A2 vs A4 Stainless Steel: Which Grade Do You Actually Need?

Walk into any hardware section, look at stainless steel nuts and bolts, and you'll see the codes: A2, A2-70, A4, A4-80, 304, 316. Confusing, right? Even experienced tradespeople sometimes buy the wrong grade and end up with rusty fasteners a year later.

This guide cuts through the jargon. You'll learn exactly when to use A2, when you need to upgrade to A4, and when either will fail you and you need something different entirely.

What Do A2 and A4 Actually Mean?

The A code comes from European standard EN ISO 3506 which classifies stainless steel fasteners by their corrosion resistance and mechanical properties.

• A2 = Austenitic stainless steel, most common grade (equivalent to AISI 304)
• A4 = Austenitic stainless steel with added molybdenum (equivalent to AISI 316)

The number after the dash (A2-70, A4-80) refers to the mechanical strength grade:

• -70 = Standard cold-worked strength
• -80 = High-strength cold-worked

For most projects the strength grade doesn't matter — corrosion resistance does. So the focus of this article is A2 vs A4.

What's Physically Different Between Them?

Both are austenitic stainless steels — non-magnetic, corrosion-resistant, work-hardenable. The difference is chemistry:

That 2-3% molybdenum in A4 is the game-changer. Molybdenum dramatically improves resistance to chloride corrosion — the type of attack that happens in salt water, salt spray, swimming pools, and industrial cleaning environments.

When A2 Stainless Steel Is Enough (90% of Uses)

A2 stainless is genuinely excellent for most applications. Don't waste money on A4 if you don't need it. A2 works fine for:

Indoor Use

• Kitchen fittings
• Furniture assembly
• Bathroom fixtures (unless salt-air coastal)
• Interior joinery
• General DIY projects

Sheltered Outdoor Use

• Garden furniture (inland UK)
• Fence and gate hardware
• Sheds and outbuildings
• Garden machinery
• Standard automotive external fasteners

Non-Marine Wet Environments

• Roof fittings
• Guttering hardware
• Chimney fastenings
• Rainwater exposed steel work

For these applications, A4 is overkill — you'll pay 40-60% more for no visible benefit.

When You MUST Use A4 Stainless Steel

A2 will fail in these environments — often within 6-24 months. Use A4 (316) instead:

Direct Marine Contact

• Boat fittings above and below waterline
• Marina and harbour installations
• Marine hardware
• Anything regularly wetted by seawater

Coastal Environments (Within 5 Miles of the Sea)

• Coastal properties in the UK — salt spray reaches inland
• Cornwall, Devon, Kent coast, Norfolk coast, Scotland coasts
• Even inland from the sea if there's regular salt-laden wind

Swimming Pool Environments

• Pool hardware
• Pool enclosures
• Poolside furniture
• Chlorine attacks A2 steel

Chemical or Industrial

• Chemical processing plants
• Food industry (where chlorine cleaners are used)
• Bleach or acid exposure
• De-icing salt environments (roadside installations)

Wastewater and Sewage

• Drainage systems
• Grease traps
• Waste treatment facilities

The Simple Test: Do I Need A4?

Ask yourself this one question:

"Will this fastener be regularly exposed to salt, chlorine, or aggressive chemicals?"

• Yes → Use A4 (316)
• No → A2 (304) is fine
• Not sure but the environment is important (safety-critical or expensive to replace) → Use A4, the extra cost is cheap insurance

What About "Marine Grade" Labels?

You'll often see products advertised as "marine grade stainless." This term is not regulated. It usually means A4/316 but not always.

Only trust the actual grade code (A2, A4, 304, 316) — not marketing terms.

At SpringFix UK, we always state the exact grade because it matters.

Cost Comparison

At the same size and quantity, A4 (316) costs roughly 40-60% more than A2 (304) — because of the added molybdenum and higher nickel content.

