6Mo Austenitic Stainless Steel: UNS S31254 vs UNS N08367 Selection

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

UNS S31254 (254 SMO) and UNS N08367 (AL-6XN) are both 6Mo super-austenitic stainless steels with excellent pitting, crevice, and chloride stress-corrosion-cracking resistance. Their performance is very close, which raises a natural question for buyers: if both are “6Mo,” why compare them at all, and what actually decides the choice? This article answers that question across chemistry, PREN, corrosion behavior, and procurement.

1. Direct Answer: S31254 or N08367?

Both grades belong to the 6Mo super-austenitic family and offer high chloride resistance, so neither is “comprehensively better.” N08367 generally carries higher nickel and a higher molybdenum range, which can give it additional stability and potentially higher localized-corrosion resistance, while S31254 is the more widely established, broadly available 6Mo choice. The decision usually comes down to the specific chemistry required, the governing product standard, availability, and whether a particular qualification already exists.

2. Quick Comparison

The table below shows typical ranges. These are representative values, not fixed figures for every product, and they must be confirmed against the applicable standard and the actual material test certificate. The key differences are nickel and the upper end of the molybdenum range.

3. Why Both Are Called 6Mo Stainless Steel

The 6Mo designation refers to a molybdenum content of roughly 6%, which is the heart of these alloys’ localized-corrosion resistance. Chromium forms the passive film, molybdenum and nitrogen resist pitting and crevice corrosion, and nickel stabilizes the austenitic structure. Together these elements produce a grade far beyond 316L — a super-austenitic stainless steel rather than a simple upgrade.

The shared 6Mo name is useful shorthand, but it hides the differences that matter: the exact chromium, nickel, molybdenum, and nitrogen balance differs between S31254 and N08367, and those differences influence both corrosion behavior and the standards under which each grade is supplied.

The term “super-austenitic” distinguishes these grades from standard austenitic stainless steels like 304 or 316. The higher chromium, molybdenum, nitrogen, and nickel push pitting and crevice resistance well into the range where the alloy competes with nickel-base materials, while still retaining the weldability and fabrication behavior of a stainless steel.

4. PREN: Similar Number, Different Alloy Design

PREN combines the key alloying elements into one ranking value:

PREN ≈ %Cr + 3.3(%Mo) + 16(%N)

The two grades’ PREN ranges are close — roughly 42–45 for S31254 and roughly 43–49 for N08367 — but close PREN does not mean the materials behave identically everywhere. PREN is only a preliminary comparison; real performance also depends on temperature, chloride concentration, surface condition, crevices, fabrication, and microstructure, none of which the formula captures.

The limitations of PREN are worth repeating: it does not address stress corrosion cracking, it ignores crevice geometry and surface condition, and it cannot reflect the effects of welding or residual stress. Two alloys with similar PREN can still differ in a hot, stressed, creviced component, so PREN should never be the sole basis of a decision.

5. Chloride and Localized Corrosion Performance

For pitting, crevice corrosion, and chloride SCC, both grades perform at the top of the austenitic range. N08367’s higher nickel and its higher molybdenum and nitrogen range can provide additional stability and corrosion resistance in demanding service, but this is an incremental difference, not a claim of immunity. Any critical pitting temperature or similar data applies only to the specific test conditions under which it was measured.

The practical point is that both alloys handle severe chloride service far better than 316L or 904L, and the choice between them often hinges on factors other than a small PREN gap — such as which grade is already qualified, approved, and readily available for the product form required.

Nickel plays a specific role in chloride SCC: higher nickel generally improves resistance to chloride stress corrosion cracking. N08367’s higher nickel content is therefore relevant not only to austenitic stability but also to SCC behavior, which is one reason the two grades are not interchangeable despite their similar PREN. This is an incremental advantage, not an immunity.

6. Where S31254 Makes Sense

S31254 is a mature, widely used 6Mo grade with an established supply chain across plate, sheet, tube, and fittings. It is a sound choice for seawater, desalination, marine equipment, heat exchangers, chemical processing, and chloride-rich process streams — environments where its already-high performance margin is more than adequate.

The maturity of S31254 matters commercially. Decades of use mean broad mill and stockist availability, established product standards, and a large body of fabricator experience. For many severe chloride applications, S31254 already offers more performance margin than the service requires, which makes it the economical default.

7. Where N08367 May Be Preferred

N08367 may be preferred in severe chloride service, demanding chemical process environments, or where its higher nickel and molybdenum balance is considered desirable. It is also chosen where a project specification, approved material list, or qualification already calls for N08367 specifically. It is important not to translate “higher nickel” into “better in every environment” — the advantage is application-specific.

