What Does Polythionic Acid Stress Corrosion Cracking (PASCC) Mean?
Polythionic acid stress corrosion cracking (PASCC) is a type of material failure that occurs when sensitized austenitic stainless steel and some nickel alloy steels develop sulfide scale on their surface during an operation that exposes them to air and moisture. The cracking typically occurs during a plant shut down, during the subsequent start up, or soon after start up.
Critical factors that influence polythionic acid stress corrosion cracking include the environment, the material's condition (whether already sensitized or susceptible for sensitization) and tensile stress. Polythionic acid stress corrosion cracking tends to propagate intergranular and usually occurs near welds or in high-stress areas. The cracking damage is localized and may not be evident until a leak appears. These cracks can quickly propagate in a matter of minutes or hours, leading to a loss of containment and damage to the surrounding environment.
Corrosionpedia Explains Polythionic Acid Stress Corrosion Cracking (PASCC)
Polythionic acid stress corrosion cracking (PASCC) occurs when intergranular cracks develop in sensitized austenitic stainless steel and nickel alloys after exposure to air and moisture during equipment shutdowns. This often occurs in a sour hydrocarbon environment.
Preventing Polythionic Acid Stress Corrosion Cracking
Preventing the formation of PASCC is easier than repairing the cracks once they have formed. Because the material's microstructure plays a large role in the initiation of PASCC, it is essential to select an appropriate austenitic stainless steel grade at the design stage. Commonly used stainless steel grades like 304/304H and 316/316H are susceptible to cracking, whereas more stabilized grades like 321/347 show high resistance to PASCC.
The following methods can prevent crack initiation during equipment shutdowns:
- Wash the equipment with soda ash or alkaline during or immediately after the shutdown to neutralize the acids formed after the shutdown.
- Purge the equipment with nitrogen/ammonia or dry nitrogen to exclude oxygen from the equipment during the shutdown to prevent polythionic acid formation.
- Use dehumidified air to prevent the formation of liquid water. This method is acceptable only if the air's dew point temperature entering the equipment is lower than the internal metal's surface temperature (minimum of 22°C).
Methods to Detect Polythionic Acid Stress Corrosion Cracking
Inspecting for polythionic stress corrosion cracking is not practical during the operational phase because cracking may not occur until well into the shutdown. The most common method to detect PASCC is to perform a nondestructive evaluation (NDE) using a dye penetrant test (PT). Note that cracks are usually filled with tightly adhering deposits, so these deposits must first be removed for the PT inspection. Fapper wheel sanding or light grinding can be used to remove the residues.
Eddy current testing (ECT) is another NDE method that can detect PASCC surface cracks on the crack initiation side. Shear wave ultrasonic technique (SWUT) and pulse array ultrasonic technique (PAUT) is useful depending on thickness, accessibility and metallurgy.
Types of Equipment that are Vulnerable to PASCC
PASCC poses a significant threat to refinery equipment. Industry experience shows that surface cracking is often found on furnace tubes, heat exchanger tubes and process vessel linings. Similar cracking is also found in hydrocracking units and catalytic cracking units where austenitic stainless steels were used to a greater extent.