Composition of 304 and 316 Stainless Steel: What the Numbers Mean

The 18/8 Baseline: Type 304

Type 304 stainless steel is the most common and widely used stainless steel, still frequently called 18/8 because of its nominal composition of 18 percent chromium and 8 percent nickel. It is an austenitic grade with excellent deep drawing and forming properties, and its typical composition is chromium 18 to 20 percent, nickel 8 to 10.5 percent, carbon 0.08 percent maximum, manganese 2.0 percent maximum, silicon 1.0 percent maximum, phosphorus 0.045 percent maximum, sulfur 0.030 percent maximum, and iron as balance, with no intentional molybdenum.

Molybdenum Makes 316 Different

Type 316 stainless steel adds 2 to 3 percent molybdenum to the chromium-nickel base, which is the single most important difference. The composition is chromium 16 to 18 percent, nickel 10 to 14 percent, molybdenum 2 to 3 percent, carbon 0.08 percent maximum, manganese 2.0 percent maximum, silicon 1.0 percent maximum, phosphorus 0.045 percent maximum, and sulfur 0.030 percent maximum. The slightly lower chromium and higher nickel maintain a balanced austenitic structure while molybdenum strengthens resistance to localized corrosion.

Why Molybdenum Matters

Molybdenum improves the stability of the passive film and resists attack by chloride ions, which are the main cause of pitting and crevice corrosion in stainless steels. This is why 316 is the preferred choice for marine hardware, coastal architecture, and chemical processing equipment, while 304 remains excellent for atmospheric and general-purpose service.

Mechanical and Physical Notes

Both grades have a density of about 0.289 lb per cubic inch, an elastic modulus near 28,000 ksi, and thermal conductivity around 112 to 113 BTU inch per hour square foot degree F. Neither is magnetic in the annealed condition. For welded structures, the low-carbon variants 304L and 316L, with carbon limited to 0.03 percent, avoid carbide precipitation and intergranular corrosion.

Typical Applications

304 is used for food processing tanks and pipes, kitchen sinks, appliances, automotive trim, medical instruments, and chemical storage. 316 is used for marine fittings and seawater piping, oil and gas pipelines and valves, pharmaceutical equipment, production of organic acids, and turbine components where chloride resistance and high-temperature strength are required.

Selection Guidance

Select 304 for general corrosion resistance, formability, and economy. Select 316 where chlorides, seawater, or aggressive process chemicals are present, and use the low-carbon L versions for heavy welded construction to preserve corrosion resistance in the heat-affected zone.

Frequently Asked Questions

Q: Why is 304 called 18/8?

A: Its nominal composition is 18 percent chromium and 8 percent nickel.

Q: What is the key difference between 304 and 316?

A: 316 contains 2 to 3 percent molybdenum for chloride resistance.

Q: Are 304 and 316 magnetic?

A: No, both are austenitic and non-magnetic in the annealed condition.

Q: What is the carbon limit of 304 and 316?

A: 0.08 percent maximum, with 0.03 percent for the L versions.

Q: Where is 316 preferred over 304?

A: In marine, coastal, and chemical environments with chlorides.

Q: What are the L variants for?

A: They prevent sensitization and intergranular corrosion in welded structures.

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