The rule that governs this page: match the grade to the acid, not to the plant. Fertilizer "acid service" isn't one corrosion problem but three. So there's no single right alloy. There's a ladder, and each rung is justified by chemistry.
What "Fertilizer Acid Service" Means Metallurgically
Fertilizer production runs on mineral acids. Sulfuric acid (H₂SO₄) digests phosphate rock, and that reaction produces wet-process phosphoric acid (WPA, H₃PO₄). Nitric acid (HNO₃) feeds the ammonium nitrate and nitrophosphate routes. None of these acids shows up clean. Phosphate rock carries chlorides and fluorides, and WPA ends up as a slurry of acid, gypsum, silica and dissolved halides, all of it running hot. That mix of a reducing acid, aggressive anions and high temperature is exactly what breaks commodity stainless. So the question this hub answers is concrete: for a given acid, concentration, temperature and halide load, which corrosion-resistant grade and form should you specify, and who ships it with a certificate? Several recognized forms of corrosion are at work here, not just one failure mode.
The Three Acids and What Each Attacks
Acids split into two families: reducing and oxidizing. That distinction decides the alloy.
Sulfuric acid is a reducing acid, and its aggressiveness follows a V-shaped curve against concentration. Dilute and mid-range sulfuric do the most damage to stainless, while very concentrated acid actually passivates steel. You buy resistance here with molybdenum and copper, and that's exactly why sulfuric-acid resistance tracks copper content.
Phosphoric acid (WPA) is reducing too, but milder than sulfuric at the same concentration. Add the fluorides and chlorides from the wet process, though, and localized attack takes over. Hot concentrated WPA is the signature fertilizer duty, and it's the one most often under-specified.
Nitric acid, by contrast, is an oxidizing acid. It doesn't behave the same way at all. It favors high-chromium austenitics, and rather than pitting, it can cause intergranular grooving on sensitized weld zones.
Across all three, you'll see the same threats show up: general corrosion, pitting, crevice corrosion under gaskets and at overlaps, chloride stress-corrosion cracking, and intergranular corrosion driven by sensitization of the heat-affected zone.
Why 316/316L Is the Baseline That Fails
316L is the go-to choice, and for dry or mildly corrosive duty it's the right call. In hot fertilizer acid, though, it's the baseline that fails. Its 2–3% molybdenum puts PREN at only around 24, so chlorides pit it. Being austenitic, it's susceptible to chloride-SCC above roughly 60 °C. And its copper-free chemistry leaves it with poor resistance to reducing sulfuric and fluoride-bearing phosphoric acid. A costly mistake we see often on incoming inquiries is 316 ordered when 904L is genuinely what WPA calls for. Stepping up at the specification stage costs far less than a bored-out heat exchanger later. See 316L for where it does belong, and pitting corrosion for the mechanism behind the failure. Once chlorides are in the acid, 904L is the choice over 316L.
Grade-to-Acid Selection Map
This is the heart of the page: the acid duty sits on the left, then the baseline that fails, the grade you should specify, and the next rung up for extreme service. Concentrations and temperatures are indicative only. Confirm against Nickel Institute / NACE isocorrosion data for your exact stream.
| Acid duty | Typical service | Baseline (fails) | Recommended grade | Step-up for severe duty |
|---|---|---|---|---|
| Dilute / mid sulfuric acid | Digestion, pickling liquors, scrubbers | 316/316L | 904L | Alloy 20 / 254 SMO |
| Concentrated sulfuric acid | Acid coolers, storage, strong-acid lines | 316/316L | 904L | Alloy 20 / titanium |
| Wet-process phosphoric acid (WPA), dilute | Reaction, filtration, clarification | 316/316L | 904L | Incoloy 825 |
| Hot concentrated WPA (fluoride + chloride) | Evaporators, heat exchangers, concentrators | 904L (marginal) | Incoloy 825 | Alloy 20 / 254 SMO / titanium |
| Nitric acid (oxidizing) | Ammonium-nitrate / nitrophosphate plant | 316 (IGC at welds) | 304L / 321 / 347 | Incoloy 825 |
| Mixed reducing + oxidizing / sour (H₂S) streams | Acid regeneration, sour process lines | 904L | Incoloy 825 | Hastelloy C-276 / C-22 |
| Vessel shells, acid-storage tanks (non-wetted / clad backing) | Pressure vessels, reactor shells, tanks | – | A516 Gr 70 (bare or clad) | SS/Ni-alloy clad or solid |
904L for Sulfuric & Phosphoric Acid Service
904L (UNS N08904, 1.4539) is a super-austenitic stainless with roughly 25% nickel, about 4.5% molybdenum, and the detail that actually matters here: close to 1.5% copper. That copper is what gives 904L its resistance to reducing acids. It suppresses active corrosion in sulfuric and phosphoric acid, exactly where molybdenum on its own falls short. A PREN near 34 lets it resist chloride pitting and crevice corrosion well past what 316L can manage, and because it carries more alloy content overall, it beats 316L on chloride stress-corrosion cracking too. For dilute-to-mid sulfuric, dilute WPA, and chloride-bearing acid service, 904L does the heavy lifting. There's a spec error that runs the opposite direction just as often: engineers reach for Alloy 20, 625, or titanium when 904L's chemistry already handles the job for less money. Check the acid concentration and temperature against the 904L isocorrosion limits before you pay for that upgrade.
