317L vs 904L: Molybdenum Content and Acid Resistance for Severe Chemical Service

What Are 317L and 904L, and How Do They Relate to 316L?

317L (UNS S31703) is 316L with its molybdenum content raised from roughly 2.5% to roughly 3.5%, while 904L (UNS N08904) is a fundamentally different, high-alloy "super austenitic" grade that adds not just more molybdenum but also far more nickel and a deliberate copper addition - these are two distinct upgrade paths from 316L, not two points on the same scale.

317L sits inside the standard 300-series austenitic family: same basic chromium-nickel-molybdenum chemistry as 316L, just with a modest molybdenum increase to buy incremental pitting resistance. 904L is built on a different design philosophy entirely. Its nickel content (23-28%) is more than double 317L's, and its 1.00-2.00% copper addition targets a corrosion mechanism - reducing acid attack - that molybdenum alone does not address. Confusing the two as "317L, but more so" is the most common mistake in selecting between them.

Sources: ASTM A240/A240M, ASTM A312/A312M, ASTM A403/A403M standard specifications.

How Do 317L and 904L Compare in Chemical Composition?

904L differs from 317L in every major alloying element - more chromium, roughly double the nickel, more molybdenum, and a copper addition that 317L simply does not have - which is why 904L is priced and classified closer to a specialty alloy than a standard stainless upgrade.

Source: ASTM A240/A240M compositional requirements for UNS S31703 and UNS N08904.

904L's nickel content is high enough that it sits near the boundary between stainless steel and nickel-iron-chromium alloy classification - a useful signal of how much further it has moved from the 316L baseline than 317L has.

Why Does Copper in 904L Matter More Than Molybdenum for Acid Resistance?

Molybdenum and copper protect against two different corrosion mechanisms, and reducing acids like sulfuric and phosphoric acid respond mainly to copper, not molybdenum - which is why 904L's copper addition, not its molybdenum edge over 317L, is the real reason it handles acid service that 317L cannot.

Molybdenum works primarily by stabilizing the passive chromium-oxide film against chloride ions in oxidizing or neutral chloride environments - the mechanism behind PREN and behind every chloride-pitting comparison in this series. Reducing acids like dilute sulfuric acid attack stainless steel differently: they can locally destabilize the passive film through active corrosion rather than pitting, and copper's specific benefit is improving the alloy's resistance to that active corrosion in reducing conditions.

This is a well-established, decades-old metallurgical principle, which is why 904L (and other copper-bearing grades such as Alloy 20) were specifically developed for sulfuric and phosphoric acid service, while 317L - despite its higher molybdenum than 316L - was not.

317L's extra molybdenum (versus 316L) improves chloride pitting resistance incrementally, but does nothing structurally different for reducing acid resistance.

904L's copper addition specifically targets sulfuric acid, phosphoric acid, and other reducing or mixed-acid environments where molybdenum-only alloys corrode actively rather than pit.

This means PREN, which excludes copper entirely, systematically understates 904L's real-world advantage over 317L in acid process service - PREN is a chloride-pitting index, not a general acid-resistance index.

How Do 317L and 904L Compare in Chloride Pitting Resistance (PREN)?

904L has a meaningfully higher PREN than 317L - roughly 34-36 versus roughly 29-31 - so even setting acid resistance aside, 904L is also the stronger choice for straightforward chloride pitting and crevice corrosion, though the gap here is smaller than the gap in reducing-acid performance.

PREN = %Cr + (3.3 x %Mo) + (16 x %N); this formula does not include copper and therefore does not capture 904L's reducing-acid advantage. *Nitrogen is not a specified minimum under ASTM A240 for either grade; 0.05% is a representative mill value used for illustrative comparison.

If chloride pitting is the only concern - no meaningful sulfuric, phosphoric, or organic acid exposure - 317L's PREN advantage over 316L may already be sufficient, and jumping straight to 904L may be more alloy than the application requires. See EETA's 254SMO comparison for a chloride-focused upgrade path from 316L that does not carry 904L's acid-service premium.

How Do 317L and 904L Perform in Sulfuric and Phosphoric Acid Service?

904L is the clear choice for sulfuric and phosphoric acid process equipment, offering resistance that standard molybdenum-bearing grades like 317L cannot match, because copper's benefit in these environments has no equivalent in 317L's chemistry.

Acid-resistance characterizations are representative of published alloy-producer and industry data; actual performance depends on concentration, temperature, aeration, and impurity levels, and should be confirmed against corrosion-rate data or coupon testing for the specific process stream.

Neither grade should be assumed suitable for concentrated, high-temperature sulfuric acid - that service typically requires higher nickel alloys such as Alloy 20 or Alloy 926, or a full nickel-based alloy. 904L's advantage over 317L is real, but it operates within a defined concentration and temperature envelope that should be checked against isocorrosion data for the specific acid, concentration, and temperature involved.

Is Either Grade a Good Choice for Nitric Acid Service?

Not necessarily - nitric acid is an oxidizing acid governed by a different selection logic than chlorides or reducing acids, and molybdenum can be mildly detrimental rather than helpful in strong, hot nitric acid, meaning neither 317L's nor 904L's molybdenum content should be assumed to help in this specific service.

