304 vs 316 Stainless Steel: General-Purpose vs Marine-Grade Performance

Standards and Designations

304 (UNS S30400) and 316 (UNS S31600) are the two most widely specified austenitic stainless steels in the world. Both are covered for plate, sheet and strip by ASTM A240/ASME SA240, for pipe by ASTM A312, for tubing by ASTM A269, and for bars by ASTM A276. The European designations are 1.4301 and 1.4401 per EN 10088, the Japanese designations are SUS304 and SUS316 per JIS G4303-G4305, and the Chinese designations are 06Cr19Ni10 and 06Cr17Ni12Mo2 per GB/T 3280. The decisive difference is a single alloying element: molybdenum.

Chemical Composition and PREN

The composition limits below follow ASTM A240.

Element (wt %)	304 (S30400)	316 (S31600)
Carbon, max	0.08	0.08
Manganese, max	2.0	2.0
Silicon, max	0.75	0.75
Phosphorus, max	0.045	0.045
Sulfur, max	0.030	0.030
Chromium	18.0-20.0	16.0-18.0
Nickel	8.0-10.5	10.0-14.0
Molybdenum	0.10, max	2.0-3.0
Nitrogen, max	0.10	0.10

The Pitting Resistance Equivalent Number, PREN = Cr + 3.3 x Mo + 16 x N, quantifies chloride pitting resistance. With typical compositions, 304 has a PREN of about 19, while 316 reaches about 24-28 because molybdenum contributes 3.3 times its weight percent. This difference determines which environments each grade can survive.

Mechanical Properties

Minimum values for annealed plate per ASTM A240 are listed below.

Property	304	316
Tensile strength, min (MPa)	515	515
0.2% yield strength, min (MPa)	205	205
Elongation in 50 mm, min (%)	40	40

At room temperature the mechanical properties are nearly identical; the choice between 304 and 316 is almost always driven by corrosion resistance, not strength. Both grades are non-magnetic in the annealed state and become slightly magnetic after cold working.

Corrosion Resistance in Real Environments

304 resists atmospheric corrosion, fresh water, food products and mild chemicals, and it performs well in dry indoor and inland outdoor service. It is not suitable for coastal or marine environments: airborne salt and chloride-laden water cause pitting and rusting, usually first at crevices and damaged surfaces. 316 resists chloride pitting and crevice corrosion far better, and it is used for seawater heat exchangers, offshore platform hardware, coastal bridges, swimming pool equipment, and chemical processing equipment handling chloride-containing fluids. In dilute acids and alkaline solutions, 316 also outperforms 304, although neither grade is suitable for concentrated acids or hot concentrated brines; for those services, a 6% molybdenum super-austenitic grade such as UNS S31254 or a duplex grade such as UNS S32205 is specified.

Applications and Cost Considerations

304 is the default grade for cost-sensitive, non-chloride service: kitchen sinks, cookware, appliances, architectural trim, handrails, food processing equipment for non-salty products, and freshwater piping. 316 is specified wherever chloride exposure is unavoidable: marine hardware, coastal building components, pharmaceutical and biotech equipment, chemical tanks and pipelines, and heat exchangers. Because of the molybdenum content, 316 typically costs 20-30% more than 304. Both grades form and weld well; 304 has a slight advantage in deep drawing, and 316 welds may require pickling and passivation after welding to restore corrosion resistance in aggressive service.

Frequently Asked Questions

Is 316 always better than 304? Only in corrosion performance. For non-chloride service, 304 is usually the more economical choice with identical strength.

Why is 316 called marine grade? Its 2-3% molybdenum gives it the pitting resistance needed to survive saltwater and coastal atmospheres.

Does 304 rust? In coastal or chloride-laden environments, yes; 304 will pit and rust where 316 would survive.

How much more does 316 cost? Typically 20-30% more than 304, reflecting the molybdenum addition.

Are 304 and 316 magnetic? Both are essentially non-magnetic in the annealed condition; cold working can induce slight magnetism.

Which grade is used for food processing? 304 is standard for food contact in non-salty service; 316 is preferred for brines, sauces and cleaning chemicals containing chlorides.

Frequently Asked Questions

Q1. Which standards cover 304 and 316?
Both are covered for plate, sheet and strip by ASTM A240/ASME SA240, for pipe by ASTM A312, for tubing by ASTM A269 and for bars by ASTM A276; the European designations are 1.4301 and 1.4401 per EN 10088, the Japanese SUS304 and SUS316 per JIS G4303-G4305, and the Chinese 06Cr19Ni10 and 06Cr17Ni12Mo2 per GB/T 3280.

Q2. How does the composition differ?
316 carries 2.0-3.0 percent molybdenum versus 0.10 max for 304, with chromium 16.0-18.0 versus 18.0-20.0 and nickel 10.0-14.0 versus 8.0-10.5; carbon, manganese, silicon, phosphorus, sulfur and nitrogen limits are the same at 0.08, 2.0, 0.75, 0.045, 0.030 and 0.10 max respectively.

Q3. How is the pitting resistance equivalent number calculated?
PREN = Cr + 3.3 x Mo + 16 x N; with typical compositions 304 reaches about 19, while 316 reaches about 24-28 because molybdenum contributes 3.3 times its weight percent, and this difference determines which chloride environments each grade can survive.

Q4. Are the mechanical properties of 304 and 316 the same?
Yes, at room temperature they are nearly identical per ASTM A240: minimum tensile strength 515 MPa, minimum 0.2 percent yield strength 205 MPa and minimum elongation 40 percent, so the choice is almost always driven by corrosion resistance, not strength.

Q5. When should duplex or super-austenitic grades be specified instead?
For concentrated acids or hot concentrated brines, neither 304 nor 316 is suitable; a 6 percent molybdenum super-austenitic grade such as UNS S31254 or a duplex grade such as UNS S32205 is specified, and 316 typically costs 20-30 percent more than 304 because of its molybdenum content.