Failure investigation of a cooling coil tube in zinc roaster furnace

The failure analysis of a cooling coil tube employed to extract the sensible heat of roasted zinc sulfide ore in a fluidised bed furnace type roasting unit was reported. The coils are prone to service degradation and subsequent failure due to the extreme operating conditions (920-960 °C). Severe corrosion attack and cracks were seen on the outer surface of the tube upon visual examination. The optical microscopy performed at the crack tip confirmed the deformation induced pearlitic banded structure. The corrosion products at the failure regions were rich in iron and zinc as revealed in the analysis by SEM with EDS. The galvanic corrosion between Fe and Zn is found to be the primary cause of the failure. It is hypothesized that the type of corrosion is possible with the zinc oxide acting as cathode with reference to the coil tube's outer surface. The selective corrosion resulted in the preferential corrosion attack and led to the premature failure of the tube.