99 Diseases of Pressure Equipment: Polythionic Acid Stress Corrosion Cracking (PASCC)

Polythionic Acid Stress Corrosion Cracking (PASCC) is an affliction of many refineries processing sulfur containing feedstocks, and since that is the norm these days, most refiners reduce their susceptibility to PASCC by selecting resistant alloys orby neutralizing exposed surfaces during shutdowns. PASCC occurs when sensitized stainless steels that have been in service, crack intergranularly after exposure to air and moisture, often during shutdowns. And that’s one of the most interesting aspects of PASCC, that is, that it typically does not occur during normal processing, but after equipment is shut down and opened up for inspection, i.e. when moist air contacts the surface of equipment that has been exposed to sour hydrocarbons in service.

Certain types of 300 series stainless steels that are exposed to elevated temperatures between 800^oF (427^oC) and 1700^oF (927^oC) during manufacture, fabrication (especially welding), or even in process environments (like furnace tubes), can become sensitized, which means their crystalline structure changes such that they become susceptible to intergranular corrosion. PASCC is just one type of intergranular corrosion that affects sensitized stainless steels. The commonly used types of stainless steels, 304/304H and 316/316H are particularly susceptible, whereas the stabilized grades like 321/347 and low carbon grades 304L/316L are much less susceptible.

Where we have equipment in service with some susceptibility to PASCC, then a common form of “prevention” is to do a soda ashwash of the equipment before or right after it is exposed to air and moisture, e.g. opened for inspection during a shutdown. NACE has some neutralization guidelines in RP0170. For furnace tubes, that have been exposed to firing conditions with sulfur laden fuels, the firebox can be kept slightly heated during shutdowns to avoid dew point formation, provided, of course, there is no need to enter the firebox for inspection or maintenance.

PASCC is one of those age old problems that refiners have faced since time immemorial, so steps are usually taken to prevent it. But it still happens from time to time. Why? For several reasons. Perhaps an inexperienced engineer selects the wrong material, or the right material is specified but not delivered or installed, e.g. sensitized exchanger tubes. Equipment can be sensitized during manufacture and fabrication without the end user knowing it. Adequate checks and QA/QC are required to make sure that we don’t inadvertently install sensitized equipment. When soda ash washing is required to reduce exposure to PASCC, there could be cases where the washing is inadequately accomplished or even inadvertently left out of the planning or execution of the shutdown.

PASCC is fairly easy to find with penetrant testing. Sometimes, it finds you by leaking, e.g. pipe or HE tubeleaks. Welds are typically most susceptible because welding heating and cooling rates can cause significant alloy sensitization.

Do you still have exposure to PASCC in your refinery because of sensitized stainless steels operating in sulfur compound containing hydrocarbon processes?