# Applications of Magnesium and Its Alloys: A Review
## 4 Magnesium and Its Alloys as a Biomaterial
### Corrosion  
Corrosion is a form of biodegradation. Magnesium's ability to biodegrade is its greatest strength, while the speed at which it degrades is its greatest weakness. Magnesium degrades rapidly in a physiological environment. Such accelerated degradation can compromise the mechanical integrity and strength of its respective applications. Besides, it can result in possible deleterious and consequences [4], such as premature failure of the implant [46], interference with the healing process, and in extreme cases, death [61]. With limited research and technologies in its early days, surgeons faced great difficulties managing magnesium corrosion in vivo. As such, they preferred the use of corrosion-resistant V2A steel in place of magnesium, and that waned interest in magnesium as a prospective biomaterial [7]. Despite significant advances today, managing magnesium corrosion is still a major technological challenge [61]. The types of magnesium corrosion can be classified into general/uniform or localized. Predominately, magnesium can be exposed to galvanic corrosion, pitting corrosion, filiform corrosion, intergranular corrosion (IGC), exfoliation corrosion (EFC), crevicle corrosion, stress corrosion cracking (SCC), corrosion fatigue (CF), and erosion-corrosion [62]. Magnesium corrosion in vivo accelerates in the presence of chloride ions, which can be found in human body fluid or blood plasma [63]. Magnesium corrodes in an aqueous environment via an electrochemical reaction to produce hydroxide and hydrogen gas [60]. The produced hydroxide layer quickly gets converted away into highly soluble magnesium chloride, and this reinforcing process loop hastens the rate of magnesium's corrosion [61]. Extensive literature has identified magnesium's rapid degradation to contribute to hydrogen evolution and alkalization [33]. Hydrogen evolution refers to the production of subcutaneous hydrogen bubbles adjacent to the implants [63]. The rate at which these hydrogen bubbles are produced is directly correlated to the corrosion rate of magnesium [64]. When hydrogen is produced faster than absorbed, it will create a balloon effect [33], which can tear and destroy the tissues and their layers as it gets separated. The destruction of tissues can delay the healing process, and in extreme cases, it can be fatal if the balloon hydrogen bubbles get too large and disrupt the bloodstream [63]. Local alkalization around a rapidly corroding magnesium implant is also a primary medical concern. While the human body can manage slight fluctuations in pH level, a greater alkaline concentration can create an adverse physiological reaction. For exam ple, if the in vivo pH value of the localized region exceeds 7.8, it can lead to alkaline poisoning [63]. A review of the literature suggests three key strategies to overcome magnesium's corrosive behaviors. They are purification, alloying, and surface modification. These strategies employ the same principle of slowing the biodegradation process. By incorporating them, it can reduce the speed of hydrogen accumulation and localized alkalinization. It will also provide room for the human body to adjust and deal with the magnesium implants [63]. However, while purifying magnesium is an option to reduce its corrosion rate materialy, it will produce a pure magnesium metal with several mechanical limitations, as discussed above, and render it unsuitable for most medical applications [59]. Instead, similar to how pure magnesium is alloyed for engineering and structural applications, alloying is widely used to improve magnesium's mechanical and corrosive performance as a biomaterial. Many variables can affect the alloy's mechanical and corrosive performance and its implications to the human body [63]. These variables include the alloying elements, their level of purity, their microstructure, and how the materials are processed and alloyed [4; 64]. Therefore, a careful and holistic assessment of the alloying element is critical.