Abstract
This paper presents a failure analysis of a boiler tube (SA-210 Grade A-1) in a low sulfur waxy residue (LSWR) oil-fired plant. A highly deformed bulged rupture was observed on the outside of the tube, and the steam from the rupture damaged the tube adjacent to the failed tube. Severe thickness reduction was observed on the inner surface, and brown and orange oxides and white scale were present in the failure region. Microstructural examination and hardness tests confirmed that material degradation (voids, microcracks, and decreased hardness) caused by overheating at the failure region had taken place. X-ray diffraction analysis indicated the presence of CaSiO2 and Na2FeO2, indicating caustic corrosion. In addition, dimples and striations were observed in scanning electron microscopy fractography results. We therefore concluded that deposits on the interior wall led to overheating, followed by evaporation of the feed water, and then NaOH in the water caused caustic corrosion. Eventually, this corrosion resulted in ductile and fatigue failure arising from internal pressure and thermal fatigue.
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