**2. Structure of layered silicates**

oxygen reduction reaction (cathodic reaction) that initiates the corrosion process. In 1952, Mayne found that the diffusion rate of water and oxygen in an unpigmented coating is too high. Consequently, he suggested that the amount of water and oxygen arriving at the metal/ coating interface is greater than the one required for corrosion to precede [2–4]. His hypothesis was based on the fact that the barrier action could not explain the effectiveness of the coating, while the conductivity of the coating is the main factor controlling the corrosion protection degree offered by the coating [2, 5, 6]. Mayne also reported that when the coating is immersed in a solution, it will gain a certain charge, negative or positive, depending on the nature of the coating. In this case, the coating will allow the opposite charge species to pass through. He believed that the permeation of these species will be through the bulk of the matrix. Corti attributed that the permeation of such species is affected by the presence of pores or imperfections in the coating. Based on that, he concluded the permeation to be through the pores and voids in the coating [2, 3]. Funke has a different understanding to the way the coating works to protect the steel. He believed that the protection degree that the coating offers depends on the degree of adhesion. The permeation of water through the coating will result in a water accumulation between the substrate and the coating, leading to coating blistering or coating delamination as a result of generating hydroxyl ions as products of the corrosion process. Such ions are believed to break the bond between the coating and the metal surface, and consequently, the coating adhesion will be significantly degraded [2, 7, 8]. Considering all the above approaches, loss of adhesion, the diffusion of water and oxygen, and the transport of the charged species can potentially result in poor corrosion protection performance. In general, mass transfer (diffusion) of a material in a specific environment is a result of a natural process tending to reach equality in concentration between two points. In the case of gases, the diffusion process will be mainly based on the difference in partial pres-

62 Current Topics in the Utilization of Clay in Industrial and Medical Applications

sure, which is the driving force pushing the gasses from one side to another.

Permeability = Solubility × Diffusion (Henry'

can reduce the adhesion strength of the coating [3].

In case of polymeric coatings, the diffusion process will mainly depend on three factors: the nature of the polymeric coating, the magnitude of the driving force, and the nature of the diffusing species. The general mechanism of the permeability through the coating will follow three main steps: solution of small molecules in polymer, migrating through the polymer, and emergence of that molecule at the outer surface. Accordingly, the permeability can be presented mathematically as the product of solubility and diffusion as shown in Eq. (1) [9, 10]:

When a coating is exposed to an electrolyte, the diffusion of the electrolyte follows two processes, which can be classified as fast and slow diffusion [11]. The fast process can occur within a few minutes of exposure, while the slow step can take weeks or months. According to Scantlebury, during the fast process, the electrolyte can reach the surface of the metal, but its electrical properties do not support the corrosion process, while the amount of electrolyte

There are many factors affecting the diffusion of the electrolyte and the corrosive species, namely water and oxygen. Considering the fact that the permeability of a coating depends on the amount of defect present, it can be concluded that the diffusion will be through areas of imperfection. These areas can be considered as free volume including pores and area of

s Law) (1)

The most commonly used layered silicates to produce the polymer-clay nanocomposites belong to a group of clays classified as 2:1 layered or phyllosilicates [18]. This structural family includes natural clays, such as montmorillonite, hectorite, and saponite. Their crystal structure consists of two-dimensional layers made up of two tetrahedral silicon sheets fused to octahedral sheet of alumina or magnesia by the tip so that the oxygen ions of the octahedral sheet also belong to the tetrahedral sheets. The layer thickness of a sheet is of the order of 1 nm, and the lateral dimensions vary from 300 Å to several microns depending on the specificity of the silicate. The relatively weak Van der Waals force ensures the stacking of the sheets between them and each layer called interlayer [19–21]. Stacking of the layers leads to a regular gap between each adjacent silicate layers called the interlayer or gallery. A negative surface charge is present on the layers due to the isomorphic substitution of tetrahedral silicon or aluminum and octahedral magnesium. The charge deficit is counterbalanced by alkali and alkaline earth cations situated inside the galleries [22].

the grafting of organic substances, such as organosilanes. In the clay, the hydroxyl functional groups are located on the surface of the layers and their edges, and so the silane agents pos-

Polymer-Clay Nanocomposites for Corrosion Protection http://dx.doi.org/10.5772/intechopen.74154 65

Variations in the strength of interfacial interactions between the polymer matrix and layered silicates result in the formation of three main kinds of nanocomposites as described below [34].

