Stainless Steel 304 vs 316 — Composition, Corrosion Resistance and Marine Selection
Standards explainer compiled by cBallast. 3-page PDF; the full content is on this page.
Compiled by
cBallast
Subject
Stainless steels
Document type
Standards explainer
Reference
2026 edition
Issued
2026-10-04
Pages
3
Format
Web page and PDF
The real difference between AISI 304/A2 and 316/A4 stainless: composition, PREN, where each survives, when 316L is mandatory, duplex upgrades, and the equivalent designations across EN, ASTM and fastener classes.
Composition and designations
Grade
EN
Fastener class
Cr %
Ni %
Mo %
PREN≈
304
1.4301
A2
18–19.5
8–10.5
—
18–20
304L
1.4307
A2 (L)
17.5–19.5
8–10.5
—
18–20
316
1.4401
A4
16.5–18.5
10–13
2.0–2.5
24–26
316L
1.4404
A4 (L)
16.5–18.5
10–13
2.0–2.5
24–26
Duplex 2205
1.4462
—
22–23
4.5–6.5
3.0–3.5
34–36
PREN (= %Cr + 3.3 × %Mo + 16 × %N) ranks resistance to pitting in chloride media: 304 ≈ 19, 316 ≈ 25, duplex 2205 ≈ 35. The molybdenum in 316 is the whole story — it slows chloride pitting and crevice attack that 304 cannot resist.
Where each grade survives
304 / A2: dry interiors, food plants, fresh water, inland atmospheres. On a ship it belongs inside — accommodation, dry stores. In coastal air it tea-stains; in seawater it pits.
316 / A4: splash zones, deck hardware, railing, fasteners outdoors near the sea — the default marine grade (see A2 bolt ranges for interior work and 316L/A4-80 jack bolts for cargo-area duty, plus 316 valves).
316L: the low-carbon (≤0.03 % C) variant, mandatory where parts are welded and then exposed to corrosives — standard 316's carbon can sensitise the heat-affected zone and invite intergranular attack.
Not even 316: permanently immersed seawater at low flow, heated chloride media and tight crevices (under washers, in rope terminals) exceed 316's limits — that is duplex, super-duplex or 6Mo territory.
Practical selection notes
Strength: A2-70 and A4-70 are mechanically identical (700 N/mm² tensile, 450 N/mm² proof) — A4 buys corrosion resistance, not strength; A4-80 adds strength (800/600 N/mm²). Magnetism is a poor field test: cold-worked 304 and 316 are both slightly magnetic. Galling: all austenitics gall — lubricate threads (see the torque chart's stainless section). Carbon steel contamination: grinding sparks and shared tooling embed free iron that rusts on the surface; pickle or passivate after fabrication. Cost difference 304→316 is typically 20–30 % — rarely worth optimising against on safety-relevant marine hardware.
Frequently asked questions
What is the main difference between 304 and 316 stainless steel?
316 contains 2–2.5 % molybdenum, which raises its pitting resistance (PREN ≈ 25 vs ≈ 19) and makes it suitable for chloride and marine exposure where 304 pits.
Are A2 and A4 bolts the same as 304 and 316?
Yes in composition terms: A2 fasteners are 304-type and A4 are 316-type austenitic stainless; the -70/-80 suffix gives the strength class.
When do I need 316L instead of 316?
Whenever the component is welded and then exposed to corrosive service: the L (≤0.03 % carbon) grade prevents sensitisation and intergranular corrosion at the weld zone.
Is 316 stainless seawater-proof?
It handles splash and spray zones well, but in stagnant or warm seawater immersion it suffers crevice and pitting attack — duplex 2205 or higher-alloyed grades are used there.
Is the magnet test reliable for identifying 304 vs 316?
No — both grades are austenitic and weakly magnetic when cold-worked; only composition (certificate or XRF analysis) separates them.