Design of Anode Systems for Cathodic Protection Of Underground and Water Submerged Metallic Structures^★

Studies on ground beds and anode configurations were undertaken by the authors as members of the Scale and Corrosion Committee of the Chief of Engineers Office. Acute soil corrosion problems at some Army installations in Southwestern and Western areas led the Committee to the consideration of cathodic protection. Previous design criteria showed that the voltage required, and therefore the power consumption, is determined almost entirely by the resistance of the anode because of its limited area. Study was made of a number of different anodes of various dimensions and geometric shapes installed both vertically and horizontally. A brief analysis of the potential gradient in the medium between the anode and the cathode is presented. Equations are developed for the resistance of a hemisphere to remote ground and for the resistance of a vertical rod to ground. Calculated resistances obtained by using the equation developed for a vertical rod are compared in tabular form with those obtained by using the equation developed for a vertical rod are compared in tabular form with those obtained by using Dwight's and Peters' equations as well as measured values using an AC Bridge. Studies were also made on the resistance to remote ground of a limited length horizontal pipe which may be used as an anode. The effect of the ground surface on the resistance of horizontal conductors were studied in experiments carried out in a slate tank having the two rear sides of the tank lined with thin sheet iron. A 1/4" steel rod which was exactly the length of the tank was supported horizontally midway between the side walls. Resistance measurements were made between the electrodes and the walls of the tank by the means of a Wheatstone Bridge and an AC galvanometer. The McCollum Earth Current Meter, developed by the National Bureau of Standards in 1927 was used for studying stray current electrolysis on an underground piping system. Experiments indicate that the measurement of soil resistivity may best be determined by using the four electrode method. An equation is given for the determination of a specific resistivity in homogeneous soil from the current and voltage readings obtained from this test. A typical design problem for the cathodic protection of pipe lines using rectifiers and graphite anodes is presented. Factors affecting required current density in the design of cathodic protection installations in large water tanks are discussed.