Material SelectionBy Ningchuan Engineering Team·November 2, 2026
Almost every stainless steel housing enquiry we receive starts with the same question: 304 or 316L? The price gap is typically 25-40 percent, so choosing wrong costs you real money - either overpaying on every vessel, or replacing pitted housings two years in. These are the decision rules we use when quoting.
The short answer
- SS304 is enough for clean municipal or well water, low chloride (below roughly 200 mg/L), ambient temperature, and no sanitary polish requirement.
- SS316L is required when chlorides are high, when the water is chlorinated or ozonated, when CIP chemicals are used, when the vessel sees RO concentrate or seawater, and for pharmaceutical or food-grade sanitary service.
What actually differs: molybdenum, not strength
Both grades are austenitic stainless steels with similar tensile strength - you are not buying a stronger vessel with 316L. The difference is chemistry:
- SS304: 18 percent chromium, 8 percent nickel. Forms a passive oxide layer that resists fresh water and mild oxidising environments.
- SS316L: 16-18 percent chromium, 10-14 percent nickel, plus 2-3 percent molybdenum. Molybdenum is what resists chloride pitting and crevice corrosion - the failure mode that kills 304 housings in coastal or high-chloride plants.
- The L means low carbon (0.03 percent maximum). Low carbon prevents chromium carbide precipitation in the weld heat-affected zone, which is where intergranular corrosion starts. On a welded pressure vessel, 316L is what you want - not plain 316.
Choose SS304 when
- Feed is municipal, well or surface water with chloride comfortably under 200 mg/L.
- Water temperature stays near ambient and there is no continuous chlorination downstream.
- Duty is general industrial prefiltration, where a cosmetic finish is acceptable.
- Budget matters and the vessel is not in a corrosion-prone location.
Choose SS316L when
- Chloride above roughly 200-250 mg/L, or any seawater, brackish water or coastal installation. Pitting risk rises sharply with chloride and temperature together.
- RO concentrate service. Reject streams concentrate chloride several-fold - a 300 mg/L feed can reach well over 1,000 mg/L in the concentrate line. This is the most common 304 mistake we see.
- Chlorinated, ozonated or chemically dosed water, and any vessel cleaned with caustic or acid CIP chemicals.
- Pharmaceutical, biotech, food and beverage sanitary duty, where internal polish to Ra 0.8 um or better, crevice-free welds and passivation documentation are specified. See our pharmaceutical water treatment equipment and food and beverage filtration systems.
- High ambient humidity, salt-laden air, or washdown with chlorinated water on the outside of the vessel.
Side-by-side decision table
| Duty | Grade | Why |
|---|
| Municipal water, chloride below 200 mg/L | SS304 | Passive layer holds, no pitting risk |
| Well water with iron and hardness | SS304 | No aggressive chloride or chemical |
| RO concentrate or reject line | SS316L | Chloride concentrated 3-5 times |
| Brackish or seawater feed | SS316L (or duplex) | High chloride, high pitting risk |
| Chlorinated or ozonated process water | SS316L | Oxidising environment attacks 304 |
| Caustic or acid CIP cleaning | SS316L | Chemical resistance plus weld integrity |
| Pharma PW / WFI, food and dairy | SS316L, Ra 0.8 um or finer | Sanitary polish and cleanability |
| Coastal site, salt air, washdown | SS316L | External chloride exposure |
Finish matters as much as grade
A 316L vessel with a rough, unpassivated weld will still pit. For sanitary duty we specify internal polish to Ra 0.8 um (0.4 um for pharma), full penetration welds with no crevices, and pickling and passivation after welding to restore the chromium oxide layer. For industrial duty a 2B mill finish is fine - spending on polish there buys nothing.
Ask for the mill certificate (EN 10204 3.1) with the heat number, so the grade on the nameplate can be traced back to the plate that was actually used. It costs nothing and settles disputes later.
Specification checklist for your purchase order
- Grade and standard: SS304, or SS316L to ASTM A240 / EN 1.4404.
- Wall thickness and design pressure, with hydrostatic test at 1.5 times design.
- Internal and external finish (Ra value, or 2B mill).
- Weld procedure, and pickling plus passivation requirement.
- Connection type and standard: tri-clamp, DIN, flange ANSI or JIS.
- Documentation: material certificate, pressure test report, surface roughness report.
Where this applies on our range
The same rule covers every wetted vessel we build. On a tight budget, specify 304 where the water is clean and 316L only on the concentrate and sanitary lines - mixing grades by duty is normal engineering practice, not a compromise. See our SS304 and SS316L membrane housings, precision cartridge filter housings, bag filter housings, sterile water tanks and sintered stainless steel cartridges - all available in either grade.
Send us your water analysis (chloride, TDS, temperature) and the duty, and we will confirm the grade, finish and pressure rating in the quotation.
Related products: SS304 / SS316L membrane housings · precision cartridge filter housings · stainless steel filter housing manufacturer · sanitary stainless steel water tanks
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