**1.1 A brief description of sol-gel synthesis process**

The sol-gel technology is a wet-chemical process where the principal chemical aspect is the transformation of compounds, known as precursors, that contain Si-OR and Si-OH to form stable colloidal particle suspensions known as sol [8]. The sol can be applied on the substrates by different deposition techniques and then sintered to obtain a coating. During the aging step, a chemical transformation of the sol occurs leading to a rigid network, resulting in a gel [9]. Generally, inorganic or organicinorganic sols are obtained via hydrolysis and polycondensation reactions between silicon alkoxides (Si(OR)4) such as: tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) and organoalkoxysilanes R′-Si (OR)n-1; where R′ is the organic functional group linked to Si through a nonhydrolyzable covalent bond. During the hydrolysis stage, alkoxide groups are replaced with hydroxyl groups. Once the hydrolysis reaction has initiated, the condensation reaction occurs simultaneously. In this stage, the hydroxyl group and residual alkoxyl group react to form a three-dimensional Si∙O∙Si network [10]. From the reaction pathway point of view, two different Si∙O∙Si formation mechanisms can take place regarding if the reaction is performed under acidic or basic conditions. Therefore, the morphology and the structure of the resulting network strongly depend on the pH of the reaction.

Under acidic conditions [11], the oxygen atom of Si∙O∙R group is protonated in the first step to form a good leaving group. The central silicon atom turns to be more electrophilic and thus more susceptible to react by water to form Si∙OH group (**Figure 1(1)**). An equilibrium condition is established between silanol groups and H+ ions, resulting in positively charged species Si∙OH2 + that interact with a silanol group to form Si∙O∙Si bonds (**Figure 1(2)**). In this case, the polymerization rate is directly proportional to the H<sup>+</sup> concentration. Therefore, a large number of monomers or small oligomers with reactive Si∙OH groups are simultaneously obtained. The hydrolysis step reaction is favored, and the condensation step reaction is the ratedetermining step. It was reported that the positively charged species, Si∙OH2 + , react preferentially with the less acidic silanols (silanols attached to the least condensed (Si∙O∙Si) end groups), giving to chain-like networks [13].

Under basic conditions [14], the hydrolysis reaction occurs directly by nucleophilic attack of OH<sup>−</sup> to the silicon atom to form Si∙OH bonds. In this case, deprotonated silanol (Si∙O− ) anion is formed and then it gets condensed with a silanol group. The condensation reaction is favored, and the hydrolysis reaction is the rate-determining

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

### **Figure 1.**

*Mechanism of acid-catalyzed sol-gel process. (1) Hydrolysis mechanism and (2) Condensation mechanism reactions. Image adapted from reference [12].*

step. The hydrolyzed species are immediately consumed because of the fast condensation. Due to the nucleophilic nature of the deprotonated silanol, the Si∙O− preferentially attacks the more acidic silanol (silanols attached to the highest condensed (Si∙O∙Si) end groups), leading to the formation of branched and highly condensed clusters.

## **1.2 Deposition methods and curing process of silane sol-gel coatings**

A typical route of formation of silane sol-gel coating is described as the following process: synthesis of sol-gel > deposition > heat treatment. A sol-gel coating can be applied to Mg metallic substrates through various techniques, such as dip-coating, spin-coating, spraying, and electrodeposition, among others. However, dipping and spinning techniques are the two most used ones, especially for flat surfaces [10]. In the case of complex shapes, uniform coating can be obtained by electro-phoretic deposition method (EPD) [15].

By dip-coating, the surface treatment is attained by immersing the substrate into the sol-gel solution. The silanol groups Si∙OH interact spontaneously with the Mg∙OH groups that existed on the alloy surface via Van der Waals interactions. Upon the heat treatment, the Si∙OH and Mg∙OH bonds are attached firmly via a condensation reaction producing metallo-siloxane (Mg∙O∙Si) covalent bonds (**Figure 2**), and the remaining Si∙OH groups of the deposited sol condense and form Si∙O∙Si bonds [16].

