Cases found in water tube blrs where due to alternating cyclic stresses set up in tube material leading to a series of fine cracks in wall. Corrosive environment aggravates. Occurs in any location where cyclic stressing of sufficient magnitude are present. Rapid start up and shut down can greatly increase susceptibility. Common in wall and supht tubes, end of the membrane on waterwall tubes, economisers, deaerators. Also common on areas of rigid constraint such as connections to inlet and outlet header. Other possible locations and causes are in grooves along partially full boiler tubes (cracks normally lie at right angle to groove ), at points of intermittent steam blanketing within generating tubes, at oxygen pits in waterline or feed water lines, in welds at slag pockets or points of incomplete fusion , in sootblower lines where vibration stresses are developed, and in blowdown lines.