For sulfuric acid service, 904L beats 316L on every metric that matters chemically: higher chromium (19.0–23.0% vs 16.0–18.0%), higher nickel (23.0–28.0% vs 10.0–14.0%), higher molybdenum (4.0–5.0% vs 2.0–3.0%), and a deliberate 1.0–2.0% copper addition that 316L simply does not carry [S3]. That composition profile, not marketing, is what makes 904L (UNS N08904, EN 1.4539) the default austenitic pick for concentrated and hot sulfuric acid streams.
The decision is not 904L-always. 316L (UNS S31603, EN 1.4404) is still specified where sulfuric acid stays dilute (typically under ~10% concentration) and temperatures stay near ambient, because at that envelope its corrosion rate sits in the <0.1 mm/yr range and the alloy is roughly one-third to one-half the price of 904L [S3]. Above those thresholds, 316L corrosion rates climb rapidly and 904L becomes the engineering-correct call.
Composition Driver: Why the Extra Ni, Mo, and Cu Matter in H2SO4
Sulfuric acid is a reducing acid at low-to-moderate concentrations and an oxidizing acid at high concentrations, and that dual behaviour is exactly why 904L carries copper. The 1.0–2.0% Cu addition stabilises passive-film behaviour in the reducing range, where 316L's Mo-only protection falls off [S1]. The 4.0–5.0% Mo in 904L (versus 2.0–3.0% in 316L) also lifts pitting resistance, with 904L's PREN in the 33–36 band versus 316L at 24–26 [S1].
Carbon is held to 0.02% max in 904L versus 0.03% max in 316L, and nitrogen to 0.10% max in both grades [S3]. The lower carbon ceiling on 904L reduces sensitization risk during welding, which matters in acid piping where a sensitized heat-affected zone becomes a preferential corrosion path. Process engineers pairing 904L tanks with instrumentation should size the wetted-side pressure transmitter bodies in the same alloy family to avoid galvanic mismatch at flanged connections.
Sulfuric Acid Concentration and Temperature Envelope
The published guidance is consistent: 904L resists concentrated sulfuric acid far better than 316L, which tends to corrode more quickly in that medium [S3]. Across the typical process envelope, 316L holds its own in dilute H2SO4 (under ~10%) below roughly 40°C; once concentration climbs into the 30–80% band, even at modest temperatures, 316L corrosion rates move above 0.5 mm/yr and 904L becomes the safe pick [S1][S3].
Temperature caps reinforce the choice. 904L is generally rated to a maximum service temperature of 450–500°C, whereas 316L is rated up to about 800°C, but that 316L temperature advantage is irrelevant in sulfuric acid because the corrosion mechanism, not creep, sets the upper limit well before 500°C [S3]. For H2SO4 storage and transfer lines, the working ceiling is a function of acid concentration and velocity, not the alloy's oxidation limit.
Mechanical Properties: Tensile Parity, Yield Gap
Tensile strength for both grades sits in a 520–720 MPa band, and both deliver ≥40% elongation, so forming and rolling schedules are interchangeable for most shop work [S3]. The difference shows up in yield strength: 904L at roughly 240 MPa versus 316L at 170 MPa, which gives 904L a meaningful advantage in pressurised acid piping where the higher 0.2% offset translates directly into thinner-wall design options for the same design pressure.
Hardness is a wash in the annealed condition. 904L comes in around 140–190 HB and 316L at 150–190 HB, with both capped near HRB 90–92 [S1][S3]. A related comparison worth noting for repair planning is the 8620 case-hardening versus through-hardened 4140 decision guide, which uses the same property-by-property logic when the question is component life rather than corrosion.
Where 904L Wins, Where 316L Still Makes Sense
Pick 904L when the service envelope includes any of: sulfuric acid above ~20% concentration; hot dilute H2SO4 above 60°C; chloride-bearing process streams that also see acid exposure (where the higher Mo and Ni raise pitting and crevice resistance); fertilizer, pharmaceutical, or pollution-control service where acid purity cannot tolerate Fe/Ni leaching [S1][S3]. Common installations include sulfuric acid coolers, absorption towers, storage tanks, and the wetted ends of industrial valve bodies on acid manifolds.