By controlling the curing temperature, the control of pore volume and size and mechanical strength can be achieved. High temperatures (more than 200°C) are normally used to cure inorganic sol-gel coatings and lower temperatures (less than 200°C) for drying/curing organic-inorganic sol-gel coatings [10]. Depending on

**Figure 2.** *Schematic representation of metallo-siloxane covalent bond formation.* the sol-gel precursors used, an optimal curing temperature should be defined since an inaccurate temperature could result in a decrement of the corrosion resistance properties of the coating and/or on the mechanical properties of the substrates. For instance, room temperature cured sol-gel coatings exhibit crack-free morphology, but a higher water sensitivity compared to coatings cured at a higher temperature. On the other hand, an increment of the curing temperature can lead to cracked coatings due to the stresses that appear during the sintering process [12]. A relatively new approach to densify sol-gel coatings is to use UV radiation [17]. Sol-gel films treated by UV radiation at room temperature can form denser sol-gel coatings able to improve the corrosion resistance of alloys.

## **1.3 Corrosion behavior of Mg alloys in aqueous environment**

The poor corrosion resistance of Mg alloys can be mainly attributed to its high electronegative potential and the poorly protective properties of the quasi-passive oxide/hydroxide layer formed upon Mg. Generally, when the Mg alloys corrode in aqueous electrolyte, the metal changes its oxidation state, forms ionic species, and releases electrons. To maintain electroneutrality, the generated electrons must be consumed by other species. Therefore, the anodic reaction must be accompanied by a reduction reaction, where a molecule, ion, or atom gains electrons. In aqueous solution, water reduction is the dominant cathodic reaction. **Figure 3(1)** illustrates the anodic and cathodic reactions and the overall reaction that takes place during the corrosion of Mg in aqueous environment. The presence of chloride ions in the aqueous solution typically leads to accelerated corrosion processes (**Figure 3(1)**). Mg(OH)2 can convert to MgCl2, with higher solubility, promoting the dissolution of the Mg alloy [18].

On the other hand, the corrosive environment in the human body has a solution consisting of 0.14 M NaCl and other inorganic species, such as Ca2+, PO4 3−, and HCO3 − . In this case, the presence of phosphates and carbonates promotes the formation of partially protective corrosion product layers [19]. It is clear that corrosion products depend on the type of the electrolyte. These corrosion products not only affect the corrosion rate but can also provide different protection properties to the substrate. **Figure 3(2)** shows a schematic representation of possible interactions between corrosion products of Mg alloy surface on a biological environment.

The deposition of a silane coating could control the corrosion of Mg alloys, although it could dissolve in contact with water due to the hydrolysis of the polysiloxane (Si∙O∙Si) network [20] that results in the release of silicic acid (Si(OH)4), which can be expressed as follows [21]:

$$\text{SiO}\_2\text{(s)} + 2\text{H}\_2\text{O} \xrightarrow{\text{s}} \text{Si(OH)}\_4 \tag{1}$$

It is important to determine the corrosion rate to explain the corrosion behavior and provide models that predict the kinetics of the corrosion in an engineering context. The most widely used technique for exploring the corrosion behavior of a coated Mg alloy involves immersing the samples in a corrosive solution, since the corrosion performance is faster than atmospheric corrosion tests [19]. To study the corrosion performance of coated Mg alloys in aqueous solution, a wide range of tests are used [2]. These tests are divided into two large groups: electrochemical and nonelectrochemical

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

### **Figure 3.**

*Schematic representation of reactions that take place between Mg alloy surface and (1) NaCl aqueous solution and (2) biological environment (reprinted from Ref. [19], Elsevier).*

tests [22]. The most common electrochemical and nonelectrochemical methods used are mentioned below.

## *1.3.1 Electrochemical techniques*

These methods are important and rapid tools for assessing the corrosion of coated Mg alloys. Between the electrochemical techniques, the most used are Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS).

## *1.3.2 Nonelectrochemical techniques*

The most common nonelectrochemical methods used for Mg corrosion research are weight loss measurements, hydrogen collection, and pH measurements.

The electrochemical techniques for exploring the corrosion behavior of coated Mg alloys can be used independently or simultaneously with the nonelectrochemical techniques. The principles of each technique and an overview of the main advantages and limitations of different techniques were provided by Durán et al. [5]. A recent review by Kirkland et al. [23] considers the methodologies used to study the corrosion of biodegradable Mg implant materials.
