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.

Subject
External surfaces, Water, Metal surfaces, Tubes, Corrosion cracking, Fissures, Stress cracking, Thermal stress, Crevices, Fingers, Boilers, Magnetite, Thermal fatigue
Keywords:
stress-assisted corrosion, superheater tubes, crevice corrosion, thermal fatigue, welded attachments
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2021
Association for Materials Protection and Performance (AMPP)
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