304 vs 316 Stainless Steel: What the Molybdenum Buys

One addition, one job

304 (EN 1.4301, 18% Cr / 8% Ni) is the general-purpose austenitic stainless. 316 (EN 1.4401) keeps the same base and adds 2–2.5% molybdenum. Molybdenum's main job is resistance to pitting and crevice corrosion in chloride-bearing environments.

Measuring the difference: PREN

The pitting resistance equivalent number, PREN = %Cr + 3.3 × %Mo + 16 × %N, gives a rough ranking:

• 304: PREN typically about 18–19.
• 316/316L: PREN typically about 23–26.

In practice, 304 pits readily in seawater and de-icing salt splash; 316 resists pitting in coastal air, food brines and many process waters, though warm stagnant seawater still attacks it under deposits and in crevices.

Where the difference does not matter

Strength, ductility, formability, weldability and heat resistance are broadly similar — neither grade is a high-temperature strength grade, and both sensitise if held in the 450–850 °C range in high-carbon form. For indoor, dry, or mildly corrosive duty, 316's molybdenum buys nothing visible.

Service	Sensible default
Indoor equipment, architecture (inland)	304
Food processing, non-brine	304 (316 for salty products)
Coastal architecture, marine hardware	316
Swimming pools, chemical dosing	316 minimum; consider duplex
Seawater immersion	Neither is ideal; duplex/super grades

Price logic

316 typically trades at a premium over 304 — commonly 20–40%, driven by molybdenum and nickel alloy surcharges. Specifying 316 everywhere "to be safe" inflates cost with no life gain in benign service; specifying 304 in chloride service saves money once and pays in pitting repairs. Match the grade to the chloride exposure, not to habit.

Details that move the boundary

A few real-world factors shift the decision more than the nominal grade does. Surface finish matters: a polished or electropolished 316 part outlasts a rough, scale-bearing 316 part, and a contaminated 316 surface with embedded free iron will rust-spot like carbon steel. Geometry matters: crevices under gaskets, deposits and lap joints trap chlorides, and crevice corrosion starts at lower temperatures than pitting — a design that drains and can be washed down protects either grade. Temperature multiplies chloride aggressiveness; warm chloride solutions that a grade tolerates cold may defeat it. Finally, the L variants (304L/316L) are the normal choice for welded construction in both families and carry the same PREN logic, since molybdenum — not carbon — drives the 304/316 difference. Where neither grade is enough in hot, stagnant seawater or strong brines, the usual next step is a duplex or super-austenitic grade rather than a thicker wall of 316.

Equivalents at a glance

304 corresponds to EN 1.4301 (X5CrNi18-10), JIS SUS304 and GB 06Cr19Ni10; 316 to EN 1.4401 (X5CrNiMo17-12-2), JIS SUS316 and GB 06Cr17Ni12Mo2. The UNS numbers S30400 and S31600 are the ones most often seen on North American certificates.