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Many coatings contain as many as 15 to 20 ingredients with their own range of functionality. Some of the main variables used to design corrosion protective coatings are:
Impermeability: The ideal impermeable coating should be completely unaffected by the specific environment it is designed to block, be it most commonly humidity, water or any other corrosive agent such as gases, ions or electrons. The ideal impermeable coating should have a high dielectric constant and also have perfect adhesion to the underlying surface in order to avoid any entrapment of corrosive agents.
Inhibition: In contrast with coatings developed on the basis of impermeability, inhibitive coatings function by reacting with a certain environment to provide a protective film or barrier on the metallic surface. The concept of adding an inhibitor to a primer has been applied to coatings of steel vessels since these vessels have been first constructed. Such coatings were originally oil based and heavily loaded with red lead.
Cathodically protective pigments: As with inhibition, cathodic protection in coatings is mostly provided by additives in the primer. The main function of these additives is to shift the potential of the environment to a less corrosive cathodic potential. Inorganic zinc based primers are good examples of this concept.
