**4. Conclusions and perspectives**

This chapter summarizes the advances of the silica sol-gel coating as a surface modification technique to control the corrosion of Mg and its alloys. The most

important advantages of the sol-gel technique are the opportunity to introduce a wide range of alkoxysilane precursors and organic molecules in the synthesis for obtaining hybrid organic-inorganic sol-gel coatings with desirable cross-linking structure and good protective corrosion behavior. The organically modified sol-gel coatings provide the possibility to obtain thick, crack-free coatings with good corrosion performance. The hybrid films can be reinforced by doping with nanoparticles to obtain denser coatings, and with inhibitors to obtain active barrier protection. Although relevant advances have been made in recent years, some aspects related to the sol-gel technique on Mg alloys should be considered before obtaining a successful industrial application, especially for aerospace, automobile, and biomedical applications.

Since the corrosion behavior of sol-gel coatings depends on the synthesis parameters, organic-inorganic precursors, and the mechanical and chemical features of the comprising organic and inorganic networks, a variety of sol-gel coating with different compositions and cross-linked structures have been developed. However, the different protective properties and the service life between those coating on Mg alloys are still not known clearly. Therefore, systematic and long-term comparisons need to be conducted in future research to better understand the corrosion mechanism, as well as the advantages and disadvantages of each coating. Furthermore, the kinetics of hydrolysis and condensation reactions, gelation kinetics, and curing process parameter should also be studied and considered to avoid cracks coating formation during the heat post-treatment. Sol-gel films treated by UV radiation at room temperature can form a denser sol-gel coating that can improve the corrosion resistance of alloys.

Although a sol-gel coating is a promising alternative, recent works show that the deposition of a single layer of sol-gel coating faces many difficulties and does not stop the corrosion of Mg alloys. On that basis, the combination of different deposition processes, such as anodization or PEO processes, and sol-gel technique could be more effective methods to mitigate the corrosion damage. However, different factors such as sealing pore effectiveness should be considered to reach a good compromise between stacking and anticorrosion properties. The preparation of efficient composite coatings for Mg-based alloys is still a huge challenge.

To achieve a practical application in the biomedical field, the design of sol-gel coatings should be more purposeful. For example, for bone implant applications, the hybrid coatings should be pro-osteogenesis and biocompatible. Moreover, the corrosion resistance studies of the silane coatings deposited on Mg alloys should be complemented with in-vitro cell viability tests to determine the bifunctionality response of the silane coatings.

However, many challenges need to be faced and solved, intelligent multilayer systems are promising alternatives to significantly increase the use of Mg alloys in many relevant applications, from corrosion protection to bioactive devices. Continues research is the best way to get them.
