High residual tensile stresses from structural welding; prone to localized sensitization.
: Using dry nitrogen to exclude oxygen and moisture from the equipment, preventing the formation of polythionic acid. Alkaline Washing
Polythionic acid stress corrosion cracking is a highly destructive, intergranular cracking mechanism that can cause catastrophic equipment failure in a matter of hours during refinery shutdowns. The phenomenon requires three conditions simultaneously:
The new edition provides updated charts for maximum H₂S partial pressure versus pH. Carbon steels that passed in 2015 may now be disqualified under the 2021 thresholds, especially in low-pH (acidic) environments. nace sp0170 pdf 2021
When the equipment is opened, air (oxygen) and moisture enter.
AMPP - The Association for Materials Protection and Performance Primary Mitigation Methods
Refining processes involving hydrotreating or hydrocracking often operate in sulfide-rich environments. The transition from operating conditions to shutdown conditions presents unique metallurgical risks that require standardized management. AMPP - The Association for Materials Protection and
Disclaimer: This article is for informational purposes only and does not constitute engineering advice. Always consult directly with a licensed materials engineer and refer to the official NACE SP0170-2021 document for specific applications.
During a , when this equipment is opened to the air, three elements combine: Sulfide Scale: Iron sulfides already present on the metal. Oxygen: Introduced from the air. Water/Moisture: Condensation from air or wash water. These three elements react to form polythionic acids ( H2SnO6cap H sub 2 cap S sub n cap O sub 6
If you need help finding a specific section of the standard, or if you'd like me to compare it to a different API standard, let me know. Share public link air (oxygen) and moisture enter.
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| Feature | NACE SP0170-2021 | ISO 15156:2020 | | :--- | :--- | :--- | | | U.S. supplement for specific alloys and hardness limits | Global general requirements | | CRA Limits | More conservative for nickel alloys | Broader, with user responsibility | | Hardness Testing | Mandatory HRC conversion table | Refers to general practices | | Welding | Stricter HAZ limits for sour gas wells | Similar but less prescriptive | | PDF Availability | AMPP website | ISO/ANSI website |