## 3 Corrosion Properties  
Since one of the critical properties of Mg alloys is controlled degradation through corrosion, we present a brief overview of corrosion principles. Programmed degradation, often referred to as controlled biodegradation, serves to meet temporary load-bearing requirements without interfering with the healing and regeneration of local tissues. However, once local wound healing has occurred, the implant gradually degrades, slowly transferring the load to the adjacent bone tissue [35]. Conversely, undesired but often normal degradation of any alloy can manifest as surface corrosion, oxidation, and mechanical wear of the implant material. Such undesirable degradation can compromise the structural integrity of the implant, release potentially toxic metallic ions, and generate wear debris particles, resulting in unfavorable biological reactions in neighboring tissues or in mechanical impediment of articulating joints. The preferred model for biodegradable implants within the body involves the degradation or corrosion of metals that are non- to minimally harmful or that can be rapidly eliminated from the body through physiological processes without local concentrations exceeding any toxic thresholds during the corrosion process [36]. A critical consideration when designing biodegradable implants is to achieve a controlled degradation rate that aligns with the timeline of tissue restoration, as depicted in Figure 1. In a controlled degradation scenario, employing biodegradable materials is intended to facilitate the restoration of tissues to their natural state, allowing for transformation and sustained growth. Throughout the degradation process, it is crucial for the implant to maintain its structural integrity until after its mechanical function diminishes. On the pathway to failure, the implant should undergo a controlled and reproducible deterioration without causing undesired side effects. The degradation rate can be regulated by selecting appropriate alloying elements. Incorporating these elements enhances degradation or corrosion resistance up to a certain threshold, beyond which the resistance will decline and controlled failure will progress [37]. Magnesium (Mg) alloys are among the materials employed for biodegradable purposes. This is particularly relevant in hard material prostheses because Mg\({}^{2+}\) ions are non-toxic at normal concentrations, and the corrosion rate can be controlled [38]. More than half of the magnesium in the human body is already found in the bones. Incorporating magnesium as a significant metallic element in the design of biomaterial alloys can aid in the favorable degradation of a biodegradable implant, as shown in Figure 2, and contribute to bone healing through cellular repair mechanisms [11]. As mentioned, for controlled degradation to be safe, the metal ions released from corrosion need to be non-toxic, but an acceptable alternate is that they are released at a slow enough \begin{table}
\begin{tabular}{l l l l l} \hline \multirow{2}{*}{**Material**} & **Young’s Modulus** & **Yield Strength** & **Tensile Strength** & \multirow{2}{*}{**Hardness (HB)**} \\  & **(GPa)** & **(MPa)** & **(MPa)** & **(MPa)** \\ \hline Ti-6 Al-4V & 55–110 & 760–1103 & 860–965 & 334 \\ Co-Cr & 240 & 500–1500 & 900–1540 & - \\ Stainless Steel & 200 & 170–310 & 540–1000 & 201 \\ Magnesium & 45 & - & 170 & - \\ AZ31 & 45 & 171–176 & 290 & 49 \\ AZ91 & 45 & 160 & 240 & 63 \\ Mg-Ca-Zn & 45 & - & - & 30–60 \\ \hline \end{tabular}
\end{table} 
Table 3: Mechanical Properties of different Magnesium alloys and non-biodegradable materials [31]. rate that the local concentration in the adjacent tissues never exceeds the safe toxic limit. Yet, this may create conflicting objectives since often one of the objectives is to degrade at the same rate as tissue is restored in vivo [39].