• **Intercalated nanocomposites**: When the polymer is not able to intercalate between the silicate layers, a biphasic composite is obtained, whose properties remain in the same range as the conventional microcomposites structure, in which the clay sheets are not swollen by the

• **Flocculated nanocomposites**: Conceptually, this is similar as intercalated nanocomposites. More polymer molecules are getting inside the layers structure to give more spacing be-

• **Exfoliated nanocomposites**: In this type of nanocomposite, the individual clay layers are separated in a continuous polymer matrix. Usually, the clay content of an exfoliated nanocomposite is much lower than that of an intercalated nanocomposite. The **Figure 1** shows

Despite the impressive progress that has been reported during the last few years regarding the use of polymer-clay nanocomposites for corrosion protection, its effect on the corrosion performance has rarely been investigated. Although most organo-modified clay nanocomposites

sess the ability to form strong covalently bonds with inorganic clays.

polymer and are in the form of clumps or agglomerates in the matrix.

illustrations of the three types of polymer-clay nanocomposites.

**Figure 1.** The main types of nanocomposite. (a) Intercalated, (b) flocculated, and (c) exfoliated.

**3. Nanocomposite structures**

tween the silicate layers.

The most used layered silicates are montmorillonite, hectorite, and saponite [23]. Their chemical formulae are illustrated in the **Table 1** [24]. All these layered silicates are characterized by a high surface area (700–800 m2 /g in the case of montmorillonite) and a moderate cation-exchange capacity (CEC) generally expressed in milliequivalents per 100 g (mequiv. per 100 g). The charge indicated by the chemical formula is an average over the whole crystal because the charge varies from layer to layer. Only a small proportion of the charges are located at the surface of the crystal, the majority being mainly in the interlayer spaces [22]. These cations are then exchangeable in solution.

To produce the nanocomposite with the required properties, the silicate layers should be exfoliated before being dispersed inside the polymer. Indeed, the hydrophilic characteristic of the clay does not allow it to be dispersed in the polymer matrix, which is generally organophilic. To render the mixture compatible, it is necessary to modify the clay before it is dispersed. For this reason, a pretreatment process should take place to weaken the forces holding the structure [24–27]. The main goal of this process is to render the surface of the layered silicate more organophilic. This improves the dispersion process of the layers inside the polymer under well-defined experimental conditions [28]. Given the structure and properties of clay, there are several modification techniques. However, the most commonly used technique for organophilic modification of clay is the ion-exchange reactions with cationic surfactants, including primary, secondary, tertiary, and quaternary alkylammonium cations to replace the hydrated cations with the protonated amine with long-chain alkylammonium cations. Alkylammonium cations in the treated clay decrease the surface energy of the inorganic host and increase interlayer spacing, rendering the modified organoclay is more compatible with organic polymer [29–33]. The above treatment process is basically an intercalating process to increase the spacing and decrease the adhesion forces between the silicate layers. The amine has the ability to achieve such target by acting as an intercalating agent. The efficiency of the intercalating agent is mainly influenced by the agent properties such as its hydrocarbon chain length [1]. It is very important to stress that there are also other original and interesting methods for modifying 2:1 phyllosilicates, such as the use of ionomers or block copolymers, and


**Table 1.** General formula and characteristic parameters of phyllosilicates 2:1.

the grafting of organic substances, such as organosilanes. In the clay, the hydroxyl functional groups are located on the surface of the layers and their edges, and so the silane agents possess the ability to form strong covalently bonds with inorganic clays.
