Sulfuric Acid Corrosion Resistance: Why It Happens and Which Grades Resist It

Key Takeaways on Sulfuric-Acid Resistance

Sulfuric acid (H₂SO₄) is a reducing acid. It attacks through general, uniform corrosion, active dissolution, rather than chloride pitting.

Chromium by itself won't do the job. The Cr₂O₃ passive film only holds up under oxidizing conditions, and it dissolves away in dilute-to-mid reducing acid.

Copper, molybdenum and nickel are what restore passivity and bring down the active corrosion rate in sulfuric service. These are the real alloying levers here.

316/316L fails across most hot or mid-concentration acid conditions. From there the ladder climbs: 316L, then 904L, then Alloy 20, then Incoloy 825, then Hastelloy.

Concentration and temperature decide everything. Dilute-hot acid is the most aggressive environment you'll face, while cold concentrated acid actually passivates the surface.

What Is Sulfuric-Acid Corrosion?

Sulfuric-acid corrosion falls under general (uniform) corrosion: the acid eats away at the whole wetted surface more or less evenly, thinning the metal and showing up as a measurable corrosion rate in mm/yr or mpy. It stands as a CorrosionMechanism in its own right, the reducing-acid, active-dissolution member of the family.

This is metallurgically distinct from chloride pitting, which is localized and governed by PREN, see PREN explained. Sulfuric attack is uniform and tracks copper, molybdenum and nickel; pitting, by contrast, is localized and tracks chromium, molybdenum and nitrogen. Keeping these two apart is the single most important idea on this page. For more, see types of corrosion and the separate chloride pitting and crevice corrosion nodes.

Why Sulfuric Acid Attacks Stainless Steel: the Mechanism

Reducing vs Oxidizing Acids and the Passive Film

Stainless steel resists corrosion because of a thin passive film (Cr₂O₃) that heals itself. That film only forms and repairs when the environment stays oxidizing enough to keep chromium locked in its protective oxide state; the chemistry behind this is covered under the passive film. Nitric acid, an oxidizing acid, helps maintain the film. Sulfuric acid, a reducing acid, does the opposite.

The Active–Passive Transition

Dilute and mid-concentration sulfuric acid is reducing: it strips the passive film faster than chromium can rebuild it. The alloy then crosses the active, passive transition into the active state, where bare metal dissolves straight into the acid. Corrosion rates jump from a fraction of a mm/yr while passive to several mm/yr once active. That's the practical reason an ordinary austenitic grade can perforate in weeks. So the engineering question comes down to this: which alloy stays passive at my acid concentration and temperature?

How Alloying Elements Buy Sulfuric-Acid Resistance

Chromium lays down the film, but in a reducing acid it's copper, molybdenum and nickel that keep it passive. That's why the role of alloying elements matters more for sulfuric service than it ever does with chloride. The chemistry spells it out.

Copper (Cu): the Sulfuric-Acid Element

Copper is the key lever here. A small copper addition (roughly 1.5% in 904L, 3–4% in Alloy 20) pushes the alloy into the passive region when it's in reducing acid, and it cuts the active corrosion rate sharply. Sulfuric service calls for copper-bearing grades precisely because of this effect. It's the whole reason 904L exists.

Molybdenum (Mo)

Molybdenum widens the range of conditions over which the alloy stays passive, which helps resistance to both reducing acid and localized attack. 904L carries roughly 4.5% Mo. Incoloy 825 carries 2.5–3.5%.

Nickel (Ni) and Chromium (Cr)

High nickel content, around 25% in 904L, 38 to 46% in Incoloy 825, stabilizes the austenite, boosts general acid resistance, and raises the threshold for stress-corrosion cracking (see stress-corrosion cracking). Chromium builds the passive film, but on its own it isn't enough in reducing acid. It's necessary. It just isn't sufficient.

What Controls the Corrosion Rate: Concentration, Temperature and Contaminants

The Concentration–Temperature Map (Iso-Corrosion)

Sulfuric acid doesn't get steadily more aggressive as it gets stronger; the relationship is non-monotonic, and that's why engineers rely on an iso-corrosion diagram, which plots corrosion rate against both concentration and temperature. The real danger zone is dilute-to-mid acid at high temperature. Cold, concentrated acid, roughly 93-98%, actually passivates the surface, and it does so well enough that plants store it in plain carbon steel tanks at ambient temperature. In between those two extremes lies a band where you've got no choice but to upgrade the alloy.

Oxidizing Contaminants, Chlorides and Fluorides

Contaminants move the line. Oxidizing additions, ferric (Fe³⁺) or cupric (Cu²⁺) ions and aeration, reduce the corrosion rate because they help the film stay passive. Chlorides and fluorides increase it, because they add a pitting mechanism on top of general attack, this is where PREN becomes relevant again, and the caution that PREN ranks chloride resistance, not acid resistance, must be kept in mind.

Velocity and Impurities

High speed and entrained solids wear the film away by sheer force, and that pushes the rate up. Stagnant, aerated, contaminated acid is a different animal altogether from clean process acid. Service data beats a single chart every time.

