2.1.1 Driving Force
This driving force for the corrosion of metals through electromechanical reactions is the free energy of the metal atoms in their metallic form. All chemical systems tend to change so that the free energy present is at a minimum. This is analogous to the flow of water downhill to minimize the free energy due to gravity. Most engineering metals are found in nature in a form with low energy. These metal ores are chemical compounds consisting of the metal atoms combined with other atoms such as oxygen or sulfur. The process of breaking up these ores into their metallic and non-metallic atoms involves an addition of energy in order to free the metal atoms from the natural, low energy content chemical compounds. The corrosion process is driven by the tendency of these metals atoms to revert to their natural state. If corrosion products are analyzed, their chemical composition is usually identical to the ore from which the metals was originally obtained.
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This manual is intended for use in the design and construction of cathodic protection systems for mitigation of corrosion of buried or submerged metallic structures. Design of cathodic protection systems is somewhat different then design of other electrical or mechanical systems because it must be based upon local environmental conditions such as soil resistivity. This manual presents criteria for cathodic protection, methodologies for the determination of required environmental conditions, methodologies for design of cathodic protection systems, examples of typical systems and design calculations, installation and construction practices, recommended initial system checkout procedures, and system maintenance requirements.