The most common fluid causing intergranular corrosion in hydrocarbon plants is polythionic acid. Both austenitic and straight chromium grades of stainless steels can be attacked by polythionic acid. This phenomenon is usually an internal problem, occurring on the process-exposed side of a piping run, vessel shell, exchanger bundle, heater tube, etc. The phenomenon usually starts with the stainless steel surface forming a thin iron sulfide film because of exposure to small amounts of sulfur, usually from hydrogen sulfide in the process stream. During a shutdown, in the presence of air and liquid water, often dew point water, the sulfides convert to polythionic acid. The polythionic acid then corrodes the chromium-depleted grain boundaries of the sensitized alloy. Since stainless steels are usually supplied to fabricators in the solution-annealed condition, sensitization is usually confined to weld affected zones.
To add to K Phan's excellent description of how polythionic acids form and what they do to sensitized stainless steels, they are weak sulfuric acids with the formula H2SxO6, where x is usually 2 to 5. ASTM G35 describes procedures for testing SS in polythonic acids at room temperature to determine relative susceptibility to intergranular SCC.
