Austenitic stainless steels are the backbone of the refining, petrochemical, and chemical processing industries. Their high chromium and nickel content provides exceptional resistance to general corrosion and high-temperature oxidation. However, these alloys harbor a critical vulnerability when exposed to specific environments during refinery shutdowns: Polythionic Acid Stress Corrosion Cracking (PTA-SCC).
To mitigate this risk during shutdowns, downtime, and equipment downtime, the Association for Materials Protection and Performance (AMPP) provides a critical standard: (formerly RP0170), titled "Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment."
The core document, NACE SP0170, must be integrated into broader integrity management systems. The following are key practical considerations for its implementation: nace sp0170 pdf
You cannot get a legal, current, and complete copy of NACE SP0170 for free. The document is copyrighted by (Association for Materials Protection and Performance), formed by the merger of NACE International and SSPC.
Identify all equipment containing austenitic stainless steels operating in the sensitization temperature range with sulfur present. Austenitic stainless steels are the backbone of the
NACE standards are copyrighted and generally require purchase. You can obtain the most recent version (reaffirmed in 2018) as a downloadable PDF through the following authorized platforms:
In some scenarios, maintaining a continuous purge of extremely dry air (with a dew point typically below -40°C/-40°F) is acceptable. Keeping the relative humidity near zero prevents the chemical reaction required to generate the acid. Material Selection as a Long-Term Solution To mitigate this risk during shutdowns, downtime, and
The NACE SP0170 standard provides structured methodologies to interrupt the PTA-SCC triangle, primarily focusing on neutralizing or isolating the corrosive environment during shutdowns. 1. Alkaline Washing (Soda Ash Solutions)
PTA SCC is a form of rapid intergranular cracking that occurs when sensitized austenitic stainless steels (those exposed to high service temperatures) are subjected to polythionic acids . These acids form when sulfide corrosion products (metal sulfides) react with oxygen and moisture
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For refinery professionals and engineers, (now maintained by AMPP ) is a critical standard practice for maintaining the integrity of stainless steel equipment during maintenance periods.