**2.1 Barrier effect of single protective silane sol-gel coatings against corrosion**

The application of single sol-gel coatings on the surface of Mg alloys has been considered as a good initial approach to provide a protective behavior to the alloy, since the deposition of an inert silane coating avoids the direct contact between substrate and corrosive environment [26]. From the point of view of synthesis, the sol-gel route offers a versatile way to synthesize effective and denser coatings with specific properties.

## *2.1.1 Passive sol-gel coating barrier*

Pure inorganic sol-gel coatings have been studied as an inert physical barrier to provide protection against corrosion. However, inorganic sol-gel coatings have some limitations such as: (i) brittleness, shrinkage, and internal stress after heat treatment process, and (ii) the high temperature required to sinter the coating that mismatches with the thermal expansion coefficient of Mg substrate [27]. A great effort has been made to incorporate organo-alkoxysilanes into the sol-gel synthesis to obtain crackfree hybrid coatings able to be sintered at lower temperatures (below 200°C, depending on Mg alloys) close to the thermal expansion coefficient of Mg substrate [28]. Due to the wide variety of organic-inorganic precursors, there is growing attention on producing hybrid-inorganic sol-gel coatings with different cross-linked structures and compositions. The final hybrid sol-gel coatings could reach fascinating mechanical and physical properties such as flexibility, hydrophobicity, exceptional dielectric properties, strength, ductility, hardness, and good thermal stability. Zucchi et al. [29] studied the protective performance of coatings obtained using organo-silanes with a long alkyl chain (octadecyl-trimethoxysilane) on AZ31 magnesium alloy. An improvement of the corrosion resistance properties of Mg alloy was observed, confirming that the modification of the siloxane network using a long aliphatic chain provides a positive effect regarding corrosion performance.

The corrosion protection of organic-inorganic hybrid thin films prepared with other organoalkoxysilane precursors such as methacryloxypropyltrimethoxysilane (MAPTMS) and tetramethoxysilane (TMOS) on AZ31 and AZ61 Mg alloys has been also studied by El-Hadaba [30]. The results showed an enhancement of the corrosion protection properties at the initial immersion time, but a quick degradation of the coated AZ31 Mg alloy after 1 day of immersion in 0.6 M NaCl aqueous solution. This behavior was attributed to microscopic pores defects in the sol-gel layer. The low pH of the sol-gel solution promoted the Mg dissolution during the deposition process together with the hydrogen evolution during the curing sol-gel coating.
