloy coating; (d) cross-section of Zn/Al–Mn alloy composite coatings; EDXS analysis of (e) Zn layer and (f) Al–Mn alloy coating; (II) (a) EIS plots, (b) modulus Bode plots and potentiodynamic curves (III) of bare AZ31B alloy, Zn layer, and Al–Mn alloy coatings in 3.5% NaCl solution [94].

Figure 1

Schematic diagram of the corrosion mechanism: (a) MgY1.72Zn2.81Zr0.17 alloy, (b) MgY3.83Zn3.03Zr0.17 alloy, (c) MgY2.58Zn1.27Zr0.15 alloy [10].

Figure 2

Schematic representation of FCC mechanism on magnesium under immersion conditions [18].

Figure 3

Schematic diagrams of the stress corrosion cracking (SCC) behavior of Mg-1Zn alloy in PBS, m-SBF, DMEM, and BCS media [23].

Figure 4

(I) corrosion rates of GW103K alloy; (II) Surface corrosion morphologies after immersed in 5 wt.% NaCl for 3 days ((a) unrefined, (b) refined by JDMJ, (c) refined by JDMJ + 5 wt.% GdCl3 additions, (d) refined by JDMJ + 10 wt.% GdCl3 additions) [35].

Figure 5

Electrochemical tests of various samples: (I) Mg-0.24Sn-0.04Mn alloy (a) EIS Nyquist plots, (b) potentio-dynamic curves; (II) Mg-0.24Sn-1.16Zn-0.04Mn alloy (a) EIS Nyquist plots and (b) potentio-dynamic curves [41].

Figure 6

(I) Hydrogen evolution (a) and corrosion rate (b) for Mg-5Al-xMn alloys immersed at 3.5 wt.% NaCl solution; (II) Macrographs of corroded surface after 3 h immersion of Mg-5Al (a), Mg-5Al-0.5Mn (b), Mg-5Al-1.4Mn (c) and Mg-5Al-3.1Mn (d) alloys [42].

Figure 7

(I) Optical microstructure of as-cast AZC alloys (a) AZC03, (b) AZC05, (c) AZC1, (d) AZC2, (e) AZC3, (f) AZC4; (II) Weight loss corrosion rate for AZC; (III) Corrosion surface morphology: (a) AZ91D, (b) AZ91D, (c) AZC2 [47].

Figure 8

Corrosion rates of selected Mg-RE alloys in 0.9%/1% NaCl, in SBF, in Hank’s [58].

Figure 9

Optical micrographs of (a) as-cast and (b) as-extruded alloys; Nyquist plots of EIS (c) as-cast and (d) as-extruded alloys [62].

Figure 10

(I) Optical and SEM micrographs (a–f) of ECAPed AZ91 Mg alloy; (II) Weight loss rates after immersion in 3.5 wt.% NaCl solution for 7 d; (III) Nyquist spectra for various times (a) 1 h, (b) 1 d, (c) 3 d and (d) 7 d [66].

Figure 11

(I) Schematic illustration of HPT processing; [67] (II) SEM images of (a) as-cast, (b) convention extrusion and (c) HPT-treated samples after immersion for 2 days; (d) potentiodynamic curves [68].

Figure 12

(I) (a) Schematic illustration of the CEC procedure; (b) 2 passes CEC specimen; (II) OCP (a) and Tafel (b)curves of various samples in SBF solution at 37 °C: (III) SEM images of samples after immersion for 48 h: (a) as-cast, (b) hot extruded, (c) CEC 2 passes [69].

Figure 13

(I) Schematic illustration of the Cu wedge mould; (II) (a) Polarization curves and (b) EIS and (c) corrosion rate of Mg–2Zn–0.5Ca alloy with different cooling rates immersed in SBF; (III) Surface morphology of various cooling rates sample immersed in SBF for 72 h: (a) C1, (b) C2, (c) C3, (d) C4 and (e) C5 [73].

Figure 14

SEM micrographs of the polished cross section (above) and as-produced surface (below) of plasma sprayed βTCP (a,c) and HA/βTCP (b,d) samples [79].

Figure 15

(I) Schematic illustration of the preparation and structure of MAO coating on Mg alloy; [82] (II) Potential polarization curves of the bare AZ31 Mg alloy and MAO coated samples; (III) the degradation and protection mechanism of three different coatings on AZ31 Mg alloy (a) SE (b) SEH (c) SEHi coatings [83].

Figure 16

(I) Schematic of PCL-LS coating on AZ31 Mg substrate; (II) SEM images of bare, alkaline treated, and coated Mg samples before and after immersion in Hank’s solution for seven days at 37 °C [85].

Figure 18

(I) tubular Mg-0.3Sr-0.3Ca stent (a) before and (b) after implantation into the femoral artery of the experimental animal; (II) Optical and histology images of vascular tissue surrounding (a,b) Mg-0.3Sr-0.3Ca and (c,d) WE43 tubular stent samples implanted in femoral artery for five weeks; (III) High-resolution SEM images from the interface of retrieved Mg sample after five weeks of implantation and the Sr-substituted HA layer. Image (b) is the magnified view of the rectangular area marked in (a). [111].