**5. Corrosion and organic coatings**

reported so far are intercalated, exfoliated structures are more desirable in property improvement of the polymeric materials. In this context, Chen and co-workers described the processing of epoxy-layered silicate nanocomposites with different properties as corrosion resistant coatings on aluminum surfaces [35]. Based on their studies, they concluded that there is a slight enhancement in anticorrosion properties for the exfoliated nanocomposites coatings and no enhancement for the intercalated nanocomposites. Finding shows that these criteria are related to the better dispersion of silicate nanosheet in some epoxy matrix than the other grades of resin matrix. However, these findings were not very conclusive, and it appears that an epoxy with a lower barrier resistance or higher permeability will help in discriminating the corrosion behavior through the introduction of clay into the matrix. Similarly, Sakai and co-workers found that the exfoliated clay structures were more fixed in polymeric matrix compared to intercalated and conventional clay structures [36]. Additionally, exfoliated clay structures exhibited better corrosion performance compared to intercalated coatings nanocomposites. This is mainly attributed to the larger clay interlayer distance, smaller clay aggregate, and uniform homogeneity of exfoliated clay structures inside the epoxy matrix. In some case, a mixture of intercalated and exfoliated nanocomposites was obtained, and it was difficult to evaluate the effect of nanocomposite structures on the corrosion performance of the prepared nanocomposites [37].

Several methods were reported to prepare clay-based polymer nanocomposites. These include in-situ polymerization, melt intercalation, and solution casting, according to the start-

**1. Intercalation of polymer from solution.** This is based on a solvent system, in which the polymer is soluble and the silicate layers are swellable. The layered silicate is first swollen in a solvent, such as water, chloroform, or toluene. When the polymer and layered silicate solutions are mixed, the polymer chains intercalate and displace the solvent within the interlayer of the silicate. Upon solvent removal, the intercalated structure remains, resulting

**2. In-situ intercalative polymerization.** In this method, the layered silicate is swollen within the liquid monomers or a monomer solution so that the polymer formation can occur between the intercalated sheets. Polymerization can be initiated either by heat or radiation [40, 41]. This method technique was used to prepare nanocomposites based on polyamide, poly(e-caprolactone), polystyrene, polyolefien, and polyethylene terephthalate [41].

**3. Melt intercalation.** The melt intercalation involves annealing, statically or under shear, a mixture of the polymer and modified clays. This method has great advantages over either in-situ intercalative polymerization or polymer solution intercalation. Firstly, this method is environmentally benign due to the absence of organic solvents. Secondly, it is compatible with current industrial process [42, 43]. This process is widely used to design nanocomposites based on polyamide, such as polyethylene terephthalate and nylon 6 [12]. **Table 2** shows the advantages and the limitations of different methods used to prepare the polymer-clay nanocomposites.

**4. Methods used for the synthesis of nanocomposites**

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

ing materials and processing techniques:

in nanocomposite [38, 39].

Corrosion is defined as the gradual oxidation of metallic materials by converting them to their original state of oxide, sulfide, carbonate, or other stable salts in the ambient environment. Corrosion results from a combination of reactions occurring at the metal-solution interface by involving electrons and the chemical species. More specifically, the oxidation of the metallic material naturally occurs at the metal-environment interface and constitutes the anodic reaction, whereas the reduction of oxygen typically takes place in solution and often constitutes the cathodic reaction [44]. The cathodic and anodic reactions form an electrical circuit, which is completed by conduction of electrons in the metal and by ionic conduction through the electrolyte. There are various forms of corrosion that can take place depending on the surrounding environment and the design of the equipment that suffers corrosion. The galvanic, pitting, intergranular, crevice, and uniform corrosions are the most investigated corrosion in the literature [45, 46]. In general, the corrosion protection methods commonly used are mainly organic and metallic coatings, inhibitors, cathodic, and anodic protection. This latter is relatively new, and it was first demonstrated and tested by Edeleanu 1954 [47].

