Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 108.—Cavitation damage to the cast-iron lining of a conduitimmediately downstream of a slide gate, caused by high velocity flow.8.1.4 Corrosion of metalsCorrosion of metals is a complex phenomenon involving many inherent structuraland environmental factors. Corrosion is commonly a result of contact betweendissimilar metals, or when metals are in contact with water, moist earthfill, or theatmosphere. Corrosion affects all types of metal and alloy pipe and reinforcing barsin concrete. Corrosion is the destructive attack on conduit materials byelectrochemical reaction to the environment. Corrosion can also be described as theprocess whereby metals return to their natural state. Certain metals, such asplatinum, gold, silver, and copper exist in nature in a stable metallic state. However,other metals require refinement by heating. Unless these refined metals areprotected from the environment, they will eventually revert from their temporaryrefined metallic state back to a more natural state. The soil and water surroundingthe conduit, and water flowing through the conduit can affect the rate of corrosion.The soil and water can contain different types of acids, alkalis, dissolved salts,organics, industrial wastes, mine drainage, etc. The rate of corrosion will vary,depending on chemical and physical properties and exposure to the environment.Factors that influence corrosion include (American Iron and Steel Institute, 1994):• Soil resistivity.—Corrosion involves the flow of current from one location toanother. The ability of soils surrounding conduits to conduct electrical particlescan affect their tendency to corrode a conduit. Resistivity is a measure of theresistance to current flow of a material, usually expressed in units of ohm-cm.Conduits surrounded by clay soils with typical resistivity values of 750 to182