Polythionic Acid Stress Corrosion Cracking in Sensitised Stainless Steel: Shutdown Risk and Weld Stabilisation
Polythionic acid stress corrosion cracking (PTA SCC) is a specific failure mode affecting sensitised austenitic stainless steel pipe fittings and piping in refinery service — particularly in hydrotreaters, hydrodesulfurisation units, and crude distillation overhead systems. The cracking occurs not during normal operation but during shutdowns and startups, when air and moisture contact a sensitised stainless surface that carries sulfur deposits from the process stream.
The PTA SCC Mechanism
The mechanism proceeds in three steps: sensitisation — during high-temperature service above approximately 425°C (or cycling through the sensitisation range 550–850°C during thermal transients), chromium carbides (M₂₃C₆) precipitate at grain boundaries in standard 304 and 316 stainless, depleting Cr below ~12% in a narrow zone adjacent to each grain boundary. The fitting remains structurally sound during high-temperature service because polythionic acid has not yet formed; sulfur deposit formation — H₂S, elemental S, and iron sulfides from the process stream deposit on the sensitised surface, stable at high temperature and not corrosive in the absence of water; and polythionic acid formation at shutdown — moisture condenses on the sulfur-bearing surface when the system cools and is opened to air. Sulfur reacts with moisture and oxygen to form polythionic acids (H₂SₓO₆, x = 2–5) that attack Cr-depleted grain boundaries, causing intergranular SCC in the presence of residual or applied tensile stress. Cracking can propagate through wall thickness in hours to days during a maintenance shutdown.
Prevention: Nitrogen Blanket and Soda Ash Wash
NACE RP0170 recommends: nitrogen blanket during cooldown — after process shutdown and before the system temperature drops below approximately 150°C, the system is isolated and purged with dry nitrogen, excluding oxygen and moisture and preventing polythionic acid formation even if sulfur deposits are present. This is the most effective preventive measure; and soda ash wash — if the system must be opened and air exposure is unavoidable, washing with 1–2% Na₂CO₃ solution neutralises the polythionic acids and raises the pH above the SCC threshold. Applied before opening the system to air where possible, or immediately after opening.
Stabilised Grades: 321 and 347
The metallurgical solution is stabilised austenitic grades: type 321 (Ti-stabilised) — TiC forms preferentially over M₂₃C₆, leaving grain boundaries Cr-enriched even after extended service in the sensitisation range; type 347 (Nb-stabilised) — NbC forms preferentially. Slightly better high-temperature strength than 321; preferred for demanding applications such as high-temperature overhead piping. A stabilisation anneal at 870–900°C after solution annealing ensures all free carbon is bound as TiC or NbC before the fitting enters service. Weld filler for 321 base metal should be 347 (ER347), not 321 — 321 weld metal has lower Nb/C content and is less stable against HAZ sensitisation. Post-weld stabilisation anneal at 870°C re-precipitates TiC/NbC for critical welds.