Metal dusting phenomena of 501 AISI furnace tubes in refinery fractional distillation unit

The purpose of this investigation was to conduct the failure analysis of 501 AISI furnace tubes places before distillation column in fractional distillation unit. The investigated furnace tubes were planned to work for ten years however after just two years of exploitation <30% of the material left. The observed corrosion process had the intense and complex character. The well-adhered shiny black deposits and deep, round pits were present after 2 years of exploitation. There is no direct explanation in the norms and standards for such abnormal intense corrosion taking place in described petroleum unit. The study based on the visual, optical and electron microscopy with chemical analysis via SEM and EDX of deposits was performed. Too high allowable operating temperature was identified as a main cause of such intense corrosion. The same measurements were performed for parallel working furnace tubes which was not subjected to premature degradation and were collected after 10 years of its normal operation in the installation. The performed measurements allowed us to determine that metal dusting as a the source of catastrophical corrosion. The work includes also an appropriate action in order to avoid the described corrosion threat.