317L vs 316L Stainless Steel: Molybdenum, PREN and Acid Service

Written by: Emma, Technical Sales Engineer  |  Reviewed by: Ethan, Materials Engineer  |  Updated: August 2026

316L and 317L are both molybdenum-bearing austenitic stainless steels, and they are frequently compared when a chloride-bearing or chemical environment demands more than plain 304L. The essential difference is chemistry: 317L carries higher chromium, nickel, and molybdenum than 316L. That extra alloying translates into generally improved resistance to localized chloride attack and, in selected environments, better performance in acidic or chemical service. But 317L is not simply “316L, but better in everything.” This article explains where the real differences are — molybdenum, PREN, and acid service — and how to decide when upgrading from 316L to 317L is actually justified.

1. What Is 317L Stainless Steel?

317L stainless steel (UNS S31703) is a low-carbon, molybdenum-bearing austenitic stainless steel. It sits in the same family as 316L but with higher chromium, nickel, and molybdenum contents. The “L” again denotes low carbon, which helps control sensitization during welding. 317L is typically considered when an application needs more localized-corrosion resistance or better performance in certain chemical media than 316L provides, while still remaining within the practical cost and availability range of standard austenitic grades.

2. 317L vs 316L: Key Chemistry Difference

The defining difference is alloy content. Relative to 316L, 317L has higher chromium, higher nickel, and — most importantly for corrosion — higher molybdenum. The exact ranges are defined by the applicable ASTM product specification, so precise values should always be confirmed against the standard and its edition rather than assumed from memory. What matters for material selection is the direction of the difference: the higher molybdenum, supported by higher chromium, shifts 317L toward better resistance to localized corrosion.

The “higher” entries in this table are qualitative and should be understood against the actual chemistry ranges in the applicable standard — not as fixed, guaranteed PREN numbers.

3. Why Molybdenum Matters

Molybdenum (Mo) is the element that most distinguishes 316/317 from plain 304. Its main contribution is to resistance against localized corrosion — specifically chloride pitting and crevice corrosion — by helping the passive film resist breakdown and supporting repassivation after local damage. It also contributes to improved performance in certain reducing chemical media.

It is important not to over-generalize. More molybdenum does not mean “better against every acid.” Corrosion behavior depends on the specific acid, its concentration, temperature, the presence of chlorides or other contaminants, and whether conditions are oxidizing or reducing. Molybdenum is a meaningful upgrade for many chloride and chemical environments, but it is not a universal corrosion-resistance multiplier.

4. PREN and Chloride Corrosion Resistance

PREN (Pitting Resistance Equivalent Number) is an empirical indicator used to compare the relative resistance of stainless steels to pitting and crevice corrosion in chloride environments. Common formulations are based on chromium, molybdenum, and nitrogen content — for example, Cr + 3.3 Mo + 16 N — although different sources may use slightly different coefficients or include other elements. Because 317L has higher chromium and molybdenum than 316L, its calculated PREN is generally higher, which is a shorthand way of saying it offers better chloride pitting resistance.

PREN has real limits. It is a comparative, ranking-style tool for localized chloride corrosion within comparable stainless-steel families — not a complete corrosion score, not a predictor of general or uniform acid corrosion, not a measure of stress corrosion cracking resistance, and not a service-life estimate. A higher PREN generally indicates greater resistance to localized chloride attack, but it does not establish a universal safe operating limit, and it cannot be used to conclude that a specific environment is automatically safe.

5. 317L in Acid and Chemical Service

“Acid service” is not one environment. Whether 316L or 317L is suitable depends on the acid type, concentration, temperature, chloride contamination, oxidizing or reducing conditions, flow, and exposure time. For this reason, there are no fixed “316L handles up to X% acid, 317L handles up to Y%” boundaries that hold across all conditions.

317L is often considered in chemical processing, process equipment, and chemical-handling applications where its higher molybdenum offers a useful margin over 316L. It is particularly relevant where localized corrosion or specific reducing media are the concern. However, 317L is not “acid-proof,” and it should not be assumed to replace dedicated high-alloy materials — such as 904L, 6% Mo super austenitic grades, or nickel-based alloys — in strongly aggressive acid service. The right material depends on the actual chemistry and conditions, not on a grade-name assumption.

6. 317L vs 316L: When Is the Upgrade Justified?

The practical question is usually about cost and margin. A useful starting logic is:

• For general process service or ordinary chloride-containing environments, 316L often remains a practical and cost-effective choice.
• When the localized-corrosion margin is thin, when higher molybdenum is needed, or when a specific chemical environment is more demanding, 317L becomes worth considering.
• When conditions already exceed what 317L can reasonably handle, the answer is not to keep climbing through the 316/317 series — it is to re-evaluate higher-alloy alternatives.

