Nace Sp0170 Pdf ✦ Instant
Polythionic acids ( H2SxO6cap H sub 2 cap S sub x cap O sub 6
The industry standard is a 1% to 2% solution of sodium carbonate ( Na2CO3cap N a sub 2 cap C cap O sub 3 ), commonly known as soda ash, dissolved in fresh water.
When refinery equipment is shut down, the materials inside are exposed to air and moisture, which can trigger a highly destructive phenomenon: Polythionic Acid Stress Corrosion Cracking (PTA SCC). (now updated as NACE SP0170-2018) is the industry-standard practice designed to mitigate this risk.
While NACE SP0170 focuses heavily on operational shutdown procedures, long-term mitigation of PTA-SCC begins during the engineering and design phase.
| Mitigation Strategy | Description | How it Works | | :--- | :--- | :--- | | | Choosing materials resistant to sensitization and implementing appropriate fabrication practices. | An assessment of the risk of PTA SCC is conducted. If the risk is judged acceptable, other mitigation methods may not be required. | | 2. Nitrogen Purging | Flowing an inert gas, specifically dry nitrogen, through the equipment. | This technique excludes oxygen, one of the key components needed for PTA formation, thus preventing the acid from ever developing. | | 3. Alkaline Washing | Washing the equipment with a soda ash (sodium carbonate) or other alkaline solution. | The alkaline solution neutralizes any PTA that may have already formed, rendering it harmless to the metal. | | 4. Dehumidification | Using dehumidified air to maintain a very low relative humidity within the equipment. | This method removes moisture, another critical element for PTA formation, thereby preventing the reaction from occurring. | nace sp0170 pdf
When equipment must be opened for inspection or maintenance, internal sulfide scales must be neutralized before air exposure. NACE SP0170 provides detailed guidelines for alkaline washing:
Formerly known as , the full title of this standard is: “Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment.”
Back in her office, Maya spread the PDF across two monitors. Between legalese and appendices she scribbled notes. The SP0170 procedures were indispensable: they provided repeatability and defensibility, the language auditors would accept. But as an engineer she needed a bridge between the routine and the rare. She sketched a complementary checklist — "human factors" — that layered onto the standard: check seals near dissimilar metal joints, map microtopography for moisture traps, interview maintenance crews about odd noises and smells. She mapped sensor blind spots and proposed mobile probes for transient conditions. To justify the changes she referenced the standard's own guidance on risk assessment and supplemental monitoring; the clauses were elastic enough to allow thoughtful extension.
: Keeping oxygen out by maintaining a positive pressure of dry nitrogen within the equipment. Polythionic acids ( H2SxO6cap H sub 2 cap
NACE SP0170 PDF: A Comprehensive Guide to Protecting Austenitic Stainless Steel from Polythionic Acid SCC
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Austenitic stainless steels are highly valued in the refining, petrochemical, and chemical processing industries for their exceptional corrosion resistance and high-temperature strength. However, these alloys face a severe and rapid degradation mechanism during maintenance shutdowns: Polythionic Acid Stress Corrosion Cracking (PTA-SCC). To combat this specific threat, the Association for Materials Protection and Performance (AMPP)—formerly NACE International—published the standard practice , titled "Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment."
When looking for a , engineering teams must ensure they are sourcing the official, most up-to-date revision from Authorized Standard Developers like the Association for Materials Protection and Performance (AMPP). While NACE SP0170 focuses heavily on operational shutdown
What is Polythionic Acid Stress Corrosion Cracking (PTA-SCC)?
Certain process units are more susceptible to PTA SCC and thus require strict adherence to NACE SP0170. These are typically units in the 'sulfur train' of a refinery, including:
NACE standards are copyrighted and generally require purchase. You can obtain the most recent version (reaffirmed in 2018) as a downloadable PDF through the following authorized platforms:
