*The Role of Silane Sol-Gel Coatings on the Corrosion Protection of Magnesium Alloys DOI: http://dx.doi.org/10.5772/intechopen.102085*

As observed, although sol-gel technology allows the preparation of different hybrid organic-inorganic sol-gel coating compositions, the obtention of effective coatings for Mg-based alloys still is a huge challenge. One of the main aspects, related to the synthesis of the sol-gel coatings on Mg alloys, is the pH of the hydrolyzed sol-gel solution. Indeed, magnesium is not stable and spontaneously degrades during sol-gel deposition step when using acidic conditions. Hernández-Barrios et al. [31] evaluated the corrosion behavior of AZ31 Mg alloy pretreated with a hybrid silica sol-gel coating prepared using acetic acid as acid-catalyst, and 3-glycidyloxypropyltrimethoxysilane (GPTMS) and TEOS as silica precursors. The results revealed that the sol synthesized with the highest acid concentration reached more stable gelation kinetics, but with the worst corrosion resistance performance (icorr: 1.3 × 10−6 A/cm2 ) compared with the sol synthesized with the slower acid concentration (icorr: 2.4 × 10−7 A/cm2 ). The decay of the corrosion resistance of the sample coated with the more acidic sol is attributed to defects on the coating's morphology and to the corrosion process advancing in the substrate. Indeed, during the sol-gel deposition, corrosion products are generated together with hydrogen evolution.

In this sense, pH of the sol is a critical parameter and should be considered to get a nondefective SiO2 coating not affecting the metallic substrate, and thus to provide a suitable corrosion resistance performance.

Another aspect to consider is related to the promotion of insulating coatings with high-density structures for blocking the penetration of electrolytes. In this case, complexing agents are added during the synthesis of the sol to react with the organic group of some organo-alkoxysilanes and therefore stimulate the organic polymerization. For instance, Qian et al. [32] prepared a hybrid sol-gel through hydrolysis and condensation reactions of TEOS and GPTMS. The opening of the epoxy group of GPTMS results in coatings with novel physical and chemical properties. The authors further incorporated triethylenetetramine (TETA) as an organic crosslinking agent to bond with the open epoxy groups. The corrosion behaviors of the coatings deposited on AZ31B magnesium alloy were evaluated by polarization curves measurements in the 3.5% NaCl solution. The results revealed that a compact and smooth silane film was formed on the substrate's surface, which provided good barrier protection, improving the corrosion resistance ability (icorr: 3.7 × 10−9 A/cm2 ) in comparison to untreated magnesium alloy substrate (icorr: 4.1 × 10−6 A/cm2 ) (**Figure 4**).

Furthermore, the corrosion resistance properties of silane films can be significantly improved by the incorporation of some nanoparticles into the sol-gel film. The beneficial effects of the addition of different nanoparticles on the corrosion resistance for Mg alloys have been reported by different researchers. For instance, the effect of incorporating SiO2 nanoparticles [33], graphene oxide [34, 35], carbon nanotubes [36], alumina, titania, zirconia [37], and Montmorillonite (MMT) [38] on the sol-gel synthesis has been evaluated.

For example, the addition of a colloidal silica nanoparticles suspension into the sol-gel coating is considered a good approach to increase the hardness, density, and wear resistance, and thus the corrosion resistance properties of hybrid silane coatings. Peres et al. [33] investigated the effect of adding different amounts of SiO2 nanoparticles into a hybrid silica sol based on TEOS and GPTMS on the corrosion resistance of AZ31 magnesium alloy. The results showed that the incorporation of nanoparticles improved the corrosion resistance of Mg alloy. However, the maximum amount of SiO2 recommended to obtain a coating with the best anticorrosive performance was between 100 and 300 mg l−1; coatings doped with a higher amount of SiO2 showed nanoparticles agglomeration and consequent defects and cracks. Thus, two critical issues should be

**Figure 4.**

*Potentiodynamic polarization curves of bare alloy and the hybrid coating in 3.5 wt.% NaCl (reprinted from Ref. [32], ESG).*

considered to avoid a detrimental effect on the anticorrosion behavior of the film: (i) the dispersion of nanoparticles into the film, and (ii) the amount of loaded nanoparticles.

On the other hand, graphene oxide (GO), which is a two-dimensional sp2 carbon material, with many inherent characteristics such as good mechanical strength, chemical inertness, and good thermal stability, has also been considered to reinforce organofunctional silane coatings for corrosion protection of Mg alloys [34]. However, the high specific surface area of graphene and the strong Van der Waals force (−stacking) between graphene layers made it to agglomerate easily, resulting in hybrid coatings with a decrease in corrosion performance and microhardness properties. As graphene-based compound [39], oxidized fullerene [40], and carbon nanotube [36] have also been considered as a novel promising reinforcement for hybrid composite silane coatings for Mg alloys due to their properties including high strength, lightweight, thermal and mechanical stability, hydrophobicity, corrosion resistance, and high specific surface area. The anticorrosion and protective action of a zeolite-filled silane sol-gel coating on AZ31 magnesium substrate was studied by Calabrese et al. [41]. The zeolite composite coating evidenced very high hydrophobicity behavior (contact angle up to 140° showed good adhesion and good barrier properties during immersion in 3.5 wt.% NaCl solution.

Although significant advances have been made regarding modified sol-gel coatings composition, there is still a large gap in this research since most of the studies only provide information about the instantaneous corrosion rate, but not about the kinetic of the sol-gel film degradation in standard aqueous solution of 3.5 wt*.*% NaCl, which is helpful for a comprehensive choice of anticorrosion strategies and a systematic control of the degradation of sol-gel films.

## *2.1.2 Active sol-gel coatings barrier*

Up to now, conventional physical barrier coatings with suitable composition designs have been considered to improve the corrosion resistance of Mg alloys; however, in very harsh environments when the aggressive agent and water reach the metal surface, the silane coatings are not capable to stop the corrosion process, reducing the lifetime of the coating protection. For this reason, smart self-healing protective coatings should be considered to provide long-term protection to the material. The smart