The final decision must reflect the actual medium, concentration, temperature, chloride level, design life, and fabrication condition, rather than a single “317L is better” rule.

7. Temperature, Welding and Fabrication Considerations

Both 316L and 317L are austenitic stainless steels and are weldable with conventional fabrication practice. The low-carbon chemistry of both grades helps control sensitization during welding. As with all stainless steel, the post-weld surface condition still matters for corrosion performance, so heat tint and contamination should be handled appropriately for the service.

For temperature, the two grades behave similarly in broad terms as austenitic stainless steels, and elevated-temperature strength or creep performance is not the primary reason to choose 317L over 316L. The decision to upgrade is usually driven by corrosion — localized chloride attack or specific chemical media — rather than by a temperature advantage. Any elevated-temperature or code-governed application should be evaluated on its own design conditions, not by extrapolating from a corrosion comparison.

Welding should follow a qualified welding procedure specification (WPS) and procedure qualification record (PQR), with attention to heat input, interpass conditions, filler metal compatibility, and post-weld cleaning. Filler metal selection — for example, whether a 316L filler is suitable with 317L base metal — should not be reduced to a single “must use this type” rule; it depends on the welding method, base material, design requirements, and the applicable AWS or ASME code. There are no WPS-independent fixed parameters.

8. ASTM Standards and How to Specify 317L

317L is supplied under product-form-specific ASTM specifications, and the applicable standard follows the product form. For example, ASTM A240 covers plate, sheet, and strip; ASTM A312 covers pipe; and ASTM A213 covers seamless boiler and heat-exchanger tube — each is a different standard, and none of them covers every 317L product form. Confirm the grade, UNS (S31703), product form, applicable ASTM specification, and its edition together on the purchase order. Where ASME code requirements apply, they belong to the design or pressure-equipment context, not to a general commercial datasheet.

Example purchase specification (illustrative only, not ASTM standard text):

“317L stainless steel plate, UNS S31703, ASTM A240/A240M, [thickness × width × length], specified finish and annealed condition, EN 10204 Type 3.1 MTC, heat-number traceability.”

This is a purchasing template, not a standard requirement. The actual grade, dimensions, finish, testing, and acceptance requirements must be confirmed against the project specification.

9. Common Selection and Purchasing Mistakes

• Treating 317L as a complete, all-around upgrade of 316L.
• Assuming 317L is better than 316L in every acid or chemical environment.
• Equating molybdenum content with overall corrosion resistance.
• Using PREN as a comprehensive corrosion score or service-life predictor.
• Using PREN to infer acid corrosion performance.
• Assuming 317L can replace 904L, 6% Mo, or nickel alloys in all aggressive chemical service.
• Applying one ASTM standard (A240, A312, or A213) to the wrong product form.
• Omitting the ASTM specification edition, finish, or delivery condition from the order.

10. FAQ

Q1: What is 317L stainless steel?
317L (UNS S31703) is a low-carbon, molybdenum-bearing austenitic stainless steel with higher chromium, nickel, and molybdenum than 316L.

Q2: What is the main difference between 316L and 317L?
The key difference is chemistry — 317L has higher chromium, nickel, and especially molybdenum, which generally improves localized-corrosion resistance.

Q3: Why does molybdenum matter?
Molybdenum helps the passive film resist breakdown and supports repassivation, improving resistance to chloride pitting and crevice corrosion and some reducing chemical media.

Q4: What is PREN?
PREN is an empirical indicator, typically based on chromium, molybdenum, and nitrogen, used to compare relative resistance to pitting and crevice corrosion in chlorides.

Q5: Does a higher PREN guarantee safe service?
No. PREN is a comparative ranking tool, not a corrosion-rate or lifetime prediction, and it says little about general acid corrosion or stress corrosion cracking.

Q6: Is 317L better than 316L in all acids?
No. Acid performance depends on acid type, concentration, temperature, chlorides, and oxidizing/reducing conditions. 317L helps in selected environments but is not universally superior.

Q7: When should I upgrade from 316L to 317L?
When the localized-corrosion margin is insufficient, when higher molybdenum is needed, or when a specific chemical environment is more demanding — not as a blanket default.

Q8: Can 317L replace 904L or nickel alloys?
No. In strongly aggressive acid or chemical service, higher-alloy materials such as 904L, 6% Mo grades, or nickel alloys may be required. 317L is not acid-proof.

Q9: Which ASTM standard applies to 317L?
The applicable standard depends on product form — for example, A240 for plate/sheet/strip, A312 for pipe, or A213 for tube. Confirm the correct standard and edition.

Q10: What should an RFQ for 317L include?
Grade/UNS (S31703), applicable ASTM specification and edition, product form, dimensions, finish, delivery condition, MTC, and heat-number traceability.