These acids, combined with residual tensile stress in the austenitic stainless steel (like 300-series stainless), cause rapid crack propagation. Key Mitigation Methods in NACE SP0170-2021
By 2021, the need for a cohesive update was clear. The industry had moved toward higher-strength materials, deeper wells, and more extreme pressure/temperature (P/T) regimes. The was released to address these gaps, clarify ambiguities, and harmonize with updated ISO standards.
: The 2021 context of this story involves sophisticated "shields." The standard outlines three primary defenses: Soda-Ash Wash : Neutralizing the acid before it can bite.
High residual tensile stresses from structural welding; prone to localized sensitization.
This creates an immediate asphyxiation hazard; therefore, rigid tagging and isolation protocols must be followed. 3. Temperature Control (Dew Point Prevention) nace sp0170 pdf 2021
: Preventing the formation of liquid water by maintaining a dry air environment.
Soda ash washing and dry purging are the primary mitigation techniques described.
A positive pressure must be maintained continuously throughout the shutdown to prevent ambient air from leaking inward.
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The standard provides specific mitigation methods to protect equipment—such as heaters, tubes, and reactors—during the downtime, shutdown, and startup phases of refinery operations. PTA SCC occurs when sulfide corrosion products react with oxygen and water to form polythionic acids, which can rapidly cause cracking in sensitized austenitic alloys.
: To prevent the formation of polythionic acids ( H2SxO6cap H sub 2 cap S sub x cap O sub 6
) and stop them from triggering intergranular stress corrosion cracking (IGSCC) in sensitized alloys.
Indirect Inspection
To lower this risk, engineers often utilize stabilized grades like (stabilized with titanium) or Grade 347 (stabilized with niobium), which undergo specialized Stabilization Post-Weld Heat Treatment (PWHT) to tie up free carbon. However, even stabilized alloys can exhibit vulnerability if operations run continuously at extreme temperatures, making the shutdown procedures in NACE SP0170 critical regardless of the exact austenitic alloy variant used. Key Shutdown Mitigation Methods
NACE SP0170-2018 remains the active standard for protecting austenitic stainless steels from polythionic acid stress corrosion cracking (PTA SCC) during refinery shutdowns as of 2021, focusing on nitrogen purging and alkaline washing to mitigate risks. The standard, now managed by AMPP, outlines critical procedures for preventing corrosion in sensitized austenitic alloys common in hydrotreating and cracking units. Access the standard through the ANSI Webstore .
NACE SP0170 (2021) serves as a critical industry standard for protecting austenitic stainless steel equipment from polythionic acid stress corrosion cracking (PTA SCC) during refinery shutdowns. The practice outlines essential mitigation techniques, including nitrogen blanketing and alkaline washing, to prevent failures in sensitized stainless steel. While foundational for safety, modern discussions analyze its application alongside advanced, resistant materials. For further technical details, visit GlobalSpec .