Getting the grade wrong is the most expensive mistake in tank procurement — either you pay for alloy you did not need, or your vessel pits and fails years early. Here is how our engineers actually decide, after nearly four decades of fabricating process equipment.
SS 304 and SS 316 look identical, weigh the same, and behave the same in most mechanical respects. The difference that matters is that SS 316 contains 2–3% molybdenum and SS 304 contains none. Molybdenum is what lets stainless survive chlorides. If your process has chlorides, salt, brine or seawater anywhere in it, you need 316. If it does not, 304 will very often do the same job for less money.
~18% Cr · 8% Ni · no Mo
PREN ≈ 18–20
The default austenitic grade. Excellent for food, dairy, beverages, potable water, and mild chemical duty. Widely available, easy to fabricate and polish, and the most economical stainless choice.
Avoid when: chlorides, brines, seawater or reducing acids are present.
~16% Cr · 10% Ni · 2–3% Mo
PREN ≈ 24–26
The molybdenum addition transforms pitting and crevice corrosion resistance in chloride environments. The standard choice for chemical processing, coastal installations, brines and many pharmaceutical duties.
Worth paying for when: chlorides, salts, acids or aggressive process chemistry are involved.
316 with carbon < 0.03%
PREN ≈ 24–26
Chemically similar to 316 but with carbon held low to resist sensitisation at welds. Because tanks and vessels are heavily welded, 316L is usually the better engineering answer for fabricated equipment.
Standard for: pharmaceutical product contact, high-purity and heavily welded fabrications.
IS 2062 · SA-516 Gr 70
Not corrosion resistant
Substantially cheaper per tonne and the correct economic answer for large-volume storage of non-corrosive fluids — many oils, fuels and benign solvents — with lining or coating where needed.
Requires: a corrosion allowance, typically ~3 mm, and protective lining or coating.
| Property | SS 304 / 304L | SS 316 | SS 316L | Mild Steel |
|---|---|---|---|---|
| Chromium | ~18% | ~16% | ~16% | — |
| Nickel | ~8% | ~10% | ~10% | — |
| Molybdenum | None | 2–3% | 2–3% | — |
| Max carbon | 0.08% (0.03% for 304L) | 0.08% | 0.03% | varies |
| PREN (approx.) | 18–20 | 24–26 | 24–26 | n/a |
| Chloride / pitting resistance | Moderate | Good | Good | Poor |
| Resistance to weld sensitisation | Moderate (good in 304L) | Moderate | Best | n/a |
| Food & dairy service | Yes | Yes | Yes | No |
| Pharmaceutical product contact | Sometimes | Yes | Preferred | No |
| Seawater / coastal exposure | Not advised | Yes | Yes | No |
| Typical corrosion allowance | Usually nil | Usually nil | Usually nil | ~3 mm |
| Relative material cost | Baseline | Higher | Higher | Lowest |
| Polishability / finish | Good | Good | Excellent | Poor |
Compositions are nominal ranges for common commercial grades and vary slightly by standard (ASTM A240, EN 10088, IS 6911). PREN is calculated as %Cr + 3.3 × %Mo + 16 × %N and is a comparative ranking tool, not a design limit.
When a client sends an enquiry without specifying a grade, this is the sequence we work through. You can run it yourself before you even call us.
| Service | Usual grade | Why |
|---|---|---|
| Potable / process water storage | SS 304 | No chloride driver; 316 rarely earns its premium |
| Milk, dairy, beverage | SS 304 / 304L | Hygienic, polishable, no aggressive chemistry |
| Edible oil, fats | SS 304 | Non-corrosive; finish and cleanability are the drivers |
| Brine, salt solutions | SS 316 / 316L | Chlorides — 304 will pit |
| Coastal or marine-air installation | SS 316 / 316L | Airborne chloride attack on external surfaces |
| Pharmaceutical product contact | SS 316L | Low carbon, fine finish, heavily welded construction |
| Agrochemical & general chemical | SS 316 / 316L | Mixed and variable chemistry; specify on the fluid |
| Solvent storage (non-chlorinated) | SS 304 or MS lined | Often non-corrosive; economics usually decide |
| Bulk fuel / lube oil storage | Mild steel | Non-corrosive bulk duty; stainless earns nothing |
| Effluent / ETP holding | SS 316L or lined MS | Highly variable — needs the actual effluent analysis |
Indicative starting points only. Confirm against your actual fluid, concentration and temperature.
Use our free calculator to get gross and working volume for any cylindrical tank, including dished ends, plus an indicative plate weight.
Chloride and acid resistant tanks from 500 L to 200,000 L with full material traceability and test certificates.
The economical workhorse for food, water and general chemical storage, built to the same fabrication standard.
Molybdenum. SS 304 is roughly 18% chromium and 8% nickel with none; SS 316 is roughly 16% chromium, 10% nickel and 2–3% molybdenum. That addition substantially improves resistance to pitting and crevice corrosion in chloride environments. If your process has chlorides, salt or brine, use 316. For food, dairy, potable water and mild chemical duty, 304 usually performs identically for less.
Pitting Resistance Equivalent Number — calculated as %Cr + 3.3 × %Mo + 16 × %N. It gives a single figure for ranking resistance to chloride pitting. SS 304 sits around 18–20, SS 316 around 24–26. That gap is exactly why 316 survives duty that pits 304. Treat it as a comparative ranking, not a design limit.
The L is low carbon — below 0.03%, against up to 0.08% for standard 316. Lower carbon reduces chromium carbide formation at grain boundaries during welding (sensitisation), which would otherwise leave the zone beside the weld open to intergranular corrosion. Since tanks are heavily welded, 316L is usually the better specification for fabricated equipment.
Only if your chemistry demands it. Where chlorides, brines or aggressive acids are present, the premium is repaid many times over — a 304 tank in that duty can pit and fail years early. Where the service is food, water or mild chemicals, 316 usually buys nothing. Specifying by process chemistry rather than habit is the biggest single cost lever in tank procurement, in both directions.
For non-corrosive bulk storage — many oils, fuels and benign solvents — particularly where the tank can be lined or coated. It is far cheaper per tonne and the right commercial answer for large volumes of harmless fluid. It is unsuitable for food, pharmaceutical or clean chemical service, and needs a corrosion allowance of typically around 3 mm that stainless generally does not.
Pharmaceutical product contact is normally SS 316L — low carbon, weldable, and able to hold a fine polished finish. Food and dairy is frequently 304 or 304L where chlorides are absent, moving to 316L where brines, salt or cleaning chemistry demand it. In both, surface finish, drainability and crevice-free construction matter as much as the grade.
SS 304, SS 304L, SS 316, SS 316L and mild steel, in tanks, reactors and pressure vessels from 500 litres to 200,000 litres, at our Bhiwadi works in Rajasthan (Delhi NCR). Material test reports and full traceability documentation are supplied against every vessel.
Tell us the fluid, concentration and maximum operating temperature. We will tell you the grade — including when a cheaper one will do the job.