Abstract
An inclined section of a carbon steel sparge line (1” OD) with intermittent flow (~ 10 sec cycles) from a steam boiler system has experienced accelerated corrosion. The principal sparge line wastage occurred near its outlet to a 4-inch condensate line; this section of the sparge line required repeated replacement every 12-18 months. The initial steam pressure reported in the sparge line was 350 psig and the condensate water temperature ranges from 120 to 180 °F and is exposed to the atmosphere. Both the steam and the condensate were treated intermittently with neutralizing amines.
Representative sections of a sparge line as connected to the condensate pipe were removed and examined by optical microscopy, scanning electron microscopy (SEM), and X-ray energy dispersive spectroscopy (EDS). The sparge line had severe, jagged pitting along the upper half of the steamside surface near the condensate pipe. Some pits had perforated the wall near the condensate pipe/sparge line joint and pit distribution gradually decreased over a one-foot length while moving away from the condensate pipe. The pits were free of any noticeable deposit. Wastage at the pipe/sparge line joint was characteristic of crevice corrosion and the presence of differential aeration cells. The pits were formed by accelerated oxygen attack and cavitation. The rapid thermal cycling of relatively deaerated steam with oxygenated condensate lead to crevice corrosion, oxygen corrosion, and cavitation damage in the affected areas. The oxygen corrosion/cavitation form pits and is related to the temperature dependence of oxygen solubility. Part of the damage may be traced to the design of the system. The corrosion will be discussed in detail and several methods will be proposed to mitigate or reduce the damage.