The higher nickel in N08367 also contributes to austenitic stability, which can be relevant where phase stability or SCC resistance is a concern. That said, the decision to specify N08367 is usually driven by a specific project requirement or qualification rather than by a general belief that more nickel is always better.

In practice, many specifications name N08367 explicitly, and in those cases the decision is already made by the project. Where both are acceptable, the comparison should then turn to availability, product form, and delivered cost rather than chemistry alone.

8. Cost, Availability and Procurement

Beyond chemistry, procurement reality often decides the choice. Raw material cost, availability, product form, applicable standards, fabrication, welding, MTC and traceability, and lifecycle cost all matter. In practice, availability, the approved material list, and the applicable product standard frequently outweigh a small PREN difference — a grade that is readily available and already qualified may be the better commercial choice even if the other shows a marginally higher PREN.

Product standards also vary by form. Plate, sheet, strip, bar, tube, and pipe fall under different specifications, and a given grade may be more readily available in one form than another. Before committing, confirm that the required product form and standard are actually obtainable for the chosen grade, since this can settle the choice faster than any chemistry comparison.

9. How to Choose Between S31254 and N08367

• Need proven 6Mo chloride resistance with established supply → consider S31254.
• Need higher-alloy 6Mo chemistry or a specific N08367 qualification → consider N08367.
• Severe chloride with elevated temperature → compare actual corrosion data.
• Combined acid-plus-chloride service → evaluate the full chemistry and process conditions.
• Extreme conditions → consider super duplex or nickel-base alloys.

These are decision starting points, not a fixed priority ranking. They direct attention to the real service and the governing requirements rather than to a single metric.

To use these guidelines, gather the actual data first: chloride concentration, temperature, pH, flow, crevices, stress, and any acid component. With that information, the governing mechanism becomes clear, and the choice between S31254 and N08367 — or the step up to a higher alloy — can be made on evidence.

10. How to Specify in an RFQ

An RFQ should state at minimum: the UNS grade, product standard, product form, dimensions, surface condition, chloride concentration, operating temperature, pH, flow or stagnant conditions, welding requirements, material test certificate (MTC), heat-number traceability, and any corrosion testing required. Do not write only “6Mo stainless steel,” because S31254 and N08367 are not a single UNS grade, and the two must be distinguished in the specification.

Where corrosion testing is required, it should be defined against the real service fluid and conditions. For critical or high-risk applications, a materials engineer may recommend specific pitting, crevice, or SCC testing to confirm the grade selection before fabrication begins.

11. Common Purchasing Mistakes

• Assuming all 6Mo grades are fully equivalent.
• Comparing only PREN.
• Assuming N08367 is always better than S31254.
• Ignoring the product standard.
• Ignoring availability.
• Comparing only material unit price.
• Ignoring welding and surface condition.
• Not providing temperature and chloride concentration.
• Treating laboratory corrosion data as a design limit.

12. FAQ

Q1: What is the difference between UNS S31254 and UNS N08367?
Both are 6Mo super-austenitic grades, but N08367 typically has higher nickel and a higher molybdenum range, giving it a potentially higher PREN.

Q2: Are S31254 and N08367 both 6Mo stainless steels?
Yes. Both are 6Mo super-austenitic stainless steels built around roughly 6% molybdenum for chloride resistance.

Q3: Which has the higher PREN, S31254 or N08367?
N08367 typically shows a higher PREN range because of its higher molybdenum and nitrogen, though the ranges overlap.

Q4: Is N08367 more corrosion resistant than S31254?
It can offer incrementally higher localized-corrosion resistance in some service, but it is not universally better in every environment.

Q5: Is S31254 equivalent to AL-6XN?
They are different UNS grades with different chemistry; S31254 is 254 SMO, while AL-6XN is N08367, and they are not exact equivalents.

Q6: Which is better for seawater, S31254 or N08367?
Both perform excellently in seawater; the choice often depends on availability, product standard, and project qualification.

Q7: Which grade is better for chloride pitting?
Both are excellent; N08367 may show a marginal edge from its higher molybdenum and nitrogen, depending on the exact chemistry.

Q8: Are S31254 and N08367 interchangeable?
Not automatically. They are different grades and should not be substituted without confirming chemistry, standards, and project requirements.

Q9: Is N08367 more expensive than S31254?
Cost depends on alloy content, form, and availability rather than grade alone; no single price relationship applies.

Q10: What should be specified when purchasing S31254 or N08367?
UNS grade, product standard, form, dimensions, surface condition, chloride concentration, temperature, pH, flow, welding requirements, MTC, and heat-number traceability.