Nandini holds third-party certification for this line. Genuine mill plate comes straight from Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE, and it's kept ex-stock for immediate dispatch across 904L plate, 904L sheet and 904L coil. For the full attribute set, look at the 904L super-austenitic stainless page. Compare grades directly with 904L vs 316L, or read the background on super-austenitic stainless. Welding 904L calls for over-alloyed filler, either ER385 or a nickel-base consumable; the full procedure is in how to weld 904L.
Incoloy 825 for Mixed Reducing/Oxidizing & Sour Acid Streams
Where the stream mixes reducing and oxidizing acids, runs hotter than 904L's comfort zone, or turns sour with H₂S, the answer steps up to a nickel alloy. Incoloy 825 (UNS N08825) is a Ni-Fe-Cr alloy, 38–46% nickel, 19.5–23.5% chromium, 2.5–3.5% molybdenum, 1.5–3% copper, titanium-stabilized. That composition resists both reducing and oxidizing acids (the copper and molybdenum cover the reducing side, the chromium the oxidizing side), while the titanium stabilization guards against sensitization the way the "L" grades do. Its high nickel also makes it resistant to sour service, and it conforms to NACE MR0175 / ISO 15156 when the heat is supplied in the qualified condition, so cite that on the purchase order for sour service. One receiving-inspection note: buyers routinely call this "Inconel 825." It is correctly Incoloy / Alloy 825, getting the designation right on the PO and MTC avoids disputes.
Nandini supplies Incoloy 825 (N08825) as plate, sheet and coil, with the NACE MR0175 / ISO 15156 condition called out where required. Compare the alloy against its neighbor on Incoloy 825 vs Inconel 625, read the fundamentals of nickel alloys, and see the joining practice in how to weld Incoloy 825, it is welded with ERNiFeCr-1 (FM 65).
A516 Gr 70 for Vessels, Storage & Structural Shells
Reactor shells, acid-storage tanks and pressure vessels don't all come into contact with acid. The corrosion barrier is a liner, clad layer or coating; the shell's job is strength and toughness, nothing more. A516 Gr 70 fills that structural role as the pressure-vessel-quality (PVQ) carbon steel: a fine-grain, carbon-manganese killed steel that conforms to ASTM A516 / ASME SA-516 (with plate quality to ASTM A20). Plants use it bare for non-aggressive structural and storage duty, and as the backing plate for clad or lined acid vessels.
Two things buyers tend to under-specify. First, impact testing: for low-temperature vessels, order normalized plate with Charpy values at the design temperature, not as-rolled material. Second, sour service, here the HIC-tested (NACE TM0284) variant is required, not plain Gr 70. State the plate thickness up front. Mechanical minimums and the normalizing requirement both shift with section size.
See A516 Gr 70 plate for grades, sizes, and the NACE-HIC and normalized options. For how PVQ plate gets specified, check out pressure-vessel steel.
The Upgrade Ladder: PREN & When to Step Up
Pitting resistance equivalent number, PREN = Cr + 3.3 × Mo + 16 × N, ranks the stainless and nickel grades by how well they shrug off chloride pitting and crevice attack. It's the single most useful figure for ranking fertilizer-acid candidates. What it doesn't capture is reducing-acid or sour-service behavior, and that gap explains why the nickel alloys land where they do.
| Grade | Family | PREN (approx.) | Choose when |
|---|---|---|---|
| 304 / 304L | Austenitic SS | ~18 | Dry / non-chloride duty only, not acid service |
| 316 / 316L | Austenitic SS | ~24 | Mild chloride duty; baseline that fails in hot acid |
| Duplex 2205 | Duplex SS | ~35 | Chloride-SCC resistance with higher strength (alternative) |
| 904L | Super-austenitic SS | ~34 | Reducing sulfuric / phosphoric acid with chlorides |
| Incoloy 825 | Ni-Fe-Cr alloy | ~31–40 | Mixed reducing + oxidizing, hot, or sour acid streams |
| Alloy 20 / 254 SMO / titanium | Super-austenitic / Ti | ~40+ | Concentrated hot WPA beyond 904L / 825 limits |
Step up a rung when concentration rises, temperature climbs, or halide load increases. Use the PREN method to rank candidates, then run the grade selector to filter by acid, temperature and chloride level. Want a structured walk-through instead? See how to select a stainless grade.
