Nace Sp0170 Pdf Jun 2026

) are formed on the metal surface during high-temperature service in a refinery environment.

By excluding oxygen, the transformation of iron sulfide scales into polythionic acid is completely halted.

In a refinery environment, equipment made from austenitic stainless steels (like 300-series alloys) often develops sulfide scales on internal surfaces during operation. When the unit is shut down and opened, these sulfides react with moisture and oxygen in the air to form . These acids can cause rapid, catastrophic cracking in sensitized metals—sometimes within hours of exposure. Key Mitigation Strategies in SP0170 nace sp0170 pdf

Air introduced into the system must be dehumidified to a very low dew point (typically below -18°C or 0°F).

When standard austenitic stainless steels (such as AISI 304 or 316) are exposed to temperatures between 425°C and 815°C (800°F to 1500°F) during service, they undergo a process called . At these elevated temperatures, carbon diffuses to the grain boundaries and combines with chromium to form chromium carbides ( ) are formed on the metal surface during

Austenitic stainless steels and other austenitic alloys (e.g., 300-series SS, Alloys 800/800H). The Threat: Polythionic Acid ( H2SnO6cap H sub 2 cap S sub n cap O sub 6 ) formed when sulfide scales ( FeScap F e cap S ) react with oxygen and water. Why is NACE SP0170 Crucial? (The PTA SCC Problem)

A positive pressure must be maintained throughout the turnaround to prevent air ingress. Personnel entry is impossible under this condition due to asphyxiation risks. 2. Alkaline Washing of Equipment Surfaces When the unit is shut down and opened,

: It emphasizes the importance of regular maintenance and monitoring of CP systems to ensure their continued effectiveness.

Ensure that any water used for rinsing or chemical blending contains less than 50 ppm chlorides to prevent secondary cracking modes.

The phenomenon requires three concurrent conditions to take place:

: Neutralizing any acidic species by washing internal surfaces with an alkaline solution, such as soda ash (sodium carbonate).