Pick 316L when the service is dilute acid below ~10% at ambient temperature, marine chloride exposure without strong acid, food/pharma lines where general corrosion resistance and lower cost matter more than acid-edge performance, or architectural and consumer applications where polishability and price drive the decision [S1][S3]. For seawater ballast, 316L remains the marine workhorse; 904L's extra alloy is wasted budget there.
Standards, Equivalents, and Specification Discipline
904L is dual-coded as UNS N08904 and EN 1.4539; 316L as UNS S31603 and EN 1.4404 [S3]. For ASTM A240 plate and sheet, A312 pipe, and A403 wrought fittings, both grades are routinely stocked, but 904L lead times run longer and minimum order quantities are higher. That logistics delta should be priced into project schedules, not just material cost.
Welding uses 904L filler (e.g. AWS A5.4 E385L or a 20Cr-25Ni-4.5Mo-1.5Cu type) rather than 316L filler; mixing fillers is a common field failure mode. Post-weld pickling and passivation are essential in H2SO4 service because heat tint disrupts the passive film. For instrumentation on acid lines, pressure sensor diaphragms specified in 904L or higher alloys (Alloy 20, Hastelloy) match the line piping and avoid bimetal corrosion at instrument tees.
Cost, Lead Time, and Total-Cost-of-Owness Reality Check
Raw 904L plate and pipe carry a price premium, typically 2–3x the per-kilogram cost of 316L for the same form and schedule. The economic case for 904L only closes when life-cycle cost is calculated: a 316L heat exchanger in hot concentrated H2SO4 may need replacement in 2–4 years, while 904L in the same service is sized for 15+ years. Specifiers should run the LCC on concentration and temperature, not on the per-tonne number, before defaulting to 316L to save capex. [S1]
For adjacent process equipment such as pumps, the same alloying logic extends to wet-end material selection; a flow meter liner specified in 316L for a 60% H2SO4 line will fail far sooner than one lined with 904L, Alloy 20, or PTFE, and the failure mode is usually liner attack rather than mechanism wear. The comparison with the broader 5000-series marine plate selection, where 5083-H116 versus 5083-H321 temper choice drives sensitization behaviour, follows the same logic: match the metallurgical state to the corrosion environment, not the other way round.
Decision Matrix: 904L vs 316L for Sulfuric Acid Service
Against the four criteria that drive most H2SO4 specifications, the comparison lines up as follows. (1) Sulfuric acid resistance: 904L superior across concentrated and hot-reducing ranges, 316L acceptable only in dilute ambient service [S3]. (2) Chloride pitting/crevice resistance: 904L PREN 33–36, 316L PREN 24–26 [S1]. (3) Mechanical headroom: 904L 240 MPa yield versus 316L 170 MPa yield, both ≥40% elongation [S3]. (4) Cost and lead time: 316L wins on both, typically 2–3x cheaper and stocked in more sizes [S1][S3].
The straightforward rule: if the line sees sulfuric acid above ~20% concentration, or any H2SO4 above 60°C, or acid streams with chloride contamination, specify 904L and accept the cost premium. If the service is dilute H2SO4 below ~10% at ambient temperature, with no chloride co-exposure, 316L remains the engineering-correct, budget-sensible choice. For services outside that envelope, jump past both grades to Alloy 20, Hastelloy C276, or PTFE-lined steel rather than overspec 904L.
Track these signals on the next sourcing cycle: 904L plate and pipe stock levels at major service centres (still tight in 2025–2026, with 8–14 week mill lead times on non-standard sizes), EN 1.4539 versus ASTM A240 904L dual-cert availability for European projects, and any move by Chinese mills to widen the 904L size range, which would compress the cost gap with 316L through 2026 [S1][S3].