Industry professionals following SP0170 typically employ one or more of the following primary protection methods: Nitrogen Purging
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Austenitic stainless steels (like Types 304 and 316) and other austenitic alloys used in high-temperature, high-pressure refinery service are often in a condition called "sensitized." This means their grain boundaries are depleted of chromium and are therefore vulnerable to attack. When PTA attacks these sensitized grain boundaries, it can initiate . nace sp0170 pdf 2021
AMPP - The Association for Materials Protection and Performance Core Objectives of SP0170
During a , when this equipment is opened to the air, three elements combine: Sulfide Scale: Iron sulfides already present on the metal. Oxygen: Introduced from the air. Water/Moisture: Condensation from air or wash water. These three elements react to form polythionic acids ( H2SnO6cap H sub 2 cap S sub n cap O sub 6 Oxygen: Introduced from the air
The standard emphasizes understanding if the material is sensitized. If not sensitized, the risk of PTA SCC is much lower, and the aggressive mitigation methods may not be necessary.
To provide proven methods for protecting refinery equipment made of austenitic stainless steel from cracking caused by polythionic acid during a shutdown. If not sensitized, the risk of PTA SCC
This method focuses on environmental control. The standard defines as using a flow of an inert gas, specifically dry nitrogen , to remove oxygen from the equipment. By eliminating oxygen, the formation of polythionic acid is chemically blocked. However, the equipment must be maintained under a positive pressure of dry nitrogen, and the gas must indeed remain dry, as moisture is the other necessary component for acid formation.