Figure 12-5. - General waterside thinning.

drum may be affected. Figure 12-5 shows general waterside thinning.

WATERSIDE BURNING may occur if the temperature exceeds about 750F in plain carbon steel tubes or about 1,000F in most alloy superheater tubes. The effect of waterside burning is the oxidation of the tube metal to a shiny, black, magnetic iron oxide known as high-temperature oxide.

WATERSIDE ABRASION is the term used to describe waterside cavities that result from purely mechanical causes rather than from corrosion. For example, tube brushes or cutters may cause abrasion spots at sharp bends in economizer, superheater, and generating tubes. The surface markings of such abrasions indicate clearly that they result from mechanical abrasion rather than from corrosion.

DIE MARKS appear as remarkably straight and uniform longitudinal scratches or folds on the watersides of the tube. They are the result of faulty fabrication. Die marks, shown in figure 12-6, may extend for the full length of the tube. Localized corrosion occurs quite often along the die mark.

TUBE CORRUGATION is a peculiar type of heat blistering that occurs when the boiler water is contaminated with oil. Corrugation may consist of closely spaced, small-diameter, hemispherical bulges, as though the tube metal had been softened and then punched from the inside with a blunt instrument. It may also exist as a herring-bone or chevron pattern on the tube wall nearest the flame, as shown in figure 12-7. It is not known exactly why oil contamination of the boiler water tends to cause this patterned corrugation.

Figure 12-6. - Die marks on the waterside of a tube.