A longitudinal groove on the internal surface along the top of the tub

A longitudinal groove on the internal surface along the top of the tub - Corrosion defects

Figure 1: Grooved window section cut from inner bend.

Figure 2: Internal surface showing wasted metal.

Figure 2: Service data.

Defect name: No data.
Record No.: 2462
Type of defect (Internal/Surface): Surface
Defect classification: Corrosion defects
Steel name: Steel
Steel composition in weight %: No data.
Note: Caustic corrosion. Generally, caustic corrosion is confined to (1) water-cooled tubes in regions of high heat flux, (2) slanted or horizontal tubes, (3) locations beneath heavy deposits, and (4) heat-transfer regions at or adjacent either to backing rings at welds or to other devices that disrupt flow.

Visual examinations revealed a longitudinal groove on the internal surface along the top of the tube (Fig. 1). Perforation had not occurred, but as much as 60% of the tube wall had been corroded.
The entire top side of the internal surface exhibited shallow metal loss in a distinct band (Fig. 2), which narrowed and ended in a "spear point" near the end of the tube in the steam drum. Sparkling black crystals of magnetite were present in and around the groove. A total of six adjacent tubes had been similarly affected.

Steam channeling along the top of the slanted section led to concentration of sodium hydroxide. Steam channeling may indicate localized or general excessive heat input. If appropriate alteration of operating parameters does not eliminate the problem, the use of rifled tubes may be effective.
The affected boiler is operated continuously during 100 to 150 days of campaign operation twice per year. The boiler is not operated during the intervening period.
Reference: Not shown in this demonstration version.