ABSTRACT
Other than metals such as gold and platinum which are discovered already in the metal state in their natural environments, many widely used metals are unstable in the atmosphere. These unstable metals are produced by reducing ores artificially; therefore, they will return to their original ores or to similar metallic compounds when exposed to the atmosphere. For example, metallic iron is oxidized to ferric oxyhydroxide in a thermodynamically stable state; that is, iron in the higher level of free energy is changed to lepidocrocite, γ - FeOOH, in the lower level:
4Fe + 302 + 2H20 4FeOOH (1.1)
AG° = -81 kcal/mole (1.2)
This behavior of a metal in a natural environment is called corrosion. The role of a coating is to prolong the time until rust forms on the substrate, as shown in Fig. 1.1. The service life of a coating de pends on the properties of the coating material and on the degree of aggressiveness of the environment. Therefore, the selection of coating
Fig. 1.1 Role of corrosion-resistant coating.
