316 Stainless Steel: The Molybdenum-Added Corrosion-Resistant Grade

316 stainless steel is the molybdenum-bearing austenitic grade based on the 18-8 chromium-nickel system. The defining difference from 304 is the addition of 2.00-3.00% molybdenum, which strengthens the passive film and makes the grade markedly more resistant to pitting and crevice corrosion in chloride ion environments. As a result, 316 is the standard selection for marine hardware, chemical process equipment and food processing plant where 304 would be at risk of localized attack.

Chemical composition (ASTM A240, wt%)

Element	Requirement
Carbon	0.08 max
Silicon	1.00 max
Manganese	2.00 max
Phosphorus	0.045 max
Sulfur	0.030 max
Chromium	16.00 - 18.00
Nickel	10.00 - 14.00
Molybdenum	2.00 - 3.00

Mechanical properties and service temperature

For plate in the solution-annealed condition, ASTM A240 requires a minimum tensile strength of 515 MPa, a minimum yield strength of 205 MPa and a minimum elongation of 40%; the annealed Brinell hardness is typically 217 HB max. The grade retains good toughness down to cryogenic temperatures (about -196 degree C) and can be used in continuous service up to approximately 870 degree C where oxidation resistance governs.

Property	Minimum value
Tensile strength	515 MPa
Yield strength (0.2% offset)	205 MPa
Elongation	40%
Hardness	217 HB max

How molybdenum improves corrosion resistance

Molybdenum enriches the passive oxide film and suppresses the initiation and growth of metastable pits in chloride solutions. The improvement is expressed by the pitting resistance equivalent number, PREN = Cr + 3.3Mo + 16N, which for 316 is approximately 24-26 versus about 18-20 for 304. The practical result is that 316 tolerates higher chloride concentrations and temperatures before pitting and crevice corrosion initiate.

Grade variants

The family includes the low-carbon variant 316L (UNS S31603, carbon 0.030% max) for welded construction, and the high-carbon variant 316H (UNS S31609, carbon 0.04-0.10%) for elevated-temperature strength. Equivalent designations: JIS SUS316, EN 1.4401 (X5CrNiMo17-12-2), GB 06Cr17Ni12Mo2.

Typical applications

Typical applications include marine components and offshore platform equipment, chemical reactors and chloride-bearing process piping, heat exchangers and condensers, food and beverage processing equipment, desalination plant components, medical device components (the low-carbon 316L variant) and coastal architectural structures.

Frequently asked questions

Q1: What is the core difference between 316 and 304? A1: The molybdenum content. 316 contains 2.00-3.00% molybdenum, which raises pitting resistance (PREN about 24-26) well above that of 304, and 316 also has a higher nickel range of 10.00-14.00%.

Q2: Can 316 be used in seawater? A2: It is widely used in marine environments and can serve in ambient seawater service with appropriate design; for warm, stagnant or highly creviced seawater service, higher-alloyed grades should be evaluated.

Q3: Why is 316 more expensive than 304? A3: The main cost driver is alloy content: 2.00-3.00% molybdenum plus a higher nickel range (10.00-14.00% versus 8.00-10.50% for 304) increase raw material cost.

Q4: Is 316 suitable for low-temperature service? A4: Yes, the austenitic structure keeps high impact toughness down to about -196 degree C, making 316 suitable for cryogenic storage and handling equipment when welding is controlled to maintain soundness.

Q5: How is a locally corroded 316 surface repaired? A5: The affected area is ground back to sound metal, weld-repaired with matching or low-carbon molybdenum-bearing filler such as ER316L, dressed smooth, and passivated to restore the protective oxide film.