Composition Snapshot of the Focus Grades
Key drivers only. Full chemistries live on each grade's money page.
| Grade | Cr % | Ni % | Mo % | Cu % | Note |
|---|---|---|---|---|---|
| 316L | 16–18 | 10–14 | 2–3 | – | C ≤ 0.03% to resist sensitization |
| 904L | 19–23 | 23–28 | 4–5 | 1–2 | Copper drives reducing-acid resistance |
| Incoloy 825 | 19.5–23.5 | 38–46 | 2.5–3.5 | 1.5–3 | Titanium-stabilized; reducing + oxidizing acids |
| A516 Gr 70 | – | – | – | – | C-Mn killed, fine-grain PVQ carbon steel |
Forms, Standards & Conformance for Acid-Service Supply
Acid-service procurement comes down to standards just as much as metallurgy. The table maps each focus grade to the forms we hold and the specs they ship against.
| Grade | Forms available | Standard | Typical MTC | Sour service |
|---|---|---|---|---|
| 904L / 316L | Sheet, plate, coil | ASTM A240 / ASME SA-240 | EN 10204 3.1 (3.2 on request) | – |
| Incoloy 825 | Sheet, plate (coil limited) | ASTM B424 / ASME SB-424 | EN 10204 3.1 (3.2 on request) | NACE MR0175 / ISO 15156 |
| A516 Gr 70 | Plate (primary) | ASTM A516 / ASME SA-516 + ASTM A20 | EN 10204 3.1 (3.2 on request) | HIC per NACE TM0284 (variant) |
Start with the specs that actually govern the material. ASTM A240 and ASME SA-240 cover stainless flat product, while ASTM B424 applies to Incoloy 825. Pressure vessel plate falls under ASTM A516. Sour service work runs through NACE MR0175 / ISO 15156 instead. To match designations across systems, there's the UNS / Werkstoff cross-reference. For the forms themselves, the sheet vs plate vs coil guide lays out where each one gets specified.
Testing, Traceability & Certification
Acid-service material is only as good as its paperwork. Every order from Nandini ships with an EN 10204 3.1/3.2 mill test certificate, tying the plate to its heat and cast with full chemistry and mechanicals. Beyond the MTC, acid-service orders often call for positive material identification (PMI) to confirm the alloy at receiving. They also call for intergranular corrosion testing to ASTM A262 to prove the grade isn't sensitized, hydrostatic or ultrasonic testing for vessel plate, and NACE MR0175 hardness / HIC verification where the stream is sour. We supply genuine mill material with its original certification intact. No re-stamping.
Our quality system carries ISO 9001:2015 certification (Certificate No. 1283438, Unitas Certification Services UCS-UK, valid to 11 January 2028). For details, check the mill test certificate standard, often just called the MTC, along with our mill-test-certificate practice. The quality-assurance program covers the rest, and you'll find every credential listed under certifications.
Fertilizer & Acid-Service Applications We Supply
Each application points to the grade that fits its particular acid duty.
Wet-process phosphoric acid plants: reactors, filters, agitators and heat exchangers go in 904L for dilute WPA, with Incoloy 825 handling the hot, concentrated, fluoride-bearing streams.
Sulfuric acid plants: acid coolers, storage and strong-acid piping run on 904L, stepping up to Alloy 20 once concentrations get most aggressive.
- DAP / MAP / NPK and superphosphate units, slurry and acid-contact parts in 904L and Incoloy 825.
- Ammonium nitrate and urea / nitric-acid service, oxidizing-acid equipment in 304L, 321, 347 or Incoloy 825.
- Acid-regeneration and mixed / sour streams, process lines and vessels in Incoloy 825 under NACE MR0175.
- Pressure vessels, reactor shells and acid-storage tanks, A516 Gr 70 plate, bare or as clad/lined backing.
All focus grades sit ex-stock, ready to ship right away. Nandini Steel's been at this since 2003, more than 20 years now, with 307+ projects completed, 260+ clients served and exports reaching over 36 countries. That track record backs every shipment of acid-service plate, sheet and coil the company sends out worldwide.
Related Industries
Fertilizer acid service draws on the same corrosion engineering we apply across the neighboring process industries we serve: chemical processing, petrochemical, and oil and gas.
Frequently Asked Questions
Which stainless steel is best for phosphoric acid service?
For wet-process phosphoric acid that's hot, concentrated, and carries fluoride, 904L isn't enough. You need to step up, to Incoloy 825, or, at the far end, to Alloy 20, 254 SMO, or titanium.
Is 316L suitable for fertilizer acid service?
What is the difference between 904L and Incoloy 825 for acid service?
Why specify Incoloy 825 instead of 316L for sour acid streams?
What certification comes with acid-service steel from Nandini?
Do you stock 904L and Incoloy 825 plate ready for dispatch?
Request a Quote for Acid-Service Steel
Reach out with the acid type, concentration, temperature and chloride/fluoride load, along with grade, form and thickness, and we'll recommend the right fit and give you live pricing. Contact the Nandini Steel team at reachus@nandinisteel.com, +91 22 43404600, or WhatsApp +91 99202 09184, or use the contact page. Live pricing runs through RFQ; see about Nandini Steel. GSTIN 27ADJPA7240C1Z0.