As mentioned above, organic coatings can retard corrosion through a barrier or inhibition mechanism. The barrier mechanism is achieved via a coating that effectively isolates the substrate from corrosive elements such as moisture, oxygen and ionic species that can react with the substrate.
Inhibition refers to a modifi cation of the organic coating that allows transport of the inhibiting species through the aqueous environment onto the metal substrate to retard corrosion via barrier, passivation or sacrifi cial mechanisms.
Organic coatings can incorporate metallic, nonmetallic (inorganic inhibitors) and organic components (inhibitors) in single or multiple layers to promote, enhance or maintain a protective or passive fi lm on the metal substrate. Multiple layer coatings are often used which consist of a metal pretreatment layer such as phosphates or chromates, primer with inhibitors
and topcoat (Fig. 1.5 ). A coating system should provide good adhesion, low moisture permeability, chemical resistance, fl exibility, impact resistance, easy application, durability and cost effectiveness.
The most common method to prevent corrosion on metallic/nonmetallic surfaces is via paints. Paints are organic/polymeric coatings that are used in atmospheric or immersion environments that are normally liquid-applied via brush, roller, doctor blade, dipping or spray [48] . Liquid-applied paints have four basic components: (a) resin or binder, (b) solvent, (c) pigments and (d) miscellaneous components (dryers, fl ow-control, gloss-control and suspension agents). The resin or binder is based on the organic/polymeric structure and can comprise the following types: acrylics [49, 50] , alkyds [51–53] , epoxy (novolacs, polyamides) [54, 55] , phenolics [56, 57] , polyesters [58, 59] , polyurethanes [60, 61] , silicone [62] , vinyl [63] , zinc-rich inorganics/
organics [64] and magnesium (Mg)-rich organics [65] . The solvent, thinner or diluent refers to their ability to dissolve or reduce the viscosity of the binder or resin in the paint. The solvent can be low molecular weight hydrocarbons, oxygenated compounds or water. Hydrocarbons include both aliphatic and aromatic compounds, whereas oxygenated compounds cover ethers, ketones, esters and alcohols [66] .
Careful consideration of solvent system, thinners and/or diluents is required in order to achieve paints that are easily applied, enabling control of the wet paint on the substrate surface and formation of an evenly distributed, smooth paint that dries at a predetermined rate. Several factors affect the choice of solvents such as solvency, viscosity, boiling point, evaporation rate, fl ash point, chemical nature, odor, toxicity and cost [67] . Once a suitable solvent has been determined via use of solvent power (solvency) and solubility parameters, evaporation rates must be considered.
Evaporation takes place in two stages
• initial stage – solvent loss is dependent on the vapor pressure of the solvent and is not affected by the pressence of a dissolved polymer;
and
• latter stage – as the polymer fi lm is formed, solvent is retained and the loss of solvent becomes diffusion limited (a slow process).
1.5 Multiple layer coating representation.
Metal substrate
Pretreatment coating (surface) Primer coating
Topcoat
Once consideration of evaporation rates is determined, the fl ash point, toxicity and environmental pollution must be taken into account. The fl ash point is defi ned as the ‘lowest temperature at which the liquid in contact with air is capable of being ignited by a spark or fl ame under specifi ed conditions’ [67] . Much legislation has been enacted in order to minimize this effect for safe handling, manufacturing, storage, transport and usage.
Finally, toxicity and environmental concerns regarding use of organic solvents has increased over the decades. This environmental concern has promoted government agencies worldwide to regulate such solvents in order to minimize volatile organic compounds (VOCs), hazadous air pollutants (HAPs) and ozone-depleting substances (ODSs). These solvents are responsible for pollution and environmental degradation which have a profound impact on the atmosphere resulting in depletion of the ozone layer and formation of acid rain.
Pigments are added to paints to improve aesthetic effects, fi re retardance, corrosion protection and weatherability [68] . Most pigments are added to provide a visual effect such as color and opacity. The pigment must remain insoluble in the solvent or diluent and not react physically or chemically within the solvating environment. The pigment will alter its appearance by selective absorption and/or scattering of light. As an example, a red pigment appears red because it refl ects the red wavelengths of incident visible white light that fall upon it and absorbs all the other wavelengths. A black pigment appears black because it absorbs all the incident white light and a white pigment appears white because it refl ects all the incident white light that fall on its surface. The performance properties of pigments in paints is determined by several factors including tinctorial strength, particle size, insolubility, opacity or transparency, durability, chemical stability, heat stability, surface area and dispersability. Finally toxicity, environmental concerns and cost all play a role in the selection and incorporation of pigments into a commercial paint formulation.
Corrosion-inhibiting pigments are a class of pigments that provide corrosion protection either singularly or in combinations without signifi cant reaction with the substrates in the operating environment [47] . These inhibitors can be both organic or inorganic depending on operating conditions and corrosion-inhibiting properties. The most effective inorganic inhibitors are chromates, nitrates, silicates, carbonates, phosphates and arsenates. The most effective organic corrosion inhibitors are amines, heterocyclic nitrogen compounds, sulfur compounds (thioethers, thioalco- hols, thioamides, thiourea and hydrazine) and natural compounds such as glue and proteins.
