Induced Alternating Current Used for Cathodic Protection of a Coated Pipe Line★

After making repairs on a leak on a coated line through a salt marsh and paralleling a high voltage high line, it was learned that although considerable gradients existed in the earth adjacent to the high line there was no correlation in the measured quantities. It had been considered that natural rectification of the induced AC voltage was occurring. After tests with coupons buried near the pipe line showed the pipeline was predominantly positive to earth at the leak location, a test ground bed was constructed to examine the feasibility of rectifying the induced alternating current. Measurement of the rectified voltage between the pipeline and the test ground bed through a 25-ampere, 18-volt selenium rectifier was 5 volts DC available for cathodic protection. A 5-ampere, 15-volt selenium stack, ten 3-inch x 60-inch graphite rods and 500 feet of 1/0 copper cable was calculated to provide circuit resistance of the required value to protect the line cathodically.