Stress-assisted corrosion (SAC) was observed on the waterside of mild steel recovery boiler tubes only at locations exhibiting the combination of a substantial external attachment weld and a significant internal oxide accumulation compared to the nominal Fe3O4 film. Penetrations of up to 30% of the tube wall thickness were observed metallographically, and the penetrations were invariably transgranular with bulbous features, rounded tips, and filled with a relatively dense oxide comprised of alternating layers of Fe2O3 and Fe3O4. Microprobe analysis also revealed the presence of trace quantities (0.1 – 1 at.%) of Cu, Cl, P, and S in the oxide, indicating at least one source of contamination in the boiler feed water. Nondestructive testing, which included two ultrasonic techniques, radiography, and visual inspection with a borescope, was not particularly successful for detecting SAC.

Subject
Materials, Recovery boilers, Oxides, Tubes, Oxide layers, Waves, Metal oxides, Inspection, Metallography, Corrosion detection, Penetration, Internal surfaces, Boilers
Keywords:
stress-assisted corrosion, SAC, waterside penetration, recovery boiler, borescope, mild steel tubing, metallography, layered iron oxide, ultrasonic inspection, radiography
Government work published by the Association for Materials Protection and Performance (AMPP) with permission of the author(s). Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2004
GOV
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