: Special considerations for refinery reactors.
When a processing unit is running normally, the environment inside the equipment is typically oxygen-free. However, when the unit is shut down for inspection, cleaning, or repairs, the introduction of air (oxygen) and moisture can trigger the formation of polythionic acids. NACE SP0170 provides the exact chemical, mechanical, and procedural steps to keep these assets safe during these critical windows. The Science of Polythionic Acid Cracking (PTA-SCC)
Convert the standard into a one-page checklist for operators. Include hold points for pH testing and visual drying inspection.
We strongly advise against it. Using an obsolete, corrupted, or unauthorized copy could lead to a catastrophic failure if a critical step is missing. Purchase the official nace sp0170 pdf from AMPP or an authorized reseller. nace sp0170 pdf
When austenitic stainless steels (such as 304, 304H, 316, or 316H) are exposed to temperatures between 425°C and 815°C (800°F to 1500°F) during operation, chromium carbides precipitate along the grain boundaries. This process, known as sensitization , leaves the zones immediately adjacent to the grain boundaries depleted of chromium, dropping below the 12% threshold required for passivation.
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By implementing the practices found in the , refiners can effectively protect their high-value equipment from sudden failure during maintenance shutdowns. A summary of specific alkaline solutions used in industry. The difference between SP0170 and other NACE standards. Guidelines on monitoring for sulfide scales. Let me know how you'd like to proceed. Share public link : Special considerations for refinery reactors
A common point of confusion for engineers is the difference between SP0170 and the more famous (ISO 15156).
Metal Sulfides+Oxygen (O2)+Liquid Water (H2O)⟶Polythionic Acid (H2SxO6)Metal Sulfides plus Oxygen open paren cap O sub 2 close paren plus Liquid Water open paren cap H sub 2 cap O close paren ⟶ Polythionic Acid open paren cap H sub 2 cap S sub x cap O sub 6 close paren
By eliminating liquid water, sulfide scales cannot hydrolyze into polythionic acid. Material Selection Considerations Under SP0170 NACE SP0170 provides the exact chemical, mechanical, and
If you'd like a real summary or key takeaways from NACE SP0170 (now often called ), let me know—I can explain what it covers and why it matters in plain language.
Begin nitrogen purging before the metal temperature drops below the dew point to prevent initial condensation. Environmental Exclusion
NACE SP0170 outlines procedures to protect austenitic stainless steels from polythionic acid stress corrosion cracking (PTA SCC) during refinery equipment shutdowns, downtimes, and startups. Key mitigation methods include dry nitrogen purging, alkaline washing, and dry air purging to prevent the formation of acids that cause rapid cracking. For the full standard, visit
Commercial desiccant dehumidifiers circulate dry air through the equipment to maintain a relative humidity (RH) well below 50% (often targeted below 30%).