A sulfur recovery unit (SRU) train in a gas processing facility went under an emergency shutdown due to the failure of a reaction furnace waste heat boiler (WHB) tube. The failed tube had been in service for approximately 18 years. The failed tube was subjected to a number of metallurgical laboratory examinations in order to determine the damage mechanism and root cause(s) of the failure. Examinations included visual inspection, scale analysis, chemical analysis, metallographic examination and mechanical testing. The examination revealed internal corrosion thinning in the tube which led to rupture since the tube could no longer withstand the pressure. Metallographic examination revealed spheroidized microstructure indicating that the tube experienced high metal temperature. This is suggesting that something was impeding heat transfer between the tube and water. Scale analysis results from a sample collected from the tube internal surface indicated the presence of iron sulfide corrosion products. Based on the aforementioned findings, it was concluded that the corrosion thinning was caused by sulfidation. Sulfidation is one of the potential damage mechanisms in WHB tubes and is caused by reactive sulfur species as a result of the thermal decomposition of sulfur compounds at high temperatures (above 500oF). Failure contributing factors as well as corrective actions to prevent recurrence of such failure are discussed in this paper.