**1. Introduction**

Organic coatings are widely used to prevent corrosion of metallic structures. However, these polymeric coatings are usually permeable to small gaseous molecules such as water vapor and oxygen, which can result in gradual corrosion of the surface. It is generally accepted that the coating efficiency is dependent on the barrier and mechanical (resistance to cracking) properties of the organic film, on the adherence of the polymeric coating to the metal substrate, and on the degree of environmental aggressiveness. Among various protective coatings, epoxy resins are commonly used as organic coatings, due to their strong adhesion capability to metallic substrates, their excellent resistance to chemicals, and their relatively high mechanical and impact strength. However, the pristine epoxy resins exhibit measureable adsorption and permeability of water vapor, which diffuses to the epoxy/steel interface and initiates corrosion of the metal substrate particularly in intensely wet conditions. Therefore, effort has been devoted in recent years to develop epoxy-based protective coatings with good barrier properties, at least with regard to water vapor and oxygen [1–3].

One way to improve the properties of a polymeric protective coating is the addition of inorganic nano-fillers, leading to the formation of nanocomposite materials. Nanoparticles with sizes in the range of ca. 1–50 nm can enhance the effectiveness of a coating by filling the micro voids and crevices in the pristine polymer coating. When the nanoparticles are in the form of 2D-nanolayers with a high aspect ratio (ca. 100–2000), they can act as barriers to the diffusion of small molecules by increasing the length of the diffusion paths (tortuous paths) of the corroding agent through the polymeric nanocomposite coating, thus inhibiting the corrosion process. The use of clays as fillers of polymeric coatings has great potential to improve the barrier properties of the coatings, provided that the nano-sized clay tactoids or the individual clay nanolayers can be dispersed effectively within the polymer matrix. However, substantial improvements of barrier properties can be achieved when the nanolayers are oriented parallel to the substrate surface [4].

During the last 20 years, considerable attention has been paid to the development of polymer-based nanocomposites, with clays being the first and most studied inorganic nanofillers used [5,6]. It has been reported that the incorporation of a small amount (1–5 wt %) of clay in a polymer matrix can lead to significant improvements in the mechanical performance, thermal stability, and barrier properties of the pristine polymer. These improvements are related to the morphology of clay micro-sized particles, which consists of tactoids with highly oriented nano-layers, as explained above. An ideal exfoliated structure of polymer-clay nanocomposites is obtained when a complete separation and dispersion of the individual clay nano-layers occurs within the polymer matrix. In this case, there is no longer any interaction between the layers, which are completely dissociated and separated by a large volume of the polymer. An intermediate case is the intercalated structure where a finite number of polymer chains penetrate the interlayer space, thus significantly increasing the spacing between the layers (ca. up to 100 Å) without destroying the ordered parallel structure of the nanolayers, at least at the level of individual tactoids.

The most widely used layered silicate is montmorillonite (MMT), which has attracted intense research interest for the preparation of polymer clay nanocomposites. The MMT-based nanocomposites exhibit enhanced physical properties compared to the pristine polymer, such as improved thermal properties (e.g., thermal stability, flame retardant, thermal conductivity), mechanical properties (e.g., mechanical strength, hardness, abrasion resistance), permeability properties (e.g., gas barrier, pervaporation), and corrosion protection properties [1,2,7–11]. The chemical structure of MMT consists of two tetrahedral silica sheets fused to a central edge-shared octahedral-based sheet of either magnesium or aluminum hydroxide [12]. In general, the surface of the clay needs to be organo-modified in order to become more compatible with the polymer matrix and to improve its dispersibility in the polymeric network. The organic modification of layered silicates can be realized through the replacement of the Na<sup>+</sup> and/or Ca2+ cations in the intragallery space, as well as on the external surfaces of the clay particles, by organic cations through a cation exchange reaction [13,14]. The improvement in the corrosion resistance of aluminium alloys and of cold rolled steel with polymeric films reinforced with organically modified clay has been clearly demonstrated. Corrosion protection was essentially related to the enhancement of the barrier properties of the coating. The formation of an organophilic environment between the clay layers is critical for the insertion of polymer chains amongst them and the formation of an intercalated structure, which seems to present the greater improvement of barrier properties. Amongst the various polymers for coating applications, epoxy resins lately have evoked intensive studies in the preparation of nanocomposite materials, due to their high tensile strength and modulus, good adhesive properties, good chemical and corrosion resistance, low shrinkage in cure, and excellent dimensional stability [1,8,15–19].

In the present study, the protection capabilities of epoxy-clay nanocomposite coatings were examined. The montmorillonite clay used has been modified with quaternary or primary octadecylammonium ions. Both the pristine glassy epoxy polymer and the epoxy-clay nanocomposites were characterized for their mechanical and thermomechanical properties, thermal stability, and barrier properties. The degree of intercalation/exfoliation of clay nanoplatelets within the epoxy polymer was determined. The corrosion behavior investigation was carried out using salt spray tests, optical and scanning electron microscopy examination, open circuit potential, and electrochemical impedance measurements.
