What is Polythionic Acid Stress Corrosion Cracking (PTA-SCC)?
This is where becomes the industry bible. If you’ve been searching for the NACE SP0170 PDF to clarify your preservation protocols, this post breaks down exactly what the standard covers, why it matters, and how to use it effectively. nace sp0170 pdf
To mitigate this catastrophic failure mechanism, NACE International (now merged into AMPP, the Association for Materials Protection and Performance) developed the standard practice , titled "Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment." What is Polythionic Acid Stress Corrosion Cracking (PTA-SCC)
If neither nitrogen blanketing nor liquid washing is feasible, keeping the environment completely dry will stop the reaction, as liquid water is a required catalyst for polythionic acid formation. The grain boundaries become highly localized zones of
Equipment is isolated, sealed, and continuously blanketed with high-purity dry nitrogen gas ( N2cap N sub 2
). This depletes the immediate adjacent areas of chromium, dropping the local chromium content below the 12% threshold required to maintain a passive oxide layer. The grain boundaries become highly localized zones of low corrosion resistance. 2. Environmental Trigger (Polythionic Acid Formation)