New Design to Provide Absolute Protection within a Certain Period for Biodegradable Magnesium Alloys

Magnesium (Mg) and magnesium alloy materials have a known potential for biodegradable implant applications, and studies based on the control of biodegradation are thus expanding in order to overcome the problems of undesirable dissolution processes and rates of Mg and Mg alloy materials [1]. In the last decade, researchers have reconnoitered that magnesium and its alloys have good mechanical properties, biocompatibility and osteogenesis [2], [3], [4], [5], [6]. Currently, there are several strategies to the defect of excessive corrosion rate of magnesium alloys, including alloy composition optimization, casting process improvement and surface modification, etc. [3], [7]. It has been demonstrated that improving the protective properties of surface coatings in high chloride ion solutions such as biological fluids is a proven effective method. Among the existing surface modification techniques, microarc fluorination (MAF) is considered to be a method of operational simplicity and high efficiency during the preparation of protective fluoride coatings for Mg materials. Fluorine, in parallel with Mg, is one of the trace elements needed by the human body, while fluorine is a component of human bones and teeth [8], [9], [10]. Notably, the coating facilitates the proliferation of osteoblasts [11], which is the main reason we therefore chose the MAF coating in this study.

The fixed pitting corrosion was firstly explored as a major cause of mechanical property loss under coating conditions. This local coating disintegration at uncontrollable fixed points implies the formation and extension of cracks, which, in the presence of stress concentrations, can consequently lead to the destruction of the mechanical integrity of the implant [1], [12], [13], [14]. Nowadays, more and more research about magnesium alloys is focusing on corrosion resistance while paying attention to the testing of mechanical properties [7], [15].Therefore, it is particularly critical concerning the testing and optimization of mechanical properties, especially the tensile properties, of Mg alloy materials when applied to biodegradable implants such as bone plates [16], [17], [18], [19].

Therefore, to avoid fixed-point corrosion became the main purpose of the new design of our research. By combining the properties of Mg and Mg alloys with multidisciplinary expansion we found that as a consequence of their good electronegativity, Mg and Mg alloys are suitable in real applications as sacrificial anodes in corrosion resistant means and rarely become protected parts [20], [21], [22], [23]. Magnesium is highly reactive and has a more negative potential than most coatings and metals. In addition, metals with a more negative potential than magnesium, such as potassium, calcium, and sodium, dissolve too quickly to meet the requirements for long-term protection. Magnesium is therefore often used as an excellent sacrificial anode material for the protection of other metals.

The innovation of our study is that it goes beyond the mindset and uses reasonable material partitioning to enable simultaneous anodic sacrifice and cathodic protection of the magnesium alloy itself for the protection of functional magnesium alloys. The optimal sacrificial anode should provide sufficient current while meeting a suitable self-dissolution rate [19], [21]. In the observation of the indentation treatment group after immersion, the corrosion localization and centralization should not be neglected. Combined with the theoretical transformation of Mg alloy material when used as a sacrificial anode, we designed the Corrosion-oriented Design (COD) model (Fig. 1) with the aim of maintaining the mechanical properties of degradable Mg alloy implants, based on the control of the overall dissolution rate of Mg alloy material, and determined its sacrificial centralized area and functional area. The design could provide absolute protection of the mechanical properties for the functional area of Mg alloys in a certain period of time. If used as bone implants, COD Mg alloys could avoid premature fracture due to the sudden change of mechanical properties in the early stage of bone healing, providing the necessary mechanical support for bone healing, and can be completely degraded at the end.

The aim of this study was to identify the effect of fixed-point corrosion on the corrosion resistance as well as the mechanical properties of magnesium alloys and to design a new corrosion-oriented model that can provide absolute protection over a period of time. The corrosion resistance and tensile strength of bare and coated magnesium alloys were measured and evaluated before and after the addition of dents. Our model design successfully utilizes the sacrificial anode principle and provides a new idea for the protection of magnesium alloys.

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