The use of organic coatings on metal is usually an effective way to protect metal surfaces from corrosion while still preserving the desirable physical and mechanical properties of the metal [47, 48]. Corrosion on a bare metal surface is very complex process in itself, as the morphology of the corrosion layers formed on the surface and corrosion rate depends on several factors [49]. To evaluate the performance of any organic coatings, several parameters must be taken into consideration such as the permeability to water and oxygen, adhesion performance to the metal, coating thickness, ionic conductivity, as well as pore size distribution [50]. Commonly, the corrosion mechanism of an organic coating to protect the metal against corrosion can be divided into three groups: the electrochemical effect, the physicochemical effect, and the adhesion to the substrate [8]. The organic coating is a complex formulation of variety of materials each having a specific function. Examples of those materials include, but not limited to the following: polymeric materials, solvents, pigments, and various additives. Organic coating simply acts as a barrier between the metal surface and the surrounding environment. The barrier ability of such coating might be attributed to its structure or due to some additives or pigments implemented inside the coating. For this reason, understanding the nature and the constituents of the coating to be applied is essential to predict the performance of this coating as a corrosion protective technique. There are several reasons that can lead to failures in the coating applied, and the most common failures and the reasons leading to such failures are the permeability of the coating, adhesion, blistering, and cathodic delamination. Importantly, it was reported that a poor coating applied to a well-prepared surface is better than a good coating applied to a poorly prepared surface [51]. Contaminations on the surface of the metal can cause a direct failure to the applied coating. These contaminations can be the reason for a poor coating adhesion, which is one of the most critical factors controlling the quality of the coating. It is also well known and documented that no matter how good the coating is, the corrosion still can take place under the coating if the surface is contaminated.

Different kinds of polymers were used to prepare nanocomposites coatings such as conjugated polymers and thermoplastic polymers [53]. Using the in-situ thermal polymerization, Yeh and colleagues prepared a series of polymer-clay nanocomposite by dispersing layered montmorillonite (MMT) clay into an organic poly(methyl methacrylate) matrix [54]. Firstly, methyl methacrylate monomers were intercalated into the montmorillonite that was exfoliated by cation-exchange reaction with quaternary alkylammonium cations or alkylphosphonium cations followed by a typical free-radical polymerization. The as-synthesized polymer-clay nanocomposites exhibited enhanced Tg compared to pure polymer. TEM analysis revealed that after the dispersion of the clay, the prepared nanocomposite displayed a mixed nanomorphology with well-exfoliated silicate layers into the polymer matrix. The electrochemical measurements using polarization resistance, corrosion current, and impedance spectroscopy revealed that nanocomposites coatings with the low clay loadings (e.g., 1 wt%) exhibited better anticorrosion protection for steel compared to the pure poly(methyl methacrylate). In an independent study, the same group has also designed several nanocomposite materials containing polyaniline (PANI) and layered montmorillonite clay and investigated their corrosion performance for cold-rolled steel [55]. Firstly, the organophilic montmorillonite was prepared via cation-exchange reaction with cocamide-propylhydroxysultaine before being mixed with aniline monomers in diluted hydrochloric acid followed by one-step oxidative polymerization. TEM analysis of as-synthesized nanocomposite revealed that the prepared materials possessed mixed nanomorphology, and the silicate layers were found to be well dispersed in the polyaniline matrix. The electrochemical measurements of potentiodynamic (e.g., Tafel plots) of a series of polyaniline nanocomposites with varying clay loadings at room temperature are illustrated in **Table 3**. Electrochemical corrosion current values of polyaniline nanocomposites were found to be decreasing progressively with further increment in clay loading. Importantly, visual inspection of the corrosion products revealed the presence of grayish oxide layer formed over polyaniline nanocomposites, showing better corrosion performance. It is very important to stress that the incorporation of the montmorillonite in polyaniline matrix resulted in a decrease in mechanical strength and in thermal decomposition temperature. This could be attributed to the significantly decreased molecular weight of polyanilines formed in the montmorillonite. Same research group evaluated the effect of adding organo-modified clay on the corrosion protection performance of conducting polymer/layered silicate, such as poly(o-methoxyaniline) and poly(3-hexylthiophene) [56]. The experimental findings revealed that the conducting polymer/layered silicate nanocomposites with low clay loading (3 wt.%) were found exhibiting

Polymer-Clay Nanocomposites for Corrosion Protection http://dx.doi.org/10.5772/intechopen.74154 69

**Figure 2.** The difference between conventional composite and nanocomposites.

better anticorrosion properties compared to the pure conducting polymer.
