UNS N08904 Stainless Steel for Sulfuric Acid: Concentration and Temperature Limits

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

UNS N08904 stainless steel — widely known as 904L and listed as EN 1.4539 — is a high-alloy austenitic stainless steel often chosen for sulfuric acid service. The real question for a chemical-process buyer is not whether 904L is “acid-resistant,” but at which combination of acid concentration and temperature it is a sound choice, and where the conditions cross into territory that demands a higher alloy or a corrosion test.

1. What Is UNS N08904 / 904L?

UNS N08904, 904L, and EN 1.4539 refer to the same family of high-alloy austenitic stainless steel. Compared with standard grades such as 316L, it carries a higher nickel content together with deliberate molybdenum and copper additions, and these alloying elements are central to its performance in acidic environments.

Typical applications include chemical processing, sulfuric acid handling, heat exchangers, tanks, and process equipment where moderate acid concentrations and temperatures are encountered. It is a corrosion-focused grade: its value lies in resisting acid attack, not in high-temperature mechanical service.

2. Why Is 904L Resistant to Sulfuric Acid?

No single element explains 904L’s acid resistance; the grade works as a package. High nickel and the copper addition help resist general corrosion in reducing acid conditions, which are characteristic of sulfuric acid at many concentrations. Chromium contributes the passive film that protects the surface, and molybdenum adds resistance to localized forms of attack such as pitting and crevice corrosion. Together these elements give 904L a broader tolerance for acid service than lower-alloy grades.

It is therefore misleading to reduce the explanation to “molybdenum makes it acid-resistant.” The useful property is the combination of nickel, copper, molybdenum, and chromium working together, and the balance matters because each element addresses a different aspect of the corrosion mechanism.

The copper addition is particularly relevant to reducing acids such as sulfuric acid, where it helps lower the general corrosion rate, while nickel contributes to overall resistance in acid conditions. Chromium and molybdenum then protect against the localized attack mechanisms that can otherwise undermine an otherwise corrosion-resistant surface, so the four elements complement rather than duplicate one another.

3. Sulfuric Acid Concentration and Temperature: The Real Selection Boundary

Sulfuric acid does not behave as “higher concentration is always more corrosive.” Corrosion behavior shifts with the combination of concentration and temperature: dilute, intermediate, and concentrated acid can each behave differently, and a material that performs well in one region may be unsuitable in another. This is why a single “maximum concentration” or “maximum temperature” figure is not a valid selection tool.

Temperature is the other half of the boundary. As temperature rises, the acceptable operating window generally narrows, often significantly, so a condition that is mild at ambient temperature can become aggressive when heated. The useful selection boundary is therefore a concentration-temperature map, not a pair of independent limits.

Broadly, the corrosion behavior of stainless steels in sulfuric acid tends to be more severe in certain concentration ranges and milder in others, and the picture shifts with temperature. Because the relationship is not monotonic, a material that survives one concentration may not survive another at the same temperature — and vice versa — which is why concentration-temperature maps replace simple limits.

4. 904L and Isocorrosion Data

An isocorrosion diagram plots, on a concentration-versus-temperature chart, the lines of equal corrosion rate — commonly a boundary such as 0.1 mm/year. These diagrams are the standard first step in screening a material for acid service: the engineer locates the intended concentration and temperature, then checks whether the point falls on the acceptable side of the relevant isocorrosion line.

Isocorrosion data is a screening and material-selection tool, not a guarantee of equipment life. The values come from specific test conditions and do not account for impurities, chlorides, oxygenation, flow velocity, or the specific geometry of the equipment. A point that sits comfortably inside the acceptable region still deserves confirmation when the service involves these additional factors.

To use an isocorrosion chart, locate the intended concentration on one axis and the operating temperature on the other, then read whether the point falls below the chosen isocorrosion line. It is good practice to leave a margin between the operating point and the boundary, because real service adds variables the laboratory chart does not capture, and to confirm critical cases with testing or a materials engineer.

5. When 904L Is a Good Choice

904L is typically a good candidate for medium-concentration sulfuric acid at moderate temperatures — the kind of service found in chemical processing, acid storage and handling, heat exchangers, and process equipment. Within that region, its combination of nickel, copper, molybdenum, and chromium provides a meaningful advantage over lower-alloy grades.

The actual choice still depends on the full picture: temperature, concentration, impurities, flow velocity, and oxygenation all influence the real corrosion rate. A condition that is nominally within 904L’s window can be pushed out of it by an oxidizing contaminant, a chloride, or an unexpected temperature excursion, so these factors should be stated alongside the acid concentration.

Flow velocity and oxygenation are two easily overlooked factors. Stagnant or low-velocity zones can allow local chemistry differences to develop, while the presence of oxygen or other oxidizers can shift the corrosion behavior of the acid. Both should be reported to the supplier when the acid concentration and temperature are specified, because they can change the outcome more than a small change in concentration.

