Analysis of degradation of a super-austenitic stainless steel for flue gas desulfurization system after a fire accident

A super-austenitic stainless steel (SASTS) which had been used for a fired flue gas desulfurization system was analyzed in terms of the microstructure, pitting resistance and hardness for the evaluation of degradation. The microstructural analysis showed that sigma phase, a metallic compound rich in Mo and Cr detrimental to corrosion resistance in SASTS, had precipitated all along the grain boundary of the alloy used for the reaction tank, while only isolated sigma phase was observed in the grain or at the grain boundary of the unused and the one from the mist eliminator. Several pits after an immersion test in a 60 °C 6%FeCl3 solution were developed only in the alloy from the reaction tank. Microscopic images around these pits indicated that the grain boundary had been selectively attacked. Hardness value was the highest for the alloy from the reaction tank. From these findings it was suggested that the degradation of the alloy used for the reaction tank was caused by the precipitation of sigma phase along the grain boundary due to an indirect heat affect from the fire accident.