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The use of an external current applied cathodically for protecting pipe lines against corrosion was first undertaken about I9IO or earlier. Since that time this system of protection has been widely extended, especially within the last Io years. According to a recent estimate, 75o cathodic protection units, protecting 3ooo miles of line, are in operation. Because of the practical importance of cathodic protection, K. H. Logan of the Bureau's Underground Corrosion Section held a number of conferences on this subject with pipe-line operators and corrosion engineers during the summer of I94 I. These al! indicated that cathodic protection is both practical and economical for the protection of transmission pipe lines. It has even proved successful in protecting distribution systems in the few cities in which the method has been tried. Because of interlacing networks, the difficulties in protecting distribution systems are, of course, much greater.
Such failures in the operation of cathodic protection as have been reported are to be attributed to poor design and poor maintenance, and since similar failures are to be expected in any pioneering undertaking, they are not to be taken too seriously. One of the major unsolved problems in cathodic protection is the measurement of the minimum current density required for protection. Otherwise, it is difficult to determine, except by experience, whether a pipe line is over or under protected.
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