For a typical bag of 10 M8 hex nuts:

• A2 (304) — £5.29
• A4 (316) — £7.99-£9.49 (estimated based on typical UK pricing)

Multiply across a whole project and the difference adds up. But if the environment demands A4, the alternative — replacing rusted A2 fasteners every 12-18 months — costs far more in time, labour, and reputation.

Common Mistakes to Avoid

1. Assuming All Stainless Steel Is Rustproof

No stainless steel is fully rustproof. Both A2 and A4 will corrode eventually in aggressive environments — just at different rates.

2. Mixing Stainless Grades

Using an A2 nut with an A4 bolt creates a small galvanic cell that speeds up corrosion at the joint. Always match grades.

3. Mixing Stainless with Regular Steel

The regular steel will corrode dramatically fast when in contact with stainless. Use stainless-only fasteners in stainless installations.

4. Believing Zinc-Plated Is Similar

Zinc-plated steel is NOT stainless steel. It's mild steel with a zinc coating. Coating wears off. Underneath, it rusts. Zinc-plated is fine for sheltered indoor use — not for outdoor, marine, or wet applications.

5. Buying No-Grade Chinese Imports

Cheap "stainless steel" fasteners from unreliable sources often don't meet A2 or A4 standards. They may be 201 grade (much lower corrosion resistance). Always buy from suppliers who declare the actual grade.

What About A5? A6? A7?

You might see A5, A6, or A7 codes:

• A5 — Same as A4 but with lower carbon (A5-70 for welding without carbide precipitation)
• A6 — Nickel-based, higher performance than A4
• A7 — Rare high-performance grade

For 99% of applications, you only need to know A2 and A4.

What Grade Does DIN Specify?

DIN standards (like DIN 934 for hex nuts) specify the shape and dimensions — not the material. You can have DIN 934 hex nuts in A2, A4, brass, mild steel, or any other material.

That's why SpringFix UK product listings always state both:

• DIN standard (dimensional compliance)
• Material grade (corrosion resistance)

For example: "M8 Hex Nut A2 304 DIN 934" — this means M8 size, A2 grade material, DIN 934 dimensional standard.

FAQs

Is A4 twice as corrosion resistant as A2? Not exactly quantifiable, but A4 lasts 5-10× longer than A2 in chloride environments.

Can I use A4 indoors where A2 is enough? Yes — no harm done. Just paying more than needed.

Do stainless bolts need a stainless washer? For best corrosion resistance and to avoid galvanic issues, yes. Match materials throughout.

Will A4 stainless attract magnets? Cold-worked A4 can be slightly magnetic due to work hardening. Fresh A4 is non-magnetic.

Does salt water damage A4 too? Very slowly over decades, yes. For truly extreme applications, upgrade to duplex or super-duplex stainless steels.

Are 304 and A2 exactly the same? Yes — they're the same grade under different naming conventions (US AISI 304, EU A2).

Do stainless nuts work with zinc-plated bolts? Yes, but not ideal. The zinc plating will corrode away where it touches the stainless. Match materials for longest life.

Where should I buy A4 stainless fasteners in the UK? From suppliers who declare the exact grade (A4-70 or A4-80) and DIN standard. Avoid vague "stainless" labelling.

Shop Stainless Steel Fasteners at SpringFix UK

We stock A2 304 grade stainless steel fasteners for general UK use:

• Stainless Steel Fasteners Collection →
• M6, M8, M10 hex nuts to DIN 934
• M6, M8, M10 nyloc lock nuts to DIN 985
• A2 304 grade — perfect for indoor, general outdoor, and sheltered marine use

Also see our related products:

• Stainless Steel Ball Studs (INX-BS3-13) →
• Stainless Steel Mounting Brackets →

Need A4 316 marine grade specifically? Contact us — we can source A4 grade fasteners for direct marine applications on request.

Same-day dispatch on orders placed before 4pm from UK stock via Royal Mail, Yodel, UPS, or DPD.

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Article by SpringFix UK — Engineering fasteners, gas struts, and precision hardware for UK trade and DIY.