**1. Introduction**

Since the discovery of copper in prehistoric times, an extensive diversity of metallic materials has emerged, as pure metals or metal alloys. Brass, bronze, steel, titanium, and aluminum alloys are currently the most applied metallic materials, notwithstanding the natural tendency to suffer corrosion under aggressive conditions and thus return to their original ore. To overcome issues related with economical losses and lack of safety, occasioned by metal corrosion, several protection methods have been developed, including the use of:

**new alloys with higher corrosion resistance**, but in addition to the high cost associated with their development, the use of new alloys requires the replacement of the metallic components;

**corrosion inhibitors**, substances which reduce or even eliminate corrosion, when present in suitable concentrations in the corrosive medium. Inhibition is accomplished by one or a combination of several mechanisms, such as adsorption, forming a ultrathin film with a thickness of only few molecular layers; in form of visible bulky precipitates, which coat the metal surface; or other common methods consisting of the combination of adsorption, conversion, and oxidation processes to form a passive layer. Some examples of most applied inhibitors are phosphates, chromates, silicates, hydroxides, carbonates, sulfates, aldehydes, amines, nitrogen heterocyclic compounds, urea, among others [1];

**cathodic protection** that uses a sacrificial metal to protect the metallic structure of interest. It is commonly used to prevent corrosion in large port structures, offshore platforms, and pipelines to transport water, oil, and gas;

**conversion layer** produced by converting the metal surface into a corrosion-resistant form. The main processes include anodizing, phosphatizing, and chromating, and they are frequently used as pretreatment for subsequent overcoats [1]. Anodizing is based on the formation of a protective surface layer, formed by oxides and hydroxides, by application of an external current. In an electrochemical cell, the surface of the metal anode is transformed into an oxide layer of defined thickness, which improves significantly the corrosion resistance and the adhesion of subsequent paints. This method is frequently applied to protect aluminum alloys but can be used also for titanium, zinc, magnesium, and other metal substrates. Besides the presence of microscopic fissures in the anodizing coating that can lead to corrosion, another drawback is the susceptibility of the oxide layer to chemical dissolution in the presence of high- and low-pH environments. Phosphatizing is mostly applied on steel substrates to produce an insoluble and porous phosphate layer that serves as an excellent base for coatings. For instance, car bodies have been phosphatized prior to the application of coatings for many years [1]. Alternatively, surface passivation using chromate conversion coatings has been used especially to protect aluminum alloys in the aerospace industry, for metal fittings and for packaging steel [2, 3]. Such coatings are formed by the reduction of Cr(VI) species to hydrated Cr2 O3 ; however, the conversion process as well as the final coating retains a small amount of unreacted Cr(VI), a highly toxic species which can be released to the environment. Recently, increasing efforts are focused on the development of innovative eco-friendly alternatives due to increasingly strict legislation regulations which demand a reduction of hexavalent chromates usage [3];

**protective coatings** applied on metal surfaces result in a barrier between the metal and the corrosive medium, thus preventing or minimizing the corrosion process.

**1. Introduction**

20 New Technologies in Protective Coatings

components;

hydrated Cr2

O3

hexavalent chromates usage [3];

Since the discovery of copper in prehistoric times, an extensive diversity of metallic materials has emerged, as pure metals or metal alloys. Brass, bronze, steel, titanium, and aluminum alloys are currently the most applied metallic materials, notwithstanding the natural tendency to suffer corrosion under aggressive conditions and thus return to their original ore. To overcome issues related with economical losses and lack of safety, occasioned by metal

**new alloys with higher corrosion resistance**, but in addition to the high cost associated with their development, the use of new alloys requires the replacement of the metallic

**corrosion inhibitors**, substances which reduce or even eliminate corrosion, when present in suitable concentrations in the corrosive medium. Inhibition is accomplished by one or a combination of several mechanisms, such as adsorption, forming a ultrathin film with a thickness of only few molecular layers; in form of visible bulky precipitates, which coat the metal surface; or other common methods consisting of the combination of adsorption, conversion, and oxidation processes to form a passive layer. Some examples of most applied inhibitors are phosphates, chromates, silicates, hydroxides, carbonates, sulfates, aldehydes, amines, nitro-

**cathodic protection** that uses a sacrificial metal to protect the metallic structure of interest. It is commonly used to prevent corrosion in large port structures, offshore platforms, and pipe-