For nitric acid, high-purity, low-carbon, molybdenum-free grades such as 304L are often the traditional and code-referenced choice, because resistance in oxidizing nitric acid is governed primarily by chromium content and metallurgical cleanliness rather than by molybdenum or copper. This is a useful caution: a higher-alloy grade is not automatically the safer choice for every acid, and nitric acid service should be evaluated on its own terms rather than assumed to favor whichever grade wins on PREN or acid resistance in other media.

How Do the Mechanical Properties of 317L and 904L Compare?

904L has a modestly higher minimum yield strength than 317L, while 317L retains a somewhat higher minimum elongation - both are practical, weldable austenitic grades with no major mechanical trade-off between them.

Source: ASTM A240/A240M minimum mechanical property requirements for UNS S31703 and UNS N08904, room temperature, annealed condition.

Mechanical properties are not the deciding factor between these two grades in almost any real selection - both are comfortably adequate for typical chemical process equipment, so the choice should be made on the corrosion-mechanism grounds covered above.

How Much More Does 904L Cost Than 317L?

904L typically costs roughly 2 to 3 times more than 317L per unit weight, driven overwhelmingly by its much higher nickel content - 904L carries more than double the nickel of 317L, and nickel is the single largest cost driver in high-alloy austenitic stainless steel.

Nickel content: 904L (23-28%) versus 317L (11-15%) - more than double, and nickel is typically the most expensive major alloying element by weight in this comparison

Copper and additional molybdenum add further cost on top of the nickel premium, though nickel remains the dominant driver

Availability: 317L is more broadly stocked as a standard 300-series upgrade grade; 904L is more specialized and often mill-order for larger projects

Total cost of ownership: in genuine sulfuric or phosphoric acid service, 317L's lower purchase price is largely irrelevant if it fails prematurely - the correct cost comparison is 904L against the alternative of frequent replacement, unplanned downtime, or a nickel-alloy upgrade that costs even more

Because the cost gap is this large, 904L should be reserved for service where its specific advantages - reducing acid resistance or a PREN meaningfully above 317L's - are actually required, rather than specified as a general-purpose upgrade.

Which Industries and Applications Use Each Grade?

317L serves as a moderate step up from 316L for chloride-heavy but non-acidic service, while 904L is reserved for genuine sulfuric acid, phosphoric acid, and severe mixed-acid or chloride process environments where 317L is known to be inadequate.

When Should You Choose 317L Instead of 904L?

Choose 317L when the service is chloride-driven but not genuinely acidic, and reserve 904L for confirmed sulfuric acid, phosphoric acid, or severe mixed-acid exposure where 317L's chemistry has no specific answer.

• Process stream is chloride-bearing but not a reducing acid (sulfuric, phosphoric, or organic acid) → 317L is usually sufficient and considerably less expensive.
• Confirmed sulfuric acid, phosphoric acid, or mixed acid/chloride process stream → 904L, since copper's benefit has no equivalent in 317L.
• Budget is a binding constraint and the corrosion margin from 316L to 317L is enough for the actual service → 317L.
• Application involves nitric acid specifically → neither grade by default; evaluate a molybdenum-free grade such as 304L or consult process-specific isocorrosion data.
• Uncertain about acid exposure and the component is costly to inspect or replace → lean toward 904L or request corrosion-rate data for the specific process chemistry before committing to 317L.

As with any acid-service decision, actual concentration, temperature, aeration, and impurity levels should be checked against isocorrosion charts or coupon test data rather than relying on grade selection alone.

Frequently Asked Questions

Q: Is 904L just a stronger version of 317L?

A: No. 904L is a fundamentally different alloy with far more nickel and an added copper content that 317L does not have. The two grades address different corrosion mechanisms - 317L is closer to a moderate upgrade from 316L, while 904L is a purpose-built acid-resistant grade.

Q: Does higher PREN mean better acid resistance?

A: Not necessarily. PREN measures chloride pitting resistance based on chromium, molybdenum, and nitrogen; it does not include copper. 904L's real advantage in sulfuric and phosphoric acid service comes from its copper addition, which PREN does not capture, so PREN alone understates the practical gap between 317L and 904L in acid process service.

Q: Can 317L be used in sulfuric acid service at all?

A: Only in dilute, low-temperature, or intermittent exposure where corrosion-rate data confirms acceptable performance. For continuous sulfuric acid process equipment, 317L is generally not recommended, and 904L or a higher nickel alloy should be evaluated instead.

Q: Is 904L considered a stainless steel or a nickel alloy?

A: 904L is classified as a stainless steel (UNS N08904, under ASTM A240), but its nickel content of 23-28% places it near the boundary with nickel-iron-chromium alloys. This high nickel content is part of why it costs substantially more than standard austenitic grades like 317L.

Q: What should I use if 904L still isn't corrosion-resistant enough?

A: For concentrated, high-temperature sulfuric acid or other extremely severe conditions beyond 904L's envelope, nickel-based alloys such as Alloy 20, Alloy 825, or Alloy C-276 are typically the next step up, offering resistance that no stainless steel grade, including 904L, can match.