Austenitic SS

Chapter from the Metallurgy & Corrosion Design Guide.

AUSTENITIC STAINLESS STEEL PROTECTION

Common Austenitic SS Grades in Refining

Grade	UNS	Cr	Ni	Mo	Key Feature	Typical Use
304	S30400	18%	8%	—	Standard grade	General corrosion resistance
304L	S30403	18%	8%	—	Low carbon (<0.03%)	Welded service (no sensitization)
316	S31600	16%	10%	2%	Mo addition for pitting	Chloride-containing, naphthenic acid
316L	S31603	16%	10%	2%	Low carbon + Mo	Welded service with Cl/acid
321	S32100	17%	9%	—	Ti stabilized	High-temp service (heater tubes)
347	S34700	17%	9%	—	Nb/Ta stabilized	High-temp, heavy-wall vessels
317L	S31703	18%	11%	3%	High Mo	Severe naphthenic acid service

Failure Mechanisms for Austenitic SS

Mechanism	Cause	Temperature Range	Prevention	Inspection	NACE/API Ref
Chloride SCC	Cl⁻ + tensile stress + T >140°F	Above 140°F	Limit Cl exposure, use duplex SS	Dye penetrant, UT	NACE, Copson curves
Sensitization	Cr carbide precipitation at grain boundaries	800-1,500°F	Use L-grade or stabilized grade	Oxalic acid etch test (ASTM A262)	ASTM A262
Polythionic Acid SCC	H2S + O2 + moisture on sensitized SS	Ambient (shutdown!)	Soda ash wash before air exposure	Visual cracking after exposure	NACE SP0170
Sigma Phase	Phase transformation, embrittlement	1,000-1,700°F long-term	Limit time at temperature, proper grade	Impact testing, metallography	API 579
475°F Embrittlement	Spinodal decomposition (duplex SS)	600-1,000°F	Avoid sustained operation in range	Impact testing	ASTM A923
Pitting/Crevice	Cl⁻ attack at passive film breaks	Any	Higher Mo content (316L min), avoid crevices	Visual, UT pit gauge	ASTM G48

Polythionic Acid Cracking Prevention (Critical for Turnarounds)

Polythionic Acid (PTA) Cracking occurs when sensitized austenitic SS is exposed to sulfur compounds + oxygen + moisture at ambient temperature. This is a SHUTDOWN phenomenon — cracking occurs when equipment is opened to atmosphere.

Prevention During Turnaround:
1. Before opening equipment containing sensitized SS to atmosphere:
• Purge with dry nitrogen to displace process gas
• Apply soda ash (Na2CO3) wash — 2% solution minimum
• Maintain alkaline environment on all SS surfaces
2. Do NOT water wash with untreated water — this provides moisture without alkalinity
3. Keep SS equipment under N2 blanket if not washing immediately
4. If PTA cracking is found: 100% NDE of all sensitized SS welds in the affected system

NACE SP0170 (formerly RP0170) is the primary reference for this topic.

Source: Metallurgy_Corrosion_Design_Guide_v1.xlsx · sheet “Austenitic SS”

© 2026 Inflection Point Engineering, LLC. All rights reserved. The content of this page — including calculation methods, reference data, written analysis, interactive tools, and source code — is the intellectual property of Inflection Point Engineering, LLC and is protected under applicable copyright, trademark, and trade secret laws. Unauthorized reproduction, redistribution, modification, or derivative use in whole or in part is prohibited without prior written consent.

Disclaimer. This material is provided for informational and educational purposes only and does not constitute professional engineering advice. Calculations, reference data, and methodologies are based on published standards and accepted engineering practice but are not a substitute for engineering judgment, site-specific analysis, or review by a licensed Professional Engineer. Inflection Point Engineering, LLC makes no warranties, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of any content presented here, and shall not be liable for any direct, indirect, incidental, or consequential damages arising from its use. Users assume all risk associated with applying this content to real-world design, operations, or decisions.