**conversion layer** produced by converting the metal surface into a corrosion-resistant form. The main processes include anodizing, phosphatizing, and chromating, and they are frequently used as pretreatment for subsequent overcoats [1]. Anodizing is based on the formation of a protective surface layer, formed by oxides and hydroxides, by application of an external current. In an electrochemical cell, the surface of the metal anode is transformed into an oxide layer of defined thickness, which improves significantly the corrosion resistance and the adhesion of subsequent paints. This method is frequently applied to protect aluminum alloys but can be used also for titanium, zinc, magnesium, and other metal substrates. Besides the presence of microscopic fissures in the anodizing coating that can lead to corrosion, another drawback is the susceptibility of the oxide layer to chemical dissolution in the presence of high- and low-pH environments. Phosphatizing is mostly applied on steel substrates to produce an insoluble and porous phosphate layer that serves as an excellent base for coatings. For instance, car bodies have been phosphatized prior to the application of coatings for many years [1]. Alternatively, surface passivation using chromate conversion coatings has been used especially to protect aluminum alloys in the aerospace industry, for metal fittings and for packaging steel [2, 3]. Such coatings are formed by the reduction of Cr(VI) species to

; however, the conversion process as well as the final coating retains a small

amount of unreacted Cr(VI), a highly toxic species which can be released to the environment. Recently, increasing efforts are focused on the development of innovative eco-friendly alternatives due to increasingly strict legislation regulations which demand a reduction of

corrosion, several protection methods have been developed, including the use of:

gen heterocyclic compounds, urea, among others [1];

lines to transport water, oil, and gas;

The use of coatings on metallic surfaces has various advantages, such as relatively low costs, environmental compatibility, and the possibility to apply them on metallic components already in use. Consequently, different kinds of protective coatings have been developed, comprising metallic, inorganic, organic, or organic-inorganic materials. The application of many metal coatings, such as chromium, zinc, nickel, aluminum, and copper, involves usually inherent pollution and toxicity-related problems. The most widely used metallic coating is zinc, commonly deposited on carbon steel by hot-dip on a molten zinc bath, process called galvanization, after which the metal substrate acquires a zinc-rich top layer with a thickness of approximately 10 μm. Inorganic coatings comprise ceramics (silica, titania, zirconia, alumina), glass, carbon, etc [1]. Although the inorganic coatings present higher corrosion resistance compared to bare substrates, they usually exhibit residual porosity and stress-induced cracks, which limit their use as efficient corrosion barrier as they allow the diffusion of corrosive species to the underlying metal [4, 5]. Organic materials such as epoxy, poly(methyl methacrylate) (PMMA), polyurethane (PU), polyesters, fluoropolymers, and related paints, combined with anticorrosive primer containing various types of pigments, are widely applied as protective coatings. This is justified by the simplicity of deposition, their dense and homogeneous structure, and consequently high corrosion resistance in aggressive environments. However, their lack of thermal stability, mechanical resistance and adhesion to metallic surfaces can result in serious restriction of their long-term stability.

Organic-inorganic hybrids stand for a class of materials formed by the combination of a polymeric and a ceramic phase, resulting in a nanocomposite material with unique properties. New functionalities result from the synergy of both components, achieved by a careful adjustment of the nature, proportion, and the type of interaction at the interface of both phases. One of the most used methodologies to produce organic-inorganic hybrid materials is the sol-gel process, which allows due to its versatility to control the structure and the functional properties. Through hydrolysis and condensations reactions, the sol-gel route allows the obtain high purity, homogeneous, and structurally tuneable materials, which have a wide range of applications such as catalysts, drug release systems, photochromic devices, biosensors, transparent insulating films, and anticorrosive coatings with excellent barrier properties [6]. The latter characteristic is related to the possibility to prepare a dense organic-inorganic network structure by linking both phases covalently on the molecular scale, and furthermore, the ability to covalently bond the inorganic phase with metallic substrates, leading to highly adherent coatings. Consequently, intense research efforts are presently focused on the development of organic-inorganic hybrid coatings in form of passive barrier layers with low permeability for corrosive species such as chloride ions, water, and oxygen.

There are various methodologies to investigate the corrosion protection efficiency of coated metals; however, the most applied electrochemical techniques are electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and chronopotentiometry. Among them, EIS allows for a deeper analysis of the electrolyte/coating/substrate systems, due to the possibility to fit the data using equivalent electrical circuits, which permit to extract important electrochemical parameters such as coating capacitance, pore resistance, double layer capacitance, charge transfer resistance, water uptake, diffusivity, among others. Additional methods like salt spray test and immersion techniques are used according to different norms for the qualitative and quantitative evaluation of corrosion zones, pitting, and for the determination of corrosion rates. To evaluate the electrochemical performance of the protective system for a given corrosive environment and coating thickness, the most important criteria are (i) the magnitude of the initial impedance modulus obtained by EIS at low frequency, defined as corrosion resistance; (ii) the values of the open circuit potential, obtained by chronopotentiometry; and (iii) the time evolution of both parameters, to evaluate the long-term stability of the coatings. For industrial application, another important aspects have to be considered such as the simplicity of the synthesis process, low costs of reagents, and their environmental compatibility.