These inhibitors are classifi ed as either anodic, cathodic, ohmic, precipitation or vapor phase and provide corrosion protection either through absorbed fi lms, formation of bulky percipitates and/or passivation
mechanisms. Anodic inhibitors include chromates, nitrites, nitrates, phosphates, tungstates and molybdates which can passivate a metal surface, producing a protective fi lm at the anode thus inhibiting the anodic reaction.
These compounds have been used for iron, steel and aluminum but the nonoxidizing inhibitors (phosphates, tungstates and molybdates) require other oxidizing species such as oxygen to be present in the environment to be effective. The oxidizing inhibitors (chromates, nitrites, nitrates) require a critical concentration in order to be effective.
Cathodic inhibitors are effective in corrosion prevention by blocking cathodic sites via precipitation. This reduces corrosion by slowing down the reduction reaction rate of the corrosion cell. Some examples of cathodic inhibitors are calcium, magnesium and zinc ions, which will precipitate as hydroxide salts onto cathodic sites as the pH increases. Cathodic poisons (e.g. arsenic, bismuth, antimony) refer to reduction of the hydrogen reaction rate which reduces the overall corrosion rate in these processes.
Ohmic inhibitors such as amines or sulfonates are fi lm-forming inhibitors.
They reduce the corrosion rate by decreasing the mobility of corrosive species between the anode and cathode on the metal surface. Precipitation inhibitors such as silicates and phosphonates promote the formation of bulky precipitate fi lms over the entire surface of the metal substrate. Finally, vapor-phase inhibitors are chemical compounds that have a high vapor pressure that can absorb onto a metal substrate. The method of corrosion inhibition is via neutralization of water, passivation and/or formation of fi lms. They are used to inhibit atmospheric corrosion and are effective in closed spaces.
Miscellaneous components or additives to paint/primer formulations are compounds that are added in very small quantities to the formulation, which have a profound effect on their properties [69] . Additives to paint formulations can comprise antifoams, antisettling agents, antiskinning agents, dispersion aids, dehydration, antigassing agents, ultraviolet (UV) absorbers, fl oating, fl ooding additives and in-fi lm or in-can preservatives.
Antifoam agents are used in paint formulations to suppress foaming of paints. This is achieved by adding either surfactants or colloids that act to lower the surface tension in the vicinity of the bubble which causes them to coalesce into larger more unstable bubbles, eventually breaking.
Antisettling agents are additives that prevent the separation or settling of pigments from the paint. They accomplish this task through a process called ‘thixotropy’ [70] . Several antisettling agents include aluminum stearate, stearate-coated calcium carbonate, modifi ed hydrogenated castor oils, modifi ed montmorillonite clay, pyrogenic silicas and other proprietary products. Antiskinning agents are compounds that prevent oxidation, drying or skinning of the paint during storage but dries properly when applied onto a substrate [71] . Dispersion aids are used to optimize the dispersion
of the pigments throughout the paint formulation. Dispersion agents can be used with water-dispersible or solvent-based systems and are generally either anionic, cationic, non-ionic and/or amphoteric. Dehydration agents are added when reaction with moisture during storage for paints is a problem. These issues occur for isocyanate-based paint formulations where adventitious moisture can degrade the properties of the polyurethane paint formulation. Antigassing agents are important for storage purposes when moisture can react with the paint producing, for example, hydrogen gas. This can be a problem for zinc-dust primers and aluminum-based paints.
UV stabilizers are added to stabilize pigments when exposed to incident radiation especially UV [72] . UV radiation is capable of degrading organic/
polymeric paint components via radical formation. This degradation mechanism results in loss of mechanical strength, elasticity, delamination, brittleness and color changes. Typically, a two-stage process is employed to retard UV degradation of paints. This consists of a UV absorber and light- stabilizer. First a UV absorber is added to the paint formulation to convert the undesirable short wavelengths (UV) into heat energy and a light stabilizer captures the resulting free radicals. Typical UV absorbers are either inorganics such as micronized iron oxides, ferrocene derivatives, organic nickel complexes and organic compounds such as oxalanilides, benzotriazoles and triazines. Light stabilizers such as hindered amine light stabilizers (HALS) are used as the radical scavengers. This two-step process insures color stability, gloss retention, crack resistance and mechanical properties over 10-year periods.
Anti-fl oating and fl ooding agents are used in paints to prevent color changes, striations, molted effects or signifi cant color changes in paints.
Flooding is the more severe form of fl oating. The surest method to inhibit both these types of coloration changes in paints is to release the pigments intimately together so that any fl occulation that takes place is co-fl occulation resulting in even dispersion. Finally in-can or in-fi lm preservatives can be added to prevent spoilage by micro-organisms or mold growth on paint surfaces. Biocides are added at very low concentrations to inhibit micro- organism/mold growth and to preseve the paint during storage and surface application/drying on substrates over time.