Localized Corrosion Behavior and the Relationship to the Second Phases in Biodegradable ZX21 Mg Alloy in Hanks' Balanced Salt Solution

Abstract

Mg alloys are considered promising biodegradable implant materials owing to their high activity, which makes Mg alloys degradable in the human body and can avoid second removal surgeries. Also, compared with other biodegradable implant materials, Mg alloy has a similar density and Young’s modulus to human bones, which minimizes the stress shielding effect. ZX series Mg alloys, which contain Zn and Ca, are designed to increase the alloy strength but maintain biocompatibility. However, even with all the advantages, the high degradation rate limits the clinical applications of ZX series Mg alloys. With fast degradation, not only the mechanical properties are affected, but also a large amount of hydrogen gas and hydroxide ions would accumulate around the implant in the patient. Therefore, it is essential to investigate the corrosion mechanism of the ZX series Mg alloys to mitigate the degradation rates.Localized corrosion is one of the noticeable features in the corrosion process of ZX series Mg alloys, which is the main reason for the low corrosion resistance and high degradation rate of the alloy. However, the origin and detailed mechanism of the initiation and propagation of localized corrosion on ZX series Mg alloys are still unclear. The second phases, which contain Zn and Ca, play complex roles in the corrosion process owing to the inhomogeneous elemental distribution and different electrochemical potentials with respect to the Mg alloy matrix. Thus, this study aims to investigate the microstructure of the localized corrosion sites and the galvanic effects around the second phases in a ZX series Mg alloy.In this study, an Mg-2 wt%Zn-1 wt%Ca (ZX21) alloy was prepared, and the localized corrosion behavior was investigated in a Hanks’ balanced salt solution (HBSS) at 37°C. By scanning electron microscope (SEM)/focused ion beam (FIB) cross-sectional observations, two kinds of second phases were identified in the ZX21 Mg alloy, one is enriched with Zn and the other is not. After immersion in HBSS, galvanic corrosion sites were found in the Mg alloy matrix next to the second phase enriched with Zn, while the Mg alloy matrix next to the second phase with lower Zn concentration was intact. Furthermore, the second phases in the ZX21 Mg alloy were found to hinder the propagation of localized corrosion. The complex roles of the second phases in the ZX21 Mg alloy on the localized corrosion behavior will be discussed based on electrochemical measurements and microstructure characterizations. Figure 1