Cathodic Protection in Water Systems
By William F. Harfst
Corrosion is best defined as a reaction between a metal and its environment. Several forms of corrosion have been identified such as general, pitting, and dissimilar metal. In these cases, the corrosion mechanism is described as an electrochemical reaction, whereby current flows from anodic to cathodic sites on the metal surface. If this process is allowed to continue, costly damage will result on the metal component or structure. Protection of metal structures from electrochemical corrosion by means of cathodic protection is achieved by making the metal object the cathode of an electric circuit. The anode of this circuit may consist of an element, such as zinc, magnesium or aluminum, which is more ﾓactiveﾔ in the electrochemical series than is the structure to be protected. Cathodic protection is only one of a number of corrosion control methods that are in common use and may not be applicable in every situation. It is only effective in controlling corrosion on metallic structures that are in contact with an electrolyte (such as water). If the structure is buried in the soil, the damp earth serves as the electrolyte. Savings in maintenance, repair, and replacement costs resulting from the use of cathodic protection systems are substantial. It is successfully used to protect above ground metal structures, buried pipes and tanks, heat exchangers, and ship hulls, among others.
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