Nace Sp0170 Pdf [new] Link
This method is a specialized form of purging that uses dry air to prevent the formation of liquid water. While seemingly contradictory to nitrogen purging, which uses an inert gas, dry air purging focuses on the second critical condition for SCC: the presence of a liquid water phase. This method relies on maintaining the relative humidity within the equipment below a certain threshold so that free water cannot condense on the metal surfaces. This denies the electrolyte necessary for the electrochemical corrosion reaction to proceed .
During normal high-temperature operation in sulfur-bearing environments, iron and chromium sulfides form on the inside of the equipment. When the unit shuts down and is opened, oxygen and moisture react with these metal sulfides to form polythionic acids ( H2SxO6cap H sub 2 cap S sub x cap O sub 6
Maintaining a dew point at least 22°C (40°F) lower than the metal temperature to prevent the formation of liquid water. nace sp0170 pdf
The committee wanted examples, not only procedures. The submission went through iterations — peer reviews, redlines, footnotes. Some resisted, arguing standards must be conservative and rigid. Others saw the same pattern Maya had: standards worked best when complemented by informed judgment.
Here is a summary of each approach and how it works: This method is a specialized form of purging
Understanding NACE SP0170: Protection of Austenitic Stainless Steels from Polythionic Acid Stress Corrosion Cracking
“Don’t clamp it,” Marina said, opening the PDF again. “I’m looking at SP0170 right now.” The committee wanted examples, not only procedures
Certain process units are more susceptible to PTA SCC and thus require strict adherence to NACE SP0170. These are typically units in the 'sulfur train' of a refinery, including:
The NACE SP0170 standard remains an indispensable tool for the refining industry. Whether you are planning a major turnaround, developing shutdown procedures, or investigating a past corrosion event, this document provides the definitive methodology for protecting austenitic stainless steel equipment from polythionic acid stress corrosion cracking.
Imagine a refinery hydrotreater. During operation, the stainless steel components are covered in iron sulfide scale. When the unit is shut down for maintenance, the temperature drops, and air (oxygen) enters. Moisture from the air combines with the sulfide scale to form polythionic acid.
