Special Cathodic Protection Applied to the Inside of Heat Exchanger Tubes in MSF Desalination Plant

Abstract

In order to enable the use of cheap ferrous materials for heat exchanger tubes in an MSF desalination plant, the application of special cathodic protection to the inside of mild steel tubes was studied by laboratory experiments. At first, the sponge ball coated with metallic zinc powder was passed intermittently at intervals of 1.5 seconds through a mild steel tube with continuous flow of deaerated 6% NaCl solution, and the whole inside of the tube was polarized to about -900mV (SCE) and was sufficiently protected. However, the durability of polarizing capacity of the ball was not appreciably long on account of dropping zinc from the ball, and further studies for ball design were necessary to fasten the zinc on the ball. In the second step of this work, deaerated 6% NaCl solution suspending metallic zinc grains ranging in size from 48 to 200 mesh was passed through the tube with and without the use of sponge ball whose diameter was slightly larger than the inside diameter of the tube, and the current generated from the dispersed anodes, namely zinc grains, protected the whole inside of the tube. The results showed that the potential of the tube was maintained below -900mV without the ball under such conditions that the extremely deaerated test solution suspending zinc grains in concentration of 1 to 2%. It was considered that satisfactory protection might be performed by dispersed anodes, and that the accelerating effect of simultaneous use of the ball was revealed when the zinc grains were very fine or dissolved oxygen content slightly increased. In air-saturated 6% NaCl solution the tube potential did not effectively polarize by the dispersed anodes even in cooperation with the ball. The self-corrosion rate of zinc grains in stagnant deaerated artificial sea water (double salinity) at 95°C and pH 7 was 0.02 to 0.03%/hr, and the life of zinc grains was expected to be about 5, 000 hours. On the other hand, zinc powder revealed severe self-corrosion and the polarizing effect by the powder was not remarkable. Then, in heat recovery section and brine heater, it is expected that satisfactory protection is performed in the inside of the steel tubes through which zinc grains having several tens mesh size are passed with brine in concentration of 1 to 2%. However, further experiments are necessary to establish the application under such severe conditions in MSF desalination plant as of 120°C and 2m/sec, and it may be necessary to test the polarizing power of larger zinc grains in order to protect sufficiently the steel tubes in heat rejection section through which air-saturated sea water is passed.