This paper discusses the results of a failure examination of superheater tubes from a boiler. The superheater tubes failed along steamside fissures associated with external surface welds attaching finger bars to adjacent tubes. The morphologies of the fissures were consistent with stress-assisted corrosion (SAC). In addition, cracks typical of thermal fatigue were present on some ID surfaces and at the tips of some fissures. The elemental compositions of corrosion products within the wide SAC fissures, determined using Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray Spectroscopy (EDXS), support the possibility that crevice conditions led to accelerated corrosion during cool down periods, resulting in pit-like areas along the fissures. Failed and unfailed superheater tubes were examined using standard metallographic techniques. The results were used to develop a theory of the role of thermal fatigue cracking and crevice corrosion in SAC fissure initiation on the steamside surfaces of the superheater tubes and propagation through the tube walls.
Stress-assisted corrosion (SAC) is a form of corrosion initiated by tensile stresses that fracture the otherwise-protective magnetite layer on water-touched surfaces of steel boiler tubes. These stresses are typically applied by external attachment welds. Steel exposed by the crack in the magnetite corrodes until a healing magnetite layer is formed. Cyclical stresses produce a fissure beneath the initial crack in the magnetite. Such fissures typically initiate within waterside pits, show signs of discontinuous growth and are blunt tipped (Figs. 1 and 2).1 Typically, multiple waterside fissures form continuous or discontinuous lines that are either parallel lines and linear or concentric ovals elongated in the long direction of the external weld. The fissures propagate radially outwards through the tube wall as shown in Figures 1 and 2.
Leaks where external attachments were welded to water-filled tubes in utility boilers were reported as early as the 1960s.2 The Electric Power Research Institute (EPRI) found that such leaks were a primary cause of availability loss in utility boilers.3