) mixed with water. A surfactant is frequently added to help the solution penetrate heavy oil or hydrocarbon films covering the sulfide scales.
) from the air react with these sulfide scales , yielding polythionic acids (
) reacts with iron in the equipment walls to form iron sulfide ( FeScap F e cap S ) scale. During a shutdown, if ambient oxygen ( O2cap O sub 2 ) and water ( H2Ocap H sub 2 cap O
This method prevents PTA formation by excluding oxygen, a key ingredient in the formation reaction. Equipment is filled and pressurized with an inert gas, typically nitrogen, after it has been properly purged. nace sp0170 pdf 2021
: Utilizing stabilized grades of stainless steel (like 321 or 347) or specific fabrication practices to reduce sensitization. Document Information NACE SP0170-2018
Neutralization or exclusion methods must be initiated before the metal temperature drops below the dew point to prevent moisture from reacting with sulfide scales. Accessing the Standard
Note: Ensure you are obtaining the latest active revision, which may be labeled 2018 or updated with technical amendments labeled through 2021. Key Takeaways ) mixed with water
) and stop them from triggering intergranular stress corrosion cracking (IGSCC) in sensitized alloys.
If left unmitigated, these acids rapidly attack the chromium-depleted grain boundaries, causing severe cracking within hours of exposure to the atmosphere. Core Mitigation Strategies in NACE SP0170
The relative humidity (RH) inside the vessel must be maintained at low levels (typically under 20% RH ). By keeping the environment completely dry, liquid water cannot condense on the metal faces, halting the chemical reaction required to generate polythionic acid. Industry Application and Implementation Matrix During a shutdown, if ambient oxygen ( O2cap
The most effective way to prevent PTA-SCC is to prevent the reaction between iron sulfide, oxygen, and water. Operators achieve this by filling and blanketing the equipment with high-purity .
Use standard-compliant checklists to document that surfaces remained covered by alkaline solutions or that nitrogen pressures never dropped during the shutdown window. Tracking the Evolution of NACE Standards