One example of an efficient corrosion protection of mild steel was recently reported for a hybrid system combining an epoxy-siloxane topcoat with an epoxy primer containing micaceous iron oxide and zinc phosphate pigments [7]. The electrochemical measurements showed a high-impedance modulus of up to 100 GΩ cm<sup>2</sup> , remaining stable for more than 1 year in contact with 3% NaCl solution. The authors attribute the excellent protection to the high resistance of the coating against water uptake provided by suitable epoxy/primer combination and the relatively high thickness (~140 μm) of this coating system. In another recent study, Ammar et al. [8] report on high-performance hybrid coatings based on acrylic-silica polymeric matrix reinforced by SiO2 nanoparticles, applied to mild steel with a thickness of 75 μm by brush coating. EIS measurements confirmed the high-corrosion protection efficiency with an impedance modulus of more than 10 GΩ cm<sup>2</sup> , decreasing one decade after 90 days of immersion in 3.5% NaCl solution. Visuet et al. [9] obtained similar results for polyurethane/polysiloxane hybrid coatings containing TiO2 as pigment. The EIS analysis showed that coatings loaded with 10-wt% TiO2 (75 μm thick) were able to withstand 263 days, in 3.5% NaCl solution, with almost unaltered corrosion resistance of about 100 GΩ cm<sup>2</sup> . Their model proposes that the TiO2 pigment works as a charge (ionic) storage surfaces, thus enhancing the barrier property of the coating against electrolyte uptake.

The above results demonstrate that elevated anticorrosive performance is usually achieved for sophisticated barrier coatings with an average thickness in the order of dozens to hundreds micrometers. For the market, however, which aims on economic and efficient solutions, elevated thickness, and complexity of the coating system, implies elevated material costs and weight increase, issues that are hardly to be accepted, especially by the aerospace industry. In this regard, dos Santos and coauthors [10] have successfully prepared highly efficient PMMAsilica coatings having a thickness of only ~2 μm, which were able to withstand aggressive saline/acid (0.05 mol L−1 NaCl + 0.05 mol L−1 H2 SO4 ) and 3.5% NaCl environments for up to 105 and 196 days, respectively, maintaining the corrosion resistance in the GΩ cm<sup>2</sup> range. The excellent performance of the primer free coating was explained by the high connectivity of reticulated sub-nanometric silica domains densely interconnected by short PMMA chain segments. Another results that confirmed the viability of thin hybrid films as efficient corrosion barrier have been reported in the study of Harb et al. [11]. The authors showed that the addition of cerium (IV) salt into PMMA-silica system results in a further improvement of the corrosion resistance and durability of the coatings applied to polished carbon steel by dip-coating. The electrochemical behavior of ~1.5 μm thick films reached for a Ce/Si molar ratio of 0.7% an impedance modulus of about 10 GΩ cm<sup>2</sup> (NaCl 3.5% solution) and remained stable within one order of magnitude for 304 days, a performance typically observed for high performance paint systems. The remarkable anticorrosive protection has been associated with the role of Ce(IV) as oxidation agent leading to an enhancement of the overall connectivity of the hybrid network, induced by the enhanced polymerization of organic and inorganic moieties.

In contrast to coating system designed as passive barrier, recent trends aim on the development of active multifunctional anticorrosive coatings with self-healing ability, high-thermal stability, and mechanical resistance, among other functionalities. Inspired by biological systems, the self-healing ability involves the complete recovery of the original properties of the material after suffering macroscopic lesions, induced by mechanical or chemical processes. Various strategies have been used to prepare self-healing coatings, usually containing an active compound, whether stored in microcapsules or incorporated into the coating. They can be activated by temperature increase, UV, pH gradient, breaking of capsules, or changes in the chemical environment [12, 13]. A number of studies report on the use of cerium salts (chloride and nitrates) and ceria nanoparticles as inhibitors, preventing corrosion by the selfhealing ability in affected areas of inorganic, organic, and hybrid coatings. The resulting substantial lifetime increase is attributed to the formation of insoluble oxides and hydroxides in the corroded zones [3, 11, 14–17]. On the other hand, significant improvements of thermal and mechanical properties have been achieved by incorporation of clays, lignin, carbon nanotubes, graphene oxide, and graphene into polymeric or organic-inorganic matrices [18–20].

This chapter reports on recent results obtained for high-performance PMMA-silica and epoxy-silica hybrids coatings, correlating their structural properties with the corrosion protection efficiency, accessed by potentiodynamic polarization and electrochemical impedance spectroscopy. Moreover, several interesting finding are presented regarding PMMA-silica hybrids reinforced with lignin, carbon nanotubes, and graphene oxide to improve their thermal and mechanical properties, as well as some recent results on active corrosion inhibition by the self-healing ability of Ce(IV) containing PMMA-silica coatings.
