External Corrosion of Furnace-Wall Tubes—II Significance of Sulphate Deposits and Sulphur Trioxide in Corrosion Mechanism

Abstract The external corrosion of furnace-wall tubes of slag-tap furnaces became a problem to operators and manufacturers in 1942. Since that time, in order to determine the mechanism of corrosion so that rational protective measures could be developed, an extensive laboratory study has been in progress on the sulphate, or “enamel,” deposits found on the tubes in areas where external corrosion occurs. These deposits are greenish-white to reddish-brown in color and are soluble in water, in which they produce an acid reaction. X-ray diffraction studies show them to consist primarily of a solid solution of sodium and potassium sulphates and alkali-metal ferric trisulphates, such as K3Fe(SO4)3. Experiments under controlled laboratory conditions with actual and synthetic “enamels” have shown that, at temperatures of 1000 F, sodium and potassium sulphates will react readily with iron oxide in an atmosphere containing a low concentration of sulphur trioxide to form the same alkali-metal ferric trisulphates that occur in “enamels.” However, under the same conditions of temperature and concentration of SO3 neither iron oxide nor the alkali-metal sulphates alone will react with SO3. These reactions suggest the mechanism of corrosion to involve the removal of the normally protective oxide on the furnace tubes by (a) the condensation on the relatively cool tubes of alkali-metal oxides which are converted to the corresponding sulphates by the SO3 in the furnace, and (b) the subsequent reaction of the iron oxide on the tubes and the alkali-metal sulphates with the SO3 evolved as the result of the slagging reactions in the coal ash deposited mechanically on the “enamel.” Thus conditions are afforded for the removal of the iron oxide on the tube with the formation of alkali-metal ferric trisulphates. Deslagging causes thermal decomposition of these compounds, and the cycle is repeated. It is suggested that prevention of corrosion by this process can be achieved by ventilating the surface of the tubes with air so as to decrease the concentration of SO3 below that necessary for the formation of the complex iron sulphates. Also, because the alkali metals required for the reaction are believed to originate principally from the flame, the addition of such air decreases the rate of deposition of alkalies. These findings are in agreement with field observations in which the maintenance of oxidizing conditions prevented further corrosion.