Cathodic Protection of Open Box Coolers

Application of cathodic protection to exterior surfaces of tubes used to cool ammonia gas at pressures of 280 to 300 atmospheres is described. Because of space and other considerations, graphite anodes and impregnated graphite anodes were employed. Heavy deposits of salts occurred initially on tube areas nearest anodes but little on surfaces farther away because of close spacing of anodes and tubes. When current was reduced, deposits diminished and heat transfer rates improved. A seasonal variation in the salt content of the cooling water resulted in an increase in resistance of the water and a corresponding decrease in the polarization of the tubes. This allowed corrosion of tubes farthest from anodes which was stopped when the anodes were relocated outward from tube surfaces to give better current distribution and output of selenium rectifiers was increased to return potential to design —850 millivolts. Success of initial experiments with the coolers resulted in application of cathodic protection to new and larger coolers when plant capacity was inreased.