304 vs 316 Stainless Steel Bolts: Which One Should You Specify?

Ask a fastener supplier for “stainless steel bolts” and you will usually be quoted 304 — the default austenitic grade. But in coastal, marine, chemical or food-processing environments, specifying 316 can be the difference between a joint that lasts twenty years and one that seizes, pits and fails within months.

This guide explains the real difference between 304 and 316 stainless steel fasteners, how to read the A2/A4 markings on the head, and how to choose the right grade for your environment.

The chemical difference: it is all about molybdenum

Both grades are austenitic stainless steels. The decisive difference is that 316 contains about 2–3% molybdenum (Mo), while 304 contains none.

Element	304 (18/8)	316
Chromium (Cr)	18.0 – 20.0%	16.0 – 18.0%
Nickel (Ni)	8.0 – 10.5%	10.0 – 14.0%
Molybdenum (Mo)	—	2.0 – 3.0%
Carbon (C) max	0.08% (304L: 0.03%)	0.08% (316L: 0.03%)

Molybdenum strengthens the protective oxide layer and gives 316 outstanding resistance to pitting and crevice corrosion in chloride environments — seawater, de-icing salts, bleach, and many industrial chemicals. This is the single most important fact in this guide.

Read the marking: A2 vs A4

Stainless steel fasteners are not marked “304” or “316” on the head. Under ISO 3506, they carry an alphanumeric code:

A2 = austenitic grade, equivalent to 304 (e.g. A2-70).
A4 = austenitic grade with molybdenum, equivalent to 316 (e.g. A4-70, A4-80).

The number after the dash is the property class in hundreds of MPa — A2-70 means a minimum tensile strength of 700 MPa, A4-80 means 800 MPa. For marine and chemical duty, specifiers usually request A4-80 for bolts and A4-70 for nuts.

Rule of thumb: if the head says A2, it is 304-class; if it says A4, it is 316-class. Never rely on colour or “shininess” — both grades look identical.

When to choose 304 (A2)

Indoor and general atmospheric use with no significant chloride exposure.
Architecture, railings, signage, furniture, electronics and light industrial equipment.
Projects where cost is a priority and the environment is mild.

304 resists atmospheric rusting well and is the most economical stainless option. For most non-coastal indoor applications it is perfectly adequate.

When to choose 316 (A4) — and pay the premium

Coastal and offshore — anything within a few kilometres of the sea, or exposed to salt spray.
Marine equipment — boat fittings, dock hardware, aquaculture and fishing gear.
Chemical and processing plants — exposure to acids, alkalis and chloride-bearing process fluids.
Road and bridge infrastructure in regions that use de-icing salt.
Food, beverage and pharmaceutical — washdown environments with chlorine-based sanitisers.

316 typically costs roughly 30–50% more than 304. Where chloride corrosion is a real risk, that premium is almost always cheaper than replacing seized or corroded fasteners later.

304 vs 316: which is stronger?

For practical purposes the mechanical properties are very similar. In the same property class (e.g. A2-70 vs A4-70) both have the same minimum tensile strength. The choice between them is driven by corrosion resistance, not strength.

If you need the highest corrosion resistance in the 316 family, specify the low-carbon 316L grade (A4 with 0.03% max carbon). The lower carbon content avoids chromium-carbide precipitation — a form of localised corrosion — after welding or service at elevated temperature. You can read more on our 316L stainless steel fasteners page, which covers highly corrosive service conditions in detail.

Three common myths about stainless fasteners

“Stainless never rusts.” Wrong. Under chloride attack, 304 can develop brown surface staining and pitting. “Stainless” means stain-resistant, not stain-proof.
“316 is magnetic, 304 is not.” Both are austenitic and normally non-magnetic, but cold working (which happens during thread rolling) can make either grade slightly magnetic. Magnet testing cannot reliably tell 304 from 316.
“Use 316 bolts with 304 structures.” Mixing is allowed mechanically, but for best galvanic behaviour keep fastener material close to the parent metal. If the structure is carbon steel, stainless fasteners (either grade) create a galvanic pair that needs consideration in wet conditions.

Selection checklist

Is the joint exposed to salt water, de-icing salt or chloride chemicals? → 316 / A4.
Is it a mild, dry indoor environment? → 304 / A2 is fine and cheaper.
Confirm the marking required (A2-70, A4-70 or A4-80) rather than just “304” or “316”.
For welded assemblies in corrosive service, specify the low-carbon L grades.
Request the material test certificate so the chemistry (especially Mo content) is verified.

Rongrun Fasteners supplies both A2 (304) and A4 (316 / 316L) stainless bolts, nuts and washers, including 316L fasteners for highly corrosive environments and other special materials. Send us your application details and drawings through the contact form — we will recommend the right grade and finish within 24 hours.