To receive the beCPG brochure, please fill out the form below:
A solution of sodium carbonate (soda ash, Na₂CO₃) dissolved in potable water, typically ranging from 1% to 2% by weight.
What is Polythionic Acid Stress Corrosion Cracking (PTA-SCC)?
Using "L-grade" stainless steels (like 304L or 316L) reduces the total carbon available to form chromium carbides, shifting the sensitization curve significantly. However, these grades have lower allowable design stresses at highly elevated temperatures. nace sp0170 pdf
Surfaces are sprayed or flushed with an alkaline solution, such as soda ash ( Na2CO3cap N a sub 2 cap C cap O sub 3 ), ammonium hydroxide ( NH4OHcap N cap H sub 4 cap O cap H
The NACE SP0170 standard remains an indispensable tool for the refining industry. Whether you are planning a major turnaround, developing shutdown procedures, or investigating a past corrosion event, this document provides the definitive methodology for protecting austenitic stainless steel equipment from polythionic acid stress corrosion cracking. A solution of sodium carbonate (soda ash, Na₂CO₃)
Austenitic stainless steels are highly valued in the refining and petrochemical industries for their excellent corrosion resistance and high-temperature strength. However, these materials are susceptible to a severe form of localized corrosion known as Polythionic Acid Stress Corrosion Cracking (PTA-SCC).
) form when sulfide scale on the interior walls of the equipment reacts simultaneously with moisture ( H2Ocap H sub 2 cap O ) and oxygen ( O2cap O sub 2 ) from the air during a shutdown. However, these grades have lower allowable design stresses
Leave the alkaline film on the metal surfaces; do not rinse it off with fresh water before exposing the asset to air. Summary of Best Practices
What is Polythionic Acid Stress Corrosion Cracking (PTA-SCC)?
During normal high-temperature operation in hydroprocessing, desulfurization, or catalytic cracking units, sulfur compounds react with iron to form sulfide scales (FeS) on the inner walls of the equipment. When the unit is shut down, opened, and exposed to moisture (H₂O) and oxygen (O₂), these sulfide scales react to form polythionic acids ( H2SxO6cap H sub 2 cap S sub x cap O sub 6 , where x ranges from 3 to 6).
Do not assume "soda ash" is generic. Verify the concentration of sodium carbonate solution (typically 2-5% by weight) using titration or conductivity.