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

self-healing effectiveness relies on a dissolving-reprecipitation interaction in the local defect, able to repair the defects entirely or partially, restoring the functionality of the coatings. The incorporation of corrosion inhibitors into the silane sol-gel coatings is the most studied strategy to obtain a self-healing ability of the silane coating thus enhancing the corrosion resistance of the metal.

Inorganic corrosion inhibitors such as rare earth inhibitors (cerium and lanthanum) have been demonstrated to be effective in the protection of magnesium alloy. For example, the rapid formation of oxygen vacancies in ceria lattice plays a crucial role in self-healing coating formulation since cerium cations interact with the OH− ion released during the corrosion process, forming stable and insoluble cerium oxide/hydroxide species that precipitate in the surface and prevent further corrosion process. Prolonging the corrosion time, the deposited film gradually grows reducing the oxygen and electron transfer [42].

The effect of adding Ce and La salts as inhibitors as well as nanoparticles in silane solution has been explored as an opportunity and a challenge for researchers. Zanotto et al. [43] studied the corrosion resistance efficiency of 3-mercapto-propyltrimethoxysilane (PropS-SH) coatings modified with cerium nitrate (Ce(NO3)36H2O) deposited on AZ31 magnesium alloy. Moreover, Qiao et al. [44] studied the corrosion resistance behavior of 3-methacryloxypropyltrimethoxysilane coatings modified by lanthanum nitrate (La(NO3)3·6H2O) deposited on AZ31 Mg alloy. Both studies demonstrated that either cerium or lanthanum ions can be added as inhibitors to the silane solutions to enhance the corrosion of the pretreatments for magnesium alloy. However, they reported that silane coatings doped with cerium nitrate salt showed poorer corrosion behavior than those doped with cerium nanoparticles, CeO2NPs. Coatings doped with CeO2NPs were more "compact," avoiding the electrolyte penetration, and therefore providing improved corrosion protection [45]. Under this perspective, Calado et al. [46] modified a hybrid epoxy-silane coating with ceria nanoparticles to improve the barrier protection of AZ31 Mg alloys. EIS results showed an improvement in corrosion resistance because the modified ceria-coating was capable to provide active corrosion protection. The ceria nanoparticles react with water and/or hydroxyl ions, producing a cerium (IV) oxide or hydroxide layer onto the AZ31 surface. Electrolyte diffusion pathways are blocked; thus, the localized corrosion activity is reduced.

The use of organic compounds with heteroatoms such as N, S, and O can provide inhibitory effects to silane coatings. The major role of heteroatoms in corrosion protection is the formation of a complex chelate with Mg2+ ions which create insoluble deposits on the metallic surface, blocking the active sites and preventing the local pH increases, which is responsible for the intensification of intermetallic dealloying [47]. Toorani et al. [48] proposed a silane coating with active corrosion properties using γ-amino propyltriethoxysilane (APS) and TEOS as silica precursors, and adding different organic inhibitors: 8-hydroxyquinoline (8-HQ ), indole-3-carbadehyde (I3C), 2-mercaptobenzoxazole (MBO), and sodium diethyldithiocarbamate (DDTC) to silane precursors. The results showed that organic inhibitors provide better active corrosion protection properties to the silane coating compared to the bare AZ91D magnesium alloy, especially when the 8-HQ inhibitor was added. In search of new organic inhibitors for corrosion protection of Mg alloys, Ashassi-Sorkhabi et al. [49] reported the effect of adding amino acids (l-alanine, l-glutamine, l-methionine, and l-aspartic amino acids) as eco-friendly inhibitors into sol-gel coating matrix. The corrosion ability of amino acids was associated with their tendency to form hydrogen bonds with the oxide or hydroxide groups on the metal surface and to the lone

pair electrons present in their heteroatoms that can complex Mg cation. The paper described that all amino acids improved the anticorrosion performance of the silane coating, but l-aspartic exhibited the best enhancement effect.

## **2.2 Barrier effect of multilayer protective coatings**

Even though silica sol-gel coatings have shown to be successful as a physical and active barrier, it is sometimes not enough for a long-term protection system in harsh environment. Some micro-defects or micro-cracks appear, allowing the penetration of corrosive agents and producing oxide-hydroxide-carbonate deposits beneath the coating, causing its rapid delamination. Thereby, the application of single-layer coating does not provide a full protection of Mg alloys. For this reason, great effort is underway to identify efficient alternative systems with desirable surface properties. In this context, the combination of different systems has been suggested, based on the deposition of a first oxide layer using conventional anodization or plasma electrolyte oxidation (PEO) processes followed by the deposition of silica sol-gel coating seems to be a good alternative.

PEO is an electrochemical process that has increasingly been employed to improve the surface properties of Mg alloys. This process produces an adhesive micro-porous oxide layer on the surface that provides a moderate protection on the metal and alloys. The ceramic-like film can be sealed with a silane coating to reduce the infiltration of the aggressive medium through the micro-pores, providing a long-term corrosion protection. Tan et al. [50] reported the preparation of a multilayer system obtained by anodizing the AZ91D Mg alloy and post-sol-gel treatment using MEMO (3-methacryloxypropyl trimethoxysilane), TPTMS (3-mercaptopropyl trimethoxysilane), and silica nanoparticles as reinforcement. The preliminary results showed that after the deposition of various silane layers by spray method, the silane coatings seal the pores of the anodized coating providing a physical corrosion protection in 3.0 wt.% NaCl.

Recently, Merino et al. [51] studied the corrosion resistant of an integrated system for AZ31B Mg alloys combining PEO and sol-gel process. In this case, the sol was prepared by using TEOS, GPTMS, colloidal SiO2 nanoparticles, and 1-methylimidazole (MI), and then deposited onto optimized oxide coating. The results revealed that the multilayer system exhibits a good corrosion performance in 3.5 wt.% NaCl, since the polarization resistance (Rp) for the integrated system samples showed a quite high value (31546.8 Ω cm2 ) compared to Mg alloy (207.3 Ω cm2 ) (**Figure 5**).

### **Figure 5.**

*Bode plot and phase angle plot for bare AZ31B Mg alloy, anodized sample and multilayer system tested in 3.5 wt.% NaCl (reprinted with permission from Ref. [51], Wiley Online Library).*

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

This is an interesting alternative to significantly improve the corrosion resistance of Mg alloys. However, only a few papers present complete and decisive results. Additionally, different factors need to be considered to reach a good compromise between stacking and anticorrosion properties, such as sealing pore effectiveness and micro-cracks formation during the deposition of multiple silane layers [52].
