Alkaline Corrosion and its Prevention on the Inner Surface of Superheater Tubes in Municipal Refuse Incinerator

Abstract

Since around 1992, some accidents involving penetration from inner side of superheater tubes during operation have occurred at municipal refuse incinerators.
This study presents that the investigation on corrosion and damage of the inner surface of superheater tubes of a municipal refuse incinerator. Pitting corrosion is observed at the lower part of surface of the inner superheater tube on horizontal type primary superheater tube. A relatively high concentration of potassium is detected in the scale which is condensed into the scale by potassium contained in the steam. It is suggested that the corrosion is classified to alkaline corrosion. When a part of boiler water contained an alkaline ion takes place carry-over owing to generating of large volume of steam exceeding separation capacity of steam water, the steam is introduced to superheater tube, so that the ion is condensed in water of the tube.
Thin membranous scale cover over the inner surface of secondary superheater tubes, in which potassium is also detected in the scale. The case is also caused by alkaline corrosion. Since potassium ion contains in the original supplied water at the start of running, desuperheating water is contaminated by the ion. The potassium ion is concentrated in the scale during the operation.