Abstract
Extended studies of the mechanism of external corrosion of furnace-wall tubes of high-capacity slag-tap furnaces, which were begun in 1942 by the Bureau of Mines in co-operation with the Combustion Engineering Company, have shown that liquid alkali-metal pyrosulphates can be formed under operating conditions from deposits of alkali-metal sulphates on the tubes. The rate of attack will be considerably higher than when the alkali-metal sulphates, or “enamel” deposits, do not form a liquid phase, which was described in a previous paper of this series. The phase boundaries for three compositions of alkali-metal sulphates in the system M2SO4–SO3 have been established. The conditions for the thermal decomposition of coal ash, leading to the formation of the SO3 necessary for the corrosion process, have been determined. Alkali-metal sulphate deposits on furnace tubes are believed to result from volatilization of alkalies in the coal, which condense as corresponding oxides on the tubes and then convert to sulphates by reaction with SO3 in the furnace atmosphere. Furnace-wall-tube attack, associated with deposits consisting mainly of FeS, is believed to be related to the deposition of pyrites on the tubes. The pyrites, originating from coarse coal, or coal that is poorly distributed in the burners, with respect to size, adheres to previously formed deposits of alkali-metal sulphates, oxidizes relatively slowly to FeS and Fe3O4, and thereby produces sufficient SO3 to react with the alkali-metal sulphates and the oxide on the metal. The net effect is to cause corrosion by the same processes that have been ascribed to corrosion by the alkali-metal-sulphate type of deposit. The relationship between the results obtained in the laboratory, and observations of furnaces before and after preventive measures have been applied, is described.