6. When 904L May Not Be Enough

904L is not suitable for every acid environment. Higher temperatures, aggressive concentration-temperature combinations, oxidizing contaminants, chlorides, mixed-acid conditions, or erosion from high flow can each push the service outside 904L’s useful range. In those cases a higher-alloy material — a higher-alloy stainless steel or a nickel-base alloy — may be required, but the specific alternative should be selected by a materials engineer against the actual conditions rather than by a blanket rule.

The important point is to recognize the limits. 904L is a high-performing acid-grade stainless steel, but it is not an acid-proof material, and pushing it beyond its concentration-temperature window invites premature failure.

It is also useful to distinguish general corrosion from localized corrosion. General corrosion is a relatively uniform loss of metal, while pitting, crevice corrosion, and stress-corrosion cracking are localized and can be more difficult to predict from a single corrosion rate. A grade that shows an acceptable general corrosion rate may still be vulnerable to a localized mechanism when chlorides or crevices are present.

7. 904L vs 316L for Sulfuric Acid

904L offers substantially better sulfuric acid resistance than 316L because of its higher nickel and molybdenum levels and its copper addition. This does not mean 316L is unusable in sulfuric acid — in very dilute or low-temperature service 316L may be adequate — and it does not mean 904L works in every sulfuric acid condition.

The upgrade decision should be made against the actual concentration-temperature condition and the cost of failure, not by assuming that a higher grade is always justified.

The cost comparison should be framed the same way: 904L costs more per kilogram, but in service where 316L would corrode faster and require earlier replacement or downtime, the premium can be justified. Where 316L already meets the requirement with acceptable life, upgrading to 904L adds cost without benefit.

8. How to Specify 904L for Acid Service

An acid-service RFQ should state, at minimum: UNS N08904 / 904L, the applicable product standard, product form, dimensions, heat treatment, sulfuric acid concentration, operating temperature, impurities, flow velocity where relevant, material test certificate (MTC), heat-number traceability, and any supplementary corrosion testing that is required.

Do not assign one ASTM product specification to every 904L item without confirmation. The correct standard depends on the product form — plate, sheet, strip, bar, tube, or pipe — and must be verified against the supplier’s documentation rather than assumed.

Supplementary corrosion testing may be worth specifying when the operating point is close to the isocorrosion boundary, when impurities or chlorides are present, or when the consequences of failure are severe. Testing against the actual process fluid is more representative than relying on generic corrosion tables, and it should be defined clearly in the RFQ.

9. Common Purchasing Mistakes

• Writing only “904L” without the UNS number.
• Providing acid concentration without temperature.
• Assuming 904L is safe at every sulfuric acid concentration.
• Applying a corrosion table directly without checking test conditions.
• Ignoring chlorides or other impurities.
• Treating a laboratory corrosion rate as an equipment design limit.
• Comparing only material unit price while ignoring failure and replacement cost.

10. FAQ

Q1: What is UNS N08904 / 904L?
UNS N08904, 904L, and EN 1.4539 are the same high-alloy austenitic stainless steel, with elevated nickel plus molybdenum and copper additions for acid corrosion resistance.

Q2: Is 904L resistant to sulfuric acid?
Yes, within certain concentration-temperature conditions. It is not acid-proof, and its usable window narrows as temperature rises.

Q3: What concentration of sulfuric acid can 904L handle?
There is no single concentration limit; suitability depends on the concentration-temperature combination and should be checked against isocorrosion data for the specific conditions.

Q4: What temperature can 904L handle in sulfuric acid?
Temperature cannot be given in isolation — the acceptable temperature depends on the acid concentration, and it generally falls as concentration or temperature becomes more severe.

Q5: Is 904L better than 316L for sulfuric acid?
Generally yes, because of its higher nickel, molybdenum, and copper content, but 316L may still be adequate in very dilute or low-temperature service.

Q6: Does sulfuric acid concentration affect 904L corrosion?
Yes. Corrosion behavior changes across dilute, intermediate, and concentrated regions, so concentration and temperature must be considered together.

Q7: Can 904L be used with hot sulfuric acid?
Only where the specific concentration-temperature combination is within the acceptable window; hot, aggressive conditions may require a higher-alloy material.

Q8: When is 904L not sufficient?
At higher temperatures, aggressive concentration-temperature combinations, or when oxidizing contaminants, chlorides, mixed acids, or high-velocity erosion are present.

Q9: What information should be included in an acid-service RFQ?
UNS N08904 / 904L, applicable product standard, product form, dimensions, heat treatment, acid concentration, temperature, impurities, flow velocity, MTC, and heat-number traceability.

Q10: Are sulfuric acid corrosion tables enough for material selection?
No. They are screening tools; actual selection should also account for impurities, chlorides, oxygenation, velocity, and confirmation testing where conditions are severe.