Which Grades Resist Sulfuric Acid: the Selection Ladder

There's an orderable grade ladder for reducing-acid capability: 304 → 316/316L → 904L → Alloy 20 → Incoloy 825 → Hastelloy B/C-276. Pick the first grade that clears your concentration and temperature with margin. Then source it.

316/316L: Where It Stops

304 and 316 both lean on chromium for their protective layer, but 316 adds something the base grade doesn't have: 2–3% molybdenum. That edge costs more. Even so, the chromium-based passivity underneath still gives way once you push into hot or mid-concentration reducing acid. So the grade's real range stays narrow, limited to cold service, very dilute acid, or acid that's cold and highly concentrated at once.

Put 904L vs 316L side by side and the gap becomes obvious. In most sulfuric acid applications, 316L isn't the endpoint. It's the grade you outgrow.

904L: Copper-Bearing Super-Austenitic

904L super-austenitic (UNS N08904) is the headline sulfuric answer: its copper-bearing, ~4.5% Mo chemistry (PREN ~34) holds passivity across a broad dilute-to-mid acid window at far lower cost than nickel alloys. It conforms to ASTM A240 and is available as 904L coil, sheet and plate. If you want the metallurgy in full, read what 904L is or the wider super-austenitic stainless steel background. The common spec error is reaching for Alloy 20, 625 or titanium when 904L already covers the duty, check your acid concentration and temperature against the 904L limits before paying for an upgrade.

Alloy 20 (Carpenter 20): the Dedicated Sulfuric Alloy

Alloy 20 (N08020) was purpose-built for sulfuric acid: 3–4% copper and ~20% chromium give it a wider working window than 904L in the mid-concentration band, which is why it is a named option for acid pumps, valves and vessels.

Incoloy 825: Ni-Fe-Cr for Mixed Reducing + Oxidizing Acids

Where the stream carries both reducing and oxidizing acids, or H₂S, the Ni-Fe-Cr Incoloy 825 (N08825) is chosen over 316L or 904L because it resists reducing and oxidizing acids and conforms to NACE MR0175 / ISO 15156 for sour (H₂S) service. It conforms to ASTM B424 and is stocked as Ni-Fe-Cr alloy 825 coil, sheet and plate. Buyers routinely call it "Inconel 825", it is correctly Incoloy / Alloy 825; getting the designation right on the PO and MTC avoids receiving-inspection disputes. See what Incoloy 825 is and the broader nickel alloys context, or compare Incoloy 825 vs Inconel 625.

When You Need Hastelloy or Higher Alloys

For hot concentrated acid, severe mixed-acid streams or the worst contaminated duty, the Ni-Mo and Ni-Mo-Cr Hastelloy grades (B-2, C-276) sit at the top of the ladder. They earn their place only when the acid genuinely beats 904L, Alloy 20 and 825. Spec them in anywhere else, and you're just paying too much.

Sulfuric Service in Industry: Worked Examples

• Acid plants and alkylation, process vessels, coolers and piping handling fresh and spent sulfuric acid; typically 904L, Alloy 20 or 825 depending on concentration. See petrochemical / refinery acid units.
• Flue-gas desulfurization (FGD), scrubbers condense dilute sulfuric with chlorides; 904L is a common wetted-surface choice.
• Pickling lines, sulfuric and mixed-acid pickling tanks and spray headers.
• Fertilizer and acid regeneration, fertilizer & acid-regeneration plants push hot, mid-concentration acid through their systems, which is why Alloy 20 and 825 tend to win out here.
• Pulp & paper bleaching, pulp & paper bleaching stages mix acid with chlorides, a combination that punishes weaker alloys fast.
• Pharmaceutical, pharmaceutical and fine-chemical process trains demand both acid resistance and clean surfaces. The wider chemical / acid processing duty falls under this category as well.

How Nandini Steel Proves the Grade for Sulfuric Service

Naming the right grade is only half the job. The metal sitting in the tank has to actually be that grade. Nandini Steel is an ISO 9001:2015-certified stockist and supplier (Cert No. 1283438, Unitas Certification Services / UCS-UK, valid to 11 January 2028), and it's also a certified specialist in 904L, Alloy 28, Incoloy 800 and Hastelloy plate, exactly the super-austenitic and nickel-alloy families this acid ladder demands.

• Real buyable stock, not a textbook list. 904L (N08904) and Incoloy 825 (N08825) are held ex-stock for immediate dispatch in sheet, plate and coil, with Alloy 20 (N08020) and Hastelloy also supplied, so the grade ladder above names options you can actually ship.
• Traceability that proves the grade. An EN 10204 3.1/3.2 mill test certificate is provided with every order, see EN 10204 3.1 mill test certificate. The material is genuine mill plate from Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE. More on how we certify the grade and the mill test certificate.
• Specify with confidence. Use the grade-selector tool to match acid concentration and temperature to a grade, read selecting a grade for the medium, or ask our metallurgy desk for a recommendation before you buy.

Over 20-plus years since 2003, Nandini Steel has delivered 307+ projects for 260+ clients across 36+ export countries. Pricing runs by RFQ; contact us for live quotes.

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