NACE SP0170 provides critical guidelines for protecting austenitic stainless steels and alloys from polythionic acid stress corrosion cracking (PTA SCC) during refinery equipment shutdowns, focusing on preventing material degradation. Mitigation strategies, including dry nitrogen purging and alkaline washing, neutralize sulfide corrosion products to maintain equipment integrity. For more details, visit ANSI Webstore .
NACE SP0170 PDF: A Comprehensive Guide to Protecting Austenitic Stainless Steel from Polythionic Acid SCC
To interrupt this destructive chemical triad (Sulfide Scale + Oxygen + Liquid Water), NACE SP0170 provides three primary defensive strategies. Refineries pick their method depending on whether the equipment needs to be physically opened. 1. Nitrogen Purging (Excluding Oxygen)
Because nitrogen blanketing creates an immediate asphyxiation hazard, strict confined space entry permits and continuous atmospheric monitoring are mandatory whenever NACE SP0170 nitrogen procedures are active. Conclusion nace sp0170 pdf
When austenitic stainless steels are exposed to high operating temperatures—typically between 425°C and 815°C (800°F to 1500°F)—chromium carbides precipitate along the grain boundaries. This depletes the adjacent areas of chromium, leaving the grain boundaries vulnerable to localized corrosion. This state is known as "sensitization."
: It outlines the best practices for the installation of CP systems to ensure their effectiveness and longevity.
You can buy the latest NACE SP0170-2018 PDF directly from the AMPP Store or ANSI Webstore . NACE SP0170 PDF: A Comprehensive Guide to Protecting
The standard outlines several strategies to prevent the formation of polythionic acid or protect the metal surface during shutdowns:
If the equipment does not need to be opened for internal inspection or maintenance, the simplest and most effective strategy is to exclude oxygen completely.
Desulfurizing, hydrocracking, hydrotreating, FCCUs, and crude/lube distillation units Polythionic Acid Stress Corrosion Cracking (PTA SCC) Where to Purchase/Download ANSI Webstore , AMPP Store, or authorized standards vendors. Understanding the Threat: What is PTA SCC? leading to SCC.
This complex acid can then attack "sensitized" austenitic stainless steel—material that has been rendered susceptible to intergranular corrosion due to thermal effects from welding or high-temperature service—while the metal is under tensile stress, leading to SCC. The cracks typically appear as a network of fine, branching lines along the grain boundaries.
NACE SP0170 is a globally recognized standard practice that outlines the operational procedures required to prevent PTA-SCC in austenitic stainless steels (such as Grades 304, 304H, 316, 316H, 321, and 347) and specific nickel-base alloys during refinery shutdowns.