in austenitic stainless steels and related alloys during equipment shutdowns. While the most recent major reaffirmed version is NACE SP0170-2018
This standard provides technical procedures to prevent Stress Corrosion Cracking (SCC) caused by polythionic acids (PTA) that form when sulfide scales on equipment react with moisture and oxygen during refinery downtime. Key Aspects of the 2021 Revision
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
Continuous during catalyst blanketing; or Dehumidified Air (Dry Air) during catalyst dumping to protect workers and prevent catalyst wetting. Refinery Fractionators & Columns
The standard (last officially updated in 2018 but widely applied through 2021) tells a "deep story" not of people, but of the silent, invisible war against Polythionic Acid Stress Corrosion Cracking (PTASCC) .
This standard is also frequently referenced in other key industry publications, such as API RP 573-2021, which confirms that the 2018 edition of SP0170 remains the current and applicable version for the industry. Any search results claiming a "2021 version" are likely referencing other related documents and not SP0170 itself.
Excludes oxygen and moisture, preventing the acid formation reaction entirely.
In summary, while NACE SP0170-2021 does not exist, the 2018 version is the current and crucial standard for anyone involved in the design, operation, maintenance, or inspection of refinery equipment containing austenitic stainless steels. The document provides the necessary framework to manage the severe risk of polythionic acid stress corrosion cracking during shutdowns. For engineers and asset managers, ensuring compliance with NACE SP0170 is not just about following a rule; it is an essential practice for ensuring safety, regulatory compliance, operational reliability, and the long-term economic viability of industrial assets.
: Choosing corrosion-resistant materials or employing specific fabrication practices to reduce susceptibility to sensitization. Inspectioneering Evolution of the Standard
The official, peer-reviewed document is available through authorized distributors like the ANSI Webstore or Accuris . Organizations should avoid unverified PDF links on third-party forums to ensure compliance with current AMPP engineering frameworks.
: Neutralizing acids by washing equipment surfaces with alkaline solutions (e.g., soda ash).
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This is the most common method specified in the standard. It involves thoroughly washing the affected equipment with a dilute alkaline solution, typically sodium carbonate. The alkaline solution neutralizes any polythionic acid that may have formed, rendering it harmless. The standard provides critical details on:
Polythionic acid cracking is an insidious mechanism because it does not happen while the refinery is running. Instead, it occurs when three distinct variables intersect:
AMPP - The Association for Materials Protection and Performance Primary Mitigation Methods
Note: Ensure you are obtaining the latest active revision, which may be labeled 2018 or updated with technical amendments labeled through 2021. Key Takeaways
is your essential playbook for preventing catastrophic equipment failure during shutdowns. ANSI Webstore What is NACE SP0170? NACE SP0170 provides specific procedures for protecting austenitic stainless steels
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