Failure analysis of sulphidation-oxidation corrosion of Incoloy 800HT superheater tube

The burst of an Incoloy 800HT superheater tube, operating at 160–690 °C steam was investigated using metallographic inspection and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). It was observed that the analyzed fracture surface has an intergranular cracks morphology. Corrosion product deposits were also found on the inner wall of the failure tube. The microstructure analysis of the tube samples showed that the inner wall of the tube was covered by a layer of loose oxide and intergranular cracks propagated from the inner surface towards the outer wall of the tube. The elemental EDS results of the corroded tube samples showed a sulfur-rich layer between the matrix and oxide layer, and the sulfur was also found in the white corrosion depositions. The performed test results revealed that the tube burst was caused by a local overheat-induced sulfidation-oxidation process. Finally, the failure mechanism was discussed and suggestions were made to prevent occurrence of similar failures.