External Corrosion of Furnace-Wall Tubes—I History and Occurrence

Abstract

Abstract Since 1942, many large central-station boiler furnaces burning pulverized coal and removing the ash as molten slag have experienced external corrosion of furnace-wall tubes. In a few cases the loss of metal was so severe as to cause failure of tubes during operation, in others, seriously thinned tubes had to be replaced to prevent unexpected interruption of service. Because the rate of corrosion under certain conditions may be so high as seriously to weaken the tubes between normally scheduled inspections, operators charged with the continuous production of power under wartime emergency conditions have been greatly concerned over prevention of further corrosion. To expedite the solution of the problem, a co-operative investigation was instituted between the Bureau of Mines and the Combustion Engineering Company. This report gives the results of the preliminary study of the occurrence of external corrosion, 16 furnaces in 13 stations being examined. Corrosion was found to occur when the temperature of the tube metal was in the normal range for boiler furnaces, usually not exceeding 700 F, while the maximum temperatures observed were less than 900 F. Deposits in corrosion areas are shown to be of two types, one having the appearance of a bluish-white porcelain enamel, being largely soluble in water in which it produces an acid reaction, and consisting principally of sodium and potassium sulphates in a complex form. The second type is iridescent blue or black, is insoluble in water, may contain significant amounts of carbon, and consists primarily of iron sulphide. Because of the greater incidence of the sulphate deposit, its study was made first and is reported in detail in a following report.