## 5 Surface Modification Used for Mg Alloys
An insoluble compound film with reasonable adhesion formed on the Mg surface can protect the Mg alloy from light mechanical stress and harsh aqueous environments; these initial layers can improve the adhesion of further coatings. These methods are easy to execute and are often used in biomedical applications. Fluoride and phosphate coatings are used for biomedical Mg alloy surfaces [65; 81; 82]. Normally, fluoride coatings are produced in hydrofluoric acid (HF) by reaction with Mg alloys [63; 83]. The main component of fluorine coating is magnesium fluoride (MgF\({}_{2}\)), which is water-insoluble and spontaneously deposits on the Mg surface. MgF\({}_{2}\) films increase corrosion resistance and improve cellular response and biocompatibility [64; 72]. For phosphate coatings, zinc phosphate and calcium phosphate are used for Mg alloys because of water insolubility, high-temperature resistance, corrosion resistance, and excellent biocompatibility [73; 84; 85; 86]. [PERSON] et al. [87] conducted a comparative investigation into the corrosion behavior and biocompatibility of various chemical conversion coatings applied to magnesium alloy surfaces. The results indicated that the NaOH coating exhibited the highest levels of hydrophilicity and corrosion resistance. Additionally, the surface treated with NaOH demonstrated the most favorable behavior for endothelial cell growth. This process simulates physiological apatite mineralization in nature and deposits bio-ceramic coatings on the substrate surface, as shown in Figure 5. The benefits of this technique are (a) ease of adjusting coating composition, phase, and crystallinity, (b) capability of coating on porous or complex-shaped implants, and (c) having a simplified method of incorporating biologically active agents or drugs into apatite coatings through copper-cipitation. Hence, the biomimetic method is widely used for metallic biomaterials [88; 89]. Biomimetic deposition can effectively enhance corrosion resistance and biocompatibility. [PERSON] et al. give an example of forming a biomimetic calcium phosphate film on a Mg alloy [90; 91]. The biomimetic deposition of CaP on the poly(dopamine) film, followed by graphene oxide coating, demonstrates a multi-step process for creating a sophisticated material with tailored mechanical, electrical, and biocompatible properties. [PERSON] et al. [92] applied a biomimetic Ca-P coating onto ZK60 magnesium alloys and observed a significant improvement in corrosion resistance when tested in simulated body fluid. * Micro-arc oxidation coating  
Micro-arc oxidation (MAO), otherwise known as plasma electrolytic oxidation (PEO), is a high-voltage plasma-assisted anodic oxidation process developed from traditional anodizing to create ceramic-like coatings [93; 94]. The details of MAO are summarized in Figure 6. MAO has been explored in a variety of fields due to its high efficiency, increased bonding strength between coating and substrate, and minimal restrictions on the surface shape of the workpiece [95; 96; 97; 98]. The main limitation of MAO is its inability to provide long-term surface protection [94]. For biocompatibility and biological activity, MAO exhibits higher bonding strength with the substrate due to its dense interior; this porous outer layer is useful in protein adsorption, osteoblast adhesion, and bone tissue regeneration, which is often desirable for biological applications [24]. [PERSON] et al. [99] regulated AZ31B alloy's corrosion and degradation behavior using MAO coatings with varying zinc phosphate concentrations. The AZ31B alloy coated with MAO coatings exhibited improved corrosion resistance in simulated physiological conditions. The addition of zinc phosphate optimized the degradation rate, reduced Mg\({}^{2+}\) and OH\({}^{-}\) release, and alleviated corrosion product formation. After a 56-day immersion, weight loss decreased from 24.37% to 5.22%.