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Polythionic Acid Stress Corrosion Cracking (PASCC)

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Polythionic Acid Stress Corrosion Cracking (PASCC) is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing feedstocks containing sulphur. This damage mechanism primarily affects standard and high carbon stainless steels, but low carbon and chemically stabilized stainless steels may also be vulnerable under certain conditions. PTA SCC typically does not occur during normal processing, but rather, after the equipment is shutdown and opened up for inspection (i.e. when moist air contacts the surface of equipment that has been exposed to sour hydrocarbons in service).

A common form of "prevention" is to perform a soda acid wash of the equipment before or right after it is exposed to air and moisture. PTA SCC can also be prevented by removing oxygen from the environment by performing a dry nitrogen purge or removing moisture by performing a dry air purge.

This damage mechanism is covered in greater detail in API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. It includes a general description of the damage mechanism, susceptible materials of construction, critical factors, inspection method selection guidelines, and control factors. PTA SCC is also covered by NACE Standard RP0170-97, Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment, which lists several methods for preventing or mitigating PTA SCC and details some of its causes.

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