Corrosion of wire screens in sulphur recovery units

The aim of this study was to provide an understanding of the mechanism of degradation of 18Cr-8Ni austenitic stainless steel (type 304) by characterization of scale and the underlying alloy after use in a S recovery unit. Sulphidation decreases chromium content in the matrix and thus lowers resistance to S attack; thus the investigation correlated the bad performance of support screens with Cr distribution in the scale and alloy. Scanning electron microscopy was used to examine four wire screen samples. A heavily sensitized microstructure indicated that the screens had experienced a temperature of > 871 deg C (upper limit for sensitization). High temperature, generated during oxygen injection, accelerated sulphidation attack, which progressed from grain boundaries to consume entire grains. Recrystallization and sensitization events also contributed to sulphidation. Sensitization in plain 304 stainless steel depletes Cr around carbide precipitates and it is conceivable that the remaining Cr content is inadequate for sulphidation resistance.