Polythionic Acid SCC and Sulphur Service: Why Sensitised Stainless Steel Cracks During Shutdown
Polythionic acid stress corrosion cracking (PTA-SCC) is a failure mechanism that is uniquely dangerous because it occurs not during normal plant operation, but during shutdown, inspection, or maintenance — when the piping is depressured and exposed to air and moisture. It affects sensitised austenitic stainless steel (316, 321, 304 — not 316L or stabilised grades) in refinery service where sulphur compounds are present. The combination of a sensitised microstructure, atmospheric moisture (which forms polythionic acid from sulphide scale), and residual stress from welding or cold working creates rapid intergranular cracking that can fracture a fitting wall without warning.
The Sensitisation Mechanism
Sensitisation occurs when austenitic stainless steel is exposed to temperatures in the range 425–870°C (the sensitisation range) for sufficient time — typically minutes to hours depending on temperature and carbon content. In this range, carbon diffuses to grain boundaries and precipitates as chromium carbide (Cr₂₃C₆), depleting the grain boundary region of chromium below 12% — the minimum for passive film stability. The sensitised structure is not immediately obvious and does not affect ambient-temperature mechanical properties, but it makes the steel highly vulnerable to intergranular corrosion in specific environments including polythionic acid.
Standard 316 stainless (C ≤0.070%) sensitises readily during welding — the HAZ spends time in the sensitisation range during cooling. Low-carbon 316L (C ≤0.030%) sensitises at a much lower rate; for short-term welding exposure, 316L is considered "sensitisation-resistant." Stabilised grades (321 with Ti, 347 with Nb) form stable titanium/niobium carbides preferentially, preventing chromium depletion — they are immune to sensitisation by definition.
When Polythionic Acid Forms
During refinery operation, sulphur compounds deposit as sulphide scale (FeS, NiS) on the internal fitting surfaces. This scale is stable and harmless during normal hot operation. On shutdown, when the fitting is cooled and exposed to air and moisture: O₂ + H₂O + sulphide scale → polythionic acids (H₂SₓO₆, where x = 3–5). Polythionic acid concentration is highest in enclosed spaces — under insulation, behind pipe supports, and inside fitting crevices — where moisture accumulates and the acid cannot dilute or flush away.
Prevention — Material Selection
- Specify 321 or 347 for refinery hydrocarbon service with sulphur: Ti- and Nb-stabilised grades are the recommended materials for reactor piping, hydrotreater internals, and reformer piping where sulphur is present and operating temperatures pass through the sensitisation range
- 316L is acceptable for short-term exposure: For piping that is never exposed above 425°C in service, 316L (low carbon) resists sensitisation adequately — but not for piping that cycles into the sensitisation range
- Standard 316 is not acceptable in sulphur service where the piping operates in or through the sensitisation range
Prevention — Operational Controls
Where sensitised material cannot be avoided (existing installations), PTA-SCC can be prevented during shutdown by: (1) purging the piping with dry nitrogen before cooldown — eliminating oxygen prevents polythionic acid formation; (2) washing with a 2% soda ash (Na₂CO₃) solution immediately on opening the system — alkaline washing neutralises any polythionic acid that has formed; (3) keeping the piping dry and sealed until inspection is complete. API RP 582 and NACE SP0170 provide detailed shutdown procedures for sensitised stainless steel in sulphur service. These procedures should be documented on the piping system QA plan.