The 2018 version of the standard (NACE SP0170-2018) is the most recent and supersedes all previous versions, including the 2012 edition . The current version has a page count of 16 pages .
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 nace sp0170 pdf
When refineries process crude oil containing sulfur compounds, iron sulfide (FeS) scale deposits form on equipment surfaces. During a shutdown, if this iron sulfide is exposed to air (oxygen) and moisture (water), it reacts to form polythionic acid (H₂SₓO₆). This acid can then rapidly attack the grain boundaries of sensitized austenitic stainless steels, leading to catastrophic intergranular stress corrosion cracking. The risk is especially high in components that have been subjected to welding or high temperatures, making them sensitized to this specific form of attack. The 2018 version of the standard (NACE SP0170-2018)
The most common practice recommended in NACE SP0170 is washing the internal surfaces of the equipment with an alkaline solution. Keeping the relative humidity near zero prevents the
Equipment is isolated, sealed, and continuously blanketed with high-purity dry nitrogen gas ( N2cap N sub 2
NACE SP0170 (formerly designated before the society renamed its “Recommended Practices” to “Standards Practices”) is titled: “Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking (SCC) During Shutdowns and Start-ups.”