Background
Polythionic acids are a concern for refineries and petrochemical plants due to the extremely rapid intergranular stress corrosion cracking that can occur in equipment during shutdowns, startups or during operation when air and moisture are present. Often, the cracking is noticed upon plant start-up causing the plant to be brought down for additional repair, resulting in additional loss of production and repair cost. Cracking is usually located adjacent to welds or high stress areas and can be rapid: reports of penetration of 12 mm (1/2 inch) wall thickness in less than 8 hours exposure are in the literature. Issues may not be noticed until the plant restarts after the shutdown period. See PHOTOS: A1 and A2 of a severe case of polythionic acid stress corrosion cracking.
Polythionic acids are a group of weak sulfur bearing acids of the form H2SnO6, (such as sulfurous acid) where n is normally between 2 and 5. See Figure 1 for schematic of the polythionic acid molecule.
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Figure 1: Polythionic Acid
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During normal plant operation in refinery and petrochemical plants, a thin chromium or iron sulfide scale is formed on fixed equipment (rotating equipment has been reported on occasion as well) during exposure to the process stream, often hydrocarbon containing H2S, under reducing conditions. Upon exposure to air during a shutdown period, the sulfide scale oxidizes (decomposes) to polythionic acid by reaction with liquid water in the presence of oxygen.
8 FeS + 11 O2 + 2 H2O = 4 Fe2O3 + 2 H2S4O6 (tetrathionic acid)
Liquid water can be present simply due to dew point water condensing from air.
