Two grades appear on almost every stainless steel enquiry: 304 and 316. Both are austenitic, easy to fabricate in common forms and corrosion-resistant in many ordinary environments. The decision is not that 316 is universally "better". It is whether the actual service conditions justify the extra alloying and its cost.
Start with the chemistry
Grade 304 is often described as 18/8 stainless steel: roughly 18% chromium and 8% nickel in a typical product. Grade 316 adds molybdenum, commonly around 2%, and typically has slightly more nickel. In Outokumpu's published examples, Core 304/4301 has typical chromium of 18.1% and nickel of 8.1%; Supra 316/4401 has 17.2% chromium, 10.1% nickel and 2.1% molybdenum. These are typical examples, not the allowed chemical limits or the measured results of any shipment. Your MTC is the record of what was supplied.
The low-carbon variants, 304L and 316L, reduce the risk of sensitization and subsequent intergranular corrosion near a weld. Do not assume that a coil, sheet or plate is dual-certified merely because a supplier says "304/304L" or "316/316L". Ask for the exact grade and certificate.
Where corrosion resistance changes the answer
304 is useful for general indoor fabrication, food equipment and many mildly corrosive conditions. Molybdenum gives 316 better resistance to localized pitting and crevice corrosion in many chloride-bearing settings. This makes 316 worth considering for coastal exposure, process equipment and certain pharmaceutical or chemical applications.
But neither grade is immune to chloride attack. Salt concentration, temperature, wet-dry cycling, crevices, cleaning agents and exposure time matter. An unwashed crevice on a seafront structure can be much harsher than a clean, freely draining surface nearby. Some hot or concentrated chloride services require a duplex or other more highly alloyed grade. Ask the project engineer to select against the actual environment rather than relying on a grade nickname.
Mechanical properties are similar, but compare the certificate
For plate, sheet and strip under the cited ASTM A240 summary, both S30400 and S31600 have minimum tensile strength of 515 MPa, minimum 0.2% yield strength of 205 MPa and minimum elongation of 40%. Those are specification minimums, not promised test values. Temper, thickness, processing and product form affect the measured material. A structural calculation should use the correct design code and allowable stresses, not this quick comparison.
When is 316 worth the premium?
Choose 316 when the environment or the project specification calls for its added resistance and suitable material is confirmed by the certificate. For ordinary indoor equipment without significant chloride exposure, 304 may meet the requirement at a lower material cost. The premium for 316 is not a fixed percentage: nickel and molybdenum markets move, and size, finish, quantity, availability and freight all affect the actual quote. Obtain like-for-like prices for the exact specification before making a commercial comparison.
For welded assemblies, decide separately whether 304L or 316L is necessary; the L suffix concerns carbon content and weld corrosion behavior, not an automatic upgrade to every property. Surface finish also matters. A smooth, cleaned, passivated fabrication may perform differently from a rough, contaminated or damaged surface of the same grade.
What to put on a purchase enquiry
State the product form (sheet, plate, coil or pipe), grade and UNS number, material and dimensional standards, thickness and dimensions, quantity, finish, tolerances, certificate type and destination. If you need 316L instead of 316, write it explicitly. Note any third-party inspection, project code or corrosion test requirement. This lets suppliers compare equivalent products and keeps grade substitutions visible.
Finally, review the MTC and the physical markings at receipt. Match the heat number to the certificate and check chemical and mechanical results against the grade's actual specification. A grade label on a quote is a starting point, not proof that the received piece matches it.
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Sources and scope
Grade figures and comparisons are drawn from Outokumpu steel grades and properties and Outokumpu Forta duplex datasheet. Confirm the current purchase standard, service design and actual MTC before ordering.