Failure of 2.25Cr–1Mo steel superheater tubes in a fluidized bed combustor due to fireside corrosion

Repeated failures have been observed in a few superheater tubes made of 2.25Cr–1Mo steel at a localized region in a circulating fluidized bed combustion boiler. This paper reports the failure analysis of two affected tubes — the ruptured part of a failed tube and another tube with localized thinning in one region. Both the tubes had grayish, thick and hard deposits of calcium sulphate on the outer diameter surfaces beneath which thick oxide layer was present. A number of intergranular cracks and internal penetration of oxides along the grain boundaries in the base material were observed emanating from the oxides present on the fireside of the tubes. The longitudinal surface adjacent to the failed region showed typical features of intergranular creep. The primary reason of the failure was the continuing thinning of the tubes caused by the alternate cycle of formation of thick calcium sulphate deposit on the fireside of the tubes and its spallation. The spallation of calcium sulphate deposits occurred along with the metal oxide leading to the exposure of the bare material. This caused rapid growth of the less protective outer oxides on the base material. Intergranular creep led to the final rupture of the tube caused by increase in the hoop stress due to the thinning of the tube.